SYNTECTONIC GROWTH STRATA IN THE CENTRAL LHASA TERRANE AND IMPLICATIONS FOR THE TECTONIC HISTORY OF TIBET PRIOR TO THE INDO-ASIAN COLLISION

Recent work suggests that crustal thickening in southern Tibet may have begun long before the Indo-Asian collision. The Lagen La conglomerate in the Nyainqentanglha Shan mountain range of southern Tibet consists of Late Cretaceous syntectonic deposits shed during movement along nearby thrust faults, and therefore provides a unique record of pre-collisional upper crustal shortening in the area. A north-verging thrust fault emplaces Paleozoic meta-sedimentary rocks over the Lagen La conglomerate, whereas to the north, a south-verging backthrust juxtaposes Lower Cretaceous limestone against the conglomerate. Paleocurrents within the conglomerate indicate a predominantly northerly flow with a wide dispersion. Facies analysis of measured sections indicates the conglomerate was deposited in a proximal alluvial fan environment, in agreement with the wide dispersion of the paleocurrent data. Clast counts and petrographic data reveal a recycled orogenic setting for these conglomerates, and varying provenance during deposition. Growth strata within the Lagen La conglomerate can be used to constrain the timing of fault movement, but complex bedding relationships resulting from inter-bed slip prevent simple restorations. Uranium-lead ages of detrital zircons from the Lagen La conglomerate, as well as U-Pb dates from an interbedded tuff, provide valuable constraints on the timing of deposition and regional deformation in southern Tibet.