Early magmatic phase in the Oslo Rift and its related stress regime

Abstract

The earliest magmatic rocks in the Oslo Graben consist of a complex of sills and dykes (intermediate to felsic syenites termed ‘mænaites’, and basic camptonites) intruded into Cambrian and lower Ordovician pre-rift sediments. No volcanism appears to be associated with this intrusive phase. The sill intrusions are regional in extent and can be found in all parts of the Oslo Graben, except in the north. Samples from six ‘mænaite’ sill occurrences, dated by the Rb-Sr method, yielded late Carboniferous ages ranging from 304 to 294 Ma. These ages place the sill intrusions in the same evolution phase as the proto-rift Asker Group sediments. Interpreting the sill complex as a paleostress indicator, and applying the tectonic model for the sill-forming mechanism, the deviatoric stress in the crust at the time of intrusion is found to be compressive at the level of the sills and shallower. In contrast the succeeding evolution of the Oslo Graben was dominated by tensile deviatoric stress. The timing of the incipient magmatic event in the Oslo Graben, and its state of deviatoric compressive stress, support the concept that the Oslo Rift developed initially in response to the build-up foreland compressional stresses in conjunction with the Hercynian orogeny. There is no evidence for an early pre-rift crustal doming stage.