Shortening tectonic structures within Mercury's two largest geological units display a clear contrast in relief, length, and spatial density. The volcanic smooth plains units are deformed by smaller‐scale structures yet host more features per area than the older intercrater plains. Although faulting in the intercrater plains is dominantly attributed to global contraction, it has been unclear whether the smooth plains faults result from volcanic loading, global contraction, or both. We use estimates of fault length and displacement to calculate spatial variations of areal strain for each unit. We find that high strain concentrations within the smooth plains suggest that global contraction has contributed to deformation in these units. The observed contrast in morphology and spatial density of structures between units may primarily reflect differences in mechanical and/or structural characteristics of the lithosphere when faulting was initiated.

Mercury's early evolution is enigmatic, marked by widespread volcanism, contractional tectonics, and a magnetic field. Current models cannot reconcile an inferred gradual decrease in the rate of radial contraction beginning at ~3.9 billion years (Ga) with crustal magnetization indicating a dynamo at ~4 to 3.5 Ga and the production of extensive volcanism. Incorporating the strong cooling effects of mantle melting and effusive volcanism into an exhaustive thermal modeling study, here, we show that early, voluminous crustal production can drive a period of strong mantle cooling that both favors an ancient dynamo and explains the contractional history of the planet. We develop the first self-consistent model for Mercury's early history and, more generally, propose an approach to assess the volcanic control over the evolution of any terrestrial planet or moon.

The major physiographic “smooth plains” units on Mercury are dominantly composed of volcanic deposits that have been deformed by horizontal compressive stresses. An open issue is whether these features formed by stresses induced by global contraction, bending stresses due to volcanic loading, or some combination of both. In this study, we model the surface expression of 12 shortening structures within several smooth plains units across Mercury to determine the geometries of the underlying faults. We implement an elastic dislocation model, using both listric and planar fault geometries, to place estimates on the depth of faulting for each feature. We show that a majority of smooth plains shortening structures penetrate the lithosphere to depths greater than 15 km. Thrust faults of this scale have not previously been recognized within the planet's smooth plains units and require a large horizontal stresses to form, which is best explained if this stress arises from global contraction. Further, our results suggest that the observed relief and length contrast between features in the smooth plains units and older intercrater plains units can be explained by interior layering of, and/or a shallower brittle‐ductile transition underlying, the smooth plains units.

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