Introduction: Palaeomagnetism in fold and thrust belts: new perspectives

E. L. PUEYO1*, F. CIFELLI2, A. J. SUSSMAN3 & B. OLIVA-URCIA4 Instituto Geológico y Minero de España, Unidad de Zaragoza c/Manuel Lasala 44, 98, 50006 Zaragoza, Spain Dipartimento Scienze, Università degli Studi di Roma TRE, Largo San Leonardo Murialdo 1, 00146 Rome, Italy Earth and Environmental Sciences Division, MS-D452, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Departamento Geologı́a y Geoquı́mica Facultad de Ciencias, Universidad Autónoma de Madrid, Campus de Cantoblanco C/Francisco Tomás y Valiente, 7, M-06, 6 planta, 28049 Madrid, Spain

Palaeomagnetism, that is, the study of the ancient magnetic field recorded in rocks, is the only vectorial indicator in the Earth sciences that is capable of associating geological bodies with their original location (primary vectors) or with intermediate locations (secondary vectors) during their geological history. For this reason, palaeomagnetism has played a key role in supporting continental drift theory.
Beyond tectonic plate-scale applications, palaeomagnetism has become a fundamental tool for assessing the evolution of mountain ranges owing to its unique potential for quantifying vertical axis rotations (VAR). Since the pioneering applications of authors such as Norris & Black (1961) and Tarling (1969), palaeomagnetism has been applied to problems at a variety of scales in many orogenic systems (e.g. Elredge et al. 1985;Kissel & Laj 1989;Weil & Sussman 2004;Elmore et al. 2012). In particular, palaeomagnetic data have been increasingly used as key quantitative information for determining the timing, distribution and magnitude of vertical axis rotations (Van der Voo & Channell 1980;McCaig & McClelland 1992;Allerton 1998).
Together with structural analyses, palaeomagnetic investigations help to unravel the deformation history of fold and thrust belts, For instance, palaeomagnetism in growth strata has shed much light not only on the kinematics of folding and thrusting but also on the deformation timing such that quantitative geological reconstructions can be accomplished even in four dimensions (Pueyo et al. 2004;Arriagada et al. 2008;Ramón et al. 2012). These approaches reduce the levels of uncertainty of other structural variables, such as shortening and internal deformation.
Overall, this Geological Society Special Publication compiles contributions presenting recent developments in palaeomagnetism, in terms of both methodology and applications of palaeomagnetic studies to fold and thrust belts. The scientific breadth of this volume includes: † compilations and overviews of fold and thrust belts with abundant palaeomagnetic data; † case studies showing how palaeomagnetic data are used in understanding the kinematic evolution of geological structures at different scales; † case studies showing how magnetostratigraphy contributes to the assessment of the magnitude and the timing of vertical axis rotations and deformation rates; † studies of complex tectonic structures, including the integration of palaeomagnetism with anisotropy of magnetic susceptibility (AMS) and/or structural analyses; and † analyses pertaining to data reliability, resolution analysis and error sources.
The volume starts with two papers that present large palaeomagnetic datasets (Fig. 1); the Western Carpathians (Márton et al. 2015) and the Western and Central Mediterranean arcs . In both cases, synthetic and robust palaeomagnetically deduced VARs are used to constrain the kinematic and geodynamic evolution in these regions. The second part of the volume presents a suite of papers focused on different structural and tectonic problems in the Andes, Pyrenees, the Iberian Range, the Alps and Anatolia. Rapalini et al. (2015) show new data focused on unravelling the primary or secondary nature of the Patagonian orocline, Japas et al. (2015) discuss Neogene rotations measured in the Andean Precordillera fold and thrust belt. Oliva-Urcia et al. (2015) present a new magnetostratigraphic section from the southwestern portion of the Pyrenean main thrust that allows for both the refinement of the chronostratigraphy of the region and dating of primary folding and thrusting events. The Pico del Aguila anticline, in the central part of the basal thrust, is the target of the paper by Anastasio et al. (2015), which uses AMS and published chronostratigraphy to decipher fold kinematics. The work by Cardello et al. (2015) in the Helvetic Alps and that by Valcárcel et al. (2015) in the Southern Iberian Range share some common features in that they represent detailed studies of individual structures and aim to obtain wellconstrained kinematic models. These papers both integrate their palaeomagnetic data with structural, tectonic and AMS data, and bracket the limitations of their palaeomagnetic analyses. Cinku et al. (2015) investigate the palaeomagnetic rotations that occurred in the Kırşehir block during the closure of the remnant of the northern branch of the Neotethys Ocean, in the İzmir-Ankara-Erzincan suture zone, from Late Cretaceous to Middle Eocene times.
The third section of this volume is devoted to new approaches for the application of palaeomagnetism to fold and thrust belts. The paper by Garcés et al. (2015) introduces a new concept for the restoration of remagnetizations to the ancient reference system in the Central Pyrenees that takes into account the geometry of unconformities that formed during the extensional-basin inversion instead of the bedding plane. Villalaín et al. (2015) present the reconstruction of highly subsident extensional basins that, in turn, reset the palaeomagnetic signal; these basins were inverted during Tertiary development of a fold and thrust belt. In this case, the geometric analysis of the remagnetized directions has allowed for reconstruction of the geometry of those basins at the time interval of the remagnetization.
While quantitative use of palaeomagnetically deduced rotations is scarce, some researchers have addressed this problem in recent years. Palinspastic restorations of orogens can be much more precise if palaeomagnetic vectors are combined with structural features in a numerical approach. This was first applied in the Bolivian orocline by Arriagada et al. (2008). In a similar way, the quantification of errors in shortening estimates deduced from restored and balanced sections in fold and thrust belts has been recently introduced (e.g. Pueyo et al. 2004;Sussman et al. 2012). Here, Ramón et al. (2015a) propose a new 3-D restoration method that combines the use of palaeomagnetic data and a parametric approach for the definition and processing of the structural surfaces. These authors demonstrate that using palaeomagnetism clearly improves the restoration result of complex geological settings. Finally, Pueyo et al. (2016) present an overview of the potential sources of error in palaeomagnetic studies applied to deformed regions, review field, laboratory and processing techniques and propose an extension of the classic reliability criteria established by Van der Voo (1990).

Future and challenges of palaeomagnetic studies in fold and thrust belts
The application of palaeomagnetism to fold and thrust belts still has many challenges that need to be met in the future: † Data resolution -resolution studies on palaeomagnetic data can be found in the literature specifically applied to tectonic problems (Demarest 1983;Bazhenov 1988). However, the definition of palaeomagnetic means at the dip-domain scale (sensu Suppe 1985;Groshong 1999) along thrust fronts is still required, perhaps with an approach similar to that of Deenen et al. (2011) but more focused on the geometry of the fold and thrust belts. † Reliability -many potential sources of error can affect palaeomagnetic datasets in fold and thrust belts (see Pueyo et al. 2016). Apart from other processing and laboratory problems, the effect of internal deformation, non-resolved overlapping of components or errors in the restoration of complex settings may limit the value of the palaeomagnetic vectors as a robust kinematic indicator. In this sense, some advances on demonstrating the quality and reliability of the palaeomagnetic data have been made (MacDonald 1980;Chan 1988;Sellés-Martínez 1988;Weinberger et al. 1995;Rodríguez-Pintó et al. 2011). However, numerical modelling of errors and the establishment of specific procedures are still to be set. † Age constraining palaeomagnetic vectors -constraining the age of the magnetization in palaeomagnetic studies is a key variable and it is usually done via the fold test (Graham 1949;McElhinny 1964), a powerful tool that is rarely used for other classic structural indicators. The statistical significance of the fold test is mature (e.g. Tauxe & Watson 1994;McFadden 1998;Enkin 2003), but mathematical approaches to the fold test do not consider phenomena such as folding kinematics. While some authors have already considered this problem (Cairanne et al. 2002;Delaunay et al. 2002;Waldhör & Appel 2006;Villalaín et al. 2015), further advances in this regard need to be accomplished. † Understanding the architecture of rotated areas -understanding of the spatial and temporal variability of rotational patterns in fold and thrust belts is still in its infancy. In this sense, integration of classic determination of VARs and magnetostratigraphic studies may shed much more light in future on these systems in 4-D. † Quantitative implications -palaeomagnetic results should play a more active role in the 2-D/3-D shortening estimation (cross-section, map view, 3-D restorations, etc.), since such data account for the out-of-plane movements biasing classic approaches (e.g. McCaig & McClelland 1992;Pueyo et al. 2004;Sussman et al. 2012).
In this sense, palaeomagnetic vectors, together with the bedding plane, are the only reliable 3-D reference frame to be used in 3-D restoration and validation techniques, since they are accurately known in both the deformed and undeformed stages. Although some recent works have demonstrated this (Ramón et al. 2012(Ramón et al. , 2015a, many challenges remain. We encourage future investigators to meet such challenges.
We acknowledge the reviewers who have helped to complete this Special Publication as well as the editorial work done by the Geological Society staff. We are particularly thankful to Massimo Mattei for writing the foreword to this volume.