Magnetic fabric analyses and paleomagnetism applied to study the mesozoic evolution of the iberian range
- Autores: Cristina García Lasanta
- Directores de la Tesis: Teresa Román Berdiel (dir. tes.), Belén Oliva Urcia (dir. tes.), Antonio María Casas Sáinz (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Martin Chadima (presid.), Andrés Gil Imaz (secret.), Francesca Cifelli (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Geología
- Materias:
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- Resumen
This thesis is a contribution to the study of the Mesozoic extension in three areas of the Iberian Range subjected to a subsequent tectonic inversion during the Cenozoic. Two main indirect techniques have been applied: anisotropy of magnetic susceptibility (AMS) and paleomagnetism. Both methods are based on the magnetic properties of minerals. AMS considers the capacity of minerals to acquire an induced magnetization in the presence of an external magnetic field (generally of low intensity for the purposes of this work); paleomagnetism uses the remanent magnetization acquired by rocks in response to the Earth’s magnetic field.
The Mesozoic period in the Iberian Basin was mainly characterized by extensional tectonics, with the occurrence of two main rifting stages: Late Permian-Triassic and Late Jurassic-Early Cretaceous. The three areas of the Iberian Range investigated in this thesis are: the northwestern part of the Castilian Branch, the northeastern part of the Cameros Basin and the eastern part of the Maestrat Basin. They were affected in a different manner by these extensional processes and also by the Cenozoic compression that originated the inversion and uplift of the Iberian Range. The basinal configuration of the Castilian Brach occurred mainly during the first rifting stage, whereas the Cameros and Maestrat Basins developed during the second stage. Cenozoic compression triggered the partial inversion of the Castilian and Maestrat domains, reactivating some of the main faults limiting these areas and favoring the development of gentle folds which affected the synrift sequence. Compression was more intense in the Cameros Basin, producing the northwards tectonic transport of the whole synrift sequence through the complete inverted main faults that limited the basin.
Three of the studies included in the thesis were developed in the Lower Cretaceous synrift units from the Cameros Basin (northern Iberia). The first one was focused in thoroughly defining the development of magnetic fabrics and the possible influence that certain synsedimentary and early diagenetic processes may have in the AMS configuration. Three types of early compactional structures (ECS) were investigated: dinosaur footprints, load structures due to differential compaction and dish-and-flame structures associated with fluid migration related to seismites. Results suggest a very early development of the magnetic fabric, disturbed by the occurrence of the synsedimentary structures. This distortion consists in an increase of the scattering degree of the three magnetic axes. Compaction related to sedimentary load reoriented the magnetic carriers to some extent thus indicating that the primary AMS imprint continued during digenesis.
In the second study, AMS analyses were combined with the structural analysis of hydroplastic synsedimentary faults and tension gashes from the Cameros Basin, in order to investigate the response of magnetic fabrics to a syn- to post-sedimentary extensional regime and their prevalence after a subsequent tectonic inversion. The combination of both groups of data indicate a general NE-SW extension direction during early and late stages of basin evolution and a NW-SE extension direction for Aptian (and probably also for Barremian) times, when secondary faults perpendicular to the main ones probably controlled the evolution of the basin. The development of axial-plane cleavage during the Albian was restricted to favorable lithologies and its role in the AMS development was to enhance the magnetic lineation orientation linked to extension where both processes were roughly coaxial with respect to the maximum horizontal extension direction.
The last study performed in the Cameros Basin applies paleomagnetic results to the geometrical reconstruction of the basin before the Cenozoic compression. It corroborates the occurrence of a generalized remagnetization, which was previously described and dated at around 100 M.a. and thus occurring between the basinal and inversion stages. The application of the SCI method to the newly obtained data and to previous paleomagnetic results allows to obtain a robust, representative orientation for this remagnetized vector. Based on this orientation, the study proposes an accurate reconstruction of the beds attitude at the remagnetization moment by applying the “asymmetric solution” method. The large gentle geometry of the Cameros Basin is ratified and the axial surface of this large-scale fold can be accurately located. According to the exposed data, a northwards hinge migration of around 5 km can be interpreted to have occurred after the remagnetization stage. This migration is in agreement with the southwards displacement registered in the southern back-thrust of Cameros and seems to be favored by a transfer of displacement through a décollement probably located between basement and cover.
Two other studies included in this thesis were developed in the Permo-Triassic red beds of the northwestern limit of the Castilian Branch (central part of the Iberian Peninsula). The first one pursues the characterization of the extensional tectonic evolution of the basin by means of AMS data collected in the Upper Permian and Triassic rocks. The effects of the Cenozoic compression in the AMS are scarce since no cleavage or other penetrative structures associated with compression are present. The most frequent shape of the obtained magnetic ellipsoids is oblate, with the magnetic foliation plane parallel to bedding, typical of synsedimentary magnetic fabrics. Magnetic lineations show a dominant ENE-WSW direction fitting with the dextral transtensional regime interpreted to control the Mesozoic rifting evolution in the Castilian Branch. Variability of results within this maximum is interpreted to be the effect of transfer faults compartmentalizing the basin and probably triggering local strain partitioning processes.
The same units were subjected to a paleomagnetic analysis revealing the coexistence of two main magnetic components within most samples. Magnetic stability tests for the high-temperature component demonstrate its primary origin and, thus, its orientation allows to define the Permo-Triassic segment of the apparent polar wander path (APWP) of the Iberian Plate, as well as to compare the paleopoles position with previous results from literature. A secondary component, of probably Mesozoic age, was also isolated and demonstrated to be acquired in a syn- to post-folding stage. The SCI method revealed a mean direction Dec.: 357º, Inc.: 36º for this component and the restitution of layers according to it suggests that paleodips (at the remagnetization moment) were mostly acquired during the basinal stage, related to normal faulting, and only locally modified by Cenozoic inversion.
Results from the application of AMS techniques to the Lower Cretaceous synrift units of the Maestrat Basin (eastern Iberia) allow to interpret that in most cases a primary (synsedimentary and early diagenetic) magnetic fabric was preserved and that the orientations of magnetic lineations record the effect of extensional processes coeval with sedimentation and diagenesis. Two main magnetic lineation orientations are found, each prevalent in a different area within the studied region. This zonation in the AMS results is interpreted to delimit two large domains influenced by two different plate-scale extensional processes: a NE-SW orientation of the magnetic lineations prevailing in the NW sector of the basin coincides with the extension direction in the Iberian Basin, whereas NW-SE to NNW-SSE magnetic lineation orientations towards the SE are parallel to the extension direction controlling the evolution of western Tethys margin. The subsequent Cenozoic tectonic inversion affected the synsedimentary magnetic fabrics at only a few sites at the borders of the basin, where compressive features are more developed.
All these studies are supported by rock magnetism results, which allowed to determine the magnetic carriers in the considered lithologies. AMS contributors are manly paramagnetic phases (mostly phyllosilicates) in Cameros and the Maestrat rocks, and a mixture of these minerals and hematite in the red beds from the Castilian Branch. Two methods of magnetic subfabrics separation were applied in order to elucidate the partial contributions of the different present phases to the total AMS: low-field AMS at low temperature (LF-ALS at LT) and high-field AMS at room temperature (HF-AMS at RT). The only carrier of the NRM detected in samples from the Castilian Brach is hematite (probably two phases with different grain-size). In the Cameros basin, the same remagnetized vector is carried by all the ferromagnetic phases present in the samples (hematite in red beds and probably magnetite in grey limestones).
