Ophir Chasma Dyke Swarm
Evidence of contribution of erosion to northern chasma formation
The whole material below was presented at the 7th International Dyke Conference. The corresponding abstract and poster can be downloaded on the V-MACS publications page.
An extensive survey of dykes in the Valles Marineris troughs (chasmata) on Mars has been undertaken. As shown in another page, most dykes are observed along the chasma walls in the spur-and-gully morphology that characterises the bedrock exposures (Lucchitta et al., 1992). In this page, a dyke swarm exposed in the central Ophir Chasma floor is reported, which has major implications for the formation mechanisms of Valles Marineris.
Grabens and ancestral basins
Figure 1. Location of the Ophir Chasma Dyke Swarm, and boundary between two Valles Marineris areas that require two distinct chasma formation mechanisms.
The southern Valles Marineris chasmata have a clear structural origin (e.g., Schultz, 1991; Peulvast et al., 2001). The northern chasmata are oval-shaped ("ancestral basins", Schultz, 1998) and for this reason, alternative mechanisms of formation have been sought, such as collapse into deep fractures (Tanaka and Golombek, 1989) or subsidence (Andrews-Hanna, 2012). Ophir Chasma belongs to the northern chasmata (Figure 1).
Zoom to an ancestral basin: Ophir Chasma
The floor of Ophir Chasma displays a dense network of dykes (Figure 2), which can be observed on CTX images in the visible spectral range (5 m/pixel) and at HiRISE resolution (25 cm/pixel). The thickest dykes are apparent on THEMIS thermal infrared images (100 m/pixel). Many of those that can be mapped at HiRISE resolution are several tens of meters thick (Figure 3).
Figure 2. The Ophir Chasma dyke swarm area seen in (a) panchromatic (CTX image mosaic, 5m/pixel); (b) thermal infrared (Themis night IR, 100 m/pixel); colour composite (HRSC, 20 m/pixel). Dykes cannot be identified at these scales, although a few E-W-trending lineaments are observed.
Figure 3 - HiRISE images (25 cm/pixel) reveal dykes oriented WNW to NW-SE on the central part of the floor of Ophir Chasma. These dykes are tens of metres thick (full analysis is in progress).
Ophir Chasma Dyke Swarm mapping
Figure 4. Half the area is currently covered by HiRISE images at a resolution of 25 cm/pixel (footprints in dashed white boxes). A dyke map could be drawn based on these images. CTX images (5 m/pixel, background map) were used in HiRISE data gaps.
Implication for northern Valles Marineris chasma formation
Dyke thickness primarily depends on the Young's modulus of the host rock (e.g., Gudmundsson and Loetveit, 2005), which increases with hydrostatic pressure, hence globally, with depth. The widespread occurrence of dykes several tens of meters thick on the floor of Ophir Chasma suggests that the current exposure level is closer to the level of neutral buoyancy of Martian mafic magmas, estimated to ca. 11 km (Wilson and Head, 1994), than to the surface. It implies that the exposed chasma floor has been intensely eroded.
Figure 5 - Observation of thick dykes requires significant erosion. Tectonic subsidence certainly occurred due to the extensional setting of Valles Marineris, and to dyke dilation itself (dilation at depth must have been counterbalanced by tectonic stretching at surface), but alone would rather mask than expose dykes.
The dykes are correlated with abundance of olivine (b) and absence of phyllosilicates (c). Their white and yellow colours in (d) and (e), respectivement, indicate that pyroxenes are more high-Ca than low-Ca. The blue area in the northeastern corner of (e) corresponds to dark sands.
Figure 6 - Spectral indexes (Pelkey et al., 2007; Salvatore et al., 2010) applied to CRISM data in the Ophir Chasma Dyke Swarm area (location on Figure 4)
Carving by erosional agents must have been significant in chasma formation
Was it liquid water?
In most geomorphological systems, in which water is the main weathering agent, large depressions are the locus of thick sedimentary infillings. Depressions that match the dimensions of the Valles Marineris chasmata on Earth include rifts as well as foreland basins, which are commonly filled by kilometers of sediments. Such systems are not adapted to Ophir Chasma, since such a system would deeply bury any dyke intruded in the basement.
Mège and Bourgeois (2011) and Gourronc et al. (2014) showed that the current Valles Marineris landscape has been primarily shaped by glacial activity, during a period which is not well constrained but could have lasted for hundreds of million years or more during the Hesperian and/or Amazonian epochs. We suggest that glacial erosion may be the main cause for Valles Marineris trough formation in Ophir Chasma. Thomson et al. (2013) reported 1600-2500 m of bed erosion since 34 Ma in the trough below the Lambert glacier East Antarctica (0.047 – 0.073 mm/yr). At a similar rate of 0.050 mm/yr in Valles Marineris, 8000 m of cumulated erosion (the elevation difference between the Ophir Chasma floor and the surrounding plateau) would be achieved in only ca. 160 my. However, on Earth, glacier bed erosion rate may be significantly faster in some cases: 300–600 mm/yr of subglacial erosion has been measured below Pine Island Glacier, West Antarctica, over 49 years, a high rate achieved by fast flowing ice and the presence of soft water-saturated sediments, (Smith et al., 2012). Up to 1 m/yr has been interpreted at the Rutford Ice Stream (Smith et al., 2007).
Glacier bed erosion by several thousands of meters in Valles Marineris troughs would therefore not be exceptional, nor irrealistic in terms of required time. However, erosion of several thousand meters of glacier bed is more easily achieved by multiple cycles of ice flow, glacier bed deepening, ice melting (Earth) or sublimation (more likely in common Mars conditions), and isostatic rebound. Such a cyclicity has been observed in Antarctica and has been attributed to orbital changes (Pekar and DeConto, 2006). Orbital cycles are exacerbated on Mars (Laskar et al., 2004), due to the absence of orbit stabilisation by a heavy natural satellite such as the Earth's Moon. Multiple glacial erosion cycles, the terms of which remain to be explored, may have vigorously contributed to erosion and deepening of the Ophir Chasma floor.
Conclusion: Chasma carving by ice stream erosion
The current V-MACS results suggest that the floor of the central Ophir Chasma may have followed an evolution similar to the bedrock of Antarctic ice streams (Figure 7). Its current low elevation would result from combination of limited tectonic stretching and subglacial erosion over several kilometres.
Figure 7 - A former ice stream may have filled Ophir Chasma (b) and eroded the bedrock over several kilometres, following processes that are currently active in Antarctica, with (a) the example of the Rutford Ice Stream, which currently has subglacial bed erosion rate up to 1 m/year (Smith et al., 2007).
This work shows that erosion, perhaps glacial erosion, may have been the main mechanism by which Ophir Chasma, and perhaps other northern chasmata of Valles Marineris, may have formed. The dyke density on the Ophir Chasma floor testifies, however, to significant crustal dilation, implying significant extensional tectonics too. The first step in the formation of Ophir Chasma is thus interpreted to have been tectonic stretching, and the second step, glacier bed erosion, resulting in several kilometers of additional topographic lowering. Other Valles Marineris northern chasmata might have formed in a similar way.
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