THE IDAHO SYNTAXIS: WESTERN IDAHO SHEAR ZONE DEFORMATION THAT HAS GONE ROUND THE OROFINO BEND
STETSON-LEE, Tor1, TIKOFF, Basil2, BYERLY, Ad1, KELSO, Paul3 and VERVOORT, Jeff D.4, (1)Department of Geoscience, University of Wisconsin-Madison, Weeks Hall, 1215 West Dayton Street, Madison, WI 53715, (2)Department of Geoscience, University of Wisconsin, 1215 W Dayton St, Madison, WI 53706, (3)Department of Geology and Physics, Lake Superior State University, 650 W. Easterday Ave, Sault Ste. Marie, MI 49783, (4)School of Earth and Environmental Sciences, Washington State University, Webster Physical Science Building 1228, Pullman, WA 99164-2812, stetsonlee@wisc.edu
A long-standing problem in the tectonics of Idaho is 90° bend (or “dog leg”) in the orientation of the Sr 0.706 isopleth that occurs near Orofino, Idaho. South of Orofino, the Sr 0.706 line is oriented North-South and is coincident with the western Idaho shear zone (WISZ). West of Orofino, the Sr 0.706 line is oriented East-West. The question is whether this feature is: 1) A primary structure; 2) A modification of an early and straighter boundary; or 3) Represents the effect of a cross-cutting geological structure. Recent paleomagnetic work suggests that ~90 Ma plutons show a consistent pole, regardless of whether they are west or south of Orofino. Recent Ar-Ar analyses of both biotite (76.06 ± 0.14 Ma; 78.03 ± 0.018 Ma) and hornblende (84.2 ± 0.13 Ma) in the granitic rocks in and west of Orofino exhibit the same cooling pattern as granitic rocks within the western Idaho shear zone to the south of Orofino. For these reasons, deformation at Orofino and directly westward occurred simultaneously with motion on the dextral, transpressional WISZ to the south.
Structural work has focused on the kinematics of deformation near Orofino. Foliation is moderate to steeply dipping, and lineations are down-dip. In addition to the reverse motion documented by earlier workers, there is a consistent sinistral component of shear observed when looking perpendicular to the lineation direction. This combination suggests that a combination of transpressive contractional tectonism occurred on the northern section continental margin (denoted by the EW orientation of the Sr 0.706 isopleth). A sinistrally resolved shear sense is consistent with kinematic analyses from the western Idaho shear zone, which indicate a ~60° angle of oblique (dextral) convergence (or 060 in present coordinates). This angle of convergence, when applied to the orientation of the shear zone near Orofino, would result in a sinistral component of deformation.
The activation of a dextral component of shear on the WISZ and a simultaneous sinistral component of shear on the EW-oriented boundary indicates that material movement is being moved into a corner: This is the geometry of a syntaxis. The WISZ deformation occurs at Orofino (it “goes around the Sr bend”), but exhibits an opposite (sinistral) sense of shear at Orofino and westward.
