Deformation in a hyperslow oceanic rift: Insights from the tectonics of the São Miguel Island (Terceira Rift, Azores)

Abstract

The evolution of hyperslow oceanic rifts, like the Terceira Rift (TR) in the Azores, is still poorly understood. Here we examine the distribution of strain and magmatism in the portion of the TR making up the Nubia-Eurasia plate boundary. We use São Miguel Island because it stretches most of the TR width, which allows to investigate the TR's architecture and shedding light on TR's age and mode of deformation. From topography and structural analysis, and new measurements of 380 faults and dikes, we show that (1) São Miguel has two main structural directions, N150 and N110, mostly concentrated in the eastern part of the island as an onshore continuation of the faults observed offshore in the NE (N110 faults) and SW (N140) TR walls; (2) a new N50-N80 fault system is identified in São Miguel; (3) fault and dike geometries indicate that eastern São Miguel comprises the TR's northern boundary, and the lack of major faults in central and western São Miguel indicates that rifting is mostly concentrated at master faults bounding the TR. Based on TR's geometry, structural observations and plate kinematics, we estimate that the TR initiated between 1.4 and 2.7 Ma ago and that there is no appreciable seafloor spreading associated with rifting. Based on plate kinematics, on the new structural data, and on São Miguel's structural and volcanic trends, we propose that the eastern two thirds of São Miguel lie along a main TR-related transform fault striking N70-N80, which connects two widely separated N130-N150 TR-trending segments.

Rifting of the oceanic Azores Plateau with episodic volcanic activity

“…1 and 7 cross section: B-B') indicates that magmas are focusing beneath this portion of the rift leading to volcanism and lateral dike intrusion, potentially with some magmatic spreading in the shallow crust. Our new age of < 1.56 Ma for the opening of Hirondelle Basin is in agreement with previous estimates of the onset of Terceira Rift extension between 1.8 and 0.8 Ma further to the west 16 , and between 2.7 and 1.4 Ma further to the east 15 . The onset of volcanic activity in the Hirondelle Basin is unknown and we assume that D. João de Castro seamount formed within the past 500 kyrs similar to the youngest volcanoes on Terceira and São Miguel 17,19,71 .…”

“…Ultraslow-spreading axes show alternating amagmatic extensional phases and magmatic phases with extension by dike intrusion 23 . Although we do not find evidence for the formation of new lithosphere by magmatic processes in the Hirondelle Basin, we agree with Sibrant et al 15 that the extension of Terceira Rift follows patterns similar to other ultraslow mid-ocean ridges 70 . The crust exposed at the Hirondelle Basin may thus represent the early magmatic phase in the building of a volcanic ridge (Fig.…”

“…The Azores Plateau formed 10 to 4 million years ago 10 and is rifted by the NW–SE striking ultraslow Terceira Rift 11 – 13 . Seismic work suggested an opening of the Terceira Rift ~ since 25–20 Ma ago 14 , whereas tectonic studies suggested rifting initiation 1to 2 Ma ago 15 , 16 . Deep submarine rift basins of the Terceira Rift are results of the extension and expose the earlier volcanic stages along the 1 to 2 km high escarpments of the rift flanks.…”

Correlated Changes Between Volcanic Structures and Magma Composition in the Faial Volcanic System, Azores

“…Note that the submarine rift zones probably cover the entire age interval of subaerial volcanic activity on Faial (∼1 Ma until recently). This contrasts observations from São Miguel and to some extent Santa Maria, which experienced distinct major stress fields during their formation due to the reconfiguration of the Africa-Eurasia plate boundary (Sibrant et al, 2015(Sibrant et al, , 2016Ramalho et al, 2017).…”

The evolution of Santa Maria Island in the context of the Azores Triple Junction

“…The topographic profile across the East Azores Fracture Zone, Santa Maria and the Terceira Rift (Fig. 15) (Vogt and Jung, 2004;Sibrant et al, 2016;Marques et al, 2018), and Santa Maria's upward movement is (at least in part) due to rift flank uplift, then some (or all) of the 200 m of uplift is younger than ca. 1.5 Ma.…”