The Global Tectonics of Mercury

The first MESSENGER flyby of Mercury revealed a large number of tectonic landforms in areas not imaged by Mariner 10. The dominant tectonic landforms in the areas imaged by MESSENGER are contractional, consisting of lobate scarps, high-relief ridges, and wrinkle ridges. Among the newly discovered tectonic landforms imaged by MESSENGER is one of the largest contractional landforms seen on Mercury to date. Wrinkle ridges not only occur on the interior smooth plains of the Caloris basin and on exterior plains but also on smooth plains that fill the interior of smaller impact basins and larger craters. In the first complete survey of the Caloris basin, MESSENGER revealed a radial graben complex, Pantheon Fossae, which converges on a zone near the center of the basin. Pantheon Fossae and basin-concentric graben located along the entire outer margin of the Caloris basin floor form a complex pattern of extension unlike anything seen in lunar maria. MESSENGER also revealed the first unmistakable evidence of extensional faulting outside of the Caloris basin, in smooth plains inside the peak ring of the relatively young Raditladi basin. Lobate scarps are the most widely distributed tectonic landform in the areas imaged by MESSENGER and Mariner 10. Areas with relatively few scarps occur in longitudinal bands that correspond to regions with less than optimal lighting geometry for recognizing low-relief morphologic features. MESSENGER images of impact craters crosscut and overthrust by lobate scarps indicate that the horizontal shortening across such features is up to 3 km. The identification of previously unrecognized lobate scarps in areas imaged by Mariner 10 indicates that earlier estimates of average global contractional strain are too low. New estimates for the average strain are about one third greater than the previous values but are still lower limits because of the likelihood that not all lobate scarps in imaged areas have been recognized. The timing of lobate scarp formation, on the basis of crosscutting and embayment relations, indicate that thrust faulting had begun before the end of Calorian- aged smooth plains emplacement and continued after the emplacement of the youngest smooth plains material. The slow, continuous radial contraction that accompanied cooling of Mercury's interior and the likely growth of a solid inner core may be expressed by relatively young lobate scarps.