Polyphase tectonothermal evolution of the Scandinavian Caledonides

Summary A polyphase tectonothermal evolution characterizes many of the allochthonous sequences which constitute the Scandinavian Caledonides. Initial Late Cambrian consumption of Iapetus oceanic crust occurred outboard of the Baltoscandian margin with development of an island-arc above a west-dipping subduction complex. Imbrication, polydeformation and variably high-grade metamorphism of Baltoscandian miogeoclinal rocks and underlying Precambrian crystalline basement occurred within an accretionary prism which developed in the subduction complex. This resulted in eastward younging, diachronous metamorphism throughout the Early and Middle Ordovician. Cooling during subsequent uplift was regionally variable, and is reflected by the wide range of radiometric dates reported for the Baltoscandian allochthons (455–535 Ma). This early Caledonian activity has been termed the Finnmarkian orogeny; however, it is best viewed as a complex, diachronous series of tectonothermal events which variably affected the Baltoscandian margin from the Late Cambrian through the Middle Ordovician. Almost complete closure of the Iapetus Ocean appears to have been accomplished by the Late Ordovician when eugeoclinal terranes which had developed along the Laurentian margin were tectonically juxtaposed with eroded remnants of the Late Cambrian-Middle Ordovician arc. Initial imbrication of the Laurentian eugeoclinal sequences with the arc occurred during the Early Silurian (Late Llandovery). These were subsequently imbricated and infolded with predeformed Baltoscandian allochthons. The resultant composite allochthon was subsequently thrust eastward onto Silurian sedimentary successions deposited over previously metamorphosed Caledonian basement. This sequence of Early Silurian to Early Devonian tectonic events has been collectively termed the Scandian orogeny; however, they are regionally diachronous and should be bracketed with the local controls available. Scandian dates of 425–430 Ma are locally recorded in allochthons along eastern portions of the orogen and they probably date initial nappe translation to higher crustal levels along the Baltoscandian margin. In lower structural units Scandian dates range from 410 Ma in central to 385 Ma in western portions of the orogen. These are interpreted to date rapid cooling during the uplift associated with a crustal rebound which followed eastward translation of overriding allochthons.