Development of a hidden fold and thrust belt in the Himalayan piedmont and distribution of active tectonics

<p><span>The pattern of active deformation of </span><span>Himalayan </span><span>frontal structures is complex with out-of-sequence reactivations in the chain and development of scarps associated to earthquake ruptures reaching the surface in the piedmont. </span><span>We analyze passive seismic records using the </span><span>Horizontal-to-Vertical Spectral Ratio method along three North-South trending profiles of the Darjeeling Himalayan piedmont, </span><span>reveal</span><span>ing</span><span> subsurface structures down to 600 meters </span><span>and</span><span> imaging the Siwalik sedimentary rocks / recent deposits interface. We </span><span>find evidence for</span><span> a thrust fold system hidden beneath the plain correlate</span><span>d</span> <span>to </span><span>geomorphologic scarp</span><span>s</span> <span>revealed</span><span> by </span><span>topographic</span><span> profiles. </span><span>These morphological surfaces are incised b</span><span>y</span><span> large rivers </span><span>of the piedmont</span><span> by several tens of meters during phases of low sedimentation, </span><span>thus slightly emerging in the plain</span><span>.&#160;</span><span>These scarps are supposedly induced by </span><span>thrust deformation related to great earthquakes propagat</span><span>ing</span><span> south of the morphological front and induc</span><span>ing</span> <span>subsurface ruptures</span> <span>in the piedmont. </span><span>Th</span><span>is </span><span>interpretation is</span><span> comforted by the lateral correlation of the imaged</span><span> thrusts and associated folds with previously evidence</span><span>d</span><span> fault scarps </span><span>associated to active thrusts</span><span> of the Darjeeling piedmont. </span><span>I</span><span>n the piedmont of</span> <span>East/Central Nepal, </span><span>oil company seismic profile</span><span>s</span><span> image </span><span>similar</span> <span>thrusts and associated folds </span><span>that can also be </span><span>correlated to </span><span>both </span><span>local incision of small rivers draining the southern flank of the Siwalik hills, </span><span>and uplift evidenced by a previously analyzed leveling profile</span><span>.</span></p><p><span>The</span><span> long-term kinematic evolution </span><span>of this hidden thrust fold belt </span><span>is slow, with a tectonic uplift of the hangingwall lower than the subsidence rate of the foreland basin, i.e., less than ~ half a millimeter per year. </span><span>The evolution of the hidden structures corresponds to that of an embryonic thrust belt affected by a layer parallel shortening (LPS) acting in the long term with a shortening rate of the order of 5-10% of the shortening rate of the whole Himalayan thrust system. </span>A<span>n aseismic deformatio</span><span>n</span><span> associated with the LPS structures </span><span>that</span><span> could absorb the entire deformation of the embryonic thrust belt in </span><span>E</span><span>ast/</span><span>C</span><span>entral Nepal </span><span>is suggested by the comparison of the long term structural evolution with geodetic and paleoseismological studies</span><span>. The amplitude of this aseismic deformation is </span><span>however</span><span> too limited to significantly reduce the seismic hazard in the </span><span>Himalayan piedmont</span><span>.</span></p>