Reconstructing the total shortening history of the NW Himalaya

Abstract

The onset of India‐Asia contact can be dated with both biostratigraphic analysis of syn‐collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction‐collision stages. These data, together with paleomagnetic data, show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intraoceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo‐Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo‐Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto‐Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50–45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm yr−1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening versus amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.