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doi:10.2204/iodp.proc.345.106.2014

Structural geology

All core pieces from Hole U1415E are relatively small (11 cm maximum length), were not cored or oriented, and comprise surficial rubble (lithologic Unit I).

Magmatic structures

The earliest history of rock in Hole U1415E is constrained by magmatic fabrics preserved in the recovered gabbroic rock. No magmatic layering was observed, possibly because of the small size of recovered pieces. Three medium- and fine-grained gabbro pieces exhibit planar, weak- to moderate-strength foliation defined by the shape-preferred orientation (SPO) of plagioclase (Sample 345-U1415E-1R-1 [Pieces 1, 5, and 6]). The plagioclase crystals are 1–2 mm long with shape anisotropy commonly 5:1 and ranging up to 10:1 in some crystals. One medium-grained olivine gabbro exhibits weak plagioclase and olivine SPO (Sample 1R-1 [Piece 2]). All other pieces show either no magmatic foliation or were too small to enable recognition of foliation. Consequently, >52% 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 1–2 mm long plagioclase crystals. The plagioclase crystals are often tabular with albite twin planes parallel to the long axes of the crystals. Microscopic observations show that plagioclase crystals in one fine-grained gabbro (Sample 1R-1, 41–43 cm [Piece 5]) are commonly elongate and exhibit weak plagioclase SPO. The crystals show only limited annealing but noticeable deformation twinning, undulose extinction, and bending (Fig. F7A, F7B), with the apparent fold hinges lying within the plane of the foliation. Occasionally, bending led to subgrain development. Minor undulose extinction, subgrain formation, and kink bands are also noted in several clinopyroxene grains. Another fine-grained gabbro (Sample 1R-1, 21–23 cm [Piece 3]) exhibits very weak foliation defined by plagioclase SPO (Fig. F7C). Plagioclase crystals in Piece 3 show rare submagmatic deformation twins and/or bent grains. In part of the thin section, seriate plagioclase grain boundaries are common, suggesting grain boundary migration that may result during subsolidus recrystallization.

The random distribution of plagioclase grains showing deformation twins in the core, the common orientation of fold hinges associated with bent grains, and the fact that some plagioclase grains appear bent around undeformed clinopyroxene crystals are suggestive that these crystal-plastic fabrics formed under hypersolidus conditions with some melt present.

Crystal-plastic deformation

No structurally continuous subsolidus crystal-plastic deformation was observed in the recovered sections.

Cataclastic deformation

Macroscopically, virtually all recovered pieces from Cores 345-U1415E-1R and 2R show limited brittle deformation restricted to several pieces hosting open fractures. These open fractures have no apparent offset and low density (<1 or 1–5 fractures per 10 cm). Section 345-U1415E-1R-1 (Piece 1) appears to be a shear phacoid with angular faces showing weakly developed slickensides.

Alteration veins

Alteration veins are present in 9 of the 14 pieces recovered in Cores 345-U1415E-1R and 2R (~65% of the pieces). When present, vein density is always low, with less than a few veins per 10 cm of recovery, and all veins are very thin (maximum thickness is <0.1 cm, in most cases <0.05 cm). Accordingly, alteration veins represent <<1% of the core volume. Where present, vein length generally exceeds the width of the core (6 cm), although vein terminations (vein tips) are frequently observed. Contacts with the gabbroic host are generally clear (Fig. F8), with only rare alteration halos observed.

The geometry of alteration veins varies from rather irregular in shape to frequently curved and commonly forms networks of crosscutting veins with no marked preferred orientation. As most pieces are not oriented and the hole is interpreted to have drilled in talus, no orientation data were collected.

In thin section, varying combinations of actinolite, chlorite, serpentine, zoisite, prehnite, zeolite, serpentine, and clay are observed as vein-filling material and as a “secondary” assemblage showing pervasive replacement of primary igneous minerals (see “Metamorphic petrology”). The same alteration mineralogy is also seen in veins with clear-cut boundaries, in veins with diffuse boundaries (alteration halos), and defining any pervasive alteration within the rock. Alteration veins cut primary igneous minerals that are, as a rule, much larger than the width of individual veins. Use of the term alteration veins implies formation of undeformed mineral veins, postdating any shearing. In contrast, sheared veins are described as cataclastic features. Veins (undeformed) are more common than the cataclastic features. In some veins, alteration minerals have a rather isotropic shape (mosaic textures), whereas in other veins alteration minerals (usually prehnite) are fibrous, with fiber orientation typically perpendicular to the vein walls.

Temporal evolution

Temporal evolution of structures recovered in Hole U1415E is, from oldest to youngest,

• Intrusion of fine- and medium-grained gabbroic and troctolitic rocks (Units 1–7),

• Magmatic fabric development (foliation),

• Limited crystal-plastic deformation in the mush and annealing of plagioclase,

• Vein formation, and

• Open fractures associated with late brittle faulting.

Details specific to structural features are illustrated with comments in STRUCTUR in “Supplementary material.”

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