PALEOMAGNETIC STUDIES ALONG THE WESTERN IDAHO SHEAR ZONE (WISZ): WHEN DID THE 90 DEGREE BEND IN THE SR 0.706 LINE OCCUR?

The Sr 0.706 isopleth demarcates the edge of cratonic North America throughout the Cordillera. This boundary is spatially coincident with the western Idaho shear zone (WISZ), and strikes approximately NS throughout most of Idaho. Near Orofino, ID, the Sr 0.706 line bends abruptly to an EW strike. Understanding the origin and timing of the 90° bend has been a recurring problem in the regional geology.

We have conducted paleomagnetic studies on undeformed metaluminous granitoids immediately cratonward (northward) of the Sr 0.706 line to address this issue. The paleomagnetic remanence carrier is relatively pure magnetite and/or titanohematite with magnetic blocking temperatures ranging from 550-580° C and 590-630° C. To constrain the timing of acquisition of the remanent magnetization, we conducted geochronology and paleomagnetic sampling on the same outcrops. The U-Pb analyses of zircons range from 90 Ma to 88 Ma (constraining crystallization), Ar-Ar analyses on hornblende provide an age of 84.2 ± 0.13 Ma (constraining cooling through ~450° C), and Ar-Ar analyses of biotite provide ages of 76.06 ± 0.14 Ma and 78.03 ± 0.018 Ma (constraining cooling through ~325° C).

The sites have an average declination of 003 and inclination of 65° (±15°) which suggests ~30° vertical axis clockwise rotation of the sites relative to the North American virtual geomagnetic pole, which was nearly stationary from 125-85 Ma. Results suggest that the bend in the WISZ is a primary feature, and that material on both sides of the Sr 0.706 isopleth experienced unified rigid body rotation after cessation of WISZ deformation. The restored geometry of the plate boundary reveals shear zones striking toward 030, a geometry coincident with the inferred orientation of Neoproterozoic transform faults associated with passive margin development. We infer that the western Idaho shear zone and simultaneous deformation near Orofino occurred at ~95 Ma, controlled by the orientation of the Precambrian structures. During post-90 Ma Sevier thrusting, the entire zone rotated as a coherent block, presumably on a sub-horizontal detachment. The Lewis and Clark line - a WNW striking zone that experienced sinistral transpression from 88 – 83 Ma and 76 – 55 Ma - may have partially accommodated this large scale rotation.