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San Andreas Fault tremor and retrograde metamorphism

Fagereng, Ake and Diener, Johann F. A. 2011. San Andreas Fault tremor and retrograde metamorphism. Geophysical Research Letters 38 (23) , L23303. 10.1029/2011GL049550

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Abstract

[1] Tectonic tremor is an enigmatic low-frequency seismic phenomenon mainly observed in subduction zones, but also documented along the deep extension of the central San Andreas Fault. The physical mechanisms behind this unusual seismic event are not yet determined for any tectonic setting; however, low effective stress conditions arising from metamorphic fluid production are commonly inferred for subduction-related tremor. We investigate the petrologic conditions at which the San Andreas tectonic tremor is inferred to occur through calculations of the pressure – temperature – time evolution of stable mineral assemblages and their water content in the dominant lithologies of the Franciscan Complex. We find that tremor locations around Parkfield and Cholame are currently experiencing retrograde metamorphic conditions. Within the temperature-depth conditions of observed tremor activity, at approximately 500°C and 20 km depth, several mineralogical transitions may occur in cooling greywacke and mafic rocks, leading to localised, significant removal of free water and an associated volume decrease. This indicates that, contrary to subduction-related tremor, tremor on the San Andreas Fault is not linked to prograde, crustal metamorphic fluid production within the fault zone; rather it might be related to mantle-derived fluids from below the tremor zone, and/or fault zone weakening that occurs as phyllosilicates replace more competent and granular mineral phases.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: San Andreas Fault; fault weakening; metamorphic petrology; retrograde metamorphism; strike-slip; tremor
Additional Information: Pdf uploaded in accordance with publisher's policy at http://www.sherpa.ac.uk/romeo/issn/0094-8276/ (accessed 20/02/2014).
Publisher: American Geophysical Union
ISSN: 0094-8276
Date of First Compliant Deposit: 30 March 2016
Last Modified: 04 Jun 2017 05:36
URI: http://orca-mwe.cf.ac.uk/id/eprint/52370

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