Abstract
Abstract. The small island of Groix in southern Brittany, France, is well known for
exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and
garnetiferous mica schists that mark a Late Devonian suture between Gondwana
and Armorica. The kinematics of polyphase deformation in these rocks is
reconstructed based on 3D microstructural analysis of inclusion trails
within garnet and pseudomorphed lawsonite porphyroblasts using differently
oriented thin sections and X-ray tomography. Three sets of inclusion trails
striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to
witness a succession of different crustal shortening directions orthogonal
to these strikes. The curvature sense of sigmoidal and spiral-shaped
inclusion trails of the youngest set is shown to be consistent with
northwest and northward subduction of Gondwana under Armorica, provided
that these microstructures developed by overgrowth of actively forming
crenulations without much porphyroblast rotation. Strongly non-cylindrical
folds locally found on the island are reinterpreted as fold-interference
structures instead of having formed by progressive shearing and fold-axis
reorientation. Six samples of a lower-grade footwall unit of the Groix
ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in
these rocks strike E–W, similar to the youngest set recognized on Groix
island. They record Carboniferous N–S shortening during continental
collision. These new microstructural data from southern Brittany bear a
strong resemblance to earlier measured in inclusion-trail orientations in the
northwestern Iberia Massif. A best fit between both regions suggests not more than about
15∘ anticlockwise rotation of Iberia during the Cretaceous opening of
the Gulf of Biscay.
Funder
Ministerio de Ciencia e Innovación
Subject
Paleontology,Stratigraphy,Earth-Surface Processes,Geochemistry and Petrology,Geology,Geophysics,Soil Science
Cited by
7 articles.
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