COMPARISONS OF THE PALEOZOIC OROGENIC HISTORY FOR THE NORTHCENTRAL NORWEGIAN CALEDONIDES TO THE SOUTHERN APPALACHIANS

The spectacularly exposed tectonic stratigraphy of the Scandinavian Caledonides has attracted and influenced the research of many Appalachian geologists. Central to northern portions of the Caledonides share many characteristic features with the southern Appalachians, including: a polygenetic, granitic Precambrian basement terrain with structures inherited by subsequent tectonic events; widespread evidences for Ordovician metamorphism and tectonics that are difficult to discern from the events of overprinting Silurian to Carboniferous tectonism; a regional unconformity that developed a karst terrane in extensive Cambrian limestones and was subsequently metamorphosed; large-scale exhumation of the composite metamorphic basement, cover and allochthonous sequences in Cordilleran core-complex styles of structures with footwalls comprising thousands of square kilometers in extent and master décollements that were active from Devonian to Permian time. Mapping by J. Tull (Ph.D. 1973) of Vestvagøy, Lofoten, northern Norway demonstrated early steeply dipping NNW-trending basement structures that are prevalent northward to the islands of Senja and Kvaløy (later inherited by the Senja Fracture Zone during Cenozoic opening of the North Atlantic) and the amphibolite facies metamorphic rocks of the Leknes synform. Initial ⁴⁰Ar/³⁹Ar dating of complex and locally polymetamorphic amphiboles in this region of Norway, and in parts of the southern Appalachians, provided evidence interpreted to reflect widespread Ordovician metamorphism (Finmarkian, Taconic) however more recent ⁴⁰Ar/³⁹Ar studies have documented dominant roles for Silurian, Devonian and Mississippian high-grade metamorphism (Scandian in the Caledonides, relative to the Acadian, Neoacadian and culminating Alleghanian metamorphism of the southern Appalachians). Early thrust faults carrying far-traveled parautochthonous to allochonous sequences (the ‘nappes’ of the Caledonides, or portions of the Appalachian Western Blue Ridge to Piedmont) were reactivated and overprinted by extensional structures that enabled exhumation of high-grade metamorphic rocks now exposed in Norway and the southern Appalachians, creating broad patterns of cooling ages that young toward structures of the nascent Atlantic crust.