ABYSSAL PERIDOTITE MYLONITES - IMPLICATIONS FOR GRAIN-SIZE SENSITIVE FLOW AND STRAIN LOCALIZATION IN THE OCEANIC LITHOSPHERE

Microstructures preserved in abyssal peridotites dredged from the ocea ns record several different physical regimes of deformation, Fabrics a ssociated with deformation processes at slow-spreading mid-ocean ridge s form two major classes of abyssal peridotites based on detailed micr ostructural observations. The most abundant class are medium- to coars e-grained tectonites with microstructures that reflect deformation pro cesses during mantle upwelling and emplacement to the base of the lith osphere. These tectonites give geothermometric temperatures of similar to 755 degrees C or higher, interpreted to represent lower temperatur e limits for diffusive exchange in coarse-grained abyssal peridotites during cooling. This conclusion is consistent with flow laws for olivi ne at these temperatures. The second class of abyssal peridotites, pre viously largely undescribed for the mid-ocean ridge environment, inclu de fine-grained mylonites associated with faulting and shear zones tha t develop during extension and cooling of the oceanic lithosphere when the brittle-plastic transition extends into mantle rocks. These mylon ites give temperatures of similar to 600 degrees C, which we suggest r epresent a lower temperature limit for plastic deformation. Reduced gr ain size in mylonites allows for diffusive exchange to continue to the se low temperatures. Relict augen in the mylonitic samples preserve eq uilibration temperatures similar to those exhibited by the coarse-grai ned tectonites. Based on flow laws for olivine, we suggest that deform ation in some fine-grained mylonites occurred by diffusion creep down to similar to 600 degrees C, Rheological data for olivine indicate tha t dislocation creep is not likely to occur at this temperature. We con clude that a reduction in grain size by cataclasis, or dynamic recryst allization, resulted in a transition in deformation mechanisms from di slocation- to diffusion-creep during uplift (and/or cooling). The obse rvation of these fine-grained mylonites indicates that shear zones tha t extend into the upper mantle will be weaker than expected if deforma tion was accommodated by brittle processes or dislocation creep. Weak faults may promote the development of long-lived detachments in the up per mantle. This inference supports mid-ocean-ridge tectonic models th at suggest that ultramafic rocks exposed at the inside corners of ridg e-axis discontinuities are exhumed along long-lived detachment faults.