The sedimentary and crustal velocity structure of Makarov Basin and adjacent Alpha Ridge

Abstract

This study examines the velocity structure of Makarov Basin and the adjacent Alpha Ridge to determine the tectonic origins of these features and link them to the larger Amerasia Basin. Seismic data from sonobuoys distributed along a 650km-long line extending from Alpha Ridge and across Makarov Basin to the Lomonosov Ridge were analyzed for this purpose. Forward modelling of traveltimes, supported by coincident multi-channel seismic reflection and shipborne gravity data, were used to determine the P-wave velocity structure along the line. The sedimentary cover averages 0.5km-thick on Alpha Ridge and 1.9km-thick in Makarov Basin, but reaches up to 5km-thick at the base of Lomonosov Ridge. Velocities in the sedimentary section range from 1.6 to 4.3kms−1. As suggested by relatively high velocities, interbedded volcaniclastic or volcanic rock may occur in the deep sedimentary section. The shallow basement of Alpha Ridge (3.3 to 3.6kms−1) is characterized by semi-continuous high amplitude reflections and is interpreted as volcanic rock possibly intercalated with sedimentary rock. Velocities do not vary significantly in the upper and mid-crustal layers between Alpha Ridge and Makarov Basin. Total crustal thickness decreases from 27km beneath Alpha Ridge to 5km-thick in Makarov Basin then thickens to >20km over a short distance as part of Lomonosov Ridge. The crustal structure of Alpha Ridge is consistent with previous studies suggesting that the Alpha-Mendeleev ridge complex is part of a large igneous province (LIP) with thick igneous crust. The lack of change in crustal velocities between Alpha Ridge and Makarov Basin suggests that the basin, at least partly, either formed during or was influenced by LIP-related magmatism. The rapid transition of crustal thicknesses from Makarov Basin to Lomonosov Ridge supports the interpretation that this section of the ridge is a transform margin.