Seismic Hazards Assessment for Radioactive Waste Disposal Sites in Regions of Low Seismic Activity

A comparative tectonic quiescence and lack of earthquakes make the stable centres of continents attractive for siting long-term radioactive waste storage facilities. The low rates of deformation in such regions, however, make it difficult to characterize their long-term seismotectonic behaviour, leading to uncertain estimates for the very low probability hazard estimates required by society. In an attempt to overcome the deficiency of both contemporary seismicity and paleoseismic data in central Canada, we have used earthquake histories from regions with similar seismotectonic characteristics from around the world. Substituting space for time, we estimate a long-term rate per 10⁶ km² of 0.004 magnitude ≥6 earthquakes per annum, of which 33–100% might rupture to the surface.     

Significance of transition between Talchir Formation and Karharbari Formation in Lower Gondwana basin evolution — A study in West Bokaro Coal basin, Jharkhand, India

Basal part of the Gondwana Supergroup represented by Talchir and Karharbari Formations (Permo-Carboniferous) records an abrupt change-over from glacio-marine to terrestrial fluviolacustrine depositional environment. The contact between the two is an unconformity. Facies analysis of the glacio-marine Talchir Formation reveals that basal glaciogenic and reworked glaciogenic sediments are buried under storm influenced inner and outer shelf sediments. Facies associations of the Karharbari Formation suggest deposition as fluvio-lacustrine deposits in fault-controlled troughs. An attempt has been made in this paper to explain the sedimentation pattern in Talchir and Karharbari basins, and the abrupt change-over from glacio-marine to terrestrial fluviolacustrine depositional environment in terms of glacio-isostacy.     

A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the ⁴⁰Ar/³⁹Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.     

Tourmaline from the Archean G.R.Halli gold deposit,Chitradurga greenstone belt,Dharwar craton （India）： Implications for the gold metallogeny

Tourmaline occurs as a minor but important mineral in the alteration zc,ne of the Archean orogenic gold deposit of Guddadarangavanahalli （G.R.Halli） in the Chitradurga greenst~ne belt of the western Dharwar craton, southern India. It occurs in the distal alteration halo of the G.R.Halli golcl deposit as （a） clusters of very fine grained aggregates which form a minor constituent in the natrix of the altered metabasalt （AMB tourmaline） and （b） in quartz-carbonate veins （vein tourmaline）. ~[＇he vein tourmaline, based upon the association of specific carbonate minerals, is further grouped as （i） albite-tourmaline-ankerite-quartz veins （vein-1 tourmaline） and （ii） albite-tourmaline-calcite-quartz veins （vein-2 tourmaline）. Both the AMB tourmaline and the vein tourmalines （vein-I and vein-2） belong to the alkali group and are clas- sified under schorl-dravite series. Tourmalines occurring in the veins are zoned while the AMB tour- malines are unzoned. Mineral chemistry and discrimination diagrams 1eveal that cores and rims of the vein tourmalines are distinctly different. Core composition of the ve：n tourmalines is similar to the composition of the AMB tourmaline. The formation of the AMB tourmaline and cores of the vein tour- malines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca. 2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-a- vis gold mineralization is spatially linked to the juvenile magmatic accretion （2.56-2.50 Ga） east of the studied area in the western part of the eastern Dharwar craton.     

Comparison of Results of Recent Seismic Profiles with Tectonic Models of the North China Craton

The geometry and timing of amalgamation of the North China craton(NCC) have been controversial,with three main models with significantly different interpretations of regional structure,geochronology,and geological relationships.The model of Zhao G C et al.suggests that the eastern and western blocks of the NCC formed separately in the Archean,and an active margin was developed on the eastern block between 2.5 and 1.85 Ga,when the two blocks collided above an east dipping subduc-tion zone.The model of Kusky ...     

Continental rift systems and anorogenic magmatism

Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Long-lived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.     

Ruffordia goeppertii (Schizaeales,Anemiaceae) – A common fern from the Lower Cretaceous Crato Formation of northeast Brazil

Extraordinarily well preserved fern macrofossils of Ruffordia goeppertii (Dunker) Seward (Schizaeales, Anemiaceae) are described from the Lower Cretaceous (late Aptian) Nova Olinda Member of the Crato Formation, northeast Brazil. The identification is based on the morphology of macrofossils and in situ spores, taken from organically preserved material. This extinct, relatively small fern exhibits dimorphic fronds with sterile and fertile pinnules and schizaeoid sporangia, including cicatricose spores. The growth form with clearly differentiated sterile and fertile pinnae may be interpreted as an ancestral state in the phylogeny of the extant genus Anemia. The abundance of this fern reflects its role as ground cover in at least partly (dry) sunny areas, possibly in fern savannah-like habitats, with adaptations to survive drought stress. Finds of Ruffordia in northern Gondwana extend the wide palaeogeographic range of this taxon.     

Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO₂ of 55–60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S^2?-rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ～80° (from SW to NW) at approximately 125–122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ?120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.     

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.     

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton:An integrated genetic perspective

The North China Craton(NCC)hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization.The diverse models for the genesis of these deposits remain equivocal,and mostly focused on the craton margin examples,although synchronous deposits formed in the interior domains.Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC.In the Archean tectonic framework of the NCC,the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents.In the Paleoproterozoic framework,they are markedly aligned along two major collisional sutures-the Trans North China Orogen and the Jiao-Liao-Ji Belt.Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold,we explore the deep roots based on available geophysical data.We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle,and that the mineralization is located above regions of high heat flow representing mantle upwelling.The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly en riched it.The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning.Based on these results,and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization,we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction,underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC.The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs,leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.