Gravity interpretation of a unified 2-D acoustic image of the central Alpine collision zone

Abstract

SUMMARY Four individual deep seismic reflection profiles across the eastern and southern Swiss Alps have been combined along the trend of the Bouguer gravity anomalies onto the European Geotraverse. The profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide-angle profiles running parallel to the strike of the Alpine arc. The contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles. The resulting acoustic image depicts the subduction of parts of the lower crust of the European plate beneath the Adriatic promontory of the African plate at a low angle of 15. Based on the interpretation of the seismic data, orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the European plate and wedging of the European and Adriatic middle and lower crusts the latter being ill-constrained by the seismic evidence alone. The south-vergent thrusting of the Southern Alps can be accounted for by the observed downbending of the Adriatic Moho and the lower crust in conjunction with the inferred wedging at mid-crustal levels. Using the geometric constraints provided by seismic data, gravity modelling of the Alpine lithosphere/asthenosphere system relative to stable central European platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth. Such a gently dipping subduction zone is at odds with prominent models on Alpine geodynamics which favour a near-vertical orientation of the subducting lithosphere ('Verschluckung'). In agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the Alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen. Both seismic and gravimetric evidence therefore suggest that late-orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper, middle and lower parts of the crust. The amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab.