Balanced Cross Sections Research Articles

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

AbstractThe Moine Thrust Zone forms the Caledonian orogenic thrust front where the Moine Supergroup metamorphic rocks have been thrust westward across the Laurentia plate stable foreland, comprising Archean-Proterozoic granulite and amphibolite facies rocks (Lewisian gneisses), with unconformably overlying Mesoproterozoic and Neoproterozoic Torridonian clastic sediments and Cambrian-Ordovician passive margin sedimentary rocks. Four major thrusts beneath the Moine thrust in the Assynt window include the (i) Ben More Thrust, which places the Loch Ailsh syenite intruded into Lewisian basement and Cambrian-Ordovician sedimentary rocks over the Sole thrust sheet, (ii) Glencoul thrust, which places Lewisian basement and folded cover rocks over Cambrian-Ordovician sedimentary rocks, (iii) Borralan thrust, which carries a large alkaline syenite intrusion beneath the Ben More roof thrust and (iv) the Sole thrust sheet, which carries imbricated Cambrian-Ordovician sedimentary rocks and lamprophyre sills over the stable foreland. Three further thrust sheets within the Lewisian basement gneisses are now recognised through restoration of balanced cross-sections, which were responsible for doming of the Assynt window. Although the Moine thrust is mapped as a single line on the map it encompasses, (a) deep ductile shear zone formed of mylonites derived from hangingwall Moine schists, footwall Cambrian quartzites and Ordovician limestones, and basement Lewisian gneisses, (b) roof thrust of the Glencoul and Ben More Thrust sheets and (c) brittle out-of-sequence motion where the Moine schists have been thrust over mylonites, which directly overlie the stable foreland (Knockan Crag).

Tectonics of the Western Internal Jura fold-and-thrust belt: 2D kinematic forward modelling

The balancing technique, called 2D kinematic forward modelling, is a powerful tool to understand the kinematic evolution of fold-and-thrust belts. This study presents a new 2D kinematic forward model for the westernmost Internal Jura fold-and-thrust belt (FTB), situated immediately adjacent to the Geneva Basin. The technique used not only provides a new valid balanced cross-section but also offers new insights regarding the kinematic evolution of the Western Internal Jura FTB. Our model proposes a pure thin-skinned style dominated by forward stepping deformation accompanied by minor back-stepping thrust sequences. A first deformation step is attributed to the thrusting of the Crêt de la Neige Anticline, followed by the Crêt Chalam Thrust and its imbrications. This is followed by thrusting along the Tacon and the Bienne thrusts. Imbricate fault-bend folding explains the steep southern limb of the Crêt de la Neige and the Bellecombe anticlines. 2D kinematic forward modelling yields a total amount of shortening by 23.6 km for the Western Internal Jura FTB. In addition to the primary décollement located at the base of the Keuper Group evaporites, three other décollements are found within the marly layers of the Aalenian “faciès de transition” units, the Oxfordian “Couches d’Effingen-Geissberg” members and the Berriasian Goldberg formation. The multiple thrust horizon approach is supported by new precise seismic interpretations. Our model provides a valid alternative to previous models that either propose local thickening of the Triassic evaporites or inversion of normal faults in the basement. This fully explains the elevated position of the Mesozoic cover in the Jura FTB.

Impact of mechanical stratigraphy on deformation style and distribution of seismicity in the central External Dinarides: a 2D forward kinematic modelling study

The External Dinarides fold-thrust belt formed during Mid-Eocene–Oligocene times by SW-propagating thrusting from the Internal Dinarides towards the Adriatic foreland. Although previously considered as structurally quite uniform, recent work reported along-strike contrasting deformation styles in two structural domains within this fold-thrust belt. The two areas with very contrasting deformation styles are separated by the N–S-striking dextral Split-Karlovac Fault, a 250 km long, transpressive transfer fault. The southeastern domain is characterized by a thin-skinned SW-vergent nappe stack in contrast to the northwestern domain, where a set of blind, thick-skinned top-SW thrust duplexes prevail underneath the passive NE-vergent backthrusts. To better understand why the External Dinarides underwent such contrasting along-strike deformation, we reconsidered a temporal and spatial along- and across-strike distribution of Paleo-Mesozoic lithofacies to both sides of the Split-Karlovac Fault and estimated the role of mechanical stratigraphy on deformation styles in this part of the fold-thrust belt. Therefore, we constructed a new 2D kinematic forward model in the western backthrust-dominated domain. Our best-fit forward-modelled balanced cross section across the central Velebit Mtn. portrays a 75 km wide triangle zone. This zone took up at least 47 km of shortening during Eo-Oligocene times. It comprises a set of thin-skinned NE-vergent backthrusts detached in the upper Paleozoic atop a SW-vergent thick-skinned antiformal stack detached in the lower Paleozoic Adriatic basement. The NE-vergent backthrusts likely nucleated at lateral facies boundaries related to extensional half grabens that locally formed during Middle Triassic and Late Jurassic passive margin extension. During the Eo-Oligocene folding and thrusting, the selective inversion of inherited Mesozoic half grabens boundary faults into the NE-vergent backthrusts in the northwestern domain led to the observed along-strike changes in the deformation style of the External Dinarides. A seismotectonic analysis of instrumentally recorded earthquakes suggests contrasting seismic behaviour along the central and southern Velebit transects within the northwestern structural domain. The central Velebit Mtn. triangle structure appears to currently accommodate dominantly strike-slip motion, with reverse faulting being confined to east of the Split-Karlovac Fault. In contrast, seismicity along the southern Velebit cross section appears to be confined to the structurally lowermost parts of the triangle zone and the foreland, while it´s structurally higher parts are less seismically active. Also, a predominance of reverse faulting along this transect suggests ongoing accommodation of shortening in this part. Our results indicate that both the variations in the mechanical stratigraphy and the pre-orogenic structural inheritance obtained during rifting and passive margin stages exert control on contractional structures within the External Dinarides, including the distribution of present-day seismicity.

Tracing the “missing shortening” in fold-and-thrust belts: Insights from structural analyses of the Hotan-Mazatagh transect in the West Kunlun foreland, NW China

Use of balanced cross-section methods, which assume length and area conservation in two dimensions, to quantitatively analyze fold-and-thrust belts is vital in structural analyses. However, layer-parallel shortening is beyond this assumption and thus indescribable by balanced cross-section methods. The contribution of layer-parallel shortening in accommodating shortening has rarely been quantitatively determined, especially in sections hundreds of kilometers in scale. In this study, we conducted structural analyses on the ∼230-km-long Hotan-Mazatagh transect along the West Kunlun foreland of the NW Tibetan Plateau, based on interpretations of seismic reflection profiles. The results reveal that the transect forms a long thrust sheet detaching along the basal Cenozoic detachment, with deformation localized in the Hotan, Manan, and Mazatagh belts. Out of the ∼23.82 km of cutoff displacement translating into the thrust sheet, only ∼1.4 km and ∼4.94 km are absorbed by the Hotan anticline and the Mazatagh detachment thrust, respectively. The remaining ∼17.48 km extent of shortening is absorbed by layer-parallel shortening in the ∼180-km-long section between the Hotan and Mazatagh belts. The layer-parallel shortening produces a strain rate of ∼2.43 × 10−8 yr−1 and absorbs ∼77.97% of the shortening of the segment. Our results argue for caution when using balanced cross-section methods to determine deformation magnitudes. We suggest that localized deformation and layer-parallel shortening function together to shape the ∼230-km-long Hotan-Mazatagh transect into a combined discrete-and-distributed deformation style.

The effects of positive tectonic inversion structures on the formation of thrust systems on the western Domeyko Cordillera, northern Chile: Implications for the orogenic development of the outer Andean forearc

This work aims to propose a novel structural setting for the outer Andean forearc. Specifically, we propose a new deformation mechanism related to thrusting belt systems induced by positive tectonic inversion processes in the western flank of the Domeyko Cordillera in northern Chile. Thrust belt systems have been rarely understood for the outer Andean forearc; in fact, the mention of thrusting is scarce and structural interpretations are restricted only to strike-slip faulting and positive tectonic inversion processes. In this study, we have constructed novel balanced cross-sections from new structural fieldwork and combined them with 2-D reflection seismic line re-interpretations, thus offering new ideas about the evolution and development of the forearc of the Central Andes. Based on this, we propose integrated kinematic models that describe the relationships between inherited extensional structures, positive tectonic inversion structures, and the formation of thrust systems. The proposed balanced cross-sections have been restored to a pre-shortening condition. In this way, two structural styles are proposed for the western flank of the Domeyko Cordillera. The first corresponds to west-directed positive tectonic inversion structures, which expose and extrude the Middle Jurassic syn-rift deposit of the ancient Mesozoic rift basin. The second consists of east-directed thin-skinned thrust systems, mainly affecting the Upper Cretaceous to Eocene volcano-sedimentary synorogenic sequences. The interaction between inversion structures and thrust system structural styles would generate a west-directed intracutaneous wedge. Finally, the minimum shortening obtained from our balanced cross-section restorations is ca. 11% for the Río Loa and the San Salvador Canyons segment and ca. 56% for the Quebrada Mala segment. The formation of thrusting structures is controlled by a major detachment fault activated from a regional-scale angular unconformity, which we have named the Quebrada Mala detachment fault. An important part of the shortening was taken up and accommodated by buttressing structures and their associated intraformational detachment-thrust levels.

Characterization of the Soapaga Fault deformation in the central part of the Eastern Cordillera, Colombia

The eastern border of the Floresta Massif is characterized by the confluence of the Soapaga Fault and the Corrales Fault, two reverse faults that are linked to each other in a simple transfer zone and whose deformation influenced the formation of the Betéitiva Syncline. The Betéitiva Syncline, located on the Soapaga Fault footwall, shows geometric features, such as upright to inverted backlimbs and thickness changes related to the fault's deformation. However, none of the previously proposed kinematic models (fault-propagation, fault-bend, and buckling–folding models) clearly explain the evolution of the Betéitiva Syncline related to the Soapaga Fault or the differentiation between this fault and the Corrales Fault. In this work, we developed a structural analysis to differentiate the Soapaga Fault from the Corrales Fault through a detailed description of the fault zones, geomorphic evidence, geometric relationships, and deformation features related to each fault. Then, we developed a structural analysis of the evolution of the aforementioned structures using five balanced cross-sections that illustrate how they are related to one another. Three kinematic models or structural processes were used to explain the evolution of the study area: the trishear fault-propagation fold model, the fault-propagation fold model, and the gravitational fold and fault model. This work presents the structural, geological, and geomorphic evidence that allowed us to differentiate the Corrales Fault from the Soapaga Fault. Additionally, our results show an increase in the Soapaga Fault displacement (4.7 km–9.5 km), the Corrales Fault displacement (0.6 km–1.3 km), and the total shortening (1.9 km–3.4 km) from north to south. In addition, we propose a structural evolution model that better explains the geometric features of the Betéitiva Syncline regarding the deformation of the Soapaga Fault, which requires 10x less shortening than previous structural models. Finally, we propose that the deformation of the Soapaga Fault is a scale-dependent variable because of the relationship that this fault has with regional structures, such as the Floresta Anticlinorium, but also with local structures, such as the Betéitiva Syncline.

Intracontinental evolution of the southern Central Asian Orogenic Belt: Evidence from Late Triassic nappe structure in the northern Alxa region, NW China

• Field mapping indicated the geometry of nappe structure in the northern Alxa region, NW China. • The crustal shortening of the southern CAOB was established during the Late Triassic. • Two large-scale strike-slip systems dominated the Late Triassic tectonic framework of the southern CAOB. The Central Asian Orogenic Belt (CAOB) experienced multistage intracontinental deformation during the Mesozoic and Cenozoic. Recent studies have shown that a large-scale nappe structure developed in the northern Alxa region. Here, we present new field investigations, structural analysis, and a new balanced cross-section in order to better constrain its geometries, displacement and activity time. Three nappe stacks with distinctive lithostratigraphic assemblies, structural characteristics and displacement were recognized: (1) the structural highest China-Mongolia boundary nappe preserving as klippen has a minimum displacement of ∼65 km; (2) the Yagan thrust sheet is dominated by an E–W-trending syncline and has a minimum displacement of ∼30 km; and (3) the structural lowest Gashunzhadegai thrust sheet is characterized by S-vergent folds and imbricated faults and has a minimum displacement of ∼14 km (30%). The youngest strata involved in the nappe structure and the cross-cutting relationships indicated that the nappe structure formed during the Late Triassic (230-215 Ma). Regionally, it occurred later than the final closure of the Paleo-Asian Ocean and marked the beginning of the intracontinental evolution of the southern CAOB. Integrating previous structural, thermochronological and sedimentological data with our studies, we suggested that the Late Triassic tectonic framework of the middle segment of the southern CAOB was dominated by two large-scale en echelon strike-slip systems. In addition, the driving force for the Late Triassic intracontinental deformation was most likely to be the collision of the Bayan Har Terrane and Yangtz Craton with the Eurasian continent.

Reconstruction of the proto-type basin and tectono-paleographical evolution of Tarim in the Cenozoic

The Tarim Basin is the largest sedimentary basin in China, and it has experienced a complex tectonic evolutionary history. Reconstruction of the proto-type basin and tectono-paleogeography is helpful to understand the different stages of evolution of the sedimentary basin and basin-mountain relationship. It is significant to combine the basin with the regional tectonic background to discuss the process of basin-mountain coupling and the tectonic evolution of the peripheral orogenic belts. With a reliable residual thickness map and lithofacies map of the Tarim Basin in the Cenozoic, based on the amount of shortening we quantified from previous works and 81 balanced cross-sections, we restored the original range and compiled the proto-type basin map of Tarim Basin. From a compilation of previous studies on the lithofacies of peripheral blocks, the tectono-paleogeography of the Tarim Basin in Cenozoic has been reconstructed. The Indian Plate collided with the Eurasian continent at ∼45–40 Ma. The remote effect of the collision led to the resurrection and reactivation of the Kunlun and Tianshan Mountains. The Southwest Tarim and Kuqa rejuvenated foreland basins separately developed along the north front of the Kunlun Mountains and the south front of the Tianshan Mountains. The tectonic evolution process of the Tarim Basin in the Cenozoic was divided into two stages: 1) in the Paleogene, the Neo-Tethys Ocean retreated stepwise westward from the Southwest of the Tarim Basin, and the sedimentary lithofacies of the Southwest Tarim Depression were bay lagoon facies and lake facies; 2) the Neo-Tethys Ocean retreat finally occurred in the Tarim Basin during the Late Oligocene to Early Miocene, and it became an almost closed terrestrial basin, with the deposition of fluvial facies and lacustrine facies. The Cenozoic tectono-paleogeography of the Tarim Basin is closely related to the closure of the Neo-Tethys Ocean and the reactivation of the Kunlun and Tianshan Mountains.

Reconstruction of proto-type basin and tectono-paleogeography of Tarim block in early Paleozoic

Tarim Basin is a large, superimposed basin rich in petroleum resources, which has experienced many stages of complex tectonic-sedimentary evolution. As the basic geological study of the Tarim Basin, the proto-type basin and tectono-paleogeographic evolution are of great significance for understanding the distribution of petroleum reservoirs in the superimposed basin and provide tectonic background and theoretical guidance for petroleum exploration. According to the residual thickness map, as well as other lithofacies and seismic data, the scopes of the proto-type basin are determined by the marginal facies method and the thickness trend method, and the shortening amounts are calculated by the balanced cross-section method. Based on these data and previous works, four proto-type basin maps of Tarim Basin in present-day geographic coordinates and four tectono-paleogeographic maps of Tarim Basin in paleogeographic coordinates during the early Paleozoic are reconstructed, which directly show the changes of sedimentary and uplift-depression pattern caused by the transformation of the tectonic environment from extension to compression. In the Cambrian, the Tarim Basin was controlled by the extensional tectonic environment, with the sedimentary framework of “carbonate platform in the west, deep-water basin in the east”. At the end of the Ordovician, the Kudi Ocean and the North Altyn Ocean were closed, and the Central and South Kunlun terrane and the Altyn-Qilian terrane were collaged with the Tarim block, which directly led to the transformation of the uplift-depression pattern in the Tarim Basin from east-west differentiation to north-south differentiation, thus changing the sedimentary environment of the Tarim Basin in the late Ordovician to Silurian.

Total Shortening Estimates Across the Western Greater Caucasus Mountains from Balanced Cross Sections and Area Balancing

The Greater Caucasus orogen forms the northern edge of the Arabia-Eurasia collision zone. Although the orogen has long been assumed to exhibit dominantly thick-skinned style deformation via reactivation of high-angle extensional faults, recent work suggests the range may have accommodated several hundred kilometers or more of shortening since its ~30 Ma initiation, and this shortening may be accommodated via thin-skinned, imbricate fan-style deformation associated with underthrusting and/or subduction. However, robust shortening estimates based upon surface geologic observations are lacking. Here we present line-length and area balanced cross sections along two transects across the western Greater Caucasus that provide minimum shortening estimates of 130-200 km. These cross sections demonstrate that a thin-skinned structural style provides a viable explanation for the structure of the Greater Caucasus, and highlight major structures that may accommodate additional, but unconstrained, shortening.

Constraining the 3-D Geometry of Fold-Thrust Belts Using Section Balancing vs. 3-D Interpolative Structural and Probabilistic Modeling

Quantitative uncertainty analysis, 2-D and 3-D modeling of the subsurface, as well as their visualization form the basis for decision making in exploration, nuclear waste storage and seismic hazard assessment. Methods such as cross-section balancing are well established and yield plausible kinematic scenarios. However, they are based on geological data with errors and subject to human biases. Additionally, kinematic models do not provide a quantitative measure of the uncertainty of structures at depth. New 3-D modeling approaches have emerged that use computational interpolation, which are less dependent on human biases. Probabilistic extensions enable the quantification of uncertainties for the modeled structures. However, these approaches do not provide information on the time evolution of structures. Here, we compare classical cross-section balancing (2-D, kinematic modeling) with 3-D computational modeling to pave the way towards a solution that can bridge between these approaches. We show the strengths and weaknesses of both approaches, highlighting areas where probabilistic modeling can possibly add quantitative structural uncertainty information to improve section balancing. On the other hand, we show where probabilistic modeling still falls short of being able to cover the observed geometric complexities. We ultimately discuss how a workflow that iteratively combines results of the approaches can improve structural and kinematic constraints. As an example, we use the fold-and-thrust belt of the northern Alpine Foreland, the so-called Subalpine Molasse, focusing on the Hausham Syncline (Bavaria) and adjacent areas. We take advantage of the fact that here the stratigraphy as well as the tectonic history are well constrained. We show that shortening within the syncline progressively increases from west to east, independent from structural uncertainties. Two equally viable models can explain this. First, strain in the west is accommodated underneath the syncline in a triangle zone that progressively tapers out, or second, the strain difference is accommodated in more internal units. This highlights the importance of introducing uncertainty modeling also in kinematic restorations, as it enables identifying key regions, where different hypotheses can be tested.

The Malolotsha Klippe: Large‐Scale Subhorizontal Tectonics Along the Southern Margin of the Archean Barberton Greenstone Belt, Eswatini

AbstractWhether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best‐preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U‐Pb zircon dating, shows that the region preserves a tightly re‐folded imbricate thrust stack in which metavolcanic and ‐volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward‐verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present‐day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late‐stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically‐ and horizontally‐dominated deformation styles that have interfered with each other at small regional and short temporal scales.

Structural inheritance and style within the Getic Depression, South Carpathians, Romania

The Carpathians are part of the Alpine orogen, and the Getic Depression (or Getic fold belt) is located to the south of the South Carpathian. Underthrust beneath the fold belt is the Moesian Platform lower plate. This foreland depocenter can reach a depth of up to 10 km and evolved in an oblique convergent setting from latest Cretaceous-Paleogene and was shortened during the Badenian-middle Sarmatian (middle to late Miocene, 16–8 Ma) and Pliocene to recent. Thus far, in previous publications, the role of the structural grain of the lower plate was poorly constrained. For the fold belt, contrasting interpretations implied a range of shortening magnitudes and contrasting fault pattern. This contribution shows seven balanced cross sections that rely on seismic reflection and well data. One section, just outside and to the south of the fold belt, gives insights into the fault network of the Moesian Platform. In that section we see 35 km of north westward directed normal sense extension that creates a set of rotated fault blocks. Growth strata within the half grabens attest to Permo-Lower Triassic extension age. The individual normal faults merge at depth into a common detachment that rises monotonously towards the east and forms a domal-shaped, arched detachment. This is the structural grain that we project down plunge towards the north beneath the fold belt. The interpreted six sections that lie in the tectonic transport direction show that structures in the western part of the Getic fold belt are compatible with the inversion on the underlying Permo-Lower Triassic extensional fault. Ubiquitous truncation of Oligocene strata by overlying units suggest inversion started during late Oligocene (intra-Burdigalian). The Badenian-Sarmatian shortening further deforms the pre-existing contractional features and the shortening increases from 5 km on the west to at least 17 km on the east. NW-SE oriented transfer zones with normal faults separate the fold belt domains with diverging displacement vectors and eastward increasing shortening. Two main salt levels within the fold belts’ sediment pile are laterally discontinuous hence they only have local significance in the deformation. The frontal part of the fold belt consists predominantly of Badenian-Sarmatian synkinematic units as the fold belt propagated into the foredeep units, this interpretation requires less shortening in the frontal structures compared to earlier solutions. Due to the lack of characteristic features on the studied sections, we infer that the Getic fold belt did not evolve as a transtensional depocenter during the lower Miocene as suggested by earlier publications.

Testing the cenozoic lower crustal flow beneath the Qinling Orogen, northeastern Tibetan Plateau

The E–W-trending Qinling Orogen lies in the transitional zone between the northeastern extent of the Tibetan Plateau and the lower-elevation portion of the continent. This region is suggested to have accommodated flow of the low-viscosity lower crust from the Tibetan Plateau northward around the Sichuan Basin during the Cenozoic Himalayan–Tibetan orogeny. To test this model, we combined field geologic mapping, balanced cross-section construction, thermochronology, and geophysical interpretation to constrain the history of crustal thickening from the Mesozoic to the present. In this study, we obtained new thermochronological data from four samples, which yielded 40Ar/39Ar ages of ∼194 Ma for hornblende and 208-151 Ma for biotite and apatite fission track ages of 69-42 Ma. The new thermochronological data and field mapping demonstrate that most shortening structures initially formed in the Mesozoic and underwent minor structural overprinting in the Cenozoic. Systematic structural analyses and restoration of balanced cross-sections demonstrate that the Qinling Orogen was shortened by a minimum of 35% strain (80 km shortening magnitude) after the Mesozoic Paleotethys Ocean closure and continental collision. The crust thickened substantially to >60 km, and the thermochronological data demonstrate that erosion and denudation were minor, such that the crust would have remained thick into the Cenozoic. An additional balanced cross-section across Cenozoic strata suggests >11% Cenozoic shortening, and the observed shortening alone is enough to thicken the crust to the presently observed ∼40–45 km. Therefore, we argue that lower crustal flow is not needed to account for the crustal thickness of the Qinling Orogen and that the data do not support the occurrence of processes associated with lower crustal flow beneath the Qinling Orogen.

Unraveling the eroded units of mountain belts using RSCM thermometry and cross-section balancing: example of the southwestern French Alps

Unraveling the eroded units of mountain belts using RSCM thermometry and cross-section balancing: example of the southwestern French Alps

Structural styles of imbricate systems involving multiple detachments: An example from the Subandean fold-thrust belt, southern Bolivia

Thin-skinned fold-thrust belts commonly contain multiple detachment horizons that exhibit first-order control on their evolution and structural styles. The Subandean fold-thrust belt of southern Bolivia is a wide (∼150 km), tectonically active, thin-skinned fold-thrust belt along the eastern margin of the Andean orogen that exhibits characteristics associated with multiple detachments. This study presents a new structural model for the region by integrating field observations, well data, and seismic reflection data. These data are interpreted to develop kinematic models, balanced cross sections, and maps and 3D representations of key structural elements. We document the dominant structural styles of the fold-thrust belt and demonstrate how these are related to structural imbrication involving multiple detachments. Many of these features are readily observable at the surface and indicative of structural architecture at depth. These include backlimbs displaced over the top of anticlinal fold crests, synclines that are bounded on their foreland side by a thrust fault and located immediately to the foreland of anticlinal crests, and numerous fore- and back-thrusts that dissect anticlinal crests and outcrop in mechanically weak units. Along-strike outcrop pattern is also shown to be indicative of coeval activity at multiple faulting levels and highlights the contributory role shallow detachment horizons play in producing the remarkable along-strike continuity of anticlines in the Subandean fold-thrust belt. These insights suggest that certain features of the Subandean fold-thrust belt are diagnostic of a multiple detachment imbricate system and can be applied to similar fold-thrust belts worldwide.

Deformation structural style of the rioni foreland fold-and-thrust belt, western greater caucasus: Insight from the balanced cross-section

The Rioni foreland fold-and-thrust belt is part of the Greater Caucasus pro-wedge and is one of the most important examples of the collision-driven far-field deformation of the Arabia-Eurasia convergence zone. Here we show the deformation structural style of the Rioni foreland fold-and-thrust belt based on seismic reflection profiles and regional balanced cross-section. The main style of deformation within the Rioni foreland fold-and-thrust belt is represented by a set of fault-propagation folds, duplexes, and triangle zone. The regional balanced cross-section shows that fault-propagation folds above the upper detachment level can develop by piggyback and break-back thrust sequences. Formation of fault-bend fold duplex structures above the lower detachment is related to piggyback thrust sequences. A balanced section restoration of compressional structures across the Rioni foreland fold-and-thrust belt provides a minimum estimate of shortening of −40%, equivalent −42.78 km. The synclines within the Rioni foreland fold-and-thrust belt are filled by the Middle Miocene-Pleistocene shallow marine and continental syn-tectonic sediments, forming a series of typical thrust-top basins. Fault-propagation folds and duplex structures formed the main structure of the thrust-top basin. The evolution of the thrust-top basins was mainly controlled by the kinematics of thrust sequences. Using end-member modes of thrust sequences, the thrust-top basins are divided into: 1) Type I-piggyback basin, 2) Type II-break-back basin, and 3) Type III—formation of thrust-top basin characterized by bi-vergent geometry and related to combined, piggyback and piggyback back thrust sequences.

Distribution, development, transformation characteristics, and hydrate prospect prediction of the rift basins of northwest Zealandia in the Southwest Pacific

As the last area with unknown oil exploration potential, Zealandia has attracted the attention of petroleum scientists. Under the constraint of the known rift distribution, the gravity and magnetic anomaly data are utilized to predict the distribution of rift basins, and the balanced cross-section technique and basin simulation technique are utilized to analyze the development of the rift basins and hydrocarbon generation history of source rocks for the hydrate prospect prediction of the rift basins of northwest Zealandia. The time of rifting in Zealandia varies greatly from place to place, and it has experienced extensive rifts, migrated to the east of the Lord Howe Ridge, migrated to the west of the Lord Howe Ridge, migrated to the west of the Dampier Ridge, and finally migrated to both sides of the Lord Howe Ridge and then experienced multiple stages of compression transformation and deep burial transformation. The Upper Cretaceous coal source rock in the rift Ⅱ stage has entered the wet gas generation stage in the deep part of the rift or deeper rift on the eastern side of the Lord Howe Ridge, and the coal-generated wet gas may migrate along the fault and form a gas hydrate in the shallow seabed stable zone.

Loa-Geo1: A field regional transect to unravel the structure of the western Central Andes

The structure of the Central Andes forearc of northern Chile has been an enigma for many years. Along this region, the preservation of a long pediplain constituted by gravels and unconsolidated sediments has hidden the structure of the region, making it difficult to understand its tectonic evolution. The traditional and regional-scale structural models are mainly supported by outcrop-scale and local observations. To unravel the structure of this Andes region, we carried out a regional-scale study based on local and regional observations, geological mapping, and the elaboration of local and regional balanced cross-sections, thereby illustrating the geometry and distribution of the first-order structural styles. We examined four specific sectors along a regional transect: the Loa and San Salvador rivers (parallel to each other), Sierra de Limón Verde, and Cerros de Tuina. Three structural styles were identified in the hybrid thick- and thin-skinned and doubly verging fold-and-thrust belt. These styles comprise partially inverted Permian to Jurassic normal faults, thick-skinned reverse faults affecting Paleozoic crystalline basement blocks, and shallow and thin-skinned fault-related folds. The shallow and thin-skinned fault-related folds usually result from the propagation of large, thick-skinned thrust ramps into stratigraphic successions. The restoration of a regionally balanced cross section constrained by field data and the previously interpreted two-dimensional (2-D) seismic profiles suggest that much of the crustal shortenings are accommodated by the reverse reactivation of Triassic and Jurassic normal faults bounding half-graben structures. It is also accommodated by blind thrust faults, whose geometry, kinematic, and depth are debatable. However, we have proposed a large, thick-skinned thrust ramp as the structure responsible for the greater shortening in the region, along which a significant thick- to thin-skinned transition occurs. The age of the synorogenic deposits identified here is unknown, specially those exposed at the Loa and San Salvador rives, however, the basal successions exposed at the Tuina sector reported Upper Cretaceous ages, thus suggesting that the orogeny in the forearc could have started during this time spam. Regional correlations with similar synorogenic deposits recognized in neighboring regions (e.g., Salar de Atacama Basin and Coastal Cordillera) also affirm an Upper Cretaceous–Paleocene age for the initiation of the crustal contraction in the region.

Transverse deformation zonality values and palinspastic reconstructions of Carpathians on the base of structural analysis of folding

On the basis of the structural analysis of folds the complex deformation profile in the Hust—Ivano-Frankivsk (central part of Ukrainian Carpathians) transsection is constructed. It characterises distribution of non-coaxial flow deformation on a spectrum of parameters: angles of an axis of compression and simple shear, sizes of horizontal compression and deformational ellipses. Horizontal shortening shows a series of peaks over more or less levelled general background of double compression. The most intensive deformation, expressed in the maximum sizes of horizontal and general compression (to 4 times), is dated for area of Internal Nappes (basically the Dukljansky zone) and to the termination of the Krosnensky zone. There are also indications on growth of deformation in back of Internal Nappes and in front of Skibovaya zone. Deformation as a whole goes down to the central part of a profile occupied by Krosnensky zone where conditions of almost horizontal main compression are marked also. Reconstruction of the primary sizes of structural zones along of Hust—Ivano-Frankivsk profile in the interval of Porkuletsky—Skibovoj zones as a result of returning in a initial condition before deformation of non-coaxial flow has increased in 2.4 times, having reached 183 km that it is necessary to consider as the bottom estimation of initial width which should be increased at the expense of components of bend folding. Possibilities which are given by model of non-coaxial flow for reconstruction on the scale of full deep crossing of Carpathians are short estimated for what approach of homogeneous deformation on depth has been used: 2-time size of horizontal reduction, and also angle of an axis of compression 20є are accepted as averages. Obtained contours of the Krosnensky zone of profile DOBRE-3 shows convergence with results of application of a method of the balanced cross-sections.