The West Ny Friesland Terrane : An exhumed mid-crustal obliquely convergent orogen : by Patrik W. Nilsson

Abstract: Ny Friesland, in northeastern Svalbard, is the type-area for transpressive orogeny. The West Ny Friesland Terrane (WNT) is composed of a c. 12 km thick basal succession of amphibolite-grade schists and gneisses, previously thought to be the lower of a c. 20 km thick succession of concordant, volcano-sedimentary strata. However, this lower succession is composed of at least five thrust intercalations, where four contain Palaeoproterozoic (c. 1750 Ma) basement gneisses and Mesoproterozoic (or younger) cover sequences. Concordance between the rock units is largely the result of ductile shearing and high Caledonian strain during thrusting. These thrust sheets were assembled sequentially during E-W orthogonal shortening ( minimum c. 80 km) and resulted in a c. 150 km long, N-S trending fold-and-thrust belt, the Atomfjella Antiform, in Ny Friesland, northeastern Spitsbergen. The geometry of this structure, with thinning of some units in the east, suggests that nappe transport was west-directed. Shortening was coupled with increasing grade of metamorphism, and changes in both vertical and lateral boundary conditions during increasing sinistral transpression. This resulted in ductile hinge-parallel elongation of the thrust-stack.Comparison of geochronological and structural data from Ny Friesland with data from the Nordauslandet Terrane (NAT) indicates that these eastern areas differ fundamentally. The Palaeoproterozoic granites in Ny Friesland have no counterparts in Nordaustlandet which is dominated by a Grenvillian-age tectonothermal history. The metamorphic grade is also lower in Nordaustlandet. The NAT Neoproterozoic and Early Palaeozoic strata show affinity with the successions of Central-East Greenland. The Ny Friesland Palaeoproterozoic gneisses are similar in age to the high-grade gneisses and metasediments in North-East Greenland, suggesting a close affinity with these areas. This thesis proposes that the WNT was the result of Caledonian transpressional deformation during collision between Baltica and Laurentia, and that tectonic escape had juxtaposed the WNT and NAT terranes north of Greenland by the end of the Silurian