Chapter 3 Active tectonics in the central and eastern Azores islands along the Eurasia–Nubia boundary: a review

Abstract

The geodynamic setting of the Azores archipelago, straddling the triple junction between the North America, Eurasia and Nubia plates, is reflected in frequent volcanic and tectonic activity. A review of neotectonics is presented for the islands forming the central and eastern groups of the Azores (Faial, Pico, São Jorge, Graciosa, Terceira, São Miguel and Santa Maria). The geometry and kinematics of active faults displacing stratigraphic and geomorphological markers of Pleistocene and Holocene age are presented. Slip-rates were determined using the available ages for the displaced markers. Maximum expected moment magnitudes were estimated using empirical correlations between magnitude and fault length, fault area and maximum observed surface displacement during surfacerupturing palaeoearthquakes. Neotectonic parameters show that the faults are in most cases very to moderately active, with slip-rates usually ranging from a few tenths of millimetres to a few millimetres per year, while maximum expected magnitudes vary from M w 6 to 7. These magnitudes are in agreement with the instrumental and historical seismic record in the region. Neotectonic data define a dextral transtensive stress regime acting on the region and contribute to characterizing the complexity of the geodynamic processes that dominate the western-most segment of the Eurasia-Nubia plate boundary.

Emergence and evolution of Santa Maria Island (Azores)—The conundrum of uplifted islands revisited

“…This final stage in the island evolution was also 666 accompanied by neotectonic activity, essentially materialized by NNW-SSE (and more rarely 667 NE-SW) nearly vertical dip-slip block faulting, which displaced some of the higher marine 668 terraces (Madeira, 1986;Madeira et al, 2015). 669…”

Soil CO2 Degassing Path along Volcano-Tectonic Structures in the Pico-Faial-São Jorge Islands (Azores Archipelago, Portugal)

“…The main tectonic features are (1) the Mid-Atlantic Ridge (MAR) that crosses the archipelago between the islands of Flores and Faial, (2) the East Azores Fracture Zone (EAFZ), which extends E-W from the MAR to south of Santa Maria, and the Azores-Gibraltar Fault Zone that includes the E-W trending Gloria Fault, and (3) its western segment, the Terceira Rift (TR), which extends from the MAR to the island of Santa Maria along a general WNW-ESE direction, and corresponds to the present-day EU-NU plate boundary (e.g., Searle, 1980;Madeira and Ribeiro, 1990;Vogt and Jung, 2004;Miranda et al, 2015; Figure 1). The main tectonic structures in the islands are normal dextral faults with a WNW-ESE trend, characteristic of the Terceira rift; the NNW-SSE conjugate fault system exhibits oblique normal left lateral displacement (Madeira and Brum da Silveira, 2003;Madeira et al, 2015). These main fault systems are also present in Faial, Pico, and São Jorge islands, with the dominant structures striking WNW-ESE and dipping 60-90 • to the NNE or to the SSW controlling the general shape of some islands (Madeira and Brum da Silveira, 2003).…”

“…Most of the identified faults in the Azores archipelago are considered active (Madeira, 1998;Madeira and Brum da Silveira, 2003;Marques et al, 2013;Madeira et al, 2015) and the major faults usually have well-developed scarps; however the volcanic nature of the islands, together with differential erosion, can attenuate or amplify the tectonic slope (Madeira and Brum da Silveira, 2003) and even make the identification of tectonic structures difficult (due to scarp burial by pumice deposits, for instance). In addition, the exuberant vegetation of some of the Azorean islands covers most of the geological structures.…”

“…In addition, the exuberant vegetation of some of the Azorean islands covers most of the geological structures. Tectonic structures used in the present study were mapped by Madeira and Brum da Silveira (2003) and Madeira et al (2015) through fieldwork and vertical air-photo interpretation. All the faults in the study islands are considered active by Madeira and Brum da Silveira (2003) as they displace volcanic sequences younger than 100 ka.…”

Chapter 1 Volcanic geology of São Miguel Island (Azores Archipelago): introduction

“…Miranda et al (2015) provide an overview of the regional tectonic setting based on geophysical and geodetic data. Madeira et al (2015) focus on active tectonics of the eastern branch of the Azores Triple Junction, as reflected in the central and eastern islands. The range and distribution of seismic and volcanic activity in the Azores region are discussed by Gaspar et al (2015b), the authors characterizing earthquakes with a magnitude .7 and the volcanic eruptions ranging from Hawaiian to explosive sub-Plinian in style that took place in historic times (broadly the last 550 years).…”