Fracture characterization in the Chalk and the evolution of the Thanet monocline, Kent, southern England

Systematic studies of fractures in the exposed Upper Cretaceous Chalk and from seismic data are used to establish the evolution of the regional structure and the related fractures on the Isle of Thanet, Kent. The results show that the main structure developed as an easterly trending, south facing, monoclinal forced fold with a WNW-ESE trending cross forced fold along a basement grain (Pegwell Bay-Minnis Bay transtension/transpression zone). The folding started during the deposition of the Chalk in the Upper Cretaceous due to an extensional tectonic phase accompanied by differential sedimentation and compaction [vertical ( σ 1 ) and horizontal ( σ 3 )] which induced vertical and steep macrofractures (mainly joints and some extensional faults) in the Chalk. This was followed late in the Upper Cretaceous by a horizontal, N-S compression ( σ 1 ) which continued in the Tertiary and caused inversion of movement on the basement faults and the occurrence of the final stages in the forced folding. The latter phase led to the rejuvenation of the pre-existing fractures in the cover (mainly by faulting) and the development of new fractures (mostly faults and joints). An uplifting/unloading phase started late in the deformation history and caused the occurrence of near horizontal (bedding-parallel) fractures (joints) and bedding-perpendicular or steep mesofractures parallel to, and perpendicular to, the burial/tectonic macrofractures. These have particularly developed at the top several metres of the exposed Chalk where a rapid increase in frequency of fractures is clear. The fracture characterization of the Chalk varies significantly in different parts of the monocline. Based on such variation a minimum of four domains (zones) have been identified. The heterogeneity in the fracture development is most probably controlled by the variation in orientation and type of the causative basement faults and the amount and type of displacement on them.