UPPER PLATE RESPONSE TO RIDGE AND PLATEAU SUBDUCTION, WASHINGTON CASCADES, USA: IMPLICATIONS FOR PLATE TECTONIC EVOLUTION OF THE PACIFIC NORTHWEST

From ~57 – 40 Ma, the Washington – Vancouver Island region lay in the upper plate of convergent to oblique convergent plate boundaries with a spreading ridge subducting off North America. This ridge was adjacent to Vancouver Island ca. 50 Ma and subsequently jumped to the south after an “Iceland-like” oceanic plateau (Siletzia) accreted to North America. The response of the upper plate supports a complex series of events. From ~57 – 53 Ma, the NW – SE trending terrestrial Swauk basin formed in the forearc region of a long lived magmatic arc that ended at ~60 Ma. There was limited strike-slip faulting and magmatism during this time. There were many changes in the upper plate from 53 - 50 Ma: (i) the Siletzia oceanic plateau formed over the Yellowstone hot spot and accreted by ~50 Ma, (ii) magmatism increased greatly to the east along the Kamloops – Challis belt accompanied by major E-W extension, (iii) magmatism migrated SW-ward across NE WA, (iv) the Swauk basin changed to NE-directed paleoflow and was deformed from ~51 – 50 Ma, (v) magmatism flared up on Vancouver Island. These events all suggest that flat slab subduction from ~60 – 53 Ma was followed by slab rollback and breakoff during accretion of Siletzia to North American from 53 – 50 Ma and immediately south of a spreading ridge off Vancouver Island and west of NW WA. The spreading ridge may have stalled off Vancouver Island until ~45 Ma as major N and NW-striking dextral faults were active from 50 – 45 Ma across the WA Cascades, and E-W extension continued in the backarc. Enigmatic magmatism across NW WA suggests some combination of continued slab rollback and/or slab window passage until 44 Ma. At 45/44 Ma, the spreading ridge jumped south to off of northern Oregon and strike-slip faulting localized on the N striking Straight Creek – Fraser fault from 45 - >35 Ma. The dramatic increase in strike-slip faulting, return of magmatism, and exhumation of the crystalline core of the WA Cascades from ~50 – 45 Ma may be explained by these complex regional plate tectonic events. However, major strike-slip faulting extends up to Alaska, and is synchronous with a Pacific basin wide plate reorganization that resulted in rapidly northward motions of the Kula plate relative to North America; thus global tectonic changes combined with local plate tectonics to produce the series of events in the Pacific NW in the Eocene.