Sediment deformation, diagenesis, and faulting at the Nankai accretionary prism
Juli Morgan (Rice University)
Maria V.S. Ask (Lulea, Sweden)
Daniel E. Karig (Cornell Univ.)
Greg Moore (University of Hawaii)
and colleagues around the world
Blanche Sunderland, TBD
NSF OCE; JOI-USSSP
I have been studying deformational processes at active convergent margins for decades, including my graduate studies with Dan Karig at Cornell University, and several recent marine surveys. This research has been facilitated by repeated drilling ventures carried out by the Ocean Drilling Program (ODP) and more recently, the Integrated Ocean Drilling Program (IODP).
My early interests involved understanding the nature of deformation in sediments accreted to the toe of the Nankai accretionary prism (Figure 1), southeast of Japan, in efforts to quantify strain and to distinguish timing, mode, and hydrologic influences in deformation. I pursued several approaches to this end, including measuring mineral preferred orientations in marine sediments as indicators of penetrative tectonic strain [Morgan and Karig, 1993, 1995b], and employing a finite element solution for estimating distributed deformation in accretionary prism toes. This latter project, which relies on the stratigraphic and structural configuration of the prism, and physical properties of the sediments obtained from seismic interval velocities, led to a numerical technique for balancing strains in domains which have experienced substantial volume change. This is a problem that has defied analysis using standard models and methods. I have applied this technique to several transects across the Nankai prism, as well as the Cascadia prism, off the central coast of Oregon [Morgan et al., 1994; Morgan and Karig, 1995a; Morgan, 1997]. The results demonstrate that deformation is accommodated by several different modes, including porosity loss and consolidation, but also by grain reorganization and rotation. In some settings, irreconcilable seismic velocities suggest that in some regions, sediments have experienced significant cementation, which has implications for the strength and behavior of these materials [Morgan, 1997].
New samples and observations from the Nankai margin were obtained during ODP Legs 190 and 196 in 2000 and 2001, respectively (Figures 2 and 3). The goals of this survey included characterizing physical properties and deformation along a transect across the prism, and quantifying pore pressures and fluid flow, and their influence on stress state and structure of the toe of the prism. Anomalies in porosity depth distributions indicate that certain regions are not undergoing the expected consolidation with burial (Figure 4).
Our group obtained samples with which we constrained clay mineral microstructure and fabrics, providing evidence for in-situ diagenesis and alteration that appear to influence physical and mechanical properties [Sunderland and Morgan, 2004]. Additionally, we carried out laboratory deformation experiments to constrain the in-situ consolidation stress of sediments in front of and beneath the accretionary prism [Morgan and Ask, 2004]. In all cases, these sediments exhibited strengths in excess of those predicted by sediment porosities (Figure 5), and also stress-strain behavior indicative of apparent "overconsolidation". These responses are thought to be induced by diagenetic hardening during burial.
The enhanced strengths of these sediments demonstrates that they are able to support greater stresses than predicted by their porosities, and thus, porosities alone cannot be used to infer in-situ effective stress state (Figures 4 and 5). This has great implications for accretionary prism mechanics and evolution, because it indicates that high porosities as observed beneath the Nankai decollement fault are not a result of pore fluid overpressures, but rather, are maintained by intergranular cementation. Such cementation may not be maintained under sudden stress pulses as might be induced during great megathrust earthquakes, leading to rapid breakdown of cementation, generation of overpressures, and dramatic weakening of the fault zone during earthquakes. Such possibilities need to be tested for during future drilling and experimental studies [Morgan et al., 2007].
A new IODP drilling venture referred to as NanTroSEIZE will begin in Fall 2007, and continue for years to come. Again, these operations will target the best known subduction zone in the world, the Nankai accretionary margin, and will involve multiple drilling legs in the frontal portions of the accretionary prism, as well as deeper sections. The ultimate target is the seismogenic zone itself, with the objectives of installing long-term monitoring devices to record pore pressures, earthquake distributions, and deformation through time and space. This will provide many student and collaborative opportunities to get involved in topical studies along active convergent margins involving fault and earthquake mechanics, stress conditions and associated physical properties, sediment deformation and diagenesis, and more.
Morgan, J.K., E.B. Sunderland, M.V.S. Ask, 2007, Deformation and diagenesis at the Nankai subduction zone: Implications for sediment mechanics, décollement initiation, and propagation, in The Seismogenic Zone of Subduction Thrust Faults, edited by Dixon, T., MARGINS Theoretical Institute Series, Columbia University Press.
Morgan, J.K. and M. Ask, 2004, Consolidation state and strength of underthrust sediments and evolution of the décollement at the Nankai accretionary margin: Results of uniaxial reconsolidation experiments, J. Geophys. Res., 109, B3, B03102, doi: 10.1029/2002JB002335.
Sunderland, E.B., and J.K. Morgan, 2004, Microstructural variations in sediments from the toe of the Nankai accretionary prism: Results of scanning electron microscope analysis, Proc. ODP, Sci. Results, 190/196. [Online: http://www-odp.tamu.edu/publications/190196SR/VOLUME/CHAPTERS/212.PDF]
Ujiie, K., Hisamitsu, T., Maltman, A.J., Morgan, J.K., Sánchez-Gómez, M., and Tobin, H.J., 2003, Deformation structures and magnetic fabrics at Site 1178: implication for deformation history recorded in accreted sediments at an evolved portion of the Nankai accretionary prism. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196. [Online: http://www-odp.tamu.edu/publications/ 190196SR/VOLUME/CHAPTERS/202.PDF].
Moore, G.F., A. Taira, A. Klaus, L. Becker, B. Boeckel, B.A. Cragg, A. Dean, C.L. Fergusson, P. Henry, S. Hirano, T. Hisamitsu, S. Hunze, M. Kastner, A.J. Maltman, J.K. Morgan, Y. Murakami, D.M. Saffer, M. Sánchez-Gómez, E.J. Screaton, D.C. Smith, A.J. Spivack, J. Steurer, H.J. Tobin, K. Ujiie, M.B. Underwood, and M. Wilson, 2001, New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg190, Geochem. Geophys. Geosyst., 2, 10.129/2001GC000166.
Morgan, J.K., 1997, Kinematic constraints on porosity change in the toe of the Cascadia accretionary prism: Evidence for cementation and brittle deformation in the footwall of the frontal thrust, J. Geophys. Res. B., 102, 15,367-15,383.
Morgan, J.K., and D.E. Karig, 1995b. Décollement processes at the Nankai accretionary prism: propagation, deformation, and dewatering. J. Geophys. Res. B. 100, 15,221-15,231.
Morgan, J.K., and D.E. Karig, 1995a. Kinematics and a balanced and restored cross-section across the toe of the eastern Nankai accretionary prism. J. Structural Geology, 17, 31-45.
Karig, D.E., and J.K. Morgan, 1994. Stress paths and strain histories during tectonic deformation of sediments. In Geological Deformation of Sediments, edited by A. Maltman, Chapman and Hall, London. p. 167-204.
Morgan, J.K., D.E. Karig, and A. Maniatty, 1994. The estimation of diffuse strains in the toe of the Nankai accretionary prism: A kinematic solution. J. Geophys. Res. B, 99, 7019-7032.
Morgan, J.K., and D.E. Karig, 1993, Ductile strains in clay-rich sediments from Hole 808C: Preliminary results using X-ray pole figure goniometry. In Proc. ODP, Scientific Results, 131, 141-155.