Rivers, re-entrants, and 3D variations in orogenic wedge development: a case study of the NW Indian Himalaya

Orogenic wedges are standard elements of collisional plate tectonics, from accretionary prisms to retro-arc basins. Recent study of orogenic wedge development has focused on links between mechanisms of internal deformation and surface processes. Models of orogenic wedges are commonly presented in the cross-section plane, which is generally effective as wedges largely develop via plane strain. The 3rd dimension can be utilized to explore effects of differences in controlling parameters on wedge evolution. We are investigating a stretch of the western Himalayan orogenic wedge that has two prominent changes in along-strike morphology: (1) a tectonic window (the Kullu Window) that appears to be strongly influenced by erosion along the 3rd largest river in the Himalayan system, the Sutlej River and (2) the Kangra Re-entrant, the largest re-entrant along the Himalayan arc. In addition to the along-strike heterogeneity, a key advantage of the proposed study area is its rich stratigraphy, with the most known diversity in the Himalayan arc. The stratigraphic wealth, combined with the along-strike heterogeneity in exposure level, offers a high resolution view of regional structural geometry. Our preliminary reconstructions suggest that the Sutlej River erosion increases the exposure depth and shortening budget across a narrow segment of the orogen, strongly warping the Kullu Window. Previous models have suggested that the out-of-sequence Munsiari thrust is the main structure associated with Kullu window formation, while our work suggests that most of this uplift and warping is accomplished by antiformal stacking of basement thrust horses. Late Miocene ages (U-Pb ages of zircons and Th-Pb ages of monazites) from a leucogranite in the core of the Kullu Window along the Sutlej River further suggests that this segment of the orogen represents a middle ground between plane strain orogenic wedge development and a tectonic aneurysm model. We have constructed a palinspastic reconstruction across the salient to the southeast of the Kangra Re-entrant (revealing ~520 km of Cenozoic shortening from the core of the Tethyan Himalayan synclinorium to the Main Frontal thrust) as well as a deformed cross-section across and to the northeast of the Kangra Re-entrant. The divergence between these two sections develops at ~5 Ma, with less footwall accretion and more overthrusting along the Kangra Re-entrant section until at least ~1-2 Ma. We speculate that the primary cause of this difference is lateral strength variations in supracrustal Indian rocks being incorporated into the growing orogenic wedge. Ongoing research into the evolution of the orogenic wedge here includes low temperature thermochronology and the development of a three-dimensional palinspastic reconstruction using 3DMove software, which provides an opportunity to better harness stratigraphic and structural complexity in restorable models.