B30307 2nd pages / 1 of 32 Modeling the collapse of Hebes Chasma, Valles Marineris, Mars M.P.A. Jackson1,†, J.B. Adams2, T.P. Dooley1, A.R. Gillespie2, and D.R. Montgomery2 1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas, Austin, Texas 78713, USA 2Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA ABSTRACT equatorial Mars (Fig. 1A). Among the northern drainage, upward reactive diapirism, and sur- chasmata is Hebes Chasma, a cavity 315 km face fl ows partly akin to allochthonous salt tec- Physical modeling and detailed mapping long and 126 km wide. The chasma is more than tonics on Earth. This balance varied over time of Hebes Chasma provide new insights into 8 km deep from its rim at +3950 m to its fl oor and from place to place within the model and the crustal composition and origin of the at −4530 m and is one of the deepest canyons probably within Hebes Chasma too. Although Valles Marineris region of equatorial Mars. on Mars. This huge cavity implies removal of the modeling and mapping provide a compre- Hebes Chasma is a 315-km-long and 8-km- 105 km3 of material (the present-day volume of hensive view of how a distinctive suite of land- deep closed depression containing distinctive the chasma estimated using Mars Orbiter Laser forms evolved in Hebes Chasma, many pieces landforms that include diapirs and extensive Altimeter [MOLA] data). The composition and of the puzzle remain hidden. allochthonous fl ows that end in pits. A central fate of the vanished material are a puzzle that puzzle of Hebes Chasma is how and where has exercised researchers trying to explain the PREVIOUS HYPOTHESES FOR 105 km3 of missing material disappeared. Our origin of Valles Marineris chasmata and similar FORMATION OF CHASMATA physical models tested the hypothesis that the canyons (Table 1). chasma formed by collapse and removal of Deep as it is, Hebes Chasma is dwarfed in Previous ideas on the origin of the chasmata material from below. Gravity-driven collapse length by other major canyons nearby in Valles have three themes: erosional excavation, tec- in the models reproduced all the chasma’s Marineris (Fig. 1A). However, Hebes Chasma tonic rifting, and karstic collapse (Table 1). main landforms as subsidence evolved from has an unusual allure for research into the origin Surface erosion entails removal of 105 km3 early sagging of the upper surface, to inward of the chasmata: This closed canyon lacks any of material from the chasma by water or wind collapse and removal of material, to emer- surface outlets, so an origin by fl uvial erosion (Fig. 2A). Although fl uvial erosion was wide- gence of diapirs and low-gradient fl ows. The is not viable. A hypothesis of downward drain- spread on Mars, removal of this volume of models and geologic evidence suggest that age has been considered for Valles Marineris material by water erosion is impossible for Hebes Mensa arched upward diapirically in general (Table 1) and for Hebes Chasma in Hebes Chasma because it lacks an outlet, and and raised deep stratigraphy almost level particular through photogeologic mapping and no fl uvial channels are visible on the fl oor of with the chasma rim. If the chasma indeed a briefl y described physical model of collapse either the chasma or canyons leading into it. collapsed by subsurface drainage as occurred (Adams et al., 2009). Here, we present the full Because of this fatal fl aw, we do not consider in the models, the upper 8–10 km of depos- results and structural analysis of physical mod- further an origin for Hebes Chasma by fl uvial its at Hebes must have been solid to depths eling designed to test and elucidate the hypoth- erosion. Some wind erosion is likely. Within of ~5 km but viscous at greater depths. The esis of downward drainage and collapse inferred the chasma, wind has scoured yardangs and materials removed could not have consisted by Adams et al. (2009). We compare specifi c deposited sparse dunes and possibly some of mainly of basalt fl ows; instead, they probably features of the models with geometrically simi- the dark smooth deposits. Windborne sediment were a mixture of hydrated and nonhydrated lar Hebes landforms illustrated in Figure 1B and has undoubtedly left and entered the chasma, salts, water ice, liquid water, and insoluble discuss these at length in the second half of the but the balance is unknown. (likely basaltic) particles. The proportions paper. This comparison leads to new insights An origin by wind erosion would have sev- of these constituents are unknown but con- into the evolution of landforms in Hebes eral implications (Fig. 2A). Collapse structures strained because the material drained from Chasma. The conclusions have implications for in the chasma walls and in Hebes Mensa would the subsiding chasma and apparently con- the composition of the upper few kilometers have formed in response to the space produced tributed to the outburst fl oods released down of deposits in Hebes Chasma and elsewhere by wind excavation. The youngest stratigraphic neighboring Echus Chasma and Kasei Valles. on Mars, but especially in the Valles Marineris layers could not underlie the fl oor or the region (Fig. 1A). benches because erosion would have removed INTRODUCTION As far as we know, this is the fi rst attempt shallow units. The layered rocks exposed along to physically model the tectonics of a Martian the edges of benches would differ from the The Valles Marineris chasmata are a strikingly canyon using scaled materials. The models rimrock of the chasma. Hebes Mensa would linear set of trenches trending east-southeast reproduce the structures in Hebes Chasma and be an erosional remnant of the same material along the north rim of the Thaumasia Plateau in reveal a complex balance of karstic downward as the walls, or it would consist of younger †E-mail: [email protected]. GSA Bulletin; MMonth/Monthonth/Month 22011;011; vv.. 11xx;xx; nno.o. XX/X;/X; pp.. 0000–000;00–000; doi: 10.1130/B30307.1; 27 fi gures; 3 tables; Data Repository item 2011021. For permission to copy, contact [email protected] 1 © 2009 Geological Society of America B30307 2nd pages / 2 of 32 Jackson et al. A Figure 1. (A) Valles Marineris region showing the location of Hebes Chasma and nearby fea- tures. Mars Orbiter Laser Altim- eter data. (B) Visible mosaic of Hebes Chasma showing informal B place names used in this paper. Mars Express High Resolution Stereo Camera. deposits fi lling a precursor chasma then left as excavated cavity; or the swale at the east end of wind was undoubtedly important in other can- an erosional remnant after a second episode of the chasma; or the preservation of the plains sur- yons (Table 1), it is unlikely to be the main agent chasma excavation. face and the youngest layers on fault-bounded that formed Hebes Chasma. Several features argue against wind erosion benches; or the arcuate headwall scarps in the The tectonic rifting hypothesis invokes an as the chief agent of chasma excavation. (1) The chasma walls; or the pits, fl ows, and diapirs in origin by north-south tectonic extension, form- canyon rim is sharp and scalloped, not rounded, the chasma fl oor. (3) Only a thin mantle of dust ing Hebes Chasma as a graben (Fig. 2B). Some hollowed, or grooved by wind erosion. (2) Wind covers the chasma rim, though wind could have features support this idea. (1) Regional struc- erosion cannot explain a wide variety of features carried and deposited the dust elsewhere on the ture obviously infl uenced the location of Hebes in Hebes Chasma: the shape and scale of the planet. So although surface erosion by water or Chasma and its parallelism with other Valles Marineris chasmata. (2) Downdropped benches of younger layers and the many scarps of nor- mal faults along the chasma walls also point to TABLE 1. PREVIOUS HYPOTHESES FOR FORMATION OF THE VALLES MARINERIS CHASMATA strong extensional control. Hypotheses Authors An origin by tectonic extension has several Erosion McCauley et al. (1972); Sharp (1973); Lucchitta (1979) Collapse by removal of water or magma Sharp (1973); Courtillot et al. (1975); Schonfeld implications (Fig. 2B). The youngest layers (1979); Tanaka and Golombek (1989); Spencer and would underlie the chasma fl oor and would Fanale (1990); Schultz (1998); Schultz and Lin (2001); Fueten et al. (2005); Rodriguez et al. (2006); Rossi et be covered by deposits derived by wasting of al. (2008) chasma walls. Hebes Mensa could be a central Tectonic rifting Sharp (1973); Blasius et al. (1977); Frey (1979); horst, and the youngest layers of its crest would Masson (1977, 1985); Schonfeld (1979); Anderson and Grimm (1998); Schultz (1991, 1995); Chadwick have been eroded. Alternatively, the mensa could and Lucchitta (1993); Peulvast and Masson (1993); consist of younger sediments deposited on the Mége and Masson (1996); Peulvast et al. (2001); down-dropped plains surface. Layered rocks in Wilkins and Schultz (2003); Bleamaster (2009) Combination of mechanisms Lucchitta et al. (1992); Tanaka (1997); Schultz (1998); the mensa extend almost as high as the chasma Lucchitta and Chapman (2002); Montgomery and rim. So if they are depositional, they must have Gillespie (2005); Montgomery et al. (2009) once fi lled the chasma. If they fi lled the chasma, 2 Geological Society of America Bulletin, Month/Month 2009 B30307 2nd pages / 3 of 32 Modeling collapse of Hebes Chasma, Mars all that fi ll except for the present mensa must of extensional structures on surrounding plains, (Sharp, 1973; Nedell et al., 1987).