Himalayan deformation and its connection to geologic hazards: cross fault examples

As a product of continental collision, the Himalaya developed major thrust and fold structures to accommodate convergence.  With the continental subduction of the Indian continent beneath Asia, the deformation front has migrated southward over time.  The result of these tectonic and structural processes is the world’s highest mountain belt that exposes rocks of a large span of ages and has had a dynamic geomorphological evolution for the past 50-60 million years.  The Himalaya is now home to more than 53 million people.  These people face the ongoing threat of earthquakes, landslides, and floods due the active landscape in which they live.  While most earthquake events are caused by thrust fault activity, it has been recognized that there are faults that cut across the range, cross faults, that also play a role in hazards.  In the central and eastern Himalaya, cross faults have been identified where the range front transitions from salients to recesses.  Examples of these structures in Sikkim and Nepal include the Gish and Kosi faults.  Similar structures have also been identified north of the range front in the Lesser and Greater Himalayan regions.  A map of aftershock data from the 2015 earthquake shows an abrupt termination of aftershocks in the region east of Kathmandu that aligns with a proposed cross fault known as the Gaurishankar lineament.  Geophysicists suggested that a cross fault could be responsible for blocking of the lateral propagation of the thrust rupture.  The Dudh Kosi valley that drains the Khumbu or Everest region has had historic and pre-historic large landslides that may have originated on the Benkar cross fault structure.  In 2021, the Melamchi valley east of Kathmandu experienced a devastating flood that was in part tied to the reactivation of a large landslide.  That landslide site is co-located with north-south striking shear zones or cross fault structures.  While cross faults are not the major structures accommodating convergence, our work suggests that there are implications for hazard occurrence due to the presence of these structures.