Fluid flow in mid-to deep crustal shear systems: Experimental constraints, observations on exhumed high fluid flux shear systems, and implications for seismogenic processes

Abstract

The aseismic parts of shear systems at mid-to deep crustal levels can localise the supply of deeply-sourced, high pressure fluids into the shallower level parts of these systems in the seismogenic regime. Even during deformation at elevated temperatures in mid-to deep crustal shear zones, high pore fluid factors promote grain-scale to macroscopic fracture growth and permeability enhancement. The evolution of permeability is governed by dynamic competition between crack growth and crack sealing/healing processes. Steady state creep below the seismic-aseismic transition leads to steady state permeability and continuous fluid flow. In contrast, within and near the base of the seismogenic regime, large cyclic changes in permeability can lead to episodic fluid flow and fluctuations in fluid pressure. At mid-crustal depths, temporal and spatial variations in pore fluid pressure and shear stress within shear networks influence rupture nucleation via cyclic changes in shear strength. Fluid pressure and shear stress cycling can also drive repeated transitions between interseismic creep and rapid, co-seismic slip. Reaction-weakening and reaction-strengthening, during hydrothermal alteration in fluid-active shear systems, can also drive transitions between seismic and aseismic behaviour on longer time-scales.