HOW MIGHT THE BOTTOM MATTER? PROVOCATIONS FROM AN ALTERNATIVE MODEL OF THE LARAMIDE OROGENY

The classroom view of compressional orogens is of continental crust pushed on from the side or scraped upon from underneath. While such stresses certainly exist and they can be used (in conjunction with erosion and flexure) to explain landward migration of fold-and-thrust belts and foredeep migration and the like, they fail us in providing self-consistent explanations of more exceptional episodes of deformation. In the Laramide orogeny, development of a deep basin far in front of frontal surface thrusts in the Maastrichtian demands some additional force. The creation of basement-cored uplifts within that basin suggests these forces are sufficient to reroute orogenic shortening, in this case largely bypassing the Colorado Plateau. Some kind of basal loading, perhaps dynamic from interaction of a shallowing slab with an Archean keel, is consistent with both events. Xenoliths from at least 100 km of arc-age lithosphere in the southern Sierra in the Miocene suggests that sub-Sierran shallow-angle subduction might not be the cause of the cessation of intrusion c. 80-85 Ma. Dextrally oblique subduction during this episode suggests that perhaps inflow of fresh asthenosphere was shutdown as crust subducting shallowly to the south passed just to the east of the Sierra Nevada. These observational differences with more typical compressional orogens (including the modern Sierra Pampeanas) provide insights into some of the more extreme interactions of continental and oceanic plates. We suggest that one possible reconciliation is that accelerated westward motion of North America shallowed all subduction zones on the western margin, but that a segment rising under the thick Archean keel of Wyoming generated dynamic stresses that pulled down the Colorado-Wyoming center of the Maastrichtian basin and pulled up further that segment of the slab that was subducting under what is now southern California. Interruption of asthenospheric flow terminated Sierran magmatism while creating conditions for convergence-parallel magmatism within the Colorado Mineral Belt, perhaps through Richter roll-type convection.