Geodynamic models for the Azores triple junction: A contribution from tectonics

Abstract

The American, Eurasian and African lithospheric plates meet at the Azores triple junction. The nature of the northern and southern branches of the junction is well known and uncontroversial, the American plate is separated from Eurasia and Africa by the Mid-Atlantic Ridge (MAR). The western group of islands of the Azores archipelago (Flores and Corvo) lies on the American plate, just west of the rift, and their tectonic features agree with that location. The nature of the third branch of the junction, to the east of the MAR, is however still controversial. Some authors believe that a ridge-ridge-ridge junction occurs in the Azores area and adopt the structure known as the “Terceira Ridge” (working as a simple rift boundary) as the third branch. Another model proposes a triangular microplate in the Azores region bounded by the Mid-Atlantic Rift to the west, the East Azores Fault Zone to the south and a fault crossing S. Miguel and the Terceira islands to the northeast; in this model the fault zones are pure dextral strike-slip boundaries. Both models would create space problems at Gloria Fault, a well-known pure dextral strike-slip structure. In the first model transtension would be necessary at Gloria Fault to accommodate plate motion, while the second would require a transpressive regime on that structure. Neotectonic and seismotectonic data in the Azores indicate a transtensile regime in the central and eastern island groups in present and recent times. This is compatible with a “leaky transform” structure acting as the third branch of the junction in the Azores area, a model which has already been presented by several authors in the past. Stress trajectories deduced from neotectonic studies in some of the islands suggest that the main plate boundary passes between the islands of S. Jorge and Pico south of S. Miguel and joins Gloria Fault east of Santa Maria. This model and boundary location is also compatible with seismotectonic and magnetic data.