Floor-fractured craters in Mare Smythii and west of Oceanus Procellarum: Implications of Crater Modification by Viscous Relaxation and Igneous Intrusion Models

Endogenic modification in lunar floor-fractured craters can constrain spatial variations in early lunar conditions. The nature of these constraints, however, depends on the assumed mechanism of crater modification. For viscous relaxation, the extent of crater modification depends on the surrounding crustal viscosity and thus provides loose constraints on the history of crustal heating within a region. For igneous intrusion models, the extent of crater modification reflects magmatically driven deformation and can be inverted to estimate both local magma pressure and intrusion depth. Both models indicate clear differences between regional conditions at Mare Smythii and in the highlands west of Oceanus Procellarum. The uniformly shallow crater depths in Mare Smythii probably indicate a long-lived period of extreme crater relaxation, whereas the wide range of modified crater depths in the western highlands suggest a much shorter period of partial crater relaxation. For comparable relaxation times, the average viscosity derived for Mare Smythii is over 10 times lower than the average viscosity inferred for the western highlands. Alternatively, if modification reflects deformation over crater-centered, laccolithlike intrusions, the derived magma pressures indicate a broad, uniform magma source beneath Mare Smythii, whereas the spatial variation of estimated magma pressures in the western highlands suggests the presence of several, smaller magma sources.

The derived intrusion depths are partly a function of crater size, but range from ~1 to 10 km in depth for both regions and may be slightly greater on average in the western highlands. While both viscous relaxation and igneous intrusions can explain the modification of individual craters, the regional variations in these derived modification conditions also allow further testing of each modification model. In particular, the correlation of the lowest viscosities or the longest relaxation times with the smallest craters in both Mare Smythii and the western highlands seems inconsistent with crater modification by the relaxation mechanism. The onset of total relaxation at progressively smaller topographic wavelengths over time might produce such a trend in Mare Smythii, but total relaxation cannot be invoked for the western highlands, where many large craters still preserve significant fraction of their initial relief. In addition, the relaxation of the smallest craters in both regions suggests the presence of exceptional high near-surface thermal gradients (~150-200 K/km). Since the observed regional variations in crater modification can be easily attributed to variations in magma pressure as a function of mantle topography, we conclude that crater floor fracturing in both regions is more consistent with the igneous intrusion mechanism than with viscous relaxation during a crustal heating event. .