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doi:10.2204/iodp.proc.334.104.2012

Structural geology

The main structural geology objectives during Expedition 334 were to describe and document style, geometry, and kinematics of structural features observed in the cores. Overall, structural data show significant changes below ~600 mbsf (i.e., increase in bedding dip, increase in fault population density, and development of highly fractured and brecciated zones) (Fig. F16). This transition corresponds broadly to the base of lithostratigraphic Unit II (see “Lithostratigraphy and petrology”). In Unit II, structures are dominated by gently dipping bedding (<20°) and normal faults. In Unit III, bedding dips and fault density increase. We identified four fault zones at 642.1–652.8, 766.2–777.5, 804.1–804.7, and 871.8–879.5 mbsf.

Structures in slope sediments

Bedding

Bedding planes were recognized as boundary surfaces of thin layers in lithologies such as sand and ash, as boundaries of compositional or grain size difference, and also as thin lamination boundaries.

Structural data from the sedimentary sequence of Hole U1379C indicate increasing bedding dips below 600 mbsf (Fig. F16). In the uppermost part of the section (0 to ~200 mbsf), bedding dips gently (mostly <20°). In the lowermost part of the sedimentary sequence, from 600 mbsf to the top of the basement, bedding dips are 0°–60°. This change in bedding dip across 600 m corresponds to an angular change and discontinuity of reflectors in the seismic profile (BGR99 Line 7).

Only 14 of the 313 measurements of bedding plane attitudes can be restored to the geologic coordinate because of the difficulties of paleomagnetic measurement, mainly because of biscuiting. The corrected data show that the bedding planes mostly strike northwest–southeast (Fig. F17), parallel to the strike of the Costa Rica subduction margin.

Brittle faults

Faults recognized in the Hole U1379C cores are characterized by striated and/or polished surfaces or by offset markers. The sense of slip is determined by offsets of markers such as lamination, bioturbation, and slickensteps on striated slip surfaces. Faults commonly show a normal sense of displacement (Figs. F16, F18, F19), although some faults show a reverse sense of shear. The first faults were recognized between 300 and 350 mbsf, and the first larger population occurs between 500 and 550 mbsf. Between 600 mbsf and the top of the basement at 881.8 mbsf, the density of faults is generally high. Intensively deformed intervals were classified as fault zones, and four fault zones are identified at this site (shaded bands in Fig. F16; see detailed description in “Fractured and brecciated zones”). Fault dips measured on discrete faults span a much wider range below 600 mbsf. This change nearly corresponds to the lithological transition from clay-dominated lithostratigraphic Unit II to the fine to medium sandstone of lithostratigraphic Unit III.

Eleven normal faults were rotated to the geographic reference frame using paleomagnetic data (Fig. F19). Although the number of corrected samples is small, two populations of normal faults are apparent. The majority trend east–west and dip to the north or south, whereas other faults dip west, striking primarily north–south. The kinematics for populations of these normal faults indicates north-northeast, south-southwest extension that is nearly parallel to the subduction vector of the Cocos plate (DeMets, 2001).

Healed faults and sediment-filled veins

Healed normal faults, which are less than a few millimeters thick and show fault cohesiveness on visual observation, can be found within cores from lithostratigraphic Units II and III. The cohesiveness of the fault plane may represent early stage, soft-sediment deformation. Examples of such faults are shown in Figure F20. The dip angle is gentle (20°–30°) for the healed normal fault recognized at 470 mbsf. The texture is characterized by an array of parallel faults, and total offsets are ~10 mm.

Sediment-filled veins aligned in arrays parallel or subparallel to the bedding plane were identified at 632–633 mbsf, just above the first fault zone (642.1–652.8 mbsf). Sediment-filled veins are frequently reported from accretionary complexes, slope basins, and forearc sediments in subduction margins (e.g., Hanamura and Ogawa, 1993; Brothers et al., 1996; Grimm and Orange, 1997).

Fractured and brecciated zones

Fractured zones are defined as moderately sheared zones fractured into a few centimeter-sized fragments. Brecciated zones represent intensively sheared zones composed of angular clasts, predominantly millimeter to 1 cm sized fragments (Fig. F21). Each fragment is angular in shape with slickenlines on its surfaces indicating a systematic direction. For the most part, these features are distinguishable from drilling-induced fractures and clasts (see “Structural geology” in the “Methods” chapter [Expedition 334 Scientists, 2012]).

The four fault zones shown in Figure F16 are characterized by alternating sequences of fractured and brecciated zones, always terminated by a weakly fractured zone in the largely undisturbed host rock below. When the interval between the last and next occurrence of fractured/brecciated zones is >20 m, they are defined as two distinct fault zones. Based on this definition, four fault zones have been identified in the lower part of sedimentary sequences at 642.1–652.8, 766.2–777.5, 804.1–804.7, and 871.8–879.5 mbsf. Slip direction and shear sense have been recorded in the fractured zone and indicate a nearly dip-slip–normal fault sense. The thickest fault zone (642.1–652.8 mbsf) is 11 m above the lithologic transition from lithostratigraphic Unit II to III and consists of an 0.8 m thick brecciated zone in the center with an average clast size of 5–10 mm and fractured zones above and below. The fault zone at 766.2–777.5 mbsf is characterized by several extensively fractured intervals and >20 normal faults. The deepest fault zone is defined by a number of normal faults between 871.8 and 879.5 mbsf (Fig. F16).

All the fault zones were developed in the silty clay interval, whereas sand layers just below the fault zone are coherent or contain considerably weaker deformation, implying a lithologic control on fault zone development. The uppermost occurrence of the fault zone corresponds to the lowermost part of lithostratigraphic Unit II (Fig. F16), just above Unit III, which is composed of fine to medium sandstone. The lowermost fault zone was identified just above the basement, corresponding to the base of Unit III.

Structures in the basement

The contact between the slope sediments and the basement is at 881.8 mbsf in Hole U1379C. Bedding planes in the basement dip gently, with angles typically <20°. Soft-sediment deformation, such as convolute lamination, was identified. No pressure-solution cleavage or slaty cleavages were observed in the basement. Mineral-filled veins developed only in the limestones and basalts. The basalts contain multiple generations of quartz and chlorite veins (see “Lithostratigraphy and petrology”).

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