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

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.

P wave anisotropy caused by partial eclogitization of descending crust demonstrated by modelling effective petrophysical properties

“…Brownlee et al (2017) have shown that off-axis anisotropy deviates systematically from elliptical symmetry for rocks with high anisotropy. However, this effect is most pronounced for rocks with high mica and/or quartz contents, which is not the case for the Holsnøy samples shown in Zertani et al (2019a). The assumption of near-elliptical anisotropy made here is thus in agreement with the scaling laws of the off-axis anisotropy proposed by Brownlee et al (2017).…”

“…anisotropy. In order to estimate effective properties for statically eclogitized areas, where the eclogite would likely have a lower initial anisotropy, we assumed a lower velocity in x direction for one of the samples (N-101 in Zertani et al, 2019a), thus giving a lower P wave anisotropy of 4%, which is in accordance with others reported from Holsnøy (Fountain et al, 1994).…”

“…The physical properties for each element representing the different material are given by the elastic tensor of the corresponding lithology, i.e., granulite or eclogite. Representative elastic tensors were calculated from the velocity measurements (x-z plane) in Zertani et al (2019a).…”

“…1). Because the eclogites measured by Zertani et al (2019a) were all collected from the main shear zones exposed on Holsnøy, they all have a high P wave…”

“…As eclogitization and deformation progress, such shear zones can widen and connect into larger shear zone networks surrounding low-strain domains (e.g., Jolivet et al, 2005) and the shear zones can ultimately reach a thickness of a few hundred meters (Angiboust et al, 2011;Boundy et al, 1997;Raimbourg et al, 2005;Zertani et al, 2019b). In exposed examples of coherent pieces of partially eclogitized crust, the preserved shear zones rarely reach scales that can be resolved with geophysical methods and the complex associations would thus act as an effective medium at depth (e.g., Zertani et al, 2019a).…”