TECTONICS, LITHOSPHERE STRENGTH AND THERMAL STRUCTURE OF WESTERN NORTH AMERICA

The western North America Cordillera is a >500 km wide mobile belt that has undergone complex surface deformation since the Mesozoic, including extensional and transcurrent faulting. In contrast, the adjacent North America craton has remained tectonically quiescent and exhibits little seismicity. The differences in tectonic regime are primarily due to contrasting thermal structures. We test this dependence regionally through a comparison of effective elastic thickness observations and lithosphere temperatures inferred from surface heat flow, mantle seismic velocity data (Vs), and mantle xenolith thermobarometry. The effective elastic thickness (Te), derived from topography-gravity coherence, provides a quantitative measure of the integrated strength of the lithosphere. The Te distribution for western North America is strongly bimodal, as previously found globally, with values less than 20 km for the Cordillera and over 100 km for the craton. Yield strength envelopes calculated using laboratory-derived rheologies show that the observed Te corresponds to Moho temperatures of 800-900C for the Cordillera and 400-500C for the craton, in good agreement with temperatures determined from Vs and other thermal indicators. Our study supports the conclusion that lithosphere elastic thickness and strength are controlled largely by temperature and that laboratory-based rheologies provide a good estimate of the deformation behaviour of the crust and upper mantle. The small Te and strength estimates indicate that the hot Cordillera lithosphere is weak and thin, such that it readily responds to time and space variable plate boundary forces, i.e. long-lived mobile belt. As much of the Cordillera is a present or recent subduction zone backarc, the high temperatures and thin lithosphere may be associated with shallow asthenosphere convection that results from a viscosity reduction by water released from the underlying subducting plate. We infer that most of the North America Cordillera is an orogenic float, where current tectonics are mainly limited to the upper crust, which is mechanically decoupled from the lower part of the lithosphere. Conversely, the total strength of the cool, thick North America craton lithosphere is too great for significant deformation under plate tectonic forces.>