Eocene intra-plate shortening responsible for the rise of a faunal pathway in the northeastern Caribbean realm

Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (northern Caribbean) has inspired the hypothesis of the GAARlandia (Greater Antilles Aves Ridge) land bridge. This landbridge, however, should have crossed the Caribbean oceanic plate, and the geological evolution of its rise and demise, or its geodynamic forcing, remain unknown. Here we present the results of a land-sea survey from the northeast Caribbean plate, combined with chronostratigraphic data, revealing a regional episode of mid to late Eocene, trench-normal, E-W shortening and crustal thickening by ∼25%. This shortening led to a regional late Eocene–early Oligocene hiatus in the sedimentary record revealing the location of an emerged land (the Greater Antilles-Northern Lesser Antilles, or GrANoLA, landmass), consistent with the GAARlandia hypothesis. Subsequent submergence is explained by combined trench-parallel extension and thermal relaxation following a shift of arc magmatism, expressed by a regional early Miocene transgression. We tentatively link the NE Caribbean intra-plate shortening to a well-known absolute and relative North American and Caribbean plate motion change, which may provide focus for the search of the remaining connection between ‘GrANoLA’ land and South America, through the Aves Ridge or Lesser Antilles island arc. Our study highlights the how regional geodynamic evolution may have driven paleogeographic change that is still reflected in current biology

Structure of the Lesser Antilles subduction forearc and backstop from 3D seismic tomography

In 2007 the Sismantilles II experiment was conducted to constrain structure and seismicity in the central Lesser Antilles subduction zone. The seismic refraction data recorded by a network of 27 OBSs over an area of 65 km×95 km provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15 km. Basement velocity gradients depict that the forearc is made up of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. Whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6 km thick sedimentary pile, in relation with the subduction of the Tiburon Ridge and associated sea floor reliefs. North, offshore Dominica Island, the outer forearc is 70 km wide. It extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop. The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera Spur. The outer forearc could be either the eastern prolongation of the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc.Peer Reviewe

Tsunami hazards in the Catalan Coast, a low-intensity seismic activity area

The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-017-2918-zThe potential impacts of tsunamis along the Catalan Coast (NW Mediterranean) are analysed using numerical modelling. The region is characterized by moderate to low seismic activity and by moderate- to low-magnitude earthquakes. However, the occurrence of historical strong earthquakes and the location of several active offshore faults in front of the coast suggest that the possibility of an earthquake-triggered tsunami is not negligible although of low probability. Up to five faults have been identified to generate tsunamis, being the highest associated possible seismic magnitudes of up to 7.6. Coastal flooding and port agitation are characterized using the Worst-case Credible Tsunami Scenario Analysis approach. The results show a multiple fault source contribution to tsunami hazard. The shelf dimensions and the existence of submerged canyons control the tsunami propagation. In wide shelves, waves travelling offshore may become trapped by refraction causing the wave energy to reach the coastline at some distance from the origin. The free surface water elevation increases at the head of the canyons due to the sharp depth gradients. The effects of potential tsunamis would be very harmful in low-lying coastal stretches, such as deltas, with a high population concentration, assets and infrastructures. The Ebro delta appears to be the most exposed coast, and about the 20% of the delta surface is prone to flooding due to its extremely low-lying nature. The activity at Barcelona port will be severely affected by inflow backflow current at the entrance of up to 2 m/s.Peer ReviewedPostprint (author's final draft

Complex structure of Piton de la Fournaise and its underlying lithosphere revealed by magnetotelluric 3D inversion

La Réunion is a large volcanic construction resting on Paleocene oceanic crust. Through the 3D inversion of a large set of magnetotelluric (MT) soundings, our results reveal the general resistivity structure of the western part of Piton de la Fournaise volcano down to its base and the first 10 km or so of the underlying lithosphere. The resistivity pattern shows a general stratification of the resistivity values. An upper resistive layer corresponds to unsaturated and water-saturated lava flows. This layer is thinner (a few hundred meters) in the Enclos and Plaine des Sables/Fond de la Rivière de l'Est areas than on the NW flank where it reaches 2000 m. Below, the rest of the edifice is distinctly more conductive and shows highly conductive patches. The origin of the globally weak resistivity of the lower part of the construction is not established but can be tentatively attributed to a higher degree of alteration. The case of the highly conductive patches is different. Resistivity values of a few Ω·m imply the presence of highly conductive fluids and/or minerals that, in this context, are generally associated with hydrothermal phenomena. In the Enclos, highly conductive patches are unambiguously attributed to the presently active hydrothermal system. Beneath the Plaine des Sables/Fond de la Rivière de l'Est, they may be associated with hydrothermal alteration developed by the ancient volcanic center. The larger highly conductive patches are found below sea level and above the crust beneath the NW flank, at depths significantly larger than the others. This area coincides with the path of the N120 rift zone, with a deep (~10-20 km) seismicity and with diffuse CO2 degassing. A relationship may therefore be inferred between deep magmatic activity, eruptive activity and postulated highly conductive hydrothermal zone. In the globally weak resistivity of the lower part of the edifice, a dome of moderate resistivity (>1 kΩ·m) is centered beneath the Plaine des Sables. It is wider than a dense intrusive complex inferred from gravity models in the same area. The present resolution of both MT and gravity models cannot exclude a same nature for both structures. Beneath 4 km b.s.l., an increase of the resistivity values underlines an interface in good agreement with the inferred location of the transition between the edifice and the oceanic crust at this depth. The upper lithosphere shows resistivity values above 250 Ω·m. This study therefore demonstrates the capability of the method to image major shallow structures such as the hydrothermally altered zones, and deeper ones such as the heterogeneity of the crust

A devastating plinian eruption at Tungurahua volcano reveals formative occupation at similar to 1100 CAL BC in Central Equador

Based on archaeological and radiometric constraints, previous studies have divided pre-Columbian times of Ecuador into a succession of cultural periods. The Paleoindian and Preceramic periods encompass the time from the first Amerindian occupation to about 4000 BC. The Formative period extends from similar to 4000 to similar to 300 BC, while the Regional Development (similar to 300 BC to similar to AD 700) and Integration periods predate the Columbian period, which starts in AD 1533 in Ecuador. The Formative cultural period is poorly known from earlier studies. Here, we bring the first documentation of Formative age occupation around Tungurahua Volcano, 120 km SW of Quito, and show that local settlements were devastated by a violent eruption around 1100 cal BC. Recent volcanological works combining lithostratigraphic, petrologic, and geochronologic analyses reveal that the steep-sloped Tungurahua edifice suffered a major Late Holocene flank failure. We show that the failure event resulted from a major explosive eruption triggered by massive magma intrusion inside the volcano. Decompression of the magma due to a flank collapse resulted in a violent, high-velocity directed blast explosion, which deposited charcoaland sherd-rich ash layers upon and near the volcano. Our C-14 results range from 2225 +/- 30 to 5195 +/- 45 BP, but most cluster between 2640 +/- 45 and 3195 +/- 45 BP. A calibration analysis indicates that the event took place at similar to 1100 cal BC, in the Formative period. We gathered 38 pottery sherds from 3 localities. The sherds show a diversity of size, shape, color, and ornamentation. Examination of pastes, surface finish, and firing indicates that our material shares many common features from site to site. The material from Tungurahua shares affinities with the Cotocollao tradition, which developed in the Quito region between 1500 and 500 BC, and with the Machalilla tradition (coastal region of Ecuador), with the occurrence of carinated bowls with punctuate decorations at 1500-1000 BC. Our study reveals that the similar to 1100 cal BC Plinian eruption of Tungurahua Volcano is among the oldest known volcanic disasters in the Andes

A Megathrust earthquake as source of a Pre-Colombian tsunami in Lesser Antilles: Insight from sediment deposits and tsunami modeling

International audienceNo megathrust earthquake similar to the Magnitude class 9 events in Sumatra in 2004 or in Japan in 2011 was firmly reported at the Lesser Antilles subduction zone. The largest known tsunamis followed either a strong intraplate earthquake (1867, Virgin Islands) or were transoceanic due to the 1755 Lisbon earthquake. In this region, where the convergence rate between the American and Caribbean plate is low, the recurrence time of large earthquakes may be long (several centuries or millennia) and the historical record of such events is short. It is thus difficult to estimate their impact and becomes crucial to gain information from longer-term geological records and tsunami modeling. An increasing number of old prehistoric tsunami deposits have been identified in recent years on several islands in the northern segment of the Lesser Antilles arc, between Antigua and Puerto-Rico, in Anegada, St-Thomas (Virgin Islands), Anguilla and Scrub islands. Here, we carefully review all those studies and evidenced that most tsunami deposits are about 500 to 800 years old (1200 to 1500 cal yrs. CE) likely suggesting a large event or a cluster of events at that time. We combined information provided by the sedimentological records (distribution and altitude of the sediment deposits) and tsunami models to discuss the origin of the middle age Pre-Colombian event(s). We listed all faults as possible sources of tsunamis in this complex tectonic region. We performed 35 run-up models by using high-resolution/topographic grids to compare the simulated wave heights and run-up distance to the sediment record. We showed that few models are able to generate tsunami waves which heights and run-up distances match the characteristic of the observed tsunami deposits. These models are Magnitude class 9 M-thrust earthquakes rupturing the subduction interface between 30 km in depth to the trench facing Anegada Island. Magnitude class 8 outer-rise earthquakes, modeled along the trench, are other candidates for the Pre-Columbian event(s) although less convincing than the mega-thrust ones. The realism of these models is discussed in the light of the recent coupling models of the subduction zone based on short-term geodetic records. Finally, considering all the results and data, the equally strong hypothesis that these sediments were deposited by one or several storms remains less convincing than our tsunamigenic earthquakes scenarios. We conclude that the occurrence of one or several large megathrust or outer-rise earthquakes in association with damaging tsunamis likely have occurred in the past in the Lesser Antilles and could occur again in the future. This opens the discussion on the threat posed by such catastrophic event in these densely populated and touristic regions

Relationships between volcano gravitational spreading and magma intrusion

Volcano spreading, with its characteristic sector grabens, is caused by outward flow of weak substrata due to gravitational loading. This process is now known to affect many present-day edifices. A volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops. Such intrusions are clearly observed in ancient eroded volcanoes, like the Scottish Palaeocene centres, or in geophysical studies such as in La Réunion, or inferred from large calderas, such as in Hawaii, the Canaries or Galapagos volcanoes. Volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice. We explore the coupling and interactions between these two processes. We use scaled analogue models, where an intrusive complex made of Golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer. We model specifically the large intrusive complex growth and do not model small-scale and short-lived events, such as dyke intrusion, that develop above the intrusive complex. The models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading. The brittle substratum strongly controls the deformation style, the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer-related gravitational spreading. In the models, intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone (i.e. grabens, folds, en échelon fractures). Therefore, simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading, except when the intrusive complex is eccentric from the volcano centre. However, the displacement fields obtained for (1) a solely gravitational spreading volcano and for (2) a gravitational spreading volcano with a growing and spreading intrusive complex are very different. Consequently, deformation fields (like those obtained from geodetic monitoring) can give a strong indication of the presence of a spreading intrusive complex. We compare the models with field observations and geophysical evidence on active volcanoes such as La Réunion Island (Indian Ocean), Ometepe Island (Nicaragua) and eroded volcanic remnants such as Ardnamurchan (Scotland) and suggest that a combination between gravitational and intrusive complex spreading has been active

Electrical conductivity and induced polarization investigations at Kilauea volcano, Hawai'i

International audienceTwo new datasets of core samples from Kilauea volcano have been used to conduct complex conductivity (induced polarization) measurements from 10 mHz to 45 kHz at 3 or 4 salinities (NaCl, 10-2 to 10 S m-1 at 25 °C) depending on the core samples. The first dataset corresponds to 15 core samples collected in 2015 at the ground surface in the caldera of Kilauea around the Halema'uma'u crater (dataset 1). The second data set (dataset 2) corresponds to 28 core samples from Keller's well (KW) in the south part of the caldera. A third dataset (dataset 3) corresponds to 28 core samples from Well PTA-2 from the Humu'ula Groundwater Research Project in Hawai'i. The electrical conductivity of these samples can be decomposed as the sum of a bulk conductivity and a surface conductivity associated with electrical conduction in the electrical double layer coating the surface of the grains. The variations of the in-phase conductivity with the pore water conductivity is used to infer the intrinsic formation factor and the surface conductivity, which is a good indicator of alteration. The surface conductivity, the normalized chargeability (determined from the dispersion of the in-phase conductivity curve as a function of frequency), and the quadrature conductivity scale with the cation exchange capacity. In addition, the normalized chargeability and the quadrature conductivity of unaltered core samples show a signature associated with the presence of (titano)magnetite. The quadrature conductivity (and therefore the normalized chargeability) depends on temperature according to an Arrhenius's law with an activation energy close to 16 kJ Mol-1. We observe that alteration yields a pronounced increase of surface conductivity, normalized chargeability, and quadrature conductivity. These data are in turn used to help in the interpretation of a DC electrical conductivity survey performed in the caldera of Kilauea. The conductivity response around Keller's KW-well is dominated by the surface conductivity contribution associated with alteration and temperature. We propose a new method to image equilibrium temperature from electrical conductivity tomography

Modelling CO2 dispersion in the air during potential limnic eruption at the lake Pavin (France)

Risk mitigation in long-dormant volcanic provinces is a challenge due to the absence of collective memory of past disasters as well as the scarcity, and subtlety, of unrest signals that can be monitored. In this study, the impact of a potential limnic eruption is assessed at the 92-m-deep lake Pavin (French Massif Central). The lake is hosted in a maar crater formed during the last eruptive event in metropolitan France (ca. 7 ka) and contains dissolved CO2 in the deepest water layer, below 60 m. Carbon dioxide (CO2) emissions measured at the lake surface (0.44 km2) reach up to 10.1 tons/day during the winter. Beyond this (limited) continuous degassing of the lake, the current CO2 budget in the monimolimnion layer (at a depth of 60 m to 92 m) was estimated at 1750 tons, of which about 450 tons are available for release in case of overturn of the lake. Scenarios for CO2 dispersion in the lower atmosphere were simulated with the DISGAS and TWODEE-2 models by varying (i) meteorological conditions, (ii) the amount of CO2 released, (iii) and the mechanisms of degassing during a potential limnic eruption. The simulations allowed identification and delimitation of areas potentially impacted by hazardous CO2 levels in the air down-valley from the lake and directly around the lake. The spatio-temporal evolution of the potential CO2 cloud raises issues regarding the impacts of such a hypothetical event in the close vicinity of the lake and, given the area is populated and highly visited, needs to be considered in future risk mitigation strategie