Eberly College of Arts and Sciences
Geology and Geography
En echelon fractures and veins are among the most common and distinctive geological structures, yet their three-dimensional forms and relationships to surrounding structures are commonly unclear. X-ray computed tomography (CT) offers an unrivaled ability to examine structures within rocks in three dimensions, and it is applied here to a sample of drill core from the Marcellus Shale of southwestern Pennsylvania (USA). CT images yield qualitative and quantitative data on the transition from a pyrite-rich planar vein to an en echelon veinlet array, and on the heterogeneity of veinlets within the array. Using a combination of three- and two-dimensional images, geometric data, and traditional petrography, we identify a range of veinlet shapes consistent with deformation during formation of an antitaxial graphite-calcite-pyrite vein system. Each of the veinlets is rooted in the underlying planar vein where it is narrowest. The transition from planar vein to en echelon array coincides with a change in bedding, suggesting that competency contrasts between adjacent beds controlled the fracture morphology. Veinlets initiated as short, lenticular fractures at ∼45° to the planar vein before lengthening, dilating, and rotating. None of the veinlets are strongly sigmoidal, nor is there measurable offset across the margins of the planar vein; therefore, finite non-shear strain was very limited, and fluid overpressure–induced fracturing during burial and diagenesis is probably the most likely process for fracturing and vein formation.
Digital Commons Citation
Andrews, Graham DM; Brown, Sarah R.; Moore, Johnathan; Crandall, Dustin; and Mackey, Paige, "The transition from planar to en echelon morphology in a single vein in shale: Insights from X-ray computed tomography scanning" (2020). Faculty & Staff Scholarship. 2231.
Graham D.M. Andrews, Sarah R. Brown, Johnathan Moore, Dustin Crandall, Paige Mackey; The transition from planar to en echelon morphology in a single vein in shale: Insights from X-ray computed tomography scanning. Geosphere ; 16 (2): 646–659. doi: https://doi.org/10.1130/GES02191.1