Subduction followed by collision: Alpine and Himalayan examples

Abstract

The high mountains of the Himalaya and Alps are both the result of ongoing continental collision following closure, by subduction, of Neo-Tethys and related ocean basins during the Mesozoic and Tertiary. The high topography is associated, in both areas, with a thickened crustal root of up to 70 km thickness as deduced from various geophysical investigations. In the Himalayan region, where several thousand kilometres of ocean crust was consumed below the Lhasa block (southern margin of the Asian continent), both continental (Trans-Himalayan batholith) and oceanic (Kohistan–Ladakh arc, accreted to Asia in the Jurassic) magmatic arcs are recognisable. In the Alps, where much smaller oceanic rifts and basins were consumed, such arc sequences are absent in the area of the high mountains. Regardless of this major difference in volume of subducted oceanic lithosphere, seismic tomography in both regions has identified several anomalous regions in the mantle interpreted as the remnants of detached slabs of oceanic crust. Metamorphosed rocks of continental crust sequences in both the Himalaya and Alps record evidence of subduction to depths exceeding 90 km (coesite stability field) as well as rapid exhumation to shallower depths at rates of 4 mm/a or more. This subduction-related metamorphism of the eclogite and blueschist-facies is clearly distinguishable from subsequent, younger, lower pressure, greenschist–amphibolite-facies metamorphic overprints related to imbricate stacking of continental crust slices, thermal relaxation, and final exhumation. Dragging of buoyant upper crust down to upper mantle depths requires an attached root of oceanic lithosphere but the rapid exhumation stage may well be a consequence of slab break-off. This process appears to have been equally effective in the Alps and the NW Himalaya, despite the significant differences in duration of subduction and the volumes of oceanic lithosphere involved.