Abstract
The continental collision process has made a large contribution to continental growth and reconfiguration of cratons throughout Earth history. Many of the mountain belts present today are the product of continental collision such as the Appalachians, the Alps, the Cordillera, the Himalaya, the Zagros, and the Papuan Fold and Thrust Belt. Though collisional mountain belts are generally elongate and laterally continuous, close inspection reveals disruptions and variations in thrust geometry and kinematics along the strike of the range. These lateral variations typically coincide with cross structures and have been documented in thrust fault systems with a variety of geometries and kinematic interpretations. In the Himalaya, cross faults provide segment boundaries that, in some cases separate zones of differing thrust geometry and may even localize microseismicity or limit areas of active seismicity on adjacent thrust systems. By compiling data on structural segmentation along the length of the Himalayan range, we find lateral variations at all levels within the Himalaya. Along the Gish fault of the eastern Indian Himalaya, there is evidence in the foreland for changes in thrust-belt geometry across the fault. The Gish, the Ganga, and the Yamuna faults all mark boundaries of salients and recesses at the mountain front. The Benkar fault in the Greater Himalayan sequence of eastern Nepal exhibits a brittle-ductile style of deformation with fabric that crosscuts the older thrust-sense foliation. Microseismicity data from several regions in Nepal shows linear, northeast-striking clusters of epicenters sub-parallel to cross faults. The map pattern of aftershock data from the 2015 Nepal earthquakes has an abrupt northeast-trending termination on its eastern side suggesting the presence of a structure of that orientation that limited slip. The orientations of the recognized cross faults and seismic patterns also align with the extensional zones to the north on the Tibetan Plateau and the Indian basement structures to the south. Results from multiple studies are consistent with a link between cross faults and either of these structural trends to the north or south and suggest that cross faults may play a role in segmenting deformation style and seismic activity along the length of the Himalaya.
Highlights
The continental collision process has been responsible for the mountain building of many modern mountain ranges as well as a number of those for which we only see the remnants
Aftershocks occurred to the SE of the main shock at depths from 12 to 50 km. These results suggest that much of the deformation was occurring within the subducting Indian plate beneath the Main Himalayan Thrust (MHT)
Geologic and geophysical data collected over the past century clearly shows that while much of the Himalaya can be characterized by a continuous series of range-parallel thrust faults, there are important lateral variations in the architecture of the range
Summary
The continental collision process has been responsible for the mountain building of many modern mountain ranges as well as a number of those for which we only see the remnants. Geologic mapping in deformed molasse and foreland sediment, known as the Siwaliks, of western Nepal demonstrated the need for an orthogonal transfer zone where the structural nature of the range front thrust systems changed abruptly along strike (Mugnier et al, 1999a,b) This transfer zone marks the western boundary of several dun structures in the Sub-Himalayan zone and has been referred to as the West Dang transfer zone, a possible east-dipping lateral ramp (Mugnier et al, 1999a) that may continue northward into the Lesser Himalaya (see section “Discussion” below). They, too, concluded that there is a likely structural change in the ramp geometry of the MHT, possibly involving a lateral ramp crossing the strike of the range These changes align with the West Dang Transfer Zone proposed by Mugnier et al (1999a) based on their work in the Sub-Himalaya to the south. These segment boundaries bound the regions of large earthquake rupture in the last millennium, though Mugnier et al (2017) suggest that segment barriers may be penetrated during the largest earthquake events
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