Paper No. 2
Presentation Time: 8:35 AM


JOHNSTON, Stephen T.1, LAHTINEN, R.2 and NIRONEN, Mikko2, (1)School of Earth and Ocean Sciences, University of Victoria, Bob Wright Centre, PO Box 3065 STN CSC, Victoria, BC V8W 3V6, Canada, (2)Geological Survey of Finland, PO Box 96, Espoo, FIN-02151, Finland,

Magmatic arcs are the factories within which continental crust is manufactured. Making equidimensional continents out of elongate, narrow magmatic arcs is problematic. Sequential accretion of parallel arcs implies unrealistic rates of crustal growth and extreme translation rates. An alternative is to shorten elongate arcs or orogens into equidimensional continental domains by buckling of the linear systems about vertical axes of rotation into one or more coupled oroclines or ‘terrane wrecks’. A prediction of the terrane wreck model for continental construction is that oroclines should be common features of Precambrian cratons. Arcuate Precambrian orogens are common, and some of these structures have been pointed to as potential oroclines. The central part of the Paleoproterozoic Svecofennian Orogen (SO) in the Fennoscandian Shield is characterized by two continuous large arcuate structures; a southerly convex to the west bend that is continuous into a northerly convex to the east bend. Here we ask do these arcuate structures constitute a pair of coupled oroclines. A test of the orocline model is to determine if tectonic vectors (TVs) change as a function of strike around the arcuate structures. TVs established in the SO include: structural vergence; metamorphic gradient; the direction oceanward as indicated by the distribution of subaerial vs submarine volcanic rocks; the component of older radiogenic crust as indicated by Epsilon Nd values; and the crustal conductivity gradient We demonstrate that TVs vary as a function of structural strike around both bends of the SO. In addition, a number of geological belts, including a MORB/EMORB volcanic rocks and Ni-bearing intrusions, are continuous around both bends. These observations are consistent with the geometry of the SO being the result of oroclinal buckling of an originally linear orogen. Palinspastic restoration of the central SO to an originally linear geometry yields a 1000 km long orogen; restores the TVs to a common orientation; explains continuity of geological belts around the bends; shows that the orogen consists of a SW-facing arc that was shortened prior to oroclinal buckling along NE-verging thrusts; and implies that rapid construction of large areas of stable continental lithosphere in the Paleoproterozoic was facilitated by terrane wrecks.
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