THE IMPORTANCE OF CLASSICAL GEODETIC OBSERVATIONS FOR ANALYZING THE GEODYNAMIC BEHAVIOR OF THE ACORES ARCHIPELAGO

Despite their relative poor precision, classical geodetic observations can provide valuable information about tectonic displacements because their lack of precision is balanced by the time span they cover. In t he case of the Acores archipelago the contribution of six decades of c lassical observations provide a historically reliable and important so urce of information in the establishment of a tectonic model for that area, complementary to GPS data, where only few years of observations are available. Based upon angular and invar wire measurements carried out in the thirties, compared with more recent classical and GPS obser vations, the authors were able to deduce some significant displacement s. For the Central Group, we conclude that the area is expanding in an approximately WSW-ENE direction, showing a component parallel to the plate boundary between Eurasia and Africa, which is about 1.5 greater than the component in the perpendicular direction. As regards the Faia l and S. Jorge islands, our results show a good agreement with the pro posed geodynamic models for Faial, but not for S. Jorge. For Faial, th e results indicate an extension perpendicular to the faults in the dir ection WNW-ESE, and a dextral strike slip along the same faults. In th e case of S. Jorge the lack of agreement with the proposed geodynamic models is probably due to the particular poor geometry and scale contr ol of the inner network. (C) 1998 Elsevier Science B.V. All rights res erved.