TEMPORAL CONSTRAINTS ON PROGRESSIVE RIFTING AND HYPEREXTENSION USING BEDROCK AND DETRITAL ZIRCON (U-TH)/(PB-HE) DATING, MAULéON BASIN, WESTERN PYRENEES

While the understanding of the structural, temporal, and thermal evolution of rifted continental margins has significantly evolved over the past several decades, outstanding questions concerning the thermal evolution and the spatial and temporal intricacies of tectonically controlled sedimentation and sedimentary provenance during progressive rifting and hyperextension remain. To constrain the proximal to distal evolution of rifted continental margins, bedrock and detrital zircon (U-Th)/(Pb-He) double dating techniques were applied to the Mauléon Basin of the western Pyrenees. This non-magmatic, asymmetric, hyperextended rift basin formed during Early Cretaceous hyperextension resulting from lateral rift propagation from the Bay of Biscay. Limited postrift shortening during Cenozoic Pyrenean inversion uplifted the basin, resulting in exposures of rift basin fill, upper and lower crustal sections, serpentinized lithospheric mantle, and basic rift-fault relationships.

New detrital zircon U-Pb ages constrain provenance and the proximal to distal evolution of the Mauléon Basin. The spatial and temporal variation of detrital zircon U-Pb ages in synrift and postrift strata defines a general model for sediment routing which switched from being regionally, to locally, and then back to regionally derived within individual structurally controlled subbasins. Additionally, coupling bedrock and detrital zircon (U-Th)/He ages from vertical transects in different structural rifting domains illustrates the thermal evolution of the basin. New thermochronometric data and inverse modeling indicates that the basin experienced synrift heating to temperatures >180°C with geothermal gradients of ~80°C/km prior to rift-related exhumation/cooling of the proximal rift margin at ~98 Ma. The distal rift basin also underwent synrift heating to >180°C and geothermal gradients of ~80-100°C/km, and remained at temperatures >180°C until the onset of Pyrenean shortening and thrust exhumation.