ALBERT

Description: This paper examines how crystalline basement thrust sheets can detach in foreland thrust belts, in terms of the deformation mechanisms and rheological evolution of the detachment fault zones. Basement thrust fault zones of the Moine Thrust Belt and the external Western Alps show relatively narrow thrust zones considering the large displacements accommodated. Microscopic examination of fault rocks from these high strain thrust zones show that syntectonic alteration of fractured feldspars to white mica of strong preferred orientation generated ultramylonites deforming by diffusion creep and other viscous deformation mechanisms, similar to documented basement thrust zones in North America. Motivated by these observations coupled with other published examinations of foreland basement thrust zones, and recent developments in crustal hydrology, a conceptual model is proposed to explain basement detachment formation and evolution. Meteoric fluid that percolated into a previously fractured upper crust is drawn into developing fault zones by dilatancy pumping during the early stages of thrust-related deformation. The generation of cataclastic fault rocks with fresh fracture surfaces by microfracturing enhances the rate of fluid-rock interaction. Syntectonic alteration causes a deformation-mechanism transition to phyllosilicate-dominated ductile fault-rock rheologies, resulting in a large ductility contrast between host rock and fault zone that inhibits growth of the zone into the wall rock and weakens the thrust. Deformation becomes focused into these weakened early thrust zones so that they become zones of high strain, preventing the development of other newer fault zones elsewhere. This model explains the detachment and continued sliding of basement thrust sheets on narrow mica-rich zones of high strain in foreland thrust belts, and suggests that reaction weakening of the basement is important in decreasing the strength of the foreland crust in orogenic wedge evolution.

Description: Detailed mapping of complex fault zones shows that secondary faults often branch off the principal slip zone. However, the effect of secondary branch faults on the hydrodynamic behaviour of fault zones has not yet been examined, largely because of a lack of hydraulic data and because numerical or analogue modelling of splay faulting is a complex issue. This contribution investigates the thermal pressurization process in cases of slip along a principal slip zone and along splay faults branching off the principal displacement zone. The study is based on porosity and permeability data presented in this paper from the principal and secondary slip zones of an active, clay-rich gouge-bearing strike-slip fault, the Usukidani fault of SW Japan. Modelling constrained by these data suggests that thermal pressurization is a viable process only as long as the rupture remains located in the central gouge zones or in mature splay fault gouge zones. Splaying of the rupture into surrounding microbreccias or into immature or newly generated splay faults of higher permeability will release fluid pressure or inhibit the generation of coseismic excess fluid pressures by thermal pressurization. The modelling results suggest that secondary fault branches can play a key role in controlling fluid pressurization during faulting. Hence, complete investigation of active fault zones needs to include secondary faults and their corresponding hydraulic behaviour, in order to establish the influence of such structures on earthquake mechanics.

Description: This paper examines the role of mechanical stratigraphy on the evolution of normal fault geometry and fault zone internal structure, using a well-exposed normal fault system from the Permian Lodeve Basin, southern France. Faults formed early during the syn-deformation tilting history of the basin tend to have steeper segments in the competent sandstone layers due to refraction, assisted by pre-existing early bedding-perpendicular joints, where displacement remained on the order of bed thickness. Faults which continued to slip during tilting have a more complex structure of splays due both to the space incompatability problem of slip at fault bends of this irregular geometry, and because tilting favours the generation of new splays at a different angle to the earlier faults experiencing rotation. Continued deformation between faults and their splays often causes both distributed deformation in between the two, and reconnection of splays to the main fault forming isolated lenses. Thus, fault zone complexity increases greatly as slip exceeds competent bed thickness, owing both to the presence of the mechanical layering, and the fact that this layering is being tilted.

Description: It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth's crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity-permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity-strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

Description: Descriptions of structural evolution across thrust belts commonly assume a transition from ductile to brittle deformation, reflecting a progressive reduction in temperature accompanying exhumation. The universality of this model is challenged here using field relationships at Ben Arnaboll, in the northern part of the Moine Thrust Belt. Deformation in the Arnaboll Thrust Sheet, an allochthonous basement body of amphibolite-facies gneisses and pegmatite sheets, carried onto Cambrian sediments, includes widely distributed, low-displacement shears developed under greenschist facies with ingress of water. These ductile deformations post-date the emplacement of the thrust sheet as they link kinematically to breaching thrust structures emanating from the footwall of the Arnaboll Thrust. The thrust itself records a transition from mylonitic (ductile) to strongly localized (brittle) deformation that pre-dates the breaching thrusts and therefore the deformation within the thrust sheet itself. The structure of breaching thrusts charts an up-dip transition from localized slip to distributed shearing analogous to the trishear in fold-thrust complexes, Therefore deformation of the Arnaboll Thrust Sheet shows a return from strongly localized translation-dominated brittle deformation to more broadly distributed ductile deformation. This is likely to have been promoted by the ingress of water and the concomitant reaction-enhanced weakening of the basement.