Palaeomagnetic studies in the British Caledonides

Morris, William Adrian (1974). Palaeomagnetic studies in the British Caledonides. PhD thesis The Open University.



In this study a large amount of new palaeomagnetic data is reported from the Paleozoic of the British Isles. The data are consistent with episodic polar shift from the vicinity of 10°N, 180°E in the Ordovician to 0°, 145°E in the Siluro-Devonian and to 25°N, 160°E in the late Devonian-early Carboniferous. The Cambrian pole is less well defined but may have lain near 30°N, 170°E. Each polar shift appears to predate a phase of deformation, i.e. crustal drift and oceanic plate consumption significantly precedes orogenesis and continental plate collision.

Large collections were made from two regions within the British Caledonides; the English Lake District, and the South Mayo Trough. Palaeomagnetic data from the Carrock Fell Gabbro Complex indicate intrusion as a dyke at some time during the Upper Ordovician subject only to slight subsequent tilting. Folding in both the Eycott and Borrowdale Volcanic Groups is found to be essentially simple and complete by the end of the Ordovician. The end-Silurian deformation which many authors have considered 'climactic' produced tight folds only in the high-level Silurian sediments. The stratigraphically lower and more competent lavas were merely tilted and cleaved. A similar sequence of deformation has affected the rocks of the South Mayo Trough. In this case however, the end-Silurian deformation accompanied major azimuthal rotation and complicated southward thrusting.

A Proto-Atlantic ocean on the site of the present Caledonian orogenic belt has been cited by many authors on a number of separate geological criteria. Palaeomagnetic evidence from the margins of this belt in the British Isles indicates that little (or no) closure has taken place across the Caledonides since early Ordovician time. The available data do however, indicate apparent discrepancies between the Baltic/Russian and North American plates, and a British sub-plate. A number of explanations is possible:
(1) Euramerica was a rigid plate, hence some of the data are invalid,
(2) the three units of Euramerica were tenuously attached:
geological evidence for postulated plate margins outside Britain is discussed
(3) the present data are inadequate to define a British sub-plate.

In this thesis the third alternative is preferred.

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