In the Margna nappe and the Malenco unit (Val Malenco, northern Italy),
a former continent-ocean transition of the Adriatic continental margin
has been recognized. Extension-related structures in lower crustal and
mantle rocks of these units reveal the following succession of events:
(1) A penetrative deformation overprints the lower crustal and mantle
rocks leading to a foliation and lineation that are subsequently
annealed under granulite facies conditions. This deformation post-dates
the intrusion of the Fedoz gabbro and is attributed to Permian
extension. (2) After a period of isobaric cooling, concentration of
strain in shear zones and mylonites under retrograde conditions marks
the initial stage of Jurassic rifting. (3) During uplift the lower
crust was disrupted. (4) Major exhumation occurred along the Margna
normal fault separating upper crustal rocks of the Margna block from
lower crustal granulites and gabbros originating from approximately 35
km depth. (5) Denudation of the subcontinental mantle and the attached
lower crust is documented by sedimentary ophicarbonates and basaltic
dikes crosscutting the ultramafic rocks.
The Malenco unit represents a complex zone that in Jurassic time was
situated between the ocean floor sequence of the Forno unit and the
Margna continental margin. This zone contains denuded subcontinental
mantle including attached lower crust, MORE-dikes, ophicarbonates and
slivers of upper crust and Mesozoic sediments interpreted as
extensional allochthons. As a consequence, a new Alpine tectonic
boundary between Penninic and Austroalpine units is postulated that is
not identical with the boundary between ultramafic rocks and rocks of
the continental crust.
An evolution model for the formation of the Austroalpine passive
continental margin is proposed in which the Permian extension is
separated from Jurassic rifting. Two stages of simple-shear-dominated
rifting with opposite sense of movement are inferred to have exhumed
the lower crust-mantle complex.