Postdating of flow in Athabasca Valles by faulting

of the Cerberus Fossae, Elysium Planitia, Mars

Joyce Vetterlein1 and Gerald P. Roberts1

Received 2 February 2009; revised 4 April 2009; accepted 4 May 2009; published 15 July 2009.

[1] Mars Orbiter Laser Altimeter (MOLA) topographic data, High Resolution ImagingScience Experiment (HiRISE), Thermal Emission Imaging System (THEMIS), and MarsOrbiter Camera (MOC) images have been used to study the timing of the formation ofAthabasca Valles relative to faulting of the southern Cerberus Fossae (SCF). Thetopography of Athabasca Valles, derived from MOLA elevation data, appears as twotroughs on the north and south footwalls of SCF. The same troughs can be identified onthe floor of the fossae, indicating that Athabasca Valles predates the accumulation of�500 m of vertical fault offset. The topographic expression of Athabasca Valles ispreserved on the graben floor because infill by sediment and lava, since formation of thevalles and fossae, amounts to less than a few tens of meters. We use this information todiscuss the mode of formation of Athabasca Valles and Cerberus Fossae.

Citation: Vetterlein, J., and G. P. Roberts (2009), Postdating of flow in Athabasca Valles by faulting of the Cerberus Fossae, Elysium

Planitia, Mars, J. Geophys. Res., 114, E07003, doi:10.1029/2009JE003356.

1. Introduction

[2] Athabasca Valles is a valley system cutting Amazo-nian terrain that is thought to be one of the youngest fluvialoutflow channels on Mars (Figure 1). It has been proposedthat Athabasca Valles formed when an aquifer was breachedby faulting and dike emplacement associated with formationof Cerberus Fossae [Burr et al., 2002a; Plescia, 2003].Others have suggested that the origin of the water wascryospheric melting associated with dike emplacement[Burr et al., 2002b; Head et al., 2003]. The released wateris thought to have flowed down the 10–30 km wide valleysystem for tens to hundreds of kilometers, discharging intoan enclosed basin, the Cerberus Palus, in Elysium Planitia[Murray et al., 2005]. Later, this fluvial channel wascovered with low-viscosity lava that appears to have eruptedfrom the vicinity of Cerberus Fossae [Jaeger et al., 2007],with individual lava flows of the order of a few metersthick. Subsidence along the �1 km wide southern CerberusFossae is located between steeply dipping north and southwalls, so the structure resembles a graben. Head et al.[2003] suggested that subsidence within the graben wasdominated by melting and collapse after dike emplacement.The subsided graben is thought to have filled with waterbefore it overspilled to form the Athabasca Valles outflowchannel [Keszthelyi et al., 2007]. Thus, in this scenario,subsidence along the southern Cerberus Fossae predatesoverspilling of water that formed Athabasca Valles.[3] However, it has been known for some time that the

interpretation of Athabasca Valles that forms following the

faulting along Cerberus Fossae is difficult to reconcile withobservations of landforms located at the junction of thesetwo features. Berman and Hartmann [2002, p. 1] pointedout that the ‘‘fractures appear to have continued developingmore recently than the most recent lavas or fluvial activity.’’They pointed out the existence of the apparently unerodededge of Cerberus Fossae where it impinges on AthabascaValles and also the existence of streamlined topographicrises, sculpted by the Athabasca floods, that appear to be cutby the faulting (Figure 2). The interpretation of Berman andHartmann [2002] is also difficult to reconcile with morerecent observations of low-viscosity lava draping the fluviallandforms [Jaeger et al., 2007] if the fluvial landformsthemselves formed following the faulting of CerberusFossae. This is because (1) the lavas appear to be cut bythe faulted walls of Cerberus Fossae [see Jaeger et al.,2007, Figure 2] (Figure 3) and (2) the lavas exist both to thenorth and to the south of Cerberus Fossae; it is difficult tosee how such thin lava flows can have crossed the faulteddepression if they formed after the faulting. The aboveinformation prompted us to reexamine the relative timing offaulting, fluvial activity, and eruption of lavas.[4] In this paper we show that Mars Orbiter Laser

Altimeter (MOLA) topographic data along strike of thesouthern Cerberus Fossae provide evidence that faultingpostdates the Athabasca floods and subsequent resurfacingby lava. The faults themselves are among the most recentlyformed features in this area. We use these findings todiscuss the modes of formation of Athabasca Valles andCerberus Fossae.

2. Method

[5] To measure the depths and offsets across the southernCerberus Fossae, MOLA data were collected from 113 MarsGlobal Surveyor orbits [Smith et al., 1999]. Each orbital

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E07003, doi:10.1029/2009JE003356, 2009

1Research School of Earth Sciences, Birkbeck and University College,University of London, London, UK.

Copyright 2009 by the American Geophysical Union.0148-0227/09/2009JE003356

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Figure 1. (a) Digital elevation map of the Cerberus Fossae and Athabasca Valles. (b) Rotated drawingof the southern Cerberus Fossae (CF2), with locations of HiRISE images marked. (c and d) Plots ofMOLA footprints along the strike of the main fault trace showing the northern footwalls (FWN) andsouthern footwalls (FWS) and hanging wall (HW). FWN is blue; FWS is green; and HW is magenta. Themarkers on each line are the MOLA footprints. The northern (CF2 N) and southern (CF2 S) arms of CF2are shown. The topography associated with Athabasca Valles preserved on the footwall as two troughs(AV) can be seen preserved in the down-thrown hanging wall block (AVHW, Athabasca Valles hangingwall). Other examples of preserved prefaulting topography are indicated by ellipses. OCT is old crateredterrain, and CW indicates a high point on a crater wall. (e) MOLA tracks that relate to points on CF2 S areshown.

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profile provides latitude, longitude, and elevation data thatconstrain the topographic relief across the north and southwalls of the southern Cerberus Fossae. The MOLA datapoints were collected between planetocentric latitudes 8.25�and 11.75� and longitudes east positive 155� and 161�.These were used as a matrix to construct a digital elevationmap (Figure 1a). Data were extracted from individual orbitsto produce a plot that shows the along-strike variation inrelief across the faults that define the north and south wallsof the southern Cerberus Fossae (Figures 1c and 1d). Thelatitude and longitude of the most northerly point on eachtraverse was recorded and used to document the positionsalong strike of the structure. We hoped to constrain theamount of sediment and/or lava infill to the graben byidentifying the extent to which features formed prior to thedeformation are blanketed by more recent sediment and/orlava. This would allow us to see the vertical offsets (throws)across the faults bounding the graben. To study details ofthe geomorphology of the region, Mars Orbiter Camera(MOC), Thermal Emission Imaging System (THEMIS), andHigh Resolution Imaging Science Experiment (HiRISE)images were examined [Malin et al., 1998; Christensen etal., 2004; McEwen et al., 2007].

3. Results

[6] The northeast end of Athabasca Valles is crossed bythe southern Cerberus Fossae (Figure 1). We were interestedin (1) whether we could measure the amount of sedimentinfill to the graben and thus constrain measured verticaloffsets (throws) and (2) whether the faults postdate orpredate formation of Athabasca Valles.[7] First, Figures 1c and 1d show that features formed

during the Noachian and/or Hesperian observed outside thegraben can also be identified on the graben floors. They areshown in the ellipses in Figures 1c and 1d. Examination ofTHEMIS and other images reveals that some of thesefeatures are crater rims or other topographic highs sur-rounded by younger lava (inliers). This observation indi-cates that prefaulting topography has been preserved on thegraben floor, which in turn suggests that (1) sediment and/orlava infill is less than the vertical extent of the prefaultingtopography and (2) disruption of surface topography on thegraben floor by melting or collapse is minimal. For exam-ple, a small peak on the southern arm of the southernCerberus Fossae (CF2 S) associated with a Noachian inlierat longitude �159� (�205 km along strike) is <70 m highbut can be observed outside the graben on the footwall ofthe bounding fault and on the floor of the graben. Sedimentand/or lava infill would thus appear to be <70 m in thislocation. Other examples with greater vertical extent areshown in Figure 1 by dashed ellipses. Given that sedimentand/or lava infill is minimal, the vertical relief across thefaults is a measure of the vertical fault offset (throw) (seeVetterlein and Roberts [2003] and J. Vetterlein and G. P.Roberts (Structural evolution of the northern CerberusFossae graben system, Elysium Planitia, Mars, submittedto Journal of Structural Geology, 2009) for similar obser-vations of throw across faults bounding the northern Cer-berus Fossae).[8] Second, Figures 1c and 1d show that the topographic

expression of Athabasca Valles observed outside the graben

Figure 2. View of faulted features in Athabasca Valles.(a) Rotated drawing of the northern and southern arms of thesouthern Cerberus Fossae showing the location of MOLAtrack 19820. (b) THEMIS image V05125011 showing astreamlined topographic rise on the floor of AthabascaValles. The box indicates the location of Figure 2c. (c) MOCnarrow-angle image SP2 53103, centered at 10.27�N,157.16�E, showing detail of a streamlined topographic risein Athabasca Valles. The rise has been faulted by thesouthern Cerberus Fossae. The MOLA orbit 19820 has beensuperimposed to show the footprints. (d) Sketch of the twograben shown in Figure 2c, with the smaller subsidiarygraben in the northeast forming a paleotip. (e) MOLA orbit19820 is the nearest track to these features, showing thesharp edges of Cerberus Fossae and the almost flat topog-raphy of the surrounding terrain.

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can be identified on the graben floors. Athabasca Vallesappears as two troughs on the footwalls of the graben(Figures 1c and 1d, labeled AV). These same troughs canbe identified on the floor of the graben (the AthabascasValles hanging wall, labeled AVHW).[9] Athabasca Valles is about 150–200 m deep and

<20 km wide on the footwalls of the graben. These 150–200 m deep valleys preserved on the graben floor areconsistent with our earlier conclusion that infill of thegraben by sediment and/or lava is minimal and in this case<150–200 m. Preservation of prefaulting topography on thegraben floor also indicates that collapse related to cryo-spheric melting was not the dominant mechanism respon-sible for subsidence, as it would have disrupted andprobably destroyed such features. Rather, subsidence isdue to down faulting of a block that is rigid enough topreserve prefaulting topography on its surface. The preser-vation of topography associated with Athabasca Valles onthe graben floor also indicates that �500 m of vertical offset(throw) has accumulated during faulting and that all thisfaulting postdates formation of Athabasca Valles. We have

found no evidence for faulting prior to or during formationof Athabasca Valles as we lack data on the offsets offeatures formed during the faulting (e.g., hanging wallgrowth strata). However, the vertical resolution of theMOLA data is a few meters averaged over the laserfootprint, so we concede that a few meters or a few tensof meters of vertical offset may have accumulated prior to orduring formation of Athabasca Valles; such offsets would berelatively minor compared to the �500 m of vertical offsetthat has accumulated after formation of Athabasca Valles.[10] To examine further whether the subsidence is best

explained by cryospheric melting and collapse or faulting,we measured the displacement gradients and throw/lengthratios of the faults bounding the southern Cerberus Fossaeand compared these values to those for well-documentedterrestrial and Martian faults [Schlische et al., 1996; Vetterleinand Roberts, 2003; Schultz et al., 2006]. The displacementsare characterized by values that decrease toward fault tips(located at �155� and 161� longitude), with a throw maxi-mum toward the center of the fault. The southern CerberusFossae fault system has a maximum throw of �0.5 km and

Figure 3. (a) Part of HiRISE image PSP_008779_1905 showing colluvial fans in the northern footwall(FW N) of the southern Cerberus Fossae offset by later faults. Faulted interfluves within early colluviumshow characteristic triangular facets. This early colluvium is being reworked from younger colluvial fansin the hanging wall of the fault. (b) Schematic cross section to indicate geological relationships. It isunclear if the dashed line is or is not a faulted contact. (c) Part of HiRISE image PSP_001408_1900,showing the relationships between low-viscosity lavas interpreted by Jaeger et al. [2007] and colluviumassociated with faulting in Cerberus Fossae.

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length of �340 km, which implies a throw/length ratio of0.0015 or 0.0029 depending on whether the north and southwalls are considered as separate faults (0.5/340 = 0.0015) orpart of the same structure (1/340 = 0.0029). Faults on theEarth and Mars exhibit values in the range 0.1–0.001[Schlische et al., 1996; Vetterlein and Roberts, 2003; Schultzet al., 2006]. Thus, the similarity between throw/lengthvalues for well-documented faults and the values we findfor the southern Cerberus Fossae suggest that the latter are notunusual, supporting our interpretation of fault-related offsetsto explain the vertical relief rather than relief produced bymelting of the cryosphere and collapse. Similar conclusionshave been reached for other faults on Mars such as thenorthern Cerberus Fossae, which has a throw/length ratio of0.002 [Vetterlein and Roberts, 2003; J. Vetterlein and G. P.Roberts, submitted manuscript, 2009].[11] Our observations also allow us to document valley

depths and widths relative to the flow direction associatedwith formation of Athabasca Valles. The northwestern endof Athabasca Valles is more deeply incised and narrowerwhere it is faulted by the northern arm of southern CerberusFossae (CF2 N) than where it is faulted by CF2 S (Figures1c and 1d). For example, Athabasca Valles is �197 m deepand �21.12 km wide where it is faulted by CF2 N but�135 m deep and �31.57 km wide where it is faulted byCF2 S. This may be consistent with the observations ofEdgett and Malin [2003], who suggested that the source ofthe flow was to the northeast of the southern Cerberus Fossaeand that the flow responsible for incision already hadsufficient energy to make this valley by the time it had flowedsouthwest to the vicinity of the southern Cerberus Fossae.[12] To test our finding that faulting of the southern

Cerberus Fossae postdates formation of Athabasca Valles,we searched for images showing the relationship betweenthese features. Figures 2b and 2c show a streamlinedtopographic rise on the floor of Athabasca Valles that hasbeen noted in the literature [Berman and Hartmann, 2002;Burr et al., 2002a]. The rise exhibits streamlined flow fromthe northeast. The topography has been subsequently offsetby faulting, clearly indicating that the faulting postdatesflow in Athabasca Valles.[13] Faulting also appears to be associated with formation

of triangular facets that indicate recent propagation of faultplanes upward to offset colluvial deposits. Figure 3 showscolluvium that has fallen from the walls of Cerberus Fossae.Colluvium is located in both the footwall and the hangingwall of the most recent fault activity, which is indicated bythe fact that the apices of triangular facets along the faultscoincide with interfluves within the footwall colluvium (see

dePolo and Anderson [2000] for examples of triangularfacets from Earth). Initial deposition of colluvium andformation of colluvial channels separated by interfluveshas been disrupted by formation of faults that have cut upthrough the colluvium. The fault planes themselves arepreserved to an extent as the triangular facet surfaces. Theseobservations indicate that the most recent faulting locallypostdates formation of colluvium that itself can have formedonly after early fault subsidence along Cerberus Fossae. Wecan use this observation to help constrain the age of the low-viscosity lava flows described by Jaeger et al. [2007]relative to faulting associated with Cerberus Fossae.HiRISE image PSP_001408_1900 used by Jaeger et al.[2007] (Figure 3c) shows that the floor of Cerberus Fossaein that location is covered by colluvium derived from thewalls of the depression rather than by the low-viscositylavas. As the low-viscosity lavas occur both north and southof Cerberus Fossae, our interpretation is that they formedprior to faulting and are thus covered by the colluvium onthe floor of the graben shown in HiRISE imagePSP_001408_1900 (see Figure 3b).

4. Discussion

[14] We have shown that subsidence associated withdown faulting of a rigid block between the north and southwalls of the southern Cerberus Fossae explains the preser-vation of prefaulting topography on the graben floor. Part ofthis prefaulting topography is two troughs associated withAthabasca Valles, whose formation thus predates the fault-ing. Cryospheric melting and collapse is not the dominantmode of subsidence, as this would have disrupted andprobably destroyed prefaulting topography on the grabenfloor. The timing of faulting after formation of AthabascaValles precludes the possibility that water ponded within thegraben prior to overspilling, as envisaged by Keszthelyi etal. [2007]. The internal volume of the graben cannot beused as a proxy for the volume of water. Low-viscositylavas thought to blanket fluvial landforms within AthabascaValles [Jaeger et al., 2007] are likely to predate the faulting,as they occur in relatively thin sheets both to the north andthe south of the southern Cerberus Fossae.[15] This evidence suggests modifications to existing

models of how Athabasca Valles relates to Cerberus Fossae(see Figure 4). We envisage that initial cracking, possiblyassociated with dike emplacement at depth, allowed therelease of water and, later, low-viscosity lava prior toaccumulation of the majority of the �500 m of verticaloffset across the graben (Figure 4a). Later (Figure 4b),

Figure 4. Schematic evolution of Cerberus Fossae and Athabasca Valles (not to scale) at (a) time 1 and(b) time 2. CF, Cerberus Fossae; AV, Athabasca Valles.

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�500 m of vertical fault offset accumulated. The essentialdifference between this model and previous models is thatAthabasca Valles formed prior to accumulation of signifi-cant vertical offset.

5. Conclusions

[16] We conclude that formation of the Athabasca Vallespredates significant vertical faulting across the faults bound-ing the north and south walls of the southern CerberusFossae graben because the topography of Athabasca Vallesis preserved on the graben floor. This is consistent withobservations of landforms in the area where CerberusFossae cuts Athabasca Valles, such as faulted streamlinedhills and faulted lava flows. Up to �500 m of verticalfaulting has accumulated since formation of AthabascaValles. Down faulting of a rigid fault block is the dominantsubsidence mechanism rather than cryospheric melting andcollapse evidenced by preservation of prefaulting topogra-phy on the graben floor. An interpretation of fault-controlledsubsidence is consistent with the throw/length ratio forCerberus Fossae (0.0015 or 0.0029) that is similar to valuesfor well-documented faults (in the range 0.1–0.001). Asfaulting and subsidence postdate flooding through Atha-basca Valles, models of flooding should not involve pond-ing of water within the morphology of the present-daygraben prior to overspilling of water. The faults formingthe walls of the southern Cerberus Fossae offset colluviumthat formed during faulting. The fault planes are preservedto an extent as triangular facets offsetting the colluvium.These triangular facet surfaces are among the youngestlandforms in the vicinity of Cerberus Fossae. Outflow ofwater through Athabasca Valles is significantly older thanthe faulting.

[17] Acknowledgments. We thank John Guest, Ian Crawford, LouiseBishop, and Dominic Fortes for discussions concerning this study andThomas Dewez for help with constructing the DEM. The research waspartly funded by Birkbeck College and benefited from provision of theRegional Planetary Imaging Facility at UCL.

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�����������������������G. P. Roberts and J. Vetterlein, Research School of Earth Sciences,

Birkbeck and University College, University of London, Gower Street,London WC1E 6BT, UK. (gerald.roberts@ucl.ac.uk; joyce_vetterlein@hotmail.co.uk)

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