Terceira Rift

The Terceira Rift is a geological rift located amidst the Azores islands in the Atlantic Ocean. It runs between the Azores Triple Junction to the west and the Azores–Gibraltar Transform Fault to the southeast. It separates the Eurasian Plate to the north from the African Plate to the south. The Terceira Rift is named for Terceira Island through which it passes.

The Terceira Rift is 550 km long, and represents the world’s slowest spreading center, with plate divergence of 2-4 mm/year. It developed from a transform fault and now operates as a hyper-slow spreading center, as recognized by the relative movement between the African and Eurasian plates. There is a strong resemblance between the Terceira Rift and other ultra- or super- slow spreading ridges, such as the Gakkel Rift and the Southwest Indian ridges. In particular, high obliquity values of 40°-65°, and a magmatic segmentation wavelength of 100 km, are similar to other very slow rifts. Rift valleys present along the rift are 1000-2200 m deep, and 30-60 km wide, similar to the Mid-Atlantic Ridge median valley. However, the amplitude of the Terceira Rift along-strike topography is 2000-4000 m, which is much larger than expected for ultraslow spreading ridges. This irregularity is thought to be due to the association of the rift with the Azores hotspot, in combination with slow spreading rates. Slow spreading results in a strong and thick axial elastic plate, and slower volcanic extrusions from the rift zone, resulting in high topography when merged with plume-related volcanism. The large volume of volcanism associated with the hotspot is mainly controlled by regional extension between Africa and Eurasia, as indicated by strikes of extension fractures. Furthermore, the majority of inter-plate deformation is focused on the Terceira Rift, although a clear deformation pattern has not yet been recognized.

The structures present along the Terceira Rift have directions likely associated with pre-rift geometry, as they do not correspond to the current direction of spreading motion, which is approximately N70°. Evidence of pre-rift structures include ancient transform directions of N110°-N125°, reactivating as transtensional fault zones, and N-S directions from former middle-oceanic rift faults, reactivating as left-lateral fault zones. It is important to note other hotspot associated islands do not display these features, and thus, the reactivated structures are most likely a result of the complex tectonic setting associated with the Azores triple junction instability. The above observations may represent earlier stages of development between the rifting of Eurasia and Africa. The first stage began in 25 Ma and lasted until 8 Ma, corresponding to the initial rifting of the oceanic plateau. At 8 Ma, extension began along a transform fault, until approximately 3 Ma, when the Terceira Rift axis was initiated.

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