THERMAL EVOLUTION OF RIFTING AND TECTONIC INVERSION IN THE NORTH PYRENEAN ZONE USING MULTI-MINERAL THERMOCHRONOLOGY (Invited Presentation)

The North Pyrenean Zone (NPZ) is regarded as an exemplar system to understand the development and evolution of an exhumed paleo-passive continental rift margin. The present day NPZ is a narrow retro-wedge fold-and-thrust belt of the Pyrenees in France that is composed of exhumed Paleozoic basement massifs, HT-LP metamorphosed sedimentary basins, and subcontinental mantle rocks. Basement massifs in the NPZ are tectonically attenuated crustal sections that provide a means for sampling different continental lithospheric levels from the upper to lower crust and subcontinental mantle. We apply multi-mineral thermochronometry and microtextural/microstructural analysis to samples from crustal transects across the Agly and Saint Barthelemy massifs to constrain the thermal and structural history of Early Cretaceous rifting and subsequent Late Cretaceous rift margin inversion. Medium temperature apatite U-Pb depth-profiling ages from amphibolite to granulite facies gneisses have Aptian-Albian ages and microstructures showing crystal plastic deformation, suggesting that these lower structural levels of the massif were rapidly exhumed from mid-lower crustal levels (T > 450°C) during crustal thinning. Sampled middle crustal zones of the massifs have apatite U-Pb ages that fall between Carboniferous and Early Cretaceous, with complex age vs. depth profiles, variable grain internal deformation, and patchy microtextures suggesting deformation and metasomatism within shear zones near the brittle-ductile transition was active during Early Cretaceous hyperextension. Low temperature (T ~180-60°C) zircon and apatite (U-Th)/He data record Late Cretaceous inversion and Paleogene Pyrenean orogenic tectonic phases record cooling associated with onset of convergence and inversion of rift related structures. Together, medium- and low-temperature thermochronometric data integrated with field observations and microstructural analysis support rapid syn-rift exhumation of crustal blocks in the distal margin related to extreme crustal thinning and exhumation of subcontinental mantle rocks in the Early Cretaceous, followed by Late Cretaceous rift margin inversion that began in the east and propagated westwards. Results from the NPZ massifs constrain thermal evolution of the continental lithosphere during rifting and inversion and provide an analog to understanding other fossil rift systems and present-day magma-poor, hyperextended rifted margins.