We present the results of detailed structural observations of the core recovered from two holes (1271A and 1271B) drilled at Site 1271 followed by a discussion of preliminary interpretations of the structural history. Holes 1271A and 1271B are located ~90 m apart and show similar structural features (Fig. F35). They are, therefore, discussed together in this section. Four categories of observations were recorded in spreadsheet format including crystal-plastic deformation, magmatic textures, brittle deformation, and orientation of metamorphic veins. These were supplemented by microstructural observations in 43 thin sections. The relatively high degree of alteration required that many high-temperature features be observed only in pseudomorphs, leading to ambiguity in their interpretation. Details of the structural classification scheme for each feature are given in "Structural Geology" in the "Explanatory Notes" chapter.
Holes 1271A and 1271B record similar events corresponding to an extended history of ductile deformation under varying thermal conditions. Crystal-plastic deformation textures in the 45-m section of largely undeformed dunite recovered from Hole 1271A contrasts with the 104-m section of variably deformed dunite, dunite with intergranular mafic material, harzburgite, orthopyroxene-bearing dunite, and spinel-bearing and orthopyroxene-bearing dunite recovered from Hole 1271B (Fig. F36). The perceived differences between these holes, however, may be due to the poor recovery and lack of crystal-plastic deformation indicators such as pyroxene grains and spinel concentrations in Hole 1271A. Where core containing magmatic veins was recovered in Hole 1271A, deformation patterns appear similar to those of Hole 1271B.
The earliest evidence of crystal-plastic deformation in the Hole 1271A and 1271B cores is found in the peridotites and comprises textures commonly associated with high-temperature, low strain–rate mantle flow. At Site 1271, spinel foliations suggest that this phase of deformation may have been concurrent with melt migration and the formation of dunite with local chromite concentrations. Peridotites recovered from Hole 1271A consist largely of dunite with very minor harzburgite. There is little evidence of an early deformation history in the dunites. However, rapid strain recovery in olivine in monomineralic dunite at high temperature would largely eliminate evidence of any early deformation history. In intervals where pseudomorphed orthopyroxene porphyroclasts are present in the orthopyroxene-bearing dunites and harzburgites, evidence of an early high-temperature, low strain–rate deformation event is preserved.
The total deformation intensity in harzburgite and orthopyroxene dunite is low in both Holes 1271A and 1271B (Hole 1271 average = 0.3 and Hole 1271B average = 0.07) (Fig. F37). Protogranular and protointergranular textures predominate in these lithologies (Fig. F38A). Orthopyroxene dunite and spinel- and orthopyroxene-bearing dunite record higher degrees of crystal-plastic deformation than the harzburgites, even though they contain significantly fewer deformation markers. There is physical evidence in several Hole 1271B orthopyroxene-bearing dunites that porphyroclastic textures developed prior to dunite formation. Elongated orthopyroxene porphyroclasts rimmed by vermicular chrome spinel are present in orthopyroxene dunites. Asymmetry of orthopyroxene porphyroclasts indicates normal sense of shear along a plane dipping 20°–30°. However, if the sample was significantly rotated after high-temperature deformation, it is possible that this was a reverse sense shear zone. Spinel is concentrated at the long ends of the grains in a manner that resembles asymmetric porphyroclast tails (Fig. F38B, F38C). The delicate vermicular spinel rimming the pyroxene is unlikely to have survived the deformation that formed porphyroclastic textures, indicating that spinel crystallized later. Higher degrees of high-temperature deformation in orthopyroxene dunites compared to harzburgites in Holes 1271A and 1271B may be evidence for strain localization in horizons of concentrated melt flow and dunite formation in the shallow mantle (e.g., Kelemen and Dick, 1995).
Well-developed spinel foliations were measured in dunites from both Hole 1271A and 1271B (e.g., Figs. F38D, F39A). These foliations define a girdle relative to paleomagnetic north in a lower hemisphere stereographic projection (Fig. F40). This contrasts with porphyroclastic foliations in Hole 1268 harzburgites (Fig. F77, in the "Site 1268" chapter), which define a local concentration of points in the stereographic projection. Site 1271 dunite spinel foliations may record folding not seen in the enclosing peridotite wallrocks. This is common in podiform dunites in ophiolitic peridotite massifs (e.g., Ramp, 1961) and may reflect a very different rheology in the dunites during their formation.
Gabbro and peridotite samples drilled at Site 1271 experienced distributed crystal-plastic deformation over a wide temperature range, beginning in granulite facies and continuing along a down-temperature path through amphibolite facies. Strain appears to have been partitioned into gabbro intrusions and veins.
Hole 1271A and 1271B dunites and harzburgites host extensive gabbroic intrusions that form magmatic net vein complexes. Because these veins were intensely altered to amphibole, chlorite, and talc, their original composition is uncertain (see "Igneous and Mantle Petrology"). However, several veins preserve sufficient primary mineralogy to be identified as troctolites and troctolitic gabbros (Figs. F39B, F41A). Several of these samples are undeformed or exhibit evidence for weak crystal-plastic deformation, including minor recrystallization along the boundaries of plagioclase (Fig. F41B) and clinopyroxene grains and formation of deformation twins in plagioclase. Other veins are strongly deformed, such as those in interval 209-1271B-11R-1 (Piece 17, 87–93 cm) (Fig. F41C, F41D). This olivine amphibole mylonite was highly deformed at granulite or upper-amphibolite facies conditions during which olivine was dynamically recrystallized into a fine-grained aggregate.
BAG (see "Igneous and Mantle Petrology") underwent varying degrees of crystal-plastic deformation. An example is shown in Figure F39C (interval 209-1271B-14R-1, 25–30 cm). This mylonite contains alternating layers of fine-grained brown amphibole and fine-grained high-relief minerals (presumably prehnite, zoisite, and/or hydrogrossular replacing plagioclase). These layers enclose coarse brown amphibole augen (Fig. F41D) that are commonly weakly deformed, with bent crystals and/or undulatory extinction.
Textures in the BAG suggest that the main phase of crystal-plastic deformation occurred under granulite facies conditions and that brown amphiboles are secondary, replacing pyroxenes. Samples 209-1271B-18R-1, 20–23 cm, and 16R-1, 24–27 cm, contain veins of brown amphibole at right angles to the brown amphibole bands that define the mylonitic foliation (Fig. F41E, F41F). Within these veins, the amphibole fibers are often randomly oriented, suggesting growth under dominantly static conditions. Other samples contain textural evidence for dominantly static overprinting of granulite facies deformation textures by colorless amphibole. Secondary colorless amphibole in Sample 209-1271B-14R-1, 32–35 cm, replaces a stretched pyroxene augen and the enclosing foliated groundmass (Fig. F42). The amphibole in the augen has cleavage at right angles to the foliation, even in the tail of the porphyroclast.
Samples 209-1271B-17R-1, 17–19 cm, and 18R-1, 114–117 cm, display textures that suggest that crystal-plastic deformation continued during amphibolite-grade metamorphism. Amphiboles in Sample 209-1271B-17R-1, 17–19 cm, contain arrays of parallel subgrains, suggesting recrystallization due to crystal-plastic strain (Fig. F43A). This sample also contains arrays of fine-grained, fibrous, secondary amphibole, talc, and chlorite that formed during deformation under greenschist facies conditions (Fig. F43B, F43C). These textures formed concurrent with minor cataclasis and are discussed in greater detail in "Brittle Features" below. The last-formed crystal-plastic deformation textures observed at Site 1271 include minor shearing along late amphibole veins (Fig. F43D).
The dunites and peridotites from Site 1271 lack the porphyroclastic deformation textures found in the peridotite samples from Holes 1268A, 1270D, and 1270C. Despite this, the overall crystal-plastic deformation intensity is relatively high, with 14% of the rocks having deformation intensities >2. Only 6.7% of the rocks from Hole 1268A have intensity >2. Hole 1271A and 1271B gabbros are substantially more deformed than those at Sites 1268 and 1270. Ductile strain at Site 1271 appears to have been preferentially localized into gabbroic veins, as it was at Sites 1268 and 1270. Downhole, the distribution of crystal-plastic deformation is quite variable, indicating that deformation was localized and not penetrative as it commonly is in mantle peridotites with well-developed porphyroclastic fabrics.
The primary magmatic features that are observed at Site 1271 include one impregnation horizon in Hole 1271A and two impregnation horizons in Hole 1271B (Fig. F35). These comprise dunite and orthopyroxene-bearing dunite containing variable proportions of gabbroic material. Interstitial clinopyroxene and plagioclase are interpreted to be cumulate phases crystallized from melt migrating along olivine grain boundaries, forming "impregnated" peridotites. Some sections of Hole 1271B that contain high proportions of gabbroic material (as much as 40% in Unit III; see "Igneous and Mantle Petrology") have the appearance of gabbroic cumulates. The textures in the impregnated sections are not uniform and may record decreasing temperatures during multiple impregnation events.
In Sections 209-1271A-3R-1 and 4R-1, the impregnating material is interstitial to olivine in the dunite, similar to near-equilibrium melt textures observed in laboratory experiments (Fig. F44A–F44C). The distribution of impregnating material on the grain scale, however, appears patchy, which contrasts with relatively uniform melt distributions observed in laboratory experiments (e.g., Faul, 2000). Within other intervals of Section 209-1271A-4R-1 (e.g., Pieces 11–13) a second generation of interstitial gabbroic material with irregular boundaries overprints the earlier formed textures (Fig. F44D).
Hole 1271B has two impregnation horizons (Fig. F35). Textures in the upper horizon suggest multiple impregnation events that followed formation of the dunite. In interval 209-1271B-11R-1, 110–113 cm, an early coarse-grained pyroxenite vein has been partially impregnated by a second crosscutting generation of gabbroic material (Fig. F45). The impregnation textures observed in Cores 209-1271B-11R to 13R do not resemble those of equilibrated partial melts. Olivine grains in the most heavily impregnated intervals are not in contact with one another. Figure F46 shows olivine that is angular in appearance, implying disaggregation of the dunite during liquid infiltration. The boundaries between olivine crystals and impregnating material are relatively sharp, but it is possible that olivine reacted with the liquid (see "Igneous and Mantle Petrology"). Figure F47 (interval 209-1271B-12R1, 42–54 cm) shows that the impregnated horizon is crosscut by a later generation of possibly more evolved gabbro dikes or veins that are very abundant within several intervals. These late magmatic veins (Fig. F39D) appear to localize ductile shear deformation in a similar fashion to the magmatic veins observed in Holes 1270C and 1270D.
Textures in portions of the lower impregnation horizon (Cores 209-1271B-19R and 20R) resemble experimentally produced melt migration textures (similar to those in Cores 209-1271A-3R and 4R). The volume fraction of impregnating material is relatively low, in contrast to the upper horizon. Figure F48A shows a large olivine crystal outlined by the surrounding impregnating material with intragranular, rounded spinel grains. Irregular veinlike impregnation textures are also present in the lower impregnation horizon over interval 209-1271B-19R-1, 10–16.5 cm (Fig. F48B). These observations indicate that this horizon may have been infiltrated several times by increasingly evolved liquids.
Orientations of two magmatic veins from Hole 1271A and two from Hole 1271B were measured using methods described in "Structural Geology" in the "Explanatory Notes" chapter. The measurements are restored to a common orientation by rotation in the core frame of reference such that the azimuth of the stable remnant magnetization points north. 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 reoriented data are plotted on a lower hemisphere stereonet in Figure F40. Poles to foliation planes are clustered relatively tightly at an orientation striking northwest and dipping ~60° southwest in the reference frame we used to plot the data, ~45° from the mean orientation of the spinel foliation measured in Hole 1271B (see "Spinel Fabrics" below).
Spinel foliations were measured in spinel-rich dunite intervals of Site 1271 core. The foliations were measured on six 4- to 5-cm-long quarter cores that were cut from the working half of cores from Hole 1271A. In Hole 1271B, it was possible to measure spinel foliations directly on the archive half of the core. Spinel foliation orientations corrected with paleomagnetic measurements are plotted in a common orientation reference frame in Figure F40. The orientations in Hole 1271A were measured over a 10-m interval beginning at Section 209-1271A-3R-1 (Piece 11, 60 cm) and ending at 4R-1 (Piece 7C, 126 cm). Poles to spinel foliation planes measured in Hole 1271A appear to loosely fall on a great circle with a near-horizontal pole trending northeast in our common reference frame. Spinel foliations in Hole 1271B were measured over an interval from Section 209-1271B-5R-1 (Piece 9, 42 cm) to 20R-1 (Piece 16, 81 cm). Poles to spinel foliation planes measured in Hole 1271B are more clustered but could lie within the great circle defined by spinel foliations in Hole 1271A. This interval covers the drilled depth range from 30 to 102 mbsf.
Foliation in serpentinite defined by anastomosing sets of serpentine and magnetite veins is present in varying degrees through much of the Site 1271 core. The most pronounced example of serpentine foliation is the yellow- and black-striped serpentinized dunite recovered from the upper half of Hole 1271A (e.g., intervals 209-1271A-1R-1, 52–142 cm, and 1R-2, 0–35 cm) (Fig. F49B). The strong subvertical foliation over this interval is defined by a mixture of planar and anastomosing serpentine and magnetite veins that have largely replaced the serpentinized dunite host rock. Serpentine fibers composing veins are commonly oriented perpendicular to vein walls and are not deformed. Veins defining the striped texture are more planar and less anastomosing than veins forming cross-fiber serpentine foliation in serpentinized harzburgite from Sites 1268 and 1270. The amplitude of anastomosing waves in the foliation at these locations appears to be a function of the size and concentration of pyroxene porphyroclasts and the degree to which veins are deflected around them. Veins at Site 1270 are likely more planar due to the limited number of pyroxene porphyroclasts in the dunite. This fabric appears to be a variant on ribbon texture serpentine as described by O'Hanley (1996) and is produced by dilational fracturing during serpentinization rather than shear deformation. It is unknown if this strong foliation mirrors a previous crystal-plastic deformation fabric.
Cross-fiber serpentine foliation in Hole 1271B appears in intervals of nonimpregnated harzburgite and dunite. Where cross-fiber serpentine foliation appears in dunite, foliation and veins are dominantly planar, as was observed in the upper portion of Hole 1271A (e.g., interval 209-1271B-17R-1, 127–135 cm). Where serpentine foliation appears in harzburgite, foliation and veins tend to form anastomosing waves similar to those observed in Hole 1268A (e.g., intervals 209-1270B-10R-1, 93–127 cm, and 13R-1, 69–74 cm). Serpentine fibers are most commonly aligned perpendicular to vein walls, indicating formation of the foliation by dilatant fracturing rather than shear deformation.
Shear zones composed of schistose amphibole and chlorite schist overprint some gabbroic veins in Holes 1271A and 1271B. Deformation occurred during alteration of the primary components of gabbroic intrusions to pale brown and colorless amphibole (see "Metamorphic Petrology"). Textures of amphibolite range from randomly oriented aggregates of bladed crystals ranging 0.1–2 mm in size (e.g., Sample 209-1271B-12R-1, 126–131 cm) to strongly foliated schistose arrays of fine-grained (<0.01 mm) amphibole (e.g., Sample 12R-1, 126–131 cm). Random aggregates of bladed crystals are interpreted to have grown under dominantly static conditions. Foliated schistose amphibole is interpreted to have grown during localized shear deformation. Highly localized strain during amphibolite alteration is suggested by variations between static and highly schistose textures on the centimeter scale. This contrasts with observations of gabbroic veins in Holes 1270C and 1270D, in which amphibolites are deformed in nearly all occurrences.
Sample 209-1271B-11R-1, 88–91 cm, contains deformation textures and mineral assemblages that suggest semibrittle deformation at amphibolite and greenschist facies conditions that overprints crystal-plastic deformation. This sample contains foliation-parallel bands of finely recrystallized olivine mylonite, intermediate grain–size amphibole, fine-grained schistose amphibole, and schistose serpentine. Intermediate grain–size amphibole comprises bladed to highly elongate pale brown and slightly pleochroic crystals (up to 0.5 mm) aligned subparallel to the shear foliation (Fig. F50). Fine-grained colorless amphibole ranges 0.03–0.06 mm in size and is elongate parallel to the shear foliation (Fig. F50). Serpentine bands comprise schistose fibrous serpentine with a long dimension ranging 0.02–0.05 mm and a short dimension ranging 0.01–0.03 mm. Serpentine schist appears to have formed concurrently with fine-grained amphibole during deformation. Intermediate grain–size amphibole may have formed concurrently with or following crystal-plastic deformation of olivine. Fine-grained colorless amphibole and schistose serpentine likely formed during lower-amphibolite to greenschist facies deformation following crystal-plastic deformation. This indicates a prolonged strain history under conditions of progressively decreasing temperature.
Two intervals of core in Hole 1271B contain semibrittle shear zones composed of schistose serpentine, chlorite, clay minerals, and possibly hydrogrossular (Figs. F51, F52). These include several pieces in the upper portion of the hole (intervals 209-1271B-1R-1, 1–16 cm, and 3R-1, 18–22 cm) and within Core 11R near strongly deformed amphibole schist (discussed above) (intervals 11R-1, 14–17 cm, and 53–57 cm). Strain was likely accommodated by diffusive mass transfer and cataclasis in the presence of aqueous fluid at temperatures <300°C based on textures indicating syndeformation, subgreenschist-grade alteration. Serpentine-chlorite schist zones may represent a low-temperature deformation overprint on the amphibole schist described above, due to continuous deformation at decreasing temperature.
The lower part of Hole 1271B experienced intense brittle deformation late in the deformation history of Site 1271. Several intervals of partially cohesive fault gouge are present in Cores 209-1271B-19R and 20R (intervals 19R-1, 3–4 cm, 49–50 cm, 72–73 cm, 65–73 cm, and 96–97 cm, and 20R-1, 34–35 cm). Gouges are matrix-supported breccias with gray clay- and/or serpentine-rich matrices. Clasts are subrounded to angular, altered serpentinite ranging 0.1–0.8 cm in size. Breccias also contain rare larger clasts of serpentinite (5 cm diameter). Intervals of core bordering gouge zones are variably fractured, including conjugate sets of planar to slightly anastomosing magnetite-filled shear fractures with 1 to <0.2 cm offset. Slickenfibers visible on fracture surfaces suggest dip slip to oblique slip but do not indicate an unambiguous shear sense. Intervals of core bordering gouge zones also appear to have a higher degree of clay alteration of serpentinite than other intervals in Hole 1271B. It is likely that the gouge intervals represent splays of a major brittle fault system. It is possible that recovery from gouge zones was low and that only a small percentage of the true thickness of the fault zone was sampled by drilling. It is also likely that characteristics of the gouge were modified by the drilling process, and thus deformation observed in the gouge may partly be a consequence of drilling.
Other brittle features recovered at Site 1271 include cohesive carbonate-matrix breccias in intervals 209-1271B-3R-1, 19–22 cm, and 48–55 cm. These breccias are composed of angular to subrounded clasts of serpentinite and rare chrome spinel crystals supported by a partially recrystallized, micritic carbonate matrix. Breccias are interpreted to be of tectonic rather than sedimentary origin because of fibrous bands and a weak foliation in the carbonate matrix (Fig. F53).
Minor fractures and small fault zones are present throughout Holes 1271A and 1271B, but in most cases do not appear to represent a significant degree of shear deformation. Minor faults are commonly associated with serpentine and carbonate veins. Slickenfibers, where visible on small faults, typically indicate dip slip but no unambiguous shear sense.
Figures F54 and F55 show the downhole distribution of brittle deformation intensity at Site 1271. The intensity of brittle deformation was measured using the intensity scale outlined in "Structural Geology" in the "Explanatory Notes" chapter. Several brittlely and semibrittlely deformed samples were recovered in Hole 1271A, but no highly deformed samples or concentrations of brittle deformation were recognized in this hole (Fig. F54).
Hole 1271B contains three zones of concentrated brittle deformation (Fig. F55). The first zone is composed of brittlely deformed rocks in Sections 209-1271B-1R-1 and 3R-1 that include amphibole schist, serpentine schist, and carbonate-matrix fault breccia. The second zone of brittle deformation is in Core 209-1271B-11R, where amphibole schist and serpentine schist shear zones overprint high-temperature ductile deformation associated with gabbroic intrusion. The last brittle shear zone in Hole 1271B is in Cores 209-1271B-18R, 19R, and 20R, where concentrated, late, pure brittle deformation forms noncohesive fault gouge and highly fractured rocks.
The BAGs contain a few quartz and quartz-amphibole-rutile veins. The dunites/harzburgites show extensive veining throughout both Holes 1271A and 1271B. Veins in peridotite include both continuous, planar veins and sigmoidal, discontinuous veinlets and range in from a few millimeters to 20 cm in length and from 0.1 mm to 0.5 cm in width. There are several generations of veins, but the veins are dominantly serpentine with subsidiary talc-tremolite, carbonate, and oxide veins. The relative timing of all of these vein sets is difficult to determine. However, a simplified order is as follows. The first generation is an almost ubiquitous, dense, anastomosing network of black serpentine + magnetite veins (Fig. F49B–F49E). This network is overprinted by later, less pervasive generations of green and white serpentine (picrolite) veins, composite black (exterior) and green (interior) serpentine/magnetite banded veins, white chrysotile veins, talc-tremolite veins, and late carbonate/aragonite and oxide veins. Figure F49 shows various examples of the different vein sets and their crosscutting relationships in Sections 209-1271A-1R-1 and 1R-2 and 209-1271B7R-1 and 2R-1.
Brittle faults offset both the anastomosing network of black serpentine + magnetite veins and the later composite black and green serpentine veins in intervals 209-1271B-7R-1, 24–32 cm, and 2R-1, 4–10 cm (Fig. F49D, F49E). Section 209-1271B-2R-1 (Piece 2) shows later green and white serpentine and carbonate veins that apparently followed a fault, and therefore postdate an earlier brittle event. This relationship is consistent with a complex series of serpentine veining events occurring synchronously with brittle faulting. Many of the larger green and white serpentine veins have slickenfibers on exposed surfaces on the edges of the pieces. These suggest shear deformation associated with the formation of these later serpentine veins. The late carbonate veins probably formed at shallow levels, associated with seawater alteration. A more detailed discussion of the mineralogy of the alteration veins can be found in "Metamorphic Petrology".
The intensity of veins was 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 total intensity of alteration veining with depth for Holes 1271A and 1271B is shown in Figures F54 and F55. The alteration vein intensity in both holes is high, with most of the dunites/harzburgites being pervasively net veined with vein intensities of 3–5. The lower alteration vein intensities of 1–2 in Hole 1271B correspond to occurrences of the BAG, for example, in Sections 209-1271B-3R-1 and 5R-1 (20–30 mbsf), to the troctolites and gabbros of Unit III from Sections 11R-1 to 18R-1 (56–89 mbsf), and to the fault rocks recovered in Section 1R-1 (0–12 mbsf).
The orientation of veins and brittle fractures was measured using the procedures outlined in "Structural Geology" in the "Explanatory Notes" chapter. Twelve brittle fractures and shear zones were measured in Hole 1271A, and nine were measured in Hole 1271B. Dips of brittle features range 0°–85° in Hole 1271A and 16°–85° in Hole 1271B. The variation of the dip of the alteration veins with depth for Holes 1271A and 1271B is shown in Figure F56. Only 12 orientations could be measured in each hole. Dips vary from 26° to 90° in Hole 1271A and from 38° to 85° in Hole 1271B. Figure F57 is a lower hemisphere plot showing the poles to the late alteration veins (green serpentine and composite black and green serpentine veins) and the brittle fractures and shear zones. Orientations were restored by rotation around a vertical axis such that the azimuth of the stable remnant magnetization for the piece measured points to the north. 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 plot shows that the veins have approximately random orientations. These orientations may simply be a consequence of volume expansion during metamorphism. Brittle fractures also have a random orientation, suggesting a complex deformation history or that the fractures formed in response to near-hydrostatic stresses created by volume expansion during metamorphism.
Core recovered from Site 1271 records a deformation history for the peridotite and intrusive gabbroic material that follows a general down-temperature path from high-temperature crystal-plastic deformation, possibly in the presence of melt, to low-temperature brittle deformation. Spinel foliations observed in dunitic intervals may record a supersolidus deformation event that occurred during porous flow of melt and formation of dunite horizons. At least two generations of magmatic features cut Site 1271 dunites and harzburgites. The first magmatic event appears to have occurred under dominantly static conditions and produced intergranular textures that resemble those observed in laboratory experiments on olivine-melt assemblages. These are inferred to result from crystallization of plagioclase and pyroxene from melt migration by porous flow along olivine grain boundaries. A later generation of magmatic veining and impregnation appears to have occurred under static conditions over some core intervals and within granulite-grade ductile shear zones over other intervals. Ductile strain is localized in peridotite only where it includes gabbroic material.
Late stages of crystal-plastic deformation occurred under hydrous, amphibolite facies conditions, during which clinopyroxene and olivine were altered to pale brown amphibole. Amphibolite alteration occurred under static conditions in some samples, where it produced random crystal arrays that partially to completely obscure granulite facies deformation fabrics. Samples in which strain was localized during late stages of crystal-plastic deformation contain schistose arrays of parallel pale brown amphibole interlayered with recrystallized olivine and/or plagioclase. In these samples, late crystal-plastic deformation occurred concurrently with semibrittle deformation in amphibole mats where strain was accommodated by cataclasis and diffusive mass transfer.
Semibrittle shear zones composed of intensely foliated colorless amphibole, serpentine, chlorite, clays, and possibly hyrogrossular cut crystal-plastic shear zones. Strain was localized into intervals of intense alteration of gabbro veins and/or host peridotite at greenschist or subgreenschist facies conditions. Core from the base of Hole 1271B contains a series of partially cohesive fault gouges. These likely formed on brittle faults late in the deformation history of Site 1271. Crosscutting relations to constrain the timing of these faults in relation to semibrittle shear zones are not present in the recovered core, but it is reasonable to assume that brittle faulting was the last deformation event.