Structure and geometry of the Interior Layered Deposits within Hebes Chasma, Valles Marineris, Mars

Introduction:The formation of Valles Marineris (VM) is known to involve tectonic extension and subsequent erosion.Isolated ancestral basins [1] were later linked by further extension faulting [2].Hebes Chasma (Fig. 1A), an isolated depression north of central VM, is thought to be one such ancestral basin.As in many of VM's chasmata, large free-standing interior layered deposits (ILDs) are located within Hebes' interior.Hebes' enclosed form is unique in that its present structure cannot be explained by erosion of the ILD through outwash channels, a feature commonly observed in other VM's chasmata.Ground water circulation [3] and salt diapirism [4] have been proposed as mechanisms for producing Hebes' present structure.This study examined small-scale fractures as well as topographic features, including landslide scars, to gain a better understanding of the structure and deformation of the main Hebes ILD mound, Hebes Mensa. Hebes Chasma ILD:The main ILD mound is 120 km across and 43 km wide, and lies within the approximate centre of the chasma.Its elevation ranges from approximately -2,000 m to 3,800 m, measured from the northern floor.Four large erosional features, or landslide scars (Green in Fig. 1B), occur on three sides of the mound.Methodology: A CTX mosaic registered to a HRSC composite DTM (orbits 0360, 2116, 2138, 2149, 5142, 5160, 5178) forms the base data for the study.Smallscale fractures were examined within HiRISE images.Attitudes of planar features were obtained using Orion software (Pangaea Scientific).Results: Fractures were examined within five HiRISE images located on the ILD.Fractures were not common; most were isolated, such as those depicted on the north slope of the ILD mound (Fig. 1C and 1D).No orthogonal fracture sets were found, as those documented in the ILDs of western Candor Chasma [7].In some cases layers have an apparent offset by several meters along fractures (Fig. 1E).The top surface of the ILD mound has a shallow dip of 3˚ to the north (Fig. 1H).The mound's northern and southern slopes have an average inclination of 17˚ and 27˚, respectively.Several major landslide scars (Fig. 1B, P1-P4) are located along the mound's edge.Each landslide scar follows the distinct geometry of a steep slope grading to an almost horizontal shelf which extends several kilometers before the slope steepens again (Fig. 1G,