The trough system of Valles Marineris, Mars contains numerous landslides in the large relief exposures (up to 8 km) of basaltic wall rock and soft interior layered deposits (ILDs). Eleven landslides, including eight circular failures, were mapped in the ILDs. Two wall rock landslide complexes and four ILD landslides were modelled using limit-equilibrium slope stability analysis to evaluate the mechanisms of slope failure. Wall rock landslide complexes in the Ophir and Hebes Chasmata required artesian fluid pressures of at least 41% of overburden pressure or ground accelerations of at least 0.19 Mars g for failure. Agreement between modelled and observed failure surface geometries and the difficulty of generating artesian pressures at a regional topographic high indicate that ground shaking from Marsquakes or impacts most likely triggered the modelled wall rock landslides. Triggering mechanisms, such as ground acceleration or fluid pressure, were also necessary for most other landslides in Valles Marineris wall rock. Given minimum rock mass strengths, ILD landslides in the Hebes and East Candor Chasmata did not require triggering mechanisms; thus, landslides in ILDs may be a result of gravitational, fluid, or seismic loading.

Pyramid Mountain is a subglacial volcano in Wells Gray Provincial Park in east-central British Columbia. Landslides deform the north and east flanks of the volcano. Field strength testing and rock mass classification designate the hyaloclastite breccia in which the landslides originated as a weak, massive rock mass: uniaxial compressive strengths (UCS) range from 24 to 35 MPa, and geologic strength index (GSI) and rock mass rating (RMR) values are 60-70. The shear strength of fracture surfaces in the hyaloclastite breccia, as measured by laboratory direct shear tests, can be characterized by a friction angle φ of 18°and cohesion c of 0.11-0.66 MPa. Limit-equilibrium slope stability analyses show that the landslides were probably triggered by the rapid drawdown of a surrounding englacial lake with no seismic ground acceleration required. Slope measurements and slope stability modeling indicate that Pyramid Mountain was asymmetric prior to failure: the north and east flanks had slope angles of 35°-40°, and the south and west flanks had slope angles of 21°-33°. Slope asymmetry may result from closer ice confinement on up-gradient (north and east) flanks due to higher ice flux in this direction relative to down-gradient (south and west) flanks. At the time of failure, the volcanic edifice was at least partially lithified, with cohesive strengths of 0.19-0.52 MPa. Failures of lithified subglacial and subaqueous volcanic edifices may be triggered by rapid drawdown of surrounding water without seismic loading.Résumé : Le mont Pyramid est un volcan infraglaciaire du parc provincial Wells Gray, dans le centre-est de la ColombieBritannique. Les flancs nord et est du volcan sont déformés par des glissements de terrain. Selon des essais de résistance effectués sur le terrain et la classification de la masse rocheuse, la brèche de hyaloclastite, où les glissements de terrain ont pris naissance, est un amas rocheux massif et faible : la résistance à la compression uniaxiale varie entre 24 MPa et 35 MPa, tandis que les valeurs fournies par l'indice GSI (« Geologic Strength Index ») et le système RMR (« Rock Mass Rating ») se situent entre 60 et 70. La résistance au cisaillement des surfaces de fracture de la brèche de hyaloclastite, telle que mesurée en laboratoire par des essais de cisaillement direct, se caractérise par un angle de frottement (φ) = 18°et une cohésion (c) = 0,11-0,66 MPa. Des analyses de stabilité des pentes selon une méthode de calcul à l'équilibre limite indiquent que les glissements de terrain ont probablement été provoqués par le rabattement rapide d'un lac intraglaciaire environnant, sans intervention de mouvement sismique. Les mesures des pentes et la modélisation de la stabilité des pentes indiquent qu'avant l'effondrement, le mont Pyramid était asymétrique : les pentes des flancs nord et est étaient comprises entre 35°et 40°et celles des flancs sud et ouest étaient comprises entre 21°et 33°. Cette asymétrie des pentes pourrait être le résultat d'une plus grande proximité du confinement de la glace sur ...