BLOCKS, POLES, EXTENSION, AND CONTRACTION - THE BACKARC STORY OF CASCADIA

Forearc and backarc regions flanking magmatic arcs define the upper plate tectonic domains of subduction plate boundaries. At the Cascadia margin in the Pacific Northwest, a stable block comprised of an over-thickened slab of oceanic crust underlies the forearc and actively translates along the margin due to clockwise rotation with respect to North America. Similarly, a clockwise rotational deformation field characterizes the backarc region. The rotation pole lies within a little-deformed stable block, the Ochoco Mountains block (OMB). Distributed deformation occurs around the margin of the OMB. East-west extension of the northwest Basin and Range (BR) tectonic domain to the south gives way to north-south contraction in the Yakima fold belt (YFB) to the north. Contacts between the BR and YFB and the OMB change systematically from a normal fault zone in the south, to a right-lateral strike slip fault system on the west side, to a depositional contact in the north, respectively. Whereas the two tectonic domains developed coevally, BR extension initiated near the Oregon border in the east at ~15 Ma and jumped to the arc after 7 Ma. Extension occurred discontinuously and is closely associated with basaltic eruptive events. Shortening in step-overs in the right-lateral shear zone and the YFB initiated after ~15 Ma. Deformation rates around the periphery measure in the sub-mm/yr range and increase with distance away from the OMB. Structural development of the Casacdia backarc bears strong similarity to the forearc in that motion of a crustal-scale stable block localizes strain around its edges. Magmatism is also associated with the margins of the OMB, which causes the margin strength to oscillate between relatively weak and relatively strong during and following magmatic pulses, respectively. Although the low tectonic rates suggest continuum-like behavior of the crust because boundaries between tectonic domains are difficult to see geodetically, discrete faults accommodate the deformation in the BR, YFB, and in the transition region between the two tectonic domains indicating that block-style deformation dominates the backarc of Cascadia.