Eastern Olympus Mons Basal Scarp: Structural and mechanical evidence for large‐scale slope instability

Abstract

The expression of the Eastern Olympus Mons Basal Scarp (EOMBS) is seemingly unique along the edifice. It exhibits two slope-parallel structures: a nearly 100 km long upslope extensional normal fault system and a downslope contractional wrinkle ridge network, a combination that is found nowhere else on Olympus Mons. Through structural mapping and numerical modeling of slope stability of the EOMBS, we show that these structures are consistent with landsliding processes and volcanic spreading. The EOMBS is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping overpressured confined aquifer and mechanical sublayer at depth. Failure of the fault-bounded portion of the flank results in estimated volumes of material ranging from 5600-6900 km 3 , or 32-39% of the estimated volume of the "East" Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains associated with the wrinkle ridges indicate that this portion of the edifice may have been tectonically active at < 50 Ma and may be coeval with estimated ages of adjacent outflow channels, 25-40 Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day.

Late Amazonian subsidence and magmatism of Olympus Mons, Mars

“…The relatively young Late Amazonian ages for the subsided basin flows generally correspond with other young ages derived for the calderas, flanks, and wrinkle ridges on and near Olympus Mons [ Neukum et al ., ; Basilevsky et al ., ; Robbins et al ., ; Hartmann and Neukum , ; Weller et al ., ]. The results of this study show that subsidence was also active during this late‐stage period of activity in response to the magmatism and that a substantial portion of the edifice was added during this period.…”

Flank vents and graben as indicators of Late Amazonian volcanotectonic activity on Olympus Mons

“…The superposed grabens on the lower flank imply a geologically recent radial extensional stress field applied to the lower portion of the edifice [ McGovern and Solomon , ; Byrne et al, ; Weller et al, ]. All observed grabens crosscut lava flows.…”

“…Alternatively, the existence of multiple magma reservoirs within the edifice cannot be ruled out. A décollement, a plane of mechanical weakness between two rock masses, has been hypothesized to underlay Olympus Mons [ Byrne et al, ; Weller et al, ]. Additionally, dike propagation, in conjunction with a detachment surface at depth, could also provide a mechanism for collapse of the volcano's base [ Elsworth and Day , ; Helgason , ; Kervyn et al, ].…”

“…The processes of gravity spreading or dike emplacement are not mutually exclusive, however. The occurrence of flank vents supports the emplacement of dikes; however, observed compressional features off the main shield suggest that gravity spreading linked to the underlying décollement is plausible also [ Weller et al, ]. Superposition relationships suggest that such an extensional stress regime occurred during waning Late Amazonian volcanic activity.…”

“…Alternatively, grabens formed in the plains surrounding Olympus Mons may have since been buried and not since reactivated. Other studies have identified recent instability within the basal escarpment in the form of flank collapse driven by radially directed extension, specifically on the eastern scarp, which is supported by volcano‐concentric shortening features in the plains surrounding Olympus Mons [ Weller et al, ]. The graben suggests that extensional deformation has occurred on Olympus Mons since the Late Amazonian, and an extensional stress field may be currently acting on the lower flanks of Olympus Mons today.…”