THE NORTH ATLANTIC CALEDONIDES --- A HIMALAYA -TIBET ANALOGUE

For the last thirty years, the Caledonide Orogen of the North Atlantic margins has been known to share many of the characteristics of the Himalaya-Tibet Orogen. Vast underthrusting of Laurentia by Baltica in the Silurian and early Devonian, and of Asia by India during the last 50 million years are comparable. In the Caledonides, allochthons of the Seve Nappe Complex, derived from the outermost parts of t Baltica margin, including the COT, were metamorphosed in a continental margin subduction system under granulite , eclogite and even, locally, UHP conditions and emplaced at least 300km eastwards (present-day coordinates) onto the Baltoscandian platform. Studies of the PTt history of emplacement of the Seve nappes indicate that isothermal decompression and migmatization occurred in the earliest Silurian (c. 440Ma) and was followed by ductile extrusion of the hot allochthon (perhaps channel flow).

A drilling program to investigate the processes of emplacement of the far-travelled Seve allochthon and its influence on the underlying nappes and basement has recently been approved by the International Continental Scientific Drilling Program. The focus will be on defining the PTt paths in detail, from the inverted metamorphic gradients beneath the granulite facies allochthon down through the underlying nappes and basal detachment into the basement. Close integration with geophysical studies, particularly shallow reflection seismic, has been essential for defining the detailed structure in the classical area around Åre in western Jämtland, where two, 2.5 km drill-holes will be located.