The role of graphite in the formation of thrust faults on Mercury

Globally, the most widespread contractional landforms on Mercury are lobate scarps. Lobate scarps are linear or curvilinear topographic features interpreted as the surface expression of thrust faults, formed as a consequence of planetary cooling and contraction. These features have been studied extensively, from the initial images captured by Mariner-10 to the more recent data acquired by MESSENGER's Mercury Dual Imaging System (MDIS). However, although several works have analyzed the global tectonics of the planet (e.g., Klimckzak et al. 2015; Watters et al., 2015), a comprehensive interpretation of thrust faults geometry and their mechanical behavior on a global scale has not yet been fully constrained. Here we show that the formation and growth of large-scale lobate scarps is facilitated by the presence of a graphite-rich layer(s), acting as fault lubricant. We studied thrust faults from seven different Mercury quadrangles and derived their geometric characteristics (relief height, amount of shortening, detachment depth) considering a fault-propagation geometry for a range of possible dip angles on isolated thrusts. Using a critical taper theory-based model (iterative mechanical model) we then estimated the basal friction coefficient for thrust-belts located in the same quadrangles. The low friction coefficients obtained under three different lithospheric initial conditions indicate the presence of a weak material that allows fault slip. Our results demonstrate the crucial role that graphite possibly plays in shaping Mercury’s lithosphere, providing new understanding on thrust faults nucleation and growth and establishing a possible connection between surface deformational processes and Mercury’s early crust composition.