燕山构造带西段宣化地区中—晚侏罗世构造变形过程

The Yanshan belt experienced multi-phase deformations in the Mesozoic. The Yanshan movement is usually divided into two contractional episodes, Episode A and Episode B. A general agreement has been reached on the timing of Episode B of the Yanshan movement (134-136 Ma), but debates remain about age and deformational processes of Episode A. The Yanshan movement was proposed on account of existence of an unconformity beneath the Tiaojishan Formation in the Xishan area in the western segment of the Yanshan belt, which was considered as the result of weak crustal shortening (Episode A of the Yanshan movement). Our recent reconnaissance of the Xuanhua area north of the Xishan led to a discovery of an angular unconformity that separate Jiulongshan Formation from the underlying intensely deformed Xiahuayuan Formation, thereby conforming the occurrence of Episode A intensively shortening in the western Yanshan belt. This project aims to put constraints on deformation age and structural features of the Episode A tectonic event based on structural analysis of the Xiahuayuan Formation and age determinations of strata beneath or above the unconformity. The results will be of significance for understanding deformational processes of the western segment of the Yanshan belt in Middle-Late Jurassic.

燕山构造带在中生代经历了多期挤压变形。根据中生代火山－沉积层序中两个区域性角度不整合面，燕山运动通常被划分为A幕和B幕两次挤压构造事件。对燕山运动B幕发生的时代已取得比较一致的认识（134~136 Ma），但对燕山运动A幕发生的时间和挤压变形过程还存在争议。燕山运动的概念首先是根据燕山构造带西段北京西山地区髫髻山组之下出现不整合面而提出的，早期认为燕山运动A幕变形强度很弱。然而，我们最近对宣化地区进行了多次野外考察，发现九龙山组与下伏强烈变形的下花园组之间存在明显的角度不整合，从而确认了燕山运动A幕为一次强烈挤压变形事件。本项目拟通过野外构造解析和同位素年代学研究，确定燕山运动A幕的变形特征并准确限定变形时代。研究成果对恢复燕山构造带西段中-晚侏罗世变形过程具有重要意义。

中生代期间，华北克拉通北缘存在多次地层挤压缩短事件，但对各个事件发生的时间、强度和原因尚无共识。本项目对侏罗纪的第一期缩短事件即燕山运动A幕进行了详细研究。野外调查发现，上侏罗世地层与下伏强烈褶皱地层之间存在一些明显的角度不整合，这表明燕山运动A幕实际上是一次强烈的收缩事件。我们通过构造分析恢复了燕山构造带侏罗纪变形过程，燕山构造带不同地区平行不整合和角度不整合并存以及在空间上的变化，这种变化受控于盆地基底的初始倾角以及地层厚度的横向变化。研究发现燕山运动A幕地壳变形强度向北西方向减弱。根据不整合上下地层的同位素年龄，将燕山运动A幕变形时间精确约束在170～165Ma较短的时间内，并推测这期变形是在华北克拉他北缘同时发生的，变形的动力来源于东南方向，古太平洋板块的低角度俯冲造成了燕山运动的发生。