Evolution of a volcanic island on the shoulder of an oceanic rift and geodynamic implications: S. Jorge Island on the Terceira Rift, Azores Triple Junction

• Published in: Tectonophysics
• Main Authors: Marques, F.O., Hildenbrand, A., Hübscher, C.
• Other Authors: Faculdade de Ciências Lisboa, Universidade de Lisboa = University of Lisbon (ULISBOA), Géosciences Paris Sud (GEOPS), Université Paris-Sud - Paris 11 (UP11)-Centre National de la Recherche Scientifique (CNRS), Institut für Geophysik Hamburg, Universität Hamburg (UHH)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2018
• Subjects: [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Jorge Island
• Online Access: https://hal.science/hal-01894661; https://doi.org/10.1016/j.tecto.2018.05.012

International audience The S. Jorge Island in the Azores lies on a peculiar setting, the southern shoulder of the Terceira Rift (TR), which raises a series of questions that we address in this study. We first established the main volcanic stratigraphy by recognizing, in the field, the main unconformities/discontinuities and their meaning (major erosion surfaces and faults), then we collected critical samples, and finally dated them by K/Ar to calibrate the stratigraphy and the age of inferred large-scale flank collapses. Based on field, geochronological and marine geophysical data: (1) we found much older rocks in S. Jorge than in previous studies (ca. 1.85 Ma), and established a new volcanic stratigraphy (from bottom to top): Old Volcanic Complex (ca. 1.9–1.2 Ma), cropping out in the eastern third of the island; Intermediate Volcanic Complex (ca. 0.8–0.2 Ma), cropping out in the western two thirds of the island and separated from the underlying complex by a major fault; Young Volcanic Complex (<ca. 0.1 Ma), unconformable on both older units. (2) We discuss the most probable mode of destruction of the successive volcanic complexes, and we conclude that a major landslide occurred between ca. 1.2 and 0.8 Ma, which was responsible for the major lateral discontinuity between the Old and the Intermediate Volcanic complexes. (3) We use previous palaeomagnetic data and new seismic data to evaluate the effects of uplift of the TR's southern shoulder on S. Jorge Island, and conclude that south-westward tilting of the oldest lava flows occurred between ca. 1.2 and 0.8 Ma due to rotation of the TR's southern shoulder during uplift. (4) Finally, we used all data to constrain the age of TR initiation to be ca. 1 Ma in this sector of the TR.