THE LATE OLIGOCENE EARLY MIOCENE HIMALAYAN BELT - CONSTRAINTS DEDUCEDFROM ISOTOPIC COMPOSITIONS OF EARLY MIOCENE TURBIDITES IN THE BENGAL FAN

We present a mineralogical and isotopic study of Early Miocene turbidi tes from the Bengal Fan, which are the oldest sediments recovered duri ng ODP Leg 116. Clay fractions are dominated by illite and chlorite an d have low delta(18)O (11-16 parts per thousand). The clays are domina ntly derived from a metamorphic precursor by physical erosion, and hav e undergone only minor low-T water-rock exchange and chemical weatheri ng. Sr and Nd isotope analysis of silt and clay fractions and coarse m uscovite, biotite and feldspar fractions are remarkably similar to tho se of the presently exposed High Himalaya Crystalline (HHC) sequence. The data show that a close analogue of the HHC was already subaerially exposed to active erosion during Early Miocene. Rb-Sr data on the bio tite separates has been used to evaluate the time elapsed between cool ing through the closure temperature for Sr in biotite in the HHC (ca. 325 degrees C) and final sedimentation in the Bengal Fan. Despite the large uncertainties in the initial Sr-87/Sr-86 values of sedimentary m ixtures, our estimates of the elapsed time between cooling and deposit ion are short (4-14 Ma). Assuming a transport time scale (less than or equal to 1 Ma), deduced exhumation rates are in the same range of mod ern values (1-3 mm/yr) since ca. 30 Ma. Given that (a) the rocks expos ed at the surface, (b) the style of weathering, and (c) rates of exhum ation were similar to the present, we infer that the factors controlli ng erosion of the Early Miocene Himalayan belt were also similar to th e present. The data suggest that the Early Miocene Himalayan belt had significant relief and induced orographic effects on precipitation sim ilar to the modern mountain range. It appears that the thrusting along the Main Central Thrust (MCT) or similar structures has uplifted meta morphic basement analogous to the present HHC at sufficiently high rat e to maintain steep relief throughout Miocene time, despite high avera ge erosion rates.