长江中下游燕山期逆冲推覆构造及成因机制

Yanshanian fold-thrust tectonics and dynamics in the Middle-Lower Yangtze River area,China

长江中下游地区为我国著名铜、铁多金属成矿带之一。本文根据长江中下游及邻近地区构造等特征,将该区构造单元自北向南划分为华北地块、大别造山带、长江中下游前陆构造带、扬子地块、华夏地块;并进一步将长江中下游前陆构造带细分为保康-武汉-宿松-巢湖褶冲带、长江中下游中生代坳陷带、通山-瑞昌-石台-宁国褶冲带三个次级构造带。在燕山早期,长江以北的保康-武汉-宿松-巢湖褶冲带逆冲构造极性指向SE,而长江以南的通山-瑞昌-石台-宁国褶冲带逆冲构造极性指向NW。长江以南褶皱样式在岳阳-通山-瑞昌一线以南由隔挡式变为隔槽式,叠瓦式逆冲断裂更发育。在九岭-幕阜山隆起及南部的白垩纪红色盆地基底中逆冲断裂多为高角度,褶皱多为隔槽式,元古宇的浅变质岩卷入逆冲作用,为典型的厚皮构造。长江以北的紧闭同斜褶皱主体形成在印支期,随后被早燕山期的逆冲推覆作用改造。结合野外地质调查,通过对已有跨长江中下游地区的深地震剖面重新解释,发现以长江为界,长江中下游地区北侧深部、浅部构造处于耦合状态;而南侧深部、浅部构造已经脱藕,形成上下地壳的"鱼骨刺"结构,深部构造可能是是印支期扬子地块向华北地块下俯冲的残余结构。长江中下游地区浅部从北向南的逆冲作用应该与大别造山带超高压变质岩挤出有关,而从东南向北西的逆冲推覆作用可能同中侏罗世古太平洋板块向亚洲大陆俯冲有关。

The Middle-Lower Yangtze River area is one of famous copper, iron and polymetallie ore metallogenic belts in China. In this paper, the Middle-Lower Yangtze River area is divided into the five tectonic units： the North China Block, the Dabie Orogenic Belt, the Yangtze Block, the Cathaysia Block, the Middle-Lower Yangtze River Foreland Belt. Furthermore, the Middle-Lower Yangtze River Foreland Belt is subdividied into three subunits： the Baokang-Wuhan-Susong-Chaohu Fold-and-thrust Belt, the Daye-Huaining- Wuhu Mesozoic Depression, the Tongshan-Ruichang-Ningguo Fold-and-thrust Belt. In the Early Yanshanian, the Wuhan-Susong- Chaohu Fold-and-thrust Belt north of the Yangtze River underwent the SE-directed thrusting; while the Rnichang-Ningguo Fold-and- thrust Belt south of the Yangtze River experienced the NW-directed thrusting. South of the Yueyang-Puji-Xianning-Yangxin, the fold styles from north to south varies from chevron anticlines to chevron synclines, and the imbricate thrust faults to the south are more developed. The Jiuling-Mufushan Basement and the basements of the Cretaceous red basins developed many high-angle thrust faults and chevron synclincs. The Proterozoic metamorphic rocks are involved in this thrusting, showing a typical thick-skinned structure. In the north of the Yangtze River, the closed asymmetric folds are mostly developed in the Indosinian. The Early Yanshanian thrusting modified the pre-exsiting structures. The deep and shallow structures north of the Yangtze River are coupling. However, the deep and shallow structures south of the Yangtze River are decoupling. The architecture of the upper and lower crust looks like ＂fish-spur＂ over the Middle-Lower Yangtze River. These deep structures should be the residual effect of the Indosinian subduction of the Yangtze Block to the North China Block. In the Middle-Lower Yangtze River, the south-directed thrusting should be related to the slow extrusion of the UHP metamorphic rocks in the Dabie Orogenic Belt. However, the northwest-directed thrusting may be related with the Middle Jurassic subduction of the Paleo-Pacific Plate to the Asian Continent.