STRUCTURAL GEOLOGY

Structural features in the cores recovered during Leg 183 are summarized on the HRVCD forms. For each section we described and sketched more detailed structural information, such as the apparent and true orientations of veins and fractures and crosscutting relationships, on a separate structural geology description form (e.g., Fig. F10). Structural data are tabulated in the vein/structure logs for each site (e.g., Table T11; see "Alteration and Weathering"). We recorded observations with reference to the structural geology checklist (Table T12). The structural data entered into the vein/structure logs include

Geometrical Reference Frame

To orient structures within the drill core, we employed the convention adopted during Leg 135 (Shipboard Scientific Party, 1992). We assumed that the core axis is always vertical within a horizontal slice of the core. Pseudo-north, 000 is defined as 90 to the cut face of the archive core; the right side is designated as 270, and the left side as 090 (Fig. F11). We measured most orientations as dip directions relative to the core reference frame on perpendicular sections of the core. To calculate the strike and dip for each pair of apparent dips measured for a structural feature, we used trigonometric functions in a Microsoft Excel spreadsheet that contained the vein/structure logs. If the structure was exposed in three dimensions, we measured the dip and working azimuth directly and entered them into the vein/structure log. We oriented structure relative to core piece and section depth. Depths are in centimeters from the top of the core section. When the vein extended over a depth interval, we recorded the top and bottom of its range.

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