Abstract
The Øygarden Complex is the westernmost basement window in the Norwegian Caledonides, yet, the age and evolution of this part of the Baltic Shield is largely unknown. We examined the eastern part of the window by detailed field mapping and SIMS U–Pb zircon geochronology, to disentangle the record of Caledonian and Sveconorwegian orogenesis and to constrain the long-term crustal evolution. The eastern Øygarden Complex comprises mainly Sveconorwegian metaigneous rocks, which intrude Telemarkian granitic basement, dated at 1506 ± 5 Ma. Sveconorwegian magmatism occurred in two distinct phases: Contemporaneous hornblende biotite granite and gabbro intrusions revealed crystallization ages of 1042 ± 3 Ma and 1041 ± 3 Ma, respectively. We dated younger leucogranitic intrusions at 1027 ± 4 Ma, 1024 ± 6 Ma and ca. 1022 Ma. The new ages clearly identify the Øygarden Complex as a part of Telemarkia and correlate it with the Sirdal Magmatic Belt. Furthermore, they show that the Precambrian evolution of the Øygarden Complex is distinctly different from the Western Gneiss Region. Bimodal magmatism at 1041 Ma and the absence of Sveconorwegian high-grade metamorphism in the eastern Øygarden Complex support the idea of an accretionary Sveconorwegian orogen. Following long-term residence at low temperatures, a temperature increase caused resetting of high-U metamict zircons at ca. 482 Ma. This early Ordovician thermal event might reflect extension of the Baltican margin or early Caledonian convergence. Caledonian ductile reworking involved top-to-E shearing and recumbent lineation-parallel folding followed by the formation of ductile-to-brittle normal-sense shear zones. We discuss this structural evolution in the light of existing and new tectonic models, including early Devonian core-complex exhumation of the Øygarden Complex.
Highlights
We examined the eastern part of the window by detailed field mapping and secondary-ion mass spectrometry (SIMS) U–Pb zircon geochronology, to disentangle the record of Caledonian and Sveconorwegian orogenesis and to constrain the long-term crustal evolution
It is impossible to quantify the amount of E‐W extension and N‐S shortening in these kinds of rocks, but conservative estimates imply at least 70% of N‐S shortening by upright folding in individual outcrops of strongly deformed gneisses.Toward the east, fabrics rotate into an orogen‐parallel trend: lineations plunge mostly toward the NE and foliations dip mostly steeply toward the SE
Our study shows that vertical metamorphic variations and lateral strain gradients during postorogenic transtension may cause a highly variable behavior of the crust, leading to differential exhumation and the formation of metamorphic core complexes (MCCs)
Summary
South Tibetan Detachment System, Nepal (view towards east). Peak of Machhapuchare is 6,993 m, bottom of photo is ~3,700 m. Domes expose parts of the deep interior, the so-called orogenic infrastructure (Eskola, 1948; Hodges, 2016; Teyssier and Whitney, 2002; Vanderhaeghe, 2012; Wegmann, 1935; Whitney et al, 2004) It is often controversial whether it is contraction or extension that drives doming and exhumation of highgrade metamorphic rocks (Ring et al, 1999; Warren, 2013). Numerous glacier-polished fjords transect the gneiss dome and provide continuous, excellent exposures that can be accessed by boat To exploit these conditions, we developed a novel semi-quantitative mapping scheme for ductile strain. The onshore-offshore correlation constrains dome and detachment geometries formed during the Devonian and their relation to Permian-Triassic rift faults
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