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Statistical Approaches |
1 Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA engelder{at}goesc.psu.edu
2 Geosciences Azur, UMR 6526, Université de Nice — Sophia Antipolisu, 250 Rue Albert Einstein, Sophia Antipolis, F-06560 Valbonne, France
Poles from line samples of systematic joint sets scatter about a mean pole because joints are neither perfectly planar nor parallel, and because measurement instruments are imprecise. Definition of a single joint set can be based solely on its orientation distribution and this distribution is assessed using two statistical parameters: square root of the circular variance (approximately equal to the standard deviation
for two-dimensional (2D) data) and cone of confidence (
95 for 3D data). The distribution for joints generated in the absence of tectonic deformation is well clustered with = 1.7° and 95 = 0.48° based on a bootstrap sample of 50. Jointing associated with various fold styles show less clustering: the kink of a fault-bend fold ( = 6.1° and 95 = 1.7°), basement-cored anticline ( = 3.5° and 95 = 1.5°), regional joint set transected by a basement-cored anticline ( = 5.2° and 95 = 1.8°) and a buttress anticline ( = 4.3° and 95 = 1.7°). Jointing associated with local faulting tends to show even less clustering: a Cretaceous marl ( = 8.3° and 95 = 2.4°) and a glauconitic sandstone ( = 8.6° and 95 = 2.2°). The latter sample was drawn from two overlapping joint sets, indicating that distribution data greater than 95 = 2.2° may signal overlapping joint sets.
