Nankai Trough Accretionary Prism Research Articles

Along-strike variations in protothrust zone characteristics at the Nankai Trough subduction margin

AbstractSignificant along-strike changes in the protothrust zone at the toe of the Nankai Trough accretionary prism were imaged in new high-resolution seismic reflection data. The width of the protothrust zone varies greatly along strike; two spatially discrete segments have a wide protothrust zone (∼3.3–7.8 km, ∼50–110 protothrusts), and two segments have almost no protothrust zone (∼0.5–2.8 km, <20 protothrusts). The widest protothrust zone occurs in the region with the widest and thickest sediment wedge and subducting turbidite package, both of which are influenced by basement topography. The trench wedge size and lithology, the lithology of the subducting section, and the basement topography all influence the rate of consolidation in the trench wedge, which we hypothesize is an important control over the presence and width of the protothrust zone. We conclude that protothrusts are fractures that form from shear surfaces in deformation band clusters as the trench fill sediment is consolidated. Strain localization occurs at sites with a high density of protothrusts, which become the probable locations of future frontal thrust propagation. The frontal thrust may propagate forward with a lower buildup of strain where it is adjacent to a wide protothrust zone than at areas with a narrow or no protothrust zone. This is reflected in the accretionary prism geometry, where wide protothrust zones occur adjacent to fault-propagation folds with shallow prism toe surface slopes.

Mud volcano possibly linked to seismogenic faults in the Kumano Basin, Nankai Trough, Japan

We investigated a mud volcano (MV) in a fault zone located at the southern edge of the Kumano Basin, the largest forearc basin along the Nankai Trough. Existing seismic sections show a truncated bottom-simulating reflection by a conduit below a topographic high, indicating the presence of an MV. New shipboard acoustic observations show that the fluid may be seeping through the seafloor, which in turn indicates that there are sufficient fluids for larger scale fluid migration in this area. Autonomous underwater vehicle-based high resolution acoustic observations and pH measurements indicate that soft sediment covers most of the MV and surrounding seafloor and that mud and small amounts of “high-backscattered materials” are sprinkled within the crater and around the MV. The MV type is different from those in the landward part of the Kumano Basin: the southern MV is smaller in size, has a steeper slope angle than the those in the northern Kumano Basin, and is located in a fault zone. The characteristics of this 14th Kumano Basin MV suggest that it is an expression of the larger scale fluid and sediment migration along the southern edge of the Kumano Basin, which may provide information regarding fluid and sediment migration along fault systems in the Nankai Trough accretionary prism.

Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High‐Pressure Experiments Using Input Site Sediments

AbstractDepth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm‐1K‐1 and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples.

Spatiotemporal Characterization of Smectite‐to‐Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3 km Below Seafloor

AbstractThe up‐dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite‐to‐illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X‐ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite‐to‐illlite diagenesis occur in samples recovered from down to 3 km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature‐ and time‐dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6 Ma led to the formation of two authigenic smectite‐illite phases consistent with a broad 1–9.3 Ma age determined by K‐Ar dating. The low K+ smectite‐illite reflects the long‐term burial history, whereas the K+‐rich smectite‐illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at ~5 km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 Mw = 8.1 Tonankai earthquake along those faults.

Continuous depth profile of the rock strength in the Nankai accretionary prism based on drilling performance parameters

A new method for evaluating the in situ rock strength beneath the seafloor is proposed and applied to the Nankai Trough accretionary prism. The depth-continuous in situ rock strength is a critical parameter for numerous studies in earth science, particularly for seismology and tectonics at plate convergence zones; yet, measurements are limited owing to a lack of drilled cores. Here, we propose a new indicator of strength, the equivalent strength (EST), which is determined only by drilling performance parameters such as drill string rotational torque, bit depth, and string rotational speed. A continuous depth profile of EST was drawn from 0 to 3000 m below the seafloor (mbsf) across the forearc basin and accretionary prism in the Nankai Trough. The EST did not show a significant increase around the forearc basin–accretionary prism boundary, but it did show a clear increase within the prism, ca. below 1500 mbsf. This result may indicate that even the shallow accretionary prism has been strengthened by horizontal compression derived from plate subduction. The EST is a potential parameter to continuously evaluate the in situ rock strength during drilling, and its accuracy of the absolute value can be improved by combining with laboratory drilling experiments.

In situ stress magnitudes at the toe of the Nankai Trough Accretionary Prism, offshore Shikoku Island, Japan

AbstractQuantifying the orientation and magnitude of tectonic stresses is essential toward understanding deformation and faulting in subduction zones. However, constraints on in situ horizontal stress magnitudes (Shmin and SHmax) are rare. We estimate Shmin and SHmax at Ocean Drilling Program Site 808 at the toe of the Nankai accretionary prism offshore Japan, using coupled constraints from (1) the width of wellbore breakouts together with estimates of rock strength and a model describing stress redistribution at the borehole wall and (2) limits on regional differential stress defined by failure on preexisting faults. Our analysis extends from 175 to 915 m below seafloor (mbsf) and spans the active frontal thrust. For an upper bound on rock unconfined compressive strength (UCS) and assuming hydrostatic formation pore pressure, Shmin and SHmax (referenced to the seafloor) increase from 6.5 MPa at 175 mbsf to 17.4 MPa at 915 mbsf, with the stress state gradually transitioning from a thrust or strike‐slip faulting regime above 800 mbsf to a normal faulting regime below. For cases with higher formation pore pressure, horizontal stresses are slightly lower but follow a similar pattern. We show that estimated Shmin and SHmax are strongly dependent on UCS, breakout width, and friction coefficient, all of which are characterized by uncertainty. Our results suggest that the prism is near thrust failure in the upper ~300 mbsf, but far from failure below. This may be reconciled with active thrusting if thrust faults are locally weaker than the surrounding rock or if SHmax fluctuates during the seismic cycle.

Insights of dielectric measurements from cuttings recovered along the deepest offshore well in the world (Nankai trough accretionary prism): IODP expedition 338, site C0002F

SUMMARY A total of 109 cuttings were recovered during the IODP expedition 338 in site C0002F down to 2005 mbsf. A special dielectric end-load probe was designed and used for the first time at sea on this sample collection to measure dielectric and electrical conductivity from 10 kHz to 6 GHz. The whole dataset was compared to specific surface area (SSA), mineralogy from XRD measurements and resistivity log while drilling acquired during the expedition to understand the relationship between fluid, clays and lithologies. The dielectric results revealed to be very powerful to: (i) understand the clay composition and content; (ii) re-calibrate cutting depths; (iii) detect unit boundaries and (iv) detect conductive and not-conductive fault systems

Frictional properties of the shallow Nankai Trough accretionary sediments dependent on the content of clay minerals

Abstract We conducted triaxial friction experiments on the shallow Nankai Trough accretionary sediments at confining pressures, pore water pressures, temperatures close to their in situ conditions, and axial displacement rates (V axial) changed stepwise among 0.1, 1, and 10µm/s. The results revealed that their frictional properties change systematically according to the content of clay minerals, smectite in particular. The steady-state friction coefficient (μ ss) at V axial = 1µm/s decreases with increasing clay mineral content, shown in weight percent, from 0.82 for a sandstone sample (6 wt%), through 0.71 for a tuff sample (≈17 wt%), and 0.53 to 0.56 for siltstone samples (29 to 34 wt%), to 0.25 for a claystone sample (42 wt%). Slip-dependent frictional behavior changes accordingly from slip hardening for the sandstone sample, through quasi steady-state slip for the tuff and siltstone samples, to distinct slip weakening for the claystone sample. Although all samples exhibit velocity-strengthening behavior upon stepwise changes in sliding velocity, the ratio of the (a − b) value to the velocity dependence of steady-state friction (∆μ ss/∆lnV sliding) decreases with increasing clay mineral content, which implies that the friction component decreases while the flow component increases accordingly. Thus, faulting in the shallow Nankai Trough accretionary prism is likely controlled by the clay mineral content, in particular the smectite content, in the sediments as well as in the fault zones.

Tracing geologically constrained fluid flow pathways using a combination of heat flow measurements, pore water chemistry, and acoustic imaging near the deformation front of the Nankai Trough off the Muroto Peninsula, Japan

Abstract Visual and acoustic surveys, heat flow measurements, and the analysis of pore water chemistry near the deformation front of the Nankai Trough accretionary prism off the Muroto Peninsula, Japan, show that fluid discharge is constrained by geological structures such as landslides. High heat-flow anomalies (up to 260 mW m− 2) exist near the base of a ca. 1.5 km ridge–trough structure, where the exit point of the secondary frontal thrust is thought to occur, but no other anomalies were detected. In contrast, on a 50-m-wide flat area on the steep slope inside the slope-scale landslide structure, where geological strata are exposed, high heat-flow anomalies of around 190 mW m− 2 exist, accompanied by biological activity and methane-rich pore water. A sub-bottom profiler recorded strata cutting through the seafloor, which also supports the existence of a landslide. Based on these results, we propose a qualitative picture of fluid flow that may explain the observed heat-flow anomalies. Upward fluid flow through the frontal thrust, fluid flow cutting through geological strata, is inhibited near the foot of the slope, while fluid discharge occurs where the landslide exposes new surfaces as a result of geologically constrained fluid flow along strata.

Magnetic fabric analyses as a method for determining sediment transport and deposition in deep sea sediments

Abstract Anisotropy of magnetic susceptibility (AMS) analyses and the characteristic relationship between the q-value (magnetic lineation/foliation) and the imbrication angle (β) of the minimum susceptibility axes from the bedding plane (q–β diagrams) provide insight into the paleocurrent direction and depositional processes. During Integrated Ocean Drilling Program (IODP) Expedition (Exp) 333 sediments were cored from within the Nankai Trough accretionary prism at Site C0018, the depositional center for six mass transport deposits (MTDs). We analyzed specimens from the two subunits, Ia and Ib, and the six mass transport deposits identified during Exp 333. The sediments that are unaffected by MTD emplacement record episodic changes in paleocurrent direction and depositional processes including current, slope gravity, viscous suspension, and grain collision over the past 1.24 Ma at Site C0018. Magnetic fabric analyses proved useful in identifying sediment disturbance and the direction of movement at the time of emplacement, especially within MTDs 1, 2, 5, and 6. The magnetic fabric analyses were also useful in characterizing undeformed sediments within MTD 3. These undeformed sediments likely record primary depositional fabrics. We propose two additional fields, deformation/disturbance and tilting, on the q–β diagram in order to account for steep imbrication angles and relatively high q-values.

Zircon and apatite thermochronology of the Nankai Trough accretionary prism and trench, Japan: Sediment transport in an active and collisional margin setting

The Nankai accretionary complex is the most recent addition to the accretionary complexes of southwest Japan and has preserved a record of sediment flux to the trench during its construction. In this study, we use U‐Pb zircon and fission track analysis of both zircons and apatites from sediments taken from the forearc and trench of the Nankai Trough, as well as rivers from southwest Japan to examine the exhumation history of the margin since the Middle Miocene. Modern rivers show a flux dominated by erosion of the Mesozoic‐Eocene Shimanto and Sanbagawa accretionary complexes. Only the Fuji River, draining the collision zone between the Izu and Honshu arcs, is unique in showing much faster exhumation. Sediment from the Izu‐Honshu collision is not found 350–500 km along the margin offshore Kyushu indicating limited along‐strike sediment transport. Sediment deposited since 2 Ma on the midtrench slope offshore the Muroto Peninsula of Shikoku (ODP Site 1176) and on the lower slope trenchward of the Kumano Basin (IODP Sites C0006E and C00007E) shares the dominant source in the Shimanto and Sanbagawa complexes seen in the modern rivers. Prior to 5 Ma, additional sediment was being sourced from further north in more slowly exhumed terrains, ~350 km from the trench axis. Around 9.4 Ma, U‐Pb zircon ages of ~1800 Ma indicate enhanced erosion from the North China Craton, exposed in northern Honshu. In the middle Miocene, at ~15.4 Ma, the sediment was being derived from a much wider area including the Yangtze Craton (U‐Pb ages ~800 Ma). We suggest that this enhanced catchment may have reflected the influence of the Yangtze River in supplying into the Shikoku Basin prior to rifting of the Okinawa Trough at 10 Ma and migration of the Palau‐Kyushu Ridge to form a barrier to transport. The restriction of Nankai Trough provenance to Mesozoic source partly reflects continued uplift of the Shimanto and Sanbagawa complexes since the Middle Miocene.

Contrasting hydrological and mechanical properties of clayey and silty muds cored from the shallow Nankai Trough accretionary prism

Abstract Two mud samples cored from the shallow (≈ 1000 mbsf) Nankai Trough accretionary prism at Site C0002 of IODP Expedition 315 are found to be distinctly different in hydrological and mechanical properties. At confining pressures, pore water pressures and temperatures close to their in situ conditions, a clayey mud sample has lower permeability of 2.92 × 10− 19 m2, while a silty mud sample has higher permeability of 2.29 × 10− 18 m2. Triaxial compression experiments at these conditions and an axial displacement rate of 10 μm/s reveal that the clayey mud sample exhibits lower failure strength of 14.2 MPa followed by a slow failure lasting for ≈ 40 s, while the silty mud sample exhibits higher failure strength of 20.1 MPa followed by a rapid failure within ≈ 5 s. Friction experiments at these conditions and axial displacement rates changed stepwise among 0.1, 1 and 10 μm/s reveal that the clayey mud sample has a much lower steady-state friction (μss ≈ 0.25) than the silty mud sample (μss ≈ 0.53). Although both samples exhibit velocity strengthening, the former has more than three times larger velocity-dependence of steady-state friction than the latter. Such contrasting hydrological and mechanical properties of the clayey and silty mud samples as revealed in this study suggest the following implications for deformation and faulting in the shallow mud-dominant Nankai Trough accretionary prism. Deformation results in a possible increase in pore pressure, and hence, in strength reduction in clayey mud, but not in silty mud. Faulting would preferentially occur in the weaker clayey mud, and its slow failure may result in a slow slip. Faults formed in clayey mud are weak and easily reactivated, but stable and not seismogenic. In contrast, once the stronger silty mud is faulted, its rapid failure may become a seismic slip. Faults formed in silty mud are strong and not easily reactivated, but possibly unstable and seismogenic.

Frictional properties of muds cored from the shallow Nankai Trough accretionary prism

Frictional properties of muds cored from the shallow Nankai Trough accretionary prism

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

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 tectonic‐induced 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 km3/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.

Structure of the Nankai Trough accretionary prism revealed by IZANAGI oceanfloor imaging sonar system

Structure of the Nankai Trough accretionary prism revealed by IZANAGI oceanfloor imaging sonar system

Possible strain partitioning structure between the Kumano fore‐arc basin and the slope of the Nankai Trough accretionary prism

A 12 km wide, 56 km long, three‐dimensional (3‐D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan, images the accretionary prism, fore‐arc basin, and subducting Philippine Sea Plate. We have analyzed an unusual, trench‐parallel depression (a “notch”) along the seaward edge of the fore‐arc Kumano Basin, just landward of the megasplay fault system. This bathymetric feature varies along strike, from a single, steep‐walled, ∼3.5 km wide notch in the northeast to a broader, ∼5 km wide zone with several shallower linear depressions in the southwest. Below the notch we found both vertical faults and faults which dip toward the central axis of the depression. Dipping faults appear to have normal offset, consistent with the extension required to form a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is difficult to determine, but the along‐strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. By considering only the along‐strike variability of the megasplay fault, we could not explain a transform feature at the scale of the notch. Strike‐slip faulting at the seaward edge of fore‐arc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollement strength variations which control the location of the fore‐arc basins may therefore play a role in the position where an along‐strike component of strain is localized. While the obliquity of convergence in the Nankai Trough is comparatively small (∼15°), we believe it generated the Kumano Basin Edge Fault Zone, which has implications for interpreting local measured stress orientations and suggests potential locations for strain‐partitioning‐related deformation in other subduction zones.

Petrophysically determined lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314

Abstract: Characterizing the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. Integrated Ocean Drilling Program (IODP) Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging-while-drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at Sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by 3D seismic profiles and initial interpretation of log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterize formations within the accretionary prism. Boundaries that correlate to the seismic-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a megasplay fault previously unresolved by log analysis and borehole images, are identified. Our study demonstrates that INCA analysis of LWD data can accurately define boundaries and characterize sediments in environments where core recovery may be incomplete.

Terrestrial organic matter controlling gas hydrate formation in the Nankai Trough accretionary prism, offshore Shikoku, Japan

Sediment core samples from Sites 1175, 1176, and 1178, ODP Leg 190, Nankai Trough were analyzed for sedimentary organic matter and inorganic elemental compositions to clarify geochemical conditions for the formation of gas hydrate. Low chloride concentrations and anomalously low core temperature infer the existence of gas hydrate at Site 1178. Trace amounts of gas hydrate are also suggested for Site 1176. Site 1175 does not have any significant evidence of gas hydrate, although all the three sites are within the gas hydrate stability zone. The sediment from Site 1178 is characterized by abundant terrigenous organic matter, older geologic ages, and comparatively higher maturity levels, suggesting high rates of CO 2 generation during diagenesis. The CO 2 generation potential of sediment may be one of the crucial conditions for the formation of gas hydrate.

Assessment of bacterial community structure in the deep sub-seafloor biosphere by 16S rDNA-based techniques: a cautionary tale

Investigations into the deep marine environment have demonstrated the presence of a significant microbial biomass buried deep within sediments on a global scale. It is now believed that this deep biosphere plays a major role in the global cycling of elements and contains a large reservoir of organic carbon. This paper reports the development of a DNA extraction protocol that addresses the particular problems faced in applying molecular ecological techniques to samples containing very low biomass. Sediment samples were collected from different geographical locations within the Pacific Ocean and include the Ocean Drilling Program (ODP) Leg 190, Nankai Trough Accretionary Prism. Seven DNA extraction protocols were tested and a commercially available DNA extraction kit with modifications was shown to produce higher yields of polymerase chain reaction (PCR)-amplifiable DNA than standard laboratory methods. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA gene diversity revealed that template DNA from these extremely low biomass sediment samples was susceptible to PCR bias and random amplification. We propose that it is essential to screen 16S rRNA gene products for bacterial diversity by DGGE or other rapid fingerprinting methods, prior to their use in establishing a representative clone library of deep sub-seafloor bacteria. This represents a cautionary approach to analysis of microbial diversity in such sub-seafloor ecosystems.

Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface: What have we learned in the past decade?

The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the décollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ 18O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ 18O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl −/high-δ 18O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.