Seismic evidence for alternative (D-type) olivine fabric in fast-spreading lithosphere

Abstract

Seismic anisotropy of the oceanic lithosphere offers a window into mid-ocean ridge processes, lithosphere evolution, and the state of Earth’s upper mantle. Natural and laboratory olivine samples deformed under various conditions exhibit a range of lattice-preferred orientation (LPO) fabrics that provide a direct link between seismic observables and upper mantle conditions. However, interpreting seismic anisotropy in the context of LPO fabric data has proven challenging due to incomplete seismic constraints and vast differences in length scale. Here, we bridge this gap by estimating the complete orthorhombic elastic tensor of unperturbed ~70 Ma Pacific lithosphere directly beneath the Moho using high-resolution ocean bottom seismic observations sensitive to Vp and Vs anisotropy. We then utilize natural and laboratory petrofabrics from the literature to infer LPO fabric type and shear strain accumulated within the shallow lithosphere. Our findings indicate alternative (D-type) fabric and high strain accumulation (>400%) in the oceanic lithosphere, challenging conventional assumptions of the prevalence of A-type fabric and suggesting D-type may be more ubiquitous in oceanic lithosphere than previously thought. Furthermore, D-type fabric indicates a dehydrated lithosphere and deformation by dislocation-accommodated grain boundary sliding (DisGBS) near the ridge. This study represents a first in situ estimate of fabric type and strain accumulation in the oceanic lithosphere that will inform future geodynamic modeling of LPO evolution in ocean basins.