STRAIN ACCOMMODATION WITHIN EMERGING SHEAR ZONES: INSIGHTS FROM THE CENTENNIAL SHEAR ZONE, MONTANA AND IDAHO

Dextral shear has been proposed as a mode of strain accommodation between the Centennial tectonic belt and the eastern Snake River Plain, yet no throughgoing strike-slip fault has been identified. A thick conglomerate of Neogene age constrains the Centennial shear zone hypothesis. Strain is distributed across numerous high angle conjugate strike-slip faults sub parallel to the orientation of maximum principle stress (σ₁). Shear experiments suggest that inherited fractures suppress formation of a throughgoing strike-slip fault. Subcritical offset and inherited structures have suppressed throughgoing strike-slip fault formation within the 2 Ma Centennial shear zone. Strain is distributed among a wide yet discrete primary deformation zone (PDZ). Results of this study illustrate fundamentally unique kinematics of emerging shear zones. Strain is distributed among many high angle, possibly conjugate, strike-slip faults that sub-parallel σ₁ and the PDZ. Deformation features are unorganized until total offset collapses strain upon a single linked fault. For these reasons, emerging and immature shear zones lack characteristic offset topography and centralized fault geometries of developed systems, bearing little resemblance to classical models of shear.