|VARISCAN FOLD BELT
The Variscan Orogen is a broad, sinuous, E-W trending fold belt, running from Portugal to Poland (Floyd et al., 1993). The orogen developed during the closure of the Rheic Ocean and subsequent collision of the Gondwana and Laurasian continents, to form the supercontinent of Pangaea, by the end of the Carboniferous (Bristow et al., 1998). Cornubia forms one small part of this fold belt. Presently a peninisula of the British Isles, Cornubia's history has been charted back to the early Ordovician, since when it has occupied positions on a number of continents and terranes and been at both destructive and passive plate margins (Bristow et al., 1998).
The Precambrian history of Cornubia is unknown but, in common with the rest of southern Britain, it is likely to have formed part of the Gondwanaland continent (Africa, Antarctica, Australia, India and South America) that occupied a position close to the South Pole (~70° S) during the Late Proterozoic/Early Paleozoic (Bristow et al., 1998). With the opening of the Rheic Ocean in the Ordovician, a section of Gondwanaland, the Avalonian Superterrane, broke away northwards carrying southern Britain with it(Durrance, 1998). Further fragmentation of this landmass gave rise to a number of terranes including Avalonia, Iberia and Armorica. Cornubia lay along the southern part of the Avalonian Terrane (Bristow et al., 1998), which had been uplifted (during Cadomian orogenic movements) prior to separation, to form an E-W trending topographic high with a series of basins and intervening shelf seas, called Pretannia. The basement comprised metamorphic, plutonic and volcanic rocks forming a magmatic arc, lying between the Rheic Ocean, to the south, and the Iapetus Ocean (extending to 20°S), to the north (Durrance, 1998).
Little is known about the character of these basement rocks apart from isolated outcrops (e.g. Eddystone Rocks in the English Channel) and studies of clasts from Devonian sediments, but rock types such as mica schists, gneisses and rhyolites have been recorded (Durrance, 1998). A small area of gneiss (the Man of War Gneiss) has been identified within the Lizard Complex (Flett, 1946; Green, 1964) of southern Cornwall. This is a relict piece of pre-Variscan basement and represents a suite of originally gabbroic to tonalitic, calc alkaline arc, plutonic rocks (Sandeman et al., 1997). They outcrop in the shoals off Lizard Head and represent an exotic suite of rocks seen nowhere else in the complex. The gneiss has been dated (Sandeman et al., 1997) at 499 Ma (Tremadoc, Lower Ordovician) which makes it the oldest dated rock in South West England.
Pretannia was deeply eroded throughout the Lower Paleozoic, supplying sediment to the Welsh Basin and to northern/central Germany (Zeh et al., 2001). The shelf sea sedimentation on the flanks of Pretannia is now recorded by exotic blocks of quartzite (of Ordovician age) that occurs trapped in Devonian melange sediments on the Lizard Peninsula (Flett, 1946), and similar pebbles found as clasts in Triassic conglomerate units in east Devon (Edmonds et al., 1975).
The accretion of Avalonia with Baltica (Fennoscandanavia and Denmark) during the Early Silurian (Late Llandovery), following the closure of the Tournquist Sea that formerly lay between them, was accompanied by considerable transcurrent movements along a series of major NW-SE trending wrench faults (Durrance, 1998). Such faults often penetrate deep within the crust and may persist as planes of weakness long after accretion has been completed. They may be reactivated by later stress fields to give vertical and horizontal shear displacements. These wrench faults (see Figure1), many of which cut across the whole Cornubian Peninsula, played a major role in the development of Devonian/Carboniferous, and later, sedimentary basins across the region (Dearman, 1963; Selwood, 1990) and have also influenced granite emplacement; their reactivation before, during and after the Variscan Orogeny has had a profound influence on the structure of southwest England that continues to the present day (Turner, 1984).
The closure of the Iapetus Ocean at the end of the Silurian saw the accretion of Avalonia/Baltica with Laurentia (North America, Greenland and northern Britain) and resulted in the Caledonian-Appalachian Orogeny (Powell and Phillips, 1985) which gave rise to the Caledonian foldbelt, the unification of northern and southern Britain, and the formation of the Old Red Sandstone continental landmass, straddling the equator. Caledonian deformation was intense across northern Britain, but decreased southwards. Across Cornubia it is difficult to locate definitive Caledonian structures, but it has been noted that many mineral lodes have a Caledonian (NE-SW) or Cadomian (E-W) trend and their host fractures may have been influenced by underlying structures in the basement (Durrance, 1998).
Throughout the Devonian Period (408-360 Ma) and the succeeding Carboniferous Period (360-286 Ma) the Rheic Ocean was in the process of closure due to the progressive northward movement of the African 'Gondwanaland' continent and its subsequent collision with Laurasia. This involved the accretion of the Avalon-Meguma and Austro-Alpine microcratons during the Middle Devonian and the Aquitaine-Iberian microcontinent during the Lower Carboniferous against the Southern margins of Laurasia (Andrews et al., 1998). By the Upper Carboniferous, the Western arm of the Rheic Ocean had been eliminated and the two macrocontinents came into collision.
The Rheic basin, trapped between the two continents, was compressed and folded to form a fold mountain belt - the Variscan Orogen, which can be subdivided into a number of tectonic zones separated by major thrusts, ranging from the northern (marginal) Rhenohercynian Zone to the (internal) Saxothuringian and Moldanubian Zones of Central Europe. The history of the Variscan Orogen is one of the staged closure of sedimentary basins by the subduction of oceanic segments, together with the development of parautochthonous and allochthonous units bounded by northwards directed thrusts. These stacked nappes were associated with low-grade (greenschist) metamorphism (Phillips, 1964; Primmer, 1985a) and later, post-orogenic granite magmatism.
Unlike much of the Variscan Orogen, the Rhenohercynian Zone (including Cornwall) developed late, in the Early Devonian, and closed rapidly, by the end of the Devonian. It comprised a series of small deepwater basins, bordered to the north by the shallow marine shelves running through modern day Somerset and North Devon. The extensional development of the orogen during the Devonian and Early Carboniferous is compatible with an ensialic back-arc setting. Northward-dipping subduction was established in Southern Europe in Lower Devonian times. Thermal doming, related to mantle convection, lead to the development of a crustal rise (composed of pre-Variscan crystalline basement mica schists and gneisses; Durrance, 1998) called 'Dumnonaea' which supplied sediment to the southern part of the Rhenohercynian Zone.