Overlapped paleomagnetic vectors and fold geometry: A case study in the Balzes anticline (Southern Pyrenees) A. Rodriguez-Pintó a,b,⇑ , E.L. Pueyo b , A. Barnolas b , A. Pocoví a , B. Oliva-Urcia a , M.J. Ramón b a Geodinámica Interna, Ciencias de la Tierra, U. de Zaragoza, Spain b Instituto Geológico y Minero de España, Spain article info Article history: Received 28 October 2011 Received in revised form 7 June 2012 Accepted 7 October 2012 Available online 23 October 2012 Edited by Chris Jones Keywords: Overlapping Primary and secondary paleomagnetic record Declination and inclination errors External Sierras Eocene Guara limestones abstract When two or more magnetic components share the same temperature and/or coercivity spectrum in a rock, the paleomagnetic components are said to be overlapping. Failure to separately identify these com- ponents in the laboratory may lead to substantial errors in the declination and inclination of the primary component (including apparent changes in polarity and with a marked impact in stability tests). Recently, numerical models have been proposed to understand this problem in relation to fold geometry; these allow the expected declination and inclination errors at different locations across the fold surface to be identified and estimated. In this paper, we present a real case study of overlapped paleomagnetic direc- tions from the northern part of the Balzes anticline (South Central Pyrenees). A new dataset is established based on 206 paleomagnetic samples taken in a wide variety of structural locations. Several facies includ- ing limestones, siltstones and marls from Eocene marine and deltaic sediments were studied. This dataset shows the occurrence of paleomagnetic overlapping controlled by lithology. That is, most paleomagnetic directions derived from marls show hardly any significant declination and inclination errors and they are consistent in all structural positions. In contrast, in most of the limestones there is strong overlapping and significant errors related to the fold geometry. The dataset also enables the evaluation of errors as a func- tion of structural position. The numerical model for Balzes anticline overlapped directions shows quali- fication and quantification of errors in all possible structural positions and the effect of overlaps on the fold test is also analyzed. Through the use of numerical modeling we propose a new tool to deal with data with a strong overlapping of components as in this case study. The identification and quantification of the overlapping ratio potentially enables data to be filtered to eliminate such errors and also allows suitable structural positions where error is minimized to be identified. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction When we demagnetize a rock in the laboratory aiming to char- acterize the paleomagnetic components, it is necessary to confirm whether the components have been correctly separated. The simultaneous removal of two or more paleomagnetic components (overlapping) may occur during laboratory procedures and this hinders the proper identification of the characteristic component. In turn, this reduces the reliability of the data (Van der Voo, 1990) leading to important declination and inclination errors, apparent changes in polarity and also geologically meaningless sta- bility test results (Pueyo, 2010; Rodriguez-Pintó et al., 2011). The problem of overlapping of components was first described many years ago (Halls, 1976), and several techniques such as the convergence of remagnetization circles have been proposed as a potential solution (Bailey and Halls, 1984; Schmidt, 1985 and McF- adden and McElhinny, 1988). Further, cases of complete overlap- ping (Dinarès-Turell and McClelland, 1991), and modeling of partially overlapped directions (Halim et al., 1996) have also been analyzed. On the other hand, very little work has been done to ex- plore in detail the nature and the effects of overlapped components in deformed areas. Recently, Rodriguez-Pintó et al. (2011) highlighted the impor- tant influence of fold geometry on the magnitude of the errors from overlapped directions. Let us consider a simple example illus- trating this problem (Fig. 1). Imagine a horizontal flat bed with a primary magnetic vector (we consider both possible polarities within the meridian plane). Later, this bed is folded. Then, a sec- ondary component is acquired and the primary component is over- lapped with that second one (normal and reverse polarities behave differently), the result is an intermediate component in both cases. Significant errors (both declination and inclination) will be found if the vectors are restored to the original coordinate system. The sys- tematic mathematical modeling of overlapped vectors in folds 0031-9201/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pepi.2012.10.005 ⇑ Corresponding author at: Geodinámica Interna, Ciencias de la Tierra, U. de Zaragoza, Spain. E-mail addresses: unaim@igme.es, adrianar@unizar.es (A. Rodriguez-Pintó). Physics of the Earth and Planetary Interiors 215 (2013) 43–57 Contents lists available at SciVerse ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi