Abstract

Eocene terrestrial strata in western North America are now dated with sufficient precision to examine the hypothesis that rollback of the Farallon flat-slab created a trenchward-migrating wave of dynamic and thermal topography. We use patterns of accumulation and unconformity, lake-type stratigraphy, and physical and isotopic provenance tools to track its migration across the Laramide foreland between 53 Ma and 47 Ma. Hydrologic ponding and coincident unconformity development began at 53 Ma, culminating at 51.8 Ma in the hydrologic closure of an ∼900,000 km2 area between the Cordilleran and Rocky Mountain divides. Four subsequent stream diversions and a southwest-expanding regional unconformity record trenchward migration of uplift. From 50 to 47 Ma, volcaniclastic detritus from magmatic centers coincident with the uplifted region progressively filled the Green River Formation lakes from north to south. These findings are consistent with recent numerical and conceptual models of slab rollback that predict initial dynamic subsidence above the slab hinge, followed by uplift and volcanism triggered by influx of asthenosphere beneath the overriding plate. Based on the surface record, rollback proceeded systematically at a rate of ∼6 cm/yr across the Wyoming craton from 53 to 47 Ma, resulting in removal of the entire Shatsky-conjugate oceanic plateau from the base of the North American lithosphere.

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