We present the results of a detailed structural analysis of the core recovered from Site 1268. Four categories of observations were recorded in spreadsheet format including magmatic structures, crystal-plastic deformation, cataclastic/brittle deformation, and alteration veins (see the "Supplementary Material" contents list). These were supplemented by microstructural observations in more than 60 thin sections. Details of the structural classification scheme for each feature are given in the "Structural Geology" in the "Explanatory Notes" chapter. We first present and discuss the observations for each category separately and then discuss the temporal and spatial relationships between them. Finally, we conclude with an initial interpretation of the structural evolution of the core in the context of the tectonic setting of Hole 1268A.
The lithologic stratigraphy of Hole 1268A is complex, consisting of a long section of highly altered peridotite, an intrusion breccia with locally intense crystal-plastic and brittle deformation, and two massive gabbro units with an intervening enclave of peridotite. The entire section has been altered to talc- and sperpentine-rich assemblages, with most primary features in the peridotite and breccia units preserved only as pseudomorphs. This complicates textural and structural interpretation, but many features are well preserved, making clear interpretation of their origin possible.
Structures preserved in the Hole 1268A cores indicate that crystal-plastic deformation occurred over a range of subsolidus conditions including high-temperature ductile flow and late, localized, high-temperature mylonitic deformation contemporaneous with gabbro intrusions in the shallow mantle section.
Gabbro and peridotite fabrics were logged using the semiquantitative deformation intensity scale that was used during Legs 118, 147, 153, 176, and 179 (see Table T4, in the "Explanatory Notes"
chapter). The orientation of these fabrics was measured in the core reference frame. In a few cases we were able to determine a shear sense. The variation in the intensity of crystal-plastic deformation with depth is shown in Figure F43. This figure represents a running downhole 20-piece average of the intensities measured on individual pieces of core. Although individual pieces vary in length, the plot will differ little from one based on average intensities for uniform length increments of the core. This is because the plot includes all 1074 pieces logged, with an average length of only 6.6 ± 8.8 cm (1
). The uppermost interval of the hole is not represented, as the first core is believed to be rubble rather than true basement. We also stopped averaging within 10 pieces of the top of Core 209-1268A-2R and within 10 pieces of the bottom of Core 29R.
The most pristine and "least deformed" high-temperature mantle textures were locally observed in sections of harzburgite with no significant crystal-plastic foliation. These are protogranular textures characterized by relatively coarse granular olivine and orthopyroxene. Orthopyroxene has smooth curved grain boundaries, commonly interstitial to the olivine matrix (Fig. F44). Protogranular textures show little sign of internal high-temperature strain and are assigned a deformation intensity grade of 0 on our scale. In the core, these textures are present in areas with little or no foliation (e.g., Fig. F44A, F44B). Minor crystal-plastic deformation is observed in the form of very weak foliations (Fig. F45), infrequent kink bands in olivine (Fig. F44G), and rare slight bending of pyroxene cleavage. Evidence of extensive dynamic recrystallization is absent, and we interpret these textures as having formed during high-temperature, low-strain mantle deformation (e.g., Mercier and Nicolas, 1975).
The Hole 1268A harzburgites also exhibit a previously undescribed variant on protogranular texture that we term protointergranular texture (Fig. F46). This texture is defined on the basis of pyroxene morphology. Orthopyroxene has a very ragged intergranular or even subophitic appearance (Fig. F44C) and generally a smaller grain size than is normal in mantle peridotites. Whereas abyssal peridotites generally have orthopyroxene grains 3–4 mm in size (Dick, 1989), the Hole 1268A protointergranular harzburgites typically have average orthopyroxene grain sizes of <2 mm. A large proportion of the protointergranular textured harzburgites appear undeformed. This texture may have developed late in the emplacement history of the peridotite as a result of pyroxene dissolution by migrating melts. This conclusion is also suggested by common large vermiform spinel intergrown with single small orthopyroxene pseudomorphs in the protointergranular harzburgites (Fig. F44D). Chromian spinel probably forms during incongruent dissolution of pyroxene (e.g., Dickey et al., 1971; Dickey, 1976; Dick, 1977). These high-temperature, low-strain textures are preserved locally in the core (e.g., Sections 209-1268A-3R-1 and 3R-2) between higher-strain regions. We grade 45% of the harzburgites at 0 or 0.5 (transitional from protogranular to weak porphyroclastic) on our intensity scale (Fig. F47).
The development of higher-strain zones at high temperatures is manifested in Hole 1268A by high-temperature porphyroclastic harzburgite. Of the harzburgites, ~55% have weak porphyroclastic (deformation intensity grade = 1) to foliated porphyroclastic (grade = 2) deformation textures (Fig. F47). This texture is generally characterized by rounded porphyroclasts of pyroxene in a matrix of recrystallized olivine. Pyroxene grains are commonly stretched, with aspect ratios varying between ~1:1 and 4:1. A weak to moderate penetrative foliation can be present over long intervals. In lower-temperature examples, kinked and strained pyroxene and olivine are abundant.
An important characteristic of the orthopyroxene pseudomorphs in the Hole 1268A porphyroclastic harzburgites is the abundance of relatively large neoblasts with polygonal grain boundaries that formed by recrystallization of porphyroclasts (Fig. F44E). This texture is not unusual in either abyssal or ophiolitic peridotites with weak to foliated porphyroclastic texture. More commonly, however, pyroxene neoblasts are finer grained and lack a well-developed polygonal texture. The coarser texture is associated with high-temperature recrystallization of orthopyroxene, often at temperatures >1000°C (e.g., Kelemen and Dick, 1995). The lower-temperature form of porphyroclastic texture is not common in the Hole 1268A harzburgites.
Porphyroclastic-textured higher-strain zones occur throughout the core, with strain varying downhole over tens of centimeters to meters (Fig. F43). Examples of coarse porphyroclastic harzburgites with varying intensities of foliation are found in Sections 209-1268A-19R-2 (Pieces 2–6, 14, 15) and 15R-1 to 15R-4. Downhole variability in the aspect ratios of orthopyroxene indicates variable strain on scales of centimeters to meters, an observation also recorded in peridotite cores from Site 920 in the Kane Fracture Zone (MARK) area (Shipboard Scientific Party, 1995). In the MARK region, however, the average overall extent of deformation appears higher. The succession of textures from protogranular to foliated porphyroclastic is well represented in virtually all mantle sections in ophiolites and is believed to represent solid-state flow and emplacement to the base of the crust.
Mylonites crosscut both the peridotites and the gabbro intrusives. The localized mylonitic deformation is superimposed on the earlier protogranular and porphyroclastic textures of the peridotites and clearly represents a later event as illustrated in Figure F48. Although the textures are only preserved in pseudomorphs, they are similar to those previously described in fresh abyssal peridotites (Jaroslow et al., 1996). Characteristic features include ribbon-textured olivine representing extreme stretching of grains, alternate enclaves of very fine and fine-grained olivine defining well-developed foliations, extreme grain size reduction, and porphyroclasts with either extreme ratios of length:width or near-perfect round or oval outlines. In the extreme, grain size reduction eliminates macroscopic foliation in the rocks. At these extents of deformation, no elements of protogranular texture are preserved.
The late mylonitic deformation occurred prior to alteration, probably shortly after gabbro intrusion, based on several observations:
Several shear sense indicators in the major mylonite zone in Cores 209-1268A-14R and 15R indicate a reverse shear sense (reverse shear sense is also recorded by two shear zones in Hole 735B, though there are others with a normal shear sense [Shipboard Scientific Party, 1999]):
Early, well-developed crystal-plastic foliations are limited in the Hole 1268A cores. Only 30 measurements were made, the majority representing high-temperature, moderate-strain porphyroclastic foliations in the harzburgites. The remainder of the foliations were developed during the late high-temperature, high-strain mylonitic deformation event. Foliation dips are plotted in Figure F43. In general there are no systematic variations in the dip of the crystal-plastic foliation with depth.
Narrow protomylonite zones (grade = 3) occur in small 0.5-cm shear zones at 14.6 and 53.3 mbsf within single pieces of harzburgite and dunite. There is also a 1-cm mylonite zone in gabbro-veined harzburgite from 88.7 to 89.2 mbsf. Mylonites occur in a 5-cm zone in harzburgite in Section 209-1268A-9R-1 (Piece 9) (49.4 mbsf), in a 40-cm zone in gabbro-veined harzburgite (64.4–64.8 mbsf), and in harzburgite-gabbro breccia (75.9–79.0 mbsf). Only a few fragments of ultramylonite (grade = 5) were found in the core in the 75.9- to 79.0-mbsf shear zone. At higher deformation grades (3–5), the transition from undeformed or weakly deformed rock to mylonite is usually quite sharp (e.g., Fig. F48), whereas the transition from lower-grade porphyroclastic deformation to mylonite is more transitional.
Both visual core description and thin section observations show that crystal-plastic deformation is not evenly distributed in Hole 1268A (Table T4). Overall, gabbros are the least deformed, with well-preserved igneous textures. They have an average crystal-plastic deformation intensity grade = 0.2. Gabbros drilled below 101 mbsf (from Core 209-1268A-20R downhole) have an average intensity grade = 0, whereas those higher in the hole have an average intensity = 0.4. The lower gabbros are not entirely undeformed, with occasional kinks in pyroxene grains, and recrystallization and polygonalization of plagioclase near the top of the lower gabbro units in Core 209-1268A-21R indicate very weak crystal-plastic deformation near the contact with the overlying peridotites (Fig. F44F). Harzburgite has an average deformation intensity = 0.9. This grade largely reflects background high-temperature deformation; therefore, the harzburgite probably underwent little penetrative deformation following intrusion of the gabbros. Dunite also appears to have a low average deformation intensity, although this may be due to the lack of deformation indicators preserved after alteration.
The harzburgite-gabbro intrusion breccias and the heavily gabbro-veined peridotites in Cores 209-1268A-15R to 20R are the most deformed rocks in Hole 1268A. These have an average deformation intensity = 1.3, and their location coincides with the three major mylonite zones in the hole (Fig. F43). This high-intensity deformation is not penetrative and is localized within the intrusion breccias, providing a sequence of textural relationships from undeformed to mylonitized intrusion breccia (Fig. F51A, F51B, F51C). The contrasts in competency created by the mixture of gabbroic rocks and peridotite in the intrusion breccia and heavily veined gabbros would provide a natural place for strain localization and failure to occur.
Taking into consideration the background high-temperature recrystallization, the Hole 1268A harzburgites and the intruding gabbros show only modest crystal-plastic deformation, compared to the more extensive deformation in the upper 500 m of Hole 735B drilled in the Atlantis Bank oceanic core complex. There, although perhaps as much as 100–200 m of the footwall was removed by erosion, 14% of the gabbros have undergone more than weak deformation (grade = >1), whereas only 6.5% of the Hole 1268A cores have undergone more than weak deformation (Fig. F47).
Much of the serpentinized peridotite recovered from Site 1268 displays a weak to moderately strong, dominantly planar fabric defined by anastomosing arrays of serpentine veins. A similar foliation was described in serpentinite core from Ocean Drilling Program (ODP) Site 920 (Shipboard Scientific Party, 1995). Serpentine and magnetite veins that define the foliation are commonly deflected around pyroxene porphyroclasts (Fig. F52), which causes anastomosing waves in the foliation. The amplitude of these waves appears to be a function of the pyroxene porphyroclast grain size and the degree to which veins deflect around porphyroclasts. In some locations, veins penetrate pyroxene porphyroclasts and define a more planar, less anastomosing foliation. Serpentine fibers forming veins are commonly aligned perpendicular to vein walls, suggesting growth during dilational opening of fractures. This suggests that the foliation is not a result of shear deformation (Fig. F53). In several samples, serpentine fibers in veins are aligned oblique to vein walls, suggesting that fractures opened by a combination of dilational and shear movement. This texture, however, does not appear to represent a large degree of shear strain accommodation. The serpentine foliation in much of the Site 1268 core is obscured and overprinted by pervasive talc alteration. Static replacement of serpentine minerals with very fine grained talc diminishes the fabric intensity, leaving only a faint trace of the original texture.
The orientation of anastomosing serpentinization foliation viewed in hand sample generally parallels that of preexisting crystal-plastic fabric. This relation was also observed in serpentinite from Site 920 (Shipboard Scientific Party, 1995). Veins defining serpentine foliation are generally aligned parallel to the long axis of pyroxene porphyroclasts. In most instances, serpentine foliation in Site 1268 core dips <40° (Fig. F54), with the highest population of dips between 20° and 30°. Serpentinization foliation is dominantly shallow dipping (<30°) at depths above 40 mbsf (Fig. F55). No serpentine foliation was present between 40 and 60 mbsf. Serpentine foliation measured at depths below 60 mbsf is more variable and generally steeper than that measured at shallow depths in the hole.
Depth intervals with the highest-intensity serpentinization foliation are located near, but not directly correlated with, intervals with highest crystal-plastic deformation intensity. Maxima in the intensity of serpentinization foliation generally appear on the shoulders of crystal-plastic deformation intensity maxima (Fig. F56). Observations of serpentine textures at Site 920 indicated a direct correlation between the intensity of crystal-plastic deformation fabric and the strength of anastomosing serpentinization foliation (Shipboard Scientific Party, 1995). This may not be the case in Site 1268 serpentinites. Depth intervals with the highest-intensity crystal-plastic deformation in Figure F56 correspond to intervals of peridotite mylonite deformed under granulite-grade conditions that do not display anastomosing serpentinization foliation. Serpentinite with the highest observed foliation strength occurs over intervals of moderate-intensity crystal-plastic deformation. Another factor that may contribute to the lack of complete correlation between serpentine foliation strength and crystal-plastic intensity is pervasive talc alteration. The crystal-plastic deformation intensity in many talc-altered peridotite samples may be estimated from the shape and alignment of pyroxene porphyroclasts that are pseudomorphed by the alteration, whereas serpentine foliation is partially to completely obscured.
Hole 1268A core contains three intervals of nonfoliated gouge/breccia that likely formed in fault zones with significant shear displacement (Fig. F57). These include an 11-cm interval in Section 209-1268A-11R-2 (Piece 8), a 23-cm interval in Section 12R-1 (Piece 15), and a 9-cm interval in Section 15R-1 (Piece 21). All three gouges are matrix-supported breccias containing clasts of serpentinized and talc-altered peridotite. The matrix of the gouges is composed dominantly of serpentine and/or talc with variable amounts of disseminated sulfides. Clasts are subrounded to angular serpentinized and talc-altered peridotites that range in size from 0.3 to 3 cm with outsize clasts as large as 15 cm. The gouges in Cores 209-1268A-11R and 12R are noncohesive, and the gouge in Core 15R contains zones of noncohesive and partially cohesive material. The gouge intervals in Core 209-1268A-12R occur at the lower boundary of an interval of peridotite mylonite deformed at granulite-grade conditions. The gouge in Core 209-1268A-15R is present at the lower boundary of an interval of mylonitized intrusion breccia. These gouge intervals represent the highest degree of brittle deformation at Site 1268.
The peridotite section in Site 1268 core is cut by cataclastic shear zones that comprise densely spaced anastomosing fractures bounding small (<0.3 cm) phacoids of undeformed serpentinite (Fig. F58). The matrix of cataclastic breccias is composed dominantly of talc and/or serpentine with fibers parallel to the shear zone orientation. Discrete fractures in cataclasite zones are filled with fibrous serpentine and/or magnetite. Cataclastic shear zones are <3 cm wide, where visible in large core pieces, or comprise the bulk of small pebbles recovered from the core barrel. Crosscutting relations and mineralization patterns suggest that cataclastic shear zones formed during or shortly after the main phase of serpentinization. Three narrow cataclasite zones in Section 209-1268A-6R-1 (Fig. F58) were more resistant to pervasive talc alteration than surrounding dunite. Only eight cataclastic shear zones appear in core pieces large enough to measure orientation. Of these, there are no statistical trends in dip magnitude (Fig. F59), nor is there any relation between dip and curated depth (Fig. F60).
Discrete shear fractures and small-offset faults ranging 0.1–0.3 cm in thickness are present throughout much of Units I, II, and III of Hole 1268A, for example (Fig. F61). Crosscutting relations indicate these are the last structural features formed at this location. Where microfaults cut distinct marker horizons, the offset is generally <1 cm. In several sections of core, groups of microfaults with the same sense of motion are aligned parallel to one another to create a composite structure with greater total shear offset. Talc veins containing fibers parallel to the direction of fracture opening typically fill fractures and microfaults. In faults with >0.5-cm offset, talc veins commonly contain slickenfibers that may indicate the direction of fault slip and sense of shear. Slickenfibers indicate that normal, reverse, strike-slip, and oblique-slip motion all occurred within these structures. Measurement of the orientation of 37 such fractures was possible in Hole 1268A cores. A histogram of fracture dips (Fig. F62) shows that the dips of the shear fractures and small faults are highly variable with no dominant preferred orientation. A plot of dip vs. depth (Fig. F63) shows that shear fractures at depths above 40 mbsf all dip <40°. Shear fractures measured at depths below 60 mbsf have slightly higher and more variable dips than the fractures at shallow depths.
Coarse-grained gabbro in the lower portion of Hole 1268A (Unit IV) contains several zones of minor to moderate cataclastic fracturing and incipient brecciation. These occur over intervals ranging from 1 to >50 cm thick and generally decrease in intensity and frequency downsection. The lowest-intensity brittle gabbro shear zones are composed of networks of fine fractures with small shear offset (<0.1 cm) cutting plagioclase and deflecting around pyroxenes in the gabbro. Higher-intensity brittle shearing of gabbro results in coalescing fractures that reduce plagioclase grain size but do not affect pyroxene porphyroclasts (Fig. F64). Unlike cataclastic shear fractures in the peridotites, fractures and breccias in Unit IV gabbros do not have any preferred orientation or create breccias with visible foliation. The relative timing of gabbro brittle deformation vs. brittle strain of overlying peridotite is not constrained by crosscutting relations in Site 1268 core.
Veins in the harzburgites and dunites range in morphology from planar and continuous to thin, irregular, discontinuous veinlets. Five distinct types of veins were measured: massive sulfide veins, talc pyrite veins, massive talc veins, serpentine veins, and chlorite veins. The relative timing of the events corresponding to these veins is complicated, and there are at least two generations of serpentine and sulfide veins. However, the observed crosscutting relationships are consistent with the following simplified sequence of events: (1) magmatic veins, (2) serpentine veins and associated serpentinization of the peridotites, (3) massive sulfide mineralization, (4) talc alteration and massive talc veins, and (5) late sulfide and serpentine veins. Evidence that the pervasive serpentinization postdates the magmatic vein event is indicated by the magmatic veins themselves, which characteristically show pale green serpentine-filled tension cracks perpendicular to their length (Figs. F65, F66). These cracks are a consequence of volume expansion of the host ultramafic rock relative to the magmatic vein during serpentinization and thus suggest that the magmatic veins were present within the ultramafics before serpentinization. Figure F65 also shows that these features predate the late serpentine veining events. A detailed discussion of the mineralogy of the alteration veins can be found in "Metamorphic Veins" in "Metamorphic Petrology." We measured the intensity and orientation of veins using the intensity scale outlined in "Fabric Intensities," in "Structural Geology" in the "Explanatory Notes" chapter. The intensity of these planar features is a measure of their average frequency in a 10-cm interval.
The total intensity of alteration veining varies considerably throughout the cored interval, as shown in Figure F67. The intensity is uniformly greater in the peridotite units (0–105 and 117–126 mbsf) than in the gabbro units (105–117 and 126 mbsf to the bottom of the hole) and is greatest in the intrusion breccia interval (63–78 mbsf). There are two other maxima in vein intensities in the ultramafic rocks. The lower maxima occurs at the contact of the Unit III harzburgites and dunites with the Unit IV gabbros (90–100 mbsf). The increase in vein intensity at the latter can be related to the intense magmatic veining and diking within Unit III. The upper maxima occurs in the upper part of the cored interval (37–48 mbsf) and corresponds to a zone of relatively high crystal-plastic deformation intensity.
Figure F68 shows plots of the downhole intensity for each of the five different types of veins. These plots show that sulfide veins are most concentrated in the upper 80 m of the hole and high intensities of serpentine veining occur at 48, 73, and 98 mbsf. The approximate inverse correlation of talc veins and serpentine veins is partly a consequence of the alteration of serpentine to talc in some of the peridotites.
The variation of dip with depth for each of the major vein types is shown in Figure F69. A total of 125 measurements were made from all the veins that could be measured within the core reference frame. The veins exhibit a wide range of dips, showing concentrations at ~30°, 60°, and 90°. The plots for the individual vein types show that the massive sulfide and massive talc veins have similar dips, with concentrations at ~30° and 60°, whereas the serpentine veins have concentrations at ~90° and 20°–50°. The steeply dipping serpentine veins correspond to the last phase of brittle lizardite veining, which is prominent in the harzburgite dunite Unit III (Fig. F70). This steeply dipping population is significant because steeply dipping veins should be undersampled by a vertical borehole.
Below, we try to assemble the pertinent crosscutting relationships in terms of time and temperature starting with the oldest features. Although the features are discussed in sequence, the structural and magmatic events they represent should be viewed as a continuum.
Bands of dunite (Fig. F58) and bands with varying modal pyroxene content (Fig. F71) are parallel to the crystal-plastic foliation in the harzburgite sections. This banding is commonly subtle and is not pervasive throughout the core. Although there are a few bands with preserved contacts, several zones of dunite were recovered without preserved contact relations (see "Site 1268 Visual Core Descriptions"). Dunite banding is similar to transposed banding subparallel to tectonic foliations in ophiolite ultramafic massifs. This probably represents source heterogeneity or banding created during melting and melt transport processes. As such, the banding is the oldest recognizable structure in the harzburgites. Where contacts are observed, their parallelism with foliation suggests complete transposition into the mantle flow direction.
There are abundant pyroxenite and gabbroic dikes or veins throughout all ultramafic sections of the core. There is a particularly high concentration of these features in the central part of the cored section. Commonly, serpentinite and talc veins follow the pyroxenite and gabbro dikes or veins and mimic their trend, creating severely altered regions in the core. Magmatic dikes or veins in Hole 1268A tend to be irregular, thickening and tapering on a small scale. Most are thin (<20 cm thick) and discontinuous on the scale of the core. The pyroxenite and gabbroic dikes or veins can be planar tabular to curviplanar sheetlike forms to highly irregular.
The pyroxenite and gabbroic veins or dikes probably represent the traces of melts that migrated along small fractures or localized porous channels. They are composed of coarse crystals and show no evidence of chilled margins. These veins vary from pyroxenite (clinopyroxenites or websterites) to gabbroic or composite veins with pyroxenitic margins and gabbroic cores. (We emphasize that these mineral compositions are inferred from alteration minerals and pseudomorphic textures.)
In general, these high-temperature magmatic veins or dikes are postkinematic (or undeformed) with respect to the ductile deformation events that formed foliation and banding in the harzburgite-dunite country rock. Examples of these postkinematic magmatic veins are shown in Figures F72, F73, and F45. Figure F72 contains a pyroxenite vein obliquely cutting the crystal-plastic foliation in a porphyroclastic harzburgite; Figure F73 shows two similar pyroxenite veins cutting the crystal-plastic foliation in porphyroclastic harzburgite, in turn cut by several serpentine-talc veins; and Figure F45 shows longitudinal magmatic veins cutting protogranular textured harzburgite. Similar veins in the MARK area of the Mid-Atlantic Ridge were observed during Leg 153 (Shipboard Scientific Party, 1995).
Late stage, high-strain, high-temperature deformation in localized mylonite zones is best developed in Sections 209-1268A-12R-1, 12R-2, 14R-2, 14R-3, 15R-1, and 17R-1. An example of mylonite crosscutting porphyroclastic harzburgite can be seen in Figure F48. The foliated mylonites contain selvages of an early pyroxenite that has been completely transposed into the foliation plane, unlike other pyroxenite and gabbroic dikes and veins. A second generation of pyroxenites crosscuts the mylonites (Figs. F48, F49). They are undeformed, indicating they are postkinematic with respect to high-temperature crystal-plastic deformation in each mylonite zone. The narrow pyroxenite veins are coarse grained and without chilled margins, which indicates that the mylonitic wallrock was still at a high temperature during magmatic intrusion.
Most mylonites occur in the section containing the intrusion breccias (Fig. F43). Intrusion breccias showing only varying degrees of local subsolidus deformation lie above the major mylonite zone at the bottom of Core 209-1286A-14R and the top of Core 15R and continue below it down to the last major mylonite zone in Core 17R. In the underlying harzburgites, which continue down into Core 209-1268A-20R, there is little evidence of late deformation and there are only a few gabbro veins.
Several lines of evidence suggest that igneous intrusive events occurred over a range of temperatures as the host peridotites cooled. For example, the postkinematic pyroxenite veins cutting the mylonites represent a younger generation than their deformed counterparts. The base of the hole is characterized by a thick section of gabbroic rocks (Unit IV) that is first encountered at the base of Unit III. Although the contact between the harzburgite and gabbro is mostly not preserved, in Section 209-1268A-20R-3 the boundary region of the core preserves a fine-grained gabbro (Piece 8) between serpentinized harzburgite (Piece 7) and coarse-grained gabbro (Piece 9). This could represent a remnant of the chilled margin of the gabbro. Coarse-grained gabbro, however, dominates Unit IV and crosscuts porphyroclastic harzburgite at the upper gabbro contact. Another contact zone is preserved in Section 209-1268A-25R-1 between Pieces 4 and 5.
The massive gabbros in Unit IV retain some of their primary phases, although they are heavily talc altered. Plagioclase contains igneous growth twins with blunt ends that are nontapering in most samples. Pyroxene appears largely undeformed with only a few kinked grains seen in thin section. The sharp crosscutting contact at the top of Subunit IVA and the lack of internal deformation suggest that the larger gabbros postdate the high-temperature, low-strain crystal-plastic deformation in the peridotites or that peridotites were weaker than the gabbroic rocks during that deformation event. Since these gabbros are neither cut by nor crosscut zones with mylonitic deformation, the timing of their intrusion cannot be established with respect to the mylonitic deformation.
As noted previously, the mantle and gabbroic sections sampled in Hole 1268A are pervasively altered under greenschist facies conditions (see "Hydrothermal Alteration" in "Metamorphic Petrology"). The deformation of the mantle section started to localize along mylonite zones under high-temperature crystal-plastic conditions (probably >900°C), perhaps synchronous with igneous intrusion. However, there is no involvement of low- to moderate-temperature alteration phases in the crystal-plastic deformation, and thus we infer that by the time alteration took place the crystal-plastic deformation of primary mantle phases had ceased and peridotites were brittle. Greenschist facies alteration phases commonly pseudomorph plastically deformed mantle phases and undeformed igneous phases in gabbroic rocks, indicating a static metamorphic overprint as documented in "Metamorphic Petrology."
The gabbroic rocks that intrude peridotite do not preserve high-temperature hydrous magmatic phases that would be expected if the intrusions stoped and incorporated material that was previously hydrated. Figures F65 and F66 show magmatic veins of pyroxenite and gabbro containing tapering cracks or tension gashes that become wider in the center of the vein and decrease in width in both directions into the surrounding olivine-rich harzburgite. This suggests that the volume expansion of the peridotites during serpentinization caused dilation in a direction parallel to the pyroxenite vein. Gabbroic veins show similar relationships throughout the core, suggesting that hydration took place after the intrusion events, consistent with the lack of chilled margins, the coarse-grained nature of the pyroxenites and gabbroic veins, and the lack of amphibolite facies metamorphism.
Strain localization in the peridotites became more intense as they passed through the brittle–ductile transition, as indicated by the presence of cataclastic shear zones, fault gouges, alteration veins, and fractures throughout the core. The first generation of brittle structures is associated with the anastomosing serpentine foliation, which is subparallel to the crystal-plastic foliation (see "Brittle Deformation"). Although most serpentinization is associated with dilation crack-seal features, cataclastic shear zone and fault zones lined with serpentine slickenfibers are locally present throughout the core. These appear to be somewhat late in the deformation history. Two significant gouge zones likely represent some of the youngest structural features that formed a long shallow-level cataclastic fault zone.
We measured a total of 75 orientations: 52 magmatic veins, 15 lithologic contacts (resulting from modal pyroxene variation from dunite to harzburgite), 2 spinel foliation planes, and 5 gabbro magmatic fabrics. There was generally no discernible magmatic fabric in the gabbros; where the measurements were taken, the fabric strength was mild and a contribution from deformation was likely. Out of the 75 measurements, 54 were on pieces with paleomagnetic declination determinations. The downhole distribution of these 75 features and corresponding dip angles are shown in Figure F74.
In order to plot the azimuthal distribution of magmatic features, the orientation of the individual pieces of the core were corrected to a common orientation using paleomagnetic measurements (see "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" in the "Leg 209 Summary" chapter). The resulting data set is shown in a plot of poles to the planes in Figure F75. The data are identified according to quality of the paleomagnetic data, but there are no obvious inconsistencies between the higher- and lower-quality paleomagnetic data.
All data were plotted in Figure F75B, keyed according to nature of the magmatic feature. Also plotted in Figure F75B are spinel foliations measured from 8-cm3 cubes (Fig. F76 shows an example). A total of 20 cubes were taken from harzburgites and dunites of oriented pieces of core throughout Hole 1268A, but the harzburgites were spinel poor, so not enough spinel grains were visible to determine the foliation. The data set for lithologic contacts and spinel foliations (Fig. F75B), all presumably predating the magmatic veins, is relatively sparse. The average dip of magmatic veins is 59°; the average dip of lithologic contacts is 49°.
Shown in Figure F77 are poles to crystal-plastic foliations reoriented to a common reference frame (see "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" in the "Leg 209 Summary" chapter). This plot clearly differentiates the early high-temperature, moderate-strain porphyroclastic foliations from those produced by the lower-temperature, higher-strain localized mylonitic events. Although there are several outliers, the high-temperature porphyroclastic harzburgite foliations cluster around a pole giving an apparent strike of ~N30°E and apparent dip of 25°E in the common reference frame. Two of the four outliers lie within Core 209-1268A-4R, which also includes one foliation lying within the main cluster. The clustered porphyroclastic foliations range 8°–56° in dip. Paleomagnetic inclinations suggest a late block rotation of ~60° for the mantle peridotites at Site 1268, assuming a rotation axis striking ~20°N, parallel to the trend of the Mid-Atlantic Ridge immediately east of Site 1268 (see "Site 1268" in "Site Summaries" in the "Leg 209 Summary" chapter). Removing this rotation would bring the dominant porphyroclastic foliation plane to near vertical, subparallel to the present-day ridge orientation.
The orientations of 22 cataclasite zones, shear fracture systems, and small faults were reoriented to a common reference frame using paleomagnetic declination measurements. This analysis assumes that the time-averaged magnetic pole for this location is true north and that the hole is vertical. Results of this analysis are shown in Figure F78, a lower hemisphere stereo plot of poles to fractures and shear zones. A moderately strong preferred orientation exists in the rotated measurements. Shear zones and fractures preferentially strike north–north-northeast in the common reference frame, with dip directions varying from east to west.
The alteration veins also exhibit a variety of azimuths downhole. Figure F79 shows the orientations of the veins restored to a common reference frame. Unfortunately, only 50% of the data set could be reoriented in this way because of the limited number of suitable paleomagnetic measurements. It should be emphasized that there is error associated with these final orientations, due to (1) the difficulty in measuring two precise orientations on the half rounds of core, (2) the uncertainties associated with the magnetic measurements, and (3) possible tectonic rotations of the recovered rocks. However, the existence of a distinct trend of poles to the brittle structures (Fig. F78) after reorientation implies that these errors are relatively small. This result suggests that the weak alignment of poles to the late serpentine veins and the talc pyrite veins is correct and, hence, that these veins have a preferred orientation. The scatter of the poles to the earlier massive talc and massive sulfide veins reveals that these veins formed an isotropic vein network.
Hole 1268A provides a glimpse into a mantle section that is tectonically exposed on the seafloor. The region is unusual because extensive peridotite exposures on the western flank of the rift valley are matched by another peridotite massif on the eastern side of the rift valley. Serpentinized harzburgites and dunites, intruded by pyroxenites and gabbroic rocks, form the section cored.
Hole 1268A has a significant range of both high-temperature crystal-plastic structures and crosscutting magmatic features that formed in a retrograde metamorphic environment. These record deformation during ascent of mantle peridotites to the surface from ductile and crystal-plastic conditions, through the brittle-ductile transition, and finally into moderate- to low-temperature brittle cataclastic and fracture regimes, accompanied by intense low-temperature hydrous alteration.
The earliest deformation in the harzburgites resulted in transposition of dunite bands into a foliation plane defined by the preferred dimensional orientation of elongate orthopyroxene in porphyroclastic and weakly foliated protogranular and protointergranular textured harzburgites. At high temperatures, coarse protogranular and "protointergranular" textures formed under low deviatoric stresses, whereas the porphyroclastic textures formed under more moderate deviatoric stress conditions. High-temperature banding in the peridotites may have been near vertical to the ridge axis and was subsequently rotated about 60° along a ridge-parallel, horizontal axis after cooling of intrusive gabbros below ~570°C. Finally, the last stages of crystal-plastic deformation led to formation of localized mylonites under high deviatoric stress conditions.
Pyroxenitic and gabbroic magmatic veins increase in density in the central part of the core in the region of mylonite formation. They are dominantly postkinematic in low- to moderate-strain regions of the harzburgite section but are pre-, syn-, and postkinematic with respect to the mylonite deformation.
Magmatic intrusion probably took place prior to significant hydration of the peridotites. Large gabbro bodies and early magmatic veins in low-strain regions of the mantle without significant crystal-plastic deformation appear to have been carried to the surface along localized shear and fault zones.
Hydration of the peridotites is associated with the formation of brittle dilational and shear features, eventually allowing nearly complete alteration of the harzburgite-dunite sections. These produced a complex series of anastomosing serpentine fabrics parallel to the crystal-plastic fabric and an array of crosscutting veins.
Late cataclastic deformation and alteration in the Hole 1268A cores is localized, varying considerably downhole. Like the late mylonitization, cataclastic deformation may have followed zones of intense crystal-plastic deformation.
The reverse shear sense found in the principal mylonite zone could result from (1) local strains caused by intrusion of gabbroic rocks or (2) normal-sense mylonites that later underwent tectonic rotations. Counterclockwise rotations of up to 60° for the mantle sections have been tentatively inferred from data on magnetic azimuth and inclination of Hole 1268A gabbroic rocks (see "Site 1268" in "Site Summaries" in the "Leg 209 Summary" chapter).
