The role of compaction contrasts in sediments in décollement initiation in an accretionary prism
Research highlights
► Experiments on mechanical properties of a sediment column approaching Nankai prism. ► The column is made of an easily deformable upper and hardly deformable lower section. ► Horizontal compression from the prism results in column differential compaction. ► Gradients in horizontal compaction guide the seaward propagation of the décollement.
Introduction
Convergent margins can be either non-accreting, where all the incoming sediment sequence is underthrust by subduction, or accreting, where part of these sediments is incorporated into a growing accretionary prism (Uyeda and Kanamori, 1979, Von Huene and Scholl, 1991). Along accreting margins, the fraction of incoming material accreted to the overriding plate is controlled by the position of the décollement, which is therefore a major factor in prism growth, dynamics and global mass balances.
The décollement can often be seismically imaged as a high amplitude, reverse polarity reflector (e.g. Moore et al. (1990) in Nankai, Shipley et al. (1990) in Costa Rica), interpreted as the interface separating high porosity/fluid pressure underthrust sediments from low porosity/fluid pressure accretionary prism material (Tobin and Saffer, 2009). The coexistence of these two independent hydrologic systems implies that the interface, or décollement, has larger permeability than the material above and below (Henry and Le Pichon, 1991). The décollement is therefore often considered as a highly fractured horizon, where the pore pressure is relatively high compared to surrounding material, as a result of channelized flow of deep fluid (Saffer and Bekins, 1998). In turn, high pore fluid pressure reduces the effective stress and promotes further fracturing or fault initiation.
Although there is an enhancement in deformation and fluid flow in a mature décollement, it is unclear how this discontinuity in physical properties of the seaward-migrating frontal thrust extends into material outboard of the deep-sea trench. In Barbados prism, the presence of a high-porosity, radiolarian-rich horizon (Moore et al., 1998, Moore, 2000) may explain décollement localization. On the other hand, such lithological heterogeneity is not relevant in Nankai area (Fig. 1), where both Upper Shikoku Basin (U.S.B.) and Lower Shikoku Basin (L.S.B.) units consist dominantly of hemipelagic silty mudstones (Shipboard Scientific Party, 2001b).
As detailed in Section 2, alternative candidates for the controlling properties are permeability (Section 2.1), shear strength (Section 2.2) or yield strength (Section 2.3). The two latter factors refer to the material mechanical state, which is usually assessed by geotechnical tests, i.e. rather by the stress necessary to trigger plastic deformation/slip than by the amount of strain for a given stress increment. In this manuscript we propose to reverse this point of view and to consider the finite strain affecting incoming material upon arrival into the prism. The amount of compaction versus depth constitutes a new possible controlling factor for décollement localization when considering the horizontal compression in the vicinity of prism toe. For the new model, we first consider the state of stress in the vicinity of the prism toe and describe, based on the analogy with cutting tool processes, how horizontal compression affects incoming material seaward of décollement tip and deformation front (Section 3). We then assess the material response to horizontal compression using mechanical testing under isotropic loading conditions up to maximum of twice in-situ effective stress (Section 4). We finally define a model combining the horizontal compression with the mechanical profile to propose a new mechanism of décollement localization based on the differential horizontal compaction of a sedimentary column approaching the Nankai prism (Section 5).
Section snippets
Permeability/pore fluid pressure hypothesis
As the décollement is the surface where the shear localizes, its propagation into incoming material is favored in horizons where the shear strength is reduced. The presence of high pore fluid pressure, which lowers the shear strength, is believed to account for the formation of the protodécollement seaward of the Nankai prism toe (Le Pichon and Henry, 1992, Le Pichon et al., 1993). In their model, the rapid loading of low-permeability hemipelagites by the deposition of high-permeability
Force balance near prism toe
In accreting margins, the material approaching the trench is eventually either accreted into the accretionary prism or underthrust beneath it (Fig. 2A). The décollement, i.e. the frontier between accreted and underthrust domains, is a mechanically weak zone, possibly because of high pore fluid pressure, so transmitted shear stresses are of low magnitude and the stress field is decoupled between accretionary prism and underthrust sequence (Westbrook et al., 1982, Taira et al., 1992, Le Pichon et
Experimental procedure
We sampled 12 small cylinders (diameter × height = 38 × 11–17 mm) in Hole 1173A, orientated so that the cylinder base is vertical (i.e. parallel to the core axis) and the height is horizontal (Fig. 1D). These samples span an interval of 250–660 mbsf (meters below sea-floor), which includes the lower part of U.S.B. and most of the underlying L.S.B. and comprises the stratigraphic equivalent of the décollement (390–420 mbsf) (Fig. 1).
The mechanical tests consist of isotropic loading of the sample under
Velocity reduction from horizontal shortening
Both the schematic stress analysis of the prism toe area as well as the analogy with metal-cutting processes show that the sedimentary column approaching the prism toe is affected by lateral compression ahead of the décollement tip. The lateral shortening enhances porosity reduction in the sedimentary column, which is already affected by the rapid loading by trench turbidites. The effect of lateral compression is nevertheless not the same as vertical loading, as in the former case the
Cement distribution
The complex pattern of cement distribution in Nankai incoming sediments, as highlighted by geomechanical testing and other data (see Section 2.3, Spinelli et al., 2007, Raimbourg et al., 2011), appears at first sight at variance with our results: the porous material of U.S.B. where we observed the most intense compaction, show evidences for silica cementation. Such contrasting behavior may be seen in the light of the Peff exceeding Peffin situ we applied, which may have triggered breakage of
Conclusion
As Nankai accretionary prism is not a perfect wedge, closed by an acute angle, the horizontal compressive stresses active there must to some extent be transmitted seaward of the décollement tip and affect diffusely the whole incoming sedimentary sequence. As a consequence, the compaction affecting this material incorporates a part of horizontal shortening.
The response of samples collected at different depths within the incoming sequence to experimentally imposed compaction show very contrasted
Acknowledgements
This research used samples and data provided by the Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP). Support for the project was provided by the Grant-in-Aid for Creative Scientific Research (19GS0211) by MEXT (Ministry of Education, Culture, Sports, Science and Technology) of Japan. We thank the staff in Kochi Core Center for their help with sampling.
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