Slope stability along active and passive continental margins ... - E-LIB

2 Geotechnical characteristics and slope stability along the Uruguayan and northern Argentine margin
(up to 100 m) and acoustically transparent sediment bodies deposited at the foot of scarps. Northward
bottom currents, which have carried the fine-grained sediments from the Rio de la Plata River, decrease
in velocity in northward flowing direction so that clayey contourites are favored on the slope of NS.
The geometry and structures of the NS slope failure complex is explained as retrogressive slide system
with subsequent failures having progressively migrated farther upslope (Winkelmann et al. 2011).
Slope stability analyses suggest that NS is currently stable and is unlikely to experience repeated
small-scale failure but may reach unstable conditions if external triggers (e.g., earthquake) are strong
enough to trigger large-scale failure. These results thus are consistent with general model of low
gradient continental slope characterized by high sedimentation of fine-grained sediments and
infrequent large-scale failures. In contrast, the continental slope between 1400-2500 m of SC is
characterized by steep slopes (3-7°) and more abundant coarse-grained sediment on the flat Ewing
terrace, which in the upper slope is dissected by the Querendi Canyon. In this area, here-in presented
slope stability analyses attest the generally low stability, where steep slopes reflect repeated small-scale
slope failures both during static conditions and certainly during infrequent seismic events. It is
interpreted that high current velocities are responsible for deposition of sandy layers on the surface of
upper slope of the canyon. Mass transport deposits are found in the mouth of the canyon but no distinct
scars and mass transport deposits were found on the upper slope of the canyon (Krastel et al. 2011).
This suggests that small slope failures continuously occur on the steep headwall, as also reveals in the
slope stability calculation. Thereby the flanks of the canyon are constantly shaped by canyon wall
incision and laterally retrograding into the open slope.
2.5.4 Slope failures along the Uruguayan and northern Argentine margin versus
slope failures on other passive margins
An important result from this study is the findings that the investigated slopes along the Uruguayan and
northern Argentine margin are mostly stable under static loading conditions and that slope failure can
be initiated by additional driving forces during earthquake shaking. These findings suggest that
earthquakes may play a critical role in initiating slope failure even on passive margin characterized by
generally low seismicity levels. Similar conclusions have been reached from infinite slope stability and
pseudostatic earthquake analysis on the slope instabilities of passive margins worldwide, such as upper
slope of northeastern Australia (Puga-Bernabéu et al. 2013), continental slope off Uruguay (Henkel et
al. 2011), northern Gulf of Mexico (Stigall and Dugan 2010), and Norwegian continental slope
(Storegga slide; Leynaud et al. 2009; Leynaud et al. 2004). According to different precondition factors
in different study areas (such as slope gradient, overpressure, grain size, weak layer etc.), critical peak
ground acceleration generated by earthquake needed to initiate slope failure range between 0.02 g (Gulf
of Mexico; Stigall and Dugan 2010), 0.1-0.2 g (Storegga slide; Kvalstad et al. 2005). These case
studies further attest that overpressure generated by rapid sedimentation rates is important to
precondition the slope towards low stability (e.g., Sultan et al. 2004), but in most cases overpressure
alone would not have driven slope failure (Stigall and Dugan 2010; Kvalstad et al. 2005). Assuming
overpressure ratios are 0.5-0.6 (estimated using Gibson’s equation integrated consideration of
sedimentation rate and permeability) on the upper 50 mbsf in the NS area of our study, PGAs
exceeding 0.19-0.2 g is required to trigger sediment failure, suggesting comparable “triggering
conditions” as for the Storegga landslide. In the Storegga region, weak layers have been identified in
contouritic deposits that formed during interglacial periods and were rapidly buried under thick glacial
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