María Luisa Osete

Preliminary palaeomagnetic results from the Subbetic Zone (Betic Cordillera, southern Spain): kinematic and structural implications

Abstract We present palaeomagnetic evidence from Jurassic limestones and basic submarine volcanic rocks (sills, pillow lavas, and intrusive material) from the central part of the Subbetic Zone of the Betic Cordillera. Isothermal remanent magnetization acquisition experiments performed on limestones and thermomagnetic curves and measurements of the magnetic susceptibility determined for the volcanic rocks demonstrate strong dependence of rock magnetic properties on lithology. In most cases a stable magnetization could be isolated after thermal and/or alternating field demagnetization. Locally, declinations of the characteristic remanent magnetization from interbedded limestones and volcanics are similar, but inclinations tend to vary. Regionally, declinations for both limestones and volcanics show significant clockwise rotation. These results cannot be explained by the anticlockwise rotation of Iberia during the Cretaceous opening of the Bay of Biscay. Considering the long and complex history of large-scale shear between Africa and Europe accommodated in southern Spain, the anomalous declinations can be explained by a model with rotation of crustal blocks within a zone of wrench tectonics.

Regional modeling of the geomagnetic field in Europe from 6000 to 1000 B.C.

We have developed a first low-degree regional geomagnetic model for the European continent valid for the period 6000–1000 B.C. from a selected compilation of sedimentary and archeomagnetic data (the SCHA.DIF.8K model). This model provides information about both direction (declination and inclination) and intensity of the Earths magnetic field. By connecting it with our previous model, SCHA.DIF.3K, valid from 1000 B.C. to 1900 A.D., and the IGRF, we furnish continuous geomagnetic field information for the last 8000 years in Europe. It has been developed using the Revised Spherical Cap Harmonic Analysis in 2 Dimensions technique (R-SCHA2D) and using the norm of the Earths magnetic field to constrain the inversion problem. The size of the cap is 22°, and the maximum degree of the expansion is 2. The linearization problem was solved by using the truncated Taylors series applied to the expressions representing the relationship between the declination, inclination, and intensity data and the Cartesian components of the geomagnetic field. We used the geocentric axial dipole (GAD) field as our initial or reference field. For time, we used the classical sliding overlapping window method. The size of the window was set to 100 years shifted by 50 years. We compared the models prediction with the input data, with the global CALS7K.2 model, and with new independent data. The regional model shows a better fit to the input and to the independent data than the global model, especially in terms of intensity, and agrees with the virtual axial dipole moment given by other studies. For the last 8000 years, the European geomagnetic field has recorded rapid changes or archeomagnetic jerks. The average field for the last 8000 years in Europe is indistinguishable from the GAD field.

Southward migration of continental volcanic activity in the Sierra de Las Cruces, Mexico: palaeomagnetic and radiometric evidence

New Palaeomagnetic data for 30 sites (271 samples) and K–Ar data from five units in the Sierra de Las Cruces, western Basin of Mexico, provide constraints on the spatial-temporal evolution of arc magmatism in the central Trans-Mexican Volcanic Belt. The normal and reversed directions show a polarity pattern with a consistent spatial zonation perpendicular to the NNW–SSE trend of the range. The magnetostratigraphy and K–Ar dates indicate that volcanic activity in the Sierra de Las Cruces migrated southeastward at a mean rate of 1.6 cm/a, between 3.6 and 1.8 Ma, and that the rate of migration may have been higher, up to 4 cm/a, during the Gauss Chron. Normal and reversed directions pass the reversal test at a 95% confidence level. The mean Plio-Quaternary palaeomagnetic direction for Sierra de Las Cruces is D=350.7°, I=30.6° (N=25, k=30.7, α95=5.3°). The declination deviates to the west of the expected direction, which suggests that small counterclockwise rotations could take place during formation of the Sierra de Las Cruces volcanics.

Oxfordian magnetostratigraphy of the Aguilón and Tosos sections (Iberian Range, Spain) and evidence of a pre-Oligocene overprint

Abstract A composite magnetic polarity sequence has been constructed for the middle and late Oxfordian (late Jurassic) from four overlapping sections situated in both limbs of an anticline. Two stable magnetisation components could be isolated in every sample analyzed. Both components pass the fold test: a low-temperature secondary component, with Dec . = 340.9° and Inc . = 44.9° ( α 95 = 1.7°), of pre-Oligocene age, showing always normal polarity, and a high-temperature primary component, with Dec. = 324.1° and Inc. = 40.6° (α 95 = 2.9°). The latter shows both normal and reversed polarities and provides the geomagnetic record for the late Jurassic. The magnetostratigraphy of the four overlapping sections has given consistent results and indicates that a high frequency of reversals characterises the pattern of the geomagnetic field during the middle to upper Oxfordian. The corresponding Oxfordian paleopole is P lat = 251.2°, P long = 55.9° ( α 95 = 3.1).

Palaeomagnetic Evidence for Block Rotations and Distributed Deformation of the Iberian-African Plate Boundary

Palaeomagnetic evidence from Jurassic limestones and basic submarine volcanic rocks from the Subbetic Zone of the Betic Cordillera indicate that block rotations have played a significant role in the tectonic evolution of this area. Two models can account for the observed clockwise rotations: 1) a shear zone generated during the Miocene thrusting and accretion of the Alboran microplate onto the southern margin of Iberia, or 2) an intracontinental domain of distributed deformation produced during the welding of Africa and Iberia since the Miocene thrusting.

Intensity of the geomagnetic field in Europe for the last 3 ka: Influence of data quality on geomagnetic field modeling

One of the main challenges of paleomagnetic research is to obtain high-resolution geomagnetic field intensity reconstructions. For the last millennia, these reconstructions are mostly based on archeomagnetic data. However, the quality of the intensity data available in the databases is very variable, and the high scatter observed in the records clearly suggests that some of them might not be reliable. In this work we investigate how the geomagnetic field intensity reconstructions and, hence, our present knowledge of the geomagnetic field in the past, are affected by the quality of the data selected for modeling the Earths magnetic field. For this purpose we rank the European archeointensity data in four quality categories following widely accepted paleomagnetic criteria based on the methodology used during the laboratory treatment of the samples and on the number of specimens retained to calculate the mean intensities. Four geomagnetic field regional models have been implemented by applying the revised spherical cap harmonic analysis to these four groups of input data. Geomagnetic field models strongly depend on the used data set. The model built using all the available data (without any preselection) appears to be the less accurate, indicating some internal inconsistencies of the data set. In addition, some features of this model are clearly dominated by the less reliable archeointensity data, suggesting that such features might not reflect real variations of the past geomagnetic field. On the contrary, the regional model built on selected high-quality intensity data shows a very consistent intensity pattern at the European scale, confirming that the main intensity changes observed in Europe in the recent history of the geomagnetic field occurred at the continental scale.

The Late Carboniferous to Late Triassic segment of the apparent polar wander path of Iberia

A palaeomagnetic study has been carried out at 16 well-dated sites from four areas in central Spain (southeastern Iberian Massif and western Iberian Ranges) in order to constrain the Late Carboniferous to Late Triassic segment of the apparent polar wander path (APWP) of Iberia. 322 samples (218 with useful results) were collected from andesitic rocks at Atienza (287 ± 12 Ma) and from Triassic continental red beds at Molina de Aragón (Anisian-Ladinian), Alcaraz (Ladinian-Carnian), Alcázar de San Juan (Ladinian-Carnian) and Cuevas de Ayllón (Carnian-Norian). Comparison of the palaeomagnetic results from the western Iberian Ranges and from the Iberian Massif indicates that the investigated area of the Iberian Ranges forms part of Stable Iberia. The palaeomagnetic poles obtained in this study and a revision of previous palaeomagnetic data, discarding poles obtained from areas of doubtful stability, show together a gradual and consistent change in latitude and longitude resulting in a coherent segment of the APWP for the Late Carboniferous to Late Triassic time span.

Palaeomagnetic rotations in opposite senses in southeastern Spain

20 volcanic and 3 sedimentary sites of upper Miocene age were sampled in the Cabo de Gata region in southeastern Spain. Palaeomagnetic and rockmagnetic investigations yielded useful results in 15 volcanic and 1 sedimentary site. Clockwise and anticlockwise block rotations of adjacent blocks could be observed. The age of these rotations is younger than Tortonian. Deformation in the eastern part of the Betics seems to follow a different pattern than in the central and western part of the Subbetic. This behaviour can be related to the crustal structure in the eastern Betics.

Palaeomagnetic results from Upper Miocene and Pliocene rocks from the Internal Zone of the eastern Betic Cordilleras (southern Spain)

Palaeomagnetic and rock-magnetic studies were carried out on samples from thirteen volcanic and nine sedimentary sites of Late Miocene to Pliocene age from the Internal Zone of the eastern Betic Cordilleras. After comparing palaeomagnetic results with the expected Pliocene/Miocene direction, rotated and unrotated areas can be recognized, rotations being clockwise and counter-clockwise. Some rotations are of great magnitude. Unlike deformation in the External Betics, Late Miocene to Present block rotations in the Internal Betic Zone are non-systematic, and related to the movement of faults as local responses to the Late Miocene-Present regional stress field, due to the N140 convergence of Africa and the Iberian Peninsula.

Oxfordian magnetostratigraphy in the Iberian Range

The magnetostratigraphy has been established in two sections of middle and late Oxfordian age from the Iberian Range (Spain). The polarity sequences correlate well with the magnetozones in sections at Aguilon and Tosos [Juarez et al., 1994]. A composite magnetostratigraphic sequence has been derived for all the sections in the Iberian Range. It correlates with the Handschumacher et al. [1988] extension of the geomagnetic polarity time scale, and ties the oceanic magnetic anomaly sequence to the Oxfordian sedimentary sections.