Dating the breccia of the base of the Jurassic sequence by means of the analysis of widespread remagnetization in the Iberian Ranges

Se ha utilizado el estudio de la reimanación regional cretácica de la Cordillera Ibérica para acotar la edad de formación de un afloramiento de brechas de la base del Jurásico (Fm. Cortes de Tajuña) del norte de la Cordillera Ibérica. Los experimentos de magnetismo de las rocas y el análisis de la NRM demuestran que los cantos de la brecha han registrado la reimanación pero muestran direcciones aleatorias, lo que implica que la formación de la brecha es post-reimanación y por tanto posterior al Cretácico Inferior. La polaridad inversa registrada por la matriz confirma que la brechificación es posterior al Supercrón Normal del Cretácico, es decir post-Santoniense.From the study of remagnetization of Cretaceous age in an outcrop of the widespread breccia unit of the Lower Jurassic sequence (Cortes de Tajuña Fm.) in the northern Iberian Range, the age of formation of this unit can be inferred. Rock magnetism experiments and NRM analyses indicate that breccia clasts are remagnetized and show paleomagnetic vectors with random orientation, what demonstrates that breccia formation post-dates the Early Cretaceous remagnetization stage. The breccia matrix records a reverse paleomagnetic vector thus confirming that brecciation must post-date the Cretaceous Normal Superchron (i.e. they must be post-Santonian in age).Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Educación y Cienciapu

Estudio paleomagnético del dique de Messejana-Plasencia

A paleomagnetic investigation of 39 sites (591 samples) across the 530 km ot the Messejana-Plasenciadike has been carried out. Rock magnetic experiments indicate PSD low Ti titanomagnetite and magnetite as the minerals carrying the NRM. The samples where mostly demagnetised by thermal demagnetisation. 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 the sites is well defined, with values ot Plat=70.5, Plong=238.0, K= 47.8 and ags=3.5. Paleomagnetic data indicates: (i) the dike had a brief emplacement time, (ii) the age of intrusion can be constrained between 180-200 Ma, (Hi) the high grouping of the VGPs directions suggest no important tectonic perturbations of the whole structure of the dike since its intrusion to the present

Datación paleomagnética de la parte sur del dique de Messejana- Plasencia

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

Late ron Age occupation at Povoado de Crestelos (Meirinhos, Mogadouro, Portugal): combustion structures and reoccupation of ditch 1 for food processing activities

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

Estudio paleomagnético de rocas de edad jurásica de la península ibérica y el sur de Marruecos

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I, leída el 28-06-2004Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu

Paleomagnetic study of the Messejana Plasencia dyke (Portugal and Spain): A lower Jurassic paleopole for the Iberian plate

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

A 2000-yr long absolute paleointensity record for South America: new data from NW Argentina

National audienceA good characterization of geomagnetic field strength changes in the Southern Hemisphere is crucial to disentangle the multi-centennial evolution of the South Atlantic Anomaly, one of the most important geomagnetic features observed on the Earth´s surface nowadays. High-quality records of the absolute paleointensity of the geomagnetic field can only be retrieved from baked archaeological materials and volcanic rocks. However, and despite recent efforts, the number of absolute paleointensities from the Southern Hemisphere remains very low, data are concentrated in the last 1000 years and the majority of available results do not meet modern standard criteria of quality. As a consequence, a considerable grade of uncertainty surrounds geomagnetic field intensity reconstructions from the Southern Hemisphere. Here, we present 41 new high-quality absolute paleointensities obtained from the study of archaeological materials from central South America (Figure 1). The new dataset reveals relatively low and steady intensities during the first millennium AD, contrasting with several periods of fast (multi-decadal) intensity changes during the second millennia AD, including the dramatic decay of field intensity observed since about two centuries and associated to the strengthening of the South Atlantic Anomaly in South America.

Revisiting the chronology of the Early Iron Age in the north-eastern Iberian Peninsula

International audienceThe 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