Thermochemical anomalies in the upper mantle control Gakkel Ridge accretion

Despite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use ⁴⁰Ar/³⁹Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle.

The forces driving the plates: constraints from kinematics and stress observations

Plate kinematics and stress observations are used to assess the nature and relative magnitudes of the forces driving the plates. Dynamical equilibrium for a purely oceanic plate determines the relative magnitudes of the active and responding forces, largely balancing drag against slab-pull. Continental plates, moving more slowly, have errors of about 30% of the estimated motions. A torque balance model is used to describe the evolution of plate dynamics over the Cenozoic Era for reconstructions of plate geometry and velocities. Torques have been fairly stable for the past 64 Ma; the misfit to the model systematically increases for earlier times, most likely due to errors in the locations of past convergent plate boundaries and velocities. Unlike kinematics, which integrates the forces acting on a plate, the stress field responds locally and can differentiate between models for forces. Stress models which incorporate the forces are compared with stress orientations for the North and South American plates.