CRETACEOUS-TERTIARY DEFORMATION HISTORY OF CENTRAL TIBET

Geologic mapping and geochronologic studies along two ~190-km-long N-S traverses provide new constraints on the Cretaceous-Tertiary deformation history of central Tibet. The regional map pattern of the Qiangtang terrane can be explained by a >600-km-long E-plunging anticlinorium, with blueschist-bearing metamorphic rocks and upper Paleozoic shelf strata in its core and Mesozoic-Tertiary strata on its limbs. Growth of the Qiangtang anticlinorium began prior to eruption of 110 to 105 Ma volcanic rocks and continued during the Tertiary, as Eocene-Oligocene redbeds and volcanic rocks are involved in the regional folding. A prominent ~E-W trending topographic divide in the northern Qiangtang terrane is parallel to and located ~120 km north of the anticlinorium axis, and may be related to post-Oligocene northward growth of the anticlinorium. The limbs of the Qiangtang anticlinorium and the Late Jurassic-Early Cretaceous Bangong-Nujiang suture zone to the south are modified by Tertiary N-dipping thrust faults. Thrusts along the Bangong-Nujiang suture zone include mid-Cretaceous and Tertiary redbeds, and 110 to 105 Ma, 65 ± 1 Ma, and 42.5 ± 0.6 Ma volcanic rocks in their footwalls. The Tertiary S-dipping Gaize-Siling Co thrust bounds the southern margin of the Bangong-Nujiang suture and defines the northernmost extent of Lower Cretaceous marine strata in central Tibet. Mesozoic metamorphic and igneous rocks in central Tibet were exhumed to upper crustal levels prior to mid-Cretaceous time. We propose that formation of the Qiangtang anticlinorium was directly linked to Cretaceous-Tertiary shortening along the Bangong-Nujiang suture zone, and may record significant northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane before and during the Indo-Asian collision.