Oblique Structures Research Articles

Numerical analysis and Schlieren visualization of gas flow dynamics inside the plasma arc cut kerf with curved cutting fronts

In plasma arc cutting (PAC), the characteristics of the gas flow exiting the nozzle significantly influence the cut quality. The high-pressure jet in PAC forms complex shockwave structures when it is incident on the metal to be cut. In this study, the flow behavior inside the kerfs of various geometries derived from an actual PAC workpiece is assessed. The shape of the cutting front in the kerf varies with the changing cutting speed. A high cutting speed yields a curved cutting front, resulting in unwanted gas flow behavior and adversely influencing the cutting performance. In this study, a computational fluid dynamics simulation model was used to analyze the effects of a curved cutting front on the gas flow behavior during the PAC process. The gas flow patterns obtained from the numerical simulations were qualitatively compared with the Schlieren experiment results. The analysis results indicated that the curvature of the cutting fronts generated oblique shockwave structures that significantly reduced the flow velocity. In particular, the weak shock structures throughout the curved cutting front gradually decreased the flow velocity. The critical flow velocity was realized in the kerf with a highly curved cutting front, beyond which the vertical penetration of the material was not possible. The shear stress lines concurred with the striation patterns on the kerf walls, thereby validating the numerical analysis results.

Complex Martinique Intermediate‐Depth Earthquake Reactivates Early Atlantic Break‐Up Structures

AbstractEarthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth.

Bragg diffraction of higher orders on oblique helicoidal liquid crystal structures

ABSTRACT For the oblique helicoidal (ChOH) structure of the chiral twist-bend nematic-forming mixture of CB7CB/CB6OCB/5CB doped by light-sensitive chiral compound based on azo-fragment, at oblique incidence of light two consequent states of Bragg reflection in the visible spectral range from 400 to 750 nm were experimentally observed in the course of decreasing the applied electric field. These states were assigned to different orders of Bragg’s diffraction occurring at ChOH layers with full pitch periodicity P. While the second and third orders of Bragg’s diffraction can be easily observed in conditions of standard electrooptic cell, the first order could be revealed only at very high electric field near the ‘red’ edge of the visible spectral range. For the second and third orders of diffraction there is a narrow range of electric field where both diffraction orders are observed (so-called hybrid state), with transmittance spectra of the ChOH structure showing two peaks at the short-wavelength and the long-wavelength edges of the visible spectral range. Model calculations of Bragg’s reflection of light were carried out using literature values of elasticity constants and dielectric permittivity, and the calculated plots of the selective reflection maximum vs. voltage were in good agreement with experimental data.

Oligocene to Miocene magmatic and hydrothermal evolution in the Maricunga belt (Región de Copiapó, Chile): A new look to a long known metallogenic belt

The Maricunga metallogenic belt, herein defined to encompass the region from 25° S to 28° S along the Cordillera Frontal of northern Chile, is known for hosting a series of gold-copper and lesser copper-molybdenum porphyries and gold-silver epithermal systems that are hosted in a segment of the late Oligocene-Miocene Andean calc-alkaline magmatic arc. Based on an up-to-date geochronological database, four episodes of magmatism and related hydrothermal deposits are proposed to characterize intensity and style of volcanism and hypabyssal intrusive rocks over time. The late Oligocene episode (26-21 Ma) is characterized by stratovolcanoes, dome complexes, pyroclastic sequences, and hypabyssal intrusives of andesitic to dacitic composition that host gold-rich porphyry systems such as Caspiche, Maricunga, and La Pepa. The early Miocene episode (21–17 Ma) has similar rock types but is volumetrically reduced, hosting epithermal and porphyry deposits like La Coipa and Caserones, respectively. A reinvigoration of volcanism occurred during the third stage between 17 and 11 Ma, giving rise to large volcanic complexes and intra-to extra-caldera ignimbritic deposits that host several gold-rich porphyry deposits such as Lobo-Marte, Luciano, and Cerro Casale; as well as epithermal deposits such as Fenix. Some of the 17 to 11 Ma and younger igneous rock belong to the high-K calc-alkaline series, showing a higher alkali content compared to previous periods. The last magmatic episode, from 11 to 5 Ma, coincides with the shallowing of the subduction angle and is characterized by an eastward migration of volcanism, and more heterogeneous rocks including trachytic compositions. During this period mineralization was emplaced at the Salares Norte epithermal deposit and El Volcan porphyry Au–Cu deposit.Porphyry and epithermal systems formed throughout the evolution of the Maricunga belt. Overprint of advanced argillic alteration on porphyry style alteration is widespread albeit commonly devoid of significant addition of epithermal Au–Ag mineralization. The Au-rich nature of the majority of the porphyry systems is attributed to the shallow crustal depths of emplacement compared to Cu- and Mo-rich porphyry systems. Conversely, high-sulfidation epithermal Au–Ag deposits such as La Coipa and Nueva Esperanza are not known to be telescoped on coeval porphyry gold deposits.The location of the mineralized centers was favored by crustal weaknesses expressed by intersection of oblique- and parallel-to arc structures, the former inherited from the construction of the continent before to the formation of the Andean range. The arc oblique structures also segment the Maricunga belt into domains with unique magmatic and hydrothermal styles.

Effect of oblique angle on the formability, crystal growth behavior and microstructure in laser direct energy deposited thin-wall structure of single-crystal superalloy

Laser directed energy deposition process shows a tremendous potential to additively repair and manufacture single-crystal components in a near-net shape. For the specific non-vertical structure such as the tilted and torsional sidewalls of single-crystal components, achieving complete monocrystalline nature in the manufactured zone is still a great challenge and hazy. In this study, oblique thin-wall structures of single-crystal superalloy were fabricated by coaxial laser directed energy deposition process. The effect of oblique angle on the formability, crystal growth behavior and microstructure formation in thin-wall structures of single-crystal superalloy was discussed. The results showed that the critical oblique angle for good formability of thin-wall structure is 30°. A poor-quality and shapeless deposited bead forms once the oblique angle researches 40°. The thin-wall structure inherits the [001]/<100> crystalline orientation from parent single-crystal substrate through the continuous epitaxial growth of ultrafine columnar dendrites. The competition and alternate development of primary trunks-secondary arms-tertiary arms achieve the multiplication and reproduction of ideal columnar dendrites along the [001]/<100> crystalline orientation in the overhanging region of the thin-wall structure. Periodic segregation forms among the columnar dendrites. Epitaxial-growth continuity of the columnar dendrites with consistent crystalline orientation of substrate can be successfully achieved in large-scale tilted and torsional thin-wall structures of single-crystal superalloy with proper processing parameters. These findings are of great importance in the microstructure control for additively manufacturing complex single-crystal components by laser directed energy deposition process.

Phase field crystal study on the size and strain rate dependent evolution of Kirkendall voids in binary alloy

The evolution of Kirkendall voids in binary alloy under a uniaxial strain rate loading is investigated by the phase field crystal (PFC) model. The simulation results show that model size and the applied strain rate greatly affect the formation and growth of Kirkendall voids. It is found that the grains are prone to rotate when the model size is smaller than 34.7a×34.7a (a is the lattice constant) and the initially set grain boundaries will disappear after relaxation, no Kirkendall voids form in this case. As the model size increases, Kirkendall voids arise and interconnect with each other, and large voids will dissociate to produce small voids. Moreover, the applied strain rate also affects the formation and growth path of Kirkendall voids. Voids only form at the grain boundaries in the absence of strain rate loading, whereas voids form both inside the grains and at the grain boundaries under strain rate loading. The growth of voids can easily form oblique structures caused by dislocation slip, and they tend to grow perpendicular to the direction of applied strain rate.

Origin of oblique structures controlled by pre-tectonic thickness variations in frictional and salt-bearing fold-and-thrust belts: Insights from analogue modelling

This work includes, first, a synthesis of experimental analogue modelling programs assessing the influence of along-strike thickness variations in the development of oblique structures in fold-and-thrust belts. As a second part, we present a new experimental program testing, systematically, the influence of tapered décollement-cover thrust wedges.Previous experiments show that frictional, brittle models feature numerous, short-wavelength structures where the cover is thinner. Deformation fronts are totally or locally oblique to the backstop when the cover thinning is progressive or sharp, respectively. Low (<30°) or higher structural obliquities occur when thickness variations are progressive or sharp, respectively. The addition of a basal or intermediate décollement, commonly entails the transference of deformation towards its external pinch-out. Consequently salients occur and transfer zones and oblique structures form over décollement boundaries. Their location and orientation strongly depend on the pre-compressional shape of the décollements. Furthermore, as demonstrated by our modelling results, tapered brittle covers over viscous décollements result in asymmetric thrust wedges formed by structures that end or change their vergence laterally, resulting in obliquities generally <30°.Two natural case studies, showing strong (South Pyrenean Central Salient) or moderately oblique (Keping Shan fold-and-thrust belt) structures are revisited and compared to the described analogue models.

Analogue modelling of the Ainsa oblique zone in the Southern Central Pyrenees, Spain

The Pyrenees are a collisional orogen striking WNW-ESE. However, in the Ainsa oblique zone in the south-central part of the Pyrenees, faults and folds rotate to ∼N–S over a westward thinning evaporitic layer. The influence of lateral change in basal friction due to declining evaporitic layer thickness in a fold-and-thrust belt is studied by analogue modelling simulating the Ainsa oblique zone. In the experiments the transition zone is simulated by a viscous silicone mixture sheet that laterally changes in thickness adjacent to frictional-plastic quartz-sand. The models confirm that a transition between weak and strong basal friction causes oblique structures to develop. Weak detachment zones are prone to fault propagation whereas strong detachment zones are prone to imbrication. At the transition zone those structural styles are combined, resulting in a complex domain. The differences in deformation style cause dextral shear and clockwise rotation in the transition zone and larger horizontal deformation propagation in the weak basal friction zone. Structures produced in the analogue models are compared to structures in the Ainsa oblique zone and found to be representative. Therefore, this study shows that transition between weak and strong detachment layers can explain the regional structural deformation style in the Ainsa oblique zone.

Structured input–output analysis of oblique laminar–turbulent patterns in plane Couette–Poiseuille flow

In this work, we employ structured input–output analysis to study the flow patterns in transitional plane Couette–Poiseuille flow (CPF). First, we focus on the well-studied intermediate laminar profile, which balances the shear and pressure effects. We show that the highest structured gain corresponds to perturbations with wavelengths associated with the oblique turbulent bands observed in experiments. In addition, the inclination angles of these structures show a Reynolds number dependence consistent with experimentally observed trends. We then examine the Reynolds number scaling of the maximal structured frequency response as the velocity profile is varied from plane Couette to Poiseuille flow. Our results demonstrate that, as expected, the scaling exponent increases over this range, but this increase is not monotonic. We attribute the variation in the shape of the scaling exponent curve as a function of the velocity profile shape to changes in flow patterns that dominate each associated flow regime. We then focus on the particular case of plane Couette flow and compute the structured response modes. Their behavior and structural features are consistent with results obtained through direct numerical simulation (DNS) studies. Finally, we employ the structured analysis framework to examine the temporal evolution of the dominant structures. For the well-studied cases of plane Couette and plane Poiseuille flows, the computed advection speeds of the oblique structures are consistent with those predicted through DNS.

A fast and efficient mesh stiffness model for spur gears considering thin-walled or oblique web

Time-varying meshing stiffness has an important contribution to the vibration control in gear systems. However, the design of gears in the electric vehicles and aerospace field is more focused on lightweight. The accurate and efficient evaluation of the time-varying meshing stiffness of lightweight spur gears has become a challenging problem, such as considering the thin-walled or oblique web. Therefore, a new approach is proposed to calculate the mesh stiffness of the lightweight spur gear by combining an analytical model and the dynamic model of the gear pair. The gear model is constructed by adopting the shell element to save computer resources and model preparation time. The validity of the presented model is investigated by comparing it with the traditional analytical and finite element models. The analytical results indicate that the proposed model has the same level of computational efficiency as the analytical model while maintaining the precision of the finite element model. The advantages of the proposed method are the ability to consider the effects of thin-walled web and oblique spoke plate structures on the mesh stiffness.

Interaction of Segmental Tunnel Linings and Dip-Slip Faults—Tabriz Subway Tunnels

In some subsurface urban development projects, bedrock faults intersecting with the tunnel path are inevitable. Due to the high costs of urban tunnel projects, it is necessary to study the behavior of such concrete structures under fault movement risks. Using an advanced 3D numerical finite difference code and a plastic hardening constitutive model for the soil, this paper examined the performance of the straight and oblique segmented structures of Tabriz Subway Line 2 under large deformations. The Tabriz Line 2 tunnel passes through a reverse fault called the Baghmisheh Fault. The fault–tunnel simulations were validated by centrifuge tests on the segmental tunnel for normal faulting. In the centrifuge tests and validation models, there was a maximum difference of 15%. According to the results of the Tabriz Line 2 tunnel under reverse faulting, segmental structures outperform no-joint linings when it comes to fault movement. During reverse fault movement, line 2 segments did not collapse but showed slight deformations. However, continuous structures collapsed under faulting, i.e., the structural forces created exceeded the section strength capacity. Among the segmental structures, the lining with oblique joints showed better behavior against faulting than the lining with straight joints. For better tunnel performance under fault movement, oblique joints should be used in segmental structures in faulting areas.

Seismic Behaviour of CFST Space Intersecting Nodes in an Oblique Mesh

The design of intersecting nodes in high-rise oblique mesh structures is a critical issue. The existing research on the intersecting nodes of oblique meshes mainly focuses on plane intersecting nodes and monotonic axial compression loads. The plane intersecting nodes cannot consider the contribution of the node’s out-of-plane angle and floor beam to the node’s out-of-plane stiffness in actual structures. In this paper, numerical analysis using ABAQUS was conducted to investigate the mechanical performance of space intersecting nodes of oblique meshes (OMSIN) under cyclic axial tension and compression loads, to provide a reference for the engineering application of oblique mesh structures in seismic regions. Six parameters were considered: the space intersecting angle, the plane angle symmetry coefficient, the plane intersecting angle, the out-of-plane constraint restraint, the steel content of the cross-section, and the concrete strength. The study showed that changes in the thickness of the steel tube wall are unfavourable for the uniform transmission of stress. Increasing the space intersecting angle significantly weakened the seismic performance, and the space angle affects the failure mode of the node. Asymmetric arrangements of the upper and lower plane angles caused nonlinear development of out-of-plane. The ultimate load and overall compressive stiffness of the specimen were positively correlated with the plane angle, and vertical constraints should be applied to the node position of components with plane angles greater than or equal to 70°. The out-of-plane constraint was a key factor affecting the seismic performance of the node, and it was proportional to the ultimate load of the component. In structural design, if the aim is to improve the mechanical performance of the component by increasing the steel content, more enormous out-of-plane constraints should be set to control plane external displacement strictly. The concrete strength is proportional to the ultimate axial load and axial stiffness, and its influence on the mechanical performance in the axial tension direction is not significant. Finally, a dimensionless skeleton curve model of the node was established. The existing formula for the bearing capacity of CFST columns was fitted to obtain the calculation formula for the axial yield and ultimate load of the OMSIN under cyclic loads.

Tectonic framework of surface and blind structures from neotectonic and geophysical (gravimetry) analyses, Central Andes of Argentina

This work describes new evidence of N–S and oblique trending surface and blind structures in the La Cantera-Gualilán tectonic depression in the Central Precordillera, San Juan Province, Argentina. Two main N–S Quaternary thrusts cross longitudinally the valley located ∼60 km west of San Juan city (30° 50′- 31° 17′ S and 68° 55′- 69° 05′ W). One of them is located in the eastern piedmont of the Sierra de La Invernada (San Juan Fault) and the other in the western piedmont of the Sierra de La Cantera (La Cantera Fault System), although the northward continuation of this structure has not yet been confirmed.Besides, a smooth water divide or “portezuelo” controlled by an oblique transpressive structure (Divisadero Coba Rubia Fault) separates two N–S elongated river basins with opposite water flow directions. One of these is the Gualilán River, which flows from south to north, and the other corresponds to the La Cantera River, running from north to south.In the present work, we describe some new natural exposures and subtle geomorphological evidence of Quaternary structures identified in the Gualilán-La Cantera tectonic depression. We also use regional and local gravity data to analyze and interpret gravity anomalies related to these surface or blind active structures. Then, based on our geologic and geophysical interpretations, we infer the presence of several, buried cross-strike structures with NW and NE trends, one of them coincident with the drainage divide between the Gualilán and La Cantera opposite river basins, possibly related to the Coba Rubia Fault. Also, we suggest the northward continuation of the La Cantera Fault System (LCFS) as a buried structure in coincidence with the Gualilán River alluvial plain. In this sense, the gravity response of the basement and these river sub-basins shows a close relationship with the surface and subsurface structures. We propose that the oblique structures could represent pre-existing crustal fabrics reactivated during the Andean orogeny, segmenting the N–S Precordillera fold and thrust belt system. Finally, according to the analyzed neotectonic and gravimetric features, we consider the La Cantera-Gualilán Valley as a potentially seismogenic source in this intraplate portion of the Andean Precordillera, crossed by several N–S and oblique surficial and buried/blind active structures. These tectonic features could play a significant role in a regional seismic hazard assessment.

Oblique helicoidal state of the twist-bend nematic doped by chiral azo-compound

ABSTRACT A novel light-sensitive chiral dopant ChD-3816 (an azo-compound containing 4-hexanoyloxyphenyl and 2-isopropyl-5-methylcyclohexylbenzoate moieties) was used for inducing helical twisting in Ntb-forming mixtures of CB7CB/CB6OCB with 5CB added to decrease the phase transition temperatures. The effects of ChD-3816 upon phase transition temperatures, as well as effects of its concentration on the measured values of helical twisting, were determined. Most of the measured parameters could be varied due to light-induced trans-cis isomerisation of ChD-3816. Under electric field, selective reflection spectra in the visible range were obtained for the emerging oblique helicoidal cholesteric structures (ChOH), with the wavelengths controllable both by electric field and appropriate UV irradiation. Possible applications for dynamic formation of contrast images are discussed.

Role of inheritance during tectonic inversion of a rift system in basement-involved to salt-decoupled transition: analogue modelling and application to the Pyrenean–Biscay system

Abstract. The reactivation of former rift systems and passive margins during tectonic inversion and their incorporation into fold-and-thrust belts result in significant structural differences not only between internal and external domains, but also along-strike. The Basque–Cantabrian and Asturian systems are among the best examples to address the role of along-strike changes in rift inheritance since they show a transition from salt to basement-inherited structures divided by a transition zone separating thick- from thin-skinned structural domains. While both domains have been widely described in the literature, the transfer system separating the two has not been sufficiently investigated due to poor seismic imaging and the lack of large-scale outcrops. This contribution aims to address the linkage between basement-controlled (i.e. thick-skinned) and salt-decoupled (i.e. thin-skinned) domains and to describe how deformation is accommodated in the transitional zone between these domains. An experimental programme based on analogue models has been designed that was inspired by the transition from the thin-skinned Basque–Cantabrian Pyrenees to the east to the thick-skinned Asturian Massif to the west. As observed in nature, experimental results show that oblique structures (at low angle with the shortening direction) form in the transitional domain, and their location depends on the linkage of the active structures occurring in both surrounding thick- and thin-skinned domains at different positions. Nevertheless, their orientation and evolution are controlled by the underlying decoupling horizon (i.e. salt). The deformation in the thick-skinned domain produces significant topography over a narrow deformation area due to the lack of effective decoupling levels. On the contrary, deformation in the thin-skinned domain is more distributed due to decoupling, resulting in a wider deformation area of less topography. As a result, syn-contractional sedimentation occurs mainly in the foreland basin in front of the thick-skinned domain, whereas it is observed in the foreland but also in piggyback basins in the thin-skinned domain.

Generation of a net flow due to fixed oblique beam structures in the nucleate boiling region

Effective utilization of unused heat below 200 °C is essential for a sustainable society. In this study, we propose a thermally driven water pump using fixed oblique beam structures with bubbles in the nucleate boiling region (approximately, 100–130 °C). Here, the oblique beam structure breaks the symmetry of the bubble force, and thus, they provide a net flow. Specifically, by using six fixed oblique beams along a circular fluidic channel, we observed a net flow of an average flow velocity of ∼40 mm/s and an average volume flow rate of ∼10 000 mm3/s (∼0.01 l/s) at the superheat of ∼22 K. Our findings should contribute to the effective use of unused heat such as factory waste heat and environmental energy.

Imaging Radar Observations of the Daytime F‐Region Irregularities in Low‐Latitudes of China

AbstractHainan COherent scatter Phased Array Radar has recorded four patches of daytime F‐region field‐aligned irregularities on 31 May 2021, and it is the first time to investigate the daytime irregularities by the radar imaging technique. In‐beam radar images are plotted on the curved plane transverse to the geomagnetic field lines. The displayed daytime irregularities present the obliquely elongated band‐like structures and the corresponding scatter regions extend to hundreds of kilometers in range and narrow to a few kilometers in the zonal direction. The diffuse and isolated structures were also observed in the radar images. The radar image sequences indicate that the irregularities were continuous in height as they drifted westward. The daytime irregularities maintained the oblique elongated structures that resemble the morphological features of the nighttime equatorial plasma bubbles. Combined with the in situ satellite measurements, these daytime F‐region irregularities are interpreted to be the remnants of the previous night's bubbles and come from the higher altitudes near the magnetic equator.

Revision of Middle Devonian stromatoporoid Climacostroma from South China

ClimacostromaYang and Dong, 1979, an endemic stromatoporoid genus, originally discovered in central Guangxi, South China, has remained enigmatic for over 40 years. Its validity and systematic position in the Order Stromatoporida (hypercalcified sponges) is here reinterpreted. In this study, the original collections of Climacostroma (including holotypes of C. guangxiense, C. microlaminatum and C. facetum) by Yang and Dong (1979) were re-examined, with improved images of the internal skeletons. We show that this genus is diagnostically characterized by thin microlaminae, dominant cassiculate structures between microlaminae, and distinct acosmoreticular to orthoreticular microstructures. Prior studies revealed similarities of Climacostroma with Stromatopora, Lineastroma, Ferestromatopora, Arctostroma, Habrostroma, and Parallelostroma. However, Climacostroma is markedly different from Stromatopora and Lineastroma by possessing thin microlaminae and microreticulate microstructure. Climacostroma is distinguished from Ferestromatopora and Arctostroma by the lack of dominant oblique or arched structures. Habrostroma differs from Climacostroma by the absence of prominent cassiculate structures. We found that Parallelostroma is most similar to Climacostroma in having dominant cassiculate structures between microlaminae and orthoreticular microstructure. Thus, we propose that Climacostroma is combined with Parallelostroma, and is reclassified under the Family Parallelostromatidae of the Order Syringostromatida, instead of its current position in the Family Stromatoporidae of the Order Stromatoporida. The three original species are here revised as two valid species, namely, P. guangxiense and P. microlaminatum.

Influence of basement fabrics on fault reactivation during rifting and inversion: a case study from the Celtic Sea basins, offshore Ireland

The evolution of rift basins is often influenced by the reactivation of pre-existing structures, a response that is partly controlled by the angular relationship between fault strike and the subsequent extension/compression axes. In the Celtic Sea basins, offshore Ireland, Caledonian and Variscan structures form a heterogeneous basement for rifting in the Triassic–Jurassic and Cretaceous and for later Pyrenean inversion in the Paleogene. Using 2D and 3D seismic reflection data, we evaluate the impact of a range of pre-existing structures on the style of deformation within basin margin and basin axis settings. The earlier Triassic–Jurassic rift phases arising from NW–SE extension exploited the NNE–SSW-, NE–SW- and ENE–WSW-trending basement structures. By contrast, Cretaceous rifting, with a dominant north–south extension direction, generated newly formed, roughly east–west-trending normal faults, which interacted with pre-existing faults that were only reactivated when they were at &gt;45° to the extension direction. During basin inversion arising from north–south compression, orthogonal extensional faults and highly oblique basement structures were reactivated as reverse and strike/oblique slip faults, and together interacted with the newly formed strike-slip faults. Combining constraints from multiple deformation phases, this study highlights the reactivation response of variably oriented faults to changing extension and compression vectors, and the associated changes in fault behaviour and mode of fault formation. Supplementary material: The location of regional 2D seismic reflection lines and a detailed description of the seismic interpretation are available at https://doi.org/10.6084/m9.figshare.c.6168279

Volcanic influence during the formation of a transform marginal plateau: Insights from wide-angle seismic data along the northwestern Demerara Plateau

Transform marginal plateaus (TMPs) are submarine seafloor highs located at the continental slope, often at the boundary of two ocean basins of different ages and associated to at least one transform or highly oblique margin. The systematic study of TMPs can, therefore, answer questions about rifting and continental margin development. The Demerara TMP (offshore Suriname and French Guiana) is located at the border between the Central Atlantic, which opened during the Lower Jurassic and the Equatorial Atlantic, which opened during the Lower Cretaceous. This study, based on wide-angle seismic data modeling from the northern and western section of the Demerara Plateau, provides information on both the lower volcanic unit of this TMP and the adjacent oceanic crust. The results confirm that the crust of the Demerara Plateau is around 30 km thick and consists of lava flows possibly mixed with crust of continental origin in its deeper layers. Seismic velocities (exceeding 7 km/s) are compatible with those of volcanic oceanic plateaus. To the west, a relatively wide transition zone separates the plateau from the Jurassic oceanic crust, which is composed of two layers, and is much thicker than normal oceanic crust (∼11 km). During the Cretaceous, the plateau was sharply cut by transform and highly oblique structures, separating the Demerara Plateau from its transform conjugate, the Guinea Plateau. As a result, the Demerara Plateau is flanked to the north by a magma-poor/strongly tectonized Cretaceous oceanic domain with thin (2–3 km) crust, likely partially consisting of serpentinized mantle. In contrast, the oceanic crust located towards the south-east appears to be more characteristic of typical oceanic crust in composition though slightly thinner than normal (4–6 km) thickness. Our analysis allows us to propose a new 3D vision of the crustal structure of the Demerara TMP and its borders.