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MANGALA VALLES, MARS: ASSESSMENT OF EARLY STAGES

OF FLOODING AND DOWNSTREAM FLOOD EVOLUTION

GIL J. GHATAN and JAMES W. HEAD IIIDepartment of Geological Sciences, Brown University, 1846Providence, 02912RI, USA

(E-mail: [email protected])

LIONEL WILSONPlanetary Science Research Group, Lancaster University, Lancaster, LA1 4YQ, UK

(E-mail: [email protected])

(Received 18 March 2005; Accepted 26 May 2005)

Abstract. The Mangala Valles system is an ~900 km fluvially carved channel system located southwest of

the Tharsis rise and is unique among the martian outflow channels in that it heads at a linear fracture within

the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse channels. Mangala

Valles is confinedwithin abroad, north–south trendingdepression, andbegins as a single valleymeasuring up

to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands

toward the northern lowlands. Approximately 600 km downstream, this single valley branches intomultiple

channels, which ultimately lose their expression at the dichotomy boundary. Previous investigations of

Mangala Vallis suggested that many of the units mapped interior to the valley were depositional, related to

flooding, and that aminimumof two distinct periods of flooding separated by tens to hundreds ofmillions of

years were required to explain the observed geology. We use infrared and visible images from the THermal

EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter

(MOLA), to investigate the nature of the units mapped within Mangala Vallis. We find that the geomor-

phology of the units, as well as their topographic and geographic distribution, are consistent with most of

them originating from a single assemblage of volcanic flow deposits, once continuous with volcanic flows to

the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south trending

depression intowhichMangalaVallis formed prior to any fluvial activity. Later flooding scoured and eroded

this volcanic assemblage north of theMangala source graben, resulting in the present distribution of the units

within Mangala Vallis. Additionally, our observations suggest that a single period of catastrophic flooding,

rather than multiple periods separated by tens to hundreds of millions of years, is consistent with and can

plausibly explain the interior geology of Mangala Vallis. Further, we present a new scenario for the source

and delivery of water to the Mangala source graben that models flow of groundwater through a sub-

cryosphere aquifer andupa fracture that cracks the cryosphere and taps this aquifer. The results of ourmodel

indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have

required less than several days to fill up prior to any spill-over of water to the north. Through estimates of the

volume of material missing from Mangala (13,000–20,000 km3), and calculation of mean discharge rates

through the channel system (~5 · 106 m3 s)1), we estimate that the total duration of fluvial activity through

the Mangala Valles was 1–3 months.

Notation:

Parameter:Definition (Units);H: water head height in aquifer above base of cryosphere fracture (m);

Earth, Moon, and Planets (2005) � Springer 2005

DOI 10.1007/s11038-005-9009-y

L: lateral length scale of aquifer (m);Patm: atmospheric pressure (500) (Pa);Pb: pressure at bottom of cryosphere fracture (Pa);Pt: pressure in unsaturated portion of hydrosphere;R: hydraulic radius of water flowing in channel (m);S: sine of slope of channel floor (none);Ua: mean flow speed of water through aquifer (m s)1);Uf: mean flow speed of water through cryosphere

fracture (m s)1);Un: mean flow speed of water through notch (m s)1);X: lateral distance traveled by water flowing through

aquifer (m);DP: resistance to flow through aquifer (Pa);R: specific storage of the aquifer (m)1);fc: Darcy–Weisbach friction factor for channel flow (none);ff: friction factor for the cryosphere fracture (none);g: acceleration due to gravity (3.74) (m s)2);ha: thickness of aquifer (m);hf: vertical height of fracture/cryosphere thickness (m);k: permeability of aquifer (m2);r: fracture wall roughness scale length (0.01) (m);w: width of cryosphere fracture (m);g: viscosity of water at temperature T (Pa s);l: shear modulus of cryosphere rock (Pa);m: Poisson’s ratio of cryosphere rock (none);qw: density of liquid water (1000) (kg m)3);s: decay time of initial transient high pressure gradient (s)

1. Introduction and Background

The Mangala Valles system, an ~900 km long north–south trending outflowchannel system located southwest of the Tharsis rise in the Memnonia regionof Mars, extends northward from one of the Memnonia Fossae graben acrossthe southern highlands, and ultimately loses its expression at the dichotomyboundary (Figures 1 and 2). Analyses of Mangala extend back to the time ofMariner 9 (Milton, 1973; Sharp and Malin, 1975; Mutch and Morris, 1979),and since then they have been of great importance for those interested indeciphering the role water has played in the evolution of the martian land-scape. While some authors have suggested that liquid CO2 or CO2–H2Omixtures resulting from decomposition of clathrates could be responsible forthe erosion of the martian outflow channels (Milton, 1974; Hoffman, 2000),thermodynamic arguments continue to support a water-related origin for the

GIL J. GHATAN ET AL.

channels (Stewart and Nimmo, 2001). Viking-based analyses suggest that thewater responsible for carving Mangala was expelled from the graben at theirhead (Carr, 1981), possibly during two distinct periods of flooding, one in theLate Hesperian and one in the Latest Hesperian/Early Amazonian (Tanakaand Chapman, 1990; Chapman and Tanaka, 1993), (or Latest Hesperian/Earliest Amazonian and Early Amazonian (Zimbelman et al., 1994; Crad-dock and Greeley, 1994)) although some investigations question the need formultiple periods of flooding to explain the observed geology (Zimbelmanet al., 1992; Craddock and Greeley, 1994). The mechanism by which waterwas transported to the graben, and ultimately to the surface, remained un-clear; however two general scenarios were proposed and developed in theliterature: (1) melting of near surface ground ice due to the emplacement ofnearby Tharsis-related lava flows (Zimbelman et al., 1992), and (2) tapping ofa near surface aquifer via faulting associated with the graben (Tanaka andChapman, 1990).

In the late 1980s and early 1990s an effort was mounted to map theMangala Valles region at a scale of 1:500,000, using high-resolution Viking

Figure 1. MOC wide-angle mosaic overlain with MOLA shaded relief regional context imageof Tharsis and the Memnonia region. Region includes eastern half of MC-16 and western halfof MC-17. Mangala heads at one of the Memnonia Fossae, a graben swarm radiating

southwest area from the Tharsis rise. A series of ridges measuring tens of kilometers wide byhundreds of kilometers long parallel the margins of the Tharsis rise to the south and west,radiating southwest from the Tharsis rise area. White box denotes the study region. Image

projection is simple cylindrical for this and all later figures.

MANGALA VALLES, MARS

images (Figure 2b) (Chapman et al., 1989, 1991; Chapman and Tanaka,1993; Craddock and Greeley, 1994; Zimbelman et al., 1994). Continuing theeffort to unravel the complicated history of Mangala and to understand itsplace in the broader context of the martian hydrological cycle, here wereexamine the Mangala Valles system and the surrounding region using newdata from post-Viking spacecraft. Specifically, we utilize the topographicdata provided by MOLA (Mars Orbiter Laser Altimeter) onboard the MarsGlobal Surveyor and the high-resolution infrared and visible images obtainedby THEMIS (THermal EMission Imaging System) onboard Mars Odyssey toinvestigate the pre-flood geology of the region, the early stages of flooding atMangala and the down-stream flood evolution. Our investigation focusesprimarily on that portion of Mangala Valles south of where the systembranches downstream (Figure 2). Additionally, we present a new model forthe source and delivery of water to Mangala Valles, which is consistent withthe global hydrologic model of Clifford (1993) (updated by Clifford andParker (2001)), and which addresses specific questions raised by the modelspresented by previous investigators.

2. Description

2.1. REGIONAL GEOLOGY

The regional geology near Mangala Valles is dominated topographically,tectonically and volcanically by the presence of the Tharsis rise located to thenortheast (Figure 1). A series of ridges tens of kilometers wide by hundreds ofkilometers long is distributed tangential to the southern and western marginsof Tharsis (Scott and Tanaka, 1986). One of these ridges trends north–southalong the eastern margin of Mangala Vallis. Dated to the early to

Figure 2. (a) MOC wide-angle mosaic overlain with MOLA shaded relief overlain withMOLA color-coded topography. The source graben for Mangala has been expanded to a

trough near the head region of the channel. Mangala is confined to a broad north–southtrending depression, flanked to the east by one of the high-standing circum-Tharsis ridges, andto the west by high-standing Noachian cratered terrain. (b) Generalized geologic map of the

Mangala Valles region, generated by merging the Viking-based geologic maps of Chapmanand Tanaka (1993), Craddock and Greeley (1994), and Zimbelman et al. (1994). At the top ofthe region, north of the dichotomy boundary, are the deposits of the Medusae Fossae For-mation. To the west and north Mangala Valles is surrounded by Noachian cratered terrain (all

Npl units are merged into a single unit in this map). To the east of Mangala are Amazonian-aged Tharsis flows. South of Mangala the broad north–south depression is filled with asmooth unit mapped as a member of the plateau sequence (HNpl3). The interior of Mangala

Vallis was mapped as being composed of five different units (Hmp, AHmp2, AHmp3, AHmch,and Amch).

b


mid-Noachian (Zimbelman et al., 1994), this ridge is composed of the oldestexposed material in the region (Nplm) (Figure 2b) and was interpreted byZimbelman et al. (1992) as having been compressionally faulted into the area,and as having served to divert flooding in Mangala to the north. Along with

Figure 3. Index map of the Mangala region, highlighting the broad north–south trending

depression.


Noachian hilly terrain (Nplh), this ridge forms the eastern flank of the broadnorth–south trending depression in whichMangalaVallis formed (Figures 2, 3and 5). Tharsis lava flows (Ahd1 and Ad2) dominate the terrain east of Man-

Figure 4. MOC wide-angle mosaic overlain with MOLA shaded relief context image for all

later figures. Five horizontal lines across image denote locations of MOLA profiles shown inFigure 5.


Figure 5. East–west MOLA topographic profiles across Mangala, extracted from 128 pixel

per degree gridded data set (see Figure 4 for location of profiles). Mangala formed within abroad north–south trending depression. South of Mangala this depression is filled withsmooth terrain, similar in smoothness at this scale to nearby Tharsis lava flows. The Noachiancratered terrain west of Mangala slopes inward toward the channel. The interior of Mangala is

deeper to the west than the east. This deeper portion is filled with the deposits of Amch (seeFigures 2b and 12). The remaining interior units form a broad northwest dipping deposit.600 km downstream from its head, Mangala forks into two branches. From its head region to

this point, the floor of Mangala decreases in elevation from 0 to 500 m below the datum.


gala forming smooth plains that stand topographically higher than the interiorof Mangala (Scott and Tanaka, 1986; Craddock and Greeley, 1994; Zimbel-man et al., 1994), and have a complicated chronology relative to theMemnonia Fossae, a series of graben that radiate away from Tharsis to thesouthwest (Wilson and Head, 2002a). Tanaka and Chapman (1990) suggesteda complicated chronology relative to graben formation, each associated with adistinct period of volcanism as well as a distinct period of flooding inMangala,one in the Late Hesperian and one in the Latest Hesperian/Early Amazonian,implying a hiatus in fluvial activity throughMangala between flood periods ofat least severalmillion years, and possibly tens to hundreds ofmillions of years.

A series of lava flows to the east of Mangala that display east-facinglobate fronts were interpreted by Zimbelman et al. (1992) and Craddock andGreeley (1994) as possibly having been extruded from the graben atMangala’s head, near where the graben intersects the western margin of thelarge north–south trending ridge. MOLA data show that these flows areactually continuous with flows emanating from Tharsis, and that the east-facing lobate margins resulted from redirection of flow due to banking upagainst the ridge (Figure 2a). To the west and north of Mangala the terrainpredominantly consists of the heavily cratered Noachian plains (Nplk andNplh), and some intermixed Hesperian cratered plains, with a northern slopeof less than a degree that abruptly increases in inclination at the dichotomyboundary.

The terrain south of Mangala is smooth, embays the surrounding Noa-chian cratered plains, fills in preexisting impact craters (Figure 6a), and isgenerally confined to low-lying areas. It fills the entire width of the broadnorth–south trending depression and was mapped as a member of the plateausequence (HNpl3; yellow in Figure 2b) (Craddock and Greeley, 1994).Craddock and Greeley (1994) suggested that the deposit formed either viaeolian deposition or via southward flooding from the Mangala Valles sourcegraben, whereas Scott and Tanaka (1986) interpreted the unit as volcanicflows. Our analysis of the topography of the source graben indicates that verylittle if any floodwater would have overtopped to the south (see Section 2.3),and thus we do not favor a fluvial origin for HNpl3. Additionally, the mannerin which the plains spread out to fill the entire width of the north–southtrending depression, as well as the way they embay impact craters and fillthem in, seems incompatible with an eolian origin. Instead we favor a vol-canic origin for this terrain due to the combination of its extremely smoothsurface texture and the manner in which the terrain embays older impactcraters, similar to the relationships observed in the mare plains on the Moon.Further, THEMIS VIS images of the terrain reveal several wrinkle ridges, aswell as possible flow fronts (Figure 6b), both consistent with a lava floworigin. Several occurrences of this member of the plateau sequence aremapped within the region (Scott and Tanaka, 1986) and generally fill in other


broad depressions associated with the circum-Tharsis ridges. Additionally,these occurrences are closely associated with younger Tharsis lava flows,further suggesting a volcanic flow origin for this unit. On the basis of theseobservations, we suggest that HNpl3 represents lava flows that appear tohave resurfaced the Noachian cratered terrain south of Mangala priorto fluvial activity and erosion of the channel system. This unit also appears tohave resurfaced at least some of the terrain north of the source graben(Figure 6a).

The excellent Viking-based mapping of the area delineates the maingeologic units in the region and the overall stratigraphy of the study area.Figure 2b provides a generalized geologic map of the regional geologyadapted from earlier studies (Craddock and Greeley, 1994; Zimbelman et al.,1994; Chapman and Tanaka, 1993). The goal of this work is not to remap the

Figure 6. (a) MOC wide-angle mosaic of the region near the Mangala source trough, showingunit HNpl3, a member of the plateau sequence. This unit is very smooth, spreads out to coverthe entire width of the north–south trending trough, embays adjacent Noachian cratered

terrain and fills in craters. The unit continues north of the source trough to the northwest. (b)Portion of THEMIS visible image (V06260004) showing wrinkle ridges and possible flowfronts (see arrows) within unit HNpl3. This and all further THEMIS visible images are of band

3 (0.654 lm).


area, but to use new data sets and new insights about the geologic history ofMars developed since the time of the Viking-based investigations to testprevious interpretations for the nature and origin of the units comprisingMangala in order to reconstruct the evolution of the Mangala Valles outflowchannel system.

2.2. MANGALA VALLES

Mangala Valles heads at one of the northernmost of the Memnonia Fossaegraben, a set of graben that radiate to the southwest, away from the nearbyTharsis rise. Within 5 km from its head Mangala expands in width fromapproximately 5.5 km to around 50 km, and continues to gradually widendownstream for another 600 km where it encounters two parallel scarpsthat bear morphological similarities to wrinkle ridges observed elsewhere onMars (arrows in Figure 2b). As revealed by MOLA data, the floor of theoutflow channel decreases in elevation from about 0 m near its head toapproximately )500 m near these ridges. From its head region to the areaof these ridges the interior of Mangala is mapped as being composed of fivedifferent units (Figure 2b) (Chapman and Tanaka, 1993; Zimbelman et al.,1994; Craddock and Greeley, 1994). A very smooth, widespread unit (Hmp;light blue in Figure 2b) occupies the easternmost region of Mangala, andembays the adjacent north–south trending ridge as well as preexisting im-pact craters. This unit is streamlined and in places displays lobate margins,although elsewhere its margins are heavily fragmented. Hmp was alsomapped along the floor of the graben at Mangala’s head. Trendingnorthwest-southeast along the western margin of Hmp is another smoothplains unit, AHmp3 (pink in Figure 2b), that displays lobate margins similarto those of Hmp. West of AHmp3 is a widespread unit that is heavilyscoured and streamlined (AHmch; medium blue in Figure 2b), and whichoccupies most of the central regions of Mangala. The very western portionof Mangala is characterized by a more deeply incised channel that trendsnorth–south along the margin of the Noachian cratered terrain to the west,and which is mapped as predominantly occupied by a unit that is smooth atlarge spatial wavelengths but blocky at finer scales (Amch; dark blue inFigure 2b). Lastly a few isolated knobs and plateaus within Mangala thatstand topographically higher than the surrounding plains along the outflowchannel floor were mapped as unit AHmp2. Together these units composethe Mangala assemblage.

As mapped by previous investigators (Chapman and Tanaka, 1993;Zimbelman et al., 1994; Craddock and Greeley, 1994), the stratigraphicsequence for the interior units of Mangala is, from oldest to youngest: Hmp,AHmch, AHmp3 and AHmp2, and Amch. Ages assigned to the units were


determined via crater counting as well as interpreted position within thestratigraphic column. However, as noted by Craddock and Greeley (1994),for some of the units within the Mangala assemblage, crater counts may bemisleading due to the small areal extent of the defined geologic units. Hmpwas interpreted by Craddock and Greeley (1994) and Zimbelman et al.(1994) as a flood plain deposit associated with the oldest flooding in Man-gala, composed of either indurated alluvium or mudflows, possibly inter-mixed with or capped in places by lava flows. AHmch (mapped as Hmch byChapman and Tanaka (1993)) was interpreted as alluvium deposited duringan early period of flooding, scoured and eroded by later flooding. BothAHmp3 and AHmp2 were interpreted as either indurated sediment or vol-canic flows emplaced after the flooding associated with AHmch. Lastly,Amch was interpreted as both alluvium deposited by the final episodes offlooding through Mangala, as well as terrain eroded by this later flooding. Itis clear from the previous interpretations for the units composing theMangala Vallis assemblage that a complex geologic history for the channelsystem was inferred from Viking-based observations. Indeed a minimum oftwo different periods of catastrophic flooding, with a period of volcanic flowemplacement in between, was considered to be necessary to explain the dis-tribution and emplacement of the units. Additionally, Hmp, AHmch, andAmch were interpreted as primarily, if not entirely, flood deposited units. Thehigh-resolution images provided by THEMIS and the detailed topographyfrom MOLA allow us to reexamine the relationships among the units of theMangala assemblage as well as their nature and origin, and to suggest analternative, simpler geologic history. We focus on the upper Mangala regionof the channel system, from Mangala’s head to where it branches 600 kmdownstream.

Figure 7. (a) THEMIS daytime infrared mosaic of unit Hmp with MOC wide-angle mosaicfilling THEMIS gaps. Visible are wrinkle ridges and stacked flow fronts. (b) Portion ofTHEMIS visible image (V05234002) showing wrinkle ridges within unit Hmp.


Hmp is the most widespread of the interior units. Ranging in elevationfrom approximately 100 m above the datum to 200 m below the datum,much of this unit stands topographically higher than the remaining interior

Figure 9. THEMIS daytime infrared mosaic of contact between regions previously mapped asHmp and AHmp3, with MOC wide-angle mosaic filling THEMIS gaps. Hmp and AHmp3 canbe seen to be continuous, displaying similar morphology and similar erosional behavior along

their margins.

Figure 8. Portion of THEMIS visible image (V06235003) showing interior of Mangala sourcetrough. Areas previously mapped as Hmp are seen to be morphology the same and continuous

with adjacent regions mapped as Amch.


units (Figure 5), although AHmp2 and AHmp3 stand at elevations within therange of those for Hmp. AHmch is topographically lower than all adjacentportions of Hmp. Amch is located within the deepest regions of Mangala, atelevations reaching down to 500 m below the datum and depths of 550 mbelow the channel rim.

THEMIS daytime IR images of unit Hmp reveal stacked flow-like featureswith lobate margins (Figure 7a), and more detailed views of the unit withTHEMIS VIS images reveal wrinkle ridges throughout the unit (Figure 7b).At both scales the unit is very smooth, similar to unit HNpl3 south ofMangala (compare with Figure 6). These characteristics, along with themanner in which the unit embays impact craters and the north–southtrending ridge support a volcanic flow origin for the unit. MOLA dataindicate that Hmp generally has a northwesterly slope, with an inclination ofless than a degree, very similar to the smooth terrain south of Mangala(Figures 2b, 6). Some portions of the interior of the graben at Mangala’shead were mapped as Hmp; however these regions are topographically lowerthan all other portions of Hmp, and appear morphologically different fromthose portions of Hmp within the main channel. Instead the portions of thegraben mapped as Hmp appear to be continuous with other regions withinthe source graben mapped as Amch (Figure 8), and thus we suggest theseanomalous portions of Hmp are actually part of unit Amch.

AHmp3 has been mapped as its own unit, although both MOC wide angleand THEMIS IR images show that AHmp3 displays the same jagged and

Figure 10. Portion of THEMIS visible image (V07009001) comparing units previously map-ped as AHmp2 and AHmp3. The two units overlie unit AHmch and display similar mor-

phologies. The exposed surface of AHmp2 stands higher than AHmp3, although along themargin of the AHmp2 outcrop are stacked layers that are at comparable elevations to nearbyAHmp3.


lobate margins as Hmp, is similarly smooth, and can be traced as beingcontinuous with Hmp (Figure 9). Additionally, MOLA data show the slopeof AHmp3 to be to the northwest, continuous with the northwest slope ofHmp (Figure 5). We thus consider Hmp and AHmp3 to be a single unit ofvolcanic flow origin.

Unfortunately the occurrences of AHmp2 are isolated mesas, so conti-nuity with Hmp is not readily apparent. However, the unit has previouslybeen interpreted as volcanic cap rock (Craddock and Greeley, 1994), andTHEMIS VIS images of the terrain show it to appear morphologicallysimilar to nearby occurrences of AHmp3 (Figure 10). Additionally, while theoccurrences of AHmp2 stand above the surrounding terrain, the elevations ofthese occurrences fall within the range for that of Hmp. Also, beneath the

Figure 11. (a) THEMIS daytime infrared mosaic of region near Mangala’s head showingmargins of unit Hmp and contact of Hmp with AHmch and Amch. In this region Hmp is very

smooth, and appears to be eroding back with irregular margins. Adjacent the smooth, high-standing portions of Hmp is blockier terrain that is heavily fragmented, which in some placeshas been mapped as Hmp and in others has been mapped as AHmch. (b) Portion of THEMISvisible image (V05511004) showing irregular margin of Hmp in more detail. At this THEMIS

resolution (18 m/pixel), the blockier terrain can be seen to clearly underlie Hmp, or to be theremnants of eroded portions of the smooth regions of Hmp. (c) Portion of THEMIS visibleimage (V06260003) of contact between Hmp and AHmch. Here AHmch appears to be the

scoured and eroded remnants of Hmp, and to be gradational with Hmp.


exposed surface of AHmp2 are a series of stacked layers that are at com-parable elevations to nearby exposures of AHmp3 (Figure 10). Thus, basedon the common volcanic origin for Hmp and AHmp2, as well as their similarmorphology and elevations, we interpret AHmp2 to have once been contin-uous with Hmp.

The relationship of AHmch to Hmp (as well as AHmp2 and AHmp3) issomewhat less clear. Although it is mapped as being stratigraphicallyyounger than Hmp, and as being alluvium deposited during flooding,AHmch often appears to be the eroded remnants of Hmp. As seen inTHEMIS daytime IR images (Figure 11a), Hmp appears to be eroding backalong its margins, revealing a more chaotic assemblage of blocky terrain.High-resolution views of these irregular margins indicate that the blockyterrain either underlies Hmp or is the remains of heavily degraded Hmp(Figure 11b). In some locations this blockier terrain has been mapped asHmp, and in other locations it has been mapped as AHmch. At other con-tacts between the two units, AHmch is clearly seen to be the eroded remnantsof Hmp, displaying a gradational relationship (Figure 11c). These observa-tions suggest that AHmch represents either the degraded remains of Hmp, oris part of the same assemblage of Hmp but lower down, and is exposed whereHmp has been sufficiently removed. In either case, AHmch does not appearto be a fluvially deposited alluvium unit within Mangala as was suggested byprevious investigators, but instead is seen to be an erosional unit of preex-isting terrain. We favor the interpretation of AHmch (at least that portionbetween the Mangala head region and where Mangala forks – within Man-gala Vallis, Figure 3) as part of the same assemblage of volcanic flows asHmp, AHmp2, and AHmp3.

Figure 12. (a) Portion of THEMIS visible image (V04762003) showing details of regionmapped as Amch. Region contains a smooth deposit that tapers to the north, and displays

cuspate margins, and which is pocked by unusual quasi-circular pits, some of which containblocks. Deposit is confined to the deep, western interior channel of Mangala Vallis. (b) Portionof THEMIS visible image (V04400003) showing an additional example of the smooth deposit

of Amch.


TABLE

IDescriptionsandinterpretationsofgeologic

unitsin

andnearMangala

Valles

Unitname/description

Previousinterpretation1

New

interpretation

Am

–MedusaeFossaeForm

ation

Post

flooddeposits

Post

flooddeposits

Amch

–Younger

flooddeposits

Alluvium

depositedin

2ndfloodperiod

Alluvium

depositedduringonefloodperiod

Ad2–Tharsislavaflows

Tharsisvolcanism

Tharsisvolcanism

AHd1–Tharsislavaflows

EarlierTharsisvolcanism

EarlierTharsisvolcanism

AHmp3–Mangala

Interiorplainsunit

Lavaflowsem

placedafter

1st

period

offlooding

More

resistantpartsofpre-floodvolcanic

assem

blagewithin

centralportionsofchannel

AHmp3–Mangala

Interiorplainsunit

Lavaflowsem

placedafter

1st

period

offlooding

LavaflowscontinuouswithHmpandpart

of

samepre-floodvolcanic

assem

blage

AHmch

–Older

flooddeposits

Alluvium

depositedin

1st

floodperiod

Volcanic

flowslower

downin

sameassem

blage

asHmp,more

eroded

andscouredbyflooding

Hmp–Mangala

interiorplainsunit

Volcanic

flowsorinduratedalluvium

Volcanic

flowsem

placedwithHNpl 3andmodified

byfloodingthateroded

Mangala

Valles

HNpl 3–Highlandplateauunit

Eolianorfluvialdeposits

Pre-floodvolcanic

flowsthatresurfacedterrain

Npl–Hilly,knobby,andcratered

Noachianunits

Basementrock

Basementrock

Nplm

–Noachianmountainousunit

Basementrock

Basementrock

1Previousinterpretationsfrom

ChapmanandTanaka(1993),Craddock

andGreeley

(1994),andZim

belmanet

al.(1994).


The deep, western portions of Mangala were mapped as Amch. THEMISVIS observations of these regions reveal smooth, lobate deposits whichextend for several tens of kilometers, that display cuspate margins, and whichare pocked by quasi-circular features (Figure 12). These deposits tend totaper to the north, are confined to the western interior channel and are notobserved elsewhere within Mangala. Their exact nature is uncertain, but theyappear consistent with being associated with the last dregs of floodingthrough Mangala, as was suggested for unit Amch.

Synthesizing the observations for the five units composing the MangalaVallis assemblage, we suggest that Hmp, as well as AHmp2, AHmp3, andAHmch, all originate from a single assemblage of volcanic flow deposits thatwere once continuous with the smooth volcanic flows south of Mangala(HNpl3). Table I shows a comparison of previous interpretations for theunits with the interpretations from this study. MOLA topography shows that

Figure 13. THEMIS daytime infrared mosaic of filled crater along the western margin ofMangala, with MOC wide-angle mosaic filling THEMIS gaps. Craters such as this along thewestern margin of Mangala appear similar to embayed and filled craters south of Mangala

filled by HNpl3.


Mangala formed within a low-lying depression, flanked to the east by thenorth–south trending ridge, and to the west by Noachian cratered plains(Figures 2a, 3 and 5). The smooth terrain south of Mangala was emplacedprior to the formation of the source graben, and thus prior to formation ofMangala. There is little about this terrain to suggest that the same flowsemplaced south of the source graben were not also emplaced to the north. Infact, a small section of the same terrain is visible north of the western area ofthe source graben (Figures 2b and 6a). Also, craters with filled interiors alongthe margin of Mangala exist that are remarkably similar in appearance tocraters embayed and filled by HNpl3 south of the source graben (Figure 13),and thus may contain remnants of the original pre-flood volcanic flows.Further, the northwest slopes of Hmp and AHmp3 are similar to thenorthwest slope of the terrain directly south of the source graben, consistentwith them having once been continuous. Thus we suggest that, prior to anyfluvial activity in Mangala, the Noachian terrain was resurfaced by volcanicflows. Those flows to the south of Mangala Valles, along with a small portionto the northwest of the source graben, remain close to their original state,since they were not eroded by fluvial activity. However, the portion of thisresurfacing unit north of the source graben was considerably eroded by thefluvial flooding that carved Mangala, resulting in the present configuration ofthe units within the outflow channel.

The erosional state, as well as geographic and topographic distribution ofthe interior units, can also be explained via this scenario. The topographiccharacteristics of Mangala suggest that as water began to flow northwardfrom the source graben massive sheet flooding occurred as the floodwaterspread out along the width of the broad depression that confines Mangala.The north–south trending Noachian-aged ridge to the east diverted theflooding northward, as did the general northerly slope of the southernhighlands in the region. Similar to the smooth volcanic terrain south ofMangala, as well as Hmp and AHmp3, the pre-flood volcanic flows north ofthe source graben likely also had a northwest-dipping slope. If this were thecase floodwater would have been deeper along the western portions ofthe region, and would have eroded for more extended periods of timethan the floodwaters to the east, explaining the presence of a deeper channelalong the western interior of Mangala. The general flow of water would havebeen to the north, with a westerly flow component that became increasinglydominant along the western portions of the valley as the western interiorchannel continued to erode and as flood discharges subsided. Thus Hmp andAHmp3 were only relatively minimally scoured and eroded by the floodingfor as soon as down cutting occurred to the west, and the flood dischargedeclined, flood levels subsided and sheet flooding over this terrain ceased. Asa result, Hmp and AHmp3 remain the highest standing units within thechannel. AHmp2 represent areas that were less easily eroded by the


floodwaters within the central portions of Mangala Vallis, and thus remainedas islands of resistant material within the outflow channel. AHmch representsportions of the pre-fluvial activity volcanic flows that were lower-lying withinthe original assemblage, and which were much more heavily scoured anderoded, resulting from concentration of floodwaters within the deeper centraland western portions of the channel. As floodwaters continued to subside, thesediment and other flood-transported materials would have been deposited inthe lowest-lying portions of Mangala, and are represented by the depositsmapped as Amch.

One complication with the above interpretation of the units of theMangala assemblage is the spread in ages assigned to the different units. Weaddress this concern as follows. Firstly, the crater statistics for Hmp are verysimilar to those for HNpl3, and even overlap each other for certain cratersizes (Craddock and Greeley, 1994), consistent with the two once having beencontinuous and part of the same volcanic resurfacing assemblage. Thesomewhat younger age for Hmp may be a result of erosion from the sheetflooding that occurred across the unit. The Amazonian/Hesperian agesassigned to AHmp2 and AHmp3 may not be reliable due to the small arealextent of these units. Stratigraphically they appear continuous with Hmp,and can thus reasonably be considered part of the same unit. AHmch was

Figure 14. (a) THEMIS daytime infrared mosaic of streamlined crater within Mangala, with

MOC wide-angle mosaic filling THEMIS gaps. (b) Portion of THEMIS visible image(V04787003) showing several stacked terraces along the margin of the streamlined cratershown in Figure 12a. Terracing is prevalent throughout Mangala along its margins as well as

along interior ‘‘islands’’ such as the streamlined crater shown here.


mapped as Amazonian/Hesperian in age, but this slightly younger age ascompared with Hmp could be due to erosion of Hmp and exposure of newmaterial to the surface during the time of flooding. However, Chapman andTanaka (1993) map the same unit as entirely Hesperian in age, in which casethere is less of an issue with AHmch being part of the same assemblage asHmp. Thus, Hmp, AHmp2, AHmp3, and AHmch can all reasonably beinterpreted as part of the same assemblage of volcanic deposits, once con-tinuous with HNpl3, and later scoured and eroded by the flooding thatdeposited Amch.

High-resolution Viking images suggested the presence of terraces alongthe Mangala Vallis margins. THEMIS images reveal these terraces in muchmore detail, indicating several tens of stacked terraces along the channelmargins as well as along the edges of streamlined islands (Figure 14). MOLAtopography indicates that these terraces are separated in height by up to tensof meters. Similar terraces are observed on Earth associated with large-scalefloods, and are interpreted as forming during the waning stages of flooding asthe flood level rapidly subsides (Thompson and Jones, 1986). The terraceswithin Mangala are thus consistent with progressive down-cutting and con-finement of flooding to narrower and deeper portions of the valley asfloodwaters subsided.

The geographic association of the head of Mangala with a MemnoniaFossae graben led to a general consensus within the literature that the

Figure 15. THEMIS daytime infrared mosaic of streamlined features trending northwest,

perpendicular to the margin of Hmp, with MOC wide-angle mosaic filling THEMIS gaps. Inthe scene the Hmp margin is irregular, and trends northeast–southwest across the image.Scour and streamline features extend across the margin of Hmp into AHmch, and transition intrend from northwest to north farther away from Hmp.


floodwater that eroded Mangala emptied from the graben. However, asnoted by Zimbelman et al. (1992), some streamlined features along thechannel floor diverge from the dominant north–south trend among the floordeposits, and instead trend northwest, tangential to and crossing the marginsof Hmp and AHmch (Figure 15). Zimbelman et al. (1992) explained thisperplexing occurrence by proposing that either (1) the floodwaters flowedover the top of these units, streamlining features marginal to the deposits, or(2) the floodwaters migrated beneath Hmp, ultimately reaching the surface atthe unit’s margin where the water proceeded to streamline adjacent features.The scenario described for the evolution of flooding within Mangalaaccounts for these perplexing features. Specifically, as sheet flooding over thetop of Hmp and AHmch subsided, flow would have been northward with awesterly component due to the northwest slope of the terrain. Thus, as thesefloodwaters retreated from the higher terrain to the east, northwest trendingstreamlined features could have formed, accounting for those featuresdescribed by Zimbelman et al. (1992), without appealing to flow of waterbeneath Hmp.

Viking-based investigations of Mangala called upon multiple periods offlooding to account for the then-proposed stratigraphy of the interior unitsand the interpretations of the nature of those units, although Craddock andGreeley (1994) suggested that a single catastrophic flood with declining dis-charge might be able to account for the observed geology. Our observationsof the approximately 600 km long region from Mangala’s head to the areawhere the channel forks (Figure 3) suggest that the units within the channelcan be reasonably explained without invoking multiple periods of flooding

Figure 16. THEMIS daytime infrared mosaic of the northern region of Mangala near wherethe channel forks, with MOC wide-angle mosaic filling THEMIS gaps. Trending northeast–southwest across the region are two wrinkle ridge-like features, separated from each other by

about 50 km. The northern ridge is more intact than the southern ridge, except near where thechannel forks into its eastern and western branches. Arrows indicate general flow paths forflooding across the area of the ridges.


separated by tens to hundreds of millions of years. In particular, previousinvestigators attributed unit AHmch to deposition by an early period offlooding later scoured and eroded by the period of flooding that depositedunit Amch. Additionally, Hmp, mapped as stratigraphically older thanAHmch, was interpreted as flood plain deposits from the earliest floodingthrough Mangala, emplaced during flooding that preceded emplacement ofAHmch. Thus, earlier studies have suggested between two and three periodsof flooding to explain the emplacement of the interior units of Mangala.However, as observations with THEMIS images reveal, Hmp, along withAHmp2 and AHmp3 appear to be pre-flood volcanic flow deposits that wereonce continuous with HNpl3, located to the south of Mangala. Additionally,AHmch appears to be part of this same assemblage of volcanic deposits,lower down in the stack, and does not appear to be a flood deposited unit.Thus only a single period of flooding would be required to account for thepresent distribution of the units within Mangala Vallis.

Early sheet flooding appears to have occurred as far north as the twowrinkle ridges located about 600 km downstream (arrows in Figure 2b). Thesouthern ridge is heavily eroded and discontinuous (Figure 16), whereas thenorthern ridge is more intact, except for a noticeable break along its westernedge where it is intersected by the deep western interior channel of Mangala.We suggest that sheet flooding banked up against each of these ridges untilthe floodwater was able to flow over the top of and down-cut through them.The more preserved state of the northern ridge suggests that there was littleflow over the top of this ridge, and that most water that flowed northward ofthe ridge did so through the break along the western region (see arrows inFigure 16).

Northward of the northern ridge, Mangala forks into two main branches(Figures 2a, b and 3), which we refer to for simplicity as the western andeastern branches, and which together form the remainder of the MangalaValles channel system. The eastern branch itself branches further down-stream, and as mapped by Chapman and Tanaka (1993) is predominantlycomposed of Amazonian-aged channel deposits, continuous with Amch fromthe southern regions of the channel system. The western branch (LabouVallis) is mapped as being filled with Hesperian-aged deposits continuouswith AHmch from the southern regions of the channel system, and disap-pears upon encountering the Medusae Fossae Formation. Again we useMOLA data to test the Viking-based observations. Specifically, MOLA datashow that the gradient along the western branch is steeper than along theeastern branch. That is, if water were to flow through Mangala today, uponencountering the fork it would preferentially travel along the western branchrather than the eastern branch. This is inconsistent with the crater statistic-determined ages assigned to the two branches by Chapman and Tanaka


(1993), which suggest that the most recent flooding through Mangala occu-pied the eastern branch.

This issue of multiple periods of flooding through Mangala Valles is sig-nificant as it bears on the total time involved in erosion of the channel systemas well as the amount of martian geologic history over which water wasavailable to feed floods at Mangala. Our observations of Mangala Vallis (theportion of the channel system prior to branching) indicate that units Hmp,AHmp2, AHmp3, AHmch of the Mangala Vallis assemblage appear morelikely to have formed via erosion of preexisting lava flows during a singleperiod of catastrophic flooding as opposed to emplacement during one per-iod of flooding followed by scouring and additional deposition during asecond period of flooding. While our study has not focused on the down-stream portion of Mangala Valles, the inconsistency between the gradients ofthe eastern and western branches and their assigned ages calls into questionwhether two or more periods of flooding are indeed required to explain theformation of those branches, and warrants reexamination with the new high-resolution data available from post-Viking spacecraft. That being the case,evidence does exist in the form of crosscutting scour relationships that mayargue for multiple floods through Mangala. Figure 17 shows an example of

Figure 17. Portion of THEMIS visible image (V06335002) of the area near where Mangalaforks showing crosscutting scour relationships along the channel floor. Relationship andrelative timing of the two sets of scours is unclear; in some place the more northerly trending

scour features appears to cut the more westerly trending features, while in other places theopposite relationship is observed.


such crosscutting scouring located near where Mangala forks, as seen inTHEMIS VIS images. Two directions of scour are observable within theimage, one trending generally northwest and the other trending a little morenortherly. It is difficult to discern the age relationship between these two setsof scour features, for in some places the northwest trending scours appear tocut the northerly trending scour features, whereas in other locations theopposite relationship is observed. Despite this ambiguity in the timing of theformation of the scour sets, the crosscutting relationship of the scours maysuggest that multiple floods have passed through Mangala. Alternatively,these crosscutting scours could plausibly be explained by multiple phaseswithin a single flood caused by intermediate-stage drainage toward the mainchannel so the floodwater broke through downstream barriers and receded inthis region. These crosscutting scours are located entirely within the terrainmapped as Amch. If the scours represent multiple floods, how do we rec-oncile them with our earlier observations and interpretations for a singleperiod of flooding within Mangala? We do so by first distinguishing betweenterminologies as used within the literature, specifically multiple periods offlooding from multiple flood events.

As the term has been used in the literature in regards to Mangala, a periodof flooding refers to an episode of flooding within a distinct geologic period(Late Hesperian versus Early Amazonian) (Tanaka and Chapman, 1990;Chapman and Tanaka, 1993), and according to the cratering chronology ofHartmann and Neukum (2001), implies a hiatus of up to several hundredmillion years between flood periods. This hiatus is a direct result of thecoarseness of the ages obtainable via crater statistics. In contrast, we use theterm event as it was used by Baker and Nummedal (1978) to describeflooding through the Channeled Scablands. As described by Baker andNummedal (1978) the large, fluvially carved Channeled Scablands wereeroded via multiple flood events, but all within a single period of floodinglasting a few thousand years during the last glacial maximum. Using thisterminology, we suggest that all the observations of Mangala Vallis can beexplained by a single period of flooding sometime during the Amazonian, theage assigned to Amch (Chapman and Tanaka, 1993; Zimbelman et al., 1994;Craddock and Greeley, 1994), and that within that period multiple eventsmay have occurred. That is, our observations of the relationship among theinterior units of Mangala are consistent with a single or several flood eventsclosely spaced in time (one period of flooding) as opposed to multiple periodsof flooding separated by millions of years. Additionally, the crosscuttingscours occur entirely within Amch, suggesting that if indeed these featureswere formed during multiple floods, these flood events occurred during asingle period. However, it should be emphasized that the crosscutting scoursmay have formed entirely within a single flood, in which case the interior


geology of Mangala is consistent with not only a single period of flooding,but also a single flood event.

As mentioned earlier, while there is a general consensus in the literatureamong those who have worked on Mangala that the water that carved thechannel emptied from the graben at the channel head, considerable ambi-guity remains surrounding the ultimate source of the water. Before assessingpast proposals for the source and delivery of floodwater we first provide adetailed description of the source trough itself in order to constrain theearliest stages of flooding at Mangala.

2.3. SOURCE TROUGH AND SURROUNDING REGION

The Memnonia Fossa at the head of Mangala trends N72E (or S72W),extends for several thousand kilometers southwest away from the Tharsisrise, and is connected to the outflow channel via a notch in its northern rim.

Figure 18. THEMIS visible and daytime infrared mosaic of the Mangala head region and

surrounding area, with MOC wide-angle mosaic filling THEMIS gaps. In the vicinity ofMangala, the source graben has been enlarged to a trough, measuring about 7 km across,approximately 220 km long, and up to 1.5 km deep. The actual graben can be seen further

along strike, and measures about 2 km wide and only tens to hundreds of meters deep. Thetrough is characterized by a left-lateral en-echelon offset, and is connected to Mangala via a5.5 km-wide notch in the northern rim. South of the trough are two impact craters, labeled Aand B. Crater A measures about 25 km in diameter, and has its ejecta transected by the source

trough indicating that it predates trough emplacement. Crater B, which measures about 80 kmin diameter, was mapped as having flood deposits along its floor (Craddock and Greeley,1994). The black line marks the location of MOLA profile 13463, shown in Figure 17.


In the region near the channel head the graben has been enlarged to atrough measuring ~7 km wide and up to ~1.5 km deep (Figure 18). Theunmodified graben can be seen further along strike and measures about2 km wide and only tens to hundreds of meters deep. The trough extendsfor about 220 km in length, its eastern end located on the flank of the northtrending ridge that parallels Mangala’s eastern margin, and its western endtapering out into the higher-standing Noachian cratered plains. The notchin the rim measures ~5.5 km wide, ~500 m deep and 10 km long and islocated directly north of a portion of the trough characterized by a left-lateral en-echelon offset.

The early stages of flooding at Mangala have profound implications forour ability to understand the later stages of flooding and its downstreamevolution. In particular, assessments of the duration of flooding at Mangalarely upon mean discharge rates for the channel. As first addressed by Komar(1979), discharges calculated for Mangala have taken advantage of the notchat the head of the channel. Such calculations rely upon the logic that all thewater that carved the channel downstream of the notch must have oncepassed through the notch itself. Therefore an estimate of the mean dischargethrough the notch, along with an estimate of the total volume, can provide anestimate of the duration of flooding. While arguably reasonable, this logicrelies upon a significant assumption, namely that there was very little over-the-top sheet flooding out of the graben prior to when flooding coalesced intothe area of the notch. MOLA and THEMIS data provide a means wherebythe validity of this assumption can now be tested.

South of the Mangala source trough is a 25 km wide impact crater (A inFigure 18), the ejecta of which is transected by the trough implying that thecrater predates graben emplacement, and by association predates Mangala

Figure 19. MOLA profile 13463 across the Mangala source trough. The southern rim of thesource trough is generally tens of meters lower than the northern rim, indicating that little ifany floodwater spilled over to the south.


Valles as well. To the east of this crater is an 80 km wide crater (B in Fig-ure 18), intersected to the north by the north–south trending ridge that flanksMangala to the east. Previous Viking-based mapping suggested the presenceof Hmp along the floor of this crater, mapped as being continuous andcontemporaneous with those portions of Hmp identified elsewhere within thesource trough. As described earlier, Hmp mapped within the source troughappears to be distinct from the widespread portions of Hmp within the mainchannel, and instead to be continuous with the flood deposits mapped asAmch. Therefore, if indeed crater B contains deposits similar to those alongthe floor of the source trough, these are more likely part of unit Amch andnot Hmp.

The detailed topography provided by MOLA shows that the northern rimof the trough is generally tens of meters lower than the southern rim (Fig-ure 19) indicating that after the trough had reached its present configurationand filled with water there was very little spill-over to the south. MOLA datacan be used to identify the lowest points along the northern rim in order todetermine where spill-over from the trough occurred. The results indicatethat initial spill-over would have occurred through two or three small valleysin the ejecta of crater A, as well as through the area currently occupied by thenotch. The ejecta distribution of crater A suggests that the pre-floodtopography of the notch area was probably occupied by ejecta as well, andthat the pre-notch area probably looked similar to one of the smaller‘‘channels’’ in the ejecta of crater A. Fortuitously, the trough transects theejecta of crater A. We can therefore compare the ejecta north and south of

Figure 20. Portion of THEMIS visible image (V04762003) of notch connecting the source

trough to Mangala Vallis. Visible on top of the surrounding plateau is a sinuous, channel-likefeature (indicated with arrows) that terminates at the margin of the notch.


the trough to assess how much flooding took place through the ejecta prior tocoalescence of flooding into the area of the notch. Such a comparison revealsthat the morphology of crater A north of the trough is indistinguishable fromthat south of the trough, indicating that there was minimal over-the-topflooding out of the trough, and that quite quickly in the early stages offlooding the floodwater coalesced into the area of the notch, followed by

Figure 21. Portion of THEMIS visible image (V06597003) of the northern wall of Crater B(see Figure 16 for location of crater). Indicated with arrows is a sinuous, channel-like featurethat extends from the rim of the crater onto the crater floor. The channel does not extend up tothe trough rim, but instead appears superposed by a lobe of material interpreted by Head et al.

(2004) as glacial deposits resulting from accumulation of snow and ice along the trough rims.


continual down cutting of that area to form the ~500 m deep notch. IndeedTHEMIS images of the area adjacent to the notch reveal channel-like fea-tures merging into the notch, supporting the conclusion that spill-over coa-lesced into the notch (Figure 20). These observations affirm the idea that thenotch can be used to determine mean discharge rates for flooding inMangala, ultimately yielding estimates of the total duration of flooding.

An additional implication of the analysis of the trough topography is thatspill-over to the north, and ultimately into the notch, would occur prior towater reaching the elevation necessary to spill into crater B. As mentioned,channel fill material was mapped on the floor of crater B, and indeed in aTHEMIS visible image of the interior of the crater a channel-like feature isobserved along the interior of the northern wall (Figure 21). This channel-like feature does not extend up to the rim of the trough, but instead appearssuperposed by a lobe-shaped deposit interpreted by Head et al. (2004) asglacial deposits resulting from accumulation of snow and ice along the rimsof the eastern portion of the trough. If water did flow within the crater alongthis channel-like feature as suggested by Craddock and Greeley (1994), thecurrent topography does not support the idea that the channel and theassociated fill material resulted from over-the-top spill-over of water from thegraben, and instead suggests a more complicated history. Two scenarios seempossible. In the first scenario water filling up the graben may have seepedthrough the graben and crater wall, into the crater itself, rather than flowingover the top of the rim. The elevation where the channel begins, 350 m, isbelow the elevation at which water within the trough would first spill over tothe north (around 400–450 m), and would therefore be consistent with thisscenario.

Alternatively, spill-over from the source trough into crater B may indeedhave occurred, but prior to the development of the present trough topog-raphy. As described below, Wilson and Head (2002a) present evidence thatthe Memnonia Fossae result from the near surface stalling of laterallypropagating dikes away from the Tharsis rise. As the dike propagated to thesouthwest and began to develop the source graben for Mangala (Wilson andHead, 2004), the eastern portion of the graben would have formed first. Thus,as the source trough formed there would have been a period of time duringwhich the eastern portion of the source graben had developed but the westernportion had not yet formed. As the eastern portion began to fill with waterthis water would have overtopped the trough and would have drained towardthe regional low point. Analysis of MOLA topography for the eastern por-tion of the source trough indicates that as that portion of the trough filled,spill-over would have been toward the floor of crater B, accounting for themapped flood deposits (Craddock and Greeley, 1994) as well as the observedchannel-like feature along the crater wall. Afterwards, as the remainder of thetrough formed, water would have ceased draining into crater B and would


instead have drained northward, eventually eroding the terrain to formMangala Valles.

Of critical importance to understanding the evolution of flooding atMangala and placing it in the broader context of the martian hydrologicalcycle is constraining where the floodwater ultimately came from, and howthat water was delivered to the source trough. We first summarize previousideas presented in the literature, outlining specific points and potentialquestions about those hypotheses. We then present a new model, constructedin the context of the Clifford (1993) model for the evolution of the martianhydrosphere, and adapted from recent work by Head et al. (2003) related tothe Cerberus Fossae and Athabasca Valles.

3. Previous Models for Delivery of Water to Mangala Valles

From the Viking-based mapping and investigations of the Mangala Vallesregion two scenarios were proposed for the ultimate source of the floodwater.Tanaka and Chapman (1990) suggested that faulting associated with thesource graben tapped a near surface aquifer under hydrostatic head, resultingin catastrophic release of groundwater onto the surface. Zimbelman et al.(1992) proposed a somewhat related origin, in which an increased geothermalgradient associated with Tharsis magmatic activity melted near-surfaceground ice along the Tharsis rise. Again, faulting associated with the grabentapped this aquifer, although Zimbelman et al. (1992) favored a less cata-strophic release rate for the water to the surface. Below we summarize thesetwo scenarios.

Tanaka and Chapman (1990) consider the flood water to have reached thesurface at the graben at Mangala’s head, and suggest that for groundwater toflow onto the surface at the high discharge rates determined for the channel(e.g. Komar, 1979), a significant hydraulic head must have existed. Theyargue that conditions in the Hesperian and Amazonian were not conducive torecharge via precipitation, and therefore the necessary head must have arisenvia elevation of preexisting groundwater, perhaps via tectonic uplift orcompressional events. The aquifer was inferred to be porous Tharsis lavaflows, and to extend along the entire length of the source graben (1000–2000 km) and up to 25 km along either side of the graben. While this modelprovides a generally reasonable scenario for the flooding at Mangala, it doesnot account for the ultimate source of the groundwater (i.e. where did thepre-existing groundwater come from). Further, the model does not attemptto explain how the water was able to remain in a liquid state in such a near-surface aquifer, nor does it place the formation of Mangala in the broadercontext of the martian hydrological cycle.


Zimbelman et al. (1992) also considered the primary source of floodwaterto be associated with the graben at Mangala’s head, and in particular wherethat graben intersects the prominent north–south trending ridge flankingMangala to the east. They interpreted this ridge as having been emplaced viacompressional faulting, and judged the intersection of this inferred north–south trending thrust fault and the east-west normal fault of the graben as anideal location for water to reach the surface. In their model, an increasedgeothermal heat flux associated with Tharsis magmatism served to meltground ice within the pores of the upper 1.5–2 km of the crust over the areadetermined by Tanaka and Chapman (1990) to be the extent of the subsur-face aquifer. This meltwater would have then flowed toward the Mangalahead region along the Tharsis gradient, confined by an overlying layer offrozen crust. The two most significant issues associated with this scenario arethe generation of sufficient volumes of meltwater to erode Mangala andmaintaining that meltwater in the liquid state for sufficient lengths of time inorder to reach Mangala’s head region.

Estimates of the minimum volume of water necessary to erode Mangalawere determined by Tanaka and Chapman (1990) following the method ofKomar (1980), whereby the water volume is estimated by assuming it carrieda sediment load of 40% by volume. Using rough Viking-based estimates ofthe Mangala Valles morphometry Tanaka and Chapman (1990) arrived at anestimate of 5 · 1012 m3 of water necessary to erode Mangala. Using MOLAdata we are able to reconstruct the original terrain with a greater degree ofaccuracy, finding that between 1.3 · 1013 and 2.0 · 1013 m3 of material hasbeen removed. Then following the method of Komar (1980) we arrive at anestimate of between 2 · 1013 and 3 · 1013 m3 of water. It is important toemphasize that, by assuming a 40% sediment load, these estimates for thetotal water that flowed through Mangala are minimum estimates, and mayunderestimate the total by several factors of ten.

Zimbelman et al. (1992) determined that the amount of water generatedvia melting of ground ice would range from 2 · 1013 to 2 · 1014 m3, andusing the estimate by Tanaka and Chapman (1990) for the total water dis-charged through Mangala considered the water budget acceptable. With ourrevised estimates of the total water discharged through Mangala, andemphasizing that these estimates are a minimum, we suggest that melting ofground ice may not be sufficient to account for the water that floodedMangala. Also, as demonstrated by Wilson and Head (2002b), the volume ofwater achievable from melting of ground ice depends upon the geometry ofthe emplaced magma and the amount of time associated with its emplace-ment. In the case of Mangala, floodwater generated via melting of ground icedue to Tharsis lava flows would require either near-simultaneous emplace-ment of large volumes of lava over a widespread region, or storage ofmeltwater generated during discrete lava emplacement events in the near-


surface crust. Additionally, the Zimbelman et al. (1992) model suggests flowof the generated meltwater through the shallow sub-surface. Under currentatmospheric conditions, and presumably those that existed during the time offlooding at Mangala, liquid water is thermodynamically unstable near thesurface, and would freeze relatively quickly (e.g. Hecht, 2002). While anincreased geothermal heat flux near Tharsis may have been sufficient tomaintain liquid water along the Tharsis rise, as the water flowed toward theMangala head region the heat flux would return to the normal planetaryaverage under which liquid water would tend to freeze in the shallow crust.

In the following section, we present a new model for the ultimate source ofwater responsible for the Mangala Valles flooding. There are many similar-ities between our model and those discussed above, namely that we tooinvoke tapping of a confined subsurface aquifer under hydraulic head, andinfer that Tharsis is the most reasonable source for the groundwater. Ofnotable difference is that we place our model within the context of the evo-lution of the martian hydrosphere as proposed by Clifford (1993).

4. New Model for Delivery of Water to Mangala Valles

4.1. MODEL BACKGROUND

The groundbreaking model put forth by Clifford (1993), and later updated byClifford and Parker (2001), provides a first-order approximation for thestructure of the martian hydrologic system. As described in these works, anywater within the pore space of the outer portion of the martian crust whichremains continuously below the freezing point of water (the cryosphere)exists as ground ice. Below this zone, and above the depth of self-compac-tion, exists another zone in which the heat flux from the planet is sufficient tokeep any water within that zone in the liquid state. Thus the martian crust isvertically layered, with a confining permafrost layer overlying a zone in whichliquid water can accumulate. As suggested by Clifford (1993), groundwaterwould accumulate within the sub-cryospheric zone (referred to here as theaquifer) after the cryosphere had become saturated with ground ice.Recharge of the groundwater system was proposed to have occurred viabasal melting of a larger south polar ice sheet. The vertical stratification ofthe crust into zones of ground ice and ground water stability is inferred tohave developed as the internal heat flux from Mars declined, and is likely tohave been in place during the time of Mangala flooding (Late Hesperian/Early Amazonian).

As discussed by several authors, if sufficient volumes of groundwateraccumulate within the sub-cryosphere aquifer, large hydraulic heads coulddevelop. If this water were suddenly provided with a pathway to the surface


via cracking of the overlying cryosphere, catastrophic release of groundwaterwould occur, which might account for the development of many of the large-scale fluvial features observed on Mars (e.g. Baker, 1982, 2001; Carr, 1996).Carr (1979) suggested that confined sub-permafrost aquifers would developon Mars as the ambient conditions on the planet cooled to their present state,and proposed that as the permafrost thickened the aquifers might becomeunstable, resulting in the catastrophic release to the surface of the trappedvolatiles. As opposed to a passive fracturing of the cryosphere due to buildupof enormous hydraulic heads (Tanaka and Chapman, 1990), we suggest anactive fracturing as a result of magmatic intrusions into the crust. As discussedby Head and Wilson (2002) the presence of the cryosphere (and the sub-cryosphere groundwater system) throughout much of martian geologic his-tory suggests that considerable interaction probably occurred betweenascending magmatic intrusions and ground ice and groundwater. Whilemelting of ground ice within the cryosphere may provide substantial volumesof meltwater (Wilson and Head, 2002b), these volumes are limited by theamount of ice held within the pore space of the cryosphere, as well as thedissipation of heat from the magmatic intrusion or surface flow. As opposedto melting of ground ice, tapping a confined liquid water aquifer that underliesthe cryosphere may be more reasonable, and could potentially yield orders ofmagnitude more water for a surface flood. Additionally, tapping such anaquifer as opposed to melting of ground ice addresses issues associated withnear-simultaneous generation of substantial volumes of meltwater to sourcethe Mangala Valles flooding, as well as the stability of that water as it flowedtoward the channel head region. Head and Wilson (2002) and Wilson andHead (2004) suggest that sub-vertical magma filled cracks (dikes) provide themost attractive means by which to tap such an aquifer.

Recently Head et al. (2003) suggested that such an event is responsible forthe relatively young fluvial activity observed along Athabasca Valles nearCerberus Fossae. Similar to Mangala, Athabasca Valles head at a graben asopposed to the circum-Chryse outflow channel which typically head nearcollapsed chaos regions. Head et al. (2003) developed a model of dike-inducedcracking of the cryosphere, resulting in catastrophic release of groundwateronto the surface, ultimately resulting in the formation of Athabasca Valles.Building on that work we develop a similar model for Mangala Valles.

The Memnonia Fossae compose one of several widespread swarms ofgraben that radiate away from the Tharsis rise. Most authors have suggestedthat these radial graben systems are entirely tectonic features, although asnoted in the literature multiple lithospheric deformation mechanisms arerequired to account for the observed distribution of the graben (Banerdtet al., 1982; Wilson and Head, 2002a). Specifically, the graben on the Tharsisrise itself are reasonably explained via isostatic models, although thesemodels prohibit development of extensional features such as graben along


the Tharsis flanks. Alternatively, flexural models can account for the grabenalong the flanks of Tharsis, but do not account for the graben on the rise.Thus a minimum of two different deformation processes must be invoked toexplain the Tharsis radiating graben systems purely via tectonic means.Drawing analogy to the Earth and Venus (e.g. Ernst et al., 1995), Wilsonand Head (2002a) present a model for the emplacement of giant dike swarmson Mars, and demonstrate that such Tharsis radiating swarms can plausiblyaccount for the formation of the radiating graben systems. Such dikespropagate radially away from a central magma reservoir, and if they stallnear the surface can create near-surface tensional stress fields that are ulti-mately compensated for by the development of graben. Thus, dikes associ-ated with overlying graben provide an attractive mechanism by which tofracture the cryosphere and tap a confined aquifer. In particular, we suggestthat the source graben for Mangala Valles is the result of a dike thatpropagated laterally away from Tharsis, stalled near the surface resulting ingraben formation, ultimately tapping the sub-cryosphere aquifer andreleasing significant volumes of water onto the surface (see also Wilson andHead, 2004). Along with volcanic flows and shield structures associated withmany of the nearby Memnonia Fossae, a series of pits within the sourcegraben at its intersection with the prominent north–south trending ridge eastof Mangala support the presence of a sub-graben dike (Figure 22). These pitsare reminiscent of collapse pits such as those associated with the East RiftZone on the Big Island of Hawaii, that result from drainage of magma andsurface collapse (Okubo and Martel, 1998). We now explore this scenario by

Figure 22. Portion of THEMIS visible image (V07321002) of the source graben where itintersects the north–south trending ridge that flanks Mangala to the east. Shown are a series ofpits within the graben, suggestive of collapse pits associated with a sub-graben dike.


the development of a model that describes the flow of water through the sub-cryosphere aquifer, and up a fracture through the cryosphere.

4.2. DEVELOPMENT OF MODEL FOR SOURCE AND SUPPLY OF MANGALA

FLOODWATER

The model developed here provides a means to determine the discharge ofgroundwater from a pressurized sub-cryosphere aquifer into the Mangalasource trough by accounting for flow of that water through the aquifer andup a vertical fracture. Figure 23 provides a perspective block diagram of themodel (note that the subsurface component of the figure and the slope of theterrain is not to scale), which assumes that steady state conditions exist, asimplification that we justify retrospectively in Section 5. The mean velocityof water flowing through porous material in the subsurface aquifer (Ua) isdescribed via Darcy’s Law, written as

Ua ¼ ðk=gÞðDP=XÞ ð1Þwhere k is the intrinsic permeability of the aquifer, g is the dynamic viscosityof water, X is the corresponding aquifer length, and DP is the pressure dif-ferential across the aquifer. The pressure differential describes the resistanceto flow through the aquifer, and is the difference between the sum of thoseforces causing water to flow toward the crack and the forces resisting flowthrough the aquifer. This force balance is written as,

Figure 23. Perspective diagram of the Mangala Valles region, showing a schematic of the

model developed for the source and supply of floodwater. A permanently frozen zone of crust(the cryosphere) overlies a zone within which liquid water is stable and can accumulate. If thelevel of water within the aquifer is above the elevation of the Mangala head region, hydraulichead can build up. The subsurface scale and the inclination of the surface are exaggerated for

visualization purposes. Model parameters are shown on the image.


DP ¼ qwgHþ Pt � Pb; ð2Þwhere qwgH represents the difference in hydraulic head between the top ofthe aquifer and the bottom of the fracture, Pt represents the pressure exertedon the underlying water via the air within the unsaturated portion of theaquifer, and Pb represents the pressure at the base of the fracture resistingflow through the aquifer. In accounting for flow from the aquifer into thefracture, the model relies upon conservation of mass, and thus the flux ofwater through the aquifer equals the flux of water passing through the ver-tical fracture. That is

Uaha ¼ Ufw ð3Þwhere ha is the thickness of the aquifer, Uf is the velocity of water traveling upthe fracture, and w is the fracture width. The flow of water through thevertical fracture is described as a force balance equating the pressure differ-ence driving water up the fracture with the excess pressure at the base of thefracture and the weight of the overlying column of water:

U2f ¼ ½ðw=hfÞðPb � qwghfÞ�=ðffqwÞ ð4Þ

where hf is the height of the fracture (i.e. cryosphere thickness), qw is thedensity of water, g is the acceleration due to gravity, and ff is the Fanningfriction factor. Combining Equations (1)–(3) yields

Ufw ¼ haðk=gXÞðqwgHþ Pt � PbÞ ð5Þ

Rearranging Equations (4) and (5) we get

Uf ¼ ðha=wÞðk=gXÞðqwgHþ Pt � PbÞ ð6Þand

w ¼ ffqwhfU2f ½1=ðPb � qwghfÞ�: ð7Þ

Equation (6) describes flow through the aquifer and Equation (7)describes flow up the fracture. While there are many unknown parameters(H, X, ff, ha, hf, k, and qc) reasonable values for them can be estimated, andthe dependence of the results upon the chosen values is explored. However,three variables remain for which values are ultimately unknown: Uf, w, andPb. To solve the system we approach the problem through two methods, eachof which can be viewed as an endmember scenario. The first approach is toassume that the fracture is held open as a result of the pressure in waterpassing through it. Thus, the fracture width can be described in terms of thepressure at the base of the fracture, and is given by


w ¼ hf½ð1� mÞ=l�ðPb � qcghfÞ; ð8Þwhere m is Poisson’s ratio for the cryosphere into which the fracture forms,and l is the shear modulus of the cryosphere rock. With Equation (8), wenow have three equations and three unknowns, and can thus solve the sys-tem. Alternatively, one could simply assign the fracture a particular width,and assume that the fracture maintains that width as the water ascends to thesurface without requiring the water to prop it open. Such an assumptionresults in only two remaining unknowns, and thus the system can be solvedusing Equations (6) and (7). Below we explore the results of these two end-member scenarios, and the dependence of those results upon the assignedvalues for the various parameters.

4.3. MODEL RESULTS – SCENARIO 1

We begin by assigning values to the model parameters (H, X, ff, ha, hf, k, qc, mand l). Assuming exponential decay of porosity with depth, and steady-stateheat conduction, Clifford (1993) estimated the thickness of the cryosphereand the depth of self-compaction for longitudinally averaged latitudes acrossMars. Using the results of the nominal case, Clifford’s (1993) model estimatesvalues of 2.5 km and 5 km, respectively, for hf and ha for the region ofMangala Valles. As noted by Clifford (1993), the determined thickness of thecryosphere depends upon the values inferred for the thermal conductivity ofthe crust, the melting point of ice, and the global heat flux. While these valuesare not known with certainty for Mars, Clifford (1993) suggests that withinthe realm of geologically reasonable values for these parameters, on aglobally averaged basis, the value determined for the cryosphere thicknesswill not vary by more than 50% from the determined nominal value. Thus wefeel comfortable using a value of 2.5 km for hf. For ha we use values of 7.5and 10 km in addition to the nominal value of 5 km.

Clifford (1993) suggested that the global aquifer was recharged via basalmelting of a once larger south polar ice sheet, although as noted by Tanakaand Chapman (1990) and Zimbelman et al. (1992) the regional topographyaround Mangala suggests that Tharsis would be a more optimal source forthe hydraulic head associated with the floodwater outbreak. However,Tanaka and Chapman (1990) argued that atmospheric conditions during theLate Hesperian/Early Amazonian were likely quite similar to today, andtherefore recharge at Tharsis was probably not feasible during that period,requiring elevation of preexisting groundwater to higher elevations along theTharsis rise in order to achieve high hydraulic heads. Recently global cir-culation models have demonstrated that precipitation is likely to occur alongthe Tharsis Montes during periods of higher obliquity (Haberle et al., 2004),


and indeed thick, widespread glacial-like fan-shaped deposits of LateAmazonian age are present along the western flanks of the Tharsis Montes(Head and Marchant, 2003; Shean et al., 2004; Parsons and Head, 2004).Additionally, as Harrison and Grimm (2004) demonstrate, if the heat fluxassociated with the Tharsis rise were to thin the cryosphere sufficiently thatgroundwater infiltration could occur, Tharsis would provide a much morereasonable source for the groundwater responsible for the martian outflowchannels. Therefore, we use Tharsis as the site for the aquifer feeding theMangala floods, and select three different heights for the top of the aquiferabove the break out point (H) and corresponding aquifer lengths (X)assuming the top of the aquifer is at different levels up the Tharsis rise. Weuse heights of 8, 10.5, and 13 km, with corresponding aquifer lengths of 1650,1700, and 1800 km, respectively.

The Fanning friction factor describing resistance to flow up the walls ofthe fracture is determined from

ff ¼ ½1=ð2:28þ 4 � logð2w=rÞÞ�; ð9Þwhere r is the wall roughness height scale. We use a value of 1 cm for r, andthus get a wall friction factor that varies with the width of the fracture.Combining Poisson’s ratio (m) and rock shear modulus (l) into the singlevariable stiffness, we assign this a value of 4 · 109 Pa.

Lastly, the permeability of the aquifer must be estimated. Attempts toestimate the permeability of the martian crust in the past have tended to resultin a conundrum. As discussed by Carr (1979), and demonstrated more re-cently by Wilson et al. (2004b), the discharge rates estimated for the martianoutflow channels (orders of magnitude higher than any known fluvial chan-nels on Earth) require values of permeability for the source aquifers of atminimum 10)9 m2 (103 darcies) (Carr, 1979), and as high as 10)6 m2 (Wilsonet al., 2004b), orders of magnitude higher than the global average perme-ability for the Earth of ~10)12 m2 (Clifford, 1993). Clifford (1993) provides anexcellent discussion of the problems incurred in achieving such high perme-abilities, and argues that while high permeabilities may be present locally, on aglobally averaged basis it is unlikely that the martian crust has a permeabilityany larger than 10)9 m2. A similar conclusion is reached by Manga (2004). Asdiscussed by Carr (1979), basalts on Earth have been recorded as havingpermeabilities as high as 10)9 m2, and in conjunction with unobstructed lavatubes higher permeabilities might be tenable. The aquifer for Mangala isinferred to be pressurized up the Tharsis rise, and thus high permeabilitiesassociated with the Tharsis basalt may be reasonable. In our calculations weuse a range of permeabilities from 10)6 m2 to 10)9 m2 in order to assess thedependence of the model results upon aquifer permeability.

Aside from the assignment of parameter values for the model, two addi-tional points must be addressed before results from the model can be


achieved. First, the discharge of water into the source trough will dependupon the length of the source trough that is actively fountaining water. Whilethe graben itself measures up to approximately 2000 km in length, the troughnear Mangala’s head is only about 220 km long. The model presented herewill yield a fracture width and a velocity for the ascending water, but thelength of the active graben is unclear. It seems likely that the active grabenlength was no greater than the 220 km of the trough, since this region istopographically isolated from the remainder of the graben to the east andwest. However, was the entire 220 km actively fountaining, or was a muchsmaller length of the trough the only portion along which water ascended tothe surface? Considering one additional point can place some further con-straints on this issue.

Conditions on Mars today are such that exposed liquid water will quicklyfreeze (Hecht, 2002), and sublimate into the atmosphere. With the workinghypothesis that conditions during the Late Hesperian/Early Amazonian atthe time of Mangala formation were like those of today, exposed liquid wateron the surface of Mars during that period of time would have experienced asimilar fate. Thus, as water discharged from the subsurface into the sourcetrough, freezing and sublimation of that water would have occurred. The rateof loss from the surface of the exposed water would have depended upon theambient temperature, the temperature-dependant saturation vapor pressure,and the molecular weight of water (Hecht, 1990; Leask et al., 2004). Assublimation progressed, the process might have stalled as the local atmo-sphere saturated with water vapor. However the shorter dimension of theexposed water surface (the graben width) is much less than the ~10 km scaleheight of the atmosphere, suggesting that the atmosphere is unlikely to havebecome saturated. Being conservative on the loss rate by assuming the watertemperature was just above freezing (1�C), water would have sublimatedfrom the surface of ponded water in the trough at a rate of 1.27 kg s)1 m)2.Dividing by the density of water, this translates into a depth reduction rate of0.69 mm s)1, or a discharge out of the trough, the total surface area of whichwe measure to be 5.13 · 108 m3, of ~354,000 m3 s)1.

Using the above determined loss rate, we can constrain the model bydetermining whether the calculated water discharge into the trough is greaterthan the vapor discharge out of the trough due to sublimation. For modelruns where the discharge in is less than the discharge out, the trough willnever fill up, in which case the parameter values assigned to that model runare inappropriate. In other cases, the model parameters may yield reasonabledischarges into the trough, but only for certain active graben lengths, inwhich case the minimum active graben length for which the model will workcan be obtained. Tables II–IV report the results for the model varying thevalues for the different model parameters. For each model configuration (agiven aquifer thickness, head drop and permeability) an active graben length


of 220 km was used. The last column in each table reports whether thedischarge into the trough is greater than the loss from sublimation. In caseswhere it is, the amount of time necessary to fill the source trough with waterprior to spillover to the north is reported, rounded to the nearest hour. Forthe cases where the trough successfully filled with an active graben length of220 km, the same model configurations were used for an active graben length

TABLE IIModel results – Scenario 1 with 8 km aquifer head drop

Permeability

(m2)

Active Graben

length (m)

Velocity

(m s)1)

Fracture

Width (m)

Discharge

(m3 s)1)

Time to fill

trough (hrs)

5 km aquifer thickness

1.00 · 10)6 16,708 (MAGL) 25.2 0.84 3.5 · 105 1.6 · 107

1.00 · 10)6 150,000 25.2 0.84 3.2 · 106 45

1.00 · 10)6 220,000 25.2 0.84 4.7 · 106 30

1.00 · 10)7 147,362 (MAGL) 10 0.24 3.5 · 105 7.1 · 107

1.00 · 10)7 150,000 10 0.24 3.6 · 105 2.0 · 104

1.00 · 10)7 220,000 10 0.24 5.3 · 105 733

1.00 · 10)8 220,000 3.79 0.064 3.5 · 104 Will not fill up

1.00 · 10)9 220,000 1.36 0.018 3.5 · 103 Will not fill up

7.5 km aquifer thickness

1.00 · 10)6 12,060 (MAGL) 28.2 1.04 3.5 · 105 4.4 · 106

1.00 · 10)6 150,000 28.2 1.04 4.4 · 106 32

1.00 · 10)6 220,000 28.2 1.04 6.5 · 106 21

1.00 · 10)7 101,629 (MAGL) 11.6 0.3 3.5 · 105 6.7 · 107

1.00 · 10)7 150,000 11.6 0.3 5.2 · 105 759

1.00 · 10)7 220,000 11.6 0.3 7.7 · 105 310

1.00 · 10)8 220,000 4.53 0.08 8.0 · 104 Will not fill up

1.00 · 10)9 220,000 1.67 0.022 8.1 · 103 Will not fill up


1.00 · 10)6 9417 (MAGL) 31.3 1.2 3.5 · 105 3.6 · 106

1.00 · 10)6 150,000 31.3 1.2 5.6 · 106 24

1.00 · 10)6 220,000 31.3 1.2 8.3 · 106 16

1.00 · 10)7 77,136 (MAGL) 13.1 0.35 3.5 · 105 8.2 · 107

1.00 · 10)7 150,000 13.1 0.35 6.9 · 105 382

1.00 · 10)7 220,000 13.1 0.35 1.0 · 106 195

1.00 · 10)8 220,000 5.07 0.094 1.0 · 105 Will not fill up

1.00 · 10)9 220,000 1.88 0.026 1.1 · 104 Will not fill up

Minimum active graben length (MAGL) necessary to fill the trough is labeled for each modelconfiguration under which the trough successfully fills.


of 150 km. Additionally, the minimum active graben length necessary to fillthe graben was determined for these model configurations.

As is clear from Tables II–IV, within the parameter space for all othervariables in the model, the trough will not fill up with aquifer permeabilitiesless than 10)7 m2. Even for certain active graben lengths, a permeability of10)7 m2 is still not feasible for filling the trough. Also, the trough fills up anorder of magnitude more quickly with an aquifer permeability of 10)6 m2

compared with a permeability of 10)7 m2 for a given active graben length.

TABLE IIIModel results – Scenario 1 with 10.5 km aquifer head drop

Permeability

(m2)

Active Graben

length (m)

Velocity

(m s)1)

Fracture

width (m)

Discharge

(m3 s)1)

Time to fill

trough (h)


1.00 · 10)6 7879 (MAGL) 33.5 1.34 3.5 · 105 6.0 · 106

1.00 · 10)6 150,000 33.5 1.34 6.7 · 106 20

1.00 · 10)6 220,000 33.5 1.34 9.9 · 106 13

1.00 · 10)7 71,333 (MAGL) 13.4 0.37 3.5 · 105 6.3 · 107

1.00 · 10)7 150,000 13.4 0.37 7.4 · 105 328

1.00 · 10)7 220,000 13.4 0.37 1.1 · 106 173

1.00 · 10)8 220,000 5.24 0.097 1.1 · 105 Will not fill up

1.00 · 10)9 220,000 1.89 0.027 1.1 · 104 Will not fill up


1.00 · 10)6 5422 (MAGL) 39.3 1.66 3.5 · 105 2.4 · 106

1.00 · 10)6 150,000 39.3 1.66 9.8 · 106 14

1.00 · 10)6 220,000 39.3 1.66 1.4 · 107 9

1.00 · 10)7 48,355 (MAGL) 15.9 0.46 3.5 · 105 8.7 · 107

1.00 · 10)7 150,000 15.9 0.46 1.1 · 106 172

1.00 · 10)7 220,000 15.9 0.46 1.6 · 106 102

1.00 · 10)8 220,000 6.16 0.12 1.6 · 105 Will not fill up

1.00 · 10)9 220,000 2.32 0.033 1.7 · 104 Will not fill up


1.00 · 10)6 4194 (MAGL) 43.7 1.93 3.5 · 105 2.2 · 106

1.00 · 10)6 150,000 43.7 1.93 1.3 · 107 10

1.00 · 10)6 220,000 43.7 1.93 1.9 · 107 7

1.00 · 10)7 36,386 (MAGL) 18 0.54 3.5 · 105 2.6 · 107

1.00 · 10)7 150,000 18 0.54 1.5 · 106 116

1.00 · 10)7 220,000 18 0.54 2.1 · 106 72

1.00 · 10)8 220,000 6.97 0.145 2.2 · 105 Will not fill up

1.00 · 10)9 220,000 2.62 0.039 2.2 · 104 Will not fill up



Rates of filling increase with increased aquifer thickness, as well as withhigher aquifer heads.

Another way to assess the results of the model is to plot the active grabenlength against the discharge for each of the four different permeabilities used(Figure 24a–c). As expected, for a given permeability, increasing the activegraben length increases the discharge into the trough. Also, for lower per-meabilities, the slope of the line decreases. One can also plot on these graphs

TABLE IVModel results – Scenario 1 with 13 km aquifer head drop

Permeability

(m2)

Active Graben

length (m)

Velocity

(m s)1)

Fracture

width (m)

Discharge

(m3 s)1)

Time to fill

trough (h)


1.00 · 10)6 5285 (MAGL) 39.6 1.69 3.5 · 105 4.9 · 106

1.00 · 10)6 150,000 39.6 1.69 1.0 · 107 13

1.00 · 10)6 220,000 39.6 1.69 1.5 · 107 9

1.00 · 10)7 48,971 (MAGL) 15.7 0.46 3.5 · 105 8.2 · 107

1.00 · 10)7 150,000 15.7 0.46 1.1 · 106 175

1.00 · 10)7 220,000 15.7 0.46 1.6 · 106 103

1.00 · 10)8 220,000 6.09 0.121 1.6 · 105 Will not fill up

1.00 · 10)9 220,000 2.25 0.033 1.6 · 104 Will not fill up


1.00 · 10)6 3624 (MAGL) 46.7 2.09 3.5 · 105 2.8 · 106

1.00 · 10)6 150,000 46.7 2.09 1.5 · 107 9

1.00 · 10)6 220,000 46.7 2.09 2.1 · 107 6

1.00 · 10)7 32,961 (MAGL) 18.5 0.58 3.5 · 105 2.8 · 107

1.00 · 10)7 150,000 18.5 0.58 1.6 · 106 102

1.00 · 10)7 220,000 18.5 0.58 2.4 · 106 64

1.00 · 10)8 220,000 7.26 0.152 2.4 · 105 Will not fill up

1.00 · 10)9 220,000 2.71 0.041 2.4 · 104 Will not fill up


1.00 · 10)6 2788 (MAGL) 52 2.44 3.5 · 105 1.7 · 106

1.00 · 10)6 150,000 52 2.44 1.9 · 107 7

1.00 · 10)6 220,000 52 2.44 2.8 · 107 5

1.00 · 10)7 24,767 (MAGL) 21 0.68 3.5 · 105 2.2 · 107

1.00 · 10)7 150,000 21 0.68 2.1 · 106 71

1.00 · 10)7 220,000 21 0.68 3.1 · 106 46

1.00 · 10)8 220,000 8.16 0.18 3.2 · 105 Will not fill up

1.00 · 10)9 220,000 3.09 0.048 3.3 · 104 Will not fill up



a horizontal line corresponding to the loss rate from the graben due tosublimation. The intersections of the permeability lines with the horizontalline correspond to the minimum active graben lengths necessary to fill thetrough. As seen, the lines for permeabilities of 10)8 and 10)9 m2 do notintersect the sublimation line, indicating that with aquifer permeabilities lessthan 10)7 m2 the trough will never fill up. Figures 24a–c step through fromthe thinnest aquifer with the lowest head drop, to the thickest aquifer andhighest head drop. As the aquifer thickness and head drop increase, theminimum active graben length necessary to fill the trough decreases, as notedby the progression of the intersection of the permeability lines toward the leftfrom Figure 24a–c. However, the permeability lines for 10)8 m)2 and 10)9

still do not intersect the sublimation line, even under conditions of very highhydraulic head.

4.4. MODEL RESULTS – SCENARIO 2

For the second model scenario we can solve the system using only Equations(6) and (7) by assigning a value for the fracture width. Assessing the fracturewidths obtained from the previous model scenario, an upper limit of about2 m is observed. Thus, we use 2 m as the fracture width for this modelscenario. The results are shown in Tables V–VII. All the same trends ob-served for the previous model scenario hold true in this case. That is, thedischarge into the trough increases with increasing aquifer thickness, headdrop and permeability. Of notable difference is that under the most favorableconditions for flow through the aquifer (7.5 or 10 km aquifer thickness, and13 km aquifer head height), permeabilities of 10)8 m2 yield discharges thatare able to fill up the source trough, although permeabilities of 10)9 m2 arestill insufficient to fill the trough under any conditions. Again, we can plot theactive graben length against discharge for the different permeabilities, alsoplotting the rate of loss from the trough due to sublimation (Figure 24d–f).As with the earlier scenario, the minimum active graben length for a givenpermeability decreases as the aquifer thickness and head drop increase. Fi-nally, for a given set of model parameters, the time necessary to fill the troughis shorter by up to several orders of magnitude for this model scenariocompared with the previous scenario.

5. Other Constraints on Discharge

The unusual circumstance of the presence of the notch connecting the en-larged trough of the Memnonia Fossae graben to the head of Mangala Vallisallows us to estimate the water discharge rate independently of the model


described in Section 4. Flow through the notch would have taken place inaccordance with the classical treatment of flow in open channels, which,using the Darcy–Weisbach equation as opposed to the Manning formula(Wilson et al., 2004a), gives the mean flow speed Un as

Figure 24. Graphs showing results from the model of active graben length versus dischargeinto the source trough, for permeabilities of 10 )6 m2 through 10 )9 m2. Horizontal line in eachplot corresponds to the loss rate from the source trough due to sublimation. Intersection of thesublimation line with a permeability line denotes the minimum active graben length necessary

to fill the trough under the model parameters corresponding to that plot. (a) Plot for an 8 kmaquifer head drop and a 5 km thick aquifer for model scenario 1. (b) Plot for a 10.5 kmaquifer head drop and a 7.5 km thick aquifer for model scenario 1. (c) Plot for a 13 km aquifer

head drop and a 10 km thick aquifer for model scenario 1. (d) Plot for an 8 km aquifer headdrop and a 5 km thick aquifer for model scenario 2. (e) Plot for a 10.5 km aquifer head dropand a 7.5 km thick aquifer for model scenario 2. (f) Plot for a 13 km aquifer head drop and a

10 km thick aquifer for model scenario 2.


Un ¼ ½ð8gRSÞ=fc�1=2;where R is the hydraulic radius, essentially equal to the depth of the flow, S isthe sine of the slope and fc is the Darcy–Weisbach friction factor. We use theDarcy–Weisbach equation as opposed to a modified Manning formula fordischarge calculations because of the non-dimensionality of the Darcy–Weisbach friction factor, and the explicit presence of gravity within theformula (Wilson et al., 2004a).

TABLE VModel results – Scenario 2 with 8 km aquifer head drop

Permeability

(m2)

Active Graben

Length (m)

Velocity

(m s)1)

Fracture

Width (m)

Discharge

(m3 s)1)

Time to Fill

Trough (hrs)


1.00 · 10)6 7308 (MAGL) 24.2 2 3.5 · 105 3.2 · 106

1.00 · 10)6 150,000 24.2 2 7.3 · 106 19

1.00 · 10)6 220,000 24.2 2 1.1 · 107 12

1.00 · 10)7 57,044 (MAGL) 3.1 2 3.5 · 105 2.2 · 107

1.00 · 10)7 150,000 3.1 2 9.3 · 105 222

1.00 · 10)7 220,000 3.1 2 1.4 · 106 126

1.00 · 10)8 220,000 0.31 2 1.4 · 105 Will not fill up

1.00 · 10)9 220,000 0.031 2 1.4 · 104 Will not fill up


1.00 · 10)6 5817 (MAGL) 30.4 2 3.5 · 105 1.9 · 107

1.00 · 10)6 150,000 30.4 2 9.1 · 106 15

1.00 · 10)6 220,000 30.4 2 1.3 · 107 10

1.00 · 10)7 38,443 (MAGL) 4.6 2 3.5 · 105 1.5 · 107

1.00 · 10)7 150,000 4.6 2 1.4 · 106 124

1.00 · 10)7 220,000 4.6 2 2.0 · 106 76

1.00 · 10)8 220,000 0.47 2 2.1 · 105 Will not fill up

1.00 · 10)9 220,000 0.046 2 2.0 · 104 Will not fill up


1.00 · 10)6 5141 (MAGL) 34.4 2 3.5 · 105 3.8 · 106

1.00 · 10)6 150,000 34.4 2 1.0 · 107 13

1.00 · 10)6 220,000 34.4 2 1.5 · 107 9

1.00 · 10)7 28,895 (MAGL) 6.12 2 3.5 · 105 1.6 · 107

1.00 · 10)7 150,000 6.12 2 1.8 · 106 86

1.00 · 10)7 220,000 6.12 2 2.7 · 106 55

1.00 · 10)8 220,000 0.62 2 2.7 · 105 Will not fill up

1.00 · 10)9 220,000 0.062 2 2.7 · 104 Will not fill up



We can estimate the flow depth by noting that the notch is cut down bywater erosion starting from the first overflow of water over the lowest pointon the north rim of the graben. As water began to overflow, it would havespread sideways on either side of the absolutely lowest point until the com-bination of the width of the region of overflow, together with the depth ofwater in the middle of the overflow zone, just accommodated the flux ofwater entering the graben from the subsurface aquifer. As long as large

Table VIModel results – Scenario 2 with 10.5 km aquifer head drop

Permeability

(m2)

Active Graben

length (m)

Velocity

(m s)1)

Fracture

width (m)

Discharge

(m3 s)1)

Time to fill

trough (h)


1.00 · 10)6 5509 (MAGL) 32.1 2 3.5 · 105 1.2 · 107

1.00 · 10)6 150,000 32.1 2 9.6 · 106 14

1.00 · 10)6 220,000 32.1 2 1.4 · 107 9

1.00 · 10)7 40,559 (MAGL) 4.36 2 3.5 · 105 1.7 · 107

1.00 · 10)7 150,000 4.36 2 1.3 · 106 134

1.00 · 10)7 220,000 4.36 2 1.9 · 106 82

1.00 · 10)8 220,000 0.44 2 1.9 · 105 Will not fill up

1.00 · 10)9 220,000 0.044 2 1.9 · 104 Will not fill up


1.00 · 10)6 4489 (MAGL) 39.4 2 3.5 · 105 1.9 · 106

1.00 · 10)6 150,000 39.4 2 1.2 · 107 11

1.00 · 10)6 220,000 39.4 2 1.7 · 107 8

1.00 · 10)7 27,206 (MAGL) 6.5 2 3.5 · 105 1.2 · 107

1.00 · 10)7 150,000 6.5 2 2.0 · 106 80

1.00 · 10)7 220,000 6.5 2 2.9 · 106 51

1.00 · 10)8 220,000 0.66 2 2.9 · 105 Will not fill up

1.00 · 10)9 220,000 0.066 2 2.9 · 104 Will not fill up


1.00 · 10)6 4029 (MAGL) 43.9 2 3.5 · 105 1.6 · 106

1.00 · 10)6 150,000 43.9 2 1.3 · 107 10

1.00 · 10)6 220,000 43.9 2 1.9 · 107 7

1.00 · 10)7 20,563 (MAGL) 8.6 2 3.5 · 105 7.7 · 106

1.00 · 10)7 150,000 8.6 2 2.6 · 106 57

1.00 · 10)7 220,000 8.6 2 3.8 · 106 37

1.00 · 10)8 220,000 0.88 2 3.9 · 105 Will not fill up

1.00 · 10)9 220,000 0.088 2 3.9 · 104 Will not fill up

Minimum active graben length (MAGL) necessary to fill the trough is labeled for each model

configuration under which the trough successfully fills.


changes in the supply flux did not occur, the width of the region of overflowwould have remained fairly stable as down-cutting began and continued toform the presently observed notch. Although, of course, this region of firstoverflow has now been destroyed, the pattern of kilometer-scale elevationfluctuations along nearby segments of the north rim (Leask, 2005) suggeststhat the notch width, ~5.5 km, would correspond to an initial water depth of

TABLE VIIModel results – Scenario 2 with 13 km aquifer head drop

Permeability

(m2)

Active Graben

length (m)

Velocity

(m s)1)

Fracture width

(m)

Discharge

(m3 s)1)

Time to fill

trough (h)


1.00 · 10)6 4594 (MAGL) 38.5 2 3.5 · 105 1.8 · 106

1.00 · 10)6 150,000 38.5 2 1.1 · 107 11

1.00 · 10)6 220,000 38.5 2 1.7 · 107 8

1.00 · 10)7 32,747 (MAGL) 5.4 2 3.5 · 105 2.1 · 108

1.00 · 10)7 150,000 5.4 2 1.6 · 106 101

1.00 · 10)7 220,000 5.4 2 2.4 · 106 63

1.00 · 10)8 220,000 0.54 2 2.4 · 105 Will not fill up

1.00 · 10)9 220,000 0.054 2 2.4 · 104 Will not fill up


1.00 · 10)6 3795 (MAGL) 46.6 2 3.5 · 105 4.7 · 106

1.00 · 10)6 150,000 46.6 2 1.4 · 107 9

1.00 · 10)6 220,000 46.6 2 2.1 · 107 6

1.00 · 10)7 21967 (MAGL) 8.05 2 3.5 · 105 7.5 · 107

1.00 · 10)7 150,000 8.05 2 2.4 · 106 62

1.00 · 10)7 220,000 8.05 2 3.5 · 106 40

1.00 · 10)8 218313 (MAGL) 0.81 2 3.5 · 105 2.1 · 109

1.00 · 10)8 220,000 0.81 2 3.6 · 105 4.7 · 104

1.00 · 10)9 220,000 0.081 2 3.6 · 104 Will not fill up


1.00 · 10)6 3428 (MAGL) 51.6 2 3.5 · 105 1.2 · 106

1.00 · 10)6 150,000 51.6 2 1.5 · 107 8

1.00 · 10)6 220,000 51.6 2 2.3 · 107 6

1.00 · 10)7 16,651 (MAGL) 10.62 2 3.5 · 105 5.3 · 108

1.00 · 10)7 150,000 10.62 2 3.2 · 106 45

1.00 · 10)7 220,000 10.62 2 4.7 · 106 30

1.00 · 10)8 162,233 (MAGL) 1.09 2 3.5 · 105 1.3 · 108

1.00 · 10)8 220,000 1.09 2 4.8 · 105 1015

1.00 · 10)9 220,000 0.11 2 4.8 · 104 Will not fill up



~20–30 m. To obtain estimates of the discharge corresponding to depths ofthis order we use these values of R=20 and 30 m in the above equation,together with the measured slope of S=0.058 and typical values of fc formartian channels with these water depths, 0.033 and 0.031, respectively(Wilson et al., 2004a). Multiplying the water speeds, 32.4 and 41.0 m s)1, bythe corresponding water depths and by the notch width we calculate dis-charges through the notch of 3.6 · 106 and 6.8 · 106 m3 s)1 for water depthsof 20 and 30 m, respectively. These values are very consistent with theanalysis of the channel system north of the notch by Komar (1979) who, aspart of a range of discharge estimates, found that for an average 50 m waterdepth the discharge was 8 · 106 m3 s)1.

As described in Section 3, we used MOLA data to calculate the missingvolume of material from Mangala, and by assuming a 40% by volume sed-iment content (Komar, 1980), calculated a minimum total volume of waterthat passed through Mangala Valles of between 2 · 1013 and 3 · 1013 m3. Asseen in Table VIII, combining these water volumes with the calculated fluxesimplies that flooding through Mangala lasted between 34 and 97 days. Theseestimates, especially the minimum of 34 days, are somewhat greater than theestimates made by previous investigators of between 6 and about 60 days(Tanaka and Chapman, 1990; Zimbelman et al., 1992), arrived at usingsimilar flow depths. Two factors contribute to this difference. First, ourestimates of the minimum total volume of flooding are between a factor offour and six times greater than previous estimates. Additionally, previousinvestigators used the Manning formula as opposed to the Darcy–Weisbachequation to calculate discharge rates and, as outlined in detail elsewhere(Wilson et al., 2004a), the Manning treatment tends to overestimate dis-charge rates by up to a factor of two. Also, to emphasize once again, the totalvolume estimated for the Mangala flooding is a minimum volume, andtherefore the estimates for duration of flooding through Mangala must alsobe considered minima.

This duration estimate is important because it allows us to address thejustification for our having used a steady-state model for water flow out ofthe aquifer. Manga (2004) has pointed out that immediately after the start of

TABLE VIIIFlood parameters through the notch, and estimates of the total flood duration through

Mangala Valles

Depth (m) Velocity (m s)1) Discharge (m3 s)1) Total water (m3) Flood duration (days)

20 32.4 3.6 · 106 2 · 1013 65

20 32.4 3.6 · 106 3 · 1013 97

30 41.0 6.8 · 106 2 · 1013 34

30 41.0 6.8 · 106 3 · 1013 51


drainage of an aquifer to the surface through a fissure, draw-down of theaquifer will create a pressure gradient much larger than that of the pre-releasehydrostatic pressure distribution. Neglect of this issue could potentially causeus to overestimate the permeability implied by any given calculated dischargerate. The characteristic time s needed for this transient high pressure gradientto decay depends on the lateral length scale L of the aquifer, which weinferred in Section 4.3 to be ~1700 km, via (Manga, 2004)

s ¼ ðL2gRÞ=ðqwgkÞ; ð11Þwhere R is the specific storage of the aquifer, estimated for martian aquifers byManga (2004) as 10)6 m)1. Inserting the relevant values we find s to be close to9 days for k = 10)6 m2, 90 days for k = 10)7 m2, 900 days for k=10)8 m2,and so on. The fact that we require 34–97 days for the formation of theMangala Valles then implies that, for permeabilites of 10)7 m2 or more, theinitial transient high pressure gradient in the aquifer would have relaxed duringthe early part of the flood event. Thus our findings in Section 4, that therequired discharge rate can only be sustained by permeabilities of order10)7 m2 or more, is self-consistent with the steady-state model used.

6. Synthesis of Model Results

As suggested earlier, the two approaches taken to modeling the source andsupply of the Mangala floodwater serve as two endmember scenarios. In thefirst scenario the fracture up which water ascends is held open entirely by thepressure exerted on it by the water, and in the second scenario the fracture isheld open regardless of the water pressure. Both cases are simplistic in thatwe are assuming a near vertical fracture that retains constant width for theentire depth of the cryosphere. However, to a first order, the two scenariosprobably span the spectrum of discharges into the graben.

Examining the results for the two model scenarios from Section 4shown in Tables II–VII, it is clear that while the minimum active grabenlengths for each model configuration technically solve the system, thetimes required to fill the source trough for such active graben lengths areunrealistic. For example, in Table II, for a 5 km thick aquifer with apermeability of 10)7 m2, a minimum active graben length of approxi-mately 147 km is calculated to fill the source trough in 7.1· 107 h,equivalent to 8100 years. Thus, while minimum active graben lengths canbe determined, the results of the model suggest that the observed 220 kmgraben length yields much more reasonable time frames for filling thesource trough. Indeed, comparison of the trough relative to the rest of thegraben to the east and west shows that significant volumes of bedrock


were removed from the graben in the vicinity of Mangala. We suggest thatthe rise of the groundwater under high hydraulic head likely served as anopportune mechanism via which to disrupt, erode and remove that missingrock, in which case the full length of the trough, 220 km, was likelyactively supplying water to the surface.

Assessing the results for the various model configurations from the firstmodel scenario, with an active graben length of the observed 220 km, wefind that discharges into the graben range from 2.8 · 107 m3/s for a per-meability of 10)6 m2 to 5.3 · 105 m3 s)1 for a permeability of 10)7 m2, withcorresponding times to fill the trough of 5 and 733 h. For the second modelscenario we find discharges for an active graben length of 220 km rangingfrom 2.3 · 107 m3 s)1 for a permeability of 10)6 m2 to 4.8 · 105 m3 s)1 fora permeability of 10)8 m2, with corresponding times to fill the trough of 6and 1015 h. Using the arguments in Section 5, we are able to greatlynarrow down the likely range of discharge rates to values in the range 3.6–6.8 · 106 m3 s)1. Taking these results together with those from Section 4suggests that k lies between 3.2 and 4.5 · 10)7 m2 and that the time to fillthe trough lay between 19 and 35 h, i.e. the order of one day.

Figure 25. Timeline of major events at the Mangala Valles region.


Figure 26. Perspective sketch diagrams outlining the evolution of flooding at Mangala Valles.(a) Initial propagation of dike, and corresponding development of eastern portion of sourcetrough. Local flooding of trough results in spill-over to the south into nearby crater. (b)

Continued propagation of the dike and complete development of the source trough. Filling oftrough results in spill-over to the north through the ejecta of crater A (see Figure 16), andthrough the area currently occupied by the notch. (c) Continued discharge from the subsurface

results in continued spill-over from the graben and widespread sheet flooding, bounded to theeast by the north–south trending ridge and to the west by the Noachian cratered plains. Sheetflooding banked up against two northeast–southwest trending wrinkle ridges. Flooding

overtopped and down cut these ridges, continuing northward. Topographic data suggestsinitial flooding north of the wrinkle ridges carved out the eastern of the two branches observedto the north. (d) Continued flooding resulted in coalescence of flooding into the area of thenotch, and subsidence of the flood level as the channel continued to down-cut the terrain. (e)

Flood levels subsided further, resulting in flooding occupying the deep, western interiorchannel of Mangala. Also, a new branch north of the two wrinkle ridges developed, west of theprevious branch. (f) The present geology of Mangala Valles and surrounding region.


7. Discussion and Implications

The results of our analysis ofMangala Valles serve to piece together a timelinefor the overall evolutionof the region (Figures 25 and26). Prior to flooding, theNoachian cratered plains were emplaced, alongwith the circum-Tharsis ridges,one of which flanks Mangala to the east. This ridge forms a portion of theeastern boundary of a broad, north–south trending depression, flanked to thewest by the Noachian plains, and into which Mangala eventually formed.During the Hesperian (or Late Noachian) the depression was resurfaced byvolcanic flows, possibly related to Tharsis. As the internal heat of Mars sub-sided, a globe-encircling cryosphere developed, which served to sequestergroundwater under high hydraulic head (Clifford, 1993). During the LateHesperian or Early Amazonian, the Memnonia Fossae formed, possibly as aresult of the lateral propagation of dikes radially away from the Tharsis rise(Wilson and Head, 2002a). As one of the graben formed, and cracked thecryosphere, extrusion of water occurred (Wilson and Head, 2004), ultimatelyenlarging the graben locally into a trough in the area of the north–southtrending valley. Formation of the troughmay have been progressive, from eastto west over the course of several days, possibly accounting for apparent flooddeposits within a crater to the southeast of the trough. Regardless, after thetrough had developed across the 220 km of the valley, it filled with water,eventually spilling over to the north through the ejecta of an impact crater.Spillover ultimately coalesced into one region of the ejecta, eventually down-cutting 500 m to form the present notch.Aswater spilled to the north out of thenotch, it spread outward, resulting in massive sheet flooding. Similar to thepresent day topography south of the source trough, the topography north ofthe trough had a northwest slope, resulting in deeper flow along the west of thevalley, and leading to greater erosion along the west as compared with the east.As the water traveled northward it eventually encountered a wrinkle ridge upagainst which it banked. Flooding overtopped this ridge, and approximately50 km further to the north encountered a second ridge, although less over-topping of the northern ridge occurred. Instead, the floodwater appears to havedown cut through the western portion of the northern ridge. Flooding north-ward of the northern ridge appears to have been more channelized. Twobranches are observed, an eastern and western branch. Topographic datasuggest that the eastern branch formed first, with continued erosion resulting indevelopment of the western branch, after which point all further floodingtraveled along the western branch. However, crater statistics from previousinvestigations suggest the opposite sequence, and thus further investigation ofthe northern reaches of Mangala Valles is required to address this apparentdiscrepancy. As the hydraulic head driving the flooding approached equilib-rium, flooding subsided, residual water eventually freezing entirely, with the


last dregs sublimating away to leave a residuewithin the deepest portions of thechannel.

The above-proposed timeline prompts an additional question. Why are nooutflow channels associated with the other Memnonia Fossae graben?

While Mangala heads at one of the Memnonia Fossae graben, there aremany tens of other graben further south of the source graben that not onlydo not have a prominent outflow channel emanating from them, but whichshow no sign of any fluvial activity whatsoever. What conditions at thesource graben were unique such that extrusion of significant volumes ofwater occurred? Zimbelman et al. (1992) suggested that the north–southtrending ridge that flanks Mangala to the east was emplaced via compres-sional faulting, and that it was the intersection of this thrust fault with thenormal faults of the source graben that made for an opportune location forwater to ascend to the surface. However, there are many additional north–south trending ridges that parallel the southwest margins of Tharsis andintersect other graben of the Memnonia Fossae, none of which led to outflowchannel development. Thus, while the ridges may have been emplaced viafaulting, the intersection of a thrust fault and normal fault does not appearsufficient to account for the location of the flooding.

We suggest two possible alternative solutions to the problem. First, thesource graben for Mangala is, with the exception of one other graben, thefurthest north of all the Memnonia Fossae. As a result, the hydraulic headassociated with that graben’s location would have been higher than the headassociated with the more southerly graben. Thus, one possible reason for thepresence of an outflow channel at the Mangala source graben, and absence ofchannels associated with the more southerly graben, is that the hydraulichead for the more southerly graben was simply insufficient to prop open thefracture through the cryosphere. A complication with this solution however,is that the driving gradient for the flooding is more likely to be up the Tharsisrise, and not south up the southern highlands. Thus, the head, while greaterfor the source graben, may not have been significantly greater.

A second possible solution appeals to the timing of development of thevarious graben of the Memnonia Fossae. As discussed by Ernst et al. (1995),the various dikes associated with large dike swarms on Earth can be em-placed over timescales of several hundred thousand years, much longer thanthe time we envision here associated with flooding at Mangala. Thus, the dikebelow the source graben for Mangala may have been emplaced prior to allthe other Memnonia Fossae, in which case the aquifer may have been de-pleted sufficiently due to the Mangala flooding such that further grabenemplacement tapped an aquifer that no longer had a large hydraulic head.The exact solution to the problem is unclear, but could conceivably be acombination of all the factors described above.


Finally, continuing the attempts of previous investigators to constrain thetotal duration of flooding at Mangala (Tanaka and Chapman, 1990;Zimbelman et al., 1992), we summarize the results of our investigation toattempt to answer this question. We emphasize again that the assumptionthat the flood carried up to 40% by volume eroded material leads to aminimum total volume estimated for the Mangala flooding, and therefore theestimates of duration of flooding through Mangala must also be consideredminima. With these caveats, we estimate that flooding through Mangalaprobably occurred over a 1–3 month period, very long compared with theone day required to fill up the source trough.

8. Summary and Conclusions

This examination of the Mangala Valles channel system indicates that thechannel’s interior geology can be reasonably explained by a single period offlooding as opposed to multiple periods separated by millions of years, as hadpreviously been proposed. Analysis of the source trough forMangala suggeststhat early spill-over from the troughquickly coalesced into the area of the notchin the trough’s northern wall, which was progressively down cut to its presentstate. Therefore, use of the notch in order to determine themeandischarge ratesof flooding through Mangala is reasonable, and our calculations indicate thatwater discharged through the notch at rates of up to ~7 · 106 m3 s)1, sug-gesting that the total duration of flooding through Mangala lasted for one tothree months. Additionally, a model developed for the source and supply ofwater to Mangala, consistent with the global hydrologic model of Clifford(1993), indicates that water discharging into the trough at the above rate tookonly about one day to entirely fill the trough and overflow to the north. Despitethe uncertainties in estimating the permeability of the aquifer feeding this flood,we do not seem to be able to escape the conclusion that the aquifer had apermeability of at least a few time 10)7 m2, ~300 times larger than the valuesassociated with even the most permeable aquifers on Earth.

Acknowledgements

Thanks are extended to Caleb Fassett, David Shean, Karl Mitchell, HaraldLeask, Jim Zimbelman, Bob Craddock, Mary Chapman, and Ken Tanakafor productive discussions about this work, and to Jim Zimbelman and BobCraddock for detailed reviews of the manuscript. Thanks to Caleb Fassettand David Shean for their help with data processing and figure preparation.JWH gratefully acknowledges grants from the NASA Mars Data AnalysisProgram and the NASA Planetary Geology and Geophysics Program. LWgratefully acknowledges PPARC grant PPA/G/S/2000/00521.


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