The Late Oligocene Early Miocene Himalayan belt: Constraints deduced from isotopic compositions of Early Miocene turbidites in the Bengal Fan

We present a mineralogical and isotopic study of Early Miocene turbidites from the Bengal Fan, which are the oldest sediments recovered during ODP Leg 116. Clay fractions are dominated by illite and chlorite and have low delta(18)O (11-16 parts per thousand). The clays are dominantly derived from a metamorphic precursor by physical erosion, and have undergone only minor low-T water-rock exchange and chemical weathering. Sr and Nd isotope analysis of silt and clay fractions and coarse muscovite, biotite and feldspar fractions are remarkably similar to those 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 biotite separates has been used to evaluate the time elapsed between cooling 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 mixtures, our estimates of the elapsed time between cooling and deposition 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 modern values (1-3 mm/yr) since ca. 30 Ma. Given that (a) the rocks exposed at the surface, (b) the style of weathering, and (c) rates of exhumation were similar to the present, we infer that the factors controlling erosion of the Early Miocene Himalayan belt were also similar to the present. The data suggest that the Early Miocene Himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range. It appears that the thrusting along the Main Central Thrust (MCT) or similar structures has uplifted metamorphic basement analogous to the present HHC at sufficiently high rate to maintain steep relief throughout Miocene time, despite high average erosion rates.