--> --> The Szamotuly Salt Diapir and Mid-Polish Trough: Decoupling During Triassic-Jurassic Rifting and Alpine Inversion

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The Szamotuly Salt Diapir and Mid-Polish Trough: Decoupling During Triassic-Jurassic Rifting and Alpine Inversion

Abstract

The Szamotuly Diapir is located on the southwestern shoulder of the Mid-Polish Trough just north of Poznan, Poland. The area underwent crustal-scale extension during the Triassic-Jurassic and Alpine-related inversion during the Late Cretaceous to Paleogene. The diapir is sourced entirely from the Permian Zechstein salt but there are also thin evaporites within the Triassic overburden. An array of 2-D time-migrated data, a regional 2-D depth-migrated seismic profile, and quantitative structural restorations were used to illustrate the effects of salt on rifting and inversion. Both extensional and contractional deformation were almost completely decoupled by the Zechstein salt. Beneath the salt, Rotliegend (lower Permian) half grabens were reactivated during the Triassic, offsetting the base salt but having no effect on the top salt. The only record of deep extension is regional thickening of Triassic-Jurassic overburden strata, with varying thickening patterns probably representing variable activity on rift faults. Inversion at the presalt level was accommodated by reverse movements on these faults. In the cover, the thickened Triassic-Jurassic strata were inverted to form the broad anticlinorium of the Mid-Polish Swell. The bulk of cover extension was accommodated at and near the Szamotuly Diapir. Minor extension during the Early-Middle Triassic created a linear reactive diapir bounded by inwardly-dipping normal faults. Increased extension during the Late Triassic caused salt to break through to the surface, forming a passive diapir that then widened as extension continued into the Jurassic. Along strike, coeval extension was recorded by ongoing reactive diapirism. Alpine inversion was also focused around the diapir because of its relative weakness. Along strike from the diapir, shortening occurred primarily by inversion of one of the reactive normal faults, whereas the diapir was simply squeezed. However, shortening was accommodated differently above and below the Upper Triassic Keuper salt. Lower and Middle Triassic strata simply moved laterally into salt, whether into the passive diapir or into the reactive diapir along strike. Younger strata were folded and thrusted, with delamination at the Keuper salt that was depositionally thicker adjacent to the reactive diapir. Zechstein salt squeezed from deeper levels flowed passively into the space created by delamination, producing an allochthonous salt wing in the subsurface.