STRUCTURAL GEOLOGY

Core from Hole 1272A provides information about the extent of crystal-plastic deformation and melt interaction, as well as the nature, extent, and geometry of brittle to brittle-ductile structures in serpentinized harzburgite encountered from 56 to 131 mbsf. The upper 56 m of the hole may represent talus including blocks as large as 5 m in maximum dimension composed of various lithologies including basalt, diabase, diorite, gabbro, harzburgite, dunite, and sedimentary breccias (see "Site 1272" in "Site Summaries" in the "Leg 209 Summary" chapter for more details).

The tectonic setting of Hole 1272A is within the inside corner, proximal to the ridge transform intersection. Hole 1272A provides a chance to more fully understand tectonic processes and fault geometries in such a region, possibly influenced by ridge and transform tectonics. FMS images and other downhole logging information, which assisted in the interpretation or definition of major brittle structures and their geometry, are presented in this context (also see "Downhole Measurements").

Sections 209-1272A-1R-1, 2R-1, 4R-1, 5R-1, 8R-1, 9R-1, and 10R-1 contain mixtures of pieces of ultramafic rock and basalts, diabase, gabbro, and/or diorite. Sections 209-1272A-3R-1, 6R-1, and 7R-1 lack ultramafic rock but do contain diabasic to basaltic rocks. In addition, Sections 209-1272A-4R-1 and 5R-1 contain carbonate-cemented breccias mixed with igneous rocks and mantle peridotites. The mafic samples contained in Cores 209-1272A-1R to 10R are characterized by rust-colored weathering stains on the surface and within joints and cracks (Fig. F32). Many of the ultramafic rocks show a weathered bright orange color typical of seafloor weathering rinds but atypical of plutonic and ultramafic rocks drilled below seafloor to basement penetration depths of 10–20 m. Many of the igneous rocks in the upper section are undeformed and require no structural analysis. Local piece-by-piece structure of the ultramafic rocks is difficult to assess in the upper cored section because of their small piece size, unoriented nature, and weathered and altered characteristics. Attempts to obtain crystal-plastic foliation directions from a few oriented pieces in the upper 56 m resulted in aberrant data points when compared with the main cluster in the harzburgite basement (see "Crystal-Plastic Fabrics and Deformation Intensity" in "Harzburgite Section [56–131 mbsf]"). The polymictic nature of the assemblage in each section of core, the fact that mantle and plutonic rocks are mixed with basaltic and sedimentary rocks, the anomalous structures, and the weathering style suggest that there may not be coherent basement in the upper 56 m of the cored interval.

Recovery of lithified breccias also suggests that a talus or rubble zone was encountered in the upper 56 m. A coarse-grained, cohesive, nonfoliated breccia was encountered in Section 209-1272A-5R-1 (Pieces 7, 9, and 10) (Fig. F33). Angular to subangular clasts of serpentinite and minor basalt are supported by a fine-grained orange carbonate-rich matrix. Some of the clasts are angular lithic fragments that have undergone fracturing, rotation, and frictional sliding with respect to a larger parent clast. These protocataclasite fragments are included in a finer-grained matrix of similar ultramafic composition. The clasts were deformed during a semibrittle process and were lithified prior to the formation of the sedimentary breccia. The enclosing carbonate matrix is relatively undeformed. The breccias may have formed in or near a fault zone based on the presence of protocataclasite and the abundance of ultramafic clasts. The matrix of the breccia is cut by a few late brittle carbonate veins, and each clast is surrounded by a coating of carbonate vein infill, caused by matrix shrinkage during tectonic activity, dewatering, and final lithification.

The paleomagnetic data are equivocal, as discussed in "Paleomagnetism" and in "Site 1272" in "Site Summaries" in the "Leg 209 Summary" chapter (Fig. F34). Below 56 mbsf, the paleomagnetic results indicate stable magnetic inclinations and declinations. Above 56 mbsf the paleomagnetic results show large variations downhole, with as much as a 90° variation in inclination orientation within a single section. Ten meters of quartz-olivine gabbro gives internally consistent results (see "Paleomagnetism" for details) that are parallel to the remanent magnetization in the harzburgites between 56 and 131 mbsf. In the context of the hypothesis that a tectonic breccia was cored in the upper 56 m of Hole 1272A, the similarity between the magnetic inclinations recorded in Cores 209-1272A-3R to 4R and the harzburgites below 56 mbsf either must be a coincidence or be explained by the quartz-olivine gabbro being intrusive dikes in the talus pile.

Hole 1272A was logged with FMS imaging starting at 46 mbsf (see "Downhole Measurements"). The vertical resolution of the FMS images is ~5 mm, allowing conductive and nonconductive features such as fractures and faults, magmatic and alteration veins, clasts within matrix, and oxide-rich layers to be recognized.

Above 56 mbsf, which we interpret as the top of basement, FMS images show typical "clast-in-matrix" geometries along the borehole wall. The clasts appear as bright spots (high resistivity) on the image with the matrix material showing darker colors (more conductive and probably water saturated). Figure F35 illustrates an example of this image pattern and shows the recovery over Section 209-1272A-11R-1, which consisted of only 36 cm of rock over the 5-m cored interval. The low recovery in this section included a polymict assemblage of basalt, diabase, and harzburgite with some clasts showing the effects of weathering and none as wide as a cut cylindrical core.

Below Core 209-1272A-10R, recovery was limited in two sections (11R-1 and 11R-2) to pebble-sized or smaller clasts and matrix composed almost exclusively of green serpentinized peridotite, pieces of cross-fibered serpentinite from veins, and fine silt-sized particles of serpentine. These two sections are interpreted to represent drill cuttings of peridotite. The first semicoherent section of ultramafic rocks was encountered just below 56 mbsf in Section 209-1272A-12R-1. The ultramafic materials in this section, however, are unusual. Section 209-1272A-12R-1 (Piece 1) recovered a basalt followed by nine subsequent pieces of highly altered harzburgite. The harzburgite, although appearing intact, had the consistency of mud (Fig. F36). When dried, this mud consolidated somewhat but became brittle and commonly friable. In addition, these unusually altered harzburgites contain iowaite, a brucitelike, Cl-bearing mineral with a layered structure, in what may be the first reported occurrence from the Mid-Atlantic Ridge (see "Metamorphic Petrology"). Serpentine minerals may form muds that are unusually weak (e.g., Phipps and Ballotti, 1992).

Below this depth, the recovery typically consisted of protogranular harzburgite and was distinctly different from the polymict assemblages above. Small pieces of basalt and gabbro were sampled at the top or base of the succeeding cored sections, but these are interpreted as fragments of fallen material from the upper part of the hole. Section 209-1272A-15R-1, which had low recovery of small pieces, did recover two allochthonous weathered basalt and diabases (Pieces 4 and 5) in the middle of the core, but all other pieces are likely to have been derived from the harzburgite section.

The cored interval of harzburgite extends from Sections 209-1272A-12R-1 to 27R-1 (~56–131 mbsf). Samples were generally completely altered to secondary phases in the upper part of the section based on thin section examination and visual inspection of the core (see "Metamorphic Petrology"). Textural classification could not be completed for the highly altered and weathered Section 209-1272A-12R-1 because pieces showed extreme plastic behavior at low temperatures and deformed on handling. Below this section, however, visual and thin section inspection allowed estimation of textures and fabrics in the samples. Also, below Section 209-1272A-21R-1 some fresh primary minerals were preserved. This is reflected in an increase in bulk density of the intervals at the base of the cored interval (see "Porosity, Density, and Seismic Velocity" in "Physical Properties").

Lithologic Variation and Contacts Measured

Primary lithologic variation in the section was minor, apart from subtle variations in pyroxene modal abundance, four samples of oxide gabbro in Section 209-1272A-19R-1 (Pieces 6–9), and a single dunite band with an upper and lower boundary of harzburgite in Section 23R-1 (Piece 2) (Fig. F37). The dunite band is bounded sharply at 11 cm by an upper harzburgite contact and at 50 cm by a lower harzburgite contact. The layer is inclined at 50° in the cut face of the core and is ~25 cm thick. The orientation of the upper and lower contacts of the dunite with adjacent harzburgite were measured, with poles plotted in Figure F38. The paleomagnetic-corrected orientations of the dunite/harzburgite compositional boundaries are close to the main cluster of crystal-plastic foliation planes, which are defined by the preferred dimensional orientation of spinel and pyroxene in the adjacent harzburgite (see below). This may be consistent with transposition of compositional layering into the foliation plane during mantle deformation.

Crystal-Plastic Fabrics and Deformation Intensity

Crystal-plastic deformation in the harzburgite section of Hole 1272A is limited. The section consists largely of protogranular harzburgite with very little recrystallization (Fig. F39). Measurements of crystal-plastic deformation intensity of a few samples from the upper 56 m of Hole 1272A are plotted, but this may not be an intact section.

Within the harzburgite, the mesoscopic and microscopic crystal shape fabrics are poorly developed (Fig. F40). A total of 15 small intervals in the core displayed a weak but measurable crystal-plastic foliation. In many cases, the shape fabric could only be identified with confidence on the cut face of the core. If the cut face was not oriented in the dip azimuth direction of the foliation plane, likely for the weakest foliations, an accurate measurement was difficult to make. Shallow foliations also tend to be the weakest foliations. In Figure F38 we show all of the data collected, but we are most confident in the steepest dips measured in the cut face of the core, which do seem to cluster. Shallower dips measured in the cut face plot farther from the main cluster and may represent an apparent dip. Some of the scatter in the plot of poles to foliations in Figure F38 may be attributed to these measurement difficulties. Nonetheless, there is a concentration of poles plunging 40° along an azimuth of 255° in the reference frame that we used to reorient the data. Note that the paleomagnetic correction applied to each piece provides an internally consistent reference frame for the core.

Harzburgite textures in Hole 1272A are almost exclusively protogranular with only a few kink bands, strained grains, and occasional coarse neoblasts found in bastites. The distribution of crystal-plastic fabrics and low intensities (Fig. F41) suggests that the Hole 1272A section is one of the least deformed of those drilled during Leg 209, with no mylonitic or strongly porphyroclastic peridotites recovered. Most sections appear undeformed, preserving equidimensional orthopyroxene grains on the cut core face. In thin sections of samples from Hole 1272A, orthopyroxene is coarse, typically 3–7 mm, with grains as large as 13 mm. Olivine grains vary from 3–8 to 15 mm in diameter. Subgrains and kink bands are present in orthopyroxene, but much of the olivine is either strain free or contains slight wandering extinction and rare kink banding. There is no significant recrystallization of olivine, so Hole 1272A harzburgites lack the bimodal grain sizes typical of porphyroclastic textures.

Spinel grain shapes were generally preserved even in altered rocks at the top of the hole. Spinel shapes in harzburgite from 18 thin sections from Cores 209-1272A-13R to 27R are vermicular, indicating that these rocks have not experienced significant strain since the formation of the spinel. The exceptions are Sample 209-1272A-17R-1, 56–58 cm, where vermicular spinels have a common elongation direction but no pyroxene foliation is observable, and Sample 23R-1, 7–11 cm, where at the harzburgite/dunite contact the spinel shapes grade from slightly vermicular to anhedral-rounded in thin section. The spinel grains in this thin section are slightly elongate and show a clear foliation parallel to the pyroxene foliation in the adjacent harzburgite. At greater depth in the same piece, the spinel shapes still range from anhedral-euhedral to vermicular, but no pyroxene or spinel foliation is observable (e.g., Sample 209-1272A-23R-1, 62–64 cm).

Shown in Figure F42 are four examples of the protogranular textures in the Hole 1272A harzburgites. These closely resemble those at other sites, with the exception that the Hole 1272A harzburgites generally have coarser pyroxene and olivine grain size, >1 cm in places. Some of the orthopyroxene bastites have a marked interstitial habit (Fig. F42A), as is characteristic of the peridotites drilled at Sites 1268, 1270, and 1271. Deeply embayed orthopyroxene grains with smooth curved grain boundaries are more common than interstitial grains (Fig. F42B, F42C) as are more equant and granular forms (Fig. F42D).

Four small fragments of clinopyroxene-rich oxide gabbro or gabbronorite (Section 209-1272A-19R-1 [Pieces 6–9]) were recovered. All appear little deformed in hand sample. Two samples yielded thin sections. One appears undeformed, whereas the other underwent a weak to moderate crystal-plastic deformation (grade = ~1), with locally extensive recrystallization of primary clinopyroxene (Fig. F42E, F42F). Plagioclase, although abundant, was replaced by large patches of chlorite. Clinopyroxene neoblasts with unstrained exsolution lamellae are enclosed by iron-titanium oxides. This indicates that recrystallization of clinopyroxene neoblasts took place at high temperature prior to exsolution. Oxides may have accommodated later deformation, shielding the clinopyroxenes from additional strain.

Brittle to Brittle-Ductile Features

Cross-Fiber Anastomosing Serpentine Foliation

Foliation in serpentinized harzburgite, defined by anastomosing sets of serpentine and magnetite veins, is present in varying degrees through much of the Site 1272 core. Within harzburgite, serpentine foliation and veins tend to form anastomosing waves similar to those observed in Holes 1268A and 1270B (e.g., intervals 209-1270B-10R-1, 93–127 cm, and 13R-1, 69–74 cm). The amplitude of anastomosing waves in the foliation appears to be partially controlled by the size of pyroxene porphyroclasts and the degree to which veins are deflected around porphyroclasts. Foliation has strong, high-amplitude anastomosing waves where it is deflected around large pyroxene grains and is near planar where there is a small degree of deflection or small pyroxene grains. Where serpentine foliation is present in harzburgite with >30% fresh olivine, the foliation is defined by very fine serpentine and magnetite veins that are concentrated into olivine-rich bands. These veins are very similar in character to ribbon texture serpentinite as defined by O'Hanley (1996). Serpentine fibers are most commonly aligned perpendicular to vein walls, indicating formation of the foliation by dilatant fracturing rather than shear deformation.

Alteration Veins

The diabase and quartz-olivine gabbros in the upper 56 m of Hole 1272A contain very few alteration veins. Conversely, the highly weathered oxidized peridotites in the uppermost 30 m of the hole from Sections 209-1272A-1R-1, 2R-2, 4R-1, and 5R-1 contain pervasive networks of black serpentine magnetite veins cut by less common green serpentine veins and by later carbonate veins. The planar carbonate veins contain aragonite and oxides and are up to 0.5 cm wide and $12 cm long. There are two crosscutting generations of carbonate veins, and they fill extensional fractures.

The peridotites from below Section 209-1272A-12R-1 (60.43 mbsf) contain four generations of serpentine veins and no carbonate veins. The first generation is composed of narrow (<1 mm wide), planar, black to dark green magnetite-serpentine veins that extend at least across the diameter of the core (Fig. F43A, F43B, F43D). These veins show complex crosscutting relationships. The second generation of veins consists of larger (as wide as 3–4 mm and as long as >30 cm) branching white- and green-banded serpentine veins (Fig. F43B, F43D). These veins are less common (<1 vein per section) and have steeper dips than the first generation of veins. The wallrock surrounding these veins commonly has dark 1- to 3-mm-wide alteration halos, and the veins themselves are always cut by small (<1 cm long) orthogonal tension cracks filled by white chrysotile (Fig. F43B, F43D). The tension cracks suggest that these serpentine veins were affected by volume expansion of the surrounding peridotite and, hence, that they formed before the last phase of pervasive serpentinization. These veins may originally have been magmatic veins that have been completely altered and serpentinized.

The third generation is represented by late talc-serpentine veins, examples of which can be seen in Sections 209-1272A-26R-1 and 27R-2. These veinlets are small (0.3 mm wide and 2–3 cm long) and relatively rare. The last generation of veins consists of small (<1 cm long) sigmoidal, en echelon chrysotile veinlets. These veinlets are ubiquitous throughout the core and commonly define a foliation (Fig. F43C) that generally dips ~45°, more rarely 80°–90° in the cut face of the core. These veinlets are similar to those that fill the tension cracks in the second generation of serpentine veins and probably formed at the same time during the last serpentinization event. A more detailed discussion of the mineralogy of the alteration veins can be found in "Vein Description" 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 total intensity of alteration veining with depth for Hole 1272A is shown in Figure F44. Alteration vein intensity is generally low (2.5) and significantly lower than that seen in the peridotites at Sites 1268, 1270, and Site 1271. The moderately high alteration vein intensities in the upper 56 m of Hole 1272A reflect the presence of the highly altered oxidized peridotites. Below 56 mbsf (Section 209-1272A-12R-1), the background vein intensity is 0.5–1 and is approximately constant with increasing depth. There are four local peaks in the intensity of veining corresponding to the location of faults and fault gouges in Cores 209-1272A-13R (62.5 mbsf), 19R (86 mbsf), and 26R (123 mbsf), and the highly veined, 25-cm-thick dunite sand in Section 23R-1 (108 mbsf).

A total of 102 vein orientations were measured in the core. Dips vary from 15° to 90° (average dip = 52°) (Fig. F45). Figure F46 shows lower hemisphere plots showing the poles to the late metamorphic veins (black to dark green magnetite serpentine and white- and green-banded serpentine veins) below 56 mbsf, restored with the declination of the stable remnant magnetization pointing 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 orientation data are clustered, and the data suggest that a population of the veins dip to the east at ~50° in the reference frame that we used to orient the data.

Serpentine Schist

Several small pieces of strongly foliated, white schistose serpentinite were recovered from the lower one-third of Hole 1272A (Samples 209-1272A-23R-1 [Piece 1, 0–3 cm], 21R-1 [Piece 6, 67–68 cm], 20R-1 [Piece 10, 59–60 cm], and 15R-1 [Pieces 10 and 11, 49–55 cm]). These samples are composed of fibrous white serpentine that fills 0.2- to 2-cm-wide veins that cut altered serpentinite. The serpentine fibers in these veins are aligned either syntaxially, indicating dilational opening; oblique to vein walls, indicating oblique shear opening; or as schistose mats parallel to vein walls. The veins that are filled by schistose serpentine contain elongate clasts of altered serpentinite with up to 10:1 length:width ratio and/or bound phacoidal shear polyhedra of altered serpentinite. A narrow (0.2 cm wide) fault zone down the centerline of a vein filled by syntaxial fibers in Sample 209-1272B-23R-1 (Piece 1, 0–3 cm) appears to have accommodated shear offset. The presence of extensive schistose and fault offset textures indicates that these samples represent a significant degree of shear strain accommodation. Although only a limited number of small pieces of white schistose serpentine were recovered from Hole 1272A, these pieces could be small portions of major fault systems that are not well represented in the recovered core. Borehole imaging by FMS logging of the Hole 1272A, presented in "Core-Log Correlations, FMS Imaging, and Orientation of Fault Zones" below, aided in interpreting the significance of these small pieces. The results suggest that poor recovery correlates with highly fractured zones.

Fault Gouge

Several intervals of brittle, partially cohesive fault gouge (Fig. F47) were recovered from Hole 1272A (intervals 209-1272A-18R-1 [Piece 16, 122–123 cm, and Piece 18, 133–148 cm], 18R-2 [Pieces 1 and 2, 0–18 cm], 19R-1 [Piece 2, 4–6 cm, Piece 5, 20–21 cm, and Piece 23, 137–150 cm], 19R-2 [Pieces 1 and 2, 0–18 cm], and 25R-2 [Pieces 11 and 12, 107–125 cm]). These gouge samples are generally similar in character, with variations in the percentage of lithic clasts and overall clast size. They are semicohesive fault breccias consisting of serpentine and/or clay-rich matrices with matrix-supported subrounded to angular serpentinite clasts, ranging 0.03–5 cm in size in most samples, and altered pyroxene crystals. Intervals bounding most gouge zones are cut by a higher percentage of shear fractures than in bulk of the core recovered from Hole 1272A. These intensely fractured zones are revealed in a downhole plot of shear fracture intensity (Fig. F44). These gouges formed in late-occurring faults. Crosscutting relations are not present to determine the relative timing between the fault gouges and serpentine schists, but it is likely that fault gouges postdate the serpentine schists. Like the altered peridotite in Section 209-1272A-12R-1, the gouges were soft, plastic, and semiconsolidated (Fig. F47) when first recovered and stiffened later as they dried. If the in situ state of hydration is similar to that when the gouges initially arrived on board, the gouges may have had a weak rheology. Alternatively, drilling activity may have enhanced their water content and plasticity.

Core-Log Correlations, FMS Imaging, and Orientation of Fault Zones

The FMS images previously discussed are oriented so that both strike and dip can be calculated utilizing sine curve fits for planar features. Because FMS images allow orientation of the structural features observed in the cores, it is possible to relate core and/or borehole features to the tectonic environment (see "Downhole Measurements" for further details). Our preliminary attempt to interpret correlations between core descriptions and FMS logs focused on fault gouges and serpentine schist where there was evidence of brittle to brittle-ductile deformation. We attempted to identify these features at similar depths in the borehole image and in the core and then to orient these structural features. In a few cases, after working with correlations of core and borehole observations and developing reliable working criteria, we extended the interpretation to regions in the borehole where similar features were observed but where there was a lack of core recovery or direct evidence of a fault zone. Unlike paleomagnetic data, in which the orientation of core in a geographical reference frame is dependent on the tectonic rotation model, FMS images with core pieces or sections provide a model-independent orientation for structural features. The FMS images can ultimately help to provide constraints on the paleomagnetic rotation model to be used.

We correlated gouges recovered in Sections 209-1272A-19R-1 and 19R-2 with the FMS log. Figure F48 shows the FMS image and segments of the two core sections containing gouges and conductive oxide gabbros. Like fractures, gouges are expected to be saturated with water at these shallow levels in the crust and thus highly conductive relative to the surrounding wallrock. The curated top of Section 209-1272A-19R-1 is located at a depth of 89.3 mbsf, but taking into account the partial recovery, the depth to the top of Section 19R-2 should be between 90.8 (unexpanded depth) and ~93 mbsf (expanded depth). The FMS image shows a conductive interval between 89 and 91.4 mbsf. After preliminary processing, the depth of particular features in the FMS image is accurate to ~1 m. Thus, the FMS image shows a strongly conductive zone, which may correspond to the level of the first significant gouge zone in the core, together with evidence for small resistive clasts between 89.6 and 90.5 mbsf. The curated top depth of the thicker gouge zone in the section is at 89.47 mbsf. A sine curve fit to the interval in the FMS image interpreted to represent the top of the thick gouge zone has a dip of 58° to the northeast striking N60°W. Thus, the fault is a high-angle fault dipping downslope in a direction oblique to the rift valley and the fracture zone. Its orientation is consistent with fault geometries at ridge/transform intersection inside corners, where the rift valley walls tend to curve away from the rift valley and toward the active transform.

A second gouge zone is present at the base of Section 209-1272A-19R-1 and the top of Section 19R-2 (Fig. F48). It may be correlated with a conductive zone with resistive clasts visible in the FMS image. It should be noted that the fault gouges sampled in the core were matrix-supported breccias with clasts, consistent with appearance of the conductive zones in the FMS image. Lastly, conductive oxide gabbros might be the cause of the highly conductive zone in the FMS image between the two fault gouges.

A small sample of serpentine schist in Section 209-1272A-20R-1 (Piece 8) at 94.89 mbsf was correlated with the borehole FMS image (Fig. F49). A 1.7-m conductive zone with resistive clasts is recorded on the FMS image between 94.3 and ~96 mbsf. The top of the zone has a strike of N18°W and a dip of 49° toward the northeast. Thus, we interpret this feature as another fault dipping toward the rift valley and the fracture zone.

In Figure F50, a zone of conductive fault breccia was identified in the FMS image, based on the appearance of abundant resistive clasts within a conductive matrix. The zone begins at ~100.6 mbsf and appears to be ~1 m thick. The orientation of the top of the zone strikes N3.9°E and dips 26.8° to the southeast. The strike of this structure is close to the strike of the Mid-Atlantic rift valley in this region.

Analysis of the FMS images yielded 10 preliminary picks of fault gouge zone orientations that give a mean strike of N24°W. The orientations of these fault zones are plotted in Figure F79. Strikes of the 10 gouge zones vary from north–south, perpendicular to the spreading direction, to more northwesterly, with a mean strike of N24°W. Thus, most are oblique to the rift axis. However, gouge zones may locally have irregular boundaries and may not always be planar. In addition, the attitude of the bottom of a gouge zone does not always match the top, as is observed in subaerial gouge zones. However, it is clear that faults dip both toward and away from the rift valley. The considerable number of gouge zones imaged in the borehole indicates that there are dense networks of faults that intersect each other in the subsurface. We have only presented some gouge zones and there are many more fault zones imaged in the FMS log that are not included in Figure F79. We selected those that provided the best orientation data. In addition, there are many other planar fractures, which will require a significant postcruise effort to delineate and orient in order to provide a more complete set of data on fault geometries at a ridge/transform intersection.

Orientation of Brittle Shear Fractures and Serpentine Foliations

Poles to shear fractures and poles to serpentine foliations rotated into a common frame of reference using the paleomagnetic data are presented in Figure F51. Fractures are generally shallow dipping, typically shallower than fault zones imaged by FMS. Most dip northeast in the reference frame that was used to reorient the data. A comparison of the brittle shear fracture intensity plot and the depth of gouge zones in the core or gouge imaged by FMS (Fig. F52) indicates a possible correspondence between the position of the gouge zones and the intensity of shear fractures. This indicates that fractures are probably related to faults corresponding to the gouge zones, perhaps antithetic or pinnate to the major fault zones. Serpentine foliations tend to have similar orientations but exhibit more of a dilational component than the shear fractures.

The earliest deformation of the serpentinized harzburgites in Hole 1272A was a mild crystal-plastic deformation, with most harzburgites retaining coarse protogranular textures that likely formed under low deviatoric stresses at high temperatures. We observed no porphyroclastic or mylonitic textures in the harzburgites. Microscopic evidence for dynamic recrystallization was found only in one small oxide gabbro interval, suggesting that high-stress, moderate-temperature crystal-plastic deformation may have been localized in scarce mafic plutonic intervals in Hole 1272A. Qualitative strain estimates usually provided by elongation of pyroxene grains in the harzburgite were rarely possible because of the lack of deformation, resulting in the lowest crystal-plastic deformation intensity estimated for any peridotite cored during Leg 209.

An anastomosing serpentine foliation forms the dominant low-temperature fabric in the serpentinized peridotites. Serpentine fibers are generally orthogonal to the margins of the anastomosing vein sets, but fibers subparallel and oblique to vein walls indicate some shear component in their development. However, these and subsequent vein sets record dominantly dilational deformation. Vein intensity may correlate with the location of brittle fault zones. Some serpentine schists are present, and we believe that they are also associated with fault zones. High densities of brittle shear fractures are present near serpentine schists and gouge zones observed in the core or inferred from FMS imaging.

Gouge and fault zones represent the latest brittle deformation event recorded in the harzburgites. They are present in recovered core and inferred from FMS imaging. They intersect the borehole every 10 m or less and are as thick as 1.5 m. The gouges had the plasticity of typical pelagic mud when they were first recovered and may have a weak rheology in the subsurface. The frequency of these gouge zones in the borehole was unexpected but may help to explain areas of poor recovery in the harzburgites. More importantly, the low strength of the gouge zones may have important implications for serpentinite-hosted fault zones, as serpentine muds are five orders of magnitude weaker than massive serpentinites, with ultimate strengths three to four orders of magnitude lower than salt (Phipps and Ballotti, 1992). This property may permit aseismic slip on major fault zones within serpentinized peridotite and allow mass wasting within the high-relief rift mountains. The densities of serpentine muds are also significantly lower than the densities of massive serpentinites, suggesting that muds may rise diapirically, given their relative buoyancy and low-temperature plasticity.

A last tectonic event may be revealed by polymict lithologies above the harzburgite basement, which include lithified breccias with peridotite clasts that originated along brittle fault zones undergoing cataclasis. The clasts included protocataclasites of harzburgite.