Formation Mechanisms of Ultradeep Sedimentary Basins: The North Barents and Some Other Basins

Eugene Artyushkov

In the North Barents basin, 16-18 km deep, the consolidated crust is attenuated by two times. However, the system of normal faults in the basement ensures crustal stretching of only ~10% which gave rise to deposition of 1-2 km of sediments at the base of the sedimentary cover. Since the Late Devonian, 14 km of new deposits were formed in the basin in the absence of significant stretching. In this part of the sequence, all the reflectors are continuous (undisrupted) except at the narrow vertical magmatic channels which were formed near the Jurassic/Cretaceous time boundary. Under such circumstances, the subsidence required a large density increase in the lithosphere. This was predominantly caused by the transformation of gabbroids into dense eclogites in the mafic lower crust which occurred at the episodes of fluid infiltration from the mantle. The transformation was enhanced by the pressure and temperature increase in course of the formation of a thick sedimentary cover. The analysis of the seismic reflection profiling and gravity data shows that the Moho in the basin is underlain not by mantle peridotites but by a layer of eclogites, ~15 km thick. Their formation from a less dense gabbroids in the lower crust was sufficient to produce the sediment loaded subsidence of 14 km in the absence of stretching. Mafic eclogites are considerably denser than mantle peridotites; however, having high P-wave velocities they are commonly placed under the Moho boundary. The same situation is typical of the North Caspian, South Caspian and North Chukchi basins ~20 km deep. As the North Barents basin, they are several kilometers deeper than it could be expected under the observed thickness of their consolidated crust. If the Moho was underlain be mantle peridotites, negative isostatic anomalies ~150-200 mGal would arise at the surface. Yet only weak positive Faye anomalies are observed above the basins. This indicates that thick lenses of dense eclogites underlie the Moho in the basins. A similar result was recently obtained for the Gulf of Mexico [Mooney, Kaban, J.G.R., 2010]. The transformation of large masses of gabbroids and basalts into eclogites in the crustal layer is possible only in highly ferruginous rocks. This is why ultradeep basins arise only in some regions. Under increased pressure and temperature and in a presence of fluid low grade metamorphism occurs in the upper crust overlain by ~20 km of sediments. This increases P-wave velocities and density of silicic rocks to the values typical of mafic oceanic crust.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013