Paleogene sedimentation, volcanism, and deformation in eastern Tibet: Evidence from structures, geochemistry, and zircon U–Pb dating in the Jianchuan Basin, SW China

Abstract

The Cenozoic tectonics of southeastern Tibet feature several large-scale strike-slip ductile shear zones along block boundaries and complex rotation and internal deformation of the blocks that are widely thought to be the result of India–Eurasia collision. The way in which we can match the rotation and internal deformation with the strike-slip movement remains an open issue. The easternmost of these shear zones, the 1000-km-long Ailao Shan/Red River sinistral shear zone, consists of three segments, and the gaps between the segments are occupied by the Chuxiong and Jianchuan Basins. Geologic data from these basins may provide positive constraints on the nature of Cenozoic deformation of continental crust of southeastern Tibet, but few data are available. This paper documents structural, geochronological, and geochemical results to constrain the tectonics of the Cenozoic Jianchuan Basin, eastern Tibet. Detailed field observations along a transect across the Jianchuan Basin revealed a horizon of volcanic rocks that overlies thick massive Eocene mudstones and siltstones, and which in turn is overlapped by coarse-grained Oligocene sandstones and conglomerates. The Late Eocene volcanics are shoshonitic and high in magnesium, and they consist of trachybasalts, trachyandesites, and trachydacites. In situ zircon U–Pb dating demonstrates that the volcanism was short-lived from 35 to 36Ma. Structural data reveal that some of the basement of the Jianchuan Basin has been uplifted, and then thrust over the Cenozoic sediments. Mudstone dikes developed along the main fault plane, and diapirs developed along other minor fractures of the hanging wall. Except for a few gentle folds, no solid-state deformation-induced structures are found in the Cenozoic strata, whereas syndepositional structures are common. These observations suggest that the basement uplift, thrusting, and sedimentation were coeval. Widespread pervasive NW–SE striking vertical fracture-cleavages in the basement suggest that the syndepositional uplift and thrusting were caused by NE–SW directed compression. These structures are distinct from ductile deformational features associated with the sinistral Ailao Shan shear zone. Available data revealed that the ductile shearing lagged ca. 10Ma behind the NE–SW compression. Our data thus suggest that deformation was widely distributed within continental block(s) at an early stage of the India–Eurasia collision, and then localized into some ductile shear zones likely due to block rotation. New zircon U–Pb geochronology and geochemistry of the igneous rocks in the basement show that these igneous rocks formed during the Early Triassic and the earliest Middle Triassic, and not as previously thought during the Late Triassic; they are therefore probably part of the Jomda–Weixi arc rather than the Yidun arc.