Insights into the magmatic architecture of the Oslo Graben by petrophysically constrained analysis of the gravity and magnetic field

A new model of the crustal structure of the Oslo Graben with a special emphasis on the location of magmatic intrusions is presented based on the interpretation of gravity and magnetic data constrained by petrophysical data, seismics, and surface geology. Three-dimensional modeling shows that the gravity high above the Oslo Rift is caused by a combination of crustal thinning and the presence of a high-density body in the upper crust, related to Precambrian gneiss complexes at the western flank of the rift. The short-wavelength gravity anomalies are correlated with the outcropping low-density magmatic rocks of Permian age that extend to the depths of 1.5-4 km. In addition, simultaneous modeling of aeromagnetic data sets collected at two flight altitudes (50 and 3400 m) using susceptibility and remanence within a range of reasonable magnetic properties suggests an underlying intrusion with a depth extent up to 15 km. Magnetization and density of the intrusion are increasing with depth, leading to a small density contrast but a large contrast in magnetization with the surrounding rocks. Therefore only modeling of the magnetic field can resolve the nature of the deep intrusions. The three-dimensional model provides a new estimate of the volume of magmatic material in the Oslo Graben system (similar to 63,000 km(3)); this is only two-thirds of the values proposed earlier (>95,000 km(3)) as we disregard high-volume magmatic underplating. Correlation with structural data, surface geology, and deep lithospheric features shows that the distribution of extrusive rocks in the Oslo Rift can be related to the timing and onset of rifting.