Grustal shortening in the Alpine Orogen: Results from deep seismic reflection profiling in the eastern Swiss Alps, Line NFP 20‐east

Abstract

The deep crustal seismic line NFP 20‐EAST crosses almost the entire Swiss Alps. Despite the complex geometry of the well‐exposed nappe structure and the considerable axial plunge of some of the units, the Vibroseis survey yielded coherent reflections from several individually identifiable nappe contacts. In the northern part of the survey the Vibroseis data closely match the internal structure of the Helvetic nappes and the underlying autochthonous‐parautochthonous Mesozoic sediments. On the northern flank of the Aar massif, an external basement uplift, these Mesozoic sediments seem to rise from a depth of approximately 7–8 km below sea level to the surface in a series of steps which is interpreted to represent crustal shortening achieved by a combination of folding and thrusting. In the southern part of the survey it was possible to image a number of thin slivers of Mesozoic carbonates pinched between slabs of Penninic basement nappes as well as nappe contacts between lithologically contrasting units. In addition, it seems that the Insubric fault zone, which marks the contact between the Penninic zone and the Southern Alps and which outcrops about 30 km to the south of the survey, shows up as steeply north dipping reflections. The lower European crust in the northern part of the survey is relatively transparent as opposed to the Adriatic lower crust, whose reflective nature may stem from shear zones associated with Mesozoic crustal stretching. The base of both the European and Adriatic crust coincides with a 1‐s‐thick band of laterally discontinuous reflections. This reflection Moho drops to greater depths going from the north toward the center of the Alpine chain, where it disappears with a steep southerly dip. The Moho reappears as a reflection band farther south. This Moho gap is situated above the lithospheric root and may be caused by perturbations related to subduction of lower crustal material. The crustal‐scale structure obtained from the Vibroseis data may be interpreted as a continent‐continent collision with wedge‐shaped indentation of a piece of Adriatic crust into the European crust and vertical escape of the material in the core of the orogen along steeply dipping faults.