The break-back thrust splay of the Main Dun Thrust (Himalayas of western Nepal): evidence of an intermediate displacement scale between earthquake slip and finite geometry of thrust systems

Numerous studies have shown that development of a forward breaking sequence in a thrust belt is accompanied by reactivation of inner faults and development of break-back thrusts. The partitioning of shortening in the Outer and Lesser Himalayan thrust belt follows these rules. Tectonic and sedimentary relationships in this thrust belt exemplify out-of-sequence thrusts in the Lesser Himalayas and break-back imbricate fans close to the MBT (Main Boundary Thrust) and MDT (Main Dun Thrust) which are transported during their development above the Himalayan basal detachment. The displacement along a single splay of the imbricate fan of the MDT was >40 m, before abandonment and nucleation of a new inner splay. The development of the break-back imbricate fan is discontinuous in time. Furthermore, superimposition of ultracataclasite, pressure-solution cleavage, extensional crenulation cleavage and tension gashes in the fault zones suggests that the 40 m displacement occurred by discrete slip events, or most probably by a succession of several seismic/aseismic deformations. These data require several scales of sequential displacement in the Outer and Lesser Himalayas consisting of forward propagation of the thrust system, out-of-sequence reactivation, break-back nucleation of thrust splays, and seismic/aseismic fault slip. Our findings require an intermediate period of tectonism between the <1000 y earthquake cycle and the >500 Ka deformation of the accretionary sediments of the Himalayas.