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Structural geology

Cores recovered from Site U1414 consist of a sedimentary sequence (~375 m) and basalt flows (~96 m) from the Cocos Ridge. Between 437 and 439.37 mbsf, a horizon of sediment is intercalated within the basalt. Bedding was determined throughout the sedimentary sequence. The dipping attitude varies from subhorizontal to gentle with a maximum value of 29° (Fig. F25). A single value of 45° was observed close to the contact with Cocos Ridge basalt. Sediment near this contact shows well-developed foliation and indications of low-grade metamorphic overprint.

Structures within the sedimentary sequence

Dipping is generally subhorizontal throughout the hole, except between 65 and 70 mbsf and between 287 and 289 mbsf, where dipping angles vary between 24° and 29° (Fig. F25). In Unit III, at ~360 mbsf, a single observation of bedding dip angle shows an inclination of 45°. After paleomagnetic correction, strikes of the bedding planes in lithostratigraphic Units I and II trend NNW–northeast (Fig. F26), subparallel to the extension of the Cocos Ridge.

Faults at various dip angles are present throughout the sedimentary sequence (Fig. F25). Some of these faults were identified as high-angle normal faults. Because most of the faults are healed, a determination of sense of shear was not possible. Although the orientations of these faults after the paleomagnetic correction are scattered, an apparent northwest orientation was identified. Given the lack of borehole breakouts at this site (see “Downhole logging”), σ2 and σ3 are expected to be close to equal. This supposition is in agreement with the observed distribution of fault plane data. However, given the small number of both corrected structural data and paleomagnetic data from the interval (see “Paleomagnetism”), shore-based analyses will be necessary.

The lower part of Unit III is characterized by well-developed foliation with dip angles ranging from subhorizontal to subvertical (Fig. F25). The foliation is characterized by millimeter-scale spacing. In distinct domains, coarse clasts derived from bioturbation structures are embedded within the fine-grained foliated matrix (Fig. F27). Around these clasts, flow structures, similar to pressure shadows, can be observed. The foliation is crosscut by calcite-filled veins with various orientations. The wall rocks along the veins are often fractured into fragments or even brecciated (Fig. F28). Additionally, high-angle normal faults are associated with the veins.

The sedimentary interval interbedded within the basalt between 437 and 439.37 mbsf is characterized by horizontal to subhorizontal sedimentary structures (lamination and foliation) and a small population of mineral veins. The sediment is mainly composed of asymmetrical sandstone elements embedded in finer grained matrix.

Structures within the basement sequence

Structures in the Cocos Ridge basalt mainly comprise mineralized veins at various orientations. A preferred orientation of strike directions was not observed. Some veins show straight boundaries; others are characterized by irregular geometries. Vein mineralization mainly consists of carbonate, quartz, and pyrite. Quartz is often characterized by the fiber growth of crystals perpendicular to vein boundaries that indicate the direction of extension.