Abstract The thermal evolution of continental rifted margins is key to understanding margin subsidence and hydrocarbon prospectivity. Observed heat-flow values, however, do not always comply with classic rifting models. Here, we use 2D numerical models to investigate the relationship between rifting, sedimentation and thermal history of margins. We find that during the synrift, the basement heat-flow and temperature are not only controlled by extension factor, but also by synrift sediment thickness and the evolution of deformation. As this progressively focuses oceanward, the proximal sectors thermally relax, while the distal sectors experience peak temperatures. In the post-rift, the lithosphere under the hyperextended margins does not return to its original state, at least for c. 100 Myr after break-up. Instead, it mimics that of the adjacent oceanic plate, which is thinner than the original continental plate. This results in heat-flow increasing oceanward at post-rift stages, when classic rifting theory predicts complete thermal relaxation. Our models also predict slightly increased heat-flows in the adjacent oceanic crust, potentially extending hydrocarbon plays into distal margins and oceanic crust, previously discarded as immature. Finally, our models indicate that commonly used temperature approximations to calculate heat-flow during rifting may strongly differ from those occurring in nature.

Abstract This study contributes to geothermal exploration in 1660–1520 Ma old, reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling-induced fractures and petal centreline fractures) and pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2–1.0 km. The results suggest a uniform NNW–SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls whether natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures steeply dip towards the WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for stimulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards engineered geothermal systems reservoir target depths. Our secondary objective is to highlight that interpretation of high-resolution acoustic data, particularly in metamorphic crystalline rocks, is subjective and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole proposes that drilling-induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however, result in over 10° differences in orientation.