RESULTS FROM GEODETIC MEASUREMENTS IN THE WESTERN PART OF THE AFRICAN-EURASIAN PLATE BOUNDARY

Since 1988 the Astronomical Observatory (University of Porto) and the Institute for Geodesy and Navigation (University of the Federal Armed Forces, Munich) have been involved in a common project to establish an d maintain a high precision geodetic network in the area of the Azores triple junction, along the Western boundary of the Eurasian plate. Th e network has stations on three of the main plates - North American, E urasian and African. This is a long-term project, with planned re-obse rvations every three years. First epoch measurements were done in 1988 , using the Global Positioning System (GPS). A second GPS campaign was conducted in 1991, during which the network was enlarged to include m ore stations on the African plate in order to monitor also the Azores- Gibraltar plate boundary. Further strength of the network was obtained by the establishment, in 1992, of a gravimetric network in the Azores archipelago; the gravimetric network comprises two absolute stations and relative measurements on all the nine islands of the archipelago. Re-observation campaigns, with GPS and gravity measurements, took plac e in 1993 and 1994. This paper focuses on comparison of the GPS result s achieved in the different campaigns. Annual values for point displac ement, direction and magnitude, are computed for some of the stations and analyzed in light of the results derived from geological and geoph ysical data interpretation. Our preliminary geodetic results show ocea n spreading across the Mid-Atlantic Ridge at rates consistent with tho se determined from 3MY averages (NUVEL-1). In addition, there is margi nal evidence for a leaky transform boundary transversing the central A zores islands. possible directions for future research on the dynamics of this plate boundary are also presented. (C) 1998 Elsevier Science B.V. All rights reserved.