Crustal structure of the Nogal basin, northern Somalia

Abstract Gravity, seismic and well data have been used to determine the crustal structure of the poorly-known Nogal basin, northern Somalia. Seismic data delineate a WNW-ESE trending graben infilled mainly by up to 6–7 km of Upper Cretaceous to Oligocene-Miocene sediments. Backstripping of well data show that the basin developed by thinning and rifting of continental crust during the Late Jurassic, Late Cretaceous and Oligocene. The Oligocene rift event is only observed at the centre of the basin, and correlates with the opening of the Gulf of Aden. The Late Jurassic rift event, which has been recognised in the northern Somalia, is largely absent in the basin. For the western part of the basin, Process Oriented Gravity and flexure Modelling (POGM) suggest a best-fit Te of 5 km, which explains both the amplitude and wavelength of the observed free-air gravity anomaly. However, the modelled anomaly in the eastern part of the basin is 30–40 mGal higher than the observed. Sediment-corrected Bouguer gravity anomalies show that the basin is associated with a relative high, which increases in amplitude to the east, towards the Kalis-1 well. We, therefore, propose that the eastern part of the basin to be locally intruded by dense magmatic material in the lower crust and/or upper mantle, which have elevated the gravity. The amount of crustal thinning inferred at Kalis-1 and Nogal-1 wells from backstripping is consistent with the results of POGM and flexure modelling. The models suggest the crust thins from about 35 km beneath the basin flanks to 32.6 km and 27.3 km at the Kalis-1 and Nogal-1 wells respectively. Moreover, the models suggest that Late Jurassic rifting was limited to the western part of the basin. The comparison of the long wavelength free-air gravity anomaly with the gravity effect of the topography based on different gravity/topography ratios suggest that while the crust is locally thinned beneath the basin it is also regionally domed, possibly by flexure due to magmatic intrusion. The doming predates the Late Cretaceous rifting and coincides with the same pre-Cenomanian uplift event during which substantial thicknesses of Upper Jurassic and Lower Cretaceous sediments were exhumed.