关键词: 纵弯叠加, 构造解析, 构造区, 褶皱属性, 前陆生长褶皱, 简池地区

Abstract: 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.

Key words: buckle superposition, structural analyses, structure domain, fold attribute, foreland growth fold, Jianchi area