Continental Masses Research Articles

The Mesozoic and Cenozoic motion of Adria (central Mediterranean): a review of constraints and limitations

This paper presents kinematic analysis on the motion of Adria, which is the continental mass that bridges Africa and Europe in the central Mediterranean. Palaeomagnetic data show a general coherence between the motion of Adria and Africa since the Late Paleozoic. This mutual motion, for the period from 120 Ma and the present, is verified by comparing inferred palaeolatitudes from relatively stable parts of Adria (Apulia, Gargano, Istria, and the Southern Alps) and the Hyblean Plateau, with latitudinal changes that are calculated from the motion of Africa with respect to hotspots. Additional constraints on the motion of Adria are provided from the Late Paleozoic-Early Mesozoic passive margin of Adria in the Ionian Sea. The seismic structure of the floor of the Ionian Sea resembles the structure of the oceanic crust in marginal back-arc basins, suggesting that it formed as a small ocean basin. Furthermore, the Ionian lithosphere in the Calabrian arc has been subjected to rapid rollback, which commonly occurs only when the subducting slab is made of oceanic lithosphere. This oceanic domain marks the Permian-Triassic to Jurassic plate boundary between Adria and Africa, suggesting that a small amount of independent motion between Adria and Africa took place at that time. Since the Jurassic, Adria and Africa have shared a relatively coherent motion path.

55 million years of continuous anatexis in Central Iberia: single-zircon dating of the Peña Negra Complex

Single-zircon evaporation and ion-microprobe dating of migmatites and anatectic granites in the Peña Negra Complex of Central Iberia reveal that the Variscan anatexis occurred continuously from 352 to 297 Ma, with a maximum at 335–305 Ma. Anatexis began coeval with the main collision of continental masses. A limited melting event, probably related to syncollision crustal-scale shear zones, produced a population of zircons with ages of c . 350 Ma. The production of new zircons decreased to a minimum at c . 343 Ma but then increased swiftly as the internal thermal evolution of the thickened Central Iberian crust led to widespread anatexis in the Peña Negra region at 332 Ma. Shortly after this, the melt resident in the migmatites was locally segregated into small bodies that crystallized as cordierite leucogranites at 321 Ma. Simultaneously, extensional subhorizontal shear zones were preferentially developed over layers of the migmatite series that, owing to their elevated heat production and fertility, had the highest melt fraction. Shearing provoked further anatexis and contributed significantly to the in situ production of high melt-fraction granodiorites and adamellites from the migmatites. This process occurred from 325 to 305 Ma, with a maximum at 309 Ma marking the peak of the Variscan extensional collapse in Central Iberia. After c . 305 Ma the melt fraction decreased quickly, so that the production of new zircons was insignificant at 300 Ma and had stopped completely by 297 Ma.

The 3.4–3.5 Ga São José do Campestre massif, NE Brazil: remnants of the oldest crust in South America

The oldest fragment of continental crust recognized in South America occurs as an isolated Archean enclave in northeastern Brazil’s Borborema Province, ca. 600 Ma Brasiliano–Pan African orogenic belt. This Archean fragment, the São José do Campestre massif, is surrounded by large tracts of 2.2–2.0 Ga Paleoproterozoic gneisses and is located more than 600–1500 km from the much larger assemblages of Archean rocks found in the São Fransciso and Amazonian cratons, located to the south and west, respectively. Geochronological studies of the São José do Campestre massif show that its oldest rocks contain zircons with U–Pb ages up to 3.5 Ga and Sm–Nd T DM model ages of more than 3.7 Ga, indicating that they represent “reworked” crust. This older “nucleus” is flanked by both reworked and juvenile 3.25 and 3.18 Ga rocks which are intruded by both 3.00 and 2.69 Ga plutonic bodies. The protracted evolution the São José do Campestre massif is consistent with that of a larger continental mass as opposed to a small crustal fragment that grew in isolation. As such, the São José do Campestre massif is interpreted as representing a detached piece of an evolved craton that became entrained with younger rocks during a subsequent Paleoproterozoic accretionary-orogenic event. This hypothesis is bolstered by the presence of Paleoproterozoic gneisses that envelop the São José do Campestre massif, as well as the existence of ca. 2.0 Ga metamorphic zircon and monazite within its rocks. The occurrence of several different Archean “cratonic basement inliers” within the greater Paleoproterozoic crustal framework of the Borborema Province suggests that cratonic slices spalled off one or more larger Archean masses prior to the ca. 2.2–2.0 Ga Paleoproterozoic orogenic collage. A important challenge is to link these older fragments to their parent cratons. Although results are not unique, the pattern of ages and isotopic signatures observed in the São José do Campestre massif is similar to that seen in parts of the São Francisco Craton, and it is possible that the São José do Campestre massif is a fragment of an Archean continental fragment formed during an episode of continental breakup prior to 2200 Ma.

Identifying important endemic areas using ecoregions: birds and mammals in the Indo-Pacific

Concentrations of large numbers of endemic species have been singled out in prioritization exercises as significant areas for global biodiversity conservation. This paper describes bird and mammal endemicity in Indo-Pacific ecoregions. An ecoregion is a relatively large unit of land or water that contains a distinct assemblage of natural communities. We prioritize 133 ecoregions according to their levels of endemicity, and explain how variables such as biome type, whether the ecoregion is on an island or continental mass, montane or non-montane, correlate with the proportion of the total species assemblage that are endemic. Following an exploratory principal components analysis we classify all ecoregions according to the relationship between numbers of endemics and overall species richness. Endemicity is negatively correlated with species richness. We show that plotting the logit transformation of the endemicity of birds and mammals against log of species richness is a more effective and useful way of identifying important ecoregions than simply ordering ecoregions by the proportion of endemic species, or any other single measure. The plot, divided into 16 regions corresponding to the quartiles of the two variables, was used to identify ecoregions of high conservation value. These are the ecoregions with the highest endemicity and lowest species richness. Further analysis shows that island and montane ecoregions, regardless of their biome type, are by far the most important for endemic species.

PERMIAN TO LOWER CRETACEOUS PLATE TECTONICS AND ITS IMPACT ON THE TECTONO-STRATIGRAPHIC DEVELOPMENT OF THE WESTERN AUSTRALIAN MARGIN

The post-Lower Permian succession of the Perth Basin and Westralian Superbasin can be directly related to the plate tectonic evolution of the Gondwanan Super-continent. In the Late Permian to Albian the northern edge of Gondwana continued to break into microplates that migrated to the north and were accreted into what is today the southeastern Asia (Burma–China) region. These separation events are recorded as a series of stratigraphically distinct transgressions (corresponding to the initial stretching of the asthenosphere and acceleration of subsidence rates) followed by rapid regressions (when new oceanic crust was emplaced in thinned continental crust causing uplifts of large continental masses). Because the events are synchronous across large regions, and may be identified from specific log and seismic signatures, the intensity of stratigraphically related transgressive/regressive cycles varies, depending on the distance from the break-up centres and these cycles allow the identification of regionally significant megasequences even in undrilled areas. The tectonic evolution and resulting stratigraphy can be described by eight plate tectonic events:Visean (Carboniferous) break-up of the southeastern Asia (Simao, Indochina and South China);Kungurian (uppermost Early Permian) break-up of Qiangtang and Sibumasu;Lowermost Norian uplift due to Bowen Orogeny in eastern Australia;Hettangian break-up of Mangkalihat (northeastern Borneo);Oxfordian break-up of Argo/West Burma, and Sikuleh (Western Sumatra);Kimmeridgian break-up of the West Sulawesi microplate;Tithonian break-up of Paternoster-Meratus (central Borneo); andValanginian break-up of Greater India/India.These events should be identifiable in all Australian Phanerozoic basins and beyond, potentially providing a template for a synchronisation of the Permian to Early Cretaceous stratigraphy.

Geologic and climatic puzzle of the Proterozoic snowball earth

P V Sukumaran took his M.Tech degree in Applied Geology from the University of Saugar and has been working with the Geological Survey of India since 1974. His interests include petrology, geochemistry, palaeoceanography and organic evolution. He has contributed several articles to Resonance. According to the hypothesis of 'snowball earth' the planet was covered in tow with ice for millions of years in two major pulses of global glaciation in the Proterozoic era of earth history. The earlier event of snowball earth is less understood but the subsequent one has overwhelming evidence in the geological record of rock strata. Accelerated chemical weathering on tropically positioned continental masses is believed to have withdrawn excess CO 2 from the atmosphere, initiating global cooling and eventually plunging the whole earth under ice cover. Accumulation of volcanically emitted CO 2 in the atmosphere during glacial ice cover melted the ice by greenhouse warming bringing back normalcy.

African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ~1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ~1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa. Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goı́ás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil. Furthermore, there is evidence for extensive granitoid magmatism in the period ~840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.

Hydrology and water resources in monsoon Asia: a consideration of the necessity of establishing a standing research community of hydrology and water resources in the Asia Pacific region

AbstractHydrological and water resources issues appear very differently in different regions, and are strongly affected by geographical conditions. Hydrological knowledge and methodologies obtained in a specific region cannot necessarily be adapted to other regions. The purpose of this paper is to clarify one way to address adequately the regional characteristics of hydrology and water resources in monsoon Asia, especially the ‘too much water’ problems in the region. For this purpose, geomorphological factors, climatic factors and human intervention in the natural environment are taken into consideration as the three major factors governing the regional characteristics of the hydrology–water resources system. To identify geomorphological features macroscopically between the Asia Pacific region and other continental regions, the concepts ‘tectonic zone’ and ‘stable region’, which are two major subdivisions of continental masses in the world, are introduced. Also, a new climatic subdivision termed ‘warm‐humid’ is proposed to express the abundant precipitation due to the Asian monsoon. Then, hydrological characteristics common or similar in ‘warm‐humid tectonic zones’ in the Asia Pacific region, contrasted with those in stable regions, are enumerated together with the human intervention corresponding to these characteristics, and research targets peculiar to warm‐humid tectonic zones are discussed. Finally, the establishment of a standing research community called ‘Asia Pacific Association of Hydrology and Water Resources’ is proposed to promote the exchange of operational knowledge and experience in water resources management, cooperative research activities, and professional education in the Asia Pacific region. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Inversion of Earth's changing shape to weigh sea level in static equilibrium with surface mass redistribution

We develop a spectral inversion method for mass redistribution on the Earth's surface given geodetic measurements of the solid Earth's geometrical shape, using the elastic load Love numbers. First, spectral coefficients are geodetically estimated to some degree. Spatial inversion then finds the continental surface mass distribution that would force geographic variations in relative sea level such that it is self‐consistent with an equipotential top surface and the deformed ocean bottom surface and such that the total (ocean plus continental mass) load has the same estimated spectral coefficients. Applying this theory, we calculate the contribution of seasonal interhemispheric (degree 1) mass transfer to variation in global mean sea level and nonsteric static ocean topography, using published GPS results for seasonal degree‐1 surface loading from the global IGS network. Our inversion yields ocean‐continent mass exchange with annual amplitude (2.92 ± 0.14) × 1015 kg and maximum ocean mass on 25 August ±3 days. After correction for the annual variation in global mean vertical deformation of the ocean floor (0.4 mm amplitude), we find geocentric sea level has an amplitude of 7.6 ± 0.4 mm, consistent with TOPEX‐Poseidon results (minus steric effects). The seasonal variation in sea level at a point strongly depends on location ranging from 3 to 19 mm, the largest being around Antarctica in mid‐August. Seasonal gradients in static topography have amplitudes of up to 10 mm over 5000 km, which may be misinterpreted as dynamic topography. Peak continental loads occur at high latitudes in late winter at the water‐equivalent level of 100–200 mm.

From Rodinia to Gondwana: A Review of the Available Evidence from South America

In this work we will examine the available evidence for the position of the cratonic masses that formed West Gondwana, in the Neoproterozoic, such as Amazonia, West Africa, São Francisco-Congo, Rio de La Plata and Kalahari. Some emphasis will be given to South America, and the smaller continental masses or microcontinents, that are usually forgotten in the exercise of global reconstruction, will also be considered. Among others, such tectonic units are the Central Goiás massif, the Borborema/Trans-Saharan block, as well as the Luiz Alves, Paraná and Pampia cratonic fragments. Taking into account the relevant paleomagnetic and geochronological data, it is necessary to consider a large time interval for the process of break up of Rodinia. Signals of extension and fracturing are as old as 1000-1100 Ma, when mafic dyke swarms are observed on the São Francisco-Congo craton, and final break out is reported as young as 750 Ma. On the other hand, the formation of intra-oceanic magmatic arcs within the large Goiás Ocean occurred prior to 930 Ma ago. Moreover, for the make up of Gondwana, the first continental collisions are reported at ca. 800 Ma, and the most important accretion episodes are the ones between 650 and 500 Ma, corresponding to the Brasiliano/Pan-African Orogenic Cycle. Therefore, it is apparent that fragmentation of Rodinia is partially synchronous with agglutination of Gondwana.

Is high obliquity a plausible cause for Neoproterozoic glaciations?

The main features of the low‐latitude Neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers. Here we use an AGCM coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations: a higher obliquity of the Earth's rotation axis. We show that high obliquity may result in an extensive glaciation during the Sturtian episode (750 Ma), due to the location of continental masses in the tropical areas, but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the Varangian‐Vendian episodes (620–580 Ma).

Paleozoic orogenic gold deposits in the eastern Central Andes and its foreland, South America

In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Córdoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Marañón Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane. The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.

A correlation between mid-ocean-ridge basalt chemistry and distance to continents.

To fully understand the structure and dynamics of the Earth's convecting mantle, the origins of temperature variations within the mantle need to be resolved. Different hypotheses have been proposed to account for these temperature variations: for example, heat coming from the decay of radioactive elements or heat flowing out of the Earth's core. In addition, theoretical studies suggest that the thermal properties of continental masses can affect mantle convection, but quantitative data that could allow us to test these models are scarce. To address this latter problem, we have examined the chemistry of mid-ocean-ridge basalt--which reflects the temperature of the source mantle--as a function of the distance of the ridge from the closest continental margin. No correlation is observed for oceanic ridges close to subduction zones or hotspots; subduction zones probably inhibit thermal transfer between the mantle beneath continents and ocean, whereas hotspots influence the major-element chemistry of ridge basalts, which makes their interpretation with respect to mantle temperature more difficult. However, we do observe a significant correlation for mid-oceanic basalts from the Atlantic and Indian oceans. From this, we conclude that the location of continental masses relative to active ridges influences the large-scale thermal structure of the mantle and we estimate that the mantle cools by 0.05 to 0.1 degrees C per kilometre from the continental margins.

A mechanism of the onset of the Australian summer monsoon

An onset mechanism of the Australian summer monsoon that incorporates possible air‐sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land‐ocean thermal contrast during the premonsoon period results in a well‐organized continental‐scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600–700 hPa level. Intensified low‐level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at ∼700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large‐scale disturbances with ascending motion (such as the Madden‐Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics.

Crustal structure of central Lake Baikal: Insights into intracontinental rifting

The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39–42.5 km; an 8‐km‐thick, laterally continuous high‐velocity (7.05–7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high‐velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by &lt;3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold‐and‐thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two‐stage rift evolution as is commonly assumed.

On bimodal differentiation by solidification front instability in basaltic magmas, part 1: basic mechanics

Extensive, coarse-grained silicic lenses are common in the upper portions of large diabase sills, lava lakes, and gabbroic intrusions. These lenses, which often contain up to 10 wt% more silica than their host rocks, record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems. The spatial, compositional, textural, and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front. This process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation. As the upper solidification front grows inward and thickens, the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself. This necessary local variation in strength dictates how crystallinity must vary to prevent failure. But the actual variation in crystallinity is determined by phase equilibria, which in general varies in a fashion that is noncovariant with that required for mechanical stability. It is this mismatch that apparently leads to failure and formation of silicic segregations. The rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening, and the overall size of the tears is regulated by the local rate of advance of the solidification front. Segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front. This process irreversibly introduces siliceous noise into basaltic systems. The silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma. This process may be fundamental to achieving the strong compositional diversity of the igneous rocks, to yielding continental masses, and to the differentiation of Earth itself.

The Nagssugtoqidian Orogen of West Greenland: tectonic evolution and regional correlations from a West Greenland perspective

The Nagssugtoqidian Orogen of West Greenland represents a belt of Palaeoproterozoic deformation and metamorphism between the North Atlantic Craton of South Greenland and a northern, lesser known continental segment that includes the Rinkian Orogen. First-order observations are interpreted to support a cycle of separation, convergence, and eventual collision of two continental masses. The emplacement of the Kangâmiut dyke swarm marked the onset of continental breakup at ca. 2040 Ma, and sedimentary basins formed between ca. 1950 and 1920 Ma. Subsequent convergence and consumption of an oceanic plate caused arc magmatism at 1920–1870 Ma. Granulite-facies peak metamorphism at 1860–1840 Ma in the centre of the orogen is related to crustal thickening by WNW-directed thrusting. Large-scale, upright folding with an east–west trend was ongoing by 1825 Ma. Sinistral strike-slip movement was concentrated along steeply dipping limbs of these large-scale folds and formed orogen-scale steep belts at ca. 1775 Ma. Close similarities between the northern and southern foreland suggest that the two cratons likely originated from one continuous continental block. Temporal and kinematic correlation of these events with adjoining orogens in Canada and Greenland shows close genetic links. The Nagssugtoqidian Orogen of West Greenland continues eastwards beneath the Greenland Ice cap to the Eastern Nagssugtoqidian belt of East Greenland (a.k.a. the Ammassalik belt). The Torngat Orogen of eastern Canada developed simultaneous with the Nagssugtoqidian Orogen with a kinematic compatibility suggesting that the two orogens formed on the west and north flanks, respectively, of a curved leading continental margin of an indenting North Atlantic Craton.

U-Pb zircon age constraint for late Neoproterozoic rifting and initiation of the lower Paleozoic passive margin of western Laurentia

A concordant U–Pb zircon age of 569.6 ± 5.3 Ma from synrift volcanic rocks of the Hamill Group, southeastern Canadian Cordillera, provides the first direct U–Pb geochronologic constraint on timing of latest Neoproterozoic rifting along western Laurentia. This age confirms a previous estimate of 575 ± 25 Ma for timing of continental breakup, as derived from the analysis of tectonic subsidence in lower Paleozoic miogeoclinal strata of the North American Cordillera. It also corresponds to the timing of passive margin deposition in the "underlying" Windermere Supergroup of the northern Cordillera, as determined by chemostratigraphic correlations. These timing relationships imply a different breakup history for the northern, as compared to the southern, Cordillera. We propose a model that attempts to explain this paradox of Cordilleran geology. The earlier Neoproterozoic (Windermere-age) rifting event probably records breakup of a continental mass from northern Laurentia followed by development of a passive margin. Accordingly, the Windermere Supergroup of the southern Canadian Cordillera was deposited in an intracontinental rift. The second Neoproterozoic rifting (Hamill–Gog) is interpreted to indicate continental breakup and establishment of a passive margin along western Laurentia.

The development of lithospheric keels beneath the earliest continents: time constraints using PGE and Re-Os isotope systematics

Abstract Continued studies of xenolith suites found in kimberlites on and around the Kaapvaal Craton, together with those from newly discovered localities on other cratons, are providing new insights into the generation and evolution of the Earth’s oldest continents. Comparison of modal abundance data with melt depletion models, together with trace element and isotope systematics in Kaapvaal low-temperature peridotites, suggest that much or all of the diopside and garnet in these rocks may have formed significantly after initial melt depletion. The Re-Os isotope system has been instrumental in providing an improved understanding of the timing of the formation of cratonic lithospheric keels. New studies that focus on carefully selected whole-rock peridotites and use combined platinum group element (PGE) and Re-Os isotope analysis provide better constraints on the significance of Re-Os model ages. The large database of Re-Os isotope analyses for peridotites for the Kaapvaal Craton indicate formation of significant amounts of lithospheric mantle in Neoarchaean time, associated with voluminous mafic magmatism. Formation of lithospheric mantle in Neoarchean time (3.0–2.5 Ga) follows the cessation of major crustal differentiation events at c . 3.1 Ga and marks the onset of craton stabilization. Some lithospheric mantle was produced in Palaeo- to Mesoarchaean time (3.8–3.0 Ga) in southern Africa, which preserved ancient crustal fragments. Large-scale preservation of Archaean continental masses was effective only after the formation of substantial, buoyant, rigid, deep lithospheric keels and their stabilization in Neoarchean time. Formation of lithospheric mantle beneath the surrounding Proterozoic crustal regions occurred in Mesoproterozoic time, with lower degrees of mantle melting than associated with the cratonic peridotites. This circum-cratonic mantle is of similar age to the oldest overlying crust and has been coupled to the margins of the craton since its formation. Major magmatic events, some coincident with the formation of circumcratonic mantle, added new lithosphere to the Kaapvaal mantle root but failed to destroy it. The mechanically strong, buoyant lithospheric keels beneath cratons protect their crust from subduction and recycling over 3 Ga time periods.

Quantifying paleogeography using biogeography: a test case for the Ordovician and Silurian of Avalonia based on brachiopods and trilobites

Despite substantial advances in plate tectonic modeling in the last three decades, the postulated position of terranes in the Paleozoic has seldom been validated by faunal data. Fewer studies still have attempted a quantitative approach to distance based on explicit data sets. As a test case, we examine the position of Avalonia in the Ordovician (Arenig, Llanvirn, early Caradoc, and Ashgill) to mid-Silurian (Wenlock) with respect to Laurentia, Baltica, and West Gondwana. Using synoptic lists of 623 trilobite genera and 622 brachiopod genera for these four plates, summarized as Venn diagrams, we have devised proportional indices of mean endemism (ME, normalized by individual plate faunas to eliminate area biogeographic effects) and complementarity (C) for objective paleobiogeographic comparisons. These can discriminate the relative position of Avalonia by assessing the optimal arrangement of inter-centroid distances (measured as great circles) between relevant pairs of continental masses. The proportional indices are used to estimate the “goodness-of-fit” of the faunal data to two widely used dynamic plate tectonic models for these time slices, those of Smith and Rush (1998) and Ross and Scotese (1997). Our faunal data are more consistent with the latter model, which we use to suggest relationships between faunal indices for the five time slices and new rescaled inter-centroid distances between all six plate pairs. We have examined linear and exponential models in relation to continental separation for these indices. For our generic data, the linear model fits distinctly better overall. The fits of indices generated by using independent trilobite and brachiopod lists are mostly similar to each other at each time slice and for a given plate, reflecting a common biogeographic signal; however, the indices vary across the time slices. Combining groups into the same matrix in a “total evidence” analysis performs better still as a measure of distance for mean endemism in the “Scotese” plate model. Four-plate mean endemism performs much better than complementarity as an indicator of pairwise distance for either plate model in the test case.