大巴山西北缘浅层次叠加褶皱变形分析:以镇巴县简池地区为例

在大巴山西北侧镇巴县简池地区开展1∶10 000的地质填图和构造解析工作，重点研究露头和区域尺度上叠加褶皱变形的时空变化、成因，确定褶皱的构造属性及变形时限。研究表明未拆离的中上三叠统-中侏罗统沉积岩系中发育两组褶皱：(1)北东近东西向褶皱(F1)，成组、分区断续相连，线性展布发育，代表了区域米仓山主背斜较陡倾南翼上的次级大型褶皱的枢纽带；(2)北西北北西向褶皱(F2)，区域呈弧形展布，发育隔挡式褶皱组合型式，构成大巴山前陆坳陷带东部边缘的复式向斜。北西北北西向褶皱向西横跨在北东近东西向褶皱之上，形成露头尺度上的2类4种基本样式，发育大角度叠加交切的两组褶皱弯滑擦痕。北东近东西向褶皱减弱消失在同造山的上三叠统-中侏罗统(Ts1Ts4岩性段)中，上被中侏罗世晚期Ts5与Ts6岩性段包络覆盖，属中生代南秦岭碰撞造山相关的前陆生长褶皱，时限约为213~178 Ma，与米仓山构造形成晚期阶段的指向南的非共轴剪切变形有关。北西北北西向褶皱将研究区的中生代及之前岩系普遍卷入了变形，属晚中生代大巴山陆内造山带的前陆构造褶皱，时限约为160~120 Ma，区域褶皱变形长期保持稳定的总体近似纯剪的应变状态。尽管两期挤压收缩褶皱事件的时间间隔不长，但两组褶皱的样式、形成时间、构造属性与形成机制都存在巨大差异，表明区域构造环境和地壳变形机制的重大变动和转换。

Superposed buckle folding at the upper structural levels in western Dabashan:example from the Jianchi area in Zhenba County

Following the 1∶10000 geological mapping and structural analysis, we focused chiefly in this paper on the spatial variations and controlling factors for the superposed fold types, and determined the tectonic nature and the age of folds in the Jianchi area of western Dabashan. Our study indicated the occurrence of two sets of tectonic folds involving undetached Triassic to Jurassic rocks. The types of folds are: (1) NE-EW trending folds (F1). They occur as interrupted NE-EW fold domains composed of outcrop or map-scale fold groups, representing the pre-existing hinge zones of parasitic macrofolds formed regionally on the steeply inclined southern limb of the Micangshan main antiform. (2) NW-NNW trending folds (F2). The F2 folds are curved folds that coincide with the Daba arc and are dominated by a unique structural style of wide spaced anticlines to form a syclinorium at the eastern margin of a foreland depression belt. The west-verging NW-NNW trending folds overprint the earlier NNE-EW ones almost orthogonally to produce two types and four modes of outcrop scale superposition, where two sets of wide-angle superposed flexural-slip striations were observed. The NE-EW folds vanish into Upper-Middle Jurassic syntectonic deposits (Ts1Ts4) and are enveloped by late Middle Jurassic sequences (Ts5 and Ts6). They are interpreted as foreland growth folds formed in association with Late Triassic collision of South Qinling orogeny around 213178 Ma, and occurred as a result of southward-directed progressive non-coaxial shearing during the late-stage development of the Micangshan structure belt. However, the NW-NNW trending folds, which affected the complete Mesozoic and pre-mesozoic rock series, developed as a consequence of the Dabashan foreland deformation during a intraplate orogenic episode (160120 Ma). And the tectonic controlling factor for the folds was to maintain a long-term stable strain field of bulk pure shear. Although the hiatus (about 10 Ma) between the two compressive folding events was brief, the superposed fold sets of different ages were markedly different in styles, tectonic nature and formation mechanism, suggesting a significant change in tectonic driver for the two sets of folds.