Thermotectonic Evolution of the North Pyrenean Agly Massif During Early Cretaceous Hyperextension Using Multi‐mineral U‐Pb Thermochronometry

Abstract

AbstractThe North Pyrenean Zone represents a fossil hyperextended passive margin, with limited orogenic overprinting, where the thermal and structural patterns associated with crustal thinning and mantle exhumation can be studied on rocks exposed at the surface. The Agly Massif is a tilted ~10‐km‐thick crustal section of Paleozoic and upper Proterozoic magmatic and greenschist to granulitic metamorphic rocks in the easternmost North Pyrenean Zone. We applied multi‐mineral geochronometry and thermochronometry on structural/crustal transects across the massif to understand the exhumation history of the upper and lower continental crust during extreme crustal thinning and mantle exhumation. Integration of zircon and apatite U‐Pb ages provides unprecedented constraints to understand the decoupled versus coupled extensional evolution, exhumation timing of the middle‐lower crust, and the age of juxtaposition of the upper crust granitic pluton with high‐grade gneisses. The Saint Arnac pluton was emplaced and cooled in the upper crust during the Carboniferous and remained at temperatures between 450 and 180 °C until the Late Cretaceous. The middle to lower crust was metamorphosed during the Carboniferous and remained at temperatures >450 °C until the Aptian, when it was rapidly exhumed along a midcrustal shear zone. Deformation was initially decoupled along a midcrustal ductile shear zone until the whole massif was in the brittle field, with structural juxtaposition of the units, and exhumation was coupled and controlled by a major southward dipping detachment fault at the southern border of the massif. The basement massif and synrift sedimentary rocks record significantly different thermal histories during rifting.