长江中下游南部逆冲变形样式及其机制

The styles and dynamics of thrust in the south of the Middle-Lower Yangtze River area,China

长江中下游长江以南地区发育燕山早期(J1-J2)逆冲推覆事件,形成的断裂-褶皱构造在纵向和横向上都表现出了鲜明的特征。纵向上,逆冲变形具有分带性,本文将长江以南的中下扬子逆冲变形划分成根部带、中部带、前锋带三个次级分带,它们的运动学特征是由南向北的逆冲,变形从根部带的厚皮构造变为中部带、前锋带的薄皮构造。野外考察和地球物理资料揭示,根部带断裂陡,叠瓦式逆断层为主,向下收敛于基底和盖层之间的深部滑脱层;中部带逆冲断裂具叠瓦式和双冲式特征,断裂产状相对较缓,深部滑脱层较根部带变浅,浅部的志留系滑脱层也十分发育,带内构造不对称性不明显,褶皱样式Ω为""和"Ω"型为主;前锋带断裂组合为叠瓦状,发育大量反冲断层及倒转褶皱,其滑脱层主要是志留系或三叠系内部的软弱层。横向上,逆冲变形具有不均一性,体现在中下扬子的差异变形上:中扬子逆冲构造在中部带、前锋带卷入变形主体地层为志留系-三叠系,而下扬子中部带和前锋带却大量出露震旦-寒武系;中扬子在前锋带主要为向北倒转的不对称褶皱,前锋带Ω到根部带都是隔槽式褶皱组合,而下扬子逆冲构造前锋带为""和"Ω"式褶皱,前锋带、中部带、根部带分别为上古生界-三叠系体现的隔档式、下古生界体现的隔档式、隔槽式褶皱组合。另外,中、下扬子深部滑脱层的深度也有显著差别。在变形的动力学上,中、下扬子燕山早期逆冲作用同古太平洋板块向东亚大陆下俯冲有关。中、下扬子逆冲变形的差异性表现可能是由于J3-K1后期赣江断裂走滑逆冲造成的不均一的隆升导致的。赣江断裂以东的下扬子逆冲抬升剥蚀严重,导致深部层次构造剥露,而中扬子保留了J1-J2时期逆冲构造的浅部构造原始样式。这种模式也符合在J3-K1东亚属于安第斯型大陆边缘的观点。

The study on the tectonics in south of the Middle-Lower Yangtze River area has confirmed that there occurred one thrusting in the Early Yanshanian （J1-J2）. The folds and faults are characterized by some transitions perpendicular to or parallel to strike of the structural belt. Perpendicular to the strike, the thrust motion senses records of the top-to-north thrusting, resulting in root belt, central belt and front belt from SE to NW. The thrust styles from the root belt to the front belt vary from thick-skinned tectonics to thin-skinned tectonics. The root belt develops a lot of imbricate thrust faults with steep-dipping angles, which converge to deep-seated regional detachment fault between the cover and the basement. However, in the central belt, faults styles mainly consist of imbricate or duplex thrusts with lower dip angles and the deep-seated regional detachment layer shallower than that of the root belt, including a Silurian-involved detachment. The thrust tectonics of the central belt has no remarkable asymmetry, developing a ＂ ＂-like or ＂Ω＂- like style of folds. In the front belt, there are a series of imbricate faults accompanying with abundant opposite-directed faults and overturned folds, and the major thrusting detachment lies in the weak layer of Silurian and Triassic. Parallel to the strike, the thrusting is different between in the Middle Yangtze River area and in the Lower Yangtze River area. In the central and front belts of the Middle Yangtze River area, Silurian to Triassic strata are involved in folding and faulting, while in the middle and front belt of the Lower Yangtze River area, abundant Sinian to Cambrian strata are mainly involved in thrusting. The front belt mainly develops some asymmetric overturned folds in the Middle Yangtze River area. However, the corresponding tectonic zone in the Lower Yangtze River area develops ＂ ＂-like and ＂Ω＂-like of folds. In the Middle Yangtze River area, the fold styles display chevron synclines from the root to the front belt. While in the Lower Yangtze River area, the fold styles from the middle to the front belt are chevron anticlines rather than chevron synclines. In addition, the depths of thrusting detachment are obvious different between in the Middle Yangtze River area and in the Lower Yangtze River area. As for the deformation dynamics, the thrusting of the Middle and Lower Yangtze River area in the Early Yanshanian is related with the subduetion of Paleo-Pacific Plate to the East-Asian Continent. The differences of thrust styles may be a contribution of the differential J3-K1 uplift processes resulting from the Ganjiang Strike-slip Fault. To the east of the Ganjiang Fault, the Lower Yangtze River area uplifted and eroded to result in denudation of deep-level thrust tectonics. While to the west of the Ganjiang Fault, the thrust tectonics remain original appearance of shallow-level structures. It shows that the Gangjiang Fault is a significant boundary. This model is consistent with a viewpoint that the East Asia area belongs to an Andeans-type continental margin during J3-K1.