Crustal Cracks and Frozen Flow in Oceanic Lithosphere Inferred From Electrical Anisotropy

Abstract

Geophysical observations of anisotropy in oceanic lithosphere offer insight into the formation and evolution of tectonic plates. Seismic anisotropy is well studied but electrical anisotropy remains poorly understood, especially in the crust and uppermost mantle. Here we characterize electrical anisotropy in 33 Ma Pacific lithosphere using controlled-source electromagnetic data that are highly sensitive to lithospheric azimuthal anisotropy. Our data reveal that the crust is ∼18–36 times more conductive in the paleo mid-ocean ridge direction than the perpendicular paleo-spreading direction, while in the uppermost mantle conductivity is ∼29 times higher in the paleo-spreading direction. We propose that the crustal anisotropy results from subvertical porosity created by ridge-parallel normal faulting during extension of the young crust and thermal stress-driven cracking from cooling of mature crust. The magnitude of uppermost mantle anisotropy is consistent with recent experimental results showing strong electrical anisotropy in sheared olivine, suggesting its paleo-spreading orientation results from sub-Moho mantle shearing during plate formation.