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SUBDUCTION ZONES: PORE PRESSURE, FAULT STRENGTH, AND FLUID FLOW

Pore Pressure & Fault Strength | NanTroSEIZE drilling program | 3-D Seismic Project

Subseafloor Observatory | Friction and Earthquakes | Low-T metamorphism | Papers

At subduction zones, tectonic loading of offscraped and underthrust sediments results in rapid compaction and high rates of fluid expulsion. Prograde metamorphism of politic marine sediments releases bound water, and also generates distinct chemical signatures that can be used to provide insight into flow pathways and rates. The resulting hydrologic and mechanical systems are intimately linked with one another, and probably change dramatically through time. The potential for devastating earthquakes and tsunamis is strong motivation for understanding how fluid processes may control fault zone strength and material properties. The fate of fluids entering subduction zones is also a key issue in balancing fluid budgets at convergent-plate boundaries, in estimating pore-fluid pressures at depths where earthquakes nucleate, and in controlling the evolution of mechanical strength and structural development.

We investigate geohydrology at convergent plate margins by combining field data collected on Ocean Drilling Program (ODP) (now the Integrated Ocean Drilling Program; IODP) expeditions, laboratory experiments, and computer models. In the laboratory, we conduct consolidation experiments to learn about sediment mechanical properties, permeability, and pore pressure. Computer models are an important part of this work, as another type of "experiment" that helps us learn about important processes we can't always observe directly. Field data collected by drilling in ocean trenches yields important information about compaction state and pore water chemistry - powerful constraints for understanding the development of fluid pressure and fluid movement.

We have several ongoing projects focused on subduction zone tectonics, fluid flow, and fault mechanics (see links at top). These include: combined numerical modeling, field, and laboratory studies aimed at quantifying fluid pressure in active subduction systems; major involvement in the IODP NanTroSEIZE drilling program; analysis of a recently collected 3-D seismic survey at the Nankai subduction zone offshore SW Japan; installation of borehole observatories to monitor pore pressure, temperature, strain, and seismicity offshore Japan; laboratory studies of frictional properties of subduction zone fault rocks; and numerical modeling studies that investigate low-T metamorphism and fluid release.

PORE PRESSURE AND FAULT STRENGTH

Our work on pore pressure and fault strength includes both numerical modeling and analysis of field and lab data. Ongoing modeling studies are investigating the magnitude and distribution of pore pressure in subduction zones, quantitatively evaluating the factors that control pore pressure, and investigating the role of permeable strata and splay faults in affecting pore pressure distribution. Some of our recent work has provided support for a conceptual model in which sediment permeability and thickness act together to control pore pressure, and thus the mechanical strength and geometry, of subduction-accretion complexes (left).

Additional work on this topic (conducted in collaboration with Harold Tobin at the Univ. of Wisconsin) has focused on analysis of P-wave velocities from seismic reflection data, in combination with laboratory consolidation tests and porosity data from drilling, to predict pore pressure to ~20 km inboard of the trench at the Nankai subduction zone. Seismic interval velocity is used to predict porosity, which in turn is used to estimate effective stress and thus pore fluid pressure (left). One key result of this work is that the strength of the plate boundary mega-thrust remains anomalously low and almost constant for ~20 km into the subduction zone, owing to nearly undrained conditions within the subjacent sediments. These results offer a plausible and quantifiable mechanism explaining the apparent absolute weakness of subduction megathrusts globally.

NanTroSEIZE DRILLING PROGRAM

As a major part of my research since 2002-2003, I have been heavily involved in NanTroSEIZE. This ambitious initiative aims to understand subduction plate boundaries through sampling and monitoring a major fault system implicated in historical great earthquakes and tsunamis. Planned boreholes will penetrate the plate boundary fault system at several depths using the Chikyu, a newly constructed Japanese riser drillship (right), and will constitute the most sustained concentration of scientific ocean drilling ever undertaken in one area. Ultimately, as part of this major international collaboration, scientific boreholes will be drilled to unprecedented depths (> 6 km) into an active plate boundary, and over 100 scientists will be involved in shipboard and shorebased science.

I am part of the NanTroSEIZE scientific leadership team, as the Specialty Coordinator for rock physical properties and hydrogeology, and as one of the lead investigators on a subseafloor observatory. As Specialty Coordinator, I provide guidance for sampling and measurements on the full suite of individual expeditions, coordinate postcruise research on rock physical properties, and will lead scientific synthesis efforts in the discipline.

Above: Description of cores on ODP expedition 205 to the Mid-America Trench offshore Costa Rica. Drillbit shown in inset.

Drilling will provide basic data to define rock physical properties, ages, composition, and pore fluid chemistry, as well as samples for laboratory studies of consolidation, permeability, friction, and fabric. We currently have funding for a lab-based project to conduct experiments on core samples obtained from Stage 1 of NanTroSEIZE (this is a collaborative effort with Mike Underwood and Bill Likos at Univ. of Missouri, Chris Marone here at Penn State, and Liz Screaton at Univ. of Florida). The drilling is closely linked with a 3-D seismic survey, which will allow analysis of tectonic and sedimentation history on the margin, as well as extrapolation of rock physical properties from individual boreholes to “map” fault and rock properties in 3-D, as shown in the example above.

3-D SEISMIC PROJECT

NanTroSEIZE drilling is closely linked with a 3-D seismic survey. This seismic survey was funded jointly by NSF and JAMSTEC, and was acquired commercially and processed in 2006-2007. The project provides a terrific opportunity to work on a high-quality seismic data set in the same region where extensive scientific drilling is underway. The seismic survey is a collaborative project with Greg Moore (Univ. of Hawaii), Harold Tobin (Univ. of Wisconsin), Nathan Bangs (Univ. of Texas Institute for Geophysics), and several colleagues at JAMSTEC. Some examples of the seismic data are shown above, in regions of particular interest for drilling at a major splay fault in the vicinity of Sites NT2-01 and NT2-03 shown on the regional seismic figure below (left) and at the trench near Site NT1-03 (right). Our component of this collaborative project includes: linking seismic reflection interpretations with the results from hydrologic models, pore pressure prediction from P-wave velocity and porosity, and detailed analysis of minor fault populations to investigate stress state.

SUBSEAFLOOR OBSERVATORY

This is under construction. Check back soon!

FRICTION AND EARTHQUAKES

This is under construction. Check back soon!

LOW-T METAMORPHISM

This is under construction. Check back soon!

Papers & Selected Abstracts from our work on Subduction Zones

Saffer, D.M., Underwood, M.B., and McKiernan, A.W. (2008), Evaluation of factors controlling smectite transformation and fluid production in subduction zones: Application to the Nankai Trough, The Island Arc, doi:10.1111/j.1440-1738.2008.00614.

Saffer, D.M. (2007), Pore pressure at plate boundaries: Insights from geohydrologic modeling, edited by Ito, H. et al., Scientific Drilling, Special Issue #1, doi: 10.2204/iodp.sd.s01.32.2007, p. 20-23.

Saffer, D.M. (2007), Pore pressure within underthrust sediments in subduction zones, in Dixon, T. et al. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, Columbia University Press, p. 171-209.

Marone, C., and Saffer, D.M. (2007), Fault friction and the upper transition from seismic to aseismic faulting, in Dixon, T. et al. (Eds.), The Seismogenic Zone of Subduction Thrust Faults, Columbia University Press, p. 346-369.

Ikari, M.J., Saffer, D.M., and Marone, C. (2007), Effect of hydration state on the frictional properties of montmorillonite-based fault gouge, J. Geophys. Res., 112, B06423, doi:10.1029/2006JB004748.

Spinelli, G.A., and Saffer, D.M. (2007), Trench-parallel fluid flow in subduction zones resulting from temperature differences, Geochem. Geophys. Geosyst., 8, doi:10.1029/2007GC001673.

Saffer, Demian M., and Bekins, B.A. (2006), An evaluation of factors influencing pore pressure in accretionary complexes: Implications for taper angle and wedge mechanics, J. Geophys. Res., doi:10.1029/2005JB003990.

Spinelli, G., Saffer, D.M., and Underwood, M.B. (2006), Effects of along-strike variability in temperature on the hydrogeology of the Nicoya margin subduction zone, Costa Rica, in press, J. Geophys. Res., doi:10.1029/2004JB003436.

McKiernan, A.W., and Saffer, D.M. (2005), Data Report: Permeability and consolidation characteristics of sediments collected during ODP Leg 205, Costa Rica, online publication, Ocean Drill. Prog. Sci. Results, 205.

Screaton, E.J., and Saffer, D.M. (2005), Fluid Expulsion and overpressure development during initial subduction at the Costa Rica convergent margin, Earth Planet Sci. Lett., 233, 361-374, 2005.

Saffer, D.M., and McKiernan, A.W. (2005), Permeability of underthrust sediments at the Costa Rican margin: Scale dependence and implications for dewatering, Geophys. Res. Lett., 32, L02302, doi:10.1029/2004GL021388.

Spinelli, G., and D. M. Saffer (2004), Along-strike varations in underthrust sediment dewatering on the Nicoya margin, Costa Rica, related to the updip limit of seismicity, Geophys. Res. Lett., 31, L04613, doi: 10.1029/2003GL018863.

Saffer, Demian M., and Marone, Chris J. (2003), Comparison of smectite- and illite-rich gouge frictional properties: Implications for the updip limit of the seismogenic zone along subduction megathrusts, Earth Planet. Sci. Lett., v. 215, p. 219-235.

Saffer, Demian M. (2003), Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with Northern Barbados and Nankai, J. Geophys. Res., 108 (B5), 2261, doi: 10.1029/2002JB001787.

Saffer, D.M., and Screaton, E.J. (2003), Fluid flow pathways at the toe of convergent margins: Interpretation of sharp geochemical gradients, Earth Planet. Sci. Lett., v. 213, p. 261-270.

Henry, P., L. Jouniaux, E. Screaton, S. Hunze, and D.M. Saffer (2003), Anisotropy of electrical conductivity records initial strain at the toe of the Nankai accretionary wedge, in press, Jour. Geophys. Res.

Saffer, D.M., and Bekins, Barbara A. (2002), Hydrologic controls on the mechanics and morphology of accretionary wedges and thrust belts, Geology, v. 30, p. 271-274.

Screaton, E.J., Saffer, D.M., Henry, Pierre, Hunze, Sabine, and Leg 190 Shipboard Scientific Party (2002), Porosity loss within underthrust sediments of the Nankai accretionary complex: Implications for overpressures, Geology, v 30, p. 19-22.

Brown, K., Saffer, D.M., and Bekins, B.A. (2001), Implications of smectite diagenesis and pore water freshening for fluid flow at the toe of the Nankai Wedge, Earth and Planet. Sci. Lett., v. 194, p. 97-109.

Moore, J.C., and Saffer, D.M. (2001), Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low-grade metamorphic processes and increasing effective stress, Geology, 29, 183–186.

Saffer, Demian M., et al. (2000), Inferred pore pressures at the Costa Rica subduction zone: Implications for dewatering processes, Earth and Planet. Sci. Lett., 177, 193-207.

Silver, E.A., M. Kastner, A. T. Fisher, J. D. Morris, K. D. McIntosh, and D.M. Saffer (2000), Fluid Flow Paths in the Crust of the Middle America Trench, Costa Rica Margin, Geology, 28, p. 679-682.

Saffer, Demian M., and Bekins, B. A. (1999), Fluid budgets at convergent plate margins: Implications for the extent and duration of fault zone dilation, Geology, 27, 1095-1098.

Saffer, Demian M., and Bekins, Barbara A. (1998), Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget, Journal of Geophysical Research, 103, no. B12, p. 30,351.

Saffer, D.M., Skarbek, R., Tobin, H.J., and McKiernan, A.W. (invited), From the trench to the seismogenic zone: Establishing links between, fluid pressure, low-T metamorphism, and fault stability, European Geophysical Union General Assembly, 2008.

Saffer, D.M., and Marone, C. (invited), Looking down the subduction conveyor: Investigating the updip limit of seismogenesis, Casey Moore Symposium, Univ. CA, Santa Cruz, 2008.

Ikari, M.J., Saffer, D.M., Marone, C., and Niemeijer, A.R., Frictional and Hydrologic Properties of Clay-Rich Fault Gouge, AGU Fall meeting, 2007.

Skarbek, R.M., and Saffer, D.M., Pore Pressure Development in Sub-Decollement Sediments in Subduction Zones: Insights From Laboratory Data and Numerical Modeling, AGU Fall meeting, 2007.

Ikari, M.J., Marone, C., and Saffer, D.M., Stability of Clay-rich Fault Gouge at Intermediate to High Shear Strain, Euro-conference of "Rock Physics and Geomechanics" on Natural hazards: Thermo-hydro-mechanical processes in rocks, 2007.

Saffer, D.M., and Kopf, A., Modeling pore water B and d11B signatures in the shallow subduction zone forearc: Examples from Barbados, Costa Rica and N. Japan, Goldschmidt Conference, 2007.

Saffer, D.M. (invited), Hydrologic processes from the trench to the seismogenic zone: Establishing links between fluid pressure, and low temperature metamorphism, fluid flow, and fault mechanics, MARGINS Workshop to Integrate Subduction Factory and Seismogenic Zone Studies, Heredia, Costa Rica, 2007.

Spinelli, G.A., and Saffer, D.M., Trench-parallel variations in subduction zone fluid pressure and fault strength resulting from temperature differences, Geological Society of America Fall meeting, 2006.

Saffer, D.M., Underwood, M.B., and McKiernan, A.W., Smectite transformation in the Nankai trough: effects on subduction zone mechanics and hydrogeology, Geological Society of America Fall meeting, 2005.

Saffer, D.M., and McKiernan, A.W., Constraints on Pore Pressure in Subduction Zones From Geotechnical Tests and Physical Properties Data, AGU Fall meeting, 2005.

McKiernan, A.W., Lockner, D., and Saffer, D.M., Frictional behavior of natural clay-rich fault gouges from ODP Leg 190, Nankai Trough, offshore Japan, AGU Fall meeting, 2005.

Spinelli, G.A. and Saffer, D.M., Influence of along-strike temperature differences on 3-D fluid flow patterns in the Nicoya margin subduction zone, Costa Rica, AGU Fall meeting, 2005.