DISTRIBUTED AND DISCRETE TRANSPRESSION ON THE OBLIQUE LARAMIDE OROGEN MARGIN AT THE NORTHERN BIGHORN ARCH, MT/WY
The central Bighorn Arch has a steeply-dipping eastern forelimb, a gently-dipping western backlimb, and is dominated by minor thrust faults. The northern Bighorn Arch is dominated by strike-slip minor faults and is more symmetric, with steep dips in the western forelimb dying northward into the Pryor Mountains. Discrete left-lateral strike-slip faults are indicated by en echelon arrays of NE-striking normal faults (e.g., Nye-Bowler Lineament in the Pryors and the Lake Basin System north of Billings), the E-striking Sage Creek Fault through the Pryors, and the ENE-striking Tongue River Lineament north of Dayton, WY.
Minor fault data (53 localities) and paleomagnetic data (23 localities) from northernmost 160 km of the arch show a dominance of strike-slip minor faulting (82%). Minor fault compression directions from conjugate bisector analysis change from N65E in the central Bighorns to N40E in the north and are confirmed by AMS analysis. Paleomagnetic pole studies have ruled out rigid-body rotations as the sole cause of this change in compression direction.
Going northward, the change from low-angle, minor thrusts to vertical, strike-slip minor faults indicates that the northern Bighorn Arch is in a broad zone of distributed transpression. Additional discrete left-lateral shear on map-scale faults have a variety of strikes, suggesting that they are reactivated basement weaknesses. The cause of the change in compression directions is uncertain, and may be due to stress refraction within the transpressional zone or the effect of late-Laramide radial spreading of the Bighorn Arch.