Tectonic and Rheologic Implications of Quartz Grain Size Variations in the Alpine Fault, New Zealand

Steven Kidder, City College New York
Virginia Toy, Institut für Geowissenschaften
David Prior, University of Otago
Lucan Mameri, Brown University
Gordana Garapic, State University of New York, New Paltz
Tim Little, Victoria University of Wellington

Abstract

I will summarize several aspects of quartz grain size distributions in Alpine Fault mylonites (New Zealand) based on an analyses of 260 rocks found along 150 km strike of the fault. 1) Quartz grain size has a bimodal distribution that may indicate two stages of recrystallization; finer sized grains likely representing conditions near the brittle ductile transition (BDT). 2) There is a systematic variation in the size of the finer-grained population along strike: Fine grains are ~40 μm in a central region and gradually decrease by a factor of four to the northeast and southwest. This along-strike size variation of the fine grains mirrors regional patterns of exhumation rate, microseismicity depth, and geothermal gradient. A coarser grain size fraction (~100 μm) remains fairly constant in size everywhere. 3) The area of quartz-rich regions occupied by fine grains (~40% on average) indicates low strains associated with the fine grains (gamma ~1), far smaller than well-documented mylonitic shear strains (gamma = ~100). Variation in the area of quartz-rich regions occupied by fine grains also does not correlate with known finite strain variations between mylonites and protomylonites. These observations point to minimal strain in mylonites near the BDT, with most BDT strain focused in a narrow (<100 m thick) ultramylonite shear zone. Large finite strains preserved in the mylonites must have accrued at deeper crustal levels, well below the BDT. 4) Constraints on strain rate, differential stress, and deformation temperature (500° and 400°, respectively, for BDT deformation in mylonites near the central and peripheral regions) suggest that commonly used quartz flow laws may underestimate the viscosity of mylonites at these conditions by at least a factor of 10. Considerably lower viscosities can be inferred, however, for the ultramylonites, where deformation involved extensive grain-boundary sliding.

Session

Magnetic fabrics or other records of deformation