燕山西段及北京西山晚中生代逆冲构造格局及其地质意义

Late Mesozoic thrust tectonics framework in the western part of the Yanshan orogenic belt and the Western Hills of Beijing:characteristics and significance.

燕山西段及北京西山晚中生代逆冲构造集中分布于三个NE向带状区域中，三个带状区域的间隔约为60km，延伸长度自东向西依次减小，呈现出明显的逆冲构造发育的三角形区域。三角形区域的北界为“内蒙地轴”南缘断裂西段，南西界与中元古代早期古盆地构造边界一致，东南部边界则与华北克拉通基底新太古代-古元古代中部碰撞造山带的东部边界大致吻合。逆冲构造具有基底卷入的厚皮构造与盖层内部的薄皮构造共存的构造属性，上盘运动方向总体指向NW，逆冲构造变形主要发生在140～130Ma。逆冲后伸展构造变形以发育在主要逆冲构造后侧为主，并利用先存构造薄弱带。先存构造薄弱带在有利区域构造应力和其他影响因素的作用下导致的构造活化，可能是燕山板内构造变形的重要机制之一。主要逆冲变形前后均有大规模岩浆活动的构造-岩浆时空组合表明，收缩构造造成地壳加厚及由此引发的深部地壳重熔，难以作为统一的机制对这些特征进行合理阐释，需要有其他方式的深部热物质与能量的参与。北京西山霞云岭-长操、教军场-大安山以及马兰-胡林等逆冲断层，是一个统一的大规模的逆冲构造的不同组成部分，具典型、连续的断坪-断坡结构，它形成于髫髻山组（148～146Ma）之后、南窖闪长岩（128Ma）侵入之前，而不是“印支期（或更早）”，它与南大寨-八宝山逆冲构造构成北京西山晚中生代逆冲构造格局。区域性的NW-SE向收缩构造作用及南大寨-八宝山逆冲构造上覆岩席的构造加载，可能是北京西山的蓝晶石带和硬绿泥石带为代表的高压动力变质作用的基本构造原因。

Late Mesozoic thrust faults in western Yanshan orogenic belt and the Western Hills of Beijing are distributed in three northeasterly trending zones, about 60 km apart from each other. The length of these three zones decreases from east to west, and as a result, they are distributed in a triangle area in map view. The western segment of the southern margin of the previously named ＂Inner Mongolia Inlier＂, served as the north edge of the triangle area. The southwestern side of the triangle coincided with the margin of an early Mesoproterozoic sedimentary basin. The southeastern boundary of the triangle, can be matched to the eastern contact between the later Archean Trans North China Orogen and the east block of North China craton. Thrust tectonics in this area are characterized by the combination of basement-involved thick-skinned structures and only cover sequence-involved thin-skinned structures, with top-to-the-northwest thrusting during 140-130Ma. Post thrusting extensional deformation developed mainly behind the thrust fronts, and most of the extensional faults reactivated and followed the existing thrust faults. Based on the observations aforementioned, we infer that the reactivation of tectonic weak zones under a preferential tectonic regime might serve as one of the important mechanisms during intraplate deformation in Yanshan belt. Large scale magmatism both before and after the major thrusting deformation has been identified in the research area. This temporal relationship between contractional deformation and magmatism in the area can not be explained reasonably only by the partial melting of the lower crust of contraction-induced thickened crust, but further thermal materials and energy probably derived from deeper asthenosphere were needed. It has been proposed that a unified thrust fault system existed in Western Hills of Beijing containing the Xiayunling-Changcao,Jiaojunchang-Da＇anshan and Malan-Hulin thrusts. The thrust faults that were once thought to be isolated with respect to each other are in fact, the different flats and ramps of a unified thrust fault system, It was formed after the Tiaojishan formation（146-148 Ma） and before the intrusion of Nanjiao diorite（128 Ma）, instead of the ＂Indosinian period （before 207M a） or earlier＂ . This newly identified unified thrust fault system together with the Nandazhai-Babaoshan thrust system constituted the later Mesozoic thrust framework, The tectonic loading of the thrust sheet above the Nandazhai-Babaoshan thrust with an inferred original front about 10 krn west to its present traces, instead of the Xiayunling thrust and associated nappes, could serve as an alternative tectonic model for dynamic metamorphism characterized by the formation of kyanites and chloritoids.