HR: 14:40h
AN: T23D-05 INVITED     [Abstracts]
TI: Tectonic evolution of subduction wedges at the Cordilleran margin
AU: * Brandon, M T
EM: mark.brandon@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06520-8109 United States
AU: Fuller, C W
EM: fullercw@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195
AU: Willett, S D
EM: swillett@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195
AB: We present here a comparison of the tectonic evolution of subduction wedges at the Cordilleran margin, as represented by the Franciscan wedge of northern California and the Cascadia wedge in the Olympic Mountains, Washington State. We use a geodynamic model to illustrate the growth and morphological evolution of these wedges with time. The Franciscan wedge was initiated at about 150 Ma and is locally active in Northern California, north of the Mendocino triple junction. The northern Cascadia wedge was initiated at about 35 Ma, at the same time that the Cascade volcanic arc started. Both wedges developed into broad bivergent wedges, some 250 km wide. The front of the wedge is marked by the trench and the rear of the wedge, by backfolding on the landward side of the forearc high. In both cases, the forearc high initiated in deep water and steadily grew by accretion and deformation. Emergence of the high above sea level marked the start of erosion. Continued erosion at rates of 0.5 to 1 km/m.y. accounts for much of the exhumation of the deep interior of the wedge. A common view is that a subduction wedge is made up entirely of materials accreted at the subduction thrust. However, the upper parts of the Cascadia and Franciscan wedges are actually made up of imbricated and backfolded rocks that originated as part of the overriding plate. In the Olympic Mountains, these units are the Crescent basalts and the Upper OSC (= Graywolf, Elwha, Grand Valley units of Tabor and Cady). In the Franciscan, these units are the Coast Range ophiolite and eastern belt of the Franciscan. There is clear evidence in both cases that the subduction zone was first initiated seaward of these units and that early wedge-related deformation was also located seaward as well. This history is expected given that when a subduction zone is first initiated, the upper plate is everywhere stronger than the subduction thrust. In other words, the taper of the upper plate is initially blunter than critical. If there were no accretion and the overriding plate remained cold and frictional, then it would be able to sustain subduction slip without deforming internally. Accretion changes this situation given that it will reduce the taper of the upper plate and cause it to deform as a critical wedge. Our numerical experiments indicate that the subduction wedge will start as a small accretionary complex. As it grows, it develops a high and the high will slow propagate rearward into the overriding plate. In this fashion, the region of active wedge deformation includes both accreted materials from the downgoing plate and captured materials from the overriding plate. A strong backstop can stop the rearward migration of the forearc high, but this does not seem to have happened at the Cascadia margin. Instead, it appears that the rearward migrate there is arrested when the wedge reaches a steady state size, where erosion is equal to accretion.
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005