José Madeira

Morpho-tectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area

The existing studies of the Azores triple junction, although based on specific geological or geophysical data, largely rely upon morphological considerations. However, there is no systematic bathymetric coverage of this area, and the quality of the available bathymetric charts does not allow consistent morpho-structural analysis.In this work we present a new bathymetric grid elaborated with all the available data sources in an area comprised between 24°xa0W to 32°xa0W and 36°xa0N to 41°xa0N. The basic data set corresponds to the merge of NGDC data with new swath profiles. This new map, included as an Appendix, combined with other results from seismology and neotectonics, is the basis for the study of the morpho-structural pattern of the Azores area, the present day stress field and its implications on the current view of the Azores geodynamics.As a major result, we conclude that the Azores region is controlled by two sets of conjugated faults with 120° and 150° strikes that establish the framework for the onset of volcanism, expressing as linear volcanic ridges or as point source volcanism. This interaction develops what can be considered as the morphological signature of the Azores Spreading axis segmentation. We argue that the Azores domain, presently in a broad transtensional regime, is acting simultaneously as a ultra slow spreading centre and as a transfer zone between the MAR and the dextral Gloria Fault, as it accommodates the differential shear movement between the Eurasian and African plates.

Geodynamic models for the Azores triple junction: A contribution from tectonics

Abstract The American, Eurasian and African lithospheric plates meet at the Azores triple junction. The nature of the northern and southern branches of the junction is well known and uncontroversial, the American plate is separated from Eurasia and Africa by the Mid-Atlantic Ridge (MAR). The western group of islands of the Azores archipelago (Flores and Corvo) lies on the American plate, just west of the rift, and their tectonic features agree with that location. The nature of the third branch of the junction, to the east of the MAR, is however still controversial. Some authors believe that a ridge-ridge-ridge junction occurs in the Azores area and adopt the structure known as the “Terceira Ridge” (working as a simple rift boundary) as the third branch. Another model proposes a triangular microplate in the Azores region bounded by the Mid-Atlantic Rift to the west, the East Azores Fault Zone to the south and a fault crossing S. Miguel and the Terceira islands to the northeast; in this model the fault zones are pure dextral strike-slip boundaries. Both models would create space problems at Gloria Fault, a well-known pure dextral strike-slip structure. In the first model transtension would be necessary at Gloria Fault to accommodate plate motion, while the second would require a transpressive regime on that structure. Neotectonic and seismotectonic data in the Azores indicate a transtensile regime in the central and eastern island groups in present and recent times. This is compatible with a “leaky transform” structure acting as the third branch of the junction in the Azores area, a model which has already been presented by several authors in the past. Stress trajectories deduced from neotectonic studies in some of the islands suggest that the main plate boundary passes between the islands of S. Jorge and Pico south of S. Miguel and joins Gloria Fault east of Santa Maria. This model and boundary location is also compatible with seismotectonic and magnetic data.

The morphology of insular shelves as a key for understanding the geological evolution of volcanic islands: Insights from Terceira Island (Azores)

Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high-resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume-Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.

The Late Triassic‐Early Jurassic Volcanism of Morocco and Portugal in the Framework of the Central Atlantic Magmatic Province: An Overview

An overview on the Late Triassic-Early Jurassic Magmatic Province of Morocco and Portugal (TJMPMP) is presented. It comprises extrusive basalts, interbedded with clastic rocks sequences preserved in elongated rift basins, and their feeder dikes and sills. Paleontologic ages range from Upper Ladinian-Lower Carnian to the Sinemurian for the sediments, while available 40 Ar/ 39 Ar analysis yield a mean age of 200±1,6 Ma for the volcanics. The volcanologic characteristics of the TJMPMP are those of continental basaltic successions. It comprises subaerial lava flows and pyroclastic deposits, sometimes deposited in lacustrine environments, and feeder dikes, constituting an interesting volcanic sub-province of the Central Atlantic Magmatic Province (CAMP) as most preserved outcrops are extrusive volcanics. These rocks correspond to Low-Ti (TiO 2 11 suggesting that their source may be within the continental lithospheric mantle. The nucleation of the rifting process may have started at two different triple junctions, an RRR junction near Florida and a RRT between Africa-Iberia-America.

Assessing landslide movements in volcanic islands using near-shore marine geophysical data : south Pico Island, Azores

Marine geophysical data from around the submarine flanks of volcanic islands can potentially help to resolve whether large-scale instability of an edifice has been geologically recently active. We use geophysical data to investigate part of the coast of Pico Island of the Azores where, above sea-level, a major slump of Topo volcano has been interpreted previously from arcuate escarpments and a rugged irregular topography seaward of them. Multibeam echo-sounder data collected offshore of this feature show remarkably little evidence for slump fault movements in the islands submarine slope. Mid-slope benches, like those associated with the Hilina Slump of Kilauea, are absent. The high-resolution data extends onto the islands shelf, allowing us to evaluate evidence for continuing activity there. In particular, as the shelfs rock platform will have been last modified by surf erosion during the postglacial period of sea-level transgression, it provides a reference surface of intermediate age (7–19xa0ka) that can potentially reveal whether any movements occurred in pre-historic times. Where the arcuate escarpments are continued offshore, the modern seabed shows no bathymetric evidence for active faults where the shelf rock platform crops out in the multibeam data. Elsewhere, mobile shelf sediments could be disguising evidence for active faulting so we examined boomer profiles able to image the rock platform beneath them. The data reveal a platform that is steep (6.6°) compared with the dips of platforms that we have studied previously around the coast of adjacent Faial Island and steeper than the platform outside the proposed slump. This suggests that it was created by coastal erosion over a shorter period and hence is consistent with a younger age of the coastline. As with the multibeam data, where escarpments are continued offshore onto the shelf, the rock surface imaged with these boomer data also shows no clear evidence of major slump-related fault displacements. This study therefore illustrates how high-resolution boomer seismic and multibeam data could usefully contribute to hazard assessment of volcanic islands, by helping to evaluate areas with no historical movements. Explanations to reconcile the onshore and offshore data here are also put forward.

Hazard potential of volcanic flank collapses raised by new megatsunami evidence

Giant tsunami triggered by catastrophic flank collapse of Fogo volcano. Large-scale gravitational flank collapses of steep volcanic islands are hypothetically capable of triggering megatsunamis with highly catastrophic effects. Yet, evidence for the generation and impact of collapse-triggered megatsunamis and their high run-ups remains scarce or is highly controversial. Therefore, doubts remain on whether island flank failures truly generate enough volume flux to trigger giant tsunamis, leading to diverging opinions concerning the real hazard potential of such collapses. We show that one of the most prominent oceanic volcanoes on Earth—Fogo, in the Cape Verde Islands—catastrophically collapsed and triggered a megatsunami with devastating effects ~73,000 years ago. Our deductions are based on the recent discovery and cosmogenic 3He dating of tsunamigenic deposits found on nearby Santiago Island, which attest to the impact of this giant tsunami and document wave run-up heights exceeding 270 m. The evidence reported here implies that Fogo’s flank failure involved at least one fast and voluminous event that led to a giant tsunami, in contrast to what has been suggested before. Our observations therefore further demonstrate that flank collapses may indeed catastrophically happen and are capable of triggering tsunamis of enormous height and energy, adding to their hazard potential.

The emergence of volcanic oceanic islands on a slow‐moving plate: The example of Madeira Island, NE Atlantic

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow-moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeiras subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

Morphology, internal architecture and emplacement mechanisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of Argana Basin (Morocco)

Abstract The morphology, internal architecture and emplacement mechanisms of the Central Atlantic Magmatic Province (CAMP) lava flows of Argana Basin in Morocco are presented. The volcanic pile was produced by two volcanic pulses. The first, represented by the Tasguint Formation, corresponds to a succession of 3–13 individual flows created by 1–8 eruptions; the second, Alemzi Formation, is composed of 2–7 individual flows formed by 1–4 eruptions. These formations, geochemically distinct, are separated by thin silty or sandy horizons or by palaeosols. They include ‘compound pahoehoe flows’ and ‘simple flows’. The first type is almost exclusive of the lower formation, while the second type dominates the upper formation. The lava flows show clear evidence of endogenous growth or ‘inflation’. The characteristics of the volcanic pile suggest slow emplacement during sustained eruptive episodes and are compatible with a continental basaltic succession facies model.

Geochemical evidence for melting of carbonated peridotite on Santa Maria Island, Azores

The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45xa0Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO2-rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume.

Radiocarbon dating Recent volcanic activity on Faial Island (Azores).

Letude de la stratigraphie volcanique de lile Faial des Acores montre 5 formations geologiques principales, dont la formation du Caldeira a lHolocene. Des echantillons de residus organiques sont dates par le 14C afin detablir une chronologie de lactivite du volcan. Les resultats obtenus permettent alors de determiner les periodes deruption dont la frequence indique lactivite intense du volcan