deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western

P. G. DeCelles* G. E. Gehrels J. Quade Department of Geosciences, University of Arizona, Tucson, Arizona 85721 T. P. Ojha P. A. Kapp† } B. N. Upreti Department of , Tribhuvan University, Tri-Chandra Campus, Ghantaghar, Kathmandu, Nepal

ABSTRACT Group manifest an upsection enrichment in slip in this duplex has been fed updip and potassium feldspar, carbonate lithic frag- southward into the Main Boundary and Main Sedimentological and provenance data ments, and high-grade metamorphic miner- Frontal thrust systems. from the lower – Dumri For- als. Modal petrographic analyses of modern We obtained 113 U-Pb ages on detrital zir- mation and Siwalik Group in western Nepal river sands provide some control on potential cons from modern rivers and Siwalik Group provide new information about the timing of source terranes for the Miocene–Pliocene sandstones that cluster at 460–530 Ma, thrust faulting and the links between erosional sandstones. The Dumri Formation was most ~850–1200 Ma, ~1.8–2.0 Ga, and ~2.5 Ga. An unroofing of the Himalaya and the likely derived from erosion of sedimentary abundance of Cambrian–Ordovician grains 87Sr/86Sr record of the ocean. In western and low-grade metasedimentary rocks in the in the Siwalik Group suggests sources of Siwa- Nepal, the Dumri Formation is an ~750–1300- Tibetan (Tethyan) Himalayan zone during lik detritus in the granites of the Dadeldhura m-thick fluvial sandstone and overbank mud- early Miocene emplacement of the Main Cen- thrust sheet and possibly the Greater Hima- stone unit. The Siwalik Group is >4200 m tral thrust. The presence in Dumri sandstones layan orthogneisses. The older ages are consis- thick and consists of a lower member (>850 of plagioclase grains suggests exposure of tent with sources in the Greater and Lesser m) of 2–12-m-thick fluvial channel sandstones crystalline rocks of the Greater Himalayan Himalayan zones. An overall upsection in- and oxidized calcareous , a middle zone, perhaps in response to tectonic unroof- crease in zircons older than 1.7 Ga suggests in- member (>2400 m) of very thick (>20 m) ing by extensional detachment faults of the creasing aerial exposure of Lesser Himalayan channel sandstones and mainly organic-rich South Tibetan detachment system. During rocks. None of the detrital zircons (even in the Histosols, and an upper member (>1000 m) of the lower Siwalik Group modern river samples) yielded a Cenozoic age composed of gravelly braided river deposits. (~15–11 Ma), emplacement of the Dadel- that might suggest derivation from the Ceno- Paleocurrent data indicate that middle dhura thrust sheet (one of the synformal crys- zoic Greater Himalayan leucogranites, but Miocene–Pliocene rivers in western Nepal talline thrust sheets of the southern Himalaya) this may be attributable to the inheritance flowed southward, transverse to the thrust on top of the Dumri Formation supplied problems that characterize the U-Pb geo- belt, throughout deposition of the Siwalik abundant metasedimentary lithic fragments chronology of the leucogranites. Group. No evidence was found for an axial to the foreland basin. A steady supply of pla- When compared with recent studies of the fluvial trunk system (i.e., the paleo- gioclase grains and high-grade minerals was 87Sr/86Sr composition of carbonate River) in Siwalik Group sandstones. A major maintained by deeper erosion into the Main nodules and detrital carbonate in paleosols increase in fluvial channel size is recorded by Central thrust sheet. From ~11 Ma to the from the Siwalik Group, the provenance data the transition from lower to middle Siwalik present, K-feldspar sand increased steadily, suggest that erosion and of meta- members at ~10.8 Ma, probably in response to suggesting that granitic source rocks became morphosed carbonate rocks in the Lesser an increase in seasonal discharge. widely exposed during deposition of the up- Himalayan zone and Cambrian–Ordovician Modal petrographic data from sandstones per part of the lower Siwalik Group. This granitic rocks of the crystalline thrust sheets in the Dumri Formation and the Siwalik provenance change was caused by erosion of in central and eastern Nepal may have passively uplifted granites and granitic ortho- played a significant role in elevating the gneisses in the Dadeldhura thrust sheet above 87Sr/86Sr ratio of synoro- *e-mail: [email protected] †Present address: Department of Earth and Space a large duplex in the Lesser Himalayan rocks. genic in the Indo-Gangetic fore- Sciences, University of California, Los Angeles, Cali- Since the onset of deposition of the conglom- land basin and the Bengal fan, as well as fornia 90024. eratic upper Siwalik Group (~4–5 Ma), fault global seawater. Data Repository item 9805 contains additional material related to this article.

GSA Bulletin; January 1998; v. 110; no. 1; p. 2–21; 15 figures; 2 tables.

2

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INTRODUCTION REGIONAL STRUCTURE AND SOURCE 5–20-km-thick sheet of amphibolite-grade TERRANE COMPOSITIONS (kyanite and sillimanite bearing) schist, para- Many studies have linked the uplift of the gneiss, and orthogneiss (Fig. 3; Hodges and Himalaya and during Cenozoic Regional Geology and Structure Silverberg, 1988; Pêcher, 1989; Schelling, 1992; time to regional and global climate changes and Macfarlane et al., 1992; Hodges et al., 1996; major changes in the oceanic 87Sr/86Sr record In Nepal and northern , the Himalaya is Coleman, 1996; and many others). In western (Ruddiman and Raymo, 1988; Quade et al., divided into four lithotectonic zones that are sep- Nepal these rocks are referred to as the Himal 1989; Richter et al., 1992; Edmond, 1992). arated by major structural discontinuities. From Group. Large tourmaline-bearing leucogranitic Weathering of metamorphic and igneous rocks, north to south, these are the Tibetan (or Tethyan) plutons of middle to late Cenozoic age are dis- rich in radiogenic Sr and widely exposed in the zone, the Greater Himalayan zone, the Lesser tributed along the highest part of the Himalaya in higher parts of the Himalaya, is proposed to have Himalayan zone, and the Subhimalayan zone the northern part of the Greater Himalayan zone

consumed atmospheric CO2 and thus contributed (e.g., Gansser, 1964; Valdiya, 1980; Fig. 1). In or- (LeFort, 1981, 1986; Ferrara et al., 1983; Schärer to global temperature decline (Raymo and der to better understand the relationships between et al., 1986; Deniel et al., 1987; France-Lanord Ruddiman, 1992). The same process is thought to unroofing of these lithotectonic zones and the and LeFort, 1988). Because these plutons, and have been responsible for the rise in oceanic foreland basin deposits, we have constructed a the gneisses they intrude, have yielded extremely 87Sr/86Sr that has occurred since ~40 Ma (Hodell balanced regional cross section on the basis of the high 87Sr/86Sr ratios (Deniel et al., 1987; France- et al., 1989; Richter et al., 1992; Hodell and 1:250 000 geologic map compiled by Shrestha et Lanord and LeFort, 1988), they are believed to be Woodruff, 1994). Geochemical data sets are now al. (1987) and our own field data from a traverse an important source of radiogenic Sr driving the available from several regions of the Himalaya along the Dadeldhura-Baitadi road (Figs. 1 and Cenozoic rise in seawater 87Sr/86Sr (Edmond, and from the erosional detritus stored in the fore- 2). The cross section is line-length balanced, but 1992). The southern boundary of the Greater land basin sediments of the Miocene Siwalik no attempt was made to incorporate the locally Himalayan zone is the Main Central thrust sys- Group along the southern flank of the orogenic intense small-scale deformation that character- tem, which is a several-kilometer-thick zone of belt and in the Bengal fan (Quade, 1993; France- izes some of the stratigraphic units (particularly numerous thrust faults and intense shear strain Lanord et al., 1993; Harrison et al., 1993; Quade the Galyang Formation). No subsurface data are (Brunel and Kienast, 1986; Macfarlane et al., et al., 1995, 1997; Derry and France-Lanord, available, the stratigraphy of the Dadeldhura 1992; Hodges et al., 1996). 1996). However, geochemical models for un- thrust sheet is poorly known, and hanging-wall Several large, generally synformal thrust sheets roofing of the Himalaya have not been tested cut-offs are generally not preserved. Therefore, composed of garnet- to biotite-grade schist and with even the most basic provenance and sedi- this cross section should be viewed only as a first- orthogneiss in their lower parts and low-grade mentological data from the foreland basin depos- order approximation that will undoubtedly be metasedimentary to unmetamorphosed sedimen- its or by considerations of the regional structural changed as new data become available. Never- tary rocks in their upper parts are located on top of development of the Himalayan thrust belt. theless, the cross section honors known surface the Lesser Himalayan zone rocks to the south of In order to help fill the gap between geo- geologic relationships, and the most important the Greater Himalayan zone (Figs. 1 and 2). chemical and geological evidence, this paper features, such as the synformal Dadeldhura thrust These are known as the crystalline, or Lesser presents data from the lower Miocene(?) Dumri sheet and the large hinterland-dipping duplex to Himalayan, nappes or thrust sheets (e.g., Gansser, Formation and Miocene–Pliocene Siwalik its north, are supported by thrust branching pat- 1964; Valdiya, 1980). Late Cambrian–Early Group in western Nepal (Fig. 1). The database terns and bedding and foliation dip data. Similar Ordovician (Schärer and Allègre, 1983; Einfalt et consists of standard sedimentological analysis large-scale structures are present on the cross sec- al., 1993) plutons of two-mica, cordierite-bearing of remarkably complete sections of the Siwalik tion of Schelling (1992) for eastern Nepal and granite are present in several of these thrust sheets Group, including more than 1200 paleocurrent that of Srivastava and Mitra (1994) for northern (e.g., LeFort et al., 1986; Einfalt et al., 1993). In measurements, standard modal petrographic India. Brief descriptions of each of the lithotec- western Nepal and adjacent Kumaon, India, the analyses of Dumri and Siwalik sandstone sam- tonic zones are provided in the following, in ad- Dadeldhura-Almora thrust sheet occupies a large ples, and U-Pb dates from single zircon grains dition to a summary of available structural and part of the source terrane directly north of the sec- from Siwalik sandstones. Modal petrographic kinematic information. tions of the Siwalik Group that we studied (Figs. 1 data and U-Pb zircon dates from modern river The Tibetan zone consists of Cambrian to and 2; Valdiya, 1980). The Dadeldhura thrust sands are presented as a means of comparing marine sedimentary to low-grade meta- sheet consists of an ~10-km-thick sequence of compositions of sands from known source ter- sedimentary rocks (mainly phyllite, quartzite, Precambrian phyllite, quartzite, slate, sericitic ranes with those of the ancient sediments. The limestone, and local volcanic rocks) in a generally quartzite, garnet-mica schist, quartz-feldspar- sedimentological and provenance data are con- southward verging thrust belt that has a minimum mica schist, chloritic schist, augen gneiss, biotite sidered within the context of a new regional bal- of ~135 km of shortening in central-southern gneiss, and local metavolcanic units (Fig. 3; anced cross section, and a preliminary assess- Tibet (Burg and Chen, 1984; Searle, 1986; Bashyal, 1986; Einfalt et al., 1993; Upreti, ment of the kinematic history of thrusting since Ratschbacher et al., 1994). These rocks were de- 1996a). Comparison with the Kathmandu crys- ~22 Ma is presented. Our purpose is to shed posited along the precollisional northern margin talline thrust sheet in east-central Nepal suggests some light on the locations and timing of expo- of the . The southern boundary of the that several kilometers of metasedimentary rock sure of source terranes for the Dumri Formation Tibetan zone is marked by the South Tibetan have been eroded from the upper part of the and the Siwalik Group, in an effort to help con- detachment system, a complex of northward- Dadeldhura sheet. A large Cambrian–Ordovician strain models for the geodynamics of the Nepal dipping normal detachment faults that has ~10 km granitic pluton, the Dadeldhura granite, crops out Himalaya and to provide information about po- of structural relief and several tens of kilometers in the southern and central parts of the thrust sheet tential sources of highly radiogenic Sr that of top-to-the-north displacement (Burg and Chen, (Fig. 1; Einfalt et al., 1993). The Dadeldhura might have contributed to the abrupt Cenozoic 1984; Burchfiel et al., 1992; Coleman, 1996). thrust sheet as a whole is folded into a northwest- rise in oceanic 87Sr/86Sr. The Greater Himalayan zone consists of a southeast–trending synform (Figs. 1 and 2). In

Geological Society of America Bulletin, January 1998 3

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T IB 100 km ET N EP AL 81°E °N 30 N X INDIA . R I N L MCT KARNALI R. ° A 30 N K A H A M

MCT B

DA DT SWK K 9°N MK ARN 2 KK A RT L I

R . Surkhet Tansen X' Dhangadhi BK MBT DB Tulsipur 81°E SK Lesser Himalayan Zone T Butwal MELPANI, BHAINSKATI, & DUMRI FMS. MF (-EARLY MIOCENE) Subhimalayan Zone GALYANG - LAKARPATA FMS. (PALEOZOIC) SIWALIK GROUP (MIOCENE - PLIOCENE) KUSHMA & RANIMATA FMS. (PRECAMBRIAN) Greater Himalayan Zone Crystalline Thrust Sheets TERTIARY LEUCOGRANITE GRANITE (CAMBRIAN-ORDOVICIAN) HIMAL GROUP GNEISSES & PARAGNEISSES (PRECAMBRIAN) DAMGAD AND MELMURA FMS. (LATE PRECAMBRIAN-EARLY PALEOZOIC) Tibetan Himalayan Zone BURHI GANDAKI & SALYANI GAD GNEISSES, PHYLLITES AND CARBONATES KALIKHOT FM. (PRECAMBRIAN) (CAMBRIAN-EOCENE)

Figure 1. Generalized geologic map of western Nepal modified from Amatya and Jnawali (1994). The locations of Macheli Khola (MK), Khutia Khola (KK), Babai Khola (BK), Swat Khola (SWK), Surai Khola (SK), and Dumri Bridge (DB) sections are shown. Major thrust faults are shown as lines with barbs on hanging-wall side (dashed where not continuously exposed), and are abbreviated as follows: MFT—Main Frontal thrust; MBT—Main Boundary thrust; RT—Ramgarh thrust; DT—Dadeldhura thrust; MCT—Main Central thrust. Star indicates location of sample from Dadeldhura granite that yielded a U-Pb age of 492 ± 6 Ma. Villages: DA—Dadeldhura; B—Baitadi. Line X–X′ indicates location of cross sec- tion shown in Figure 2.

Nepal and northern India, most workers have in- thrust sheet is structurally distinct from the Greater Himalayan zone and the Dadeldhura terpreted the crystalline thrust sheets as the south- Greater Himalayan rocks in the Main Central thrust sheet is not straightforward on the basis of ern continuations of the Greater Himalayan zone thrust hanging wall, displaying a lower grade of these criteria, because granitic orthogneisses in rocks (e.g., Gansser, 1964; Stöcklin, 1980; metamorphism (garnet and lower) and an associ- the Greater Himalayan zone have produced Late Valdiya, 1980; Schelling, 1992). Upreti (1996a) ation with the Cambrian–Ordovician granites. Cambrian Rb/Sr ages (Ferrara et al., 1983; LeFort argued that in western Nepal, the Dadeldhura However, the distinction between the rocks of the et al., 1986) and U-Pb data that are consistent with

4 Geological Society of America Bulletin, January 1998

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GREATER LESSER HIMALAYA TIBETAN HIMALAYA HIMALAYA DADELDHURA MCT A. THRUST SHEET STDS MBT X' DADELDHURA X SUBHIMALAYA RT SOUTH 5 DT 5 NORTH

MFT DT 0 km 0 km -5 -5 -10 -10 -15 MBT B.

20 km DEFORMED LENGTH = 115 km RESTORED LENGTH = 343 km TOTAL SHORTENING = 228 km (66%) MCT 20 km

MAIN FRONTAL DADELDHURA-RAMGARH TIBETAN (TETHYS) SIWALIK GROUP MELMURA & DAMGAD FMS.

MAIN BOUNDARY SALYANI GAD GNEISS SWAT & DUMRI FMS. DADELDHURA GRANITE LAKARPATA GROUP KALIKHOT FM. GALYANG FM. MAIN CENTRAL RANIMATA FM. VAIKRITA (HIMAL) GROUP KUSHMA FM.

Figure 2. Regional structural cross section X–X′ of the Himalayan fold-thrust belt in western Nepal (see Fig. 1 for location). MFT—Main Frontal thrust; MBT—Main Boundary thrust; RT—Ramgarh thrust; DT—Dadeldhura thrust; MCT—Main Central thrust; STDS—South Tibetan detachment system. Eroded rocks in the lower part of the Dadeldhura thrust sheet are shown above present Lesser Himalayan topogra- phy in order to emphasize the minimum amount of derived from the Dadeldhura sheet that was available to Miocene–Pliocene deposi- tional systems of the Indo-Gangetic foreland basin system. Note the difference in scale between deformed- and restored-state cross sections.

an ~500 Ma age in the Annapurna Range of north- structure to the north of the crystalline thrust sheets Zhao et al., 1993; Pandey et al., 1995). This ramp central Nepal (Hodges et al., 1996). It is therefore is characterized by a large antiformal duplex (in is located below the duplex in Lesser Himalayan plausible that the Dadeldhura thrust sheet is the central Nepal; Schelling, 1992) or hinterland- rocks (Fig. 2; Pandey et al., 1995). The maximum southward, structurally shallower, continuation of dipping duplex (in western Nepal and northern rates of surface uplift and horizontal convergence the hanging wall of the Main Central thrust. From India; Srivastava and Mitra, 1994; Fig. 2). The de- between India and southern Tibet are also located the standpoint of source terrane composition, velopment of this duplex probably was responsible above this zone of active seismicity, which projects however, the Dadeldhura thrust sheet, with its for folding of the overlying crystalline thrust vertically upward to the surface trace of the Main thick metasedimentary stratigraphic section and sheets (Dhital and Kizaki, 1987; Schelling, 1992; Central thrust (Bilham et al., 1997). Whereas some lower grade of metamorphism, is different from Srivastava and Mitra, 1994). South of the Dadel- of the uplift may be a response to Main Central the MCT sheet (Fig. 3). dhura thrust sheet, the Lesser Himalayan rocks thrust displacement, the microseismic data suggest To the south of the Greater Himalayan zone, crop out in a narrow belt between the Main that displacement may also be occurring along the and surrounding the crystalline thrust sheets, is the Boundary thrust and the northward-dipping ramp in the basal thrust (Pandey et al., 1995). This Lesser Himalayan zone, which consists of Prot- Ramgarh thrust below the base of the Dadeldhura would imply that the duplex is still growing. The erozoic(?) to Devonian phyllite, quartzite, lime- thrust sheet (Fig. 2; Valdiya, 1980; Shrestha et al., kinematics implied by the cross section (Fig. 2) stone, and dolostone (Valdiya, 1980; Stöcklin, 1987; Srivastava and Mitra, 1994). Active seis- transfer displacements on individual thrusts within 1980; Bashyal, 1986; Upreti, 1990, 1996b; Fig. 3). micity in the Himalayan thrust belt is concentrated the duplex updip and southward into the frontal The southern boundary of the Lesser Himalayan at depths of 5–20 km along a major ramp in the parts of the Ramgarh, Main Boundary thrust, and zone is the Main Boundary thrust, and its internal basal Himalayan thrust (Ni and Barazangi, 1984; Main Frontal thrust systems.

Geological Society of America Bulletin, January 1998 5

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LITHIC SANDSTONE, RECYCLED METASED. CLASTS KEY GRAIN KEY TYPES

MIO-PLIOCENE

SANDSTONE, CONGLOMERATE; SANDSTONE, SIWALIK GROUP: MUDSTONE, GROUP: SIWALIK ROCK TYPES MFT SYSTEM: ZONE SUBHIMALAYAN TOP ERODED OR TOP THRUST BY CUT PHYLLITE QUARTZ KEY GRAIN KEY TYPES QUARTZ FELDSPAR MICA QUARTZ CARBONATE PHYLLITE CHLORITE QUARTZ CHLORITE MICA PHYLLITE VOLCANICS PATA ROCK TYPES ULLERI FM. AUGEN GNEISS SYANGJA FM. SYANGJA QUARTZITE MELPANI FM. MELPANI QUARTZITE FM. GALYANG PHYLLITE, QUARTZITE LAKAR FM. CARBONATES PHYLLITES RANIMATA FM. RANIMATA PHYLLITE & QUARTZITE; BASIC INTRUSIVES FM. KUSHMA & QUARTZITE PHYLLITE SURKHET GR. SURKHET SS, LS, SHALE SANGRAM FM. PHYLLITE, QUARTZITE

MBT SYSTEM: LESSER ZONE HIMALAYAN

(UPPER PRECAMB-UPPER PALEOZ) PRECAMB-UPPER (UPPER

TOP ERODED OR TOP THRUST BY CUT TERT. MIDLAND GROUP MIDLAND CRET. 0 km 8 4 2 6 poorly documented, Nepal is de- Schelling (1992) in eastern the section compiled by the Main Central thrust system; lithostratigraphic names used in this section picted for in western Himalayan zone) rocks (Greater not strictly applicable to Himal Group are 2. as in Figure Abbreviations Nepal. 10 PHYLLITE MUSCOVITE BIOTITE CHLORITE STAUROLITE FELDSPARS KYANITE GARNET QUARTZ BIOTITE MUSCOVITE FELDSPARS QUARTZ CARBONATE MINOR PHYLLITE KEY GRAIN KEY TYPES ROCK TYPES KALIKHOT FM. SCHIST DAMGAD FM. QUARTZITE SALLYANI GAD GNEISS BURHI GANDAKI GNEISS MELMURA FM. PHYLLITE DADELDHURA GRANITE: TOURMALINE GRANITE; 492 Ma DT SYSTEM: CRYSTALLINE THRUST SHEET

TOP ERODED TOP

(PALEOZOIC) DADELDHURA GROUP (PRECAMBRIAN) GROUP DADELDHURA

GROUP

PHULCHAUKI

MICAS, GARNET MICAS,

FELDSPARS

KYANITE SILLIMANITE

MICAS FELDSPARS KEY GRAIN KEY TYPES BIOTITE MUSCOVITE FELDSPARS SILLIMANITE

BIOTITE FELDSPARS SILLIMANITE PRECAMBRIAN

GNEISS; PRECAMBRIAN GNEISS; AMPHIBOLITES; PRECAMBRIAN AMPHIBOLITES; GRANITIC AUGEN GNEISS; AUGEN GRANITIC

GRANITIC GNEISS & AUGEN & GNEISS GRANITIC INTERCALATED PARAGNEISS AND LOCAL AND PARAGNEISS INTERCALATED QUARTZITE;AMPHIBOLITE

SILICATE GNEISS; MARBLE; GNEISS; SILICATE GRANITIC, MIGMATIC ORTHOGNEISS WITH ORTHOGNEISS MIGMATIC GRANITIC, CALC-SILICATE SCHIST; CALC-SILICATE

BIOTITE PARAGNEISS; CALC- PARAGNEISS; BIOTITE MIGMATITES: FELDSPATHIC GNEISS; FELDSPATHIC

PARAGNEISSES:

ROLWALING-KHUMBU-KANGCHENJUNGA

ROCK TYPES SCHIST; GARNET-BIOTITE ROLWALING-KHUMBU JUNBESI GNEISS: FOLIATED GNEISS: JUNBESI TWO-MICA TOURMALINE- BEARING LEUCO- GRANITES; MIOCENE MCT SYSTEM: GREATER ZONE HIMALAYAN TOP ERODED TOP BY OR CUT STDS Figure 3. The lithological composition of each of the major thrust sheet systems in The lithological composition of each 3. Figure 8 4 2 6 0 km western Nepal Himalaya,western and ages. Com- type assemblages and grain mineral with key Stöcklin (1980),piled from (1980), Valdiya et al. (1987), Shrestha Schelling (1992), and Nepal are in western rocks Himalayan and Mitra (1994). Because Greater Srivastava 18 12 16 14 10

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The Main Frontal thrust is generally mapped at combined Almora (equivalent to the Dadeldhura) ters, equal to the bed length of the preserved syn- the frontal topographic break of the Himalaya, and Main Central thrust systems in northern In- formal part of the thrust) after deposition of the where folded and thrusted Siwalik Group rocks dia. Combining our estimates for the Sub- Dumri Formation. The Main Boundary thrust in overlie gravels (e.g., Nakata, 1989; himalaya and Lesser Himalaya with the estimates Nepal must postdate the emplacement of the Schelling, 1992; Mugnier et al., 1993). Reflection by Srivastava and Mitra (1994) for the Almora crystalline thrust sheets because the latter were seismic data and low-relief hills in the Quaternary thrust and Main Central thrust in Kumaon, and folded by growth of the Lesser Himalayan du- alluvium south of the topographic front of the the estimates by Ratschbacher et al. (1994) for plex, which fed slip southward into the Main range indicate that the locus of active shortening the Tibetan Himalaya, yields a total of ~556–623 Boundary thrust. The Main Boundary thrust cuts along the front of the thrust belt is in the subsur- km of shortening for the Himalayan fold-thrust rocks as young as the Pliocene upper Siwalik face beneath the northern Indo-Gangetic plain belt in western Nepal. This value is comparable Group and carries rocks as old as early middle (Mugnier et al., 1993). Between the Main Bound- to the total estimated by Srivastava and Mitra Miocene; thus its age is between ~15 and ~2 Ma. ary thrust and the Main Frontal thrust are two to (1994), but is considerably greater than Schelling’s Meigs et al. (1995) reported provenance and age four thrust faults that carry north-dipping, folded estimate in eastern Nepal. The difference be- data from the Siwalik Group in northern India panels of the Siwalik Group, suggesting that a re- tween the estimates in India and western Nepal that suggest that the Main Boundary thrust was gional decollement exists in the lower Siwalik compared with eastern Nepal arises mainly from active by ~9 Ma and possibly as early as ~11 Ma. rocks at a depth of ~5–6 km (Fig. 2; Dhital and the large amounts of shortening required to build We present data in this paper that support an ~11 Kizaki, 1987; Schelling et al., 1991; Schelling, the Lesser Himalayan duplex in the western cross Ma age of initiation of the Lesser Himalayan du- 1992; Srivastava and Mitra, 1994; Mugnier et al., sections and the absence of comparable internal plex. However, the Main Boundary thrust is only 1993; Acharyya, 1994). shortening in the duplex in Schelling’s (1992) the most recently active of the thrusts that root cross section. beneath the duplex, and our data indicate that it Estimates of Shortening and Timing of The ages of major displacement events on was not active until Pliocene time. The Main Thrust Faults Himalayan thrusts are well known only for the Frontal thrust and associated hanging-wall imbri- Main Central thrust system in north-central cates that cut the Siwalik Group must, at least in Quantitative estimates of thrust displacements Nepal. Thermochronologic studies of the Main part, postdate the Pliocene upper member, which and crustal shortening in western Nepal have not Central thrust document a major cooling event at is the youngest unit in the hanging walls of these been made. Any cross section through the ~22–20 Ma (Hubbard and Harrison, 1989; thrusts. The ~45° of progressive rotation of bed- Himalaya is likely to underestimate shortening Copeland et al., 1991; Harrison et al., 1992; ding in the upper Siwalik Group in the Surai because of the general absence of hanging-wall Macfarlane et al., 1992; Macfarlane, 1993; Khola section suggests the presence of a progres- cutoffs and the pervasive penetrative deformation Coleman, 1996). Hodges et al. (1996) demon- sive unconformity related to displacement on one and large volume losses incurred by the meta- strated that faults in the Main Central thrust sys- of the intra-Siwalik thrusts during Pliocene time. morphic rocks north of the Main Boundary tem experienced several displacement events be- That Quaternary deposits are deformed by the thrust. Nevertheless, minimum estimates are use- ginning ~22.5 Ma in the Annapurna Range of Main Frontal thrust system shows that the thrust ful in developing palinspastic restorations of the north-central Nepal, and presented evidence for system is still active (Nakata, 1989). orogenic wedge and the Indo-Gangetic foreland significant out-of-sequence displacements on basin system, and in assessing provenance data. thrusts within the Greater Himalayan zone. TERTIARY FORELAND BASIN DEPOSITS The cross section in Figure 2 exhibits ~228 km of Macfarlane (1993) presented evidence for late horizontal shortening in Lesser Himalayan and Miocene–Pliocene reactivation of the Main Cen- Sedimentology of the Dumri Formation Subhimalayan rocks, exclusive of Dadeldhura tral thrust. Recent Th-Pb dating by Harrison et al. thrust and Main Central thrust displacements. (1997) of synkinematic monazite inclusions The terms Dumri and Suntar Formations are The resulting 66% shortening is essentially iden- within garnet crystals proximal to the Main Cen- used synonymously in western Nepal for a thick tical to the 65%–70% shortening determined by tral thrust suggests that a major displacement unit of fluvial sandstone and red mudstone that Srivastava and Mitra (1994) in the Subhimalaya event occurred on the Main Central thrust system constitutes the youngest stratigraphic unit in the and Lesser Himalaya of northern India, ~100 km as recently as ~5.5 Ma, although some of the up- hanging wall of the Main Boundary thrust sys- west of our study area. Displacements on thrusts lift necessary to convey these rocks to the surface tem. Although a detailed analysis of the Dumri within the duplex were fed into the Ramgarh could have been facilitated by passive transport Formation is beyond the scope of this paper, we thrust, which is interpreted as the roof thrust of above the Lesser Himalayan duplex. Srivastava briefly discuss the sedimentology of the unit in the duplex. Because the Dadeldhura thrust sheet and Mitra (1994) also reported crosscutting rela- order to provide some context for Dumri petro- is above the Ramgarh thrust, it also would have tionships that indicate that the Main Central graphic data and hypotheses for Neogene ero- undergone significant southward displacement thrust may have had a major episode of out-of- sional unroofing of the Himalayan thrust belt. and folding into a large antiform-synform pair sequence (break-back) displacement. The South Our reconnaissance of the Dumri Formation was during growth of the duplex. Simultaneously, a Tibetan detachment system was active between restricted to exposures in five areas: (1) along the section of rock ~15 km thick was eroded from the ~21 and 16 Ma, approximately synchronous with Dadeldhura road, where the folded Dadeldhura hanging wall of the Dadeldhura thrust between Main Central thrust emplacement (Burchfiel et thrust juxtaposes Precambrian(?) schist against its present northern trace and the trace of the al., 1992; Hodges et al., 1996; Coleman, 1996). the Dumri; (2) along Patu Khola, northwest of Main Central thrust (Fig. 2). We have not at- In western Nepal, along the north limb of the Tulsipur; (3) in Swat Khola, northwest of Surkhet tempted to restore the Main Central and Dadel- Dadeldhura syncline, the southward-dipping (Birendranagar); (4) in road cuts north of dhura thrust systems, but Schelling (1992) esti- Dadeldhura thrust cuts the Dumri Formation, of Surkhet; and (5) at the type section of the Dumri mated ~175–210 km of shortening on the Main probable early Miocene age. This means that the along the Tansen-Butwal road on either side of Central thrust in eastern Nepal, and Srivastava Dadeldhura thrust must have had a major phase the Dumri bridge (Fig. 1). and Mitra (1994) estimated ~193–260 km on the of displacement (at least several tens of kilome- The Dumri Formation in the Surkhet area is at

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least 1300 m thick, but only ~750 m of the unit are the Himalayan foreland basin (Fig. 4). The thick measured section at Khutia Khola in far exposed in the Tansen area (Sakai, 1989). The younger age limit of the Dumri Formation is western Nepal, and from sections at Macheli, Dumri consists of gray and greenish sandstone also poorly known, mainly because the top of Babai, and Surai Kholas (Figs. 1 and 4). The and interbedded red mudrocks. The proportion of the formation is everywhere either faulted or Siwalik Group of far western Nepal can be di- sandstone relative to mudstone decreases up- eroded. The oldest Siwalik Group rocks are of vided into informal lower, middle, and upper section. The sandstone bodies are typically 10–40 middle Miocene age (~14 Ma; Quade et al., members as defined by Quade et al. (1995). Al- m thick and are composed of medium- to fine- 1995) and are lithologically similar to upper though this scheme is useful for broad regional grained, trough cross-stratified, and horizontally Dumri Formation lithofacies. These combined comparisons, it is merely lithostratigraphic and laminated sandstone. Bedding is lenticular and observations, therefore, suggest that the Dumri cannot be used for detailed chronostratigraphy. In basal surfaces of sandstone bodies are erosional. is of early to early-middle Miocene age. the Khutia Khola section, the measured thick- Fossil logs and rip-up clast conglomerates are nesses of the lower and middle Siwalik members present in the lower parts of some sandstone units, Sedimentology of the Siwalik Group are, respectively, 862 m and 2468 m. The base of and upward-fining sequences are common. We the Khutia section is not exposed, and the top of interpret these sandstone bodies as the deposits of Stratigraphy and Age. Data reported in this the section is truncated by a thrust fault that large fluvial channels. Less-abundant, finer- paper are derived mainly from a new ~3.4-km- places the lower member on top of the upper grained, rippled, tabular sandstone beds are inter- preted as crevasse-splay deposits. The associated red mudrocks are generally structureless and mot- tled, and contain rooted zones, plant fragments, Mn/Fe-rich mottles, and occasional carbonate 6 nodules; these deposits probably represent paleo-

sols. The Dumri Formation is therefore inter- UPPER preted as fluvial channel, , and over- bank deposits (Sakai, 1989). Sakai’s (1989) sparse paleocurrent data from unspecified types 5 of cross-stratification in the Dumri indicate gener- ally southward paleoflow. Our paleocurrent mea- CANDE & KENT (1995) surements (~500 measurements) from trough 7 cross strata in the Tansen and Surkhet areas show 4 a strong west-southwestward vector mean. 8 The Dumri Formation has yet to be reliably MIDDLE dated. In western Nepal and northern India, the 9 unit overlies well-dated middle to late Eocene nummulitic limestone and black shale of the 3 10 Bhainskati Formation (also referred to as the Swat or Subathu Formation). At the Dumri 10.8 Ma Bridge locality, the contact between the Dumri 11 and the underlying Bhainskati is marked by a 3–4-m-thick, extremely mature paleosol— (LOWER MIOCENE-QUATERNARY) GROUP SIWALIK LOWER 2 12 probably an oxisol—that has conspicuous Fe- rich mottles. We interpret this as a deeply 13 weathered soil profile at a major unconformity between the Dumri and Bhainskati Formations. 14 Regional lithostratigraphic correlations suggest 1 Ma

an early Miocene age for the Dumri Formation DUMRI FM. (LOWER MIOCENE ?) (Sakai, 1983, 1989; Kayastha, 1992), although FLUVIAL CONGLOMERATE Gautam (1989) inferred a late Eocene age on the FLUVIAL SANDSTONE basis of paleomagnetic inclination data. In BHAINSKATI (SWAT) FM. FLUVIAL MUDSTONE northern India, the Eocene Subathu Formation (EOCENE) 0 NUMMULITIC LIMESTONE is overlain by the poorly dated Dagshai Forma- km tion and the early–middle Miocene Kasauli For- MARINE BLACK SHALE mation (Sahni, 1953; Najman et al., 1994). In , similar fluvial red beds in the Muree Formation are considered to be of early Figure 4. Simplified stratigraphic section of the Tertiary rocks of western Nepal, based on Miocene age (Burbank et al., 1996). Regardless measured sections at Dumri Bridge and Khutia and Swat Kholas. The contact between the of the exact regional correlation, it appears that Dumri Formation and Siwalik Group is unknown because it is either eroded or removed by a major unconformity, spanning perhaps most faulting at all exposures we have studied. The magnetostratigraphy of the Siwalik Group is of the , exists between well- based on unpublished data of T. P. Ojha and D. Richards. The magnetic polarity time scale of dated Eocene rocks and poorly dated lower Cande and Kent (1995) is shown for reference, and the position of lower-middle Siwalik Group Miocene rocks throughout the northern part of boundary is highlighted at ~10.8 Ma.

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member. The upper member in the area is at least ternate with crevasse splay (Fig. 5C) and paleosol mentary structures and textures typical of grav- 1000 m thick. deposits (Fig. 5D), and the paludal facies typi- elly braided river deposits (e.g., Smith, 1974; The age range of the Siwalik Group in Nepal cally are at the bases of channel bodies. On the Rust, 1978; Miall, 1996; Fig. 5B). These features is known to be between ~14 Ma and ~1 Ma on basis of sandstone body thicknesses, channel are also common in the modern gravel-bed rivers the basis of vertebrate fossils (West et al., 1978, depths ranged from a few meters to ~12 m during and streams that drain the Himalaya in western 1991; Corvinus, 1994) and magnetostratigraphic deposition of the lower Siwalik units and up to Nepal. Some of these modern rivers (e.g., the studies (Tokuoka et al., 1986; Harrison et al., several tens of meters during deposition of the Karnali) exhibit anastomosing, rather than classi- 1993; Appel and Rössler, 1994; Quade et al., middle Siwalik units. The limited lateral expo- cal braided, channel-belt morphologies. During 1995). Studies to develop the precise ages of sure in our sections prevents estimation of bank- the dry season, when flows are many meters be- boundaries between lower, middle, and upper full discharges, but these channel deposits are low bankfull stage, these channels have internally Siwalik members are in progress, but work thus probably comparable with those documented by braided networks of anabranches. It is plausible far indicates that the age of the lower-middle Willis (1993a, 1993b) in the Siwalik rocks of that some Siwalik rivers had similar anastomos- boundary in the Khutia Khola section is ~10.8 northern Pakistan, which indicate channel-belt ing morphologies. Ma (Fig. 4), whereas the same boundary in the discharges in the range of 103–104 m3/s. Paleocurrent Directions. Limbs of trough Surai Khola section is ~8 Ma (T. P. Ojha and The paleosols are mostly red and orange in the cross-strata were measured in 50 of the channel D. Richards, unpub. data). At Bakiya Khola in lower Siwalik Group and gray and yellow in the deposits throughout the Khutia Khola section and eastern Nepal, the lower-middle boundary is ~9 middle and upper members. Lower Siwalik in 6 channel deposits in the Surai Khola section. Ma and the middle-upper boundary is <4.5 Ma paleosols are rich in pedogenic carbonate nod- Each measurement station consists of a single (Quade et al., 1995). ules, similar to lower Siwalik paleosols in central channel sandstone from which 15–25 individual Lithofacies Descriptions and Interpreta- Nepal (Quade et al., 1995). The dark gray paleo- trough limbs were measured, yielding an average tions. The Siwalik Group is characterized by al- sols of the upper and middle Siwalik units repre- trough-axis orientation for each station (according ternating siltstone and gray sandstone units. sent pedogenesis in poorly drained conditions, to method 1 of DeCelles et al., 1983). Wherever Siwalik lithofacies are divisible into five assem- analogous to Histosols in the modern soil nomen- possible, trough axes were measured directly on blages (Table 1) that characterize fluvial deposits clature. Paleosols formed in better drained condi- the outcrop; however, such exposures are ex- and have been documented in the Siwalik Group tions are typified by the bright yellow paleosols tremely rare in the sections we have studied. In the of Pakistan, India, and central Nepal (e.g., Willis, of the upper middle and upper Siwalik units, and conglomeratic upper member, we measured 10 1993a, 1993b; Hisatomi and Tanaka, 1994; by darker yellow to red paleosols in the lower imbricated clasts per station in the Surai, Khutia, Tanaka, 1994; Burbank et al., 1996). Although a Siwalik units. Macheli, and Babai Khola sections. detailed treatment of the sedimentology of the The paludal deposits, although relatively mi- Average trough axes for the Khutia Khola sec- Siwalik Group is beyond the scope of this paper, nor in volume, are distinctive because they are tion are consistently within the azimuth range of we provide general descriptions and interpreta- well laminated and contain abundant fossil 120°–200° (Fig. 6, A and B); the overall average tions in order to assess major changes in the na- leaves, occasional bone fragments, coal, and lig- is 160°. Only one channel deposit produced an ture of Siwalik fluvial systems through time. nite seams. That they occur almost exclusively azimuth outside of this range. The average paleo- The five major lithofacies assemblages docu- directly beneath fluvial channel deposits suggests current azimuths of the lower and middle mem- mented in the Khutia Khola section consist of that the paludal facies may have formed in poorly bers are indistinguishable. At Surai Khola, the sandy fluvial channel, gravelly fluvial channel, drained, topographically low areas that ulti- mean paleocurrent direction is 208° (Fig. 6C). crevasse splay, paludal flood plain, and paleosol mately were the targets of channel avulsions. The data from the upper member exhibit more deposits (Table 1). Channel sandstones (Fig. 5A) The gravelly channel deposits of lithofacies as- scatter; the mean azimuth is ~200° (Fig. 6D). and imbricated conglomerate bodies (Fig. 5B) al- semblage 5 (Table 1) are characterized by sedi- These data demonstrate that the rivers that de-

TABLE 1. SUMMARY OF SIWALIK GROUP LITHOFACIES Lithofacies Description Stratigraphic Interpretation assemblage occurrence 1 2–42 m thick, lenticular bodies of medium- to coarse-grained sandstone; Present mainly in lower and middle Fluvial channel deposits; size of erosional basal surfaces; upward fining; trough and planar cross- Siwalik members; thicknesses of channels increases from stratification; parallel lamination; ripples; epsilon cross-stratification; units and degree of amalgamation average 8–10 m deep in lower dewatering structures. increase upsection. Siwalik Group to more than 20 m in middle Siwalik Group. 2 Several-meter-thick packages of fine- to very fine-grained sandstone Present throughout lower and middle Crevasse splay deposits. beds, 10–50 cm thick, interbedded with thin beds of siltstone; beds are Siwalik Group, but most abundant tabular, rippled, horizontally laminated, and bioturbated (Scoyenia in lower member. ichnofacies). 3 Variegated, mottled, red, yellow, gray, and orange siltstone; no primary Red and orange colors are prevalent Paleosols; reddish units are more bedding; root traces, carbonate nodules, Fe nodules, burrows; basal in lower Siwalik Group; gray and oxidized and less organic rich contacts gradational with underlying lithofacies; thickness ranges from yellow more abundant in middle than gray units. 20 to 200 cm. and upper Siwalik Group; overall, this lithofacies most abundant in lower member. 4 5–150-cm-thick beds of laminated, organic-rich siltstone; fossil leaves Most abundant in lower and middle Paludal deposits in overbank and bones, coal and lignite are common; characteristically occur on Siwalik Group. areas. top of units of lithofacies assemblage 3 and below units of assemblage 1. 5 Pebble- to cobble-conglomerate; clast-supported; beds 50–150 cm Characteristic of upper Siwalik Group. Large gravelly braided-river thick; imbricated; trough and planar cross-stratification and crude deposits. horizontal stratification; well-organized, often upward-fining.

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Figure 5. The Siwalik Group lithofacies. (A) Stacked channel sandstone bodies (lithofacies assemblage 1) in the middle Siwalik Group. Note person for scale. (B) Imbricated conglomerate of lithofacies assemblage 5 in the upper Siwalik Group. (C) Tabular beds of lithofacies assemblage 2. Beds are 20–50 cm thick. (D) Paleosol deposits in lower Siwalik member, showing carbonate nodules and large tubular burrows. Scale in B and D is 20 cm long.

posited the Siwalik Group in western Nepal in the lower member suggests that the middle levels in different areas along the Himalayan flowed consistently southward, essentially identi- Miocene flood plain was relatively stable and well thrust belt (Tokuoka et al., 1986; Sah et al., 1994). cal to the modern fluvial drainage pattern in the drained, but subject to occasional flooding. The The evidence discussed herein suggests that northern Indo-Gangetic flood plain. We have prevalence of crevasse splay deposits implies that the middle and upper Siwalik rivers were similar found no evidence for axial (i.e., parallel to the lower Siwalik channels had well-developed lev- in morphology to the modern transverse tribu- mountain front) or northward drainage in any part ees, which in turn suggests that they were anasto- taries of the Ganges, wherein gravel deposition is of the Siwalik Group of western Nepal. Hisatomi mosing and/or meandering. In contrast, the mid- restricted to within ~20 km of the topographic and Tanaka (1994) found no evidence for axial dle member is dominated by dark colored front of the mountain belt. The modern Indo- drainage in the Siwalik Group of central Nepal. Histosols and laterally extensive, thick (com- Gangetic flood plain north of the Ganges River is Stratigraphic Trends in Lithofacies and Flu- monly >20 m) channel deposits. At bankfull dominated by two types of rivers that flow gener- vial Characteristics. As in other areas of the stage, middle Siwalik Group rivers must have had ally south-southeastward, transverse to the trend Himalayan foreland basin system (Tokuoka et al., discharges comparable to those of large modern of the mountain front: (1) large rivers (discharges 1986; Harrison et al., 1993; Quade et al., 1995; rivers draining the thrust belt, about five times of ~104 m3/s) that have catchments extending Burbank et al., 1996), the Siwalik Group of west- greater than those of lower member rivers. into the interior of the mountain belt and ern Nepal exhibits an overall upward-coarsening Crevasse splays are not well developed, implying (2) smaller rivers (discharges of ~102–103 m3/s) trend (Fig. 4). The lower member is characterized that the middle Siwalik channels lacked promi- that have catchments limited to the frontal part of by alternating red and yellow paleosols and rela- nent and were more laterally mobile than the mountain belt. The former produce fluvial tively thin (~10 m) and laterally restricted channel lower Siwalik channels. The upward transition megafans with areas of 104–105 km2, but topo- deposits, and associated crevasse splay deposits. from middle to upper Siwalik members is gradual graphic relief of only ~20 m (Wells and Dorr, The abundance of oxidized, calcareous paleosols and probably takes place at different stratigraphic 1987; Gohain and Parkash, 1990; Mohindra et

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A. KHUTIA KHOLA C. SURAI KHOLA MIDDLE SIWALIKS LOWER & MIDDLE TROUGH LIMBS SIWALIKS n=621 TROUGH LIMBS 33 STATIONS n=124 6 STATIONS

D. SURAI, KHUTIA, BABAI, & MACHELI KHOLAS B. KHUTIA KHOLA UPPER SIWALIKS LOWER SIWALIKS IMBRICATIONS TROUGH LIMBS n=80 n=376 8 STATIONS 20 STATIONS

Figure 6. Rose diagrams with vector means (arrows) and 95% confidence intervals (arc lines) summarizing paleocurrent data from the Khutia, Surai, Babai, and Macheli Khola sections. See Figure 1 for locations of sections. Exact stratigraphic locations of measurement stations are avail- able from DeCelles.

al., 1992; Sinha and Friend, 1994; Gupta, 1997). ing upper Siwalik deposition (e.g., Awasthi et al., transition, because although subsidence rate may The rivers on these fluvial megafans are highly 1994). Carbon isotopic data from Siwalik Group influence channel density, it cannot influence flu- mobile and have variable morphologies, includ- paleosol carbonate nodules also indicate a bio- vial discharge. The most plausible explanation of

ing anastomosing, braided, and meandering. mass transition from C3 to C4 plants at ~7–8 Ma, the lithofacies transition from lower to middle Si- Highly sinuous meandering rivers flow parallel which Quade et al. (1995) interpreted as a shift walik Group is an increase in channel bankfull to braided or anastomosing rivers over large dis- from semitropical evergreen forests to mon- discharge, at least on a seasonal basis (Hisatomi tances. Shallow ponds and oxbows are locally soonal grasslands. However, the carbon isotopic and Tanaka, 1994). Because the paleobotanical abundant. The flood plain receives large amounts transition postdates the lithostratigraphic transi- data indicate that overall effective moisture de- of precipitation (1000–1500 mm/yr) and major tion from lower to middle Siwalik Group by 1–3 creased from the lower to middle Siwalik Group, floods during the monsoon. During the dry sea- m.y.; thus it does not appear to coincide with any channel sizes must have increased in response to son, however, the water level in channels of ma- major sedimentological changes in this part of concentration of discharge in a rainy season, sim- jor transverse rivers drops more than 10 m. The the foreland basin. ilar to that of today in the Indo-Gangetic foreland middle and upper Siwalik Group fluvial deposits Because the paleocurrent data indicate that basin system. The sedimentological evidence can most likely accumulated under a similar mon- flow directions were generally southward be explained by gradual intensification of sea- soonal climatic regime (Quade et al., 1995). throughout Siwalik Group deposition, the up- sonality beginning ~11 Ma, eventually sufficient

Independent evidence from fossil leaves and section changes in sedimentology from the lower to trigger C4 grass expansion on the Indo- carbon and oxygen isotopic studies of paleosol to the middle members cannot be explained sim- Gangetic flood plain by ~8 Ma. carbonate and organic material support the con- ply as a result of southward progradation of the To summarize, the Siwalik Group in western tention that the rivers that deposited the middle alluvial system in front of the southward- Nepal records a change from small channels and upper Siwalik Group were influenced by propagating Himalayan thrust front. Had this and well-drained flood plains to much larger monsoonal discharge. In most sections, fossil been the case, the lower member rivers should channels and more poorly drained flood plains leaves indicative of tropical rainforest vegetation have been larger than the upper member rivers, from middle to time. Rivers had a are abundant in the lower member, but disappear because the former would have been farther variety of morphologies, but highly sinuous within a few hundred meters above the base of downstream in the overall depositional mosaic of rivers seem to have dominated during early the middle member (Quade et al., 1995). Paleo- the foreland. The paleocurrent data also exclude Miocene time, whereas anastomosing and botanical studies indicate the presence of wide- the possibility that the transition into the middle braided rivers prevailed during middle and late spread, broad-leaved tropical evergreen forests member took place in response to a change from Miocene time. The changes in fluvial style are during lower Siwalik deposition, which were re- transverse to axial-trunk drainage. Neither can an best explained as a response to increased rainy placed by dry subtropical plants and grasses dur- increase in subsidence rate explain the lithofacies season discharge.

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Provenance TABLE 2. PETROGRAPHIC PARAMETERS Qm Monocrystalline quartz Methods. We cut samples of medium-grained, Qp Polycrystalline quartz Qpt Foliated polycrystalline quartz fluvial channel sandstones from the Dumri Bridge Qms Monocrystalline quartz in sandstone/quartzite lithic grain and Khutia Khola sections into standard petro- C Chert graphic thin sections, stained them for Ca-plagio- S Siltstone Qt Total quartzose grains (= Qm + Qp + Qpt + Qms + C + S) clase and K-feldspar, and point-counted (mini- mum of 450 grains per slide) according to the K Potassium feldspar (including perthite, myrmekite, microcline) P Plagioclase feldspar (including Na and Ca varieties) Gazzi-Dickinson method (e.g., Ingersoll et al., F Total feldspar grains (= K + P) 1984). Traditional parameters were also docu- mented in order to distinguish certain coarse- Lvm Mafic volcanic grains Lvf Felsic volcanic grains grained rock fragments (Table 2). Samples of Lvv Vitric volcanic grains sand from modern rivers draining specific litho- Lvx Microlitic volcanic grains tectonic zones of the thrust belt were point- Lvl Lathwork volcanic grains Lv Total volcanic lithic grains (= Lvm + Lvf + Lvv + Lvx + Lvl) counted in order to provide a basis for interpreta- tion of the Tertiary sandstones in terms of known Lsh Mudstone Lph Phyllite source terranes. In addition, nine samples of Si- Lsm Schist (mica schist) walik Group sandstone were crushed, sieved, and Lsc Schist (calc schist) panned for dense minerals in the field, and three Lma Marble (foliated, coarse-grained) Lls Limestone samples of modern river sand were panned for Ld Dolostone dense minerals. Zircons were separated from the Lm Total metasedimentary lithic grains (= Lsh + Lph + Lsm + Lsc + Lma + Lls + Ld) panned concentrates using heavy liquids, a mag- Dense minerals, typically monocrystalline: netic separator, and disposable sieve screens. Biotite Uranium-lead ages of zircons >145 φ in sieve size Chlorite were determined by isotope dilution–thermal ion- Cordierite Epidote ization mass spectrometry. The larger grains were Garnet separated into populations on the basis of their Hornblende Kyanite color, shape, and clarity, and representatives of Muscovite each population were then selected for analysis. Phosphate All of the zircons analyzed were abraded to about Pyroxene Sillimanite two-thirds of their original diameter prior to dis- Staurolite solution, and were analyzed as individual grains Tourmaline following the methods of Gehrels et al. (1991). Zircon Petrographic Results. Dumri Formation sand- stones have average modes1 of QmFLt = 68, 2, 30; QtFL = 78, 2, 20; and QmPK = 98, 2, 0 (Figs. Hisatomi (1990) and Critelli and Ingersoll (1994). recycled sandstone clasts from the middle Siwa- 7 and 8). Lower Siwalik Group sandstones have Lithic fragments are represented in all samples lik member (Figs. 5B and 10). Notably lacking in modes of QmFLt = 54, 10, 36; QtFL = 72, 10, 18; by phyllite, quartz-mica schist (Fig. 9A), the upper Siwalik conglomerates in western and QmPK = 84, 13, 3. Middle and upper Siwalik quartzite, sericitic quartzite (Fig. 9B), limestone, Nepal are clasts of medium- to high-grade meta- Group sandstones have modes of QmFLt = 53, dolostone, marble (Fig. 9C), and minor amounts morphic and igneous rocks. 17, 30; QtFL = 68, 17, 15; and QmPK = 76, 11, of volcanic grains (Fig. 9D). Foliated polycrys- Modal petrographic data from grain mounts of 13. In ternary QFL space, framework composi- talline quartz grains are common in some sam- modern river sands provide comparative infor- tions shift from the Q-L binary in Dumri and ples. Stratigraphic trends in lithic grain content mation about sands derived from known lithotec- lower Siwalik sandstones toward more feld- are also evident: phyllite grains decrease and car- tonic zones (Fig. 11; see Appendix for sample lo- spathic compositions in middle and upper Siwalik bonate grains generally increase upsection (Fig. cations). Samples from the Sun Kosi River, sandstones. This trend away from relatively 8, B and C). Both muscovite and biotite are abun- which drains the Greater Himalayan zone in the quartz-rich (during Dumri time), to plagioclase- dant in most middle and upper Siwalik Group hanging wall of the Main Central thrust, are en- rich (during lower Siwalik time), to K-feldspar- sandstone samples. Accessory grains include riched in quartz, plagioclase, micas, and lithic rich (during middle and upper Siwalik time) com- garnet, kyanite, zircon, epidote, staurolite, silli- fragments of garnet-mica schist and sillimanite positions is most clearly seen in the mono- manite, pyroxene, hornblende, chlorite, and schist; accessory minerals include cordierite, mineralic populations (QmPK) (Figs. 7B and cordierite. The higher grade minerals (kyanite sillimanite, garnet, staurolite, and kyanite (Fig. 8A). Samples of the Dumri Formation contained and sillimanite) first appear in the upper part of 11). The Sun Kosi samples are remarkably defi- virtually no K-feldspar, but significant amounts of the lower Siwalik Group (Fig. 8E). Cements are cient in K-feldspar (Fig. 11, B and C). In contrast, plagioclase (Fig. 8A). Similar results were ob- mainly quartz (overgrowths and pore-filling sands from Rao and Lamikheti Kholas (see Ap- tained in studies of the Siwalik Group by granular aggregates) in the Dumri Formation and pendix), which are derived entirely from rocks of calcite, kaolinite, quartz, and local anhydrite in the Dadeldhura thrust sheet, contain abundant K- 1 GSA Data Repository item 9805, tables for re- the Siwalik Group. feldspar, schist, phyllite, and coarse-crystalline, calculated modal point-count data and for U-Pb iso- tope data, is available on request from Documents Sec- Conglomerates in the upper Siwalik member quartzo-feldpathic (granitic) lithic fragments. Ac- retary, GSA, P.O. Box 9140, Boulder, CO 80301. are dominated by quartzite and contain subordi- cessory minerals are dominated by amphiboles E-mail: [email protected]. nate amounts of micritic limestone, marble, and and garnets. Sands from the Rapti River in cen-

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ages reported herein. Eight individual zircon grains from a sample of the Dadeldhura granite Qm Qm MIDDLE & UPPER collected along the Dadeldhura road (Fig. 1) were SIWALIK GR. analyzed in this study. Four grains yield a concor- LOWER SIWALIK dant age of 492 ± 6 Ma, and the other four grains GROUP have significant inherited components of ~1687 DUMRI FM. Ma (n = 2, MSWD [mean square of weighted de- viates] = 0.4) and ~2465 Ma (n = 2, MSWD = 13) (Fig. 14). Parrish and Hodges (1996) reported Qt Qt single-grain U-Pb ages from Lesser Himalayan rocks and the Main Central thrust zone in central

FLtFLtNepal. Lesser Himalayan ages range between ~1867 and ~2657 Ma, and clusters are in the ranges ~1867–1884 Ma and ~1921–1962 Ma; a few early and ages were also reported (Fig. 13). Ages associated with the Main Qm Central thrust zone are scattered between ~967 and ~1710 Ma; a cluster of three slightly discor- dant analyses is in the ~967–973 Ma range. The Qm FLFLMain Central thrust zone zircons are entirely younger than the zircons from Lesser Himalayan rocks. In our study, zircons from the modern rivers yielded age clusters that nearly match the ages reported by Parrish and Hodges (1996) and reflect derivation from both the Greater and 60% Qm Lesser Himalayan zones: the ~750–1000 Ma ages K PKP indicate sources in Greater Himalayan rocks, A. B. whereas the dominant ~1700–1900 Ma and ~2500 Ma ages and subordinate ~1900–2400 Ma Figure 7. Ternary diagrams. (A) All point-count data. (B) Means and standard deviations. ages reflect sources in Lesser Himalayan rocks. Each point represents a single modal analysis of 450 grains. Qm—monocrystalline quartz; Qt— All of the Siwalik sandstone samples from the total quartzose grains; F—total feldspar; Lt—total lithic grains; L—lithic grains exclusive of Khutia Khola section contained zircons having quartzose lithics; P—plagioclase feldspar; K—potassium feldspar. ages consistent with sources in Greater Hima- layan (~800–1000 Ma) and Lesser Himalayan (~2500 Ma) rocks (Fig. 13). The sands and sand- tral Nepal, which cuts through the Mahabharat are used as interpreted crystallization ages for stones sampled in this study provide a distribution thrust sheet (another of the crystalline thrust these grains, whereas the more highly discordant of detrital zircons from a much larger region than sheets containing large Cambrian–Ordovician grains are excluded from further discussion. Zir- the area covered by the Parrish and Hodges granite bodies, similar to the Dadeldhura sheet), cons from the Narayani, Karnali, and Mahakali (1996) study. The close match between the detri- are similar to those from the Dadeldhura thrust Rivers exhibit age clusters in the 750–1000 Ma, tal ages and the in situ ages suggests that the range sheet and include abundant K-feldspar, schist, 1700–1900 Ma, and 2200–2600 Ma ranges of ages reported by Parrish and Hodges (1996) phyllite, and medium-grade metamorphic miner- (Figs. 12 and 13). The Karnali River yielded one may be generally representative of the Himalayan als (especially garnet). Samples collected at the grain in the 480–500 Ma range, and a few ages of fold-thrust belt in Nepal, with the important topographic front of the range from large rivers 1900–2400 Ma were obtained from zircons from exception of Cambrian–Ordovician zircons that that drain the entire thrust belt (Mahakali and all three rivers. All of the Siwalik sandstone sam- are present in the sand and sandstone samples. Karnali Rivers) contain a mixture of grain types ples contained grains of ~800–1000 Ma and The ~460–530 Ma grains in the Siwalik Group that can be assigned to each of the major lithotec- ~2500 Ma (Figs. 12 and 13). The lower and mid- sandstones suggest sources in the Cambrian– tonic zones (Fig. 11): plagioclase, schist, abun- dle Siwalik samples contained grains of 460–530 Ordovician Dadeldhura granite in the hanging dant mica, and high-grade metamorphic minerals Ma, but only one grain in this age range was wall of the Dadeldhura thrust, and possibly the from the Greater Himalayan zone; K-feldspar, found in the upper member sample. Conversely, sparsely dated Cambrian–Ordovician (Ferrara et phyllite, chlorite schist, and foliated polycrys- the middle Siwalik sample lacked grains in the al., 1983; LeFort et al., 1986; Hodges et al., 1996) talline quartz from the crystalline thrust sheets 1800–2000 Ma age range, whereas lower and up- orthogneisses in the Greater Himalayan zone. The (the Almora and Dadeldhura sheets); and carbon- per Siwalik samples contained several grains of number of older (>1700 Ma) grains increases ate, quartzite, and phyllite grains from the Lesser this age range. steadily from the lower through upper Siwalik Himalayan zone. The detrital zircon ages are consistent with sandstones and into the modern rivers. None of Isotopic Results. Of the 130 detrital zircon what little is known about the ages of zircons in the zircon grains analyzed in this study has a grains from Siwalik Group sandstones and mod- metamorphic and plutonic sources in the Greater Cenozoic age, but this may be attributable to the ern rivers that were analyzed (Table DR2, see Himalayan and Lesser Himalayan zones. Few inheritance problems that characterize the Ceno- footnote 1), 113 grains yielded concordant to geochronological data are available from Hima- zoic leucogranites of the Greater Himalayan zone slightly discordant ages. The 207Pb*/206Pb* ages layan terranes for comparison with the zircon (e.g., Hodges et al., 1996).

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E. A. B. C. D. Ky Si

4000

3000 MIDDLE SIWALIK

2000 ~10.8 Ma LOWER SIWALIK

1000

87Sr/ 86 Sr

DUMRI FM. K/(K + P) PHYLLITE CARBONATE 0.728 0.722 0.730 0.732 0.724 0.718 0.726 0.720 0.716 0 m 0.714 0 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6

Figure 8. Plots showing the stratigraphic distributions. (A) K-feldspar and plagioclase. (B) Phyllite grains. (C) Carbonate lithic grains, in- cluding dolostone, limestone, and marble. (D) The 87Sr/86Sr composition of paleosol carbonate (diamonds) and carbonate fraction of associated detrital silt (stars) from Quade et al. (1997). (E) Occurrence of kyanite (Ky) and sillimanite (Si). Phyllite and carbonate contents shown in B and C are normalized to total volcanic, carbonate, and phyllitic lithic grains.

Interpretation and Kinematic History sponse to tectonic unroofing by the South Ti- zone, plus metasedimentary lithic grains from the betan detachment system. This is supported by upper part of the Dadeldhura thrust sheet. Abun- Figure 15 illustrates a hypothetical, struc- Nd isotopic data that indicate that since ~17 Ma, dant plagioclase and accessory kyanite and silli- turally controlled unroofing sequence that could sediment deposited on the Bengal fan has been manite seem to typify sands derived from Greater account for the petrographic and zircon prove- derived predominantly from the Greater Hima- Himalayan zone rocks in Nepal (Fig. 11). The U- nance indicators in the Dumri Formation and layan zone (France-Lanord et al., 1993). Pb zircon dates are consistent with sources in Siwalik Group. Thrusting and uplift in the The initial phase of displacement on the Greater Himalayan high-grade metamorphic Tibetan (Tethyan) Himalayan zone took place Dadeldhura thrust occurred after deposition of rocks and Lesser Himalayan metasedimentary during Eocene to early Miocene time (Searle, the Dumri Formation, because the fault cuts the rocks, probably in the upper parts of the Dadel- 1991; Ratschbacher et al., 1994). The high-grade Dumri along its northern trace (Figs. 1 and 2). dhura thrust sheet. The Cambrian zircon ages metamorphic rocks of the Main Central thrust This phase of thrusting must have involved at could reflect derivation from the granites of the sheet were being shortened and uplifted in the least 60 km of southward displacement (the ap- Dadeldhura thrust sheet and/or the Greater Him- ductile regime by ~22–20 Ma (e.g., Hubbard and proximate bed length of the Dadeldhura sheet alayan orthogneisses. Harrison, 1989; Harrison et al., 1992; Hodges et that is above the Dumri Formation; Fig. 2). Fur- The increase in coarse-grained, unaltered K- al., 1996). Initial displacement along strands of ther evidence of major early to middle Miocene feldspar in sandstones of the upper part of the the South Tibetan detachment system was un- displacement of the crystalline thrust sheets in lower Siwalik Group is an indication of wide- derway by ~19–18 Ma (Burchfiel et al., 1992; Nepal comes from the Copeland (1996) study of spread erosion of granitic rocks beginning at ~11 Hodges et al., 1996). Erosion of sedimentary and 40Ar/39Ar cooling ages of rocks in the synformal Ma. K-feldspar is sparse in modern sands derived low-grade metasedimentary cover rocks in the Kathmandu thrust sheet, where muscovite cool- from the Greater Himalayan zone in Nepal, but Tibetan and Greater Himalayan zones probably ing ages span ~22–13 Ma. Thus, lower Siwalik abundant in sands derived from the Dadeldhura produced the quartzose sedimentary and meta- Group sandstones should record the early un- thrust sheet. Most of this K-feldspar is derived sedimentary lithic detritus of the Dumri Forma- roofing history of the crystalline thrust sheets as from the Dadeldhura granite and the granitic tion (Fig. 15A). The small amounts of plagio- well as continued erosion of the Main Central gneisses that surround it. We therefore tentatively clase may imply that crystalline basement rocks thrust hanging wall, yielding plagioclase and interpret the influx of K-feldspar in the upper part had been exposed in the Main Central thrust high-grade metamorphic minerals such as kya- of the lower Siwalik Group to be an indication of hanging wall by Dumri time, perhaps in re- nite and sillimanite from the Greater Himalayan widespread, deep erosion of the Dadeldhura

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Figure 9. Photomicrographs of Siwalik Group sandstones, all under crossed polarizers. (A) Quartz-mica schist (QM), monocrystalline quartz (Qm), and foliated polycrystalline quartz (Qpt). (B) An orthoquartzite grain in the lower Siwalik Group. (C) A metacarbonate grain, probably a fragment of marble derived from the Lesser Himalayan zone, in the middle Siwalik Group. (D) Volcanic lithic (Lv) and plagioclase feldspar (P) grains, in the lower Siwalik Group. Widths of A, B, and D are ~1 mm; width of C is ~0.5 mm.

dhura thrust sheet at about the time of deposition CLAST COUNTS of the upper part of the lower Siwalik Group (~11 KK2 CARBONATE Ma). Horses of the Galyang and Lakarpata For- KK1 CHERT mations were imbricated upon each other and SK2 QUARTZITE fault slip was fed updip and southward into a roof thrust (the Ramgarh thrust) near the base of the SK1 SS/SILT Dadeldhura thrust sheet (Fig. 15C). As the duplex 0% 50% 100% gained structural relief, the overlying Dadeldhura thrust sheet was folded into its present synformal Figure 10. Chart showing composition of upper Siwalik Group conglomerates, based on clast shape, and much of the thrust sheet was eroded. counts of ~100 each at Khutia Khola (KK1, KK2) and Surai Khola (SK1, SK2). The timing of this displacement event is synchro- nous with the initiation of Main Boundary thrust displacement in northern India as interpreted by thrust sheet. The zircon ages provide support of sediment from the Greater Himalayan zone. Meigs et al. (1995). However, in the model pre- for a significant source of K-feldspar in the The increased amounts of older (>2.0 Ga) zir- sented here, the Main Boundary thrust (in the Cambrian–Ordovician granites, and the presence cons also suggest erosion of Lesser Himalayan strict sense) was the last thrust carrying Lesser of accessory high-grade metamorphic mineral rocks. Together, the data imply that a major dis- Himalayan rocks to be emplaced, and thus its grains (kyanite and sillimanite) and abundant placement event began on the system of thrust main phase of displacement may have been 800–1000 Ma zircons attests to continued supply faults that built the duplex beneath the Dadel- somewhat later.

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Upper Siwalik Group conglomerates were de- rived from nearby high-relief sources of low- Qm grade metasedimentary and sedimentary rocks in A. the Main Boundary thrust sheet, probably during its main phase of emplacement. Clasts of lithic sandstone, undoubtedly derived from the middle Siwalik Group, constitute a significant propor- KR1 tion of some clast counts. These were probably SK5 SK3 derived from the hanging walls of intra-Siwalik thrusts and/or from now completely eroded Si- RR2 MK walik outcrops on the hanging wall of the Main KR2 Boundary thrust. By the time of deposition of the RK upper Siwalik Group, the thrust front had ad- LK SR vanced into the preserved outcrop belt of foreland RR1 basin deposits and, as a result, middle Siwalik sandstones were recycled (Fig. 15D). The persis- tence of Cambrian–Ordovician, ~800–1100 Ma, FLtand >1700 Ma zircons in the upper Siwalik Qm Group, as well as K-feldspar and high-grade metamorphic minerals, indicates continued sup- B. ply of sand-sized material from the Greater Him- KR2 alayan zone and the Dadeldhura thrust sheet. SK3 MK Some of the older (Precambrian) zircons also KR1 RR1 could have been recycled from Dumri and/or SK5 RR2 Siwalik sandstones. RK LK SR Earthquake seismic evidence indicates that the large ramp beneath the Lesser Himalayan duplex is still active (Pandey et al., 1995), and this dis- GHS, SUN KOSI RIVER placement may be accommodated in the frontal DT/MT SHEETS thrust belt by displacement in the Main Frontal LHS, SETI AND RAPTI RIVERS thrust system (Nakata, 1989). Recent global po- ENTIRE THRUST BELT sitioning satellite (GPS) studies suggest that the majority of convergence measurable at the sur- PKface is occurring between the Greater Himalayan zone and the northern part of the Lesser Hima-

0.90 layan zone (Bilham et al., 1997). The overall rate C. 0.80 K/(K + P) of crustal shortening accommodated within the 0.70 0.60 Lesser Himalaya and Subhimalaya since ~11 Ma 0.50 is ~21 mm/yr, which is comparable to modern 0.40 0.30 rates of underthrusting of the northern Indian 0.20 plate beneath the Himalaya calculated by Bilham 0.10 0.00 et al. (1997). KR2 KR1 MK LK RK SK5 RR1 RR2 SR SK3 DISCUSSION 0.80 D. 0.70 CARBONATE 0.60 GRAINS Axial Paleodrainage 0.50 0.40 The data presented here have direct bearing on 0.30 the timing of development of a large axial (i.e., 0.20 0.10 eastward-flowing) river system during Miocene– 0.00 Pliocene time in the Indo-Gangetic foreland basin. It has been inferred that an axial river sys- tem developed during deposition of the middle Si- Figure 11. Ternary diagrams showing the modal framework compositions of modern sands walik Group in response to rapid subsidence in from the Sun Kosi (SK3, SK5), Karnali (KR1, KR2), Mahakali (MK), Seti (SR), Rao (RK), the proximal foreland basin during emplacement Rapti (RR1, RR2), and Lamykheti (LK) Rivers of Nepal. See Appendix for locations of sample of the Main Boundary thrust sheet (Burbank et al., sites. In A and B, solid symbols are for sands in rivers draining the entire thrust belt and only 1996), and that the present transverse drainage the Greater Himalayan zone, and open symbols are for sands in rivers draining the crystalline pattern in the proximal foreland is a relatively re- thrust sheets and the Lesser Himalayan zone. Shaded columns in C and D are for rivers drain- cent (post-Pliocene) development, due to ero- ing the Lesser Himalayan zone and crystalline thrust sheets. sional unloading and isostatic rebound of the fore-

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sol carbonate nodules in Siwalik flood-plain de- posits occurred at ~9–10 Ma (Quade et al., 1997; Fig. 8D). The shift in paleosol carbonate 87Sr/86Sr is mirrored by data from detrital carbonate silt in paleosol parent material (Fig. 8D). In paleosol carbonate from other sections of the Siwalik Group in Nepal and in resedimented pedogenic clays in the Bengal fan (Derry and France- Lanord, 1996), the 87Sr/86Sr ratio rises abruptly at ~9–7 Ma and then falls to values found in mod- ern Himalayan rivers after ~3 Ma (Quade et al., 1997). A similar fall in the 87Sr/86Sr ratio in the Khutia Khola section has not been documented because the upper part of the section has been re- moved by faulting. Can the trends in the 87Sr/86Sr ratio in the Si- walik Group and Bengal fan be coupled to trends in petrographic data and kinematic-erosional events in the thrust belt? In the Khutia Khola sec- tion, it is tempting to correlate several short-term peaks in the 87Sr/86Sr ratio of paleosol carbonate with peaks in the carbonate and K-feldspar grain contents of associated sandstones (Fig. 8). That a correlation with carbonate grain content should exist is suggested by the general coherence of the limited isotopic data from detrital carbonate silt (only 10 measurements) and paleosol carbonate (Fig. 8D). In addition, 87Sr/86Sr ratios of waters from springs in the Dadeldhura granite outcrop belt are >0.730 (Quade, unpub. data) and whole- rock initial Sr ratios from the granite itself are greater than 0.726 (Einfalt et al., 1993). However, the major rise in 87Sr/86Sr since ~9 Ma postdates the onset of increases in detrital carbonate and K- Figure 12. U-Pb concordia diagrams for detrital zircons from: (a–c) three modern river sands feldspar grains in Siwalik sandstones by ~1–2 in western (Mahakali) and central (Karnali and Narayani) Nepal, and (d–e) lower, middle, and m.y. (Fig. 8, A, C, and D). The modern sand pet- upper Siwalik Group sandstones at Khutia Khola. Each square represents a single detrital zir- rographic data strongly suggest that the main con grain; the solid squares are considered to be concordant enough to represent crystallization sources of K-feldspar for Siwalik sandstones ages and are included in summary histograms in Figure 13. were granitic rocks in the crystalline thrust sheets, and the carbonate grains were clearly de- rived from metamorphosed dolostone and lime- land basin (Burbank, 1992). The paleocurrent and were larger. However, paleocurrent data from the stone in the Lesser Himalayan zone. Low oxygen lithofacies data indicate that the transverse Dumri Formation indicate predominantly west- isotopic values (δ18O [Peedee belemnite] = drainage pattern in the Indo-Gangetic foreland southwestward paleodrainage (Sakai, 1983; au- –15‰ to –9‰) and low carbon isotopic values basin has existed since at least middle Miocene thors’ unpub. data), as would be expected for an (–6‰ to –2‰) from the paleosol detrital carbon- time. Whereas the data presented in this paper axial system flowing into the Indus portion of the ate also suggest a metamorphosed carbonate support the interpretation that the middle Siwalik foreland basin. Until more data are available on source terrane (Quade et al., 1997). Thus, the key Group was deposited during the growth of the the sedimentology, age, and regional paleo- to understanding the 87Sr/86Sr record is the bulk- Lesser Himalayan duplex (of which the Main drainage patterns for the Dumri, this interpreta- rock kinematic history of the Himalayan thrust Boundary thrust is an imbricate), Siwalik sand- tion will remain speculative. The deposits of an belt and the evolving distribution of rock types stones that crop out in the frontal thrust belt in axial system correlative with the Siwalik trans- and sources of radiogenic Sr. western Nepal contain no evidence for axial verse systems are probably in the subsurface be- Consideration of the structural geometry and drainage. If the deposits of such a river system are neath the Indo-Gangetic plain. kinematic history of the Himalayan thrust belt in preserved at the latitude of our study, they must be western Nepal (Figs. 2 and 15) provides a plausi- in older rocks. An obvious candidate in the appro- Implications for Geochemical Models of ble explanation for the observed Neogene trends priate stratigraphic position is the Dumri Forma- Himalayan Unroofing in the 87Sr/86Sr ratio of flood-plain and Bengal tion. Channel sandstones in the lower Dumri For- fan sediments. Deep erosion of the Dadeldhura mation are generally much thicker than those in The data presented herein provide new con- thrust sheet and the other crystalline thrust sheets the lower Siwalik Group, suggesting that the straints on geochemical models of Himalayan must have commenced with growth of the Lesser rivers that deposited the early Dumri Formation unroofing. A major increase in 87Sr/86Sr of paleo- Himalayan duplex, beginning ~11 Ma (Fig.

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(Rea, 1992), which suggests a linkage with rapid erosion of the crystalline thrust sheets and the growing Lesser Himalayan duplex. The increase of 87Sr/86Sr preserved in Neo- gene foreland basin deposits and the Bengal fan may have been caused by the development of the Lesser Himalayan duplex, erosion of the crys- talline thrust sheets, and incision into the under- lying Lesser Himalayan rocks. However, the ef- fect of this major kinematic-erosional event on the Neogene marine 87Sr/86Sr record (Hodell and Woodruff, 1994) is not clear. The marine record exhibits a nearly inverse correlation with mass accumulation rate in the northern Indian Ocean (Rea, 1992), and with the 87Sr/86Sr records of the Bengal fan (Derry and France-Lanord, 1996) and Siwalik Group (Quade et al., 1997). Changes in Sr flux as well as 87Sr/86Sr ratio must have played important roles in controlling the seawater ratio. It is also quite likely that Greater Himalayan rocks provided radiogenic Sr throughout the Neogene, because the petrographic data suggest Figure 13. Summary histograms of U-Pb zircon ages from Siwalik Group sandstone samples, that these rocks were exposed by early Miocene sand samples from modern rivers, and for comparison, ages reported by Parrish and Hodges time during deposition of the Dumri Formation. (1996) from the Main Central thrust (MCT) zone and Lesser Himalayan source terranes in the The 87Sr/86Sr ratio of seawater has been rising Langtang area of north-central Nepal. Star is age of Dadeldhura granite reported in this paper. rapidly since ~40 Ma, so it is probable that multi- ple sources of radiogenic Sr, each coming into play at a time dictated by the kinematic history of the thrust belt, contributed to the long-term trend in Sr composition.

CONCLUSIONS

(1) The three-fold, informal lithostratigraphic subdivision of the Siwalik Group used in other parts of the Indo-Gangetic foreland basin is applicable to the Siwalik Group in western Nepal, the lower member consisting of >850 m of fluvial channel sandstones alternating with ox- idized calcic paleosols, the middle member con- sisting of >2400 m of very thick (>20 m) channel sandstones and drab-colored Histosols, and the upper member comprising at least 1000 m of mainly gravelly braided river deposits. The in- Figure 14. U-Pb concordia diagram for eight individual zircon grains from the Dadeldhura crease in fluvial channel size from the lower to granite. Four grains are concordant at an age of 492 ± 6 Ma, and the other four have inherited middle Siwalik Group probably occurred in re- components of about 1687 Ma and 2465 Ma when regressed through 492 Ma. sponse to increased seasonal discharge. (2) Paleocurrent data indicate that the overall drainage pattern in the northern part of the Indo- 15C). This would have delivered abundant K- flux of radiogenic Sr. As more of the crystalline Gangetic foreland basin has been similar to the feldspar grains to the foreland basin, and carbon- thrust sheets were eroded, however, their contri- present-day drainage pattern since middle ate grains would have become increasingly avail- butions to the flux of radiogenic Sr would have Miocene time. able as the crystalline sheets were erosionally diminished, and the overall ratio preserved in the (3) Provenance data from the Dumri Forma- breached and the underlying Lesser Himalayan Siwalik Group and Bengal fan would have de- tion and the lower-middle Siwalik Group duplex was exposed to weathering (Fig. 15, C creased. If correct, this scenario would imply that demonstrate an overall upsection enrichment in and D). For ~5–6 m.y., the tandem high 87Sr/86Sr the regional rate of erosion of the 10–15-km- feldspar, carbonate, and high-grade metamor- sources in the Cambrian–Ordovician granites of thick Dadeldhura thrust sheet must have been phic minerals at the expense of quartzose grains the crystalline thrust sheets and the highly solu- ~2–3 mm/yr. In the northern Indian Ocean, deep- and low-grade metasedimentary and sedimen- ble metamorphosed carbonate rocks of the Lesser sea sediments record an abrupt increase in mass tary lithic grains. K-feldspar grains increase Himalayan duplex would have supplied a large accumulation rate during the period ~11–6 Ma abruptly in the upper part of the lower Siwalik

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Group, suggesting that granitic rocks became DUMRI FM. (EARLY MIOCENE) TIBETAN widely exposed at ~11 Ma. Modal point counts THRUSTS PLAG. of modern river sands derived from known QTZ. + + PHYLLITE STDS source terranes suggest that the main sources of MCT THZ K-feldspar were the Cambrian–Ordovician gran- GHZ ite and associated orthogneiss of the Dadeldhura thrust sheet. Provenance data from the upper Si- A. walik Group indicate local low-grade metasedi- mentary source terranes in the Lesser Himalayan rocks of the Main Boundary thrust sheet, in ad- dition to continued influx from the Dadeldhura sheet and the Greater Himalayan zone. LOWER SIWALIK GR. (~15 -11 Ma) (4) Combined provenance data and crosscut- PLAG. + QTZ. + M ting relationships outline the history of major ETAMOR TA PHIC LHS ME SEDS. S thrust displacements in western Nepal. The Main THZ Central thrust was active during early Miocene DT SHEET GHZ time and probably was the major source of the DT Dumri Formation. The Dadeldhura thrust cuts the B. DADELDHURA GRANITE Dumri Formation and probably was emplaced ~15–11 Ma, mainly during deposition of the lower Siwalik Group. Beginning ~11 Ma, the trailing portion of the Dadeldhura sheet was MIDDLE SIWALIK GR. (~11-5 Ma) folded into a regional antiform by the growth of a large duplex in underlying Lesser Himalayan GHS METAMO RPH rocks. Slip on faults within the duplex was fed ICS M into a roof thrust near the base of the Dadeldhura AR + METASEDS C K-SP . T sheet (the Ramgarh thrust) and eventually (by ~4–5 Ma) into the Main Boundary thrust. Con- GHZ THZ LHZ tinued duplex growth is demonstrated by concen- RT-DT trated microseismicity, and slip is transferred C. LESSER HIMALAYAN DUPLEX southward into the Main Frontal thrust system. On the basis of minimum estimates of shortening in Lesser Himalayan and Subhimalayan rocks, UPPER SIWALIK GR. (<5 Ma) the overall rate of crustal shortening since ~11 GHS METAMO Ma is ~21 mm/yr, which is comparable to mod- RPH ICS ern rates of underthrusting of the northern Indian ME PAR + TASED RECYCLED SIWALIK GR. K-S S. plate beneath the Himalaya. + LHS METASEDS (5) Comparison of U-Pb ages of detrital zir- GHZ THZ LHZ cons from the Siwalik Group and modern rivers MFT MBT with available zircon dates from Himalayan D. LESSER HIMALAYAN DUPLEX source terranes indicates that this method may be a powerful provenance tool in Himalayan stud- ies. Cambrian–Ordovician (460–530 Ma) detrital Figure 15. Schematic cross sections showing a hypothetical early Miocene–Pliocene unroof- zircons were probably derived from the Dadel- ing history for the Himalayan thrust belt in western Nepal. Abbreviations: THZ—Tibetan Him- dhura granite (which yielded a U-Pb zircon age alayan zone; GHZ—Greater Himalayan zone; LHZ—Lesser Himalayan zone; STDS—South of 492 ± 6 Ma) in the hanging wall of the Dadel- Tibetan detachment system; DT—Dadeldhura thrust; RT—Ramgarh thrust; other abbrevia- dhura thrust, and perhaps from poorly dated tions as used in text. (A) Deposition of the Dumri Formation: Sediment is derived mainly from Greater Himalayan orthogneisses. There are the THZ and the cover rocks of the GHZ during emplacement of Main Central thrust. Minor 800–1000 Ma zircons, probably derived from the amounts of plagioclase could have been derived from GHZ gneisses exposed by initial displace- Greater Himalayan zone, present throughout the ment on the South Tibetan detachment system. (B) Lower Siwalik Group: Sediment is derived Siwalik Group, and these indicate that the rocks mainly from the GHZ and cover section of the active Dadeldhura thrust sheet. (C) Middle in the hanging wall of the Main Central thrust Siwalik Group: Growth of large duplex in LHZ folds and passive uplifts trailing part of DT system have been a major source of sediment sheet (large vertical arrow). K-feldspar-rich sediment is derived from metamorphic and granitic since early middle Miocene time. Zircons older rocks of the DT sheet, including the Dadeldhura granite, and from continued erosion of GHZ than ~1.8 Ga were probably derived from Lesser rocks. Carbonate grains increase as LHZ duplex is erosionally breached. (D) Upper Siwalik Himalayan rocks and perhaps the upper part of Group: Main Boundary thrust is emplaced as a southernmost imbricate of the LHZ duplex, and the Dadeldhura sheet. None of the 113 detrital slip is transferred to thrusts in the Main Frontal thrust system. Conglomerate provenance is zircons dated in this study has an age consistent dominated by local sources in the frontal part of the Main Boundary thrust sheet, but sand-sized with derivation from the Cenozoic leucogranites sediment is continually supplied from the DT sheet, the LHZ duplex, and the GHZ. of the higher Himalaya.

Geological Society of America Bulletin, January 1998 19

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