The Structural Character And Meso-cenozoic Evolution Of Micang Mountain Structural Zone, Northern Sichuan Basin, China

The couple between orogen and sedimentary basin has been one of hotspots in tectonic geology since 21st century. It is based on the dynamic evolution processes of thrust belt and its periphery sedimentary basin, and the material-energy exchanging process between them. As a typical example, we conduct structural and sedimentary investigations on the Micang Ms. and Northern Sichuan basin located in the north Yangtze plate, guided by the theories of continental dynamics and plate tectonics. Our methods are included of fields, boreholes, seismic data, geochronology, etc. Thus, the objects of our study are focus on three points. (1) Multistage deformation. (2) The dynamic evolution processes in orogen-basin system during Meso-Cenozoic. (3) The evolutional mechanism of Micang Ms.Micang Ms. Structural Zone is located in the northern margin of Yangtze plate and Sichuan basin, connected with Longmen Ms. orogen belt to the west, Daba Ms. foreland-thrust belt to the east, and Qinling orogen belt and Hannan uplift to the north, respectively. Based on field investigation, Micang Ms. is characteristic by typical superimposed structures with obvious three-episode. The first stage is characteristic with E-W striking structures (e.g., anticlines, thrust-faults), under the control of the collision between the Yangtze and the North China plates. The second stage is characteristic with NE-strike structures controlled by the wedging process of the Yangtze plate into Qinling orogen. Furthermore, the last stage is characteristic with typical superimposed structures on the former structures, and penetratively reworked older structures.The seismic data and field investigations show that it developed typical thick-skinned structures with involved basement, duplex-thrust with roof thrust and floor thrust in the Lower Triassic and the basement with depth of ~20km, respectively. Of which the structural style is much similar to Alberta-style triangle zone. Moreover, paleostress analysis indicates there are at least 5 episodes stress field in region, including two episodes of N-S trending, one episode of NW-SE, one episode of NE-SW and one episode of E-W trending. Microstructures and finite strain measurement suggest that an elongation of triaxial-prolate ellipsoid pattern predominantly take place in Micang Ms. Meanwhile, the results of multiple thermochronology analysis prove that MiCang Ms tectonic belt and Hannan uplift process is asynchronous. The rapid uplifting time of MiCang Ms began in ca.138Ma, 22Ma later than Hannan.Sedimentology investigations (e.g., stratigraphic contacts, basin filling sequence, provenance) in Micang Ms. and its periphery suggest that the Northern Sichuan basin which located in the south edge of Micang Ms. and don't have characters of typical forland basin. Moreover, the basin has not any exchange of material with Micang Ms. since Mesozoic in space. On the contrary, there is an obvious coupling process between Qinling orogen with Hanan uplift during the same time.According to the comparative researches among Longmen Ms., Micang Ms. and Daba Ms., it indicates that the formation of Micang Ms. is under the control both of irregular geometry boundaries in the northern margin of Yangtze plate, and diachronous collision process between the Yangtze and the North China plates. Thus, there are two significant roles of the Micang Ms., one is as a rheologically strong basement to block the deformation of its peripheral orogens, and another is a superimposed block by the multistage deformation from its peripheral orogens.By comprehensive analysis of sedimentology, geochronology, cross-cutting relations in structures and palaeostress, it suggests that the Micang Ms. has been experienced six episodes of tectonic evolution. (1) Pre-collision stage before Late Triassic, Micang Ms. was a passive continental margin of the north Yangtze plate located at the south of Qinling Paleotethys. (2) Syn-collision stage from Late Triassic to Early Jurassic, because of diachronous collision between the Yangtze and the North China plates from east to west, Hannan area and Qinling orogen underwent strong tectonism. However, Micang Ms. slightly underwent tectonism, more probably, to form an underwater uplift and partially deposit. (3) Tectonic Quiescence from Middle Jurassic to Early Cretaceous, Micang Ms. was characteristic with a faintly N-S ward tilting process. (4) Intracontinental deformation stage during Cretaceous, under the control of Yangtze plate wedging into Qinling orogen, strong intracontinental deformation took place here. Firstly, the E-W trending thrust structures formed, and then to form strike-slip structures with NE-striking that superimposed the former structures. The main structures of Micang Ms. were basically stereotyped until now. (5) Superimposed deformation stage from Paleocene to Early Eocene, the tectonic belts on both sides of Micang Ms. was expanded and deformed to within the basin, which superimposed on the earlier structures of sedimentary cap rock around its periphery. (6) Uplift stage since Late Eocene, influenced by eastward growth of the Qinghai-Tibet plateau, Micang Ms. and its periphery was entirely uplifted from west to east and suffered from denudation.