Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal 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 Geology, 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 Miocene–Pliocene 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 Cenozoic 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 paleosols, 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) deposition 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-Ganges gioclase grains and high-grade minerals was 87Sr/86Sr composition of paleosol 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 weathering 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 middle Miocene synoro- *e-mail: [email protected] †Present address: Department of Earth and Space a large duplex in the Lesser Himalayan rocks. genic sediments 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/110/1/2/3382696/i0016-7606-110-1-2.pdf by guest on 28 September 2021 KINEMATIC HISTORY OF THE HIMALAYAN FOLD-THRUST BELT 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 Tibetan Plateau 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 India, 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.