Final Breakup Research Articles

Evidence of post-Gondwana breakup in Southern Brazilian Shield: Insights from apatite and zircon fission track thermochronology

Apatite and zircon fission track thermochronology studies are applied to basement and sedimentary rocks from the Sul-Rio-Grandense Shield to unravel the tectonic history of the onshore southernmost Brazilian margin. The Sul-Rio-Grandense Shield is a major geotectonic feature of southernmost Brazil that includes Paleoproterozoic basement areas and Neoproterozoic fold belts linked to the Brasiliano/Pan-African orogeny. Crustal reworking and juvenile accretion events related to this cycle were dated in the region between 900 and 500Ma and were responsible for the assembly of southwestern Gondwana in southeastern South America. Apatite fission track (AFT) ages range from 340±33 to 77±6Ma and zircon fission track (ZFT) ages range from ca. 386 to 210Ma. Based on thermal history modeling, the most part of the samples record an early cooling event during the Carboniferous, which reflect the main tectonic activity of the final stages of the Gondwanides at the Pacific margin of West Gondwana. Subsequently, the Permo-Triassic cooling event is related to the last stages of the Gondwanides, with convergence along the southern border of Western Gondwana and consequent reactivation of N-S and NE-SW trending basement structures. The onset of initial breakup of southwestern Gondwana with opening of the South Atlantic Ocean is mostly recorded in the eastern terrain and ZFT ages show that the temperature during this period was high enough for total or at least partial resetting of fission tracks in zircon. The last cooling event of the Sul-Rio-Grandense Shield records the final breakup between South America and Africa, which began during the Late Cretaceous. However, the Cenozoic rapid cooling episode probably is a result of plate adjustment after breakup and neotectonic reactivation of faults associated with South Atlantic rift evolution.

A stem acrodontan lizard in the Cretaceous of Brazil revises early lizard evolution in Gondwana.

Iguanians are one of the most diverse groups of extant lizards (>1,700 species) with acrodontan iguanians dominating in the Old World, and non-acrodontans in the New World. A new lizard species presented herein is the first acrodontan from South America, indicating acrodontans radiated throughout Gondwana much earlier than previously thought, and that some of the first South American lizards were more closely related to their counterparts in Africa and Asia than to the modern fauna of South America. This suggests both groups of iguanians achieved a worldwide distribution before the final breakup of Pangaea. At some point, non-acrodontans replaced acrodontans and became the only iguanians in the Americas, contrary to what happened on most of the Old World. This discovery also expands the diversity of Cretaceous lizards in South America, which with recent findings, suggests sphenodontians were not the dominant lepidosaurs in that continent as previously hypothesized.

Seasonal Variability of the Polar Stratospheric Vortex in an Idealized AGCM with Varying Tropospheric Wave Forcing

Abstract The seasonal variability of the polar stratospheric vortex is studied in a simplified AGCM driven by specified equilibrium temperature distributions. Seasonal variations in equilibrium temperature are imposed in the stratosphere only, enabling the study of stratosphere–troposphere coupling on seasonal time scales, without the complication of an internal tropospheric seasonal cycle. The model is forced with different shapes and amplitudes of simple bottom topography, resulting in a range of stratospheric climates. The effect of these different kinds of topography on the seasonal variability of the strength of the polar vortex, the average timing and variability in timing of the final breakup of the vortex (final warming events), the conditions of occurrence and frequency of midwinter warming events, and the impact of the stratospheric seasonal cycle on the troposphere are explored. The inclusion of wavenumber-1 and wavenumber-2 topographies results in very different stratospheric seasonal variability. Hemispheric differences in stratospheric seasonal variability are recovered in the model with appropriate choices of wave-2 topography. In the model experiment with a realistic Northern Hemisphere–like frequency of midwinter warming events, the distribution of the intervals between these events suggests that the model has no year-to-year memory. When forced with wave-1 topography, the gross features of seasonal variability are similar to those forced with wave-2 topography, but the dependence on forcing magnitude is weaker. Further, the frequency of major warming events has a nonmonotonic dependence on forcing magnitude and never reaches the frequency observed in the Northern Hemisphere.

“Примирение состоялось?”: О том, как Андрей Белый “ждал”, но “не просил” прощения Эмилия Метнера

The article deals with Belyi's attitude to Emil Medtner after their final break-up in 1915. Since emigration to Berlin Belyi not only wanted to finish the once interrupted debate and prove himself right but also to make amends with his friend and teacher. The second tendency – the desire to make amends – is analysed in the article. It can be seen in Vospominaniia o Bloke, in the memoirs Nachalo veka, in the poem ‘Starinnyi drug’. Proof can also be found in memoirs of Belyi's contemporaries (P. Zaitsev, E. Makaev, G. Kireevskaia).

Single drop breakup experiments in stirred liquid–liquid tank

The breakup process has been investigated by single drop experiments in a stirred liquid–liquid tank. The time taken to deform, stretch and produce two or more fragments was measured. For each of the observed breakup events, the number of drops in the resulting population was registered. The occurrence of equal and unequal breakup events was also noticed. Four oil-in-water systems were studied: toluene, petroleum, n-dodecane, and octanol. The mean energy dissipation rate of the tank was 1.14m2/s3 and the diameter of the observed mother drops that breakup were mainly in the range of 1–4mm. Multiple breakup events were the most frequent outcome in the experiments. It was observed that the number of daughter drops produced in a breakup was dependent on the mother drop size. The breakup time increased with the mother drop size and was in the approximate range of 10–100ms. A large uncertainty in the measurements is associated with the determination of the final breakup time, particularly in the cases of high order breakup events. Most breakup events took place in the large velocity field close to the radial impeller. It is not evident whether the breakup events were due to the time average shear or pressure and velocity fluctuations.

REE and Lu-Hf systematics of zircons from rapakivi granites and associated rocks of supercontinent Nuna (Columbia)

The last of the rifting events at 1.37–1.45 Ga that is considered to have led to the final breakup of Nuna was recorded in most of its building blocks (Laurentia, Baltica, and Siberian craton). At the type locality for the Riphean on the western slope of the South Urals, this event was recognized as the Mashak igneous event (∼1380 Ma) and can be classified as a LIP based on the volumes of magma generated (total igneous volumes >0.05 × 106 km3). The emplacement of large volumes of A-type granitic plutons in Laurentia and northwestern Baltica (in the present coordinates) was almost synchronous with this event. In the South Urals, within the Bashkirian meganticlinorium, the processes of granite generation and emplacement during this time took place on a much smaller scale. Anorogenic granites of this region are represented by rapakivi granites of the Berdyaush massif. The first REE and Lu-Hf isotope data for zircons from rapakivi granites and associated rocks of the Berdyaush massif reveal almost identical Lu-Hf isotope ages (∼1383 Ma) of rapakivi granites, quartz syenitic diorites, and nepheline syenites. It was shown that these rocks, which were emplaced into the heterogeneous Proterozoic crust between the Early and Middle Riphean, have the Hf isotopic compositions and Lu-Hf model ages reflecting the contribution from crustal sources with ages of 2013–2219 Ma, whereas the associated gabbros indicate derivation from a source with the Lu-Hf ages of 1658 ± 1685 Ma and moderately depleted ɛHf(1383) of (4.1 ± 0.7–4.9 ± 0.6). The REE characteristics (ΣREE, Eu/Eu*, Ce/Ce*, Pr/Pr*, La/Lu) and Lu-Hf isotope systematics of zircons can be used to constrain the history and genetic evolution of supercontinents, e.g., supercontinent Nuna (Columbia) in particular.

The Cretaceous opening of the South Atlantic Ocean

The separation of South America from Africa during the Cretaceous is poorly understood due to the long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted between ∼121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Africa during that period. The kinematic solutions are generally consistent with fracture zone traces and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (<100 km) within South America, west of the Rio Grande fracture zone. This process has lasted until ∼92 Myr ago after which both Africa and South America (south of the equator) behaved rigidly. The clearing of the continental–oceanic boundaries within the Equatorial Atlantic Gateway was probably completed by ∼95 Myr ago. The clearing was followed by a progressive widening and deepening of the passageway, leading to the emergence of north–south flow of intermediate and deep-water which might have triggered the global cooling of bottom water and the end for the Cretaceous greenhouse period.

Detrital zircon U–Pb dating and whole-rock geochemistry from the clastic rocks in the northern marginal basin of the North China Craton: Constraints on depositional age and provenance of the Bayan Obo Group

Located in the northern margin of the North China Craton (NCC), the Bayan Obo Group is composed of low-grade metamorphic clastic rocks and carbonates unconformably overlying the Archean–Paleoproterozoic basement and is famous for its hosting of the world's largest REE ore deposit. It has long been debated whether the Bayan Obo Group was correlated with the Proterozoic sedimentary records of the Xiong’er and Yanliao rifts or whether it was an analog of the Paleozoic Sailinhudong Group. In this study, we present new U–Pb data from detrital zircons which constrain the maximum depositional ages of the Dulahala and Halahuogete formations of the Bayan Obo Group as 1822±9Ma and 1710±29Ma, respectively. Our geochronological and geochemical data suggest that the major provenance of the Dulahala and Jianshan formations was the Paleoproterozoic khondalites which were deposited during 2.0–1.95Ga and metamorphosed during 1.95–1.82Ga, whereas the sediments of the Halahuogete and Bilute formations were derived from various sources in the NCC. Combining our data with the pioneer's researches, it appears that the northern marginal rift system of the NCC resulted from two discrete rifting events, which were probably related to the initial intra-continental extension and the final breakup of the Columbia Supercontinent.

The Sytykanskaya kimberlite pipe: Evidence from deep-seated xenoliths and xenocrysts for the evolution of the mantle beneath Alakit, Yakutia, Russia

Mantle xenoliths (>150) and concentrates from late autolithic breccia and porphyritic kimberlite from the Sytykanskaya pipe of the Alakit field (Yakutia) were analyzed by EPMA and LAM ICP methods. In P-T-X-f(O2) diagrams minerals from xenoliths show widest variations, the trends P-Fe#-CaO, f(O2) for minerals from porphyric kimberlites are more stepped than for xenocrysts from breccia. Ilmenite PTX points mark moving for protokimberlites from the lithosphere base (7.5 GPa) to pyroxenite lens (5–3.5 GPa) accompanied by Cr increase by AFC and creation of two trends P-Fe#Ol ∼10–12% and 13–15%. The Opx-Gar-based mantle geotherm in Alakit field is close to 35 mW/m2 at 65 GPa and 600 °C near Moho was determined. The oxidation state for the megacrystalline ilmenites is lower for the metasomatic associations due to reduction of protokimberlites on peridotites than for uncontaminated varieties at the lithosphere base. Highly inclined linear REE patterns with deep HFSE troughs for the parental melts of clinopyroxene and garnet xenocrysts from breccia were influenced by differentiated protokimberlite. Melts for metasomatic xenoliths reveal less inclined slopes without deep troughs in spider diagrams. Garnets reveal S-shaped REE patterns. The clinopyroxenes from graphite bearing Cr-websterites show inclined and inflected in Gd spectrums with LREE variations due to AFC differentiation. Melts for garnets display less inclined patterns and Ba-Sr troughs but enrichment in Nb-Ta-U. The 40Ar/39Ar ages for micas from the Alakit mantle xenoliths for disseminated phlogopites reveal Proterozoic (1154 Ma) age of metasomatism in early Rodinia mantle. Veined glimmerites with richterite – like amphiboles mark ∼1015 Ma plume event in Rodinia mantle. The ∼600–550 Ma stage manifests final Rodinia break-up. The last 385 Ma metasomatism is protokimberlite-related.

Structural analysis of extended Australian continental crust: Capel and Faust basins, Lord Howe Rise

Abstract The Capel and Faust basins (northern Lord Howe Rise) are located in the SW Pacific between Australia, New Zealand and New Caledonia. New seismic, gravity, magnetic and bathymetry data and rock samples have enabled the construction of a three-dimensional geological model providing insights into the crustal architecture and basin stratigraphy. Multiple large depocentres up to 150 km long and 40 km wide, containing over 6 km of sediment, have been identified. These basins probably evolved through two major Early Cretaceous rifting episodes leading to the final break-up of the eastern Gondwanan margin. Pre-break-up plate restorations and potential field data suggest that pre-rift basement is a collage of several discrete terranes, including a Palaeozoic orogen, pre-rift sedimentary basins and rift-precursor igneous rocks. It is likely that a pre-existing NW-trending basement fabric, inherited from the New England Orogen (onshore eastern Australia), had a strong influence on the evolution of basin architecture. This basement fabric was subjected to oblique rifting along an east–west vector in the ?Early Cretaceous to Cenomanian and NE–SW-oriented orthogonal rifting in the ?Cenomanian to Campanian. This has resulted in three structural provinces in the study area: Eastern Flank, Central Belt and Western Flank.

Detrital zircon U–Pb, Hf isotopes, detrital rutile and whole-rock geochemistry of the Huade Group on the northern margin of the North China Craton: Implications on the breakup of the Columbia supercontinent

Supracrustal successions in the Zhaertai–Bayan Obo–Huade rift zone on the northern margin of the North China Craton include the Zhaertai, Bayan Obo and Huade Groups from west to east. The Huade Group, which has been subdivided into four formations, is composed of basal pebbled quartzites, quartzites, schists, phyllites, marbles, slates and diopsidites. Geochemistry of the metasedimentary succession indicates that source rocks of the lower two formations are felsic, displaying passive margin signatures, whereas those of the upper two formations have continental island arc or active continental margin signatures. U–Pb ages of detrital zircons from the group yielded three age populations of 2690–2450, 2150–1710 and 1660–1330Ma, of which the youngest age population only exist in the upper Huade Group. The dominant 2690–2450 detrital zircons were likely sourced from the granitic rocks in the Yinshan Block. The subordinate 2150–1710Ma detrital zircons were probably recycled from the metasedimentary units from the Khondalite Belt and/or derived from the similar aged granitic plutons of the Hongqiyingzi “Group”, which are also evidenced by the similar geochemical features of rutiles from the Huade Group and the Khondalite Belt. Minor amounts of 1660–1330Ma detrital zircons may have derived from the North American and/or Baltica cratons connected to the northern margin of the North China Craton in the Columbia supercontinent. The youngest detrital zircon age peak of ∼1337Ma in the upper Huade Group suggests that the sedimentation began after ∼1.34Ga. In combination with the ∼1320Ma granitoids cross-cutting it, depositional ages of the upper Huade Group can be constrained between ∼1.34 and ∼1.32Ga. Taking into account the lithostratigraphic features, provenances and depositional ages, a continental rift basin deposit represented by the upper Huade Group on the northern margin of the North China Craton developed between ∼1.34 and ∼1.32Ga, which indicates that the final breakup of the North China Craton from the Columbia supercontinent happened in the middle Mesoproterozoic.

Mid-Mesoproterozoic (∼1.32 Ga) diabase swarms from the western Liaoning region in the northern margin of the North China Craton: Baddeleyite Pb–Pb geochronology, geochemistry and implications for the final breakup of the Columbia supercontinent

Diabase swarms widely intrude the late Paleoproterozoic to Mesoproterozoic sedimentary rocks in the northern margin of the North China Craton (NCC). In this paper, we present new baddeleyite SIMS Pb–Pb ages and whole-rock geochemical data for representative samples of the diabase swarms intruding the Mesoproterozoic Wumishan and Xiamaling formations in the western Liaoning region. Our aim is to elucidate the age and origin of the diabase swarms and their relationship to the fragmentation of the Columbia supercontinent. The baddeleyite SIMS Pb–Pb dating results show emplacement ages varying from 1325.9±4.1Ma to 1318.6±3.7Ma, indicating voluminous basic magmatism leading to the generation of the diabase swarms during the Mid-Mesoproterozoic time. Geochemically, the Mesoproterozoic diabasic rocks have relatively low REEs contents and show weak positive Eu anomalies. Moreover, these diabase samples are characterized by enrichment in light rare earth elements (LREEs), large ion lithophile elements (LILEs; e.g., Rb and Pb) and some high field strength elements (HFSEs; e.g., Th, U, Zr, Ti and Hf), and depletion in heavy rare earth elements (HREEs) and several high field strength elements (HFSEs; e.g., Nb, Sm, Gd and P), with affinities to basalts belonging to within-plate tholeiite series. Taken together, these geochemical characteristics suggest that the primary magmas of the diabases in the western Liaoning region could have been derived by partial melting of a transitional mantle or slightly enriched mantle in a continental extensional setting. In the context of assembly, growth and breakup of the Columbia supercontinent, together with the recognition of large volumes of ∼1.32Ga diabase swarms in the northern NCC as well as the nearly coeval rift-related anorogenic magmatism recorded elsewhere on the globe, such as Africa, Baltica, Siberia, North America and Greenland, we conclude that the northern margin of NCC experienced the global-scale Mid-Mesoproterozoic rifting events, corresponding to the final breakup of the Columbia supercontinent, probably at around 1.32Ga.

Final Gondwana breakup: The Paleogene South American native ungulates and the demise of the South America–Antarctica land connection

The biogeographic hypothesis more accepted today is that Antarctica (West Antarctica) and southern South America (Magellan region, Patagonia) were connected by a long and narrow causeway (Weddellian Isthmus) between the Antarctic Peninsula and South America since the Late Cretaceous (Campanian) until the Early Paleogene allowing terrestrial vertebrates to colonize new frontiers using this land bridge. Stratigraphically calibrated phylogenies including large, terrestrial native ungulates Litopterna and Astrapotheria taxa reveal long ghost lineages that extended into the Late Paleocene and provide evidence for the minimum times at which these “native ungulates” were present both on Antarctica and South America. Based on these results we estimate that the Weddellian Isthmus was functional as a land bridge until the Late Paleocene. Our data place the disconnection between Antarctica and South America in the Late Paleocene, indicating that the terrestrial faunistic isolation (Simpson's “splendid isolation”) in South America begun at the end of the Paleocene (~56 to 57m.y.). This faunistic isolation is documented to have occurred at least 25Ma before the existence of deep-water circulation conditions in Drake Passage (~30m.y.) based on the onset of seafloor spreading in the west Scotia Sea region. We hypothesize that in the early stages of extension (Late Paleocene, ~55m.y.) a wide and relatively shallow epicontinental sea developed between the Antarctic Peninsula and South America drowning the Weddellian Isthmus and preventing the faunal interchange for obligate cursorial terrestrial forms.

A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8–30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.

Historical biogeography of a new antitropical clade of temperate freshwater fishes

AbstractAimAny discovery of new biogeographical pattern in landlocked animals may challenge hypotheses that account for the evolution of intercontinental faunas. In this study, we examined the evolutionary origin and historical biogeography of temperate, freshwater perch‐like fishes within a molecular phylogenetic framework.LocationTemperate Eastern Eurasia, the Americas and Australia.MethodsData from five nuclear markers obtained from 15 targeted species and a broad sampling of percomorphs were assembled to investigate their phylogeny and historical biogeography. We (1) applied a fossil‐calibrated Bayesian reconstruction to provide a phylogenetic time‐scale for the diversification of the taxa, (2) inferred the evolution of habitat preference through a tree‐based character reconstruction method, and (3) reconstructed the evolution of range distributions using a dispersal–extinction–cladogenesis model.ResultsWe recovered a new antitropical freshwater clade, Percichthyoidea, composed of Sinipercidae, Percichthyidae (including Percilia but excluding Macquaria colonorum), Elassomatidae and Centrarchidae. Our time‐scale analysis indicates that the Percichthyoidea originated around the Cretaceous–Palaeogene transition. Ancestral habitat reconstructions revealed only one marine‐to‐freshwater transition that preceded the origin of the percichthyoids; the warm‐to‐temperate transition occurred earlier. Ancestral‐area inference indicates a wide‐ranging pan‐American and Eastern Eurasian distribution of the most recent common ancestor of Percichthyoidea. The initial diversification event occurred between the southern (Percichthyidae) and northern (remaining percichthyoids) clades. A second divergence occurred between Eastern Eurasia (Sinipercidae) and North America (Centrarchidae plus Elassomatidae). The trans‐Pacific distribution of the Percichthyidae may have preceded the final breakup of south Gondwana.Main conclusionsThe present‐day distribution of the Percichthyoidea is unique within extant fishes, comprising four trans‐Pacific continental blocks across the two hemispheres. Current geological and palaeoenvironmental reconstructions may well explain the longitudinal but not the latitudinal pattern of this freshwater fish clade, as it requires a transequatorial freshwater system connecting temperate regions of South and North America during the Late Cretaceous/Palaeogene. An alternative hypothesis is discussed, in which the ancestor of the Percichthyoidea was not completely adapted to freshwater.

Age and paleomagnetism of the 1210 Ma Gnowangerup–Fraser dyke swarm, Western Australia, and implications for late Mesoproterozoic paleogeography

Dolerite dykes of the Gnowangerup–Fraser Dyke Suite are subparallel to the southern and southeastern margins of the Yilgarn Craton. We collected oriented samples for paleomagnetic study from 19 dykes along the Phillips and Fitzgerald Rivers and north of Ravensthorpe. Alternating-field (AF) demagnetization revealed a stable two-polarity remanence in 14 dykes, and the primary nature of the magnetic directions is supported by a positive baked-contact test and by rock-magnetic evidence. U–Pb zircon and baddeleyite ages for two dykes confirm that the Gnowangerup–Fraser dykes are part of the 1210Ma Marnda Moorn Large Igneous Province. The mean paleomagnetic pole, at 55.8°N, 323.9°E, A95=6.5°, is almost identical to the previously reported VGP of the 1212Ma Fraser dyke, also supported by a positive baked-contact test. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time. We present a paleomagnetically permissible drift model for these two continents between 1210 and 1070Ma. One dyke yields a stable remanence with a VGP similar to the paleopole for the 755Ma Mundine Well dykes indicating that this dyke may have been emplaced during the same event at c. 755Ma. Differences in lengths and shapes of late Mesoproterozoic Apparent Polar Wander Paths of several continents suggests that a large supercontinent did not exist between about 1300 and 1050Ma. This may have been a transitional time between the final breakup of Nuna and the assembly of Rodinia.

Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton.

Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean

Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ∼20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.

The Faroe–Shetland Basin: a regional perspective from the Paleocene to the present day and its relationship to the opening of the North Atlantic Ocean

Abstract The Faroe–Shetland Basin is located offshore NW Scotland on the SE margin of the Atlantic Ocean and comprises numerous sub-basins and intra-basin highs that are host to a number of significant hydrocarbon discoveries. The principal hydrocarbon discoveries are in Paleocene–Eocene strata, although earlier strata are known, and their existence is therefore intimately linked to the opening and evolution of the North Atlantic from 54 Ma. The final rifting and separation of Greenland from Eurasia is commonly attributed to the arrival of a mantle plume which impacted beneath Greenland during early Tertiary time. Moreover, the ensuing plate separation is commonly described in terms of instantaneous unzipping of the North Atlantic, whereas in reality proto-plate boundaries were more diffuse during their inception and the linked rift system which we see today, including connections with the Arctic, was not established until Late Palaeogene–Early Neogene time. From a regional analysis of ocean basin development, including the stratigraphic record on the adjacent continental margins, the significance of the Greenland–Iceland–Faroe Ridge and the age and role of Iceland, we propose a dual rift model whereby North Atlantic break-up was only partial until the Oligo-Miocene, with true final break-up only being achieved when the Reykjanes and Kolbeinsey ridges became linked. As final break-up coincides with the appearance of Iceland, this model negates the need for a plume to develop the North Atlantic with rifting reliant on purely plate tectonic mechanisms, lithospheric thinning and variable decompressive upper mantle melt along the rifts.

Genesis of the 1.21 Ga Marnda Moorn large igneous province by plume–lithosphere interaction

The 1.21Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup–Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (<75km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about −12, strong depletion of Nb–Ta–Zr–Hf–Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia.