Polyphasal foreland-vergent deformation in a deep section of the 1 Ga Sveconorwegian orogen

Abstract

Metamorphic belts in Precambrian shields expose deep interiors of orogens and are often challenging to interpret in tectonic terms. The Eastern Segment of the 1.1–0.9Ga Sveconorwegian orogen is a metamorphic belt, which was metamorphosed at high-pressure granulite and upper amphibolite facies at 35–40km depth and shows highly complex fold patterns. We use detailed structural analysis in combination with U–Pb SIMS dating of complex zircon to identify the structural and tectonic evolution in a composite migmatitic orthogneiss complex of the Eastern Segment. We link four fold phases to late-orogenic foreland-vergent flow, and date D2 and D3 at 0.97–0.95Ga. Leucosome and mesosome of felsic metasupracrustal migmatitic gneiss contain igneous zircon that dates the crystallization of the source rock or protolith at 1695±8Ma, and 1690±8Ma, respectively, demonstrating a temporal link to unmetamorphosed or little metamorphosed igneous rocks east of the Sveconorwegian orogen (the Transscandinavian Igneous Belt). Early migmatization attributed to Hallandian orogenesis is dated by formation of secondary zircon in two leucosome samples at 1402±12 and 1386±7Ma. The pre-Sveconorwegian structure (Sc), which is strongly overprinted by Sveconorwegian deformation and migmatization, is a composite coarse gneissic layering made up of a primary compositional layering and (variably present) Hallandian leucosome veins. The dominant foliation, a pervasive gneissic banding (S1), is axial planar to intrafolial F1 folds and developed as a result of tectonic overprint of Sc; S1 is associated with a strong ESE-trending aggregate stretching lineation (L1). S1 and L1 were folded by asymmetric SE-vergent F2 folds during foreland-vergent flow. Crystallization of Sveconorwegian zircon in syn-F2 leucosome dates this phase at 970±5Ma. The sequence was subsequently deformed by symmetric and asymmetric F3 folds that are S- to SE-vergent. Syn-F3 leucosome, mineral parageneses and microtextures associated with D3 show that this deformation occurred under still high temperatures. The last ductile phase (D4) also involved the generation of leucosome synkinematic with N-S trending folds that deformed all previous structures under amphibolite facies conditions. K-feldspar-rich, originally coarse-grained and strongly deformed metapegmatite contain two generations of zircon: texturally old 1414±5Ma cores and fragments, and voluminous Sveconorwegian envelopes, and new grains that demonstrate the presence of melt as late as 958±7Ma. Ductile structures are similar in metasupracrustal and metaplutonic orthogneiss complexes. Likely, these units were tectonically juxtaposed during D1, while D2–D4 structures reflect a common tectonic evolution after their emplacement. We interpret D1–4 structures as recording WNW-ESE convergence (D1) and ESE-vergent flow (D2 and D3), followed by E-W gentle upright folding (D4). Sveconorwegian 0.98–0.96Ga foreland-vergent deformation, accompanied by migmatization at all four stages, was responsible for formation of the polyphasal deformation pattern in this part of the orogen.