a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

influence of the tectosphere upon plate motion this paper tests two hypotheses for the influence of thick lithosphere the tectosphere upon plate motion the first is that the tectosphere reaches down into high viscosity regions of the asthenosphere effectively slowing the motion of continental plates the second is that the tectosphere reaches down below the slow moving convective boundary layer into regions of rapid mantle convection and effectively speeds or slows the motion of continental plates our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis

seismic evidence for accumulated oceanic crust above the 660 km discontinuity beneath southern africa 1 high pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle thus subducted oceanic crust may accumulate at the base of the upper mantle direct observational evidence for this hypothesis however remains elusive we present an analysis of a negative polarity shear wave converted from compressional wave at a seismic discontinuity near 570 600 km depth beneath southern africa the negative polarity of the converted phase indicates a 2 2 0 2 s velocity decrease with depth at the seismic discontinuity this velocity reduction is associated however with a low velocity contrast at the 660 km discontinuity the exsolution of ca perovskite in former oceanic crust at depth greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660 km discontinuity

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

precambrian blueschist terranes of the world blueschists are most common in post paleozoic orogenic belts higher geothermal gradients and a thinner lithosphere in the precambrian are widely thought to have disfavored blueschist formation however several precambrian blueschist belts have been reported from eurasia on the basis of established geologic and geochronologic constraints these belts are grouped as suspect possible and definite precambrian blueschists suspect ones include those in alaska in anglesey u k in the scandinavian caledonides and in yunnan china possible precambrian blueschist belts are those in delhi india in central china in heilongjiang china and in the urals and altai sayan tien shan u s s r definite precambrian blueschist terranes are those in western africa and in aksu china these blueschist belts are described with emphasis on the age constraints the recently investigated aksu blueschist terrane is presented as an example this high p t coherent blueschist greenschist terrane at aksu is unconformably overlain by a thick unmetamorphosed sedimentary sequence ranging in age from late sinian ca 600 800 ma through eocambrian to paleozoic the basal sinian conglomerates contain subrounded to rounded clasts of the underlying schist clearly demonstrating the pre sinian age of blueschist facies metamorphism characteristic features of the above mentioned blueschist belts include 1 1 the blueschist terranes are coherent and primary compositional lithological layers are well preserved melange belts are not common 2 2 protoliths include pelite sandstone bimodal volcanics limestone and small quantities of pillow basalt and pelagic sediment ultramafics are rare 3 3 the protoliths were subjected to multiple deformation and transitional blueschist greenschist facies metamorphism typical precambrian blueschist contains the assemblage crossitic amphibole epidote chlorite albite quartz actinolite rare lawsonite jadeitic pyroxene and aragonite have been described in few belts 4 4 the metamorphic facies series belongs to the high pressure intermediate type of miyashiro 1961 the precambrian blueschist and other pre mesozoic intracontinental blueschist of eurasia formed before rather than during final closure of intervening ocean basins between pairs of continents they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of accretionary wedges including both oceanic and continent derived elastics the rare preservation of precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high rather than reflecting lack of production their existence in china and africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 ma ago

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

the teleseismic signature of fossil subduction northwestern canada 1 between june 2003 and september 2005 20 broadband three component seismometers were deployed along the mackenzie liard highway in canada s northwest territories as part of the joint lithoprobe iris canada northwest experiment canoe these stations traverse a paleo proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling teleseismic receiver functions computed from 250 earthquakes clearly reveal the response of the ancient subduction zone on the radial component the suture is evident as a direct conversion from the moho the depth of which increases from 30 km to 50 km over a horizontal distance of 70 km before its signature disappears the structure is still better defined on the transverse component where the moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth the seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients respectively anisotropy can be characterized by a 5 variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane the identification of this structure provides an unambiguous connection between fossil subduction and fine scale anisotropic mantle layering previous documentation of similar layering below the adjacent slave province and from a range of precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction

an integrated study of the grain size dependent magnetic mineralogy of the chinese loess paleosol and its environmental significance 1 to investigate the grain size dependent properties of the magnetic minerals in chinese loess paleosol samples touxiangdao xining qinghai province china magnetic extracts were divided into two size fractions by gravitational settling on the basis of hysteresis measurements thermal demagnetization of low temperature saturation isothermal remanent magnetization and nonmagnetic studies sem and xrd we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples the results show that the oxidation degree nonstoichiometry of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered in ascending order of the oxidation degree of our samples we find 1 loess coarse particles lc are multidomain md magnetite with slight oxidation 2 paleosol coarse pc particles are also md magnetite but with a higher oxidation degree compared to lc 3 loess fine lf particles are pseudo single domain psd magnetite with a high oxidation degree and 4 paleosol fine pf particles are psd maghemite single domain sd and superparamagnetic sp maghemite mainly stay in the residues further thermomagnetic analysis of pf psd maghemite revealed that this natural maghemite has a curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50 after a 700 c heating cooling cycle these new results identify the sources of multicomponent nrm in chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components

teleseismic studies of the canadian landmass lithoprobe and its legacy national research council of canada nrc research press http www nrc cnrc gc ca notices e html copyright permission to reproduce materials on this web site were produced and or compiled by the national research council for the purpose of providing canadians with direct access to information about the programs and services offered by the government of canada the material on this site is covered by the provisions of the copyright act by canadian laws policies regulations and international agreements such provisions serve to identify the information source and in specific instances to prohibit reproduction of materials without written permission

combined effects of the mjo and the arctic oscillation on the intraseasonal eastern china winter temperature variations abstractthe present study reveals that the madden julian oscillation mjo related temperature anomalies over east asia have notable differences among positive neutral and negative arctic oscilla

petrogenesis of the archean tonalite trondhjemite granodiorite ttg and granites in the lushan area southern margin of the north china craton implications for crustal accretion and transformation abstract tonalite trondhjemite granodiorite ttg assemblages and abundant high k calc alkaline magmatism are considered to be closely related to craton stabilization and usually record a tectonic transition three episodes of granitic magmatism are identified in the lushan area at the southern margin of the north china craton ncc on the basis of new la icp ms zircon u pb ages and whole rock geochemical data they are ttg gneisses sodic potassic granite spg suite and potassic granite dated at 2840 ma 2760 ma and 2570 ma respectively the ttg gneisses have high sio 2 variable mg cr and ni contents with high la yb n and sr y ratios suggesting that the parental magmas were derived by partial melting of subducted oceanic crust they show no obvious correlations between sr y and dy yb or la yb n and dy yb and all have negative nb and ta but positive zr and hf anomalies and variable dy yb nb ta zr hf and zr sm ratios suggesting melting of a deep source 45 km with rutile eclogite and garnet amphibolite in the residue the ttg samples have positive hf t values 2 6 to 7 0 with the youngest t dm c 2896 ma close to the crystallization age indicating that they were produced by partial melting of juvenile crustal materials in an arc setting rocks of the spg suite have high sio 2 contents variable na 2 o k 2 o ratios 0 41 10 91 and low y and yb contents with high sr y ratios indicating an affinity to i type granites they display depleted and concave upward ree patterns between middle and heavy rees and higher zr sm than ttg gneisses possibly resulting from amphibole removal by fractional crystallization or as a residual phase during melting meanwhile the rocks have a large variation of dy yb without obvious correlations between sr y and dy yb and la yb n and dy yb in combination with variable but uniformly low y contents therefore we suggest that the spg samples were derived from partial melting with garnet in the residue magmatic zircons have hf t values from 4 3 to 4 2 with t dm c ages from 3200 ma to 2877 ma indicative of older mafic lower crustal sources the rocks formed within a short time interval with the ttg and their emplacement was accompanied by regional granulite and amphibolite metamorphism implying that they may also have formed in an arc setting the potassic granite has high sio 2 74 79 wt and k 2 o na 2 o 9 22 wt contents with a low 10 000ga al 1 89 ratio and t zr 670 c magmatic zircons have a narrow range of hf t values 2 0 to 4 1 with t dm c ages 2942 2692 ma older than the formation age but consistent with the formation age ca 2760 ma of the lushan spg suite the potassic granite has a hf isotope composition identical to that of the spg suite and is enriched in lree and depleted in y with a positive eu anomaly suggesting that the spg suite is the main source one zircon yielded an inherited age of 2837 36 ma consistent with the age of the ttg gneisses indicating that the gneisses could also be the source of the potassic granite the two types of source rocks indicate a shallow crust depth hence the heat required for partial melting of granodioritic rocks to form the potassic granite could have been provided by the upwelling of mantle or mantle derived melts in an extensional setting therefore the ttg gneisses spg suite and potassic granite in the lushan area record a tectonic transition from a compressive to an extensional setting in the southern margin of the ncc between 2 85 and 2 50 ga

comparing crustal and mantle fabric from the north american craton using magnetics and seismic anisotropy a central target in earth sciences is the study of deformation at various depth levels within the earth seismology has offered a remarkable tool for doing this via seismic anisotropy it is however not always clear how to interpret those observations a question of interest is to understand the relation between the deformation of the mantle and the crust and in studying the relation between the two mantle deformation is expressed in seismic anisotropy in this paper we seek an objective way of extracting information about crustal fabric as well to be able to compare with seismic anisotropy the magnetization of crustal rocks offers an attractive possibility for doing this we thus explore the use of magnetic data and we compare magnetic crustal fabric orientation with mantle fabric observations from seismic anisotropy we apply our technique to the north american craton for which we have an excellent magnetic dataset and we show that there is a clear relation between crustal and mantle fabric for the cratonic region this has important implications for crustal formation and for interpreting seismic anisotropy observations

fundamental uniformity in the trace element patterns of the volcanics of the kaapvaal craton from 3000 to 2100 ma evidence for the lithospheric origin of these continental tholeiites

late neoarchean magmatism and tectonic evolution recorded in the dengfeng complex in the southern segment of the trans north china orogen abstract the dengfeng complex exposed in songshan area in the southern segment of the trans north china orogen tnco mainly consists of late neoarchean tonalite trondhjemite granodiorite ttg gneiss metadiorite and amphibolite the sanhuangzhai amphibolite records the earliest mafic magmatism 2658 5 ma in the region which is enriched in lree and lile with negative nb ta and ti anomalies and positive initial e nd t and e hf t values suggested to be the remnant of an arc related juvenile mafic crust the shipaihe amphibolite at the end of neoarchean 2492 7 ma is characterized by flat ree pattern slightly negative nb ta and ti anomalies and positive initial e nd t and e hf t values which is interpreted as large degree partial melt of metasomatised mantle source in a back arc basin zircon u pb dating shows two episodes of ttg magmatism 2 57 2 55 ga and 2 55 2 50 ga within the dengfeng complex all ttg gneisses have high sio 2 and na 2 o but low mg cr ni yb n and y with high la yb n and sr y the early ttg gneiss 2 57 2 55 ga in the eastern songshan area has variable whole rock hf nd isotopes e hf t 2 14 4 64 e nd t 0 11 3 35 respectively and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source but the later ttg gneiss 2 55 2 50 ga in the western songshan area has depleted whole rock hf nd isotopes e hf t 5 59 6 08 e nd t 2 42 3 86 respectively suggesting an overall juvenile source and its protolith is interpreted as the partial melt of flat subducting oceanic crust the shipaihe metadiorite is characterized by high mg cr ni sr ba highly fractionated ree patterns and strong negative nb ta and ti anomalies resembling to sanukitoid suite which is interpreted as the partial melt of a mantle wedge metasomatised by subduction derived fluids melts integrated geochronology and geochemical data of the dengfeng complex in songshan area reveal the late neoarchean crustal accretion process from subduction to arc continental collision accretionary orogeny followed by post collisional extension back arc basin implying an active continental margin arc environment in the southern segment of the north china craton

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

anisotropic upper mantle stratigraphy and architecture of the slave craton the earth s physical properties show a dominantly radial structure which is the result of compositional differentiation isochemical phase changes1 and rheological layering2 rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property elastic anisotropy may closely mimic the effects of isotropic heterogeneity3 nonetheless an improved characterization of earth s rheological structure promises important insights into such processes as plate dynamics and continental evolution here i present a methodology for effectively characterizing sharp transitions in anisotropic upper mantle structure using the coda of teleseismic p waves application to seismograms from the slave craton reveals a well developed stratigraphy at least in part anisotropic with major boundaries occurring at nominal depths of 75 135 and 195 km the geometry and sharpness of these discontinuities suggest a structural origin perhaps involving shallow subduction

the effects of wildfires on the magnetic properties of soils in the everglades we present results of a rock magnetic study of soils that were affected by wildfires that burned portions of the everglades in the spring of 2008 soils at sites that were extensively burned exhibit a pronounced surface magnetic enhancement effect with magnetizations of surface samples up to 16 times greater than that observed at depth 7 cm at these sites the increase in magnetization results from an increased abundance of a low coercivity phase maghemite that occurs at the expense of the abundance of a high coercivity phase goethite these results indicate that fire induced heating caused goethite in the surface soils to convert into a more magnetic low coercivity phase such as maghemite goethite is an excellent adsorber of phosphorus and therefore we hypothesize that the destruction of goethite as a result of burning may have important implications for phosphorus cycling in the everglades ecosystem copyright 2010 john wiley sons ltd

the case for accretion of the tectosphere by buoyant subduction this paper tests three hypotheses for the origin of the tectosphere continental collision cannot explain the low metamorphic grade of crust that predates the tectosphere halfspace cooling and buoyant underlating can both fit the diamond age data although underplating by buoyant subduction is the favored model thermal models provide a further test if half space cooling formed the tectosphere diamonds from 150km depth will be at least 200 m y younger than diamonds from 190km if buoyant subduction formed the tectosphere diamonds from 150km depth will be the same age or older than diamonds from 190km

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

the crust and uppermost mantle structure of southern peru from ambient noise and earthquake surface wave analysis abstract southern peru is located in the northern central andes which is the highest plateau along an active subduction zone in this region the nazca slab changes from normal to flat subduction with the associated holocene volcanism ceasing above the flat subduction regime we use 6 s to 67 s period surface wave signals from ambient noise cross correlations and earthquake data to image the shear wave velocity v s v structure to a depth of 140 km a mid crust low velocity zone is revealed and is interpreted as partially molten rocks that are part of the andean low velocity zone it is oblique to the present trench and possibly indicates the location of the volcanic arcs formed during the steepening of the oligocene flat slab beneath the altiplano plateau the recently subducted slab beneath the forearc shows a decrease in velocity from the normal to flat subduction regime that might be related to hydration during the formation of the nazca ridge which in turn may contribute to the buoyancy of the flat slab the mantle above the flat slab has a comparatively high velocity which indicates the lack of melting and thus explains the cessation of the volcanism above a velocity contrast from crust to uppermost mantle is imaged across the cusco vilcanota fault system and is interpreted as the boundary between two lithospheric blocks

paleoproterozoic subduction in northwestern canada from near vertical and wide angle seismic reflection data near vertical incidence and refraction wide angle reflection seismic data recorded as part of lithoprobe studies in the paleoproterozoic archean domains of canada s northwest territories show remarkable reflections from within the upper mantle a parallel pair of reflectors imaged by the near vertical data can be traced from moho levels 33 km down to 70 km depth in a previous study the reflectors were interpreted as the top and bottom of an 1 8 ga subducted oceanic crust beneath the hottah terrane further inboard where the seismic line changes its direction from east west to nearly north south another pair of reflectors extends subhorizontally for about 100 km at 70 km depth before dipping downward the subhorizontal reflectors were not correlated with the dipping slab instead they were interpreted as a separate feature however they roughly coincide with a horizontal interface modeled from wide angle data by an earlier study considering the crooked line acquisition geometry we re exami

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

length of the global plate boundary at 2 4 ga phanerozoic data show that during continental dispersal the total length of the global plate boundary is directly related to the size distribution and the surface area of continental fragments we use this observation to test theories about the length of ancient plate boundaries and how the increased mantle heat production in the precambrian was manifested by the crust using monte carlo simulations of plate boundaries on a sphere we calculate estimates of global plate boundary length for the present day continents and plates the mean plate boundary lengths are within 1 of each other 3 40 x 10 5 km and 3 43 x 10 5 km from the size distribution and normalized area of archean cratons estimates of the length of plate boundary at 2 4 ga range from 1 7 to 2 7 times the present length the preferred mean value is 2 2 times the present length this result favors a model of sea floor tectonics in which global plate velocities and sea floor spreading rates were about 16 slower than at present and in which the oceanic crust was more than 20 km thick