We present the results of a detailed structural analysis of core recovered from Holes 1270A, 1270B, 1270C, and 1270D. Four categories of observations were recorded in spreadsheet format including magmatic fabrics, crystal-plastic deformation, brittle deformation, and alteration veins (see the "Supplementary Material" contents list). These were supplemented by microstructural observations from 46 thin sections, although the relatively high degree of alteration in the peridotites meant that some of the features are preserved only as pseudomorphs, leading to potential ambiguity in their interpretation. Details of the structural classification scheme for each feature are given in "Structural Geology" in the "Explanatory Notes" chapter. We conclude with a discussion of the temporal and spatial relationships of the structures and then present an initial interpretation of the evolution of each of the four holes in the context of their tectonic setting.
The three cores from Hole 1270A recovered largely green "popcorn" serpentinite (Fig. F52). This informal name reflects the puffed up, expanded appearance of the pyroxene pseudomorphs in hand sample. It can be seen in thin section that the pyroxene bastites (pseudomorphed orthopyroxene grains) have a halo of clear serpentine around them in a mass of green serpentine heavily veined by magnetite. Because of this type of serpentinization, these protogranular peridotites may appear to have a porphyroclastic fabric. Thus, caution should be taken when relying on visual core descriptions of these serpentinites. Instead, we rely heavily on five thin sections in our analysis. In addition to the serpentinites, a small volume of highly altered gabbro was found (3.6 vol%). The alteration commonly obscured the extent of deformation, but they generally appear to be undeformed.
Bastite pseudomorphs of orthopyroxene permit the identification of the serpentinite protoliths as harzburgite and allow characterization of the primary protogranular texture. Two examples of protogranular texture are shown in Figure F53. The rocks contain isolated orthopyroxene pseudomorphs with smooth curved grain boundaries, deep embayments, and irregular lobes locally extending into a serpentinized olivine matrix. Orthopyroxene cleavage pseudomorphs show that most grains are unstrained, although a few samples have slight kinks in them. Although some pyroxene pseudomorphs have an aspect ratio >4:1 (porphyroclastic by definition in "Fabric Intensities" in "Structural Geology" in the "Explanatory Notes" chapter), suggesting crystal-plastic elongation, the highest crystal-plastic intensity seen in the thin sections is at most 0.5 (very weakly deformed).
As noted above, well-developed crystal-plastic foliations are rare in the serpentinized harzburgites. An example of a primary foliation is shown in the upper core piece in Figure F52B. Here there is a clear foliation, defined by the pyroxene shape fabric that is not the product of porphyroclastic deformation. Individual pyroxenes can be seen to have irregular lobes and embayments characteristic of protogranular texture, but overall they are elongate in the foliation plane. Unfortunately, foliations are difficult to identify, but we estimate the average foliation dips ~45° in the cut face.
Figure F54 is a downhole plot of fabric intensity made using a running average (weighted by piece length) of the deformation intensities measured on seven contiguous core pieces. This plot suggests that the harzburgites in the upper 16 m of the hole have a weak porphyroclastic texture, whereas those deeper in the hole range from undeformed (protogranular) to very weakly porphyroclastic. As noted earlier, because of the unusual texture of these serpentinites, estimated deformation intensities during visual core description are not entirely reliable and the intensity of the porphyroclastic texture may be overestimated.
The Hole 1270A core is generally devoid of extensive magmatic veining. Only Section 209-1270A-4R-1 contains minor volumes of veins, and none of these pieces is oriented. Two thin veins of altered pyroxenite cut harzburgite in Piece 5 of this section, and a thin altered gabbroic vein crosscuts partially serpentinized (95%) harzburgite in Pieces 14 and 15. The gabbroic veins are similar and are probably contiguous (Fig. F55). The veins cut orthopyroxene grains and consist of plagioclase pseudomorphed by prehnite and hydrogrossular and olivine replaced by serpentine. They appear undeformed and are oblique to the foliation and thus are considered postkinematic with respect to the crystal-plastic deformation. The presence of thin magmatic veins suggests that the host peridotites were near magmatic temperatures during vein formation. Two small fragments of pyroxenite without contact relationships in Section 209-1270A-4R-1 (Pieces 1 and 2) are the only other pieces that could be magmatic vein material. The amount of plutonic rock in Hole 1270A is particularly low compared to Holes 1270C and 1270D.
Some of the serpentinized peridotite recovered from Hole 1270A displays a weak to moderately strong, dominantly planar fabric defined by anastomosing arrays of serpentine veins and elongate mesh cells. Similar foliations were described in serpentinite from Ocean Drilling Program (ODP) Site 920 (Shipboard Scientific Party, 1995), and the texture was termed "ribbon texture serpentinite" by O'Hanley (1996) based on observations of serpentinite from ophiolites. Serpentine and magnetite veins that define the foliation are commonly deflected around pyroxene porphyroclasts, which causes anastomosing waves in the foliation. Serpentine fibers in veins are commonly aligned perpendicular to vein walls, suggesting growth during dilational fracture opening. This indicates that the foliation is not a result of shear deformation but is a purely dilational feature.
Two intervals of fine-grained breccia interpreted as fault gouge were recovered from Section 209-1270A-3R-1 (Pieces 1 and 3). Both breccias contain subrounded to angular lithic clasts and individual crystals ranging 0.03–0.3 cm in size within a gray-green carbonate-rich serpentine matrix (Fig. F56). The primary difference between the fault gouge zones is that Piece 1 appears to be supported by the extremely fine grained matrix and is slightly cohesive, whereas Piece 3 is clast supported and is noncohesive. The gouge reacts with hydrochloric acid, but it is unknown if this reaction is wholly with carbonate clasts or also with a carbonate fraction within the matrix. Some of the gouge characteristics described here could be a result of disturbance by drilling, and the in situ equivalent of these pieces may have different characteristics. In addition to fault gouge zones in Section 209-1270A-3R-1, there is also a small fragment of fine-grained, carbonate-matrix, cohesive cataclasite (Section 2R-1, Piece 15). This piece may have formed during an earlier phase of brittle deformation, predating formation of the gouge zones described above.
Section 209-1270A-1R-1 (Piece 7) contains a shear zone that was identified in thin section but was not apparent during visual observation of the core. The zone consists of a schistose mat of fibrous white serpentine (Fig. F57). These serpentine fibers terminate along a diffuse boundary into a very fine granular-texture serpentinite that contains clasts of pseudomorphed pyroxene and normal mesh-texture serpentine. This zone, which resembles a breccia, is several millimeters wide and lies between the schistose serpentine and normal mesh serpentinite. Its contact with the mesh serpentinite is irregular and undulatory. The foliation of the schistose serpentine is aligned at a 30° angle oblique to the contact between the breccia serpentine and the normal mesh serpentine.
The alteration veins in the harzburgites and dunites range from planar and continuous to thin, irregular, discontinuous veinlets. They form a relatively small proportion of the core and have maximum widths of 2 mm. The veins are dominantly serpentine with subsidiary talc and oxide veins. There are at least three generations of serpentine veins: dark magnetite-rich serpentine veins, pale green serpentine veins, and late white chrysotile veins. Interval 209-1270A-2R-1, 88–93 cm (Fig. F58), shows an early generation of green serpentine and the later generation of white chrysotile veins oriented parallel to the foliation in the harzburgite. Chrysotile veins are transposed into the foliation of the sheared serpentinite (interval 209-1270A-1R-1, 34–39 cm) described in "Brittle Deformation" above, indicating that the early phase of serpentinization occurred prior to the phase of semibrittle deformation. Interval 209-1270A-1R-1, 43–50 cm, contains normally faulted chrysotile veins, indicating that serpentinization also occurred before low-temperature brittle deformation. Two gabbroic veins are present within this core, one is cut by serpentine-filled tension cracks perpendicular to its length (interval 209-1270A-4R-1, 48–56 cm), providing evidence that the pervasive serpentinization postdates gabbroic veins. A detailed discussion of the mineralogy of the metamorphic veins can be found in "Metamorphic Petrology."
Crystal-plastic deformation in the oxide gabbronorites of Hole 1270B occurred over a range of conditions. High-temperature, moderate-strain fabrics (deformation intensity = 1–2.5) predominate (74.9 vol%) (Fig. F59). Low-strain fabric deformation (intensity = <1) is also important (23.8 vol%), but higher-strain fabrics are rarely present (1.3%). Altered peridotite, largely soapstone and serpentinite, makes up 1.7% of the recovered material and displays only low strain–rate fabrics formed at high temperatures. Gabbro and peridotite deformation intensities were logged using separate, but equivalent, deformation intensity scales as explained in "Structural Geology" in the "Explanatory Notes" chapter. The orientation of the fabrics was measured in the core reference frame.
High-temperature, low-strain fabrics are found in the altered peridotites. They are completely altered but contain numerous talc and bastite pseudomorphs of orthopyroxene, allowing us to characterize the extent of deformation. The grain boundaries are generally rounded with few protrusions, and the shapes correspond to a weak to moderate porphyroclastic texture similar to those seen in the Hole 1268 altered peridotites (see earlier discussion in "Structural Geology" in the "Site 1268" chapter).
The majority of gabbros found in Hole 1270B have high-temperature, moderate-strain fabrics and range from weakly foliated and well-foliated porphyroclastic gneiss to protomylonite. Recrystallization of plagioclase is extensive, usually >40%, with the development of ribbon porphyroclasts (Fig. F60A) and bent and kinked clinopyroxene (Fig. F60B, F60C) and extensive development of clinopyroxene porphyroclasts with well-developed tails of neoblasts (Fig. F60D, F60E). Low-strain fabrics are also common and are characterized by undeformed clinopyroxene, bent or kinked tabular orthopyroxene grains, and only incipient recrystallization and development of tapered deformation twins in plagioclase. Nearly undeformed oxide gabbros are locally present, particularly in the lower sections of the hole (Fig. F61D).
Long sections of the core show well-developed crystal-plastic foliations (Fig. F62A). In some cases the fabric appears to be uniformly developed at a scale of tens to hundreds of centimeters, and in other cases the deformation of the rock can be heterogeneous down to the millimeter scale. Locally, more than one foliation can be seen in a single piece of core, with one crosscutting the other, as in Section 209-1270B-7R-2 (Piece 13), where a 3-cm protomylonite zone cuts weakly deformed coarse-grained oxide gabbro (Fig. F62B). The protomylonitic band contains a high concentration of iron-titanium oxide. In Section 209-1270B-1R-1 (Piece 17), a foliated microgabbro obliquely crosscuts a foliated coarse oxide gabbronorite. The foliated microgabbro is, in turn, crosscut by a nearly undeformed pyroxenite band (Fig. F63). Thus, the high-temperature deformation accompanied the magmatic construction of the gabbroic rocks.
Late Fe-Ti oxide fills cracks in broken plagioclase and orthopyroxene grains (Fig. F61A, F61B) and forms mantles and tails to clinopyroxene porphyroclasts, where oxides appear to cement neoblasts of clinopyroxene and extend out into the enclosing matrix of recrystallized plagioclase subgrains (Figs. F61C, F60D, F60E). In porphyroclastic gabbros, oxides can be seen on crosscutting microscopic shear zones, suggesting that the oxide precipitation occurred along several different planes in an anastomosing shear zone (Fig. F64C). The unusual structure of many of the lenticular oxide aggregates, parallel to the foliation plane but extending into the recrystallized plagioclase matrix in all directions (Fig. F64A, F64B), suggests that the oxide aggregates may have formed by late-stage migration of iron-titanium–rich liquid (for further discussion, see Fig. F24 and associated text in the "Leg 209 Summary" chapter).
Rocks with crystal-plastic deformation intensity >2.5 are rare (Fig. F59), with mylonitic gabbros composing only 1.3% of the recovered core. In gabbro mylonites, recrystallization of all phases including clinopyroxene is extensive (Fig. F65). These rocks were recovered in two sections near the base of the hole: Sections 209-1270D-8R-1 and 10M-1. Other than the mylonites, there is little evidence of amphibolite or greenschist facies deformation.
Shown in Figure F66 is a running 10-piece average for total crystal-plastic deformation intensity (noting that the poorest recovery came in intervals with low total intensity). Peridotites with high-temperature, low strain–rate fabrics completely altered to soapstone and serpentine were recovered near the bottom of Section 209-1270B-6R-1 and from the top and middle of Section 7R-1. The lack of deformation, and character of alteration, suggest that the high-temperature downhole stratigraphy may have been emplaced a by brittle fault. Similar altered peridotites were recovered in the upper three cores in Hole 1270B, but here they may represent debris from the top of the hole.
Overall, the highest average deformation intensities are found in the upper portion of the hole above ~20 mbsf in the oxide gabbronorites/gabbronorites, where there is a nearly uniform penetrative foliation. Deformation intensity decreases from ~20 to ~23 mbsf and then spikes in Core 209-1270B-7R before rising again in strongly foliated rocks in Core 8R and the top of Core 9R. This variation in intensity is accompanied by a systematic downward decrease in the dip of the foliation. There may be a positive correlation between deformation intensity and the proportion of Fe-Ti oxides (Fig. F24 in the "Leg 209 Summary" chapter) and a negative correlation between deformation intensity and the proportion of plagioclase in the gabbros (Fig. F66).
Shown in Figure F67 are the orientations of the crystal-plastic foliations reoriented into a common orientation using the magnetic declination data as described in "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" in the "Leg 209 Summary" chapter. The data in Figure F67 are plotted with the declination of the stable remanent magnetization pointing south because the paleomagnetic data suggest a reversed polarity. In this plot we have color-coded samples from the same core. Samples from individual cores cluster together, and the data as a whole define a partial girdle in the stereo plot. This probably does not correspond to a fold axis, as demonstrated by the three different foliations measured in Section 209-1270B-1R-1 (Piece 17) that share a common paleomagnetic declination and inclination. The three foliations in this piece crosscut each other but they appear to have developed penecontemporaneously, suggesting the partial girdles represent various orientations of shear planes in an anastomosing shear zone. The systematic downhole decrease in the average dip of foliation suggests that the overall dip of this shear zone may decrease with depth. Caution is warranted, however, because the presence of soapstone and serpentinites in Sections 209-1270B-6R-1 and 7R-1 and the variations of deformation intensity with depth suggest that deformation might have occurred in several distinct episodes, that the section may be cut by a significant fault zone, and/or that there may be more than one gabbroic intrusion represented in the core (see "Igneous and Mantle Petrology" and "Geochemistry").
There is strong evidence of strain localization in the oxide gabbronorites in Hole 1270B. Textures suggest that the oxide in some of these deformed rocks may have crystallized after deformation. Similar observations have been made at the Atlantis Bank oceanic core complex (Site 735B and vicinity), where a long-lived detachment fault exposed gabbroic rock (Dick et al., 1991, 1999, 2000; Natland and Dick, 2001). Similarly, at Sites 921–924 in the MARK area of the Mid-Atlantic Ridge, a large detachment fault exposes predominantly gabbroic rock at the eastern inside-corner high of the Kane Fracture Zone (Cannat et al., 1997; Agar et al., 1997; Agar and Lloyd, 1997). In Hole 735B and at Sites 921–924, investigators have suggested that strain localization occurs within partially molten gabbros and controlled the flow and crystallization of late iron-titanium–rich liquids in these intrusions. Deformation and strain localization continued to lower temperatures in the MARK and Atlantis Bank oxide gabbros, with the formation of numerous cataclastic and mylonitic intervals. Localized cataclastic and mylonitic deformation is rare in Hole 1270B, suggesting that such later, lower-temperature strain localized elsewhere after cooling of the Hole 1270B oxide gabbronorites and gabbro.
Minor brittle deformation overprints ductile deformation in several core intervals (e.g., interval 209-1270B-8R-1, 114–116 cm). Brittle deformation is concentrated near discrete planar fracture surfaces. On a centimeter scale, brittle deformation is preferentially localized into narrow zones of fine-grained, recrystallized plagioclase neoblasts between pyroxene porphyroclasts (Fig. F68) and away from coarse plagioclase and pyroxene porphyroclasts. Brittle deformation is accomplished by fracturing of plagioclase neoblasts to produce variably sized but generally fine angular grains. Minor brittle fractures cut plagioclase porphyroclasts but in most cases do not have slip >0.1 mm. Several zones of brittle shear overprint the ductile deformation and are spatially associated with concentrations of Fe-Ti oxides (e.g., interval 209-1270B-1R-1, 90–98 cm). In some cases it appears that oxides fill voids created by dilatant brittle deformation (Fig. F69). Oxides within many of these zones contain undeformed exsolution lamellae of ilmenite within titanomagnetite. Ilmenite lamellae in similar oxide-rich gabbros collected from other locations on the Mid-Atlantic Ridge and Southwest Indian Ridge formed at temperatures >600°C (T. Schroeder, unpubl. data). If ilmenite lamellae from Hole 1270B formed under similar conditions, the brittle overprint of ductile shear zones likely occurred under amphibolite grade conditions. The lack of greenschist grade alteration of these shear zones also suggests higher-temperature brittle deformation.
Very few zones of low-temperature (<500°C) deformation were recovered in Hole 1270B. These include several intervals containing slight brecciation/semibrittle deformation in which fracturing occurred concurrently with alteration and replacement of primary igneous minerals by chlorite and other greenschist facies alteration products, for example, Sections 209-1270B-5R-1 (Piece 3) (greenschist deformation of gabbro) (Fig. F70) and 7R-1 (Piece 2) (greenschist deformation of peridotite). These zones are characterized by slightly fibrous to elongate chlorite, talc, and possible amphibole with remnant clasts of pyroxene and plagioclase.
Several core intervals were encountered with moderate to high intensities of shear fracturing. These fractures generally have little or no offset, are typically filled with alteration vein material (see below), and are not thought to represent significant faults.
A plot of average deformation intensity vs. expanded depth (Fig. F71) shows that the cataclastic intensity correlates with the intensity of alteration veining in the top two cores, but the correlation is not as good in the rest of the hole. The orientations of four brittle shear zones were measured and have a wide range of dips (22°–80°). They show a trend from high dips at shallow depths to low dips deeper in the hole (Fig. F72), which corresponds with the trend observed for the dip of the crystal plastic fabric.
The alteration veins in the gabbros are planar, weakly developed, and relatively thin (1–2 mm). The veins mainly contain chlorite, with subsidiary talc, amphibole, serpentine, and sulfide and oxide veins. Crosscutting relationships in which chlorite-amphibole veins predate talc-chlorite veins are observed in the gabbros in intervals 209-1270B-2R-1, 3.5–6.5 cm; 7R-1, 133–149 cm; and 7R-2, 0–14 cm. These greenschist facies veins crosscut and therefore postdate all ductile and brittle deformation in the gabbros. The altered peridotites from the bottom of Section 209-1270B-6R-1 and the top of 7R-1 show serpentine and talc veining similar to the peridotites in the other holes at this site and are not discussed further here. A detailed discussion of the mineralogy of the alteration veins can be found in "Metamorphic Petrology."
The intensity and orientation of veins were measured using the intensity scale outlined in "Structural Geology" in the "Explanatory Notes" chapter. The intensity of these veins is a measure of their average frequency in a 10-cm piece of core. The vein intensity averages 0.5–0.7 over the length of the core (Fig. F71), indicating that on average there is 1 vein per 14–20 cm in the recovered core. There are regions of increased vein intensity (1 vein per 7–10 cm) in Sections 209-1270B-2R-1, 7R-1, 10R-1, and 10R-2.
The dip distribution and dip variation with depth for the alteration veins are shown in Figure F73. Only 14 dip measurements could be made from all the veins within the core and they vary from 14° to 87°. Figure F74 is a lower hemisphere plot showing the poles to both the metamorphic veins and the brittle shear zones. The data are plotted with the azimuth of the stable remanent magnetization pointing south because the negative inclination of the remanent magnetization suggests reversed polarity for this hole. The reorientation of the veins using paleomagnetic data is discussed in "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" in the "Leg 209 Summary" chapter. After reorientation, the data are loosely clustered.
Holes 1270C and 1270D are located within meters of each other and are structurally similar, but more dunite was recovered in Hole 1270C (Fig. F75). This difference may not be significant, as recovery was sparse in both holes. Two features distinguish these holes from Hole 1270A, drilled 450 m downslope to the west. Harzburgites in Holes 1270C and 1270D, which constitute the large majority of the recovered core, are substantially more deformed than the harzburgites in Hole 1270A (see "Crystal-Plastic Deformation" in "Hole 1270A"). Typical harzburgite from Holes 1270C and 1270D has porphyroclastic texture and average deformation intensities of 1.4 and 1.8, respectively. Observation of textures in 6 thin sections from Hole 1270C and 13 from Hole 1270D correspond well to the visual core descriptions of deformation intensities from the same samples.
There are numerous highly deformed magmatic veins in Holes 1270C and 1270D that crisscross the cores (discussed further in the next section). Strain was partitioned mainly into these gabbroic veins, as can be seen in Figure F76. The gabbroic veins have an average deformation intensity of 2.8, close to protomylonitic textures, whereas the harzburgites have an average deformation intensity of 1.4–1.8. In general, the peridotites are only affected by the deformation in the veins within a few centimeters of vein contacts. In some cases, protogranular orthopyroxene is preserved in the peridotite right at the contact. Where the peridotites form enclaves between favorably oriented veins, the peridotites may be transposed into the foliation plane and show local deformation up to protomylonite intensity (Fig. F77A). Visual inspection of the cores shows that total deformation intensity increases substantially in areas with abundant magmatic veins. This is difficult to quantify but is subtlely present in the downhole total deformation intensity plot as a series of sharp peaks in the most heavily veined regions, rising above the background level of the high-temperature porphyroclastic deformation (Fig. F75).
Unlike the Hole 1270A harzburgites, which were generally protogranular, the Hole 1270C and 1270D harzburgites are generally porphyroclastic. Protogranular textures are abundant in some samples with deformation grades of 1 (weakly porphyroclastic) or less, with orthopyroxene grains and pseudomorphs exhibiting the characteristic smooth curved lobate and commonly interstitial grain boundaries (Fig. F77B). However, in most samples, large grains have recrystallized into smaller subgrains and porphyroclasts (Fig. F77C, F77D). Numerous rounded porphyroclasts and strain-free, coarse subgrains are present, and porphyroclastic textures with broken, kinked, and strained orthopyroxene grains and pseudomorphs are more common (Fig. F77F). The margins of large porphyroclasts are commonly surrounded by broken fragments of pyroxene (e.g., Fig. F77E). Pyroxene neoblasts commonly lack 120° grain boundary angles at triple junctions. In some samples, favorably oriented pyroxenes have undergone stretching to form elongate grains due to sliding on the 001 crystallographic plane (Fig. F78). One oriented sample with a well-defined foliation gives a normal shear sense, though the foliation plane is nearly horizontal (~10°) (Fig. F78).
The early high-temperature, low strain–rate protogranular and porphyroclastic textures predate the intrusion of the gabbroic veins and reflect high-temperature mantle flow. This is evident from crosscutting relationships between the veins and these textures in thin section and hand sample. It was observed that where a porphyroclastic foliation has been measured in the harzburgites, its orientation is somewhat different than that of the foliations within the gabbroic veins (Fig. F79). Locally, porphyroclastic textures with granulation around orthopyroxene, strained grains, and poorly developed strained subgrains can be seen in proximity to the sheared gabbroic veins. Thus, porphyroclastic deformation of peridotites at least locally overlapped in time with deformation of the veins.
Magmatic veins in Holes 1270C and 1270D vary in composition from pyroxenitic to gabbroic to felsic, although it is difficult to reconstitute the original composition because of alteration of their primary phases. The veins are generally <0.5 cm to 1 mm in thickness. They have typically undergone extensive dynamic and/or static metamorphism at amphibolite through greenschist facies and have been crosscut by serpentine veins. These magmatic veins are important for establishing the relative timing of deformation events. Peridotites in Holes 1270C and 1270D are cut by veins that appear to be intruded synkinematically. The veins commonly appear to localize deformation, isolating enclaves of less deformed harzburgites.
An example of strain localization in magmatic veins is present in Section 209-1270C-1R-1 (Pieces 7 and 13). These veins were deformed during high-temperature crystal-plastic deformation. A thin section from Piece 13 (Fig. F80) includes harzburgite enclaves isolated within intrusive gabbroic veins that are transformed into porphyroclastic mylonite zones. Within the mylonites, remnants of the primary mineralogy can be observed. Pyroxene and plagioclase neoblasts and smaller porphyroclasts are segregated into compositional bands. Plagioclase has undergone grain size reduction, dominantly by dynamic recrystallization but also, in part, by cataclasis. Clinopyroxene has been partially to completely replaced by brown amphibole crystals, which are now the predominant porphyroclasts in the mylonitic zones. The harzburgite at the edge of the gabbroic veins is also affected by the mylonitization, with entrainment of recrystallized orthopyroxene and olivine into the high-strain zones. Subgrains and subgrain rotation are common in olivine adjacent to the high-strain zone. Subgrain rotation appears to be the dominant mechanism of recrystallization within olivine. Away from the margins of mylonitized gabbroic zones, the harzburgites have not been dynamically recrystallized, retaining their earlier porphyroclastic deformation fabric and grain size (olivine and orthopyroxene grains >4 mm). At the edge of some mylotinized gabbroic veins, some large clinopyroxene grains have not undergone grain size reduction. These clinopyroxenes are partially replaced by brown amphibole. Their grain size is >4 mm, probably representing the original grain size of the vein material. This suggests that the harzburgite wallrock was at high temperatures during intrusion. One vein contains subhedral apatite and zircon, indicating an evolved magmatic composition, as confirmed by geochemical data (see "Geochemistry"). Amphibolitization is likely to have occurred by synkinematic hydration of the shear zone. Amphibole is not present in the harzburgite enclaves. Other mylonitized gabbroic veins in Hole 1270C include those found in intervals 209-1270C-2R-1 (Piece 14, 88–81 cm) and 3M-1 (Piece 3, 16–21 cm).
All of Hole 1270D except Section 209-1270D-11R-1 contains deformed magmatic veins. An example of a highly veined interval is 3R-1, 53–76 cm (Fig. F81). Crosscutting veins in Hole 1270D help establish the timing of intrusion and deformation events. Figure F82 illustrates an undeformed planar pyroxenite vein in interval 209-1270D-3R-1, 10–16 cm, that cuts the crystal-plastic fabric in the harzburgite. However, the undeformed vein is crosscut by a deformed gabbroic vein with a reverse sense of shear. The crosscutting deformed vein is roughly planar, but the texture inside the vein is porphyroclastic with vein parallel foliation. Figure F83 shows a composite gabbro-pyroxenite vein that merges with a lens of deformed gabbro and pyroxenite. The vein is mylonitized and folded where it merges with the lens. The vein is oriented subparallel to the crystal-plastic foliation defined by elongate pyroxene porphyroclasts in the harzburgite, unlike the undeformed vein in Section 209-1270D-3R-1, which is oriented at a high angle to the crystal-plastic foliation plane in the harzburgite. These relationships suggest an early pyroxenitic intrusive event prior to intrusion of the gabbroic veins.
Several of the ductilely deformed gabbroic veins present in Holes 1270C and 1270D (see "Magmatic Veins" above) are overprinted by semibrittle deformation characterized by fibrous bands of amphibole, chlorite, and/or talc (see "Metamorphic Petrology"). These minerals are present in nearly monomineralic bands (0.5–3 mm wide) parallel to the dominant shear foliation and, in many cases, appear to have grown synkinematically. Brown amphibole is present in shear zones as porphyroclasts with shape-preferred orientation parallel to the foliation. Colorless amphibole is present as fibrous mats or stringers between brown amphibole porphyroclasts. Talc is present as fibrous masses subparallel to foliation in shear zones or as bands of extremely fine grained aggregates with little foliation (Fig. F84). Chlorite is commonly intergrown with colorless amphibole with crystals parallel to the foliation or as random or radial masses.
Textures in greenschist facies shear zones indicate possible strain accommodation via cataclasis and diffusive mass transfer during alteration following the cessation of crystal-plastic deformation. Brown amphibole is boudinaged along cleavage planes, with colorless, fibrous amphibole between fractured grains aligned subparallel to foliation in shear zones (Fig. F85). This strain accommodation process has been referred to as incongruent pressure solution (Brodie and Rutter, 1985) and paracrystalline microboudinage (Misch, 1969, 1970). Section 209-1270D-1R-1 (Pieces 1–5) was highly deformed at greenschist-grade conditions by these processes. This interval of core is composed of amphibole-talc schist cut by phacoidal shear fractures (Fig. F86).
Serpentine surrounding semibrittle shear zones does not show evidence of significant deformation. In most occurrences, serpentine in peridotite near the boundaries of gabbroic dikes displays mesh texture, indicating static alteration (O'Hanley, 1996). There are no crosscutting relations present to determine with certainty if serpentinization of harzburgite preceded or followed semibrittle deformation of the gabbroic veins.
The intensity of cataclastic/semibrittle deformation is highest at shallow levels in Hole 1270D (Fig. F87). Section 209-1270D-1R-1 (Pieces 1–5) is composed dominantly of intensely deformed semibrittle tremolite-chlorite schist. Cataclastic intensity drops rapidly with depth beneath the upper interval of Core 209-1270D-1R and is variable in the lower part of the hole. The intensity of brittle deformation is distinctly low from ~20 to 37 mbsf, where the concentration of magmatic veins is high. Orientation of brittle features, including semibrittle shear zones, minor fault zones, and cross-fiber serpentine foliation, was measured at eight locations in Hole 1270D and three locations in Hole 1270C. Unfortunately, many brittlely deformed pieces from Holes 1270C and 1270D are small fragments that are not oriented. Dips of features in Cores 209-1270D-3R, 4R, 5R, and 6R range 19°–48° (Fig. F88).
In serpentinite enclaves between the mylonitized gabbroic veins, cross-fiber serpentine foliation is commonly oblique to shear foliation by as much as 30°. The formation of cross-fiber serpentine foliation postdates the peak of crystal-plastic deformation in the gabbroic veins. The timing relations between greenschist-grade semibrittle overprint of gabbroic shear zones and formation of cross-fiber serpentine foliation are not known.
The alteration veins in harzburgites and dunites range in morphology from continuous veinlets to sigmoidal or en echelon, discontinuous veinlets to isolated segments. Alteration veins are more numerous than in Hole 1270A and are larger, as wide as 5 mm. The veins are dominantly serpentine with subsidiary oxide, rare talc, and late carbonate/aragonite. There are at least three generations of serpentine veins based on crosscutting relationships: dark magnetite-rich veins, pale green serpentine veins, and late white chrysotile veins. Serpentine veins appear to be localized near the gabbroic veins. Figure F89 shows examples of the last two generations of these serpentine veins. Interval 209-1270C-1R-1, 33–40 cm (Fig. F89A), shows tension cracks filled with white serpentine, crossing a thin gabbroic vein perpendicular to its length. Similar features are commonly associated with the gabbroic veining in Holes 1270C and 1270D. Good examples can be seen in intervals 209-1270C-2R-1, 35–47 cm, and 58–72 cm, 209-1270D-3R-1, 104–120 cm, and 3R-2, 1–29 cm, and throughout Core 4R. These features provide evidence that the pervasive serpentinization postdates the gabbroic vein event (Fig. F89B, F89D). Some en echelon, sigmoidal serpentine veins appear to be transposed and rotated into talc shear zones in Hole 1270D, indicating that some serpentinization occurred before or synchronously with ductile to semibrittle deformation. Figure F89C shows late serpentine veins cutting a semibrittle shear zone in interval 209-1270D-4R-1, 39–40 cm, illustrating that serpentinization also postdates some of the brittle deformation. Other intervals show brittle faulting of the serpentine veins themselves (intervals 209-1270D-10R-1, 0–9 cm, and 4R-1, 38–52 cm). All of these observations together suggest that serpentinization occurred in a number of events synchronously with, and subsequent to, the semibrittle deformation.
Oxyhydroxide veins commonly crosscut serpentine veins in cores from Hole 1270D, commonly using the preexisting serpentine veins as conduits. Oxyhydroxide veins and late carbonate veins that are concentrated in Hole 1270D in Sections 209-1270D-6R-1 and 7R-1 represent late brittle events postdating the magmatic, crystal-plastic, and semibrittle deformation. A detailed discussion of the mineralogy of the alteration veins can be found in "Metamorphic Petrology."
The alteration vein intensity in Hole 1270D decreases with depth from 3.5 near the top of the hole to <2 at the bottom of the hole (Fig. F87). The average vein intensity in the uppermost 20 m is broadly similar to that in Hole 1270C (see the "Supplementary Material" contents list). In the uppermost part of Hole 1270D, the vein intensity is only 2.3. This value may be low because these rocks are very cataclastically deformed and fewer veins are visible. The alteration vein intensity in Hole 1270D varies in the 20- to 30-mbsf interval, corresponding to the region of most intense gabbroic veining. Overall, the metamorphic vein intensity appears to be more strongly correlated with crystal-plastic intensity than cataclastic fabric intensity, suggesting that the majority of the veining events might have occurred before brittle deformation.
The variation of the dip of the alteration veins with depth for Holes 1270C and 1270D is shown in Figure F73. Only a few orientations could be measured in each hole. Dips vary from 15° to 35° in Hole 1270C and from 27° to 58° in Hole 1270D. Figure F90 is a lower hemisphere plot showing the poles to the alteration veins and the brittle shear zones restored with the azimuth of the stable remanent magnetization reoriented to a common reference frame. The reorientation of the veins using paleomagnetic data is discussed in "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" in the "Leg 209 Summary" chapter. The data for Hole 1270C are clustered. However, the poles to the veins in Hole 1270D are more scattered.
Following the same procedure as for Hole 1268A, 8-cm3 cube samples were cut from Site 1270 harzburgites to determine the spinel foliation. One cube each from Holes 1270A and 1270C and three from Hole 1270D were examined. The cubes from Holes 1270A and 1270C as well as two cubes from Hole 1270D show vermicular spinel in at least one of the faces of the cubes. The vermicular spinel grains are 1–3 mm in size and were intergrown with pyroxene, now pseudomorphed by serpentine. Additionally, some smaller rounded spinel grains (~0.1 mm in size) are isolated in the olivine matrix, away from pyroxene pseudomorphs. One cube from Hole 1270D (Sample 209-1270D-4R-1 [Piece 12A, 91–93 cm]) shows two compact, relatively large (4 mm) grains along with small grains. None of the cubes showed any spinel foliation. Outside of the sections of the harzburgite, including mylonitized gabbroic veins, the shape of the vermicular spinel grains (Fig. F91) indicates that they probably have not been deformed since their crystallization.
The four holes drilled at Site 1270 define a 450-m-long east–west traverse across a westerly dipping topographic slope. This slope has previously been interpreted to be an exposed west-dipping detachment fault surface on the east side of the axial valley (Fujiwara et al., 2003). Holes 1270B, 1270C, and 1270D recovered some deformed rock that is interpreted to have formed within a fault zone. The recovered rocks reveal structures that have mean dips of 40°–50°. However, their dip direction cannot be determined from the core alone. In order to determine their present-day dip directions, we oriented the core using paleomagnetic measurements of the mean magnetic azimuth and inclination. This method of orientation requires knowledge of the polarity of the Earth's magnetic field when the rocks were magnetized and assumptions about the orientation of the plausible axes about which the rocks have been rotated. We made the following assumptions:
This leads to the conclusion that the present-day dip direction of the deformation fabrics is approximately east-northeast (for further discussion, see Fig. F28 and accompanying text in the "Leg 209 Summary" chapter). If this result is correct, then the deformation fabrics, including the mylonitic foliation, are not parallel to the topographic slope interpreted to be a fault surface (Fig. F92). We note, however, that this result is nonunique, especially considering the confidence bounds on the mean inclination (±10° for the gabbros and ±13° for the peridotites) and the possibility that more complex tectonic rotations may have taken place.
Figure F93 shows the downhole locations of all of the major fault zones recognized at this site next to recovery plots for each hole. This plot emphasizes the uncertainty in reconstructions of stratigraphic successions at this site and raises the possibility that much of the poor recovery may be due to the presence of fault zones.
Based on the preceding discussion we suggest the following structural evolution of this site: