Plate kinematic evidence for the existence of a distinct plate between the Nubian and Somalian plates along the Southwest Indian Ridge

Abstract

Previously, we estimated the angular velocity of the Nubian plate relative to the Somalian plate from an updated set of spreading rates and transform fault azimuths. We found that the Nubia-Somalia plate boundary intersects the Southwest Indian Ridge (SWIR) between ≈26°E and ≈32°E if both the Nubian and Somalian plates are rigid and if the boundary between them is narrow. These prior results are not completely satisfactory mainly for two reasons: (1) The four-plate circuit Somalia-Antarctica-Nubia-Arabia does not close. (2) The largest (Mw ≤ 6.8) recorded "African" earthquakes that are near, but not along, the SWTR occur near 48°E, well east of the intersection with the Nubia-Somalia boundary. Here we investigate these problems through detailed analysis of plate motion data, especially those along the SWIR east of the Andrew Bain Transform Fault Complex. We find an improved fit to the data and improved plate circuit closure if a region of the African lithosphère is interpreted as a new component plate. This new plate lies between the Nubian and Somalian plates along the SWIR and is separated from the latter by a diffuse boundary that includes the locations of the largest off-ridge earthquakes. Following C. J. H. Hartnady, we call this new plate "Lwandle." Use of this new plate geometry shifts the Nubia-Somalia pole of rotation northeastward to just south of South Africa and thus alters estimates of current India-Eurasia plate motion. Copyright 2007 by the American Geophysical Union.