Re-evaluation of the kinematics of victoria block using continuous GNSS data

Abstract

The divergent boundary between the Somalia and Nubia plates is a complex tectonic domain where extensional processes are localized along narrow rift structures, isolating small blocks imbedded within the East African Rift. One of these tectonic units is the Victoria Block, which is the subject of this study. Here we process space-geodetic data for 37 permanent GNSS stations distributed along Nubia, Somalia and Victoria to (1) compute the motion of the three tectonic units in the ITRF2008 reference frame and (2) deduce the relative motion of Victoria with respect to its neighbouring plates. The Nubia Plate motion is computed from a set of 25 stations, the Somalia Plate motion from a set of 7 stations and the Victoria motion from a set of 5 stations. Although the number and distribution of the used stations is still not optimal, the good adjustment between observed and predicted motions confirms that Victoria acts as a rigid tectonic block. The instantaneous relative Euler poles for the Nubia-Victoria and Somalia-Victoria pairs are now evaluated as 10.66oN, 32.98Eo, 0.120o Myr-1 and 8.02oS, 32.29oE, 0.159o Myr-1, respectively. The computation of the relative interplate velocities along Victoria's boundary is straightforward in most situations because the western and northeastern boundary segments correspond towell-developed rift basins,where extension ismostly normal to rift basin flanks and seismicity concentrates along narrow structures. This is particularly evident on the Western Branch between Victoria and Nubia. The southeastern limit of the Victoria Block is poorly defined, and geodetic data indicate that differential motion between Somalia and Victoria may be accommodated by a complex boundary area, which roughly encompasses the Masai Terrain. Geodetic observations of the Victoria-Somalia boundary along the Eastern Branch, particularly in the Manyara Rift, reveal highly oblique horizontal extension. In this region seismicity is sparse which suggests that strain is accommodated by magmatic processes. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.