Cooling related to heat-flow decrease in post-rift sequences

Reconstruction of the thermal history is vital to maturity and kinetic modelling as maturity and hydrocarbon generation are temperature-controlled phenomena. In reconstructing the thermal history of sedimentary sequences, many studies are often based on borehole temperatures (BHTs) and assume heat flow constant through time and equal to the modern one. During its life from rifting to collision, the sedimentary sequence of a continental margin, experiences a thermal and diagenetic evolution under changing burial and thermal conditions reflecting the evolving geodynamic setting. In this work, three case histories are presented to give evidences that cooling can occur not only because of exhumation but also during burial due to heat flow variations. The first case refers to the thermal evolution of Lower Cretaceous Upper Sarir Sandstone (Libya). A fluid inclusions study in quartz and dolomite cements highlight a chronological trend in homogenization temperatures recording a decrease in the geothermal gradient from around 50 to 27 °C/Km completely unrelated to uplift events. The two latter cases refer to the thermal evolution from rifting (Late Triassic-Early Jurassic) and drifting (Middle Jurassic-Early Cretacous) to collision (Upper Cretaceous-Tertiary) of the Mesozoic succession of the passive margin developed on the northwestern edge of the Adriatic block (Northern Italy) that led to the opening of the oceanic basin closed during Cretaceous-Tertiary alpine convergence. Within this passive margin, two different situations were investigated: the Lombardian Basin and the Trento Plateau. The thermal evolution of the Lombardian basin was investigated by the integrated study of fluid inclusions and organic matter maturity. The thermal history of Trento Plateau was studied by a combined study of organic matter maturity and apatite fission track thermochronology on units ranging from Permian to Cretaceous. In both cases the model fits well experimental data if a quite high heat-flow (about 95-100 mW/m2) is assumed in the last stage of the Late Triassic-Early Jurassic rifting, followed by a strong decrease reaching values similar to present-day one at the end of Cretaceous (about 50 mW/m2). This study evidence how the occurrence of cooling events are not necessarily related to a decrease in depth by exhumation, but can be associated to the drastic heat flow decreasing connected to the end of the extensional phases, even during periods of increasing burial depth.