MIGRATING MANTLE AND SLAB-DRIVEN DRAINAGE DISRUPTION AND LAKE FORMATION IN THE HINTERLAND AND FORELAND OF THE PALEOGENE NORTH AMERICAN CORDILLERA

The hinterland and foreland of the North American Cordilleran both experienced ponded continental drainage during the Paleogene that has been interpreted to result from removal of the flattened Farallon slab. Its removal and subsequent replacement by asthenosphere have been invoked to explain a southwestward migrating wave of Eocene to Oligocene volcanism and topographic uplift. However, until recently, the timing of slab removal from beneath North America was only ambiguously related to the surface record. We apply recent radioisotopic geochronology and terrestrial facies analysis to reconstruct Paleogene landscape evolution and assess competing interpretations of Cenozoic tectonics. Though there are significant structural geophysical differences between the hinterland and foreland, both experienced a migrating, time-transgressive succession of events at multiple locations during the Paleogene: (1) basin-wide transitions from fluvial to increasingly evaporative lacustrine conditions, which largely post-dates formation of Laramide basement structures; followed by (2) volcanism; then (3) pronounced regional unconformity. We interpret this succession, which migrates from NE to SW over time, to reflect the damming of drainages by a broad wave of low magnitude (< 1 km) uplift triggered by replacement of cold shallow slab by hot asthenosphere and, exclusively in the hinterland, the delamination of Cordilleran mantle lithosphere. Mantle lithosphere remains in place beneath the foreland, but geophysical evidence suggests that it was removed from the hinterland lithosphere sometime between the Paleogene and the present day. Initial developmental of metamorphic core complexes in the hinterland occurred simultaneously with rollback-magmatism and, interestingly, is spatially coincident but post-dates the formation of lake basins. We hypothesize that this spatial coincidence and temporal progression of events suggests a shared mechanism: the initiation and detachment of Raleigh-Taylor instabilities at the base of the North American lithosphere following flat slab removal and its replacement by asthenosphere. Were this mechanism were proven true, the spatial pattern of metamorphic core complexes across the western USA would essentially map the initiation of lithospheric delamination.