4.1. Horizontal Shortening
Comparisons of the deformed and undeformed configurations of the three cross sections along the Muroto transect provide estimates of along strike variations in horizontal shortening at the prism toe (Table 1 and Figure 6). The difference in total length between the porosity-corrected (Lt) and deformed states (Ld) indicates a total horizontal shortening (Table 1) range of 5.37–6.38 km or horizontal strains of 39–43% across the outer wedge. Restoring the displaced strata based strictly on area and bed length balancing techniques recovers only1.3–2.0 km or ∼24–35% of the total shortening, implying that the majority of shortening is accommodated by porosity reduction. This indicates that structural restorations applied to contractionally deformed, unconsolidated sediments must account for substantial dewatering and consolidation during the initial stages of deformation. We estimate that the total horizontal shortening results have an error of about ±10% based on the uncertainty in computed porosity. The error is constrained by inserting a range of initial and deformed porosities into equation (1) based on a weighted average estimate per stratigraphic unit.
Figure 6. Interpreted PSDM profiles and corresponding porosity-corrected structural restorations across the Muroto transect region of the Nankai Trough shown at 2:1 vertical exaggeration. Main stratigraphic facies correlated within the protothrust zone, first, second, and third thrust sheets are shown in their defined colors. Trench fill sediments are orange and yellow, facies transition is green, and Shikoku Basin sediments are blue and purple. The locations of ODP Hole 808 and 1174 are also indicated along line 284. Restored sediment thicknesses represent the compaction state found at the reference location (crossline 685). The apparent loss of the outer trench wedge facies in the thrust sheets is attributed to submarine erosion. Restored estimates of the eroded surface and the slope sediment facies are shown with dashed lines.
Download figure to PowerPoint
Table 1. Horizontal Shortening Estimates Along Three Seismic Inlines Across the Muroto Transect
|Region||Ld Deformed Length (km)||Sfs Fault Slip Shortening (km)||Sc Compactive Shortening (km)||St Total Shortening (km)|| (%)||La Area Balance Restored Length (km)||Lt Total Restored Length (km)||Sfs Fault Slip Shortening (%)||Sc Compactive Shortening (%)||St Total Shortening (%)|
The length difference between total horizontal shortening (Lt), which includes both a porosity loss correction and slip along faults, and a restoration that only accounts for fault slip (La) allows us to determine the proportion of total shortening that is specifically attributed to the diffuse compactive strain (i.e., porosity loss) within the sediments. This proportion varies along the margin, ranging from 65% at line 215 in the SW to 76% estimated at lines 260 and 284. The proportion of shortening accommodated by compactive strain also decreases systematically with distance landward, from 86 to 97% in PTZ-1, to values as low as 60–77% in TS2 and 3 (Figure 7 and Table 1). This corresponds to a decrease in the average porosity within the accretionary wedge from ∼50% to ∼32%. For all of the individual thrust slices, this proportion is >60%, and generally >70%.
Figure 7. Proportion of shortening accommodated by dewatering/compactive porosity loss as a function of structural position relative to the trench.
Download figure to PowerPoint
Previous studies in this region estimated a minimum total shortening of 31% within the first two thrust sheets at ODP Site 808 [Morgan and Karig, 1995]. Of this, the component attributed to compactive strain (about 68% in the vicinity of drill Site 808) was determined by the ratio between the length of the décollement after the complete restoration and the length of the décollement after the restoration of only the folded and displaced strata (similar to the area balancing restoration in Table 1) [Morgan and Karig, 1995]. In comparison, along Line 284, we calculate 48% shortening within TS-1 and −2, with 76% of the total attributed to compactive strain. Two possible explanations for the slightly lower values reported by Morgan and Karig  include (1) the assumption of no volume change along the base of the prism, which provides a minimum limiting solution, and (2) sparse velocity logs and 2-D seismic data used in the study limit the resolution of porosity distribution. For our purposes, the PSDM velocity-porosity transforms used in this study provided increased precision in applying a porosity correction. Henry et al.  estimate 10–15% strain associated with porosity loss at ODP Site 1174 and infer only minor shortening due to slip along faults. Our calculations for Line 284 in PTZ-1 and −2 similarly show only minor fault-related shortening (∼4%), and somewhat larger (22%) total strain from compactive shortening. We consider this difference to be remarkably small, given the significant difference in the two techniques.
4.2. Porosity Loss and Dewatering Rates
The seismically derived porosity distribution for the outer accretionary wedge allows estimation of both the porosity loss within each stratigraphic unit, and the rate of dewatering of the accreted strata as a function of distance from the trench. Here, we report porosity reduction and dewatering rates using line 284 as an example. The average porosity of the accreted strata is reduced from ∼50% at the trench reference location (CDP 685), to ∼32% by TS-3 (Figure 8), with most of the reduction (from ∼50% to 35%) occurring in the outer ∼4 km between the trench and TS-1. The trench sediments and hemipelagic Shikoku Basin facies exhibit similar patterns of porosity loss with progressive thrust deformation (Figure 8). The rate of porosity reduction in the outermost ∼2–4 km of the wedge is higher than that for the rapidly buried (and presumably lower permeability) underthrust sediment section (Figure 8b), but the two are comparable arcward of TS-1. The pattern of decreased porosity loss with distance arcward mirrors the pattern of decreasing diffuse compactive shortening (see Figure 7) and is consistent with the strain hardening behavior of sediment during consolidation.
Figure 8. (a) Porosity and (b) change in porosity with distance into the subduction zone, for the trench wedge facies sediments (TW; boxes with crosses), accreted Shikoku Basin facies sediment (SB; open circles), and averaged values for the accreted sediment (closed circles), and underthrusting section (open boxes). Sign convention for porosity change in Figure 8b is that positive values indicate porosity loss. Although the underthrust section is undercompacted relative to the sediments immediately above [Moore et al., 2001], it is not evident as plotted because the porosities reported for the Shikoku Basin facies are averaged over the lower several hundred meters of the accretionary wedge.
Download figure to PowerPoint
Using the porosity distribution (Figure 8a), we calculate dewatering rates within each zone by assuming (1) conservation of solid mass (or in this case, solid area) and (2) that the initial thickness of each unit at the trench is the same as the current-day trench section (this assumption is also used for the reconstruction, as discussed in section 3) [e.g., Bekins and Dreiss, 1992]. Under these assumptions, the residence time of sediment in each region is given by the solid area divided by the delivery rate of solids at the trench:
where γ and A are the porosity and area of the strata in a given region (e.g., PTZ, TS1, TS2), γ0 is the initial porosity at the trench reference location, H0 is the thickness of the section at the reference location, and vp is the plate convergence rate. The dewatering flux (Q) is then obtained by dividing the change in water content between regions by the residence time:
where e is void ratio, the subscript n refers to the zone of interest, and (n-1) to the adjacent trenchward zone. The dewatering flux is expressed in units of Vol/t per km along strike, and reflects the rate at which water is expelled due to consolidation as sediments are progressively incorporated into more arcward thrust slices.
The computed dewatering rates are largest in the trench sediments, and in the outermost ∼2–4 km of the accretionary wedge. Our results show that 5 km3/Ma H2O (per km along strike) are expelled from the trench sediment between the trench and 2 km arcward, and 6.75 km3/Ma between the trench and 6.5 km, whereas the accreted hemipelagic Shikoku Basin sediment releases 1.7 km3/Ma and 2.6 km3/Ma by 2 km and 6.5 km from the trench, respectively (Figure 9). When normalized by the volume of sediments undergoing dewatering, the fluid source terms (units of VolH2O/Volsed per s) range from 1.1 × 10−14 s−1 to 1.2 × 10−13 s−1, and are in good agreement with those computed in numerical modeling studies of the Muroto transect [Bekins and Dreiss, 1992; Saffer and Bekins, 1998]. For comparison, the underthrust Shikoku Basin section yields <0.4 km3/Ma H2O in the outer 2 km, and ∼1.3 km3/Ma in the outer 6.5 km.
Figure 9. Cumulative dewatering as a function of distance from the trench, with the same legend as in Figure 8. For comparison, dewatering rates reported for the Alaska margin (gray squares), Costa Rican margin (green squares), and underthrusting sediment at the Barbados margin (blue squares) are also shown.
Download figure to PowerPoint
The dewatering rates along the Muroto transect are similar, though slightly higher than those reported for the eastern Alaska margin [von Huene et al., 1998] and along the Barbados margin [Screaton et al., 1990], and slightly lower than those reported for sediments underthrust at the Costa Rican margin [Saffer, 2003](Figure 9). Normalized dewatering rates for the Peru margin range from 3 × 10−17 to 3 × 10−13 [Kukowski and Pecher, 1999]. The rates of fluid loss from the underthrusting section are comparable for the Muroto transect and the northern Barbados margin [Zhao et al., 1998].
The differences in dewatering fluxes between units along our transect, and between margins, can be explained by four primary factors: (1) initial sediment thickness, where larger sediment thickness leads to higher dewatering fluxes, (2) initial porosity, where higher initial porosity results in larger total pore volume and higher compressibility, both leading to higher dewatering rates, (3) plate convergence rate, which controls burial and tectonic loading rate, and (4) permeability, which governs the rate of fluid expulsion (as manifested in observed porosity loss) in response to loading. For example, the trench sediments in our study area are thicker than the accreted and underthrust Shikoku Basin sediment sections, and thicker than the underthrusting section at Barbados, leading to higher overall volumes of water expulsion (Figures 3–4 and 9). In addition, the initial porosity, and thus compressibility, of the trench sediments is higher than that of the Shikoku Basin sediment or the underthrust section at Barbados, owing to their shallower burial state at the trench. This leads to higher rates of compaction early in the loading history, as documented by the rapid porosity loss within the trench sediments in the outer few km of the accretionary wedge (Figures 7–8). The trench sediments also contain abundant silty and sandy turbidites that should allow for efficient drainage and dewatering in comparison to the uniformly clay-rich Shikoku Basin facies or underthrust pelagic claystones at Barbados [e.g., Moore et al., 2001; Steurer and Underwood, 2003]. At Costa Rica, the sediment section is only ∼380 m thick, but the convergence rate of 88 km/Ma is higher than that at Nankai or Barbados (29 km/Ma), and the entire sediment column is highly porous, compressible, and permeable [e.g., Saffer, 2003], leading to efficient dewatering and rapid fluid expulsion there. Overall, our analysis is consistent with observations from several other well-studied margins, and provides additional insights into the interplay between sediment thickness, permeability, and plate convergence rate in controlling porosity loss and dewatering fluxes.