A crustal study off Lofoten, N. Norway, by use of 3-component Ocean Bottom Seismographs

Abstract

Twenty-four 3-component Ocean Bottom Seismographs (Obs) were used in August 1988 in a combined seismic refraction and reflection study off Lofoten, N. Norway. The purpose of the experiment was to map the crustal structure from the continental shelf to the oceanic crust off Lofoten. The very high data quality is demonstrated by the strong P-wave and shear-wave reflections, as well as converted waves from the Moho observed on the continental shelf. These arrivals are observed continuously from near vertical to wide angle incidence. Very high seismic sea-floor velocities in this area (3.1 km/s to 5.1 km/s) indicates absence or very thin sequences of Mesozoic sediments. The 5.1 km/s refractor coincides with the base Cretaceous reflection interpreted from the multichannel reflection data. The crystalline continental crust is here divided in layers with velocities of 6.0 km/s, 6.4 km/s and 6.8 km/s, respectively. On the seaward side of the escarpment Tertiary sediments varying in thickness from 1.0–1.5 km are situated on top of a 1.5-2.0-km-thick layer of flood-basalt containing seaward-dipping reflectors. A layer with velocity of 6.7 km/s is observed above the lower crust, which in this area is found to have a velocity as high as 7.3 km/s. These high velocities indicate that the crust in this area is of oceanic origin or, alternatively, the high-velocity layer in the lower crust might represent a magmatic body underplating highly thinned and intruded continental crust. Seven obss were deployed in the area that was covered by landward-flowing basalt deposited during the early Eocene breakup between Norway and Greenland. The survey was partly performed to investigate whether this method can be used to map structures below the basalt, which is impenetrable with conventional seismic reflection techniques. The obs data contain considerable information about structures below the flood-basalt; pre-opening sediments up to 4.0 km thick are indicated below the 1.0–2.5-km-thick landward-flowing basalt. The velocities of the crystalline portion of the crust are found to be similar to those observed under the continental shelf (6.0–6.8 km/s), which implies that the crust east of the escarpment is of continental origin. The crystalline crust is strongly thinned in this area, showing a minimum thickness of about 6 km. The depth to the Moho increases from about 15 km in the western part of the area to about 26 km on the continental shelf. The success of the obs survey indicates that such measurements can become an important tool in investigations on passive volcanic margins, and, potentially, in other areas where highly reflective surfaces make the reflection technique inefficient.