A Lithosphere‐Asthenosphere Boundary and Partial Melt Estimated Using Marine Magnetotelluric Data at the Central Middle Atlantic Ridge

Abstract

The differential motion between the lithosphere and the asthenosphere is aseismic, so the magnetotelluric (MT) method plays an important role in studying the depth and nature of the lithosphere‐asthenosphere boundary (LAB). In March 2016, we deployed 39 marine MT instruments across the Middle Atlantic Ridge (MAR), 2,000 km away from the African coast, to study the evolution of the LAB with ages out to 45 million years (My). The MT acquisition time was limited to about 60 days by battery life. After analyzing dimensionality and coast effects for the MT data, determinant data were inverted for two‐dimensional resistivity models along two profiles north and south of the Chain Fracture Zone (CFZ). The imaged thickness of the lithospheric lid (>100 Ωm) ranges from 20 to 80 km, generally thickening with age. In the north of CFZ, punctuated low‐resistivity anomalies (<1 Ωm), likely associated with potential partial melts, occur along its base. In the south of CFZ, the base of the resistive lid is demarcated by a low‐resistivity channel (<1 Ωm) most likely fed by deeper melts. Sensitivity analyses and structural recovery tests indicate the robustness of these features. Resistivity models are in good agreement with results of seismic data. These results imply that partial melt is persistent over geologic timescales and that the LAB is dynamic features fed by upward percolation of mantle melt. The melt fraction is about 1–7% based on the resistivity, temperature, pressure, and hydrous basalt models, which is consistent with petrophysical observations.

Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small‐Scale Convection From the PI‐LAB Experiment

“…Melt may pond at the base of the lithosphere in channels (Sim et al, 2020; Sparks & Parmentier, 1991), as suggested from active source studies in the Pacific (Stern et al, 2015) and near our study region further to the north (Mehouachi & Singh, 2018). Low‐resistivity channels, consistent with the presence of partial melt, have also been imaged within our study region using MT (Wang et al, 2020). The channels are likely fed by the buoyant upwellings we observe at greater depths.…”

The Nature of the Lithosphere‐Asthenosphere Boundary

“…Beneath 70 Myr old Pacific lithosphere, there is general agreement between the 1‐D seismic velocity profile and the 1‐D resistivity profile, where the LVZ is characterized by a small volume of interconnected melt (Selway & O'Donnell, 2019). Beneath young seafloor near the equatorial Mid‐Atlantic Ridge there is good agreement between the location of the slowest seismic velocity anomalies and the lowest resistivity anomalies, which also generally occur beneath the strongest LAB phases in Sp imaging (Harmon et al, 2020; Rychert et al, 2019; Wang et al, 2020). Beneath the Piedmont and Coastal Plain in the southeastern United States, the electrical LAB is ~50 km deeper than the seismic LAB obtained from seismic tomography (Murphy & Egbert, 2019).…”

“…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.…”

“…The electrical LAB is estimated to be 50–80 km in northeastern China (Wei et al, 2008) and 80–120 km in the Qiangtang Terrane, central Tibet (Vozar et al, 2014). Beneath young seafloor estimates for the resistive lithospheric lid are thin, <80 km, near the East Pacific Rise, ~17°S (Baba et al, 2006; Evans et al, 1999) and 9°N (Key et al, 2013), beneath the area near the Mid‐Atlantic Ridge ~2°S (Wang et al, 2020), beneath the area near the ultraslow‐spreading Mohns Ridge (Johansen et al, 2019), and beneath the region offshore Nicoya Peninsula, Costa Rica ~9°N (Worzewski et al, 2011). A conductive LAB channel was reported beginning at 45 km depth beneath 30–40 Myr seafloor off central America (Naif et al, 2013) and beneath <30 Myr seafloor at Mid‐Atlantic Ridge (Wang et al, 2020).…”

“…No original data were used in this manuscript. The data on which this article is based are available in Artemieva (2006), Harmon et al (2009), Audet and Burgman (2011), Ekstrom (2011), French et al (2013), Kumar and Kawakatsu (2011), Schmerr (2012), Steinberger and Becker (2018), Rychert and Shearer (2009, 2011), Tharimena, Rychert, and Harmon (2017), Tharimena, Rychert, Harmon, and White (2017), Rychert et al (2018b, and references therein), Harmon et al (2020), Wang et al (2020), Watts et al (2013), Shito et al (2015), Forsyth et al (1998), Hawley et al (2016), Kawakatsu et al (2009), Lin et al (2016), Takeo et al (2013), Gaherty et al (1996), Tan and Helmberger (2007), Mehouachi and Singh (2018), Stern et al (2015), Lizarralde et al (1995), Schultz et al (1993), Bills et al (1994, 2007), Behn et al (2009), Mitrovica and Forte (2004), Hu et al (2016), Key et al (2013), Naif et al (2013), Turner et al (2017), and Johansen et al (2019).…”