Magnetotelluric Studies at the San Andreas Fault Zone: Implications for the Role of Fluids

Abstract

Fluids residing in interconnected porosity networks have a significant weakening effect on rheology of rocks and can strongly influence deformation along fault zones. The magnetotelluric (MT) technqiue is sensitive to interconnected fluid networks, and can image these zones on crustal and upper mantle scales. MT has imaged several prominent electrically conductive anomalies at the San Andreas fault which have been attributed to the presence of saline fluids within such networks, and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle portion of the crust, and where these fluids are kept at high pressures, fault creep is supported. In response to fluid flux from deeper levels in combination with meteoric and crustal metamorphic fluid supply, and in response to fault creep, high-conductivity zones develop as fault zone conductors in the brittle portion of crust. In turn, the absence of crustal fluid pathays may be characteristic for mechanically locked segments. MT models suggest that those fluids are trapped at depth and kept at high pressures. It is speculated that fluids may infiltrate neighbouring rocks and induce non-volcanic tremor. MT has imaged several prominent electrically conductive anomalies at the San Andreas fault which have been attributed to the presence of saline fluids within such networks, and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle portion of the crust, and where these fluids are kept at high pressures, strain provides a means to develop and maintain permeability structure, and thus electrical conductivity, within the damage zone. Accordingly, these conductivity models played an important role in pin-pointing the SAFOD drilling location near Parkfield. *abstractClick here to download abstract: abstract.txt Surv. Geophys. manuscript No.In this paper, we consider MT studies at the SAF near Hollister and Carrizo The SAF has also been a target for telluric (Madden et al. 1993;Park et al. 2007) and magnetotelluric monitoring (e.g. Kappler et al. 2010). These installations attempted to monitor long-term electric (and magnetic) field variations of internal origin due to resistivity variations assiciated with earthquakes, large scale fluid flow or other causes. To date, it was not possible to identify significant field variations which can be attributed to tectonic processes. In this paper, we don't discuss details of this issue. 7The main objective of this paper is to review the MT results from the SAF in combination with other geophysical, geological and geochemical data and models in order to address fluid-related processes and their potential implications on the geodynamic setting, active tectonic processes and the mechanical ...

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