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doi:10.2204/iodp.proc.344.105.2013

Structural geology

Site U1412 is located on the prism-toe of the Costa Rica margin ~2.5 km from the frontal thrust. The primary goal at Site U1412 was to describe and document the style, geometry, and kinematics of the prism units, décollement zone, and underthrusted sediments. Hole stability issues limited recovery. In particular, Holes U1412B and U1412D yielded few structural data. Structural observations were discontinuous. We divided the structural data into two intervals corresponding to Holes U1412A and U1412B and Holes U1412C and U1412D.

Holes U1412A and U1412B (0–200.3 mbsf)

Bedding dip varies from subhorizontal to intermediate dip angles as large as 35° (Fig. F13). After orienting the dip directions and dip angles into geographic coordinates, the mean bedding attitude is west-southwest, with some surfaces dipping northwest. The bedding orientation is not depth dependent. The bedding trends (NNE–SSW and southwest–northeast) are subparallel to the trench and the proximal flank of the Cocos Ridge, respectively. The combination of the trench and the subducting Cocos Ridge probably results in the two preferential orientations of the bedding planes.

Hole U1412A cores are characterized by the presence of normal faults. These structures form distinct shear planes with little displacement but well-preserved striations (Fig. F14). After orienting the dip directions, dip angles, and striations into geographic coordinates, the faults appear to form two conjugate sets with northwest–southeast and NNE–SSW trends (Fig. F15A). This data set reveals a subvertical orientation of the maximum principal stress axis, σ₁, and a subhorizontal, ENE–WSW orientation of the minimum principal stress axis σ₃ (Fig. F15B), indicating a stress regime characteristic of normal faulting. We interpret these faults to be compaction-related features. The interval where these conjugate normal faults occur ranges from 28 to ~143 mbsf within lithostratigraphic Unit I. As the stratigraphic age within this interval is Pleistocene, the deduced formation age of the normal faults is younger than Pleistocene.

Holes U1412C and U1412D (300–387 mbsf)

Cores from Hole U1412C are both highly deformed and characterized by drilling disturbance. Hole U1412D was located 50 m trenchward of Hole U1412C, making direct correlation of the structures with Holes U1412A and U1412B difficult. Paleomagnetic orientation of the structural data is particularly difficult and cannot be provided at this time. Bedding planes measured in Hole U1412C show dip angles between 45° and 65° (Fig. F13). This hole is characterized by the presence of two fault zones with prominent brecciation. These fault zones are localized between 330 and 342 mbsf and 358 and 365 mbsf. The upper fault zone is located at the boundary where sedimentary age reverses from Miocene to Pleistocene (see “Paleontology and biostratigraphy”). The age reversal may be due to large accumulated displacement on the fault and suggests high activity and multiple faulting episodes. Adjacent to and within these fault zones, well-developed foliation with varying dip angles is observed (Fig. F13). Foliation is not clearly developed in Hole U1412D cores, suggesting less intense deformation. Interestingly, some normal faults similar to those present in Hole U1412A (Fig. F16) are observed in Hole U1412D and are interpreted to be compaction-related features.

Hole U1412C is additionally characterized by distinct fault planes mainly developed as reverse faults with moderately steep to steep dip angles (Fig. F13). Normal faults were also observed.

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