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FOOTWALL SITE (continued)

Structural Relationships

Within the context of deformation expected for the footwall of a detachment at an oceanic core complex, the structural observations for Site U1309 show a somewhat surprising lack of deformation immediately below the hypothesized detachment fault exposed at the seafloor. Some localized high temperature deformation fabrics are observed. However, the amount of rock that is strongly deformed at high temperatures is relatively modest. The majority of the recovered cores show only evidence for greenschist grade semibrittle/brittle deformation associated with basaltic intrusions and hydrothermal alteration. In many cases, this deformation amounts to no more than a pervasive static alteration of the rock; pseudomorphs of igneous texture remain largely unmodified. These observations indicate that the exhumation of the uppermost Central Dome of Atlantis Massif was dominated by brittle and semibrittle processes associated with magmatic intrusions and extension.

Five structural units are identified in the uppder 400 m of Hole U1309D. The boundaries between these units coincide with boundaries between gabbroic zones defined by igneous relationships. The characteristic structural features that differ between the units, or approaching the boundary between units, are the intensity of cataclastic fabrics, the abundance of vein alteration, the intensity of crystal plastic fabric, and the amount of magmatic foliation (Fig. F16). Several aspects of the downhole logging data correlate wtih the stucturally defined unit. Caliper data indicate less clean hole conditions within many of the boundary zones (Fig. F16H), which are located in Hole U1309D at 135, 160, 260, and 285 mbsf. The boundary zone at ~160 mbsf is associated with relatively good hole conditions, but neutron porosity is high (Fig. F16I) and density is low (Fig. F16J) within this 5–10 m section of the borehole. The fact that several of the stuctural unit boundaries coincide with the thin screens of ultramafic rock leads us to suggest that they acted as zones of weakness during alteration and may have controlled the late denudation history of Atlantis Massif.

The early history of deformation recorded at Site U1309 is constrained by magmatic and crystal plastic deformation fabrics in ultramafic and gabbroic rocks. Crystal plastic deformation is localized into narrow shear zones (up to 30 cm thick) in both Holes U1309B and U1309D, with no significant change in style or distribution with depth. The highest density of shear zones is at 35–80 mbsf in Hole U1309D. Intervals deeper in the hole have discrete shear zones only a few centimeters thick. Crystal plastic deformation is clearly partitioned into more-fractionated gabbroic rock types but is generally lacking (only ~5% of the core showed such high-temperature deformation) in the 400 m section of the footwall transected in Hole U1309D during Expedition 304. This is in marked contrast to the significant deformation recorded in the upper 500 m of core recovered from ODP Hole 735B on the Southwest Indian Ridge (Dick, Natland, Miller, et al., 1999). The dip of the majority of crystal plastic shear zones at Site U1309 ranges from 20°–60° toward the west; at various depths subhorizontal dips occur as well.

Microstructures indicative of deformation by semibrittle processes at amphibolite-grade conditions in some parts of the section suggest small strains; the majority of the recovered core shows evidence of only greenschist-grade semibrittle/brittle deformation associated with basaltic intrusion and hydrothermal alteration. In many instances, this deformation amounts to no more than a pervasive static alteration; pseudomorphs of igneous texture remain largely unmodified. Alteration veins, fractures, cataclasite, and breccia record low-temperature brittle deformation. Tentative reorientation of structures using paleomagnetic and logging data indicate that the majority of veins dip toward the east and several faults strike east-west. These observations indicate that denudation of the upper central dome of Atlantis Massif was dominated by brittle and semibrittle processes associated with magmatic intrusion and extension. No structures indicative of high displacement by either ductile or brittle processes have been recovered to date. This result is significant, and it severely limits the possible thickness of fault zones that could comprise a detachment system on the central zone. Poor recovery of the the uppermost 20 m of the footwall leaves open the possibility that this narrow zone accommodated a very high strain along a brittle fault. Such extreme localization of strain has been documented to occur in continental detachments (John, 1987; Miller, 1996). If this is the case here, the central dome differs from the southern ridge of Atlantis Massif, where seafloor mapping and sample analysis suggest a detachment zone thickness on the order of 50–100 m (Schroeder and John, 2004).

The relatively undeformed nature of the plutonic section of the massif provides an unprecedented opportunity to study emplacement processes associated with formation of oceanic lithosphere over a wide range of magmatic conditions. The orientation of intrusive contacts, where preserved, are commonly subhorizontal to gently dipping—gabbro against peridotite and diabase against gabbro—suggesting a sheet or sill-like geometry of emplacement. In contrast, thin basaltic dikes typically have subvertical (65°–80°) contacts with chilled margins. Layering in the gabbro sequences from the upper parts of both Holes U1309B and U1309D is defined by variations in modal composition, grain size, or both (Fig. F17). Modal proportions of olivine and plagioclase vary over vertical intervals of centimeters. The observation of this layering in more primitive possibly cumulate-textured rocks indicates that melt migration was initially dominated by porous flow and compaction. In contrast, the observations of gabbro dikes and late magamatic leucocratic veins that cut more primitive rock types indicate that melt migration was controlled by brittle mechanisms during the late-stage fractionation and crystallization process. At the grain scale, the role of melt in promoting brittle processes is indicated by pyroxene crystals in oxide gabbros that are cut by veins filled with magmatic oxide and hornblende (Fig. F18). Finally, the observation of crystal-plastic shear zones within narrow intrusions/dikes and at the contacts between gabbroic intervals suggests that the presence of melt promoted strain localization during extension.

Paleomagnetic measurements on samples from Holes U1309B and U1309B show dominantly negative inclinations that represent reversed polarity remanence (Fig. F19). However, there are important downhole fluctuations in inclination. Average inclinations determined for core from <180 mbsf are approximately –45°, similar to the reversed polarity dipole inclination expected for this site. A pronounced reduction in inclination angle is evident in the interval from ~180 to 260 mbsf, where values are typically shallower than –30°; some rocks in this interval have shallow positive magnetic inclinations. At depths >260 mbsf, the inclinations again are steeper, though the average inclination is ~5°–10° shallower than those in the upper 180 m of the hole.

The reversed polarity magnetization components found at Site U1309, particularly from gabbro, provide a robust estimate of the geomagnetic field at the time the magnetization was attained. The mean inclination of the most reliable samples is –49.4° (+3.4°/–2.1°). Crosscutting relations between steeply dipping basaltic dikes and gently to moderately dipping faults, taken with the paleomagnetic data showing little difference in magnetic inclination (mean from most reliable gabbro samples = –51°) from the expected (–49°), suggests little horizontal axis rotation of the upper ~180 m at Site U1309, the presumed upper part of the footwall to the fault system exposed at this location in the central dome.

The multicomponent remanence signatures discussed in the previous section potentially contain information on the thermal and tectonic history at Site U1309. The highest stability reversed polarity magnetization is shallower on average than the normal polarity overprint, although data scatter is also greater; the two components are not antipodal. The possible difference between the normal and reversed polarity directions may reflect the influence of tectonic tilting of the 180–260 mbsf interval after acquisition of the highest stability reversed polarity magnetization. In contrast, the mean inclination from the uppermost 180 m of Site U1309 (–48°) is essentially identical to that expected from a geocentric axial dipole (sequence experienced little tectonic rotation about horizontal axes. These shipboard results raise intriguing questions about the validity of the rolling hinge model for core complex formation. More complete onshore studies are necessary before final conclusions can be drawn.

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