6.1. Regional Trends
 The ILDs of West Candor have clearly been subjected to deformation in which the resulting small-scale structures developed non-random trends. Data presented inTable 2 indicates that there is no obvious relationship between the orientation of dominant fractures and local slope. The consistent orientation and wide distribution throughout West Candor Chasma suggest regional control, rather than the localized effects of topography. There also appears to be no clear relationship between fracture orientation and elevation. Dominant fracture orientations (Figure 6; A), suggest two clusters of 120° fracture orientation at ∼1600 m and ∼3000 m. However, this apparent relationship is most likely a sampling bias for these elevations, as result of the large number of data collected in HiRISE images F, H and I. The bias for these elevations is also evident in the 40° clusters within the secondary fractures versus elevation graph (Figure 6, image B). The graphs for unidirectional and curved fractures (Figure 6, images C and D) also present a sampling bias at 3000 m elevation. Sampling biases are also present for 1000 m and 3000 m in the graph of fault orientations versus elevations (Figure 6, image E) for a large range of orientations. The small sample size for the en echelon fractures (Figure 6, image F) does not provide enough data to draw any definitive conclusions about their genesis. No data set shows clear isolated clusters of the same orientation at different elevations. In the larger data sets, the full range of orientations is present at all elevations with sufficient data. Hence we suggest that the fractures are present throughout the entire ILD stratigraphy and not governed by particular stratigraphic units.
 Within the ILD of West Candor, several regional trends were recognized in the orientations of deformational features and interpretations of their genesis are given. The most prominent trend is found within the unidirectional fractures and orthogonal fracture sets, reflecting the postulated orientations of the border (101°–107°) and cross (40°) faults. The second most prominent trend is 70° and was displayed by faults, unidirectional fractures and orthogonal fractures. A third trend is observed within the faults of West Candor, oriented 135°–150°, roughly perpendicular to the inferred cross fault orientations. Assuming the simplest scenario of ILD deformation, a continuation of the imposed stress field which initially produced the chasma-forming faults, with a similar orientation, would produce small-scale deformational features and corresponding orientations. The number and distribution of deformational features may have been influenced by heterogeneities within the ILD (e.g., bed thickness, particle size, competency, etc.) and could account for their inhomogeneous distribution throughout the study area.
 During a continuation of the imposed stress field, unidirectional and dominant orthogonal fracture sets formed in direct response to the imposed stresses, parallel to the maximum horizontal compressive stress. Where a mutually crosscutting relationship is present, both orthogonal fracture orientations (dominant and secondary) may be directly linked to the stresses which acted to produce the blunted terminations of West Candor. However, where no crosscutting relationship is present, the alignment of the stresses producing the secondary fracture sets is more ambiguous and may have resulted directly from the imposed stress or indirectly by visco-elastic relaxation or warping [Rives et al., 1994].
 Along Ceti Mensa's southeast border scarp, orthogonal and unidirectional fracture histograms display several consistent trends parallel to the chasma-forming faults and the 70° trend which is discussed later. The alignment of orthogonal (Figures 8 and 9, images K, L below, O, Q, R, and M) and unidirectional fractures (Figure 13, images L, O, R, and M) parallel to the southeast border scarp may indicate that a large fault underlies the southeast border scarp. The interaction of the imposed regional stress and an underlying fault could generate the observed fracture orientations along the southeast border scarp. This proposed fault may be another preexisting blind thrust (below a wrinkle ridge) aligned concentrically about the Tharsis region that was exploited later to form Ceti Mensa's southeast border scarp. The distance from the southeast border scarp to the SCF is 42 km, roughly the same distance between the NCF to the SCF. The frequency distribution of wrinkle ridge spacing reported by Watters based on 2934 measurements displays an average spacing of 30 km (the mode, instead of the mean was used to obtain the most reliable measure because of the strongly skewed distribution). The value of ∼42 km between cross-faults and between the SCF and southeast border scarp is well within the skewed frequency distribution range of ridge spacing. Hence we suggest that a buried basement fault is located beneath the southeast border scarp of Ceti Mensa and that the visible scarp is the erosional surface feature of that fault within the ILD. The fault may have been active during basin formation.
 The prominent 133–151° trend of the small-scale faults in areas B and H ofFigure 15 is slightly more than 90° from the inferred orientation of the nearby cross fault. In both locations, relatively large populations of short faults with very uniform orientations are present at the intersection of a border and cross fault. Assuming normal displacement, these highly uniform faults of short length are consistent with small cross faults termed “release faults” by Destro et al. . The faults perpendicular to the southeast border scarp identified in area L of Figure 15 may also be release faults that accommodated bending stresses from displacement along the large underlying cross fault proposed above.
 Release faults are cross faults that form to alleviate bending stresses accumulated by differential vertical displacement in the hanging wall of a normal fault (Figure 17) and die out before intersecting another normal fault [Destro, 1995]. Release faults form near the fault tip and do not cut normal fault planes or detachment surfaces [Destro, 1995]. Generally, only one or two major release faults form over strike ramps or at the margins of a normal fault but smaller release faults may occupy intermediate positions. If a major release fault does not form, numerous small release faults may form along the length of the normal fault to accommodate the bending stresses [Destro, 1995].
Figure 17. Block diagram of a release fault (modified from Destro ). Release faults accommodate predominantly vertical/dip-slip displacement and die out within the hanging wall before connecting to a second normal fault [Destro, 1995].
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 The 70° trend displayed by faults, unidirectional fractures and orthogonal fractures (Figures 13 and 15, images C, D, E, H, I, J, L, N, O, and Q) is most abundant in the southwest of the study area (Figures 13 and 15, images C, D, and E). The material in which they form is described by Okubo et al.  and Okubo as slump material shed off the southern flank of Ceti Mensa and eroded by aeolian processes. However, the 70° trend displayed by faults and unidirectional fractures is also present in the other parts of the study area outside the area covered by the slumped material. The 70° trend is prominent in faults atop northwestern Ceti Mensa, through central Ceti Mensa and north of the northern re-entrant. In these areas, the fault trend is almost parallel to the northern border scarp and is therefore more likely to have resulted from the same deformation event and not from slumping. This connection may suggest that none of the deformational features with a 70° orientation resulted from a slumping event but that it records a significant trend of a regional scale. It does however not appear to correlate with the regional trends within West Candor, nor is it reflected by any obvious trends of faults within the chasm walls of Candor, which record the orientation of faulting during the initial collapse. Hence one possibility is that these features result from a regional stress that post-dates the collapse of the chasm. However, recent modeling of stresses during the formation of Valles Marineris byAndrews-Hanna [2012b, Figure 4e] suggests a broad band of high tensile stresses oriented approximately SW-NE in the area of Candor and Ophir chasma. The predicted tensile failure orientation in this band would be approximately 70°. If the features observed here can be attributed to the stress orientation predicted by this model, they would correspond to stresses related to Valles Marineris formation. Outside West Candor, the 70° orientation appears in several features, such as a graben complex south of Echus Chasma (78°–84°), the southern border of Juventae Chasma (83°) and a pit chain that intersects the SW border of Hebes Chasma (74°). All of these regions are within Andrews-Hanna's band of NW-SE-oriented tensile stress.
 The locations of the left and right stepping en echelon fracture sets are in close proximity and of similar orientation to the southeast border scarp and the proposed fault along it. While the number of fracture sets is limited, the two clusters of shear orientations could be interpreted as a conjugate set with a common maximum compressive stress direction of ∼40° (Figure 18). This orientation for the maximum compressive stress is consistent with one of the primary orthogonal fractures orientations.
Figure 18. Diagram for the formation of the conjugate en echelon arrays clustered about two shear zone orientations and a common maximum compressive stress orientation. Dashed line labeled A shows the average orientation (22°) of the sinistrally offset en echelon fractures, produced by a dextral shear sense. Dashed line labeled B shows the average orientation (58°) of the dextrally offset en echelon fractures, produced by a sinistral shear sense. The orientation of the individual fractures that compose the en echelon arrays is 40°, parallel to the maximum compressive stress (modified from Twiss and Moores ).
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 Curved fractures display broad, multimodal orientation trends, only weakly subparallel to the border and cross faults. Hence, their origin cannot directly be linked to the movement of chasma-forming faults or to any other processes.
 Other than the curved fractures, all of the trends discussed above indicate their development is the result of a pervasive underlying mechanism and not local topography or material properties.
6.2. Timing and Significance of Small-Scale Deformational Features
 The occurrence of small-scale deformational features within ILD implies ILD had already formed and lithified before deformation. For the purpose of further discussion, we accept the prevalent view that ILD formed as ancestral basin fill [Lucchitta, 1990, 1999; Witbeck et al., 1991; Lucchitta et al., 1994; Schultz, 1998; Chapman and Tanaka, 2001; Fueten et al., 2006, 2008; Okubo et al., 2008; Okubo, 2010]. The orientations of the deformational features within the ILD of West Candor record three strong trends, one of 70° and two that are parallel to the chasma-forming faults, suggesting the study area has undergone two distinct periods of deformation. The small displacement of the exposed deformational features indicates that they record minor amounts of strain (consistent with the new modeling results ofAndrews-Hanna [2012a, 2012b]. Deformational features can form in response to applied stresses without the need for reactivation of major faults. However, the model of Wilkins and Schultz requires slip on the border faults to produce slip on cross faults. Slip on the cross faults may be the origin of the nearby cross-fault-parallel fractures. This would imply that that some slip on the border faults is required to produce cross-fault – parallel fractures. This slip may be at depth resulting in minor surface strain. Along the southern blunted termination, within the material described byOkubo et al. as an eroded slump, the small number of orthogonal fractures oriented parallel to the border and cross faults appears to represent a period of deformation that post-dates the eroded slump. Fractures which formed before the onset of slumping would have been obliterated during transport or modified to display highly variable orientations. The small number, immature nature and orientations of these fractures are consistent with fractures formed recently in response to stresses parallel to the border and cross faults.
 The 70° fractures and faults are enigmatic. It is possible that they were formed by a local band of tensile stress that crossed the main extension structure at the same time, as suggested by the model of Andrews-Hanna [2012b]. A key result of his modeling is the apparent complexity of the local stress orientation during a seemingly simple process of regional extension. It is also possible that this set of fractures and faults formed by an unknown later event.