FLUX TECTONICS: THE ROLE OF ACCRETION AND EROSION AT SUBDUCTION WEDGES

I review the influence of accretionary and erosional fluxes on deformation and growth of subduction wedges, using the Cascadia wedge as an example. Subduction wedges grow due to the accretion of mainly sedimentary materials from the downgoing plate. The wedge must maintain a stable taper during growth, so the accretionary flux into the wedge represents the most important factor controlling deformation rates. This conclusion runs counter to the widely-held idea that deformation rates scale with convergence velocities. As an example, the northern Cascadia subduction zone has a convergence velocity of 30 km/m.y. and the accreted sedimentary section is equivalent to a solid-rock thickness of 1.7 km, giving an accretionary flux of 52 km^2/m.y. In comparison, the Cenozoic convergence velocity in the Alps was much slower, 5 km/m.y., but the accreted section was much thicker, 15 km, resulting in an accretionary flux of 75 km^2/m.y. The velocities are very different but the fluxes are similar. In fact, both wedges have similar cross sectional dimensions, and similar structural styles. They both have a deformed structural lid and an underlying accretionary complex. Both are doubly vergent, with a pro-shear zone marked by the subduction thrust, and a more slowly deforming retro-shear zone defining the back of the wedge. In the Alps, the retro-shear zone corresponds to the large backfold that underlies the south side of the Alps. At the Cascadia margin, the retro-shear zone is also a large backfold, which underlies the eastern flank of Cascadia forearc high, corresponding to the Coast Ranges of Oregon, Washington, and Vancouver Island. For Cascadia, the entire region from the trench to the east side of the Cascadia forearc high is actively deforming and shows the topographic form expected of a doubly-vergent wedge. The main difference is that the entire wedge in the Alps was subject to subaerial erosion, whereas only the subaerial part of the forearc high has been eroded at the Cascadia wedge. As a result, the time to steady state is much longer for the Cascadia wedge, and accreted materials tend to show large horizontal motion as they move from initial accretion at the front of the wedge, to the main site of erosion, the forearc high, located some 150 km rearward in the wedge.