The formation of the Lepontine gneiss dome

Abstract

Episodes of horizontal extension of the Alpine orogenic crust were recognised decades ago, yet their influence on the thermal and tectonic structure has been either marginalised or neglected in models that were brought forward to explain the evolution of the Central European Alps. In this thesis, I present data from three extensional shear zones at the margins of the Lepontine gneiss dome in the Central Alps that allowed a quantification of the influence that horizontal extension had during the Cenozoic history of this part of the Alpine orogen. A method based on microstructurally controlled 40Ar/39Ar geochronology and Ar-diffusion modelling from samples across the three shear zones was used to date the activity of these shear zones. Furthermore, modelling diffusion of Ar from minerals permitted estimates of the thermal effect that displacement along each shear zone had on rocks in their respective hanging and footwalls. Based on this data, the timing and amount of exhumation associated with each of the investigated shear zones was calculated, and consequently integrated into the large geological data set readily available for the Central Alps. I found that the Cenozoic tectonometamorphic structure of the Central Alps, and in particular the Lepontine gneiss dome, must be the result of repeated tectonic mode switches from horizontal contraction to horizontal extension and vice versa. Two major extensional events could be identified, each of which were preceded and followed by episodes that were dominated by horizontal shortening. The first major extensional event in the Late Eocene/Early Oligocene marks the onset of melting and Barrovian metamorphism within the Lepontine gneiss dome, and was related to a far-field extensional stress regime that led to lithospheric thinning at that time. Structures of this Late Eocene/Early Oligocene extensional episode were subsequently overprinted by a shortening event that lasted throughout the Mid and Late Oligocene. In the Early Miocene, a second large-scale extensional event exhumed the core of the Lepontine dome and led to the cessation of Barrovian metamorphism and melting within the dome. The driving force for this Early Miocene extensional event is not absolutely clear, but might be related to the indentation of the Adriatic sub-plate into the thickened orogenic crust of the Alps. Renewed horizontal compression followed this last major extensional event, rotating some of the extensional structures into an overturned orientation, misleading to the previous interpretation of these structures as the result of horizontal shortening. The exhumation related to the two temporally distinct extensional phases in the Late Eocene/Early Oligocene and in the Miocene is in the order of at least 20 km. This value was previously recognised by other authors, but then generally attributed to horizontal compression. The temporal correlation between significant movements along extensional shear zones and major geological events within the dome suggests that the tectonometamorphic structure of the Central Alps is indeed primarily the result of horizontal extension. Subsequent horizontal shortening overprinted these earlier structures, but was not able to obliterate the thermal and structural signature of the Lepontine gneiss dome that was established during the episodes of horizontal extension. Show more