T321 Paleocene-Eocene Tectonic and Magmatic Evolution of the Pacific Northwest

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Tahoma 4

LIFE AND DEATH OF THE RESURRECTION PLATE: STILL THE MOST COMPELLING EXPLANATION FOR COEVAL FOREARC MAGMATISM IN THE PACIFIC NORTHWEST AND ALASKA

Paper number: T321-3
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

In 2003 we proposed that a plate is missing from reconstructions of the northeastern Pacific in Paleocene-Eocene time (Haeussler et al. 2003, GSAB). This plate, the Resurrection plate, was located east of the Kula and north of the Farallon plate. We interpreted coeval near-trench or forearc magmatism in southern Alaska and the Cascadia margin as evidence for two slab windows associated with trench-ridge-trench (TRT) triple junctions, located at the western and southern boundaries of the Resurrection plate. In Alaska, the Sanak-Baranof belt of near-trench intrusions records a west to east migration, from 61-50 Ma, of the northern TRT triple junction along a 2100-km-long section of coastline. In the Pacific Northwest, voluminous forearc volcanism of the Siletzia terrane occurred between 66-48 Ma in rift basins at the subduction margin. Synchronous near-trench magmatism from SE Alaska to Puget Sound at 50 Ma documented the subduction of a spreading center, the crest of which was subparallel to the SE AK-BC margin. We interpreted this ~50 Ma event as recording the subduction zone consumption of the last of the Resurrection plate. This hypothesis provides an explanation for a number of aspects of northern Cordilleran geology, including northward terrane transport, the cessation of arc magmatism along the SE Alaska and British Columbia margin around 50 Ma, exhumation of core complexes and Challis and related volcanism in the backarc region after 50 Ma.

New studies on both margins confirm and clarify details of this hypothesis. New dates from intrusions on Baranof Island in SE Alaska show initiation of slab window magmatism as early as 51 Ma and extending to as young as 41 Ma, which suggests there were significant ridge-transform offsets on the subducting slab. New analyses on Siletzia rocks showed they were rapidly erupted between 56-49 Ma, and point to their origin as an oceanic plateau, likely from the Yellowstone hot spot interacting with the subducting Resurrection-Farallon spreading center. The plateau was then accreted onto the continental margin between 50.5 and 49 Ma in Oregon.

The Resurrection Plate hypothesis, nuanced slightly to accommodate new data, remains a viable explanation for the forearc magmatism, and still provides the most inclusive interpretation of many aspects of Paleogene geology along the NE Pacific Rim.

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

doi: 10.1130/abs/2017AM-307987

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