Tectonism of Mercury
Mercury is a so-called one-plate planet, with no evidence for having ever undergone plate tectonics. Nonetheless, the innermost planet boasts a long record of tectonic deformation. The most prominent manifestation of this history are long scarps that widely occur throughout the planet’s cratered terrains; some of these scarps rise kilometers above the surrounding landscape. Mercury’s volcanic plains, occupying about a quarter of the planet, abound with low ridges that often interconnect with one another. Both the scarps and ridges are underlain by thrust faults, and they point to a tectonic history dominated by crustal shortening. At least some of the shortening strain recorded by the ridges reflects subsidence of the lavas in which they formed. But the widespread distribution of scarps attests to a planet-wide process: global contraction, wherein Mercury has experienced a reduction in volume as its interior cooled through time.
The onset of this phenomenon placed the lithosphere into a net state of horizontal compression, and it accounts for why Mercury hosts only a few instances of extensional structures. These landforms, shallow troughs that form complex networks, occur only in volcanically flooded impact craters and basins and developed as those lavas thermally contracted. Tellingly, widespread volcanism on Mercury ended at around the same time the population of scarps began to form; explosive volcanism endured beyond this point, but almost exclusively at sites of lithospheric weakness, where large thrust and normal faults penetrate deep into the interior. Together, these observations are consistent with decades-old predictions that global contraction would shut off major volcanic activity, and they illustrate how closely Mercury’s tectonic and volcanic histories are intertwined.
The tectonic character of Mercury is thus one of sustained crustal shortening with only localized extension, which started almost four billion years ago and extends into the geologically recent. This character somewhat resembles that of the Moon, but differs substantially from those of Earth, Venus, or Mars. It may be, then, that Mercury represents how sub-Mars-sized terrestrial planets tectonically evolve, and it could provide a basis for understanding the geological properties of similarly small worlds yet to be discovered in other solar systems.