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doi:10.2204/iodp.proc.344.107.2013

Structural geology

Site U1413 is located on the upper slope of the Costa Rica forearc, offshore Osa Peninsula. Hole U1413A was drilled to a depth of 189.1 mbsf. Hole U1413B was drilled to a depth of 25.6 mbsf and was devoted to geochemistry sampling. Hole U1413C has a total depth of 582.2 mbsf, with a cored interval of 404.2 m.

We divided the entire section into two structural domains (Figs. F14, F15): (1) Domain I, from 0 to 180 mbsf, is characterized by subhorizontal to gently northeast-dipping bedding with few fault structures, and (2) Domain II, from 180 to 582.2 mbsf, shows an increase of bedding dip angles and abundance of fault structures with depth. In Domain II, the few measurements that were oriented to the geographic coordinates show a general northeast-dipping attitude.

Faulting-related deformation is abundant from ~180 mbsf to the bottom of the drilled section (Fig. F14). Normal faulting is more abundant than reverse faulting. Dip angles of normal faults and reverse faults vary from subhorizontal to subvertical with some dips >75°. The frequency of normal and reverse faults varies along the entire hole. Bedding planes are more abundant in intervals of less pronounced faulting. The deeper parts of Hole U1413C, starting with Core 344-U1413C-21R, are characterized by high-angle reverse faults with unusually steep dip angles >75° (Fig. F16). Because of these unusually high dip angles of reverse faults and the variation in fault frequency related to depth (especially the reverse faults), selected cores were double-checked for consistency. Structural data on faults were then ranked in terms of reliability as A (reliable) or B (less reliable) (see the structural data set in STRUCTURE in “Supplementary material”). B-faults comprise ~25% of the checked structures.

The measurements oriented in true geographic coordinates show a complex distribution that is indicative of multiple fault sets (Fig. F17). Future spatial analysis on faults will be carried out on the B-faults to further determine their reliability.

Brittle shear deformation is localized along brecciated fault zones at 181, 230, 237–239, 365, 529–532, and 567 mbsf.

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