Crustal structure beneath the northern margin of the Swiss Alps

Abstract

The structure of the Earth's crust under the northern margin of the Swiss Alps was determined by means of seismic refraction and reflection measurements. The profile runs northeast from Montreux on Lake Geneva along the Helvetic nappes on the southern edge of the Molasse basin to the Rhine valley south of Lake Constance with two shotpoints 165 km apart. We derived a two-dimensional P-wave velocity model from these data. Using data from two fans perpendicular to the profile at a distance of 110 km from the shotpoints we migrated the overcritical Moho reflections laterally to their proper position. The complicated sedimentary structure was mainly taken from various exploration surveys and earlier shallow research. Main features of the crust are: the 3-km-thick upper basement has an average velocity of 6.0 km/s, which is followed by 2 km with a reduced velocity of 5.6 km/s. At 10 km depth the velocity increases to 6.15 km/s. No significant structural changes occur in the depth range between 10 and 26 km with an average velocity of 6.2 km/s. At a depth of 26 km in the west and 24 km in the east the velocity increases to 6.6 km/s for about 3 km followed by a low-velocity zone of 6.4 km/s with an average thickness of 6 km. The crust—mantle boundary is reached at 34 km depth under shotpoint Jaunpass and deepens to 35 km near shotpoint Saentis. The southeastward downdip of the Moho observed on the P m P fans migrates the derived deeper crustal structure laterally to the northwest by about 9 km from the surface position of the profile. The velocity-depth structure derived from the refraction and wide-angle reflection data agrees well with a short near-vertical reflection profile near shotpoint Jaunpass.