Structural analysis of the Valles Marineris fault zone:Possible evidence for large-scale strike-slip faultingon MarsAn Yin...
Valles Marineris fault zone       | RESEARCH               A                                                              ...
YINspacecraft with a typical spatial resolution of      generate faults and folds similar to those caused                 ...
Valles Marineris fault zone     | RESEARCH                               TABLE 1. EXISTING MODELS AND THEIR PREDICTIONS FO...
YIN             σ1        A     Right-slip fault model                               σ3                       σ3          ...
Valles Marineris fault zone   | RESEARCH                                                                                  ...
YINdivided by the Geryon Montes into the northernand southern subtrough zones (Fig. 5A). The                              ...
Valles Marineris fault zone                | RESEARCH  B                  CTX: P05_002815_1720_XN_08S084W                 ...
YIN                                        NE-trending              Fig. 6C                                        joints ...
Valles Marineris fault zone   | RESEARCH                                                                                  ...
YIN                                                                              K                  Avfs1                 ...
Valles Marineris fault zone   | RESEARCHsame time as right-slip faulting. The junctions       left-lateral sense, suggesti...
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars
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Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars

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Structural analysis of the valles marineris fault zone possible evidence for large scale strike-slip faulting on mars

  1. 1. Structural analysis of the Valles Marineris fault zone:Possible evidence for large-scale strike-slip faultingon MarsAn Yin*DEPARTMENT OF EARTH AND SPACE SCIENCES AND INSTITUTE FOR PLANETS AND EXOPLANETS (iPLEX), UNIVERSITY OF CALIFORNIA, LOS ANGELES, CALIFORNIA 90095-1567, USA, ANDSTRUCTURAL GEOLOGY GROUP, CHINA UNIVERSITY OF GEOSCIENCES (BEIJING), BEIJING 100083, CHINAABSTRACTDespite four decades of research, the origin of Valles Marineris on Mars, the longest trough system in the solar system, remains uncertain.Its formation mechanism has been variably related to rifting, strike-slip faulting, and subsurface mass removal. This study focuses on thestructural geology of Ius and Coprates Chasmata in southern Valles Marineris using THEMIS (Thermal Emission Imaging System), ContextCamera (CTX), and HiRISE (High Resolution Imaging Science Experiment) images. The main result of the work is that the troughs and theirplateau margins have experienced left-slip transtensional deformation. Syntectonic soft-sediment deformation suggests the presence ofsurface water during the Late Amazonian left-slip tectonics in Valles Marineris. The total left-slip motion of the southern Valles Marineris faultzone is estimated to be 150–160 km, which may have been absorbed by east-west extension across Noctis Labyrinthus and Syria Planum inthe west and across Capri and Eos Chasmata in the east. The discovery of a large-scale (>2000 km in length and >100 km in slip) and rathernarrow (<50 km in width) strike-slip fault zone by this study begs the question of why such a structure, typically associated with plate tecton-ics on Earth, has developed on Mars. LITHOSPHERE; v. 4; no. 4; p. 286–330. | Published online 4 June 2012. doi: 10.1130/L192.1INTRODUCTION et al., 2000; Anguita et al., 2001, 2006; Webb DATA AND METHODS and Head, 2001, 2002; Bistacchi et al., 2004; Although the 4000-km-long Valles Marin- Montgomery et al., 2009). Some combination Satellite Dataeris trough zone is the longest canyon system of these processes and the role of preexistingin the solar system (Fig. 1A), its origin remains weakness in controlling its developmental his- The main objective of this study was to useelusive. The following hypotheses have been tory have also been proposed (Lucchitta et al., the shape, orientation, spatial association andproposed: (1) rifting (e.g., Carr, 1974; Blasius et 1994; Schultz, 1998; Borraccini et al., 2007; crosscutting relationships to infer fault kine-al., 1977; Frey, 1979; Sleep and Phillips, 1985; Dohm et al., 2009). The purpose of this study matics and the deformation history of the lin-Masson, 1977, 1985; Banerdt et al., 1992; Luc- is to test the above models by analyzing satel- ear and continuous Ius-Melas-Coprates troughchitta et al., 1992, 1994; Peulvast and Masson, lite images across two of the longest and most system in the southern Valles Marineris region1993; Schultz, 1991, 1995, 1998, 2000; Mège linear trough zones in the Valles Marineris (Fig. 1). To achieve this goal, the followingand Masson, 1996a, 1996b; Schultz and Lin, system: Ius Chasma in the west and Coprates data, available publically via web access at2001; Anderson et al., 2001; Mège et al., 2003; Chasma in the east across the southern Valles http://pds.jpl.nasa.gov, were used for recon-Golombek and Phillips, 2010), (2) subsurface Marineris region (Figs. 1B and 1C). As shown naissance and detailed mapping: (1) Thermalremoval of dissolvable materials or magma here, the two trough zones are dominated by Emission Imaging System (THEMIS) satel-withdrawal (e.g., Sharp, 1973; Spencer and trough-parallel normal and left-slip faults; left- lite images obtained from the Mars OdysseyFanale, 1990; Davis et al., 1995; Tanaka and slip zones are associated with en echelon folds, spacecraft with a typical spatial resolution ofMacKinnon, 2000; Montgomery and Gillespie, joints, bookshelf strike-slip faults, and thrusts ~18 m/pixel (Christensen et al., 2000), (2) the2005; Adams et al., 2009), (3) massive dike typically seen in a left-slip simple shear zone Context Camera (CTX) images (spatial reso-emplacement causing ground-ice melting and on Earth. In total, this study indicates that the lution of ~5.2 m/pixel), (3) High-Resolutionthus catastrophic formation of outflow chan- Valles Marineris fault zone is a left-slip trans- Imaging Science Experiment (HiRISE) (spatialnels (McKenzie and Nimmo, 1999), (4) inter- tensional system and its development was resolution of 30–60 cm/pixel) satellite imagesaction among Tharsis-driven activity and an similar to that of the Dead Sea left-slip trans- collected by the Mars Reconnaissance Orbiterancient Europe-sized basin (Dohm et al., 2001a, tensional fault zone on Earth. The magnitude of spacecraft (Malin et al., 2007; McEwen et al.,2009), and (5) large-scale right-slip or left- the left-slip motion across the Valles Marineris 2007, 2010), (4) Mars Obiter Camera (MOC)slip faulting related to plate tectonics, lateral fault zone is estimated to be ~160 km. The lack images from Mars Global Surveyor space-extrusion, or continental-scale megalandslide of significant distributed deformation on both craft with a spatial resolution of 30 cm/pixelemplacement (Courtillot et al., 1975; Purucker sides of the Valles Marineris fault zone suggests to 5 m/pixel (Malin and Edgett, 2001), and that rigid-block tectonics is locally important (5) High-Resolution Stereo Camera (HRSC) *E-mail: yin@ess.ucla.edu. for the crustal deformation of Mars. satellite images obtained from the Mars Express286 For permission to copy, contact editing@geosociety.org| |Volume 4Geological Society of America www.gsapubs.org © 2012 | Number 4 | LITHOSPHERE
  2. 2. Valles Marineris fault zone | RESEARCH A (km) km 60oN 12 30oN 8 300oW 240oW 180oW 120oW 60oW 0o 4 0 30oS Valles Marineris -4 60oS -8 100°W 90°W 80°W 70°W 60°W 50°W 40°W 30°W 20°W 10°N B 0 100 200 300 400 500 km Fig. 1C and Fig. 29A Noctis 0° Labyrinthus Thaumasia Capri thrust fault Ius Ch asma Coprates 10oS Geryon Chasma Montes Copra tes C atena Fig. 28 Melas Capri Chasma Chasma 20oS Eos Chasma Eos Syria Planum Sinai Thaumasia fault Planum Thaumasia zone Planum thrust 30oS (B) North Ius C fault Fig. 2a Fig. 4A Thrust Normal fault Hsu Fig. 15 Fig. 25 Hpl2 Fig. 5A Fig. 4C Inferred offset Hpl2 Thaumasia Fig. 11A Hr thrust West Capri South Ius Hpl3 fault fault Valles Marineris Fig. 23 fault zone Fig. 24 Fig. 26A 0 100 200 300 400 500 km North Coprates Fig. 17 fault Fig. 16 Fig. 26C Fault scarps examined by this study. Hpl2 Hr Dot indicates scarp-facing direction Thaumasia thrust Hpl2Figure 1. (A) Global topographic map of Mars and location of Valles Marineris. (B) Topographic map of Valles Marineris and locations of Figures 1C, 28,and 29A. The Ius-Melas-Coprates (IMC) trough zone is bounded by a continuous and nearly linear fault system at the bases of the trough walls. The faultsystem terminates at northeast-striking normal faults bounding Capri and Eos Chasmata in the east and a complexly extended region across NoctisLabyrinthus and Syria Planum. The Ius-Melas-Coprates trough zone also terminates the north-striking Thaumasia thrust in the south and may have offsetthe thrust to the north for 150–160 km in a left-lateral sense (see text for detail). Note that Melas Chasma is much wider and its southern rim is higherthan the surrounding region. Also note that the eastern part of the Melas depression has a semicircular southern rim. (C) Geologic map of southern VallesMarineris from Witbeck et al. (1991). Locations of detailed study areas described in this study are also shown. Note that the location of the Thaumasiathrust, not mapped by Witbeck et al. (1991), is defined by the Hesperian plain deposits (units Hpl2 and Hpl3) in the west and the Hesperian wrinkle ridgeterrane (unit Hr) in the east. This contact, truncated by the Ius-Melas-Coprates trough zone, corresponds to the Thaumasia thrust belt shown in Figure 1B.LITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 287
  3. 3. YINspacecraft with a typical spatial resolution of generate faults and folds similar to those caused First-order fold~12–13 m/pixel (Neukum et al., 2004). The util- by tectonic processes involving deformation of Second-orderity of these data in planetary geologic mapping the entire lithosphere (e.g., Okubo et al., 2008; foldscan be found in an excellent review by Schultz Metz et al., 2010; Okubo, 2010), care must beet al. (2010). Following the approach of Fueten taken to separate the two types of structures. Cri-et al. (2008, 2011) and Schultz et al. (2010), teria used in this study for identifying, mapping,the THEMIS and HRSC images were used for and characterizing tectonically induced featuresreconnaissance to locate key regional structures. include: (1) structural features not restricted toThis was followed by the use of high-resolution a single unit, (2) deformation patterns that canCTX and HiRISE images for detailed mapping be explained by a uniform stress field across theand structural analysis. Mapping on HRSC studied region with a sufficient arterial cover-images has been carried out by geologists with age (e.g., across a segment of the whole troughthe aid of HRSC digital terrain models (DTM) zone), and (3) structural patterns involvingusing ORION software available from Pangaea recent strata in the trough zones and their asso-Scientific (http://pangaeasci.com/) (Fueten et ciation with those observed in older bedrock ofal., 2005). This approach allows determination the trough walls and plateau margins.of attitudes of planar structural features such asbedding and faults (Fueten et al., 2006, 2008, Map Symbols2010, 2011). Because mapping conducted inthis study used mostly high-resolution CTX For map symbols in the geologic maps gener-images, for which digital topographic maps are ated by this study, I followed the standard nam- Figure 2. Relationship between trends of minornot widely available to the public, the “rule of ing practice employed by the U.S. Geological parasitic fold axes and the main fold axis. NoteV’s” was applied for inferring the bed dip direc- Survey. Capital letters N, H, and A denote the that the orientation of minor folds could varytions. This is a general practice of structural geologic time of Noachian, Hesperian, and Ama- as much as 90°. Thus, using fold trends for infer-geologists mapping on Earth; it can be done zonian during which the map units were depos- ring regional shortening directions without abecause the shading of the images, particular ited or emplaced. Units with lowercase names are structural context can be misleading.with low incident angles of sunlight, provides informal, with the intention of being descriptive.a general sense of topography (i.e., locations of For example, trough-fill, gully-fill, and surfaceridges and depressions) (Tanaka et al., 2009). deposits of dust and talus with unknown ages holistic approach of analyzing structures at vari- may be designated as units tf, gf, and sd. When ous scales independently without assuming anyPhotogeologic Mapping two trough-fill units are mapped, subscripts genetic linkages was considered in this study. “y” and “o” (i.e., tfy, tfo) are used to denote the Photogeologic mapping is inherently uncer- younger and older fill units. For multiple land- GEOLOGIC BACKGROUNDtain due to the lack of direct field checks. As slides, subscripts of 1, 2, 3, …, are used to indi-such, it is important to separate observations cate the older-to-younger sequence (e.g., ls1, ls2). The formation of Valles Marineris started infrom interpretations. To do so, each interpreted the Late Noachian (Dohm and Tanaka, 1999;geologic map is presented in companion with an Methods and Procedure of Structural Dohm et al., 2001a, 2001b, 2009) and lasteduninterpreted satellite image used for map con- Analyses after the end of the Hesperian (Schultz, 1998)struction. Key features are also marked on the or as late as the Late Amazonian (younger thanuninterpreted images so that the readers can eas- An important procedure in the structural 0.7 Ga) (Witbeck et al., 1991). The base of theily see how the suggested interpretations in this analysis of folds adopted in this study is that the trough walls is commonly marked by linearstudy were reached. In addition, assumptions first-order structures at the scale of the trough and high-angle escarpments interpreted as faultand mapping criteria are listed when presenting width are mapped first. This is followed by sys- scarps (Sengör and Jones, 1975; Blasius et al.,geologic maps. Finally, the strengths and weak- tematic mapping of smaller secondary structures 1977; Witbeck et al., 1991; Lucchitta et al.,nesses of competing interpretations of the same within the first-order features. For example, a 1992; Schultz, 1998) (Figs. 1B and 1C). Blasiusset of observations are compared, and, when large flexural-slip fold may contain numerous et al. (1977) considered that the inferred faultspossible, their predictions for future investiga- secondary parasitic folds, which typically fan out may still be active as some of the scarps cuttions are discussed. from the main fold axial plane (Fig. 2). In such a recent sediments and landslides. This argument The mapping presented in this study focuses case, the trends of the secondary folds could dif- was corroborated in a recent study by Spagnu-mainly on the trough-wall and trough-floor fer significantly from that of the main fold axis. olo et al. (2011).structures. Identifying structures on trough walls A statistic tabulation of all fold trends without The existing hypotheses for the origin ofis difficult in places, as young talus deposits and a clear differentiation of their actual structural Valles Marineris may be divided into (1) thosescreens of dusts obscure bedding attitudes. Map- positions and size-order relationships could related to erosion, (2) those related to tectonicping structures in Valles Marineris trough floors lead to grossly wrong interpretations, such as an processes, and (3) those related to gravity-drivenis equally challenging, as they are generally inference of multiple phases of shortening under processes (Table 1). In the erosion models, thecovered by young landslides and recent trough variable regional stress states. That approach is Valles Marineris trough system was inducedfills (mostly sand and dust deposits) (Witbeck most effective for a simple structural system that by surface collapse via mass withdrawal fromet al., 1991; Quantin et al., 2004). Given that formed under a single stress regime. However, below (magma, ice, or carbonate rocks) (e.g.,landslide emplacement, possibly triggered by for structures formed by superposition of several Sharp, 1973; Spencer and Fanale, 1990; Davisimpacts, climate change, and tectonics, can also tectonic events under different stress regimes, a and Golombek, 1990; Tanaka and MacKinnon,288 www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE
  4. 4. Valles Marineris fault zone | RESEARCH TABLE 1. EXISTING MODELS AND THEIR PREDICTIONS FOR THE ORIGIN OF VALLES MARINERIS (VM)Model types Erosion Tectonics Gravity-driven (submodels below) (submodels below) (submodels below)List of predicted Collapse Antecedent Simple rift Complex rift Right-slip Left-slip Spreading Megaslidefeatures (below) (Ref. 20–26) (This study) (Ref. 1–11) (Ref. 12–13) (Ref. 14–16) (Ref. 17–19) (Ref. 18) (Ref. 19) 1. Relation btw None Could be related Steep trough- Steep trough- Vertical trough- Vertical trough- Vertical trough- Vertical trough- troughs and to preexisting bounding faults bounding faults bounding faults bounding faults bounding faults bounding faults faults joints 2. Fault or trough- Highly irregular N/A Curvilinear, Curvilinear, Straight and Straight and Straight and Straight and edge geometry trough margins discontinuous, discontinuous, continuous continuous continuous linear continuous linear in map view overlapping overlapping linear fault linear fault fault traces fault traces fault traces fault traces traces traces 3. Normal faulting Locally N/A Only structures Required NW-striking en NE-striking en NE-striking en NE-striking en developed echelon normal echelon normal echelon normal echelon normal faults faults faults faults 4. Strike-slip Not required N/A Not required Not required Required Required Required Required faulting 5. Along-strike Localized N/A Decrease Decrease Variable Variable Variable Variable variation of extension across laterally laterally extension along extension along extension along extension along fault motion collapse sites releasing bends releasing bends releasing bends releasing bends 6. Attitude of Inward dipping Flat Outward dipping Inward tilt No need for No need for No need for No need for trough-margin due to rift- predating VM bed tilt bed tilt bed tilt bed tilt beds shoulder uplift rifting 7. Secondary Normal faults N/A Normal faults Normal faults Normal, Normal, Normal, reverse, Normal, reverse, structures reverse, strike- reverse, strike- strike-slip and strike-slip and slip and folds slip and folds folds folds 8. Relation btw Not required Not required Not required Not required Possible, with Possible, with Possible, with Not required VM faults and strike-slip faults strike-slip faults strike-slip faults NNE-trending as transfer as transfer as transfer grabens structures structures structures 9. Relation No relation No relation No relation No relation VM faults must VM faults must VM faults VM faults between VM have outlasted have outlasted terminate at terminate at faults and thrusting thrusting thrust belt thrust belt Thaumasia thrust belt10. Predict closed Yes No No No Possible by Possible by Possible by Possible by basins forming pull- forming pull- forming pull- forming pull- apart basins apart basins apart basins apart basins11. Linking Not required Not required Not required Not required Fault zone Fault zone Fault zone Fault zone structures at as extension as extension terminates at terminates at terminates at terminates at ends of the VM vanishes vanishes NNE-trending NNE-trending NNE-trending NNE-trending trough zone contractional extensional extensional extensional structures at structures at structures in the structures in the two ends two ends east and a thrust east and a thrust belt in the west belt in the west Note: References: (1) Blasius et al. (1977), (2) Masson (1977, 1985), (3) Frey (1979), (4) Wise et al. (1979), (5) Melosh (1980), (6) Plescia and Saunders (1982),(7) Lucchitta et al. (1992), (8) Peulvast and Masson (1993), (9) Schultz (1998), (10) Peulvast et al. (2001), (11) Schultz and Lin (2001), (12) Lucchitta et al. (1992),(13) Schultz (1998), (14) Courtillot et al. (1975), (15) Anguita et al. (2001), (16) Bistacchi et al. (2004), (17) Purucker et al. (2000), (18) Webb and Head (2002), (19)Montgomery et al. (2009), (20) Sharp (1973), (21) Spencer and Fanale (1990), (22) Davis and Golombek (1990), (23) Tanaka and MacKinnon (2000), (24) Montgomeryand Gillespie (2005), (25) Adams et al. (2009), and (26) Jackson et al. (2011).2000; Montgomery and Gillespie, 2005; Adams (2001, 2002) and (2) the thin-skinned megaland- eastern scarp is about two times wider than thatet al., 2009; Jackson et al., 2011) or development slide model of Montgomery et al. (2009).The in the west. The trough-floor elevation of Melasof an antecedent drainage system. The tectonic contrasting predictions by the competing mod- Chasma decreases northward, reaching the maxi-hypotheses may be divided into (1) the right- els for the development of the Valles Marineris mum depth against the broadly curved northernslip model (Courtillot et al., 1975; Anguita et al., trough zone are summarized in Table 1. trough margin (Fig.1). Coprates Chasma is the2001; Bistacchi et al., 2004) (Fig. 3A), (2) the The 2400 km linear trough zone consisting longest linear trough zone in Valles Marineris.left-slip model (Purucker et al., 2000; Webb and of Ius-Melas-Coprates Chasmata is the most Its width decreases systematically from west toHead, 2002; Montgomery et al., 2009) (Fig. 3B), dominant feature in southern Valles Marineris east, opposite to the width-variation trend of Ius(3) the simple-rift model (e.g., Blasius et al., (Fig. 1B). Overall, the elevation of the trough zone Chasma. Discontinuous and trough-parallel lin-1977; Masson, 1977, 1985; Frey, 1979; Wise et floor decreases from west to east. Ius Chasma has ear ridges with various lengths are present withinal., 1979; Plescia and Saunders, 1982; Lucchitta two linear subtrough zones separated by a linear the trough zone (Fig. 1).et al., 1992; Peulvast and Masson, 1993; Schultz, ridge. The central ridge terminates in the west-1998; Peulvast et al., 2001; Schultz and Lin, ernmost part of Ius Chasma where two subtrough GEOLOGY OF IUS CHASMA2001) (Fig. 3C), and (4) the complex-rift model zones merge. In the east, the central ridge termi-(i.e., the ancestral basin model) (see Lucchitta nates at much wider Melas Chasma. The overall Trough-Bounding Structureset al., 1992; Schultz, 1998). Finally, the gravity- width of Ius Chasma increases gradually east-driven hypotheses consist of (1) the thick-skinned ward. Melas Chasma has an irregular southern The Geryon Montes separate Ius Chasmagravitational spreading model of Webb and Head margin marked by two spoon-shaped scarps; the into the northern and southern trough zones. TwoLITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 289
  5. 5. YIN σ1 A Right-slip fault model σ3 σ3 B Left-slip fault model σ1 Conjugate Riedel shear (R′) Fold Fold Conjugate Riedel shear (R′) Riedel shear (R) Thrust Thrust Riedel shear (R) MAIN TREND of MAIN TREND of the shear zone the shear zone Normal fault Normal fault Primary shear (P) Primary shear (P) Joint set Joint set Instantaneous strain ellipse Instantaneous strain ellipse Future cross faults Future cross faults σ3 σ1 σ1 σ3 (minimum (maximum (maximum (minimum compressive compressive compressive compressive stress) stress) stress) stress) Closed (pull-apart) basin Closed (pull-apart) basin Trough-bounding faults C Rift model Closed basin Cross fault Extension vanishes at the end of the fault Trough-bounding fault Strike-slip fault Thrust fault Normal fault Basin FoldFigure 3. Three end-member structural models for the formation of Valles Marineris and their predicted structural associations and basin development:(A) right-slip fault zone model, (B) left-slip fault zone model, and (C) rift zone model. Note that box-shaped closed basins may be explained as pull-apartbasins in strike-slip fault models, whereas the rift model requires orthogonal development of coeval normal faults. Symbols σ1 and σ3 represent themaximum and minimum compressive-stress directions.main linear traces of escarpment were mapped Head scarps are mostly buried by younger talus no strike-slip offsets are apparent across localalong the north side of the two subtrough zones deposits coming from above, leaving the toes of short fault scarps and the regional escarpment,by Witbeck et al. (1991) (Fig. 1C). Along the the older slumping structures sticking out at the the trough-bounding structure of this segment ofnorth-wall base of the northern subtrough, the base of younger talus slopes (Fig. 4B). Ius Chasma appears to be a normal fault.linear escarpment typically truncates spurs and Some of the debris flows coming out of the A prominent escarpment is also well devel-gullies and is expressed by prominent rocky channel together with large fan deposits at the oped along the base of the north wall of thecliffs with their bases covered by unconsoli- base of the escarpment are cut by fault scarps (see southern subtrough, as seen from a CTX imagedated talus cones. This can be seen from a CTX the lower-right corner of Fig. 4B). The young (CTX: P12_005795_1730_XI_07S082W)image (CTX: B03_010713_1739_XN_06S082W) fault scarp in turn is buried by younger slump in Figure 4C. Landslides and talus depositsshown in Figures 4A and 4B. The talus cones structures to the west (Fig. 4B). Since sediments were derived from spurs, whereas long-runoutcommonly exhibit multiphase slump structures cut by the fault scarp appear unconsolidated, the debris flows originated from canyons cuttingwith sharp semicircular breakaway scarps in scarp marking the trace of a trough-bounding the trough wall. These deposits were emplacedthe upslope source regions and crescent-shaped fault may be still active, as postulated by the onto the trough floor, which is dominated bytoes at the downslope distal edges (Fig. 4B). classic study of Blasius et al. (1977). Because consolidated layered sediments (Fig. 4C). The290 www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE
  6. 6. Valles Marineris fault zone | RESEARCH landslides and debris flows appear to be much older than those seen in the northern subtrough in that their surfaces are highly incised and N irregular in comparison to the smooth surfaces seen in the northern subtrough (cf. Fig. 4B). Because the sunlight was shed from the left side of the image in Figure 4C, the left banks of canyons on the trough wall and trough floor are well defined by the shade (Fig. 4C). The banks of the trough-wall canyons are nearly linear, whereas the banks of the trough-floor Fig. 4B Base canyons have rather irregular shapes (thin white of es carp men arrows in Fig. 4C). An interesting observation t 2 km is that the west banks of linked trough-wall CTX: B03_010713_1739_XN_06S082W (North Ius Scarp) A and trough-floor canyons are not continuous but appear to be offset by the basal escarpment (indicated by thick white arrows in Fig. 4C). CTX: B03_010713_1739_XN_06S082W (North Ius Scarp) If the trough-floor canyon banks (i.e., risers as defined in tectonic geomorphology; see Burbank and Anderson, 2001) were once con- Scarps of slump structures tinuous and linear, the misalignment of the N trough-wall and trough-floor risers may indi- Debris flow cate a left-slip sense of offset along the trough- bounding fault. Alternatively, the trough-floor canyons may have originated by a different mechanism such as wind erosion rather than flowing water. The third possibility is that the trough-wall and trough-floor canyons were parts of a large drainage system. In this case, Toes of collapse Scarp stream channels bend westward coming out 500 m structures of the mountain range as a result of tributaries B merging with the eastward-flowing main stream along the axis of the trough floor; this may have caused an apparent left-lateral stream deflec- tion (e.g., Burbank and Anderson, 2001). This latter explanation seems unlikely because the N Truncated and discontinuities of canyon banks are abrupt and offset risers Truncated occur at the exact location of the escarpment. risers Also, the truncation of the canyon banks by the escarpment is consistent with the presence of a fault offsetting the risers. The fault-offset inter- Landslide Landslide Talus pretation implies ~500 m of left-slip motion after the formation of the risers as seen from 4 km two canyon systems (Fig. 4C). As debris flows Debris flow C and landslides are not offset by the escarpment (i.e., the trough-bounding fault), the inferredFigure 4. (A) Context Camera (CTX) image B03_010713_1739_XN_06S082W shows a linear scarp left-slip fault motion must have occurred beforebounding the north trough wall of the northern Ius subtrough zone. The location of the escarp- the emplacement of these units.ment is marked by white triangles. See Figure 1C for location. (B) Close-up view of scarps at thebase of the north wall of the northern Ius subtrough zone. Numerous recent slump structures are Trough-Floor Structures: The Westernpresent on talus slopes. Their presence requires the operation of geologic processes that have con-tinuously built up the basal talus slope, causing it to exceed the angle of repose periodically and Traversethus occurence of slumping. Alternatively, slumping may have been triggered by seismic activityon nearby faults. (C) Context Camera (CTX) image P12_005795_1730_XI_07S082W shows a linear In order to understand the structural evolu-topographic scarp bounding the north trough wall of the southern Ius subtrough zone. See Fig- tion of Ius Chasma, two traverses were mappedure 1C for location. Two south-flowing channels display steep risers on the west sides that are trun- via photogeologic analysis across the troughcated and offset left laterally by the escarpment. The offset risers are indicated by white arrow pairs zone. The eastern structural transect is ~45 kmpointing in opposite directions. Also note that the highly eroded drainage channel on the trough long in the north-south direction and ~30 kmfloor has an apparent left-lateral deflection, curving progressively eastward from north to south.The south side of this inferred channel is marked by a series of thin and long white arrows. Several wide in the east-west direction (Fig. 5A). Thislandslides originating from the north trough wall overlie the trace of the escarpment, indicating overall trough zone is bounded by flat plateauthat the formation of the escarpment predates the emplacement of the landslides. margins in the north and south. In detail, it isLITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 291
  7. 7. YINdivided by the Geryon Montes into the northernand southern subtrough zones (Fig. 5A). The Plateau region Anorthern subtrough floor exposes a large land- Nslide complex derived from the north troughwall in the north and a narrow strip of layeredtrough-fill sediments in the south. The southernsubtrough exposes several smaller landslidesthat were originated from the south troughwall and layered trough-fill strata display anorthwest-trending fold complex associated with Fig. 5Bcomplex minor secondary and tertiary folds (seedetails in the following sections). The geologyof the northern and southern subtrough zones isdescribed separately later herein.Northern Subtrough Zone A narrow strip of thinly bedded unit (gfo inFigs. 5B and 5C) lies in the north across a transi-tion zone between the north trough wall and the Fig. 7main landslide (unit As of Witbeck et al., 1991). Northern troughTo the south, a narrow strip of folded trough-fill strata is exposed along the southern marginof the northern subtrough (units Atf1 and Atf2 Fig. 6Ain Figs. 5B and 5C). Unit gfo is separated fromthe main landslide by a zone of unconsolidated Fig. 6Iand fine-textured unit (sdy in Figs. 5B and 5C).This unit is interpreted as surface deposits ofwindblown dust and talus. Beds in unit Atf2 are Fig. 6Ecomposed of flat-lying, light-toned layers thatcan be traced into two deep-cut valleys acrossthe south wall (Figs. 5B and 5C). This relation-ship suggests that unit Atf2 lies depositionally on Geryon Montes Fig. 6Gtop of preexisting topography of the south wall.Embayment relationships indicate that foldedstrata of unit Atf1 are younger than the mappedlandslide complex (unit As in Figs. 5B and 5C). Fig. 6KThus, landslide emplacement predated deposi- Fig. 10 Fig. 8tion of unit Atf1 and folding. Trough-floor structures. In order to deci-pher the effect of tectonic deformation across Fig. 9the northern trough zone, one must first establish Southern troughthe primary structures related to the emplace-ment of the large landslide complex. For land-slides that occur in Valles Marineris withoutpostemplacement deformation, their surfacescommonly display continuous grooves andridges with rounded geometry in cross-sectionviews (e.g., Lucchitta, 1979; also see fig. 6a inBarnouin-Jha et al., 2005). Fractures associatedwith emplacement of landslides typically trendperpendicular to the landslide transport directions(see fig. 2 in McEwen, 1989). In contrast to theseexpectations, the northeast-trending and north- 10 kmnortheast–trending ridges with rounded morphol-ogy within the landslide block are highly broken Plateau regionand are systematically cut by northwest-trending Figure 5. (A) Index map across a segment of western Ius Chasma basedfractures. In addition to having rounded top mor- on Context Camera (CTX) image P05_002815_1720_XN_08S084W. The Iusphology (Figs. 6A–6D), the interpreted primary trough can be divided into the northern and southern subtrough zonesridge surfaces exhibit characteristic (1) rough divided by the Geryon Montes. See Figure 1C for location. (Continued onsurface morphology possibly induced by erosion following page.)292 www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE
  8. 8. Valles Marineris fault zone | RESEARCH B CTX: P05_002815_1720_XN_08S084W C HNu sdy sdy N HNu HNu HNu ls HNu N HNu gfy ls sdy Fig. 7 HNu HNu HNu sdy gfo HNu gfd ls HNu Original ridges sdy ls of landslide Original ridges of landslide sdy Embayment As relationship As between As sl and Atf1 Fig. 6A As Fig. 6E sl Joint sets HNu Embayment relationship Atf1 Fig. 6I 5 km between As and Atf-1 Asd Buried Fault scarp? fault (?) Fig. 6G Atf2 HNu Asd Fig. 6K ls Embayment of sdy HNu HNu trough-fills into 5 km sdy HNu valleys sdy sdy Young landslide deposits derived from steep cliffs in mountainous regions; D ls surface morphology is well preserved. sdy Young unconsolidated surficial deposits (dust and talus), fine- textured surface and lack of bedding. Asd Sand dune fields gfy Young gully fills restricted in narrow valleys in trough walls. gfo Thinly bedded and consolidated sedimentary strata at the base of north trough wall. Beds can be traced into deep-cut valleysand form mesas in places. They are interpreted as older valley and trough fill deposits. E As Landslide sheet covering most northern trough floor. Atf2 Younger trough-fill strata, light-toned and fine-textured on surface. Beds are not folded. Atf1 Older trough-fill strata, dark-toned and rougher textured than Atf2. Beds are folded. HNu Trough-wall rock, thickly bedded. Ridge formed during emplacement of the landslide blockFigure 5 (continued ). (B) A portion of uninterpreted CTX image P05 Inferred fault contact now buried below young sediments_002815_1720_XN_08S084W. See A for location. (C) Interpreted geologicmap of the northern Ius trough zone based on image shown in B. Detailedlithologic division is shown in map legend and also described in detail Strike-slip fault Fold Jointsin the text. (D) Development of northwest-striking right-slip faults inter-preted as a result of bookshelf faulting. (E) Development of northwest- Landslide,striking right-slip faults interpreted as a result of formation of Riedel Normal fault Thrust rock avalanche,shears. See text for details. sl or debris flowLITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 293
  9. 9. YIN NE-trending Fig. 6C joints 1 km A 300 m C N N Offset ridges Lobate Fig. 6D scarps Sand NE-trending joints dunes Sand Fault dunes trace CTX: P05_002815_1720_XN_08S084W 400 m Offset ridges with D round tops sdy B N Joints ? As N 1 km Fault ls traces ls sdy Young landslide deposits derived from steep cliffs in ls Strike-slip fault Joints mountainous regions; surface morphology is well preserved. sdy Young unconsolidated surficial deposits (dust and talus), fine- textured surface and lack of bedding. Normal fault Impact crater As Landslide sheet covering most northern trough floor. Landslide (unit ls) , rock avalanche, Thrust or debris flow sl Figure 6. (A) Uninterpreted Context Camera (CTX) image P05_002815_1720_XN_08S084W. See Figure 5B for location. (B) Interpreted geologic map based on image shown in A. (C) Close-up image showing possible left-lateral offset of a ridge across an interpreted fault. See A for location. (D) Close-up view showing possible left-lateral offset of a ridge across a fault. See A for location. (Continued on fol- lowing page.)294 www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE
  10. 10. Valles Marineris fault zone | RESEARCH Offset E ridges F N Offset ridges N N Joint sets 1 km ? Strike-slip fault Normal fault Thrust Joints Fold Impact crater CTX: P05_002815_1720_XN_08S084W G N H 500 m N Offset marker bed CTX: P05 002815_1720_XN_08S084W TX: P05_002815_1720 XN 08S084W : P05_002815_1720_XN_08S084W 84W I J Joi Jo nts o Joints J nt Joints Joints Rule of Vs indicating north-dipping beds 500 m R le indicating Rule of Vs ind ating indic ndicating d south-dipping beds south- so h-d pping beds ou out i d N Dunes Dunes Estimated strike/dip N directions Dunes ) Dunes (?) Interpreted bedding Joints Estimated strike and dip directions Strike-slip fault Normal fault Thrust Joints Fold Impact crater of layered rocksFigure 6 (continued). (E) Uninterpreted CTX image; see Figure 5B for location. White triangles point to the traces of left-slip faults. (F) An interpretedfault map corresponding to image shown in E. (G) Uninterpreted CTX image; location shown in Figure 5B. (H) Interpreted geologic map showing rela-tionships between a left-slip fault and a series of oblique folds. (I) Uninterpreted CTX image; location shown in Figure 5B. (J) Interpreted geologic mapshowing relationships between a right-slip fault and a fold. (Continued on following page.)LITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 295
  11. 11. YIN K Avfs1 Joint sets L N ? Dark-toned marker bed ? Offset marker bed Offset marker bed O set arker d Asd Avfs1 Asd Offset marker bed O set marker Offset m rker bed 350 m West-facing scarp West-facing scar West-facing scarp a Avfs1 (folded and highly fractured) Asd East-facing scarp East-facing scarp East facin carp st-f Avfs2 Avfs3 Asd Sand dune deposits Avfs2 Valley-fill sediments Strike-slip fault Fold Youngest valley-fill Avfs3 Avfs1 Oldest valley-fill sediments Joints sedimentsFigure 6 (continued). (K) Uninterpreted CTX image indicating the traces of a folded marker bed and a linear scarp interpreted as a fault; location shownin Figure 5B. (L) Interpreted geologic map showing relationships between a left-slip fault and a fold.after landslide emplacement (Figs. 6B–6D) and the landslide itself but also younger unit Atf1 in mal faults includes (1) their irregularly curved(2) a gradual change in ridge trends (Figs. 6B and the south (Figs. 6B and 6C). They also offset the fault traces, and (2) fault scarps dying out in6C). The latter is in sharp contrast to the north- contact between the landslide and unit sdy right scarp height away from their intersectionswest-trending linear and smooth scarps inter- laterally along the northern margin of the trough with strike-slip faults indicating diminishingpreted as post-landslide fractures in this study. zone (Figs. 6B and 6C). The northwest-striking normal-slip motion (Figs. 6F and 6G). In con-The observation that the rounded ridges display right-slip faults and east-trending left-slip faults trast, the east-striking left-slip faults terminatecurved geometry is common for all landslides in must have occurred simultaneously, as indicated at northwest-striking thrusts that are expressedValles Marineris due to lateral spreading of the by their mutual crosscutting relationship shown by lobate scarps (see Figs. 5A and 6B).landslide materials (e.g., Lucchitta, 1979). in Figures 6B and 6C. Thus, interpretation of the The northwest-striking right-slip faults The recognition of originally continu- origin of any one set of the fractures must take across the northern subtrough margin exhibitous ridges induced by landslide emplacement this relationship into account. nearly regular spacing at 3.5–4 km. This struc-allows determination of fault kinematics. First, The northwest-striking right-slip faults tural arrangement is quite similar to the classicthe offsets of north-northeast–trending ridges display an en echelon fault pattern, typically bookshelf fault pattern in a broad shear zone.in the western part of the landslide require left- related to the initiation of strike-slip fault zones That is, northwest-striking right-slip faultsslip faulting on several east-striking fractures (e.g., Naylor et al., 1986) (Figs. 6F and 6G). The were generated by east-trending left-slip shear(Figs. 6A–6D). Second, the offset of northeast- observed right-slip faults may have normal-slip (Fig. 6D). Alternatively, the northwest-strikingtrending ridges in the eastern part of the land- components, as they all bound linear basins right-slip faults may have been generated asslide requires right-slip faulting along northwest- parallel and adjacent to the faults. The north- Riedel shears in a broad east-trending right-striking fractures (Figs. 6E–6D). The strongest west-striking faults terminate at north-striking slip shear zone (Fig. 6E). This interpretation,evidence for the tectonic origin of the north- normal faults at their southern ends (Figs. 6F however, contradicts the observation thatwest-striking fractures is that they cut not only and 6G). The evidence for the interpreted nor- east-striking left-slip faults were active at the296 www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE
  12. 12. Valles Marineris fault zone | RESEARCHsame time as right-slip faulting. The junctions left-lateral sense, suggesting that this structure expressed through cliff-forming morphology.between the trough zone and the northwest- has moved left laterally. Together, these obser- It also has a lower albedo, contrasting with unitstriking right-slip faults, with angles ranging vations suggest that the fault may have been gfo1. An anticline trending west-northwest wasfrom 45° to 50°, are also too large to be Riedel reactivated from a preexisting joint fracture. recognized based on bedding dip directionsshears, as Riedel shear fractures typically form Notice that the linear sand dunes in this area determined by the use of the rule of V’s. Theat ~15°–25° from the main trend of the shear can be clearly distinguished from the north- anticline is bounded by the western scarp alongzone (Sylvester, 1988). As shown in the fol- east-striking linear ridges interpreted as joints its northern edge. This fault is interpreted tolowing, the right-slip interpretation for trough- in bedrock; the sand dunes are restricted to have offset a thin and light-toned marker bedfloor deformation shown in Figure 5E is also topographic depressions, have variable ridge left laterally. This same marker bed is foldedinconsistent with the west-northwest trend of trends, and display curvilinear geometry along into a smaller anticline to the north and is itselffolds in the trough zone, which indicate left- individual ridges (Figs. 6G and 6I). in turn offset by a curved fault interpreted as aslip shear (Fig. 5B) (see more details in the fol- Northern trough margin. Three linear thrust, because its geometry in map view indi-lowing sections). fracture-scarp zones are recognized across the cates a low dip angle. The west-northwest–trending folds involv- northern margin of the northern subtrough zone Southern trough margin. The presence ofing unit Atf1 are expressed by the distribu- (Figs. 7A and 7B). The northern scarp zone has a linear alignment of north-facing scarps alongtion of several weathering-resistant layers an east strike in the east and a west-northwest the southern edge of the northern subtrough(Figs. 6G–6J). The antiformal and synformal strike in the west. The two segments are linked wall in Ius Chasma indicates that the trough-fold shapes can be inferred from the dip direc- by a northwest-striking right-step bend. The wall boundary may be controlled by a buriedtions of the fold limbs using the “rule of V’s” southern scarp zone strikes mostly to the east fault below unit Atf2 (Figs. 5B and 5C). Theand the fold shapes in map view can be inferred except along its westernmost segment, where shape of the scarp indicates north side down,from the distribution of marker beds with dis- it strikes in a west-northwest direction. Finally, and thus implies a normal-slip componenttinctive tones (e.g., dark-toned bed in Fig. 6I). the western scarp zone trends dominantly in across this inferred fault. However, this obser-The fold in Figure 6I could have been offset by the west-northwest direction. vation alone does not preclude the fault froma left-slip fault trending northeast. This inferred Figures 7A and 7B (CTX image P05_002815 having strike-slip motion (i.e., it could be afault has clearly defined scarps in the south; its _1720_XN_08S084W) show two scarp zones transtensional structure).southern segment faces to the west, whereas its along the base of the north trough wall of thenorthern segment faces to the east, as inferred northern Ius subtrough zone. Small black and Southern Subtrough Zonefrom the shading of the topographic scarps. white triangles in Figure 7 show the traces of From younger to older ages, the follow-Such geometry implies along-strike changes in interpreted left-slip faults. A right-bank riser of ing major lithologic units are recognized inthe sense of relative vertical motion across the a trough-wall canyon appears to be offset left the southern trough zone (Figs. 8A and 8B):fault that can only be associated with strike-slip laterally (Fig. 7). This interpretation implies (1) young surface deposits of mostly dust (unitfaults. Although this inferred fault appears to that the curved segments of the faults (labeled sdy), (2) recent landslides (unit ls), (3) sandoffset a dark-toned bed defining the southern as right-step bend) trending northwest are dune deposits (unit Asd), (4) two phases offold limb, its extent to the north is unclear. thrust faults. Also note that both the northern postfolding landslide deposits (units ptf-sl1 and The northeast-striking linear ridges and and southern fault scarps truncate stream chan- ptf-sl2), (5) flat-lying postfolding trough fillselongate depressions are prominent morpho- nels in their hanging walls. No corresponding (unit pst-fd), (6) pre- and synfolding sedimentslogical features across the folds (Figs. 6G–6J). channels can be found in the footwalls, which (unit pre/syn-fd), and (7) thickly bedded strataThese features could either be erosional or may have been buried. The large white tri- exposed on the north trough wall (unit HNu)depositional features such as linear sand ridges angles in Figure 7A also indicate an oblique (Figs. 8A and 8B).or expression of joints. The joint interpreta- northwest-trending scarp in the western part The pre- and synfolding strata consist oftion is favored here as the linear ridges have of the image. In close-up view, both the north- highly fragmented broken beds in the easternremarkably even spacing and are distributed ern and southern scarp zones consist of an part of the main depression, which has beenacross major topographic highs forming narrow en echelon array of short fractures (Figs. 7C incised by erosion. The broken blocks vary fromresistant bends, possibly due to fluid flow and and 7D), which are interpreted as high-angle a few hundreds of meters to less than 15 m in themineralization along the joint fractures (Okubo extensional fractures as they only display dip- longest dimension (Figs. 8C and 8D). Most largeand McEwen, 2007). Thus, the formation of the slip offset and have straight surface traces. blocks have rounded edges, and their internalfolds and joints in the area is interpreted to have The latter imply high angles of fracture dips. beds are folded (Figs. 8C and 8D). This obser-occurred during the same phase of tectonic The arrangement of the extensional fractures vation suggests that folding occurred before thedeformation. An additional piece of evidence to implies left-slip motion on the scarps that are fragmentation of the beds. Individual brokensupport the joint interpretation is that the north- interpreted as faults. The left-slip interpreta- beds appear to have slid from the margins toeast-trending linear ridges occur in all trough- tion is consistent with the interpreted possible the center of the depression, as they commonlyfloor units except the youngest unconsolidated left-slip offset of canyon banks. display long and straight lateral ridges markingdeposits, unit gfy (Figs. 6B and 6C). Interpreted geologic relationships adjacent the edges of transport flows linking with lateral In the southeastern corner of Figure 6B, to the western scarp in Figure 7A are shown spreading heads in the interior of the depressiona northeast-striking fault offsets two sets of in Figures 7E and 7F in which two units (gfo1 (Figs. 8E and 8F).marker beds in a left-lateral sense (Figs. 6I and and gfo2) are recognized. The older unit gfo1 has Similar broken bed units were also observed6J). As this fault is subparallel to nearby joint a ripple-like surface morphology and locally by Metz et al. (2010) in many parts of Vallessets, it is interpreted here as a normal fault. high albedo. In contrast, the younger unit Marineris. These authors suggested that theyHowever, the northern extent of the fault also gfo2 displays a smooth surface morphology, is were induced by emplacement of landslidesoffsets or deflects the northern fold limb in a well layered, and is resistant to weathering as from trough walls, and thus were unrelated toLITHOSPHERE | Volume 4 | Number 4 | www.gsapubs.org 297

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