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Architecture and early evolution of the Oslo Rift |
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| Authors: | B Sundvoll BT Larsen |
| Institution: | a Mineralogical-Geological Museum, Sarsgt. 1, N-0562, Oslo, Norway b Statoil A /S P.O. Box 300, N-4001, Stavanger, Norway |
| Abstract: | A revised assessment of architecture and pre-rift fabric connections of the Oslo Rift has been undertaken and linked to a new appraisal of observations and data related to the initial phase of the rift evolution. In addition to half-graben segmentation, accommodation zones and transfer faults are readily identified in the linking sectors between the two main grabens and between graben segments. Axial flexures are proposed between facing half-grabens. The accommodation zones were generally sites of volcanism during rifting. Pre-rift tectonic structures played an influential role in the rift location and development. The deviant N-S axis of the Vestfold graben segment is viewed as related to pre-rift structural control through faults and shear zones. This area was probably a site of Proterozoic/Palaeozoic crustal and lithospheric attenuation. Field evidence suggests that the rift started as a crustal sag with no apparent surface faulting in a flat and low-lying land at a time about 305–310 Ma. Volcanism, sub-surface sill intrusion and faulting started about simultaneously some time after the initial sag (300–305 Ma). Faulting and basaltic volcanism were initially localized to transfer faults along accommodation zones and a NNW-SSE transtensional zone along the eastern margin of the incipient Vestfold graben segment. This transtensional zone was probably created by right-lateral simple shear tracing pre-rift structures in response to a regional stress field with the tensional axis normal and the maximum compressional axis parallel to the NNE-SSW-trending rift axis. |
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