Tectonic Controls on Pyroclastic Volcanism on Mercury

Abstract:

Over much of Mercury’s geologic history the planet has contracted as a response to cooling of its interior. Such contraction is evident as landforms formed by thrust faults, which have accommodated a radius decrease of ~5 km. Stresses from global contraction imposed on the lithosphere are not favorable for and prevent volcanism. Yet, there are examples on Mercury where pyroclastic deposits superpose thrust faults, indicating that explosive volcanism has occurred after the onset of global contraction. To better understand the spatial relationships of thrust faults with the pyroclastic vents, we used MESSENGER image data to categorize 343 vents by their occurrence either (1) within 30 km, (2) within 100 km, or (3) farther than 100 km from a thrust fault, using ArcGIS. Vents were also classified by their association with impact craters. Results show that 75% of all vents are located within impact structures, with 36% of vents within 30 km of thrust faults, 41% located farther than 30 but within 100 km of thrust faults, and 23% of vents are farther than 100 km from a thrust fault. To investigate whether this geospatial relationship is tectonically controlled, three areas —representing the three categories of vents— were mapped, and the locations and orientations of vents and faults were recorded. Stress changes around these faults were then numerically modeled with the COULOMB 3.4 software, using elastic rock properties, a background stress field, and fault size- and dislocation parameters applicable to conditions of Mercury’s global contractional tectonic environment. Preliminary results indicate that stress changes can locally produce conditions beneficial for volcanism. Further modeling will determine if such beneficial conditions are geospatially correlated with the pyroclastic vents and thus enable a better understanding of pyroclastic volcanism on Mercury after the onset of global contraction.