龙门山南段山前天全-乐山剖面磁组构研究及其对新生代构造变形的指示意义

四川盆地西南部西侧为龙门山冲断带,南面紧挨川西南褶皱带,其新生代构造变形特征对于认识青藏高原东南缘的变形机制具有一定的指示意义.磁组构是一种灵敏的应变指示计,在变形微弱的沉积岩地区尤为适用.本文在雅安-乐山剖面选取12个采样点进行磁组构分析,结合已有的天全-雅安飞仙关剖面的27个采样点数据,综合讨论川西南地区的构造变形特征.所有采样点的磁组构测试结果显示出3种弱变形的磁组构类型:沉积磁组构、初始变形磁组构和铅笔状磁组构.雅安-乐山剖面采样点的磁线理绝大部分为北东-南西走向,和龙门山南段的整体延伸方向一致,表明四川盆地西南缘新生代构造变形主要受控于龙门山的构造作用.飞仙关剖面的磁组构测试结果显示44%的采样点表现出磁线理和地层走向斜交的特征,由初始变形磁组构演变而来,并且所有异常磁组构仅局限在断层上盘,本文认为这是雅安地区新生代期间局部逆时针旋转引起变形叠加的结果.

Magnetic fabric investigation in Tianquan-Leshan section in front of Longmenshan fold-thrust belt and its indicative significance for the Cenozoic deformation

The characteristic of Cenozoic deformation of the southwestern Sichuan basin, which is located in the east of Longmenshan fold-thrust belt and north of southwest Sichuan fold belt, is important in recognizing the deformation regime of the southeastern margin of Tibetan plateau. Magnetic fabric is a sensitive strain indicator, and is particularly useful in weak deformed sedimentary region. An integrated magnetic fabric investigation, combined with 12 sites in the Ya'an-Leshan section and 27 sites in the Feixianguan section, was carried out to discuss the Cenozoic deformation characteristic of southwestern Sichuan basin. Three weak deformed magnetic fabrics were observed in the study area: sedimentary magnetic fabric, initial deformation magnetic fabric and pencil structure magnetic fabric. The direction of most of magnetic lineation in the Ya'an-Leshan section is NE-SW, which is consistent with the extension direction of the southern Longmenshan. It suggests that Cenozoic deformation of the southwestern Sichuan basin is mainly controlled by the Longmenshan fold-thrust belt. It revealed one type of atypical magnetic fabric in about 44% sites located in the hanging wall of Feixianguan fault. The atypical magnetic fabric, characterized by a magnetic lineation that is oblique to the bedding strike, is evolved from initial deformation magnetic fabric. And it was the result of local counterclockwise rotation occurred in Cenozoic.