Intrusive and depositional constraints on the Cretaceous tectonic history of the southern Blue Mountains, eastern Oregon

Abstract

We present an integrated study of the postcollisional (post-Late Jurassic) history of the Blue Mountains province (Oregon and Idaho, USA) using constraints from Cretaceous igneous and sedimentary rocks. The Blue Mountains province consists of the Wallowa and Olds Ferry arcs, separated by forearc accretionary material of the Baker terrane. Four plutons (Lookout Mountain, Pedro Mountain, Amelia, Tureman Ranch) intrude along or near the Connor Creek fault, which separates the Izee and Baker terranes. High-precision U-Pb zircon ages indicate 129.4-123.8 Ma crystallization ages and exhibit a north-northeast-younging trend of the magmatism. The 40 Ar/ 39 Ar analyses on biotite and hornblende indicate very rapid (<1 m.y.) cooling below biotite closure temperature (~350 °C) for the plutons. The (U-Th)/He zircon analyses were done on a series of regional plutons, including the Lookout Mountain and Tureman Ranch plutons, and indicate a middle Cretaceous age of cooling through ~200 °C. Sr, Nd, and Pb isotope geochemistry on the four studied plutons confirms that the Izee terrane is on Olds Ferry terrane basement. We also present data from detrital zircons from Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These detrital zircons record only Paleozoic-Mesozoic ages with only juvenile Hf isotopic compositions, indicating derivation from juvenile accreted terrane lithosphere. Although the Blue Mountains province is juxtaposed against cratonic North America along the western Idaho shear zone, it shows trends in magmatism, cooling, and sediment deposition that differ from the adjacent part of North America and are consistent with a more southern position for terranes of this province at the time of their accretion. We therefore propose a tectonic history involving moderate northward translation of the Blue Mountains province along the western Idaho shear zone in the middle Cretaceous.

A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

“…5). Triassic and later Mesozoic igneous rocks and detrital zircon are present on the Wallowa and Olds Ferry Terranes of Washington, Idaho, and Oregon (LaMaskin et al, 2011;LaMaskin, 2012;LaMaskin et al, 2015;Gaschnig et al, 2017a). Triassic to Early Jurassic magmatic assemblages are reported from the Mojave Desert and the Eastern Coast Plutonic Complex (Barth and Wooden, 2006;Gehrels et al, 2009).…”

Cooling and exhumation of the southern Idaho batholith

“…Fourth, Giorgis et al (2005) hypothesized that a ~80-100-km-wide, subduction-related magmatic arc was active from ca. 125 to 91 Ma and was located entirely west of the current Idaho batholith; this arc was shortened to a ~8-km-wide zone by subsequent tectonism associated with the western Idaho shear zone (see also Gaschnig et al, 2017). The other coastal batholiths lack this type of major shear zone with intense shortening on their margins, suggesting that Cretaceous igneous rocks in Idaho might also record fundamentally different intrusive and cooling paths, reflecting a different tectonic environment relative to other coastal batholiths.…”

Introduction: EarthScope IDOR project (deformation and magmatic modification of a steep continental margin, western Idaho–eastern Oregon) themed issue

“…The Kurz et al (2016) interpretation resolves the issue of the isotopic compositions of the Neogene volcanic rocks in a manner consistent with the seismic results of Davenport et al (2017). Gaschnig et al (2017) examine Cretaceous igneous rocks and sediments on the southernmost part of the exposed Blue Mountain terranes in easternmost Oregon. A series of four plutons intrude the Blue Mountain terranes on or near the Conner Creek fault, which separates the Izee and Baker terranes.…”

“…The (U-Th)/ He zircon studies on these plutons indicate a middle Cretaceous cooling age through ~200 °C. Gaschnig et al (2017) also present a detrital zircon Introduction: EarthScope IDOR project | THEMED ISSUE analysis of Late Cretaceous sedimentary rocks at Dixie Butte, Oregon. These results indicate Paleozoic-Mesozoic ages with subtle, but important, differences from the ages of magmatism in the Blue Mountains and WISZ region.…”