NO FLAT-SLAB NEEDED: THE TRANSITION FROM THIN- TO THICK-SKINNED THRUSTING IN THE IDAHO-MONTANA FOLD-THRUST BELT WAS CONTROLLED BY THE PRE-OROGENIC STRATIGRAPHY

The eastward transition from thin-skinned, Sevier belt thrusting to thick-skinned Laramide belt thrusting is commonly attributed to inception of shallow subduction. However, invoking a flat-slab trigger for the northeastward transition in structural style within the Idaho-Montana fold-thrust belt is problematic: in southwestern Montana, thick-skinned thrusts were activated prior to initiation of shallow subduction along the plate boundary and the Idaho batholith remained active from mid Cretaceous to Eocene time. We present new field results from central Idaho to southwestern Montana that implicate a non flat-slab, pre-orogenic control on the transition in structural style. Within Mississippian and younger sedimentary rocks, a regional décollement horizon kinematically linked thin-skinned thrusts in central Idaho with those in southwestern Montana. In central Idaho, directly northeast of the Idaho batholith, lower Paleozoic rocks were similarly shortened in a thin-skinned style with a basal décollement horizon near the base of a newly documented >2.5 km thick section of fine-grained, Neoproterozoic-Cambrian rocks. However, to the northeast in east-central Idaho, Neoproterozoic to Cambrian basal décollement horizons utilized elsewhere within the Sevier belt are missing within the Lemhi arch. Instead, Ordovician sandstone unconformably overlies tilted Mesoproterozoic quartzite and crystalline basement. Within this region, structurally deeper thrusts behaved similarly to thick-skinned thrusts of southwestern Montana, including: local inversion of older rift faults; formation of major (>15 km wavelength) anticlines that were truncated by thrusts; and crosscutting of older and structurally shallower thin-skinned thrusts by later, deeper thrusts. Our model for evolution of the Idaho-Montana fold-thrust belt involves early, thin-skinned thrusting. Subsequently, the basal detachment horizon stepped downward into previously deformed rocks within the Lemhi arch, which lacked favorable décollement horizons. This activated a transition to thick-skinned thrusting. Thus, the transition from a thin- to a thick-skinned structural style corresponds with a major change in the pre-orogenic stratigraphy near the rifted margin of Laurentia, without the need to invoke shallow subduction.