CRUSTAL-SCALE SHORTENING ACCOMMODATION AND STRAIN PARTITIONING IN COLLISIONAL OROGENS: MAJOR ADVANCES AND PERSISTENT CHALLENGES IN HIMALAYAN-TYPE SYSTEMS

Understanding what controls shortening accommodation and strain partitioning in large collisional orogens like the Himalayan-Tibetan (HT) system remains as a major challenge for tectonics research. Critical-wedge models, wherein shortening is accommodated by slip along discrete thrusts/shear zones, can describe the geometry and kinematics of the lower grade frontal parts of many collisional systems. The channel flow model, wherein melt-weakened crust flows toward the surface (orogen-normal, ON) or out of the collision zone (orogen-parallel “escape”, OP) in response to stress gradients, provides an elegant mechanism to explain observations in the high-grade HT hinterland. However, as the dynamic and rheologic conditions for these models are distinct, it is likely that they operate in spatially and/or temporally distinct parts of the orogen. These realizations led to an HT model where channel flow transitions from ON to OP in the late Miocene, and geophysical evidence that indicates low velocity mid-crust beneath the Tibetan plateau corroborates this idea. Simultaneous with OP flow, ongoing shortening is accommodated through critical-wedge thrusting along the Main Boundary and Frontal thrusts, leading to development of a hybrid critical wedge-escape flow system.

Critically, we still don’t understand what drives this transition from ON to OP flow + critical wedge behavior. In the HT1 numerical model, ON flow persists until a significant erosion reduction is introduced, but more recent evidence suggests that such an erosion reduction may be in consistent with HT evolution. This also highlights our need to better understand any linkage between shortening accommodation and climate, as erosion rates in models such as HT1 are likely too high (11-13 mm/a) to be sustained over time intervals required to establish channel flow (10-30 Ma). Additionally, it remains unclear how the HHT or MHD contribute to the shortening budget. In most regions, this boundary represents the largest thermal-metamorphic break in the entire HT system, yet recent work in the Annapurna region indicates that little to no mylonite is associated with this zone, potentially indicating a more recent (reactivated?) history. This contribution will present new results that highlight these emergent challenges for characterizing HT evolution.