Structural orientations were measured on the archive half of the core using a contact goniometer (Fig. F10). The strike and dip for the plane derived from the two apparent dips was calculated using the geocalculator program in Georient 4.2 (Holcombe, 1996). If the structure could be measured directly, then the strike and dip were entered directly into the vein/structural geology log (Table T8).
All data were recorded with reference to the structural geology checklist (Table T9) on an ODP standard structural VCD form, of which an example is shown in Figure F11. Data from the descriptions were entered in the structural log (Table T8) under the following headings:
Several problems are inherent in any structural study of drilled core. In most cases, only part of the drilled interval in any hole is recovered, and thus, the results are not fully representative. In addition, there may be preferential loss of core because faulted rocks are commonly weaker than the adjacent bedrock and, therefore, may be lost during the drilling.
It is rarely possible to determine the original orientation of the observed structures. The structures initially can only be oriented relative to local reference coordinates based on the orientation in the core tray. The drilling process undoubtedly caused fracturing and rotation of the individual pieces of the core, so this reference orientation may be consistent over only a few tens of centimeters if correlations cannot be made between structures in the separate pieces of the core. Thus, in most instances each piece of core would need to be oriented back to its real position according to an external reference frame, such as by downhole logging tools and paleomagnetic measurements. This has not yet been attempted.
In some pieces of volcanic rocks found throughout the recovered cores, the orientation of elongated flattened and stretched vesicles was used to define the approximate orientation of the volcanic layering. This assumes that flattening of the vesicles is by pure shear alone. Were simple shear involved during flow of these lavas, then the elongation direction of the vesicles would show the approximate orientation of the of the XY plane of the strain ellipsoid in the cut faces of the core (Fig. F12). At high shear strains, however, this would rotate close to the most likely layering orientation. As the vesicles in the cores intersected were generally flattened, albeit tending toward prolate, it was more reasonable that the flattening was by loading normal to and/or shear parallel to the bedding surface of the lava. Therefore, the orientation of the flattening plane of the vesicles was interpreted to give a fair indication of the general orientation of the volcanic layering in the cases where flowbanding was not present.