The crustal structure of the Alpha Ridge at the transition to the Canadian Polar Margin: Results from a seismic refraction experiment

The crustal structure of the Alpha Ridge and its connection to the Canadian Polar Margin was studied by a seismic refraction experiment consisting of a 350-km-long line almost perpendicular to the margin and of a 175-km-long cross line on the ridge. Explosive shots spaced 22 km apart were recorded by geophones deployed on the ice and spaced about every 1.5 km. P wave velocity models were developed by forward and inverse modeling of travel times and by tomographic inversion. In proximity to the coast, the models show a 30-km-thick continental crust with velocities from 5.5 to 6.6 km/s. The continent-ocean transition zone is characterized by thinned and intruded continental crust; a high-velocity lower crustal body (7.5 km/s) indicates magmatic underplating. An up to 5-km-thick layer with velocities of 4.7-5.4 km/s can be correlated from the transition zone onto the Alpha Ridge where reflection seismic facies may indicate an upper crustal layer of extrusive volcanics. The transition zone is characteristic for volcanic-style continental margins. The crust on Alpha Ridge consists of the volcanics, a 4-km-thick layer with intermediate velocities of 6.1-6.6 km/s and a lower crustal layer with velocities of 6.8 to 7.3 km/s; Moho depth varies between 26 and 32 km. Velocities on the ridge are similar to other large igneous provinces. The Alpha Ridge and the contiguous Mendeleev Ridge are interpreted to result from interaction between a Cretaceous plume and a seafloor spreading center parallel to the Canadian Polar Margin with the Lomonosov Ridge acting as a shear margin.