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A Brittle Failure Mode Plot Defining Conditions for High-Flux Flow

Richard H. Sibson

Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand

E-mail, rick.sibson{at}stonebow.otago.ac.nz

Stress and fluid-pressure conditions for the initial formation within intact rock of faults (shear fractures), extension fractures, and extensional shears, and for the reshear of existing faults, may all be represented on a generic failure plot of differential stress (₁ – ₃) versus least effective compressive stress, ₃' = (₃ – P_f), scaled to nominal tensile strength, T (~ half the cohesive strength). Plots of this kind may be used to define maximum sustainable overpressure in different tectonic environments and the structural conditions under which the flow of large fluid volumes may occur through fault-fracture meshes containing gaping extension and extensional shear fractures. They may also be adapted to different tectonic regimes and correlated to depth for particular fluid-pressure conditions. High-flux flow through distributed fault-fracture meshes requires the tensile overpressure condition, ₃' < 0, to be met (generally involving P_f > ₃), which can only be achieved in the absence of throughgoing cohesionless faults that are well oriented for frictional reactivation in the prevailing stress field. High-flux flow through distributed fault-fracture meshes, intrinsically a transient, pulsing phenomenon, may therefore occur as follows: (1) in effectively intact low-pemeability crust devoid of throughgoing favorably oriented faults, (2) where existing faults have become severely misoriented in the prevailing stress field, and (3) where existing faults have regained cohesive strength through hydrothermal cementation.

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