Dynamic Stress at Martian Surface in the Model of Rotation of the Lithosphere

Introduction: The crustal dichotomy and the Tharsis volcanic province are dominant features at Martian surface. Both exogenic (giant impact) and endogenic (e.g., degree-1 convection) were proposed to explain the dichotomy. In a recent study Zhong [1] proposed a unified model for the Tharsis rise and the dichotomy. A lithospheric thickness variations of hemispherical pattern is assumed, consistent with the formation of the crustal dichotomy. Assuming the dichotomy is due to extensive melting, a melt residue cap of angular radius 90◦ and variable thickness (shallowing towards the edges) of stiff (i.e., devolatilized) material is imposed. A single upwelling forms below the center of the insulating cap. The interaction between the plume head and the thick melt residue results in motion of the entire lithosphere with respect to the plume, which continues until the plume is positioned below the edge of the melt residue cap. This model of rotation of the lithosphere (ROL) may explain the time–space patern of volcanism in the western hemisphere and the position of Tharsis province. In this study we investigate the pattern and evolution of the dynamic stress in the shallow lithosphere in the model of ROL, and the implications for surface tectonic features on Mars. No melt residue: We first calculate the stress field for a reference model with no melt residue cap (corresponding to Model 1 in [1]). We solve three-dimensional incompressible mantle convection in spherical shell using CitcomS [2]. The temperature-dependent viscosity and weak asthenosphere result, after a transient period, in a degree-1 thermal structure dominated by a single upwelling, which remains stationery over time (Fig. 1a). The stress pattern at the surface of the lithosphere is characterized by a region of strong extension above the upwelling and a broad antipodal compressional domain. The extension is concentric around the plume, with the fault lines pointing radially from the plume center. This result is consistent with findings of Harder and Christensen [3] (in their model the degree-1 convective pattern arises from an endothermic phase transition in the deep mantle, rather than from T-dependent viscosity and weak asthenosphere as in our model). Stress in model of ROL: We calculate the surface stress field for a model with a melt residue cap (Model 2 in [1]). The upwelling initially forms below the center of the melt residue cap (Fig. 1b). The interaction of the plume head with the melt residue cap subsequently drives rotation of the lithosphere with respect to the upwelling (Fig. 1c). This differential motion stops when the plume is located slightly past the cap boundary (Fig. 1d). The stress pattern shown in Fig. 3 differs significantly a b