Open Session - SWISS GEOSCIENCE MEETINGs

Symposium 1: Structural Geology, Tectonics and Geodynamics
tion of underlying units in the core of the Lepontine dome may have been minor, they mark the onset of orogen-parallel
extension of the Penninic nappe stack at c. 35 Ma. This orogen-parallel extension was replaced by orogen-perpendicular southeast-vergent
extensional movements (older than c. 30 Ma) in and above the Monte Rosa (Mischabel phase, D 3 ) and Adula
nappes (Claro phase, D 3 ) but continued later in deeper levels of the nappe stack at the Simplon fault in the west and at the
Forcola fault in the east (from c. 19 Ma onwards and at c. 25 Ma, respectively).
Neither the ductile nor the brittle deformation at these two faults continued towards south through the meanwhile present
Southern Steep Belt until the Periadriatic fault. The renewed orogen parallel-stretching in the flat-lying part of the nappe
stack was probably instead accommodated by Olino-phase shearing in the Southern Steep Belt. While the Penninic nappe
stack in the block north of the Periadriatic fault was stretched more or less parallel to that fault, there was no significant
stretching of the south block parallel to the Periadriatic fault. This stretching-fault character of the Periadriatic fault may
explain the variations in displacement (including locally sinistral shearing) that can be observed along the strike of the
fault.
Although the kinematics of all deformation phases postdating D 1 (i.e. northwest-vergent nappe stacking) differed considerably,
it is remarkable that all these deformation phases contributed to the formation of the Lepontine dome in that they
effected thinning of the units between the European basement now exposed in the Lepontine Dome and the Periadriatic
fault which developed its backthrust geometry only after D 3 .
Fig1: Strongly schematized diagrams illustrating the exhumation of the Central Alps. (a) Situation after D1. (b) Orogen-parallel stretching
of D2 in the western and eastern flank of the Lepontine dome accommodate a first stage of unroofing. (c) D3 SE-vergent shearing effects
further exhumation by normal faulting. (d) Uplift of the nappe stack in the core of the Vanzone antiform. (e) Final orogen-parallel stretching
and overturning of the Southern Steep Belt are accommodated by Olino-phase shearing in the Southern Steep Belt while extensional
shearing, followed by brittle normal-faulting, in the flat-lying part of the nappe stack accommodates further unroofing.