| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.345.113.2014 Structural geologyMagmatic structuresMacroscopic observations: lithologic Unit IThe uppermost core (345-U1415P-2G) is a rubble/sediment unit comprising mostly pelagic mud, sand, some olivine gabbro rubble, one piece of foliated olivine gabbro, and one basalt clast. Macroscopic observations: lithologic Unit IICoring in Unit II (Cores 345-U1415P-3R through Sample 345-U1415P-15R-1 [Piece 6] and ghost Core 4G) recovered a ~50 m thick sequence (Fig. F37) of olivine gabbro and orthopyroxene-bearing olivine gabbro hosting a diversity of textures. This sequence is called the Multitextured Layered Gabbro Series (see “Igneous petrology”). This heterogeneous interval reveals an extraordinary variety of textures that include variations in grain size (0.1 mm to >5 cm), crystal morphology (from equant annealed to disequilibrium skeletal and lobate grains), and mineral fabrics (from isotropic to strong) and consists of a sequence of interfingered intervals that range from homogeneously textured primitive orthopyroxene-bearing olivine gabbro to dramatically banded or layered olivine gabbro. Layering/BandingLayering in Unit II can be divided into two broad categories: (1) more common, steeply dipping asymmetric and sometimes diffuse leucocratic banding (Fig. F38) and (2) less common, more regular, gently dipping grain size and modal layering (Fig. F39). Variation in the steeply dipping leucocratic banding/layering recorded in Unit II ranges from distinct (Fig. F38D) to weak (Fig. F38A, F38E). In the best examples, banding/layering can be traced through the entire diameter of the core, therefore confirming planar versus tubular geometry. Diffuse but locally subplanar plagioclase/orthopyroxene-rich leucocratic bands can be interpreted as layering, either originally horizontal or normal to a cooling surface. As significant rotation is likely in the history of these rocks, we use the term banding to avoid genetic connotations. The banding is defined by modal, grain size, and shape variations in plagioclase, olivine, orthopyroxene, and clinopyroxene and to a first order manifests as variably distinct leucocratic and melanocratic bands. These bands often display asymmetric distribution of minerals and mineral shapes/habits (Fig. F38B, F38D). The banding has variable dips (4°–90°) and is often curved or folded but generally steeply dipping (mean = 63°; mode = 80°; standard deviation = 24°) (Fig. F37). In Unit II, 51% of the core (by length) contains some element of this style of banding, whether distinct or diffuse. This percentage is likely a reasonable estimate given the relatively good core recovery in Unit II. The banding extends discontinuously through the entire thickness of the cored unit, from Sample 345-U1415P-4G-1, 62–93.5 cm (Piece 8; 13.2 mbsf), to Sample 15R-1, 16.5–22 cm (Piece 3; 64 mbsf). It seems unlikely that Hole U1415P intersected a single, variably natured anastomosing band. Several cycle boundaries appear to be crossed, some more distinct than others, so it is more likely that Unit II samples a steeply dipping multilayered unit that is at least 8.6 m thick (assuming the mode of the dip is the most representative dip of the unit). Bending or folding of the layers is suggestive of magmatic slumping. The original orientation of the banding is, at this point, unconstrained. About 3% of the core in this unit exhibits simpler, gently dipping grain size and modal layering. This layering has planar, sharp, single boundaries between different rock types, such as those shown by orthopyroxene-bearing olivine gabbro and anorthositic gabbro in Figure F39. Other examples can be found in Sample 345-U1415P-4G-1, 8.5–18.0 cm (Piece 3), in which a 1.5 cm wide gabbronorite band cuts olivine gabbro (Fig. F40B), and in Sample 6R-1, 70.5–72.5 cm (Piece 7), in which a troctolite layer cuts olivine gabbro. It is likely that this layering formed under magmatic conditions. Enclaves and magma mixingRarely, small (5–10 cm long) patches of homogeneous medium-grained orthopyroxene-bearing olivine gabbro reside within coarse-grained orthopyroxene-bearing olivine gabbro (e.g., Sample 345-U1415P-7R-2, 9.5–54 cm [Piece 2]); these are interpreted to be enclaves, perhaps indicative of magma mixing or assimilation of already solidified material (see “Igneous petrology”” for further discussion). Magmatic foliationThe majority of olivine gabbro in Unit II is isotropic with no mineral shape-preferred orientation (SPO) (Fig. F40A, F40B). Only 6% of the core pieces (by length) of Unit II exhibit recognizable foliation defined by olivine and/or plagioclase SPO. Where present, foliation strength is weak, and only rarely (1.5% by length) does rock show moderate to strong foliation. The presence of foliation seems to be restricted to finer grained olivine gabbro. Where possible, estimates of foliation strength in core pieces were validated by examination of foliation in thin section. Foliation, where present (13 measurements), is predominantly planar, with dips ranging from 17° to 88° (mean = 49°; standard deviation = 14°) (Fig. F37). Microscopic observations: lithologic Unit IIThe majority of Unit II is isotopic and shows no, or at best weak, magmatic foliation. In contrast to gabbroic rock from Unit II in Hole U1415J, plagioclase shows less evidence for hypersolidus deformation, although some deformation twins, subgrain boundaries, and undulose extinction are present in most samples. Interestingly, Hole U1415P Unit II core also shows a greater range of plagioclase grain size (from 1 cm to <0.1 mm) within a single piece, with extreme degrees of annealing of the small grain sizes. In addition, the larger plagioclase crystals are commonly zoned, more so than in Hole U1415J Unit II core. Lastly, skeletal olivine in various geometries is much more common in Hole U1415P Unit II than in Hole U1415J Unit II. Figure F40 shows two examples of gabbros that have no magmatic foliation and give an indication of the isotropic nature of most of the gabbros in Unit II. Sample 345-U1415P-3R-1, 120–123 cm (Piece 20A), is olivine gabbro with variable grain size (plagioclase ranges from 0.5 to 5.0 mm in diameter), hosting only weakly annealed plagioclase and minor deformation twins, undulose extinction, and locally developed subgrains (Fig. F40C). Sample 4G-1, 10–13 cm (Piece 3), reveals a sutured boundary between orthopyroxene-bearing gabbro and olivine-bearing gabbro. This orthopyroxene gabbro is isotropic and has weakly annealed, zoned plagioclase crystals that show bending and deformation twins (Fig. F40D), undulose extinction, and locally developed subgrains with curved boundaries. In contrast, the olivine-bearing gabbro hosts only weak magmatic foliation defined by plagioclase SPO yet is moderately annealed. Plagioclase crystals also show bending, deformation twins, undulose extinction, and rare subgrains. Olivine crystals show subgrain development. As discussed earlier, Unit II only contains rare examples of distinct magmatic foliation (Fig. F41 shows one such example). Sample 345-U1415P-6R-2, 119–122 cm (Piece 14), contains a relatively coarse grained orthopyroxene and plagioclase vein that appears to intrude along the foliation and truncate the grains of a troctolitic gabbro (Fig. F41A, F41B). Very strong magmatic foliation is defined by both the preferred orientation and shape anisotropy of plagioclase crystals, which are commonly tabular and as long as 3 mm but normally 1–2 mm in length, with aspect ratios of as high as 10:1. Their [010] albite twin planes typically run parallel to the long axes of the crystals (Fig. F41C, F41D). Plagioclase crystals are locally bent with deformation twins, and rare subgrains are weakly annealed with scalloped grain boundaries. The skeletal olivine crystals show subgrains. Microstructures of layering/bandingFigure F42 shows the microstructure of the banding observed in Sample 345-U1415P-6R-1, 101–103 cm (Piece 7C). Despite some alteration, the nature of the boundary is clearly heterogeneous, hosting a wide range of grain sizes. Coarse-grained clinopyroxene and (partially altered) equant olivine passing diagonally across the thin section form one side of an isotropic, medium-grained gabbro layer. Plagioclase shows rare deformation twins, undulose extinction, and partial annealing. Interstitial clinopyroxene in the gabbro layer increases away from the boundary with the coarse-grained olivine and clinopyroxene and appears to originate from another, but more restricted (Fig. F38B), coarse-grained clinopyroxene and olivine layer. The fine-grained troctolite layer beneath the banding contains equant, fine-grained (0.5–0.1 mm) plagioclase crystals, associated with delicate 1–10 mm diameter skeletal olivine crystals (Fig. F42C). These olivine crystals exhibit undulose extinction and subgrains. Plagioclase crystals inside the skeletal olivine are strongly annealed, a feature that is commonly seen in other cores from this unit (Fig. F42B). Figures F42D and F42E show other examples of fine-grained annealed plagioclase crystals. Crystals in Figure F42E are enclosed by an orthopyroxene oikocryst. Figure F39 shows an example of the simple layering/boundaries that are relatively rare in Unit II and indeed rare in Hole U1415P. Sample 345-U1415P-14R-1, 16–25 cm (Piece 3), hosts a layer of orthopyroxene-bearing olivine gabbro within an anorthositic gabbro (Fig. F39A). The boundaries are sutured and defined by modal and grain size variation; the lower boundary of that layer is shown in Figure F39B. The orthopyroxene-bearing olivine gabbro has weak magmatic foliation defined by plagioclase SPO. Plagioclase shows significant grain size variation (0.1–3 mm), sometimes occurring in large clusters (Fig. F39C) that show undulose extinction, subgrains, and deformation twins. The olivine is skeletal, locally with subgrains, and the orthopyroxene is interstitial. The anorthositic gabbro has weak to moderate magmatic foliation defined by plagioclase SPO. Plagioclase shows significant grain size variation (0.1–1 mm) similar to that seen in the orthopyroxene-bearing olivine gabbro, with the larger grains showing zonation, rare undulose extinction, and deformation twins. In both the orthopyroxene-bearing olivine gabbro and the anorthositic gabbro, plagioclase occurs in small, highly annealed patches similar to those shown in Figures F42C–F42F. These textures suggest the rocks have undergone some recrystallization, perhaps as a result of a thermal input associated with intrusion of a new magma. Figure F39D shows another example of a plagioclase texture that shows subgrain development that may have captured partial annealing during a reheating event. Macroscopic observations: lithologic Unit IIIUnit III (Pieces 7, 8, and 9 in Section 345-U1415P-15R-1 and Cores 345-U1415P-16R through 23R, 19G, 21G, 24G, and 25G) comprises a 38 m thick sequence of coarse-grained troctolite with little magmatic layering (2% of the piece length). Two end-member examples of troctolite are shown in Figure F43. Figure F43A shows an isotropic, clinopyroxene (5%)-bearing troctolite with near cotectic proportions of olivine (20%) and plagioclase (75%). Figure F43B shows a weakly to moderately well foliated troctolite with more olivine (35%), less plagioclase (64.5%), and minimal clinopyroxene (0.5%). Of the core pieces from Unit III, 54% (by length) exhibit magmatic foliation defined by olivine and plagioclase SPO (Fig. F43B). The foliation strength is weak and occasionally moderate and increases downhole. Some troctolite at the top of Unit III is isotropic. Magmatic foliation in Unit III is planar, with dips ranging from 12° to 90° (mean = 66°; standard deviation = 19°) (Fig. F37). The mean dip of Unit III is similar to that recorded by magmatic banding in Unit II (63°); however, no significance should be attached to this similarity, given the large standard deviations associated with these means and paucity of distinct, easily measured foliations. Figure F43C shows troctolite in Hole U1415J Unit III for comparison. The Hole U1415J Unit III troctolite is, on average, slightly more olivine-rich and has slightly stronger magmatic foliation but is interpreted as a lithologic equivalent of troctolite in Hole U1415P Unit III. MicrostructuresUnit III is dominated by coarse-grained (5 mm) troctolite that is not seen in Unit II or Hole U1415J. This troctolite is generally more altered than olivine gabbro of Unit II and often contains mostly serpentinized olivine and pseudomorphs after plagioclase. Plagioclase crystals appear to be tabular and zoned and show undulose extinction, subgrains, and rare deformation twins. Olivine varies from tabular to rarely skeletal, sometimes showing subgrains and/or undulose extinction, and clinopyroxene is interstitial, occasionally forming rims around olivine grains. Intrusive contactsTwo pieces from Unit III show discordant intrusive contacts between relatively fine grained olivine gabbro and troctolite. Figures F44B and F44E show the contact preserved in unoriented Sample 345-U1415P-18R-1, 78–83 cm (Piece 7), truncating both olivine grains in the troctolite and is discordant to the steeply dipping olivine foliation in the same unit. The intrusive orthopyroxene-bearing olivine gabbro exhibits moderate magmatic foliation defined by plagioclase SPO that parallels the contact (Figs. F44E, F45F). Plagioclase crystals in the orthopyroxene-bearing olivine gabbro show subgrains, deformation twins, bent grains, and local patches of annealing. Tabular olivine crystals locally show undulose extinction. Figures F44C and F44F show the altered contact preserved in oriented Piece 8 (Sample 345-U1415P-22R-2, 113–115 cm). The boundary between troctolite and olivine gabbro again truncates steeply dipping olivine foliation and individual olivine grains in the troctolite (Fig. F44C). Olivine gabbro is isotropic in terms of magmatic foliation (Figs. F44F, F45D). Plagioclase crystals in olivine gabbro commonly show undulose extinction, deformation twins, and local patches of annealing. Both of these contacts are clearly intrusive and indicate that Unit III was intruded by magma after sufficient crystallization of the troctolite to fracture. The intrusive bodies must be <3 m thick because although a second contact for each was not recovered, core recovery limits their possible thickness. One piece of anomalous, very fresh, fine-grained, moderately foliated olivine gabbro was also recovered near the top of Hole U1415P (Fig. F44A) in Sample 345-U1415P-3R-1, 31–34 cm (Piece 7). No contacts with the coarse-grained olivine gabbro of Unit II were recovered, but the similarity in mineralogy, grain size, alteration, and magmatic foliation raises the possibility that this piece also records either the same or similar late intrusive event. Figures F44D and F45A illustrate the moderate magmatic foliation of the olivine gabbro defined by plagioclase SPO that wraps locally around relatively large clinopyroxene crystals. Plagioclase crystals show common deformation twins, undulose extinction, and rare subgrains and moderately annealed grain boundaries. Clinopyroxene oikocrysts often have an amoeboid character and locally show subgrain development (Fig. F45B) and kinked cleavage with interfingering grain boundaries. Near-cumulus orthopyroxene and clinopyroxene host elongate and deformed plagioclase (Fig. F45C). Correlation of lithologic units in Holes U1415J and U1415PBased on primary mineralogy, rock types and textures, and magmatic fabric development (foliation, layering, and/or banding), three distinctive lithologic units were recovered at Site U1415 during Expedition 345. Coring in all holes recovered a surficial rubble unit, not considered here. Holes U1415I and U1415J were sufficiently close together that Unit II in both holes is interpreted to be contiguous (see “Structural geology” in the “Hole U1415J” chapter [Gillis et al., 2014e]). Troctolite in Unit III in both Holes U1415J and U1415P is lithologically similar, commonly has olivine-plagioclase foliation (~50% of the recovered rocks), contains little layering (<6%), and is termed the Troctolite Series. The Multitextured Layered Gabbro Series of Unit II in Hole U1415P is the third lithologic unit. This unit might also correspond with the few olivine gabbro pieces recovered in Hole U1415J (Piece 15 from Section 345-U1415J-9R-1, and Pieces 1, 2, and 3 from Section 10R-1) between the Hole U1415J layered Unit II gabbro and Unit III troctolite. This interpretation is supported by the recognition that one of these olivine gabbro intervals has a different magnetic remanence direction from that of Units II and III in Hole 1415J, suggesting that the olive gabbro comes from a different block (see “Paleomagnetism” in the “Hole U1415J” chapter [Gillis et al., 2014e] for further discussion). In Hole U1415P, a fourth lithologic unit is defined to include the late intrusive fine-grained gabbro recognized in Unit III. The thickness of the cored units (30–60 m) in Holes U1415I, U1415J, and U1415P; the lateral distance between boreholes (10–100 m); and the variable dip of their foliation/layering suggests that the units described above were recovered from large (30–60 m diameter) slump blocks. Crystal-plastic deformationA planar, weakly foliated zone of subsolidus crystal-plastic deformation is noted over a thin interval (<1 cm) in one piece from Unit II (Sample 345-U1415P-15R-1, 43.5–44.25 cm [Piece 8]). No structurally continuous subsolidus crystal-plastic deformation was observed in the recovered section. Cataclastic deformationBrittle structures are only locally developed in Units II and III of Hole U1415P and comprise <11% of the recovered pieces (Rank 1–5), with <0.5% showing Ranks 3–5. Dip measurements of cataclastic fabrics in core from Hole U1415P were only possible in three short intervals and range from 48° to 78°, with a mean of 62°. Because of the very localized nature of cataclastic deformation, only three small pieces of cataclasite were recovered from Unit II (interval 345-U1415P-10R-1, 121–134 cm) (Fig. F46). Macroscopically, the cataclastic deformation exhibits heterogeneous grain sizes and strong alteration associated with veins. Each piece shows strong preferred orientation and development of cataclastic foliation (Fig. F46A). Unfortunately, because of the small size of recovered pieces, none have measurable dips. Microscopically, this cataclastic deformation is characterized by angular to subrounded clasts of variably altered plagioclase and pyroxene, amphibole, and prehnite/chlorite in a fine-grained clast-derived matrix (Fig. F46B). This microstructure involves a zone of ultracataclasite comprising rounded clasts in a clay-dominated matrix (Fig. F46C) and is commonly cut by prehnite and minor chlorite veins in turn cut by another period of cataclastic deformation (Fig. F46A). Crosscutting relations indicate a complex succession of vein formation and brittle deformation. In Unit III, cataclastic deformation is also very localized and preserved as cracked carbonate vein fill. In a sheared chlorite vein hosted in Piece 6 (Thin Section 129; Sample 345-U1415P-16R-1, 47–49 cm), the carbonate fibers were formed normal to the vein wall and subsequently pulled apart by later minor deformation (Fig. F47A, F47B). Alteration veinsAlteration veins found in Hole U1415P fill existing cracks with metamorphic minerals including amphibole (tremolite-actinolite), chlorite, serpentine, prehnite, carbonates, zeolite, and clays (see “Metamorphic petrology”). These veins bear witness to the late-stage fluid circulation in Hess Deep gabbroic rock. In Hole U1415P, however, vein thickness is typically <1 mm and together represents an insignificant proportion of the core volume. Alteration vein densityA 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., 2014f]). The alteration vein density in Hole U1415P is quite low in recovered intervals, in contrast with Hole U1415J. This observation is consistent with the absence of well-developed intervals of intense cataclasis in Hole U1415P core. Rare exceptions include (1) the few small pieces of fully serpentinized/chloritized troctolite and (2) highly altered edges of a few large pieces. About half of the cored length is devoid of veins (Fig. F48). Where present, the vein density is either very low (1 vein per 10 cm) or low (<5 veins per 10 cm). Only 5% of the core has medium to high vein abundance (i.e. >5 veins per 10 cm). No significant contrast in vein density exists between Units II and III. It is likely that a sampling bias exists because of the brittle nature of densely veined rocks. The frequent occurrence of core pieces bound at both ends by zones of alteration, some with slickenside striations, allows us to infer that intensely altered intervals (thick veins or intervals with high vein density) were not recovered. Examples of this relationship are common in troctolite from Unit III (Fig. F49). Vein shape and structure of vein-filling materialAlteration veins in Hole U1415P form networks of regularly spaced planar features or are gently curved. Two crosscutting networks can locally be observed in the same piece. Irregular veins and anastomosing networks are much less frequent than in Hole U1415J. Most veins have clear-cut relationships with their igneous host; alteration halos are uncommon. One exception concerns the edge of some troctolite pieces corresponding to what we interpret as alteration and cracking fronts (Fig. F49), in which dense networks of subvertical veinlets are spatially correlated to a higher than average alteration degree; where the veins stop (vein tips), the alteration intensity drops abruptly. Two faults are recognized in Unit III of Hole U1415P along alteration fronts (Samples 345-U1415P-16R-1, 63 cm [Piece 7], and 20R-1, 87 cm [Piece 8]). The serpentine at the edge of these pieces is strongly sheared with development of a well-defined lineation. Both faults have moderate dips (~30°) and show a downward-dip slickenside lineation. Alteration vein dipBecause of the reasonable recovery in Hole U1415P (~30%), vein orientation was measurable in many pieces. In 145 measurements, we observed slight preferred orientation in the vein dip distribution. Veins with a shallow to intermediate dip (<60°) and subvertical veins (>75°) are more abundant than veins with dips between 60° and 75°. This bimodal distribution is observed in both Units II and III (Fig. F50). No relationship between vein mineralogy and dip was observed (Fig. F51). We note that serpentine veins are more common in the lower part of the hole (olivine-rich troctolite in Unit III), whereas colorless amphibole is more common in the shallow part of the hole (gabbro of Unit II). Multiple stages of vein filling were frequently observed, with chlorite veins later filled with prehnite (see “Metamorphic petrology”). Crosscutting relationships are consistent with the inferred crystallization temperature of the minerals (e.g., chlorite veins crosscutting amphibole veins). Temporal evolutionTemporal evolution of structures recovered in Hole U1415P is, from oldest to youngest,
|
|||
