Structural Dynamics of Salt Systems

Physical modeling and detailed mapping of Hebes Chasma provide new insights into the crustal composition and origin of the Valles Marineris region of equatorial Mars.Hebes Chasma is a 315-km-long and 8-kmdeep closed depression containing distinctive landforms that include diapirs and extensive allochthonous fl ows that end in pits. A central puzzle of Hebes Chasma is how and where 10 5 km 3 of missing material disappeared. Our physical models tested the hypothesis that the chasma formed by collapse and removal of material from below. Gravity-driven collapse in the models reproduced all the chasma's main landforms as subsidence evolved from early sagging of the upper surface, to inward collapse and removal of material, to emergence of diapirs and low-gradient fl ows. The models and geologic evidence suggest that Hebes Mensa arched upward diapirically and raised deep stratigraphy almost level with the chasma rim. If the chasma indeed collapsed by subsurface drainage as occurred in the models, the upper 8-10 km of deposits at Hebes must have been solid to depths of ~5 km but viscous at greater depths. The materials removed could not have consisted mainly of basalt fl ows; instead, they probably were a mixture of hydrated and nonhydrated salts, water ice, liquid water, and insoluble (likely basaltic) particles. The proportions of these constituents are unknown but constrained because the material drained from the subsiding chasma and apparently contributed to the outburst fl oods released down neighboring Echus Chasma and Kasei Valles.

Section: Role Of Saltsmentioning

confidence: 67%

Modeling the collapse of Hebes Chasma, Valles Marineris, Mars

Jackson¹,

Adams²,

Dooley³

et al. 2011

“…Stratigraphically, the basin is divided into fi ve depositional megasequences: prerift, rift, transitional, transgressive marine, and regressive marine (Fiduk et al, 2004). Aptian-age salt has been deforming into diapirs, rollers, canopies, and thrusts (Jackson et al, 1994) from Albian until the present day. Since the Eocene, the Espirito Santo Basin has been a prograding shelf and slope environment (Fiduk et al, 2004).…”

Section: Geologic Settingmentioning

confidence: 99%

Trails of depressions and sediment waves along submarine channels on the continental margin of Espirito Santo Basin, Brazil

Heiniö

Davies

2009

Trails of depressions and associated sediment waves that follow the courses of underlying erosional submarine channels in Neogene strata in the Espirito Santo Basin on the Brazilian continental margin are described here. The features were imaged using threedimensional seismic data, and they have a wavelength of several hundred meters and heights of tens of meters. Individual depressions are also found above knickpoints, scarps, and irregular top surfaces of masstransport complexes. We propose that the depressions form initially as fl ute-shaped troughs of channel-confi ned sediment waves and develop into more circular depressions as a result of deposition on the shallow, upslopefacing stoss fl anks and erosion or nondeposition in the lee fl anks and the troughs. Depressions above topographical irregularities are interpreted to form when downslope-fl owing currents encounter a break in slope, leading to excavation of a depression at the break and deposition of sediment downslope. This mechanism may be analogous to the formation of plunge pools and base-of-slope depressions. Similar features have been observed previously in several bathymetric data sets as well as created in numerical models and are thus considered ubiquitous in submarine settings.

“…Hamblin (1965) stated what has become the most widely accepted conceptual model for reverse drag wherein a hanging-wall fold develops in kinematic response to a fault with decreasing dip with depth (listric profi le). The common occurrence of hanging-wall folds in extending deltas where faults have listric profi les associated with salt or shale detachments (Jackson et al, 1994;Morley and Guerin, 1996) has often been cited as support for this conceptual model. The presence of reverse-drag folds has thus been used as evidence to support the interpretation of listric normal fault geometry at depth when the fault geometry is unknown (Shelton, 1984), and a number of authors have developed kinematic models of normal fault deformation that relate the geometry of hanging-wall strata to fault shape through geometric constructions (Dula, 1991;Gibbs, 1983;Groshong, 1994;Medwedeff and Krantz, 2002;Verrall, 1981;White et al, 1986;Xiao and Suppe, 1992).…”

Section: Introductionmentioning

confidence: 99%

Deformation associated with a continental normal fault system, western Grand Canyon, Arizona

Resor

2008

Reverse-drag folds are often used to infer subsurface fault geometry in extended terrains, yet details of how these folds form in association with slip on normal fault systems are poorly understood. Detailed structural mapping and global positioning system (GPS) surveying of the Frog Fault and Lone Mountain Monocline in the western Grand Canyon demonstrate a systematic relationship between elements of the normal fault system and fold geometry. The Lone Mountain Monocline, which parallels the Frog Fault, is made up of two half-monoclinal fl exures: a hanging-wall fold in which dips gradually increase toward the fault over ~1.5 km reaching a maximum dip of 25° and a footwall fold in which dips decrease away from the fault over ~0.5 km from a maximum of 12°. The highest dips associated with folding are found where throw on the Frog Fault and a synthetic fault are at a maximum. Lower dips are found where there is less throw on the Frog Fault and antithetic faults are present. This relationship between fault and fold geometry suggests that the folding is associated with Basin and Range extension rather than Laramide contraction. Mechanical models of normal faultrelated deformation predict similar patterns of folding over planar faults of fi nite extent and corroborate the important role of subsidiary fault geometry in the overall pattern of deformation. Application of these models in an inverse sense yields ~1 km estimates of down-dip extent, a result that may indicate fault confi nement within the sedimentary section or weakening effects associated with folding layered strata. A general analysis illustrates that reverse-drag folds of moderate dip are expected to form in association with slip on planar faults of fi nite extent-a result that has the potential to impact our estimates of hydrocarbon volumes, crustal extension, and earthquake hazards associated with continental normal faults.