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

Abstract

TheCenozoictectonicsofsoutheasternTibetfeatureseverallarge-scalestrike-slipductileshearzonesalongblock boundaries and complex rotation and internal deformation of the blocks that are widely thought to be the result ofIndia–Eurasia collision.The wayin which we canmatch 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 docu- ments 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 wasshort-lived from 35to 36 Ma. Structural data reveal that some of the base- ment ofthe Jianchuan Basinhas been uplifted, and then thrust over theCenozoicsediments. Mudstonedikes de- velopedalongthemainfaultplane,anddiapirsdevelopedalongotherminorfracturesofthehangingwall.Except for a few gentle folds, no solid-state deformation-induced structures are found in the Cenozoic strata, whereas syndepositionalstructures are common.These observationssuggest that the basement uplift, thrusting,and sed- imentation were coeval. Widespread pervasive NW–SE striking vertical fracture-cleavages in the basement sug- gestthatthesyndepositionalupliftandthrusting werecaused by NE–SWdirectedcompression. Thesestructures aredistinctfromductiledeformationalfeaturesassociatedwiththesinistralAilaoShanshearzone.Availabledata revealed that the ductile shearing lagged ca. 10 Ma 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 TriassicandtheearliestMiddleTriassic,and notaspreviouslythoughtduring theLateTriassic; theyaretherefore probably part of the Jomda–Weixi arc rather than the Yidun arc.