北京南观地区构造组合特征及成因机制

北京西山地区广泛发育SE-NW向近乎同时期的逆冲推覆构造和伸展构造，为理解华北克拉通东部构造演化至关重要，而其构造组合特征、变形时代、形成机制和构造动力学背景却没有明确的认识.精细的野外构造研究表明北京西山的南观地区，发育逆冲推覆构造+变质核杂岩的构造组合，形成一种新的构造组合型式:“楔沉式”.“楔沉式”构造组合型式的特征:（1）产状相近的一套逆冲断层和正断层，共同构成上窄下宽的楔状褶皱-断层构造；（2）楔状体斜向向下楔入；（3）中间为比较新的地层（倒转向斜），两侧为老的地层.通过对卷入逆冲推覆构造呈“布丁状”透镜体产出的花岗岩脉和闪长玢岩脉进行锆石U-Pb年代学研究，其形成年龄分别为141±2 Ma和139±2 Ma.结合前人对侵入逆冲推覆构造的（南窖）石英二长岩岩枝的形成时代（128~136 Ma），认为南观逆冲推覆构造的形成时代为140~130 Ma，稍早于、或与房山变质核杂岩的形成时代近乎同时.逆冲推覆方向为320°~335°，与变质核杂岩代表的伸展构造的最大拉伸方向（SE-NW向）近乎一致.因此，北京西山地区（乃至华北克拉通东部）晚中生代逆冲构造-岩浆活动-伸展构造，这种独特的构造组合型式是构造体制从挤压向伸展转换的体现，也是华北克拉通破坏的构造体现，其形成机制可能与太平洋板块的俯冲紧密相关. 

Tectonic Pattern and Genetic Mechanism at Nanguan Area,West Hill of Beijing

A series of tectonic association of the thrust nappe structure and the extensional tectonics are widely developed in the West Hill of Beijing, which are very important for understanding the tectonic evolution of the eastern part of North China craton (NCC). However, their deformed age, tectonic dynamic background and genetic mechanism are poorly understood. The Nanguan area in the West Hill of Beijing is an ideal place to decipher such a tectonic pattern. The detailed field structure observations show a tectonic association of the thrust nappe structure and the metamorphic core complex structure in the Nanguan area, generating a new type of tectonic pattern: the downward diapiric tectonics. It has the following characteristics: (1) wedge-shaped fold-fault structure with a normal fault along one side of the fold and a thrust fault along another side; (2) the fold (overturned syncline) is centered and both sides are older strata or basements, and (3) downward diapiric structure of the wedge-shaped body. Pudding-shaped lens of granitic and dioritic dykes were found in the thrust nappe. The granitic and dioritic lens give two weighted zircon U-Pb age of 141±2 Ma and 139±2 Ma, respectively. The previous studies suggest that a quartz monozite, which intruded into the thrust nappe, has zircon U-Pb age of 128-136 Ma. Thus, the deformation time of the thrust nappe is between ca. 140 and 130 Ma, which is a little earlier than or nearly coeval with the extensional tectonics represented by the metamorphic core complex structure. The orientation of the thrust nappe structure was generally W320°-335°N. The principal stress orientation of the extensional tectonics was also SE-NW trending. Thus, the tectonic pattern of compression-magmatism-extension in the West Hill of Beijing (and even in the eastern part of NCC) might be closely related to SE-NW-trending subduction of the Pacific plate. The tectonic pattern also might be an expression to the destroy or lithospheric thinning of the NCC.