Open Session - SWISS GEOSCIENCE MEETINGs

1.12
Dynamics of orogenic metamorphism: Diachronic evolution in the Central
Alps
Engi Martin*, Janots Emilie**, Allaz Julien* and Berger Alfons***
*Institut für Geologie, Baltzerstrasse 3, CH-3012 Bern (engi@geo.unibe.ch)
** Institut für Mineralogie, Corrensstrasse 24, D-48149 Münster
*** Institut for Geografi og Geologi, Øster Voldgade 10, DK-1350 København K
The Central Alps arguably represent Earth’s best studied collisional orogen. Despite many decades of research, fundamental
aspects of the PTt-evolution across the Lepontine belt and its margin have not been satisfactorily resolved. Recent studies
combining petrology and mineral chronometry have focussed on several of the critical tectonic units and have confirmed
substantial differences in age across the orogen. Integrating the results within the structural framework yields a consolidated
basis to interpret the geodynamic evolution of the metamorphic belt now surfacing in the Central Alps. A brief outline
is presented here.
Early convergence is now known to have yielded HP metamorphic sequences typical of subduction in a few units only. In
northern parts of the Lepontine, accretionary wedge units attained LT-HP conditions (e.g. carpholite in Valaisan units;
Bousquet et al. 2002); further south, portions of the tectonic accretion channel (TAC; Engi et al. 2003) comprise a tectonic
mélange, with eclogite facies fragments. Much of the HP-stage was reached in the Eocene, but prograde lawsonite-blueschist
facies conditions in the TAC (Brouwer & Engi, 2005) were attained prior to 55 Ma ago. Eclogite facies garnet growth in some
parts of the TAC initiated >70 Ma ago, in others it lasted to 36 Ma (Brouwer et al., 2005). Tectonic extrusion of portions of the
TAC (from Mantle depths) and their incorporation as thrust sheets (Adula) or tectonic slivers (e.g. Mergoscia-Arbedo zone)
into the stack of gneiss nappes (e.g. Simano, Maggia) must have preceded intrusion of tonalite melts of the Bergell suite, i.e.
prior to ≈32 Ma ago (Oberli et al., 2004), at pressures of 5-6 kbar. This marks the important transition in the metamorphic
evolution from HP-conditions (experienced by a small and well defined subset of the tectonic units) to the regional Barrovian
phase, which encompassed all of the Lepontine units, at mid-crustal depths (15-20 km).
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Symposium 1: Structural Geology, Tectonics and Geodynamics