Characterizing the Mid-Faeroe Ridge using seismic velocity measurements

Abstract

The character of the basement and overlying sedimentary sequences in the vicinity of the Mid-Faeroe Ridge is investigated through a synthesis of seismic velocity measurements. A finite difference tomographic algorithm was applied to a wide-aperture seismic profile acquired across the center of the Faeroe Basin to produce a velocity model free from a priori artifacts. A series of parameterizations and smoothing constraints were run to assess the uniqueness of the final model. Uncertainties in the velocity structure were estimated at ±5%. Comparison of the in situ velocity of the Mid-Faeroe Ridge with deterministic lithology-velocity data extracted from nearby boreholes strongly implies that Tertiary to Cretaceous shale and sandstone sequences extend to depths of 4.5 km. The occurrence of pre-Cretaceous sedimentary lithologies at 4.5-8.0 km depth is supported by in situ velocities of 4.0-4.5 km/s which are best explained by well-consolidated sediments and are incompatible with likely constituents of the basement. Evidence for a major structural discordance across the southeastern flank of the Mid-Faeroe Ridge is provided by a prominent fault whose location demarcates a lateral velocity variation of ∼0.5 km/s. A rapid increase of the in situ velocity at 8 km depth is strongly suggestive of a transition into the basement, whose velocity (5.5 km/s) is comparable with high-pressure laboratory measurements of basalts from the Faeroe Islands. This basaltic basement in the center of the Faeroe Basin lies above a Moho depth of 18 ± 3 km which suggests cumulative stretching factors of about 3.0 and implies substantial igneous activity. We conclude that in the region of the Mid-Faeroe Ridge the prerift (Lewisian?) basement has been highly attenuated, pervasively intruded, and possibly wholly replaced by basaltic material. Copyright 1997 by the American Geophysical Union.