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Structural geology

The main structural geology goal during Expedition 334 was to describe and document style, geometry, and kinematics of structural features observed in the cores. At Site U1381, cores from the sedimentary sequence (~95 m) and the underlying oceanic basement basalt of the Cocos plate were retrieved. Gently dipping (mostly <15°) bedding is the only structure observed in the cores retrieved from the sedimentary sequence. The basement basalt contains fractures, some of which are filled with vein minerals. The dip angle of the fractures and of the veins shows a scattered distribution from >0° to 86° (Fig. F11).

Structures in sediment

Bedding dips were the only structures observed in the sediment cores retrieved at Site U1381. No brittle deformation such as faults or fractures was recognized in the sediment cores. Bedding planes were recognized as boundaries between different sediment compositions or grain sizes. Bedding in the sediments at Site U1381 dips gently (<15°), as shown in Figure F11.

Structures in basalt

Many fractures and mineral-filled veins (mostly calcite) were identified in the basement basalt (Fig. F12). The population frequency of the fractures was estimated as <10 fractures/10 m in the uppermost part of the basalt sequence (95–140 mbsf) and >10 fractures/10 m in the lowermost part (140–164 mbsf). It appears that the density of the veins increases downhole (Fig. F11). Some of the fractures exhibit an alteration zone filled with greenish clay minerals and pyrite (Fig. F12; see “Lithostratigraphy and petrology” for a detailed description). Vein thicknesses are mostly <1 mm. The dip angle of the fractures shows a scattered distribution from 0° to 86° (Fig. F11). Sedimentary sequences interbedded in the basement basalt have bedding planes with gentle dips (<20°).