THE AZORES TRIPLE JUNCTION EVOLUTION SINCE 10-MA FROM AN AEROMAGNETICSURVEY OF THE MID-ATLANTIC RIDGE

In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction . These models were mainly based on morphological analysis of sea bott om topography, sparse magnetic profiling, sidescan sonar surveying ove r the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detail ed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge b etween 37-degrees-N and 40-degrees-30'N the magnetic anomalies up to a nomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a c oherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fou rier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more conti nuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the r idge at 39-degrees-30'N, 29-degrees-40'W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38-degr ees-23'N, 30-degrees-15'W) and by the Princess Alice Fracture Zone (at 38-degrees-00'N, 30-degrees-50'W). Anomaly identifications using a tw o-dimensional model and plate tectonics reconstruction techniques allo wed the calculation of rotation pole parameters. The results thus obta ined reveal that, at least between anomalies 5 and 3 (approximately 10 -3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved at tached to the Eurasian plate. The triple junction (Azores-North Americ a-Africa, or Eurasia-North America-Africa) moved northward from approx imately 38-degrees-00'N, 30-degrees-50'W to approximately 38-degrees-2 0'N, 30-degrees-15'W (between 4 and 3A) and probably to 38-degrees-50' N, 30-degrees-00'W at anomaly 2A time. A detailed reconstruction model is presented.