Accretion of the Archean Slave Province

Detailed field studies of selected areas in the greenstone belts of the Slave Province of Canada were presented. This area was long cited as a type area by supporters of the (now generally abandoned) rift model of greenstone belts. It was shown that a plate tectonic interpretation accounted more successfully for the regional geology and identified four terranes that had experienced complex divergent and convergent histories between 2.7 and 3.4 Ga. A dismembered ophiolite was identified and a late episode of widespread granitic intrusion was recognized

Geology of the Frozen Ocean Lake - New Bay Pond area, north-central Newfoundland

Rocks of the Frozen Ocean Group outcrop in the Frozen Ocean Lake-New Bay Pond Area, Newfoundland, and are here divided into four formations. The Lewis Lake Formation forms the base of the Frozen Ocean Group, and is composed predominantly of mafic volcanic flows; this is conformably overlain by the Blue Star Formation which is entirely sedimentary in nature. The Blue Star Formation is in turn conformably overlain.by the Bursey Point Formation, which is a mixed volcanic and sedimentary unit. The top of the Frozen Ocean Group is marked by the Lynx Pond Formation, a mixed silicic- and mafic-volcanic formation, with minor sedimentary intercalations. The Frozen Ocean Group is everywhere in fault contact with structurally underlying, medial to late Ordovician, rocks of the Shoal Arm Group and Point Leamington Formation: structural analysis of this fault zone reveals that the present juxtaposition is a result of an early Silurian back-thrusting event, although several later episodes of deformation have modified the original geometry. The back-thrusting event is believed to be a result of continued, post-Taconic convergence between the Taconic-modified margin of North America, and the remaining, open part of the Appalachian Ocean. The later deformation episodes which have affected rocks of the area include right-lateral strike-slip faulting associated with pull-apart basin formation, upright regional folding, batholith intrusion, and a second generation of dextral strike-slip faulting

Encyclopedia of earth and space science

Buku ini menekankan pembahasan pada proses fisik yang terlibat dalam pembentukan dan evolusi Bumi dan alam semesta, menjelaskan banyak contoh dari berbagai jenis fenomena geologi dan astrofisika, memberikan perspektif sejarah, dan memberikan wawasan tentang proses penyelidikan ilmiah dengan memasukkan profil biografi orang-orang yang telah memberikan kontribusi signifikan terhadap perkembangan ilmu pengetahuan. Teori big bang disajikan bersama dengan evaluasi prinsip-prinsip fisik dan hukum dasar. Struktur yang dihasilkan dari alam semesta, galaksi, tata surya, planet, dan tempat-tempat di Bumi juga dibahas

A Paradigm Shift: North China Craton's North Margin Orogen Is the Collisional Suture With the Columbia Supercontinent

Abstract In a new study, Wu, Wang, Zhou, Zhao, Haproff, et al. (2022, https://doi.org/10.1029/2022GC010662) present a comprehensive study of the North Margin Orogen of the North China Craton (NCC), showing that older accreted rocks in this belt preserve a record of active margin magmatism from 2.2 to 2.0 Ga, followed by collisional tectonics, marked by mélange and mylonitic shear zones, then granulite facies metamorphism at 1.9–1.8 Ga, marking the final collision of the NCC with the Columbia Supercontinent. The multidisciplinary studies presented in this work support earlier suggestions that the North China Craton amalgamated during accretionary orogenesis in the Neoarchean to earlier Paleoproterozoic, and that the late widespread 1.85 Ga high‐grade metamorphism is craton‐wide in scale, and not confined to a narrow orogen in the center of the craton. This new understanding creates new possibilities for refining reconstructions of one of Earth's earliest, best documented supercontinents, showing a globally linked plate network at 1.85 Ga, and suggests drastic new correlations and models for mineral resource exploration

The importance of a weak mid-lithospheric layer on the evolution of the cratonic lithosphere

Seismically detectable discontinuities at mid-depths of some cratonic lithospheric mantle define mid-lithosphere discontinuities (MLD), demonstrating that the lithospheric mantle is layered. The genesis and strength of the MLD are still in debate, most proposed models suggest the MLD is likely not weaker than the normal lithosphere, whereas other proposed models suggest that some metasomatised MLD rocks are weaker than the normal lithospheric mantle rocks. Thus, the weak MLD is likely a weakly-coupled layer at mid-depths in some cratonic lithosphere blocks, possibly influencing their stabilities. We assess the geodynamic significance of the MLD using geodynamic modeling. We propose that a weak MLD, with lower effective viscosity, can be connected to thinned cratonic margins during the evolution of some cratons and form continuously connected weak zones from cratonic margins to craton interiors, which can lead to lithospheric thinning or removal by extension, basal drag, delamination, thermochemical erosion, and other actions. Through analyzing different scenarios, we propose that some samples of weak MLDs can be found in a composite ophiolite profile formed on the Precambrian Karelian continental margin, with both continental and oceanic lithosphere, which is supported by chronological, petrological, and structural architectures of the profile. This creates new opportunities to directly study the properties of the MLD, which could help understand and settle the controversies on the origin of the MLD and its physical, chemical, and geophysical properties

P–T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

The southern Alaska convergent margin contains several small belts of sedimentary and volcanic rocks metamorphosed to blueschist facies, located along the Border Ranges fault on the contact between theWrangellia and Chugach terranes. These belts are significant in that they are the most inboard, and thus probably contain the oldest record of Triassic–Jurassic northward-directed subduction beneath Wrangellia. The Liberty Creek HP-LT schist belt is the oldest and the innermost section of the Chugach terrane.Within this belt lawsonite blueschists contains an initial high-pressure assemblage formed by lawsonite+phengite+chlorite+sphene+albite± apatite±carbonates and quartz. Epidote blueschists are composed of sodic, sodic–calcic and calcic amphiboles+ epidote+phengite+chlorite+albite+sphene±carbonates and quartz. P–T pseudosections computed from four representative samples constrain maximumpressures at 16 kbar and 250–280 °C for the Lawsonite-bearing blueschists, and 15 kbar and 400–500 °C for the epidote-bearing blueschists, suggesting a initial subduction stage of 50–55 km depth. The growth of late albite porphyroblasts in all samples suggests a dramatic decompression fromca. 9 kbar to 5 kbar. The Liberty Creek schists can be correlated with the Seldovia blueschist belt on the Kenai Peninsula.Metamorphismin both terranes took place in the Early Jurassic (191–192 Ma), recording an early stage of subduction beneathWrangellia. In the nearby terranes of the same margin, the age ofmetamorphismrecords an early stage of subduction at 230 Ma. Based on this difference in age, a maximum of 40Ma were necessary to subduct the protoliths of the Seldovia and Liberty Creek blueschists to depths of circa 50–55 km, suggesting a minimum vertical component of subduction of 1.2–1.5 cm/year

Density and viscosity changes between depleted and primordial mantle at ∼1000 km depth influence plume upwelling behavior

Earth's mantle is more than 2800 km deep, compositionally heterogenous, and potentially stratified. However, understanding of its heterogeneities and stratification is limited. Recently, plume-like anomalies were detected to exhibit different types of anomalous behavior at ∼1000 km depth, where they laterally pond, neck or broadly penetrate to the upper mantle, suggesting that their behavior may be influenced by possible viscosity or density stratification near this depth. However, the specific key reasons are unclear. Here, we use 2D thermal-mechanical-chemical modeling to constrain the key factors causing these large-scale plume-related anomalies. Upward mantle plume penetration at ∼1000 km depth is mainly caused by large source volume and excess temperature, whereas its necking and lateral extension at this depth can only be caused by viscosity and composition-related density stratification, respectively. Considering the various plume behaviors at this proposed boundary, we show that Earth's mantle is likely heterogeneously stratified at ∼1000 km depth, with regional viscosity and/or density changes of different lateral scales and vertical gradients. We speculate that this boundary separates the upper depleted mantle from the primordial mantle domain below. This fundamental boundary has been progressively evolving during stratification of Earth's mantle through melt extraction and mantle stirring throughout Earth history. Highlights • Different plume behaviors suggest regional mantle layering at ∼1000 km depth. • The viscosity layering can lead to plume necking at ∼1000 km depth without ponding. • A composition-induced density stratification can explain the ponding phenomenon. • The regional mantle layering is related to density and/or viscosity stratification. • This likely indicate the boundary between depleted mantle and primordial mantle