HR: 1340h
AN: T33C-0559    [Abstracts]
TI: Rheological controls on the growth of subduction wedges
AU: * Fuller, C W
EM: fullercw@u.washington.edu
AF: Dept. Earth and Space Sciences, Univ. Washington, Seattle, WA 98195 United States
AU: Brandon, M T
EM: mark.brandon@yale.edu
AF: Dept. Geology and Geophysics, Yale Univ., New Haven, CT 06520 United States
AU: Willett, S
EM: swillett@u.washington.edu
AF: Dept. Earth and Space Sciences, Univ. Washington, Seattle, WA 98195 United States
AB: Accreting subduction zones are associated with broad regions of active deformation in the overriding plate that we refer to as the subduction wedge. We focus here on how subduction wedges evolve with time with an emphasis on how the morphological evolution varies between a wedge dominated by frictional deformation mechanisms and one dominated by both frictional and viscous mechanisms. For this study we use a thermo-mechanical model with thermally activated viscous and frictional rheologies that has been adapted to allow for the accretion of a thin sedimentary layer and slip on a discrete subduction thrust at the base of the wedge. Other critical elements of the model are flexural compensation, sedimentation and erosion In our numerical experiments, we infer that subduction zones initiate with the upper plate having a frictionally stable wedge taper where the upper plate is strong enough to slip on the subduction thrust without deforming internally. Our results show two different evolutionary behaviors following the initiation of accretion depending on the rheology considered. In a wedge dominated by frictional deformation mechanisms, as may occur in a cold subduction setting, accretion leads to the development of a relatively small, seaward vergent accretionary complex with a topographic high that migrates landward. This landward migration continues until the high rests above the intersection point of the upper plate mantle and the subducting plate, which acts as the ultimate backstop. Alternatively, the migration can be arrested through either sedimentation landward of the high or erosion of the high. In contrast, a subduction wedge with both frictional and viscous deformation mechanisms, as would be expected in warmer settings, immediately forms a topographic high above the mantle intersection point. The high develops in this case due to viscous weakening of the landward portions of the subduction wedge allowing deformation to propagate into the frictionally stable region of the wedge. The development of viscous deformation within a subduction wedge, and thus the morphological evolution of the wedge, is largely temperature controlled. In natural settings it is likely that subduction wedges transition from being dominated by frictional deformation mechanisms to mixed frictional and viscous mechanisms. This transition is largely controlled by the amount of material that has been accreted into the wedge since larger amounts of accreted material result in greater radiogenic heating and warmer subduction wedges. The thermal dependence on the amount of accreted material can potentially explain the differences observed in wedge morphology between segments of subduction wedges along the same margin as observed in the Aleutians. Also, we can distinguish between these evolutionary paths in natural examples by looking at the temporal evolution of the forearc high position. Within Cascadia it appears that the forearc high has migrated landward suggesting that the deformation within the subduction wedge has been dominated by frictional mechanisms.
DE: 0545 Modeling (4255)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8169 Sedimentary basin processes
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005