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doi:10.2204/iodp.proc.308.212.2009

Orientation of breakouts

Although the breakouts occur in pairs on opposite sides of the borehole, we have projected occurrences in the western half of the borehole to the eastern half (following the convention of the GMI-Imager software). Hence, all values range between 0° and 180° and are plotted as a single histogram. The histogram for Hole U1322A is particularly symmetrical and shows little downhole variation in azimuth (mean = 85°, standard deviation = 9.3°) (Fig. F3). The breakouts were less well developed in IODP Hole U1324A but show the same general east–west orientation (mean azimuth = 91°, standard deviation = 15.6°) (Fig. F4). According to accepted quality ranking criteria (Zoback and Zoback, 1991) the breakouts in Hole U1322A are of highest or “A” quality, whereas breakouts in Hole U1324A are of “B” quality. Although the downhole occurrence of the breakouts is concentrated into particular zones, a cursory inspection indicates no consistent correlations to lithologic or log properties. For example, the breakouts appear to occur more often within the mass transport deposits at Site U1322 but not at Site U1324.

Breakout orientation represents SHmin at 85°–265° and 91°–271°, respectively, at Sites U1322 and U1324. SHmin vectors for each hole are plotted on the Ursa-Mars region bathymetric map (Fig. F1).