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

2 Geotechnical characteristics and slope stability along the Uruguayan and northern Argentine margin
(Sosa 1998). Recent documented earthquakes occurred in the years of 1849, 1888, and 1988 A.D. (for
locations of epicenters, see Fig. 2.1A, Sosa 1998). The epicenter and magnitude of the 1988 earthquake
is not conclusively defined. Seismological observatory of the University of Brazil showed the epicenter
located 36.5°S, 53.5°W, +/- 100 km with the regional magnitude of 3.9, whereas, NEIC (National
Earthquake Information Center) recorded the epicenter of 36.27°S, 52.73°W with a body wave
magnitude of 5.1 m b (Sosa 1998). Here we choose the seismic parameters as the epicenter of 36.27°S,
52.73°W with a body wave magnitude of 5.2 m b following Assumpção (1998).
Based on the seabed morphology, our study area is separated into two distinct subareas: northern mass
movements dominated area (NS) on the Uruguayan slope and southern canyons dominated area (SC)
on the northern Argentine slope. Krastel et al. (2011) refer to NS as the “northern slide area” and
“drift-and-scarp area”. NS is characterized by a smooth topography and gentle slopes (1-3°) typical of
margins where deposition prevails over erosion (Ewing and Lonardi 1971). Several scarps (Fig. 2.1B,
C) and mass transport deposits (MTDs) (Figs. 2.2A, A’, B, B’) at 1200-2800 m were found in NS
(detail descriptions see Krastel et al. 2011). SC is characterized by the flat Ewing terrace, which is
dissected by canyons at ~1400 m (e.g., Mar del Plata Canyon and Querendi Canyon, Fig 2.1D). The
slope below the Ewing terrace is steep (3-7°), suggesting that erosion has been the main process
shaping the slope of SC (Ewing and Lonardi 1971). No clear MTDs were found in the sedimentary
sequences upper slope and flanks of the Querendi Canyon (Fig. 2.2C, D). However, MTDs were
imaged in the thalweg of the Querendi Canyon, which suggests headward erosion is common in this
canyon (Fig. 9 of Krastel et al. 2011)
At present, sedimentation in the study area is strongly influenced by high fluvial discharge of the Rio
de la Plata River and strong bottom current forces. The sediments of the study area are generally
divided into a clayey silt NS part and sand SC part (Huppertz 2011). The boundary between the two
environments is located at the position of the Brazil Malvinas confluence (BMC, ~37°S). North of the
BMC, high quantities of terrigenous sediments (~80×10 6 ton/yr) discharged by Rio de la Plata River,
containing 75% coarse to medium silt, 15% fine to very fine silt and 10% clay (Giberto et al. 2004), are
swept northwards by alongshore currents and deposited at the Uruguayan continental shelf and slope
(Piola et al. 2005); sediment transport is dominated by the southward flow of North Atlantic Deep
Water (NADW) at the Uruguayan slope in water depths between 2000-4000 m. South of the BMC,
coarse fluvial sediments are trapped with the estuary and occasionally carried directly down slope by
turbidity current (Garming et al. 2005); most of the surficial sediments at the northern Argentine slope
have been carried and reworked northward by water masses in different depths including the Antarctic
Intermediate Water (AAIW, ~500-1000 m), the Circumpolar Deep Water (~1000-3500 m) and the
Antarctic Bottom Water (AABW, >3500 m) (more details, see Piola and Matano 2001).
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