A Lithospheric Cross-Section Through the Swiss Alps-I. Thermokinematic Modelling of the Neoalpine Orogeny

Abstract

In this paper we develop a forward 2-D thermokinematic model to investigate the Neoalpine 35-0 Ma phase of orogeny along the European Geotraverse (EGT) through the Swiss Alps on a crustal and lithospheric scale. Using a divergence-free kinematic model (div v=0), we define mass displacements, which subsequently serve as input to a transient thermal model. the thermal model uses critically assessed material prorameters and accounts for the depth dependence of the thermal properties in processes such as crustal thickening and mantle-lithospheric subduction. Based on the presentday density pattern of the deep seismic image and estimated exhumation and shortening rates, we derive, in a first modelling step, a mass-displacement field describing the Neoalpine orogeny as a uniform process in time. In a second—thermal—modelling step, this kinematic scenario is further refined by modelling the non-uniform cooling histories of the southern Lepontine in the Penninic domain. For that purpose we adopt lithospheric shortening rates—and consequently exhumation rates—to agree with total Neoalpine shortening, while keeping the geometry of the kinematic model fixed. the resultant thermokinematic model reflects the main characteristics of Neoalpine tectonics, and shows a good overall agreement with combined geological and geophysical data. the asymmetric feature of the present-day tectonic structure along the profile is strongly reflected in the thermal structure of the lithosphere. This demonstrates the need for a kinematic model to investigate the deep-temperature field in active tectonic provinces. For further refinement of the model, the amounts of shortening have to be more precisely estimated, and a higher spatial density in geochronological and metamorphic data is required. Furthermore, surface heat-flow values are, up to now, too uncertain to constrain the predicted surface heat flow. In summary, our results show that we need, in particular, data constraining the horizontal component of the tectonic and thermal evolution. the results of the Neoalpine orogeny modelling demonstrate that the presented thermokinematic procedure yields a good first-order approximation to investigate crustal-scale and lithospheric processes. We conclude. therefore, that the approach presented provides the potential for application not only to continent-continent collision zones, but also to any active tectonic province.