Fault-Segment Rupture, Aftershock-Zone Fluid Flow, and Mineralization (original) (raw)
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The where and how of faults, fluids and permeability –
Fault stepovers are features where the main trace of a fault steps from one segment to the next in either an underlapping or overlapping manner. Stepovers exert a critical influence on crustal permeability and are known to control phenomena such as the migration of hydrocarbons and the location of geothermal fields. In the Kalgoorlie-Ora Banda greenstone district, Western Australia, we demonstrate a spatial association between stepovers and gold deposits. It is shown that although underlapping stepover geometries are typically rare in fault systems, they are anomalously associated with gold deposits. Further, the along-strike and across-strike dimensions of both underlapping and overlapping fault stepovers fit, to a first-order approximation, the same self-similar trend. Boundary element modelling of Coulomb failure stress changes is used to explain these observations in terms of damage generated by rupture events on the bounding fault segments and associated aftershock sequences. Our models indicate that a larger region of damage and permeability enhancement is created around underlapping stepovers than around overlapping stepovers. By taking into account both the enhancement and decay of permeability during the seismic cycle, it is estimated that a 5 Moz goldfield could feasibly form in 1–16 earthquake-aftershock sequences, potentially representing durations of just 10–8000 years. The existence of supergiant gold deposits is evidence that crustal permeability attains transiently high values on the order of 10 12 m2. It should be expected that transient and time-integrated permeability values have a distinct threedimensional structure in continental crust due to stepover-related channels.
Earth and Planetary Science Letters, 2006
In strike-slip fault systems, Coulomb failure stress changes due to mainshocks can trigger large aftershocks or further earthquakes. The combination of static stress changes from mainshocks and large aftershocks potentially has a profound influence on the final distribution of aftershocks and crustal-scale fluid redistribution. Because mineralization acts as a high fluid flux indicator the interaction of static stress changes, fault triggering and fluid flow can be studied from mineralized fossil fault systems. Two examples are presented from separate fault systems in the Kalgoorlie greenstone terrane,Western Australia (the Black Flag and Boulder-Lefroy Fault systems). Using mapped fault geometries, slip directions and the known distribution of fault-hosted gold mineralization we show that the repeated arrest of mainshock ruptures, at both dilational and contractional fault step-overs, controlled aftershock-related fluid flow. Importantly, the largest aftershocks or subsequent triggered earthquakes exerted a very strong control on where the highest fluid fluxes occurred through small-event aftershock fault networks (at distances up to ∼15 km away from the step-overs). Fluid flow through mid-crustal fault systems in crystalline rock is spatially localised in regions where repeated clusters of aftershocks cause permeability enhancement. It is dependent on the seismogenic behaviour of the system, rather than a passive exploitation of the internal structure and fabrics developed by faults or damage zones. Field evidence implies that high pore fluid factors were repeatedly attained in the aftershock-related mineralized faults and that the fluids were derived from deep-level, overpressured reservoirs, rather than local wall rock porosity. It is apparent that high-pressure fluids, possibly released in a pulse after a mainshock, contribute to the rupture of structures already promoted towards failure from static stress changes.
Geofluids
Fault stepovers are features where the main trace of a fault steps from one segment to the next in either an underlapping or overlapping manner. Stepovers exert a critical influence on crustal permeability and are known to control phenomena such as the migration of hydrocarbons and the location of geothermal fields. In the Kalgoorlie-Ora Banda greenstone district, Western Australia, we demonstrate a spatial association between stepovers and gold deposits. It is shown that although underlapping stepover geometries are typically rare in fault systems, they are anomalously associated with gold deposits. Further, the along-strike and across-strike dimensions of both underlapping and overlapping fault stepovers fit, to a first-order approximation, the same self-similar trend. Boundary element modelling of Coulomb failure stress changes is used to explain these observations in terms of damage generated by rupture events on the bounding fault segments and associated aftershock sequences. O...
Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand
Geology, 2012
Rock damage during earthquake slip affects fl uid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the fi rst two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200-400 yr earthquake cycle, reveal a >50-m-thick "alteration zone" formed by fl uid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-fi lled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fl uid pressure measured across this zone confi rms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10-20 m 2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ~10-14 m 2 , implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confi rmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fl uid fl ow and sites of high fl uid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.
Active Fault and Shear Processes and Their Implications for Mineral Deposit Formation and Discovery
Journal of Structural Geology, 2010
Mineralisation associated with fault, vein and shear zone systems can be related to processes that operated when those systems were active. Despite the complexity of processes that operate in faults, veins and shear zones, there are typically systematic patterns in geometry (e.g. segmentation and stepovers) and scaling, which are the cumulative result of multiple slip events. In turn, there are systematic patterns in individual slip events (e.g. earthquake-aftershock sequences, shear zone creep transients,earthquake swarms) with implications for permeability enhancement and mineral deposit formation. This review identifies three avenues for future research: (1) a need to improve constraints on the scaling characteristics of faults, shear zones and veins specifically related to mineralisation. (2) The integration of stress change and damage concepts with 3-D lithological observations and reactive transport modelling. (3) Understanding the impact of multiphase fluids (e.g. H2O–CO2–NaCl fluids) on fault mechanics and permeability. Static stress change modelling, damage mechanics modelling and fault/vein scaling concepts have promising predictive capabilities for the future discovery of mineral deposits. The review mostly refers to epithermal, mesothermal, and carlin-type gold deposits, but the principles could extend to any hydrothermal mineral deposit formed during faulting, fracturing and shearing.
Journal of Structural Geology, 2009
The geometries, kinematics, and failure mechanisms of epithermal fault-vein networks are examined at the Cracow goldfield, Queensland, Australia, and compared with observations of active geothermal areas. Quartz–carbonate cementation and precious metal mineralisation is confined to a network of steeply dipping faults (>70). Breccia textures indicate fault rock formed from repeated events, involving large components of dilation during fracture, wall-rock fragmentation, and mineral precipitation. New fault rock tended to form on the margins of pre-existing fault rock, generating thick zones up to 10 m wide (fault cores) and accumulating up to 300 m normal offsets. The fault-vein networks are segmented, corrugated over tens of metres along strike, and contain complex fracture networks in step-over zones. Mineralisation is not associated with any specific fault location, but occurs along planar segments of faults, in step-over zones, at fault tips and where fault dips change. The wall rocks surrounding the faults contain networks of shear and extension veins, with a large range of orientations. Furthermore, kinematic indicators on the faults are broadly normal dip-slip, but vary to oblique and strike-slip with no obvious relationship to geometry or location along the fault. Inconsistent fault kinematics and the range of wall rock vein orientations are attributed to transient changes in stress state, due to intrusion of nearby dykes. As a result, permeability would have been enhanced by dilatancy in fault rock and wall rock fractures, when corrugated normal faults temporarily failed in oblique or strike-slip events. Other permeability enhancement mechanisms likely included failure driven by high fluid pressures, and failure where faults steepen (near the Earth’s surface or at jogs). Mineralisation was associated with repeated, transient pulses of fluid flow rather than a steady-state process.
Mineral precipitation as a mechanism of fault core growth
Journal of Structural Geology, 2020
Faults vary in structural style, from simple planes to complex systems composed of fault cores and damage zones. Increased fault complexity results from the interaction of mechanical and chemical processes, including fracture growth, shear, and linkage, and mineral dissolution and precipitation. Although water-rock interaction is traditionally associated with fault rock weakening and shear localization, we investigate processes of fault core widening by water-rock interactions that resulted in quartz precipitation. We combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the Dixie Comstock epithermal gold deposit, Nevada, USA. Mineralized portions of the fault contain strong, thick, silicified fault cores and wide, weak damage zones, with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement, dilation, and cementation. We present a model of fault zone evolution in which the hydrothermal regimes favoring either alterationweakening or precipitation-strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems. Alteration-weakening favors localization of the fault into thinner, clay-rich, low permeability fault cores. Precipitation-strengthening promotes thick, strong, and low permeability fault cores, with mineralization-embrittlement enhancing transient permeability following coseismic failure.
Damage and permeability around faults: Implications for mineralization
Mineral deposits are commonly hosted by small-displacement structures around jogs in major faults, but they are rarely hosted by the major fault itself. This relationship may be explained by time-dependent fracturing and healing in and around major faults and associated permeability evolution. A damage mechanics formulation is used here to explore the spatial-temporal evolution of damage in and around a fault following a fault-slip event. We show that regions of increased damage rate correspond to the location of mineral deposits and that these areas correspond to areas of aftershocks predicted by stress-transfer modeling. The fault itself enters a healing regime following the slip event; hence, it is expected to become less permeable than the fracture network outside the fault. Our results support the hypothesis that mineralization occurs in a fracture network associated with aftershocks; this may be due to the higher time-integrated permeability of the fracture network relative to the main fault.
Damage, Stress Transfer and Mineralisation Around Major Faults
Mineral deposits are commonly hosted by small-displacement structures around jogs in major faults, but they are rarely hosted by the major fault itself. This relationship may be explained by time-dependent fracturing and healing in and around major faults and associated permeability evolution. A damage mechanics formulation is used here to explore the spatial-temporal evolution of damage in and around a fault following a fault-slip event. We show that regions of increased damage rate correspond to the location of mineral deposits and that these areas correspond to areas of aftershocks predicted by stress-transfer modeling. The fault itself enters a healing regime following the slip event; hence, it is expected to become less permeable than the fracture network outside the fault. Our results support the hypothesis that mineralization occurs in a fracture network associated with aftershocks; this may be due to the higher time-integrated permeability of the fracture network relative to the main fault.