HR: 0800h
AN: T21C-0496    [Abstracts]
TI: Himalayan-style escape tectonics vs late-orogenic extension in the Variscan Belt of Western Europe
AU: * Gebelin, A
EM: agebelin@scimail.uwaterloo.ca
AF: University of Waterloo, Department of Earth Sciences, Waterloo, ONT N2L3G1 Canada
AU: Brunel, M
EM: brunel@dstu.univ-montp2.fr
AF: University of Montpellier II, LDL-Place E. Bataillon, Montpellier, 34095 France
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, IL 62901-4324 United States
AB: The Variscan Orogenic Belt in Western Europe was formed mainly during the Carboniferous (Mississipian-Pennsylvanian) continental collision between the Gondwana and the Laurentia lithospheric plates. This major orogen, now deeply eroded, shows numerous geodynamic similarities with the Himalayan orogen, such as lateral continental-scale ductile wrench zones.
New structural, geophysical and geochronological data on the NW Massif Central (France) provide new constrains on the timing and tectonic setting of late-Variscan granite magmatism during this orogeny. Previous investigations have emphasized the role of late-orogenic extension in the emplacement of granite plutons in the Limousin region.
In contrast, the new dataset supports ascent and intrusion in a strike-slip regime. The Variscan granite plutons in the Limousin region are emplaced in a transpressive setting. They are spatially associated with major synmagmatic strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone (SASZ). A continuous band of orthogneisses and mylonites attests of the right-lateral strike-slip deformation that affected plutons over a width of 1 to 5 km. The shear zones are characterized by steeply dipping foliation and shallowly plunging stretching lineations. A few granite plutons exhibit steep lineations suggesting local northward reverse movement. The Anisotropy of Magnetic Susceptibility (AMS) was used to delineate magmatic fabrics. The Millevaches pluton, a representative example of Limousin syntectonic granites, shows a sigmoidal map pattern of magnetic foliations and lineations consistent with right-lateral shearing. Magnetic lineations rotate gradually from NNW-SSE in the center of the shear zone to NW-SE outside. Magnetic foliations are steep in the center of the shear zone and gradually become sub-horizontal outside.
New ⁴⁰Ar/³⁹Ar data shows that in Limousin, wrench tectonics started around 350 Ma and ended around 300 Ma. New U/Pb and microstructural data on leucogranite mylonites indicates that the leucogranites were emplaced and sheared simultaneously at 313 ñ 4 Ma in a right-lateral strike-slip regime.
The 3-D shape at depth of these granite bodies, investigated using gravimetric data inversion, is that of laccoliths, less than 4 km thick. In conjunction with structural data, this suggests that the Limousin granites have a flat-bottom shape. Vertical shear zones may have helped channeling magmas from a deep source to their middle crust emplacement level. Then, magmas were trapped in the subhorizontal micaschist host-rock, sub-horizontal foliation. This mode of emplacement would explain the large aspect ratio of the Limousin plutons and their large horizontal extent.
This granitic magmatism took place in a transpressional context, demonstrated by strike slip shearing and thrusting, during continental collision between Laurentia and Gondwana. The Limousin region can be described as a ductile, right-lateral, wrenching ``pop-up'' zone within which granitic magmas were channeled. The Galician region, along the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones, also associated with voluminous granite magmatism. The Galician shear zones can be interpreted as conjugate structures to the Limousin shear zones, hence forming a set of escape tectonic shear zones similar to their Himalayan counterparts.
DE: 1140 Thermochronology
DE: 1518 Magnetic fabrics and anisotropy
DE: 3625 Petrography, microstructures, and textures
DE: 8035 Pluton emplacement
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005