Site 1277 was located on a basement ridge beneath thin sediment cover (Figs. F8B, F9). The main objective was to recover samples of basement, so the site was drilled without coring from 0 to 103.90 mbsf. It was predicted that the bit would be some tens of meters above the sediment-basement contact at that depth. During drilling, a "wash" core barrel was in place. When retrieved, this core barrel contained 2.29 m of fractured igneous rock and associated volcaniclastic sediments (Core 210-1277A-1W) that are indistinguishable from rock in the top of the next core, Core 2R. During drilling, hard zones were encountered at 8589 and 97.5100 mbsf. Thus, it is believed that recovery in Core 210-1277A-1W comes from depths between 85 and 103.90 mbsf, and the top of basement (Unit 1) is set at 85 mbsf.
Two lithologic units are recognized. Unit 1 is a succession of alternating basalt flows (~50%), coarse breccia units containing a wide variety of clasts of gabbro and serpentinite (~20%), and minor volcaniclastic and ferruginous sediments (~10%). There are also variably deformed gabbroic rocks (~20%). Interpreted depth of this unit is 85142.10 mbsf. The principal characteristic of Unit 1 is that it is a sedimentary and volcanic succession. The sedimentary units are derived entirely from rock types characteristic of the underlying basement at this site.
Lithologic Unit 2 (142.18 mbsf to the bottom of the hole at 180.30 mbsf) consists almost entirely of tectonized, altered ultramafic rocks including harzburgite, dunite, and serpentinite mylonite. There is a gabbro cataclasite at the top of the unit. The entirety of Unit 2 is cut pervasively by secondary veins that record several stages of veining and mineral precipitation. Many of the smaller veins cutting the serpentinized peridotite are composed of talc, magnetite, and calcite. There are no sediments or lavas in Unit 2 that would indicate proximity to the seafloor. The rocks are interpreted as mantle that was exposed by tectonic extension and that was associated with pervasive deformation and hydrothermal alteration. After their exhumation, these serpentinites were buried by the lavas, allochthonous debris, and coarse sediments of Unit 1. The allochthonous debris was shed from local seafloor topography along the crest of the basement ridge.
No shipboard biostratigraphic analysis was conducted on the few sedimentary rocks recovered at Site 1277 because no suitable materials were recoved.
We made pass-through magnetometer measurements and magnetic susceptibility measurements on all split-core archive sections at 2-cm intervals. To isolate the ChRM, cores were subjected to alternating-field demagnetization up to 60 mT. We analyzed the results in Zijderveld diagrams (Zijderveld, 1967) and calculated ChRM direction using principal component analysis (Kirschvink, 1980). We also determined magnetic susceptibility on all whole cores at 2.5-cm intervals as part of the multisensor track analysis, and we measured split-core sections at 2-cm intervals with the point-susceptibility meter. No shipboard discrete samples were taken because of time constraints at the end of the leg.
Paleomagnetic data exhibit significant variations in demagnetization behavior among recovered lithologies. As at Site 1276, there was drilling-induced remagnetization, but it was reduced compared to Site 1276 sediments. Greenish volcaniclastic breccia in Core 210-1277A-2R has the lowest NRM intensity (~0.020.3 A/m), contrasting with relatively fresh aphyric basalt in Cores 1R and 3R (~14 A/m), gabbro in Cores 4R and 5R (0.5 A/m), and serpentinite in Core 9R (19 A/m). The basalts appear to record a stable component of magnetization with normal inclinations (~45°). The green breccia, on the other hand, displayed variable inclinations (from positive shallow to negative shallow). Gabbros and adjacent sediments have the same stable inclination values (~40°), similar to those of the basalt.
The serpentinized peridotites in different parts of long, coherent core pieces in Core 210-1277A-9R showed ChRM inclinations that cluster around a mean of 40°, generally in agreement with inclinations in the basalt and gabbros. Because NRM intensities of the serpentinized peridotites are on the order of 19 A/m, they could contribute significantly to the regional magnetic anomaly. There is a distinct difference between gabbro, with relatively low magnetic susceptibility and high median destructive field (MDF), and serpentinite, with high susceptibility and low MDF. This difference may be explained by either the finer grain size of the gabbros or their higher degree of low-temperature alteration, or both.
Because of time constraints at the end of the leg, no shipboard geochemical analyses were conducted on rocks from Site 1277.
Evaluation of physical properties at Site 1277 included nondestructive measurements of density, porosity, velocity, and thermal conductivity. Porosity varied between 2% and 25%, bulk densities varied between 2.0 and 3.8 g/cm3, and grain densities varied between 2.18 and 4.36 g/cm3. Compressional wave velocity varied between 3300 and 6300 m/s. Velocity was lowest in highly altered and veined breccias (Section 210-1277A-4R-2), and it was highest in a coarse-grained gabbro (Section 5R-3). Thermal conductivity ranged from 1.6 to 2.3 W/(m·K), with no obvious trend in the values.
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