Study of the sedimentary cover and oceanic basement of the subducting Pacific plate and the décollement zone in the overriding Middle American plate during Leg 205 focused on the interaction between deformation and fluid processes at convergent margins. Differences in recovery of hard rock at Site 1253 and sediment at Site 1254 required integration of two different approaches for the description of deformation features. Procedures for documenting the structural geology of Leg 205 cores closely followed those used during previous drilling of the Costa Rica convergent margin during Leg 170 (Kimura, Silver, Blum, et al., 1997), which in turn is based on the approach first developed at the Nankai Trough during Leg 131 (Taira, Hill, Firth, et al., 1991). Techniques for hard rock description were those used during Leg 185 (Plank, Ludden, Escutia, et al., 2000) and Leg 187 (Christie, Pedersen, Miller, et al., 2001).
Each structural feature observed in cores was annotated on the Visual Structural Description forms, using the structural geology symbols shown in Figure F9 (e.g., Lundberg and Moore, 1986). Structural features were then manually sketched on a separate structural geology description sheet (Fig. F10). A Structural Description Sheet (SDS) was used to document more detailed description of the structural information, such as the apparent and true orientations of veins and fractures and crosscutting relationships (Fig. F11).
For structures observed in hard rock cores or in broken sections of sediment cores, the piece number was recorded on the SDS. Recording the piece number, even in sediments, preserves the information needed during paleomagnetic reorientation. Features such as horizontal bedding planes are located by using two identical interval depth values, whereas inclined structures, such as fault zones, are located by an interval top and bottom. The thickness of a dipping structure differs from the length of the interval over which it occurs and is therefore documented in a separate column titled "Thickness."
The orientations of structural features were measured using the protractor-goniometer method pictured in Figure F12 and explained in detail in the "Explanatory Notes" chapter of the Leg 131 Initial Reports volume (Taira, Hill, Firth, et al., 1991). For linear features, such as those seen on broken surfaces within the core, a direct measurement of plunge and trend was usually possible, although it was often more convenient in practice to measure the orientation of a toothpick inserted into the core and aligned with the linear structure in order to avoid additional cutting of the spilt core to measure in the third dimension. In most instances, determining the orientation of planar structures required the measurement of two apparent dips: the intersection of the structure with the core face and a second intersection, commonly that seen in a surface cut perpendicular to the core face. Again, it was helpful in practice to measure the orientation of a toothpick inserted in line with this second intersection. The true spatial orientation was then derived, using a stereographic projection program (STEREONET version 4.25 by R.W. Allmendinger), by finding the great circle that fit the two apparent dips. The two apparent measurements were recorded in columns 7-10 of the SDS (Fig. F11). Fault planes with striations were recorded with the trend and plunge of the lineation and the sense of movement ("+" = reverse; "-" = normal) and a confidence level that indicates the quality of this determination (high confidence; very probable; possible, but not certain; or no indication).
The explicit reference to orientation within the core is necessary because the actual geographic orientation of most cores is not known, having been broken into pieces and differentially rotated during RCB drilling. The length of the core was taken to represent the vertical; therefore, the direction at right angles to the core axis was the horizontal, relative to which dip angles and plunges were measured (Fig. F13). All structural data are reported using the double line at the back of the working half of the core as a reference azimuth at 000°; the back of the archive half, therefore, represents the 180° azimuth.
Data and some commentary were extracted from the SDS and entered into a structural spreadsheet, as necessary for statistical manipulation, and to produce graphical representations of the structural geology presented in the site reports. We placed particular importance on attempting to restore the orientations of structures in the cores to their unrotated positions before drilling, using paleomagnetic data.
Reorientation to true north in the geographic reference frame was remarkably successful during Nankai Leg 190, when most of the reorientations in RCB cores were carried out using paleomagnetic records (Moore, Taira, Klaus, et al., 2001). Because RCB drilling rotates individual core segments differentially relative to one another, magnetic declination of natural remanent magnetism (NRM) were used to reorient the individual pieces of core back to their in situ orientation (Taira, Hill, Firth, et al., 1991, p. 44, table 6). However, rather than passing individual pieces of core through the cryogenic magnetometer specifically for this purpose, we used the paleomagnetic declination and inclination data (after removing the drilling-induced overprint) that are now readily accessible in the ODP Janus database (see "Paleomagnetism").
We initially recorded the interval of an intact segment of rotated core (or several adjacent and clearly similarly oriented pieces) in which a structure of interest occurred as a "piece interval" on the SDS. Two or more NRM declination measurements per piece (taken from the ODP Janus paleomagnetic database) are needed to give reliable results. Because the routine measurements were made at 5-cm intervals, viable segments must be at least ~7 cm in length. Moreover, only those pieces where the paleomagnetic declinations were reasonably consistent, normally <10° difference between two measurements, were considered as suitable. A check on the integrity of this procedure was performed during Leg 190 on several APC cores by making both Tensor tool and NRM corrections on the same structural data (Moore, Taira, Klaus, et al., 2001); the results were found to be very consistent, generally to within 10° and in most cases to <2°. The relevant declination value was entered into our structural spreadsheet. A simple algorithm based on the steps given in Taira, Hill, Firth, et al. (1991, p. 44, table 6) was used to determine the true geographic orientation of each structure.
As during all legs, a persistent problem in describing structural features in the cores was the distinction between natural structures and those induced or modified by the drilling process. Core disturbance is not reported here unless it reflected original structure or sediment rheology. Also following the practice of previous legs, steeply inclined fractures, especially those lacking strongly and regularly developed slickensides, were generally considered to be drilling induced.