Low displacement fault zones often comprise a mesh of
interlinked faults, extensional-shear and purely extensional vein fractures.
Hydrothermal mineralisation localised around such structures testifies to their
importance as conduits for fluid flow. Mesh structures are most commonly
developed in situations approximating bulk pure shear but predominantly simple
shear meshes also occur. They appear to develop through the infiltration of
pressurised fluids into a heterogeneous rock mass with varying material
properties. In some circumstances, mesh formation appears to be a precursor to
the development of major through going faults. Mesh formation generally involves
hydrofracturing (Pf > 3, at least locally) and the
condition Pf ~ 3 to be maintained for the mesh to remain a high
permeability conduit, requiring fluid over pressuring at other than shallow
depths in extensional-transtensional regimes. The volumetric character of
earthquake swarm activity appears to result from the passage of hydrothermal
fluids through mesh structures, representing a form of distributed fault-valve
action along suprahydrostatic hydraulic gradients arising from magmatic
intrusion, compaction overpressuring, metamorphic dewatering, etc.
Fluid redistribution in the crust is influenced by
maximum hydraulic gradient (not necessarily vertical), by existing permeability
anisotropy arising from bedding and other forms of crustal layering, and by
structural permeability developed under the prevailing stress field. Strong
directional permeability develops in the 2 direction, parallel to
fault-fracture intersections within the mesh and orthogonal to fault slip
vectors. In particular tectonic settings, stress-controlled permeability allows
strongly focused flow through fault-fracture meshes with high potential for
mineralisation. Favoured localities for mesh development include short-lived
link structures along large-displacement fault zones such as dilational jogs,
lateral ramps, and transfer faults.