Major and minor structural features of a bedding shear zone along a coal seam and related gas outburst, Pingdingshan coalfield, northern China

Abstract

Abstract A fractured coal band, traditionally called a soft-coal band, or structurally referred to as a bedding shear zone, within the Wu coal seam in the Pingdingshan coalfield, northern China, was structurally studied. Based on field examination and observations by reflectance microscopy and scanning electron microscopy (SEM), the bedding shear zone is not consistent with a simple bedding fault defined by pioneering geologists, but rather a complexly deformed band locally overprinted by ductile shearing fabrics. Dominant small-scale structures within the bedding shear zone are thrusts, folds, cleavage duplexes and related structures, but local, refolded folds, kink bands, and S–C band structures also indicate ductile shear deformation, typically developed in areas of steep dip such as within the limb of a subordinate fold. Bed-parallel shearing within the coal seam has been considered a serious influence on gas outburst in underground mining production in addition to thickening the seam and reducing the quality of coal. Locally, abnormal thickening of a shear zone and extensive compressive structures where ductile fabrics frequently occur are closely related with the site of an outburst. Thickening of a shear zone increases the capacity of methane storage, and compressive structures within a shear zone may act as a tectonic screen, blocking methane migration, which can result in a pocket of high methane pressure. A potential risk of outburst is indicated as a working face approaches an area of high methane pressure. Three abnormal phenomena encountered during a mining operation presage methane outburst for seams with a bedding shear zone.