TheOslo Graben orOslo Rift is agraben formed during a geologicrifting event inPermian time, the last phase of theVariscan orogeny.[1] The main graben forming period began in the lateCarboniferous, which culminated with rift formation andvolcanism, with associatedrhomb porphyrylava flows. This activity was followed byuplifting, and ended withintrusions about 65 million years after the onset of the formation. It is located in the area around theNorwegian capitalOslo.
The lava production was high when the rhomb porphyry lavas were deposited. The lavas reflect a period of abundant earthquake-related movements, whentectonic forces tore the crust apart.
In theVestfold district, one lava flow was deposited on average every 250,000 years, resulting in a 3000-metre thick sequence of mainly volcanic material. In theOslo area, lavas were deposited on average every 800,000 years. Only a few plant remains have been found between these lavas. The bedrock in this area, roughly fromSkien to Oslo andMjøsa, results in soil rich in nutrients important for plant growth.
Since the Permian,erosion has removed the volcanic peaks and indeed most of the lava layer and laid bare themagma chambers andvolcanic pipes deep below, allowing scientist a rare view of what goes on beneath arift valley. Several of the old magma plumes are now quarried, the rich blacklarvikite (named fromLarvik, a town south of Oslo) being one.
TheSärna alkaline complex in western Sweden, also of Late Carboniferous age, is thought to be related to the Oslo Graben as it is aligned to it.[2]
59°48′04″N10°33′23″E / 59.8011°N 10.5564°E /59.8011; 10.5564
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