Crystal-plastic, magmatic, and brittle deformation identified in rocks recovered during Expedition 305 from ~401.3 to 1415.5 mbsf show a low overall degree of deformation (Fig. F27). The temporal evolution of the deformation history in Hole U1309D may be summarized as follows:
Magmatic fabric. About 22% of all recovered rocks show a generally weak magmatic foliation (Fig. F28). The formation of the magmatic foliation is attributed to processes related to igneous emplacement of the gabbro bodies. The growth of clinopyroxene megacrysts, commonly poikilitic, appears to have occurred late in the crystallization sequence of the gabbros, as in many samples they embed equilibrated and/or corroded plagioclase grains. Clinopyroxene growth may locally overprint early hypersolidus crystal-plastic recystallization.
High-temperature plastic deformation. Whereas core descriptions suggest an absence of plastic deformation except in local shear zones, microstructural observations frequently indicate a weak to moderate crystal-plastic strain overprint in samples (Fig. F29). Except for a few cases, there is no strong foliation development coupled to this phase of plastic strain; therefore, it is likely to have been weak.
Lower temperature and/or higher strain rate deformation. Clearly overprinting the high-temperature plastic fabric when present, and much more localized, are discrete, relatively thin (generally a few centimeters up to 1 m) porphyroclastic and mylonitic shear zones. The shear zones commonly, but not systematically, show a close spatial association with concentrations of oxide minerals (Figs. F12, F30A). Overall, crystal-plastic foliation has been identified in <3% of the core.
Magmatic veins. Magmatic veins seem to be the expression of late magmatic intrusions, and they do not show any associated high-temperature deformation. Magmatic vein types appear close to intrusive bodies of the same rock type, suggesting a local, parental relationship.
Serpentinization. Serpentinization occurred along microcracks, and the products of this process appear in all olivine-bearing rocks, most prominently and highly heterogeneously in olivine-rich rock types.
Fault zones, cataclastic deformation, and veining. All fault gouges (Fig. F31), zones of cataclasis (Fig. F32), and late, silica-rich hydrothermal veins formed under greenschist facies or lower conditions. Crosscutting relationships indicate a complex succession of events involving fluid flow and deformation. The amount of strain recorded is negligible overall, except for four fault zones (at ~695, 756, 785, and 1107 mbsf) identified by the occurrence fault gouges. These faults, however, are relatively thin (no more than a few tens of centimeters), unlikely to have accommodated large displacement, and more likely reflecting late deformation internal to the overall gabbroic body rather than major regional tectonic event(s). One of the most pervasive indicators of internal deformation is the presence of fibrous light green veins with fibers indicating the direction of vein movement (vein faults). In most cases (>75%), these fault veins display strike-slip movement, which may be associated with the deformation of structural blocks in the footwall. The relative timing of serpentinization and late brittle events is not clearly defined, although they are probably distinct events based on the difference in the fluid chemistry required for serpentinization versus the formation of talc- and tremolite-rich veins.
Open fractures. Late fracturing of the rock results in fractures that are open and have no apparent offset. Sulfides are present along these fractures only in the lower part of the drilled section (>600 mbsf).
Similar to the metamorphic record, the deformation history is essentially separated into high-temperature and low-temperature events, with no significant deformation occurring under the amphibolite facies conditions. Only very locally was synkinematic brown amphibole observed (in oxide gabbros). Neither the high- nor the low-temperature deformation recorded by the Expedition 305 Shipboard Scientific Party is geometrically consistent with (too steeply dipping) or directly related to a major detachment system such as the one inferred to be responsible for exposing the corrugated domal surface of Atlantis Massif. The latter is probably essentially brittle and highly localized in the uppermost part of the section at Site U1309.
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