Model of syn‐convergent extrusion of orogenic lower crust in the core of the Variscan belt: implications for exhumation of high‐pressure rocks in large hot orogens

Abstract

International audienceReflection seismic section, field structural analysis and gravimetric modelling of orogenic lower crust in the core of a Carboniferous orogenic root reveal details of the polyphase process of exhumation. Subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the NW. The fabrics are developed in both mid-crustal and lower crustal high-pressure granulite units as a result of intensive NW-SE intraroot horizontal shortening driven probably by the west-directed collision. In granulites, the steep fabrics originated as a result of extrusion of orogenic lower crust in a similar to 20 km wide vertical ascent channel from lower crustal depths at 350-340 Ma. The large granulite bodies preserve older granulite facies fabrics documenting a two-stage evolution during the exhumation process. Surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth, suggesting preservation of the similar to 20 km wide subvertical tabular structure reaching Moho depths. Horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units. This structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid-crustal levels (765 degrees C and 0.76 GPa) at 342-337 Ma. Large massifs of extruded and progressively dismembered felsic granulites disturbed mid-crustal fabrics in the surrounding horizontally flowing partially molten crust. The horizontal mid-crustal flow resulted in collapse of the supra-crustal Tepla-Barrandian Unit (interpreted as the orogenic lid) along a large-scale crustal detachment above the extruded lower crustal dome. The presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens

Contrasting mafic to felsic HP-HT granulites of the Blanský les Massif (Moldanubian Zone of southern Bohemia): complexity of mineral assemblages and metamorphic reactions

“…Moreover, we will show that the history and original chemistry of the most common calc-alkaline granulites (termed type 1 here) can be particularly well assessed in relict S 1 /S 2 domains in the southern part of the BLGM (Franěk et al (2011a). Contrary to previous ideas, some of the most siliceous and Ky-rich granulites, designated as type 2, seem to be residual, i.e.…”

Intrusive and deformation history of the Ševětín Pluton, Moldanubian Batholith: record of polyphase tectonic evolution of the Blanice Graben, Bohemian Massif

“…The metamorphic foliation (S 1 ; this study) in Moldanubian rocks to the W and SE of the SP can be, on regional scale, correlated with the subhorizontal foliation S 4 of Franěk et al (2011). Its formation was related to the LP-MT stage of deformation of the Moldanubian Unit at c. 340 Ma (Franěk et al 2011).…”

“…Its formation was related to the LP-MT stage of deformation of the Moldanubian Unit at c. 340 Ma (Franěk et al 2011). Further to the E of the SP, the metamorphic foliation becomes gradually steeper due to diapiric formation of the Pelhřimov granite-dome Complex (Verner et al 2014) at c. 328-327 Ma.…”

“…Recent structural studies documented that many of the high-pressure lithologies within the Gföhl Unit represent vertical extrusions of lower crustal rocks (e.g. Finger et al 2007;Franěk et al 2011;Schulmann et al 2014). The granulite-facies metamorphism occurred at about 340 Ma (e.g.…”

“…This study focuses on the identification of tectonic regimes that controlled the evolution of central and Holub 2007;Franěk et al 2011) and eclogite (e.g. Medaris et al 2005) massifs and into the medium-pressure mid-crustal Drosendorf and Ostrong units (Dallmeyer et al 1995;Franke 2000).…”