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doi:10.2204/iodp.proc.309312.205.2009 Downhole distribution of chilled marginsThe top of the SDC is defined by the conspicuous change with depth from sheet flows to massive basalts ~1060.9 meters below seafloor (mbsf) (see the “Expedition 309/312 summary” chapter) (Fig. F1). The base of the SDC is defined by the uppermost gabbroic intrusion at 1404.6 mbsf (Wilson et al., 2006). Recovery within the SDC was highly variable but averaged <12%. Above 1348 mbsf, 25 chilled margins and igneous contacts (dike boundaries without clear chilled margins) were recovered, 13 of these associated with densely fractured or veined (brecciated) basalt. The structure log tabulates the depths and orientations of these structures (see LOGS in “Supplementary material”). The dikes are mostly steeply dipping, with 15 of the 25 oriented samples containing igneous contacts dipping 70°–90° in the core reference frame. However, several igneous contacts dip <70° and some dip <50° (Fig. F1) (see Fig. F312 in the “Site 1256” chapter). A recent study of logging data by Tominaga et al. (2009) indicates that the dikes dip to the northeast. Samples from core recovered from the SDC below ~1348 mbsf were pervasively recrystallized to a granoblastic texture inside the contact aureole of the underlying gabbro body (see the “Expedition 309/312 summary” chapter; Koepke et al., 2008). No chilled margins of dikes were recognized in this lowermost interval, presumably owing to a combination of low recovery and a strong metamorphic overprint. |
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