HOW DOES THE MID-CRUST ACCOMMODATE DEFORMATION IN LARGE, HOT COLLISIONAL OROGENS? INSIGHT FROM THE HIMALAYA-TIBET SYSTEM

Our understanding of how the lithosphere deforms in orogenic systems is currently undergoing a paradigm shift. The recognition that hot, weak crust may play an important role in accommodating convergence in the Himalayan-Tibetan system has formed the basis for new hypotheses about collisional orogenesis. Models of shortening accommodation via gravity-driven flow of semi-viscous mid-crustal rocks towards the Himalayan orogenic front (‘channel flow’) are able to account for many geologic observations in the Himalaya. However, alternative models exist, including ‘thrust-wedge taper’ in which flow of a low-viscosity mid-crustal layer is not required to satisfy the observed kinematic framework. Deciphering whether these models are mutually exclusive or if they represent a continuum of deformation within a single orogenic system is of crucial importance for defining the evolution of the Himalayan orogen and, by extension, our understanding of the evolution of continent-continent collisions. This presentation will summarize key datasets from the Himalaya that bear directly on this issue, including: 1) the location, nature and potential relationships of thrust faults within the Himalayan foreland; 2) the timing, duration and magnitude of slip on detachments in the hinterland; 3) the distribution and pressure-temperature-time evolution of mid- and lower-crustal metamorphic and anatectic rocks and 4) the mechanisms by which strain is accommodated in both the deep and shallow crust. Integration of these data suggests that the Himalaya were constructed through elements of both channel flow and thrust-wedge taper. We argue that much of the current debate regarding applicability of end-member models reflects comparison of observations from different parts of the orogenic system at different stages in its evolution.