Role of the foredeep evaporites in wedge tectonics and formation of triangle zones: Comparison of the Carpathian and Pyrenean thrust fronts

Syntectonic evaporitic levels within foredeep basin play an important role during the last stages of development of fold-and-thrust belts. These evaporites easily transmit tectonic stresses acting as preferred detachment levels. Both within the Spanish Pyrenees as well as within the Polish Carpathians syntectonic evaporites developed in their foredeep basins that controlled fi nal stages of thrusting within the orogenic wedges. Within the SE frontal Pyrenees, thick and laterally extent evaporites of the Cardona and Barbastro formations controlled the position of ramps and backthrusts as well as of triangle zones developed within the Ebro basin. Overlap zones of successive evaporitic levels defi ned the location of anticlines on the hangingwall of thrust ramps, whereas the external pinch outs of the uppermost evaporitic level defi ne the position of backthrusts. Frontal Polish Carpathian orogenic wedge including a zone of deformed Miocene foredeep sediments (the Zgłobice unit) is also characterized by the presence of Middle Miocene foredeep evaporites, that strongly infl uenced Miocene stages of the Carpathian thrusting. In the Tarnów area, where foredeep evaporites are thinner and consist mainly of anhydrite, a triangle zone developed within the Zgłobice unit between the north-directed frontal thrust of the fl ysch (pre-Miocene) Outer Carpathians and the southdirected backthrust related to the Miocene anhydritic layer. The tip point of this triangle zone may have been controlled either by location of a zone of rapid thinning of anhydrites above the northern slope of the pre-Miocene erosional paleovalley, or by an overlap zone of two anhydritic units above this slope. The Zgłobice unit in the Wieliczka area is characterized by a system of north-directed tectonic slices carrying thick rock salt interlayered by siliciclastics. The Wieliczka triangle zone might have formed where lateral changes of evaporitic facies occurred at the transition from thick rock salt to thinner anhydrite faciesComparative study of the Carpathian and the Pyrenean orogenic fronts was supported by the Polish (the Ministry of Science and Higher Education project No 6.12.0002.00.0), and the Spanish (Programa de Cooperación Científico-Técnica hispano-polaco 2004-2005) funds for bilateral scientifi c cooperation. Interpretation of seismic data from the Polish central Carpathians that formed a basis for this paper and was presented in Krzywiec et al. (2004) was supported by the Polish Oil & Gas Company