OBSERVATIONS OF SEISMIC REFLECTORS IN THE LOWER LITHOSPHERE BENEATH THE SKAGERRAK

THE lithosphere-the rigid outer shell of the Earth-includes both the crust and the upper mantle. In the past decade, the complex continental crust and underlying shallower mantle lithosphere have been extensively studied using reflection seismology, but the lower lithosphere has not been successfully imaged. Here we report the observation of continuous and virtually horizontal reflectors in the lower continental lithosphere down to depths of 100-110 km below the Skagerrak (Scandinavia). These are apparently not structurally associated with the crust, whereas previously reported mantle reflectors, such as those observed by the BIRPS group around the British Isles1,2, are often located near large faults in the overlying crust. Like the crust, the mantle lithosphere shows a seismically transparent upper and a reflective lower part. Thus, the processes that form seismic reflectors seem to have an affinity for ductile rheological environments in the mantle as well as in the crust. Our observations are consistent with a model consisting of a mechanically strong upper lithosphere, underlain by a thermal boundary layer which separates it from the convecting asthenosphere. © 1990 Nature Publishing Group.