Crustal structure near the Arctic Mid-Ocean Ridge

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 Pn 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 Pn 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.