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Extensive Secondary Chaos Formation in Chryse Chaos and Simud Valles, MarsJ. Alexis. P. RodriguezPlanetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, Arizona, USAKen L. TanakaAstrogeology Team, U.S. Geological Survey, Flagstaff, Arizona, USAJeffrey S. KargelHydrology & Water Resources, University of Arizona, Tucson, Arizona, USADavid CrownPlanetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, Arizona, USADaniel C. BermanPlanetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, Arizona, USAIntroductionThe southern circum-Chryse region along the highlandlowlandboundary of Mars displays the planet’s largest andmost complex assemblage of interconnected canyons andchannels. The geologic history of this region is key to theunderstanding of the evolution of crustal volatile release aswell as of the nature of the largest condensed surface fluidflows on the planet. Here, large plateau zones have undergonecollapse, forming low-lying depressions floored by brokenupand morphologically diverse blocks.These collapsed terrains, traditionally referred to aschaotic terrains, commonly occur in close spatial associationwith Martian outflow channels. Martian chaotic terrainsand outflow channels have been intensively studied sincethe 1970s. The consensus is that chaotic terrains representzones where aquifer destabilization led to ground collapseand to the rapid release of vast amounts of fluids at thesurface, which subsequently carved the outflow channels(Sharp 1973). Impact crater densities and geologic relationsindicate that both the chaotic terrains and the outflowchannels formed during the Late Hesperian Epoch (Scott &Tanaka 1986). An obvious implication of this hypothesis isthat the formation of chaotic terrains necessarily pre-dated,but was penecontemporaneous with, the excavation of theirassociated outflow channels.Yet, some chaotic terrains formed within the floors ofoutflow channels, and thus must post-date the excavationof the channel floors they modify. These chaotic terrains,known as secondary chaotic terrains, have been previouslydescribed as occurring in the higher outflow channel floors(Rodriguez et al. 2005) . Here, we present a synthesis of themorphologic attributes of a secondary chaotic terrain knownas Chryse Chaos that apparently destroyed the southernreaches of the lower outflow channel system of Simud Valles(Fig. 1).Morphology of Chryse ChaosChryse Chaos is located along the lower outflow channelfloor of Simud Valles. Its maximum length and width are700 km and 270 km, respectively, and its surface area is~0.12 million km 2 . Yet, the floor of Chryse Chaos is locateda mere ~200 m below adjacent channel floors. Chryse Chaosexhibits a large population of knobs, which are locallyclosely spaced into clusters (zones outlined by yellow linein Fig. 1b).The northern margin of Chryse Chaos consists of aprominent break in slope (black, teethed line in Fig. 1b). Ourmapping shows that no channels cut down to the chaos floorlevel and extend north from this break in slope, which formsthe downstream margin of the chaotic terrain, suggestingthat the formation of Chryse Chaos was not associated withgeneration of catastrophic floods. In addition, we have notidentified any landforms indicative of water ponding such asequipotential terraces (shorelines) along the margins of theFigure 1. (a) View of Chryse Chaos (part of THEMIS IR mosaiccentered at -10.06˚N; 322.31˚E). (b) View of northern margin ofChryse Chaos. Perspective view of MOLA-based shaded DEM(128 pixels/degree) centered at 13.81˚N; 321.15˚E and relatedelevation profile (A-A'). The hachured line shows the location ofthe break in slope that marks the northern margin of Chryse Chaos.The yellow lines outline collapsed mesas, one of which is flankedby two erosional channels (red arrows). Shown is the location ofFig. 2a.257