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

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

AN INTEGRATED TECTONIC AND MAGMATIC HISTORY OF THE INTERIOR REGION OF THE PACIFIC NORTHWEST: THE SIGNIFICANCE OF SYRINGA EMBAYMENT

Paper number: T321-9
Program: Technical Sessions
Time: 10:05 AM – 10:25 AM

Authors

Abstract

The abrupt, boundary between cratonic North American and accreted terranes occurs in western Idaho – eastern Washington, designated by geochemical gradients (Sr, Nd, O) that are spatially coincident with a major, regional-scale (western Idaho-Ahsahka) shear zone system. The boundary is oriented NS in central-southern Idaho, and then changes 90° near Orofino, ID, to become EW oriented: The boundary forms the eastern and northern boundaries of the Syringa embayment. We interpret the isotopic boundary and shear zones to follow a rift-transform system associated with the rifted, Precambrian margin of western Laurentia. The IDOR (Idaho-Oregon) EarthScope project demonstrated that the western Idaho shear zone is a dextral, transpressional feature that continues through the crust and produces a ~7 km Moho step. Northward (right-lateral) translation of accreted terranes along the western Idaho shear zone would produce a structural syntaxis at Orofino. Two NE-trending dextral shear zones – Mt Idaho and Limekiln – formed in the accreted terrane portion of the Syringa embayment during the Late Cretaceous. These shear zones formed after cessation of western Idaho-Ahsahka shear zones and may result from continued convergence into the Orofino syntaxis. This deformation history relates to the magmatic history. The voluminous Atlanta lobe peraluminous magmatism occurred after cessation of the western Idaho shear zone, and resulted from crustal thickening (not subduction) during orogenic plateau formation. Younger (< 70 Ma) Bitteroot lobe magmatism occurs exclusively in the northern Idaho batholith, inboard from the Orofino syntaxis. (U-Th)/He data from the continental side of the Syringa embayment has a unique and younger cooling history as compared to rest of accreted terrane-continent boundary. We propose that late Bitteroot lobe magmatism results from late contraction and crustal thickening only in the syntaxis. The Orofino syntaxis also exerts a major influence on current deformation, with Orofino as the fulcrum for Tertiary-present rotational geodetic motion in the northern US Cordillera.

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

doi: 10.1130/abs/2017AM-304892

© 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.