Crustal structure near the Arctic Mid‐Ocean Ridge

Abstract

Seismic reflection and crustal refraction studies were carried out at the FRAM I ice station near the Arctic Mid‐Ocean Ridge crest. This ridge is spreading at the very slow rate of 5 mm yr −1 . Upper mantle P n arrivals with apparent velocities averaging 7.9 km/s are observed at distance ranges less than 15 km, with corresponding intercept times of typically 1.1 s of which 0.5 s can be attributed to the travel path through a sedimentary layer. Anisotropy of about 6% appears to be present. These early P n arrivals are observed on seven of the eight refraction lines studied, but crustal velocities are not well constrained. Modeling was done by computing travel time curves for a number of possible velocity structures for comparison with the data and suggests a crustal thickness between 2 and 3 km. This thin crust is associated with low‐amplitude magnetic anomalies. The remaining refraction line was shot within a region of enhanced magnetic anomaly amplitudes the Yermak H zone, where the data interpretation indicates a crustal thickness of about 8 km. This thicker crust and associated strong magnetic anomalies may be due to the ‘Yermak hot spot,’ a region of high magmatic activity. The generally thin crust is probably due to low basaltic productivity at the ridge crest, as a result of increased cooling at the very slow spreading rate.