Structural restoration of thrusts at the toe of the Nankai Trough accretionary prism off Shikoku Island, Japan: Implications for dewatering processes

Abstract

[1] A three-dimensional prestack depth-migrated seismic reflection data volume acquired off Shikoku Island, Japan covers the seaward portion of the Nankai Trough accretionary prism. We calculate and interpret total horizontal shortening lengths along three cross-sectional profiles through the volume, incorporating a technique addressing the significant amount of water volume sediments lose during accretion, constrained by porosity values derived from seismic interval velocities. The results reveal a total horizontal shortening of ∼40% within sediments of the first three thrust sheets in the wedge. This indicates that structural restorations applied to water-saturated young sediments, or other domains displaying large tectonicinduced porosity changes (e.g., accretionary prisms, subaerial and submarine fold and thrust belts), must account for the substantial amount of distributed compactive strain that affects the sediment during the initial stages of accretion. Our analysis of the porosity reduction also allows an estimate of dewatering rates across the outer accretionary wedge. We find that porosity loss and associated dewatering decrease with distance landward from the trench and correspond to a progressively decreasing contribution of diffuse compactive strain to the total shortening. We compute a dewatering rate of 10.5 km 3 /Ma (per km along strike) over the outer ∼7 km of the accretionary wedge. This relatively high rate of dewatering when compared to other well-studied subduction systems probably reflects the large thickness of accreted sediment and high sediment permeability that allows efficient consolidation. These results highlight the importance of considering distributed compactive strain in structural restorations for any setting where deformation occurs in sediments.

The last 2 Myr of accretionary wedge construction in the central Hikurangi margin (North Island, New Zealand): Insights from structural modeling

“…The internal deformation and imbrication of sedimentary rock units in the upper plate have been validated through retro‐deformation and kinematic analyses of high‐quality 2‐D and 3‐D seismic reflection data across different margins worldwide [e.g., Nankai, Costa Rica, and Hikurangi, cf. Morgan et al ., ; Morgan and Karig , ; Sitchler et al ., ; Moore et al ., ; Burgreen‐Chan et al ., ; Boston et al ., ]. Restoration techniques help to discriminate between geometrically and kinematically feasible versus unlikely deformation paths, but cannot provide a unique reconstruction of the kinematic history of an accretionary wedge [ Hossack , ].…”

“…Progressive restoration of faulting and folding, together with back‐stripping and decompaction of sedimentary units, give important insights on strain accumulation on macroscopic structures, but can only account for a fraction of total shortening when ductile deformation, mass transfer processes, and layer‐parallel shortening affect volumetric changes [ Hossack , ; Mitra , ]. Significant amounts of internal deformation by layer‐parallel shortening of soft sediments at the toe of accretionary wedges have been documented by drill cores [ Lundberg and Moore , ; Morgan and Karig , ] and modeled by lateral variations in seismic velocity controlled by porosity [e.g., Moore et al ., ]. These studies attribute up to 30–40% of shortening to diffuse, though heterogeneous, distributed strain [ Morgan et al ., ; Morgan and Karig , ; Moore et al ., ], compatible with estimates independently derived from field studies and sandbox models [cf.…”

Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

“…Variations in both the incoming sediment column thickness (Figure 1) and the amount of long-term sediment accretion cause potential difficulties in calculating the amount of sediment subducted (e.g., Clift and Vannucchi 2004). Additionally, instead of underthrust sedimentfilled graben blocks being only vertically loaded, initial sediment accretion may affect overall sediment porosity due to the addition of both vertical and horizontal strains on the subducting sediment (e.g., Moore et al 2011). This may affect the sediment dehydration further landward and, as a result, whether or not subduction erosion might occur at depth.…”

Origin, structural geometry, and development of a giant coherent slide: The South Makassar Strait mass transport complex

“…7,16,and 17). However, the effects of structural compaction in deep-water fold and thrust belts can contribute as much as 40% shortening (Moore et al, 2011), so the correlation method may be a signifi cant underestimate. Another way to estimate shortening is by using the depth to detachment method (e.g., Epard and Groshong, 1993).…”