The edge of failure: critical stress overpressure states in different tectonic regimes
Earth’s seismogenic crust is partitioned into the three Andersonian stress domains critically organized to the edge of failure both along plate boundaries and within plate interiors. Brittle/frictional failure in rocks (the formation and reactivation of faults and fractures) may be induced by two principal drivers: increasing differential stress (σ1–σ3) and/or pore fluid pressure, Pf, defined relative to vertical stress by λv = Pf/σv. Borehole measurements suggest the presence of hydrostatic-Byerlee conditions (the stress governed by the frictional strength of optimally oriented faults with Byerlee friction (0.6 < μs < 0.85) under hydrostatic fluid pressure), sometimes postulated as the standard state for fractured seismogenic crust with a bulk permeability too high to allow fluid overpressuring. However, especially in areas of crust undergoing shortening and fluid release under compression, pore fluids are likely to be overpressured above the hydrostatic pressure (i.e. λv > 0.4) in the lower seismogenic zone (c. T > 200°C), where hydrothermal circulation and cementation reduces fracture permeability. In such regions, critical stress overpressure states prevail with differential stress inversely related to the degree of fluid overpressuring. Failure criteria on plots of (σ1–σ3) v. λv constructed for particular depths can be used to explore critical stress overpressure states, loading paths to failure and potential mineralizing scenarios in different settings.