川东北-大巴山盆山体系岩石圈结构及浅深变形耦合

Lithospsheric structure of northeastern SichuanDabashan basinrange system and topdeep deformation coupling

近年来,盆山体系研究已经成为大陆动力学的热点和前沿领域之一。为了进一步理解大巴山前陆的构造演化,解决该区油气勘探的关键问题,中国地质科学院与中国石化南方公司合作,2007年完成了一条300km长的深地震反射剖面。基于深地震反射剖面提供的岩石圈结构的几何图式和深地震测深剖面提供的速度数据,作者分析了川东北-大巴山盆山体系的岩石圈结构特征,探讨了变形样式与岩石圈结构的关系,提出了对大巴山造山带形成的新认识。川东北-大巴山盆山体系继承了扬子克拉通基底。较大的岩石圈厚度和强度,导致扬子克拉通在与华北克拉通拼合后的陆内造山过程中,将其收缩变形集中在其顶部而不是中下地壳,区域性的拆离层使盖层和基底解耦,结晶地壳保持弹性只出现大尺度的挠曲几乎没有横向缩短,故大巴山造山带表现为"薄皮"、"无根"的特征。大巴山造山带的席卷深度和变形样式主要受区域性的深部滑脱面控制,该滑脱面发育于寒武系底部泥岩层内,由TWT4.0s反射所指示。沿该滑脱面,城口断裂将南秦岭震旦系和古生界地层逆冲到浅表并向南西推覆60km叠置于四川盆地中-古生界地层之上;且镇巴断裂和城口断裂均收敛于该滑脱层,其下伏的变质岩层基本未卷入变形。

Recently, the research of basin-mountain system has become one of the hotspots of the frontier research fields in the study of continental geodynamics. For deeper understanding tectonic evolution of Dabashan foreland and to resolve key problem of petroleum exploration this area, a deep seismic reflection profile with a length of 300km was conducted in 2007 by cooperation of Chinese Academy of Geological Sciences （CAGS） and Southern Exploration and Development Division Company of SINOPEC （SEDDC）. Based on the geometry of lithosphere structure from deep seismic reflection profile and the velocity data from Deep Seismic Sounding profiles （DSS）, the authors have made an analysis on the feature of lithosphere structure of northeastern Sichuan-Dabashan basin-range system and some discusses on the relationship between deformation style and lithosphere structures, and then update understanding Dabashan orogen. The basement of Yangtze craton extends beneath whole northeastern Sichuan-Dabashan basin-range system. The larger thickness and strength of lithosphere result in Yangtze craton focus its contract deformation rather at the top of crust than middle-to-lower crust in the process of intracontinental orogeny after Yangtze spliced with North China craton. Due to regional deep decollement decoupled the sedimentary cover and basement, the crystalline rock crust keep rigidity and just behave in large scale bending hardly evident lateral shorting,so that the Dabashan orogen shows us feature of ‘thin-skinned structure’ and rootless. The deformation styles and involved depth of Dabashan orogen is dominated by a regional deep decollement existing within the base of mudstone rock layer of lower Cambrian system and marked by TWT 4.0 second reflectors. Along this decollement, the Chengkou fault thrust the strata Sinian System-Lower Paleozoic to shallow and surface moreover overthrust them about 60km to SW onto strata Mesozoic-Paleozoic of northeastern Sichuan basin. The Zhenba fault and Chengkou fault also get gently and vanished at it in deep. The underlying metamorphic rock layers are nearly not involved into deformation.