THE PROGRESSION FROM THIN- TO THICK-SKINNED THRUSTING IN THE IDAHO-MONTANA FOLD-THRUST BELT RESULTED FROM STEP-DOWN OF THE BASAL DÉCOLLEMENT INTO BASEMENT ROCKS TOWARD THE CONTINENTAL INTERIOR

Structural style in the North American Cordillera ranges from thin- to thick-skinned endmembers, which are usually attributed to a steeply vs. shallowly dipping subducting slab, respectively. However, thick-skinned thrusting in the Idaho-Montana fold-thrust belt developed adjacent to the concomitant Idaho batholith and preceded the ca. 85 Ma cessation of magmatism in the Sierra Nevada batholith. Recent mapping at 1:24,000 scale demonstrates that thin-skinned thrusts structurally overlie thick-skinned thrusts of the same age in the Idaho-Montana fold-thrust belt, defining a double-decker fold-thrust belt. We document a depth-dependent transition in structural style near the Lemhi arch, which is a basement high developed within the older rift and passive margin succession. We hypothesize an alternative to a flat slab control on the initiation of thick-skinned thrusting in the Idaho-Montana fold-thrust belt. Integrating field mapping, compiled geologic maps, and Raman Spectroscopy of Carbonaceous Material data, we estimate the maximum temperatures and construct a maximum burial profile along a ~ 500 km long balanced and restored cross section. The deepest regionally continuous horizon of the passive margin section served as the early thin-skinned décollement, with a long regional flat above the Lemhi arch. Shortening above this décollement from ~ 145 to 90 Ma resulted in burial of the Lemhi arch to ~6.5 km depth. The décollement geometry limited the ability of the surface slope of the wedge to increase, forcing the basal slope to increase by activating a deep décollement within the middle crust, resulting in a transition to thick-skinned thrusting at ~90-70 Ma. Early exhumation of the thick-skinned Blacktail-Snowcrest uplift in the foreland demonstrates rapid transmission of strain inboard, while contemporaneous exhumation of the more inboard Patterson culmination was related to progressive abandonment of the upper décollement from the rear of the wedge. The structural style and kinematics of the Idaho-Montana fold-thrust belt do not require specific tectonic events at the adjacent plate margin. Instead, the observed spectrum of structural styles developed during progressive propagation of a fold-thrust belt through thin passive margin rocks into the continental interior.