The angular velocity of Nubia relative to Somalia and the location of the Nubia-Somalia-Antarctica triple junction

Abstract

SUMMARY An ew analysis of geologically current plate motion across the Southwest Indian ridge (SWIR) and of the current location of the Nubia‐Antarctica‐Somalia triple junction is presented. Spreading rates averaged over the past 3.2 Myr are estimated from 103 well-distributed, nearly ridge-perpendicular profiles that cross the SWIR. All available bathymetric data are evaluated to estimate the azimuths and uncertainties of transform faults; six are estimated from multibeam data and 12 from precision depth recorder (PDR) data. If both the Nubian and Somalian component plates are internally rigid near the SWIR and if the Nubia‐Somalia boundary is narrow where it intersects the SWIR, that intersection lies between ≈26 ◦ E and ≈32 ◦ E. Thus, the boundary is either along the spreading ridge segment just west of the Andrew Bain transform fault complex (ABTFC) or along some of the transform fault complex itself. These limits are narrower than and contained within limits of ≈24 ◦ Et o≈33 ◦ E previously found by Lemaux et al. from an analysis of the locations of magnetic anomaly 5. The data are consistent with a narrow boundary, but also consistent with a diffuse boundary as wide as ≈700 km. The new Nubia‐Somalia pole of rotation lies ≈10 ◦ north of the Bouvet triple junction, which places it far to the southwest of southern Africa. The new angular velocity determined only from data along the SWIR indicates displacement rates of Somalia relative to Nubia of 3.6 ± 0.5 mm yr −1 (95 per cent confidence limits) towards 176 ◦ (S04 ◦ E) between Somalia and Nubia near the SWIR, and of 8.3 ± 1.9 mm yr −1 (95 per cent confidence limits) towards 121 ◦ (S59 ◦ E) near Afar. The new Nubia‐Somalia angular velocity differs significantly from the Nubia‐Somalia angular velocity estimated from Gulf of Aden and Red sea data. This significant difference has three main alternative explanations: (i) that the plate motion data have substantial unmodelled systematic errors, (ii) that the Nubian component plate is not a single rigid plate, or (iii) that the Somalian component plate is not a single rigid plate. We tentatively prefer the third explanation given the geographical distribution of earthquakes within the African composite plate relative to the inferred location of the Nubia‐Somalia boundary along the SWIR.