Flexure Zone Research Articles

Beam flexure and hinge zone detailing in reinforced concrete ductile frames requiring beam sway mechanisms

This paper is the resuIt of deliberations of the Society's discussion group on seismic design of ductile moment resisting reinforced concrete frames.

Geology of Outstanding Arctic Aerial Photographs 3. Margin of Sverdrup Basin, Lyall River, Devon Island

ABSTRACT An outstanding aerial photograph of northern Grinnell Peninsula, Devon Island is reproduced with a geological interpretation. The area was occupied by three successive depositional basins: the Franklinian Miogeosyncline, the Arctic Platform and the Sverdrup Basin. Remnants of all three are preserved. The exposed section is more than 3300 m (11,000 ft) thick, including 12 Ordovician to Cretaceous map units and six unconformity surfaces. Three major deformations produced two intersecting structural trends. The Cornwallis Fold Belt developed first, with deformation ending in Late Devonian time. Its northerly trend was rejuvenated and influenced all younger structures. The Ellesmerian Orogeny occurred between Late Devonian and mid-Pennsylvanian time and included two phases. The first phase included a southward compression that put an east-west overprinting on northern parts of the Cornwallis Fold Belt. It apparently occurred before deposition of the Mississippian Emma Fiord Formation. This is a shaly and coaly transitional unit representing the earliest stages of the Sverdrup Basin. It apparently filled topographic lows in a fresh-water lake, as subsidence followed the first phase of folding. The second phase of the Ellesmerian Orogeny was contemporaneous with marked regional downwarping that developed the Sverdrup Basin. The phase was marked at Lyall River by folding and faulting near the zone of flexure. The topographic relief created provided coarse conglomerate to the contemporaneous Canyon Fiord Formation. A narrow east-trending, furrow-shaped high within the Sverdrup Basin was emergent through Canyon Fiord deposition. South of this high and largely cut off from the main basin was the narrow Lyall River Embayment of the Sverdrup Basin. The early history of the Sverdrup Basin was influenced greatly by pre-existing structure. The flexure that produced the basin caused an abrupt increase in the northward plunge of the Cornwallis Fold Belt. Numerous normal faults exposed in the area probably were active in the Cretaceous-Tertiary Eurekan Orogeny. They were controlled by older structures.

Structure of Hawaiian volcanoes from seismic refraction data

Numerous seismic refraction traverses were carried out over the Hawaiian Islands to outline the structure of the volcanoes and determine the thickness of the layers of the crust. Results indicate that there is a progressive thickening of the crust in the area from younger to older volcanic islands. High velocity mantle-like material was detected under rift zones and in volcanic plugs. Volcanic regions were found to be intersected by flexure zones in the crust and in the mantle.

Oil and Geology in Cuanza Basin of Angola

The Cuanza basin is in northwestern Angola on the Atlantic Coast of West Africa. This basin is about 300 km. long north-south and 170 km. wide east-west, and contains an Early Cretaceous carbonate-evaporite sequence and a Late Cretaceous and Tertiary argillaceous-arenaceous sequence. The Precambrian crystalline basement is partly covered by extrusive rocks and granite-wash type sediments. Surface and subsurface sediments of the basin consist of Lower and Upper Cretaceous, Paleocene, Eocene, and Miocene strata. Occurrences of oil and gas have been reported in almost all of the stratigraphic units in the Cuanza basin, and there is major production from the Cretaceous rocks. Study of these hydrocarbon occurrences and of the geological history of the basin shows that close relationships exist between sources, migration, and entrapment of oil, and environment of deposition controlled by the basement and salt tectonics. During Early Cretaceous time, subsidence of the central part of a restricted basin determined the regional cyclical deposition of a carbonate-evaporite sequence providing a favorable situation for genesis and entrapment of oil. Thus, the deposition during Aptian time of a very fine crystalline limestone, interbedded with argillaceous limestone and overlain by an oolitic sandy calcarenite, itself underlying evaporites, had an important influence on the subsequent extent of oil accumulations in the Binga Formation. During Aptian-Albian time, differential subsidence on the western margin of the basin caused lateral interfingering of back-reef calcarenite, argillaceous carbonate, and evaporite. This interfingering is believed to be related closely to oil accumulations in this area. Very i portant vertical development of reef deposits in the Longa area is related to lateral migration of the underlying Massive Salt, which flowed with the help of the excess of weight introduced by the growing reef. On the eastern margin, upper Albian reef buildups capped by marine shale also provided a favorable situation for generation and accumulation of oil. During Late Cretaceous and Tertiary time, a major basement flexure or fault zone appears to have been associated genetically downdip with deposits that accumulated with greater thickness than elsewhere. This flexure and the loci of maximum deposition moved eastward during Late Cretaceous and Paleocene, then westward during Eocene and Miocene. These thick formations, which are mainly argillaceous-arenaceous and which were deposited partly in deltaic and lagoonal environments, grade westward into thinner marine deposits and eastward into thinner continental deposits. During each particular epoch corresponding with a stabilization of this moving flexure, favorable conditions for genesis of hydrocarbons seem to be related to these transitional environments. Oil production is located above the Massive Salt at the crest of salt anticlines, and one small oil field has been discovered below the Massive Salt along a ridge of the Basement Complex in a pinch-out of sandstones between Precambrian mica-schist below, and salt above.

Le Neocomien de la region du Bas-Verdon

Abstract The Neocomian (lower Cretaceous) of the lower Verdon region (Basses-Alpes, southern France) represents a transition facies between deep-water marine deposits to the northwest and continental deposits to the southeast. The facies change between the marly limestone series of the flexure zone to the west and the white limestone series of the platform to the southeast can be traced. Fluctuations of the ancient shoreline resulted in alternations of bioclastic marine limestones and lacustrine sediments.

Biostratigraphy of South-Central Lafourche Parish, Louisiana: ABSTRACT

ABSTRACT A biostratigraphic study of south-central Lafourche Parish, Louisiana, was undertaken to solve some of the structural and stratigraphic problems of the area. Wells were examined paleontologically from Valentine, Bully Camp, Golden Meadow, Leeville, and Bayou Raphael Fields in Lafourche Parish. One well was examined from Bayou Jean LaCroix Field in eastern Terrebonne Parish. Results of these paleontological examinations served as the principal source of regional correlations; electrical logs were also used at key locations when samples were not available, or were not collected from high enough in a well for the uppermost regional occurrence of index forms to be observed. A restored stratigraphic dip section illustrating downdip thickening, and a stratigraphic strike section demonstrating the essentially horizontal nature of the beds, were constructed. A structural cross-section through the Golden Meadow Field illustrates a graben and fault pattern typically associated with a deep-seated salt mass. As defined in this study, the Eggerella Wedge and the second Cibicides carstensi zone are useful horizons south of a zone of flexure found between the west flank of the Golden Meadow Field and the south flank of the Bully Camp Field. This zone of flexure may be traced downdip. Sediments above the trace are plate-like continental shelf deposits and are easily correlated; those below the trace are continental slope deposits and extremely difficult to correlate because of the great thickening and gross lithologic changes which take place in this zone.

Comparison of Plio-Miocene Sedimentation of Gulf Coast with Atokan Sedimentation of Arkoma Basin: ABSTRACT

The Plio-Miocene sediments of the Gulf Coast and the Atoka sediments of the Arkoma basin represent similar stratigraphic sequences deposited in quite different tectonic settings. The Plio-Miocene units are associated with the orogenically placid Gulf Coast geosyncline. The pattern of deposition has been development of load-produced basins (depocenters) during cyclic offlap. The Atokan units are associated with the Ouachita orogeny and represent shelf and trough suites. Certain aspects of these stratigraphic sequences are comparable with the modern sediments of the northwestern Gulf of Mexico. The depositional patterns of the Plio-Miocene and the Atokan sedimentary prisms reflect structural-sedimentation interrelationships. In each prism, flexure zones demark abrupt thickening of the sedimentary units. The Atokan sediments were deposited on more competent sub-strata than were the Plio-Miocene sediments so that fewer major flexures developed. The depositional axis of the Atoka probably was tectonically controlled (a facies scarp) and the depositional axis of the Plio-Miocene (if properly located) was determined by sedimentary processes. The Plio-Miocene and the Atoka have other features in common. Each is very predominantly clastic, represents a new area of maximum sediment accumulation in the depositional basin, and displaced a carbonate-shale facies. Prograding deltaic facies dominate the depositional environments but cyclic deposition is a prominent aspect of the sedimentation. Both the Atoka and the Plio-Miocene thicken at comparatively rapid rates and attain greater thicknesses than the associated older and younger sediments. Kuendig (1959) has suggested that geosynclines should be classified by structural configuration, not sedimentary content. However, the similarities of the Plio-Miocene and the Atoka indicate that sedimentary patterns reflect source areas, transport and depositional processes, and topography. They do not reflect the structure of the catchment basin. End_of_Article - Last_Page 279------------

EXPERIMENTAL ANALYSIS OF FRACTURE PATTERNS

All existing fractures can be imitated in four scale models which show tension or shear fractures with or without rotational deformation. These four experiments can easily be produced by deforming soft clay which rests on a moving square of wire cloth. En echelon zones are easily produced by placing clay above the edge of a moving tin plate. Tension and shear fractures open in the flexure zone and are unfailing indicators of relative displacement. Normal or gravity faults occur preferably above domes and anticlines or other areas of distension. Salt-dome patterns may readily be imitated by pushing a plug into a clay model from below. A systematic study of fracture anomalies may prove useful in the detection of areas of uplift above salt domes. Longitudinal and transverse normal faults in North dome of the Kettleman Hills are readily imitated in a clay model and may be due to vertical uplift with or without lateral compression. The en echelon arrangement of the Kettleman Hills is probably not due to north-south movement toward a weak zone trending northwest. True en echelon folds would trend west-northwest but not nearly north-northwest. The echelon arrangement may be only the alternation of folds or may be due to northeast-southwest movements, possibly in combination with subcrustal blocks and faults. The San Andreas-Garlock fault system can be imitated only if considerable rotation has opened the angle between shear planes from near 60° to 90° or to 120°. Asymmetry of the fault pattern suggests clockwise rotation. A more complicated experiment in which three blocks are placed as suggested by Ralph Reed seems to come nearer to a fitting imitation but may still include too large an area. The value of experimentation lies in experience gained by the experimenter rather than in the exact duplication of known fracture patterns. The intense occupation with mechanical principles and the close visible relationship between cause and effect is sobering as well as stimulating.

Supplementary Report

Abstract The leading ideas of Giese, Stein and Lauer's work may be expressed by the following statements. 1. In a rolling wheel-and-tire assembly, there are electric fields: (a) Between road and tire surface. (b) Between outside and inside tire surface. (c) Between inside tire surface and tube. (d) Between inside tire surface and rim. 2. Special attention is devoted to the so-called “inner field” between inside tire surface and tube; an attempt is made to measure and control it by suitable means. Regarding these fields it may be remarked that electrical boundary surfaces, and hence charge accumulations due to intimate contact, occur at the following points : 1. Instantaneous contact area between tire and road. 2. Points between casing and tube, especially in the flexure zone, where casing and tube may be displaced somewhat from each other. In the latter case—that of the field between casing and tube, henceforth to be called the “inner field”, the casing and tube section is to be regarded as a capacitance whose magnitude may be estimated as follows: area about 500 sq. cm., plate interval about 10−3 cm., due to elevations and depressions along casing at cords ; hence, capacitance of 5×104μμf. For very intimate contact, the plate interval may be reduced to about 10−6 cm., in which case the capacitance rises to 5×107μμf.