TIMING AND RATE OF MERCURY'S GLOBAL CONTRACTION

Mercury likely underwent an early phase of expansion caused by interior heating followed by a phase of contraction caused by global cooling. The long-lived cooling process reduced the planet’s radius, which engaged the entirety of Mercury’s surface in the development of thrust fault-related landforms. Although thermal models have predicted the onset time and rate of contraction, these predictions have not been tested with global-scale geologic observations. Impact cratering intensely altered Mercury’s surface during the late heavy bombardment of the inner solar system. Following this, impacts decreased exponentially in size and frequency, the rate of which has been studied in detail, also resulting in the development of a morphologic classification scheme whereby craters are grouped into classes corresponding to distinct time-stratigraphic systems. From oldest to youngest they are Pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. Over geologic time, impact craters and thrust faults have formed two main stratigraphic relationships, where craters are either crosscut by or superpose thrust faults. The ages of superposing craters indicate that thrust fault activity had locally ceased there, whereas crosscut craters provide evidence of tectonic activity after crater emplacement. We analyzed the stratigraphic relationships of all thrust faults and craters larger than 20 km to determine the timing and rate of contraction. We calculated the geometric probabilities of thrust faults to crosscut craters of the different time-stratigraphic systems, and, from that, determined the onset time of thrust faulting, as well as the amount of radial contraction and the average strain and strain rate during each of Mercury’s time-stratigraphic systems. We find that thrust faulting globally commenced during the Calorian with contraction initially being a rapid process characterized by high strain rates. These strain rates, and thus global contraction, gradually slowed toward the Kuiperian. These results provide important clues for thermal models and also allow for the calculation of slip rates of thrust faults and timescales of development of structural relief of their corresponding landforms on Mercury.