DETRITAL GEOCHEMICAL FINGERPRINTS OF RIVERS ALONG THE INDUS-YALU SUTURE ZONE IN TIBET: IMPLICATIONS FOR DRAINAGE EVOLUTION, TIMING OF ARC DEVELOPMENT AND EROSION

The Indus-Yalu Suture (IYS) zone between India and Asia comprises today a central depression occupied by the eastward flowing Yalu River, flanked to the north and to the south by high-elevation ranges. Our detrital geochemical study focuses on modern sand samples collected from tributaries of the Yalu River and includes the poorly studied western part of the drainage system. Modern sands from these rivers provide an opportunity to broadly sample source rocks present within the suture zone and study their regional geochemical fingerprints. Seven sand samples from rivers along the suture zone in Tibet between Xigatze to the east and Mt. Kailas to the west were collected for detrital zircon U-Pb geochronologic and apatite fission track (AFT) thermochronologic analyses. Zircon U-Pb ages for all rivers range between 15 and 3568 Ma. Rivers draining the northern side of the suture zone mainly yield ages between 40 and 60 Ma, similar to the age of the Gangdese magmatic arc. Samples from rivers draining the southern side of the suture zone record a Tethyan Himalayan signal characterized by age clusters at 500 Ma and 1050 Ma. Our results indicate that the ages and proportion of U-Pb zircons ages of downstream samples from tributaries of the Yalu River directly reflect source area ages and relative area of source rock exposure in the catchment basin. Significant age components at 37 – 40 Ma, 47 – 50 Ma, 55 – 58 Ma and 94 – 97 Ma reflect episodicity in Gangdese arc magmatism. Our AFT ages show two main signals at 23 – 18 Ma and 12 Ma, which are in agreement with accelerated exhumation of the Gangdese batholith. The 23 – 18 Ma signal partly overlaps with deposition of the Kailas Formation along the suture zone and may be related to exhumation due to upper plate extension in southern Tibet in response to Indian slab rollback and/or break–off events.