Transition from extensional to compressional magnetic fabrics in the Cretaceous Cabuérniga basin (North Spain) Belén Oliva-Urcia a, 1 , Teresa Román-Berdiel a, * , Antonio M. Casas a , M. Felicidad Bógalo b , M. Cinta Osácar a , Cristina García-Lasanta a a Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain b Departamento de Física, Escuela Politécnica Superior, Universidad de Burgos, Avda Cantabria s/n, 09006 Burgos, Spain article info Article history: Received 15 February 2012 Received in revised form 13 August 2012 Accepted 3 September 2012 Available online 26 September 2012 Keywords: Anisotropy of magnetic susceptibility Syn-sedimentary faults Inverted sedimentary basin BasqueeCantabrian basin abstract The transition between extensional and compressional-driven magnetic fabrics in sedimentary rocks is explored in this paper through the study of an example of the BasqueeCantabrian basin. In the area where extensional structures prevail and no superimposed deformation is observed, except for gentle large-scale folds, the magnetic fabric is interpreted as extensional, in consistency with mesostructural (tension gashes) and macrostructural (large-scale faults) data. Compressional tectonic fabrics are unequivocally interpreted in the area with cleavage development related to the buttressing of the syn-rift sequence against faults located near the northern basin margin. In this area, k max is oriented according either to the intersection lineation or the dip direction of cleavage planes. In the area located in-between, where no macroscopic evidence of either compression or extension exist, there is a transitional fabric between compressional (resulting from the modification during inversion of a previous sedimentary or extensional fabric) and extensional (inherited from the extensional stage) magnetic fabrics that correlate with subtle evidences at the microscopic scale (pressure shadows, deformation and re-orientation of nodules). Therefore, the magnetic fabric is revealed as an exceptionally sensitive marker of deformation in sedimentary rocks. This transition in the magnetic fabric occurs within a length of 6.25 km along the cross-section that correlates with a thickness of 200 m of the stratigraphic pile. These results indicate that even in the absence of clear structural markers of compressional deformation, extensional magnetic fabrics can be only interpreted when there is a minimum separation (in the vertical or the horizontal) to the cleavage front. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In the past decades, many works have aimed to interpret the anisotropy of magnetic susceptibility (AMS) in sedimentary rocks in order to decipher their deformational history. However, the complexity of AMS results and the different parameters that influence them make sometimes difficult a correlation between the magnetic fabric of a rock and its deformational history. In the case of inverted basins, this complexity is even higher, since there can be more than one parameter involved, as it is the influence of the intensity of both the extensional and the compressional deforma- tion. In a sedimentary rock that has been deformed at low temperature (less than 150  C), Graham (1966) proposed an early record of the magnetic fabric. He also assumed that a change from sedimentary to tectonic fabric is easier within unconsolidated rocks. In the case of the analyses of AMS in compressional settings and fold-and-thrust belts, several key works have been developed (Hrouda and Janak, 1976; Borradaile and Tarling, 1981; Kliegfield et al., 1981; Rochette and Vialon, 1984; Kissel et al., 1986; Lowrie and Hirt, 1987; Lee et al., 1990; Averbuch et al., 1992; Sagnotti et al., 1998; Parés et al., 1999; Saint-Bézar et al., 2002; Parés and Van der Pluijm, 2004; Robion et al., 2007; Oliva-Urcia et al., 2009; Pueyo Anchuela et al., 2010). In those cases, the magnetic fabric is directly related to the intensity of deformation, and interferences to the straightforward interpretation of the magnetic fabric come from composite fabrics (magnetic fabrics from rocks with multiple mineral-preferred orientations may have ellipsoids whose shape and orientation arise from the addition of two or more fabric components, Housen et al., 1993; Aubourg et al., 1995; Aranguren et al., 1996; Debacker et al., 2004), or relatively complex magnetic mineralogy (Larrasoaña et al., 2004; Oliva-Urcia et al., 2009). In the * Corresponding author. Tel.: þ34876553469. E-mail address: mtdjrb@unizar.es (T. Román-Berdiel). 1 Present address: Instituto Pirenaico de Ecología, CSIC, Avda. Montañana 1005, 50059 Zaragoza, Spain. Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2012.09.001 Journal of Structural Geology 46 (2013) 220e234