The transformation of smectite-group clay minerals to illite has garnered considerable interest as a potentially important process affecting both the mechanical and hydrologic behavior of subduction zones. Illitization can generate fluid overpressure by release of bound water, and the mineralogical change and associated cementation may increase intrinsic frictional strength while decreasing the sliding stability of faults. Released bound water also contributes to pore water freshening observed in boreholes at numerous margins. Here the authors combine data from Ocean Drilling Program drill sites along two transects at the Nankai subduction zone with numerical models of smectite transformation, to (i) quantify the distribution of smectite transformation and fluid production downdip of the trench; and (ii) evaluate its hydrologic and mechanical implications. High heat flow (ca 180 mW/m 2 ) along the axis of the Kinan Seamount Chain (Muroto transect) initiates clay mineral transformation outboard of the trench, whereas lower heat flow (70-120 mW/m 2 ) 100 km to the SW (Ashizuri transect) results in negligible presubduction diagenesis. As a result, considerably more bound fluid is subducted along the Ashizuri transect; simulated peak fluid sources down-dip of the trench are considerably higher than for the Muroto transect (ca 1.2-1.3 ¥ 10 -14 /s vs ca 6 ¥ 10 -15 /s), and are shifted ca 10 km further from the trench. More generally, sensitivity analysis illustrates that heat flow, taper angle, incoming sediment thickness, and plate convergence rate all systematically affect reaction progress and the distribution of bound water release down-dip of the trench. These shifts in the loci and volume of fluid release are important for constraining fluid flow pathways, and provide insight into the links between clay transformation and fault mechanics.

[1] Pore water freshening has been observed within sediments near the trench at numerous subduction zones. Constraining the relative contributions of long-distance updip flow of freshened fluids and in situ clay dehydration holds important implications for margin-scale fluid flow but remains unresolved because the evolution of pore water chemistry expected due to in situ clay dehydration and the budget of fresh water release have generally not been carefully quantified. Here, we address this problem at the wellstudied Nankai Trough offshore SW Japan, where Deep Sea Drilling Project and Ocean Drilling Program drilling have sampled the sedimentary section along two transects: the Muroto transect and the Ashizuri transect, located $100 km southwest along strike. Broad low-chloride anomalies with minimum concentrations significantly below typical seawater values were documented along the Muroto transect at Sites 1173, 1174, and 808 and indicate progressive freshening with increasing burial. In contrast, freshening at Site 1177 along the Ashizuri transect ($24 km seaward of the trench) is characterized by a series of sharp excursions from seawater values. We use a one-dimensional model to simulate sedimentation, thermal history, clay dehydration, and chemical diffusion, in order to quantify the component of pore water freshening attributable to in situ smectite dehydration. We find that simulated reaction progress is in close agreement with observations. The peak magnitude of pore water freshening is reasonably well predicted within the lowermost $150 m of the section but consistently underpredicted at intermediate depths. The residual pore water freshening profiles (not explained by smectite dehydration) reach peak magnitudes of $50-75 mM and approximately straddle the décollement at Sites 1174 and 808. We suggest that these residuals may reflect either an additional in situ dehydration process or a contribution from updip fluid migration along the fault. Our conclusion differs from that of previous studies, which have suggested that most of the observed geochemical anomaly could be generated by in situ smectite dehydration with as little as 15 wt% initial smectite. This substantial discrepancy is readily explained by the high sensitivity of pore water freshening to assumed porosity at the time of dehydration.

[1] Sediment permeability is a key parameter controlling overpressure development and fluid egress at subduction zones. Existing data compilations for argillaceous sediments illustrate that scale effects on permeability are generally minor, although this is a topic of active debate. Here, we report laboratory permeability measurements from sediments incoming to the Costa Rican subduction zone, at porosities from 26% to 78%. We then apply a method for directly estimating average permeability at the scale of sediment layer thickness, constrained by published estimates of excess pore pressure. Permeability values from both methods are in excellent agreement, and exhibit a trend consistent with permeability-porosity relationships used in numerical models at scales of several to 10's of km. Our results indicate no scale dependence of permeability, and strengthen previous interpretations drawn from more limited data sets. The permeability-porosity relationship we derive is comparable to those inferred at the Nankai and Barbados subduction zones, indicating that the high porosity, and hence high permeability, of incoming sediments at Costa Rica is the most likely explanation for comparatively rapid dewatering there. Citation: Saffer, D. M., and A. W. McKiernan (2005), Permeability of underthrust sediments at the Costa Rican subduction zone: Scale dependence and implications for dewatering, Geophys. Res. Lett., 32, L02302,

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