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doi:10.2204/iodp.proc.345.111.2014

Structural geology

Core from Holes U1415K–U1415N comprises a collection of aphyric basalt; olivine basalt; dolerite, diorite, and gabbro clasts; and sand. All pieces were relatively small (5 cm maximum length), not cored or oriented, and from loose rubble.

Magmatic structures

Hole U1415K

Ghost core recovered in Hole U1415K consists of four sections of clay and sand with some cuttings and gravel in the lowermost part of Section 345-U1415K-2G-4. Section 2G-5 contains an interval of gravel followed by three cobbles: oxide-quartz diorite, olivine gabbro, and aphyric basalt. None of the pieces show macroscopic magmatic foliation. Microscopic examination of diorite collected from the gravel (Sample 345-U1415K-2G-5, 10–12 cm [Piece 1]) shows rare undulose extinction in plagioclase crystals and a lack of a magmatic foliation. Similarly, microscopic examination of oxide-quartz diorite (Sample 2G-5, 13–15 cm [Piece 2]) confirms the lack of magmatic foliation; however, the sample shows aligned Fe-Ti oxides and rare undulose extinction in plagioclase.

Hole U1415M

Ghost core recovered in Hole U1415M (Core 345-U1415M-2G) consists of four sections of sand with some cuttings and an aphyric basalt gravel in the lowermost part of Section 2G-4. No magmatic flow features were observed.

Hole U1415N

Coring in Hole U1415N recovered five cores consisting of basalt and dolerite. The basalts are mostly olivine phyric, with no macroscopic magmatic flow structures observed. Microscopically, a sparsely plagioclase phyric aphanitic basalt (Sample 345-U1415N-4R-1, 12–15 cm [Piece 3]) contains plagioclase microlites that define a moderate magmatic flow fabric parallel to the chilled margin of the sample (Fig. F10A, F10B). Microscopic observations indicate that most phaneritic basaltic clasts show a local and weak alignment of matrix plagioclase crystals.

Olivine xenocrysts in phaneritic olivine phyric basalt frequently occur as glomerocrysts, which can include plagioclase crystals (e.g., Sample 1R-1, 52–54 cm [Piece 10]). The xenocrysts also occasionally host subgrains (Fig. F10C), indicating that they have undergone limited crystal-plastic deformation. Perhaps the xenocrysts were inherited from crystal mush forming in a crustal magma chamber. In rare cases, plagioclase crystals in the matrix of the basalts may show deformation twinning and undulose extinction (Fig. F10D).

Macroscopically, phaneritic dolerites do not exhibit magmatic foliation. In thin section, however, a subophitic dolerite (Sample 4R-1, 52–54 cm [Piece 12]) shows weak magmatic foliation flowing around an anorthositic troctolite xenolith/enclave (Fig. F11A). This piece is similar to several pieces recovered in Sections 345-U1415N-4R-1 and 5G-1 and is therefore presumed to also have weak magmatic foliation. Plagioclase crystals within dolerite show sparse undulose extinction, bending, and deformation twinning (Fig. F11B). The smallest plagioclase crystals within the anorthositic troctolite xenolith also exhibit crystal-plastic deformation in the form of subgrains, deformation twinning, and undulose extinction (Fig. F11C).

Crystal-plastic deformation

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

Cataclastic deformation

Macroscopically, pieces recovered from cores in Holes U1415K and U1415N show limited brittle deformation restricted to several pieces hosting open fractures. These open fractures are low density (<1 or 1–5 fractures per 10 cm) and have no apparent offset.

Alteration veins

Alteration veins are present in the majority of pieces recovered in Holes U1415K and U1415N and represent a ubiquitous, although volumetrically insignificant, component of the recovered rock types. Veins are generally rare (less than a few veins per 10 cm of recovery), with only one piece characterized by a high density of tiny veins forming an anastomosing network. The alteration veins are all very thin (<0.1 cm maximum thickness, in most cases <0.05 cm) and together represent <1% of the core volume.

Aphyric and olivine phyric basaltic rock, gabbro, and dolerite in both Holes U1415K and U1415N host veins filled with various secondary minerals, including chlorite, clays, and prehnite. Prehnite veins typically cut chlorite veins. In a few examples, chlorite and prehnite are found in the same vein, with prehnite reopening the chlorite vein and forming zoned veins with prehnite in the middle and chlorite on the walls. Basaltic clasts in Hole U1415N rarely show pale green halos variably developed around chlorite veins. Most veins are clear-cut; form networks of relatively planar, parallel, and/or curved sets; and crosscut each other, showing no preferred orientation at the piece scale. Because all pieces are unoriented, no orientation data were collected.

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