Gravity and lithospheric stress on the terrestrial planets with reference to the Tharsis Region of Mars

Abstract

An analytical theory for isostatic compensation in one‐plate planets, including membrane stresses in the lithosphere, self‐gravitation, and rotational ellipticity, is applied to Mars. Interpretation of the results depends on whether the outer radial fractures around Tharsis are old or young features. If the outer fractures are old, Mars is inferred to be basically isostatic and the Tharsis region is inferred to have thin crust over lower‐density lithospheric mantle. For an average crustal thickness of 50 km and a lithospheric thickness of 450 km, the computed crust is essentially absent near the center of Tharsis. Calculations indicate that a pronounced directionality of the stress field should occur only as the radial fractures around Tharsis, in agreement with observations. The greater extent of radial graben early in Martian history is attributed to general thermal expansion of the planetary interior which adds extension to the strains produced by the isostatic stresses. Conversely, later planet‐wide contraction enhances circumferential thrusts around Tharsis. For planet‐wide expansion and isostatic stresses to interact in this way, the strain rates from each process must be comparable. This implies an average lithospheric viscosity of 1028 P or 1027 Pa s. Long wavelength loads, such as those associated with extrusive volcanism, produce strains similar to those associated with expansion of the interior. These flexural strains may have been a significant, but not dominant, contribution to Martian tectonics earlier in the planet's history when load‐forming processes were active. That is, some of the older fractures may have been produced by the relaxation of flexure at that time. It is likely that most of the stresses associated with flexure have now relaxed and the present stresses, crustal thicknesses, and lithospheric densities are similar to those predicted assuming isostasy. In contrast, if the outer radial fractures are young, a couple of kilometers of the volcanic load on Tharsis is probably supported by flexure. As in the model for older radial fractures, the lithospheric mantle is less dense beneath Tharsis and the relaxation time for flexure in the lithosphere is quite long. The flexurally supported loads probably formed late in the planet's history, and the compensation was probably more nearly isostatic before that time.