The décollement zone is a plate boundary fault that initiates at the seaward edge of the accretionary prism and increases in displacement with subduction and evolves into the subduction trace between the North American and Caribbean plates. Studying this fault, therefore, provides unique insights on plate-boundary deformation processes. Critical questions include: What is the nature of this plate boundary surface? What is the fluid pressure along it? How does it evolve with time? The accessibility of the décollement zone here was the original motivation for the initial DSDP leg (Leg 78A). The décollement zone was not penetrated until ODP Leg 110, and extensive logs through this difficult-to-drill interval were not obtained until the completion of Leg 171A.
In situ physical properties measurements through the negative-polarity reflections of the décollement zone are critical to calibrate the nature of this reflection. Leg 156 attempted to penetrate and log through a strong negative-polarity reflection at Site 947. This site failed in a zone of hole instability above the décollement zone. Site 1045 was specifically located where the depth to the décollement zone is less than at Site 947, but the negative-polarity reflection remains strong. Site 1045 showed that the low-density interval in the décollement zone correlates with the strong negative-polarity and is probably the most significant result of Leg 171A. This low-density interval is consistent with the low seismic impedance predicted by the waveform modeling (Bangs et al., 1996). This result indicates that the large northeasterly trending negative-polarity area (Shipley et al., 1994) of the décollement zone is also of low density and high porosity.
The LWD data constrain density well and therefore allow seismic models that output impedance (velocity × density) to be interpreted in terms of density. The seismic data at each drill site can be effectively "forward" modeled using the LWD density distribution and a linear increase in seismic velocity (Bangs et al., 1999). Conversely, modeling or "inversion" of the seismic waveforms everywhere through the décollement zone produces density maps of this surface (Fig. F9) (Bangs et al., 1999; Zhao et al., 2000) that provide a regional basis for interpreting processes in the décollement zone. The above authors have converted the density maps to porosity, but the actual amount of pore water is not exactly known because of the water that remains within the opal and smectite mineral structures (Brown and Ransom, 1996). Therefore, viewing the porosity maps as representative of total water content is more appropriate. Using their inversion of the seismic data, Zhao et al. (2000) also have mapped the thickness of the low-density interval that characterizes the décollement and proto-décollement zones. Figure F4 shows Bangs et al.'s (1999) visualization of the low-density interval associated with the décollement and proto-décollement zones.
The individual LWD penetrations of the décollement zone plus the density maps produced by the seismic inversions show that the density of the décollement zone increases from the deformation front with depth beneath the accretionary prism (Fig. F9). As the décollement zone densifies, the contrast between it and the immediately overlying accretionary prism is erased. Because the density contrast at the base is larger than at the top, the basal density contrast remains after the overlying contrast is lost. The residual basal density contrast provides a positive impedance contrast that results in the positive seismic reflection that characterized the décollement zone after dewatering.
A northeasterly trending area of reduced density that mimics the area of strong negative polarity (see Figs. F2, F9; Frontispiece 2) is the prominent exception to the trend of densification along the décollement zone west of the deformation front. The penetration at Site 1045 shows that the low-density interval is thinner and slightly higher in density than that observed at either of the sites east of the deformation front. Therefore, the density anomaly in the northeasterly trending area can be interpreted as a residual of the primary density profile. This northeasterly trending area of reduced density is interpreted as an interval of arrested consolidation (Bangs et al., 1999; Moore et al., 1998b; Zhao et al., 2000). The lower density and higher fluid content of this zone requires that the fluid support a greater portion of the overburden than adjacent more dewatered areas. Hence, the area of negative polarity probably has a higher fluid pressure. The occurrence of high amplitude reflections or "bright spots" above the northeasterly trending zone is consistent with leakage of fluids from the zone of negative polarity (and high fluid pressure) into the overlying prism (Shipley et al., 1997; Bangs et al., 1999).
Studies of shear (S)-wave velocities and anisotropy suggest that the base of the accretionary prism is underconsolidated (Peacock and Westbrook, in press). The low S-wave velocities and high Poisson's ratios of these sediments near Sites 948 and 949 confirm direct observations of low porosity of the sediments above the décollement zone. S-wave splitting studies indicate little alignment in pore space in sediments above the décollement zone, suggesting that the sediment is underconsolidated, not hydrofractured. Underconsolidation above the décollement zone is also indicated by the density distribution at Site 947 and the high-amplitude seismic reflections just above the décollement zone (Bangs et al., 1999).
The arrested consolidation of the décollement zone could be achieved by features that reduce the permeability of the décollement zone conduit or its drainage to adjacent sediments or by enhancement of flow from deep sources. Simulations of bulk density distributions in the décollement zone show that the low-density, apparently arrested consolidation examples can be maintained by reasonable rates of fluid flux from depth (Stauffer and Bekins, in press). Moreover, the documentation of high-angle faults compartmentalizing the décollement zone (DiLeonardo et al., in press) suggests that flow from depth along the décollement zone may be focused laterally and retarded at it updip termination, helping to arrest consolidation.
A popular previous view interpreted the northeasterly area of negative-polarity seismic reflections and low density as hydrofractures created by true physical dilation and superlithostatic fluid pressures (Bangs et al., 1996; Brown et al., 1994; Shipley et al., 1994; Moore et al., 1995b). Extensional veins in the décollement zone (Labaume et al., 1997; Vrolijk and Sheppard, 1991) indicate localized extensional fractures that could lower the impedance. However, the fact that the LWD densities in the décollement zone at Site 1045 are slightly higher than that at the reference sites argues against dilation and hydrofracture. Because the density is measured ~40 min after the hole is cut, some have argued that density values lower than those measured may have been eliminated as a result of the collapse of the hydrofractures. However, the seismic inversions (Bangs et al., 1999; Zhao et al., 2000) indicate that the density in the areas of negative-polarity reflections in the décollement zone is higher everywhere than the density in the proto-décollement zone. This observation supports the view that the negative-polarity reflections are explained by arrested consolidation, not hydrofracture.
In summary, we favor the interpretation of the northeasterly-trending zone of negative polarity as a result of arrested consolidation, as it represents a simpler interpretation of the available data than the dilative hydrofracture option.
Processes that may cause the systematic densification of the décollement zone (Fig. F9) include collapse of radiolarian porosity, collapse of intergranular porosity at all scales, cementation, and mineral phase transitions.
Analysis of the radiolarian porosity shows that it is largely gone because of the physical collapse of the radiolarians, infilling tests with authigenic mineral phases, and dissolution of tests for décollement zone sediments at Sites 671 and 948, the farthest west penetration of the décollement zone (Wallace and Moore, unpubl. data; T. Steiger, pers. comm., 1998; Ogawa, 1993). However, as the radiolarian porosity averages only ~5% in the incoming section, its complete destruction contributes little to the overall densification of the décollement zone (Wallace and Moore, unpubl. data). More significantly, scanning electron microscope (SEM) studies indicate a significant decrease in intergranular porosity due to the collapse of clay mineral fabric, which can account for 65% to 90% of the observed density change (Wallace and Moore, unpubl. data).
Densification caused by phase changes occurs because of the loss of interlayer water in smectite, phase changes in opal fossil tests, and precipitation of zeolites. Experiments suggest that the smectite in the décollement zone is dewatering to some degree (Fitts and Brown, 1999), and X-ray diffraction studies suggest that the opal is transforming to opal-CT and quartz in the décollement zone (T. Steiger, pers. comm., 1998). Zeolites also fill radiolarian tests and are in the clay matrix (Wallace and Moore, unpubl. data). All of the above processes densify the décollement zone but are less important than the collapse of intergranular porosity between clay-sized particles.
We believe that the décollement zone initiates in the radiolarian mudstone because it is of low density and, therefore, weaker than adjacent sediments (Fig. F5). However, densification with underthrusting eliminates any preferential weakness caused by low density. The correlation of the décollement zone and radiolarian mudstone is strong, as viewed in the drill holes (Fig. F8) and, arguably, can be continued beyond the boreholes with the seismic reflection data (Wallace and Moore, unpubl. data). If the décollement zone densifies with underthrusting, then why does it remain localized?
Experiments with clays indicate they decrease in strength from 25% to 75% of their peak shear strength because of the orientation of the platy clay minerals (Lupini et al., 1981). SEM images of the décollement zone document well the zonal orientation of clay minerals (Labaume et al., 1997; Wallace and Moore, unpubl. data). Thus, this strain-softening phenomena in clays apparently overcomes any tendency toward increased strength caused by low density. Moreover, any overpressuring of the décollement zone associated with its localized fluid flow would weaken it and favor its persistence. Finally, even though the décollement zone densifies, the sediments surrounding it also densify, tending to partially perpetuate the relatively low density of the décollement zone.
Leg 171A resulted in an unanticipated measurement of permeability over the 45 m separating Sites 1046 and 949 (Screaton et al., in press). Site 949, with a previously installed borehole monitoring system (CORK), showed a pressure response caused by LWD packer tests at Site 1046. Modeling of this response yields a permeability of 10-14 m2, which is 1–2 orders of magnitude higher than that expected from previous single-hole operations at Site 949. This two-hole measurement is considered robust; if it is representative of the entire décollement zone, then the décollement zone could drain fluid at the rate that it is produced (Screaton et al., in press). Although the measurement may correctly represent the permeability in the vicinity of Site 1046, continuing densification with underthrusting or compartmentalization of the décollement zone by high-angle faulting (DiLeonardo et al., in press) may account for a further decrease in décollement zone permeability.