Development of a hidden fold and thrust belt in the Himalayan piedmont and distribution of active tectonics
Authors: Large Etienne , Huyghe Pascale , Mugnier Jean-Louis , Jouanne François , Guillier Bertrand , Chakraborty Tapan 
Published: March 2022

&lt;p&gt;&lt;span&gt;The pattern of active deformation of &lt;/span&gt;&lt;span&gt;Himalayan &lt;/span&gt;&lt;span&gt;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. &lt;/span&gt;&lt;span&gt;We analyze passive seismic records using the &lt;/span&gt;&lt;span&gt;Horizontal-to-Vertical Spectral Ratio method along three North-South trending profiles of the Darjeeling Himalayan piedmont, &lt;/span&gt;&lt;span&gt;reveal&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; subsurface structures down to 600 meters &lt;/span&gt;&lt;span&gt;and&lt;/span&gt;&lt;span&gt; imaging the Siwalik sedimentary rocks / recent deposits interface. We &lt;/span&gt;&lt;span&gt;find evidence for&lt;/span&gt;&lt;span&gt; a thrust fold system hidden beneath the plain correlate&lt;/span&gt;&lt;span&gt;d&lt;/span&gt; &lt;span&gt;to &lt;/span&gt;&lt;span&gt;geomorphologic scarp&lt;/span&gt;&lt;span&gt;s&lt;/span&gt; &lt;span&gt;revealed&lt;/span&gt;&lt;span&gt; by &lt;/span&gt;&lt;span&gt;topographic&lt;/span&gt;&lt;span&gt; profiles. &lt;/span&gt;&lt;span&gt;These morphological surfaces are incised b&lt;/span&gt;&lt;span&gt;y&lt;/span&gt;&lt;span&gt; large rivers &lt;/span&gt;&lt;span&gt;of the piedmont&lt;/span&gt;&lt;span&gt; by several tens of meters during phases of low sedimentation, &lt;/span&gt;&lt;span&gt;thus slightly emerging in the plain&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;span&gt;These scarps are supposedly induced by &lt;/span&gt;&lt;span&gt;thrust deformation related to great earthquakes propagat&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; south of the morphological front and induc&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt; &lt;span&gt;subsurface ruptures&lt;/span&gt; &lt;span&gt;in the piedmont. &lt;/span&gt;&lt;span&gt;Th&lt;/span&gt;&lt;span&gt;is &lt;/span&gt;&lt;span&gt;interpretation is&lt;/span&gt;&lt;span&gt; comforted by the lateral correlation of the imaged&lt;/span&gt;&lt;span&gt; thrusts and associated folds with previously evidence&lt;/span&gt;&lt;span&gt;d&lt;/span&gt;&lt;span&gt; fault scarps &lt;/span&gt;&lt;span&gt;associated to active thrusts&lt;/span&gt;&lt;span&gt; of the Darjeeling piedmont. &lt;/span&gt;&lt;span&gt;I&lt;/span&gt;&lt;span&gt;n the piedmont of&lt;/span&gt; &lt;span&gt;East/Central Nepal, &lt;/span&gt;&lt;span&gt;oil company seismic profile&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; image &lt;/span&gt;&lt;span&gt;similar&lt;/span&gt; &lt;span&gt;thrusts and associated folds &lt;/span&gt;&lt;span&gt;that can also be &lt;/span&gt;&lt;span&gt;correlated to &lt;/span&gt;&lt;span&gt;both &lt;/span&gt;&lt;span&gt;local incision of small rivers draining the southern flank of the Siwalik hills, &lt;/span&gt;&lt;span&gt;and uplift evidenced by a previously analyzed leveling profile&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;The&lt;/span&gt;&lt;span&gt; long-term kinematic evolution &lt;/span&gt;&lt;span&gt;of this hidden thrust fold belt &lt;/span&gt;&lt;span&gt;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. &lt;/span&gt;&lt;span&gt;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. &lt;/span&gt;A&lt;span&gt;n aseismic deformatio&lt;/span&gt;&lt;span&gt;n&lt;/span&gt;&lt;span&gt; associated with the LPS structures &lt;/span&gt;&lt;span&gt;that&lt;/span&gt;&lt;span&gt; could absorb the entire deformation of the embryonic thrust belt in &lt;/span&gt;&lt;span&gt;E&lt;/span&gt;&lt;span&gt;ast/&lt;/span&gt;&lt;span&gt;C&lt;/span&gt;&lt;span&gt;entral Nepal &lt;/span&gt;&lt;span&gt;is suggested by the comparison of the long term structural evolution with geodetic and paleoseismological studies&lt;/span&gt;&lt;span&gt;. The amplitude of this aseismic deformation is &lt;/span&gt;&lt;span&gt;however&lt;/span&gt;&lt;span&gt; too limited to significantly reduce the seismic hazard in the &lt;/span&gt;&lt;span&gt;Himalayan piedmont&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;