Mesozoic tectonics west of the accretionary boundary in west-central Idaho

Abstract

The late Mesozoic accretionary boundary in west-central Idaho has played a critical role in tectonic models proposed for the northwestern U.S. Cordillera. From west-to-east, major elements include the Permian to Jurassic Wallowa island-arc terrane, a poorly understood transition zone consisting of the Riggins Group assemblage and deformation belt along the west side of the island arc-continent boundary, Late Jurassic to Cretaceous arc-continent boundary, and Precambrian North American margin intruded by the Cretaceous–Paleogene Idaho batholith. We focus on the transition zone in the area between White Bird and Riggins, Idaho, which includes a contractional belt in variously deformed and metamorphosed rocks of island-arc affinity. We propose that the rocks of the entire transition zone, including those originally defined as the Riggins Group, are likely of Wallowa terrane origin and/or related basinal assemblages. Ultramafic rocks in the transition zone are possibly related to a Jurassic or Cretaceous basinal assemblage that includes the Squaw Creek Schist of the Riggins Group. Our recent work addresses the kinematic history of structures in the contractional belt. The belt was reactivated in the Neogene to accommodate mostly brittle normal faulting that strongly influenced preservation of the Miocene Columbia River Basalt Group at this location along the eastern margin of the flood basalt province. This field guide provides a road log for examining the geology between Moscow and New Meadows, Idaho, along U.S. Highway 95.

Orogenic link ∼41°N–46°N: Collisional mountain building and basin closure in the Cordillera of western North America

“…Maps and sections are accompanied by stereographic projection plots compiled across four fault-bounded domains (I-IV; Fig. 3) bridging accreted island-arc rocks of the eastern Blue Mountains Province (Wallowa terrane + correlative units; Hamilton, 1963a;Vallier, 1977;Schmidt et al, 2016b) and the western margin of Laurentia (Hamilton, 1969a;Blake, 1991). From structurally lowest to highest levels (west to east across the SRSZ), major domain-bounding structures include the Heavens Gate fault (Gray and Oldow, 2005;Kauffman et al, 2014;Schmidt et al, 2016b), Rapid River thrust/Riggins normal fault (Hamilton, 1969a;Onasch, 1977Onasch, , 1987Hooper, 1982;Lund and Snee, 1988), and Pollock Mountain thrust (Aliberti, 1988;Blake, 1991;Selverstone et al, 1992;McKay et al, 2017).…”

“…Given its age, location, and bulk chemical composition, sample 06KG15 may represent a small satellite body associated with weakly metamorphosed (greenschist) and deformed (local magmatic fabric) hornblende-bearing tonalite of the Fish Hatchery stock (130.0 ± 1.9 Ma; 238 U/ 206 Pb locality AM-05 of Aliberti, 1988;Snee et al, 1995;Gray and Isakson, 2016). Both intrusions are hosted by volcanogenic rocks of the Wild Sheep Creek Formation (Sarewitz, 1983;Schmidt et al, 2016b) and are proximal to east-northeast-dipping imbricate faults (U-Pb locality 06KG15, upper plate of Heavens Gate fault [ Fig. 7B], and U-Pb locality AM-05, footwall of Morrison Ridge-Rapid River fault system [ Fig.…”

“…The youngest and most consistent population produced a weighted mean date of 90.1 ± 1.2 Ma. We interpret older populations as xenocrysts incorporated from enclosing island-arc-supracrustal rocks, i.e., late Paleozoic to mid-Mesozoic volcanic cover and/or associated hypabyssal intrusions (Vallier, 1977;Schmidt et al, 2016b). CA-ID-TIMS analysis on six zircon grains showing unzoned to well-developed growth rings yielded a weighted mean U-Pb date of 90.62 ± 0.03 Ma, which is interpreted here as a magmatic crystallization age ( Figs.…”

“…Blake et al, 2009Blake et al, , 2016. West of locality ID48, late Early Cretaceous shortening was accommodated by east-dipping imbricate structures of the Rapid River thrust-Morrison Ridge fault system (small-scale duplexing in Schmidt et al, 2016b;Shingle Creek structures of Onasch, 1977), which crosscut ca. 113-111 Ma garnet-bearing metamorphic tectonites (Wilford, 2012;McKay et al, 2017; see truncated isograd of Hamilton, 1963bHamilton, , 1969afigure 2 Research Paper of Gray et al, 2012) Figs.…”

“…According to LaMaskin et al (2015), exposed rocks of the Wallowa terrane occupied a fluvial to deep-marine setting during pre-160 Ma terrane accretion. Because the sedimentary assemblages at Pittsburg Landing have not produced detrital zircon of pre-late Paleozoic age (3% of grains older than 260 LaMaskin et al, 2015) and were sourced from crystalline basement/volcanic cover rocks of the Wallowa arc (Walker, 1986;Vallier, 1995;Schmidt et al, 2016aSchmidt et al, , 2016bKurz et al, 2017), we question terrane accretion prior to 160 Ma sedimentation (Coon Hollow basin [CHB] of Figs. 2A and 14).…”