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 plates imbedded within the East African Rift. One of these tectonic units is the Victoria plate, which is the subject of this study. Here we process space geodetic data for 37 permanent GNSS stations distributed along the Nubia, Somalia and Victoria plates to (1) compute the motion of the three plates in the ITRF2008 reference frame and (2) deduce the relative motion of Victoria with respect to its neighboring 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 plate motion from a set of 5 stations. The instantaneous relative Euler poles for the Nubia-Victoria and Somalia-Victoria pairs are now evaluated as 10.6582oN, 32.9752Eo, 0.1196o/Myr and 8.0164oS, 32.2869oE, 0.1589o/Myr, respectively. The computation of the relative inter-plate velocities along Victoria s plate boundary is straightforward in most situations because the western and northeastern boundary segments correspond to well-developed rift basins, where extension is mostly normal to rift basin flanks and seismicity concentrates along narrow structures. This is particular evident on the western branch between Victoria and Nubia. The southeastern limit of the Victoria plate is poorly defined, and geodetic data indicates that differential motion between Somalia and Victoria may be accommodated by a complex plate 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.