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Structural geology

All core pieces from Hole U1415H were relatively small (8 cm maximum length), not cored or oriented, and likely from loose rubble that comprises lithologic Unit I.

Magmatic structures

Samples recovered from Hole U1415H show no magmatic layering. One piece of olivine gabbronorite (Sample 345-U1415H-1R-1, 18–22 cm [Piece 4]) exhibits weak plagioclase and pyroxene shape-preferred orientation (SPO) (Fig. F7A), and four pieces of olivine gabbro (Sample 1R-1 [Pieces 3, 8, 9, and 10]) (Fig. F7B) exhibit planar, weak to moderate plagioclase and olivine SPO. The olivine gabbro is similar to Pieces 1 and 3 in Section 345-U1415G-1R-1. Most medium-grained pieces that were cut and large enough show magmatic foliation; however, the core sample of the partially altered, coarse-grained olivine gabbro (Piece 1) and the altered gabbro (Piece 7) may not be large enough to reveal weak SPO. More than 70% of the core has magmatic foliation.

Microscopic observations show that magmatic foliation is defined by both the preferred orientation and shape anisotropy of plagioclase crystals. The plagioclase crystals are often tabular and 1–4 mm in length, with aspect ratios as high as 4:1 and traces of [010] albite twin planes parallel to the long axes of the crystals. An olivine gabbronorite (Sample 345-U1415H-1R-1, 18–22 cm [Piece 4]) exhibits weak magmatic foliation defined by the SPO of plagioclase and pyroxene (Fig. F7A). Plagioclase crystals show gently curved grain boundaries with 120° grain junctions and very rare deformation twins and/or bent grains.

An olivine gabbro (Sample 345-U1415H-1R-1, 51–53 cm [Piece 10]) has moderate and parallel plagioclase SPO, clinopyroxene SPO, and olivine SPO (Fig. F7B). The olivine SPO is defined by sometimes tabular, partially skeletal olivine crystals that show minor undulose extinction and subgrain development (Fig. F7C). Plagioclase commonly shows deformation twinning (Fig. F7B–F7E), occasionally with subgrain development (Fig. F7D). Annealed plagioclase grain boundaries are common, with some regions of the sample developing equilibrated clusters of polygonal plagioclase with 120° grain junctions (Fig. F7E). Some of the plagioclase crystals show seriate bulging grain boundaries indicative of grain boundary migration (Fig. F7F). Locally, clinopyroxene crystals appear to be partially interstitial to the plagioclase and olivine crystals and thus form SPO. Again, the crystal-plastic deformation likely occurred under hypersolidus conditions during mush formation.

Crystal-plastic deformation

No structurally continuous subsolidus crystal-plastic deformation is observed in the recovered section except for one thin zone (3 mm thick) of protomylonitic deformation along one side of Sample 345-U1415H-1R-1, 0–8 cm (Piece 1).

Cataclastic deformation

Two types of brittle deformation are recognized in Hole U1415H: (1) cohesive cataclasites with associated alteration and veining (Fig. F8) and (2) diffuse fracturing with no vein fill or alteration.

Brittle deformation associated with cohesive cataclasite was identified in several pieces (Sample 345-U1415H-1R-1 [Pieces 1, 3, and 8]) that show localized cataclastic deformation with intensity from moderate to well developed. Although fracturing and/or brecciation is noted throughout each deformed gabbro piece, microstructural observations illustrate strain localization into areas with dense anastomosing fractures and incipient brecciation/cataclasis, overprinted by thin zones of cataclasite (or ultracataclasite) (Fig. F8D, F8E). Locally, these zones involve deformation of prehnite (and/or zeolite) veins in Sample 345-U1415H-1R-1, 33–36 cm (Piece 7) (Fig. F9), suggesting synchronous cracking/cataclasis, low-temperature vein formation, and cataclasis of these newly formed “weak” minerals.

Diffuse fracturing of many recovered pieces, showing only open fractures (i.e., no vein fill or alteration), implies a second period of brittle deformation.

Alteration veins

Alteration veins are present in 6 of the 12 pieces recovered in Hole U1415H (~50% of the pieces). 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. 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. Vein lengths generally exceed the width of the cores (6 cm), although vein terminations (vein tips) are frequently observed; their contact with the host gabbroic rocks is generally clear cut (no alteration halo).

Most of the veins are curved and crosscut each other, showing no preferred orientation at piece scale. Because most pieces are not oriented, no orientation data were collected.

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 and as “secondary” material where pervasive replacement of primary igneous minerals (plagioclase, olivine, and pyroxene) occurs in the rock (see “Metamorphic petrology”). Alteration veins cut primary igneous minerals that are, as a rule, much thicker than the width of individual veins. Undeformed veins are more common than cataclastic/deformed veins. In some veins, alteration minerals have an isotropic shape (mosaic textures), whereas in others, alteration minerals (usually prehnite) are fibrous, with the orientation of the fibers typically perpendicular to the vein walls.

Temporal evolution

Temporal evolution of structures recovered in Hole U1415H is, from oldest to youngest,

  • Intrusion of gabbroic and anorthositic rocks,

  • Formation of magmatic fabric (foliation),

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

  • Cataclasis associated with low-temperature faulting and low-temperature vein formation, and

  • Open fractures.

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