Abyssal peridotite mylonites: implications for grain-size sensitive flow and strain localization in the oceanic lithosphere
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Microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation, Fabrics associated with deformation processes at slow-spreading mid-ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations. The most abundant class are medium- to coarse-grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere. These tectonites give geothermometric temperatures of similar to 755 degrees C or higher, interpreted to represent lower temperature limits for diffusive exchange in coarse-grained abyssal peridotites during cooling. This conclusion is consistent with flow laws for olivine at these temperatures. The second class of abyssal peridotites, previously largely undescribed for the mid-ocean ridge environment, include fine-grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle-plastic transition extends into mantle rocks. These mylonites give temperatures of similar to 600 degrees C, which we suggest represent a lower temperature limit for plastic deformation. Reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures. Relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse-grained tectonites. Based on flow laws for olivine, we suggest that deformation in some fine-grained mylonites occurred by diffusion creep down to similar to 600 degrees C, Rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature. We conclude that a reduction in grain size by cataclasis, or dynamic recrystallization, resulted in a transition in deformation mechanisms from dislocation- to diffusion-creep during uplift (and/or cooling). The observation of these fine-grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep. Weak faults may promote the development of long-lived detachments in the upper mantle. This inference supports mid-ocean-ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge-axis discontinuities are exhumed along long-lived detachment faults.