Sediment deformation, diagenesis, and faulting at
the Nankai accretionary prism
Principal Investigator: |
Juli Morgan (Rice University) |
Collaborators: |
Maria V.S. Ask (Lulea, Sweden) Daniel E. Karig (Cornell Univ.) Greg Moore (University of Hawaii) and colleagues around the world |
Graduate Students: |
Blanche Sunderland, TBD |
Funding Sources: |
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).
Figure 1
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].
Figure
2
Figure
3
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).
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.
Figure
5
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.
Papers:
Morgan, J.K., E.B. Sunderland, M.V.S. Ask, 2007,
Deformation and diagenesis at the Nankai subduction zone: Implications for
sediment mechanics, dŽcollement 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 dŽcollement 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., S‡nchez-G—mez, 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. S‡nchez-G—mez, 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. DŽcollement
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.