Teleseismic studies of the North American Cordillera: Evaluating the changing structure, composition, and fabric after subduction.

摘要

The cessation of wide-scale subduction and orogenic compression during the early to mid-Cenozoic radically altered the North American Cordillera. This dissertation summarizes the results of three seismic studies, conducted in different regions of western North America, aimed at understanding how the structure and character of the crust and upper mantle relate to regions of post-subduction magmatism and persisting high elevations. Across the southern Basin and Range and Colorado Plateau teleseismic receiver functions show that only the Colorado Plateau contains thick crust commensurate with its high elevation. In contrast the southern Basin and Range has a relatively uniform crustal thickness of ∼30 km, which is inadequate to support the high elevations of some of its metamorphic core complexes. We conclude that local variations in the density of the crust or upper mantle may support at least some high elevations in the southern Basin and Range. A large dataset of receiver functions collected across the Sierra Nevada show a complicated crust-mantle boundary which varies geographically, transitioning from thin crust beneath the eastern Sierra to thick crust underlying the western foothills. The thicker crust coincides with xenoliths sampling a remnant mafic-ultramafic residue produced during arc magmatism in the late Cretaceous. Modeling of receiver functions suggests that recent volcanism throughout the elevated eastern Sierra and nearby Basin and Range results from continued foundering of this dense material and its replacement with asthenosphere at relatively shallow depths in the upper mantle. In the Canadian Cordillera, regional observations of shear-wave splitting constrain the orientation and magnitude of seismic anisotropy. A pronounced and unusual trend of shear-wave splitting across the central British Columbia suggests that eastward directed flow of mantle asthenosphere fuels recent, widespread and geochemically distinct post-subduction volcanism within the northern slab window. These observations show how local and dynamic processes contribute to the support of lingering high elevations across western North America and that regions formerly associated with subduction may experience renewed magmatism due to inflow and subsequent melting of asthenospheric mantle.