Investigating inter-relationships among kinematic vorticity, strain, and minimum translations from shear zones associated with internal thrusts of major fold-thrust belts

We investigate kinematic evolutionary paths of seventeen major shear zones associated with internal thrust faults from the Himalayan, Appalachian, Caledonian, Zagros, Alpine, and Sevier fold-thrust belt (FTB), by assessing published minimum strain, kinematic vorticity number, and minimum translation. We estimate the pure shear component from the recorded vorticity and reconstruct a first-order kinematic path based on multiple strain markers, wherever possible. The studied shear zones follow decelerating strain paths. In general, sub-simple shear more effectively accumulates translation than pure/simple shear deformation. Shear zones with relatively high pure shear component record higher strain.Incremental strain markers from these shear zones record a progressive evolution from an earlier simple shear dominated to a pure shear dominated flow. These results are in agreement with earlier theoretical studies. Internal shear zones that act as roof thrusts of duplexes or have stacked imbricate structures in their immediate footwall, generally record relatively higher strain, greater translation and greater pure shear component toward the later stage than similar shear zones without such footwall structures. We interpret that slip-transfer and structural culmination formed during growth of immediate footwall structures contribute to the kinematic evolution of internal shear zones. The same shear zone records along-strike variation in its kinematic path as a result of its varying immediate footwall geometry. Thus, deciphering comprehensive kinematic evolutionary paths of internal shear zones also requires understanding of immediate footwall structures. Additionally, studying kinematic paths of internal shear zones may provide insights into the geometry of immediate footwall structures when they are not exposed.

Automated summary

The study reveals that the kinematic paths of internal shear zones are influenced by multiple factors, with immediate footwall structures playing a key role in the evolution of shear zones. Shear zones tend to evolve progressively from simple shear domination to pure shear domination, with shear zones having higher pure shear components recording higher strain. Additionally, there is variation along the strike in the kinematic paths of shear zones due to the changing geometries of immediate footwall structures.