Evidence of remagnetized red-beds in the Middle Atlas (Morocco)

Se ha realizado un estudio paleomagnético en el límite entre el Atlas Medio Plegado y el Atlas Medio Tabular, en el entorno de los sinclinales de Skoura y Tighboula y el anticlinal de Boulemane. La finalidad de este trabajo es tratar de reconocer una remagnetización que afecte a los materiales sedimentarios de dicha área. Para ello se han realizado diferentes test de estabilidad: (i) dos test del pliegue, uno en el anticlinal de Boulemane muestreando cinco estaciones paleomagnéticas en calizas jurásicas, y otro en el sinclinal de Tighboula, con tres estaciones en capas rojas del Cretácico Inferior; (ii) un test del conglomerado en un nivel basal del Cretácico. Los resultados de dichos test indican que en capas rojas la componente característica es una remagnetización sinpliegue, mientras que la señal magnética en las calizas es débil y no permite aislar ninguna componente paleomagnética. La similitud en el comportamiento paleomagnético de las capas rojas (propiedades de la magnetización remanente natural, probable dirección NNW de la remagnetización, polaridad normal sistemática) con la observada en cuencas de similares características del Alto Atlas podrían indicar que ambas remagnetizaciones están relacionadas.A paleomagnetic study has been carried out in the boundary between the Folded Middle Atlas and the Tabular Middle Atlas, around Skoura and Tighboula synclines and Boulemane anticline. The goal of this work is to test the existence of a remagnetization affecting the sedimentary rocks in this area. For this purpose different stability tests were done: (i) two fold tests, one in the Boulemane anticline by studying five paleomagnetic sites located in Jurassic limestones, and the other in the Tighboula syncline in three sites from Cretaceous red-beds; (ii) a conglomerate test was done sampling a Cretaceous basal level and the overlying red-beds. The results of these tests show a synfolding remagnetization in the redbeds, whereas the magnetic signal of the limestones is too weak and it is not possible to isolate a magnetic component. Red-beds shows similar paleomagnetic properties than in other basins of the High Atlas (properties of the natural remanent magnetization, a probably NNW trend of the remagnetization and a systematic normal polarity), which could indicate a relationship between both.Proyecto CGL2012-38481 de la DGICyT, MINECO y por fondos FEDER de la Unión Europea

Magnetic extractions and electronic microscopy observations in Jurassic remagnetized carbonates

IX Congreso Geológico de España, Huelva, 12-14 de septiembre, 2016El estudio de la remagnetización en rocas carbonatadas viene realizándose durante los últimos 40 años, tanto por medio del análisis de sus propiedades magnéticas como tratando de realizar observaciones por medio de microscopía electrónica (SEM/TEM). Los resultados obtenidos con el primer análisis han proporcionado numerosos datos y hoy en día sabemos que generalmente la remagnetización es portada por magnetita. Ésta es de tamaño nanométrico generada durante la diagénesis, correspondiéndose con una remagnetización química. Sin embargo, los numerosos intentos de observación de dichos cristales por medio de SEM/TEM siempre han sido infructuosos y los cristales observados siempre presentan tamaños dentro del rango multidominio. En un nuevo intento de observación directa de dichos cristales mediante microscopía electrónica, hemos realizado nuevos ensayos sobre extractos magnéticos en calizas jurásicas remagnetizadas de las cuencas del Alto Atlas (Marruecos), Cameros y Vasco-Cantábrica (España) según las últimas técnicas propuestas en la literatura. Las propiedades magnéticas de los diferentes subproductos provenientes del proceso de extracción indican una pérdida de material magnético durante el mismo, principalmente las partículas de grano más fino (granos de tamaño monodominio y superparamagnético). Igualmente, mediante SEM/TEM únicamente han podido observarse cristales de magnetita multidominio.Since the last 40 years the analysis of remagnetized carbonates is under debate, by studying their magnetic properties and by using electronic microscopy (SEM/TEM). Results obtained from the first analyses allowed to infer that remagnetization is carried generally by nanometric magnetite and generated during the diagenesis, therefore it corresponds to a chemical remagnetization. However, several attempts to observe these magnetites by SEM/TEM were unsuccessful and in all cases the observed magnetite was in the range of multidomain size. In a new attempt to observe these crystals by electronic microscopy we did new magnetic extracts (according the last techniques proposed in the literature) in Jurassic remagnetized limestones of the High Atlas (Morocco), Cameros and Basque-Cantabrian (Spain) basins. Magnetic properties of the different sub-products of the extracts show a lost of magnetic material during the procedure, mostly of the smallest size (superparamagnetic and single-domain size). Likewise, only multi-domain magnetite grains were observed by SEM/TEM.proyecto CGL2012-38481 de la DGICyT, MINECO y fondos FEDER de la Unión Europea, así cómo por el programa de financiación de la NSF (EEUU) al Institute for Rock Magnetis

The carriers of AMS in remagnetized carbonates. Insights for remagnetization mechanism and basin evolution

Magnetic fabrics are usually studied to unravel the evolution of sedimentary basins, mainly focusing the attention in paramagnetic minerals. However, since basins are sometimes affected by burial-related chemical remagnetizations, magnetic fabrics can also be carried by authigenic ferromagnetic minerals related to remagnetization processes. Consequently, the study of the different types of fabrics can give complementary and valuable information about the evolution of sedimentary basins. Here, we explore, in the Jurassic carbonates of the Central High Atlas (Morocco), the role in magnetic fabrics of authigenic magnetite that grew during the Cretaceous (ca. 100 Ma) widespread remagnetization event. Magnetic fabrics are studied in 53 sites using the anisotropy of the magnetic susceptibility measured at room temperature (RT-AMS) and comparing results with sub-fabrics that alternatively enhance the paramagnetic and the ferromagnetic signal. Furthermore, an innovative analysis is proposed, comparing the magnetic fabrics before and after bedding correction (the common procedure) with fabric orientation after partial bedding correction. We use the paleomagnetic information to reconstruct the attitude of bedding at the remagnetization time and to restore the magnetic fabrics at this time. The performed analysis allows interpreting RT-AMS in terms of the contribution of different subfabrics, and the relationship between these, tectonic processes and the magnetic mineralogy. Four RT-AMS types are defined: T1 is carried by superparamagnetic magnetite that grew during the remagnetization stage and shows a horizontal lineation parallel to the extension direction at this time; T3 and T4 are carried by paramagnetic minerals and show compressive fabrics with the magnetic lineation parallel to intersection (bedding-cleavage) lineation; finally, T2 shows a mix between ferrimagnetic and paramagnetic fabrics.MINECO (Spanish Ministry of Economy and Competitiveness) cofinanced by the ERDF (European Union) (research projects CGL2012-38481 and CGL2016-77560)

Extensional vs. compressional deformation in the Central High Atlas salt province: a paleomagnetic approach

In this paper we address the problem of the distinction between diapiric, salt-driven and compressional structures, using the outstanding example of the Central High Atlas (Morocco). A remagnetized component carried by magnetite has been isolated in 32 new paleomagnetic sites. It is characterized by: maximum unblocking temperatures around 450 °C, syn-folding behavior and normal polarity. These 33 mean paleomagnetic directions were analyzed together with other 68 from published works around the study area to construct a robust paleomagnetic dataset along a cross-section perpendicular to the main structures. The remagnetization direction (n: 100, Dec: 332.2°, Inc: 34.5°, η: 6.2°, ξ: 2.0°, A/n: 6.427°) and the paleo-dip of beds (the attitude of the beds at the remagnetization occurrence) were calculated through small circle methods. The remagnetization can be dated as ca. 100 Ma. Because of its occurrence between the extensional and compressional periods, this remagnetization offers the possibility of restore the basin to its pre-inversion geometry. Comparison between present-day and pre-inversion structure allows discriminating three different evolutionary patterns: (i) thrusted and welded salt-walls mainly structured during the extensional stage (Ikkou ridge) with steep limbs close to the salt-wall core. (ii) Jurassic salt-walls with weaker deformation, restricted to the areas adjacent to the structure (Tadaghmamt and Timedouine); in this case, Cenozoic compression is limited to welding of the salt-walls and buttressing of the sedimentary sequences against faults. (iii) salt-rollers gently initiated during the Jurassic (Toumliline diapir), thrusted during the Cenozoic compression. Results show the importance of salt tectonics both during extension and compression, as well as the control of the compressional features by the inherited extensional structures. The performed restorations prove that paleomagnetism is a useful, independent tool to obtain palinspastic restorations and to separate, and quantify, the imprint generated during the basinal stage from the inversional features.MINECO (Spanish Ministry of Economy and Competitiveness) cofinanced by the ERDF (European Union) (research projects CGL2012-38481 and CGL2016-77560

Is it possible to identify temporal differences among combustion features in Middle Palaeolithic palimpsests? The archaeomagnetic evidence: A case study from level O at the Abric Romaní rock-shelter (Capellades, Spain)

Archaeomagnetic dating is probably one of the most known applications of magnetic methods to archaeology but there are others still underutilized and of particular interest to Palaeolithic archaeology. Here, we report a novel application of archaeomagnetism as a technique to determine temporal diachronies among combustion features from the same surface within palaeolithic palimpsests. The approach is based on the subtle directional changes of the Earth's magnetic field through time (secular variation, SV) and on the ability of burned materials to record such variations under certain conditions. Three Middle Palaeolithic hearths from level O (ca. 55 ka BP) at the Abric Romaní rock-shelter (NE Spain), were archaeomagnetically investigated. The studied surface (black homogeneous carbonaceous facies), recorded the magnetic enhancement produced by fire with a tenfold increase in concentrationdependent magnetic parameters in the uppermost centimetre with respect to its unburned or deeper counterparts. Pseudo-single domain (PSD) Ti-low titanomagnetite was identified as the main remanence carrier. The irreversibility of thermomagnetic curves suggests that these samples did not undergo enough high temperatures as to record a full thermoremanence (TRM). Additionally, the occasional occurrence of maghaemitized magnetite is interpreted as an indication of a thermochemical remanent magnetization (TCRM), making these samples unsuitable for absolute palaeointensity determinations. Two well-defined (a95 < 5 ) and statistically indistinguishable archaeomagnetic directions were obtained with their mean directions within their respective confidences circles at the 95% level. The lack of directional changes and the similarity in the magnetic properties suggest that these hearths recorded simultaneously or closely confined in time the Earth's magnetic field direction at the time of cooling. These results agree well with archaeological evidence which indicates a synchronic occupation of this activity area. The possibility of determining temporal differences among combustion features in prehistoric sites arises as a promising tool in palimpsest dissection studies and may help to reconstruct occupation patterns of prehistoric groups. The practical limits of the method are discussed as well as its potential to identify post-depositional mechanical alteration processes.Spanish Ministry of Economy and Competitiveness, (MINECO) and European Regional Development Fund (projects CGL2012-38481 and CGL2012-32149

La preservación del registro arqueomagnético en estructuras de combustión experimentales con 2 y 5 años de antigüedad

Trabajo presentado en: Magiber XI, 4-7 de septiembre de 2019, Condeixa a NovaJCYL-EFRD project BU235P18 and the MINECO-EFRD projects HAR2015-68321-P and CGL2016-77560, as well as the ERC Consolidator Grant project PALEOCHAR (648871

Paleo-watertable definition using cave ferromanganese stromatolites and associated cave-wall notches (Sierra de Arnero, Spain)

The steeply-dipping-dolostone-hosted caves of the Sierra de Arnero (N Spain) contain low-gradient relict canyons with up to ten mapped levels of ferromanganese stromatolites and associated wall notches over a vertical range of 85 m, the highest occurring ~ 460 m above base level. Despite a plausible speleogenetic contribution by pyrite oxidation, and the irregular cave-wall mesomorphologies suggestive of hypogenic speleogenesis, the Arnero relict caves are dominantly epigenic, as indicated by the conduit pattern and the abundant allogenic sediments. Allogenic input declined over time due to a piracy-related decrease in the drainage area of allogenic streams, explaining the large size of the relict Arnero caves relative to the limited present-day outcrop area of the karstified carbonates. Allogenic-sediment input also explains the observed change from watertable canyons to phreatic conduits in the paleo-downstream direction. Stromatolites and notches arguably formed in cave-stream passages at the watertable. The best-defined paleo-watertables show an overall slope of 1.7°, consistent with the present-day relief of the watertable, with higher-slope segments caused by barriers related to sulfide mineralization. The formation of watertable stromatolites favored wall notching by the combined effect of enhanced acidity by Mn–Fe oxidation and shielding of cave floors against erosion. Abrasive bedload further contributed to notch formation by promoting lateral mechanical erosion and protecting passage floors. The irregular wallrock erosional forms of Arnero caves are related partly to paragenesis and partly to the porous nature of the host dolostones, which favored irregular dissolution near passage walls, generating friable halos. Subsequent mechanical erosion contributed to generate spongework patterns. The dolostone porosity also contributes to explain the paradox that virtually all Arnero caves are developed in dolostone despite being less soluble than adjacent limestone. U-series dating of carbonate speleothems and paleomagnetic data from ferromanganese stromatolites and clastic sediments indicate that the paleo-watertables recorded ~ 320 m above the present-day watertable formed during the Matuyama Chron but prior to ~ 1.5 Ma, implying long-term base-level-lowering rates from ~ 125 to ~ 213 m/Ma. To our knowledge, this is the first attempt of paleomagnetic dating of cave ferromanganese stromatolites. These deposits are excellent geomagnetic recorders and offer a direct way to delineate and date paleo-watertables, especially in caves developed in dolostone.Financial support was provided by grants ICT-Soplao-53.5.00.12.00 (IGME - Provincial Government of Cantabria - Turismo del Nansa) and CGL2012-38481 (Ministerio de Economía y Competitividad - European Regional Development Fund

Gravity and magnetic anomalies in the allochthonous Órdenes Complex (Variscan belt, northwest Spain): Assessing its internal structure and thickness

The Órdenes Complex is the largest Variscan allochthonous structure of NW Iberia, and preserves the suture of a long-standing Paleozoic ocean, probably the Rheic. Gravity and magnetic data, the latter specifically acquired on land for this study, show that the complex occupies the core of an open synform with a maximum depth of 9–10 km, which contrasts with the flat geometry of the lower crust and Moho discontinuity beneath. The maximum depth reached by the ophiolitic rocks marking the suture is around 7 km. The allochthonous units formed by basic and ultrabasic rocks are lens-shaped in section, and occur not only at the periphery of the complex, but form wide ribbons trending NE-SW to N-S. The Bouguer anomaly related with the longest of them, the Fornás Unit, previously used to support an autochthonous interpretation of the complexes, is modeled as a rootless, massive amphibolite body with a maximum thickness of 6 km located at the downthrown block of a large normal fault cutting across previous thrust faults and extensional detachments. The main magnetic anomalies are associated with ultrabasic rocks cropping out in the NW and SE, but a weak, wide anomaly in the central part of the complex is related with one or more thin layers of amphibolite partly mineralized with massive sulphides. The weakly arcuate geometry of this anomaly and of the Bouguer anomaly caused by the Fornás Unit may reflect the NE flank of the Central Iberian arc, an orocline whose core is occupied by the allochthonous complexes.FEDER, Ministerio de Ciencia e Innovación (Ramón y Cajal

pySCu: A new python code for analyzing remagnetizations directions by means of small circle utilities

The Small Circle (SC) methods are founded upon two main starting hypotheses: (i) the analyzed sites were remagnetized contemporarily, acquiring the same paleomagnetic direction. (ii) The deviation of the acquired paleomagnetic signal from its original direction is only due to tilting around the bedding strike and therefore the remagnetization direction must be located on a small circle (SC) whose axis is the strike of bedding and contains the in situ paleomagnetic direction. Therefore, if we analyze several sites (with different bedding strikes) their SCs will intersect in the remagnetization direction. The SC methods have two applications: (1) the Small Circle Intersection (SCI) method is capable of providing adequate approximations to the expected paleomagnetic direction when dealing with synfolding remagnetizations. By comparing the SCI direction with that predicted from an apparent polar wander path, the (re)magnetization can be dated. (2) Once the remagnetization direction is known, the attitude of the beds (at each site) can be restored to the moment of the acquisition of the remagnetization, showing a palinspastic reconstructuion of the structure. Some caveats are necessary under more complex tectonic scenarios, in which SC-based methods can lead to erroneous interpretations. However, the graphical output of the methods tries to avoid ‘black-box’ effects and can minimize misleading interpretations or even help, for example, to identify local or regional vertical axis rotations. In any case, the methods must be used with caution and always considering the knowledge of the tectonic frame. In this paper, some utilities for SCs analysis are automatized by means of a new Python code and a new technique for defining the uncertainty of the solution is proposed. With pySCu the SCs methods can be easily and quickly applied, obtaining firstly a set of text files containing all calculated information and subsequently generating a graphical output on the fly.CGL2012-38481 and CGL2016-77560 of the MINECO (Spanish Ministry of Economy and Competitiveness) with also FEDER founding (European Union). PC acknowledges the MINECO for the F.P.I. research grant BES-2013-062988. LT acknowledges support from National Science Foundation grant # EAR 1345003

Analysing non-coaxial folding effects in the Small Circle Intersection method

The Small Circle (SC) tools analyse the stereographic tracks (small circles) followed by the palaeomagnetic vectors during folding processes. Working with interfolding and synfolding remagnetizations, the Small Circle Intersection (SCI) method allows finding the best solution of grouping that should correspond with the remagnetization direction. Once this is known, it is possible to determine the magnetization age as well as the degree of bed tilting at this moment. The SC tools are based on some assumptions, among which the coaxiality between the different deformation events is the one addressed in this work (i.e. absence of vertical axis rotations, VARs, or differential horizontal axis rotations, dHARs). This assumption is based on the necessity of knowing the rotation axis for folding after the acquisition of the remagnetization, and SC tools consider the bedding strike as this axis, something that is only accomplished under coaxial folding. In order to explore how non-coaxiality affects the solutions derived from the SC methods, we first (i) identify the variables that control these errors through simple models that only consider two theoretical palaeomagnetic sites, after that it is possible (ii) to derive the mathematical relationships between them. Finally, we (iii) simulate errors derived from the use of SC tools using a population of 30 palaeomagnetic sites recreating different possible scenarios with VARs and dHARs in nature.Research financed by the projects DR3AM- CGL2014-55118 and CGL2016-77560-C2 from the Spanish Ministry of Science (MINECO) and by the Applied Geology (GeoAP-E0117R) and Geotransfer groups of the Government of Aragon, as well as by the project BU235P18 (Junta de Castilla y Leon, Spain) and the European Regional Development Fund (ERD). It has also benefited from the MAGIBER-II network (CGL2017-90632-REDT) funded by the Ministry of Science, Innovation and Universities