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

TIKOFF, Basil¹, KAHN, Maureen¹, FAYON, Annia², SCHMIDT, Keegan L.³, KELSO, Paul⁴, GASCHNIG, Richard⁵, VERVOORT, Jeffrey⁶ and HOLE, John A.⁷, (1)Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, (2)Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, (3)Division of Natural Science, Lewis - Clark State College, Lewiston, ID 83501, (4)Department of Geology and Physics, Lake Superior State University, 650 W. Easterday Ave, Sault Ste. Marie, MI 49783, (5)Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts Lowell, Lowell, MA 01854, (6)School of the Environment, Washington State University, Webster Physical Science Building 1228, Pullman, WA 99164, (7)Geosciences, Virginia Tech, Blacksburg, VA 24061, basil@geology.wisc.edu

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.

Session No. 321

T229. Paleocene-Eocene Tectonic and Magmatic Evolution of the Pacific Northwest

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

Tahoma 4 (The Conference Center)

Geological Society of America Abstracts with Programs. Vol. 49, No. 6
doi: 10.1130/abs/2017AM-304892

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GSA Annual Meeting in Seattle, Washington, USA - 2017

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