Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon

Abstract

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic-early Mesozoic) accretionary complex with an asso ciated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of metaigneous and metasedimentary rocks faulted into chert-argillite mélange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial 87 Sr/ 86 Sr ratios (0.7033-0.7034) and positive initial ε Nd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite mélange have initial 87 Sr/ 86 Sr values ranging from 0.7073 to 0.7094 and initial ε Nd values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analog to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacifi c region.

Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

“…The only direct evidence available for the facing direction of an early Mesozoic arc in the Canadian Cordillera comes from the west-facing arc on Quesnellia in southern British Columbia (Mortimer 1987), although the case is made that the early Mesozoic Talkeetna arc in southern Alaska faced towards Panthalassa with the accompanying accretionary complex preserved in the innermost Chugach terrane (Plafker and Berg 1994). 'Molucca Sea models' have been invoked by Mihalynuk et al (1994) for Early -Middle Jurassic enclosure of the Cache Creek terrane between arcs on Stikinia and Quesnellia, by Lynch (1995) for Early Cretaceous enclosure of the Tyaughton-Methow basin between arcs in the southwestern Coast Mountains and the southern interior of British Columbia, by Schwartz et al (2010) for Late Jurassic enclosure of the Baker terrane between the Wallowa and Olds Ferry arcs in eastern Oregon, and by Dickinson and Lawton (2001) in Mexico for Jurassic -Cretaceous enclosure of the Mezcalera Ocean between the Guerrero terrane and a continental margin arc.…”

U-Pb geochronology and geochemistry of intrusive rocks from the Cougar Creek Complex, Wallowa arc terrane, Blue Mountains Province, Oregon-Idaho

“…Rocks of the Blue Mountains Province include an assemblage of volcanic arc terranes, sedimentary basins, and mélange complexes that record a protracted history of Permian to Cretaceous subduction, magmatism, and accretion of crustal fragments to western North America (Brooks and Vallier, 1978;Dickinson, 1979;Avé Lallemant et al, 1980;Wilson and Cox, 1980;Hillhouse et al, 1982;White et al, 1992;Vallier, 1995;Gray and Oldow, 2005;Dorsey and LaMaskin , 2007;LaMaskin et al, 2008;Schwartz et al, 2010). The focus of this study is the Wallowa arc terrane, which lies north of the Baker mélange terrane, and northwest of the Olds Ferry continental fringing arc terrane.…”

“…Previously, the timing of magmatic activity in the Wallowa terrane was based on limited geochronologic, thermo chrono logic, and paleontologic data (Vallier, 1977;Balcer, 1980;Avé Lallemant et al, 1980, 1985Walker, 1986Walker, , 1995Schwartz et al, 2010). Furthermore, the major-and trace-element geochemistry of igneous rocks from the Wallowa arc terrane remains inadequately characterized.…”

East-derived strata in the Methow basin record rapid mid-Cretaceous uplift of the southern Coast Mountains batholith

“…Terranes of the Blue Mountains province include a Permian island-arc sequence overlain by Triassic volcanic and volcaniclastic rocks, a Middle to Late Triassic island-arc assemblage containing mostly andesitic volcanic rocks with intercalated sedimentary rocks, disrupted fragments of ocean floor, island-arc volcanic, plutonic, and sedimentary rocks ranging in age from Middle Devonian to Early Jurassic, and a Triassic-Jurassic sedimentary overlap succession that contains abundant Precambrian detrital zircon (Vallier 1995;LaMaskin et al 2008LaMaskin et al , 2011Schwartz et al 2010). The Blue Mountains Province is intruded by Triassic, Jurassic, and Cretaceous plutons ranging in age from ca.…”