East-west extension and Miocene environmental … of Geology, Tribhuvan University, ......

For permission to copy, contact [email protected] 2003 Geological Society of America 3

GSA Bulletin; January 2003; v. 115; no. 1; p. 3–20; 12 figures.

East-west extension and Miocene environmental change in thesouthern Tibetan plateau: Thakkhola graben, central Nepal

Carmala N. Garzione†

Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA

Peter G. DeCellesDamian G. HodkinsonTank P. OjhaDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

Bishal N. UpretiDepartment of Geology, Tribhuvan University, Tri-Chandra Campus, Ghantaghar, Kathmandu, Nepal

ABSTRACT

East-west extensional basins are distrib-uted across the southern half of the Tibetanplateau at an elevation of ;4 km. These ba-sins have generated much interest becauseof their potential implications for the re-gional tectonics and force distribution inthe plateau. This study documents the sed-imentology of the Miocene–Pliocene Thak-khola graben fill in order to reconstruct ba-sin evolution and paleoenvironment.Analysis of depositional systems, paleo-drainage patterns, and conglomerate clastprovenance of the .1-km-thick graben fillsets limits on the timing of activity of thebasin-bounding faults and the developmentof southward axial drainage in the basin.During the deposition of the oldest basin fill(Tetang Formation, ca. 11–9.6 Ma), prob-ably in a restricted basin, minor motion oc-curred on the basin-bounding fault systems.An angular unconformity separates the Te-tang and overlying Thakkhola Formations,where this contact can be observed in thesouthern part of the basin. Southward axialdrainage was established by ca. 7 Ma withthe onset of deposition of the ThakkholaFormation. Several episodes of damming ofthis drainage system are recorded by wide-spread lacustrine deposits in the southernpart of the basin. Facies distribution andthe progressive rotation of strata in theThakkhola Formation indicate that theDangardzong fault on the western edge ofthe basin was active at this time, and drain-

†E-mail: [email protected].

age ponding may have been related to dis-placement on normal faults associated withthe South Tibetan detachment system to thesouth of Thakkhola graben.

Contrasts between deposits of the Tetangand Thakkhola Formations provide evi-dence for environmental change in the ba-sin. In the Tetang Formation, the abun-dance of lacustrine facies, the pollen record,and the absence of paleosol carbonate sug-gest that conditions were more humid thanduring subsequent deposition of the Thak-khola Formation. Environmental change inthe Thakkhola graben coincided with en-vironmental change observed in the Siwalikforeland basin sequence, Arabian Sea, andBay of Bengal at ca. 8–7 Ma. Although thisclimate change event has been previouslyattributed to intensification of the Asianmonsoon in response to uplift of the Tibet-an plateau, paleoaltimetry data indicatethat this region had already attained a highelevation by ca. 11 Ma. Thus, the Thakkho-la graben stratigraphic record suggests thatuplift of the southern Tibetan plateau andthe onset of the Asian monsoon as inferredfrom paleoclimatic indicators were not di-rectly related in a simple way.

Keywords: extension, Nepal, Thakkholagraben, Tibetan plateau.

INTRODUCTION

Normal faults and grabens indicating gen-erally east-west extension are distributedthroughout the southern half of the Tibetan

plateau (Fig. 1). Since their discovery (Molnarand Tapponnier, 1978; Ni and York, 1978), ge-ologists and geophysicists have speculated onthe origins of these seemingly enigmatic fea-tures in a regionally contractional tectonic set-ting between the colliding Indian and Eurasianplates. The list of possible causes includes (1)upper-crustal gravitational failure in responseto the attainment of high elevation on the pla-teau (Molnar and Tapponnier, 1978; Molnarand Lyon-Caen, 1988), (2) extension in re-sponse to regional, conjugate strike-slip fault-ing associated with eastward extrusion of Ti-bet (Armijo et al., 1986), (3) isostatic responseto erosion of the mantle lithosphere below Ti-bet (England and Houseman, 1989), (4) lower-crustal flow (Royden et al., 1997), (5) obliqueconvergence between India and Asia (Mc-Caffrey and Nabelek, 1998), (6) arc-parallelextension (Seeber and Pecher, 1998), and (7)middle Tertiary mantle upwelling in easternAsia that induced thermal weakening of thelithosphere (Yin, 2000).

Indirect evidence of the timing of initiationof east-west extension comes from the datingof mineral and whole-rock samples from veinsand north-trending dikes thought to be asso-ciated with east-west extension. These datessuggest older ages of ca. 14 Ma for a vein inthe Tethyan Himalaya (Coleman and Hodges,1995), 18.3 6 2.7 Ma for ultrapotassic dikesin the Lhasa terrane (Williams et al., 2001),and ca. 13.5 Ma for fault zone mineralizationin the Shuang Hu graben in the Qiangtang ter-rane in central Tibet (Blisniuk et al., 2001). Ayounger estimate of ca. 4 Ma for the initiationof extension in the Shuang Hu graben was ob-

4 Geological Society of America Bulletin, January 2003

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Figure 1. General tectonic map of southern Tibetan plateau and Nepal Himalaya, modified from Hodges (2000) and Hauck et al. (1998).Features: Main Central thrust (MCT), Main Boundary thrust (MBT), Main Frontal thrust (MFT), Thakkhola graben (TG), Gyironggraben (GG), Yadong-Gulu rift (YGR), Kangmar dome (KD). Box shows location of study area detailed in Figure 2. Inset is a smaller-scale political map of the region; the gray rectangle shows the location of the tectonic map.

tained by using Holocene slip rates to deter-mine the time required to produce the totaldisplacement on the major graben-boundingfault (Yin et al., 1999). More direct chrono-logical constraints on the beginning of east-west extension come from a thermochronol-ogical study of the shear zone that bounds thewestern edge of the Yangbajian graben in thesoutheastern part of the plateau (Fig. 1), wherea phase of rapid exhumation, inferred to in-dicate extension, took place between ca. 8 Maand 3 Ma (Pan and Kidd, 1992; Harrison etal., 1995). The link between high elevationand late Miocene east-west extension (begin-ning ca. 8 Ma) is supported by a regionallydocumented paleoclimate event that has beeninferred to correlate with the intensification ofthe south Asian monsoon (Quade et al., 1989;Hoorn et al., 2000) and changes in oceanicupwelling conditions (Kroon et al., 1991; Prellet al., 1992). However, the relationship be-tween high-elevation–driven climate changeand regional east-west extension has not beenseriously examined in any of the graben fills.

The argument linking extension to lateMiocene climate change led Garzione et al.(2000a, 2000b) to document the paleoeleva-tion history of the Thakkhola graben in north-central Nepal (Fig. 1) with oxygen isotope

data from lacustrine and paleosol carbonates.Those data suggest that the basin floor wasalready at an elevation of ;4 km by ca. 11Ma. In this paper we document the sedimen-tology and provenance of the Thakkhola basinfill in order to reconstruct the paleogeographyof the basin fill and shed new light on thenature of east-west extension in the southernpart of the plateau.

GEOLOGIC SETTING

The Thakkhola graben, in north-central Ne-pal, formed in Paleozoic to Cretaceous rocksof the Tethyan Series between the South Ti-betan detachment system to the south and theIndus suture zone to the north (Fig. 1). TheSouth Tibetan detachment system is a low-angle,north-dipping, normal-fault system located inthe highest part of the Himalaya (Burg andChen, 1984; Burchfiel et al., 1992; Godin etal., 1999). On the basis of the uniformly highelevation north of the South Tibetan detach-ment system (Fielding et al., 1994), this areais considered to be part of the Tibetan plateau.However, the Indus suture zone marks theboundary between the Indian and Eurasiaplates, so the area south of the suture is also

part of the Himalayan fold-thrust belt (Gans-ser, 1964; Searle, 1986).

The kinematic history of the Himalayanfold-thrust belt followed a generally south-ward progression of emplacement of the majorthrust systems (Schelling, 1992; Srivastavaand Mitra, 1994; DeCelles et al., 1998, 2001)(Fig. 1). Tibetan thrusts were active mainlyduring Eocene and Oligocene time (Searle,1986; Ratschbacher et al., 1994) and producedregional Barrovian metamorphism of the un-derlying Greater Himalayan rocks (Hodgesand Silverburg, 1988; Vannay and Hodges,1996; Coleman, 1998; Guillot, 1999; Catlos etal., 2001). Thermochronologic data from theGreater Himalayan rocks suggest rapid cool-ing during early Miocene time (since ca. 22.5Ma), presumably during emplacement of theMain Central thrust system (Hubbard and Har-rison, 1989; Hodges et al., 1996; Coleman,1998). At about the same time, the South Ti-betan detachment system became active, drop-ping Tethyan Series rocks downward to thenorth against Greater Himalayan rocks in thefootwall (Hodges et al., 1996; Coleman, 1998;Godin et al., 1999; Murphy and Harrison,1999). Since middle Miocene time the frontof the fold-thrust belt has propagated south-ward into the Lesser Himalayan and Subhi-

Geological Society of America Bulletin, January 2003 5

EAST-WEST EXTENSION AND MIOCENE ENVIRONMENTAL CHANGE, THAKKHOLA GRABEN, NEPAL

Figure 2. Geologic map of Thakkhola graben modified from Fort et al. (1982) and Colchenet al. (1986), showing locations of measured sections (stars and thick gray lines, nearChele).

malayan zones (Mugnier et al., 1993; De-Celles et al., 2001). Several major thrustsystems, including the Ramgarh thrust, havebuilt a large duplex in Lesser Himalayan rocksto the south of the Main Central thrust. Situ-ated on top of the Lesser Himalaya are isolat-ed, synformal thrust sheets commonly calledthe crystalline nappes. These are interpreted tobe the southern continuations of the GreaterHimalayan zone (Gansser, 1964; Stocklin,1980; Valdiya, 1980; Johnson et al., 2001),which have been folded into synformal struc-tures during growth of the Lesser Himalayanduplex (Schelling, 1992; Srivastava and Mitra,1994; DeCelles et al., 2001). The MainBoundary thrust places Lesser Himalayanquartzite and phyllite against the Neogene Si-walik Group of the Subhimalayan zone (Dhi-tal and Kizaki, 1987; Schelling, 1992; Srivas-tava and Mitra, 1994). Provenance data fromthese foreland basin deposits indicate large-scale unroofing of the crystalline nappes be-ginning at ca. 11 Ma, which suggests that theLesser Himalayan duplex that folded the crys-talline nappes began growing at that time(DeCelles et al., 1998).

The Thakkhola graben is bounded by Pa-leozoic Tethyan Series rocks in its westernfootwall and Mesozoic Tethyan Series rocksin its eastern footwall (Fig. 2). Different levelsof exposure between the eastern and westernfootwalls reflect greater displacement alongthe western basin-bounding fault (Fort et al.,1982; Colchen et al., 1986; Colchen, 1999).The Mustang and Mugu granites are exposedalong the western edge of the basin. These arepart of the North Himalayan granite belt, ex-posed midway between the South Tibetan de-tachment system and the Indus suture zone(Fig. 1) (Le Fort, 1986; Le Fort and France-Lanord, 1995). The North Himalayan granitesrange in age from ca. 10 Ma to ca. 17 Ma,and the Mugu pluton has been dated by Th-Pb monazite at 17.6 6 0.3 Ma (Harrison etal., 1997).

Two formations have been mapped in Thak-khola graben. The older Tetang Formation isup to 240 m thick and crops out in the south-eastern part of the basin, where the entire ba-sin fill has been incised (Fig. 2). The TetangFormation rests unconformably on previouslyfolded and faulted Tethyan Series rocks. Thebest estimate for the age range of the TetangFormation is between ca. 11 and 9.6 Ma,based on magnetostratigraphy (Garzione et al.,2000a). An older age (ca. 14 Ma) for exten-sion in the Thakkhola graben was inferredfrom an 40Ar/39Ar mica date from a vein as-sociated with a north-striking normal fault

along the Marsyangdi valley in central Nepal(Coleman and Hodges, 1995).

An angular unconformity marks the contactbetween the Tetang Formation and the over-lying Thakkhola Formation (Fort et al., 1981).The Thakkhola Formation crops out through-out the basin and is up to ;1000 m thick.Stable carbon isotopic data from paleosol car-bonate nodules of the Thakkhola Formationindicate the presence of C4 grasses in the basinthroughout deposition of the Thakkhola For-mation (Garzione et al., 2000a). Global ex-pansion of C4 grasses took place between 8

and 6 Ma (Cerling et al., 1997) and did notappear in the south Asian soil carbonate rec-ord until ca. 7 Ma (Quade et al., 1989, 1995),which suggests an age of younger than 7 Mafor deposition of the Thakkhola Formation.Oxygen isotopes of lacustrine and paleosolcarbonate rocks in the Thakkhola graben in-dicate elevations consistent with modern ele-vations since deposition began in the basin(Garzione et al., 2000a, 2000b).

Today, the Kali Gandaki river locally drainsthe southern Tibetan plateau, flowing south-ward along the axis of the Thakkhola graben,


GARZIONE et al.

through the fold-thrust belt to the Himalayanforeland. Terrace deposits in the Kali Gandakivalley document multiple phases of Quater-nary damming and incision (e.g., Fort et al.,1982; Iwata, 1984; Hurtado et al., 2001).

SEDIMENTOLOGY OF THE TETANGAND THAKKHOLA FORMATIONS

Environments of deposition are interpretedon the basis of six, bed-by-bed measured sec-tions in the Thakkhola graben (Figs. 2, 3, and4). The ;240-m-thick Tetang Formation cropsout in the southeastern part of the basin andfines upward from alluvial-fan and braidedfluvial conglomerates into lacustrine mudstoneand carbonate (Fig. 3). The Thakkhola For-mation, which is distributed throughout thebasin, is at least ;800 m thick and generallyfines upward from alluvial-fan and braidedfluvial conglomerate to lacustrine mudstoneand carbonate and braided fluvial conglomer-ate (Fig. 4). Each measured section is separat-ed by .5 km distance. Minor faults, syntheticto the eastern and western basin-boundingfaults, have displacements of several tens tohundreds of meters and offset sections of theTetang and Thakkhola Formations, precludingprecise correlations between sections. Withinthese sections, we counted .2600 clasts to de-termine provenance and measured .710 pa-leocurrent indicators. Paleocurrent directionswere determined from imbricated gravels. Tenor more clast orientations were measured inindividual beds. Conglomerate provenancewas determined by counting 100 or moreclasts from each site. Clasts were recorded byrock type and description and classified as Pa-leozoic, Mesozoic, and Tertiary granite, on thebasis of our observations of these units alongthe margins of the basin. Paleocurrent andprovenance data are shown on the stratigraph-ic columns (Figs. 3 and 4). Excellent lateralcontinuity of exposures allowed for two-dimensional observations of depositional ar-chitecture and facies associations over hun-dreds of meters to several kilometers.Stratigraphic sections indicate three geneticlithofacies associations: alluvial fan, braidedstream, and lacustrine.

Alluvial-fan Lithofacies Association

DescriptionDominantly conglomeratic sections of the

alluvial-fan lithofacies association, up to 400m thick, are exposed along the western andsoutheastern edges of the basin. The alluvial-fan lithofacies association consists of five lith-ofacies: A1–A5.

Lithofacies A1 is moderately sorted, clast-supported, granule-boulder conglomerate (Fig.5A). Clasts are subangular to subrounded.Conglomerate beds are 0.3–2 m thick, haveerosive bases, and can be traced laterally fortens to hundreds of meters. Deposits are gen-erally massive but also have local clast imbri-cation and crude horizontal stratification. Nooverall vertical trends in grain size are evidentin these conglomerate bodies.

Lithofacies A2 consists of poorly sorted,matrix- or clast-supported, cobble-boulderconglomerate (Fig. 5A). Clasts are angularand are supported by a matrix of silt- to sand-sized grains. Individual beds are 0.5–4 mthick, extend laterally for tens of meters, andhave nonerosive bases. Beds of lithofacies A2lack stratification but locally exhibit inversegrading.

Lithofacies A3 comprises imbricated tomassive, moderately well sorted, pebble-cobbleconglomerates. These conglomerates are clastsupported and consist of rounded clasts in asand-sized matrix. The deposits occur in 1–20-m-thick, multistory lenticular bodies witherosive bases that continue laterally for up to100 m or more. Lenticular beds are 0.2–3 mthick and extend laterally for tens of metersand are sometimes deeply inset within adja-cent deposits.

Lithofacies A4 consists of wedge-shapedbodies of structureless or horizontally lami-nated, fine-grained to very coarse grainedsandstone (Fig. 5B). Scour and fill structuresare common. These deposits form 0.1–1-m-thick lenticular beds that continue laterally forup to 10 m and are associated with imbricated,pebble-cobble conglomerates of facies A3.

Lithofacies A5 is a minor component of thealluvial-fan facies that is associated with all ofthe above lithofacies and consists of massive,completely unstratified, red sandy mudstonewith angular, matrix-supported clasts up topebble size. These beds can be traced laterallyfor hundreds of meters and are generally ,1m thick. They are often mottled and rarelycontain diffuse nodular carbonate (,1 cm di-ameter) horizons.

InterpretationWe interpret lithofacies A1 as unconfined

sheetflood deposits. Horizontal stratificationand imbrication suggest that clasts were trans-ported in traction flows. Many authors haveinterpreted similar alluvial-fan facies as thedeposits of hyperconcentrated flood flows(e.g., Nemec and Steel, 1984; Horton andSchmitt, 1996).

Lithofacies A2 is interpreted as debrisflows. Clast-supported, inversely graded beds

indicate that dispersive forces were significant,perhaps during transport as density-modifiedgrain flows and/or clast-rich debris flows (e.g.,Nemec and Steel, 1984). Another possibilityis that these inversely graded beds were de-posited as gravelly traction carpets, a type ofshear modified grain flow (Todd, 1989; Sohn,1997).

Lithofacies A3 has channel-fill geometriesthat suggest it is fluvial channel fill depositedin fan channel networks or incised fanheadtrenches (e.g., DeCelles et al., 1991). In as-sociation with channel-fill deposits, lithofaciesA4 is interpreted as sand wedges that accu-mulated on bar flanks, deposited during wan-ing flow.

We interpret lithofacies A5 as paleosols thatformed on temporarily abandoned parts of thealluvial fan.

Braided River Lithofacies Association

DescriptionThe braided stream lithofacies association

consists of three lithofacies. Sections of inter-bedded clast-supported conglomerate (B1),sandstone (B2), and mudstone (B3), between10 and 400 m thick, are exposed throughoutthe Thakkhola basin. The primary braidedstream lithofacies (B1) consists of moderatelysorted, clast-supported, pebble-cobble con-glomerate. Clasts are well rounded and areusually imbricated or horizontally stratified(Fig. 5C). Conglomerate deposits form 0.1–2-m-thick lenticular beds that extend laterallyfor 3 to .50 m. These beds stack to formlenticular bodies that extend laterally for tensof meters to .1 km. Thicker, more laterallyextensive pebbly beds display trough cross-stratification. Beds have sharp or scoured ba-ses and commonly fine upward. Coarser cob-ble conglomerates are concentrated in the axisof the Thakkhola basin.

Lithofacies B2 is moderately to poorly sort-ed, massive or weakly horizontally laminatedsandstone/mudstone and commonly displaysscour and fill structures or cross-stratification.Grain size varies from silt to very coarse sand,and floating pebbles are common. These sand-stone beds are ,0.1–0.5 m thick, continue lat-erally for tens of meters, and are interbeddedwith conglomerate deposits of lithofacies B1.Sandstone grain size usually fines upward, andsandstone often caps a fining-upward con-glomerate. Conglomerate and sandstone formmultistory lenticular bodies between 1 and 12m thick.

Lithofacies B3 separates bodies of lithofa-cies B1 and B2, consisting of laminated ormassive mudstone. Laminated mudstone is



Figure 3. Logs of measured stratigraphic sec-tions of the Tetang Formation at (A) Tetangvillage, and (B) along Ghidiya Khola (see Figs.2 and 10 for locations).


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Figure 4. Logs of measured stratigraphic sections of the Thakkhola Formation near (A) Chele village, (B) Giling village, (C) Dakmarvillage, and (D) Dhi village. (E) Paleoflow directions based on imbricated clasts measured in axial braided river facies along TsarangKhola (see Fig. 2 for location).



Figure 4. (Continued.)


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Figure 5. Examples of lithofacies; 35-cm-long hammer for scale. (A) Sheetflood and debris-flow deposits of facies A1 and A2 of alluvial-fan facies association. Thakkhola Formation, near Chele village (Fig. 4A). Arrow points to contact between water-transported depositsbelow and debris-flow deposits above. Lower bed has dark red paleosol development in upper 20 cm of bed. (B) Imbricated pebblyfluvial conglomerate of facies B1 of braided fluvial lithofacies association. Tetang Formation, near Tetang village (Fig. 3A). Arrows pointto examples of imbricated clasts. (C) Imbricated cobble conglomerate of facies B1 and sandy bar flank deposits of facies B2 of braidedfluvial lithofacies association. Thakkhola Formation, near Chele village (Fig. 4A). Note the presence of granite clasts (arrows), a commonclast-type in axial river deposits of the Thakkhola Formation. (D) Paleosol of facies B3 of braided fluvial lithofacies association withnodular pedogenic carbonate horizon. Thakkhola Formation, near Chele village (Fig. 4A). Black line indicates thickness of massiveoxidized mudstone in upper part of paleosol. Arrows point to two nodular carbonate horizons.

gray and grades up into massive gray to red/yellow/gray mudstone, forming a 0.1–1.5-m-thick deposit. These deposits are often stackedto form bodies up to 5 m thick. These depositsusually contain one or more of the following:root traces, bioturbation, plant fossils, hema-tized zones, and nodular carbonate horizons(Fig. 5D).

InterpretationOn the basis of bedding thickness, shape,

lateral continuity, and stacking patterns, theimbricated and horizontally bedded conglom-erate deposits of lithofacies B1 are interpretedas longitudinal bar deposits that filled rela-tively shallow gravelly channels (e.g., Heinand Walker, 1977). Normal grading of indi-vidual beds may have developed during wan-ing flow.

Interbedded sandstones/mudstones of litho-facies B2 may have been deposited on the topsor flanks of bars during waning flow or in

slack-water ponds, such as abandoned chan-nels (Rust, 1972; Miall, 1977).

Lithofacies B3 contains features that areconsistent with overbank deposition and sub-sequent soil development (Miall, 1985).Stacked sequences of alternating overbank de-posits and paleosols suggest that river bankswere sometimes stable over long periods oftime.

Lacustrine Lithofacies Association

DescriptionThe lacustrine lithofacies association com-

prises mudstone (C1) with interbedded sand-stone and conglomerate (C2) and carbonate(C3 and C4). These deposits are most abun-dant in the axial and southeastern parts of thebasin. Lithofacies C1 is poorly consolidated,laminated to massive calcareous mudstone(Fig. 5E). Some massive calcareous mudstonebeds exhibit exfoliation weathering around

nodular masses (5–12 cm diameter), biotur-bation, and plant fossils.

Lithofacies C2 occurs within the mudstonefacies as 0.1–0.5-m-thick beds of normallygraded or structureless sandstone and clast-supported, pebble conglomerate. These bedsare tabular, have sharp erosional bases, andcan be traced laterally for tens to .100 m overthe length of the outcrop belt. Asymmetric rip-ple cross-laminae are sometimes preserved inthe upper parts of these sandstone beds.

Lithofacies C3 consists of laminated tomassive carbonate up to 1.4 m thick and lat-erally continuous for tens of meters. Thesecarbonate beds are interbedded within braidedfluvial lithofacies. The carbonates are micriticand contain ostracods, charophytes, and shellfragments. Petrographic inspection revealsthat carbonates contain 10%–50% blocky orsparry calcite.

Lithofacies C4 is massive to laminated car-bonate that forms beds from 0.05 to 0.5 m



Figure 6. Paleocurrent map showing average paleoflow direction of axial drainage facies(large arrows) and transverse drainage facies (small arrows) from each measured sectionin the Tetang Formation (gray arrows) and Thakkhola Formation (black arrows).

thick. These deposits are rich in organic mat-ter and often release a petroliferous odor whenbroken. In thin section, the carbonate is mi-critic; sparry or blocky recrystallized calciteconstitutes up to 50% of the rock. Carbonatebeds of lithofacies C4 are stacked to form de-posits up to 100 m thick and laterally contin-uous for as far as 2 km. These carbonates arelaterally contiguous with organic-rich mud-stones of lithofacies C1.

InterpretationThe mudstone and sandstone/conglomerate

of lithofacies C1 and C2 form packages from2 to 80 m thick that continue laterally for ki-lometers. Lithofacies C1 was deposited byfallout of suspended sediment carried bystreams feeding into the lakes. The exfoliation-weathering, bioturbated massive mudstonesare interpreted as paleosols that developed onthe margin of the lake during regression. Lith-ofacies C2 is interpreted as the deposits of tur-bidity flows within the lake (Lowe, 1982; Ne-mec and Steel, 1984); ripples formed duringwaning stages of the flow.

Carbonate beds of lithofacies C3 are inter-preted as slack-water pond deposits associatedwith fluvial deposits, on the basis of their lim-ited lateral extent and contiguity with thebraided river lithofacies.

We suggest that lithofacies C4 representsprofundal lacustrine deposits. Accumulationsof organic material are probably associatedwith seasonal accumulations of phytoplanktonthat remained unoxidized as a result of stag-nant bottom conditions.

SEDIMENT DISPERSAL PATTERNS

Paleocurrent Directions

Paleocurrent indicators across the basin aresummarized in Figure 6. Paleocurrent datafrom imbricated cobble-boulder conglomeratesin the Tetang Formation in the Ghidiya Kholasection (khola means river in Nepali) indicatedominantly westward paleoflow (Fig. 3B). Inthe Tetang Village section, imbricated pebbleconglomerates indicate east-southeastward pa-leoflow below 80 m in the section, shifting tothe south-southeastward up to 100 m andsouthwestward above 120 m (Fig. 3A).

Along the western edge of the basin, cobble-boulder conglomerates of the Thakkhola For-mation indicate dominantly southeast- andeastward paleoflow (Fig. 4A [lower 150 m]and Fig. 4C). Above 150 m in the Chele sec-tion, pebble-cobble conglomerates indicatesouth- to westward paleoflow. Along the axisof the basin, cobble conglomerates indicate

southeast- to southwestward paleoflow (Figs.4B and 4E). Paleoflow indicated by pebbleconglomerates in the northeastern part of thebasin was generally toward the west-northwest(Figs. 4D and 6).

Provenance

Conglomerate provenance indicates a sim-ple unroofing history from Mesozoic and Pa-leozoic clasts in the Tetang Formation to dom-inantly Paleozoic, some Mesozoic, and granite

clasts in the Thakkhola Formation. In the Te-tang Formation, along the southeastern edgeof the basin, clast counts indicate a source re-gion in the Mesozoic rocks exposed in the ad-jacent footwall that bounds the basin on theeast (Fig. 3B). However, in the central part ofthe basin, conglomerate clasts consist of;60% Paleozoic phyllites and ;40% Triassicto Jurassic calcareous clastic and carbonaterocks, reflecting rocks exposed in the westernfootwall (Fig. 3A). Paleocurrent indicators inthe Tetang Formation are consistent with the


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Figure 7. Schematic cross section through the southern Thakkhola graben, showing important features used to interpret the tectonichistory of the basin.

provenance data (Figs. 3 and 6) and also sug-gest that clasts deposited in the southeasternpart of the basin were derived from the east,whereas clasts deposited in the central part ofthe basin were derived from the west andnorth.

In the Thakkhola Formation, along the axisof the basin, cobble conglomerates consist of60%–75% Paleozoic clasts, 10%–25% Meso-zoic clasts, and 10%–25% granite clasts (Fig.3B [above unconformity] and Fig. 4B). Alongthe eastern edge of the basin near the town ofDhi, conglomerate clasts are almost entirelyMesozoic (Fig. 4D). Although the basin’seastern margin north of Tetang village has notbeen mapped, the composition of these con-glomerates combined with paleocurrent infor-mation suggests that Mesozoic rocks are ex-posed in the eastern footwall near Dhi (Figs.4D and 6). Along the fault that bounds thebasin on the west, boulder-cobble conglom-erates of the Thakkhola Formation containdominantly Paleozoic and some Mesozoicclasts, consistent with sources in the westernfootwall. Clasts consist of 60%–98% Paleo-zoic rocks, and most of the remaining clastsare Mesozoic rocks. In both the Chele andDakmar sections, a brief incursion of graniteclasts is found near the base of the ThakkholaFormation, where granite clasts make up be-tween 10% and 50% of the total clast popu-lation (84 m–150 m levels near Chele and 15m–33 m levels near Dakmar) (Figs. 4A and4C).

BASIN EVOLUTION

A model of basin evolution must explainthe following features: paleotopography priorto initial sedimentation, growth faults and fa-cies patterns in the Tetang Formation, the an-gular unconformity between the Tetang andThakkhola Formations, and growth strata andfacies patterns in the Thakkhola Formation.These features are shown in Figure 7 and dis-cussed in the ensuing sections.

Tetang Formation

The Tetang Formation displays complexstratigraphic patterns associated with remnantpaleotopography, with no significant thicken-ing toward the eastern or western basin-bounding faults (Fig. 7). Along the easternflank of the basin, where the contact relation-ships are especially well exposed along theNarsing and Ghidiya Kholas, the Tetang For-mation tapers and onlaps the Mesozoic base-ment (Figs. 8 and 9). Thus, the evidence link-ing deposition of the Tetang Formation withmajor east-west extension along the presentbasin-bounding faults is not clear-cut. The ap-parent restriction of alluvial-fan deposits tothe eastern part of the Tetang outcrop areasuggests local high-relief topography alongthe eastern margin of the basin. However,these deposits could also be associated withlocal, remnant topography that resulted fromshortening in the Tethyan fold-thrust belt. Ex-amples in which the Tetang Formation over-

laps remnant topography are superbly exposedalong Narsing Khola southeast of Tetang vil-lage, where the upper carbonate-rich part ofthe Tetang Formation thickens abruptly west-ward off of the crest of a large fold in Me-sozoic rocks (Fig. 8). Similarly, a large (;2km2) paleotopographic high directly south ofTetang village separated the two southern out-crop areas from the three northern areas (Fig.10).

The Tetang Formation records several pa-leoenvironments, including alluvial fans,small braided rivers and associated flood-plains, and large lakes. Early Tetang deposi-tion was dominated by fluvial environmentsalong the axis of the basin and alluvial-fanenvironments along the eastern margin of thebasin. Fluvial conglomerates are pebbly andconsist of beds between 0.2 and 0.8 m thick,which are interpreted as bar deposits. Bedthickness indicates that channels were as deepas 0.8 m. Both grain size and channel depthsuggest that this river system was much small-er than the modern Kali Gandaki river.

The late Tetang deposition was in an exten-sive lake system (5 10 km2) dominated bycarbonate deposition (facies C4) (Figs. 3 and9). Extremely positive d13C values (d13CVPDB

up to 12‰ [VPDB 5 Vienna Peedee belem-nite isotope standard]) in diagenetically al-tered carbonates of the Tetang Formation(Garzione, 2000) may have resulted from me-thanogenic processes (e.g., Talbot and Kelts,1990), which suggests that these lakes hadproductive surface waters that led to the de-



Figure 8. Stratigraphic relationships near Narsing Khola (see Fig. 10 for location). (A) Tetang Formation (Tt) onlaps Cretaceous ChukFormation (K), whereas Cretaceous rocks and Tetang Formation were beveled before deposition of the Thakkhola Formation (Th). (B)Tetang Formation overlying folded Cretaceous rocks.

Figure 9. Stratigraphic relationships along Ghidiya Khola. Cretaceous Chuk Formation (K), Tetang Formation (Tt), and ThakkholaFormation (Th) (see Fig. 10 for location). Letters indicate lithofacies discussed in text: A1, B1, C1, C4. Thin, solid, subhorizontal linesshow lithofacies and formation boundaries, and dashed lines are inferred boundaries. Thick, subvertical lines are minor normal faults,and very thick line shows approximate location of measured section. Note the eastward thinning of Tetang Formation and the lateralextent of axial drainage facies of the Thakkhola Formation.

velopment of dysaerobic bottom conditions.Tetang lacustrine deposits display shoaling-upward cycles on the order of ,5 m and be-come more widespread through time. Thepredominance of carbonate deposition, fluc-tuating lake levels, and increasing lake sizesuggest balanced-filled to underfilled lakeconditions, with limited throughgoing riversystems (e.g., Carroll and Bohacs, 1999).

Paleocurrent, provenance, and lithofaciesdata suggest that Tetang conglomerates werederived from both margins of the basin anddeposited in small southward-flowing riversand small, coarse-grained alluvial fans. Prov-enance data from the central part of the basinindicate both Paleozoic and Mesozoic sourceterranes to the north and west. These rockswere almost certainly exposed throughout the

fold-thrust belt prior to deposition of the Te-tang Formation, and inherited drainage pat-terns could have transported clasts from distalregions into the basin.

The strongest evidence for syndepositionalnormal faulting during Tetang depositioncomes from detailed mapping in Narsing andGhidiya Kholas. Tetang strata have fairly uni-form dips of 188–248 toward the north-northwest(Fig. 10). Variations of dip are localizedaround minor normal growth faults (Fig.11A). These growth faults record several me-ters to 10 m of offset and are roughly parallel(6308) to the eastern and western basin-bounding faults. Larger-offset, northwest-striking normal faults cut both the Tetang andThakkhola Formations and do not appear tobe associated with syntectonic thickness

changes in either unit, suggesting that theypostdate deposition (Fig. 10).

Given the absence of evidence for majorsyndepositional normal faulting during depo-sition of the Tetang Formation, we suggestthat much of the accommodation space for Te-tang deposition may have developed as a re-sponse to normal faulting and footwall upliftassociated with the South Tibetan detachmentsystem (Fig. 12A). Vannay and Hodges (1996)and Godin (1999) reported muscovite 40Ar/39Ar ages from the footwall of the Annapurnadetachment (part of the South Tibetan detach-ment system in the Kali Gandaki drainage)that indicate cooling through the muscoviteclosure temperature between ca. 15 and 13Ma. Younger muscovite 40Ar/39Ar ages be-tween 12.7 6 0.4 Ma and 11.8 6 0.4 Ma from


GARZIONE et al.

Figure 10. Geologic map of southern part of the Thakkhola graben showing the distribution of the Tetang Formation with respect toMiocene paleotopography developed in the Mesozoic Tethyan Series rocks. Locations of Figures 8, 9, and 11A are shown.

the immediate hanging wall of the detachmenthave been interpreted as reflecting a hydro-thermal event associated with late brittle ex-tension on the Annapurna detachment (Godin,1999). These ages coincide with initial depo-sition of the Tetang Formation and suggestthat footwall uplift could have produced astructural dam that was capable of pondingdrainage to the north and creating large lakeson top of the fold-thrust belt (Fig. 12A). Thismechanism could account for the locally thickaccumulations of the Tetang Formation andthe absence of significant thickening towardeither of the basin-bounding faults. Growthfaults in the upper Tetang Formation (Fig.11A) herald the onset of east-west extension,which was subsequently recorded by deposi-tion of the overlying Thakkhola Formation.

Development of the Tetang-ThakkholaAngular Unconformity

The Thakkhola Formation overlies the Te-tang Formation in angular unconformity (Fig.7). Tetang strata dip ;188 to 248 to the north-northwest. These strata were rotated into thewestern basin-bounding fault and beveled pri-or to deposition of the Thakkhola Formation,which dips ;158 to the northwest at the baseof the section, above the unconformity. Fortet al. (1981, 1982) recognized the angular un-conformity between the Tetang and ThakkholaFormations near the village of Tetang. Thisstudy documents the same unconformity alongGhidiya Khola, which was previously mappedentirely as Tetang Formation (Fort et al.,1981). The contact and facies in the cliffsabove Ghidiya Khola (Figs. 9 and 11B) cor-

relate better with imbricated cobble conglom-erates at the base of the Thakkhola Formation.Recognition of this unconformity is significantbecause overlying Thakkhola strata representthe first southward-flowing axial river system,consistent in size with the Kali Gandaki riverand containing the first granite clasts (Fig. 3B,top of stratigraphic column). This angular un-conformity also represents a temporal gap of$2.6 m.y., beginning at ca. 9.6 Ma and endingafter 7 Ma (Garzione et al., 2000a), whichmay have resulted from incision of the TetangFormation and Tethyan Series by this largesouthward-flowing river system. The TetangFormation was rotated ;58 to the northwestprior to deposition of the Thakkhola Forma-tion, suggesting that significant displacementoccurred on the Dangardzong fault during thishiatus.



Figure 11. (A) Normal growth faulting in the Tetang Formation (see Fig. 10 for location). Subhorizontal lines show single horizons,broken by normal faults. Note that Tetang deposits are thickened on the downthrown side of the faults. (B) Angular unconformity(arrow) between the Tetang Formation carbonate (lithofacies C4) and Thakkhola Formation cobble conglomerates (lithofacies B1). Blackbar represents 1 m. (C) Widespread lacustrine deposits of the Thakkhola Formation. Shown here are two lacustrine intervals thatcorrespond with 285–370 m and 400–430 m in the Chele stratigraphic section (Fig. 4A).

Thakkhola Formation

The base of the Thakkhola Formation canonly be observed above the Tetang Formationand where there are paleotopographic highs inunderlying Cretaceous rocks. Along the axisof the basin, the Thakkhola Formation con-sists of braided fluvial deposits. We gatheredclast composition and paleocurrent data fromthis paleo–river system in three locations: (1)overlying the Tetang Formation along GhidiyaKhola, (2) near the town of Giling, and (3)along Tsarang Khola (Figs. 3B, 4B, and 4E).Cobble conglomerate beds from these depos-its, interpreted as longitudinal bars, are 0.2–2m thick and continue laterally for tens of me-ters. The thickness of bar deposits indicatesthat channels were as deep as 2 m. These bedsare amalgamated to form lenticular bodiesperpendicular to flow that can be traced lat-erally for hundreds of meters to several kilo-meters. Grain size, lateral extent, and channeldepth suggest that this paleo–river system wascomparable in size to the modern Kali Gan-daki river. The generally southward paleoflow

recorded in these deposits is the same as themodern Kali Gandaki river. The axial river de-posits can be distinguished from small trans-verse river deposits in the basin by their lateralextent, bedding thickness, and the presence ofgranite clasts in the axial river conglomerates,probably derived from the Mustang-Mugugranite bodies. We interpret these deposits asevidence for the existence of the paleo–KaliGandaki river by late Miocene time (ca. 7 Ma)(Fig. 12B). The thickness of the ThakkholaFormation increases markedly from east towest, and bedding generally dips toward thewest. In the western part of the basin, adjacentto the Dangardzong fault, the dip of the Thak-khola Formation decreases up-section from asmuch as 20 8 at the base of the section to ,38at the top as a result of growth folding of stra-ta during displacement on the Dangardzongfault (Fig. 7).

Along the western edge of the basin, theThakkhola Formation consists of alluvial-fanconglomerates, up to 400 m thick, depositedby braided streams, debris flows, and sheetfloods (Figs. 4A and 4C), also providing evi-

dence that the Dangardzong fault was activeduring deposition of the Thakkhola Formation(Fig. 12B). In the northeastern part of the ba-sin, conglomerates with west-northwestwardpaleoflow directions are interpreted as trans-verse braided fluvial drainages that flowed to-ward the axis of the basin (Fig. 4D). Interbed-ded lacustrine deposits of mudstone andcarbonate probably represent paludal depositsassociated with these transverse drainages.Conglomerate clasts in these small tributarydrainages were derived entirely from Meso-zoic rocks, presumably in the eastern footwall.

Three widespread lacustrine units occurthroughout the southern part of the basin. Inthe Chele section, lacustrine intervals occurbetween 285–370 m, 400–430 m, and 640–685 m (Figs. 4A and 11C) and can be tracedlaterally for up to 5 km. Axial drainage faciesoccur below and above these lacustrine de-posits. Episodes of lacustrine deposition alter-nating with axial fluvial deposition indicateperiodic damming of this southward-flowingriver system (Fig. 12C).

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GARZIONE et al.

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thirds of the section measured near Chele (Fig.4A) exhibit generally southwestward paleo-flow directions. These facies lack graniteclasts and were deposited by rivers muchsmaller than the modern Kali Gandaki river.The predominance of Paleozoic clasts in thesedeposits indicates that the rivers were derivingmaterial from the western margin of the basin.We interpret these deposits as tributary drain-ages that flowed parallel to the basin axis be-fore joining the paleo–Kali Gandaki river.

Conglomerate provenance in the ThakkholaFormation provides some constraints on ex-humation rates in the western footwall. Thefirst granite clasts appear at the base of theThakkhola Formation in the axial drainage fa-cies. These granites were most likely derivedfrom the Mugu-Mustang plutons along thenorthwestern margin of the basin (Fig. 2). LeFort and France-Lanord (1995) determinedthat the Mugu granite was emplaced at pres-sures between 2.6 and 4.1 kbar, which indi-cates depths between ;8 and 12 km. Harrisonet al. (1997) obtained a Th-Pb monazite ageof 17.6 6 0.3 Ma for the Mugu pluton. If aconservative age estimate of ca. 5 Ma is as-sumed for the base of the Thakkhola Forma-tion, exhumation rates of the Mugu plutonwere fairly rapid, averaging between ;0.6 and1 mm/yr between the pluton crystallizationage and the depositional age of the ThakkholaFormation.

DISCUSSION

Climate Trends

The sedimentary fill of the Thakkhola gra-ben records paleoclimate trends in the south-ern Tibetan plateau. Several lines of evidencesuggest that conditions have become increas-ingly arid in the Thakkhola graben since de-position began at ca. 11 Ma. Pollen data fromthe Tetang Formation indicate the presence ofabundant pine (Pinus), alder (Alnus), fir (Abi-es), oak (Quercus), hemlock (Tsuga), andbirch (Betula) (Yoshida et al., 1984). Althoughsome pollen may have been transported intothe basin by wind, Tsuga pollen, which doesnot persist in the basin today, is less prone towind transport. Today the graben floor is cov-ered by steppe vegetation, with restrictedhigh-elevation pockets of juniper, pine, and fir(Fort, 1987). Currently, no natural stands ofdeciduous trees exist in the basin, and the co-nifers that do persist are restricted to higherelevations, which receive more precipitation.

Paleosol characteristics suggest that condi-tions were more humid during Tetang depo-sition than during Thakkhola deposition. The

presence of pedogenic carbonate is generallyassociated with semiarid or arid climates (e.g.,Slate et al., 1996). Lack of pedogenic carbon-ate in Tetang Formation paleosols associatedwith fluvial and alluvial-fan lithofacies, incontrast to the abundance of paleosol carbon-ate nodules in the Thakkhola Formation, sug-gests that climate may have been more humidduring Tetang deposition. In addition, histichorizons and gleyed horizons are more com-mon in the Tetang Formation, suggesting wa-terlogged conditions.

Oxygen isotopes from carbonates in theThakkhola graben suggest that during Mio-cene time this region was as elevated as it istoday (;3800–5900 6 500 m, Garzione et al.,2000a, 2000b). Environmental changes anddepositional changes observed at ca. 8–7 Main the Indo-Gangetic foreland, Bengal fan, andArabian Sea have been attributed to intensifi-cation of the Asian monsoon at that time(Quade et al., 1989; Kroon et al., 1991; Prellet al., 1992; Burbank et al., 1993; Derry andFrance-Lanord, 1996). More recently, stableisotopes of mollusks in Siwalik foreland basindeposits have been interpreted to indicate anintense Asian monsoon prior to 8 Ma, withoverall greater annual precipitation, much likemodern Burma (Dettman et al., 2001). Inmonsoon climates, summer monsoon precipi-tation usually has more negative d18O valuesthan winter precipitation because the intensityof the rainfall causes a rapid depletion of 18Oin the cloud mass (Rozanski et al., 1993).Very large seasonal variations in d18O valuesof mollusks between winter and summer rain-fall reflect variations that are greater thanmodern foreland precipitation (Dettman et al.,2001). Prior to 7 Ma, the d18O value of winterrainfall was similar to modern rainfall, where-as summer rainfall was more negative thanmodern by up to several per mil. This findingsuggests that the intensity of the summer mon-soon was even greater than today. In addition,the middle member of the Siwalik Group (be-ginning at ca. 11 Ma in western Nepal) dis-plays an increase in channel size and changein channel morphology that suggest an ap-proximately five-fold increase in discharge(DeCelles et al., 1998). This increase in dis-charge recorded in the middle member of theSiwalik Group is consistent with observationsof a more humid climate during deposition ofthe Tetang Formation. Greater humidity in theHimalaya and Tibet during the late Miocenemay explain the presence of large mammalianfauna and more humid-climate flora in otherbasins on the Tibetan plateau (e.g., Chen,1981; Xu, 1981, Li et al., 1995).

Perhaps the most important implication of

an elevated and more humid Tibetan plateauprior to 8 Ma is that our current ideas aboutthe timing of uplift of the plateau and relatedclimate change are too simple. It is generallyaccepted that intensification of the Asian mon-soon coincided with uplift of the plateau at ca.8 Ma (e.g., Prell and Kutzbach, 1992, Raymoand Ruddiman, 1992; Harrison et al., 1992;Molnar et al., 1993). However, paleoclimateand paleoelevation prior to 8 Ma are still poor-ly understood. Future studies will likely revealthat the Tibetan plateau had a more compli-cated uplift history than we currently under-stand and that the intensification of the Asianmonsoon occurred earlier than 8 Ma.

Southward Drainage of the Kali GandakiRiver

Facies distributions and paleocurrent direc-tions in the Thakkhola graben, at the base ofthe Thakkhola Formation, document the onsetof southward axial drainage, which was sim-ilar to the size of the modern Kali Gandakiriver. Paleocurrent indicators from the lowerTetang Formation in the central part of the ba-sin also indicate southward-flowing rivers, butfacies distributions in the upper Tetang For-mation suggest that drainage became increas-ingly restricted.

Several intervals of widespread lacustrinedeposition within the Thakkhola Formation(Fig. 11C) indicate intermittent damming ofthe axial drainage system or rapid subsidencealong the Dangardzong fault. A few mecha-nisms could explain the lack of a large south-ward axial drainage system during Tetang andThakkhola deposition: (1) glacial damming,(2) northward paleoslope and paleodrainage,and (3) structural damming. Glacial dammingor a northward paleoslope are possible, butunlikely, mechanisms for the lack of a south-ward axial drainage system during depositionof the Tetang Formation. Glacial advance andretreat have been suggested as a mechanismfor Quaternary sediment damming and inci-sion along the Kali Gandaki river (e.g., Fortet al., 1982; Iwata, 1984). However, becausethere is no evidence for glacial deposition inthe Tetang Formation or Thakkhola Forma-tion, we view this as an unlikely mechanismfor sediment damming during that time. Anorthward paleoslope also seems unlikely inview of the absence of northward paleoflowindicators and clasts derived from the GreaterHimalayan rocks. On the contrary, southwardpaleoflow of braided rivers in both formationsand the development of lakes in the southernpart of the graben indicate a southwardpaleoslope.


GARZIONE et al.

Abundant structural and thermochronologi-cal evidence suggests growth of structuressouth of Thakkhola graben that could havecaused structural damming. Initial displace-ment on the Main Central thrust in central Ne-pal occurred by ca. 22.5 Ma at about the sametime as initial extension on the South Tibetandetachment system (Hodges et al., 1996; Co-leman, 1998). Younger muscovite 40Ar/39Arages between 12.7 6 0.4 Ma and 11.8 6 0.4Ma from the immediate hanging wall of thedetachment (Godin, 1999) and the identifica-tion of Quaternary normal faulting along thedetachment (Hurtado et al., 2001) are evi-dence of recent brittle reactivation of the de-tachment system, which could have been re-sponsible for the closed-basin conditionsduring Tetang deposition. Either (1) headwarderosion through the Greater Himalaya south ofthe detachment system or, (2) the rupture ofthe detachment system and Greater Himalayaby the Dangardzong fault could have breachedthis structural dam. The mapping of Colchenet al. (1986) shows the southern terminationof east-west extensional faults associated withthe Thakkhola graben within Greater Hima-layan rocks, whereas mapping by Hurtado etal. (2001) terminates Thakkhola graben nor-mal faults at the South Tibetan detachment.Determining the validity of either of these in-terpretations is crucial to understanding thedevelopment of southward drainage of theKali Gandaki river and the relationship be-tween grabens in the southern Tibetan plateauand the South Tibetan detachment system.

CONCLUSIONS

1. Basin fill of the Thakkhola graben pro-vides a record of paleoenvironment and thedevelopment of east-west extension in thesouthern Tibetan plateau. The Tetang Formation(ca. 11–9.6 Ma) is up to 230 m thick and con-sists of pebbly braided fluvial deposits, whichgrade up into lacustrine deposits. Along the east-ern edge of the basin, sheetflood-dominated al-luvial fans constitute the lower part of the Te-tang Formation. The Tetang and overlyingThakkhola Formations are separated by an an-gular unconformity represented by an ;58northwest rotation and beveling of Tetangstrata, possibly related to initial activity on theDangardzong fault and development of asouthward, axial, throughgoing drainage sys-tem. The younger Thakkhola Formation(younger than 7 Ma) is up to 800 m thick andconsists of cobbly, braided fluvial deposits andintermittent widespread lacustrine deposits alongthe axis of the basin. Fluvial and debris-flowalluvial-fan deposits are restricted to the west-

ern edge of the basin, and pebbly cobbly de-posits show that braided rivers fed into thebasin off of its eastern edge.

2. North-striking normal growth faultingwithin the Tetang Formation indicates that mi-nor east-west extension occurred during Te-tang deposition, although stratigraphic thick-ening and rotation associated with the majorbasin-bounding faults was not observed. TheDangardzong fault that bounds the westernedge of the basin was active prior to andthroughout deposition of the ThakkholaFormation.

3. Deposition of the Tetang Formation tookplace in several restricted basins, owing to pa-leotopography developed in deformed TethyanSeries rocks. During deposition of the lowerTetang Formation, small braided rivers in thecentral part of the basin flowed mainly south-ward. Lacustrine deposition became morewidespread later, as shown up-section in theTetang Formation. Late Miocene brittle mo-tion on the Annapurna detachment, south ofthe Thakkhola graben, may have formed astructural dam, which caused the ponding ofdrainage to the north and led to the develop-ment of large lakes within the Tethyan fold-thrust belt. By the base of the Thakkhola For-mation, a large axial river system withsouthward paleoflow had developed. South-ward drainage of the paleo–Kali Gandaki rivercould have developed by headward erosion ofthe Greater Himalaya that breached the SouthTibetan detachment system or by extensionalfaults that cut through the detachment systemand greater Himalayan rocks. Several episodesof widespread lacustrine deposition indicatethat this river system experienced periodicdamming perhaps by renewed activity on theAnnapurna detachment.

4. A more humid climate during depositionof Tetang Formation is inferred from thegreater abundance of conifer and deciduouspollen compared to today, the lack of pedo-genic carbonate, and common histic andgleyed paleosol horizons. Environmentalchange in the Thakkhola graben between de-position of the Tetang and Thakkhola forma-tion correlates with changes in climate ob-served in the Siwalik foreland basin, ArabianSea, and Bay of Bengal. Both evidence fromthe Thakkhola graben that the southern Tibet-an plateau was already elevated by ca. 11 Ma(Garzione et al., 2000a, 2000b) and evidencefor intense summer rainfall and dry winters by10.7 Ma in the Himalayan foreland (Dettmanet al., 2001) suggest that by this time a strongsouth Asian monsoon had developed.

ACKNOWLEDGMENTS

This project was supported by the National Se-curity Education Program, the Geological Society ofAmerica Graduate Research Fund, the GeoStructurePartnership at the University of Arizona (includingBP, ExxonMobil, and Conoco), and the Institute forthe Study of Planet Earth at the University of Ari-zona. We thank Jay Quade and David Dettman forinsightful discussions and for the use of their labsto carry out the stable isotope analyses. We alsothank Jose Hurtado for helpful discussions on thegeology of the Thakkhola graben. We thank PaulHeller, Brad Ritts, and an anonymous reviewer forhelping us improve the manuscript.

REFERENCES CITED

Armijo, R., Tapponnier, P., Mercier, J.L., and Han, T., 1986,Quaternary extension in southern Tibet: Field obser-vation and tectonic implications: Journal of Geophys-ical Research, v. 91, p. 13,803–13,872.

Blisniuk, P.M., Hacker, B.R., Glodny, J., Ratschbacher, L.,Wu, Z., McWilliams, M.O., and Calvert, A., 2001,Normal faulting in central Tibet: Nature, v. 412,p. 628–632.

Burbank, D.W., Derry, L.A., and France-Lanord, C., 1993,Reduced Himalayan sediment production 8 Myr agodespite an intensified monsoon: Nature, v. 364,p. 48–50.

Burchfiel, B.C., Zhiliang, C., Hodges, K.V., Yuping, L.,Royden, L.H., Changrong, D., and Jiene, X., 1992,The south Tibetan detachment system, Himalayan or-ogen: Extension contemporaneous with and parallel toshortening in a collisional mountain belt: GeologicalSociety of America Special Paper 269, 51 p.

Burg, J.P., and Chen, G.M., 1984, Tectonics and structuralzonation of southern Tibet: Nature, v. 311,p. 219–223.

Carroll, A.R., and Bohacs, K.M., 1999, Stratigraphic clas-sification of ancient lakes: Balancing tectonic and cli-matic controls: Geology, v. 27, p. 99–102.

Catlos, E.J., Harrison, T.M., Kohn, M.J., Grove, M., Ryer-son, F.J., Manning, C., and Upreti, B.N., 2001, Geo-chronologic and thermobarometric constraints on theevolution of the Main Central thrust, central NepalHimalaya: Journal of Geophysical Research, v. 106,p. 16,777–16,204.

Cerling, T.E., Harris, J.M., MacFadden, B.J., Leakey, M.G.,Quade, J., Eisenmann, V., and Ehleringer, J.R., 1997,Global vegetation change through the Miocene/Plio-cene boundary: Nature, v. 389, p. 153–158.

Chen, W.Y., 1981, Natural environment of the Pliocene ba-sin in Gyirong, Xizang, in Liu, D.S., ed., Geologicaland ecological studies of the Quinghai-Xizang pla-teau, Volume I: Beijing, Science Press, p. 343–352.

Colchen, M., 1999, The Thakkhola-Mustang graben in Ne-pal and the late Cenozoic extension in the HigherHimalayas: Journal of Asian Earth Sciences, v. 17,p. 683–702.

Colchen, M., LeFort, P., and Pecher, A., 1986, Annapurna-Manaslu-Ganesh Himal: Centre National de la Re-cherche Scientifique, Paris, 136 p.

Coleman, M.E., 1998, U-Pb constraints on Oligocene–Miocene deformation and anatexis, Marsyangdi Val-ley, central Nepalese Himalaya: American Journal ofScience, v. 298, p. 553–571.

Coleman, M., and Hodges, K., 1995, Evidence for Tibetanplateau uplift before 14 Myr ago from a new minimumage for east-west extension: Nature, v. 374, p. 49–52.

DeCelles, P.G., Gray, M.B., Ridgeway, K.D., Cole, R.B.,Pivnik, D.A., Pequera, N., and Srivastava, P., 1991,Controls on synorogenic alluvial-fan architecture,Beartooth Conglomerate (Paleocene), Wyoming andMontana: Sedimentology, v. 38, p. 567–590.

DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp,P.A., and Upreti, B.N., 1998, Neogene foreland basindeposits, erosional unroofing, and kinematic history of



the Himalayan fold-thrust belt, western Nepal: Geo-logical Society of America Bulletin, v. 110, p. 2–21.

DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Gar-zione, C.N., Copeland, P., and Upreti, B.N., 2001,Stratigraphy, structure, and tectonic evolution of theHimalayan fold-thrust belt in western Nepal: Tecton-ics, v. 20, p. 487–509.

Derry, L.A., and France-Lanord, C., 1996, Neogene Hi-malayan weathering history and river 87Sr/86Sr: Impacton the marine Sr record: Earth and Planetary ScienceLetters, v. 142, p. 59–74.

Dettman, D.L., Kohn, M.J., Quade, J., Ryerson, F.J., Ojha,T.P., and Hamidullah, S., 2001, Seasonal stable isotopeevidence for a strong Asian monsoon throughout thepast 10.7 m.y.: Geology, v. 29, p. 31–34.

Dhital, M.R., and Kizaki, K., 1987, Structural aspect of thenorthern Dang, Lesser Himalaya: University of theRyukyus, Bulletin of the College of Science, v. 45,p. 159–182.

England, P.C., and Houseman, G.A., 1989, Extension dur-ing continental convergence, with application to theTibetan plateau: Journal of Geophysical Research,v. 94, 17,561–17,579.

Fielding, E.J., Isacks, B.L., Barazangi, M., and Duncan, C.,1994, How flat is Tibet? Geology, v. 22, p. 163–167.

Fort, M., 1987, Geomorphic and hazards mapping in thedry, continental Himalaya: 1:50,000 maps of Mustangdistrict, Nepal: Mountain research and development,v. 7, p. 222–238.

Fort, M., Bassoullet, J.P., Colchen, M., and Freytet, P.,1981, Sedimentological and structural evolution of theThakkhola-Mustang graben (Nepal Himalaya) duringlate Neogene and Pleistocene: Calcutta, India, Geo-logical Survey of India, Neogene/Quaternary Bound-ary Field Conference, India, 1979, Proceeding, p. 25–35.

Fort, M., Freytet, P., and Colchen, M., 1982, Structural andsedimentological evolution of the Thakkhola Mustanggraben (Nepal Himalayas): Zeitschrift fur Geomor-phologie, v. 42, p. 75–98.

Gansser, A., 1964, Geology of the Himalayas: London, In-terscience, 289 p.

Garzione, C.N., 2000, Tectonic and paleoelevation historyof the Thakkhola graben and implications for the evo-lution of the southern Tibetan plateau [Ph.D. thesis]:Tucson, University of Arizona, 146 p.

Garzione, C.N., Dettman, D.L., Quade, J., DeCelles, P.G.,and Butler, R.F., 2000a, High times on the Tibetanplateau: Paleoelevation of the Thakkhola graben, Ne-pal: Geology, v. 28, p. 339–342.

Garzione, C.N., Quade, J., DeCelles, P.G., and English,N.B., 2000b, Predicting paleoelevation of Tibet andthe Himalaya from d18O vs. altitude gradients in me-teoric water across the Nepal Himalaya: Earth andPlanetary Science Letters, v. 183, p. 215–229.

Godin, L., 1999, Tectonic evolution of the Tethyan sedimen-tary sequence in the Annapurna area, central NepalHimalaya [Ph.D. thesis]: Carleton University, 219 p.

Godin, L., Brown, R.L., and Hammer, S., 1999, High strainzone in the hanging wall of the Annapurna detach-ment, central Nepal Himalaya, in Macfarlane, A.,Sorkhabi, R.B., and Quade, J., eds., Himalaya and Ti-bet: Mountain roots to mountain tops: Geological So-ciety of America Special Paper 328, p. 199–210.

Guillot, S., 1999, An overview of the metamorphic evolu-tion in central Nepal: Journal of Asian Earth Sciences,v. 17, p. 713–725.

Harrison, T.M., Copeland, P., Kidd, W.F.S., and Yin, A.,1992, Raising Tibet: Science, v. 255, p. 1663–1670.

Harrison, T.M., Copeland, P., Kidd, W.F.S., and Lovera,O.M., 1995, Activation of the Nyainqentanghla shearzone: Implications for uplift of the southern Tibetanplateau: Tectonics, v. 14, p. 658–676.

Harrison, T.M., Lovera, O.M., and Grove, M., 1997, Newinsights into the origin of two contrasting Himalayangranite belts: Geology, v. 25, p. 899–902.

Hauck, M.L., Nelson, K.D., Brown, L.D., Zhao, W., andRoss, A.R., 1998, Crustal structure of the Himalayanorogen at ;908 east longitude from Project INDEPTHdeep reflection profiles: Tectonics, v. 17, p. 481–500.

Hein, F.J., and Walker, R.G., 1977, Bar evolution and de-velopment of stratification in the gravelly braided

Kicking Horse River, British Columbia: CanadianJournal of Earth Sciences, v. 14, p. 562–570.

Hodges, K.V., 2000, Tectonics of the Himalaya and south-ern Tibet from two perspectives: Geological Societyof America Bulletin, v. 112, p. 324–350.

Hodges, K.V., and Silverburg, D.S., 1988, Thermal evolu-tion of the Greater Himalaya, Garhwal, India: Tecton-ics, v. 7, p. 583–600.

Hodges, K.V., Parrish, R.R., and Searle, M.P., 1996, Tec-tonic evolution of the central Annapurna Range, Nep-alese Himalayas: Tectonics, v. 15, p. 1264–1291.

Hoorn, C., Ojha, T., and Quade, J., 2000, Palynologicalevidence for vegetation development and climaticchange in Sub-Himalayan zone (Neogene, central Ne-pal): Palaeogeography, Palaeoclimatology, Palaeoe-cology, v. 163, p. 133–161.

Horton, B.K., and Schmitt, J.G., 1996, Sedimentology of alacustrine fan-delta system, Miocene Horse Camp For-mation, Nevada, USA: Sedimentology, v. 43,p. 133–155.

Hubbard, M.S., and Harrison, T.M., 1989, 40Ar/39Ar con-straints on deformation and metamorphism in theMain Central thrust zone and Tibetan slab, eastern Ne-pal Himalaya: Tectonics, v. 8, p. 865–880.

Hurtado, J.M., Hodges, K.V., and Whipple, K.X., 2001,Neotectonics of the Thakkhola graben and implica-tions for recent activity on the South Tibetan fault sys-tem in the central Nepal Himalaya: Geological Societyof America Bulletin, v. 13, p. 222–240.

Iwata, S., 1984, Geomorphology of the Thakkhola-Muktinathregion, central Nepal, and its late Quaternary history:Geographical Reports of the Tokyo Metropolitan Uni-versity, v. 19, p. 25–42.

Johnson, M.R.W., Oliver, G.J.H., Parrish, R.R., and John-son, S.P., 2001, Synthrusting metamorphism, cooling,and erosion of the Himalayan Kathmandu Complex,Nepal: Tectonics, v. 20, p. 394–415.

Kroon, D., Steens, T., and Troelstra, S.R., 1991, Onset ofmonsoonal related uplift in the western Arabian seaas revealed by planktonic foraminifera, in Prell, W.L.,Niitsuma, N. et al., Proceedings of the Ocean DrillingProgram, Scientific results, Volume 117: College Sta-tion, Texas, Ocean Drilling Program, p. 257–263.

Le Fort, P., 1986, Metamorphism and magmatism duringthe Himalayan collision, in Coward, M.P., and Ries,A.C., eds., Collision tectonics: Geological Society[London] Special Publication 19, p. 159–172.

Le Fort, P., and France-Lanord, C., 1995, Granites fromMustang and surrounding regions (central Nepal):Journal of Nepal Geological Society, v. 11, p. 53–57.

Li Jijun, editor, 1995, Uplift of Qinghai-Xizang (Tibet) pla-teau and global change: Lanzhou, China, LanzhouUniversity Press, 207 p.

Lowe, D.R., 1982, Sediment gravity flows: II. Depositionalmodels with special reference to the deposits of high-density turbidity currents: Journal of Sedimentary Pe-trology, v. 52, p. 279–297.

McCaffrey, R., and Nabelek, J., 1998, Role of oblique con-vergence in the active deformation of the Himalayasand southern Tibetan plateau: Geology, v. 26,p. 691–694.

Miall, A.D., 1977, A review of the braided river deposi-tional environment: Earth-Science Reviews, v. 13,p. 1–62.

Miall, A.D., 1985, Architectural-element analysis: A newmethod of facies analysis applied to fluvial deposits:Earth-Science Reviews, v. 22, p. 261–308.

Molnar, P., and Lyon-Caen, H., 1988, Some simple physicalaspects of the support, structure, and evolution ofmountain belts, in Clark, S., Burchfiel, B.C., and Sup-pe, J., eds., Processes in Continental Lithospheric De-formation: Geological Society of America Special Pa-per 218, p. 179–207.

Molnar, P., and Tapponnier, P., 1978, Active tectonics ofTibet: Journal of Geophysical Research, v. 83,p. 5361–5375.

Molnar, P., England, P., and Martinod, J., 1993, Mantle dy-namics, uplift of the Tibetan plateau, and the Indianmonsoon: Reviews of Geophysics, v. 31, p. 357–396.

Mugnier, J.L., Mascle, G., and Faucher, T., 1993, Structureof the Siwaliks of western Nepal: An intracontinental

accretionary prism: International Geology Reviews,v. 35, p. 32–47.

Murphy, M.A., and Harrison, T.M., 1999, The relationshipbetween leucogranite magmatism and the Qomolung-ma detachment in the Rongbuk Valley, south Tibet:Geology, v. 27, p. 831–834.

Nemec, W., and Steel, R.J., 1984, Alluvial and coastal con-glomerates: Their significant features and some com-ments on gravelly mass-flow deposits, in Kloster,E.H., and Steel, R.J., eds., Sedimentology of gravelsand conglomerates: Canadian Society of PetroleumGeologists Memoir 10, p. 1–31.

Ni, J., and York, J., 1978, Late Cenozoic tectonics of theTibetan plateau: Journal of Geophysical Research,v. 83, p. 5377–5384.

Pan, Y., and Kidd, W.F.S., 1992, Nyainqentanghla shearzone: A late Miocene extensional detachment in thesouthern Tibetan plateau: Geology, v. 20, p. 775–778.

Prell, W.L., and Kutzbach, J.E., 1992, Sensitivity of theIndian monsoon to forcing parameters and implica-tions for its evolution: Nature, v. 360, p. 647–652.

Prell, W.L., Murray, D.W., Clemens, S.C., and Anderson,D.M., 1992, Evolution and variability of the IndianOcean summer monsoon: Evidence from the westernArabian Sea drilling program, in Duncan, R.A. et al.,eds., Synthesis of results from scientific drilling in theIndian Ocean: American Geophysical Union Geo-physical Monograph 70, 447–469.

Quade, J., Cerling, T.E., and Bowman, J.R., 1989, Devel-opment of Asian monsoon revealed by marked eco-logical shift during the latest Miocene in northern Pak-istan: Nature, v. 342, p. 163–166.

Quade, J., Cater, J.M.L., Ojha, T.P., Adam, J., and Harrison,T.M., 1995, Late Miocene environmental change inNepal and the northern Indian subcontinent: Stableisotopic evidence from paleosols: Geological Societyof America Bulletin, v. 107, p. 1381–1397.

Ratschbacher, L., Frisch, W., Liu Guanghua, and ChenChengsheng, 1994, Distributed deformation in south-ern and western Tibet during and after the India-Asiacollision: Journal of Geophysical Research, v. 99,p. 19,917–19,945.

Raymo, M.E., and Ruddiman, W.F., 1992, Tectonic forcingof late Cenozoic climate: Nature, v. 359, p. 117–122.

Royden, L.H., Burchfiel, C., King, R.W., Wang, E., Chen,Z., Shen, F., and Liu, Y., 1997, Surface deformationand lower crustal flow in eastern Tibet: Science,v. 276, p. 788–790.

Rozanski, K., Araguas-Araguas, L., and Gonfiantini, R.,1993, Isotopic patterns in modern global precipitation,in Swart, P., McKenzie, J.A., and Lohman, K.C, eds.,Continental indicators of climate, Proceedings ofChapman Conference, Jackson Hole, Wyoming:American Geophysical Union Geophysical Mono-graph 78, p. 1–36.

Rust, B.R., 1972, Structure and process in a braided river:Sedimentology, v. 18, p. 221–245.

Schelling, D., 1992, The tectonostratigraphy and structureof the eastern Nepal Himalaya: Tectonics, v. 11,p. 925–943.

Searle, M.P., 1986, Structural evolution and sequence ofthrusting in the High Himalaya, Tibetan Tethys, andIndus suture zones of Zanskar and Ladakh, westernHimalaya: Journal of Structural Geology, v. 8,p. 923–936.

Seeber, L., and Pecher, A., 1998, Strain partitioning alongthe Himalayan arc and the Nanga Parbat antiform: Ge-ology, v. 26, p. 791–794.

Slate, J.L., Smith, G.A., Wang, Y., and Cerling, T.E., 1996,Carbonate-paleosol genesis in the Plio–Pleistocene St.David Formation, southeastern Arizona: Journal ofSedimentary Research, v. 66, p. 85–94.

Sohn, Y.K., 1997, On traction-carpet sedimentation: Journalof Sedimentary Research, v. 67, p. 502–509.

Srivastava, P., and Mitra, G., 1994, Thrust geometries anddeep structure of the outer and lesser Himalaya, Ku-maon and Garhwal (India): Implications for evolutionof the Himalayan fold thrust belt: Tectonics, v. 13,p. 89–109.

Stocklin, J., 1980, Geology of Nepal and its regional frame:Geological Society of London Journal, v. 137,p. 1–34.


GARZIONE et al.

Talbot, M.R., and Kelts, K., 1990, Paleolimnological sig-natures from carbon and oxygen isotopic ratios in car-bonates from organic carbon-rich lacustrine sedi-ments, in Katz, B.J., ed., Lacustrine basinexploration—Case studies and modern analogs:American Association of Petroleum Geologists Mem-oir 50, p. 99–112.

Todd, S.P., 1989, Stream-driven, high-density gravelly trac-tion carpets: Possible deposits in the Trabeg Conglom-erate Formation, southwest Ireland and some theoret-ical considerations of their origin: Sedimentology,v. 36, p. 513–530.

Valdiya, K.S., 1980, Geology of the Kumaon Lesser Him-alaya: Wadia Institute of Himalayan Geology, 291 p.

Vannay, J.C., and Hodges, K.V., 1996, Tectonometamorph-ic evolution of the Himalayan metamorphic core be-

tween Annapurna and Dhaulagiri, central Nepal, Jour-nal of Metamorphic Geology, v. 14, p. 635–656.

Williams, H., Turner, S., Kelley, S., and Harris, N., 2001,Age and composition of dikes in southern Tibet: Newconstraints on the timing of east-west extension andits relationship to postcollisional volcanism, v. 29,p. 339–342.

Xu Ren, 1981, Vegetational changes in the past and the upliftof the Qinghai-Xizang Plateau, in Geological and eco-logical studies of the Qinghai-Xizang plateau, VolumeI, Geology, geological history, and origin of Qinghai-Xizang Plateau: Beijing, Science Press, p. 139–144.

Yin, A., 2000, Mode of Cenozoic east-west extension inTibet suggesting a common origin of rifts in Asia dur-ing the Indo-Asian collision, v. 105, p. 21,745–21,759.

Yin, A., Kapp, P.A., Murphy, M.A., Manning, C.E., Har-

rison, T.M., Grove, M., Ding Lin, Deng Xi-Guang,Wu Cun-Ming, 1999, Significant late Neogene east-west extension in northern Tibet: Geology, v. 27,p. 787–790.

Yoshida, M., Igarashi, Y., Arita, K., Hayashi, D., and Shar-ma, T., 1984, Magnetostratigraphic and pollen analyticstudies of the Takmar series, Nepal Himalayas: NepalGeological Society Journal, v. 4, p. 101–120.

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