2016 PSPA – Cristina Biete Castells – Effects of inherited continental margin structures in the south-central Taiwan fold-and-thrust belt

Orogens and fold-and-thrust belts (FTB) commonly present along-strike changes like changes in the structural architecture and/or different deformation styles. The generation and evolution of the geological architecture is primarily controlled by the presence of pre-existing structures and/or stratigraphy horizontal changes. With the idea to keep advancing in the knowledge of how this factors affect, we study the central-south Taiwan FTB.

Taiwan is an oblique Arc-continental collision between the Luzon Arc, striking N-S, and the Eurasian continental margin, striking in south Taiwan roughly E-W. This margin presents several extensional basins that now are involved in the Taiwan FTB

In this study we combine surface geology and balanced cross sections, which will help to understand the FTB, with seismic tomography and seismicity, that are used to trace structures from the continental margin offshore western Taiwan into the FTB. We use a Vp of 5.2 km/s as a proxy for the basement-cover interface

We found that the FTB includes significant along strike changes in structure and stratigraphy. Major N to S changes in seismic velocities are interpreted as basement highs and lows and these correlate with areas were changes in the structural grain of the FTB take place, including some localized variation in the strike of thrusts and folds that are evident on the map. Several seismicity clusters align along the borders of these basement blocks. We interpret this to be related with reactivation of basement structures.

Figure 1: a) Tomography of 6km depth slice showing with grey transparent rectangles the main areas with velocity changes. b) Tomography E-W sections with seismicity and main geologic structures. c) Geological map showing with grey transparent rectangles the main areas were structural and stratigraphic changes are present. d) N-S tomography section with seismicity showing a seismicity cluster at the south of a basement high. e) Block diagram showing roughly the possible structure of the basement on the fold-and-thrust belt, with gray transparent rectangles showing the areas were structural, stratigraphic, velocity and seismicity changes are.

This work is supervised by Joaquina Álvarez-Marrón and Dennis Brown (ICTJA-CSIC). Economic support has been provided through CGL2014-4377-P project funded by the Spanish Ministry of Science and Innovation. We acknowledge National Geographic Institute for providing the seismic data for the study.

2016 PSPA – Pilar Sánchez - Advantages using Phase Cross-Correlation for searching temporal structural changes. Application to 2011 El Hierro eruption

Ambient seismic noise is a continuous vibration of the ground due to natural and artificial sources although the main origin is the interaction between the atmosphere, the ocean and the solid Earth. This noise is recorded by seismic stations located everywhere in the world at any time and it does not depend on earthquake activity. On other hand, the knowledge of the seismic wave field has been improving during the last decades; this allows it to study characteristics of its sources and the ground. For all of these reasons, its importance has increased in some areas of the Geophysics such as volcanic monitoring and tomography.

Figure 1. Summary sketch of a
seismic wave path  (grey  line).
The  red  triangle is  a  seismic
 station and the orange rectangle

 shows the recorded signal.        

Seismic waves created by noise sources travel through the Earth and suffer multiple scattering during their path until arriving to seismic stations (Figure 1). So we cannot know this path but if the ground and sources are equal in time we have to obtain coherent and stable signals after a while, i.e., we can obtain a stable trace that will be directly related to seismic response of the medium.

In order to obtain that trace, it is necessary a coherence measure that can be reached with the correlation between two temporal series. During this work, we present the advantages to use the phase correlation instead of classical correlation for different approaches and explain the constraints we have to take into account for the data processing.

We further apply these concepts to field data of nine seismic stations installed in El Hierro (Spain). We focus on finding temporal structural changes due to the 2011 eruption that happened in this island. It is important to work in a frequency band where the noise source not change its properties and where tremors not be recorded. To do so, we calculate spectrograms with which we can analyse the energy distribution in function of frequency and time (Figure 2).

Figure 2. Spectrogram for ten days of CCUM station. The white arrow marks the onset of the eruption. 

Once all the parameters are chosen, we calculate correlations with different methods of the whole available data and extract preliminary but promise results about the 2011 El Hierro eruption.

This work is supervised by Martin Schimmel (ICTJA-CSIC). Economic support has been provided through CGL2013-48601-C2-1-R project funded by the Spanish Ministry of Science and Innovation. We acknowledge National Geographic Institute for providing the seismic data for the study.

2016 PSPA - Mireia Peral – Analogue modelling of double polarity subduction

Reproducing geological processes in the laboratory has been of a great interest among scientists for years. Laboratory experiments, scaled in time and space, may help us to improve our knowledge of some geological processes that are impossible to study in nature. For example, we may reproduce large tectonic processes and study their evolution in the order of millions of years.

This study is based on laboratory simulations of a specific tectonic process, called double polarity subduction. In a subduction system, which occurs on convergent margins of plate boundaries, two plates collide and one moves under the other (overriding plate). In the Mediterranean Vergés and Fernàndez (2012) hypothesized the presence of two subductions retreating in opposite directions since 30 million years.

Our experiments consist in simulating in the laboratory the evolution of a double polarity subduction process, but without taking into account the overriding plate. We realized the experiments in a square Plexiglas tank full of high viscous syrup, representing the mantle. The setup contains two oceanic plates (Figure 1), made with silicone putty, and subduction is started by deflecting manually the leading edge of the plates. Each centimeter corresponds to 60 km. The experiments are monitored with two cameras that take photographs in time intervals of 30 seconds from the top of the experiment and from an oblique position, so we can study the system in function of time. Different setups were designed to test the influence of two variables in the system: i) the width of the plates and ii) the lateral distance between the two subducting plates.

The evolution of the models is characterized by three different phases: (1) initial stage of subduction, corresponding to the evolution of the system until the plates reach the base of the model box; (2) approaching trenches, until they pass each other; (3) diverging trenches, until the retreating of plates is over.

Our results indicate that velocities of the trench (marking the position at which the subducting plate begins to descend) increase during phase 2 and then decrease during phase 3. We explain this trendline as due to the interaction of the mantle flow induced by both plates in the contact area. When plates are wider the same process is active. Nevertheless, when lateral distance between plates increases we do not observe any change of the velocities during the evolution and the interaction of the two plates become negligible when they are 10 cm spaced (600 km in nature).

Figure 1. Top view of a double-polarity subduction experiment. The images correspond to three different stages of the evolution. The plates are 10 cm (600 km in nature) width and initially spaced 0,5 cm (30 km in nature).

This work is supervised by Manel Fernandez (ICTJA-CSIC) and Sergio Zlotnik (UPC, Barcelona). The experiments have been carried out in collaboration with Ágnes Király, Francesca Funiciello and Claudio Faccenna from the Laboratory of Experimental Tectonics (Roma Tre University, Italy). This study is part of the Project “Testing the geodynamic evolution of the Western Mediterranean (We-Me) financed by CSIC (PIE-CSIC-201330E111). We also thank to the project AECT-2016-1-0002 of the Barcelona Supercomputing center (BSC-CNS).

4ª edición del PhD Student Presentation Award

Hola a todos:
El próximo lunes 19 de Diciembre a las 11:00 h se celebrará en el Salón de Actos del ICTJA-CSIC el 4º PhD Student Presentation Award (PSPA) de nuestro instituto. 

En este mismo blog ya se pueden empezar a leer (y compartir) los resúmenes de las diferentes presentaciones que optan al PSPA de este año. 


A lo largo de esta semana los pósters que se presentan también se irán exponiendo en el pasillo de la planta baja (al lado del Laboratorio Polivalente, el antiguo LARX).

Salud!

2016 PSPA - Juvenal Andrés - Curie-depth and thermal gradient map of the Iberian Peninsula and surrounding areas

The distribution of temperature at depth is a combination of past and present processes such as, collisions, thickening and thinning of the crust or subduction. Furthermore, the thermal structure of an area is dependent on the physical properties of the rocks (e.g. petrology, radiogenic heat production and thermal conductivity), hindering its understanding. The Curie point (CP) is the temperature at which magnetic minerals become paramagnetic. For the upper part of the lithosphere, the most abundant magnetic mineral is the magnetite, which has a CP of 580°C. Therefore, if we can calculate the depth at which we lose the magnetic signal we can get the depth of 580ºC isotherm. 

In this study, we have calculated a complete map of the Curie-Depth Point (CDP) (Fig.1) from a compilation of aeromagnetic data for the Iberian Peninsula and surrounding offshore areas by means of spectral analysis. The final magnetized layer appears on the range of 13 km to 27 km depth below topography onshore and bathimetry offshore. As expected, this isotherm is shallow in offshore zones, where the crust is thinner while in continental areas, the CDP appears deeper. 

We have compared our results with a Moho depth map of the same area. Offshore, the CDP is usually located beneath the Moho which may imply a magnetic upper mantle, partly formed by serpentinites. This serpentinized upper mantle might have played an important role in the evolution of some areas like the Western Mediterranean. On the contrary, for continental areas the CDP is located above the Moho, with NW Iberia featuring the deepest CDP values. We correlate these values with the late orogenic Variscan evolution that led to crustal thinning and intense thermal metamorphism that melted and re-equilibrated the crust. Finally, we have derived a complete map of the thermal gradient of the Iberian Peninsula and offshore areas.

Figure 1. Final distribution of calculated CDP (black dots) overlapped on ETOPO1 with major geological boundaries.

This work is supervised by Ramon Carbonell (ICTJA-CSIC) and Puy Ayarza (USAL). Economic support has been provided through CGL2014-56548-P project funded by the Spanish Ministry of Science and Innovation. We acknowledge GETECH Group Plc. for providing the magnetic data for the study.