Cenozoic extension and magmatism in the North American Cordillera: the role of gravitational collapse

Abstract

Following a protracted phase (∼155–60 Ma) of crustal shortening and mountain building, widespread extension and magmatism have dominated the tectonic history of the North American Cordillera since early Tertiary. Although decades of intensive investigations have made North American Cordillera one of the best studied continental regions in the world, many fundamental issues, including the cause of metamorphic core complexes and basin-and-range extension and their relationship with the overlapping magmatism, remain controversial. Recent studies have emphasized the role of gravitational collapse in causing both extension and magmatism in the Cordillera, but the geodynamics of gravitational collapse are not well understood. Using simple thermal–rheological and thermomechanical modeling, we address the following questions. (1) Could gravitational collapse of the thickened Cordilleran crust have formed the metamorphic core complexes? (2) Did gravitational collapse induce the intensive mid-Tertiary volcanism in the Cordillera? (3) What caused basin-and-range extension and the associated basaltic volcanism? Our results show that, although a thickened crust at isostatic equilibrium is dynamically unstable and tends to collapse, major postorogenic extension happens only when the lithosphere is sufficiently weakened by thermal processes associated with orogenesis, including thermal relaxation, radioactive heating, shear heating, and mantle thermal perturbations. Ductile spreading within the lower crust plays a major role in postorogenic gravitational collapse. This mechanism can explain most metamorphic core complexes and the associated plutonism. However, it is unlikely to have induced major mantle upwelling required by the voluminous silicic eruption during the mid-Tertiary. The cause of the mid-Tertiary mantle upwelling remains speculative, but strong mantle thermal perturbations under the northern Basin and Range province may have persisted since mid-Miocene. Thermomechanical modeling shows that this mantle upwelling may have caused significant ductile deformation within the surrounding lithosphere, with lithospheric material being pushed away and downward from the upwelling asthenosphere. The loci of maximum lithospheric thinning are near the margins of the upwelling mantle and have migrated outward during basin-and-range extension. This process can explain some of the spatial–temporal evolution of extension and volcanism in the Cordillera since mid-Miocene. The resultant lithospheric structures are consistent with geophysical observations in the Basin and Range province.