Inverted Fluvial Features in the Aeolis-Zephyria Plana, Western Medusae Fossae Formation, Mars: Evidence for Post-Formation Modification Alexandra Lefort,1 Devon M

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E03007, doi:10.1029/2011JE004008, 2012

Inverted fluvial features in the Aeolis-Zephyria Plana, western Medusae Fossae Formation, Mars: Evidence for post-formation modification Alexandra Lefort,1 Devon M. Burr,1 Ross A. Beyer,2,3 and Alan D. Howard4 Received 20 October 2011; revised 6 January 2012; accepted 13 January 2012; published 16 March 2012.

[1] The Aeolis and Zephyria Plana contain the western-most portion of the Medusae Fossae Formation (MFF), an enigmatic and extensive light-toned deposit located in the Martian equatorial region and dated from the Hesperian to Amazonian epochs. This area hosts a large population of sinuous ridges (SRs), interpreted as inverted fluvial features, formed by precipitation, indurated by chemical cementation, buried by subsequent deposition, and finally exhumed. This interpretation of SRs as uniformly fluvial represents a modification to an earlier hypothesis for one particular SR of possible glaciofluvial (i.e. esker) formation. These SRs provide a tool to investigate the degree and character of post-fluvial modification processes in this region. We combined digital terrain models made from Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) stereo image pairs with individual data points from the Mars Orbiter Laser Altimeter (MOLA) to estimate relief, cross-sectional profiles, longitudinal profiles and slope directions of selected SRs. Longitudinal profiles of several SRs display undulations with amplitudes of up to order 100 m. While some of the lower amplitude undulations may be due to differential erosion, undulations having amplitudes in excess of SR relief require alternative explanations. Our combined morphologic and topographic analysis suggests that multiple post-flow processes, including compaction of the deposits and tectonic displacements, have modified the original SR profiles. Specification of the type(s) and magnitudes of these modification processes will contribute to understanding both the potential of post-flow modification of fluvial profiles elsewhere on Mars as well as the nature and properties of the MFF. Citation: Lefort, A., D. M. Burr, R. A. Beyer, and A. D. Howard (2012), Inverted fluvial features in the Aeolis-Zephyria Plana, western Medusae Fossae Formation, Mars: Evidence for post-formation modification, J. Geophys. Res., 117, E03007, doi:10.1029/2011JE004008.

1. Introduction formation deformation of aqueous features. On Earth, (paleo-) river profiles that show pronounced convexities or local [2] Because of its negligible internal strength, water seeks topographic maxima have been used to infer tectonic uplift the lowest equipotential level. This characteristic makes during or after fluvial flow [Burnett and Schumm, 1983; aqueous features a useful medium for detecting post-formation King and Stein, 1983; Cox, 1994; Boyd and Schumm, 1995; deformation of water-formed features. On Earth, for example, Roberts and White, 2010; Hartley et al., 2011]. Martian nonhydrostatic shorelines of paleolakes have shown locations negative relief channels and valleys tend to trend downhill and amounts of post-lake uplift [e.g., Gilbert, 1890; Påsse, [e.g., Aharonson et al., 2002], which suggests that the 1990; Caskey and Ramelli, 2004]. Likewise, studies of the topography along the course of those channels has not been hypothesized Martian north polar sea [Parker et al., 1989; strongly altered. However, the floors of these valleys are Head et al., 1999] indicate post-formation deformation of often modified by mass wasting events and infilled by shorelines [Perron et al., 2007; Di Achille and Hynek, 2010]. aeolian material [Baker and Partridge, 1986; Howard et al., Profiles of fluvial features can also be diagnostic of post- 2005], hiding parts of the local topography. In contrast, the topographic profiles of paleo-river channels in inverted 1University of Tennessee Department of Earth and Planetary Sciences, relief – formed through preferential induration of the channel Knoxville, Tennessee, USA. beds and regional erosion of the surrounding landscape – are 2Sagan Center at the SETI Institute, Mountain View, California, USA. no longer susceptible to modification by infilling processes 3NASA Ames Research Center, Moffet Field, California, USA. 4 and therefore are more likely to provide information on Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA. regional post-flow modification such as tectonic uplift or sediment compaction. Thus, although negative relief chan- Copyright 2012 by the American Geophysical Union. nels provide some information on flow conditions, inverted 0148-0227/12/2011JE004008

E03007 1of24 E03007 LEFORT ET AL.: DEFORMATION OF SINUOUS RIDGES ON MARS E03007 channels provide information both on flow conditions and lowland terrains and the Noachian highlands [Bradley et al., on post-flow modification. 2002]. It is one of the youngest Martian surficial deposits, [3] The Aeolis and Zephyria Plana (AZP) regions of the dated by several authors to the Amazonian epoch [e.g., western Medusae Fossae Formation (MFF) are ideal locations Schultz and Lutz-Garihan, 1981; Scott and Tanaka, 1986; to investigate past flow conditions and post-flow modifications Tanaka, 1986; Head and Kreslavsky, 2001, 2004; Werner, of inverted channels. Indeed, these regions host a dense popu- 2006], although some recent work estimates that a signifi- lation of sinuous ridges (SRs), largely interpreted as inverted cant part of the MFF may already have been present during fluvial features [Burr et al., 2009; Zimbelman and Griffin, the Hesperian [Kerber and Head, 2010]. The present extent 2010]. Although the AZP provides the largest dataset of of the MFF is estimated to be 2.1–2.5•106 km2 though it SRs, similar ridges have been observed in other regions of may have once covered up to 5•106 km2, and its present Mars [e.g., Williams, 2007], including plateaus surrounding volume is estimated as 1.4•106–1.9•106 km3 [Bradley et al., Valles Marineris, and notably Juventae, Ius and Ganges 2002; Hynek et al., 2003]. The material of the MFF is Chasmata [Mangold et al., 2004; Le Deit et al., 2010; Weitz highly friable, layered, and heavily eroded by aeolian pro- et al., 2010], as well as in Arabia and Meridiani Terrae, cesses [e.g., Scott and Tanaka,1986;Greeley and Guest, Lunae Planum [e.g., Williams, 2007; Williams et al., 2007], 1987; Schultz and Lutz, 1988; Zimbelman et al.,1997;Malin Gale, Eberswalde, Myamoto and Terby craters [e.g., Malin et al.,1998;Frey et al., 1998; Takagi and Zimbelman, 2001; and Edgett, 2003; Moore et al., 2003; Anderson and Bell, Bradley et al.,2002;Head and Kreslavsky, 2001, 2004; Hynek 2010; Newsom et al., 2010; Thomson et al., 2011; Ansan et al., 2003]. At large scale, the formation has been pervasively et al., 2011] and Dorsae Argentae [Head, 2000a, 2000b; sculpted into yardangs, elongated ridges with a streamlined Head and Hallet, 2001a, 2001b; Head and Pratt, 2001; shape produced by aeolian erosion in indurated material [e.g., Ghatan and Head, 2004; Banks et al., 2009]. Most of these Ward, 1979; Scott and Tanaka, 1982; Bradley et al., 2002; SRs are interpreted as inverted fluvial channels [Newsom Mandt et al., 2008; Zimbelman and Griffin, 2010]. Other et al., 2010; Le Deit et al., 2010; Weitz et al., 2010; erosional morphologies in the MFF include pedestal craters Thomson et al., 2011; Ansan et al., 2011] or as eskers and windstreaks [e.g., Schultz and Lutz, 1988; Malin and [Head, 2000a, 2000b; Head and Hallet, 2001a, 2001b; Head Edgett, 2000; Hynek et al., 2003]. The Mars Advanced and Pratt, 2001; Ghatan and Head, 2004; Banks et al., Radar for Subsurface and Ionosphere Sounding (MARSIS, 2009], which are long ridges of stratified material, generally 150 m vertical resolution in the free space) on the Mars sand and gravel, formed by deposition of sediment in Express spacecraft revealed that the dielectric properties in meltwater channels on, within, or beneath glaciers [e.g., the MFF are consistent with either dry and very porous Brennand, 2000; Knight, 2005]. In the AZP, the large sediments or ice-rich deposits [Watters et al., 2007b], majority of SRs are interpreted as inverted fluvial channels although the Shallow Radar (SHARAD, 15 m vertical reso- or floodplains [Burr et al., 2009, 2010]. These fluvial SRs lution in the free space) on the Mars Reconnaissance Orbiter provide a dataset for investigating the paleo-topography of (MRO) spacecraft has not found compelling evidence for multiple fluvial features, allowing us to assess the occur- ice preserved within the MFF materials [Carter et al., 2009]. rence, extent and type of post-flow processes. The tops of the [6] Several hypotheses have been suggested regarding fluvial SRs, if unmodified, should correspond to the surface the origin of the MFF. Terrestrial ignimbrites originating from of the original channel beds. In this case, they would retain multiple pyroclastic flows have been proposed by several the original flow paleoslope and decrease monotonically in researchers as being the best analog [e.g., Scott and Tanaka, the same direction, which would correspond to the original 1982, 1986; Mandt et al., 2008; Zimbelman et al., 1997]. In direction of flow. particular, observations of caprock over yardangs eroding [4] In this paper, we combine several datasets to investi- into discrete boulders suggest the presence of competent gate the profiles of a distributed sample of inverted fluvial layer sediments within the western MFF deposits [Zimbelman features in the AZP. We first give background information on and Griffin, 2010]. Other hypotheses include primary volca- the MFF, the AZP SRs and the study area. Next, we explain niclastic airfall deposits [Hynek et al., 2003], as well as mixed the data and methods used for this analysis. We then present volcanic sedimentary flow and fall deposits [Bradley et al., the results of the topographic analysis by showing a few 2002; Hynek et al., 2003]. Earlier hypotheses, including selected examples of undulating SRs. We next review pos- aeolian deposits [Ward, 1979; Carr, 1981; Scott and Tanaka, sible hypotheses regarding the processes involved in the 1986; Carr, 1996; Wells and Zimbelman, 1997] and layered- formation of those undulations and discuss in turn the like- polar deposits emplaced as a result of true polar wander lihood of each of those hypotheses. In the final part, we dis- [Schultz and Lutz, 1988], are not well supported by more cuss the implications of the presence of those topographic recent data [e.g., Bradley et al., 2002; Mandt et al., 2008; undulations, including the implication that post-flow modi- Zimbelman and Griffin, 2010]. Although data from a single fication has deformed the MFF. MARSIS track suggest that the surface underlying the MFF is flat and horizontal [Watters et al., 2007b], another study 2. Background using SHARAD suggests that, at higher resolution, the underlying surface may actually not be flat [Carter et al., 2.1. MFF 2009]. [5] The MFF is one of the most extensive of several light- [7] Our study area is the western MFF, coinciding largely toned friable layered deposits that ring the Martian equator with the AZP region. This area between 2N and 8S and [e.g., Hynek et al., 2003; Weitz et al., 2010]. The MFF is 149 and 158E is south of Elysium Mons and adjacent to the  situated along the global dichotomy boundary between 130 southern highlands. It contains numerous SRs on and  and 240 E and overlies both the northern Amazonian between the two western-most lobes of the MFF (Figure 1).

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Figure 1. (a) MOLA shaded relief context image with black outlines showing MFF as mapped by Greeley and Guest [1987]; see Scott and Tanaka [1986] and Zimbelman [2011] for alternative mapping. AP, Aeolis Planum; ZP, Zephyria Planum; CP, Cerberus Palus; AV, Athabasca Valles; MV, Marte Vallis. Black box shows the study area. (b) Study area with distribution of the fluvial SR networks studied so far. Background: THEMIS day IR mosaic with MOLA DTM and CTX and HiRISE DTM overlay. The dotted lines represent MOLA data points along SRs. White boxes show areas where topographic undulations have been observed (SRs with MOLA points but without white boxes do not show undulations); the boxes with figure numbers are discussed in the text. Black box shows the location of (c) the THEMIS visible- wavelength image illustrating multiple SR morphologies and stratigraphic complexity. A, B, thin; D, flat; E, F, multilevel; C, wispy.

2.2. Sinuous Ridges (SRs) [9] SRs are mostly interpreted as inverted fluvial features, [8] The SRs of the AZP (Figure 1) are positive-relief either inverted paleochannels or inverted floodplains [Burr elongated features with sinuosities up to a value of 2.4, up to et al., 2009; Zimbelman and Griffin, 2010]. Based on geo- hundreds of kilometers in length, up to a kilometer in width, logic and topographic context, spatial distribution, and ter- and heights of up to several tens of meters [Burr et al., 2009]. restrial analogs [Maizels, 1987; Pain and Ollier, 1995; The SRs are partially exhumed by erosional processes, Williams et al., 2007; Pain et al., 2007], these fluvial SRs probably mostly aeolian abrasion, based on the presence of were inferred to have formed by orographic precipitation yardangs and other erosional morphologies prevalent in the with resultant surface runoff [Burr et al., 2009]. Orographic MFF [Burr et al., 2010]. SRs may be isolated or organized in effects created by the dichotomy boundary may have branching or sub-parallel networks. A previous study [Burr strongly influenced the formation of fluvial valleys on the ‐ et al., 2009] documented about 150 SRs in AZP and classi- northward slope of the crustal dichotomy at the Noachian fied them into 5 morphological crest types (flat-topped, Hesperian transition [Irwin et al., 2010]. If the western MFF thin, rounded, multi-level and wispy) and 4 network types dates to the early Hesperian [Kerber and Head, 2010], this (isolated, sub-parallel, branched, and random). Another study process may also have caused the orographic precipitation [Zimbelman and Griffin, 2010] used a derivative classification hypothesized to have created the AZP SRs [Burr et al., scheme but combined individual and network classes (flat- 2009], which are located preferentially in the lower, hence crested, narrow-crested, round-crested, branching, non- older, member of the MFF. Subsequent induration, burial by branching, and multilevel features). deposition, and finally exhumation by aeolian abrasion of the

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Figure 2. Examples of SR erosion. CTX image (P03_002279_1737_XI_06S208W). (a) Large white box: segment of SR with fractured capping layer. (b) Boulders originating from detached parts of the capping layer are visible next to the SR. (c) The northern part of this segment appears thinner, due to the complete loss of the capping layer and partial erosion of the underlying substrate. Impacts at the sur- face of the SR may, for the larger ones, obliterate portions of the SR and create a gap or, for the smaller ones, contribute to removal of part of the capping layer. Small white box: close-up on fractures and boulders. White ellipse: thinning of the SR due to complete removal of the capping layer followed by partial erosion of the less consolidated underlying substrate. Black ellipse: gap in the SR due to complete erosion of both the indurated capping layer and the underlying material.

(less resistant) surrounding landscape resulted in the inver- to be linked in regional image mosaics, has been tentatively sions of the fluvial features into SRs [Pain et al., 2007; Burr interpreted as an esker, based on its rounded morphology, et al., 2009, 2010]. Out of the several possible induration undulating profile and absence of tributaries [Nussbaumer et mechanisms (armoring, lava capping, chemical cementation), al., 2003; Nussbaumer, 2005, 2007; Burr et al., 2009; multiple lines of evidence including SR morphology and Zimbelman and Griffin, 2010]. thermal inertia [Burr et al., 2009, 2010; Zimbelman and [11] SRs interpreted as paleochannels have so far been Griffin, 2010] indicate that induration by chemical cementa- found only on the two western-most lobes of the MFF, with tion of the fluvial sediments is most likely, although some the exception of a single SR network in a depression in the SRs may have been indurated through lava capping [Burr central MFF, also hypothesized to be fluvial [Harrison et al., et al., 2009]. We interpret the flat top of several of the SRs 2011]. We performed an extensive survey in the eastern and as a capping layer corresponding to the layer of indurated central MFF in order to identify other SRs, based on mor- fluvial sediments, which overlies pre-existing nonindurated phological and textural criteria (continuous, sinuous ridge, material. High-resolution HiRISE images show erosion of presence of meanders, scroll bars, oxbow lakes) but did not this capping layer by fracturing and fragmentation into indi- find any other convincingly fluvial examples. vidual boulders, exposing the underlying less consolidated material (e.g., Figure 2). These observations support the 2.3. Hypotheses Regarding SR Topographic Profiles hypothesis that this top layer is indurated. The presence of [12] Inverted fluvial features should, if unmodified, retain this flat capping layer suggests that, for those particular some characteristics of the original fluvial features, such as SRs, any nonfluvial material that may have overlaid the scroll bars and meanders, which have indeed been observed indurated fluvial sediments has now been largely eroded [e.g., Burr et al., 2009, 2010]. In addition, the slope of the away. Several SRs also display features similar in appearance SR surface should correspond to the original downhill slope to features typical of terrestrial fluvial channels and flood- of the channel bed. Thus, our null hypothesis for this study plains such as meanders, oxbow lakes and scroll-bars [Burr was that the profiles of the fluvial SRs monotonically et al., 2009, 2010]. decrease in elevation in one direction consistent with other [10] A single 400 km-long SR (Figure 3a), first identi- SRs in the network and presumed to be the downstream fied as several individual segments which were later shown direction. During our investigations, we developed the

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Figure 3. (a) “Rounded” SR (black) with possible tributaries (white) (THEMIS 512 ppd mosaic overlaid on MOLA DTM). (b) Close-up of the SR showing lateral ridges (HiRISE image). Black arrows in Figures 3b, 3c, and 3d all point to lateral ridges. (c) Close-up on a possible tributary. (d) Possible terrestrial analog: inverted channel with lateral ridges in the Atacama Desert, Chile. alternative hypothesis that fluvial SR profiles do not decrease wavelength and several tens to hundreds of meters in monotonically in a single direction, indicating post-flow amplitude. modification. [13] In this paper, “longitudinal profile” means a topo- 3. Data and Methodology graphic profile along the top of the SR, and “undulation” refers to increasing and decreasing elevations along a lon- [14] The previous studies on AZP SRs [Burr et al., 2009, gitudinal profile. “Cross-sectional profile” means a topo- 2010] used THEMIS VIS and IR images [Christensen et al., graphic profile across the width of the SR, and “relief” refers 2004], with a spatial resolution of, respectively, 19 m/pixel to the elevation differences in a cross-sectional profile. and 100 m/pixel, and isolated images from the Context “Short wavelength, low amplitude undulation” refers to a Camera (CTX, [Malin et al., 2007], 6 m/pixel) and from the topographic undulation that is a few meters to a few tens of High Resolution Imaging Science Experiment (HiRISE, meters in wavelength and up to 20 meters in amplitude. [McEwen et al., 2007], 25 cm/pixel), to provide geomor- “Long wavelength, high amplitude undulation” refers to a phologic information. The MOLA digital terrain model topographic undulation that is several hundreds of meters in (DTM) provided topographic data [Smith et al., 2001]. For

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Table 1. Summary of CTX and HiRISE Stereo Pairs and Processing Methods for Each Area Area Instrument Image Pair Figures Stereo Processing Software Area 43_B CTX P03_002279_1737_XI_06S208W 4, 10 ASP P02_002002_1738_XI_06S208W Area 40_D CTX B20_017548_1739_XI_06S206W 6 ASP G02_019104_1740_XI_06S206W Area 34_A HiRISE PSP_009623_1755 8 SOCET Set PSP_007975_1755 Area 34_B CTX B17_016348_1749_XI_05S204W 9, 11 ASP B21_017904_1749_XI_05S204W this study, we use HiRISE images (10% coverage of our over the CTX mosaic. As the CTX images were previously study area) and CTX images (95% area coverage) for co-registered to the MOLA DTM, we consider that the pos- high-resolution geomorphic information. We also create and sible mis-registration errors between the individual MOLA analyze HiRISE and CTX digital terrain models (DTMs) in data points and the CTX and HiRISE images are minimal. addition to the MOLA DTM, for high-resolution topo- MOLA data points located on top of the SRs as seen in the graphic analyses. Available data have enabled us to look at CTX and HiRISE images were manually selected and two dozen well-preserved SRs (Figure 1) within that area. exported into new ArcGIS layers (one layer for each SR). The We chose these SRs on which to focus based on (1) the good large footprint of the MOLA data points is another possible state of preservation of the SRs; (2) the availability of stereo source of error. For narrow SRs, the MOLA footprint may pair images from CTX and HiRISE, from which to derive overlap the SR top, sides and the surrounding terrain, there- DTMs; and (3) the desirability of a broad geographic dis- fore providing an inaccurate topographic elevation value for tribution of SRs. the top surface of the SR. To minimize these errors, we selected MOLA data points that are located as centrally on 3.1. Nontopographic Data top of the SR (i.e. as far from the sides of the SR) as possible. [15] All the CTX and HiRISE images available in the Those data points for each SR were then plotted on a graph in study area (acquired from the Planetary Data System, PDS, ArcGIS to create the SR profiles. http://pds.nasa.gov/) were radiometrically corrected and [18] 2. CTX and HiRISE images have a ground scale of projected using the Integrated Software for Imagers and 6 m/pixel and 25 cm/pixel, respectively. After CTX and Spectrometers (ISIS [Gaddis et al., 1997; Torson and Becker, HiRISE stereo pair images were radiometrically corrected 1997; Anderson et al., 2004]). They were then imported into and projected using ISIS, the CTX DTMs were produced ArcGIS where all the data were co-registered using the using the Ames Stereo Pipeline (ASP, [Moratto et al., 2010]) georeferencing tool. A shaded relief of the MOLA DTM was and the HiRISE DTMs were produced using SOCET Set used as the first base layer. Over the MOLA shaded relief, [Kirk et al., 2008]. The DTMs were then imported into we overlaid the THEMIS day IR 512 ppd mosaic (100 m/ ArcGIS. Table 1 specifies the CTX and HiRISE image pairs pixel), which was accurately co-registered to the MOLA and used in this study and the software used to process them. For did not require any adjustment. The CTX images were then each of the DTMs produced with the ASP, we resampled the co-registered to the THEMIS mosaic and the HiRISE images 16 ppd MOLA areoid (acquired from the PDS, http://pds. to the CTX images. A CTX mosaic covering 95% of the nasa.gov/) to the resolution of the DTM, then subtracted study area was created using the individual CTX images. the resampled MOLA areoid from the DTM. We did not perform bundle adjustment or incorporate MOLA ground 3.2. Topographic Data control points when using the ASP, and as a result the [16] Three different and independent types of topographic derived models were offset from (lower than) the MOLA data were used in these investigations: (1) individual MOLA data by about 300 m vertically. This offset was simple to data points; (2) three DTMs created from CTX stereo image correct a posteriori by creating a difference map between the pairs; and (3) one DTM created from a HiRISE stereo image MOLA and CTX DTMs, then adding the median (300 m) pair. For each of the SR networks investigated in detail, of the difference histogram to the CTX DTM values. The topographic data from two of these three sources were used, result is general alignment between the CTX and HiRISE with one exception where only MOLA data were available DTMs and the MOLA DTM (Figures 4, 6, 8, 9, and 11). (Figure 5). As shown in the images and analysis below, the These DTMs have a vertical accuracy of 10–20 m (CTX) and data from the multiple independent sources generally agreed 20 cm (HiRISE, [Kirk et al., 2008]), and a horizontal accu- well. racy of about 18 m (CTX) and 1 m (HiRISE), respectively. [17] 1. Individual MOLA data points have a 150-m- Small mis-registrations with the CTX and HiRISE images footprint and 300-m-along-track spacing [Smith et al., were also corrected in ArcGIS using the georeferencing tool. 2001; Neumann et al., 2001, 2003]. Because of their large Elevations and locations were collected in ArcGIS to create footprint and between-track spacing (up to kilometers at this longitudinal and cross-sectional profiles. To acquire the equatorial location), these data have a relatively low reso- topographic profiles, the CTX and HiRISE DTMs were lution compared to the CTX and HiRISE DTMs, but they stretched to render the SRs visible and the transparency of cover most of the study area, whereas CTX and HiRISE the DTMs was increased so that the corresponding, under- DTMs are available over only certain SR networks (Figure 1). lying, CTX or HiRISE images showed through the DTMs. MOLA data tracks where imported into ArcGIS and projected The profiles were acquired over the stretched DTMs, by

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Figure 4. Branching network in Area 43_B. (a) CTX image (P03_002279_1737_XI_06S208W) of the NE part of the SR network in Area 43. (b) CTX DTM (stereo pair P03_002279_1737_XI_06S208W/ P02_002002_1738_XI_06S208W) over MOLA DTM with location of the topographic profiles between A and B and between C and D. Black rectangle: location of upper left image. (c) Longitudinal profile derived from MOLA data point and CTX DTM along the top of the SR between A and B. Green dots: MOLA data points, brown line: CTX DTM profile, vertical red lines: SR relief. (d) Longitudinal profile derived from MOLA data points and CTX DTM along the top of the SR between C and D. Red dots: MOLA data points, blue line: CTX DTM profile, red vertical lines: SR relief. Black line: polynomial smoothing of the MOLA data points. Green Box: close-up on part of the topographic profile, illustrating short-wavelength undulation (here 15 m in amplitude), which is less than the relief of the SR in that area (20 m). In those examples, the long wavelength topographic undulations have amplitudes of 90 m and 80 m. using the underlying morphology visible by transparency slope. For each SR, we use the same locality designations for additional guidance and in order to minimize uncertain- (i.e. area numbers) as used in Table 1 of Burr et al. [2009]. ties in the measurements. The SR profiles were then analyzed See Figure 1 for the locations of each area. comparatively with the morphology of the SRs as shown by the CTX and HiRISE images. 4.1. Area 43 [20] Area 43 is located in the south-western part of the   4. Results of the Topographic and Morphological study area (centered at 6.5 S/151 E). It displays several Analysis branched SRs, mainly flat, thin and multilevel, showing various degrees of preservation. [19] The results of our investigations are not consistent 4.1.1. Area 43_B with our null hypothesis. That is, the derived SR profiles do [21] Area 43_B is a subset of Area 43 showing an SR not show a monotonic change in elevation. Rather, SR network characterized by Burr et al. [2009] as a branched topographic profiles show local maxima with undulation network of mostly thin SRs with some multilevel and flat   amplitudes from 10 m up to 150 m. In all the following reaches (Figure 4a). The flow direction here is difficult to examples of undulating SRs, the original flow direction was infer. Based on the overall fan-like shape, other researchers inferred based on the plan-view morphology of the networks have interpreted it as a distributary network with a north-east (e.g., branching characteristics) and on the current regional flow direction [e.g., Irwin et al., 2005; McMenamin and

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Figure 5. Area 43_E. (a) CTX image (P05_003136_1746_XN_05S208) showing SRs crossing over a scarp. Dotted black and white line= scarp location. White dots= MOLA data points. (b) Longitudinal profile along the top of a SR using MOLA data points.

McGill, 2005; Williams and Edgett, 2005]. However, our rises by 90 m if the paleo-flow had been to the SW and by topographic analysis shows that the main regional slope at least 150 m if the paleo-flow direction had been to the NE. direction is northeast to southwest, which suggest that the Longitudinal profiles on top of the SRs and on each side of main flow direction has been toward the southwest. There- the SRs both show similar undulations, indicating that the fore, for this analysis, we consider both flow directions as undulations in the SRs are represented in the topography of possibilities. A pair of CTX stereo images covers the the plateau as a whole. northeastern part of the network. Both MOLA and CTX 4.1.2. Area 43_E DTMs show lower elevation terrain to the southwest (light [23] To the southeast of Area 43_B is Area 43_E blue and yellow on Figure 4b), higher elevation terrain to (Figure 5). At the time of this study, CTX DTMs were not the northeast (orange on Figure 4b), and then a short ‘apron’ available in that area so the topographic study used only or narrow swath of lower elevation terrain even farther to the MOLA data points. This area is characterized by SRs that northeast (yellow on Figure 4b), around the edge of the seem continuous but whose morphology changes along their fan-shaped plateau. course, from flat, wide and very sinuous (Figure 5a), with 30 [22] Topographic profiles along the best preserved SRs to 80 m in relief, to thinner, narrower, straighter (Figure 5a), within the network were created both with individual MOLA with 50 m to 100 m in relief. The change in morphology data points and with the CTX DTM, and the two datasets occurs as the SR cross a 220 m high scarp (Figure 5b). Flow show good agreement. These profiles display two types of direction here is also difficult to infer and we consider both undulations: (1) short wavelength (a few meters to a few tens flow directions (northeastward and southwestward) to be of meters) and low amplitude undulations (a few meters) and plausible. The longitudinal profile for either flow direction (2) long wavelength (several hundreds of meters), high rises significantly. For a paleo-flow direction toward the amplitude undulations (several tens to hundreds of meters). southwest, the longitudinal profile rises by 200 m; if the The short wavelength undulations are generally less than the paleo-flow had been to the opposite direction (southwest relief of the individual SRs (Figure 4d), whereas the long to northeast) then the longitudinal profile rises by 70 m wavelength undulations are far more than their maximum and 220 m. relief. In the example in Figure 4c, the longitudinal profile

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Figure 6. Topographic profile along SR in Area 40_B. (a) CTX image (P01_001395_1742_XN_05S205W, grayscale, right side) and CTX DTM (B20_017548_1739_XI_06S206W / G02_019104_1740_XI_06S206W, color, left side) showing thin meandering SRs with the location of the topographic profile with MOLA points (dots) from A to C and CTX DTM (line) from B to C. The profile shows a good match between the MOLA elevations and the CTX elevations. Black box: close-up on the thin, meandering SRs and the surrounding floodplain. Black arrows: possible oxbow lakes. (b) Longitudinal profile with CTX DTM (blue line) and MOLA data points (red squares). Vertical red lines: relief of the SR at different locations along its path. Thin black line: moving average of the MOLA data points with a window of 2. We observe here low relief SRs (highest relief measured: 25 m) with undulating topographic profiles (up to 100 m amplitude). (c) Cross-sectional profiles of the SR in locations CS1, CS2 and CS3, showing variations in width, height and shape of the SR.

4.2. Area 40_B profiles in Figure 6) and appear to be well-preserved, consis- [24] In the central part of the study area (Figure 1, centered tent with low to moderate exhumation and minimal erosion of at 5.5S, 154E), we observe examples of branched, sub- the capping layer. In the surrounding material on each side of parallel thin and very sinuous SRs (Figure 6). The SRs are the SRs, we observe morphologies consistent with ancient low-relief (10 to 20 m high, based on the cross-sectional scroll-bars and ox-bow lakes (Figure 6), suggesting that those

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Figure 7. Area 34_A and 34_B networks. Background: CTX (stereo pair B17_016348_1749_ XI_05S204W/B21_017904_1749_XI_05S204W) and MOLA DTMs. Black lines mark the location of eastern-facing scarps. The small white arrows show areas where thin SRs seem to cross over or between scarps. The white squares show the locations of Figures 8, 9 and 11. thin SRs are located in the center of ancient floodplains that the average relief of those SRs is 15 m and that the that grade into the surrounding deposits. Topographic data relief is relatively constant along their course (Figure 8c). available here are a CTX DTM and MOLA individual data Evidence for flow direction comes from elevations, which points. The topographic profile shows a good match between are 160 m higher in the northeastern part of the network MOLA and CTX elevations. Based on the regional slope, than in the southwestern part (Figure 8). In the northeastern the hypothesized original flow direction is toward the west- part of the hourglass, the individual channels are well sepa- northwest. For that flow direction, the longitudinal profile rated, have high relief and a convergent branching to the rises by 100 m; if the flow were to the east-southeast, southwest. By contrast, in the southwestern part of the hour- then the longitudinal profile would rise by 170 m. glass, the channels cross (possibly anastomose) and locally splay as broad drapes, suggesting a depositional fan envi- 4.3. Area 34   ronment. The channels there are also in lower relief, sug- [25] This area, centered at 5 S, 155 E, displays a complex gesting a broad covering of coarser or cemented sediment. network of branched sub-parallel SRs, mostly thin or multi- For these reasons, we infer a flow direction toward the level. It is also characterized by several east-facing scarps southwest. The profile shows a local maximum with an trending north-south or northwest-southeast and 100 m in 85 m amplitude undulation for presumed flow toward height (Figure 7). Thin SRs having 10–20 m of relief cross the southwest. The elevations start rising near the center of over or between those scarps. Because the regional slope is the northeast fan-shaped network and the highest point of the generally toward the west and since the SRs of that area undulation is situated in the center of the second fan-shaped converge to the west or southwest, we consider that the network (Figure 8a). If the flow direction were toward the original flow direction was also toward the west [Burr et al., northeast, the longitudinal profile would rise by 180 m. 2009]. 4.3.2. Area 34_B 4.3.1. Area 34_A [27] The SR in Figure 9 is part of the sub-parallel network. [26] This network of thin SRs has a plan-view morphology It is a multilevel SR displaying a thin, well preserved SR reminiscent of an hourglass with branching networks with a relatively consistent morphology on top of a wider connected together in a narrower central part by continuous flat SR. For this example, we had only two MOLA data SRs (Figure 8a). The central part of the network is located points located on top of the SR. We are considering here the between two of the northwest-southeast scarps. The SRs are topography of the thin SR. The relief of that SR varies well-preserved, with a relatively consistent morphology between 10 and 25 m. The longitudinal profile displays a along their course (Figure 8b). For the topographic analysis 120 m amplitude undulation, assuming a flow toward the of that network, we use a HiRISE DTM and MOLA data west, based on the regional slope and westward convergence points. Here again, we observe a good fit between the two of the SR. SR relief is lowest in the lowest part of the lon- types of data, with a maximum difference of 10 m. Cross- gitudinal profile (10 m) versus 25 m in the highest part of sectional profiles acquired along the course of the SRs show the topographic profile. This correlation of low relief with

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Figure 8. SR network in Area 34_A. (a) CTX image (P18_008120_1754_XI_04S205W) with MOLA DTM overlay and HiRISE DTM (stereo pair PSP_009623_1755/PSP_007975_1755) with location of the topographic profile between A and B including MOLA data points (green dots) and HiRISE DTM (blue line). Ellipses indicate the location of the two branching networks. (b) Close-up on the morphology of the SR network. (c) Topographic profile with MOLA data points (green dots) and HiRISE DTM (blue line), showing 85 m topographic undulation. Black line: polynomial smoothing of the MOLA data points. Vertical red lines represent the relief of the SR. low elevation suggests increased erosion of the capping en- or supraglacial deposits may be lowered directly onto layer and underlying sediments in that low relief area. underlying topography; thus, eskers may cross topographic However, erosion of 15 m of material is not sufficient to highs [Shreve, 1985; Benn and Evans, 1998]. As a result, explain the 120 m topographic undulation. For flow their topographic profile may display undulations similar to direction toward the east, this particular profile would not those observed in the SRs. Studies of SRs in other regions display any significant rise in elevation. of Mars [e.g., Head, 2000a, 2000b; Head and Hallet, 2001a, 2001b; Head and Pratt, 2001; Ghatan and Head, 5. Hypotheses to Explain SR Profile Undulations 2004; Banks et al., 2009] have concluded that those rid- ges were eskers based at least in part on their undulatory [28] Based on the types of data sources and the complex profiles. geologic history of the MFF, multiple causes could explain [31] 3. Different generations of SRs: The MFF underwent the observed profile undulations. These causes include: multiple episodes of deposition and erosion, as inferred by [29] 1. Data artifacts: The undulations could be data arti- complex stratigraphy [Head and Kreslavsky, 2001, 2004], facts in the derived terrain models due to any number of variations in yardang orientations between layers [Scott and error sources, from spacecraft motion or camera noise to Tanaka, 1982; Bradley and Sakimoto, 2001; Hynek et al., errors in the stereo correlation process itself. 2003] and varying crater densities between layers [Sakimoto [30] 2. Formation as Eskers: Some of the undulating SRs et al.,1999;Hynek et al., 2003]. Regional MOLA topogra- may be eskers rather than inverted fluvial channels. Sub- phy shows that the SRs occur at multiple elevations, and glacial flow is not only controlled by the topography but also plan-view images show that SRs are frequently superposed by the water pressure in relation to the overlying ice, and (Figure 1), demonstrating formation of multiple generations

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Figure 9. SR network in area 34_B. (a) CTX DTM (stereo pair B17_016348_1749_XI_05S204W/ B21_017904_1749_XI_05S204W) overlaid on a CTX image (B17_016348_1749_XI_05S204W) showing athinSR“climbing” onto a plateau for the hypothesized flow direction to the west. Blue squares represent individual MOLA data points. (b) White rectangle: close-up on the SR. An impact crater is visible on the SR. On each side of this crater, the SR is continuous. (c) Topographic profile. Black line: CTX profile, between A and B. Blue squares represent individual MOLA data points. Red vertical lines represent the relief of the SR at different positions along the SR. The amplitude of the topographic undulation for west- erly flow is 120 m. of SRs. This formation of superposed SRs suggests that an MFF material that is variably indurated to poorly con- elsewhere, even in circumstances where superposition is not solidated and easily eroded by aeolian abrasion [e.g., Ward, obvious and SRs may appear co-linear (i.e. contiguous in a 1979; Scott and Tanaka, 1982; Bradley et al., 2002; Mandt linear fashion, or apparently connected from a plan view et al., 2008; Zimbelman and Griffin, 2010]. Moreover, the perspective), multiple distinct generations of SR formation presence of fossil dune fields in some areas of the MFF may have occurred separated in time. If those SRs are located [Kerber and Head, 2011] also suggests mobilization and at very different stratigraphic levels, this co-linear superpo- accumulation of loose fine material, possibly originating sition could result in (apparent) undulations in the SR topo- from erosion of the MFF [Burr et al., 2010]. Differential graphic profile that in fact are derived from exhumation of erosion along an SR may result in an SR profile that does not two SRs of different generations. slope consistently downward. Erosion of SRs has also [32] 4. Differential erosion: The MFF has experienced resulted in discontiguous segments and/or terminal trunca- obvious and uneven erosion at all scales, from meter-scale to tion [e.g., Burr et al., 2009, Figure 5]. kilometer-scale yardangs, deflation depressions and knobs. [33] 5. Differential settling: The MFF deposits are sedi- The widespread occurrence of yardangs in the MFF points to mentary and thus may have undergone settling at rates that

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Table 2. Summary of the Hypothesized Processes Leading to the Formation of the Undulations, the Criteria Used to Discriminate Between the Different Processes, and Examples of Undulating SRs to Which These Processes May Apply Process Criteria Examples Data artifact • High frequency, narrow width, low amplitude undulations Area 43_B (Figure 10) • Different on each dataset Esker • Esker morphology (low sinuosity, low integration, isolated, Rounded SR, Burr et al. no scroll bars, no oxbow lakes, no multilevel) [2009, Figures 3 and 14] • Variation along trend of SR relief and overall • Increase in elevation of the surrounding terrain is correlated with decrease in relief of the ridge • Sharp-crested ridge in ascending slope or level surface, and lower, broader, more rounded or flat on descending • Follow contours and, when they are sinuous, wander back and forth across the valley irrespective of slope Erosion • The amplitude of the undulations is less than the maximum relief of the SR Area 34_B (Figure 11) • Correlation between the relief and the overall elevation of the SR (i.e. lower parts of the SR corresponds to area where the relief of the SR is the lowest) • Correlation between the morphology and elevation of the SR (i.e. lower parts of the SR appear more eroded than higher parts of the SR) Differential settling • The amplitude of the undulations is more than the maximum relief of the SR Area 43_B (Figure 4) • No correlation between SR morphology and elevation Area 40_B (Figure 6), (i.e. lower parts of the SR do not correspond to area Area 34_A (Figure 8), where the relief of the SR is the lowest) Area 34_B (Figure 9) • No correlation between SR morphology and elevation. (i.e. lower parts of the SR do not appear more eroded than higher parts of the SR) • Correlation between MFF thickness and undulation amplitudes: Regions of thicker MFF should have undergone greater settling because i) greater the weight in regions of thicker MFF would produce more compaction/settling, ii) thicker MFF would have more pore space for compaction/settling Tectonism • No correlation between SR morphology and elevation Area 40_B (Figure 6), (i.e. lower parts of the SR do not correspond to area Area 34_A (Figure 8), where the relief of the SR is the lowest) Area 34_B (Figure 9) • No correlation between SR morphology and elevation. (i.e. lower parts of the SR do not appear more eroded than higher parts of the SR) • Presence of lobate scarps, asymmetrical ridges indicative of the presence of faults • Undulations aligned along those scarps/ridges Multi-generational SRs • The upper surface of the SRs is at significantly different elevations Area 40_B (Figure 5) • The thickness and widths of SRs is anomalously large due to overlap / superposition / adjacency, • The morphology along the SR appears different. varied spatially. This hypothesized differential settling could be caused by crustal tectonism within the basement material have resulted either from compaction of porous MFF material, inferred to underlie the MFF [Watters et al., 2007b; Carter i.e., removal of void space from beneath now eroded over- et al., 2009]. Such tectonism might originate from pro- burden material, or it could have resulted from removal of cesses associated with the formation of the dichotomy material at depth, i.e., by sublimation of interstitial ice. The boundary or related to the wrinkle ridges from the Cerberus inference of settling is consistent with the finding from plains. MARSIS [Watters et al., 2007b] that the unit may contain a significant amount of water. In this case, compaction may 6. Hypothesis Testing: Results have occurred by removal of that water through sublimation of subsurface ice or ground water withdrawal. On the other [35] Each of these potential causes listed in the section hand, MARSIS and SHARAD data may also suggest that above constitutes a hypothesis for the observed SR undula- the MFF deposits are highly porous today [Watters et al., tions, and they are not necessarily mutually exclusive. We 2007b; Carter et al., 2009], which may imply that those tested each of these hypotheses as described below. Table 2 deposits have not been compacted. However, only a few summarizes each process and the criteria used to differenti- MARSIS and SHARAD tracks have been studied so far in ate between the different processes and lists examples of SRs the AZP and they do not cover the undulating areas. Thus, to which those processes may apply. the MFF may be very porous along the studied MARSIS and SHARAD tracks, but compaction may have occurred locally 6.1. Data Errors and Artifacts in the undulating areas. [36] In push-broom cameras, like CTX and HiRISE [34] 6. Crustal tectonics: Tectonic features such as blind [McEwen et al., 2007; Malin et al., 2007], distortions and thrust faults, deep-seated normal faulting or igneous intru- waviness may occur along the scan direction. However, the sions, may create folds or undulations in the overlying land undulating SRs all trend roughly east-west (Figure 1), per- surface. Thus, the topographic undulations in the SRs may pendicular to the flight path/image direction, so push-broom

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Figure 10. Area 43_B. Possible artifacts in the CTX DTM (P02_002002_1738_XI_06S208W, P03_002279_1737_XI_06S208W). The topographic profile of the raw DTM shows undulations of various wavelengths and amplitudes. The three spikes in the middle (white ellipse) are most likely artifacts, and could be eliminated by filtering or resampling to a lower spatial resolution. Regardless, the undulations on which this work focuses are longer-wavelength, like those in Figures 4, 5, 6, 8, 9, and 11.

distortions are most likely not the cause of the undulations. [39] Further investigations with more recent Mars data and HiRISE and CTX DTMs can have other geometric dis- terrestrial analogs have given evidence weighing against tortions caused by a number of instrument and spacecraft these earlier conclusions. CTX and HiRISE images show effects [e.g., Mattson et al., 2009; Shean et al., 2011], as well that this SR, instead of possessing a rounded morphology, as mis-matches and offsets in the stereo correlation process displays in places two lateral paired ridges (Figure 3b). itself. At short wavelengths, without a reference, it can be Although the cause for this morphology is still under inves- difficult to discriminate noise from signal in this regard tigation, field work shows that lateral paired ridges can form (e.g., Figure 10). However, we observe long-wavelength on terrestrial paleochannels. A possible terrestrial analog is undulations in CTX and HiRISE DTMs which are consistent observed in the Atacama, where inferred paleochannels with those found in the MOLA data set (Figures 4, 6, 8, 9, with sorted and cross-bedded sediments show lateral paired and 11). We therefore consider that data artifacts are not a ridges (Figure 3d) [Jacobsen et al., 2011]. A second pos- likely explanation for the long-wavelength undulations. sible analog is the Mirackina paleochannel in southern [37] Errors in co-registration of MOLA data points with Australia, where the lateral ridges are interpreted as resulting the THEMIS, HiRISE and CTX data, as well as the large from local cementation of the channel margins by silcrete footprint of MOLA data points, may also contribute to errors from groundwater discharge [Williams et al., 2011; R. M. E. in the acquired topographic values. We consider that the Williams et al., Variability in Martian sinuous ridge form: method used to co-register these different datasets and to Case study in the Aeolis/Zephyria Plana region and lessons select the individual MOLA data points (see paragraphs 3.1 from the Mirackina paleoriver, South Australia, manuscript and 3.2) minimizes the possibility of such errors. in preparation, 2012]. Moreover, a survey that we conducted using HiRISE and CTX images revealed a few possible 6.2. Formation as Eskers tributaries for this SR, with high junction angles more remi- [38] In our study area, one SR was previously considered niscent of a fluvial network than of an esker (Figure 3a). as an esker candidate, based on its unusual rounded mor- Although the tributaries are fainter than the main SR phology, lack of observed tributaries, and undulatory profile (Figure 3c) and may be discontinuous, this difference is [Burr et al., 2009; Zimbelman and Griffin, 2010], each of reasonable given their smaller size. They also transition in which is more characteristic of eskers than of inverted fluvial places into negative relief, which is consistent with other features. However, the relatively high sinuosity of this SR SRs on Mars [Williams and Edgett, 2005]. The hypothesis was noted to be unlike that of terrestrial eskers and no other that this SR is of fluvial origin is consistent with the glaciogenic landforms were observed in the region. inferred fluvial origin of the other AZP SRs, the lack of other

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Figure 11. Area 34_B. (a) Background: CTX DTM (stereo pair B17_016348_1749_XI_05S204W/ B21_017904_1749_XI_05S204W) over CTX image (B17_016348_1749_XI_05S204W). Red squares: individual MOLA data points. (b) Black rectangle: close-up on the SR. (c) Topographic profile. Black line: CTX profile. Red squares: individual MOLA data points. CS= cross-sectional profile. Red solid line= average relief of the SR at different locations along its course. Blue dashed line= hypothesized original paleosurface of the SR. observed glaciogenic features, and the newly discovered are characteristics that are inconsistent with an esker origin. possible tributaries. We therefore consider that this “rounded On the basis of these observations, we conclude that the SR”, like the other SRs, is most likely an ancient fluvial undulatory SRs shown here are not eskers. feature. 6.3. Multigenerational SRs [40] Regarding the other undulatory SRs, their morphology was found to be inconsistent with formation as eskers [Burr [41] The lack of fine stratigraphic layers in this area makes et al., 2009; Zimbelman and Griffin, 2010]. High sinuosity, the identification of stratigraphic relations between the SRs semi-concentric lineations inferred to be paleo-scroll bars, difficult. However, a few other criteria may help in identi- distinct SR arcs adjacent to SRs inferred to be paleo-oxbow fying different generations of SRs. lakes, multilevel SRs and the high integration of SR networks

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[42] If this hypothesis were true, then: (1) the upper sur- amplitudes of the topographic undulation are limited to (has a faces of the co-linear SRs would be at significantly different maximum value of) the relief of the sinuous ridge. elevations; (2) the morphology along the SR could appear [51] Thus, differential erosion is most reasonable where different; (3) the thickness and widths of SRs could be undulation amplitude is less than SR relief. Analyzing con- anomalously large due to overlap/superposition/adjacency; jointly both cross-section relief and undulation amplitude for and (4) the shape of the profile could display one or more each SR network is important for testing this hypothesis, as steep changes in elevation instead of undulating smoothly. SR relief varies by a factor of a few from one network to [43] This hypothesis may notably apply to the SRs that are another. For example, the average SR relief in Area 43 is associated with scarps (Figures 5, 8 and 9). It may especially 30 m (Figure 4), while in Area 34_B (Figure 11) it is apply for the SRs in Figure 5. The change in SR morphology 100 m. This difference may be due to different types of and the contrary slopes on each side of the scarp may be fluvial processes, to different degrees/depths of induration of explained as resulting from two co-linear SRs that formed at the sediments, or to different depths of erosion of the sur- different stratigraphic levels. The straighter, narrower SR on rounding material. Undulations over 100 m in Area 34_B the western side of the scarp may have formed first, at a may be interpreted as the result of erosion, although other lower stratigraphic level, while the wider, more sinuous processes may have contributed. In Area 43, where SR relief SR on the eastern side of the scarp may have formed is less than 100 m, another process is required. subsequently, at a stratigraphic level 220 m higher than [52] A particular SR in Area 34_B (Figure 11), located the previous SR. north of the SR shown in Figure 9, is a good example of SR where undulations are most demonstrably the result of 6.4. Differential Erosion differential erosion. In that example, the topographic profile [44] Differential erosion of an SR constitutes localized undulates as the elevation of the SR surface first decreases removal of the SR capping layer, exposing the underlying by about 240 m over about 7.5 km (i.e., a 3.2% slope), and material and causing erosional stripping of the less resistant then increases again by about 100 m over 9.5 km (i.e., substrate. At the scale of meters to tens of meters, we observe 1.2% slope). The relief of this SR is highest in the eastern various evolutionary stages of SR erosion (Figure 2), with the part (up to 150 m in cross-section 1, or CS1) and its wide, following interpretations: nonfragmented morphology in that area indicates good [45] 1. The capping layer of some parts of the SRs is preservation. The elevation of the surrounding terrain in that fractured. Tabular boulders, inferred to be fallen parts of the area is around À2330 m. Between CS4 and CS8, the SR capping layer, are seen on the side of some of these fractured appears narrower, more fragmented, and has a lower relief SRs. Interpretation: fractures likely formed during desicca- (35 m). The elevation of the surrounding terrain in that area tion, contraction during induration of the material, and/or decreases to around À2450 m. Past CS8, the SR widens, and settlement of the underlying material. fluvial textures become visible. In that section, the SR is [46] 2. Where a tabular capping layer is not visible, the notably multilevel, as we observe a thin (a few meters high) SRs commonly appear narrower. Interpretation: These nar- SR superposed over the flat SR (starting around CS10), row sections are reaches where, after fracturing, the capping which suggests that only meager erosion has occurred on that layer has been completely removed and the less resistant surface. The elevation of the surrounding terrain in that area underlying substrate has been progressively eroded. increases again to about À2370 m. Therefore, undulations on [47] 3. Some SR segments are missing. Interpretation: the SR also follow undulations in the surrounding terrain. These gaps result from the complete removal of both the Differential erosion in the surrounding terrain is indicated by capping layer and the less resistant underlying substrate in the fact that the relief of the SR at certain cross-sections is these locations. different on each side (e.g., in CS2, CS4, CS9 and CS10, the [48] 4. In some sites, SRs transition into lineations of relief of the SR is higher on the southern side, than on the adjacent flat-topped knobs and mesas. Interpretation: These northern side). We interpret this difference to be the result of co-linear mesas are highly eroded SR remnants [Burr et al., less exhumation on the northern side of the SR than on the 2009, Figure 5a]. Where observed, these knobs likely con- southern side. In this case, we consider as a reference level stitute the last stage before complete SR removal. the lower elevation (i.e. higher SR relief). The morphology [49] 5. While the erosional sequence outline above is likely of this surrounding terrain is relatively homogeneous, a mix due largely to aeolian abrasion [e.g., Pain et al., 2007; of smooth terrain and occasional dunes. Higher dunes are Burr et al., 2009, 2010], erosion is also produced by impacts, sometimes visible in the depression between two SRs or as suggested by the presence of craters on or adjacent to SRs very close to the side of the SR, areas that probably act as (e.g., Figures 2 and 9), which also modify the SR mor- traps for loose material leading to the creation of higher phology. Large impacts may destroy segments of the SR, dunes. while smaller impacts may remove part of the capping layer. [53] The fact that we observe a similar undulation both in [50] The erosional features shown in Figure 2 are closely the topographic profile of the SR and in the surrounding spaced and must have relief less than the SR relief, for the terrain suggests either that differential erosion has occurred SR to remain visible. Therefore, differential erosion is both in the central part of the SR and in the surrounding inferred for locations where the SR appears less distinct due terrain, or that some other process affecting both SR and to reduction of the SR width and/or height, fragmentation surrounding terrain have led to the lowering of that surface into mesa or knobs, or for multilevel SRs, disappearance of as a whole. In this particular case, the morphology of the the thinner superposed SRs. Differential erosion is also a central part of the SR does appear very eroded compared to the likely cause for local minima in SR profiles where the eastern and western extremities of the SR and the amplitude

16 of 24 E03007 LEFORT ET AL.: DEFORMATION OF SINUOUS RIDGES ON MARS E03007 of the undulation (100 m) is less than the maximum relief of amount of material. Given the pervasive presence of a thin the SR (150 m). It is therefore plausible that 100 m of material capping layer on most SRs, such deep and confined indu- has been removed in the central part of the SR, leaving SR ration seems unlikely. Therefore, in the situations where the remnants 30 to 50 m in relief, and erasing surficial fluvial amplitude of the undulations is greater than the maximum textures such as thinner superposed SRs (which then reappear relief of the SR (which generally corresponds to the long- around CS10, in the less eroded eastern extremity of the SR). wavelength undulations) and where there is no correlation The higher elevations of the surrounding terrain between CS8 between morphology and overall elevation of the SR (i.e. and CS11 are probably due to the fact that, like the SR sur- lower parts of the SR do not appear more eroded than higher face, the surrounding terrain has been less eroded in that area. parts of the SR), we have to consider alternative hypotheses. [54] A dashed curve represents our reconstruction of the SR after inversion but before modification (i.e., post-inversion 6.5. Differential Settling erosion), assuming a maximum SR relief of about 150 m. [58] Some of the topographic undulations may be consis- Erosion occurred mostly in the central part of the SR and in tent with differential settling of the western MFF subsequent the surrounding terrain, perhaps due to enhanced aeolian to creation of the SRs. Differential settling could produce abrasion in that particular area. While other processes such as undulations that are greater in amplitude than the relief of compaction of the material may also have contributed to the the sinuous ridges, since the drop in elevation is produced in formation of that depression, the undulation observed here the subsurface. The spatial variability in the amplitude of the can for the most part be explained by differential erosion. undulations suggests that this hypothesized settling has not [55] Differential erosion affects not only the SRs but also been uniform within the MFF, although a uniform or very the surrounding terrain, as shown by the variation of texture long wavelength settling in the region would not be detectable and elevations of the terrain immediately adjacent to the in SR profiles. To estimate the likelihood that undulations are ridges. Previous studies of the MFF [e.g., Schultz, 2007; due to settling, we examine the amplitude of the undulations Kadish et al., 2009; Kerber and Head, 2010; Zimbelman and and the variations of the SR relief along course. Griffin, 2010] suggest that one hundred meters or more of [59] Differential settling may apply when either one or erosion, as observed in Figure 11, is entirely plausible. both of the following apply: Based on estimates of the relief of yardangs in the lower [60] a. the amplitude of the undulations is more than the member of the MFF, Zimbelman and Griffin [2010] con- maximum relief of the SR. This condition applies to all sider that a minimum of 19,000 km3 of lower member MFF examples presented in part 4 except for Area 43_E for which deposits must have been removed by aeolian erosion. More- DTMs are not available, so SR relief is not known. Other- over, the presence in the MFF of pedestal craters, whose wise, we cannot rule out the more obvious possibility that ejecta deposits are perched on average 115 m above the the observed undulations of that particular SR are due to surrounding terrain [Kadish et al., 2009], with maximum erosion. heights up to 2 km [Schultz, 2007], suggests that the [61] b. there is not a correlation between SR morphology thickness of material eroded must have been equivalent to and elevation. Lower elevations may correspond to areas at least these pedestal heights [Kerber and Head, 2010]. where the upper surface of the sinuous ridge is still distinct Therefore, differential erosion is supported when the ampli- (preserved surficial sinuous ridge morphology). A lack of tude of the undulations is less than the relief of the sinuous correlation between erosional morphology (e.g., narrow plan ridges, even in cases where the amplitude of the undulations view width) and elevation indicates that the low elevation is is 100 m or more. not due to erosion at the surface and therefore is likely due to [56] Counter-indications of differential erosion: In some mechanism beneath the surface (e.g., compaction, Area43_B (Figure 4), SRs get thinner where the profile tectonic faults). This condition applies to all examples pre- exhibits the steepest overall slope, attributed to removal of sented in part 4. the capping layer and subsequent deep erosion of the [62] For example, in the case of Area 43_B, the north- underlying uncemented sediments [Burr et al., 2009]. How- eastern reaches have both high relief and a decrease in ele- ever, the relief along the northeastern reaches tends to be vation (Figure 4), suggesting removal of material from depth, higher than the SR relief in the center of the plateau, while at possibly volatiles as/after the scarp formed, providing an exit. the same time the northeastern SR surfaces appear at least [63] We discuss two possible causes for differential settling as well preserved as those of the SRs in the center of the to determine which may be more likely in the AZP region. plateau. This comparison makes erosion alone an unlikely 6.5.1. Compaction of Low Density Material process to explain the profile dip toward the northeast. The [64] MARSIS data suggest that the MFF material is a low average relief of the ridges over this area is 35 m, with density, highly porous material [Watters et al., 2007b]. Such extreme values between 15 and 90 m. Dips in the topo- a material is prone to compaction when overlain by subse- graphic profile do not necessarily correspond to areas of low quent deposits. Pyroclastic flows are one of the favored SR relief (Figure 4), which is unexpected if the undulations hypotheses proposed for the origin of the MFF and terrestrial are due only to erosion. ignimbrites are considered one of the most likely analogs [57] Differential erosion would also not be plausible [e.g., Scott and Tanaka, 1982, 1986; Mandt et al., 2008; where the amplitude of the undulation is greater than the Zimbelman et al., 1997]. Terrestrial ignimbrites are gen- relief of the SR, as is the case for the thin SRs in area 43 erally characterized by a high porosity (up to 70% porosity (Figure 4), for Areas 40 and 34_A (Figures 6 and 8), and for [Ross and Smith, 1961; Moon, 1993]), a low bulk density, a the multilevel SRs in Area 34_B (Figure 9), unless those thin low cohesion and low tensile strength, and weakness in SRs were indurated over depths of several tens of meters so compression. Ignimbrites may become welded, generally that they are still visible even after removal of a similar

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Figure 12. Broad linear ridges. Background: MOLA DTM overlaid on THEMIS IR mosaic. White lines: center of the broad ridges number 1 to 5. Black line: path of the MOLA DTM topographic profile. Topo- graphic profile (MOLA DTM) from A to B. X-axis: distance (m), Y-axis: elevation (m). Some of the ridges appear steeper on the western side than on the eastern side (ridges number 2 and 3).

within a relative short time after deposition (e.g., minutes to [65] The compaction hypothesis is also plausible if the days) [Quane et al., 2009]. The degree of welding varies MFF is made of ash-fall deposits [Hynek et al., 2003]. Geist laterally and vertically within the deposits and may cause and Cochran [1991] show that volcanic ash soils in northern significant variations in the porosity of the material [Guest, Idaho are prone to compaction because of their low weight- 1969; Rust and Russell, 2000]. For example, Ross and Smith to-volume; since their bulk density is 0.70 g/cm3, their [1961] found, within the same ignimbrite unit, porosity porosity can be 77 percent, and they may have only 20% values as high as 70% for unwelded ignimbrites and only up coarse fragments in the soil profile. Page-Dumroese [1993] to 24% for welded ignimbrites. In general, extreme changes showed extensive compaction of those soils under various in material characteristics (notably porosity, density, cohesion overburdens. and compressive strength) may be encountered over small [66] Due to significant lateral variation in degrees of distances within ignimbrites, both laterally and vertically. porosity and induration, the MFF deposits may have been Ignimbrite facies vary from massive, indurated and jointed to differentially compacted under the weight of subsequently weak and poorly indurated [Selby et al., 1988; Crown et al., deposited material, now eroded. That overlying mass may 1989; de Silva,1989;Johnston, 1992; Moon, 1993]. These correspond to the large amount of material already eroded extreme facies “may occur individually, and may be locally from the MFF [e.g., Zimbelman and Griffin, 2010]. If the vertically associated, or regionally grade into each other MFF material was deposited over preexisting relief (mesa laterally” [Moon, 1993]. Most ignimbrites may lose strength and depressions, possibly wrinkle ridges from the underlying when saturated and may undergo significant plastic deforma- Cerberus Plains), compaction of the porous, low density tion [Moon, 1993]. Laboratory experiments show that initial ignimbrites over this preexisting relief may also cause compaction and welding of hot pyroclastic deposits leading to undulations. the formation of massive indurated ignimbrite facies occurs [67] Most of the topographic undulations of inferred flu- within days (and up to a couple of months) after deposition vial SRs seem to occur in the southern part of the AZP [Cas and Wright,1987;Quane et al., 2009]. The SRs must (Figure 1). Several of the undulations (Figures 6, 8 and 9) have required a geologically significant period of time to seem to coincide with locations where SRs cross broad form, as suggested by the presence of meanders, ox-bow lakes (30 km wide) roughly linear ridges with north-south/ and scroll bars in several SRs, which are features indicative of northwest-southeast direction which are visible in the MOLA mature drainage systems [e.g., Knighton, 1998]. Compaction DTM (Figure 12). These broad ridges may be topographic due to such rapid welding cannot explain the topographic features located in the material underlying the MFF, pos- undulations of the SRs. However, terrestrial ignimbrites sibly preexisting wrinkle ridges [e.g., Fuller and Head, 2002] facies may change over relatively short distances, and regions from an extension of the Cerberus Plains under the MFF with lower density and higher porosity material would more [Zimbelman, 2011] and over which the MFF has differen- easily undergo compaction compared to areas with denser, less tially settled. porous material. [68] The characteristics of pedestal craters within the MFF are another indication that these deposits may be compacted

18 of 24 E03007 LEFORT ET AL.: DEFORMATION OF SINUOUS RIDGES ON MARS E03007 more easily than other deposits on Mars. Barlow [1993] period of time. Moreover, modeling by Kite et al. [2011] noted the unusual depth to diameter ratio of MFF craters suggests that, under conditions of high obliquity, moderate- (approximately 79% deeper than similar-sized simple craters to-high eccentricity, and longitude of perihelion aligned elsewhere). Kadish et al. [2009] observed that the base of with equinox, the MFF is one of the equatorial areas where MFF pedestal craters can extend hundreds of meters below snowmelt is most likely to occur. Other possible remnants the surface of the surrounding terrain, unlike middle and of ice-rich deposits have been observed in the equatorial high-latitude pedestal craters, whose base is generally located region, including potential glaciers on the western side of above the surface of the surrounding plains. A similar Tharsis [e.g., Shean et al., 2005; Milkovich et al., 2006] and observation is made by Kerber and Head [2010]. A possible inferred ice-rich material within equatorial craters [Shean, interpretation for these observations is that impact projectiles 2010]. In the Mangala Valles, Levy and Head [2005] inter- may penetrate deeper into the MFF compared to other geo- preted the surface to be a sublimation lag deposit within logical units on Mars, possibly because the MFF low-density which ancient frozen floodwaters from the subsurface may material is more easily compacted [Barlow, 1993]. still be preserved. Pedestal craters are also often associated 6.5.2. Release of MFF Volatiles with potentially volatile-rich terrain [e.g., Schultz and Lutz, [69] Another process that may cause compaction of the 1988; Wrobel et al., 2006; Kadish et al., 2009], although MFF is the release of volatiles. If the MFF material is anal- the morphology of the pedestal craters observed in the MFF ogous to terrestrial ignimbrites [e.g., Scott and Tanaka, 1982, [e.g., Schultz and Lutz, 1988] suggests that, unlike polar 1986; Mandt et al., 2008], magmatic and atmospheric pedestal craters, the exhumation process was more likely volatiles could have been trapped in the pyroclastic flows. to be aeolian erosion than ground ice sublimation [e.g., The MFF may be made of pyroclastic material produced by Kadish et al., 2009; Kerber and Head, 2010]. This hypoth- the interaction of lava from Elysium with near-surface vola- esis is consistent with the MFF being polar layered deposits tiles, which may have led to the incorporation of a signifi- [Schultz and Lutz, 1988] but does not dismiss other origins. cant amount of volatiles into the pyroclastic deposits 6.5.3. Comparison of the Two Scenarios [Keszthelyi et al., 2000]. While some of the volatiles would [71] Of the two scenarios (compaction and volatile release), have been released during the flow, post-depositional escape compaction of porous material under a now-eroded over- of volatiles has been observed years after deposition of ter- burden appears more likely. Observations of yardangs and restrial ignimbrites and it may take decades to centuries for pedestal craters [e.g., Zimbelman and Griffin, 2010; Kerber thick deposits (many tens of meters) to lose all their volatiles and Head, 2010] indicate that a significant amount of [e.g., Ross and Smith, 1961; Sheridan and Ragan, 1976; material has already been eroded from the area. Moreover, Riehle et al., 1995]. McColley et al. [2005] suggested that the suggested presence of ice within these deposits [Schultz loss of volatiles from the subsurface may be the origin for and Lutz, 1988; McColley et al., 2005; Watters et al., formation of the mesa and buttes interpreted as collapsed 2007b; Kite et al., 2011] has not so far been demonstrated features in the eastern MFF. This type of scenario would and morphologies indicative of subsurface sublimation, such require that the hypothesized orographic precipitation that as pitted surfaces, fretted terrains and thermokarstic textures led to formation of the fluvial channels and floodplains [e.g., Levrard et al., 2004; Levy and Head, 2005; Levy et al., [Burr et al., 2009] was coeval with the emplacement and 2008; Morgenstern et al., 2007; Lefort et al., 2009], have cooling of the pyroclastic deposits since, based on Earth not been observed in the AZP. Recent radar observations, analog observations [e.g., Ross and Smith, 1961; Sheridan while allowing for either porous or ice-rich material, do and Ragan, 1976; Riehle et al., 1995], the cooling of the not give conclusive evidence of the nature of these deposits deposits, and therefore their capacity for compaction would [Watters et al., 2007b; Carter et al., 2009]. Therefore, removal have lasted no more than a few centuries. of volatiles from depth appears a less likely hypothesis, but [70] If, as suggested by MARSIS data, the MFF is ice-rich, it cannot be ruled out at this point of the study. sublimation of this ice may account for the SR undulations. Removal of the interstitial ice by sublimation, possibly 6.6. Tectonic Processes preferentially along fractures and joints within the material, [72] Some undulations occur over broad linear ridges that would lead to collapse of the remaining dry material. display an asymmetric profile, the western side being steeper Although spacecraft data cannot distinguish between the than the eastern side (Figure 12). Those broad ridges may dialectic properties of a dry, porous and low density deposit be the surface expression of thrust faults and may have and those of an ice-rich deposit [Watters et al., 2007b; Carter induced the undulations that we observe in the SR topo- et al., 2009], the presence of ice in equatorial regions has graphic profiles in Figures 6, 8 and 9. been suggested by other studies. Results from GRS data [73] Most tectonic activity associated with the dichotomy analysis [Jakosky et al., 2005] imply that ground ice may still boundary is considered to have lasted no longer than the early be present at low latitudes, although not especially in the Hesperian [Frey and Schultz, 1988; McGill and Squyres, MFF. This low-latitude ground ice would be unstable today 1991; Watters, 2003a; Frey, 2004; Nimmo,2005;Andrews- and in the process of sublimating. A recent study by Clifford Hanna et al., 2008; Roberts and Zhong,2006;Watters et al., et al. [2011] considers the behavior of volatiles in an ice-rich 2007a]. If, as estimated by Kerber and Head [2010], the crust buried by an initially dry porous mantle of sediment. initial deposition of the MFF occurred by the Hesperian, it is This study suggests that thermal reequilibration of the crust plausible that tectonic processes linked to the dichotomy will cause the volatiles to migrate from an initial depth of boundary occurred during the emplacement of the MFF, several meters to a final depth of only a few meters below especially during the emplacement of the lower and there- the surface of the mantle. Therefore, originally dry MFF fore older MFF member that coincides with the AZP. Lobate deposits could have become ice-rich in a geologically short scarps interpreted as compressional features have been

19 of 24 E03007 LEFORT ET AL.: DEFORMATION OF SINUOUS RIDGES ON MARS E03007 identified a few hundred kilometers south of the dichotomy and provided specific examples of undulatory SRs that may boundary [Watters and Robinson, 1999; Watters, 2003a, have been affected by each of these processes. However, it is 2003b]. Those scarps cut late Noachian and early Hesperian not yet possible to attribute with certainty a process to each materials, which suggests that tectonic processes were still of the observed undulatory profiles, or to identify a specific active at that time [Watters and Robinson, 1999]. Moreover, trend in the magnitude of the undulations and the most likely extensional features dated, at the earliest, to the late [McGill mechanisms for each area. These tasks are complicated by and Dimitriou, 1990] or middle [McGill et al., 2004] the fact that in some cases, the undulations are likely not Noachian have also been found along the boundary. Nimmo caused by a single process but by a combination of different [2005] also suggested that tectonic processes linked to the processes, as synthesized in Table 2. The main trend we formation of the dichotomy boundary may have caused the observe is the fact that most undulations (with the exceptions reversing of local slopes that may be identified by analyzing of the undulations of the “rounded” SR in the central part of ancient drainage channels. However, those compressional or the AZP) occur in the southern part of the AZP (Figure 1). extensional features are parallel to the dichotomy boundary Further work is required to discriminate between the different and therefore inconsistent with the north-south directions of potential processes that led to the formation of the topo- the broad linear ridges. Tanaka et al. [2005] mapped tectonic graphic undulations and to say whether or not a particular structures with a similar orientation as those broad ridges but trend exists. did not propose hypotheses regarding their origin. [74] The broad north-south ridges have a similar orienta- 7. Implications and Future Work tion as the fractures along the Gordii Dorsum escarpment and Apollinaris Patera [Forsythe and Zimbelman, 1988; 7.1. Implications Forsythe et al., 1991; Sleep, 1994]. Forsythe and Zimbelman [79] Our study shows that the longitudinal profiles of [1988] and Forsythe et al. [1991] suggest that north-south inverted fluvial channels have topographic undulations, oriented features and other linear valleys within MFF, some of which point to post-formation modifications of the including the area of Apollinaris Patera, may be transcurrent SRs. These undulations may have been produced by a variety faults. Sleep [1994] also proposed that the Gordii Dorsum of causes, which may be inferred based on comparison of and other similarly oriented (north-south) structures may be high-resolution morphologic and topographic data (Table 2). transform faults related to plate tectonic activity in the Topographic undulations have been observed on terrestrial northern plains of Mars. Martian plate tectonic activity has SRs and attributed to fault displacements [Williams et al., been proposed more recently by other studies [e.g., Connerney 2009], but their amplitude (less than 15 m) is much less et al., 2005]. However, such a process seems unlikely with than the amplitude of the undulations observed in the AZP regard to mapping by Pruis and Tanaka [1995] that showed SRs. stratigraphic issues in relation to that hypothesis. [80] These findings have three broad implications: [75] Thrust faults may also be related to the wrinkle ridges [81] 1. They demonstrate that neither the surface topog- in the Cerberus Plains. The Cerberus region hosts extensive raphy nor the topography of strata within the MFF can be lava plains that were emplaced over a period of 200+ million assumed to be original, and therefore are additional evidence years from the Cerberus Fossae fissures [e.g., Hartmann, of the complex history of the MFF. If the differential settling 1999]. Fluvial, volcanic, and tectonic activity on or around hypothesis is verified, then determining the amount and the Cerberus Plains may have occurred as recently as a few precise locations of differential settling would help in esti- million years ago [Berman and Hartmann, 2002; Burr et al., mating the amount and distribution of porosity in the MFF 2002; McEwen et al., 2005]. Movement of magma to the and would provide information on the original emplacement fissures may have caused tectonic movement of the crust mechanism of the MFF, possibly including its content in beneath the MFF, leading to the formation of thrust faults in volatiles. Further study on erosion patterns of the SRs may the MFF material or in the material underlying the MFF. also bring more information on the type of material of the [76] Local tilt of the Martian crust may also explain the Western MFF. undulations. However, based on the MARSIS observations [82] 2. In flood/fluvial modeling studies, topographic that the Martian crust at the poles is not deformed [Phillips profiles are assumed to have remained unchanged since the et al., 2008], we consider it unlikely that the undulations in time of fluid flow and therefore useful for estimating basic the AZP may be due to crustal deformation. fluvial characteristics such as paleodischarge [e.g., Irwin [77] Among those scenarios, we consider that the forma- et al., 2005; Williams et al., 2009] or even flow direction. tion of thrust faults due to either tectonic processes related to Another consequence of these findings is that the SR slopes the dichotomy boundary or to the Cerberus Plains to be a may not be original and so should be used with caution as more likely explanation for the undulations than transform input into fluvial models (for example, for paleodischarge faults related to plate tectonic activity or regional or local tilt calculations). of the crust. However the fact that no north-south oriented [83] 3. Although previous studies have used undulatory thrust faults have been identified in the western MFF is an slopes in support of an esker interpretation for SRs, our work issue and it is therefore not possible to be conclusive at this shows that the undulatory slopes are not exclusively diag- time on the occurrence in this area of tectonic processes nostic of esker formation. associated with the observed undulations. [84] 4. This study may also have implications for other deposits outside of the MFF. It notably suggests that slopes 6.7. Conclusion in other light-tone, layered and friable deposits similar to [78] At this stage of the study, we have identified a set of the MFF, for example light-tone deposits on some of the processes that may have caused the observed undulations, plateaus surrounding Valles Marineris [Hynek et al., 2003;

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Le Deit et al., 2010; Weitz et al., 2010], in Meridiani and helpful discussion. This work was funded by a Mars Data Analysis Arabia Terra or in-crater deposits (e.g., Gale, Spallazani, Program grant to D.M.B. Terby crater) [e.g., Hynek et al., 2003; Thomson et al., 2011; Ansan et al., 2011] may not be original either and may also References have been modified by compaction or tectonic processes. Aharonson, O., M. T. Zuber, D. H. Rothman, N. Schorghofer, and SRs are widely distributed on Mars [Williams, 2007] and K. X. Whipple (2002), Drainage basins and channel incision on Mars, Proc. have been observed in several of these light-toned deposits, Natl. Acad. Sci. U. S. A., 99(4), 1780–1783, doi:10.1073/pnas.261704198. including notably Juventus, Ius and Ganges Chasma [Le Deit Anderson, J. A., S. C. Sides, D. L. Soltesz, T. L. Sucharski, and K. J. Becker et al., 2010; Weitz et al., 2010], Miyamoto crater [Newsom (2004), Modernization of the integrated software for imagers and spectro- meters, Lunar Planet. Sci., XXXV, Abstract 2039. et al., 2010], and Gale crater [Thomson et al., 2011], the Anderson, R. B., and J. F. Bell (2010), Geologic mapping and characteriza- selected site for the Mars Science Laboratory. Therefore, tion of Gale Crater and implications for its potential as a Mars Science in the same way as in terrestrial studies and as demon- Laboratory landing site, Mars, 5,76–128, doi:10.1555/mars.2010.0004. Andrews-Hanna, J. C., M. T. Zuber, and B. Banerdt (2008), The Borealis strated in the present study for the MFF, SRs may be used as basin and the origin of the Martian crustal dichotomy, Nature, 453, a tool to diagnose post-flow modification in other layered 1212–1215, doi:10.1038/nature07011. deposits on Mars. Ansan, V., et al. (2011), Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars, Icarus, 211, 273–304, doi:10.1016/ 7.2. Future Work j.icarus.2010.09.011. Baker, V. R., and J. B. Partridge (1986), Small Martian valleys: Pristine and [85] Several types of analyses may allow precise determi- degraded morphology, J. Geophys. Res., 91(B3), 3561–3572, doi:10.1029/ nation of the extent and amplitude range of the topographic JB091iB03p03561. Banks, M. E., N. P. Lang, J. S. Kargel, A. S. McEwen, V. R. Baker, undulations and identification of the process or combination J. A. Grant, J. D. Pelletier, and R. G. Strom (2009), An analysis of of processes that led to their formation. Future work may sinuous ridges in the southern Argyre Planitia, Mars using HiRISE involve a more comprehensive survey of the topography of and CTX images and MOLA data, J. Geophys. Res., 114, E09003, the SRs in the western MFF and mapping of the location and doi:10.1029/2008JE003244. Barlow, N. G. (1993), Increased depth-diameter ratios in the Medusae amplitude of the topographic undulations. A detailed analysis Fossae Formation deposits of Mars, Lunar Planet. Sci., XXIV, Abstract of the MFF will help to identify whether undulatory SRs are 1031. clustered within particular stratigraphic units, geographic Benn, D., and D. Evans (1998), Glaciers and Glaciation, 734 pp., John Wiley, New York. locations, or geologic contexts. Berman, D. C., and W. K. Hartmann (2002), Recent fluvial, volcanic, and [86] Identifying the SR paleo-flow directions is also tectonic activity on the Cerberus Plains of Mars, Icarus, 159(1), 1–17, important in order to accurately estimate the amplitude of the doi:10.1006/icar.2002.6920. deformations. Morphometric analysis of the SR networks, Boyd, K., and S. Schumm (1995), Geomorphic evidence of deformation in the northern part of the New Madrid seismic zone, U.S. Geol. Surv. especially of the branching networks, will help to rigorously Prof. Pap. 1538-R, 35 pp. identify the paleo-flow directions. Bradley, B. A., and S. E. H. Sakimoto (2001), Relationships between the [87] The differential settling hypothesis may be tested Medusae Fossae Formation (MFF), fluvial channels, and the dichotomy boundary southeast of Nicholson Crater, Mars, Lunar Planet. Sci., XXXII, using SHARAD data by identifying possible deformations Abstract 1335. in subsurface layering associated with surface undulations. Bradley, B. A., S. E. H. Sakimoto, H. Frey, and J. R. Zimbelman (2002), SHARAD and MARSIS data may also help in comparing Medusae Fossae Formation: New perspectives from Mars Global Surveyor, J. Geophys. Res., 107(E8), 5058, doi:10.1029/2001JE001537. material porosity between the undulating and the non- Brennand, T. A. (2000), Deglacial meltwater drainage and glaciodynamics: undulating areas. If compaction occurred, we would expect Inferences from Laurentide eskers, Canada, Geomorphology, 32, the subsurface material in the lower part of the undulations 263–293, doi:10.1016/S0169-555X(99)00100-2. to be less porous than the material in the higher part of the Burnett, A. W., and S. A. Schumm (1983), Alluvial river response to Neotectonic deformation in Louisiana and Mississippi, Science, 222, undulations and in the surrounding terrains, since compac- 49–50, doi:10.1126/science.222.4619.49. tion would have caused a decrease in the porosity. Burr, D. M., J. A. Grier, A. S. McEwen, and L. P. Keszthelyi (2002), [88] Data from MARSIS and SHARAD may also help test Repeated aqueous flooding from the Cerberus Fossae: Evidence for very recently extant, deep groundwater on Mars, Icarus, 159,53–73, the hypothesis that differential settling occurred over pre- doi:10.1006/icar.2002.6921. existing underlying topographic features. Carter et al. Burr, D. M., M.-T. Enga, R. M. E. Williams, J. R. Zimbelman, A. D. [2009] observe a radar reflection on one of the SHARAD Howard, and T. A. Brennand (2009), Pervasive aqueous paleoflow fea- – tracks in the north of the AZP that suggests the presence of tures in the Aeolis/Zephyria Plana region, Mars, Icarus, 200,5276, doi:10.1016/j.icarus.2008.10.014. an undulation in a subsurface interface beneath the MFF. Burr, D. M., R. M. E. Williams, K. D. Wendell, M. Chojnacki, and Similar studies using SHARAD tracks over the broad north- J. P. Emery (2010), Inverted fluvial features in the Aeolis/Zephyria Plana south ridges and other areas where the SR profiles undulate region, Mars: Formation mechanism and initial paleodischarge estimates, J. Geophys. Res., 115, E07011, doi:10.1029/2009JE003496. may provide valuable information regarding the topography Carr, M. H. (1981), The Surface of Mars, 232 pp., Yale Univ. Press, of the material underlying the MFF and the origin of the New Haven, Conn. broad north-south ridges. This information may help assess Carr, M. H. (1996), Water on Mars, Oxford Univ. Press, New York. Carter, L. M., et al. (2009), Shallow radar (SHARAD) sounding observa- the hypothesis that the MFF has been compacted over pre- tions of the Medusae Fossae Formation, Mars, Icarus, 199, 295–302, existing topographic features. doi:10.1016/j.icarus.2008.10.007. [89] Finally, extending the search for topographic undula- Cas, R. A. F., and J. V. Wright (1987), Volcanic Successions, Modern and tions to SRs in other light-toned deposits on Mars would allow Ancient, 528 pp., Allen and Unwin, London, doi:10.1007/978-94-009- 3167-1. for determination of whether or not this phenomenon is Caskey, S. J., and A. R. Ramelli (2004), Tectonic displacement and far- localized to the MFF or widespread among light-tone deposits. field isostatic flexure of pluvial lake shorelines, Dixie Valley, Nevada, J. Geodyn., 38, 131–145, doi:10.1016/j.jog.2004.06.001. [90] Acknowledgments. 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