The Margna nappe represents the deepest unit of the Lower Austroalpine
nappe pile and tectonically overlies the Penninic Malenco-Serpentinite.
It could be proved that a large recumbent syncline with a core of
permomesozoic sediments links the Margna nappe with the Sella nappe.
The mesozoic sediments show a strong affinity to those of the
Austroalpine environment. The basement of the Sella nappe contains
country rocks with a pre-Alpine metamorphic overprint and intrusions
representing the most acid member of the calcalcaline Late Paleozoic
intrusive suite of the Bernina nappe.
The following main events related to Alpine tectonics can bei
distinguished: (A) The main deformation phase of late Cretaceous age
has formed the recumbent folds of the nappe pile, an axial plane
schistosity and a stretching lineation. The facing direction of the
sedimentary syncline indicates west-directed movement during nappe
emplacement. The associated metamorphism led to mineral assemblages of
upper greenschist to lower amphibolite facies. The nappe pile is a
product of deformation above an E-dipping subduction zone. (B) Ductile
top east-directed extensional structures accommodated thinning of both
the Mesozoic sediments and the Margna basement and demonstrate the
collapse of the nappe pile. (C) During a first backfolding event under
middle to upper greenschist facies conditions, isoclinal folds with
N-dipping axial planes and thrusts towards south were developed. The
main schistosity in the roof of the Malenco Serpentinite is attributed
to this pre-Bergellian event. These compressive structures may be
linked with the beginning of the continent-continent-collision. (D) A
second backfolding event which is tectonically separeted from the first
one caused more open folds and a pressure solution schistosity. These
structures may be related to vertical movements along the Insubric line.