Masters Thesis

Structural style and shortening magnitude across the Kepintage fold and thrust belt in the Tian Shan foreland, northwest China

The Tian Shan represents intracontinental deformation related to the Indo-Asian collision, and thus provides an ideal location to study orogenic growth and the controls on foreland structural styles. The southern piedmont is dissected by faults and folds that vary in structural style along strike. Structural analysis, combined with magnetostratigraphy of Miocene and Pliocene sediments deformed in the foreland, provide data about the structural evolution of the mountain front. Mapping along the southernmost structure within the Kepintage thrust zone indicates the structure is a fault-propagation fold with a trishear component that has developed since the Pliocene. The gently dipping (25-35) north limb contains ~8 km of Paleozoic and Cenozoic strata. The core of the anticline exposes intensely faulted Paleozoic strata. Total shortening across the structure, based on our cross section interpretation, is 10.5 +/- 1.1 km for a total shortening of approximately 33%. Preliminary magnetostratigraphy from Neogene strata on the northern flank of the fold indicate ages of deformed strata are ~14.5 - 1.0 Ma, and growth strata at the top of the section are dated ~0.9 Ma. Growth strata provide maximum shortening rate estimates of 7.5 +/- 0.8 mm/yr. This rate is several times faster than the long-term geologic rates determined for this part of the foreland, but similar to short-term geodetically determined rates across the thrust belt at this location. Total shortening within the Kepintage thrust belt is ~35.3 +/- 3.5 km, which is more robust than several previous studies have indicated. However, our new observations suggest that over 35% of this shortening has occurred in only the last 2 million years, despite initial uplift of the range at approximately 20 Ma. Additionally, structural style appears to change from north to south. Fault-bend folds comprise the northern foreland and trishear fault-propagation folds the southern part. Furthermore, structural style varies along strike to the east and west of the study area. Contributing factors to controls on structural style are elucidated, including: thickness of Neogene strata, lithological variations, and location of décollement horizon. Our new interpretations suggest that shortening rates must have increased dramatically during the Quaternary at this location within the Tian Shan. Moreover, our data suggest that the Kepintage fault is a quite nascent feature, and illuminates the complex uplift history of the Tian Shan.

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