HR: 0800h
AN: T21B-0530    [Abstracts]
TI: The influence of the basalt-eclogite transition on incipient subduction dynamics
AU: * Hall, C
EM: chall@gps.caltech.edu
AF: Cal Tech, Seismo Lab, MS 252-21, Pasadena, CA 91125
AU: Gurnis, M
EM: gurnis@gps.caltech.edu
AF: Cal Tech, Seismo Lab, MS 252-21, Pasadena, CA 91125
AB: Subduction becomes self-sustaining when the negative buoyancy of the slab is larger than the sum of elastic bending, fault friction, and viscous drag. The oceanic lithosphere, consisting of basaltic crust overlying mantle which is depleted in iron, is compositionally buoyant relative to the asthenosphere and will resist subduction. This is not a critical impediment for mature subduction zones because the basaltic crust should metamorphose into denser eclogite, with the result that the slab has nearly neutral compositional buoyancy at depths greater than this phase transition. During subduction initiation, however, compositional buoyancy may offer significant resistance to subduction and initially limit subduction to plates which have gained sufficient negative buoyancy from cooling. We have shown that while including a 6 km thick basaltic crust does not greatly affect the maximum force required to bend the lithosphere, it substantially increases the net work required to reach a self-sustaining state. These earlier models did not consider the basalt-eclogite transition. We use a visco-elastoplastic numerical method to model the evolution of the force balance during incipient subduction occurring at a fixed convergence rate. Phases are tracked using a particle advection scheme. In some cases, the buoyant basaltic crust buoyantly detaches from the mantle lithosphere and rises to the base of the over-riding lithosphere. Buoyant detachment removes most of the compositional impediment to subduction initiation, but will deny the slab of the additional driving force that would occur after the transformation of basalt to eclogite. Crustal detachment is encouraged by mechanical thickening during convergence, and therefore may depend on the degree of coupling and fault strength at the plate interface.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3210 Modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting