HR: 1340h
AN: T13B-0481    [Abstracts]
TI: Hydration and Flat-Plate Subduction Stability
AU: * Patel, P I
EM: warlord@mail.utexas.edu
AF: University of Texas, Department of Geosciences, 1 University Station, Austin, TX 78712 United States
AU: Lavier, L
EM: luc@utig.ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd. #600, Austin, TX 78759 United States
AU: Grand, S
EM: steveg@speer.geo.utexas.edu
AF: University of Texas, Department of Geosciences, 1 University Station, Austin, TX 78712 United States
AB: Approximately 10% of the Earth's subduction zones are experiencing sub-horizontal subduction. This behaviour is commonly attributed to several factors including low plate density, increased plate convergence velocity, and slab suction. In addition, mantle hydration may also be a controlling factor in both the onset and ultimate failure of sub-horizontally subducting systems. Here, we present 2D numerical experiments aimed at studying the onset and stability of flat subduction in a scenario roughly reminiscent of the Farallon plate's sub-horizontal subduction beneath the western US from 80 to 40 mya. In these experiments, the lithosphere and mantle are modeled as a visco-elasto-plastic medium. The brittle parts of the lithosphere are modeled as a frictional and cohesional material. The ductile lithosphere is modeled as a non-Newtonian Maxwell visco-elastic material. Faults in the brittle parts of the model are formed by locally decreasing the cohesion and friction as a function of plastic strain. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions. A proxy to emulate phase changes allows for "instantaneous" transformations between phases at specified pressures and temperatures. In these experiments, both crustally-thickened (representing oceanic plateaus) and archetypical oceanic lithosphere are subducted beneath continental lithosphere. These simulations begin with subduction initiation along a pre-weakened zone at the contact between oceanic and continental lithospheres. Various parameters including subduction velocity, mantle viscosity, and bulk lithospheric density are varied (within realistic ranges) in order to promote large scale, stable, flat subduction. In these stable flat-slab systems, local mantle viscosities and densities will vary according to a preliminary phase change model proxy in order to simulate alteration of the lithosphere via hydration. This hydration is due to water and other volatiles released from the subducted slab that fails to trigger surface volcanism and remains trapped in mantle phases. Crustal phase densities also evolve via an eclogite phase change proxy. As the viscosity of the lithospheric mantle between the horizontally coupled plates degrades due to hydration, flat or flattened subduction begins to fail. This results in slab rollback until a more typical subduction geometry arises. Preliminary results show that the retreat ultimately leads to catastrophic delamination of material from the continental lithosphere resulting in a region of attenuated lithospheric thickness similar to that observed in the Basin and Range Provinces of North America. This outcome is consistent with, though certainly not a proof of, a possible role for a build-up of hydrated phases acting as a destabilization mechanism for flat slab subduction.
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005