The Caledonian-Variscan-Appalachian orogenic belts
The Variscides of Central Europe are the result of the convergence of the plates of Gondwana and Laurussia in the Paleozoic. This orogen is characterized by the juxtaposition of blocks of continental crust that were little affected by the Variscan orogeny against blocks of intense Variscan deformation and metamorphism. These low strain domains principally consist of a Neoproterozoic/Cambrian Cadomian basement overlain by volcano-sedimentary successions of an extended peri-Gondwana shelf. These Cadomian blocks are separated by high strain zones containing the record of subduction-related processes. Traditionally the high strain zones are interpreted as sutures between one or more postulated lithospheric microplates sandwiched between the two major plates. Paleobio-geographic constraints in combination with geochemical and isotopic fingerprints of the protoliths, however, imply that the Variscides are the result of the exclusive interaction of the two plates of Gondwana and Laurussia.
We explain the Variscan orogen in a two-plate scenario, reasoning that the complexity of the Variscan orogen (multitude of high-grade metamorphic belts, compositional diversity of coeval magmatism, and arrangement of foreland basins) is the result of the distribution of crustal domains of contrasting physical properties. Post-Cadomian rifting along the Cadomian-Avalonian belt, which culminated in the opening of the Rheic Ocean, resulted in vast coeval intracontinental extension and the formation of extended peri-Gondwana shelf areas, namely the Avalonian shelf and the Armorican Spur to the north and south of the evolving Rheic Ocean, respectively. Both shelf areas affected by heterogenous extension consist of stable continental blocks separated by zones of thinner continental crust.
During Variscan collisional tectonics the continental blocks behave as unsubductable crust, whereas the thinner continental crust was subductable and eventually constituted the high strain domains of the orogen. The variable interplay between both crustal types in space and time is seen as the principal cause for the observed sequence of orogenic processes. The first collisonal contact along the convergent Gondwana-Laurussia plate boundary occurred between Brittany and the Midland microcraton causing the early Devonian deformation along the Anglo-Brabant Fold Belt. This process is coeval with the initiation of continental subduction along the Armorican Spur of the Gondwana plate and the formation of back arc and transtensional basins to both sides of the Armorican Spur (e.g., Lizard, Rheno-Hercynian, Careón, Sleza) on the Laurussia plate. As further subduction along this collision zone is blocked, the plate boundary zone between the Gondwana and Laurentia plates is reorganized, leading to a flip of the subduction polarity and a subduction zone jump outboard of the already accreted blocks.
The following Devonian-Early Carboniferous subduction accretion process is responsible for the juxtaposition of additional Cadomian blocks against Laurussia and a second suite of high-pressure rocks. The final collision between Gondwana and Laurussia is marked by an intracontinental subduction event affecting the entire internal zone of the orogen. Subduction stopped at 340 Ma and the following isothermal exhumation of the deeply subducted continental crust is primarily responsible for Late Variscan high-temperature metamorphism and cogenetic voluminous granitic magmatism. During this final transpressional stage the irregular shape of the Variscan orogen was established by the highly oblique motion of the decoupled lithospheric blocks of Iberia and Saxo-Thuringia. Rapid overfilling of synorogenic marine basins in the foreland and subsequent folding of these deposits along vast external fold and thrust belts finally shaped the Variscides, feigning a relatively simple architecture.
In terms of plate tectonics, the model places the opening of the Paleotethys in the Devonian with a rotational axis of the spreading centre just east of the Variscan orogen. The movement of Gondwana relative to Laurussia follows small circle paths about this axis from 370-300 Ma. As a consequence of the incomplete closure of the Rheic Ocean after the termination of the Variscan orogeny, Gondwana decoupled from the European Variscides along the dextral Gibraltar Fault Zone. The relative motion between Gondwana and Laurussia after 300 Ma is associated with a shift of the rotational axis to a position close to the Oslo Rift, and is related to the opening of Neotethys and the evolution of the Central European Extensional Province. The Permian convergence of Gondwana and Laurussia led to Permian collisional tectonics along the Mauritanides/Alleghanides, eventually resulting in the assembly of the “Wegenerian” Pangea by the end of the Paleozoic.
Kroner, U., Roscher, M., Romer, R.L. (2016) Ancient plate kinematics derived from the deformation pattern of continental crust: Paleo- and Neo-Tethys opening coeval with prolonged Gondwana-Laurussia convergence. Tectonophysics, 681: 220-233. DOI: 10.1016/j.tecto.2016.03.034
Romer, R.L. and Kroner, U. (2015) Sediment and weathering control on the distribution of Paleozoic magmatic tin–tungsten deposits. Mineralium Deposita, 50: 327–338
Kroner, U. and Romer, R.L. (2013) Two plates - many subduction zones: the Variscan orogeny reconsidered. Gondwana Research, 24: 298-329.