The influence of synorogenic extension on the crustal architecture of North Gondwana during the assembly of Pangaea (Ossa–Morena Zone, SW Iberia)

Abstract

A new structural study of a D 2 -M 2 tectono-thermal structure in SW Iberia (Ponte de Sor-Seda gneiss dome) characterized by a spatial distribution of telescoping isograds providing a record of Buchan-type metamorphic conditions is presented. It comprises an infrastructure made up of the Lower Gneiss (LGU) and the Intermediate Schist (ISU) units separated by early D 2 ductile extensional shear zones. The LGU and the ISU are composed of Ediacaran-Cambrian rocks that experienced the M 2 highest-grade metamorphic conditions (amphibolite facies). Late Ediacaran-Early Terreneuvian and Late Miaolingian-Early Furongian protolith ages for LGU (496 ± 3 Ma) and ISU (539 ± 2 Ma) orthogneisses are reported. The superstructure made of Cambrian-Devonian rocks (Upper Slate Unit. USU) deformed under M 2 greenschist facies, tectonically overlies the ISU across a D 2 extensional shear zone. Kinematic criteria associated with D 2 -M 2 fabrics indicate top-to-ESE-SE sense of shear. A late-D 2 brittle-ductile high-angle extensional shear zone (Seda shear zone) crosscut the gneiss dome. D 3 upright folds, thrusts and transpressive shear zones caused the steepening of D 2 structures and the local crenulation of S 2 foliation. The Mississippian D 2 -M 2 event recorded in the OMZ may be regarded as a regional scale phenomenon that markedly influenced the crustal architecture of North Gondwana during the assembly of Pangea. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6828875