Highly variable lithospheric structure and associated magmatic accretion at the ultraslow-spreading Gakkel Ridge

The mid-ocean ridge (MOR) is the longest volcanic chain on the Earth (∼75,000 km), with spreading rates varying from fast (>80 mm/yr) to ultraslow (<20 mm/yr). It is generally believed that mantle beneath MORs upwells passively due to viscous drag from the diverging tectonic plates, leading to pressure-release melting. While passive mantle upwelling models explain the uniform crustal thickness observed at fast-spreading ridges, they fail to account for the complexities at ultraslow-spreading ridges. At these ridges, enhanced conductive cooling and hydrothermal circulation thicken the ocean lithosphere, shrinking the melting zone and inhibiting melt production. The fundamental dynamics governing crustal accretion at ultraslow-spreading ridges remain elusive. In 2021, we conducted a high-resolution active-source ocean-bottom seismometer (OBS) experiment along the eastern ultraslow-spreading Gakkel Ridge between 76° and 100° E using the icebreaker ‘Xuelong 2’, during the Joint Arctic Scientific Mid-ocean ridge Insight Expedition (JASMInE). Our new seismic model reveals highly variable crustal thickness, which ranges from 3.3 km to 8.9 km along the ridge axis. Meanwhile, this thickness increases from ~4.5 km to ~7.5 km over the past 5 Myr across the ridge axis. In addition, the magnetotelluric data reveals prominent low-resistivity zones at depths 20–45 km beneath volcanic centers, but high resistivities (>100 Ω m) down to ~ 50 km at volcanic ends, indicating highly variable electrical lithosphere (eLAB). Microearthquakes recorded by the OBSs occurred at depths of <10 below the seafloor along the ridge axis, suggesting a relatively shallow brittle lithosphere and a high magma supply. These observations contradict the passive upwelling models and are instead consistent with buoyant active mantle flow model that is driven by thermal and compositional density changes due to melt extraction. Active mantle upwelling is predicted to play a more significant role as the spreading rate decreases, which is highly sensitive to the mantle temperature and composition. This implies that the observed variability in crustal and lithospheric thickness is likely an inherent characteristic of ultraslow-spreading ridges.