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doi:10.2204/iodp.proc.345.109.2014 Structural geologyMagmatic structuresThe uppermost core sections (345-U1415I-1R-1, 1R-2, and 1R-3) compose a rubble unit with cobbles and five cored pieces of olivine gabbro (upper part of Unit I). All pieces from these three sections are relatively small (11 cm maximum length), and no magmatic layering was recognized (possibly because of the small size of the pieces). Sections 4R-1 and 4R-2 of the Layered Gabbro Series of Unit II recovered 1.6 m of relatively continuous core, including one 42 cm long piece (Sample 345-U1415I-4R-1 [Piece 8]) with centimeter- to decimeter-scale layering defined by differences in modal mineralogy and more rarely by grain size (Figs. F17, F18A). Layers are largely defined by variation in modal olivine and, to a lesser extent, pyroxene and have planar boundaries with sharp contacts at <1 cm scale. The dip of the layers is consistently between 29° and 38° (mean dip = 32°; standard deviation = 7.5°). Layering is evident in 32% (by length) of the recovered core, but this must be regarded as a minimum because layering thicker than piece length is only recognized if contacts are preserved. Boundaries between layers observed in both core pieces and in thin section are parallel to the magmatic foliation. No abrupt intrusive contacts were observed, which is consistent with the interpretation that the layering formed under hypersolidus conditions. More than 77% of the recovered core pieces exhibit foliation defined by planar, weak to strong plagioclase, and olivine shape-preferred orientation (SPO) (Fig. F19A). This estimate should also be regarded as a minimum because weak fabrics are hard to recognize in pieces that have also undergone cataclasis, are small, or are coarse grained. All estimates of foliation strength in core pieces were validated by examination of the foliation in thin section, where possible. Foliation intensity seems to increase in strength with increasing olivine content (Fig. F19B). Olivine gabbro, the dominant rock type (see “Igneous petrology”), has weak to moderate foliation (Fig. F19C). Too few pieces of gabbro, gabbronorite, and troctolite were recovered from this hole to allow for meaningful analysis of their characteristic fabric strength. Weak olivine lineation was observed within the plane of the foliation in a few of the olivine gabbro pieces. Microscopic observations show that magmatic foliation is defined by plagioclase crystals but may also be defined by olivine and, to a lesser extent, orthopyroxene and clinopyroxene when the crystals have suitable habits. In all cases, foliation is defined by both the preferred orientation and the shape anisotropy of the crystals. The plagioclase crystals are commonly tabular and as long as 5 mm (normally 1–2 mm in length), with aspect ratios of as much as 8:1. Their [010] albite twin planes typically parallel the long axes of the crystals (Fig. F18C). Olivine crystals are often tabular and 1–3 mm in length, with aspect ratios as high as 6:1 and [010] olivine axes likely perpendicular to the foliation, and sometimes exhibit distinct skeletal shapes (Donaldson, 1976) (Fig. F18E). Less commonly, orthopyroxene and clinopyroxene crystals (3–5 mm in length) also exhibit elongation of as much as 4:1 parallel to the foliation (Fig. F18B, F18C). However, in many cases clinopyroxene appears as relatively large (0.5–2 cm diameter) subequant oikocrysts around which the plagioclase-defined foliation may be deflected. Figure F20 shows four examples over the full range of foliation strength observed in the core: isotropic, weak, moderate, and strong (as defined in “Structural geology” in the “Methods” chapter [Gillis et al., 2014e]). Figures F18B and F18C are close-up images of the strong plagioclase, olivine, and clinopyroxene foliation observed in Sample 345-U1415I-4R-1A, 35–38 cm (Piece 6). Figure F18 also shows examples of microstructures within Piece 6 that are typical of those seen throughout the Hole U1415I core, including deformation twins in a plagioclase crystal (Fig. F18D), skeletal olivine crystals (Fig. F18E), and subgrain development with straight grain boundaries in olivine (Fig. F18F). Deflection of magmatic foliation is common around relatively large (0.5–2.0 cm) pyroxene oikocrysts developed in Sections 345-U1415I-4R-1 and 4R-2. Figure F21 illustrates how a moderate-strength foliation defined by plagioclase and olivine SPO (Fig. F21B) wraps around and between clinopyroxene oikocrysts in troctolite (Sample 345-U1415I-4R-2, 9–13 cm [Piece 2]). No obvious rotation of the oikocryst with respect to the foliation could be discerned. Plagioclase crystals hosted by oikocrysts are typically sparse, show random orientation (i.e., do not show foliation), and are characteristically smaller and more elongate than those outside the oikocryst (Fig. F21A). In this example, rare plagioclase crystals within the oikocrysts are bent (Fig. F21E), suggesting that crystal-plastic deformation occurred above the solidus before the final crystallization of clinopyroxene. Above-solidus crystal-plastic deformation likely explains subgrain development in olivine and deformation twinning in plagioclase away from the oikocryst, as shown in Figure F21C. Figure F22 highlights representative igneous microstructures seen in samples from Hole U1415I that provide insights into the development of the magmatic foliation. Variations in deformation twinning, bending, and undulose extinction shown by plagioclase crystals away from clinopyroxene oikocrysts are highlighted in Figures F22A and F22B. Figures F22C and F22D show the ubiquitous annealed plagioclase grain boundaries and 120° grain triple junctions. Figure F22E illustrates plagioclase subgrain development, and Figure F22F shows curious interfingering/bulging grain boundaries between plagioclase and a large clinopyroxene. This texture suggests dissolution and grain boundary migration (Passchier and Trouw, 2005). The presence of rare to common crystal-plastic deformation in both plagioclase and olivine suggests deformation during crystallization of the crystal mush. Annealed plagioclase grain boundaries, subgrain development, and evidence for grain-boundary migration hints that even more crystal-plastic deformation may have been obscured/removed during near-solidus cooling. The presence of apparently relatively few deformed olivine crystals with a skeletal morphology, however, argues against significant recrystallization (Donaldson, 1976). The observation that the magmatic foliation wraps around the large pyroxene oikocrysts (and crystal-plastic deformation) suggests that some component of compaction was involved during formation of the magmatic foliation. It remains unclear from these rocks alone, however, how the magmatic foliation was formed. Crystal-plastic deformationNo significant, structurally continuous subsolidus crystal-plastic deformation was observed in the recovered section, except for one thin (0.5–1 cm) subvertical zone of protomylonitic deformation along the margin of a prehnite vein in Sample 345-U1415I-2R-1, 14.4–35 cm (Pieces 5, 7, and 8). Cataclastic deformationBrittle structures in Hole U1415I are minor and restricted to a few thin zones of cataclasite and ultracataclasite recovered in intervals 345-U1415I-2R-1, 0–57.5 cm (Fig. F23); 4R-1, 0–143 cm; and 4R-2, 0–17 cm (Fig. F24A), comprising roughly 4%–5% of recovered pieces. Cataclastic deformation in interval 2R-1, 24.5–31.5 cm, shows subvertical orientation (Fig. F23B). Crosscutting relationships indicate a complex succession of vein formation and cataclastic deformation. One sample (Thin Section 13; Sample 345-U1415I-2R-1, 25–31 cm [Piece 7]) shows a deformed prehnite vein along with the cataclasite (Fig. F24B). Undeformed prehnite veins also cut cataclasite (Fig. F24C), suggesting repeated fracturing and fluid flow events, at least locally, under low-grade metamorphic conditions. Alteration veinsMacroscopic alteration veins were observed during core description. Data on the location, mineralogy, and morphology of veins and associated wall rock alteration were recorded for each vein. Roughly 85% of the 35 pieces of gabbroic rock recovered from Hole U1415I host alteration veins. Despite being common, alteration vein abundance is variable. In 65% of the pieces, fewer than a few veins per 10 cm of recovery were observed. In the remaining 20%, a high number of veins was observed (from ~10 veins to significantly >10 veins per 10 cm of recovery). All alteration veins observed in Hole U1415I are very thin, with a maximum thickness is of 0.5 cm; veins thicker than 0.1 cm are not frequent. Many veins are no more than 0.01 cm in thickness (thinner features were not recorded as veins), with a mean vein thickness of 0.075 cm (average = 44 measurements). Accordingly, alteration veins represent <1% of the volume of the cores. Vein length typically exceeds the core width (6 cm), although vein terminations (vein tips) are frequently observed. Short, thin veins may branch to longer and thicker ones. Contact relations with host gabbroic rock ranges from diffuse to clear. No well-developed alteration halos were observed (macroscopically). No systematic correlation between the mineralogy of the vein-filling material and the vein abundance or thickness was observed, although the prehnite veins are definitely the most common and often cut other vein types (see below). Prehnite vein shape is typically irregular but commonly curved, and locally veins form anastomosing networks. En echelon and overlapping veins dominate in the largest pieces. Apparent crosscutting relationships were frequently observed, but clear, measurable offsets are infrequent, and where observed, determining if these are real offsets or veins branching from each other is difficult. A hierarchy of crosscutting relationships based on vein mineralogy has not been established for Hole U1415I (barring a single example of one albite vein crosscut by a prehnite vein), implying such a hierarchy likely does not exist at this location. Longer pieces show two vein orientations: (1) a shallow-dipping vein population subparallel to the igneous layering, although still cutting the primary igneous fabric at a low angle, and (2) a vein population emplaced at a high angle relative to the igneous layering. However, other veins in smaller pieces have intermediate dips. Globally, vein orientations can be described as random. However, because of the limited number of measured veins in this hole (19), this conclusion is not statistically robust. Microscopic observations illustrate the two populations. Examples include irregular fractures hosting chlorite veins roughly orthogonal to magmatic foliation (Thin Section 22; Sample 345-U1415I-4R-1, 48–50 cm [Piece 8A]) and a second example showing minor curved to irregular veins subparallel to the magmatic layering (Thin Section 12; Sample 2R-1, 2–6 cm [Piece 1]). Temporal evolutionTemporal evolution of structures recovered in Hole U1415H is, from oldest to youngest,
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