Fold and thrust belts: structural style, evolution and exploration – an introduction

Abstract Using the geologist’s interpretation of 6 published balanced cross-sections in the fold and thrust belts of the northwestern Mediterranean, a comparative analysis of the interpreted subsurface structural architecture is used to address the links between the structural style and the mechanics of fold and thrust emplacement. For each cross-section example, the geo-dataset and the methods used by the interpreters are different in quantity and quality. Here we have examined how useful is the content of information of each cross-section to constrain the structural evolution scenario. Each interpretation is examined according to considerations of the mechanics of sedimentary basin deformation and how uncertain is the extrapolation of fault trajectory at depth. It is shown that each case reveals a particular type of structural style: thin-skin or thick skin tectonics, fault-related folding, pre-existing fault pattern. The present structural analysis is used to determine for each cross-section the nature of the mechanical problem to address that will reduce uncertainty on the geologic scenario reconstruction. The proposed mechanical boundary conditions could serve to develop analog or numerical models that aim at testing the mechanical validity of the structural scenario of fold and thrust emplacement.

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The role of inherited structures and basin geometry during the 3D inversion of a passive continental margin: the case of the Doldenhorn-Aar Massif system (Central Swiss Alps)

10.5194/egusphere-egu21-2609 ◽

2021 ◽

Author(s):

Ferdinando Musso Piantelli ◽

David Mair ◽

Marco Herwegh ◽

Alfons Berger ◽

Eva Kurmann ◽

...

Keyword(s):

Continental Margin ◽

Large Scale ◽

Passive Continental Margin ◽

Swiss Alps ◽

Cylindrical Shape ◽

Structural Position ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts ◽

Aar Massif

Inversion of passive margins and their transportation into fold-and-thrust belts is a critical stage of mountain building processes and their structural interpretation is fundamental for understanding collisional orogens. Due to the multitude of parameters that influence their formation (e.g. the interaction between sedimentary cover and basement, the mechanical stratigraphy or the rheology of different rock types) as well as along-strike internal variations, a single cross-sectional view is insufficient in exploring the 3D evolution of a fold-and-thrust belt. Hence, a 3D geological characterization is required to better comprehend such complex systems. Based on a detailed digital map, a 3D structural model of the current tectonic situation and sequential retrodeformation, we elaborate the 3D evolution of a part of the former European passive continental margin. In this setting, we focus on the Doldenhorn Nappe (DN) and the underlying western Aar massif (external Central Alps, Switzerland). The DN is part of the Helvetic nappe system and consists of a large-scale recumbent fold with a thin inverted limb of intensively deformed sediments (Herwegh and Pfiffner 2005). The sedimentary rocks of the DN were deposited in Mesozoic-Cenozoic times in a small-sized basin, which has been inverted during the compression of the Alpine orogeny (Burkhard 1988). Along NNW-SSE striking geological cross-sections, restoration techniques reveal the original asymmetric triangular shape of the DN basin and how the basin has been exhumed from ~ -12 km (Berger et al. 2020) to its present position at 4km elevation above sea level throughout several Alpine deformation stages. Moreover, the model allows to visualize the current structural position of the DN and the massif as well as the geometric and overprinting relationships of the articulated deformation sequence that shaped the investigated area throughout the Alpine evolution. Here we document that: (i) the DN is a strongly non-cylindrical recumbent fold that progressively pinches out toward the NE; (ii) significant along-strike (W-E) stratigraphy thickness variations are reflected in structural variations from a single basal thrust deformation (W) to an in-sequence thrust deformation (E); and (iii) the progressive exhumation of the basement units towards the E and thrusting towards the N. In this context, special emphasis is given to illustrate how three-dimensional geometry of inherited pre-orogenic structures (e.g., Variscan-Permian and rifting related basement cover structures) play a key role in the structural style of fold-and-thrust belts. In summary, today&#8217;s structural position of the DN is the result of the inversion of a small basin in an early stage of thrusting, which was followed by sub-vertical buoyancy driven exhumation of the Aar massif and subsequent thrust related shortening. All three stages are deeply coupled with an original non-cylindrical shape of the former European passive continental margin.

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Multiple detachments of thin-skinned fold-and-thrust belts in the eastern Sichuan Basin, China

10.5194/egusphere-egu2020-20903 ◽

2020 ◽

Author(s):

Zhidong Gu

Keyword(s):

Seismic Data ◽

Sichuan Basin ◽

Structural Deformation ◽

Kinematic Evolution ◽

Structural Style ◽

Thrust Belts ◽

Eastern Sichuan Basin ◽

Fold And Thrust Belts ◽

Eastern Sichuan ◽

Stratigraphic Succession

The eastern Sichuan Basin, South China, is characterized by approximately parallel thin-skinned fold-and-thrust belts with exposed narrow anticlines and wide synclines. The structural deformation, however, has remained controversial due to the previous poor seismic data. In this study, the new collected pre-stack long-offset 2D- and 3D seismic data have been applied, and a 200-km long cross section perpendicular to the fold-and-thrust belts has been constructed to analyze the structural style and geometric and kinematic evolution. The stratigraphic succession is composed of competent layers separated by three main incompetent layers being multiple detachments, which are the Cambrian evaporites, the Lower Silurian shales, and the Middle-Lower Triassic evaporites, respectively. The basal detachment, the Cambrian evaporites, played a dominant role in the structural deformation, above which the fold-and-thrust belts were generated, and the middle and top detachments accommodated the displacement during the deformation. The main structural styles are detachment folds, fault propagation folds, back thrusts and basement-involved folds. The evolution succession of the fold-and-thrust belts should be kink band, detachment folds, and sequential thrusts of the forelimb and backlimb of the folds. The style of deformation is dependent on the mechanical characters of stratigraphic succession, i.e., the thickness variation of competent and incompetent layers in the stratigraphic units.

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Impact of mechanical stratification on the structural style of the Lublin Basin, SE Poland: results of seismic interpretation and implications for quantification of deformation within the frontal parts of thin-skinned fold-and-thrust belts

International Journal of Earth Sciences ◽

10.1007/s00531-021-02140-7 ◽

2021 ◽

Author(s):

Mateusz Kufrasa ◽

Piotr Krzywiec

Keyword(s):

Seismic Interpretation ◽

Upper Devonian ◽

Strain Partitioning ◽

Se Poland ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts ◽

Internal Homogeneity ◽

Lithological Composition ◽

Seismic Reflectors

AbstractWe demonstrate how lithological and mechanical stratification of Ediacaran–Carboniferous sedimentary package governs strain partitioning in the Lublin Basin (LB) which was incorporated in the marginal portion of the Variscan fold-and-thrust belt. Based on the geometry of seismic reflectors, the pre-Permian–Mesozoic sedimentary sequence was subdivided into two structural complexes differing in structural style. The lower one reveals forelandward-vergent imbrication, while the upper one comprises fold train, second-order deformations, and multiple local detachments. Lithological composition of the upper structural complex controlled geometry, kinematics, and position of compressional deformations in stratigraphic profile. System of foreland-vergent thrusts which links lower and upper detachment developed due to efficiency of simple shear operating in heterogeneous clastic-carbonate-evaporitic strata of the Lower–Upper Devonian age. Internal homogeneity promoted the formation of conjugate sets of thrusts in Silurian shales and Upper Devonian limestones. Structural seismic interpretation combined with sequential restoration revealed localised thickening of Devonian strata and up to 5% difference in length of Devonian horizons. This mismatch is interpreted as a manifestation of distributed shortening, including layer-parallel shortening (LPS), which operated before or synchronously to the initiation of folding. The amount of distributed strain is comparable with numbers obtained in external parts of other fold-and-thrust belts. The outcomes derived from this study may act as a benchmark for studying variability in a structural style of multilayered sequences which were incorporated in the external portion of other fold-and-thrust belts.

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Using outcrop data and analog models to aid seismic interpretation in fold and thrust belts

Interpretation ◽

10.1190/int-2018-0014.1 ◽

2018 ◽

Vol 6 (4) ◽

pp. SM51-SM61

Author(s):

Sandro Serra

Keyword(s):

Seismic Data ◽

Seismic Interpretation ◽

Complex Structures ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts ◽

Analog Models

Seismic data in fold and thrust belts (FTBs), especially in onshore areas, are often difficult to interpret due to poor imaging of complex structures. Outcrops, either in the same area as the seismic or in FTBs with similar stratigraphy, provide direct and essentially unfiltered views of structures that can form in this structural style. In addition, physical and mathematical analog models can provide insights on the development of compressional structures and the parameters that most strongly influence their shape. The task of seismic interpretation in FTBs can be aided substantially by using well-exposed outcrops and analog models of compressional structures as templates.

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Varying thrust geometry along the Central Atlas fronts: structural criteria for 3-D reconstruction

10.5194/egusphere-egu2020-7670 ◽

2020 ◽

Author(s):

Antonio M. Casas ◽

Pablo Calvín ◽

Pablo Santolaria ◽

Tania Mochales ◽

Hmidou El-Ouardi ◽

...

Keyword(s):

Cross Sections ◽

Large Scale ◽

Thickness Ratio ◽

Basin Inversion ◽

Structural Style ◽

Thrust Belts ◽

Igneous Intrusions ◽

Fold And Thrust Belts ◽

Thrust Sheets ◽

Cover Thickness

Multiple constraints, including poorly known parameters, determine along-strike changes of frontal thrust structures in fold-and-thrust belts. Along the 400 km long, continuous Central Moroccan Atlas belt, structural style shows significant changes, preserving similar figures of shortening. This implies the absence of large-scale vertical-axes rotations, as demonstrated by paleomagnetic studies accomplished during the development of this project. The main factors controlling thrust geometry are:- the geometry of Triassic-Jurassic extensional basins subsequently inverted during Cenozoic compression, with especial mention to changes of cover thickness and orientation of structures- transfer of displacement between the northern and southern thrust systems- transfer of displacement between the basement (Paleozoic) units and the Mesozoic cover through the Upper Triassic detachment. This factor strongly determines the width of the belt in each transect, as it occurs in other basement-and-cover fold-and-thrust belts- cover/detachment thickness ratio.- localization and partitioning of deformation between different structures in the inner part and the borders of the massif- amount of superposition between different cover thrust sheets, including folded thrusts- structural style, changing from thin-skinned style to large recumbent folds along strike, probably depending on P-T conditions and cover thickness- backthrusts related to low cover thickness/detachment thickness ratio, especially frequent in the northern Atlas thrusts- differential shortening between sections related to layer-parallel shortening and folds associated with cleavage development in the central part of the chain- influence of previous structures, such as individual diapirs, salt walls or igneous intrusions that modify the pre-compressional geometry of the detachment level, nucleate structures and favor buttressing. This feature can also be a source of errors in the calculation of shortening. All these factors result in strong along-strike changes such as branching of thrust surfaces, progression of deformation towards the foreland and differential cleavage development. Influence of structures developed during the basinal/diapiric/igneous stage results in a variability of trends that varies between from less than 10&#176; to more than 30&#176;, what allows in some cases to distinguish between structures controlled by basinal features and newly formed thrusts.In spite of the different techniques for cross-sections reconstruction, and in some cases, the different interpretations for the origin of structures, the shortening figures obtained along the chain are remarkably constant, on the range of 35 km, thus implying a 18 to 30% of shortening for most of the transects what attests for the reliability of the results.Recognition and quantification of factors controlling the development of structures is the fundamental step to determine the main thrust surfaces, and the secondary backthrusts in a region where basin inversion is one of the main constraints. Structural criteria point to a dominant southward vergence and secondary northwards-directed thrusts. Minor strike-slip components were probably localized in the core of the chain. Present-day 3-D reconstruction of the Atlas is currently being done considering all these inputs as well as those obtained from merging the vast dataset obtained.

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Prediction uncertainties in salt detached fold and thrust belts – examples from the surface and subsurface of the Northern Calcareous Alps (Austria)

10.5194/egusphere-egu2020-9969 ◽

2020 ◽

Author(s):

Klaus Pelz ◽

Pablo Granado ◽

Michael König ◽

Elizabeth P. Wilson ◽

Philipp Strauss ◽

...

Keyword(s):

Northern Calcareous Alps ◽

Deformation Localization ◽

Well Control ◽

Northern Calcareous ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts ◽

Thrust Sheets ◽

Lateral Extent ◽

Thickness Variations

As shown for fold and thrust belts worldwide and for the Northern Calcareous Alps (NCA) in particular, the initial thickness and spatial distribution of autochthonous salt exerts fundamental control on deformation localization and structural style. The initial sedimentary geometries of mini-basins formed by downbuilding into or rafting on salt do influence the geometries of thrust sheets during subsequent shortening. The lateral extent and spacing of individual thrust sheets and the overall cylindricity of structures is governed by initial facies changes and thickness variations within and across mini-basins and salt ridges between them. During convergence, remaining inflated salt localizes shortening whereas mini-basins may react as rather rigid blocks. As deformation culminates at these secondary welds that eventually become thrusted and squeezed, apparent structural closures might become exploration targets but potentially yield more complex internal geometries and less predictable facies distribution.In this contribution we show several cross sections constrained by surface and subsurface data in the eastern NCA and below the Vienna Basin. We compare areas with abrupt changes in stratigraphic thickness, limited lateral extent of individual thrust sheets and highly non-cylindrical structural style along strike to areas where thrust sheets extend over several tens of kilometers along strike with remarkably cylindrical structures, little thickness variations and less abrupt facies changes. Predictive capabilities in underconstrained areas (i.e., insufficient seismic imaging and/or resolution, lack of well control, bad outcrop conditions) are analyzed and compared to closures with well control and pre-drill expectations. Evidently, culminations can be predicted with more confidence in areas with little variation in facies and sedimentary thicknesses. Reliability of predictions generally degrades with decreasing thrust sheet size, observable non-cylindricity within and in between thrust sheets, and with increased complexities at the edges of mini-basins (e.g., squeezed and thrusted flaps). Internal geometries of mini-basins need to be imaged and analyzed properly to narrow down these uncertainties at potential culminations along the edges.

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Introduction: tectonic evolution and mechanics of basement-involved fold-and-thrust belts

Geological Magazine ◽

10.1017/s0016756816000650 ◽

2016 ◽

Vol 153 (5-6) ◽

pp. 759-762 ◽

Cited By ~ 1

Author(s):

OLIVIER LACOMBE ◽

JONAS RUH ◽

DENNIS BROWN ◽

FARAMARZ NILFOUROUSHAN

Keyword(s):

Seismic Hazard ◽

Cross Section ◽

Tectonic Evolution ◽

Economic Potential ◽

Short Term ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts

Defining the structural style of fold-and-thrust belts is an important step for understanding the factors that control their long- and short-term dynamics, for comprehending seismic hazard associated with them, and for assessing their economic potential. While the thin-skinned model (no basement involvement) has long been the driving methodology for cross section construction and restoration of foreland fold-and-thrust belts, a wealth of new geological and geophysical studies have shown that they are often thick-skinned, that is, basement-involved.

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