EXHUMATION OF CRETACEOUS ROCKS AFFECTED BY THE WESTERN IDAHO SHEAR ZONE, OWYHEE MOUNTAINS, IDAHO

The northern Owyhee Mountains of Idaho, located south of the western Snake River Plain, contain granitoid plutonic rocks that are locally affected by deformation associated with the western Idaho shear zone (WISZ). One unit, the Wilson Peak granodiorite, lacks the well-developed and consistently oriented (020) foliation and is interpreted to post-date deformation. The area is also overlain by significant thicknesses of basalt flows. We present new biotite ⁴⁰Ar/³⁹Ar and apatite fission-track thermochronologic data that constrain the cooling from > 300 °C to < 60 °C during Cretaceous through Miocene time.

Biotite ⁴⁰Ar/³⁹Ar analyses obtained from granitic samples collected along an east-west transect range from 71.5 ± 0.2 Ma in the east to 82.7 ± 0.2 Ma in the west. Apatite fission-track ages form the same samples also increase from east to west ranging from 23.4 ± 1.2 Ma to 57.4 ± 2.6 Ma, respectively. This trend in cooling ages is similar to what has been observed in other exposures of the WISZ further north near McCall, ID. In the Owyhees, however, two samples yield significantly younger apatite fission-track ages of 15.6 ±0.9 Ma and 4.8 ± 0.4 Ma. These ages likely record resetting by the overlying Miocene to present basalts. Rocks were also sampled for (U-Th)/He zircon analyses, to determine if this system was also reset by voluminous basalt magmatism. The consistency of this new thermochronologic data from the Owyhee Mountains and the pre-existing thermochronologic data from the WISZ near McCall support the throughgoing nature of the WISZ across the western Snake River plain.