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Abstract:
Subduction zone processes and the resulting geometries at depth are widely studied bylarge‐scale geophysical imaging techniques. The subsequent interpretations are dependent oninformation from surface exposures of fossil subduction and collision zones, which help to discernprobable lithologies and their structural relationships at depth. For this purpose, we collected samplesfrom Holsnøy in the Bergen Arcs of western Norway, which constitutes a well‐preserved slice ofcontinental crust, deeply buried and partially eclogitized during Caledonian collision. We derived seismicproperties of both the lower crustal granulite‐facies protolith and the eclogite‐facies shear zones byperforming laboratory measurements on cube‐shaped samples.PandSwave velocities were measured inthree perpendicular directions, along the principal fabric directions of the rock. Resulting velocitiesagree with seismic velocities calculated using thermodynamic modeling and confirm that eclogitizationcauses a significant increase of the seismic velocity. Further, eclogitization results in decreasedVP/VSratios and, when associated with deformation, an increase of the seismic anisotropy due to thecrystallographic preferred orientation of omphacite that were obtained from neutron diffractionmeasurements. The structural framework of this exposed complex combined with the characteristicvariations of seismic properties from the lower crustal protolith to the high‐pressure assemblage providesthe possibility to detect comparable structures at depth in currently active settings using seismologicalmethods such as the receiver function method.
