Constraints on lithospheric mantle and crustal anisotropy in the NoMelt area from an analysis of long-period seafloor magnetotelluric data

Abstract

Despite strong anisotropy seen in analysis of seismic data from the NoMelt experiment in 70 Ma Pacific seafloor, a previous analysis of coincident magnetotelluric (MT) data showed no evidence for anisotropy in the electrical conductivity structure of either lithosphere or asthenosphere. We revisit the MT data and use 1D anisotropic models of the lithosphere to demonstrate the limits of acceptable anisotropy within the data. We construct 1D models by varying the thickness and the degree of anisotropy within the lithosphere and conduct a series of tests to investigate what types of electrical anisotropy are compatible with the data. We find that electrical anisotropy is possible in a sheared and/or hydrous mantle within the lower lithosphere (60-90 km depth). The data are not compatible with pervasive electrical anisotropy in the crust. Causes of anisotropy within the highly resistive upper and mid-lithosphere, as seen seismically, are not expected to cause measurable impacts on MT response.

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

“…The uppermost mantle anisotropy creates an even smaller and more difficult to detect split in the MT responses. The crustal and uppermost mantle anisotropy signals are well below the 5-20% data error levels used for the long-period data interpreted as lacking lithospheric anisotropy in Sarafian et al (2015) and Matsuno and Evans (2017). Broadband MT systems (Naif et al, 2013) capable of measuring periods as short as 20 s in deep water are better suited for anisotropy studies, but they would still have difficulty resolving the maximum signal level of anisotropy found at periods less than 100 s. Additionally, marine MT data are subject to the long ranging effects of the large conductivity contrasts at coastlines as well as regional bathymetry.…”

“…However, relatively few electrical anisotropy studies have been attempted in oceanic settings. Of these, several have employed the low‐frequency MT method to study the deep and conductive asthenosphere (Baba et al, ; Evans et al, ; Naif et al, ; Sarafian et al, ), and in some cases the resistive lithospheric mantle (Matsuno & Evans, ; Sarafian et al, ). Authors in these studies estimate electrical anisotropy in the asthenospheric mantle to range in magnitude from 0 (Sarafian et al, ) to a factor of 4 (Evans et al, ) and have interpreted the anisotropy as having resulted from aligned melt tubes (Naif et al, ) or anisotropic hydrogen diffusion in preferentially oriented olivine grains (Evans et al, ).…”

Seismic and numerical constraints on the formation and evolution of oceanic lithosphere

“…We interpret this feature as the G discontinuity. The conductivity structure for the NoMelt site is not consistent with the presence of melt, indicating that G does not represent melt accumulation (Matsuno & Evans, 2017;Sarafian et al, 2015). The discontinuity could be explained by a dehydration boundary, the presence of which is also supported by the local conductivity structure (Sarafian et al, 2015).…”

“…An array of 31 short-period OBS was temporarily deployed alongside the broadband instruments to record long-offset active-source seismic refractions, and seismic reflection data were also collected with a 6 km multi-channel seismic streamer. The measurements presented in Chapters 2 and 3 of this thesis are part of a larger body of work from NoMelt that has provided constraints on the seismic and electrical structure through the crust and upper 300 km of the oceanic mantle beneath the NoMelt array (Lin et al, 2016;Lizarralde et al, 2018;Ma et al, 2018;Matsuno & Evans, 2017;Russell et al, 2019;Sarafian et al, 2015).…”

“…However, melt is not expected to be stable beneath old, cold oceanic lithosphere far from the ridge axis although seismic discontinuities are still observed in these regions. azimuthal anisotropy, attenuation, and electrical conductivity through the shallow upper mantle (Lin et al, 2016;Ma et al, 2018;Matsuno & Evans, 2017;Russell et al, 2019;Sarafian et al, 2015).…”