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doi:10.2204/iodp.proc.314315316.124.2009

Structural geology

The overall objective of drilling at Site C0002 was to sample the sediments that comprise the lower sections of Tertiary Kumano Basin as well as the more deformed sediments beneath the basal unconformity. These buried sediments are considered to be part of the Shimanto accretionary prism, which is exposed onshore in southwest Japan only a few tens of kilometers from the drill site. With this goal in mind, Expedition 315 scientists cored sediments from 0 to 200 m CSF and 475 to nearly 1100 m CSF and retrieved samples and data from both above and below the unconformity. Structural geologists identified and documented over 500 structural features in the cored sediments, of which nearly 80 were oriented with shipboard paleomagnetic data. The results are consistent with observations and interpretations of LWD data collected during Expedition 314, which indicate north–south extension and suggest that the preliminary conclusions from Site C0001 (i.e., in the slope apron sequence) also extend to Site C0002. Specifically, Site C0002 records an early phase of northeast–southwest extension, similar to the deformation recorded in the slope apron at Site C0001, as well as a later period of north–south extension not recorded at Site C0001.

Types and distribution of structures

Structural geologists working at Site C0002 identified five main structural features and one relatively minor feature (Fig. F7). The main features observed were steepened bedding, faults, breccia, shear zones, and vein structures (Figs. F7, F8, F9). In Hole C0002D, bedding was the only observed structure, and no deformational structures were observed in Hole C0002C. Fissility occurred only in the upper sections of Hole C0002B and was consistently oriented parallel to bedding; thus, it was grouped with bedding. All of these features, except fissility, were also identified and described at Site C0001; therefore, only differences in textures and distributions of structures at this site are noted here.

Steepened bedding

Bedding is subhorizontal to 80 m CSF, dipping mostly <5° (Fig. F9) in Hole C0002D. From 80 to 160 m CSF the range of bedding dip increases to as much as 10°, and the range increases as much as 17° at 203 m CSF close to the bottom of the hole (Fig. F9). It is uncertain whether this relatively high dip at 203 m CSF is meaningful or not, because no cores were recovered between 160 and 203 m CSF. Combined with data in Hole C0002B from 475 to 1057 m CSF, the dip at 203 m CSF may be exceptional because dip is generally <10° from 475 to 600 m CSF (Fig. F9). In Hole C0002B, the mean dip of bedding increases progressively from near 0° at 500 m CSF to an average of 45° in the accretionary prism at the bottom of the hole (bedding, Fig. F7A). The range in bedding dip also increases with depth such that near the bottom of the hole bedding ranges from 20° to 80°. This wide range in dip with depth and the absence of clear peaks in dip magnitude limits correlation of specific changes in bedding dip with other features. However, subtle peaks in bedding dips occur at 750, 850, 950, and 1025 m CSF.

Faults

Faults (Fig. F7B) are relatively rare in the upper 400 m of Hole C0002B. Although a small cluster of moderately dipping faults occurs at ~700 m CSF, most of the observed faults are present in two clusters below 900 m CSF (Fig. F8). One cluster ranges from 920 to 950 m CSF, whereas the other cluster shows more scatter in terms of depth distribution and ranges from 1000 to 1050 m CSF. These two clusters of faults also record the greatest range in dip as well as the highest dip magnitude (i.e., 90° at 920 m CSF) of all of the faults at this site. The upper cluster of faults is just below the top of Unit IV, although a few faults are also present above the boundary. LWD data (i.e., resistivity) and shipboard physical properties show that this boundary records the largest contrast in physical properties and probably represents the top of the accretionary prism. Cores also show well-developed stratal disruption below this boundary (Fig. F7C). The lower cluster of faults occurs deeper within Unit IV. Although much of the deformation represented by faulting at Site C0002 occurs below the Unit III/IV boundary, faults are also present above the boundary. Normal faults are particularly well organized close to and just above the boundary, consistent with core observations indicating that they are the youngest structures. Systematic analyses of the fault geometries, histories, and kinematics of the faults just above and below the boundary at 920 m CSF, however, have not been completed.

Shear zones

Shear zones (Fig. F7D) are less common than faults but show a similar distribution with depth, although only three shear zones were recognized above 900 m CSF (Fig. F8). Shear zones occur in two clusters, one between 920 and 940 m CSF and the second between 1025 and 1050 m CSF, similar to the distribution of faults.

Breccia

Breccia (Fig. F7B) is particularly well developed in the lower sections of Hole C0002B, especially near the top and bottom of Unit III and in the accretionary prism beneath the unconformity. The extent of brecciated zones was probably larger than indicated by core-based observations, as recovery was generally poor in these intervals.

Vein structures

Vein structures are particularly well developed between 850 and 920 m CSF (Fig. F8), and a wide variety of forms and shapes are displayed (Fig. F7D). This interval generally correlates with lithostratigraphic Unit III, which is a clay-rich hemipelagic mud sandwiched between accreted sediments below and silty-clay hemipelagic sediments above. Unit III also has higher CaCO₃ content than the overlying and underlying units (see “Lithology” and “Organic geochemistry”). Vein structures, although similar to vein structures observed at Site C0001, are generally larger in both height and width, often extending parallel to the core axis for tens of centimeters. S-shaped (or sigmoidal) vein structures are also present but are more elongate in the direction of the core axis. Although many of the vein structures appear perpendicular to bedding, they are more widely spaced than those observed at Site C0001. These more widely spaced veins are also generally thicker, suggesting that the width, or size, of the vein structures is a function of the availability of vein-filling material in the wall rock.

Crosscutting relations and kinematics

Three phases of deformation are suggested by the textures and crosscutting relations observed in the cores. An early phase of thrust faulting (and possibly strike-slip faulting) is followed by two phases of normal faulting with the first recorded by shear zone formation and associated with northeast–southwest extension and the second recorded in normal fault formation and associated with north–south extension concentrated in Unit II at ~750 m CSF. Importantly, because of the limited time spent at this site and the limited number of structural geologists on board at the time of Site C0002 coring, this progressive history is based on only a few observations; thus, future work on the cores may refine the various relations or substantially change the progressive history. The proposed history, however, is consistent with the known or inferred geologic history of the region and with the interpretation of seismic reflection profiles that show late-stage normal faults associated with north–south extension.

Of the planar and linear features observed in the cores, ~20% (i.e., 80 of the over 450 features recorded) were reoriented to true north using nearly 50 individual paleomagnetic poles obtained from the shipboard paleomagnetic laboratory. Of these features, ~30 were bedding and 20 were vein structures with the remainder comprising faults, shear zones, fissility, and breccia. The remaining data could not be reoriented because of extensive whole-round sampling, magnetic overprint related to the magnetic properties of the mud used and/or generated during drilling, and drilling-related disturbances. Reoriented data provide key insights into the kinematics and the state of stress in sediments cored at Site C0002.

Bedding orientations corrected for drilling-related rotations provide a fundamental guide to the degree of deformation at Site C0002 (Fig. F10). Bedding is generally horizontal at this site, although a few poles trend west–southwest, suggesting east–northeast tilting.

The geometries and kinematics of the structural features observed in the two intervals of deformation at Site C0002 vary significantly. The upper interval, at the top of Unit IV, contains both shear zones and faults with normal offset, whereas the lower interval contains faults and shear zones with a variety of offsets, including thrust, normal, and strike slip. When corrected for drilling-related rotations, the normal faults and shear zones in the upper interval define two populations. The youngest population, based on limited crosscutting relations, is a set of variably oriented normal faults. Although there are relatively few data, the available reoriented slickenlines generally trend north–south (Fig. F11). Two additional reoriented faults with normal offset also have geometries consistent with north–south extension, whereas a third fault dips west, suggesting east–west extension. Shear zones represent the older populations of structures from this interval, and although they also record normal displacements, they display a different geometry than normal faults. Shear zones generally strike northwest and dip steeply west or gently east (Fig. F12). Although slip lineations were not observed for these structures, five of the eight shear zones have normal displacements, suggesting northeast–southwest extension.

The lower interval of deformation (i.e., below ~1000 m CSF) records a variety of structures, although only a few have been reoriented. Reoriented structures with known kinematics include two thrust faults, two strike slip faults, and two zones of relatively intense brecciation (Fig. F13). Visual inspection of these limited data indicates that they are all consistent with northwest–southeast shortening, suggesting that they are kinematically related. Kinematics for the group as a whole show north–south shortening (Fig. F13).

Discussion

Drilling at Site C0002 successfully penetrated and sampled the lower Kumano Basin sequence, as well as the underlying accretionary sequences considered to be part of the Tertiary Shimanto belt. Note that fossils retrieved from this site (see “Biostratigraphy”) are younger than the Shimanto belt exposed on land. Structural data collected from this site show three phases of deformation that are consistent with

1. Northwest–southeast shortening concentrated primarily in Unit IV and interpreted to be the accretionary prism;
2. A middle phase of northeast–southwest extension along the margin, which is similar to results obtained from Site C0001; and
3. A final, and still ongoing, phase of generally north–south extension recorded primarily in structures in Unit II from the Kumano forearc basin.

The kinematics of the last phase of deformation are consistent with LWD borehole breakout data obtained during Expedition 314. Additional data obtained at the Kochi Core Center or through future drilling programs are necessary, however, to support these preliminary results.

These preliminary results also provide new data for developing and testing hypotheses related to the early history of the accretionary prism and to better understanding the transition from accretion to extension as is seen across the Kumano Basin transect. For example, do the similarities in progressive deformation histories we have identified in Sites C0001 and C0002 represent the advection of the accreted rocks through a steady strain field, or has the strain field changed through time? Also, both sites record extensional deformation near the basin/prism contact where unconformities appear to be present. Is it possible that the time gaps indicated by unconformities are due to the structural removal of the section (e.g., through regional scale sliding or slumping)? These questions, among others, will be investigated in collaboration with Expedition 314 and 315 shipboard parties as NantroSEIZE experiments continue.

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