The tectonics of Mercury: The view after MESSENGER's first flyby

During its first flyby of Mercury, MESSENGER imaged many tectonic landforms, most of which are contractional in nature and include lobate scarps, high-relief ridges, and wrinkle ridges. Wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the Caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters. MESSENGER revealed a radial graben complex, Pantheon Fossae, nearly co-centered with the Caloris basin. Pantheon Fossae and a network of mostly basin-concentric graben in the outer portions of the Caloris basin interior form a pattern of extension not seen elsewhere on Mercury. The first clear example of extensional faults outside of the Caloris basin has been documented on smooth plains inside the peak ring of the relatively young Raditladi basin. A map of the distribution of tectonic landforms imaged by MESSENGER and Mariner 10 shows that lobate scarps are the most widespread type of feature. Estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by MESSENGER range from ~ 1 to 3 km. Previously unrecognized lobate scarps detected in areas imaged by Mariner 10 indicate that earlier estimates of contractional strain are low. A new estimate of the average areal contractional strain (~ 0.06%) accommodated by scarps is at least one third greater than comparable previous estimates and corresponds to a decrease in Mercury's radius of at least 0.8 km since the end of heavy impact bombardment of the inner solar system. These figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date. Crosscutting and embayment relations revealed by MESSENGER suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits. Relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of Mercury's interior and the growth of the planet's solid inner core.