Mechanics, structure and evolution of fault zones

SM2.1/NH4.7/TS2.7 Mechanics, structure and evolution of fault zones The mechanics of fault zones is an open subject of central importance in better understanding the seismic cycle. Fault zones have a complex internal structure, suggesting mechanical behaviour that spans a wide range of spatio-temporal scales: from gouge particle size distribution to outcrop scale roughness to large scale fault segmentation, and from rapid dynamic rupture events to the tectonic loading time scale. The structure of fault zones is constantly evolving with time and accumulated slip. Damage processes, such as fragmentation of host rock, intense cataclasis of fault gouge in preferred localized zones and asperity deformation, compete with time dependent healing processes that recover mechanical properties, and tectonic loading dynamics that ultimately drive the system. The presence of fluid in some faults can also play a role on their mechanics and the evolution of their structure, on heat and mass transport, and on healing processes. The interplay of these complex components and their evolution at all scales represent the basic ingredients of fault mechanics. This session will focus on the mechanics and structural expression of processes operating in active fault zones including: effects of fluids and/or of heat transport and heat generation, morphology of asperities, role of developing damage, dynamics of granular gouges, field observations of seismic slip criteria in outcrops or in boreholes. Contributions on the scaling laws describing gouge particle size, fault morphology or slip distribution and processes of strain localization are also welcome. We invite contributions from field geologists, laboratory experimentalists, computer modelers and theoreticians. TS2.2 Deformation mechanisms, microstructures, and frictional properties of upper-crustal fault zones Tectonic faults in the brittle part of the Earth’s lithosphere are typically complex zones of fracturing and fluid-rock interactions. Fault zone structure and evolution are closely linked to fault slip behavior and permeability throughout the seismic cycle, including the nucleation, propagation and arrest of earthquake ruptures. Of particular importance are the frictional properties of constituent fault rock materials and the effects of thermally- and chemically-activated processes on the evolution of frictional strength, both on short (dynamic) and long (static) timescales. We welcome field, microstructural, and laboratory contributions addressing the relationships between deformation mechanisms, friction, and the rheology and structure of fault zones in the upper crust. Key topics to be addressed include: • Linking fault zone microstructure to fault zone mechanics • Interactions between mechanical and chemical processes in the evolution of friction and permeability • The importance of fault rock fabrics and fault rock distribution • Dynamic and static fault weakening mechanisms TS2.3 Fault zone properties and growth mechanisms in the upper crust: insights from field, laboratory and modelling studies Understanding of fault zone properties in different geological contexts is important from a scientific point of view, to better assess the processes of fault nucleation and subsequent fault development, as well as for the exploration and management of fluid-related geo-resources. Over a wide range of scales, geologists employ several methodologies to assess the geometry, kinematics, mechanics and spatial properties of fault zones. Large-scale deformation is typically assessed by the interpretation of subsurface geophysical data, whereas at an outcrop-scale it is studied directly in the field or, alternatively, from well cores and/or well logs. Thin-section scale deformation, on the contrary, is analyzed by optical microscopy, SEM and microprobe investigation of key microstructures. In the laboratory, experimental studies are conducted on natural and/or artificial samples to quantify both petrophysical and mechanical fault zone properties. The results of these studies are often integrated to assess the fault architecture and, hence, the control exerted by fault zones on subsurface fluid flow. On this regard, a great contribution has been provided by analogue and numerical modelling in layered and massive media. All these aspects require a good understanding of the specific factors controlling the faulting processes (i.e. pressure-temperature conditions, fluid content, etc.) and, possibly, of the structural complexities and heterogeneities present within individual fault zones (i.e. inherited structural features, linkage processes, etc.). To address the aforementioned topics, this session aims to discuss the (1) short- and long-term processes related, at all scales, to the initiation and growth of individual faults and fault systems, and (2) the impact of these processes on porosity, permeability and overall transmissibility of fault zones. We encourage contributions from various disciplines in order to fuel the integration of field, laboratory and modelling studies to address the multi-scale relationships among deformation mechanisms, fault architecture and fault hydraulic properties. Solicited speakers: S. Abe, N. Brantut, L. Goren, B. Grasemann, S. Miller, D. Moore, B.A. Van der Pluijm, C. Viti