Modification of the Seismic Properties of Subducting Continental Crust by Eclogitization and Deformation Processes

Zertani, Sascha ORCIDiD
John, Timm ORCIDiD
Tilmann, Frederik ORCIDiD
Motra, Hem B. ORCIDiD
Keppler, Ruth ORCIDiD
Andersen, Torgeir B. ORCIDiD
Labrousse, Loic

DOI: https://doi.org/10.1029/2019JB017741
Persistent URL: http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/8530
Zertani, Sascha; John, Timm; Tilmann, Frederik; Motra, Hem B.; Keppler, Ruth; Andersen, Torgeir B.; Labrousse, Loic, 2019: Modification of the Seismic Properties of Subducting Continental Crust by Eclogitization and Deformation Processes. In: Journal of Geophysical Research: Solid Earth, 124, 9, 9731-9754, DOI: https://doi.org/10.1029/2019JB017741. 
 
John, Timm; 1 Institute of Geological Sciences Freie Universität Berlin Berlin Germany
Tilmann, Frederik; 1 Institute of Geological Sciences Freie Universität Berlin Berlin Germany
Motra, Hem B.; 3 Department of Geosciences, Marine and Land Geomechanics and Geotechnics University of Kiel Kiel Germany
Keppler, Ruth; 4 Institute of Geosciences and Meteorology University of Bonn Bonn Germany
Andersen, Torgeir B.; 5 The Centre of Earth Evolution and Dynamics (CEED), Department of Geosciences University of Oslo Oslo Norway
Labrousse, Loic; 6 Sorbonne Université, CNRS‐INSU, Institut des Sciences de la Terre Paris, ISTeP, UMR 7193 Paris France

Abstract

Subduction zone processes and the resulting geometries at depth are widely studied by large‐scale geophysical imaging techniques. The subsequent interpretations are dependent on information from surface exposures of fossil subduction and collision zones, which help to discern probable lithologies and their structural relationships at depth. For this purpose, we collected samples from Holsnøy in the Bergen Arcs of western Norway, which constitutes a well‐preserved slice of continental crust, deeply buried and partially eclogitized during Caledonian collision. We derived seismic properties of both the lower crustal granulite‐facies protolith and the eclogite‐facies shear zones by performing laboratory measurements on cube‐shaped samples. P and S wave velocities were measured in three perpendicular directions, along the principal fabric directions of the rock. Resulting velocities agree with seismic velocities calculated using thermodynamic modeling and confirm that eclogitization causes a significant increase of the seismic velocity. Further, eclogitization results in decreased VP/VS ratios and, when associated with deformation, an increase of the seismic anisotropy due to the crystallographic preferred orientation of omphacite that were obtained from neutron diffraction measurements. The structural framework of this exposed complex combined with the characteristic variations of seismic properties from the lower crustal protolith to the high‐pressure assemblage provides the possibility to detect comparable structures at depth in currently active settings using seismological methods such as the receiver function method.


Key Points:

Eclogitization of continental crust increases seismic velocities (isotropic averages up to 8.21 km/s) and decreases VP/VS ratios by ~0.04. Eclogitization coeval with deformation causes a high P wave anisotropy of up to 9%. Shear zone formation coeval with eclogitization causes changes of the seismic response of the structure.