Analysis of the fold-thrust zone in the southern Junggar Basin, northwestern China

Four topics related to the fold-thrust zone in the southern Junggar basin are discussed in this paper. (1) Based on structural characteristic, the fold-thrust zone in the southern Junggar basin is divided into several parts longitudinally, latitudinally and sectionally.Firstly, along the longitudinal direction (or from south to north), the fold-thrust zone is classified into three belts, i.e. the overthrust belt, the basement-involved belt and the decollement belt. Secondly, along the latitudinal direction (or from west to east), the basement-involved fold-thrust belt can be divided into five sections, such that backthrust'displacements increase and thrust detach surfaces deepen gradually from west to east. The decollement belt is separated into western and eastern sections by the Hongche fault, and has the structural characteristics that the eastern section had greater deformation and produced large backthrusts with fault planes oriented towards the hinterland. Finally, in cross- sectional profile, the western portion of the decollement belt preserves double-deck structures, whereas the eastern holds three-deck structures. (2) In terms of fold-forming mechanisms, the thrust-folds in the studied area are classified into three categories-the basement-involved, the decollement and the combination thrust-fold. Among these three categories, basement-involved thrust-folds have the main characteristics that folding occurred prior to thrusting and the decollement thrust-folds have the common feature that folding accompanied thrusting. (3) Transverse and strike-parallel accommodation zones are present in the studied area. The transverse accommodation zones generally formed within the basement-involved fold-thrust belt and its styles are mainly sinistral strike-slip faults. In contrast, the strike-parallel accommodation zones are developed mostly within the decollement fold-thrust belt and have the characteristics that the distribution and the displacement transportation of their thrusts both are relaying. (4) The controlling factors of the fold-thrust zone in the southern Junggar basin are compression with N-S direction,structural deformation occurred at the end of the Pliocene to the early Pleistocene and decollement related to clay with abnormal pressure during the Paleogene, the lower Cretaceous and the lower-Middle Jurassic.