Crustal extension in the Oslo Graben, SE Norway: a method incorporating magmatism and erosion

Abstract

Three methods for determining crustal extension were examined for use in the Late Paleozoic Oslo Graben of southeast Norway: 1. (1) analysis of faults and dikes, 2. (2) volume budget/isostasy, 3. (3) analysis of crustal thickness distribution assuming constant crustal volume during extension. These methods were found not to be suitable mainly due to rift age and magmatism. Instead, a new integrated model which combines the effect of Moho contours, volumes of mantle injecta and erosion, was used. Volumes of intracrustal mantle injecta are found from a model combining data on upper crustal felsic magmatic rocks with petrological and gravity inversion models. For the Vestfold Graben Segment in the south this implies twice the stretching estimate as compared to stretching derived from Moho contours alone, and two to three times the estimate from an (inadequate) set of fault and dike information. The batholiths mark zones of high crustal thinning, and magmatism was most likely caused by a combination of a thermal anomaly and lithospheric stretching.