Frequency-size distribution of competent lenses in a block-in-matrix mélange: Imposed length scales of brittle deformation?

Abstract

This study explores the frequency-size distribution of competent lenses (phacoids) in the Chrystalls Beach Complex, a mélange shear zone within the Otago Schist accretion-collision assemblage, New Zealand. The distribution of phacoids within a cleaved, pelitic matrix follows a power law distribution with exponent 1.18 < D < 2.37 (2-D) at outcrop scale, whereas distributions at thin section scale have higher D than the outcrop they were sampled from. D relates to volume fraction and distribution of competent material, where high D correlates with small aspect ratio phacoids and high matrix volume fraction. In outcrops which experienced mixed continuous/ discontinuous deformation, D = 1.4 ± 0.2. It is suggested that D ≫ 1.4 indicates dominant continuous deformation, whereas D ≪ 1.4 is indicative of localized deformation on shear discontinuities. In this mélange, D therefore appears a function of dominant deformation style. Actively deforming mélange shear zones likely exhibit a range of seismic styles and earthquake magnitudes at length scales limited by lithological layering and shear zone thickness. A hypothesis is suggested where strongly coupled shear zone segments may be distinguished by a high volume fraction of competent material (low D), while aseismically creeping regions are characterized by a high proportion of incompetent rock (high D). As the b value in the Gutenberg-Richter frequency-magnitude relationship also tends to be high in coupled and low in creeping fault segments, it may be that D is analogous to b, and the b value is also related to the ratio of incompetent/competent material and dominant fault zone deformation style. Copyright 2011 by the American Geophysical Union.