Introduction
The geological sciences have been connected with the visual arts from their infancy as a discipline. Geological studies use visual information to complement text in order to convey complex ideas about solid Earth processes. Until the mid-20th century, illustrations in scientific papers in the natural sciences consisted of sketches, line drawings, or other art forms, such as watercolours or oil paintings (Merriam, 2009). Critical observations such as field relationships, which underpin many important geological interpretations, were presented in this fashion and thus had an inherent subjectivity, or bias. Although a geologist's field notebook is still a collection of sketches and drawings, in modern papers art work has been replaced by digital photographs and computer generated figures. Nonetheless, even in modern papers, the vast majority of figures consist of static representations (sequences of discrete events) that do not accurately portray dynamic geological processes in time and space. However, owing to the rapid development of computer animation technologies, it is now possible to construct dynamic illustrations in the form of animations, videos or 3D representations that provide robust renditions of the dynamic processes that have shaped the Earth’s evolution.
In this paper, we use animations to illustrate the development of curved orogenic belts (in plan view) (e.g. Marshak, 2004; van der Voo, 2004; Sussman and Weil, 2004; Weil and Sussman, 2004). The evolution of curved orogens, which are a ubiquitous feature of recent and ancient mountain belts, has been debated since the 19th century (Suess, 1885; Wegener, 1929; Wilson 1949; Carey, 1955, 1958; Eldredge et al., 1985). Following the kinematic classification of Weil and Sussman (2004), which relies on Carey’s original definition (1955), arcuate orogenic belts that formed by buckling of an originally linear orogen about a vertical axis of rotation are classified as oroclines. Oroclines are amongst the largest geological structures on Earth and have formed from Archean to recent times.
The Ibero-Armorican orocline (Fig. 1) is a curved orogenic system characterized by a 180° bend of the Variscan structural grain (Weil et al., 2001). It was referred to as the "Asturian Knee" by Eduard Suess in the late 19th century in “Das Antlitz der Erde (1885-1908)” (translated into English in 1909). Suess recognized the bend in northern Iberia of structures that are now attributed to the Carboniferous collision between Laurussia and Gondwana during Pangea amalgamation. Since Suess' initial observations, the Ibero-Armorican orocline has been the object of many studies (Brun and Burg, 1982; Dias and Ribeiro, 1995), especially at its core (e.g. Julivert, 1971; Julivert and Arboleya, 1984, 1985; Pérez-Estaún et al., 1988; Weil et al., 2000, 2002; Weil, 2006). The aforementioned studies have attempted to decipher the curved mountain belt kinematics, and a wealth of different hypotheses, spanning the entire classification of Weil and Sussman (2004), have been proposed: (1) a primary arc inherited from a Neoproterozoic embayment (Lefort, 1979); (2) a progressive arc resulting from indentation of a point-shaped block situated either in Gondwana (e.g. Matte and Ribeiro, 1975; Brun and Burg, 1982; Dias and Ribeiro, 1995) or in Avalonia (Simancas et al., 2009), (3) an oblique collision producing a non-cylindrical orogen (Martínez-Catalán, 1990), (4) a thin-skinned origin produced by a progressive change in the transport direction of the thrust units similar to a photographic iris, (Pérez-Estaún et al., 1988), and more recently (5) an orocline formed by the rotation around a vertical axis of an originally linear orogen (Weil, 2006; Weil et al., 2000; 2010; Gutiérrez-Alonso, 2004, 2008, Martínez-Catalán, 2011; Gutiérrez-Alonso et al., 2012). The latter is the only proposed mechanism that conforms to the definition of a true orocline (Weil and Sussman, 2004).
Figure 1. Ibero-Armorican orocline
Tectonostratigraphic location of the Ibero-Armorican orocline (after Weil et al. 2010, and Pastor-Galán, 2011) showing the orocline trace and the main structures related to its formation.
The Ibero-Armorican orocline is a central component of the Western European Variscan Belt, a complex continental-scale orogen (1000 km wide and 8000 km long; Fig. 1) that formed through a series of protracted collisional events extending from 420 to 300 Ma (e.g., Franke, 2006, Martínez Catalán et al., 2007 and references therein). Variscan deformation represents the closing of at least two, and possibly four, oceans between Laurentia, Baltica, Gondwana, and several micro-continents during the late Paleozoic amalgamation of the Pangea supercontinent (e.g., Van Staal et al., 1998; Martínez-Catalán et al., 1997, 2007; Matte, 2001). The Ibero-Armorican orocline is characterized by arcuate structural trends that trace an arc from Brittany across the Cantabrian Sea into western Iberia, where it is truncated by the Cenozoic Betics Alpine front in southern Spain. New interpretations, based on the ideas of du Toit (1937), consider the Ibero-Armorican orocline as part of a coupled bend together with the southern Central-Iberian arc (Martínez-Catalán, 2011; Shaw et al., 2012)
Ries and Shackleton (1976) divided the Ibero-Armorican orocline into three structural zones based on a tangential-longitudinal strain model: the outer arc that underwent extension, the inner arc that underwent compression, and a narrow (ca. 10 km wide) neutral zone characterized by low strain. In their model, stretching parallel to the outer arc increases away from the core (Ries and Shackleton, 1976), whereas shortening in the inner arc increases towards the core (Julivert and Marcos, 1973; Pastor-Galán et al., 2012a). Outer arc extension was accommodated by dextral strike-slip faulting in the upper crust, and ductile elongation in the lower crust (Gutiérrez-Alonso et al., 2004). At the core of the Ibero-Armorican orocline is the Cantabrian Arc (Weil et al., 2001), which consists of crust considered to have originated along southern (Gondwanan) margin of the Rheic Ocean during the Paleozoic (e.g., Martínez-Catalán, 2002; Robardet, 2003; Murphy et al., 2006).
In this paper we use compilations of data from recent studies that constrain the timing, kinematics, lithospheric geometry and the possible causes of oroclinal bending in the Ibero-Armorican orocline. These constraints are used to develop a virtual tour of the orocline.