Temporal Constraints And Pulsed Late Cenozoic Deformation During The Structural Disruption Of The Active Kashi Foreland, Northwest China

摘要

In response to the ongoing Indo-Asian collision, structural deformation has encroached into the Tian Shan foreland in western China since the early Miocene. In order to reconstruct a detailed history of foreland deformation along the southern margin of the Tian Shan, we have synthesized extensive mapping, analysis of seismic sections, magnetostratigraphy of the foreland fill and associated growth strata, apatite fission track dating, changes in sediment accumulation rates, and geodetic surveys of deformed fluvial terraces. Across an area exceeding 6500 km~2 that spans the Kashi foreland in the northwestern Tarim Basin, we use these data to place temporal constraints on individual structures that delineate at least 4 distinct stages of deformation. Stage 1 involved initial uplift of hinterland structures beginning at 20-25 Ma. Stage 2 began at ～16.3 Ma when the basement-involved deformation front stepped south to the Kashi Basin Thrust that bounds the foredeep strata. Stage 3 occurred from 13.5 to 4 Ma, as the deformation front migrated south in episodic steps above thin-skinned detachment and ramp-flat folding. Finally, stage 4 began after 4 Ma with growth of the Keketamu Anticline followed by rapid southward migration of the deformation front to the Atushi and then Kashi detachment anticlines. On the basis of the deformed Tertiary strata and available seismic data, calculations of the minimum magnitude of shortening yield totals ranging from10 to 32 km since the early Miocene, of which 7-12 km occurred since ～4 Ma. Although average shortening rates since 16.3 Ma were 0.8-1.3 mm/a, rates were faster from 16.3 to 13.5 Ma (1.1-3.2 mm/a), after which they decreased to 0.5 mm/a between 13.5 and 4 Ma before increasing to 2.2-2.7 mm/a after 4 Ma. Similarly, the rate of southward propagation of the deformation front was initially 3-7 mm/a for the first 10 Ma of basin development, slowed to 1.5 mm/a between 13.5 and 4 Ma, and increased to more than 10 mm/a since 4 Ma. These changes in shortening and structural migration rates over time suggest that the loci of orogenic deformation shifted in and out of the foreland during the Neogene. Surveyed stream terraces suggest that, although most current deformation is located along the southern front of the foreland, modest deformation and shortening continue in the hinterland. Structural style within the foreland has also changed over time from imbricate thrust faults with hanging wall folds along the northern margin, to ramp-flat faults and detachment folds along the southern margin. Where the basement rocks are previously deformed, such as in the hinterland of the Kashi foreland, thick-skinned fault ramps and related fault propagation folds dominate the structural style. As syntectonic sedimentation within the foreland produced thick, subhorizontal stratigraphy, the structural style evolved to detachment and fault bend folding above these footwall flats within the Neogene Strata.