Deformation of the Pannonian lithosphere and related tectonic topography: a depth-to-surface analysis

Abstract

Fingerprints of deep-seated, lithospheric deformation are often recognised on the surface, contributing to topographic evolution, drainage organisation and mass transport. Interactions between deep and surface processes were investigated in the Carpathian-Pannonian region. The lithosphere beneath the Pannonian basin has formerly been extended, significantly stretched and heated up and thus became extremely weak from a rheological point of view. From Pliocene times onward the ‘crème brulee’ type lithosphere has been subject to compressional stresses and, albeit at low strain rate, continuously shortened. Field data and numerical models suggested lithospheric folding as a primary mode of intraplate deformation resulting co-existence of subsiding and uplifting areas. Complementing previous studies, analogue tectonic models were performed, which enabled the simulation of the physical properties of the Pannonian lithosphere under laboratory conditions. Model results support that far-field tectonic and gravitational stresses could trigger folding of the low-strength Pannonian lithosphere. The amplitude of folding was comparable to the inferred amount of Pliocene-Quaternary subsidence and uplift. The dominant wavelength of folding was ~400 km in all experiments. However, shorter wavelength folding were also observed and found to be amplified by initial thickness variations of the brittle crust. This multi-wavelength behaviour of folding is in line with field data and numerical predictions. 3D analogue models shed light on the strain partitioning between lithospheric folding and activation of strike-slip zones in the upper crust. Horizontal shortening was absorbed by the folding of the ductile lithosphere and distributed among multiple shear zones. As a consequence of the ‘crème brulee’ structure, the low strain rate and the number of fault zones, the individual seismo-active surface is reduced and only low slip rates can occur. These observations explain the recorded low seismicity in the Pannonian basin interior. The analogue models gave an independent verification of the concept that folding of the weak Pannonian lithosphere controls basin inversion and moulds topography. Surface expressions are presented by case studies from the western part of the basin, where inversion has the most pronounced imprints on the landscape. The investigation focused on landforms that had been thought to be pre-defined by brittle tectonics. Revision of these hypotheses was accomplished by the interpretation of high-resolution, near-surface geophysical data. Geophysical imaging of the sub-surface provided further evidence of smaller-scale folding. Instead of fault control, a combination of Aeolian erosion and folding was found to play a key role in the formation of the analysed morphology. Rivers are elements of the surface that are sensitive recorders of differences in subsidence or uplift rates. Effects of crustal movements on drainage network development were investigated in a feasibility study. The catchment area of the Carpathian hinterland was analysed by means of a fractal study. The results confirmed that stochastic behaviour of erosional processes has been overprinted by alterations related to differences in the uplift history. Fractal dimensions reflected tendencies that correlated to different amounts of uplift. In summary, close relationship between lithospheric to crustal deformation and surface processes has been confirmed in many new aspects in the evolution of the Pannonian basin.