Dgam. Aerden et J. Malavieille, Origin of a large-scale fold nappe in the Montagne Noire, Variscan belt, France, J STRUC GEO, 21(10), 1999, pp. 1321-1333
Detailed structural analysis of a large recumbent fold in low-grade Palaeoz
oic sediments of the Montagne Noire, and the previously established tectono
metamorphic history of its crystalline substratum, indicate a two-stage dev
elopment history. First, relatively homogeneous crustal thickening produced
folds with steep axial planes associated with a regional cleavage (S-1). S
ubsequently, these structures became rotated and amplified in a subhorizont
al flow with pure shear and simple shear components related to low-angle th
rusting. The development of an associated horizontal crenulation cleavage (
S-2) with a component of vertical shortening of S-1, plus evidence for a sy
nchronous decompression of autochthonous units allows this flow regime to b
e placed in the context of a thrust wedge undergoing gravitational spreadin
g above a continuously contracting footwall. Decoupling of an extending thr
ust wedge from a contracting footwall by a basal thrust with low shear resi
stance, explains bulk shortening, uplift and exhumation of mid- to lower-cr
ustal autochthonous basement during thrusting and recumbent folding at high
er levels. Structural relationships in the southern Montagne Noire demonstr
ate that traditional tectonic units recognized in this area, the so called
'nappes', are in fact unrelated to the recumbent folding as inferred previo
usly. These 'nappes' postdate not only the recumbent folding, but also a la
ter phase of refolding (D-3) associated with the formation of a gneiss dome
. The traditional nappe division can be simplified to a hanging wall and fo
otwall domain of a major decollement that formed during a second syn-collis
ional collapse (D-4) associated with the formation of intermontane Stephani
an basins. The late timing of the 'nappes' and simplified tectonic structur
e of the southern Montagne Noire imply a thinner D-2 recumbent fold structu
re with only one major anticline-syncline pair, and is in better agreement
with the low metamorphic grade of the area. (C) 1999 Elsevier Science Ltd.
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