Thrust fault nucleation due to heterogeneous bedding plane slip: Evidence from an Ohio coal mine

Abstract

Shales often form the roof and floor rocks of coal mines in the Appalachian Basin. From a geologic perspective, these mines offer outstanding 3D exposures of fresh rocks, otherwise only accessible via boreholes or heavily weathered surface outcrops, and present an excellent opportunity to directly observe pristine in situ natural fractures in shale. In Carroll County, Ohio, small thrust faults that predate mining operations are well exposed within the roof rocks and develop near areas where gradients of coal–shale contacts are steepest, causing ground instability. We hypothesize that bedding contact surfaces served as natural displacement discontinuities that perturbed the local stress state, resulting in localized thrust fault development. Using borehole and in-mine survey data, we digitally constrain bedding contact geometry to model slip along this interface and resolve related stress perturbations. Our modeling results suggest that inherited non-planarity of contact surfaces influenced the nucleation of secondary faults. Simulation results as constrained by the integrated field and modeling approach taken in this study help us to better understand the relationship between subsurface stress perturbations and the formation of secondary fractures. For mining purposes, recognizing subsurface stress heterogeneities due to variable bedding geometry within a regional stress field can improve our predictive capabilities of underground structural hazards.