Vertical Foliation Research Articles

Patterns of total and incremental strain in subsiding troughs: experimental centrifuged. models of inter-diapir synclines

The experimental technique of centrifuge modelling has been used to elucidate patterns of total strain and progressive deformation associated with troughs that form by gravitational subsidence of a dense cover into a less dense basement. Model structures are analogous to synclines found between mantled gneiss domes, and the models provide data with which to test the hypothesis that Archean greenstone belts are products of vertical gravity tectonics.Experimental results include the following. (1) Strain within the main body of subsiding belts is dominated by strong horizontal contraction and vertical extension. In natural structures this strain pattern would produce vertical foliation and steeply plunging mineral lineation. (2) Within cover units near trough margins the flattening plane trends parallel with or at a low angle to the cover–basement interface at all levels in the trough. (3) Patterns of strain within subsiding troughs, at least as expressed in surface deformation, reflect the geometry of the cover–basement interface at depth. Flow within the cover units converges towards points of maximum trough subsidence to produce local vertical constrictive strain. Deformation above trough saddles is characterized by vertical flattening strain. (4) Horizontal shortening within a subsiding, stratified cover sequence produces early folds with horizontal axes and steep axial surfaces. Where the stratified sequence forms a relatively low-density unit within the cover, folds and thrusts verge consistently away from the trough axis. Continued subsidence and horizontal contraction tend to rotate initially shallowly dipping structures into steeply dipping attitudes.

Considerações sobre a origem dos grãos "olhos-de-quartzo" nas rochas porfiroidais do distrito de Western Mines, Ilha de Vancouver, Canadá

Quartz-eye bearing porphyroidal occur in straight association with the vent ore, in the vent zone, of the base metal massive sulphide deposits of the Western Mines District. Microscopic observations on these indicated two stages of development, which are part of a continuous process of rock alternation. In the earlier stage (first generation quartz) there was a total or quasi-total replacement of chloritecalcite spherulites or a filling of vesicles by quartz. It was accompanied by development of strong vertical foliation which superimposed to the primary bedding. In the final stage (second generation quartz) the quartz overprinted the vertical schistosity, and deformation was minimum. Quartz-eye typical of the earlier stage, termed first generation quartz, is surrounded by a bowed micaceous matrix; this is not the case when it is examined in a section perpendicular to the foliation. Composite grains and recrystallized single round grains may be in association with deformed or broken grains of original quartz and with latter or second generation grains. A preserved nucleus of calcite is occasionally observed. Hypidiomorphic and idiomorphic coarse grains of quartz are typical of the final fase of the vent alteration process, they are termed second generation quartz. Overgrowth of second generation quartz without optical character of continuity upon first generation quartz is commonly observed. In addition most of the round rims of quartz represent the outer zone of the crystallized second generation quartz. It becomes distinguishable from the rest of the quartz grain because of the abundance of microinclusions. Since both stages of quartz generation affected either mafic-intermediate or intermediate-acidic rocks, it is suggested that the term quartz-eye bearing porphyroidal rocks might be better regarded as a textural terminology than a compositional one.

Geology of the Ketcherside Mountain area, southeastern Missouri, and the source of Grassy Mountain Ignimbrite

The St. Francois Mountains igneous complex is a shallow batholith about 1,500 m.y. old which has intruded a roof formed by its own volcanic ejecta. It is similar to other shallow-roofed batholiths such as the Boulder batholith. However, it has been tilted and beveled by erosion so that the roof is available for study at a wide range of stratigraphic levels. The volcanic roof is composed mostly of ash-flow tuff sheets, implying that caldera-collapse eruptions were the major mechanism of formation of the roof. Only two calderas have been proposed in the St. Francois Mountains thus far, the Taum Sauk caldera and the Butler Hill caldera. The southern ring fault of the Taum Sauk caldera is located in the Ketcherside Mountain area of southeastern Missouri. The fault is nearly vertical, strikes about N50°E, and is intruded by a small body of porphyritic granite with vertical flow foliation. Rocks in the down-dropped block are broken by high-angle faults as much as 3 km from the ring fault, but extracaldera units are little deformed. Associated with the ring fault are bedded tuffs and volcaniclastic sedimentary units, some of which are mineralized. These data support the hypothesis that the Taum Sauk caldera is a trap-door caldera with major faulting along the eastern and southern walls. There is little evidence of faulting along the western wall. The Butler Hill caldera occurs in the eastern St. Francois Mountains and is centered approximately on the Butler Hill Granite. The caldera, one of the earliest features to develop, has been obscured by later structural modification, but the major collapse ash-flow tuff of the caldera, the Grassy Mountain Ignimbrite, is well exposed. Analysis of flow directions of the ignimbrite indicates a source for the unit in the eastern St. Francois Mountains, as predicted by the Butler Hill caldera model.

Structure and stratigraphy of the Dalradian rocks of the Bennabeola area, Connemara, Eire

Summary The geology of an area of 400 sq km of Lower and Middle Dalradian rocks is described. It consists mainly of rocks belonging to the Appin and Argyll Groups which have been strongly deformed during four stages (F 1 –F 4 ) with the major development of tectonic slides occurring before F 3 . Eighteen major F 3 folds, whose geometry is described, are correlated across the Connemara antiform (F 4 ); they refold an earlier nappe-like (? F 2 ) antiform, the Derryclare fold. All of the Appin Group rocks and the Cleggan Boulder Bed Formation are contained within the core of the Derryclare fold, which locally has attenuated limbs. A zone of vertical foliation, the ‘steep belt’, is present in the south of the area. It is superimposed upon the originally steeply inclined, south-facing Derryclare fold and appears to have developed during F 4 .

Cornwallis Fold Belt and the mechanism of basement uplift

Cornwallis Fold Belt is a north-trending anticlinorium more than 650 km (400 mi) long, that extends from the Precambrian Shield to the Sverdrup Basin. It is the folded and faulted sedimentary suprastructure that overlies Precambrian crystalline basement rocks of the Boothia Horst. The horst and fold belt represent lower and intermediate levels of the Boothia Uplift. Evolution of the Cornwallis Fold Belt includes two phases, formation and modification.Formation. The basic structure of the Cornwallis Belt, a relatively simple, steep-sided, north-plunging anticlinorium, was formed in the interval from Proterozoic to Late Devonian time during several discrete phases of deformation that involved a similar stress pattern. These phases can be attributed to pulses of differential vertical uplift of the underlying Boothia Horst. The earliest phases involved periods of gentle arching of the crystalline basement and sedimentary cover in late Proterozoic and early Paleozoic times. The fold belt was formed mainly by the Cornwallis Disturbance (new name) which involved further differential vertical uplift, and comprised several pulses: (1) Early Silurian, mild, affecting only part of the belt; (2) Early Devonian, very strong, affecting the entire belt; (3) late Early Devonian, moderately strong, affecting the entire belt; (4) Late Devonian, moderately strong, affecting the entire belt. Each pulse was a cycle that began with uplift and erosion of the fold belt and shedding of detritus into the adjacent basins, and was followed by broader regional subsidence and the resumption of deposition on the belt. Between pulses of uplift there was regional subsidence, during which the fold belt subsided less than the flanking basins and received less sediments.Differential vertical displacement originated in the crystalline basement, occurring along fault zones that define the Boothia Horst, and are parallel to and controlled by a steep to vertical north-trending foliation. Faults extend into the sedimentary suprastructure comprising the overlying Cornwallis Fold Belt, and change gradually upward from vertical faults to high-angle reverse faults, overturned anticlines, and finally to asymmetric anticlines. Because the fold belt plunges north, this gradational sequence occurs from south to north in the exposed part of the fold belt. Structures formed by early pulses were rejuvenated by later pulses with the same sense of movement.Modification. The basic structure of the Cornwallis Fold Belt was modified by other types of deformation during the interval from Late Devonian to the present. Many of the preexisting faults were reactivated, but with a different sense of movement. During the Late Devonian to Middle Pennsylvanian Ellesmerian Orogeny, southward overriding of upper levels of the sedimentary succession produced folds in the rocks east and west of the Cornwallis Fold Belt which had not been previously deformed and could easily be displaced southward on an underlying décollement surface. The north-trending Cornwallis Fold Belt, however, was an obstacle to southward overriding in which the effects of overriding were reduced. Zones of interference structures developed near the margins, guided by older basement-controlled structures. Left-lateral faults were developed on the western margin and right-lateral movement is probable on the eastern margin.The Cornwallis Fold Belt extends an unknown distance northward beneath the younger rocks of the Sverdrup Basin. These younger rocks were deposited during a long period of northward downwarping that began in mid-Mississippian time. This same downwarping caused an abrupt increase in the northward plunge of the fold belt.During the Cretaceous–Tertiary Eurekan Rifting Episode the Cornwallis Fold Belt was fragmented by block faulting. The horsts form islands, and the grabens form submarine channels, some of which contain thick sections of semiconsolidated Cretaceous–Tertiary sediments. Numerous other normal faults that occur within the fold belt probably formed at this time. Cretaceous–Tertiary faults within the Cornwallis Fold Belt have a rectilinear pattern that was inherited from preexisting structures.

A reconnaissance of the Wadi Beihan, South Yemen: With notes on basement control of gully alignment in superficial deposits

The Wadi Beihan is a relatively little known, isolated region in the north of South Yemen. It is flanked by basement mountains made up of acidic and hornblendic gneisses and schists, with syntectonic granites and with consistent north-northeast strike and near vertical foliation. The superficial deposits, confined to the wadi, consist of various categories of alluvium and areas of dunes. The most interesting feature is the marked rectilinear pattern of gully systems eroded in low terrace alluvium, and the almost exact parallelism between these alignments and trends of basement foliation and master joints. It is suggested that the gully alignment is related to basement structures and a mechanism is discussed. It is also suggested that certain joints of other regions, thought to be posthumously related to fractures in underlying older rocks, may have originated in a similar way.

Structural analysis of complex fold belts in the Lewisian north of Kylesku, Sutherland, Scotland

Synopsis Complex patterns, of folds with horizontal axes perpendicular to the foliation in gneiss, occur on vertical foliation surfaces. Similar complex patterns, of corresponding folds with vertically disposed axes, occur on horizontal surfaces. The vertical and horizontal surfaces represent cross-sections near to or within the steep limb of N.W.-S.E. monoclinal fold belts which cut rocks of a crystalline basement. Heterogeneous flow appears to have been operative in the formation of the complex folds. Local intensely developed small-scale folding along narrow N.W.-S.E. fold belts possibly resulted from their coincidence with pre-existing N.W.-S.E. master joints in the basement.

Mylonization of hybrid rocks near Philadelphia, Pennsylvania

The pre-Cambrian banded gneiss of a part of the Piedmont Plateau near Philadelphia is interpreted as a hybrid rock, resulting from the thorough injection of felsic material into a plagioclase amphibolite. The composition of the injecting solutions probably ranged from mostly water to that of an aqueous potash-silica-alumina magma. In the western part of the area vertical, winding mylonite zones cut these rocks. These zones outline lenticular masses of less sheared rocks and in some places attain a thickness of a quarter of a mile. East of Wissahickon Creek schistose shear zones run parallel to the nearly vertical foliation of the gneiss. A post-Ordovician high-angle fault forms the northern border of the gneiss west of Wissahickon Creek, crosses it and forms the southern border of the gneiss east of Wissahickon Creek. A branch of this fault bounds the western part of the gneiss on the south. The paper presents the results of a structural and petrographic study of a belt of the gneiss 37 miles long, extending from Trenton to Bryn Mawr.