New plate-tectonic reconstructions of the Gondwana margin suggest that
the location of Gondwana-derived terranes should not only be guided by
the models, but should also consider the possible detrital input from
some Asian blocks (Hunia), supposed to have been located along the
Cambrian Gondwana margin, and accreted in the Silurian to the
North-Chinese block. Consequently, the Gondwana margin has to be
subdivided into a more western domain, where the future Avalonian blocks
will be separated from Gondwana by the opening Rheic Ocean, whereas in
its eastern continuation, hosting the future basement areas of Central
Europe, different periods of crustal extension should be distinguished.
Instead of applying a rather cylindrical model, it is supposed that
crustal extension follows a much more complex pattern, where local
back-arcs or intra-continental rifts are involved.
Guided by the age data of magmatic rocks and the pattern of subsidence
curves, the following extensional events can be distinguished:
During the early to middle Cambrian, a back-arc setting guided the
evolution at the Gondwana margin. Contemporaneous intra-continental rift
basins developed at other places related to a general post-PanAfrican
extensional phase affecting Africa
Upper Cambrian formation of oceanic crust is manifested in the
Chamrousse area, and may have lateral cryptic relics preserved in other
places. This is regarded as the oceanisation of some marginal basins in
a context of back-arc rifting. These basins were closed in a
mid-Ordovician tectonic phase, related to the subduction of buoyant
material (mid-ocean ridge?)
Since the Early Ordovician, a new phase of extension is observed,
accompanied by a large-scale volcanic activity, erosion of the rift
shoulders generated detritus (Armorican Quartzite) and the rift basins
collected detrital zircons from a wide hinterland. This phase heralded
the opening of Palaeotethys, but it failed due to the Silurian collision
(Eo-Variscan phase) of an intra-oceanic arc with the Gondwana margin.
During this time period, at the eastern wing of the Gondwana margin
begins the drift of the future Hunia microcontinents, through the
opening of an eastern prolongation of the already existing Rheic Ocean.
The passive margin of the remaining Gondwana was composed of the
Galatian superterranes, constituents of the future Variscan basement
areas. Remaining under the influence of crustal extension, they will
start their drift to Laurussia since the earliest Devonian during the
opening of the Palaeotethys Ocean. (C) 2008 Elsevier B.V. All rights
reserved.