Plate motion and thermal instability in the asthenosphere |
| |
| Authors: | HJ Melosh |
| |
| Institution: | Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 U.S.A. |
| |
| Abstract: | This paper investigates the effect of shear heating in the asthenosphere on the thermal structure of the upper mantle. Equations describing the motion of the lithosphere over the asthenosphere in the presence of a strongly temperature-dependent stress-strain rate relation are derived and solved with the help of several approximations. These approximations are shown to be valid under conditions appropriate for the earth.Two sets of solutions are found. For one set (the “subcritical” solutions) a normal shear stress—velocity relation is found for small stresses. The velocity increases as the stress increases, reaching a maximum velocity σc for a critical stress σc. The subcritical solutions have a negligible effect on the thermal structure of the earth, even at the critical stress. The other set of solutions (the “supercritical” solutions) has the bizarre property that a decrease of applied shear stress leads to an increase of velocity. Thus, as the shear stress goes to zero, the velocity becomes infinite. At larger shear stresses the velocity decreases until it reaches σc at a stress σc (the two sets of solutions share this point in common). There are no steady solutions of any kind for shear stresses in excess of σc. We discard the supercritical solutions as candidates for the thermal structure of the earth on the basis of their instability to small perturbations of applied stress and temperature.The realm of subcritical solutions (stress less than σc, velocity less than σc) thus defines a regime of plate motion in which the thermal effects of shear heating are negligible. If the shear stresses acting on plates exceed σc, however, new physical processes must come into play to dissipate the excess heat generated. Assuming that the velocities of plates on the earth today are less than σc, relative to the deep mantle, a strict upper limit of a few tens of bars can be derived for σc, corresponding to effective viscosities of ca. 1019 poise in the asthenosphere. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
