T. P. Ojha

Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust belt in western Nepal

Regional mapping, stratigraphic study, and 40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold-thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of 40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an ∼8-km-high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold-thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small-scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

Late Miocene environmental change in Nepal and the northern Indian subcontinent : Stable isotopic evidence from paleosols.

Neogene sediments belonging to the Siwalik Group crop out in the Himalayan foothills along the length of southern Nepal. Carbon and oxygen isotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past ∼11 m.y. The carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca. 7.0 Ma, marking the displacement of largely C3 vegetation, probably semi-deciduous forest, by C4 grasslands. By the beginning of the Pliocene, all the flood plains of major rivers in this region were dominated by monsoonal grasslands. The floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenous organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of ∼4‰ in the oxygen isotopic composition of soil carbonate, as observed previously in Pakistan. The reasons for this are unclear; if not diagenetic, a major environmental change is indicated, perhaps related to that driving the carbon isotopic shift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-deciduous and dry deciduous forest between 11 and 6 Ma. The gradual nature of this floral shift, which culminated in the rapid expansion of C4 grasses starting ∼7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pCO2 alone (Cerling et al., 1993) but fits well with a gradual onset of monsoonal conditions in the late Miocene in the northern Indian subcontinent. Himalayan uplift, driving both monsoonal intensification and consumption of CO2 through weathering, may be the common cause behind major late Miocene environmental change globally. However, the decline of effective moisture associated with monsoon development has probably slowed, not increased, the rate of consumption of CO2 by chemical weathering of Himalayan sediments.

Seasonal stable isotope evidence for a strong Asian monsoon throughout the past 10.7 m.y

O of wet-season rainfall was significantly morenegative (29.5‰ SMOW) prior to 7.5 Ma than after ( 26.5‰SMOW). If this change is attributable to a lessening of the amounteffect in rainfall, this agrees with floral and soil geochemical datathat indicate increasing aridity beginning at 7.5 Ma.Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology,seasonality.INTRODUCTIONThe Tibetan Plateau is the engine that drives the modern Asianmonsoon by generating a high-altitude region of low pressure in thesummer as the plateau heats, and a region of high pressure in the winteras the plateau cools (Hastenrath, 1991). During the summer, warm airrises from the plateau, pulling moist air off the ocean, across the Indiansubcontinent, and into the highlands; this results in heavy summer rain-fall on the subcontinent. The opposite occurs in the winter, resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent. Thus, the presence of a strong wet-season–dry-season alternation implies the presence of a plateau broad and highenough to drive the monsoon.The timing of the uplift of the plateau remains a matter of con-siderable debate because there are few direct indicators of paleotopog-raphy in the geologic record. Consequently, past workers in Tibet, re-lying on indirect indicators of uplift, have proposed dates ranging from40 to 3.4 Ma, on the basis of initiation of potassic volcanism (Chunget al., 1998; Turner et al., 1993) or extension on the plateau (Harrisonet al., 1995; Coleman and Hodges, 1995), changes in marine sedimen-tation rates (Burbank et al., 1993), sediment types (Rea et al., 1998),or biota (Nigrini and Caulet, 1992; Kroon et al., 1991), and changesin stable carbon isotope and palynological patterns on the Indian sub-continent (Quade et al., 1989; Chen, 1981). Although different areasof the plateau may have risen at different times, many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7–8Ma by a process such as lithospheric delamination (Molnar et al.,1993). This inference was based on the following approximately coevalphenomena: a major change in plant communities of the Indian sub-continent (Quade et al., 1989), and shifts in marine upwelling patternsthat are linked to an intense monsoon (Kroon et al., 1991). Althoughthere is strong evidence for significant climate change at 7–8 Ma, it isunclear whether this is the onset of the monsoon. The floral transitionseems to have been a global rather than local phenomenon (Cerling etal., 1997) and monsoonally driven upwelling may have already beenpresent by 10–12 Ma (Nigrini and Caulet, 1992; Kroon et al., 1991).Because there is an intimate association between the intense sea-sonality of the modern monsoon and a high Tibetan Plateau and be-cause evaporation can be unambiguously recognized in the d

Initiation of the Himalayan Orogen as an Early Paleozoic Thin-skinned Thrust Belt

Research by many workers in various regions of the Himalaya, combined with our recent geologic and geochronologic studies in Nepal, indicate that fundamental aspects of the Himalayan orogen originated in an early Paleozoic thrust belt and are unrelated to Tertiary IndiaAsia collision. Manifestations of early Paleozoic tectonism include ductile deformation, regional moderate- to highgrade metamorphism, large-scale southvergent thrusting, crustal thickening and the generation of granitic crustal melts, uplift and erosion of garnet-grade rocks, and accumulation of thick sequences of synorogenic strata. Determining the relative contributions of early Paleozoic versus Tertiary tectonism constitutes a significant challenge in understanding the Himalayan orogen.

Eocene‐early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal

Sedimentologic, petrographic, and U-Pb detrital zircon ages from middle Eocene through early Miocene sedimentary rocks in the Lesser Himalayan zone of western and central Nepal indicate that a peripheral foreland basin system had developed in the eastern Himalayan collision zone by middle Eocene time. The shallow-marine, Eocene Bhainskati Formation accumulated in a back-bulge depozone between a southward migrating forebulge and the Indian craton. Migration of the forebulge through this region during Eocene-Oligocene time produced a regional unconformity that spans ∼15–20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone, represented by fluvial deposits of the Dumri Formation. Continued southward migration of the foredeep during the Neogene accommodated the fluvial Siwalik Group. Light mineral provenance data and U-Pb detrital zircon ages suggest that the Bhainskati was derived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zone during initial growth of the Himalayan fold-thrust belt. The Dumri was derived from metasedimentary and crystalline rocks of the Greater Himalayan zone during emplacement of the Main Central thrust and contemporaneous tectonic unroofing by normal faulting along the South Tibetan detachment system. The Lesser Himalayan crystalline thrust sheets were emplaced soon after deposition of the Dumri Formation, ∼15–10 Ma. Paleocurrent and lithofacies data from the Dumri Formation indicate deposition by west-southwestward flowing rivers that drained into the Indus portion of the Himalayan foreland basin system during the early Miocene. Thick channel sandstones in the lower Dumri may represent the early Miocene counterpart of the modern Ganges River. Eastward diversion of the Ganges drainage system to near its present location had occurred by ∼15 Ma, as the high-standing Aravalli Range on the northern Indian shield approached the front of the fold-thrust belt. Assuming reasonable values for the flexural rigidity of Indian lithosphere, the time span of the forebulge unconformity yields a velocity of ∼14–33 mm/yr for the southward migration of the fold-thrust belt relative to India. This range of values is consistent with Neogene and present-day estimates and suggests that only one third to one half of India-Eurasia convergence has been accommodated by shortening in the Himalayan fold-thrust belt since the onset of collision.

Isotopic Preservation of Himalayan/Tibetan Uplift, Denudation, and Climatic Histories of Two Molasse Deposits

Two distinctive molasse deposits within the Indo-Asian collision zone have been investigated to help understand the post-Oligocene evolution of the Himalaya and southern Tibetan plateau. The Siwalik Group (predominantly fluvial sandstones and siltstones), is widespread throughout the foothills of the Himalaya from Pakistan to eastern India. Paleomagnetic analysis of a measured section in the Bakiya Khola, southeastern Nepal, constrains depositional ages (

Impulsive alluviation during early Holocene strengthened monsoons, central Nepal Himalaya

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