The Age And Its Evolution Of Nappu Bashan Nappe Structure

This paper took the latest progresses of plate tectonics and continental dynamics as academic guiding ideology, and took existing geological materials and field observation and experimental results as basis, and mainly focused on formation age of dominant structures of north Dabashan tectonic belt and huge Bashan nappe structures as well as their composites evolution. A practical investigation on tectonic framework and basic characteristics of main faults inside north Dabashan and of nappes between main faults and of Bashan main arcuate fault were carried out, and existing chronology data of north Dabashan were comprehensively collected and systematically analyzed, also thermochronology testing were made together with observations related with structure and sedimentary in south Dabashan, hence, formation age and its composite evolution of huge double layer thrust nappes of Dabashan were comprehensively explored and discussed as well as basin and mountain relationship.Consequently, the main conclusions can be made as follows:(1) North Dabashan tectonic belt was mainly overlapped by two two stages of consisted of two stages of tectonic deformations. The early-stage tectonic deformation was collisional orogen structure in Indosinian (including thrust nappe structure) occurring from230to200Ma, and the late-stage tectonic deformation was huge multilevel thrust nappe structure of intracontinental orogen in middle-late Yanshanian mainly forming from165to100Ma. Thereinto, the early-stage tectonic was dominantly controlled structure inward north Dabashan, and the late-stage tectonic happened on the basis of the early-stage tectonic, namely, the late-stage tectonic was Dabashan huge double layer nappe structure in middle-late Yanshanian along Bashan main arcuate nappe fault zone, and overlapped and contained and reformed the early-stage tectonic which originally belonged to the collisional orogen structure in Indosinian. North Dabashan thrusted along Bashan main arcuate fault zone as main napping interface, and contained early-stage tectonic deformation inward north Dabashan, mainly including early-stage nappe faults such as Ankang-Qingfeng fault, Hongchunba-Zengjiaba fault and Gaoqiao fault which inherited to reactivate, and the secondary thrust nappe occurred which divided north Dabashan into secondary nappes such as Ankang-Wudang nappe, Ziyang-Pingli nappe, Gaoqiao nappe and Gaotan nappe. So the north Dabashan huge thrust nappes thrusted on a large scale southwestward and overlaid onto the north margin of Yangtze block.(2) Jurassic rifted basin along Hongchunba fault of north Dabashan belonged to post main orogenic extensional collapse of Qinling orogenic belt, and it is reflected that north Dabashan tectonic belt underwent post main orogenic extensional collapse in advance, and the main faults of north Dabashan were controlled by post main orogenic extensional collapse in middle and shallow level of crust of orogen, which mainly manifested as extensional tectonics and control deposition of basin. The deformation of middle-upper Jurassic didn’t only reflect that Qinling shifted to dynamics adjustment of intraplate tectonic evolution after it finished plate subduction and collision orogen in Indosinian, but more importantly, rocks of middle-upper Jurassic suffered intracontinental orogen in middle-late Yanshanian, which were thrusted and deformed and metamorphosed. So a tectonic movement was revealed which was characterized by orogenesis, and it recorded tectonic events of intracontinental orogen in middle-late Yanshanian in shallow level of crust, and it reflected that north Dabashan turned into tectonic evolution of post main orogenic extensional collapse ranging from late Indosinian to early Yanshanian after main collisional orogen in Indosinian, then strong intracontinental orogen occurred in middle-late Yanshanian ranging from late Jurassic to early Cretaceous, and the huge multilevel thrust nappe structures were main pattern.(3) Bashan main arcuate fault mainly shaped up in middle-late Yanshanian (J3-K1) mainly ranging from140to100Ma and was main nappe fault, along which north Dabashan huge nappe structure thrusted onto north margin of Yangtze block on a large scale in middle-late Yanshanian under uniform dynamics mechanism of intracontinental orogen. The Bashan main arcuate fault cut all early-stage tectonics inward north Dabashan, and it was of sinistral and dextral fault but is mainly characterized by dextral shear strike-slip. North Dabashan huge thrust nappe structure presented as thrusting and dextral shear strike-slip as a whole in late stage. Taking Chengkou-Fangxian fault as main nappe fault, north Dabashan huge thrust nappe structure moved southwestward on a large scale in middle-late Yanshanian, because of inhomogeneous resistance and tectonism of Chuanzhong-Hannan block in west and Huangling uplift in east, the arc-shaped structure with southwestward eccentricity of Bashan main arcuate fault formed, accompanied lateral extrusion escape of compositions within orogenic belt to both east and west. Sedimentology and chronology research on south Dabashan tectonic belt revealed that late Jurassic to early Cretaceous (J3-K1) was very tectonic evolution stage of south Dabashan, and many characteristics of the tectonic stage were just response to intracontinental orogen that north Dabashan huge nappe structure thrusted and overlaid onto north margin of Yangtze block in middle-late Yanshanian. Nanhuan system became main detachment layer in middle-upper part of north Dabashan huge nappe structure when it thrusted onto Yangtze block on a large scale in middle-late Yanshannian.(4) North Dabashan and south Dabashan thrust nappe structure integrated into Dabashan double layer huge thrust nappe structure system, which were on the basis of originally relevant different early-stage tectonics, respectively. There existed apparent differences in involved deformation between the northern and southern south Dabashan. The northern belt developed strong thrust tectonics, the newest involved rock was middle Triassic, and received terrestrial foreland deposits of T3-J1-2which were overlaid by terrestrial rifted deposits of J1-2with unconformity. The similarity of structural deformation between terrestrial foreland deposits of T3-J1-2and terrestrial rifted deposits of J1-2, and correlatability between northern belt of south Dabashan and north Dabashan, jointly reflected that the northern belt of south Dabashan was overlaid by Meso-cenozoic thrust nappe structure on the basis of foreland thrust fold structures of original Mianlue palaeo-suture zone. The foreland fold belt of southern belt of south Dabashan developed continuously transitional deposits of T3-J-K from foreland basin to Yangtze craton basin, and the rocks of T3-J-K1were involved into deformation, and the southern belt became propagation front deformation belt when Bashan huge thrust nappe structure thrusted southwestward.