forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

efficient cooling of rocky planets by intrusive magmatism the earth is in a plate tectonics regime with high surface heat flow concentrated at constructive plate boundaries other terrestrial bodies that lack plate tectonics are thought to lose their internal heat by conduction through their lids and volcanism hotter planets io and venus show widespread volcanism whereas colder ones modern mars and mercury are less volcanically active however studies of terrestrial magmatic processes show that less than 20 of melt volcanically erupts with most melt intruding into the crust signatures of large magmatic intrusions are also found on other planets yet the influence of intrusive magmatism on planetary cooling remains unclear here we use numerical magmatic thermo mechanical models to simulate global mantle convection in a planetary interior in our simulations warm intrusive magmatism acts to thin the lithosphere leading to sustained recycling of overlying crustal material and cooling of the mantle in contrast volcanic eruptions lead to a thick lithosphere that insulates the upper mantle and prevents efficient cooling we find that heat loss due to intrusive magmatism can be particularly efficient compared to volcanic eruptions if the partitioning of heat producing radioactive elements into the melt phase is weak we conclude that the mode of magmatism experienced by rocky bodies determines the thermal and compositional evolution of their interior rocky planets dominated by intrusive magmatism can cool more efficiently than those dominated by extrusive volcanism according to numerical simulations of mantle convection

emergence of silicic continents as the lower crust peels off on a hot plate tectonic earth the processes for crustal recycling during the archaean are unclear numerical simulations suggest that dense lower crust would have peeled off into the mantle leading to a rapid concentration of buoyant silicic rocks that formed the continents

geological archive of the onset of plate tectonics plate tectonics involving a globally linked system of lateral motion of rigid surface plates is a characteristic feature of our planet but estimates of how long it has been the modus operandi of

recent sedimentation and organic carbon burial in a shelf sea the north sea to obtain a carbon budget of the north sea recent sedimentation rates and organic carbon and nitrogen concentrations were determined on 27 box cores from the shallow north sea with a water depth 200 m and a surface area of 500 103 km2 the sedimentation rates in these box cores range from 5 to 35 cm 100 year 1 corg concentrations are 0 05 1 based on these numbers and published data the total amount of corg buried in the shallow north sea is calculated as 1 105 tons year 1 in large parts of the shallow north sea recent sedimentation is nearly absent reworking of relict deposits and partial mixing of these deposits with small amounts of recent fine grained sediments by biological and physical processes and human activities beam trawl fishery result in apparent sedimentation rates which are likely to exceed the real net sedimentation rates considerably the results of this study suggest that the majority of the organic matter is exported from the shelf over the shelf edge into the norwegian sea and into inner shelf deeps skagerrak and norwegian channel coarse grained shelf sediments are only of minor significance for the preservation of organic matter

a case of post depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the madeira abyssal plain abstract a set of oxidized and unoxidized sediment intervals from the f turbidite identified in two piston cores from the madeira abyssal plain map was analyzed for calcium carbonate total organic carbon toc total nitrogen tn stable carbon composition of toc 13c toc total hydrolyzable amino acids thaa total neutral sugars tsug lignin phenols lig two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n alkanes and n acids comparison shows changes for all properties depicting the effect of post depositional aerobic oxidation the oxidation process re mineralized approximately 80 of the toc and approximately 60 of the tn in the original deposit and left the residual organic matter depleted in 13c by 1 7 2 9 thaa and tsug account for 10 or less of the toc in all samples leaving approximately 90 of the toc chemically unidentified thaa tsug lig marine phytoplankton biomarkers and plantwax n acids were lost from the oxidized deposit to an extent as great as that for toc two non protein amino acids alanine and aminobutyric acid and plant wax n alkanes displayed much less degradation 9 37 and concentrated 3 3 4 8 times relative to toc by the oxidation process these observations show that both marine and terrestrial components contribute to the re mineralized toc fraction a binary mixing approach was used to model quantitatively the change in 13c toc and plantwax n alkane concentration results suggest that terrestrial organic carbon contributes approximately 15 to the toc content of the original turbidite its abundance increases approximately 2 4 times as a consequence of the oxidation process and approximately 40 and 90 of the terrestrial and marine component of toc in the original turbidite respectively was destroyed by the oxidation process although selective preservation of terrestrial relative to marine organic carbon is a well documented phenomenon in sedimentary processes this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment

formation of ridges in a stable lithosphere in mantle convection models with a viscoplastic rheology numerical simulations of mantle convection with a viscoplastic rheology usually display mobile episodic or stagnant lid regimes in this study we report a new convective regime in which a ridge can form without destabilizing the surrounding lithosphere or forming subduction zones using simulations in 2 d spherical annulus geometry we show that a depth dependent yield stress is sufficient to reach this ridge only regime this regime occurs when the friction coefficient is close to the critical value between mobile lid and stagnant lid regimes maps of convective regime as a function of the parameters friction coefficients and depth dependence of viscosity are provided for both basal heating and mixed heating situations the ridge only regime appears for both pure basal heating and mixed heating mode for basal heating this regime can occur for all vertical viscosity contrasts while for mixed heating a highly viscous deep mantle is required

a novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids we propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high performance liquid chromatography mass spectrometry hplc ms analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol the characteristic membrane lipid of non thermophilic crenarchaeota which is especially abundant in the marine and lacustrine environment based on these findings an index was developed the so called branched and isoprenoid tetraether bit index based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol this bit index was applied to surface sediments from the angola basin where it was shown to trace the outflow of the congo river furthermore analyses of particulate organic matter from the north sea showed relatively higher bit indices in water column particulate organic matter near large river inputs a survey of globally distributed marine and lacustrine surface sediments shows that the bit index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable the new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 ma old

field observations of the isotopic composition of particulate organic carbon in the southern north sea and adjacent estuaries abstract the carbon isotopic composition of the organic matter in fluvial sediments and in fluvial suspended matter falls in the range valid for continental organic matter 25 to 28 the 13 c values in sediments from marine sedimentation areas and in marine suspended matter varies between 20 and 25 the rather low mean values for the deposited sediments of 13 2 suggest the presence of relatively large amounts of continental organic matter this may be a direct supply by the present day rivers and or organic matter derived from the erosion of peat layers even in the marine environment a distinction between these two sources of detrital continental organic matter is not possible with stable isotopes studies in the estuaries studied the organic matter contents decrease and the 13 c values of the organic matter increase in the seaward direction these changes are primarily caused by the mixing of fluvial and marine sediments

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

crenarchaeotal membrane lipids in lake sediments a new paleotemperature proxy for continental paleoclimate reconstruction paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy a novel sea surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota this proxy will provide an independent continental paleotemperature tool that will allow paleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

a new class of equilibrium geotherms in the deep thermal lithosphere of continents abstract equilibrium lithospheric geotherms are generally assumed to have a self similar shape with a nearly linear thermal gradient in the upper portions of the thermal lithosphere and a smoothly decreasing gradient in its lowermost portions large thermal gradient increases are thought to be unsustainable because they would seem to imply a local heat flux imbalance that would be smoothed away over time simulations of mantle convection allowing for continents lead us to suggest that this may not be the case and that kinked geotherms may exist in thermal equilibrium within specific continental regions this new class of geotherms arises from the coexistence of two heat loss modes within the continental lithosphere oceanic heat loss is dominated by convection i e plate creation and subduction continental heat transfer on the other hand is of a conjugate nature with conduction dominating in stable chemically buoyant portions of continental lithosphere and convection operating in its weaker lower portions at the ocean continent boundary horizontal heat transfer occurs within a zone of accommodation where oceanic and continental heat loss are reconciled if continents are underlain by mantle downwellings then this horizontal heat transfer can involve a convective pattern that allows weak lower portions of the thermal lithosphere to flow differentially below a passive continental margin horizontal advection can thus be low in the near surface regions of the margin leading to mild thermal gradients but can outweigh vertical conduction at depth leading to steep gradients this allows inflected i e kinked geotherms to be generated downstream of the margin the thermal structure of the lower lithosphere adjusts to that of the continental interior and inflections decay however within a finite downstream region inflected geotherms can be maintained indefinitely by continued sub continental flow a similar situation can occur when weak lower portions of the continental thermal lithosphere flow from thinner to thicker regions of chemically stabilized upper lithosphere

generation of felsic crust in the archean a geodynamic modeling perspective abstract as a consequence of secular cooling of the earth there is generally no modern analog to assist in understanding the tectonic style that may have operated in the archean higher mantle temperatures and higher radiogenic heat production in the archean earth would have impacted the thickness and composition of the crust for this reason well constrained numerical modeling based on the fragmentary evidence preserved in the geological record is the most appropriate tool to evaluate hypotheses of archean crust formation the main lithology of archean gray gneiss complexes is the sodic tonalite trondhjemite granodiorite ttg suite melting of hydrated basalt at garnet amphibolite granulite or eclogite facies conditions is considered to be the dominant process that generated the archean ttgs taking into account geochemical signatures of possible mantle contributions to some ttgs models proposed for the formation of archean crust include subduction melting at the bottom of thickened continental crust and fractional crystallization of mantle derived melts under water saturated conditions we evaluated these hypotheses using a 2d coupled petrological thermomechanical tectono magmatic numerical model with initial conditions appropriate to the eoarchean mesoarchean based on the result of our experiments we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust 1 delamination and dripping of the lower mafic crust into the mantle 2 local thickening of the crust and 3 small scale crustal overturns in the context of a stagnant deformable lid tectono magmatic geodynamic regime that is terminated by short lived subduction we identify two distinct types of continental crust the first type is a pristine granite greenstone like crust with dome and keel geometry formed over delaminating upwelling mantle which is mostly subjected to vertical tectonics processes by contrast the second type is a reworked accreted crust comprising strongly deformed granite greenstone and subduction related sequences and subjected to both strong horizontal shortening and vertical tectonics processes thus our study has identified a possible spatial and temporal transition from pristine granite greenstone like crust with dome and keel geometry to reworked accreted crust forming more felsic gneiss terranes in the archean we suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the archean geological record

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

continental crust formation on early earth controlled by intrusive magmatism the global geodynamic regime of early earth which operated before the onset of plate tectonics remains contentious as geological and geochemical data suggest hotter archean mantle temperature and more intense juvenile magmatism than in the present day earth two crust mantle interaction modes differing in melt eruption efficiency have been proposed the io like heat pipe tectonics regime dominated by volcanism and the plutonic squishy lid tectonics regime governed by intrusive magmatism which is thought to apply to the dynamics of venus both tectonics regimes are capable of producing primordial tonalite trondhjemite granodiorite ttg continental crust but lithospheric geotherms and crust production rates as well as proportions of various ttg compositions differ greatly which implies that the heat pipe and plutonic squishy lid hypotheses can be tested using natural data here we investigate the creation of primordial ttg like continental crust using self consistent numerical models of global thermochemical convection associated with magmatic processes we show that the volcanism dominated heat pipe tectonics model results in cold crustal geotherms and is not able to produce earth like primordial continental crust in contrast the plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various ttg rocks using a systematic parameter study we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data low medium and high pressure ttg our study thus suggests that the pre plate tectonics archean earth operated globally in the plutonic squishy lid regime rather than in an io like heat pipe regime

recent sediment accumulation organic carbon burial and transport in the northeastern north sea abstract organic carbon and nitrogen and sedimentation rate determinations were made of boxcores from the norwegian channel north sea the geographical distribution of recent sedimentation areas were defined by analysis of published data and 3 5 khz penetrating echo sounder data the annual dry bulk sediment accumulation in the northeastern north sea is established at 74 10 6 tons as the average organic carbon content for the norwegian channel and skagerrak kattegat sediments is 0 6 and 1 8 respectively the organic carbon accumulation rate in the northeastern north sea could be calculated the total organic carbon accumulation in the norwegian channel and skagerrak northern kattegat is 0 17 10 6 tons yr 1 and 0 83 10 6 tons yr 1 respectively less than 10 of this is accounted for by local primary production this means that more than 90 of the organic carbon buried in the sediments of the northeastern north sea must have been supplied from different sources terrigenous sources supply 20 of the organic carbon the remainder is marine organic matter produced elsewhere in the north sea or imported from the atlantic ocean norwegian sea and the baltic sea storm wave and current induced near bottom nepheloid layers are the main likely mechanism to transport fine grained sediments and associated organic matter from the north sea plateau into the kattegat skagerrak and norwegian channel

characterization of transport and deposition of terrestrial organic matter in the southern north sea using the bit index we have applied a recently introduced proxy the bit branched and isoprenoid tetraether index to determine terrestrial organic matter tom transport from the rivers rhine and meuse and their tributaries to the southern north sea this index is based on crenarchaeol an isoprenoidal glycerol dialkyl glycerol tetraether gdgt predominantly derived from aquatic crenarchaeota and branched gdgts produced by soil bacteria up to 1 6 ng l 1 of branched gdgts were measured in seawater demonstrating the presence of tom in the southern north sea bit indices were inversely correlated with salinity indicating that this tom was recently supplied by rivers the substantial amount of branched gdgts 35 ng l 1 detected in river water and the high branched gdgt concentration measured in the water of a swiss peat bog 5 900 ng l 1 suggest fluvial transport of branched gdgts from peats and soils to the oceans the high crenarchaeol concentration measured in river water 4 4 ng l 1 was probably derived from crenarchaeota living in rivers and organic material from soil and peat the bit index 13c value and c n ratio of surface sediments deposited in the southern north sea were compared to determine tom deposition bit index 0 07 to 0 26 and d13c 220 6 to 222 9 both showed substantial small scale differences in tom deposition in the southern north sea but this pattern was not obvious from the c n ratios a good correlation was found between 13c and branched gdgt concentrations indicating that the absolute gdgt concentrations give additional information to the bit index

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

peel back controlled lithospheric convergence explains the secular transitions in archean metamorphism and magmatism abstract the preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the mesoarchean and neoarchean 3 2 2 5 ga these transitions which are imprinted in the metamorphic and magmatic rock record include the emergence of paired metamorphism an increasing depth of tonalite trondhjemite granodiorite ttg formation the widespread appearance of granites and changes in the composition of mafic rock previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre plate tectonic to plate tectonic regimes however their tectonic driver remains elusive in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present day here we demonstrate that lithospheric convergence driven by peel back peeling process under warmer mantle conditions termed as peel back convergence may explain the late archean secular transitions this tectonic phenomena features large scale peeling a form of delamination but not dripping of the mantle lithosphere with or without lower crustal rocks during convergence we simulated numerical models of peel back convergence under archean crust mantle conditions and predicted the pressure temperature p t conditions of crustal metamorphism and melting the evolution of peel back convergent setting features juxtaposition of a colder compressional regime with thickened crust that forms at the site of peeling and a warmer extensional regime with thinned crust forming behind it the metamorphic and magmatic p t conditions prevalent in these two tectonothermal sites replicate the features of the late archean rock record including the appearance of coeval high t p and intermediate t p metamorphic rocks greater abundance of high t p relative to intermediate t p metamorphic rocks dominant formation of ttgs at higher pressures where garnet rutile is stable in the residue reworking of pre existing felsic rocks sediments to produce potassic granites and interaction of crustal melts and mantle to form hybrid granitoids thus peel back convergence can explain the thermobaric bimodality of late archean rocks within an asymmetric tectono thermal framework however unlike modern convergent plate boundaries the asymmetry is controlled by lithospheric peeling

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

propagation velocity of shockwaves and pressure waves in soil and rock a variable pressure wave propagation velocity is developed and analyzed in ground materials which may be characterized by an exponential density pressure relationship the peak overpressure dependence is minor in the low pressure range and the propagation velocity of the pressure wave is essentially a constant in fact it is the seismic p wave propagation velocity pressure dependence becomes increasingly more significant at peak overpressures exceeding 1 kbar 108 pa at very high pressure levels encountered only in shockwaves at and above 10 kbar 109 pa the initial mechanical characteristics of the ground materials become less significant and the propagation velocity depends primarily upon the peak overpressure

crustal evolution and mantle dynamics through earth history resolving the modes of mantle convection through earth history i e when plate tectonics started and what kind of mantle dynamics reigned before is essential to the understanding of the evolution