A LATE CRETACEOUS SINISTRAL REVERSE SHEAR ZONE AT GREEN KNOB, IDAHO ALONG THE EAST-WEST TRENDING SEGMENT OF A CORNER IN THE CORDILLERAN MESOZOIC ACCRETIONARY BOUNDARY

The normally north-south trending Mesozoic accretionary boundary that occurs in western Idaho forms a corner in north-central Idaho. The east-west trending segment of the corner is poorly exposed and little researched. We collected new structural data at Green Knob, Idaho, an area that has been previously mapped by Anderson (1991) to establish details of its kinematics and timing. The basement geology at Green Knob consists of Mylonitized Neoproterozoic quartzite and schist which have been extensively intruded by undated biotite tonalite that is also variably mylonitized. These rocks are intruded by hornblende biotite quartz diorite which has been dated at 84.9 +/- 1 Ma (U-Pb zircon, Tikoff et al, 2023), and contain magmatic fabrics that parallel the mylonitic fabrics. We interpret this age as close to the age of cessation of shear. The mylonitic fabric consists of well-developed dominantly shallowly-to-moderately north-dipping foliation. Lineation is more common than previously mapped and is dominantly shallow east-northeast plunging. Asymmetric fabrics are moderately well-developed along directions parallel to lineation and foliation and indicate east-northeast over west-southwest (reverse sinistral) sense of shear. The timing and kinematics of the east-west trending mylonite shear zone at Green Knob, Idaho suggest that it is coeval with dextral transpression that occurred on the north-south striking western Idaho shear zone to the south, and likely kinematically compatible, within the context of transpression in a corner of the boundary.