Optimally oriented “fault‐valve” thrusts: Evidence for aftershock‐related fluid pressure pulses?

A thrust‐vein network from the Triumph gold deposit, Western Australia, is explained in terms of an extremely high rate of fluid‐pressure increase, prior to failure, relative to the rate of stress increase. The thrust fault is a small‐displacement fault characterized by a thick, fault‐parallel shear vein, plus multiple low‐angle extension veins, with orientations that demonstrate the thrust was optimally oriented relative to the locally imposed crustal stresses. Large extension veins have irregular margins, are dominantly composed of coarse milky quartz with no obvious laminations or solid inclusion trails, and are regularly spaced along the thrust (1–2 m). The fault‐vein geometries indicate the Triumph thrust is a rare candidate for “fault‐valve” failure of an optimally oriented thrust, and it is possible the structure formed in a small number of failure events, during load weakening of the thrust. An analysis using the Coulomb criterion shows that load weakening of a thrust occurs when fluid pressure increases relative to tectonic stress by a factor dependent on the orientation of the thrust. Thrust and reverse faults in dry crust load strengthen prior to failure, but the poroelastic behavior of sealed, fluid‐saturated crust is enough to induce load weakening in compressive environments; thus poroelastic load weakening is expected to be an important failure mechanism in hydrothermal environments. However, in the case of the Triumph thrust, dilatant shear failure necessitates a fluid pressure increase which is an order of magnitude larger still. The observations and results are consistent with a pulse of high fluid pressure migrating up through fault or fracture networks that have elevated permeability relative to the wall rock, under conditions of transiently low differential stress. Fluid pressure differences resulted between the fault and wall rock, leading to extension fracture and fault failure. Such conditions may occur when adjacent large earthquakes induce damage, breach overpressured fluid reservoirs, and generate fluid‐pressure driven aftershocks.