Axial Planes Of Folds Research Articles

Superimposition and timing of deformations in the Mount Morrison roof pendant and in the central Sierra Nevada, California

The Mount Morrison roof pendant, the only roof pendant in the central Sierra Nevada containing Paleozoic fossils, is complexly deformed and contains three generations of structures, including folds, reverse faults, schistosities, and lineations. All three generations of structures occur in the Ordovician-Silurian(?) metasedimentary rocks, whereas only the younger two are recorded in the Pennsylvanian-Permian(?) metasedimentary rocks and the Permian(?)-Jurassic(?) metavolcanic rocks. The average strike directions of axial planes of folds are north-south, N25°W, and N60°W in the first, second, and third generations, respectively. Generations of structures having similar styles, orientations, and relative age relations occur in other pendants of the central Sierra Nevada. The pendant is interpreted as a thin sequence with tight isoclinal folds instead of a thick homoclinal sequence. The first deformation occurred during Devonian or Mississippian time, perhaps during the Antler orogeny. Uplift, erosion, and volcanism occurred in Late Permian time between the first and second deformations, perhaps as an expression of the Sonoma orogeny. The second generation structures formed in several pulses between Early Triassic and Early Cretaceous time, as indicated by temporal relations between deformed wall rocks and younger, crosscutting granitic plutons. The third generation structures formed between Early and Late Cretaceous time, during which these structures were crosscut by granitic rocks. The wall rocks of the batholith may form an anticlinorium instead of a synclinorium. Other roof pendants in the axial portion of the batholith may be relatively old, because they contain the same three sets of structures as found in the Ordovician-Silurian(?) rocks of the Mount Morrison roof pendant. Locally, various age belts of granitic rocks have shielded roof pendants from subsequent deformation.

Evolution of Folds around Broken Bow Uplift, Ouachita Mountains, Southeastern Oklahoma

Folds in the studied area range from a few centimeters to 3 km in wavelength and are asymmetric with sharp angular hinge zones. Anticlines are overturned southward away from the continental interior. Fold hinges trend east-west and are arched over the Broken Bow uplift which trends northeast across the fold hinges. Asymmetric minor folds are present on the limbs of larger structures, but many minor folds do not exhibit a change in symmetry from one limb of the larger fold to the other. Slaty cleavage is common in the southern part of the Broken Bow uplift and rare in the north. Cleavage is distributed symmetrically about axial planes in many folds. However, in the southern part of the uplift, slaty cleavage cuts across axial planes of folds at angles up to 30°. This cross-cutting relation is present in both outcrop-size folds and folds with wavelengths of a kilometer or more. The Broken Bow uplift has at least three recognizable phases of deformation. The first phase produced buckle folds that probably were generated by gravitational body forces during the rapid deepening of the basin axis in the north associated with flysch sedimentation during the Carboniferous. The second phase was associated with the principal Ouachita diastrophism in the Late Pennsylvanian or Early Permian. The northward dipping slaty cleavage was superimposed on the earlier folds and cut across their hinge surfaces. Original fold shapes were modified greatly and anticlines were overturned producing north-dipping fold axial planes. These north-dipping axial planes are inconsistent with previous suggestions that a mobile core or infrastructure is buried beneath the Mesozoic coastal pla n sediments in the south. Additional small-scale folds probably were generated during this second phase. The second phase may have contained a large regional shear-strain component produced by underthrusting the basement toward the continental interior. The last deformation phase produced the Broken Bow uplift by arching east-west-trending folds and slaty cleavage about a northeast-southwest axis. Movements were essentially vertical with little internal deformation of the rocks.

Structural zones of the Variscan fold belt in SW England, their location and development

The D evonian and Carboniferous strata of SW England were deformed during the Variscan orogeny. Differing amounts of polyphase deformation imposed upon rocks of differing rheological character gave rise to a variety of attitudes and styles of folding. The present account attempts to outline the location and development of structural zones within which the attitudes of folds are constant. Dearman (1969a) suggested a zonal scheme based on the recognition of areas of upright primary folds passing southwards into inclined and recumbent folds on the north Cornish coast between Bude and Padstow. Continuity of this arrangement of structures eastwards to the western margins of Dartmoor can be demonstrated (Dearman 1971a). The zone of recumbent folding was extended, without subdivision, into south Cornwall. Simpson (1970) , recognising a zone of upright folding to the south of Brixham in south Devon, correlated this with the inclined folding in the Newquay area and thus subdivided the southern zone of recumbent folds. A line between Brixham and Newquay separates recumbent folding to the north from upright or inclined folding to the south, but this simple subdivision is unsatisfactory as recumbent first phase folds ( F 1 ) are common in the Mylor and Gramscatho Beds to the south of Perranporth ( Smith 1965 ). Neither zonal scheme, based solely on the attitude of fold axial planes, attempts to separate the north facing primary folds of south Cornwall and south Devon from the south facing primary folds of north Cornwall. In a discussion of the spatial relationships of the folding the following criteria

Structural Geology, Cochas to Yauricocha, Central High Andes, Peru

The study area is along the continental divide, within the Mesozoic fold-and-fault belt of western Peru. The rocks exposed are Early Jurassic to early Tertiary sedimentary strata; neither pre-Early Jurassic nor Tertiary volcanic rocks are exposed. Structurally the area can be divided into two zones separated by the Tranca reverse-fault zone. The eastern zone has northwest-trending folds and reverse faults; the overturn of the fold axial planes and the direction of overthrusting are toward the northeast. The faults are in dextral en echelon array. The western zone shows tight folding, and imbricate reverse faulting. In both zones decollement at depth may control the structural pattern. The age of the tectonism is early to middle Tertiary. Comparisons are shown between the types of deformation in the study area and those elsewhere in the same fold-and-fault belt. A series of post-tectonic stocks of granodiorite-diorite composition of middle Tertiary age or younger is present in the area.

Deformation and Metamorphism in the Lukmanier Region, Central Switzerland

The Lukmanier region in the central-southern Swiss Alps contains parts of three important structural and stratigraphic units: the Gotthard Massif in the north, the parautochthonous Mesozoic cover of this massif, and the Lucomagno Massif, a frontal part of the Lower Pennine nappe complex to the south. Two successive phases of Alpine folding have been recognized in the Mesozoic cover, both of which occurred after the main northward horizontal transport of the Lower Pennine nappes. The earlier phase, called Phase B, produced three large-scale fold zones in the cover, with a penetrative axial-plane schistosity that strikes east-west and dips steeply north; fold hinge lines are markedly curved in the plane of the schistosity due to differential strain effects. Deformed fossils and ooids indicate the principal axes of Phase B strain and show that the steeply plunging, micaceous mineral lineation, which is found on most schistosity surfaces, is parallel to Phase B maximum strain extension. Regional Phase B folds were modified to various extents by Phase B movements on tectonic slides that lie parallel to the axial-plane schistosity and form the east-west contacts of both massifs with the Mesozoic rocks. The subsequent phase of deformation, called Phase V, produced no large-scale structures, but corrugated bedding and Phase B schistosity and mineral lineation mainly in the southern half of Lukmanier. Fold axial planes strike southeast and dip gently northeast, and fold axes plunge gently southeast. Complex systems of joints and rare, local strike-slip faults developed after Phase V, probably during postorogenic regional uplift. Effects of both phases of Alpine folding are found in the Gotthard and Lucomagno Massifs. Phase B produced the schistosity that strikes east-west and dips steeply north in both massifs, and the shapes of augen, together with deformed schlieren in the Hercynian granodiorite in the Gotthard Massif, indicate that the orientations of Phase B strain axes are similar to those in the Mesozoic cover, with the mineral lineation on the schistosity in both Massifs being parallel to Phase B maximum strain extension. Very intense Phase B deformation in the Gotthard Massif was restricted mainly to mylonitization in narrow east-west zones, and such movements in one zone of ancient paragneisses appear to have controlled the position of the Scopi synclinal zone, the most northerly large-scale Phase B fold zone in the Mesozoic cover. Phase V deformation had little effect in the Gotthard Massif, but Phase V folds are found everywhere in the Lucomagno Massif: their orientation is similar to Phase V folds in the Mesozoic cover. Complex systems of joints in both Massifs probably developed at about the same time as similar joints in the Mesozoic cover. Alpine metamorphism, generally of lower almandine-amphibolite facies, produced a diversity of porphyroblasts in the Mesozoic cover, and they show a variety of textural relations with phases of deformation. These relations change across the area and are due to migration of phases of deformation from south to north relative to stationary metamorphic conditions. Porphyroblast textures also show that shape and predeformational orientation may be important factors controlling amounts of porphyroblast rotation. Regional considerations of Alpine events in Lukmanier and adjacent areas lead to the conclusion that the main northward horizontal transport of the Lower Pennine nappes preceded Phase B deformation in Lukmanier, and in turn Phase B preceded and initiated, by uplift of the Gotthard Massif, the main northward horizontal transport of the Ultrahelvetic and Helvetic nappes.

Experimental tests of buckling folds in relation to strain ellipsoid in simple shear deformations

A series of experimental deformations with plastic materials, such as modelling clay, putty and wax, was carried out by simple shear in order to determine the geometrical relationships of buckling folds with the strain ellipsoid. It was found that the fold axis is not necessarily parallel to the intermediate or major axis of the strain ellipsoid. The fold axis develops parallel to the major axis of the strain ellipse (i.e., a section through the strain ellipsoid) on the enveloping surface of the bed. The orientation of the fold axis is partially controlled by the viscosity of the layer and its host because in rotational strain the orientation of the strain ellipsoid is dependent on the relative viscosity of the two materials. The axial planes of folds also are not necessarily parallel to the AB plane of the strain ellipsoid. However, there is a tendency of the strain ellipsoids to be roughly symmetrical about the axial plane. There is a resistance of isolated layers of competent material embedded in an incompetent host to form asymmetrical folds by simple shear alone. Asymmetrical folds, however, readily form by a combination of pure shear and simple shear, and by shearing strain parallel to the enveloping surface of a series of pre-existing folds. In some of the tests a cleavage-like structure developed symmetrical about the axial plane of the folds. The structures were strictly parallel to the plane of maximum extension of the strain ellipsoid.

Structure of the Glenarm Series in Chester County, Pennsylvania

Research Article| April 01, 1961 Structure of the Glenarm Series in Chester County, Pennsylvania HUGH McKINSTRY HUGH McKINSTRY R.O.T.C. Building, Harvard University, Cambridge 38, Mass Search for other works by this author on: GSW Google Scholar GSA Bulletin (1961) 72 (4): 557–577. https://doi.org/10.1130/0016-7606(1961)72[557:SOTGSI]2.0.CO;2 Article history received: 05 Jun 1958 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation HUGH McKINSTRY; Structure of the Glenarm Series in Chester County, Pennsylvania. GSA Bulletin 1961;; 72 (4): 557–577. doi: https://doi.org/10.1130/0016-7606(1961)72[557:SOTGSI]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 The rocks of the Glenarm series in southeastern Pennsylvania, of probable Paleozoic age, show strongly overturned and locally recumbent folds. Detailed mapping of structural features indicates that not only the bedding but also the cleavage and axial planes of folds have undergone deformation which increases in intensity from northwest to southeast along with increasing grade of metamorphism. These observations, together with the outcrop pattern, point to large-scale refolding of earlier folds; recumbent structures result from the warping of axial planes that originally had steeper dips. This structure accords with the pattern of folding of unquestioned Paleozoic rocks in the adjoining Philadelphia area without recourse to the hypothetical “Martic overthrust.” The writer suggests that the late-stage deformation is related mechanically to the change in strike of the Appalachian belt; the belt strikes generally north-south in New England and Virginia but is nearly east-west in Pennsylvania. 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.

STRUCTURE AND METAMORPHISM OF THE KEENE-BRATTLE-BORO AREA, NEW HAMPSHIRE-VERMONT

Metamorphosed sedimentary and volcanic rocks, ranging in age from middle Ordovician (?) to lower Devonian, occupy about 55 per cent of the Keene-Brattleboro area of southwestern New Hampshire and southeastern Vermont. Rocks of the low-grade zone of metamorphism are present in a north-south band near the western edge of the area; rocks of the middle-grade zone occur on either side of the low-grade zone; and rocks of the high-grade zone occupy the central and eastern parts of the area. The lithology of the stratified rocks is similar to that farther north. The Bernardston formation of north-central Massachusetts is correlated with the Littleton formation. Foliated igneous rocks of middle Devonian (?) and late Devonian (?) age occupy about 45 per cent of the area. These include three plutons of the Oliverian magma series and five members of the New Hampshire magma series. The stratified rocks were severely folded, broken by a major thrust fault, and metamorphosed during the late Devonian (Acadian) orogeny. The resulting structural features in this area, trend almost north. The Bronson Hill anticline occupies most of the area. Three elongate domes, each with a plutonic core of the Oliverian magma series and a surrounding envelope of stratified rock, are present on this anticline. A pluton of Kinsman quartz monzonite occurs in a structural basin on the anticline, near the center of the area. The steeply dipping western limb of the Brattleboro syncline occupies the western one-eighth of the area. The Northey Hill thrust, a major fault of western New Hampshire, separates the Bronson Hill anticline from the Brattleboro syncline. This thrust, which cuts out the eastern limb of the Brattleboro syncline, has a stratigraphic separation of about 10,000 feet. Foliation and bedding are mutually parallel in most of the stratified rocks east of the Northey Hill thrust, whereas foliation is parallel to the axial planes of folds in most of the rocks west of the thrust. Minor folds of two ages are present west of the thrust and in a few places just east of thrust. The older minor folds are isoclinal, or nearly so, and have a well-developed axial-plane flow cleavage. The younger minor folds are more open and have an incipient to strong axial-plane slip cleavage. It is believed that the Oliverian magma series was forcefully injected as a thick composite sheet during the very early stages of the orogeny, while the enclosing rocks were essentially flat. The New Hampshire magma series was emplaced during the late stages of the orogeny. Initial movements along the Northey Hill thrust may have occurred during early stages of the orogeny, but a considerable part of the movement took place after the metamorphic intensity in the rocks along the fault had reached its maximum. The Triassic border fault, which forms the eastern border of the Triassic rocks of the Connecticut valley in Massachusetts, enters the area 1 mile east of the Connecticut River and trends northeasterly to the vicinity of Keene, where it apparently ends. The maximum stratigraphic displacement along the fault is about 15,000 feet. Numerous small high-angle normal faults are present. In three localities, graben, in which the roof of one of the plutons of the Oliverian magma series is preserved, form very striking structural features. The normal faults which bound the graben have a maximum stratigraphic separation of 3000 to 4000 feet. The age of the faulting associated with the graben, and also much of the normal faulting of western New Hampshire, is believed to be Triassic. Fluorite deposits of the Keene-Brattleboro area are also believed to be Triassic.