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The geology of Formigas Islets and its significance to our comprehension of the Terceira Rift in the Azores Triple Junction

Ramalho, Ricardo, Quartau, Rui, Madeira, José and Rebelo, Ana C. 2018. The geology of Formigas Islets and its significance to our comprehension of the Terceira Rift in the Azores Triple Junction. Presented at: 2018 American Geophysical Union Fall Meeting, Washington DC, United States, 10-14 December 2018.

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Formigas in the Azores constitute a group of rocky islets located in between Santa Maria and São Miguel Islands. The islets constitute the emerged portion of a much larger submarine volcanic edifice standing on the southeastern-most margin of the Azores Plateau and on the western shoulder of the Terceira Rift. These islets are located atop of a large submarine bank that extends 9 km to the SE, to the Dollabarat Reef. Despite its minute onshore size, the Formigas-Dollabarat volcanic edifice is key to our comprehension of the geodynamic evolution of the Azores Triple Junction and the onset of Terceira Rift. We have recently mapped in high detail these outcrops, showing that Formigas mostly comprise a basaltic submarine effusive sequence dipping 25-30º to the west. Unconformably on top of this sequence, consolidated beachrock comprising rounded boulders of subaerial lava flows occur, together with fossiliferous calcarenites. The sequence is thus interpreted as the foreset unit of a coastal lava delta, whose subaerial topset was removed by marine erosion, but whose existence is attested by the presence of boulders of subaerial lavas included in the beachrock. The passage zone of this delta was located above the islets' present-day maximum elevation (11 m), but not far given the presence of the beachrock. This suggest that Formigas constitute the remains of the submarine outer flank of a westward-progradating lava delta, formed along the coast of a subaerial shield volcano that extended significantly to the E. These observations, in conjunction with inferences from a recent high-resolution bathymetric survey, suggest that the Formigas-Dollabarat volcanic edifice was built before or during the onset of the Terceira Rift, and that the volcanic edifice was himself affected and submerged by vertical tectonics associated to the rifting process. Effectively, the morphology of the whole bank suggests a horst-and-graben structure, defined by NNW-SSE trending normal faults and their conjugate pair NNE-SSW, and with blocks progressively downthrown towards the east. Formigas thus constitutes the vestigial remains of a much larger island, deformed and submerged by the transtensive kinematics associated with the development of Terceira Rift.

Item Type: Conference or Workshop Item (Paper)
Date Type: Published Online
Status: Unpublished
Schools: Earth and Environmental Sciences
Last Modified: 23 Jun 2021 15:00

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