Magnetic Structure of Fast‐Spread Oceanic Crust at Pito Deep

Abstract

Magnetic surveys at tectonic windows that expose magnetic polarity boundaries provide the unique opportunity to explore the pattern of magnetization variations within the oceanic crust and determine the spatially averaged magnetizations of source layers that contribute to marine magnetic anomalies. Here we investigate the C2An.2n/C2An.2r polarity boundary in the tectonic window of Pito Deep, which has exposed a cross‐section through lavas, dikes, and the uppermost kilometer of gabbros at fast‐spread ocean crust. Near‐bottom magnetic anomaly surveys from two expeditions have been incorporated into a penalized least squares inversion method. The application of this method to magnetic data allows us to account for complex bathymetry and differing observation altitudes. When correlated with rock type, the magnetization solution shows median values of 4.4 ± 2.7 A/m for lavas, 2.0 ± 1.9 A/m for dikes, and 1.9 ± 1.9 A/m for gabbros. On a regional scale, lavas and dikes have a different polarity of magnetization than the underlying gabbros. The geometry of the polarity boundary is compatible with a large (~6 km) horizontal offset or very shallow dip of isotherms at the dike/gabbro boundary, and indicates that the zone of melt is significantly wider across axis than predicted from seismic tomography models that suggest pervasive cooling throughout the lower crust within a few kilometers of the spreading center.

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Petrography and Petrology

“…In addition, ship‐based in situ sampling was performed very locally as a function of depth, enabling the construction of "pseudo"‐sections, both for Hess Deep (e.g., Lissenberg et al., 2013) and for Pito Deep (e.g., Brown et al., 2019; S. Maher et al., 2020; S. M. Maher et al., 2021) as summarized in Coogan (2014, and references therein). However, these "sections" cannot serve as a "reference" for deep fast‐spreading oceanic crust for the following reasons: (a) data sets are highly incomplete; (b) possible faults related to ridge tectonics cause many problems; (c) it is not clear whether magmatism is representative, due to the special tectonic situation (propagating rifts); and (d) there is no upward stratigraphic continuation into the volcanic sequence.…”

“…In addition, ship-based in situ sampling was performed very locally as a function of depth, enabling the construction of "pseudo"-sections, both for Hess Deep (e.g., Lissenberg et al, 2013) and for Pito Deep (e.g., Brown et al, 2019;S. Maher et al, 2020; S. M. Maher et al, 2021) as summarized in Coogan (2014, and references therein).…”

Influence of the oceanic crust structure on marine magnetic anomalies: Review and forward modelling

“…The magnetization of the dike layer is relatively weak (between 1 and 5 A/m; Table 1) (Gee & Kent, 2007;Maher et al, 2020), whereas the thickness is roughly the same as that of the lava layer (between 500 and 2000 m) (Christeson et al, 2007;Gee & Kent, 1994). The gabbro layer is the thickest among these oceanic crustal layers, with a thickness of 3000-5000 m (Kodaira & Fujie, 2015).…”

“…According to the results of oceanic crust drilling and seismic methods, the lava layer comprises basalt with relatively high magnetization varying from 1 to 20 A/m or even higher (Table 1) (Gernigon et al, 2015; Mallows & Searle, 2012; Searle et al, 2019) and a thickness of 300–2000 m (Christeson et al, 2010; Gernigon et al, 2015; Hussenoeder et al, 1996; Pouliquen & Sailhac, 2003). The magnetization of the dike layer is relatively weak (between 1 and 5 A/m; Table 1) (Gee & Kent, 2007; Maher et al, 2020), whereas the thickness is roughly the same as that of the lava layer (between 500 and 2000 m) (Christeson et al, 2007; Gee & Kent, 1994). The gabbro layer is the thickest among these oceanic crustal layers, with a thickness of 3000–5000 m (Kodaira & Fujie, 2015).…”

“…Previous simulations typically assumed a lava layer with vertical polarity boundary as the magnetic source layer (Mendel et al, 2005; Zhang et al, 2012) and then fitted the simulation results to the observed data by adjusting the topography, magnetization, or layer thickness (Bronner et al, 2013; Hey et al, 2010; Mallows & Searle, 2012). To compensate for the deficiency of this method, the contributions of the dike and gabbro layers with stable magnetization and thickness values was incorporated into the simulation (Dyment et al, 2015; Maher et al, 2020); however, the magnetic structure models for oceanic crust are extremely monotonous and lack of systematic analysis (Hosford et al, 2003). In addition, few studies have investigated the influence of polarity boundaries on the simulation of magnetic anomalies.…”