THE RELATIVE TIMING OF GLOBAL CONTRACTION AND PLAINS VOLCANISM ON MERCURY

Mercury experienced episodes of flood volcanism evinced by large expanses of smooth plains, and compressional tectonics apparent as thrust-fault-related landforms interpreted to have resulted primarily from interior cooling and global contraction. The timing between the two processes is not well understood, but this relationship is critical for evaluating how smooth plains were emplaced, because global contraction imposes a stress state on Mercury’s lithosphere that tends to impede the ascent of magma to the surface. Size–frequency distributions of fresh and volcanically buried impact craters in smooth plains give emplacement times between ~4.1 and ~3.7 Ga. Direct crosscutting relations between plains and faults are scarce, but some long fault scarps with substantial relief that cut into plains units show no evidence of embayment by volcanic material. Such relations indicate that the development of the majority of relief on, and thus the tectonic activity underlying, these landforms took place after smooth plains emplacement. Relatively recent tectonic activity is also manifest as crisp surface breaks of large scarps and minor graben along scarp crests, both of which cut small, presumably young craters. Dating the onset of scarp formation is more difficult because cratering and resurfacing hinder the detection of older structures, and fault reactivation or extended tectonic activity overprints earlier records of deformation. Even so, several large craters that superpose pronounced scarps have degradation states indicating that they are concurrent with or predate the last stage of smooth plains emplacement, showing that some tectonic activity must have occurred during plains volcanism. We therefore find that global contraction operated throughout an extended period in Mercury’s past and overlapped with plains volcanism. As the horizontal compression induced by global contraction favors sill formation over the emplacement of expansive plains formed by magma erupted through vertical conduits, coeval flood-mode volcanism fed by surface-breaking dikes should be restricted to areas shielded from those stresses, such as within large impact structures that were comparatively recent at the time of volcanism.