The nature and significance of rift-related, near-surface fissure fill networks in fractured carbonates below regional unconformities

Hosted in carbonate and crystalline basement rocks below regional unconformities, fissure-fill networks are a widely recognized, but relatively little described near-surface phenomenon (< 1-2km). Faults and fractures in otherwise tight Devonian carbonate basement rocks of the Tor Bay region, Devon, are associated with the development of millimetre- to decametre-wide fissures containing red early Permian sedimentary material, vuggy calcite mineralisation and wall-rock collapse breccias. These features preserve evidence concerning the style and history of fault deformation and reactivation in near surface settings, and on fluid-related processes such as elutriation and/or mineralization. Field observations, palaeostress analysis, and fracture topology analyses show that the rift-related faults and fractures created a network of long-lived open cavities during the early Permian development of the Portland-Wight Basin. Once formed, they were subjected to episodic, likely seismically-induced fluid fluxing events and local karstification. The large, well-connected networks of naturally propped fractures were - and possibly still are - important fluid migration pathways within otherwise low permeability host rocks. These structures are likely equivalent to those observed in many other rift-related, near-surface tectonic settings, and suggest that the Tor Bay outcrops can be used as a global analogue for sub-unconformity open fissure systems hosted in low permeability basement rocks.