TERTIARY STRUCTURAL EVOLUTION OF THE GANGDESE THRUST SYSTEM, SOUTHEASTERN TIBET

Structural and thermochronological investigations of southern Tibet (X izang) suggest that intracontinental thrusting has been the dominant c ause for formation of thickened crust in the southernmost Tibetan plat eau since late Oligocene. Two thrust systems are documented in this st udy: the north dipping Gangdese system (GTS) and the younger south dip ping Renbu-Zedong system (RZT). West of Lhasa, the Gangdese thrust jux taposes the Late Cretaceous forearc basin deposits of the Lhasa Block (the Xigaze Group) over the Tethyan sedimentary rocks of the Indian pl ate, whereas east of Lhasa, the fault juxtaposes the Late Cretaceous-E ocene, Andean-type are (the Gangdese batholith) over Tethyan sedimenta ry rocks. Near Zedong, 150 km southeast of Lhasa, the Gangdese thrust is marked by a >200-m-thick mylonitic shear zone that consists of defo rmed granite and metasedimentary rocks. A major south dipping backthru st in the hanging wall of the Gangdese thrust puts the Xigaze Group ov er Tertiary conglomerates and the Gangdese plutonics north of Xigaze a nd west of Lhasa. A lower age bound for the Gangdese thrust of 18.3+/- 0.5 Ma is given by crosscutting relationships. The timing of slip on t he Gangdese thrust is estimated to be 27-23 Ma from Ar-40/Ar-39 thermo chronology, and a displacement of at least 46+/-9 km is indicated near Zedong. The age of the Gangdese thrust (GT) is consistent with an upp er age limit of similar to 24 Ma for the initiation of movement on the Main Central thrust. In places, the younger Renbu-Zedong fault is thr ust over the trace of the GT, obscuring its exposure. The RZT appears to have been active at circa 18 Ma but had ceased movement by 8+/-1 Ma . The suture between India and Asia has been complexly modified by dev elopment of the GTS, RZT, and, locally, strike-slip and normal fault s ystems.