Thrust Faulting and the Late-stage Contraction of the Moon

Lunar Reconnaissance Orbiter Camera images have revealed thousands of young thrust fault scarps in the highlands and mare. These relatively small-scale faults are the expression of a period of late-stage lunar global contraction. Here we present the most comprehensive assessment of recent thrust faulting on the Moon and its implications for lunar radius change. Combining the estimate of the fractional areal strain of the highlands from the population of lobate thrust fault scarps with the fractional areal strain of the maria from the population of small mare ridge thrust faults, the total areal contractional strain is estimated to be ∼0.0048% to 0.0053%. This areal strain corresponds to a radial contraction of ∼42 to 46 m. Assuming a uniform rate of radius change over the last 400 Ma, the rate of change is estimated to be a maximum of ∼11 cm Ma −1 . The fractional areal strain of the maria contributes little to the total radius change, thus the late-stage radius change is dominated by the highland thrust faults. The isotropic compressional stress induced by this amount of radial contraction is estimated to be ∼4 MPa, consistent with stress drop estimates from recorded shallow moonquakes. Inconsistency between thermal history models that predict much larger amounts of radial contraction and the absence of degraded, ancient highland fault scarps supports a nonmonotonic lunar thermal evolution, possibly due to the insulating effects of a thick, porous megaregolith. Alternatively, the presence of a strong elastic lithosphere may have inhibited thrust faulting until stresses reached a threshold level in the Copernican era.