Organic and inorganic thermal indicators throughout the Late Paleozoic sedimentary fill of the Aragón-Béarn Basin in the western Pyrenees, integrated with stratigraphic and structural data, point to a two-stage tectonic evolution:
High heat flow values and widespread magmatic intrusions driven by deeply rooted strike-slip faults characterized the first stage. This regime accompanied the rapid and localized deposition of the lower sedimentary succession and ended with uplift and erosion.
The second stage was characterized by lower heat flow values, shallower strike-slip faults active during the deposition of an upper transgressive sedimentary succession and minor magmatic intrusions.
The lower portion of the succession (Unidad Gris, Unidad de Transito and Unidad Roja Inferior, mainly Stephanian-Early Permian in age) records maximum temperatures between 170°C and 220°C as shown by clay mineral-based geothermometers (KI between 0.40 and 0.52°Δ2θ, conversion of mixed layer illite-smectite to discrete illite), vitrinite reflectance (about 2.5%), and total homogenization temperatures from host-rock quartz cements between 160 and 180°C.
The upper portion of the succession (Unidad Roja Superior, mainly Upper Permian in age) records maximum temperatures between 100–110° and 150°C as indicated by KI values between 0.57 and 0.94°Δ2θ, illite content in mixed layer illite-smectite of 85–92%, and fluid inclusions in host-rock quartz-calcite cements.
Local thermal maturity anomalies affected selected portions of the Late Paleozoic sedimentary succession with organic matter and clay minerals reaching respectively Ro%: 4.2–4.7% and KI up to 0.32°Δ2θ in the lower sequence, and Ro% 2.9% in the upper sequence. These localized anomalies with respect to the basin thermal signature are the result of contact metamorphism related to the emplacement of magmatic intrusions across the sedimentary fill during two different magmatic events.