Site 1117

Hole 1117A (RCB):
9°46.526´S, 151°32.945´E; 1663.2 mbsl
0—111.10 mbsf cored; 6.42 m recovered (6%)
Hole 1117B (RCB):
9°46.527´S, 151°32.951´E; 1663.2 mbsl
0—9.50 mbsf cored; 0.05 m recovered (0.5%)
Hole 1117C (RCB):
9°46.520´S, 151°32.943´E; 1663.2 mbsl
0—9.50 mbsf cored; 0.10 m recovered (1%)

Site 1117 is located on the upper slope of the northern flank of Moresby Seamount, 3.5 km to the northwest of Site 1114. It was a successful attempt to bare-rock spud into, and drill through, the main detachment fault where it crops out.

The base of the cored section consists of a noncumulate, quartz-magnetite gabbro that passes upward into brecciated and then mylonitized equivalents, with a fault gouge at the surface. The upward- increasing shearing and alteration confirm that the northern flank of the seamount is an outcropping fault surface. The first undeformed gabbro occurs at 86 mbsf, but brecciated gabbro was found deeper in the section, at 96 mbsf. Therefore, the minimum thickness of the shear zone preserved within the footwall is about 100 m.

In the surficial core we recovered 4 m of soft, light-colored clayey material with a soapy feel, which we interpret as a fault gouge. This material contains talc, chlorite, calcite, ankerite, and serpentine, which is consistent with hydrothermal alteration of the underlying deformed gabbro. It has low porosity (~30%), bulk density of ~2.2 g/cm3, unconfined compressive strength in the range of 65—90 kPa, thermal conductivities of 1.3—1.8 W/(m•K), and transverse sonic velocities of ~2 km/s. These physical properties strongly contrast with those common to near-seafloor deposits and to the gabbro protolith below (porosity 3%, bulk density 2.76 g/cm3, thermal conductivity up to 3.8 W/(m•K), and sonic velocity 6.0—6.4 km/s). Although the fault gouge has been exposed at the seafloor, its characteristics still reflect its deformational origin. The shape parameter of the ellipsoid of magnetic susceptibility (T = 0.2—0.8) indicates an oblate magnetic fabric and the degree of anisotropy of the magnetic susceptibility is maximum in the fault gouge (Pj = 1.1—1.2), both indicative of flattening as a result of high shear strain.

The deformation textures in the gabbro range from brecciated to cataclastic, to mylonitic, almost totally obscuring the initial subautomorphic texture. Mylonite clasts recovered down to 57 mbsf show a well-developed foliation with S-C structures. Within the foliation, the association of epidote-rich and very fine grained layers of quartz, epidote, and chlorite reveals greenschist facies conditions during deformation. Asymmetrical fibrous quartz pressure shadows present around pyrite also attest to syntectonic metamorphism. The gabbro between 62 and 86 mbsf shows evidence of increasing brecciation upward, passing progressively to the mylonite. Quartz + epidote veins, reoriented parallel to the foliation in the mylonite, suggest that the brecciation was assisted by silica-rich fluids. Late veins of epidote and calcite cut the rock, attesting to more carbonate-rich fluids in the late stage of shearing. Late alteration, associated with fluid flow within the shear zone, has produced chlorite, talc, and fibrous amphibole replacing primary plagioclase and clinopyroxene.

The mineralogy and texture of the gabbro are similar to those of high-level gabbros occurring in ophiolites. These gabbros, together with the presence of diabases reminiscent of a sheeted diabase complex at Site 1114, suggest that Moresby Seamount may be part of an ophiolitic complex exhumed by extension along the northward-dipping low-angle normal fault bounding the seamount to the north.


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