A key fold structure within a Sveconorwegian eclogite-bearing deformation zone in Halland, south-western Sweden : geometry and tectonic implications

Abstract: Retro-eclogites and associated high-pressure rocks are found in the Ullared area, southwest Sweden. These rocks are evidence of the high-pressure deformation which occurred during the formation of the c. 500 km long Sveconorwegian orogenic belt around one billion years ago. Their presence is restricted to a domain north of the towns Ätran, Gällared and Ullared. Detailed structural, petrographic and geochronologic studies of the Ätran area were made with the aim of further understanding the regional structure and metamorphic history, with particular focus on a fold closure suggested by airborne magnetic anomaly maps. A cylindrical inclined south-vergent isoclinal fold with fold axis parallel stretching lineations forms a c. 4 km wide fold closure around the town of Ätran. The southern demarcation of the Ätran closure is a lithotectonic boundary where eclogites are restricted to the area north of the boundary. U-Pb zircon SIMS geochronology of a granitic meta-intrusion (Ätran granite) found just south of the Ätran closure yields an igneous intrusive age of 1388±7 Ma and a Sveconorwegian migmatization & amphibolitization age of 955±15 Ma. The intrusive age of the Ätran granite is coeval to those of the nearby Torpa and Tjärnesjö granitic metaintrusions, which are now recognized as tectonostratigraphic markers to the aforementioned lithotectonic boundary. P-T estimates for microdomains with biotite+hornblende+garnet indicate post-eclogite re-equilibrium at conditions of 8.5±1.1 kbars and 690±50°C. Thermodynamic modelling using Domino-Theriak software, combined with petrological data from samples within the Ätran area suggest decompression from a peak metamorphic pressure of c. 17 kbars. Pseudosection calculations describe a c. 2 kbar (6 km Δ depth) zone where eclogite assemblages are stable in mafic and not felsic rocks. The recognition of this eclogite facies variable zone serves as a plausible explanation for the observation of eclogite mineral relicts in the mafic but not the felsic rocks found north of the lithotectonic boundary. The parallelism of the fold axes and stretching lineations of the cylindrical folding near Ätran prompt a wrench shear component in the regional strain model. Following the regional strain model, the Ätran fold is interpreted as the lateral tip of a proposed ≥50 km wide fold nappe that propagated at least 75 km into the Eastern Segment. A probable tectonic scenario for the formation of this fold nappe and related regional deformation is a Sveconorwegian late-orogenic deformation with a present day E-W shortening.