MESOZOIC DEFORMATION RECORDED IN SEDIMENTARY BASINS ON TOP OF THE NORTHERN YIDUN ARC, EASTERN TIBETAN PLATEAU

Models describing the evolution of the Tibetan Plateau do not account for the Mesozoic crustal configuration of southern Asia because age and spatial data from Mesozoic rocks prove difficult to survey. Two Mesozoic sedimentary basins in the northern part of the Yidun Arc Terrane, eastern Tibetan Plateau, are examined to resolve the geologic nature of the region during Mesozoic time. The West Ganzi basin (WGB) and East Ganzi basin (EGB) lie nonconformably on top of the Triassic Yidun Arc between elevations of 3,990-4,930 m and exhibit distinctive red mudstone, sandstone, and conglomerate packages. The WGB spans an area 19.5 km² and is located 80 km west of Garze, while the EGB encompasses 32 km² and is located 15 km southwest of Garze. The thickness of stratigraphy for the WGB and EGB are ~400 m and ~600 m, respectively. Both have 5-20 m thick basal conglomerates with interceded mudstone. Overlying the interceded conglomerates is sandstone (1-10 m thick) and interceded mudstone layers, producing an overall fining upward trend to both basins.

The WGB is bound to the north and south by reverse faults that place Yidun Group rocks on top of basin red rocks. On the southern margin of the WGB, reverse faulting produced fanning growth strata. The EGB is bound to the south by a high- angle reverse fault, juxtaposing Yidun Group rocks on top of basin red rocks. In addition, the EGB contains an anticline-syncline-anticline series with fold axes trending northwest-southeast. In the WGB and EGB, provenance analyses using detrital zircons show age peaks at ~215 Ma, ~230 Ma, 450-550 Ma, 700-900 Ma, ~1.9 Ga, and ~2.5 Ga. These data show that the youngest zircons in the basins are derived from the Yidun Arc and not from Cenozoic rocks in the region, suggesting these basins are Mesozoic in age and record deformation, which preceded the India-Asia collision.