RESEARCH Topographic expressions of lunar graben Melanie B. Callihan* and Christian Klimczak STRUCTURAL GEOLOGY AND GEOMECHANICS GROUP, DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GEORGIA 30602, USA ABSTRACT Graben, defined as landforms produced by normal faulting, have long been recognized on the Moon, but their map patterns, as well as topographic expressions, have not been studied systematically. The topography across graben and its along-strike variations reveal details about the growth of the normal faults forming the graben. Individual normal faults grow in length by the propagation of fault tips during slip events, which can also enlarge the displacement along the fault plane. Displacement and length accumulate and grow larger over time with more slip events, fault interaction, and linkage. We measured fault lengths and vertical offsets and then calculated the displacement for lunar graben using data from the camera and laser altimeter onboard the Lunar Reconnaissance Orbiter. Our study systematically investigated 14 graben systems across the lunar surface. Graben lengths were found to range from ~43 to 453 km, and displacements ranged from ~127 to 1115 m. These displacements were plotted against graben fault length to produce slip distributions, which revealed growth patterns involving mechanical interaction and fault linkage. Displacement-to-length scaling was used to further study the evolu- tion of graben-bounding normal faults. We observed a sublinear growth pattern for lunar graben-bounding normal faults, consistent with growth of faults via segment linkage, where different stages of linkage are present on the lunar surface. Lunar graben-bounding faults show higher scaling ratios than previously estimated, likely due to variations in host-rock properties and mechanical stratigraphy. LITHOSPHERE; v. 11; no. 2; p. 294–305; GSA Data Repository Item 2019082 | Published online 31 January 2019 https://doi.org/10.1130/L1025.1 INTRODUCTION offset considering horizontal and vertical movement (Fig. 1), and it can be calculated using the fault dip and throw, or it can be measured along slip. Planetary bodies lacking major erosional processes and atmospheres, Graben that show similar offset on both faults are symmetric. Frequently, such as the Moon, preserve the morphologies and topographic expressions however, the two graben-bounding faults show differences (asymmetry) in of major landforms over long time scales. This enables us to study faults displacement, where the fault with the higher displacement is considered in terms of their lateral extent, segmentation, offset, fault displacement, the master fault, and the fault with lower displacement is the antithetic and overall fault evolution. In recent years, the collection of high-resolu- fault. The symmetry of a graben reveals information about its maturity. tion topographic measurements from the Lunar Orbiter Laser Altimeter Asymmetric graben or half graben typically indicate the early stages of (LOLA; Smith et al., 2010) on board the Lunar Reconnaissance Orbiter graben development, but as they grow larger, they become more symmetric. (LRO) gathered the data essential to create an in-depth understanding of Generally, fault growth is governed by preexisting discontinuities, lunar faults and structures. Using lunar graben geomorphology, geometry, the local or regional stress field, rock strength, and pore-fluid pressure displacement-to-length scaling, and overlap-to-spacing ratios, this study (Arthur et al., 1963). Faults generally grow by accumulation of seismic aimed to create a better understanding of the growth of graben-bounding slip (Walsh and Watterson, 1987; Cowie and Scholz, 1992a, 1992b) and normal faults on the Moon. fault linkage (Peacock and Sanderson, 1991; Cartwright et al., 1995; Man- sfield and Cartwright, 2001). Isolated faults grow via continued slip events Graben Geomorphology and enable the formation of fault populations, where groups of smaller faults occur in the same region. In the second case, faults can increase Normal faults, inclined planar discontinuities along which extensional their length and displacement over time by linking or coalescing with strains are accommodated via frictional sliding, are commonly found across other smaller faults in the region (Cartwright et al., 1995). These arrays the surface of the Moon (Golombek, 1979; Hiesinger and Head, 2006; of subparallel (en echelon) small faults consist of fault segments rather Smith et al., 2010; Watters and Johnson, 2010; Klimczak, 2014; Nahm, than one single fault slip plane and form different characteristic structures 2016). Graben are linear landforms marked by multiple, oppositely dipping in map and outcrop view (Willemse, 1997; Fossen et al., 2010). When (antithetic) normal faults that create a downdropped block in the center two faults grow toward one another, they begin to mechanically interact (Fig. 1; Schultz et al., 2007; Fossen, 2009). Each displacement along a (Peacock and Sanderson, 1991; Willemse, 1997; Gupta and Scholz, 2000; normal fault can be decomposed into its horizontal and vertical displace- Peacock, 2002; Soliva and Benedicto, 2005). Two segmented faults that ment components, heave and throw, respectively. Displacement is the total approach another, but that are not physically linked, are said to underlap. Underlap reaches a critical point when fault segments and their stress fields Melanie B. Callihan http://orcid.org /0000 -0002 -3314-5172 start to interact with each other (Willemse, 1997). When fault tips have *Corresponding author: [email protected] moved past one another, the faults are considered to be overlapping. Once Geological© 2019 The SocietyAuthors. of Gold America Open |Access: LITHOSPHERE This paper | Volume is published 11 | underNumber the 2 terms| www.gsapubs.org of the CC-BY-NC license. 294 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/11/2/294/4659924/294.pdf by guest on 26 September 2021 CALLIHAN AND KLIMCZAK | Topographic expressions of lunar graben RESEARCH properties and regional stresses (Cowie and Scholz, 1992b) for future VE 8x NSWidth research on lunar normal faults. Previous studies suggested that maximum displacement (Dmax) and fault length (L) are related to one another by: -800 Master Fault c DLmax =γ , (1) Throw where γ is a constant dependent on rock type and regional stresses (Cowie and Graben Scholz, 1992b), and c is the scaling exponent. In scenarios where faults are isolated, the scaling exponent was found to be near unity, whereas the effect -1200 Throw of fault linkage or restriction at depth resulted in lower exponents. Previous Elevation (m) studies on faults have revealed D /L (γ) ratios of ~0.001–0.05 (Muraoka Displacemen max and Kamata, 1983; Walsh and Watterson, 1987; Krantz, 1988; Opheim and Antithetic Fault w Gudmundsson, 1989; Peacock and Sanderson, 1991; Cowie and Scholz, 1992a; Dawers et al., 1993; Cartwright et al., 1995; Dawers and Anders, Thro t 1995; Clark and Cox, 1996; Watters et al., 2000; Mansfield and Cartwright, -1600 Heave 2001; Schultz and Fossen, 2002; Schultz et al., 2006; Polit et al., 2009; Wat- 0 5 10 ters and Johnson, 2010; Gudmundsson et al., 2013; Roggon et al., 2017). Length (km) Graben on the Moon Figure 1. Topographic profile with eight times vertical exaggeration (VE) across a graben forming Rima Ariadaeus. Graben showing identified master and antithetic faults based on amount of throw/displacement. The majority of graben on the Moon occur along the periphery of the lunar maria (Fig. 2A; Golombek and McGill, 1983; Watters and Johnson, 2010; Nahm, 2016), but they are also found in floor-fractured craters (Schultz, 1976; Jozwiak et al., 2012, 2015), in large impact basins (Wil- helms et al., 1979; Nahm, 2016), and on very local scales (Watters et al., interaction and linkage between segments begin, the fault tip propagation 2000; Watters and Johnson, 2010; French et al., 2015). Graben occur in will temporarily arrest, and fault tips begin curving toward each other (Pea- a variety of terrains, including basaltic mare, anorthositic highlands, and cock and Sanderson, 1991; Willemse, 1997; Fossen and Rotevatn, 2016). in mare-highland transitions. The majority of graben analyzed in this Overlapping segments and underlapping segments are considered to be study were found at the mare-highland transitions and in Schrödinger soft-linked when individual faults mechanically interact but do not form basin (Figs. 2B and 2C). a continuous fault plane. Once the faults fully coalesce to form a single Large-scale lunar graben have not yet been systematically investigated fault plane, the two original fault segments are considered hard-linked for their along-strike characteristics in order to fully understand their geo- and can be identified by jogs or abrupt changes in fault orientation in map morphologic expressions. Our investigation examines these relationships view (Willemse, 1997; Soliva and Benedicto, 2004). Graben that have and thus reveals growth mechanisms involving fault linkage and interac- grown by linkage of smaller segments as opposed to growth of a single tion and their displacement-to-length scaling relationships. Previous stud- isolated fault exhibit a much more complex set of structures and variety ies have investigated smaller lunar graben for geomorphological patterns of map patterns. These map patterns show changes in spacing, overlap, (Watters and Johnson, 2010), and an in-depth study of an isolated lunar and sudden changes in fault strike.