CRUSTAL REFLECTIVITY UNDERNEATH THE CENTRAL SCANDINAVIAN CALEDONIDES

We have studied data from a c. 160 km long explosive/vibroseis reflection seismic profile, part of the Central Caledonian Transect, acquired in the Central Scandinavian Caledonides between 1988 and 1992. Extended correlation has been used to increase the record length to 20 s for the vibroseis component. The reflectivity, interpreted as primarily due to dolerites, is strong at depths less than 15 km in the Precambrian basement, throughout the profile. These Precambrian rocks are largely covered by thin sedimentary thrust sheets, transported during the Caledonian orogeny to their present locations. Aeromagnetic surveying shows a large positive anomaly in this area, suggesting that the dolerites are located in a homogeneous, highly magnetized, Ratan type granite, a granite belonging to the Transscandinavian Igneous Belt. In the middle/lower crust, weak east-dipping reflectivity is observed. To the east, partly in the Svecofennian Domain, a c. 60 km segment of sub-horizontal reflectivity at 14-15 s suggests a flat reflection Moho at a depth of c. 50km. Underneath the central part of the profile, coinciding with the location of the TIB granite, the reflection Moho appears to shallow to c. 45 km, or to be less reflective, while further west it appears to again deepen to c. 50 km. Estimates of signal penetration depth, together with studies of fold variations, indicate that these variations in reflection Moho topography and deep reflectivity are real. We propose that much of the deep reflectivity was erased during emplacement of the TIB granite at 1.85-1.65 Ga. Extension at c. 1.0 Ga led to the thinning of the crust and allowed dolerites to intrude the granitic upper/middle crust. Later Caledonian compression sheared this granite-dolerite system.