IA )W cli',hla cn1cnt fault 7011(-s 0 11Cn cOrnprise a nrc,II of inrcIIinkc(I faults, cvtcnqonal shear and In]rck cxtcnsir>nal 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 throughgoing faults. Mesh formation generally involves hydrofracturing (Pf> a3, at least locally)
and the condition Pf - 63 to be maintained for the mesh to remain a high permeability conduit, requiring fluid
overpressuring 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.