Sonar images of the path of recent failure events on the continental margin off Nice, France

Abstract

When examined in plan view with side-looking sonar, the path of recent sub-sea failure events on the continental margin off Nice displays morphologic features indicative of the passage of sediment flows that interacted with the sea-bed substrate. The path begins as chutes incised on a steep prodelta slope. These chutes feed into the floor of a submarine canyon, which is covered with trains of bedforms oriented at right angles to the flow direction. The bedforms have wavelengths between 35 and 100 m and heights estimated at less than 5 m. The waves are made of coarse material ranging from sand to boulders. Giant bedforms of similar character and scale in the Channeled Scabland (eastern Washington) were produced by a sudden catastrophic flood. Near the base of the slope, where the canyon floor widens and the thalweg gradient decreases, the path becomes a scoured surface with depressions up to 70 m deep produced by the erosion of bedded sediments. As the bottom slope decreases, the width of the area scoured and the cross section of the flows increase. Along the 80 km of the path that was imaged, which was entirely upslope from the location where submarine cables were disrupted by a failure event in 1979, there is little sign of deposition, but extensive evidence of substrate reworking. Our plan-view study of a present-day surface and its bedforms offers an instantaneous picture of an unconformity in development that contrasts with the classical study of similar unconformities in cross section where the emphasis is placed on the time dimension.

Evidence of slope instability in the Southwestern Adriatic Margin

“…In these cases, submarine failure may provide a mechanism to enhance the tsunami wave with respect to what would be generated solely by pure co-seismic dislocation (Tappin et al, 2001;Synolakis et al, 2002). In some cases, tsunami are known to occur in response to sediment failure only, in the lack of a concurrent seismic shock (Malinverno et al, 1988;Gardner, 2001).…”

Heat flow and tectonics of the Ligurian Sea basin and margins

“…Recent submarine landslides documented on the Var upper canyon suggest that canyon cutting is recent, estimated to be late Quaternary age (Guillocheau et al, 1983;Gennesseaux et al, 1980;Malinverno et al, 1987). The Var canyon walls have gullies and spurs which set up small drainage basins with mass-wasting processes independent of the main channel activity (Malinverno et al, 1987).…”

“…One scale has relief with a wavelength less than a few heat flow probe lengths and gives rise to nonlinear gradient measurements (Lachenbruch, 1968). The other scale of topography is that which ranges from the large relief of canyons (200 to 400 m) to the short-wavelength 10 to 100 m relief of gully systems on the canyon walls (Malinverno et al, 1987) and possibly on the relatively unexplored intercanyon highs.…”

“…Sea Beam bathymetry (Pautot et al, 1984) indicates small ridges of less than 50 m relief within the platforms. Negative topographic features on the intercanyon highs tend to be wide bowl-shaped gullies; therefore, no apparent topographic counterpart to these acute ridges is found which could give are avenues of downslope sediment transport and that mass-wasting is presently active within gullies observed on canyon walls (Malinverno et al, 1987;Gennesseaux et al, 1980;Guillocheau et al, 1983;Pautot et al, 1984;Pautot, 1981;Bellaiche et al, 1978). Moreover, regional seismicity (Rehault and Bethoux, 1984) provides an excellent triggering mechanism for destabilization of sediments resting on steep slopes (e.g., Booth et al, 1985).…”

Seafloor mapping for geohazard assessment: state of the art

“…For instance, the ever-increasing use of the seafloor for cable route and drilling facilities (extending more and more into deep water), make marine geohazards a major concern for industries and public agencies dealing with marine infrastructures. Also onshore, for coastal communities and structures, marine geohazards may produce either direct effects such as submarine landslides retrogressively propagating onshore as in Finneidfjord in 1996 (Longva et al 2003) or Stromboli in 2002 (Chiocci et al 2008b) or indirect effects such as submarine landslides generating or contributing to tsunamis, as in Grand Banks in 1929 (Piper et al 1999) or Nice in 1979 (Malinverno et al 1988).…”