Abstract - Faults are zones of extreme internal complexity and heterogeneous strain distribution over a wide range of scales. Four active opencast lignite fields in the Ptolemais Basin, where the lignite-marl sequence is displaced by numerous of normal faults with throws in range from a few meters up to 40 m, are one of the best outcrop places in the world for characterising and understanding the fault zone structure over a range of scales. Every few months fresh fault outcrops are exposed as part of the mining operations, and serial visits to the mines allows a truly 3D analysis of the fault zones. We have visited and mapped the mines 9 times from June 2010 to July 2012, at intervals of ca. 3 months, during which time each face is taken back between 20 and 50 m. Various resolutions of photographs and structural measurements were imported into a fully georeferenced 3D structural interpretation package, resulting in a seismic scale and outcrop resolution 3D fault volume with outcrop and panoramic view photographs acting as the seismic sections in equivalent seismic surveys. 2D and 3D investigation of the fault zone structure shows that the faults form soft-linked systems, characterised by a prevalence of fault tips as opposed to branch-points, with ductile bed rotations between faults accommodating transfers of strain between adjacent faults. Quantitative analysis of the faults indicates that these systems are extremely soft and that for a given throw, these faults are both shorter and more segmented than many other fault systems. There are no significant differences between the throw gradients in the coal and marl in contrast with the interactive faults which have higher throw gradients than the simple faults with largest differences between the fault tips. In terms of displacement partitioning, the data show that the main slip-surface accommodates a higher percentage of the total throw as throw increases.
Abstract of talk given to:
Irish Geological Research Meeting, University of Ulster, March 2013.