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

Abstract

S U M M A R YA new 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 • E to ≈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.

The Mw7 Machaze, Mozambique, Earthquake of 23 February 2006

“…The Rift marks the boundary between the Nubian and Somalian plates (figure 2) and extends more than 3,000 km from the Gulf of Aden/Afar triple junction in the north to Mozambique in the south. The Somalian plate is moving southeast relative to the Nubian plate (Horner-Johnson et al 2005). Although it appears that the Somalian plate does not behave in a completely rigid manner, the majority of the plate motion is accommodated by extensional faulting in the rift (HornerJohnson et al 2005).…”

“…The geology and geomorphology of Mozambique indicates that the Urema and Dombe troughs may be the southern continuation of the western branch through northern Mozambique (Chorowicz and Sorlien 1992). Persistent seismicity in the Mozambique Channel (Horner-Johnson et al 2005) indicates that the eastern branch continues to the south to intersect the Southwestern Indian Ridge (SWIR) somewhere between 26.3°E and 32.2°E. Current estimates of opening across the rift range from 8.3 ± 1.9 mmyr -1 in the Afar region at the northern end of the rift and 3.6 ± 0.5 mmyr -1 near the intersection with the SWIR (Horner-Johnson et al 2005).…”

“…Persistent seismicity in the Mozambique Channel (Horner-Johnson et al 2005) indicates that the eastern branch continues to the south to intersect the Southwestern Indian Ridge (SWIR) somewhere between 26.3°E and 32.2°E. Current estimates of opening across the rift range from 8.3 ± 1.9 mmyr -1 in the Afar region at the northern end of the rift and 3.6 ± 0.5 mmyr -1 near the intersection with the SWIR (Horner-Johnson et al 2005). Midzi et al 1999).…”

Motion of Nubia relative to Antarctica since 11 Ma: Implications for Nubia-Somalia, Pacific–North America, and India-Eurasia motion

“…Fitting magnetic anomaly and fracture zone crossings also shows that the flanks of the Southwest Indian Ridge record the motion of two distinct plates, Nubia and Somalia, relative to the Antarctic plate (Lemaux et al, 2002). Surprisingly, however, prior estimates of the rotation between Nubia and Somalia have resulted in disparate poles of rotation for the motion since 3.16 Ma compared with that since 11.03 Ma, with the pole since 3.16 Ma being southwest of South Africa (Horner-Johnson et al, 2005) and the pole since 11.03 Ma being near the east tip of Brazil (Lemaux et al, 2002). Here we use new profiles collected along the western part of the Southwest Indian Ridge (Ligi et al, 1999) to improve the estimate of Nubia-Antarctica relative motion since the old end of chron 5, denoted as chron 5o herein.…”

“…1); it is bounded on the east by the Andrew Bain Fracture Zone (FZ) complex (Fig. 2), composed of a series of large transform offsets that appear to be within or near the Nubia-Somalia-Antarctica triple junction (Chu and Gordon, 1999;Horner-Johnson et al, 2005;Lemaux et al, 2002). Between the BTJ and Andrew Bain FZ complex, the Southwest In-on August 1, 2015 geology.gsapubs.org Downloaded from dian Ridge extends over 2000 km with only 5 ridge segments (Fig.…”