TIMING OF EXHUMATION OF LARAMIDE RANGES IN MONTANA AND WYOMING AND IMPLICATIONS FOR FLAT-SLAB SUBDUCTION PROCESSES (Invited Presentation)

Flat-slab subduction of oceanic plateaus and aseismic ridges has been associated with basement block uplifts, broken foreland basin development, dynamic topography and subsidence, regional plateau uplift, slab melting and high seismicity. Flat-slab subduction has been used to explain thick-skinned deformation through intraplate coupling and increasing basal shear stress or by increasing horizontal compressional stresses near the trench that propagate >500 km inboard from the plate margin.

The timing and patterns of Laramide uplifts have been constrained using the ages of synorogenic conglomerates, changes in basin subsidence, and timing of exhumation of basement block uplift through thermochronology. In particular, the history of magmatism in Montana and Wyoming has been used traditionally to track the location of the flat-slab between ca. 80 and 50 Ma, yet numerical models of flat-slab subduction do not include the Laramide region in Montana. These models predict NE-ward younging of dynamic subsidence and exhumation of Laramide structures as a response to subduction of the buoyant Shatsky plateau conjugate. The history of uplift, basin subsidence, and erosion suggest that exhumation in the Laramide of Wyoming is mostly early Cenozoic and possibly represents thermal processes related to westward roll-back of the subducting slab or accretion and subsequent removal of Shatsky conjugate depleted mantle lithosphere under the Wyoming craton. Whereas the exhumation history in Wyoming may record a second phase of Laramide tectonics related to slab roll-back or removal, the earlier history associated with flat-slab subduction seems to have been overprinted. Preliminary thermochronological data from SW Montana indicate that exhumation of Laramide structures there started as early as ca. 90 Ma, much earlier than in Wyoming and before the magmatic sweep through the region. If exhumation of Laramide structures in Montana is associated with flat-slab subduction, and if flat-slab subduction is associated with subduction of the Shatsky plateau conjugate, our new data suggest that Montana preserves the earliest record of flat-slab subduction. Moreover, magmatism may be out of sync with flat-slab subduction in Montana, potentially calling for a different plate configuration in future numerical modeling.