Abstract: Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation. This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers. The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).

Abstract The 16 chapters presented in this memoir cover some of the recent advances made in the descriptions and analysis of thrust-related fold systems. The chapters include kinematic and geometric analyses of fault-propagation folding, trishear folding, detachment folding, wedge-thrust fold systems, and basement-involved thrust systems. Examples are given from the Zagros fold belt of Iran; the sub-Andean fold belt of western Argentina; the frontal fold belt of the Tianshan, northern margin of the Tarim Basin in western China; from the fold and thrust belts of the Spanish Pyrenees, Taiwan, Wyoming, and southern California; as well as the deep-water fold belts of offshore Brazil and the Niger Delta. In particular, several chapters focus on new analyses of detachment folding as well as improved kinematic and geometric models of thrust-related folding. Improved seismic imaging combined with theoretical, numerical, and analog modeling, plus the detailed field studies as presented in this volume, indicates the range of challenges and the strategies that can be brought to bear in integrating the well-established geometric and kinematic models of thrust fault-related folding with mechanical models to account for the natural complexities of real-world structures. I hope that the readers of this memoir will find these new ideas and concepts relevant for the exploration and exploitation of hydrocarbon systems in fold and thrust belts worldwide.

Abstract Detachment folds seem conceptually the simplest of all fault-related folds, yet they continue to offer substantial and rich challenges that are symptomatic of fault-related folding in general. The standard picture of detachment folds goes back at least to August Buxtorf's widely reproduced 1916 regional cross sections of the Swiss Jura in which he interpreted the Jura box folds to be regionally detached from the basement along a weak layer of Triassic evaporites. Most theoretical and conceptual models of detachment folding have followed Buxtorf's model with a twofold mechanical stratigraphy composed of a competent flexural lid conserving bed length overlying a weak basal detachment layer that conserves only volume. However, this standard model of detachment folding has a fundamental and classic problem from a balancing perspective, which is outlined in this chapter. Furthermore, the classic Jura box folds drawn by Buxtorf were in fact not at all constrained by subsurface data, except along the Grenchenberg railroad tunnel where a surprising and much more complex structure was encountered, involving a strongly folded thrust fault that was intersected three times in the 8.5-km (5.3-mi)-long tunnel. Hans Laubscher argued that such complexly folded thrust faults are in fact typical of many Jura box folds. Analogous structures have been observed in Canada, Mexico, and Spain. I argue here on the basis of mass-balance considerations that detachment folds with folded thrusts are one of several theoretically expected modes of detachment folding. More broadly, the classical models of detachment folding assume a closed-system behavior in which all shortening is locally consumed in the fold. These classical models include (1) local flow of a weak basal layer, for example, salt (Wiltschko and Chapple mechanism) and (2) pure-shear detachment folding (Groshong and Epard mechanism). Here we discuss several additional models of detachment folding, including open-system behavior with (3) far-field flow of the weak basal layer, (4) roof detachments above the basal layer, and (5) roof ramps above the basal layer such as the folded thrust fault of the Grenchenberg anticline. In this final mechanism, the flexural lid or other more competent stratigraphic intervals shorten by a combination of fault slip and flexure, whereas the less competent intervals, including the basal layer, shorten by flow. All of these mechanisms are end members capable of existing in combination within actual folds; several examples combine two or three of these mechanisms.

Abstract The thickness variations and shapes of strata deposited over actively growing folds provide a convolved record of the interaction of deformation and sedimentation. Here we show how key elements of the history of shortening can be extracted using area-of-relief measurements on many stratigraphic horizons in well-imaged structures. In pregrowth strata, the shortening is equal to the vertical gradient in the area of structural relief, S = d A /d z . In growth strata, this relationship must be modified because the observed gradient in the area of relief now includes the effects of both structural thickening and stratigraphic thinning caused by deposition over the growing structure, S = (d A /d z ) obs — (d A /d z ) strat . This stratigraphic thinning cannot be measured directly from thickness variations because of later structural thickening but can be determined through consideration of the ratio of the mean shortening rate to the sedimentation rate, , which is a key parameter. We apply these concepts to a set of actively growing detachment folds: Nankai Trough, Japan; Cascadia, offshore Oregon; Yaken anticline, western China; and Agbami anticline, Niger Delta. These examples show a considerable diversity and complexity, including the effects of excess area in weak basal detachment layers, multiple detachment levels, and large differences in . All show not only constant shortening as a function of height in pregrowth strata and approximately constant shortening rates, , over substantial stratigraphic thicknesses, but also abrupt and large increases in shortening rate by factors of 5-10 and significant hiatuses in fold growth. The typical shortening rates in these frontal structures are in the range of 0.1-3 mm/yr (0.003-0.12 in./yr) and represent only 1-10% of the regional plate tectonic rates of their larger tectonic settings (2-6 cm/yr [0.8-2.4 in./yr]), indicating that shortening is not currently concentrated in these frontal zones of these mountain belts and accretionary wedges.

Abstract Field data in combination with interpretation of old seismic lines across the Pusht-e Kuh arc and northwest Dezful embayment in the Zagros fold belt allow the investigation of the geometry of folding at different structural levels. Folds in the Pusht-e Kuh arc are exposed along the upper part of the 7-km (4.34-mi)-thick Competent Group level, which is folded between the main detachment at or near the base of the cover sequence (lower Mobile Group) and the Gachsaran evaporites (upper Mobile Group). Intermediate detachment levels (Triassic Dashtak, middle Cretaceous Garau and Kazhdumi, Paleocene Amiran, and middle Miocene Kalhur formations) within the Competent Group control the geometry of anticlines at surface as well as their variations with depth. The Kabir Kuh anticline is the largest and highest anticline in the Pusht-e Kuh arc and has been selected to construct a geometrical model to explore the variations of folding style with depth. Results from this anticline constituted a backbone for both a regional study to link the mechanical stratigraphy to the structure and to build up a conceptual model for folding that may apply to the Pusht-e Kuh arc as well as to the Dezful embayment tectonic domains. The Kabir Kuh anticline in its central part displays box fold geometry, characterized by a wide and rounded crestal domain, which is slightly tilted to the southwest. This geometry developed above an intermediate detachment at the level of 1.3-km (0.81-mi)-thick Triassic Dashtak evaporites within the Competent Group. Below this detachment, the geometry of the fold changes to more acute with a narrower crestal region. Although conjectural, we propose low-angle thrusting forming a tectonic wedge to accommodate shortening at the deepest part of the anticline (Paleozoic sequence). Subsidiary thrusts, related to fold tightening, may reactivate axial surfaces as well as local detachment levels to propitiate displacements of the crest of the anticline above its forelimb as inferred along the southeastern segment of the Kabir Kuh anticline. A conceptual model of folding characterized by changes in fold geometry at depth is proposed. A significant outcome of this model is that anticlines in the Passive Group may be displaced, sometimes few kilometers, from anticlines in the Competent Group. In the same way, the internal mechanically weak layers of the Competent Group may form intermediate detachments that can produce a significant change in fold style with depth and displace the upper part of the structure shifting again the position of the anticline crests. The uplifted Pusht-e Kuh arc is an excellent natural laboratory to investigate potential relationships among mechanical stratigraphy, tectonic structure, time of oil generation, and time of trap development. Fold and thrust geometries investigated in this study are directly applicable to petroleum exploration of recently awarded exploration areas within the Pusht-e Kuh arc and may apply to the rest of the Zagros fold belt.

Abstract Trishear is a kinematic model of fault-propagation folding in which the decrease in displacement along the fault is accommodated by deformation in a triangular shear zone radiating from the tip line. This model has garnered increasing acceptance, particularly for cases where parallel kink-fold models do not work (e.g., footwall synclines, lateral and vertical changes in bedding thickness, and orientation). The articulation of the model in terms of velocity fields has enabled systematic explorations of the parameters controlling the trishear geometry; rapid, objective application of trishear to the simulation of real structures; and application of the model in three dimensions. The model has highlighted the importance of a parameter not unique to trishear, the propagation to slip ratio, which has a profound effect on fold geometry and is fundamental to understanding all types of fault-related folds. The drive to understand the significance of such parameters has instigated the application of several mechanical modeling strategies. Block-motion viscous, finite-element, and discrete-element analyses have all provided insight into trishearlike fault-propagation folds. Clearly, from these models, trishear most successfully simulates fold geometries where significant layered anisotropy is absent and the material is incompressible. Despite these modeling efforts, the significance of the trishear apical angle remains elusive. Trishear has been applied to a variety of real-world problems, including growth strata analysis, potential fracture distribution, paleoseismology, and even seismic hazard analysis.

Abstract Pressure-solution cleavage is frequently among the most abundant mesostructures in carbonate thrust wedges. It can exert a primary function in fluid migration, and consequently, understanding its time-space evolution can significantly impact the reservoir modeling and performance. The evidence that pressure-solution cleavage is commonly at a high angle to bedding and, in many cases, displays a frequency distribution relating to the host-fold geometry indicates a partial synfolding development driven by fault-fold kinematics. Double-edge fault-propagation folding assumes layer-parallel shortening during fold evolution. Accordingly, this model can provide a tool for inferring the distribution of pressure-solution cleavage within thrust-related folds that, under appropriate stress conditions, can significantly improve secondary porosity and permeability in reservoirs. We summarize the pressure-solution cleavage pattern in three anticlines that have possibly developed by double-edge fault-propagation folding, and then we analyze the deformation patterns associated with double-edge fault-propagation folding, investigating the influence of different model parameters (i.e., ramp propagation history, shape, and initial length) onto the cross-sectional deformation pattern in fault-propagation anticlines. Modeling results indicate that, in carbonate thrust wedges, forelimb panels and footwall sectors close to the thrust ramp can provide promising targets for hydrocarbon exploration.

Abstract This chapter shows, from the theoretical point of view, that growth strata geometries above angular unconformities depend on the modification of initial unconformity angles due to different folding mechanisms operating in underlying layers. This theory is applied to the Sant Llorenç de Morunys growth structure, where the analysis of the unconformity angles shows that classic folding mechanisms, such as layer-parallel simple shear in both angular and curved folds, and tangential-longitudinal strain operated in the forelimb of a fault-propagation fold. These folding mechanisms, inferred from changes in unconformity angles, are consistent with the minor structures and lithological features of the involved stratigraphic units. Forward modeling, including limb rotation, hinge migration, and the above-mentioned folding mechanisms, explains the main structural features of the syntectonic sediments in the Sant Llorenç de Morunys fault-propagation fold.

Abstract Two adjacent active thrust ramps in western Taiwan show contrasting hanging-wall structural geometries that suggest different kinematics, although they involve the same stratigraphic section and basal detachment. The Chelungpu thrust shows a classic fault-bend folding geometry, which predicts folding by kink-band migration without limb rotation, whereas the hanging wall of the Changhua thrust shows the characteristic geometry of a shear fault-bend folding, which predicts a progressive limb rotation with minor kink-band migration. We test the kinematic predictions of classical and shear fault-bend folding theories by analyzing deformed flights of terraces and co-seismic displacements in the 7.6 moment magnitude scale Chi-Chi earthquake. In particular, differences in terrace uplift across active axial surfaces are used to show that the assumptions of classical fault-bend folding are closely approximated, including constant fault-parallel displacement, implying conservation of bed length, and hanging-wall uplift rates that are proportional to the sine of the fault dip. This provides a basis for precise determination of total fault slip because the formation of each terrace, combined with terrace dating, gives long-term fault-slip rates for the Chelungpu thrust system. Even the coseismic displacements of 3 to 9 m (10 to 29 ft) in the Chi-Chi earthquake are approximately fault parallel but have additional transient components that are averaged out over the time scale of terrace deformation, which represents 10–100 large earthquakes. In contrast, terrace deformation in the hanging wall of the Changhua thrust ramp shows progressive limb rotation, as predicted from its shear fault-bend folding geometry, which combined with terrace dating allows an estimation of the long-term fault-slip rate of 21 mm/yr (0.83 in./yr) over the last 31 ka. A combined shortening rate of 37 mm/yr (1.46 in./yr) is obtained for this part of the western Taiwan thrust belt, which is about 45% of the total plate-tectonic shortening rate across Taiwan. The Changhua shear fault-bend fold ramp is in the early stages of its development with only 1.7 km (1.06 mi) total displacement, whereas the Chelungpu classical fault-bend fold ramp in the same stratigraphy has nearly an order of magnitude more displacement (∼14 km [8.7 mi]). We suggest that shear fault-bend folding may be favored mechanically at low displacement, whereas classical fault-bend folding would be favored at large displacement.

Abstract The present bathymetry, basin geometries, and spatial earthquake distribution in the inner California borderlands reflect complex basin inversion processes that reactivated two low-angle Miocene extensional detachments as blind thrust faults during the Pliocene to Holocene. The Oceanside and the Thirtymile Bank detachments comprise the inner California blind thrust system. These low-angle detachments originated during Neogene crustal extension that opened the inner California borderlands, creating a rift system that controlled the deposition of early to late Miocene sedimentary units and the exhumation of the metamorphic Catalina schist. During the Pliocene, a transpressional regime induced by oblique convergence between the Pacific and the North American plates reactivated the Oceanside and the Thirtymile Bank detachments as blind thrust faults. This reactivation generated regional structural wedges cored by faulted basement blocks that inverted the sedimentary basins in the hanging wall of the Miocene extensional detachments and induced contractional fold trends along the coastal plain of Orange and San Diego counties. Favorably oriented high-angle normal faults were also reactivated, creating zones of oblique and strike-slip faulting and folding such as the offshore segments of the Rose Canyon, San Diego, and the Newport-Inglewood fault zones. We evaluate several different styles of geometric and kinematic interactions between these high-angle strike-slip faults and the low-angle detachments, and favor interpretations where deep oblique slip is partitioned at shallow crustal levels into thrusting and right-lateral strike-slip faulting. Analyses of seismic reflection profiles, well data, earthquake information, and sea-floor geology indicate that the Oceanside and the Thirtymile Bank blind thrust faults are active and represent important sources of earthquakes in this region. Restored balanced cross sections provide a minimum southwest-directed slip of 2.2–2.7 km (1.4–1.8 mi) on the Oceanside thrust and illustrate the function of this detachment in controlling the processes of basin inversion and the development of the overlying fold and thrust belt.

Abstract The east–west-trending late Cenozoic Kuqa fold belt is a part of the compressive southern margin of the Tianshan Mountains in western China. Approximately 20,000 km (12,000 mi) of two-dimensional seismic reflection profiles are integrated with surface geology and well data to examine the deformation style and structural evolution of the Kuqa fold belt. Mesozoic through Holocene strata in the northern Tarim Basin have been deformed in a thrust system that roots northward into the Paleozoic basement of the southern Tianshan. The south-vergent deformation is characterized by a series of forward-breaking thrust faults, fault-related folds, and detachment folds. Two major decollement levels exist: an upper detachment in salt-gypsum lithologies in the Paleogene–Miocene Kumgeliem, Suweiyi, and Jidike formations, and the lower detachment mostly within Jurassic coal and mudstone strata. Fault-propagation folds developed above both detachments and have been refolded in some cases by displacement on the lower thrust faults. Imbricate thrust faults and duplex structures linking the two detachments developed with salt that apparently flowed into the cores of the duplex structure. Near the high Tianshan mountain front, Mesozoic and Cenozoic strata are involved in deformation that began at approximately 25–26 Ma as documented by growth strata north of Kuqa. Toward the southward limit of the fold belt, Miocene through Holocene strata are folded in the Quilitage and Yaken anticlines, which began growing above a thrust system that propagated at about 5.5 Ma. The Yaken anticline at the south edge of the eastern Kuqa fold belt has only emerged as a topographic anticline in the last 0.2–0.3 Ma associated with an acceleration of the Quilitage-Yaken thrust system. Structural restoration suggests a shortening of 15–20 km (9–12 mi) across the eastern Kuqa fold belt. Considering that this shortening began about 25 Ma, the average shortening rate was about 0.7 mm/yr (0.03 in./yr). Because the frontal thrust system underlying the Quilitage and Yaken anticlines has a shortening of 6 km (3.7 mi) that began approximately 5.5 Ma, their average shortening rate is about 1.1 mm/yr (0.04 in./yr). However, the shortening rate on this frontal system from about 5.5 Ma to about 0.2–0.3 Ma is approximately 0.6 mm/yr (0.02 in./yr) followed by an acceleration to about 4–5 mm/yr (0.16–0.19 in./yr) at approximately 0.2–0.3 Ma, causing the topographic emergence of these structures. These results indicate that shortening rates in the Kuqa fold belt have increased in the late Pleistocene, which is consistent with more regional present-day geodetic shortening rates of about 9 mm/yr (0.35 in./yr) across the southern Tianshan, which also indicate a substantial acceleration relative to Neogene shortening rates.

Abstract In this chapter, we study the geometry and evolution of the Andean fold and thrust belt located between 35° and 36°S. The main goal of this chapter is twofold; the first is to propose a new Neogene chronostratigraphic scheme based on outcrop studies and a new 39Ar/40Ar and K-Ar dating of volcanic intervals. The second is to integrate the structural kinematics deduced from the age and internal architecture of Neogene strata with two balanced cross sections to define the main pulses of fold-belt activity during the Miocene and Pliocene. Fold-belt kinematics is interpreted applying the Coulomb critical wedge model, and a succession of at least two cycles of activity is proposed. The hybrid thick-thin-skinned structural style of the fold belt is characterized by deep basement structures transferring shortening to the sedimentary cover, which is deformed into forethrust, back thrusts, and duplexes complicated by local salt tectonics. A net shortening of 30 km (19 mi) was calculated via restoration of two balanced cross sections. Clastic, pyroclastic, and volcanic synorogenic deposits form two megasequences called the Pincheira-Ventana of Miocene age (16–7 Ma) and the Malargüe of Pliocene age (5–1 Ma) composed of four tectonosequences, S1, S2, S3, and S4, respectively. Active structures acted as barriers bounding local independent synorogenic depocenters as is shown by palinspastic and paleogeographic reconstructions. Two main pulses of deformation and a general foreland migration of deformation are evidenced by the spatial and temporal arrangement of synorogenic depocenters. Fold-belt kinematics is interpreted as a succession of two cycles of activity applying the Coulomb critical wedge model. Each cycle included a supercritical stage with out-of-sequence reactivation of basement structures and foreland migration of deformation followed by a subcritical stage with relative low rates of structural uplift and consequent high relative depositional rates in synorogenic basins. The results of this work agree with a progressive north to south deactivation of the Neogene deformation front between 33 and 37° being still active to the north but fossilized during the Pliocene and Miocene in the central and southern parts, respectively.

Abstract In a typical wedge structure developed in layered sedimentary rocks, displacement is transferred from a deep fault to a shallow fault. The deep and shallow faults dip in opposite directions and merge with one another to form a wedge-shaped indentor. Displacement within the wedge fault system is transferred vertically into structures overlying the faults instead of being transferred laterally toward the foreland as in a typical fault-bend fold. Wedge structures associated with basement-involved contractional uplifts are commonly observed where faults that uplift a basin margin emanate from competent basement rocks into layered sedimentary rocks within the basin. Two categories of basement-involved wedge structures have been identified based on the level of the wedge point with respect to the basement surface. In the first category, the wedge point occurs within the sedimentary section overlying the basement, and in the second category, the wedge point occurs within the basement material. Interpretations of both categories of basement-involved wedge structures imaged in long-offset seismic data from the Hanna Basin in south-central Wyoming are presented.

Abstract This work is based on a three-dimensional (3-D) reconstruction methodology for geologic structures from field and subsurface data. The methodology consists of several steps: (1) collection and georeferencing of data, i.e., 3-D digitalization; (2) analysis of data and definition of a 3-D geometric and stratigraphic model, i.e., structural and stratigraphic analysis; and finally, (3) the reconstruction of key surfaces that form the structure, i.e., reconstruction of the reference surface and reconstruction of additional surfaces, both honoring the defined geometric model. The methodology has been applied to a natural example, the Sant Corneli anticline, a thrust-related fold located in the southern Pyrenees. This fold, oriented approximately east–west, has a complex 3-D geometry, with stratigraphic and structural variations, both laterally and vertically. This chapter focuses on the study of the Late Cretaceous postrift series. Outcrop information has been collected to reconstruct the superficial geometry of the Sant Corneli anticline. Seismic profiles that cross the area have been interpreted to reconstruct the geometry of the thrust fault at depth. The reconstruction methodology at surface and at depth is made following the same workflow, adapting it to the different nature of the original data sets. At the same time, the use of 3-D forward numerical modeling allows us to explore the relationship between the fold and the thrust, the main factors that influence fold development through space and time, and to find the kinematic model(s) that best fit the proposed 3-D reconstruction. The main contribution of this work is the incorporation of forward modeling techniques in the 3-D reconstruction workflow to independently establish the relationship between the various reconstructed geologic surfaces. Once this relationship is known, it is possible to refine the reconstructed surfaces and integrate them in a single and comprehensive 3-D model that honors both the available data and the established kinematic model. This technique is also used to analyze the possible kinematic evolution of a fault-related structure that best reproduces the deformed geometry obtained through the 3-D reconstruction process. Moreover, this study also improves the developed 3-D reconstruction methodology as it incorporates the use of isopach maps.

Abstract The effects of syncontractional sedimentation and erosion on simple, critically tapered Coulomb wedges were evaluated by conducting twelve two-dimensional analog model sandbox experiments. All 12 models produced critically tapered Coulomb wedges with topographic slopes of 6–10° above horizontal basal detachments. The model without syncontractional sedimentation or erosion exhibited a general forward-breaking sequence with synchronous thrust activity. Syncontractional sedimentation produced longer wedges composed of fewer major forward-vergent thrusts and lowered thrust activities in the foreland. Syncontractional erosion inhibited forward propagation of the deformation front, decreased the number of major thrusts, and increased thrust activities in the hinterland. Where combined, the effects of syncontractional sedimentation and erosion were complementary. At the scale of individual folds, syncontractional sedimentation altered fold evolution by producing limb rotation and a front-limb trishear zone formed by tip-line thrust splays. At this scale, syncontractional erosion did not cause significant changes to the fold geometries as they developed. Comparisons of the model thrust wedges with natural fold and thrust wedges indicate that the Nankai accretionary prism, with its well-ordered array of closely spaced thrusts, would be typical of fold and thrust belts with low rates of surface processes. In contrast, the fold and thrust belts of offshore Niger Delta, the central Apennines, and the sub-Andes are characterized by buried, widely spaced, low-activity thrusts in the foreland that would be typical of high syncontractional sedimentation. High syncontractional erosion would produce very active hinterland thrusts resembling the present-day Taiwan fold and thrust belt. Changes in thrust-wedge dynamics caused by increased syncontractional erosion in the model wedges imply that subaerial fold and thrust belts, with higher erosion, would evolve differently from their submarine counterparts.

Abstract Two gravitational fold and thrust belts (GFTBs) from the deep waters of the Pará-Maranhão and Barreirinhas basins were interpreted in two-dimensional seismic sections and analyzed using the concepts of fault-related folding and taper-wedge mechanics. These basins lie in the equatorial Atlantic Ocean margin of Brazil, a classic example of transform to oblique (transtensional) continental margin. There, deep-water anoxic shales of probably Turonian age served as decollement zones to Late Cretaceous and Paleogene predominantly siliciclastic wedges to slide down from upper slope (extension) to lower slope (contraction) realms. The structural style of both fold belts is typical thin-skinned tectonics with imbricate thrust faults branching upward from the detachment level with associated fault-related folding. The direction of tectonic transport is from the coast to offshore (southwest to northeast). The similarities and differences between the GFTBs were highlighted. The most striking difference regards the distribution of contractional strain throughout the fold belt. In the Pará-Maranhão GFTB, a single major contraction event was achieved via eight regularly spaced imbricate thrust faults. Intervening slices present all types of classic fault-related folding (fault-bend, fault-propagation, and detachment folds). In the Barreirinhas GFTB, an earlier minor contraction was also spread over regularly spaced faults; however, a major late contraction was achieved by one main thrust fault with one major fault-related fold associated to it. This fold is predominantly a shear fault-bend fold whose geometry varies slightly along strike. Their main similarities regard the discrete nature of the detachment level, their structural coherence and narrowness, their low to moderate wedge taper, and their noncritical nature of the taper wedge. Both present syntectonic growth strata that record variations in the balance between the rates of sedimentation and structural uplift of the fold, and folding by limb rotation. The fault-related folding thus determined and the parameters established for wedge taper are slightly different from those presented by active submarine fold and thrust belts at convergent margins and passive margins throughout the world. The decollement dip of the GFTBs is significantly lower, and the bathymetric slope is somewhat higher than elsewhere around the globe.

Abstract The evolution of thrust structures in the toe-thrust belt of the Niger Delta has been analyzed and reveals a common growth history. Once initiated, structures propagate rapidly along strike with minimal slip. Subsequently, the propagation slows or ceases, and slip accumulation begins to dominate. In the waning stage, deformation retreats toward structural culminations. The pattern of slip evolution across the deep-water thrust belt indicates that neighboring structures are active simultaneously. A structural restoration across the western Niger Delta suggests that a series of detachment advances in the late Miocene initiated when the Akata shale detachment horizon was buried to a depth of around 4 km (2.5 mi) below mud line. These produced a deep-water fold and thrust belt accommodating more than 12 km (7 mi) of shortening. Deformation combined with compaction disequilibrium may have helped produce elevated pore pressures approaching the fracture gradient in the detachment zone. Once the thrust structures began to attain structural height, pore-pressure transfer developed in interbedded sands and silts within the long backlimb of the fault bend folds. This process may have influenced the behavior of the fault zones. A simple model of pore-pressure evolution within the growing structures indicates that pressure transfer can help explain the localization of out-of-sequence thrust growth and might facilitate the focusing of slip toward the central high-relief parts of thrust structures.

ABSTRACT A new method is presented for unraveling some aspects of the kinematic evolution of thrust-related folds. This technique consists of measuring crestal structural relief, shortening, and the area of a fold for different amplification stages, and then plotting the crestal structural relief and the fold area versus the shortening. One of the main advantages of this technique is that the data can be obtained from different sources: a section across a fold with associated syntectonic sediments, different sections across a fold that underwent a lateral shortening gradient, or different sections across a fold at different amplification stages. This technique has been applied to theoretical, natural, and experimental thrust-related folds. The analyses carried out show that each fold had a different kinematic evolution. They show also that, except for the theoretical examples, the kinematic evolution may be very complex, because different folding mechanisms may operate during fold amplification and increases/decreases of fold area and thickening/thinning of beds may occur. Therefore, to model or sequentially restore natural/experimental thrust-related folds, we recommend the application of techniques such as the one we propose here, when possible. This would avoid the automatic application of forward models based on kinematic assumptions and geometric resemblance between the model and the actual example.