Depth-dependent seismic anisotropy in the oceanic crust of the Nazca Plate offshore northern Chile

Abstract. Seismic anisotropy preserves information about the tectonic, structural, and hydrothermal evolution of oceanic lithosphere, yet direct observations from mature oceanic crust remain sparse. Here, we present constraints on azimuthal P-wave anisotropy from an active source seismic experiment conducted on a ~45 Ma old segment of the Nazca Plate at ~25.5 °S before the plate is affected by outer-rise bending prior to its subduction. Traveltime residuals show a coherent twofold azimuthal pattern for rays with maximum penetration depths within the upper ~2 km of the crust, with faster propagation approximately E–W and slower propagation approximately N–S. Apparent anisotropy inferred from the analysis reaches ~4.5 %–5.5 % for the shallowest ray group down to 1 km below seafloor, decreasing to 3 %–4 % for rays penetrating to ~1–2 km depth. Below ~2 km depth, in the lower crust, no coherent anisotropic signature is resolved. The fast symmetry axis strikes obliquely to the paleo-spreading fabric and is parallel to E–W oriented horst and graben structures related to N–S extension. We therefore interpret the observed azimuthal velocity variation as being primarily associated with the younger fault and fracture architecture. The weakening of the coherent anisotropic signal with increasing ray penetration depth is consistent with decreasing fracture compliance and /or density with depth, potentially related to increasing confining pressure and progressive mineral sealing. To place these observations in a broader geodynamic context, a comparison with previous oceanic-crust studies indicates that sampling depth and tectonic fabric strongly influence reported anisotropy, whereas the available data are insufficient to isolate a robust dependence on plate age or spreading rate.