We present the results of a detailed structural study of the core recovered from Site 1275 followed by a discussion of preliminary interpretations of the structural history. Holes 1275A and 1275C recovered minimal quantities of core so it was not possible to make significant structural observations and therefore these holes are not discussed here. Holes 1275B and 1275D show similar lithologic and structural features and are discussed together. Four categories of observations were recorded including magmatic foliation, crystal-plastic deformation, brittle deformation, and alteration vein intensity (see the "Supplementary Material" contents list). These were supplemented by microstructural observations from 120 thin sections. Details of the structural classification scheme for each feature are given in "Structural Geology" in the "Explanatory Notes" chapter.
Holes 1275B and 1275D show evidence of limited crystal-plastic deformation at irregular intervals throughout both cores. The crystal-plastic deformation intensity does not exceed grade 2 (well foliated) in either hole (Fig. F54). Core from Hole 1275B (average crystal-plastic foliation grade = 0.02) (Fig. F55) has less crystal-plastic deformation than core from Hole 1275D (average grade = 0.07). No measurable crystal-plastic fabric orientations were found in Hole 1275B, but in Hole 1275D 14 foliations were measured in shear zones typically no more than a centimeter wide. Dips of foliations average 50° ± 18° with no systematic variation with depth.
Most of the 120 thin sections from this site show the effects of a pervasive, very weak, high-temperature, crystal-plastic deformation. Plagioclase displays undulose extinction, deformation twins, and subgrain formation. Some olivine displays undulose extinction. A few gabbros have local patches of plagioclase neoblasts on the margins of larger grains, particularly where the larger grains impinge on each other. Samples with minor localized dynamic recrystallization textures and common bent plagioclase grains were graded 0.5 on the crystal-plastic deformation intensity scale.
A small minority of the gabbros from Hole 1275D include more pronounced dynamic recrystallization near boundaries of large plagioclase grains and completely recrystallized small plagioclase grains. This deformation may have enhanced the magmatic foliation present in several samples. These samples are graded 1 or 2 on the crystal-plastic intensity scale depending on the intensity of foliation produced. Plagioclase neoblasts are polygonal, coarse grained, have near 120° grain boundaries, and show nearly complete strain recovery. These are features of very high temperature recrystallization or crystal-plastic deformation in the presence of melt. Several of the most highly deformed samples contain plagioclase porphyroclasts with asymmetric tails of polygonal neoblasts.
Two different styles of brittle deformation are recognized. The first is gabbro and diabase cataclasites where the dominant strain accommodation process was fracturing and cataclastic flow. The second is semibrittlely deformed schistose rocks where strain was accommodated by schistose growth of chlorite and other alteration minerals.
Holes 1275B and 1275D contain several intervals of cataclastically deformed gabbro and diabase. Cataclastic zones rich in green amphibole form branching bands across many brittlely deformed gabbros (Fig. F56). Cataclasites typically contain magmatic fabrics, alteration veins, and minor crystal-plastic fabrics that are overprinted by brittle deformation including fracturing of plagioclase and amphibole. Cataclasites and protocataclasites typically comprise clasts of gabbro, microgabbro, diabase, and amphibole schist within a matrix of fractured plagioclase and amphibole (Fig. F57). The clasts are subangular to subrounded and generally range from 1 to 10 mm in size. The matrix has grain sizes ranging from 0.01 to 0.5 mm. Amphibole within the matrix is generally finer grained than fractured plagioclase and sometimes has a slightly schistose fabric indicating growth during deformation. Discrete fractures cutting plagioclase porphyroclasts are typically filled with green amphibole.
Cataclastic deformation in gabbros appears to have occurred after minor crystal-plastic deformation and/or magmatic deformation. Fractured plagioclase in most cataclastically deformed gabbros contains patchy subgrain boundaries and in some cases minor dynamic recrystallization textures. In some grains, brittle fractures appear to be localized on subgrain boundaries. The presence of pleochroic green amphibole that appears to have partially crystallized in cataclasites suggests that brittle deformation occurred within lower amphibolite facies to upper greenschist facies
Several cataclasite samples contain subrounded to subangular diabase clasts that are either undeformed or weakly deformed (e.g., Sample 209-1275B-6R-1[Piece 2, 6–16 cm]) (Figs. F56, F58). Several of these shear zones also contain elongate diabase clasts parallel to the shear foliation (Fig. F59). In some locations, diabase clasts in cataclastic shear zones contain possible preserved remnants of chilled zones parallel to current margins of the clasts (Figs. F56, F58) indicating that diabase may have intruded the cataclasite during active deformation. It is also possible that these features are alteration rinds and not related to magmatic processes.
In Sample 209-1275B-3R-1, 57–60 cm, gabbro and diabase are in fault contact, separated by a shear zone. The shear zone consists of a ~0.5-cm-wide zone of chlorite schist that borders the diabase across a sharp, planar contact. Plagioclase microlites near the margin of the diabase are aligned parallel to the margin but are randomly oriented away from it. The chlorite schist grades into gabbro on the opposite shear zone margin across a zone of cataclastically deformed gabbro. These textures suggest a complex structural relationship between gabbro and diabase that included both magmatic intrusion and faulting.
Several samples from Holes 1275B and 1275D contain schistose shear zones composed of fine- to medium-grained chlorite, amphibole, and talc. These are discrete zones cutting gabbro and diabase (e.g., Sample 209-1275D-1R-1, 9–14 cm) (Fig. F59) commonly within alteration veins that follow the path of gabbroic veins in troctolite. Fibrous chlorite, talc, and amphibole form an anastomosing foliation that is cut by anastomosing shear fractures (Fig. F60). This type of deformation appears to postdate and partially overprint the cataclastic deformation in Sample 209-1257D-1R-1, 9–14 cm, and may represent a later, lower-temperature phase of strain localization. Amphibole cataclasites are more common in core than semibrittle schists in Holes 1275B and 1275D. This may be because fewer are present or because recovery of semibrittle schists was relatively poor.
Brittle deformation is concentrated in the upper portions of Holes 1275B and 1275D. The upper 15 m of Hole 1275B is composed dominantly of undeformed diabase. Gabbro beneath the diabase has been strongly deformed by cataclasis that formed the amphibole and plagioclase matrix cataclasites and lesser semibrittle talc-chlorite schists (Fig. F61). Brittle deformation decreases below 30 mbsf in the troctolites and lower gabbros. The upper 50 m of Hole 1275D shows extensive brittle deformation. This portion of the hole is composed dominantly of troctolite with lesser gabbro. Nearly all gabbros in the upper 55 m of Hole 1275D contain significant amphibole and plagioclase matrix cataclasites and/or talc-chlorite-amphibole schists. The troctolite is strongly fractured and veined. Serpentine veins in troctolite appear to have accommodated some shear deformation and have schistose textures. Some show offsets. The significant decrease in brittle deformation below ~55 mbsf corresponds to the base of the primary troctolite unit. This depth may represent the thickness of a detachment fault system. It is possible that the troctolite is weaker than gabbro, and strain from a detachment fault occurs over a wider interval where troctolite is present. This might explain why strong brittle deformation textures are present at greater depths in Hole 1275D than in Hole 1275B. Peaks in brittle deformation intensity in the lower sections of Holes 1275B and 1275D represent minor localized fracturing, faulting, and weak brecciation. These include narrow (<0.3 cm) normal, reversed, and oblique slip faults with <4 cm total offset found within each hole.
Orientations of brittle deformation features were measured in the core reference frame. Faults, brittle shear zones, and fractures have a wide range of dips, but the magnitude of the dips show no systematic trends with depth (Fig. F62).
The distribution and intensity of metamorphic veins in Hole 1275D are strongly correlated with the lithology. The troctolites are more intensely veined than any of the other lithologies (Figs. F61, F63). They are traversed by altered magmatic veins and contain several generations of alteration veins. The earliest generation of veins is white and green picrolite veins, which either form discrete veins (Fig. F64B) or fill sigmoidal expansion cracks orthogonal to the magmatic veins (Fig. F64A). Later carbonate-clay-Fe oxyhydroxide veins (Fig. F64A, F64D) cut these veins and are often located within or parallel to the early magmatic veins (Fig. F64B, F64C). The magmatic veins themselves occasionally contain green amphibole-chlorite veins. Rare chlorite veins are also present within the troctolites.
The gabbros show three distinct generations of veins. The first generation is a set of relatively rare planar black amphibole veins. These are cut by planar green amphibole and green amphibole-chlorite veins that are uniformly developed throughout the gabbros (Fig. F65C). These veins are up to 2 mm thick, are also planar, and occur every 10–20 cm in the core. Later carbonate-clay-Fe oxyhydroxide veins (Fig. F65A, F65B) crosscut these green veins. This last generation of veins are larger (up to 1 cm wide) brittle fractures that acted as conduits for low-temperature seawater circulation.
The diabases contain sparse green amphibole-chlorite veins. The cataclastic rocks are cut by amphibole and serpentine veins, at least some of which are deformed, indicating that these veins were pre- and syndeformational (e.g., interval 209-1275B-1R-2, 20–26 cm).
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 variation in total intensity of alteration veining with depth for Holes 1275B and 1275D is shown in Figures F61 and F63. Metamorphic vein intensity is generally low in the gabbros and diabases (vein intensity = ~1) but higher in the troctolites (vein intensity = 2–3). Hole 1275D has a higher intensity of veining in the troctolites than Hole 1275B. This is consistent with the higher degree of alteration in Hole 1275D compared to Hole 1275B, as discussed in "Metamorphic Petrology." The correlation of increased cataclastic deformation in the uppermost 55 m of Hole 1275D with increased metamorphic veining and alteration suggests a causal relationship. The high-frequency variation in the lower parts of the holes, shown by the alteration vein intensity curves (Figs. F61, F63), is a function of lithologic variation and grain size. For example, diabases and finer-grained gabbros are less veined than coarser-grained gabbros.
Figure F66 shows that there is no systematic variation in dip with either depth or vein type for both Holes 1275B and 1275D. This figure illustrates the correlation of vein types with lithology and, hence, with depth. Serpentine veins are only found in the troctolite units. Carbonate-clay veins are also predominant in the troctolites. Conversely, amphibole veins are largely absent from the troctolite units, except where they are associated with the magmatic veins cutting the troctolites. The veins in all the lithologies are undeformed except in cataclasites. They are interpreted to be a consequence of brittle fracturing as a result of cooling and unroofing of the footwall of a detachment fault and are thus interpreted to be syndeformational.
The gabbros from Holes 1275B and 1275D have a distinctive magmatic layering/foliation defined by variations in plagioclase and pyroxene crystal size and their shape-preferred orientation. The boundaries between the layers can be sharp or diffuse, vary from planar to irregular, and define layers that are between 10 and 30 cm thick. Figures F67 and F68 show poles to the layer boundaries measured in Holes 1275B and 1275D. The measurements are corrected to a common orientation using paleomagnetic azimuth measurements (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 dip of these layers is similar in both holes and varies from 0° to 50°, with a mean dip of 17°–22°. Figures F67 and F68 also show the orientations of granophyric veins/segregations. Their dips range from horizontal to vertical; however, they do define a girdle that trends approximately northeast–southwest in the reference frame we used to orient the data. In both holes, there is a population of granophyric veins that have orientations approximately perpendicular to the mean orientation of the magmatic layering. The orientation of fourteen diabase/gabbro contacts is also shown in these figures. Eleven are subhorizontal and only three are steep, suggesting that the recovered diabase might be sills rather than dikes, unless there has been considerable tectonic rotation. Figure F68 also shows that the magmatic veins in the troctolite are randomly oriented.
The first deformation event took place under hypersolidus to possibly high-temperature subsolidus conditions and produced a weak crystal-plastic deformation. Samples in which limited polygonal neoblasts and porphyroclasts show very minor internal stain and complete recovery probably underwent hypersolidus deformation. Plagioclase grains that show moderate internal strain textures including undulose extinction, deformation twins, and subgrain boundaries were likely deformed at temperatures near the solidus. Lack of significant grain size reduction by dynamic recrystallization in all samples suggests that only a small degree of shear strain was accommodated by crystal-plastic flow.
Samples from the upper portions of both holes are highly deformed by brittle and semibrittle processes. Formation of amphibole and plagioclase matrix cataclasites began at the end of crystal-plastic deformation and continued during down-temperature deformation into upper greenschist facies conditions. The last deformation event formed greenschist-grade talc-chlorite-amphibole schists found in both holes. This extremely localized deformation occurred together with alteration and veining.
Site 1275 was drilled on the surface of a large domal massif core complex or "megamullion" that has been hypothesized to have formed by low-angle normal faulting (Escartin and Cannat, 1999; Fujiwara et al., 2003; McLeod et al., 2002; Escartin et al., 2003). In this hypothesis, the upper surface of the massif is an exposed normal fault. Brittle deformation localized in the upper portion of Holes 1275B and 1275D would therefore represent the thickness of a detachment fault system at this site. If this is the case, strain began to localize on the fault at lower amphibolite or upper greenschist facies conditions over a thickness of 30–50 m.