Flexural Slip Mechanism Research Articles

Measuring the research impact of the book Folding and Fracturing of Rocks by John G. Ramsay

Abstract On the 50th anniversary of the publication of the book Folding and Fracturing of Rocks by John G. Ramsay we use an analysis of citations to assess the book's impact on research in the field of structural geology. Five topics dealt with in the book have had special success in terms of the number of citations: (1) superimposed folding; (2) fold classification based on layer thickness variation; (3) simple shear deformation; (4) the R f / φ method; and (5) the flexural-slip mechanism of folding. The great impact of this book can be seen in several examples where the book has inspired new lines of research, such as: progressive deformation, strain analysis, shear zones and folding analysis. Ramsay's great merit in writing his book was to have wisely known to apply the quantitative methodology of continuum mechanics to the analysis of the geological structures. We conclude that Folding and Fracturing of Rocks is the most influential textbook in the field of structural geology research.

Flexural-slip during visco-elastic buckle folding

Flexural-slip is considered as an important mechanism during folding and a general conceptual and qualitative understanding has been provided by various field studies. However, quantitative evidence of the importance of the flexural-slip mechanism during fold evolution is sparse due to the lack of suitable strain markers. In this study, 2D finite element analysis is used to overcome these disadvantages and to simulate flexural-slip during visco-elastic buckle folding. Variations of single and multilayer layer fold configurations are investigated, showing that flexural-slip is most likely to occur in effective single layer buckle folds, where slip occurs between contacts of competent layers. Based on effective single layer buckle folds, the influence of the number of slip surfaces, the degree of mechanical coupling (based on the friction coefficient), and layer thickness, on the resulting slip distribution are investigated. The results are in agreement with the conceptual flexural-slip model and show that slip is initiated sequentially during the deformation history and is maximum along the central slip surface of the fold limb. The cumulative amount of slip increases as the number of slip surfaces is increased. For a lower degree of mechanical coupling increased slip results in different fold shapes, such as box folds, during buckling. In comparison with laboratory experiments, geometrical relationships and field observations, the numerical modeling results show similar slip magnitudes. It is concluded that flexural-slip should represent a significant contribution during buckle folding, affecting the resulting fold shape for increased amounts of slip.

Tectonic evolution of kid metamorphic complex and the recognition of Najd fault system in South East Sinai, Egypt

A low-to medium-grade metamorphic belt of a volcano-sedimentary succession occurs in the eastern side of South Sinai as a part of the northernmost extension of the Arabian–Nubian Shield in Egypt. The belt is known as the Kid metamorphic complex. It is considered as one of the major belt among the other exposed metamorphic belts in South Sinai. Here, we detect and investigate the signature of the Najd Fault system in South Sinai based on detailed structural analysis in field and digital image processing. The enhanced satellite image and the geo-spatial distributions confirm that the Kid belt is essentially composed of nine Precambrian units. Field relations and geometrical analysis of the measured structural data revealed that the study area underwent four successive deformational phases (D1–D4). D1 is an upright tight to isoclinal large-scale folds that caused few F1 small-scale folds and a steeply dipping S1 axial plane foliation. The second deformational event D2 produced dominant of sub-horizontal S2 foliation planes accompanied with recumbent isoclinal folds and NW–SE trending L2 lineations. The main sense during D2 was top-to-the-NW with local reversals to the SE. The third folding generations F3 is recorded as axial plane S3-surfaces and is characterized by open concentric folding that overprinting both F1 and F2 folds and has a flexural-slip mechanism. F3 fold hinges plunge to the west–northwest or east–southeast indicate north–northeast–south–southwest shortening during D3. The fourth deformational event D4 is characterized by NE plunging open concentric folding overprint the pre-existing fold generations and formed under flexural slip mechanism reflecting coaxial deformation and indicating change in the stress regime as a result of the change in shortening from NE–SW to NW–SE. This phase is probably accompanied with the final assembly of east and west Gondwana. The dextral NW–SE shear zone that bounded the southwestern portion of the metamorphic belt is probably related to reactivation of the Najd fault system during Oligo-Miocene in South Sinai.

Kinematics of orocline-parallel faults in the Texas and Coffs Harbour oroclines (eastern Australia) and the role of flexural slip during oroclinal bending

The Texas and Coffs Harbour oroclines are defined by a Z-shaped curvature in the southern New England Orogen (eastern Australia), but the geometry and kinematics of faults around these oroclines, as well as their possible role during oroclinal bending, have hitherto not been understood. Using aeromagnetic and open file seismic data, as well as field observations, the pattern, geometry and kinematics of fault systems, have been investigated. Fault traces with a strike-slip component are oriented parallel to the curved magnetic and structural fabrics of the Texas and Coffs Harbour oroclines. Our observations show evidence for sinistral or sinistral-reverse, dextral (or dextral-reverse) and normal kinematics along NW-striking faults. The dominant kinematics along NNE- and NE-striking faults is dextral or dextral-reverse. The timing of faulting is not well constrained, but the ubiquitous recognition of orocline-parallel faults may suggest that a flexural slip mechanism operated during oroclinal bending in the early–middle Permian (ca 299–265 Ma). Our observations indicate that many of the orocline-parallel faults, with strike-slip separation, were reactivated during the Mesozoic and Cenozoic, as indicated by the recognition of displaced Triassic granitoids, Mesozoic sedimentary rocks and Cenozoic basalts.

Timing and structural evolution in the limb of an orocline: The Pisuerga–Carrión Unit (southern limb of the Cantabrian Orocline, NW Spain)

Abstract Oroclines are the largest scale folds on Earth, and the process of oroclinal formation is a key topic in tectonics. However, most studies of oroclines have focused on the hinge areas, where the changes in strike, and therefore the orocline shape, are most obvious. In this paper, we investigate the deformation mechanisms, the timing, and the structural and tectonic evolution of the Pisuerga–Carrion Unit, situated on the southern limb of the Cantabrian orocline at the NW of the Iberian Peninsula. The Cantabrian Orocline located in the Variscan Belt of Western Europe has been recently defined as a secondary orocline, constraining kinematics and deformation timing. Our study in the Pisuerga–Carrion Unit reveals that an out-of-sequence thrust system developed and reactivated existing structures by a flexural-slip mechanism that was diachronous with respect to oroclinal formation. Joint analysis of unconformity-bounded rock sequences provide a late Moscovian age for oroclinal initiation (ca. 308 Ma), at least locally. Additionally, comparing those joint sets found in different series we quantify a minimum of 40° counterclockwise vertical axis rotation for the Pisuerga–Carrion Unit during the Late Pennsylvanian.

Method and technique of 3-D dynamic structural evolution modelling of fault basin

The important and difficult point in the 3-D basin structural evolution modelling is how to make the modelling conform to the real geological process. In this paper, the rectification of formation boundary deformation is implemented by 3-D flexural slip mechanism and shearing mechanism. In compaction correction, by taking account of the phase boundary and diagenetic effect on porosity, a formwork of the porosity-depth change characteristic is obtained by ‘plane divisional and longitudinal zoning’ method, and then the porosity data of grid cells in different geological times are calculated using compaction correction algorithm, and finally the decompaction correction calculation is finished based on volume-balance technique. Using this modelling method to simulate the structural evolution of Niuzhuang-Wangjiagang region in Dongying Sag, the 3-D dynamic evolution models of geologic body from the upper section of the 4th member of Shahejie formation to present are figured out and they are according with the process of the historical and spatial modality of geological structure development. It demonstrates that this method has certain practicality and can express the process of geological structure development effectively.

Mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at Sheep Mountain, Wyoming

We use finite element methods to investigate the reactivation of fractures (opening and shearing) and the development of bedding-surface slip during the deformation of the asymmetric anticline at Sheep Mountain, Wyoming. A series of numerical simulations were run to show the effect of mechanical stratigraphy, layer thickness, slip on bedding surfaces, and tectonic shortening of the fold on the response of a representative three-layer (ductile, brittle, ductile) two-dimensional system. The model uses large deformation frictional contact mechanics to capture the response of existing fractures and slip along bedding surfaces, and considers both elastic and elastoplastic layer properties. The computational results demonstrate the relationships among overall configuration of the multilayer, slip on bedding surfaces, and the sequence and mode of deformation (opening versus shearing) of bed-perpendicular fractures. We show that fractures located in the hinge are mainly reactivated as joints and that those in the forelimb are predominantly reactivated as thrust faults. A flexural-slip mechanism develops during folding when the layers bounded by frictional bedding surfaces have similar stiffnesses. In contrast, when the difference in the layer stiffnesses is significant (softer outer layers) the deformation is accommodated within the softer units without exceeding the frictional strength of the bedding surfaces. A reduction of the middle layer thickness from 100 m to 10 m has a minor quantitative effect on the slip along the bedding surfaces. We compare the numerical results with fracture data collected at Sheep Mountain Anticline, and discuss the similarities and differences between the field observations and the model results.

Joint Pattern in Precambrian Rocks Around Galudih (India): Implications for Fold Mechanism

In this paper, analysis of joints developed in folded quartzites of Galudih area, Jharkhand State (India) is presented. The joint pattern indicates that NE–SW joints are the dominant set. These have developed perpendicular to the NW–SE trending axial planes and hinges of the folds in quartzites implying that they are cross-joints. Plumose structures are observed on NE–SW joint surfaces that indicate their formation by extension perpendicular to the joint face. It is inferred that this extension in NW-SE direction is related to NE–SW compressional stresses that resulted in folding of the quartzite by flexural-slip mechanism.

Interaction of flexural shear, S– C fabrics, and oblique shear during folding of micaceous quartzite

This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S– C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S– C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane).

Flexural Slip in an Anticlinal Plungeout as a Mechanism for Dike Offsets: Nahal Ardon Valley, Ramon National Geological Park, Israel

This paper investigates the flexural-slip mechanism in a dome-shaped termination of an anticlinal fold. The flexural slip in the area is expressed by regular offsets of dikes at the contact with a sill, and by many small-scale detachment structures in layered strata. The flexural slip structures are localized in two stratigraphic levels, marking two slip horizons. The lower horizon is located at the top of the Triassic sequence, at the contact between the competent Triassic dolostone and the soft Jurassic strata, which are rich in clay. The upper slip horizon is located within the Jurassic strata at the level of a basaltic sill. The interval between the two slip horizons (32 m) is almost undeformed. Kinematic indicators demonstrate that the slip verges toward the center of the dome structure of the Ramon anticline, and coincides with the directions of the dike offsets. The structural observations suggest that beddingplane slip is the result of flexural-slip folding of the Ramon anticline. The directions i...

A palaeomagnetic study of recumbently folded and thermally metamorphosed Torridon Group sediments, Eishort anticline, Skye, Scotland

Two geologically significant palaeomagnetic components have been isolated in samples of Torridon Group sandstones from the Kishorn nappe of NW Scotland. The samples were collected from the Eishort anticline, a large recumbent fold of Caledonian age. The softer component (which is demagnetized by low temperatures and weak magnetic fields), when in situ, displays a steep upward direction typical of reversed Tertiary magnetizations. Both polarities of the harder component (which is demagnetized by high temperatures only) are present and, when dip corrected, display either northwesterly upward or southeasterly downward directions characteristic of Torridonian remanences. It is suggested that the Tertiary component is of chemical origin and the result of alteration of magnetite (titanomagnetite) grains. The geographical distribution of these grains is irregular and as a result the distribution of the Tertiary overprint is irregular. The Torridonian palaeomagnetic components from both limbs of the fold can be restored by rotation about the fold axis to internally consistent directions which lie close to previously published data from the Caledonian foreland. No correction is necessary for finite strain which suggests that the Eishort anticline formed by a flexural slip mechanism. However, an anticlockwise rotation of 26° is required to reconcile the Torridonian directions from the Kishorn nappe with those of the Caledonian foreland which suggests that, during emplacement, the thrust sheet rotated on the underlying fault surface.

The mid-Paleozoic deformation in the Hazen fold belt, Ellesmere Island, Arctic Canada

The mid-Paleozoic deformation of lower Paleozoic subgreenschist-facies sediments of the Hazen fold belt in northern Ellesmere Island is represented predominantly by chevron-style folding. Folded multilayers display cleavage fans suggesting synchronous fold and cleavage formation. Bedding-parallel slip indicates a flexural slip mechanism of folding. The geometry of several large-scale anticlinoria has been interpreted as being due to formation of these structures over detachments and thrust ramps.The constant fold geometry, the parallel orientation of faults and large- and small-scale folds, and the axial-plane foliation are related to a single phase of folding with a migrating deformation front in the Hazen fold belt during the mid-Paleozoic orogeny. The minimum amount of shortening in the Hazen and Central Ellesmere fold belts has been estimated from surface geology to increase from 40–50% of the original bed length in the external southeastern part to 50–60% in the more internal northwestern part of the belts.The convergent, thin-skinned nature of the Hazen and Central Ellesmere fold belts indicates that the postulated transpressive plate motions during the accretion of Pearya did not affect the study area.

The flexural-slip mechanism

Detailed field studies of turbiditic sequences from South Georgia (South Atlantic), North Devon (England) and Cardigan Bay (Wales) show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts. Stair-stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding-parallel quartz veins. These veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers; they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface. Complex slickenfibre patterns, and departures from ‘ideal’ behaviour, in which slip occurs orthogonal to the fold hinge, probably result from changes in slip vector on the limbs of growing non-cylindrical folds. Movement horizons show many of the features associated with large-scale thrusting, such as ramps, duplexes and imbricate structures, and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically-restricted packages changes across fold hinges. Chevron folds are thought to develop mainly by flexural flow in the early stages, with flexural slip becoming dominant later as the beds become lithified; new slip surfaces are generated as the dip of the fold limbs increases.

Deformation of chlorite in naturally deformed low-grade rocks

The intracrystalline deformation of chlorite in naturally deformed low-grade rocks was investigated with transmission electron microscopy (TEM). As in other phyllosilicates, the deformation of chlorite is dominated by the (001) slip plane. Slip along this plane is very easy through the generation and movement of (partial) dislocations with Burgers vector b = 1/ n[ uv0]. After initial bending of the (001) planes through a flexural slip mechanism, accommodated by slip along (001), the misorientation is concentrated in tiltwalls subnormal to (001), which are formed by (partial) dislocations with b = 1/ n[ uv0]. Tiltwalls subparallel to (001) are formed by arrays of dislocations with a total Burgers vector [001]. These boundaries are either initiated at the grain boundary and move inwards by dislocation climb, or form by movement of free dislocations towards the tip of propagating boundaries by glide along planes inclined to (001); once a boundary is formed, the increasing misfit is accommodated by climb of these dislocations along (001). Due to the combination of very easy slip along (001) and the ability to form subgrain boundaries subparallel to (001) by dislocation climb and activation of hard slip systems, chlorite deforms plastically even at temperatures below 300°C.

Characteristics and Alternative Mechanisms of Late Quaternary Faulting, Proposed LNG Terminal Site, Point Conception, California

Late Quaternary faults at the proposed Point Conception LNG site consist of westerly-trending reverse faults that are parallel to bedding in the Pliocene bedrock beneath the terrace deposits, and southwesterly-trending cross-faults that offset displacements along the bedding-plane faults. These faults exhibit a number of unusual characteristics. The bedding-plane faults appear to be spaced at near-regular intervals of approximately 60–120 m, and the areas between the faults are relatively undisturbed. The zone of disruption along each fault is normally only a few centimeters, but a single-event displacement of 1.4 m can be demonstrated for the Arroyo fault and a 2.3 m displacement is suggested for the SF-1 fault. Such large displacements are normally associated with major fault zones and major earthquakes. Also, each fault appears to have experienced but one or two events, and “new faulting” along any of the numerous zones of weakness in the bedrock section beneath the site appears as likely as recurrent movement on a previously established fault. Flexural slip and backthrust models have been proposed as mechanisms for site faults. While the faults exhibit some characteristics expected from the flexural slip model, data on shoreline-angle elevations on both flanks of the Government Point syncline clearly prohibit flexure of this structure during the period of fault movements. In the absence of flexure, a flexural slip mechanism is untenable. A simple backthrust model does not explain the spacing of site faults, but an antithetic model taken from ramp regions of overthrusts could explain this characteristic. Whether such a model is appropriate is unknown, as the applicant9s consultants obtained no data that might be utilized in considering mechanisms other than flexural slip.

Stewart Peak Culmination, Idaho-Wyoming Thrust Belt, as Compared with Other Fold-and-Thrust Belt Culminations: ABSTRACT

The Stewart Peak culmination, located in the northern Salt River Range of the Idaho-Wyoming thrust belt, is an anticlinorium in the hanging wall of the Absaroka thrust fault. The culmination is topographically and structurally higher than areas to the north or south and consists of the oldest rocks exposed in the thrust belt. Rocks from the lower part of the Absaroka thrust sheet, ranging in age from Middle Cambrian to Mississippian, are stacked by an anastomosing network of imbricate thrust faults. Fold geometries include kink, chevron, and open concentric forms which deformed by a flexural-slip mechanism. Structural culminations are an important and predictable component of most fold-and-thrust belts. Down-plunge projections from culminations into adjacent depressions are often the key to unraveling complex structural relations. Several factors may contribute to the development of a culmination. Surface geologic mapping, integrated with down-plunge projections and geophysical data, indicate that the Stewart Peak culmination is the result of polyphase uplift and arching of the Absaroka thrust sheet by motion on younger and structurally lower thrust faults, End_Page 736------------------------------ namely the Murphy and Firetrail thrusts. These younger faults are interpreted to sole into the overlying Absaroka thrust, forming a subsurface duplex zone which may have considerable oil and gas potential. In addition, magnetic data suggest that the Stewart Peak culmination may be positioned over the northwest continuation of the Moxa arch, an anticlinal flexure of authochthonous basement which formed contemporaneously with thin-skinned thrusting. The Stewart Peak culmination is compared to structural culminations in the Canadian Rockies and the southern Appalachian orogene of Virginia and Tennessee. Culminations are prospective areas for oil and gas, and traps may be formed in a variety of geometric configurations. End_of_Article - Last_Page 737------------

Kink Band Structures in the Valley and Ridge Province, Central Pennsylvania

The dominant structures of the Valley and Ridge province in central Pennsylvania are traditionally thought to be open, concentric folds of great length and parallel to one another. However, the majority of the folds possess planar limbs and narrow hinges and therefore do not conform to the concentric fold geometry. Furthermore, many folds change radically in profile along trend, and even vanish. Parallel-sided zones of rotated bedding, as much as 50 feet in width, have been observed transecting bedding at a large angle. The zone boundaries are defined by the abrupt flexure of the bedding, which is continuous across the structures and of constant thickness. Slickensides on bedding surfaces indicate that the flexural slip mechanism was operative in the development of these structures. Because of this geometry and because of the inferred deformational mechanism, these structures are interpreted as large kink bands. Structural models which utilize the kink band concept accurately represent the fold geometry both in the cross sections and the changes along trend. Because folds ranging in size from a few feet to the largest in the province can be represented by such models, it is suggested that kink banding was an important deformational process in the development of the fold geometry.