An unstable kinematic state of the Himalayan tectonic wedge: Evidence from experimental thrust-spacing patterns

The frontal tectonic wedge in the NW Himalayas shows a sequence of imbricate thrusts with increasing spacing in the foreland direction. We used analogue model experiments to analyze the thrust pattern in relation to the kinematics of wedge evolution. Experimental findings reveal two kinematic states of the foreland-ward propagation of a wedge: unstable state and stable state. The unstable state is characterized by continuous vertical growth of the wedge with progressive horizontal contraction. On the other hand, in the stable state the wedge ceases to grow vertically, but propagates laterally in the foreland direction. In the experimental runs thrust wedges remained in the unstable state even after large horizontal shortening (>50%) when the basal friction was high (∼0.46). Our analysis suggests that successive thrusting occurs always with increasing spacing in unstable wedges, and the rate of increase is larger for larger basal friction. It can maintain uniform spacing only when the wedges turn into the stable state. Using finite element models, we determined the stress distribution in the deforming wedge to find the potential locations of new thrusts in front of a wedge, which also show larger spatial distances with increasing vertical thickness of the wedge. Both physical and numerical models suggest that the tectonic wedge in the NW Himalayan frontal belt has evolved in an unstable state.