T. Román-Berdiel

Analogue models of transpressive systems

This paper presents three series of analogue models of transpressional deformation in a brittle/ductile system: (1) simple transpression, (2) transpression combined with erosion of uplifted areas of the hanging wall above the deformation front, and (3) transpression combined with erosion of the hanging wall and sedimentation at the foot of uplifted zones. In each series of experiments, different convergence angles α, from 0° (pure wrenching) to 90° (pure thrusting) were applied to the models. Results show a sharp contrast between structures formed at α≤15° (wrench-dominated transpression) and α≥30° (thrust-dominated transpression). For a low convergence/strike-slip ratio (0°≤α≤15°), the deformation is localized and structures are typical of a strike-slip regime (R and Y faults). For higher convergence angles (30°≤α≤90°), the deformation is similar for all models, with an elongate asymmetric uplift showing fault-propagation-fold geometries and flanked by thrust-wrench faults. Fault dips also show a significant change from more than 70° for α≤15° to less than 40° for α>30°. For α≥30°, the geometry of the main faults at the borders of the uplift zone is modified by P faults. In experiments with erosion and sedimentation, and thrust-dominated transpression, new faults with increasing dips form during progressive deformation, branching on the main fault at the base of the model.

Granite laccolith emplacement in the Iberian arc: AMS and gravity study of the La Tojiza pluton (NW Spain)

A laccolithic geometry is inferred from geological and geophysical data for the La Tojiza granitic body emplaced in an extensional regime. Nearly circular in outcrop, this 16 km diameter pluton, one of the so-called ‘post-tectonic granodioritic plutons’ of Galicia, is located in the northern Variscan arc of Iberia, within the Mondoñedo Nappe (Western Asturian–Leonese Zone). From anisotropy of magnetic susceptibility measurements and extensive microstructural data, the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric. Magnetic foliations strike NE–SW and dip gently SE. Magnetic lineations trend NW–SE and plunge gently SE. Gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border. This pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile Vivero normal fault. It is suggested that the emplacement of most post-tectonic plutons of this part of the Variscan orogenic belt may have been controlled by the late extensional collapse of the Iberian Massif.

Granite emplacement during contemporary shortening and normal faulting: structural and magnetic study of the Veiga Massif (NW Spain)

Abstract The Veiga Massif belongs to the calc-alkaline series of Hercynian granitic rocks of the Ibero-Armorican arc The Veiga granodiorite intruded during the Upper Carboniferous into the core of the WNW-ESE N-verging ‘Ollo de Sapo’ antiform, formed by Precambrian and Palaeozoic metasediments. Internal fabrics show that magma intrusion was contemporary with shortening. Measurements of feldspars orientations and anisotropy of magnetic susceptibility (AMS) throughout the granite are consistent and indicate a foliation striking WNW-ESE (parallel-to-folding), with a constant dip of 75–85 °N. The zonation of bulk low-field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders. The difference in elevation between outcrops (more than 600 m) allows us to infer the three-dimensional attitude of granite fabrics throughout the Massif. Syn-magmatic fabric folds are preserved in the inner part of the igneous body. The highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion. At the scale of the Massif, foliation is convergent toward the bottom of the intrusion, along a line located at its northern border, where the magma source is interpreted to be located. In the western border of the Massif, the presence of C and S structures indicates that magma cooling was coeval with movement of the Chandoiro fault, a N-S striking normal fault with a N290E hanging wall displacement direction. These results indicate that emplacement of the Veiga granite is coeval with NNE-SSW shortening and with an WNW-ESE extension direction, parallel to the trend of the late folds.

Tethyan versus Iberian extension during the Cretaceous period in the eastern Iberian Peninsula: insights from magnetic fabrics

This work investigates how anisotropy of magnetic susceptibility (AMS) recorded the strain related to the Early Cretaceous extensional processes in synrift sediments of the Maestrat basin (eastern Spain). Forty-two sites, distributed throughout the Lower Cretaceous sequence with dominant gentle dips, were sampled. Minerals contributing to the AMS are mainly phyllosilicates. The parallelism between magnetic and sedimentary foliation seems to indicate that a primary (synsedimentary and early diagenetic) magnetic fabric was preserved at 84% of sites. Consequently, preferred orientations of magnetic lineations are interpreted to record the effect of extensional processes coeval with sedimentation and diagenesis during this period. At these 35 sites, two main magnetic lineation orientations are found, delimiting two large domains: a NE–SW orientation prevailing in the NW sector of the basin (parallel to the extension direction of the Iberian basin), and NW–SE to NNW–SSE orientations to the SE (parallel to the extension direction controlling the western Tethys margin). Directional variability demonstrates that the Maestrat basin is located at the boundary between two domains (Iberian and Tethyan) undergoing different plate-scale extensional processes. 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.

Anisotropy of magnetic susceptibility of the Pyrenean granites

ABSTRACT In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.