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Quantified Fracture (Joint) Clustering in Archean Basement, Wyoming: Application of Normalized Correlation Count Method


We demonstrate statistically significant self-organized clustering over a length scale range from 10-2to 101m for north-striking opening-mode fractures (joints) in Late Archean Mount Owen Quartz Monzonite. Spatial arrangement is a critical fracture network attribute that until recently has only been qualitatively assessed. We use normalized fracture intensity plots and the Normalized Correlation Count (NCC) method of Marrett et al.to quantitatively discriminate clustered from randomly placed or evenly spaced patterns over a wide range of length scales and to test the statistical significance of the resulting patterns. We propose a procedure for interpreting cluster patterns on NCC diagrams generated by the freely available spatial analysis software CorrCount.Results illustrate the efficacy of NCC to measure clustering patterns in fractures in texturally homogeneous Archean granitic rock in a setting distant (>2 km) from folds or faults. These regional fractures are conduits for water flow in their current geological setting, and their patterns—and the NCC approach to defining clusters—may be useful guides to spatial arrangement style and clustering magnitude in other, less accessible fractured basement rocks.