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doi:10.2204/iodp.proc.344.106.2013

Structural geology

The primary objective of structural geology studies during Expedition 344 was to describe and document the style, geometry, and kinematics of structures observed in the cores. Site U1380, located on the middle slope of the Costa Rica forearc, offshore the Osa Peninsula, was drilled for the first time during Expedition 334. Hole U1380A was drilled to 397.0 mbsf with a cored interval of 52.37 m. During Expedition 344, Site U1380 was reoccupied. Coring in Hole U1380C began at 438 mbsf and continued to 800 mbsf. Bedding dips vary from subhorizontal to moderately steep, with a mean dip of ~20°. Moderate to strong foliation is present between 496 and 551 mbsf (Fig. F15). A total of 154 fault planes were identified throughout the cored interval. Areas of particularly localized faulting and intense fracturing and brecciation were defined as fault zones. The sense of shear was determined mainly by slicken steps and the attitude of fault planes. Fault planes were divided into normal, reverse, and strike-slip faults. Because of intense fracturing and drilling disturbance through the entire hole, only a few orientation data were paleomagnetically corrected (Fig. F16). Paleomagnetically corrected bedding planes show a general strike northwest–southeast with gentle dipping angles between 10° and 30° northeast and southwest, respectively. Although data are slightly scattered, fault kinematics analyses inferred a normal fault stress regime with σ₁ and σ₃ oriented vertically and in a northeast–southwest direction (Fig. F16).

Structures in slope sediment

The general deformational fabrics of the slope sediment cored in Hole U1380C allow us to distinguish two structural domains: Domain I ranges from Core 344-U1380C-2R (438 mbsf) to Section 13R-6, 46 cm (551.48 mbsf), and Domain II ranges from Section 13R-6, 46 cm (551.48 mbsf), through Core 52R (797.42 mbsf).

Domain I (438–551.48 mbsf)

Abundant faults and fractures (the latter without obvious displacement) and relatively steeply dipping bedding planes characterize the upper part of Hole U1380C. This interval is characterized by the downward progressive development of a penetrative planar fabric that partly forms foliation overprinting the sedimentary structures. This development can be observed from Core 344-U1380C-7R to Section 13R-6, 46 cm, where the sediment mass easily splits along foliation surfaces. The progressive increase in foliation intensity involves both closer spacing and a dominant planar geometry of its surfaces downhole toward 551.48 mbsf. Hand lens observations show that the most intense foliation develops in Core 344-U1380C-13R (Fig. F17) where the surfaces are characterized by the preferred alignment of platy minerals. In the upper part of the interval, the orientation of foliation is mostly parallel to bedding/laminations (Fig. F15). Moving downhole toward 551.48 mbsf, foliation becomes less steep (4°–5°) but bedding maintains a moderate to steep dip (30°). The foliated interval (496.37–548.98 mbsf) is also characterized by deformation bands and veins. Core 344-U1380C-13R records the simultaneous occurrence of posttilting of both deformation bands and veins.

Fault planes show normal, strike-slip, and reverse sense of shear. Reverse faults increase in abundance downhole toward Core 344-U1380C-13R, where they are as abundant as normal faults. In Core 344-U1380C-13R, we also observe intense brecciation. The brecciated domains are composed of centimeter- to subcentimeter-sized angular fragments aligned along a preferred orientation and with polished surfaces. We interpret these foliated brecciated domains to be fault zones. The bottom of Domain I coincides with the strong reflector identified in seismic Line BGR99-7, located at ~550 mbsf (Fig. F1).

Domain II (551.48–797.42 mbsf)

The lower part of Hole U1380C is characterized by a downhole trend of decreasing bedding dip angles. Dip angles change from an average of 40° above 630 mbsf to an average of 10° in the lowermost 100 m of the hole. The decrease in bedding dip values is not linear but shows steps associated with brecciated zones at ≈620–625, ≈697–705, ≈750–755, ≈760–764, ≈770–773, ≈775–778, and ≈790–793 mbsf (Fig. F15). The zones between 750 and 778 mbsf define a particularly tectonized interval where bedding dip angles are higher than immediately above and below. This interval also corresponds to a relative increased frequency of faults. Faults with both a normal and reverse sense of shear are common throughout the structural domain and equally present, and their abundance increases downhole. Strike-slip faults increase in abundance downhole as well.

Domain II includes well-consolidated sediment (see “Physical properties”). Mud and sand dikes are injected upward into cemented intervals, occasionally leading to formation of a jigsaw-puzzle structure indicative of in situ brecciation. Mineral veins, mostly calcite, are also identified in the cemented interval in this domain. These structures indicate that high fluid pressure was generated just below the consolidated interval.

The paleomagnetically corrected bedding planes generally strike northwest–southeast, almost parallel to the trench axis (Fig. F16B). The beddings dip gently between 10° and 30° both northeast and southwest, corresponding to the landward and trenchward directions, respectively. Fault kinematics analyses were conducted based on paleomagnetic data using the “FaultKin” software (Allmendinger, 2011; www.geo.cornell.edu/​geology/​faculty/​RWA/​programs/​faultkin-5-beta.html) (Fig. F16B). A normal fault stress regime with σ₁ and σ₃ oriented to the vertical and northeast–southwest, respectively, is inferred from the analyses. This indicates an extension stress regime that is quite similar to that at Site U1379 (Expedition 334 Scientists, 2012b).

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