Stress in the Earth's crust: the link between crustal fluids and extensive-dilatancy anisotropy

Abstract

Observations of seismic shear-wave splitting (birefringence) along almost all raypaths in the Earth's crust have led to the recognition that the fluid-filled cracks, microcracks and pores known to pervade most rocks in the crust have been preferentially aligned by the prevailing stress-field. This phenomenon is known as extensive-dilatancy anisotropy or EDA, and the fluid-filled microstructures as EDA-cracks because the seismic effects can be modelled by distributions of parallel cracks. The effects of these stress-aligned EDA-cracks have been observed in many different areas, and they appear ubiquitous in the upper 10 to 20 km of the crust. Many details of the crack geometry and the current stress-field can be readily determined from records of the shear-waves which have propagated through these aligned cracks. In separate studies, geochemists have confirmed the presence of microscopic fluid-filled cavities in almost all geological material. These are referred to as fluid inclusions, and provide unequivocal evidence of crustal fluid circulation, often along stress-controlled fractures. Different stress episodes can be recognized in many rock samples and studies of fluid inclusions can be used to interpret the palaeo-stress history of a rock, where the in situ morphology of these inclusions is related to the current regional stress-field. This paper discusses the relationship between the fluid inclusions of the geochemical literature and EDA-cracks, and concludes that the former may be regarded as one of the constituent inclusion types of extensive-dilatancy anisotropy.