Breaking the Grenville–Sveconorwegian link in Rodinia reconstructions

The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple‐junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre‐1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville–Sveconorwegian orogenesis. Moreover, palaeomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and palaeomagnetic data, of which a “classic” triple‐junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga

The vesicular Sainte-Sophie dykes: a chemically distinct, near-surface facies of the Grenville Dyke Swarm?

Undeformed and unmetamorphosed diabase dykes up to 5 m wide occur in an area of 100 km2 about 50 km northwest of Montreal near the village of Sainte-Sophie. The dykes are subvertical and oriented east–west. The most common phenocryst phase is plagioclase (1–2 mm, exceptionally 10 mm), followed by olivine, and in one dyke, pyroxene. Most dykes have vesicles 0.1–10 mm in diameter and fine-grained margins. Alteration is extensive: plagioclase is partly sericitized, olivine is serpentinized, and zeolites and carbonate are developed in the vesicles and matrix. Ar–Ar dating was challenging, but the weighted mean of two plateau ages indicates intrusion and cooling at 591.6 ± 2.5 Ma. The overall characteristics of these dykes suggest that they may be a facies of the much larger Grenville dyke swarm to the west. However, there are distinct chemical differences that suggest they were fed from a different, generally more enriched mantle source. This zone may be related to the one that produced the alkaline Monteregian intrusions some 470 million years later in the same region. If the Sainte-Sophie dykes are part of the Grenville dyke event, then the use of chemical fingerprints to identify components of this swarm must be used with caution. Another difference between the SainteSophie and main Grenville dykes is the presence of vesicles and zeolites in the former, which suggests the dykes were emplaced close to the surface; hence, this crustal block has seen little erosion during the last 600 million years. Des dykes de diabase non déformés et non métamorphisés atteignant jusqu’a` 5 m de largeur sont présents dans une région de quelque 100 km2 située a` environ 50 km au nord-ouest de Montréal, près du village de Sainte-Sophie. Ces dykes sont subverticaux et d’orientation est–ouest. La phase phénocristique la plus abondante est le plagioclase (1–2 mm, exceptionnellement 10 mm), suivi de l’olivine et, dans un dyke, du pyroxène. La plupart des dykes présentent des vésicules de 0,1–10 mm de diamètre et des bordures a` grains fins. Ils présentent en outre une altération intense, le plagioclase étant partiellement séricitisé, l’olivine, serpentinisée, et des zéolites et des carbonates étant présents dans les vésicules et la matrice. La datation Ar–Ar s’est avérée difficile, mais la moyenne pondérée de deux âges plateau indiquent une intrusion et un refroidissement a` 591,6 ± 2,5 Ma. Les caractéristiques générales de ces dykes portent a` croire qu’ils pourraient constituer un faciès du beaucoup plus grand essaim de dykes de Grenville plus a` l’ouest. Des différences claires sur le plan chimique indiqueraient toutefois qu’ils proviennent d’une source mantellique distincte généralement plus enrichie, qui pourrait être reliée a` la zone qui a produit les intrusions alcalines montérégiennes, il y a quelque 470 millions d’années dans la même région. Si les dykes de Sainte-Sophie découlent effectivement du même épisode que les dykes de Grenville, l’utilisation de signatures chimiques pour identifier des éléments de cet essaim nécessiterait donc une certaine prudence. Une autre différence entre les dykes de Sainte-Sophie et les dykes de l’essaim de Grenville est la présence de vésicules et de zéolites dans les premiers, qui donne a` penser que ces dykes se sont mis en place près de la surface et, donc, que ce bloc crustal a subi peu d’érosion au cours des dernières 600 millions d’années

The Sveconorwegian orogeny: reamalgamation of the fragmented southwestern margin of Fennoscandia

The Sveconorwegian orogeny encompasses magmatic, metamorphic and deformational events between ca. 1140 and 920 Ma at the southwestern margin of Fennoscandia. In recent years, the tectonic setting of this nearly 200 Myr-long evolution has been debated, with some workers arguing for collision with an unknown continent off the present-day southwest coast of Norway, and others advocating accretionary processes inboard of an active margin. Recently, it has been suggested that orogeny may have been gravity-driven by delamination and foundering of heavy subcontinental lithospheric mantle in an intraplate setting, in some ways similar to proposed sagduction processes in the Archaean. Resolving the tectonic setting of the Sveconorwegian orogen has implications for correlation with other orogens and Rodinia supercontinent reconstructions and for assessments of the evolution of plate tectonics on Earth, from the Archaean to the present. Here, we present new mapping and geochronological data from the Bamble and Telemark lithotectonic units in the central and western Sveconorwegian orogen – the former representing a critical region separating western parts of the orogen that underwent long-lived high- to ultrahigh-temperature metamorphism and magmatism from parts closer to the orogenic foreland that underwent episodic high-pressure events. The data show that the units constituting the Sveconorwegian orogen most likely formed at the southwestern margin of Fennoscandia between ca. 1800 and 1480 Ma, followed by fragmentation during widespread extension between ca. 1340 and 1100 Ma marked by bimodal magmatism and sedimentation. A summary of Sveconorwegian magmatic, metamorphic and depositional events in the different units shows disparate histories prior to their assembly with adjacent units. The most likely interpretation of this record seems to be that episodic, Sveconorwegian metamorphic and deformational events in the central and eastern parts of the orogen represent accretion and assembly of these units. This process most likely took place behind an active margin to the southwest that sustained mafic underplating in the proximal back-arc, resulting in high- to ultrahigh-temperature metamorphism in the western parts. In this interpretation, all features of the Sveconorwegian orogen are readily explained by modern-style plate tectonic processes and hypotheses involving some form of vertical, intraplate tectonics are not supported

Anorthosite formation and emplacement coupled with differential tectonic exhumation of ultrahigh-temperature rocks in a Sveconorwegian continental back-arc setting

The tectonic setting and mechanisms and duration of emplacement of Proterozoic massif-type anorthosites and the significance of typically associated ultrahigh-temperature (UHT) host rocks have been debated for decades. This is particularly true of the Rogaland Anorthosite Province (RAP) in the SW Sveconorwegian Orogen. Earlier studies suggest that the RAP was emplaced over 1–3 Myr around 930 Ma towards the end of orogenesis, resulting in an up to 15–20 km-wide contact metamorphic aureole. However, our structural observations show that the RAP is located in the footwall of a 15 km-wide extensional detachment (Rogaland Extensional Detachment, RED), separating the intrusions and their UHT host rocks from weakly metamorphosed rocks in the hanging wall. U–Pb zircon dating of leucosome in extensional pull-aparts associated with the RED yields ages of 950–935 Ma, consistent with Re–Os molybdenite ages from brittle extensional structures in the hanging-wall block that range between 980 and 930 Ma. A metapelite in the immediate vicinity of the RAP yields a 950 Ma U–Pb age of matrix-hosted monazite, and part of the RAP was intruded by the Storgangen norite dike at ca. 950 Ma, providing a minimum age of emplacement. These ages are consistent with Ar–Ar hornblende and biotite ages that show rapid cooling of the footwall before 930 Ma, but slow cooling of the hanging wall. Field and geochronologic data suggest that the RAP formed and was emplaced over a long period of time, up to 100 Myr, with different emplacement mechanisms reflecting an evolving regional stress regime. The distribution of UHT rocks around the RAP reflects differential extensional exhumation between 980 and 930 Ma, not contact metamorphism. The duration and style of orogenic activity and externally (as opposed to gravitationally) driven extension suggest that the RAP formed in a continental back-arc setting

Tectonic evolution and paleogeography of the Kırşehir Block and the Central Anatolian Ophiolites, Turkey

In Central and Western Anatolia two continent-derived massifs simultaneously underthrusted an oceanic lithosphere in the Cretaceous and ended up with very contrasting metamorphic grades: high pressure, low temperature in the Tavsanli zone and the low pressure, high temperature in the Kirsehir Block. To assess why, we reconstruct the Cretaceous paleogeography and plate configuration of Central Anatolia using structural, metamorphic, and geochronological constraints and Africa-Europe plate reconstructions. We review and provide new Ar-40/Ar-39 and U/Pb ages from Central Anatolian metamorphic and magmatic rocks and ophiolites and show new paleomagnetic data on the paleo-ridge orientation in a Central Anatolian Ophiolite. Intraoceanic subduction that formed within the Neotethys around 100-90 Ma along connected N-S and E-W striking segments was followed by overriding oceanic plate extension. Already during suprasubduction zone ocean spreading, continental subduction started. We show that the complex geology of central and southern Turkey can at first order be explained by a foreland-propagating thrusting of upper crustal nappes derived from a downgoing, dominantly continental lithosphere: the Kirsehir Block and Tavsanli zone accreted around 85 Ma, the Afyon zone around 65 Ma, and Taurides accretion continued until after the middle Eocene. We find no argument for Late Cretaceous subduction initiation within a conceptual "Inner Tauride Ocean" between the Kirsehir Block and the Afyon zone as widely inferred. We propose that the major contrast in metamorphic grade between the Kirsehir Block and the Tavsanli zone primarily results from a major contrast in subduction obliquity and the associated burial rates, higher temperature being reached upon higher subduction obliquity.European Research Council ; Netherlands Organization for Scientific Research (NWO

A 60-million-year Cenozoic history of western Amazonian ecosystems in Contamana, eastern Peru

Weprovide a synopsis of ~60million years of life history in Neotropical lowlands, based on a comprehensive survey of the Cenozoic deposits along the Quebrada Cachiyacu near Contamana in PeruvianAmazonia. The 34 fossilbearing localities identified have yielded a diversity of fossil remains, including vertebrates,mollusks, arthropods, plant fossils, and microorganisms, ranging from the early Paleocene to the lateMiocene–?Pliocene (N20 successive levels). This Cenozoic series includes the base of the Huchpayacu Formation (Fm.; early Paleocene; lacustrine/ fluvial environments; charophyte-dominated assemblage), the Pozo Fm. (middle + ?late Eocene; marine then freshwater environments; most diversified biomes), and complete sections for the Chambira Fm. (late Oligocene–late early Miocene; freshwater environments; vertebrate-dominated faunas), the Pebas Fm. (late early to early late Miocene; freshwater environments with an increasing marine influence; excellent fossil record), and Ipururo Fm. (late Miocene–?Pliocene; fully fluvial environments; virtually no fossils preserved). At least 485 fossil species are recognized in the Contamana area (~250 ‘plants’, ~212 animals, and 23 foraminifera). Based on taxonomic lists from each stratigraphic interval, high-level taxonomic diversity remained fairly constant throughout themiddle Eocene–Miocene interval (8-12 classes), ordinal diversity fluctuated to a greater degree, and family/species diversity generally declined, with a drastic drop in the early Miocene. The Paleocene–?Pliocene fossil assemblages from Contamana attest at least to four biogeographic histories inherited from (i) Mesozoic Gondwanan times, (ii) the Panamerican realm prior to (iii) the time of South America’s Cenozoic “splendid isolation”, and (iv) Neotropical ecosystems in the Americas. No direct evidence of any North American terrestrial immigrant has yet been recognized in the Miocene record at Contamana.Facultad de Ciencias Naturales y Muse

Interpreting and reporting ⁴⁰Ar/³⁹Ar geochronologic data

The ⁴⁰Ar/³⁹Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing ⁴⁰Ar/³⁹Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of ⁴⁰Ar/³⁹Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting ⁴⁰Ar/³⁹Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of ⁴⁰Ar/³⁹Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity

Sedimentology and the facies architecture of the Ghaggar-Hakra Formation, Barmer Basin, India: Implications for early Cretaceous deposition on the north-western Indian Plate margin

Fluvial strata of the Lower Cretaceous Ghaggar-Hakra Formation are exposed in fault blocks on the central-eastern margin of the Barmer Basin, Rajasthan. The sedimentology of these outcrops is described from 114 logs (thicknesses up to 100 m) and 53 two-dimensional correlation panels. The formation comprises three distinct channel-belt sandstone packages defined as the Darjaniyon-ki Dhani, Sarnoo and Nosar sandstones separated by thick siltstone-dominated floodplain successions. The sediments were deposited in a sub-tropical, low sinuosity fluvial system that matures into a highly sinuous fluvial system. The Nosar Sandstone, the youngest of the three packages, exhibits a significant increase in energy and erosive power compared to those underlying it. This distinct change in fluvial style is interpreted to be rejuvenation due to an actively developing rift network forming accommodation space, rather than climatic controls acting on part of the depositional system. Consequently, the Ghaggar-Hakra Formation at outcrop represents Lower Cretaceous syn-rift deposition within the Barmer Basin with active localised fault movement from Nosar Sandstone times onward. These findings provide sedimentological evidence in support of pre-Palaeogene northwest-southeast extension in the Barmer Basin. Moreover, they imply Cretaceous extension took place widely along the northern extremity of the West Indian Rift System consistent with plate tectonic models of the break-up of Gondwana and evolution of the Indian Ocean. Outcrops of Lower Cretaceous strata are patchy across India and Pakistan. This study provides valuable material which, when combined with the available published data, facilitates a re-evaluation of Lower Cretaceous palaeogeography for the northwest Indian Plate. The reconstruction demonstrates a complex fluvial system, where the sediments are preserved sporadically as early syn-rift strata. The findings imply a high preservation potential for early Cretaceous fluvial successions within rifted fault blocks near Saraswati and Ashiwarya of the Barmer Basin beneath the Palaeogene fill that likely have significant potential for further hydrocarbon exploration

Causes and Consequences of Diachronous V-Shaped Ridges in the North Atlantic Ocean

In the North Atlantic Ocean, the geometry of diachronous V-shaped features that straddle the Reykjanes Ridge is often attributed to thermal pulses which advect away from the center of the Iceland plume. Recently, two alternative hypotheses have been proposed: rift propagation and buoyant mantle upwelling. Here, we evaluate these different proposals using basin-wide geophysical and geochemical observations. The centerpiece of our analysis is a pair of seismic reflection profiles oriented parallel to flowlines that span the North Atlantic Ocean. V-shaped ridges and troughs are mapped on both Neogene and Paleogene oceanic crust, enabling a detailed chronology of activity to be established for the last 50 million years. Estimates of the cumulative horizontal displacement across normal faults help to discriminate between brittle and magmatic modes of plate separation, suggesting that crustal architecture is sensitive to the changing planform of the plume. Water-loaded residual depth measurements are used to estimate crustal thickness and to infer mantle potential temperature which varies by 25◦C on timescales of 3–8 Ma. This variation is consistent with the range of temperatures inferred from geochemical modeling of dredged basaltic rocks along the ridge axis itself, from changes in Neogene deep-water circulation, and from the regional record of episodic Cenozoic magmatism. We conclude that radial propagation of transient thermal anomalies within an asthenospheric channel that is 150 50 km thick best accounts for the available geophysical and geochemical observations

Lineamenter Brudd og seismisitet i relasjon til grunnvann på Bømlo, Vestlandet

Dette arbeidet er et resultat av feltarbeid på øygruppen Bømlo i Sunnhordland, Vestlandet, samt fjernanalyse av topo- og fotolineamenter, numeriske og analytiske modeller. En analyse av 389 topolineamenter viser en distribusjon av orienteringer fra NNV - SSØ til Ø - V, men ingen markante trender. Fotolineamentene (3415) viser en nær sirkeldistribusjon av orienteringer fra hele tolkningen. Dette indikerer at det er store variasjoner i lineamentsorienteringer på Bømlo, men en analyse foretatt på mindre områder gir generelt at de tektoniske enhetene på sørlige Bømlo (Siggjogruppen, Vikafjord/Grutlefjordgruppa og Langevåggruppa) har to populasjoner av lineaments- orienteringer; NNV - SSØ og fra Ø - V til ØNØ - VSV, mens de tektoniske enhetene på nordlige Bømlo (Rolvsnesgranodioritten, Bremneskomplekset, Vardafjell-gabbroen, Geitungenheten og Lyklingofiolitten) har tre populasjoner; NNØ - SSV, ØNØ - VSV og NØ - SV. Feltarbeidet er konsentrert på nordlige Bømlo, samt spredte bakgrunnslokaliteter og viser at det er store variasjoner i bruddorienteringer (1344) ved de ulike lokalitetene. Variasjonen er størst i de plutonske bergartene i nord. Forkastingsarkitekturer er undersøkt ved forkastninger i ulike bergarter. Resultatene viser to ulike forkastnings- arkitekturer hvor den ene forkastningen har en forventet potenslov avtagning av brudd fra forkastningskjernen. Den andre forkastningen viser at deformasjonen hovedsakelig har foregått i de lokale heterogenitetene i bergarten (pegmatittganger). Observasjonene er anskueliggjort ved numeriske modeller hvor pegmatittgangene er fremstilt med høyere Youngs modul (100 GPa) enn vertsbergarten (1 GPa), og påført 10 MPa horisontal kompresjon. Resultatet viser høye spenningskonsentrasjoner i de stive legemene (pegmatittgangene) og kan trolig forklare feltobservasjonene. Feltresultatene viser at det er en lineær sammenheng mellom bruddfrekvens og tykkelse av pegmatittgangene, med en økning i bruddfrekvens i tynne pegmatittganger. Numeriske modeller bekrefter også dette og kan tyde på at observasjonene skyldes forskjeller i mekaniske egenskaper. Analytiske modeller gir også en indikasjon på at pegmatittgangene er viktige vannledere. Største horisontalspenning (σH) er funnet til å være N117ºØ. Det er en svak antydning til at fjellbrønner som er i nærheten av lineamenter som er orientert parallelt med denne vektoren har en forhøyet brønnkapasitet. Det er også en svak antyding til at småskala brudd som er orientert parallelt med denne spenningsvektoren har en forhøyet apertur. Gjentakelsesintervallet for jordskjelv i Sunnhordland kan fra Gutenberg & Ritcher sammenhengen beskrives ved en logN = 4,58 - 1,13M, som tilsvarer et gjentakelsesintervall på jordskjelv med magnitude 4 eller større hvert 16 år, 61 år for magnitude 4,5, og ca. 220 for et jordskjelv med magnitude 5