Detachment folding, growth mechanism and seismic potential in the Jammu Sub-Himalaya

The Surin Mastgarh anticline (SMA) marks the active deformation front in the northwest Sub-Himalaya near Jammu. The southern limb of the SMA is not truncated by an emergent thrust, unlike other frontal folds along the Himalayan front. We investigate the structural data, deformation pattern, and seismic potential of the SMA using field surveys and re-interpretation of seismic profile. Strath profiles along the Chenab and the Munavar Tawi river valleys indicate active growth of the SMA by layer parallel shortening, and limb rotation accompanied by flexural slip, suggesting an early phase of its development. Lateral migration of the southern limb suggesting limb lengthening and bending at the hinge are the dominant mechanisms of fold amplification with strain localization at the fold core above the floor thrust tip. Bending moment faulting at the hinge led to the formation of crestal grabens or lakes in the southeastern section of the SMA. Our results suggest the existence of a weak, less viscous layer beneath a brittle sedimentary detachment. The SMA initiated as a detachment fold and sequentially deformed by passive roof thrusting. Therefore, the seismotectonic model of the Jammu Sub-Himalaya is different from that of the central and eastern Himalaya.