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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.

Plain Language Summary Slow cooling of Mercury's mantle and core has decreased the radius of the planet by as much as 7 km. This radial contraction put the crust in a state of horizontal compression resulting in widespread thrust faulting. Extensive flood volcanism resurfaced portions of the crust, the most recent deposits of which are termed producing the smooth plains. Thrust faults are also observed within these plains, but it is not clear whether these features formed from global contraction or from bending of the crust due to the weight of the emplaced lavas. In this study, we measure the length of the faults and their relief (height) to calculate the spatial distribution of strain (horizontal shortening) in both the smooth plains and intercrater plains of Mercury. We find that the amount of strain recorded in the smooth plains is best explained by at least some contribution from stresses due to global contraction. Therefore, we suggest that all geological units on Mercury have been deformed by global contraction, indicating that substantial radius decrease occurred after the bulk of volcanic activity had ended.