2015 SPA - Mireia Peral - Dynamics of double-polarity subduction: application to the Western Mediterranean

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

Plate tectonics describes large-scale Earth’s lithosphere motions through a number of thin rigid lithospheric fragments (plates) that are in motion relative to each other. The relative velocities of the plates are around 5 cm/year and most earthquakes, volcanic eruptions and orogenic belts occur in regions where different plates are in contact (plate boundaries). This study focuses on convergent margins, in particular on subduction zones, that place where two plates collide and one (commonly known as slab) moves under the other sinking into the mantle.

Figure 1. Scenario of the Western Mediterranean

 evolution (figure from  Chertova et al., 2014).

 Blue triangles indicate  the direction of 

subduction and the yellow arrows indicate the 

movemet of the plates.

The motivation of this work lies on the recent geodynamic model proposed by Vergés and Fernàndez (2012) to explain the evolution of the Western Mediterranean since 85 Ma. This model is based on the interaction of two plates which are characterized by opposite direction of subduction (double-polarity subduction). Our objective is to analyze the dynamic feasibility of this process and its consequences through a 3D numerical model.

The dynamic evolution of the Earth at large temporal and spatial scales, as subduction processes, is modeled as a flow problem and requires advance numerical techniques and high computation times. Numerical models of viscoelastic flow in 2D/3D have been developed to understand the dynamics of tectonic plates in large timeframes. The final model consists of two plates subducting into the upper mantle and the problem is driven by the density contrast between the lithosphere and the mantle beneath.

Our numerical results indicate that 2D and 3D single (one plate) subduction models with the same configuration result in similar slab morphologies. Anyway 3D models (because of taking into account the three dimensions) produce a faster subduction. In addition, a preliminary double subduction model, in which the two plates are separated 100 km one from the other, has been calculated. Comparing it with a single subduction model we observe slight differences in the subduction velocity and in the slab morphology near the contact area. We are currently checking the sensitivity of the double subduction models to modification of the space between the two plates and to different velocity boundary conditions. Moreover the lateral contact between the plates, the effect of temperature and geometries applicable to the Western Mediterranean region will be considered in future models. Finally, analogue models will be done in order to compare them with numerical solutions.

Figure 2. Time evolution of a 3D double polarity subduction model with a lateral separation of 100 km between plates.

This work is supervised by Manel Fernandez (ICTJA-CSIC) and Sergio Zlotnik (UPC, Barcelona) and is part of the project “Testing the geodynamic evolution of the Western Mediterranean (We-Me), financed by the CSIC as “Proyecto Intramural Especial” PIE-CSIC-201330E111.

2015 SPA - Mireia Peral - The use of gravity gradients from GOCE satellite data in interpreting major crustal and lithospheric structures: application to the Iberian Peninsula

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

Gravity is used in geophysics to study the Earth’s interior. Gravity prospecting measures variations of the Earth’s gravity field in order to find out density contrasts. In 2009 the European Space Agency (ESA) launched GOCE satellite (Gravity field and steady-state Ocean Circulation Explorer) aiming at determine  both Earth’s mean gravity field and geoid with high accuracy and spatial resolution. The satellite was equipped with an innovative Electrostatic Gravity Gradiometer that measures spatial variation of gravity (gravity gradients) in the three dimensions. This study has been done with the objective of analyze the information provided by gravity gradients and finding out the minimum features of geological structures that may be studied using GOCE data.

Gravity gradients, although characterized by less power signal, carry more information about geological structures than gravity measures alone. Gravity gradients are calculated over the Iberian Peninsula from a global gravity field model. We study the correlation between these signals and the topography of the region, as each gravity gradient component provides us specific information about geological structures.

On the other hand, the potential of GOCE data is studied by forward modelling of gravitational fields using the Tesseroids software. Gravity gradients produced by several synthetic models are computed and compared with the uncertainty of GOCE data. Varying some parameters of these prisms such as dimensions, depth and density, we identify the main characteristics of geological structures that could be detected by GOCE satellite. We find that the smallest dimensions of the structures that still generate enough signal in the vertical component vary from around 22.5 x 7.5 x 2.5 km if the density contrast with the surrounding media is 500 kg/m³ to 49.5 x 16.5 x 5.5 km if the density contrast is 50 kg/m³, with a maximum burial depth of 40 km. Moreover, a synthetic rift model (divergent plate boundary) is implemented in order to observe the gravitational gradient field generated by a common geological structure. Our results indicate that typical rift structures can be detectable by GOCE satellite.

Figure. Gravity gradients of the three main components computed at 255 km mean satellite altitude over the Iberian Peninsula region. a) g_xx: horizontal component in the North-South direction. b) g_yy: horizontal component in the East-West direction. c) g_zz: vertical component.

This work is supervised by Manel Fernandez and Montserrat Torne from the ICTJA-CSIC, Barcelona.

2015 SPA - Maria Jesus Rubio de Inglés. The Evolution of the North Atlantic Oscillation for the last 700 years inferred from D/H isotopes in the sedimentary record in Lake Azul (Azores Archipelago, Portugal)

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

Spanish people are concerned about the existence of the Azores High Pressure since Mariano Medina (“weather man”) introduced this term on the TV weather forecast around 1958. At these southern latitudes, this expression has been associated to pleasant weather conditions but, a persistence of these conditions are linked to severe droughts in the westernmost areas of the Iberian Peninsula. But, what does it mean?

The Azores high pressure is a part of a complicated atmospheric system formed by 2 centers of action. This dipole is formed by a high pressure cell in Azores and a low pressure cell in Iceland. This atmospheric pattern is called the North Atlantic Oscillation (NAO) and it is defined as the pressure difference between Azores and Iceland. The NAO is responsible of the winter weather in Europe and North America.

Owing to the importance of this climatic phenomenon in Europe and nearby areas, we have gone to the crux of the southern center of action to reconstruct the NAO index for the last 700yr from lake sediments. If the high pressure cell is intensified and over Azores archipelago, the precipitation decreases. And, the precipitation increase with a weaker or shifted high pressure cell.

The precipitation gets recorded in the sediment in many ways but, in the present work the hydrogen isotopes have been used. Water molecule is composed by two atoms of hydrogen and one atom of oxygen. The hydrogen contained in the water can be lighter or heavier depending on the number of neutrons. The gravity force makes heavy water molecules fall first than light molecules. But, if the rainy episode continues, the light molecules will fall. In other words, short rainy periods record heavy hydrogen isotope signal and long rainy periods record light signal. Since the positive phase of NAO is related to driest periods, these periods will be marked by a heavy hydrogen isotope signal.

The analyses every half centimeter in the sediment core retrieved from Lake Azul disentangle the NAO effects over Azores. This reconstruction shows a multidecadal oscillation of the NAO phase for the last 700 year. Other authors (such as Trouet et al., 2009) found a persistence of a positive phase during the Medieval Climate Anomaly (MCA) followed by a trend towards negative phase during the Little Ice Age (LIA). We do not observe these patterns in our record despite that, similar fluctuations are observed. This could be because, since all the NAO reconstruction reflects the effects not the NAO itself (which is defined as pressure), those effects can vary between sites or be affected by other patterns. Then, we can conclude saying that we are reconstructing the NAO effects for the southern dipole of the climatic phenomenon.  

This is part of the project PaleoNAO. The supervisors of this phD thesis are Santiago Giralt (ICTJA- Environmental changes in the Geological Record department) and Alberto Sáez (UB- Stratigraphy, paleontology and marine geoscience department)

2015 SPA - David Cruset - Crestal graben fluid evolution during late growth stage of the Puig-reig anticline (South Pyrenean fold and thrust belt)

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

Hello, my name is David Cruset and I am in the first year of my PhD in the Institute of Earth Sciences Jaume Almera - CSIC. The topic of my thesis is the fluid migration in the South Pyrenean fold and thrust belt, from Late Cretaceous to Oligocene. To deal with this objective, I will do petrographic studies and geochemical analyses of the products related with fluid flow and the structural characterization of the structures where these products were formed. The interest of this thesis is that fluids are important because they are responsible of heat and matter transport and are involved in ore deposition and hydrocarbon accumulations. In addition, the study of the diagenetic products related with fluid migration can shed light on the geodynamic evolution of a basin or orogen.

Fig. 1 Geological cross section of the frontal part of the South Pyrenean fold and thrust belt.

As a preliminary step, during the last year I have been studying the fluid flow in the Puig-reig anticline, one of the structures of the frontalmost part of the South Pyrenean fold and thrust belt (Fig. 1). This work shows how during the early folding deep hot meteoric fluids circulated along inverse and strike-slip faults, whereas during the main stage of folding cooler meteoric waters percolated downwards the normal faults formed by collapse of the crest of the anticline (Fig.2). This study reports the controls of fracturing on the palaeohydrology of folds during different stages of their evolution.

The comparison of the obtained results with those obtained in previous works by other authors in nearby areas will allow us to perform the fluid flow model of the frontal part of the South Pyrenean fold and thrust belt.

Fig. 2 Schemes of the structural and fluid flow evolution of the Puig-reig anticline. Red and blue arrows indicate fluid movement. No vertical exaggeration. A) Fluid flow during the early folding. Hot evolved meteoric fluids migrated along the main faults and more permeable sedimentary units. B) Fluid flow during the main stage of folding. During this event, local meteoric fluids circulated downwards the normal faults formed by outer arc extension and mixed with the evolved meteoric fluids. 

This work is supervised by Dr. Jaume Vergés (Group of Dynamics of The Litosphere - GDL, ICTJA - CSIC) and by Dr. Anna Travé (Grup de Geologia Sedimentària, Department of Geochemistry, Petrology and Geological Prospecting - UB). 

2015 SPA - Mar Moragas Rodriguez - Influence of diapir growing in carbonate deposition: The Central High Atlas Jurassic Rift Basin (Morocco)

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

All of us use oil, gas, and plastic, which is a product deriving from petroleum. Oil and gas (hydrocarbon) are stored in the subsurface (between 1 and 6 km); in rocks (reservoirs) that have voids in which these fluids can be contained. At these depths, it is difficult to know where we can find these raw materials. Thus, we carry out field studies to have a better understanding of geological systems comparable to those where hydrocarbons might be emplaced.

In the present work we focus on a specific reservoir type: Carbonate deposits. Imagine a coral reef that you know very well from wildlife documentaries, they produce carbonate which later on will result in carbonate rocks; but are they distributed everywhere in our planet? No, nowadays they are mainly located in shallow marine environments with warm waters. Light, temperatures, water depth, amongst a large amount of other factors, control their development and demise and of other carbonate production factories. Patterns of carbonate platform development and deposition are generally complicated on diapiric settings. Diapirs are a type of geological structures formed due to the upward movement of mobile and less dense material (salt or shales) through more brittle rocks. Diapir growth and rapid salt movements cause high variability of carbonate distribution patterns and hampers the predictability of where we can find them.

In order to have a complete picture of the evolution of diapiric basins and the distribution of carbonate deposits, different study methods (structural, sedimentological, diagenetic, amongst others) are applied on Jurassic-aged diapiric structures located in the Central High Atlas, Morocco. The study of this area allows us to characterize the carbonate deposit associated to diapirs (carbonate type, thickness and length of carbonate units,...) and to understand the influence of diapir growth on the carbonate deposition.

This study is funded by Statoil Research Centre, Bergen (Norway), by the Spanish Ministry of Education and Science (MEC) through the projects Intramural Especial (CSIC 201330E030), MITE (CGL 2014-59516). We are grateful to Statoil for its support and permission to publish this research.