SLAB-DRIVEN FORMATION OF THE KAILAS BASIN WITHIN THE INDIA-ASIA SUTURE ZONE, SOUTHERN TIBET

The Oligo-Miocene Kailas Formation is preserved within the southern Lhasa Terrane just north of the India-Asia suture zone and is exposed semi-continuously over ~1300 km. Kailas Formation facies assemblages and stratigraphic architecture vary somewhat along strike, but in its most complete exposures, the basal Kailas Formation consists of coarse-grained (up to boulder) alluvial fan deposits resting in buttress unconformity on Gangdese Batholith rocks. Above this, the rocks transition to fluvially dominated facies; organic rich lacustrine facies are present above the fluvial section in western Tibet (81°E) but are absent east of ~85°E. The upper Kailas Formation consists of alluvial fan conglomerates interbedded with red, occasionally calcic paleosols. Paleocurrent and provenance data indicate that most Kailas sediment was derived from the granitic and volcanic rocks of the Gangdese Batholith and Linzizong Volcanics to the north. Coarse-fine-coarse stratigraphic architecture, basal buttress unconformity, extensional growth-strata, and interbedded volcanic rocks all suggest an extensional origin for the Kailas Basin. New U-Pb data from interbedded tuffs and andesite flows show that the age of the Kailas Formation is oldest in western Tibet and is progressively younger to the east. Deposition occurred between 26 and 24 Ma in western Tibet (81°E), at 25–23 Ma north of Lazi (87.8°E), at 23–22 Ma near Dazhuka (89.8°E), and as late as 18 Ma southwest of Lhasa (92°E). This spatial-temporal pattern is nearly identical to, although several million years preceding, the spatial-temporal distribution of adakitic and ultrapotassic rocks within the Lhasa terrane, which have been interpreted as the results of Indian continental slab breakoff. Geophysical studies of southern Tibet have interpreted a slow anomaly present near 660 km depth beneath continental India as the detached Indian slab; this anomaly shallows to the east. Together, these data suggest that the formation of the Kailas basin was driven by Indian continental slab shearing that began in western Tibet and migrated east. Tearing and breakoff of this slab likely terminated extension and subsidence within the Kailas basin.