The conference on magnitude of deviatoric stresses in the Earth's crust and uppermost mantle

Abstract

The magnitude of deviatoric stresses in the earth's crust and uppermost mantle, and especially the magnitude of shear stresses that resist plate motions across the major plate boundaries, is a matter of central importance to solid earth geophysics. Since currently available estimates vary by at least a factor of 10, even when the same features or set of observations are involved, such fundamental issues as the driving mechanism(s) of plate tectonics and the energetics of crustal faulting are entirely unresolved. More generally, there is very little in the study of active geologic processes of geophysical interest that does not involve material deformation of some kind or another; in understated terms the analysis of material deformation to reveal causative processes is poorly constrained when the magnitude of the deviatoric stresses involved is so uncertain.Apart from the obvious problem of our inability to measure directly deviatoric stresses in the earth's crust and uppermost mantle through all but a small fraction of its depth—or sample directly this region in any other way at the present time—a confounding difficulty exists in the great range of time scales involved in geophysical processes of interest, from fractions of seconds for the spontaneous failure of small crustal earthquakes to billions of years for the relaxation of certain topographic and gravimetric features of the continental crust. It is probably naive then to expect that the same magnitude of deviatoric stresses should exist for all these geologic processes, given the great range of time scales over which they operate. Moreover, the earth's crust and uppermost mantle are heterogeneous enough, especially in continental regions, that significant spatial variations probably exist as well. Certainly, an important problem is whether the actual variations in space and time of deviatoric stresses in the earth's crust and uppermost mantle can be large enough to accommodate the order of magnitude differences as presently perceived or whether our present perceptions are only imagined, in one sense or another. Even so, we can be assured that at least some of our perceptions must be imagined, when deviatoric stress estimates differing by an order of magnitude or more are offered to explain the same basic processes or the same observations of them.