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

Abstract

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 (t(dep)) to between 10.8 and 4.9 Ma. The average accumulation rate during this interval was 0.4-0.5 mm/yr. 40Ar/39Ar dating of 126 detrital K-feldspars from nine horizons in the Bakiya Khola section (t(dep) = 8.25 to approximately 2.2 Ma) provide snapshots in time of the cooling ages exposed at the earth's surface in this drainage basin. The minimum age in the spectra from seven of the nine horizons averages only 3 m.y. older than t(dep), indicating exceptionally rapid unroofing in the Himalaya throughout that interval. Half of the grains from these nine sections yield minimum ages between 5 and 21 Ma. This latter age corresponds to the onset of a widespread unroofing event throughout the Himalaya and southern Tibet. Carbonates in paleosols exposed along Bakiya Khola yield carbon isotopic values averaging - 10.5 +/- 0.8 parts per thousand deltaC-13(PDB) between 11 and 7.0 Ma, and about 0 parts per thousand deltaC-13 between 7.4 Ma to the top of the section at about 2 Ma. This shift, previously recognized in Siwalik Group sediments in Pakistan, is interpreted to result from a change from dominantly C3 plants (i.e., trees) to dominantly C4 plants (i.e., grasses). The first appearance of C4 grasses favored by a warm growing season marks a significant ecological shift that may be related to intensification of the Asian Monsoon brought about by uplift of the Tibetan Plateau. In south-central Tibet, the Kailas conglomerate developed a thickness of over 3 km due to uplift and erosion of the Gangdese belt. Age assessment of this unit has been problematic (Late Eocene to Miocene?). 40Ar/39Ar K-feldspar results from a cobble from the Kailas conglomerate, presumed to be from the Gangdese volcanic suite, yields an age spectrum characterized by relatively slow cooling (5-degrees-C/Ma) between 27-19 Ma followed by very rapid cooling (100-degrees-C/Ma) between 19-18 Ma. The feldspar microstructure indicates that it has been heated to >400-degrees-C subsequent to eruption. This thermal history is consistent with deep burial in the volcanic pile subsequent to extrusion followed by rapid unroofing beginning at 20 to 19 Ma. These results suggest an Early Miocene upper limit for the depositional age of the Kailas conglomerate, and extends the evidence for a substantially enhanced unroofing at this time in the Gangdese-Himalaya system.