HR: 16:05h
AN: V44A-01    [Abstracts]
TI: Detachment and Exhumation of Ultra-high-pressure Rocks During Continental Subduction
AU: Warren, C J
EM: clare.warren@dal.ca
AF: Department of Earth Sciences, Dalhousie Univerity, Halifax, NS B3H 3J5, Canada
AU: Beaumont, C
EM: chris.beaumont@dal.ca
AF: Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada
AU: * Jamieson, R A
EM: beckyj@dal.ca
AF: Department of Earth Sciences, Dalhousie Univerity, Halifax, NS B3H 3J5, Canada
AU: Lee, B
EM: bonny.lee@dal.ca
AF: Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada
AB: In many Phanerozoic collisional orogens, ultra-high-pressure (UHP) rocks are inferred to have been formed and exhumed during the transition from oceanic subduction to continental collision. We use fully coupled, thermal- mechanical, upper-mantle-scale models to investigate how detachment and exhumation of UHP material in the subduction channel are affected by the competition between down-channel shear traction and up-channel buoyancy forces. Our reference model involves constant velocity (5 cm/y) subduction of a weak continental margin and stronger continental interior, and includes strain weakening and reversible density changes accompanying metamorphic phase transformations. During subduction, decoupling of margin material initiates at depth along a shear zone that propagates upwards as subducted material progressively weakens, leading to diachronous detachment and exhumation of UHP and HP rocks. UHP exhumation begins first, as a buoyant plume that tunnels upwards along the weak shear zone. HP material initially forms a stagnant nappe, which is then exhumed by, and folded over, the rising UHP plume. Model predictions are consistent with observed PTt paths and timing constraints from natural examples. The results can be interpreted in terms of a lubrication theory of channel flow in which the exhumation number, E, expresses the relative contributions of Poiseuille (up-channel) and Couette (down-channel) components. The value of E depends on the pressure gradient (dominated by density contrasts), channel thickness, effective viscosity, and subduction velocity, and varies with both time and distance down the channel. Although buoyancy is the main driving force for exhumation, viscous strain weakening plays a critical role by first reducing the downward Couette drag and then promoting upward Poiseuille flow of low-viscosity UHP material. In natural systems, other weakening mechanisms may also play a role. Wide- continental-margin models produce a low-viscosity channel that tunnels up to lower crustal levels, suggesting a link between subducted margin properties and exhumation style, with implications for UHP exhumation in different parts of the Himalayan-Tibetan system.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting