LOGS AND STRUCTURAL GEOLOGY

Structural data were determined from RAB images using Schlumberger's Geoframe software. Full details of structural interpretation methods from resistivity images are outlined "Interpreting Structure from RAB Images" in the "Explanatory Notes" chapter. Structural interpretations reported here were made using RAB images of medium-focused button resistivity, which images at a penetration depth of 3 in (7.6 cm). Comparisons between shallow-, medium-, and deep-focused resistivity images indicated little variation for structural analysis. In situ stress orientation measurements determined from borehole breakouts utilized all three depth images (shallow, medium, and deep).

Hole 808I successfully penetrated both the frontal thrust and décollement zones in the toe of the accretionary prism, allowing detailed structural analysis of these zones and the state of deformation between them. Unfortunately, insufficient penetration of the underthrust section below the décollement zone prevented structural analysis of this zone. In addition, strong borehole breakouts were recorded in the RAB images throughout much of the hole, allowing an initial determination of in situ horizontal stress orientation at Hole 808I and direct comparison with other structural data. An increase in deformation and fracturing is noted at Site 808 in comparison with reference Site 1173. Direct comparisons with structural data from core analysis of Leg 131 Site 808 reveal many similarities. The frontal thrust and décollement zones are easily identified and correlated and, in addition, several more minor fractured intervals can be correlated between Leg 131 cores (Shipboard Scientific Party, 1991) and Leg 196 RAB images (Fig. F11). Methods for interpreting structure and bedding differ considerably between Legs 131 (core analysis) and 196 (RAB image analysis) (see "Interpreting Structure from RAB Images" in the "Explanatory Notes" chapter. This should be considered when comparing the two data sets directly.

Fractures were differentiated according to resistivity as conductive vs. resistive (Fig. F12). Conductive fractures are generally confined to major deformation zones and are interpreted to represent open fractures or alternatively fault planes coated with conductive pyrite (as tentatively identified on small fault planes through XRD analysis at Leg 131 Site 808; Shipboard Scientific Party, 1991). Resistive fractures occur mostly between distinct deformation zones and may signify increases in fault gouge (low-conductivity clays), brecciation, or mineralization (although a complete absence of mineral veining was noted in Leg 131 cores). Deformation zones are characterized by generally high resistivity, possibly signifying intense deformation and brecciation with microfaulting (microfaulting is unidentifiable at the resolution of RAB images) or porosity collapse due to overall compactive deformation, cut by discrete conductive fractures (e.g., Fig. F13).

Fractures

As expected, an increase in deformation and fracturing is noted at Site 808 in comparison with reference Site 1173. Fracture frequency (Figs. F11, F12) increases significantly at major deformation zones, such as the frontal thrust zone (~400 mbsf), a major fractured interval at ~560 mbsf, and the décollement zone (~940-960 mbsf). These zones also correspond to increases in deformation seen in Leg 131 Site 808 cores. Fractures at the RAB image scale are relatively sparse between these zones. Deformation (in the form of identifiable fractures) is almost absent between the frontal thrust zone and the fractured interval at 560 mbsf, whereas an increase in background fracture frequency (predominantly resistive fractures) occurs below this depth. These trends are in agreement with patterns of fault frequency recorded at Leg 131 Site 808 (Fig. F11).

A dominant fracture strike of ~northeast-southwest (more specifically, east-northeast-west-southwest) is present throughout the borehole (Figs. F11, F14). This trend deviates slightly from the Nankai plate convergence vector of ~310°-315° (Seno et al., 1993; Fig. F14), but this deviation is within the error of RAB azimuth measurements and may be insignificant. This orientation is particularly clear within the major deformation zones (frontal thrust and 560-mbsf fractured interval) but is more variable between them (e.g., above 400 and at 600-900 mbsf; Fig. F11). A dominant strike of north-south occurs within and just above the décollement zone. Fracture dips range from ~25° to 90° with highest dips recorded within the frontal thrust zone. No evidence of displacement was imaged at this site in contrast to Site 1173 where normal offset was observed. The broad range of fracture dips does not rule out any style of faulting. Fracture dip direction shows some variability throughout Hole 808I with distinct patterns developing within the discrete deformation zones.

Bedding

Bedding dips are predominantly low angle at Hole 808I (almost all dip <25°), with the majority <5°-7° (the minimum resolvable dip on RAB images collected here, shown as 0° on Fig. F15). The total range of bedding dips is 0°-50°. Increases in bedding dip cannot be correlated with zones of deformation (as was observed at Leg 131 Site 808) as bedding planes were difficult to identify due to masking by a generally high resistivity deformation signature. However, an absence of subhorizontal dips (<5°-7°) is observed within these zones and a slight increase in average bedding dip in their vicinity. Predominantly shallow bedding dips were also observed in Leg 131 core data (0°-15°).

Bedding orientation appears to be fairly random above ~650 mbsf (Fig. F15) but with a more dominant northeast-southwest trend below this depth (particularly within the interval 650-800 mbsf) and a north-south trend at the décollement zone. This northeast-southwest trend is similar to fracture orientations in the same interval and throughout the borehole (Figs. F11, F12). A north-south trend of fractures within the décollement zone is also observed.

Identification of bedding planes and differentiation between fractures and beds was problematic within RAB images at Hole 808I, partly because bedding and fracture strike and dip were likely to be similar at this location within the accretionary prism. In addition, the inaccuracy of bedding strike from low-dipping bedding planes is likely to be high due to the resolution of RAB images. The possibility of misinterpretation of bedding planes and fractures is discussed at length elsewhere (see "Interpreting Structure from RAB Images" in the "Explanatory Notes" chapter and immediately below). This is particularly relevant for highly conductive steeply dipping features observed within the frontal thrust zone and the 560-mbsf fractured interval (see "560-mbsf Fractured Interval"), which may represent conductive open fractures or steeply dipping conductive beds.

Frontal Thrust Zone

The frontal thrust zone (389-414 mbsf) represents the most strongly deformed interval in Hole 808I RAB images and in the cores of Leg 131 Site 808 (Shipboard Scientific Party, 1991). In RAB images, the zone is characterized by overall high resistivity with mostly conductive (open?) fractures (Fig. F13). The high resistivity is interpreted as a response to porosity collapse due to overall compactive deformation in the frontal thrust zone. The majority of fractures are oriented ~east-northeast-west-southwest and are south-dipping. A progression from a few north-dipping fractures in the upper part of the deformation zone to dominantly south-dipping in the middle and lower parts of the zone is observed. These south-dipping fractures are antithetic to the northward dip of the main frontal thrust zone (imaged in seismic data). Figure F16 shows a three-dimensional (3-D) image of part of the frontal thrust zone (location indicated on Fig. F13) showing the original cylindrical form of the borehole RAB image and south-dipping conductive fractures. The view is slightly elevated and from the south-southwest (200°). Fractures are mostly steeply dipping (50°-90°) throughout the frontal thrust zone (Figs. F11, F13), with significant increases in dip compared to sections above and below. The frontal thrust zone occurs at a slightly greater depth (389-414 mbsf) than at Leg 131 Site 808 (357-395 mbsf), which may reflect, in part, the horizontal separation of the two boreholes.

We cannot rule out the possibility that some interpreted fractures within this zone may represent conductive bedding planes within a highly deformed high-resistivity zone, as steeply dipping beds were recognized in this interval at Leg 131 Site 808. However, the anomalously high conductivity of the planar features (relative to the surrounding stratigraphy) suggests that they are mostly fractures. Fractures and bedding are likely to have similar strikes and dips in this interval and are therefore likely to be difficult to differentiate. If the conductive features with progressive changes in dip represent bedding planes, this interval provides evidence for folding. Evidence of folding and steep (50°-90°) to overturned beds was also reported within this zone at Leg 131 Site 808 (pp. 115 and 139 in Shipboard Scientific Party, 1991).

The general northeast-southwest strike of fractures is in agreement with measurements from Leg 131 and with the plate convergence vector of ~310°-315° (Seno et al., 1993). Fracture orientations are specifically closer to an east-northeast-west-southwest trend (Figs. F11, F13) as also observed for all fractures within Hole 808I (Fig. F14). This orientation deviates slightly from the convergence vector (Fig. F14) and orientations of borehole breakouts (described in detail below). However, this deviation may be within the error of RAB orientation and convergence vector measurements.

A prominent fractured interval (similar in intensity to the frontal thrust zone ~150 m above) occurs at 559-574 mbsf (Figs. F11, F12). This zone is similar to the other major deformation zones at Hole 808I by being characterized by high-conductivity (open?) fractures within a zone of overall high resistivity. Fracture orientations are very similar to those of the frontal thrust zone (east-northeast-west-southwest) and are steeply dipping to the south. Core analysis of Leg 131 Site 808 also revealed an increase in fault frequency at this interval (Fig. F11). Initial attempts to identify this apparent fault zone within seismic data across Hole 808I have proved inconclusive. As discussed in "Frontal Thrust Zone," we cannot rule out the possibility that several of the conductive features interpreted as fractures may be steeply dipping bedding planes within this interval.

Décollement Zone

In general, the décollement zone and fractured intervals above it show minimal deformation in comparison with the frontal thrust zone and 560-mbsf fractured interval. In the vicinity of the décollement zone, a series of discrete fractured intervals composed of conductive fractures are separated by relatively undeformed sections. This deformed interval occurs between 897 and 965 mbsf, with fractured intervals at 897-904, 937-942, and 959-965 mbsf. This entire deformed zone is characterized by patchy intervals of mottled high resistivity. Analysis of cores from the décollement zone at Leg 131 Site 808 (Shipboard Scientific Party, 1991) observed marked brecciation, which may be represented by the high-resistivity mottled texture at Leg 196 Hole 808I. We define the décollement zone itself as the base of this interval, at 937-965 mbsf (orange dashed lines on Fig. F17). The base of the décollement zone is sharply defined as the maximum extent of conductive fracturing (Fig. F18) and is marked by changes in physical properties (Fig. F17), including a sharp high in density and a low in porosity. The upper boundary of the décollement zone is more difficult to define in RAB images. An alternative interpretation of the décollement zone may constitute only the lowest fractured interval (959-965 mbsf) or it may include the entire deformed 897- to 965-mbsf interval. Our choice of the 937- to 965-mbsf interval is marked by a general increase in fracture frequency and by marked variability in physical properties (Fig. F17). The décollement zone as defined from core data at Leg 131 Site 808 showed both clear upper and lower boundaries at 945-964 mbsf (blue lines on Fig. F17). The Leg 196 décollement zone (937-965 mbsf) is a slightly broader zone than seen in Leg 131 cores (945-964 mbsf) but the bases of the two zones are coincident. Some of these differences may be in part caused by the 107-m horizontal offset between Holes 808C and 808I. It should also be noted that the discrete fractured intervals (containing conductive fractures) show clear lows in resistivity (e.g., 937-942 and 959-965 mbsf; Fig. F17).

Borehole Breakouts and In Situ Stress

Borehole breakouts were recorded throughout much of Hole 808I and indicate a consistent orientation. Breakouts are indicated by two vertical lines of low resistivity (high conductivity) running along the RAB image (Figs. F19, F20). The high-conductivity breakouts are due to elongation of the borehole in the direction of the minimum horizontal compressive stress orientation (Barton, 2000; ₂ in this case) and represent the opening or enlargement of the borehole (hence conductive) in this direction. The two breakout "lines" occur opposite each other (~180° apart) in the borehole, indicating a vertical plane. A consistent northeast-southwest orientation is recorded, with an overall range of N30°-70°E down the borehole and an average of N40°-50°E. This trend is consistent with the plate convergence vector of ~310°-315° (Seno et al., 1993), which would be expected to be in alignment with the maximum horizontal compressive stress orientation, ₁. The orientation of breakouts deviates slightly from the dominant trend of fractures (east-northeast-west-southwest, see preceding discussion and Fig. F14), but whether this deviation is real or within measurement error remains unknown.

In the upper part of the borehole, continuous strong breakouts are recorded between ~270 and 530 mbsf, coincident with log Unit 2 and bracketing the frontal thrust zone (Fig. F12). The development of continuous borehole breakouts through this interval may either be caused by (1) a distinctive stress regime in the vicinity of the frontal thrust zone or (2) the presence of a particular lithology and mechanical state conducive to breakout formation in this interval (see also "Discussion and Synthesis"). Borehole breakouts occur if the tangential stress exceeds the compressive sediment strength; we, therefore, infer with the latter hypothesis that the strength of log Unit 2 is reduced relative to overlying and underlying lithologies. Below this depth (530 mbsf) borehole breakouts are patchy and intermittent with possible examples of conjugate breakouts, and may be induced by changes in drilling parameters.

Figures F19 and F20 show similarities and differences in breakout patterns between shallow (recorded at 1-in depth from the borehole), medium (3-in depth) and deep (5-in depth) images. Figure F19 shows a decrease in breakout resistivity from shallow to deep, suggesting minimal deformation and invasion 5 in away from the borehole. In contrast, Figure F20 illustrates a consistent resistivity signature in all depth images, suggesting a similar degree of deformation and invasion at 1, 3, and 5 in from the borehole. These differences may be related to variable drilling parameters, but their consistency within certain sections of the borehole and the contrasting resistivity of the two intervals (Fig. F19 images generally high resistivity interbedded sediments, whereas Fig. F20 images low-resistivity sediments) suggests that the degree of breakout invasion may be lithologically controlled.