Cenozoic Salt Structures In The Western Kuqa Depression, Southern Tianshan:Structural Analysis And Physical Modeling

The Western Kuqa depression (WKD) located in the northern Tarim Basin and developed the Cenozoic thick-skinned contractional salt structures. In the overburden, various salt structural styles were formed, particularly salt diapirs, which are unique in the western China. Subsalt basement deformed strongly and several giant gas-fields have discovered. The study of the Cenozoic salt tectonics in the WKD is the key issue both for structural geology research and for oil-and-gas exploration.In this thesis, we combined field observation, well data and high-quality reflection seismic profiles to constrain the distribution and styles of salt structures, evolution of salt structures in the WKD. Based on physical modeling and previous studies, the mechanisms of salt structures were discussed. The main conclusions are summarized as follows:1. Physical modeling confirmed that only the regional compression can't trigger salt diapirs, structural analysis suggested that salt diapirs in the WKD were established prior to regional compression and would be triggered by differential sedimentary loading in the overburden.2. Under regional compression, pre-shortening salt diapirs located the deformation in overburden and controlled the strike of both Kelasu and Qiulitage structural belts. In the Kelasu structural belts, Tuzimaza diapir formed above strongly deformed basement structures, we suggested its geometry was affected by patterns of salt diapir, shortening in overburden and differential basement-uplift:1) Both emergent diapir and buried diapir formed before strong regional compression commenced on WKD.2) The north limb of diapir was uplifted higher than south limb.3) Buried salt wall, salt stock and thrusted salt diapir were formed in the western section, middle section, and eastern section, respectively.4) The structural segmentation of Tuzimaza diapir is mainly formed since late Pliocene and controlled by shortening in the overburden. Quele salt dome was a thin-skinned salt structure in Qiulitage structural belt and its deformation was controlled by regional compression.3. Salt accumulated mainly in the Kelasu and Qiulitage structural belts, and between western and eastern depocenters in Baicheng sag, while the thickness of salt layer below two depocenters is very thin. We suggested that differential sedimentary loading played an important role in the WKD:1) under bulk shortening and differential sedimentary loading, salt flow out of depocenter in three-dimensional and2) salt flow dominantly along the direction of regional compression (N-S) but also obvious along strike (E-W).4. Salt structures formed in three growth stages from Oligocene to present:1) during Oligocene to early Miocene, salt withdrawal minibasin and passive diapirs formed;2) from late Miocene to early Pliocene, early folding developed under weak regional compression, meanwhile, salt diapirs continue to growth passively; and (3) since late Pliocene to present, large-scale minibasins, allochthonous salt sheet, salt nappe, salt pillow, and salt anticline were formed. This is the major period of folding, thrusting, and squeezing diapirs in the WKD.5. The major controlling factors of salt structures in the WKD include regional tectonic stress and its strength, differential sedimentary loading and pre-shortening diapirs.