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Influence of Gravity on the Geometry of Martian Normal Faults

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Abstract

Consideration of the effects of gravity on lithostatic stress on Mars indicates that dilational faulting found in the upper 2 km on Earth may extend to depths of 5 km on Mars.

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... [4] We have identified seven candidate cave entrances (skylights) located on or near the flanks of Arsia Mons (southernmost of the three massive shield volcanoes of Tharsis Montes,Figure 1). This region has widespread pit craters and grabens, suggesting an abundance of subsurface void spaces [Ferrill et al., 2003; Wyrick et al., 2004]. These candidates may have formed in a manner comparable to pit craters (which are usually found nearby) except that an area of competent surface materials may have remained intact to form a ceiling as subsurface materials collapsed and drained into the subterranean voids below (seeFigure 2 and auxiliary material). ...
... [12] Most of the candidates are either adjacent to pit craters, or are directly in-line with pit-crater chains, suggesting similar formation processes, though pit craters consistently show distinct walls that slope inward at the angle of repose ($30°). Recent investigations and some terrestrial analogues suggest these pits are likely caused by the drainage of loosely consolidated surface materials into deep extensional fractures or faults that could reach down to 5 km into the crust [Ferrill et al., 2003; Wyrick et al., 2004]. [13] Some terrestrial pit craters found in Hawaii are visibly similar to our candidates [Okubo and Martel, 1998]. ...
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1] Seven possible skylight entrances into Martian caves were observed on and around the flanks of Arsia Mons by the Mars Odyssey Thermal Emission Imaging System (THEMIS). Distinct from impact craters, collapse pits or any other surface feature on Mars, these candidates appear to be deep dark holes at visible wavelengths while infrared observations show their thermal behaviors to be consistent with subsurface materials. Diameters range from 100 m to 225 m, and derived minimum depths range between 68 m and 130 m. Most candidates seem directly related to pit-craters, and may have formed in a similar manner with overhanging ceilings that remain intact.
... Fig. 10. Schematic illustration of steep dilated faults and their potential relationship to pit chain development (after Fig. 2 of Ferrill et al., 2003). Compare pits in illustration to pits (cavus) and pit chains (cavi) shown in Figs. ...
... Collapse pits likely formed at the source during or after the fluvial episode as surface material dropped into subsurface cavities, was lowered by graben formation, or was carried away with the flow of water . Fig. 10 (after Ferrill et al., 2003) schematically illustrates steep dilated faults on Mars and their potential relationship to pit chain development. This kind of fault system would likely have created highly anisotropic conditions in the aquifer system , greatly enhancing the eastward flow of groundwater. ...
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A number of martian outflow channels were carved by discharges from large dilational fault zones. These channels were sourced by groundwater, not surface water, and when observed on high-standing plateaus they provide indicators of elevated paleo-groundwater levels. We identify three outflow channels of Hesperian age that issued from a 750-km-long fault zone extending from Candor Chasma to Ganges Chasma. Two of these channels, Allegheny Vallis and Walla Walla Vallis, have sources >2500 m above the topographic datum, too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region. The indicated groundwater levels likely required regional sources of recharge at low latitudes. The floodwaters that erupted from Ophir Cavus to form Allegheny Vallis encountered two ridges that restricted the flow, forming temporary lakes. The flow probably breached or overtopped these obstructions quickly, catastrophically draining the lakes and carving several scablands. After the last obstacle had been breached, a single main channel formed that captured all subsequent flow. We performed hydrologic analyses of this intermediate phase of the flooding, prior to incision of the channel to its present depth. Using floodwater depths of 30–60 m, we calculated flow velocities of 6–15 m s−1 and discharges in the range of 0.7–3×106 m3 s−1. Locally higher flow velocities and discharges likely occurred when the transient lakes were drained. Variable erosion at the channel and scabland crossing of MOLA pass 10644 suggests that the upper 25–30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock. We infer that an ice-covered lake with a surface elevation >2500 m probably existed in eastern Candor Chasma because this canyon is intersected by the Ophir Catenae fault system from which Allegheny Vallis and Walla Walla Vallis originated. We introduce a new hydrology concept for Mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in Tharsis by effusions of flood basalts. This model could help to reconcile the expected presence of a thick cryosphere during the Hesperian with the abundant evidence for groundwater as a source for some of the circum-Chryse outflow channels.
... The presence of a large quantity of pits in proximity to the radarbright mountains suggests the two may be related (Adams and Jurdy, 2012). Both extension fracturing and dilation faulting result in subsurface voids into which loose material on the surface can drain, creating linear assemblages of pits known as "pit chains" (Tanaka and Golombek, 1989;Ferrill et al., 2003;Wyrick et al., 2004;Martin et al., 2017). Radebaugh et al. (2007) describe the sub-parallel mountain ranges of T8 as possible extension features, although Liu et al. (2016aLiu et al. ( , 2016b supersedes this idea and calls for contraction using methane ground fluids as lubricant. ...
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We mapped in detail Titan's South Belet region which spans from longitude 60°E to 120°E and from latitude 60°S to 0°, encompassing both equatorial and southern mid-latitude regions. We used Cassini RADAR in its Synthetic Aperture Radar (SAR) mode data as our basemap, which covers 31.8% of the region, supplemented with data from the RADAR's radiometry mode, the Imagining Science Subsystem (ISS), the Visual and Infrared Mapping Spectrometer (VIMS), and topographic data. This mapping work is a continuation of the detailed global mapping effort introduced in Malaska et al. (2016a) and continued in Lopes et al. (2020). We followed the mapping procedure described in Malaska et al. (2016a) for the Afekan Crater region and identified four major terrain classes in South Belet: craters, hummocky/mountainous, plains, and dunes. Each terrain class was subdivided into terrain units by characteristic morphology, including border shape, texture, general appearance, and radar backscatter. There are two terrain units that were not included in previous studies but were identified in our mapping of South Belet: “bright alluvial plains” and “pitted hummocky”. Similar to the Afekan Crater region, we find that plains dominate the surface make-up of South Belet, comprising ~47% of the mapped area. Unlike Afekan, the areal extent of the dunes closely rivals the dominance of plains, making up 43% of the mapped area. The next most widespread unit by area in the region following the dunes are the mountains/hummocky terrains (10%), and finally, crater terrains (0.01%). The introduction of two new units, “bright alluvial plains” and “pitted hummocky”, are necessary to capture the full range of morphologies seen in South Belet and expands our understanding of processes typical of Titan's equatorial and mid-latitude regions. For example, the presence of alluvial fans indicates a period in Titan's past where discharges and slopes were such that sediment could be mobilized and deposited. Similarly, the pits associated with the “pitted hummocky” may represent an important erosional feature, with implications for the removal of volatiles from Titan's crust. However, analysis of our geomorphological mapping results suggests the geology of South Belet is consistent with the narrative of organics dominating the equatorial and mid-latitudes. This is similar to the conclusion we arrived at through our mapping and analysis of the Afekan region. Lastly, the applicability of the terrain units from our mapping of the Afekan region, which bears a similar latitude but in the northern hemisphere, to our mapping of South Belet suggests latitudinal symmetry in Titan's surface processes and their evolution.
... The possibility of caves on Mars came much later. In 2003, planetary scientists with the Southwest Research Institute and the University of Texas, San Antonio hinted at the possibility of speleogenesis on Mars while discussing geological faulting and associated processes [2]. Over the past 10 years, caves have been discovered nearly everywhere in our solar system. ...
... The possibility of caves on Mars came much later. In 2003, planetary scientists with the Southwest Research Institute and the University of Texas, San Antonio hinted at the possibility of speleogenesis on Mars while discussing geological faulting and associated processes [2]. ...
... In this case, we would expect to find graben and other extensional features in the area. Ferrill et al. (2003) and Wyrick et al. (2004) described how dilational faulting can form pit chains similar to extensional fractures where collapses occur into voids created by such faulting. ...
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We will be examining the distribution of pits in the equatorial region of Titan by using chi-squared and fractal analyses. This could lead to the development of a model for pit formation.
... As part of our proof-of-concept study, several possible cave-like features (a.k.a. the seven sisters) were identified on the northern flank of the Arsia Mons volcanic field [Cushing et al., 2007]. Surrounding topography for most of the features is characterized by collapse pits and grabens [Ferrill et al., 2003;Wyrick et al., 2004]. On Earth, collapse pits are often associated with lava tubes and form when the cap rock subsides and the rock and sediment collapse into the void. ...
... Halliday (1966) suggests lunar caves may also provide access to water ice, as well as mineral deposits developed by volcanic activity. On Mars, features associated with speleogenesis (e.g., Ferrill et al., 2003; Wyrick et al., 2004; Halliday, 2007) and actual cave-like features (e.g., Cushing et al., 2007) have been confirmed. These features are considered important for the exploration of life (Mazur et al., 1978; Boston et al., 1992; Klein, 1998; Grin et al., 1998 Grin et al., , 2001 Boston et al., 2001) because they offer protection from low surface temperatures, unfiltered ultraviolet radiation (Mazur et al., 1978; Klein, 1998) and violent windstorms, which may degrade and decompose organic materials . ...
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Inversion of Martian topography, using a three-dimensional boundary element model, permits revised estimates of fault dip angle, depth of faulting, paleogeothermal gradient, and extensional strain in the Valles Marineris region. The major normal faults dip at 40°-55° to depths of ∼60-75 km with comparatively small footwall uplifts (< 500 m for throws to 10 km), implying that paleogeothermal gradients during faulting were ∼10 K km-1 or less, assuming relatively rapid strain rates appropriate to the extending Martian crust. Accumulation of dip-slip offsets along the main normal faults likely was associated with anticlinal flexure and deformation of preexisting strata within the troughs. The magnitudes of the predicted vertical and horizontal displacements outside the troughs, as well as extensional strains, are spatially variable, depending on both cross- and along-strike position relative to the troughs. The predicted displacements and strains attain maximum values at the troughs and decay to background values at cross-strike distances of ∼250 km, corresponding to 3-4 times the depth of faulting, with an average province-wide strain of 4-15%. By implication, the aggregate strain field surrounding Tharsis is a highly variable spatial and temporal composite of the inhomogeneous strain fields, each associated and scaling in size with an individual graben, from the smallest structures through the largest Valles Marineris troughs.