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.Plain Language Summary A major goal in geoscience is to understand the creation and evolution of oceanic lithosphere. To that end, geophysicists study how properties like electrical conductivity vary with direction and depth in the oceanic lithosphere. We call such directional variation "electrical anisotropy." Since electrical conductivity is particularly sensitive to fluids, certain minerals, and past deformation, the patterns of electrical anisotropy in the crust and mantle provide evidence for how the lithosphere forms and evolves. For the first time, we use an active-source electromagnetic technique to constrain the electrical anisotropy of Pacific oceanic crust and the shallowest portions of the mantle. Our model shows that the oceanic crust and uppermost mantle are highly anisotropic. We interpret the electrical anisotropy in the crust as fluid-filled cracks that parallel the paleo mid-ocean ridge. This suggests that crustal electrical structure begins to form at the mid-ocean ridge and continues to evolve over time through those early weaknesses. If such cracks are also present in other tectonic plates, then the oceanic crust may be a more important reservoir of water than previously thought. Uppermost mantle electrical anisotropy is consistent with strong shear deformation of olivine that freezes into the mantle early in its formation.

Permeability Profiles Across the Crust‐Mantle Sections in the Oman Drilling Project Inferred From Dry and Wet Resistivity Data

“…Although our core‐scale measurements do not show a systematic correlation in resistivity in the discrete orientations, recent active‐source electromagnetic measurements have shown a clear electrical anisotropy in the oceanic lithosphere, whereby conduction at crustal depths is enhanced in a direction subparallel to the paleo mid‐oceanic ridge (Chesley et al, 2019). Fisher (1998) noted that the permeability of oceanic crust varies extensively due to the presence of fractures, and cannot simply be determined from core measurements.…”

Crustal Structure of the Greenland‐Iceland Ridge from Joint Refraction and Reflection Seismic Tomography

“…The maximum depth of thermal cracks grows with the cooling of oceanic lithosphere, and it can reach >20 km depth at 50‐Ma‐old lithosphere (Korenaga, 2007). The pervasive existence of thermal cracks in the oceanic lithosphere has some observational support as well (Chesley et al., 2019; Korenaga, 2017; Korenaga & Korenaga, 2016); we also note that the V p / V s ratio observed for the oceanic crust part of the Hatton Bank transect (Eccles et al., 2011) is consistent with the prediction of crack‐like porosity (Korenaga, 2017). Thus, a positive correlation between crustal velocity and thickness may simply result from the closure of porosities at greater pressures (see, e.g., Figure 14 of Behn & Kelemen, 2003 and related references), especially when oceanic crustal thickness is not very different from the standard value of 7 km (White et al., 1992); note that, with thermal gradient fixed, thicker crust leads to higher average temperature, which acts to reduce crustal velocity, but this effect is small (only ∼0.025 km s −1 difference between crustal thicknesses of 5 and 10 km) because of a counteracting pressure effect on seismic wave speed (White & McKenzie, 1989).…”

The Nature of the Lithosphere‐Asthenosphere Boundary

“…Thus, for example, Naif et al (2013) used MT data to detect enhanced conductivity in the plate‐motion direction for partial melts at the LAB, interpreted as shearing. Chesley et al (2019) observed a similar anisotropy in subsolidus lithospheric mantle using a related electrical method (controlled‐source sounding), and Wang et al (2020) estimated melt fraction at the LAB beneath Mid‐Atlantic Ridge using MT data.…”