SUPPLEMENTARY INFORMATION

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1 Recent extensional tectonics on the Moon revealed by the Lunar Reconnaissance Orbiter Camera Thomas R. Watters 1, Mark S. Robinson 2, Maria E. Banks 1, Thanh Tran 2, Brett W. Denevi 3 1. Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC 20560, USA. 2. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85251, USA. 3. Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. Correspondence and requests for materials should be addressed to 1 T.R. Watters ( watterst@si.edu). NATURE GEOSCIENCE 1

2 SUPPLEMENTARY NOTES S1. MARE BASIN AND FLOOR-FRACTURED CRATER GRABEN The graben associated with mare basins generally exhibit flat-floored, symmetric crosssectional geometry with walls maintaining roughly the same relief as they extend from mare basalts into basin rim or highlands material. This consistency of form suggests the bounding antithetic normal faults of the graben have roughly the same fault plane dips 1-4. The extensional and compressional stresses that formed the pattern of basin-related graben (extensional landforms) and wrinkle ridges (contractional landforms) result from loading of the lithosphere by thick sequences of relatively dense uncompensated mare basalts that induce subsidence and flexure of the lunar lithosphere 5-9. Evidence of extension beyond the mare basins is limited to the floors of some impact craters. Graben in these floor-fractured craters are smaller in scale than typical basin-related graben and often form polygonal patterns 1, 2. The source of the extensional stresses is generally thought to result from uplift of the crater floor due to either relaxation of crater topography or from shallow-depth volcanic intrusions confined to the craters 1, S2. DISTRIBUTION OF SMALL-SCALE GRABEN Small-scale graben are widely distributed on the Moon, found on both the nearside and farside (Supplementary Fig. 1). Because of the scale of the graben, they can only be detected in meter scale images. As of the time of this survey, only about 35% of the surface of the Moon was covered by NAC images. Thus, the young graben described here are likely an incomplete sample of the total population; the total number of such features is not known at this time. The graben are spatially correlated, but not always co-located, with contractional tectonic landforms 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION and deform both mare basalts and highland material. The Virtanen graben are largest of the newly detected graben and occur in the floor material of a heavily degraded impact basin in the farside highlands (~17.8 N, E) (Supplementary Fig. 1). S3. TOPOGRAPHIC SETTING OF BACK-SCARP GRABEN Lunar Orbiter Laser Altimeter (LOLA) profiles across the Pasteur and Madler graben indicate that, like those associated with the Lee-Lincoln scarp, graben occur in elevated backscarp terrain. The Madler graben are found on a low-relief mound, one of many hills that appear to be remnants of heavily degraded craters. The Pasteur graben cut across a series of gently undulating hills and valleys in the back-limb area. S4. VIRTANEN GRABEN A digital terrain model (DTM) covering part of the Virtanen graben (Supplementary Fig. 2) derived from LROC NAC stereo pairs. Stereo pairs were initially processed with the U.S. Geologic Survey's Integrated Software for Imagers and Spectrometers (ISIS) and the terrain was derived using the BAE Systems SOCET SET photogrammetry system with custom libraries provided by the USGS Astrogeology Science Center in Flagstaff, AZ. The DTM reveals the Virtanen graben are located on a topographic rise with several hundred meters of relief (Supplementary Fig. 2, 3). The graben extend from elevations of ~950 m to >1100 m along the crest of a ridge-like landform (Supplementary Fig. 2). The rise is flanked to the southwest by the rim of a ~2.5 km diameter degraded crater and to the northeast and by the rims of a string of smaller-diameter, more heavily degraded craters. A topographic profile across the widest graben (~500 m) shows that it is ~17 m deep with a relatively flat floor (Supplementary NATURE GEOSCIENCE 3

4 Fig. 4A). To the southeast, the graben narrows (to ~200 m) and the maximum depth decreases to ~5 m (Supplementary Fig. 4B). The smallest of the Virtanen graben measured is ~26 m wide and has a maximum depth of ~1 m (Supplementary Fig. 4C). S5. STRESS TO INITIATE NORMAL FAULTING The isotropic compressional stress due to lunar radial contraction is estimated to be ~10 MPa, based on the areal contractional strain expressed by the population of young lobate scarps 18. The extensional stress needed to initiate normal faulting on the Moon at a depth of 500 m (consistent with the approximate depth of faulting of a 500 m wide symmetric graben where antithetic normal faults with dips of ~60 intersect at a common depth), in the absence of compressional background stresses, is ~2 MPa based on frictional strength criteria (see 19). However, the graben appear to be asymmetric (see Supplementary Fig. 4) and thus their associated faults may extend to greater depths 20. S6. DEPTH TO LACCOLITH The minimum depth to the laccolith can be estimated assuming the vertical load q is slightly greater than the lithostatic pressure ρgh where ρ is the density, g is the acceleration due to gravity, and h is the thickness of the plate and the minimum depth of the laccolith. Assuming the maximum deflection w o is approximated by the maximum relief of the rise, the thickness of the plate is given by h = [ρgl 4 (1-2 )/32E w o ] 1/2 where L is the width of the plate, is poisson s ratio, and E is the Young s modulus (see 19). In the case of the Vitello graben, since the maximum relief of the topographic rise is ~16 m, the thickness of the plate or minimum depth of the laccolith is estimated to be ~80 m. This depth may correspond to the local thickness of the 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION mare basalts that fill the valley, and may be a sufficient depth to account for the pit chains associated with some of the Vitello graben if pits formed by dilational faulting confined to the basalts. Supplementary Note Fig. 1. Locations of small-scale graben (red stars) with previously known (black dots) and newly detected (white dots) lobate scarps 21. The locations of 26 shallow moonquakes 22 are also shown (black triangles). Plot is centered on the lunar farside (180.0 E). Graben and lobate scarp locations are plotted on a shaded relief map merged with a global LROC WAC stereo derived DTM. NATURE GEOSCIENCE 5

6 Supplementary Note Fig. 2. Digital terrain model (DTM) from NAC stereo images (frames M LE, M RE and M LE, M RE) shows that the Virtanen graben are located on a ridge-like topographic feature. The DTM has a horizontal spatial scale of 2 m/pixel (these NAC stereo images have a resolution of ~0.7 m/pixel) and a vertical precision of ~0.5 m 15. Elevations are referenced to a sphere of 1,737,400 m. 6 NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION Supplementary Note Fig. 3. Perspective view of the Virtanen graben generated by draping NAC images over the digital terrain model shown in Fig. S2, vertical exaggeration is 5:1. NATURE GEOSCIENCE 7

8 Supplementary Note Fig. 4. Topography of the Virtanen graben. (A) Topographic profile across the widest graben. Profile location is shown in Fig. S2 (vertical exaggeration is ~11:1). (B) Topographic profile across the widest graben where it a narrows. Profile location is shown in Fig. S2 (vertical exaggeration is ~11:1). (C) Topographic profile across a narrow graben. Profile location is shown in Fig. S2 (vertical exaggeration is ~11:1). Elevations are referenced to a sphere of 1,737,400 m. 8 NATURE GEOSCIENCE

9 SUPPLEMENTARY INFORMATION REFERENCES 1. Wilhelms, D. E. The Geologic History of the Moon. U.S. Geol. Surv. Prof. Paper 1348 (1987). 2. Watters, T. R. & Johnson, C. L. Lunar Tectonics. in Planetary Tectonics, T. R. Watters, R. A. Schultz, Eds. (Cambridge Univ. Press, New York, NY, 2010), pp McGill, G. E. Attitude of fractures bounding straight and arcuate lunar rilles Icarus, 14, (1971). 4. Golombek, M. P. Structural analysis of lunar grabens and the shallow crustal structure of the Moon. J. Geophys. Res., 84, (1979). 5. Phillips, R. J. et al., Mascons: Progress toward a unique solution for mass distribution. J. Geophys. Res., 77, (1972). 6. Melosh, H. J. The tectonics of mascon loading. Proc. Lunar Planet. Sci. Conf. 9, (1978). 7. Solomon, S. C. & Head, J. W. Vertical movement in mare basins relation to mare emplacement, basin tectonics and lunar thermal history. J. Geophys. Res., 84, (1979). 8. Solomon, S. C. & Head, J. W. Lunar mascon basins: Lava filling, tectonics, and evolution of the lithosphere. Rev. Geophys. Space Phys. 18, (1980). 9. Freed, A. M., Melosh, H. J. & Solomon, S. C. Tectonics of mascon loading: Resolution of the strike-slip faulting paradox. J. Geophys. Res. 106, (2001). 10. Pike, R. J. Genetic implications of the shapes of martian and lunar craters. Icarus, 15, (1971). 11. Schultz, P. H. Floor-fractured lunar craters. The Moon, 15, (1976). 12. Hall, J. L., Solomon, S. C. & Head, J. W. Lunar floor-fractured craters: Evidence for viscous relaxation of crater topography. J. Geophys. Res., 86, (1981). 13. Wichmanand, R. W. & Schultz, P. H. Floor-fractured craters in Mare Smythii and west of Oceanus Procellarum: Implications of Crater Modification by Viscous Relaxation and Igneous Intrusion Models. J. Geophys. Res., 100, (1995). 14. Dombard, A. J. & Gillis, J. J. Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor-fractured craters. J. Geophys. Res., 106, (2001). NATURE GEOSCIENCE 9

10 15. Tran T. et al. Generating digital terrain models using LROC NAC images. ISPRS Technical Commission IV & AutoCarto, ASPRS/CaGIS 2010 Fall Specialty Conference (2010). 16. Miller S. B. and Walker, Further Developments of Leica Digital Photogrammetric Systems by Helava. A. S. ACSM/ASPRS Ann. Convention Exposition, Tech. Papers, (1993). 17. Miller S. B. and Walker, A. S. Z. Die Entwicklung der digitalen photogrammetrischen Systeme von Leica and Helava. Photogramm. Fernerkundung, 195, 4-16 (1995). 18. Watters, T.R. et al., Evidence of recent thrust faulting on the Moon revealed by the Lunar Reconnaissance Orbiter Camera. Science, 329, (2010). 19. Turcotte, D. L. & Schubert, G.Geodynamics: Application of Continuum Physics to Geological Problems (Cambridge University Press, Cambridge, UK, 2002). 20. Schultz, R. A., Moore, J. M., Grosfils, E. B., Tanaka, K. L. & Mége D. The Canyonlands model for planetary grabens: revised physical basis and implications. in The Geology of Mars: Evidence from Earth-Based Analogs (ed Chapman M.), (Cambridge Univ. Press, New York, 2007). 21. Banks, M. A. et al. The search for lunar lobate scarps using images from the Lunar Reconaissance Orbiter camera. 42st Lunar Planet. Sci. Conf., Abstract #2736 (2011). 22. Nakamura, Y. Shallow moonquakes: How they compare with earthquakes. Proc. Lunar Planet. Sci. Conf. 11, (1980). 10 NATURE GEOSCIENCE