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Seismic and well studies have shown that thrust faults, high-angle reverse faults, and transpressive faults are three typical styles of syndepositional faults that developed in the Quaidam basin during its Tertiary compressive and transpressive evolution. These faults are characterized by intervals that are thicker within the footwall than within the hanging wall, as well as by clear differences in lithology of contemporary sediments across the faults.
Syndepositional thrust faults with listric geometry generally act as the leading front of the thrust belt and control the wedge-shaped geometry of basin or compressive fault-depression thickening toward the fault. The hanging-wall folds were formed by horizontal shortening after the main faulting period, during which the upthrown side of the fault was tilted upslope due to the faulting along listric plane. Syndepositional reverse faults are planar, having steeper (60-80°) planes rooting into basement; the strata on the upthrown and downthrown blocks generally are flat. The syndepositional transpressive faults have either a steep-dipping, more planar fault plane or a plane steepening downward. Transpressive deformation after the faulting created the hanging-wall folds.
Wedge-shaped sediment that thickens toward the fault, flat sediment, or wedge-shaped sediment that thins toward the fault are three typical geometries of compressive fault depressions, which are likely to be bordered by syndepositional thrust faults, reverse faults, and transpressive faults, respectively. Lithologically, the sides of the syndepositional faults of the inner Quaidam basin have coarser sediments on the hanging wall than on the footwall. The eroded hanging wall of the marginal fault supplied the sediments for the footwall. These sediments change quickly basinward from coarse clastic deposits into finer grained deep-water sediments.
In the Quaidam basin, the hanging-wall anticlines have proven to be good hydrocarbon traps. The syndepositional faults can act as a migrating path connecting the reservoirs on the hanging wall and the source rocks on the footwall.
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