LARGE SCALE DEFORMATION IN THE CORDILLERA AND OTHER BACKARCS; THE ROLE OF TEMPERATURE (Invited Presentation)

The large-scale controls of continental deformation are: (a) in the upper crust there is brittle frictional deformation on faults and fractures, controlled by the normal stress mediated by pore pressure. (b) In the lower crust/upper mantle, there is ductile temperature-controlled deformation (also intermediate complex mylonite deformation). In the frictional crystalline crust, there appears to be little excess pore pressure, so fluid pressure is near hydrostatic, and strength increases nearly linearly with overburden load and therefore with depth (high pore pressure is important in thick sediment sections). At some depth, the temperature is high enough for ductile deformation and the temperature is the parameter that controls the strength. Continental thermal regimes are remarkably bimodal, hot and weak in subduction zone backarcs like the Cordillera, and cold and strong in cratons and other stable areas. The average temperatures at the Moho are 800-850 C for the Cordillera and 400-500 C for the cratons from a number of constraints. Applying these thermal regimes to laboratory data, the brittle-ductile transition (about 450 C but strain rate dependent) is at about 15 km for the Cordillera and about 60 km for the craton. The effective elastic thickness, Te from gravity-topography coherence agrees well. Several large-scale consequences are: (a) the craton and other stable areas are too strong by a factor of 20 to be deformed by normal plate tectonic forces. That is why they are so stable. The Cordillera and other backarcs are weak enough to be readily deformed by these forces, and are tectonic mobile belts. In continental collision, the weak backarc side of the closing ocean accommodates most of the deformation. The lower crust of the Cordillera is sufficiently hot and weak for there to be widespread lower crust detachment (viscosities of 10*19 Pa s). The upper crust may move independently of the mantle. There also is lower crust lateral flow that flattens the Moho which is has a remarkably constant depth of ~35 km, Mexico to Alaska, in spite of many episodes of large extension (e.g., Basin and Range; SW B.C.) and shortening (e.g., Laramide). The backarc Moho is a “liquid-liquid” gravitation equipotential boundary that flattens by lower crust flow over geological time scales.