Provenance of Block Fields Along Lunar Wrinkle Ridges

Block fields and associated relatively high reflectance material along wrinkle ridge summits are revealed in meter‐scale Lunar Reconnaissance Orbiter Camera images. Wrinkle ridges with block fields in Mare Australe, Crisium, Fecunditatis, Frigoris, Nubium, the northern half of Oceanus Procellarum, Serenitatis, and Tranquillitatis are evenly distributed, and block density generally increases with wrinkle ridge slope. The median cross‐sectional area (proxy for diameter) of the 1,368 blocks measured in this study is ~7 m2, and 75% of these blocks are ≤~14 m2. We propose that the largest variation in cross‐sectional area of the blocks is related to substrate physical properties of the mare basalt and not a function of the slope on which they occur, suggesting that physical properties rather than slope are a stronger control on block size. The maximum length of blocks may constrain the minimum basalt flow thickness or joint width; our block measurements suggest basalt flows ~2–14 m thick, agreeing with previous estimates. The data suggest that blocks originate from mare basalt layers that buckle and break as a result of movement along ridge‐forming thrust faults. High reflectance material associated with wrinkle ridge blocks likely represents freshly exposed rock and soil. Meter‐scale blocks may erode relatively quickly due to collisional disruption, indicating recent downslope movement of regolith exposing preexisting blocks or blocks formed and exposed by recent activity on ridge‐forming faults. Plain Language summary Wrinkle ridges are contractional tectonic landforms found in mare basalts. First detected by telescope observation of the lunar nearside, they have been extensively photographed and imaged by pre‐Apollo and post‐Apollo lunar missions. One characteristic of lunar wrinkle ridges that went undetected until the Lunar Reconnaissance Orbiter mission was the presence of thousands of boulders or blocks found in fields on the ridge slopes. In this study, Lunar Reconnaissance Orbiter Camera images were used to analyze the spatial distribution of wrinkle ridge block fields in all the major nearside mare, and over a thousand individual blocks were measured. We find that the median cross‐sectional area of the measured blocks is about 7 m2. The dimension of measured blocks is consistent with estimates of the thickness of basalt flows. This suggests that the blocks originate from individual mare basalt flows that buckle and break due to the contraction and upl