Folding Of Rocks Research Articles

Timing of Slaty Cleavage During Folding of Precambrian Rocks, Northwest Tasmania

Detailed analysis of folds and cleavage in five small areas in the Precambrian Burnie Quartzite and Slate near Sulphur Creek, in northwest Tasmania, reveals that slary cleavage can be interpreted as an essentially planar structure initiated in a short interval during protracted folding. Cleavage initiated late during folding appears to be largely superimposed on fold profiles, and where rotation of early folds occurs during folding, cleavage may cut across the profile from one limb to the other. Cleavage initiated partway through folding is refracted in sandy beds forming cleavage fans. The largest cleavage-fan angles develop in folds where cleavage is initiated early in folding. Geometrical relations show that, for the area as a whole, either the onset of folding or the initiation of slaty cleavage was diachronous.

Structure of Miocene Rocks in Sierra Madre, Northeastern Santa Barbara County, California: ABSTRACT

Folds in Miocene rocks of the central Sierra Madre commonly are northwest-trending, curvilinear, concentric, and symmetrical; asymmetry and overturning occur in the vicinity of reverse faults. Fold hinges undulate across the area and in places bifurcate or merge. Disharmonic folds and sandstone dikes occur locally in the Monterey Shale. Faults are of three types: (1) along the northeastern edge of the range is a distributive fault zone consisting of parallel, longitudinal, reverse-slip faults which join both westward and downward and which have a subsurface distributive component that is now obscured by more recent deposits on the northeast; (2) southwest of the reverse-slip faults is a group of parallel, longitudinal, normal-separation faults; and (3) the remaining faults are mostly small, diagonal faults of different types; many are hinged at one or both ends and have slips or separations that indicate contemporaneous folding and faulting. Folding began during late Miocene or Pliocene while Miocene sediment was still unconsolidated and water laden. Diagonal faulting, intrusion of sandstone dikes, and possibly disharmonic folding accompanied early deformation. Asymmetrical and overturned folds formed later and large reverse-slip faults developed along their hinges. Normal-separation faults possibly formed last as lag faults. Northeast-southwest shortening of the area averages about 22%. End_of_Article - Last_Page 433------------

Sense of Displacement along Foothills Fault System: New Evidence from the Melones Fault Zone, Western Sierra Nevada, California

Sense of displacement along the Foothills fault system, western Sierra Nevada, California, has been interpreted in terms of reverse, thrust, and strike-slip faulting. Within the Melones fault zone (part of the Foothills fault system) near Downieville, synoptic fault-plane intersections, in cataclastically deformed metasedimentary rocks and serpentinite, plunge steeply northwest, as do axes of minor flexure folds in metasedimentary rocks. Fold axes in serpentinite plunge steeply to the northeast and southeast; slickenside orientations are varied, but plunge chiefly southward at steep to moderate angles. Cleavage in slate on either side of the fault zone strikes approximately north and dips steeply eastward. These relationships imply a complicated chronology of events characterized by possible early emplacement of serpentinite and a late phase of strike-slip displacement. I conclude that present fault dips near Downieville probably approximate initial dips, that the pattern of faulting suggests a conjugate relationship best explained in terms of strike-slip displacement, that the latter interpretation is supported by attitudes of minor-fold axes, but not by those of slickensides, and that such an interpretation does not preclude earlier oblique-slip or dip-slip displacements.

Basic magma tectonics contrasted to folding of basic rocks

In the Precambrian gneisses of SE Norway, several generations of amphibolitised basic dykes/sills occur. While prekinematically emplaced dykes and sills show “normal” (but complex) fold structures, other metabasic intrusions have imposed a deformation on to the host rock. Apparent fold structures which have developed due to fabric control exerted by the host rocks, are recorded from late kinematic intrusions. Attention is drawn to the problem of distinguishing primary intrusive structures from simple and multiple fold-structures.

Folding of laterally compressed multilayers in the field of gravity, II numerical examples

Based on the mathematical theory of folding of stratified rocks and other layered global structures as developed in part 1 of this work, numerical calculations are presented for a number of systems exposed to lateral compression. In some of these systems the lateral compressive force is the only force operating, in others gravity is also acting. The models treated vary in complexity from a single layer embedded between infinite half spaces to structures of up to ten unlike strata which are welded together, have a free upper surface and are supported by a thick viscous substratum in the field of gravity. The purpose of the study is threefold: 1. (1)to determine the characteristic (or dominant) wavelength(s) of systems with selected thickness ratios, viscosity ratios and density-contrast ratios among the unlike layers; 2. 2)to determine the relative amplitudes of the folds on the different interfaces in multilayered systems; 3. 3)to determine the relation between shortening by buckling ( buckle-shortening) on the one hand, and shortening by homogeneous strain ( layer-shortening) on the other. The results of the 16 unlike models treated numerically are expressed in dimensionless quantities thus making the models applicable equally well to small- and medium-scale folding of stratified rocks as to deformations of continental dimension of global shells such as the various layers in the Earth's crust and upper mantle.

The finite-amplitude instability in the folding of layered rocks

Examination and comparison of several computational studies of finite-amplitude single-layer folding bring to light an important unifying concept, that of the finite-amplitude instability. If the fold-causing stresses are constant throughout the development of a fold, the overall shortening strain-rate due to the folding will first show a marked increase with increasing fold amplitude and then decrease as the fold becomes tightly appressed; on the other hand, if the shortening strain-rate is held constant, the stresses necessary to maintain the constant shortening rate will decrease and then increase again. Considering the layer-in-medium system as a whole, this instability results in a bulk-effective viscosity with respect to layer-parallel compression, which is first high, then decreases to a minimum, and then rises again, These changes in bulk-effective viscosity are large, nearly two orders of magnitude.Effects of the finite-amplitude instability on the internal stresses within the layer and surrounding medium are as follows. First, there will probably be an early, low-amplitude stage with high layer-parallel stresses within the competent layer. The major development of deformation fabrics may occur during (his early stage. Second, development of plastic yield in the crestal region of the fold is strongly controlled by the details of external load pattern and therefore by the mechanism causing the folding. Third, the increase in effective bulk viscosity of the layer-in-medium system corresponds to a major rearrangement of the deformation patterns in both the medium and the layer.Consideration of décollement folding and multi-layer folding suggests that the concept of finite-amplitude instability may be applicable, at least qualitatively, to these more common fold types. If this is true, then the concept is important in considering more general geologic questions such as the dating of an orogenic episode by angular unconformity.

The structure and stratigraphy of the Dalradian rocks of north Achill Island, County Mayo, Ireland

Achill Island is underlain by a mixed suite of low-grade metamorphic rocks. The lithological units of the northern part, the subject of this paper, are systematically described. The deformation history can be divided into five separate phases. The first phase, D 1, produced isoclinal F 1 folds and S 1 flattening schistosity in mica schists; and an L -tectonite fabric associated with some tight F 1 folds in psammitic rocks. The second phase, D 2, produced tight to isoclinal reclined F 2 folds and S 2 strain-slip schistosity in mica schists; and S 2 flattening schistosity or an L–S fabric in psammitic rocks. This was followed by the third phase, D 3, which produced tight to close upright folds and S 3 strain-slip schistosity. The folds formed during these phases plunge gently between north-east and south-east. This variation in the trend of the plunge is the result of open F 4 refolding. Finally, the latest deformation, D 5, resulted in the development of kink bands. Three tectonic slides, of D 1 age, are recognized in the area, one of which separates rocks of two separate stratigraphical successions. This slide can be mapped around the major southward-facing F 2, folds of the area. One large dextral strike-slip fault is present. The two stratigraphical successions are compared with the Scottish Dalradian; one with the Islay Succession, and the other with the Moine Series. In North Achill large-scale F 2 folding places the equivalents of the Moine Series structurally above the Islay succession in the core of an F 2. anticline.

Synorogenic Quartz Sandstone in the Jurassic Mobile Belt of Western Nevada: Boyer Ranch Formation

The province of Lower Mesozoic layered rocks in western Nevada contains a diversity of marine lithologies whose deposition continued locally as late as Middle Jurassic time. At places in the province, mature quartz sandstone constitutes all or part of the highest stratigraphic units in sections of non-volcanic rocks and is believed to record the last deposition of terrigenous sediments before complete effacement of the marine basin by widespread orogeny. The sandstone was deposited early in the erogenic episode and at least locally in troughs created by folding of subjacent rocks. The sands are anomalously mature with respect to co-deposited clastic components and to coarse detrital materials in nearly all earlier Mesozoic rocks of the province. The problems are the source of the quartz sand and the reasons the sand was deposited synorogenically. The name, Boyer Ranch Formation, is formally proposed for a lithesome of homogeneous Jurassic quartz sandstone and basal conglomerate and limestone in the Dixie Valley region which is approximately in the northern third of the outcrop area of Jurassic quartz sandstone in western Nevada. The Boyer Ranch Formation contains up to 500 ft of sandstone, largely fine-grained calcareous quartz arenite, whose granulometric properties suggest eolian sorting, but whose bedding indicates quiet-water deposition. The sandstone lies above limestone and carbonate-pebble conglomerate with interstitial quartz sand. The inferred early Mesozoic geographic and tectonic histories in the Dixie Valley region suggest that the sands of the Boyer Ranch Formation accumulated at the eastern shoreline of the Mesozoic basin in western Nevada in late Early or Middle Jurassic time. Until the onset of orogeny, the sands remained unlithified, probably owing to eolian saltation, and followed a generally westerly regression of the shoreline. Postulated strong wave action prevented seaward movement of the sand. The late Early Jurassic or Middle Jurassic (or both) orogeny created local troughs which the sea reinvaded and provided an irregularly configured and low-energy shoreline environment such that movement of the sand into the water was no longer impeded. The sands may have evolved locally through the action of water and wind at the beach of the Early Mesozoic sea in western Nevada, or they may have been largely co-derived from a distant source with sands in Jurassic rocks of the eastern Cordillera and the Colorado Plateau.

The Origin and Deformation of Some "Molar-Tooth" Structures in the Precambrian Belt-Purcell Supergroup

The most characteristic type of "molar-tooth" structure in the Belt-Purcell succession of western North America is briefly described and illustrated. It is interpreted as a system of originally linear, vertical calcite sheets or ribbons, locally united by horizontal members to form an open framework and attributed to some form of algal or allied organic activity. Subsequent to their formation in water-rich silts and silty clays at depths of less than 1.3 m. below the sedimentary interface, compaction crinkled the vertical sheets and, in some cases, produced structures resembling the ptygmatic veins in metamorphic rocks. The geometry and mechanism of deformation of isolated vertical sheets approximate to theoretical models used to investigate folding in rocks. Although these models can in principle be used to determine various properties of the calcite sheets, the available data permit an estimate of the "viscosity" only-of the order of 400-40,000 poise at the time of their formation. Further investigation of these structures is needed to establish their organic origin with certainty; to determine the environmental conditions controlling their formation; to explain their restriction to later Precambrian sequences; to investigate their possible use as a Precambrian form-fossil; and to compare their deformation more fully with theoretical models.

Folding and Fracturing of Rocks. John G. Ramsay. McGraw-Hill, New York, 1967. xvi + 568 pp., illus. $17.50. International Series in the Earth and Planetary Sciences

Folding and Fracturing of Rocks. John G. Ramsay. McGraw-Hill, New York, 1967. xvi + 568 pp., illus. $17.50. International Series in the Earth and Planetary Sciences

A Mathematical Theory of Finite–Amplitude Rock–Folding

The problem of the finite-amplitude folding of an isolated, linearly viscous layer under compression and imbedded in a medium of lower viscosity is treated theoretically by using a variational method to derive finite difference equations which are solved on a digital computer. The problem depends on a single physical parameter, the ratio of the fold wavelength, L, to the dominant wavelength of the infinitesimal-amplitude treatment, L_d. Therefore, the natural range of physical parameters is covered by the computation of three folds, with L/L_d = 0, 1, and 4.6, up to a maximum dip of 90°. Significant differences in fold shape are found among the three folds; folds with higher L/L_d have sharper crests. Folds with L/L_d = 0 and L/L_d = 1 become fan folds at high amplitude. A description of the shape in terms of a harmonic analysis of inclination as a function of arc length shows this systematic variation with L/L_d and is relatively insensitive to the initial shape of the layer. This method of shape description is proposed as a convenient way of measuring the shape of natural folds. The infinitesimal-amplitude treatment does not predict fold-shape development satisfactorily beyond a limb-dip of 5°. A proposed extension of the treatment continues the wavelength-selection mechanism of the infinitesimal treatment up to a limb-dip of 15°; after this stage the wavelength-selection mechanism no longer operates and fold shape is mainly determined by L/L_d and limb-dip. Strain-rates and finite strains in the medium are calculated f or all stages of the L/L_d = 1 and L/L_d = 4.6 folds. At limb-dips greater than 45° the planes of maximum flattening and maximum flattening rat e show the characteristic orientation and fanning of axial-plane cleavage.

Rheological stability with couple stresses and its application to geological folding

The author’s theory of elasticity and viscoelasticity of initially stressed anisotropic solids is extended to include couple stresses and an additional refinement represented by a dependence of the deformation on the second gradient of the stress. The theory is intended to provide an approximate continuous model valid over a wide range for the mechanics of laminated media. The equations derived for the continuous model are rigorous for a medium initially at rest under the initial stress but are applicable as an approximation to stability problems of viscous and plastic media which exhibit initial strain rate with finite strain. The effect of gravity is included. A ‘couple-stress analogy’ renders immediately applicable to the medium with couple stresses a large class of solutions obtained earlier for plates and multilayers under initial stress. The problem of internal buckling is discussed. The theory is particularly suited to the analysis of folding and gravity instability of stratified sedimentary rock and remarkably simple results are shown to provide striking verification when applied to a wide variety of geological structures.

Gravity sliding and cross folding in Carboniferous rocks, Nova Scotia

Gravity sliding and cross folding in Carboniferous rocks, Nova Scotia

Sequential Multivariate Regression Methods and Scalars in the Study of Fold-Geometry Variability

Quantitative descriptions of the location, orientation, and style of folds are based on scalar attributes rather than on the usual vectoral and qualitative data. These scalars are used (a) to map the areal variability of fold geometry and (b) as a basis for sequential multivariate regression analyses in a search for those geological factors that control the nature and variability of folds in sedimentary or metamorphic rocks.

Palynological Evidence for Folding, Faulting, and Erosional Contacts in Subsurface: ABSTRACT

Palynological studies, conducted on exposed rocks illustrating various degrees of folding, faulting, and other expressions, have been used to interpret conditions observed in subsurface samples. Palynological evidence of rock folding or faulting is shown by degrees of spore and pollen preservation and progressive color changes from yellow to black. Folded structures generally can be distinguished from fault structures by more uniform and widespread occurrence of specific palynological physical changes. Regions containing igneous intrusions show similar palynological changes, but differ in proportion to type and distribution of igneous bodies. Unconformities are detected by marked palynological assemblage changes and in many instances by recycled fossils above an erosional surface. Recognition of palynological physical aspects can be useful in the identification of paleogeographic features, direction and approximate location of stratigraphic traps, paleotemperature conditions within an area, and probable oil or gas potentials. End_of_Article - Last_Page 1564------------

Deformation of the Ross Ice Shelf, Antarctica

The structural features in the Camp Michigan area on the northeastern margin of the Ross Ice Shelf, Antarctica, include anticlines, synclines, drag folds, crevasses (some similar to strike-slip faults, others to joints) and rifts (extraordinarily wide crevasses). Strain rates were measured on the surface of the ice shelf by determining the relative positions of stakes in the ice at various times by triangulation. Nye9s flow law for ice is obtained by a systematic simplification of the generalized flow law proposed by Glen. Glen9s laboratory data on ice creep were used to determine the necessary constants in Nye9s flow law, and the stresses were calculated on this basis. A fracture criterion for ice is formulated on theoretical grounds. This criterion may be described as a surface of revolution about the hydrostatic stress line in stress space. The stresses calculated from strain rates measured in actively crevassing areas are considered “unstable” and are greater than the actual stresses present. They would cause rupture in the material if they were actually present. Stresses in noncrevassing areas are designated as “stable.” A plot of stable versus unstable stresses shows good agreement with the theory. Folding in the area was analyzed in accordance with the method of Biot. Even though an exact stability analysis is used, the predicted wave lengths agree only with the wave length of the youngest folds. The fracture criterion is extended to rocks, but the available data are insufficient to definitely establish its validity for rocks. The principle of least work indicates that faults do not necessarily include the intermediate principal stress axis in the plane of the fault, but the unavailability of data precludes formulation of a usable criterion. Folding in rocks is amenable to study by Biot9s theory, but lack of data prohibits wave-length prediction for geologic examples.

Presa de Grandval

This important project is situated on the top section of the river Truyere, within the region of the French Central Massif. The dam has been constructed by the Electricite de France. The total hydroelectric development project in this zone involves six dams and six power stations, to exploit an overall hydrostatic head of 520 ms. The water accumulated by these dams has a maximum volume of 540.10 6 cubic metres, and will produce a yearly output of 1450 .10 6 kWh. The Grandval dam rests on strong microschistous rock folds. Its length is 330 ms, and its maximum height 88 ms, measured from the foundation base. Structurally it is formed by a series of vaults supported by abutments, which are spaced at 50 ms intervals. The mean thickness of the vaults at the base is 4.90 ms, and 1.50 at the crown. The vertical slope on the upstream side is 0.70. A feature of this design is that the vaults are not uniformly cylindrical up to the top, as is customary. The top part of the dam consists, in fact, of a small gravity dam, resting on the underlying vaulted structure. The dam is straight at the top, and a road runs along it, on an overhanging structure. The dam has a surface spillway, which is ski jump shaped, and runs over the two abutments on either side of the power station.

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.

Development of Folds in Sedimentary Strata

Information concerning the mechanics of rock folding based on theoretical considerations, laboratory experiments, and field observations is presented in this paper. Theoretical considerations are based on the theory of elastic stability. Photoelastic model experiments that illustrate the theory and qualitatively describe complex cases are included. The wave lengths of folds are compared with the dimensional arrangement of sedimentary strata. A plot of this relationship shows the wave length to be approximately equal to 27 times the thickness of the dominant member. The importance of the stratification of sedimentary rocks in determining the geometry of folds is emphasized. The concept of describing a sedimentary section in terms of structural lithic units is discussed.

Flow Folding in Rocks

PROF. L. U. DE SITTER1 considers that in flow folding “internal movement itself is no longer orientated but can take any direction”, and that the position of shear planes is random. He believes it developed most typically in incompetent rocks like salt, or at very high confining pressures or high temperatures. It is further maintained that this is the only true plastic deformation of rocks. These beliefs have recently been reiterated and enlarged on2.