Manuel Julivert

Paleomagnetic results in support of a model for the origin of the Asturian arc

Abstract Paleomagnetic directions have been determined at 17 Cambrian and 28 Carboniferous limestone sites distributed along the Asturian arc of the Hercynian fold belt in northwestern Spain. The stable vectors are carried by hematite in all cases, and also by magnetite in some grey Carboniferous limestones. A secondary component of magnetization is usually removed well below 400°C, and the higher-temperature component is taken to be the characteristic direction. The structure at each site was carefully evaluated in order to make optimum local tectonic corrections. In addition the data must be corrected for rotations about a vertical axis to allow for a phase of radial folding superimposed on earlier structures. The corrected paleomagnetic declinations are found to vary systematically along the arc of the fold belt. Consequently, paleomagnetic data from the Asturian arc should not be included in compilations used for the construction of a Paleozoic apparent polar wander path. The paleomagnetic data allow us to distinguish between different tectonic models for the evolution of the Asturian arc. A two-stage model for the development of the present curvature is favored. Part of the curvature appears to be primary, preceding the Hercynian deformation. The first stage of the model involved rotations of thrust sheets during their emplacement, producing a more tightly curved arc than the original form. In a second stage, the development of radial folds further tightened the curvature of the arc. Both stages result in clockwise rotations in the north and anticlockwise rotations in the south.

Magnetic fabric and deformation in the Navia–Alto Sil slate belt, northwestern Spain

Abstract Ries and Schackleton (Ries, A.C., Schackleton, R.M., 1976. Patterns of strain variation in arcuate fold belts. Philos. Trans. R. Soc. London A. 283, 281–288) discussed the distribution of finite strain in the Ibero-Armorican arc and suggested that the arc was produced by tectonic bending of a straight or gently curved fold belt. Later investigations by others revealed different strain patterns in the fold-thrust belt, and so its evolution continues to be controversial. This study focuses on the strain pattern in the Navia–Alto Sil slate belt. The low-field anisotropy of magnetic susceptibility (AMS) was measured in Ordovician black slates and Cambrian to lower Ordovician quartzites, sampled around the arcuate belt. The magnetic fabric is controlled primarily by the slaty cleavage in both lithologies. The AMS was also measured with a high-field torsion magnetometer, and the results indicate that paramagnetic minerals are responsible for the magnetic fabric in the slates and diamagnetic and paramagnetic minerals in the quartzites. Strain markers, i.e. pressure shadows around pyrite grains and rutile distribution, are found in the slates, and the orientation of the principal axes of strain ellipsoids agrees with those of the AMS ellipsoid. A systematic variation in the orientation of the strain ellipsoid and anisotropy ellipsoid suggests that there is strain heterogeneity along the slate belt, and that the arc did not form by tectonic bending.

Relationship of Mesozoic graben development, stress, shortening magnitude, and structural style in the Eastern Cordillera of the Colombian Andes

Abstract We use the Eastern Cordillera of Colombia as an example in early stages of inversion orogen showing still modest values of shortening. The style of deformation recorded in this orogenic chain seems to be strongly influenced by two main factors. The first is the pre-compression geometry of the rift basin, conditioning the strong heterogeneity imparted by a trough filled with Jurassic to Neocomian sediments limited by Precambrian and Palaeozoic high-angle walls. The second factor is the orientation of the stress regime with respect to the main normal faults during the inversion. If the stress field is of pure compression, the normal faults are not extensively inverted and the deformation is accommodated mainly in terms of footwall shortcuts. On the other hand, in transpressive regimes the inversion of the former normal faults is more common and the footwall shortcuts are not dominant structures. No significant lateral variations in tectonic shortening are found in the Eastern Cordillera. Finally we emphasize the role of buckle folds in the internal parts of the inversion orogens and give a cautionary note when interpreting these structures in terms of fault-related folding using the well-documented example of the Soapaga fault area.