Tangential Longitudinal Strain Research Articles

Microtectonics and fluid evolution in the Katanga Supergroup (Tenke Fungurume Mining District): Evidence of pre- to post-orogenic Cu-Co mineralization in the Central Africa Copperbelt

The sediment-hosted stratiform copper-cobalt deposits in the Central African Copperbelt (CACB) are renowned for their resource potential, encompassing Cu-Co (Ni, U) and Cu-Zn-Pb (Ag, Ge, Mo, Cd) mineralization. Microstructural and microthermometric analyses of carbonate-quartz veins from the Tenke Fungurume mining district, situated in the central part of the Katanga Copperbelt (KCB), have delineated a pre- to post-folding paragenesis of vein formation and hypogene Cu-Co ore mineralization. The mineralization primarily consists of chalcopyrite, chalcocite, carrollite, and bornite.Pre-folding veins comprise bedding-normal veins related to the extension stage, occasionally linked with mullion development during the following compressional stage, as well as bedding-parallel beef veins. Syn-folding mineralization manifests as saddle reef veins within fold hinge zones, fracture-filling veins aligned with tangential longitudinal strain in hinge zones of folded bedding-parallel veins, thin veinlets forming parallel to an axial-planar and shear-band cleavage, and bedding-parallel shear veinlets with oblique fiber orientations. The post-folding Cu-Co mineralization consists of veins that cross-cut earlier veins and folds.Microthermometric analysis of fluid inclusion assemblages from the pre-, syn-, as well as post-folding veins reveals the presence of different fluid types. A high-temperature (180–340 °C) and high-salinity (27–52.5 wt% NaCl + (KCl) eq.) fluid was responsible for the Cu-Co mineralization during the pre- to syn-folding stages. A hypersaline fluid (27.0–42.1 wt% NaCl eq.) with a broad range in temperature (45–387 °C) circulated during the post-folding stage of the Lufilian orogeny. A fluid exhibiting low to moderate homogenization temperatures (28–215 °C) and low to moderate salinity (3.4–23.0 wt% NaCl eq.) is related to fluid circulation during the post-Lufilian epoch. The wide range and variations in salinity of these three fluid types associated with Cu-Co mineralization provide evidence for a multistage fluid flow and a complex hydrothermal system linked to the deposition of Cu-Co ores within the Tenke Fungurume Mining District and the broader Katanga basin. This hydrothermal system has operated during diagenesis, the basin development stage to the Lufilian orogeny and the subsequent post-Lufilian period.

Evolution of fracture networks and connectivity during fault–bend folding: Insights from the Sinon Anticline in the southwestern Hongseong–Imjingang Belt, Korea

Crustal shortening in an elastico-frictional regime is mainly accommodated by contractional fault–fold systems with fracture networks. According to recent research, fracture networks in fold–thrust belts express complex internal strain states in response to thrusting and related folding. Furthermore, their connectivity and fluid flow characteristics likely depend on the structural positions and mechanical stratigraphy that control heterogeneous deformation processes. This study provides characteristics of fold-related fracture networks in the Sinon Anticline, which was formed by fault–bend folding in the southwestern Hongseong–Imjingang Belt, Korea. The fracture networks in the metamorphosed turbidites characterized by interbedded competent metasandstone layers and relatively thin incompetent schist layers have evolved through pre-, syn-, and post-folding fracturing events. Their complexity reflects the spatiotemporal variation in the strain pattern related to early layer-parallel shortening and subsequent fault–bend folding. Based on insights from detailed mapping and topological analysis of the fracture network, we conclude that strain partitioning that occurs during flexural folding results in a superposed tangential longitudinal strain expressed by fractures with a high (hydraulic) connectivity in the hinge zones. Strain partitioning is caused by flexural interlayer slip along incompetent schist layers in the fold limbs. Bed-parallel slip localization zones probably have low porosity and permeability and may act as barriers to fluid migration across beds. We suggest that heterogeneous vertical axis rotation, which occurred as the system's hanging wall slid over the footwall ramp, increased the complexity of fracture networks within the Sinon Anticline. Our findings indicate that the evolution, connectivity, and fluid flow properties of fracture networks can be characterized through careful interpretation of folding mechanisms and related strain states during formation of fault–bend fold systems.

Understanding the Relationship between Large-Scale Fold Structures and Small-Scale Fracture Patterns: A Case Study from the Oman Mountains

Considering the foreland fold belt of the Salakh Arch in the northern Oman Mountains, predictions made from two-dimensional (2D) restorations and geometrical analyses are tested here to assess the relationship between large-scale folds and small-scale fractures. The Salakh Arch is composed of six anticlines that are interpreted as faulted detachment folds. They have an overall stratigraphy of a 2-km-thick carbonate platform underlain by more than 1.5 km of interbedded sandstone and shale sequences. These sequences are most likely detached on a regionally extensive evaporite horizon. The folding of the Salakh Arch structures most likely occurred during the Neogene Period, and perhaps partly in the early Quaternary Period. This is evident from the thrusting of the Late Neogene Barzaman Formation which was deposited during the Late Neogene Period. Robust outcrop and subsurface fracture data are used to test these predictions. The results from the study indicate that most fractures are related to the orientation of the local structure, with some sets parallel and some sets perpendicular to local hinge lines. Prefolding regional fractures are also widely distributed, and these were mostly formed during the Late Cretaceous Period. Many pre-existing fractures are associated with faults that formed during the Late Cretaceous Period under a NW–SE compression. The local fractures generally have orientations that are consistent with being formed by the flexural slip/flexural flow of fold limbs and tangential longitudinal strains on fold hinges. These structures can be predicted from finite stratal dips, simple curvatures, and three-dimensional (3D) folding restoration maps. The Gaussian curvatures and 3D faulting restoration maps can be used as proxies for fault-related fractures. Local hinge-related fractures may reflect local tangential longitudinal strain during large-scale fold tightening. Fold structures that have formed at an oblique orientation to the regional shortening direction show additional fracture arrays perpendicular to the hinge, indicating weak axial extension. This is presumed to develop as the arcuate thrust belt of Salakh Arch was amplified. The analysis here illustrates the importance of taking a 3D approach, especially for noncylindrical folds. The protocols developed in this study and their results may have general applicability to investigations of fracture patterns in other folds.

P5974Prognostic value of global tangential strain by three-dimensional echocardiography heart failure patients with intermediate ECG criteria for cardiac resynchronization therapy

Abstract Background In the current clinical guidelines, cardiac resynchronization therapy (CRT) is recommended in heart failure (HF) patients with left bundle branch block (LBBB) with QRS width ≥150ms as Class I indication. In terms of HF patients with intermediate ECG criteria, prediction for benefit of CRT is still controversial in routine echocardiography. Purpose The aim was to assess whether three-dimensional (3D) echocardiographic indexes of global LV function at baseline has a prognostic value following CRT implantation in patients who fulfilled with intermediate ECG criteria. Methods We studied 62 HF patients who fulfilled with the indication criteria of CRT implantation according to current the clinical guidelines. In addition to routine two-dimensional echo, 3D echo dataset was acquired for determination of 3D global tangential strain (GTS) and 3D global longitudinal strain (GLS). We tracked predefined unfavorable outcomes for 3 years after CRT implantation: death, hospitalization due to worsening HF. Results LBBB with QRS width ≥150ms was evident in 26 of 62 patients (aged 68±11 years with 160±26 ms of QRS duration and 29±7% of LV ejection fraction), and the other 36 patients only fulfilled intermediate ECG criteria (QRS width 120–149ms or non-LBBB). Unfavorable events occurred in 21 patients (34%). The median GTS was −15.4%. Although GLS was not predictive, GTS greater than −15.4% had high probability of unfavorable outcomes over 3 years (Log-rank, p<0.05). There is no difference in the probability of unfavorable outcomes between LBBB and intermediate ECG criteria. Baseline GTS in patients with intermediate ECG criteria was associated with unfavorable outcomes: −12.6±2.6% vs. −17.3±3.8% (p<0.05). Outcome was better in the intermediate ECG criteria patients with GTS ≤−15.4% than in those with LBBB and in those with intermediate ECG criteria patients with GTS >−15.4% (Log-rank: p<0.05, p<0.0001, respectively). Conclusions Baseline GTS by 3D echocardiography is useful for predicting outcome over 3 years after CRT implantation regardless of the ECG criteria for CRT indication.

Timing of Cleavage Development in Relation to Folding and its Implications: An Example from the Deformed Mesoproterozoic Sedimentary Cover of Kaladgi Basin, Southwestern India

ABSTRACT Detailed analysis of cleavage-fold relationships in the Mesoproterozoic cover sediments of Kaladgi basin, south western India, revealed three types of timing relationships between cleavage and folds: (1) cleavage developed earlier than folds. (2) Cleavage developed later at some stage of folding and (3) non-development of cleavages in folds. In the first case cleavage developed earlier than the folding. Evidences that support such an interpretation include high cleavage-bedding angle throughout the fold, asymmetric fanning of cleavage in the folds with large fan angles and rotation of cleavage more in the steep and short limbs of asymmetric fold. In the second case, cleavage initiated later at some stage during folding which is evident from low cleavage fan angles, relatively low cleavage–bedding angles in the limbs of folds and the offset of bedding trace against cleavage trace due to pressure solution. Even though pressure solution is ubiquitous throughout the fold the effects of offset are more pronounced in the limbs compared to that in the hinge due to changing angular relationship of bedding and cleavage throughout the fold. In the third case, cleavage is absent in the folds resulting in its sporadic occurrences and reflects its diachronous nature. The final geometry of cleavage in relation to folds at any location is a combined effect of relative timing of cleavage development and mechanism of folding involving layer parallel shortening, flexural slip, tangential longitudinal strain, syn- to postfold flattening and hinge migration.

Fracture patterns associated with the evolution of the Teton anticline, Sawtooth Range, Montana, USA

Abstract The Teton anticline and adjacent structures, in the Sawtooth Range, Montana, USA, are fractured in such a way that may be taken as a model for fractures propagating during buckle folding. However, advances in understanding both the process of folding in forelands and the evolution of fracture patterns found within these folds suggest that it is time to reinterpret the nexus between fracturing and folding within these classic structures. With the benefit of seismic lines, the Teton anticline is best described as a fault-propagation fold. Joint propagation initiated with the formation of two major sets whose orientation is controlled by pre-folding, regional stresses. Two more joint sets propagated in local stress fields, developed in response to anticline growth. Some early joints were reactivated as wrench faults during amplification and tightening of the anticlines. The fracture sets identified are consistent with: (a) propagation in a regional stress field, which may be related to stretching in the Sawtooth Range orocline; and (b) tangential longitudinal strain of the backlimb and forcing or trishear of the forelimb during anticline development. Thus, we suggest that fracture networks across folded structures should be interpreted and characterized in the light of the geological history of the entire system.

Active Bending‐Moment Faulting: Geomorphic Expression, Controlling Conditions, Accommodation of Fold Deformation

AbstractBending‐moment faults and flexural‐slip faults (FSFs), as two basic fault styles due to bending‐related tangential longitudinal strain, extensively and prominently crop out as surface scarps in the Pamir‐western Kunlun and southern Tian Shan regions, northwestern China. Characteristic geomorphic expression, favorable formation conditions, and the role in folding accommodation of active FSFs have been systematically summarized in our recent studies. Here we investigate similar properties for well‐developed bending‐moment normal fault (BMnF) scarps at four sites. Our study concludes the following: (i) BMnF scarps are relatively sinuous compared to FSF scarps and probably trend obliquely to the fold hinge. A group of BMnF scarps can delineate a single asymmetric graben or create grabens alternating with horsts. (ii) BMnF scarps primarily overlie poorly‐layered conglomerates. The fold's interlimb angle can range from ~160° to <40°, and the folding kinematics can vary from active‐hinge migration to fixed‐hinge rotation. (iii) The fault‐zone width, fault spacing, and efficiency in folding accommodation significantly decrease with (a) thinner conglomerate beds, (b) a smaller interlimb angle, and (c) the transition of the hinge from migrated to fixed. (iv) Different bed lithologies and fold geometries beneath the surface account for the predominance of BMnF scarps on the western Kunlun piedmont and FSF scarps in the Pamir‐Tian Shan convergent zone. (v) Presence of BMnF scarps on the western Kunlun piedmont indicates that ~4 km of fault slip is transferred northward along a detachment at the base of the Cenozoic and is ultimately absorbed by the Mazatagh Thrust in the Tarim Basin.

A new mechanism for producing cleavage in preexisting folds: The translation mechanism. An example in the Burela section (Variscan belt, NW Spain)

An outcrop on the Cantabrian coast (Burela section) shows a long train of tight meter-scale folds developed in Cambrian siliciclastic rocks. These folds have been shortened in the axial trace direction on the fold profile, developing a cleavage in the incompetent layers which obliterates the primary cleavage and crosscuts the folds. Several mechanisms have been analyzed to explain the development and attitude of this cleavage, some of them being the same as those that have previously been proposed to form folds but operating in a reverse sense. They are: anti-flexural flow, anti-reverse tangential longitudinal strain and homogeneous strain. The sole operation of these mechanisms cannot explain this cleavage and a new one has been defined with this aim. This mechanism consists of deformation of the incompetent layers by translation of the competent ones (translation mechanism), and it involves an area decrease within the incompetent layers in the fold profile plane and, if there is no important volume decrease, a stretching in the hinge direction that must affect both competent and incompetent layers. The geometrical properties of this mechanism have been analyzed in detail and it is concluded that, combined with a small amount of homogeneous flattening, this mechanism can explain the distribution of the cleavage through the folds.

Plio-Quaternary thin-skinned tectonics along the crustal front flexure of the southern Central Andes: a record of the regional stress regime or of local tectonic-driven gravitational processes?

We present here a record of Plio-Pleistocene deformations above the flexural front of the southern Central Andes of Argentina. We combine a seismic profile with structural and geomorphological observations to show that thin-skinned extension located on top of the crustal front flexure is coeval with thin-skinned shortening at the toe of the topographic bulge. The seismic line shows that a flat zone with no internal deformation separates the stretched and shortened domains. Such features are usually interpreted as the result of strike-slip faulting along basement faults, or tangential longitudinal strain folding in the soft sedimentary cover above crustal bending. We propose an alternative linking extension at the apex of the crustal anticline, to basal contraction by the downslope translation of a rigid thin nappe of sediments (30 × 30 km2 in area) above evaporites at a depth of 700–900 m. The size of such a process is unusually large onshore (630–810 km3) but mimics the gravity gliding observed in deltas and passive margins. Since this process disconnects zones with a shallow stress field from deeper crustal levels, it could allow extension above a compressive deformation front and should not be interpreted merely as a record of the crustal stress regime. Large-scale gravity gliding of the cover down the slope of a structural high could also explain some of the extension observed in mountain hinterlands.

Fracture patterns in successive folding in the western Sichuan basin, China

A detailed field survey of fractures was carried out in the Pingluoba and adjacent anticlines in the western Sichuan basin to discuss fracture development during progressive folding, including the relative timing of fracture formation, the successive deformation in a rock layer, the distribution of fracture sets in folds, and the variation of fractures along with the subsequent deformation of rock layers. Based on a comparison of the diverse fracture patterns in folded strata at different stages of fold growth (horizontal, low-angle dipping, high-angle dipping, and vertical layers), a model of fracture development related to fold evolution was built up. It is suggested that the strata were initially under uniform compression, and two sets of planar conjugate shear fractures were formed, the orientations of which would vary along with the subsequent folding of strata. Then, during progressive folding, the deformation of rheologically hard layers was controlled by tangential longitudinal strain, inducing the development of a set of tensile fractures that struck parallel to the fold axis. Layer parallel shear was predominant in the soft layers, and two sets of bedding-oblique dextral shear fractures intersecting with the bedding at angles of 25–40° and 160° were developed. In the progressive folding process, the shear fractures were developed later than the tensile fractures. A polar stereonet of fractures was generated and applied to improve the efficiency and accuracy of fracture identification in the field, providing precise evidence for the exact analysis of fracture patterns and rock deformation.

Structural characterization of Turtle Mountain anticline (Alberta, Canada) and impact on rock slope failure

This paper proposes a structural investigation of the Turtle Mountain anticline (Alberta, Canada) to better understand the role of the different tectonic features on the development of both local and large scale rock slope instabilities occurring in Turtle Mountain. The study area is investigated by combining remote methods with detailed field surveys. In particular, the benefit of Terrestrial Laser Scanning for ductile and brittle tectonic structure interpretations is illustrated. The proposed tectonic interpretation allows the characterization of the fracturing pattern, the fold geometry and the role of these tectonic features in rock slope instability development. Ten discontinuity sets are identified in the study area, their local variations permitting the differentiation of the study zone into 20 homogenous structural domains. The anticline is described as an eastern verging fold that displays considerable geometry differences along its axis and developed by both flexural slip and tangential longitudinal strain folding mechanisms. Moreover, the origins of the discontinuity sets are determined according to the tectonic phases affecting the region (pre-folding, folding, post-folding). The localization and interpretation of kinematics of the different instabilities revealed the importance of considering the discrete brittle planes of weakness, which largely control the kinematic release of the local instabilities, and also the rock mass damage induced by large tectonic structures (fold hinge, thrust). • Remote methods such as TLS and ALS are powerful tools for detailed structural analyses. • Five sets of fractures occurred during folding event, three are post-folding. • Fold geometry variation along axis induces variation of the fractures properties. • Small-scale instabilities mainly controlled by fold geometry and related fracturing • Large rock slope failure allowed by tectonic induced rock mass strength reduction

Fracture patterns in the Zagros fold-and-thrust belt, Kurdistan Region of Iraq

Fracture data have been collected in the Kurdistan Region of Iraq, which is a poorly accessible and unexplored area of the Zagros. Pre to early folding NE–SW striking extensional fractures and NW–SE striking contractive elements represent the older set affecting the exposed multilayer of the area. These latter structures are early syn-folding and followed by folding-related mesostructural assemblages, which include elements striking parallel to the axial trend of major folds (longitudinal fractures). Bedding perpendicular joints and veins, and extensional faults belonging to this second fracture set are located in the outer arc of exposed anticlines, whilst longitudinal reverse faults locate in the inner arcs. Consistently, these elements are associated with syn-folding tangential longitudinal strain. The younger two sets are related to E–W extension and NNE–SSW to N–S shortening, frequently displaying reactivation of the older sets. The last shortening event, which is described along the entire Zagros Belt, probably relates with the onset of N–S compression induced by the northward movement of the Arabian plate relative to the Eurasian Plate. In comparison between the inferred palaeostrain directions and the kinematics of recent GPS measurements, we conclude that the N–S compression and the partitioning into NW–SE trending folds and NW to N trending strike-slip faults likely remained unchanged throughout the Neogene tectonic history of the investigated area.

Analogue modeling of lithospheric-scale orocline buckling: Constraints on the evolution of the Iberian-Armorican Arc

We report on a series of analogue modeling experiments that study the oroclinal buckling process as a thick-skinned process involving the entire lithosphere. The results obtained in the experiments suggest that, during oroclinal buckling, extension in the outer arc and significant shortening in the inner arc are produced by tangential longitudinal strain as the main mechanism of deformation. The models also reveal that the mantle lithosphere thickens in different noncylindrical ways depending on the initial lithospheric mantle thickness: from almost recumbent to folding with subvertical axial planes for the thinnest to the thickest mantle lithosphere, respectively. The results provide useful insights into thick-skinned orocline buckling as it is interpreted to have happened in the Iberian-Armorican Arc.

Saw-tooth structures and curved veins related to folds in the south-central Pyrenees (Spain)

Two generation of folds ( F 1 and F 2) and associated structures, developed in the Eocene turbidites of the south-central Pyrenees, are analyzed in this paper. F 1 folds are close, have sub-horizontal axes and southwards vergence. They have an associated cleavage S 1. Competent layers were folded by layer-parallel shortening, tangential longitudinal strain, some possible flexural flow and an obliquely superimposed homogeneous strain due mainly to simple shear. Flexural slip is also an important mechanism in the whole multilayer. F 2 folds are gentle and scarce; they fold the S 1 cleavage. Among the structures associated with F 1 folds, there are sets of veins with curved form in the competent layers. The displacement of each vein gave rise usually to a step in the layer boundary, so that a set of veins produces a structure that is named “saw-tooth structure”. The veins initiated as small faults that made flexural slip difficult and gave rise to a concentration of stress on the steps, leading to an opening of the fractures and a propagation of them along a curved path, as suggested by a simple mechanical model. This propagation agrees with finite element models developed by other authors.

The neutral lines in buckle folds

The neutral line in a buckle fold, dividing areas of outer-arc extension from areas of inner-arc shortening, is a fundamental concept in structural geology. In the past, folds have been constructed kinematically from a given neutral line geometry using the tangential longitudinal strain pattern. In this study, a mechanical finite element model is used to numerically buckle single-layer folds with Newtonian and power-law viscous rheology. Two neutral lines can be distinguished, the incremental neutral line (zero layer-parallel strain rate) and the finite neutral line (zero finite layer-parallel strain). The former develops first and migrates through the layer from the outer towards the inner arc ahead of the latter. Both neutral lines are discontinuous along the fold and terminate either at the bottom or top interface of the layer. For decreasing viscosity ratio between layer and matrix and for decreasing initial amplitude, the neutral lines develop later during folding and, for some cases, no neutral line develops. The dynamical behaviour of the neutral lines is similar for Newtonian and power-law viscous rheology if the viscosity ratio is large, but substantially different for small viscosity ratios. The results are discussed in light of fold-related structures, such as outer-arc-extension structures and inner-arc-shortening structures.

Kinematic analysis of asymmetric folds in competent layers using mathematical modelling

Mathematical 2D modelling of asymmetric folds is carried out by applying a combination of different kinematic folding mechanisms: tangential longitudinal strain, flexural flow and homogeneous deformation. The main source of fold asymmetry is discovered to be due to the superimposition of a general homogeneous deformation on buckle folds that typically produces a migration of the hinge point. Forward modelling is performed mathematically using the software ‘FoldModeler’, by the superimposition of simple shear or a combination of simple shear and irrotational strain on initial buckle folds. The resulting folds are Ramsay class 1C folds, comparable to those formed by symmetric flattening, but with different length of limbs and layer thickness asymmetry. Inverse modelling is made by fitting the natural fold to a computer-simulated fold. A problem of this modelling is the search for the most appropriate homogeneous deformation to be superimposed on the initial fold. A comparative analysis of the irrotational and rotational deformations is made in order to find the deformation which best simulates the shapes and attitudes of natural folds. Modelling of recumbent folds suggests that optimal conditions for their development are: a) buckling in a simple shear regime with a sub-horizontal shear direction and layering gently dipping towards this direction; b) kinematic amplification due to superimposition of a combination of simple shear and irrotational strain with a sub-vertical maximum shortening direction for the latter component. The modelling shows that the amount of homogeneous strain necessary for the development of recumbent folds is much less when an irrotational strain component is superimposed at this stage that when the superimposed strain is only simple shear. In nature, the amount of the irrotational strain component probably increases during the development of the fold as a consequence of the increasing influence of the gravity due to the tectonic superimposition of rocks.

Compaction‐induced stress variations with depth in an active anticline: Northern Apennines, Italy

It is shown that the stress field can vary with depth, even within a homogeneous tectonic setting, as documented for the active Mirandola fault‐related fold along the buried front of the northern Apennines. Analyses of borehole breakouts and other well data, integrated with seismological information and field evidence, show that extension perpendicular to the fold axis, above approximately 1200 m, changes to a strike‐slip stress field and finally to compression near the main detachment at depth. Similar along‐depth strain variations recognized in non‐active anticlines are usually explained by invoking tangential longitudinal strain folding (i.e., stretching above and shortening below a neutral surface) or gravitational instabilities. However, in this study, we propose that differential compaction may play a significant and generally overlooked role. With the aid of numerical modeling, it is shown that where fold limbs are onlapped by highly compactable deposits (as in the Mirandola area), differential compaction induces stretching at shallow (<1–2 km) depths. The amount of stretching is a function of the shape of the fold and of the thickness of the syn‐tectonic sediments. We conclude that in the Mirandola case study, the stress variations observed with depth are the result of a combination of a regional compression at depth and local tension driven by differential compaction of growth strata on the limbs of the anticline with respect to its crest.

An approach to folding kinematics from the analysis of folded oblique surfaces

Two-dimensional analysis of folded surfaces oblique to the mechanical layering can shed light on the kinematic mechanisms that operated during the development of folds. A new version of the program ‘FoldModeler’, developed in the Mathematica™ environment, is used to obtain the deformed configuration of an initial pattern of oblique surfaces deformed by any combination of the most common kinematic folding mechanisms: flexural flow, tangential longitudinal strain, with or without area change and heterogeneous simple shear. The layer can also undergo any form of homogeneous strain at any moment of the folding process. The outputs of the program provide complete information about the strain distribution in the folded layer that includes graphs of the angle between the oblique surfaces as a function of the inclination of the layering through the fold. These graphs can be very useful to discriminate between the mechanisms that operate in the development of natural folds, and they have been obtained and discussed for the most common combinations of strain patterns. The program is applied to obtain theoretical folds that give a good fit of some natural examples of folded oblique surfaces.

Evidence for coupled reverse and normal active faulting in W Iberia: The Vidigueira–Moura and Alqueva faults (SE Portugal)

The Vidigueira–Moura fault (VMF) is a 65 km long, E–W trending, N dipping reverse left-lateral late Variscan structure located in SE Portugal (W Iberia), which has been reactivated during the Cenozoic with reverse right-lateral slip. It is intersected by, and interferes with the NE–SW trending Alentejo–Plasencia fault. East of this intersection, for a length of 40 km the VMF borders an intracratonic tectonic basin on its northern side, thrusting Paleozoic schists, meta-volcanics and granites, on the north, over Cenozoic continental sediments preserved in the basin, on the south. West of the faults intersection, evidence of Cenozoic reactivation is scarce. In the eastern sector, Plio-Quaternary VMF reactivation is indicated by geomorphologic, stratigraphic, and structural data, showing reverse movement with a right-lateral strike-slip component, in response to a NW–SE trending compressive stress. An average vertical displacement rate of 0.06 to 0.08 mm/yr since late Pliocene (roughly the last 2.5 Ma) is estimated. The Alqueva fault (AF) is a subparallel, northward dipping, 7.5 km long anastomosing fault zone that affects Palaeozoic basement rocks, and is located 2.5 km north and on the hanging block of the VMF. The AF is also a reverse left-lateral late Variscan structure, which has been reactivated during the Tertiary with reverse right-lateral slip; however, Plio-Quaternary reactivation was normal left-lateral, as shown by abundant kinematical criteria (slickensides) and geomorphic evidence. It shows an average displacement rate of 0.02 mm/yr for the vertical component of movement in the approximately last 2.5 Ma. It is proposed that the normal displacements on the AF result from tangential longitudinal strain on the upthrown block of the VMF above a convex ramp of this main reverse structure. According to this model of faults interaction, the AF is interpreted to work as a bending-moment fault sited above the VMF thrust ramp. Consequently, it is expected that the displacements on the AF increase towards the topographic surface with the increase in the imposed extension, declining downwards until they vanish above or at the VMF ramp. In order to constrain the proposed scheme, numerical modeling was performed, aiming at the reproduction of the present topography across the faults using different geodynamic models and fault geometries and displacements.

Some considerations on the kinematics of chevron folds

Geometrical modelling and field analysis of chevron folds suggest that these structures are a result of the combination of several kinematical mechanisms, whose magnitude and order of application vary within certain limits. A possible mechanism operating early in the fold growth is homogeneous layer shortening, whose contribution is restricted by the high competence contrast that the multilayers developing chevron folds usually exhibit. In the early stages of folding, when the curvature is small, equiareal tangential longitudinal strain (ETLS) is an essential mechanism, since the operation of parallel tangential longitudinal strain (PTLS) or flexural flow (FF) would give rise respectively to area changes or strain in the limbs of the final fold that are too high to be geologically realistic. After folding by ETLS, probable mechanisms are PTLS and FF, which can operate in this order or simultaneously. In general, FF is necessary at the last stage of buckling, although the increment of folding due to this mechanism can be very small. High values of slip between layers and area change produced in the later stages of chevron folding can bring an end to buckling, probably at an interlimb angle value of 60–70°, and induce the onset of homogeneous strain (HS). This strain is not coaxial in many cases, with simple shear playing an important role, and gives rise to asymmetrical folds.