Elongated gneiss domes are common features of orogenic core zones. They are characterized by 1) alignment of their length axis parallel to the trend of the orogenic belt; 2) gneiss or migmatite core mantled by deformed metasediments; 3) discrete contact between gneiss and metasedimentary rocks; 4) steeply dipping foliation in marginal zones and shallowly dipping foliation in the core of the dome; and 5) increase in metamorphic grade towards the core.
Some gneiss domes were rigid bodies when they attained their present position in the structural framework of an orogen, but their origin is still a matter of conjecture. Models that attempt to explain the tectonic evolution of gneiss domes in the Variscan Pyrenees include diapirism (Soula, 1982), compression followed by extension (Zwart, 1986), and transpression (Carreras & Capella, 1994; Gleizes et al., 1998). In order to test existing or develop new models, we present results from field studies and analogue modelling.
Detailed structural field studies were undertaken in the Variscan core of the Pyrenees, the Axial zone, and concentrated on the Aston-Hospitalet and Bossost domes. The domes are cored by a pre-Variscan (early Cambrian?) orthogneiss and Variscan migmatites, respectively. Their structural characteristics are similar: a) schistosity associated with the major D2 deformation wraps around the domes, forming a girdle around a subhorizontal E-trending fold axis; b) mineral lineation plunges gently, trending NW-SE to NW-ESE. Complex geometries with mineral lineations oriented orthogonal to stretching direction are also observed. Orientation of gneissic foliation and quartz-rodding in the gneiss core of the Aston-Hospitalet dome are similar to schistosity and mineral lineation in the adjacent schist, implying coeval development. Kinematic indicators in the schist of the Aston-Hospitalêt dome display complex sense of shear with predominant sinistral strike-slip component. Top to the east thrusting is also observed. These patterns are not concordant with dextral shearing in the late stage of D2 as proposed by Gleizes et al. (1998), and may imply an earlier (or later ?) phase with sinistral shear component.
Analogue modelling of dome structures are performed with a shear
box apparatus that allows homogeneous deformation in general flow regimes
(Piazolo et al., in press). Experiments include deformation of viscoelastic
fluid polymer (PDMS) with inserted rigid dome-shaped objects, and domes and
layers of different viscosity (Rhodorsil Gomme) under various stress regimes.
The resulting geometries of schistosity and lineation are compared to the
observed natural orientations, and help constrain the deformational parameters
that caused
development of the gneiss domes of the Pyrenean Axial zone. Preliminary
experiments under sinistral transpressional stress yielded orientation of
lineations similar to what is observed in the field.
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