Structural Geometrics, Kinematics And Formation Mechanism Of Southwestern Sichuan Basin

The southwestern Sichuan area is located on the southeastern Tibetan, including the south segment of Longmenshan Mountains and some secondary structural belt. The study area is a key zone to revealing southeastward extension deformation of the southeastern Tibetan and interaction with Sichuan block, which is of significance to discussing the “5?12” Wenchuan Mw 8.0 earthquake in 2008 and the “4?20” Lushan Mw 7.0 earthquake in 2013. We may say in general that the structural belts in the southwestern Sichuan area represent the Cenozoic tectonic activities and as a window of neo-tectonic activities on the transformation of the old structures. So we conducted the structure geometrics and kinematics research in the southwestern Sichuan area.Based on structural geology, low temperature chronology and fault-related fold theory, integrated with the latest surface geology, magnetic, seismic and drilling data, the paper well interpreted seismic reflection profiles, constrained the tectonic deformation time, analyzed the structural geometrics and kinematics, restored the structural evolution, and discussed the genetic mechanism.The southwestern Sichuan area was divided into three tectonic belts from northwest to southeast: the Longmenshan thrust belt, Xiongpo belt, and the Longquanshan thrust belt. Each tectonic belt produced stratified deformation with the basement ductile shear layer and the Middle-Lower Triassic salt layer as the detachment fault.(1) The tectonic deformation in front of the south segment of Longmenshan Mountains varies along the structural trend. Wedge structures developed in mid-depth of the north part of Gaojiachang-Sanhechang belt, while the shallow dominated by the combination of fault and fold. The deep and mid-shallow layer of the south part characterized by duplex structure and superposed wedge structures. The crustal shortening of south part is much larger than the north, and the shallow layer is larger than the deep, with maximum shortening of 14.5 km(38%). AFT data revealing the Gaojiachang-Sanhechang belt experienced two period uplift history at ~30 Ma and 8-7 Ma respectively. The Lushan-Lianhuashan tectonic belt present with multi-layer thrust controlled by multiple slippage, the mid-shallow layer characterized by imbricate fan, and the deep layer dominated by superposed wedge structures. Structural recovery process revealing the front tectonic deformation zone of south segment of Longmenshan is piggy-back propagation.(2) The Xiongpo anticline is controlled by east-thrust fault, with the shortening amount about 6.8-8.5 km based on pure-shear wedge fault-bend fold model. The uplift history varies between the north and south segment. The north segment experienced slow uplift(the Late Cretaceous to 50 Ma) �' relative rapid uplift(50-5 Ma) �' rapid uplift(~5 Ma); the south experienced slow uplift(the Late Cretaceous to 68 Ma) �' quiet epoch(68-30 Ma) �' rapid uplift(~30 Ma).(3) The Longquanshan is characterized by structural segmentation, zonation and differential uplifting across and along the anticline. The south segment is controlled by fore-thrust based on forelimb breakthrough fault-bend fold mixed with folding mechanisms-“kink-band migration”and “limb rotation” The middle segment has similar characteristics with Xiongpo anticline with “kink-band migration” folding mechanism. The north segment presented with stratified deformation structure. The growth strata indicates tectonic deformation began at ~175 Ma, integrate with AFT data supposed two period rapid uplift 60―42 Ma and 20―10 Ma after Late Cretaceous. However, different segmentation, zonation varies in uplifting time.(4) The tectonic deformation experienced sea-land changes in the Late Triassic �' the rise of the south segment of Longmenshan Mountains and formation of Longquanshan thrust belt in the Middle Jurassic �' the formation of Xiongpo thrust belt during the Late Eocene and Miocene �' the expansion of the south segment of Longmenshan Mountains after the Pliocene.(5) By comparing the physical models, the paper presents some constraints on the deformation mechanism of southwest Sichuan area.