Fault Zone Processes Research Articles

Are feldspar-to-mica reactions necessarily reaction-softening processes in fault zones?

Reaction-softening by mineralogical changes from feldspars to sericite has been documented from many fault zones. During external crystalline basement deformation in the Alpine orogeny, the Ser Barbier thrust and splay faults in the Pelvoux Massif experienced ultracataclasis and sericitisation. Microstructural information and geochemical data from the fault rocks suggest that different muscovitisation reactions occurred at different times within the evolution of the fault zone, and each reaction had its own impact on fault rheology. Early cataclasis aided chemical breakdown of orthoclase feldspars to muscovite, yet quartz release accompanying this process resulted in local cementation and consequent hardening of the ultracataclasite. Continued deformation was accompanied by muscovitisation of the albite feldspar, and resulted in the formation of mica-rich fault rocks which experienced progressive silica removal by the fluid with increasing deformation. At this stage, reaction-enhanced ductility dominated. Much of the early cemented ultracataclasites escaped later deformation, and their low permeability allowed preservation of their early geochemical characteristics by preventing later fluid access. Such findings demonstrate how the complex interplay between deformation processes and geochemical reactions may result in a changing rheology during fault zone evolution.

The recognition of reactivation during continental deformation

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

Introduction to Special Section: Mechanical Involvement of Fluids in Faulting

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects.The United States Geological Survey sponsored a Conference on the Mechanical Effects of Fluids in Faulting under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, California, from June 6 to 10, 1993. The purpose of the conference was to draw together and to evaluate the disparate evidence for the involvement of fluids in faulting; to establish communication on the importance of fluids in the mechanics of faulting between the different disciplines concerned with fault zone processes; and to help define future critical investigations, experiments, and observational procedures for evaluating the role of fluids in faulting. This conference drew together a diverse group of 45 scientists, with expertise in electrical and magnetic methods, geochemistry, hydrology, ore deposits, rock mechanics, seismology, and structural geology. Some of the outstanding questions addressed at this workshop included the following:1. What are fluid pressures at different levels within seismically active fault zones? Do they remain hydrostatic throughout the full depth extent of the seismogenic regime, or are they generally superhydrostatic at depths in excess of a few kilometers?2. Are fluid pressures at depth within fault zones constant through an earthquake cycle, or are they time‐dependent? What is the spatial variability in fluid pressures?3. What is the role of crustal fluids in the overall process of stress accumulation, release, and transfer during the earthquake cycle? Through what mechanisms might fluid pressure act to control the processes of rupture nucleation, propagation, and arrest?4. What is the chemical role of fluids in facilitating fault creep, including their role in aiding solid‐state creep and brittle fracture processes and in facilitating solution‐transport deformation mechanisms?5. What are the chemical effects of aqueous fluids on constitutive response, fractional stability, and long‐term fault strength?6. What are the compositions and physical properties of faultfluids at different crustal levels?7. What are the mechanisms by which porosity and permeability are either created or destroyed in the middle to lower crust? What factors control the rates of these processes? How should these effects be incorporated into models of time‐dependent fluid transport in fault zones?8. What roles do faults play in distributing fluids in the crust and in altering pressure domains? In other words, when and by what mechanisms do faults aid in or inhibit fluid migration? What are the typical fluid/rock ratios, flow rates, and discharges for fault zones acting as fluid conduits?9. Are fluids present in the subseismogenic crust, and by what transformation and/or transport processes are they incorporated into the shallower seismogenic portions of faults?

Multimechanism friction constitutive model for ultrafine quartz gouge at hypocentral conditions

Occurrence of instability in crustal faults depends in part on the small‐magnitude dependence of frictional strength on slip rate and slip history. Rate dependence of friction reflects the operation of thermally activated mechanisms at points of contact along fault surfaces and is expected to change in space and time owing to variations in environmental conditions and slip rates during the seismic cycle. Several lines of evidence suggest solution‐precipitation processes in fault zones may be activated during interseismic periods when slip rates are small and may contribute to fault healing. We develop a constitutive model for faulting at hypocentral conditions that is capable of describing the variation in frictional properties as different slip mechanisms are activated in response to changes in temperature or slip rate. This model is based on the assumption that slip mechanisms are thermally activated and follow an Arrhenius relationship between temperature and slip rate, which allows the addition of temperature dependence to existing rate‐ and state‐dependent friction constitutive laws. Multiple slip mechanisms are treated as operating independently and concurrently, where each mechanism is described by the rate‐, state‐, and temperature‐dependent friction constitutive relation. The constitutive model is used to analyze triaxial friction experiments on ultrafine‐grained quartz gouge at temperatures to 600°C, effective confining pressure of 150 MPa, and water‐saturated or room‐dry conditions. These experiments investigated the stress relaxation response and slip history effects during slide‐hold‐slide tests with hold times up to 105s. The microstructure of the deformed quartz gouge and the transient friction behavior define at least two distinct frictional slip regimes: a low‐temperature regime characterized by cataclastic mechanisms with significant slip history effects, and a high‐temperature regime characterized by solution‐precipitation‐aided cataclastic flow with large‐magnitude rate dependence and insignificant slip history effects. In the model the parameters of the friction constitutive relation (e.g., a, b, and L) are treated as constants for each slip mechanism but are different for the different mechanisms. This model accurately describes the frictional behavior within each regime and across the transition between regimes. The analysis suggests that the greatest‐magnitude rate weakening behavior occurs at 100° to 300°C under wet conditions at laboratory slip rates. Significant solution‐precipitation is activated at temperatures above 300°C at laboratory slip rates or at lower slip‐rates and lower temperatures. The high‐temperature solution‐precipitation regime is described by a large‐magnitude rate strengthening (a − b = 0.03) and an apparent activation energy of approximately 44 kJ mol−1. The constitutive analysis suggests that the solution‐precipitation‐aided flow mechanism could be important during interseismic periods at hypocentral conditions and low shear stress but apparently is not characterized by significant slip history effects.

Cathodoluminescence observations on low-temperature mylonites: potential for detection of solution-precipitation microstructures

This note represents cathodoluminescence (CL) images, obtained using a scanning electron microscope, of a low-temperature granitic mylonite from central Japan. It is argued that CL observations offer a simple but powerful tool for elucidating the importance of solution-transfer processes during mylonitization. Cataclastic deformation such as fracturing of feldspars, accompanied by solution-precipitation processes, were found to be much more clearly observable in CL images than using other techniques. It is also shown that quartz grains precipitated from solution can be distinguished from original quartz grains in the granite, even after being deformed plastically. More detailed CL work on mylonites is needed in the future, since quantitative evaluation of the role of solution-precipitation processes in deep fault zones is essential for constraining the rheological properties of faults at depth.

Tectonic stress orientation and the seismic regime of a single fault

The seismic regime of tectonic faults was studied by the method of physical modelling. This method is based on the assumption that the physical processes in actual seismo-active fault zones can be scaled in laboratory conditions. The results of our laboratory experiments on plate models proved the great importance of tectonic stress orientation on the seismic regime of tectonic faults. Changes in the fault inclination led to distinct changes both in the recurrence time intervals between individual seismic events and in the amount of seismic energy released. For some fault inclinations the seismic activity of faults vanished altogether. Only a limited zone in the fault plane was found to participate in the process of seismic energy release. The extent of this so-called seismo-active zone depends not only on the contact conditions, but also on the value of the acting load and on the fault inclination; this presumption is supported by the fact that for each inclination there exists an upper limit to the seismic energy that can be released in one seismic event.

Brecciation processes in fault zones: Inferences from earthquake rupturing

Surface-rupture patterns and aftershock distributions accompanying moderate to large shallow earthquakes reveal a residual brittle infrastructure for established crustal fault zones, the complexity of which is likely to be largely scale-invariant. In relation to such an infrastructure, continued displacement along a particular master fault may involve three dominant mechanical processes of rock brecciation: (a)attrition brecciation, from progressive frictional wear along principal slip surfaces during both seismic and aseismic sliding, (b)distributed crush brecciation, involving microfracturing over broad regions when slip on the principal slip surfaces is impeded by antidilational jogs or other obstructions, and (c)implosion brecciation, associated with the sudden creation of void space and fluid-pressure differentials at dilational fault jogs during earthquake rupture propagation. These last, high-dilation breccias are particularly favorable sites for hydrothermal mineral deposition, forming transitory low-pressure channels for the rapid passage of hydrothermal fluids. Long-lived fault zones often contain an intermingling of breccias derived from all three processes.