Surface Strains Associated with the Evolution of Mercury’s Domical Swells

Abstract:

Topography and gravity data recovered by the MESSENGER mission have revealed the existence of 3–4 domical topographic swells on Mercury that are characterized by diameters of ~1000 km and coincident geoid highs. A number of mechanisms may explain the formation of these swells, including uplift associated with deep-seated buoyancy or voluminous intrusion of magma. A domical swell within the northern smooth plains (NSP), informally called the northern rise (NR), is perhaps a type example because, unlike the others, it is not obscured by large variations in crustal thickness. The floors of impact craters partially infilled by volcanic plains deposits are tilted by amounts that match the long-wavelength topographic slope, indicating that the growth of the rise largely postdates plains emplacement. Further, high values of low-degree admittance indicate that the NR is either supported by buoyancy near the base of the mantle or is elastically supported under top loading of the lithosphere. The latter of these two scenarios predicts compression at the NR and extension elsewhere in the NSP. The magnitudes of these surface stresses decrease with increasing lithospheric thickness.

We use horizontal surface strain, as accommodated by contractional landforms (primarily wrinkle ridges), as a proxy for the stress state subsequent to the emplacement of the NSP. Specifically, we consider the areal density of wrinkle ridges to reflect the distribution of horizontal stress-induced strains in the region. We do not observe higher ridge densities on the NR than across the rest of the NSP, which indicates that any stresses associated with top loading of the lithosphere were less than the global compressive stresses associated with the radial contraction of Mercury. If the NR did form through top loading, the elastic lithosphere must have been at least 100 km thick at the time of NR formation.

Fig. 1. Wrinkle ridge densities, calculated with a 200-km smoothing radius and plotted in a truncated Mollweide projection. The NSP is outlined in white.