The Variscan structure of the Herrera unit (Iberian Chain, NE Spain) is characterized by a system of NNW–SSE-striking, east-verging, foreland-dipping thrusts, generated in a thin-skinned context, whose formation was favoured by the presence of two main detachment levels (Precambrian and Silurian shales). During the formation of the thrust system (first phase of deformation, D1), the thrust sheets were deformed internally, mainly by asymmetrical folds with axial surface cleavage and east-verging thrusts; the normal evolution of the thrust system and the emplacement of an underlying thrust sheet resulted in progressive tilting and stacking, thus generating a foreland-dipping thrust system. These structures were folded and tilted by the emplacement of the Datos thrust (second phase of deformation, D2), generating an associated deformation characterized by subvertical folds with axial surface cleavage, with slight east vergence. These structures can obliterate the previous D1 structures in the sector near the Datos thrust. D1 and D2 structures have not been observed in the same outcrops, but the relationship between bedding and cleavage as well as cleavage relationship with other structures allows discrimination between the two cleavage sets. Based on the features of Palaeozoic rocks of the Iberian Range and the Iberian Massif, we support the idea that the Herrera unit belongs to the Cantabrian Zone.
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. ARTICLE HISTORY
By means of fault population analysis (using the right dihedra, y-R diagram and Etchecopar's methods), the states of stress in a number of sites of the Cameros Massff (Northwestern Iberian Chain, Spain) and nearby areas of the Ebro Basin have been determined. These states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the Alpine compression (Oligocene-lower Miocene). The three main compression directions obtained (NW-SE, NNE, NE-SW)can be explained by a single compression field with horizontal o, trending NNE and vertical 02, whose primary trajectories are deflected due to (1) largescale strike-slip faults or lateral ramps of thrusts or (2) frontal ramps of thrusts oblique to the main compressire axis. The reliability of this scheme has been checked by means of a two-dimensional mathematical model based upon the finite element method and applied on a sheet whose theological properties are similar to those of a standard limestone. This model simulates the main features of the regional stress field, and it was constructed after determining the average parameters of the possible external or "primary" field and simulating the major faults that could deflect the stress trajectories. INTRODUCTION Palaeostresses can be determined by means of analysis of fault populations on an outcrop scale. Several methods for this kind of analysis have been performed in the last l0 years [Angelier and Mechler, 1977' Armijo et al., 1982; Carey, 1979; Etchecopar et al., 1981; Sim6n G6mez, 1986]. These methods can be used in order to reconstruct both the orientation and shape of the local deviatoric stress ellipsoid. Using this analysis in a number of fault measurement stations within an area the state of stress responsible for the fault movements in each site can be determined. By integrating all the stress states of the same age we can draw a picture of the palaeostress field in this area during that time. An important condition for the correct determination of the palcostress field in a given area by means of microfault analysis is to determine the presence of geological structures of greater size, such as hectometric to kilometric strike-slip or reverse faults, that can deflect the stress trajectories. In the recent years some research has been made in order to define two-dimensional models of deviations in stress trajectories and magnitudes caused by these fractures in a given stress field [Rispoli, 1981; Xiaohan, 1983].These models have been made using real examples from map to outcrop scales and by means of mathematical simulation, usually based upon the finite element method. From these studies it can be seen that in certain areas, different stress tensors that had been previously interpreted as several primary stress episodes can be considered as deflections in the trajectories of an only regional stress field, caused either by deep or surface faults.In this paper, we make an attempt to draw a picture of the stress field that operated during the ...
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