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 D2–M2 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 unit (LGU) and the intermediate schist unit (ISU), separated by early D2 ductile extensional shear zones. The LGU and the ISU are composed of Ediacaran–Cambrian rocks that experienced the M2 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 M2 greenschist facies, tectonically overlies the ISU across a D2 extensional shear zone. Kinematic criteria associated with D2–M2 fabrics indicate topto-ESE–SE sense of shear. A late-D2 brittle-ductile high-angle extensional shear zone (Seda shear zone) crosscut the gneiss dome. D3 upright folds, thrusts and transpressive shear zones caused the steepening of D2 structures and the local crenulation of S2 foliation. The Mississippian D2–M2 event recorded in the Ossa–Morena Zone may be regarded as a regional-scale phenomenon that markedly influenced the crustal architecture of North Gondwana during the assembly of Pangaea.