The effect of overburden thickness on deformation mechanisms in the Keping fold-thrust belt, southwestern Chinese Tian Shan Mountains: Insights from analogue modeling

The structural evolution of the Keping fold-thrust belt in the South Chinese Tian Shan, NW China is poorly understood. Here, we present the results of a series of scaled sandbox models based on the structural architecture of the Keping fold-thrust belt in our investigation of the effect of overburden thickness on deformation mechanisms in fold-thrust belts. These models simulated the evolution of the Keping fold-thrust belt, where the western segment was shortened with a thicker overburden relative to the eastern segment. Model results confirm that fewer thrust faults with wider spacing develop in thicker cover units, whereas more thrust faults with narrower spacing develop in thinner cover units. Transfer zones also form as a result of faster and farther propagation in a segment with thicker overburden, consistent with the prediction of the critical taper theory. Our modeling further shows that due to the episodic accretion of thrust wedges and non-simultaneous formation of thrust faults, the deformation front may locally propagate farther in the thinner overburden. Alternatively, there may not be so much difference between two parts of sand layers with different thicknesses, an observation that is inconsistent with the theoretical prediction. The similarities between the model and real geological observations suggest that the variations in the overburden thicknesses across the Piqiang Fault transfer zone strongly influenced the development of the structural architecture of the Keping fold-thrust belt. They further indicate that the growth of fold-thrust belts in nature is episodic, and that the propagation of deformation front cannot be simply predicted by the critical taper theory. The overburden thickness variations across the Piqiang Fault transfer zone were produced by high angle reverse faulting along the pre-existing Piqiang-Selibuya Fault during the late Pliocene and associated crustal uplift and erosion of the hanging wall strata.