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Mancilla F.L.,University of Granada | Diaz J.,CSIC - Institute of Earth Sciences Jaume Almera
Tectonophysics | Year: 2015

Crustal thickness maps at regional scales are typically compiled using estimations inferred from different geophysical datasets providing a variable coverage of the investigated area. Consequently, spurious effects related to changes in data resolution or artifacts in grid interpolation may affect significant zones of those maps. The TopoIberia-IberArray broad-band seismic network, covering the Iberian Peninsula and Northern Morocco with stations distributed on a regular 60 × 60 km grid provides a unique opportunity to avoid such technical problems and to obtain a crustal thickness map derived from a same method sampling evenly all the region. Data from more than 340 stations has been gathered and analyzed using the P-to-S conversion phases at the Moho discontinuity (receiver functions). The crustal thickness has been inferred applying the classical H-κ stacking technique, though in regions of complex crustal structure, we have preferred to estimate the thickness directly from the arrival time of the converted phase at some sites.The topography of the Moho discontinuity is strongly correlated with tectonic processes. The investigated area, extending from the Sahara platform to the Bay of Biscay, has a great geodynamic diversity, including, North to South, crustal imbrication in the Pyrenean and Cantabrian range, a large and relatively undisturbed Variscan Massif in the center of Iberia and areas of complex and still not completely understood geodynamics in the Alboran crust domain and the Atlas range. The crustal thickness map reflects this diversity, showing variations reaching 30. km between the thickest and thinnest zones of continental crust. The final map has an overall similarity with previous estimations of the crustal thickness using independent data, as those coming from more sparse deep seismic sounding profiles, but provides further constraints at regional scale. © 2015 Elsevier B.V.


Rull V.,CSIC - Institute of Earth Sciences Jaume Almera
Quaternary Science Reviews | Year: 2016

Easter Island (Rapa Nui) is a remote Pacific island known for its megalithic statues, the moai, built by an ancient culture which disappearance is still debated. Theories claiming for either self-destruction (ecocide) of this ancient culture or an eventual genocide after the European contact have been the most popular. Anthropogenic drivers have been traditionally preferred as causes of this major cultural shift, whereas climatic changes have been dismissed or underrated. However, the latest findings suggest that the topic is more complex than formerly thought and demand a more holistic perspective. This paper reviews the main paleoclimatic, paleoecological, archaeological and historical evidence of the major Rapanui cultural shift leading to the end of the moai-building civilization and uses an integrated approach to analyze its timing and potential causes. The disappearance of the ancient Eastern Island culture that erected the moai was a dramatic cultural shift with significant changes in lifestyle, socio-political organization, religious performance, art and also in the geographical settlement of the cultural core of the Rapanui society. The ancient society, represented by the so called Ancient Cult (or moai cult) was centered on the Rano Raraku crater, to the east of the island, whose soft volcanic rocks (tuff) where suitable for moai carving. This society was replaced by the Birdman-Cult society, based on Rano Kao, to the westernmost end of the island. The assumed date for such shift is uncertain ranging between mid-16th and late-18th centuries. It is suggested that such geographical change, as well as the associated societal transformations, may have been the result of a combination of climatic, ecological and cultural drivers and events. The latest paleoecological reconstructions show that the Rano Raraku catchment was deforested by AD 1450 and the lake inside dried out by AD 1550 owing to an intense climatic drought. This would have caused a landscape deterioration transforming the Raraku catchment into a wasteland devoid of freshwater and unsuitable for human life and the cultural flourishment that characterized the Ancient-Cult society. The drought lasted for about a century and a half and would have forced the Rapanuis to look for alternative freshwater sources. The only feasible option was the freshwater lake inside the then forested Rano Kao crater, where the ceremonial village of Orongo, the center of the Birdman Cult, was funded by AD 1600. The Kao crater is made of hard volcanic rocks (basalts) unsuitable for moai carving by the Neolithic Rapanui culture, unaware of metals, which would have contributed to the end of the moai-building phase. Deforestation and drought would have led to a general demographic decline. The shift from the rigid socio-political organization of the Ancient Cult to the more flexible system characteristic of the Birdman Cult could be viewed as a cultural adaptation to changing environmental conditions. The occurrence of a further, rather catastrophic, genocide caused by slave trading and epidemic diseases, occurred shortly after the European contact (AD 1722), has been documented historically. Therefore, the Rapanui civilization has undergone at least two cultural crises caused by natural and anthropogenic drivers. A complex synergistic scenario like that proposed here can conciliate multidisciplinary lines of evidence formerly used to defend more simplistic and apparently contradictory hypotheses of cultural change. © 2016 Elsevier Ltd


Mouthereau F.,CNRS Institute of Earth Sciences | Lacombe O.,CNRS Institute of Earth Sciences | Verges J.,CSIC - Institute of Earth Sciences Jaume Almera
Tectonophysics | Year: 2012

The Zagros Mountains are the result of the Arabia/Eurasia collision initiated at ~. 35. Ma as the rifted Arabian lithosphere was underthrusted beneath the Iranian plate due to its negative buoyancy. The onset of crustal thickening started at ~. 25. Ma, as recorded by the hinterland exhumation and foreland clastic deposition. Deformation throughout the Arabia/Eurasia collision zone and the uplift of the Iranian plateau occurred after 15-12. Ma, as a result of shortening/thickening of the thin Iranian crust. We emphasize that only 42% of the post-35. Ma convergence is partitioned by shortening within central Iran. Tomographic constraints show ongoing slab steepening or breakoff in the NW Zagros, whereas underthrusting of the Arabian plate is observed beneath central Zagros. The current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in NW Zagros and underthrusting in central Zagros. Critical wedge approach applied to the Zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust. In contrast, the weak sedimentary cover deforms by buckling of a thick multilayered cover. Thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes. There is evidence that the role of the slab pull force in driving the Arabian plate motion was reduced after ~. 12. Ma. Large-scale mantle flow induced by mantle upwelling at the Afar plume appears to be the main driver of the Arabia plate motion. We stress that the main kinematic change in the Zagros region occurred at 15-12. Ma as the Zagros uplifted, before the Arabian slab detached. The Zagros appears key to investigate coupling between continental rheology, plate driving forces and mountain building, in which the role of rift inheritance appears to be central. © 2012 Elsevier B.V.


Marti J.,CSIC - Institute of Earth Sciences Jaume Almera | Felpeto A.,Instituto Geografico Nacional
Journal of Volcanology and Geothermal Research | Year: 2010

A new method to calculate volcanic susceptibility, i.e. the spatial probability of vent opening, is presented. Determination of volcanic susceptibility should constitute the first step in the elaboration of volcanic hazard maps of active volcanic fields. Our method considers different criteria as possible indicators for the location of future vents, based on the assumption that these locations should correspond to the surface expressions of the most likely pathways for magma ascent. Thus, two groups of criteria have been considered depending on the time scale (short or long term) of our approach. The first one accounts for long-term hazard assessment and corresponds to structural criteria that provide direct information on the internal structure of the volcanic field, including its past and present stress field, location of structural lineations (fractures and dikes), and location of past eruptions. The second group of criteria concerns to the computation of susceptibility for short term analyses (from days to a few months) during unrest episodes, and includes those structural and dynamical aspects that can be inferred from volcano monitoring. Thus, a specific layer of information is obtained for each of the criteria used. The specific weight of each criterion on the overall analysis depends on its relative significance to indicate pathways for magma ascent, on the quality of data and on their degree of confidence. The combination of the different data layers allows to create a map of the spatial probability of future eruptions based on objective criteria, thus constituting the first step to obtain the corresponding volcanic hazards map. The method has been used to calculate long-term volcanic susceptibility on Tenerife (Canary Islands), and the results obtained are also presented. © 2010 Elsevier B.V.


Geyer A.,University of Bristol | Marti J.,CSIC - Institute of Earth Sciences Jaume Almera
Tectonophysics | Year: 2010

One of the most characteristic features of volcanic islands is the existence of rift zones defined commonly as orientated eruptive fissures or parallel rows of elongate cinder cones and dyke swarms. Occasionally, these rifts can appear at the birth of the volcanic island and persist until the last episodes of its constructions, controlling the form and structure of the island (e.g. Azores Islands). In the case of Tenerife (Canary Islands), it is possible to observe two rift zones (Santiago del Teide and Dorsal rifts) running NW-SE and ENE-WSW, marked by parallel rows of aligned cones and eruptive fissures. Additionally, at the southern part of the island (Southern Volcanic Zone) basaltic volcanism is characterized by scattered vents and apparently non-coherently orientated eruptive fissures. Some authors relate the existence of the latter volcanism to a N-S running rift zone that defines the third branch of a three-armed rift system in the island. In the present paper, we first investigate the tectonic controls on the distribution of basaltic volcanism at the Southern Volcanic Zone, and their relation with the NW-SE and ENE-WSW rifts. The numerical results obtained suggest that basaltic volcanism of the southern part of Tenerife can be easily explained as the result of a extensional stress field derived from the combined effects of the NW-SE and ENE-WSW rifts. As a second objective, we have also investigated the origin of the Santiago del Teide and Dorsal rift zones and their role on the formation of the original shield volcano and the subsequent evolution of the whole island. Our numerical results contrast with previously published explanations on the origin of the Tenerife rifts that included fracturing due to volcano spreading or to deformation of the volcano due to magma intrusion. We consider that volcanic activity in Tenerife began throughout fissural volcanism along these structures that were already present in the oceanic basement, progressively accumulating the basaltic series that gave rise to the construction of the composite shield volcano. © 2009 Elsevier B.V. All rights reserved.


Verges J.,CSIC - Institute of Earth Sciences Jaume Almera | Fernandez M.,CSIC - Institute of Earth Sciences Jaume Almera
Tectonophysics | Year: 2012

Initial SE-dipping slow subduction of the Ligurian-Tethys lithosphere beneath Africa from Late Cretaceous to middle Oligocene twisting to a later faster E-dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate Betic-Rif orogenic system. This new subduction-related geodynamic scenario is supported by a kinematic model constrained by well-dated plate reconstructions, tectonic, sedimentary and metamorphic data sets. The slow initial SE-dipping subduction of the Ligurian-Tethys realm beneath the Malaguide upper plate unit is sufficient to subduct Alpujarride and Nevado-Filabride rocks to few tens of kilometers of depth in middle Eocene times. The shift from SE- to E-dipping subduction during latest Oligocene-early Miocene was possibly caused by both the inherited geometry of the highly segmented Ligurian-Tethys domain and by the fast roll-back of the subducted lithospheric slab. The early Miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the Betic front, the exhumation of the HP/LT metamorphic complexes, the opening of the Alboran basin, its flooring by HP Alpujarride rocks and subsequent HT imprint, can be explained by the fast NW- and W-directed roll-back of the Ligurian-Tethys subcrustal lithospheric slab. The W retreat of the Ligurian-Tethys lithosphere in middle-late Miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes. From latest Miocene onward the Betic-Rif system evolved under both the northerly push of Africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab. The SW-directed scape of the Rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed Betic-Rif system between Africa and Iberia. © 2012 Elsevier B.V.


Diaz J.,CSIC - Institute of Earth Sciences Jaume Almera | Gallart J.,CSIC - Institute of Earth Sciences Jaume Almera
Earth and Planetary Science Letters | Year: 2014

The regional mantle flow beneath the westernmost Mediterranean basin and its transition to the Atlantic domain is addressed by inspecting the anisotropic properties of the mantle. More than 100 new sites, from the Variscan core of Iberia to the northern rim of the Western African Craton, are now investigated using the data provided by different temporary and permanent broad-band seismic arrays. Our main objective is to provide a larger regional framework to the results recently presented along the Gibraltar Arc in order to check the validity of the different geodynamic interpretations proposed so far. The significant variations in the retrieved anisotropic parameters suggest that different processes must be invoked to explain the origin of the observed anisotropy. Beneath the Variscan units of the Central Iberian Massif the new results show a moderate amount of anisotropy with fast polarization directions (FPD) oriented close to E-W. Those results can only be explained in terms of global mantle flow if models accounting for contributions from surface plate motion, net lithosphere rotation and density variations are taken into consideration. One of the major results presented is the significant number of good quality data without evidence of anisotropy ("nulls") observed beneath permanent stations in southern Portugal. Those "nulls" can be explained by the presence of a predominantly vertical mantle flow associated to large variations in the lithospheric thickness. Beneath the Gibraltar Arc the FPD show a spectacular rotation, evidenced by the results presented by Díaz et al. (2010) and Miller et al. (2013). Those results are reviewed here taking also into consideration the geodynamic modeling presented recently by Alpert et al. (2013) and other geophysical and geodetic results. Further South, the analysis of new broad-band stations installed in the Moroccan Meseta and the High Atlas show a small degree on anisotropy and a large number of "null" events, pointing again to the presence of vertical flow in the mantle. The results favor an asthenospheric origin related to present-day mantle flow for the anisotropy observed from the Variscan core of Iberia to the northern rim of the West African Craton. This flow is deflected around the high velocity slab beneath the Gibraltar Arc and seems affected locally by vertical flow associated to edge-driven convective cells. The presence of significant backazimuthal variations in the anisotropic parameters retrieved from single events suggests that a second order contribution from an anisotropic layer within the lithosphere may also exist. © 2014 Elsevier B.V.


Diaz J.,CSIC - Institute of Earth Sciences Jaume Almera | Gil A.,CSIC - Institute of Earth Sciences Jaume Almera | Gallart J.,CSIC - Institute of Earth Sciences Jaume Almera
Geophysical Journal International | Year: 2013

In the last 10-15 years, the number of high quality seismic stations monitoring the Euro- Mediterranean region has increased significantly, allowing a corresponding improvement in structural constraints. We present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath this complex area, compiled from inversion of Pn and Sn phases sampling the whole region. The method of Hearn has been applied to the traveltime arrivals of the International Seismological Center catalogue for the time period 1990-2010. A total of 579 753 Pn arrivals coming from 12 377 events recorded at 1 408 stations with epicentral distances between 220 km and 1 400 km have been retained after applying standard quality criteria (maximum depth, minimum number of recordings, maximum residual values...). Our results show significant features well correlated with surface geology and evidence the heterogeneous character of the Euro-Mediterranean lithosphere. The station terms reflect the existence of marked variations in crustal thickness, consistent with available Moho depths inferred from active seismic experiments. The highest Pn velocities are observed along a continuous band from the Po Basin to the northern Ionian Sea. Other high velocity zones include the Ligurian Basin, the Valencia Trough, the southern Alboran Sea and central part of the Algerian margin. Most significant low-velocity values are associated to orogenic belts (Betics, Pyrenees, Alps, Apennines and Calabrian Arc, Dinarides-Hellenides), and lowvelocity zones are also identified beneath Sardinia and the Balearic Islands. The introduction of an anisotropic term enhances significantly the lateral continuity of the anomalies, in particular in the most active tectonic areas. Pn anisotropy shows consistent orientations subparallel to major orogenic structures, such as Betics, Apennines, Calabrian Arc and Alps. The Sn tomographic image has lower resolution but confirms independently most of the features evidenced in the Pn tomography ©The Authors 2012.


Rull V.,CSIC - Institute of Earth Sciences Jaume Almera
Holocene | Year: 2016

A number of informal terms (e.g. Anthropocene, Anthropozoic, Psychozoic, Noozoic, and Technogene) have been used to designate the rock unit and time interval where the impact of collective human action on the Earth system is clearly recognizable (called here the Humanized Earth System (HES)). Presently, Anthropocene is the most commonly used, and the International Commission on Stratigraphy is considering its acceptance as a formal stratigraphic unit. Despite their informal character, all of these terms contain suffixes (i.e. -cene, -zoic, or -gene) that define formal chronostratigraphic/geochronologic (C/G) units (e.g. series/epoch, erathem/era, and system/period), which is misleading. In addition, the use of these terms involves unsupported evolutionary assumptions and may lead to conflicting stratigraphic settings. Therefore, it is recommended that these terms are avoided until there is sufficient scientific support to unequivocally define its C/G rank, which is not expected to occur in the near future. © 2016, © The Author(s) 2016.


Geyer A.,CSIC - Institute of Earth Sciences Jaume Almera | Marti J.,CSIC - Institute of Earth Sciences Jaume Almera
Geology | Year: 2012

Benford's law predicts that the distribution of first digits of real-world observations is not uniform, but instead the lower digits (1, 2, and 3) occur more frequently than the higher ones (..., 8, 9). It has been shown that the use of Benford's law may help as a validity check on databases, and that the first-digit rule may provide new ways to detect anomalous signals in data sets. In fact, nonconformity to Benford's law could be an indicator of (1) incompleteness, (2) excessive data round-off, or (3) data errors, inconsistencies, or anomalies. This law has long been known, but has received little attention, in earth sciences. In this work, we first test the conformity of three volcanology-related data sets, and then we consider the relevance and potential utility of using Benford's law to assess the integrity and authenticity of the presented volcanological data. The first two data sets are the area in square kilometers and age in years of collapse calderas extracted from the Collapse Caldera Database (CCDB), covering areas from 0.03 to 4700 km2 and ages from a few years to 2000 Ma. The third data set is the duration in days of the volcanic eruptions that occurred between A.D. 1900 and 2009 according to the Smithsonian's Global Volcanism Program catalogue (http://www.volcano.si.edu). Results obtained indicate that the volcanological data sets of this study follow Benford's law. The present analysis shows that excessive data round-off, data errors, or anomalies may be detected when comparing the data with Benford's law expected frequencies. © 2012 Geological Society of America.

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