Collaborative Research: Thermomechanical Models of Forearc Deformation at the Cascadia Subduction Zone

University Of Washington, Seattle WA

Investigators

Abstract

There is much disagreement about the width of the actively deforming forearc wedge at the Cascadia subduction zone. The narrow wedge interpretation maintains that the continental shelf is underlain by a strong backstop that limits wedge deformation to the ~50 km wide continental slope. This view is compatible with the fact that the shelf is flat and appears to deform relatively slowly. The strength of the shelf backstop is usually attributed to the more lithified character of older accreted rocks within the back of the wedge. The alternative interpretation is that actively deforming wedge is some 150 to 225 km wide, and is delimited by a seaward-vergent deformation front at the Cascadia trench and a landward-vergent deformation front at the east flank of the Oregon-Washington Coast Ranges, the Olympics and the Vancouver Island Insular Range. The change in topographic slope at the crest of this forearc high represents a reversal in structural vergence in the wedge. In this model, the relatively strong lithospheric mantle of the overriding plate represents a deep-seated flat-lying backstop. The greater strength of the mantle backstop allows wedge deformation to involve both accreted sedimentary rocks and the older crustal lid of the subduction zone (e.g. Silitez and Crescent basalts). This model accounts for the development of the forearc high along the entire length of the Cascadia margin with remarkable uniformity irrespective of local crustal geology. Active permanent uplift is recognized everywhere along the forearc high, with the fastest rates (~0.8 km/m.y.) occurring in the Olympic Mountains. The PI's propose a 2 year study that will use thermomechanical modeling to test the wide wedge hypothesis at Cascadia. The timing is ideal for this work given recent seismic and geodetic studies that provide detailed information about the structure and short-term deformation of the forearc, and recent thermochronologic, geomorphic, and geologic studies that provide local information about long-term deformation and uplift across the Olympics and Corvallis sectors of the margin. The proposed work will examine 3 issues where the PI's hypothesis is most likely to fail: 1) How is the shelf able to remain flat lying and relatively undeformed within an actively deformed wedge? In the Olympics, the trench slope, shelf, and forearc high are all underlain by accreted sedimentary rocks, so variations in wedge strength seems an unlikely explanation. The PI's will test the idea that the shelf part of the wedge is stabilized by deposition in shelf basins, which are 2 to 3 km thick. 2) What causes the thick structural lid of the subduction zone to uplift and fold into the forearc high observed today? Thermomechanical modeling will allow the PI's to determine the role of ductile flow in controlling the growth of the forearc high. They will also explore if uplift and folding of the lid can occur by frontal accretion alone, or if underplating is required. 3) How is the pattern of wedge deformation influenced by the distribution of rock strength? The Cascadia margin includes soft accreted sediments, older lithified accreted sediments, and a structural lid of older igneous rocks (e.g., Siletz, Crescent, Wrangellia terranes). Using realistic constitutive relationships, the PI's will determine how these units deform above and seaward of a much stronger mantle backstop. These process-oriented studies will provide the basis for building a full thermomechanical model to test if the long-term evolution of a wide forearc wedge is consistent with the known tectonic evolution of the Cascadia forearc. This will allow the PI's to test if the wedge will retain a steady evolution in the face of large changes in sediment fluxes. This research will contribute towards a more realistic understanding of the thermal structure and long-term velocity field within the Cascadia forearc. This information is essential for improving resolution of the width of the seismogenic zone for the Cascadia subduction zone.