Strain localization along the Atlantis Bank oceanic detachment fault system, Southwest Indian Ridge

Abstract

[1] Microstructural observations and mineral thermometry from in situ samples collected from the Atlantis Bank oceanic core complex (SW Indian Ridge) indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile, semibrittle, and brittle regimes as strain localized along the exposed, now subhorizontal fault surface. Ductile, semibrittle, and brittle fabrics are developed within dominantly gabbroic rocks. Footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor, whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault, respectively. Sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910°C and 650°C, by amphibole-accommodated dissolution-precipitation creep at temperatures $750°C-450°C, by chlorite-accommodated reaction softening at temperatures $450°C-300°C, and by brittle fracturing and cataclasis at temperatures <300°C.

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“…Here, lower temperature fault rocks are present in addition to the amphibolite facies mylonite. These chloritized and weathered fault gouge and talc-serpentine schists, preserved locally on the fault surface Miranda and John, 2010), are very similar to fault rocks found on the Mid-Atlantic Ridge core complexes (e.g., MacLeod et al, 2002;Escartín et al, 2003, Schroeder andJohn, 2004;Dick et al, 2008).…”

“…The degree of greenschist grade deformation is, however, much less in Hole U1473A than in Hole 735B and in previously reported sample suites from the surface of Atlantis Bank (e.g., Miranda and John, 2010). …”

“…Many of the gabbroic rocks in the shallow drill cores and seen in ROPOS ROV images from the surface of the platform are mylonitic or porphyroclastic, displaying intense crystal-plastic deformation with shallow fabrics. In Hole 735B, crystal-plastic deformation of oxide gabbro and olivine gabbro was likewise found to be strong in the uppermost 500 m but diminished markedly downsection (Robinson, Von Herzen, et al, 1989;Cannat et al, 1991;Dick et al, 1991a;Miranda and John, 2010). Figure F5.…”

The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U–Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

“…Strain is localized in the Hole 735B core in zones of well-developed crystal plastic and brittle fabrics; the thickest and most significant zone is located in the upper 100 m of the hole Dick et al, 2000;Miranda and John, 2010). A key question is whether the downhole variation in dates for Hole 735B can be explained by brittle or ductile shearing along the detachment fault?…”

Oceanic core complex development at the ultraslow spreading Mid‐Cayman Spreading Center

“…Many of the bathymetric features of the MCSC resemble those observed on other slow‐and ultraslow spread MORs such as the MAR or the SWIR. For example, Mount Dent has a curviplanar surface similar to that observed on the Atlantis Massif on the MAR at 30°N [e.g., Cann et al , 1997] and the Atlantis Bank off the SWIR axis [e.g., Miranda and John , 2010], and is flanked to the west by linear ridges similar to those observed at 13°19′N on the MAR [ Smith et al , 2006, 2008; MacLeod et al , 2009]. The apparent similarities between these massifs are reinforced in two‐dimensional profiles across these different features (Figure 4).…”