Floor‐fractured craters in Mare Smythii and west of Oceanus Procellarum: Implications of crater modification by viscous relaxation and igneous intrusion models

Abstract

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 fractions of their initial relief. In addition, the relaxation of the smallest craters in both regions suggests the presence of exceptionally 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.

Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor‐fractured craters

“…Another possibility not specifically addressed in this analysis is topographic relaxation working in conjunction with active volcanism, and indeed, such volcanic/relaxation hybrid models have been suggested [e.g., Wichman and Schultz, 1995;Yingst and Head, 1998]. For instance, a forming laccolith would heat the surrounding country rock.…”

“…We begin by selecting parameters that maximize the degree Figure 4 is that even when the parameters are hedged to accentuate relaxation, the modeled diabase is simply too stiff to predict large degrees of relaxation for smaller craters ((60 km in diameter), as is observed in the floor-fractured crater population [see Wichman and Schultz, 1995]. To test the robustness of this conclusion, we artificially enhance the strain rate predicted by the diabase flow law by a factor of 100, in a simulation of a relaxing crater 20 km in diameter.…”

Recent extensional tectonics on the Moon revealed by the Lunar Reconnaissance Orbiter Camera

“…Laccolith intrusions can result in uplift and flexural bending of the overlying material (Supplementary Note S6). This mechanism has been proposed to account for uplift and extension in lunar floor-fractured craters [22][23][24][25] (Supplementary Note S1). It must be noted, however, that there is currently no direct evidence of young (<100 Myr) extrusive volcanism on the Moon 17,18,26 that would substantiate this mechanism.…”

Crater-Centered Laccoliths on the Moon: Modeling Intrusion Depth and Magmatic Pressure at the Crater Taruntius

“…floor-fractured craters (Wichman and Schultz 1995). Such behavior contrasts sharply with the predicted effects of Over 200 craters on the Moon contain distinctive fracviscous relaxation, where floor uplift preferentially reture patterns in and around their crater floors.…”

“…Thus, ure only at geometric stress concentrations near the sill while such intrusions should reflect local magmatic condiedge (Pollard and Johnson 1973). As the sill expands, howtions (Wichman and Schultz 1995), crater structures may ever, the net upward force on the overburden grows in influence both the dimensions and the locations of the proportion to the (increasing) intrusion floor size (Gilbert modeled intrusions (Wichman 1993). 1877).…”