UPPER CRUSTAL DEFORMATION IN SOUTHERN TIBET BEFORE AND DURING THE INDO-ASIAN COLLISION

Early Tertiary volcanic sequences of the Linzizong Formation are widely preserved above an angular unconformity in southern Tibet. Field mapping and geo/thermochronologic studies in areas where this unconformity is preserved have unique potential to constrain the history of crustal shortening, burial, and denudation before and during the Indo-Asian collision. Below the unconformity, Upper Jurassic-Lower Cretaceous marginal marine strata occur in the hanging wall of a north-dipping thrust fault which involves strongly folded Upper Cretaceous red beds of the Takena Formation in the footwall; both thrusting and folding predate the unconformity. Zircons separated from an andesite collected near the base of the Linzizong Formation yielded a U-Pb LA-ICP-MS age of 68.7 ± 2.4 Ma. This age is older than previously determined 40Ar/39Ar and K-Ar ages of 49-60 Ma, and together with the field relations, confirm the conclusions of previous workers that southern Tibet experienced significant shortening during the Cretaceous (Pan, 1993; Murphy et al., 1997). There is also evidence for shortening in southern Tibet during the Cenozoic Indo-Asian collision. Approximately 15 km north of Penbo, the upper part of the Linzizong Formation occurs in the footwall of a north-dipping thrust system, which involves Carboniferous-Triassic strata in the hanging wall. Additionally, at a locality where Linzizong volcanic rocks are absent due to denudation, a U-Pb zircon age of 51.7 ± 2 Ma for a folded dike that cuts the Takena Formation provides a maximum age for some of the folding. We hope to quantify how much shortening in southern Tibet occurred before vs. during the Indo-Asian collision through more regional geologic mapping and U-Pb studies of dikes and Linzizong volcanic rocks, and construction of balanced cross sections. Based on previous studies, Linzizong volcanic rocks may have been regionally buried to >10 km depth and subsequently denuded during the Tertiary (Pan, 1993; Copeland et al., 1995; Miller et al., 2000). This possibility suggests that our future thermochronologic studies along N-S transects across southern Tibet should reveal N-S variations in the timing and magnitude of Tertiary denudation. These variations may record the timing and magnitude of crustal underthrusting beneath southern Tibet during the Indo-Asian collision.