A lithospheric cross-section through the Swiss Alps—II. Constraints on the mechanical structure of a continent-continent collision zone

The calculation of strength profiles along the European Geotraverse (EGT) through the Swiss Alps yields constraints on the large-scale vertical and lateral mechanical structure through the Alpine continent-continent collision zone. Strength profiles are evaluated for different assumptions on petrological stratification and strain rate and are based on temperature-depth profiles derived from transient thermo-kinematic modelling of the Neoalpine orogeny. The main contribution to the total strength results from the mantle lithosphere, which is strongly controlled by temperature. In contrast, the crustal contribution is mainly determined by variations in petrological stratification. A direct correlation between surface heat flow and the total strength of the crust, the mantle lithosphere and the entire lithosphere (crust and mantle lithosphere) is not observed. Our results demonstrate that in tectonically active areas a transient thermal model, along with detailed knowledge of the deep structure and petrology, is necessary to evaluate lithospheric strength envelopes. Inside the collision zone, strain rate has a strong control on the bottom of the mechanically strong crust, whereas outside the collision zone the effect is less significant. The cut-off depth of seismicity along the profile, which correlates largely with the bottom of the mechanically strong crust, deviates from the 300-400°C isotherm. The inferred effective elastic thickness for the Molasse Basin north of the Alps is in agreement with flexural modelling results, whereas for the Southern Alps the predictions deviat

A lithospheric cross-section through the Swiss Alps—I. Thermokinematic modelling of the Neoalpine orogeny

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 provinc

Three dimensional interface modelling with two-dimensional seismic data: the Alpine crust-mantle boundary

We present a new approach to determine the 3-D topography and lateral continuity of seismic interfaces using 2-D-derived controlled-source seismic reflector data. The aim of the approach is to give the simplest possible structure consistent with all reflector data and error estimates. We define simplicity of seismic interfaces by the degree of interface continuity (i.e. shortest length of offsets) and by the degree of interface roughness (least surface roughness). The method is applied to structural information of the crust—mantle boundary (Moho) obtained from over 250 controlled-source seismic reflection and refraction profiles in the greater Alpine region. The reflected and refracted phases from the Moho interface and their interpretation regarding crustal thickness are reviewed and their reliability weighted. Weights assigned to each reflector element are transformed to depth errors considering Fresnel volumes. The 2-D-derived reflector elements are relocated in space (3-D migration) and interpolation is performed between the observed reflector elements to obtain continuity of model parameters. Interface offsets are introduced only where required according to the principle of simplicity. The resulting 3-D model of the Alpine crust—mantle boundary shows two offsets that divide the interface into a European, an Adriatic and a Ligurian Moho, with the European Moho subducting below the Adriatic Moho, and with the Adriatic Moho underthrusting the Ligurian Moho. Each sub-interface depicts the smoothest possible (i.e. simplest) surface, fitting the reflector data within their assigned errors. The results are consistent with previous studies for those regions with dense and reliable controlled-source seismic data. The newly derived Alpine Moho interface, however, surpasses earlier studies by its lateral extent over an area of about 600 km by 600 km, by quantifying reliability estimates along the interface, and by obeying the principle of being consistently as simple as possibl

An object-oriented approach to hybrid structured/unstructured grid generation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77270/1/AIAA-1996-32-959.pd