FLAT SLAB SUBDUCTION IN THE GEOLOGICAL RECORD: CONSIDERATION OF MODERN ANALOGUES

Modern flat subduction zones correlate spatially and temporally with the subduction of oceanic plateau and anomalously warm oceanic crust. The Andean margin, for example, has several flat-slab segments, up to 500 km wide, that are each correlated with subduction of anomalously warm oceanic crust, represented by oceanic plateau. The Late Cretaceous-Eocene Laramide orogeny of the western United States is characterized by an absence of magmatism, widespread hinterland deformation and thick-skinned tectonics, characteristics that have been attributed to flat-slab subduction. However the origin of the flat-slab in this region is controversial. We attribute the generation of the flat slab to subduction of an elongate swell and associated oceanic plateau associated with the ancestral Yellowstone hotspot. Reconstructions in a hot spot reference frame indicate that the Yellowstone hot spot was located beneath the oceanic Kula or Farallon plate until about 50 Ma. Evidence for the existence of the plume beneath the oceanic plate prior to this time is derived from 70-50 Ma accreted terranes such as the Carmacks and Crescent mafic complexes of western Canada. These terranes have geochemical and paleomagnetic data compatible with an origin as seamounts above the Yellowstone plume. Reconstructions indicate that the Crescent seamount was emplaced into oceanic crust that was 20 to 30 million years old and buoyancy flux calculations yield a value similar to those for the modern Yellowstone hotspot. Therefore a plume of comparable vigor and at the same paleolatitude as Yellowstone was responsible for generation of these seamounts. Such a plume would have generated a swell up to 2400 km in length and an oceanic plateau that would have been subducted as North America drifted westward in a hotspot reference frame. Assuming the present distribution of hotspots and underlying plumes is representative of the past, then overriding of plumes and their buoyant swells at convergent margins should be common in the geologic record. This orogenic activity profoundly changes the geometry of subduction zones, and hence the style of orogenic activity, and may provide a unified interpretation for a number of individually enigmatic events that characterize the Laramide orogeny.