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Abstract

Lithospheric-scale analogue experiments have been conducted to investigate the influence of strength heterogeneities on the distribution and mode of crustal-scale deformation, on the resulting geometry of the deformed area, and on its topographic expression. Strength heterogeneities were incorporated by varying the strength of the crust and upper mantle analogue layers and by implementing a weak plate or part-of-a-plate between two stronger ones. Three (brittle crust/viscous crust/strong viscous upper mantle) and four (brittle crust/viscous crust/brittle upper mantle/strong viscous upper mantle) layer models were confined by a weak silicone layer on one side in order to contain but not oppose lateral extrusion. Experimental results show that relative strength contrasts between converging plates and intervening weak plates control the location and the shape of deformation sites taken as ‘collision orogens’. If the contrast is small, internal deformation of the strong plates through fore- and backthrusting occurs early in the deformation history. However, the bulk system is dominated by buckling that nucleates on the weak plate whose antiformal topography is highest; model Moho of the bordering stronger plates is deepest under these conditions. If the contrast is large, deformation remains localized within the weak plate for a larger amount of shortening and develops a root zone below a narrow deformation belt, which coincides with the locus of maximum topography. Implementing a buoyant, low-viscosity layer above the model Moho of the weak plate favours the development of asymmetric model orogens notwithstanding the initial symmetric setup. Once the asymmetry is established strain remains localized in thrust faults and ductile shear zones documenting foreland directed displacement of the model orogen. Such laterally and vertically irregular configurations have applications in continent-continent collision settings such as the Eastern Alps. First-order mechanical boundary conditions recognized from modelling to be favourable to the post early Oligocene tectonics of the Eastern Alps include: (1) subtle rather than high-strength contrasts between the Adriatic indentor and the strongly deformed region comprising Penninic and Austroalpine units to the north of it; (2) decoupling of Penninic continental upper crust from its substratum to allow for crustal-scale buckling of the Tauern Window; (3) weak mechanical behaviour of the European lower crust during collision to account for its constant thickness along the TRANSALP deep seismic transect; and (4) the direct continuation of the basal detachment underlying the fold and thrust belt in the hangingwall of the European plate with a wide ductile shear zone in the core of the orogen, which separates the European from the Adriatic plate.

The mechanical properties of the continental lithosphere are non-uniform in space and time (Ranalli 1997). This heterogeneity is primarily due to changes in composition and thermal conditions expressed in the rheological stratification of the lithosphere with the Moho being the most important discontinuity (e.g. Ranally & Murphy 1987). Laterally, the rheology of the continental lithosphere may be modified because of tectonics, leading for example to the separation or collision of continents. Such processes may result in changes of composition (e.g. continental next to oceanic rheology) and lithospheric thicknesses, both in compression as well as extension, and are usually associated with a pronounced thermal perturbation, which influences the strength of the lithosphere transiently. In that way, the thermo-mechanical age of the lithosphere is reset, which emphasizes the strong time-dependence of lithospheric strength (Cloetingh & Burov 1996). Additionally, the lithosphere is affected by faulting and shearing producing a number of metastable rheological discontinuities that are prone to reactivation (Ranalli 2000). Subsequent deformation of the lithosphere will be steered by pre-existing lateral strength variations (e.g. Ziegler et al. 1998) with relative strength differences among deforming minerals (Handy 1990), rock layers (Hudleston & Lan 1993), crustal-scale layers (Gerbault & Willingshofer 2004), or lithospheric plates (e.g. Molnar & Tapponnier 1975) as controlling factor in terms of strain distribution, structure and style of deformation.

Our study focuses on this strength contrast across plate boundaries. In particular, we investigate differences in the structural evolution of collision zones, their deep structure, the relationship to higher-level deformations and the resultant topographic expression for conditions of continental convergence as a function of the relative strength contrast of the colliding plates. For this purpose we use a fully mechanical approach, namely lithospheric-scale analogue modelling, which is not restricted by the amount of imposed strain and allows incorporating lateral material transfer. We subsequently discuss implications of our modelling results on aspects of the tectonic evolution of the Eastern Alps in Europe, from where a wealth of surface and subsurface data allow constraining the large-scale geometry of the mountain range as well as its evolution through time.

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