Relationship Between Lateral/Basal Shear Stress Ratio and Structural Vergence of Thrust Wedges: Results From Analogue Modeling and Implications for the Origin of Eastern Sichuan-Xuefeng Fold-Thrust Belt in South China

Thrust wedges are common in nature, but not all of their origins can be properly explained by the classic Coulomb wedge theory, which considers only a 2D perspective. A few analogue models, however, revealed that lateral shear stress (inherent in both analogue models and nature) may strongly affect the development of thrust wedges and the lateral/basal shear stress (tau(s)/tau(d)) ratio might have controlled the structural vergence of thrust wedge, suggesting that origin of thrust wedges needs to be considered from a 3D perspective. The origin of the Eastern Sichuan-Xuefeng fold-thrust belt (ESXFTB), which is characterized by two oppositely vergent thrust systems at different crustal levels and may contain another large purely backward-vergent thrust wedge on the world, is still problematic. Revealing the relationship between tau(s)/tau(d) ratio and structural vergence may prompt a deep understanding of the origin of thrust wedges as well as the ESXFTB. This relationship, however, is still not very clear due to the lack of systematic experiments. For these reasons, we performed systematic experiments of two types of analogue models under different tau(s)/tau(d) ratios. Our model results revealed, for the first time, that structural vergence of thrust wedges lying on a weak decollement may progressively change from frontward (when tau(s)/tau(d) <0.09) to dual (when tau(s)/tau(d) = 0.09-1.15) and then to backward (when tau(s)/tau(d) >1.15) and the two oppositely vergent thrust systems in the ESXFTB can be produced simultaneously by the single northwestward Mesozoic shortening probably due to their apparently different tau(s)/tau(d) ratios.

Automated summary

The article discusses the origin of thrust wedges and the case study of the Eastern Sichuan-Xuefeng fold-thrust belt. The experimental results reveal, for the first time, the variation of structural vergence of thrust wedges under different shear stress ratios.