THE EVOLUTION OF OROGEN-PARALLEL SHEAR WITHIN A DYNAMIC THRUST WEDGE

An Yin’s insights into map relationships and kinematic sequencing continues to inspire new generations of geoscientists. We observe active orogen-parallel shear on the Western Nepal Fault System (WNFS) that adds a new component to the dynamic evolution of the Himalayan thrust wedge. During periods of duplex development and wedge growth, active vertical faults are predicted to exhibit either shortening-parallel strikes or flexural slip kinematics. By contrast, the northwestern segment of the WNFS truncates the wedge obliquely and exhibits kinematics consistent with dextral slip. Where active faults within the WNFS do strike parallel to shortening, the geomorphic signature is consistent with active surface uplift and segmentation of frontal ramps in the Main Himalayan Thrust. Quaternary slip rates on the WNFS are similar to those reported on the southern reach of the Karakorum Fault, consistent with slip transfer. Our results, in conjunction with observations from Yin and his collaborators, are also consistent with a southeastward propagation of the Karakorum Fault after ~13 Ma and initiation of the WNFS sometime after 3 Ma, which is synchronous with slip on the Main Frontal Thrust and ~5 Ma later than the initiation of Gurla Mandhata-Humla Fault System. The new temporal constraints indicate an episodic propagation of orogen-parallel faults that coincides with foreland-ward propagating thrusts, like the Main Frontal Thrust.

The close agreement between thickening of the Greater Himalaya Sequence and the growth of the Lesser Himalaya Duplex with a cessation of orogen-parallel fault propagation suggests that orogen-parallel strain from the Karakoram Fault either bleeds into the shortening structures or is accommodated by some other mechanism (i.e., extension) when the wedge is sub-critical. Similarly, the subsequent activation of the foreland propagating thrusts and initiation of the WNFS suggests a continuation of southeastward propagation and a kinematic linkage with the Main Himalayan Thrust. The evolution of fault systems, like the WNFS, that accommodate orogen-parallel shear within dynamic thrust wedges may be limited to periods characterized as critically tapered Coulomb wedges. If correct, this scenario emphasizes the critical influence that kinematic sequences have on the structural architecture of thrust systems.