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AAPG Bulletin, V.
Brittle-failure controls on maximum sustainable overpressure in different tectonic regimes Richard H. Sibson1
1Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand; email: [email protected]
Richard H. Sibson graduated with B.Sc. (hons) degree in geology from the University of Auckland, New Zealand, and with an M.Sc. degree and a Ph.D. in structural geology from Imperial College, London. He now teaches structural geology at the University of Otago after previous appointments at Imperial College and University of California Santa Barbara. His research focuses on the structure and mechanics of fault zones and fluid redistribution coupled to fault processes.
Thanks are due to F. Ghisetti, B. Stump, J. Watkins, and H. Tobin for suggesting improvements to the manuscript.
Formation or reactivation of brittle faults and fractures within low-permeability rocks capping regions of overpressured crust creates drainage conduits limiting the degree of overpressuring. Maximum sustainable overpressure is therefore affected by the local state of stress within the capping layer and by any existing architecture of faults and fractures. Reshear of existing cohesionless faults that are favorably oriented for frictional reactivation within the stress field provides the lower limiting bound to overpressures and inhibits development of other brittle structures. Formation of drainage conduits by hydraulic extension fracturing is important only in the case of intact caprock under low differential stress. Brittle-failure-mode plots demonstrate that maximum overpressure is inversely related to the level of differential stress and that high overpressures are easier to sustain in compressional regimes. Changes in the regional stress state in areas of overpressuring (for example, during tectonic inversion) may induce significant fluid redistribution.
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