REPLY

I thank Buck [this issue] for the opportunity to discuss further the mechanical origin of crustal-scale low-angle normal faults. Buck raises three main objections: (1) the magnitude of basal shear traction (10-50 MPa) selected in my calculations is too high, (2) the magnitude of shear stress predicted in the model with low basal shear traction (10 MPa) is too low to initiate low-angle normal faults, and (3) the predicted tensile stress near the surface in the model is too high for rocks to sustain. Because the concept of shearing at the base of the upper crust is essential to my models, I will address this point first. MAGNITUDE OF BASAL SHEAR TRACTION On the basis of recent geological and geophysical studies of metamorphic core complexes in the U.S. Cordillera, I proposed that a subhorizontal shear traction applied at the base of the brittle upper crust could be responsible for the formation of mid-Tertiary, crustal-scale low-angle normal faults in the North American Cordillera. I assumed that the magnitude of the basal shear traction is between 10 and 50 MPa. Buck considers these magnitudes too high because the widespread mid-Tertiary volcanic activity implies that the lower crus