A TWO-DIMENSIONAL, LINEAR–ELASTIC MODEL TO EXPLAIN RADIAL EXTENSIONAL FRACTURES, PANTHEON FOSSAE, CALORIS BASIN, MERCURY

In this study, two-dimensional linear elasticity theory is used to model the lithospheric stress field that produces radial extensional fractures observed at Pantheon Fossae in the Caloris Basin of Mercury. These fractures were imaged by the MESSENGER mission flyby of Mercury on January 14, 2008 and show radial fractures extending outward from a 40-kilometer impact crater named Apollodorus. Recent studies have proposed several different mechanisms to explain these fractures, including magmatic processes, central basin uplift, and stresses produced by the formation of the impact crater itself.

The model presented here attempts to describe the state of the stress field, independent of the physical mechanism that produced it. The first part of the analysis uses a model with azimuthal symmetry, consisting of a two-dimensional infinite plate with a hole in the center to represent the crater and a constant horizontal pressure applied along the crater wall. This model produces a stress field with compressive stresses in the radial direction and tensional stresses in the azimuthal direction, which is consistent with the formation of radial extensional fractures. However, this simple model cannot explain the observed asymmetry of the fractures distribution, where fractures extend further and are more abundant along a preferred azimuth of approximately N30°E. The second part of the analysis superposes a regional stress field, with the maximum horizontal compressive stress aligned with this direction of maximum fracture extent. This analysis shows that the yield strength of the lithosphere is minimal along the direction of the maximum compressive stress. Therefore, a stress field with constant pressure applied horizontally along the crater wall superimposed upon a regional stress field with maximum horizontal compressive stress aligned along a N30°E azimuth can adequately explain the observed fracture distribution.