Previous   |    Next

doi:10.2204/iodp.proc.345.107.2014

Structural geology

All core pieces from Holes U1415F and U1415G are relatively small (maximum length = 6 cm), were not cored or oriented, and comprise a surficial rubble unit (lithologic Unit I).

Magmatic structures

The earliest history of the rock in Hole U1415G is constrained by magmatic fabrics preserved in the recovered gabbroic rock. No magmatic layering was observed, possibly because of the small size of the pieces. One gabbro piece exhibits planar moderate plagioclase SPO (Sample 345-U1415G-1R-1 [Piece 5]; Fig. F6A). Two olivine gabbro pieces exhibit moderate to strong plagioclase and olivine SPO (Sample 1R-1 [Pieces 1 and 3]; Fig. F6B). All cut pieces of sufficient size show moderate to strong magmatic foliation; consequently, 100% of the core has magmatic foliation.

Thin section observations show that the magmatic foliation is defined by both the preferred orientation and shape anisotropy of the plagioclase crystals. The plagioclase crystals are often tabular and 1–5 mm in length, have shape aspect ratios as high as 5:1, and show traces of [010] albite twin planes running parallel to the long axes of the crystals. Microstructural observations in thin section show that plagioclase crystals in the gabbro (Sample 345-U1415G-1R-1, 21–26 cm [Piece 5]) exhibit minor deformation twinning and gently curved grain boundaries with 120° grain junctions; the latter indicates significant grain boundary annealing. Olivine gabbro hosted in Sample 1R-1, 10–16 cm (Piece 3) (Table T2), has strong plagioclase SPO and moderate olivine SPO defined by sometimes tabular, partially skeletal olivine crystals (Fig. F6B) that show minor undulose extinction and incipient subgrain development. Plagioclase crystals show rare deformation twinning, common annealed grain boundaries, and, in patchy regions, equilibrated clusters of polygonal plagioclase with 120° grain junctions (Fig. F6C). The plagioclase SPO appears to wrap at least partly around the clinopyroxene oikocrysts (Fig. F6D), whereas plagioclase crystals within the oikocryst are smaller with a higher shape aspect ratio (as high as 8:1) than those outside the grain and have no preferred orientation. Both of these observations and the relatively minor crystal-plastic deformation present in both plagioclase and olivine crystals suggest that foliation development occurred under hypersolidus conditions by compaction and/or shearing. Plagioclase crystals in clinopyroxene oikocrysts occur in glomerocrystic clusters (Fig. F6E). These clusters may be preserved by growth of the clinopyroxene, albeit with some dissolution, and thus record the geometry of plagioclase nucleation and crystal accumulation at an early stage of crystal mush formation.

Crystal-plastic deformation

A planar, weakly foliated zone of subsolidus crystal-plastic deformation is noted over a thin interval (<1 cm) in one piece (Sample 345-U1415G-1R-1, 10–16 cm [Piece 3]). No other structurally continuous subsolidus crystal-plastic deformation was observed in the recovered section.

Cataclastic deformation

Macroscopically, all recovered pieces show no brittle deformation. Minor open fractures are apparent from microstructural observations, with no apparent offset and very low density (<1 fracture per 10 cm).

Alteration veins

Alteration veins are present in four of the five pieces recovered in Hole U1415G (i.e., ~80% of recovered pieces). When present, vein density is always low and is less than a few veins per 10 cm of recovery. All veins are very thin (maximum thickness = 0.02 cm). Accordingly, alteration veins represent no more than 0.1% of the volume of the cores. Vein length generally exceeds the width of the core (6 cm), although vein terminations (vein tips) are frequently observed. Their contact with the host gabbroic rocks is generally clear with no alteration halos. Alteration veins are, in most cases, curved. Very few planar veins are observed; they form networks of crosscutting veins with no marked preferred orientation.

In thin section, the same mineral assemblages (varying combinations of amphibole, chlorite, serpentine, zoisite, prehnite, zeolite, serpentine, and clay) are observed as alteration vein-filling material. Alteration veins cut primary igneous minerals that are, as a rule, much larger than the width of individual veins. Undeformed veins are more common than cataclastic/deformed veins. In some veins, alteration minerals show no preferred orientation (mosaic textures), whereas in others, alteration minerals (usually prehnite) are fibrous with the orientation of the fibers typically perpendicular to the vein walls.

Details specific to structural features were illustrated with comments and sketches in STRUCTUR in “Supplementary material.”

Temporal evolution

Temporal evolution of structures recovered in Hole U1415G is, from oldest to youngest,

• Intrusion of fine- and coarse-grained gabbroic rocks;

• Magmatic foliation development and limited crystal-plastic deformation in the crystal mush and annealing of plagioclase;

• Very localized subsolidus crystal-plastic deformation;

• Vein formation; and

• Open fracture formation associated with late, brittle faulting.

Top of page   |    Previous   |    Next