Trans-lithospheric diapirism documented in the Variscan Bohemian Massif – comparison with numerical models

Abstract

<p>In the easternmost part of the European Variscan collisional belt, the Bohemian Massif, strongly metamorphosed felsic rocks crop out at several locations in a current distance of up to several hundreds of kilometers from the supposed contact of the subducting and overriding plates. These rocks were interpreted to originate from the subducting plate (now the Saxothuringian domain), which means that the orogenic root (the Moldanubian domain) consists of rocks that originate from both upper and lower plate. More specifically, the root domain is composed of (U)HP granulites and orthogneiss, garnet peridotites, eclogites and ultra-potassic plutons that alternate with the less metamorphosed rocks of the upper plate.</p><p>Such a process of subduction and emplacement of the subducted crust into the upper plate is called relamination. In order to better constrain the dynamics of relamination, we set up a numerical thermal-mechanical model and compare the modeling results with the data from the Bohemian Massif. The model simulates oceanic and continental subduction and takes into account non-linear visco-plastic rheology, percolation of fluids, melting and melt extraction. For different parameter values, the models show different styles of behavior, namely (i) exhumation of the subducted crust along the plate interface, and (ii) flow of the subducted crust beneath the upper plate and then incorporation into its crust (i.e. relamination).</p><p>In the former case, the material records heterogeneous peak metamorphism sampling the conditions along the subduction zone, and cooling during decompression. Similar features are typical for the metamorphic complex in the Saxothuringian domain of the Bohemian Massif.</p><p>In the latter case, the typical feature is the development of diapirs that grow from the subducted continental crust, pierce the overlying lithosphere and intrude into the middle crust of the upper plate. We show that growth of such trans-lithospheric diapirs results in a similar rock assemblage as observed in the orogenic root in the Bohemian Massif. The pressure-temperature-time paths obtained in the modeled diapirs mimic those of the Moldanubian granulites. The flow of crustal material through the mantle wedge results into mixing, hydration of the mantle and melting of both materials. Emplacement of the resulting melt into crust can explain formation of the Moldanubian ultra-potassic plutons.</p>

The Effect of Melt Infiltration on Metagranitic Rocks: the Snieznik Dome, Bohemian Massif

“…In a larger-scale tectonic context, according to the subduction-relamination-extrusion model of Chopin et al (2012a) and Maierová et al (2014Maierová et al ( , 2018, the original shallowdipping fabric is related to progressive burial. During burial, solid-state deformation and incipient melting occurred at 1.9 GPa and 700-770ºC (Chopin et al, 2012b), whereas relatively low-strain domains remained melt-free (Fig.…”

“…The growth of the dome, 60km across, was associated with unroofing and sliding of the upper plate crustal rocks along localized extensional detachments, thus enhancing final exhumation of the buoyant HP core (e.g. Skrzypek et al, 2011b;Maierová et al, 2014). The vertical extrusion of the dome was accompanied by intrusions of kilometer-scale granitoid dykes emplaced along both axial-planar zones of sub-domes and detachment fabrics (Parry et al, 1997;Lehmann et al, 2013;Závada et al, 2017).…”

“…This involves Late Devonian subduction of the Cambro-Ordovician oceanic crust and passive margin followed by underthrusting of buoyant felsic crust that resulted in its stacking beneath dense upper plate rocks (Mazur et al, 2012), a process called 'relamination' (e.g. Hacker et al, 2011;Maierová et al, 2014Maierová et al, , 2018. The continental rocks were subducted to asthenospheric depths as testified by evidence for (U)HP eclogite-and granulite-facies conditions (Bröcker & Klemd, 1996;Kryza et al, 1996;Klemd & Bröcker, 1999;Jedlicka & Faryad, 2017), whereas the middle crustal rocks achieved amphibolite facies conditions (Murtezi, 2006;Jastrzebski, 2009;Skrzypek et al, 2011aSkrzypek et al, , 2011bSkrzypek et al, , 2014Jastrzebski et al, 2019).…”