Structural and kinematic analysis of the Corredoiras detachment: evidence for early Variscan synconvergent extension in the Ordenes Complex, NW Spain

Abstract

An important detachment is described in the
allochthonous Ordenes Complex, in the NW Iberian
Massif, and its meaning is related to the kinematics of
contemporaneous convergent structures. The Corredoiras
Detachment (CD) separates a hangingwall unit,
characterised by a medium-pressure metamorphic
gradient, from a footwall high-pressure and hightemperature
unit and an underlying ophiolitic unit. An
associated ductile shear zone, nearly 2000 m thick,
developed in the lower part of the hangingwall unit,
where the Corredoiras Orthogneiss, a Lower Ordovician
metagranite, was progressively transformed into
augengneisses, mylonitic and ultramylonitic gneisses.
The attitude of the stretching and mineral lineation in
the mylonites varies due to late refolding at map scale,
but the sense of movement can be estimated, being
roughly top to the SE. According to crosscutting relationships,
the CD developed subsequent to the
thrusting of the high-pressure/high-temperature unit
onto the ophiolitic unit, and prior to younger extensional
detachments, upright folding and strike-slip
tectonics. The geometric relationships of the CD with
the previous structures in the footwall unit, the subtractive
character of the metamorphic gap between its
hangingwall and footwall, and the available isotopic
data suggest that the CD is an early Variscan, ductile
extensional detachment, the movement of which was
roughly simultaneous with the onset of thrusting of the
allochthonous complexes over their relative autochthon.