STRUCTURAL GEOLOGY

Introduction

Seafloor spreading at slow-spreading centers such as the SWIR (0.8 cm/yr) is accommodated both by adding cross-sectional area to the crust via new magmatic material and by mechanical extension and deformation of the crust. During periods of high magma supply, spreading is probably dominantly accommodated by magmatic processes, whereas when magma supplies are low, spreading is accommodated dominantly by tectonic processes. The gabbroic massif exposed on the Atlantis Bank and drilled at Hole 1105A during Leg 179 formed along the ridge segment at the northern RTI of the Atlantis II Fracture Zone. The ridge segment is likely to have evolved in a magma-starved setting near the termination of the ridge axis at the transform. A large part of the extension was likely to have been accommodated by mechanical extension. Thus, as gabbroic rocks solidified and cooled in the subaxial environment, fault and ductile shear zones were continuously forming in the new lithosphere as it was created. Subsequently, the gabbroic massif was unroofed and exposed to the seafloor, probably along a low-angle detachment fault at the northern RTI (Dick et al., 1991c) in a manner similar to the one proposed in the tectonic models for the MARK region south of the Kane Fracture Zone (Karson and Dick, 1983; Karson et al., 1987; Karson, 1990). After its initial exposure to the seafloor, the massif was transported to its present position northward along the transverse ridge directly to the east of the Atlantis II Transform during the last 11.5 m.y. (Dick et al., 1991b; Dick, Natland, Miller, et al., 1999). It has likely suffered further deformation by transform-related processes after being transported from the ridge axial environment in which it formed. Oceanic gabbroic rocks, such as those recovered from Hole 1105A, crystallized, compacted, solidified, and cooled over a range of temperatures and in two tectonic regimes (ridge and transform) leading to a variety of both ductile and brittle structures.

Hole 1105A penetrated to a depth of 158.00 m, and the cored interval measured 143 m starting at 15.0 mbsf. Core recovery included 118.43 m of gabbroic rocks for a total recovery of 82.82%. This high recovery provides a rather complete coverage of the rock types and pseudostratigraphy of the gabbroic section cored, as well as a detailed view of the structures in the gabbroic section. The rocks recovered in Hole 1105A record a wide variety of structural styles ranging from brittle to ductile. Each structure or structural interval in the core was logged in the structural geology log (see the "Appendix" contents list).

The discussion of structural observations in Hole 1105A proceeds from high- to low-temperature structures because the age progression in the deformational events represents a down-temperature retrograde tectonic history. This history is predicted by evolution of a plutonic assemblage originating in an RTI tectonic setting. Temperatures ranged from magmatic to seafloor temperatures during the massif's evolution and transport to its present position along the Atlantis II Transform Fault, which is about halfway between the two SWIR segments terminating the transform. At the highest temperature end of the spectrum, distinguishing between magmatic and dynamically metamorphosed rocks can be difficult (e.g., Thayer, 1963). In oceanic gabbros, the distinctions between igneous cumulate textures and dynamically recrystallized metamorphic texture are often blurred. This is because the gabbroic rocks solidify and cool in a deforming plate boundary zone, and most display at least some crystal-plastic microstructure. Deformation can initiate in a hypersolidus state before the last remaining liquid has solidified or just after complete solidification, when the rock is near magmatic temperatures (e.g., Cannat, 1991; Cannat et al., 1991). Textures can be annealed considerably and resemble igneous xenomorphic granular textures if cessation of deformation is at high subsolidus temperatures. There is commonly little distinction between the xenomorphic granular texture of many adcumulates and a granulitic gneissic texture when grains are coarse, equant, or polygonal, and these rocks lack strong tectonite foliations, even though the gneisses may have completely recrystallized under dynamic conditions. Another factor that compounds the confusion between magmatic and tectonic textures is that strain is typically localized in small intervals of the gabbro section. Transitions from completely undeformed magmatic textures and tectonite fabrics can occur rapidly at an interval contact or grade more subtly into the shear zone through an entire interval. These tectonic boundaries are not at any regular intervals or spacing (see Fig. F26; "Igneous and Metamorphic Petrology and Geochemistry"). Lastly, recrystallization and metamorphism in oceanic gabbros are generally retrograde in nature, but if dynamic recrystallization takes place at high temperature near the solidus, the metamorphic mineralogy may be nearly identical to that of the original igneous rock. Generally, however, thin sections provide sufficient microstructural information to distinguish between magmatic and dynamically metamorphosed tectonite textures.

Magmatic Textures and Fabrics

The majority of gabbroic rocks from Hole 1105A contain fine- to coarse-grained igneous or cumulate textures with random orientation of those primary igneous minerals. They are generally little affected or very weakly affected by crystal-plastic deformation (e.g., see Fig. F53). Some rare pegmatitic gabbros with random orientation textures are also present in the core. These random orientation textures are present even when there are inequant, elongate, or tabular igneous mineral habits, which are prerequisite for forming igneous lamination. Other rocks of gabbroic composition cored in Hole 1105A, however, show a distinct preferred dimensional orientation of those mineral grains that possess shape anisotropy. These preferred orientations can result from (1) crystal-plastic deformation or (2) the orientation of primary minerals above the solidus temperature by self-nucleation phenomenon, crystal accumulation, compaction, or reorientation by magmatic flow. Crystal-plastic and igneous fabrics are distinguished by the shapes and characteristics of the constituent minerals of the gabbroic rocks. The term igneous lamination is reserved for those rocks that show little to no evidence of crystal-plastic strain including distortion, stretching, or flattening of mineral grains, or evidence for grain-size reduction by dynamic recrystallization. In addition, definitive igneous textures or structures must be observed. For example, magmatic twins in plagioclase are readily distinguishable from their deformational tapered counterparts. Likewise, magmatic growth twinning {100} of augite is common in igneous samples and is a criteria for igneous origins because these twins will not form during dynamic recrystallization of pyroxene. Similarly, subhedral and euhedral shapes of plagioclase are common where intergrown subophitically or included as chadocrysts within pyroxene oikocrysts. Plagioclase also tends to display zoned rims that can be readily recognized to be of igneous origin.

The igneous origins of certain laminations or linear-preferred dimensional orientation are clear from the nature of the minerals displaying the fabric. In thin section, the characteristics of some rocks with magmatic laminations or lineations verify that shape-preferred orientation because of igneous processes exists locally in the section. These rocks are devoid or nearly devoid of recrystallization or other crystal-plastic deformation microstructure. For example, a fine-grained gabbroic rock displayed in thin section, Sample 179-1105A-12R-2, 50-53 cm (Figs. F54, F55), shows well-developed igneous lamination defined by the preferred dimensional orientation of tabular plagioclase and elongate pyroxene. There is some deformation twinning in plagioclase locally in the thin section, but most twinning is magmatic with straight margins of the twin plane and sharp blunt ends to each twin. Plagioclase displays distinct albite growth twins in most grains, and subhedral crystal shapes are common (e.g., Fig. F55). Plagioclase also shows core to rim magmatic zoning and sometimes complex zoning (Fig. F54) demonstrating its igneous origin. Pyroxene also commonly shows {100} magmatic twinning parallel to the elongation direction (Fig. F54) and preservation of euhedral to subhedral shapes of included plagioclase (Fig. F56). Overall, the Hole 1105A core lacked significant evidence of magmatically aligned fabrics. Weak foliations of apparent ambiguous origin, which were noted in the VCDs, on further inspection under the microscope, generally consisted of crystal-plastic deformation fabrics. Fine-grained microgabbros tend to show more examples of magmatically aligned fabric when compared to their coarse-grained counterparts.

Igneous Banding, Layers, and Interval Contacts

In general, igneous layers were observed throughout the core. Of 141 intervals defined in the core, ~30% of these display sharp, well-defined contacts. Igneous contacts consist of gradual or abrupt changes in modal mineralogy, modal proportions, grain size, and/or texture, or some combination of these (see "Igneous and Metamorphic Petrology and Geochemistry"). Other interval boundaries are structural in nature and mark boundaries between undeformed and ductilely deformed rocks. These boundaries can be abrupt with sharp shear-zone contacts between intervals of deformed rocks and intervals of undeformed rocks, or there may be a gradation with progressively increasing intensity of deformation and foliation as the high-strain portion of the shear zone is approached. For example, interval 179-1105A-29R-1, 95-118 cm (displayed in Fig. F57 ), shows a sharp, sheared contact between a coarse-grained gabbro above and a pegmatitic gabbro below. The contact is marked by a narrow ductile shear zone. On a smaller scale, igneous banding is present and consists of thin layers or bands that differ in lithology, modal proportions, grain size, and/or texture. All of these are less than several centimeters in thickness. Opaque-oxide mineral banding (discontinuous planar zones of oxide concentration) is the most common manifestation of banding and seems to reflect, in some cases, incursions of melts and their crystallization products into pre-existing cumulate gabbros with significant residual porosity. The orientation of oxide bands can locally lack consistency in the section, and the banding typically has irregular margins (e.g., see Fig. F58). It should be noted that, except for igneous contacts with felsic to leucocratic gabbro veins (see below), there are no igneous bands, layer, or interval contacts in which an intrusive contact with the adjacent gabbroic rock is clearly demonstrated. Lastly, the Formation MicroScanner (FMS) logs (see "Downhole Logging") supports observations made in the core that the gabbroic section is strongly layered throughout and on a scale that ranges from meters to centimeters. FMS data also show, as in the core, that igneous layers have a significant range of dips (0-75), similar to dipping layers noted in ophiolite complexes (e.g., Casey and Karson, 1981).

Transitional Magmatic-Tectonite Textures

Most of the samples with igneous textures in Hole 1105A core are not completely free of at least some minor crystal-plastic deformation. Most commonly, deformation is restricted to mechanical plagioclase twins that are typically tapered and pinch out, undulose extinction of plagioclase, and some development of subgrain boundaries. Olivine is also mildly strained and commonly kinked. Clinopyroxene tends to be undeformed in mildly strained rocks and retains obvious igneous textures. Plagioclase included within pyroxene is generally shielded from deformation typical of nonincluded plagioclase. The transition from igneous textures to crystal-plastic textures and fabrics is most commonly marked with the first appearance of thin bands (two to three grains thick) of plagioclase neoblasts along grain boundaries or subgrain boundaries (Fig. F59), abundant mechanical twinning of plagioclase, kinking, subgrain formation and rotation in olivine and plagioclase, and the appearance of serrated grain boundaries. These rocks, however, preserve much of their original igneous texture.

High-Temperature Crystal-Plastic Fabrics

Equigranular Gneissic Textures

Penetrative grain-size reduction in which all phases have undergone dynamic recrystallization to produce a finer grained, nearly equigranular gneissic mosaic texture with less than 0%-10% porphyroclasts is present locally in the section cored in Hole 1105A. This type of texture may be the highest temperature, lowest stress crystal-plastic textural type present because it implies a lack of significant ductility contrast between clinopyroxene, olivine, and plagioclase. The coarseness of the dynamically recrystallized plagioclase, olivine, and clinopyroxene implies higher temperature and lower stress conditions than porphyroclastic metagabbros with finer grain sizes. Each of the phases in the rock is dynamically recrystallized. The grain size of the recrystallization (average 0.75 mm) is not as fine as porphyroclastic rocks, and localization of grain-size reduction has not been as severe. The photomicrograph in Figure F60 of thin section 179-1055A-30R-2, 109-113 cm, shows an example of even dispersion of grain sizes in a dynamically recrystallized, fine-grained, gneissic gabbroic rock. The sample shows highly strained plagioclase (complex undulose extinction and subgrain structure) and moderately well-developed shape-preferred orientation. The fabric is defined by the preferred dimensional orientation of both plagioclase and clinopyroxene. Elongate aggregates of clinopyroxene and plagioclase also help to define the foliation.

Porphyroclastic Textures and Fabrics

Nonpenetrative grain-size reduction of minerals by dynamic recrystallization results in a strongly bimodal grain-size distribution. Grain-size reduction is present along anastomosing high-strain zones that outline porphyroclastic remnants of the original igneous rock. Porphyroclastic fabrics are defined by shape-preferred dimensional orientation of flattened and/or elongated pyroxene, olivine, or plagioclase porphyroclasts (e.g, see Fig. F61). Textures are characterized by bimodal grain sizes with finer grained polygonal neoblasts that often show less strain than porphyroclasts of the same phase. Grain-size reduction varies across individual shear zones and becomes more pronounced toward the center of the shear zone where the strain is the highest. Some highly localized deformation bands in the porphyroclastic rocks show extreme grain-size reduction (mylonitic bands). Porphyroclastic textures are present locally in much of the core, but appear to be more prevalent in the lower portion of the core (from below 90 mbsf). Examples of porphyroclastic textures are present in Sections 179-1105A-5R-1, 7R-1, 7R-2, 8R-1, 12R-1, 16R-1, 16R-2, 22R-1, 23R-1, 25R-1, 25R-3, 30R-1, and 30R-2. Where the minerals defining the fabric have elongated anhedral shapes with evidence of stretching and the development of asymmetric recrystallized tails, the structure is interpreted to be a crystal-plastic fabric. In core from Hole 1105A, porphyroclastic phases show different extents of crystal-plastic behavior. For example, plagioclase tends to be the most highly strained phase and one of the first to recrystallize to fine-grained neoblasts; olivine subsequently recrystallizes and finally, pyroxene. Pyroxene, which is rheologically the strongest of the three primary minerals under ductile conditions, may show little sign of deformation microstructure even though plagioclase and olivine are internally strained and/or dynamically recrystallized to fine-grained neoblasts. Largely strain-free pyroxene porphyroclasts commonly preserve chadocrysts or inclusions of euhedral to subhedral plagioclase, whereas the nonincluded plagioclase in the same rock has completely recrystallized under dynamic conditions (Figs. F62, F63). Pyroxene tends not to show significant strain when included in a matrix of opaque oxides, presumably because the oxides take up most of the strain as the weakest of all phases. In higher strain facies where clino-pyroxene exceeds 50% of the rock, it does show higher strain and dynamic recrystallization, commonly in the form of mantles or asymmetric tails around the porphyroclasts. Most porphyroclastic clinopyroxenes are partially replaced by postkinematic patches of brown amphibole, indicating that some of the dynamic recrystallization ceased prior to alteration temperature ranging from 700-900C (Spear, 1981).

Because of the localized nature of the porphyroclastic textures, it is likely that most have formed along discrete ductile high-strain zones. A number of examples exist in the core in which porphyroclastic textures grade to porphyroclastic mylonite and to ultramylonite toward the center of a discrete shear zone where strains are high. Decimeter-thick mylonite zones with high oxide-mineral content were also observed in the core from ~53 and 71 mbsf (e.g., see Fig. F64). Other mylonites were observed on thin-section scales as discrete high-strain zones within porphyroclastic rocks and were generally marked by very fine-grained neoblast sizes (<0.03-0.01 mm). In general all of the porphyroclast phases are internally strained, bent, or kinked and may contain subgrains or intracrystalline fractures in the mylonites. Pyroxene, the strongest phase, is also generally affected by internal strain. In some shear zones, the crystal-plastic fabric shows a gradually changing orientation of the foliation away from the center of the shear zone, defining classic Ramsay-Ghram ductile shear-zone geometries. Shear criteria all yield normal displacement along inclined shear zones where measurements were possible.

High-Stress Crystal Plastic Deformation and Strain Localization

As temperature decreased, ductile shear became more localized into narrower shear zones, some of which are on thin-section scales. Grain-size reduction is more pronounced, and coarse porphyroclastic textures give way to porphyroclastic mylonites, mylonites, and ultramylonites along narrow bands as the grain-size reduction becomes more pronounced (see Fig. F64). Porphyroclasts are more strongly deformed, and dynamic recrystallization is accompanied by the formation of micro-cracks (e.g., see Fig. F65). Plagioclase porphyroclasts tend to form elongate ribbons in the neoblastic matrix (Fig. F66). Olivine porphyroclasts tend to recrystallize to polygonal neoblasts, and clinopyroxene is recrystallized as asymmetric tails or mantles around porphyroclasts (Fig. F67).

Oxide gabbros or oxide-bearing gabbros appear to localize strain on both a small and large scale. Good examples of strain localization occur at several interval boundaries throughout the core. For example, Section 179-1105A-25R-3 (Fig. F68) displays a contact between oxide gabbro and olivine gabbro. The oxide gabbro is generally dynamically recrystallized and possesses a porphyroclastic texture, but it also contains mylonitic bands. The olivine gabbro displays an igneous texture, but, immediately at the contact between olivine gabbro and oxide gabbro, rocks are affected by ductile deformation where oxides minerals are present.

In one of the oxide-rich mylonite zones, foliated oxide-free gabbro residing adjacent to undeformed gabbro has a porphyroclastic texture with strongly bimodal grain sizes. Clinopyroxene forms the major porphyroclastic phases, and plagioclase is dynamically recrystallized to finer neoblasts. An oxide band appears to localize mylonitic deformation as can bee seen in thin section (Fig. F69). Clinopyroxene porphyroclasts within the oxide-mineral matrix appear to be little strained, and this may indicate a high ductility contrast between the opaque oxides and the pyroxene, effectively shielding undeformed pyroxene porphyroclasts. Microfractures oriented at 35-40 to the shear zone margins appear to be Riedel shears (Fig. F65). The effect of a migrating oxide-rich melt into the shear zone may be to localize the strain in the rheologically weak oxide-mineral-bearing regions. The fact that the host rock seems to be oxide free and deformed to a lesser extent than the oxide-rich region across an abrupt boundary may indicate that the oxide-rich melt invaded or infiltrated the shear zone after deformation was initiated. Once solidified, the oxide-mineral-rich intervals acted to localize deformation as the rheologically weakest part of the rock and focused ductile deformation, whereas the adjacent plagioclase had entered into the domain of brittle-ductile behavior (e.g., Riedel shears). Alternatively, as temperatures dropped and plagioclase and clinopyroxene increased in ductile strength, a contrasting and pre-existing oxide-rich zone localized strain to the rheologically weakest part of the rock.

Experimental data on the lower temperature ductility of opaque oxides supports the notion that opaque oxides will represent zones of strain localization during high to moderate temperature deformation. Whether the late-stage oxide-rich melts and their infiltration into gabbroic rocks initiated deformation or the solidified cumulus oxide minerals in the cumulates simply localized the strain in the weakest rheology as deformation proceeded is unknown. Given that oxides are likely to be ductile at lower temperatures than clinopyroxene, olivine, and plagioclase (Agar and Lloyd, 1997), it is unlikely that the distinction between hypersolidus or subsolidus initiation of the deformation can be assessed by microstructure. The deformation proceeds to the solid state and obliterates any evidence of hypersolidus deformation.

Brittle Structures

The two classes of brittle structures recognized are (1) magmatic veins (MV) and (2) faults and fractures (F) and vein-filled fractures (AV). Each of these structures is indicated on the hard-rock VCDs.

Magmatic Veins

Brittle magmatic deformation is represented by the intrusion of veins of felsic compositions on a variety of scales. Veins range from 1 mm to ~3 cm in width. Their composition ranges from leucogabbro with assemblages of plagioclase + clinopyroxene brown hornblende to trondhjemite. The more silicic end-members are more common. Although numerous in some sections of the core (e.g., Sections 179-1105A-1R-1 to 1R-5, 6R-2 and 6R-3, 9R-1, 10R-1, and 27R-1 to 27R-4), they are volumetrically a small fraction of the total recovery. The downhole presence of veins are plotted in Figure F26 (see "Igneous and Metamorphic Petrology and Geochemistry"). Most veins are not affected by ductile deformation, especially the most felsic veins. However, there are some leucocratic gabbro veins that appear to have been affected by ductile deformation shortly after intrusion (Fig. F70). The strain appears localized in the vein and not in the country rock into which it intrudes. The clearest example is in Sample 179-1105A-4R1, 77-85 cm, in which crystal-plastic fabrics are oriented such that the foliation plane is parallel to the vein's margins. Veins have a range of orientations from near vertical to horizontal.

Fractures and Alteration Veins

Most joints and fractures are filled by vein minerals along fracture walls, and they are logged in the Structural Geology spreadsheet as alteration veins and marked as AV on the core barrel sheets. There is significant variation in the density of veins throughout the section, but, in general, they tend to be scarcer in penetratively deformed sections of the gabbro (e.g., Sections 179-1105A-25R-1, 29R-1, and 30R-2,) and where the grain sizes are very coarse in undeformed gabbro (Sections 179-1105A-28R-1 and 4). Most of the veins observed are planar; however, curviplanar and irregular fractures were also observed. The majority of the planar fractures are characterized by actinolite and/or chlorite vein-fill mineral assemblages. These higher temperature veins have average dips of ~35, but their dips range from 0-80. Most of the vein fill has extensional characteristics, but several veins displayed slicken-fibers in the true-dip direction of the vein. The remainder of the veins also have variable dips and contain either higher temperature brown amphibole or lower temperature altered calc-silicate, carbonate, and smectite-zeolite.

In regions where smectite-calcite veins are in the core, the veins tend to be highly irregular, clustered, and wide (up to 0.4 cm), and the rocks appear altered and oxidized. Pieces 3, 4, and 5 of Section 179-1105A-21R-1 (~111 mbsf) and Pieces 1 and 2 of Section 26R-2 (~135-136 mbsf) were characterized by this type of alteration (Fig. F71A). Calcite veins were commonly partially open or consisted of calcite with well-developed crystal terminations grown into open vugs in veins (Fig. F71C). This evidence indicates that there may be active fluid flow through these fracture systems, and they may be associated with or found in the vicinity of active or recently active fault or fracture systems. Supporting this evidence of open subsurface fracture systems and currently active hydrological systems are temperature anomalies discovered in the borehole during logging at depths of ~104-105 and 135-136 mbsf, at or near the altered core regions. Although irregular in shape, these fractures are steeply dipping. This orientation may indicate that the deformation is transform related. Likewise, an open vertical fracture system was observed in Section 179-1105A-16R-2 (~92 mbsf), where a subvertical fracture was imaged with FMS and a temperature anomaly was found during downhole logging (see "Downhole Logging"). These brittle structures appear to represent the latest features in the structural evolutionary scheme.

Implications of Magnetic Data for Low-Temperature Structural Evolution

Preliminary magnetic data indicates that the core possesses a single coherent direction with an average inclination of ~69. This value is uncorrected for borehole deviation measured at 3. This is compared with an inclination of -52 expected for the site, significantly below the observed inclination. The difference of ~17 is larger than the discrepancy predicted by secular variation and plate movement in the last 11.5 m.y. As in Hole 735B paleomagnetic data, these results indicate a consistently reversed polarity for the section and may indicate a significant block rotation (~17) of the gabbroic massif. The reversed polarity and proposed block rotation is strikingly similar to the block rotation of ~18 shown for Hole 735B gabbroic rocks (Pariso et al., 1991). The consistency of the magnetic inclination downhole suggests that any relative internal rotations along ductile shear zone in the section must have occurred before cooling below the blocking temperature for magnetite (~580C) and are necessarily high temperature in origin. This is consistent with microstructural observations. In addition, gabbroic rocks from Holes 735B and 1105A have undergone approximately the same rotation after magnetic acquisition with little evidence of internal relative block rotation within the massif. This rotation is likely to have occurred during or after unroofing of the massif on the rift valley walls at the inner corner of the northern RTI and during the massif's subsequent transport along the transform. The steepening of the inclination would suggest a clockwise rotation along a low-angle normal detachment fault at the inner corner of the RTI (Fig. F72). This rotation is consistent with a low-angle normal fault detachment model for unroofing and exposure of the gabbroic massif at the seafloor.

Summary

The structure of the Hole 1105A core is complex, and structural styles and intensities range from ductile to brittle. Most of the gabbroic samples cored possess igneous textures, but several intervals of the core display crystal-plastic fabrics, and most samples display at least some crystal-plastic microstructure. Mylonitic zones with minimum thicknesses of 30 cm and characterized by high oxide-mineral content were observed at ~53 and 71 mbsf. Centimeter- to decimeter-thick zones of ductile shear that are coarser grained with porphyroclastic textures are restricted to the upper 90 m of core, whereas thicker zones (>1 m) of ductile deformation zones with weak to strong crystal-plastic fabrics become more prevalent at depths >90 mbsf. Some intervals of penetrative ductile deformation in the lower portion of the core exceed 2 m in thickness. Zones of ductile deformation are commonly oxide rich, as are the contact regions between undeformed and ductilely deformed gabbroic rocks, as if oxides lined the outer margins of ductile shear zones. Oxide gabbro-rich zones tend to be strain localizers based on the fact that many, but not all, of the crystal-plastic shear zones are rich in oxide minerals. Inclinations of the ductile foliations measured on the core face vary from ~18 to 75 in the cored intervals and average ~30-35. Thin sections show a range of textures from strictly igneous to slightly deformed igneous, to dynamically recrystallized metamorphic textures with crystal-plastic fabrics. As deformation intensity increases, the effect can be most easily observed in plagioclase, where there is a progression from strain-free plagioclase, to plagioclase with deformation twins, undulose extinction, and kink bands, to dynamically recrystallized grains. Minor recrystallization of neoblasts along grain margins progresses to porphyroclastic textures with bimodal grain sizes and small neoblasts of plagioclase and highly strained and kinked plagioclase, pyroxene, or olivine porphyroclasts. Olivine is also likely to have recrystallized to neoblast grain sizes prior to pyroxene, which tends to be preserved as the dominant porphyroclastic phase unless the intensity of deformation is most severe.

Brittle fractures are generally filled with vein material such as actinolite and chlorite, but no large faults zones were recovered in the core. Smectite and calcite alteration veins are associated with oxidized gabbroic zones that may represent recently active fracture systems. Based on temperature, sonic, resistivity, and porosity logs, these fractures observed in the core may be associated with active shallow hydrological systems along the transform margin. There are several regions of low recovery, which could correspond to fault or fracture zones based on logging data.

Rotations associated with ductile shear zones within the section occurred before magnetic acquisition in the samples. However, magnetic measurements indicate that a late rotation of the gabbroic massif occurred at temperatures below ~580C. This rotation is consistent with that determined at Hole 735B, indicating both sites show coherent behavior.

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