45th Lunar and Planetary Science Conference (2014) 2396.pdf DEEP-SEATED CONTRACTIONAL TECTONICS IN MARE CRISIUM, THE MOON. Paul K. Byrne1,2, Christian Klimczak1, Sean C. Solomon1,3, Erwan Mazarico4, Gregory A. Neumann5, and Maria T. Zuber4, 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 ([email protected]); 2Lunar and Planetary Institute, Universities Space Research Association, Houston, TX 77058; 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964; 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; 5Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771. ▪ Mare Crisium is more tectonically deformed dated by wrinkle ridges may be due to global contrac- than previously recognized tion resulting from interior cooling [6], a process that ▪ Basin-circumferential faults within Crisium pe- the Moon began to experience ~3.6 Ga [3]. netrate ~20 km into the lunar lithosphere The recent availability of high-resolution topo- ▪ GRAIL data suggest that these deep-seated graphic and gravity field data for the Moon allows for faults bound the Crisium mascon the tectonics of lunar maria to be investigated anew. Here we combine Lunar Reconnaissance Orbiter Cam- Introduction. Crisium is an elongate, 450- by 550- era (LROC) [7] Digital Terrain Model (DTM) data km-wide Nectarian [1] impact basin on the lunar near- (118 m/px) and gravity field data (to spherical harmon- side that hosts expansive mare basalt deposits. Like ic degree and order 660; spatial block size 8.3 km) other lunar maria [e.g., 2], tectonic deformation in Cri- from the Gravity Recovery and Interior Laboratory sium is characterized by abundant wrinkle ridges [3], (GRAIL) mission [8]. We (i) remap the tectonics of landforms interpreted as folds over faults that may be Mare Crisium, (ii) use elastic dislocation models to blind or surface breaking [4]. These ridges have been determine the depths to which prominent basin- ascribed to subsidence of the mare deposits [5], al- circumferential faults penetrate, and (iii) compare the though on Mercury a portion of the strain accommo- distributions of faults with that of the Crisium mascon. Fig. 1. Structural map of Mare Crisium, shown with color-coded elevation from the LROC DTM. The map is in an orthographic projection, centered at 17.1°N, 59.4°E. 45th Lunar and Planetary Science Conference (2014) 2396.pdf Photogeological Mapping. The basin interior is We find that the large, basin-concentric ridges replete with wrinkle ridges, consistent with previous within Crisium have accumulated substantial lateral observations [3]; we document more than 170 (Fig. 1). shortening (e.g., >1 km for Dorsum Oppel, Fig. 2), and The most prominent such structures follow the basin that their underlying faults have planar to listric geo- outline and verge toward the interior (most notably metries. Intriguingly, despite variations in fault dip and from 30°–180° and 260°–330° azimuth, measured architecture, these structures all penetrate ~20 km into clockwise from north), along the inner boundary of a the lunar crust, far below the base of the mare deposits. circumferential, elevated “bench.” Gravity Data. GRAIL Bouguer gravity anomaly Artificially illuminated DTM hillshade maps, for data indicate that Crisium’s mascon occupies almost solar azimuth angles of 0° and 180°, reveal ~east– the entire interior, with gravity values of 100 mGal at west-orientated structures that are not readily visible in the perimeter rising to ~500 mGal inward of 0.7–0.8 of photogeological data. We identify 10 partially buried the basin radius. Further, as noted earlier [10], the large craters within Crisium, but we note a further five (the ridges that follow the inner edge of the “bench” within largest of which is ~95 km in diameter) that are demar- the basin are collocated with the boundary of the high- cated by wrinkle ridges but have no other surface ma- est values of the Bouguer anomaly (Fig. 3). nifestation. The DTM also reveals subtle ridge-like changes in relief across the mare that are virtually im- possible to detect otherwise. We interpret these 13 ridges, ~30–100 km in length, as additional shortening structures that have no surficial faulted component. Elastic Modeling. We used the open-access elastic dislocation program COULOMB [9] to match solu- tions for surface displacements to topographic profiles, taken from the LROC DTM, across the most promi- nent ridges in Crisium. By varying the dip, penetration depth, and direction and amount of displacement of a model fault, derived surface displacements were com- pared with five profiles taken across the large circum- ferential ridges within Crisium, and each model was Fig. 3. Bouguer gravity anomaly map for Mare Crisium. The modified until a satisfactory match was found. map has the same projection as that in Fig. 1. This approach is applicable to the Moon because, Crustal thickness [11] within Crisium varies from with virtually no erosion, topographic relief across a ~20 km at the basin perimeter to ~0–4 km inward of fault is essentially equal to the vertical displacements the prominent ridges. Given our elastic modeling re- on that structure. A sufficiently accurate match be- sults, then, it appears that Crisium’s mascon is structu- tween model results and observations allows the geo- rally bound by shallow- and outward-dipping thrusts. metric parameters of the lunar fault (e.g., cumulative Outlook. Extending this analysis to other maria displacement, fault dip angle, and penetration depth) to with “bench”-like features (e.g., Nectaris and Serenita- be determined reliably. tis) will help determine the extent to which deep-seated tectonic deformation, a process not previously recog- nized for the Moon, has operated on that body. References. [1] Fassett, C. I. et al. (2012) JGR, 117, E00H06. [2] Bryan, W. B. (1973) Geochim. Cos- mochim. Acta, 1, 93–106. [3] Solomon, S. C. & Head, J. W. (1980) Rev. Geophys., 18, 107–141. [4] Mueller, K. & Golombek, M. P. (2004) Annu. Rev. Earth Pla- net. Sci., 32, 435–464. [5] Maxwell, T. A. et al. (1975) Geol. Soc. Am. Bull., 86, 1,273–1,278. [6] Solomon, S. C. et al. (2013) AGU Fall Meeting, abstract P11A-08. [7] Scholten, F. et al. (2012) JGR, 117, E00H17. [8] Lemoine, F. G. et al. (2013) JGR, 118, 1,676–1,698. [9] Lin, J. & Stein, R. S. (2004) JGR, 109, B02303. Fig. 2. Topographic profile (teal) and COULOMB model fit [10] Zuber, M. T. & James, P. B. (2013) AGU Fall (lime) for Dorsum Oppel, a prominent eastward-verging Meeting, abstract P13B-1753. [11] Wieczorek, M. A. ridge in the western part of Mare Crisium. et al. (2013) Science, 339, 671–675. .