Time-delay interpretation of seismic refraction data in the Caledonian front, Jämtland, central Scandinavian Caledonides

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Abstract
Seismic refraction studies have been carried out along several profiles through the Caledonian front in Jämtland in order to investigate the configuration of the Precambrian crystalline basement beneath the Caledonian sedimentary cover. The seismic sections were calculated according to the time-delay method and by assuming a linear increase of velocity with depth. Recordings from each of the 3 shotpoints along the profiles allowed the construction of continuous profiles. A method was applied whereby the basement velocity and the velocity gradients were varied until the best fit with respect to the calculated depths from different shotpoints was obtained. The seismic models are compared with drill-hole data, interpretation of aeromagnetic data and surface geology. Depths calculated from the seismic data compare well with the information from the few deep drillholes that reach down to the autochthonous basement. The magnetic and seismic models concur well along most of the profiles but differ greatly locally, thus demonstrating the importance of the application of various geophysical techniques, particularly when investigating complex stratigraphies, as in the Caledonides. Both the seismic and the magnetic models, suggest the existence of an undulating basement surface in the Caledonian front that coincides with some of the major folds in the sedimentary cover. East of the Offerdal synform the basement surface has previously been interpreted to dip gently (about 2°) westwards. The new seismic and magnetic data suggest a more irregular basement configuration than has previously been demonstrated.

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