长江中下游地区中生代陆内构造作用与成因分析

长江中下游地区多期构造叠加、复合联合作用明显,中生代以来地壳运动频繁、构造活动强烈,地表为桐柏-大别造山带,九岭-江南隆起带,其间为对冲样式构造组合的长江复合构造带。大别造山带前陆深地震反射剖面揭示长江中下游深部为“双鳄鱼构造”状态,大量前陆断褶带的物质被掩盖在大别造山带之下,岩片叠置、断裂交错的镶嵌构造极为发育,下地壳切过莫霍面向北俯冲,是后期岩浆的活动通道和就位空间。综合地球物理剖面揭示下扬子地区南北岩石圈结构差异明显,上地壳南北对冲构造发育;下地壳为向北倾斜的构造带,北部仍向南逆冲,南部切过莫霍面向北俯冲。长江中下游地区上述构造的形成受控于长江陆内异化带的作用,该带是一条陆内岩石圈规模的向北倾斜的破裂带,发生在中国大陆形成之后,不受早期板块界线的控制,是岩石圈规模的大陆异化、物质重建、结构重组的产物;主要结构特征为上地壳对冲式逆断层组合,中地壳为水平流变层,下地壳切过莫霍面由南向北俯冲;形成过程是印支晚期-燕山早期中国陆内近南北向挤压,地壳破碎,岩石圈加厚;燕山晚期大规模伸展拉张,岩浆活动,壳幔混熔。特殊成因机制和演化过程的综合效应是长江陆内异化带的地质内涵。深部物质沿长江陆内异化带上升,在上部地壳内多期叠加构成的构造格架中就位,岩体形态在深部近东西向呈带状,中部状态复杂,浅表受控于多种构造组合形式,见空即灌。

Mesozoic intracontinental tectonism and its genesis analysis of the middle-lower reaches of the Yangtze River

The middle-lower reaches of the Yangtze River has a characteristic of multiphase structural superposition and combination. It has experienced frequent crustal movement and intense tectonic activity since Mesozoic. It manifests as Tongbai-Dabie orogenic belt, Jiuling-Jiangnan uplift and Yangtze River compound structural belt. The deep structures of the middle-lower reaches of the Yangtze River show as a double "crocodile structure" which is revealed by Dabie orogenic belt foreland deep seismic reflection profile. Many material of foreland fault fold belt are covered by the Dabie orogenic belt. Mosaic structures are well developed as a result of tectonic slices and fault superimposed. The lower crust cut the Moho surface and subducted toward north and make this place be the channel of upwelling magma. The lithospheric structures are obviously different between the south and north Lower Yangtze area which is revealed by the comprehensive geophysical profiles. The face to face vergencing structures are well developed in the upper crust. The lower crust is a north-dipping tectonic zone and north part southward thrusting, south curst cut the Moho surface and northward subduction. The structures of the Yangtze River region are controlled by the intracontinental alienation belt of the Yangtze River. This belt is a north-dipping crushed zone in lithospheric scale, which is formed after the formation of China continent, but uncontrolled by early plate boundaries. It is the production of physical reconstruction and structure reorganization. The major structural features of Yangtze River compound structural belt are showed as face to face vergencing fault assemblages in the upper crust, the horizontal rheology in middle crust and north-dipping tectonic zone cut the Moho face and northward subduction in the lower crust. It is result of SN-trending compressive stress in Late Indosinian-Early Yanshanian, crust broken and thickened lithosphere, then magmatic activities and crust-mantle immiscible occurred under the extensional structural setting in Late Yanshanian. The combined effect of special genetic mechanism and the evolution is the geological meaning of the alienation band in Yangtze River intracontinent. Deep-seated material is upwelling along the alienation band and in place of the tectonic framework which was formed by the multiphase structural superposition in the upper crust. Rock in the deep crust is EW-trending, that in the middle is complex, while close to the surface is determined by a variety of structural combinations.