Along-strike Structural Variations Research Articles

Structural geometry and geomorphic evolution of the Trans Indus Manzai Ranges, Western Himalayas, Pakistan

At the leading edge of fold-thrust belts, structural deformation can either be accommodated by thick-skinned faulting, and development of structural wedges near the mountain fronts, or by foreland propagation into the basins along the detachment, forming thin-skinned structures. Manzai Ranges at the western extremity of the Trans Indus Ranges were chosen as a case study to describe the structural deformation accommodated in the foreland basin and to study the folded mountain front. These ranges also provide an opportunity to compare along-strike structural variations of the Himalayan frontal thrust system. Both surface (field structural measurements and digital elevation model) and subsurface (2D seismic reflection and well) data were integrated to document different structural styles and their corresponding geomorphic expressions. Based on seismic reflection profiles, the results of structural interpretations constrain westward dipping basement (2.5o) at a depth of ∼10 km in the foreland, which dips deeper in the hinterland. In addition, our interpretation shows a basin-wide Eocene-Miocene unconformity that marks the Himalayan orogenic events. The structural geometry of the frontal Surqamar anticline is quantitatively characterized to be detachment fold, while Jandola, Kundi and Manzai anticlines demonstrate staircase geometry and characterized as fault-bend folds. The kinematics of development varies along-strike in the Manzai Ranges as displacement transfers along the detachment from typical fault-bend folding in the Manzai anticline to detachment folding in the Surqamar anticline, forming thin-skinned structures. The results of this study highlight the structural styles and their evolution in the context of active Himalayan tectonics.

Interpretation and analysis of seismic and analog modeling data of triangle zone: a case study from the frontal part of western Kura foreland fold-and-thrust belt, Georgia

2D seismic reflection profiles revealed the presence of a triangle zone at the frontal part of the western Kura foreland fold-and-thrust belt of the pro-wedge of the Greater Caucasus. To understand the triangle zone geometry, seismic interpretations should be substantiated by forward kinematic modeling, supported by analog experiments. This study presents a new structural model for the region by integrating field observations, well data, and seismic reflection data. East-West directed along-strike structural variation of the frontal thrust is observed on the interpreted seismic profiles which affected the fold geometry. The Bitsmendi breakthrough fault-propagation fold gradually transits into a wedge structure in the W-E direction and is represented by the triangle zone. The seismic profiles interpretation results completely match with analog models of similar triangle zones. The analysis of the experimental results helps us to further understand the kinematic evolution of natural systems and improve seismic interpretation. The triangle zone developed in the western part of the Bitsmendi breakthrough fault-propagation fold is related to double fault-bend fold structural wedges and is characterized by the presence of passive, and active wedges, and passive-backthrust and passive-forethrust.

Structural Significance of the Mid-level Décollement Within the Western Sichuan Fold-And-Thrust Belt (WSFTB), Insights From Sandbox Modeling

The WSFTB is located outboard of the eastern Tibetan Plateau, western China. It has received great attention due to high earthquake risks and rich resources of oil and gas. For both issues, the detailed structural configuration and deformation mechanism behind it are of great importance, but remain unclear due to the complexity created by the presence of multiple décollements. The effect of regionally distributed shallow Triassic salt décollement (SD) and the basal one (BD) has been well understood. In this paper, we focus on the third décollement situated between them. We conducted three sandbox experiments by varying this mid-level décollement (MD) from absence to presence, and from frictional to viscous, to test the effect on diversity of regional structural configuration. Our experimental results illustrated that 1) Absence of MD facilitated decoupling on SD, forming the greatest contrast between subsurface deformation front and the blind one beneath SD; 2) Frictional MD itself showed little decoupling, while its weakness reduced the bulk strength of deep structural level, lowering decoupling effect on SD and leading to approximating deformation fronts in the shallow and deep; 3) The viscous MD, along with SD relieved the resistance on their interbed layer. Consequently, the fastest deformation propagation rate and farthest deformation front (in all the experiments) occurred in the middle structural level. The modeled fold and thrust structures are comparable with the southern, central and northern WSFTB respectively, suggesting that varied MD may control the along-strike structural variations presented. The results also indicate that MD can alter the deformation partition in depth of any other multiple décollement system.

Along-strike structural variation in a salt-influenced fold and thrust belt: Analysis of the Kuqa depression

The Kuqa depression is associated with complex salt structures. Variation in fold geometries and subsalt fault activities were investigated using parallel cross-sections to reveal detailed along-strike variations within the Keshen 5 segment and their controlling factors. Along-strike variations were identified in both suprasalt and subsalt units. Linkages, such as the location of a transfer zone, were apparent, while the distribution of variations above and below the salt layer were decoupled. The results indicate that the pre-existing salt wall is the key factor driving along-strike variations because it acts as a weak zone that influences deformation propagation. Suprasalt structural variation results in different sediment depths, which have a significant impact on subsalt fault activity. In the eastern segment, fault activity decreases from the hinterland to the basin. In contrast, owing to the pre-existing salt wall and deep sedimentation near the hinterland in the western segment, the fault beneath the salt wall is characterized by maximum displacement. This study reveals the connection between suprasalt and subsalt deformation and the role of that a salt diapir plays in along-strike structural evolution variations for the Keshen 5 segment of the Kuqa depression. These findings are relevant for understanding along-strike deformation propagation and structural variation in other fold and thrust systems.

Investigating the Goban Spur rifted continental margin, offshore Ireland, through integration of new seismic reflection and potential field data

The Goban Spur, offshore Ireland, is a magma-poor rifted continental margin conjugate to the well-studied Newfoundland margin, offshore Canada. Published studies demonstrated that a 70-km-wide zone of exhumed serpentinized mantle lies between oceanic crust and stretched continental crust at the seaward limit of Goban Spur. However, the along-strike extent of this serpentinized zone has, until now, been unknown due to insufficient data coverage. The crustal architecture of the margin is complicated due to its multi-staged tectonic history. Here, six newly acquired multi-channel seismic reflection lines are processed and interpreted, along with vintage seismic profiles, to characterize its structure and evolution. These seismic profiles reveal significant along-strike structural variations along the Goban Spur margin, and allow us to delimit five distinct crustal zones related to different rifting stages and their regional extents. The geometries of each crustal domain are variable along the margin strike, probably suggestive of different extension rates during the evolution of the margin or inherited variations in crustal composition and rheology. The transitional zone between oceanic crust and stretched continental crust consists of both shallow peridotite ridges and deeper exhumed serpentinized mantle, much like the conjugate Iberian and Newfoundland margins. Above the top basement in the exhumed domain, the syn-exhumed sediments show strikingly weak reflectivity, rarely seen at other magma-poor margins. Magmatic events occur coincident with each rifting stage, and the volume of magmatic accretions increases from NW to SE, more than previously interpreted. Plate reconstruction of the Goban Spur and its possible conjugate – the Flemish Cap, shows asymmetry in the crustal architectures, likely due to rift evolution involving more 3-D complexity than can be explained by simple 2-D extensional kinematics.

Strain analysis of the Xuefengshan Belt, South China: From internal strain variation to formation of the orogenic curvature

The Early Mesozoic Xuefengshan Belt, located in the central part of the South China Block, East Asia, developed as an intracontinental belt during the Middle Triassic. Analyses of structural and strain patterns in Neoproterozoic conglomerates constrain the internal deformation and three-dimensional kinematic evolution of this fold-and-thrust belt. Finite strain data (RXZ) range from 1.1 to 3.2 and generally increase eastwards towards the deep units of the orogenic core. Our results also show that at basal décollement zone the D1 schistosity-forming deformation has a gradient of stronger fabrics, higher strain ratios, and a dominant thrust-normal flattening mechanism. High tectonic levels exhibit lower strain ratios and later structural overprinting that locally modified the D1 strain patterns. Parallel to the belt, enhanced strain and tectonic uplifting along the northern and southern sections of the belt indicate an along-strike structural variation that corresponds to the formation of the orogenic curvature. During the thrusting propagation, large igneous intrusions escaped deformation due to their stiffness and facilitated strain accumulation on the peripheries, whereas stress in the central part migrated progressively and formed a tectonic salient towards the west without significant strain localization.

Duplex and along-strike structural variation: A case study from Sikkim Himalayan fold thrust belt

The Sikkim Lesser Himalayan fold thrust belt records along-strike structural variations over ∼15 km. The lower Lesser Himalayan (Rangit) duplex (LHD) is blind, and the overlying Pelling and Ramgarh thrust sheets are more intensely folded and preserved in eastern Sikkim than in western Sikkim. A retrodeformable, regional balanced cross-section reveals a minimum shortening of ∼403 km (∼80%) with an average long-term shortening rate of ∼18 mm/yr in the eastern transect. Comparison with published data indicates that eastern Sikkim records ∼47 km lesser shortening. In western Sikkim, an ∼2 km high footwall Main Himalayan thrust (MHT) ramp below the Rangit duplex caused higher structural culmination exposing the duplex. In contrast, an ∼0.35 km high ramp below the Rangit duplex in eastern Sikkim did not create sufficient culmination to expose the duplex, causing along-strike discontinuous duplex exposure. The LHD with a blind component, and having fewer (11) but gentler dipping (∼40°) horses transferred more displacement to the roof thrust in eastern Sikkim than the twelve, steeper dipping (∼60°) horses in western Sikkim. Lateral variations in location and height of footwall MHT ramp, initial width of the Lesser Himalayan basin, and presence of a lateral ramp explain the structural variation in Sikkim.

Using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts

We investigate changes in shortening, displacement and fold geometry to understand the detailed along-strike structural variation within fold-thrust belts, and infer thrust growth and linkage mechanisms. Field observations from the Vercors in SE France are used to characterise deformation style in the region. Parallel cross sections are constructed, analysed and used to create shortening and thrust displacement profiles from the northern to southern Vercors. Sections show changes in structural style and shortening accommodation from thrust-dominated in the north to fold-dominated in the south. The total shortening distance in the Vercors does not change significantly along strike (3400–4650 m), however displacements along individual thrust zones do vary significantly and displacement profiles show a range in displacement gradients (16–107 m/km). Despite relatively simple shortening patterns in the Vercors, sections show a more complex 3D internal structure of the fold-thrust belt. Thrust displacements and geometries suggest both large-scale thrust zones and small-scale thrusts are soft linked, transferring displacement along strike through transfer zones. Short, soft-linked thrust segments indicate an intermediate stage of thrust growth and linkage, well documented for normal fault systems, which form prior to the formation of thrust branches and hard-linked displacement transfer.

Wavelet spectral probe for seismic structure interpretation and fracture characterization: A workflow with case studies

Conventional amplitude images are routinely and often exclusively used to interpret subsurface structures; however, the amplitude images are composite seismic response to various geologic features at different scales. The interference makes it difficult to investigate structural details that are not visually discernible in the amplitude images. This paper presents a new algorithmic workflow to unravel seismic structural details by differentiating waveform characteristics at varying scales. Using a wavelet model as a spectral probe, the algorithm performs a least-squares linear regression between the model and data at each sample location to calculate the absolute correlation coefficient. The resulting absolute correlation coefficient attribute, which is indicative of instantaneous waveform similarity relative to the wavelet spectral probe, provides a superior image with enhanced precision and resolution of the structural grain. Two case studies in the Powder River and the Appalachian Basins demonstrate that the new algorithm helps investigate kinematic relationships among structural elements and differentiate between structural styles. In particular, the algorithm helps better define the along-strike structural variation and segmentation associated with cross-strike lineaments in the foreland basins. Comparative analysis indicates that the wavelet spectral probe algorithm helps shed new light on subtle but critical structural fabrics not easily discernible from conventional amplitude images. Revealing such information has important implications for better defining basin structural complexities and reservoir heterogeneity, detecting faults and fractures at different scales, and modeling fractured reservoirs.

Along-strike structural variations in the Southern Patagonian Andes: Insights from physical modeling

The Southern Patagonian Andes between 48° and 53° SL offers a unique opportunity to study the results of orogenic growth superimposed over a previously rifted region. In this sector the northern Austral or Magallanes basin was affected by late to middle Jurassic extension followed by Late Cretaceous to Neogene tectonic compression. The resultant fold-thrust belt displays significant along-strike variations in width and lateral position of the structural domains that may be reflecting a first order control of the Jurassic extensional depocenters, as proposed in previous works. This hypothesis is tested using a series of scaled sandbox analog models involving the positive inversion of two adjacent depocenters with different extensional rates accommodated by a transfer fault. The correlation between structures in the Southern Patagonian Andes and the features in the model have important first order similarities supporting the theory of a positively inverted extensional basin with a previous history of southward increase in extension. Moreover structural cross-sections analyzed from field and seismic data have a geometry that is consistent with serial cross sections resulting from the models.

Mechanical analysis of controls on strain partitioning in the Himalayas of central Nepal

We present a mechanical analysis of the problem of slip partitioning between the major thrust systems in a collisional range. We focus on two structures in the Himalayas of central Nepal: the Main Himalayan Thrust (MHT) and the Main Central Thrust (MCT). We use finite element modeling to test the influence of various parameters, such as friction coefficients and surface processes, and we investigate how they affect the distribution of deformation between these two faults. We observe that reproduction of the late Quaternary kinematic pattern across the range with our model requires strict conditions on the friction coefficients, such that the MHT is very weak, whereas the MCT is significantly stronger. The most important parameter that controls slip partitioning appears to be the dip angle of the MCT, with a gentler or steeper MCT promoting or inhibiting slip, respectively. We also show that transient loading and unloading through focused glacial erosion in the higher part of the range can unclamp the MCT and allow a significant increase in slip rates. The results of this mechanical sensitivity investigation have important implications for the dynamics of the Himalayan wedge and point toward along-strike structural variations as a first-order control on slip partitioning.

Along-strike structural variations in the foreland thrust and fold belt of the southern Urals

The foreland thrust and fold belt of the southern Urals has a number of along-strike structural changes in which regional-scale structures terminate abruptly. One important along-strike structural change is the southern termination of the Precambrian-cored Bashkirian Anticlinorium, which plunges southward beneath the synformal Zilair Nappe. Balanced and restored dip cross-sections, a N-S-oriented strike section, together with a geological interpretation of a CDP reflection seismic profile have been used to determine the 3D structure and the amount of shortening in the foreland thrust and fold belt in the area of the Bashkirian Anticlinorium and the Zilair Nappe. The geometry of the foreland thrust and fold belt is that of a west-vergent, basement-involved thrust stack with shortening on the order of 13% to 17% (approximately 17 km). The small amount of shortening allows a correlation to be made between the along-strike structural changes and pre-existing basement structures. In particular, the southern termination of the Bashkirian Anticlinorium coincides with a Precambrian-age aulacogen, the Sernovodsk-Abdulino Aulacogen, developed in the basement. An E-W-oriented tear fault, the Belaya tear fault, appears to have developed in the basement, along the northern margin of the Sernovodsk-Abdulino Aulacogen. The Belaya tear fault was active during much of the history of the foreland thrust and fold belt, and exerted strong control on its structural and tectonosedimentary development. Because of the small amount of shortening involved, the foreland thrust and fold belt of the southern Urals provides an important example for understanding the effects of pre-existing basement structures on the development of foreland thrust and fold belt's.

Centrifuge modelling of thrust faulting: structural variation along strike in fold-thrust belts

The three-dimensional geometry and the kinematic evolution of fold-thrust structures are investigated by analog modelling using the centrifuge technique. We focus on along-strike structural variation and transfer of displacement between faults and folds. The total horizontal shortening across the model fold-thrust belts is constant, yet there are systematic variations in shortening by the mechanisms of thrusting, folding and layer-parallel shortening. These along-strike changes in structural geometry and strain mechanism are manifestations of displacement transfer. The model thrust faults have different amounts of thrust displacement at different points along strike. The variation is accommodated by transfer of displacement to over- or underlying faults or to en-echelon faults, by transfer of shortening into folding, or by transfer of displacement into layer-parallel shortening. In the models it is clear that faults nucleate at points that are localized by earlier folding of competent units. Faults nucleated at different points along a single fold may propagate along the fold and link together to form major thrusts; the thrust displacement in such cases varies from smaller values at linkage points to larger values at nucleation points. The models have a stratigraphie succession like that of the central Appalachian Valley and Ridge province. After an equivalent amount of structural shortening the models exhibit structural features analogous to those of the Valley and Ridge. The lowest competent unit (equivalent to the Cambro-Ordovician lithotectonic unit) forms a large-scale duplex like that of the Waynesboro sheet). The middle competent unit (equivalent to the Siluro-Devonian clastic units) forms anticlinoria and synclinoria like those of the Martinsburg sheet). These structures are spatially associated with underlying culminations and depressions that reflect, respectively, large and small displacements on thrusts that ramp through the lowest competent unit. However, in the models it appears that the amplitude of buckling of the middle unit controls the amount of displacement on underlying thrusts, whereas in the Appalachians the structural culminations of the middle unit are interpreted to have been caused by large displacement on underlying faults.