Seismic Refraction Crustal Study Along the Sognefjord, South-West Norway, Employing Ocean-Bottom Seismometers

Abstract

SUMMARY This paper presents a crustal structure model derived from an extensive oceanbottom seismographic refraction experiment conducted on a 150 km profile line along the Sognefjord, south-west Norway. The main part of the profile was located in the Western Gneiss Region (WGR) which is characterized by deformation and metamorphic overprinting during the Caledonian Orogeny. The western and eastern ends of the profile crossed the Solund Devonian Basin and the allochthonous unit of the Jotun Nappe, respectively. Within the WGR, the fjord bottom is covered with 200-250m thick glacial sediments with a low P-wave velocity (1600m) and a high Poisson's ratio (0.48-0.49). The P-wave velocity and velocity gradient of the uppermost crystalline basement is 6.05 km s-' and 0.03 s-', respectively. The Poisson's ratio within the upper 12 km crust has an almost constant value of 0.26 in spite of the westward increase in the Caledonian metamorphism. This may indicate that the uniform bulk composition of the WGR is the predominant factor on the seismic wave velocities rather than the 'fossil' Caledonization. The P-wave velocity and the Poisson's ratio in the lower crust (deeper than 19-20 km) are 6.6-7.0 km s-' and 0.27, respectively, which are comparable to those for an amphibolitic-granulitic rock type. The Moho gently deepens eastward from 31 to 36km. The velocity contrast at the Moho is large (1.0-1.2kmsp1), as seen in the other areas of the WGR. At the eastern end of the profile (the Jotun Nappe), the P-wave velocity at the uppermost basement shows a higher value (6.20 km s-') than that within the WGR (6.05 km s-'), representing relatively mafic rock components in the Jotun Nappe. Such a lateral velocity change becomes obscure at depths deeper than 6-7km, which indicates that the detachment between the WGR and the Jotun Nappe is situated at a very shallow depth. This strengthens the hypothesis that the WGR is a westward extension of the Precambrian Baltic Shield buried beneath the Caledonian Nappes. A mid-crustal interface with a velocity jump of 0.2-0.3 km s-' was observed at a depth of 19-20 km. This interface disappears under the western part of the profile, indicating the intense deformation of the WGR during the Caledonian Orogeny.