Evaluation of the Origin Hypotheses of Pantheon Fossae, Mercury

Introduction: Since its discovery during the first flyby of the MESSENGER spacecraft, the origin of Pantheon Fossae (PF), a complex structure consisting of radial graben in the center of the Caloris Basin, Mercury, has been intensely debated. Several studies looked at the larger context for its formation, where three different formation theories have been suggested, i.e. basin-interior uplift [1, 2, 3], graben formation as surface expressions of dike intrusions [4], and an origin associated with the 40km-diameter Apollodorus crater [5], located near the center of PF. Basin-interior uplift was proposed to have occurred as the result of loading by an external annular load [2, 6] with a bull´s-eye gravity anomaly (central positive anomaly surrounded by a ring of lower gravity and another ring of high gravity) [2]. Furthermore, lateral crustal flow [3], caused by a horizontal pressure gradient associated with thinner crust beneath the basin than beneath the surrounding areas, is also suggested to have caused basin-interior uplift. Graben formation as a result of dike intrusions would be explained by the formation of a magma reservoir below the surface of PF. Over-pressurization of the reservoir would have resulted in the radial intrusion of dikes, which then caused the grabens to form at the surface [7]. The location of the Apollodorus crater near the center of the PF structure raised the question of a connection between them. The Apollodorus impact is believed to have caused a sudden change of the stress state. Stress changes traveling at seismic speeds are suggested to have caused the faulting to propagate with rupture propagation speeds (in km per second). The relative timing of the impact and graben faulting at PF are demonstrated by the observable ejecta cover on the graben closest to the crater [5]. We test these hypotheses with a detailed study of the graben pattern on MESSENGER image PIA10397, a strain analysis along 5 concentric traverses, and an analysis of the loading of the Mercurian lithosphere [8]. Graben Geometry: An inspection of the graben pattern shows two distinct generations of normal faults (Fig. 1). The first (set 1; red) consists of evenly spaced graben with uniform graben width, consistent with stratigraphic restriction of these faults [9]. This shows that the crater floor of the Caloris basin is layered stratigraphically and a calculation of graben depths from throws (calculated from shadow-length