Magma-induced tectonics at the East Pacific Rise 9º50’N: Evidence from high-resolution characterization of seafloor and subseafloor 

Abstract

Mid-ocean ridges host the most extensive magmatic system on Earth, where ~60% of the lithosphere is formed. Fast spreading segments such as the East Pacific Rise (EPR) 9º50’N (full spreading rate >80 mm/yr) represent only ~20% of the global ridge network, but contribute ~50% of the total oceanic crustal accretion.  At these ridge segments, magma accumulates in on-axis, quasi-steady-state axial magmatic lenses (AML), typically found 1-2 km below the seafloor, 2 km wide on average, and <0.1 km thick. AMLs are highly three-dimensional in geometry, marked by alternating lineated ridges and troughs where dikes originate and connect with the seafloor through the systems of the faults and fissures (Marjanović et al., 2023). A couple of kilometers away from the ridge axis, the seafloor is dominated by abyssal hills, which are bounded by faults resulting from unbending, cooling, and extension of the lithosphere. Within the critical region between the axial summit trough (AST) and the first abyssal hill bounding faults, sparse mapping has shown that prominent faults can exist, but the mechanism for their origin and contribution to tectonic strain has remained elusive. At the EPR 9º50’N, we combine meter-scale bathymetric mapping with the highest-resolution seismic imagery of an AML to date (horizontal resolution 25 x 25 m2) to reveal a remarkable vertical alignment between the AML and seafloor fault scarps. This genetic link we observe for four distinct cases. Each AML-fault pair is aligned asymmetrically with respect to the ridge axis and is associated with confirmed and possible records of hydrothermal venting observed in the results of recent seafloor and water column mapping (Wu et al., 2023). Along most of such faults’ scarps, the emplacement of magma through various eruption episodes is evident, helping build the crust outside the AST. Our observations at 9º48’N support a mechanism by which these asymmetric, tectonic-magmatic features originate from shallow magma injection sites. After initial magma injection, the surrounding crustal stresses are perturbed thus promoting further crack propagation aligned with the orientation of the underlying magma body. Finally, by joint analyses of faults exposed on the seafloor and seismically-imaged in the subsurface, we also show that their collective contribution to the overall tectonic component of seafloor spreading is less than 0.5%, with a close to negligible role of the lava-covered faults, much smaller than previously proposed. Marjanović, M. et al., 2023, Insights into dike nucleation and eruption dynamics from high-resolution seismic imaging of magmatic system at the East Pacific Rise: Science advances, v. 9, p. eadi2698, doi:10.1126/sciadv.adi2698. Wu, J.-N., Parnell-Turner, R., Fornari, D.J., Barreyre, T., and McDermott, J., 2023a, Oceanic heat transfer by diffuse and focused flow through off-axis vents at 9°50’N, East Pacific Rise, in AGU Fall Meeting Abstracts, v. 2023, p. V43B-0174.