On the origin of seismic anisotropy in the shallow crust of the Northern Volcanic Zone, Iceland

• Published in: Journal of Geophysical Research: Solid Earth
• Main Authors: Bacon, Conor R, Johnson, Jessica H, White, R. S., Rawlinson, Nicholas
• Format: Article in Journal/Newspaper
• Language: English
• Published: AGU 2021
• Subjects: 02 - Geodynamics; Geophysics and Tectonics; Askja; Mid-Atlantic Ridge; Iceland; North Atlantic
• Online Access: http://eprints.esc.cam.ac.uk/6035/; http://eprints.esc.cam.ac.uk/6035/1/JGR%20Solid%20Earth%20-%202021%20-%20Bacon%20-%20On%20the%20Origin%20of%20Seismic%20Anisotropy%20in%20the%20Shallow%20Crust%20of%20the%20Northern%20Volcanic%20Zone%20.pdf; http://eprints.esc.cam.ac.uk/6035/2/2021jb022655-sup-0001-supporting%20information%20si-s01.pdf; https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JB022655?af=R; https://doi.org/10.1029/2021JB022655

Title: Authors: Conor Andrew Bacon, Jessica Helen Johnson, Robert Stephen White, Nicholas Rawlinson Journal: Journal of Geophysical Research: Solid Earth Plain Language Summary Iceland is well known for its earthquakes and volcanoes, which have helped to produce an awe-inspiring primordial landscape over the last 20 million years or so. The emergence of Iceland in the North Atlantic Ocean can be attributed to the interaction of the Mid-Atlantic Ridge, where new oceanic crust forms by rifting between the North American and Eurasian plates, and a rising conduit of hot mantle from deep in the Earth, known as a mantle plume. The confluence of these two phenomena has produced excessive melting of mantle rocks, with the resultant melt accreted and cooled to form the Icelandic crust. We investigate how extensional stresses related to the divergence of the two tectonic plates have influenced the upper 3–4 km of the crust around Askja volcano, in the deep interior of Iceland. To do so, we exploit information contained in recordings of earthquakes from the neighborhood of Askja, which suggests that cracks formed parallel to the Mid-Atlantic Ridge, which permeate the upper crust, gradually close with depth. This relationship between the regional stress field associated with rifting and brittle deformation in the uppermost crust breaks down around Askja itself, where magmatic processes likely cause local changes in the stress field. Abstract The Icelandic crust is a product of its unique tectonic setting, where the interaction of an ascending mantle plume and the Mid-Atlantic Ridge has caused elevated mantle melting, with the melt accreted and cooled in the crust to form an oceanic plateau. We investigate the strength and orientation of seismic anisotropy in the upper crust of the Northern Volcanic Zone using local earthquake shear-wave splitting, with a view to understanding how the contemporary stress field may influence sub-wavelength structure and processes. This is achieved using a data set comprising urn:x-wiley:21699313:media:jgrb55395:jgrb55395-math-000150,000 earthquakes located in the top 10 km of the crust, recorded by up to 70 stations over a 9 year period. We find that anisotropy is largely confined to the top 3–4 km of the crust, with an average delay time of 0.10 ± 0.05 s, and an average orientation of the fast axis of anisotropy of N014°E ± 27°, which is perpendicular to the spreading direction of the Eurasian and North American plates (N106°E). These results are consistent with the presence of rift-parallel cracks that gradually close with depth, the preferential opening of which is controlled by the regional stress field. Lateral variations in the strength of shear wave anisotropy (SWA) reveal that regions with the highest concentrations of earthquakes have the highest SWA values (∼10%), which reflects the presence of significant brittle deformation. Disruption of the orientation of the fast axis of anisotropy around Askja volcano can be related to local stress changes caused by underlying magmatic processes.