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East-west extensional basins are distributed across the southern half of the Tibetan plateau at an elevation of ~4 km. These basins have generated much interest because of their potential implications for the regional tectonics and force distribution in the plateau. This study documents the sedimentology of the Miocene-Pliocene Thakkhola graben fill in order to reconstruct basin evolution and paleoenvironment. Analysis of depositional systems, paleodrainage patterns, and conglomerate clast provenance of the >1-km-thick graben fill sets limits on the timing of activity of the basin-bounding faults and the development of southward axial drainage in the basin. During the deposition of the oldest basin fill (Tetang Formation, ca. 11-9.6 Ma), probably in a restricted basin, minor motion occurred on the basin-bounding fault systems. An angular unconformity separates the Tetang and overlying Thakkhola Formations, where this contact can be observed in the southern part of the basin. Southward axial drainage was established by ca. 7 Ma with the onset of deposition of the Thakkhola Formation. Several episodes of damming of this drainage system are recorded by widespread lacustrine deposits in the southern part of the basin. Facies distribution and the progressive rotation of strata in the Thakkhola Formation indicate that the Dangardzong fault on the western edge of the basin was active at this time, and drain age ponding may have been related to displacement on normal faults associated with the South Tibetan detachment system to the south of Thakkhola graben. Contrasts between deposits of the Tetang and Thakkhola Formations provide evidence for environmental change in the basin. In the Tetang Formation, the abundance of lacustrine facies, the pollen record, and the absence of paleosol carbonate suggest that conditions were more humid than during subsequent deposition of the Thakkhola Formation. Environmental change in the Thakkhola graben coincided with environmental change observed in the Siwalik foreland basin sequence, Arabian Sea, and Bay of Bengal at ca. 8-7 Ma. Although this climate change event has been previously attributed to intensification of the Asian monsoon in response to uplift of the Tibetan plateau, paleoaltimetry data indicate that this region had already attained a high elevation by ca. 11 Ma. Thus, the Thakkhola graben stratigraphic record suggests that uplift of the southern Tibetan plateau and the onset of the Asian monsoon as inferred from paleoclimatic indicators were not directly related in a simple way. Keywords: extension, Nepal, Thakkhola graben, Tibetan plateau.

Fossil shells and paleosol carbonate from ancestral Himalayan river deposits provide a [sup.87]Sr/[sup.86]Sr record of lowland Himalayan river water during the late Neogene. Reconstructed [sup.87]Sr/[sup.86]Sr river values increased sharply in [...]

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 [approximately]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 [C.sub.3] vegetation, probably semi-deciduous forest, by [C.sub.4] 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 terrigenons organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of [approximately]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 [C.sub.4] grasses starting [approximately]7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pC[O.sub.2] 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 C[O.sub.2] 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 C[O.sub.2] by chemical weathering of Himalayan sediments.

Stable isotope profiles of fossil freshwater bivalve shells and mammal teeth provide a record of the seasonal [[Delta].sup.18]O variation in surface waters of the Himalayan foreland over the past 11 m.y. Between 3.1 and 10.7 Ma the [[Delta].sup.18]O of surface waters approached or exceeded 0%0 standard mean ocean water (SMOW) in the dry season. Since 9.5 Ma the magnitude of seasonal variability in [[Delta].sup.18]O has remained essentially unchanged. Both observations imply that the Tibetan Plateau had attained sufficient elevation and area prior to 10.7 Ma to support a strong Asian monsoon. These data also imply that the [[Delta].sup.18]O of wet-season rainfall was significantly more negative (-9.5% [salinity] SMOW) prior to 7.5 Ma than after (-6.5% [salinity] SMOW). If this change is attributable to a lessening of the amount effect in rainfall, this agrees with floral and soil geochemical data that indicate increasing aridity beginning at 7.5 Ma. Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology, seasonality.