Spatial distribution of brittle strain in layered sequences

摘要

A new method of spatial analysis of brittle deformation is proposed that can be applied to both opening mode fractures (joints and veins) and faults. The method has been developed to provide a measure of heterogeneity based on both the position and displacement of individual fractures sampled along a linear traverse. It is based on a non-parametric comparison of the cumulative frequency and strain with that for a uniform distribution. In addition the method provides a statistic that may be used to test a cumulative data-set for significant departures from a uniform distribution. Two areas of lower Jurassic strata in England provide exceptional outcrops of several kilometre lengths displaying groups of tensile fractures (veins) and normal faults with displacements ranging over 5 to 6 orders of magnitude. The strata consist of shales interbedded with carbonates (decimetres to metres thick), having shale/carbonate ratios of 5/1 and 13/1. Data collected along 25 scan-lines of different length and resolution were analysed. The results show that strain is highly localized at the vein-scale in the more carbonate-rich sequence whilst it is uniformly distributed in the mudstone-dominated sequence. Fault-strain is fairly homogenously distributed in both study areas. These differences may be due to mechanically different behaviours of the sedimentary successions during early deformation history.