Distributed deformation close to the Azores Triple “Point”

Abstract Terceira Rift and the northern and southern branches of the Mid-Atlantic Ridge (MAR) form a triple junction close to 39°N known as the Azores Triple Junction. New swath bathymetric data are used to investigate the surface expression of faulting close to the triple junction, by the systematic mapping of MAR-generated abyssal hills. It is shown that close to the geometrical intersection between the three spreading axes there exists no single transform fault connecting Terceira Rift to the MAR but a distributed tectonic deformation area characterized by mesoscale brittle deformation close to the surface, covering approximately 90 km by 100 km, and almost no volcanism, which links Terceira Rift to the MAR, accommodating the relative displacement of the three plates close to the geometrically triple point. Magnetic chrons are used to compute the spatial variation of spreading velocity at the Mid-Atlantic Ridge and confirm the above interpretation: they show a progressive increase of spreading velocity along a single MAR segment, between pure “Nubian” at 38°30′N and pure “Eurasian” at 39°25′N, without the development of a transform fault that would integrate the Eurasia–Nubia plate boundary. The comparison of similar triple junctions where a slower axis joins two faster ridges, shows that the slower arm does not reach the “triple point” and that there is always a finite triple junction area, highly tectonized, in which size is dependent on the angle between the two faster arms and, consequently, on the relative spreading velocity of the slower arm.