Temporal constraints on progressive rifting of a hyper-extended continental margin using bedrock and detrital zircon (U-Th)/(Pb-He) dating, Mauléon Basin, western Pyrenees
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While the understanding of the structural, temporal, and thermal evolution of rifted continental margins has significantly evolved over the past several decades, critical outstanding questions remain, especially concerning the thermal evolution as well as the spatial and temporal intricacies of tectonically controlled sedimentation and sedimentary provenance during progressive rifting and hyper-extension. To constrain the proximal to distal tectonic, stratigraphic and thermal evolution of rifted continental margins, bedrock and detrital zircon (U-Th)/He (ZHe) and zircon U-Pb double dating techniques were applied to the Mauléon Basin of the western Pyrenees. This non-magmatic, asymmetric, hyper-extended rift basin formed during Early Cretaceous hyper-extension of Iberian lithosphere, as a result of lateral propagation of rifting in the Bay of Biscay, and experienced a limited magnitude of shortening during post-rift Pyrenean inversion. This resulted in the preservation of outcrops of sedimentary cover, upper and lower crustal sections, serpentinized lithospheric mantle, and the basic rift-relationships; making the Mauléon Basin an ideal locality to constrain rift-related processes during hyper-extension. Detrital zircon U-Pb analyses indicate that the proximal rift margin is primarily composed of pre-rift strata with Pan-African zircon U-Pb signatures and primary age peaks at ~615 Ma and ~1000 Ma. In contrast, the distal rift margin is composed of exhumed mid-lower crustal granulites, which have a similar Pan-African signature but with additional Variscan (Permian) overgrowths. Detrital zircon U-Pb analyses of syn- to post-rift strata indicate compartmentalized, local sourcing from the pre-rift strata in the proximal margin and the exhumed lower crust in the distal margin. Late syn- to post-rift strata show a shift to non-compartmentalized, regional sourcing from the proximal rift margin and hinterland. These observations are combined to present a sediment dispersal model for the progression of rifting. ZHe analyses shows preservation of two distinct age domains: an elevation-invariant age cluster at ~98 Ma, interpreted as rift-related cooling due to major exhumation along the SMD, and an elevation-invariant age cluster at ~50 Ma, interpreted as rapid cooling related to Pyrenean inversion. These results indicate the Mauléon Basin experienced early syn-rift heating prior to the exhumation of parts of the proximal domain to <180°C; these are the only localities that record rift-related timing. Based on thermochronometric modeling and burial estimates the syn-rift geothermal gradients in the necking domain were as high as ~80°C/km and ~80-100°C/km in the hyper-thinned domain. From the early syn-rift until Pyrenean inversion at ~50 Ma, most of the distal rift margin remained at temperatures >180°C. These observations of heating and high geothermal gradients are compared to geologic and numerical rifting models to give insight into preferred models for the rifting evolution of hyper-extended margins.