Adriano Pimentel

Volcanic, tectonic and mass-wasting processes offshore Terceira Island (Azores) revealed by high-resolution seafloor mapping

Terceira Island, in the Azores Archipelago, lies at the intersection of four submarine volcanic ridges. New high-resolution bathymetric and seismic reflection data have been used to analyze the main volcanic, tectonic and mass-wasting features of the island offshore. Volcanic features such as linear volcanic centers, and pointy and flat-topped cones are mainly concentrated on the narrow western and north-western ridges, characterized by an overall rugged morphology. Fault scarps dominate mainly the broad eastern and south-eastern ridges, which are characterized by an overall smooth and terrace-like morphology. On the eastern ridge, faults form a series of horsts and grabens related to the onshore Lajes Graben. The strikes of the fault scarps, linear volcanic centers and alignment of volcanic cones on the ridges reveal two main structural trends, WNW–ESE and NNW–SSE, consistent with the main tectonic structures observed on the Azores Plateau. In contrast, a large variability of strike was observed in inter-ridge areas, reflecting the relative importance of regional and local stresses in producing these structures. Mass-wasting features are subordinate and mostly represented by hundred meter-wide scars that indent the edge of the insular shelf surrounding the island, apart from two large, deeper scars identified on the southern steep flank of the western ridge. Finally, the remarkable morpho-structural differences between the western and eastern ridges are discussed in the framework of the evolution of the Terceira volcanic edifice and hypothesized to reflect successive stages of ridge evolution.

Spatio-temporal constraints on magma storage and ascent conditions in a transtensional tectonic setting: The case of the Terceira Island (Azores)

Abstract The mafic magmatism of the last 50 ka on Terceira Island, Azores archipelago, occurred along three segments of the fissure zone that crosses the island. The two subaerial segments developed with different trends over pre-existing, quiescent or extinct, central volcanoes. The Serreta submarine ridge is the offshore segment of the fissure zone that erupted recently in 1998-2001. The combined study of CO2+H2O fluid inclusions hosted in mafic minerals and rock geochemistry of the magmas, reveals different storage and ascent conditions among the fissure zone segments. The maximum pressure of fluid trapping for all the fissure systems occurred at the Moho Transition Zone, between 498 and 575 MPa (20.3-21 km deep). At this depth interval all magmas stagnated for some time, before ascending toward the surface, experiencing fractional crystallization and degassing. Magmas of the southeastern and Serreta segments of the fissure zone ascended rapidly through the crust without further stops. Those of the central segment experienced a multi-step ascent, with fluid trapping at 406 and 209 MPa (16.5-8.5 km deep) and associated geochemical evolution toward trachybasalt. The magma ascent below the different segments of the fissure zone varies from almost isochoric at the submarine segment, associated with minimum re-equilibration of the inclusions, to polybaric slow ascent at the central segment, associated to almost complete re-equilibration of the inclusions. Variable degrees of re-equilibration and multi-step ascent may be linked to both the presence of preexisting intracrustal crystallized bodies of more evolved composition and the stress field acting on this area. The latter responds to the local and shallow conditions related to the presence of older central volcanoes and to the main regional spreading direction of the Terceira Rift, which at regional scale, is approximately orthogonal to the fissure zone axis.

Constraining chronology and time-space evolution of Holocene volcanic activity on the Capelo Peninsula (Faial Island, Azores): The paleomagnetic contribution

Faial is one of the most volcanically active islands of the Azores Archipelago. Historical eruptions occurred on the Capelo Peninsula (westernmost sector of the island) during A.D. 1672–1673 and more recently in A.D. 1957–1958. The other exposed volcanic products of the peninsula are so far loosely dated within the Holocene. Here, we present a successful attempt to correlate scoria cones and lava flows yielded by the same eruption on the Capelo Peninsula using paleomagnetic data from 31 sites (10 basaltic scoriae, 21 basaltic lava flows). In the investigated products, we recognize at least six prehistoric clusters of volcanic activity, whereas 11 lava sites are correlated with four scoria cones. Dating was conducted by comparing our paleomagnetic directions with relocated Holocene reference curves of the paleosecular variation of the geomagnetic field from France and the UK. We find that the studied volcanic rocks exposed on the Capelo Peninsula are younger than previously believed, being entirely formed in the last 8 k.y., and that the activity intensified over the last 3 k.y. Our study confirms that paleomagnetism is a powerful tool for unraveling the chronology and characteristics of Holocene activity at volcanoes where geochronological age constraints are still lacking.

Enhancement of eruption explosivity by heterogeneous bubble nucleation triggered by magma mingling.

We present new evidence that shows magma mingling can be a key process during highly explosive eruptions. Using fractal analysis of the size distribution of trachybasaltic fragments found on the inner walls of bubbles in trachytic pumices, we show that the more mafic component underwent fracturing during quenching against the trachyte. We propose a new mechanism for how this magmatic interaction at depth triggered rapid heterogeneous bubble nucleation and growth and could have enhanced eruption explosivity. We argue that the data support a further, and hitherto unreported contribution of magma mingling to highly explosive eruptions. This has implications for hazard assessment for those volcanoes in which evidence of magma mingling exists.