Field evidence of Eros-scale asteroids and impact-forcing of Precambrian geodynamic episodes, Kaapvaal (South Africa) and Pilbara (Western Australia) Cratons

NASA Astrophysics Data System (ADS)

Glikson, Andrew Y.

2008-03-01

The role of asteroid and comet impacts as triggers of mantle-crust processes poses one of the fundamental questions in Earth science. I present direct field evidence for close associations between impact ejecta/fallout units, major unconformities and lithostratigraphic boundaries in Archaean and early Proterozoic terrains, including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries, with implications for the nature and composition of their provenance terrains. As originally observed by D.R. Lowe and G.R. Byerly, in the Barberton Greenstone Belt, eastern Kaapvaal Craton, South Africa, 3.26-3.24 Ga asteroid mega-impact units are closely associated with the abrupt break between an underlying simatic mafic-ultramafic volcanic crust and an overlying association of turbidites, banded iron formations, felsic tuff and conglomerates of continental affinities. Contemporaneous stratigraphic relationships are identified in the Pilbara Craton, Western Australia. Evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains, likely dominated by mafic compositions, attributed to impact-triggered oceanic volcanic activity. Relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with ~ 2.48 Ga and possibly the 2.56 Ga mega-impact events.

Lithospheric-Mantle Structure of the Kaapvaal Craton, South Africa, Derived from Thermodynamically Self-Consistent Modelling of Magnetotelluric, Surface-Wave Dispersion, S-wave Receiver Function, Heat-flow, Elevation and Xenolith Observations

NASA Astrophysics Data System (ADS)

Muller, Mark; Fullea, Javier; Jones, Alan G.; Adam, Joanne; Lebedev, Sergei; Piana Agostinetti, Nicola

2013-04-01

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained - the tLAB) is at least 220 km deep, to account for mantle geotherms in the range 35-38 mWm-2. Richly diamondiferous kimberlites across the Kaapvaal Craton require a lithospheric thickness substantially greater than 160 km - the depth of the top of the diamond stability field. In this paper we use the recently developed LitMod software code to derive, thermodynamically consistently, a range of 1-D electrical resistivity, seismic velocity, density and temperature models from layered geochemical models of the lithosphere based on mantle xenolith compositions. In our work, the "petrological" lithosphere-asthenosphere boundary (pLAB) (i.e., the top of the fertile asthenospheric-mantle) and the "thermal" LAB (tLAB) are coincident. Lithospheric-mantle models are found simultaneously satisfying all geophysical observables: MT responses, new surface-wave dispersion data, published SRFs, surface elevation and heat-flow. Our results show: 1. All lithospheric-mantle models are characterised by a seismic LVZ with a minimum velocity at the depth of the petrological/thermal LAB. The top of

Seismic evidence for depth-dependent metasomatism in cratons

NASA Astrophysics Data System (ADS)

Eeken, Thomas; Goes, Saskia; Pedersen, Helle A.; Arndt, Nicholas T.; Bouilhol, Pierre

2018-06-01

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100-150 km. The metasomatic component can consist of 0.5-1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100-200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Lithospheric-Mantle Structure of the Kaapvaal Craton, South Africa, Derived From Thermodynamically Self-Consistent Modelling of Seismic Surface-Wave and S-wave Receiver Function, Heat-flow, Elevation, Xenolith and Magnetotelluric Observations

NASA Astrophysics Data System (ADS)

Muller, M. R.; Fullea, J.; Jones, A. G.; Adam, J.; Lebedev, S.; Piana Agostinetti, N.

2012-12-01

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained) is at least 220 km deep, to account for mantle geotherms in the range 35-38 mWm-2. Richly diamondiferous kimberlites across the Kaapvaal Craton require a lithospheric thickness substantially greater than 160 km - the depth of the top of the diamond stability field. In this paper we use the recently developed LitMod software code to derive, thermodynamically consistently, a range of 1-D seismic velocity, density, electrical resistivity and temperature models from layered geochemical models of the lithosphere based on mantle xenolith compositions. In our work, the "petrological" lithosphere-asthenosphere boundary (pLAB) (i.e., the top of the fertile asthenospheric-mantle) and the "thermal" LAB (tLAB as defined above) are coincident. Lithospheric-mantle models are found simultaneously satisfying all geophysical observables: new surface-wave dispersion data, published SRFs, MT responses, surface elevation and heat-flow. Our results show: 1. All lithospheric-mantle models are characterised by a seismic LVZ with a minimum velocity at the depth of the petrological/thermal LAB. The

Craton stability and continental lithosphere dynamics during plume-plate interaction

NASA Astrophysics Data System (ADS)

Wang, H.; Van Hunen, J.; Pearson, D.

2013-12-01

Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. A high cratonic root strength is generally considered to be the most important factor. We first perform and discuss new numerical models to investigate craton stability in both Newtonian and non-Newtonian rheology in the stagnant lid regime. The results show that only a modest compositional rheological factor of Δη=10 with non-Newtonian rheology is required for the survival of cratonic roots in a stagnant lid regime. A larger rheological factor (100 or more) is needed to maintain similar craton longevity in a Newtonian rheology environment. Furthermore, chemical buoyancy plays an important role on craton stability and its evolution, but could only work with suitable compositional rheology. During their long lifespan, cratons experienced a suite of dynamic, tectonothermal events, such as nearby subduction and mantle plume activity. Cratonic nuclei are embedded in shorter-lived, more vulnerable continental areas of different thickness, composition and rheology, which would influence the lithosphere dynamic when tectonothermal events happen nearby. South Africa provides a very good example to investigate such dynamic processes as it hosts several cratons and there are many episodic thermal events since the Mesozoic as indicated by a spectrum of magmatic activity. We numerically investigate such an integrated system using the topographic evolution of cratons and surrounding lithosphere as a diagnostic observable. The post-70Ma thinning of pericratonic lithosphere by ~50km around Kaapvaal craton (Mather et al., 2011) is also investigated through our numerical models. The results show that the pericratonic lithosphere cools and grows faster than cratons do, but is also more likely to be effected by episodic thermal events. This leads to surface topography change that is significantly larger around the craton than within

Evidence for Depth-Dependent Metasomatism in Cratonic Lithosphere

NASA Astrophysics Data System (ADS)

Eeken, T.; Goes, S. D. B.; Pedersen, H.; Arndt, N. T.; Bouilhol, P.

2017-12-01

The long-term stability of the cratonic cores of continents has been attributed to low temperatures and depletion in iron and water. However, a long-standing enigma is that steady-state thermal models based on heat flow measurements and xenoliths systematically overpredict the seismic velocities in Archean lithospheric mantle. We perform a Monte-Carlo inversion for thermal parameters and water content (leading to metasomatism) to fit 1-D geotherms to average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To satisfactorily match the seismic profiles, we need a significant amount of hydrous and/or carbonated minerals starting between the Moho and 70 km depth and extending down to at least 100-150 km depth (if distributed over this depth range, this requires 0.5 and 1 wt% water for amphiboles, or 0.2 wt% water plus sufficient potassium to form phlogopites or 5 wt% CO2 and sufficient Ca to make carbonate, or a combination thereof). Lithospheric temperatures that lead to a good fit of the seismic constraints are commonly lower than those inferred from xenoliths, but consistent with heat flow constraints. The dispersion data also require differences in Moho heatflux between regions and 100-200°C lower sublithospheric mantle temperatures below Yilgarn, Slave and Finland than below Kaapvaal, consistent with regional tectonic settings inferred from global tomography. Thus, significant upward-increasing metasomatism by water and CO2-rich fluids is a plausible mechanism to explain the average seismic structure of cratonic lithosphere. Such metasomatism would also contribute to the positive chemical buoyancy of cratonic roots.

Constant average olivine Mg# in cratonic mantle reflects Archaean mantle melting to the exhaustion of orthopyroxene

NASA Astrophysics Data System (ADS)

Bernstein, S.; Kelemen, P. B.; Hanghoj, K.

2006-12-01

Shallow (garnet-free) cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has high Mg# (100x Mg/(Mg+Fe)>92) and is poor in Al and Ca compared to off-cratonic mantle. Many xenoliths show rhenium-depletion age of > 3 Ga, and are thus representative of depleted mantle peridotite that form an integral part of the stable nuclei of Archaean (2.5-3.8 Ga) cratons. Accordingly, the depleted composition of the xenolith suites is linked to Archaean melt extraction events. We have compiled data for many suites of shallow cratonic mantle xenoliths worldwide, including samples from cratons of Kaapvaal, Tanzania, Siberia, Slave, North China and Greenland, and encompassing both the classic orthopyroxene-rich peridotites of Kaapvaal and orthopyroxene-poor peridotites from Greenland. The suites show a remarkably small range in average olivine Mg# of 92.8 +/- 0.2. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# in the shallow cratonic mantle as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine, or dunitic, residue. Experimental data for peridotite melting at pressures less than 4 GPa and data on natural rocks suggest that mantle olivine has a Mg# of about 92.8 at the point of orthopyroxene exhaustion. If the melt extraction was efficient, no further melting could take place without a considerable temperature increase or melt/fluid flux through the dunite residue at high temperatures. While the high Mg#, dunite-dominated xenolith suites from e.g. Greenland represent simple residues from mantle melting, the orthopyroxene-rich xenolith suites with identical Mg# as known from e. g. Kaapvaal must reflect some additional processes. We envisage their derivation from dunite protoliths via subsequent melt/rock reaction with silica-rich melts or, in some cases, possibly as residues at higher average melting

The Role of Water in the Stability of Cratonic Keels

NASA Technical Reports Server (NTRS)

Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina

2011-01-01

Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

Chemical stratification of cratonic lithosphere: constraints from the Northern Slave craton, Canada

NASA Astrophysics Data System (ADS)

Kopylova, Maya G.; Russell, James K.

2000-08-01

We describe the mineralogical and chemical composition of the Northern Slave mantle as deduced from xenoliths of peridotite within the Jericho kimberlite, Northwest Territories. Our data set includes modal, major, trace and rare earth element compositions of bulk samples of spinel peridotite, low-T and high-T garnet peridotite and minor pyroxenite. Compared to primitive upper mantle, Jericho peridotite shows depletion in the major elements and enrichment in incompatible elements (except for HREE). The Slave mantle is also uniquely stratified. Older, depleted spinel peridotite extends to a depth of 80-100 km and is underlain by garnet peridotite which shows a gradual decrease in Mg# with depth to 200 km. The youngest layer of fertile garnet peridotite, enriched in clinopyroxene and garnet, is underlain by a pyroxenite-rich horizon at the base of the petrological lithosphere. The Northern Slave is further distinguished from the Kaapvaal and Siberian upper mantle by a marked vertical stratification in Mg#, lower abundances of orthopyroxene and higher abundances of clinopyroxene. In addition, a deeper layer of garnet peridotite below Jericho shows less depletion than low-T peridotite from other cratons. The Northern Slave peridotite results from a series of chemical events that include: (i) high-degree melting of pyrolite at P>3 Gpa for low-T peridotite and lower pressure melting for high-T peridotite, (ii) enrichment of low-T spinel peridotite in orthopyroxene, and (iii) pervasive metasomatic enrichment in alkali and LREE's by kimberlite-related fluids. The chemical stratification described for two of the three lithospheric domains of the Slave craton makes this craton an exception among cratons with commonly unstratified lithospheres. The gradual increase in fertility with depth below the Slave craton is related to age stratification and may have formed by incremental downward growth of mantle lithosphere with time, and/or later re-fertilization of deeper mantle

Mantle metasomatism in the Kaapvaal Craton lithosphere: constraints on the composition of the metasomatic agent from fluid inclusions in MARID-type xenoliths

NASA Astrophysics Data System (ADS)

Konzett, J.; Krenn, K.; Hauzenberger, Ch.

2012-04-01

The emplacement of both group I and group II kimberlites in the Kaapvaal Craton of the Kimberley region in South Africa is associated with an intense metasomatic alteration of the country rocks as evidenced by a diverse suite of xenoliths sampled by the kimberlites mainly comprising metasomatized peridotites and minor MARID-type xenoliths. These are characterized by hydrous potassic silicates and LILE-HFSE-rich titanates. Because the metasomatic agent is not preserved in these rocks its composition has to be inferred from that of the metasomatic assemblages. Here we present for the first time data on fluid inclusions from two MARID-xenoliths sampled by group-I kimberlites of the Kimberley cluster. They provide direct evidence for the nature of the metasomatic fluids involved in kimberlite-related metsomatism. The xenoliths contain phlogopite+K-richterite+diopside+ilmenite±rutile±apatite±zircon. Fluid inclusions with 4-10 µm in size were found in diopside, K-richterite and zircon and contain L+V+one-to-several daughter phases. Investigations with the freezing and heating stage indicate two different chemical systems for the fluids: (1) H2O-NaCl dominant fluids found as L+V+S inclusions in zircon together with abundant needle-like apatite, rutile and phlogopite solid inclusions. The fluid inclusions in part occur along zircon host-rutile/apatite inclusion grain boundaries which indicates that the fluids were trapped during zircon growth. They contain 30-32 mass% NaCl and show a density of 0.87-0.94 g/cm3. Halos of tiny fluid inclusions, however, indicate that most if not all zircon inclusions are decrepitated during ascent from depth and/or superheating during entrainment of the xenoliths into the kimberlite. Using EMPA, enstatite and a SiO2 polymorph were identified in opened fluid inclusions exposed at the surface of polished thin sections. Because these phases were exclusively found in the fluid inclusions, they are considered daughter crystals. The enstatite

Craton destruction by subduction, collision or plume impingement? Comparisons of some representative cratons in the world

NASA Astrophysics Data System (ADS)

Wang, Zhensheng; Kusky, Timothy; Li, Xiaoyong; Wang, Xu; Fu, Jianmin; Yuan, Yuefeng; Zhu, Peimin

2015-04-01

The fact that cratonic lithosphere can be destructed has been demonstrated by numerous studies. However, the driving force of craton destruction and its mechanism are still unclear1,2. Subduction, collision and mantle plumes, the most important driving forces for most geological events, may also be responsible for craton destruction. However, their relationship in the destruction process including which of them is the major driving force and how they interact with each other is not understood sufficiently1,2. In this article, the North China Craton, North Atlantic Craton, Tanzania Craton, Wyoming Craton, Kaapvaal Craton, Yangtze Craton, Yilgarn Craton and Superior Craton are used as examples to study their difference and similarities during subduction, collision, or mantle plume impingement. The work is mainly based on comparison of their tectonic history, geophysical data, as well as xenolith chronology. It is suggested that large-scale craton destruction can be influenced by the interaction of subduction, collision and mantle plumes, acting to different degrees in different examples. Mantle plume related thermal action enhances the chemical stratification or layering of subcontinental lithospheric mantle (SCLM), which can form a weak-coupling mid lithosphere discontinuity (MLD) 3,4,5 and a lower denser SCLM below the MLD. Convergence (subduction and collision), especially when associated with slab rollback 6, leads to the regional thinning of the cratonic margin, which is subsequently linked by extension (mantle plume or slab rollback) related discontinuities. Continuous extension-related discontinuities extend upward to the MLD depth and cut off the shearing resistance from adjacent blocks. Next, the lower part of the chemically stratified SCLM in the cratonic interior is decoupled along the weakly coupled MLD and founders into the deep asthenosphere 7. Then the destruction of the rest of the lithosphere can be affected by upwelling related decompression melting

Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle.

PubMed

Wilson, A H; Shirey, S B; Carlson, R W

2003-06-19

Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30-50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO(2)/Al(2)O(3) ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.

Regional Variations in Composition of Cr-spinel Xenocrysts From Kimberlite

NASA Astrophysics Data System (ADS)

Schulze, D. J.

2001-05-01

Important information on the composition of the upper mantle can be obtained by studying mantle xenocrysts in kimberlite, especially in situations in which intact mantle xenoliths are rare to absent. Spinel-group minerals are especially useful as they can coexist with garnet or represent regions of the mantle shallower than garnet-facies rocks, and chromites can exist in rocks too Al-depleted to form garnet. Xenolith studies have shown that along most typical cratonic geothermal gradients, the maximum Cr/(Cr+Al) (cr#) of spinel coexisting with garnet is 0.88. Cr-spinels with cr# > 0.88 are from Al-depleted rocks or from assemblages in which Al is partitioned into another phase (e.g., metasomatic phlogopite). Approximately 2500 Cr-spinel xenocrysts from 36 kimberlites in southern Africa and North America have been analysed (and some published data used) and evaluated, primarily in terms of cr# and Fe2/(Fe2+Mg) (fe#). Differences from pipe to pipe within and between cratons reflect variations in geologic history and fertility/depletion, only some of which can be related to mantle age. Within southern Africa, pipe average values of spinel xenocryst cr# are highest on the Kaapvaal Craton (0.80-0.89) where fe# varies from 0.36 to 0.47. Suites from the craton margin (e.g., in Lesotho) indicate a less depleted mantle (cr# = 0.75-0.80), similar to those from the Zimbabwe Craton (Orapa and Letlhakane, cr# = 0.80-0.81). Jwaneng (Kaapvaal Craton) is similar to the Zimbabwe Craton pipes (cr# = 0.83). Off-craton South African suites (Kalkput and Rietfontein) have lower cr# (0.72-0.75). Most southern African suites contain a significant population of Cr-spinel with cr# > 0.88 (including off-craton Rietfontein) except Liqhobong on the craton margin in Lesotho. Cr-spinel suites from North American kimberlites are quite different, with most suites being significantly more aluminous than African populations. Most Kirkland Lake kimberlites on the Superior Craton have a very

Terrestrial heat flow in east and southern Africa

NASA Astrophysics Data System (ADS)

Nyblade, Andrew A.; Pollack, Henry N.; Jones, D. L.; Podmore, Francis; Mushayandebvu, Martin

1990-10-01

We report 26 new heat flow and 13 radiogenic heat production measurements from Zimbabwe, Zambia and Tanzania, together with details and some revisions of 18 previous heat flow measurements by other investigators from Kenya and Tanzania. These measurements come from Archean cratons, Proterozoic mobile belts, and Mesozoic and Cenozoic rifts. Heat flow data from eight new sites in the Archean Zimbabwe Craton are consistent with previous measurements in the Archean Kaapvaal-Zimbabwe Craton and Limpopo Belt (Kalahari Craton) and do not change the mean heat flow of 47±2 mW m-2 (standard error of the mean) in the Kalahari Craton based on 53 previous measurements. Eight new sites in the Archean Tanzania Craton give a mean heat flow of 34±4 mW m-2. The mean heat flow from nine sites in the Proterozoic Mozambique Belt to the east of the Tanzania Craton in Kenya and Tanzania is 47±4 mW m-2. Twelve measurements in the Mesozoic rifted continental margin in east Africa give a mean heat flow of 68±4 mW m-2; four measurements in the Mesozoic Luangwa and Zambezi Rifts range from 44 to 110 mW m-2 with a mean of 76±14 mW m-2. In comparing heat flow in east and southern Africa, we observe a common heat flow pattern of increasing heat flow away from the centers of the Archean cratons. This pattern suggests a fundamental difference in lithospheric thermal structure between the Archean cratons and the Proterozoic and early Paleozoic mobile belts which surround them. Superimposed on this common pattern are two regional variations in heat flow. Heat flow in the Tanzania Craton is lower by about 13 mW m-2 than in the Kalahari Craton, and in the Mozambique Belt in east Africa heat flow is somewhat lower than in the southern African mobile belts at similar distances from the Archean cratonic margin. The two regional variations can be explained in several ways, none of which can as yet be elevated to a preferred status: (1) by variations in crustal heat production, (2) by thin

Water in the Cratonic Mantle Lithosphere

NASA Technical Reports Server (NTRS)

Peslier, A. H.

2016-01-01

The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents (<10 ppm wt H2O) at > 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (<20 ppm wt H2O). The xenoliths may represent a biased sample of the cratonic lithosphere with an over-­abundance of metasomatized peridotites with

Constraining the Protolith of Large, Macroscopically Layered Kyanite-bearing Eclogite Xenoliths from the Kaapvaal Craton, South Africa

NASA Astrophysics Data System (ADS)

Rebelo, C. C.; Gurney, J. J.; Richardson, S. H.; Shaw-Kahle, B.

2015-12-01

We describe the geochemistry of a suite of ten layered, kyanite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal Craton, South Africa. All samples are characterized by clear zonation corresponding to the presence or absence of kyanite and the state of preservation of clinopyroxene. The sampled zones are defined as KF (free of kyanite, with well-preserved garnet and clinopyroxene), KZ (preserved clinopyroxene is absent; garnet occurs with kyanite); and TZ (partially preserved clinopyroxene and more altered garnet; kyanite may or may not be present). We report on results of petrographic observations, mineral chemistry, and trace element and oxygen isotope analyses for the different zones. Results from adjacent zones are presented as geochemical transects perpendicular to the layering for each xenolith. We use the results to infer the geochemical evolution of likely protoliths for the various samples. Mineral chemistry of garnets across the different zones shows decreasing FeO*, Cr2O3, MgO and MnO and increasing in CaO from KF into KZ. Clinopyroxene shows increasing in Al2O3 and Na2O from KF into KZ. Clinopyroxenes approach a more jadeitic composition towards the KZ and are more diopsidic in KF. Trace element analyses were conducted with laser ablation ICP-MS on hand-picked mineral separates. Garnets show subchondritic LREE depletion and superchondritic HREE enrichment in both KF and KZ. Positive europium anomalies in garnet are present in all zones, with relatively larger anomalies in garnet grains from KZ. Preserved clinopyroxenes show complementary REE patterns to garnet grains, with superchondritic LREEs and subchondritic HREEs. For the suite of xenoliths, results from mass spectrometry on oxygen extracted by laser fluorination on mineral separates show that δ18O values are above the accepted mantle value and fall within the range of hydrothermally altered oceanic crustal material. At this stage of the research, the bulk protolith is interpreted to

The birth, growth and ageing of the Kaapvaal subcratonic mantle

NASA Astrophysics Data System (ADS)

Brey, Gerhard P.; Shu, Qiao

2018-06-01

The Kaapvaal craton and its underlying mantle is probably one of the best studied Archean entity in the world. Despite that, discussion is still vivid on important aspects. A major debate over the last few decades is the depth of melting that generated the mantle nuclei of cratons. Our new evaluation of melting parameters in peridotite residues shows that the Cr2O3/Al2O3 ratio is the most useful pressure sensitive melting barometer. It irrevocably constrains the pressure of melting (melt separation) to less than 2 GPa with olivine (ol), orthopyroxene (opx) and spinel (sp) as residual phases. Garnet (grt) grows at increasing pressure during lithosphere thickening and subduction via the reaction opx + sp → grt + ol. The time of partial melting is constrained by Re-depletion model ages (TRD) mainly to the Archean (Pearson and Wittig 2008). However, only 3% of the ages are older than 3.1 Ga while crustal ages lie mainly between 3.1 to 2.8 Ga for the W- and 3.7 to 2.8 Ga for the E-block. Many TRD-ages are probably falsified by metasomatism and the main partial melting period was older than 3.1 Ga. Also, Nd- and Hf- model ages of peridotitic lithologies from the W-block are 3.2 to 3.6 Ga old. The corresponding very negative ɛNd (-40) and ɛHf values (-65) signal the presence of subducted crustal components in these old mantle portions. Subducted components diversify the mantle in its chemistry and thermal structure. Adjustment towards a stable configuration occurs by fluid transfer, metasomatism, partial melting and heat transfer. Ages of metasomatism from the Lu-Hf isotope system are 3.2 Ga (Lace), 2.9 Ga (Roberts Victor) and 2.62 Ga (Finsch) coinciding with the collision of cratonic blocks, the growth of diamonds, metamorphism of eclogites and of Ventersdoorp magmatism. The cratonic lithosphere was stabilized thermally by the end of the Archean and cooled since then with a rate of 0.07 °C/Ma.

Synchroneity of cratonic burial phases and gaps in the kimberlite record: Episodic magmatism or preservational bias?

NASA Astrophysics Data System (ADS)

Ault, Alexis K.; Flowers, Rebecca M.; Bowring, Samuel A.

2015-01-01

A variety of models are used to explain an apparent episodicity in kimberlite emplacement. Implicit in these models is the assumption that the preserved kimberlite record is largely complete. However, some cratons now mostly devoid of Phanerozoic cover underwent substantial Phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions. Here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the Slave craton, Superior craton, and cratonic western Australia. A similar pattern exists in the Kaapvaal craton, although its magmatic, deposition, and erosion history differs in key ways from the other localities. One explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism. Another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts. Even if kimberlites occurred during cratonic burial phases, the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced. In this model, kimberlite magmatism was more continuous than the preserved record suggests, implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects. Either way, the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns.

Garnet lherzolites from Louwrensia, Namibia: Bulk composition and P/T relations

USGS Publications Warehouse

Boyd, F.R.; Pearson, D.G.; Hoal, Karin O.; Hoal, B.G.; Nixon, P.H.; Kingston, M.J.; Mertzman, S.A.

2004-01-01

Bulk, mineral and trace element analyses of garnet lherzolite xenoliths from the Louwrensia kimberlite pipe, south-central Namibia, together with previously published Re-Os isotopic data [Chem. Geol. (2004)], form the most extensive set of chemical data for off-craton suites from southern Africa. The Louwrensia suite is similar to those from the Kaapvaal craton in that it includes both predominantly coarse-grained, equant-textured peridotites characterised by equilibration temperatures 1200 ??C. Redepletion ages range back to 2.1 Gy, concordant with the age of the crustal basement and about 1 Gy younger than the older peridotites of the adjacent Kaapvaal craton root. The coarse, low-temperature Louwrensia peridotites have an average Mg number for olivine of 91.6 in comparison to 92.6 for low-temperature peridotites from the craton. Orthopyroxene content averages 24 wt.% with a range of 11-40 wt.% for Louwrensia low-temperature peridotites, in comparison to a mean of 31.5 wt.% and a range of 11-44 wt.% for low-temperature peridotites from the Kaapvaal craton. Other major, minor and trace element concentrations in minerals forming Louwrensia lherzolites are more similar to values in corresponding Kaapvaal peridotite minerals than to those in lithospheric peridotites of Phanerozoic age as represented by off-craton basalt-hosted xenoliths and orogenic peridotites. Proportions of clinopyroxene and garnet in both the Louwrensia and Kaapvaal lherzolites overlap in the range up to 10 wt.% forming a trend extending towards pyrolite composition. Disequilibrium element partitioning between clinopyroxene and garnet for some incompatible trace elements is evidence that some of the trend is caused by enrichment following depletion. The disequilibrium is interpreted to have been caused by relatively recent growth of diopside, as previously suggested for cratonic peridotites. Attempts to constrain the depth of melting required to produce the Louwrensia peridotites suggests

Geochemistry and mineralogy of sediments from the Ventersdorp and Transvaal Supergroups, South Africa: Cratonic evolution during the early Proterozoic

NASA Astrophysics Data System (ADS)

Wronkiewicz, David J.; Condie, Kent C.

1990-02-01

Approximately 100 pelite and 12 quartzite samples from the Ventersdorp (~2.7 Ga) and Transvaal Supergroups (~2.6-2.1 Ga) have been analyzed to monitor the early Proterozoic evolution of the Kaapvaal Craton, southern Africa. From oldest to youngest, pelites were sampled from the Ventersdorp-Bothaville (BOT), Transvaal-Selati (SEL), Black Reef (BR), Timeball Hill (TH), Strubenkop (STR), and Silverton (SIL) Formations. Paleocurrent measurements in Transvaal quartzites indicate sources lying predominantly to the north and east. Relative to the BOT-SEL-BR, pelites from the TH-STR-SIL are enriched in heavy-REE, LILE, Zr, Hf, Nb, and Ta, depleted in K 2O, MgO, Ni, and Cr, and have lower Cr/Zr, Sc/Th, K 2O/Na 2O, and K/ Rb ratios. Compared to SEL-BR, BOT-TH-STR-SIL pelites have higher light-REE contents and La/Yb ratios, and lower Eu/Eu∗ ratios (0.61-0.66). Relative to NASC (North American Shale Composite), THSTR-SIL pelites are enriched in light-REE, Th, U, Ta, Nb, Sc, Cs, have higher La/Yb ratios, and are depleted in K 2O and MgO. BOT-SEL-BR pelites are enriched in K 2O, MgO, Cr, and Ni, have higher K 2O/Na 2O, Sc/Th, and Eu/Eu∗ ratios, and are depleted in Th, U, heavy-REE, and High Field Strength Elements (HFSE) relative to NASC. Compositions of TH-STR-SIL pelites suggest a provenance similar to average Phanerozoic uppercontinental crust. This source is more evolved than that of BOT-SEL-BR pelites, indicating a transformation from primitive (mafic-rich) to evolved (felsic-rich) upper-crust at 2.2 Ga. This transition follows earlier primitive to evolved trends in Moodies-Pongola (3.3-3.0 Ga) and Witwatersrand (~2.8 Ga) successions. These data suggest that several cycles of changing upper-continental crust occurred in the Kaapvaal craton between 3.3-2.1 Ga.

A billion years of metasomatic alteration of the Kaapvaal SCLM encapsulated in fibrous diamonds

NASA Astrophysics Data System (ADS)

Weiss, Yaakov; Goldstein, Steven; Class, Cornelia; Winckler, Gisela

2017-04-01

Constraining the nature and timing of metasomatic events by C-O-H fluids in the sub-continental lithospheric mantle (SCLM) is an ongoing challenge in our understanding of the SCLM history. 'Fibrous' diamonds, which rapidly grow during such fluid-rock interaction, commonly encapsulate the C-O-H fluid metasomatic agents, which are trapped and encapsulated as μm-scale high-density fluid (HDF) inclusions and can be directly sampled. They thus offer a unique opportunity to investigate metasomatic events involving C-O-H fluids in the SCLM. We analyzed major and trace elements and the helium content and isotopic composition of the HDFs included in a set of diamonds from the DeBeers-Pool and Finsch kimberlites in the Kaapvaal craton, South Africa. Saline HDF micro-inclusions in 3 of the DeBeers diamonds and in 2 from Finsch have very similar major and trace element compositions, suggesting they formed in related metasomatic events. They are characterized by high alkalis (K, Rb and Cs), Ba and LREEs compared to Th, U, Nb and Ta, and by Ti, Zr, Hf and Y negative anomalies relative to REEs of similar compatibility. Their 3He/4He ratios vary between 3-4 Ra. Plotting the 3 DeBeers-Pool diamonds on an 4He/3He vs 238U/3He diagram defines an 'isochron' with an age of 96±45 Ma. This result represents the first radiometric age reported for fibrous diamonds and the C-O-H mantle fluids they carry. In addition, 1 diamond from DeBeers-Pool has silicic microinclusion compositions while 2 diamonds from Finsch carry carbonatitic HDFs. These diamonds display more radiogenic 3He/4He ratios between 0.07-0.6 Ra, which suggests formation during earlier and different metasomatic events. Using the measured U, Th, 4He and 3He content of these diamonds, and the equation for 4He production by U and Th, we calculate 3He/4He ratios as a function of time. Assuming that the HDFs initial R/Ra values varied between 3-11, representing common values for MORB, the SCLM and subducted components, the silicic

Southern hemisphere craton modification by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, J.; Liu, L.; Faccenda, M.; Zhou, Q.; Fischer, K. M.; Marshak, S.; Lundstrom, C.

2017-12-01

The longevity of cratons is generally attributed to neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Large portions of the cratonic lithospheres in South America and Africa, however, have experienced significant modification since the Mesozoic, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We show that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered lithosphere deformation and deep lithospheric loss during Late Cretaceous to early Tertiary, as further evidenced by positive lithosphere residual topography, negative lithosphere residual gravity and the realignment of seismic anisotropy in the cratonic roots. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow and high seismic velocities. We conclude that lowermost cratonic lithospheres is compositionally denser than the asthenospheric mantle and can be episodically removed when perturbed by underlying mantle dynamics, while the shallower buoyant lithosphere helps to stabilize cratonic crust over billions of years. We further propose that zones where lithosphere was lost would take tens of millions of years to recover thermally, but the density of the new thermal root would remain less than that of the intact root.

Destroying a Craton by Plate Subduction, Small-scale Convection, and Mantle Plume: Comparison of the Wyoming Craton and the North China Craton

NASA Astrophysics Data System (ADS)

Li, A.; Dave, R.

2016-12-01

A typical craton has a thick, strong, and neutrally buoyant lithosphere that protects it from being destructed by mantle convection. The Wyoming craton and the North China craton are two rare representatives, where the thick Archean lithosphere has been significantly thinned and partially removed as revealed in seismic tomography models. The Wyoming craton in the west-central US experienced pervasive deformation 80-55 Ma during the Laramide orogeny. It has been subsequently encroached upon by the Yellowstone hotspot since 2.0 Ma. Recent seismic models agree that the northern cratonic root in eastern Montana has been broadly removed while the thick root is still present in Wyoming. Our radial anisotropy model images a VSV>VSH anomaly associated with the deep fast anomaly in central Wyoming, indicating mantle downwelling. Continuous low velocities are observed beneath the Yellowstone hotspot and the Cheyenne belt at the craton's southern margin, suggesting mantle upwelling in the sub-lithosphere mantle. These observations evidence for small-scale mantle convection beneath the south-central Wyoming craton, which probably has been actively eroding the cratonic lithosphere. The small-scale mantle convection is probably also responsible for the observed, localized lithosphere delamination beneath the eastern North China craton. In addition, a plume-like, low-velocity feature is imaged beneath the central block of the North China craton and is suggested as the driving force for destructing the cratonic root. Like the Wyoming craton that was subducted by the Farallon plate during the Laramide orogeny, the North China craton was underlined by the ancient Pacific plate before the root destruction in Late Jurassic. In both cases, the subducted slab helped to hydrate and weaken the cratonic lithosphere above it, initiate local metasomatism and partial melting, and promote small-scale convection. The craton's interaction with a mantle plume could further strengthen the small

Synthesis of proterozoic data as a prerequisite for tectonic and thermal modeling

NASA Technical Reports Server (NTRS)

Burke, K. C.

1984-01-01

Rocks of the Pongola supergroup form an elongate belt in the Archean Kaapvaal Caton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, it is suggested that the Pongola supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in Earth history reveals striking similarities. Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago. It is suggested that it was this collision which initiated the Ventersdorp rifting. The Ventersdorp rift province is related to an extension in the Kaapval Craton associated with the collision.

Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

NASA Astrophysics Data System (ADS)

Pilet, S.; Müntener, O.; Jean, G.; Schoene, B.; Schaltegger, U.

2016-12-01

The temporal coincidence between LIPs and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here, we present a synthesis of stratigraphic constraints on the Triassic-Jurassic and Pliensbachian-Toarcian boundaries combined with geochronological data demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. As current hypothesis for LIPs seems unable to produce these successive climatic changes, we evaluate an alternative suggesting that the initial cooling could be due to gas release during the initial thermal erosion of the cratonic lithosphere due to emplacement of the CAMP and Karoo-Ferrar volcanic provinces. Karoo and CAMP areas were underlain by thick lithosphere (>200 km) prior to continental break up. Even in presence of abnormal potential mantle temperature, the presence of thick lithosphere excludes significant melting of the asthenospheric mantle without initial stage of thermal erosion of the cratonic lithosphere. Various studies on Kaapvaal craton have shown that sulfide minerals are enclosed in the basal part of the cratonic lithosphere. We argue that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere causing global cooling and eustatic regression, which was followed by warming/transgression associated with the progressive increase of CO2 in the atmosphere associated to LIPs emission. We suggest that the nature of the underlying lithosphere during large LIP eruption exerts an important control on the consequences at the Earth's surface. This model offers an explanation for why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

Thrust exhumation of the Southern Marginal Zone of the Limpopo Complex in the Neoarchaean: link of distinct high-grade shear zones with DC and IC P-T-t paths

NASA Astrophysics Data System (ADS)

Smit, C. Andre; van Reenen, Dirk D.

2010-05-01

The Limpopo Complex is a ~750km long E-W trending zone of predominantly granulite facies rocks situated between the Archaean Kaapvaal and Zimbabwe cratons of southern Africa. Large ductile shear zones are an integral part of the Limpopo architecture, defining the boundaries between the belt and the adjacent cratons and are interpreted to have been responsible for uplift (exhumation) of over thickened crust during the Neoarchaean [10 and references therein; 1]. The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal craton in the south and the high-grade Southern Marginal Zone (SMZ) of the Limpopo Complex in the north. Integrated structural, metamorphic, magmatic and age data collected over a period of more than 30 years provide convincing evidence for a Neoarchean high-grade tectono-metamorphic event that affected the SMZ in the interval ~2.72 - 2.60 Ga [4; 5, 6; 7; 2; 8; 9; 11]. The thrust-controlled exhumation of the SMZ is demonstrated by the convergence of a retrograde P-T path in the hanging wall (SMZ) and a prograde P-T loop in the footwall (Kaapvaal Craton) of the steeply SW-verging Hout River Shear Zone [4; 5]. The coeval ages (~2.69 Ga) of the two contrasting metamorphic histories are indicated by geochronological data [2; 3]. In addition, the establishment of a retrograde isograd and zone of rehydrated granulites in the hanging wall by hydrous CO2-rich fluids derived by dehydration of the low-grade rocks in the footwall provides another convincing link between the two contrasting metamorphic environments [10]. Distinct retrograde P-T paths [4; 6; 8] linked to distinct shear deformational events document evidence for a two-stage post-peak exhumation history of the SMZ: (i) granulites sampled far from the contact with the cool rocks of the Kaapvaal Craton are characterized by P-T paths with two distinct decompression-cooling (DC) stages (DC=>DC paths), (ii) granulites sampled close to this contact are

Olivine water contents in the continental lithosphere and the longevity of cratons.

PubMed

Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

2010-09-02

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

Water and Metasomatism in the Slave Cratonic Lithosphere (Canada): An FTIR Study

NASA Technical Reports Server (NTRS)

Kilgore, McKensie; Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; Pearson, D. Graham; O'Reilly, Suzanne Yvette; Kopylova, Maya G.; Griffin, William L.

2017-01-01

Water in the mantle influences melting, viscosity, seismic velocity, and electrical conductivity. The role played by water in the long-term stabilization of cratonic roots is currently being debated. This study focuses on water contents of mantle minerals (olivine, pyroxene and garnet) from xenoliths found in kimberlites of the Archean Slave craton. 19 mantle xenoliths from central Lac de Gras, and 10 from northern Jericho were analyzed by FTIR for water, and their equilibration depths span the several compositional layers identified beneath the region. At both locations, the shallow peridotites have lower water contents in their olivines (11-30 ppm H2O) than those from the deeper layers (28-300 ppm H2O). The driest olivines, however, are not at the base of the cratonic lithosphere (>6 GPa) as in the Kaapvaal craton. Instead, the deepest olivines are hydrous (31-72 ppm H2O at Lac de Gras and 275 ppm H2O at Jericho). Correlations of water in clinopyroxene and garnet with their other trace element contents are consistent with water being added by metasomatism by melts resembling kimberlite precursors in the mantle approx.0.35 Ga ago beneath Lac de Gras. The northern Jericho xenoliths are derived from a region of the Slave craton that is even more chemically stratified, and was affected at depth by the 1.27 Ga Mackenzie igneous events. Metasomatism at Jericho may be responsible for the particularly high olivine water contents (up to 300 ppm H2O) compared to those at Lac de Gras, which will be investigated by acquiring trace-element data on these xenoliths. These data indicate that several episodes of metasomatic rehydration occurred in the deep part of the Slave craton mantle lithosphere, with the process being more intense in the northern part beneath Jericho, likely related to a translithospheric suture serving as a channel to introduce fluids and/or melts in the northern region. Consequently, rehydration of the lithosphere does not necessarily cause cratonic root

Establishing and Validating Empirically-Based Ground Truth Criteria for Seismic Events Recorded on Regional Networks (Postprint)

DTIC Science & Technology

2011-12-30

which data sets containing GT0 events (explosions and mine tremors) are available, local crustal structure is well known, and hand-picked arrival...available, local crustal structure is well known, and hand-picked arrival times have been obtained. Boomer et al. (2010) describes the development of...local criteria for the simple crustal structure of the Archean Kaapvaal Craton in southern Africa. Continuing the development of local criteria in

The Carrancas Formation, Bambuí Group: A record of pre-Marinoan sedimentation on the southern São Francisco craton, Brazil

NASA Astrophysics Data System (ADS)

Uhlein, Gabriel J.; Uhlein, Alexandre; Halverson, Galen P.; Stevenson, Ross; Caxito, Fabrício A.; Cox, Grant M.; Carvalho, Jorge F. M. G.

2016-11-01

The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuí Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitaí Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have δ13C values ranging from +7.1 to +9.6‰, which is a unique C isotopic composition for the lowermost Bambuí Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaí fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ13C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ13C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ13C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaí Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuí Group strata and has implications for the development of the Bambuí basin.

Water and metasomatism in the Slave cratonic lithosphere (Canada): an FTIR study

NASA Astrophysics Data System (ADS)

Kilgore, M.; Peslier, A. H.; Brandon, A. D.; Schaffer, L. A.; Pearson, D. G.; O'Reilly, S. Y.; Kopylova, M. G.; Griffin, W. L.

2017-12-01

Water in the mantle influences melting, viscosity, seismic velocity, and electrical conductivity. The role played by water in the long-term stabilization of cratonic roots is currently being debated [1]. This study focuses on water contents of mantle minerals (olivine, pyroxene and garnet) from xenoliths found in kimberlites of the Archean Slave craton. 19 mantle xenoliths from central Lac de Gras, and 10 from northern Jericho were analyzed by FTIR for water, and their equilibration depths span the several compositional layers identified beneath the region [2]. At both locations, the shallow peridotites have lower water contents in their olivines (11-30 ppm H2O) than those from the deeper layers (28-300 ppm H2O). The driest olivines, however, are not at the base of the cratonic lithosphere (>6 GPa) as in the Kaapvaal craton [1]. Instead, the deepest olivines are hydrous (31-72 ppm H2O at Lac de Gras and 275 ppm H2O at Jericho). Correlations of water in clinopyroxene and garnet with their other trace element contents are consistent with water being added by metasomatism by melts resembling kimberlite precursors in the mantle 0.35 Ga ago beneath Lac de Gras [1]. The northern Jericho xenoliths are derived from a region of the Slave craton that is even more chemically stratified, and was affected at depth by the 1.27 Ga Mackenzie igneous events [3,4]. Metasomatism at Jericho may be responsible for the particularly high olivine water contents (up to 300 ppm H2O) compared to those at Lac de Gras, which will be investigated by acquiring trace-element data on these xenoliths. These data indicate that several episodes of metasomatic rehydration occurred in the deep part of the Slave craton mantle lithosphere, with the process being more intense in the northern part beneath Jericho, likely related to a translithospheric suture serving as a channel to introduce fluids and/or melts in the northern region [5]. Consequently, rehydration of the lithosphere does not necessarily

Structures of the West African Craton Margin across southern Mauritania inferred from a 450-km geoelectrical profile

NASA Astrophysics Data System (ADS)

Ritz, M.; Robineau, B.; Vassal, J.; Bellion, Y.; Dukhan, M.

1989-04-01

Magnetotelluric (MT) measurements were carried out at 20 sites, extending 450 km across southern Mauritania in order to study lithospheric structures related to the West African craton (WAC) margin. The MT profile starts to the west on the Senegal-Mauritania basin (S-M basin), traverses across the Mauritanides orogenic belt, and terminates on the western border of the WAC (Taoudeni basin). Distortion effects due to local shallow inhomogeneities are present in nearly all of the basin data. In such a situation, the preliminary interpretation of the data was done by using 1D inversions based upon rotationally invariant parameters. Such distortion is not apparent for the belt and craton sites, and 1D inversions were followed by 2D modeling. The models produced reveal a clear crustal subdivision into a resistive upper crust underlain by a two-layer lower crust with two conductors, one at mid-crustal depths (supposed fluid-produced) beneath the S-M basin and the second at the base of the crust beneath the WAC. The 14-km-thick conductive material below the Mauritanides belt is interpreted as large imbricated thrusts representing the deep roots of the Mauritanides nappes. The models also show that significant contrasts in resistivity extend deep in the lithosphere between the cratonic area and the Senegal microplate.

Sm-Nd isotopic systematics of the ancient Gneiss complex, southern Africa

NASA Technical Reports Server (NTRS)

Carlson, R. W.; Hunter, D. R.; Barker, F.

1983-01-01

In order to shed some new light on the question of the absolute and relative ages of the Ancient Gneiss Complex and Onverwacht Group, a Sm-Nd whole-rock and mineral isochron study of the AGC was begun. At this point, the whole-rock study of samples from the Bimodal Suite selected from those studied for their geochemical characteristics by Hunter et al., is completed. These results and their implications for the chronologic evolution of the Kaapvaal craton and the sources of these ancient rocks are discussed.

Archean crustal evolution in the Southern São Francisco craton, Brazil: Constraints from U-Pb, Lu-Hf and O isotope analyses

NASA Astrophysics Data System (ADS)

Albert, Capucine; Farina, Federico; Lana, Cristiano; Stevens, Gary; Storey, Craig; Gerdes, Axel; Dopico, Carmen Martínez

2016-12-01

In this study we present U-Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920-2850 Ma), Rio das Velhas II (2800-2760 Ma) and Mamona (2750-2680 Ma). The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct εHf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium- to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ18O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ18O values (down to 2.50‰) throughout the Meso- and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism. We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65 Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9 Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision.

Le craton ouest-africain et le bouclier guyanais: un seul craton au Protérozoïque inférieur?

NASA Astrophysics Data System (ADS)

Caen-Vachette, Michelle

Geochronological and paleomagnetism data for southern West African craton and Guyana shield in South America, are concordant and suggest the existence of a large unit grouping them during Archean and Lower Proterozoic times. The paleomagnetism data allow to put on a single line, the Zednes (Mauritania), Sassandra (Ivory Coast) and Guri (Venezuela) fault zones, the mylonites of which were dated 1670 Ma. This age reflects the end of the eburnean-transamazonian shearing tectonic, which affected the large West Africa-Guyana unit. This line separates the western Archean domain from the eastern lower Proterozoic one; thence it is possible to correlate the Sasca (Ivory Coast) and Pastora (Venezuela) areas. Archean relics have been found in mobile pan-african-bresiliano zones which surround the Precambrian cratons; this fact suggests the existence of still more extended Archean craton than defined above.

Super-deep low-velocity layer beneath the Arabian plate

NASA Astrophysics Data System (ADS)

Vinnik, L.; Ravi Kumar, M.; Kind, R.; Farra, V.

2003-04-01

S and P receiver functions reveal indications of a low S velocity layer at 350-410 km depths beneath the Arabian plate. A similar layer was previously found beneath the Kaapvaal craton in southern Africa and Tunguska basin of the Siberian platform. We hypothesize, that the boundary at 350 km depth may separate dry mantle root of the Arabian plate from the underlying wet mantle layer. This boundary is not found beneath the Gulf of Aden, where the root is destroyed by sea-floor spreading.

Neoproterozoic stratigraphic framework of the Tarim Craton in NW China: Implications for rift evolution

NASA Astrophysics Data System (ADS)

Wu, Lin; Guan, Shuwei; Zhang, Shuichang; Yang, Haijun; Jin, Jiuqiang; Zhang, Xiaodan; Zhang, Chunyu

2018-06-01

The Tarim Craton is overlain by thick Neoproterozoic sedimentary successions in rift tectonic setting. This study examines the latest outcrop, seismic, and drilling core data with the objective of investigating the regional stratigraphy to deeply recognize the evolution of rifting in the craton. Cryogenian to Lower Ediacaran successions are mainly composed of clastic rocks with thicknesses of 2000-3000 m, and the Upper Ediacaran successions are composed of carbonate rocks with thicknesses of 500-800 m. The rift basins and stratigraphic zones are divided into northern and southern parts by a central paleo-uplift. The northern rift basin extends through the northern Tarim Craton in an E-W direction with two depocenters (Aksu and Kuruktag). The southern rift basin is oriented NE-SW. There are three or four phases of tillites in the northern zone, while there are two in the southern zone. Given the north-south difference of the stratigraphic framework, the northern rift basin initiated at ca. 740 Ma and the southern rift basin initiated at ca. 780 Ma. During the Cryogenian and Ediacaran, the northern and southern rift basins were separated by the central paleo-uplift, finally connecting with each other in the early Cambrian. Tectonic deformation in the Late Ediacaran led to the formation of a parallel unconformity in the rift basins and an angular unconformity in the central paleo-uplift. The Neoproterozoic rift basins continued to affect the distribution of Lower Cambrian hydrocarbon source rocks. The north-south distribution and evolution of the rift basins in the Tarim Craton have implications for reconstructions of the Rodinia supercontinent.

Kimberlites of the Man craton, West Africa

NASA Astrophysics Data System (ADS)

Skinner, E. M. W.; Apter, D. B.; Morelli, C.; Smithson, N. K.

2004-09-01

The Man craton in West Africa is an Archaean craton formerly joined to the Guyana craton (South America) that was rifted apart in the Mesozoic. Kimberlites of the Man craton include three Jurassic-aged clusters in Guinea, two Jurassic-aged clusters in Sierra Leone, and in Liberia two clusters of unknown age and one Neoproterozoic cluster recently dated at ∼800 Ma. All of the kimberlites irrespective of age occur as small pipes and prolific dykes. Some of the Banankoro cluster pipes in Guinea, the Koidu pipes in Sierra Leone and small pipes in the Weasua cluster in Liberia contain hypabyssal-facies kimberlite and remnants of the so-called transitional-facies and diatreme-facies kimberlite. Most of the Man craton kimberlites are mineralogically classified as phlogopite kimberlites, although potassium contents are relatively low. They are chemically similar to mica-poor Group 1A Southern African examples. The Jurassic kimberlites are considered to represent one province of kimberlites that track from older bodies in Guinea (Droujba 153 Ma) to progressively younger kimberlites in Sierra Leone (Koidu, 146 Ma and Tongo, 140 Ma). The scarcity of diatreme-facies kimberlites relative to hypabyssal-facies kimberlites and the presence of the so-called transitional-facies indicate that the pipes have been eroded down to the interface between the root and diatreme zones. From this observation, it is concluded that extensive erosion (1-2 km) has occurred since the Jurassic. In addition to erosion, the presence of abundant early crystallizing phlogopite is considered to have had an effect on the relatively small sizes of the Man craton kimberlites.

New paleomagnetic results on ˜ ˜2367 Ma Dharwar giant dyke swarm, Dharwar craton, southern India: implications for Paleoproterozoic continental reconstruction

NASA Astrophysics Data System (ADS)

Babu, N. Ramesh; Venkateshwarlu, M.; Shankar, Ravi; Nagaraju, E.; Parashuramulu, V.

2018-02-01

Here we report new paleomagnetic results and precise paleopole position of the extensional study on ˜ 2367 Ma mafic giant radiating dyke swarm in the Dharwar craton, southern India. We have sampled 29 sites on 12 dykes from NE-SW Karimnagar-Hyderabad dykes and Dhone-Gooty sector dykes, eastern Dharwar craton to provide unambiguous paleomagnetism evidence on the spectacular radiating dyke swarm and thereby strengthening the presence of single magmatic event at ˜ 2367 Ma. A total of 158 samples were subjected to detailed alternating field and thermal demagnetization techniques and the results are presented here along with previously reported data on the same dyke swarm. The remanent magnetic directions are showing two components, viz., seven sites representing four dykes show component (A) with mean declination of 94{{}°} and mean inclination of - 70{{}°} (k=87, α_{95}=10{{}°}) and corresponding paleopole at 16{{}°}N, 41{{}°}E (dp=15{{}°} and dm=17{{}°}) and 22 sites representing 8 dykes yielded a component (B) with mean declination of 41{{}°} and mean inclination of - 21{{}°} (k=41, α_{95}=9{{}°}) with a paleopole at 41{{}°}N, 200{{}°}E (dp=5{{}°} and dm=10{{}°}). Component (A) results are similar to the previously reported directions from the ˜ 2367 Ma dyke swarm, which have been confirmed fairly reliably to be of primary origin. The component (B) directions appear to be strongly overprinted by the 2080 Ma event. The grand mean for the primary component (A) combined with earlier reported studies gives mean declination of 97{{}°} and mean inclination of - 79{{}°} (k=55, α_{95}=3{{}°}) with a paleopole at 15{{}°}N, 57{{}°}E (dp=5{{}°}, dm=6{{}°}). Paleogeographical position for the Dharwar craton at ˜ 2367 Ma suggests that there may be a chance to possible spatial link between Dharwar dykes of Dharwar craton (India), Widgemooltha and Erayinia dykes of Yilgarn craton (Australia), Sebanga Poort Dykes of Zimbabwe craton (Africa) and Karelian

Coexistence of enriched and modern-like 142Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa

NASA Astrophysics Data System (ADS)

Schneider, Kathrin P.; Hoffmann, J. Elis; Boyet, Maud; Münker, Carsten; Kröner, Alfred

2018-04-01

The short-lived 146Sm-142Nd isotope system is an important tool for tracing Hadean crust-mantle differentiation processes and constraining their imprint on much younger rocks from Archean cratons. We report the first comprehensive set of high-precision 142Nd analyses for granitoids and amphibolites of the Ancient Gneiss Complex (AGC; Swaziland) and the oldest metavolcanic units of the Barberton Greenstone Belt (BGB; South Africa). The investigated samples span an age range from 3.66 Ga to 3.22 Ga and are representative of major geological units of the AGC and the lower Onverwacht Group of the BGB. Measured samples yielded μ142Nd values in the range from -8 ppm to +3 ppm relative to the JNdi-1 terrestrial standard, with typical errors smaller than 4.4 ppm. The distribution of the μ142Nd values for these 17 measured samples is bimodal with ten samples showing a tendency towards slightly negative μ142Nd anomalies, whereas seven samples have 142Nd similar to the terrestrial reference. The only confidently resolvable μ142Nd anomalies were found in a 3.44 Ga Ngwane Gneiss sample and in amphibolites of the ca. 3.45 Ga Dwalile Greenstone Remnant, revealing μ142Nd values ranging from - 7.9 ± 4.4 to - 6.1 ± 4.3 ppm. The μ142Nd deficits do not correlate with age, lithological unit, or sample locality. Instead, our results reveal that two distinct mantle domains were involved in the formation of the AGC crust. The two reservoirs can be distinguished by their μ142Nd signatures. Mantle-derived rocks tapped the enriched reservoir with negative μ142Nd at least until 3.46 Ga, whereas the granitoids preserved a negative μ142Nd signature that formed by incorporation of older AGC crust at least until 3.22 Ga. The oldest gneisses with no μ142Nd anomaly are up to 3.64 Ga in age, indicating that a modern terrestrial 142Nd reservoir was already present by early Archean times.

Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly.

PubMed

Hanson, Richard E; Crowley, James L; Bowring, Samuel A; Ramezani, Jahandar; Gose, Wulf A; Dalziel, Ian W D; Pancake, James A; Seidel, Emily K; Blenkinsop, Thomas G; Mukwakwami, Joshua

2004-05-21

We show that intraplate magmatism occurred 1106 to 1112 million years ago over an area of two million square kilometers within the Kalahari craton of southern Africa, during the same magnetic polarity chron as voluminous magmatism within the cratonic core of North America. These contemporaneous magmatic events occurred while the Rodinia supercontinent was being assembled and are inferred to be parts of a single large igneous province emplaced across the two cratons. Widespread intraplate magmatism during Rodinia assembly shows that mantle upwellings required to generate such provinces may occur independently of the supercontinent cycle.

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

NASA Astrophysics Data System (ADS)

James, D. E.; Boyd, F. R.; Schutt, D.; Bell, D. R.; Carlson, R. W.

2004-01-01

We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of

Metasomatism and the Weakening of Cratons: A Mechanism to Rift Cratons

NASA Astrophysics Data System (ADS)

Wenker, Stefanie; Beaumont, Christopher

2016-04-01

The preservation of cratons is a demonstration of their strength and resistance to deformation. However, several cratons are rifting now (e.g. Tanzania and North China Craton) or have rifted in the past (e.g. North Atlantic Craton). To explain this paradox, we suggest that widespread metasomatism of the originally cold depleted dehydrated craton mantle lithosphere root can act as a potential weakening mechanism. This process, particularly melt metasomatism, increases root density through a melt-peridotite reaction, and reduces root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. Using 2D numerical models, we model silicate-melt metasomatism and rehydration of cold cratonic mantle lithosphere that is positioned beside standard Phanerozoic lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by the initially weaker adjacent standard Phanerozoic lithosphere. Melt is added to specified layers in the cratonic mantle lithosphere at a uniform volumetric rate determined by the duration of metasomatism (3 Myr, 10 Myr or 30 Myr), until a total of ~30% by volume of melt has been added. During melt addition heat and mass are properly conserved and the density and volume increase by the respective amounts required by the reaction with the peridotite. No extensional boundary conditions are applied to the models during the metasomatism process. As expected, significant refertilization leads to removal and thinning of progressively more gravitationally unstable cratonic mantle lithosphere. We show that the duration of metasomatism dictates the final temperature in the cratonic upper mantle lithosphere. Consequently, when extensional boundary conditions are applied in our rifting tests in most cases the Phanerozoic lithosphere rifts. The craton rifts only in the models with the hottest cratonic upper mantle lithosphere. Our results indicate rifting of cratons

Ambient noise tomography of the East African Rift in Mozambique

NASA Astrophysics Data System (ADS)

Domingues, Ana; Silveira, Graça; Ferreira, Ana M. G.; Chang, Sung-Joon; Custódio, Susana; Fonseca, João F. B. D.

2016-03-01

Seismic ambient noise tomography is applied to central and southern Mozambique, located in the tip of the East African Rift (EAR). The deployment of MOZART seismic network, with a total of 30 broad-band stations continuously recording for 26 months, allowed us to carry out the first tomographic study of the crust under this region, which until now remained largely unexplored at this scale. From cross-correlations extracted from coherent noise we obtained Rayleigh wave group velocity dispersion curves for the period range 5-40 s. These dispersion relations were inverted to produce group velocity maps, and 1-D shear wave velocity profiles at selected points. High group velocities are observed at all periods on the eastern edge of the Kaapvaal and Zimbabwe cratons, in agreement with the findings of previous studies. Further east, a pronounced slow anomaly is observed in central and southern Mozambique, where the rifting between southern Africa and Antarctica created a passive margin in the Mesozoic, and further rifting is currently happening as a result of the southward propagation of the EAR. In this study, we also addressed the question concerning the nature of the crust (continental versus oceanic) in the Mozambique Coastal Plains (MCP), still in debate. Our data do not support previous suggestions that the MCP are floored by oceanic crust since a shallow Moho could not be detected, and we discuss an alternative explanation for its ocean-like magnetic signature. Our velocity maps suggest that the crystalline basement of the Zimbabwe craton may extend further east well into Mozambique underneath the sediment cover, contrary to what is usually assumed, while further south the Kaapval craton passes into slow rifted crust at the Lebombo monocline as expected. The sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active NNE-SSW Urema rift, while further south the Mazenga graben adopts an N-S direction

The P wavespeed structure below and around the Kaapvaal craton to depths of 800 km, from traveltimes and waveforms of local and regional earthquakes and mining-induced tremors

NASA Astrophysics Data System (ADS)

Simon, R. E.; Wright, C.; Kgaswane, E. M.; Kwadiba, M. T. O.

2002-10-01

An average P-wavespeed model from the surface to depths of 800 km was derived for southern Africa using traveltimes and waveforms from earthquakes recorded at stations of the Kaapvaal and South African National networks. In this first study of the transition zone for the central part of the African superswell, a damped least-squares inversion was used to minimize effects of origin time errors. Triplications were observed for both the 410 and 670 km discontinuities, with crossover points between first arrival branches at average distances of 19.61° and 23.92°, respectively. The Herglotz-Wiechert method combined with ray tracing was used to derive a preliminary model, followed by refinements using phase-weighted stacking and synthetic seismograms to yield the final model BPI1A. This model shows a prominent 410 km discontinuity, but a weakly-defined 670 km discontinuity, in agreement with the SATZ model for a region of southern Africa to the north of the region covered by the present study. The wavespeeds of BPI1A from the base of the crust to 270 km depth lie between those of the SATZ model and the IASP91 model, which have higher and lower wavespeeds respectively. Between depths of 270 km and the 410 km discontinuity, models BPI1A, IASP91 and SATZ have similar wavespeeds but the 410 km discontinuity for BPI1A is about 10 km deeper than in IASP91. Model GNEM for Eurasia has lower wavespeeds than the other three models above the 410 km discontinuity. Within the transition zone models BPI1A and SATZ converge as the depth increases, with wavespeeds that exceed those of IASP91 below 500 km depth. These models and model GNEM all have similar wavespeeds below 750 km depth. The seismic results indicate no regions of anomalous low wavespeeds within the uppermost 800 km of the mantle that could be associated with high temperatures and the uplift of the African superswell. However, higher seismic wavespeeds in the transition zone than elsewhere are suggested for the southern

Palaeomagnetism of the Palaeoproterozoic Boonadgin Dyke Suite, Yilgarn Craton: Possible connection with India

NASA Astrophysics Data System (ADS)

Liu, Y.; Li, Z. X.; Pisarevsky, S.; Kirscher, U.; Mitchell, R.; Stark, J. C.

2017-12-01

A palaeomagnetic study was carried out on the newly identified 1.9 Ga Boonadgin dyke swarm in the Yilgarn Craton, Western Australia. Ten dykes revealed a high-temperature characteristic remanent magnetisation (ChRM) with dual polarities, directing either SW shallow downward (4 sites) or NE shallow upward (6 sites). Our results reveal that the Yilgarn Craton was at an equatorial palaeolatitude at 1.9 Ga. Meanwhile, a paleopole from the ca. 1.9 Ga Dharwar dykes of South India, supported by a positive baked-contact test, puts India at a similar paleolatitude. The Boonadgin dyke swarm can be interpreted to represent an arm of a radiating dyke swarm that shared the same plume centre with coeval mafic dykes in the Dharwar and Bastar cratons of southern India. We therefore propose that the Western Australia Craton (WAC, consisting of the the Yilgarn and Pilbara cratons) and South India were connected at ca. 1.89 Ga.

Formation and Evolution of the Continental Lithospheric Mantle: Perspectives From Radiogenic Isotopes of Silicate and Sulfide Inclusions in Macrodiamonds

NASA Astrophysics Data System (ADS)

Shirey, S. B.; Richardson, S. H.

2007-12-01

Silicate and sulfide inclusions that occur in diamonds comprise the oldest (>3 Ga), deepest (>140 km) samples of mantle-derived minerals available for study. Their relevance to the evolution of the continental lithosphere is clear because terrestrial macrodiamonds are confined to regions of the Earth with continental lithospheric mantle keels. The goals of analytical work on inclusions in diamond are to obtain paragenesis constraints, radiogenic ages, and initial isotopic compositions. The purpose is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere and to relate the source of the C and N in diamond-forming fluids to understanding the Earth's C and N cycles in the Archean. Although sulfide and silicate inclusions rarely occur in the same diamond, they both can be grouped according to their geochemical similarity with the chief rock types that comprise the mantle keel: peridotite and eclogite. Silicate inclusions are classified as harzburgitic (depleted; olivine > Fo91, garnet Cr2O3 > 3 wt% and CaO from 0 to 5 wt%), lherzolitic (fertile), or eclogitic (basaltic; garnet Cr2O3 < 2 wt% and CaO from 3 to 15 wt%, clinopyroxene with higher Na2O, Al2O3, and FeO); they are amenable for trace element study by SIMS and for Sm-Nd and Rb-Sr analysis by conventional P-TIMS after grouping by mineralogical similarity. Sulfide inclusions (chiefly FeS with lesser Ni, Cu, and Co) are classified as peridotitic (Ni > 14 wt%; Os > 2 ppm) versus eclogitic (Ni < 10 wt%; Os < 200 ppb); single sulfides are amenable for S isotopic study by SIMS or TIMS, and Re-Os analysis by N-TIMS. Work on inclusions in diamonds depends on the distribution of mined, diamond-bearing kimberlites, and the generosity of mining companies because of the extreme rarity of inclusions in suites of mostly gem-quality diamonds. Most isotopic work has been on the Kaapvaal-Zimbabwe craton with lesser work on the Slave, Siberian

Results of paleomagnetic study of Early Proterozoic rocks in the Baikal Range of the Siberian craton

NASA Astrophysics Data System (ADS)

Vodovozov, V. Yu.; Didenko, A. N.; Gladkochub, D. P.; Mazukabzov, A. M.; Donskaya, T. V.

2007-10-01

This paper presents paleomagnetic results obtained from the study of Early Proterozoic rocks in the Baikal Range of the Siberian craton, namely, the 1850 1880-Ma volcanicalstic rocks of the Akitkanskian series of the North Baikal volcanic-plutonic belt) and 1674-Ma basic dikes of the Chaya complex within the massif. The data of this work are used to reconstruct the development of the Siberian craton structure in the Early Precambrian. The projections of the inferred paleomagnetic directions onto a sphere form S (southern) and W (western) groups of vectors of characteristic magnetization components. The S group consists of three clusters representing primary magnetization components belonging to different time levels of the end of the Early Proterozoic. The W group is represented by directions associated with a metachronous magnetization probably acquired during the Riphean. Four paleomagnetic poles are obtained. Two of them that can be regarded as key poles correspond to time levels of 1875 and 1670 Ma (the Early Proterozoic). The two other poles can be used for a detailed reconstruction of the Proterozoic segment of the Siberian apparent polar wander path. The data presented in the paper indicate that the formation of the southern Siberian craton structure was accomplished at the end of the Early Proterozoic, which resulted in a synchronous motion of different blocks composing the southern flank of the craton (in particular, the Sharyzhalgai and Baikal Ranges).

The Ufa indenter: stratigraphic and geophysic evidences for an actual indentation of the Southern Urals by the East European craton

NASA Astrophysics Data System (ADS)

Lefort, Jean-Pierre; Danukalova, Guzel

2014-07-01

Study of the altitudes of the lowest part of the Upper Cretaceous-Eocene and Aktschagylian-Quaternary stratigraphic ensembles known on the western slope of the Southern Urals evidences the existence of an East-West elongated dome which follows the N53° latitude. This ridge is superimposed at depth with the remnants of the Sernovodsk-Abdulino Aulacogen and with the Belaya tear fault, which support the existence of a recent rejuvenation of these old structures. North of these disruptions the Southern Urals display a clear bent towards the East. Detailed microstructural studies show that this curvature is associated with a typical stress pattern which suggests the existence of an indentation of the fold belt by the East European craton. The hypothesis of an Ufa indenter is not supported by an equivalent East-West deep fault north of the bend. However, a long N100° magnetic anomaly, interpreted as a shear zone, suggests that the indenter is a reality. Quaternary uplift and crustal thickening at its front as well as seismological data support our interpretation. It is not stressed that the curvature of the Urals observed at 56° latitude results solely from this recent indentation. It is only assumed that the actual indentation is rejuvenating a former unevenness which existed before in the East European craton. Study of the inner part of the indenter shows that this type of structure is not necessarily rigid and undeformed. Some of the structures described on the URSEIS deep seismic line could be much younger than previously expected.

Assessment of undiscovered oil and gas resources of the southern Siberian craton (Baykit High, Nepa--Botuoba High, Angara--Lena Terrace, and Cis--Patom Foredeep Provinces), Russia, 2011

USGS Publications Warehouse

Klett, T.R.; Schenk, Christopher J.; Wandrey, Craig J.; Charpentier, Ronald R.; Brownfield, Michael E.; Pitman, Janet K.; Pollastro, Richard M.; Cook, Troy A.; Tennyson, Marilyn E.

2012-01-01

Using a geology-based assessment methodology, the U.S. Geological Survey estimated volumes of undiscovered, technically recoverable, conventional petroleum resources for the southern Siberian craton provinces of Russia. The mean volumes were estimated at 3.0 billion barrels of crude oil, 63.3 trillion cubic feet of natural gas, and 1.2 billion barrels of natural gas liquids.

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

NASA Astrophysics Data System (ADS)

James, David E.; Niu, Fenglin; Rokosky, Juliana

2003-12-01

during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.

Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

NASA Astrophysics Data System (ADS)

Tang, Yan-Jie; Zhang, Hong-Fu; Ying, Ji-Feng; Su, Ben-Xun

2013-03-01

Studies of mantle xenoliths have confirmed that Archean subcontinental lithospheric mantle (SCLM) is highly depleted in basaltic components (such as Al, Ca and Na) due to high-degree extraction of mafic and ultramafic melts and thus is refractory and buoyant, which made it chronically stable as tectonically independent units. However, increasing studies show that ancient SCLM can be refertilized by episodic rejuvenation events like infiltration of upwelling fertile material. The North China Craton is one of the most typical cases for relatively complete destruction of its Archean keel since the eruption of Paleozoic kimberlites, as is evidenced by a dramatic change in the compositions of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, reflecting significant lithospheric thinning and the change in the character of the SCLM. The compositional change has been interpreted as the result of refertilization of Archean SCLM via multiple-stage peridotite-melt reactions, suggested by linear correlations between MgO and indices of fertility, covariations of Al2O3 with CaO, La/Yb, 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and Re-depletion ages (TRD), high Re abundances, scatter in Re-Os isotopic plot, variable in situ TRD ages of sulfides, and correlation between TRD ages and olivine Fo of peridotite xenoliths in Paleozoic kimberlites and Cenozoic basalts on the craton. By integrating major and trace element, Sr, Nd and Os isotopic compositions of peridotite xenoliths and orogenic massif peridotites from the continents of Europe, Asia, America, Africa and Australia, together with previous studies of petrology and geochemistry of global peridotites, we suggest that (1) refertilization of cratonic and circum-cratonic lithospheric mantle is widespread; (2) Archean SCLM worldwide has experienced a multi-stage history of melt depletion and refertilization since segregation from the convecting mantle; (3) cratonic SCLM may be more susceptible to compositional change caused by

Craton destruction and related resources

NASA Astrophysics Data System (ADS)

Zhu, Rixiang; Zhang, Hongfu; Zhu, Guang; Meng, Qingren; Fan, Hongrui; Yang, Jinhui; Wu, Fuyuan; Zhang, Zhiyong; Zheng, Tianyu

2017-10-01

Craton destruction is a dynamic event that plays an important role in Earth's evolution. Based on comprehensive observations of many studies on the North China Craton (NCC) and correlations with the evolution histories of other cratons around the world, craton destruction has be defined as a geological process that results in the total loss of craton stability due to changes in the physical and chemical properties of the involved craton. The mechanisms responsible for craton destruction would be as the follows: (1) oceanic plate subduction; (2) rollback and retreat of a subducting oceanic plate; (3) stagnation and dehydration of a subducting plate in the mantle transition zone; (4) melting of the mantle above the mantle transition zone caused by dehydration of a stagnant slab; (5) non-steady flow in the upper mantle induced by melting, and/or (6) changes in the nature of the lithospheric mantle and consequent craton destruction caused by non-steady flow. Oceanic plate subduction itself does not result in craton destruction. For the NCC, it is documented that westward subduction of the paleo-Pacific plate should have initiated at the transition from the Middle-to-Late Jurassic, and resulted in the change of tectonic regime of eastern China. We propose that subduction, rollback and retreat of oceanic plates and dehydration of stagnant slabs are the main dynamic factors responsible for both craton destruction and concentration of mineral deposits, such as gold, in the overriding continental plate. Based on global distribution of gold deposits, we suggest that convergent plate margins are the most important setting for large gold concentrations. Therefore, decratonic gold deposits appear to occur preferentially in regions with oceanic subduction and overlying continental lithospheric destruction/modification/growth.

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

USGS Publications Warehouse

Cooper, J.A.; Stacey, J.S.; Stoeser, D.G.; Fleck, R.J.

1979-01-01

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820-760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.

When Rifts Meet Cratons

NASA Astrophysics Data System (ADS)

Chen, W. P.; Ning, J.

2017-12-01

The longevity of cratons and the evolution of rifts are two outstanding issues in continental dynamics. Intriguingly, there are several active cases where the two seemingly antithetical tectonic settings abut each other. In most instances, rifting is not accompanied by widespread destruction of adjacent cratons. In the case of the East African rift system (EARS), the most prominent active rift system in the world, its western branch clearly circumvents the Tanzania craton and continues southward along the narrow Malawi rift. Meanwhile, a broad zone of scattered seismicity associated with normal faulting extends westward for about 1,000 km, as accentuated by the recent earthquake of Mw 6.8 in Botswana. Along the eastern branch of the EARS, the well-defined Kenya rift terminates against the Tanzania craton as a diffuse zone of extension (the northern Tanzania divergence.) Yet, farther southward, a band of concentrated seismicity follows the trace of the Davie ridge off the east coast of Africa for another 1,300 km. Similarly, the Ordos plateau (the western portion of the north China craton, NCC), comparable in size to the Tanzania craton, is straddled by the active Yinchuan and Shanxi rifts on its western and eastern flanks, respectively. Along the edges of the Colorado plateau, the very broad Basin and Range province of extension and the narrow Rio Grande rift surround the stable plateau. Therefore, it seems that rifting is not an effective process to destabilize cratons en masse. Widespread, low-angle detachment faulting and the intrusion of Mesozoic granitic plutons characterize the eastern portion of the NCC, an often-cited example of a craton's demise. Here we propose that these features are the consequence, not the cause of the destruction of the NCC. The exact cause(s) of this destruction process remain enigmatic, as the spatial extent of this event apparently reaches as far north as Lake Baikal.

Craton Heterogeneity in the South American Lithosphere

NASA Astrophysics Data System (ADS)

Lloyd, S.; Van der Lee, S.; Assumpcao, M.; Feng, M.; Franca, G. S.

2012-04-01

We investigate structure of the lithosphere beneath South America using receiver functions, surface wave dispersion analysis, and seismic tomography. The data used include recordings from 20 temporary broadband seismic stations deployed across eastern Brazil (BLSP02) and from the Chile Ridge Subduction Project seismic array in southern Chile (CRSP). By jointly inverting Moho point constraints, Rayleigh wave group velocities, and regional S and Rayleigh wave forms we obtain a continuous map of Moho depth. The new tomographic Moho map suggests that Moho depth and Moho relief vary slightly with age within the Precambrian crust. Whether or not a correlation between crustal thickness and geologic age can be derived from the pre-interpolation point constraints depends strongly on the selected subset of receiver functions. This implies that using only pre-interpolation point constraints (receiver functions) inadequately samples the spatial variation in geologic age. We also invert for S velocity structure and estimate the depth of the lithosphere-asthenosphere boundary (LAB) in Precambrian South America. The new model reveals a relatively thin lithosphere throughout most of Precambrian South America (< 140 km). Comparing LAB depth with lithospheric age shows they are overall positively correlated, whereby the thickest lithosphere occurs in the relatively small Saõ Francisco craton (200 km). However, within the larger Amazonian craton the younger lithosphere is thicker, indicating that locally even larger cratons are not protected from erosion or reworking of the lithosphere.

How to make a craton

NASA Astrophysics Data System (ADS)

Lee, C.; Chin, E. J.; Erdman, M.; Gaschnig, R. M.; Lederer, G. W.; Savage, P. S.; Zhong, S.; Zincone, S.

2013-12-01

Most Archean cratons are underlain by long-lived 200-300 km thick thermal boundary layers, significantly thicker than oceanic boundary layers, which eventually subduct. The longevity of cratons is perplexing because cold thermal boundary layers should be gravitationally unstable or should thermally erode with time. However, it is agreed that thermal contraction of the cratonic root is compensated by intrinsic compositional buoyancy due to extreme melt depletion. This melt depletion is also thought to have dehydrated the peridotitic residue, strengthening the cratonic mantle, making it resistant to thermo-mechanical erosion. Exactly how cratonic mantle arrives at this chemically buoyant and dehydrated state is unknown. Possible scenarios include formation by melting within a large plume head, accretion of oceanic lithosphere, and accretion of sub-arc mantle. The high degrees of melting would seem to imply formation in hot plume heads, but low Al and heavy rare earth element contents suggest formation in the spinel stability field, implying formation at shallower depths than their current equilibration pressures. We present a new thermobarometer designed to estimate the average melting pressures and temperatures of residual peridotites using whole rock major element compositions. We find that the average melting pressures and temperatures of cratonic peridotites range between 3-4 GPa and 1600 °C. If cratonic peridotites melted via adiabatic decompression, these average pressures represent maximum bounds on the final pressures of melt extraction. Currently, cratonic peridotites derive from 4-7 GPa, implying that the building blocks of peridotites experienced an increase of 1-3 GPa, equivalent to 30-90 km of overburden. Our results thus imply that cratonic mantle most likely formed by tectonic thickening of oceanic or arc lithospheres. But because both arc and oceanic lithospheres might be expected to be wet due to hydrous flux melting and serpentinization

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

NASA Astrophysics Data System (ADS)

Shirey, Steven B.; Harris, Jeffrey W.; Richardson, Stephen H.; Fouch, Matthew; James, David E.; Cartigny, Pierre; Deines, Peter; Viljoen, Fanus

2003-12-01

The Archean lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150-250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re-Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm-Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2-3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted

Paleomagnetism of the Wyoming Craton: A Pre-Laurentian Puzzle

NASA Astrophysics Data System (ADS)

Kilian, T.; Chamberlain, K.; Mitchell, R. N.; Evans, D. A.; Bleeker, W.; Lecheminant, A. N.

2010-12-01

The Archean Wyoming craton is mostly buried beneath Phanerozoic sediments in the Rocky Mountains of the west central United States. Exposures of the craton are entirely in thrust-bounded Laramide uplifts and contain numerous swarms of Neoarchean-Proterozoic mafic dikes. U-Pb ages from these dikes include ~2685 Ma from a dike in the Owl Creek Mountains (Frost et al., 2006) as well as another in the Bald Mountain region of the Bighorn Mountains (this study), ~2170 Ma from the Wind River Mountain quartz diorite (Harlan et al., 2003), ~2110 Ma from a dike in the Granite Mountains (Bowers and Chamberlain, 2006), ~2010 Ma from a Kennedy dike in the Laramie Range (Cox et al., 2000), and ~780 Ma for dikes in the Beartooth and Teton Mountains (Harlan et al., 1997). These possible age ranges of magmatic events will allow a detailed comparison with other cratons, especially Superior and Slave. Prior to the assembly of Laurentia, Wyoming may have been connected with Slave in supercraton Sclavia (Bleeker, 2003; Frost et al., 2007), or alternatively, Wyoming may have been attached to the present southern margin of Superior in the supercraton Superia, as judged by similarities of the thrice-glaciated Huronian and Snowy Pass sedimentary successions (Roscoe and Card, 1993). Paleomagnetic results will be presented from over 150 dikes in the Wyoming craton. All dikes were from the basement uplifts of the Beartooth Mountains, Bighorn Mountains, Owl Creek Mountains, Granite Mountains, Ferris Mountains and Laramie Range. Dikes range in widths from 1 to >100 meters, and trends vary across all orientations. Stable remanence is observed in majority of sites with at least 8 different directions from the various uplifts. Structural corrections are applied when necessary to restore shallowly dipping Cambrian strata to horizontal. The paleomagnetic study is being integrated with precise U-Pb geochronology of dikes that bear stable remanence directions. Results will eventually allow a

Timing and tempo of the Great Oxidation Event

PubMed Central

Chamberlain, Kevin R.; Bleeker, Wouter; Söderlund, Ulf; de Kock, Michiel O.; Larsson, Emilie R.; Bekker, Andrey

2017-01-01

The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude. PMID:28167763

Uniformity in sulfur isotope composition in the orogenic gold deposits from the Dharwar Craton, southern India

NASA Astrophysics Data System (ADS)

Sakthi Saravanan, C.; Mishra, B.

2009-07-01

The sulfur isotope composition of sulfides (mainly pyrite and arsenopyrite) from gold deposits/prospects of the Dharwar Craton such as Hutti, Hira-Buddini, Uti, Kolar (Chigargunta), Ajjanahalli, and Jonnagiri has a narrow range (δ34S = +1.1 to +7.1‰). Such craton-scale uniformity of the above gold camps is noteworthy, in spite of the wide diversity in host rock compositions and their metamorphic conditions, and suggests a magmatic or average crustal source of sulfur for all deposits studied. In addition, our study points towards gold precipitation from reduced ore fluids, with near-homogeneous sulfur isotope compositions.

The Río de la Plata Craton: a review of units, boundaries, ages and isotopic signature

NASA Astrophysics Data System (ADS)

Oyhantçabal, Pedro; Siegesmund, Siegfried; Wemmer, Klaus

2011-04-01

A review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm-Nd TDM model ages between 2.6 and 2.2 Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1 Ga for the metamorphic protoliths and 2.1-2.0 Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm-Nd TDM model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0-2.6 and 2.3-1.6 Ga). The crystallization ages on zircon ranges between 3.1 and 0.57 Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton.

207Pb-206Pb zircon ages of eastern and western Dharwar craton, southern India : Evidence for contemporaneous Archaean crust

NASA Astrophysics Data System (ADS)

Maibam, B.; Goswami, J. N.; Srinivasan, R.

2009-04-01

Dharwar craton is one of the major Archaean crustal blocks in the Indian subcontinent. The craton is comprised of two blocks, western and eastern. The western domain is underlain by orthogneisses and granodiorites (ca. 2.9-3.3 Ga) collectively termed as Peninsular Gneiss [e.g., 1] interspersed with older tracts of metasedimentary and metamorphosed igneous suites (Sargur Group and Dharwar Group; [2]). The eastern part of the craton is dominated by Late Archaean (2.50-2.75 Ga) granitoids and their gneissic equivalents. They are interspersed with schist belts (also of Sargur Group and Dharwar Group), which are lithologically similar to the Dharwar Supergroup in the western block, but are in different metamorphic dress. Here we report 207Pb-206Pb age of zircons separated from the metasedimentary and gneissic samples from the two blocks to constrain the evolution of the Dharwar craton during the early Archaean. Detrital zircons of the metasedimentary rocks from both the blocks show a wide range of overlapping ages between ~2.9 to >3.5 Ga. Zircon ages of the orthogneisses from the two blocks showed that most of the analysed grains of the eastern Dharwar block are found to be of the age as old as the western Dharwar gneisses. Imprints of younger events could be discerned from the presence of overgrowths in zircons from the studied samples throughout the craton. Our data suggest that crust forming cycles in the two blocks of the Dharwar craton occurred contemporaneously during the Archaean. References [1] Beckinsale, R.D., Drury, S.A., Holt, R.W. (1980) Nature 283, 469-470. [2] Swami Nath J., Ramakrishnan M., Viswanatha M.N. (1976) Rec. Geol. Surv. Ind., 107, 149-175.

Heat flow, heat production, and crustal temperatures in the Archaean Bundelkhand craton, north-central India: Implications for thermal regime beneath the Indian shield

NASA Astrophysics Data System (ADS)

Podugu, Nagaraju; Ray, Labani; Singh, S. P.; Roy, Sukanta

2017-07-01

Heat flow and heat production data sets constrain the crustal thermal structure in the 2.5-3.5 Ga Bundelkhand craton, the oldest cratonic core in northern Indian shield, for the first time and allow comparisons with the southern Indian shield. Temperature measurements carried out in 10 boreholes at five sites in the craton, combined with systematic thermal conductivity measurements on major rock types, yield low heat flow in the range of 32-41 mW m-2, which is distinct from the generally high heat flow reported from other parts of the northern Indian shield. Radioelemental measurements on 243 samples of drill cores and outcrops reveal both large variability and high average heat production for the Neo-Archaean to Palaeo-Proterozoic granites (4.0 ± 2.1 (SD) μW m-3) relative to the Meso-Archaean tonalite-trondhjemite-granodiorite (TTG) gneisses (2.0 ± 1.0 (SD) μW m-3). On the basis of new heat flow and heat production data sets combined with available geological and geophysical information, a set of steady state, heat flow-crustal heat production models representative of varying crustal scenarios in the craton are envisaged. Mantle heat flow and Moho temperatures are found to be in the range of 12-22 mW m-2 and 290-420°C, respectively, not much different from those reported for the similar age Dharwar craton in southern India. This study reveals similar mantle thermal regimes across the northern and southern parts of the Indian shield, in spite of varying surface heat flow regimes, implying that much of the intraprovince and interprovince variations in the Indian shield are explained by variations in upper crustal heat production.

Shear wave velocity and radial anisotropy beneath the Wyoming craton: craton destruction and lithospheric layering

NASA Astrophysics Data System (ADS)

Dave, R.; Li, A.

2016-12-01

The Wyoming craton has evolved under an intriguing geological history with suture zones, accreted margins, flat-slab subduction, orogeny and an encroaching hotspot. Whether and how the cratonic root has been widely destroyed by the series of tectonic events remain controversial. Aiming to address these questions using a craton-wide model, we have analyzed Rayleigh and Love wave data from 75 earthquakes recorded by 103 USArray TA stations in the Wyoming craton. 2-D phase velocity maps are constructed for 18 periods from 20 s to 166 s using the two-plane-wave tomography. The Yellowstone hotspot and the Cheyenne belt are characterized by low velocity anomalies at all periods in both Rayleigh and Love wave models. The northern craton in Montana is broadly fast at periods < 70 s and is relatively slow at longer periods, suggesting a shallower lithosphere. The fast anomaly in Wyoming has a NE-SW trend and extends to more than 200 km in the VSV model. However, such a fast anomaly is largely absent in the Love wave images at long periods. The association of VSV>VSH with this deep fast anomaly indicates mantle downwelling beneath south-central Wyoming. Mantle upwelling likely happens in slow regions at the hotspot, the Cheyenne belt, and the northeastern craton. The overall pattern of velocity anomaly and radial anisotropy suggests that small-scale mantle convection is vigorously acting beneath the Wyoming craton and continuously destructing the cratonic lithosphere. In addition, the average VSV and VSH models show a strong positive radial anisotropy of 5% (VSH>VSV) above 100 km and a weak negative anisotropy (VSV>VSH) below 120 km. Such a significant change in radial anisotropy could contribute to the observed mid-lithosphere discontinuity (MLD) from receiver functions. Both VSV and VSH reveal a fast lid above 100 km and a large velocity reduction at the depths of 115-190 km, corresponding with a lithosphere-asthenosphere boundary (LAB) at 150 km. These observations

EAG Eminent Speaker: Two types of Archean continental crust: plume and plate tectonics on early Earth

NASA Astrophysics Data System (ADS)

Van Kranendonk, M. J.

2012-04-01

Over 4.5 billion years, Earth has evolved from a molten ball to a cooler planet with large continental plates, but how and when continents grew and plate tectonics started remain poorly understood. In this paper, I review the evidence that 3.5-3.2 Ga continental nuclei of the Pilbara (Australia) and Kaapvaal (southern Africa) cratons formed as thick volcanic plateaux over hot, upwelling mantle and survived due to contemporaneous development of highly depleted, buoyant, unsubductable mantle roots. This type of crust is distinct from, but complimentary to, high-grade gneiss terranes, as exemplified by the North Atlantic Craton of West Greenland, which formed through subduction-accretion tectonics on what is envisaged as a vigorously convecting early Earth with small plates. Thus, it is proposed that two types of crust formed on early Earth, in much the same way as in modern Earth, but with distinct differences resulting from a hotter Archean mantle. Volcanic plateaux provided a variety of stable habitats for early life, including chemical nutrient rich, shallow-water hydrothermal systems and shallow marine carbonate platforms.

Rifting an Archaean Craton: Insights from Seismic Anisotropy Patterns in E. Africa

NASA Astrophysics Data System (ADS)

Ebinger, C. J.; Tiberi, C.; Currie, C. A.; van Wijk, J.; Albaric, J.

2016-12-01

Few places worldwide offer opportunities to study active deformation of deeply-keeled cratonic lithosphere. The magma-rich Eastern rift transects the eastern edge of the Archaean Tanzania craton in northeastern Tanzania, which has been affected by a large-scale mantle upwelling. Abundant xenolith locales offer constraints on mantle age, composition, and physical properties. Our aim is to evaluate models for magmatic fluid-alteration (metasomatism) and deformation of mantle lithosphere along the edge of cratons by considering spatial variations in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics. Waveforms of teleseismic earthquakes (SKS, SKKS) recorded on the 39-station CRAFTI-CoLiBREA broadband array in southern Kenya and northern Tanzania are used to determine the azimuth and amount of shear-wave splitting accrued as seismic waves pass through the uppermost mantle and lithosphere at the craton edge. Lower crustal earthquakes enable evaluation of seismic anisotropy throughout the crust along the rift flanks and beneath the heavily intruded Magadi and Natron basins, and the weakly intruded Manyara basin. Our results and those of earlier studies show a consistent N50E splitting direction within the craton, with delay times of ca. 1.5 s, and similar direction east of the rift in thinner Pan-African lithosphere. Stations within the rift zone are rotated to a N15-35E splitting, with the largest delay times of 2.5 s at the margin of the heavily intruded Magadi basin. The short length scale of variations and rift-parallel splitting directions are similar to patterns in the Main Ethiopian rift attributed to melt-filled cracks or oriented pockets rising from the base of the lithosphere. The widespread evidence for mantle metasomatism and magma intrusion to mid-crustal levels suggests that

Origin and evolution of the Amazonian craton

NASA Technical Reports Server (NTRS)

Gibbs, A. K.; Wirth, K. R.

1986-01-01

The Amazonian craton appears to be formed and modifed by processes much like those of the better-known Precambrian cratons, but the major events did not always follow conventional sequences nor did they occur synchronously with those of other cratons. Much of the craton's Archean style continental crust formation, recorded in granite-greenstone and high-grade terranes, occurred in the Early Proterozoic: a period of relative quiescence in many other Precambrian regions. The common Archean to Proterozoic transition in geological style did not occur here, but an analogous change from abundant marine volcanism to dominantly continental sedimentary and eruptive styles occurred later. Amazonian geology is summarized, explaining the evolution of the craton.

Dynamics of cratons in an evolving mantle

NASA Astrophysics Data System (ADS)

O'Neill, C. J.; Lenardic, A.; Griffin, W. L.; O'Reilly, Suzanne Y.

2008-04-01

The tectonic quiescence of cratons on a tectonically active planet has been attributed to their physical properties such as buoyancy, viscosity, and yield strength. Previous modelling has shown the conditions under which cratons may be stable for the present, but cast doubt on how they survived in a more energetic mantle of the past. Here we incorporate an endothermic phase change at 670 km, and a depth-dependent viscosity structure consistent with post-glacial rebound and geoid modelling, to simulate the dynamics of cratons in an "Earth-like" convecting system. We find that cratons are unconditionally stable in such systems for plausible ranges of viscosity ratios between the root and asthenosphere (50-150) and the root/oceanic lithosphere yield strength ratio (5-30). Realistic mantle viscosity structures have limited effect on the average background cratonic stress state, but do buffer cratons from extreme stress excursions. An endothermic phase change at 670 km introduces an additional time-dependence into the system, with slab breakthrough into the lower mantle associated with 2-3 fold stress increases at the surface. Under Precambrian mantle conditions, however, the dominant effect is not more violent mantle avalanches, or faster mantle/plate velocities, but rather the drastic viscosity drop which results from hotter mantle conditions in the past. This results in a large decrease in the cratonic stress field, and promotes craton survival under the evolving mantle conditions of the early Earth.

Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy

NASA Astrophysics Data System (ADS)

Dongre, Ashish N.; Viljoen, K. S.; Rao, N. V. Chalapathi; Gucsik, A.

2016-04-01

similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).

Predicting and testing continental vertical motion histories since the Paleozoic

NASA Astrophysics Data System (ADS)

Zhang, Nan; Zhong, Shijie; Flowers, Rebecca M.

2012-02-01

Dynamic topography at the Earth's surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry, sea-level change, continental flooding, sedimentation and erosion. These observations provide important constraints on and test of mantle dynamic models. Based on global mantle convection models coupled with the surface plate motion history, we compute dynamic topography and its history for the last 400 Ma associated with Pangea assembly and breakup, with particular focus on cratonic regions. We propose that burial-unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models. Our models show that there are currently two broad dynamic topography highs in the Pacific and Africa for the present-day Earth that are associated with the broad, warm structures (i.e., superplumes) in the deep mantle, consistent with previous proposals of dynamical support for the Pacific and African superswells. Our models reveal that Pangea assembly and breakup, by affecting subduction and mantle upwelling processes, have significant effects on continental vertical motions. Our models predict that the Slave craton in North America subsides before Pangea assembly at 330 Ma but uplifts significantly from 330 Ma to 240 Ma in response to pre-Pangea subduction and post-assembly mantle warming. The Kaapvaal craton of Africa is predicted to undergo uplift from ~180 Ma to 90 Ma after Pangea breakup, but its dynamic topography remains stable for the last 90 Ma. The predicted histories of elevation change for the Slave and Kaapvaal cratons compare well with the burial-unroofing histories inferred from thermochronology studies, thus supporting our dynamic models including the development of the African superplume mantle structure. The vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models.

Predicting and testing continental vertical motion histories since the Paleozoic

NASA Astrophysics Data System (ADS)

Zhang, N.; Zhong, S.; Flowers, R. M.

2011-12-01

Dynamic topography at the Earth's surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry, sea-level change, continental flooding, sedimentation and erosion. These observations provide important constraints on and test of mantle dynamic models. Based on global mantle convection models coupled with the surface plate motion history, we compute dynamic topography and its history for the last 400 Ma associated with Pangea assembly and breakup, with particular focus on continental cratonic regions. We propose that burial-unroofing histories of continental cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models. Our models show that there are currently two broad dynamic topography highs in the Pacific and Africa for the present-day Earth that are associated with the broad, warm structures (i.e., superplumes) in the deep mantle, consistent with previous proposals of dynamical support for the Pacific and African superswells. Our models reveal that Pangea assembly and breakup, by affecting subduction and mantle upwelling processes, have significant effects on continental vertical motions. Our models predict that the Slave craton in North America subsides before Pangea assembly at 330 Ma but uplifts significantly from 330 Ma to 240 Ma in response to pre-Pangea subduction and post-assembly mantle warming. The Kaapvaal craton of Africa is predicted to undergo uplift from ~180 Ma to 90 Ma after Pangea breakup, but its dynamic topography remains stable for the last 90 Ma. The predicted histories of elevation change for the Slave and Kaapvaal cratons compare well with the burial-unroofing histories inferred from thermochronology studies, thus supporting our dynamic models including the development of the African superplume mantle structure. The vertical motion histories for other cratons can provide further tests and constraints on our mantle

The Pongola structure of southeastern Africa - The world's oldest preserved rift?

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Kusky, T. M.

1985-01-01

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.

Magmatic zircon Lu-Hf isotopic record of juvenile addition and crustal reworking in the Gawler Craton, Australia

NASA Astrophysics Data System (ADS)

Reid, Anthony J.; Payne, Justin L.

2017-11-01

the northern Gawler Craton and, to a lesser extent, the Kalinjala Shear Zone in the southern Gawler Craton, broadly correspond to crustal and even lithospheric-scale discontinuities evident in geophysical studies. Therefore, these shear zones may approximate some of the first order crustal domains within the Gawler Craton.

Geological evolution of the Antongil Craton, NE Madagascar

USGS Publications Warehouse

Schofield, D.I.; Thomas, Ronald J.; Goodenough, K.M.; De Waele, B.; Pitfield, P.E.J.; Key, R.M.; Bauer, W.; Walsh, G.J.; Lidke, D.J.; Ralison, A.V.; Rabarimanana, M.; Rafahatelo, J.-M.; Randriamananjara, T.

2010-01-01

The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U-Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ?? 14. Ma to 3231 ?? 6. Ma and 3187 ?? 2. Ma to 3154 ?? 5. Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ?? 2. Ma and subject to migmatisation at around 2597 ?? 49. Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ?? 18. Ma and 2542 ?? 5. Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ?? 13. Ma and 2513 ?? 0.4. Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ?? 6. Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ?? 130. Ma to 763 ?? 13. Ma and 553 ?? 68. Ma. These events are

Paleomagnetism and geochronological studies on a 450 km long 2216 Ma dyke from the Dharwar craton, southern India

NASA Astrophysics Data System (ADS)

Nagaraju, E.; Parashuramulu, V.; Kumar, Anil; Srinivas Sarma, D.

2018-01-01

Paleomagnetic and geochronological studies were carried out on a ∼ 450 km long (from 17 sites) N-S striking Paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km (increasing from North to South) in the Dharwar Craton, to study the characteristics of paleomagnetism and geochronology in vertical dimension. U-Pb/Pb-Pb dating on baddeleyite gives a crystallisation age of 2216.0 ± 0.9 Ma for long dyke AKLD. Paleomagnetic data from this well dated ∼ 2216 Ma dyke swarm in Dharwar Craton are of excellent quality. High coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions. Combined normal and reverse polarity remanences on AKLD and other N-S dykes define the most reliable paleomagnetic pole for ∼ 2216 Ma at latitude 36°S and longitude 312°E (A95 = 7°). Though paleomagnetic data is unavailable on other N-S dykes below the Cuddapah basin, high precision geochronology suggest that they are of similar age within errors. Though there is a variation in the crustal depth of Dharwar craton from north to south, consistent Pb-Pb/U-Pb baddeleyite geochronology and paleomagnetic studies along the AKLD established its continuity and preservation along its entire strike length. The virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test. Positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. Updated apparent polar wander path of Dharwar craton indicates relatively low drift rate during 2.21-2.08 Ga interval. Magnetogranulometry and SEM studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals.

Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland Nunavut, Canada

NASA Astrophysics Data System (ADS)

Spratt, Jessica E.; Skulski, Thomas; Craven, James A.; Jones, Alan G.; Snyder, David B.; Kiyan, Duygu

2014-03-01

New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to 10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of 500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.

The Yilgarn Craton western Australia: A tectonic synthesis

NASA Technical Reports Server (NTRS)

Fripp, R. E. P.

1986-01-01

The Yilgarn Craton in Western Australia is one of the larger contiguous preserved Archaean crustal fragments, with an area of about 650,000 square kilometres. Of this, by area, about 70% is granitoid and 30% greenstone. The Craton is defined by the Darling Fault on its western margin, by Proterozoic deformation belts on its southern and northwestern margins, and by unconformable younger sediments on its eastern and northeastern margins. A regional geotectonic synthesis at a scale of 1:500,000 is being prepared. This is based largely upon the 1:250,000 scale mapping of the Geological Survey of Western Australia together with interpretation using geophysical data, mainly airborne magnetic surveys. On a regional basis the granitoids are classied as pre-, syn- and post-tectonic with respect to greenstone belt deformation. The post-tectonic granitoids yield Rb-Sr isochrons of about 2.6 b.y., close to Rb-Sr ages for the greenstones themselves which are up to about 2.8 b.y. old, although data for the latter is sparse. Contacts between earlier granitoids and greenstones which are not obscured by the post-tectonic granitoids are most commonly tectonic contacts, intensely deformed and with mylonitic fabrics. The general concensus however is that there is a pre-tectonic, pre-greenhouse sialic gneiss preserved in places. A discussion follows.

Construction and destruction of some North American cratons

NASA Astrophysics Data System (ADS)

Snyder, D. B.; Humphreys, G.

2015-12-01

Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of, by definition, its rarity. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slabs similar to modern oceanic lithosphere in these construction histories whereas underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities. Archean continental building blocks may resemble the modern lithosphere of Ontong-Java-Hikurangi oceanic plateau. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons comprise smaller continental terranes that 'cratonized' during a granitic bloom at 2.61-2.55 ga. Cratonization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and conductive by introducing or concentrating sulfides or graphite throughout the lithosphere at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. The arrival of the subducted Shatsky Rise conjugate at the Wyoming craton at 65-75 Ma appears to have eroded and displaced the thus weakened base of the craton below 140-160 km. This replaced old refertilized continental mantle with new depleted oceanic mantle. Is this the same craton?

Barberton greenstone belt volcanism: Succession, style and petrogenesis

NASA Technical Reports Server (NTRS)

Byerly, G. R.; Lowe, D. R.

1986-01-01

The Barberton Mountain Land is an early Archean greenstone belt along the eastern margin of the Kaapvaal Craton of southern Africa. Detailed mapping in the southern portion of the belt leads to the conclusion that a substantial thickness is due to original deposition of volcanics and sediments. In the area mapped, a minimum thickness of 12km of predominantly mafic and ultramafic volcanics comprise the Komati, Hooggenoeg, and Kromberg Formations of the Onverwacht Group, and at least one km of predominantly pyroclastic and epiclastic sediments derived from dacitic volcanics comprise the Fig Tree Group. The Barberton greenstone belt formed primarily by ultramafic to mafic volcanism on a shallow marine platform which underwent little or no concurrent extension. Vents for this igneous activity were probably of the non-constructional fissure type. Dacitic volcanism occurred throughout the sequence in minor amounts. Large, constructional vent complexes were formed, and explosive eruptions widely dispersed pyroclastic debris. Only in the final stages of evolution of the belt did significant thrust-faulting occur, generally after, though perhaps overlapping with, the final stage of dacitic igneous activity. A discussion follows.

A magnetotelluric study from over Dharwar cratonic nucleus into Billigiri Rangan charnockitic massif, India

NASA Astrophysics Data System (ADS)

Pratap, A.; Kusham; Pradeep Naick, B.; Naganjaneyulu, K.

2018-07-01

The electrical resistivity structure of the crust beneath the Dharwar craton in southern India was investigated by magnetotelluric method. In the present study, a northwest-southeast oriented 220 km long profile of 18 stations with a station spacing of ∼10-15 km is used. The profile extends from Dharwar cratonic nucleus in the north to Billigiri Rangan charnockitic massifs in the south. Time series data are processed to get the apparent resistivity and phase. The dominant geoelectric strike direction (75°) was calculated in a period range of 0.01-1000 s. The data are rotated to 75° strike direction. Two-dimensional inversion is carried out by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. Our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile. The mid-lower crust in southern part of the profile shows less resistive (<300 ohm-m) nature in the depth range of 20-50 km and is related with the Chitradurga shear zone and Billigiri Rangan charnockite massif. These zones were interpreted as CO2 flushed terranes. Regional-scale carbonation occurred in Late Archaean is associated with Chitradurga shear zone and in Late Proterozoic is associated with Salem-Attur shear zone. The CO2 rich fluids derived during that time might have exhausted in dehydration reactions. Later events such as the Indian plate passing over several hotspots and the metasomatized fluids associated with the Cretaceous-Tertiary magmatism in the region is the reason for observed low resistivity near Billigiri Rangan massif and surrounding regions in the south.

Thermal state, oxygen fugacity and COH fluid speciation in cratonic lithospheric mantle: New data on peridotite xenoliths from the Udachnaya kimberlite, Siberia

NASA Astrophysics Data System (ADS)

Goncharov, A. G.; Ionov, D. A.; Doucet, L. S.; Pokhilenko, L. N.

2012-12-01

Oxygen fugacity (fO2) and temperature variations in a complete lithospheric mantle section (70-220 km) of the central Siberian craton are estimated based on 42 peridotite xenoliths in the Udachnaya kimberlite. Pressure and temperature (P-T) estimates for the 70-140 km depth range closely follow the 40 mW/m2 model conductive geotherm but show a bimodal distribution at greater depths. A subset of coarse garnet peridotites at 145-180 km plots near the "cold" 35 mW/m2 geotherm whereas the majority of coarse and sheared rocks at ≥145 km scatter between the 40 and 45 mW/m2 geotherms. This P-T profile may reflect a perturbation of an initially "cold" lithospheric mantle through a combination of (1) magmatic under-plating close to the crust-mantle boundary and (2) intrusion of melts/fluids in the lower lithosphere accompanied by shearing. fO2 values estimated from Fe3+/∑Fe in spinel and/or garnet obtained by Mössbauer spectroscopy decrease from +1 to -4 Δlog fO2 (FMQ) from the top to the bottom of the lithospheric mantle (˜0.25 log units per 10 km) due to pressure effects on Fe2+-Fe3+ equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO2 distribution with depth similar to those in the Slave craton. Published fO2 estimates for Udachnaya xenoliths based on C-O-H fluid speciation in inclusions in minerals from gas chromatography are similar to our results at ≤120 km, but are 1-2 orders of magnitude higher for the deeper mantle, possibly due to uncertainties of fO2 estimates based on experimental calibrations at ≤3.5 GPa. Sheared peridotites containing garnets with u-shaped, sinusoidal and humped REE patterns are usually more oxidized than Yb, Lu-rich, melt-equilibrated garnets, which show a continuous decrease from heavy to light REE. This further indicates that mantle redox state may be related to sources and modes of metasomatism.

Garnet pyroxenite from Nilgiri Block, southern India: Vestiges of a Neoarchean volcanic arc

NASA Astrophysics Data System (ADS)

Samuel, Vinod O.; Kwon, Sanghoon; Santosh, M.; Sajeev, K.

2018-06-01

Southern peninsular India preserves records of Late Neoarchean-Early Paleoproterozoic continental building and cratonization. A transect from the Paleoarchean Dharwar Craton to the Neoarchean arc magmatic complex in the Nilgiri Block across the intervening Moyar Suture Zone reveals an arc-accretionary complex composed of banded iron formation (BIF), amphibolite, metatuff, garnet-kyanite schist, metagabbro, pyroxenite and charnockite. Here we investigate the petrology, geochronology and petrogenesis of the pyroxenite and garnet-clinopyroxenite. The pyroxenite is mainly composed of orthopyroxene and clinopyroxene with local domains/pockets enriched in a clinopyroxene-garnet assemblage. Thermobarometric calculations and phase equilibria modeling suggest that the orthopyroxene- and clinopyroxene-rich domains formed at 900-1000 °C, 1-1.2 GPa whereas the garnet- and clinopyroxene-rich domains record higher pressure of about 1.8-2 GPa at similar temperature conditions (900-1000 °C). Zircon U-Pb SHRIMP dating show weighted mean 207Pb-206Pb age of 2532 ± 22 Ma, with metamorphic overgrowth at 2520 ± 27 Ma and 2478 ± 27 Ma. We propose a tectonic model involving decoupling and break-off of the oceanic plate along the southern flanks of the Dharwar Craton, which initiated oceanic plate subduction. Slab melting eventually built the Nilgiri volcanic arc on top of the over-riding plate along the flanks of the Dharwar Craton. Our study supports an active plate tectonic regime at the end of the Archean Era, aiding in the growth of paleo-continents and their assembly into stable cratons.

3D Fault Network of the Murchison Domain, Yilgarn Craton

NASA Astrophysics Data System (ADS)

Murdie, Ruth; Gessner, Klaus

2014-05-01

The architecture of Archean granite-greenstone terranes is often controlled by networks of 10 km to 100 km-scale shear zones that record displacement under amphibolite facies to greenschist facies metamorphic conditions. The geometry of such crustal scale 'fault networks' has been shown to be highly relevant to understand the tectonic and metamorphic history of granite-greenstone terranes, as well as the availability of structural controlled fluid pathways related to magmatic and hydrothermal mineralization. The Neoarchean Yilgarn Craton and the Proterozoic orogens around its margins constitute one of Earth's greatest mineral treasure troves, including iron, gold, copper and nickel mineral deposits. Whereas the Yilgarn Craton is one of the best studied Archean cratons, its enormous size and limited outcrop are detrimental to the better understanding of what controls the distribution of these vast resources and what geodynamic processes were involved the tectonic assembly of this part of the Australian continent. Here we present a network of the major faults of the NW Yilgarn Craton between the Yalgar Fault, Murchison's NW contact with the Narryer Terrane to the Ida Fault, its boundary with the Eastern Goldfields Superterrane. The model has been constructed from various geophysical and geological data, including potential field grids, Geological Survey of Western Australia map sheets, seismic reflection surveys and magnetotelluric traverses. The northern extremity of the modelled area is bounded by the Proterozoic cover and the southern limit has been arbitrarily chosen to include various greenstone belts. In the west, the major faults in the upper crust, such as the Carbar and Chundaloo Shear Zones, dip steeply towards the west and then flatten off at depth. They form complex branching fault systems that bound the greenstone belts in a series of stacked faults. East of the Ida, the far east of the model, the faults have been integrated with Geoscience Australia

The Chara-Sina dyke swarm in the structure of the Middle Paleozoic Vilyui rift system (Siberian Craton)

NASA Astrophysics Data System (ADS)

Kiselev, A. I.; Konstantinov, K. M.; Yarmolyuk, V. V.; Ivanov, A. V.

2016-11-01

The formation of the Vilyui rift system in the eastern Siberian Craton was finished with breakdown of the continent and formation of its eastern margin. A characteristic feature of this rift system is the radial distribution of dyke swarms of basic rocks. This peculiarity allows us to relate it to the breaking processes above the mantle plume, the center of which was located in the region overlain in the modern structure by the foreland of the Verkhoyan folded-thrust belt. The Chara-Sina dyke swarm is the southern part of a large area of Middle Paleozoic basaltic magmatism in the eastern Siberian Craton. The OIB-like geochemical characteristics of dolerite allow us to suggest that the melting substrate for Middle Paleozoic basaltic magmatism was represented by a relatively homogeneous, mid-depleted mantle of the plume with geochemical parameters similar to those of OIB.

Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

NASA Astrophysics Data System (ADS)

Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

2011-12-01

The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo<90, suggest that the xenoliths are non-cratonic spinel-peridotite (cratonic peridotite olivine Fo> ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from

Is Nubia plate rigid? A geodetic study of the relative motion of different cratonic areas within Africa.

NASA Astrophysics Data System (ADS)

Njoroge, M. W.; Malservisi, R.; Hugentobler, U.; Mokhtari, M.; Voytenko, D.

2014-12-01

Plate rigidity is one of the main paradigms of plate tectonics and a fundamental assumption in the definition of a global reference frame as ITRF. Although still far for optimal, the increased GPS instrumentation of the African region can allow us to understand how rigid one of the major plate can be. The presence of diffused band of seismicity, the Cameroon volcanic line, Pan African Kalahari orogenic belt and East Africa Rift suggest the possibility of relative motion among the different regions within the Nubia. The study focuses on the rigidity of Nubia plate. We divide the plate into three regions: Western (West Africa craton plus Nigeria), Central (approximately the region of the Congo craton) and Southern (Kalahari craton plus South Africa) and we utilize Euler Vector formulation to study internal rigidity and eventual relative motion. Developing five different reference frames with different combinations of the 3 regions, we try to understand the presence of the relative motion between the 3 cratons thus the stability of the Nubia plate as a whole. All available GPS stations from the regions are used separately or combined in creation of the reference frames. We utilize continuous stations with at least 2.5 years of data between 1994 and 2014. Given the small relative velocity, it is important to eliminate eventual biases in the analysis and to have a good estimation in the uncertainties of the observed velocities. For this reason we perform our analysis using both Bernese and Gipsy-oasis codes to generate time series for each station. Velocities and relative uncertainties are analyzed using the Allan variance of rate technique, taking in account for colored noise. An analysis of the color of the noise as function of latitude and climatic region is also performed to each time series. Preliminary results indicate a slight counter clockwise motion of West Africa craton with respect to South Africa Kalahari, and South Africa Kalahari-Congo Cratons. In addition

The 3.26-3.24 Ga Barberton asteroid impact cluster: Tests of tectonic and magmatic consequences, Pilbara Craton, Western Australia

NASA Astrophysics Data System (ADS)

Glikson, Andrew; Vickers, John

2006-01-01

The location in the Barberton Greenstone Belt (Kaapvaal Craton) of ∼3.26-3.24 Ga asteroid impact ejecta units at, and immediately above, a sharp break between a > 12 km-thick mafic-ultramafic volcanic crust (Onverwacht Group ∼3.55-3.26 Ga, including the ∼3.298 > 3.258 Ga Mendon Formation) and a turbidite-felsic volcanic rift-facies association (Fig Tree Group ∼3.258-3.225 Ga), potentially represents the first documented example of cause-effect relations between extraterrestrial bombardment and major tectonic and igneous events [D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, Geological and geochemical record of 3400 Ma old terrestrial meteorite impacts, Science 245 (1989) 959-962; D.R. Lowe, G.R. Byerly, F.T. Kyte, A. Shukolyukov, F. Asaro, A. Krull, Spherule beds 3.47-3.34 Ga-old in the Barberton greenstone belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution, Astrobiology 3 (2003) 7-48; A.Y. Glikson, The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2 ± 0.1 Ga bombardment of the Earth-Moon system, J. Geodyn. 32 (2001) 205-229]. Here we correlate this boundary with a contemporaneous break and peak magmatic and faulting events in the Pilbara Craton, represented by the truncation of a 3.255-3.235 Ga-old volcanic sequence (Sulphur Springs Group-SSG) by a turbidite-banded iron formation-felsic volcanic association (Pincunah Hill Formation, basal Gorge Creek Group). These events are accompanied by ∼3.252-3.235 Ga granitoids (Cleland plutonic suite). The top of the komatiite-tholeiite-rhyolite sequence of the SSG is associated with a marker chert defined at 3.238 ± 3-3.235 ± 3 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250 × 150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics (Pincunah Hill Formation-basal Gorge

Lithospheric magnetic field modelling of the African continent

NASA Astrophysics Data System (ADS)

Hemant, K.; Maus, S.

2003-04-01

New magnetic satellite missions in low-earth orbit are providing increasingly accurate maps of the lithospheric magnetic field. These maps can be used to infer the geological structure of regions hidden by Phanerozoic cover, taking into account our knowledge of crustal structure from surface geology and seismic methods. A GIS based modelling technique has been developed to model the various geological units of the continents using the UNESCO geological map of the world, supported by background geological information from various sources. Geological units of each region are assigned a susceptibility value based on laboratory values of the constituent rock types. Then, using the 3SMAC seismic crustal structure, a vertically integrated susceptibility (VIS) model is computed at each point of the region. Starting with this VIS model, the total field anomaly is computed at an altitude of 400 km and compared with the MF2 lithospheric magnetic field model derived from CHAMP data. The modelling results of the Precambrian units of the West African cratons agree well with MF2. The anomaly in the Central African cratonic region also correlates well, although part of it is unaccounted for as yet. Furthermore, the anomalies over the Tanzanian craton and surrounding region agree very well. Most of the regions around the South African cratons are hidden by Phanerozoic cover, yet the results above the Kaapvaal craton and the southern Zimbabwe craton around the Limpopo belt show good correspondence with the observed anomaly map. The results also suggest a probable extension of the Precambrian units below the sediments of younger age. In general, the lower crust is likely to be more mafic than presumed in our current understanding of Central Africa. Deviations in the magnitude of the anomalies in some regions are likely to be due to incomplete seismic information in those regions. Thus, the thickness of crustal layers derived from magnetic anomalies for these locations may help to

Water Distribution in the Continental and Oceanic Upper Mantle

NASA Technical Reports Server (NTRS)

Peslier, Anne H.

2015-01-01

Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

Composition of the lithospheric mantle in the Siberian craton : New constraints from fresh peridotites from the Udachnaya-East Kimberlite

NASA Astrophysics Data System (ADS)

Doucet, Luc-Serge; Ionov, Dmitri A.; Ashchepkov, Igor

2010-05-01

contents. The broad range of heavy REE appears to be controlled by the presence and the abundance of garnet and is also related to microstructures such that granular spinel harzburgites have lower HREE contents than "fertile" porphyroclastic garnet lherzolites. Trace elements in cpx and garnet have equilibrated patterns in porphyroclastic peridotites and complex sinusoidal shapes in granular peridotites. Bulk-rock major element compositions show important variations in Mg# (0.89 - 0.93), SiO2 (41.5 - 46.6%), Al2O3 (0.3 - 4%) and CaO (0.3 - 4%). As for compatible trace elements, the major element compositions appear to be related to microstructures. Calculated modal compositions show highly variable opx contents (4.5 - 24%), which are generally lower than in Kaapvaal peridotites but are similar to those from the North Atlantic craton [3]. Overall, modal compositions and the contents of low-mobility elements, are consistent with an origin by variable degrees of partial melting of fertile mantle [1-3]. The range in FeO contents (6-8.5%) may indicate either variable melting depths [2] or post-melting enrichments. Enrichments in SiO2 show some similarities to those in supra-subduction xenoliths [4]; enrichments in highly incompatible elements can be explained by metasomatism with possible involvement of subduction-related fluids. Strong correlations between chemical compositions and microstructures indicate the involvement of tectonic processes in melt percolation and metasomatism. We suggest that the cratonic lithosphere in Siberia was formed in three stages: (1) formation of proto-cratonic mantle by high-degree melting at variable depth, (2) accretion of the proto-craton domains in subduction-related settings, (3) metasomatism commonly accompanied by deformation. [1] Boyd et al (1997) Contrib. Mineral. Petrol. 128, 228-246. [2] Herzberg (2004) J. Petrol. 45, 2507-2530. [3] Wittig et al (2008) Lithos 71, 289-322. [4] Ionov (2009) J. Petrol. In press

Lithospheric Layering beneath Southern Africa Constrained by S-to-P Receiver Functions

NASA Astrophysics Data System (ADS)

Liu, L.; Liu, K. H.; Gao, S. S.

2016-12-01

To investigate the existence of intra-lithospheric interfaces in an area of active rifting of ancient lithosphere, we stack S-to-P receiver functions (SRFs) recorded by broadband seismic stations in the vicinity of the non-volcanic sections of the East African Rift System and the stable Kaapvaal and Zimbabwe cratons. The data set was recorded by about 200 permanent and portable seismic stations installed over the past 30 years. The SRFs are computed using frequency-domain deconvolution, and are stacked in consecutive circles with a radius of 2 degrees. They are converted to depth series after moveout corrections using the IASP91 Earth model. In the upper mantle , a robust negative arrival is found in virtually all the stacked traces in the depth range of 50-100 km. Comparison with results from seismic tomography and mantle xenolith studies suggests that this discontinuity represents a mid-lithospheric discontinuity (MLD), similar to what was observed beneath the North American continent. The absence of observable negative arrivals in the anticipated depth of 250 km or greater beneath the study area suggests a gradual instead of sharp transition from the lithosphere to the asthenosphere. No significant shallowing of the MLD is observed beneath the young rift segments, suggesting that rifting is limited in the crust, an observation that is consistent with recent results from the SAFARI (Seismic Arrays for African Rift Initiation) project. The shallowest MLD of about 65 km in the study area is found in a NW-SE trending zone across central Zimbabwe and western Zambia. The MLD may reflect a low velocity zone caused by metasomatism, a process commonly found beneath ancient cratons.

Modification of the Western Gondwana craton by plume-lithosphere interaction

NASA Astrophysics Data System (ADS)

Hu, Jiashun; Liu, Lijun; Faccenda, Manuele; Zhou, Quan; Fischer, Karen M.; Marshak, Stephen; Lundstrom, Craig

2018-03-01

The longevity of cratons is generally attributed to persistence of neutrally-to-positively buoyant and mechanically strong lithosphere that shields the cratonic crust from underlying mantle dynamics. Here we show that large portions of the cratonic lithosphere in South America and Africa, however, experienced significant modification during and since the Mesozoic era, as demonstrated by widespread Cretaceous uplift and volcanism, present-day high topography, thin crust, and the presence of seismically fast but neutrally buoyant upper-mantle anomalies. We suggest that these observations reflect a permanent increase in lithospheric buoyancy due to plume-triggered delamination of deep lithospheric roots during the Late Cretaceous and early Cenozoic periods. Lithosphere in these regions has been thermally reestablished since then, as confirmed by its present-day low heat flow, high seismic velocities and realigned seismic anisotropy. We conclude that the original lowermost cratonic lithosphere is compositionally denser than the asthenospheric mantle and can be removed when perturbed by underlying mantle upwelling. Therefore, it is the buoyancy of the upper lithosphere that perpetuates stabilization of cratons.

Formation of cratonic lithosphere: An integrated thermal and petrological model

NASA Astrophysics Data System (ADS)

Herzberg, Claude; Rudnick, Roberta

2012-09-01

The formation of cratonic mantle peridotite of Archean age is examined within the time frame of Earth's thermal history, and how it was expressed by temporal variations in magma and residue petrology. Peridotite residues that occupy the lithospheric mantle are rare owing to the effects of melt-rock reaction, metasomatism, and refertilization. Where they are identified, they are very similar to the predicted harzburgite residues of primary magmas of the dominant basalts in greenstone belts, which formed in a non-arc setting (referred to here as "non-arc basalts"). The compositions of these basalts indicate high temperatures of formation that are well-described by the thermal history model of Korenaga. In this model, peridotite residues of extensive ambient mantle melting had the highest Mg-numbers, lowest FeO contents, and lowest densities at ~ 2.5-3.5 Ga. These results are in good agreement with Re-Os ages of kimberlite-hosted cratonic mantle xenoliths and enclosed sulfides, and provide support for the hypothesis of Jordan that low densities of cratonic mantle are a measure of their high preservation potential. Cratonization of the Earth reached its zenith at ~ 2.5-3.5 Ga when ambient mantle was hot and extensive melting produced oceanic crust 30-45 km thick. However, there is a mass imbalance exhibited by the craton-wide distribution of harzburgite residues and the paucity of their complementary magmas that had compositions like the non-arc basalts. We suggest that the problem of the missing basaltic oceanic crust can be resolved by its hydration, cooling and partial transformation to eclogite, which caused foundering of the entire lithosphere. Some of the oceanic crust partially melted during foundering to produce continental crust composed of tonalite-trondhjemite-granodiorite (TTG). The remaining lithosphere gravitationally separated into 1) residual eclogite that continued its descent, and 2) buoyant harzburgite diapirs that rose to underplate cratonic nuclei

An Integrated Geochronological, Petrological, Geochemical and Paleomagnetic Study of Paleoproterozoic and Mesoproterozoic Mafic Dyke Swarms in the Nain Craton, Labrador

NASA Astrophysics Data System (ADS)

Sahin, Tugce

The Nain craton comprises the western, Labrador segment of the larger North Atlantic craton (NAC) which exposes Early through Late Archean gneisses. The NAC is bounded on all sides by Paleoproterozoic collisional orogens that involved either considerable structural reworking (Torngat-Nagssugtoqidian-Lewisian) or the accretion of juvenile arc magmas (Ketilidian-Makkovik). The NAC remains poorly understood compared to other Archean crustal blocks now dispersed globally. Compounding this problem is a lack of reliable paleomagnetic poles for NAC units that predate its assembly into the supercontinent Laurentia by ca. 1800 Ma, which could be used to test neighboring relationships with other cratonic fragments. In order to understand the history of the NAC as part of a possible, larger supercontinent, the record of mafic dyke swarms affecting the craton, particularly those that postdate the Late Archean terrane assembly, were examined in this study. Diabase or gabbroic dyke swarms are invaluable in such studies because their geometries offer possible locus points, they often have a punctuated emplacement and precisely datable crystallization histories, and they have cooling histories and oxide mineralogy amenable to recovering robust paleopoles. Coastal Labrador exposes a number of mafic dykes, some of which are demonstrably Paleoproterozoic (e.g. 2235 Ma Kikkertavak dykes; 2121 Ma Tikkigatsiagak dykes) or Mesoproterozoic (e.g. 1280-1270 Ma Nain and Harp dykes) in age (U-Pb; baddeleyite or zircon). The southern half of the Nain craton (Hopedale block) in particular preserves a rich array of mafic dykes. Dyke cross-cutting relationships are numerous and relatively well exposed, permitting multiple opportunities for paleomagnetic field tests (e.g. baked contact). The results presented here allow understanding of the tectonic evolution of the NAC with implications for strengthened Labrador-Greenland correlations, and testing possible Paleoproterozoic supercontinent

Formation of cratonic lithosphere during the initiation of plate tectonics

NASA Astrophysics Data System (ADS)

Moresi, L. N.; Beall, A.; Cooper, C. M.

2017-12-01

The Earth's oldest near-surface material, the cratonic crust, is typically underlain by unusually thick Archean lithosphere (<300 km). This cratonic lithosphere likely thickened in a high compressional stress environment. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition, from a stagnant lid Earth to the onset of plate tectonics, generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing harzburgite and felsic crust, can thicken and stabilize during the lid-breaking event. The peak compressional stress experienced by lithosphere is 3-4 higher than for the stagnant lid or mobile lid regimes immediately before and after. It is plausible that the cratonic lithosphere has still not returned to this high stress-state, explaining its stability. The lid-breaking thickening event reproduces craton features previously attributed to subduction: thrust structures, assembled crustal fragments and transport of basaltic upper crust to depths required to generate felsic melt. Palaeoarchean `pre-tectonic' structures can also survive the lid-breaking event, acting as strong crustal rafts. Together, the results indicate that the signature of a catastrophic switch, from a stagnant lid Earth to the initiation of plate tectonics, has been captured and preserved in the unusual characteristics of cratonic crust and lithosphere.

The Southern Cone: A critical element in North American geology

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dalziel, I.W.D.

1993-02-01

The Pacific and Atlantic-Gulf of Mexico continental margins converge towards southern Mexico, delimiting the Southern Cone of North American. The margins are controlled by late Precambrian to early Paleozoic rift systems. The Neoproterozoic rifts along the Pacific margin truncate the 1.3--1.0 Ga Grenville-Llano front and still older structural boundaries within the craton, such as the Snowbird line. The Atlantic margin originated by separation from another continent within the Grenville orogen near the time of the Precambrian-Cambrian boundary. The Gulf of Mexico margin was initiated with rifting at that time, but appears to truncate the Ordovician Taconian orogen in Georgia. Themore » continental margins of the Southern Cone may prove critical in understanding the origin of North America as a discrete continent. A possible continuation of the Grenville-Llano front has now been identified along the Pacific margin of the East Antarctic craton; the opposite side of the Grenville orogen may be present in South America and East Antarctic; a southern continuation of the Taconic Appalachians may have been identified in southern South American and Antarctica (L. Dalla Salda et al., Geology, 1992 a;b: I. Dalziel, Geology, 1991, and GSA Today, 1992; P. Hoffman, Science, 1991; E. Moores, Geology, 1991). Thus the geology of the Southern Cone of North America provides opportunities for critical testing of these globally important hypotheses, notably through geochronometry, isotope geochemistry, stratigraphy, and paleobiogeography. Conversely, East Antarctica, southern Africa, and the proto-Andean margin of South America may offer exciting opportunities to further understanding of pre-Pangea geology across southern North America.« less

Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent

PubMed Central

Wan, Bo; Windley, Brian F.; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

2015-01-01

The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic. PMID:26388458

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

NASA Astrophysics Data System (ADS)

Betts, Peter G.; Valenta, Rick K.; Finlay, Jim

2003-05-01

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S 1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north-south shortening (D 2), open to isoclinal south-southeast-oriented F 2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D 2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D 2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.

Interaction of the Siberian craton and Central Asian Orogenic Belt (CAOB) recorded by detrital zircons from Transbaikalia

NASA Astrophysics Data System (ADS)

Powerman, V.; Shatsillo, A.; Chumakov, N.; Kapitonov, I.; Hourigan, J. K.

2015-12-01

The goal of this study is to pinpoint the beginning of interaction of two gigantic crustal structures: the Siberian Craton and the Central Asian Orogenic Belt (CAOB). We hypothesize that the beginning of convergence should be recorded in the Neoproterozoic passive margin strata of Siberian Craton by the first appearance of extraregional Neoproterozoic zircons. In order to test this hypothesis, we have acquired U-Pb zircon age distributions from twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited atop accreted CAOB crust. Stratigraphically lower strata from the Siberian margin yield Archean - Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present. The provenance shift happens in the upper portion of the section and is marked by a strong influx of extra-regional Neoproterozoic sediments. The youngest grains of 610 Ma constrain the sedimentation age and confine the timing of interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary unit, suggesting the continuance of the convergence. The coeval volcanoclastic unit on the CAOB side has a similar U-Pb detrital age distribution, strengthening the provenance link. Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635 Ma (?) "Snowball Earth" tillites of Dzhemkukan Fm. The absence of Neoproterozoic zircons in Dzhemkukan Fm. is probably explained by a thin-skinned tectonics that did not result in massive orogenesis . Our data are in good correlation with other Neoproterozoic sedimentary basins of southern Siberian Craton, including Cisbaikalia and Bodaibo Synclinorium.

Identifying the Transition Zone Between East and West Dharwar Craton by Seismic Imaging

NASA Astrophysics Data System (ADS)

Ashish; Parvez, Imtiyaz A.

2018-01-01

The data from 12 temporary broadband seismic stations operated across east-west corridor in Dharwar region of Indian Peninsula along with ten other seismic stations operated by CSIR National Geophysical Research Institute (NGRI) in the region have been analysed that provide high-resolution image of southern Dharwar crust. Crust along the corridor is imaged by receiver function H-k stacking, common conversion point stacking using data from 22 sites in combination with joint inversion modeling of receiver functions and Rayleigh wave group velocity dispersion curves. The velocity image reveals thinner crust (36-38 km) except one site (coinciding with Cuddapah basin on the surface) in East Dharwar Craton (EDC), while crust beneath the West Dharwar Craton (WDC) is thicker (46-50 km). This study also observed a transition zone between EDC and WDC starting west of Closepet granite to the east of Chitradurga Schist Belt (CSB), which shows diffused Moho with a thickness of 40-44 km. Chitradurga Schist Belt is identified as the contact between Mesoarchean (WDC) and Neoarchean (EDC) crustal blocks. The lowermost part of the crust (V_s > 4.0) is thin (2-6 km) beneath EDC, intermediate (6-8 km) beneath transition zone and thicker (14-30 km) beneath WDC across the profile.

On the relations between cratonic lithosphere thickness, plate motions, and basal drag

USGS Publications Warehouse

Artemieva, I.M.; Mooney, W.D.

2002-01-01

An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140-350 km), with a bimodal distribution for Archean cratons (200-220 km and 300-350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the "critical" surface area for cratons to have thick (>300 km) keels is >6-8 ?? 106 km2 . Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350-450 km decreasing to 280-400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small

The 3-dimensional construction of the Rae craton, central Canada

NASA Astrophysics Data System (ADS)

Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

2015-10-01

Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

Sedimentary petrography of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

NASA Astrophysics Data System (ADS)

Schreiber, U. M.; Eriksson, P. G.; van der Neut, M.; Snyman, C. P.

1992-11-01

Sandstone petrography, geochemistry and petrotectonic assemblages of the predominantly clastic sedimentary rocks of the Early Proterozoic Pretoria Group, Transvaal Sequence, point to relatively stable cratonic conditions at the beginning of sedimentation, interrupted by minor rifting events. Basement uplift and a second period of rifting occurred towards the end of Pretoria Group deposition, which was followed by the intrusion of mafic sill swarms and the emplacement of the Bushveld Complex in the Kaapvaal Craton at about 2050 Ma, the latter indicating increased extensional tectonism, and incipient continental rifting. An overall intracratonic lacustrine tectonic setting for the Pretoria Group is supported by periods of subaerial volcanic activity and palaeosol formation, rapid sedimentary facies changes, significant arkosic sandstones, the presence of non-glacial varves and a highly variable mudrock geochemistry.

Regional sub-cratonic mantle heterogeneities under the Kaapvaal craton recorded by sulfide inclusions in diamond.

NASA Astrophysics Data System (ADS)

Thomassot, E.; Lorand, J. P.

2016-12-01

Sulfide inclusions in diamonds (SID) have been trapped by their host at depth greater than 150 km and then protected from alteration or reequilibration processes. Subsequently, depending on their initial composition, there have experienced multiple exsolution events at temperatures between 650°C and 200°C. Mineralogical and isotopic composition of SID thus directly reflects their sources whereas their texture give information on the exhumation history of their diamond host. Our samples originating from Koffiefontein, De Beers Pool and Jwaneng kimberlites, include both peridotitic (P-type, n=6) and eclogitic (E-type, n=151) sulfides. P-type sulfides are monosulfide solid solutions (exsolved from a nickeliferous monosulfide solid solution, mss, stable at T>300°C). Their Ni and Cr content indicate that the sublithospheric mantle under Koffiefontein (Ni=26wt%; Cr=0.36 wt%) is more refractory than De Beers Pool (Ni=24wt%; Cr=0.28wt%) and Jwaneng (Ni=17wt%; Cr=0.22wt%). Sulfides from De Beers Pool have S/Se (3900±100) greater than the chondritic values ( 2540) pointing to a metasomatic overprint predating the formation of diamonds. S-isotopic composition (d34S=-0.9 ± 0.5‰, Δ33S=0‰) are homogeneous, and consistent with the composition of MORB. E-type sulfides are unmixed assemblage of chalcopyrite (crystallized from an intermediate solid solution formed at T 650°C) pentlandite (T° from 260 to >200°C) and pyrrhotite. Comparing the chemical composition of the unmixed phases with experimental data, we determined a blocking T° for E-type assemblages ranging from 540°C at Koffiefontein, to less than 200°C at Jwaneng and DeBeersPool. E-type sulfides thus confirm the chemical heterogeneity from one mine to the other, with d34S extended to fractionated values (-9 to +11‰). Our study indicates that SID are robust probes to explore regional scale mantle heterogeneities likely associated to discrete depletion events and possible recycling of ancient sediment, predating the formation of diamonds. Major, trace element and isotopes data will be presented at the conference in order to support this conclusion.

Destruction of the North China Craton: Lithosphere folding-induced removal of lithospheric mantle?

NASA Astrophysics Data System (ADS)

Zhang, Kai-Jun

2012-01-01

High heat flow, high surface topography, and widespread volcanism indicate that the lithospheric mantle of typical cratonic character of the North China Craton has been seriously destroyed in its eastern half. However, the mechanism of this process remains open to intense debate. Here lithosphere folding-induced lithospheric mantle removal is proposed as a new mechanism for the destruction of the craton. Four main NNE-SSW-striking lithospheric-scale anticlines and synclines are recognized within North China east of the Helan fold-and-thrust belt. The lithosphere folding occurred possibly during the Late Triassic through Jurassic when the Yangzi Craton collided with the North China Craton. It was accompanied or followed by lithospheric dripping, and could have possibly induced the lithosphere foundering of the North China Craton. The lithosphere folding would have modified the lithosphere morphology, creating significant undulation in the lithospheric base and thus causing variations of the patterns of the small-scale convection. It also could have provoked the formation of new shear zones liable to impregnation of magma, producing linear incisions at the cratonic base and resulting in foundering of lithospheric mantle blocks. Furthermore, it generated thickening of the lithosphere or the lower crust and initiated the destabilization and subsequent removal of the lithospheric mantle.

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

NASA Astrophysics Data System (ADS)

Aulbach, Sonja; Massuyeau, Malcolm; Gaillard, Fabrice

2017-01-01

Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities. LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to 200-250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton). Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past

Architectural elements from Lower Proterozoic braid-delta and high-energy tidal flat deposits in the Magaliesberg Formation, Transvaal Supergroup, South Africa

NASA Astrophysics Data System (ADS)

Eriksson, Patrick G.; Reczko, Boris F. F.; Jaco Boshoff, A.; Schreiber, Ute M.; Van der Neut, Markus; Snyman, Carel P.

1995-06-01

Three architectural elements are identified in the Lower Proterozoic Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) of the Kaapvaal craton, South Africa: (1) medium- to coarse-grained sandstone sheets; (2) fine- to medium-grained sandstone sheets; and (3) mudrock elements. Both sandstone sheet elements are characterised by horizontal lamination and planar cross-bedding, with lesser trough cross-bedding, channel-fills and wave ripples, as well as minor desiccated mudrock partings, double-crested and flat-topped ripples. Due to the local unimodal palaeocurrent patterns in the medium- to coarse-grained sandstone sheets, they are interpreted as ephemeral braid-delta deposits, which were subjected to minor marine reworking. The predominantly bimodal to polymodal palaeocurrent trends in the fine- to medium-grained sandstone sheets are inferred to reflect high-energy macrotidal processes and more complete reworking of braid-delta sands. The suspension deposits of mudrocks point to either braid-delta channel abandonment, or uppermost tidal flat sedimentation. The depositional model comprises ephemeral braid-delta systems which debouched into a high-energy peritidal environment, around the margins of a shallow epeiric sea on the Kaapvaal craton. Braid-delta and tidal channel dynamics are inferred to have been similar. Fine material in the Magaliesberg Formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early Proterozoic.

Did the circum-Rodinia subduction trigger the Neoproterozoic rifting along the Congo-Kalahari Craton margin?

NASA Astrophysics Data System (ADS)

Konopásek, Jiří; Janoušek, Vojtěch; Oyhantçabal, Pedro; Sláma, Jiří; Ulrich, Stanislav

2017-12-01

Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko-Dom Feliciano-Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820-785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800-770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740-710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este-Coastal Terrane is interpreted as an axial part of a Neoproterozoic "Adamastor Rift". Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Pérez-Luís Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.

The Intraplate Earthquakes of SE Africa

NASA Astrophysics Data System (ADS)

Fonseca, J. F. B. D.; Domingues, A.

2014-12-01

Southeast Africa is a region of complex tectonic inheritance, combining early Archean cratonic blocks - the Zimbabwe and Kaapvaal cratons - and several collision belts ranging in age from ~2.6Ga (Limpopo Belt) to the Pan-African Orogeny, 800-500 Ma ago (Zambezi Belt, Mozambique Belt). Mesozoic rifting was the last tectonic event to leave an imprint in the lithosphere of the region. Tertiary deformation is mild and related to the extensional regime of the East African Rift System (EARS) further north. Instrumental seismology started in Johannesburg in 1910 (Saunders et al., 2008). Ambraseys and Adams (1991) conducted a reappraisal of the seismicity of Sub-Saharan Africa from 1900 to 1930, and the compilations by Krige and Maree (1948) and Gutenberg and F. Richter (1949) are main sources for the two subsequent decades. The available data can be considered complete since 1900 for magnitudes above M6 (Ambraseys and Adams, 1991), although major epicentral errors are likely to affect the early decades. Earthquakes above this threshold in the region to the south of Lake Malawi and to the east of the Okavango Rift are reported in 1919 (MS6.5, NE South Africa), 1932 (MS6.8, Natal Bay), 1940 (MS6.2, SE Mozambique), 1951 (MS6.0, Central Mozambique), 1957 (M6.0, Central Mozambique), again in 1957 (M6.2, Central Mozambique), 1958 (M6.0, Southern Zimbabwe), 1959 (M6.1, Southern Zambia) and 2006 (M7.0, Central Mozambique). The four M>6 earthquakes from 1957 to 1959 configure a cluster in time, followed by a shutdown (at M>6) that was to last nearly five decades. While a coincidence is unlikely, a causal link is challenged by the large spatial scatter of about 500 km. However, the fact that the four earthquakes occurred on the border of the Zimbabwe craton may provide a clue to the mechanism promoting the clustering. We combine these data with recent seismicity results (Fonseca et al., 2014) to discuss possible factors controlling the seismotectonics of the region.

3D Numerical Model of Continental Breakup via Plume Lithosphere Interaction Near Cratonic Blocks: Implications for the Tanzanian Craton

NASA Astrophysics Data System (ADS)

Koptev, A.; Calais, E.; Burov, E. B.; Leroy, S. D.; Gerya, T.

2014-12-01

Although many continental rift basins and their successfully rifted counterparts at passive continental margins are magmatic, some are not. This dichotomy prompted end-member views of the mechanism driving continental rifting, deep-seated and mantle plume-driven for some, owing to shallow lithospheric stretching for others. In that regard, the East African Rift (EAR), the 3000 km-long divergent boundary between the Nubian and Somalian plates, provides a unique setting with the juxtaposition of the eastern, magma-rich, and western, magma-poor, branches on either sides of the 250-km thick Tanzanian craton. Here we implement high-resolution rheologically realistic 3D numerical model of plume-lithosphere interactions in extensional far-field settings to explain this contrasted behaviour in a unified framework starting from simple, symmetrical initial conditions with an isolated mantle plume rising beneath a craton in an east-west tensional far field stress. The upwelling mantle plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of a magma-rich branch above the plume head and a magma-poor one along the opposite side of the craton, the formation of a rotating microplate between the two rift branches, and the feeding of melt to both branches form a single mantle source. The model bears strong similarities with the evolution of the eastern and western branches of the central EAR and the geodetically observed rotation of the Victoria microplate. This result reconciles the passive (plume-activated) versus active (far-field tectonic stresses) rift models as our experiments shows both processes in action and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments.

Metamorphic P-T-t path retrieved from metapelites in the southeastern Taihua metamorphic complex, and the Paleoproterozoic tectonic evolution of the southern North China Craton

NASA Astrophysics Data System (ADS)

Lu, Jun-Sheng; Zhai, Ming-Guo; Lu, Lin-Sheng; Wang, Hao Y. C.; Chen, Hong-Xu; Peng, Tao; Wu, Chun-Ming; Zhao, Tai-Ping

2017-02-01

The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9 Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite + plagioclase + quartz + ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite + biotite + plagioclase + quartz + K-feldspar + ilmenite + rutile + pyrite. The retrograde mineral assemblage (M3), biotite + plagioclase + quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P-T conditions of 12.2 kbar and 830 °C, whereas garnet rims record P-T conditions of 10.2 kbar and 840 °C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P-T path can be retrieved. High resolution SIMS U-Pb dating and LA-ICP-MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92 Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83 Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous

The ambient noise and earthquake surface wave tomography of the North China Craton

NASA Astrophysics Data System (ADS)

Pan, J.; Obrebski, M. J.; Wu, Q.; Li, Y.

2010-12-01

The North China Craton (NCC) is unique for its unusual Phanerozoic tectonic activity. The NCC was internally tectonically stable until Jurassic when its southern margin collided with the Yangzte Craton. Subsequently, the eastern and central part of the NCC underwent distinctive evolutions during the Late Mesozoic and Cenozoic. In contrast to the Erdos block located in the western part of NCC and that seems to have preserved the typical features of a stable craton, the eastern NCC has experienced significant lithospheric thinning as evidenced by widespread magmatism activity and normal faulting, among other manifestations. The eastern part of the NCC is also one of the most seismically active intracontinental regions in the world. Here we focus on the region that comprises the North China Basin and the Yanshan-Taihang Mountains, two major tectonic units located to the east and in the center of the NCC, respectively. We combine ambient noise data and ballistic surface wave data recorded by the dense temporary seismic array deployed in the North China to obtain phase velocity maps at periods ranging from 5s to 60s. 1587 and 3667 ray paths were obtained from earthquake surface waves and ambient noise correlations, respectively. The phase velocity distribution was reconstructed with grid size 0.25x0.25 degrees and 0.5x0.5 degrees from ambient noise tomography and earthquake surface wave tomography. For periods shorter than 15s, the phase velocity variations are well correlated with the principal geological units in the NCC, with low-speed anomalies corresponding to the major sedimentary basins and high-speed anomalies coinciding with the main mountain ranges. Within the period range from 20s to 30s, the phase velocity variations seem to be related to the local variations of the crustal thickness. For the periods above 30s, the strength of the phase velocity variations decreases with increasing periods, which may imply that the uppermost mantle is much more homogeneous

Petrologically-constrained thermo-chemical modelling of cratonic upper mantle consistent with elevation, geoid, surface heat flow, seismic surface waves and MT data

NASA Astrophysics Data System (ADS)

Jones, A. G.; Afonso, J. C.

2015-12-01

The Earth comprises a single physio-chemical system that we interrogate from its surface and/or from space making observations related to various physical and chemical parameters. A change in one of those parameters affects many of the others; for example a change in velocity is almost always indicative of a concomitant change in density, which results in changes to elevation, gravity and geoid observations. Similarly, a change in oxide chemistry affects almost all physical parameters to a greater or lesser extent. We have now developed sophisticated tools to model/invert data in our individual disciplines to such an extent that we are obtaining high resolution, robust models from our datasets. However, in the vast majority of cases the different datasets are modelled/inverted independently of each other, and often even without considering other data in a qualitative sense. The LitMod framework of Afonso and colleagues presents integrated inversion of geoscientific data to yield thermo-chemical models that are petrologically consistent and constrained. Input data can comprise any combination of elevation, geoid, surface heat flow, seismic surface wave (Rayleigh and Love) data and receiver function data, and MT data. The basis of LitMod is characterization of the upper mantle in terms of five oxides in the CFMAS system and a thermal structure that is conductive to the LAB and convective along the adiabat below the LAB to the 410 km discontinuity. Candidate solutions are chosen from prior distributions of the oxides. For the crust, candidate solutions are chosen from distributions of crustal layering, velocity and density parameters. Those candidate solutions that fit the data within prescribed error limits are kept, and are used to establish broad posterior distributions from which new candidate solutions are chosen. Examples will be shown of application of this approach fitting data from the Kaapvaal Craton in South Africa and the Rae Craton in northern Canada. I

Towards a Holistic Model for the Tectonic Evolution of the North China Craton

NASA Astrophysics Data System (ADS)

Kusky, T. M.; Polat, A.; Windley, B. F.; Wang, J.; Deng, H.

2016-12-01

The North China Craton (NCC) consists of distinctly different tectonic elements assembled during the late Archean - early Proterozoic. We propose a new tectonic evolution of the NCC. The Eastern Block (EB) consists of small microblocks that resemble a collage of accreted arc-rocks from a sutured archipelago similar to the SW Pacific, accreted between 2.6 and 2.7 Ga. An Atlantic-type margin developed on the western side of the EB by 2.5 Ga, and a >1,300 km long arc/accretionary prism collided with this passive margin at 2.5 Ga, obducting ophiolites and ophiolitic mélanges, and forming a foreland basin. This was followed by arc-polarity reversal, and injection of mantle wedge-derived melts. By 2.43 Ga, the ocean behind the accreted arc closed through the collision of an oceanic plateau. Rifting of the amalgamated craton followed at 2.4-2.35 Ga, with a failed rift arm preserved in the center of the craton, and two that successfully made an ocean along the northern margin. By 2.3 Ga an arc built on older cratonic material collided with this passive margin which soon converted to an Andean-type margin. Andean margin tectonics affected much of the craton from 2.3-1.9 Ga, forming a broad E-W swath of continental margin magmas, and retro-arc sedimentary basins including a superimposed basin over the passive margin on the northern margin. From 1.88-1.79 Ga the craton experienced a craton-wide granulite facies metamorphism and basement reactivation event with high-pressure granulites and eclogites in the north, and medium-pressure granulites across the craton. Early Proterozoic granulites and anatectic melts were generated by high-grade metamorphism and partial melting at mid-crustal levels beneath a collisionally-thickened plateau. This collision of the NCC on its northern margin was with the Columbia (Nuna) Continent. The NCC broke out in the period 1753-1673 Ma, as indicated by the formation of a suite of anorthosite, mangerite, charnockite, and alkali-feldspar granites

Recycling lower continental crust in the North China craton.

PubMed

Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

2004-12-16

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

Spatial distribution of eclogite in the Slave cratonic mantle: The role of subduction

NASA Astrophysics Data System (ADS)

Kopylova, Maya G.; Beausoleil, Yvette; Goncharov, Alexey; Burgess, Jennifer; Strand, Pamela

2016-03-01

We reconstructed the spatial distribution of eclogites in the cratonic mantle based on thermobarometry for 240 xenoliths in 4 kimberlite pipes from different parts of the Slave craton (Canada). The accuracy of depth estimates is ensured by the use of a recently calibrated thermometer, projection of temperatures onto well-constrained local peridotitic geotherms, petrological screening for unrealistic temperature estimates, and internal consistency of all data. The depth estimates are based on new data on mineral chemistry and petrography of 148 eclogite xenoliths from the Jericho and Muskox kimberlites of the northern Slave craton and previously reported analyses of 95 eclogites from Diavik and Ekati kimberlites (Central Slave). The majority of Northern Slave eclogites of the crustal, subduction origin occurs at 110-170 km, shallower than in the majority of the Central Slave crustal eclogites (120-210 km). The identical geochronological history of these eclogite populations and the absence of steep suture boundaries between the central and northern Slave craton suggest the lateral continuity of the mantle layer relatively rich in eclogites. We explain the distribution of eclogites by partial preservation of an imbricated and plastically dispersed oceanic slab formed by easterly dipping Proterozoic subduction. The depths of eclogite localization do not correlate with geophysically mapped discontinuities. The base of the depleted lithosphere of the Slave craton constrained by thermobarometry of peridotite xenoliths coincides with the base of the thickened lithospheric slab, which supports contribution of the recycled oceanic lithosphere to formation of the cratonic root. Its architecture may have been protected by circum-cratonic subduction and shielding of the shallow Archean lithosphere from the destructive asthenospheric metasomatism.

Cretaceous potassic intrusives with affinities to aillikites from Jharia area: Magmatic expression of metasomatically veined and thinned lithospheric mantle beneath Singhbhum Craton, Eastern India

NASA Astrophysics Data System (ADS)

Srivastava, Rajesh K.; Chalapathi Rao, N. V.; Sinha, Anup K.

2009-11-01

Cretaceous potassic dykes and sills at the Jharia area intrude the Permo-carboniferous coal-bearing Gondwana sediments of the Eastern Damodar Valley, Singhbhum craton. These intrusives are widely regarded as a part of the Mesozoic alkaline and Rajmahal flood basalt magmatism in the Eastern Indian shield. Jharia intrusives display a wide petrographic diversity; olivine, phlogopite and carbonate are the predominant phases whereas apatite and rutile constitute important accessories. Impoverishment in sodium, silica and alumina and enrichment in potassium, titanium and phosphorous are the hallmark of these rocks and in this aspect they are strikingly similar to the rift-related aillikites (ultramafic lamprophyres) of Aillik Bay, Labrador. Crustal contamination of the Jharia magmas is minimal and the incompatible trace element ratios demonstrate (i) their generation by greater degrees of partial melting of a sub-continental lithospheric mantle (SCLM) source similar to that of the kimberlites of Dharwar craton, southern India, and (ii) retention of long-term memories of ancient (Archaean) subduction experienced by their source regions. We infer that a metasomatically veined and thinned lithosphere located at the margin of the Singhbhum craton and the inheritance of an ancient (Archaean) subducted component has played a significant role in deciding the diverging petrological and geochemical characters displayed by the Jharia potassic intrusives: those of kimberlites (orangeites) and lamproites (cratonic signature) and those of aillikites (rift-related signature). A substantial melt component of Jharia potassic intrusives was derived from the SCLM and the melt contribution of the Kerguelen plume is inferred to be minimal.

Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton-example from the North China Craton

NASA Astrophysics Data System (ADS)

Chen, Ling; Wei, Zigen; Jiang, Mingming; Ling, Yuan

2016-04-01

Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton - example from the North China Craton Ling Chen1,2, Zigen Wei3, Mingming Jiang1, Yuan Ling1 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing100101, China 3. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an integrated lithospheric structural image along an E-W profile across the North China Craton (NCC) derived from the teleseismic data recorded at two dense seismic arrays in combination with other geophysical and geological observations. Our S- and P-receiver function images show substantial undulations of the lithosphere-asthenosphere boundary (LAB), from 60-100 km in the eastern NCC to ~160-200 km in the central-western NCC, and <150-km in the Qilian orogenic belt further to the west, accompanying marked lithospheric structural variations. This agrees with previous studies that suggest the occurrence of fundamental destruction in the eastern NCC but localized lithospheric thinning and modifications in the central-western NCC. A negative velocity discontinuity is identified at the depth of ~80-100 km within the thick lithosphere of the central-western NCC, spatially coincident with the top interface of a relatively low velocity layer in the overall high velocity mantle root imaged by surface wave tomography. Detailed data analyses show that this mid- or intra-lithospheric discontinuity has considerably larger S-to-P and P-to-S conversion amplitudes than the LAB below, which provides observational constraints to further decipher the origin of the discontinuity. Our imaging results corroborate

Magnetostratigraphy of displaced Upper Cretaceous strata in southern California

NASA Astrophysics Data System (ADS)

Fry, J. Gilbert; Bottjer, David J.; Lund, Steve P.

1985-09-01

A magnetostratigraphic study of Upper Cretaceous marine strata from the Santa Ana Mountains in southern California has identified a Campanian reversed magnetozone. This reversed interval, corresponding to marine magnetic anomaly 33 34 (Chron 33r) of Campanian age, can be correlated with a Campanian reversed magnetozone that has been reported from strata of the Great Valley Sequence in central California. The Late Cretaceous paleolatitude of the Santa Ana Mountains is estimated from this study to be 26.6°N. This is significantly different from the region's expected Cretaceous paleolatitude of 43.8°N as part of the North American stable craton, and indicates that this region (part of the Peninsular Ranges terrane) was 1900 km farther south in Cretaceous time relative to the stable craton. *Present address: Mobil Oil Corp., P.O. Box 900, Dallas, Texas 75221

Construction and destruction of some North American cratons

NASA Astrophysics Data System (ADS)

Snyder, David B.; Humphreys, Eugene; Pearson, D. Graham

2017-01-01

Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of its rarity, but metasomatic weakening is an essential precursor. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slab-like geometries similar to modern oceanic lithosphere in these construction histories. Underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities, emphasizing the role of lateral accretion. Archean continental building blocks may resemble the modern lithosphere of oceanic plateau, but they better match the sort of refractory crust expected to have formed at Archean ocean spreading centres. Radiometric dating of mantle xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences, and these ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons stabilized during a granitic bloom at 2.61-2.55 Ga. This stabilization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and more conductive by introducing or concentrating sulfides or graphite at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. Late Cretaceous flattening of Farallon lithosphere that included the Shatsky Rise conjugate appears to have weakened, eroded and displaced the base of the Wyoming craton below 140-160 km. This

Magnetotelluric characterization of the northern margin of the Yilgarn Craton (Western Australia)

NASA Astrophysics Data System (ADS)

Piña-Varas, Perla; Dentith, Michael

2017-04-01

The northern margin of the Yilgarn Craton (Western Australia) was deformed during the convergence and collision with the Pilbara Craton and the intervening Glenburgh Terrain that created the Capricorn Orogen. The Yilgarn Craton is one of the most intensively mineralised areas of continental crust with world class deposits of gold and nickel. However, the region to its north has surprisingly few deposits. Cratonic margins are considered to be key indicators of prospectivity at a regional scale. The northern limit of the Yilgarn Craton within the Capricorn Orogen is not well resolved at date because of overlying Proterozoic sedimentary basins. We present here some of the results of an extensive magnetotelluric (MT) study that is being performed in the area. This study is a component of large multi-disciplinary geoscience project on the 'Distal Footprints of Giant Ore Systems' in the Capricorn Orogen. The MT dataset consists of a total of 240 broadband magnetotelluric stations (BBMT) and 84 long period stations (LMT). Analysis of the dataset reveals a clear 3-D geoelectrical behaviour and extreme complexity for most of the sites, including an extremely high number of sites with phases out-of-quadrant at long periods. 3-D inverse modelling of the MT data shows high resistivity Archean units and low resistivity Paleoproterozoic basins, including very low resistivity structures at depth. These strong resistivity contrasts allow us to successfully map northern margin of the Yilgarn Craton beneath basin cover, as well as identifying major lateral conductivity changes in the deep crust suggestive of different tectonic blocks. Upper crustal conductive zones can be correlated with faults on seismic reflection data. Our results suggest MT surveys are a useful tool for regional-scale exploration in the study area and in area of thick cover in general.

Building Archean Cratons From Hadean Crust

NASA Astrophysics Data System (ADS)

O'Neil, J.; Carlson, R.

2016-12-01

Geologic processing of Earth's surface has removed most of the evidence concerning the nature of Earth's first crust. The largest volumes of ancient crust, the so-called Archean cratons, are dominated by felsic Tonalite-Trondhjemite-Granodiorite (TTG) rocks. These felsic rocks, however, are most likely derived by melting of an older mafic precursor. Although in part dictated by survivability, the scarcity of Hadean zircons also suggests that felsic rocks may have not been a prominent component of the earliest crust. Both points raise questions about the nature of the primordial crust and how, or if, it was involved in the formation of stable Archean cratons. The Hudson Bay Terrane of the Northeastern Superior Province is one of such Archean cratons, mainly composed of 2.88 to 2.69 Ga TTG. New data show these Neoarchean granitoids to be the youngest to yield significantly low 142Nd/144Nd, down to 15 ppm lower than that of the terrestrial Nd standard. 142Nd is the decay product of short-lived radioactive 146Sm and because of the short 103 Ma half-life of 146Sm, deviations in 142Nd/144Nd ratio can only be produced by Sm-Nd fractionation prior to 4 Ga. The variability in 142Nd/144Nd ratios in 2.7 Ga felsic rocks from the Hudson Bay Terrane shows conclusively that this large block of Archean crust was formed by reworking of much older > 4.2 Ga crust over a 1.5 billion year interval of early Earth history. Reworking of pre-existing crust likely is an important mechanism contributing to the stabilization of Earth's first continents.

The Victor Mine (Superior Craton, Canada): Neoproterozoic lherzolitic diamonds from a thermally-modified cratonic root

NASA Astrophysics Data System (ADS)

Stachel, Thomas; Banas, Anetta; Aulbach, Sonja; Smit, Karen V.; Wescott, Pamela; Chinn, Ingrid L.; Kong, Julie

2018-05-01

The Jurassic Victor kimberlite (Attawapiskat Field) was emplaced into an area of the central Superior Craton that was affected by a lithosphere-scale thermal event at 1.1 Ga. Victor diamonds formed ca. 400 million years after this event, in a lithospheric mantle characterized by an unusually cool model geotherm (37-38 mW/m2; Hasterok and Chapman 2011). The bulk of Victor diamonds derives from a thin (<10 km thick) layer that is located at about 180 km depth and represents lherzolitic substrates (for 85% of diamonds). Geothermobarometric calculations (average pressure and temperature at the 1 sigma level are 57 ± 2 kbar and 1129 ± 16 °C) coupled with typical fluid metasomatism-associated trace element patterns for garnet inclusions indicate diamond precipitation under sub-solidus (lherzolite + H2O) conditions. This conclusion links the presence of a diamond-rich lherzolitic layer in the lithospheric mantle, just above the depth where ascending melts would freeze, to the unusually low paleogeotherm beneath Attawapiskat, because along an average cratonic geotherm (40 mW/m2) lherzolite in the presence of hydrous fluid would melt at depths >140 km.

Timing of mafic magmatism in the Tapajós Province (Brazil) and implications for the evolution of the Amazon Craton: evidence from baddeleyite and zircon U Pb SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Santos, João Orestes Schneider; Hartmann, Léo Afraneo; McNaughton, Neal Jesse; Fletcher, Ian Robert

2002-09-01

The precise timing and possible sources of the mafic rocks in the Amazon craton are critical for reconstruction of the Atlantica supercontinent and correlation of mafic magmatism worldwide. New SHRIMP U-Pb baddeleyite and zircon ages and the reinterpretation of 207 existing dates indicate one orogenic (Ingarana) and four postorogenic (Crepori, Cachoeira Seca, Piranhas, and Periquito) basaltic events in the Tapajós Province, south central Amazon craton. Orogenic gabbro dikes that host gold mineralization are 1893 Ma and interpreted as associated with the Ingarana gabbro intrusions of the bimodal calk-alkalic Parauari intrusive suite. The age of 1893 Ma can be used as a guide to discriminate older and mineralized orogenic dikes from younger and nonmineralized Crepori- and Cachoeira Seca-related mafic dikes. The baddeleyite U-Pb age of the postorogenic Crepori dolerite (gabbro-dolerite sills and dikes) is 1780±9 Ma, ˜150 my older than the ages provided by K-Ar. This value correlates well with the Avanavero tholeiitic intrusions in the Roraima group, in the northern part of the craton in Guyana, Venezuela, and Roraima in Brazil. Early Statherian tholeiitic magmatism was widespread not only in the Amazon craton, but also in the La Plata craton of southern South America, where it is known as the giant Piedra Alta swarm of Uruguay and the post-Trans-Amazonian dikes of Tandil in Argentina. The Cachoeira Seca troctolite represents laccoliths, Feixes, and São Domingos, whose baddeleyite U-Pb age is 1186±12 Ma, 120-150 my older than the known K-Ar ages. This age is comparable to other Stenian gabbroic rocks with alkalic affinity in the craton, such as the Seringa Formation in NE Amazonas and the basaltic flows of the Nova Floresta formation in Rondônia. Dolerite from the giant Piranhas dike swarm in the western Tapajós Province has a Middle Cambrian age (507±4 Ma, baddeleyite) and inherited zircons in the 2238-1229 Ma range. The Piranhas dikes fill extensional NNE and

African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

PubMed

King, S D; Ritsema, J

2000-11-10

Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

Seismicity and lithospheric structure of Central Mozambique: implications for the southward propagation of the East African Rift System

NASA Astrophysics Data System (ADS)

Fonseca, J. F. B. D.; Domingues, A.

2017-12-01

South of latitude 5ºS, there is scarce support for a single preferred location of continental rifting in SE Africa. Building on the complexity already displayed further north around the Victoria microplate, the structures associated with rifting activity are now distributed over three branches: one directed towards the SW through Zambia and into the Okawango rift in Botswana; one running offshore along the Mozambique Channel; and a central rift system through lake Malawi and Central Mozambique. Our investigation focuses on this central branch, whose tectonic relevance was highlighted by the M7 Machaze earthquake in 2006. Through the temporary deployment of 30 broadband stations in central Mozambique we were able to document that the Shire and Urema grabens linking the Malawi rift to the Machaze epicentral area are seismically active, correlating with a 300 km long narrow band of seismicity reaching the lower crust. No significant seismicity was recorded along the Mazenga graben, south of the Machaze epicentral area. A tomographic model derived from ambient noise analysis showed a strong correlation between the seismicity and a sharp NNE-SSW boundary between the fast crust of the Zimbabwe and Kaapvaal cratons and slower crust underneath the Mozambique Coastal Plains. The seismicity shuts down were this trend rotates to a more N-S direction as the Lebombo monocline is approached. 20th Century seismicity of SE Africa shows a clear cluster in time, with five M>6 earthquakes concentrated in the 1950's, distributed along the edges of the Zimbabwe craton and spanning distances of 600 km. Spatial correlation with such range is hard to reconcile with stress transmission in the crust and may point to the interaction of the cratonic root with asthenospheric flow. Under this light, the M6.5 Central Botswana earthquake of April 2017 and the M7 Machaze earthquake of 2006, both located in the vicinity of the borders of the Kaapvaal craton, may bear a similar correlation. The

Quantum phases and phase transitions in disordered low-dimensional systems: thin film superconductors, bilayer two-dimensional electron systems, and one-dimensional optical lattices

NASA Astrophysics Data System (ADS)

Zhang, Nan

-spatial evolutions of the surface and CMB heat fluxes, and the dynamic topography since the Paleozoic. My result shows that the surface heat flux increases by ~16% from 200 to 120 Ma ago as a result of Pangea breakup and the equatorial CMB heat flux has two minima that coincide with the Kiaman (316-262 Ma) and Cretaceous (118-83 Ma) Superchrons, respectively, and may be responsible for the Superchrons. My results of the dynamic topography show that the Slave Craton subsided when the major downwelling occupied the mantle beneath North America, while Sao Francisco Craton, Kaapvaal Craton, and Yilgarn Craton were supported by the large scale upwellings in the mantle beneath the very south of Pangea around 330 Ma during Pangea formation. After Pangea formed, Slave Craton started to uplift as the major downwelling heated up with time and were controlled by the subductions close to it. Sao Francisco Craton and Kaapvaal Craton kept uplifting due to the returning African Superplume. My reconstructed dynamic topography history compares well with the vertical motion history of Slave Craton indicated by the thermochronometry study.

Cyclic Cratonic Carbonates and Phanerozoic Calcite Seas.

ERIC Educational Resources Information Center

Wilkinson, Bruce H.

1982-01-01

Discusses causes of cyclicity in cratonic carbonate sequences and evidence for and potential significance of postulated primary calcite sediment components in past Paleozoic seas, outlining problems, focusing on models explaining existing data, and identifying background. Future sedimentary geologists will need to address these and related areas…

New constraints on the upper mantle structure of the Slave craton from Rayleigh wave inversion

NASA Astrophysics Data System (ADS)

Chen, Chin-Wu; Rondenay, Stéphane; Weeraratne, Dayanthie S.; Snyder, David B.

2007-05-01

Rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We invert for phase velocities at periods between 20 s-142 s (with greatest sensitivity at depths of 28-200 km) using crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. Phase velocities obtained for the Slave province are comparable to those from other cratons at shorter periods, but exceed the global average by ~2% at periods above 60 s, suggesting that the Slave craton may be an end member in terms of its high degree of mantle depletion. The one-dimensional inversion of phase velocities yields high upper-mantle S-wave velocities of 4.7 +/- 0.2 km/s that persist to 220 +/- 65 km depth and thus define the cratonic lithosphere. Azimuthal anisotropy is well resolved at all periods with a dominant fast direction of N59°E +/- 20°, suggesting that upper mantle anisotropy beneath the Slave craton is influenced by both lithospheric fabric and sub-lithospheric flow.

Instability of the southern Canadian Shield during the late Proterozoic

NASA Astrophysics Data System (ADS)

McDannell, Kalin T.; Zeitler, Peter K.; Schneider, David A.

2018-05-01

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (∼150-350 °C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic "Middle Age." Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5 °C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ∼200 °C - signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (≪60 °C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the

Upper-mantle tectonic compartmentalization beneath Amazonian Craton from P-wave seismic tomography

NASA Astrophysics Data System (ADS)

Rocha, M. P.; Azevedo, P. A. D.

2017-12-01

The Amazonian Craton (AC) is one of the largest cratonic areas in the world, with more than 4.4 million square meters, defined as a cratonic nucleus composed of Archean to Mesoproterozoic provinces. Among the questions that remain open about AC, is that of its formation. Geochronological studies suggest that smaller blocks would have assembled in successive collisions until they stabilize in the current configuration, being AC formed by six provinces with different ages. Recent results using the P-wave seismic tomography method suggest that the geochronological boundaries between the Archean blocks exist and reach the upper mantle. These limits appear as low-velocity anomalies with NW-SE direction inside AC, differently from the expected velocity signatures for cratonic regions (high-velocities). The Archean blocks can be interpreted as high-velocity anomalies between the low-velocity anomalies, and are consistent with previous geochronological models. These results were achieved mainly by the installation of the stations of the Brazilian Seismographic Network in recent years, improving coverage especially in the northern region of Brazil. However, they are still preliminary since the seismographic stations in AC region are very distant from each other, which impairs the resolution of such structures. New stations would need to be installed in the region to confirm these results.

On the origin of cratonic `high-mu' isotopic signatures

NASA Astrophysics Data System (ADS)

Reimink, J. R.; Carlson, R.; Shirey, S. B.; Pearson, D. G.; Kamber, B. S.

2017-12-01

Some Archean cratons (i.e. Slave, Wyoming) contain Neoarchean granitoids with initial Pb isotopic compositions indicative of derivation from sources characterized by high time-integrated U/Pb ratios (high-mu [1]). Single-stage high-m precursor source reservoir separation from the depleted mantle occurred no later than 3.9 Ga [2]. However, multi-stage separation could have occurred in the Hadean, suggesting that recycling or reworking of Eoarchean/Hadean crust played a significant role in the generation of Neoarchean granitic crust in many cratons. The Sm-Nd system is similar to the U-Pb system in that it has a short-lived parent-daughter pair (146Sm-142Nd) that is sensitive to very early differentiation events, as well as a long-lived parent-daughter pair (147Sm-143Nd) that is sensitive to differentiation throughout all of Earth history. The 103 Ma half-life of 146Sm makes it sensitive only to Sm/Nd fractionation that occurred in the Hadean, providing a useful tracker for very early differentiation events. Indeed, evidence for Neoarchean remelting of ancient crust in another craton has come from analyses of the paired Sm-Nd isotope systems from the Hudson Bay terrane of the northeastern Superior Province. These results indicate that the source of 2.7 Ga Hudson Bay terrane granitoids was Hadean mafic crust, and not Eoarchean felsic crust [3]. Here, we present new data from Neoarchean granites located in the Slave and Wyoming cratons, along with modeling of the dual paired-isotope systems of U-Pb and Sm-Nd to achieve a tighter constraint on the composition of the precursors and the timing of their melting. Combining our newly collected 142Nd data with the high-m signature of these Neoarchean rocks, we evaluate precursor source separation ages along with the source Sm/Nd and U/Pb compositions. In the simplest end-member scenarios, use of the 142Nd system allows us to test whether the cratonic high-mu signature was created by melting of Hadean mafic crust or Eoarchean

MARID-type Glimmerites from Kimberley, South Africa: Metasomes or high-pressure cumulates?

NASA Astrophysics Data System (ADS)

Förster, Michael W.; Prelevic, Dejan; Buhre, Stephan; Jacob, Dorrit E.

2015-04-01

fractionation of Al between melt and mineral and are not realistic for low-Al diopsides. Calculations by Konzett et al. (2014) yielded 4.2 GPa (155 km) by using a Ca-in-opx thermometer and a cratonic geotherm of 40 mW/m² and seem to be more realistic. By applying a sandwich experimental approach, mixing glimmerite samples with harzburgitic peridotites, we hope to achieve deeper insights into the origin of MARID-type glimmerites. References Dawson, J. B., & Smith, J. V. (1977). The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite. Geochimica et Cosmochimica Acta, 41(2), 309-323. Dawson, J. B. (1987). The MARID suite of xenoliths in kimberlite: relationship to veined and metasomatised peridotite xenoliths. Mantle Xenoliths. Chichester: John Wiley, 465-474. Konzett, J., Krenn, K., Rubatto, D., Hauzenberger, C., & Stalder, R. (2014). The formation of saline mantle fluids by open-system crystallization of hydrous silicate-rich vein assemblages-Evidence from fluid inclusions and their host phases in MARID xenoliths from the central Kaapvaal Craton, South Africa. Geochimica et Cosmochimica Acta, 147, 1-25. Nguuri, T. K., Gore, J., James, D. E., Webb, S. J., Wright, C., Zengeni, T. G., Gwavava, O. & Snoke, J. A. (2001). Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons. Geophysical Research Letters, 28(13), 2501-2504. Putirka, K. D. (2008). Thermometers and barometers for volcanic systems. Reviews in Mineralogy and Geochemistry, 69(1), 61-120. Sweeney, R. J., Thompson, A. B., & Ulmer, P. (1993). Phase relations of a natural MARID composition and implications for MARID genesis, lithospheric melting and mantle metasomatism. Contributions to Mineralogy and Petrology, 115(2), 225-241. Waters, F. G. (1987). A suggested origin of MARID xenoliths in kimberlites by high pressure crystallization of an ultrapotassic rock such as lamproite. Contributions to Mineralogy and Petrology

Microstructural analysis of calcite-filled fractures inherited from basement structures, southern Ontario, Canada: long term instability of the craton?

NASA Astrophysics Data System (ADS)

Spalding, Jennifer; Schneider, David

2016-04-01

Intra-cratonic regions are generally characterized by tectonic stability and low seismicity. In southern Ontario, Canada, moderate levels of seismicity have been recorded over the last few decades reaching magnitudes of 5 MN, indicating that the geosphere is not as stable as predicted. The stratigraphy of the region consists of Ordovician limestone with a thickness of ~200 m that unconformably overlays the Mesoproterozoic crystalline Grenville Province. Subsequent tectonism including repeated Paleozoic orogenies and rifting along the east coast of North America has reactivated Proterozoic structures that have propagated into the overlying carbonate platform forming mesoscopic-scale brittle structures. Exposed along the shores of Lake Ontario are decameter-scale fracture zones, with a fracture spacing of 0.5 to 10 meters. The dominant fracture set trends E-W, and often forms conjugate sets with less prominent NNE-oriented fractures. More locally, an older NW-oriented fracture set is cross cut by the E-W and NNE oriented fractures. Regionally, there have been six directions of maximum horizontal stress in southern Ontario since the Precambrian, with the current orientation of maximum stress oriented ENE as a consequence of far field Atlantic ridge-push forces generated at distant plate boundaries. Calcite mineralization along fractured surfaces locally form sub-horizontal slickenside fabrics which are covered by a layer of euhedral calcite crystals, suggesting that fracture dilation (and fluid flow) occurred after fracture slip to allow the growth of calcite crystals. Due to the proximity of the carbonate units to the crystalline basement, we expect the calcitic veins to be enriched in rare earth elements and are presently conducting geochemical analyses. The calcite veins and surfaces vary from 2.5 cm to 1 mm thicknesses, often with larger calcite crystals in the center of the vein and smaller crystals at the vein boundaries, likely representing nucleation on small

Paleomagnetic study of 1765 Ma dyke swarm from the Singhbhum Craton: Implications to the paleogeography of India

NASA Astrophysics Data System (ADS)

Shankar, Ravi; Srinivasa Sarma, D.; Ramesh Babu, N.; Parashuramulu, V.

2018-05-01

We report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765.3 ± 1.0 Ma WNW-ESE trending dyke swarm from Singhbhum Craton. This pole has been used in this study to propose the paleogeographic reconstruction of India with Baltica Craton and North China Craton. Incremental alternating field (AF) and thermal demagnetization, isolated high coercivity components with north to north-westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes. The primary origin of the ChRM is supported by the positive baked contact test. The WNW-ESE trending dykes yield a mean paleomagnetic direction with a declination = 329.2° and an inclination = -22.8° (k = 31.6; α95 = 9.3°). The positive bake contact test proves the primary nature of remanence. The pole position of Singhbhum Craton at 1765 Ma is 45°N, 311°E (dp = 5.2 and dm = 9.9). Paleogeographic reconstruction at ca. 1770 Ma, supported by geological, tectonic and metallogenic evidences indicate that the Baltica Craton and India linkage can be stable for at least ∼370 Ma (∼1770-1400 Ma). There is also reasonable evidence in support of India-North China Craton spatial proximity at ∼1770 Ma.

Water transportation ability of flat-lying slabs in the mantle transition zone and implications for craton destruction

NASA Astrophysics Data System (ADS)

Wang, Zhensheng; Kusky, Timothy M.; Capitanio, Fabio A.

2018-01-01

Water transported by deep subduction to the mantle transition zone (MTZ) that is eventually released and migrates upwards is invoked as a likely cause for hydroweakening and cratonic lithosphere destruction. The destruction of the North China Craton (NCC) during the Mesozoic has been proposed to be related to hydroweakening. However, the source of water related to large-scale craton destruction in the NCC is poorly constrained. Some suggest that the water was mainly released from a flat-lying (or stagnating) slab in the MTZ, whereas others posit that most water was released from a previously existing strongly hydrous MTZ then perturbed by the stagnating subduction in the MTZ layer. In this study, we use numerical modeling to evaluate the water carrying ability of flat-lying slabs in the MTZ with different slab ages and water contents to simulate its maximum value and discuss its potential role on large-scale hydroweakening and craton destruction. Our results reveal that a single flat-lying slab in the MTZ cannot provide enough water for large-scale cratonic lithosphere hydroweakening and thinning. Water estimates invoked for craton destruction as experienced by the NCC can only be the result of long-term piling of multiple slabs in the MTZ or penetrating deeper into the lower mantle.

Seismic Refraction & Wide-angle Reflection Experiment on the Northern Margin of North China Craton -Data Acquisition and Preliminary Processing Result

NASA Astrophysics Data System (ADS)

Li, W.; Gao, R.; Keller, G. R.; Hou, H.; Li, Q.; Cox, C. M.; Chang, J. C.; Zhang, J.; Guan, Y.

2010-12-01

The evolution history of Central Asian Orogen Belt (CAOB) is still the main tectonic problems in northeastern Asia. The Siberia Craton (NC), North China Craton (NCC) and several blocks collided, and the resulting tectonic collage formed as the Paleo-Asian Ocean disappeared. Concerning the northern margin of North China Craton, many different geological questions remain unanswered, such as: the intracontinental orogenic process in the Yanshan orogen and the nature and location of the suture between the southern NC and the northern NCC. In Dec 2009, a 400 km long seismic refraction and wide-angle reflection profile was completed jointly by Institute of Geology, CAGS and University of Oklahoma. The survey line extended from the west end of the Yanshan orogen, across a granitoid belt to the Solonker suture zone. The recording of seismic waves from 8 explosions (500~1500 kg each) was conducted in four deployments of 300 Reftek125 (Texan) seismic recorders, with an average spacing of 1 km. For the calculations, we used the Rayinvr, Vmed and Zplot programs for ray tracing, model modification and phase picking. The initial result show that: 1)the depth of low velocity sediment cover ranges from 0.6 to 2.7 km (velocity: 2.8~5.6 km/s); 2)the depth of basement is 5.6~10 km (the depth of basement under the granitoid belt deepens to 10 km and velocity increases to 6.2 km/s); 3)the upper crust extends to a depth of 15.5~21 km and has the P-wave velocities between 5.6 and 6.4 km/s; 4)the thickness of the lower crust ranges from 22~28 km(velocity: 6.4~6.9 km/s); and 5)the depth of Moho varies from 39.5 km under the granitoid belt to 49 km under the Yanshan orogen. Based on these results, we can preliminarily deduce that: 1) the concave depression of the Moho observed represents the root of the Yanshan orogen, and it may prove that the orogen is dominated by thick-skinned tectonics; 2) the shape of velocity variations under the granitoid belt is suggestive of a magma conduit. It

Geology of the Terre Adélie Craton (135 – 146˚ E)

USGS Publications Warehouse

Ménot, R.P.; Duclaux, G.; Peucat, J.J.; Rolland, Y.; Guillot, S.; Fanning, M.; Bascou, J.; Gapais, D.; Pêcher, A.

2007-01-01

More than 15 years of field and laboratory investigations on samples from Terre Adélie to the western part of George Vth Land (135 to 146°E) during the GEOLETA program allow a reassessment of the Terre Adélie Craton (TAC) geology. The TAC represents the largest exposed fragment of the East Antarctic Shield preserved from both Grenville and Ross tectono-metamorphic events. Therefore it corresponds to a well-preserved continental segment that developed from the Neoarchean to the Paleoproterozoic. Together with the Gawler Craton in South Australia, the TAC is considered as part of the Mawson continent, i.e. a striking piece of the Rodinia Supercontinent. However, this craton represents one of the less studied parts of the East Antarctic Shield. The three maps presented here clearly point out the extent of two distinct domains within the Terre Adélie Craton and suggest that the TAC was built up through a polyphased evolution during the Neoarchean-Siderian (c.a. 2.5Ga) and the Statherian (c.a. 1.7Ga) periods. These data support a complete re-assessment of the TAC geology and represent a valuable base for the understanding of global geodynamics changes during Paleoproterozoic times.

Small-scale upper mantle flow during the initiation of craton destruction

NASA Astrophysics Data System (ADS)

Zhao, Liang; Wang, Kun; Xu, Xiaobing

2017-04-01

The North China Craton (NCC) is an old craton which has experienced multi-episodic tectonism with surrounding plates. Bordered to the north by Xing'an-Mongolian Orogenic Belt, to the south by Qingling-Dabie-Sulu Orogen and to the far-east by (Paleo-) Pacific plate, the NCC has lost the cratonic properties within its eastern part. Evidently, the initiation and mechanism of craton destruction attract tremendous attention and remain hot debated. During the Mesozoic to the Cenozoic, the northeastern part of the NCC has been intensively revoked, along with the transition from NE shortening to NW-SE extension. The subduction of Paleo-Pacific plate becomes the prime suspect due to the same kinematic direction. Here we present a hybrid shear wave splitting measurement to investigate the mantle deformation of the NCC, and intend to constrain geodynamic process during the initiation of craton destruction. The SKS waveform data is recorded from 60 broadband stations with an average spacing of 15 km. We employ the traditional routine method to obtain fast polarization directions (FPDs, Φ) and delay times (δt) for the teleseismic events with epicenter range in 85°-115°. One may often have troubles in delimiting SKS and S wave with regard to the events at distances <85°. Waveform modeling has the advantages through repeated compatibility tests which thus can help us acquire more accurate Φ and δt. Combining these two methods, we depict the major FPDs overlapping on the geological map. Three segments along the profile exhibit NW-SE trending, which are parallel to the extension direction recorded in Erlian Basin, Songliao Basin and metamorphic core complex in Liaoning Peninsula. However, the in-between E-W trending FPDs cannot be neglected. Our previous tomography results show a high-velocity anomaly extend to the depth greater than 200 km beneath the Yanshan belt (118°-120°E). In comparison with other SKS observations in the NCC, the east-end nearly E-W FPD is possibly

Cratons are from Earth, Planum are from Venus

NASA Astrophysics Data System (ADS)

Cooper, C. M.; Lenardic, A.; Moresi, L.

2004-12-01

Both the Earth and Venus exhibit ancient features that are associated with long-term stability from deformation after their initial formation. On the Earth, these features are referred to as cratons. On Venus, a classic example of such a feature is Lakshmi Planum, a large plateau that sits 4 km above the surface. Both cratons and the Lakshmi Planum have been proposed to form through some form of crustal thickening over mantle downwellings, though the physical viability of these models have not been tested. Here we present the work of numerical simulations and scaling analysis, which suggest that the formation and preservation of such features can be achieved through crustal thickening via localized deformation (i.e., thrust stacking) even in the presence of a high viscosity crust, which would inhibit viscous deformation. We choose to present this work in such a way that will highlight the similarities and differences between the two formation histories using an alternative poster format.

Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

PubMed Central

Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

2016-01-01

The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

Asymmetry and polarity of the South Atlantic conjugated margins related to the presence of cratons: a numerical study

NASA Astrophysics Data System (ADS)

Andrés-Martínez, Miguel; Pérez-Gussinyé, Marta; de Monserrat Navarro, Albert; Morgan, Jason P.

2015-04-01

Tectonic asymmetry of conjugated passive margins, where one margin is much narrower than the conjugate one, is commonly observed at many passive margins world-wide. Conjugate margin asymmetry has been suggested to be a consequence of lateral changes in rheology, composition, temperature gradient or geometries of the crust and lithosphere. Here we use the South Atlantic margins (from Camamu/Gabon to North Santos/South Kwanza) as a natural laboratory to understand conjugate margin asymmetry. Along this margin sector the polarity of the asymmetry changes. To the North, the Brazilian margin developed in the strong Sao Francisco craton, and this constitutes the narrow side of the conjugate pair. To the South, the Brazilian margin developed in the Ribeira fold belt, and the margin is wide. The opposite is true for the African side. We have thus numerically analysed how the relative distance between the initial location of extension and the craton influences the symmetry/asymmetry and polarity of the conjugate margin system. Our numerical model is 2D visco-elasto-plastic and has a free surface, strain weakening and shear heating. The initial set-up includes a cratonic domain, a mobile belt and a transition area between both. We have run tests with different rheologies, thickness of the lithosphere, and weak seeds at different distances from the craton. Results show asymmetric conjugated margins, where the narrower margin is generally the closest to the craton. Our models also allow us to study how the polarity is controlled by the distance between the initial weakness and the craton, and help to understand how the presence of cratonic domains affects the final architecture of the conjugated margins.

Absolute age Determinations on Diamond by Radioisotopic Methods: NOT the way to Accurately Identify Diamond Provenance

NASA Astrophysics Data System (ADS)

Shirey, S. B.

2002-05-01

Gem-quality diamond contains such low abundances of parent-daughter radionuclides that dating the diamond lattice directly by isotopic measurements has been and will be impossible. Absolute ages on diamonds typically are obtained through measurements of their syngenetic mineral inclusions: Rb-Sr in garnet; Sm-Nd in garnet and pyroxene; Re-Os and U-Th-Pb in sulfide; K-Ar in pyroxene; and U-Pb in zircon. The application of the first two isotope schemes in the list requires putting together many inclusions from many diamonds whereas the latter isotope schemes permit ages on single diamonds. The key limitations on the application of these decay pairs are the availability and size of the inclusions, the abundance levels of the radionuclides, and instrumental sensitivity. Practical complications of radioisotope dating of inclusions are fatal to the application of the technique for diamond provenance. In all mines, the ratio of gem-quality diamonds to stones with datable inclusions is very high. Thus there is no way to date the valuable, marketable stones that are part of the conflict diamond problem, just their rare, flawed cousins. Each analysis destroys the diamond host plus the inclusion and can only be carried out in research labs by highly trained scientists. Thus, these methods can not be automated or applied to the bulk of diamond production. The geological problems with age dating are equally fatal to its application to diamond provenance. From the geological perspective, for age determination to work as a tool for diamond provenance studies, diamond ages would have to be specific to particular kimberlites or kimberlite fields and different between fields. The southern African Kaapvaal-Zimbabwe Craton and Limpopo Mobile Belt is the only cratonic region where age determinations have been applied on a large enough scale to a number of kimberlites to illustrate the geological problems in age measurements for diamond provenance. However, this southern African example

Platinum group elements in a 3.5 Ga nickel-iron occurrence - Possible evidence of a deep mantle origin

NASA Technical Reports Server (NTRS)

Tredoux, Marian; Hart, Rodger J.; Lindsay, Nicholas M.; De Wit, Maarten J.; Armstrong, Richard A.

1989-01-01

This paper reports the results of new field observations and the geochemical analyses for the area of the Bon Accord (BA) (the Kaapvaal craton, South Africa) Ni-Fe deposit, with particular consideration given to the trace element, platinum-group element, and isotopic (Pb, Nd, and Os) compositions. On the basis of these data, an interpretation of BA is suggested, according to which the BA deposit is a siderophile-rich heterogeneity remaining in the deep mantle after a process of incomplete core formation. The implications of such a model for the study of core-mantle segregation and the geochemistry of the lowermost mantle are discussed.

Cold cratonic roots and thermal blankets: How continents affect mantle convection

USGS Publications Warehouse

Trubitsyn, V.P.; Mooney, W.D.; Abbott, D.H.

2003-01-01

Two-dimensional convection models with moving continents show that continents profoundly affect the pattern of mantle convection. If the continents are wider than the wavelength of the convection cells (???3000 km, the thickness of the mantle), they cause neighboring deep mantle thermal upwellings to coalesce into a single focused upwelling. This focused upwelling zone will have a potential temperature anomaly of about 200??C, much higher than the 100??C temperature anomaly of upwelling zones generated beneath typical oceanic lithosphere. Extensive high-temperature melts (including flood basalts and late potassic granites) will be produced, and the excess temperature anomaly will induce continental uplift (as revealed in sea level changes) and the eventual breakup of the supercontinent. The mantle thermal anomaly will persist for several hundred million years after such a breakup. In contrast, small continental blocks (<1000 km diameter) do not induce focused mantle upwelling zones. Instead, small continental blocks are dragged to mantle downwelling zones, where they spend most of their time, and will migrate laterally with the downwelling. As a result of sitting over relatively cold mantle (downwellings), small continental blocks are favored to keep their cratonic roots. This may explain the long-term survival of small cratonic blocks (e.g., the Yilgarn and Pilbara cratons of western Australia, and the West African craton). The optimum size for long-term stability of a continental block is <3000 km. These results show that continents profoundly affect the pattern of mantle convection. These effects are illustrated in terms of the timing and history of supercontinent breakup, the production of high-temperature melts, and sea level changes. Such two-dimensional calculations can be further refined and tested by three-dimensional numerical simulations of mantle convection with moving continental and oceanic plates.

New Constraints on Upper Mantle Structure Underlying the Diamondiferous Central Slave Craton, Canada, from Teleseismic Body Wave Tomography

NASA Astrophysics Data System (ADS)

Esteve, C.; Schaeffer, A. J.; Audet, P.

2017-12-01

Over the past number of decades, the Slave Craton (Canada) has been extensively studied for its diamondiferous kimberlites. Not only are diamonds a valuable resource, but their kimberlitic host rocks provide an otherwise unique direct source of information on the deep upper mantle (and potentially transition zone). Many of the Canadian Diamond mines are located within the Slave Craton. As a result of the propensity for diamondiferous kimberlites, it is imperative to probe the deep mantle structure beneath the Slave Craton. This work is further motivated by the increase in high-quality broadband seismic data across the Northern Canadian Cordillera over the past decade. To this end we have generated a P and S body wave tomography model of the Slave Craton and its surroundings. Furthermore, tomographic inversion techniques are growing ever more capable of producing high resolution Earth models which capture detailed structure and dynamics across a range of scale lengths. Here, we present preliminary results on the structure of the upper mantle underlying the Slave Craton. These results are generated using data from eight different seismic networks such as the Canadian National Seismic Network (CNSN), Yukon Northwest Seismic Network (YNSN), older Portable Observatories for Lithospheric Analysis and Reseach Investigating Seismicity (POLARIS), Regional Alberta Observatory for Earthquake Studies Network (RV), USArray Transportable Array (TA), older Canadian Northwest Experiment (CANOE), Batholith Broadband (XY) and the Yukon Observatory (YO). This regional model brings new insights about the upper mantle structure beneath the Slave Craton, Canada.

Updating the Geologic Barcodes for South China: Discovery of Late Archean Banded Iron Formations in the Yangtze Craton.

PubMed

Ye, Hui; Wu, Chang-Zhi; Yang, Tao; Santosh, M; Yao, Xi-Zhu; Gao, Bing-Fei; Wang, Xiao-Lei; Li, Weiqiang

2017-11-08

Banded iron formations (BIFs) in Archean cratons provide important "geologic barcodes" for the global correlation of Precambrian sedimentary records. Here we report the first finding of late Archean BIFs from the Yangtze Craton, one of largest Precambrian blocks in East Asia with an evolutionary history of over 3.3 Ga. The Yingshan iron deposit at the northeastern margin of the Yangtze Craton, displays typical features of BIF, including: (i) alternating Si-rich and Fe-rich bands at sub-mm to meter scales; (ii) high SiO 2  + Fe 2 O 3total contents (average 90.6 wt.%) and Fe/Ti ratios (average 489); (iii) relative enrichment of heavy rare earth elements and positive Eu anomalies (average 1.42); (iv) and sedimentary Fe isotope compositions (δ 56 Fe IRMM-014 as low as -0.36‰). The depositional age of the BIF is constrained at ~2464 ± 24 Ma based on U-Pb dating of zircon grains from a migmatite sample of a volcanic protolith that conformably overlied the Yingshan BIF. The BIF was intruded by Neoproterozoic (805.9 ± 4.7 Ma) granitoids that are unique in the Yangtze Craton but absent in the North China Craton to the north. The discovery of the Yingshan BIF provides new constraints for the tectonic evolution of the Yangtze Craton and has important implications in the reconstruction of Pre-Nuna/Columbia supercontinent configurations.

Tectonic evolution of greenstone-Gneiss association in Dharwar Craton, South India: Problems and perspectives for future research

NASA Technical Reports Server (NTRS)

Rao, Y. J. B.

1986-01-01

The two fold stratigraphic subdivision of the Archean-Proterozoic greenstone-gneiss association of Dharwar craton into an older Sargur group (older than 2.9 Ga.) and a younger Dharwar Supergroup serves as an a priori stratigraphic model. The concordant greenstone (schist)-gneiss (Peninsular gneiss) relationships, ambiguities in stratigraphic correlations of the schist belts assigned to Sargur group and difficulties in deciphering the older gneiss units can be best appreciated if the Sargur group be regarded as a trimodal association of: (1) ultrabasic-mafic metavolcanics (including komatiites), (2) clastic and nonclastic metasediments and paragneisses and (3) mainly tonalite/trondhemite gneisses and migmatites of diverse ages which could be as old as c. 3.4 ga. or even older. The extensive occurrence of this greenstone-gneiss complex is evident from recent mapping in many areas of central and southern Karnataka State.

Neoarchean Subduction Recorded in the Northern Margin of the Yangtze Craton, South China

NASA Astrophysics Data System (ADS)

Zhang, S. B.; Zheng, Y. F.

2016-12-01

The Neoarchean is an important era during which plate tectonics began to operate widely on the earth and the continental crust compositions changed dramatically. However, reliable record of plate subduction has never been reported yet in the Yangtze Craton. Here we report geochemical studies on gneissic tonalite, trondhjemite and amphibolite in the Yudongzi Complex in the northern margin of the Yangtze Craton, which suggests that there is a plate subduction recorded in this area at about 2.7 Ga.The rocks in the Yudongzi Complex are gneissic granite, gneissic tonalite, amphibolite gneiss and magnetite quartzite. Most rocks are enriched in sodic. The gneissic granites show positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Ybcn and Y, resembling typical TTG. The amphibolite and tonalite gneiss show less fractionated REE patterns. SHRIMP zircon U-Pb dating on one gneissic trondhjemite, one amphibolite and one tonalite gave crystallization ages of 2667±21 Ma, 2701±10 Ma and 2697±9 Ma, respectively. They all recorded a metamorphic event at about 2.48 Ga. The SHRIMP zircon oxygen isotope analysis for a trondhjemite and an amphibolite gave δ18O values of 6.2±0.3‰ and 6.3±0.4‰, respectively. The oxygen isotope ratios higher than normal mantle values suggest a source experienced low temperature alteration. The laser fluoration analysis of bulk minerals gave δ18O values of 6.4-8.8‰ for zircon and 12.5-15.2‰ for quartz. The zircon Lu-Hf isotope analysis on the trondhjemite and amphibolite gave similar ɛHf(t) values of 0.08±0.48 and 0.07±0.63, respectively. Whole-rock ɛNd(t) values range from -1.5 to +1.0. These trondhjemite and tonalite can be interpreted as derivation from partial melting of subducted oceanic slab with a garnet-amphibolite residue.Considering the 2.67 Ga A-type granitic rocks at Huji in the interior of the craton, plate subduction took place in the northern edge of the Yangtze Craton. The Yudongzi trondhjemite and tonalite were

Contrasting cratonal provenances for upper Cretaceous Valle Group quartzite clasts, Baja California

USGS Publications Warehouse

Kimbrough, D.L.; Abbott, G.; Smith, D.P.; Mahoney, J.B.; Moore, Thomas E.; Gehrels, G.E.; Girty, G.H.; Cooper, John D.

2006-01-01

Late Cretaceous Valle Group forearcbasin deposits on the Vizcaino Peninsula of Baja California Sur are dominated by firstcycle arc-derived volcanic-plutonic detritus derived from the adjacent Peninsular Ranges batholith. Craton-derived quartzite clasts are a minor but ubiquitous component in Valle Group conglomerates. The source of these clasts has implications for tectonic reconstructions and sediment-dispersal paths along the paleo-North American margin. Three strongly contrasting types of quartzite are recognized based on petrology and detrital zircon U-Pb geochronology. The first type is ultramature quartz arenite with well-rounded, highly spherical zircon grains. Detrital zircon ages from this type are nearly all >1.8 Ga with age distributions that closely match the distinctive Middle-Late Ordovician Peace River arch detrital signature of the Cordilleran margin. This type has been previously recognized from prebatholithic rocks in northeast Baja California (San Felipe quartzite). A second quartzite type is subarkosic sandstone with strong affinity to southwestern North America; important features of the age spectra are ~1.0-1.2 Ga, 1.42 and 1.66 Ga peaks representing cratonal basement, 500-300 Ma grains interpreted as recycled Appalachian-derived grains, and 284- 232 Ma zircon potentially derived from the Early Permian-Middle Triassic east Mexico arc. This quartzite type could have been carried to the continental margin during Jurassic time as outboard equivalents of Colorado Plateau eolianites. The third quartzite type is quartz pebble conglomerate with significant ~900- 1400 Ma and ~450-650 Ma zircon components, as well as mid- and late Paleozoic grains. The source of this type of quartzite is more problematic but could match either upper Paleozoic strata in the Oaxaca terrane of southern Mexico or a southwestern North America source. The similarity of detrital 98 zircon spectra in all three Valle Group quartzite types to rocks of the adjacent Cordilleran

An alternative model for the development of the allochthonous southern Appalachian Piedmont.

USGS Publications Warehouse

Zen, E.-A.

1981-01-01

The recent deep-seismic-reflection data across the S Appalachian Piedmont require rethinking of the tectonic relations in that area. Some of the traditional tectonic-lithostratigraphic belts of the Piedmont may be 'doubly allochthonous', that is, they may be terranes that are exotic mutually and with respect to the N American craton. These terranes may have been brought to the edge of the craton by plate-tectonic processes, in a manner similar to that proposed for the post-Triassic 'Wrangellia' in southeastern Alaska, and then obducted onto the craton as traditional thrust allochthons. If this idea is correct, then there is no compelling need for an intercontinental suture in the lower crust under the exposed southern Appalachian Piedmont; however, multiple sutures may obtain under the Coastal Plain overlap or farther off shore. The location of the Paleozoic Iapetus Ocean may also be off the present shore. The tectonic units now exposed in the Appalachian Piedmont not only may not be continuous with those of the N Appalachian region that have been considered by many authors to be the same on a cylindrical model but could have had different geologic origins. The nature of the ultramafic rocks spatially associated with the Kings Mountain belt and the Raleigh and Kiokee belts, as well as the paleomagnetic orientations of rocks of the various Piedmont belts, may provide useful tests for this microplate model.-Author

Mg-Fe Isotope Systems of Mantle Xenoliths: Constrains on the Evolution of Siberian Craton

NASA Astrophysics Data System (ADS)

An, Y.; Kiseeva, E. S.; Sobolev, N. V.; Zhang, Z.

2017-12-01

Mantle xenoliths bring to the surface a variety of lithologies (dunites, lherzolites, harzburgites, wehrlites, eclogites, pyroxenites, and websterites) and represent snapshots of the geochemical processes that occur deep within the Earth. Recent improvements in the precision of the MC-ICP-MS measurements have allowed us to expand the amount of data on Mg and Fe isotopes for mantle-derived samples. For instance, to constrain the isotopic composition of the Earth based on the study of spinel and garnet peridotites (An et al., 2017; Teng et al., 2010), to trace the origin and to investigate the isotopic fractionation mechanism during metamorphic process using cratonic or orogenic eclogites (Li et al., 2011; Wang et al., 2012) and to reveal the metasomatism-induced mantle heterogeneity by pyroxenites (Hu et al., 2016). Numerous multi-stage modification events and mantle layering are detected in the subcontinental lithospheric mantle under the Siberian craton (Ashchepkov et al., 2008a; Sobolev et al., 1975, etc). Combined analyses of Mg and Fe isotopic systems could provide new constraints on the formation and evolution of the ancient cratonic mantle. In order to better constrain the magnitude and mechanism of inter-mineral Mg and Fe isotopic fractionations at high temperatures, systematic studies of mantle xenoliths are needed. For example, theoretical calculations and natural samples measurements have shown that large equilibrium Mg isotope fractionations controlled by the difference in coordination number of Mg among minerals could exist (Huang et al., 2013; Li et al., 2011). Thus, the Mg isotope geothermometer could help us trace the evolution history of ancient cratons. In this study we present Mg and Fe isotopic data for whole rocks and separated minerals (clinopyroxene (cpx) and garnet (grt)) from different types of mantle xenoliths (garnet pyroxenites, eclogites, grospydites and garnet peridotites) from a number of kimberlite pipes in Siberian craton (Udachnaya

Idetification of the chemical sedimentary protolish of the early Paleoproterozoic banded iron formation from Wuyang area, in the southern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Lan, C.; Zhao, T.

2016-12-01

The Paleoproterozoic banded iron formation (BIF) from Wuyang area in the southern margin of the North China Craton (NCC) were metamorphosed under granulite facies, and are characterized with an assemblage of clinopyroxene, magnetite and orthopyroxene. Two types of iron ores can be identified on the basis of macro- and micro-textures: banded quartz-clinopyroxene (±othopyroxene) -magnetite ores and massive clinopyroxene-magnetite ores. Two-pyroxene geothermometry indicates that the primary counterparts of these ores have undergone metamorphism with a peak temperature of about 762±9°. Both the banded and massive ores have also similarly BIF-like REE+Y features, and thus are proposed to have all formed from chemical sediments. Similarly, clinopyroxenes from both types have BIF-like rare earth element compositions and are rich in Fe (16-23 wt.% FeOtotoal), further suggesting that they are primary Fe-Mg-Ca-rich chemical sediments during metamorphism. Slight enrichments of TiO2, Al2O3, Zr, Hf, Ta and Th of the Wuyang IF suggest relatively low detritus input. The massive ore have magnetite containing V, Cr and Ti much higher than those of the banded ores, suggesting that they may have undergone stronger secondary alteration possibly related to the intrusion of nearby pyroxenite plutons. Different ores have seawater-like REE+Y patterns with LREE depletions and positive anomalies of La, Eu, and Y, showing that granulite facies metamorphism did not essentially modify the primary compositions of the Wuyang IF deposited from paleo-seawater. Our results suggest less than 0.1% contribution from high-temperature hydrothermal fluids.

Dating Kimberlite Eruption and Erosion Phases Using Perovskite, Zircon, and Apatite (U-Th)/He Geochronology to Link Cratonic Lithosphere Evolution and Surface Processes

NASA Astrophysics Data System (ADS)

Stanley, J. R.; Flowers, R. M.

2015-12-01

In many cases it is difficult to evaluate the synchronicity and thus potential connections between disparate geologic events, such as the links between processes in the mantle lithosphere and at the surface. Developing new geochronologic tools and strategies for integrating existing chronologic data with other information is essential for addressing these problems. Here we use (U-Th)/He dating of multiple kimberlitic minerals to date kimberlite eruption and cratonic erosion phases. This approach permits us to more directly assess the link between unroofing and thermomodification of the lithosphere by tying our results to information obtained from mantle-derived clasts in the same pipes. Kimberlites are rich sources of information about the composition of the cratonic lithosphere and its evolution over time. Their xenoliths and xenocrysts can preserve a snapshot of the entire lithosphere and its sedimentary cover at the time of eruption. Accurate geochronology of these eruptions is crucial for interpreting spatiotemporal trends, but kimberlites can be difficult to date using standard techniques. Here we show that the mid-temperature thermochonometers of the zircon and perovskite (U-Th)/He (ZHe, PHe) systems can be viable tools for dating kimberlite eruption. When combined with the low temperature sensitivity of (U-Th)/He in apatite (AHe), the (U-Th)/He system can be used to date both the emplacement and the erosional cooling history of kimberlites. The southern African shield is an ideal location to test the utility of this approach because the region was repeatedly intruded by kimberlites in the Cretaceous, with two major pulses at ~200-110 Ma and ~100-80 Ma. These kimberlites contain a well-studied suite of mantle xenoliths and xenocrysts that document lithospheric heating and metasomatism over this interval. Our ZHe and PHe dates overlap with published eruption ages and add new ages for undated pipes. Our AHe dates constrain the spatial patterns of Cretaceous

Application of multi-dimensional discrimination diagrams and probability calculations to Paleoproterozoic acid rocks from Brazilian cratons and provinces to infer tectonic settings

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.; Oliveira, Elson P.

2013-08-01

In present work, we applied two sets of new multi-dimensional geochemical diagrams (Verma et al., 2013) obtained from linear discriminant analysis (LDA) of natural logarithm-transformed ratios of major elements and immobile major and trace elements in acid magmas to decipher plate tectonic settings and corresponding probability estimates for Paleoproterozoic rocks from Amazonian craton, São Francisco craton, São Luís craton, and Borborema province of Brazil. The robustness of LDA minimizes the effects of petrogenetic processes and maximizes the separation among the different tectonic groups. The probability based boundaries further provide a better objective statistical method in comparison to the commonly used subjective method of determining the boundaries by eye judgment. The use of readjusted major element data to 100% on an anhydrous basis from SINCLAS computer program, also helps to minimize the effects of post-emplacement compositional changes and analytical errors on these tectonic discrimination diagrams. Fifteen case studies of acid suites highlighted the application of these diagrams and probability calculations. The first case study on Jamon and Musa granites, Carajás area (Central Amazonian Province, Amazonian craton) shows a collision setting (previously thought anorogenic). A collision setting was clearly inferred for Bom Jardim granite, Xingú area (Central Amazonian Province, Amazonian craton) The third case study on Older São Jorge, Younger São Jorge and Maloquinha granites Tapajós area (Ventuari-Tapajós Province, Amazonian craton) indicated a within-plate setting (previously transitional between volcanic arc and within-plate). We also recognized a within-plate setting for the next three case studies on Aripuanã and Teles Pires granites (SW Amazonian craton), and Pitinga area granites (Mapuera Suite, NW Amazonian craton), which were all previously suggested to have been emplaced in post-collision to within-plate settings. The seventh case

Chemical transfers along slowly eroding catenas developed on granitic cratons in southern Africa

USGS Publications Warehouse

Khomo, Lesego; Bern, Carleton R.; Hartshorn, Anthony S.; Rogers, Kevin H.; Chadwick, Oliver A.

2013-01-01

A catena is a series of distinct but co-evolving soils arrayed along a slope. On low-slope, slowly eroding catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~ 5 mm ky− 1) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr− 1) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr− 1) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscapes.

Phanerozoic burial and unroofing history of the western Slave craton and Wopmay orogen from apatite (U-Th)/He thermochronometry

NASA Astrophysics Data System (ADS)

Ault, Alexis K.; Flowers, Rebecca M.; Bowring, Samuel A.

2009-06-01

Low temperature thermochronometry of cratonic regions can illuminate relationships among burial and unroofing patterns, surface subsidence and uplift, and lithosphere-asthenosphere interactions. The Slave craton, initially stabilized by the development of a thick lithospheric mantle root in late Archean time, is an excellent location in which to examine these connections. Although the Slave craton currently lacks Phanerozoic cover, Phanerozoic sedimentary xenoliths entrained in ca. 610 to 45 Ma kimberlites indicate that the region underwent a more dynamic history of burial and unroofing than widely recognized. We report new apatite (U-Th)/He thermochronometry data along a southeast to northwest transect from the interior of the Slave craton into the adjacent Paleoproterozoic Wopmay orogen to resolve the region's depositional and denudational history. Six samples from the western Slave craton and three samples from Wopmay orogen yield mean dates from 296 ± 41 Ma to 212 ± 39 Ma. Individual apatite dates are broadly uniform over a wide span of apatite [eU], and this pattern can be used to more tightly restrict the spectrum of viable temperature-time paths that can explain the dataset. When coupled with geologic and stratigraphic information, temperature-time simulations of the thermochronometry results suggest complete He loss from the apatites at minimum peak temperatures of ~ 88 °C in Devonian-Pennsylvanian time, cooling to near-surface conditions by the Early Cretaceous, followed by reheating to ≤ 72 °C during Cretaceous-Early Tertiary time. Consideration of modern and ancient geotherm constraints implies ≥ 3.3 km of burial during the first Phanerozoic heating phase, with an ancillary phase of reburial in late Mesozoic-Cenozoic time. The uniformity of the apatite (U-Th)/He dates indicates that the rocks encompassed by our > 250 km-long sample transect experienced similar Phanerozoic thermal histories. Despite the distinctly different lithospheric

Reprocessing Seismic Data - Using Wits Seismic Exploration Data to Image the Karoo Basin

NASA Astrophysics Data System (ADS)

Webb, S. J.; Scheiber-Enslin, S. E.; Manzi, M. S.

2016-12-01

During the heyday of seismic exploration of the Witwatersrand Basin, Anglo American's Gold Division acquired several thousand kilometres of Vibroseis reflection seismic data. These data, acquired from 1983-1994, were collected with the goal of finding extensions to the Witwatersrand Basin. In a prescient move, over 500 line kilometres were collected at 16 s two way travel time (TWT), extending to depths of 50 -70 km and have provided critical insight into the formation of the Kaapvaal Craton. In addition to these deep seismic lines, Anglo American acquired an extensive network of heretofore unpublished seismic lines that were collected at 6 sec TWT extending well beyond the known limits of the Witwatersrand Basin. The South African government as part of the national geophysical program in the late 1980s acquired six research reflection seismic lines in varied geological settings accruing another 700 km of data. Many of these data are now hosted at the University of the Witwatersrand's newly established Seismic Research Centre and represent unprecedented coverage and research opportunities. With recent global interest in shale gas, attention focused on the Karoo Basin in South Africa. Early exploration seismic data acquired by Soekor in the 1970s has been lost; however, digitized paper records indicate clear reflection targets. Here we examine one of the AngloGold seismic lines that was acquired in the middle of the Karoo Basin just south of Trompsburg extending to the southeast towards Molteno. This 150 km long line crosses the edge of the Kaapvaal Craton and shows clear reflectors throughout the Karoo Basin. These include the well-defined base of the Karoo and a number of dolerite sills within it. Nearby gas escape structures have been identified on surface and it is likely that several disruptions along this line are related to these or to dykes associated with the sills.

Earthquake triggering in southeast Africa following the 2012 Indian Ocean earthquake

NASA Astrophysics Data System (ADS)

Neves, Miguel; Custódio, Susana; Peng, Zhigang; Ayorinde, Adebayo

2018-02-01

In this paper we present evidence of earthquake dynamic triggering in southeast Africa. We analysed seismic waveforms recorded at 53 broad-band and short-period stations in order to identify possible increases in the rate of microearthquakes and tremor due to the passage of teleseismic waves generated by the Mw8.6 2012 Indian Ocean earthquake. We found evidence of triggered local earthquakes and no evidence of triggered tremor in the region. We assessed the statistical significance of the increase in the number of local earthquakes using β-statistics. Statistically significant dynamic triggering of local earthquakes was observed at 7 out of the 53 analysed stations. Two of these stations are located in the northeast coast of Madagascar and the other five stations are located in the Kaapvaal Craton, southern Africa. We found no evidence of dynamically triggered seismic activity in stations located near the structures of the East African Rift System. Hydrothermal activity exists close to the stations that recorded dynamic triggering, however, it also exists near the East African Rift System structures where no triggering was observed. Our results suggest that factors other than solely tectonic regime and geothermalism are needed to explain the mechanisms that underlie earthquake triggering.

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U Pb and TDM ages and host-rock affinity from detrital zircons

NASA Astrophysics Data System (ADS)

Veevers, J. J.; Saeed, A.

2007-12-01

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U-Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a "crustal" model age ( TDMC). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone. Zircons from the Ellisras Basin are aged 2700-2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite-greenstone terrane of the Kaapvaal Craton. The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160-880 Ma, host magma mafic granitoid (< 65% SiO 2) derived from juvenile depleted mantle sources ( ɛHf positive) at 1.65 Ga and 1.35 Ga, with TDMC of 2.0-0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock-carbonatite), derived from mixed crustal and juvenile depleted mantle sources ( ɛHf positive and negative) at 1.50 Ga and 1.35 Ga, with TDMC of 2.0-0.9 Ga. Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ˜ 1000 Ma (Grenville) cratons embedded in 700-500 Ma (Pan

The geological record of base metal sulfides in the cratonic mantle: A microscale 187Os/188Os study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

NASA Astrophysics Data System (ADS)

Bragagni, A.; Luguet, A.; Fonseca, R. O. C.; Pearson, D. G.; Lorand, J.-P.; Nowell, G. M.; Kjarsgaard, B. A.

2017-11-01

We report detailed petrographic investigations along with 187Os/188Os data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. When devoid of alteration, BMS grains mainly consist of pentlandite + pyrrhotite ± chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement. The 187Os/188Os measured in the twenty analysed BMS grains vary from 0.1084 to >0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in 187Os/188Os is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock TRD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution. The TRD ages determined in BMS grains are clustered around 2.8-2.7, ∼2.2 and ∼1.9 Ga. The 2.8-2.7 Ga TRD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic TRD ages can be explained by

Moho Depth Variations in the Northeastern North China Craton Revealed by Receiver Function Imaging

NASA Astrophysics Data System (ADS)

Zhang, P.; Chen, L.; Yao, H.; Fang, L.

2016-12-01

The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of the region. In this study, we used two-year teleseismic receiver function data from the North China Seismic Array consisting of 200 broadband stations deployed in the northeastern NCC to image the Moho undulation of the region. A 2-D wave equation-based poststack depth migration method was employed to construct the structural images along 19 profiles, and a pseudo 3D crustal velocity model of the region based on previous ambient noise tomography and receiver function study was adopted in the migration. We considered both the Ps and PpPs phases, but in some cases we also conducted PpSs+PsPs migration using different back azimuth ranges of the data, and calculated the travel times of all the considered phases to constrain the Moho depths. By combining the structure images along the 19 profiles, we got a high-resolution Moho depth map beneath the northeastern NCC. Our results broadly consist with the results of previous active source studies [http://www.craton.cn/data], and show a good correlation of the Moho depths with geological and tectonic features. Generally, the Moho depths are distinctly different on the opposite sides of the North-South Gravity Lineament. The Moho in the west are deeper than 40 km and shows a rapid uplift from 40 km to 30 km beneath the Taihang Mountain Range in the middle. To the east in the Bohai Bay Basin, the Moho further shallows to 30-26 km depth and undulates by 3 km, coinciding well with the depressions and uplifts inside the basin. The Moho depth beneath the Yin-Yan Mountains in the north gradually decreases from 42 km in the west to 25 km in the east, varying much smoother than that to the south.

A modern analogue for tectonic, eustatic, and climatic processes in cratonic basins: Gulf of Carpentaria, northern Australia

USGS Publications Warehouse

Edgar, N. Terence; Cecil, C. Blaine; Mattick, R.E.; de Deckker, Patrick; Djajadihardja, Yusuf S.

2003-01-01

The Gulf of Carpentaria is a tropical, silled epicontinental sea and may be a modern analogue for ancient cratonic basins. For the purpose of this study, the Gulf of Carpentaria is compared to Pennsylvanian cratonic basins of the United States. During the Pennsylvanian, the North American continent moved from the Southern Hemisphere, through the Equator, into the Northern Hemisphere. Today, the Gulf of Carpentaria–New Guinea region is a few degrees south of the Equator and is moving towards it. During the Pennsylvanian, the world was subjected to major glaciations and associated sea-level changes. The island of New Guinea and the Gulf of Carpentaria have undergone similar processes during the Quaternary. A reconnaissance seismic survey of the gulf conducted by the USGS and the Australian National University (ANU), combined with oil-exploration well data, provided the first step in a systematic evaluation of a modern tropical epicontinental system. During the Cenozoic, the region was dominated by terrestrial sedimentation in a temperate climate. At the same time, carbonates were being deposited on the northern shelf edge of the Australian Plate. During the Miocene, carbonate deposition expanded southward into the gulf region. Then in the Late Miocene, carbonate sedimentation was replaced by terrigenous clastics derived from the developing Central Range of the island of New Guinea, which developed a wetter climate while moving northwards into the tropics. At least 14 basin-wide transgressive–regressive cycles are identified by channels that were eroded under subaerial conditions since about the Miocene. Comparison of the modern Gulf of Carpentaria sequences with those of the Pennsylvanian reveals many similarities.

LA-SF-ICP-MS zircon U-Pb geochronology of granitic rocks from the central Bundelkhand greenstone complex, Bundelkhand craton, India

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.; Verma, Surendra P.; Oliveira, Elson P.; Singh, Vinod K.; Moreno, Juan A.

2016-03-01

The central Bundelkhand greenstone complex in Bundelkhand craton, northern India is one of the well exposed Archaean supracrustal amphibolite, banded iron formation (BIF) and felsic volcanic rocks (FV) and associated with grey and pink porphyritic granite, tonalite-trondhjemite-granodiorite (TTG). Here we present high precision zircon U-Pb geochronological data for the pinkish porphyritic granites and TTG. The zircons from the grey-pinkish porphyritic granite show three different concordia ages of 2531 ± 21 Ma, 2516 ± 38 Ma, and 2514 ± 13 Ma, which are interpreted as the best estimate of the magmatic crystallization age for the studied granites. We also report the concordia age of 2669 ± 7.4 Ma for a trondhjemite gneiss sample, which is so far the youngest U-Pb geochronological data for a TTG rock suite in the Bundelkhand craton. This TTG formation at 2669 Ma is also more similar to Precambrian basement TTG gneisses of the Aravalli Craton of north western India and suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Aravalli craton of the North-western India.

Syntectonic remagnetization in the southern Methow block: Resolving large displacements in the southern Canadian Cordillera

USGS Publications Warehouse

Enkin, R.J.; Mahoney, J.B.; Baker, J.; Kiessling, M.; Haugerud, R.A.

2002-01-01

The Upper Cretaceous Ventura Member of the Goat Wall unit in the southern Methow block of southern British Columbia and northern Washington State holds a syntectonic magnetization. Eight new sites from Manning Park in British Columbia give a mean direction of D = 27.5??, I = 60.1??, k = 304.7, ??95 = 3.2?? after optimal partial tilt correction. Of five groups of bedded sites from farther south in the basin reported by Bazard et al. [1990], four have a syntectonic remanence with a direction similar to what we observe. The exception is one group which has optimal concentration of remanence directions on >100% untilting and an abherent direction which must be rejected. Combining the accepted sites, the optimal differential syntilting direction is D = 11.8??, I = 61.5??, k = 39.3, ??95 = 3.4?? (N = 47), giving a mean pole of 79.8??N, 359.2??E, K = 19.5, and A95 = 4.8??. The age of the remagnetization is constrained to be between 88 and 80 Ma. Compared to cratonic North America, this result indicates that the southern Methow block was displaced from the south by 1800 ?? 500 km, meaning it lay south of the Sierra Nevada subduction zone but well north of other paleomagnetically constrained Cretaceous rock units from the Insular superterrane, including correlative strata of the Mount Tatlow area in the northern Methow block. Among several possibilities to reconcile this discrepancy, the most plausible has the whole Methow block translated coherently but with the southern Methow block strata remagnetized during transit.

Mesozoic crustal thickening of the eastern North China craton: Evidence from eclogite xenoliths and petrologic implications

NASA Astrophysics Data System (ADS)

Xu, Wenliang; Gao, Shan; Wang, Qinghai; Wang, Dongyan; Liu, Yongsheng

2006-09-01

A suite of xenoliths of eclogite, garnet clinopyroxenite, and felsic gneiss is found in Early Cretaceous high-Mg [Mg# >45, where Mg# = molar 100 × Mg/(Mg + Fetotal)] adakitic intrusions from the Xuzhou-Huaibei (Xu-Huai) region along the southeastern margin of the North China craton. The primary mineral assemblage of garnet + omphacite/augite + quartz + rutile ± pargasite of the eclogite and garnet clinopyroxenite xenoliths defines a minimum pressure of >1.5 GPa, while the estimated peak metamorphic temperatures range from 800 to 1060 °C. An Sm-Nd whole-rock garnet isochron and zircon U-Pb dates show that timing of the eclogite facies metamorphism took place ca. 220 Ma. This Triassic age agrees with the age of eclogites from the Dabie-Sulu ultrahigh-pressure metamorphic (UHPM) belt. The ages of abundant Late Archean to early Paleoproterozoic (2.3 2.6Ga) inherited zircons correspond to the most prominent crustal growth event in the North China craton. In addition, these xenoliths and their host high-Mg adakitic intrusions have complementary major and trace element compositions, suggesting that the adakites formed by partial melting of Archean metabasalts that were the protoliths of the Xu-Huai eclogite and garnet clinopyroxenite xenoliths. Trace element and Sr-Nd isotopic modeling shows that the high-Mg adakitic intrusions can be modeled as melts from ˜40% partial melting of the metabasalts in the eclogite facies, followed by interaction with the convecting mantle and variable degrees of crustal assimilation. Together with the similar zircon age populations between the xenoliths and the host rocks, these lines of evidence strongly suggest their genetic link via thickening, foundering, and partial melting of the Archean North China craton mafic lower crust, followed by adakitic melt-mantle interaction. The crustal thickening resulted from Triassic collision between the Yangtze craton and the North China craton, which produced the Dabie-Sulu UHPM belt in the

Lithospheric structure, composition, and thermal regime of the East European Craton: Implications for the subsidence of the Russian platform

USGS Publications Warehouse

Artemieva, I.M.

2003-01-01

A new mechanism for Paleozoic subsidence of the Russian, or East European, platform is suggested, since a model of lithosphere tilting during the Uralian subduction does not explain the post-Uralian sedimentation record. Alternatively, I propose that the Proterozoic and Paleozoic rifting (when a platform-scale Central Russia rift system and a set of Paleozoic rifts were formed) modified the structure and composition of cratonic lithosphere, and these tectono-magmatic events are responsible for the post-Uralian subsidence of the Russian platform. To support this hypothesis, (a) the thermal regime and the thickness of the lithosphere are analyzed, and (b) lithospheric density variations of non-thermal origin are calculated from free-board constraints. The results indicate that Proterozoic and Paleozoic rifting had different effects on the lithospheric structure and composition. (1) Proterozoic rifting is not reflected in the present thermal regime and did not cause significant lithosphere thinning (most of the Russian platform has lithospheric thickness of 150-180 km and the lithosphere of the NE Baltic Shield is 250-300 km thick). Paleozoic rifting resulted in pronounced lithospheric thinning (to 120-140 km) in the southern parts of the Russian platform. (2) Lithospheric density anomalies suggest that Proterozoic-Paleozoic rifting played an important role in the platform subsidence. The lithospheric mantle of the Archean-early Proterozoic part of the Baltic Shield is ??? 1.4 ?? 0.2% less dense than the typical Phanerozoic upper mantle. However, the density deficit in the subcrustal lithosphere of most of the Russian platform is only about (0.4-0.8) ?? 0.2% and decreases southwards to ???0%. Increased densities (likely associated with low depletion values) in the Russian platform suggest strong metasomatism of the cratonic lithosphere during rifting events, which led to its subsidence. It is proposed that only the lower part of the cratonic lithosphere was

Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain

NASA Astrophysics Data System (ADS)

Miller, Meghan S.; O'Driscoll, Leland J.; Butcher, Amber J.; Thomas, Christine

2015-12-01

The westernmost Mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the Neo-Tethys Ocean, continental collision of Africa and Eurasia, and the Canary Island mantle plume. This study utilizes S receiver functions (SRFs) from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern Spain through Morocco and the Canary Islands. The lithospheric thickness ranges from ∼65 km beneath the Atlas Mountains and the active volcanic islands to over ∼210 km beneath the cratonic lithosphere in southern Morocco. The common conversion point (CCP) volume of the SRFs indicates that thinned lithosphere extends from beneath the Canary Islands offshore southwestern Morocco, to beneath the continental lithosphere of the Atlas Mountains, and then thickens abruptly at the West African craton. Beneath thin lithosphere between the Canary hot spot and southern Spain, including below the Atlas Mountains and the Alboran Sea, there are distinct pockets of low velocity material, as inferred from high amplitude positive, sub-lithospheric conversions in the SRFs. These regions of low seismic velocity at the base of the lithosphere extend beneath the areas of Pliocene-Quaternary magmatism, which has been linked to a Canary hotspot source via geochemical signatures. However, we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses. We propose that the low velocity structure beneath the lithosphere is material flowing sub-horizontally northeastwards beneath Morocco from the tilted Canary Island plume, and the small, localized volcanoes are the result of small-scale upwellings from this material.

Composition of the lithospheric mantle in the northern part of Siberian craton: Constraints from peridotites in the Obnazhennaya kimberlite

NASA Astrophysics Data System (ADS)

Sun, Jing; Liu, Chuan-Zhou; Kostrovisky, Sergey I.; Wu, Fu-Yuan; Yang, Jin-Hui; Chu, Zhu-Yin; Yang, Yue-Heng; Kalashnikova, Tatiana; Fan, Sheng

2017-12-01

The character of the lithospheric mantle of the northern Siberian craton is not well established; nearly all published data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report major elements of the whole rock, trace elements data of clinopyroxene and Re-Os isotope and PGE concentration of mantle xenoliths from the Obnazhennaya kimberlite pipe (160 Ma) in the northern part of Siberian craton. The Obnazhennaya mantle xenoliths include spinel harzburgites, spinel dunites, spinel lherzolites and spinel-garnet lherzolite. The spinel harzburgites and dunites have refractory compositions, with 0.23-1.35 wt% Al2O3, 0.41-3.11 wt% CaO and 0.00-0.09 wt% TiO2, whereas the lherzolites (both spinel- and spinel-garnet-) have more fertile compositions, containing 2.16-6.55 wt% Al2O3, 2.91-7.55 wt% CaO and 0.04-0.15 wt% TiO2. The trace element compositions and mineralogical textures of the Obnazhennaya xenoliths indicate the occurrence of metasomatic enrichments, including carbonatite melts, basaltic melts from Siberian Trap and kimberlitic melts. The spinel harzburgites and dunites have 187Os/188Os of 0.11227-0.11637, giving a TRD age of 1.6-2.2 Ga. This suggests that old cratonic mantle still existed beneath the Obnazhennaya. In contrast, both spinel and spinel-garnet lherzolites have more radiogenic 187Os/188Os ratios (0.11931-0.17627), enriched P-PGEs. But the higher Al2O3 and Os character of these lherzolites suggest that they were not juvenile mantle but the refertilized ancient mantle. Therefore, our results suggest that the cratonic mantle beneath the northern part of Siberian craton contain both ancient and reworked lithospheric mantle, and the metasomatism may not be effective at overprinting/eroding the pre-existing lithosphere.

Southern Appalachian Regional Seismic Network

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chiu, S.C.C.; Johnston, A.C.; Chiu, J.M.

1994-08-01

The seismic activity in the southern Appalachian area was monitored by the Southern Appalachian Regional Seismic Network (SARSN) since late 1979 by the Center for Earthquake Research and Information (CERI) at Memphis State University. This network provides good spatial coverage for earthquake locations especially in east Tennessee. The level of activity concentrates more heavily in the Valley and Ridge province of eastern Tennessee, as opposed to the Blue Ridge or Inner Piedmont. The large majority of these events lie between New York - Alabama lineament and the Clingman/Ocoee lineament, magnetic anomalies produced by deep-seated basement structures. Therefore SARSN, even withmore » its wide station spacing, has been able to define the essential first-order seismological characteristics of the Southern Appalachian seismic zone. The focal depths of the southeastern U.S. earthquakes concentrate between 8 and 16 km, occurring principally beneath the Appalachian overthrust. In cross-sectional views, the average seismicity is shallower to the east beneath the Blue Ridge and Piedmont provinces and deeper to the west beneath the Valley and Ridge and the North American craton. Results of recent focal mechanism studies by using the CERI digital earthquake catalog between October, 1986 and December, 1991, indicate that the basement of the Valley and Ridge province is under a horizontal, NE-SW compressive stress. Right-lateral strike-slip faulting on nearly north-south fault planes is preferred because it agrees with the trend of the regional magnetic anomaly pattern.« less

Crustal development of the North China Craton constrained by geochemical and isotopic data on Neoarchean and Paleoproterozoic granitoids, Inner Mongolia

NASA Astrophysics Data System (ADS)

Hui-Chun Chen, Nancy; Zhao, Guochun; Cawood, Peter A.

2017-04-01

The North China Craton is the oldest continental fragment in China. It contains magmatic rocks as old as 3.8 Ga, but is dominated by crustal components that formed in the Neoarchean at ca. 2.7 and 2.5 Ga, and also includes Paleoproterozoic rocks dated at 1.9-1.8 Ga. The craton has been incorporated into Precambrian supercontinents, although it's exact position within, as well as the overall configuration of, these supercontinents is poorly understood. New geochemical and geochronological data on granitoids from the northern margin of the craton at Siziwangqi in central Inner Mongolia further constrain craton evolution with respect to Neoarchean and Paleoproterozoic supercontinent cycles. The granitoids comprise a tonalite-trondhjemite-granodiorite (TTG) association with crystallization ages of 2.52-2.49 Ga and inherited zircon crystals as old 2.7 Ga, and alkali feldspar granites with ages of 2.47 and 1.87 Ga. Geochemically, the rocks are metaluminous to peraluminous and belong to the calc-alkaline (TTG) and subalkaline to alkaline (alkali feldspar granite) series. The TTG granitoids are characterized by light LREE enrichment, a weak positive Eu anomaly, and flat heavy HREE profiles. The alkali granite is also enriched in the LREE but has a strong positive chondrite-normalized Eu anomaly and displays weak HREE enrichment. Our compositional and geochronological data, integrated with regional data, indicate that in the Neoarchean the North China Craton constituted part of an accretionary convergent plate margin that lay on the edge of a an older continental mass (possibly within the Kenor supercraton). The Paleoproterozoic alkali feldspar granite was associated with collisional assembly of the craton into the Nuna (Columbia) supercontinent.

Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: What Is the Message?

NASA Astrophysics Data System (ADS)

El Atrassi, F.; Debaille, V.; Mattielli, N. D. C.; Berger, J.

2014-12-01

While Archean terrains are mainly composed of a TTG (Tonalite-trondhjemite-granodiorite) suite, more mafic lithologies such as amphibolites are also a typical component of those ancient terrains. Although mafic rocks represent only ~10% of the Archean cratons, they may provide key evidence of the role and nature of basaltic magmatism in the formation of the Archean crust as well as the evolution of the Archean mantle. This study focuses on the Archean crust from the West African Craton in Mauritania (Amsaga area). The Amsaga Archean Crust mainly consists of TTG and thrust-imbricated slices of mafic volcanic rocks, which have been affected by polymetamorphic events from the amphibolite to granulite facies. Our main objectives aim to the identification of the mafic lithology origin and a better understanding of their role in the continental crust emplacement. Our petrological observations show that these amphibolites have fine to medium granoblastic and nematoblastic textures. The amphibolites are dominated by amphibolite-facies mineral assemblages (mainly amphibole and plagioclase), but garnet and clinopyroxene occur in a few samples. Two groups are distinct in their geochemical characteristics (major and trace elements), although both have tholeiitic basalt composition. The first group show LREE-enriched patterns and negative Nb-Ta anomalies. The second group is characterized by near-flat LREE patterns and flat HREE patterns. This second group clearly shows no Nb-Ta anomalies. The first group could be related to arc-like basalts, as it is many similarities with some Archean amphibolites probably formed in a supra-subduction zone, for instance the volcanic rocks from the southern edge of the Isua Supracrustal Belt. On the contrary, the second group has a MORB-like signature which is more unusual during the Archean. Different scenarios will be discussed regards to the Archean geodynamics.

Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation

NASA Astrophysics Data System (ADS)

Tappe, Sebastian; Romer, Rolf L.; Stracke, Andreas; Steenfelt, Agnete; Smart, Katie A.; Muehlenbachs, Karlis; Torsvik, Trond H.

2017-05-01

Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr-Nd-Hf-Pb-Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative ΔεHf coupled with > + 5 ‰ δ7Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05-3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (-3.6 to - 2.4 ‰ δ13C for carbonatites versus -5.7 to - 3.6 ‰ δ13C for kimberlites) require open-system fractionation at magmatic temperatures. Given that the oxidation state of Earth's mantle at >150 km depth is too reduced to enable larger volumes of 'pure' carbonate melt to migrate, it is reasonable to speculate that percolating near-solidus melts of carbonated peridotite must be silicate-dominated with only dilute carbonate contents, similar to the Tikiusaaq kimberlite compositions (e.g., 16-33 wt.% SiO2). This concept is supported by our findings from the North Atlantic craton where kimberlite and other deeply derived carbonated silicate melts, such as aillikites, exsolve their carbonate components within the shallow lithosphere en route to the Earth's surface, thereby producing carbonatite magmas. The relative abundances of trace elements of such highly

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

NASA Astrophysics Data System (ADS)

Jian, Ping; Kröner, Alfred; Shi, Yuruo; Zhang, Wei; Liu, Yaran; Windley, Brian F.; Jahn, Bor-ming; Zhang, Liqao; Liu, Dunyi

2016-06-01

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction-accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431-99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449-113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436-242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605-741 Ma; n = 40) exhibits a prominent age peak at 2520 Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800 Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone-forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian-Triassic composite granitoid-gabbroic pluton (8-20 km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean-continent convergent margin.

Subduction beneath Laurentia modified the eastern North American cratonic edge: Evidence from P wave and S wave tomography

NASA Astrophysics Data System (ADS)

Boyce, A.; Bastow, I. D.; Darbyshire, F. A.; Ellwood, A. G.; Gilligan, A.; Levin, V.; Menke, W.

2016-07-01

The cratonic cores of the continents are remarkably stable and long-lived features. Their ability to resist destructive tectonic processes is associated with their thick (˜250 km), cold, chemically depleted, buoyant lithospheric keels that isolate the cratons from the convecting mantle. The formation mechanism and tectonic stability of cratonic keels remains under debate. To address this issue, we use P wave and S wave relative arrival-time tomography to constrain upper mantle structure beneath southeast Canada and the northeast USA, a region spanning three quarters of Earth's geological history. Our models show three distinct, broad zones: Seismic wave speeds increase systematically from the Phanerozoic coastal domains, through the Proterozoic Grenville Province, and to the Archean Superior craton in central Québec. We also recover the NW-SE trending track of the Great Meteor hot spot that crosscuts the major tectonic domains. The decrease in seismic wave speed from Archean to Proterozoic domains across the Grenville Front is consistent with predictions from models of two-stage keel formation, supporting the idea that keel growth may not have been restricted to Archean times. However, while crustal structure studies suggest that Archean Superior material underlies Grenvillian age rocks up to ˜300 km SE of the Grenville Front, our tomographic models show a near-vertical boundary in mantle wave speed directly beneath the Grenville Front. We interpret this as evidence for subduction-driven metasomatic enrichment of the Laurentian cratonic margin, prior to keel stabilization. Variable chemical depletion levels across Archean-Proterozoic boundaries worldwide may thus be better explained by metasomatic enrichment than inherently less depleted Proterozoic composition at formation.

The Cambrian-Ordovician rocks of Sonora, Mexico, and southern Arizona, southwestern margin of North America (Laurentia): chapter 35

USGS Publications Warehouse

Page, William R.; Harris, Alta C.; Repetski, John E.; Derby, James R.; Fritz, R.D.; Longacre, S.A.; Morgan, W.A.; Sternbach, C.A.

2013-01-01

The most complete sections of Ordovician shelf rocks in Sonora are 50 km (31 mi) northwast of Hermosillo. In these sections, the Lower Ordovician is characterized by intraclastic limestone, siltstone, shale, and chert. The Middle Ordovician is mostly silty limestone and quartzite, and the Upper Ordovician is cherty limestone and some argillaceous limestone. A major disconformity separates the Middle Ordovician quartzite from the overlying Upper Ordovician carbonate rocks and is similar to the disconformity between the Middle and Upper Ordovician Eureka Quartzite and Upper Ordovician Ely Springs Dolomite in Nevada and California. In parts of northwestern Sonora, Ordovician rocks are disconformably overlain by Upper Silurain rocks. Northeastward in Sonora and Arizona, toward the craton, Ordovician rocks are progressively truncated by a major onlap unconformity and are overliand by Devonian rocks. Except in local area, Ordovician rocks are generally absent in cratonic platform sequences in northern Sonora and southern Arizona.

Accretionary Tectonics of Rock Complexes in the Western Margin of the Siberian Craton

NASA Astrophysics Data System (ADS)

Likhanov, I. I.; Nozhkin, A. D.; Savko, K. A.

2018-01-01

The geological, geochemical, and isotope-geochronological evidence of the events at the final stage of the Neoproterozoic history of the Yenisei Range is considered (beginning from the formation of fragments of the oceanic crust in the region and their accretion to the Siberian Craton until the postaccretionary stage of crustal tension and onset of the Caledonian orogeny). Based on an analysis of new data on the petrogeochemical composition, age, and geodynamic nature of the formation of contrasting rocks in the composition of tectonic mélange of the Near-Yenisei (Prieniseiskaya) regional shear zone, we have found the chronological sequence of events that marks the early stages of the Paleoasian Ocean evolution in the zone of its junction with the Siberian Craton. These events are documented by the continental marginal, ophiolitic, and island-arc geological complexes, each of which has different geochemical features. The most ancient structures are represented by fragments of oceanic crust and island arcs from the Isakovka terrane (700-620 Ma). The age of glaucophane-schist metamorphic units that formed in the paleosubduction zone corresponds to the time interval of 640-620 Ma. The formation of high-pressure tectonites in the suture zone, about 600 Ma in age, marks the finishing stage of accretion of the Isakovka block to the western margin of the Siberian Craton. The final events in the early history of the Asian Paleoocean were related to the formation of Late Vendian riftogenic amygdaloidal basalts (572 ± 6.5 Ma) and intrusion of postcollisional leucogranites of the Osinovka massif (550-540 Ma), which intruded earlier fragments of the oceanic crust in the Isakovka terrane. These data allow us to refine the Late Precambrian stratigraphic scheme in the northwestern Trans-Angarian part of the Yenisei Range and the evolutionary features of the Sayan-Yenisei accretionary belt. The revealed Late Neoproterozoic landmarks of the evolution of the Isakovka terrane are

Paleomagnetism of Neoproterozoic cap carbonates of the Sao Francisco and Amazonian cratons, Brazil

NASA Astrophysics Data System (ADS)

Trindade, R. I. F.; Dagrella-Filho, M. S.; Figueiredo, F. T.; Font, E.; Babinski, M.; Nogueira, A. C. R.; Riccomini, C.

2003-04-01

The low-latitude paleomagnetic record of some Neoproterozoic glacial successions has long been used as an evidence for dramatic changes in Earth climate (Snowball Earth events). But the test for such events demands a global-scale paleomagnetic database in the Neoproterozoic glacial/carbonate successions. In order to better constrain these extreme climatic events, an extensive paleomagnetic survey has been conducted in carbonates that cap the Neoproterozoic glacial rocks of the Sao Francisco (SFC) and Amazonian (AMC) cratons, Brazil. In the SFC, sampling was performed at 104 sites (horizontal beds) distributed at two sectors (north and south), about 1.000 km apart, within the stable area of the craton. In the AMC, sampling included 15 sites in undeformed, sub-horizontal beds, and 25 sites along the limbs of regional folds in the margin of the craton. After paleomagnetic cleaning, most samples from both SFC and AMC yielded similar PGVs (mean SFC: 32°N, 322°E (A95=2.8, K=65.3); mean AMC: 31°N, 336°E (A95=6.8, K=46.2)). Pb-Pb ages around 520 Ma for SFC samples, a negative fold-test for AMC samples, and the coincidence of these poles with 520 Ma Gondwanan reference poles after rotation of South America to Africa, suggest a continental-scale remagnetization by this time. Some sites from both cratons, however, seem to have not been affected by the Cambrian remagnetization. Two mean poles were calculated for SFC and AMC for which consistent reversals could be recorded, suggesting a primary nature for the characteristic magnetization (mean SFC pole: 66°S, 198°E (A95=5.4, K=47.5); mean AMC pole: 77°S, 66°E (A95=8.8, K=47.7)). These results indicate a moderate latitude (51°) for the SFC Bebedouro (Sturtian) glaciation and a low latitude (24°) for the AMC Puga (Varanger) glaciation, and reinforces the hypothesis of Neoproterozoic global-scale ice-ages.

The Kimberlites and related rocks of the Kuruman Kimberlite Province, Kaapvaal Craton, South Africa

NASA Astrophysics Data System (ADS)

Donnelly, Cara L.; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.; Shee, Simon R.

2011-03-01

The Kuruman Kimberlite Province is comprised of 16 small pipes and dikes and contains some of the oldest known kimberlites (>1.6 Ga). In this study, 12 intrusions are subdivided into three groups with distinct petrology, age, and geochemical and isotopic compositions: (1) kimberlites with groundmass perovskites defining a Pb-Pb isochron age of 1787 ± 69 Ma, (2) orangeite with a U-Pb perovskite age of 124 ± 16 Ma, and (3) ultramafic lamprophyres (aillikite and mela-aillikite) with a zircon U-Pb age of 1642 ± 46 Ma. The magma type varies across the Province, with kimberlites in the east, lamprophyres in the west and orangeite and ultramafic lamprophyres to the south. Differences in the age and petrogenesis of the X007 orangeite and Clarksdale and Aalwynkop aillikites suggest that these intrusions are probably unrelated to the Kuruman Province. Kimberlite and orangeite whole-rock major and trace element compositions are similar to other South African localities. Compositionally, the aillikites typically lie off kimberlite and orangeite trends. Groundmass mineral chemistry of the kimberlites has some features more typical of orangeites. Kimberlite whole-rock Sr and Nd isotopes show zoning across the Province. When the kimberlites erupted at ~1.8 Ga, they sampled a core volume (ca 50 km across) of relatively depleted SCLM that was partially surrounded by a rim of more metasomatized mantle. This zonation may have been related to the development of the adjacent Kheis Belt (oldest rocks ~2.0 Ga), as weaker zones surrounding the more resistant core section of SCLM were more extensively metasomatized.

Mantle heat flow and thermal structure of the northern block of Southern Granulite Terrain, India

NASA Astrophysics Data System (ADS)

Manglik, Ajay

2006-07-01

Continental shield regions are normally characterized by low-to-moderate mantle heat flow. Archaean Dharwar craton of the Indian continental shield also follows the similar global pattern. However, some recent studies have inferred significantly higher mantle heat flow for the Proterozoic northern block of Southern Granulite Terrain (SGT) in the immediate vicinity of the Dharwar craton by assuming that the radiogenic elements depleted exposed granulites constitute the 45-km-thick crust. In this study, we use four-layered model of the crustal structure revealed by integrated geophysical studies along a geo-transect in this region to estimate the mantle heat flow. The results indicate that: (i) the mantle heat flow of the northern block of SGT is 17 ± 2 mW/m 2, supporting the global pattern, and (ii) the lateral variability of 10-12 mW/m 2 in the surface heat flow within the block is of crustal origin. In terms of temperature, the Moho beneath the eastern Salem-Namakkal region appears to be at 80-100 °C higher temperature than that beneath the western Avinashi region.

Crustal architecture and tectonic evolution of the Cauvery Suture Zone, southern India

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Yellappa, T.; Santosh, M.

2016-11-01

The Cauvery suture zone (CSZ) in southern India has witnessed multiple deformations associated with multiple subduction-collision history, with incorporation of the related accretionary belts sequentially into the southern continental margin of the Archaean Dharwar craton since Neoarchean to Neoproterozoic. The accreted tectonic elements include suprasubduction complexes of arc magmatic sequences, high-grade supracrustals, thrust duplexes, ophiolites, and younger intrusions that are dispersed along the suture. The intra-oceanic Neoarchean-Neoproterozoic arc assemblages are well exposed in the form of tectonic mélanges dominantly towards the eastern sector of the CSZ and are typically subjected to complex and multiple deformation events. Multi-scale analysis of structural elements with detailed geological mapping of the sub-regions and their structural cross sections, geochemical and geochronological data and integrated geophysical observations suggest that the CSZ is an important zone that preserves the imprints of multiple cycles of Precambrian plate tectonic regimes.

An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.

The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons frommore » several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.« less

Accretion of Grenvillian terranes to the southwestern border of the Río de la Plata craton, western Argentina

NASA Astrophysics Data System (ADS)

Varela, Ricardo; Basei, Miguel A. S.; González, Pablo D.; Sato, Ana M.; Naipauer, Maximiliano; Campos Neto, Mario; Cingolani, Carlos A.; Meira, Vinicius T.

2011-04-01

A comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Río de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic-ultramafic rocks of 1,244-1,027 Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846-570 Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640-580 Ma) and Cambro-Ordovician (~550-470 Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480-450 Ma), late phase (440-420 Ma), and Chanic phase (400-360 Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt.

Chemical composition of the continental crust: Insights from a quantitative interpretation of the Vp/Vs ratio

NASA Astrophysics Data System (ADS)

Guerri, M.; Youssof, M.; Fullea, J.

2017-12-01

The processes driving continental crust formation are not yet fully understood. One of the fundamental keys necessary to investigate the enigma is represented by crustal composition. The Vp/Vs ratio from seismic receiver functions or tomography studies is a powerful tool to constrain the crustal composition. However, to date only qualitative relationships between Vp/Vs and composition have been proposed. We present a quantitative interpretation of the Vp/Vs in terms of major oxide components, based on thermo-elastic constrained modelling of rock phase equilibria and physical properties. The geophysical-petrological approach is implemented in the new release of the software package LitMod, which now allows for integrated and self-consistent modeling of the entire lithosphere (crust + lithospheric mantle) and upper mantle. Forward modelling of the Vp/Vs, based on petrology and thermodynamics, reveals that, as expected, mafic compositions have higher Vp/Vs than felsic ones. However, in high temperature settings (surface heat flow > 75 mW/m2), the quartz alpha / quartz beta transition strongly increases Vp, leaving Vs almost unaltered, leading to SiO2-rich compositions displaying Vp/Vs values higher than those associated with mafic compositions. Additionally, we highlight the importance of H2O, the presence of which stabilizes amphibole (in place of pyroxene), characterized by a relatively low Vp/Vs. If H2O is present, mafic compositions show Vp/Vs ratios that are comparable to those produced by anhydrous SiO2-rich compositions. The destabilization of amphibole (in favour of pyroxene) generates a sharp seismic discontinuity, potentially detectable by, for example, seismic refraction and receiver function investigations. We invert the Vp/Vs ratio for composition and hydrous state of the crust in the Southern African cratons. Our results show that the Kaapvaal craton, Archean in age, has an intermediate (SiO2 60 wt%) composition. The finding has implications on our

Ichnologic evidence of a Cambrian age in the southern Amazon Craton: Implications for the onset of the Western Gondwana history

NASA Astrophysics Data System (ADS)

Santos, Hudson P.; Mángano, M. Gabriela; Soares, Joelson L.; Nogueira, Afonso C. R.; Bandeira, José; Rudnitzki, Isaac D.

2017-07-01

Colonization of the infaunal ecospace by burrowing bilaterians is one of the most important behavioral innovations during the Ediacaran-Cambrian transition. The establishment of vertical burrows by suspension feeders in high-energy nearshore settings during Cambrian Age 2 is reflected by the appearance of the Skolithos Ichnofacies. For the first time, unquestionable vertical burrows typical of the Skolithos Ichnofacies, such as Skolithos linearis, Diplocraterion parallelum and Arenicolites isp., are recorded from nearshore siliciclastic deposits of the Raizama Formation, southeastern Amazon Craton, Brazil. Integration of ichnologic and sedimentologic datasets suggests that these trace fossils record colonization of high-energy and well-oxygenated nearshore sandy environments. Chronostratigraphically, the presence of these vertical burrows indicates an age not older than early Cambrian for the Raizama Formation, which traditionally has been regarded as Ediacaran. Therefore, the Raizama ichnofauna illustrates the advent of modern Phanerozoic ecology marked by the Agronomic Revolution. The discovery of the Skolithos Ichnofacies in these shallow-marine strata suggests possible connections between some central Western Gondwana basins.

Cenozoic extension along the reactivated Aurora Fault System in the East Antarctic Craton

NASA Astrophysics Data System (ADS)

Cianfarra, Paola; Maggi, Matteo

2017-04-01

The East Antarctic Craton is characterized by major intracontinental basins and highlands buried under the 34 Ma East Antarctic Ice Sheet. Their formation remains a major open question. Paleozoic to Cenozoic intraplate extensional tectonic activity has been proposed for their development and in this work the latter hypothesis is supported. Here we focus on the Aurora Trench (AT) within the Aurora Subglacial Basin (latitude 75°-77°S, longitude 117°-118°E) whose origin is still poorly constrained. The AT is an over 150-km-long, 25-km-wide subglacial trough, elongated in the NNW-SSE direction. Geophysical campaigns allowed better definition of the AT physiography showing typical half-graben geometry. The rounded morphology of the western flank of the AT was simulated through tectonic numerical modelling. We consider the subglacial landscape to primarily reflect the locally preserved relict morphology of the tectonic processes affecting the interior of East Antarctica in the Cenozoic. The bedrock morphology was replicated through the activity of the listric Aurora Trench Fault, characterized by a basal detachment at 34 km (considered the base of the crust according to available geophysical interpretations) and vertical displacements ranging between 700 and 300 m. The predicted displacement is interpreted as the (partial) reactivation of a weaker zone along a major Precambrian crustal-scale tectonic boundary. We propose that the Aurora Trench Fault is the southern continuation of the > 1000 km long Aurora Fault independently recognized by previous studies. Together they form the Aurora Fault System, a long lived tectonic boundary with poly-phased tectonic history within the EAC that bounds the eastern side of the Aurora Subglacial Basin. The younger Cenozoic reactivation of the investigated segment of the Aurora Fault System relates to the intraplate propagation of far-field stresses associated to the plate-scale kinematics in the Southern Ocean.

An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

NASA Astrophysics Data System (ADS)

Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

2017-01-01

The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

Primary differentiation in the early Earth: Nd and Sr isotopic evidence from diamondiferous eclogites for both old depleted and old enriched mantle, Yakutia, Siberia

NASA Technical Reports Server (NTRS)

Snyder, Gregory A.; Jerde, Eric A.; Taylor, Lawrence A.; Halliday, Alex N.; Sobolev, Vladimir N.; Sobolev, Nickolai V.; Clayton, Robert N.; Mayeda, Toshiko K.; Deines, Peter

1993-01-01

Ancient, stable, continental cratons possess thick, subcontinental-lithospheric mantle 'keels' which favor particularly the emplacement of diamondiferous kimberlites and included peridotites and eclogites. These refractory mantle samples of the roots provide hard constraints on the theories of formation, growth, and evolution of these cratons. Xenoliths containing only primary garnet and clinopyroxene (eclogites), although rare in most kimberlites, can retain the geochemical signatures of their parent protoliths (e.g., subducted oceanic crust, ancient mantle) thus offering the opportunity to address mantle processes which may have taken place at earlier times in the Earth's history. In fact, it has been postulated that some eclogites are residues from the accretion of the early Earth. Nd and Sr isotopic data are presented which may be interpreted as evidence of an early (greater than 4 Ga) mantle differentiation event. The kimberlites of Yakutia are located both marginal and central to the Siberian craton, and a wide variety of xenoliths are present within them. The Siberian mantle samples have received little attention in the western world, largely because suitable suites of Yakutian samples have not been readily available. Importantly, there is evidence that metasomatism of the Siberian lithosphere has been considerably less intense or extensive than for the Kaapvaal craton. Therefore, it should be considerably easier to elicit the igneous/metamorphic histories of Siberian kimberlitic xenoliths. One of the notable features of the Siberian eclogites is the common appearance of diamonds, especially in the Mir and Udachnaya pipes. In all, eight eclogite samples (eight garnet separates and eight clinopyroxene separates) have been analyzed to date on the Udachnaya pipe, seven from our group.

Archean foreland basin tectonics in the Witwatersrand, South Africa

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Kusky, T. M.

1986-01-01

The Witwatersrand Basin of South Africa is the best-known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch-type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean-type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes above zones of shallow subduction. It is suggested that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision-related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau. The geologic evidence does not agree with earlier suggestions that the Witwatersrand strata were deposited in a rift or half-graben.

Exploring Moho sharpness in Northeastern North China Craton with frequency-dependence analysis of Ps receiver function

NASA Astrophysics Data System (ADS)

Zhang, P.; Yao, H.; Chen, L.; WANG, X.; Fang, L.

2017-12-01

The North China Craton (NCC), one of the oldest cratons in the world, has attracted wide attention in Earth Science for decades because of the unusual Mesozoic destruction of its cratonic lithosphere. Understanding the deep processes and mechanism of this craton destruction demands detailed knowledge about the deep structure of this region. In this study, we calculate P-wave receiver functions (RFs) with two-year teleseismic records from the North China Seismic Array ( 200 stations) deployed in the northeastern NCC. We observe both diffused and concentered PpPs signals from the Moho in RF waveforms, which indicates heterogeneous Moho sharpness variations in the study region. Synthetic Ps phases generated from broad positive velocity gradients at the depth of the Moho (referred as Pms) show a clear frequency dependence nature, which in turn is required to constrain the sharpness of the velocity gradient. Practically, characterizing such a frequency dependence feature in real data is challenging, because of low signal-to-noise ratio, contaminations by multiples generated from shallow structure, distorted signal stacking especially in double-peak Pms signals, etc. We attempt to address these issues by, firstly, utilizing a high-resolution Moho depth model of this region to predict theoretical delay times of Pms that facilitate more accurate Pms identifications. The Moho depth model is derived by wave-equation based poststack depth migration on both Ps phase and surface-reflected multiples in RFs in our previous study (Zhang et al., submitted to JGR). Second, we select data from a major back azimuth range of 100° - 220° that includes 70% teleseismic events due to the uneven data coverage and to avoid azimuthal influence as well. Finally, we apply an adaptive cross-correlation stacking of Pms signals in RFs for each station within different frequency bands. High-quality Pms signals at different frequencies will be selected after careful visual inspection and adaptive

Mechanisms for strain localization within Archaean craton: A structural study from the Bundelkhand Tectonic Zone, north-central India

NASA Astrophysics Data System (ADS)

Sarkar, Saheli; Patole, Vishal; Saha, Lopamudra; Pati, Jayanta Kumar; Nasipuri, Pritam

2015-04-01

The transformation of palaeo-continents involve breakup, dispersal and reassembly of cratonic blocks by collisional suturing that develop a network of orogenic (mobile) belts around the periphery of the stable cratons. The nature of deformation in the orogenic belt depends on the complex interaction of fracturing, plastic deformation and diffusive mass transfer. Additionally, the degree and amount of melting during regional deformation is critical as the presence of melt facilitates the rate of diffusive mass transfer and weakens the rock by reducing the effective viscosity of the deformed zone. The nature of strain localization and formation of ductile shear zones surrounding the cratonic blocks have been correlated with Proterozoic-Palaeozoic supercontinent assembly (Columbia, Rodinia and Gondwana reconstruction). Although, a pre-Columbia supercontinent termed as Kenorland has been postulated, there is no evidence that supports the notion due to lack of the presence of shear zones within the Archaean cratonic blocks. In this contribution, we present the detailed structural analysis of ductile shear zones within the Bundelkhand craton. The ductlile shear zone is termed as Bundelkhand Tectonic Zone (BTZ) that extends east-west for nearly 300 km throughout the craton with a width of two-three kilometer . In the north-central India, the Bundelkhand craton is exposed over an area of 26,000 sq. The craton is bounded by Central Indian Tectonic zone in the south, the Great Boundary fault in the west and by the rocks of Lesser Himalaya in the north. A series of tonalite-trondjhemite-granodiorite gneiss are the oldest rocks of the Bundelkhand craton that also contains a succession of metamorphosed supracrustal rocks comprising of banded iron formation, quartzite, calc-silicate and ultramafic rocks. K-feldspar bearing granites intrude the tonalite-trondjhemite-granodiorite and the supracrustal rocks during the time span of 2.1 to 2.5 Ga. The TTGs near Babina, in central

Geochemical fingerprinting of ∼2.5 Ga forearc-arc-backarc related magmatic suites in the Bastar Craton, central India

NASA Astrophysics Data System (ADS)

Asthana, Deepanker; Kumar, Sirish; Vind, Aditya Kumar; Zehra, Fatima; Kumar, Harshavardhan; Pophare, Anil M.

2018-05-01

The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.

Intraplate deformation on north-dipping basement structures in the Northern Gawler Craton, Australia: reactivation of original terrane boundaries or later intra-cratonic thrusts?

NASA Astrophysics Data System (ADS)

Baines, G.; Giles, D.; Betts, P. G.; Backé, G.

2007-12-01

Multiple intraplate orogenic events have deformed Neoproterozoic to Carboniferous sedimentary sequences that cover the Archean to Mesoproterozoic basement of the northern Gawler Craton, Australia. These intraplate orogenies reactivated north-dipping basement penetrating faults that are imaged on seismic reflection profiles. These north-dipping structures pre-date Neoproterozoic deposition but their relationships to significant linear magnetic and gravity anomalies that delineate unexposed Archean to Early Mesoproterozoic basement terranes are unclear. The north-dipping structures are either terrane boundaries that formed during continental amalgamation or later faults, which formed during a mid- to late-Mesoproterozoic transpressional orogeny and cross-cut the original lithological terrane boundaries. We model magnetic and gravity data to determine the 3D structure of the unexposed basement of the northern Gawler Craton. These models are constrained by drill hole and surface observations, seismic reflection profiles and petrophysical data, such that geologically reasonable models that can satisfy the data are limited. The basement structures revealed by this modelling approach constrain the origin and significance of the north-dipping structures that were active during the later intraplate Petermann, Delamerian and Alice Springs Orogenies. These results have bearing on which structures are likely to be active during present-day intraplate deformation in other areas, including, for example, current seismic activity along similar basement structures in the Adelaide "Geosyncline".

SKS Anisotropy Measurements in Mid-Plate South America: a Test of Subduction-Induced Upper Mantle Flow and the Effect of Cratonic Keels

NASA Astrophysics Data System (ADS)

Assumpcao, M.; Melo, B. C.

2017-12-01

Shear-wave splitting from core-refracted (SKS) waves indicates the amount and orientation of seismic anisotropy in the upper mantle, and is used to infer past and present mantle dynamics and continental evolution. Previous SKS studies in South America concentrated mainly in the Andes and in SE Brazil. Although effects of frozen anisotropy in the lithospheric mantle were suggested in some parts of SE Brazil, the main contribution to the orientation of the fast polarization directions have been attributed to asthenospheric flow around cratonic keels, especially around the São Francisco craton in eastern Brazil (Assumpção et al., 2006,2011). We added extra SKS splitting measurements in the area of the Pantanal and Paraná-Chaco basins (FAPESP-funded "3-Basins" Project). Results from 47 new stations will be presented, both from the temporary deployments and from the Brazilian permanent net. This data set partly fills the gap in SKS measurements between the Andes and SE Brazil, providing a more complete and robust anisotropy map of the S. American stable platform. On average, over most of the mid-continent, the fast polarization orientation tends to be close to the absolute plate motion given by the hotspot reference frame HS3-NUVEL-1A. Nevertheless, the new and previously published fast polarizations results suggest mantle flow around the Amazon and São Francisco cratons. A comparison with recent modeling of upper mantle flow induced by the Nazca plate subduction (Hu et al., 2017) shows good agreement with the predictions of mantle flow around the Amazon craton. Further south, however, especially in the Pantanal Basin, the observed SKS fast orientations are ENE-WSW, deviating from the general ESE-WNW predicted orientations. We propose that the observed ENE-WSW orientation may be due to flow around a possible cratonic nucleus beneath the northern part of the Paraná Basin ("Paranapanema block"). This cratonic block (inferred from geological observations) is also

Mass Independent Fractionation of Sulphur Isotopes in Precambrian Sedimentary Rocks: Indicator for Changes in Atmospheric Composition and the Operation of the Global Sulphur Cycle

NASA Astrophysics Data System (ADS)

Peters, M.; Farquhar, J.; Strauss, H.

2005-12-01

Large mass independent fractionation (MIF) of sulphur isotopes in sedimentary rocks older than 2.3 Ga and the absence of this isotopic anomaly in younger rocks seem to be the consequence of a change in Earth's atmospheric composition from essentially oxygen-free or to oxygen-rich conditions. MIF is produced by photochemical reactions of volcanogenic sulphur dioxide with UV radiation in the absence of an ozone shield. The products of such processes are elemental sulphur with positive and sulphate with negative Δ33S values. Here we present isotope data (32S, 33S, 34S) for sedimentary pyrites from Archaean and Palaeoproterozoic rocks of the Kaapvaal Craton (South Africa), the Pilbara Craton (Australia) and the Greenland Shield (Isua Supercrustal Belt). Their ages range from 3.85 to 2.47 Ga. Large positive Δ33S values up to +9.13 ‰ in several Archaean units from the Kapvaal and Pilbara Cratons are attributed to low atmospheric oxygen at that time. Interestingly, very low Δ33S values between -0.28 and +0.57 ‰ appear to characterize the Witwatersrand succession of South Africa (3.0 Ga). This rather small MIF signature was previously detected in rocks of the same age in Western Australia (OHMOTO et al., 2005). The signature is interpreted as a global signal, which could be the consequence of a shielding effect induced by one or more atmospheric components. The most probable chemical compounds for this process are methane and carbon dioxide. Rocks of the Kameeldoorns Fm. (2.71 Ga), Kaapvaal Craton, display also low values between -0.46 and +0.33 ‰, which are consistent with the small (absent) MIF signal in rocks of the Hardey Fm. (2.76 Ga) of Western Australia (OHMOTO et al., 2005). Very low carbon isotope values between -51 and -40 ‰ in late Archaean kerogens (2.6 - 2.8 Ga) indicate a high concentration of methane in the atmosphere (PAVLOV et al., 2001). This high methane level could produce an organic haze, which absorbed most of the UV radiation and prevented

Seismic structure of the Slave craton crust

NASA Astrophysics Data System (ADS)

Barantseva, O.; Vinnik, L. P.; Farra, V.; van der Hilst, R. D.; Artemieva, I. M.; Montagner, J. P.

2017-12-01

We present P- and S-receiver functions for 20 stations along a 200-km-long NNW-SSE seismological profile across the Slave craton, and estimate the average crustal Vp/Vs ratio which is indicative of rock composition. We observe high Vp/Vs ratio ( 1.85-2.00) for the bulk crust and elevated Vp values at a depth range from 20-30 km to 40 km. High Vp values (>7.0 km/s) suggest mafic composition of the lower crust. In case of dry lower crustal rocks, the Vp/Vs ratio is expected to range from 1.6 to 1.8, which is lower than the observed values of 1.9-2.0. Laboratory studies show that Vp/Vs 1.9-2.0 can be explained by the presence of numerous cracks saturated with an incompressible fluid. Our results are at odds with the structure of the cratonic crust in many regions worldwide, and may suggest a unique geodynamic evolution of the Slave crust. Possible explanations for the observed crustal structure include the presence of an underplated mafic material, possibly related to intraplate magmatism or paleosubduction. Receiver functions are highly sensitive to the change of acoustic impedance and S-wave velocities, but do not resolve the internal seismic structure with a high precision. We extend our study of the crustal structure by using ambient noise tomography (ANT). We measure Rayleigh wave dispersion from Green's functions that are estimated from one-year noise cross-correlation (NCF). The phase velocity maps are inverted for 1D wave speed profiles which are then combined to form 2D and 3D models of the crust of the Slave Province. The combined results of RF analyses and ANT are interpreted in terms of crustal structure, composition, and evolution.

Cratonic roots and lower crustal seismicity: Investigating the role of deep intrusion in the Western rift, Africa

NASA Astrophysics Data System (ADS)

Drooff, C.; Ebinger, C. J.; Lavayssiere, A.; Keir, D.; Oliva, S. J.; Tepp, G.; Gallacher, R. J.

2017-12-01

Improved seismic imaging beneath the African continent reveals lateral variations in lithospheric thickness, and crustal structure, complementing a growing crust and mantle xenolith data base. Border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors and weakly magmatic sectors, providing constraints on crustal rheology and, in some areas, magmatic fluid migration. We report new seismicity data from magmatic and weakly magmatic sectors of the East African rift zone, and place the work in the context of independent geophysical and geochemical studies to models for strain localization during early rifting stages. Specifically, multidisciplinary studies in the Magadi Natron rift sectors reveal volumetrically large magmatic CO2 degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO2 degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust (>30 km), and they are prevalent in areas with high Vp/Vs in the lower crust. Within the southern Tanganyika rift, focal mechanisms are predominantly normal with steep nodal planes. Our comparative studies suggest that pervasive metasomatism above a mantle plume, and melt extraction in thin zones between cratonic roots, lead to

Geochemistry of Precambrian carbonates - 3-shelf seas and non-marine environments of the Archean

NASA Technical Reports Server (NTRS)

Veizer, Jan; Clayton, R. N.; Hinton, R. W.; Von Brunn, Victor; Mason, T. R.

1990-01-01

Samples from the Pangola and Ventersdorp Supergroups (Kaapvaal Craton, South Africa) and from the Fortescue and Hamersley Groups (Pilbara Block, Australia) were analyzed, using XRF, AAS, and isotope-analysis techniques to investigate the mineralogical, chemical, and isotopic features of these representatives of contemporary shelf carbonates (Pangola and Hamersley samples) and nonmarine carbonates (the Ventersdorp and Fortescue samples). Results show that, mineralogically, the shelf carbonates are almost exclusively dolostones, while the lacustrine facies are predominantly limestones. Geological, trace-element, and oxygen-isotope results of the shelf carbonates suggest that their original mineralogy may have been aragonite, and that the Pangola dolostones may represent a direct dolomitization product of this precursor. By contrast, the stabilization of the Hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone.

Magmatism during the accretion of the late Archaean Dharwar Craton (South India): sanukitoids and related rocks in their geological context.

NASA Astrophysics Data System (ADS)

Moyen, J.-F.; Martin, H.; Jayananda, M.; Peucat, J.-J.

2003-04-01

The South Indian Dharwar Craton assembled during the late-Archaean (ca. 2.5 Ga). This event was associated with intense granite genesis and emplacement. Based on petrography and geochemistry, 4 main types of late Archaean granitoids were distinguished: (1) Anatectic granites (and diatexites), formed by partial melting of TTG gneisses; (2) Classical TTGs; (3) Sanukitoids, generated by interaction between slab melts (TTG) and mantle peridotite; (4) The high HFSE Closepet granite, interpreted as derived from partial melting of a mantle metasomatized by slab melts (TTG). While the 3 later groups all are interpreted as resulting from slab melt/mantle wedge interactions, their differences are related to decreasing felsic melt/peridotite ratios during the ascent “slab melts” in the mantle wedge above an active subduction zone. Field data together with geochronology and isotope geochemistry allow to subdivide the Dharwar craton into three main domains: (1) The Western Dharwar Craton (WDC) is an old (3.3 2.9 Ga ), stable continental block with limited amounts of 2.5 Ga old anatectic granites. (2) The Eastern Dharwar Craton (EDC) is subdivided into two parts: (2a) West of Kolar Schist Belt, a region of 3.0-2.7 Ga old basement intruded by 2.5 Ga old anatectic granites; (2b) East of Kolar, an area featuring mainly 2.5 Ga old diatexites and granites, derived of partial melting of a newly accreted TTG crust. Anatectic granites are ubiquitous, and late in the cratonic evolution; they witnessed generalized melting of a juvenile crust. In contrast, deep-originated granites emplaced before this melting and are restricted to the boundaries between the blocks. This structure of distinct terranes separated by narrow bands operating as channels for deep-originated magmas provides independent evidences for a two-stage evolution: an arc accretion context for the TTG, sanukitoids and related rocks, immediately followed by high temperature reworking of the newly accreted craton

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

NASA Astrophysics Data System (ADS)

Ionov, Dmitri A.; Prikhodko, Vladimir S.; Bodinier, Jean-Louis; Sobolev, Alexander V.; Weis, Dominique

2005-08-01

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870 1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al2O3 and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (≤3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na2O (2 3.1%) in clinopyroxene, LREE enrichments in whole-rocks.

Laboratory derived constraints on electrical conductivity beneath Slave craton

NASA Astrophysics Data System (ADS)

Bagdassarov, Nikolai S.; Kopylova, Maya G.; Eichert, Sandrine

2007-04-01

The depth profile of the electrical conductivity, σ(d), beneath the Central Slave craton (Canada) has been reconstructed with the help of laboratory measurements carried out on peridotite xenoliths. σ(T) of xenoliths was determined in the piston-cylinder apparatus at 1 and 2 GPa and from 600 to 1150 °C. σ(T) of xenoliths follows the Arrhenius dependence with the activation energy, E, varying from 2.10 to 1.44 eV depending on temperature range and the Mg-number. The calculated xenolith geotherm and the suggested lithology beneath the Central Slave have been used to constrain σ(d) as follows: σ(d) in the crust varies between 0.5×10-5 and 10-3 S/m; the lithospheric σ(d) sharply decreases below the Moho at 39.4 km to 0.5×10-8 S/m, which corresponds to 460 °C, and then gradually increases with the depth d to 0.5×10-2 S/m. The modeled MT-response of the constrained σ(d) profile has been compared with MT-observations [Jones, A.G., Lezaeta, P., Ferguson, I.J., Chave, A.D., Evans, R.L., Garcia, X., Spratt J., 2003. The electrical structure of the Slave craton. Lithos, 71, 505-527]. The general trend of the calculated MT-response based on the σ(d) model mimics the MT-inversion of the field data from the Central Slave.

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

NASA Astrophysics Data System (ADS)

Schandelmeier, H.; Richter, A.; Harms, U.

1987-09-01

The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.

Early Archean sialic crust of the Siberian craton: Its composition and origin of magmatic protoliths

NASA Astrophysics Data System (ADS)

Vovna, G. M.; Mishkin, M. A.; Sakhno, V. G.; Zarubina, N. V.

2009-12-01

This study demonstrates that the base of the Archean deep-seated granulite complexes within the Siberian craton consists of a metabasite-enderbite association. The major and trace element distribution patterns revealed that the protoliths of this association are represented by calc-alkaline andesites and dacites, containing several minor sequences of komatiitic-tholeiitic volcanic rocks. The origin of the primary volcanic rocks of the metabasite-enderbite association is inferred on the basis of a model of mantle plume magmatism, which postulates that both andesitic and dacitic melts were derived from the primary basitic crust at the expense of heat generated by ascending mantle plumes. The formation of the protoliths of the Archen metabasite-enderbite association of the Siberian craton began at 3.4 Ga and continued until the late Archean.

Chapter 27: Geology and petroleum potential of the north and east margins of the Siberian Craton, north of the Arctic Circle

USGS Publications Warehouse

Klett, T.R.; Wandrey, C.J.; Pitman, Janet K.

2011-01-01

The Siberian Craton consists of crystalline rocks and superimposed Precambrian sedimentary rocks deposited in rift basins. Palaeozoic rocks, mainly carbonates, were deposited along the margins of the craton to form an outwardly younger concentric pattern that underlies an outward-thickening Mesozoic sedimentary section. The north and east margins of the Siberian Craton subsequently became foreland basins created by compressional deformation during collision with other tectonic plates. The Tunguska Basin developed as a Palaeozoic rift/sag basin over Proterozoic rifts. The geological provinces along the north and east margins of the Siberian Craton are immature with respect to exploration, so exploration-history analysis alone cannot be used for assessing undiscovered petroleum resources. Therefore, other areas from around the world having greater petroleum exploration maturity and similar geological characteristics, and which have been previously assessed, were used as analogues to aid in this assessment. The analogues included those of foreland basins and rift/sag basins that were later subjected to compression. The US Geological Survey estimated the mean undiscovered, technically recoverable conventional petroleum resources to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 103 trillion cubic feet of natural gas and 3 billion barrels of natural gas liquids. ?? 2011 The Geological Society of London.

Paleomagnetic Results of the 925 Ma Mafic Dykes From the North China Craton: Implications for the Neoproterozoic Paleogeography of Rodinia

NASA Astrophysics Data System (ADS)

Zhao, X.; Peng, P.

2017-12-01

Precambrian mafic dyke swarms are useful geologic records for Neoproterozoic paleogeographic reconstruction. We present a paleomagnetic study of the 925 Ma Dashigou dyke swarm from 3 widely separated locations in the central and northern parts of the North China Craton, which are previously unsampled regions. Stepwise thermal and alternating field demagnetizations were successful in isolating two magnetic components. The lower unblocking temperature component represents the recent Earth magnetic field. The higher unblocking temperature component is the characteristic remanent magnetization and yields positive baked contact test. Results from detailed rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier in these rocks. There was no regional event that has reset the remanent magnetization of all the dyke sites, as indicated by the magnetization directions of both overlying and underlying strata. The similarity of the virtual paleomagnetic poles for the 3 sampled regions also argues that the characteristic remanent magnetizations are primary magnetization when the dykes were emplaced. The paleomagnetic poles from the Dashigou dyke swarm of the North China Craton are not similar to those of the identical aged Bahia dykes from the São Francisco Craton, Brazil, indicating that these mafic dykes may be not parts of a common regional magmatic event that affected North China Craton and NE Brazil at about 925 Ma.

Cratonic roots under North America are shifted by basal drag: new evidence from gravity and geodynamic modeling

NASA Astrophysics Data System (ADS)

Kaban, M. K.; Petrunin, A.; Mooney, W. D.

2013-12-01

The impact of basal drag on the long-lived cratonic roots has been debated since the discovering of plate tectonics. Previously, evidence for a shifted mantle structure under North America was postulated from a comparison of the surface expression of the Great Meteor hotspot track versus its location at 200 km depth as inferred from seismic tomography (Eaton and Frederiksen, 2007). We present new results that are based on the integrative modeling of gravity and seismic data. The starting point is the residual gravity anomaly and residual topography, which are computed by removing of the crustal effect and of the effect of temperature variations in the upper mantle from the observed fields (Mooney and Kaban, 2010). After the temperature correction both residual fields chiefly reflect compositional density heterogeneity of the upper mantle. The residual gravity and topography are jointly inverted to determine the 3D density structure of the upper mantle. The inversion technique accounts for the fact that although these parameters are controlled by the same factors, the effect depends on depth and wavelength. Therefore, we can resolve the vertical distribution of density more reliable than by interpreting only one parameter. We found a strong negative anomaly under the North American craton, as expected for a depleted mantle. However, starting from a depth of about 200 km the depleted root is shifted west-southwest. The maximal shift reaches about 1000 km at a depth of 300 km. The direction agrees with the North American plate movement and with the anisotropy pattern in the upper mantle (e.g. Bokelmann, 2002). The results of the gravity modeling are confirmed by geodynamic modeling. The mantle flow is estimated from the density and temperature distribution derived from seismic tomography models. A 3D viscosity model is supplemented with weak boundaries based on an integrated model of plate boundary deformations. The calculated plate velocities are in a good agreement

Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African craton

NASA Astrophysics Data System (ADS)

Bertrand-Sarfati, Janine; Moussine-Pouchkine, Alexis

1988-08-01

The Atar Group, part of the Upper Proterozoic sequence covering the West African craton, stable since 2000 Ma, is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies. The carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments. The mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions. The settings of the mixed facies are not always clear but they do not contain thick sequences of high-energy facies. Few obvious facies sequences are discernable; those that are present are considered to be punctuated aggradational cycles (PACs) and they always start with biostromes of columnar stromatolites with very few sediments. Composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous, despite the fact that the contacts are not gradational. However, much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies, bound by sharp boundaries. The basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea-level rise. Consequently, their appearance can be considered as a time-line. We describe, in the Atar Group and its equivalents, three sedimentation trends, all of which are interpreted to be of shallowing upward character. The Atar Group appears to have been deposited in an epeiric sea (i.e. an extremely flat ramp). There are two contrasting styles of sedimentation: (1) after the submergence of the whole area, columnar stromatolites built extensive biostromes; (2) during the stable phase, sediments are deposited in a mosaic of laterally-discontinuous facies. Tidal influence cannot be recognized in the sequence, neither can a salinity increase toward the land; both common features in published epeiric sea

Looking for the Edge: Does Lateral Change in Azimuthal Anisotropy Mark the Limit of the North American Craton?

NASA Astrophysics Data System (ADS)

Chen, X.; Levin, V. L.; Li, Y.; Yuan, H.

2017-12-01

Thickness of the lithosphere in eastern North America decreases from nearly 250 km beneath the central craton to 90 - 110 km at the coast. The rapid thinning suggested by tomographic images of shear wave velocity takes place beneath the Proterozoic Grenville province. Shear wave splitting measurements of core refracted waves on a 1300 km long array from James Bay to the Fundy Basin show that the average delay time increases from 0.5 s in the Superior Province to 1 s in the Appalachians. Also, azimuthal anisotropy constrained by the joint inversion of surface and body waves (Yuan and Romanowicz, 2011) has smaller amplitude under the older Superior Province than the younger Appalachians. Significantly, the increases in anisotropy amplitude and the splitting delay times happen at nearly the same place, beneath the Grenville Province. Due to the limited lateral resolution of 500 km of the tomographic model, it is difficult to relate changes in seismic wave speed and anisotropy to tectonic boundaries on the surface. On the other hand, our new measurements of shear wave splitting are done with lateral step of 50 km or less, and thus offer us a way to detect the exact location where upper mantle fabric changes. We carry out forward modeling using a 1D anisotropic layered reflectivity method of Levin and Park (1997) and taking Yuan and Romanowicz (2011) model as a starting point. Our working hypothesis is that the upper mantle volume characterized by high seismic velocity and small amplitude of anisotropy represents old cratonic lithosphere of North America. Through our detailed modeling of closely spaced observations we seek to delineate its southern edge, and to characterize its internal structure. Figure Caption: All splitting results plotted on a background of distribution of shear wave velocity and contour map of azimuthal anisotropy amplitude at 160 km depth. Orientations of sticks which are centered at the stations represent fast polarizations, while the lengths are

Imaging the lithosphere-asthenosphere boundary across the transition from Phanerozoic Europe to the East-European Craton with S-receiver functions

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, Brigitte; Krüger, Frank

2013-04-01

Cratons are characterized by their thick lithospheric roots. In the case of the Eastern European Craton, high seismic velocities have been imaged tomographically to more than 200 km depth. However, the exact depth extent of the cratonic lithosphere and especially the properties of the transition to a much thinner lithosphere beneath Phanerozoic central Europe still remain under discussion. Whereas a number of recent seismic campaigns has significantly increased the knowledge about crustal structure and Moho topography in central Europe, comparably detailed, 3-D information on upper mantle structure, e.g. the lithosphere-asthenosphere boundary (LAB), is yet missing. The international PASSEQ experiment, which was conducted from 2006 to 2008, strived to fill this gap with the deployment of 196 seismological stations, roughly a quarter of which were equipped with broad-band sensors, between eastern Germany and Lithuania. With a mean inter-station distance of 60 km, reduced to about 20 km along the central profile, PASSEQ offers the densest coverage for a passive experiment in this region yet. Here, we present first S-receiver function results for this data set, complemented by additional data from national and regional networks and other temporary deployments. This increases the number of available broad-band stations to almost 300, though mostly located to the west of the Trans-European Suture Zone (TESZ). Besides, we also process data from short-period (1 s and 5 s) sensors. The visibility of mantle-transition zone phases, even in single-station data, provides confidence in the quality of the obtained S-receiver functions. Moho conversions can be confidently identified for all stations. In case of a low-velocity sedimentary cover, as found for example in the Polish Basin, the S-receiver functions even provide clearer information on Moho depth than the P-receiver functions, which are heavily disturbed by shallow reverberations. For stations west of the TESZ, a clear

APW path traced for the Guiana Shield (2070-1960 Ma) and Paleogeographic Implications: Paleomagnetic data from the 1.98-1.96 Ga Surumu Group (Northern Amazonian Craton)

NASA Astrophysics Data System (ADS)

Bispo-Santos, F.; Dagrella Filho, M. S.; Reis, N. J.; Trindade, R. I.

2013-05-01

Definition of continental paleogeography for times prior to formation of Columbia Supercontinent (1900-1850 Ma) is very complex, since amalgamation of some continental blocks of Earth was still in progress, as in the case of Laurentia, Baltica and Amazonian Craton. So, paleogeographic models proposed for this time are still very speculative and/or subjective. The use of the paleomagnetic technique tracing apparent polar wander (APW) paths for the various cratonic blocks can contribute to understand the continental amalgamation and breakup, especially for times where all created oceanic lithosphere was fully consumed. In this study, we present the paleomagnetic data obtained for samples collected from 39 sites from the well-dated 1980-1960 Ma (U-Pb) volcanic rocks belonging to the Surumu Group, cropping out in the northern Roraima State (Guiana Shield, Amazonian Craton). AF and thermal treatment revealed northwestern directions with moderate downward inclinations on samples from 20 out of the 39 analyzed sites. Site mean directions cluster around the mean, Dm = 298.6°; Im = 39.4° (N = 20; α95 = 10.1°), which yielded a key paleomagnetic pole (SG) for the Guiana Shield, located at 234.8°E, 27.4°N (A95 = 9.8°). Magnetic mineralogy experiments show that the magnetization of these rocks, probably of primary origin, is carried by magnetite and/or hematite. The SG pole contributes to a better fit of the APW path traced for Guiana Shield during the Paleoproterozoic (2070-1960 Ma). Comparison with the APW path traced for the West-Africa Craton for the same time interval suggests that these cratonic blocks were linked at 2000-1960 Ma ago, forming a paleogeography in which the Guri (Guiana Shield) and Sassandra (West-Africa Craton) shear zones were aligned as suggested in previous geologic models. KEYWORDS: Paleoproterozoic, Paleomagnetism, APWP, Amazonian Craton, Surumu Group.

Depositional environments and tectonic significance of the Wajid Sandstone of southern Saudi Arabia

NASA Astrophysics Data System (ADS)

Dabbagh, Mohamed E.; Rogers, John J. W.

The Wajid Sandstone, of probable Early Paleozoic age, rests disconformably on crystalline rocks of the southern part of the Arabian shield. Scattered outcrops extend over an area about 450 km north-south and 300 km east-west. The southern part of the formation, near the Yemen border, consists of fluvial sandstones and very minor siltstones and silty shales. The fluvial origin is demonstrated by the presence of fining-upward cycles, channels, trough cross bedding, and absence of all organic traces. The northern part of the outcrop area consists of internally homogeneous, tabular cross-bedded, horizontally bedded sandstones apparently formed in a shallow marine environment. These marine rocks contain trace fossils broadly similar to Skolithos. Abundant cross bedding in both facies of the Wajid indicates a northward transport direction, toward what is now the center of the Arabian shield. The southern part of the Arabian shield, which was cratonized about 500 to 600 Ma ago (Pan-African age), was apparently still a depositional area receiving sediments from a southern source in Early Paleozoic time. Other, older, shields also show a tendency to be areas of deposition shortly after their apparent age of stabilization, becoming sources of clastic sediments only after several hundreds of millions of years. The conversion from basin to uplifted source may indicate a prolonged process of shield maturation after initial stabilization.

Heat flow, heat generation and crustal thermal structure of the northern block of the South Indian Craton

NASA Astrophysics Data System (ADS)

Gupta, Mohan L.; Sharma, S. R.; Sundar, A.

Heat flow values and heat generation data calculated from the concentration of heat producing radioactive elements, U, Th and K in surface rocks were analyzed. The South Indian Craton according to Drury et al., can be divided into various blocks, separated by late Proterozoic shear belts. The northern block comprises Eastern and Western Dharwar Cratons of Rogers (1986), Naqvi and Rogers (1987) and a part of the South Indian granulite terrain up to a shear system occupying the Palghat-Cauvery low lands. The geothermal data analysis clearly demonstrates that the present thermal characteristics of the above two Archaean terrains of the Indian and Australian Shields are quite similar. Their crustal thermal structures are likely to be similar also.

Heat flow, heat generation and crustal thermal structure of the northern block of the South Indian Craton

NASA Technical Reports Server (NTRS)

Gupta, Mohan L.; Sharma, S. R.; Sundar, A.

1988-01-01

Heat flow values and heat generation data calculated from the concentration of heat producing radioactive elements, U, Th and K in surface rocks were analyzed. The South Indian Craton according to Drury et al., can be divided into various blocks, separated by late Proterozoic shear belts. The northern block comprises Eastern and Western Dharwar Cratons of Rogers (1986), Naqvi and Rogers (1987) and a part of the South Indian granulite terrain up to a shear system occupying the Palghat-Cauvery low lands. The geothermal data analysis clearly demonstrates that the present thermal characteristics of the above two Archaean terrains of the Indian and Australian Shields are quite similar. Their crustal thermal structures are likely to be similar also.

High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

USGS Publications Warehouse

Runkel, Anthony C.; Miller, J.F.; McKay, R.M.; Palmer, A.R.; Taylor, John F.

2007-01-01

Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings. Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a

Large igneous provinces (LIPs) and carbonatites

NASA Astrophysics Data System (ADS)

Ernst, Richard E.; Bell, Keith

2010-03-01

There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Paraná-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615-555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114-1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.

The Archaen volcanic facies in the Migori segment, Nyanza greenstone belt, Kenya: stratigraphy, geochemistry and mineralisation

NASA Astrophysics Data System (ADS)

Ichang'l, D. W.; MacLean, W. H.

The Migori segment is an 80 by 20 km portion of the Nyanza greenstone belt which forms the northern part of the Archean Tanzanian Craton in western Kenya, northern Tanzania and southeastern Uganda. It consists of two volcanic centres, each with central, proximal and distal volcanic facies, comprising the Migori Group, the Macalder and Lolgorien Subgroups, and eleven volcano-sedimentary formations. The centres are separated by a basin of tuffs and greywacke turbidites. The volcanics are bimodal mafic basalt and dolerite ( Zr/Y = 3.8 - 6.5, La N/Yb N = 1.0 - 2.4) , and felsic calc-alkaline dacite-rhyolite ( Zr/Y = 10 - 21, La N/Yb N = 19 - 42 ) and high-K dacite ( Zr/Y = 9 - 16, La N/Yb N = 21 - 22 ). Felsic units form approximately three-fourths of the volcanic stratigraphy. Basalts, calc-alkaline dacites and rhyolites were deposited in a submarine environment, but the voluminous high-K dacites were erupted subaerially. The turbidites contain units of iron-formations. Granitic intrusions are chemically continuous with the high-K dacites. The felsic volcanics are anologous to those found at modern volcanic arc subduction settings involving continental crust. The Macalder ZnCuAuAg volcanogenic massive sulphide deposits is in central facies basalts-greywacke-rhyolite. Gold mineralisation occurs in proximal facies tuffs and iron formation, and in oblique and semi-conformable quartz veins. Greenstones in the Nyanza belt are dominated by calc-alkaline felsic volcanics in constrast to the komatiite-tholeiitic basalt volcanism in the Kaapvaal Craton of South Africa, and a mixture of the two types in the Zimbabwe Craton.

Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

USGS Publications Warehouse

Bauer, W.; Walsh, G.J.; De Waele, B.; Thomas, Ronald J.; Horstwood, M.S.A.; Bracciali, L.; Schofield, D.I.; Wollenberg, U.; Lidke, D.J.; Rasaona, I.T.; Rabarimanana, M.H.

2011-01-01

The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian

Granulites from Northwest Indian Shield: Their differences and similarities with Southern Indian granulite terrain

NASA Technical Reports Server (NTRS)

Sharma, R. S.

1988-01-01

Granulite facies suite in the NW Indian Shield is exposed at Sand Mata, Udaipur district, Rajasthan, as an oval-shaped massif within amphibolite facies rocks of the Banded Gneissic Complex (3.5 to 2.6 b.y. old) - a possible analogue of the Peninsular gneiss of Dharwar craton. On the basis of quantitative P-T estimates, combined with the textural evidence for the crystallization sequence of the Al-silicate polymorphs (kyanite to sillimanite to kyanite) in the pelitic granulite, the deduced P-T path for the Sand Mata granulites is the reverse of that characterizing the Plate tectonic collision zone. It, however, agrees with the P-T path inferred in the case of the southern Indian granulitic rocks.

Proterozoic evolution of the western margin of the Wyoming craton: Implications for the tectonic and magmatic evolution of the northern Rocky Mountains

USGS Publications Warehouse

Foster, D.A.; Mueller, P.A.; Mogk, D.W.; Wooden, J.L.; Vogl, J.J.

2006-01-01

Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from ???2.4 to ???1.8 Ga. Rock-forming events in the area as young as ???1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages ???1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions. ?? 2006 NRC Canada.

Paleoproterozoic andesitic volcanism in the southern Amazonian craton (northern Brazil); lithofacies analysis and geodynamic setting

NASA Astrophysics Data System (ADS)

Roverato, Matteo; Juliani, Caetano; Capra, Lucia; Dias Fernandes, Carlos Marcelo

2016-04-01

Precambrian volcanism played an important role in geological evolution and formation of new crust. Most of the literature on Precambrian volcanic rocks describes settings belonging to subaqueous volcanic systems. This is likely because subaerial volcanic rocks in Proterozoic and Archean volcano-sedimentary succession are poorly preserved due to erosive/weathering processes. The late Paleoproterozoic Sobreiro Formation (SF) here described, seems to be one of the rare exceptions to the rule and deserves particular attention. SF represents the subaerial expression of an andesitic magmatism that, linked with the upper felsic Santa Rosa F., composes the Uatumã Group. Uatumã Group is an extensive magmatic event located in the Xingú region, southwestern of Pará state, Amazonian Craton (northern Brazil). The Sobreiro volcanism is thought to be related to an ocean-continent convergent margin. It is characterized by ~1880 Ma well-preserved calc-alkaline basaltic/andesitic to andesitic lava flows, pyroclastic rocks and associated reworked successions. The superb preservation of its rock-textures allowed us to describe in detail a large variety of volcaniclastic deposits. We divided them into primary and secondary, depending if they result from a direct volcanic activity (pyroclastic) or reworked processes. Our study reinforces the importance of ancient volcanic arcs and rocks contribution to the terrestrial volcaniclastic sedimentation and evolution of plate tectonics. The volcanic activity that produced pyroclastic rocks influenced the amount of detritus shed into sedimentary basins and played a major role in the control of sedimentary dispersal patterns. This study aims to provide, for the first time, an analysis of the physical volcanic processes for the subaerial SF, based in field observation, lithofacies analysis, thin section petrography and less geochemical data. The modern volcanological approach here used can serve as a model about the evolution of Precambrian

Limiting depth of magnetization in cratonic lithosphere

NASA Technical Reports Server (NTRS)

Toft, Paul B.; Haggerty, Stephen E.

1988-01-01

Values of magnetic susceptibility and natural remanent magnetization (NRM) of clino-pyroxene-garnet-plagioclase granulite facies lower crustal xenoliths from a kimberlite in west Africa are correlated to bulk geochemistry and specific gravity. Thermomagnetic and alternating-field demagnetization analyses identify magnetite (Mt) and native iron as the dominant magnetic phases (totaling not more than 0.1 vol pct of the rocks) along with subsidiary sulfides. Oxidation states of the granulites are not greater than MW, observed Mt occurs as rims on coarse (about 1 micron) Fe particles, and inferred single domain-pseudosingle domain Mt may be a result of oxidation of fine-grained Fe. The deepest limit of lithospheric ferromagnetism is 95 km, but a limit of 70 km is most reasonable for the West African Craton and for modeling Magsat anomalies over exposed Precambrian shields.

Probing the edge of the West African Craton: A first seismic glimpse from Niger

NASA Astrophysics Data System (ADS)

Di Leo, Jeanette F.; Wookey, James; Kendall, J.-Michael; Selby, Neil D.

2015-03-01

Constraints on crustal and mantle structure of the Eastern part of the West African Craton have to date been scarce. Here we present results of P receiver function and SK(K)S wave splitting analyses of data recorded at International Monitoring System array TORD in SW Niger. Despite lacking in lateral coverage, our measurements sharply constrain crustal thickness (˜41 km), VP/VS ratio (1.69 ± 0.03), mantle transition zone (MTZ) thickness (˜247 km), and a midlithospheric discontinuity at ˜67 km depth. Splitting delay times are low with an average of 0.63 ± 0.01 s. Fast directions follow the regional surface geological trend with an average of 57 ± 1°. We suggest that splitting is due to fossil anisotropic fabrics in the crust and lithosphere, incurred during the Paleoproterozoic Eburnean Orogeny, with possible contributions from the later Pan-African Orogeny and present-day mantle flow. The MTZ appears to be unperturbed, despite the proximity of the sampled region to the deep cratonic root.

A multidisciplinary study on the crustal architecture and tectonic evolution of the Biligiri Rangan Block, southern India: Implications for Neoarchean plate tectonics

NASA Astrophysics Data System (ADS)

Raveendran Thankamoni, Ratheesh Kumar

2017-04-01

Southern India is comprised of a collage of crustal blocks ranging in age from Archean to Neoproterozoic. Previous studies considered the Archean high-grade granulite terrain to the north of the Southern Granuilte Terrain (SGT) of southern India as the part of the Dharwar Craton and hence subdivided this craton into western, central and eastern provinces. This contribution presents my detailed examinations on the least studied Central Dharwar Province, comprising the Biligiri Rangan (BR) - Male Mahadeshwara (MM) Hills domain composed predominantly of charnockites. One of my recent study (Ratheesh-Kumar et al., 2016) for the first time provided necessary evidence for Neoarchean subduction-accretion-collision tectonic evolution of this domain as a separate crustal block which has been named as Biligiri Rangan Block (BRB) by using a multidisciplinary approach involving field investigation, petrography, mineral chemistry, thermodynamic modeling of metamorphic P-T evolution, and LA-ICPMS U-Pb and Lu-Hf analyses of zircons on representative rocks together with regional-scale crustal thickness model derived using isostatic gravimetric geophysical method. The important findings of this study are: (1) The BRB preserves the vestiges of a Mesoarchean primitive continental crust as indicated by the age (ca. 3207) and positive ɛHf value (+2.7) of quartzofeldspathic gneiss occurred in the central part of the block (2) The charnockites and associated mafic granulites and granites provide ages between ca. 2650 Ma and ca. 2498 Ma with large negative ɛHf values are suggestive of Neoarchean charnockitization and crustal remelting (3) New geochemical data of charnockites and mafic granulites from BRB are consistent with arc magmatic rocks generated through oceanic plate subduction (4) Delineation of a suture zone along the Kollegal structural lineament bounding the BRB and the Western Dharwar Craton surmised from the occurrences of quartzite-iron formation intercalations and also

The Origin of the Chinese Central Tianshan Block in the Southern Central Asian Orogenic Belt: Evidence from Detrital Zircon Study

NASA Astrophysics Data System (ADS)

Huang, Z.; Long, X.; Yuan, C.

2016-12-01

The Chinese Central Tianshan Block (CTB) is one of the oldest continental fragments in the southern Central Asian Orogenic Belt (CAOB). Although it is vital for understanding the evolution of the CAOB, its origin has been poorly studied. The CTB was previously suggested to have been originated from the North China, the South China, the Tarim cratons or the East European Craton (Baltica). A total of 165 concordant U-Pb and Hf isotopic analyses of detrital zircon are obtained from three meta-sediments in the CTB, including one meta-sandstone from Xingxingxia formation and one meta-sandstone as well as one quartzite from Kawabulake formation. Detrital zircon grains from the Xingxingxia and Kawabulake formations are dominated by respective youngest age populations at 1002 Ma and 930-960 Ma, providing constraints on the maximum depositional ages for these two formations. Zircon grains from the meta-sediments have very similar age distributions, with two dominant peaks at 0.93-1.0 Ga and 1.0-1.6 Ga and a minor peak at 2.3-2.7 Ga. They have similar Hf isotopic signatures, suggesting that the meta-sediments in the CTB share similar sedimentary provenance. The early Neoproterozoic detrital zircon grains are mainly local-derived, whereas the Paleo-Mesoproterozoic grains are both autochthonous and allochthonous. The occurrence of these Mesoproterozoic and Neoproterozoic zircon grains are coincident with the Nuna breakup and the Rodinia assembly. This suggests that the CTB might experience the tectonic switching of the Nuna to the Rodinia. The distinct Meso-Neoproterozoic age patterns and Hf isotopic compositions of these detrital grains from the CTB and the surrounding blocks indicate that the CTB was not located close to the North China, the South China or the Tarim cratons in Precambrian. Our new data suggest that the CTB was most likely once a part of the East European Craton before the Neoproterozoic. This study was supported by National Basic Research Program of China

Geochemistry of Mesozoic plutons, southern Death Valley region, California: Insights into the origin of Cordilleran interior magmatism

USGS Publications Warehouse

Ramo, O.T.; Calzia, J.P.; Kosunen, P.J.

2002-01-01

Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with ??Ndi of -5 to -16, 87Sr/86Sri of 0.707-0.726, and 206Pb/204Pbi of 17.5-20.0. Cretaceous granitoids of the region are mainly monzogranites with ??Ndi of -6 to -19, 87Sr/86Sri of 0.707-0.723, and 206Pb/204Pbi of 17.4-18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust - some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unuasually juvenile composition (??Ndi -3.2, 87Sr/86Sri 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordillera plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane.

Palaeointensity determinations on rocks from the Achaean- Paleoproterozoic dykes from the Karelian craton

NASA Astrophysics Data System (ADS)

Shcherbakov, Valera; Shcherbakova, Valentina; Lubnina, Natalia; Zhidkov, Grigory; Tsel'movich, Vladimir

2017-04-01

The Karelian craton was a fragment of either an earlier late Archean supercontinent, sometimes referred to as Kenorland. Now the craton is a large Archean composite granite-greenstone terrane in the eastern part of the Fennoscandian Shield bounded by Paleoproterozoic Svecofennian orogen in the south-west and by Lapland-Kola orogen in the north-east and Belomorian province in the east-north-east. Mafic dykes, volcanic rocks, sills, and layered intrusions with ages of ca. 2.51-2.45 Ga and ca. 2.06-1.95 Ga are widespread and well-studied in the Karelian Craton. Paleointensity (Banc) results obtained on the Shala dike (age of 2504 Ma by U-Pb, ID TIMS) tracked near vl. Shala and on the Deda island are discussed here. Eighteen block samples of gabbronotires were collected in two sites in the Shala quarry. Stepwise thermal demagnetization (≤ 20 steps, up to 600 C) and stepwise AF demagnetization were done. To monitor possible mineralogical changes during thermal cleaning, magnetic susceptibility was measured after each heating step. Intensive rock magnetic investigations and thermal palaeointensity experiments using the Thellier-Coe (with check-points) and Wilson procedures were carried out. Electronic microscopy study of two samples was made too. For the exception of a viscous component some specimens from the contact zone of the gabbronorite dyke with thin dolerite dyke show two distinct components. The first E-NE intermediate-down direction component was separated at fields up to 50-60 mT and unblocking temperatures up to 520-540 C. The other S-SE low-down direction component is separated at fields from 60 to 100 mT and unblocking temperatures from 540 to 590-600 C. Based on the positive contact tests for the gabbronorite dyke, the S-SE shallow inclination remanence (I = -5.7 degrees) is interpreted to be of primary origin. Reliable palaeointensity determinations Banc fitting a set of selection criteria were determined on 13 samples from 2 sites carrying

Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

NASA Astrophysics Data System (ADS)

Yinshuang, A.; Zhang, Y.; Chen, L.

2016-12-01

The central and western NCC(CWNCC) only experienced localized lithospheric modification and has remained relatively stable since the Pre-Cambrian in contrast to the fundamental destruction in the east. For better unraveling the tectonic evolution and dynamics of CWNCC, detailed knowledge of lithospheric structure is thus important. However, most of the available seismological observations are dominated by regional seismic tomography and the resolutions are rather low due to the limited data coverage or intrinsic limitation of the methods. S receiver function(RF) contains information from deep velocity discontinuities and is free from the interference of crustal multiples, so it is widely used in subcontinental lithospheric structural studies. We collected teleseismic data from 340 broadband stations in CWNCC, and adopted 2-D wave equation-based poststack migration method to do S-receiver function CCP imaging. Finally, we get 8 migrated profile images in CWNCC and adjacent areas and integrate them for an overview. The most prominent feature of the LAB beneath central NCC is an sudden subsidence to 160km in the southern portion, and the dimension and extension of this deep anomaly is correlated to the lithosphere in Ordos, so we interpret it as a remnant cratonic mantle root. The LAB beneath western NCC can extend to the depth of 150-180 km but appears laterally variable. Western Ordos becomes shallower than its eastern counterpart and there are two obvious deep anomalies beneath the eastern Ordos, divided by a geological boundary at 37°N, which reflects that the lithosphere of Ordos is not so homogeneous or rigid as people thought before. Furthermore, a negative velocity discontinuity is widely identified at the depth of 80- 110 km within the thick lithosphere of CWNCC, and the location is spatially coincide with the modified LAB in ENCC. Although the cause of this mid-lithospheric discontinuity(MLD) is still controversial, mechanically, it may indicate an ancient

Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

NASA Astrophysics Data System (ADS)

Tohver, Eric; van der Pluijm, B. A.; Van der Voo, R.; Rizzotto, G.; Scandolara, J. E.

2002-05-01

A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondônia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40Ar/39Ar age dating of biotite and plagioclase yield ages of ∼1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062±3 Ma, similar to the ∼1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n=16 sites, Plat.=24.6°N, Plong.=164.6°E, A95=5.5°, Q=5), the first reported pole for the Amazon craton for the 1200-600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at ∼1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia's Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have

Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event.

PubMed

Luo, Genming; Junium, Christopher K; Izon, Gareth; Ono, Shuhei; Beukes, Nicolas J; Algeo, Thomas J; Cui, Ying; Xie, Shucheng; Summons, Roger E

2018-03-07

The marine nitrogen cycle is dominated by redox-controlled biogeochemical processes and, therefore, is likely to have been revolutionised in response to Earth-surface oxygenation. The details, timing, and trajectory of nitrogen cycle evolution, however, remain elusive. Here we couple nitrogen and carbon isotope records from multiple drillcores through the Rooihoogte-Timeball Hill Formations from across the Carletonville area of the Kaapvaal Craton where the Great Oxygenation Event (GOE) and its aftermath are recorded. Our data reveal that aerobic nitrogen cycling, featuring metabolisms involving nitrogen oxyanions, was well established prior to the GOE and that ammonium may have dominated the dissolved nitrogen inventory. Pronounced signals of diazotrophy imply a stepwise evolution, with a temporary intermediate stage where both ammonium and nitrate may have been scarce. We suggest that the emergence of the modern nitrogen cycle, with metabolic processes that approximate their contemporary balance, was retarded by low environmental oxygen availability.

Electrical Conductivity Model of the Mantle Lithosphere of the Slave Craton (NW Canada) and its tectonic interpretation in the context of Geochemical Results

NASA Astrophysics Data System (ADS)

Lezaeta, P.; Chave, A.; Evans, R.; Jones, A. G.; Ferguson, I.

2002-12-01

The Slave Craton, northwestern Canada, contains the oldest known rocks on Earth, with exposed outcrop over an area of about 600x400 km2. The discovery of economic diamondiferous kimberlite pipes during the early 1990s motivated extensive research in the region. Over the last six years, four types of deep-probing magnetotelluric (MT) surveys were conducted within the framework of diverse geoscientific programs, aimed at determining the regional-scale electrical structures of the craton. Two of the surveys involved novel acquisition; one through frozen lake ice along ice roads during winter, and the second deploying ocean-bottom instrumentation from float planes during summer. The latter surveys required one year of recording between summers, thus allowing long period transfer functions that lead to mantle penetration depths of over 300 km. Two-dimensional modeling of the MT data from along the winter road showed the existence of a high conductivity zone at depths of 80-120 km beneath the central Slave craton. This anomalous region is spatially coincident with an ultradepleted harzburgitic layer in the upper mantle that was interpreted by others to be related to a subducted slab emplaced during the mid-Archean. A 3-D electrical conductivity model of the Slave lithosphere has been obtained, by trial and error, to fit the magnetic transfer and MT response functions from the lake experiments. This 3-D model traces the central Slave conductor as a NE-SW oriented mantle structure. Its NE-SW orientation coincides with that of a late fold belt system, with the first phase of craton-wide plutonism at ca 2630-2590 Ma, three-part subdivision of the craton based on SKS results, and with a G10 (garnet) geochemical mantle boundaries. All of these highlight a NE-SW structural grain to the lithospheric mantle of the craton, in sharp contrast to the N-S grain of the crust. Constraints on the depth range and lateral extension of the electrical conductive structure are obtained

Extensive crustal melting during craton destruction: Evidence from the Mesozoic magmatic suite of Junan, eastern North China Craton

NASA Astrophysics Data System (ADS)

Yang, Fan; Santosh, M.; Tang, Li

2018-05-01

The cratonic destruction associated with the Pacific plate subduction beneath the eastern North China Craton (NCC) shows a close relationship with the widespread magmatism during the Late Mesozoic. Here we investigate a suite of intrusive and extrusive magmatic rocks from the Junan region of the eastern NCC in order to evaluate the role of extensive crustal melting related to decratonization. We present petrological, geochemical, zircon U-Pb geochronological and Lu-Hf isotopic data to evaluate the petrogenesis, timing and tectonic significance of the Early Cretaceous magmatism. Zircon grains in the basalt from the extrusive suite of Junan show multiple populations with Neoproterozoic and Early Paleozoic xenocrystic grains ranging in age from 764 Ma to 495 Ma as well as Jurassic grains with an age range of 189-165 Ma. The dominant population of magmatic zircon grains in the syenite defines three major age peaks of 772 Ma, 132 Ma and 126 Ma. Zircons in the granitoids including alkali syenite, monzonite and granodiorite yield a tightly restricted age range of 124-130 Ma representing their emplacement ages. The Neoproterozoic (841-547 Ma) zircon grains from the basalt and the syenite possess εHf(t) values of -22.9 to -8.4 and from -18.8 to -17.3, respectively. The Early Paleozoic (523-494 Ma) zircons from the basalt and the syenite also show markedly negative εHf(t) values of -22.7 to -18.0. The dominant population of Early Cretaceous (134-121 Ma) zircon grains presented in all the samples also displays negative εHf(t) values range from -31.7 to -21.1, with TDM of 1653-2017 Ma and TDMC in the range of 2193-3187 Ma. Accordingly, the Lu-Hf data suggest that the parent magma was sourced through melting of Mesoarchean to Paleoproterozoic basement rocks. Geochemical data on the Junan magmatic suite display features similar to those associated with the arc magmatic rocks involving subduction-related components, with interaction of fluids and melts in the suprasubduction

Neoarchean ductile deformation of the Northeastern North China Craton: The Shuangshanzi ductile shear zone in Qinglong, eastern Hebei, North China

NASA Astrophysics Data System (ADS)

Liu, Boran; Neubauer, Franz; Liu, Junlai; Jin, Wei; Li, Weimin; Liang, Chenyue

2017-05-01

Archean granitic gneiss domes and greenstone belts are well-preserved in eastern North China Craton (NCC), one of the oldest Archean terrains in the world. The Shuangshanzi ductile shear zone in Qinglong, eastern Hebei Province is located between an Archean granitic gneiss dome and a greenstone belt within an uplift in eastern NCC. Supracrustal rocks from the Neoarchean Shuangshanzi and Zhuzhangzi Groups, and some Archean granitic gneisses were involved in the shearing along the eastern margin. In the southern part, the narrow NE-trending shear zone dips NW with dip angles of 40-60° and, in the northern part, the shear zone dips NWN with dip angles of 70-85°. Microstructural and EBSD fabric analyses suggest that the shear zone was developed at upper greenschist facies to lower amphibolite facies conditions with deformation temperatures of 400-550 °C. LA-ICP-MS zircon U-Pb dating of mylonitized granitic rocks and undeformed quartz diorite cutting the shear zone suggest that the Shuangshanzi ductile shear zone was formed between 2550 Ma and 2452 Ma. Detailed kinematic studies of the shear zone show a clear sinistral shear sense with a slightly oblique-slip component in the northern part and a sinistral transtensional slip component in the southern part. It is therefore suggested that the shear zone was formed during the Anziling doming with respect to the down-slipping Neoarchean Shuangshanzi and Zhuzhangzi Groups. The difference in kinematics along the southern and the northern sections is interpreted to be caused by the doming with an uneven clockwise spiral rotation. The BIF-rich supracrustal rocks have higher density than their neighboring granitic gneisses, and therefore can easily sink to form synclines by sagduction processes. The sagduction is mainly triggered by gravitational inversion of high density supracrustal rocks with respect to relatively light granitic gneisses within the dome. As a result, the gneisses synchronously moved upward. A shear zone

Provenance analysis of heavy minerals in beach sands (Falkland Islands/Islas Malvinas) - A view to mineral deposits and the geodynamics of the South Atlantic Ocean

NASA Astrophysics Data System (ADS)

Dill, Harald G.; Skoda, Radek

2017-10-01

Beach sands are ideal traps to collect heavy minerals (HM) from different geodynamic settings and mineral deposits. The coastal sediments contain a mixture of HM derived from the submarine shelf and from source rocks in the hinterland. This is true in a transgressive periglacial regime, where drowned valleys and estuaries are instrumental in draining HM to the arenaceous beach sediments from more distal basement lithologies. A scenario like this can be found in the Falkland Islands/Islas Malvinas. The site under study is the missing link between South Africa and South America, the splitting-apart of which is mirrored by the HM distribution predominantly concentrated in the backshore and dune belt along the coast. The HM are subdivided into three HM associations reflecting the geodynamic evolution of the South Atlantic Ocean and of some of the prominent mineral deposits on the Gondwana Continent: (1) Gondwana cratons and Proterozoic orogens, with Cr and BIF deposits (rutile, zircon, ilmenite, tourmaline, garnet, Cr spinel), (2) rift-related and break-apart magmatic lithologies with mantle-derived pipe rocks such as kimberlites (zircon, pyroxene, spinel, Mg ilmenite), (3) Cordillera-type lithologies with polymetallic stratabound deposits (tourmaline, amphibole, chlorite, REE phosphates). The variation of the major HM from the stable craton (Kalahari-Kaapvaal Craton) in the East to the mobile fold belt (Andes) in the West follows the order of stability of HM. In addition to these 3 geodynamic HM groups, sporadic occurrences of HM originating from alteration (leucoxene, chlorite s.s.s. (= solid solution series)) are part of armored relics such as "nigrine" which on transport disintegrated and thereby released these HM. The major ultrastable and stable HM zircon, rutile, tourmaline s.s.s., spinel s.s.s., and garnet s.s.s. are displayed in a synoptical x-y plot showing the mantle and crustal trends of fractionation and formation of cumulates by means of particular

Lithospheric Deformation Along the Southern and Western Suture Zones of the Wyoming Province

NASA Astrophysics Data System (ADS)

Nuyen, C.; Porritt, R. W.; O'Driscoll, L.

2014-12-01

The Wyoming Province is an Archean craton that played an early role in the construction and growth of the North American continent. This region, which encompasses the majority of modern day Wyoming and southern Montana, initially collided with other Archean blocks in the Paleoproterozoic (2.0-1.8 Ga), creating the Canadian Shield. From 1.8-1.68 Ga, the Yavapai Province crashed into the Wyoming Province, suturing the two together. The accretion of the Yavapai Province gave way to the Cheyenne Belt, a deformational zone that exists along the southern border of the Wyoming Province where earlier studies have found evidence for crustal imbrication and double a Moho. Current deformation within the Wyoming province is due to its interaction with the Yellowstone Hotspot, which is currently located in the northwest portion of the region. This study images the LAB along the western and southern borders of the Wyoming Province in order to understand how the region's Archean lithosphere has responded to deformation over time. These results shed light on the inherent strength of Archean cratonic lithosphere in general. We employ two methods for this study: common conversion point (CCP) stacking of S to P receiver functions and teleseismic and ambient Rayleigh wave dispersion. The former is used to image the LAB structure while the latter is used to create a velocity gradient for the region. Results from both of the methods reveal a notably shallower LAB depth to the west of the boundary. The shallower LAB west of the Wyoming Province is interpreted to be a result of lithospheric thinning due to the region's interaction with the Yellowstone Hotspot and post-Laramide deformation and extension of the western United States. We interpret the deeper LAB east of the boundary to be evidence for the Wyoming Province's resistance to lithospheric deformation from the hotspot and tectonic processes. CCP images across the Cheyenne Belt also reveal a shallower LAB under the western

Early Neoarchaean A-type granitic magmatism by crustal reworking in Singhbhum craton: Evidence from Pala Lahara area, Orissa

NASA Astrophysics Data System (ADS)

Topno, Abhishek; Dey, Sukanta; Liu, Yongsheng; Zong, Keqing

2018-04-01

Several volumetrically minor ˜ 2.8 Ga anorogenic granites and rhyolites occur along the marginal part of the Singhbhum craton whose origin and role in crustal evolution are poorly constrained. This contribution presents petrographic, geochemical, zircon U-Pb and trace element, and mineral chemical data on such granites exposed in the Pala Lahara area to understand their petrogenesis and tectonic setting. The Pala Lahara granites are calc-alkaline, high-silica rocks and define a zircon U-Pb age of 2.79 Ga. These granites are ferroan, weakly metaluminous, depleted in Al, Ca and Mg and rich in LILE and HFSE. They are classified as A2-type granites with high Y/Nb ratios. Geochemical characteristics (high SiO2 and K2O, very low MgO, Mg#, Cr, Ni and V, negative Eu anomaly, flat HREE and low Sr/Y) and comparison with melts reported by published experimental studies suggest an origin through high-temperature, shallow crustal melting of tonalitic/granodioritic source similar to the ˜ 3.3 Ga Singhbhum Granite. Intrusion of the Pala Lahara granites was coeval with prominent mafic magmatism in the Singhbhum craton (e.g., the Dhanjori mafic volcanic rocks and NNE-SSW trending mafic dyke swarm). It is suggested that the ˜ 2.8 Ga A-type granites in the Singhbhum craton mark a significant crustal reworking event attendant to mantle-derived mafic magmatism in an extensional tectonic setting.

Recognized Multiple Rifts of the Neoproterozoic in the Initiation of the Tarim Craton (NW China) and Their Tectonic Implications

NASA Astrophysics Data System (ADS)

He, B.; Jiao, C.; Huang, T.; Zhou, X.; Cai, Z.; Cao, Z.; Jiang, Z.; Cui, J.; Yu, Z.; Chen, W.

2017-12-01

The Tarim Basin is the largest, oil-bearing and superimposed basin in the northwest of China. The development and tectonic property of the initial Tarim basin have been acutely disputed and remain enigmatic. Urgently need to reveal the origin and formation dynamics of the Tarim Carton and evaluate the potential of the deep energy resources. However, covered by vast desert and huge-thickness sedimentary strata, suffered by multiple tectonic movements, seismic data with low signal- to- noise ratio in the deep are the critical difficulties. We analyse 4 field outcrops, 18 wells, 27 reprocessed seismic reflection profiles with high SNR across the basin and many ancillary ones and aeromagnetic data. We find about 20 normal fault-controlled rift depressions of the Cryogenian and Ediacaran scattered in the Tarim basin, which developed on the Precambrian metamorphic and crystalline basements and covered by the epeiric sea and basin facies sediments of the Lower Cambrian. The structural styles of the rifts are mainly half grabens, symmetrical troughs and horst-grabens. The regional differences exist obviously in spatial and temporal. The WNW-ESE-trending faults occur in the central part and northern of the basin and the NE, and the NEE-trending faults occur in the southern parts, which response with the anomaly of aeromagnetic. Some main faults of the Ediacaran inherited from the Cryogenian and some occurred newly, the more rifting depressions occurred during the Ediacaran. The extensional NNW-SSE-oriented and NNE-SSW-oriented paleostress field occurred simultaneously during rifting, and accompanied with the clockwise shearing. According to the activities of syn-sedimentary faults, magmatic events and sediments, the tectonic properties of the rifts are different depending on their locations in the Tarim craton. The rifting phases mainly occurred from 780 Ma to 615 Ma. The formation of rifts were associated with the opening of the South Tianshan Ocean and the South Altun

East-China Geochemistry Database (ECGD):A New Networking Database for North China Craton

NASA Astrophysics Data System (ADS)

Wang, X.; Ma, W.

2010-12-01

North China Craton is one of the best natural laboratories that research some Earth Dynamic questions[1]. Scientists made much progress in research on this area, and got vast geochemistry data, which are essential for answering many fundamental questions about the age, composition, structure, and evolution of the East China area. But the geochemical data have long been accessible only through the scientific literature and theses where they have been widely dispersed, making it difficult for the broad Geosciences community to find, access and efficiently use the full range of available data[2]. How to effectively store, manage, share and reuse the existing geochemical data in the North China Craton area? East-China Geochemistry Database(ECGD) is a networking geochemical scientific database system that has been designed based on WebGIS and relational database for the structured storage and retrieval of geochemical data and geological map information. It is integrated the functions of data retrieval, spatial visualization and online analysis. ECGD focus on three areas: 1.Storage and retrieval of geochemical data and geological map information. Research on the characters of geochemical data, including its composing and connecting of each other, we designed a relational database, which based on geochemical relational data model, to store a variety of geological sample information such as sampling locality, age, sample characteristics, reference, major elements, rare earth elements, trace elements and isotope system et al. And a web-based user-friendly interface is provided for constructing queries. 2.Data view. ECGD is committed to online data visualization by different ways, especially to view data in digital map with dynamic way. Because ECGD was integrated WebGIS technology, the query results can be mapped on digital map, which can be zoomed, translation and dot selection. Besides of view and output query results data by html, txt or xls formats, researchers also can

From Texas to the Northwest Territories: Low temperature history of the North American craton using a radiation damage model for apatite He diffusion

NASA Astrophysics Data System (ADS)

Flowers, R. M.; Ault, A. K.; Wolin, E.; Kelley, S.; Bowring, S. A.

2009-12-01

The radiation damage accumulation and annealing model (RDAAM) for apatite He diffusion helps resolve previously enigmatic characteristics of apatite (U-Th)/He data in cratonic regions. First, nonlinear positive date-eU correlations are predicted for many T-t paths, thus explaining excessive scatter in some (U-Th)/He datasets. Second, under common circumstances, the RDAAM predicts (U-Th)/He dates that are older than corresponding apatite fission-track (AFT) dates, helping reconcile previous data in which (U-Th)/He dates were older than expected using Durango He diffusion kinetics. We present five apatite (U-Th)/He datasets, three with co-existing AFT data, from the North American craton that can quantitatively be explained by the RDAAM. These datasets include three from the Canadian shield (Trans-Hudson Orogen, Lake Athabasca region, Slave Craton) and two from the U.S. midcontinent (Kansas, Texas panhandle). All samples are Precambrian (4.0-1.6 Ga) basement, except for Triassic-Jurassic sandstones analyzed in the Texas study. We use the results of these studies to evaluate broad thermal history patterns across the North American craton. Although each dataset yields a distinct thermal history, all can be accounted for by varying the magnitudes of two well-documented episodes of burial and unroofing in Paleozoic-Mesozoic and Cretaceous-Tertiary times. The oldest consistent (U-Th)/He and AFT dates of these studies are early Paleozoic and are preserved in the Trans-Hudson Orogen. Together with a strong (U-Th)/He date-eU correlation and dates as young as Jurassic in the Lake Athabasca region, as well as widespread Permo-Triassic dates from the Slave craton, the three Canadian shield datasets are most simply explained by increased magnitudes of burial toward the northwest in Paleozoic-Mesozoic time, with less significant burial in the Cretaceous. In contrast, (U-Th)/He data from Kansas yield a date-eU correlation and a cluster of Cretaceous dates, (U-Th)/He dates from the

Large-scale removal of lithosphere underneath the North China Craton in the Early Cretaceous: Geochemical constraints from volcanic lavas in the Bohai Bay Basin

NASA Astrophysics Data System (ADS)

Sun, Jing; Liu, Zheng; Zhang, Shuai; Li, Xiaoguang; Qi, Jiafu

2017-11-01

Cratons are generally considered as the most stable tectonic units on the Earth. Rare magmatism, seismic activity, and intracrustal ductile deformation occur in them. However, several cratons experienced entirely different fates, including the North China Craton (NCC), and were subsequently destroyed. Geodynamic mechanisms and timing of the cratonic destruction are strongly debated. In this paper, we investigate a suite of Mesozoic intermediate to felsic volcanic rocks which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC. These volcanic rocks have Precambrian basement-like Sr-Nd isotopic characteristics, consistent with derivation from the lower continental crust underneath the NCC. The Late Jurassic ( 165 Ma) intermediate volcanic rocks don't exhibit markedly negative Eu anomalies, which require a source beyond the plagioclase stability field. And the low heavy rare earth elements (HREEs) contents of these samples indicate that their source has garnet as residue. The Early Cretaceous ( 122 Ma) felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies, suggesting that their source have both garnet and plagioclase. The crust thicknesses, estimated from the geochemistry of the intermediate and felsic rocks, are ≥ 50 km at 165 Ma and 30-50 km at 122 Ma, respectively. The crustal thinning is attributed to lithospheric delamination beneath the NCC. Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous, between 140 and 120 Ma.

Late orogenic processes between Baltica and Siberia cratons during the building of Pangea

NASA Astrophysics Data System (ADS)

Puchkov, V.

2003-04-01

Since the Middle Carboniferous, the territory between the Baltica, Kazakhstania and Siberia plates have been part of the continental crust, the last oceanic crust having by then been subducted. At this time continent-continent collision started and proceeded until the latest epochs of the Paleozoic, forming the Uralide orogen. Two features of this collision standout: 1) The northward movement of the Kazakhstanian block between the Baltica and Siberia plates resulted in a large-scale oroclinal bend of structures in West Siberia, which is well expressed in the magnetic field, as well as in the exposed Kazakhstanian structures, and 2) The orogenic event affected the whole territory between the Baltica and Siberia cratons, including the Kazakhstanian continent. A series of intermontane molasse basins are distributed throughout the orogen and in some places, like Bolshoi Karatau, are affected by Late Paleozoic deformation. Continent-continent collision had ceased by the end of the Permian, and in many places Triassic sediments unconformably overly the orogen. Triassic sedimentary sequences are typically coarse-grained (eventually proluvial), evidencing a new stage of mountain building, though many researchers have attributed this to a superplume event. The reason being the widespread flood basalt eruptions that took place not only between, but also on the cratons under discussion. There is a point of view that the break-up of the supercontinents (e.g., Pangea and Rodinia) was preceded by such a superplume event, passing to formation of linear graben systems and then to oceanic spreading. In the case under discussion, the process was arrested (the West Siberian basin with its grabens is often called a "failed ocean"). There is reason to suspect that the Paleozoic collision that led to formation of the Uralides left an enclosed ocean basin in the North, between the Baltica and Siberia plates. Only in the Early Jurassic, with the advent of the Old Kimmerian collision did

Paleomagnetism of the 765 Ma Luakela Volcanics in NW Zambia and Implications for Neoproterozoic Positions of the Congo Craton

NASA Astrophysics Data System (ADS)

Wingate, M. T.; Pisarevsky, S. A.; de Waele, B.

2004-12-01

Owing to the scarcity of reliable paleopoles, the Neoproterozoic position of the Congo craton (incorporating the Sao Francisco, Tanzania, and Bangweulu blocks) is very poorly known. We report new paleomagnetic data for the 765 ± 5 Ma Luakela volcanics, a NE-trending belt of basaltic to andesitic flows in NW Zambia (Key et al., 2001, J. Afr. Earth Sci., 33, 503-528). The volcanics are up to 0.8 km thick and occur within a 2 km thick succession of siliciclastic rocks that unconformably overlies Neoarchean and Paleoproterozoic rocks of the Congo craton margin, and is correlated with the Roan and Mwashia Groups of the Katanga Supergroup (Key et al., 2001). The strata are essentially undeformed, and either subhorizontal or dip shallowly to the SE. Although no metamorphic mineral growth is observed in fine-grained sedimentary rocks, alteration has strongly affected plagioclase and pyroxene in the volcanic rocks, and magnetite has been partially altered to hematite (Key et al., 2001). AF and thermal analysis of 65 samples from nine sites isolated three magnetisation components. Component A, carried mainly by SD magnetite, is directed very shallowly to the SE. Component B, carried mainly by hematite, is oriented shallowly SW-up. A low stability component C is directed very steeply downward. Some samples contain only component A, others only component B, and some contain both A and B. Component A is likely to be primary, because it is carried by SD magnetite (which petrography indicates is primary), does not resemble younger magnetisations from the Congo craton, and because the rocks have not been thermally metamorphosed. Component B, carried by hematite, we consider to be an overprint, possibly acquired during Pan-African deformation in the Lufilian Arc. Component C is similar to Permo-Carboniferous paleodirections from the region, and may have been acquired at that time. Paleopoles for components A and B (LVA and LVB) are about 90° apart, and similar to those from the

Contrast of lithospheric dynamics across the southern and eastern margins of the Tibetan Plateau: a numerical study

NASA Astrophysics Data System (ADS)

Sun, Yujun; Fan, Taoyuan; Wu, Zhonghai

2018-05-01

Both of the southern and eastern margins of the Tibetan Plateau are bounded by the cratonic blocks (Indian plate and Sichuan basin). However, there are many differences in tectonic deformation, lithospheric structure and surface heat flow between these two margins. What dynamics cause these differences? With the constraints of the lithospheric structure and surface heat flow across the southern and eastern margins of Tibetan Plateau, we constructed 2-D thermal-mechanical finite-element models to investigate the dynamics across these two margins. The results show that the delamination of mantle lithosphere beneath the Lhasa terrane in Oligocene and the rheological contrast between the Indian and Tibetan crust are the two main factors that control the subduction of the Indian plate. The dynamics across the eastern margin of the Tibetan Plateau are different from the southern margin. During the lateral expansion of the Tibetan Plateau, pure shear thickening is the main deformation characteristic for the Songpan-Ganzi lithosphere. This thickening results in the reduction of geothermal gradient and surface heat flow. From this study, it can be seen that the delamination of the mantle lithosphere and the rheological contrast between the Tibetan Plateau and its bounding blocks are the two main factors that control the lithospheric deformation and surface heat flow.

Devonian-Carboniferous unconformity in Argentina and its relation to the Eo-Hercynian orogeny in southern South America

NASA Astrophysics Data System (ADS)

López-Gamundí, O. R.; Rossello, E. A.

1993-04-01

The Devonian-Carboniferous contact in southern South America, characterized by a sharp unconformity, has been related to the Late Devonian-Early Carboniferous Eo-Hercynian orogeny. The Calingasta-Uspallata basin of western Argentina and the Sauce-Grande basin (Ventana Foldbelt) of eastern Argentina have been selected to characterize this unconformity. The Eo-Hercynian movements were accompanied in western Argentina by igneous activity related to a Late Devonian—Early Carboniferous magmatic arc mainly exposed today along the Andean Cordillera. This magmatic activity is partly reflected also in eastern Argentina (Ventana Foldbelt), where isotopic dates suggest a thermal event also related to the intrusions present to the west in the North Patagonian Massif and Sierras Pampeanas. The scarcity of Lower Carboniferous deposits in the stratigraphic record of southern South America suggests that the Early Carboniferous was a time interval dominated by uplift and erosion followed by widespread subsidence during the Middle and Late Carboniferous. The origin of the Eo-Hercynian orogeny can be linked with the convergence between the Arequipa Massif, and its southern extension, and the South American continent. Its effects are best represented along the ‘Palaeo-Pacific’ margin, although distant effects are discernible in the cratonic areas of eastern South America.

Geochemistry of Archean Mafic Amphibolites from the Amsaga Area, West African Craton, Mauritania: Occurrence of Archean oceanic plateau

NASA Astrophysics Data System (ADS)

El Atrassi, Fatima; Debaille, Vinciane; Mattielli, Nadine; Berger, Julien

2015-04-01

While Archean terrains are mainly composed of a TTG (Tonalite-trondhjemite-granodiorite) suite, more mafic lithologies such as amphibolites are also a typical component of those ancient terrains. Although mafic rocks represent only ~10% of the Archean cratons, they may provide key evidence of the role and nature of basaltic magmatism in the formation of the Archean crust as well as the evolution of the Archean mantle. This study focuses on the Archean crust from the West African craton in Mauritania (Amsaga area). The Amsaga Archean crust mainly consists of TTG and thrust-imbricated slices of mafic volcanic rocks, which have been affected by polymetamorphic events from the amphibolite to granulite facies. We report the results of a combined petrologic, Sm-Nd isotopic, major element and rare earth element (REE) study of the Archean amphibolites in the West African craton. This study was conducted in order to characterize these rocks, to constrain the time of their formation and to evaluate their tectonic setting and their possible mantle source. Our petrological observations show that these amphibolites have fine to medium granoblastic and nematoblastic textures. They are dominated by amphibolite-facies mineral assemblages (mainly amphibole and plagioclase), but garnet and clinopyroxene occur in a few samples. These amphibolites have tholeiitic basalt composition. On a primitive mantle-normalized diagram, they display fairly flat patterns without negative anomalies for either Eu or Nb-Ta. We have shown using Sm-Nd whole rock isotopic data that these amphibolites formed at 3.3 ±0.075 Ga. They have positive ɛNdi values (+5.2 ± 1.6). These samples show isotopically juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source and that they have a

Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data

NASA Astrophysics Data System (ADS)

Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

2016-12-01

Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. Recent studies (Bodin et al., 2015, Calo et al. 2016) confirmed the presence of a structural boundary under the north American craton at 100-140 km depths by taking advantage of the power of a trans-dimensional Monte Carlo Markov chain (TMCMC). They generated probabilistic 1D radially shear wave velocity profiles for selected stations in North America by jointly inverting 2 different data types (PS Receiver Functions, surface wave dispersion for Love and Rayleigh waves), which sample different volumes of the Earth and have different sensitivities to structure. The resulting 1D profiles include both isotropic and anisotropic discontinuities in the upper mantle (above 350 km depth). Here we extend this approach and include the vp/vs ratio as an unknown in the TMCMC algorithm to avoid artificial layers induced by multiples of the receiver functions. Additionally, we include SKS waveforms in the joint inversion and invert for azimuthal anisotropy to verify if layering in the anisotropic structure of the stable part of the North American continent involves significant changes in the direction of

Sequence stratigraphy of the Upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: Transgressing assumptions of cratonic flooding

USGS Publications Warehouse

Eoff, Jennifer D.

2014-01-01

New data from detailed measured sections permit comprehensive analysis of the sequence framework of the Furongian (Upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. The sequence-stratigraphic architecture of the lower part of the Sunwaptan Stage at the base of the Tunnel City Group, at the contact between the Wonewoc Formation and Lone Rock Formation, records the first part of complex polyphase flooding (Sauk III) of the Laurentian craton, at a scale smaller than most events recorded by global sea-level curves. Flat-pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation. Thinly-bedded, fossiliferous sandstone represents the most distal setting during earliest highstand. Subsequent deposition of sandstone characterized by hummocky or trough cross-stratification records progradational pulses of shallower, storm- and wave-dominated environments across the craton before final flooding of Sauk III commenced with carbonate deposition during the middle part of the Sunwaptan Stage. Comparison of early Sunwaptan flooding of the inner Laurentian craton to published interpretations from other parts of North America suggests that Sauk III was not a single, long-term accommodation event as previously proposed.

Intraplate Earthquakes and Deformation within the East Antarctic Craton

NASA Astrophysics Data System (ADS)

Lough, A. C.; Wiens, D.; Nyblade, A.

2017-12-01

The apparent lack of tectonic seismicity within Antarctica has long been discussed. Explanations have ranged from a lack of intraplate stress due to the surrounding spreading ridges and low absolute plate velocity (Sykes, 1978), to the weight of ice sheets increasing the vertical normal stress (Johnston, 1987). The 26 station GAMSEIS/AGAP array deployed in East Antarctica from late 2008 to early 2010 provides the first opportunity to study the intraplate seismicity of the Antarctic interior using regional data. Here we report 27 intraplate tectonic earthquakes that occurred during 2009. Depth determination together with their corresponding uncertainty estimates, show that most events originate in the shallow to middle crust, indicating a tectonic and not a cryoseismic origin. The earthquakes are primarily located beneath linear alignments of basins adjacent to the Gamburtsev Subglacial Mountains (GSM) that have been denoted as the East Antarctic rift system (Ferraccioli et al, 2011). The geophysical properties of the `rift' system contrast sharply with those of the GSM and Vostok Subglacial Highlands on either side. Crustal thickness, seismic velocity, and gravity anomalies all indicate large lateral variation in lithospheric properties. We propose the events outline an ancient continental rift, a terrain boundary feature, or a combination of the two where rifting exploited pre-existing weakness. It is natural to draw parallels between East Antarctica and the St. Lawrence depression where rifting and a collisional suture focus intraplate earthquakes within a craton (Schulte and Mooney, 2005). We quantify the East Antarctic seismicity by developing a frequency-magnitude relation, constraining the lower magnitudes with the 2009 results and the larger magnitudes with 1982-2012 teleseismic seismicity. East Antarctica and the Canadian Shield show statistically indistinguishable b-values (near 1) and seismicity rates as expressed as the number of events with mb > 4 per

The Rae craton of Laurentia/Nuna: a tectonically unique entity providing critical insights into the concept of Precambrian supercontinental cyclicity

NASA Astrophysics Data System (ADS)

Bethune, K. M.

2015-12-01

Forming the nucleus of Laurentia/Nuna, the Rae craton contains rocks and structures ranging from Paleo/Mesoarchean to Mesoproterozoic in age and has long been known for a high degree of tectonic complexity. Recent work strongly supports the notion that the Rae developed independently from the Hearne; however, while the Hearne appears to have been affiliated with the Superior craton and related blocks of 'Superia', the genealogy of Rae is far less clear. A diagnostic feature of the Rae, setting it apart from both Hearne and Slave, is the high degree of late Neoarchean to early Paleoproterozoic reworking. Indeed, following a widespread 2.62-2.58 Ga granite bloom, the margins of Rae were subjected to seemingly continuous tectonism, with 2.55-2.50 Ga MacQuoid orogenesis in the east superseded by 2.50 to 2.28 Ga Arrowsmith orogenesis in the west. A recent wide-ranging survey of Hf isotopic ratios in detrital and magmatic zircons across Rae has demonstrated significant juvenile, subduction-related crustal production in this period. Following break-up at ca. 2.1 Ga, the Rae later became a tectonic aggregation point as the western and eastern margins transitioned back to convergent plate boundaries (Thelon-Taltson and Snowbird orogens) marking onset of the 2.0-1.8 Ga assembly of Nuna. The distinctive features of Rae, including orogenic imprints of MacQuoid and Arrowsmith vintage have now been identified in about two dozen cratonic blocks world-wide, substantiating the idea that the Rae cratonic family spawned from an independent earliest Paleoproterozoic landmass before its incorportation in Nuna. While critical tests remain to be made, including more reliable ground-truthing of proposed global correlations, these relationships strongly support the notion of supercontinental cyclicity in the Precambrian, including the Archean. They also challenge the idea of a globally quiescent period in the early Paleoproterozoic (2.45-2.2 Ga) in which plate tectonics slowed or shut down.

The Western Edge of Cratonic North America and Topography of the Northern U.S. Rocky Mountains

NASA Astrophysics Data System (ADS)

Foster, D. A.; Russo, R. M.; van der Lee, S.; Mueller, P. A.

2009-12-01

We used seismic structure of the upper mantle determined via waveform inversions of surface and regional shear waves (Beadle and van der Lee, 2007) to examine the 3-D geometry of the base of North American lithosphere at the junction between thick, stable cratonic eastern North America and the thinner, recently tectonized western part of the continent. This boundary has been affected by long-term subduction beneath North America. Variability in convergence rates and directions, and especially in slab dip, have been postulated as important controls on the configuration of the transition from thick to thin lithosphere, and on the distribution and degree of crustal deformation and volcanism in the western U.S. We show that the lithospheric thickness transition at depths of 70-130 km - defined as contours of zero shear velocity anomaly - correlates strongly with the high topography of Laramide uplifts in the northern Rockies, which lie west of this seismically defined craton edge. The transition from thick to thin lithosphere also includes an embayment symmetrically centered on the Yellowstone hotspot, offset cratonward from the surface position of the hotspot by ca. 140-180 km at depths of 130-150 km. We interpret this structure as a reduction of cratonic seismic velocities reflecting the thermal halo around the hotspot, and perhaps associated with the separation of the lower lithosphere. The steep velocity gradient (boundary) east of the hotspot occurs along the Big Horn Mountains, and distributed mountain ranges of southwestern Montana. The steep transition between thin and thick lithosphere turns sharply west along the northern margin of the Helena thrust salient-Lewis and Clark fault zone, where it may reflect the edge of the Archean Medicine Hat Block and/or the northern termination of the influence shallow Farallon slab subduction the during Laramide time. Laramide-style basement uplifts are absent north of this zone and the eastern front ranges of the Rockies

Bouguer images of the North American craton and its structural evolution

NASA Technical Reports Server (NTRS)

Arvidson, R. E.; Bowring, S.; Eddy, M.; Guinness, E.; Leff, C.; Bindschadler, D.

1984-01-01

Digital image processing techniques have been used to generate Bouguer images of the North American craton that diplay more of the granularity inherent in the data as compared with existing contour maps. A dominant NW-SE linear trend of highs and lows can be seen extending from South Dakota, through Nebraska, and into Missouri. The structural trend cuts across the major Precambrian boundary in Missouri, separating younger granites and rhyolites from older sheared granites and gneisses. This trend is probably related to features created during an early and perhaps initial episode of crustal assembly by collisional processes. The younger granitic materials are probably a thin cover over an older crust.

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

NASA Astrophysics Data System (ADS)

Owona, Sébastien; Tichomirowa, Marion; Ratschbacher, Lothar; Ondoa, Joseph Mvondo; Youmen, Dieudonné; Pfänder, Jörg; Tchoua, Félix M.; Affaton, Pascal; Ekodeck, Georges Emmanuel

2012-10-01

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207Pb/206Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624 ± 2 Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87Rb/86Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87Rb/86Sr isochron ages differ from sample to sample (599 ± 3, 572 ± 4, 554 ± 5, 540 ± 5 Ma) yielding the oldest Neoproterozoic age (~600 Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540 Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87Rb/86Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540 Ma) more closely to the border of the Congo Craton.

Eclogites and garnet pyroxenites from Kimberley, Kaapvaal craton, South Africa: their diverse origins and complex metasomatic signatures

NASA Astrophysics Data System (ADS)

Shu, Qiao; Brey, Gerhard P.; Pearson, D. Graham

2018-06-01

We describe the petrography and mineral chemistry of sixteen eclogite and garnet pyroxenite xenoliths from the reworked Boshof road dump (Kimberley) and define three groups that stem from different depths. Group A, the shallowest derived, has low HREE (heavy rare earth element) abundances, flat middle to heavy REE patterns and high Mg# [= 100·Mg/(Mg + Fe)]. Their protoliths probably were higher pressure cumulates ( 0.7 GPa) of mainly clinopyroxene (cpx) and subordinate orthopyroxene (opx) and olivine (ol). Group B1 xenoliths, derived from the graphite/diamond boundary and below show similarities to present-day N-MORB that were modified by partial melting (higher Mg# and positively inclined MREE (middle REE) and HREE (heavy REE) patterns of calculated bulk rocks). Group B2 samples from greatest depth are unique amongst eclogites reported so far worldwide. The calculated bulk rocks have humped REE patterns with very low La and Lu and prominent maxima at Sm or Eu and anomalously high Na2O (up to 5 wt%) which makes protolith identification difficult. The complex trace element signatures of the full spectrum of Kimberley eclogites belie a multi-stage history of melt depletion and metasomatism with the introduction of new phases especially of phlogopite (phlog). Phlogopite appears to be characteristic for Kimberley eclogites and garnet peridotites. Modelling the metasomatic overprint indicates that groups A and B1 were overprinted by volatile- and potassium-rich melts probably by a process of chromatographic fractionation. Using constraints from other metasomatized Kimberley mantle rocks suggest that much of the metasomatic phlogopite in the eclogites formed during an intense episode of metasomatism that affected the mantle beneath this region 1.1 Gyr ago.

Rayleigh Wave Phase Velocities Beneath the Central and Southern East African Rift System

NASA Astrophysics Data System (ADS)

Adams, A. N.; Miller, J. C.

2017-12-01

This study uses the Automated Generalized Seismological Data Function (AGSDF) method to develop a model of Rayleigh wave phase velocities in the central and southern portions of the East African Rift System (EARS). These phase velocity models at periods of 20-100s lend insight into the lithospheric structures associated with surficial rifting and volcanism, as well as basement structures that pre-date and affect the course of rifting. A large dataset of >700 earthquakes is used, comprised of Mw=6.0+ events that occurred between the years 1995 and 2016. These events were recorded by a composite array of 176 stations from twelve non-contemporaneous seismic networks, each with a distinctive array geometry and station spacing. Several first-order features are resolved in this phase velocity model, confirming findings from previous studies. (1) Low velocities are observed in isolated regions along the Western Rift Branch and across the Eastern Rift Branch, corresponding to areas of active volcanism. (2) Two linear low velocity zones are imaged trending southeast and southwest from the Eastern Rift Branch in Tanzania, corresponding with areas of seismic activity and indicating possible incipient rifting. (3) High velocity regions are observed beneath both the Tanzania Craton and the Bangweulu Block. Furthermore, this model indicates several new findings. (1) High velocities beneath the Bangweulu Block extend to longer periods than those found beneath the Tanzania Craton, perhaps indicating that rifting processes have not altered the Bangweulu Block as extensively as the Tanzania Craton. (2) At long periods, the fast velocities beneath the Bangweulu Block extend eastwards beyond the surficial boundaries, to and possibly across the Malawi Rift. This may suggest the presence of older, thick blocks of lithosphere in regions where they are not exposed at the surface. (3) Finally, while the findings of this study correspond well with previous studies in regions of overlapping

Geology and mineralization of the Wyoming Province

USGS Publications Warehouse

Hausel, W.D.; Edwards, B.R.; Graff, P.J.; ,

1991-01-01

The Wyoming Province is an Archean craton which underlies portions of Idaho, Montana, Nevada, Utah, and much of Wyoming. The cratonic block consists of Archean age granite-gneiss with interspersed greenstone belts and related supracrustal terranes exposed in the cores of several Laramide uplifts. Resources found in the Province and in the adjacent accreted Proterozoic terrane include banded iron formation, Au, Pt, Pd, W, Sn, Cr, Ni, Zn, Cu, and diamonds. The Province shows many similarities to the mineral-rich cratons of the Canadian shield, the Rhodesian and Transvaal cratons of southern Africa, and the Pilbara and Yilgarn blocks of Western Australia, where much of the world's precious and strategic metal and gemstone resources are located.

A mantle origin for Paleoarchean peridotitic diamonds from the Panda kimberlite, Slave Craton: Evidence from 13C-, 15N- and 33,34S-stable isotope systematics

NASA Astrophysics Data System (ADS)

Cartigny, Pierre; Farquhar, James; Thomassot, Emilie; Harris, Jeffrey W.; Wing, Bozwell; Masterson, Andy; McKeegan, Kevin; Stachel, Thomas

2009-11-01

In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ13C-value of - 5.2‰ and range from - 6.9 to - 3.0‰, with one extreme value at - 14.1‰. The associated δ15N-values range from - 17.0 to + 8.5‰ with a mean value of - 4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ13C-values ranging from - 11.2 to - 4.4‰ with one extreme value at - 19.4‰. Their δ15N ranges from - 2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ13C values, between - 4.5 and - 3.5‰. The δ34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from - 3.5 to +5.7‰. None of them provide evidence for anomalous δ33S-values; observed variations in δ33S are from +0.19 to - 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur ( δ33S = 0‰). At Panda, the N contents and the δ13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ34S, Δ 33S, δ15N and δ13C, we find no

Attenuation Characteristics of High Frequency Seismic Waves in Southern India

NASA Astrophysics Data System (ADS)

Sivaram, K.; Utpal, Saikia; Kanna, Nagaraju; Kumar, Dinesh

2017-07-01

We present a systematic study of seismic attenuation and its related Q structure derived from the spectral analysis of P-, S-waves in the southern India. The study region is separated into parts of EDC (Eastern Dharwar Craton), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT). The study is carried out in the frequency range 1-20 Hz, using a single-station spectral ratio technique. We make use of about 45 earthquakes, recorded in a network of about 32 broadband 3-component seismograph-stations, having magnitudes ( M L) varying from 1.6 to 4.5, to estimate the average seismic body wave attenuation quality factors; Q P and Q S. Their estimated average values are observed to be fitting to the power law form of Q = Q 0 f n . The averaged power law relations for Southern Indian region (as a whole) are obtained as Q P = (95 ± 1.12) f (1.32±0.01); Q S = (128 ± 1.84) f (1.49±0.01). Based on the stations and recorded local earthquakes, for parts of EDC, WDC and SGT, the average power law estimates are obtained as: Q P = (97 ± 5) f (1.40±0.03), Q S = (116 ± 1.5) f (1.48±0.01) for EDC region; Q P = (130 ± 7) f (1.20±0.03), Q S = (103 ± 3) f (1.49±0.02) for WDC region; Q P = (68 ± 2) f (1.4±0.02), Q S = (152 ± 6) f (1.48±0.02) for SGT region. These estimates are weighed against coda Q ( Q C) estimates, using the coda decay technique, which is based on a weak backscattering of S-waves. A major observation in the study of body wave analysis is the low body wave Q ( Q 0 < 200), moderately high value of the frequency-exponent, ` n' (>0.5) and Q S/ Q P ≫ 1, suggesting lateral stretches of dominant scattering mode of seismic wave propagation. This primarily could be attributed to possible thermal anomalies and spread of partially fluid-saturated rock-masses in the crust and upper mantle of the southern Indian region, which, however, needs further laboratory studies. Such physical conditions might partly be correlated to the active seismicity and

Density of the continental roots: Compositional and thermal contributions

USGS Publications Warehouse

Kaban, M.K.; Schwintzer, P.; Artemieva, I.M.; Mooney, W.D.

2003-01-01

The origin and evolution of cratonic roots has been debated for many years. Precambrian cratons are underlain by cold lithospheric roots that are chemically depleted. Thermal and petrologic data indicate that Archean roots are colder and more chemically depleted than Proterozoic roots. This observation has led to the hypothesis that the degree of depletion in a lithospheric root depends mostly on its age. Here we test this hypothesis using gravity, thermal, petrologic, and seismic data to quantify differences in the density of cratonic roots globally. In the first step in our analysis we use a global crustal model to remove the crustal contribution to the observed gravity. The result is the mantle gravity anomaly field, which varies over cratonic areas from -100 to +100 mGal. Positive mantle gravity anomalies are observed for cratons in the northern hemisphere: the Baltic shield, East European Platform, and the Siberian Platform. Negative anomalies are observed over cratons in the southern hemisphere: Western Australia, South America, the Indian shield, and Southern Africa. This indicates that there are significant differences in the density of cratonic roots, even for those of similar age. Root density depends on temperature and chemical depletion. In order to separate these effects we apply a lithospheric temperature correction using thermal estimates from a combination of geothermal modeling and global seismic tomography models. Gravity anomalies induced by temperature variations in the uppermost mantle range from -200 to +300 mGal, with the strongest negative anomalies associated with mid-ocean ridges and the strongest positive anomalies associated with cratons. After correcting for thermal effects, we obtain a map of density variations due to lithospheric compositional variations. These maps indicate that the average density decrease due to the chemical depletion within cratonic roots varies from 1.1% to 1.5%, assuming the chemical boundary layer has the same

The Acasta Gneiss - a Hadean cratonic nucleus

NASA Astrophysics Data System (ADS)

Sprung, P.; Scherer, E. E.; Maltese, A.; Bast, R.; Bleeker, W.; Mezger, K.

2016-12-01

The known terrestrial rock record lacks undisputed, chemically intact Hadean crust. Direct evidence from this eon has been restricted to zircon grains within younger rocks [1]. The Acasta Gneiss Complex (AGC; NT, CA) has yielded zircon with Hadean domains [e.g., 2,3], but the time at which AGC rocks became closed chemical systems is unclear [4,5]. Determining this `time of last disturbance' (tld) would provide a minimum protolith age, and is crucial for using radiogenic isotope compositions of bulk rocks to trace crust-mantle evolution. Recent studies mostly focused on the `low-strain' eastern AGC [e.g., 6, 7], which records an evolving, early-mid Archean cratonic nucleus [7]. We also studied the `high-strain' banded gneiss in the western AGC, which hosts >4 Ga zircon domains [2,3], too. Our focusing lay on adjoining, lithologically distinct bands [8] of two distinct chemical groups: A) Mafic, chondrite-normalized LaN/YbN ≦20, slightly HFSE- depleted, and B) TTG-like, LaN/YbN up to 145, markedly HFSE-depleted. Six adjacent bands yield a well-defined 4 Ga Sm-Nd isochron with a ɛNd4Ga of +2 and ɛHf4Ga values from +1 to +6. Within-band Sm-Nd and Lu-Hf systematics imply younger mineral re-equilibration [9]. We interpret the 4 Ga Sm-Nd isochron to date the physical juxtaposition of bands in the gneiss unit and to define tld among bands for elements less mobile and diffusive than Sm and Nd. Contrasting Sm-Nd results from the same unit [10] likely are due to sampling at too fine a scale. Digestion of metamict pre-tld zircon likely caused the scatter in Lu-Hf. Both decay systems hint at the existence of a possibly local, strongly depleted Hadean mantle domain. The TTG-like bands are 0.4 Gyr older than similar rocks in the `low-strain' eastern AGC [7]. The AGC was thus an evolved cratonic nucleus already at 4 Ga, possibly with a depleted lithospheric keel. [1] Cavosie et al. (2004) Prec. Res. 135, 251-279 [2] Bowring & Williams (1999) CMP 134, 3-16 [3] Iizuka et al

The joint inversion of phase dispersion curves and receiver functions at the margin of East European Craton

NASA Astrophysics Data System (ADS)

Chrapkiewicz, Kajetan; Wilde-Piórko, Monika; Polkowski, Marcin

2017-04-01

For the first time a joint inversion of Rayleigh-wave phase velocity dispersion curves and P receiver functions has been applied to study the south-western margin of East European Craton (EEC) in Poland. The area of investigation lies in the vicinity of Trans-European Suture Zone (TESZ) regarded as the most prominent lithospheric boundary in Europe separating the Precambrian EEC from assemblage of Phanerozoic-accreted terranes (e.g. Pharaoh, 1999). While the sedimentary and crystalline crust of EEC's margin has been precisely recognized with the borehole and refraction data compilation (Grad et al., 2016), the structure of lithosphere-asthenosphere boundary (LAB) underneath remains poorly understood. To address this issue, the passive seismic experiment „13 BB Star" (2013-2016) was carried out in northern Poland - just at the margin of EEC. For each station of „13 BB Star" network we obtained a credible 1-D shear-wave velocity model with linearized damped least-squares inversion (Herrmann, 2013) down to the depth of 250 km. The joint inversion of receiver functions and surface-wave dispersion curves has proved to be a natural approach when inferring the nature of cratonic LAB (e.g. Bodin et al., 2014). It's sensitive to both absolute velocities and sharp discontinuities and thus provides a better vertical resolution compared to surface wave data alone. The results indicate the presence of steady 4 per cent grow in the shear-wave velocity between 120 and 180 km depth and gradual 6 per cent drop over 180-220 km depth range. The latter may be interpreted as the LAB with depth and absolute-velocity change similar to those reported for other cratons (Kind et al., 2012). National Science Centre Poland provided financial support for this work by NCN grant DEC-2011/02/A/ST10/00284.

A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

NASA Astrophysics Data System (ADS)

Halls, Henry C.; Li, Jianghai; Davis, Don; Hou, Guiting; Zhang, Baoxing; Qian, Xianglin

2000-10-01

A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China's largest dyke swarm, which trends for about 1000km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U-Pb dating of primary zircon, is 1769.1+/-2.5Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D=36°, I=-5°, k=63, α95=4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp=2°, dm=4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800Ma

Lithosphere mantle density of the North China Craton based on gravity data

NASA Astrophysics Data System (ADS)

Xia, B.; Artemieva, I. M.; Thybo, H.

2017-12-01

Based on gravity, seismic and thermal data we constrained the lithospheric mantle density at in-situ and STP condition. The gravity effect of topography, sedimentary cover, Moho and Lithosphere-Asthenosphere Boundary variation were removed from free-air gravity anomaly model. The sedimentary covers with density range from 1.80 g/cm3 with soft sediments to 2.40 g/cm3 with sandstone and limestone sediments. The average crustal density with values of 2.70 - 2.78 g/cm3 which corresponds the thickness and density of the sedimentary cover. Based on the new thermal model, the surface heat flow in original the North China Craton including western block is > 60 mW/m2. Moho temperature ranges from 450 - 600 OC in the eastern block and in the western block is 550 - 650 OC. The thermal lithosphere is 100 -140 km thick where have the surface heat flow of 60 - 70 mW/m2. The gravity effect of surface topography, sedimentary cover, Moho depth are 0 to +150 mGal, - 20 to -120 mGal and +50 to -200 mGal, respectively. By driving the thermal lithosphere, the gravity effect of the lithosphere-asthenosphere boundary ranges from 20 mGal to +200 mGal which shows strong correction with the thickness of the lithosphere. The relationship between the gravity effect of the lithosphere-asthenosphere boundary and the lithosphere thickness also for the seismic lithosphere, and the value of gravity effect is 0 to +220 mGal. The lithospheric mantle residual gravity which caused by lithospheric density variation range from -200 to +50 mGal by using the thermal lithosphere and from -250 to +100 mGal by driving the seismic lithosphere. For thermal lithosphere, the lithospheric mantle density with values of 3.21- 3.26 g/cm3 at in-situ condition and 3.33 - 3.38 g/cm3 at STP condition. Using seismic lithosphere, density of lithosphere ranges from 3.20 - 3.26 g/cm3 at in-situ condition and 3.31 - 3.41 g/cm3 at STP condition. The subcontinental lithosphere of the North China Craton is highly heterogeneous

The peculiar case of Marosticano xenoliths: a cratonic mantle fragment affected by carbonatite metasomatism in the Veneto Volcanic Province (Northern Italy)

NASA Astrophysics Data System (ADS)

Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo; Florencia Fahnestock, M.; Bryce, Julia G.; Marzoli, Andrea

2017-04-01

The Tertiary Magmatic Province of Veneto, known as Veneto Volcanic Province (VVP), in the Northern Italy, represents one of the most important volcanic provinces of the Adria Plate. It is composed by five volcanic districts: Val d'Adige, Marosticano, Mts. Lessini, Berici Hills and Euganean Hills. Most of the volcanic products are relatively undifferentiated lavas, from nephelinites to tholeiites in composition. Commonly VVP nephelinites and basanites carry mantle xenoliths. This study presents a petrological characterization of the new xenolith occurrence of Marosticano and comparison with previously studied VVP xenolith populations (i.e. from the Lessinean and Val d'Adige areas), which represent off-craton lithospheric mantle fragment affected by Na-alkaline silicate metasomatism (Siena & Coltorti 1989; Beccaluva et al., 2001; Gasperini et al., 2006). Marosticano (MA) peridotites are anhydrous spinel-bearing lherzolites and harzburgites, which are geochemically well distinguishible from the other VVP mantle xenoliths. Primary minerals record the "most restitic" composition of the VVP sampled mantle, even calling the geochemical features of a sub-cratonic mantle. Olivines in both lherzolites and harzburgites show high Ni contents compared with the Fo values (Ni→ lherzolite: 2600-3620 ppm; harzburgite: 2600-3540 ppm; Fo → lh: 91-92; hz: 90-93) that follow the trend of olivine from a cratonic area (Kelemen, 1998). Orthopyroxenes have mg# values with 1:1 ratio with coexisting olivines and Al2O3 contents always <4 wt%, even for the most fertile lherzolite. Low Al2O3 (<5 wt%) associated with high Cr2O3 (>0.5 wt%) contents are also the chemical characteristics of the clinopyroxenes. On the whole both MA pyroxenes show major element contents that recall the characteristics of those from cratonic (sp-bearing) peridotites (e.g. from Greenland, South Africa and Tanzania; Downes et al., 2004). In addition, the relationship between the high Fo content of olivine and the

Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with late Proterozoic intracratonic troughs

NASA Astrophysics Data System (ADS)

Alvarez, Philippe

1995-09-01

During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession. The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29 30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650 Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in

Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with Late Proterozoic intracratonic troughs

NASA Astrophysics Data System (ADS)

Alvarez, Philippe

During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession.The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29-30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in

Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the Arabian Shield

USGS Publications Warehouse

Fleck, Robert J.; Greenwood, W.R.; Hadley, D.G.; Anderson, R.E.; Schmidt, D.L.

1980-01-01

Rubidium-strontium studies of Precambrian volcanic and plutonic rocks of the Arabian Shield document an early development of the Arabian craton between 900 and 680 m.y. (million years) ago. Geologic studies indicate an island-arc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north or northwest-trending axes. Magmatic trends show consistent variation in both composition and geographic location as a function of age. The oldest units belong to an assemblage of basaltic strata exposed in western Saudi Arabia that yield an age of 1165:!:110 m.y. The oldest andesitic strata studied yield an age of 912:!:76 m.y. The earliest plutonic units are diorite to trondhjemite batholiths that range from 800 to 9,00 m.y. in age and ,occur along the western and southern parts of Saudi Arabia. Younger plutonic units, 680 to 750 m.y. in age, range from quartz diorite to granodiodte and become more abundant in the central and northeastern parts of the Arabian Shield. Initial 'Sr/ 86 Sr ratios for both dioritic groups range from 0.7023 to 0.7030 and average 0.7027. The absence of sialic detritus in sedimentary units and the evidence for an island-arc environment suggest the early development of the Arabian craton at a convergent plate margin between plates of oceanic lithosphere. Active subduction apparently extended from at least 900 m.y. to about 680 m.y. Subsequent to this subduction-related magmatism and tectonism, called the Hijaz tectonic cycle, the Arabian craton was sutured to the late Precambrian African plate in a collisional event. This period of orogeny, represented in Arabia and eastern Africa by the Mozambiquian or Pan-African event, extended from some time before 650 m.y. to at least 540 m.y. and perhaps 520 m.y. B.P. Although the tectonic processes of subduction and continental collision during the 900+ to 500-m.y. period require similar directions of plate convergence, the

Fluid Characteristics in the Giant Quartz Reef System of the Bundelkhand Craton, India: Constraints from Fluid Inclusion Study

NASA Astrophysics Data System (ADS)

Rout, D.; Panigrahi, M. K.; Pati, J. K.

2017-12-01

Giant quartz reefs are anomalous features indicating extensive mobilization of silica in the crust. Such reefs in the Abitibi belt, Canada and elsewhere are believed to be the result of activity of fluid of diverse sources on terrain boundaries. The Bundelkhand granitoid complex constituting a major part of the Bundelkhnad Craton in north-Central India is traversed by numerous such quartz reefs all across for a length of about 500 km. There are about 20 major reefs having dimensions of 35 to 40 km in length, 50 to 60 m in width standing out as prominent ridges in the region. Almost all are aligned parallel to each other in a sub-vertical to vertical manner following the NE-SW to NNE-SSW trend. Fluid inclusion petrography in quartz from these reefs reveal four types of inclusions viz. aqueous biphase (type-I), pure carbonic (type-II), aqueous carbonic (type-III) and polyphase (type-IV) inclusions. The type-I aqueous biphase inclusions are the dominant type in all the samples studied so far. Salinities calculated from temperature of melting of last ice (Tm) values are low to moderate, ranging from 0.18 to 18.19 wt% NaCl equivalents. Temperature of liquid-vapor homogenization (Th) values of these inclusions show a wide range from 101 ºC to 386 ºC (cluster around 150-250 ºC) essentially into liquid phase ruling out boiling during its course of evolution. Besides, aqueous Biphase inclusions, some data on pure CO2 inclusions furnish a near constant value of TmCO2 at -56.6 ºC in the Bundelkhand Craton indicating absence of CH4. Bivariate plot between Th and salinity suggest three possible water types which are controlling the overall activity of fluid in quartz reefs of Bundelkhand Craton viz. low-T low saline, high-T low saline and moderate-T and moderate saline. A low saline and CO2-bearing and higher temperature nature resembles a metamorphic fluid that may be a source for these giant quartz reefs. The low temperature low-salinity component could be a meteoric

Crustal accretion and exhumation of the Rio de la Plata Craton

NASA Astrophysics Data System (ADS)

Girelli, T. J.; Chemale, F., Jr.; Lavina, E.; Laux, J. H.; Bongiolo, E.; Lana, C.

2017-12-01

The Rio de la Plata is one key area for the reconstruction of the Paleoproterozoic Supercontinent in Western Gondwana. We present U-Pb-Hf isotopes, chemistry on minerals and whole-rock geochemistry from para and orthogneisses of the Santa Maria Chico Granulite Complex, one of the Rio de la Plata fragments partially affected by the Brasiliano Orogeny. U-Pb and Lu-Hf isotopes allowed the characterization of two main events: an oceanic juvenile crustal accretion (i) 2430 - 2290 Ma (ɛHf(t) -3.17 to +7.00); a continental arc (ii) 2240 - 2120 Ma (ɛHf(t)= -4 to +2.4). We recognized two main high-grade metamorphic events in the region linked to an arc volcanic setting (830 - 870 °C - 6.7 - 7.2 kbar, 2.3 Ga) and later to continent-continent collision (770 - 790 °C and 8.7 - 9.1 kbar, 2.1 - 2.0 Ga). The development of orogenic sedimentary basins (fore-arc and intra-arc) occurred during the last cycle with the maximum depositional age of 2.12 Ga and were metamorphosed during 2.06 Ga main granulitic event. The granulitic rocks were cut by 1.8 Ga alkaline granitic dikes related to crustal extension recognized in the different segments of the craton and widespread in the adjacent paleoplates at the time. The present data point to that Paleoproterozoic granulitic rocks of the Santa Maria Chico Granulite Complex and adjacent Nico Pérez and Rivera terranes, formed in a multi-stage volcanic arc to continental collision environment along 370 Ma (2430 to 2060 Ma). These terranes were amalgamated during the Paleoproterozoic to the core of the Rio de la Plata Craton as part of Columbia Supercontinent and later partially reworked during the amalgamation of Western Gondwana in the Neoproterozoic.

Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

USGS Publications Warehouse

Downes, H.; Macdonald, R.; Upton, B.G.J.; Cox, K.G.; Bodinier, J.-L.; Mason, P.R.D.; James, D.; Hill, P.G.; Hearn, B.C.

2004-01-01

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzbugites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0??5121 (close to the host minette values) to 0??5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd

Platinum-group element contents of Karelian kimberlites: Implications for the PGE budget of the sub-continental lithospheric mantle

NASA Astrophysics Data System (ADS)

Maier, W. D.; O'Brien, H.; Peltonen, P.; Barnes, Sarah-Jane

2017-11-01

We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38 ppb Pt and 1.33 ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd > 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3-27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. 10.1016/j.chemgeo.2016.08.019).

Brittle Deformation in the Ordos Basin in response to the Mesozoic destruction of the North China Craton

NASA Astrophysics Data System (ADS)

Wang, Q.; Jiang, L.

2012-12-01

Craton is continental block that has been tectonically stable since at least Proterozoic. Some cratons, however, become unstable for some geodynamic reasons. The North China Craton (NCC) is an example. Structure geological, geochemical, and geophysical works have revealed that the NCC was destructed in Cretaceous and that lithosphere thickness beneath the eastern NCC were thinned by 120 km. The present study will focus on deformation of the western NCC, and to understand the effect of the Mesozoic destruction of the North China Craton (NCC). Structural partitioning of the Ordos Basin, which is located in the western NCC, from the eastern NCC occurred during the Mesozoic. Unlike the eastern NCC where many Cretaceous metamorphic core complexes developed, sedimentary cover of the NCC remains nearly horizontal and deformation is manifested by joint. We visited 216 sites of outcrops and got 1928 joints measurements, among which 270 from Jurassic sandstones, 1378 from the Upper Triassic sandstones, 124 from the Middle and Lower Triassic sandstones, and 156 from Paleozoic sandstones. In the interior of the Ordos Basin, joints developed quite well in the Triassic strata, while joints in the Jurassic stata developed weakly and no joint in the Cretaceous strata. The Mesozoic stratigraphic thickness are: 1000 meters for the Lower Triassic, the Middle Triassic sandstone with thickness of 800 meters, 3000 meters for the Upper Triassic, 4000 meters for the Jurassic, and 1100 meters for the Lower Cretaceous. The vertical difference in joint development might be related to the burying depth of the strata: the higher the strata, the smaller the lithostatic stress, and then the weaker the joint. Joints in all stratigraphic levels showed a similar strain direction with the sigma 1 (the maximum pressure stress) vertical and the sigma 3 (the minimum pressure stress) horizontal and running N-S. The unconformity below the Cretaceous further indicates that joints in Jurassic and Triassic

Long wavelength magnetic anomalies over continental rifts in cratonic region

NASA Astrophysics Data System (ADS)

Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

2017-12-01

New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost (<80 km depth) lithospheric mantle in these regions in the spinel-peridotite and plagioclase-peridotite stability fields. The most common lithology by far (95% of samples) is a spinel-lherzolite indicating relatively low oxygen fugacities (FMQ -1). Chrome spinel in these peridotites is non-magnetic (Al + Mg > 0.2 or Fe < 0.3) and primary magnetite (Fe3O4) occurs only in trace amounts, typically yielding low natural remanent magnetizations (NRM < 10-2 A/m). The low Koenigsberger ratios (Qn < 1) of these materials, combined with high geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also <1, the new data presented here suggests that relatively young rifts would display a central negative magnetic anomaly surrounded by two broad positive anomalies. The latitude of the rift is predicted to exert a primary control on the magnitude of such anomalies, while the steepness of the magnetic gradient across the rift would primarily reflect thermal equilibration over time.

U-Pb thermochronology of the lower crust: producing a long-term record of craton thermal evolution

NASA Astrophysics Data System (ADS)

Blackburn, T.; Bowring, S. A.; Mahan, K. H.; Perron, T.; Schoene, B.; Dudas, F. O.

2010-12-01

The EarthScope initiative is focused on providing an enhanced view of the North American lithosphere and the present day stress field of the North American continent. Of key interest is the interaction between convecting asthenosphere and the conducting lithospheric mantle that underlie the continents, especially the cold ‘keels’ that underlie Archean domains. Cratonic regions are in general characterized by minimal erosion and or sediment accumulation. The Integration of seismic tomography, and mantle xenolith studies reveal a keel of seismically fast and relatively buoyant and viscous mantle; physical properties that are intimately linked with the long-term stability and topographic expression of the region. Missing from this model of the continental lithosphere is the 4th dimension--time--and along with it our understanding of the long-term evolution of these stable continental interiors. Here we present a thermal record from the North American craton using U-Pb thermochronology of lower crustal xenoliths. The use of temperature sensitive dates on lower crustal samples can produce a unique time-temperature record for a well-insulated and slowly cooling lithosphere. The base of the crust is insulated enough to remain unperturbed by any plausible changes to surface topography, yet unlike the subadjacent lithospheric mantle, contains accessory phases amenable to U-Pb dating (rutile, apatite, titanite). With near steady state temperatures in the lower crust between 400-600 °C, U-Pb thermochronometers with similar average closure temperatures for Pb are perfectly suited to record the long-term cooling of the lithosphere. Xenoliths from multiple depths, and across the craton yield time-temperature paths produced from U-Pb thermochronometers that record extremely slow cooling (<0.25 °C/Ma) over time scales of billions of years. Combining these data with numerical thermal modeling allow constraints to be placed on the dominant heat transfer mechanisms operating

U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India

NASA Astrophysics Data System (ADS)

Chauhan, Hiredya; Saikia, Ashima; Kaulina, Tatiana; Bayanova, Tamara; Ahmad, Talat

2015-04-01

The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have experienced multiple deformation and metamorphic events followed by emplacement of voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been inferred that the whole magmatic episode leading to the formation of the basement spanned from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by 2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably deposited. However, no comprehensive age data on the basement gneisses from the study area spanning the entire magmatic episode is available. This work attempts to provide a time frame work for evolution of the basement gneisses of the Aravalli Craton. We present here U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of north western India. Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The temperatures of measurements were 1300°C for Pb and 1500°C for U. Pb isotope ratios were corrected for mass fractionation with a factor of 0.10% per amu, based on repeat analyses of the standard NBS SRM 982. The U analyses were corrected for mass fractionation with a factor of 0.003% per amu, based on repeat analyses of the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5% for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991, 2008). Four zircon fractions corresponding to four

Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons

NASA Astrophysics Data System (ADS)

Bolhar, Robert; Hofmann, Axel; Kemp, Anthony I. S.; Whitehouse, Martin J.; Wind, Sandra; Kamber, Balz S.

2017-10-01

Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8-3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral

Development of the Archaean Mallina Basin, Pilbara Craton, northwestern Australia; a study of detrital and inherited zircon ages

NASA Astrophysics Data System (ADS)

Smithies, R. H.; Nelson, D. R.; Pike, G.

2001-06-01

SHRIMP U-Pb zircon dates are combined with an examination of the age distribution patterns and provenance of both detrital zircons and of zircon xenocrysts in granites to investigate the development of the Archaean Mallina Basin, in the granite-greenstone terrain of the Pilbara Craton, northwestern Australia. The oldest dated components of the basin are c. 3010 Ma volcaniclastic rocks in the western part of the area. New data indicate that siliciclastic turbidites that dominate the southern and eastern part of the basin were deposited at or after c. 2970 Ma but before c. 2955 Ma. Linking both the detrital zircon populations as well as zircon xenocrysts from granites that intruded the Mallina Basin to well-dated areas of the Pilbara granite-greenstone terrane indicates that the sediment was derived from the south, north, northwest, and east. The basin probably evolved primarily in an intracontinental setting between two elevated land masses to the southeast and northwest. Most of the rocks within the basin were folded before intrusion of granites, the oldest of which has been dated at 2954±4 Ma. Evidence of a second depositional cycle is provided by a maximum depositional age of 2941±9 Ma, indicated by a detrital zircon population from a sample of wacke from the southeast part of the Mallina Basin. This second depositional phase may have been related to renewed extension, and recycling of sedimentary rocks within the basin.

Reassessing Geophysical Models of the Bushveld Complex in 3D

NASA Astrophysics Data System (ADS)

Cole, J.; Webb, S. J.; Finn, C.

2012-12-01

dense, thicker crust underneath the Bushveld Complex necessitates the presence of dense material in the central area between the eastern and western lobes. The simplest way to achieve this is to model the mafic component of the Bushveld Complex as a single intrusion. This is similar to what the first students of the Bushveld Complex suggested. Conceptual models are by definition simplified versions of the real situation, and the geometry of the Bushveld Complex is expected to be much more intricate. References Cawthorn, R.G., Cooper, G.R.J., Webb, S.J. (1998). Connectivity between the western and eastern limbs of the Bushveld Complex. S Afr J Geol, 101, 291-298. Cousins, C.A. (1959). The structure of the mafic portion of the Bushveld Igneous Complex. Trans Geol Soc S Afr, 62, 179-189. Du Plessis, A., Kleywegt, R.J. (1987). A dipping sheet model for the mafic lobes of the Bushveld Complex. S Afr J Geol, 90, 1-6. Nguuri, T.K., Gore, J., James, D.E., Webb, S.J., Wright, C., Zengeni, T.G., Gwavava, O., Snoke, J.A. and Kaapvaal Seismic Group. (2001). Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons. Geoph Res Lett, 28, 2501-2504. Webb, S.J., Cawthorn, R.G., Nguuri, T., James, D. (2004). Gravity modelling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results, S Afr J Geol, 107, 207-218.

Mantle Flow Implications across Easter and Southern Africa from Shear Wave Splitting Measurements

NASA Astrophysics Data System (ADS)

Ramirez, C.; Nyblade, A.; Bagley, B. C.; Mulibo, G. D.; Tugume, F.; Wysession, M. E.; Wiens, D.; van der Meijde, M.

2015-12-01

In this study, we present new shear wave splitting results from broadband seismic stations in Botswana and Namibia, and combine them with previous results from stations in Kenya, Uganda, Tanzania, Malawi, Zambia, South Africa, Mozambique, Zimbabwe, and Angola to further examine the pattern of seismic anisotropy across southern Africa. The new results come from stations in northern Namibia and Botswana, which help to fill in large gaps in data coverage. Our preliminary results show that fast polarization directions overall trend in a NE orientation. The most noticeable measurements that deviate from this pattern are located around the Archean Tanzania Craton in eastern Africa. The general NE pattern of fast polarization directions is attributed to mantle flow linked to the African superplume. Smaller scale variations from this general direction can be explained by shape anisotropy in the lithosphere in magmatic regions in the East African rift system and to fossil anisotropy in the Precambrian lithosphere.

Paleoproterozoic volcanic centers of the São Félix do Xingu region, Amazonian craton, Brazil: Hydrothermal alteration and metallogenetic potential

NASA Astrophysics Data System (ADS)

da Cruz, Raquel Souza; Fernandes, Carlos Marcello Dias; Villas, Raimundo Netuno Nobre; Juliani, Caetano; Monteiro, Lena Virgínia Soares; Lagler, Bruno; Misas, Carlos Mario Echeverri

2016-06-01

Geological, petrographic, scanning electron microscopy, and X-ray diffraction studies revealed hydrothermalized lithotypes evidenced by overprinted zones of potassic, propylitic, sericitic, and intermediate argillic alterations types, with pervasive and fracture-controlled styles, in Paleoproterozoic volcano-plutonic units of the São Félix do Xingu region, Amazonian craton, northern Brazil. The Sobreiro Formation presents propylitic (epidote + chlorite + carbonate + clinozoisite + sericite + quartz ± albite ± hematite ± pyrite), sericitic (sericite + quartz + carbonate), and potassic (potassic feldspar + hematite) alterations. The prehnite-pumpellyite pair that is common in geothermal fields also occurs in this unit. The Santa Rosa Formation shows mainly potassic (biotite + microcline ± magnetite), sericitic (sericite + quartz + carbonate ± chlorite ± gold), and intermediate argillic (montmorillonite + kaolinite/halloysite + illite) alterations. These findings strongly suggest the involvement of magma-sourced and meteoric fluids and draw attention to the metallogenetic potential of these volcanic units for Paleoproterozoic epithermal and rare and base metal porphyry-type mineralizations, similar to those already identified in other portions of the Amazonian craton.

Crustal structure and tectonics of the northern part of the Southern Granulite Terrane, India

USGS Publications Warehouse

Rao, V.V.; Sain, K.; Reddy, P.R.; Mooney, W.D.

2006-01-01

Deep seismic reflection studies investigating the exposed Archean lower continental crust of the Southern Granulite Terrane, India, yield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of granulites. Seismic reflection images along the Kuppam-Bhavani profile reveal a band of reflections that dip southward from 10.5 to 15.0??s two-way-time (TWT), across a distance of 50??km. The bottom of these reflections beneath the Dharwar craton is interpreted as the Moho. Further south, another reflection band dipping northward is observed. These bands of reflectivity constitute a divergent reflection fabric that converges at the Moho boundary observed at the Mettur shear zone. Reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks, which we infer resulted in crustal thickening and the formation of granulites. Anomalous gravity and magnetic signatures are also observed across the Mettur shear zone. The gravity model derived from the Bouguer gravity data corroborates seismic results. The tectonic regime and seismic reflection profiles are combined in a 3-D representation that illustrates our evidence for paleo-subduction at a collision zone. The structural dissimilarities and geophysical anomalies suggest that the Mettur shear zone is a suture between the Dharwar craton in the north and another crustal block in the south. This study contributes significantly to our understanding of the operation of Archean plate tectonics, here inferred to involve collision and subduction. Furthermore, it provides an important link between the Gondwanaland and global granulite evolution occurring throughout the late Archean. ?? 2006 Elsevier B.V. All rights reserved.

Proterozoic orogens in southern Peninsular India: Contiguities and complexities

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Santosh, M.

2013-12-01

The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a

Sedimentary facies analysis of the Mesozoic clastic rocks in Southern Peru (Tacna, 18°S): Towards a paleoenvironmental Redefinition and stratigraphic Reorganization

NASA Astrophysics Data System (ADS)

Alván, Aldo; Jacay, Javier; Caracciolo, Luca; Sánchez, Elvis; Trinidad, Inés

2018-07-01

The Mesozoic rocks of southern Peru comprise a Middle Jurassic to Early Cretaceous sedimentary sequence deposited during a time interval of approximately 34 Myr. In Tacna, these rocks are detrital and constitute the Yura Group (Callovian to Tithonian) and the Hualhuani Formation (Berriasian). Basing on robust interpretation of facies and petrographic analysis, we reconstruct the depositional settings of such units and provide a refined stratigraphic framework. Accordingly, nine types of sedimentary facies and six architectural elements are defined. They preserve the record of a progradational fluvial system, in which two styless regulated the dispersion of sediments: (i) a high-to moderate-sinuosity meandering setting (Yura Group), and a later (ii) incipient braided setting (Hualhuani Formation). The Yura Group (Callovian-Tithonian) represents the onset of floodplain deposits and lateral accretion of point-bar deposits sited on a semi-flat topography. Nonetheless, the progradational sequence was affected by at least two rapid marine ingressions occurred during Middle Callovian and Tithonian times. Such marine ingressions reveal the proximity of a shallow marine setting and incipient carbonate deposition. In response to increase in topographic gradient, the Hualhuani Formation (Berriasian) deposited as extensive multistory sandy channels. The mineralogy of the Mesozoic sediments suggests sediment supplies and intense recycling from a craton interior (i.e. Amazon Craton and/or plutonic sources) located eastward of the study area.

Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

NASA Astrophysics Data System (ADS)

van der Meer, Quinten H. A.; Klaver, Martijn; Reisberg, Laurie; Riches, Amy J. V.; Davies, Gareth R.

2017-11-01

Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from ∼50 to ∼170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on

New U-Pb zircon geochronology of the Choma-Kalomo Block (Zambia) and the Dete-Kamativi Inlier (Zimbabwe), with implications for the extent of the Zimbabwe Craton.

NASA Astrophysics Data System (ADS)

Glynn, Sarah; Wiedenbeck, Michael; Master, Sharad; Frei, Dirk

2015-04-01

The Choma-Kalomo Block is a north-east trending, Mesoproterozoic terrane located in southern Zambia. It is composed of as yet undated gneissic basement with a high-grade metamorphosed supracrustal metasedimentary sequence, which is intruded by hornblende granites and gneisses of the Choma-Kalomo Batholith, that is dated between ca. 1.37 and 1.18 Ga. Our new zircon U-Pb age data on metasedimentary rocks of the Choma-Kalomo Block identifies samples of different ages, with slightly different provenances. The oldest metasedimentary rock is a muscovite-biotite schist, which has only Palaeoproterozoic detrital zircons, the two age clusters around 2.03-2.02 Ga and 1.8-1.9 Ga, correspond to the ages of granitic intrusion, and metamorphism, in the Magondi Mobile Belt on the western side of the Archaean Zimbabwe Craton. The second sample is a garnetiferous paragneiss, which contains both Palaeoproterozoic (2.04 Ga), and Mesoproterozoic zircons, ca. 1.36 Ga, derived from the granites of the Choma-Kalomo Batholith. The third sample is a biotite-muscovite schist, in which the detrital zircon ages fall into four separate clusters: ca. 3.39 Ga, ca. 2.7-2.6 Ga, ca. 2.1-1.7 Ga (with a peak at ca. 1.18 Ga), and 1.55 - 1.28 Ga. The Archaean zircons in this sample are derived from the Zimbabwe Craton, while the Palaeoproterozoic samples come from the Magondi belt, and the youngest zircons come from both phases of the Choma-Kalomo Batholith. A possible connection between the Choma-Kalomo Block and the Dete-Kamativi Inlier - some 150 km to the south-east in western Zimbabwe - has been proposed on the basis of similarities in the nature of their Sn-Ta-muscovite pegmatite mineralisation. The Dete-Kamativi Inlier, which is part of the Magondi Mobile Belt, is a window into Palaeoproterozoic north-east trending belts of deformed and metamorphosed supracrustal rocks. By dating localities which we suspect form the basement to the surrounding younger sediments, along with selected pegmatites

Structure and development of the southern Moroccan continental shelf

USGS Publications Warehouse

Dillon, William P.

1974-01-01

The structure of the continental shelf off southern Morocco was studied by means of 2,100 km of seismic reflection profiles, magnetic and bathymetric surveys, and dredge samples. The research area lies off four geologic divisions adjacent to the coast: the Atlas Mountains; the Souss Trough; the Anti-Atlas Mountains; and the Aaiun Basin. The continental shelf, along with the western Atlas Mountains, the western Souss Trough, and the entire Aaiun Basin, has subsided along a normal fault-flexure system. This system runs along the shore at the Anti-Atlas Mountains, and cuts off this cratonic block from the shelf subsidence. The shelf is narrow and characterized by out-building off the Anti-Atlas range, whereas it is broader and characterized by upbuilding to the north and south. Deposition was essentially continuous at least from Early Cretaceous through Eocene time. Published work suggests that the last cycle of sedimentation began during Permian rifting. After Eocene time, most sediments carried to the shelf must have bypassed it and gone to construct the slope and rise or to the deep sea. Tertiary orogenies caused extensive folding of Mesozoic and early Tertiary deposits off the Atlas Mountains. ?? 1974.

Silurian sequence stratigraphy in the North American craton, Great Lakes area

USGS Publications Warehouse

Shaver, R.H.; ,

1996-01-01

A notable circumstance of late Early through Late Silurian sedimentation on the Great Lakes area craton is that at least two and possibly three cycles of third-order duration (if eustatically considered) are recognized in basin and shallow-platform settings alike. Both virtually pure and siliciclastic-rich carbonate rocks exist in parts of platform-situated sections in contrast to siliciclastic-rich to evaporite-dominated basin sections. Knowledge of the reef history, together with evidence of incidental periodic incursions of siliciclastic sediments, permitted understanding of a regional event or sequence stratigraphy more than 15 years ago before conventional biostratigraphic and physical stratigraphic evidence became adequate to corroborate. This midwestern US and Ontario Silurian record has become strategic for testing different schools of thought that champion either tectonism or eustasy to explain cyclical sequences.

Integrated Numerical Model for the East African Rift System: Plume-induced Rifting and Continental Break-up from Lake Malawi to Red Sea

NASA Astrophysics Data System (ADS)

Koptev, A.; Leroy, S. D.; Calais, E.; Gerya, T.

2016-12-01

We present numerical experiments that target to reveal the role of active mantle plume, far-field tectonic forces and pre-existing lithospheric heterogeneities in structural development of the East African Rift system (EARS). Starting with models capturing the essential geophysical features of the central and southern parts of the EARS (two «cratonic» bodies (Tanzanian craton and Bangweulu block) embedded into a «normal» surrounding lithosphere) we show that development of the magmatic Eastern branch, the amagmatic Western branch and its southern prolongation (Malawi rift) can be the result of non-uniform splitting of some hot plume material that has been initially seeded underneath the southern part of Tanzanian craton. The second series of experiments has been designed in order to investigate northern segment of the EARS where Afro-Arabian plate separation is supposed to be related with the impact of Afar mantle plume. These models permit us to reproduce observed orientation and relative position of two spreading axes (Red Sea, Gulf of Aden) and rifting (Main Ethiopian rift) one. All are joining at Afar triple junction. Finally, for laterally extended experiment we have used parameters of the best-fit models for the southern and northern segments of the EARS in order to define the position of Kenyan plume and the velocity boundary conditions. This model cover all rifting and spreading structure associated with both Afar and Kenyan plumes: Red Sea Rift and the Aden Ridge to the north of the Afar Triple Junction; Main Ethiopian Rift running to the south that continues as the Kenyan Rift; Western Rift and its southern prolongation corresponding to Malawi rift.We argue that main features of the EARS can be reproduced in a relatively simple context of the interaction between two mantle anomalies corresponding to Afar and Kenyan plumes and pre-stressed rheologically stratified continental lithosphere containing only first-order structural heterogeneities (such as

Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

NASA Astrophysics Data System (ADS)

Xia, B.; Thybo, H.; Artemieva, I. M.

2017-07-01

We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

Blueschist facies fault tectonites from the western margin of the Siberian Craton: Implications for subduction and exhumation associated with early stages of the Paleo-Asian Ocean

NASA Astrophysics Data System (ADS)

Likhanov, Igor I.; Régnier, Jean-Luc; Santosh, M.

2018-04-01

The tectonic evolution of the Siberian Cratonic margins offers important clues for global paleogeographic reconstructions, particularly with regard to the complex geological history of Central Asia. The Yenisey Ridge fold-and-thrust belt at the western margin of the Siberian Craton forms part of the Central Asian Orogenic Belt (CAOB) and is a key to understand the Precambrian tectonic evolution of the Siberian Craton and crustal growth in the CAOB, the world's largest Phanerozoic accretionary orogenic belt. Here we report for the first time, the occurrence of glaucophane schist relics in tectonites within the Yenisey shear zone which provides insights on Chilean-type convergent boundary. We present results from isotope geochronology (SHRIMP zircon analysis and mica 40Ar/39Ar dating), coupled with P-T calculations derived from conventional geothermobarometry and pseudosections in the system NCKFMASH that suggest two superimposed metamorphic events. During the first stage, glaucophane schists formed at around 640-620 Ma at P-T conditions of 8-10 kbar and 400-450 °C. In the second stage, the rocks experienced dynamic metamorphism (c. 600 Ma) at 11-15 kbar/550-640 °C. The differences in P-T parameters between weakly deformed rocks and intensely deformed tectonites and P-T paths suggest distinct tectonic processes. Geochemical features of the mafic tectonites suggest N-MORB and E-MORB affinity, and the zircon U-Pb ages suggest formation of the protoliths at 701.6 ± 8.4. The sequence of spreading, subduction and shear deformation identified in our study correlate with the early stages of development of the Paleo-Asian Ocean at the western margin of the Siberian Craton and supports the spatial proximity of Siberia and Laurentia at 700-600 Ma, as proposed for the Late Neoproterozoic paleogeographic reconstructions and as robustly constrained from large igneous province (LIP) record.

At the Cratonic Crossroads: A geochronologic and geochemical perspective on the Little Rocky Mountains, Montana

NASA Astrophysics Data System (ADS)

Gifford, J. N.; Mueller, P. A.; Foster, D. A.; Mogk, D. W.

2012-12-01

The Medicine Hat Block (MHB) is a poorly constrained structural element in the Paleoproterozoic amalgamation of Laurentia. It lies between the Wyoming and Hearne cratons along the northern margin of the Great Falls Tectonic Zone. The block was caught between the Hearne and Wyoming cratons during the Paleoproterozoic closure of an ocean and subsequent continental collision. The majority of the MHB is concealed by younger material, and it is recognized primarily by its seismic signature and its influence on the geochemistry of younger igneous rocks. The MHB appears to be composed of Archean (2.6-3.1 Ga) and Proterozoic (1.75 Ga) continental crust based on limited data from drill holes and xenoliths. The Little Rocky Mountains (LRM) are the only potential exposure of Precambrian basement rocks in the northeastern GFTZ, and represent unique surface exposure of the MHB. The LRM is cored by a dome-shaped Tertiary syenite intrusion, with Precambrian metamorphic units exposed along the margins of the dome. Limited previous geochronology from the LRM includes K/Ar ages of 1.7-1.75 Ga and a Rb/Sr age of c. 2.55 Ga from a quartzofeldspathic paragneisses. These data leave the affinity of the LRM uncertain, either representing reworked Archean crust and/or Paleoproterozoic material generated during the subduction of oceanic lithosphere and formation of the GFTZ. New U/Pb ages of zircons from the Precambrian meta-igneous rocks in the LRM range from 2.2 - 3.3 Ga, with prominent peaks between 2.6 - 2.8 Ga. Outliers clustering around 1.7 - 1.8 Ga are rare and likely reflect Paleoproterozoic reworking of older material. These ages are consistent with a MHB affinity for the LRM. Pb-isotope data define a 3.1 Ga model age, which suggests some influence of older Wyoming Craton or MHB crust. The dominance of 2.6-2.8 Ga U/Pb ages suggests that the Paleoproterozoic igneous arc was constructed on pre-existing MHB crust. Models for reconciling the high angle junction between the GFTZ and

Reworking of Archean mantle in the NE Siberian craton by carbonatite and silicate melt metasomatism: Evidence from a carbonate-bearing, dunite-to-websterite xenolith suite from the Obnazhennaya kimberlite

NASA Astrophysics Data System (ADS)

Ionov, Dmitri A.; Doucet, Luc S.; Xu, Yigang; Golovin, Alexander V.; Oleinikov, Oleg B.

2018-03-01

The Obnazhennaya kimberlite in the NE Siberian craton hosts a most unusual cratonic xenolith suite, with common rocks rich in pyroxenes and garnet, and no sheared peridotites. We report petrographic and chemical data for whole rocks (WR) and minerals of 20 spinel and garnet peridotites from Obnazhennaya with Re-depletion Os isotope ages of 1.8-2.9 Ga (Ionov et al., 2015a) as well as 2 pyroxenites. The garnet-bearing rocks equilibrated at 1.6-2.8 GPa and 710-1050 °C. Some xenoliths contain vermicular spinel-pyroxene aggregates with REE patterns in clinopyroxene mimicking those of garnet. The peridotites show significant scatter of Mg# (0.888-0.924), Cr2O3 (0.2-1.4 wt.%) and high NiO (0.3-0.4 wt.%). None are pristine melting residues. Low-CaO-Al2O3 (≤0.9 wt.%) dunites and harzburgites are melt-channel materials. Peridotites with low to moderate Al2O3 (0.4-1.8 wt.%) usually have CaO > Al2O3, and some have pockets of calcite texturally equilibrated with olivine and garnet. Such carbonates, exceptional in mantle xenoliths and reported here for the first time for the Siberian mantle, provide direct evidence for modal makeover and Ca and LREE enrichments by ephemeral carbonate-rich melts. Peridotites rich in CaO and Al2O3 (2.7-8.0 wt.%) formed by reaction with silicate melts. We infer that the mantle lithosphere beneath Obnazhennaya, initially formed in the Mesoarchean, has been profoundly modified. Pervasive inter-granular percolation of highly mobile and reactive carbonate-rich liquids may have reduced the strength of the mantle lithosphere leading the way for reworking by silicate melts. The latest events before the kimberlite eruption were the formation of the carbonate-phlogopite pockets, fine-grained pyroxenite veins and spinel-pyroxene symplectites. The reworked lithospheric sections are preserved at Obnazhennaya, but similar processes could erode lithospheric roots in the SE Siberian craton (Tok) and the North China craton, where ancient melting residues and

Granite-hosted molybdenite mineralization from Archean Bundelkhand craton-molybdenite characterization, host rock mineralogy, petrology, and fluid inclusion characteristics of Mo-bearing quartz

NASA Astrophysics Data System (ADS)

Pati, J. K.; Panigrahi, M. K.; Chakarborty, M.

2014-06-01

The dominantly high-K, moderate to high SiO2 containing, variably fractionated, volcanic-arc granitoids (± sheared) from parts of Bundelkhand craton, northcentral India are observed to contain molybdenite (Mo) in widely separated 23 locations in the form of specks, pockets, clots and stringers along with quartz ± pyrite ± arsenopyrite ± chalcopyrite ± bornite ± covellite ± galena ± sphalerite and in invisible form as well. The molybdenite mineralization is predominantly associated with Bundelkhand Tectonic Zone, Raksa Shear Zone, and localized shear zones. The incidence of molybdenite is also observed within sheared quartz and tonalite-trondhjemite-granodiorite (TTG) gneisses. The fluid inclusion data show the presence of bi-phase (H2O-CO2), hypersaline and moderate temperature (100°-300°C) primary stretched fluid inclusions suggesting a possible hydrothermal origin for the Mo-bearing quartz occurring within variably deformed different granitoids variants of Archean Bundelkhand craton.

Late Pan-African and early Mesozoic brittle compressions in East and Central Africa: lithospheric deformation within the Congo-Tanzania Cratonic area

NASA Astrophysics Data System (ADS)

Delvaux, D.; Kipata, M. L.; Macheyeki, A. S.

2012-04-01

Tectonic reconstructions leading to the formation of the Central-African part of Gondwana have so far not much taken into account constraints provided by the evolution of brittle structures and related stress field. This is largely because little is known on continental brittle deformation in Equatorial Africa before the onset of the Mesozoic Central-African and Late Cenozoic East-African rifts. We present a synthesis of fault-kinematic data and paleostress inversion results from field surveys covering parts of Tanzania, Zambia and the Democratic Republic of Congo. It is based on investigations along the eastern margin of the Tanzanian craton, in the Ubendian belt between the Tanzanian craton and Bangweulu block, in the Lufilian Arc between the Kalahari and Congo cratons and along the Congo intracratonic basin. Paleostress tensors were computed for a substantial database by interactive stress tensor inversion and data subset separation, and the relative succession of major brittle events established. Two of them appear to be of regional importance and could be traced from one region to the other. The oldest one is the first brittle event recorded after the paroxysm of the Terminal Pan-African event that led to the amalgamation Gondwana at the Precambrian-Cambrian transition. It is related to compressional deformation with horizontal stress trajectories fluctuating from an E-W compression in Central Tanzania to NE-SW in the Ubende belt and Lufilian Arc. The second event is a transpressional inversion with a consistent NW-SE compression that we relate to the far-field effects of the active margin south of Gondwana during the late Triassic - early Jurassic.

Mantle eclogites and garnet pyroxenites - the meaning of two-point isochrons, Sm-Nd and Lu-Hf closure temperatures and the cooling of the subcratonic mantle

NASA Astrophysics Data System (ADS)

Shu, Qiao; Brey, Gerhard P.; Gerdes, Axel; Hoefer, Heidi E.

2014-03-01

The Earth's mantle is a huge metamorphic complex which undergoes permanent changes ruled by plate tectonics. It also has enclaves underneath Archean crust which are exempt from the convecting mantle since at least 2.5 Ga. Since then, this mantle may cool slowly as a result of diminishing heat input from the asthenosphere, of declining heat production from radioactive decay and of denudation of the crust. Under such circumstances, two point garnet-clinopyroxene isochrons from mantle xenoliths (here garnet pyroxenites and eclogites) reflect either cooling ages or eruption ages depending on whether the mantle portion under consideration was below or above the closure temperature of a radiogenic system. Available literature data from the Slave and Kaapvaal craton for the Sm-Nd and Lu-Hf isotope systems and our own new data from Bellsbank (Kaapvaal) provide a whole range of two-point isochron ages from younger than the kimberlite eruption age to early Proterozoic. The meaning of ages other than the kimberlite eruption age is unclear. We use here a compilation of Sm-Nd and Lu-Hf two-point isochron age data from this study and the literature, to assess the meaning of such isochrons. This is achieved by plotting the temperature of last equilibration as derived from the Fe-Mg exchange between garnet and clinopyroxene versus the two-point isochron age. There is a low temperature alignment for both systems of increasing age with decreasing temperature and an alignment around the kimberlite eruption ages at high temperatures. We interpret the intersect between the low temperature limb and the kimberlite eruption age as closure temperature which gives about 920 °C for the Lu-Hf system and about 850 °C for the Sm-Nd system. The differences of the cooling ages between the two isotope systems for individual samples combined with the closure temperatures from this study are used to deduce the cooling of the Slave and Kaapvaal subcratonic mantle from the early Proterozoic until

Gravity and magnetic modelling in the Vrancea Zone, south-eastern Carpathians: Redefinition of the edge of the East European Craton beneath the south-eastern Carpathians

NASA Astrophysics Data System (ADS)

Bocin, A.; Stephenson, R.; Matenco, L.; Mocanu, V.

2013-11-01

A 2D gravity and magnetic data model has been constructed along a 71 km densely observed profile, called DACIA PLAN GRAV MAN's. The profile crosses part of the nappe pile of the south-eastern Carpathians and includes the seismically active Vrancea Zone and was acquired with the objective to illuminate the basement structure and affinity in this area. The modelling approach was to create an initial model from well constrained geological information, integrate it with previous seismic ray tracing and tomographic models and then alter it outside the a priori constraints in order to reach the best fit between observed and calculated potential field anomalies. The results support a realignment of the position of the TTZ (Tornquist-Teisseyre Zone), the profound tectonic boundary within Europe that separates Precambrian cratonic lithosphere of the East European Craton (EEC) from younger accreted lithosphere of Phanerozoic mobile belts to its west. The TTZ is shown to lie further to the south-west than was previously inferred within Romania, where it is largely obscured by the Carpathian nappes. The crust of the EEC beneath the south-eastern Carpathians is inferred to terminate along a major crustal structure lying just west of the Vrancea seismogenic zone. The intermediate depth seismicity of the Vrancea Zone therefore lies within the EEC lithosphere, generally supporting previously proposed models invoking delamination of cratonic lithosphere as the responsible mechanism.

The Archean kalsilite-nepheline syenites of the Awsard intrusive massif (Reguibat Shield, West African Craton, Morocco) and its relationship to the alkaline magmatism of Africa

NASA Astrophysics Data System (ADS)

Haissen, Faouziya; Cambeses, Aitor; Montero, Pilar; Bea, Fernando; Dilek, Yildirim; Mouttaqi, Abdellah

2017-03-01

More than 40% of the known alkaline complexes are reported from Africa. Most are ring complexes composed of syenites and associated or not, lithotypes as carbonatites, granites and mafic rocks. Radiometric dating indicates the presence of alkaline complexes with ages spanning from Precambrian to the present. In terms of outcrops, alkaline complexes are reported from cratonic zones and from belts embedded between cratonic areas. Because of the high economic potential for associated REE deposits, these alkaline complexes have received much attention from Earth scientists. These studies aim mainly to constrain the role of the mantle and the crust (and the interaction between them) in the genesis of this peculiar magmatism, and also to explain the variability observed in lithotypes and geotectonic settings. Among those alkaline complexes, Precambrian occurrences are rare. Up-to-date only a few Proterozoic examples were cited in Africa. The recently studied Awsard complex in Southern Morocco is a peculiar one with a crystallization age of 2.46 Ga and an unusual rock assemblages. This paper is a first approximation to a comparison of geochemical and isotopic fingerprints of the Awsard magmatism (as the oldest one) with other known different ages African complexes from different geotectonic settings, aiming to detect if there is any evolution in this alkaline magmatism through time. A first conclusion is that magma sources for this alkaline magmatism has been probably evaluating over geological time, from parental magmas compositions close to that of primitive mantle in these early geological time to compositions holding more and more depleted mantle and continental crust components. However, to go further in this debate more modern isotopic, geochemical and geochronological data from all these complexes are needed. Nevertheless, this comparison highlighted the peculiar character of the Awsard magmatism with an isotopic composition very close to that of Primitive mantle

Velocity model of the crust and upper mantle at the southern margin of the East European Craton (Azov Sea-Crimea-Black Sea area), DOBRE-2 & DOBRE'99 transect

NASA Astrophysics Data System (ADS)

Starostenko, Vitaly; Janik, Tomasz; Stephenson, Randell; Gryn, Dmytro; Tolkunov, Anatoliy; Czuba, Wojciech; Środa, Piotr; Sydorenko, Grigoriy; Lysynchuk, Dmytro; Omelchenko, Victor; Grad, Marek; Guterch, Aleksander; Kolomiyets, Katerina; Thybo, Hans; Dannowski, Anke; Flűh, Ernst R.; Legostaeva, Olga

2013-04-01

The southern part of the eastern European continental landmass consists mainly of a thick platform of Vendian and younger sediments overlying Precambrian basement, part of the East European Craton (EEC). The Scythian Platform (SP) lies between the EEC and the (mainly Alpine) deformed belt running from Dobrudja (Romania) to Crimea (Ukraine) and the Greater Caucasus (Russia), along the northern margin of the Black Sea. Hard constraints on the Palaeozoic history on the SP are very sparse and little is known of its crustal structure in this area. The poster presents the seismic results of a multidisciplinary project that fills some of this gap. The project is called DOBRE-2 (as it forms a prolongation of the successful DOBRE project executed in 1999-2001). The main objectives of DOBRE-2 were to elucidate the deep-seated structure of the lithosphere and geodynamic setting of the shelf zones of the Azov and Black seas and the Crimean peninsula and to study the deep controls on the structure of basement and sedimentary cover. DOBRE-2 traverses a number of major faults and suture zones separating the EEC from the SP, the Crimean Mountains, and the Black Sea depression. Significant hydrocarbon reserves occur in the basins traversed by DOBRE-2. Deep seismic reflection profiling (30 second, Vibroseis) has been completed on a 100-km segment of the profile on the Azov massif (part of the Ukrainian Shield) as well as a 47-km segment in Crimea. These are complemented by refraction profiling on the shelf zones of the Azov (~53 km) and Black (~160 km) seas and coincident near-vertical (CDP) in the Black Sea, using a combination of onshore seismograph stations, ocean-bottom seismometers, onshore explosive energy sources (6 shot points), as well as ship-borne seismic acquisition. We present a 2-D seismic velocity model (Vp in the crust, depth to the Moho and depth to the intracrustal reflectors) along (~780 km) the DOBRE-2 & DOBRE'99 transect. Our model extends the model published

A petrological view of early Earth geodynamics

NASA Astrophysics Data System (ADS)

Herzberg, C.

2003-04-01

Xenoliths of low T Archean cratonic mantle consist mostly of harzburgite and lherzolite with geochemical depletions that are characterisitc of igneous residues. Many authors have identified the complementary magmas as komatiites. This model is re-examined in light of work presented in Herzberg & O'Hara (2002) and found to be problematic. Munro-type alumina-undepleted komatiites from Alexo, Pyke Hill, and other locations often contain olivine phenocrysts with maximum Mg# \\cong 94. Residues of fractional melting would consist of pure dunite having Mg# = 97-98, but these are not observed. Residues of equilibrium melting would also be pure dunite with Mg# = 94, but these are also not observed. Olivines with Mg# = 94 are found in rare harzburgites, indicating that residues of alumina-undepleted komatiite have either been overprinted by subsequent magmatism or they have been geodynamically eroded. Alumina-undepleted komatiites can be successfully modeled with a primary magma containing 30% MgO produced by 0.5 mass fractions of equilibrium melting of depleted peridotite. A hot plume interpretation is consistent with both the petrology and helium isotopic compositions of alumina-undepleted komatiites. But what about cratonic mantle? The FeO and MgO contents of residues of fertile mantle peridotite formed by both equilibrium and fractional melting can be predicted and applied to xenoliths of cratonic mantle in most cases. Application to xenoliths from the Kaapvaal and Slave cratons is not possible owing to a second stage of Opx enrichment, but results can be applied to most xenoliths from Siberia, Tanzania, Somerset Island, and east Greenland as they contain less than 45% SiO_2. These xenoliths are very similar to residues produced by fractional melting. Pressures of initial melting were mostly 3 to 5 GPa, but can be as high 7 GPa. Pressures of final melting were highly variable and can be as low as 1 GPa. Potential temperatures (T_P) were typically 1450 to 1600oC and

The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere

NASA Astrophysics Data System (ADS)

Day, James M. D.; Barry, Peter H.; Hilton, David R.; Burgess, Ray; Pearson, D. Graham; Taylor, Lawrence A.

2015-03-01

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (<10 kyr) emplaced xenoliths, to 0.05 RA in olivine from cratonic peridotite xenoliths of the 1179 Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1 ± 2.1 RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on

Cordilleran hingeline: Late Precambrian rifted margin of the North American craton and its impact on the depositional and structural history, Utah and Nevada

NASA Astrophysics Data System (ADS)

Picha, Frank; Gibson, Richard I.

1985-07-01

The structural pattern set by late Precambrian rifting and fragmentation of the North American continent is apparent in both sedimentary and tectonic trends in western Utah and eastern Nevada. The late Precambrian cratonic margin (Cordilleran hingeline) displays several prominent structural features, such as the Wasatch and Ancient Ephraim faults, Fillmore arch and northeast-trending lineaments, which were repeatedly reactivated as structural uplifts, ramps, strike-slip faults, and extensional detachments. The renewed activity affected, among others, the geometry of the late Paleozoic Ancestral Rocky Mountain uplifts and basins, the extent of the Jurassic Arapien basin, the sedimentary pattern of the Cretaceous foreland basin, the geometry of the Sevier orogenic belt, and the extent and type of Basin-and-Range extensional tectonics. The rifted cratonic margin has thus remained a major influence on regional structures long after rifting has ceased. *Present address: Everest Geotech, 10101 Southwest Freeway, Houston, Texas 77074

Sedimentation and tectonics in the southern Bida Basin, Nigeria: depositional response to varying tectonic context

DOE Office of Scientific and Technical Information (OSTI.GOV)

Braide, S.P.

1990-05-01

The Upper Cretaceous Bida basin of central Nigeria is sandwiched between the Precambrian schist belts of the Northern Nigerian massif and the West African craton. Of interest is the southern part of the basin, which developed in continental settings, because the facies architecture of the sedimentary fill suggests a close relation between sedimentation dynamics and basin margin tectonics. This relationship is significant to an understanding of the basin's origin, which has been controversial. A simple sag and rift origin has been suggested, and consequently dominated the negative thinking on the hydrocarbon prospects of the basin which were considered poor. Thismore » detailed study of the facies indicates rapid basin-wide changes from various alluvial fan facies through flood-basin and deltaic facies to lacustrine facies. Paleogeographic reconstruction suggests lacustrine environments were widespread and elongate. Lacustrine environments occurred at the basin's axis and close to the margins. This suggests the depocenter must have migrated during the basin's depositional history and subsided rapidly to accommodate the 3.5-km-thick sedimentary fill. Although distinguishing pull-apart basins from rift basins, based solely on sedimentologic grounds, may be difficult, the temporal migration of the depocenter, as well as the basin architecture of upward coarsening cyclicity, show a strong tectonic and structural overprint that suggests a tectonic framework for the Southern Bida basin similar in origin to a pull-apart basin.« less

Interaction of coeval felsic and mafic magmas from the Kanker granite, Pithora region, Bastar Craton, Central India

NASA Astrophysics Data System (ADS)

Elangovan, R.; Krishna, Kumar; Vishwakarma, Neeraj; Hari, K. R.; Ram Mohan, M.

2017-10-01

Field and petrographic studies are carried out to characterize the interactions of mafic and felsic magmas from Pithora region of the northeastern part of the Bastar Craton. The MMEs, syn-plutonic mafic dykes, cuspate contacts, magmatic flow textures, mingling and hybridization suggest the coeval emplacement of end member magmas. Petrographic evidences such as disequilibrium assemblages, resorption textures, quartz ocelli, rapakivi and poikilitic textures suggest magma mingling and mixing phenomena. Such features of mingling and mixing of the felsic and mafic magma manifest the magma chamber processes. Introduction of mafic magmas into the felsic magmas before initiation of crystallization of the latter, results in hybrid magmas under the influence of thermal and chemical exchange. The mechanical exchange occurs between the coexisting magmas due to viscosity contrast, if the mafic magma enters slightly later into the magma chamber, then the felsic magma starts to crystallize. Blobs of mafic magma form as MMEs in the felsic magma and they scatter throughout the pluton due to convection. At a later stage, if mafic magma enters the system after partial crystallization of felsic phase, mechanical interaction between the magmas leads to the formation of fragmented dyke or syn-plutonic mafic dyke. All these features are well-documented in the study area. Field and petrographic evidences suggest that the textural variations from Pithora region of Bastar Craton are the outcome of magma mingling, mixing and hybridization processes.

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

USGS Publications Warehouse

Tosdal, R.M.

1996-01-01

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link

Lithospheric structure of the Northern Ordos and adjacent regions from surface wave tomography: implications to the tectonics of the North China Craton

NASA Astrophysics Data System (ADS)

LI, S.; Guo, Z.; Chen, Y. J.

2017-12-01

We present a high-resolution upper mantle S velocity model of the northern Ordos block using ambient noise tomography and two-plane-wave tomography between 8 and 143 s. The Ordos block, regarded as the nuclei of the Archean craton of North China Craton, is underlain by high velocity down to 200 km, indicating the preservation of cratonic root at the interior. However, thick lithospheric keel (≥ 200 km) is not observed outside the Ordos, suggesting craton reworking around the Ordos. The most important findings is the prominent low velocity shown beneath the Datong volcano that migrates westward with depth. At 200 km depth, the low velocity locates almost 500 km west to the leading edge of the flat-lying Pacific slab in the mantle transition zone. This observation is in conflict with the previous interpretation that the Datong volcano is fed by the deep upwelling related to the subduction of the Pacific plate. The westward tilted low velocity beneath the Datong volcano, however, is in agreement with the predominant NW-SE trending alignment of fast direction revealed by SKS splitting in this area, suggesting the Datong volcano is likely due to the asthenospheric mantle flow from west. Two possible scenarios could be related to this mantle process. First, the low velocity beneath the Datong volcano may link to the large-scale, deep-rooted mantle upwelling beneath the Mongolia, northwest to the Datong volcano at deeper depth revealed by Zhang et al. (2016). We postulate that when the raising mantle materials reaches the shallow depth, it would be forced bent by the thick lithosphere beneath the Gobi in Mongolia and flow southeastward to Datong volcano. Second, it is also worth noting that the low velocity beneath the Datong volcano connects to the low velocity zone (LVZ) beneath the Ordos block below 200km, which further links the LVZ beneath the northeastern Tibet to the west. Therefore, the Datong volcano could be fed by the mantle flow from northeastern Tibet. The

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

NASA Astrophysics Data System (ADS)

Tappe, Sebastian; Foley, Stephen F.; Kjarsgaard, Bruce A.; Romer, Rolf L.; Heaman, Larry M.; Stracke, Andreas; Jenner, George A.

2008-07-01

New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites ( 87Sr/ 86Sr i = 0.70323-0.70377; ɛNd i = +1.2-+1.8; ɛHf i = +1.4-+3.5; 206Pb/ 204Pb i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures ( 87Sr/ 86Sr i = 0.70388-0.70523; ɛNd i = -0.5 to -3.9; ɛHf i = -0.6 to -6.0; 206Pb/ 204Pb i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO 2, Al 2O 3, Na 2O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region. Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO 2-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and

Direct dating of paleomagnetic results from Precambrian sediments in the Amazon craton: Evidence for Grenvillian emplacement of exotic crust in SE Appalachians of North America

NASA Astrophysics Data System (ADS)

D'Agrella-Filho, Manoel S.; Tohver, Eric; Santos, João O. S.; Elming, Sten-Åke; Trindade, Ricardo I. F.; Pacca, Igor I. G.; Geraldes, Mauro C.

2008-03-01

We apply a new diagenetic dating technique to determine the age of magnetization for Precambrian sedimentary rocks in the SW Amazon craton. Two new paleomagnetic poles are reported from the rocks of the Aguapeí Gp.: red beds of the Fortuna Fm. (Plat = 59.8°N, Plon = 155.9°E, A95 = 9.5, K = 14, 18 sites, N/n 128/115, Q = 5) and the reverse-polarity mudstones of the overlying Vale da Promissão Formation (Plat = 49.5°N, Plon = 89.3°E, A95 = 12.5, K = 30, 6 sites, N/n = 94/80, Q = 4). The Fortuna Fm. magnetization is hosted by massive, interstitial hematite cement and constitutes a post-depositional remanence. The age of diagenesis of the red beds is well-constrained by the 1149 ± 7 Ma U-Pb age of authigenic xenotime rims on detrital zircons determined by SHRIMP analysis. The magnetite-hosted remanence of the Vale da Promissão Fm. may be detrital in origin, but the age of deposition is poorly constrained. The reliable and precisely-dated Fortuna Fm. paleomagnetic pole fixes the paleogeographic position of the Amazon craton near the SE Appalachians portion of North America at 1.15 Ga. These data demonstrate a mobile Grenvillian link between these two cratons, and support the recent identification of Amazon crust in the Blue Ridge province region of North America.

The System Forsterite-Diopside-Enstatite up to 70 kbar and its Significance to the Genesis of Komatiites

NASA Astrophysics Data System (ADS)

Dasgupta, S.; Gupta, A. K.

2011-12-01

Liquidus phase relations in the system forsterite-diopside-enstatite has been made at 70 kbar under anhydrous conditions using a Walker-type multi-anvil high pressure apparatus. Positions of the pseudoeutectic/ invariant, minimum points and amount of solid solutions of appearing phases are summarized in table 1. Comparison of these phase relations with those conducted by previous investigators at lower pressures and temperatures shows that the fosterite-pyroxene liquidus boundary shifts toward forsterite and away from the diopside apex with increasing pressure. Microprobe analyses indicate that the maximum amount of MgSiO3 that can be incorporated in diopside increases with pressure, and at the solidus (70 kbar, 2010°C), it is about 82%. On the basis of EPMA analyses of coexisting liquid and crystalline phases, three-phase triangles have been constructed. It is observed that at 70 kbar, the early partial melt generated from a model peridotite does not precipitate orthopyroxene. If such a melt instead of crystallizing in-situ, ascend to the surface, then the polybaric-polythermal crystallization path should never intersect the liquidus phase field of orthopyroxene, enstatitess may then appear in the solidus as an exsolution product. Our calculation shows that at 31% partial melting of a model mantle, orthopyroxene should appear as a liquidus phase. With further increase in the degree of partial melting (42-60%), proportion of orthopyroxene crystallizing from the melt progressively increases. With reference to the above discussion we propose that the Gorgona komatiites which are primarily orthopyroxene-deficient komatiites, are an outcome of low degree of partial melting, whereas the orthopyroxene-bearing Commondale komatiites of the southern Kaapvaal Craton, South Africa, are the outcome of a larger degree of partial melting, both generated from melting of an anhydrous mantle.

The dehydration, rehydration and tectonic setting of greenstone belts in a portion of the northern Kaapvaal Craton, South Africa

NASA Technical Reports Server (NTRS)

Vanreenen, D. D.; Barton, J. M., Jr.; Roering, C.; Vanschalkwyk, J. C.; Smit, C. A.; Debeer, J. D.; Stettler, E. H.

1986-01-01

High-grade gneiss terranes and low-grade granite-greenstone terranes are well known in several Archaean domains. The geological relationship between these different crustal regions, however, is still controversial. One school of thought favors fundamental genetic differences between high-grade and low-grade terranes while others argue for a depth-controlled crustal evolution. The detailed examination of well-exposed Archaean terranes at different metamorphic grades, therefore, is not only an important source of information about the crustal levels exposed, but also is critical to the understanding of the possible tectonic and metamorphic evolution of greenstone belts with time. Three South African greenstone belts are compared.

Refertilized mantle keel below the Southern Alps domain (North-East Italy): Evidence from Marosticano refractory mantle peridotites

NASA Astrophysics Data System (ADS)

Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo; Fahnestock, M. Florencia; Bryce, Julia G.; Marzoli, Andrea

2018-02-01

The Veneto Volcanic Province (VVP), a Cenozoic magmatic province in northeastern Italy, is one of the widest volcanic areas of the Adria plate. It consists of five main magmatic districts, and its most primitive products commonly host mantle xenoliths. In this study, we present a newly discovered xenolith suite from the Marosticano district that contains peridotites with compositional characteristics of mineral assemblages that provide insight into an unexpected nature of the sub-continental lithospheric mantle (SCLM) of the Adria plate. In contrast to xenoliths from other VVP sites previously studied (i.e., Val d'Adige and Lessini Mts.), Marosticano xenoliths exhibit highly refractory compositions typical of on-craton peridotites. High olivine forsteritic contents (Fo: 91-93) indicate high degrees of partial melting (> 25%) that should have been associated with the complete consumption of clinopyroxene. Major and trace element compositions further link these peridotite fragments to early Proterozoic cratonic mantle. The occurrence of clinopyroxene within such rocks suggests Marosticano clinopyroxene testify to a metasomatic legacy. The i) LREE-enrichments of Marosticano clinopyroxene and ii) the dissolved CO2 mole fractions (up to 1.0) for the inferred clinopyroxene-forming melt are consistent with carbonatite/CO2-rich silicatic melts as metasomatic agents. The latter could be responsible for the equilibrium temperatures (1033-1117 °C) and oxidizing conditions [ΔlogfO2 (FMQ) = - 0.6 - + 1.1], anomalously high for a cratonic environment but similar to the off-craton VVP xenoliths. The cratonic signature and carbonatite/CO2-rich silicate metasomatism found together in the Marosticano mantle xenoliths reveal that ancient features can be preserved in SCLM in a young, active geodynamic setting such as the Adria plate boundary. In this framework Lessini Mts. and Val d'Adige xenoliths could be interpreted as circumcratonic reminiscent domains affected by

Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada

NASA Astrophysics Data System (ADS)

de Stefano, Andrea; Lefebvre, Nathalie; Kopylova, Maya

2006-02-01

A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline lamprophyric breccias were emplaced at 2.68-2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation. Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and aggregates. The diamonds contain 0-740 ppm N and show two modes of N aggregation at 0-30 and 60-95% B-centers suggesting mantle storage at 1,100-1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7-2.9 Ga cratonic mantle and experienced subsequent alteration of syngenetic inclusions

A review of structural patterns and melting processes in the Archean craton of West Greenland: Evidence for crustal growth at convergent plate margins as opposed to non-uniformitarian models

NASA Astrophysics Data System (ADS)

Polat, Ali; Wang, Lu; Appel, Peter W. U.

2015-11-01

The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite-trondhjemite-granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West

Petroleum geology and resources of the Nepa-Botuoba High, Angara-Lena Terrace, and Cis-Patom Foredeep, southeastern Siberian Craton, Russia

USGS Publications Warehouse

Ulmishek, Gregory F.

2001-01-01

Three structural provinces of this report, the Nepa-Botuoba High, the Angara-Lena Terrace, and the Cis-Patom Foredeep, occupy the southeastern part of the Siberian craton northwest of the Baikal-Patom folded region (fig. 1). The provinces are similar in many aspects of their history of development, stratigraphic composition, and petroleum geology characteristics. The sedimentary cover of the provinces overlies the Archean?Lower Proterozoic basement of the Siberian craton. Over most of the area of the provinces, the basement is covered by Vendian (uppermost Proterozoic, 650?570 Ma) clastic and carbonate rocks. Unlike the case in the more northwestern areas of the craton, older Riphean sedimentary rocks here are largely absent and they appear in the stratigraphic sequence only in parts of the Cis-Patom Foredeep province. Most of the overlying sedimentary section consists of Cambrian and Ordovician carbonate and clastic rocks, and it includes a thick Lower Cambrian salt-bearing formation. Younger rocks are thin and are present only in marginal areas. 1 A single total petroleum system (TPS) embraces all three provinces. The TPS is unique in two aspects: (1) its rich hydro-carbon reserves are derived from Precambrian source rocks and (2) preservation of oil and gas fields is extremely long owing to the presence of the Lower Cambrian undeformed salt seal. Discovered reserves of the TPS are about 2 billion barrels of oil and more than 30 trillion cubic feet of gas. The stratigraphic distribution of oil and gas reserves is narrow; all fields are in Vendian to lowermost Cambrian clastic and carbonate reservoirs that occur below Lower Cambrian salt. Both structural and stratigraphic traps are known. Source rocks are absent in the sedimentary cover of the provinces, with the possible exception of a narrow zone on the margin of the Cis-Patom Foredeep province. Source rocks are interpreted here to be Riphean and Vendian organic-rich shales of the Baikal-Patom folded region

Fe-based redox state of mantle eclogites: Inherited from oceanic protoliths, modified during subduction or overprinted during metasomatism?

NASA Astrophysics Data System (ADS)

Aulbach, S.; Woodland, A. B.; Vasilyev, P.; Viljoen, F.

2016-12-01

Kimberlite-borne mantle eclogite xenoliths of Archaean and Palaeoproterozoic age are commonly interpreted as representing former oceanic crust. As such, they may retain a memory of the redox state of the convecting mantle source that gave rise to their magmatic protoliths and which controls the speciation of volatiles in planetary interiors. Mantle eclogite suites commonly include both cumulate and variably evolved extrusive varieties [1], which may be characterised by initial differences in Fe3+/Fetotal. However, in the warmer ancient mantle, they were also subject to modification due to partial melt loss upon subduction (if a plate tectonic regime existed) and, after capture in the cratonic mantle lithosphere, may be overprinted by interaction with metasomatic melts and fluids. Data are as yet sparse, but new Fe-based oxybarometry shows mantle eclogites to have highly variable fO2 (FMQ-3 to FMQ), whereby low fO2 relative to modern MORB may relate to subduction of more reducing Archaean oceanic crust or loss of ferric Fe during partial melt loss [2,3]. Indeed, using V/Sc as a redox proxy, it was recently shown that Archaean mantle eclogites are more reduced than modern MORB (ΔFMQ-1.3 vs. ΔFMQ -0.4), leading to a shallower depth of redox melting [4]. Although higher Fe contents of eclogites compared to peridotites may translate into greater robustness during metasomatism after emplacement into the cratonic lithosphere, it is possible that this is at least in part responsible for their highly variable Fe-based fO2. In order to help further constrain the redox state of mantle eclogites and unravel the effect of primary and secondary processes, we are currently measuring Fe3+/Fetotal by Mössbauer in garnet from two compositionally well-characterised mantle eclogite suites (Kaapvaal craton and West African craton), with the aim to use recently calibrated oxybarometers [2,3] to calculate fO2. The results will bear on the speciation and hence mobility of carbon during

Mantle structure and composition to 800-km depth beneath southern Africa and surrounding oceans from broadband body waves

NASA Astrophysics Data System (ADS)

Simon, R. E.; Wright, C.; Kwadiba, M. T. O.; Kgaswane, E. M.

2003-12-01

Average one-dimensional P and S wavespeed models from the surface to depths of 800 km were derived for the southern African region using travel times and waveforms from earthquakes recorded at stations of the Kaapvaal and South African seismic networks. The Herglotz-Wiechert method combined with ray tracing was used to derive a preliminary P wavespeed model, followed by refinements using phase-weighted stacking and synthetic seismograms to yield the final model. Travel times combined with ray tracing were used to derive the S wavespeed model, which was also refined using phase-weighted stacking and synthetic seismograms. The presence of a high wavespeed upper mantle lid in the S model overlying a low wavespeed zone (LWZ) around 210- to ˜345-km depth that is not observed in the P wavespeed model was inferred. The 410-km discontinuity shows similar characteristics to that in other continental regions, but occurs slightly deeper at 420 km. Depletion of iron and/or enrichment in aluminium relative to other regions are the preferred explanation, since the P wavespeeds throughout the transition zone are slightly higher than average. The average S wavespeed structure beneath southern Africa within and below the transition zone is similar to that of the IASP91 model. There is no evidence for discontinuity at 520-km depth. The 660-km discontinuity also appears to be slightly deeper than average (668 km), although the estimated thickness of the transition zone is 248 km, similar to the global average of 241 km. The small size of the 660-km discontinuity for P waves, compared with many other regions, suggests that interpretation of the discontinuity as the transformation of spinel to perovskite and magnesiowüstite may require modification. Alternative explanations include the presence of garnetite-rich material or ilmenite-forming phase transformations above the 660-km discontinuity, and the garnet-perovskite transformation as the discontinuity.

Seismic anisotropy beneath the southern Ordos block and the Qinling-Dabie orogen, China: Eastward Tibetan asthenospheric flow around the southern Ordos

NASA Astrophysics Data System (ADS)

Yu, Yong; Chen, Yongshun John

2016-12-01

SKS wave splitting analysis is performed to estimate the seismic anisotropy in the upper mantle using teleseismic data recorded by a temporary seismic array of 180 stations called SOSArray deployed in the southern Ordos block and the Qinling-Dabie orogen. The most important finding is that large delay times with NW-SE fast polarization directions in the northeastern Tibet are continuous across the boundary into the southwestern part of the Ordos block, where the SKS wave splitting results are significantly different from those in the rest of the Ordos block. Based on our SKS wave splitting results in addition to the results from previous studies, we propose an asthenospheric flow model for the eastward extrusion of the Tibetan upper mantle. The model consists of two corner flows around the southwestern corner and the southeastern corner of the Ordos block and the eastward flow along the Weihe graben and the Qinling-Dabie orogen for the escaping Tibetan upper mantle. Finally, the clockwise turning flow of the asthenosphere around the southwestern corner of Ordos block has currently extended laterally into the interior of the Ordos block, suggesting that the thick cold lithospheric root of the southwestern Ordos block there is currently being replaced with hot Tibetan asthenosphere at depths, that is, we observed an on-going process of thermal erosion of a cratonic lithosphere by lateral hot asthenospheric flow.

Paleointensity determination on Neoarchaean dikes within the Vodlozerskii terrane of the Karelian craton

NASA Astrophysics Data System (ADS)

Shcherbakova, V. V.; Lubnina, N. V.; Shcherbakov, V. P.; Zhidkov, G. V.; Tsel'movich, V. A.

2017-09-01

The results of paleomagnetic studies and paleointensity determinations from two Neoarchaean Shala dikes with an age of 2504 Ma, located within the Vodlozerskii terrane of the Karelian craton, are presented. The characteristic components of primary magnetization with shallow inclinations I = -5.7 and 1.9 are revealed; the reliability of the determinations is supported by two contact tests. High paleointensity values are obtained by the Thellier-Coe and Wilson techniques. The calculated values of the virtual dipole moment (11.5 and 13.8) × 1022 A m2 are noticeably higher than the present value of 7.8 × 1022 A m2. Our results, in combination with the previous data presented in the world database, support the hypothesized existence of a period of high paleointensity in the Late Archaean-Early Proterozoic.

Tracing the Trans-European Suture Zone into the Mantle Transition Zone

NASA Astrophysics Data System (ADS)

Knapmeyer-Endrun, B.; Krueger, F.

2012-12-01

The depth to which lithospheric roots of cratons influence the surrounding mantle has important consequences for our understanding of the thermal structure of the mantle and its geodynamics. For example, it has been proposed that the cratonic keel of the Kaapvaal craton extends even into the mantle transition zone (MTZ). Here, we use more than 19,000 P-receiver functions from 479 stations in central and eastern Europe to map the MTZ discontinuities beneath the western boundary of the East European Craton (EEC) and adjacent Phanerozoic Europe, across the Trans-European Suture Zone (TESZ), the most fundamental lithospheric boundary in Europe. The main data source in our study is the PASSEQ experiment, which achieved the densest coverage across the TESZ yet with about 200 temporary stations installed from 2006 to 2008 in Germany, the Czech Republic, Poland and Lithuania. These recordings are supplemented by national and regional networks and broad-band data from older temporary deployments. Receiver functions use P-to-S converted teleseismic waves, which yield relative travel times to the 410 km and 660 km discontinuities. They are especially useful in mapping variations in MTZ thickness. We observe significantly shorter travel times, by as much as 2 s compared to standard Earth models, for conversions from both MTZ discontinuities within the craton. This is an effect of the cratonic root and can be explained by about 5% higher velocities in the upper mantle, compared to IASP91, beneath the craton. By contrast, the differential travel time across the MTZ is normal to slightly raised. This implies that any insulating effect of the continental root does not reach the MTZ, comparable to recent observations for the North American Craton. In contrast to earlier observations in Siberia, we do not find any trace of a discontinuity at 520 km depth. If this discontinuity is caused by the transition of wadsleyite to ringwoodite, this means that the MTZ is rather dry beneath the

2.6-2.7 Ga continental crust growth in Yangtze craton, South China

NASA Astrophysics Data System (ADS)

Chen, K.; Gao, S.; Wu, Y.

2013-12-01

A combined study of zircon U-Pb and Lu-Hf isotopes and whole rock major and trace elements and Sr-Nd isotopes has been conducted for 10 granitic and tonalitic-trondhjemitic-granodioritic (TTG) gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ~2.6-2.7 Ga (Fig. 1), in addition to the previously reported ~2.9 Ga and ~3.2-3.3 Ga magmatism (Zhang et al., 2006; Jiao et al., 2009; Gao et al., 2011). The ~2.6-2.7 Ga rocks show relatively high REE (530-1074 ppm), apparently negative Eu anomaly (Eu/Eu* = 0.22-0.35), low #Mg (19.51-22.63) and low LaN/YbN (10.3-24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O+Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10000 × Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ~2.6-2.7 Ga zircon grains have ɛHf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ~2.6-2.7 Ga granitoids compared to the ~3.2-3.3 Ga and ~2.9 Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65-2.76 Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛNd(t) values (-3.74 to 1.59) and the zircon ɛHf(t) values (-11.18 to 3.55) for the ~2.9 Ga TTG and the Hf isotopes of ~3.2-3.3 Ga igneous zircons (-7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions. References Gao, S., Yang, J., Zhou, L., Li, M., Hu, Z.C., Guo, J.L., Yuan, H.L., Gong, H.J., Xiao, G.Q., Wei, J.Q., 2011. Age and growth of the Archean Kongling terrain

Emplacement and deformation of the A-type Madeira granite (Amazonian Craton, Brazil)

NASA Astrophysics Data System (ADS)

Siachoque, Astrid; Salazar, Carlos Alejandro; Trindade, Ricardo

2017-04-01

The Madeira granite is one of the Paleoproterozoic (1.82 Ga) A-type granite intrusions in the Amazonian Craton. It is elongated in the NE-SW direction and is composed of four facies. Classical structural techniques and the anisotropy of magnetic susceptibility (AMS) method were applied to the study of its internal fabric. Magnetic susceptibility measurements, thermomagnetic curves, remanent coercivity spectra, optical microscopy and SEM (scanning electron microscopy) analyses were carried out on the earlier and later facies of the Madeira granite: the rapakivi granite (RG) and the albite granite (AG) respectively. The last one is subdivided into the border albite granite (BAG) and the core albite granite (CAG) subfacies. AMS fabric pattern is controlled by pure magnetite in all facies, despite significant amounts of hematite in the BAG subfacies. Microstructural observations show that in almost all sites, magnetic fabric correlates to magmatic state fabrics that are defined by a weak NE-SW orientation of mafic and felsic silicates. However, strain mechanisms in both subfacies of AG also exhibit evidence for solid-state deformation at high to moderate temperatures. Pegmatite dyke, strike slip fault (SFA-B-C), hydrothermal vein, normal fault (F1-2) and joint (J) structures were observed and their orientation and kinematics is consistent with the magmatic and solid-state structures. Dykes, SFA-C and F1, are usually orientated along the N70°E/40°N plane, which is nearly parallel to the strike of AMS and magmatic foliations. In contrast, veins, SFB, F2 and some J are oriented perpendicular to the N70°E trend. Kinematic analysis in these structures shows evidence for a dextral sense of movement in the system in the brittle regime. The coherent structural pattern for the three facies of Madeira granite suggests that the different facies form a nested pluton. The coherence in orientation and kinematics from magmatic to high-temperature solid-state, and into the brittle

Numerical validation of the 'Pop-Down tectonics' as a structural frame for hot lithospheres with particular reference to the Hearne craton (Canadian Shield)

NASA Astrophysics Data System (ADS)

Poh, Jonathan; Yamato, Philippe; Gapais, Denis; Duretz, Thibault; Ledru, Patrick

2017-04-01

The formation of the architecture of the main cratons of the Canadian Shield has been debated over the past three decades. Understanding the role of tangential Vs. vertical tectonics in the Rae craton is of great interest as the role of inherited structure is fundamental for the subsequent drainage of fluids and the formation of high to ultra-high grade unconformity-type uranium deposits. These deposits are located in the vicinity of the intersection between the unconformity at the base of the Paleoproterozoic Athabasca sedimentary basin (1.75-1.5 Ga) and the graphite-rich metasediments of the Wollaston-Mudjatik transition zone, one of the main fault system of the Rae Craton related to the Trans-Hudson orogeny (1.82-1.78 Ga). A new tectonic model, Pop-down tectonics, was proposed as the primary driving process to concentrate supracrustal materials, strains, fluid transfers and metal transport in permeability enhanced deformation zones. The sub-vertical structural patterns with regional horizontal shortening seen in the tectonic model appear to be consistent with field evidences and has the potential for sustaining strong fluid-rock interactions. In the light of previous analogue modelling studies, we test the viability of the Pop-down tectonics model in a thermo-mechanical framework. The numerical experiments are based on a series of 2D visco-elasto-plastic thermo-mechanical models. We employ various thermal and rheological parameters derived from laboratory experiments. The geometry, thermicity and kinematics of the models are further constrained by applying existing geophysical and geological data. We impose a fixed upper mantle temperature of 1330 °C and a thin crust ranging from 30 - 40 km. The outcome of the models will provide insights into the mechanical processes controlling the deformation of hot lithospheres in convergent settings.

Pn wave velocities beneath the Tanzania Craton and adjacent rifted mobile belts, east Africa

NASA Astrophysics Data System (ADS)

Brazier, Richard A.; Nyblade, Andrew A.; Langston, Charles A.; Owens, Thomas J.

2000-08-01

P wave travel times from regional earthquakes recorded by the Tanzania Broadband Seismic Experiment have been inverted for long wavelength (>100 km) Pn velocity variations beneath Tanzania using a generalized inverse algorithm. Pn velocities, on average, are 8.40 to 8.45 km/s beneath the center of the Tanzania Craton, 8.30-8.35 km/s beneath the terminus of the Eastern Branch of the rift system, and 8.35-8.40 km/s beneath the Western Branch. These velocities indicate that there are no broad (>100 km wide) thermal anomalies in the uppermost mantle beneath areas of rifting in Tanzania, and suggest that thermal anomalies present deeper in the mantle have not yet reached the base of the crust.

Low water contents in diamond mineral inclusions: Proto-genetic origin in a dry cratonic lithosphere

NASA Astrophysics Data System (ADS)

Taylor, Lawrence A.; Logvinova, Alla M.; Howarth, Geoffrey H.; Liu, Yang; Peslier, Anne H.; Rossman, George R.; Guan, Yunbin; Chen, Yang; Sobolev, Nikolay V.

2016-01-01

The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., Bell and Rossman, 1992; Peslier et al., 2010; Peslier, 2010; Nestola and Smyth, 2015), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.

Structural evolution and tectonic context of the Mfongosi Group, Natal thrust front, Tugela terrane, South Africa

NASA Astrophysics Data System (ADS)

Basson, I. J.; Watkeys, M. K.; Phillips, D.

2005-11-01

The Mesoproterozoic Natal Metamorphic Province of Kwazulu-Natal in South Africa is an assemblage of several tectonic units, including accreted oceanic island arcs, obducted oceanic crust and deformed basin material. The highly deformed Mfongosi Group occurs at the leading edge of collision (the Natal thrust front), against and directly overlying the southern margin of the Kaapvaal Craton. Structures within the Mfongosi Group record "local" D1 and D2 events, the first of which was "oblique obduction", with predominantly N- to NNE-verging thrusting ( D1). This was followed by sinistral transpression combined with vertical constriction, forming SW-plunging kink folds and SW-plunging prolate pillow basalts ( D2). The third and final event ( D3) was E-W to ESE-WNW extension in a post-thrusting phase, defined by fibrous antitaxial quartz-calcite veining. The westernmost portion of the Mfongosi Group, the Ngubevu area, shows significantly higher finite strains (up to Rf = 12) compared to central Mfongosi and eastern Nkandlha areas ( Rf = 1.5 and less), suggesting highly oblique, largely NE-directed initial collision. Deformation of the NTF in the context of nappe emplacement is constrained by 40Ar/ 39Ar dating of post-cataclastic nematoblastic/porphyroblastic hornblende of the Manyane amphibolite close to the thrust between the Tugela nappe and the Mfongosi Group in the Mfongosi area. Hornblende overgrew the products of low-temperature deformation during the "local" D1 and D2. A minimum age of 1171 ± 16 Ma (95% conf., including J-error; weighted by √MSWD; MSWD = 4.3) is obtained for the tectonic juxtaposition of the Tugela nappe against the southern portions of the "Mfongosi Basin". This "local" D1 and D2 of the Mfongosi Group pre-dates the regional "oblique D1" and "left-lateral D2" previously determined for the central and southern terranes of the Natal Metamorphic Province by other researchers. Comparison of the 1171 ± 16 Ma age, with ages for shearing and

Nouveau pôle paléomagnétique Stephanien inférieur pour le craton saharien (formation de Merkala, bassin de Tindouf, Algérie). New Lower Stephanian palaeomagnetic pole for the Sabaran craton (Merkala formation, Tindouf basin, Algeria)

NASA Astrophysics Data System (ADS)

Henry, Bernard; Merabet, Nacer-Eddine; Bouabdallah, Hamza; Maouche, Said

1999-08-01

A palaeomagnetic study carried out in the Lower Stephanian Merkala formation (Tindouf basin) pointed out two juxtaposed neighbouring components of the magnetization. The oldest one allowed a new Stephanian pole located at 32.4°S and 56.6°E ( K= 399, A9.5 = 2.3°) to be obtained. The other one results from the superimposition of this old component on a Permian remagnetization. The new Stephanian pole, associated with the previous data from the Saharan craton, allows the Stephano-Autunian segment of the African apparent polar wander path to be specified.

U-Pb geochronology of Martín García, Sola, and Dos Hermanas Islands (Argentina and Uruguay): Unveiling Rhyacian, Statherian, Ectasian, and Stenian of a forgotten area of the Río de la Plata Craton

NASA Astrophysics Data System (ADS)

Santos, João O. S.; Chernicoff, Carlos J.; Zappettini, Eduardo O.; McNaughton, Neal J.; Greau, Y.

2017-12-01

The Río de la Plata Craton is one of the three major cratons of South America. The craton is largely covered by sedimentary basins where its most exposed area is Buenos Aires-Piedra Alta Province (Chernicoff et al., 2014). This province includes the Martín García Island in the confluence of Uruguay River and the Río de la Plata estuary. Despite to be a reference area for the craton the Martín García Island lacks modern geological investigation. We present U-Pb SIMS (secondary ion mass spectrometer) geochronological data on zircon and titanite, as well as Hf isotope determinations on zircon, from rocks of Martín García Island (Argentina), Sola and Dos Hermanas Islands (Uruguay) and from Paso Severino Formation (Uruguay). We investigated: 1) Rhyacian intermediate-to acidic plutonic, arc-type rocks of the Florida Belt dated between 2090 Ma and 2115 Ma, derived from juvenile Neoarchean crust (TDMHf: 2.52 Ga; average εHf: +3.62); 2) Rhyacian metadacite (San José metamorphic belt) of 2127 Ma; 3) Statherian metagabbros of 1724-1734 Ma, with Transplatense inheritance; 4) Early Ectasian metagabbro of 1392 Ma, with Rhyacian inheritance; and 5) Stenian metagabbros of 1193 Ma (TDMHf: 2.00 Ga, εHf: 0.1). Most of the dated orthogneisses show Brasilian-age (from 778 to 550 Ma) Pb loss in the zircons, interpreted to be caused by shearing or uplifting during Neoproterozoic. The results show that the area is not exclusively Rhyacian in age but encompasses Statherian, Ectasian, and Stenian gabbros. The last two are interpreted as distal intrusions associated to the Sunsás Orogen. All post-Transplatense metagabbros have geochemical signature of island arc basalt derived from primitive mantle with enrichment of LILE and depletion of HFSE. These characteristics point to the recurrence of magma chambers intermittently active during the Rhyacian, Statherian, Ectasian, and Stenian, all with a similar source. The term "Transplatense" is used to replace "Trans-Amazonian" events

Cambrian ophiolite complexes in the Beishan area, China, southern margin of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shi, Yuruo; Zhang, Wei; Kröner, Alfred; Li, Linlin; Jian, Ping

2018-03-01

We present zircon ages and geochemical data for Cambrian ophiolite complexes exposed in the Beishan area at the southern margin of the Central Asian Orogenic Belt (CAOB). The complexes consist of the Xichangjing-Xiaohuangshan and Hongliuhe-Yushishan ophiolites, which both exhibit complete ophiolite stratigraphy: chert, basalt, sheeted dikes, gabbro, mafic and ultramafic cumulates and serpentinized mantle peridotites. Zircon grains of gabbro samples yielded 206Pb/238U ages of 516 ± 8, 521 ± 4, 528 ± 3 and 535 ± 6 Ma that reflect the timing of gabbro emplacement. The geochemical data of the basaltic rocks show enrichment in large-ion lithophile elements and depletion in the high field strength elements relative to normal mid-oceanic ridge basalt (NMORB) in response to aqueous fluids or melts expelled from the subducting slab. The gabbro samples have higher whole-rock initial 87Sr/86Sr ratios and lower positive εNd(t) values than NMORB. These geochemical signatures resulted from processes or conditions that are unique to subduction zones, and the ophiolites are therefore likely to have formed within a supra-subduction zone (SSZ) environment. We suggest that the Cambrian ophiolite complexes in the Beishan area formed within a SSZ setting, reflecting an early Paleozoic subduction of components of the Paleo-Central Asian Ocean and recording an early Paleozoic southward subduction event in the southern CAOB along the northern margin of the Tarim and North China Cratons.

Mesoproterozoic rapakivi granites of the Rondonia Tin Province, southwestern border of the Amazonian craton, Brazil-I. Reconnaissance U-Pb geochronology and regional implications

USGS Publications Warehouse

Bettencourt, Jorge S.; Tosdal, R.M.; Leite, W.B.; Payolla, B.L.

1999-01-01

Rapakivi granites and associated mafic and ultramafic rocks in the Rondonia Tin Province, southwestern Amazonian craton, Brazil were emplaced during six discrete episodes of magmatism between ca 1600 and 970 Ma. The seven rapakivi granite suites emplaced at this time were the Serra da Providencia Intrusive Suite (U-Pb ages between 1606 and 1532 Ma); Santo Antonio Intrusive Suite (U-Pb age 1406 Ma); Teotonio Intrusive Suite (U-Pb age 1387 Ma); Alto Candeias Intrusive Suite (U-Pb ages between 1346 and 1338 Ma); Sao Lourenco-Caripunas Intrusive Suite (U-Pb ages between 1314 and 1309 Ma); Santa Clara Intrusive Suite (U-Pb ages between 1082 and 1074 Ma); and Younger Granites of Rondonia (U-Pb ages between 998 and 974 Ma). The Serra da Providencia Intrusive Suite intruded the Paleoproterozoic (1.80 to 1.70 Ga) Rio Negro-Juruena crust whereas the other suites were emplaced into the 1.50 to 1.30 Ga Rondonia-San Ignacio crust. Their intrusion was contemporaneous with orogenic activity in other parts of the southwestern Amazonian craton, except for the oldest, Serra da Providencia Intrusive Suite. Orogenic events coeval with emplacement of the Serra da Providencia Intrusive Suite are not clearly recognized in the region. The Santo Antonio, Teotonio, Alto Candeias and Sao Lourenco-Caripunas Intrusive Suites are interpreted to represent extensional anorogenic magmatism associated with the terminal stages of the Rondonian-San Ignacio orogeny. At least the Sao Lourenco-Caripunas rapakivi granites and coeval intra-continental rift sedimentary rocks may, in contrast, represent the products of extensional tectonics and rifting preceding the Sunsas/Aguapei orogeny (1.25 to 1.0 Ga). The two youngest rapakivi suites, the Santa Clara Intrusive Suite and Younger Granites of Rondonia, seemingly represent inboard magmatism in the Rondonian-San Ignacio Province during a younger episode of reworking in the Rio Negro-Juruena Province during the waning stages of the collisional 1.1 to 1.0 Ga

Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

NASA Astrophysics Data System (ADS)

Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

2016-07-01

This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some

Petrology and tectonic development of supracrustal sequence of Kerala Khondalite Belt, Southern India

NASA Technical Reports Server (NTRS)

Kumar, G. R. Ravindra; Chacko, Thomas

1988-01-01

The granulite terrain of southern India, of which the Kerala Khondalite belt (KKB) is a part, is unique in exposing crustal sections with arrested charnockite growth in different stages of transformation and in varied lithological association. The KKB with rocks of surficial origin and incipient charnockite development, poses several problems relating to the tectonics of burial of vast area and mechanisms involved in expelling initial H2O (causes of dryness) for granulite facies metamorphism. It is possible to infer the following sequence of events based on the field and laboratory studies: (1) derivation of protoliths of KKB from granitic uplands and deposition in fault bounded basin (cratonic rift); (2) subhorizontal deep burial of sediments; (3) intense deformation of infra and supracrustal rocks; (4) early granulite facies metamorphism predating F sub 2 - loss of primary structure in sediments and formation of charnockites from amphibole bearing gneisses and khondalites from pelites; (5) migmatisation and deformation of metasediments and gneisses; (6) second event of charnockite formation probably aided by internal CO2 build-up; and (7) isothermal uplift, entrapment of late CO2 and mixed CO2-H2O fluids, formation of second generation cordierites and cordierite symplectites.

The 1.33-1.30 Ga Yanliao large igneous province in the North China Craton: Implications for reconstruction of the Nuna (Columbia) supercontinent, and specifically with the North Australian Craton

NASA Astrophysics Data System (ADS)

Zhang, Shuan-Hong; Zhao, Yue; Li, Xian-Hua; Ernst, Richard E.; Yang, Zhen-Yu

2017-05-01

The Yanliao rift zone in the northern North China Craton (NCC) is the location of the standard section for late Paleoproterozoic-Mesoproterozoic stratigraphy in China and is associated with the emplacement of large volumes of diabase sills. Detailed field investigations show that the sills are distributed over a region that is >600 km long and >200 km wide, with areal extent > 1.2 ×105 km2 and cumulative thickness of the sills in any one area ranging from 50 m to >1800 m. High-resolution secondary ion mass spectrometry (SIMS) baddeleyite dating shows that emplacement of these sills occurred between about 1330 and 1305 Ma with a peak age of 1323 Ma. Emplacement of these diabase sills was accompanied by pre-magmatic uplift that started at about 1.35-1.34 Ga as indicated by the disconformity between the Changlongshan and Xiamaling formations and absence of sedimentation after the Xiamaling Formation in some areas. All the diabase sills exhibit similar geochemical features of tholeiitic compositions with intraplate characteristics. Given a relatively short duration of emplacement at 1.33-1.30 Ga, along with the large areal extent and volume, as well as intraplate character, this magmatic province constitutes a large igneous province (LIP). This Yanliao LIP and the accompanying pre-magmatic uplift were related either to a mantle plume and/or continental rifting during breakup of the NCC from the Nuna (Columbia) supercontinent. Paleomagnetic, ash bed and LIP data and other geological constraints suggest that the NCC had a close connection with Siberia, Laurentia, Baltica, North Australia and India crustal blocks. In particular, the most direct age match between the 1.33-1.30 Ga Yanliao LIP and the 1.33-1.30 Ga Derim Derim-Galiwinku LIP of the North Australian Craton (NAC), as well as the similarities between the late Paleoproterozoic-Mesoproterozoic stratigraphic units of the Yanliao rift in the NCC with the southeastern McArthur Basin in the NAC, indicate that the

Ca. 2.7 Ga ferropicritic magmatism: A record of Fe-rich heterogeneities during Neoarchean global mantle melting

NASA Astrophysics Data System (ADS)

Milidragovic, Dejan; Francis, Don

2016-07-01

Although terrestrial picritic magmas with FeOTOT ⩾13 wt.% are rare in the geological record, they were relatively common ca. 2.7 Ga during the Neoarchean episode of enhanced global growth of continental crust. Recent evidence that ferropicritic underplating played an important role in the ca. 2.74-2.70 Ga reworking of the Ungava craton provides the impetus for a comparison of ca. 2.7 Ga ferropicrite occurrences in the global Neoarchean magmatic record. In addition to the Fe-rich plutons of the Ungava craton, volumetrically minor ferropicritic flows, pyroclastic deposits, and intrusive rocks form parts of the Neoarchean greenstone belt stratigraphy of the Abitibi, Wawa, Wabigoon and Vermillion domains of the southern and western Superior Province. Neoarchean ferropicritic rocks also occur on five other Archean cratons: West Churchill, Slave, Yilgarn, Kaapvaal, and Karelia; suggesting that ca. 2.7 Ga Fe-rich magmatism was globally widespread. Neoarchean ferropicrites form two distinct groups in terms of their trace element geochemistry. Alkaline ferropicrites have fractionated REE profiles and show no systematic HFSE anomalies, broadly resembling the trace element character of modern-day ocean island basalt (OIB) magmas. Magmas parental to ca. 2.7 Ga alkaline ferropicrites also had high Nb/YPM (>2), low Al2O3/TiO2 (<8) and Sc/Fe (⩽3 × 10-4) ratios, and were enriched in Ni relative to primary pyrolite mantle-derived melts. The high Ni contents of the alkaline ferropicrites coupled with the low Sc/Fe ratios are consistent with derivation from olivine-free garnet-pyroxenite sources. The second ferropicrite group is characterized by decisively non-alkaline primary trace element profiles that range from flat to LREE-depleted, resembling Archean tholeiitic basalts and komatiites. In contrast to the alkaline ferropicrites, the magmas parental to the subalkaline ferropicrites had flat HREE, lower Nb/YPM (<2), higher Al2O3/TiO2 (8-25) and Sc/Fe (⩾4 × 10-4) ratios, and

Seismically imaged shallow and deep crustal structure and potential field anomalies across the Eastern Dharwar Craton, south Indian shield: Possible geodynamical implications

NASA Astrophysics Data System (ADS)

Pandey, O. P.; Chandrakala, K.; Vasanthi, A.; Kumar, K. Satish

2018-05-01

The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60 km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500 °C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95 Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart

Did clockwise rotation of Antarctica cause the break-up of Gondwanaland? An investigation in the 'deep-keeled cratons' frame for global dynamics

NASA Astrophysics Data System (ADS)

Osmaston, M. F.

2012-04-01

Introduction. The 'deep-keeled cratons' frame for global dynamics is the result of seeking Earth-behaviour answers to the following outside-the-box proposition:- "If cratons have tectospheric keels that reach or approach the 660 km discontinuity, AND the 660 level is an effective barrier to mantle circulation, then obviously (i) when two cratons separate, the upper mantle to put under the nascent ocean must arrive by a circuitous route and, conversely, (ii) if they approach one another, the mantle volume that was in between them must get extruded sideways." Surprisingly it has turned out [1 - 4] that Earth dynamical behaviour for at least the past 150 Ma provides persuasive affirmation of both these expectations and that there is a rational petrological explanation for the otherwise-unexpected immobility of subcratonic material to such depths [5 - 7]. Clockwise rotation of Antarctica? This contribution greatly amplifies my original plate dynamical arguments for suggesting [8] that such rotation is ongoing. Convection is unsuited to causing rotation about a pole within the plate so, as noted then, a gearwheel-like linkage to Africa at the SWIR would provide its clearly CCW (Biscay-Caucasus) relationship to the Mediterranean belt for the past 100 Ma, also seen in its separation from South America. Gearwheel-like linkage of motion requires the presence of some kind of E-W restraint further north. In that case it was the N Africa/Arabia involvement in the Alpide belt, but the earlier opening of the central Atlantic by the eastward motion of Africa, suggests its rigid Gondwanan attachment to Antarctica rotation at that time, with little constraint in the north. Further east, the seafloor data show that Australia-Antarctica separation involved no such opposite rotational linkage, so, with no E-W mechanical constraint in the north by Indonesia, they must have rotated together, as is recorded by Australia's eastward motion to generate the Mesozoic seafloor at its western

Late Triassic (Carnian) lamproites from Noril'sk, polar Siberia: Evidence for melting of the recycled Archean crust and the question of lamproite source for some placer diamond deposits of the Siberian Craton

NASA Astrophysics Data System (ADS)

Ivanov, Alexei V.; Demonterova, Elena I.; Savatenkov, Valery M.; Perepelov, Alexander B.; Ryabov, Viktor V.; Shevko, Artem Y.

2018-01-01

Two typical lamproitic dykes were found in Noril'sk region of the north-western Siberian Craton, which according to mineralogical, geochemical and isotopic criteria belong to anorogenic, non-diamondiferous type of lamproites. According to the geologic relationships, they cut through the Noril'sk-1 intrusion of the Siberian flood basalt province and thus are younger than 251 Ma. 40Ar/39Ar dating of the two dykes yielded ages of 235.24 ± 0.19 Ma and 233.96 ± 0.19 Ma, showing that they were emplaced in Carnian of the Late Triassic, about 16 Ma after the flood basalt event. There are some indications that there were multiple lamproitic dyke emplacements, including probably emplacement of diamondiferous lamproites, which produced Carnian-age diamond-rich placer deposits in other parts of the Siberian Craton and in adjacent regions. Lead isotope modelling shows that the source of the studied lamproites was formed with participation of recycled crust, which underwent modification of its U/Pb ratio as early as 2.5 Ga. However, the exact mechanism of the recycling cannot be deciphered now. It could be either through delamination of the cratonic crust or subduction of a mix of ancient terrigenous sediments into the mantle transition zone.

Three-dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

NASA Astrophysics Data System (ADS)

Patro, Prasanta K.; Sarma, S. V. S.; Naganjaneyulu, K.

2014-01-01

crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an 500 km long N-S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3-D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well-defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3-D model also brought out a well-defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur-Oddanchatram-Kodaikanal Shear Zone and Karur-Kamban-Painavu-Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction-collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat-Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well-defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of

Supercontinents, True Polar Wander, and Paleogeography of the Slave Craton

NASA Astrophysics Data System (ADS)

Mitchell, Ross Nelson

evidence in support of a long-lived, Ediacaran-aged hotspot. The consistency of paleocurrent directions derived from the Great Slave Supergroup argues against significant regional vertical-axis rotations and for large and rapid TPW to explain discordant paleomagnetic directions observed within section. The last frontier for paleomagnetic constraints on supercontinents, TPW, and the antiquity of plate tectonics is earliest Proterozoic time. Laurentia, one of Earth's oldest continents that formed at the core supercontinent Nuna, contains several cratons that have adequate paleomagnetic and geochronologic data with which to test for evidence of early TPW and relative plate motion, the hallmark of tectonics. Although past comparisons have been made between the Slave and Superior cratons at each "bookend" of Laurentia, new paleomagnetic data supported by baked contact tests allow for conclusive early Proterozoic reconstructions. Similar to periods following Proterozoic supercontinents Nuna and Rodinia, early Proterozoic time is characterized by large TPW oscillations and large-scale plate reorganizations prior to amalgamation, possibly indicating the presence of `Kenorland', an Archean supercontinent.

Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Zian; Zhang, Lu; Lin, Ge; Zhao, Chongbin; Liang, Yingjie

2018-06-01

The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a `hot crust but cold mantle' structure. At the eastern NCC, the crust and the mantle characterized by a `cold crust but hot mantle.' The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas

Lithospheric thermal evolution and dynamic mechanism of destruction of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Zian; Zhang, Lu; Lin, Ge; Zhao, Chongbin; Liang, Yingjie

2017-09-01

The dynamic mechanism for destruction of the North China Craton (NCC) has been extensively discussed. Numerical simulation is used in this paper to discuss the effect of mantle upward throughflow (MUT) on the lithospheric heat flux of the NCC. Our results yield a three-stage destruction of the NCC lithosphere as a consequence of MUT variation. (1) In Late Paleozoic, the elevation of MUT, which was probably caused by southward and northward subduction of the paleo-Asian and paleo-Tethyan oceans, respectively, became a prelude to the NCC destruction. The geological consequences include a limited decrease of the lithospheric thickness, an increase of heat flux, and a gradual enhancement of the crustal activity. But the tectonic attribute of the NCC maintained a stable craton. (2) During Late Jurassic-Early Cretaceous, the initial velocity of the MUT became much faster probably in response to subduction of the Pacific Ocean; the conductive heat flux at the base of the NCC lithosphere gradually increased from west to east; and the lithospheric thickness was significantly decreased. During this stage, the heat flux distribution was characterized by zonation and partition, with nearly horizontal layering in the lithosphere and vertical layering in the underlying asthenosphere. Continuous destruction of the NCC lithosphere was associated with the intense tectono-magmatic activity. (3) From Late Cretaceous to Paleogene, the velocity of MUT became slower due to the retreat of the subducting Pacific slab; the conductive heat flux at the base of lithosphere was increased from west to east; the distribution of heat flux was no longer layered. The crust of the western NCC is relatively hotter than the mantle, so-called as a `hot crust but cold mantle' structure. At the eastern NCC, the crust and the mantle characterized by a `cold crust but hot mantle.' The western NCC (e.g., the Ordos Basin) had a tectonically stable crust with low thermal gradients in the lithosphere; whereas

3D Crustal Velocity Structure Model of the Middle-eastern North China Craton

NASA Astrophysics Data System (ADS)

Duan, Y.; Wang, F.; Lin, J.; Wei, Y.

2017-12-01

Lithosphere thinning and destruction in the middle-eastern North China Craton (NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. Up to 42 wide-angle reflection/refraction deep seismic sounding (DSS) profiles have been completed in the middle-eastern NCC, we collect all the 2D profiling results and perform gridding of the velocity and interface depth data, and build a 3D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, using the Kriging interpolation method. In this model, four layers are divided by three interfaces: G is the interface between the sedimentary cover and crystalline crust, with velocities of 5.0-5.5 km/s above and 5.8-6.0 km/s below. C is the interface of the upper and lower crust, with velocity jump from 6.2-6.4 km/s to 6.5-6.6 km/s. M is the interface between the crust and upper mantle, with velocity 6.7-7.0 km/s at the crust bottom and 7.9-8.0 km/s on mantle top. Our results show that the first arrival time calculated from HBCust1.0 fit well with the observation. It also demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C. The depth of interface Moho varies beneath the source area of the Tangshan earth-quake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin (NCB) in the east of the craton is obviously higher than the regional average, this high-velocity probably results from longterm underplating of the mantle magma. This research is founded by the Natural Science

A remanent and induced magnetization model of Magsat vector anomalies over the west African craton

NASA Technical Reports Server (NTRS)

Toft, P. B.; Haggerty, S. E.

1986-01-01

Scalar and vector Magsat anomalies over the west African craton are analyzed by forward and inverse models. A forward model of the Man shield is based on Liberia. Induced magnetization contrasts due to sporadic iron-formations and to regional metamorphic rocks, and a contrast in remanent magnetization within the lower crust are included. This combination reproduces the location, magnitude and adopted local zero level of anomalies in the initial Magsat maps. An inverse model of the Reguibat shield estimates the magnetization contrast of its lithosphere, and when magnetism is restricted to shallower than 75 km both shields can be represented by a susceptibility contrast of +0.02. A residual anomaly between the shields involves a relative deficiency of induced magnetization along with other causes.

A remanent and induced magnetization model of Magsat vector anomalies over the west African craton

NASA Astrophysics Data System (ADS)

Toft, P. B.; Haggerty, S. E.

1986-04-01

Scalar and vector Magsat anomalies over the west African craton are analyzed by forward and inverse models. A forward model of the Man shield is based on Liberia. Induced magnetization contrasts due to sporadic iron-formations and to regional metamorphic rocks, and a contrast in remanent magnetization within the lower crust are included. This combination reproduces the location, magnitude and adopted local zero level of anomalies in the initial Magsat maps. An inverse model of the Reguibat shield estimates the magnetization contrast of its lithosphere, and when magnetism is restricted to shallower than 75 km both shields can be represented by a susceptibility contrast of +0.02. A residual anomaly between the shields involves a relative deficiency of induced magnetization along with other causes.

The Presence of a Stable Block bounded by Active Zones (Mobile Belts) in the southwestern North American Proterozoic craton

NASA Astrophysics Data System (ADS)

Goodell, P.; Martinez P, C.; Mahar, M. A.

2014-12-01

Bouguer gravity data, initial Sr isotope values, zircon U-Pb, and multiple occurrences of felsic Proterozoic rocks, have revealed an elevated, less deformed, felsic cratonic block in the northern Mexico. The block is situated in western Chihuahua and is bounded by active zones or mobile belts on three sides, and is here referred to as the Western Chihuahua Cratonic Block (WCCB). Bouguer gravity data clearly indicate a region of a highly negative anomaly (< -200 mgal) in contrast to adjoining areas. The region is large and the anomaly is relatively smooth over broad areas; the WCCB appears as a smaller version of the Colorado Plateau. The block is characterized by high initial Sr isotope ratios (<0.706). Several occurrences of Proterozoic rocks are located within or next to the WCCB, and they reveal the character of the Bouguer anomaly. On the east, at Los Filtros, Proterozoic rocks crop out in a basement cored uplift interpreted to having been derived from the WCCB during the Ouachita orogeny. At Sierra La Mojina boulders of 1.1 Ga granites are found in Permian conglomerates. And at Basasiachic, xenoliths of 1.1 Ga granites are present in ash flow tuffs. Establishment of the Precambrian character of the WCCB is of importance, and these multiple occurrences are evidence. Prior studies of the Sierra Madre Occidental suggest that the region was uplifted because of a vast Cenozoic batholith presumed to lie under the SLIP (Silicic Large Igneous Province), the Upper Volcanic Series. The present study challenges that conclusion and maintains the SMO is underlain by Proterozoic silicic crust. The geology of age dated samples supports this. The WCCB is surrounded on three sides by Active Zones or Mobile Belts, which have been active extensional and translational zones periodically over a long period of time. On the east are the Paleozoic Pedrogosa Basin, Mesozoic Chihuahua Trough and Cenozoic Rio Grande Rift, the first two of which also continue around the northern border

Petrogenesis of a Mesoproterozoic shoshonitic lamprophyre dyke from the Wajrakarur kimberlite field, eastern Dharwar craton, southern India: Geochemical and Sr-Nd isotopic evidence for a modified sub-continental lithospheric mantle source

NASA Astrophysics Data System (ADS)

Pandey, Ashutosh; Chalapathi Rao, N. V.; Chakrabarti, Ramananda; Pandit, Dinesh; Pankaj, Praveer; Kumar, Alok; Sahoo, Samarendra

2017-11-01

, USA. Fluid-related subduction enrichment of the mantle source is apparent from the enriched ratios of La/Nb, Ba/Nb and (Hf/Sm)N, (Ta/La)N < 1. Petrogenetic modelling reveals melt generation from 1 to 2% partial melting of an enriched mantle source that subsequently underwent fractional crystallization. Our study provides geochemical and isotopic evidence for a sub-continental lithospheric mantle (SCLM) modified by subduction and asthenospheric upwelling in the Eastern Dharwar Craton. The partial melting of a resulting heterogeneous Eastern Dharwar Craton SCLM to generate Udiripikonda lamprophyre and Wajrakarur kimberlites has been attributed to the Mesoproterozoic regional lithospheric extension event.

Granulite sulphides as tracers of lower crustal origin and evolution: An example from the Slave craton, Canada

NASA Astrophysics Data System (ADS)

Aulbach, Sonja; Krauss, Cristen; Creaser, Robert A.; Stachel, Thomas; Heaman, Larry M.; Matveev, Sergei; Chacko, Thomas

2010-09-01

We carried out a detailed study of sulphide minerals, a ubiquitous mineral group in lower crustal mafic to peraluminous granulite xenoliths from the Diavik kimberlites, to assess their use in constraining the origin and tectonothermal evolution of the deep crust, and to obtain additional data on the composition of lower crust beneath ancient continents. Sulphides are overwhelmingly pyrrhotite with minor Ni (0.7-3.9 at.%), Co (0.1-0.7 at.%), and Cu contents (0.4-3.9 at.%). Sulphide modes in mafic granulites range from 0.14 to 0.55 vol%, translating into bulk rock S contents from ˜600 to 2000 ppm, similar to S contents in other mafic igneous rocks and indicating preservation of primary igneous S contents. In mafic granulites, Re and Os abundances in sulphides range from 42.5 to 726 ppb and 3.2 to 180 ppb, respectively, whereas those in peraluminous granulites are distinctly lower (36.1-282 ppb and 1.8-7.2 ppb, respectively), suggestive of Re and Os loss to fractionating sulphides in the more evolved precursors of these rocks. The significant within-sample variability of 187Os/ 188Os and correlation with 187Re/ 188Os indicates the preservation of primary Re-Os isotope systematics and time-integrated decay of the measured 187Re. Within the large uncertainties inherent in the nature of the samples and technique, sulphides in some granulites may record major tectonothermal events in the central Slave craton spanning several billion years of evolution. Multiple generations of sulphide can occur in a single sample. These data attest to the heterogeneous composition and complex history of the Slave craton lower crust.

High-quality heat flow determination from the crystalline basement of the south-east margin of North China Craton

NASA Astrophysics Data System (ADS)

Jiang, Guangzheng; Tang, Xiaoyin; Rao, Song; Gao, Peng; Zhang, Linyou; Zhao, Ping; Hu, Shengbiao

2016-03-01

Very few of heat flow data have come from the crystalline basement in the North China Craton but rather from boreholes in the sedimentary cover of oil-gas basins. Explorations for hot dry rock (HDR) geothermal resources and porphyry gold deposits in eastern China offer now valuable opportunities to study the terrestrial heat flow in the crystalline basement. In this study, we obtained continuous temperature logs from two boreholes (the LZ borehole with a depth of 3471 m and the DR borehole with a depth of 2179 m) located in the south-east margin of the North China Craton. The boreholes have experienced long shut-in times (442 days and 261 days for the LZ borehole and DR borehole, respectively); thus, it can be expected that the temperature conditions have re-equilibrated after drilling and drill-mud circulation. Rock thermal conductivity and radiogenic heat production were measured for 68 crystalline rock samples from these two boreholes. The measured heat-flow density was determined to be 71.8 ± 2.3 mW m-2 (for the LZ borehole) and 91.5 ± 1.2 mW m-2 (for the DR borehole). The heat flow for the LZ borehole is close to the value of 75 mW m-2 determined in the Chinese Continental Scientific Drilling main hole (CCSD MH), both being in the Sulu-Dabie orogenic belt and thus able to verify each other. The value for the DR borehole is higher than the above two values, which supports former high heat-flow values determined in the Bohai Bay Basin.

Crustal and mantle structure beneath the Terre Adélie Craton, East Antarctica: insights from receiver function and seismic anisotropy measurements

NASA Astrophysics Data System (ADS)

Lamarque, Gaëlle; Barruol, Guilhem; Fontaine, Fabrice R.; Bascou, Jérôme; Ménot, René-Pierre

2015-02-01

The Terre Adélie and George V Land (East Antarctica) represent key areas for understanding tectonic relationships between terranes forming the Neoarchean-Palaeoproterozoic Terre Adélie Craton (TAC) and the neighbouring lithospheric blocks, together with the nature of its boundary. This region that represents the eastern border of the TAC is limited on its eastern side by the Mertz shear zone (MSZ) separating more recent Palaeozoic units from the craton. The MSZ, that recorded dextral strike-slip movement at 1.7 and 1.5 Ga, is likely correlated with the Kalinjala or Coorong shear zone in South Australia, east of the Gawler Craton and may therefore represent a frozen lithospheric-scale structure. In order to investigate the lithospheric structure of the TAC and the MSZ, we deployed from 2009 October to 2011 October four temporary seismic stations, which sampled the various lithospheric units of the TAC and of the neighbouring Palaeozoic block, together with the MSZ. We used receiver function method to deduce Moho depths and seismic anisotropy technique to infer the upper mantle deformation. Results from receiver functions analysis reveal Moho at 40-44 km depth beneath the TAC, at 36 km under the MSZ and at 28 km beneath the eastern Palaeozoic domain. The MSZ therefore delimits two crustal blocks of different thicknesses with a vertical offset of the Moho of 12 km. Seismic anisotropy deduced from SKS splitting at stations on the TAC shows fast polarisation directions (Φ) trending E-W, that is, parallel to the continental margin, and delay times (δt) ranging from 0.8 to 1.6 s. These results are similar to the splitting parameters observed at the permanent GEOSCOPE Dumont D'Urville station (DRV: Φ 95°N, δt 1.1 s) located in the Palaeoproterozoic domain of TAC. On the MSZ, the small number of good quality measurements limits the investigation of the deep signature of the shear zone. However, the station in the Palaeozoic domain shows Φ trending N60°E, which is

Structure of the subduction system in southern Peru from seismic array data

NASA Astrophysics Data System (ADS)

Phillips, Kristin; Clayton, Robert W.; Davis, Paul; Tavera, Hernando; Guy, Richard; Skinner, Steven; Stubailo, Igor; Audin, Laurence; Aguilar, Victor

2012-11-01

The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and Pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30° and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back-arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west.Vp/Vs ratios estimated from receiver function stacks show average values for this region with a few areas of elevated Vp/Vs near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data

NASA Astrophysics Data System (ADS)

Roy, Corinna; Calo, Marco; Bodin, Thomas; Romanowicz, Barbara

2016-04-01

Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. On the other hand, several recent studies documented significant changes in the direction of azimuthal anisotropy with depth that suggest layering in the anisotropic structure of the stable part of the North American continent. In particular, Yuan and Romanowicz (2010) combined long period surface wave and overtone data with core refracted shear wave (SKS) splitting measurements in a joint tomographic inversion. A question that arises is whether the anisotropic layering observed coincides with that obtained from receiver function studies. To address this question, we use a trans-dimensional Markov-chain Monte Carlo (MCMC) algorithm to generate probabilistic 1D radially and azimuthal anisotropic shear wave velocity profiles for selected stations in North America. In the algorithm we jointly invert short period (Ps Receiver Functions, surface wave dispersion for Love and Rayleigh waves) and long period data (SKS waveforms). By including three different data types, which sample different volumes of the Earth and have different sensitivities to 
structure, we overcome the problem of

High-grade iron ore at Windarling, Yilgarn Craton: a product of syn-orogenic deformation, hypogene hydrothermal alteration and supergene modification in an Archean BIF-basalt lithostratigraphy

NASA Astrophysics Data System (ADS)

Angerer, Thomas; Hagemann, Steffen G.; Danyushevsky, Leonid

2013-08-01

Banded iron formation (BIF)-hosted iron ore deposits in the Windarling Range are located in the lower greenstone succession of the Marda-Diemals greenstone belt, Southern Cross domain, Yilgarn Craton and constitute a total hematite-martite-goethite ore resource of minimum 52 Mt at 60 wt.% Fe (0.07 P). Banded iron formation is interlayered with high-Mg basalts at Windarling and precipitated during episodes of volcanic quiescence. Trace element content and the rare earth element (REE) ratios Y/Ho (42 to 45), Sm/Yb (1.5), together with positive La and Gd anomalies in `least-altered' hematite-magnetite-metachert-BIF indicate the precipitation from Archean seawater that was fertilised by hydrothermal vent fluids with a basaltic HREE-Y signature. Hypogene iron ore in sub-greenschist facies metamorphosed BIF formed during three distinct stages: ore stage 1 was a syn- to post-metamorphic, syn-D1, Fe-Ca-Mg-Ni-Co-P-REE metasomatism that produced local Ni-REE-rich Fe-dolomite-magnetite alteration in BIF. Hydrothermal alteration was induced by hot fluid flow controlled by brittle-ductile reactivation of BIF-basalt margins and crosscutting D1 faults. The Ni-Co-rich content of dolomite and a shift in REE ratios in carbonate-altered BIF towards Archean mafic rock signature (Y/Ho to 31 to 40, Sm/Yb to 1 to 2 and Gd/Gd* to 1.2 to 1.4) suggest that high-Mg basalts in the Windarling Range were the primary source of introduced metals. During ore stage 2, a syn-deformational and likely acidic and oxidised fluid flow along BIF-basalt margins and within D1 faults leached carbonate and precipitated lepidoblastic and anhedral/granoblastic hematite. High-grade magnetite-hematite ore is formed during this stage. Ore stage 3 hydrothermal specular hematite (spcH)-Fe-dolomite-quartz alteration was controlled by a late-orogenic, brittle, compressional/transpressional stage (D4; the regional-scale shear-zone-related D3 is not preserved in Windarling). This minor event remobilised iron oxides

The formation and rejuvenation of continental crust in the central North China Craton: Evidence from zircon U-Pb geochronology and Hf isotope

NASA Astrophysics Data System (ADS)

Li, Qing; Santosh, M.; Li, Sheng-Rong; Guo, Pu

2014-12-01

The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U-Pb and Lu-Hf data on TTG (tonalite-trondhjemite-granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450-1900 Ma suggesting a long-lived convergent margin. The εHf(t) values of these zircons range from -11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120-130 Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the εHf(t) values of these zircons show wide a range from -30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.

Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

USGS Publications Warehouse

Fanning, C.M.; Flint, R.B.; Parker, A.J.; Ludwig, K. R.; Blissett, A.H.

1988-01-01

Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ??? 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ?? 2 Ma, and were intruded by anorogenic granites at ??? 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia. ?? 1988.

Tectonic evolution of the Archaean high-grade terrain of South India

NASA Technical Reports Server (NTRS)

Ramakrishnan, M.

1988-01-01

The southern Indian shield consists of three major tectonic provinces viz., (1) Dharwar Craton, (2) Eastern Ghat Mobile Belt, and (3) Pandyan Mobile Belt. An understanding of their mutual relations is crucial for formulating crustal evolution models. The tectonic evolution of these provinces is summarized.

Linking craton stability and deep earth processes using thermochronology; a case study in the Superior Province of the Canadian Shield.

NASA Astrophysics Data System (ADS)

Sturrock, C. P.; Flowers, R. M.; Zhong, S.; Metcalf, J. R.; Kohn, B. P.

2017-12-01

Ancient, cratonic continental interiors are often presumed to be stable in the long term, neither accumulating nor shedding significant amounts of overlying sediment. However, recent low-temperature thermochronologic work suggests that such long term stability is an overly simplistic view and that forces besides plate tectonics, such as dynamic topography, may play a significant role. New apatite (U-Th)/He (AHe) and apatite fission track (AFT) data from Archean-Proterozoic basement rocks along a 1400km NW-SE transect in the Superior Province of the Canadian Shield record a spatially variable thermal history for the craton in Paleozoic through the end of Mesozoic time. Dates range from 600­­­­­­±60 Ma (AHe) and 529­±48 Ma (AFT) in the west to 184±14 Ma (AHe) and 174±9 Ma (AFT) in the east. Tectonic activity within the Superior Province ceased by 1.8 Ga, with the latest activity at the margins ending at 1 Ga. Widespread resetting of both AHe and AFT systems post 1 Ga is most likely due to regional scale burial at one or more times since the Cambrian. The temperature sensitivity of the AHe and AFT systems (30-90°C and 60-120°C, respectively) require at least a few km of burial across the craton that has since been stripped away. Preliminary inverse thermal history models, utilizing geologic constraints and radiation damage effects on He diffusion in apatite, indicate significant reheating in the Paleozoic-early Mesozoic (37 to >120°C) and a possible lesser reheating event since the mid Mesozoic (<100°C). Making the simplified assumption of a 25°C/km geothermal gradient and 0°C surface temperature, burial in some areas must have been at least 2-5km in the Paleozoic and was <4km in the Mesozoic. These burial and denudation patterns do not correlate with global sea level changes, making dynamic topography a good candidate for a driving mechanism. New AHe data from kimberlites emplaced in the early to mid-Jurassic will provide an important new constraint

The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

NASA Astrophysics Data System (ADS)

Plavsa, Diana; Collins, Alan S.; Foden, John D.; Clark, Chris

2015-05-01

Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1-D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1-D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the

Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle

NASA Astrophysics Data System (ADS)

Hughes, Hannah S. R.; McDonald, Iain; Faithfull, John W.; Upton, Brian G. J.; Loocke, Matthew

2016-01-01

Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranes vs. Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF). Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations > 2.9 wt.% and Co/Ni ratios > 0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (< 3600 ppm) and low Co/Ni ratios (< 0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd ≥ chondrite whilst Palaeozoic sulphides have Pt/Pd < chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

USGS Publications Warehouse

Carlson, R.W.; Irving, A.J.; Schulze, D.J.; Hearn, B.C.

2004-01-01

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm-Nd and Lu-Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic-a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous-early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm-Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger "ages" suggesting

Origin of a classic cratonic sheet sandstone: Stratigraphy across the Sauk II-Sauk III boundary in the Upper Mississippi Valley

USGS Publications Warehouse

Runkel, Anthony C.; McKay, R.M.; Palmer, A.R.

1998-01-01

The origin of cratonic sheet sandstones of Proterozoic and early Paleozoic age has been a long-standing problem for sedimentologists. Lower Paleozoic strata in the Upper Mississippi Valley are best known for several such sandstone bodies, the regional depositional histories of which are poorly understood. We have combined outcrop and subsurface data from six states to place the Upper Cambrian Wonewoc (Ironton and Galesville) Sandstone in a well-constrained stratigraphic framework across thousands of square kilometers. This framework makes it possible for the first time to construct a regional-scale depositional model that explains the origin of this and other cratonic sheet sandstones. The Wonewoc Sandstone, although mapped as a single contiguous sheet, is a stratigraphically complex unit that was deposited during three distinct conditions of relative sea level that span parts of four trilobite zones. During a relative highstand of sea level in Crepicephalus Zone time, quartzose sandstone lithofacies aggraded more or less vertically in nearshore-marine and terrestrial environments across much of the present-day out-crop belt around the Wisconsin arch. At the same time, finer grained, feldspathic sandstone, siltstone, and shale aggraded in deeper water immediately seaward of the quartzose sand, and shale and carbonate sediment accumulated in the most distal areas. During Aphelaspis and Dunderbergia Zones time a relative fall in sea level led to the dispersal of quartzose sand into a basinward-tapering, sheet-like body across much of the Upper Mississippi Valley. During early Elvinia Zone time a major transgression led to deposition of a second sheet sandstone that is generally similar to the underlying regressive sheet. The results of this investigation also demonstrate how subtle sequence-bounding unconformities may be recognized in mature, cratonic siliciclastics. We place the Sauk II-Sauk III subsequence boundary at the base of the coarsest bed in the Wonewoc

U-Pb dating and isotopic signature of the alkaline ring complexes of Bou Naga (Mauritania): its bearing on late proterozoic plate tectonics around the West African craton

NASA Astrophysics Data System (ADS)

Blanc, A.; Bernard-Griffiths, J.; Caby, R.; Caruba, C.; Caruba, R.; Dars, R.; Fourcade, S.; Peucat, J. J.

1992-04-01

In the West African fold belt of Mauritania, high-grade metamorphic series, similar to those of Amsaga (Reguibat shield-West African Craton), are exposed in a window. At Bou Naga-Mauritania (19° N, 13° 15' W) in the South of this window, an alkaline ring complex has intruded the metamorphic country rocks. This complex consists of two geological formations: the Eastern formation is mainly composed of red rhyolite sills, whereas the Western formation is made up of several kinds of alkaline rocks both saturated and under-saturated which cross cut the earlier saturated units. Three U-Pb zircon age measurements have been made on the alkaline complex, and one on an orthogneiss from the metamorphic country rocks. The syenite and the alkaline granite of the Western block are 676 ± 8 and 687 ± 5 Ma old. The orthogneiss is Archaean with an age of 2709 ± 136 Ma, but the lower intercept of discordia on concordia, shows an age of 756 ± 25 Ma linked with the genesis of the alkaline complex. A major crustal contribution is recorded by Nd and O isotopes in the SiO 2-saturated rocks. These results provide evidence for the correlation of the metamorphic country rocks with the Reguibat Archaean basement and for an early Pan-African continental rifting phase in this area before the tectonometamorphic events in the Mauritanide belt. Furthermore, with regards with previous geodynamic works of the West African Craton, our results leads us to suggest a significant diachronism between late Proterozoic crustal evolution to the West and to the East of the West African Craton. This is a further evidence for modern-type plate tectonics at this time.

Surface Curvature in Island Groups and Discontinuous Cratonic Structures

NASA Astrophysics Data System (ADS)

McDowell, M. S.

2002-05-01

The Canadian Archipelago includes eight major islands and a host of smaller ones. They are separated by water bodies, of varying widths attributable to glacial activity and ocean currents. Land form varies from relatively rugged mountains (~2000 m) in eastern, glacial, islands, to low lying western, similar to the continental topography adjacent. The Arctic region is thought to have been low average elevation before the Pleistocene. To a picture puzzler, it all looks like it fit together. Experimentally cutting apart the islands from large scale maps shows that the rough edges match fairly well. However, when those independent pieces are sutured together, without restraint, as in free air, the fit is far better. Far more importantly, they consistently form a noticeably concave surface. This tendency is not at all apparent in flat surface or computer screen manipulation; the pieces need to be "hand joined" or on a molded surface to allow the assembly to freely form as it will. Fitting together the coastlines above 60 \\deg north, from 120 \\deg west to 45 \\deg east, and comparing the resulting contracted area to the original, obtains an 8 percent area reduction. The curvature "humps" a trial planar section of 15 cms by 1.6 cm, a substantial difference in the radius of curvature. If you rashly suggest applying that formula globally, the resulting sphere would have a surface area of 4.7 x108,(down from 5 x108), and therefore radius of 6117 km, down from 6400, which is a rather preposterous conclusion. As nobody would believe it, I tested the idea elsewhere. The Huronian succession of six named cratons is adjacent on the south. I cut this map apart, too, and fit it together, once again getting a curvature, this time more pronounced. I am trying it with the Indonesian Archipelago, although this area has volcanic complications, and with Precambrian Basins in western Australia and Nimibia, Africa. Indications are - an essentially similar pattern of fit, but non uniform

Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

NASA Astrophysics Data System (ADS)

Chen, Yunfeng; Gu, Yu Jeffrey; Hung, Shu-Huei

2017-02-01

The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (<- 1%) at comparable depths. The P velocity increases from - 0.5% above 70 km depth to 1.5% at 330 km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160 km) and Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

The formation of Laurentia: Evidence from shear wave splitting

NASA Astrophysics Data System (ADS)

Liddell, Mitch V.; Bastow, Ian; Darbyshire, Fiona; Gilligan, Amy; Pugh, Stephen

2017-12-01

The northern Hudson Bay region in Canada comprises several Archean cratonic nuclei, assembled by a number of Paleoproterozoic orogenies including the Trans-Hudson Orogen (THO) and the Rinkian-Nagssugtoqidian Orogen. Recent debate has focused on the extent to which these orogens have modern analogues such as the Himalayan-Karakoram-Tibet Orogen. Further, the structure of the lithospheric mantle beneath the Hudson Strait and southern Baffin Island is potentially indicative of Paleoproterozoic underthrusting of the Superior plate beneath the Churchill collage. Also in question is whether the Laurentian cratonic root is stratified, with a fast, depleted, Archean core underlain by a slower, younger, thermally-accreted layer. Plate-scale process that create structures such as these are expected to manifest as measurable fossil seismic anisotropic fabrics. We investigate these problems via shear wave splitting, and present the most comprehensive study to date of mantle seismic anisotropy in northern Laurentia. Strong evidence is presented for multiple layers of anisotropy beneath Archean zones, consistent with the episodic development model of stratified cratonic keels. We also show that southern Baffin Island is underlain by dipping anisotropic fabric, where underthrusting of the Superior plate beneath the Churchill has previously been interpreted. This provides direct evidence of subduction-related deformation at 1.8 Ga, implying that the THO developed with modern plate-tectonic style interactions.

Geophysical constraints for terrane boundaries in southern Mongolia

NASA Astrophysics Data System (ADS)

Guy, Alexandra; Schulmann, Karel; Munschy, Marc; Miehe, Jean-Marc; Edel, Jean-Bernard; Lexa, Ondrej; Fairhead, Derek

2014-05-01

The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes corresponding to magmatic arcs, back arcs, continental basement blocks, accretionary wedges and metamorphic blocks. These terranes should be in theory characterized by contrasting magnetic and gravity signatures thanks to their different petrophysical properties. To test this hypothesis, the stratigraphically defined terranes in southern Mongolia were compared with potential field data to constrain their boundaries and extent. The existence of terranes in southern Mongolia cannot be attested by the uniform geophysical fabrics due to the lack of systematic correspondence between the high/low amplitude and high/low frequency geophysical domains and major terranes. Processed magnetic and gravity grids show that both gravity and magnetic lineaments are E-W trending in the west and correlate with direction of some geological units. In the east, both magnetic and gravity lineaments are disrupted by NE-SW trending heterogeneities resulting in complete blurring of the geophysical pattern. Correlation of magnetic signal with geological map shows that the magnetic highs coincide with late Carboniferous-early Permian volcanic and plutonic belts. The matched-filtering shows good continuity of signal to the depth located along the boundaries of these high magnetic anomalies which may imply presence of deeply rooted tectono-magmatic zones. The axes of high density bodies in the western and central part of the studied CAOB are characterized by periodic alternations of NW-SE trending high frequency and high amplitude gravity anomalies corresponding to late Permian to Triassic cleavage fronts up to 20 km wide. The matched-filtering analysis shows that the largest deformation zones are deeply rooted down to 20 km depth. Such a gravity signal is explained by the verticalization of high density mantle and lower crustal rocks due to localized vertical shearing

Denudation rates of the Southern Espinhaço Range, Minas Gerais, Brazil, determined by in situ-produced cosmogenic beryllium-10

NASA Astrophysics Data System (ADS)

Barreto, Helen N.; Varajão, César A. C.; Braucher, Régis; Bourlès, Didier L.; Salgado, André A. R.; Varajão, Angélica F. D. C.

2013-06-01

To investigate denudation rates in the southern part of the Espinhaço Range (central-eastern Brazil) and to understand how this important resistant and residual relief has evolved in the past 1.38 My, cosmogenic 10Be concentrations produced in situ were measured in alluvial sediments from the three main regional basins, whose substratum is composed primarily of quartzites. The long-term denudation rates (up to 1.38 My) estimated from these measurements were compared with those that affect the western (São Francisco River) and eastern (Doce and Jequitinhonha Rivers) basins, which face the West San Francisco craton and the Atlantic, respectively. Denudation rates were measured in 27 samples collected in catchments of different sizes (6-970 km2) and were compared with geomorphic parameters. The mean denudation rates determined in the northern part are low and similar to those determined in the southern part, despite slightly different geomorphic parameter values (catchment relief and mean slope). For the southern catchments, the values are 4.91 ± 1.01 m My- 1 and 3.65 ± 1.26 m My- 1 for the Doce and São Francisco River basins, respectively; for the northern catchments, they are 4.40 ± 1.06 m My- 1 and 3.96 ± 0.91 m My- 1 for the Jequitinhonha and São Francisco River basins, respectively. These low values of denudation rates suggest no direct correlation if plotted against geomorphic parameters such as the catchment area, maximum elevation, catchment relief, average relief and mean slope gradients. These values show that the regional landscape evolves slowly and is strongly controlled by resistant lithology, with similar erosional rates in the three studied basins.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

NASA Astrophysics Data System (ADS)

Kitano, Ippei; Osanai, Yasuhito; Nakano, Nobuhiko; Adachi, Tatsuro; Fitzsimons, Ian C. W.

2018-05-01

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500-1500 Ma from garnet-biotite gneiss, garnet-cordierite-biotite gneiss, some samples of garnet-orthopyroxene-biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000-1800 Ma from garnet-hornblende-biotite gneiss, other samples of garnet-orthopyroxene-biotite gneiss, garnet-two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100-700 Ma from paragneisses and igneous protolith ages of ca. 1100-800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000-1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500-1500 Ma craton, and in the case of the WC precursor sediments there was possibly

Crust structure of the Northern Margin of North China Craton and adjacent region from Sinoprobe-02 North China seismic WAR/R experiment

NASA Astrophysics Data System (ADS)

Li, W.; Gao, R.; Keller, G. R.; Li, Q.; Cox, C. M.; Hou, H.; Guan, Y.

2011-12-01

The Central Asian Orogen Belt (CAOB) or Altaids, situated between the Siberian craton(SC) to the north and north China craton (NCC) with tarim to the south, is one of the world's largest accretionary orogens formed by subduction and accretion of juvenile material from the Neoproterozoic through the Paleozoic. The NCC is the oldest craton in China, which suffered Yanshan intercontinental orogenic process and lithosphere thinning in Mesozoic. In the past 20 years, remarkable studies about this region have been carried out and different tectonic models were proposed, however, some crucial geologic problems remain controversial. In order to obtain better knowledge of deep structure and properties of crust on the northern margin of north China craton, a 450 km long WAR/R section was completed jointly by Institute of Geology, CAGS and University of Oklahoma. Our 450 km long NW-SE WAR/R line extends from west end of the Yanshan orogen, across the Bainaimiao arc, Ondor sum subduction accretion complex to the Solonker suture zone. The recording of seismic waves from 8 explorations was conducted in 4 deployments of 300 reftek-125A records and single-channel 4.5Hz geophones with station spacing of 1km. The shooting procedure was employ 500 or 1500kg explosives in 4-5 or 15-23 boreholes at 40-45m depth. The sampling rate was 100 HZ, and recording time window was 1200s. The P wave field on the sections got high quality data for most part of the profile, but have low signal-to-noise for the south end, where closed to Beijing with a lot of ambient noise from traffic, industry and human activity. Arrivals from of refracted and reflected waves from sediments and basement (Pg), intracrust (Pcp, Plp) and Moho (Pmp) were typically observed, but Pn phase through the upper most mantle was only observed for 2 shots. Identification and correlation of seismic phases was done manually on computer screen Zplot software. Each trace has been bandpass filtered (1-20Hz) and normalized with AGC

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation.

PubMed

Krüner, A; Byerly, G R; Lowe, D R

1991-04-01

others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.

The Intraplate Seismogenic Zone of Porto dos Gauchos in the Amazon Craton, Brazil

NASA Astrophysics Data System (ADS)

Barros, L. V.; Assumpcao, M. S.; Quintero, R. Q.

2007-05-01

The Porto dos Gauchos Seismogenic Zone (PGSZ), in the center north of Mato Grosso State, in the contact between the southern Amazonian Craton and northern Parecis Basin, represents one of the most important area of seismic activity in Brazil, with the largest magnitude ever observed in the stable continent of the South America plate (6.2 mb on January 31, 1955). Focal mechanism studies indicated a pure reverse faulting regime with compressional SHmax. oriented in SE-NW direction. After the 1955 earthquake, located in Serra do Tombador, a recurrent seismicity has been detected in Porto dos Gauchos, 100 km northeast of Serra do Tombador. No recent events have been detected in the area of the 1955 epicenter, suggesting a long recurrence time or mislocation of Serra do Tombador earthquake. The Porto dos Gauchos recurrent seismicity has been observed since 1959, when a 4.5 macroseismic estimated magnitude was felt by local inhabitants settled in that remote area two years earlier. In subsequent years, with deployment of regional stations in Brazilian Amazon region earthquakes were detected in 1981 (3.8 mb), 1989 (3.3 mb), 1993 (3.8 mb), 1996 (4.4 mb), 1997 (3.3 mb), and finally on March 10, 1998 (5.1 mb). The aftershocks of 1998 main shock were studied with a local network with up to eight 3- component stations. Such network detected more than 2500 events until December of 2002, when the network was deactivated, but only 100 were accurately located. Based on this set of events and a controlled source experiment we determined a 1-D velocity model for the area, a composite focal mechanism with P wave polarities, spectral analysis studies to estimate the source dimension, stress drop and moment magnitudes for the main shock and some others events of the set. On March 23, 2005 another shock occurred in the same seismogenic area of Porto dos Gauchos, with magnitude 4.7 to 5.0. One week later five stations were installed again to monitor the aftershock activity, detecting

Magnetic susceptibility and AMS of the Bushveld alkaline granites, South Africa

NASA Astrophysics Data System (ADS)

Ferré, Eric C.; Wilson, Jeff; Gleizes, Gérard

1999-06-01

The Bushveld Complex in South Africa includes one of the world's largest anorogenic alkaline granite intrusions (66,000 km 2). The granite forms a composite laccolith, of 350 × 250 km in area and about 2 km in thickness, which was emplaced at about 5 km depth into sediments overlying the Kaapvaal craton, at 2054 Ma. The Bushveld granite and its roof-rocks have long been mined for Sn, W and F. The Bushveld granites have high magnetic susceptibilities ( Km from 1000 to 4000 μSI), and a quantitative model is presented, suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals. The measured AMS foliations coincide with observed subhorizontal mineral lineations and compositional layering. Magnetic lineation trends vary considerably within the horizontal plane. The existence of a weak planar fabric and, an almost absent linear component may reflect (a) laccolithic emplacement by roof uplift, causing flattening magmatic fabrics, or (b) emplacement of largely crystal-free magma crystallizing in-situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity-driven mechanisms. Weak magnétic fabrics, like those identified in the Bushveld granites require specific sampling schemes and procedures, in addition to rigorous constraint of magnetic mineralogy and crystallization sequence.

Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics

NASA Astrophysics Data System (ADS)

Luguet, Ambre; Behrens, Melanie; Pearson, D. Graham; König, Stephan; Herwartz, Daniel

2015-09-01

The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the "mantle" perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS ± PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS ± PGM did not disturb

Geology, distribution, and classification of gold deposits in the western Qinling belt, central China

USGS Publications Warehouse

Mao, J.; Qiu, Yumin; Goldfarb, R.J.; Zhang, Z.; Garwin, S.; Fengshou, R.

2002-01-01

Gold deposits of the western Qinling belt occur within the western part of the Qinling-Dabie-Sulu orogen, which is located between the Precambrian North China and Yangtze cratons and east of the Songpan-Ganzi basin. The early Paleozoic to early Mesozoic orogen can be divided into northern, central, and southern zones, separated by the Shangdan and Lixian-Shanyang thrust fault systems. The northern zone consists of an early Paleozoic arc accreted to the North China craton by ca. 450 Ma. The central zone, which contains numerous orogenic gold deposits, is dominated by clastic rocks formed in a late Paleozoic basin between the converging cratonic blocks. The southern zone is characterized by the easternmost exposure of Triassic sedimentary rocks of the Songpan-Ganzi basin. These Early to Late Triassic turbidities, in part calcareous, of the immense Songpan-Ganzi basin also border the western Qinling belt to the west. Carlinlike gold deposits are abundant (1) along a westward extension of the southern zone defined by a window of early Paleozoic clastic rocks extending into the basin, and (2) within the easternmost margin of the basinal rocks to the south of the extension, and in adjacent cover rocks of the Yangtze craton. Triassic and Early Jurassic synkinematic granitoids are widespread across the western Qinling belt, as well as in the Songpan-Ganzi basin. Orogenic lode gold deposits along brittle-ductile shear zones occur within greenschist-facies, highly deformed, Devonian and younger clastic rocks of the central zone. Mainly coarse-grained gold, along with pyrite, pyrrhotite, arsenopyrite, and minor base metal sulfides, occur in networks of quartz veinlets, brecciated wall rock, and are dissminated in altered wall rock. Isotopic dates suggest that the deposits formed during the Late Triassic to Middle Jurassic as the leading edge of the Yangtze craton was thrust beneath rocks of the western Qinling belt. Many gold-bearing placers are distributed along the river

Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)

NASA Astrophysics Data System (ADS)

Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

2014-05-01

The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding

Petrology, geochemistry and zirconology of impure calcite marbles from the Precambrian metamorphic basement at the southeastern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Liu, Yi-Can; Zhang, Pin-Gang; Wang, Cheng-Cheng; Groppo, Chiara; Rolfo, Franco; Yang, Yang; Li, Yuan; Deng, Liang-Peng; Song, Biao

2017-10-01

Impure calcite marbles from the Precambrian metamorphic basement of the Wuhe Complex, southeastern margin of the North China Craton, provide an exceptional opportunity to understand the depositional processes during the Late Archean and the subsequent Palaeoproterozoic metamorphic evolution of one of the oldest cratons in the world. The studied marbles are characterized by the assemblage calcite + clinopyroxene + plagioclase + K-feldspar + quartz + rutile ± biotite ± white mica. Based on petrography and geochemistry, the marbles can be broadly divided into two main types. The first type (type 1) is rich in REE with a negative Eu anomaly, whereas the second type (type 2) is relatively poor in REE with a positive Eu anomaly. Notably, all marbles exhibit remarkably uniform REE patterns with moderate LREE/HREE fractionation, suggesting a close genetic relationship. Cathodoluminescence imaging, trace elements and mineral inclusions reveal that most zircons from two dated samples display distinct core-rim structures. Zircon cores show typical igneous features with oscillatory growth zoning and high Th/U ratios (mostly in the range 0.3-0.7) and give ages of 2.53 - 2.48 Ga, thus dating the maximum age of deposition of the protolith. Zircon rims overgrew during granulite-facies metamorphism, as evidenced by calcite + clinopyroxene + rutile + plagioclase + quartz inclusions, by Ti-in-zircon temperatures in the range 660-743 °C and by the low Th/U (mostly < 0.1) and Lu/Hf (< 0.001) ratios. Zircon rims from two dated samples yield ages of 1839 ± 7 Ma and 1848 ± 23 Ma, respectively, suggesting a Palaeoproterozoic age for the granulite-facies metamorphic event. These ages are consistent with those found in other Precambrian basement rocks and lower-crustal xenoliths in the region, and are critical for the understanding of the tectonic history of the Wuhe Complex. Positive Eu anomalies and high Sr and Ba contents in type 2 marbles are ascribed to syn-depositional felsic

Mountain building at northeastern boundary of Tibetan Plateau and craton reworking at Ordos block from joint inversion of ambient noise tomography and receiver functions

NASA Astrophysics Data System (ADS)

Guo, Zhen; Chen, Yongshun John

2017-04-01

We have obtained a high resolution 3-D crustal and uppermost mantle velocity model of the Ordos block and its surrounding areas by joint inversion of ambient noise tomography and receiver functions using seismic recordings from 320 stations. The resulting model shows wide-spread low velocity zone (Vs ≤ 3.4 km/s) in the mid-to-lower crust beneath northeastern Tibet Plateau, which may favor crustal ductile flow within the plateau. However, our model argues against the eastward crustal ductile flow beneath the Qinling belt from the Tibetan Plateau. We find high velocities in the middle part of Qinling belt which separate the low velocities in the mid-to-lower crust of the eastern Qinling belt from the low velocity zone in eastern Tibetan Plateau. More importantly, we observe significant low velocities and thickened lower crust at the Liupanshan thrust belt as the evidence for strong crustal shortening at this boundary between the northeastern Tibetan Plateau and Ordos block. The most important finding of our model is the upper mantle low velocity anomalies surrounding the Ordos block, particularly the one beneath the Trans North China Craton (TNCO) that is penetrating into the southern margin of the Ordos block for ∼100 km horizontally in the depth range of ∼70 km and at least 100 km. We propose an on-going lithospheric mantle reworking at the southernmost boundary of the Ordos block due to complicated mantle flow surrounding the Ordos block, that is, the eastward asthenospheric flow from the Tibet Plateau proposed by recent SKS study and mantle upwelling beneath the TNCO from mantle transition zone induced by the stagnant slabs of the subducted Pacific plate.

Timing and implications for the late Mesozoic geodynamic settings of eastern North China Craton: Evidences from K-Ar dating age and sedimentary-structural characteristics records of Lingshan Island, Shandong Province

NASA Astrophysics Data System (ADS)

Li, Jie; Jin, Aiwen; Hou, Guiting

2017-12-01

The Lingshan Island in Shandong Province in the eastern North China Craton, well known for the Late Mesozoic multi-scale slide-slump structures is related to paleo-earthquake. Terrigenous clastic rocks, volcanic clastic rocks and volcanic lavas are extensively exposed in the Lingshan Island and its adjacent regions of the Shandong Province, which led to fierce debates on their ages, sedimentary characteristics and tectono-sedimentary evolution. In this contribution, we present the characteristics of the Late Mesozoic stratigraphy in the Lingshan Island. Whole-rock K-Ar dating of dyke at Beilaishi and rhyolites at Laohuzui of the Lingshan Island yielded ages of 159 Ma and 106-92 Ma which coincides with the Laiyang Period rifting and the Qingshan Period rifting in the Jiaolai Basin, respectively. On the basis of the analysis to the Late Mesozoic sedimentary environment of `flysch' and `molasse'-like formations as well as tectonic stress fields reconstruction, four episodes of the tectono-sedimentary evolution were established in the Lingshan Island and its adjacent regions in the eastern North China Craton. They consist of two episodes of extensional events for the syn-rift, and two episodes of compression events for the inversion of the post-rift. The entire episodes can be summarized as follows: (1) the first syn-rift NW-SE extension in Laiyang Period can be identified by the `flysch' formation (Unit 1) and by emplacement of the NE-trending dyke in the Laiyang Group. This syn-rift episode can be related to the NW-SE post-orogenic extension resulted from the gravity collapse of the thickened lithosphere along the Sulu Orogen. (2) The first post-rift NW-SE inversion, which was caused by the NW-directed subduction of Izanaqi Plate, can be well documented by the `X' type conjugate joints as well as slide slump folds in Unit 1. (3) The second syn-rift NW-SE extension in Qingshan Period is characterized by rhyolite rocks (Unit 2). This syn-rift episode can be considered

Petrology of Teofilândia granitoids: An example of 2.1 Ga crustal accretion in the São Francisco Craton (Bahia, Brazil)

NASA Astrophysics Data System (ADS)

Nascimento, H. S.; Nédélec, Anne; Bouchez, Jean-Luc

2017-07-01

Teofilândia granitoids are representative of the Paleoproterozoic plutonic rocks, which intruded the Serrinha block, an Archean crustal fragment of the Sao Francisco Craton (Bahia, Brazil). Three plutons were emplaced, the Teofilândia granodiorite, the Barrocas trondhjemite and the Santa Rosa granite, respectively dated at 2130, 2127 and 2073 Ma. The two first plutons are calc-alkaline rocks following a trondhjemitic trend. They resemble Archean TTGs (tonalites-trondhjemites-granodiorites) by their major and trace element compositions and especially by their fractionated REE patterns, with very low HREE contents. These juvenile magmas resulted from partial melting of a young mafic protolith, likely represented by the nearby Rio Itapicuru greenstone belt. Barrocas trondhjemite and Teofilândia granodiorite derive from similar sources, possibly at different depths and with a different degree of melting. The rocks were deformed at high temperature during the Trans-Amazonian collision and are therefore pre-collisional and ascribed to a subduction stage. The younger Santa Rosa pluton is a small, syn-to post-collisional granite that derived from anatexis of the Archean crust. It is representative of a second, volumetrically minor, plutonic episode of potassic, shoshonitic or alkaline affinities. The large amount of 2.1 Ga granitoids emplaced in Brazil as well as in the West African craton, suggests that, at that time, a global event of possible mantle origin was responsible for the intense magmatic activity that involved both crustal accretion and crustal reworking in many places of the world.

Insights Into Layering in the Cratonic Lithosphere Beneath Western Australia

NASA Astrophysics Data System (ADS)

Sun, Weijia; Fu, Li-Yun; Saygin, Erdinc; Zhao, Liang

2018-02-01

The characteristics of internal lithospheric discontinuities carry crucial information regarding the origin and evolution of the lithosphere. However, the formation and mechanisms of the midlithosphere discontinuity (MLD) are still enigmatic and controversial. We investigate the midlithospheric discontinuities beneath the Archean Western Australian Craton, which represents one of the oldest continents on the globe, using a novel receiver-based reflectivity approach combined with other geophysical information comprising tomographic P and S wave velocity, radial anisotropy, electrical resistivity, and heat flow data. The MLD is rather shallow with a depth of 68-82 km. Multiple prominent discontinuities are observed in the lithospheric mantle using constructed high-frequency (0.5-4 Hz) P wave reflectivities. These multiple discontinuities coincide well with the broad-scale reduction of relative P and SV wave velocities at the top of the graded transition zone from the lithosphere to the asthenosphere. Strong radial anisotropy in the upper lithosphere mantle tends to be weak across the MLD, which might reflect quasi-laminar lithospheric heterogeneity behavior with a horizontal correlation length that is greater than its vertical correlation length. Broad-scale electrical resistivity variations show little coherence with the MLD. Given these various geophysical observations, the upper lithosphere exhibits rigid and elastic properties above the MLD, while the lower lithosphere tends to be ductile and rheological or viscous. A model comprising quasi-laminar lithospheric heterogeneity could effectively represent the MLD characteristics beneath the Archean continent.

Mesoproterozoic evolution of the Río de la Plata Craton in Uruguay: at the heart of Rodinia?

NASA Astrophysics Data System (ADS)

Gaucher, Claudio; Frei, Robert; Chemale, Farid; Frei, Dirk; Bossi, Jorge; Martínez, Gabriela; Chiglino, Leticia; Cernuschi, Federico

2011-04-01

Mesoproterozoic volcanosedimentary units and tectonic events occurring in the Río de la Plata Craton (RPC) are reviewed. A belt consisting of volcanosedimentary successions exhibiting greenschist-facies metamorphism is exposed in the eastern RPC (Nico Pérez Terrane) in Uruguay. The Parque UTE Group consists of basic volcanics and gabbros at the base (1,492 ± 4 Ma, U-Pb on zircon), carbonates in its middle part and interbedded carbonates, shales and acid volcanics (1,429 ± 21 Ma, U-Pb on zircon) at the top. The Mina Verdún Group is made up of rhyolites and acid pyroclastics at its base and top, and Conophyton-bearing limestones and massive dolostones in the middle. A U-Pb LA-ICP MS zircon age of 1,433 ± 6 Ma is reported here for lapilli-tuffs at the base of the Mina Verdún Group (Cerro de las Víboras Formation). This age shows that the Mina Verdún Group immediately postdates the Parque UTE Group, a fact supported by carbon isotope chemostratigraphy. Both units were deformed and metamorphosed between 1.25 and 1.20 Ga, as shown by K-Ar and Ar-Ar ages. This tectonic event affected most of the RPC and led to the accretion of the Nico Pérez Terrane to the remainder of the RPC along the Sarandí del Yí megashear. We report a U-Pb LA-ICP MS zircon age (upper intercept) of 3,096 ± 45 Ma for metatonalites of the La China Complex (Nico Pérez Terrane), which yield a lower intercept age of 1,252 Ma. A proto-Andean, Mesoproterozoic belt is envisaged to account for abundant Mesoproterozoic detrital zircon ages occurring in Ediacaran sandstones of the RPC. If the RPC is fringed at both sides by Mesoproterozoic, Grenville-aged belts it is likely that it occupied a rather central position in Rodinia. A possible location between Laurentia and the Kalahari Craton, and to the south of Amazonia, is suggested.

The Penokean orogeny in the Lake Superior region

USGS Publications Warehouse

Schulz, K.J.; Cannon, W.F.

2007-01-01

The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine-Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine-Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. The Penokean orogen was strongly overprinted by younger tectonic and thermal events, some of which were previously ascribed to the Penokean. Principal among these was a

A S-wave Tomography Study for the First Order of Tectonic Unit Boundaries nearby Eastern Region of Tibetan Plateau

NASA Astrophysics Data System (ADS)

Zhang, F.; Wu, Q.; Li, Y.; Zhang, R.

2017-12-01

The eastern region of Tibetan plateau (ERTP) and North-South seismic zone (NSSZ), which look like an N-S trending rectangle shape, consist of the Sichuan basin in its center, the Yungui plateau to the south, the Qinling mountain range and Ordos plateau range to the north. In the field of geosciences, Sichuan basin and Yungui plateau belong to Yangtze Craton while Ordos plateau belongs to western part of Sino-Korean Craton, and Qinling mountain is the suture zone where Sino-Korean Craton and Yangtze Craton collide between each other. To the west, ERTP is bounded by the Songpan-Ganzi terrane where a branch of the Paleotethys ocean, namely Songpan-Ganzi ocean, closed at late Triassic as a result of convergence between Qiangtang terrane, Sino-Korean Craton and Yangtze Craton. To the east, ERTP is bounded by South China block which was separated from Rodinia supercontinent by rifting processes during Neoproterozoic and was made up of two major crustal blocks-the Yangtze block in the north and the Cathysia block in the south. Tomography results using Chinaarray data from 2011 to 2015 show prominent heterogeneity beneath ERTP and NSSZ to 600 km depth. At depth range from 10 km to 100 km, the patterns of high-veloicty (high-V) and low-velocity (low-V) are well correlated with the tectonic features. The broad high-V anomalies mainly underlie Alashan, Ordos, Sichuan basin blocks, as well as part of the southern Chuandian fragment. In contrast, the broad low-V anomalies are approximately located beneath the Kunlun-Qilian fold zone, Songpan-Ganzi blocks, northern Chuandian fragment and southwestern part of Yunnan. At depth of 200 km and 300 km, the high-V zone beneath the southern Chuandian fragment slightly enlarges and is connected to Sichuan basin high-V zone. A weak striped low-V zone, located between Ordos and Sichuan basin blocks, is revealed in the Qinling fold zone. Both the amplitudes of high-V and low-V anomalies decrease to within -2% and 2%, comparing with that

The Diversity and Evolution of Different type Granitoids in Eastern Dharwar Craton: Evidence from "Arc-Plume" Collision in Neoarchaean

NASA Astrophysics Data System (ADS)

Nandy, J.; Dey, S.

2017-12-01

Neoarchaean crustal growth, role of plate tectonics and potential secular changes is still disoriented in Dharwar craton. To provide constraints on these questions, geochronological and geochemical data are presented on the unstudied granitoids associated with Kadiri greenstone belt, eastern Dharwar craton. Five diverse type of granitoids suites are identified in that area. Field setting, petrography, whole rock geochemistry study with Sm-Nd isotopes and zircon dates help to identify their source and petrogenesis. Along the eastern margin of the Kadiri belt a sanukitoid-like granitoid body is exposed which was probably derived from a metasomatised mantle wedge above a subduction zone followed by some older crustal contamination. Along the western margin transitional TTG is exposed displaying an intrusive relation with Kadiri dacite-rhyolite. This suite is interpreted to be derived from a mafic source with some enriched crustal component. Further east and west vast area is occupied highly silicic biotite monzogranite which is enriched in LILE, high K2O/Na2O. These granitoids are product of intracrustal shallow melting. At the south-eastern tip of the Kadiri belt occurs a well-foliated and banded transitional TTG which was probably derived from melting of mafic source with some contribution of felsic crustal material. At the north-eastern tip of the belt a highly silicic ferroan granitoid is exposed. Geochemical characteristics indicate that it is A-type granite, produced from shallow melting of a felsic crustal source. The basalt in greenstone belt is generated in oceanic plateau setting and granitoids in arc setting in different time. A tectonic scenario envisaging collision between an arc and oceanic plateau followed by repeated slab break-off and crustal recycling is proposed to explain the evolution of the terrain.

Velocity Structures underneath NRIL seismic station, Russia: Imaging the difference between the Siberian Craton and the West Siberian Basin

NASA Astrophysics Data System (ADS)

Youssof, M.; Thybo, H.; Artemieva, I. M.; Vinnik, L. P.

2016-12-01

This study discusses analysis of seismic P- and S-wave receiver functions (RFs). We construct RFs using records from NRIL seismic station, which is located between the northwestern edge of the Siberian Craton and the northeastern corner of the West Siberian Basin. We select 511 P- and S-RFs during a span of 7 years of recordings. Analysis of these records is very important as it might reveal the structural difference between these two tectonic settings at the crustal and lithospheric mantle scales as we split the analysis of arrivals from the east and west. The crust seems on average felsic, as concluded by Poisson's rations of 0.25 - 0.27, from the surface to the shallow Moho signal at 32 km. A prominent doublet Moho-like signal is seen in the observation as well as the inversion results. This might be a suggestive of a zone of underplating signature. The inversion evidences some lithospheric mantle stratifications within a frequency of 50 km thickness, in the range from 80 to 240 km depth. These layers observed beneath the region at depths of 85, 130 and 180 and 230 km. The observation images the deep discontinuities within the transition zone (410, 520 and 660 km). We find these converted phases are relatively shallower than the standard timing with earlier signals by 1.5 and 4.5 s, for P and S relative residuals respectively. We present here an extra interpretation based on dividing the directionality of rays into two clusters. This division simulates the present topographic/tectonic division exposed on the surface of Norilsk, due to the major NNW-SSE (150° azimuth from North) main divide. From this analysis, we find that the western division (West Basin mantle) has more distinct mantle discontinuities than the case in the eastern side (Cratonic mantle).

Intersection of Southern Parkway and Southern Heights, looking toward the ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

Intersection of Southern Parkway and Southern Heights, looking toward the Beechmont Historic District, showing changes in landscaping, northeast - Southern Heights-Beechmont District Landscapes, Louisville, Jefferson County, KY

Rhyacian A-type tholeiitic granites in southern Brazil: Geochemistry, U-Pb zircon ages and Nd model ages

NASA Astrophysics Data System (ADS)

Mesquita, Maria José; Bitencourt, Maria de Fátima; Nardi, Lauro Stoll; Picanço, Jefferson; Chemale, Farid, Jr.; Pimenta, Vanessa de Almeida

2017-04-01

In the southern South American platform, 2.5 to 2.0 Ga terranes, probably related to the Atlantica supercontinent, occur mainly as minor reworked inliers within Neoproterozoic, Brasiliano/Pan-African orogenic belts, as the Ribeira Belt in southern Brazil. The dispersion of such fragments has generated uncertainties about their geotectonic reconstruction, and their study has been supported mainly by elemental and isotope geochemistry. The southern Ribeira Belt lies between the Paranapanema and Luiz Alves cratons and contains reworked Neoarquean and Paleoproterozoic terranes which outcrop as basement nuclei in supracrustal sequences, as the Setuva Complex. The Água Comprida Suite, situated in the northern part of the Setuva Complex, consists of Amphibole-Biotite Syenogranite (ABS), Porphyritic Biotite Syenogranite (PBS), and Equigranular Biotite Syenogranite (EBS). All granites are foliated and intensively deformed. The oldest foliation (Sn) is marked by augen feldspars set in a recrystallized matrix, followed by a crenulation cleavage (Sn + 1) which evolves to discrete shear zones. ABS is a metaluminous, reduced A-type granite with FeOt / (FeOt + MgO) > 0.9, with high HFSE and REE contents, corresponding to magmas related to continental medium to high-K tholeiitic series. PBS and specially EBS are highly differentiated, metaluminous to peraluminous (EBS), oxidized granites. The increase of Al2O3 and Rb, and decrease of HFS and RE elements relative to ABS indicate their evolution from tholeiitic magmas. The Água Comprida Suite granites are cogenetic rocks evolved from a within-plate mantle source, marked by high Nb, Ta, and Y. The influence of previously metasomatised mantle sources is evidenced by negative Nb, Ti, and P anomalies. The age of ABS is 2187 ± 26 Ma, and that of PBS is between 2180 ± 13 to 2186 ± 22 Ma. The Nd model age of 2.4 Ga, and εNd(2.18 Ga) between - 0.23 and - 0.27 support the interpretation of ABS being formed from juvenile material with a

A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

NASA Astrophysics Data System (ADS)

Ikenne, Moha; Souhassou, Mustapha; Arai, Shoji; Soulaimani, Abderrahmane

2017-03-01

Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760-700 Ma), syn-collisional Bou-Azzer magmatism (680-640 Ma), followed by widespread Ediacaran magmatism (620-555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma. Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330-300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins. Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in

The Mackenzie River magnetic anomaly, Yukon and Northwest Territories, Canada-Evidence for Early Proterozoic magmatic arc crust at the edge of the North American craton

USGS Publications Warehouse

Pilkington, M.; Saltus, R.W.

2009-01-01

We characterize the nature of the source of the high-amplitude, long-wavelength, Mackenzie River magnetic anomaly (MRA), Yukon and Northwest Territories, Canada, based on magnetic field data collected at three different altitudes: 300??m, 3.5??km and 400??km. The MRA is the largest amplitude (13??nT) satellite magnetic anomaly over Canada. Within the extent of the MRA, source depth estimates (8-12??km) from Euler deconvolution of low-altitude aeromagnetic data show coincidence with basement depths interpreted from reflection seismic data. Inversion of high-altitude (3.5??km) aeromagnetic data produces an average magnetization of 2.5??A/m within a 15- to 35-km deep layer, a value typical of magmatic arc complexes. Early Proterozoic magmatic arc rocks have been sampled to the southeast of the MRA, within the Fort Simpson magnetic anomaly. The MRA is one of several broad-scale magnetic highs that occur along the inboard margin of the Cordillera in Canada and Alaska, which are coincident with geometric changes in the thrust front transition from the mobile belt to stable cratonic North America. The inferred early Proterozoic magmatic arc complex along the western edge of the North American craton likely influenced later tectonic evolution, by acting as a buttress along the inboard margin of the Cordilleran fold-and-thrust belt. Crown Copyright ?? 2008.

Petrogenesis of the middle Jurassic appinite and coeval granitoids in the Eastern Hebei area of North China Craton

NASA Astrophysics Data System (ADS)

Fan, Wenbo; Jiang, Neng; Xu, Xiyang; Hu, Jun; Zong, Keqing

2017-05-01

An integrated study of zircon U-Pb ages and Hf-O isotopic compositions, whole rock elemental and Sr-Nd isotope geochemistry was conducted on three lithologically diverse middle Jurassic plutons from the Eastern Hebei area of the North China Craton (NCC), in order to reveal both their petrogenesis and possible tectonic affinity. The three plutons have consistent magmatic zircon U-Pb ages from 167 ± 1 Ma to 173 ± 1 Ma. The Nianziyu pluton has typical characteristics of appinite with low SiO2 (43.7-52.6%), high Ca, Mg, Fe and H2O contents. It possesses subduction-related trace element patterns, enriched Nd-Hf isotopic signatures as well as elevated zircon δ18O values (6.2-7.2‰), arguing for an enriched mantle source metasomatized by fluids related to subduction. The Shuihutong monzogranites have high silica (SiO2 = 75.4-75.9%) and alkali contents, low Ca contents and striking negative Ba, Sr and Eu anomalies. Samples from the pluton have more evolved Nd-Hf isotopic values and are considered to be most likely derived from anatexis of ancient lower continental crust. Hybridization between mantle- and ancient lower crust-derived magmas is proposed for the mafic microgranular enclave-bearing Baijiadian granitoids, which are characterized by variable εNd (t) and εHf(t) values. Integrated with the regional geologic history, we suggest that the formation of the three middle Jurassic plutons were related to the subduction of the Paleo-Pacific ocean plate beneath the NCC. Their petrogenetic differences reflect complex magmatic processes in subduction settings involving melting of multiple sources, possible partly facilitated by fluid metasomatism and water-rich magma injection, accompanied with various degrees of magma mixing. The appearance of middle Jurassic appinitic rocks leads us to propose that the NCC destruction and lithosphere thinning were facilitated and controlled by the weakening of the lithospheric mantle after hydration because of the subduction of the

Data science implications in diamond formation and craton evolution

NASA Astrophysics Data System (ADS)

Pan, F.; Huang, F.; Fox, P. A.

2017-12-01

Diamonds are so-called "messengers" from the deep Earth. Fluid and mineral inclusions in diamonds could reflect the compositions of fluids/melts and wall-rocks in which diamond formed. Recently many diamond samples are examined to study the water content in the mantle transition zone1, the mechanism of diamond formation2 and the mantle evolution history3. However, most of the studies can only explain local activities. Therefore, an overall project of data grouping, comparison and correlation is needed, but limited progress has been made due to a lack of benchmark datasets on diamond formation and effective computing algorithms. In this study, we start by proposing the very first complete and easily-accessible dataset on mineral and fluid inclusions in diamonds. We rescue, collect and organize the data available from papers, journals and other publications resources ([2-4] and more), and then apply several state-of-the-art machine learning methods to tackle this earth science problem by clustering diamond formation process into distinct groups primarily based on the compositions, the formation temperature and pressure, the age and so on. Our ongoing work includes further data exploration and training existing models. Our preliminary results show that diamonds formed from older cratons usually have higher formation temperature. Also peridotitic diamonds take a much larger population than the ecologitic ones. More details are being discovered when we finish constructing the database and training our model. We expect the result to demonstrate the advantages of using machine learning and data science in earth science research problems. Our methodology for knowledge discovery are very general and can be broadly applied to other earth science research problems under the same framework.[1] Pearson et al, Nature (2014); [2] Tomlinson et al, EPSL (2006); [3] Weiss et al, Nature (2016); [4] Stachel and Harris, Ore Geology Reviews (2008); Weiss et al, EPSL (2013)

Paleomagnetism of the 1210 Ma Gnowangerup-Fraser dyke swarm, Western Australia

NASA Astrophysics Data System (ADS)

Pisarevsky, S. A.; Li, Z. X.; Wingate, M. T. D.; Tohver, E.

2012-04-01

The Gnowangerup-Fraser mafic dyke swarm is part of the Marnda Moorn LIP and subparallel to the southern and southeastern margins of the Yilgarn Craton. Some dykes become progressively recrystallized towards the craton margin and others are strongly deformed within the orogen, implying that at least some dykes were emplaced prior to the youngest deformation in the Albany-Fraser Orogen. Five dykes have previously yielded U-Pb ages between 1203 and 1218 Ma, and the primary nature of the magnetic directions in a 1212 Ma Fraser dyke is supported by a positive baked-contact test. We collected paleomagnetism samples from 19 dykes, along the Phillips and Fitzgerald Rivers, and near Ravensthorpe. AF demagnetisation revealed a stable bipolar remanence in 13 dykes. The mean paleomagnetic pole is almost identical to the VGP of the 1212 Ma Fraser dyke. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210 Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time.

Palaeoproterozoic tectonic evolution of the Alto Tererê Group, southernmost Amazonian Craton, based on field mapping, zircon dating and rock geochemistry

NASA Astrophysics Data System (ADS)

Lacerda Filho, J. V.; Fuck, R. A.; Ruiz, A. S.; Dantas, E. L.; Scandolara, J. E.; Rodrigues, J. B.; Nascimento, N. D. C.

2016-01-01

New geochemical and geochronological U-Pb and Sm-Nd data from amphibolites of the Alto Tererê Group, which are of Palaeoproterozoic age, are presented. The amphibolites are exposed in the central-eastern portion of the Rio Apa Block, southern Amazonian Craton, Mato Grosso do Sul, Brazil, and are composed of hornblende, plagioclase, quartz, biotite, cummingtonite and epidote. The amphibolites are subdivided into three lithofacies: (i) thinly banded amphibolites (metabasalts), (ii) coarse- and medium-grained amphibolites with relic subophitic texture (metagabbros), and (iii) amphibolites with relic cumulate texture (metapyroxenites). Chemical data also suggest the subdivision of the amphibolites into three different types. These rocks yield a U-Pb zircon age of 1768 ± 6 Ma and are therefore older than rocks of part of the Rio Apa Complex. Their Sm-Nd model ages range between 2.89 and 1.88 Ga, and their εNd (T) values range between -3.40 and + 3.74. Chemical analyses of these rocks indicate SiO2 concentrations between 45.23 and 50.65 wt.%, MgO concentrations between 4.34 and 8.01 wt.%, TiO2 concentrations between 0.91 and 1.74 wt.%, weakly fractionated rare-earth element (REE) patterns with mild depletion in heavy REEs, enrichment in large-ion lithophile elements (LILEs) and high-field-strength element (HFSEs), negative Nb, Ta and Co anomalies, positive Ba and Pb anomalies, low Ce concentrations, high Rb/Y ratios and low Th/La and Hf/Sm ratios. These features reflect metasomatism of the mantle wedge produced by sediments from the subducted plate. Various degrees of melting mark the evolution of the parent basic magmas, although subordinate crustal contamination may also have occurred. The geochemical signature of the amphibolites corresponds to that of tholeiitic basalts generated in an extensional back-arc-basin environment. The deposition in the basin apparently ceased during the first episode of compression and deformation at approximately 1.68 Ga, and the main

Geodynamic Setting of Proterozoic Dyke Swarms of the Leo-Man Craton, West Africa, Based on New U-Pb Dating and Geochemistry

NASA Astrophysics Data System (ADS)

Baratoux, L.; Jessell, M.; Söderlund, U.; Ernst, R. E.; Benoit, M.; Naba, S.; Cournede, C.; Perrouty, S.; Metelka, V.; Yatte, D.; Diallo, D. P.; Ndiaye, P. M.; Dioh, E.; Baratoux, D.

2016-12-01

Over 20 sets of dolerite dykes crosscutting Paleoproterozoic basement in West Africa were distinguished via the interpretation of regional and high-resolution airborne magnetic data available over the West African Craton. Some of the dykes reach over 300 km in length and are considered parts of much larger systems of mafic dyke swarms which form the plumbing system of Large Igneous Provinces (LIPs). Five different dyke swarms in Burkina Faso, Niger, Ghana and Senegal were investigated. In terms of petrography and composition, the mafic dykes correspond to tholeiitic basalts and are typically composed of plagioclase + clinopyroxene ± orthopyroxene ± olivine. They display a doleritic texture of variable grain size. Eleven ID-TIMS U-Pb ages obtained on baddeleyite define five generations of Proterozoic age. The N10 Libiri dyke swarm, found in western Niger, yielded an age of ca. 1790 Ma. The N40 Bassari swarm in Senegal was dated at ca. 1764 Ma, and is potentially linked to the 1790 Ma Libiri swarm, 1400 km away. The 300 by 400 km Korsimoro N100 dyke swarm transects central Burkina Faso and was dated at ca. 1575 Ma. Five ca. 1520 Ma ages were obtained for dykes of the Essakane swarm, three in Burkina Faso, one from Ghana (N130 orientation) and one from Senegal (E-W orientation), and document a large extent (600 km wide and 1500 km long) and short duration of dyke emplacement. The Manso N350 dyke swarm in southern Ghana, which is about 400 km long and about 200 km wide, yields a preliminary age of ca 870 Ma. A mantle plume origin is suggested for these swarms, especially the 1790-1765 Ma Libiri-Bassari swarm and the 1520 Ma Essakane swarms (which have lithosphere-contaminated E-MORB chemistry), whose scale is similar to largest giant radiating swarms (e.g. CAMP and Mackenzie). The 870 Ma Manso swarm has composition closer to OIB, consistent with a plume/hotspot origin. The 1575 Ma Korsimoro swarm has composition between EMORB and NMORB, which suggests a rift setting.

In-situ trace element and Sr isotopic compositions of mantle xenoliths constrain two-stage metasomatism beneath the northern North China Craton

NASA Astrophysics Data System (ADS)

Wu, Dan; Liu, Yongsheng; Chen, Chunfei; Xu, Rong; Ducea, Mihai N.; Hu, Zhaochu; Zong, Keqing

2017-09-01

Subduction and collision are the key processes triggering geochemical refertilization of the lithospheric mantle beneath cratons. However, the way that the subducted plate influences the cratonic lithospheric mantle remains unclear. Here, in-situ major and trace-element and Sr isotopic compositions of peridotite and pyroxenite xenoliths carried by the Dongbahao Cenozoic basalts, located close to the northern margin of North China Craton (NCC), were examined to investigate the effects of the subducted Paleo-Asian oceanic plate on the lithospheric mantle of the NCC. Based on petrographic and geochemical features, peridotites were subdivided into two types recording two-stage metasomatism. Clinopyroxene (Cpx) in both types of peridotites show chemical zoning. In those peridotites we refer to as Type 1 peridotites, Cpx exhibit uniform convex-upward rare earth element (REE) patterns but core-rim variations in 87Sr/86Sr ratios (0.7065-0.7082 in the cores and 0.7043-0.7059 in the spongy rims), and have high (La/Yb)N ratios (> 1.12) (N means normalized to chondrite), relatively low Ti/Eu ratios (< 3756) and negative high field strength element (HFSE) (Nb, Ta, Zr, Hf and Ti) anomalies in the cores, indicating early-stage metasomatism by carbonatitic melts derived from the subducted sedimentary carbonate rocks. Cpx in the Type 2 peridotites have highly variable REE patterns (from light rare earth element (LREE)-depleted to LREE-enriched) and feature zoned Sr isotopic compositions contrasting to those in Type 1, i.e., increasing 87Sr/86Sr ratios from the cores (0.7020-0.7031) to the spongy rims (0.7035-0.7041). Accompanying variations of 87Sr/86Sr ratios, Cpx in both types of peridotites display increasing Nb/La ratios from the cores to the spongy rims. In addition, Cpx in the Type 2 peridotites show remarkably increased (La/Yb)N, Ca/Al, Sm/Hf and Zr/Hf ratios but decreased Ti/Eu and Ti/Nb ratios from the cores to the spongy rims. These features imply a later

Lithospheric layering in the North American craton revealed by including Short Period Constraints in Full Waveform Tomography

NASA Astrophysics Data System (ADS)

Roy, C.; Calo, M.; Bodin, T.; Romanowicz, B. A.

2017-12-01

Recent receiver function studies of the North American craton suggest the presence of significant layering within the cratonic lithosphere, with significant lateral variations in the depth of the velocity discontinuities. These structural boundaries have been confirmed recently using a transdimensional Markov Chain Monte Carlo approach (TMCMC), inverting surface wave dispersion data and converted phases simultaneously (Calò et al., 2016; Roy and Romanowicz 2017). The lateral resolution of upper mantle structure can be improved with a high density of broadband seismic stations, or with a sparse network using full waveform inversion based on numerical wavefield computation methods such as the Spectral Element Method (SEM). However, inverting for discontinuities with strong topography such as MLDS's or LAB, presents challenges in an inversion framework, both computationally, due to the short periods required, and from the point of view of stability of the inversion. To overcome these limitations, and to improve resolution of layering in the upper mantle, we are developing a methodology that combines full waveform inversion tomography and information provided by short period seismic observables. We have extended the 30 1D radially anisotropic shear velocity profiles of Calò et al. 2016 to several other stations, for which we used a recent shear velocity model (Clouzet et al., 2017) as constraint in the modeling. These 1D profiles, including both isotropic and anisotropic discontinuities in the upper mantle (above 300 km depth) are then used to build a 3D starting model for the full waveform tomographic inversion. This model is built after 1) homogenization of the layered 1D models and 2) interpolation between the 1D smooth profiles and the model of Clouzet et al. 2017, resulting in a smooth 3D starting model. Waveforms used in the inversion are filtered at periods longer than 30s. We use the SEM code "RegSEM" for forward computations and a quasi-Newton inversion

Very early Archean crustal-accretion complexes preserved in the North Atlantic craton

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nutman, A.P.; Collerson, K.D.

1991-08-01

The North Atlantic craton contains very early Archean supracrustal rocks, orthogneisses, and massive ultramafic rocks. Most units of supracrustal rocks are dominated by mafic volcanic rocks, layered gabbros, and banded iron formations, bust some also contain abundant felsic volcanic-sedimentary rocks, quartzites, and marbles. Some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic-sedimentary rocks in these sequences (ca. 3800 Ma) and also from older (ca. 3850 Ma) sources. The presence of the ca. 3850 Ma detrital zircons suggests that the supracrustal units containing them were deposited on, or close to, ca. 3850 Ma sialic crust. Themore » massive ultramafic rocks have chemical affinities to upper mantle rocks. The voluminous suites of tonalitic gneisses are dominated by 3700-3730 Ma bodies that intrude the supracrustal sequences, but they also locally contain components with ages between 3820 and 3920 Ma. The diverse supracrustal units, upper mantle rocks, and {ge} 3820 Ma components in the gneisses were tectonically interleaved in very early Archean convergent plate boundaries, giving rise to accretion complexes. In the period 3700-3730 Ma, voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels, forming juvenile continental crust and leaving behind a refractory lower crustal to upper mantle substrate.« less

Paleosols as Archives of Environmental Change in Deep Time

NASA Astrophysics Data System (ADS)

Crowley, Quentin

2015-04-01

Paleosols develop at the geosphere-atmosphere interface and potentially provide an archive of environmental conditions at the time of their formation. Although paleosols from deep time can be difficult to recognize due to the masking of pedogenic features by metamorphism and deformation, they may record transient (i.e. time-dependent) events which are often difficult to recognize in other geological proxies. Paleosols from the Archean and Paleoproterozoic are rare and complex to study, but offer an opportunity to gain insight into what may be relatively short-scale temporal variations in the Earth's atmospheric composition. For instance, it is widely believed that atmospheric oxygen saturation rose from <10E-05 present atmospheric level (PAL) in the Archean to >10E-02 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga. Until recently however, chemical or physical evidence from paleosols for earlier oxygenation events were generally thought to be lacking. Recent studies of paleosols from eastern India (Keonjhar Paleosol, Singhbhum Craton) and South Africa (Nsuze Paleosol, Kaapvaal Craton) have provided chemical evidence for transient Mesoarchean atmospheric oxygenation at ca. 3.0 Ga. These paleosols are considered to preserve the earliest known vestiges of terrestrial oxidative weathering, signifying a transient, early oxygen accumulation in the Earth's atmosphere. This has far-reaching implications from both atmospheric and biological evolutionary perspectives in that chemical signatures preserved in these Mesoarchean paleosols are thought to signify the presence of molecular oxygen at levels higher than those attributable to photo-dissociation of atmospheric water alone. Such elevated levels of atmospheric oxygen could only be due to the presence of a sufficiently large biomass of micro-organisms capable of oxidative photosynthesis. Although the Archean-Paleoproterozoic paleosol geological record is fragmentary and geochemical signatures are not necessarily

Lithospheric Delamination or Relict Slab Beneath the Former North American Cratonic Margin in Idaho and Oregon? New Constraints From Seismic Tomography.

NASA Astrophysics Data System (ADS)

Stanciu, A. C.; Russo, R. M.; Mocanu, V. I.; VanDecar, J. C.; Hongsresawat, S.; Bremner, P. M.; Torpey, M. E.; Panning, M. P.

2016-12-01

We present a new high-resolution P-wave velocity model of the upper mantle beneath the former passive margin of the North American craton in Oregon and Idaho. We identify high velocity anomalies in the central part of the model and low velocity anomalies to the northwest and southeast. Our results derive from an integrated data set of teleseismic P waves recorded at 145 broadband stations, 85 deployed between 2011 and 2013 as part of the IDOR Passive experiment, and 60 USArray-TA stations. We determined 15,000 travel-times using multi-channel cross-correlation (VanDecar and Crosson, 1990). Phanerozoic tectonic events that affected upper mantle seismic structure here include subduction of Farallon and Juan de Fuca lithosphere, accretion of Blue Mountains terranes, Sevier and Laramide orogenies, Idaho batholith formation, Yellowstone and Columbia River volcanism, and Basin and Range extension. Our results indicate a high P-wave velocity anomaly located beneath the Idaho Batholith in west-central Idaho traceable down to 150-200 km depth. A similar anomaly identified by Schmandt and Humphrey (2011) beneath Washington and Montana was interpreted as a slab remnant from the accretion of Siletzia to North America. Alternatively, the fast Vp anomalies are delaminated North American craton lithosphere. Thickened lithosphere may have formed during Farallon subduction, terrane collision and accretion. Crust as much as 55 km thick present during Late Cretaceous (Foster et al., 2001; Gaschnig et al., 2011) is potentially indicative of lithospheric thickening leading to delamination. To the southeast, upper mantle low velocity anomalies occur beneath the Western Snake River Plain. We associate these low velocities with high temperatures generated by the Yellowstone mantle plume system. We observe a low velocity anomaly beneath the Wallowa Mountains starting at 150-200 km extending to depths below the resolution of our model.

Three-Dimensional Rheological Structure of North China Craton Determined by Integration of Multiple observations: Controlling Role for Lithospheric Rifting

NASA Astrophysics Data System (ADS)

Xiong, X.; Shan, B.; Li, Y.

2017-12-01

The North China Craton (NCC) has undergone significant lithospheric rejuvenation in late Mesozoic and Cenozoic, one feature of which is the widespread extension and rifting. The extension is distinct between the two parts of NCC: widespread rifting in the eastern NCC and localized narrow rifting in the west. The mechanism being responsible for this difference is uncertain and highly debated. Since lithospheric deformation can be regarded as the response of lithosphere to various dynamic actions, the rheological properties of lithosphere must have a fundamental influence on its tectonics and deformation behavior. In this study, we investigated the 3D thermal and rheological structure of NCC by developing a model integrating several geophysical observables (such as surface heatflow, regional elevation, gravity and geoid anomalies, and seismic tomography models). The results exhibit obvious lateral variation in rheological structure between the eastern and western NCC. The overall lithospheric strength is higher in the western NCC than in the east. Despite of such difference in rheology, both parts of NCC are characterized by mantle dominated strength regime, which facilitates the development of narrow rifting. Using ancient heatflow derived from mantle xenoliths studies, and taking the subduction-related dehydration reactions during Mesozoic into account, we constructed the thermal and rheological structure of NCC in Ordovician, early Cretaceous and early Cenozoic. Combining the evidence from numerical simulations, we proposed an evolution path of the rifting in NCC. The lithosphere of NCC in Ordovician was characterized by a normal craton features: low geotherm, high strength and mantle dominated regime. During Jurassic and Cretaceous, the mantle lithosphere in the eastern NCC was hydrated by fluid released by the suduction of the Pacific plate, resulting in weakening of the lithosphere and a transition from mantle dominated to crust dominated regime, which

Reinterpretation of Paleoproterozoic accretionary boundaries of the north-central United States based on a new aeromagnetic-geologic compilation

USGS Publications Warehouse

Holm, D.K.; Anderson, R.; Boerboom, Terrence; Cannon, W.F.; Chandler, V.; Jirsa, M.; Miller, J.; Schneider, D.A.; Schulz, K.J.; Van Schmus, W. R.

2007-01-01

The Paleoproterozoic crust in the north-central U.S. represents intact juvenile terranes accreted to the rifted Archean Superior craton. A new tectonic province map, based on the interpretation of a new aeromagnetic compilation, published geologic maps, and recent geochronologic data, shows progressive accretion of juvenile arc terranes from ca. 1900-1600 Ma. Contrary to earlier models, geon 18 Penokean-interval crust is primarily confined to a ???2100 Ma tectonic embayment of the rifted Superior craton. The newly defined Spirit Lake tectonic zone, characterized by a sharp magnetic discontinuity that marks the southern limit of Archean and Penokean-interval rocks, is here interpreted to represent an eastern analog of the Cheyenne belt suture zone in southern Wyoming. South of this boundary, geon 17 Yavapai-interval rocks form the basement upon which 1750 Ma rhyolite and succeeding quartzite sequences were deposited. Substantial portions of the Penokean and Yavapai terranes were subsequently deformed during the 1650-1630 Ma Mazatzal orogeny. The northern boundary of the Mazatzal terrane is obscured by abundant 1470-1430 Ma "anorogenic" plutons that stitched the suture with the older Yavapai terrane rocks. These data reveal a progressive tectonic younging to the south as the Laurentian craton grew southward and stabilized during the Proterozoic. Late Mesoproterozoic rift magmatism produced pronounced geophysical anomalies, indicating strong, but localized crustal modification. In comparison to the western U.S., little tectonism has occurred here in the last 1 billion years, providing a uniquely preserved record of the Precambrian evolution of the continental U.S. lithosphere. ?? 2007 Elsevier B.V. All rights reserved.

Metallogeny of precious and base metal mineralization in the Murchison Greenstone Belt, South Africa: indications from U-Pb and Pb-Pb geochronology

NASA Astrophysics Data System (ADS)

Jaguin, J.; Poujol, M.; Boulvais, P.; Robb, L. J.; Paquette, J. L.

2012-10-01

The 3.09 to 2.97 Ga Murchison Greenstone Belt is an important metallotect in the northern Kaapvaal Craton (South Africa), hosting several precious and base metal deposits. Central to the metallotect is the Antimony Line, striking ENE for over 35 km, which hosts a series of structurally controlled Sb-Au deposits. To the north of the Antimony Line, hosted within felsic volcanic rocks, is the Copper-Zinc Line where a series of small, ca. 2.97 Ga Cu-Zn volcanogenic massive sulfide (VMS)-type deposits occur. New data are provided for the Malati Pump gold mine, located at the eastern end of the Antimony Line. Crystallizations of a granodiorite in the Malati Pump Mine and of the Baderoukwe granodiorite are dated at 2,964 ± 7 and 2,970 ± 7 Ma, respectively (zircon U-Pb), while pyrite associated with gold mineralization yielded a Pb-Pb age of 2,967 ± 48 Ma. Therefore, granodiorite emplacement, sulfide mineral deposition and gold mineralization all happened at ca. 2.97 Ga. It is, thus, suggested that the major styles of orogenic Au-Sb and the Cu-Zn VMS mineralization in the Murchison Greenstone Belt are contemporaneous and that the formation of meso- to epithermal Au-Sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated Cu-Zn VMS mineralization.

Proterozoic Bushveld-Vredefort catastrophe: Possible causes and consequences

NASA Technical Reports Server (NTRS)

Elston, W. E.; Twist, D.

1988-01-01

Bushveld Complex and Vredefort Dome are unique features, formed in close proximity during the same time interval, approximately 2 Ga. Both show evidence of catastrophic events in the shallow marine environment of the otherwise stable Kaapvaal Craton. Explanation by multiple impacts of an asteroid, brecciated by an inter-asteroidal collision and disintegrating in Earth's gravity field is supported by pseudotachylite, shatter cones, coesite, and stishovite at Vredefort but these shock phenomena were not found in the Bushveld Complex. The Bushveld Complex was formerly interpreted as a lopolith, a view incompatible with gravity, electrical resistivity, magnetic, and seismic-reflection data. It is outlined by five inward-dipping lobes of layered ultramafic-mafic plutonic rocks that partly coalesce to form a basin-like feature 400 km in diameter and 65,000 sq. km. in area, equivalent to a small lunar mare. The Bushveld Complex is orders of magnitudes larger than other proposed terrestrial impact structures and differs from them in important ways. Its principal members, in order of age, are Rooiberg Felsite, RLS, and Lebowa Granite. The Bushveld-Vredefort events occurred during the interval from neutral or reducing atmosphere to oxidizing atmosphere. This transition is usually related to the evolution of photosynthesizing organisms. If the impact hypothesis for Bushveld-Vredefort can be confirmed, it may represent a global catastrophe sufficient to contribute to environmental changes favoring aerobic photosynthesizing eukaryotes over anaerobic prokaryotes.

Magnetic susceptibility of the South African Agouron scientific drillcores quantifies iron and sulfur alteration relevant to geochemical oxygenation proxies

NASA Astrophysics Data System (ADS)

Raub, T. D.; Nayak, P. M.; Tikoo, S. M.; Johnson, J. E.; Peek, S.; Fischer, W. W.; Kirschvink, J. L.

2010-12-01

Various geochemical characteristics of sedimentary iron- and sulfur-bearing minerals motivate early- to late-oxygenation hypotheses from South African and Australian scientific drillcores. Most intervals of these drillcores appear to be remagnetized (in some cases multiple times); and ~2.0 Ga magnetic sulfide crystallization is particularly pervasive in carbonaceous siltstones of the ca. 2.7-2.2 Ga Griqualand margin of Kaapvaal craton. Robust interpretation of trace element abundances suggesting “whiffs” to “pervasive” levels of late Archean oxygen depends upon systematics of presumed depositional iron speciation; so multiple iron- and sulfur-mineral-altering events affecting existing drillcore records call straightforward interpretations into question. We report ca. 10,000 magnetic susceptibility measurements and associated detailed rock-magnetic results from all lithologies of Archean basinal and slope facies in drillcores GKP and GKF and relatively younger and shallower facies in Paleoproterozoic drillcores GEC and GTF. Specific carbonaceous siltstone and carbonate intervals are less-altered as revealed by coherent and relatively low magnetic susceptibilities: geochemical and biomarker interpretations based upon data from these intervals should be preferred to those from others. Magnetic susceptibility tracks subtle facies variation in drillcore GTF diamictite and suggests highly-structured Paleoproterozoic glacioeustatic sequence architecture consistent with assignment of Makganyene glaciation and its associated geochemical signature to a ca. 2.2 Ga “Snowball Earth” ice age.

Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

NASA Astrophysics Data System (ADS)

Mandal, Prantik

2017-02-01

In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (∼2.5-3.6 Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (∼1.0-2.5 Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7-70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6 Ga Dalma and ∼117 Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (Vs) of 3.0 km/s and a 26-km thick lower-crust (LC) of average Vs of 3.6 km/s. The present study detects a sharp drop in Vs (∼-2 to 3%) at 120-260 km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120 km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88-103 km depths, suggesting a 88-103 km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2-3% drop in Vs could be attributed to the presence of partial melts in the

Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

NASA Technical Reports Server (NTRS)

Haggerty, Stephen E.; Toft, Paul B.

1988-01-01

Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

Morphostructural evidence for Recent/active extension in Central Tanzania beyond the southern termination of the Kenya Rift.

NASA Astrophysics Data System (ADS)

Le Gall, B.; Rolet, J.; Gernigon, L.; Ebinger, C.; Gloaguen, R.

2003-04-01

The southern tip zone of the Kenya Rift on the eastern branch of the East African System is usually thought to occur in the so-called North Tanzanian Divergence. In this region, the narrow (50 km-wide) axial graben of southern Kenya splays southwards, via a major EW-trending volcanic lineament, into a 200 km-wide broad rifted zone with three separate arms of normal faulting and tilted fault blocks (Eyasi, Manyara and Pangani arms from W to E). Remote sensing analysis from Central Tanzania demonstrates that rift morphology exists over an area lying 400 km beyond the southern termination of the Kenya Rift. The most prominent rift structures are observed in the Kilombero region and consist of a 100 km-wide range of uplifted basement blocks fringed to the west by an E-facing half-graben inferred to reach depths of 6-8 km from aeromagnetic dataset. Physiographic features (fault scarps), and river drainage anomalies suggest that the present-day rift pattern in the Kilombero extensional province principally results from Recent/Neogene deformation. That assumption is also supported by the seismogenic character of a number of faults. The Kilombero half-graben is superimposed upon an earlier rift system, Karoo in age, which is totally overprinted and is only evidenced from its sedimentary infill. On the other hand, the nature and thickness of the inferred Neogene synrift section is still unknown. The Kilombero rifted zone is assumed to connect northwards into the central rift arm (Manyara) of the South Kenya Rift via a seismically active transverse fault zone that follows ductile fabrics within the Mozambican crystalline basement. The proposed rift model implies that incipient rifting propagates hroughout the cold and strong crust/lithosphere of Central Tanzania along Proterozoic (N140=B0E) basement weakness zones and earlier Karoo (NS)rift structures. A second belt of Recent-active linked fault/basins also extends further East from the Pangani rift arm to the offshore

Southern Pine Beetle

Treesearch

Robert C. Thatcher; Patrick J. Barry

1982-01-01

The southern pine beetle (Dendroctonus frontalis Zimmermann) is one of pine's most destructive insect enemies in the Southern United States, Mexico, and Central America. Because populations build rapidly to outbreak proportions and large numbers of trees are killed, this insect generates considerable concern among managers of southern pine forests. The beetle...

The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

NASA Astrophysics Data System (ADS)

Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

2009-05-01

The Renard kimberlite cluster consists of nine pipes located within a 2km2 area in the northern Otish Mountains of Quebec. The pipes are named Renards 1 to 10, with subsequent investigation revealing Renards 5 and 6 to join at depth (now Renard 65). The pipes are located within the eastern portion of the Superior craton, emplaced into Archean granitic and gneissic host rocks of the Opinica Subprovince (Percival, 2007). Amphibolite grade metamorphism, locally passing into the granulite facies (Percival et al., 1994) occurred in late Archean time (Moorhead et al., 2003). Radiometric dating of the hypabyssal Renard 1 kimberlite indicates Neoproterozoic emplacement, with a 206Pb/238U model age of 631.6±3.5 Ma (2σ) (Birkett et al., 2004). A later study on the main phases in Renard 2 and 3 gave a similar emplacement, with a 206Pb/238U model age of 640.5±2.8Ma (Fitzgerald et al., 2008). This makes this kimberlite district one of the oldest in Canada, similar in eruption age to the Wemindji kimberlites (629±29Ma: Letendre et al., 2003). These events are broadly coeval with the conversion from subduction magmatism to rifting in northern Laurentia (Birkett et al., 2004). The bodies are part of a late Neoproterozoic to Cambrian kimberlite field in eastern Canada (Girard, 2001; Moorhead et al, 2002; Letendre et al., 2003) and fit into the north-east of the Eocambrian/Cambrian Labrador Sea Province of Heaman et al. (2004). To better understand the diamondiferous lithospheric mantle beneath the Renard kimberlites, 116 microxenoliths and xenocrysts were analysed. The samples were dominantly peridotitic, composed primarily of purple garnet, emerald green clinopyroxene and olivine, with a few pink and red garnets. A minor eclogitic component comprises predominantly orange garnets and lesser amounts of clinopyroxene. A detailed study on the major, minor and trace element composition of xenolith minerals is currently underway. All but three of the clinopyroxenes analysed to date

The significance of the Medicine Hat Block (southern Alberta, northern Montana) in the assembly of Laurentia: New interpretations from recent single grain zircon geochronological and geochemical data

NASA Astrophysics Data System (ADS)

LaDouceur, B. O.; Gifford, J.; Malone, S.; Davis, B.

2017-12-01

Keywords: Medicine Hat Block, Zircon, U/Pb ages, Hf isotopes, Laurentia The Medicine Hat Block (MHB) is one of the core cratonic elements that amalgamated in the Paleoproterozoic to form Laurentia. However, unlike many of the other cratons, the role of the MHB in the formation of Laurentia is poorly constrained. Virtually all of the MHB is concealed by Proterozoic and younger supracrustal sequences, limiting the data collected from this craton. The primary source of samples from the MHB comes from two sources: 1) xenoliths of variably metamorphoses gneisses, amphibolites, and meta-plutonic rocks collected from Eocene volcanic rock, and 2) similar lithologies recovered from boreholes that penetrate to the MHB basement. Multigrain zircon TIMS analyses yielded U/Pb ages ranging from 1.70 Ga to 3.26 Ga. Recent zircon single-grain LA-ICPMS U-Pb ages revealed a slightly older range of Archean ages, 2.63 Ga to 3.27 Ga, and two samples yielding Paleoproterozoic ages at 1.78 and 1.82 Ga. Whole-rock Sm/Nd data indicated that the samples formed from crustal sources, with model ages ranging between 1.80 Ga to 3.48 Ga. In-situ zircon Hf isotopic results revealed that Archean-aged zircon are generally suprachondritic, with eHf(t) values between 8.3 and -8.7. In contrast, the Paleoproterozoic grains yielded negative eHf(t) values ranging from -6.8 to -21.2, suggestive of a reworked Archean crustal component in their genesis. In particular, the Sweetgrass Hill xenolith suite is characterized solely by Paleoproterozoic ages, with evolved eHf(t) suggesting that any older U-Pb ages were reset by granulite facies metamorphism and zircon recrystallization. The combined U-Pb and Hf isotopic data from these samples helps illuminate the character of the MHB and its relationships to the Wyoming and Hearne cratons, as well as the Great Falls Tectonic Zone (GFTZ). The ages overlap between cratonic elements; however, the abundance of positive eHf(t) values of the 2.8 Ga ages suggests that the

Tectonics and distribution of gold deposits in China - An overview

USGS Publications Warehouse

Zhou, T.; Goldfarb, R.J.; Phillips, G.N.

2002-01-01

present along the northern, southeastern and southern margins of the North China craton, along the southwestern and northwestern margins of the Yangtze craton, in the Tianshan and Altayshan orogenic belts in northern Xinjiang, and throughout the southeastern China fold belt. Gold-placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern China and along the northerwestern margin of the Yangtze craton. The major provinces with significant gold in porphyry-related copper systems and base metal skarns are present in the Yangtze River area along the northeastern and southeastern margin of the Yangtze craton, in the fold belt in southwestern China, and scattered through northern China. Three-quarters of the Chinese gold-only deposits occur within the North China craton margins. Half are located in the uplifted Precambrian metamorphie rocks and most of the remainder are hosted in the Phanerozoic granitoids that intruded the reworked Precambrian terranes. The abundance of granite-hosted gold contrasts the North China craton with other Precambrian cratons, such as those in Western Australia, central Canada, and Zimbabwe, where gold is mainly hosted in the Archean greenstone belts. This difference may be explained by the multiple episodes of Phanerozoic tectonism along the North China craton margins resulting from the collision of the Angara, North China, and South China cratons, and from subduction of the Izanagi-Pacific oceanic plates underneath the eastern China continent.

Resolving the tectonic transition between ancestral North America and the northern Cordillera

NASA Astrophysics Data System (ADS)

Schaeffer, A. J.; Audet, P.; Lebedev, S.

2015-12-01

The northern Cordillera, situated in the Canadian northwest, is one of the most actively deforming regions in Canada and host to the highest earthquake activity in the country. Furthermore, it presents a largely contiguous snapshot through almost 4 Gyr of Earth's history across a zone <2000 km in linear extent. Deformation is thought to be driven by tectonic forces transferred from the Alaska-Pacific plate collision eastwards to the Cordilleran Deformation Front (CDF), where the westward edge of the Canadian Shield acts as a rigid backstop. Past studies in the southern Yukon indicate a sharp transition into the craton underlying the CDF and evidence of craton growth through shallow subduction. Further north the proximity of the craton edge to the CDF remains largely unresolved; based on studies of the southern Cordillera and Alaska, significant variations in lithospheric architecture are expected. Additionally, significant seismicity is observed further north off the Beaufort Shelf; however, its relationship to the regional stress fields and associated tectonic forcing is unclear. Despite the high seismicity levels across, detailed study of this region has been limited by insufficient coverage of seismological infrastructure, hindering resolution in past models. With the deployment of the USArray Transportable Array in Alaska over the last several years, combined with regional arrays such as the Yukon-Northwest Seismic Network (YNSN), Banks Island Seismic Network (BISN) and Mackenzie Mountains Experiment, new studies will leverage these datasets enabling more detailed imaging of the structure and seismicity across the region. Here we present a new high-resolution, vertically polarized shear speed and azimuthal model of northwestern Canada and Alaska, constrained by vertical component seismogram fits computed using the Automated Multimode Inversion of Surface, S, and multiple-S waveforms. With this new model, we aim to address key questions relating to the dynamics

Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei

2017-08-01

There is a wide support that the Inner Mongolia Palaeo-uplift on the northern margin of the North China Craton has undergone an uplifting history. However, when and how did the uplift occurred keeps controversial. Extensive field-based structural, metamorphic, geochemical, geochronological and geophysical investigations on the Inner Mongolia Palaeo-uplift, which suggested that the Inner Mongolia Palaeo-uplift was an uplifted region since the Early Precambrian or range from Late Carboniferous-Early Jurassic. The geochemical characteristics of the Late Paleozoic to Early Mesozoic intrusive rocks indicated that the Inner Mongolia Palaeo-uplift was an Andean-type continental margin that is the extensional tectonic setting. To address the spatial and temporal development of the Inner Mongolia Palaeo-uplift, we have carried out provenance analysis of Permian sedimentary rocks which collected from the Panyangshan basin along the northern margin of the North China Craton. The QFL diagram revealed a dissected arc-recycled orogenic tectonic setting. Moreover, the framework grains are abundant with feldspar (36-50%), indicating the short transport distance and unstable tectonic setting. Detrital zircon U-Pb analysis ascertained possible provenance information: the Precambrian basement ( 2490 and 1840 Ma) and continental arc magmatic action ( 279 and 295 Ma) along the northern margin of the North China Craton. The projection in rose diagrams of the mean palaeocurrent direction, revealing the SSW and SSE palaeoflow direction, also shows the provenance of the Panyangshan basin sources mainly from the Inner Mongolia Palaeo-uplift. The andesite overlying the Naobaogou Formation has yielded U-Pb age of 277.3 ± 1.4 Ma. The additional dioritic porphyry dike intruded the Naobaogou and Laowopu Formations, which has an emplacement age of 236 ± 1 Ma. The above data identify that the basin formed ranges from Early Permian to Middle Triassic (277-236 Ma). Accordingly, the Inner Mongolia

Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton

NASA Astrophysics Data System (ADS)

Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei

2018-06-01

There is a wide support that the Inner Mongolia Palaeo-uplift on the northern margin of the North China Craton has undergone an uplifting history. However, when and how did the uplift occurred keeps controversial. Extensive field-based structural, metamorphic, geochemical, geochronological and geophysical investigations on the Inner Mongolia Palaeo-uplift, which suggested that the Inner Mongolia Palaeo-uplift was an uplifted region since the Early Precambrian or range from Late Carboniferous-Early Jurassic. The geochemical characteristics of the Late Paleozoic to Early Mesozoic intrusive rocks indicated that the Inner Mongolia Palaeo-uplift was an Andean-type continental margin that is the extensional tectonic setting. To address the spatial and temporal development of the Inner Mongolia Palaeo-uplift, we have carried out provenance analysis of Permian sedimentary rocks which collected from the Panyangshan basin along the northern margin of the North China Craton. The QFL diagram revealed a dissected arc-recycled orogenic tectonic setting. Moreover, the framework grains are abundant with feldspar (36-50%), indicating the short transport distance and unstable tectonic setting. Detrital zircon U-Pb analysis ascertained possible provenance information: the Precambrian basement ( 2490 and 1840 Ma) and continental arc magmatic action ( 279 and 295 Ma) along the northern margin of the North China Craton. The projection in rose diagrams of the mean palaeocurrent direction, revealing the SSW and SSE palaeoflow direction, also shows the provenance of the Panyangshan basin sources mainly from the Inner Mongolia Palaeo-uplift. The andesite overlying the Naobaogou Formation has yielded U-Pb age of 277.3 ± 1.4 Ma. The additional dioritic porphyry dike intruded the Naobaogou and Laowopu Formations, which has an emplacement age of 236 ± 1 Ma. The above data identify that the basin formed ranges from Early Permian to Middle Triassic (277-236 Ma). Accordingly, the Inner Mongolia

Neoarchean high-pressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India: Petrology and zircon SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Saitoh, Yohsuke; Tsunogae, Toshiaki; Santosh, M.; Chetty, T. R. K.; Horie, Kenji

2011-08-01

.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530-2540 Ma) and metamorphic (2470-2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P- T data and exhumation path with those reported for Late Neoproterozoic-Cambrian HP-UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.

Origin and tectonic evolution of early Paleozoic arc terranes abutting the northern margin of North China Craton

NASA Astrophysics Data System (ADS)

Zhou, Hao; Pei, Fu-Ping; Zhang, Ying; Zhou, Zhong-Biao; Xu, Wen-Liang; Wang, Zhi-Wei; Cao, Hua-Hua; Yang, Chuan

2017-12-01

The origin and tectonic evolution of the early Paleozoic arc terranes abutting the northern margin of the North China Craton (NCC) are widely debated. This paper presents detrital zircon U-Pb and Hf isotopic data of early Paleozoic strata in the Zhangjiatun arc terrane of central Jilin Province, northeast (NE) China, and compares them with the Bainaimiao and Jiangyu arc terranes abutting the northern margin of the NCC. Detrital zircons from early Paleozoic strata in three arc terranes exhibit comparable age groupings of 539-430, 1250-577, and 2800-1600 Ma. The Paleoproterozoic to Neoarchean ages and Hf isotopic composition of the detrital zircons imply the existence of the Precambrian fragments beneath the arc terranes. Given the evidences from geology, igneous rocks, and detrital zircons, we proposed that the early Paleozoic arc terranes abutting the northern margin of the NCC are a united arc terrane including the exotic Precambrian fragments, and these fragments shared a common evolutionary history from Neoproterozoic to early-middle Paleozoic.

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution.

PubMed

Fritz, H; Abdelsalam, M; Ali, K A; Bingen, B; Collins, A S; Fowler, A R; Ghebreab, W; Hauzenberger, C A; Johnson, P R; Kusky, T M; Macey, P; Muhongo, S; Stern, R J; Viola, G

2013-10-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

NASA Astrophysics Data System (ADS)

Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

2013-10-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

Eclogite formation beneath the northern Slave craton constrained by diamond inclusions: Oceanic lithosphere origin without a crustal signature

NASA Astrophysics Data System (ADS)

Smart, Katie A.; Chacko, Thomas; Stachel, Thomas; Tappe, Sebastian; Stern, Richard A.; Ickert, Ryan B.; EIMF

2012-02-01

craton. Hence, the Jericho DIs and host high-MgO eclogites may represent an example of eclogite formation in an oceanic setting without the diagnostic 'crustal signatures' that are typically observed in cratonic eclogite xenolith suites worldwide.

The Hadean to Paleoarchean geodynamo: microconglomerate tests from siliciclastic metasedimentary rocks from the Southern Cross Terrane of Western Australia

NASA Astrophysics Data System (ADS)

Cottrell, R. D.; Tarduno, J. A.; Bono, R. K.; Thern, E. R.; Chhibber, S. K.

2016-12-01

Detrital zircons found within metasedimentary rocks of the Yilgarn Craton (Western Austrlia) contain information about the early history of the geodynamo. Recently reported single crystal paleointensity (SCP) values obtained from zircon grains of the Jack Hills (JH) belt (Tarduno et al., 2015) suggest the presence of a core dynamo to times as old as 4.2 Ga. Magnetizations carried by zircons within these rocks have been preserved despite multiple reheating events of the host metasedimentary unit. Two positive conglomerate tests (Tarduno and Cottrell, 2013; Tarduno et al., 2015) as well as interlaboratory studies reproducing a prior positive conglomerate test (Dare et al., 2015; 2016) attest to the primary nature of the remanence carried by these zircons. Moreover, new Li data limit heating after zircon formation to < 500 oC. Similarly, the metasedimentary rocks of the Southern Cross Terrane, more than 400 kilometers away from the Discovery Site of the Jack Hills, contain detrital Eoarchean to Hadean age zircons (Nelson, 2005; Wyche, 2007). Following Tarduno et al. (2015), a micro-conglomerate test of oriented, small ( 500-700 mm) samples centered on single zircons ( 200-350 mm) was performed using the University of Rochester's ultra-high resolution 3-component DC SQUID magnetometer that affords an order of magnitude greater sensitivity than other high-resolution SQUID rock magnetometers. The characteristic remanences from oriented zircons (N=10; R=0.81) that unblock between 565 and 580 °C, consistent with a magnetite carrier, are well defined but together cannot be distinguished from those drawn from a random distribution (Ro=5.03) at the 95% confidence level; this indicates a positive microconglomerate test. Preliminary paleointensity determinations range between 4-27 μT. Additional studies of hand samples from the Southern Cross Terrane are underway.

Tectonic and sedimentary linkages between the Belt-Purcell basin and southwestern Laurentia during the Mesoproterozoic ca. 1.60-1.40 Ga

USGS Publications Warehouse

Jones, James V.; Dainel, Christohper G; Doe, Michael F

2015-01-01

Mesoproterozoic sedimentary basins in western North America provide key constraints on pre-Rodinia craton positions and interactions along the western rifted margin of Laurentia. One such basin, the Belt-Purcell basin, extends from southern Idaho into southern British Columbia and contains a >18-km-thick succession of siliciclastic sediment deposited ca. 1.47–1.40 Ga. The ca. 1.47–1.45 Ga lower part of the succession contains abundant distinctive non-Laurentian 1.61–1.50 Ga detrital zircon populations derived from exotic cratonic sources. Contemporaneous metasedimentary successions in the southwestern United States–the Trampas and Yankee Joe basins in Arizona and New Mexico–also contain abundant 1.61–1.50 Ga detrital zircons. Similarities in depositional age and distinctive non-Laurentian detrital zircon populations suggest that both the Belt-Purcell and southwestern successions record sedimentary and tectonic linkages between western Laurentia and one or more cratons including North Australia, South Australia, and (or) East Antarctica. At ca. 1.45 Ga, both the Belt-Purcell and southwest successions underwent major sedimentological changes, with a pronounced shift to Laurentian provenance and the disappearance of the 1.61–1.50 Ga detrital zircon. Upper Belt-Purcell strata contain strongly unimodal ca. 1.73 Ga detrital zircon age populations that match the detrital zircon signature of Paleoproterozoic metasedimentary rocks of the Yavapai province to the south and southeast. We propose that the shift at ca. 1.45 Ga records the onset of orogenesis in southern Laurentia coeval with rifting along its northwestern margin. Bedrock uplift associated with orogenesis and widespread, coeval magmatism caused extensive exhumation and erosion of the Yavapai province ca. 1.45–1.36 Ga, providing a voluminous and areally extensive sediment source–with suitable zircon ages–during upper Belt deposition. This model provides a comprehensive and integrated view of

The evolution of Gondwana: U-Pb, Sm-Nd, Pb-Pb and geochemical data from Neoproterozoic to Early Palaeozoic successions of the Kango Inlier (Saldania Belt, South Africa)

NASA Astrophysics Data System (ADS)

Naidoo, Thanusha; Zimmermann, Udo; Chemale, Farid

2013-08-01

The provenance of Neoproterozoic to Early Palaeozoic rocks at the southern margin of the Kalahari craton reveals a depositional setting and evolution with a significant position in the formation of Gondwana. The sedimentary record shows a progression from immature, moderately altered rocks in the Ediacaran Cango Caves Group; to mature, strongly altered rocks in the Early Palaeozoic Kansa Group and overlying formations; culminating below very immature quartzarenites of Ordovician age. Petrographic and geochemical observations suggest the evolution of a small restricted basin with little recycling space towards a larger continental margin where substantial turbidite deposition is observed. For the southern Kalahari craton, a tectonic evolution comparable to supracrustal rocks in southern South America, Patagonia and Antarctica is supported by similarities in U-Pb ages of detrital zircons (Mesoproterozoic, Ediacaran and Ordovician grain populations); Sm-Nd isotopes (TDM: 1.2-1.8 Ga); and Pb-Pb isotopes. The maximum depositional age of the Huis Rivier Formation (upper Cango Caves Group) is determined at 644 Ma, but a younger age is still possible due to the limited zircon yield. The Cango Caves Group developed in a retro-arc foreland basin syntectonically to the Terra Australis Orogeny, which fringed Gondwana. The Kansa Group and overlying Schoemanspoort Formation are related to an active continental margin developed after the Terra Australis Orogen, with Patagonia being the ‘missing link’ between the Central South American arc and Antarctica during the Ordovician. This explains the occurrence of Ordovician detritus in these rocks, as a source rock of this age has not been discovered in South Africa. The absence of arc characteristics defines a position distal to the active continental margin, in a retro-arc foreland basin. The similarity of isotope proxies to major tectonic provinces in Antarctica and Patagonia, with those on the margins of the Kalahari craton

Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

NASA Astrophysics Data System (ADS)

Aulbach, S.; Creaser, Robert A.; Stachel, Thomas; Heaman, Larry M.; Chinn, Ingrid L.; Kong, Julie

2018-05-01

The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re-Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n = 14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs -3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012-0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic. The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution

Southern Living and Southern Voices: Models of Regional Magazine Success and Failure.

ERIC Educational Resources Information Center

English, John W.

This paper examines the phenomenon of magazine success and failure as demonstrated by two regional magazines, "Southern Living" and "Southern Voices." The former, a combination of articles about food, travel, sports, and other positive aspects of southern life, was quickly accepted by its readers and advertisers and began…

Deformation ages within the Klong Marui continental wrench fault, southern Thailand

NASA Astrophysics Data System (ADS)

Kanjanapayont, P.; Grasemann, B.; Edwards, M. A.

2009-04-01

The Klong Marui Fault is a ductile to brittle dextral strike-slip shear zone characterized by strong NNE-SSW geomorphic ridges trending up to 150 km. from Thai Gulf to Andaman Sea. At it southern part in the Phung Nga region, the ductile core forms a 40km long ridge. The geology within this wrench zone consisted of steep strongly deformed layers of migmatitic gneisses, mylonitic granites/pegmatites and phyllonitic metapelites. Brittle cataclasitc zones were localized in the eastern and western margin of this ductile core zone. The first deformation stage was dextral ductile strike-slip movement at mid to upper crustal levels and formed the main mylonitic foliation (c), secondary synthetic foliations (c'), and lineation in the migmatitic gneisses, mylonitic granites and metapelites. Locally sillimanite-clasts in high-temperature recrystallization quartz fabric fabric suggest deformation at amphibolite facies condition. More typically, quartz dynamically recrystallize by subgrain rotation and grain boundary migration under greenschist facies conditions. Microstructure of myrmekite and "V"-pull-apart clearly indicates dextral sense of shear. Pegmatites cross-cut the main mylonitic foliation but were sheared at the rims indicating syn-kinematic emplacement. Dynamically recrystallizing quartz mainly by basal gliding, bulging and low-temperature subgrain rotation record the latest stage of ductile dextral strike-slip deformation during decreasing temperature conditions. The NNE-SSW trending dextral strike-slip deformation accommodated the E-W transpression as a result of the differential movement of the northward drifting Indian craton and Asia. The brittle/ductile deformation produced cataclasites and minor faults which overprint the higher temperature fabric causing exhumation and juxtaposition of fault rocks developed under different metamorphic conditions in a positive flower structure.

Depth to the Moho in Southern New England and Eastern New York State from Seismic Receiver Functions

NASA Astrophysics Data System (ADS)

Cipar, J. J.; Ebel, J.

2016-12-01

The thickness of the Earth's crust is a fundamental parameter of geophysics and geology. The eastern New York/southern New England area encompasses the suture between the Paleozoic Appalachian orogen and the Proterozoic Laurentian craton. The recent installation of the IRIS Traveling Array (TA) in 2013-2014 coupled with stations operated by Boston College, Lamont-Doherty, and the US National Seismic Network provide an unprecedented source of data for seismic studies of crustal structure. We use the receiver functions complied by the EarthScope Automated Receiver Survey (EARS) to measure crustal thickness. Our procedure is to stack receiver functions (RFs) at each station using the correct moveout for the P-to-S conversion at the Moho (Ps phase). The time difference between the Ps and direct P arrivals (Ps-P time) is dependent on crustal thickness (H) and crustal S-wave velocity (Vs). To get an estimate of H, we assume that the mean P-wave velocity (Vp) in the crust is 6.5 km/s, and determine the range of Vs for a range of Poisson's ratio (0.23-0.27). We then solve for H using the P-Ps times measured from the RF stacks (at Δ=60°) and our estimates for Vp and Vs. The uncertainty in S-wave velocity translates to approximately ±2 km uncertainty in crustal thickness. Our crustal thickness map shows the well-known general progression from shallow crust near the Atlantic coast line ( 30 km) to deeper crust (45+ km) in the Laurentian craton. However, some detailed features become evident on our map. Most notably, thin crust ( 30 km) extends inland from the coast to the Connecticut River valley in eastern-central Massachusetts and southeastern New Hampshire. The Berkshire Hills of western Massachusetts have thick crust (43 km), reaching as deep as 46 km in extreme northwestern Massachusetts. Thus, there is a 13-15 km increase in crustal thickness over a distance of about 60 km. Currently, no stations are located in that zone. We find that the eastern Adirondacks have

The southern cape conductive belt (South Africa): Its composition, origin and tectonic significance

NASA Astrophysics Data System (ADS)

De Beer, J. H.; Van Zijl, J. S. V.; Gough, D. I.

1982-03-01

Magnetometer array studies have led to the discovery and mapping of the Southern Cape Conductive Belt (SCCB) crossing the southern tip of Africa from west to southeast coasts. The SCCB lies just south of the Namaqua-Natal Belt of cratonic rocks remobilized about 1000 m.y. B.P. It is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m.y. Relationships between the SCCB and the basement geochronology, geology and tectonics are considered in detail. These relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a Proterozoic subduction which stopped about 1000 to 800 m.y. B.P. Associated with this subduction we propose a Proterozoic range of Andean mountains, whose roots are now exposed in the Namaqua-Natal Belt. Later subduction further south, near the present south coast, is proposed to account for the intrusion, between the south coast and the SCCB, of the Cape Granites in the time interval 600-500 m.y. B.P. There is some evidence for a third, yet more distant, subduction episode off Permian Gondwanaland. After outlining this tectonic history, the paper turns to a closer examination of the hypothesis that the Southern Cape Conductive Belt consists of partly serpentinized basalt accumulated at the top of a Proterozoic subduction. A large static magnetic anomaly, which correlates with the SCCB over most of its length, is well fitted by a model which strongly supports this hypothesis. Bouguer gravity anomalies along western and central profiles likewise support the hypothesis. A discussion follows of the process of formation of the proposed block of serpentinized marine rocks, beginning with serpentinization of the crust near oceanic ridges by reaction of warm, porous, newly-extruded basalt with seawater convecting through it. The serpentinized basalt is stable at crustal temperatures and pressures and so

The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

NASA Astrophysics Data System (ADS)

Kaminsky, Felix V.; Sablukov, Sergei M.; Sablukova, Ludmila I.; Zakharchenko, Olga D.

2009-10-01

In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area - Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos - this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the 'Transbrasiliano Lineament'. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic.

Paleozoic tectonics of the Ouachita Orogen through Nd isotopes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gleason, J.D.; Patchett, P.J.; Dickinson, W.R.

1992-01-01

A combined isotopic and trace-element study of the Late Paleozoic Ouachita Orogenic belt has the following goals: (1) define changing provenance of Ouachita sedimentary systems throughout the Paleozoic; (2) constrain sources feeding into the Ouachita flysch trough during the Late Paleozoic; (3) isolate the geochemical signature of proposed colliding terranes to the south; (4) build a data base to compare with possible Ouachita System equivalents in Mexico. The ultimate aim is to constrain the tectonic setting of the southern margin of North America during the Paleozoic, with particular emphasis on collisional events leading to the final suturing of Pangea. Ndmore » isotopic data identify 3 distinct groups: (1) Ordovician passive margin sequence; (2) Carboniferous proto-flysch (Stanley Fm.), main flysch (Jackfork and Atoka Fms.) and molasse (foreland Atoka Fm.); (3) Mississippian ash-flow tuffs. The authors interpret the Ordovician signature to be essentially all craton-derived, whereas the Carboniferous signature reflects mixed sources from the craton plus orogenic sources to the east and possibly the south, including the evolving Appalachian Orogen. The proposed southern source is revealed by the tuffs to be too old and evolved to be a juvenile island arc terrane. They interpret the tuffs to have been erupted in a continental margin arc-type setting. Surprisingly, the foreland molasse sequence is indistinguishable from the main trough flysch sequence, suggesting the Ouachita trough and the craton were both inundated with sediment of a single homogenized isotopic signature during the Late Carboniferous. The possibility that Carboniferous-type sedimentary dispersal patterns began as early as the Silurian has important implications for the tectonics and paleogeography of the evolving Appalachian-Ouachita Orogenic System.« less

Infrared spectral and carbon isotopic characteristics of micro- and macro-diamonds from the Panda kimberlite (Central Slave Craton, Canada)

NASA Astrophysics Data System (ADS)

Melton, G. L.; Stachel, T.; Stern, R. A.; Carlson, J.; Harris, J. W.

2013-09-01

One hundred and twenty-one micro-diamonds (< 1 mm) and 90 macro-diamonds (2.5 mm to 3.4 mm) from the Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm- 2). Micro-diamond nitrogen concentrations range from < 10 at. ppm to 1696 at. ppm (median = 805 at. ppm) and the median aggregation state is 23%B. Macro-diamonds range from < 10 at. ppm to 1260 at. ppm (median = 187 at. ppm) nitrogen and have a median nitrogen aggregation of 26%B. Platelet and hydrogen peaks were observed in 37% and 79% of the micro-diamonds and 79% and 56% of the macro-diamonds, respectively. Nitrogen based time averaged residence temperatures indicate that micro- and macro-diamonds experienced similar thermal mantle residence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130 °C and 1160 °C (at 3.5 Ga residence). In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ13C compositions range from - 6.9‰ to + 1.8‰ (median = - 4.3‰). CL imaging reveals distinct growth layers that in some samples differ by > 2‰, but mostly vary by < 0.5‰. Comparison of only the “gem-quality” samples (n = 49 micro- and 90 macro-diamonds) between the two diamond sets, indicates a statistically significant shift of + 1.3‰ in average δ13C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fractionation process or growth histories. A broad transition to heavier isotopic values is also observed in connection to decreasing mantle residence temperatures. The bimodal mantle residence temperature distribution may coincide with the transition from highly depleted shallow to more fertile deep lithospheric mantle observed beneath the Central Slave Craton. The increase in δ13C with decreasing residence temperature (proxy for decreasing depth) is interpreted to reflect diamond

Hotspots in Hindsight

NASA Astrophysics Data System (ADS)

Julian, B. R.; Foulger, G. R.; Hatfield, O.; Jackson, S.; Simpson, E.; Einbeck, J.; Moore, A.

2014-12-01

Torsvik et al. [2006] suggest that the original locations of large igneous provinces ("LIPs") and kimberlites, and current locations of melting anomalies (hot-spots) lie preferentially above the margins of two Large Lower-Mantle Shear Velocity Provinces" (LLSVPs), at the base of the mantle, and that the correlation has a high significance level (> 99.9999%). They conclude the LLSVP margins are Plume-Generation Zones, and deep-mantle plumes cause hotspots and LIPs. This conclusion raises questions about what physical processes could be responsible, because, for example the LLSVPs are likely dense and not abnormally hot [Trampert et al., 2004]. The supposed LIP-hotspot-LLSVP correlations probably are examples of the "Hindsight Heresy" [Acton, 1959], of basing a statistical test upon the same data sample that led to the initial formulation of a hypothesis. In doing this, many competing hypotheses will have been considered and rejected, but this fact will not be taken into account in statistical assessments. Furthermore, probabilities will be computed for many subsets and combinations of the data, and the best-correlated cases will be cited, but this fact will not be taken into account either. Tests using independent hot-spot catalogs and mantle models suggest that the actual significance levels of the correlations are two or three orders of magnitude smaller than claimed. These tests also show that hot spots correlate well with presumably shallowly rooted features such as spreading plate boundaries. Consideration of the kimberlite dataset in the context of geological setting suggests that their apparent association with the LLSVP margins results from the fact that the Kaapvaal craton, the site of most of the kimberlites considered, lies in Southern Africa. These observations raise questions about the distinction between correlation and causation and underline the necessity to take geological factors into account. Fig: Left: Cumulative distributions of distances from

Quantifying denudation of the West African passive-transform margin: implications for Cenozoic erosion budget of cratons and source-to-sink systems

NASA Astrophysics Data System (ADS)

Grimaud, Jean-Louis; Chardon, Dominique; Rouby, Delphine; Beauvais, Anicet

2014-05-01

We develop an approach based on the differential elevation of dated successive topographies of the onshore part of the West African margin to calibrate in-situ volumetric denudation over a 3.9 million km2 cratonic surface for the past 45 Ma. We obtain a regionally averaged volumetric erosion rate of 5 x 10-3 km3/km2/m.y. corresponding to a total average denudation of 300 m and a denudation rate of 6 m/m.y., which remained nearly constant over the three time spans (45- 24, 24-11 and 11-0 Ma) despite spatial variations related to epeirogenic movements. Denudation is converted into a minimum yield of 12 +/- 2 t/km2/yr with a minimum solute component of 4 +/- 2 t/km2/yr accounting for the porosity of the eroded regoliths. Our results would imply a minimum contribution of 1.6 +/- 0.4 Gt/yr of the non-orogenic landmass to the global continental yield since the last peak greenhouse. Reconstruction of two incision stages of West Africa landscape from the reconstructed topographies combined with paleogeographic data shows that the current river catchments of the sub region have acquired their current configuration by the end of the Oligocene at the latest (24 Ma ago). The fairly steady geometry of the West African drainage since then offers the opportunity to effectively link the inland geomorphic record to offshore sedimentation. Volumetric denudation analysis applied to West African sub-drainage areas attests to the role of drainage reorganization and epeirogenic mouvements (flexural growth of the marginal upwarp and amplification of the Hoggar intraplate swell) on the spatial and temporal distribution of continental denudation and yield. Onshore denudation and clastic sediments accumulation in the post-24 Ma Niger catchment - delta system are within the same order of magnitude. These results suggest that cratonic-type erosion fluxes estimated from the West African margin may be used to estimate the size of drainage basins from the fossil sedimentary record.

The significance of 24-norcholestanes, triaromatic steroids and dinosteroids in oils and Cambrian-Ordovician source rocks from the cratonic region of the Tarim Basin, NW China

USGS Publications Warehouse

Li, Meijun; Wang, T.-G.; Lillis, Paul G.; Wang, Chunjiang; Shi, Shengbao

2012-01-01

Two oil families in Ordovician reservoirs from the cratonic region of the Tarim Basin are distinguished by the distribution of regular steranes, triaromatic steroids, norcholestanes and dinosteroids. Oils with relatively lower contents of C28 regular steranes, C26 20S, C26 20R + C27 20S and C27 20R regular triaromatic steroids, dinosteranes, 24-norcholestanes and triaromatic dinosteroids originated from Middle–Upper Ordovician source rocks. In contrast, oils with abnormally high abundances of the above compounds are derived from Cambrian and Lower Ordovician source rocks. Only a few oils have previously been reported to be of Cambrian and Lower Ordovician origin, especially in the east region of the Tarim Basin. This study further reports the discovery of oil accumulations of Cambrian and Lower Ordovician origin in the Tabei and Tazhong Uplifts, which indicates a potential for further discoveries involving Cambrian and Lower Ordovician sourced oils in the Tarim Basin. Dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24-norcholestanes for dinoflagellates and diatoms. Therefore, the abnormally high abundance of these compounds in extracts from the organic-rich sediments in the Cambrian and Lower Ordovician and related oils in the cratonic region of the Tarim Basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the Tarim Basin during the Cambrian Period. Steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms, as well as define petroleum systems.

Integrated elemental and Sr-Nd-Pb-Hf isotopic studies of Mesozoic mafic dykes from the eastern North China Craton: implications for the dramatic transformation of lithospheric mantle

NASA Astrophysics Data System (ADS)

Liu, Shen; Feng, Caixia; Santosh, M.; Feng, Guangying; Coulson, Ian M.; Xu, Mengjing; Guo, Zhuang; Guo, Xiaolei; Peng, Hao; Feng, Qiang

2018-02-01

Evolution of the lithospheric mantle beneath the North China Craton (NCC) from its Precambrian cratonic architecture until Paleozoic, and the transformation to an oceanic realm during Mesozoic, with implications on the destruction of cratonic root have attracted global attention. Here we present geochemical and isotopic data on a suite of newly identified Mesozoic mafic dyke swarms from the Longwangmiao, Weijiazhuang, Mengjiazhuang, Jiayou, Huangmi, and Xiahonghe areas (Qianhuai Block) along the eastern NCC with an attempt to gain further insights on the lithospheric evolution of the region. The Longwangmiao dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 4.3) and EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i > 0.706; ε Nd (t) < -6.3, (206Pb/204Pb) i > 16.6, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.8, ε Hf (t) < -22.4). The Weijiazhuang dykes are sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7), and display similar EM1-like isotopic features ((87Sr/86Sr) i > 0.706; ε Nd (t) < -7.0, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) < -23.3). The Mengjiazhuang dykes are also sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 2.4) and EM1-like isotopic features((87Sr/86Sr) i > 0.706; ε Nd (t) < -18.4, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) < -8.6). The Jiayou dykes also display sub-alkaline affinity with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7) and EM1-like Sr-Nd-Pb-Hf isotopic features ((87Sr/86Sr) i > 0.706; ε Nd(t) < -15.3, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i > 15.4, (208Pb/204Pb) i > 36.9, ε Hf (t) < -18.4). The Huangmi dykes are alkaline (with Na2O + K2O ranging to more than 5.9 wt.%)) with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 9.3) and EM1-like isotopic composition ((87Sr/86Sr) i > 0.705; ε Nd (t) < -15.1, (206Pb/204Pb) i > 16.9, (207Pb/204Pb) i > 15.5, (208Pb/204Pb) i > 36

Coupled Fe and multiple-S isotope systematics of pyrite and evidence of increasing atmospheric oxygen in 2.5 Ga sediments of the Kaapvaal Craton

NASA Astrophysics Data System (ADS)

Bauer, A.; Ono, S.; Romaniello, S. J.; Anbar, A. D.

2017-12-01

Using combined iron and sulfur isotopic data from black shale-hosted pyrite grains of 2.5 Ga samples from the GKP-01 drill core of the Griqualand West Basin, South Africa, we untangle the pathways of pyrite formation for distinct morphologies of pyrite and evaluate the role of these pyrites as recorders of atmospheric S-MIF signals. The analysis of subsamples at stratigraphic intervals allows us to document the characteristic time scale of change in S-MIF signatures resulting from atmospheric photochemical reactions with respect to residence time of the seawater sulfate reservoir. Disseminated pyrite grains are characterized by a range of Δ33S (-1 to +8‰) and 56Fe (-2.5 to 0‰) values. Pyrite laminae are predominantly characterized by relatively homogeneous and negative Δ33S (-2 to 0 ‰) and 56Fe (-2 to -1‰) isotope signatures. These correlated Fe-S systematics suggest distinct pathways of pyrite formation: 1) pyrite laminae formed below the sediment-water interface via diffusion of dissolved oceanic Fe2+ and sulfate; and 2) disseminated pyrite formed at the chemocline by reaction of reduced and elemental sulfur with a reservoir of Fe2+ affected by removal of Fe oxides. Recognition of distinct mechanisms of pyrite formation for these morphologies is a critical step in deconstructing the pathways for S-MIF production, transfer, and preservation in the Archean sedimentary record. Our results have implications for mass balance and atmospheric modeling studies that rely on the Δ33S record as well as for studies attempting to document larger-scale, lithofacies-specific trends in sulfur isotopic signals. Finally, our results are consistent with locally increasing sulfate concentrations along this Archean continental shelf and may correspond to an increase in low-level O2 production prior to the Great Oxygenation Event.

76 FR 35508 - Alabama Southern Railroad, L.L.C.-Temporary Trackage Rights Exemption-Norfolk Southern Railway...

Federal Register 2010, 2011, 2012, 2013, 2014

2011-06-17

... DEPARTMENT OF TRANSPORTATION Surface Transportation Board [Docket No. FD 35510] Alabama Southern Railroad, L.L.C.--Temporary Trackage Rights Exemption--Norfolk Southern Railway Company Norfolk Southern... grant nonexclusive overhead temporary trackage rights to Alabama Southern Railroad, L.L.C. (ABS) over a...

Source field effects in the auroral zone: Evidence from the Slave craton (NW Canada)

NASA Astrophysics Data System (ADS)

Lezaeta, Pamela; Chave, Alan; Jones, Alan G.; Evans, Rob

2007-09-01

We present an investigation of source field effects on the magnetic fields from multiple long period magnetotelluric (MT) data collected on the floors of lakes throughout the Slave craton (NW Canada) from 1998 to 2000. Monthly and daily power spectra of the magnetic fields suggest a dynamic and seasonally varying source, with atypical geomagnetic activity in year 2000. Bounded influence MT and GDS responses were obtained for periods ranging between 80 and 25,000 s over selected monthly time segments. The responses at periods over 4000 s vary, suggesting source field effects. A frequency domain principal component (PC) method was applied to the array to investigate the spatial form of the source field variations. The PC analysis was tested with synthetic data from a regional 3D model with a uniform external source to study the sensitivity of the eigenvectors to conductivity structure, demonstrating a negligible influence with increasing penetration depth. We conclude that magnetic fields at periods near one half day are subject to a 1D polarized source of relatively homogeneous morphology over the survey area during any month recorded, except for the summer month of July 2000 that had particularly high geomagnetic activity. In general, the source space approaches two polarizations at periods below one half day, with the dominant NS component seen quasi-homogeneous over the survey area at periods over 1000 s.

Southern Pine Beetle Information System (SPBIS)

Treesearch

Valli Peacher

2011-01-01

The southern pine beetle (SPB) is the most destructive forest insect in the South. The SPB attacks all species of southern pine, but loblolly and shortleaf are most susceptible. The Southern Pine Beetle Information System (SPBIS) is the computerized database used by the national forests in the Southern Region for tracking individual southern pine beetle infestations....

Southern Pine Beetle Competitors

Treesearch

Fred M. Stephen

2011-01-01

When southern pine beetles mass attack a living pine tree, if colonization is successful the tree dies and its phloem becomes immediately available to a complex of other bark beetles and long-horned beetles, all of which, in order to reproduce, compete for the new resource. In southern pines the phloem-inhabiting guild is composed of the southern pine beetle...

The India and South China cratons at the margin of Rodinia — Synchronous Neoproterozoic magmatism revealed by LA-ICP-MS zircon analyses

NASA Astrophysics Data System (ADS)

Hofmann, M.; Linnemann, U.; Rai, V.; Becker, S.; Gärtner, A.; Sagawe, A.

2011-04-01

The palaeogeographic position of South China in relation to India in the Neoproterozoic is controversial. Resolution of this controversy constrains the reconstruction of Rodinia during its breakup and contributes to our understanding of Snowball Earth. This work compares the Neoproterozoic histories of the Lesser Himalaya in northern India and the Yangtze block in southern China. We present U-Pb LA-ICP-MS ages of detrital zircon grains from six Indian and three Chinese siliciclastic sedimentary rocks, such as sandstones or diamictites/tillites. In total, 1148 grains were analysed from which 833 measurements gave ages with a degree of concordance between 90 and 110%. The correlation of the Indian and the Chinese sections is possible using the tillites of both areas purportedly deposited during the Snowball Earth time interval: the Blaini tillite from India and the Nantuo tillite from China. The U-Pb ages confirm the Marinoan age of the Chinese Liantuo tillite. Although the youngest zircon age for the Indian Blaini tillite is about 678 Ma, the Marinoan age is indicated by the presence of a typical Marinoan white to bright yellowish overlying cap carbonate. In addition to the tillites, representative detrital zircon ages from over- and underlying clastic rocks were determined. The Chinese samples are dominated by zircons with Neoproterozoic ages with a main peak between ca. 750 Ma and ca. 950 Ma and are characterised by the absence of Archaean ages. The Indian samples contain abundant Neoproterozoic zircon grains, but also contain Mesoproterozoic to Archaean zircons. For all samples, a local source area that provided the Neoproterozoic zircons is likely. A synchronous Neoproterozoic magmatic event in both cratons probably reflects the breakup of the supercontinent Rodinia and therefore the same tectono-magmatic event. Our results indicate a similar history for India and South China which both underwent at least one synchronous episode of crustal growth during the

High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province

USGS Publications Warehouse

Schmitz, M.D.; Bowring, S.A.; Southwick, D.L.; Boerboom, Terrence; Wirth, K.R.

2006-01-01

High-precision U-Pb ages have been obtained for high-grade gneisses, late-kinematic to postkinematic granitic plutons, and a crosscutting mafic dike of the Archean Minnesota River Valley tectonic subprovince, at the southern ramparts of the Superior craton of North America. The antiquity of the Minnesota River Valley terranes is confirmed by a high-precision U-Pb zircon age of 3422 ?? 2 Ma for a tonalitic phase of the Morton Gneiss. Voluminous, late-kinematic monzogranites of the Benson (Ortonville granite) and Morton (Sacred Heart granite) blocks yield identical crystallization ages of 2603 ?? 1 Ma, illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the Minnesota River Valley terranes. Postkinematic, 2591 ?? 2 Ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the Minnesota River Valley subprovince. Monazite growth from 2609 to 2595 Ma in granulitic paragneisses of the Benson and Montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism. Neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the Mesoarchean continental Minnesota River Valley terranes to the southern margin of the Superior Province, along the western Great Lakes tectonic zone. Subsequent Paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion, expressed in the Minnesota River Valley by major WNW-trending tholeiitic diabase dikes dated at 2067 ?? 1 Ma, only slightly younger than the structurally and geochemically similar 2077 ?? 4 Ma Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada. ?? 2006 Geological Society of America.

The nature and location of the suture zone in the Rokelide orogen, Sierra Leone: Geochemical evidence

NASA Astrophysics Data System (ADS)

Lytwyn, Jennifer; Burke, Kevin; Culver, Stephen

2006-12-01

The boundaries of the West African Craton mark the location of a continuous suture zone that records Neoproterozoic to Early Cambrian oceanic closure. The western part of the circum-West African suture zone extends through the line of outcrop of the Mauritanide, Bassaride and Rokelide mountain belts. Our geochemical analyses are consistent with the idea that igneous and metamorphic rocks of the Rokelide and Southern Mauritanide mountain belts of West Africa occupy a suture zone that records the closing of a Neoproterozoic to Early Cambrian ocean basin during the Pan-African orogeny and final assembly of Gondwana. The closing of that basin was marked by the collision between Archean rocks of the Leo massif of the West African Craton and reactivated Archean and Paleoproterozoic rocks that now outcrop nearer to the coast of Africa in Sierra Leone and Liberia. Within the Rokelides, the geochemistry of the Kasewe Hills volcanic rocks and Marampa amphibolite indicate that remnants of an arc system are caught up in the suture zone. The geochemistry of Guingan schists that outcrop along strike of the Rokelides is compatible with the idea that the metamorphosed equivalents of the Marampa and Kasewe Hills arc volcanic rocks extend through the Bassarides and into the Southern Mauritanides.

Radiation damage-He diffusivity models applied to deep-time thermochronology: Zircon and titanite (U-Th)/He datasets from cratonic settings

NASA Astrophysics Data System (ADS)

Guenthner, W.; DeLucia, M. S.; Marshak, S.; Reiners, P. W.; Drake, H.; Thomson, S.; Ault, A. K.; Tillberg, M.

2017-12-01

Advances in understanding the effects of radiation damage on He diffusion in uranium-bearing accessory minerals have shown the utility of damage-diffusivity models for interpreting datasets from geologic settings with long-term, low-temperature thermal histories. Craton interiors preserve a billion-year record of long-term, long-wavelength vertical motions of the lithosphere. Prior thermochronologic work in these settings has focused on radiation damage models used in conjunction with apatite (U-Th)/He dates to constrain Phanerozoic thermal histories. Owing to the more complex damage-diffusivity relationship in zircon, the zircon (U-Th)/He system yields both higher and, in some cases, lower temperature sensitivities than the apatite system, and this greater range in turn allows researchers to access deeper time (i.e., Proterozoic) segments of craton time-temperature histories. Here, we show two examples of this approach by focusing on zircon (U-Th)/He datasets from 1.8 Ga granitoids of the Fennoscandian Shield in southeastern Sweden, and 1.4 Ga granites and rhyolites of the Ozark Plateau in southeastern Missouri. In the Ozark dataset, the zircon (U-Th)/He data, combined with a damage-diffusivity model, predict negative correlations between date and effective uranium (eU) concentration (a measurement proportional to radiation damage) from thermal histories that include an episode of Proterozoic cooling (interpreted as exhumation) following reheating (interpreted as burial) to temperature of 260°C at 850-680 Ma. In the Fennoscandian Shield, a similar damage model-based approach yields time-temperature constraints with burial to 217°C between 944 Ma and 851 Ma, followed by exhumation from 850 to 500 Ma, and burial to 154°C between 366 Ma and 224 Ma. Our Fennoscandian Shield samples also include titanite (U-Th)/He dates that span a wide range (945-160 Ma) and are negatively correlated with eU concentration, analogous to our zircon He dataset. These results support

Timescales of Crustal Cooling of the Superior Craton near Attawapiskat, Ontario, Canada, and Implications for Extent of Keweenawan Plume Heating.

NASA Astrophysics Data System (ADS)

Edwards, G. H.; Blackburn, T.; Smit, K.

2017-12-01

The thermal history of the Superior Craton was punctuated by a period of mantle plume heating at 1.1 Ga associated with the Keweenawan Rift, though the plume's spatial extent, temperature, and duration of heating remain unresolved. Kimberlites of Mesoproterozoic and Jurassic age in the Attawapiskat area, Northern Ontario contain lithospheric mantle and lower crustal xenoliths that record the thermal history 600km to the north of exposed 1.1 Ga Keweenawan volcanics and the topographically and gravimetrically defined plume center. Previous work on Attawapiskat kimberlites identified two populations of diamonds with differing thermal histories, suggesting two distinct phases of diamond growth. Corresponding geothermobarometric data indicate geotherm relaxation and broadening of the diamond stability field between the Mesoproterozoic and Jurassic. These data, however, do not uniquely resolve whether the region experienced significant heating coincident with Keweenawan rifting ( 1.1 Ga) or prolonged, unperturbed cooling since amalgamation of the Superior Craton ( 2.6 Ga). To discern between these two possible histories, we use accessory phase U-Pb thermochronology to construct a continuous thermal record of the lower crust. Here we present a dataset of U-Pb ID-TIMS measurements of rutile and apatite from xenoliths (n=8) sourced from the Jurassic age Victor Kimberlite. The U/Pb and Pb isotopic compositions of rutile and apatite from shallow-residing amphibolite xenoliths exhibit Proterozoic dates with a high degree of U-Pb discordance, reflecting slow cooling of the middle crust prior to 1.1 Ga. Granulite and eclogite xenoliths record younger dates consistent with their deeper sample residence, but with a high degree of U-Pb concordance that is inconsistent with continuous cooling through the Proterozoic. Reproducing the measured trend with numerical models requires a reheating event at 1.1 Ga. Imposing a 60-70mW/m2 geotherm at 1.1 Ga is high enough to replicate the

Gold deposits of the northern margin of the North China craton: Multiple late Paleozoic-Mesozoic mineralizing events

USGS Publications Warehouse

Hart, C.J.R.; Goldfarb, R.J.; Qiu, Yumin; Snee, L.; Miller, L.D.; Miller, M.L.

2002-01-01

The northern margin of the North China craton is well-endowed with lode gold deposits hosting a resource of approximately 900 tonnes (t) of gold. The ???1,500-km-long region is characterized by east-trending blocks of metamorphosed Archean and Proterozoic strata that were episodically uplifted during Variscan, Indosinian, and Yanshanian deformational and magmatic events. At least 12 gold deposits from the Daqinshan, Yan-Liao (includes the Zhangjiakou, Yanshan, and Chifeng gold districts), and Changbaishan gold provinces contain resources of 20-100 t Au each. Most deposits are hosted in uplifted blocks of Precambrian metamorphic rocks, although felsic Paleozoic and Mesozoic plutons are typically proximal and host ???30% of the deposits. The lodes are characterized by sulfide-poor quartz veins in brittle structures with low base metal values and high Au:Ag ratios. Although phyllic alteration is most common, intensive alkali feldspar metasomatism characterizes the Wulashan, Dongping, and Zhongshangou deposits, but is apparently coeval with Variscan alkalic magmatism only at Wulashan. Stepwise 40Ar-39Ar geochronology on 16 samples from gangue and alteration phases, combined with unpublished SHRIMP U-Pb dates on associated granitoids, suggest that gold mineralizing events occured during Variscan, Indosinian, and Yanshanian orogenies at circa 350, 250, 200, 180, 150, and 129 Ma. However, widespread Permo-Triassic (???250 Ma) and Early Jurassic (???180 Ma) thermal events caused variable resetting of most of the white mica and K-feldspar argon spectra, as well as previously reported K-Ar determinations. Compiled and new stable isotope and fluid inclusion data show that most ??18O values for ore-stage veins range from 8 to 14???, indicating a fluid in equilibrium with the Precambrian metamorphic basement rocks; ??D values from fluid inclysions range widely from -64 to -154???, which is indicative of a local meteoric component in some veins; and highly variable ??34S data

Crustal and uppermost mantle S-wave velocity below the East European Craton in northern Poland from the inversion of ambient-noise records

NASA Astrophysics Data System (ADS)

Lepore, Simone; Polkowski, Marcin; Grad, Marek

2018-02-01

The P-wave velocities (V p) within the East European Craton in Poland are well known through several seismic experiments which permitted to build a high-resolution 3D model down to 60 km depth. However, these seismic data do not provide sufficient information about the S-wave velocities (V s). For this reason, this paper presents the values of lithospheric V s and P-wave-to-S-wave velocity ratios (V p/V s) calculated from the ambient noise recorded during 2014 at "13 BB star" seismic array (13 stations, 78 midpoints) located in northern Poland. The 3D V p model in the area of the array consists of six sedimentary layers having total thickness within 3-7 km and V p in the range 1.85.3 km/s, a three-layer crystalline crust of total thickness 40 km and V p within 6.15-7.15 km/s, and the uppermost mantle, where V p is about 8.25 km/s. The V s and V p/V s values are calculated by the inversion of the surface-wave dispersion curves extracted from the noise cross correlation between all the station pairs. Due to the strong velocity differences among the layers, several modes are recognized in the 0.021 Hz frequency band: therefore, multimodal Monte Carlo inversions are applied. The calculated V s and V p/V s values in the sedimentary cover range within 0.992.66 km/s and 1.751.97 as expected. In the upper crust, the V s value (3.48 ± 0.10 km/s) is very low compared to the starting value of 3.75 ± 0.10 km/s. Consequently, the V p/V s value is very large (1.81 ± 0.03). To explain that the calculated values are compared with the ones for other old cratonic areas.

Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar

USGS Publications Warehouse

Thomas, Ronald J.; De Waele, B.; Schofield, D.I.; Goodenough, K.M.; Horstwood, M.; Tucker, R.; Bauer, W.; Annells, R.; Howard, K. J.; Walsh, G.; Rabarimanana, M.; Rafahatelo, J.-M.; Ralison, A.V.; Randriamananjara, T.

2009-01-01

The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1:100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of 14 U-Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern "terranes", each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of "cratonic" Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U-Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high-grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720-740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U-Pb SHRIMP ages between 705 and 718 Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

NASA Astrophysics Data System (ADS)

Toteu, Sadrack Félix; Penaye, Joseph; Deloule, Etienne; Van Schmus, William Randall; Tchameni, Rigobert

2006-04-01

Ion microprobe U-Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700-665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100-950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa-Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west-central Africa.

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

PubMed Central

Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

2013-01-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

Age and composition of Archean crystalline rocks from the southern Madison Range, Montana. Implications for crustal evolution in the Wyoming craton

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mueller, P.A.; Shuster, R.D.; Wooden, J.L.

1993-04-01

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the metasupracrustal sequence ([approximately]2.6 Ga)-along with an intercalated meta-andesite ([approximately]2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group ([approximately]3.3 Ga) and a younger, mostlymore » granitic group ([approximately]2.7 Ga). Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crust in their petrogenesis. These comparisons suggest that the juxtaposition of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes. 41 refs., 6 figs., 2 tabs.« less

Magmatism and Tectonics in the Meso-Archean Pongola Supergroup, South Africa

NASA Astrophysics Data System (ADS)

Wilson, Allan

2013-04-01

The Pongola Supergroup is one of the most extensive and well preserved volcano-sedimentary successions emplaced in a continental setting in the Meso-Archean (c. 2.95 Ga). It contrasts with both the older (Barberton type c.3.5 Ga) and younger (Belingwe type c.2.7 Ga) greenstone belts in southern Africa in that the sequence has not undergone the strong horizontal compressional tectonics typically related to greenstone belt-TTG environments. However, it is appropriate to compare this sequence with rocks of the Barberton greenstone belt by which the final phase of deposition preceded that of the juxtaposed Pongola basin with a relatively small time interval. The Pongola succession, which commenced with the first major magmatic event after the Barberton greenstone belt, overlies granitoids and remnants of greenstone belts in SE South Africa and in SW Swaziland. Formation was not in a continental rift environment but most likely in a marginal epicontinental basin with syn-depositional subsidence in a half-graben fault system in the type area. The Pongola rocks occur in two domains related to a NW-trending central basement high in the Kaapvaal Craton and achieving a maximum thickness of 8 km in the northern areas. The lower section (Nsuze group 3.7 km thick) is made up mainly of lavas and pyroclastic rocks and the upper section (Mozaan Group 4.3 km thick) is aranaceous sediments and argillites with a thick volcanic unit observed in the south-eastern facies. Chemical affinities of the lavas include tholeiite and calc-alkaline over the compositional range of basalt to rhyolite. There is a preponderance of andesites in the compositional array. The preservation of these rocks gives insight into the range of volcanic processes that took place at this stage of Earth history and in some areas it is possible to identify eruptions from a single source over several kilometres, as well as feeder-dyke systems to the lava flows. Simultaneous eruption of contrasting magmas from several

Lower Devonian paleomagnetic dating of a large mafic sill along the western border of the Murzuq cratonic basin (Saharan metacraton, SE Algeria).

NASA Astrophysics Data System (ADS)

El-M. Derder, Mohamed; Maouche, Said; Liégeois, Jean-Paul; Henry, Bernard; Amenna, Mohamed; Ouabadi, Aziouz; Bellon, Hervé; Bruguier, Olivier; Bayou, Boualem; Bestandji, Rafik; Nouar, Omar; Bouabdallah, Hamza; Ayache, Mohamed; Beddiaf, Mohamed

2017-04-01

The Murzuq basin located in central North Africa, in Algeria, Libya and Niger is a key area, delineating a relictual cratonic area within the Saharan metacraton (Liégeois et al., 2013). On its western border, we discovered a very large sill ("Arrikine" sill), with a thickness up to 250m and a minimum length of 35 km. It is made of mafic rocks and is interbedded within the Silurian sediments of the Tassilis series. In the vicinity, the only known post-Pan-African magmatism is the Cenozoic volcanism in the In Ezzane area. Further south in Niger, also along the SW border of the Murzuq basin, large Paleozoic dolerite (Carte géologique du Sahara central, 1962) are probably related to the "Arrikine" sill magmatism, as they are in the same stratigraphical position. Several hundred kilometers westward and southwestward of Arrikine, Paleozoic magmatic products are known: Carboniferous basic intrusives (346 Ma; Djellit et al., 2006) are located in the Tin Serririne basin and Devonian ring complexes (407 Ma; Moreau et al, 1994) in the Aïr Mountains. For the Arrikine sill, K/Ar data gave a rejuvenation age (326 Ma) related to a K-rich aplitic phase and the LA-ICP-MS U-Pb method on zircon showed that only inherited zircons are present (0.6 to 0.7, 2.0 and 2.7 Ga ages), pointing to ages from the underlying basement corresponding to the Murzuq craton covered by Pan-African sediments (Derder et al., 2016). By contrast, a well-defined paleomagnetic pole yielded an age of 410-400 Ma by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age, similar to that reported for the Aïr complexes (Moreau et al., 1994), can be correlated with the deep phreatic eruption before Pragian time thought to be at the origin of sand injections, which gave circular structures observed on different borders of the Murzuq basin (Moreau et al,. 2012). This Lower Devonian magmatism had therefore a regional extension and can be related to a "Caledonian" transtensive reactivation of the

The Formation of Laurentia: Evidence from Shear Wave Splitting and Seismic Tomography

NASA Astrophysics Data System (ADS)

Liddell, M. V.; Bastow, I. D.; Gilligan, A.; Darbyshire, F. A.; Pugh, S.

2016-12-01

Earth conditions during the Precambrian, and their effect on plate tectonic processes during that era, are not fully understood; how the fast wave-speed cratonic roots, or keels, often found beneath these regions were formed is also debated. The geological record of northern Hudson Bay includes the 1.8Ga TransHudson Orogen (THO) terrane, a remnant of the Paleoproterozoic collision of the Archean Rae and Churchill domains that overlies one of Earth's largest cratonic keels. This region is thus an ideal natural laboratory for the study of Precambrian processes. We use broadband seismological data recorded at 65 stations in northern Hudson Bay to address questions regarding the manner and scale of plate deformation during Precambrian assembly of the region. To explore these questions, we conduct a study of mantle seismic anisotropy using SKS splitting. Our study constitutes the most complete shear wave splitting analysis of northern Canada to date utilising up to 11 years of data for some stations. Anisotropic fast directions (φ) and delay times (δt) are determined using a modified Silver and Chan (1991) method. In the Hudson Strait, φ directions parallel the THO, while the islands in northern Hudson Bay show changes in φ over length scales short enough to indicate lithospheric origin. Complex anisotropy patterns are observed in the central Rae craton and southern Baffin Island, suggesting multiple sources. Several possible sources of anisotropy are explored, including basal drag of the North American plate, mantle flow deflected by the Laurentian keel, and discontinuities associated with possible two-stage keel development P and S-wave relative arrival-time tomographic velocity models are also presented. Waveforms are aligned using the adaptive stacking routine of Rawlinson et al. (2004), and models are produced using the Fast Marching Tomography inversion code of Rawlinson et al. (2006). The THO is modeled as a low velocity feature relative to the neighbouring

Age and composition of the UHP garnet peridotites in the Dabie orogenic belt (central China) record complex crust-mantle interaction in continental margin

NASA Astrophysics Data System (ADS)

Zhao, Y.; Zheng, J.; Wang, B.

2017-12-01

The Dabie-Sulu UHP belt was created by the collision between the North and South China cratons in Middle Triassic time (240-225 Ma). There are lots of garnet-bearing ultramafic body occurs as a lens in the belt. Age and composition of the Maowu garnet peridotites in the Dabie orogenic belt are reported. The garnet harzburgites are main moderately refractory (Mg#Ol=92) and minor fertile (Mg#Ol=88) with high Ni (2344-2603 ppm) and low Al2O3 (0.35-0.54 wt.%), CaO (0.76-2.19 wt.%) and TiO2 (˜0.01 wt.%). Zircons in the harzburgites mainly document metamorphism at 230 ± 2 Ma, 275 ± 5 Ma, 357 ± 4 Ma, and complex minor populations of ages including: 1.8 Ga, 1.3 Ga, and Neoproterozic-early Paleozoic ages (901-420 Ma). The early Meszosic and late Paleozoic zircons have similar trace-element patterns and ranges in ɛHf(t) (+0.6 to +3.4), Th/U ratio (0.2-0.7) and Hf depleted-mantle model ages (TDM ) mainly cluster in the interval 1.2-0.9 Ga. In contrast, the Paleo-Mesoproterozoic zircons have negative ɛHf(t) (-24.9 to -2.7) and oldest Hf TDM = 3.4Ga. Zircons of Neoproterozic-early Paleozoic have a wide range of Hf depleted-mantle model ages (2.4-0.7Ga) and ɛHf(t) (-15.3 to +9.5). Above of the all, we suggest that the Maowu garnet harzburgites are interpreted as a fragment of the metasomatized ancient lithospheric mantle beneath the southern margin of the North China Craton. They experienced the Proterozoic thermal event (1.9-1.8Ga), which is coeval with the assembly of the supercontinent Columbia. And then 1.3Ga mantle metasomatism with asthenospheric materials resulted in the final breakup of the Columbia supercontinent. Neoproterozic-early Paleozoic (901-420 Ma), deep parts of the south margin of the craton were metasomatized during the assembly and breakup of the Rodinia supercontinent. Then, the southern margin of the craton occurred oceanic crust subduction ( 357 Ma), subsequent continental deep subduction and final continent-continent collision in Triassic.

Russian aeromagnetic surveys of the Prince Charles Mountains and adjacent regions into the 21st century

NASA Astrophysics Data System (ADS)

Golynsky, Alexander; Golynsky, Dmitry; Kiselev, Alexander; Masolov, Valery

2014-05-01

Russian aeromagnetic investigations in the Prince Charles Mountains (PCM) and surrounding areas, seek to contribute data on the tectonics of Precambrian igneous belts and cratonic fragments, the crustal structure of the Lambert Rift system and other major aspects of Antarctic geology, critical to understanding continental growth processes (Golynsky et al., 2006). Over the past decade, the Polar Marine Geoscience Expedition projects acquired approximately 77,400 line-km of aeromagnetic data over the largely ice-covered regions of MacRobertson Land and Princess Elizabeth Land. The airborne surveys were performed with a standard profile spacing of 5 km and tie-line interval of 15-25 km. The total amount of the Russian aeromagnetic data collected in this region exceeded more than 165,000 line-km. Together with the PCMEGA and AGAP surveys (Damaske and McLean, 2005; Ferraccioli et al., 2011) the PMGE dataset forms the longest transect ever mapped in East Antarctica exceeding 1950 km in length. Several distinct crustal subdivisions are clearly differentiated in the magnetic data. The high-amplitude positive anomalies that extend around the Vestfold Hills and Rauer Islands are likely be attributed to the southern boundary of high-grade metamorphic Late Archean craton. The northern PCM that are composed by ~1 Ga orthogneiss and charnockite display a predominantly northeasterly trending magnetic fabric that continues to the eastern shoulder of the Lambert Rift. The aeromagnetic data from the Southern PCM reveal the spatial boundary of the Archaean Ruker Terrane that is characterized by a short-wavelength anomalies and the prominent Ruker Anomaly that is associated with a banded iron formation. The prominent alternating system of linear NE-SW positive and negative anomalies over the eastern shoulder of the Lambert Rift may reflect the western boundary of the Princess Elizabeth Land cratonic(?) block, although its relationships and tectonic origin remained largely ambiguous

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pasyanos, M

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a wide period range. Including short period group velocities allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems. Crustal shear wave velocities are generally faster in oceanic regions and cratons,more » and slower in more recent crust and in active and formerly active orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell has had a significantly different impact in the north and the south, indicating specifics of the feature (temperature, time of influence, etc.) to be dissimilar between the two regions. Factoring in other information, it is likely that the southern portion has been active in the past, but that shallow activity is currently limited to the northern portion of the superswell.« less

Crustal Structure Beneath India and Tibet: New Constraints From Inversion of Receiver Functions

NASA Astrophysics Data System (ADS)

Singh, Arun; Ravi Kumar, M.; Mohanty, Debasis D.; Singh, Chandrani; Biswas, Rahul; Srinagesh, D.

2017-10-01

The Indian subcontinent comprises geological terranes of varied age and structural character. In this study, we provide new constraints to existing crustal models by inverting the P-to-s receiver functions (RFs) at 317 broadband seismic stations. Inversion results fill crucial gaps in existing velocity models (CRUST1.0 and SEAPS) by capturing regions which are less represented. The final model produced is much more heterogeneous and is able to capture the structural variations between closely spaced seismic stations. In comparison to the global models, major differences are seen for seismic stations located over various rift zones (e.g., Godavari, Narmada, and Cambay) and those close to the coastal regions where transition from oceanic to continental crust is expected to create drastic changes in the crustal configuration. Seismic images are produced along various profiles using 49,682 individual RFs recorded at 442 seismic stations. Lateral variations captured using migrated images across the Himalayan collisional front revealed the hitherto elusive southern extent of the Moho and intracrustal features south of the Main Central Thrust (MCT). Poisson's ratio and crustal thickness estimates obtained using H-k stacking technique and inversion of RFs are grossly similar lending credence to the robustness of inversions. An updated crustal thickness map produced using 1,525 individual data points from controlled source seismics and RFs reveals a (a) thickened crust (>55 km) at the boundary of Dharwar Craton and Southern Granulite Terrain, (b) clear difference in crustal thickness estimates between Eastern Dharwar Craton and Western Dharwar Craton, (c) thinner crust beneath Cambay Basin between southwest Deccan Volcanic Province and Delhi-Aravalli Fold Belt, (d) thinner crust (<35 km) beneath Bengal Basin, (e) thicker crust (>40 km) beneath paleorift zones like Narmada Son Lineament and Godavari Graben, and (f) very thick crust beneath central Tibet (>65 km) with maximum

Azimuthal Anisotropy beneath the Contiguous United States Revealed by Shear Wave Splitting

NASA Astrophysics Data System (ADS)

Liu, K. H.; Yang, B.; Liu, Y.; Dahm, H. H.; Refayee, H. A.; Gao, S. S.

2017-12-01

We have produced a uniformly-measured XKS (including SKS, SKKS, and PKS) splitting database for the contiguous United States and adjacent areas. The database consists of about 30,000 pairs of splitting parameters from 3185 stations. Both the fast orientations and splitting times show systematic spatial variations. The vast majority of the fast orientations are in agreement with the absolute plate motion (APM) direction computed under a fixed hot-spot reference frame. Spatial coherency analysis of the splitting parameters indicates that for the majority of the study area, where a single layer of anisotropy with a horizontal axis of symmetry is inferred, the source of anisotropy is located in the rheologically transitional zone between the lithosphere and asthenosphere. Beneath the western U.S., the previously recognized semi-circular feature of the fast orientations has a much greater spatial coverage, extending to northern Mexico and the Rio Grande Rift. The fast orientations are parallel to the western, southern, and southeastern edges of the North American Craton and can be interpreted by simple shear strain associated with mantle flow around the cratonic keel. The combination of anisotropy induced by this around keel flow and the APM can effectively explain the E-W fast orientations beneath the southern margin of the North American Craton and NE U.S., as well as the nearly N-S fast orientations and small splitting times observed in the SE U.S. The splitting times show a systematic decrease from both the western and eastern U.S. toward the central U.S., where the thickness of the lithosphere is the largest in the study area. This trend can be explained by the reduced efficiency of anisotropy development at greater depth, as well as by the lack of around keel flow in the continental interior.

Transition Metal Systematics of Opx-Enriched Harzburgites From the Cascades Arc With Implications for the Origin of Cratonic Peridotites

NASA Astrophysics Data System (ADS)

Turner, S. J.

2007-12-01

A number of peridotite xenoliths collected from the Simcoe volcanic field region of the Cascades arc exhibit notable enrichment of modal orthopyroxene. The process driving this enrichment is most likely metasomatism of the mantle wedge by Si-rich fluids derived ultimately from the underlying slab. By investigating the resultant elemental systematics associated with subduction zone metasomatism of this type, we hope to shed light on the origin of other opx-rich peridotites, such as those seen in many cratonic xenolith suites. The xenoliths found in the Simcoe volcanic field provide a rare opportunity to examine the composition of sub arc mantle, as it is unusual to find mantle xenoliths in volcanic arc lavas. The samples were analyzed using laser ablation ICPMS and their bulk compositions were reconstructed from point-counted mineral modes. Two-pyroxene mineral thermometry of the samples yield temperatures of approximately 1000 degrees C, corresponding to a depth of origin at uppermost mantle pressures if typical arc geotherms are assumed. Most of the peridotites are harzburgites or olivine-orthopyroxenites (Mg#s 0.88-0.9; opx mode 0.15-0.9), with small amounts of clinopyroxene (<0.02). Clinopyroxenes are significantly enriched in the light rare earths, consistent with a metasomatic origin for these opx-rich harzburgites. Of note is the counterintuitive systematics of Zn. Whole-rock Zn decreases with opx, but Zn in olivine also decreases with opx mode while Zn in opx increases with opx mode, hence the decrease in whole- rock Zn is not simply due to mechanical segregation of harzburgite into opx- and ol-rich zones. In summary, the REE signatures suggest the subducting slab as the most likely candidate for the source of the fluids that caused the opx enrichment. The opx-enrichment itself and the unusual trends in Zn suggest a reaction between a silicic fluid and normal harzburgite. Moreover, the concomitant decrease in olivine and whole-rock Zn with opx mode suggests

77 FR 17122 - Indiana Southern Railroad, LLC-Temporary Trackage Rights Exemption-Norfolk Southern Railway Company

Federal Register 2010, 2011, 2012, 2013, 2014

2012-03-23

... Railroad, LLC--Temporary Trackage Rights Exemption--Norfolk Southern Railway Company Norfolk Southern Railway Company (NSR), pursuant to a written trackage rights agreement (Agreement), has agreed to grant overhead temporary trackage rights to Indiana Southern Railroad, LLC (ISRR) over NSR's line of railroad...

Tracking the Progress of EarthScope/USArray: The crust and upper mantle beneath the transition region between tectonic western US and cratonic eastern US

NASA Astrophysics Data System (ADS)

Shen, W.; Lin, F.; Ritzwoller, M. H.

2010-12-01

The transition region between the tectonic western US and the cratonic eastern US contains numerous significant geological regions (e.g., the Rocky Mountains, the Colorado Plateau, and the Rio Grande Rift), and also, unknowns (e.g, the location or extent of the east-west US dichotomy, the compensation of the high topography of the western Great Plains, the extensional mechanics of the Rio Grande Rift, and the structure of the mantle beneath the Colorado Plateau). The answers to these questions and others are critical to an understanding of the tectonics and tectonic history of this region and its impact on the cratonic eastern US. The recent deployments of seismic stations, particularly the EarthScope USArray Transportable Array (TA), provide an opportunity to construct a detailed 3-D structural model of the crust and upper mantle beneath this transition region, and thus allow us to address some of the questions listed above. We present results from ambient noise tomography (ANT) and teleseismic earthquake tomography by using data from TA stations within the western and central US. We processed continuous seismic noise data from ~600 TA stations from August 2008 to March 2010, which after data selection produces a data set with ~100,000 inter-station paths. Rayleigh wave phase speed maps between 6 and 40 sec period and Love wave phase speed maps between 8 and 30 sec with a resolution of ~60 km are constructed using eikonal tomography. In addition, we applied eikonal tomography (ET) to about 300 teleseismic earthquakes to obtain long-period (30 - 100 sec) Rayleigh wave phase speed maps and Love wave phase speeds maps (30 - 60 sec). By jointly inverting Rayleigh and Love phase speeds maps from ANT and earthquake tomography, we constructed a 3-D isotropic and radially anisotropic shear velocity model of the crust and upper mantle to ~150 km depth together with model uncertainties constrained by a Monte-Carlo inversion. The 3-D isotropic model reveals a variety of

Fertilizing Southern Hardwoods

Treesearch

W. M. Broadfoot; A. F. Ike

1967-01-01

If present trends continue, fertilizing may soon be economically feasible in southern hardwood stands. Demands for the wood are rising, and the acreage alloted for growing it is steadily shrinking. To supply anticipated requests for information, the U. S. Forest Service has established tree nutrition studies at the Southern Hardwoods Laboratory in Stoneville,...

Adakite-gabbro-anorthosite magmatism at the final (576-546 Ma) development stage of the Neoproterozoic active margin in the south-west of the Siberian craton

NASA Astrophysics Data System (ADS)

Vernikovskaya, A. E.; Vernikovsky, V. A.; Matushkin, N. Yu.; Kadilnikov, P. I.; Romanova, I. V.; Larionov, A. N.

2017-12-01

In the late Neoproterozoic a prolonged active continental margin mode dominated the southwestern margin of the Siberian craton. Based on results of geological, petrological-geochemical, U-Th-Pb and Sm-Nd, Rb-Sr isotope investigations, for the first time we established that on the final evolution stage of this margin 576-546 Ma, intrusions of adakites and gabbro-anorthosites of the Zimoveyniy massif were emplaced in the South Yenisei Ridge. These new data indicate genetic relationships of the studied adakites and host NEB-metabasites. The formation of adakites could have been due to a crustal or a mantle-crustal source in a setting of transform sliding of lithospheric plates after the subduction stopped.

Magmatism and Epithermal Gold-Silver Deposits of the Southern Ancestral Cascade Arc, Western Nevada and Eastern California

USGS Publications Warehouse

John, David A.; du Bray, Edward A.; Henry, Christopher D.; Vikre, Peter

2015-01-01

� hornblende, biotite, and pyroxene phenocrysts. Seven epithermal gold-silver deposits with >1 Moz gold production, several large elemental sulfur deposits, and many large areas (10s to >100 km2) of hydrothermally altered rocks are present in the southern ancestral arc, especially south of latitude 40°N. These deposits are principally hosted by intermediate to silicic lava dome complexes; only a few deposits are associated with mafic- to intermediate-composition stratovolcanoes. Large deposits are most abundant and well developed in volcanic fields whose evolution spanned millions of years. Most deposits are hundreds of thousands to several million years younger than their host rocks, although some quartz-alunite deposits are essentially coeval with their host rocks. Variable composition and thickness of crustal basement is the primary control on mineralization along the length of the southern ancestral arc; most deposits and large alteration zones are localized in basement rock terranes with a strong continental affinity, either along the edge of the North American craton (Goldfield, Tonopah) or in an accreted terrane with continental affinities (Walker Lake terrane; Aurora, Bodie, Comstock Lode, Paradise Peak). Epithermal deposits and quartz-alunite alteration zones are scarce to absent in the northern part of the ancestral arc above an accreted island arc (Black Rock terrane) or unknown basement rocks (Modoc Plateau). Walker Lane structures and areas that underwent large magnitude extension during the Late Cenozoic (areas with Oligocene-early Miocene volcanic rocks dipping >40°) do not provide regional control on mineralization. Instead, these features may have served as local-scale conduits for mineralizing fluids.

A geological synthesis of the Precambrian shield in Madagascar

USGS Publications Warehouse

Tucker, Robert D.; Roig, J.Y.; Moine, B.; Delor, C.; Peters, S.G.

2014-01-01

Available U–Pb geochronology of the Precambrian shield of Madagascar is summarized and integrated into a synthesis of the region’s geological history. The shield is described in terms of six geodynamic domains, from northeast to southwest, the Bemarivo, Antongil–Masora, Antananarivo, Ikalamavony, Androyan–Anosyan, and Vohibory domains. Each domain is defined by distinctive suites of metaigneous rocks and metasedimentary groups, and a unique history of Archean (∼2.5 Ga) and Proterozoic (∼1.0 Ga, ∼0.80 Ga, and ∼0.55 Ga) reworking. Superimposed within and across these domains are scores of Neoproterozoic granitic stocks and batholiths as well as kilometer long zones of steeply dipping, highly strained rocks that record the effects of Gondwana’s amalgamation and shortening in latest Neoproterozoic time (0.560–0.520 Ga). The present-day shield of Madagascar is best viewed as part of the Greater Dharwar Craton, of Archean age, to which three exotic terranes were added in Proterozoic time. The domains in Madagascar representing the Greater Dharwar Craton include the Antongil–Masora domain, a fragment of the Western Dharwar of India, and the Neoarchean Antananarivo domain (with its Tsaratanana Complex) which is broadly analogous to the Eastern Dharwar of India. In its reconstructed position, the Greater Dharwar Craton consists of a central nucleus of Paleo-Mesoarchean age (>3.1 Ga), the combined Western Dharwar and Antongil–Masora domain, flanked by mostly juvenile “granite–greenstone belts” of Neoarchean age (2.70–2.56 Ga). The age of the accretionary event that formed this craton is approximately 2.5–2.45 Ga. The three domains in Madagascar exotic to the Greater Dharwar Craton are the Androyan–Anosyan, Vohibory, and Bemarivo. The basement to the Androyan–Anosyan domain is a continental terrane of Paleoproterozoic age (2.0–1.78 Ga) that was accreted to the southern margin (present-day direction) of the Greater Dharwar Craton in pre

Seismic crustal structure of the Limpopo mobile belt, Zimbabwe

NASA Astrophysics Data System (ADS)

Stuart, G. W.; Zengeni, T. G.

1987-12-01

A 145 km N-S seismic traverse was deployed to determine the crustal structure of the Limpopo mobile belt in southern Zimbabwe and the nature of its northern boundary with the Zimbabwean craton. Rockbursts from South African gold mines to the south and regional seismicity from the Kariba-South Zambia belt to the north were used as seismic sources. P-wave relative teleseismic residuals were also measured to assess whether any velocity contrast between the craton and the mobile belt extended into the upper mantle. Interpretation of reduced travel times from the local Buchwa iron-ore mine blasts, which were broadside to the traverse, revealed an upper crustal interface in the Limpopo mobile belt at a depth of 5.8 ± 0.6 km, dividing material with a velocity of about 5.8 km/s from that of about 6.4 km/s. On the craton, arrivals from the same source showed a 4.4 ± 0.5 km thick 5.5 km/s layer overlying crust of about velocity 6.5 km/s. P-wave arrivals from the regional seismicity were used to construct a crustal cross-section. Absolute crustal thickness was tentatively estimated from the identification of a Moho reflection on the mine blast recordings. To the south of Rutenga, the crust thins from around 34 km to 29 km in association with a positive gravity anomaly centred over the late-Karoo Nuanetsi Igneous Province and Karoo Tuli Syncline. North of Rutenga to the boundary with the Zimbabwean craton, the crust is about 34 km thick. The craton boundary was found to be a steeply southerly dipping zone associated with high-velocity material, which could either be deep-seated greenstones or mafic material associated with the margin in the region studied. This zone divides cratonic crust, which was found to be about 40 km thick, from that typical of the mobile belt and implies a step in the Moho of around 6 km. Analysis of relative teleseismic residuals showed that the velocity contrasts are not confined to the crust but extend into the uppermost upper mantle with the

Inclusions of crichtonite-group minerals in Cr-pyropes from the Internatsionalnaya kimberlite pipe, Siberian Craton: Crystal chemistry, parageneses and relationships to mantle metasomatism

NASA Astrophysics Data System (ADS)

Rezvukhin, Dmitriy I.; Malkovets, Vladimir G.; Sharygin, Igor S.; Tretiakova, Irina G.; Griffin, William L.; O'Reilly, Suzanne Y.

2018-05-01

Cr-pyrope xenocrysts and associated inclusions of crichtonite-group minerals from the Internatsionalnaya kimberlite pipe were studied to provide new insights into processes in the lithospheric mantle beneath the Mirny kimberlite field, Siberian craton. Pyropes are predominantly of lherzolitic paragenesis (Cr2O3 2-6 wt%) and have trace-element spectra typical for garnets from fertile mantle (gradual increase in chondrite-normalized values from LREE to MREE-HREE). Crichtonite-group minerals commonly occur as monomineralic elongated inclusions, mostly in association with rutile, Mg-ilmenite and Cr-spinel within individual grains of pyrope. Sample INT-266 hosts intergrowth of crichtonite-group mineral and Cl-apatite, while sample INT-324 contains polymineralic apatite- and dolomite-bearing assemblages. Crichtonite-group minerals are Al-rich (1.1-4.5 wt% Al2O3), moderately Zr-enriched (1.3-4.3 wt% ZrO2), and are Ca-, Sr-, and occasionally Ba-dominant in terms of A-site occupancy; they also contain significant amounts of Na and LREE. T-estimates and chemical composition of Cr-pyropes imply that samples represent relatively low-T peridotite assemblages with ambient T ranging from 720 to 820°С. Projected onto the 35 mW/m2 cratonic paleogeotherm for the Mirny kimberlite field (Griffin et al., 1999b. Tectonophysics 310, 1-35), temperature estimates yield a P range of 34-42 kbar ( 110-130 km), which corresponds to a mantle domain in the uppermost part of the diamond stability field. The presence of crichtonite-group minerals in Cr-pyropes has petrological and geochemical implications as evidence for metasomatic enrichment of some incompatible elements in the lithospheric mantle beneath the Mirny kimberlite field. The genesis of Cr-pyropes with inclusions of crichtonite-group minerals is attributed to the percolation of Ca-Sr-Na-LREE-Zr-bearing carbonate-silicate metasomatic agents through Mg- and Cr-rich depleted peridotite protoliths. The findings of several potentially

Southern Pine Bark Beetle Guild

Treesearch

T. Evan Nebeker

2011-01-01

Dendroctonus frontalis (southern pine beetle), D. terebrans (black turpentine beetle), Ips avulsus (small southern pine engraver or four-spined engraver), I. grandicollis (five-spined engraver), and I. calligraphus (six-spined engraver) comprise the southern pine bark beetle guild. Often they are found sharing the same hosts in the Southeastern United States. They...

Southern cascades bioregion

Treesearch

Carl N. Skinner; Alan H. Taylor

2006-01-01

The Cascade Range extends from British Columbia, Canada, south to northern California where it meets the Sierra Nevada. The Southern Cascades bioregion in California is bounded on the west by the Sacramento Valley and the Klamath Mountains, and on the east by the Modoc Plateau and Great Basin. The bioregion encompasses the Southern Cascades section of Miles and Goudey...

Implications of new ^{40}Ar/^{39}Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India

NASA Astrophysics Data System (ADS)

Pillai, Shilpa Patil; Pande, Kanchan; Kale, Vivek S.

2018-04-01

The Kaladgi Basin on the northern edge of the Dharwar craton has characters diverse from the other epicratonic Purana basins of Peninsular India. Sedimentological studies in the basin have established the presence of three cycles of flooding separated by an event of intra-basinal deformation accompanied by low grade incipient metamorphism. The overall structural configuration of the basin indicates its development by supracrustal extension accompanied by shearing in a trans-tensional regime during the Mesoproterozoic. This was followed by sagging that yielded Neoproterozoic sedimentation in a successor nested basin. ^{40}Ar/^{39}Ar dating of an intrusive mafic dyke along the axial plane of a fold has yielded a plateau age of 1154{± }4 Ma. This helps constraint the age of the various events during the evolution of this basin.

P-T-t paths from polyphased garnets of the Yenisey Ridge: evidence for three tectonothermal events along the western margin of Siberian craton

NASA Astrophysics Data System (ADS)

Likhanov, Igor

2015-04-01

Studies of pelitic gneisses and schists within the Yenisey regional shear zone (Garevka complex) at the western margin of the Siberian craton provide important constraints on the tectonothermal events and geodynamic processes in the Yenisey Ridge. In situ U-Th-Pb geochronology of monazite and xenotime from different growth zones of the garnet porphyroblasts coupled with P-T path calculations derived from garnet zoning patterns records three superimposed metamorphic event [1]. The different field gradients reflect contrasting tectonic settings. The first stage occurred as a result of the Grenville-age orogeny during late Meso-early Neoproterozoic (1050-850 Ma) and was marked by low-pressure zoned metamorphism at c. 4.8-5.0 kbar and 565-580 °C with a metamorphic field gradient of dT/dZ = 20-30 °C/km. At the second stage, the rocks experienced middle Neoproterozoic (801-793 Ma) collision-related medium-pressure metamorphism at c. 7.7-7.9 kbar and 630 °C with dT/dZ < 10 °C/km. The final stage evolved as a synexhumation retrograde metamorphism (785-776 Ma) at c. 4.8-5.4 kbar and 500 °C with dT/dZ < 14 °C/km and recorded uplift of the rocks to upper crustal levels in shear zones. The duration of post-collisional thrust exhumation does not exceed 16 Myr, which gives an exhumation rate of the metamorphic rocks of about 500-700 m/Myr [2]. This is in good agreement with the rate of exhumation (400 m/Myr) calculated for coeval collision-related metamorphic events in the Teya complex of the Yenisey Ridge [3] resulted from crustal thickening due to overthrusting [4] and also agrees with the results of thermomechanical numerical modeling (350 m/Myr) [5]. The final stages of collisional orogeny were followed by the development of rift-related bimodal dyke swarms of the Baikal-Yenisey belt, resulting from Neoproterozoic (790-780 Ma) extensional processes along the western margin of the Siberian craton and the onset of Rodinia's breakup [6]. Post-Grenville metamorphic

Southern rim of Pacific Ocean basin: southern Andes to southern Alps

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dalziel, I.W.D.; Garrett, S.W.; Grunow, A.M.

1986-07-01

Between the southern Andes of Tierra del Fuego and the southern Alps of New Zealand lies the least accessible and geologically least explored part of the Pacific Ocean basin. A joint United Kingdom-United States project was initiated in 1983 to elucidate the geologic history and structure of the Pacific margin of Antarctica from the Antarctic Peninsula to Pine Island Bay at approximately lone. 105/sup 0/W. The first season (1983-1984) of this West Antarctic Tectonics Project was spent in the Ellsworth-Whitmore crustal block, and the second (1984-1985) in the Thurston Island crustal block. The project involves structural and general field geology,more » petrology, geochemistry, paleomagnetism, and airborne geophysics (magnetics and radar ice echo sounding). A final geologic season will be spent in the Pensacola Mountains of the Transantarctic Range in 1987-1988.« less

The African Lithosphere

NASA Astrophysics Data System (ADS)

Priestley, K.; Debayle, E.; McKenzie, D.; Pilidou, S.

2007-12-01

There have been a number of prior, large scale surface wave studies of Africa, the majority of which rely on fundamental mode observations. In this study we use a large data set of multi-mode surface waves recorded over epicentral distances most of which are shorter than 6000 km, to investigate the Sv wave speed heterogeneity of the upper mantle beneath Africa. The inclusion of the higher mode data allow us to build an upper mantle model for the African plate with a horizontal resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers extending to about 400 km depth. Our tomographic images of the upper mantle beneath Africa displays significant shear velocity features, much of which correlate with surface geology. High velocity mantle persists beneath the West African and Congo cratons to 225-250 km depth, but the high velocity root beneath Kalahari Craton extends to only about 175 km depth. Low velocity upper mantle underlies the Pan- African terranes of Africa with the exception of the Damara mobile belt separating the Congo and Kalahari Cratons. The Damara mobile belt is underlain by a thick high velocity upper mantle lid which is indistinguishable from that beneath the Congo Craton to the north and the Kalahari Craton to the south. Low velocity upper mantle underlie the Hoggar, Tebesti and Darfur volcanic areas of northern Africa, and very low velocities underlie the Afar region to at least 400 km depth. We use the relationship between shear velocity and temperature of Priestley & McKenzie (2006) to derive a model for the African thermal lithosphere. Two types of lithosphere underlie Africa. Thick lithosphere underlies most of western Africa and all of southern Africa; in the latter the extent of the thick lithosphere is significantly different from the distribution of Archean crust mapped at the surface. Thick lithosphere forms one continuous structure beneath the Congo and Kalahari Cratons. Other than the Pan-African Damara

Seismic evidence for mantle suture and a collisional origin for the Canadian Cordillera

NASA Astrophysics Data System (ADS)

Chen, Y.; Gu, Y. J.; Currie, C. A.; Johnston, S. T.; Hung, S. H.; Schaeffer, A. J.; Audet, P.

2017-12-01

The North American Cordillera is a Phanerozoic orogenic belt that extends from Mexico to Alaska. Its eastern boundary is marked by pronounced changes in geophysical observations (e.g., mantle seismic velocity, surface heat flow, and effective elastic thickness) indicating a steep structural gradient beneath the Cordilleran foreland and the adjacent North American Craton. Seismological constraints on this boundary zone have been highly uneven: on the one hand, the knowledge of subsurface structures of the US Cordillera has been greatly enhanced by the USArray; on the other hand, detailed surveys of the northern counterpart, the Canadian Cordillera, are limited due to relatively sparse broadband data coverage. Questions pertaining to where and how Cordillera-Craton transition occurs in the upper mantle remain debated. Here, we utilize new teleseismic travel-time data from recently deployed networks in the Alberta foreland basin and nearby USArray stations and invert for mantle seismic velocities using finite-frequency tomography. The resulting high-resolution 3D model shows a dramatic increase in lithosphere thickness (>200 km) from the Cordillera to Craton. Additionally, independent calculations of mantle temperature from P (4.3%) and S (7.0%) velocity contrasts yield a consistent eastward 200-300 °C decrease at 150 km depth. We attribute the sharp structural and temperature/velocity gradients to the Cordillera-Craton boundary (CCB) established since at least the Late Cretaceous (ca. 100 Ma). The CCB dips steeply to the west beneath a carbonate belt that delineates a cryptic orogenic suture near the southern Rocky Mountain Trench, which provides strong evidence for an upper mantle suture between North America and an allochthonous Cordillera. The westward-dipping CCB may be a preserved structure associated with partial subduction of the leading edge of the North American Craton during its terminal collision with a microcontinent (Cordillera); this would require a

Les granitoïdes de la couverture protérozoïque de la bordure nord du craton du Congo (Sud-Est du Cameroun et Sud-Ouest de la République centrafricaine), témoins d'une activité magmatique post-kibarienne à pré-panafricaineGranitoids of the Proterozoic cover of the Congo craton northern edge (South-East of Cameroon and South-West of the Central African Republic), witnesses of a post-Kibarian to pre-Pan-African magmatic activity

NASA Astrophysics Data System (ADS)

Vicat, Jean-Paul; Moloto-A-Kenguemba, Gaétan; Pouclet, André

2001-02-01

Granitoid bodies dated from the Late Mesoproterozoic intrude the Palaeoproterozoic cover of the northern edge of the Congo craton. They line up a north-south left-lateral shear zone related to the Late Kibaran tectonics. They originated from crustal melting, may be due to the thermal anomalies, that were responsible of the large basaltic production during the pre-Pan-African extension of the Central Africa rift system.

Large Calcium Isotopic Variation in Peridotitic Xenoliths from North China Craton

NASA Astrophysics Data System (ADS)

Huang, S.; Zhao, X.; Zhang, Z.

2016-12-01

Calcium is the fifth most abundant element in the Earth. The Ca isotopic composition of the Earth is important in many aspects, ranging from tracing the Ca cycle on the Earth to comparing the Earth to other terrestrial planets. There is large mass-dependent Ca isotopic variation, measured as δ44/40Ca relative to a standard sample, in terrestrial igneous rocks: about 2 per mil in silicate rocks, compared to 3 per mil in carbonates. Therefore, a good understanding of the Ca isotopic variation in igneous rocks is necessary. Here we report Ca isotopic data on a series of peridotitic xenoliths from North China Craton (NCC). There is about 1 per mil δ44/40Ca variation in these NCC peridotites: The highest δ44/40Ca is close to typical mantle values, and the lowest δ44/40Ca is found in an Fe-rich peridotite, -1.13 relative to normal mantle (or -0.08 on the SRM 915a scale). This represents the lowest δ44/40Ca value ever reported for igneous rocks. Combined with published Fe isotopic data on the same samples, our data show a positive linear correlation between δ44/40Ca and δ57/54Fe in NCC peridotites. This trend is inconsistent with mixing a low-δ44/40Ca and -δ57/54Fe sedimentary component with a normal mantle component. Rather, it is best explained as the result of kinetic isotopic effect caused by melt-peridotite reaction on a time scale of several hundreds of years. In detail, basaltic melt reacts with peridotite, replaces orthopyroxene with clinopyroxene, and increases the Fo number of olivine. Consistent with this interpretation, our on-going Mg isotopic study shows that low-δ44/40Ca and -δ57/54Fe NCC peridotites also have heavier Mg isotopes compared to normal mantle. Our study shows that mantle metasomatism plays an important role generating stable isotopic variations within the Earth's mantle.

Metallogenesis and tectonics of the Russian Far East, Alaska, and the Canadian Cordillera

USGS Publications Warehouse

Nokleberg, Warren J.; Bundtzen, Thomas K.; Eremin, Roman A.; Ratkin, Vladimir V.; Dawson, Kenneth M.; Shpikerman, Vladimir I.; Goryachev, Nikolai A.; Byalobzhesky, Stanislav G.; Frolov, Yuri F.; Khanchuk, Alexander I.; Koch, Richard D.; Monger, James W.H.; Pozdeev, Anany I.; Rozenblum, Ilya S.; Rodionov, Sergey M.; Parfenov, Leonid M.; Scotese, Christopher R.; Sidorov, Anatoly A.

2005-01-01

subduction zones formed near continental margins. (4) From about mainly the mid-Cretaceous through the present, a succession of continental-margin igneous arcs (some extending offshore into island arcs) and contained metallogenic belts, and tectonically paired subduction zones formed along the continental margins. (5) From about the Jurassic to the present, oblique convergence and rotations caused orogen-parallel sinistral, and then dextral displacements within the plate margins of the Northeast Asian and North American Cratons. The oblique convergences and rotations resulted in the fragmentation, displacement, and duplication of formerly more continuous arcs, subduction zones, passive continental margins, and contained metallogenic belts. These fragments were subsequently accreted along the margins of the expanding continental margins. (6) From the Early Jurassic through Tertiary, movement of the upper continental plates toward subduction zones resulted in strong plate coupling and accretion of the former island arcs, subduction zones, and contained metallogenic belts to continental margins. In this region, the multiple arc accretions were accompanied and followed by crustal thickening, anatexis, metamorphism, formation of collision-related metallogenic belts, and uplift; this resulted in the substantial growth of the North Asian and North American continents. (7) In the middle and late Cenozoic, oblique to orthogonal convergence of the Pacific Plate with present-day Alaska and Northeast Asia resulted in formation of the present ring of volcanoes and contained metallogenic belts around the Circum-North Pacific. Oblique convergence between the Pacific Plate and Alaska also resulted in major dextral-slip faulting in interior and southern Alaska and along the western part of the Aleutian- Wrangell arc. Associated with dextral-slip faulting was crustal extrusion of terranes from western Alaska into the Bering Sea.

Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

USGS Publications Warehouse

Cox, R.; Coleman, D.S.; Chokel, C.B.; DeOreo, S.B.; Wooden, Joseph L.; Collins, A.S.; De Waele, B.; Kroner, A.

2004-01-01

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

Jurassic magmatism in Dronning Maud Land: synthesis of results of the MAMOG project

USGS Publications Warehouse

Leat, P.T.; Curtis, M.L.; Riley, T.R.; Ferraccioli, F.

2007-01-01

The Jurassic Karoo large igneous province (LIP) of Antarctica, and its conjugate margin in southern Africa, is critical for investigating important questions about the relationship of basaltic LIPs to mantle plumes. Detailed aerogeophysical, structural, anisotropy of magnetic susceptibility (AMS), geochronological and geochemical investigations completed under the British Antarctic Survey’s MAMOG project have provided some of the answers. Across most of the area, magma volumes were small compared to those in southern Africa. Jurassic dikes intruding the Archean craton are sparse and the Jutulstraumen trough, a Jurassic rift, is interpreted, from aerogeophysical data, as largely amagmatic. The largest volumes of magma were emplaced along the margin of the craton and close to the Africa-Antarctica rift. Although dikes were emplaced by both vertical and horizontal flow, overwhelmingly magmas in Dronning Maud Land were locally derived, and not emplaced laterally from distant sources. Basaltic magmatism was protracted in Dronning Maud Land (several dike emplacement episodes between ~206 and 175 Ma), and the small magma volumes resulted in highly diverse magma compositions, including picrites and ferropicrites interpreted to have been derived from hot mantle in a mantle plume. The protracted magmatism before the locally ~177 Ma flood lava eruptions, and evidence for a radiating dike swarm, favor a model of mantle plume incubation for 20-30 million years before flood lava eruption.

Southern pulpwood production, 1982

Treesearch

James F. Rosson

1982-01-01

The annual report of southern pulpwood production is based on a canvass of all pulpmills that fall within the 12 states of the Southern and Southeastern Forest Experiment Station regions. Neighboring experiment stations exchange information on mills located beyond the region but utilizing wood from the region.

The Recent Rise of Southern Magazines.

ERIC Educational Resources Information Center

Hynds, Ernest C.

During the past decade states in the southern United States have produced an increasing percentage of the nation's "city" magazines. Three magazines illustrate what the southern metropolitan magazines are doing to serve their readers and their communities. "Southern Living" provides information about its readers' interests as…

Southern pulpwood production, 1993

Treesearch

Michael Howell; Andrew J. Hartsell

1993-01-01

In 1993, Southern pulpwood production declined 2 percent to 66.3 million cords. Roundwood production decreased 2 percent to 46.3 million cords, and wood residue production fell 4 percent to 19.9 million cords. Pulping capacity of the 102 Southern pulpmills was 132,327 tons per day.

How unique is the Udachnaya-East kimberlite? Comparison with kimberlites from the Slave Craton (Canada) and SW Greenland

NASA Astrophysics Data System (ADS)

Kamenetsky, Vadim S.; Kamenetsky, Maya B.; Weiss, Yakov; Navon, Oded; Nielsen, Troels F. D.; Mernagh, Terrence P.

2009-11-01

The origin of alkali carbonates and chlorides in the groundmass of unaltered Udachnaya-East kimberlites in Siberia is still controversial. Contrary to existing dogma that the Udachnaya-East kimberlite was either contaminated by the crustal sediments or platform brines, magmatic origin of the groundmass assemblage has been proposed on the basis of melt immiscibility textures, melt inclusion studies, and strontium and neon isotope compositions. We further tested the idea of alkali- and chlorine enrichment of the kimberlite parental melt by studying olivine-hosted melt inclusions and secondary serpentine in kimberlites from the Slave Craton, Canada (Gahcho Kué, Jericho, Aaron and Leslie pipes) and southern West Greenland (Majuagaa dyke). Host olivine phenocrysts closely resemble groundmass olivine from the Udachnaya-East kimberlite in morphology, compositions (high-Fo, low-Ca), complex zoning with cores of varying shapes and compositions and rims of constant Fo. Melt inclusions in olivine consist of several translucent and opaque daughter phases and vapour bubble(s). The daughter crystals studied in unexposed inclusions by laser Raman spectroscopy and in carefully exposed inclusions by WDS-EDS are represented by Na-K chlorides, calcite, dolomite, magnesite, Ca-Na, Ca-Na-K and Ca-Mg-Ba carbonates, bradleyite Na 3 Mg(CO 3)(PO 4), K-bearing nahpoite Na 2(HPO 4), apatite, phlogopite and tetraferriphlogopite, unidentified sulphates, Fe sulphides, djerfisherite, pyrochlore (Na,Ca) 2Nb 2O 6(OH,F), monticellite, Cr-spinel and Fe-Ti oxides. High abundances of Na, K (e.g., (Na + K)/Ca = 0.15-0.85) and incompatible trace elements in the melt inclusions are confirmed by LA-ICPMS analysis of individual inclusions. Heating experiments show that melting of daughter minerals starts and completes at low temperatures (~ 100 °C and 600 °C, respectively), further reinforcing the similarity with the Udachnaya-East kimberlite. Serpentine minerals replacing olivine in some of the studied

Southern pulpwood production, 1990