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

1 Introduction
1.2 Submarine slope stability processes
1.2.1 Types of submarine mass movements
Sediments transport processes in marine environments are more complicated than those on land
because of different driven forces including gravity, suspension, geostrophic circulation and so on (Fig.
1.2). Among them, submarine mass movements are the dominant processes transporting large amounts
of sediment across continental slope to the deep ocean. Submarine mass movements are classified as
slide (brittle deformation), slump (plastic deformation), debris flow (plastic or laminar flow) and
turbidity current (fluid turbulent flow) according to the mechanical behavior of process (Mulder and
Cochonat 1996; Mulder 2011; Fig. 1.2).
Submarine slides and slumps involve the movement of coherent masses of sediments bounded on all
sides by distinct failure planes (Mulder et al. 1996). Differentiation of slides and slumps is based on the
value of the Skempton ratio D/L (where D is the maximum depth of the slip surface, and L is the total
length of the slump). Slides are translational with a D/L ratio generally < 0.15, whereas slumps are
rotational and deep-seated with a D/L ratio between 0.15-0.33 (Skempton and Huchinson 1969). Most
submarine slides appear to be translational and are characterized by a flat, slope-parallel basal failure
surfaces (Canal et al. 2004). The failure surface is predetermined and corresponds to a discrete layer
with low shear strength, such as permeable sand layers (e.g., Afen slide; Wilson et al. 2004), high
porous tephra layers (e.g., submarine slide along the Middle American Trench; Harders et al. 2010) or
sand and clay interbeds (e.g., hemipelagic and contouritic deposits offshore Norway; Laberg and
Camerlenghi 2008). Submarine slides are commonly associated with a variety of extensional and
compressional features. Extensional features are common in the upper part of the slide, especially in
the headwall area, where listric faults are predominantly orientated perpendicular to the transport
direction (Martinsen and Bakken 1990). In contrast, compression features are common at the front of
the slide deposit where dominated by imbricate thrust slices of chaotically deformed strata (Martinsen
2005). Slides and slumps are not isolated processes and often form multiple phases of failures.
Retrogressive slides are one of most common multiple phases of failure, which form because of
upslope propagation of the failure (Mulder 2011).
Slides and slumps can transform to debris flows or turbidity currents through gradually increasing
ambient water and disintegration of coherent blocks. Debris flows are flows in which the sediment is
heterogeneous and may include lager clasts supported by a matrix of fine sediment (Lee et al. 2007).
Turbidity currents involve the downslope transport of a relatively dilute suspension of sediment grains
that are supported by an upward component of fluid turbulence (Parson et al. 2007). Turbidity currents
are often formed by the disintegration of slides of debris flows, although they also may be generated
independently of other gravity-driven processes (Mulder 2011).
Different types of submarine mass movements are represented different degrees of material
transformation. To achieve it, sufficient energy was needed to reach a given degree of remolding (Fig.
1.3; Locat and Lee 2009). When the remolded energy available (E rA ) is larger than the remolded energy
needed (E rN ), it is easily transferred to debris flows and turbidity currents. When smaller, it may tend to
generate slides or slumps. However, the exact cause of transformation from slides to debris flows then
to turbidity currents is still not fully understood. It is, however, out of the scope of this thesis, which
focuses on the precondition factors and triggering mechanisms of initiation of submarine slope failure.
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