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Caledonides |
Department of Geology, Trinity College, Dublin 2, Ireland
By reference to the Caledonide Orogen, a method is illustrated by which quantitive estimates may be obtained of lateral movements between opposing forelands of a collision orogen. The procedure involves: 1: Identification of a rigid foreland frame (2 plates) bounding the collision orogen and establishing the orientation of the main movement direction between forelands, 2: Measurement along strike of diachroneity in the ending of arc volcanism and of cooling ages for granites; relating this to a rate of migration of collision along strike of the collision suture, and calculation of the rate of plate convergence from this, knowing the angle between the collision suture and the plate movement direction, 3: Recognition of swings in strike which predate collision, and 4: Measurement of lateral offset of dated collision points in the strike slip sector of the curved orogen and calculation from this of the components of lateral shortening around the curve. Application of these methods to the Caledonide Orogen of the British Isles, Scandinavia and E Greenland, gives values for foreland convergence on a trend of 115–295° of 0.88 cm/year between the Lower Caradoc (430 Ma) and Lower Ludlow (c. 400 Ma). Convergence slowed to 0.75 cm/year on this trend during the Caledonian orogenesis between Lower Ludlow (c. 400 Ma) and the end of the Middle Devonian (c. 365 Ma), giving lateral convergences of about 267 km in the E Greenland W Norway region (c. 22%) and 216 km (c. 31%) in the paratectonic Caledonides of the British Isles.
