Structural Vergence Research Articles

Influence of multiple detachments on structural vergence and evolution of the thin-skinned fold-and-thrust belt in the eastern Sichuan Basin: Insights from numerical modeling

The thin-skinned fold-and-thrust belt with multiple detachments in the eastern Sichuan Basin is characterized by a series of fault-detachment folds with a backward preferred structural vergence, which is rare in nature and can't be explained well by the classic critically taper theory. Here, three discrete element simulations were conducted to study the influence of multiple detachments on the structural vergence and evolution of the eastern Sichuan thin-skinned fold and thrust belt. The experimental results reveal that multiple detachment layers affected the bulk mechanical strength and facilitated a dually vergent thrust wedge with a backward preferred structural vergence. The addition of intermediate detachment layers influences structural vergence and fault activity of thrust wedges. With the increase in intermediate weak layers, the preferred structural vergence of these model results progressively moved from forward to nearly symmetric and eventually to backward. These results exhibit first-order structural similarities to the structural vergence and fault activity in the eastern Sichuan fold and thrust belt, which indicates that the influence of the internal distribution of weak layers within the eastern Sichuan Basin should not be ignored. The limited Cambrian evaporite layer and the widespread Silurian shale and Lower-Middle Triassic evaporite layers had significant impacts on the deformation of the fold-and-thrust belt in the eastern Sichuan basin. Consequently, we propose a kinematic model for the coupled geodynamic processes of the eastern Sichuan fold-and-thrust belt. The framework of the eastern Sichuan Basin with a number of backward fault-detachment folds was formed by Mesozoic northwestward propagation, and Cenozoic eastward propagation reactivated the folding and thrusting. The Huayingshan fault was affected by the limited Cambrian evaporite layer.

Deformational in curved fold-and-thrust belts: As a function of backstop shape and basal friction: Insights from analogue modeling and application to the Pamir salient, Hindu Kush region

Curved fold-and-thrust belt (CFTB) is a common structural phenomenon in orogenic belts and accretionary wedges, which has attracted the attention of several geologists. However, the formation mechanism of CFTB, particularly the cause of Pamir salient, has been greatly controversial. Therefore, based on previous studies, CFTB was studied by 12 groups of analogue modeling using the different shapes of the backstop and variable basal friction properties, and different compression velocities. The results show that the formation of CFTB is closely associated with the initial geometric characteristics of CFTB, the strength of the basal ductile layer, and the influence of lateral friction. Although the variation in backstop geometries is dominated by the pure Coulomb wedge, CFTB with completely different structural styles can form because of different boundary shapes. The CFTB formed by lower basement friction strength has a larger deformation range, and the CFTB formed by high strength of basal friction, which is narrow, has a high uplift amplitude, forming a tensile structure around its uplift. A high compressional rate forms the higher arc curvature, and a lower compressional rate forms the smaller arc curvature. Models of a brittle basement with triangular backstop and models with ductile basement and boundary parallel to the compression direction may provide a more reliable explanation for the formation of the forward thrust, backward thrust, and symmetrical pop-up structures of the Pamir salient and its structural vergence from south to north. This study further reveals that initial boundary geometry, stratigraphic structure, and rheology with a difference in brittle and ductile layers will yield more complex structural styles and different kinematic characteristics.

Relationship Between Lateral/Basal Shear Stress Ratio and Structural Vergence of Thrust Wedges: Results From Analogue Modeling and Implications for the Origin of Eastern Sichuan–Xuefeng Fold‐Thrust Belt in South China

AbstractThrust wedges are common in nature, but not all of their origins can be properly explained by the classic Coulomb wedge theory, which considers only a 2D perspective. A few analogue models, however, revealed that lateral shear stress (inherent in both analogue models and nature) may strongly affect the development of thrust wedges and the lateral/basal shear stress (τs/τd) ratio might have controlled the structural vergence of thrust wedge, suggesting that origin of thrust wedges needs to be considered from a 3D perspective. The origin of the Eastern Sichuan–Xuefeng fold‐thrust belt (ESXFTB), which is characterized by two oppositely vergent thrust systems at different crustal levels and may contain another large purely backward‐vergent thrust wedge on the world, is still problematic. Revealing the relationship between τs/τd ratio and structural vergence may prompt a deep understanding of the origin of thrust wedges as well as the ESXFTB. This relationship, however, is still not very clear due to the lack of systematic experiments. For these reasons, we performed systematic experiments of two types of analogue models under different τs/τd ratios. Our model results revealed, for the first time, that structural vergence of thrust wedges lying on a weak décollement may progressively change from frontward (when τs/τd <0.09) to dual (when τs/τd = 0.09–1.15) and then to backward (when τs/τd >1.15) and the two oppositely vergent thrust systems in the ESXFTB can be produced simultaneously by the single northwestward Mesozoic shortening probably due to their apparently different τs/τd ratios.

Foreland migration of orogenic exhumation during nappe stacking: Inferences from a high-resolution thermochronological profile over the Southeast Carpathians

The exhumation mechanism of orogens showing one structural vergence, or single-sided orogens, is still not fully understood due to their common transition from periods of high convergence to slab retreat. The overprinting of inherited nappe stacks by subsequent back-arc extension makes it difficult to assess the low amounts of contractional exhumation observed, which are furthermore difficult to interpret due to long residence times in partial retention zones of typical low-temperature thermochronology markers. This is the case in many orogens worldwide and is often observed in the Mediterranean area as well. One notable exception is the Romanian segment of the Carpathian Mountains, where the back-arc extension took place far from the location where the nappe stack formed. To understand the mechanics of contractional exhumation in this single-sided orogen, we generated a new high-resolution low-temperature (apatite fission-track and U-Th/He) thermochronological transect across the Southeast Carpathian nappe stack and its foreland. This new dataset and its correlation with previous exhumation and kinematic studies demonstrate that the gradual accretion of sediments and crustal material was associated with an exhumation that migrated progressively towards the foreland throughout the entire contractional history. The exhumation style changed when the thin-skinned deformation was interrupted by periods of crustal accretion, associated either with the late Eocene onset of continental collision or with the late Miocene locking of the subduction system. The generally accepted moments of onset and termination of slab retreat are not directly visible in available exhumation data of the Southeast Carpathians. Our results are in agreement with modelling studies, which inferred that the formation of single-sided collisional wedges is primarily controlled by the pre-existing rheology of continental plates and is associated with large amounts of continental subduction. • Formation of the Southeast Carpathian orogen is associated with a pattern of migration of exhumation towards the foreland; • The initial exhumation pattern is modified by the onset of continental collision and locking of the subduction system; • The formation of such orogens is primarily controlled by plates rheology and prolonged periods of continental subduction.

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

AbstractStudies of recent destructive megathrust earthquakes and tsunamis along subduction margins in Japan, Sumatra, and Chile have linked forearc morphology and structure to megathrust behavior. This connection is based on the idea that spatial variations in the frictional behavior of the megathrust influence the tectono-morphological evolution of the upper plate. Here we present a comprehensive examination of the tectonic geomorphology, outer wedge taper, and structural vergence along the marine forearc of the Cascadia subduction zone (offshore northwestern North America). The goal is to better understand geologic controls on outer wedge strength and segmentation at spatial scales equivalent to rupture lengths of large earthquakes (≥M 6.7), and to examine potential linkages with shallow megathrust behavior.We use cross-margin profiles, spaced 25 km apart, to characterize along-strike variation in outer wedge width, steepness, and structural vergence (measured between the toe and the outer arc high). The width of the outer wedge varies between 17 and 93 km, and the steepness ranges from 0.9° to 6.5°. Hierarchical cluster analysis of outer wedge width and steepness reveals four distinct regions that also display unique patterns of structural vergence and shape of the wedge: Vancouver Island, British Columbia, Canada (average width, linear wedge, seaward and mixed vergence); Washington, USA (higher width, concave wedge, landward and mixed vergence); northern and central Oregon, USA (average width, linear and convex wedge, mixed and seaward vergence); and southern Oregon and northern California, USA (lower width, convex wedge, seaward and mixed vergence). Variability in outer wedge morphology and structure is broadly associated with along-strike megathrust segmentation inferred from differences in oceanic asthenospheric velocities, patterns of episodic tremor and slow slip, GPS models of plate locking, and the distribution of seismicity near the plate interface. In more detail, our results appear to delineate the extent, geometry, and lithology of dynamic and static backstops along the margin. Varying backstop configurations along the Cascadia margin are interpreted to represent material-strength contrasts within the wedge that appear to regulate the along- and across-strike taper and structural vergence in the outer wedge. We argue that the morphotectonic variability in the outer wedge may reflect spatial variations in shallow megathrust behavior occurring over roughly the last few million years. Comparing outer wedge taper along the Cascadia margin to a global compilation suggests that observations in the global catalog are not accurately representing the range of heterogeneity within individual margins and highlights the need for detailed margin-wide morphotectonic analyses of subduction zones worldwide.

Detachment fold duplexes within gravity-driven fold and thrust systems

Fold duplexes transfer displacement from a lower to an upper bounding detachment system via trains of folds with broadly parallel geometries. While they have been previously recognised in orogenic systems where they are considered to be kinematically equivalent to imbricating thrust ramps, we here describe the first example from a gravity-driven fold and thrust system (FATS) developed within late-Pleistocene mass transport deposits (MTDs) that formed around the Dead Sea Basin. The recognition in this study of basal and upper detachments that bound the FATS, together with later thrust ramps that imbricate the previously folded sequence, indicates that a fold duplex model is applicable in this case. Truncation of synclinal hinges, together with trapping of duplex roof stratigraphy in synclinal fold cores indicates that initiation of buckling precedes detachments, which then propagated along the upper and lower boundaries of the FATS to create a fold duplex. Downslope-verging folds, which are bound by the detachments, are subsequently cut by thrust ramps with greatest displacement recorded where ramps branch from the basal detachment. As thrust displacement increases then ramp angles generally reduce, which allows thrusts to continue to move and accrue larger displacements. Sequential flattening of lower thrusts in overstep sequences may create apparent ‘back-steepening’ up the slope in what superficially resembles ‘pseudo-piggyback’ sequences. Flattening of thrusts is achieved through tightening, rotation and expulsion of wet sediment and fluid from the cores of footwall synclines and is a consequence of loading from overlying thrust sheets. We speculate that expelled fluids may pond directly beneath overlying detrital-rich units that act as baffles and locally increase fluid pressures thereby facilitating further movement along the upper detachment. We establish a new model, whereby the vergence of structures formed above the upper detachment depends on the relative rates of roof and FATS translation, with slower downslope translation of the roof generating upslope verging folds in a ‘sub-active’ roof, while more rapid movement of a ‘super-active’ roof creates downslope verging folds. The observation that such patterns of minor fold vergence in the roof still largely correspond with the position of folds and thrusts in the underlying FATS indicates that only limited relative translation subsequently occurred between the roof and the FATS. This suggests that displacement must have transferred upwards to new upper detachments shortly after the folds in the roof were created, thereby ‘fixing’ the spatial correlation. As older detachments are folded and ‘lock up’, displacement migrates to new upper detachments that develop along pristine ‘easy-slip’ laminations at higher stratigraphic levels, thereby thickening the deforming FATS towards the sediment free surface. The creation of these new upper detachments at higher stratigraphic levels, together with the development of local overstep imbricate sequences are the principal differences between fold duplexes observed in orogenic settings and those in surficial gravity-driven FATS.

Geometric and kinematic analysis of structural elements along north front of Bagharan Kuh Mountain, NE Iran

At the end of the western part of Bagharan Kuh Mountain in the northeast of Iran, mountain growth has been stopped toward the west because of the stress having been consumed by the thrusting movements and region rising instead of shear movement. Chahkand fault zone is situated at the western part of this mountain; this fault zone includes several thrust sheets that caused upper cretaceous ophiolite rocks up to younger units, peridotite exposure and fault related fold developing in the surface. In transverse perpendicular to the mountain toward the north, reduction in the parameters like faults dip, amount of deformation, peridotite outcrops show faults growth sequence and thrust sheets growth from mountain to plain, thus structural vergence is toward the northeast in this fault zone. Deformation in the east part of the region caused fault propagation fold with axial trend of WNW-ESE that is compatible with trending of fault plane. In the middle part, two types of folds is observed; in the first type, folding occurred before faulting and folds was cut by back thrust activity; in the second type, faults activity caused fault related folds with N60-90W axial trend. In order to hanging wall strain balance, back thrusts have been developed in the middle and western part which caused popup and fault bend folds with N20-70E trend. Back thrusts activity formed footwall synclines, micro folds, foliations, and uplift in this part of the region. Kinematic analysis of faults show stress axis σ1 = N201.6, 7, σ2 = N292.6, 7.1, σ3 = N64.8, 79.5; stress axis obtained by fold analysis confirm that minimum stress (σ3) is close to vertical so it is compatible with fault analysis. Based on the results, deformation in this region is controlled by compressional stress regime. This stress state is consistent with the direction of convergence between the Arabian and Eurasian plates. Also study of transposition, folded veins, different movements on the fault planes and back thrusts confirm the progressive deformation is dominant in this region that it increases from the east to the west.

Effects of lateral friction on the structural evolution of fold-and-thrust belts: Insights from sandbox experiments with implications for the origin of landward-vergent thrust wedges in Cascadia

Various factors are inferred to influence the structural evolution of fold-and-thrust belts or thrust wedges, but the effects of lateral friction (such as that exerted by the opposite block of a strike-slip fault in nature or by sidewalls in analogue models) have rarely been given sufficient attention and hence are still far from being fully understood. Up to now, seaward-vergent and dually vergent thrust wedges have been satisfactorily explained by the classic Coulomb wedge theory and can be well reproduced in analogue and numerical models. In contrast, landward-vergent thrust wedges, which are uncommon in nature (e.g., those occurring in the Cascadia subduction zone), have not yet been adequately explained by the classic Coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions. In order to look closer at the effects of lateral friction on the structural evolution of fold-and-thrust belts and to explore the origin of landward-vergent thrust wedges, we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress (τ s /τ d ). Results of our models revealed that in the presence of a weak or ductile basal decollement, the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction, but also the structural vergence of fold-and-thrust belts. Fold-and-thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction. According to the results of models and the relationships between structural vergence and lateral friction direction occurring both in Cascadia and the analogue models, we propose that an extremely high τ s /τ d ratio due to an extremely weak basal decollement and landward-directed lateral friction exerted by the opposite blocks of basement-involved strike-slip faults are probably the two key factors responsible for the origin of landward-vergent thrust wedges in Cascadia. This work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold-and-thrust belts in the presence of weak or ductile basal decollements.

The Hongliuhe fold-and-thrust belt: Evidence of terminal collision and suture-reactivation after the Early Permian in the Beishan orogenic collage, Northwest China

The Early Permian strata of the Hongliuhe Formation are located in the Beishan orogenic collage, a tectonically significant region between the Tianshan orogen and the Southern Mongolian accretionary system, of the Central Asian Orogenic Belt (also called the Altaids). Stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates, all unconformably overlying an earlier mid-Paleozoic volcanic assemblage. The clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid-section, illustrating the unroofing sequence of an arc-related provenance. The intra-oceanic Gongpoquan composite arc that comprises most of the central Beishan fits the clast-lithotype profile of the conglomerates, and is the most probable provenance. The Hongliuhe Formation stratigraphy experienced a thin-skinned fold-and-thrust belt style of deformation that has a consistent southward vergence. Deformation in the lower stratigraphic levels created thrust imbrication of formation-scale blocks, each separated by steep, north-over-south brittle–ductile shear zones. The blocks are internally deformed with large-scale folding, with one significant block containing a sheath fold 15km in breadth. The upper stratigraphic levels accommodated strain through low-angle thrust ramping, shear folding, imbricate fanning and duplexing. This indicates that the Hongliuhe Formation was syn-orogenically deposited in a progressively deepening terrestrial- to nearshore environment within the Gongpoquan composite arc. Our observations and data indicate that the basin overlies an earlier Paleozoic suture zone. This implies that the northern half of the Gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness, and created a stratigraphic and structural response consistent with a subduction- or collision-driven fold-and-thrust belt. Its southward structural vergence is supporting evidence of a north-dipping subduction-related fault system under the northern Gongpoquan. The timing of both the genesis of the Hongliuhe Formation and this deformation event coincides with the subduction of the Liuyuan Ocean or docking of the Dunhuang block further south, at the southern margin of the Gongpoquan composite arc.

Antarctic/Scotia plate convergence off southernmost Chile

The southern tip of South America off Chile has suffered a long phase of ocean-continent convergence which has shaped the continental margin through different phases of accretion and tectonic erosion. The present accretionary wedge is a discontinuous geological record of plate convergence and records only part of the accretionary processes resumed after Chile ridge consumption (14 Ma). The structural style of the subduction complex, such as rates of sediment accretion and tectonic erosion, structural vergence, width of the accretionary wedge, taper angle and deformation in the forearc basin, varies along the margin. Large taper values are related to narrow wedges and seaward vergent structures. Low tapers occur where deformation at the toe of the accretionary complex is spread over wide areas and is related both to landward and seaward vergent thrust faults. Seismic data interpretation contributes to define more accurately frontal wedge morphology and geometry of subduction and suggests that different modes of accretion together with tectonic erosion may be active concurrently along the trench at different locations. In areas of subduction driven accretionary processes the majority of trench sediments are involved in accretionary processes and sediments are uplifted and piled up in the form of imbricate thrust sheets. In areas where the wedge is non-accretionary the continental margin shows steeper continental slopes associated with narrow accretionary wedges, more intense sediment disruption and very shallow decollement levels. Variation in structural style and in the geometry of the forearc region setting off Southernmost Chile, has been interpreted as related to the existence of different structural domains: the nature of their boundaries is still unclear mainly for the lack of high resolution bathymetric data. They have been tentatively related to tectonic lineaments belonging to the Magellan Fault system and/or to the character and morphology of the converging plates (lateral heterogeneities, sea-mounts and fracture zones), which produce a segmentation of the margin.

STRATIGRAPHIC, STRUCTURAL AND TECTONIC ENIGMAS ASSOCIATED WITH THE CAPE FOLD BELT: CHALLENGES FOR FUTURE RESEARCH

The Cape Fold Belt has been a topic of investigation for over 100 years, and with each renewed surge of interest, new ideas on the origin of the fold belt as a whole have come to the fore. During the last two decades researchers who have focused on the structural geology of the Cape Supergroup, in particular, have provided greater insight into local characteristics of the fold belt, but a broader understanding of the tectonic development of the fold belt is not without controversy. Research focus has been on documentation and interpretation of mainly thin skinned structural characteristics in the central and eastern part of the main “arm” of the fold belt. Identification of thrust sheets, particularly in lower units of the Cape Supergroup coincident with the thickest accumulations of sediment, led researchers to propose a thrust stacking mechanism to account for the abnormal thickness of quartzitic units, rather than previously accepted models. The significance of thrust stacking and the disruptive effect it has had on sediments of the entire Cape Supergroup is emphasized. The stratigraphic order of Cape Supergroup rocks, as currently accepted, is therefore called into question, especially in areas of pervasive thrusting, and needs revision. Some tectonic models proposed to date account only for local characteristics of the fold belt, whereas other models do take into account the entire fold belt. Examples of the latter include the Andean type model which adequately explains the general northward vergence of structures, but thickness estimates of stratigraphic packages of Cape and Karoo Supergroup cover rocks revealed through recent seismic surveys across the central part of the fold belt do not corroborate this model. Proponents of the Transpression model, on the other hand, advocate that right-lateral strike-slip motion produced en-echelon folds and flower structures in some parts of the fold belt. The latter model lacks refinement in that it does not explain the presence of structures interpreted to have formed through left-lateral movement, especially in the western part of the fold belt. This paper outlines unanswered questions relating to the stratigraphy, structural geology and proposed tectonic models that remain enigmas, and these need resolving before the tectono-thermal history of the fold belt can be fully understood.

Influence of synkinematic sedimentation in a thrust system with two decollement levels; analogue modelling

Compressive systems in foreland domains are characterised by fold and thrust belts linked to the presence of one or several ductile layers in depth acting as a decollement level. The main parameters controlling the structural evolution are: the presence of a decollement level, the amount and rate of shortening, and the amount of synkinematic sedimentation. The effect of these parameters has only been studied on a thrust belt scale. Furthermore, only the effect of synkinematic sedimentation on a simple system with one decollement level has been studied at the scale of a single structure. The aim of this study was to use analogue modelling to test the effect of shortening rate, velocity and the localization of sedimentation on a single system characterised by the presence of two prekinematic decollement levels. The main results showed variations in the structural vergence, folding geometry (symmetric or asymmetric), the evolution of the deformation (horizontal propagation versus vertical uplift), and the decoupling of the lower and upper brittle structures in relation with the main parameters (shortening rate and mass transfer). The results of the experiments were then compared to natural examples from the sub-Andean thrust belt.

The role of syntectonic sedimentation in the evolution of doubly vergent thrust wedges and foreland folds

This paper shows the evolution of time-constrained two-dimensional scaled analogue models of doubly vergent thrust systems in the presence of syntectonic sedimentation. Two sets of experiments were considered: (1) the addition of a syntectonic layer composed of a polymer and overlying sand in the prowedge; and (2) the addition to the previous condition of a progradational sedimentary load. Results from the first set of experiments indicate that the foreland fold-and-thrust belt has a strong relationship with the competence of the syntectonic layers. When the competence is low, the deformation produces tight asymmetric detachment-folds. As the competence increases, the fold-and-thrust belt shows break-trough folds with longer and better defined foreland-vergence. Results from the second set of experiments indicate that structural vergence is determined by the sense of progradation of the syntectonic layers, and in the case of strong aggradation at the prowedge, extension and reactive diapirism form contemporaneously with the contraction. Three end-members are proposed for mountain front thrust systems formed in the presence of syntectonic polymer and sand sedimentation: (1) outcropping fold-and-thrust belt sequence, in the case of no syntectonic sedimentation; (2) long displacement blind-thrust sheets, in the case of under-filled basins and (3) short displacement blind-thrust sheets, in the case of over-filled basins. All results indicate that ductile units at the base of syntectonic layers increase the displacement of the underlying frontal thrusts at the prowedge, and reduce the critical taper. Results also indicate that at very high sedimentary rates and hyper-critical taper conditions the prowedge collapses. Conclusions drawn from this research may be applied as an analogue to foreland evolution and to evaluate hydrocarbon generation, migration, and entrapment in thrust belts in areas where seismic imaging is generally poor.

Passive‐roof thrusting in the Messinian Vena del Gesso Basin (Northern Apennines, Italy): constraints from field data and analogue models

AbstractWe present the results of a study of the Vena del Gesso Basin (Romagna Apennines, Italy) integrating field analyses and analogue modelling. This basin represents one of the best‐preserved top‐thrust basins in the Northern Apennines foreland and is one of the few examples where primary evaporites, related to the Messinian salinity crisis of the Mediterranean, widely crop out. The structural style affecting the Messinian gypsum is examined to get insights into the mechanism responsible for the overall deformation features recognizable in the area. The evaporites are completely detached at the base and widespread back‐thrusts, repeatedly doubling these deposits, strongly contrast with the regional forelandward vergence of structures in the Apennines. On the basis of the comparison between field data and experimental results, the features characterising this area can be described as the result of the deformation linked to the sequential activation of an obliquely propagating passive‐roof duplex. Analogue models evidenced the major role played (1) by syntectonic erosion that promoted the development of passive‐roof duplex style, as well as (2) the role of décollement level pinch‐out that determined an oblique progression of deformation. Finally our data lead to reconsider the palaeoenvironmental reconstruction concerning the onset of the Messinian salinity crisis in the Mediterranean. Copyright © 2007 John Wiley & Sons, Ltd.

Deformation patterns and structural vergence in brittle–ductile thrust wedges: An additional analogue modelling perspective

Scaled physical models of brittle–ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold-and-thrust belts and accretionary prisms. This study focused on some potential controlling factors, such as (1) relative strength between the brittle overburden and the viscous décollement layer (brittle–ductile coupling), (2) the effect of very low basal friction, and (3) pre-shortening rheological stratigraphy, including décollement strength. Type 1 models had a single basal décollement, whereas in Type 2 models the décollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence. Other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous décollement (Type 1a), and the role of décollement offset (Type 3). The experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland- or foreland-verging thrust faults, or dually-verging thrusts. By controlling the orientation of the σ 1 axis, basal friction and décollement offset is found to effectively influence the deformation pattern and structural vergence, and could explain the development of dominating backthrusts in Cascadian-type margins. The decoupling of a frontal monocline via the upper décollement (Type 2 models) produced passive-roof duplex structures, termed “composite-roof duplex” when the frontal monocline coexists with outward fold propagation.

Geologic correlation between the Neoproterozoic Sergipano belt (NE Brazil) and the Yaoundé belt (Cameroon, Africa)

The Yaoundé belt (Cameroon) and the Sergipano belt (NE Brazil) belonged to a major and continuous Neoproterozoic orogen at the northern margin of the ancient Congo-São Francisco craton. The Yaoundé belt comprises schists, quartzites, gneisses and migmatitic gneisses grouped into three domains; the low-grade Mbalmayo Group in south and the medium- to high-grade Yaoundé and Bafia Group in north. The Sergipano belt is divided into six domains, the three southernmost of which are mostly made up of clastic and chemical metasedimentary rocks whereas the others are more diverse with a migmatite–gneiss complex, and two metavolcanicplutonic complexes. In general, the two belts show structural vergence and decrease of metamorphic grade towards the craton; three main deformation phases are recognized in the Sergipano belt in contrast with two described in the Yaoundé belt. The minimum age of Pan-African-Brasiliano collision in the Sergipano belt is constrained at 628 ± 12 Ma on syn-collision granites, whereas in the Yaoundé belt collision took place between 620 and 610 Ma, i.e. the age of granulite facies metamorphism. Sm–Nd isotope geochemistry and U–Pb age dating indicate that most clastic metasedimentary rocks in both belts were derived from sources to the north and, to a lesser degree, from the cratons to the south.

Physical modelling of deformation in the Tasman Orogenic Zone

Structural vergence within the Western Subprovince of the Lachlan Fold Belt is towards the hinterland rather than the foreland, in contrast to many well-known orogenic belts. High angle-reverse faults and upright folds verge eastwards, away from the Australian craton, towards the inferred centre of orogenic and magmatic activity. We designed a series of analogue models to test the anomalous vergence in the western Lachlan Fold Belt, particularly the interaction of a stable Australian craton with Tasman Line geometry, interacting with weaker oceanic or transitional lithospheric material. We found consistently that vergence direction in the models was towards the hinterland, not the foreland, as in the western Lachlan Fold Belt, irrespective of the way the model was deformed. Strength gradients between the oceanic and cratonic lithosphere control the deformation patterns. An important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere.

Ophiolite obduction and the Samail Ophiolite: the behaviour of the underlying margin

Abstract Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high- P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72–76 Ma is at the time the ophiolite is emplaced finally onto the margin (80–70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82–80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding (‘retrocharriage’) associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.

Proterozoic – Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn Block and its tectonic implications

Aeromagnetic and field data suggest that meta‐igneous rocks exposed on the south coast of central Victoria at Waratah Bay, Phillip Island, Barrabool Hills and inland near Licola, are continuous—beneath Bass Strait—with Proterozoic/Cambrian igneous rocks in King Island and Tasmania. This correlation is supported by a pre‐Early Ordovician unconformity above gabbro protomylonite at Waratah Bay, age equivalent to the Tasmanian Tyennan unconformity. Cambrian volcanics at Licola and unusual features of the Melbourne Zone sequence indicate that Tyennan continental crust extends north as basement to the central Victorian portion of the Lachlan Fold Belt. In contrast, adjacent parts of the Lachlan Fold Belt in Victoria contain conformable sea‐floor sequences that span the Early Cambrian to Late Ordovician, with no evidence of either Cambrian deformation or underlying continental basement. The block of Tyennan continental crust beneath central Victoria—the Selwyn Block—is fundamentally different, and has influenced temporal and spatial patterns of sedimentation, deformation, metamorphism and plutonism. Palaeogeographical reconstructions suggest that the block was a submarine plateau that lay outboard of the Australian craton, upon which a condensed Ordovician sequence was deposited. The sequence above the Selwyn Block unconformity at Waratah Bay is similar to widespread post‐Tyennan sediments in western Tasmania. During Late Ordovician and Early Silurian deformation, the Selwyn Block protected much of the overlying sedimentary sequence. Instead, shortening was focused into the Stawell and Bendigo Zones to the west. These zones were sandwiched between the Selwyn Block and the Australian craton in a ‘vice’ scenario reminiscent of some Appalachian orogenic events. The region above the Selwyn Block was downwarped adjacent to the overthrust Bendigo Zone as a foreland deep, into which a conformable clastic wedge of sediment was deposited in Late Ordovician to Devonian time, prior to final Middle Devonian deformation. The Selwyn Block includes the Cambrian calc‐alkaline Licola and Jamieson Volcanics that are correlated with the Tasmanian Mt Read Volcanics. In Victoria, these form a basement high controlling the unusual down‐cutting thrusts in the overlying Melbourne Zone and explaining the major structural vergence reversal between the Melbourne and Tabberabbera Zones. The Selwyn Block has exerted some control on the timing, chemistry and distribution of post‐orogenic granites, and on central Victorian gold mineralisation. Reactivated faults in the block influenced deposition, and continue to control the deformation of the portions of the Otway and Gippsland Basins that lie above it.

Thrust faults and fold vergence in the Palaeozoic middle and upper Witteberg Group, Cape Supergroup (Cape Fold Belt), Steytlerville: an interpretation of their relationship

Research Article| March 01, 2002 Thrust faults and fold vergence in the Palaeozoic middle and upper Witteberg Group, Cape Supergroup (Cape Fold Belt), Steytlerville: an interpretation of their relationship P.W.K. Booth P.W.K. Booth Department of Geology, University of Port Elizabeth, P.O Box 1600, Port Elizabeth, 6000, Republic of South Africa., e-mail: glapwb@upe.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information P.W.K. Booth Department of Geology, University of Port Elizabeth, P.O Box 1600, Port Elizabeth, 6000, Republic of South Africa., e-mail: glapwb@upe.ac.za Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2002 Geological Society of South Africa South African Journal of Geology (2002) 105 (1): 25–38. https://doi.org/10.2113/1050025 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P.W.K. Booth; Thrust faults and fold vergence in the Palaeozoic middle and upper Witteberg Group, Cape Supergroup (Cape Fold Belt), Steytlerville: an interpretation of their relationship. South African Journal of Geology 2002;; 105 (1): 25–38. doi: https://doi.org/10.2113/1050025 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Four sections through middle and upper Witteberg Group strata north of Steytlerville were investigated in detail to ascertain the phenomenon of opposite vergence of folds, as well as the relationship between folding and thrusting. Strata of the middle Witteberg Group are thinly-bedded and predominantly arenaceous, whereas those of the upper Witteberg Group show variation in bed thickness, lithological composition and proportion of arenaceous to argillaceous material. East-southeast striking thrust faults are commonly developed throughout the study area, and have a complex history of development. They display characteristic ramp and flat geometry, with south-southwest-dipping forethrusts predominating over north-northeast-dipping backthrusts. Thrust/fold relationships show that thrust faults formed prior to, during and after folding. Fold styles vary from gentle to tight, and fold axes of folds plunge shallowly east-southeast and west-northwest. The orientation of east-southeast-striking axial planes of folds and their predominantly northward vergence indicate that folds formed during a north-northeastward-directed compression event, during the Cape orogeny. The few examples of southward vergence of fold structures are explained in some cases by formation of folds during backthrusting, and in other cases folds formed prior to fore-and backthrusting. The implications of these findings are that strata of the middle and upper Witteberg Group have inherent structural complexities that make it difficult to define the lithostratigraphy. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.