EOCENE PALEOGEOGRAPHY OF THE WASHINGTON – VANCOUVER ISLAND REGION DURING STRIKE-SLIP FAULTING BEFORE AND AFTER A MAJOR 50 MA TRANSITION

We present tectonic reconstruction maps for the Washington – Vancouver Island region using ArcGIS software that tracks plate boundary deformation and the general paleogeography at 60, 55, 50, 45, and 40 Ma. We use a custom Add-In tool for the ESRI ArcGIS Desktop software suite. The tool takes a set of polygons depicting present day locations of tectonic blocks and sequentially restores each polygon by displacement of their centroid along time slice-specific vectors. Polygons are tectonic blocks defined by known faults and the geology as depicted on the State of Washington geologic map from Schuster et al. (2005). The main geologic constraints we used were the reconstruction of Wells and McCaffrey (2013) back to 16 Ma, no deformation from 16 to 35 Ma, and ages of basins and offsets on strike-slip faults from new U-Pb zircon geochronology reported in Eddy et al. (in review). The results of Eddy et al. include: a 60-51 Ma regional Swauk basin showing little or no evidence for syn-depositional strike-slip faulting; a deformation event at 51 – 50 Ma interpreted to be the collision of Siletzia with the margin of North America; activity on the regional N-striking Straight Creek fault from 51 to 35 Ma; formation of the Chumstick basin at 49 – 44 Ma during major strike-slip faulting on both NW and N striking faults; and a regional overlap sequence of the Roslyn – Deadhorse Canyon units at about 44 – 40 Ma when strike-slip faulting localized on the Straight Creek fault. The strike-slip faults used in our reconstructions and their offset include the Straight Creek fault with 150 km offset from 50 – 35 Ma, the Leavenworth fault zone with 30 km offset from 49 – 45 Ma, the Entiat fault with ca. 30 km offset from 49 – 40 Ma, and the Ross Lake fault zone with unknown, but likely many 10’s km offset from ca. 65 – 40 Ma. In the GIS-based reconstructions, we investigate a hypothesis that a major transition occurred at 50 Ma that involved the synchronous collision of Siletzia and possible demise of the Resurrection plate, and subsequent rapid northward translation of terranes along the coast and inboard within Washington and British Columbia. The strike-slip faults in our reconstructions represent 15 – 25% of the plate motion and formed within a former long lived magmatic arc and its proximal forearc basin in Washington, but in the arc to backarc in British Columbia.