The Caledonian-Appalachian Orogen

The tectonic development, division and position of the slate regions

The Caledonian-Appalachian Orogen has a general NE-SW strike and runs from Scandinavia, the British Isles to the east coast of Northern America up to Alabama. Before the opening of the Atlantic, the orogen showed a length of about 4,650 mi. and a width of about 310 mi. and corresponds to a typical continental orogen. The investigations of the tectonic development is difficult because of lateral movements of about 1,250 mi. at the end of the mountain building. In addition, the later Variscan orogeny overprinted the old mountains and the following Atlantic opening masked former relations (Barker & Gayer, 1985).

The tectonic frame of the Caledonian-Appalachian Orogen during the Early-Paleozoic was given by three continental blocks separated by oceans (Windley, 1995): Gondwana, Laurentia and Baltica.

The Iapetus-Ocean was situated between Laurentia and Baltica whereas the Tornquist ocean was situated between the continental blocks of Baltica and Gondwana. During the Arenigian time a continental fragment drifted away from the Gondwana continent. It is named Avalonia and led to the development of the Rheic-Ocean between the southern edge of Avalonia and the northern edge of Gondwana (MacKerrow et al. 1991 cited in Windley, 1995).
Windley (1995) assumes an opening of the Iapetus-Ocean within the late Precambrian about 600 million years ago which is based on isotopic and paleontologic data as well as the occurrence of pillow lava in Scotland (‘Tayvallich volcanic rocks’).

The closure took place from north to south and the collision began at the edge of Laurentia from Middle-Cambrian to Middle-Ordovician time (Windley, 1995). There are several opinions about the closure and the development of the suture and among all authors the time of the closure or collision varies between 435 to 412 million years ago (Pickering et al., Soper, Thirwall).

The mountain building was characterized by the docking of terranes that means small continental fragments and island arcs of Avalonia and Armorica slowly built a greater orogen with a complicated tectonic division of the mountain chains. Barker & Gayer (1985) assume 20 identifiable terranes from which five are associated with the southern-east edge and eight with northern-west edge. Both believe that all the other terranes evolved from micro-continental fragments within the ‘oceanic domain’ of the Iapetus.
The Caledonian-Appalachian mountains developed in form of several mountain buildings with different main deformation phases (Fig. 1), resulting in a orogen with four segments. In the Appalachian three main phases of mountain building are distinguished: the Taconic, Acadian and Alleghanian phases and all phases are of different importance within the various slate regions.


Caledonian-Appalachian Orogen

Fig. 1: Map of the Caledonian-Appalachian Orogen before the beginning of the Mesozoic drifting and the main deformation phases (click to enlarge).


The Taconic mountain building took place from Ordovician to Silurian and was related to the docking of volcanic island arcs and terranes (Avalonia) between Northern America and Baltica. Deformation of the Taconic mountain building can be seen in central Pennsylvania and Newfoundland. To the south a westward directed thrusts and a tectonic overprinting of the Alleghanian mountain building masks the former deformation (Hatcher, 1989).

The following Acadian mountain building (Silurian-Devonian) is considered as the result of the closure of the Iapetus-Ocean and the collision of Northern America with Baltica. Due to the occurrence of deformed and metamorphosed sedimentary and volcanic rocks as well as Acadian plutonic rocks, the Acadian mountain building can be reconstructed within the central Appalachian through New England up to the Canadian Appalachian whereas it is less evident in the southern part (Hatcher, 1989).

From Pennsylvanian to Permian the final part took place with the Alleghanian mountain building which was characterized by the collision of Africa with Northern America. In contrast to the two first mountain building phases which mainly formed the Northern Appalachian, the third one effected the Central- and Southern Appalachians. It led to folds and faults as well as to the deformation of the molasse within the Valley and Ridge Province. In addition, it overprinted the structures of the former two phases.


Cross section composite orogenic belt

Fig. 2: Cross section through a composite orogenic belt (after HATCHER & WILLIAMS from MOORES & TWISS, 1995). Click to enlarge.


As in Fig. 2 seen the Caledonian-Appalachian Mountains with its structural symmetry, foredeeps, thin-skinned fold- and thrust belts as well as the core with its crystalline thrusts sheets correspond to a typical orogen. (Moores & Twiss, 1995). Following this drawing, the slates of Pennsylvania would have been in ‘A’ and the slates of Newfoundland in ‘B’.
The tectonic constitution of the British Caledonian mountains and the Appalachians can be devised in several terranes or zones which differ in their geological history.


Geographic division Caledonian terranes

Fig. 3: Geographic division of the Caledonian terranes within the British Isles and the occuring slate deposits (Source: GIBBONS & GAYER, 1985). Click to enlarge.


The Caledonian mountains of the British Isles can be devised in nine terranes which show a general strike of ENE-WSW and differ by their specific pre-Devonian development (Fig. 3).
Fig. 3 shows that the Caledonian slate deposits of the British Isles are within the ‘Southern Britain Terrane’ (Wales) and the ‘Lakesman Terrane’ (Lake District).
The ‘Southern Britain Terrane’ consist of a Precambrian basement which is also known as ‘Avalonia fragment’. This rocks can be directly correlated with rocks of the ‘Avalon Zone’ of Newfoundland (King 1980 in Gibbons & Gayer, 1985). The basement is covered by Early- to Middle Cambrian sediments in which the most important slate deposits of Wales can be found.
Within the ‘Lakesman Terrane’ one can observe Early-Cambrian to Late-Silurian sediments and Ordovician volcanic rocks which are related to subduction (Gibbons & Gayer, 1985). The roofing slate deposits in this zone are situated within Ordovician and Silurian rock units with the Lake District and Cumbria.


Physiographic division Appalachian Orogen

Fig. 4: Physiographic division of the Appalachian Orogen with the position of the slate deposits (click to enlarge).


The Appalachian are often separated in ‘Physiographic Zones’ which primarily follow geomorphological criteria (Fig. 4). In general, the central and southern part is devised in following Zones: ‘Piedmont’, ‘Blue Ridge’, ‘Valley and Ridge’ and the ‘Appalachian Plateau’. The northern part is separated in anticlinoria and synclinoria. Williams (2002) classified the Appalachian in tectonostratigraphic zones which are characterized by their own structures and stratigraphy and thus reflect special geotectonic environments and paleogeographical relation during Ordovician time (Rast, 1989). The position of the slate deposits is shown in Fig. 4.

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