North Atlantic Crustal Genesis in the Vicinity of the Azores

Summary In terms of current knowledge of crustal genesis in the Atlantic Ocean, several unique or highly anomalous features exist in the vicinity of the Azores. These include the seismically active East Azores Fracture Zone extending from Gibraltar to the Mid‐Atlantic Ridge; the seismically inactive West Azores Fracture Zone which is offset northwards from the trend of the East Azores Fracture Zone; the transverse island chain of the Azores islands which trends southeast‐northwest across the Mid‐Atlantic Ridge; the marked change in direction of the Mid‐Atlantic Ridge from northeast‐southwest to north‐south; and the broadening of the Mid‐Atlantic Ridge to the east. Bathymetric and magnetic data from surveys of the Mid‐Atlantic Ridge in the Azores area by R.V. Trident , the U.S.Naval Oceanographic Office, and other sources have been compiled. These, together with the previously published data are compatible with a basic model consisting of a Mid‐Atlantic Ridge migrating eastwards at the local crustal spreading rate, which is greater to the north than to the south of an east‐west transverse fracture system. Superimposed on this is the development of a northwest‐southeast trending secondary spreading centre or triple junction, within a ‘leaky transform’ system (Menard & Atwater 1968) which would have developed as the result of a change in the local crustal spreading direction south of the east‐west transverse fracture system from mainly east‐west to mainly northwest‐southeast. Simple geometrical considerations, when combined with the length of a newly proposed ‘Terceira Rift’ and independent estimates of local crustal spreading rates suggest that the change in crustal spreading direction and triple junction development began at least 45 My ago.