Margin Formation Research Articles

2‐D Geodynamic Modeling of the Central South Atlantic Wide Rifted Margins, Implications for Evaporite Deposition

AbstractThe thick late syn‐ to early post‐rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn‐rift evaporites in the central South Atlantic are not well constrained and remain to be quantified. We use forward 2‐D thermo‐mechanical modeling coupled with melt prediction and surface processes to assess the contribution of lithospheric and mantle processes on the distal margin topography and subsidence history during continental rifting. Models show that (a) counter‐flow of depleted lower lithospheric mantle during rifting explains the magma‐poor nature of these margins and (b) weak crust and syn‐rift sediment control the wide crustal necking and subsidence history of the distal margin. Integration of our modeling results with quantified geophysical and geological observations suggests that (a) base level was down to −600 m below present‐day global sea level (bsl) during distal margin formation in the Aptian before sag and evaporite deposition, (b) base level was about −300/−400 m bsl at the end of evaporite deposition, and (c) scenarios with a fixed shallow base level (−400 m bsl) or with an increasing base level from an initially deep position (−1,600 m bsl) during evaporite deposition can both fit the observed evaporite distribution. However, erosional features along the base of evaporites suggest a deep initial base level.

Sedimentary characteristics, lithofacies, and paleogeography of the Middle–Lower Cambrian in the Tarim Basin, NW China

The Cambrian pre-salt dolomite sequence in the Tarim Basin is a target zone of great strategic significance for hydrocarbon exploration in the basin. Using the results of the interpretation of 3D seismic data from Lunnan and 2D seismic data covering the whole basin, and based on a synthesis of outcrop data, drilling data, well logs, core data, and thin-section data, and the findings from previous studies, this paper studies the characteristics of the facies of the Middle–Lower Cambrian in the platform area in the Tarim Basin, the formation and evolution of platform margins, and the sedimentary characteristics, lithofacies, and paleogeographic characteristics of the Middle–Lower Cambrian series. Based on the types of lithofacies and the seismic facies analysis, the sedimentary facies of the Middle–Lower Cambrian strata can be classified into the five types. From bottom to top, the Middle–Lower Cambrian have experienced several development stages, including the deposition of continental shelf sediments of the Yuertusi Formation, gently sloping non-rimmed platform margin sediments of the Xiaoerbulake Formation, weakly rimmed platform margin sediments of the Wusongger and Shayilike formations, and strongly rimmed platform margin sediments of the Awatage Formation. In each stage, the platform margin shifted further east relative to its location in the previous stage. Based on these results and previous studies, the lithofacies and paleogeography maps of the Middle–Lower Cambrian in the Tarim Basin are modified, and the implications for hydrocarbon exploration in this area are discussed.

Neoproterozoic passive margin formation and evolution during the Rodinia–Gondwana supercontinent cycle at the eastern margin of the West African Craton

Abstract Petrographical and geochemical data from the Togo structural unit (TSU), also referred to as the Atacora structural unit, are presented together with the existing dataset; geochemical and age data from the sedimentary and metasedimentary rocks from the passive margin sequences of the Dahomeyide belt in Ghana to infer their provenance and depositional setting and expand the discussion on the Rodina–Gondwana supercontinent assembly during the Pan-African orogeny. The metasedimentary rocks of the TSU are quartzites and phyllites. The framework grains of the quartzites consisting dominantly of quartz and small amounts of feldspar grains and relict lithic fragments classify them as quartz arenite, subarkose and sublitharenite. Generally, the studied rocks show similar rare-earth element and multi-element patterns, which imply derivation from similar sources. Elemental ratios, including (La/Lu)N, Th/Sc and La/Sc, suggest sediments sourced from intermediate to felsic rocks. Provenance and depositional setting indicators of the TSU suggest deposition in a passive margin setting, with the West African and Amazonian cratons’ granitoids and granitic gneisses as possible provenance, akin to siliciclastic rocks of the Buem structural unit and the Voltaian Supergroup of the Volta Basin. The deformational history of the TSU is similar to those of the Buem structural unit and the eastern margin of the Voltaian Supergroup, indicating the effect of the Pan-African orogeny on the passive margin of the Dahomeyide belt. We, therefore, propose the formation and evolution of a Neoproterozoic passive margin unit, which was tectonically deformed during the Rodinia–Gondwana supercontinent cycle.

An overview of Rodinia-Gondwana Supercontinents, India - Asia Collision, emergence of Himalaya and Paleobiogeography of the eastern Tethys in Indian Subcontinent, South Asia

The existence of Rodinia Supercontinent during Meso-Neoproterozoic and its subsequent breakup and reassembly as Gondwana Supercontinent is a widely accepted hypothesis in geological history of the Earth. The breakup of the Rodinia resulted in formation of rift basins and passive continental margins around 650 Ma. Major palaeoclimatic events like Neoproterozoic global glaciation (Snowball Earth) followed by global warming have been recorded from different continents including Indian Lesser Himalaya (Blaini -Krol Cryogenian-Ediacaran Period). The emergence of multicellular Ediacaran life in the Upper Krol Formation is consistent with an increase in the atmospheric oxygen. The base of Ediacaran System in the Lesser Himalaya is established in the cap carbonate, the Blaini Formation overlying the glacial diamictites (Blainian). The pink cap carbonate of the Blaini Formation shows negative δ13C value (-3‰ PDB) and invariably correlate with the Marinoan glacial event. Large continents of Gondwanaland Supercontinent South America, Africa, India, Australia and Antarctica were located over the South Polar region and global equatorial to polar oceanic circulation was blocked and may have been the main cause of glaciation. Paleogeographically Carboniferous–Permian diamictite, coal beds and plant fossil bearing Gondwana outcrops are found in South Sikkim and Arunachal Lesser Himalaya of Northeast India. The Late Paleocene – Middle EoceneTethyan foraminiferal – algal carbonate biofacies are well developed after Late Cretaceous – Paleogene mass extinction event in the eastern Tethys, Meghalaya, east India. These Tethyan benthic orthophragmine foraminiferal –algal limestones were deposited in the passive continental margin setting of a shallow marine carbonate shelf in eastern Tethys and Standard Benthic Zones (SBZ) of the Alpine- Adriatic western Tethys are recorded from Meghalaya. The Cretaceous – Tertiary boundary is well marked in these basins by biotic mass extinction. Late Cretaceous mega ammonoid fossils and Sauropod dinosaur bones are abundantly found in the Maastrichtian Mahadeo Formation. The present δ13C and δ18O isotope data is comparable with the marine values of the limestone The Umlatodoh Limestone contains foraminiferal taxa of early Ilerdian and Middle Eocene (early Lutetian) age. The Indo-Myanmar Orogenic Belt (IMOB) represents the eastern suture of Indian plate and it was formed due to the collision of the Indian plate with the Myanmar plate. The Naga-Manipur ophiolites have been assigned to range in age from Upper Cretaceous to Eocene on the basis of faunal assemblages (radiolarian, planktonic foraminefera) in the olistolithic blocks of pelagic limestone and cherts. Pan Indian Pre and Post India–Asia Collision, emergence of Himalaya and Paleogeography of the Indian Subcontinent has been discussed in South Asia.

Seismically constrained gravity inversions reveal magmatic and metamorphic processes at a major lithospheric boundary in northwestern Australia

Magmatic and metamorphic processes modifying the lower crust and uppermost mantle are key to understanding continental growth and evolution. However, our ability to identify and quantify such processes is challenged by inaccessibility, data sparsity, and uncertainties in defining properties at depth. As a result, knowledge of composition, metamorphic grade, and the nature of the continental crust-mantle transition is often limited. Here, we model the structure and properties of the crust and uppermost mantle between the West and North Australian Cratons to infer tectonic processes at this major lithospheric boundary. Seismically constrained 3D gravity inversions were used to estimate crustal and mantle densities and Moho depths. The application of isostatic principles and the joint interpretation of gravity, active and passive seismic models, well data, and outcrop data are used to identify and characterize anomalous masses that are otherwise unresolvable using supracrustal datasets. The results suggest a thick, dense crust with a mafic bulk composition, a high-density, reflective lower crust, and a low-density uppermost mantle beneath the southern Canning Basin. We interpret these crustal and mantle features as reflecting mafic mantle-derived magmatism and crustal anatexis related to Proterozoic-Cambrian Large Igneous Provinces. In contrast, the Proterozoic Paterson Orogen exhibits a thick, dense, high-grade metamorphic bulk crust with a felsic to intermediate composition, a high-density, low-reflectivity lower crust, and low-density uppermost mantle, inferred to result from convergent tectonics. The northern Canning Basin displays a thinner, less dense crust with an intermediate to mafic composition, and mafic, high-density, reflective lower crust, associated with mafic magma emplacement during rifting and passive margin formation. These variations in crustal structure reveal a complex interplay of Proterozoic terrane amalgamation to the cratonic margin, repeated intraplate high-volume mafic magmatic events, and rifting of the offshore basin, which have collectively shaped the deep crust and uppermost mantle in this region.

The heterogeneous crustal architecture of the Falkland Plateau Basin

AbstractContinental break‐up can be oftentimes associated with intracontinental wrenching that can lead to the generation of transform margins and transform marginal plateaus. The wrenching phase can be accompanied by complicated processes, which result in heterogeneous structural and crustal architectures. This makes understanding the evolution of such tectonic settings challenging. The Falkland Plateau is an example of a transform marginal plateau where regional wrenching accompanied the incipient stages of Gondwanan continental break‐up to result in a mosaic of crustal types underlying its largest basin: the Falkland Plateau Basin (FPB). The uncertainties in crustal boundaries have led to several models for the evolution of the plateau, which hinder the development of a reliable plate reconstruction of Southern Gondwana. We integrate seismic reflection, gravity and magnetic data to propose an updated crustal architecture of the FPB. The results show that extended continental crust underlies the basin in the west and north. The eastern and central parts consist of a juxtaposition of intruded and underplated continental crust which transitions southwards to a thick oceanic domain. The basin is crosscut by three main NE–SW trending shear zones which facilitated the development of the contrasting crustal and structural domains interpreted across the plateau. This integrated reassessment of the FPB provides new insights into the tectonic evolution of the plateau, the deformation associated with wrenching and transform margin formation and our understanding of the tectono‐stratigraphic evolution of such areas.

The Middle Jurassic Cheshmeh-Ghasaban High-Mg gabbro (Hamedan, NW Iran): A plume-influenced intracontinental rifting event?

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

Thermochronometry constraints on south West Greenland passive continental margin development

Passive continental margins (PCMs) represent the interface between the marine and terrestrial realms. However, topographic evolution of PCMs is often difficult to decipher due to paucity of the preserved geological record. Here, we report uranium-thorium-helium ((U-Th)/He) analysis of the Precambrian crystalline basement from southern West Greenland that help constrain the process of rifting between Greenland and North America and contributes to the debate about the West Greenland PCM development. The majority of zircon (U-Th)/He dates (220-580 Ma) imply several kilometres of burial of the basement by Paleozoic (and potentially Mesozoic) sediments. Apatite (U-Th)/He dates (80-230 Ma) record thermal processes associated with extensional tectonism starting in the Late Triassic and passive margin formation in the Early Cretaceous. Our data provide no evidence of thermal activity during Cenozoic times, suggesting that the thermal effects of Paleogene rifting and break-up were negligible and the magnitude of Cenozoic erosion was <3.5 km in the study area.

Intraplate records of a transform margin formation: Brittle deformation in the basement of the basins from the northeastern extremity of the Brazilian equatorial margin

The diachronous rifting of the Pangea supercontinent, with the dispersion of the continental masses, resulted in the formation of numerous rift systems, which primarily evolved into passive margins, such as the Brazilian Equatorial Margin (BEM). In the South American counterpart, basement structures serve as the primary markers for direct analysis of the nucleation and development of the BEM since much of the rift record occurs in the subsurface. The geodynamic models that explain the evolution of this margin are largely based on geophysical and numerical modeling data. In this paper, we carried out an onshore, multitool, and multiscale structural analysis of the brittle deformation overprinted on the Precambrian basement of the Potiguar and Ceará basins, which led to the proposition of four superimposed events. The D1 event (Late-Brasiliano) is mainly represented by the NE-SW (dextral) stepped fracture systems and the unfilled and quartz- or chalcedony-filled NE-SW to E-W-trending extensional joints. This event is related to NE-SW intraplate transpression, associated with the progressive exhumation of the Brasiliano-Pan African orogenic belt. The D2 event (Jurassic to early Aptian) represents the intracontinental deformation related to the initial rupture of the Pangea. The brasiliano-age ductile shear zones were reactivated (brittle) along with the development of NE-SW normal faults, NNE-SSW en échelon fractures, NE-SW joints, as well as the emplacement of the basic dykes (NE-SW and E-W) related to the Rio Ceará-Mirim magmatism. This event is associated with the NW-SE extension responsible for the origin of the Cariri-Potiguar trend basins. At the beginning of the installation of the equatorial proto-margin, a significant cataclastic process took place, marking the D3 event (post-early Aptian). This enabled the nucleation of hybrid (WNW-ESE, E-W, and NW-SE), shear fractures (WNW-ESE, NNW-SSE, E-W, NNE-SSW, and ENE-WSW), and NW-SE unfilled or filled (chalcedony, iron oxide and hydroxide, and calcite) dilatational joints, which express an E-W dextral transtensional regime related to this event. E-W and NW-SE pull-apart dilatational jogs are characteristic of this event and may be analog to the E-W Messejana and Jacaúna transtensional grabens (western part of the Potiguar Basin). The younger event (D4) is characterized by N–S normal faults and joints, and dry or filled (calcite and iron oxide/hydroxide) NNW-SSE hybrid shear fractures. A clockwise rotation of the South American plate with respect to the African plate and the paleostress field change to N–S compression and E-W extension, are responsible for the formation of D4 structures. The brittle structures developed in the crystalline basement of the Potiguar and Ceará basins represent the intracontinental record of the deformation related to the formation of the Atlantic equatorial margin under a dextral transtensional regime.

Formation and geophysical character of transitional crust at the passive continental margin around Walvis Ridge, Namibia

Abstract. When interpreting geophysical models, we need to establish a link between the models' physical parameters and geological units. To define these connections, it is crucial to consider and compare geophysical models with multiple, independent parameters. Particularly in complex geological scenarios, such as the rifted passive margin offshore Namibia, multi-parameter analysis and joint inversion are key techniques for comprehensive geological inferences. The models resulting from joint inversion enable the definition of specific parameter combinations, which can then be ascribed to geological units. Here we perform a user-unbiased clustering analysis of the two parameters electrical resistivity and density from two models derived in a joint inversion along the Namibian passive margin. We link the resulting parameter combinations to breakup-related lithology and infer the history of margin formation. This analysis enables us to clearly differentiate two types of sediment cover. The first type of sediment cover occurs near the shore and consists of thick, clastic sediments, while the second type of sediment cover occurs further offshore and consists of more biogenic, marine sediments. Furthermore, we clearly identify areas of interlayered massive, and weathered volcanic flows, which are usually only identified in reflection seismic studies as seaward-dipping reflectors. Lastly, we find a distinct difference in the signature of the transitional crust south of and along the supposed hotspot track Walvis Ridge. We ascribe this contrast to an increase in magmatic activity above the volcanic centre along Walvis Ridge and potentially a change in the melt sources or depth of melting. This change of the predominant volcanic signature characterizes a rift-related southern complex and a plume-driven Walvis Ridge regime. All of these observations demonstrate the importance of multi-parameter geophysical analysis for large-scale geological interpretations. Additionally, our results may improve future joint inversions using direct parameter coupling, by providing a guideline for the complex passive margin's parameter correlations.

Quantification and Restoration of the Pre‐Drift Extension Across the NE Atlantic Conjugate Margins During the Mid‐Permian‐Early Cenozoic Multi‐Rifting Phases

AbstractThe formation of the NE Atlantic conjugate margins is the result of multiple rifting phases spanning from the Late Paleozoic and culminating in the early Eocene when breakup was accompanied with intense magmatic activity. The pre‐breakup configuration of the NE Atlantic continental margins is controlled by crustal extension, magmatism, and sub‐lithospheric processes, all of which need to be quantified for the pre‐breakup architecture to be restored. Key parameters that need to be extracted from the analysis of crustal structures and sediment record include stretching factors, timing of rifting phases, and nature of the deep crustal structures. The aim of this study is to quantify the pre‐drift extension of the NE Atlantic conjugate margins using interpreted crustal structure and forward basin modeling. We use a set of eight 2D conjugate crustal transects and corresponding stratigraphic models, constrained from an integrated analysis of 2D and 3D seismic and well data. The geometry and thickness of the present‐day crust is compared to a reference thickness which has experienced limited or no crustal extension since Permian time allowing the quantification of crustal stretching. Based on the eight conjugate crustal transects, the total pre‐drift extension is estimated to range between 181 and 390 km with an average of 270–295 km. These estimates are supported by the results of forward basin modeling, which predict total extension between 173 and 325 km, averaging 264 km. The cumulative pre‐drift extension estimates derived from basin modeling are in turn used to calculate the incremental crustal stretching factors at each of the three main rifting phases between the conjugate Greenland‐Norwegian margins. The mid‐Permian early Triassic rifting phase represents 32% of the total extension, while the equivalent values are 41% for the mid‐Jurassic to mid‐Cretaceous and 27% for the Late Cretaceous‐Paleocene rifting phases. These values are used to establish and present at first, a full‐fit palinspastic plate kinematic model for the NE Atlantic since the mid‐Permian and will be the base for future work on more elaborated models in order to build accurate paleogeographic and tectonic maps.

The Lobed-Leaf Phenotype in Brassica juncea Is Associated with the BjLMI1 Locus as Evidenced Using GradedPool-Seq

The shape of the leaf is the primary phenotype which determines the commercial value of leaf mustard (Brassica juncea). However, there arefew reports on the lobed-leaf gene of B. juncea, and the molecular regulatory mechanisms underlying leaf margin formation are unknown. In this study, an F2 population derived from ‘MN001’ and ‘MU056’ was constructed. Genetic analysis revealed that the lobed-leaf trait is controlled by a major gene, and lobed leavesare dominant compared to round leaves. The GradedPool-Seq analysis combined with the re-sequencing results of parents identified a major interval on chromosome 10 of B. juncea’s genome A. The BjLMI1 gene (BjuA040054) was confirmed to be a candidate gene by gene ontology (GO) analysis, and it is homologous with LMI1 and encodes HD-Zip protein ATHB-51. A base substitution was observed in the conserved domain, and a 63 bp fragment deletion was found in the exon region between the two parents in the CDs region. The expression of BjLMI1 was significantly higher in the lobed-leaf parent than in the round-leaf parent. These findings provide insights into the molecular mechanism underlying leaf margin formation and will be valuable in the development of an ideal leaf shape in B. juncea.

Craton Formation in Early Earth Mantle Convection Regimes

AbstractHow the geological record of cratons reconciles with the tectonic environments in which they formed has remained debated. We use 2D Cartesian geometry numerical models of mantle convection varying temperatures from present day to Archaean‐inferred values, to address the formation of cratons, accounting for melt depletion‐dependent rheological stiffening. For mantle temperatures comparable to present day, melting is negligible and the convective regime depends on the strength of the thermal lithosphere. For mantle potential temperatures higher than present day, high depletion degree and large depleted mantle volumes are formed at low lithospheric strength and high surface mobility, whereas these are negligible beneath a poorly mobile lithosphere. When compared to the models, the record of tectonics and large volumes of high‐degree depleted mantle in Archaean cratons is best explained by a lithosphere initially prone to yielding and mobility. At high mobility, large depletion favors the progressive differentiation of the thermochemical lithosphere, which stiffens and thickens with increasing mantle temperatures. The ensuing reduced heat flow atop a hotter mantle is in agreement with the inferred Archaean thermal evolution, and may rule out the viability of a stagnant lid for the early Earth. Large‐scale depletion stiffening resists plate margin formation and this wanes as heat production decreases, thus may hold the key for the establishment of plate tectonics during secular cooling.

Magmatism at Passive Margins: Effects of Depth‐Dependent Wide Rifting and Lithospheric Counterflow

AbstractRifted passive margins exhibit a large variety in the timing, distribution, and amount of magmatism. The factors controlling magmatism during rifted margin formation, remain, however, incompletely understood, partly owing to the complex rifting styles. In this study, we use 2‐Dimensional numerical models to investigate the effects of depth‐dependent wide rifting and lithospheric counterflow on magmatism during rifted margin formation. Results show that a strong crust promotes narrow margins with a sharp transition to normal thickness oceanic crust whereas a weak crust promotes depth‐dependent wide rifting, with preferential removal of mantle lithosphere, leading to formation of wide margins with over‐thickened (&gt;18 km) igneous crust in the distal margin. Counterflow of depleted lithospheric mantle, in contrast, may delay syn‐rift magmatism, and result in exhumed a‐magmatic continental mantle at narrow margins. The combination of wide rifting and lithospheric counterflow results in magma‐poor wide margins, with in some cases a transition to excess magmatic activity at breakup. Our models provide an explanation for the contrasting magmatic productivity at narrow and wide rifted margins, such as observed in the Lofoten‐Vesterålen, Newfoundland, Kwanza Basin, and Orange Basin margins.

Evolution of Rift Systems and Their Fault Networks in Response to Surface Processes

AbstractContinental rifting is responsible for the generation of major sedimentary basins, both during rift inception and during the formation of rifted continental margins. Geophysical and field studies revealed that rifts feature complex networks of normal faults but the factors controlling fault network properties and their evolution are still matter of debate. Here, we employ high‐resolution 2D geodynamic models (ASPECT) including two‐way coupling to a surface processes (SP) code (FastScape) to conduct 12 models of major rift types that are exposed to various degrees of erosion and sedimentation. We further present a novel quantitative fault analysis toolbox (Fatbox), which allows us to isolate fault growth patterns, the number of faults, and their length and displacement throughout rift history. Our analysis reveals that rift fault networks may evolve through five major phases: (a) distributed deformation and coalescence, (b) fault system growth, (c) fault system decline and basinward localization, (d) rift migration, and (e) breakup. These phases can be correlated to distinct rifted margin domains. Models of asymmetric rifting suggest rift migration is facilitated through both ductile and brittle deformation within a weak exhumation channel that rotates subhorizontally and remains active at low angles. In sedimentation‐starved settings, this channel satisfies the conditions for serpentinization. We find that SP are not only able to enhance strain localization and to increase fault longevity but that they also reduce the total length of the fault system, prolong rift phases and delay continental breakup.

Influence of Surface Processes on Postrift Faulting During Divergent Margins Evolution

AbstractWe used thermomechanical numerical models to simulate the formation and evolution of divergent continental margins since continental rifting, taking into account surface processes of erosion of the continental escarpment. We found that the degree of crust–mantle coupling, the magnitude and extent of erosion of the coastal landscape, and the preexistence of weakness zones in the continental crust are important elements that control the reactivation of faults along divergent margins during the postrift phase. The numerical experiments indicated that the presence of a lower crust with a relatively low viscosity, facilitating the decoupling of the upper crust and the development of hyperextended margins, could also contribute to the development and/or reactivation of normal faults in the interior of the continent when the margin is continually subjected to differential denudation and consequent flexural response of the lithosphere. This effect is suppressed in scenarios where the lower crust presents a relatively high viscosity, consequently inducing the coupling of the upper crust with the lithospheric mantle. In this case, the long wavelength of the flexural response of a coupled lithosphere to erosional unloading has a minor impact on the reactivation of faults in the upper crust. As an application of our model, we propose that the combination of a decoupled continental lithosphere, escarpment retreat due to erosion, and the preexistence of shear zones parallel to the coast contributed to the development of postrift Cenozoic tectonism in southeastern Brazil.

Localized Repression of Two bHLH Genes is Involved in the Formation of White Margins and White Abaxial Surfaces in Carnation Petals by Inducing the Absence of Anthocyanin Synthesis

Localized Repression of Two bHLH Genes is Involved in the Formation of White Margins and White Abaxial Surfaces in Carnation Petals by Inducing the Absence of Anthocyanin Synthesis

Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China

AbstractHigh-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr–Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47–7.02 wt%) contents and high Mg# values (67.27–68.77), with SiO2 (58.57–62.13 wt%), Al2O3 (14.49–16.07 wt%) and CaO (5.05–6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (87Sr/86Sr)i ratios (0.7041–0.7057) and ϵNd(t) (3.73–5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe–Niujuanzi–Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting.

Kinematic Boundary Conditions Favouring Subduction Initiation at Passive Margins Over Subduction at Mid-oceanic Ridges

We perform numerical modelling to simulate the shortening of an oceanic basin and the adjacent continental margins in order to discuss the relationship between compressional stresses acting on the lithosphere and the time dependent strength of the mid-oceanic ridges within the frame of subduction initiation. We focus on the role of stress regulating mechanisms by testing the stress–strain-rate response to convergence rate, and the thermo-tectonic age of oceanic and continental lithospheres. We find that, upon compression, subduction initiation at passive margin is favoured for thermally thin (Palaeozoic or younger) continental lithospheres (&amp;lt;160 km) over cratons (&amp;gt;180 km), and for oceanic basins younger than 60 Myr (after rifting). The results also highlight the importance of convergence rate that controls stress distribution and magnitudes in the oceanic lithosphere. Slow convergence (&amp;lt;0.9 cm/yr) favours strengthening of the ridge and build-up of stress at the ocean-continent transition allowing for subduction initiation at passive margins over subduction at mid-oceanic ridges. The results allow for identifying geodynamic processes that fit conditions for subduction nucleation at passive margins, which is relevant for the unique case of the Alps. We speculate that the slow Africa–Europe convergence between 130 and 85 Ma contributes to the strengthening of the mid-oceanic ridge, leading to subduction initiation at passive margin 60–70 Myr after rifting and passive margin formation.

The tectono-stratigraphic architecture of the Falkland Plateau basin; implications for the evolution of the Falkland Islands Microplate

Commonly, intra-continental wrenching is associated with a high degree of crustal faulting and fragmentation. The resulting continental blocks can undergo vertical-axis rotations, which in turn can lead to the generation of intricate fault networks within and along their boundary regions. Investigations into these structural complexities can support understanding of when and how these continental blocks rotate, and what their position was prior to transform margin formation. In the case of the Falkland Islands Microplate (part of the Falkland Plateau transform margin), its position between South Africa, South America, and East Antarctica prior to the break-up of Gondwana is still debatable. This uncertainty affects the reliability of plate models for this region. Here we integrate gravity and 2D and 3D seismic reflection data from the eastern margin of the microplate (west side of the Falkland Plateau Basin) to provide insights into the tectono-stratigraphic architecture of this area from Jurassic onwards, and into the evolution of the Falkland Islands Microplate. Our findings show that the western part of the Falkland Plateau Basin is an integral part of the microplate, and it underwent deformation in a relatively fast-changing stress regime. Stress field configuration estimates across the Falkland Islands Microplate support an alternation between a NE-SW and NW-SE/WNW-ESE orientation of σ3 during the Jurassic and an ENE-WSW oriented σ3 during the Lower Cretaceous. Correlations of this local stress configuration with the regional support a Middle to Upper Jurassic rotation of the microplate in a predominantly extensional setting facilitated by the early fragmentation of south-western Gondwana.