The structural and stratigraphic evolution of the Neogene Kashi foreland basin, northwest China

Growth of the Tian Shan in central Asia has occurred since at least the early Miocene as a product of the Indo-Asian continental collision. The strata within the Kashi foreland along the Tian Shan's southern flank reveal depositional environments and tectonic changes during range growth. Until now, however, the magnitude, timing, pace, and style of deposition and deformation have been largely unconstrained. This study combines 6500 km2 of geologic mapping with stratigraphic analysis, seismic sections, and magnetostratigraphy to define the depositional history, structural disruption, and shortening of the Kashi foreland over the past 20 Ma. First, new magnetostratigraphies spanning ∼13 km of strata from 11 locations are correlated with the geomagnetic polarity timescale to provide precise ages for the entire suite of foreland strata. Neogene deposition initiated at ∼19 Ma in the proximal Kashi foreland and continues to the present. Capping the Tertiary stratigraphy, the widespread Xiyu Conglomerate represents a southward-prograding, coarse-grained wedge with newly defined basal ages ranging from 15.5 Ma to <1.0 Ma. Second, the structural disruption and shortening of the foreland is analyzed. Younger, more southerly structures are detachment folds where Neogene strata are thickest. In contrast, northerly structures are dominated by "thick-skinned" fault ramps that cut previously deformed Paleozoic rocks. Thus, deposition of relatively weak, subhorizontal strata led to favorable conditions for detachments to form in the near surface (2-6 km), and a progressive change from thick- to thin-skinned structures. Surface observations combined with seismic sections underpin new balanced sections that define the style and magnitude of shortening. An overall west-to-east decrease in Tertiary shortening is consistent with the gradient predicted by clockwise rotation of the Tarim basin. Magnetostratigraphic ages of growth strata and thermochronology constrain the timing and rates of deformation on specific faults and folds. Overall, deformation has migrated south since 19 Ma, but minimum rates of shortening have varied 5-fold within the foreland. Shortening rates over the last 1-2 Ma are less than one-half the ∼8 mm/yr geodetically determined rates. Variable deformation rates imply that the loci of Neogene deformation may have migrated into and out of the basin.