VARIATION OF LOCAL JOINT STRIKE ORGANIZATION IN A DIABASE SILL SVALBARD, NORWAY

This study explores the variation in the local degree of organization of joint orientation within a geologic unit. The study area is a Cretaceous diabase sill located in central eastern Svalbard that is extensively exposed (≈ 60-65 km²) due to glacial and shoreline erosion. The joints formed due to cooling, and to tectonic stresses that varied through time. There are at least four dominant fracture sets or preferred orientations present. We obtained satellite imagery of the sill from Landinfo with ≈fifty-centimeter resolution. With this high-resolution imagery, joint patterns in the sill were traced using ArcGIS. We split the traces into constant line segments and used the open source software “MODO ko MODO” to map the local degree of joint organization within the sill. MODO (measure of the degree of organization) quantifies the local degree of fracture orientation organization. Two MODO measures were used: the Multinomial and the Beta functions. For the multinomial function, we treat a collection of n proximal joints as a sample chosen from a uniform distribution. We use the multinomial probability mass function to calculate the probability that the sample was drawn from a uniform distribution. We report the inverse log of this probability, which ranges from ca. 3-20. The Beta function is a measure of angular dispersion of sample orientations: its values range from zero (high dispersion) to one (low dispersion). Observed spatial variation within the sill spans about 60% of the range of the beta measure. Areas of similar MODO values define organization domains. The sill domains have a scale from tens of meters to hundreds of meters, and are often elongate with a long axis parallel to a dominate fracture direction. Different proportions, concentrations, and mixes of the preferred orientations in the diabase sill contribute to the local degree of organization and domain character at any one locality. Fracture MODO provides an additional tool to capture within-unit fracture variability. We propose fracture MODO will have implications for fluid flow and rock mass strength variables.