ALBERT

Description: The timing of development of the magnetic fabric is a major issue in the application of anisotropy of magnetic susceptibility (AMS) as a strain marker. Analysis of AMS in unconcealed synsedimentary structures can be a sound approximation to this task. In this work, three types of early compactional structures (ECS) were studied by means of AMS, since they can help to understand the timing of development of the magnetic fabric. All three types of ECS are found in fine-grained detrital rocks (to avoid other influences such as palaeocurrents), claystones and marls of the Enciso Group within the Cameros Basin (NE Spain): dinosaur footprints, load structures due to differential compaction and dish-and-flame structures associated with fluid migration related to seismites. In addition, to determine possible influences of lithology on the magnetic fabric, different rock types (siltstones and limestones) were also sampled. In general, the influence of ECS results in scattering of the three magnetic axes, higher at the margins of the structure than at its centre. This fact suggests that ECS occurs during the development of the magnetic fabric, disturbing the incipient magnetic fabric stages, and strongly conditions its later evolution during diagenesis. The later homogeneous compaction process due to sedimentary load and physicochemical processes reorient the susceptibility carriers to some extent (i.e. the magnetic fabric is still under development), but not totally, since AMS still records the previous scattering due to ECS imprint. For the Enciso Group deposits, the magnetic fabric begins to develop at the earliest stages after deposition and it stops when diagenetic processes have finished.

Description: 〈span〉〈div〉Summary〈/div〉Of the several factors involved in the development of magnetic fabrics in fault zones at shallow crustal levels, lithology and deformation intensity have probably the most important consequences for the reconstruction of their kinematic history. The basement-involved Cenozoic thrusts in the Demanda Massif (N Spain) provide the opportunity for testing the applicability of Anisotropy of Magnetic Susceptibility (AMS) to the study of deformation in cataclastic fault rocks belonging to shallow fault zones. The Rastraculos thrust is a relatively minor basement thrust (dip-slip movement of 2 km defined from cross-sections and geological maps) of Cenozoic age. This thrust contains a re-activated fault zone involving different rock types both belonging to its hangingwall (Paleozoic) and its footwall (Triassic sandstones and dolostones and Jurassic limestones). AMS results show magnetic foliations parallel or slightly oblique to the fault zone, and both transport-parallel (projected onto the foliation plane) and transport-perpendicular (parallel to the observed intersection lineation) magnetic lineations. The two types of strain/magnetic fabric relationships can be related to deformational and mineralogical features inferred from the direct analysis of thin and polished sections under the microscope and the naked eye, respectively. Analysis of fault rocks in the Rastraculos fault zone indicates that in cataclasites, magnetic fabrics are particularly dependent on lithology and hence magnetic mineralogy. The results obtained prove the usefulness of AMS in fault zones where kinematic indicators are scarce and also give clues on the number of samples necessary to define magnetic susceptibility axes, depending on grain size, ellipsoid shapes and magnetic mineralogy.〈/span〉

Description: The Internal Sierras (IS) in the southern margin of the Western and Central Axial Zone (Southern Pyrenees) are affected by a syn-orogenic remagnetization that provides information to reconstruct deformation geometries at the time of acquisition of magnetization. Furthermore, the IS structure changes strike along its structural trend, from ~N120 to 130°E in the western and eastern margins to ~N070–090°E in the central part. Palaeomagnetic techniques have been used to (i) accurately define the timing of remagnetization with regard to deformation and (ii) determine if the along-strike trend variation in the IS was induced by deformation and thrust emplacement during the Pyrenean compression or, on the contrary, was the result of a primary orientation controlled by structures inherited from pre-orogenic times. From 23 new palaeomagnetic sites, collected in Upper Cretaceous marls and marly limestones, two meaningful and stable palaeomagnetic components were resolved, principally carried by magnetite: (1) a lower-temperature component (B) that unblocks between 200 °C and 325–400 °C and (2) a higher-temperature component (C) that has been successfully isolated by means of combined thermal (up to 400 °C) and AF demagnetization (generally up to 50–100 mT). The B component is a late remagnetization that post-dates folding and emplacement of basement thrust sheets in the IS (mainly the Gavarnie thrust). It supports small but statistically significant clockwise rotations in the western part of the IS (from +18 to +26°). These rotations can be attributed to the westwards shortening decrease in the thrust system below the Gavarnie unit that results from its along-strike structural change, with a higher number of basement thrusts to the east. The C component has been interpreted as an early remagnetization, based on the results of conglomerate and fold tests. This component predates basement thrusting and is diachronous across the study area: reverse and normal polarities dominate in the eastern and western margins of the IS, respectively. New and previous palaeomagnetic data point out that curvature in the IS is probably a primary feature and the along-strike change in their trend could be interpreted as the result of basement geometrical features inherited from Variscan, Late Variscan or Mesozoic times. A complex, multi-episodic remagnetization probably related to burial and deformation processes occurred during Eocene times.