The Alps — a transpressive pile of peels

Abstract

One of the striking features of recent reflection seismograph traverses in the Alps is the strong reflection band of the ‘Penninic front’. It cuts discordantly through the complex geometry of the Pennine nappes and ties laterally into the Miocene Simplon line. The early Miocene ductile component of this line continues into the base of the roof zone of the Lepontine dome with its retrograde EW shearing and stretching. A kinematic model compatible with the data may be constructed on the concept of an orogenic lid subjected to dextral transpression by the early Miocene Adriatic indenter (Insubric-Helvetic phase). In this model, the Penninic front reflections mark the shear zone at the base of the orogenic lid. The frontal part of this lid is the fold-and-thrust belt of the Helvetic nappes and equivalent contemporaneous thrusts such as those of the Prealps. The lid was deformed and partly eroded during the subsequent Windows phase which pushed up the External Massifs. The Insubric-Helvetic lid was subdivided into a mosaic of sublids which accommodated the divergence of translation at the NW-corner of the early Miocene Adriatic indenter. The movement of this indenter had a component of about 150 km translation to the west (dextral strike-slip along the Insubric line), and 100 km to the north (estimated translation and shortening of the Helvetic nappes). As a consequence, that part of the brittle orogenic lid north of the Insubric line (Silvretta sublid) was pushed in a northerly direction, while that at the western front of the Adriatic indenter (Gran Paradiso sublid) moved more to the west. In between, the Lepontine sublid was stretched axially by normal faulting and uplifted by tectonic underplating due to particularly intensive N-S compression in the narrowest part of the central Alps. It participated as a pull-apart domain in the dextral motion along the Insubric line and was subject to rapid tectonic denudation comparable to that in metamorphic core complexes. Adjacent to the west (external side of the lid), the Prealps sublid accommodated axial stretching largely through complementary strike-slip along its bordering transverse zones (Giffre and Kander lines). Below the orogenic lid, the middle and lower crust were deformed disharmonically. In the western Alps, parts of the lower crust and even the upper mantle were peeled off from the subducted slab and wedged into the middle crust, whereas in the central Alps, during this particular phase, subduction of lower and some middle crust seems to have been dominant, except for the most intensely squeezed Lepontine domain, where middle crust was piled up und produced an uplift of up to 20 km. The pile of Alpine nappes participating in this phase therefore had three components: those inherited from previous phases and forming passively the orogenic lid; those developing at the base and the front of the lid; and those forming disharmonically in the middle and lower crust below the lid.