Syrtis Major and Isidis Basin contact: Morphological and ... Major and Isidis Basin contact:...

Syrtis Major and Isidis Basin contact: Morphological and topographic

characteristics of Syrtis Major lava flows and material of the Vastitas

Borealis Formation

Mikhail A. IvanovVernadsky Institute for Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

James W. Head IIIDepartment of Geological Sciences, Brown University, Providence, Rhode Island, USA

Received 18 October 2002; revised 10 March 2003; accepted 14 April 2003; published 28 June 2003.

[1] The floor of Isidis Basin is covered by materials of the Vastitas Borealis Formation(VBF) that appear to be emplaced essentially as a single unit. Along its westernboundary, Isidis Basin is in contact with volcanic flows from Syrtis Major Planum. Thecontact between the Isidis unit and volcanic flows from Syrtis Major is sharp togradational and in places is characterized by a high (�500 m) scarp or by a network offaults that separate pieces of lava plains off the main plateau of Syrtis. Clusters of knobsand mesas, sometimes arranged in flow-like features, are also typical features of thetransition zone. Several important characteristics of the transition from Syrtis Major toIsidis Basin are documented. (1) The small-scale surface texture seen in MOC imagesappears to be the same for both the Syrtis lava plateau and the knobs and mesas thatcharacterize the transition. (2) There is strong evidence for the breakup of the coherentsurface of Syrtis Major where it is in contact with materials in Isidis Basin. (3) Theplateau breakup (the knobby terrain) occurs basinward after the major break of slope ofSyrtis Major where it enters the Isidis Basin. (4) There is no evidence for plateaubreakup anywhere up on the slopes of Syrtis Major Planum. (5) The lavas of Syrtisremain morphologically intact where they are in contact with other units, such as theNoachian cratered terrain or where lava flows are stacked within Syrtis Major itself.These characteristic features of the transition zone from Syrtis to Isidis are readilyexplained if the zone of plateau breakup consists of relatively young lava flows that havebeen superimposed onto the surface of a volatile-rich substratum, such as the interiorunit of Isidis Basin (the Vastitas Borealis Formation). Thus simple superposition ofvolcanic materials on top of volatile-bearing sediments can explain the key geologicaland topographic aspects of the transition zone from Syrtis Major to Isidis Basin. On thebasis of our findings, we outline the following scenario for the evolution of this region.In the Early Hesperian, volcanic plains are emplaced in Syrtis Major (the lower part ofthe Syrtis Major Formation), and wrinkle ridges deform their surfaces soon thereafter.Concurrently, volcanic plains are emplaced on the floor of the Isidis Basin, and wrinkleridges deform their surfaces soon thereafter. The apparent simultaneity of these unitsmay mean that Syrtis Major was the source of many of the flows in the Isidis Basin. Inthe early part of the Upper Hesperian, subsequent to the formation of most of thewrinkle ridges, the Vastitas Borealis Formation was emplaced in the Isidis Basin andelsewhere in the northern lowlands. Following the emplacement of the Vastitas BorealisFormation, the upper part of the Syrtis Major Formation was emplaced, erupting fromthe eastern margins of Syrtis Major Planum and flowing down into the westernmost partof the Isidis Basin on top of the recently emplaced Vastitas Borealis Formation.Modification of the superposed lavas by degradation and evolution of the VBF formedthe scarps and unusual morphology of the marginal areas. We found no compellingevidence for massive or sudden erosion from Syrtis Major to produce the plainscurrently on the surface of the floor of the Isidis Basin (the Vastitas Borealis

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E6, 5063, doi:10.1029/2002JE001994, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JE001994$09.00

17 - 1

Formation). INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5480 Planetology: Solid

Surface Planets: Volcanism (8450); 5470 Planetology: Solid Surface Planets: Surface materials and

properties; 5464 Planetology: Solid Surface Planets: Remote sensing; 5415 Planetology: Solid Surface

Planets: Erosion and weathering; KEYWORDS: Mars, Isidis Basin, Vastitis Borealis Formation

Citation: Ivanov, M. A., and J. W. Head III, Syrtis Major and Isidis Basin contact: Morphological and topographic characteristics of

Syrtis Major lava flows and material of the Vastitas Borealis Formation, J. Geophys. Res., 108(E6), 5063, doi:10.1029/2002JE001994,

2003.

1. Introduction

[2] The largest ancient impact structures on Mars mayhave potentially served as first-order sinks for volatiles.Among these, the Isidis Basin [Wilhelms, 1973; Schultzand Frey, 1990] could be a unique site because (1) it is alarge basin at the very end of the southern highlands with ahighly degraded and lowered north-eastern rim and (2) it isin direct contact with the extensive volcanic plains of SyrtisMajor. Specific features at the contact between lava flowsfrom Syrtis and materials covering the floor of Isidis mayillustrate key aspects of the nature of materials in Isidis Basinand the timing of emplacement of both Syrtis Major andIsidis Basin material units. The history of the Isidis Basin isof sufficient interest since it has been designated the landingsite for the Beagle 2 lander [Bridges et al., 2003].[3] According to the geological mapping of the eastern

equatorial region of Mars by Greeley and Guest [1987], fiveprincipal units are represented in the area of the transitionfrom Syrtis Major Planum to Isidis Basin. The oldest is theunit HNu (plateau and high-plains undivided material) ofNoachian/Hesperian age that occurs as a relatively smallarea directly at the boundary between Syrtis Major andIsidis Basin. The surface of the unit appears as a tight andchaotic collection of knobs, hills, and mesas of different sizeand orientation. The next unit, Hs (Syrtis Major Formation,Late Hesperian), covers the surface of Syrtis Planum andforms vast, morphologically smooth plains with individuallava flows occasionally seen in places [Schaber, 1982;Greeley and Guest, 1987]. Three units make up the Isidislowland. The oldest is the ridged member of the VastitasBorealis Formation, unit Hvr, Late Hesperian in age, thatoccupies the central portion of the basin and is surroundedby an annulus of two Amazonian units, Aps (smooth plainsmaterial) and Apk (knobby plains material).[4] The new topography and image data provided by the

Mars Global Surveyor (MGS) spacecraft have led toreevaluation of the stratigraphic scheme for the northernlowlands and Isidis Basin as well [Tanaka and MacKinnon,1999; Tanaka and Kolb, 2001; Tanaka et al., 2002a]. Thenew stratigraphy includes eight units, and the four oldestones appear to be most important in the understanding ofthe late Noachian - late Hesperian evolution of the northernplains of Mars. The units (from the older to younger) are asfollows [Tanaka et al., 2002a]: (1) The knobby plateau unitthat occurs at the transition from the southern crateredhighlands to the northern lowlands. The unit is interpretedto be formed due to massive erosion of the highlandmaterials; (2) The boundary plains material, which istopographically lower than the previous unit and is inter-preted to be due to the erosion and redeposition of highlandmaterials; (3) The channeled plains materials that occurwithin the floor of the large circum-Chryse outflow chan-

nels. This unit is absent in the area of our study; (4) TheVastitas Borealis Formation, which is the most widespreadunit in the northern lowlands and within Isidis Basin aswell. In the earlier stratigraphic schemes, the VastitasBorealis Formation had been divided into grooved, ridged,mottled, and knobby members [Scott and Tanaka, 1986;Greeley and Guest, 1987; Tanaka and Scott, 1987]. Theridged member dominates in the central portion of IsidisBasin [Greeley and Guest, 1987; Grizzaffi and Schultz,1989; Kargel et al., 1995].[5] The new stratigraphic scheme is closely related to a

specific mechanism for the formation of the lowland units.This is the sedimentary filling of the northern plains due tomassive erosion of southern highlands [Tanaka, 1997;Tanaka and Kolb, 2001; Tanaka et al., 2000, 2001a,2002a, 2002b]. One of the specific elements of this mecha-nism is the catastrophic erosion of material from rims oflarge impact basins where large areas of the Hesperianridged plains occur, such as Hesperia and Malea Plana atHellas basin, Syrtis Major Planum at Isidis Basin [Tanaka etal., 2001b, 2002b], and within Chryse Planitia [Dohm et al.,2001].[6] Tanaka et al. [2000] describe the nature of the interior

unit presently covering the Isidis Basin (predominantly theVastitas Borealis Formation) and its relation to the wrinkle-ridged volcanic plains materials of Syrtis Major Planum.They describe the character of the interior plains unit andconclude that the ‘‘unit grades into knobby plains materialsin western Isidis Planitia, which appears to have formed bythe degradation of the wrinkle-ridged, Hesperian plateaurocks of Syrtis Major Planum and the underlying andadjacent Noachian cratered materials of Arabia Terra.’’They describe a scenario in which ‘‘catastrophic breakupof Syrtis Major Planum rocks produced a series of massflows that become deposited within Isidis Basin as essen-tially one unit’’ (the present Vastitas Borealis Formation).They further speculate that ‘‘magmatic activity at SyrtisMajor Planum may have instigated hydrothermal activityalong the Isidis margin’’ to produce the catastrophic event.[7] Tanaka et al. [2002b] outline a more detailed scenario

for the Hellas Basin rim in which they propose a three stagemodel (see their Figure 3): 1) sills intrude shallow friablerocks of the basin rim and mobilize volatile rich sediment;2) surface rocks are fluidized and flow into the adjacentbasin, removing massifs from the basin margin, and pro-ducing volatile-rich sedimentary deposits across the basinfloor; 3) subsequent eruptions cover eroded basin rimsurface and basin inner slopes with lava plains, but leavedeposits in the basin interior floor exposed and largelyundisturbed. Tanaka et al. [2002b] also point out that thismodel can be applied to the Syrtis Major Planum-IsidisBasin origin [e.g., Tanaka et al., 2000].

17 - 2 IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT

[8] In our study, we analyzed in detail the transition zonefrom Syrtis Major Planum to Isidis Basin in order to addressthe following questions: (1) What is the general sequence ofevents in the transition zone; (2) How does this sequencecorrelate with the proposed origin of the Vastitas BorealisFormation [Greeley and Guest, 1987; Grizzaffi and Schultz,1989; Scott et al., 1995; Tanaka and Kolb, 2001; Tanaka etal., 2000, 2001a, 2002b]; (3) What is the possible nature ofthe Vastitas Borealis Formation; (4) What is the place androle of the Hesperian ridged plains that make up themajority of Syrtis Major?[9] We analyzed the contact between Syrtis Major and

Isidis Basin using the new MGS data such as the MOLAtopography (gridded into the 1/64 degree topographic mapand individual profiles as well) and MOC images with theresolution several meters per pixel. The geological contextof the area of our study was provided by the Viking Orbiter(VO) images of medium (180–200 m/px) and high (25–28m/px) resolution that cover significant and contiguous areasat the contact.[10] In the beginning of the paper we describe the general

characteristics of the transition zone that may have playedthe key role in interpretation of the morphology andgeologic history of the area under study. Then we describeand analyze two areas of detailed study using the completedata set available for this area. In the discussion section ofthe paper the above questions are addressed with theinformation collected during the study and a summary ofour work is given as conclusions.

2. Major Features of the Transition Zone

[11] The area of our study (Figure 1) is between 5–19�Nand 277–287�W, in the western portion of Isidis Basinwhere it is in contact with the lava plateau of Syrtis Major.The floor of the basin (Figure 1) is approximately outlinedby the �3500 m contour line below which the floor is veryflat with the variations in elevations less than 500 m over anarea about 800 � 800 km across. The floor is gently slopedtoward the southwestern sector of the basin. The wholeinterior portion of the basin is characterized by a largepositive gravitational anomaly up to 400–500 mGal [Smithet al., 1999; Zuber et al., 2000].[12] The floor of Isidis Basin is covered by material

which appears either as morphologically smooth to hillockyor as hosting numerous low and narrow curvilinear andarcuate ridges [Grizzaffi and Schultz, 1989]. The ridgedmember of the Vastitas Borealis Formation [Greeley andGuest, 1987; Tanaka et al., 2002a] characterizes the centralportion of the basin and it is the reference locality of theso-called ‘‘thumbprint terrain’’ [Lockwood et al., 1992] thatalso occurs elsewhere within the northern plains [Kargel etal., 1995]. The ridges are morphologically prominent and,as a rule, consist of dense chains of small mounds, many ofwhich have a central pit. The ridges are about 10–40 kmlong and 0.5–1 km wide and the individual mounds areabout 400 m wide on the average [Grizzaffi and Schultz,1989]. There is a variety of interpretation of the ridges andmounds ranging from a volcanic origin [Frey and Jarose-wich, 1982; Hodges and Moore, 1994], to ice-related land-forms such as pingos and moraines [Rossbacher andJudson, 1981; Lucchitta, 1981; Grizzaffi and Schultz,

1989; Kargel et al., 1995]. The thumbprint terrain, whichis the most important feature of the Isidis interior, does notoccur, however, within the transition zone from SyrtisMajor to Isidis Basin where the knobby terrain is the mostcommon feature.[13] Sinuous narrow ridges about 50–60 km long and

0.5–1.5 km wide represent another type of ridge withinIsidis. These ridges appear as continuous structures andtypically inserted in a narrow depression. This type of ridgeoccurs at the periphery of the Isidis floor (particularly, nearthe contact of Isidis and Syrtis) and commonly they areradial to the center of the basin. By their morphology, theseridges strongly resemble the ridges at the mouth of TiuVallis in Chryse Planitia and have been interpreted either assqueeze-ups between two ice floes [Lucchitta et al., 1986]or as esker-like features [Kargel and Strom, 1992; Kargel etal., 1995].[14] The high-resolution MOLA topographic data have

revealed that the floor of Isidis Basin is complicated by asystem of broad sinuous to arcuate ridges [Head et al.,2002] that represent the third type of ridge structures withinIsidis Basin (Figure 2). The ridges are about 50 m high witha few structures as high as about 150 m and several tens ofkm wide. They occur throughout the whole area of the Isidisfloor, and divide it into series of secondary basins about150–180 km across [Hiesinger and Head, 2002; H. Hie-singer and J. W. Head, The Syrtis Major volcanic province,Mars: Synthesis from Mars Global Surveyor data, manu-script in preparation, 2003 (hereinafter referred to as Hie-singer and Head, manuscript in preparation, 2003)]. Thefloor of these basins is typically flat or slightly concavedownward so the apparent depth of the basins mostlydepends on the height of the bordering ridges.[15] Ridges with the same topographic characteristics

typically occur within the northern plains where they arecovered by materials of the Vastitas Borealis Formation.There the ridges make a broad circumferential patternaround the Tharsis Rise and have been interpreted as buriedwrinkle ridges analogous to those exposed elsewhere onMars within the Hesperian ridged plains (Hr) [Head et al.,2002]. These ridges, both in Isidis and within the northernplains, have almost no morphologic expression on thesurface in Viking images, however, and have not beenincluded into the global data set of wrinkle ridges byChicarro et al. [1985]. Both the mascon within Isidis Basinand the presence of the broad ridges (probable wrinkleridges) on its floor are consistent with the interpretation oflava filling of the central portions of Isidis Basin from SyrtisMajor Planum.[16] The eastern portion of Syrtis Major Planum (Figure 1)

is on the inner wall of the Isidis Basin where the regionalslope toward the basin is about 0.7–1.0�. In the medium tolow-resolution VO images which are available for this area,the surface of Syrtis appears as morphologically smooth andrelatively featureless and only in some places are a fewshort and morphologically pristine lava flows visible. Theeastern portion of Syrtis Major is significantly differentfrom the rest of the Planum because it displays almost nomorphologic or topographic signatures of wrinkle ridgeswhile these are common structures elsewhere in SyrtisMajor Planum [Schaber, 1982; Hiesinger and Head,2002; Hiesinger and Head, manuscript in preparation,

IVANOV AND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT 17 - 3

2003]. The disappearance of wrinkle ridges within theeastern portion of Syrtis Major may mean that they arecovered and that this area has been flooded with lavarelatively recently in its evolution.[17] We performed crater counts within the eastern por-

tion of Syrtis in the area where wrinkle ridges are absent.The counts show that the number of craters larger than 5 kmper 106 km2 is one hundred sixty five (±25, Figure 3). Thisnumber of craters is very close to, and statistically indistin-guishable from, the crater density for the rest of SyrtisMajor [Hiesinger and Head, 2002; Hiesinger and Head,manuscript in preparation, 2003] and for the surface ofAmenthis trough to the southeast of the basin [Maxwell andMcGill, 1988; Grizzaffi and Schultz, 1989]. The craterdensity in eastern Syrtis and for the rest of it as well isequivalent to an Early Hesperian crater retention age[Tanaka, 1986]. Thus, if the late episode of volcanicresurfacing in eastern Syrtis indeed took place, it appearsto correspond to a rather short phase of volcanism that wasclose in time to the final stages of the formation of SyrtisMajor Planum.[18] The high-resolution MOLA data reveal that the

eastern portion of Syrtis hosts a number of very prominenttopographic ridges (Figure 4). The length of the ridges

varies from �100 up to 200–250 km. Their mean cross-sectional area is about 3 km2 (one sigma standard deviation,±1 km2). In several important aspects these ridges aredifferent from typical wrinkle ridges in the rest of SyrtisMajor Planum and in other areas of ridged plains on Mars:(1) The ridges in eastern Syrtis are oriented in W-E directionalong the higher topographic gradient and the distribution ofwrinkle ridges in the rest of Syrtis is less correlative, if at all,with the topography; (2) The ridges in eastern Syrtis are lesssinuous than typical wrinkle ridges in Syrtis and elsewhereon Mars; (3) The ridges in eastern Syrtis do not produce ananastomosing pattern of structures that is typical of wrinkleridges in other parts of Syrtis Major [Hiesinger and Head,2002; Hiesinger and Head, manuscript in preparation, 2003]and elsewhere [Head et al., 2002]; (4) There is no evidencefor the ridges in eastern Syrtis to be arranged en echelonwhile this pattern appears to be characteristic of wrinkleridges in the rest of Syrtis Major Planum; (5) The ridges ineastern Syrtis are systematically higher (100–400 m high)and broader (25–50 km wide) than the wrinkle ridgesnearby the eastern portion of the Syrtis lava plateau (50–80 m high, 15–30 km wide); (6) Some of the ridges ineastern Syrtis become broader down the regional slope;(7) The two largest ridges in the eastern Syrtis have a

Figure 1. (opposite) The distribution of topography within Isidis Planitia (blue and purple colors) and adjacent SyrtisMajor Planum (red-orange-yellow colors). The width of the map is about 1500 km. The floor of Isidis below �3.5 km isflat with variations in the relief less than 500 m over an area about 800 � 800 km. The eastern portion of Syrtis Majordisplays a steady slope toward Isidis with a mean slope of about 0.3–0.4�. MOLA gridded topography map with spatialresolution 1/64 of a degree. Contour interval is 500 m. Map projection is simple cylindrical. Red boxes indicate areas ofdetailed study. Color-coded topography the areas of detailed study, each about 240 km in width. a) Northern area. Thearcuate scarp between 281�W and 282�W and two flow-like features that start at about 16�N, 281.5�W and are oriented ineastern and northeastern directions are seen. The southern flow crosses a topographic ridge within Isidis Planitia (at about16�N, 280.5�W) without deflection. Note that both flows spread on top of two topographic ridges (buried wrinkle ridges[Head et al., 2002], right side of the map above the center). Red arrow indicates a mesa (remnant of Syrtis plateau). b) Themajor topographic features within the southern area are longitudinally oriented troughs within the HNu unit (between279�W and 280�W) and equidimensional (at 12.5�N, 278.5�W) and arcuate (at 11�N, 278.2�W) depressions at the easternboundary of this unit. Where a topographic ridge (a lava tube?) in the eastern portion of Syrtis Major comes to the plateauedge, there is a distinct topographic ridge within the HNu unit (center of the map).

Figure 2. Detrended topographic map of Isidis Planitia. The sinuous topographic ridges (contrastingbright features) make a radial-concentric pattern the center of which is shifted toward the western portionof Isidis. The center of the map is at 13�N, 272�W. Simple cylindrical projection. The high-pass filtercore is 50 km. From Head et al. [2002].


triangle-shaped cross-section and a narrow (<1 km) summitdepression. Distinct lava flows appear to emanate fromthese depressions.[19] The above distinctions strongly suggest a different

mode of origin for the ridges in eastern Syrtis and wrinkleridges within the rest of Syrtis. The many distinctivecharacteristic features of the eastern Syrtis ridges supportthe interpretation that they are either lava flows or tubes orperhaps both [Schaber, 1982]. Direct evidence for superpo-sition of lava flows associated with the ridges onto thesurrounding terrains (Figure 5) suggests that the ridges areamong the youngest features in eastern Syrtis Major.[20] The contact between the lava plains of Syrtis Major

and the floor of Isidis Basin (the transition zone) is the keyarea of our study. This territory illustrates the most impor-tant aspects of the interaction between lavas from Syrtis andmaterials on the floor of Isidis Planitia and the majorepisodes of the sequence of events during this interaction.The transition zone in the area of our study is sharp togradational and is characterized either by a prominent singlescarp or by a network of narrow troughs. A very character-istic feature of the transition zone is the knobby terrain that

consists of numerous small knobs and mesas. They aretypically arranged in elongated and equidimensional clus-ters and, in places, in flow-like features. A significantportion of the transition zone is occupied with unit HNu(undivided materials of the plateau and high-plains assem-blage) [Greeley and Guest, 1987]. Dense collections ofmesas, small knobs, and plates make up the surface of thisunit and, in fact, it represents a large and contiguous area ofknobby terrain. The details of the structure of the transitionzone are described in the following section.

3. Transition From Syrtis Major Planum toIsidis Basin

[21] We analyzed in detail the transition zone from SyrtisMajor Planum to the Isidis Basin in two neighboring areasthat appear to portray the key features of the transition. Thenorthern area is from �14� to 17.5�N and the southern areais from �10�N to 14�N.3.1. Northern Area

[22] A significant portion of the transition there is char-acterized by a scarp that extends as a wide arc convex

Figure 3. Results of crater counting in the eastern portion of Syrtis Major Planum and in the westernportion of Isidis Planitia where the ridged member of the Vastitas Borealis Formation is exposed. Thenumber of craters larger than 5 km in diameter is 165 (Early Hesperian) and 118 (Late Hesperian) for theeastern Syrtis and western Isidis, respectively. In both regions the crater count was done on the MDIMphotobase with a resolution about 200 m/px within a region of the same area (about 0.23 106 km2) andshape. For clarity, error bars are shown as light gray (Syrtis) and hatched (Isidis) areas.


toward Syrtis Major for about 125–300 km from 14�N toabout 15.5�N (Figures 1a, 6a, and 6b). The MOLA topo-graphic map de-trended with a hi-pass filter about 22 kmwide (Figure 7) shows that the zone of the scarp occurswhere the surface of Syrtis Major forms a gentle depressionconfined between two topographic ridges at about 16�N and14�N (Figure 8). This characteristic of the plateau topogra-phy weakly depends on the size of the filtering windowsand appears within their broad range from about 15 up to 50km [Head et al., 2002]. The floor of Isidis Basin adjacent tothe scarp is among the lowest areas within the entire basin(Figures 1a, 6a, and 6b) and is topographically flat andslightly sloped to the south.[23] We have measured the elevation of the scarp base

and rim in nine points corresponding to the major breaks inslope along five MOLA orbits that correspond to the MOCimages available for the scarp area. The difference betweenthe rim and the base elevation gives the height of the scarpat a given scarp exposure. The mean height of the scarp isabout 530 m with variations from 370 up to about 650meters (Table 1). The elevation of the scarp baseline is closeto �3800 m (mean elevation is about �3830 m) and thevariations of the elevation are as much as about 260 m overa distance of 125 km (Table 1).[24] Almost along its entire strike, the scarp cuts the

surface of Syrtis Major cleanly with little or no evidence forprogressive plateau breakup (Figure 8). A typical charac-teristic of the scarp is that its edges are scalloped atdifferent scales. The largest scale is represented by thearc-like shape of the scarp itself and has the width about

100 km (Figure 8a). The next scale of the sinuosity of thescarp is due to broad (about 20 km wide) alcoves at its edge(Figure 8b). The alcoves are cut by smaller niches that areabout 5 km wide and those are deformed by scallops withtypical width about 1.5 kilometers (Figure 8c). Due to this,the scarp appears as a sinuous feature in plan view and itsgeneral morphology does not favor a tectonic explanation ofits origin. In many cases, numerous large (about 700–800and up to 1000 m across), equidimensional, and angularblocks are present on the floor of Isidis Basin near the baseof the scarp (Figures 8a, 8c, and 8d). It is likely that only aportion of the blocks is exposed and that their actual size islarger and may correspond to the dimension of small-scalescallops at the scarp rim. In this case, the blocks may be dueto destruction of the scarp by small-scale landsliding andfall of large individual blocks. Direct evidence for fallenboulders and blocks (a few meters up to a few tens ofmeters) is seen in each MOC image of the scarp area. Thereis no evidence, however, on the floor of Isidis Basinadjacent to the scarp for the larger contiguous bodies oflandslide that may be responsible for the formation of thelarger-scale niches and alcoves of the scarp.[25] At about 14.5�N a large highstanding mesa is visible

on the floor of Isidis Basin (Figures 1a, 6a, and 6b). Themesa is separated from the mainland of Syrtis MajorPlanum by a gap about 11 km wide (Figure 8d). Morpho-logically, the surface of the mesa is indistinguishable fromthe surface of Syrtis Major lava plateau and continues thetopographic trend of the neighboring surface of Syrtis.These characteristics of the mesa strongly suggest that it

Figure 4. MOLA shaded relief map of the transition zone from Isidis Planitia (left) to Syrtis MajorPlanum. Width of image is �600 km. The appearance of the coarse and fine topographic details isemphasized by different directions of illumination. The sinuous and arcuate ridges are the mostprominent topographic features within Isidis (right side of the maps). The eastern portion of Syrtis Major(center and left side of the maps) is characterized by a series of topographic ridges oriented normal to thegeneral strike of the Isidis/Syrtis contact (central portions of the maps). To the west and south, ananastomosing pattern of narrow and sinuous wrinkle ridges in Syrtis Major is seen (right and lowerportions of the maps). The maps are based on the MOLA (1/64 of a degree) topographic map, projectionis simple cylindrical.


Figure 5. The surface of the eastern portion of SyrtisMajor Planum where one of the most prominent topo-graphic ridges occurs (center of the image from left toright). Width of image is �110 km. The crest area of theridge is occupied by a rille-like elongated depression (at theleft edge of the image). Distinct small lava flows (arrowsindicate lava fronts) emanate from the ridge which wasinterpreted [Schaber, 1982] as a lava tube. Fragment of theVO image 377S58 (resolution 218 m/px).

Figure 6. a) The morphology of the transition zone in the northern area with superposed contour lines,b), showing the variations in elevation along specific features of the transition zone (contour interval is50 m). An arrow indicates a mesa; the solid lines crossing the image are tracks of MOLA profiles. Widthof area is about 240 km. See text for details. Figures a) and b) are fragments of the MDIM photobase.

Figure 7. Detrended topographic map of the northernportion of the transition zone from Isidis Planitia to SyrtisMajor Planum. Width of image is about 300 km. The areawithin Syrtis Major bounded by the arcuate scarp isconfined by two topographic ridges (arrows) from the northand the south. The flow-like features in the northern part ofthe transition zone are the extensions of the northerntopographic ridge (presumably, a lava tube) into IsidisBasin. The high-pass filter core is 22 km. See Head et al.[2002] for details of the technique.


is a large piece of Syrtis Major. The floor of the gapbetween the mesa and the plateau is peppered with numer-ous large and small blocks (Figure 8d) and the edges of boththe mesa and the Syrtis plateau are etched with larger nichesand smaller scallops.[26] MOLA orbit 13813 crosses the southern portion of

the scarp almost normal to its strike (Figure 9). Suchgeometry allows for direct measurements of the topographicgradients at the scarp. MOC image M11-02034 (resolution2.94 m/px) corresponds to the MOLA orbit 13813 andshows the fine details of the scarp structure. The mostimportant feature of the scarp is that it consists of twodistinct parts, lower and upper, separated by a relativelynarrow (about 300 m wide) shelf-like feature (Figure 9a).

The surface of both parts of the scarp displays a lowbrightness and crisp morphology with evidence for layer-ing; this clearly represents the exposed cross-section of theSyrtis Major lava plateau. The surface of the shelf isbrighter and featureless and likely represents a debrisapron. The slope within the lower part of the scarp isabout 10.5�, within the shelf it is about 7�, and the slope isas high as about 25� for the upper portion of the scarp(Figure 9b).[27] The uppermost portion of the upper part of the scarp

appears to be slightly different (it is darker and has lesssubdued morphology) from the rest of the scarp and mayrepresent either a single lava flow or a stack of thin flowsindistinguishable at this resolution. There is a simple

Figure 8. Morphology of the large arcuate scarp in the northern area. a) A large alcove characterizesthis segment of the scarp. The typical size of these feature along the scarp is a few tens of km. A graben-like feature within Syrtis Major (left lower portion of the image) is cut by the scarp at the base of whichlarge angular blocks are seen on the floor of Isidis Basin (upper portion of the image). Fragment of theVO image 152S05 (26 m/px). b) Niches at the scarp (lower left corner) have typical width of about 5 km.Fragment of the VO image 151S01 (26 m/px). c) Scalloped edge of Syrtis Major Planum (left half of theimage) along the arcuate scarp. The scallops have a typical width of about 1–2 km. Angular andelongated blocks of about the same size as the size of the scallops are seen on the floor of Isidis Basin(right half of the image). Fragment of the VO image 152S06 (25 m/px). d) Gap between the main portionof Isidis Basin (lower left corner of the image) and a large mesa within Isidis (upper right corner of theimage). The edges of both the main plateau and the mesa are scalloped and a large number of rectangularand angular blocks are seen on the floor of the gap. Fragment of the VO image 152S07 (26 m/px). Allimages show the area about 13 � 13 km.

IVANOVAND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT 17 - 9

geometric relationship between width of a layer as it is seenin a map (Lv) and the true thickness of this layer (Ht):

Ht ¼ Lv= cos að Þð Þ* sin a� gð Þ;

where a and g are angles of the slope that cut the layer andthe dip of the layer, respectively. If an assumption is madethat the uppermost layer (or layers) within a lava plateau isparallel to the surface of the plateau, then it is possible toestimate the true thickness of lava flows and stacks. For theupper layer seen in the MOC image M11-02034 the meanthickness is about 95 m (±11 m). In our study we assumethat the steep slopes (25�) occur in other places along thescarp in its upper sections and estimated the true thicknessof lava stacks (relatively thick coherent layer with littleevidence for internal layering) or individual flows in fourother localities along the scarp (Table 2). The individuallayers exposed in the scarp have a thickness about 10–15 mand the mean thickness of the stacks is about 80 m varyingfrom about 40 up to 115 m (Table 2).

[28] To the north of the scarp, the transition from Syrtisto the Isidis Basin appears as two elongated flow-likefeatures (Figures 1a, 6a, and 6b). They are oriented normalto the general strike of the contact, and extend for 110–150km into the basin. In plan view, the features have amushroom-like shape and are about 8–12 km across wherethey are close to Syrtis and about 30–60 km at their distalends. These features represent a continuation of a topo-graphic ridge (interpreted to be of volcanic origin) at thenorthern margin of Syrtis Major. Near Syrtis, along aboutone third of the length of the features, both their sides arebounded by distinct scarps 300–450 m high on the average.There, the surface of the features is morphologically smooth,coherent, and similar to the surface of Syrtis Major at theresolution of both the VO and MOC images (Figure 10). Inthe direction from the main volcanic plateau of Syrtis towardthe east inside Isidis Basin, however, there is clear observa-tional evidence for the progressive plateau breakup andtransformation into knobby terrain, consisting of numerousknobs, plates, and mesas (Figures 10b–10d). The typical

Table 1. Elevations at the Base and Brim and Scarp Height Along the Arcuate Scarp and the Flow-Like Features in the Northern Areaa

MOLA Orbit

Break in Slope at the Scarp Base Break in Slope at the Scarp Brim

Scarp Height, mLatitude (N) Longitude (W) Elevation, m Latitude (N) Longitude (W) Elevation, m

Scarp12329 13.99 281.04 �3877.29 13.92 281.03 �3506.15 371.1413813 14.00 281.12 �3882.85 13.97 281.11 �3443.16 439.6911260 14.03 281.39 �3811.85 14.08 281.39 �3167.38 644.4711260 14.28 281.42 �3772.25 14.23 281.41 �3212.06 560.1911260 15.57 281.59 �3832.38 15.64 281.60 �3265.85 566.5316605 14.58 281.58 �3831.18 14.53 281.57 �3279.81 551.3716605 15.42 281.69 �3868.17 15.47 281.70 �3309.01 559.1612656 14.73 281.79 �3819.74 14.65 281.78 �3228.95 590.7912656 15.29 281.87 �3756.49 15.35 281.87 �3289.96 466.53MEAN: �3828.02 �3300.26 527.76

Southern Flow-like Feature, Southern Scarp11260 281.59 15.57 �3832.38 281.60 15.64 �3265.85 566.5313813 281.35 15.71 �3786.45 281.35 15.74 �3396.30 390.1512329 281.28 15.75 �3789.48 281.28 15.78 �3305.35 484.1313846 281.04 15.82 �3725.94 281.04 15.85 �3369.89 356.0515209 280.15 15.76 �3857.84 280.16 15.79 – –MEAN: �3798.42 �3334.35 449.22

Southern Flow-like Feature, Northern Scarp11260 281.65 15.96 �3779.14 281.64 15.89 �2922.59 856.5513813 281.38 15.91 �3739.59 281.38 15.89 �3292.49 447.1012329 281.31 16.02 �3807.70 281.30 15.94 �3406.83 400.8713846 281.06 15.99 �3832.54 281.06 15.96 �3672.83 159.7115209 280.21 16.21 �3762.05 280.21 16.18 – –MEAN: �3784.20 �3323.685 466.06

Northern Flow-like Feature, Southern Scarp11260 281.66 16.17 �3790.95 281.68 16.21 �3230.62 560.3313813 281.43 16.27 �3783.10 281.43 16.30 �3253.87 529.2312329 281.33 16.17 �3819.11 281.34 16.20 �3353.32 465.7913846 281.12 16.41 �3764.38 281.13 16.45 �3402.97 361.4115209 280.29 16.77 �3772.45 280.32 16.93 – –MEAN: �3786.00 �3310.19 479.19

Northern Flow-like Feature, Northern Scarp13813 281.46 16.47 �3726.76 281.45 16.44 �3284.29 442.4712329 281.39 16.57 �3704.97 281.38 16.52 �3315.38 389.5913846 281.17 16.78 �3678.53 281.17 16.76 �3540.91 137.6215209 280.34 17.10 �3771.87 280.32 16.93 – –MEAN: �3720.53 �3380.19 323.23aNote: The MOLA orbit 15209 crosses the flow-like features where they do not have distinct scarp.

17 - 10 IVANOVAND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT

size of the features of the knobby terrain is about 2 km(±0.8 km).[29] As in the case of the arcuate scarp to the south, the

edges of the flow-like features are scalloped at different

wavelengths. The larger alcoves in the bounding scarpshave an average width of about 12 km, twice as small asthe typical width of alcoves at the arcuate scarp. In places,large angular blocks appear to be rubble within and in close

Figure 9. The high-resolution image (MOC image M11-02034, 2.94 m/px) and the correspondingMOLA orbit 13813 show details of the structure of the arcuate scarp. See text for description. Fragmentof MOC image shows area about 1.2 � 3.4 km.

Table 2. True Thickness of Uppermost Lava Stacks at the Eastern Edge of Syrtis Major Planum

MOCImage

Resolution,m/px Latitude, N Longitude, W Profile Layer/Stack Lv, m Ht, m

M07-02742 4.42 14.37 281.1 PR1 stack1 168.8 78.8PR1 stack2 196.9 92.0PR1 stack3 209.6 97.9

MEAN: 89.6M11-02034 2.94 14.23 281.17 PR1 stack1 226.4 105.7

PR2 stack1 220.5 103.0PR3 stack1 173.5 81.0PR4 stack1 199.1 93.0

MEAN: 92.3M08-02589 2.95 14.93 281.82 PR1 stack1 82.6 38.6

PR1 stack2 103.3 48.2PR2 stack1 82.6 38.6PR2 stack2 135.7 63.4PR3 stack1 102.9 51.0PR3 stack2 102.9 51.0

MEAN: 55.1M10-01296 2.95 15.08 281.91 PR1 stack1 215.4 100.6

PR2 stack1 247.8 115.7MEAN: 108.2

TOTAL MEAN: 77.2


proximity to alcoves (Figure 10b) on the floor of Isidisproviding direct evidence for the collapse of the flowedges.[30] We have measured the elevations of the rims and

bases on both sides of the flow-like features in places wherethey are crossed by five MOLA orbits corresponding to theavailable MOC images for this area (Table 1). As it is seenfrom the table, the elevation of the rims of each boundingscarp becomes progressively lower basinward whereas theelevations of the scarp base are much more even. This isbecause the surface of the flow-like features continues thetopographic trend of Syrtis Major and leads to progressivediminishing of the height of the bounding scarps. Due tothis, both flow-like features appear to be thinning outtoward the interior of the Isidis Basin. Although the

elevation of the baseline of the features is more stable, itshows significant variation from baseline to baseline andfrom feature to feature. The maximum differences in eleva-tion along the bounding scarps are about 100 m over adistance of about 80 kilometers.[31] A specific characteristic of the flow-like features is

that their surface texture changes along strike. Near Syrtis,where the flows are coherent and bounded by the scarpfeatures their surface has about the same slope as the surfacewithin Syrtis. The regional slope within Syrtis in thevicinity of the flows is about 0.3� over a distance of about40 km and the slope within the flows is about 0.2� over thesame distance. At some point, however, at �16�N, 280.7�Wfor the southern flow and �16.5�N, 281.1�W for thenorthern flow, the topographic profile of the flows becomes

Figure 10. Progressive changes of texture of the surface of Syrtis Major Planum along the southernflow-like feature in the northern portion of the transition zone. Width of each image is about 14 km.a) The surface of Syrtis Major within the plateau is morphologically intact. Material on the surface of theplateau embays old cratered terrain. Fragment of the VO image 155S19 (28 m/px). b) The area where theflow-like feature begins. The surface of the flow appears to be morphologically intact and analogous tothe surface of Syrtis Major. At the northern side of the feature (center of the image), however, a largealcove with additional smaller scallops cuts the flow. Large angular blocks are rubble on the floor ofIsidis Basin within the alcove. Fragment of the VO image 153S15 (27 m/px). c) The surface of the flow atabout its middle (large diagonal feature in the lower left portion of the image) where the flow is disruptedinto plates. Fragment of the VO image 153S17 (28 m/px). d) At the distal end of the southern flow, itappears as a cluster of knobs, plates, and mesas. Fragment of the VO image 152S14 (27 m/px). Allimages show the area about 13 � 13 km.


nearly horizontal (Figure 11). Starting from this point, thesurface of the flow-like features breaks up and consists ofnumerous tightly spaced knobs (Figure 10d) that becomeprogressively less dense outward. The total height of thefeatures within these disrupted parts is about 100–200 mand remains approximately the same along the strike of thefeatures. It is important to note that the height of the flow-features in their horizontal part is either almost equal ordouble the thickness of the uppermost lava stacks within thearcuate scarp to the south.[32] MOLA orbit 15209 crosses the southern flow-like

feature where it shows the dense cluster of knobs and mesasand MOC image M15-00469 (resolution is 2.95 m/px)corresponding to this orbit shows a single small (about1.9 km across and about 140 m tall) mesa. A debris apronsurrounds the mesa but on its northern slope at least twothick layers or layer stacks are exposed. The fine-scale

texture of the mesa surface is the same as within the lavaplateau of Syrtis Major leaving little doubt that the mesa is apiece of the plateau. The surface of the mesa slopes slightly(at about 1.3�) to the north and appears to be broken withinits northern third. The high-resolution MOLA topographyand MOC images allow us to estimate the thickness of theuppermost layer (or possible stack of layers) exposed at themesa walls. The estimated thickness is about 75–80 m,which is very close to the estimates of the thickness oflayers exposed within the arcuate scarp to the south.

3.2. Southern Area

[33] The southern area (Figures 1b and 12) is dominatedby material of the unit HNu [Greeley and Guest, 1987]that consists of a great number of knobs, plates, and mesas(Figure 12). These features have typical sizes of about 3–5 km across (and up to a few tens of km), and thus they are

Figure 11. Topographic profile along the southern flow-like feature. The surface of the flow, where it iscoherent, continues the topographic trend of the surface of Syrtis Major to the west. Starting from thepoint where the coherent surface of the flow breaks up and appears as a cluster of knobs and mesas theoverall topographic profile of the flow becomes almost horizontal. Points along the profile indicateindividual measurements of elevation taken from the MOLA topographic map with resolution 1/64 of adegree. Width of image is about 170 km. The image showing the position of the profile is a fragment ofthe MDIM photobase.


noticeably larger than knobs characterizing the knobbyterrain in the northern area. The largest plates in thesouthern area occur either at the edge of Syrtis Major orwithin the northeastern portion of the unit HNu where thefloor of Isidis Basin is relatively high (�3700, �3600m).The unit HNu, in fact, represents a large and contiguousarea of knobby terrain at the transition from Syrtis Major toIsidis Basin. This area runs along the edge of Syrtis forabout 350 km in the S-N direction and extends for about150 km from the edge of Syrtis into Isidis Planitia.[34] To the west, the knobby terrain display a gradational

transition to Syrtis Major following, however, the regionalbreak in slope along the edge of the coherent portion ofSyrtis that is characterized by a steady slope (about 1.3�)toward the Isidis Basin (Figure 13). The break in slopeoccurs at different elevations, which are close, on theaverage, to the �3700 m contour line (Figure 1b). Thediffuse contact with the Vastitas Borealis Formation definesthe eastern edge of the unit HNu that roughly coincides withan elevation of about �3900 meters (Figure 1b). The high-resolution topographic map shows, however, that the varia-tions in the elevation of the unit baseline along its easternboundary are about 200 m over a distance many tens ofkilometers.[35] The topographic profiles across the area of knobby

terrain show that its surface is flat on the average andfollows the regional slope of the floor of Isidis Basin towardthe southwest. Although the individual MOLA topographicprofiles show that the surface of the unit HNu is highlyirregular, the typical height of individual knobs and mesaswithin it is about 100 m with a few knobs that are taller, upto 250 m (Figure 14). The value of 100 m, which defines thevisible thickness of the HNu unit, is close to that for the

knobby terrain in the northern area of our study. In a mannersimilar to the northern area, there are no pitted mounds ortheir chains visible in close proximity to unit HNu.[36] The important characteristic of unit HNu visible in

the topographic map (Figure 1b) is that its territory iscomplicated by elongated and equidimensional depressionstens of kilometers across. The depth of the depressions isabout 50–100 m and, in places, up to 150 m and theyappear to be organized in linear and arcuate troughs. Theinterior of the troughs has more subdued morphology andnoticeably less abundant knobs (Figure 12). The two mostprominent enter the area of knobby terrain from the southand north along about 279.5�W. Due to this, the centralportion of unit HNu (between about 12–13�N and 279–280�W) appears as a short and broad topographic ridgeextending in the east direction from the edge of Syrtis Majorinto Isidis Basin for about 60 km (Figure 1b). It is importantto note that two of the largest lava flows (or possibly lavatubes) within Syrtis come to the edge of the plateau at about12�N where the broad ridge within the knobby terrainbegins. This suggests that the ridge within the unit HNuis a continuation of the lava flows in Syrtis. Another large(about 60 km across) equidimensional depression withinunit HNu occurs at the eastern edge of the unit (betweenabout 12–13�N and 278–279�W). The surface of thedepression is also characterized by few knobs and mesasand those that are visible appear to be smaller, less crispmorphologically, and more rounded in plan view.[37] One of the most obvious differences between the

southern area and the northern one is that no prominentsingle scarp outlines the main plateau of Syrtis Major in thesouthern area. All scarps there are relatively low featuresthat either outline individual mesas or occur in series at the

Figure 12. The morphology of the transition zone in the southern area of detailed study, a), withsuperposed contour lines, b), showing the variations in elevation along specific features of the transitionzone (contour interval is 50 m). Width of image is about 260 km. See text for details. Figures a) and b) arefragments of the MDIM photobase. The solid line crossing the image is the track of the MOLA orbit13989.


edge of Syrtis Major (Figure 15). Along the contact betweenthe unit HNu and the Syrtis lava plateau there is abundantevidence for plateau breakup (Figures 15a and 15b). Forexample, at about 13.5�N, 280.5�W the morphologicallysmooth and coherent surface of Syrtis is cut by a series ofcurvilinear graben. The larger graben separate big elongatedfragments of the plains off the main body of the plateau andsubordinate graben further divide these fragments intosmaller pieces. At about 12.2�N, 280�W a large triangle-shaped block of the Syrtis plateau is outlined from the westby curvilinear narrow troughs and the surface of the block isalmost horizontal and lower while the edge of the plateauimmediately to the west of the block is higher and slopedtoward the east. This suggests that the block is a detachedpiece of Syrtis. The eastern edge of the block is broken intosmaller pieces. The progressive fragmentation of largerpieces of the Syrtis plateau into smaller small knobs andmesas is the typical feature of the transition zone from Syrtis

Major to Isidis Basin in the southern area of our study(Figures 15c and 15d).[38] There are varieties of surface textures within the

knobby terrain of the HNu unit, which depend on the sizeand arrangement of knobs and mesas. In the central portionof the unit, at about 12.2�N and between 279–280�W, theapproximately rectangular plates about 3.5 km across arearranged in bands that are about 4–4.5 km wide and 30–40 km long (Figure 15e). These bands characterize thesurface of the broad topographic ridge that appears as acontinuation of the largest lava flows (possible lava tubes)within Syrtis Major. At the southern edge of the knobbyterrain area (at 10–10.5�N, 278–279�W) a similar arrange-ment of plates into bands occur (Figure 15f ). There is aseries of subparallel curvilinear narrow troughs there that arecharacterized by distinct narrow rims on both sides and someof the troughs are transformed into ridges (with medialdepression, in places) along their strike (Figure 15f ). The

Figure 13. Topographic profile across the southern area of the detailed study. Width of image is about240 km. The morphologically coherent surface of Syrtis Major Planum slopes toward Isidis Basin atsteady topographic gradient about 0.4�. Where the plateau is breaking up (at the contact with the knobbyterrain of the unit HNu to the East) the gradient abruptly diminishes (0.1�) and the surface of the unitHNu is essentially horizontal. The profile is taken from the MOLA topographic map with resolution 1/64of a degree. The image showing the position of the profile is a fragment of the MDIM photobase.


troughs continue into the plateau for about 30–60 km andcut it into slices about 8–12 km wide. At the edge of theplateau the slices are abruptly disrupted and continue intoIsidis Basin as bands of tightly spaced plates that arecharacterized by softened morphology and cut by narrowand apparently shallow elongated depressions in two direc-tions. This creates a hummocky texture of the surface that ischaracteristic of the southern edge of the knobby terrain.

4. Discussion

[39] The specific large- and small-scale features of thetransition zone from Syrtis Major to Isidis Basin that havebeen described in the previous sections allow us to addresstwo major questions about the character of interactionbetween the volcanic province of Syrtis Major and thelowland of Isidis Basin: (1) What is the timing and sequenceof events during the emplacement of materials present at thecontact of Syrtis Major and Isidis Planitia?; (2) What is thepossible nature of the Vastitas Borealis Formation materialsthat cover the floor of Isidis Basin?

4.1. Timing and Sequence of Events

[40] The crater count within Syrtis Major Planum [Max-well and McGill, 1988; Hiesinger and Head, 2002; Hie-singer and Head, manuscript in preparation, 2003] showsthat about 150 to 165 craters larger than 5 km in diameteroccur in 106 km2 of the territory of Syrtis. This craterdensity corresponds to an Early Hesperian age [Tanaka,1986]. Our crater count (Figure 3) within the eastern portionof Syrtis (area is about 230,000 km2) that has evidence forrelatively late resurfacing shows the density about 165craters larger than 5 km per 106 km2. Taking into accountthe error bars of the crater count, our results do not differ

from the previous data meaning that the eastern portion ofSyrtis Major is indistinguishable from the rest of the Planumby the crater statistics. Grizzaffi and Schultz [1989] deter-mined the same crater density for the surface of plainswithin the Amenthis trough that cuts the southeasternportion of the rim of Isidis Basin. The crater counts withinthe basin [Grizzaffi and Schultz, 1987, 1989; Maxwell andMcGill, 1988] shows that the surface of Vastitas BorealisFormation on the floor of the basin is younger (LateHesperian) than the surface of Syrtis Major. We havecounted craters on the floor of Isidis Basin near thetransition zone from Syrtis to Isidis within the territory thatis equivalent both in area and shape to the territory wherewe have counted craters in the eastern Syrtis. Our data (118craters >5 km per 106 km2, Figure 3) show that the craterretention age of the surface of Vastitas Borealis Formationnear the transition zone also correspond to the Late Hespe-rian time. Grizzaffi and Schultz [1987] have pointed out,however, that the plains within Isidis Basin interior possiblyexhibit two ages related to the formation and modificationof the plains.[41] Both the crater statistics data and characteristic struc-

tures visible in MOLA topography imply that the floor of theIsidis Planitia basin was covered with lava plains before theemplacement of materials of Vastitas Borealis Formation[Grizzaffi and Schultz, 1987, 1989; Maxwell and McGill,1988]. The recent results from the high-resolution MOLAdata [Head et al., 2002] show that a system of large sinuousridges complicates the floor of Isidis Basin (Figure 2).These ridges appear to be similar in plan view to thecommon wrinkle ridges deforming the Hesperian lavaplains (unit Hr) elsewhere on Mars. Several observationsstrongly suggest that the large ridges in Isidis are, in fact,buried wrinkle ridges [Head et al., 2002]: (1) The basin of

Figure 14. Part of MOLA orbit 13989 that crosses the eastern portion of knobby terrain of the unit HNuin the southern area of the detailed study. The typical height of the knobs is about 100–150 meters. Trackof the orbit is shown in Figure 12a. Latitude is in degrees North.


Figure 15. Plateau breakup and progressive fragmentation of the Syrtis Major materials within thesouthern area of the detailed study. a) and b) The initial stages of the breakup when series of linear andcurvilinear graben cut the morphologically coherent surface of the Syrtis Major plateau (central portionsof both images). c) and d) At these stages of the breakup, large fragments of the plateau (plates) arecompletely separated from the Syrtis Major mainland. e) and f) At these stages of the breakup, the largeplateau blocks are further disrupted into smaller knobs, plates, and mesas. All images are fragments of theVO image 377S58 (218 m/px) and each covers an area about 55 � 55 km.

IVANOVAND HEAD: SYRTIS MAJOR AND ISIDIS BASIN CONTACT 17 - 17

Isidis Planitia is a large (up to 400–500 mGal) mascon[Smith et al., 1999] known since the Viking mission[Sjogren, 1979; Sjogren and Wimberly, 1981]. The presenceof a mascon suggests lava filling of Isidis Basin anddeformation associated with subsidence. Furthermore, wrin-kle ridges are common structures of the Hesperian lavaflows elsewhere on Mars [e.g., Scott and Tanaka, 1986;Greeley and Guest, 1987]. (2) The lowland of Isidis Basinneighboring the large lava province of Syrtis Major, thesurface of which is deformed by wrinkle ridges [Schaber,1982; Greeley and Guest, 1987; Hiesinger and Head, 2002;Hiesinger and Head, manuscript in preparation, 2003]. (3)The large ridges in Isidis are geometrically similar to thetrue wrinkle ridges exposed elsewhere on the surface ofMars. (4) The large ridges in Isidis appear to be similar inwidth, length, and sometimes in arrangement to the‘‘stealth’’ wrinkle ridge-like structures that make up acircum-Tharsis pattern in the northern lowlands of Mars[Head et al., 2002].[42] The most prominent topographic features within

eastern Syrtis (Figure 4) are lava flows and possibly lavatubes [Schaber, 1982]. Although the eastern portion appearto be of the same age (the Early Hesperian) as the rest of theplateau, the prominent lava flows there occupy a smallportion, about 10–12% of the area of our crater counting.Such a small total area of these features prevents reliableestimates of their age with the available images. Observa-tional evidence such as small lava flows emanating from thelarger features and superposed on the surrounding surfacesuggests, however, that these lava flows and tubes aremanifestations of the latest volcanic activity in the region.The flows extend down the regional slope of Syrtis Majortoward Isidis and enter the basin. In the northern area of ourdetailed study the two large flow-like features within Isidisoccur as continuations of a large topographic ridge (pre-sumably, a lava tube) within Syrtis. In the southern area, thehighest portion of the unit HNu is in the area where the mostprominent lava flows (putative tubes) in Syrtis come to theedge of the plateau.[43] There is abundant evidence for internal layering

within the large scarp at the edge of Syrtis Major lavaplateau similar to the layering characterizing scarps ofValles Marineris and interpreted as lava flows [McEwen etal., 1999; Malin and Edgett, 2001]. The layering means thatlava from Syrtis has been delivered into Isidis Basin not as asingle unit but by discrete events. Our estimates show thatthe thickness of layers within the Syrtis lava suite variesfrom a few tens of meters up to 100–115 m and is about 80m on the average. The measurements of unembayed wrinkleridges in the Lunae Planum/eastern Solis Planum region[Head et al., 2002] show that the mean height of thesefeatures is about 65 m, which is less than the meanmeasured thickness of the lava stacks (Table 2). Wrinkleridges are absent in the eastern portion of Syrtis but arecommon features in the rest of the Planum [e.g., Greeleyand Guest, 1987; Hiesinger and Head, 2002; Hiesinger andHead, manuscript in preparation, 2003]. There, the ridgesappear to be narrower and lower (less prominent) than in theLunae Planum area. We interpret this to mean that thecharacteristic lack of wrinkle ridges in eastern Syrtis isconsistent with their burial by later lava flows of sufficientthickness.

[44] An important observation in this respect is that thelarge wrinkle-type ridges in Isidis apparently did not controlthe flow path of the flows from Syrtis (Figure 1a). This isconsistent with the suggestion that the materials of theVastitas Borealis Formation covered the possible wrinkleridges in Isidis by the time of the late volcanic activity andthe later flows from Syrtis were emplaced on top of thesematerials.[45] The knobby terrain appears to be one of the most

common features along the transition from Syrtis Major toIsidis Basin. The knobby terrain in both areas of our detailedstudy (the knobby component of the unit Aps [Greeley andGuest, 1987] in the northern area and the unit HNu in thesouthern area) share three important characteristics: (1) Thistype of terrain begins basinward after the regional break inslope at the contact between Syrtis Major and Isidis Basin(Figures 11 and 13); (2) The overall surface of the occur-rences of knobby terrain is horizontal on average andparallel to the surrounding surface (Figure 14); (3) Thevisible thickness (the average height of individual knobsand mesas) of knobby terrain is about 100–200 meters(Figures 6b and 12b). These characteristics of the knobbyterrain are also consistent with the emplacement of latestportions of lava from Syrtis on the already existing flat andhorizontal surface of the Vastitas Borealis Formation.[46] Another typical feature of the transition zone are

scarps that occur in the northern area preferentially as highsingle structures (Figure 8) and as a series of lowerstructures in the southern area (Figure 15). The scarps arehigh (a few hundred meters) structures that maintainapproximately the same height if oriented parallel to thestrike of the transition zone or become progressively lower iforiented normally to the general orientation of the transitionzone. The typical feature of the larger scarps is that they aresinuous at different scales and are characterized by sets ofalcoves, niches, and scallops of a variety of widths, from afew up to tens of kilometers. At some niches the evidence forrock avalanches is presented and, typically at the base ofscarps, numerous angular blocks are visible (Figure 10).There is direct evidence for small boulders and blocks (froma few meters up to a few tens of meters across) slumped fromthe large scarps in the MOC images for the northern area ofour study (Figure 9). All these features of the scarps areinconsistent with their tectonic origin but readily explainedby scalloping of the scarp edges during listric faulting andmass-wasting that occurs at different scales. In contrast tothis, in the southern area there are no large single scarps but,instead, abundant evidence for Syrtis Major plateau breakupis seen (Figure 15). The plateau breakup typically occursexactly at the major break in slope from the steady-slopedsurface of Syrtis to the almost horizontal floor of IsidisBasin. The formation of scarps and the network of faultscutting and breaking up the plateau represent the youngestprocess in the sequence of events that shaped the transitionzone from Syrtis Major Planum to Isidis Basin.[47] The contiguous areas of knobby terrain and scarps

show significant variations in elevation over relatively shortdistances (typically, a hundred meters to a hundred km,Figures 6b, 12b, and 16). Such variations, which are twoorders of magnitude larger than the variations at the mouthsof the large circum-Chryse outflow channels [Ivanov andHead, 2001], appear to be inconsistent with the late


embayment of the knobby terrain or scarps by the materialsof the Vastitas Borealis Formation. Tanaka and Banerdt[2000] and Watters [2003] have proposed that the broadtilting of the floor of Isidis Basin is due to loading in thenorthern lowlands. We see no evidence that such broad-scale tilting has influenced the formation of small-scalemorphologic and topographic features characterizing thetransitional zone from Syrtis to Isidis.[48] The above characteristics of the typical features of

the transition zone such as knobby terrain and borderinglarge single scarps suggest that these features representhighly modified remnants of the late lava flows from SyrtisMajor that were superimposed on the surface of materials ofthe Vastitas Borealis Formation. Thus the generalizedsequence of events in the area of transition from the high-standing lava plains of Syrtis Major to the lowland of IsidisBasin appears as follows (Figures 17a and 17b): (1) Lavaflows from Syrtis Major extended into Isidis Basin andpartly filled its volume. Apparently, the volcanic activitywithin the eastern portion of Syrtis Major continued longerthan the volcanism in the rest of the plateau; (2) The lavaplains on the floor of Isidis Basin were deformed by wrinkleridges that have separated the floor of the primary basin intosecondary basins; (3) Materials of the Vastitas BorealisFormation filled the basin blanketing the surface of thepreexisting lava plains and partly covered the wrinkleridges; (4) The very late lava flows from the eastern Syrtiswere emplaced on top of the Vastitas Borealis Formationmaterials, were disrupted, and presently appear as knobbyterrain.[49] In order to estimate the largest possible area of the

knobby terrain we outlined it along the most distant knobsand mesas. The area within this contour includes the voids

where no knobs or mesas are seen and is about 50,000 km2.The mean height of the scarp in the northern area, about 0.5km, is likely an upper limit of the thickness of the late lavaflows from the eastern Syrtis. The visible thickness ofknobby terrain, about 100 m, likely represents a lowerestimate of the thickness of the eastern Syrtis lava flows.Thus the total volume of the late volcanic material fromSyrtis delivered into Isidis Basin is likely from �5,000 to25,000 km3.[50] The total cross-section of all topographically prom-

inent ridges within eastern Syrtis, assuming that all of themare lava flows and tubes that delivered lavas to the transi-tional zone, is about 13 km2. This is a maximum estimatebecause some ridges may not be lava flows and this wouldreduce the effective cross-section of the lava feeders. Wealso assumed a range of velocities of the lava flows to befrom 0.1 up to 10 km per hour, which should incorporate allreasonable velocities of lava flows. At the above numbers,the necessary volume of lava (from 5,000 to 25,000 km3)would be emplaced on the floor of Isidis during a very shorttime, from about 2 days to about 800 days. It is clear thatsuch duration is far beyond the ‘‘resolution’’ of the agedetermination by the standard technique based on theimages available for the area of our study.

4.2. Possible Nature of Vastitas Borealis Formation

[51] There are three principal situations in which lavaflows of Syrtis Major are in contact with other materials: (1)The surface of the unit Hs that makes up the Syrtis MajorPlanum is characterized by flow fronts and margins. Thereis good evidence within the eastern portion of Syrtis Majorfor the superposition of lava flows on the surface of theplateau (Figure 5). Also, the stacked lava flows of different

Figure 16. Elevations of the baseline of the southern flow-like feature in the northern area of detailedstudy. The elevations have been determined for the major breaks in slope at the base of the flow where itis crossed by five MOLA orbits that correspond to the available MOC images for this area. The MOLAtracks are shown in Figure 6. The difference in the elevations is as large as about 100 m per 100 km of thefeature length. The numbers in the lower part of the plot indicate individual MOLA orbits.


thickness are exposed in any scarp that cut the surface ofSyrtis; (2) Along the southern, western, and northern edgesof Syrtis Major the lava flows from the plateau are incontact with heavily cratered Noachian terrains (Figure 18a)and occur there at a variety of topographic gradients. Forinstance, where lava flows from Syrtis enter the craterAntoniadi (Figure 18b) through the narrow gap in the craterrim [Hiesinger and Head, 2002; Hiesinger and Head,manuscript in preparation, 2003], the topographic gradientis changed from about 0.07� within Syrtis to about 0.5� inthe gap, and back to almost horizontal on the crater floor;(3) Along the eastern edge of the plateau, in the area of ourstudy, the lava flows are in contact with the materials ofVastitas Borealis Formation on the floor of Isidis Basin.[52] Neither in case (1) nor (2) is there any evidence for

plateau breakup or progressive disruption of lava flows intosmall knobs, plates, and mesas. These specific featuresoccur exclusively at the eastern edge of Syrtis Major whereits lava flows entered into Isidis Basin (Figures 6 and 12).Such contrasting characteristics of the contact of the Syrtislavas with their surroundings suggest that either the lavaflow sequence within Syrtis Major itself, or rocks of theNoachian highlands, provided firm and resistant basementfor the formation of the morphologically intact suite of lava

plains. In contrast, materials of Vastitas Borealis Formationon the floor of Isidis Basin served as a weak, non-resistantbasement for the lava flows from Syrtis Major. In order toprovide this, the basement as a whole or, more likely, somepart of it should be mobilized and evacuated during theinteraction with superposing lava flows. The phenomenaobserved at the transition zone from Syrtis Major Planum toIsidis Basin is readily explained by interaction of hot lavawith materials enriched with volatile components (H2O,CO2, or both) on the floor of the basin. During (and perhapsfollowing) the interaction with lavas, these componentsmobilized and escaped, which softened the non-volatileresiduum, and allowed the lava flows to subside, breakup, and eventually to be completely destroyed [Squyres etal., 1987; Hoffman, 2000]. The superposition of hot volca-nic materials from Syrtis Major on top of volatile-bearingsediments of the Vastitas Borealis Formation within IsidisBasin may explain the key features of the contact such asthe breakup of the Syrtis Major plateau, formation ofknobby terrain along the contact and the other specificfeatures.[53] The large sinuous ridges that are covered within

Isidis Basin with the material of Vastitas Borealis Forma-tion, regardless of their origin, form secondary basins on the

Figure 17. a) The interpreted cross-section of the transition zone from Isidis Basin to Syrtis MajorPlanum (not to scale) and b) the suggested correlation chart of units and structures for this area. See textfor details.


floor of Isidis. These basins, which are 150–180 km across[Hiesinger and Head, 2002; Hiesinger and Head, manu-script in preparation, 2003] should introduce a majorinhomogeneity in the distribution of the Vastitas BorealisFormation material. Its varying thickness could play a keyrole in the appearance of specific features of the transitionzone from Syrtis Major to Isidis Basin. For example, thelarge flow-like features that characterize the northern area ofour study extend into Isidis on top of two low and broadtopographic ridges separating the floor of Isidis into a seriesof secondary basins (Figure 1). The scarps at the edges ofthe flows (Figure 10) are facing toward the deeper portionof the secondary basins (Figure 1). The large arcuate scarpthat cuts the edge of Syrtis Major to the south of the flow-like features also outlines the western portion of a secondarybasin within Isidis (Figure 1). The thickness of the materialsof the Vastitas Borealis Formation should be larger in suchbasins. Potentially, a larger amount of volatiles could bestored there, enough to destroy superimposing lava flows,undermine them, cause the scarp retreat, and create highbounding scarps at the edges of the flows where the layerenriched in volatile components is pinching out.[54] In contrast to the northern area, in the southern area

of our study there are no single prominent scarps and broadand shallow topographic troughs and equidimensionaldepressions characterize the area of unit HNu (Figure 1),which dominates the southern area. This difference could bedue to the smaller thickness of the volatile-bearing layer ofthe Vastitas Borealis Formation within the southern area orsmaller amount of volatile components or both. Anotherexplanation of the characteristic features within the southernarea (Figure 15, but still in the framework of the interactionof lavas with the volatiles) is the most rapid and/or more

voluminous emplacement of volcanic materials there. Thisis supported by the presence of the two largest lava flows(tubes) within the Syrtis Major plateau converging on thecentral area of the extension of knobby terrain in thesouthern area (unit HNu). The other important featuresthat occur in the southern area and could be indicative ofthe lava/volatile interaction are the softened morphology ofthe knobs and plates and the narrow troughs with rims at thesouthern edge of the unit HNu. One of the plausibleexplanations of the unusual morphology of these featuresis the interaction of hot lava flows with an underlyingvolatile-rich substratum, rapid release of the volatiles andtheir ventilation through weak zones in a relatively thicklayer of lavas.[55] The interaction of hot lava flows with the volatile-

rich layer must lead to diminishing of the amount of thevolatile components and maybe also to their complete loss ifthe volatiles are not very abundant. Thus the superpositionof lava flows from Syrtis Major on the materials of VastitasBorealis Formation may explain the typical lack of smallpitted mounds and cones on the floor of Isidis Basin withinand in the vicinity of the knobby terrain if the moundsrepresent either pseudocraters [Frey et al., 1979] or cryo-volcanic cones [Hoffman et al., 2001; Hoffman and Tanaka,2002].

5. Summary and Conclusions

[56] In our study, we analyzed in detail the transition zonefrom Syrtis Major Planum to Isidis Basin in order to addressthe following questions: (1) What is the general sequence ofevents in the transition zone and what is the place and roleof the Hesperian ridged plains that make up the majority of

Figure 18. Character of interaction of lava flows from Syrtis Major Planum with the surroundingmaterials. a) Syrtis lavas are in contact with the relicts of ancient cratered uplands in the southern portionof Syrtis Major. The lava embays the old terrains without any evidence for breakup and/or fragmentation,which is the characteristic of the contact of the Syrtis lavas with the materials on the floor of Isidis Basin.b) Lava flows from Syrtis Major enter the crater Antoniadi at the northwestern portion of Syrtis. The lavaflows remain morphologically intact within the Syrtis plateau (a), within the breached rim of theAntoniadi crater where the flows are on steep slopes (arrow), and on almost horizontal floor of the crater(b). Both images are fragments of the MDIM photobase and show areas about 140 � 140 km. SeeHiesinger and Head [2002] and Hiesinger and Head (manuscript in preparation, 2003) for details of theserelationships.


Syrtis Major? (2) How does this sequence correlate with theproposed origin of the Vastitas Borealis Formation [Griz-zaffi and Schultz, 1989; Scott et al., 1995; Tanaka and Kolb,2001; Tanaka et al., 2000, 2001a, 2002b]? (3) What is thepossible nature of the Vastitas Borealis Formation?[57] Our results and interpretations are summarized in the

schematic cross-section of the transition zone from IsidisBasin to Syrtis Major Planum (Figure 17). In this scenario,in the Early Hesperian, volcanic plains are emplaced inSyrtis Major (the lower part of the Syrtis Major Formation,Hs, lower) and wrinkle ridges deform their surfaces soonthereafter. Concurrently, volcanic plains are emplaced onthe floor of the Isidis Basin and were deformed by anetwork of wrinkle ridges. The apparent simultaneity ofthese units may mean that Syrtis Major was the source ofmany of the flows in the Isidis Basin. In the early part of theUpper Hesperian, subsequent to formation of most ofwrinkle ridges, the Vastitas Borealis Formation (VBF) wasemplaced in the Isidis Basin and elsewhere in thenorthern lowlands [e.g., Tanaka and Scott, 1987]. Fol-lowing the emplacement of the VBF, flows of the upperpart of the Syrtis Major Formation were emplaced, eruptingfrom the eastern margins of Syrtis Major Planum andflowing down into the westernmost part of the Isidis Basinon top of the recently emplaced VBF. Modification of thesuperposed lavas by degradation of apparently volatile-richmaterial of the VBF formed the scarps and unusual mor-phology of the unit HNu that characterizes the transitionalzone. The Vastitas Borealis Formation continued to undergomodification and evolution during the Late Hesperian tocreate the presently observed facies (ridged, knobby, etc.)and deposit. Our interpretation is largely based on strati-graphic relations documented in this paper, which include 1)the detection of underlying Hesperian-aged ridged plainsbelow the Vastitas Borealis Formation in the Isidis Basin[Head et al., 2002], 2) the crater ages of major units, and 3)the detailed documentation of the characteristic features andstratigraphic relationships within the transition zone fromSyrtis Major to the Isidis Basin described above.[58] In a recent paper, Tanaka et al. [2002b] outlined a

detailed three-stage model for the evolution of the HellasBasin rim and the filling of the basin interior which theyalso applied to Syrtis Major Planum and the Isidis Basininterior [see also Tanaka et al., 2000]. In this model theyenvisioned the following steps. 1) Prior to emplacement ofSyrtis Major lavas, sills intrude shallow friable rocks of thebasin rim and mobilize volatile-rich impact breccias.2) Surface rocks are fluidized and flow into the adjacentbasin, removing massifs from the basin margin, and pro-ducing volatile-rich sedimentary deposits across the basinfloor. 3) Subsequent volcanic eruptions cover the erodedbasin rim surface and basin inner slopes with lava plainsforming Syrtis Major Planum, but leave the volatile-richdeposits in the basin interior floor exposed and largelyundisturbed.[59] The results of our study suggest an alternative

sequence of events (Figure 17) that is not consistent withthe model outlined by Tanaka et al. [2000, 2001a, 2001b,2002b]. The unit presently exposed on the basin floor (theVBF) postdates both stratigraphically and from crater countdata the Early Hesperian volcanic ridged plains in SyrtisMajor Planum [Hiesinger and Head, 2002; Hiesinger and

Head, manuscript in preparation, 2003] and in the subsur-face of Isidis Planitia [Head et al., 2002]. Stratigraphically,it predates the latest phases of Late Hesperian volcanismoriginating from the edge of Syrtis Major Planum andextending down onto the western edge of the Isidis Basinfloor. These findings suggest that material of the VBF is notderived from intrusions of pre-Syrtis Major sills. Thus thespecific morphology and stratigraphic relationships typicalof the transitional zone from Syrtis Major to Isidis Basinsuggest a different source for the material of the VastitasBorealis Formation and later age than proposed by Tanakaet al. [2002a, 2002b]. We do agree, however, that the VBFrepresents a volatile-rich deposit on the floor of the basin.[60] Grizzaffi and Schultz [1989] proposed that a thick

(up to 2 km) glacier-like air fall deposit was emplaced onthe floor of Isidis Basin. In this model, the remnants of thedeposit, the ridged member of the Vastitas Borealis Forma-tion [Greeley and Guest, 1987], form the youngest materialswithin Isidis and represent a lag deposit left after thesublimation of the volatile component. Although this modeloffers a possible source for the volatile-rich material, it doesnot explain why the air fall occurred precisely within thebasin. Also, in the model of Grizzaffi and Schultz [1989], alllava plains from Syrtis Major are considered to be relativelyold and predate the air fall deposit. Our findings, however,place the formation of material of the Vastitas BorealisFormation between two major episodes of volcanism inSyrtis Major Planum.[61] The model of Scott et al. [1995] based on detailed

mapping of a range of morphologies within Isidis Basinsuggests the existence of a large lake-like body of waterwithin the basin. This model provides another explanationfor the formation of a volatile-rich deposit (the VastitasBorealis Formation) and its stratigraphic relationships withthe lavas from Syrtis Major. The stratigraphic order of theVBF described by Scott et al. [1995] corresponds closely toour scheme of the sequence of events at the transitional zonefrom Syrtis to Isidis. The model of Scott et al. [1995],however, does not specify the sources for water that couldaccumulate in the basin and the map showing the arealdistribution of channels and possible paleolakes clearlyindicates the lack of feeding channels around Isidis.[62] Another possible explanation of the volatile-rich

nature of material of the VBF on the floor of Isidis Basinis the existence of a large standing body of water (an ocean)within the northern lowlands, the water of which could fillIsidis through its lowered northeastern rim [Parker et al.,1989, 1993]. This hypothesis is consistent with the MOLAtopography data [Head et al., 1999; Ivanov and Head,2001; Kreslavsky and Head, 2002], the sequence of eventsfound in the transitional zone from Syrtis to Isidis, andreadily explains the nature of material of the VBF. In thisscenario, water emplaced on the floor of the basin is likelyto have frozen and sublimed away geologically rapidly[e.g., Kreslavsky and Head, 2002] and the VBF is inter-preted to be the sublimation residue of the frozen water inthe basin. The late stage Syrtis Major lava flows super-imposed on top of the VBF material prior to its advancedsublimation interacted with its volatile component andcaused its sublimation and escape. The evacuation of thevolatiles left behind a soft non-cemented deposit that wasunable to support the load of lava flows. The flows were


broken, disrupted, and underwent an enhanced mass wast-ing due to both the relaxation of the underlying deposit andenforced ventilation of volatiles. These processes ultimatelyformed the unit HNu and other specific features of mor-phology and topography in the transitional zone from SyrtisMajor to Isidis Basin.

ReferencesBridges, J. C., et al., Selection of the landing site in Isidis Planitia of Marsprobe Beagle 2, J. Geophys. Res., 108(E1), 5001, doi:10.1029/2001JE001820, 2003.

Chicarro, A. F., P. H. Schultz, and P. Masson, Global and regional ridgepatterns on Mars, Icarus, 63, 153–174, 1985.

Dohm, J. M., J. C. Ferris, V. R. Baker, R. C. Anderson, T. M. Hare, R. G.Strom, N. G. Barlow, K. L. Tanaka, J. E. Klemaszewski, and D. H. Scott,Ancient drainage basin of the Tharsis region, Mars: Potential source foroutflow channel systems and putative oceans and paleolakes, J. Geophys.Res., 106, 23,924–23,958, 2001.

Frey, H., and M. Jarosewich, Subkilometer Martian volcanoes: Propertiesand possible terrestrial analogs, J. Geophys. Res., 87, 9867–9879, 1982.

Frey, H., B. L. Lowry, and S. A. Chase, Pseudocraters on Mars, J. Geophys.Res., 84, 8075–8086, 1979.

Greeley, R., and J. Guest, Geologic map of the eastern equatorial region ofMars, U. S. Geol. Surv. Misc. Invest. Ser., Map I-1802-B, scale1:15000,000, 1987.

Grizzaffi, P., and P. H. Schultz, Evidence for a thick transient layer in theIsidis impact basin (abstract), Lunar Planet. Sci., XVIII, 371–372, 1987.

Grizzaffi, P., and P. H. Schultz, Isidis Basin: Site of ancient volatile-richlayer, Icarus, 77, 358–381, 1989.

Head, J. W., H. Hiesinger, M. A. Ivanov, M. A. Kreslavsky, S. Pratt, andB. Thomson, Possible ancient oceans on Mars: Evidence from MarsOrbiter Laser Altimeter data, Science, 286, 2134–2137, 1999.

Head, J. W., III, M. A. Kreslavsky, and S. Pratt, Northern lowlands of Mars:Evidence for widespread volcanic flooding and tectonic deformation inthe Hesperian Period, J. Geophys. Res., 107(E1), 5003, doi:10.1029/2000JE001445, 2002.

Hiesinger, H., and J. W. Head, Syrtis Major, Mars: Results from MarsGlobal Surveyor data, Lunar Planet. Sci. [CD-ROM], XXXIII, abstract1063, 2002.

Hodges, C. A., and H. J. Moore, Atlas of volcanic landforms on Mars, U. S.Geol. Surv. Prof. Pap., 1534, 1994.

Hoffman, N., White Mars: A new model for Mars’ surface and atmospherebased on CO2, Icarus, 146, 326–342, 2000.

Hoffman, N., and K. L. Tanaka, Coexisting ‘‘flood’’ and ‘‘volcanic’’morphologies in Elysium as evidence for cold CO2 of warm H2O out-bursts, Lunar Planet. Sci. [CD-ROM], XXXIII, abstract 1505, 2002.

Hoffman, N., J. S. Kargel, and K. L. Tanaka, Isidis Basin—A potentialfocus of cryovolcanic activity on Mars, Lunar Planet. Sci. [CD-ROM],XXXII, abstract 1493, 2001.

Ivanov, M. A., and J. W. Head, Chryse Planitia, Mars: Topographicconfiguration, outflow channel continuity and sequence, and tests forhypothesized ancient bodies of water using Mars Orbiter Laser Altimeter(MOLA) data, J. Geophys. Res., 106, 3275–3278, 2001.

Kargel, J. S., and R. G. Strom, Ancient glaciation on Mars, Geology, 20,3–7, 1992.

Kargel, J. S., V. R. Baker, J. E. Beget, J. F. Lockwood, T. L. Pewe, J. S.Shaw, and R. G. Strom, Evidence of ancient continental glaciation in theMartian northern plains, J. Geophys. Res., 100, 5351–5368, 1995.

Kreslavsky, M. A., and J. W. Head, Fate of outflow channel effluents in thenorthern lowlands of Mars: The Vastitas Borealis Formation as a subli-mation residue from frozen ponded bodies of water, J. Geophys. Res.,107(E12), 5121, doi:10.1029/2001JE001831, 2002.

Lockwood, J. F., J. S. Kargel, and R. B. Strom, Thumbprint terrain on thenorthern plains: A glacial hypothesis (abstract), Lunar Planet. Sci., XXIII,795–796, 1992.

Lucchitta, B., Mars and Earth: Comparison of cold-climate features, Icarus,45, 264–303, 1981.

Lucchitta, B., H. M. Fergusson, and S. Summers, Sedimentary deposits inthe northern lowland plains, Mars, Proc. Lunar Planet. Sci. Conf. 17th,Part 1, J. Geophys. Res., 91, suppl., E166–E174, 1986.

Malin, M. C., and K. S. Edgett, Mars Global Surveyor Mars Orbiter Cam-era: Interplanetary cruise through primary mission, J. Geophys. Res., 106,23,429–23,570, 2001.

Maxwell, T. A., and G. E. McGill, Age of fracturing and resurfacing in theAmenthes Region, Mars, Proc. Lunar Planet. Sci. Conf. 18th, 701–711,1988.

McEwen, A. S., M. C. Malin, M. C. Carr, and W. K. Hartmann, Volumi-nous volcanism on early Mars revealed in Valles Marineris, Nature, 397,584–586, 1999.

Parker, T. J., R. S. Saunders, and D. M. Schneeberger, Transitional mor-phology in West Deuteronilus Mensae, Mars: Implications for modifica-tion of the Lowland/Upland boundary, Icarus, 82, 111–145, 1989.

Parker, T. J., D. S. Gorsline, R. S. Saunders, D. C. Pieri, and D. M.Schneeberger, Coastal geomorphology of the Martian northern plains,J. Geophys. Res., 98, 11,061–11,078, 1993.

Rossbacher, L. A., and S. Judson, Ground ice on Mars: Inventory, distribu-tion, and resulting landforms, Icarus, 45, 39–59, 1981.

Schaber, G. G., Syrtis Major: A low-relief volcanic shield, J. Geophys. Res.,87, 9852–9866, 1982.

Schultz, R. A., and H. V. Frey, A new survey of multiring impact basins onMars, J. Geophys. Res., 95, 14,175–14,189, 1990.

Scott, D. H., and K. L. Tanaka, Geologic map of western equatorial regionof Mars, U. S. Geol. Surv. Misc. Invest. Ser., Map I-1802-A, scale1:15000,000, 1986.

Scott, D. H., J. M. Dohm, and J. W. Rice Jr., Map of Mars showingchannels and possible paleolake basins, U. S. Geol. Surv. Geol. Invest.Ser., Map I-2461, scale 1:30000,000, 1995.

Sjogren, W. L., Mars gravity: High resolution results from Viking Orbiter 2,Science, 203, 1006–1010, 1979.

Sjogren, W. L., and R. N. Wimberly, Mars: Hellas Planitia gravity analysis,Icarus, 45, 331–338, 1981.

Smith, D. E., W. L. Sjogren, G. L. Tyler, G. Balmino, F. G. Lemoine, andA. S. Konopliv, The gravity field of Mars: Results from Mars GlobalSurveyor, Science, 286, 94–96, 1999.

Squyres, S. W., D. E. Wilhelms, and A. N. Moosman, Large-scale volcano-ground ice interactions on Mars, Icarus, 70, 385–408, 1987.

Tanaka, K. L., The stratigraphy of Mars, Proc. Lunar Planet. Sci. Conf.17th, Part 1, J. Geophys. Res., 91, suppl., E139–E158, 1986.

Tanaka, K. L., Geologic mapping of sedimentary materials in the northernplains of Mars, (abstract), Lunar Planet. Sci., XXVII, 1411–1412, 1997.

Tanaka, K. L., and W. B. Banerdt, The interior lowland plains unit of Mars:Evidence for a possible mud ocean and induced tectonic deformation,Lunar Planet. Sci. [CD-ROM], XXXI, abstract 2041, 2000.

Tanaka, K. L., and D. J. MacKinnon, Basins and sedimentation within theMartian northern plains, Lunar Planet. Sci. [CD-ROM], XXX, abstract1217, 1999.

Tanaka, K. L., and E. J. Kolb, Geologic history of the polar regions of Marsbased on Mars Global Surveyor data, Icarus, 154, 3–21, 2001.

Tanaka, K. L., and D. H. Scott, Geologic map of the polar regions of Mars,U.S. Geol. Surv. Misc. Invest. Ser., Map I-1802-C, scale 1:15000,000,1987.

Tanaka, K. L., T. Joyal, and A. Wenker, The Isidis plains unit, Mars:Possible catastrophic origin, tectonic tilting, and sediment loading, LunarPlanet. Sci. [CD-ROM], XXXI, abstract 2023, 2000.

Tanaka, K. L., W. B. Banerdt, J. S. Kargel, and N. Hoffman, Huge, CO2-charged debris-flow deposit and tectonic sagging in the northern plains ofMars, Geology, 29, 428–430, 2001a.

Tanaka, K. L., J. S. Kargel, and N. Hoffman, Evidence for magmaticallydriven catastrophic erosion on Mars, Lunar Planet. Sci. [CD-ROM],XXXII, abstract 1898, 2001b.

Tanaka, K. L., J. A. Skinner, T. M. Hare, T. Joyal, and A. Wenker, Resurfa-cing of the northern plains of Mars by shallow subsurface, volatile-drivenactivity, Lunar Planet. Sci. [CD-ROM], XXXIII, abstract 1406, 2002a.

Tanaka, K. L., J. S. Kargel, D. J. MacKinnon, T. M. Hare, and N. Hoffman,Catastrophic erosion of Hellas basin rim on Mars induced by magmaticintrusion into volatile-rich rocks, Geophys. Res. Lett., 29(8), 1195,doi:10.1029/2001GL013885, 2002b.

Watters, T. R., Lithospheric flexure and the origin of the dichotomy bound-ary on Mars, Geology, 31, 271–274, 2003.

Wilhelms, D. E., Comparison of Martian and Lunar multiringed circularbasins, J. Geophys. Res., 78, 4048–4095, 1973.

Zuber, M. T., et al., Internal structure and early thermal evolution of Marsfrom Mars Global Surveyor topography and gravity, Science, 278, 1788–1793, 2000.

��J. W. Head III and M. A. Ivanov, Department of Geological

Sciences, Brown University, Box 1846, Providence, RI 02912, USA.([email protected])


Syrtis Major and Isidis Basin contact: Morphological and ... Major and Isidis Basin contact:...

Documents

Transcript of Syrtis Major and Isidis Basin contact: Morphological and ... Major and Isidis Basin contact:...