PALEOGEOGRAPHIC RECORD OF EOCENE FARALLON SLAB ROLLBACK BENEATH NORTH AMERICA

Eocene terrestrial strata in western North America are now dated and characterized with sufficient precision to examine the hypothesis that rollback of the Farallon flat-slab created a trenchward-migrating wave of dynamic and thermal topography. The Green River Formation lake basins occur east of the North American Cordillera and have long been ascribed to flexural subsidence and river blockages created by basement structures. However, the timing of basement faulting (80-50 Ma) and hydrologic ponding (53-47 Ma) in the foreland are offset, and only partly overlap. Ponding began at 53 Ma and culminating in the hydrologic closure at 51.8 Ma of a ~900,000 km² area between the Cordilleran and Rocky Mountain divides previously flowed to Hudson Bay. From 53-47 Ma, a broad area of surface uplift migrated trenchward, and is recorded by progressive inboard expansion of a regional unconformity across basin floors and a series of four major hinterland-directed stream diversions. In some cases (the Challis and Absaroka centers), surface uplift coincided with voluminous volcanism, whereas across much of the Wyoming craton, only scattered small magmatic centers (Black Hills, Devils Tower, Rattlesnake Mountains) and hydrothermal vents record its passage.

Expansion and closure of the Green River Formation lakes and synchronous formation and inward migration of a ringing unconformity suggest a basin-forming mechanism of broader-scale than flexural moats surrounding individual basement uplifts. Recent numerical and conceptual models of slab rollback predict broad initial dynamic subsidence above the slab hinge followed by uplift and volcanism triggered by the advection of asthenosphere beneath the overriding plate, and provide an attractive explanation for the Eocene as a whole in the North American foreland. A typical vertical ‘rollback basin’ succession should progressively record deposition of fluvial, lacustrine, then volcaniclastic lithofacies, followed by regional unconformity. Based on the surface record, we interpret that rollback proceeded systematically across the Wyoming craton at ~6 cm/yr from 53-47 Ma, resulting in removal of the entire Shatzke-conjugate oceanic plateau from the base of the North American lithosphere.