The western Idaho shear zone: a transpressional self-exhuming, intra-arc shear zone

The mid-Cretaceous Salmon River suture zone of west-central Idaho marks the boundary between the North American craton and the accreted terranes to the west. The arc-craton boundary in this part of the Cordillera is defined by sharp gradients in the Sr and O isotope ratios, indicating that oceanic lithosphere is juxtaposed directly against continental lithosphere. The granitic units that carry this isotopic gradient span a 10 km wide zone. This is in contrast to other Cordilleran arcs, such as the Sierra Nevada, where the isotopic gradient occurs over a much wider zone (40-50 km). Additionally, this narrow band of arc rocks is characterized by mid-crustal mineral assemblages, suggesting an origin at depths greater than 20 km. To the west, the accreted terranes have volcanics and volcaniclastic sediments that display lower greenschist facies assemblages. Thus the Salmon River suture zone exposes a much deeper level of crust than is observed on the arc side of the boundary. The isotropic gradient, the width of the arc, and the level of exposure in west-central Idaho may reflect a tranpsressional self-exhuming, intra-arc shear zone. The western Idaho shear zone (WISZ) is a dextral transpressional feature that reactivated the Salmon River suture zone in the Late Cretaceous. U-Pb zircon ages from a syn-deformational tonalitic pluton indicate that deformation was active by 92.2 +/- 1.3 Ma. The solid-state fabric of the WISZ is cut by several low-angle, mesoscale shear zones. 40Ar-39Ar ages on recrystallized biotite and hornblende from these shear zones suggests that deformation ceased 80 Ma. Biotite and hornblende thermochronology indicates that these rocks were initially rapidly exhumed between 90 and 80 Ma, however they did not pass thought the apatite fission track closure temperature until much later (44-28 Ma). Thus exhumation was rapid during transpressional deformation and slowed significantly afterwards. We hypothesize that transpression involved a significant contractional component, resulting in shortening across the WISZ and a steepening of the isotropic gradient across the arc-craton transition. This narrowing of the arc was accommodated by vertical extrusion, causing the exhumation of mid-crustal level rocks in the shear zone. Since exhumation was driven by deformation the exposure of rocks from deeper in the crust is centered on the WISZ.