Dating Short-Lived Late-Orogenic Extensional Events in the Variscan Belt of Central Europe

Urs Schaltegger Institute of Isotope Geology and Mineral Resources, ETH Zentrum, 8092 Zürich, Switzerland

Fernando Corfu Dept of Geology, Royal Ontario Musem, 100 Queen's Park, Toronto, Ont. M5S2C6 Canada


The Variscan orogen in Europe was built during several collisional and subduction events throug Upper Paleozoic times. The late-orogenic stage lasted for about 50 million years and was dominated by oblique collision, dextral wrenching and extension. This is Indicated by dextral strike-slip faulting, the formation of extensional or pull-apart basins, exhumation of deep crustal units and magmatism.

The timing of these events was investigated by high-
resolution U-Pb geochronology within four key areas: The Southern Vosges volcano-sedimentary basin, the Southern Black Forest (Badenweiler-Lenzkirch zone), the North Swiss Permocarboniferous Trough (drilled by the Swiss Cooperative for Radioactive Waste Disposal, NAGRA) and the Central Alpine basement (Aar massif).


Precise U-Pb dating of zircon, titanite and monazite from volcanic, plutonic and metamorphic rocks revealed that tectonic and magmatic events in different areas of the orogenic belt were contemporan-eous and had a duration of only few million years. A first generation of volcano-sedimentary basins formed in the time from 345 to 340 Ma (i.e. Visean). Sedimentation was coeval with volcanism, which started with extrusion of a bimodal suite and ended with large ignimbrite eruptions, dated at 340 Ma in the Vosges and Black Forest areas, at the top of the sedimentary pile. Basin subsidence was followed by granitoid plutonism at 340 Ma (Vosges), 334 Ma (Central Alps) and 330 Ma (Black Forest) These granites are mainly crust-derived
but also incorporated lithospheric mantle components in some places. The magmatism is broadly contemporaneous
to exhumation tectonics and concomitant HT/LP metamorphism, which brought granulitic crust into contact with non-metamorphic sediments. This exhumation was dated at
ca. 334 Ma in the Black Forest and at ca. 330 Ma in the Central Alpine basement.

A younger generation of volcano-sedimentary basins, occurring in the Aar massif and below the Alpine molasse cover of northern Switzerland, was dated at 303 to 299 Ma (i.e. Stephanian). The sedimentary facies was dominated by continental clastic deposits interlayered with explosive acid volcanics (tuffs, ignimbrites). The basin deposits were intruded by subvolcanic rhyolites and voluminous granites at 298 Ma (Aar massif), which show geochemical signatures of a lithospheric mantle source.

Geotectonic environment

Several geotectonic models have been proposed previously, invoking syn-convergent crustal escape parallel to the orogenic axis or orogenic collapse scenarios, for the Visean and Stephanian generation of basins, repectively. The diachronism of these two distinct extensional events
separated by ca. 30 million years of apparent tectonic
inactivity can be found in large parts of the Variscan belt and points to a far-field and large-scale (i.e. plate tectonic) control on Late Variscan tectonic processes.