3D STRAIN AT TRANSITIONS IN FORELAND ARCH GEOMETRY: STRUCTURAL MODELING OF THE SOUTHERN BEARTOOTH ARCH TRANSITION ZONE, NW BIGHORN BASIN, NW WYOMING

The upper crustal expression of Laramide low-angle subduction in the Central Rocky Mountain foreland is an anastamosing network of basement arches whose structural culminations are linked by complex transition zones. These zones between arches of differing orientation and vergence commonly exhibit suites of second-order structures including strike-slip, backthrust, and diversely-oriented faults and folds whose roles in strain accommodation are enigmatic. End-member hypotheses to explain 3D strain at foreland arch transitions include 1) uniformly-directed slip on diversely-oriented pre-existing basement structures, and 2) diversely-directed slip necessary for 3D strain accommodation. These opposing views of arch transition zones were tested by analyzing inferred shortening directions from minor fault slip data throughout the southern Beartooth arch transition zone. This transition from the NW-trending Beartooth arch to the NNW-trending western Bighorn Basin margin is characterized by a decrease in arch amplitude, a southerly jog in trend, and slip transfer along blind master thrusts. The results indicate that the area's three principal, diversely-oriented structural features were obliquely reactivated within a regionally consistent ~N65E shortening field and experienced ~uniformly-directed slip.

Strain within this relatively uniform slip field was modeled in 3D by combining serial, restorable cross sections aligned N65E, parallel to the inferred slip direction. Geometric and kinematic insights from the model include: 1) slip transfers between en echelon splays of the arch-bounding master fault; 2) along-strike variations in fault-related folding can be dramatic; 3) material appears to rotate within zones of oblique-slip; and 4) fault block compartmentalization along a NE-SW tear fault acts as the abrupt termination of four separate reverse faults, isolating fault blocks within the transition zone. The southern Beartooth arch transition offers a window into the process of foreland arch linkage in which oblique-slip faulting accommodated an arch jog and amplitude change via ~uniformly-directed slip on reactivated basement structures.