| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.345.110.2014 Structural geologyMagmatic structuresMacroscopic observations: lithologic Unit IThe uppermost cores (345-U1415J-1R, 2G, 3R, and 4R) comprise a surficial rubble unit (Unit I; see “Igneous petrology”) with 11 cobbles and 6 cored pieces of gabbro, olivine gabbro, and troctolite. All pieces from these four cores are relatively small (13 cm maximum length), and weak modal layering was only recognized in one piece (Sample 345-U1415J-3R1, 49–55 cm [Piece 11]). Of the plutonic pieces, 82% (by length) exhibit planar magmatic foliation defined by weak to strong plagioclase and olivine SPO. The dip of the magmatic foliation in the five oriented pieces recovered ranges from 15° to 47° (mean = 32.4°; standard deviation = 13°), which is consistent with dip values seen in the higher recovery cores of Unit II. Distinct clinopyroxene lineation was observed in interval 345-U1415J-3R-1, 17.5–28.5 cm (Pieces 5 and 6). Macroscopic observations: lithologic Unit IIDrilling in Unit II (Cores 345-U1415J-5R, 6G, 7G, 8R, 9R, 14G, and 15G) recovered a layered sequence of troctolite, olivine gabbro, gabbro, and gabbronorite showing centimeter- to decimeter-scale layering (Fig. F60) defined by differences in modal mineralogy and sometimes by grain size. Figures F61A and F61B show examples of the layering observed in Unit II. The most conspicuous layering is indicated by the boundaries between gabbro and the more olivine-rich rock types (olivine gabbro and troctolite) and is therefore primarily defined by variations in modal olivine accompanied by changes in modal pyroxene and, to a lesser degree, plagioclase. Some of the boundaries show grain size variation caused by the appearance of large (2–5 mm) pyroxene crystals and other variations caused by increases in olivine grain size from 1–2 to >5 mm. Plagioclase grain size changes less dramatically across these boundaries, with small increases in mean grain size from 1–2 to 3–4 mm. The layers have planar boundaries with sharp contacts at the centimeter scale but are gradational and sutured at the 1–5 mm (grain size) scale (see “Igneous petrology” for more details). Of the recovered pieces, 23% exhibit layering, but this must be regarded as a minimum because layering thicker than piece length will only be recognized if boundaries are preserved. The dip of the layers in all cores in the unit, except for Core 345-U1415J-7G, is consistently between 30° and 49° (mean dip = 36.5°; standard deviation = 6°) (Fig. F60). Boundaries between layers that were observed in both core pieces and in thin sections are parallel or near parallel to the magmatic foliation. No abrupt intrusive contacts were observed, which is consistent with the interpretation that the layering formed under hypersolidus conditions. Ghost Core 345-U1415J-7G comprises gabbro, olivine gabbro, and troctolite very similar to that found in Cores 5R and 8R (see “Igneous petrology”); however, layering of the entire Core 7G has a mean dip of 59° (standard deviation = 21°). If the lower two pieces are excluded, the mean dip of the upper part of the core is 76° (standard deviation = 5°). Given this core is a ghost core recovered after redrilling through the same depth interval as Core 8R, the simplest interpretation of the data is that Core 7G was recovered from a block that had rotated >40° into a large cavity at 34–40 mbsf during drilling. Of the Unit II core pieces, 97% (by length) exhibit foliation defined by plagioclase, olivine, and rarely pyroxene SPO (Fig. F61A–F61C), compared to ~75% for the entire hole. These estimates should be regarded as minima because weak fabrics are hard to recognize in pieces that have also undergone cataclasis, are small, or are coarse grained. Estimates of foliation strength in core pieces were validated by examination of the foliation in thin section, where possible. Figure F62 illustrates that >60% of the core from Unit II has moderate to strong foliation strength compared to 48% for the whole core and only 30% for Unit III. Therefore, Unit II generally exhibits a stronger foliation compared to the rest of the core. Magmatic foliation intensity appears to increase in strength with increasing olivine content (Fig. F63), although it should be noted that the color contrast between olivine and plagioclase crystals on the outer surface of the core means that magmatic foliation is more easily recognized in olivine-rich rock. Gabbro tends to have weak magmatic foliation and olivine gabbro shows moderate foliation, whereas troctolite, the dominant rock type in this unit (see “Igneous petrology”), exhibits moderate to strong foliation (Fig. F63). Too few pieces of gabbronorite were recovered from this hole to allow for meaningful analysis of its characteristic fabric strength. Foliation is predominantly planar, although it can be seen to anastomose around large (1–2 cm diameter) clinopyroxene oikocrysts (Fig. F61C), a feature that is also seen around smaller (<1 cm diameter) oikocrysts in thin section. Very rarely, foliation can vary in dip by as much as 15° on a core piece scale. Dip of magmatic foliation for Unit II (excluding the ghost cores) ranges from 10° to 57° (mean = 35°; standard deviation = 9°) (Fig. F60). A weak olivine lineation was observed within the plane of foliation in a few intervals of troctolite and olivine gabbro. Similar depth of recovery, lithology, and dip of magmatic layering and foliation seen in Core 345-U1415J-5R to that seen in Core 345-U1415I-4R recovered in Hole U1415I (see “Structural geology” in the “Hole U1415I” chapter [Gillis et al., 2014d]) suggests that these units can be correlated (Fig. F60). Therefore, this unit has a subsurface horizontal extent of at least 10 m and a vertical thickness of ~30 m. Macroscopic observations: lithologic Unit IIIDrilling in Unit III (Cores 345-U1415J-10R through 26R and ghost Cores 17G, 22G, 24G, and 25G) recovered a sequence of coarse-grained, troctolite, olivine gabbro, and gabbro that show only sparse magmatic layering (3% of the piece length). Again, this must be regarded as a minimum because layering thicker than piece length will only be recognized if boundaries are preserved and because of the large proportion of cataclastic rock in this unit. The best two of four recovered pieces exhibiting layering are shown in Figure F64. All recognized boundaries are planar at the >1 cm length scale and parallel to magmatic foliation based on both core and thin section observation. Boundaries are defined by variations in modal proportions of olivine accompanied by changes in proportions of plagioclase and pyroxene but additionally may be defined by grain size (Fig. F64; see “Igneous petrology” for more details). Boundaries may be sharp or gradational; the two oriented examples from RCB (nonghost) cores have a steeply dipping orientation (66° and 90°). No abrupt intrusive contacts were observed, which is consistent with the interpretation that the layering formed under hypersolidus conditions. Of the core pieces from Unit III, 55% (by length) exhibit foliation defined by plagioclase, olivine, and rarely pyroxene SPO (Fig. F64) compared to ~75% for the entire hole. This estimate should be regarded as minima because the core has a large proportion of cataclasites that make recognition of primary magmatic foliation impossible. However, no strong magmatic foliations were recognized in this unit (Fig. F66), suggesting that Unit III has generally weaker fabric strength than Unit II. Figure F63 shows that troctolite, the dominant rock type, has mostly moderate foliation strength. Too few pieces of the other rock types were recovered from this hole to allow for meaningful analysis of their characteristic fabric strength. Magmatic foliation recorded in Unit III is planar (Fig. F64A, F64B), with dips ranging from 57° to 90° (mean dip = 78°; standard deviation = 11°) (Fig. F60). One piece (Sample 345-U1415J-13R-1, 37.5–51.5 cm [Piece 6]) exhibits an anomalously shallow dip of 17°. We suggest that this piece is likely from a small, <1 m scale rotated block within a fault zone cutting this unit. The mean dip of Unit III is significantly steeper than the mean dip recorded in Unit II (35°). This difference considered with the different compositions of Units II and III, the freshness of the rock, lack of recovery and poor drilling conditions at the depth of the boundary between Units II and III, and the character of the seafloor bathymetry typical of submarine mass wasting suggests that Unit II is a tens of meters–scale slump block lying on top of Unit III. The vertical dip of magmatic foliation in Unit III together with the difficult drilling conditions raises the possibility that it, too, might be a tens of meters thick slump block (see “Paleomagnetism”). Microscopic observationsThe similarity of the core recovered from lithologic Units I and II in Hole U1415J to that found in Hole U1415I means that the Hole U1415J Unit I and II core shows many of the microstructural features associated with the magmatic foliation discussed in the “Hole U1415I” chapter (Gillis et al., 2014d). Many of the common features are therefore only briefly discussed here. Significant differences are shown in Unit III and include larger olivine and plagioclase mean grain size in troctolites, weaker magmatic foliation, and a relative paucity of equant clinopyroxene oikocrysts. Magmatic foliation in all units 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, the foliation is controlled by both the preferred orientation and shape anisotropy of the crystals. Plagioclase crystals are commonly tabular and as long as 8 mm but normally 1–2 mm in length, with aspect ratios as high as 8:1. The [010] albite twin planes typically run parallel to the long axes of the crystals. Olivine crystals defining the foliation are often tabular or weakly skeletal and 1–5 mm in length, with aspect ratios as high as 6:1 and [010] olivine axes likely perpendicular to foliation. Less commonly, orthopyroxene and clinopyroxene crystals (3–8 mm in length) also exhibit elongation of up to 10:1 parallel to foliation (Fig. F65A, F65B). However, in many cases clinopyroxene appears as relatively large (0.5–2 cm in diameter) equant to subequant oikocrysts, around which the plagioclase-defined foliation may be deflected. Lithologic Unit IUnit I consists of troctolite and olivine gabbro with magmatic foliation similar to that observed in Unit II and in Hole U1415I (discussed below). This unit also contains gabbro with distinctive magmatic fabrics that are relatively rare in the core. Interval 345-U1415J-3R-1, 17.5–28.5 cm (Pieces 5 and 6), contains large (as long as 8 mm), high aspect ratio (10:1) twinned clinopyroxenes that define a very strong lineation within weak plagioclase foliation and more numerous smaller, equant clinopyroxene crystals (Fig. F65A). The elongate pyroxenes are largely undeformed, show only rare undulose extinction and seriate grain boundaries with adjacent plagioclase crystals (Fig. F65B), and have a few plagioclase chadocrysts compared to the more equant clinopyroxene crystals. Plagioclase crystals are commonly annealed with 120° triple junctions and gently curved grain boundaries and are therefore typical in this respect in comparison with much of the core. Figures F65C and F65D illustrate an interval of gabbro (interval 345-U1415J-3R-1, 78.5–85.5 cm [Piece 16]) with relatively coarse plagioclase grain size (2–5 mm) and moderate plagioclase foliation. The plagioclase crystals show significant deformation twinning and bending but show less well developed grain-boundary annealing than elsewhere in the core. This piece supports the assertion that plagioclase grain-boundary annealing is less well developed in the coarser grained rocks. Lithologic Unit IIAs discussed above, Unit II correlates with Unit II of Hole U1415I, and therefore only selected examples of magmatic foliation are presented here. Figure F66 shows examples of the typical tabular olivine- and plagioclase-defined magmatic foliation developed within and around equant clinopyroxene oikocrysts in (Fig. F66A) and at the margin of (Fig. F66B) the troctolite. In both cases, magmatic foliation within the relatively fine grained (1–3 mm) troctolitic portion of the rock is strong and wraps around parts of the oikocrysts. Note that the most distinctive wrapping of foliation is around the right side and bottom of the oikocrysts in Figure F66A; it is not solely confined to the bottom of the oikocrysts. Locally, the undeformed oikocrysts include bent plagioclase crystals (Fig. F66B), indicating that crystal-plastic deformation associated with mush development was occurring above the solidus. Magmatic foliation outside of the oikocrysts also contains large, bent plagioclase crystals with deformation twins and subgrains (Fig. F66D–F66F), which likely also records crystal-plastic deformation within the mush. Figure F66F additionally shows typical, small (<1 mm) plagioclase crystals immediately adjacent to and within the margins of a large, bent plagioclase crystal, illustrating how parts of the plagioclase network of crystals are commonly annealed and strain-free. Figure F66D shows a relatively unstrained, large (2–3 mm) plagioclase crystal oriented almost perpendicular to magmatic foliation. The survival of this crystal within the strong foliation perhaps suggests that foliation was not formed solely by recrystallization. Figure F67 shows examples of textural features relevant to the origin of magmatic foliation not seen in thin sections of rock recovered in Hole U1415I. Figure F67A shows an olivine gabbro with large (up to 2 cm), elongate (and twinned) clinopyroxene oikocrysts aligned with the olivine and plagioclase foliation. These oikocrysts partially enclose relatively large (2–3 mm) plagioclase crystals aligned with the dominant foliation, indicating that the pyroxene margins continued to grow after/during foliation development (Fig. F67B). Conversely, just above these large plagioclase crystals and elsewhere in the thin section, moderate plagioclase foliation shown by smaller (<1 mm) plagioclase crystals appears to wrap around the oikocrysts, suggesting that some compaction of foliation occurred against clinopyroxene oikocrysts formed earlier. The larger, perhaps earlier formed, plagioclase crystals commonly show deformation twins in this thin section, again suggesting hypersolidus deformation within a crystal mush. Figure F67D shows a clinopyroxene oikocryst-bearing troctolite with strong magmatic foliation defined by annealed plagioclase and tabular olivine crystals (Fig. F67E). The foliation wraps around the uppermost oikocryst in Figure F67D. Interestingly, that oikocryst is deformed and exhibits subgrain development (Fig. F67F), suggesting that oikocrysts can be deformed under near-solidus conditions, likely toward the end of foliation development. Figures F67B and F67C show an example of northeast–southwest imbricated or tiled plagioclase crystals within magmatic foliation that runs approximately horizontally in the figure. Imbrication suggests that a shear component was involved during foliation development. Lithologic Unit IIIUnit III is dominated by coarse-grained (≥5 mm) troctolite not seen in Unit II or Hole U1415I. Unfortunately, this troctolite is generally more altered than the finer grained troctolite seen elsewhere and mostly contains only serpentinized olivine and pseudomorphed plagioclase crystals. Plagioclase crystals appear to be tabular, and olivine varies from tabular to highly skeletal morphologies (Fig. F68E). The moderate foliation of these rocks is mostly defined by the SPO of plagioclase and, to a lesser extent, by tabular olivine crystals when present. Figures F64A and F64C show that the olivine gabbro on the right side of Sample 345-U1415J-10R-1, 43–51 cm (Piece 6), contains large (0.5 cm), altered, skeletal olivine crystals that do not define SPO. However, the plagioclase crystals between the skeletal olivine crystals do define moderate magmatic foliation (Figs. F64C, F68A). The observation that there is plagioclase foliation between larger, apparently only weakly deformed olivine crystals again supports the suggestion that foliation started to form at high melt fractions early in the history of the formation of these cumulates, perhaps as a deposit of crystals from a magmatic current. The finer grained (0.5–1.0 mm) troctolitic portion of Piece 6 shows features that are typical of those found in other units. Strong magmatic foliation is defined by tabular plagioclase and olivine crystals (Figs. F65C, F68B). Olivine crystals also exhibit skeletal morphologies with limited deformation (subgrains but no kinking), but likely with some annealing, to form smooth grain boundaries (Fig. F68B, F68C). Small plagioclase crystals (<0.5 mm) are typically annealed, with gently curved grain boundaries and 120° triple junctions (Fig. F68D). Interesting in this sample is a large clinopyroxene oikocryst extending across the boundary between olivine gabbro and troctolite, suggesting the boundary formed under hypersolidus conditions and was subsequently overgrown by late crystallizing clinopyroxene (Fig. F64C). Sample 345-U1415J-11R-1, 22–24 cm (Piece 4), is a relatively weakly altered troctolite with isotropic magmatic foliation (Fig. F68E). Large (as wide as 1 cm) skeletal olivine shows some undulose extinction and subgrain development but no kinking. Tabular plagioclase crystals have a bimodal grain size distribution, with the larger grains (3–5 mm) showing deformation twins, bent grains, undulose extinction, and subgrain formation and the smaller grains (<2 mm) showing greater grain-boundary annealing and less deformation. The presence of this isotropic troctolite within a sequence of foliated troctolite argues against large-scale, melt-poor shearing of Unit III. The existence of small annealed plagioclase crystals within this sample suggests that annealing is not a consequence of foliation development or strain-induced recrystallization and may simply develop passively during near-solidus cooling of the units in Hole U1415J. Figure F69 shows photomicrographs of an olivine gabbro (Sample 345-U1415J-25G-1, 11–15 cm [Piece 2]) that, although from a ghost core, exhibits high-temperature, near-solidus deformation within and between clinopyroxene oikocrysts. The piece also exhibits a dramatic range in grain size, with large (1–3 cm) clinopyroxene oikocrysts, 0.5–1.0 cm skeletal olivine crystals, and 0.5–5.0 mm long plagioclase crystals (Fig. F69A). Similar to other samples, larger (1–5 mm) plagioclase crystals commonly show deformation twinning, whereas finer grained (<1 mm) plagioclase crystals are commonly annealed (Fig. F69B, F69C). The piece shows no magmatic foliation, but the smaller plagioclase crystals within the clinopyroxene oikocrysts often occur in chain-like clusters resembling relict grain boundaries within the oikocrysts (Fig. F69A, F69E). We speculatively suggest that these clusters may represent the boundaries of former 0.5 cm diameter olivine crystals (similar to those found outside of the oikocryst) that have since been replaced by clinopyroxene. Figure F69D shows a narrow zone (0.5–2 mm wide) of submillimeter plagioclase and clinopyroxene crystals between two large clinopyroxene oikocrysts. Here the grain boundaries are not annealed, and the tiny (<0.5 mm) fragments of clinopyroxene suggest that this zone might be a recrystallized, high-temperature, near-solidus deformation zone between two clinopyroxene oikocrysts. The presence of subgrains within the oikocrysts elsewhere in the thin section (Fig. F69A) indicates that the sample did undergo near-solidus strain/deformation. Crystal-plastic deformationVery little significant, structurally continuous, subsolidus crystal-plastic deformation was observed in the recovered section, barring narrow (1–2 mm thick) zones of incipient crystal-plastic deformation (undulose extinction and subgrain formation in plagioclase and olivine and bent grains of clinopyroxene) associated locally with margins of zones of intense cataclasis/ultracataclasis. In addition, four thin (≤5 cm) intervals of deformation likely associated with alteration minerals and reaction weakening (see also “Cataclastic deformation”) were recovered in Hole U1415J. These zones are 2–40 mm thick and include intervals at 65.440 mbsf (Sample 345-U1415J-11R-1, 44–49.5 cm [Piece 7]), 69.805 mbsf (Sample 12R-1, 11–13 cm [Piece 3A]), 98.965 mbsf (Sample 21R-1, 16–23 cm [Piece 4]), and 101.895 mbsf (Sample 23R-1, 10–12 cm [Piece 3]). Each zone and the associated foliation is defined by anastomosing ultrafine-grained dark to light brown layers (chlorite and clay?) separated by lenses hosting porphyroclasts of plagioclase, prehnite, epidote, clinozoisite/zoisite, and carbonate that have themselves undergone subgrain formation and/or twinning and locally significant grain size reduction (Fig. F70). Macroscopic observations show rotation of foliations into the fine-grained zones, microfaulting, and asymmetric porphyroclasts or “fish” (Passchier and Trouw, 2005). Although sense-of-shear indicators are recognized in thin section, none of the recovered pieces hosting these microstructures are oriented; therefore, no shear sense was determined. Cataclastic deformationBrittle structures are locally well developed in Hole U1415J and comprise roughly 37% of recovered pieces (Fig. F71). Although nondisplacive microfracturing and veining are by far the most dominant brittle deformation processes recorded in Hole U1415J, distinct zones of cataclastic deformation were recovered. Cores from lithologic Units I and II (Cores 345-U1415J-2G through 9R) record only minor brittle deformation, whereas cores from Unit III reveal thicker zones of variable fracture intensity and cataclasite/ultracataclasite, especially Cores 12R (69.70–71.25 mbsf) and 21R (98.80–100.42 mbsf) (Figs. F71, F72). Cataclastic deformation intensity rankings in Hole U1415J range from 0 (>60% recovered core length) to 5 (~3% recovered core length). Figures F73 and F74 show the macroscopic character of three examples of brittle deformation displaying the range of cataclastic fabric strength observed in the core (see “Structural geology” in the “Methods” chapter [Gillis et al., 2014e]):
Macroscopic estimates of cataclastic fabric intensity in core pieces were validated by examination of the fabric in thin section, where possible. The dip of zones of cataclastic deformation measured in core from Hole U1415J varies from gentle to subvertical (mean = 59°; n = 19) (Fig. F75), with no clear relationship between downhole depth and dip. Macroscopic observations show that the cataclastic deformation zones exhibit heterogeneous grain sizes and degrees of alteration and reflect the variable intensity of cataclasis over a centimeter scale. The majority of pieces in Core 345-U1415J-12R are characterized by brittle fabrics of intensity Ranks 3–5 (Fig. F73A–F73C) and host several types of veins. Most pieces show little preferred orientation, whereas a few pieces show strong preferred orientation and development of cataclastic foliation (Fig. F73C). Unfortunately, because of the small size of recovered pieces, none have measurable dips (Fig. F75). Microscopically, zones of cataclastic deformation in Core 345-U1415J-12R show very heterogeneous grain sizes and degrees of alteration. Cataclastic fabrics are characterized by grain size reduction by microcracking and rotation of the primary igneous and metamorphic minerals, producing angular to subrounded porphyroclasts of variably altered plagioclase, pyroxene, prehnite/chlorite, and/or compound gabbroic clasts in a fine-grained clast-clay(?) matrix (Fig. F73). This microstructure is commonly cut by prehnite and minor chlorite veins in turn cut by another period of cataclastic deformation (Fig. F73F). Crosscutting relations indicate a complex succession of vein formation and brittle deformation. As with Core 345-U1415J-12R, most pieces from Core 21R host brittle structures ranging from dense anastomosing fractures to cataclasite (Fig. F74A–F74C). Pieces recovered in Core 21R also contain deformed/fractured prehnite and chlorite veins. The subvertical orientation in these pieces (Fig. F76) is likely related to veining and associated cataclastic deformation. One piece shows very well developed cataclastic foliation (Figs. F70, F74C). The microstructure locally hosts both angular and rounded clasts of plagioclase, clinopyroxene, and prehnite in a fine-grained matrix (Fig. F74E). Narrow zones of ultracataclasite locally cut the less-fractured host, with chlorite veins both cutting and deformed by zones of fracture and/or cataclasis (Fig. F74D, F74F). Crosscutting relationships in Core 21R therefore record a complex succession of vein formation and cataclastic deformation and suggest that vein formation is at least locally synchronous with cataclastic deformation. In Hole U1415J, unique brittle structures were recovered in several pieces from Cores 345-U1415J-13R, 19R, and 26R. Section 345-U1415J-13R-1 contains a cream-colored string of fractured anorthosite (Sample 345-U1415J-13R-1, 18.5–32.0 cm [Piece 4]) (Fig. F76A) indicating a subvertical zone of localized, relatively “dry” (associated with little alteration) brittle strain. Microscopic observation illustrates an almost “shattered” texture characterized by angular plagioclase clasts locally hosted in a chlorite and/or prehnite matrix (Fig. F76B–F76D). Several pieces of fractured dolerite adjacent to altered cataclasite were recovered (e.g. Sample 345-U1415J-19R-1, 63.5–70 cm [Piece 10]) (Fig. F77A). Microstructural observations suggest that the dolerite hosts a chilled/undercooled margin that was likely emplaced into cohesive cataclastic gabbro hosting variable intensity prehnite/chlorite alteration. Although subsequently fractured/sheared, grain size in the dolerite decreases toward the contact between the two rocks types (Fig. F77B). The cataclasite zone (interval 345-U1415J-19R-1, 63.5–70 cm [Piece 10]) shows both angular and rounded clasts of plagioclase, prehnite aggregates, and clinopyroxene within a partially chlorite filled matrix (Fig. F77C). Away from the contact with the cataclastic gabbro, dolerite shows an intersertal texture characterized by tabular laths of plagioclase hosting anhedral grains of clinopyroxene (Fig. F77D) cut locally by branching chlorite-filled zones of cataclasis. The contact zone between the cataclasite zone and dolerite zone shows two distinct ultracataclasites (Fig. F77E). These ultracataclasites contain minerals from each rock type, indicating that it could be derived from both dolerite and cataclasite (Fig. F77F). Although both epidote and chlorite veins cut the dolerite, contact zone, and cataclastic gabbro, these veins were deformed by later cataclasis (Fig. F77B, F77E, F77F), implying deformation and fluid flow likely were repeated events under medium- and low-grade metamorphic conditions (≤400°C; see “Metamorphic petrology”). A thin interval of dolerite cataclasite was recovered in Core 23R (Sample 345-U1415J-23R-1, 9.5–13 cm [Piece 3]) with widely developed epidote and prehnite veins (Fig. F78A). This zone of intense cataclasis comprises clasts of epidote, prehnite, and altered minerals in a fine-grained matrix of prehnite (Fig. F78B, F78C). Clasts of epidote, prehnite, and other fine-grained alteration minerals form aggregates that locally include chlorite. The dolerite hosts epidote, clinozoisite, and prehnite veins that show mutually crosscutting relations (Fig. F78D). Between veins, dolerite locally shows ultracataclastic texture. Furthermore, dolerite contains a chilled margin, again cut by a zone of cataclasis, with a sharp boundary between the two (Fig. F78A, F78E). Crosscutting relationships indicate a complex succession of dolerite and vein intrusion and cataclasis, suggesting that the successive cataclastic deformation and fluid flow events occurred after dolerite intrusion under relatively medium and low grade metamorphic conditions. In the deepest core recovered, Core 345-U1415J-26R, cataclastic rock appears less altered with angular and rounded plagioclase clasts in a fine-grained matrix derived from plagioclase, clinopyroxene, and prehnite, within which a narrow ultracataclasite zone has developed (Fig. F79). Prehnite only partially replaces plagioclase, with fractured plagioclase locally filled by chlorite (Fig. F79C). Crosscutting relationships suggest that cataclasis of the gabbro occurred prior to infiltration by prehnite and chlorite, similar to relations seen above in other sections. Alteration veinsAlteration veins represent a ubiquitous, although volumetrically insignificant, component of the rock types recovered in Hole U1415J and reflect the cracking and fluid circulation experienced by the gabbroic rocks exposed at the Hess Deep Rift. Alteration veins are filled with various types of “secondary” minerals or mineral assemblages, including talc, amphibole, epidote, chlorite, serpentine, prehnite, carbonate, zoisite, zeolite, and clay minerals (see “Metamorphic petrology”). Vein shape and structure of the vein-filling materialAlteration veins recovered in Hole U1415J are all very thin; their maximum thickness rarely exceeds 5 mm. Veins thicker than 1 mm are not frequent, with the large majority being only a few hundred micrometers across. The thinnest veins identified with the naked eye, and thus systematically logged, are ~100 µm thick, but thin section observation reveals a population of much thinner alteration veins. A continuum exists between “macroscopic” and “microscopic” veins and what is described as pervasive alteration. Vein length generally exceeds the width of the core (~6 cm), but vein tips are quite common. Therefore, we inferred that veins are rather short features, likely not much longer than a few decimeters. Most veins have irregular geometry (lightning-bolt shapes; Fig. F80A), with many veins curved, and form intricate networks with complex branching and crosscutting relationships. Networks with regularly spaced parallel veins are less common; they were only observed in lithologic Unit III (Fig. F80B). En echelon vein networks, diagnostic of crack-opening in a shear-stress regime, are virtually absent. Texture of the vein-filling material depends on mineralogy; some minerals preferentially develop a fibrous texture, whereas others are more blocky. Most veins show evidence of progressive filling from their walls and are termed crack-seal veins. This type of vein is best illustrated by chlorite veins that commonly show two sets of fibers that grew from each vein wall and join in the center of the vein (Fig. F81A). This relationship shows that veins were previously open cracks where hydrothermal fluids circulated and eventually sealed with alteration minerals. Some veins show evidence of multiple filling with different minerals. In some rare examples, the filling was not completed and geode-like microcavities are preserved. Many veins show evidence of weak to moderate shearing parallel to their walls (Fig. F81B–F81E). This shearing may be locally intense in intervals affected by postvein emplacement cataclasis. Filled tension gashes were observed in some cataclasites (Fig. F81F), potentially affording an estimate of paleostress orientation in oriented blocks. Alteration vein densityAlteration veins are found in ~75% of the cored length, with an average density of a few veins per decimeter and average vein thickness of <1 mm. Accordingly, veins represent <<1% of the volume of the recovered cores. A sampling bias likely exists because of the brittle nature of the veins. In fact, some pieces are limited at both ends by thick alteration halos, allowing us to infer that intervals of intensely altered veins (thick veins or intervals with high vein density) were not recovered. A semiquantitative scale ranging from 0 (no veins) to 5 (>20 intercepts with veins per 10 cm) was used to describe downhole variations in vein density (see “Structural geology” in the “Methods” chapter [Gillis et al., 2014e]). When considering the entire hole, the different density ranks occur at approximately equal frequency (Fig. F82). However, veins are not evenly distributed throughout Hole U1415J; vein density presents a marked and rather progressive increase downhole (Fig. F83). In summary, the upper two units of the hole (above 55.3 mbsf; Cores 345-U1415J-3R–5R, 8R, and 9R) have overall low vein density. Almost 40% of the recovered core length in these units has no veins, and <10% of the recovered length has high vein density (Classes 4 and 5). In contrast, Unit III (55.30–104.55 mbsf) is characterized by ~50% of the cored length exhibiting a high to very high alteration vein density. Alteration vein dipMany pieces recovered from Hole U1415J are long enough to allow measurement of vein dip. The entire range of dips, from shallow to vertical, is observed over the length of the hole (Fig. F84). At the scale of single pieces, we observed that the scatter in vein azimuth is also quite high. Accordingly, the orientation of alteration veins can be described as globally random; they form a network with no preferred orientation, consistent with hydraulic fracturing. When comparing vein dips from the upper two units (I and II) with those from Unit III, dips preserve random orientation in the upper half of the hole and show two maxima, one at intermediate dips and a second subvertical maximum in Unit III (Fig. F85). This tendency is confirmed by observations at the scale of some larger pieces of core showing two preferred vein orientations (Fig. F80A). No relationship between vein dip and vein mineralogy is recognized (Fig. F86). Chlorite and prehnite veins are present throughout the entire hole, whereas epidote and talc veins have a more restricted distribution primarily correlated with one of the zones of cataclasis in Core 345-U1415J-12R. Low-temperature zeolite veins are most common in Units I and II, with rare zeolite veins recognized as deep as 85 mbsf (Core 18R). Crosscutting relationships and apparent vein chronologyAlteration veins are observed both in rock where the background alteration is poorly developed (Fig. F81B) and in highly altered core (Fig. F81A). Emplacement of alteration veins may therefore both predate and postdate an early stage of pervasive alteration. The mineralogy of the vein-filling material may change according to the nature of the crosscut mineral (e.g., the same vein may be filled with prehnite through plagioclase and serpentine when through olivine). Apparent crosscutting relationships were frequently observed, but in detail it is hard to distinguish between actual crosscutting and branching relationships. Very few measurable offsets were identified. A systematic hierarchy in crosscutting relationships between veins with different fillings is hard to establish. Epidote, amphibole, and talc veins are generally early and predate the cataclastic deformation (if present). Chlorite veins are also generally affected by late deformation events but can be contemporaneous with cataclasis, as attested by tension gashes filled with chlorite. Prehnite veins can be deformed but most cut previous structures, although chlorite veins crosscutting prehnite veins were observed in some thin sections. Zeolite veins are clearly late, as they crosscut all other features. Temporal evolutionTemporal evolution of structures recovered in Hole U1415J is, from oldest to youngest,
|
|||
