Graphite lubricates Mercury’s global contraction

Abstract

Secular cooling of Mercury’s interior drove planetary contraction, forming widespread lobate scarps that are the surface expressions of thrust faults and fold-and-thrust belts. Despite their importance to Mercury’s global tectonics, the mechanics and rheology of these features remain poorly understood. Using Critical Taper analysis calibrated by friction experiments, we estimate the maximum basal friction (µb) from observed wedge geometry and show that the topography of large fold-and-thrust belts is consistent with weak graphite-bearing, low-angle fault zones. This finding indirectly supports models proposing a graphite flotation crust during Mercury’s early differentiation. We infer that remnants of this carbon-rich layer, heterogeneously distributed within Mercury’s crust because of early impacts, caused the weakening of large portions of the crust during later contraction, allowing the nucleation of weak, low-angle thrust faults. Since the gently dipping, frictionally weak thrusts require greater horizontal shortening to build the observed relief than steeper, stronger faults, Mercury’s cumulative radial contraction may exceed previous, more conservative estimates. Laboratory experiments and mechanical modeling show that Mercury’s widespread low-angle thrust faults likely formed due to graphite-rich layers in its crust, which acted as a solid lubricant and weakened fault zones during planetary contraction. These findings suggest that remnants of an early graphite flotation crust facilitated the development of weak, shallow-dipping faults, indicating Mercury’s total radial contraction may be greater than previously estimated.