Chapter 18 Brace-Goetze Strength Profiles, The Partitioning of Strike-slip and Thrust Faulting at Zones of Oblique Convergence, and the Stress-Heat Flow Paradox of the San Andreas Fault

Abstract

Abstract The laboratory work of W.F. Brace, C.G. Goetze, and their colleagues provided much of the basis for the now common image of strength in the crust and upper mantle: a brittle upper crust obeying Byerlee's law, a weaker ductile lower crust, and a strong, but also ductile, uppermost mantle. Accordingly, for most plausible geotherms in continental crust and upper mantle, the strongest region may lie in the uppermost mantle. Although the lower crust is likely to be relatively weak, it couples the deformation of the upper mantle to that in the upper crust in most regions. Insofar as the strongest region lies in the uppermost mantle and deforms by ductile flow, the overall large-scale deformation of continential regions should be described best as that of a viscous continuum. Thus, the principal stresses and strain rates throughout the continental crust and upper mantle should be nearly perpendicular and parallel to the Earth's surface, and oblique-slip faulting should be a minor process. Insofar as blocks of upper crust rotate about vertical axes because they are coupled to the vorticity field of the underlying viscous substratum, sets of oblique-slip faults should not be stable under finite deformation. Consequently, with finite deformation, oblique convergence is likely to become partitioned into pure strike-slip parallel to the boundary between rigid plates or blocks, and pure convergence on thrust faults also with strikes parallel to the boundary. If, indeed, blocks of upper crust were effectively carried by the stronger, continuously deforming substratum, then orientations of stress in the upper crust would provide little insight into the dynamics of crustal deformation, much less insight than measurements of the full deformation gradient tensor or velocity gradient tensor. Moreover, dissipative heating in the strong upper mantle could provide the source of excess heat flow in the Coast Ranges of California.