龙门山南段邛西断层转折褶皱磁组构及其有限应变

龙门山南段位于四川盆地以西，其新生代构造变形特征对于认识青藏高原东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。在龙门山南段邛西断层转折褶皱不同构造部位选取48个采样点开展磁组构研究，分析断层转折褶皱的有限应变特征及区域构造变形机制。实验结果表明，邛西地区上白垩统中主要载磁矿物为高矫顽力的赤铁矿，背斜整体应变较弱，且存在3种类型的磁组构，以沉积磁组构和初始变形磁组构为主，铅笔状磁组构少见，主要存在于靠近褶皱中段的前翼部位，说明断层转折褶皱前翼较后翼和核部应变强，且中段地层应变较其他部位更为强烈。此外，各采样点磁线理的优势方位为近南北向（N10°E），表明邛西断层转折褶皱的形成与龙门山南段晚新生代近东西向的地壳水平缩短有关，暗示龙门山南段的最大主压应力方向在晚新生代存在转变的可能。

Magnetic fabric and finite strain analysis of the Qiongxi fault-bend fold in the ;southern Longmen mountain

The characteristics of Cenozoic deformation in the southern Longmen Mountain fold-thrust belt, which is located to the west of Sichuan basin, is important in recognizing the deformation regime of the eastern margin of the Tibetan Plateau. Magnetic fab-ric is a sensitive strain indicator particularly useful in weak deformed sedimentary areas. In this paper, a magnetic fabric investigation was carried out into the Qiongxi fault-bend fold, which is located to the south of Longmen Mountain fold-thrust belt on the basis of 48 sampling sites scattered in different parts so as to analyze the finite strain of the bending fold and discuss the regional deformation regime. The results show that hematite is the main magnetic carrier mineral in the upper Cretaceous of Qiongxi area, and the fold suf-fered weak strain;in addition, there exist three types of magnetic fabrics in the study area, and most of the sampling sites show sedi-mentary and initial deformation magnetic fabrics, with pencil structure magnetic fabrics only existent on the forelimb of the middle part of the fold. These phenomena indicate that the strain on the forelimb is relatively higher, and the strain on the layer of the mid-dle part is somewhat stronger than that of other parts of the fold-bend fold. In addition, the stereonet map of magnetic lineation shows that the orientation of the maximal susceptibility axial cluster is in nearly SN direction （N 10°E） , which indicates that the defor-mation of the bending fold of Qiongxi fault is related to the late Cenozoic EW-trending crustal shortening in the southern Longmen Mountain, and this may imply a change of the direction of the maximal stress in the southern Longmen Mountain in late Cenozoic.