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Joined 2026-09-21 Message
Incoming links: Fault rock
Cataclasite 2026-10-25
A cohesive fault rock produced predominantly by mechanical grain fragmentation and granular rearrangement. It differs from a loose fault breccia or fault gouge in cohesion, while sharing a brittle crushing mechanism rather than requiring bulk melting.
Fault gouge 2026-10-25
Fine-grained fault rock produced by comminution and alteration; its mineralogy and fluid state can strongly affect friction.
Geological joint 2026-10-25
An opening fracture in rock with little or no shear displacement across its faces. A geological joint is approximately normal to the least compressive principal stress when it forms in a homogeneous medium. It can remain open or become a mineral vein. Unlike a fault, a geological joint need not contain crushed fault rock.
Probably yes in connected fractures or transiently opened pore space, especially where a transform fault admits circulating seawater; a uniform water-filled porous layer several kilometres thick is not guaranteed. Porosity and permeability of a porous medium generally decrease as cracks close or seal, while ongoing faulting can reopen pathways. Fluid can occur at high pressure and temperature, including a supercritical state; it need not resemble cold surface water.
Several complementary tests address presence and origin:
- Geological: recover fresh cores or dredged fault rocks. Hydrous alteration assemblages, veins, serpentinite replacing peridotite, and altered gabbro record water-rock interaction. Sealed fluid inclusions can preserve former fluids. These establish fluid access in the past; they do not by themselves demonstrate currently connected free pore fluid.
- Geochemical: analyse sampled fluids and fluid inclusions for salinity and chlorine, oxygen and hydrogen isotope ratios, and strontium or boron isotope ratios in alteration products. Compare with seawater, host rocks, and possible magmatic or metamorphic sources. A seawater-like input modified systematically by water-rock interaction supports deep circulation from the ocean. Exchange, isotope fractionation, mixing and phase separation mean that altered fluids need not retain the exact surface composition.
- Geophysical: combine seismic tomography, Compressional-to-shear wave speed ratio and seismic attenuation with magnetotelluric sounding or controlled-source electrical measurements. A connected saline fluid network can reduce electrical resistivity and change elastic wave speeds. Sample hydrothermal vents, measure heat flux and, if drilling is possible, make direct pore pressure and permeability measurements.
No one anomaly uniquely identifies water. Seismic velocity can also reflect cracks, composition and temperature; electrical conductivity can reflect melt or conductive minerals. Serpentinization demonstrates incorporation of water into minerals, which must be distinguished from fluid currently occupying pore space. At an oceanic transform fault, an external seawater supply is the natural first hypothesis, but isotope ratios and chemical budgets can test contributions from deep fluids rather than presuming the source. A research example of coupled deformation and hydrothermal alteration is Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif by Chiara Boschi and others; a detachment system illustrates the methods without proving identical flow at every transform fault.
Within a shallow fault zone undergoing brittle fracture, repeated sliding crushes and grinds grains. Expect fault gouge, potentially rich in fine clay minerals, and coarser angular fault breccia; a cohesive, cemented crushed rock is a cataclasite. Slickensides or displaced grains can record the sliding direction.
An opening geological joint need not contain any fault rock: it may be an open crack. If fluids circulated, it can contain a mineral vein, for example calcite or quartz, grown from the fracture walls. Thus comminuted fault rock supports shear, while open space or a crack-filling mineral vein supports opening. Mineral filling is conditional on fluid chemistry and timing; its precise composition cannot be inferred from the photograph.