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Regional structural domains in western North America (NA) (e.g. Basin and Range, Colorado River core complexes, southern British Columbia core complexes) that are characterized by systems of normal and strike-slip faults record temporally distinct extensional episodes (∼15-0 ma, ∼31-20 ma, 55-42 ma). The structures commonly are attributed to gravity-driven post-orogenic collapse. However, remarkable overall consistency in the directions of tectonic transport within the domains (280o; 240o, 285o) supports a previous hypothesis of convergence followed by collision and plate capture. In this model arrival of buoyant oceanic lithosphere, commonly part of a spreading center, impedes subduction and leads to collision with the overriding continental plate. During collision the formerly converging plates couple as the buoyant lithosphere binds against the base of the overriding continental plate. The length of the sutured former plate boundary will vary depending upon the length of the recently subducted ridge segment. The length of the segment of plate boundary that must be coupled in order to induce crustal stretching is uncertain. Coupling of the ridge segment to the overriding plate leads to "capture", after which movements of the newly coupled plates are integrated. The movement of the Pacific plate coupled to NA adds a northwesterly component that affects the southwestward-moving North America plate. The net effect of the integration of the Pacific and NA plate motions has been westward extension accommodated by normal and lateral faults, some quite large.
Strain related to the effects of coupling may be widespread, extending hundreds of km away from the former plate boundary as shown by characteristic fault patterns with distinct ages. The fault patterns record deformation related to integrated plate motions following each episode of coupling. The differences in tectonic transport directions suggest plate motions vary more than expected. The diverse movements of the Pacific and NA plates also suggest that the motions of oceanic and continental plates are independent and that movements take place in response to different forces. Oceanic plates that passively move in response to gravitational forces acting upon the coolest parts of the slabs differ from large continental plates such as NA and Africa that may move in response to the effects of mantle acting upon their lithospheric keels or some other process.
The similarity of ages of extension and related movements along large lateral faults in western NA to ages of comparable events within the Himalayas-Alps orogenic belt is remarkable. The correlation leads to the speculation that changes in Pacific-North America plate motions may be transmitted through the coupled plates, that is, a megaplate, on a global scale. Hiatuses between extensional episodes record decoupling of oceanic lithosphere from the overlying plate, a process that ta kes at least 10 m.y.
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