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Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England

Jon P.T. Tarasewicz^,1, Nigel H. Woodcock^,1 and J. Anthony D. Dickson^,1

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

The obliqu-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudston-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to singl-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution.

Key Words: fault breccia • coseismic • damage zone • cementation • Variscan

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