PALEOALTIMETRY OF THE TIBETAN PLATEAU

Paleoaltimetry estimates from various archival records and basins on the Tibetan Plateau imply that the plateau, at least south of the Banggong Co-Nujiang suture, has been topographically high (>4km) since possibly late Eocene time. Paleoaltimetry estimates exist for Late Eocene, Oligocene and Miocene lacustrine and paleosols from the Lunpola Basin and late Early Miocene and younger lacustrine and paleosols from the Oiyug Basin. Thermal maturation indices from these units are uniformly very low, indicating minimal to non-existent post-depositional alteration of these materials. From both of these basins combined d¹⁸Oc and dD composition of n-alkanes from epicuticular plant wax-derived estimates of water isotopic compositions are concordant as measured relative to the Global Meteoric Water Line. Calculated early-middle Miocene (~20-15 Ma) paleoelevation estimates for the Oiyug basin range from 3000-5400 m and average of ~4700 m. Late Miocene-Pliocene (~5-8 Ma) paleoelevation estimates range from 5300-6400 m with an average of ~5600 m. The modern hypsometric mean elevation of the Oiyug basin is ~5000 m, our results thus allow for a possible decrease in the average elevation of the southern part of the Tibetan Plateau since the late Miocene, compatible with widespread east-west extension in Tibet since at least that time.

Evidence for the paleoelevation of the pre-collisional Andean margin arc sequence, and particularly the Linzizhong volcanics, are as yet equivocal, due to significant post-depositional diagenetic alteration of both their d¹⁸Oc and thermal destruction of n-alkanes. d¹⁸Oc (PDB) average about -15‰ from both lacustrine and paleosol carbonates and hence are relatively depleted. This might be taken as evidence for a relatively high pre-collisional height (>3 km) of this margin. However, various thermal alteration indices from these units imply temperatures of >100°C indicating complete diagenetic overprinting of all materials thus far analyzed. At T >100°C even very modest water-rock exchange between relatively undepleted waters (d¹⁸Ow >-7‰) altering relatively undeplete d¹⁸Oc ( >-7‰ ) could explain these isotopic compositions. This would imply low elevations at the time of alteration. Unfortunately the age of alteration, other than being early Eocene or younger is not yet constrained.