Summary of Scientific Results: Geochemical Gradients, Muroto Transect | Table of Contents


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 landward‹thus presumably more structurally evolved‹position 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