Search

The archaeological excavation undertaken at Quinta de Crestelos (Merinhos, Mogadouro, Portugal), revealed a large occupation site dated between Late Iron Age and Early Roman periods. On the ridge top a small fortified site with elaborated defensive structures, consisting of large stone walls and ditches, showed several different phases of construction and large modifications. The different phases of the site evidenced the ‘Romanization’ Process of this area and the large changes occurring in the Late Iron Age sites, with defensive structures, such as the original ditches, reused as functional and production areas. Paleomagnetism studies and analysis of the combustion structures of the later phases of the site intended to clarify nature, main components and changes during the time when they were used. Los trabajos de excavación que se han llevado a cabo en el yacimiento de Quinta de Crestelos (Merinhos, Mogadouro, Portugal), han puesto de manifiesto la presencia de una importante ocupación durante la II Edad del Hierro y la romanización de esta zona. En la zona alta de este enclave se ha registrado un poblado fortificado, con complejos sistemas defensivos, entre los que se encuentran unas potentes murallas y fosos, que van a sufrir diferentes reestructuraciones a lo largo del tiempo. La evolución que se ha podido identificar en esta zona muestra el proceso de romanización de esta región, y la adaptación que se produce en los poblados de la Edad del Hierro, a través de la reformulación de espacios defensivos, como los fosos, en áreas funcionales y productivas. Se ha llevado a cabo un estudio de paleomagnetismo, y análisis de todas estas estructuras de combustión, con la finalidad de poder determinar la función de las mismas, sus principales características, y su uso a lo largo del tiempo.

The chronology of the Late Bronze Age and the earliest stages of the Iron Age in the Mediterranean is an important topic of debate since the study of both local social dynamics and trade and colonial activity around the Mediterranean requires obviously well-established chronological frameworks. However, the exact chronology of the Early Iron Age in the Mediterranean region is still a problematic issue today since different, and in some cases unbalanced, sources of information (historical texts, material culture sequences, and radiocarbon dating results) are used. The NE Iberian Peninsula is not an exception and different time periods are proposed in the literature for the Early Iron Age. Here, and in order to provide a new and an independent input to feed this debate, we apply the archeomagnetic dating method to four archeological hearths from the Sant Jaume Complex, a set of several Early Iron Age archeological sites located in the north-eastern part of the Iberian Peninsula. The archeomagnetic dating results suggest that the abandonment of the studied structures (that can be linked to the abandonment of the archeological sites) most probably occurred before 650 BC, and therefore invalidate the age commonly ascribed to the Early Iron Age sites in this area. Our study provides, hence, new evidences that the traditional view of human settlement development in the NE Iberian Peninsula should be revisited., This project was supported by the CGL2015-63888-R (MINECO/FEDER) project and by the Ramón y Cajal program (MGP) of the Spanish Ministry of Economy and Competitiveness. Financial support was also given by the PICS International Program for Scientific Cooperation (CNRS-France and CSIC-Spain).

vEGU21: Gather Online | 19-30 April 2021, Recent archeomagnetic studies performed in different regions of the world have revealed unusual periods of sharp changes in intensity during the first millennium. Here we focus on the study of intensity variations between 600 BCE and 600 CE in central Asia, where an important intensity decrease seems to be present during the second half of the 1st millennium BCE. For this purpose, we present a new paleosecular variation (PSV) curve obtained from 51 new archeointensities and the selected previous data located within a radius of 1000 km around Termez (Uzbekistan). The new curve shows an intensity maximum around 400 BCE followed by a rapid decrease. When the virtual axial dipole moment (VADM) values are compared with the Dipole Moment estimations derived from different global geomagnetic models key differences are observed, suggesting an important non-dipolar effect for this feature. Finally, in order to constrain the spatial behaviour of this phenomenon and its global implications, we investigate the PSV intensity and VADM trends from twelve regions distributed among Central America, Europe and Asia. A VADM maximum is observed in Western Europe (Iberia and Germany) around 450 BCE, associated to rates of change of about 9 µT/century. This feature is also observed eastwards, in the Caucasus and the Levant, but associated to lower rates of changes. In Central Asia (Uzbekistan) our new study suggests that maximum values of about 14 µT/century, between 400-300 BCE, were achieved. In other regions, as Eastern Asia and Central America, rapid variations of the intensity are not observed during the targeted period.

X Congreso Geológico de España, 5-7 Julio 2021, Vitoria - Gasteiz, Reciente estudios arqueomagnéticos han detectado una caída en la intensidad del campo en torno al 3000 BP (Shaar et al., 2016). Este fenómeno se ha observado en distintas partes del mundo, como en Israel, Europa, y Corea. El principal objetivo de este estudio es ampliar nuestro conocimiento acerca del comportamiento espacial y temporal de dicho evento en la región de Asia central durante el I milenio AEC. Para esto se ha utilizado el protocolo de paleointensidad de Thellier en el tratamiento de 141 fragmentos cerámicos procedentes del sur de Uzbekistán. Los experimentos de magnetismo de rocas indican que los principales portadores magnéticos de las muestras son la magnetita y la titanomagnetita. Los resultados obtenidos confirman una caída drástica de la intensidad entre el 400 y el 100 AEC en la zona de Asia central. Por último, se han comparado los valores del Momento Dipolar Virtual Axial (VADM) con el valor del momento dipolar obtenido a partir del modelo global SHA.DIF.14k (Pavon-Carrasco et al., 2014).

Absolute past geomagnetic intensity values can mainly be recovered by fired archaeological materials and volcanic rocks. Here, we present 10 new archeointensities from the Mediterranean region that help to better constrain geomagnetic field intensity changes in Europe over the last two millennia. The new archeointensity results were obtained from the Thellier classical method including thermoremanent magnetization (pTRM) checks and both the TRM anisotropy and cooling rate corrections and were derived from at least three specimens. The new data presented, together with a selection of previous archeointensities satisfying a set of quality criteria, confirm the presence of several intensity maxima in Europe over the last 2000 years. In particular, the new archeointensities allow to better define the starting point of the double-oscillation feature that occurred in Europe during the second half of the first millennium CE, and reinforce the existence of a relative maximum at the end of the 14th century - beginning of the 15th century in Western Europe. From selected European archeointensities two new paleosecular variation curves are constructed for Western and Eastern Europe using temporal cubic b-splines in a bootstrap approach. The obtained curves suggest that the occurrence of the intensity maxima is characterized by a period of about 300 ± 50 years. In addition, our results suggest that the maxima do not occur simultaneously in Western and Eastern Europe, pointing out an intensity eastward drift with a mean lag-time of about 100 years.

Recent studies indicate the existence of rapid fluctuations of the geomagnetic field intensity over the last 3000 years. These features have been observed during the first millennium BCE in the Mediterranean area (from Western and Central Europe to the Near East), and in other regions as the Canary and the Azores Islands, China and Korea. Nevertheless, due to the difficulty to obtain precise secular variation intensity curves, the spatial and temporal extension of these events, and the potential link between the different regional events observed are still under discussion. In this study, we focus on the rapid intensity decrease that seems to take place in central Asia during the second half of the 1st millennium BCE. We present 51 new archeointensity from South Uzbekistan, obtained by Thellier-Thellier method, also, rock magnetism experiments have been conducted in order to identify the principal magnetic minerals responsible for the magnetization. The results of those experiments show that the maghemite and/or magnetite, in some cases with low titanium content, are the main magnetic carriers. New data together with 73 previous archaeointensity data, allow us to better defined geomagnetic field intensity changes in South Uzbekistan between 600 BCE and 600 CE. The results indicate a rapid drop out in intensity between 500 BCE and 200 BCE with a minimum of 37.2 µT around 200 BCE ¿ 50 CE. A comparison with different VADMs values available for different regions indicates that similar intensity trends are observed during this period at the continental scale. Finally, the results are compared with Axial Dipole Moment estimations derived from different global models, suggesting that this sharp continental decrease is controlled by non-dipolar sources.

[EN] The occurrence of ε-FeO in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-FeO is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-FeO has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-FeO is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated., The authors also acknowledge the financial support from the Spanish Ministry of Science, Innovation and Universities under the projects RTI2018-095856-B-C21, CGL2017-87015-P, CGL2017-92285-EXP, CGL2017-92285-EXP/BTE, MAT2017-86540-C4-1-R, MAT2017-87072-C4-2-P and RTI2018-095303-A-C52, from Comunidad de Madrid NANOFRONTMAG S2013/MIT-2850 and NANOMAGCOST S2018/NMT-4321, and from the European Commission under H2020 frame by AMPHIBIAN Project ID: 720853. APO thanks the Ministry of Economy, Industry and Competitiveness (PTA Contract).

This study presents new archaeomagnetic results from 33 combustion structures (kilns and hearths) from the archaeological sites of Castelinho, Crestelos, Olival Poço da Barca and Fonte do Milho in NE Portugal. The age of the investigated structures ranges from 1210 BC to 200 AD according to calibrated radiocarbon dating, thermoluminescence dating and archaeological constraints. Stepwise thermal and alternating field demagnetization isolate a single, stable, characteristic remanence component with very well defined directions. Rock magnetic analyses suggest low-Ti titanomagnetite/maghemite as the main magnetic carrier of the remanence. Mean directions are well grouped in most structures. The effect of thermoremanent anisotropy on mean directions has been evaluated and was found to be important. Inclination increases of between 2° and 13° after applying the anisotropy correction at specimen level. This highlights the requirement of evaluating this effect on the directions of small and flattened thin kilns and hearths. The 31 new directional data improve both the temporal and spatial distribution of the Iberian archaeomagnetic dataset from Late Bronze Age to Roman Times. Finally, a new directional palaeosecular variation curve for Iberia for the last twelve centuries BC is proposed. The curve has been computed using the bootstrap method and includes data coming from sites within 900 km of Madrid. The new palaeodirectional secular variation curve for Iberia is consistent with the Western European palaeosecular variation curve and with the prediction of regional European models., The authors are Grateful to the Spanish Research Project CGL2014-54112-R which has supported the field and laboratory costs, as well as the post-doctoral contract of APO.

Reunión MAGIBER XI Portugal ,Condeixa a Nova (4 al 6 de septiembre de 2019), Los estudios arqueomagnéticos se han convertido en una herramienta imprescindible para caracterizar las variaciones del campo geomagnético de los últimos milenios. Los datos que proporcionan este tipo de estudios permiten obtener curvas locales de variación paleosecular para las tres componentes del campo geomagnético: declinación, inclinación y, más esporádicamente, la paleointensidad. Un ejemplo de dichas curvas son las recientemente publicadas por Molina-Cardín et al., (2018) para Iberia de los últimos 3000 años. Sin embargo, se requiere un continuo aporte de nuevos datos arqueomagnéticos para mejorar la resolución espacial y temporal de las bases de datos arqueomagnéticas globales para refinar este tipo de curvas. Los datos arqueomagnéticos permiten, además, obtener modelos geomagnéticos regionales o globales cada vez más robustos (Pavón-Carrasco et al. 2009, 2014; Campuzano et al., 2019). En este trabajo presentamos nuevos datos arqueomagnéticos que se han obtenido recientemente gracias al estudio de varias estructuras arqueológicas calentadas a alta temperatura, muestreadas en varios yacimientos del levante español correspondientes al periodo Ibérico pleno. En concreto se han obtenido nuevos datos direccionales (declinación e inclinación) y de paleointensidad en los yacimientos de Pontós y Ullastret (Girona), además de los de Nulles y Coll del Moro (Tarragona) y Vinaroz (Castellón) que permitirán mejorar la resolución de la curva para la segunda mitad del primer milenio antes de Cristo. A pesar de que este periodo está bastante bien caracterizado gracias a estudios anteriores, los nuevos datos, especialmente los de paleointensidad, contribuyen a una mejor definición de las variaciones locales respecto a regiones cercanas donde las curvas de variación secular están mejor definidas (Hervé et al., 2013a y b).

The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.

Variations of geomagnetic field in the Iberian Peninsula prior to Late Iron Age times are poorly constrained. Here we report 14 directional and 10 palaeointensity results from an archaeomagnetic study carried out on 17 combustion structures recovered from six archaeological sites in eastern Spain. The studied materials have been dated by archaeological evidences and supported by radiocarbon dates (8th-5th centuries BC). Rock magnetic experiments indicate that the characteristic remanent magnetization is carried by a low coercivity magnetic phase with Curie temperatures of 500-575 °C, most likely titanomagnetite/maghemite with low titanium content. Archaeointensity determinations were carried out by using the classical Thellier-Thellier experiment including pTRM-checks and magnetic anisotropy corrections. A new full vector Iberian Paleosecular Variation Curve for the Iron Age is presented. High fluctuation rates on both directions and intensities are observed during the Early Iron times that seems to be related with the Levantine Iron Age Anomaly (LIAA), the most prominent anomaly of the geomagnetic field of the last three millennia. Two intensity maxima were observed at Iberian coordinates, the oldest around 750 BC (associated with easterly declinations of around 23°) and the second 275 yrs later (475 BC) with northerly directions. The related virtual axial dipole moment was up to 14 ⋅ 10 22 Am2 for the oldest materials (750 BC) and reaching 16 ⋅ 10 22 Am2 for the materials corresponding to the end of the Early Iron Age. In order to investigate the origin of the unusually high fluctuations of the palaeofield we have developed a new global geomagnetic field reconstruction, the SHAWQ-IronAge model, which is based on a critical revision of the global archeomagnetic and volcanic dataset. The new model provides an improved description of the evolution of the LIAA, which is related to a normal flux patch at the core-mantle boundary (CMB) below Arabian Peninsula clearly observed at around 950 BC. This flux patch expanded towards the north-west, while decreasing in intensity, reaching Iberia at around 750 BC. Around 600-500 BC, it underwent a revival below the European continent after that it seems to vanish in situ.

In this work, we present 16 directional and 27 intensity high‐quality values from Iberia. Moreover, we have updated the Iberian archeomagnetic catalogue published more than 10 years ago with a considerable increase in the database. This has led to a notable improvement of both temporal and spatial data distribution. A full vector paleosecular variation curve from 1000 BC to 1900 AD has been developed using high‐quality data within a radius of 900 km from Madrid. A hierarchical bootstrap method has been followed for the computation of the curves. The most remarkable feature of the new curves is a notable intensity maximum of about 80 μT around 600 BC, which has not been previously reported for the Iberian Peninsula. We have also analyzed the evolution of the paleofield in Europe for the last three thousand years and conclude that the high maximum intensity values observed around 600 BC in the Iberian Peninsula could respond to the same feature as the Levantine Iron Age Anomaly, after travelling westward through Europe., Authors acknowledge the Spanish Ministry of Economy (CGL2014-54112-R and CGL2017-87015-P projects; BES-2015- 074575 and BES-2016-077257 grants), the Spanish Ministry of Education (FPU14/02422 grant), and the Autonomous Community of Madrid through the European Youth Employment Initiative (PEJ15/AMB/AI- 0203). M. G. P. and M. R. acknowledge the Ramón y Cajal program and the CGL2015-63888-R project of the Spanish Ministry of Economy and Competitiveness. Financial support was also given by the PICS International Program for Scientific Cooperation (CNRS-France and CSIC-Spain).

Trabajo presentado en el Symposium "El Castillón: un centro de poder a orillas del esla. De la prehistoria reciente a la edad media", celebrado en Salamanca (España), los días 8 y 9 de Junio de 2017

Trabajo presentado en el 41st International Symposium on Archaeometry (ISA), celebrado en Kalamata (Grecia) del 15 al 21 de mayo de 2016, Archaeomagnetic investigations have been carried out on 25 combustion structures from Portugal and Spain, along the Duero River Valley at the archaeological sites of Castelinho, Crestelos and Olival poço da Barca (NE Portugal). The age of most of the investigated structures ranges from the 3rd century BC up to Roman times according to archaeological information. Stepwise thermal and alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions. Mean site directions were well grouped in most sites. Only the two most poorly preserved structures showed a relatively high directional dispersion. Classical Thellier palaeointensity experiments were conducted on pilot specimens. Successful results were obtained in specimens from Olival poço da Barca. In contrast, alterations occurred during thermal treatment at Crestelos and Castelinho kilns and hearths. Archaeomagnetic dating was conducted in all sites by using six reference Palaeosecular Variation (PSV) curves provided by local studies (from Iberia and France) and by regional (SCHA.DIF.3k) and global models (ARCH3k.1 and SHA.DIF.14k). The master curve that provides the most accurate dating is the French curve. The new archaeomagnetic dating results suggest that the Crestelos site was active since 265±87 BC up to 252±138 AD, with a mean occupation period between 198 BC-43 AD, in agreement with recent radiocarbon ages and archaeological information. There could have been a previous occupation of the site but it could not be precisely dated. Distributions of the directions from Crestelos and Castelhino describe a consistent PSV trend, suggesting that the archaeomagnetic dating tool could be improved in Iberia in the near future. A PSV curve for Iberia for the Iron Age is presented

Trabajo presentado en el 41st InternationalSymposium on Archaeometry (ISA), celebrado en Kalamata (Grecia), del 15 al 21 de mayo de 2016, Archaeomagnetic studies can either be used to construct and improve palaeosecular variation curves (PSVC) with new data from well dated archaeological sites or be used to date archaeological structures comparing the measurements of remanent magnetization with the evolution of the geomagnetic field by means of existing PSVCs for that region. In both cases some investigations about the samples behaviour should be carried out in order to correct the measurements and get reliable values, curves and dates. A major correction is that of anisotropy of thermoremanent magnetization which has been observed to produce an important underestimation in the inclination (effect called inclination shallowing). Combustion structures from 3 different archaeological sites (Crestelos and Olival poço da Barca in NE Portugal and El Castillón in NW Spain) have been analysed. The anisotropy of thermoremanent magnetization has produced variations of about ±5º in declination and up to +13º in inclination of site means. Palaeointensity values have been obtained from El Castillón, showing anisotropy corrections between -1% and -15%. The effect of these errors in dating has been analysed finding that it can lead to age differences between -1 and +5 centuries. That highlights the importance of systematically including the anisotropy correction in any archaeomagnetic study of this kind of materials even when only directions are investigated. Due to the time cost of calculating the anisotropy of thermoremanent magnetization we have also measured the anisotropy of magnetic susceptibility, anhisteretic remanent magnetization and isothermal remanent magnetization. The suitability of these alternatives to correct the anisotropy effect is discussed.

Trabajo presentado en el 9ª Asamblea Hispano Portuguesa de Geodesia y Geofísica, celebrada en Madrid (España), del 28 al 30 de junio de 2016, [ES] La datación arqueomagnética está basada en dos principios básicos: la variación secular del campo magnético terrestre y el magnetismo termorremanente que adquieren las estructuras de combustión durante su último enfriamiento. El campo magnético terrestre se describe a partir de sus componentes: declinación, inclinación e intensidad. Estos parámetros sufren una variación de largo periodo, de origen interno, denominada Variación Secular. Dicha variación puede ser determinada en el pasado a partir del estudio de estructuras bien datadas. Una vez obtenida la “Curva de Variación Paleosecular” (PSVC) para una región, ésta puede utilizarse como curva de calibración para datar otras estructuras. En el caso de la Península Ibérica, disponemos de aproximadamente 200 datos arqueomagnéticos direccionales que, junto con los datos del sur de Francia y del norte de Marruecos permitirán en un futuro próximo establecer una PSVC de Iberia para los últimos 3000 años. En el momento actual, la curva direccional de referencia que mejor definición tiene para la península es la curva sintética generada por el modelo regional europeo SCHA.DIF.3k. Con ella es posible realizar dataciones arqueomagnéticas direccionales, utilizando la herramienta archaeo_dating. Las estructuras que se pueden datar mediante esta técnica deben haber sufrido calentamientos moderados (300ºC o superiores) y los materiales deben muestrearse in situ. Se presenta en este trabajo detalles del trabajo de campo y de laboratorio así como los resultados de los estudios realizados hasta el momento y las perspectivas futuras de la técnica., [EN] Archaeomagnetic dating is based on two basic principles: Earth’s magnetic field’s secular variation and thermoremanent magnetization acquired by combustion related structures over their last cooling. Earth’s magnetic field can be described through its components: declination, inclination and intensity. These parameters show a variation of internal origin over long time periods, known as Secular Variation. This variation can be determined in the past by studying well-dated structures. Once the Paleosecular Variation Curve (PSVC) for a given region is obtained, it can be used as a calibration curve for dating other structures. For the Iberian Peninsula, approximately 200 archaeomagnetic directional data points are available. Together with data from southern France and northern Morocco, they will allow for an Iberian PSVC for the last 3000 years. Currently, the best-defined directional reference curve for the Peninsula is the one defined by the European regional SCHA.DIF.3k model, which allows us to perform archaeomagnetic dating through the archaeo_dating tool. Structures to be dated must have been undergone moderate heating (300ºC or above) and materials must have been sampled in situ. In this work, details of field and laboratory work are presented, as well as results from studies performed to date and insights on the future of this technique.

Available apparent polar wander (APW) paths for the 200 Ma configuration of Pangea, just prior to the opening of the Central Atlantic Ocean, differ as much as 10o in arc length. Here, we add new data from northwest Africa for this time, obtained from the northeasttrending Foum-Zguid and Ighrem dykes (ca. 200 Ma). These dykes form part of the northern domain of the Central Atlantic Magmatic Province (CAMP), and crosscut the Anti-Atlas Ranges in Morocco, and compositionally correspond to quartz-normative tholeiites intruded in continental lithosphere shortly before the opening of the Central Atlantic Ocean. The Foum-Zguid dyke has been intensively studied, whereas the Ighrem dyke has received less scientific focus. We sampled both dykes for paleomagnetic investigation along 100 km of each dyke (12 sites for Foum-Zguid and 11 for Ighrem, 188 samples included in the final analyses). Rock magnetic experiments indicate a mixture of multidomain and single-domain magnetite and/or low-Ti titanomagnetite particles as the principal remanence carriers. In both dykes, the primary nature of the characteristic remanent magnetization is supported by positive contact tests, related to Fe-metasomatism or baked overprints of the corresponding sedimentary country rocks. The directions of the characteristic magnetization exhibit exclusively normal polarity. Site-mean virtual geomagnetic poles are differently grouped in each dyke, suggesting distinct geomagnetic secular variation records. The Foum-Zguid paleomagnetic pole (N = 12, PLat = 67.9◦N, PLon = 247.9◦E, κ = 125, A95 = 3.9◦) plots close to that of Ighrem (N = 11, PLat = 78.4◦N, PLon = 238.2◦E, κ = 47, A95 = 6.7◦), confirming those mineralogical and geochemical evidences supporting that they represent dissimilar magmatic stages. Virtual geomagnetic poles dispersion from both dykes (S = 10.5◦13.0◦ 8.1◦ ) is in line with those obtained from recent studies of a CAMP-related dyke in Iberia and results from CAMP lavas in the Argana basin. These three new estimates of paleosecular variation at low latitudes around the Triassic–Jurassic boundary are concordant with a recently proposed dispersion curve for the Jurassic but suggest a slightly lower geomagnetic scatter than considered so far. After combining results from both dykes, the resulting paleomagnetic pole (PLat = 73.0◦N, PLon = 244.7◦E, N = 23, κ = 55, A95 = 4.1◦) is statistically compared with existing and coeval African paleopoles, and with global synthetic 200 Ma running mean poles in northwest Africa coordinates.

A revision of Jurassic palaeomagnetic data from Iberia has been carried out in order to investigate the consistency between the palaeomagnetic information and the tectonic models proposed for the western Mediterranean. Due to the presence of a widespread (but partial) remagnetization which affected most Jurassic sediments in Iberia, selection criteria have been designed to avoid completely remagnetized sites. A total of 72 sites have been considered for the palaeomagnetic discussion (35 sites from the Messejana–Plasencia dolerite dyke, 14 sedimentary sites from the Iberian Range and 23 sites from the Betic Cordillera). Three palaeopoles for Iberia have been selected (from the Iberian Massif and from the Iberian Range) for the period around 200 Ma, the Toarcian–Aalenian and the Oxfordian. Data from the Subbetic Zone (Betic Cordillera) are used to constrain the palaeolatitude of the Iberian microplate. Iberian data are in general agreement with the BC02 master curve and the reconstruction parameters used to transfer the Iberian data to Europe, but lower palaeolatitudes than predicted by BC02 master curve are observed in Iberia for the Late Jurassic. Iberia reached a maximum in palaeolatitude during the Toarcian–Aalenian (the reference point of Madrid was at about 37°), and since then, the palaeolatitude decreased (Madrid was at 22° by the Kimmeridgian). Tectonic reference models for the Western Mediterranean (Stampfli and Borel, 2002, 2004) do not fit the Iberian declinations and palaeolatitudes. New palaeogeographic reconstructions are proposed for the Hettangian–Sinemurian, the Toarcian–Aalenian and the Oxfordian.

An early Turonian ( c. 93 Ma) anoxic, cyclic marine deposition is registered in the unfolded outcrops from the Tarfaya coastal basin, where very high sedimentation rates enable the investigation of past geomagnetic field record at high temporal resolution. One hundred and fourteen samples have been sampled along a 10.5 m vertical profile ( c. 200–500 ka) of orbital-scale forced sedimentation. Rock magnetic investigations reveal mineralogy principally controlled by diamagnetic and paramagnetic behaviour, along with very low concentrations of low-coercivity ferromagnetic material which is probably magnetite. A well-defined magnetic fabric can be seen with the minimum susceptibility axis perpendicular to the foliation plane, and magnetic lineation compatible with NW African palaeostress since sedimentation times and/or the palaeocurrent associated with upwelling system deposition. Magnetic signature has the potential for performing reliability checks of reversed tiny wiggles, which were found in four samples not considered for the tectonic analysis. Alternating field demagnetization shows a single, stable, low-coercivity directional component. The new palaeopole ( N =88; PLat=64.3°, PLon=256.3°, A 95 =2.5°; K =38.7), obtained after moderate ( f =0.8) inclination flattening correction, is the first early Turonian palaeopole for the NW African Craton. It can contribute to the 90 Ma-centred sliding window of the different proposed synthetic Apparent Polar Wander Paths.

The Almonacid de la Cuba section, located in the Iberian Range, in central-eastern Spain, contains an outstanding record of the Pliensbachian-Toarcian boundary (Early Jurassic). Four ammonite assemblages, characterized respectively by the presence of Pleuroceras, Canavaria, Dactylioceras (Eodactylites) and Dactylioceras (Orthodactylites) have been distinguished. The base of the Toarcian is located at level CU35.2, based on the first occurrence of Dactylioceras. The presence of Boreal and Mediterranean taxa allows correlation between both bioprovinces. Magnetostratigraphy shows the most complete record of reversals of the Earth magnetic field for the base of Toarcian. The onset of the positive δ13C excursion which has been found in the Toarcian of several sections in Europe has been recorded. Average paleotemperatures for the latest Pliensbachian Spinatum Zone was about 12.5°. Seawater temperature rise during the lowermost Toarcian, reaching average temperatures of 16.7°C at the Tenuicostatum Zone. The 87 Sr/ 86 Sr curve fits with the LOWESS calibration curve.

The only Iberian lower Jurassic paleomagnetic pole come from the “Central Atlantic Magmatic Province”-related Messejana Plasencia dyke, but the age and origin of its remanence have been a matter of discussion. With the aim of solving this uncertainty, and to go further into a better understanding of its emplacement and other possible tectonic features, a systematic paleomagnetic investigation of 40 sites (625 specimens) distributed all along the 530 km of the Messejana Plasencia dyke has been carried out. Rock magnetic experiments indicate PSD low Ti-titanomagnetite and magnetite as the minerals carrying the NRM. The samples were mostly thermally demagnetized. Most sites exhibit a characteristic remanent component of normal polarity with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from a total of 35 valid sites is representative of the whole structure of the dyke, and statistically well defined, with values of PLa=70.4°N, PLo=237.6°E, K=47.9 and A95=3.5°. Paleomagnetic data indicates that: (i) there is no evidence of a Cretaceous remagnetization in the dyke, as it was suggested; (ii) most of the dyke had a brief emplacement time; furthermore, two dyke intrusion events separated in time from it by at least 10,000 y have been detected; (iii) the high grouping of the VGPs directions suggests no important tectonic perturbations of the whole structure of the dyke since its intrusion time; (iv) the pole derived from this study is a good quality lower Jurassic paleopole for the Iberian plate; and (v) the Messejana Plasencia dyke paleopole for the Iberian plate is also in agreement with quality-selected European and North American lower Jurassic paleopoles and the magnetic anomalies data sets that are available for rotate them to Iberia.

Magma flow in dykes is still not well understood; some reported magnetic fabrics are contradictory and the potential effects of exsolution and metasomatism processes on the magnetic properties are issues open to debate. Therefore, a long dyke made of segments with different thickness, which record distinct degrees of metasomatism, the Messejana–Plasencia dyke (MPD), was studied. Oriented dolerite samples were collected along several cross-sections and characterized by means of microscopy and magnetic analyses. The results obtained show that the effects of metasomatism on rock mineralogy are important, and that the metasomatic processes can greatly influence anisotropy degree and mean susceptibility only when rocks are strongly affected by metasomatism. Petrography, scanning electron microscopy (SEM) and bulk magnetic analyses show a high-temperature oxidation-exsolution event, experienced by the very early Ti-spinels, during the early stages of magma cooling, which was mostly observed in central domains of the thick dyke segments. Exsolution reduced the grain size of the magnetic carrier (multidomain to single domain transformation), thus producing composite fabrics involving inverse fabrics. These are likely responsible for a significant number of the ‘abnormal’ fabrics, which make the interpretation of magma flow much more complex. By choosing to use only the ‘normal’ fabric for magma flow determination, we have reduced by 50 per cent the number of relevant sites. In these sites, the imbrication angle of the magnetic foliation relative to dyke wall strongly suggests flow with end-members indicating vertical-dominated flow (seven sites) and horizontal-dominated flow (three sites).

A Paleomagnetic investigation of 11 sites across the southern part of Messejana-Plasencia dike has been carried out. The magnetic behaviour o f pilot samples indicates that thermal demagnetisation is more effective than alternating field cleaning (AF) in isolating the Characteristic Remanent Magnetisation (ChRM). Therefore thermal demagnetisation was systematically used with the remaining samples. The ChRM is well grouped. Most sites exhibit normal polarities with the exception of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from the sites is well defined, with values of Plat=72.5, Plong=243.7 and Al);=6.0. This coincides with the pole calculated in a previous investigation (Schott et at. 1981) o f the dike. It is also in agreement with Toarcian poles from the Iberian Range (Osete et al. 2000) and statistically different from the Oxfordian Pole obtained in the same region. Previous radiometric studies suggested that the age o f dike intrusion is 160-200Ma. Paleomagnetic data indicates that the age o f intrusion can be constrained between 180-200Ma.