Abstract
Fold-and-thrust (FAT) belts occur worldwide and have long been the focus of research of structural geologists who have devised a variety of techniques to image, characterize and model their main structural features. This introductory chapter reviews the principal geological features of FAT belts formed in different settings, emphasizing aspects related to their kinematic evolution and structural styles. Despite great advances, challenges remain, particularly in the understanding of the spatial and temporal evolution (4D) of FAT belts and their controlling factors. These research efforts are being assisted by the growing availability to researchers of relatively new tools to collect field data, high quality 3D seismic data, and computer and laboratory modelling tools. This volume includes technical papers presented in the conference ‘International Meeting of Young Researchers in Structural Geology and Tectonics (YORSGET-08)’ held in Oviedo (Spain), together with other papers on the same theme. These papers deal with FAT belts in different parts of the world and cover a broad range of different aspects, from detailed structural analysis of single structures to regional issues, and from studies based on classical field structural geology to modelling.
Fold-and-thrust belts, or FAT belts for short, have a worldwide distribution (see surveys in Nemcok et al. 2005; Cooper 2007), have formed in all eras of geological time, and are widely recognized as the most common mode in which the crust accommodates shortening. Generations of geologists have struggled to understand their origin, geometry, evolution and the control exerted on them by different structural, tectonic, stratigraphic and petrological parameters (see for instance monographic books such as McClay & Price 1981; MacQueen & Leckie 1992; McClay 1992a, 1994; Mitra & Fisher 1992; Nemcok et al. 2005; Lacombe et al. 2007).
A number of factors have contributed to our greater understanding of the structure of FAT belts; some of them derive from detailed analyses of prevalent patterns of faulting and folding and their related structural features, whereas others come from other earth science disciplines. Many important concepts were developed and applied to FAT belts as early as the 1970s, or even before, and have been subsequently modified. Since a comprehensive historical review of research progress is almost impossible here, only a few important landmarks are briefly described, starting with structural methods from small- to large-scale and following with techniques applied to these regions furnished by related disciplines such as petroleum geology, geophysics, geomorphology, petrology and sedimentology.
The application of classical methods of studying deformation mechanisms in rocks and of quantifying geological strain (e.g. Ramsay 1967; Durney & Ramsay 1973; Fry 1979a, b) to FAT belts has led to greater understanding of deformation on a small scale and has given insights into the mechanisms responsible for the development of individual structures such as folds and faults. A number of techniques were initially devised to analyze the structure of FAT belts, for example: (a) balanced and restored cross-sections (e.g. Dahlstrom 1969; Mitra & Namson 1989); (b) construction of geological cross-sections using techniques such as depth to detachment estimations (e.g. Chamberlin 1910; Mitra & Namson 1989) and the Busk and dip domain methods (Busk 1929; Suppe 1985, respectively); and (c) advances in the understanding of quantitative relationships between thrusts and their related folds and the rules to help to constrain the structural geometries (Suppe 1983; Jamison 1987; Mitra 1990; Suppe & Medwedeff 1990). These techniques have enabled construction and validation of admissible and retro-deformable geological sections across single structures, structural units and entire FAT belts. They have also provided guidelines for seismic interpretation, and proved to be essential tools in structural interpretation widely applied elsewhere, particularly in areas of scarce and/or poor quality data. The study of accretionary wedges played a key role in the development of the Coulomb wedge theory (e.g. Davis et al. 1983; Dahlen et al. 1984). This theory, which incorporates the best of the gravity-driven v. surface force-driven motion hypotheses, was a major step forward in the understanding of the kinematics and dynamics of FAT belts because it allowed the inclusion of the effects of gravity and topography and provided answers to the problems of structure sequencing in these regions.
The contribution of geophysics has been hugely beneficial to FAT belt research. In particular, since many belts contain major oil and gas accumulations in structural traps, the geophysical explorations carried out by the hydrocarbon industry have supplied an enormous amount of subsurface data. From the early seismic experiments in the late 1920s in the Zagros FAT belt and in Oklahoma until the present day, seismic imaging has furnished additional constraints on the geological interpretation of the deep geometry of structures, which had previously relied on geological data collected at the surface. Seismic has, in some cases, also allowed mapping of subsurface structures that are decoupled from their surface structural expression. Without forgetting the important contribution made by some other branches of geophysics, such as gravimetry for constraining of the deep structural configuration, or palaeomagnetism in the understanding of rotations around vertical axes, one of the most important boons to mapping of FAT belts has been the development of 3D seismic survey methods. 3D seismic data volumes provide a continuous and more accurate image of the subsurface than can be obtained with 2D seismic methods (Hart 1999). Aided by the development of structural tools, for example, Geosec 3D, 3D Move, Lithotect and Gocad software packages to visualize, characterize and model the 3D structure of folds and thrusts, 3D seismic is starting to supply answers, particularly with respect to the questions of the geometry of structures along strike and whether structures evolve in a self-similar fashion, that is, whether observed spatial variations in fold geometry reflect temporal geometric evolution (Elliott 1976; Means 1976), or whether they evolve through different structural forms.
The contributions of tectonic geomorphology and palaeo-seismology of mountain fronts (Bull 2007 and references therein) have helped to determine the development of structures through the study of the landscape evolution and offset/position of landforms. Furthermore, P–T–t data (e.g. Spear & Selverstone 1983; England & Thompson 1984; Thompson & England 1984) collected from the interiors of FAT belts have been employed to interpret the burial, thermal and subsequent uplift histories operating during emplacement of thrust sheets. In addition, geochronology studies using isotopes, magnetostratigraphy, fission tracks, cosmogenic nuclides, and so on (e.g. Vance & Müller 2003; Allègre 2008; Lisker et al. 2009; Dunai 2010) have enabled estimates to be derived for both the timing and the short- and long-term rates of motion of single structures and larger-scale tectonic processes. These have demonstrated, for instance, that synchronous movement of different thrusts is a significant feature in the kinematic evolution of FAT belts, which, in turn, has important implications for thrust sequences and the balancing/restoration of cross-sections and forward modelling of thrust terrains. A great deal of recent research has been focused on the dynamic interaction between FAT belt evolution and surficial processes such as syn-kinematic sedimentation, erosion, uplift and subsidence from lithosphere-scale (e.g. Beaumont et al. 1992; Kooi & Beaumont 1996) to individual structure-scale (e.g. Riba 1976; Suppe et al. 1992; Hardy & Poblet 1994). Thus, whereas the geometry of the syn-tectonic sediments and basins within the FAT belts is influenced by development of structures, the latter in turn are themselves strongly affected by sedimentation and erosion. In short, studies based on tectonic geomorphology and palaeoseismology, P–T–t paths, geochronology and surficial processes in FAT belts sparked an increasing interest in quantitative modelling of the evolution of these belts at various scales.
Types of FAT belts
Summarizing the main features of FAT belts is not an easy task because they are remarkably diverse. Although they exhibit a number of common characteristics, no single map or cross-section can provide a universal portrayal of a FAT belt because many parameters exert an important influence on them (see, for instance, Fitz-Diaz et al. 2011). These factors include the plate tectonics setting in which they developed, whether only the cover or both the cover and basement rocks are involved in the structures, the role of mechanical stratigraphy, the presence, distribution and thickness of a salt/shale detachment, the occurrence of syn-orogenic erosion and deposition leading to burial, the depth to detachment and the effective elastic thickness of the lithosphere (e.g. Royden 1993), the occurrence of pre-existing basement structures, the timing and deformation rates, and so on. All of these factors are important, though a discussion of their effect is beyond the scope of this paper.
FAT belts are typical regions in most orogenic belts controlled by compressional tectonics and have been documented in environments resulting from plate convergence such as those formed at plate collision boundaries (e.g. Himalayas, Apennines), at plate subduction boundaries (e.g. Andes, Zagros) and at intraplate locations influenced by neighbour plate convergence (e.g. Yinshan belt and Western Ordos belt in China). Accretionary prisms are a special type of FAT belt developed in subduction zones such as the Barbados prism. FAT belts known as toe thrust belts develop in deep water at the leading edge of large-scale gravitationally driven sedimentary prisms on continental margins such as those in the Gulf of Mexico and offshore Brazil, or in thick delta complexes such as the Niger delta or in deltas in northwest Borneo. FAT belts may also form due to transpression at oblique plate boundaries, where the overall plate convergence involves strike‐slip components of motion, such as part of the southern Carpathians and the Central-South Trinidad thrust belt, or along large-scale transform or transcurrent faults at restraining bends or oversteps and at fault splays, such as the Transverse Ranges of the San Andreas transform fault and perhaps the Palmyride belt in Syria in relationship to the Dead Sea transform fault.
Foreland FAT belts and crystalline thrusts
One of the most well-known types of FAT belts are the foreland FAT belts that are 10 to 1000 kilometres wide and constitute the external zones of orogens (Figs 1 & 2). The Canadian Rocky Mountains zone (Price 1981) is one of the best examples of a foreland FAT belt. These FAT belts typically involve an unmetamorphosed or low-grade metamorphic sedimentary cover, whose thickness decreases towards the foreland (interior of a continent), deposited over a metamorphic/igneous basement, whose top usually dips towards the hinterland (ancient sea), that constitute the passive continental margin. They usually exhibit a wedge geometry in cross-sectional view and this shape is maintained throughout the deformational history of the belt. Deformation is confined to the uppermost part of the crust bounded by a sole thrust (basal detachment) that dips gently towards the hinterland and rises stratigraphically upwards towards the foreland; the sole thrust may be blind or may reach the topographic surface in some cases (Figs 1, 2, 3). There may be various detachments above the sole thrust that tend to rise up towards the foreland, but all of them ultimately branch off from the main sole fault. Many thrusts surfaces are asymptotic at depth, forming imbricate thrust systems in which thrust surfaces may maintain an approximately regular spacing. However, other types of thrust systems such as duplex, antiformal stacks, triangle zones and intercutaneous wedges (Butler 1982; McClay 1992b) are common as well (Figs 1, 2, 3, 4). The loading effect of a FAT belt creates accommodation space in the foreland basin developed in front of the belt, which is usually filled in with debris eroded from the mountain belt and in some cases affected by the prograding thrust system which carries forward the older foreland basin. This uplifted, shortened and transported basin is known as a piggy-back basin.
Balanced geological section across the Rocky Mountains Foothills, Cordilleran FAT belt. Note the sub-vertical to overturned thrusts in the southwestern part of the cross-section, probably due to underlying folding and thrusting (modified from Price 1981).
(a) Simplified structural map of the foreland FAT belt of the Variscan Orogen in northwest Iberian Peninsula (Cantabrian Zone), which exhibits an orocline geometry (Ibero-Armorican or Asturian arc). Dominant thrusting style in the SW portion of the belt changes to a dominant folding style in the NW part of the belt. (b) Balanced geological section across the Cantabrian Zone. The cross-section line is located in Figure 2a. Note the sub-vertical to overturned thrusts in the east part of the cross-section probably due to thrust sheet stacking. Both figures are modified from Pérez-Estaún et al. (1988).
Geological section across the south end of one of the largest structures within the leading edge of the southern Canadian Cordillera, called the Highwood structure, SW Alberta, which is relayed by the Turner Valley structure to the north (modified from MacKay 1996). The Highwood structure is interpreted as a sort of intracutaneous wedge including imbricate thrust systems, an antiformal stack, duplexes and backthrusts.
Folded repetitions of Devonian limestones caused by thrust sheet stacking in the Somiedo nappe (Juan Luis Alonso, pers. com.), foreland FAT belt, Variscan orogen, NW Iberian Peninsula (Cantabrian Zone).
Some FAT belts include a slate belt (Hobbs et al. 1976; Twiss & Moores 1992) such as that of Wales (e.g. Smith & George 1961), which mainly consists of a monotonous series of relatively unfossiliferous deep-water sediments, such as shales and slates. These sediments were deposited in relatively internal positions of the belt, such as the edge of passive continental margins or offshore volcanic environments. Slate belts are narrow, elongate features parallel to the orogenic belt. The stratigraphy and structure of slate belts is usually difficult to reconstruct due to poor exposure conditions, the lack of fossils and stratigraphic markers as well as the metamorphic conditions up to low grade. The low competence contrasts within these sedimentary piles results in tight to isoclinal flattened folds with thickened hinges which can possess near-similar geometry. They exhibit variably inclined axial surfaces, from upright to recumbent. They include parasitic folds and can form fold nappes, with more than one fold generation present in some cases. Pervasive continuous foliation such as a slaty cleavage and phyllitic foliations, overprinted by later spaced foliations in some cases accompanied by folding, and by kink bands, become dominant. Although thrusts are present, the lack of marker beds makes them difficult to map.
On the thickened side of the orogenic belts in the transition to the crystalline core of a mountain chain, metamorphic rocks, either as basement slices or as progressively metamorphosed sedimentary rocks, appear. The thrusts involving metamorphic and/or igneous rocks are usually known as crystalline thrusts (Hatcher 1995) and have been known for many decades in the Alps, Appalachians, Scandinavian and British Caledonides. Whereas the thrusts closer to the undeformed regions form under brittle conditions, those close to the metamorphic core develop under ductile to brittle conditions. Thus, the thrust surface may be initiated due to a local ductile behaviour, whereas the thrust sheet may be transported by brittle translation. In contrast to those FAT belts developed in regions occupied by sedimentary rocks where bedding is the main structural discontinuity, appropriately oriented pre-existing faults, well-developed foliations and ductile-brittle transitions are the main mechanical weaknesses that allow the propagation of crystalline thrusts. Crystalline thrust sheets are among the largest structures in orogenic belts and may form as large slabs or on the overturned limbs between recumbent folds because of continued transport (fold nappes). Usually, multiple generations of folds, accompanied by foliations, produce a variety of fold interference structures. Thus, apart from thrusts, a frequent sequence of deformations observed in these regions includes recumbent isoclinal folds refolded by upright, more open folds, and finally deformed by smaller-scale kink bands.
Accretionary prisms
Accretionary prisms (e.g. Casey Moore & Silver 1987; von Huene & Scholl 1991) are the main locus of deformation in subduction zones, where the rock assemblages are mechanically scraped off the downgoing oceanic slab and accreted to the seaward edge of the upper advancing plate forming thrust sheets (Fig. 5). Many earthquakes that take place in subduction zones display thrust fault focal mechanisms. The Nankai accretionary prism (Morgan & Karig 1995) is a classical example of this type of belt. Because they are composed of heterogeneous material, the internal structure of the prism is highly variable. Accretionary prisms have a wedge shape in cross-sectional view and their fronts are usually scalloped in map view. The bulk geometry of the prism and its detailed structure are strongly controlled by the thickness of the sedimentary pile of the subducting oceanic plate. The wedge is underlain by a detachment that ramps up to progressively shallower levels towards the trench, and propagates seaward decoupling mass from the downgoing slab that accretes to the overriding slab usually in the form of duplex (tectonic underplating). While this happens, the upper part of the trench fill or offscrapped sequence is incorporated in the accretionary prism at the toe of the wedge (frontal accretion). Seaward of the emergent frontal thrust, blind thrusts emanate upwards from the detachment level while the detachment propagates leading to an imbricate thrust system with related folds. Sediments deposited in the synclinal troughs form piggy-back basins that usually evolve into asymmetrical tilted basins due to the general wedge tilting and may become incorporated into the prism as a result of thrust activity. In addition, deformation may reach the inner boundary of the accretionary prism leading to folding and backthrusting (rear accretion). Large-scale long-term margin subsidence observed in many wedges requires thinning of the upper plate, and since sedimentation continued during subsidence, erosion along the upper plate's base must have been occurred (subduction erosion). The processes that caused the erosion are still being discussed (see von Huene et al. 2004 for a summary): hydrofracturing of the upper plate due to elevated pore-fluid pressure or physical abrasion caused by horst and graben entering the trench axis. Accretionary prisms exhibit abundant small-scale folds, cleavages, boudins and veins. They usually include mud or serpentinite diapirs in sediment-dominated or igneous-basement prisms respectively. Thrust systems are often strongly affected by the collision of seamounts or fracture zones. Lateral faults commonly cut across the prisms and faulting of the lower lithospheric plate sometimes reflects pre-existing structure and may have some effect on the structure of the upper plate. Blueschists may form at the base of large accretionary prisms if pressures are high due to substantial lithostatic overburden and temperatures are low due to the relatively cold downgoing slab. However, if the subducting slab is not cool, then blueschists do not form and the wet sediment may melt creating small granitic intrusions. One of the main differences between the conventional FAT belts and those developed in accretionary prisms is that in the latter there is a lack of rheological contrast among the different sedimentary beds and the generally weak, water-rich sediments of the accretionary prism become pervasively disrupted. Such chaotic deposits are called melanges.
(a) Line drawing of a seismic profile through the Barbados accretionary prism in which the detachment truncates some underlying horizons (modified from Westbrook et al. 1988). (b) Seismic profile showing the toe of the seaward frontal portion of the accretionary wedge in the North Iberian Atlantic margin, Bay of Biscay, developed as the western prolongation of the Pyrenees during Tertiary times (data courtesy of F. J. A. Pulgar and Marconi team, from Fernández-Viejo et al. in press). The section displays growth ramp anticlines and synclines including Mesozoic and Tertiary sediments deposited before, during (growth strata) and after fold development.
Toe thrust belts
Toe thrust belts (e.g. Worrall & Snelson 1989; Cobbold et al. 1995) exhibit distinctive features. Unlike other FAT belts, they do not require lithospheric shortening, their deformation and transport are achieved entirely by gravity. They are generally detached on salt or overpressured shales and are, therefore, unrelated to the basement. In some cases, the down-slope advance of the glide complex is balanced by shortening in the frontal, lower portion located on the flat basin floor and by extension in the rear, upper part located on the inclined basin margin (Fig. 6). However in other cases the ductile substrate migrates forwards under the differential load to give extension on the delta top and contraction at the delta toe (Fig. 7). The contractional structures developed include folds and thrusts as well as structures involving salt or shale such as tongues, wedges and canopies, and may accommodate significant amounts of shortening (100 km in the case of the Campos Basin, Brazil; Demercian et al. 1993) and may have travelled considerable distances (60–160 km in the case of the Campos Basin; Nemcok et al. 2005). These FAT belts are controlled by the interaction of several parameters such as the width and dip of the basin margin, the distribution, thickness and rheology of the detachment horizon, the temporal and spatial variation of the sedimentary loading, and the occurrence of barriers to the gliding movement such as seamounts or tilted fault blocks. The main cause of the instability is the strongly reduced basal traction caused by an extremely weak layer at the base of the wedge.
Line drawing of a seismic profile through the offshore Eastern Venezuela basin deformed during Neogene times due to gravity collapse superimposed on a Mesozoic passive margin. Normal faulting and associated rollover anticlines in the rear part of the complex are balanced by thrusting and related folding in the frontal portion (modified from Di Croce 1995).
(a) Top view and (b) cross-sectional view of an experimental sandbox model designed to simulate differential sedimentary loading in a progradational delta system developed on top of a ductile substrate. The structures developed during delta progradation are delta-top extensional faults giving rise to a graben and a FAT belt at the foot of the delta slope (modified from McClay et al. 2000).
Structural styles and evolution of FAT belts
Relationships between FAT belts and basement-involved belts
The style of deformation of FAT belts (see for instance the Canadian Rocky Mountains in Bally et al. 1966; Price 1981), in which thrusting involves only the sedimentary cover whereas the basement remains unaffected by the thrusting, is known as thin-skinned deformation (Figs 1, 2, 3 & 8b1). As we trace the thrusts back into the hinterland, basement rocks become involved in the thrust sheets (crystalline thrusts). These basement rocks may have been transported in a ‘thin-skinned’ manner on sub-horizontal thrusts over other basement or cover rocks. Eventually, the thrusts root down into crystalline basement rocks and lose their thin-skinned character. The thin-skinned structural style contrasts with the thick-skinned style of basement-involved belts (Coward 1983), in which both the basement and the cover are deformed due to contraction (Fig. 8b3), such as the Laramide uplifts (e.g. Schmidt et al. 1993). Usually, in contrast to FAT belts, rocks have not been transported over long distances in basement-involved belts. In these belts the basement is incorporated into the thrust sheets due to: (a) steep faults that penetrate the basement and create basement uplifts; and/or (b) reactivated inherited basement fabrics that control the subsequent thrust architectures such as inverted half-grabens. The basement uplifts of the thick-skinned belts may consist of fault blocks whose uplift and rotation caused forced folding of the cover sequences that drape relatively unfolded basement rocks (e.g. USA Rocky Mountain foreland, Prucha et al. 1965; Stearns 1971) or blocks formed by folded basement and cover rocks (Berg 1962; Blackstone 1983). The thick-skinned belts caused by reactivation of pre-existing faults exhibit different features depending mainly on whether all the faults were reactivated or only a few of them, the degree of reactivation, the angle between the direction of compression and the strike of the old faults, the main features of the inherited faults and the rheology of the involved sequences.
FAT and basement-involved belts coexist and are somehow related in many orogenic belts. These different structural styles may be distributed:
In different portions of the belt following a transversal pattern, such as the Rocky Mountains-USA Cordillera that exhibits thin-skinned styles in the interior and thick-skinned styles in the outer part (Hamilton 1988); in some cases the different structural styles are developed in adjacent regions, so that the steep faults that penetrate the basement rocks become sub-horizontal when they reach the cover and are responsible for its detachment such as in the Alps (e.g. Hayward & Graham 1989).
Along strike of the belt, such as the Andes (Fig. 8a) in which the structural style varies from thin-skinned styles in regions over inclined slab segments and thick sedimentary basins filled with cover rocks (Fig. 8b1) to transitional thin/thick-skinned styles (Fig. 8b2) to thick-skinned over flat segments of the subducted plate and little sedimentary cover on top of the crystalline basement (Fig. 8b3) (e.g. Mingramm et al. 1979; Allmendinger et al. 1983).
Superimposed in the same region but developed during different times, such as the Bohemian Massif where an initial thin-skinned event is followed by a thick-skinned phase in which thrusts involve crystalline rocks and cross-cut older structures (Rez et al. 2011).
Some shortening-dominated regions formed by reactivation of inherited structures include elements from both FAT belts (thin-skinned) and basement-involved belts (thick-skinned). Thus, folds are related to steep faults that do not emanate from a detachment and cause relatively small displacements of the rocks. However, these belts are not proper thick-skinned belts because the basement rocks are not involved in the deformation similarly to thin-skinned belts (Fig. 9).
(a) Simplified structural map of the frontal portion of the Andes, NW Argentina, showing the transition between the Subandean Ranges and the Santa Barbara System (modified from Uliana et al. 1995). (b) Three regional-scale geological cross-sections (modified from Mingramm et al. 1979) whose cross-section lines are located in Figure 1a. Section 1 across the Subandean Ranges exhibits a classical thin-skinned structural style, section 3 across the Santa Barbara System illustrates the typical thick-skinned style, and section 2 across the transition between both ranges interfered by the oblique Lomas de Olmedo Trough shows an intermediate structural style.
Geological section across the Mahakam delta, Kalimantan, Indonesia, showing tight, anticlines whose steeper limb is bounded by a fault and broad synclines developed in sand, shale and coal-rich sequences of Miocene age. The folds developed due to regional-scale contractional reactivation of delta-top extensional growth faults produced during delta progradation. The faults root into the overpressured shale delta sequence (modified from McClay et al. 2000).
Amongst other factors, the presence of evaporites or shales at depth has a crucial impact on the structural style of the orogenic belt as a result of the efficiency of the detachment. The occurrence of basement fabrics exerts an essential control on the structural style as well; thus, passive margin basins filled by post-rift sedimentary prisms tapering onto the cratons favour the FAT belts (thin-skinned style), whereas intra-cratonic rift systems tend to give rise to basement-involved belts (thick-skinned structural style).
Along-strike geometry
On a large-scale in map view, FAT belts may be: (a) linear; (b) sinuous, when they show geometrical variations along-strike in the form of salients or virgations where the belt bulges into the undeformed region, and recesses, reentrants or syntaxis where the belt has not propagated far into the undeformed region (salients and recesses may have experienced different kinematic evolutions); and (c) curved or arcuate, usually known as oroclines (Fig. 2a). The term ‘orocline’ was originally applied to curved mountain belts which were initially straight, or at least straighter than they are today. However, in the last few years, the definition has been broadened to include any curved mountain belt, regardless of its original shape. The Appalachians of eastern North America are a classical example of a sinuous mountain belt (e.g. Mitra 1997), whereas the Ibero-Armorican Variscan belt and its prolongation in the Variscan Pyrenees (García-Sansegundo et al. 2011) is a well-known example of an orocline. Many parameters influence the occurrence of curved geometries in map-view, for example, interaction of the propagating belt with basement highs, along-strike pinch-outs of favourable detachment horizons, lateral variations in stratigraphic thicknesses and/or lithologies, interaction of the belt with strike-slip faults, superposition of second deformation events with tectonic transport directions oblique or perpendicular to that responsible for the belt formation, and so on.
The non-straight geometry in map-view of some belts may be also caused by the fact that individual thrust sheets are not necessarily continuous along strike. Thrust surfaces may merge or be truncated by another thrust, but they can also transfer slip to another thrust fault through a transfer or relay zone, splay up and distribute movement among several smaller-scale thrusts, strike into the axial zones of folds that accommodate shortening or become segmented by sub-vertical faults, called tear or transfer faults, that separate different parts of the thrust sheet with distinct displacement, differential types of structures responsible for shortening accommodation (e.g. folding dominated in one tear fault block v. thrusting dominated in the other tear fault block) or connect non coplanar parts of a thrust surface.
Structural evolution
Most FAT belts exhibit an orogenic polarity which may vary along the belt, like the Himalayas (e.g. Antolín et al. 2011). Thus, folds verge usually towards the foreland in the case of foreland FAT belts (Figs 1, 2, 3 & 8b1), towards the trench in the case of accretionary wedges (Fig. 5b) and towards the basin in the case of toe thrust belts (Figs 6 & 7b), and thrusts show a sense of movement of top-to-the-foreland, and deformation becomes younger towards the foreland with thrusts developing in a break-forward or piggy-back sequence (Figs 10 & 11) defining a regional foreland-directed tectonic transport direction. Although a dominant orientation of the tectonic transport vector prevails, in detail the sense of motion of each thrust sheet may vary in its orientation spatially and/or through time (e.g. Simón & Liesa 2011). In addition to folds and thrusts vergent and directed respectively towards undeformed regions following a break-forward propagation sequence, other types of structures (backthrusts and back-vergent folds, out-of-sequence thrusts and/or reactivated thrusts) occur in many belts behind the deformation front (Figs 3 & 11). As new thrusts and related folds develop, unless the thrust ramp spacing is relatively large compared to the displacement along each thrust, early thrusts steepen due to rotation of the thrust imbricates (Figs 1, 2 & 10) accompanied by a certain amount of slip on the thrusts. Tilting and folding of the earliest thrusts makes continued slip on them increasingly difficult, and eventually they become inactive and too difficult to reactivate. New thrusts with their associated folds may propagate breaching, cutting through and/or folding the older thrust surfaces and related folds.
Schematic development of an imbricate thrust system and related fault-propagation folds following a break-forward sequence and steepening of the earliest (hinterlandward) thrusts (modified from Mitra 1990).
(a) Cross-sectional view of an experimental sandbox model designed to simulate an orogenic wedge. The experiment was performed at the Fault Dynamics Laboratories conducted by K. R. McClay and is displayed at the Geological Museum–University of Oviedo (photograph courtesy of L. M. Rodríguez-Terente). (b) Schematic cross-section of an experimental thrust wedge derived from analysis of a series of scaled sandbox models showing three different regions and the general sequence of development of thrust faults. The backthrusts at the rear portion of the wedge were active during various growth stages. The scale is approximate (modified from Huiqi et al. 1992).
The kinematic evolution briefly described above is satisfactorily explained by the critical-taper theory. This requires the thrust wedges (Fig. 11) to maintain a critical taper angle (equal to the detachment surface dip plus surface-slope angle) to produce a dynamic equilibrium among different stresses: traction at the wedge base, compressive push at the back of the wedge, and the slope stress at the topographic surface. The wedge is thickened and shortened internally as thrusts override each other, folds develop, penetrative strain occurs and/or duplexes bounding rock packages are accreted from beneath the wedge. When the wedge reaches a critical taper angle, it slides stably along the detachment towards undeformed areas. During translation, the thrust wedge is lengthened as thrusts propagate towards undeformed regions and new material is added to its toe causing the taper angle to decrease because it distributes topographic elevation over a longer distance. When a subcritical taper angle is reached, stable sliding stops and internal deformation occurs again within the wedge. If the wedge reaches a temporary supercritical taper angle, the surface slope is decreased by wedge thinning due to erosion, collapses in a series of extensional faults, similar to large landslides, and/or slumps that remove material from the surface (Fig. 12). The subcritical, critical and supracritical stages may occur more than once during the evolution of a thrust wedge (e.g. Torres-Carbonell et al. 2011).
Geological section across the Ainsa Basin, southern Pyrenees, showing thrusts and related folds (fault-bend, fault-propagation and detachment folds) partly truncated by extensional faults, which in turn, are cut and offset by part of the displacement along some thrusts. Since the relationships between thrusts and extensional faults indicate that they developed synchronously, the extensional surfaces are interpreted to reflect the episodic extensional collapse of the Pyrenean thrust wedge that advanced progressively into a marine foreland basin (modified from Muñoz et al. 1994).
Fold-thrust interaction
The alternation of competent and incompetent lithologies exerts an important control on the geometry of thrust surfaces. Thus, thrust surfaces exhibit bedding-parallel segments, known as thrust flats, usually within the incompetent horizons such as shales and evaporites, and bedding-oblique parts, called thrust ramps, so that they step up and cut across competent beds such as sandstones and limestones (Fig. 13a). After thrust movement, all sorts of situations are possible: hanging wall flats over footwall flats, hanging wall flats over footwall ramps, hanging wall ramps over footwall flats and hanging wall ramps over footwall ramps. Since the flats, that usually exceed the ramps in length, and the ramps are linked, overall the thrusts acquire stairstep geometries. The occurrence of thrust ramps with strikes perpendicular to the tectonic transport vector (frontal ramps), forming an acute angle to the transport direction (oblique ramps) or with strikes parallel to the transport direction (lateral ramps) (Fig. 13b), leads to development of fault-bend folds (Rich 1934; Suppe 1983) due to the necessity of beds to conform to the thrust surface geometry (Fig. 14) and fault-propagation folds (Mitra 1990; Suppe & Medwedeff 1990) in those cases in which shortening is accommodated by both thrust ramp propagation and synchronous folding (Fig. 15). Although different types of ramp folds are the most typical modes of fold-thrust interaction in FAT belts, for example, the Iberian Variscan Massif (Mantero et al. 2011) or the Carpathians (Poul et al. 2011), not all folds in FAT belts are underlain by thrust ramps. Thus, simultaneously with propagation of a bedding-parallel thrust surface (detachment or décollement) or when a displacement gradient occurs along a detachment, detachment folds (Jamison 1987) form above or below the detachment surface as in the case of the Pico del Águila detachment anticline in the southern Pyrenees (Vidal-Royo et al. 2011) (Fig. 16). Apart from ramp folds and detachment folds, other types of thrust-related folds develop for example, drag folds, and so on. All these folds can develop in the frontal part of thrust sheets (leading edge folds), in the rear portion of the thrust sheets (trailing edge folds), or within the thrust sheets (intraplate folds). As deformation increases, folds become tighter and overturned limbs may develop, and when they are too tight to accommodate more shortening they become locked-up and can be cut across by reverse faults (Fig. 17) giving rise to hybrid thrust-related folds (breakthrough and transported fault-propagation and detachment folds) or to break-thrust folds such as the Maiella Mountain anticline in the Apennines (Masini et al. 2011) if folding developed before faulting. The dominant fold style is class 1B to 1C (Ramsay 1967) and most folds are asymmetric with a vergence towards undeformed regions. Usually in the hinge zones of folds cored by incompetent units such as argillaceous sediments, slaty cleavage may develop.
(a) Geological section across the Nittany anticlinorium, Central Appalachians (section originally from Perry 1978 simplified by Geiser 1988) showing a staircase geometry of the thrust surfaces and all sorts of hanging wall and footwall ramp and flat situations. (b) 3D sketch showing the geometry of a thrust surface including frontal, oblique and lateral ramps.
Geological section across the Pine Mountain thrust system, southern Appalachians, in which the Powell Valley anticline is interpreted as a fault-bend fold cored by a duplex structure (modified from Suppe 1985).
Geological section across the Meilin anticline, western Taiwan, interpreted as a simple-step fault-propagation fold (modified from Suppe 1985).
Satellite image of the External Sierras in the frontal part of the south Pyrenean foreland FAT belt showing a detachment fold train formed by narrow, rounded-hinge anticlines separated by wide, flat-bottom synclines affecting Triassic, Cretaceous, Paleocene, Eocene and Oligocene sedimentary rocks.
Geological section across the Puri anticline, a long structure located in the leading edge of the Papua New Guinea FAT belt, showing out-of-sequence thrusting cutting across the steep forelimb of the fold and involving Eocene to Miocene rocks (modified from Medd 1996).
The folds described so far form under non-metamorphic or low-grade metamorphic conditions. In regions where the basement is involved in the thrust sheets and deformation takes place under medium to high-grade metamorphic conditions, large recumbent folds form with vergence towards the undeformed regions. These are accompanied by a pervasive cleavage. Various fold generations occur, for example, in the Iberian Variscan Massif (see Fernández et al. 2011).
Folding is a subordinate phenomenon related to thrusting in many belts in which imbricate thrust faults dominate, for example, in the Valley and Ridge province of the Appalachian Chain (Mitra 1988) or in the Southeastern Pyrenees (Muñoz 1992). However in other belts, folds dominate the tectonic architecture, for example, in Papua New Guinea (Smith 1965) or in the Western Irish Namurian Basin (Tanner et al. 2011). Along-strike transitions from dominant folding to dominant thrusting have been reported in the Appalachians (Gwinn 1964), in the Canadian Rocky Mountains (Wheeler et al. 1972) and in the Cantabrian Mountains (Julivert & Arboleya 1984) amongst other FAT belts.
Contributions in this book
Much current research on the structure of FAT belts is focused on structural studies of regions or individual structures, and on the geometry and evolution of these regions employing kinematic, mechanical and experimental modelling. In keeping with the main trends of current research, this special publication is devoted to the kinematic evolution and structural styles of FAT belts. The topics of the papers included in this volume range from detailed structural analysis of individual structures to large-scale regional studies of FAT belts and from classical structural geology studies based on data collected in the field to numerical modelling. The papers included in this Special Publication are a selection of the works on FAT belts presented in various sessions at the ‘International Meeting of Young Researchers in Structural Geology and Tectonics (YORSGET-08)’ held at Oviedo (Asturias, Spain) in June–July 2008 together with others on the same topic matured over a similar period. This meeting was organized by M. Gutiérrez-Medina, C. López-Fernández, D. Pedreira and J. Poblet to jointly celebrate the 400 years anniversary of the foundation of the University of Oviedo and the 50 years anniversary of the Geology Faculty at Oviedo. The meeting attracted 53 oral presentations and 90 posters, and an audience of 201 participants from 25 countries, and included a field excursion to the Variscan FAT belt of NW Iberian Peninsula. This conference included a special session in memory of the structural geologist Martin Casey from the University of Leeds (UK) who died in 2008 and was a recognized authority on the modelling of natural deformation features and the author of papers fundamental to the understanding of the deformation responses of rock. In addition, this Special Publication includes the last article written by the structural geologist Florentino Díaz-García from the University of Oviedo (Spain) who devoted his research efforts to the study of the Variscan belt in northwestern Iberian Peninsula and who, unfortunately, died in 2009. This volume does not attempt to provide a comprehensive coverage of FAT belts, but puts together some contributions on specific topics of interest within this research theme, some of them presenting new concepts and techniques and others applying well-known concepts and techniques to new regions.
Internal deformation
The first two articles of this special publication describe the application of two different methods to analyze the internal deformation undergone by thrust-related anticlines. Masini et al. carried out field mapping and collected structural data in order to construct two sections across the Maiella Mountain anticline, a structure located in the Central Apennines, Italy, that resulted from Messinian–Early Pliocene extension and subsequent Late Pliocene shortening. They employed a strain simulation technique based on the inclusion of circular strain markers in the cross-sections, subsequent sequential restoration of the cross-section and the strain markers, and application of a mathematical model to obtain strain ellipses for each strain marker. According to their structural model, the Maiella structure is a break-thrust fold that underwent small amounts of extension associated with two main normal faults and subsequent shortening due to a major thrust and folding. The simulation of the strain distribution shows high strain intensity in both limbs and low deformation in the anticline crest and in part of the thrust footwall. In the anticline forelimb, where the strain is greater, high deformation is concentrated in two symmetric triangular zones separated by the thrust ramp. The distribution of strain intensity predicted in different structural positions of the Maiella structure is in broad agreement with the distribution of fracture density obtained from field data by previous authors. Vidal-Royo et al. employ 2D discrete-element modelling to explore the evolution of the Pico del Águila anticline, a detachment fold formed during Eocene–Oligocene in the Spanish side of the Pyrenean FAT belt. This numerical technique treats the different units as an assemblage of circular elements that mutually interact with elastic forces influenced by gravity and obey Newton's equations of motion. Regarding the role of the mechanical stratigraphy, the competent beds experience rigid-body translation/rotation, localized faulting and minor shearing, whereas incompetent beds suffer high strain and are deformed by complex structures. The occurrence of extensional faults, stretching and gravitational instabilities in the crest of the anticline becomes substantially reduced when the model incorporates growth strata, and deformation is confined to the core of the structure leading to a tighter, narrower fold than in those situations in which growth strata is lacking.
3D data applications
This section contains two articles focussed on the determination of the kinematic evolution of folds and thrusts employing 3D data. Tanner et al. collected structural and stratigraphic data along the western Irish coast using high resolution GPS, constructed 3D geological surfaces and plotted the data onto a north–south vertical plane in order to obtain a balanced section parallel to the tectonic transport direction across the western Irish Namurian Basin. This basin is interpreted as a FAT belt developed in front of the northward propagating Variscan orogenic front in which folding predominates over thrusting. Subsequently, passive markers were included in the cross-section that was sequentially restored and decompacted to various Carboniferous syn-tectonic horizons. This allowed them to visualize variations in the folds’ wavelength along the cross-section and to deduce how fold uplift evolved through time. Since accurate ages of the horizons are available, they were able to estimate folding- and thrusting-induced shortening rates and concluded that although the basin was subjected to comparatively very little shortening with respect to similar tectonic settings, the rates of orogenic shortening are within the typical ranges for FAT belts. They also found that the Western Irish Namurian Basin underwent an anomalous strong subsidence during its evolution. Simón & Liesa illustrate how construction of various geological sections through the Utrillas thrust sheet in the Iberian Chain, Spain, perpendicular and oblique to the frontal thrust trace, allows a 3D reconstruction of the geometry of the main thrust surface formed by ramps oriented in different directions. Detailed structural analysis and tectono-sedimentary relationships with the deposits of the adjacent foreland and piggy-back Tertiary basins furnished data on the Eocene-Oligocene incremental motion history involving different tectonic transport directions. A schematic, plan-view retro-deformation of the distinctly oriented thrust ramps enabled estimation of the finite horizontal displacement along the thrust. The successive transport directions towards the ENE, NNE and north obtained are consistent with the development of two superposed main sets of folds and with the evolution of the intraplate stress fields through Tertiary times.
Tectonic and magnetic fabrics
This section deals with the information supplied by the magnetic fabrics to decipher the tectonic evolution of structures. Antolín et al. combined the anisotropy of magnetic susceptibility and structural analysis of a Triassic flysch to decipher the tectonic evolution of the Tethyan Himalaya FAT belt, Tibet. These authors defined a southern domain characterized by a magnetic foliation parallel to the first tectonic foliation in accordance with the south vergence of the belt, and a northern domain in which the magnetic foliation and the second tectonic foliation show a vergence opposite to that of the Himalayan system. Both domains are separated by a zone where both tectonic foliations coexist and where an intermediate magnetic fabric is developed. The magnetic lineation supplied information on the north–south transport direction of the thrust sheets. The different orientation of the magnetic foliations suggest a Middle Miocene clockwise rotation around a vertical axis which could be explained as a result of large-scale dextral shearing caused by eastward extrusion of the Tibetan Plateau or block rotations due to intrusion and exhumation of the North Himalayan domes.
Thrust wedges
The two articles in this section discuss the structural style and evolution of thrust wedges in various American FAT belts. The geological map and sections across the eastern Fuegian Andes FAT belt, Argentina, constructed by Torres-Carbonell et al. reveal that the structure of this region consists of various thrusts and backthrusts rooted at the base of the Cretaceous and within the Paleocene rocks. The occurrence of syn-tectonic sequences bounded by unconformities allowed them to perform a sequential cross-section restoration, decipher the kinematic evolution and timing of structural development and unravel the behaviour of the Coulomb wedge from Eocene to Oligocene times. The oldest stage was characterized by propagation of the basal detachment and formation of foreland-directed thrusts leading to a taper angle decrease. During the subsequent period, corresponding to a subcritical Coulomb wedge stage, backthrusting accommodated significant shortening. The last period consisted of a new critical stage caused by renewed foreland-directed thrusting at the wedge front. Fitz-Diaz et al. compare the structural style of one traverse across the southern Canadian Rocky Mountains with another one across the Mexican FAT belt. Although the age of deformation, the overall structural pattern and the total amount of shortening are similar, the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation in the former and individual thrust sheets with much more internal deformation, such as buckle folds, in the latter. One of the reasons for the differences in tectonic style is the facies distribution; massive platform limestone separated by thinly-bedded basinal limestone in the Mexico section, so that strain is concentrated toward the margins between platforms and basins, and thick platform carbonates forming continuous resistant units in Canada. Other possible reasons for the differences in tectonic style between the two sections include the taper angle of the tectonic wedges and the amount of friction along the basal detachment.
Structural evolution-case studies
The last section of this publication contains five articles that unravel the sequence of deformations in various belts located in Spain and in the Czech Republic. Based on field mapping and structural analysis of the Palaeozoic basement of the Central Pyrenees, France-Andorra-Spain, García-Sansegundo et al. propose a new division of this portion of the Variscan FAT belt into two different zones: non-metamorphic and metamorphic units, and a new sequence of Variscan deformations. The non-metamorphic units include thrust systems and related folds with a poorly developed cleavage, whereas in the metamorphic units two fold sets with axial plane cleavage and thrusts approximately coeval with the second fold generation occur. The structure of the Pyrenean non-metamorphic units has Variscan foreland affinities and is comparable to that of the Cantabrian Zone (foreland FAT belt of the Iberian Massif ), whereas the deformation observed in the Pyrenean metamorphic units is characteristic of the Variscan hinterland and is consistent with the features of the West Asturian-Leonese Zone or Central-Iberian Zone. The relative position of the foreland and hinterland, with the non-metamorphic units located southwards of the metamorphic ones, and the south-directed motion of the Variscan thrusts, suggest that the Variscan Pyrenees may be equivalent to northern branch of the Ibero-Armorican or Asturian arc. Structural analysis in the Palaeozoic rocks that constitute the Forcarei synform, located in the northwest part of the Iberian Variscan Massif, Spain, allowed Fernández et al. to conclude that two main Variscan deformation events occurred. The first event in this region caused a pervasive cleavage, a stretching lineation, the Forcarei Thrust and other related structures such as minor folds. The second event produced the large-scale Forcarei Synform and a sub-vertical crenulation cleavage parallel to its axial plane, an intersection lineation and minor folds which caused fold interference patterns with previous ones. The dominantly sinistral shear deduced from the analysis of quartz fabrics and kinematic indicators related to the first deformation is interpreted as a result of top-to-the-south displacement along the Forcarei Thrust combined with clockwise rotation along a sub-vertical axis evolving to dominant clockwise rotation in the last stages of thrust motion, subsequently folded into a synform during the second deformation event. Mantero et al. document two deformation phases in the Devonian and Carboniferous rocks of the Puebla de Guzmán antiform located in the Iberian Pyrite Belt in the south part of the Iberian Variscan Massif, Spain, which could be the result of a progressive deformation related to the growth of the South Portuguese Zone orogenic wedge. The first deformation phase gave rise to a widespread penetrative cleavage linked to thrusts at deep crustal levels and folds above them. The second deformation phase consists of thrusts, which constitute the most pronounced cartographic-scale structures, and two fold sets with axial plane crenulation cleavages. Despite the widespread presence of folds, the enveloping surfaces of bedding are sub-horizontal, and steep or overturned dips are restricted to the vicinity of frontal or lateral thrust ramps due to fault-propagation folds. The thrust displacement is distributed into a large number of thrusts and the important thickening produced in this region is due to thrust sheet stacking. Geological mapping, cross-section construction, structural analysis, and sedimentological and biostratigraphical data of Upper Devonian and Lower Carboniferous rocks allowed Rez et al. to establish a new Variscan sequence of deformations for the southern part of the Moravian Karst (Bohemian Massif, Czech Republic). Both deformation events involved northeast-directed thrusting suggesting progressive deformation during a constant stress orientation. The first event consisted of ‘thin-skinned’ thrusting and related folding and caused tectonic juxtaposition of two distinct facies of coeval sequences. The second deformational event led to ‘thick-skinned’ thrusting, which involved the crystalline rocks of the Brno Massif and cross-cut first phase thrusts, and folding of previous structures. Later normal and strike‐slip faults of probable Alpine age were responsible for the compartmentalization of the region into numerous small-scale blocks. Poul et al. present a new model for the controversial isolated ridges made up of Upper Jurassic limestones that constitute the Pavlov Hills located in the Outer Western Carpathians, Czech Republic. These uplands have been interpreted in the past as relicts of island ridges, exotic bodies derived from the basement, klippes, olistoliths and blocks bounded by faults. However, according to their new model based on geological mapping and geological interpretation of seismic lines, the present-day geometry and position of these ridges resulted from fault-bend folds related to antiformal stacks detached at the base of the Upper Jurassic limestones, subsequently offset by strike‐slip faults sub-perpendicular to the thrusts trace. Unlike previous models for this region, this new interpretation including northwest-directed thrusts with flats and ramps is consistent with the structural style described for the Outer Western Carpathians.
Concluding remarks
If progress is to be made in the understanding of the evolution and styles of FAT belts, from small-scale to lithosphere-scale sections, and from both the pure scientific and applied points of view, detailed multidisciplinary studies are required. These studies will need to integrate information from structural geology, stratigraphy–sedimentology, petrology, geochronology, geomorphology and geophysics, and take maximum advantage of all the new data that are being acquired by recently developed technologies. Accurate analyses of many aspects of individual fold-and-thrust structures located in different FAT belts, such as their 2D and 3D geometry, strain and fracture patterns, formation mechanisms and history (timing, uplift and shortening magnitudes and rates), have been carried out in recent times employing field and/or subsurface data, experimental and/or numerical models. If integrated into large-scale models of each FAT belt, these separate data collected in different parts of the belt would have an extraordinary impact on the understanding of the mechanisms responsible for the origin and evolution of these regions.
Due perhaps to interest from the economical point of view and the abundance of data derived, it seems that whilst the FAT belts located in external zones of cordilleras are relatively well studied, the FAT belts in interior parts of orogens have received less attention and require more detailed research on the more complex structures found therein, on the degree of basement involvement in the structures, on the control exerted by pre-existing structures, and on the structural and temporal relationships between the inner and outer portions of FAT belts. A great deal of research is being done in offshore FAT belts because, although they are inaccessible, seismic imaging is excellent especially with modern technology. However seismic data may provide an incomplete picture. For instance, the initial attempts to contrast measurements of extension and contraction in a particular submarine toe thrust belt yielded different figures. This suggests that deformation is accommodated by other mechanisms apart from structures displayed in the seismic data. Existing kinematic algorithms for fold-thrust systems are not able to consider this, and therefore, they are likely to yield poor predictions of subseismic deformation and fault zone architecture. This means that current models need to incorporate the strain undergone by rocks amongst other parameters. Much less research is carried out in many onshore FAT belts because they are developed in severe terrains, the structures are complex and the subsurface data are poor in quality and/or quantity. This is the reason why understanding them has been strongly guided by geometrical models, fed by surface geological maps and sparse subsurface data, and which, in some cases, involve an important level of uncertainty rarely addressed in modern studies. Nowadays, since many geometrical models have been developed to predict the subsurface and 3D features of the structures, it would be essential to test them on well-known natural and experimental structures in which the complete geometry of the structure are available; this would permit magnitudes of errors in the structural interpretations to be quantified and would guide future improvements of these models. In addition, onshore FAT belts have to be revisited and detailed work carried out in order to revise the old interpretations in the light of the new concepts which, in turn, will provide additional conceptual and numerical models for this type of regions.
Contrasting the insights gained from analyzing active FAT belts, in which the structures are relatively well preserved and processes can be observed and quantified with a certain degree of confidence, with those gained from examining exhumed belts would contribute substantially to better constraint the 4D evolution of these regions. For instance, in recent years, the role of surficial processes has been emphasized in several studies on the evolution of FAT belts. The impact of erosion and sedimentation in controlling geomorphology, fold-thrust growth, exhumation processes and deformation history has been explored through numerical and experimental methods. More data, such as uplift estimates, and so on, collected in different FAT belts are needed to further elucidate the interactions between the tectonic processes responsible for mountain building and climate-dependant surface processes on various time and space scales to maintain the dynamic equilibrium of orogenic wedges.
3D seismic data and precise analyses of folds and thrusts show that even simple structures are often far more complex than expected. For instance, many FAT belts exhibit faulted folds not well described by current theories; while some sections across individual structures may conform to proposed theories, the structures can exhibit rapid changes in 3D leading to a variety of complex fold-thrust architectures along strike. The capabilities of end-member fold-thrust interaction models, especially those embedded within many structural analysis software packages, are relatively restricted because they assume that deformation mainly causes displacement along faults and fold amplification. The models do not entertain other type of behaviours. However, discrete element modelling is beginning to allow us to build geologically realistic models. Geomechanics has to be the next step in structural geology research, to provide the additional constraints that geometry and kinematics do not, and needs to be applied at all scales, from small-scale deformational features to entire FAT belts. The last decades have seen a proliferation of quantitative approaches to studying FAT belts by means of geometrical, kinematical and some mechanical numerical modelling and physical experiments proving to be powerful tools for simulating the structural evolution of these belts. Although the present-day models of FAT belts have reached high levels of sophistication and are the subject of many present-day publications, detailed field and subsurface observations on FAT belts are still essential because natural examples based research is the ultimate test of the theoretical and physical models. We hope that this Special Publication will serve as a reference on future research and understanding of both the spatial and temporal evolution (4D) of FAT belts.
Acknowledgments
Comments by J. Turner substantially improved the initial version of this manuscript. We are indebted to the reviewers of the articles included in this Special Publication and of those manuscripts that, unfortunately, could not be part of this book. It was a pleasure to work with the efficient staff of the Geological Society of London, in particular A. Hills, T. Anderson and H. Floyd-Walker. Thanks to Carlos Olivares for reviewing the format of the manuscripts included in this publication and figure drafting, and Mayte Bulnes for critical reading of this manuscript and figure drafting. We acknowledge financial support by research grant CGL2008-03786/BTE (Mechanical analysis of deformation in folds) funded by the Spanish Ministry for Science and Innovation. J. Poblet is grateful to the Consolider programme project CSD2006-0041 (Topo-Iberia).
- © The Geological Society of London 2011
