The nature and tectonic significance of fault-zone weakening: an introduction

Abstract

Fault zones control the location, architecture and evolution of a broad range of geological features, act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards. In general, the repeated localization of displacements along faults and shear zones, often over very long time scales, strongly suggests that they are weak relative to their surrounding wall rocks. Geophysical observations from plate boundary faults such as the San Andreas fault additionally suggest that this fault zone is weak in an absolute sense, although this remains a controversial issue. Our understanding of fault-zone structure and mechanical behaviour derive from three main sources of information: (1) studies of natural fault zones and their deformation products (fault rocks); (2) seismological and neotectonic studies of currently active natural fault systems; (3) laboratory-based deformation experiments using rocks or rock-analogue materials. These provide us with a basic understanding of brittle faulting in the upper crust of the Earth where the stress state is limited by the frictional strength of networks of faults under the prevailing fluid-pressure conditions. Under the long-term loading conditions typical of geological fault zones, poorly understood phenomena such as subcritical crack growth in fracture process zones are likely to be of major importance in controlling both fault growth and strength. Grain-size reduction in highly strained fault rocks produced in the plastic-viscous and deeper parts of frictional regime can lead to changes in deformation mechanisms and relative weakening that can account for the localization of deformation and repeated reactivation of crustal faults. Our understanding the interactions between deformation mechanisms, metamorphic processes and the flow of chemically active fluids is a key area for future study. An improved understanding of how fault-or shear-zone linkages, strength and microstructure evolve over large changes in finite strain will ultimately lead to the development of geologically more realistic numerical models of lithosphere deformation that incorporate displacements concentrated into narrow, weaker fault zones.In continental and oceanic regions, the deformation of the Earth's crust (and lithosphere) is characteristically heterogeneous, with most displacements being localized into linked systems of faults and shear zones. In both intraplate and plate margin settings, these approximately planar or tabular deformation zones influence strongly the location, architecture and evolution of a broad range of geological features, including rift basins, orogenic belts and transcurrent fault systems. Many fault zones are known to act as conduits for the focused migration of fluids and clearly play a central role in determining the location, modes of transport and emplacement of economically important hydrocarbon reservoirs, hydrothermal mineral d...

Tectonic evolution of the Ganos segment of the North Anatolian Fault (NW Turkey)

“…The angle between the σ 1 orientation and the strike of the GF is <45° suggesting that 505 the fault is not anomalously weak (Rutter et al 2001). At local (sub-area) scale, the maximum 506 principal stress σ 1 varies between NW-SE and E-W. 507…”

Continental reactivation and reworking: an introduction

“…Thus, many continental fault zones have very long-lived movement histories (e.g. see Butler et al 1997;Rutter et al 2001 and associated papers).…”

“…It is well established that many faults exhibit stick-slip behaviour, with short-lived periods of inactivity (c. 0.01 Ma) within what is essentially a continuous slip history (e.g. Rutter et al 2001). In this case, reactivation can be viewed as being the accommodation of displacement along a structure formed during an earlier tectonic regime .…”

Subsurface Structure, Physical Properties, and Fault Zone Characteristics in the Scientific Drill Holes of Taiwan Chelungpu-Fault Drilling Project

“…On the other hand, the presence of clay-rich fault gouges (Wintsch et al 1995;Vrolijk and van der Pluijm 1999;Warr and Cox 2001) and/or permanently high pore pressure (Hubbert and Rubey 1959;Suppe and Wittke 1977) within the fault zone can effectively reduce fault strength and the coefficient of friction . Other solution-transport mechanisms such as pressure solution and fluid-assisted mineral reactions may be important to determine the rheology of fault zones and the time scale of inter-seismic strength recovery (Blanpied et al 1998;Rutter et al 2001). The role of these mechanisms in determining the fault strength and earthquake instability mechanisms are unknown because of uncertainties regarding the mineralogy, microstructures and physical properties of fault zone materials and the nature and distribution of fluids at focal depths.…”