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

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

ACCRETING SILETZIA: REGIONAL MAGMATIC AND TECTONIC CONSEQUENCES

Paper number: T321-4
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

Teleseismic tomography images a large high-velocity slab extending vertically beneath the ~50 Ma Challis magmatic trend from the base of North America to depths of 230–600 km. The only reasonable origin of this slab is the leading portion of the Farallon lithosphere that accreted to North America 50-55 Ma, along with the Siletzia ocean plateau that resides on the western portion of accreted Farallon lithosphere. We infer flat-slab subduction beneath most of western North America by 55 Ma, to which we attribute Laramide-age magmatic quiescence and Rocky Mtn contraction. Accretion then requires a tear in the flat slab separating the accreted Farallon lithosphere to the north of central Oregon and the subducting flat slab to the south. With Siletzia accretion, the ignimbrite flareup (Challis) and core complex extension began in Idaho and westernmost Montana, beneath the imaged mantle slab. This indicates flat-slab foundering. Simultaneously, a switch from trench-normal compression to tension occurred as the new Cascadia subduction zone rolled backward. That is, Siletzia accretion and the resulting reconfiguration of subduction ended the Laramide orogeny, initiated core-complex extension and the ignimbrite flareup. This tectonic-magmatic transition began in the Pacific Northwest and then propagated to the south as the torn northern margin of the subducting flat slab foundered beneath the Great Basin.

Siletzia accretion occurred as British Columbia moved northward, creating the Columbia Embayment. The accreted, Farallon slab that resided beneath the Columbia Embayment (and regions to the north and south) must have been ripped apart as the ocean plateau portion of Farallon lithosphere rotated clockwise many 10s of degrees. We suggest that Clarno-Pasco-Okanogan volcanism is a product of this slab rupture, with the Pasco Basin forming where continental crust was absent.

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

doi: 10.1130/abs/2017AM-306059

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.