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

Hardman, Kit and Holdsworth, Robert E. and Dempsey, Edward and McCaffrey, Ken (2020) 'The nature and significance of rift-related, near-surface fissure fill networks in fractured carbonates below regional unconformities.', Journal of the Geological Society, 177 (6). pp. 1168-1185.


Fissure-fill networks are a widely recognized, but relatively little described, near-surface phenomenon (<1–2 km) hosted in carbonate and crystalline basement rocks below regional unconformities. Faults and fractures in otherwise tight Devonian carbonate basement rocks of the Tor Bay region, Devon, SW England are associated with the development of millimetre- to decametre-wide fissures containing red-coloured early Permian sedimentary material, vuggy calcite mineralization and wall rock collapse breccia. These features preserve evidence about 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 development of the Portland–Wight Basin in the early Permian. Once formed, they were subjected to episodic, probably 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 probably 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.

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Publisher statement:© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License ( licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer:
Date accepted:15 June 2020
Date deposited:26 May 2021
Date of first online publication:19 June 2020
Date first made open access:26 May 2021

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