Competence contrasts in ductile deformation as illustrated from naturally deformed chert-mudstone layers

Abstract

A sample of chert-mudstone layers provides a rare opportunity for quantitative analyses of competence contrasts in naturally deformed rocks. This sample contains deformed quartz veins, possibly formed subperpendicular to bedding during initial bedding-normal compaction, and abundant radiolarians as strain markers whose viscosity during deformation is approximated by that of chert. Under several assumptions these allow quantitative analyses of two types of competence contrast in ductile deformation, layer competence contrast and inclusion/matrix competence contrast. Layer competence contrast results in strain and cleavage refraction through the layers, while inclusion/matrix competence contrast is illustrated by the difference in strain state between radiolarians and their matrix. A quartz vein presumed to be initially perpendicular to bedding provides estimates of layer-parallel shear strains and chert/mudstone layer viscosity ratios which were also used as radiolaria/matrix viscosity ratios in different layers. Radiolarian shapes are factorized into cleavage strains and pre-cleavage shapes. Cleavage strains in matrix are then determined from radiolarian strains applying Eshelby's equations, and their variations as well as the refraction of cleavage through the layers are compared with Treagus's theoretical modeling of strain refraction. With decreasing layer competence, cleavage refracts toward bedding and increases in intensity so that cleavage-bedding intersections vary slightly but systematically through the layers, suggesting a threedimensional strain refraction. All three principal axes of radiolarian and matrix cleavage strain ellipsoids estimated indeed refract from layer to layer. Strain magnitude systematically increases with decreasing layer competence. These results are consistent with the strain refraction theory. However, no systematic change in strain ellipsoid shape associated with strain refraction is recognized.