Peter Robinson, Geological Survey of Norway (Norway)
Arne Solli, Geological Survey of Norway (Norway)
Kurt Hollocher, Union College (United States)
Michael P. Terry, South Dakota School of Mines (United States)
Robert D. Tucker, Washington University (United States)
Emily Walsh, Cornell College (United States)
Per Terje Osmundsen, Geological Survey of Norway (Norway)
Problems of geometric/metamorphic evolution of major Late Silurian - Early Devonian thrust nappes, involving imbrication and later thinning/extension of Baltican basement, continental margin strata, and rocks of magmatic arcs and ophiolites from Iapetus, are difficult in the foreland region of Scandinavia, but compounded in the hinterland. A key is in understanding the nature and sequence of nappe units, using every tool available, and their dogged tracing from simple layer-cake geometry to areas of increasing complexity, metamorphic intensity with partial melting, and tectonic thinning. Such tracing, applying broadest concepts to narrowest of outcrops in difficult terrain, has progressed for several decades, but hampered by few and decreasing numbers of adequately trained geologists to do field work and guide application of increasingly sophisticated igneous geochemistry, metamorphic petrology, structural fabric analysis, and geochronology. Without it, there is no chance to construct the geometry, much less kinematics and dynamics of this superbly exposed and accessible orogen.
A Late Neoproterozoic mafic dike swarm cutting Neoproterozoic sandstones of the Baltican extended margin, exposed in the 2 km thick Särv Nappe, Middle Allochthon, central Sweden, is now traced by geochemistry into the hinterland, where the Nappe consists of quartzites and interlayered amphibolites (some as eclogite) 3-1m thick. The same swarm is detected in slices of Mesoprotereozoic basement now included with the Middle Allochthon.
The Støren Nappe, Upper Allochthon, contains extensive Ordovician granitoid gneisses, intruding equivalent volcanics, unrelated to adjacent Mesoproterozoic Baltican basement, and with greatly less intense metamorphism. These calc-alkaline gneisses appear to be the core of a magmatic arc from the W. margin of Iapetus, like arc intrusives in the N.E. Appalachians. The arc gneisses are separated from highly metamorphosed Baltican basement and lower nappes by the early extensional Agdenes Detachment, predating final imposition of subhorizontal sinistral shear fabrics on both sets of units.
Narrow synclines exposing nappe sequences trace into regions dominated by Baltican basement. In Moldefjord syncline, 600m separates the base of nappes against basement and the base of Støren Nappe, correlating to Early Ordovician strata with Laurentian fossils. Helleneset syncline, with Risberget, Stra and Blhø Nappes, is only 100 m wide. On Lepsøy isoclinal synclines 50-200m wide, exposing Stra and Blhø Nappes, are recumbently refolded across the whole island, then refolded in upright folds related to strong subhorizontal constrictional and top-west extension, the latter cut by extensive ductile mylonite zones. Onshore studies of "brittle" faults and offshore seismic studies indicate a key role of late Scandian extension, also Mesozoic detachment faults, to be accounted for in reconstructing geometry of older structural features.