The Tornquist Zone northwest of the Carpathians: An intraplate pseudosuture

Abstract

Deep seismic surveys, teleseismic tomography and other geophysical studies all suggest that the Tornquist Zone is an old plate boundary or suture, but geological relations indicate an intraplate origin and Permo-Carboniferous to Early Tertiary age. From the North Sea, the Tornquist Zone is developed on the Baltica side of the Caledonian collisional suture (Thor suture, formed by closure of the Thor Ocean/Tornquist Sea). To satisfy both geological and geophysical data, a new model is presented for the formation of the Late Carboniferous-Early Permian NW-SE fault zones that dismember the northern Variscan foreland and outline its border against the Baltic Shield and East European Platform. They are interpreted to have formed in relation to deep, normal/oblique sense, Wernicke-type, listric detachments that sole below the rigid litho-sphere. Detachment induced attenuation of the foreland's Caledonian and Proterozoic lithosphere triggered endogenic pro-cesses that eventually produced a more shallow asthenosphere-lithosphere boundary and a levelled, more high lying, seismic Moho. The thick lithosphere below the Baltic Shield and East European Platform was sheltered by a NE-dipping boundary detachment, which absorbed also ensuing extension and compression. When, by Early Tertiary, the inverted Tornquist Zone rose from the proximal part of its hanging wall, a strong contrast had been established between the attenuated and reset lithosphere southwest of the Tornquist Zone, and the unaffected, thick and cool lithosphere of the neighbouring Shield and Platform.