Wedge-shaped Unit Research Articles

Modular All-Metal Ultrawideband Cylindrical Array for Multifunction Operation

Legacy cylindrical phased array systems are constrained to narrowband operation due to antenna element limitations amid a growing need to support advanced multifunctional platform requirements. In this article, a notch-based all-metal ultrawideband (UWB) antenna element suitable for cylindrical arrays is introduced. The proposed element is designed to operate over 2.75–10 GHz while being highly amenable to a cylindrical architecture via modularized assembly of compact scalable column facets. The primary enabling technical innovations are: 1) a new shunt-loaded stepped-impedance (SI) gap design scheme to improve low-frequency performance without increasing profile and 2) the design of a tongue-and-groove azimuthal crosswall that precisely conforms to the wedge-shaped unit cell for simple yet robust cylinder assembly. For a proof-of-concept demonstration, a 120° sector array is built having a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12^{\prime \prime }$ </tex-math></inline-formula> outer diameter and 21 columns—each comprising eight single-polarized elements (168 total feed points). Measurements are presented for active phase-mode impedance and embedded element patterns of a central element, demonstrating good agreement with full-wave simulations.

Seismic evidence for crustal architecture and stratigraphy of the Limpopo Corridor: New insights into the evolution of the sheared margin offshore southern Mozambique

The Gondwana breakup along the Mozambique continental margin caused the formation of extensional and/or sheared margins. One of these sheared margins offshore southern Mozambique, the Limpopo Corridor (LC), is a key area for understanding the evolution of the southward movement of Eastern Gondwana with respect to Western Gondwana. However, crustal architecture and stratigraphy in this area have been inadequately studied, mainly due to the lack of deep seismic data.In this study, we present interpretations of new multichannel seismic reflection profiles oriented west to east across the LC and Mozambique Fracture Zone (MFZ). Our results show that the basement of the LC is characterized by dipping and alternating strong and weak reflectors, revealing a thick Volcano-Sedimentary Sequence (VSS). To the west of the LC, a marginal basement high zone extends from north to south, consisting of an eroded and fractured VSS. Besides, the acoustic basement is characterized by an eastward dipping or flat VSS in the LC. Sediments overlying the top of the VSS reveal an onlap geometry against the basement high. Five major stratigraphic units (U1-U5) with well-layered sedimentary sequences are mainly constrained from the western boundary of the basement high to the eastern MFZ, and dated to Neocomian, Middle Cretaceous, Late Cretaceous, Paleogene, and Neogene times respectively.We propose that the VSS in the acoustic basement likely formed in pre-Neocomian time. An uplift event occurred continuously from the Oxfordian-Kimmeridgian to Middle Cretaceous, resulting in a basement high that represents the western limit of the LC. Between this limit and the MFZ, the LC experienced the intracontinental rifting, wrench faulting, and discrepant subsidence associated with the southward strike-slip movement of East Gondwana along the MFZ. The earliest sediments overlying the fractured and deformed VSS consist of a wedge-shaped unit (U1) that deposited along the entire LC during the active transform stage. When the spreading centre passed the adjacent segment of the LC in the Mozambique Basin, the depocentre rapidly shifted to the basin, and the LC entered a passive transform margin stage. We propose that the LC results from three successive stages: (1) intracontinental rifting stage with substantial uplift and erosion, (2) active transform margin stage with strike-slip movement along the MFZ, and (3) passive transform margin with a different sedimentary setting.

Structure and development of the Changliangshan ductile shear zone, North Tibet: implications for the initial closure of the Paleo-Tethys Ocean in the central Qiangtang region

The Changliangshan ductile shear zone extends ~80 km in an east–west direction in central Qiangtang, Tibetan Plateau. Structural and kinematic analyses show that the shear zone comprises mainly protomylonite, mylonite, and phyllite with dextral-dominated strike–slip motion in the northern branch of the shear zone and sinistral strike–slip shear in the southern branch. Concordant U–Pb ages on the metamorphic rims of zircons from mylonite indicate that deformation along the shear zone started at 249.1 ± 6.7 Ma during the initial collision between the North and South Qiangtang terranes. The 40Ar/39Ar ages of sericite (221–215 Ma), combined with available kinematic and structural data, indicate that the metamorphic rocks of the shear zone experienced transpressional exhumation and uplift during the Late Triassic. In addition, the E–W-trending Changliangshan ductile shear zone marks the contact between Yangtze-type (warm-water) and Gondwana-type (cold-water) deposits. The initial NE–SW-directed collision gave rise to shearing along the Changliangshan ductile shear zone under lower- to middle-greenschist facies conditions during the Early–Middle Triassic. Subsequently, the Yangtze-type sediments passively pierced the Gondwana-type deposits as a wedge-shaped unit along the shear zone. Furthermore, the data reveal that the Changliangshan shear zone formed during oblique convergence between the North and South Qiangtang terranes during initial closure of the Paleo-Tethys.

New insights into the tectono-stratigraphic evolution of the Malvinas Basin, offshore of the southernmost Argentinean continental margin

Abstract A detailed tectono-stratigraphic analysis of the Malvinas Basin was achieved by the interpretation of around 65,000 km of 2D seismic reflection profiles. Five main seismo-stratigraphic units and their sub-units, informally named units U1 to U5 a/b, bound by major unconformities were identified and correlated with the Mesozoic to Cenozoic main tectonic phases of the basin. Unit U1 (Pre-168 Ma) represents the seismic basement. Unit U2 (168–150.5 Ma, syn rift phase) thickens and deepens southwards. Units U1 and U2 are affected by several syn-rift normal faults that exhibit a main NE–SW strike direction in the south of the basin and a NW–SE strike direction in the centre of the basin. This suggests that the Malvinas Basin may have developed initially as a rift basin with two different extensional directions: (1) a NW–SE directed extension probably linked with the opening of the Weddell Sea (Early Mid-Jurassic), and (2) a NE–SW directed extension most likely linked with the Jurassic back-arc extension of Gondwana and probably later with the onset of the opening of the South Atlantic during Late Jurassic to Early Cretaceous time. Unit U3 (150.5–68 Ma, sag phase) is mainly an aggradational wedge-shaped unit. Unit U4 (68–42.5 Ma, transtensional foredeep phase) overlies unconformable unit U3. It deepens in the south because of an N–S/NW–SE directed extensional regime. Accumulation rates decrease during units U2, U3, and U4 from 4.84, to 1.23 to 0.8 km3/Ma × 103. Units U5a and U5b (42.5–5.5 and 5.5–0 Ma, transpressional foredeep phase) finally represent a change from aggradation to progradation sedimentary pattern and to a left-lateral transpressional regime in the south. The sediment supply was considerably higher than before and a thick sedimentary wedge has been deposited until today. Accumulations rates increased in units U5a and U5b from 2.28 to 8.91 km3/Ma × 103.

Geological constraints on the eruption of the Jwaneng Centre kimberlite pipe, Botswana

Geological mapping has allowed constraints to be placed on the eruption mechanisms involved in the formation of the Late Permian–Early Triassic Jwaneng Centre kimberlite pipe, Botswana. Twelve lithofacies and three lithofacies associations (LFA 1–3) are recognised. LFA 1 comprises massive to bedded volcaniclastic kimberlite and marginal shale breccias and outcrops over 65% of the surface area of the pipe. It is characterised by a lithic population dominated by Transvaal shale clasts. LFA 1 grades into LFA 2, which comprises massive and bedded volcaniclastic kimberlite and volcaniclastic breccias and outcrops over 19% of the surface area of the pipe. The lithic population of LFA 2 is dominated by contorted and fluidal-outlined Karoo-age mudstones and siltstones. LFA 3 comprises a wedge-shaped unit in the north of the pipe and consists of a series of allochthonous megablocks of sedimentary and volcaniclastic strata. The juvenile clast type and matrix mineral assemblages of the volcaniclastic deposits in the Centre Pipe differ from those in many other southern African kimberlite pipes. Various emplacement models for kimberlite pipes are discussed and evaluated in the light of the new geological data. Both a maar–diatreme model and an explosive volatile-driven eruption model could account for much of the geology of the Centre Pipe and distinguishing between the two models based on deposits alone is difficult. There is strong circumstantial evidence for ambient conditions favourable to phreatomagmatism at the time of the eruption, and the influence of external water may explain the differences between the Jwaneng Centre Pipe and the Class 1 kimberlites common across Southern Africa in terms of both the juvenile clasts and of the inter-clast cement. However, low abundances of some types of lithic inclusions derived from major country rock units pose an unresolved problem for a classic maar–diatreme model of pipe formation.

Crustal structure across the Pacific margin of Nicaragua:evidence for ophiolitic basement and a shallow mantle sliver

Seismic wide-angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots. The structure and the P-wave velocity of the subducting Cocos and overriding Caribbean Plates were determined by modelling wide-angle data and further constrained by coincident seismic reflection, borehole and gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic sediment cover. The plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network. A major feature of the upper plate is an 80-km-wide high-velocity, high-density rock unit reaching from the front of the margin to about the middle of the shelf. This wedge-shaped unit is 15 km thick beneath the shelf edge and is composed of a 5-km-thick upper part with velocities increasing from 3.5 km s−1 near the trench to 5.2 km s−1 at the shelf break overlying a 10-km-thick lower part with velocities of 6.0 km s−1. Analysis of the gravity field requires densities of 2.6–2.7 g cm−3 for the upper part and 2.9 g cm−3 for the lower part of this unit. These velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore Guatemala and exposed on the Nicoya Peninsula, Costa Rica. The velocity structure of this ophiolitic unit ends about 50 km offshore. Landward, the basement underneath the Sandino Basin is formed by older igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Nicaragua. Beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12–18 km and coincides with the down-dip limit of the seismogenic zone. Mantle densities are required for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Farallon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with the late Cretaceous to Palaeocene deep-water sediment of the Rivas Formation at the base of the Sandino Basin. Suture of the Farallon Plate and Chortis Block might have initiated the strike-slip movement along the Motagua–Polochic Fault System. The development of a new subduction zone in Eocene–Oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the Chortis Block and shifted the volcanic arc about 70 km southwestwards, close to its present position.

Orogenic mass transfer and reflection seismic patterns — evidence from DEKORP sections across the European Variscides (central Germany)

Orogenic wedges evolve by different mass transfer modes which control different styles of deformation, metamorphic pattern, and bulk reflectivity. Structural patterns and reflectivity strongly vary depending mainly on the length/thickness ratio of thrust sheets and on their displacement/length ratio. Reevaluation of DEKORP seismic sections across the central European Variscides (lines DEKORP 1A-C, 9N, 2N-2S, 3B) underlines the usefulness of the integrated interpretation strategy in establishing orogenic growth mode and evolution. The results are in contrast with reflectivity patterns predicted from critical taper theory. The northern flank of the Mid-European Variscides is formed by a doubly vergent orogenic wedge system which evolved from collision of a passive margin (Rhenohercynian zone) with a continental arc (Mid-German Crystalline Rise, MGCR, of the Saxothuringian zone). The two fold and thrust wedges show inward-dipping shear zone-related reflections above subhorizontal mid-crustal reflection bands, evidence of dominantly frontal accretion of upper crustal material in both wedges. In contrast, the sections crossing the axial zone (MGCR) either show arched reflection stacks related to underplated amphibolite-facies rocks in exposed antiformal nappe stacks, or a transparent wedge-shaped unit related to low-grade calcalkaline magmatics and steeply dipping Devono-Carboniferous sediments of the former upper-plate magmatic arc. Analysis suggests that part of this arc was tectonically eroded prior to collision. In combination with isograd and structural patterns the observed features indicate that heterogeneous basal accretion was the dominant accretion mode in the axial zone during collision. The discussed patterns are suggested to reflect the material transfer site into the orogenic wedge and therefore also growth mode and material provenance in collisional belts.

Facies architecture and geometry of a prograding carbonate ramp during the early stages of foreland basin evolution: Lower Eocene sequences, Sierra del Cadí, SE Pyrenees, Spain

Abstract This paper describes the Early Eocene sedimentological evolution of the Sierra del Cadí carbonate platform during the early development of the southeastern Pyrenean foreland basin, Spain. The platform developed as a south-facing ramp system located either across the southern margin of a growth anticline within the newly formed basin or along the axial margin of the basin. Sequences I and II both comprise basal transgressive systems tracts (TSTs) dominated by a thick succession of outer ramp lime mudstones, shallowing upwards into a highstand systems tract (HST) of mid- and inner ramp bioclastic shoals, sand banks and lagoonal deposits. In Sequence III the TST comprises a thick, wedge-shaped unit of east to west backstepping, inner and mid-ramp Nummulites shoals and lagoonal deposits, progressively overlain by a thick succession of outer ramp lime mudstones and marls of the HST. No lowstand systems tracts are recognized. Sequence boundaries correspond to flooding surfaces which grade basinwards into correlative conformities, interpreted to represent the onset of high tectonic activity in the orogenic wedge and resulting high flexural subsidence across the foreland and drowning of inner ramp regions. TSTs represent continued high flexural subsidence creating accommodation space infilled by high rates of outer ramp sediment accumulation. In Sequences II and III, TSTs show a pronounced west to east thickening, interpreted to represent the effect of variable subsidence across the margins of an uplifting growth anticline within the otherwise flexurally subsiding foreland. Progradational successions that characterize HSTs developed during periods of reduced tectonic subsidence, both within the foreland and in the orogenic belt, allowing the growth and expansion of carbonate ramp facies belts.

Ambiente sedimentario y edad de la unidad superior de brechas de la Cuenca de Quesa (provincia de Valencia)

The miocene deposits of the Quesa basin (Betic foreland) outcrops in the central part of the Valencia province (Spain). Quesa basin is a subsiding hanging-wall basin related to an ENE-WSW listric fault system. The deposits of this basin are formed by a 440 metres thick sequence of red clays, containing few intercalations of sandstones and micritic white limestones, and breccias. The upper 70 metres of this sequence is a wedge-shaped unit composed by breccias containing megablocks, wich interfinger with tabular limestones towards the central part of the basin. The breccias are foot-wall derived debris fiow deposits formed during normal faulting. A mammal site close to the base of the breccias (Quesa 2), has yielded a fossil micrommamal association of Middle to Upper Aragonian age. This allow us to consider as Middle Miocene the sediments and tectonic events in Quesa Basin.

Pollen morphology and ultrastructure of the corystospermales: permineralized in situ grains from the triassic of Antarctica

Corystosperms, represented by Dicroidium leaves and Pteruchus-like pollen organs, are major components of the Early-Middle Triassic silicified flora from the Fremouw Formation of Antarctica. The micromorphology and ultrastructure of the in situ pollen contained within these organs are described. Pollen sacs of varying ontogenetic ages have been isolated. Mature grains are monosulcate and bisaccate, with large, crescent-shaped eusacci. The exine is relatively thick in the cappa region and thins toward the distal sulcus; surface ornamentation is psilate. In medial positions of the proximal wall, the exine is homogeneous but becomes tectate-alveolate in more lateral regions of the cappa. The alveolar units extend into the sacci forming an endoreticulum; however, the endoreticulations are discontinous and only attach to the outer walls of the sacci. A wedge-shaped unit, where the sacci attach to the corpus, characterizes both the proximal and distal poles. The sulcus is broad, extends the entire width of each grain, and is longitudinally flanked by elevated lips. The structural features of these grains are discussed with respect to other fossil and extant saccate pollen. The grains are systematically compared with those of other bisaccate pollen-producing plants with which the Corystospermales have been suggested to be closely related, including Glossopteridales, Caytoniales, and angiosperms (Lactoridaceae). The permineralized in situ grains are also compared with other compressed Pteruchus species known at the ultrastructural level and with morphologically similar dispersed palynomorphs known from Antarctic sediments.

Evidence for Wisconsinan Glaciations in the Verrill Canyon Area, Scotian Slope

AbstractA composite thickness of about 25 m of sediment has been cored from the Verrill Canyon on the Scotian Slope. It is interpreted that the majority of this sequence was deposited in a glaciomarine environment during oxygen isotopic stage 2 and the top of stage 3. These sediments, as seen in high-resolution seismic reflection profiles, are well stratified, become thicker upslope, are laterally variable in thickness, and pass upslope into possible outer shelf tills. Three wedge-shaped units of incoherent reflections interfinger with the parallel reflections and terminate in water depths greater than 700 m. These wedge-shaped units are interpreted as slumped diamict and outwash deposits. The age of the uppermost wedge-shaped unit is 26,000–21,000 yr based on extrapolation of radiocarbon dates. This unit documents a late Wisconsinan glacier readvance on the outer Scotian Shelf. The underlying wedge-shaped unit, estimated to be 70,000 yrs old, extends further west along the continental slope, and may represent a more extensive early Wisconsinan ice advance. A third wedge-shaped unit, inferred to have formed during isotopic stage 6, is possibly a remnant of the first glaciation in the study area.

The Fulmar Oil-field (Central North Sea): geological aspects of its discovery, appraisal and development

The Fulmar Oil-field (Central North Sea): geological aspects of its discovery, appraisal and development

Deformation of poorly consolidated sediment during shallow emplacement of a basalt sill, Coso Range, California

A 150-m-long, wedge-shaped unit of folded and faulted marly siltstone crops out between undeformed sedimentary rocks on the north flank of the Coso Range, California. The several-meter-thick blunt end of this wedge abuts the north margin of a basaltic sill of comparable thickness. Chaotically deformed siltstone crops out locally at the margin of this sill, and at one locality breccia pipes about one meter in diameter crosscut the sill. The sill extends about 1 km south up the paleoslope, where it merges through continuous outcrop with a lava flow that in turn extends 1.4 km to a vent area marked by more than 100 m of agglutinate and scoria. Apparently, lava extruded at this vent flowed onto unconsolidated sediments, burrowed into them, and fed a sill at about 40 m depth within the sedimentary sequence. The sill initially propagated by wedging between sedimentary beds, but eventually began to push some beds ahead of itself, forming a remarkable train of folds in the process. The sediments apparently were wet at the time of sill emplacement, because hydrothermal alteration is common near the contact between the two rock types and because the breccia pipes that crosscut the sill apparently resulted from phreatic explosions of pore water heated at the base of the cooling sill. Comparison of deformation of the host material at the Coso locality with that reportedly caused by emplacement of sills elsewhere indicates that the character of deformation differs greatly among the various localities. The specific response of host material depends upon such parameters as initial properties of magma and host material, rate of sill growth and attendant rate of strain of host material, and depth of sill emplacement. Some properties may change considerably during an intrusive-deformational episode, thus complicating accurate reconstruction of such an event.

Depositional Systems in Naknek Formation, Late Jurassic, Lower Cook Inlet, Alaska: ABSTRACT

The Naknek Formation is a thick wedge-shaped unit of Late Jurassic age. It accumulated along the western edge of a Jurassic fore-arc basin. The formation was examined by BP Alaska geologists along the western shore of Lower Cook Inlet between Chisik Island and Cape Douglas. Correlation of stratigraphic sections has identified three sandstone/conglomerate sequences separated by two siltstone units. The conglomerate units are thick, lenticular, and occur as both organized and disorganized conglomerate facies. The sandstone units exhibit the following sedimentary structures and bedding types: (1) massive, graded, and amalgamated beds; (2) thickening and coarsening-upward sequences; (3) slump structures; (4) load and flute casts; (5) soft-sediment faulting; (6) rip-up clasts; (7) ripple lamination; and (8) minor cross-bedding. The siltstone units are parallel laminated and have thin sandstone beds with partial and complete density current sequences. Bioturbation is not common although some grazing burrows are present. Ammonites, belemnites, plant debris, and radiolarians are the most common fossils, although forams, gastropods, and pelecypods (dominantly Buchia sp.) are also present. Previous workers have described the Naknet Formation as a shallow-marine deposit, but did not identify specific sedimentary facies. This study indicates that the Naknek Formation was deposited in moderate to deep water as a submarine fan complex. End_of_Article - Last_Page 551------------