Abstract The outer parts of collision mountain belts are commonly represented by fold and thrust belts. Many of the key concepts in the structural geology of fold and thrust belts have origins in ancient orogens such as the Appalachians and Caledonian chains of Europe, together with the Alps. Impetus in thrust belt research then came from the desire to exploit geological resources that reside in the subsurface, especially arising from hydrocarbon exploration in the foothills of the Canadian Cordillera in the 1960s and 1970s. Notwithstanding decades of exploitation, continental fold and thrust belts are still estimated to hold reserves of 700 billion barrels of oil equivalent. But exploration will focus increasingly on small, hard-to-resolve structures. Basic geological understanding remains as important today as it did for the pioneering explorers in the Canadian foothills. It is a theme that runs throughout this Special Publication.

Abstract In 1888, inspired by fieldwork in what has become known as the Moine Thrust Belt, NW Scotland, Henry Cadell conducted a pioneering series of analogue deformation experiments to investigate the structural evolution of fold–thrust belts. Some experiments showed that imbricate thrusts build up thrust wedges of variable form, without requiring precursor folding. Others demonstrated a variety of fold–thrust structures and how heterogeneities in basement can localize thrust structures. These experiments are described here and used to draw lessons on how analogue deformation experiments are used to inform the interpretation of fold–thrust structures. Early adopters used Cadell's results as guides to structural styles when constructing cross-sections in thrust belts. His models and the host of others created since serve to illustrate part of the range of structural geometries in thrust belts. However, as with much subsequent work, Cadell's use of a deformation apparatus, with a fixed basal slip surface, biases perceptions of fold–thrust belts to be necessarily ‘thin-skinned’ (experimental design bias) and can simply reinforce established interpretations of natural systems (confirmation bias). So analogue deformation experiments may be unreliable guides to the deterministic interpretations of specific fold–thrust structures in the sub surface of the real world.

Abstract It has long been recognized that the structures that accommodate shortening within fold-and-thrust belts exhibit a wide variety of styles that reflect the mechanical behaviour of the stratigraphic units that are being deformed. The ability to characterize these different structural styles, and to understand the factors that control their variability, is essential to many applications, including petroleum geology, earthquake hazard assessment and regional geological studies. The relative contributions of different aspects of the mechanical stratigraphy and boundary conditions to determining fault-related folding style are investigated through the use of the discrete-element modelling (DEM) method in this study. Modelling emergent contractional structures within a shortening wedge with this method demonstrates that (a) The major different styles of shortening structures can all be reproduced under different mechanical circumstances within the range of realistic mechanical conditions, and (b) Different aspects of the mechanics of the deforming rock units (for example, peak strength, strain weakening, layer strength anisotropy) exert various degrees of control on the styles of structures that emerge from the models as shortening progresses. These analyses inform our understanding of the relative importance of these different factors in determining the style of structures which accommodate shortening in different fold-and-thrust belt systems.

Abstract Whether thrusts are ramp-dominated and form imbricate fans or run out onto the syn-orogenic surface, forming ‘thrust-allochthons’, is governed by the activity of secondary ‘upper’ detachments along the syn-orogenic surface, activations of which are inhibited by syn-kinematic sedimentation at the thrust front. In the northern Apennines, where thrust systems are ramp-dominated and form an emergent imbricate fan, syn-kinematic sedimentation was abundant and accumulated ahead and above each thrust. In the southern Apennines, the far-travelled Lagronegro allochthon achieved its high displacements (>65 km) while the foredeep basin received little sediment. The imbricate fan at the front of the main Himalayan arc developed within a foredeep that experienced high rates of syn-kinematic sedimentation. In contrast, further west, the Salt Range Thrust emerged into a distal, weakly developed foredeep with significantly reduced rates of sediment accumulation. Displacements were strongly localized onto this thrust (c. 25 km displacement) which activated an upper detachment along the syn-orogenic surface. It is an arrested thrust-allochthon. Lateral variations into the adjacent, ramp-dominated but still salt-detached, Jhelum fold-belt are marked by increases in syn-kinematic sedimentation. As sedimentation styles can vary in space and time, individual thrusts and thrust systems can evolve from being allochthon prone to imbricate dominated.

Abstract The genetic analysis of fold and thrust belts is facilitated by tracking the evolution of their organic endowment (petroleum tectonics). Petroleum tectonic analysis of convergent orogenic systems provides an audit of the processes that control the deformation and kinematics of orogenic belts. The distribution and deformation paths of the organic endowment intervals are key factors in determining the petroleum system evolution of fold and thrust belts. This comparison of orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper, mechanical stratigraphy and inherited architecture on the creation, preservation and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean convergent systems share key characteristics of fold and thrust belts, with major differences in scale, degree of incorporation of organic endowment in evolution of the fold and thrust belt and its foreland, and preservation of fold and thrust belt wedge-top deposits. The Zagros is an orogen dominated by flexural processes that is a perfect storm for hydrocarbon generation and preservation. Its multiple stacked sources ensure continuous hydrocarbon generation while stacked detachments foster a low taper and thick wedge-top basins. The Pyrenees is also a flexurally dominated orogen, but the early consumption of its source rocks led to minimal survival of hydrocarbon accumulations during exhumation in a long lasting, high-taper orogenic wedge. The Sevier was initially a flexural orogen that was later dominated by dynamic uplift of the fold and thrust belt and distal foreland subsidence with foreland deformation. The consumption of its pre-orogenic sources during the early low-taper phase indicates a probable robust petroleum system at that time. However, the late high-taper phase exhumed and destroyed much of the early petroleum system. The addition of syntectonic foreland sources to be matured by both local and dynamic subsidence created an additional later set of petroleum systems. Post-orogenic events have left only remnants of world-class petroleum systems. The Beni segment of the Sub-Andean Orogen is a flexural system with probable dynamic overprints. Its most robust petroleum system probably occurred during its early low-taper flexural phase, with dynamic subsidence enhancement. Its late high-taper phase with possible dynamic uplift shuts down and stresses the petroleum systems. Comparison of these orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper kinematics, mechanical stratigraphy, distribution of source rocks relative to shortening and inherited architecture on the creation, preservation and destruction of petroleum accumulations in fold and thrust belts.

Abstract This paper describes the results of an empirical one-dimensional model for toe thrust structures that enables both fluid pressure expectations and seal integrity to be evaluated. The growth of a deepwater thrust anticline is forward modelled with calibration made to a present-day structure. Two locations are used to describe the structural evolution: the crest of the anticline and the floor of the adjacent syncline. These locations are followed through structural growth and burial. Pore pressures in the evolving toe thrust are modelled by employing a centroid model for a pre-tectonic target reservoir interval, with shale pressures modelled using the concept of a variable fluid retention depth. A shale compaction model based on critical state soil mechanics is used to model top seal porosity and to aid the evaluation of the capillary top seal. Together with mechanical seal analysis this enables the controls on trapped hydrocarbon column heights to be assessed. Application of the model to two recently drilled toe thrusts is discussed, and results compared to model predictions. The model can be used to offer a generic seal integrity scheme for a toe thrust that may help focus attention on the key exploration risks.

Abstract Raman spectroscopy allows thermal maturation of carbonaceous sediments to be determined. The technique has been employed on metamorphic samples exceeding temperatures of 270°C, but recently has been shown to be effective at lower temperatures. Thermal maturation techniques commonly depend on sample size, have varying efficacies at different temperatures and in different conditions. The underlying processes are not well understood, thus data interpretation can be ambiguous. Here we show the efficacy of Raman as a low-temperature thermal marker in a thrust belt. The Bornes region, in a French Subalpine chain, provides an opportunity to test the technique against published vitrinite reflectance data and thermal modelling for the first time. In doing so we show that Raman is an effective thermal marker to temperatures as low as 75°C, has a small error and is consistent with previous work. The Raman data allow us to postulate the relative thickness of the sedimentary succession across the chain, the timing of thermal maturation and the timing and thickness variations of the over-thrust Prealpine nappe. The work establishes Raman as a low-temperature thermal marker for correlation with other techniques to ensure effective and robust interpretation, that can readily be applied to fold–thrust belts in hydrocarbon provinces.

Abstract The Ribblesdale fold belt, representing the Variscan inversion of the Bowland Basin, is a well-known geological feature of northern England. It represents a crustal strain discontinuity between the granite-underpinned basement highs of the northern Pennines and Lake District in the north, and the Central Lancashire High/southern Pennines, in the south. Recent seismic interpretation and mapping have demonstrated that the Ribblesdale fold belt continues offshore towards Anglesey via the Deemster Platform, beneath the Permo-Triassic sedimentary cover of the southern part of the East Irish Sea Basin. The Môn–Deemster fold–thrust belt (FTB) affects strata of Mississippian to late Pennsylvanian age. Variscan thrusts extend down into the pre-Carboniferous basement but apparently terminate at a low-angle detachment deeper in the crust, here correlated with the strongly sheared Penmynydd Zone exposed in the adjacent onshore. Up to 15% shortening is observed on seismic sections across the FTB offshore, but is greater in the strongly inverted onshore segment. Pre-Carboniferous thrusting post-dates formation of the Penmynydd Zone, and is probably of Acadian age, when basement structures such as the southward-vergent Carmel Head Thrust formed. Extensional reactivation of the Acadian structures in early Mississippian time defined the northern edge of the offshore Bowland Basin. The relatively late brittle structures of the Menai Strait fault system locally exhume the Penmynydd Zone and define the southern edge of the basin. The longer seismic records from the offshore provide insights to the tectonic evolution of the more poorly imaged FTB onshore.

Abstract Critical gravity and magnetic data suggest the presence of a continuous zigzag exhumed mantle body inside the attenuated crust of the north Iberia continental margin. We propose that this body greatly conditioned the structural domains of the Cantabrian–Pyrenean fold-and-thrust belt during their evolution from hyperextension in Early Cretaceous times to shortening and inversion during the Cenozoic. This may be seen as a new line for cross-section construction and balancing, because previous cross-sections do not incorporate comparable volumes of exhumed mantle. Five structural cross-sections, constrained by the results of 3D gravity inversion, feed our discussion of the complexities of the doubly vergent Pyrenean orogen in view of the inversion of a precursor hyperextended rifted margin. In all sections, crustal rocks underthrust the lithospheric mantle in the hyperextended region, supporting that the near-surface exhumed mantle lithosphere acts as a more rigid buttress, allowing weaker continental material to be expelled outwards and upwards by thrusting during the Alpine collision; thus giving rise to two uplifted crustal triangular zones at the boundaries with the exhumed mantle. Contractional slip is localized in lithospheric-scale thrusts, which in turn reactivate parts of the extensional system. The NE–SW transfer zones that offset the rift therefore behave as compartmental faults during the orogenic phase. The amount of shortening increases from 34 km in the Cantabrian Cordillera, where the Basque–Cantabrian Basin partially preserves its original extensional geometry, to 135 km in the nappe stack of the central Pyrenees.

Abstract A detailed analysis of the Late Miocene Mandach Thrust, a key tectonic structure of the easternmost Jura Mountains (northern Alpine foreland), is presented providing insights into the modes of along-strike structural cover–basement interactions in a classical foreland setting. Our study builds on the construction, restoration and forward modelling of eight closely spaced cross-sections constrained by depth-migrated 2D seismics and geological maps. The results indicate predominantly thin-skinned thrust tectonics without significant inversion of underlying basement structures. However, inherited pre-thrusting normal faults exerted a strong control on the observed thrusting style, changing along-strike from a comparatively simple geometry to a complex, partly overthrust, partly reactivated normal fault. The observed variations relate to changes in the relief of the mechanical basement and the characteristics of pre-thrusting normal faults. The thrust's complexity is further increased by the local activation of secondary detachment horizons and possibly along-strike sedimentary facies changes within the thrust-faulted sedimentary sequence. The variations in thrusting style go along with subtle changes in shortening that may point towards as yet undetected transfer structures. As such, our structural analysis of the Mandach Thrust provides an improved understanding of the fault's kinematics and serves to highlight existing exploration uncertainties.

Abstract Hydrocarbon distribution in the Southern Apennines thrust belt of Italy is directly related to the geological characteristics and complex evolution of the thrust tectonic pile involving units from different palaeogeographical domains. Within this structural–stratigraphic context, the main exploration target is represented by the carbonate units of the Apulian Platform, which contain the largest and deepest oilfields in the region. By integrating different types (e.g. wells, seismic, maps, reports) of legacy public and confidential data of various vintages, the subsurface structural setting of the contractional Apulian structures around the Benevento Field in the Southern Apennines hydrocarbon province is reconstructed. The discovery dates back to the early 1970s. The reservoir consists of Cretaceous–Lower Miocene carbonates at a depth of around 3000 m below sea level bearing both oil and gas. A new digital interpretation and integration, which takes into account the most recent understanding of the evolution of this thrust belt, has allowed reconstruction of the trap style of the Benevento Field and of the prospective (undrilled) structures in the surrounding area. A history of positive inversion tectonics is interpreted from a 2D kinematic restoration; this has important implications for both the structural style and the hydrocarbon prospectivity of the Southern Apennines thrust belt.

Abstract The superposition of structures produced by different tectonic phases is common in sedimentary basins. Yet the earlier structures often remain overlooked with potentially negative exploration consequences. In the Maiella anticline, Pliocene compression has folded carbonate sequences containing Cretaceous extensional structures. The geometry and evolution of the Apulian carbonate platform margin outcropping on the Maiella Mountain are described by two opposing groups of models. One proposes a structurally controlled platform margins cut by syn-sedimentary Cretaceous faults; the other assumes a passive Cretaceous palaeo-escarpments progressively filled by Cretaceous to Tertiary sediments later deformed by the Pliocene compression. Assuming models in line with either one of these two groups has significant implications for exploration plays on both platforms and adjacent basins of analogous subsurface systems. These include: hypothesized margin geometries; sediment transport mechanisms (directions and distribution); size of sequences; type and size of traps and associated exploration targets, risks and uncertainties. We demonstrate that during the Late Cretaceous the platform margin was cut by normal faults which controlled the palaeogeography of the platform and the sediment input into the adjacent basin in which thick, resedimented, carbonate megabreccia and turbidites were deposited. These carbonates represent exploration targets in similar settings worldwide.

Abstract The Diapir Fold Zone of the Carpathians is the most prolific onshore hydrocarbon area in Romania. Structural complexity, mainly due to the presence of salt, combined with poor seismic quality near and below the salt lead to contrasting structural models in the area. To gain insights into the mid-Miocene tectonic evolution, structural geometries and the effects of penetrative strain, we ran dual décollement scaled sandbox models with layered brittle and ductile materials. Results of two analogue models (20 and 33% shortening) revealed that the onset of the deformation sequence was mainly characterized by layer-parallel shortening. As shortening continued, a foreland-verging sequence of supra-salt detachment folds and sub-salt duplexes evolved. The sub-salt duplexes are located directly below the crests of the detachment folds, as the development of these large wavelength anticlines was related to sub-salt deformation. Salt flow was another controlling factor of the deformation style, as salt accumulated in the anticlinal cores and increased the coupling in the supra-salt synclinal axis. Our results offer insights into the effects of salt on the kinematic evolution of this area, help to predict geometries in areas of poor seismic quality, and highlight the important contribution of penetrative strain on deformation and reservoir quality.

Abstract The structure and distribution of accommodation in fold and thrust belts vary both laterally and longitudinally. Here we integrate gravity, bathymetry and 2D seismic datasets to investigate the structural and stratigraphic variation in the southern part of the Hikurangi subduction wedge, onshore and offshore North Island, New Zealand. Three morphostructural portions are recognized: The inner portion demonstrates reactivation of inherited structures, producing thick-skinned deformation. Pre-subduction rocks are represented by kilometres of acoustically chaotic seismofacies. Thick-skinned deformation and readily deformable substrate lead to the development of wide trench-slope sub-basins, infilled with >5 km of syn-subduction sediments. The mid portion typically demonstrates thrust faults with connections to deeper structures, leading to the development of an imbricate system with asymmetrical sub-basins typically <5 km thick developed on the back-limb of thrust related folds. An antiformal stack marks the transition from the thick-skinned interior of the basin to the thin-skinned accretionary prism. Beyond this, the relatively non-deformed outer portion demonstrates frontal folds, propagating thrusts and up to 3 km thickness of syn-subduction strata. Structural variation across the subduction wedge controls the generation of accommodation with implications for sediment distribution within fold and thrust belts and for petroleum system development.

Abstract We defined the folding sequence of the fold–thrust belt of the Western Papua New Guinea Highlands by analysing the surface expression of the structures and the response of the drainage system to the active fold–thrust belt. The interaction between structures is typically assessed by examining the syn-kinematic strata preserved; however, in our study area, this is problematic as these strata are poorly imaged on seismic lines. This study found common morphological features that allowed grouping and mapping of three different structural settings: group I, basement-involved tectonics; group II, thin-skinned anticlines which sole near the Koi–Iangi Sandstone; and group III, thin-skinned folds associated with the intra-Ieru Formation detachment. Fold Front Sinuosity analysis supports the idea that the fold–thrust belt propagates from the NW to the SE. Considering the detailed morphotectonics and drainage analyses, we interpret that the group II folds developed as out-of-sequence thrusting and folding, associated with buttressing against the group I larger structures.

Abstract The Timor Orogen comprises the island of Timor, a narrow offshore area to the north and a wider offshore fold-and-thrust belt to the south. This orogen formed by jamming and subsequent collision of the Banda Sea subduction system by the Australian Plate. The BandaSeis seismic survey has revealed excellent images of the deep-water fold-and-thrust belt. Seismic interpretation of the dataset demonstrated structural and tectonic features not previously described, including regional geological features on the Australian continental crust and two regional NE–SW sinistral strike-slip faults, and a prominent Middle Permian palaeogeographical high (Timor Plateau). Moreover, since the Middle–Late Triassic and Middle Jurassic, the two NE-trending strike-slip faults governed the formation of the West Timor and Cova-Lima sub-basins. The location along the Australian margin plays a dominant role in controlling the structural style and shaping of the Timor Orogen. Vertical loading and the southerly motion of the orogenic wedge are the main driving forces responsible for its building, illustrating a thin-skinned tectonic framework. Thrust faults nucleate in a forward-breaking sequence in the motion of thrust transport, with younger thrusts developing in front of older thrusts. Most of the collisional deformation has been classified into two styles: shallow thin-skinned and deep-seated deformation.

Abstract The complicated geology of the Banda region results from complex collision between the Eurasia, Australia and Pacific plates, ongoing in the region since the Late Oligocene but particularly in the study area since the Middle Miocene (from 15 Ma). Regional 2D broadband seismic data have provided improved imaging of Mesozoic and Cenozoic sedimentary successions in the region. The region comprises the deep and ultra-deep Banda Sea enclosed by a magmatic inner arc and an outer deformed zone, comprising a series of orogens. This outer orogenic zone comprises islands with extended and sometimes hyperextended continental crust, a series of marginal foredeeps and intervening fold-and thrust belts. This paper illustrates how the offshore fold-and-thrust belts that bound the fore-deeps change in size, shape and degree of basement reactivation in a clockwise sense around the Banda Arc.

Abstract The Sivas Basin in central-eastern Anatolia is a north-verging salt-bearing fold-and-thrust belt including synorogenic salt tectonics. It formed between the northern leading edge of the Taurides platform and the Kırşehir block since Late Cretaceous time. We have constructed five regional cross-sections supported by field data and 2D seismic to constrain the structure of the basin and its evolution. The area is divided into three tectonic domains from south to north: (1) a Maastrichtian to Eocene north-verging fold-and-thrust belt, which terminates by a regional Eocene evaporitic level; (2) an Oligo-Miocene salt domain which contains two generations of minibasins separated by a salt canopy, forming a salt-and-thrust belt; and (3) a late Miocene to present day foreland basin. The cross-sections show the along-strike variations and the increasing shortening in the fold-and-thrust belt from west ( c. 15 km) to east ( c. 25 km). The thick salt allows for the intracutaneous propagation of the fold-and-thrust belt below a domain of salt withdrawal minibasins, decoupled as the initial salt thickness increases. In that case, the salt domain is thrusted both frontward and backward. Efficient exhumation followed by erosion of the fold-and-thrust resulted in synorogenic salt tectonics in the foreland and thus increased the mechanical resistance between them.