Global Rift Research Articles

A new formation model of the Atlantic-Arctic heterochronous rifting system: A concept and basic provisions

A new formation model of the global Atlantic-Arctic heterochronous rifting system is substantiated, according to which the Central and North Atlantics, Labrador-Baffin seas, and Arctic Ocean represent morpho-tectonic elements of different orders of the united recent Atlantic-Arctic Ocean. Evolution of the global rift system of this ocean includes three stages: the first stage (D–J1) was expressed by establishment of a tectonic zone in the lithosphere of Pangea with formation of the ophiolite ultrabasite-gabbro association; the second stage (J1–₽1) represented stretching of the continental crust to form depressions and uplifts with areal manifestation of trap magmatism of Cretaceous and other ages; and the third one (₽2–present) is neotectonic-magmatic reactivation with formation of a middle ridge, that is being accompanied by outpouring of glassy basalts and by hydrothermal manifestations. Within the framework of this model, the history of formation is reconstructed of the Eurasian Basin and the Gakkel Ridge, that were included in the Russia's updated application for expansion of the outer continental shelf border.

Early Devonian (415–400 Ma) A-type granitoids and diabases in the Wuyishan, eastern Cathaysia: A signal of crustal extension coeval with the separation of South China from Gondwana

Abstract The evolution of the South China continental crust and its linkage to the assembly and rifting of eastern Gondwana are key issues in the understanding of the early Paleozoic evolution of eastern Asia. We report U-Pb zircon ages and geochemical and Lu-Hf isotopic data for the South Fufang and Yingshang granitoids and the Mayuan diabases from the Wuyishan of eastern South China. The zircons yielded U-Pb ages of ca. 414–404 and ca. 409–401 Ma for the granitoids and diabases, respectively. Petrographic and geochemical features indicate that the granitoids are peraluminous A-type granites, expressed by high Ga/Al ratios and high Zr, Nb, Ce, Y, and rare earth element contents. They show negative zircon εHf(t) values (–15.4 to –5.8), consistent with the derivation from a crustal source. The granitoids likely originated from partial melting of dry granulite residues in the lower crust. The diabases show depletion in Ti, and negative correlations between FeOt and Mg#, and SiO2 and TiO2/FeOt, reflecting clinopyroxene, olivine, and Fe-Ti oxide fractionation. Their negative zircon εHf(t) values (–4.5 to –0.4) indicate an ancient enriched-mantle origin. The diabases likely originated from partial melting of a sub-continental lithospheric mantle. We interpret these A-type granitoids and diabases as post-orogenic, formed during extensional collapse of thickened crust. Their generation indicates that South China experienced crustal extension during the Early Devonian. The extension occurred coevally with global rifting that led to the separation of the continental blocks of eastern Asia from eastern Gondwana, which was associated with the Early Devonian opening of the paleo–Tethys Ocean.

Hidden tectonism on Miranda's Elsinore Corona revealed by polygonal impact craters

We mapped the spatial distribution and orientations of polygonal impact craters (PICs) on the classical Uranian satellite Miranda to gain insight into the complex tectonic history of this moon. PIC straight rim segments in regions without visible fractures can be used to infer sub-regolith and/or sub-resolution fracture orientations. We identified and analyzed 14 PICs on Miranda, nine of which are located on its trailing hemisphere, overprinting Elsinore Corona and the nearby cratered terrain. These PICs do not follow the E-W orientations of the ridges within Elsinore's bounding terrain, indicating that these ridges are not underlain by deep-rooted (at least as deep as the crater bowl), sub-vertical normal or strike-slip faults. Instead, the ridges may be contractional features, consisting of folds and possibly thrust faults, or cryovolcanic features without fault structures. Furthermore, the PICs in this region indicate that a previously undetected NW oriented fracture system is present over much of Miranda's trailing hemisphere. Another PIC, somewhat distal from Elsinore on Miranda's anti-Uranus hemisphere, might be associated with the NW fractures as well. Additionally, three PICs are present proximal to Verona Rupes on Miranda's Uranus-facing hemisphere. These PICs exhibit N-S orientations consistent with the surrounding fractures that make up Miranda's Global Rift System. One other PIC is proximal to Arden and might be associated with the Arden bounding terrain. Identification of the NW fracture system provides new information on Miranda's complex geologic history. Based on cross cutting relationships with Miranda's ancient subdued crater population, the NW fracture system is older than Elsinore. However, this fracture system also overprints Elsinore, suggesting reactivation of the NW fractures after this corona had formed. Along with discussion of Miranda's geologic history, we consider the possibility that Miranda has undergone true polar wander, and we compare its terrains and geologic features to similar features on the Saturnian satellite Enceladus.

Continental crustal evolution and synchronous metallogeny through time in the North China Craton

The generation and evolution of continent crust have long been matters of debates. There is evidence to support the view that the first formation of TTGs of continent was possibly before 4.4 Ga, followed by the sizable continent crust associated with BIF-bearing supra-crustal rocks at ca. 3.8 Ga. Multiple stages of continental growth include continental nuclei at ~3.1 Ga, enormous crustal growth at 2.9–2.7 Ga and stabilization and cratonization at ~2.5 Ga. Subsequently a stable super-continent was formed and the Earth was evolved into the Proterozoic eon. In the Proterozoic eon, major geological events include the great oxygen event (GOE), global rift and mobile belt event, and the Earth’s middle age event with a 1.0 billion years long period of Earth’s adjustment or silence. After that, the Rodinian super-continental break up event (the Nanhua rift) and subsequent Gondwana amalgamation have been suggested to be involved with mantle-plume and the modern style-like plate tectonics, both of which are still under debates. Recent studies suggest that the continental metallogeny has secular changes with the evolving Earth, showing synchronous evolution with the continental crust through time. The North China Craton (NCC) is one of the oldest cratons in the world, and has undergone six major tectono-metallogenic cycles: (1) the Neoarchean crustal growth and Algoman-type BIFs-greenstone belt and gold metallogenic system, (2) Paleoproterozoic GOE associated with global (Huronian) rifting- Glaciation and Superior-type BIFs-Mg-B metallogenic system, (3) Paleoproterozoic rifting-subduction-accretion-collision event (mobile belt) and Cu-Pb-Zn metallogenic system; (4) Late Paleoproterozoic – Neoproterozoic multi-stage rifting and REE-Fe-Pb-Zn metallogenic system; (5) Paleozoic orogenesis at the margins of the craton and porphyry Cu-Mo metallogenic system; and (6) Mesozoic extensional tectonics associated with lithosphere thinning and intracontinental Au metallogenic systems. Ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities. Changes of metallogenic styles with time, from the dominant BIFs in Archean to polymetallic deposits in Proterozoic and Phanerozoic, demonstrate that the nature and style of metallogeny through time change with different stages of the Earth’s evolution and are the expression of variable materials at the different stages. The changes of metallogeny through time are controlled by the irreversible continental tectonic evolution. The connotation and signification of synchronous evolution of continental crust and mineral deposit systems through time are beyond the study of mineral resources but would provide new insights into the geodynamics of the generation and evolution of the continent. The NCC provides an excellent venue to address the linkage between metallogenic processes and secular changes in the Earth System, including mantle dynamics and crust-mantle interaction on a global perspective.

Late Imperialism

The globalization of production (and finance)—which emerged along with neoliberalism out of the economic stagnation of the mid–1970s and then accelerated with the demise of Soviet-type societies and China's reintegration into the capitalist world system—has generated a more generalized monopoly capitalism, ushering in what can be called late imperialism. Late imperialism refers to the present period of monopoly-finance capital and stagnation, declining U.S. hegemony and rising world conflict, accompanied by growing threats to the ecological bases of civilization and life itself. It stands at its core for the extreme, hierarchical relations governing the capitalist world economy in the twenty-first century, which is increasingly dominated by mega-multinational corporations and a handful of states at the center of the world system. Just as it is now common to refer to late capitalism in recognition of the end times brought on by simultaneous economic and ecological dislocations, so it is necessary today to speak of late imperialism, reflecting the global dimensions and contradictions of that system, cutting across all other divisions, and posing a "global rift" in human historical development: an epochal crisis posing the question of "ruin or revolution."

Notes from the Editors, July-August 2019

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 This special issue of Monthly Review is meant both to celebrate the fiftieth anniversary of Harry Magdoff's The Age of Imperialism: The Economics of U.S. Foreign Policy, which was devoted to the analysis of imperialism at the height of U.S. hegemony, and to carry this analysis forward to address the present era of late imperialism in the twenty-first century. In bringing together work on the political economy of imperialism in the current era of globalized production, we seek to transcend the now fashionable view within the Western academic left that the concept of imperialism is obsolete. The imperialist world system stands not only for capitalism at its most concrete historical level, but also for the entire dynamic structure of power constituting accumulation on a world scale, which can only be understood in terms of a developing global rift between center and periphery, global North and global South. Failure to attend to this fissure would be fatal for humanity.

The Origin of Carbonatites from Amba Dongar within the Deccan Large Igneous Province

Abstract There are disparate views about the origin of global rift- or plume-related carbonatites. The Amba Dongar carbonatite complex, Gujarat, India, which intruded into the basalts of the Deccan Large Igneous Province (LIP), is a typical example. On the basis of new comprehensive major and trace element and Sr–Nd–Pb isotope data, we propose that low-degree primary carbonated melts from off-center of the Deccan–Réunion mantle plume migrate upwards and metasomatize part of the subcontinental lithospheric mantle (SCLM). Low-degree partial melting (∼2%) of this metasomatized SCLM source generates a parental carbonated silicate magma, which becomes contaminated with the local Archean basement during its ascent. Calcite globules in a nephelinite from Amba Dongar provide evidence that the carbonatites originated by liquid immiscibility from a parental carbonated silicate magma. Liquid immiscibility at crustal depths produces two chemically distinct, but isotopically similar magmas: the carbonatites (20% by volume) and nephelinites (80% by volume). Owing to their low heat capacity, the carbonatite melts solidified as thin carbonate veins at crustal depths. Secondary melting of these carbonate-rich veins during subsequent rifting generated the carbonatites and ferrocarbonatites now exposed at Amba Dongar. Carbonatites, if formed by liquid immiscibility from carbonated silicate magmas, can inherit a wide range of isotopic signatures that result from crustal contamination of their parental carbonated silicate magmas. In rift or plume-related settings, they can, therefore, display a much larger range of isotope signatures than their original asthenosphere or mantle plume source.

Unsolved problems of the global tectonics and possibility of their solutions

The deep geological and geophysical studies of the continents and oceans have revealed a number of well-defined new regularities in the structure of the crust and upper mantle that do not find a clear explanation in the modern geodynamic concepts. The regularities are the following. The Earth is divided into two hemispheres with different structure of the lithosphere: the Pacific and Indo-Atlantic hemisphere. The Pacific hemisphere is surrounded by a ring of the tectonically active zones with high seismicity (Benioff Zones). The system of the mid-ocean ridges with approximately equal distances between them, 90°, is symmetrical relatively to the South Pole. The crust in the oceans is different in age and composition, it identified the remnants of an ancient (Archaean) crust and large areas of subcontinental crust. The continents are characterized by the large thickness of the lithosphere (more than 200 km), composed of the lower density depleted matter. Experimental data on petrophysical properties of the crust and upper mantle matter at high pressure and temperature, the data on deep xenoliths, geochemical studies of natural gases have showen a large role of deep energy-intensive fluids in the formation of the sialic crust and depleted mantle rocks. These data give possibility to explain the continents and oceans origin. The irregular in space the Earth degassing results in different lithosphere types formation: the thick granite-gneiss crust and the lower density depleted mantle of the continents were created in the areas of the higher deep fluids flows; in the areas of the lower fluids advection the primary oceanic crust was preserved and only some separate spots of the transition crust appeared. The lower density of the continental «roots» was the main factor in the formation of continents: the lower density lithosphere led to its emerging in respect to the oceanic lithosphere. Two hemispheres with different lithosphere structure were formed may be due to the elliptical form of the orbit, causing periodic changes of the planet accelerations. The structural symmetry of the global rift system relative to the planet poles gives possibility to explain its origin by the expansion of the planet.

Early resonances of Tethys and Dione: Implications for Ithaca Chasma

We investigate the tidal evolution of Tethys identifying several possible resonance scenarios that could be relevant for intense tidal heating and the formation of Ithaca Chasma early in the satellite’s history. By studying the phase-space we identify initial conditions for which Tethys and Dione can be trapped in a 2:1 mean motion resonance. From evolution models we show that capture of Tethys in this resonance is indeed possible. This scenario would imply a small semi-major axis of Tethys early in its history and small Q-values (high dissipation rates) of Saturn. Various resonance scenarios are obtained including later evolution out of resonance. As a result of the early 2:1 resonance tidal heating can be significantly enhanced in Tethys. Depending on the assumed dissipation factor of Saturn at the time of the resonance, the resulting global dissipation rate ranges between 20 and several hundred GW and could provide the heat pulse associated with the formation of Ithaca Chasma, a global rift system on Tethys. In addition, the orbital expansion due to tides will lead to surface stresses that could trigger the formation of Ithaca Chasma. The stress pattern is consistent with the orientation of faults of Ithaca Chasma at high latitudes but the orientation does not match for the equatorial regions. Expansion due to freezing of an early subsurface ocean as suggested in previous studies could be an additional factor required to explain the formation and orientation of the rift system at low latitudes.

Oldest lamproites from Peninsular India track the onset of Paleoproterozoic plume-induced rifting and the birth of Large Igneous Province

Potassic and ultrapotassic magmatism from deep lithospheric sources in intra-cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of Large Igneous Provinces (LIPs) triggered by mantle plumes. Here we report for the first time, precise zircon U–Pb age data from a suite of lamproites in the Bastar Craton of central India that mark the onset of Paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major LIP events during the Precambrian evolution of the Indian shield. The lamproites from the Nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile, apatite and zircon. The rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite. Geochemical features of the lamproites correlate with their counterparts in Peninsular India and other similar suites elsewhere in the world related to rift settings, and also indicate OIB-like magma source. The associated syenite shows subduction-related features, possibly generated in a post-collisional setting. Magmatic zircon grains with high Th/U ratios in the syenite from the Nuapada lamproite form a coherent group with an upper intercept age of 2473±8Ma representing the timing of emplacement of the magma. Zircon grains in three lamproite samples yield four distinct age groups at ca. 2.4Ga, 2.2Ga, 2.0Ga and 0.8Ga. The 2.4Ga group corresponds to xenocrysts entrained from the syenite whereas the 2.2Ga group is considered to represent the timing of emplacement of the lamproites. The ca. 2.0Ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern Bastar and the adjacent Dharwar Cratons. A few young zircon grains in the syenite and lamproites show a range of early to middle Neoproterozoic ages from 879 to 651Ma corresponding to younger thermal event(s) as also represented by granitic veins cutting across these rocks and extensive silicification. Zircon Lu–Hf isotope data suggest magma derivation from a refertilized Paleo-Mesoarchean sub-continental lithospheric mantle, or OIB-type sources. The differences in Hf-isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale. A combination of the features from geochemical and zircon Hf isotope data is consistent with asthenosphere-lithosphere interaction during the lamproite magma evolution.The timing of lamproite emplacement in central India correlates with the global 2.2Ga record of LIPs. We link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction-accretion history prior to the Neoarchean cratonization, as well as the thermal blanket effect of the Earth's oldest supercontinent. Pulsating plumes and continued rifting generated voluminous dyke swarms across the Bastar and Dharwar Cratons, forming part of a major global rifting and LIP event.

Underground hydrosphere and geofluids of the Earth’s crust and upper mantle

The physicochemical state of the water at various temperature and pressure conditions of the Earth’s interior has been characterized. Two groups of geofluidic systems for water have been established: subcritical ( T 374 ¸ 450 °C), which together with the hydrogeological system of structured waters constitute hydrophysical, hydro- and fluid-geological lithosphere zonation. Based on the systematization of data on the decrepitating of gas-liquid inclusions in minerals of rocks of various facies of metamorphism, as well as fluid manifestations in zones of global rifting, a high content of mineral substances in the composition of dense supercritical fluids and a high electrical conductivity of such fluids is established. Recognition of high “salt content” and high electrical conductivity of supercritical fluids opens new possibilities in interpreting local high-speed anomalies and anomalies of electrical conductivity in the Earth’s crust, revealed from deep seismic and magnetotelluric sounding data. It is shown that with the water condensate — a kind of liquid-phase “restite” from the sphere of supercritical fluids separation — the genesis of highly mineralized waters and brines is connected in the crystalline basement of ancient platforms. On the example of the lithosphere of Belarus, for the first time, deep boundaries of localization of both structured water and supercritical geofluids have been determined. The hydro- and fluid-geological zonation of the lithosphere and upper mantle has been established as a basis for solving the scientific and practical problems of tectonophysics, mineralogy, hydrogeology, genesis and distribution of different type’s minerals, including hydrogenogenic minerals. A thermophysical method for determining the depth and thickness of a zone of underground structured waters is proposed. The method is based on comparison of seismic and thermal characteristics of rocks with sub- and supercritical temperatures for water.

Factors influencing seismic wave attenuation in the lithosphere in continental rift zones

Attenuation of seismic waves in the crust and the upper mantle has been studied in three global rift systems: the Baikal rift system (Eurasia), the North Tanzanian divergence zone (Africa) and the Basin and Range Province (North America). Using the records of direct and coda waves of regional earthquakes, the single scattering theory [ Aki, Chouet, 1975 ], the hybrid model from [ Zeng, 1991 ] and the approach described in [ Wennerberg, 1993 ], we estimated the seismic quality factor ( Q C ), frequency parameter ( n ), attenuation coefficient (δ), and total attenuation ( Q T ). In addition, we evaluated the contributions of two components into total attenuation: intrinsic attenuation ( Q i ), and scattering attenuation ( Q sc ). Values of Q C are strongly dependent on the frequency within the range of 0.2–16 Hz, as well as on the length of the coda processing window. The observed increase of Q C with larger lengths of the coda processing window can be interpreted as a decrease in attenuation with increasing depth. Having compared the depth variations in the attenuation coefficient (δ) and the frequency ( n ) with the velocity structures of the studied regions, we conclude that seismic wave attenuation changes at the velocity boundaries in the medium. Moreover, the comparison results show that the estimated variations in the attenuation parameters with increasing depth are considerably dependent on utilized velocity models of the medium. Lateral variations in attenuation of seismic waves correlate with the geological and geophysical characteristics of the regions, and attenuation is primarily dependent on the regional seismic activity and regional heat flow. The geological inhomogeneities of the medium and the age of crust consolidation are secondary factors. Our estimations of intrinsic attenuation ( Q i ) and scattering attenuation ( Q sc ) show that in all the three studied regions, intrinsic attenuation is the major contributor to total attenuation. Our study shows that the characteristics of seismic wave attenuation in the three different rift systems are consistent with each other, and this may suggest that the lithosphere in the zones of these different rift systems has been modified to similar levels.

Fault geometries on Uranus’ satellite Miranda: Implications for internal structure and heat flow

Miranda, a ∼470-km-diameter uranian icy satellite, has a surface that exhibits evidence of a complex tectonic history. Tectonic structures are mostly localized in three regions termed coronae, but also form a rift system inferred to be global in extent. Ridges within the boundary of Arden Corona, and those that make up the 340° Chasma, part of the global rift system, have been interpreted as normal fault blocks. Using Voyager data, we test the hypothesis that these Arden Corona faults, as well as those at the northern edge of the 340° Chasma, are listric in geometry. For this testing, we use four geometric criteria for listric faults: (1) progressive down-dip decrease in fault scarp dip, (2) progressive down-dip increase in back-tilted face slope, (3) concavity of the exposed scarp surface, and (4) presence of a rollover structure. Results of this analysis support the hypothesis that the faults within the Arden Corona boundary are listric in geometry, but do not strongly support the hypothesis for the faults within the 340° Chasma. By analogy with terrestrial structures, the listric character of faults within the Arden Corona boundary suggests the presence of a subsurface detachment. This detachment likely occurred at Miranda’s brittle–ductile transition zone at the time of faulting. Measurements of the Arden Corona fault system geometry are used to estimate depths to the proposed brittle–ductile transition zone at the time of faulting, resulting in values of 6.7–9.0km. Those depths in turn are used to estimate a thermal gradient of 6–25Kkm−1 and a surface heat flux of 31–112mWm−2. The weaker evidence of a listric geometry for the faults of the 340° Chasma suggests that those faults did not interact with a brittle–ductile transition at the time of their formation. Our estimated thermal gradient of the Arden Corona region is consistent with a previous heating event on Miranda that was as significant as Europa’s current resonance-induced tidal heating. This heating event may be associated with a hypothesized previous tidal resonance of Miranda with Umbriel and/or Ariel.

Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

Whose numbers count? Probing discrepant evidence on transgenic cotton in the Warangal district of India

A B S T R A C T The relationship between poverty and transgenic agricultural crops has created a global rift in develop- ment studies. Some, but not all, questions in this debate should be amenable to empirical treatment. Field studies have generated divergent numbers on yields and other agronomic outcomes. Studies from India come to diametrically opposed findings about Bt cotton: either the technology is scale-neutral and prof- itable for farmers of all size classes, or produces rural catastrophe - reaching the characterization 'genoci- dal' in one prominent critique. This essay suggests a method learned from field investigation of data volatility across studies in the most controversial district in India. The method involves a multi-discipli- nary team concentrating on plausible mechanisms for data distortion at the field level. Interpolating among studies and field results, this essay concludes that widespread reports of 'the failure of Bt cotton in India' - on agronomic, economic and environmental grounds - are not sustainable scientifically but do have plausible origins in methodology and in interests connected to the contentious politics around 'GMOs' globally.

Return to rifts – the next wave: fresh insights into the petroleum geology of global rift basins

Rift basin exploration has provided the oil and gas industry with almost one third of discovered global hydrocarbon resources. The maturity of prolific passive margin sequence plays has necessitated a shift in technical emphasis to understanding petroleum geology of the deeper precursor rift basin megasequences. Modelling of these petroleum systems relies extensively on theoretical stretching hypotheses for whole-crust evolution and heat flow prediction. Several alternatives have emerged to account for anomalous rift-related thermal stress, asymmetry of structural styles and origin of thick syn-rift ‘sag-basin’ subsidence patterns at passive continental margins. These observations cannot be predicted by the pure shear uniform stretching model first introduced by McKenzie in 1978. Newly acquired, 2D long cable deep seismic records provide empirical evidence that supports more complex, polyphase, depth-dependent stretching origins for rift-basin formation. Heterogeneities are believed to be inherent in the brittle upper crust, in the ductile lower crust and lithospheric mantle and so result in the complex distribution of accommodation space recorded by syn-rift megasequence deposition during episodes of orthogonal or oblique extension. No single stretching model uniquely describes the varied structural response of the anisotropic crust to plate-scale extension. A hybrid of phased simple and pure shear deformation mechanisms separated by regional lateral, and possibly vertical crustal discontinuities, may explain the apparent paradox of along-strike co-existence of uniform versus depth-dependent stretching structural geometries.

Global Rift: Robert Kagan and the Europe/America Divide

ConstellationsVolume 10, Issue 3 p. 358-363 Global Rift: Robert Kagan and the Europe/America Divide Eli Zaretsky, Eli ZaretskySearch for more papers by this author Eli Zaretsky, Eli ZaretskySearch for more papers by this author First published: 17 October 2003 https://doi.org/10.1111/1467-8675.00338Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume10, Issue3September 2003Pages 358-363 RelatedInformation

Oxygen isotopic fluxes associated with high-temperature processes in the rift zones of Iceland

High-temperature reactions in the rift zones in Iceland, involving magma, crust and hydrothermal fluids, cause the lowering of the 18O/ 16O ratio of the crust relative to the mantle. A significant flux of 18O to the hydrosphere results from these reactions. We use the available oxygen isotope data to estimate the isotopic composition of the crust produced in this environment. This enables us to place limits on the flux of 18O from the mantle to the hydrosphere. The rift zones in Iceland are efficient in transferring 18O to the hydrosphere; we find that they release up to five times the amount of 18O as does an equivalent length of mid-ocean ridge. In spite of this large flux the net depletion of the Icelandic crust is relatively small, the data show that the δ 18O of the crust in the rift zone, has been lowered by about 1 to 3‰. Our findings have implications for the 18O-history of seawater. We argue that the rift zones in Iceland are a model analog for a global rift system above sea level, a proposed tectonic configuration that perhaps was prevalent in the geological past. Contrary to the proposed termination of the 18O-flux from the mantle to the hydrosphere, we demonstrate that a significant 18O-flux still prevails. This particular tectonic configuration, therefore, does not allow depleted seawater to persist.

Neoproterozoic Tectonics of Australia‐Antarctica and Laurentia and the 560 Ma Birth of the Pacific Ocean Reflect the 400 M.Y. Pangean Supercycle

Unequivocal evidence for the Proterozoic reconstruction of Australia‐Antarctica and Laurentia remains elusive, although various authors have interpreted sedimentary and igneous events in terms of initial (Neoproterozoic) rifting, and final (Neoproterozoic/Cambrian) drifting. The synchronous rifting and drifting reflect the tectonics of a late Neoproterozoic Pangea (East Gondwanaland and Laurentia) that amalgamated along the Mozambiquean belt ∼720 Ma and broke up at 560 Ma by growth of the Paleo‐Pacific and lapetus oceans. In this paper these events are interpreted as part of a 400 m.y. supercycle comparable to the 320 Ma amalgamation of Pangea A and its 160 Ma breakup during Supercycle A (320 Ma to present). Events near the end of the Cambrian (500 Ma) in East Gondwanaland included epeirogenic uplift of cratons, shown by widespread K/Ar and apatite fission‐track dates, and the inception of quartz turbidite fans, intense deformation and intrusion by granite of the Antarctic and south‐eastern Australian margins, followed by a global sea‐level maximum. These events are comparable to the mid‐Cretaceous (100‐90 Ma) epeirogenic uplift of Australia, intrusion by granite of the western USA and elsewhere, followed by a second global sea‐level maximum. Both supercycles were driven by pulses of supercontinent‐induced heat that caused global rifting by extension, subsidence, and filling of sedimentary basins, followed by continental drifting and seafloor spreading. Our detailed compilation of the tectonic effects of Supercycle B (720‐320 Ma) suggests that Pangea‐induced heat has punctuated earth history since at least 720 Ma. The previous Pangean amalgamation, indicated by the global 1100 Ma (Grenvillian) deformation, suggests a third supercycle that introduced the modern regime of plate tectonics.

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

Recent work on the Olympic Dam CuUAuAg deposit, South Australia, the Wernecke Mountain breccias, Yukon, the Kiruna iron ore district, Sweden, and the southeast Missouri iron ore district, and a review of literature on other iron-rich mineral deposits in Proterozoic rocks, suggest that these occurrences constitute a distinct class of ore deposits characterized by low-titanium, iron-rich rocks formed in extensional tectonic environments. Other examples of this class may include the mineral deposits of the Great Bear magmatic zone of northwest Canada, the Bayan Obo district of China, and perhaps the Redbank breccia pipes of the Northern Territory, Australia. We designate this class of deposits as Proterozoic iron oxide (CuUAuREE) deposits, and propose that the ore deposits generally referred to as ‘Kiruna-type’ should be considered a subset of this larger class. Salient characteristics of this class of deposits are as follows: 1. (1) Age. The majority of known deposits, particularly the larger examples, are found within Early to mid-Proterozoic host rocks (1.1–1.8 Ga). 2. (2) Tectonic setting. The deposits are located in areas that were cratonic or continental margin environments during the late Lower to Middle Proterozoic, and in many cases there is a definite spatial and temporal association with extensional tectonics. Most of the districts occur along major structural zones, and many of the deposits are elongated parallel to regional or local structural trends. The host rocks may be igneous or sedimentary; many of the deposits occur within silicic to intermediate igneous rocks of anorogenic type. However, mineralization in many deposits is not easily related to igneous activity at the structural level of mineralization. 3. (3) Mineralogy. The ores are generally dominated by iron oxides, either magnetite or hematite. Magnetite is found at deeper levels than hematite. CO3, Ba, P, or F minerals are common and often abundant. The deposits contain anomalous to potentially economic concentrations of REEs, either in apatite, or in distinct REE mineral phases. 4. (4) Alteration. The host rocks are generally intensely altered. The exact alteration mineralogy depends on host lithology and depth of formation, but there is a general trend from sodic alteration at deep levels, to potassic alteration at intermediate to shallow levels, to sericitic alteration and silicification at very shallow levels. In addition, the host rocks are locally intensely Fe-metasomatized. In spite of these similarities, many variations occur between and within individual districts, particularly in deposit morphology. Individual deposits occur as strongly discordant veins and breccias to massive concordant bodies. Both the morphology and the extent of alteration and mineralization appear to be largely controlled by permeability along faults, shear zones and intrusive contacts, or by permeable horizons such as poorly welded tuffs. Thus, the variations of morphology are explicable in terms of local wall-rock and structural controls. Similarly, local variations in mineralogy and geochemistry may be largely attributable to wall-rock composition, and to P, T, and fo2 controls related to depth of formation. We believe that these deposits formed primarily in shallow crustal environments (<4–6 km), and that they are expressions of deeper-seated, volatile-rich igneous-hydrothermal systems, tapped by deep crustal structures. The global occurrence of this type of deposit at approximately 1.8 to 1.4 Ga suggests a relation to a global rifting events effecting continental crust, possibly the break-up of a Proterozoic supercontinent. Secular cooling of the Earth insured that subsequent rifting and mineralizing events might generate deposits similar in kind but smaller in magnitude.