Incompetent Layers Research Articles

The northwestern termination of the Himalayan Mountain Front: Active tectonics from microearthquakes

The seismicity of the northwestern Himalayan syntaxial bend and the geologically complex area to the west, between the Hazara thrust system (HTS) and the higher mountains of Indus‐Kohistan, is examined in a wider tectonic context by using data from about 1,800 microearthquakes. The microearthquake data were obtained from a telemetered seismic network in northern Pakistan centered at Tarbela dam on the Indus River and were collected during an 11‐month period prior to impounding of the Tarbela reservoir. The observed seismicity indicates that a branch of the main boundary thrust (MBT) traverses the region as a straight northwesterly extension of the Murree thrust, the mapped section of the MBT southeast of the syntaxial bend along the Kashmir Himalayas. Seismic release on this extension of the MBT, here named the Indus‐Kohistan seismic zone (IKSZ), is highest in the upper 25 km of the crust and correlates with a pronounced topographic step. Deeper activity on the IKSZ extends to a depth of 70 km. Seismicity in the lower crust defines a second lineation 100 km southwest of the IKSZ and parallel to it. The syntaxial bend and the eastern HTS, microseismically virtually inactive at present but associated with recent and historical microseismicity, overprint the two northwesterly seismic lineaments. The two tectonic regimes may be simultaneously active at different depths separated by an incompetent layer. A set of steeply dipping faults, either parallel or perpendicular to the IKSZ, is active in the region between the HTS and the IKSZ. Seventeen composite fault plane solutions show a predominant pattern of either reverse or strike slip faulting with the inferred slip vectors oriented such that north‐south compressional stresses are relieved. Such a stress field is compatible with the north‐south convergence between the Indian and Eurasian plates inferred from plate tectonics.

Discussion

The President (Sir Peter Kent) commented that a full scale case of the effect of varying thickness of an incompetent layer was thought to be provided by the Zagros orogenic belt. In the southeast, where abundant salt extrusions over a long period indicated a thick mobile layer at depth, folds were short and irregular. In the middle stretches (the ‘Fars Platform’) the characteristic feature was of very long relatively simple cylindrical folds. The latter were clearly related to abscherung, but the rarity of salt extrusions and their pattern was compatible with a halite layer of limited thickness, sufficient to facilitate simple folding without complex flowage. Dr. R. Cave remarked that the laboratory models of gravity tectonics described closely simulate the concept in Davies & Cave (1967) that much of the folding and related structure in the Lower Palaeozoic rocks of mid-Wales may be related to sliding under gravity. In a recent survey of part of mid-Wales by the Institute of Geological Sciences it was concluded that the pile of Lower Palaeozoic sediment was still wet and largely non-lithified when deformation commenced. This deformation was not confined to an early Devonian orogenic spasm and the concept relates it to mass movement of sediment down a westward declivity, from the shelf margin in east Wales. Structures in the extreme west, for instance near Aberystwyth in the Aberystwyth Grits and in underlying formations further north, are thought to have resulted from collapse of a pile of sediment which had accumulated on, or at

Fold development in the Jura

The term “folding” encompasses a wide range of processes, most of which are poorly understood. Jura folds, though comparatively simple, have developed by a superposition of different types of instabilities both in space and time. They are never periodic and sinusoidal and are more realistically approximated by kink bands with rounded hinges. Thrusting and kinking instabilities had closely similar thresholds, with kinks usually following and deforming thrusts. An analysis of embryonic folds shows that instabilities in the sedimentary cover were initiated primarily at inherited flaws of the basal décollement layer. They thence spread upward, often following stratigraphically higher incompetent layers in secondary décollement and there nucleating secondary instabilities before reaching the surface (disharmonic folding). Embryonic folds thus are usually narrow, emanating from secondary décollement layers that are connected with the basal décollement zone by thrusts nucleated at inherited obstacles. These are eventually overcome, permitting basal décollement to coalesce with kinking instabilities that grow downward from nuclei in higher décollement intervals. In this way folds centered in the basal décollement layer, and consequently of normal width, may be superposed on the narrow embryonic folds. The sequence and importance of the different elements may vary from place to place to result in a vast catalog of fold shapes.

Stress-induced diffusion during folding

The application of thermodynamics to folding shows that mechanical disequilibria also correspond to chemical disequilibria and that they can be adjusted by the diffusion of elements down the free-energy gradient. Mechanical disequilibria in the form of heterogeneous stress distribution for the two-dimensional, large-amplitude folding of competent layers in a less competent matrix have been determined by a series of finite-element analyses. Folds with high viscosity contrast ( 100 1 ) have a gradient of the mean stress (pressure) in the competent layer which is directed perpendicularly to the layer. The incompetent layers in a folded multi-layer sequence show a mean stress gradient directed parallel with the layer, and the pressure minima are located at the hinge zone. Folds with low viscosity contrast have the isopachs (lines of constant mean stress) oriented almost parallel to the direction of compression and a minimum mean stress on the outer arc. The variations of mean stress in experimentally produced folds are in qualitative agreement with the variation in: 1. (1) the mineral composition; 2. (2) the bulk chemical composition; 3. (3) the chemical composition of a mineral; and 4. (4) the grain-size variation of natural folds.

Strain distribution in superposed buckling folds and the problem of reorientation of early lineations

Two series of experiments were carried out with soft model-materials in order to assess the relative importance of initial homogeneous strain, external rotation and late-stage strain in reorienting early lineations during superposed buckle-folding. In the first series cylindrical buckling folds were produced in embedded planar sheets containing a “lineation”. In the second series noncylindrical folds were produced by compression of a set of cylindrical folds. The experiments indicate that the ratio of buckle shortening to layer-parallel strain is much smaller when the principal extension is parallel to the fold-axis than in the case when the principal extension is perpendicular to the fold-axis. In very competent rocks, the reorientation of old lineations is mainly by external rotation and by the associated concentric longitudinal strain. In moderately competent rocks, the orientation of early lineations always changes by initial homogeneous strain before buckling becomes significant. Because of the unlike amounts of initial strain in layers of different competences, orientations of unrolled lineations may not be parallel in disharmonically folded layers of unlike competences. Under certain conditions the early lineation may become virtually parallel to the later fold-axis. The experiments indicate that the effects of late-stage strain in buckle-folding are largely restricted to the incompetent layers of a multilayer. Hence, if orientation data of early lineations in both competent and incompetent rocks are lumped together, the pattern of orientation may become quite complex. Even for a single competent layer, the pattern of early lineations can locally become complex because of the complex nature of concentric longitudinal strain (and strain resulting from stretching of middle surface of the layer), development of conical folds, development of shear strain along hinge zones of deformed early folds and also because of the development of different orders of folds in both the first and the second deformations.

Development of Chevron Folds

Multilayered rock complexes with regular alternations of competent and incompetent layers of thickness t 1 and t 2, respectively, and with high ductility contrast form folds of the chevron style when subjected to compression along the layering. The geometric forms of progressively developing chevron folds are analyzed using a model whose properties are based on the geometric forms of naturally deformed rock layers. It is found that the chevron fold style is only stable where no strongly marked variations in competent layer thickness t 1 exists. The thickness of the incompetent layer exerts no influence on fold model stability. Slight variations of competent layer thickness can be accommodated by local modifications of the fold style, such as limb faults, bulbous hinge zones, or layer boudinage, but if any strongly marked variation exists, the fold limbs become curved. The chevron model involves dilation at the hinge zones, and saddle reef formation, incompetent layer flow into the hinge, or slow hinge collapse generally results from actual or potential dilation. The speed of development of chevron folds is calculated under conditions of constant stress and of constant load. Folding starts slowly but rapidly accelerates; the later stages are characterized by a progressive slowing in shortening rate and fold growth, leading either to a stage of locking up of the fold or to modification of its geometry by limb thinning and hinge thickening; toward a more similar geometric style. The strains in the hinge zone region are related to the rates of shear taking place in the fold limbs; the geometric model is not completely stable throughout the fold development, and complex progressive strain increments can occur in the hinge zones and lead to the development of superposed small-scale structures indicating reversals of principal axes of incremental strain.

A comparison of natural and experimental strain patterns around fold hinge zones

Patterns of finite strain around fold hinges in incompetent layers in buckled interlayered materials have been investigated in a series of experimental tests conducted with a view to comparing the results with a recently described anomalous slaty cleavage pattern from northern Norway. As well as compressing rubber, putty and plasticene models, a photoelastic strain analysis of buckled rubber strips in gelatine has been carried out. Strain patterns from the various tests are strikingly similar to the natural cleavage patterns, and a discussion of the factors favouring their development is presented.

Influence of Sampling on Geologic Interpretation Based on Piston Coring in Gulf of Mexico: ABSTRACT

Examination of samples taken on the northwest slope of the Gulf of Mexico with a newly developed flexible liner coring device indicates a possible source End_Page 1830------------------------------ of error in the interpretation of recent basin deposition. Visual and radiographical comparisons of samples from a piston corer with samples from a gravity corer used at the same stations confirms the existence of highly incompetent layers previously only suspected. The detection of these layers in a more consolidated sedimentary column, as well as undetected shortening of samples obtained by piston and gravity corers, is important if the sedimentary history and the engineering properties are to be examined. End_of_Article - Last_Page 1831------------

STRAIN AND MOVEMENT PICTURES IN COMPETENT LAYERS IN FLEXURAL FOLDING:Deformation of Heterogeneously Layered Rocks in Flexural Folding (II)

STRAIN AND MOVEMENT PICTURES IN COMPETENT LAYERS IN FLEXURAL FOLDING:Deformation of Heterogeneously Layered Rocks in Flexural Folding (II)

FURTHER DEVELOPMENT OF THE THEORY OF INTERNAL BUCKLING OF MULTILAYERS

The approximate theory of internal buckling of multilayers is obtained by a direct method and generalized to nonsinusoidal deformations. The numerical discussion includes the case of unequal thickness of competent and incompetent layers. Simple expressions are derived for the dominant wave length for rigid confinement or self-confinement showing the influence of interstitial flow. The theory is presented in the context of viscous media using both solid and fluid mechanics and is valid for large compressive strain and moderate slopes. Its applicability to elastic and viscoelastic media is indicated.

THEORY OF SIMILAR FOLDING OF THE FIRST AND SECOND KIND

Research Article| February 01, 1965 THEORY OF SIMILAR FOLDING OF THE FIRST AND SECOND KIND M. A BIOT M. A BIOT SHELL DEVELOPMENT COMPANY, HOUSTON, TEXAS Search for other works by this author on: GSW Google Scholar Author and Article Information M. A BIOT SHELL DEVELOPMENT COMPANY, HOUSTON, TEXAS Publisher: Geological Society of America Received: 02 Jun 1964 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1965, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1965) 76 (2): 251–258. https://doi.org/10.1130/0016-7606(1965)76[251:TOSFOT]2.0.CO;2 Article history Received: 02 Jun 1964 First Online: 02 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 M. A BIOT; THEORY OF SIMILAR FOLDING OF THE FIRST AND SECOND KIND. GSA Bulletin 1965;; 76 (2): 251–258. doi: https://doi.org/10.1130/0016-7606(1965)76[251:TOSFOT]2.0.CO;2 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 SocietyGSA Bulletin Search Advanced Search Abstract It is shown that in the initial phase there are two basic types of similar folding of multi-layered structures, referred to here as folding of the first and second kind, depending on the prevalence of over-all flexure or shear. A simple theory brings to light the underlying mechanics and the controlling parameters which govern these two types of folding and the transition from one to the other. The dominant wave length obtained in folding of the second kind is the same as derived previously (Biot, 1961) assuming the incompetent layers to act as lubricants. In folding of the first kind the system behaves approximately as a single layer (Biot, 1957; 1961). The same theory solves at the same time the problem of folding of a single anisotropic layer. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Salt Plugs of Southern Iran: ABSTRACT

The distribution of the south Persian salt plugs has encouraged observers in the past to connect these plugs with diagrammatic lines, suggesting deep-seated linear structural weaknesses. Little has been said about the associated anticlines, particularly as to whether or not they reflect the presence of an extensive, deep-seated salt body. It is suggested in this paper, that the nature of the prominent 'whale-back' anticlines in the salt-plug region of the southern part of the province of Fars reflects the existence of an incompetent layer of salt at depth. Anomalous swings of strike and possible rim synclines give a characteristic pattern to the salt-plug area. This pattern contrasts strongly with an area in N.W. Iran (Lurestan) which, although having similar 'whale-back' anticlines, lacks anomalies of strike, and shows no surface outcrops of salt. End_of_Article - Last_Page 356------------

Fluid Dynamics of Viscous Buckling Applicable to Folding of Layered Rocks

For slow creep under relatively small differential stress rocks may in a first approximation be treated as very viscous fluids. Under such conditions it is therefore possible to apply the theory of fluid dynamics to the evolution of strain in complexes consisting of coherent assemblages of rock bodies with unlike shape and unlike effective viscosities. In this paper fluid dynamics is applied to models simulating layered rocks exposed to compression parallel with layering. As basis for discussion and experimental tests presented in the paper the fluid dynamic theory of the following models is briefly reviewed. (1) Single layer enclosed in uniform surroundings whose dimension normal to the layer is large relative to the characteristic wavelength. (2) Multilayers consisting of alternating competent and incompetent layers; (a) unrestricted by surrounding media, and (b) restricted by surrounding media. In all cases the contacts between adjacent layers are considered welded to prevent free slip. Layer-parallel shear is consequently included in the models. This unfortunately makes the mathematics somewhat cumbersome but also makes the models more realistic representatives of rock structures than free-slip models. The welded-contact model of an embedded single layer gives a somewhat smaller wavelength/thickness ratio than the free-slip model of Biot and others. For multilayers (unrestricted, case b) consisting of many alternating competent and incompetent layers, the welded-contact model gives results very different from the free-slip model. If the spacing between the competent layers, i.e., the thickness of the incompetent layers, is greater than a certain limit, the multilayer will buckle in many folds, whereas at smaller spacing the multilayer will buckle to 1 or ½ wave. Many of the theoretical predictions of the behavior of viscous models apply to elastic models if viscosity coefficients are replaced by rigidity modula in the equations. All important predictions of the theory have been tested by rubber models. Excellent agreement between theory and experiment has been established. The multilayers which buckle to a single wave if unrestricted (by adjacent bodies) will buckle to many waves with a characteristic wavelength if surrounded by uniform viscous materials. Equations for enclosed multilayers of this kind are also developed. A theory is developed for viscous buckling of a thin competent layer embedded in an incompetent layer of limited thickness which in turn is sandwiched between thick competent layers. The latter layers remain straight during compression whereas the middle layer buckles. The wavelength of the buckles is shown to be a function of the spacing between the buckling layer and the straight walls. The case of a multilayer taking the place of the thin central layer in the foregoing case is discussed. The approach throughout the work has been to develop the stream function ^psgr and the derivative velocity components U and V which satisfy the particular boundary conditions of the models. When the correct stream function is found for a particular situation, the situation is completely known from a fluid dynamic point of view because the velocities, the rate of change of strain, the stress variations and the variations of pressure throughout the system all follow by way of the general fluid dynamic equations. The models have been chosen with application to common rock structures in mind.

The Deformation of Confined, Incompetent Layers in Folding

AbstractShearing stresses acting on an incompetent layer confined between competent beds during its deformation in the development of folds, differ from those commonly accepted as the cause of a number of minor structures associated with the deformation. It is suggested that the effect of bedding drag is negligible, and that the structures described as slaty cleavage, fracture cleavage, and drag folds can be explained by the laminar flow of the incompetent material whilst accommodating itself to the interspaces determined by the more competent layers.

Structure of the Westport Concordant Pluton in the Grenville, Ontario

Structural analysis in the Grenville of southeastern Ontario shows at least two successive directions of movement during folding. The first movement (northwest-southeast) gave rise to folds about northeast-trending, subhorizontal axes. The second movement (northeast-southwest) caused rotation of these preexisting fold-axes about vertical axes, expressed by buckling of competent bands along the strike, and, in certain places, flowage of incompetent layers into vertically plunging cylindrical folds, or vortices. These formed natural vertical channels for the uprise of magmas and/or granitic emanations and, since they were also dilatant areas, are the sites of concordant batholiths. The Westport pluton is one of these, gabbro and, later, monzonite being emplaced within the vortex.

Pyritic mineralization in the Goudreau area of Algoma, Ontario

The pyritic ores of this area are believed to be of hydrothermal origin. They represent the mineralization by hydrothermal solutions of the sheared and brecciated incompetent layer lying between the competent acid volcanic footwall and the basic volcanic hangingwall. Evidence for these relationships is to be found in the origin of the ottrelite schist of the footwall and in the shearing and veining of the hangingwall. These events preceded the emplacement of the Algoma granite from which the mineralizing solutions are believed to come.

Construction of Geological Sections of Folds with Steep-Limb Attenuation

A simplified form of the Method of Tangents (Coates, 1945) is considered to be the most direct and the most flexible method of deriving geometrical control from the observed dips in the construction of accurate geological sections. The method is first described as applied to equal-thickness projections of folds in competent beds. The causes of steep-limb attenuation in folds of orogenic belts are then analyzed. Without regard to the complexities introduced by plastic flow of mobile beds, it is considered that the major factor producing attenuation in a layered sequence of competent and incompetent beds is the varying compaction of the incompetent members. The compaction is greatest where the beds are vertical, where the resistance of the competent members offers no protection to the incompetent layers. A force resolution shows that the compaction for any attitude of dip (k1) can be calculated from the formula, k1 = K (1 - Cos dip), where K is the compaction in vertical beds expressed as proportion of original stratigraphical thickness. The fold profile does not become identical in depth u til K = 100 per cent, but it is apparent that identical folding can be produced only by plastic flow. There is empirical evidence to support the deduction that compaction has a maximum value of about 50 per cent. The writer proceeds to demonstrate that the compaction factor as computed from the versed cosine formula can be allowed for in section construction by moving the projected ray, which on equal-thickness projections bisects the angle of tangent intersection, progressively toward the steeper dip. The position of such compensated boundary is determined by thinning ratios computed from the versed cosine formula. To eliminate elaborate graphical constructions, the positions of these rays have been calculated for all possible adjacent dips in 5° units, and for values of K ranging from K = 10 per cent, at 5 per cent intervals to K = 50 per cent.

Relict fracture cleavage in paraschists

Research Article| October 01, 1939 Relict fracture cleavage in paraschists J. T. Stark; J. T. Stark Search for other works by this author on: GSW Google Scholar J. H. Maxson J. H. Maxson Search for other works by this author on: GSW Google Scholar Author and Article Information J. T. Stark J. H. Maxson Publisher: Geological Society of America Received: 13 Jan 1939 First Online: 02 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 © 1939 Geological Society of America GSA Bulletin (1939) 50 (10): 1529–1534. https://doi.org/10.1130/GSAB-50-1529 Article history Received: 13 Jan 1939 First Online: 02 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 J. T. Stark, J. H. Maxson; Relict fracture cleavage in paraschists. GSA Bulletin 1939;; 50 (10): 1529–1534. doi: https://doi.org/10.1130/GSAB-50-1529 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 SocietyGSA Bulletin Search Advanced Search Abstract To the commonly known criteria suggesting that metamorphosed rocks were originally sedimentary may be added relict fracture cleavage, when confined to certain thin beds. Thin beds in the Archean Vishnu series of the Grand Canyon possess cross structures at high angles to their bounding surfaces. These transverse planes presumably were developed in incompetent layers during crustal deformation. Later they became the loci of biotite crystallization and thus were preserved. Close folding of parts of the Vishnu series on Vishnu Creek indicated by reversals in attitude of fracture cleavages is corroborated by the testimony of drag folds. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.