THE ACTIVE WARWICK STRIKE-SLIP FAULT AND THE COLUMBIA HILLS THRUST FAULT OF THE YAKIMA FOLD AND THRUST BELT ACCOMMODATE VERTICAL-AXIS ROTATION IN THE CASCADIA BACKARC

Contractional and strike-slip structures accommodate clockwise vertical-axis rotation of the North America plate above the Cascadia Subduction Zone. New geological mapping, geochemical and geochronologic data, and structural analyses of the Columbia Hills anticline (CHA) and thrust fault (CHTF) and the dextral Warwick fault zone (WFZ) in southern Washington reveal how transpressional structures partition this deformation. The CHA is a S-verging anticline, with a shallow (7-18°) N-dipping back limb and a poorly-expressed S-dipping forelimb. The NW-trending WFZ cuts the CHA and CHTF. West and east of the WFZ, the CHTF juxtaposes Columbia River Basalt Group units. West of the WFZ, Grande Ronde Basalt is thrust over the younger Wanapum Basalt, Priest Rapids member, Basalt of Rosalia (15.9 ± 0.02 1σ Ma) flow. Line-balanced cross-sections to the west and east of the WFZ using CHTF dips of 30°N and 45°N both indicate ~1 km of post-15.9 Ma shortening. Post-15.9 Ma shortening rates on the CHTF west and east of the WFZ are 0.04-0.06 ± 0.01 mm/yr and 0.05-0.09 ± 0.01 mm/yr, respectively. Whether the CHTF is active in the Quaternary is uncertain. East of the WFZ, the Haystack Butte basalt (⁴⁰Ar/³⁹Ar age = 0.834 ± 0.004 Ma) flowed down a paleochannel across the CHTF. Roughly 19 m relief marks the basal contact elevation change across the fault. If the change in elevation reflects fault uplift, offset of the Haystack Butte basalt yields a post-0.8 Ma shortening rate of 0.02 ± 0.01 mm/yr. Deformed loess deposits dated by luminescence indicate WFZ activity and fault slip after ~110 ka. Active and abandoned channels incised into Quaternary sediments are offset as much as 0.5 km across the WFZ. The ~50-km-length of the WFZ exceeds the ~4 km width of the CHA, indicating that the WFZ accommodates shear regionally. The Brocher et al., (2017) regional model predicts a WFZ slip rate of ~0.1 mm/yr, similar to the ~0.12 mm/yr estimated rate from separation of the CHA across the WFZ. Post-Miocene CHTF shortening rates account for only ~50% of the predicted 0.1 mm/yr regional shear. Depending on whether the CHTF is active in the Quaternary, the WFZ thus absorbs 50 to 100% of the predicted regional shear. Earthquakes up to Mw 7.2 on the WFZ or CHTF are possible given fault dimensions and historical seismicity.

Brocher, et al., 2017, Tectonics, doi:10.1002/2016TC004223.