Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

Abstract

27 p.International audience[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Teplá-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material

From Cadomian arc to Ordovician passive margin: geochemical records preserved in metasedimentary successions of the Orlica-Śnieżnik Dome in SW Poland

“…Following the model of linnemann et al (2007), it may be hypothesized that deposition of the older monotonous MF preceded events related to the Cadomian orogeny and occurred in a back-arc setting during the time span preceding 540 Ma. Thus, the chemical affinity of the deformed and metamorphosed volcano-sedimentary successions of the OSD, as reported here, with the neoproterozoic to Early Palaeozoic volcano-sedimentary successions widespread in the Saxo-Thuringian Zone (linnemann and Romer 2002) corroborate the isotopic data and the interpretations that the OSD represents an exhumed fragment of the Saxo-Thuringian Zone (Chopin et al 2012;Mazur et al 2012).…”

“…according to Fischer (1936), the discontinuity between the MF and SF was intruded by the granitic precursor of the Śnieżnik orthogneiss, a view that has never been questioned. However, more modern models suggest that the Śnieżnik granite intruded the volcano-sedimentary succession along the pre-Variscan subhorizontal planar structure (e.g., Don et al 1990Don et al , 2003Chopin et al 2012). The entire metasedimentary sequence is believed to form a continuous succession of either neoproteorozic to Cambrian age (micropalaeontological evidence, e.g., Gunia 1974Gunia , 1989Gunia and Wierzchołowski 1979) or of Cambrian to Ordovician age (Jastrzębski et al 2010; detrital zircon geochronology).…”

“…The tectonometamorphic evolution of the Polish part of the OSD involved between three and five tectonometamorphic events related mainly to the Variscan collisional episode and crustal-scale folding (e.g., Murtezi 2006;Jastrzębski 2009;Szczepański 2010aSzczepański , 2011aSkrzypek et al 2011a, b;Chopin et al 2012). Variscan metamorphism is partly characterized by a Barrovian zonal pattern with biotite-, garnet-, staurolite-, kyanite-and sillimanite zones developed (Opletal and Domečka 1983;Jastrzębski 2009;Szczepański 2010a;Chopin et al 2012).…”

“…Variscan metamorphism is partly characterized by a Barrovian zonal pattern with biotite-, garnet-, staurolite-, kyanite-and sillimanite zones developed (Opletal and Domečka 1983;Jastrzębski 2009;Szczepański 2010a;Chopin et al 2012). …”

“…The Upper Cretaceous nysa Kłodzka Graben serves to divide the OSD into the eastern Śnieżnik Massif (SM) and the western Bystrzyckie and Orlickie Mountains Massif (BOM). Recently, the OSD has been interpreted as a mantled gneiss dome formed in response to crustal-scale folding initiated during Variscan collision of the Saxo-Thuringia and Brunia terranes (Chopin et al 2012). In this model, the OSD represents Saxo-Thuringian crust underthrust beneath the Teplá-Barrandian terrane (TB) and subsequently exhumed in front of the Brunia rigid butress.…”