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Geophysics of the Caledonian-Appalachian Orogen |
Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada
Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
The Appalachian-Caledonide orogen extending from the southeastern USA to northeastern Canada and Britain now has more crustal-scale seismic reflection profiles recorded across it than any other orogen in the world. From SW to NE these include the COCORP Ouachita traverse, the COCORP southern Appalachian traverse, the COCORP northern Appalachian traverse, the BIRPS Western Isles-North Channel (WINCH) offshore profile, the BIRPS Moine-Outer Isles Seismic Traverse (MOIST) profile and the BIRPS SW Approaches Traverse (SWAT) profile. In addition there are several seismic profiles in the eastern USA and Canada with data to mid-crustal (15–20 km) depth within the crystalline ‘core’ of the Appalachians. In eastern N America, where the Caledonian (Taconian) deformation is overprinted by Acadian and Alleghanian deformation, the seismic reflection data clearly indicate large-scale (several hundred kilometres) cratonward thrusting of shelf, off-shelf and basinal facies sediments and metasediments. The eastern limit of undeformed Precambrian (Grenville) basement is probably represented by a zone of E-dipping reflectors which, in New England, can be correlated with surface exposures of faults and metasediments. Two profiles offshore from Scotland cross the Caledonide orogenic front. The BIRPS MOIST profile indicates that the Moine thrust zone is also characterized by E-dipping reflectors which extend to mid-crustal depths. In contrast with the data from the Appalachians, the BIRPS MOIST, WINCH and SWAT data show remarkably continuous reflections from the crust-mantle transition, as identified on seismic refraction data both across strike (LISPB) and along strike (ICSSP and CSSP). The Moho is at a depth of about 30 km throughout much of Britain and is commonly overlain by a thick (4–5 s travel time or about 12–15 km) layered lower crust at depths between 15 and 30 km. In some areas the data offshore from Scotland show deep reflections which extend into the mantle. Additionally, many of the profiles in eastern N America and Britain provide strong evidence that late normal faults (Mesozoic) may be reactivated older compressional structures (Caledonian and Variscan in Britain; Alleghanian in the southern Appalachians).
