Fluid overpressures and strength of the sedimentary upper crust

The classic crustal strength-depth profile based on rock mechanics predicts a brittle strength sigma(1) - sigma(3) = kappa((rho) over bar gz - P-f) that increases linearly with depth as a consequence of [1] the intrinsic brittle pressure dependence kappa plus [2] an assumption of hydrostatic pore-fluid pressure, P-f = rho(w)gz. Many deep borehole stress data agree with a critical state of failure of this form. In contrast, fluid pressures greater than hydrostatic (rho) over bar gz > P-f > rho(w)gz are normally observed in clastic continental margins and shale-rich mountain belts. Therefore we explore the predicted shapes of strength-depth profiles using data from overpressured regions, especially those dominated by the widespread disequilibrium-compaction mechanism, in which fluid pressures are hydrostatic above the fluid-retention depth z(FRD) and overpressured below, increasing parallel to the lithostatic gradient (rho) over bar gz. Both brittle crustal strength and frictional fault strength below the z(FRD) must be constant with depth because effective stress ((rho) over bar gz - P-f) is constant, in contrast with the classic linearly increasing profile. Borehole stress and fluid-pressure measurements in several overpressured deforming continental margins agree with this constant-strength prediction, with the same pressure-dependence kappa as the overlying hydrostatic strata. The role of z(FRD) in critical-taper wedge mechanics and jointing is illustrated. The constant-strength approximation is more appropriate for overpressured crust than classic linearly increasing models. (C) 2014 The Author. Published by Elsevier Ltd.