A STATISTICAL INVESTIGATION INTO THE SPATIAL DISTRIBUTION OF MERCURY’S PYROCLASTIC ACTIVITY

The tectonics of Mercury are dominated by global contraction, with the resultant planetary radius decrease accommodated in the lithosphere by thrust faulting. Geological observations show that the earliest preserved thrust faulting coincided with the waning stages of effusive volcanism, but that explosive volcanism continued beyond this point. Stresses from global contraction, however, would preclude efficient vertical magma ascent. Notably, pyroclastic activity—manifest as flat-rimmed depressions surrounded by diffuse, spectrally distinct halos—spatially coincides with lithospheric discontinuities such as faults and impact craters. In particular, > 85% of pyroclastic vents are situated on the floors, rims, central peaks, or peak rings of impact structures. A substantial portion of vents is also proximal to thrust faults: they are most spatially concentrated at or within 20 km of faults, with ever fewer vents progressively farther from tectonic structures. Very few sites are not found near any identifiable structural discontinuities. To test if pyroclastic activity occurred randomly across Mercury or if it is tied to faulting and cratering, we generated datasets of random point locations of equal count to those volcanic sites, computed their spatial relationship to the mapped faults and craters, and compared them to our observations. We find that, although the observed proximity of vents to faults is indistinguishable from a random distribution, their spatial association with craters is non-random. To examine additional ties between lithospheric weaknesses and pyroclastic activity, we performed a multivariate analysis that tested correlations between vent size, vent distance from mapped faults, the length of the nearest fault to a vent, the location of vents within crater(s), and the diameters and degradation states of those craters. Our results show that vents are larger in more heavily degraded craters and smaller in morphologically fresh craters, vents are generally small when they are not in a crater (irrespective of whether or not a fault is nearby), and vents are larger in craters where faults are nearby. These findings demonstrate that pyroclastic activity on Mercury has been tied to lithospheric weaknesses, implying a structural control of magma ascent after the onset of global contraction.