Tectonic underplating versus out-of-sequence thrusting beneath the Lesser Himalaya: Insights from the analogue modeling of the Nepal Himalaya fold-and-thrust belt

The Nepal Himalaya fold-and-thrust belt is a classical two-décollement thrust–fold system. The belt has been extensively studied by analogue and numerical modeling, but the formation mechanism of the mid-crustal duplex structure beneath the northern part of the Lesser Himalaya remains debated. The formation of this thrust duplex has been attributed either to tectonic underplating along new ramps in response to the subducting Indian Plate, or to out-of-sequence thrusting along preexisting ramps. We conducted a series of experiments using analogue models, employing a model setup that was faithful to the geological setting of the Nepal Himalaya. We used industrial computed tomography (CT) and particle image velocimetry (PIV) to generate detailed deformation maps. The analogue models successfully reproduce the main structure of the Nepal Himalaya fold-and-thrust belt, showing that the two competing processes (i.e., underplating along new ramps and out-of-sequence thrusting along preexisting ramps) operate alternately to form the duplex structure. The models also show that underplating is associated with extensive foreland sedimentation, analogous to the foreland molassic sedimentation in the Sub-Himalaya, which leads to a systematic recession of the deformation front and a smaller horse. Finally, the recognition that out-of-sequence thrusting occurs in the interval between major underplating events provides important insights into the origin of the 2015 MW 7.8 Gorkha earthquake.