PRE-COLLISIONAL PALEOCENE ALTIPLANO-STYLE PALAEO-TOPOGRAPHY OF SOUTHERN TIBET: CROC SUGGESTS NO!

The topography of the Andean-type continental margin of southern Tibet prior to collision with India has been a matter of considerable uncertainty. Strong early Late Cretaceous contractional deformation of the Takena Formation and older stratigraphic units of southern Tibet have long been argued as indicating a pre-collisional Altiplano-style of high (>3 km) topography for southern Tibet. This would imply a considerably thickened crust for this region prior to collision that would have significant implications for models of syn-collisional crustal mass balance and rheology of crust and potentially mantle lithosphere behavior during collision. We report the recent finding of a probable crocodilian tooth within Paleocene paleosols in the Penbo (=Linzhou) area of southern Tibet. The Linzhou Basin lies well within the andesite-dominated arc complex characteristic of the pre-collisional “Andean” margin of Asia. The probable crocodilian tooth was recovered from a sequence of paleosols that overlie a basal conglomerate that defines the local base of the post-62.6 Ma and pre-53.9 Ma Nianbo Formation. These paleosols are overlain by arc-related andesitic agglomerates and ash flow tuffs among other volcanic and volcaniclastic sediments. The Nianbo unconformably overlies strongly folded and metamorphosed late Early Cretaceous Takena and overlying early Late Cretaceous Shexing sandstones and mudstones. Modern crocodilians are restricted in elevation both stream gradients and temperature. The hypsometry based on geographic ranges of all 25 species and subspecies of crocodilian yields a median elevation of about 240 m, a hypsometric mean elevation of 430 m, and that 95% of their ranges lie below 1250 m and 99% below 2135 m. These overestimate the actual elevation ranges reflecting the generalized ranges. It seems highly improbable, even accounting for potentially warmer Paleocene climatic conditions, that Paleocene crocodilians would have adapted to high elevation (>3 km) habitats requiring migrations up high gradient river systems unlike any crocodilian habitat today. The most parsimonious conclusion is that the paleo-topography of southern Tibet just prior to collision was more comparable to Java and Sumatra than to the Altiplano.