Ar SINGLE-CRYSTAL DATING OF DETRITAL MUSCOVITE AND K-FELDSPAR FROM LEG 116 , SOUTHERN BENGAL FAN : IMPLICATIONS FOR THE UPLIFT AND EROSION OF THE HIMALAYAS

Detrital K-feldspars and muscovites from Ocean Drilling Program Leg 116 cores that have depositional ages from 0 to 18 Ma have been dated by the Ar/Ar technique. Four to thirteen individual K-feldspars have been dated from seven stratigraphic levels, each of which have a very large range, up to 1660 Ma. At each level investigated, at least one K-feldspar yielded an age minimum which is, within uncertainty, identical to the age of deposition. One to twelve single muscovite crystals from each of six levels have also been studied. The range of muscovite ages is less than that of the K-feldspars and, with one exception, reveal only a 20-Ma spread in ages. As with the K-feldspars, each level investigated contains muscovites with mineral ages essentially identical to depositional ages. These results indicate that a significant portion of the material in the Bengal Fan is first-cycle detritus derived from the Himalayas. Therefore, the significant proportion of sediment deposited in the distal fan in the early to mid Miocene can be ascribed to a significant pulse of uplift and erosion in the collision zone. Moreover, these data indicate that during the entire Neogene, some portion of the Himalayan orogen was experiencing rapid erosion (< uplift). The lack of granulite facies rocks in the eastern Himalayas and Tibetan Plateau suggests that very rapid uplift must have been distributed in brief pulses in different places in the mountain belt. We suggest that the great majority of the crystals with young apparent ages have been derived from the southern slope of the Himalayas, predominantly from near the main central thrust zone. These data provide further evidence against tectonic models in which the Himalayas and Tibetan plateaus are uplifted either uniformly during the past 40 m.y. or mostly within the last 2 to 5 m.y. INTRODUCTION Determination of the timing and rates of vertical tectonic activities are fundamental to assessment of virtually all geophysical processes during orogeny. Although modern geodetic methods can precisely constrain recent movements in tectonically active regions, other approaches requiring geochronology, either direct or indirect, are needed to extract these important parameters from the geologic record. For example, isotopically based age studies of minerals from differing elevations can reveal apparent uplift rates (e.g., Wagner et al., 1977; Harrison et al., 1979) and, in certain cases, the age of the onset of erosion (e.g., Copeland et al., 1987a). However, these studies are usually limited to assessment of uplift relative to the earth's surface (unroofing) and may not record the onset of unroofing due to the thermal inertia of the overburden. In contrast, the stratigraphic record preserved in the Bengal Fan has the potential for yielding information about uplift and erosion of the Himalayas that can not easily be obtained from land-based studies. One objective of the drilling of Leg 116, located at the distal end of the Bengal Fan, was to investigate the relationships between the stratigraphy and sedimentology of the fan and the tectonics of the India-Asia collision. Although the recovered core from Site 718 contains thick sections of early and mid Miocene sediment (Cochran, Stow, et al., 1989) suggesting rapid erosion in the Himalayas at about this time, sedimentation at the distal portion 1 Cochran, J. R. , Stow, D.A. V. , et al., 1990. Proc. ODP, Sci. Results, 116: College Station, TX, U.S.A. (Ocean Drilling Program). Department of Geological Sciences, State University of New York at Albany, Albany, NY 12222, U.S.A. 3 Present address: Department of Geosciences, University of Houston, Houston, TX 77204-5503, U.S.A. 4 Present address: Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90024, U.S.A. of the fan may not reflect tectonic activity in the source area 3000 km away but may rather be the result of redeposition of material from an older part of the fan. Our approach is to test this hypothesis as well as to determine the provenance age of these turbidites and relate this information to the tectonics of the Himalayas and southern Tibetan Plateau where substantial regional geochronology is now available (e.g., Kai, 1981; Ferrara et al., 1983; Copeland et al., 1987a, b; Maluski et al., 1988; Hubbard and Harrison, 1989). The Ar/Ar dating method (see McDougall and Harrison, 1988) was used to date single crystals and aggregates of detrital K-feldspar and muscovites from several stratigraphic intervals of the core recovered from Leg 116. A companion study (Corrigan and Crowley, this volume) has dated apatites from these samples by the fission-track method, with similar goals in mind. Various models have been proposed for the tectonic evolution of the Himalayas and Tibetan Plateau (e.g., Dewey and Burke, 1973; Powell and Conagahan, 1979; Powell, 1986; Zhao and Morgan, 1987). Our results place constraints on these models as they provide further information concerning the timing, rates, and periodicity of uplift and erosion resulting from the India-Asia collision. ANALYTICAL METHODS Samples used in this study are the same as those used by Corrigan and Crowley (this volume). Individual samples were obtained by combining coarse silt and coarser material over 100-m intervals to obtain sufficient apatites for their study. Samples are referred to by site number followed by the number of meters below the sea floor at which the interval began (e.g., 718-560). The fossils used to date the stratigraphic horizons at Sites 717 and 718 were not ideal for the task (Gartner, this volume). Two problems contribute to the uncertainty of these age assignments: (1) the lack of fossiliferous beds in some intervals made extrapolation from other depths necessary, and (2) the potential occurrence of trans-