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MAGMATISM (NOT GROUNDWATER RELEASE) CAUSED SURFACE COLLAPSE in GANGES CAVUS, and LIKELY OTHER LOCATIONS on MARS. J. R. Michalski

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MAGMATISM (NOT GROUNDWATER RELEASE) CAUSED SURFACE COLLAPSE in GANGES CAVUS, and LIKELY OTHER LOCATIONS on MARS. J. R. Michalski 51st Lunar and Planetary Science Conference (2020) 1173.pdf MAGMATISM (NOT GROUNDWATER RELEASE) CAUSED SURFACE COLLAPSE IN GANGES CAVUS, AND LIKELY OTHER LOCATIONS ON MARS. J. R. Michalski, Division of Earth and Planetary Science, University of Hong Kong, Hong Kong, China. [email protected] Introduction: Mars contains a large number of yet islands [4-5] but these features could potentially be unexplained collapse features, sometimes spatially caused by flowing low-viscosity lava or pyroclastic linked to large channels. These pits and cavi are often flows [6]. In fact, deposits of Elaver Vallis with Ganges taken as evidence for collapse due to the release of large Chasma are also enriched in olivine, suggesting they are volumes of pressurized groundwater [1-3]. One such either mobilized volcanics or actual deposits of primary feature, Ganges Cavus, is an extremely deep (> 6 km) volcanic flows. collapse structure nested on the southern rim of Morella Though Colman [5] championed the catastrophic Crater, a 78-km-diameter impact structure breached on flooding model in this location, he points out that it its east side by the Elaver Vallis outflow channel [4] would not be possible to reach the necessary hydraulic (Figure 1). Previous workers have concluded that Gan- head required in the current topographic setting because ges Cavus, and other similar collapse features in the the ~6 km-deep, adjacent Ganges Chasma would have Valles Mariners area formed due to catastrophic release likely resulted in outbursts in the canyon floor or walls. of pressurized groundwater that ponded and ultimately This is less problematic in a magmatic model, driven by flowed over the surface [5]. That hypothesis seems un- surface collapse (Figure 3) and I point out that the floor likely and possibly untenable based on detailed obser- of Ganges Chasma also contains volcanic materials sim- vations. Here I present the case that collapse was driven ilar to those observed in Morella crater. by magmatic/volcanic processes, and suggest that chan- In the case of Ganges Cavus-Morella Crater, I show nel erosion might have been linked to volcanism. that the groundwater hypothesis is not necessary, and Methods: The geology of Morella Crater was stud- likely untenable as a sole or driving factor in the for- ied primarily using remote sensing data from the follow- mation of collapse features – though it is certainly pos- ing missions: Mars Global Surveyor (MGS), Mars Od- sible that magmatism in the presence of ground ice yssey (MO), Mars Express (MEx), and Mars could have mobilized meltwater [7]. Instead, collapse of Reconnaissance Orbiter (MRO). Data were obtained Ganges Cavus and the geology of Morella Crater can be and organized using the JMARS software developed explained in terms of its volcanic and magmatic history. and operated by Arizona State University. Topography Morella crater contains >3400 km3 of olivine-rich lava was analysed using MOLA data merged with elevation and ash, which postdates channel features and predates derived from the MEx HRSC data. Geomorphology was the collapse of Ganges Cavus. The cavus is, in effect, an evaluated using visible and thermal infrared images unusual type of caldera. It is likely that the large col- from THEMIS, HRSC, CTX and HiRISE. Surface min- lapse volume (>2100 km3) can be explained in terms of eralogy was investigated using data from OMEGA and magma extraction (Figure 3), both by volcanic eruption CRISM. and by lateral migration of magma in the subsurface to Results and Conclusions: The geology of Morella the north where it may have been erupted into Ganges crater and Ganges cavus is dominated by evidence for Chasma. The geologic puzzle of Morella Crater and volcanism. The minerals/lithologies present all point to Ganges Cavus has important implications for the origins olivine-bearing or olivine-rich mafic to ultramafic ma- of other collapse structures on Mars and challenges the terials. A clear view of the Morella crater fill deposits is idea of pressurized groundwater release on Mars (Figure exposed on the north wall (south-facing wall) of Ganges 4). Throughout the region, the association of NW- cavus, where a >1 km-thick deposit of olivine-rich ma- striking fracture systems, extension, collapse, and vol- terial crops out. Within this deposit, HiRISE shows lay- canic mineralogy area all indisputable. It is likely that ering (Figure 2) and high-resolution thermal inertia of collapse is driven by magmatism, but that magmatism the material shows many packages of low-inertia and also drives permafrost melting, which plays a role in high-inertia layers. Elsewhere in Morella crater, a series shaping the landscape. of low-lying as well as perched, draping deposits of var- References: [1] Carr, M. H. (1979) JGR, 84, 2995- iable thermal inertia and olivine-rich composition are 3007. [2] Baker, V. R. et al. (1974), Icarus 23, 27-41. observed suggesting a series of episodes of volcanism. [3] Marra W. A. et al. (2015), GeoResJ 8, 1-13. [4 ] Col- The morphology, mineralogy, thermal inertia and man et al. (2007), Icarus, 189 (2), 344-361. [5] Colman, context all suggest a combination of explosive and effu- N. M. (2013). JGR, 118 (2) ,263-277. [6] Leverington, sive volcanic units. The main evidence for water seems D. W. (2004), JGR 109 (E10011). [7] Rodriguez, J.A.P to rest in the occurrence of channels and streamlined et al. (2003), 8th Int. 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