Quantitative structural analyses and numerical modelling of ...

J. metamorphic Geol., 2008, 26, 273–297 doi:10.1111/j.1525-1314.2007.00755.xVertical extrusion and horizontal channel flow of orogenic lowercrust: key exhumation mechanisms in large hot orogens?K. SCHULMANN, 1 O. LEXA, 1,3 P. ŠTÍPSKÁ, 1 M. RACEK, 2 L. TAJČMANOVÁ, 2,4 J. KONOPÁSEK, 2,3J.-B. EDEL, 1 A. PESCHLER 1 AND J. LEHMANN 11 Université Louis Pasteur, EOST, UMR 7516 - 7517, 1 Rue Blessig, Strasbourg 67 084, France2 Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic3 Institute of Petrology and Structural geology, Charles University, Albertov 6, 128 43, Prague, Czech Republic4 Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, I-35 137 Padova, ItalyABSTRACTA large database of structural, geochronological and petrological data combined with a Bougueranomaly map is used to develop a two-stage exhumation model of deep-seated rocks in the eastern sectorof the Variscan belt. An early sub-vertical fabric developed in the orogenic lower and middle crustduring intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These eventswere responsible for exhumation of the orogenic lower crust from depths equivalent to 18)20 kbar todepths equivalent to 8)10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub-horizontal fabrics developed at mediumto low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record theearly vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root inresponse to SE-directed Saxothuringian continental subduction. Fabrics that record the later subhorizontalexhumation event relate to an eastern promontory of the Brunia continent indenting into therheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogeniccrust over the Brunia continent in a north-directed sub-horizontal channel. This sub-horizontal flowoperated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks andboudins of lower and middle crustal rocks and for their progressive thermal re-equilibration. Theerosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow oflower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin.Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SEorientedSaxothuringian continental subduction in the Early Visean, suggesting a major switch of plateconfiguration during the Middle to Late Visean.Key words: Bohemian Massif; channel flow; exhumation; orogenic lower crust; Variscan belt.INTRODUCTIONCurrent concepts of exhumation of deep-seated rocksin convergent orogens are generally based on the styleof the pressure–temperature–time (P–T–t) pathretrieved from high-pressure (HP) to ultra-highpressure(UHP) rocks (e.g. Duchene et al., 1997). Onegroup of exhumation mechanisms for these rocks hasbeen inferred from conceptual or numerical modelsdriven by subduction–accretion processes that result ineither corner flow circulation within an accretionarywedge (Platt, 1986, 1993; Allemand & Lardeaux, 1997;Gerya & Stockhert, 2006) or buoyancy-driven exhumationof subducted continental crust (Chemendaet al., 1995). Another group of conceptual models hasbeen developed for gravity-driven exhumation of HProcks in thickened orogenic root systems. In thesemodels, processes such as convective removal of atectospheric root (England & Houseman, 1988;Andersen et al., 1991) or lateral variations in gravitationalpotential energy of thickened continentallithosphere (Milnes & Koyi, 2000; Rey et al., 2001;Vanderhaeghe & Teyssier, 2001) drive the exhumation.Recently, an alternative model has been developed forthe large-scale horizontal movement of melt-bearingmiddle crust based on channel flow (Beaumont et al.,2001, 2006; Godin et al., 2006), and has been appliedto explain the ductile extrusion of medium-pressuremetamorphic rocks along the Himalayan front (Grujicet al., 1996; Jamieson et al., 2002, 2004). Whereas thefirst group of models focuses on explaining verticaldisplacements of UHP and HP rocks, the second groupof models emphasizes the importance of large-scalehorizontal displacements in orogens.Monocyclic continuous models of orogenic lowercrust exhumation may be supported by structural andkinematic field studies that emphasize the 2D characterof the exhumation process (e.g. Milnes & Koyi, 2000).However, more commonly there are complexities inthe 3D character of the tectonic evolution of largeÓ 2007 Blackwell Publishing Ltd 273133