The Davie Ridge, Northern Mozambique: The Crustal Structure from Wide-angle Seismic Data

Abstract:

The breakup of Gondwana created the Somali and the Mozambique basins, two of the oldest ocean basins on earth. The relative movements between Africa and Antarctica-India-Madagascar created a sheared margin along the present day coastline of northern Mozambique and Tanzania. The N-S oriented offshore Davie Ridge is believed to have formed during these shear movements. Whether the Davie Ridge is of continental origin or has been formed by magmatic processes during the continental drift is unknown, because deep seismic sounding data are missing to describe the crustal fabric. Previous geophysical studies in this area are rare. From the 1970s and 1980s mainly seismic reflection data were acquired by French institutions across the ridge. Few dredge samples provided first evidence for a continental origin of the southern Davie Ridge.

In 2014, four east-west-orientated seismic refraction profiles as well as gravity and magnetic field data were collected across the Davie Ridge. Here, we present two P-wave velocity models across the Mozambican margin. The profiles are situated in the northern part (11.5° S) of the Davie Ridge and the transition (13° S) to the southern part where the topography vanishes. Each profile consists of 20 OBS/OBH systems with a spacing of about 9.5 km.

Along both profiles the total thickness of the sediments is about 5 km. The sediments show unusual high seismic velocities of 4.0-4.6 km/s below 3 to 4 km depth. In the northern profile, east of the Kerimbas Basin the crust is about 6 km thick with velocities from 5.5 to 6.6 km/s, which is most likely of oceanic origin. Towards the Mozambican coast the crust thickens to 15 km. The data indicate that the transition from stretched continental to oceanic crust is below the Kerimbas Basin. The southern profile shows a thinned crust (4 km) close to the shelf break. In the East, the crustal thickness is similar to the northern profile with slightly higher velocities up to 6.8 km/s.