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
This pa per re-as sesses the tectono-ther mal evo lu tion of the fron tal Infratatricum part of the In ner West ern Carpathians orogen based on new geo log i cal-pet ro log i cal and zir con fis sion track (ZFT) data and pub lished 40 Ar-39 Ar and K-Ar data. The study area is in the Považský Inovec Mts. in West ern Slovakia, where the Infratatric Inovec Nappe com prises a micaschist-gneiss base ment and Up per Car bon if er ous-Lower Cre ta ceous cover with the Ju ras sic-Lower Cre ta ceous Humienec Suc ces sion; this lat ter is re con structed from olistoliths in the Horné Belice Suc ces sion Up per Cre ta ceous flysch. The Infratatric Inovec Nappe pos te rior part is thrust over its fron tal Humienec tec tonic Slice with infolded Up per Cre ta ceous sed i ments. This nappe ex hib its very low-tem per a ture tectono-ther mal over print ing, and this is doc u mented by elec tron probe microanalysis of meta mor phic phases and P-T es ti mates. The late Early Cre ta ceous age of this D1 stage event at approx i mately 115-95 Ma is re corded by 114 ±2 and 106 ±4 Ma phengitic white mica 40 Ar-39 Ar ages from a Lower Cre ta ceous cherty slate, and by 101 ±3 Ma 40 Ar-39 Ar age and 102 ±4 Ma ZFT age from Perm ian meta-sand stone. The Inovec Nappe was thus de rived from an Albian-Early Cenomanian accretionary wedge. The hemipelagic Up per Turonian to Lower Santonian red marls and up ward syn-orogenic Up per Santonian to Maastrichtian flysch were de pos ited on the fron tal at ten u ated part of the Inovec Nappe-type base ment in the in ferred Pieninic-Váhic (~South-Penninic) zone. This flysch con tains olistolithic to clastogeneous meta mor phosed ma te rial, in clud ing the Lower Cre ta ceous slates, sup plied from the pos te rior part of the Inovec Nappe. The 40 Ar-39 Ar age of 86 ±2 Ma from this nappes' Perm ian meta-sand stone is con sis tent with D2 thrust ing at ap prox i mately 95-85 Ma and Horné Belice fore land flysch ba sin sup ply in the D3 stage at ~85-65 Ma. The Inovec Nappe fron tal Humienec tec tonic Slice with infolded Up per Cre ta ceous sed i ments sug gests for ma tion of a Paleocene-Eocene accretionary wedge at ap prox i mately 65-40 Ma in the D4 stage. This event is con strained by 40 Ar-39 Ar age of 48 ±2 Ma from the Tatricum hang ing wall blastomylonites and whole-rock K-Ar age of 46 ±3 Ma from a Perm ian ba salt olistolith in the footwall Infratatricum Horné Belice flysch. The Infratatricum fi nally be came part of an Eocene accretionary wedge north of the Hrádok-Zlatníky thrust-fault, and re heat ing and ex hu ma tion cool ing is doc u mented by 57-37 Ma ZFT ages. Al though the Infratatricum ex hib its Late Cre ta ceous and Eocene tectono-ther mal im prints, it is a rem nant of the Early Cre ta ceous structure at the In ner West ern Carpathian front. This im poses the Infratatricum as a dis tal con ti nen tal mar gin of the Penninicum.
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