T215 Subduction Zone Response to Ridge and Plateau Collision: Perspectives from the Archean to the Cenozoic

Tuesday, 24 October 2017
8:00 AM - 12:00 PM
Tahoma 5

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

Paper number: T215-9
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306118

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