Timing of Ordovician deformation in the Caledonian-Appalachian orogen

Abstract

Summary Evidence for Ordovician deformation is variably present throughout the Caledonides. The Grampian (late Cambrian-early Ordovician) and Taconian (middle to late Ordovician) major orogenies occurred during this time period and involved plutonism and metamorphism as well as deformation. The Taconian is particularly important in N America, whereas the Grampian (Penebscot), although locally identified, seems less important. Conversely, the Grampian appears to be more important in Scotland and in Scandinavia, where it is termed the Finnmarkian and constituted a major orogenic phase. Subduction and oceanic closure seems to have been an important Ordovician process, as indicated by abundant ophiolites. In Scandinavia the principal phase of thrust emplacement of the ophiolites occurred in the early to middle Ordovician. Major deformation in central E Greenland is of late Ordovician age.

Thrusting and Extension in the Scandian Hinterland, Norway: New U-Pb Ages and Tectonostratigraphic Evidence

“…4) that are intrusive into a sequence of greenschist facies, weakly folded, Ordovician fossil-bearing sedimentary and volcanic rocks. The metamorphism and deformation have been considered to represent a Mid-Ordovician, Taconian equivalent event (Hall and Roberts, 1988). Like the Early Ordovician fossils in the Støren Nappe at Hølonda, the fossils on Smøla also have Laurentian faunal affinities (Bruton and Bockelie, 1979).…”

Did the Taconic Appalachians continue into southern South America?

“…3). Both the Taconic and Famatinian belts have a well-defined structural fabric and corresponding metamorphic zonation (Hall and Roberts, 1988;Dalla Salda et al, 1991), In both the Appalachians and the Famatinian belt, thrust planes affected the lower crust and upper mantle (Haworth et al, 1988;Dalla Salda et al, 1991). The dominant vergence in the Taconic orogen is westward, toward the craton, although easterly vergence at an early stage has been recognized (Hatcher et al, 1989).…”

Timing of Silurian to middle Devonian deformation in the Caledonides of Scandinavia, Svalbard and E Greenland

“…Another important dating study is that of Dallmeyer et al (1985) and Dallmeyer (1988) who employed the 4°Ar-39Ar method. Plateau ages for hornblendes have revealed a significant Ordovician tectonothermal record in parts of the Khli and Seve Nappes (see also Hall & Roberts 1988 The work of Dallmeyer et al (1985) has shown that, as in northern Norway, the upper allochthon contains imbricated slices of polydeformed rocks carrying an early Caledonian thermal history. Again, this favours a two-stage development of the Scandian orogeny with early ductile thrusting succeeded by later imbrications.…”

“…The Western Province, in W Spitsbergen,: provides good evidence of middle Ordovician deformation and high-pressure metamorphism (Ohta et al 1983, Hall & Roberts 1988, with the unconformably overlying sedimentary sequence extending up into the Wenlock. While there are no Devonian strata known from this western province, comparison with neighbouring areas has led to the assumption that the second Caledonian tectonothermal event occurred in the period late Wenlock to Pridoli (Harland 1985).…”

“…In recent years isotopic age-determination data have proved invaluable in allowing a more comprehensive assessment of the tectonothermal evolution of the orogen, particularly in Scandinavia. There, increasing recognition has been given to the important late Cambrian to early Ordovician Finnmarkian deformation event (Hall & Roberts 1988), but the present disposition of the major nappe complexes is by and large a result of Silurian orogenesis arising from collision of the continents Laurentia and Baltica. In addition it is possible to recognize an orogen-transverse diachronism of Siluro-Devonian deformation from NW to SE.…”