CORNER TECTONICS: KINEMATICS OF A LATE CRETACEOUS TRANSPRESSIONAL TERMINATION IN THE SYRINGA EMBAYMENT OF NORTH-CENTRAL IDAHO

Following Jurassic collision of outboard terranes and Early Cretaceous contraction on the nearly north-south trending Mesozoic accretionary boundary in western Idaho, Late Cretaceous dextral transpression initiated on the western Idaho shear zone (WISZ) as the estimated convergence direction changed to azimuth 060 (present orientations). In the Syringa embayment of north-central Idaho, the north-south trending WISZ abruptly bends into the northwest-southeast striking contractional Ahsahka shear zone (ASZ) that was coeval and kinematically coupled with the WISZ. North of the ASZ, the boundary then bends again into an extensively covered and poorly known east-west trending segment described by early studies as a mylonite shear zone with rare lineation and little kinematic or chronological information. Our re-examination of the exposed mylonite zone at Green Knob, Idaho on the east-west trending segment indicates that it consists of shallowly north-dipping foliation and more poorly developed, shallowly-east-northeast plunging lineation in Neoproterozoic metasedimentary and intruding Cretaceous tonalite rocks. Kinematic indicators are developed parallel to foliation and lineation and suggest top-west-southwest (sinistral-reverse) shear sense. This assemblage is intruded by 84.6 +/- 1 Ma (U/Pb zirc; Tikoff et al., 2023) hornblende quartz diorite with magmatic fabrics that parallel the mylonite. We interpret this age to date cessation of shear on the sinistral-reverse shear zone and further hypothesize that it is coeval and kinematically compatible with shear on the WISZ-ASZ structural system. Fabrics along this zone suggests it is oriented ~30 degrees to the estimated 060 azimuth convergence direction in the embayment. In contrast the WISZ, with steeply-dipping foliation, steeply-pitching lineation, and shear direction parallel to foliation and perpendicular to lineation, has been well-characterized as a contraction-dominated transpressional zone, consistent with its orientation at a 60 degree angle to the convergence direction. The ASZ, with moderately-dipping foliation, steeply-pitching lineation, and shear direction parallel to foliation and lineation, is characterized as a reverse shear zone, consistent with its orientation at 90 degrees to convergence.