Flow Of Partially Molten Crust Controlling Construction, Growth And Collapse Of The Variscan Orogenic Belt

Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c.

• Within our regional conceptual scheme, some of these proposals may be checked, discussed, and eventually ruled out.
• Single Ocean model of Paris and Robardet ; Martínez Catalán et al. ; Nance et al. .
• The early U2 plutons were emplaced at shallow depths P ≤ 0.4 GPa forming elliptical bodies NW–SE trending characterized by a sub-horizontal foliation, and magmatic lineation (Casini et al. 2015).
• In the French Massif Central, until the middle of the 20th century, the prevailing model attributed the high-grade granitic-gneisses to a crystalline basement and the paragneisses and schists to a sedimentary cover deposited from the Neoproterozoic throughout the Paleozoic (Jung, 1953; Roques, 1971).

Moreoever, geochronological data available for the Najac clogites point to burial coeval with the UGU central nappe exposed in the Haut-Allier, Marvejols and Rouergue, but also for a much older exhumation. These data suggest that the Najac eclogites are part of a nappe equivalent to the MAU, which has a different P-T-t https://best-of-istanbul.com/ history than the UGU. In the northern part of the E-FMC, the position of the late Devonian Brévenne back-arc, to the south of the Morvan arc represented by the late Devonian Somme Unit, has been used to infer a southward subduction of the Rheic Ocean along a continental active margin (Faure et al., 1997, 2008).

Properties Groningen Gas

In addition to the thermal input at the base of the orogenic crust, partial melting might also be enhanced by exhumation, as dehydration melting reactions are crossed during decompression . The preservation of platform carbonates of Visean age in the northern part of the French Massif Central and around the Montagne Noire, indicates that these regions were below sea level at this time. Other sedimentary sequences of this period corresponds to the erosion products of subcontemporaneous volcanics and plutonics traping organic matter in continental basins delimited by strike-slip shear zones or normal faults (Bertaux et al., 1993; Bruguier et al., 1998; Thiéry et al., 2009). In the North-Eastern Massif Central, the extrusive equivalents of the granitoids record a drastic shift in composition during the middle Carboniferous. They change from mafic, low to medium-K calc-alkaline to felsic and high K , broadly similar to MPG in composition.

2 The Middle Crustal Units

This model provides a potential explanation for the enigmatic high geothermal gradient identified in the external zone and for propagation of crustal thickening in the foreland beneath supracrustal rocks. Our model is also compatible with the shallow northward plunge (0–30°) of the mineral and stretching lineation displayed along the Sillon Houiller (Grolier and Letourneur, 1968; Bonijoly and Castaing, 1984).