Paleostress reconstruction from kinematic indicators in the Oslo Graben, southern Norway: new constraints on the mode of rifting

Abstract

The Oslo Graben, southern Norway, is a N-S-trending Carboniferous-Permian rift system, characterized by major mafic to silicic magmatism, N-S-trending faults, reactivation of preexisting Precambrian faults and formation of half grabens. Magmatism is expressed by the presence of lavas, dyke injections, cauldron formation and the intrusion of batholiths. Paleostress analyses, mainly based on slickensides, have been performed in the area, both within and outside the rift structure. Combined with the current tectono-magmatic model of the Oslo Graben area, the analyses show the following stress evolution from the Caledonian Orogeny, with its compressional tectonics in the Silurian to the late stages of the Carboniferous-Permian rifting. NW-SE compression occurred in Silurian times, due to the continental collision of the Caledonian Orogeny. After a long period of a missing geological record, the area was affected by N-S compression during the Late Carboniferous. A shallow sedimentary basin developed, indicative for the pre- and proto-rift phases. The transition from the proto-rift phase to the initial rift phase, is marked by a transition from a transpressional regime into a transtensional regime. During the Early Permian, the stress regime changed from pure extension to radial extension. We suggest that the Oslo Graben can not simply be explained in terms of, or passive, or active rifting, but that a combination of both, evolving through time, is more suitable to explain the observations. We propose a model in which the Oslo Graben is initially triggered by far field stresses in the latest Carboniferous and earliest Permian, which infers opening due to passive rifting. These far field stresses are suggested to be linked to the Hercynian Orogeny, active to the south in central Europe in Carboniferous times and the reorganisation of the Pangea supercontinent. In Early Permian times, the stress regime caused radial extension, indicative for uplift and a change to an active mode of rifting. Simultaneously with the radial extension, large volumes of magma are emplaced at near surface levels. Despite all alternative models, an anomalous thermal gradient is necessary to create these large volumes of magmatic material. We propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active (plume-related), although far field stresses might still be present.