SUMMARY OF SCIENTIFIC RESULTS
Development of the Décollement Zone, Muroto Transect
Leg 190 completed a transect of the basal décollement of the Nankai accretionary prism from an undeformed state at Site 1173 to the well developed detachment fault landward of the deformation front documented at Sites 1174 and 808 (Fig. 36).
At Site 1173, there is little structural or physical properties evidence for
a proto-décollement zone, i.e., incipient deformation indicative of a
detachment fault. The stratigraphic equivalent to the Site 1174 décollement
interval indicated in Figure 36 is based on correlation of core magnetic
susceptibility (Fig. 8). This interval is part of a thicker domain of elevated
bedding dip, but shows no localized increase in observed deformation.
However, a marked downhole decrease in P-wave velocity and a slight
porosity increase at the top of the interval (~389 mbsf) suggest that a
subtle mechanical strength discontinuity could contribute to the localization
of the décollement in this interval. Pore fluid chlorinity also shows a small
low-chloride excursion above this interval and an abrupt transition to higher
values at ~390 mbsf; however, there is not a clearly corresponding feature
in the Sites 1174 or 808 chlorinity data. It is unknown, of course, whether in
the future the décollement actually will propagate along this particular
stratigraphic horizon to the position of Site 1173.
The hallmark of the décollement zone at Sites 808 and 1174 is intense
brittle deformation, manifested as finely spaced fracturing that breaks the
mudstone into millimeter- to centimeter-scale fragments (Fig. 19). The
fragments have polished and slickenlined surfaces, showing complex and
heterogeneous slip directions, but they do not exhibit obvious internal
deformational structures at the core scale. At Site 1174, the upper limit of
the décollement zone is marked by a sharp increase in the intensity of
brecciation, although the lowermost prism section above exhibits distributed
fracturing as well. Within the décollement zone, there is a downward increase
in intensity of the brecciation, peaking in a 7-m-thick zone of fine
comminution of the mudstone just above the very sharp base of the
décollement zone (Fig. 20). Within the fault zone, there are several intervals,
up to tens of centimeters thick, of unbroken mudstone, which are
interpreted as intact blocks in a multistranded shear zone.
It is remarkable that the décollement zone at Site 1174 appears to be at
least equally well developed as it is at Site 808. It is thicker at Site 1174
than it is at Site 808 (32.6 vs. 19.2 m thick, respectively). It is also
brecciated to a finer scale, despite the more landwardthus presumably
more structurally evolvedposition of Site 808. Differences in the observed
structures could be explained by differences in core recovery; however, the
greater thickness at Site 1174 could not. Notable at both sites is the
complete absence of veins, alteration zones, or other evidence of past fluid
rock interaction specific to the décollement.
The development of the décollement and the strain discontinuity across it
are clearly exhibited in the core physical properties data. At Site 1174,
there is a sharp porosity increase and P-wave velocity decrease right at the
base of the structurally defined décollement. These same features are even
more pronounced at Site 808, and both Sites 808 and 1174 exhibit evidence
of a porosity minimum within the décollement. At both sites, however, the
most prominent feature is the discontinuity across the base of the zone
crossing into the underthrust section. This discontinuity is likely due to a
combination of undercompaction of the rapidly loaded underthrust section
(e.g., Saffer et al., 2000) and enhanced tectonic compaction of the prism
and décollement caused by the imposition of lateral tectonic stress (Morgan
and Karig, 1995).
In summary, the décollement beneath the toe of the Nankai accretionary
prism develops from an unremarkable and homogeneous interval of
hemipelagic mudstone into a 20- to 32-m-thick zone of strong brittle
deformation, the base of which marks a boundary between the distinct
physical/mechanical regimes of the prism and the underthrust section. The
two drilling penetrations of the fault zone suggest an anastomosing system
of discrete brittle shears, reminiscent of faults observed in mudrocks on
land. Despite a major effort to detect fault-channeled fluid flow, there is no
evidence that a chemically distinct fluid is being channeled in the décollement
zone along the transect defined by these three sites.
Correlation of the décollement horizon between Muroto and Ashizuri Transects imposes an intriguing question on the localization of the décollement in the lower Shikoku Basin mudstone. Although DSDP/ODP drilling has not penetrated the décollement at the Ashizuri Transect, a clear and continuous seismic reflector allows us to correlate the décollement horizon at the toe region to Site 1177 (Fig. 37). At Site 1177 this reflector is at 420 mbsf and coincides with the identical horizon of the décollement of the Muroto Transect based on chronological and magnetic susceptibility correlations. It raises an important question as to why the décollement stays at the same stratigraphic horizon despite the fact that there is a major difference in the thickness of turbidites and the lithology and diagenesis of the Shikoku Basin sediments between the two transects. This question should be challenged by further shore-based study.
Summary of Scientific Results: Geochemical Gradients, Muroto Transect | Table of Contents