Young thrust-fault scarps in the highlands: evidence for an initially totally molten moon

Thermoelastic stress calculations show that if only the outer few hundred kilometers of the Moon was initially molten and if it had a cool interior, i.e., the magma ocean model of the Moon, the highlands should not have any young, compressional tectonic features. In contrast, if the Moon was initially totally molten, the highlands should have 10-km- scale, ?0.5- to 1 × 10⁹-year-old thrust faults. Observations using the Apollo panoramic imagery show that young thrust faults do exist in the highlands. Extrapolation of the data suggests that some 2000 thrust-fault scarps, whose average length is 9 km, are in the highlands. The fault scarps generally occur in series or complexes of four or five scarps. The average length of these complexes is 50 km; the largest observed complex is 120 km long. Extrapolation of the data suggests that there are about 400 such complexes. The ages of the scarps range from 60±30 to 680±250 my, with a possible bias of up to plus a factor of 2 or minus a factor of 4. These scarps are by far the youngest endogenic features on the Moon. The selenographical, size, age, morphological, and azimuth frequency distributions of the scarps can be explained by the effects of the kilobar-level thermoelastic stresses, the 100-bar-level tidal and rotational stresses, and influence by preexisting structures. These results show that the Moon has recently entered an epoch of late stage, global tectonism and favor the concept that the Moon was initially totally molten.