an algorithm for automated tsunami warning in french polynesia based on mantle magnitudes we have developed a new magnitude scale mm based on the measurement of mantle rayleigh wave energy in the 50 to 300 sec period range and directly related to the seismic moment through mm log10 m 20 measurements are taken on the first passage of rayleigh waves recorded on scale on broadband instruments with adequate dynamical range this allows estimation of the moment of an event within minutes of the arrival of the rayleigh wave and with a standard deviation of 0 2 magnitude units in turn the knowledge of the seismic moment allows computation of an estimate of the high seas amplitude of a range of expectable tsunami heights the latter combined with complementary data from t wave duration and historical references have been integrated into an automated procedure of tsunami warning by the centre polynesien de prevention des tsunamis cppt in papeete tahiti

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

the sholl shear zone west pilbara evidence for a domain boundary structure from integrated tectonostratigraphic analyses shrimp upb dating and isotopic and geochemical data of granitoids abstract the pilbara block provides a record of archaean continental growth involving the tectonic accretion of outboard island arcs and collisions with other continental scale fragments this record of continental growth is balanced by breakup and strike slip dismemberment of the continent new shrimp u pb in zircon ages and sm nd data provide evidence in the west pilbara which demonstrates that subduction related and tectonic accretion processes at the western margin of that ancestral continent between 3 15 2 78 ga were coeval with and genetically related to crustal scale tectonics and basin formation inboard of that margin the tectonic division of the west pilbara is defined by integrated tectonic analyses geochronology geochemistry and isotopic analyses geochronological studies clearly indicate that the western pilbara comparises two domains with different recorded geohistories whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time in combination these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the pilbara block the supracrustal rocks of the northern roebourne lithotectonic complex domain 6 in a pilbarawide scheme were formed in an island arc setting facing an ocean to the north west prior to 3260 ma the time of emplacement of voluminous granitoids into the complex in contrast the supracrustal rocks of the southern sholl belt pilbara domain 5 were formed in a back arc setting behind a north west facing arc between 3125 and 3112 ma with more or less synchronous granite emplacement at about 3115 ma the two domains were tectonically juxtaposed between 2991 and 2925 ma by the sholl shear zone a largely sinistral shear zone with subsequent volcanism in both domains to about 2925 ma the roebourne lithotectonic complex domain 6 is interpreted to be an allochthonous terrane which formed north east relative to its present position but indigenous to the pilbara block rather than an exotic terrane the east pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two west pilbara domains

polydiapirism of the archean mount edgar batholith pilbara block western australia structural analysis of the mount edgar batholith and the enveloping greenstone belt indicates that the batholith and rocks around its margin were subjected to similar deformation events that involved progressive and stepwise diapiric uplift of the batholith these events may be correlated on the basis of fold style orientation and fabric development early recumbent to reclined structures d1 including thrusts were buckled d2 by rounded upright folds prior to shearing d4 along normal faults parallel with the batholith margin d4 deformation was responsible for uplifting the batholith to its present structural position a deep seated linear deformation zone d3 within the batholith did not affect the supracrustal belt syntectonic granite intrusion amphibolite facies metamorphism and migmatisation were associated with d1 in the batholith whereas greenschist facies metamorphism was associated with d1 in the lowermost unit of the greenstone belt the talga talga subgroup amphibolite facies conditions were maintained within the batholith throughout the structural history but this grade was reached in the supracrustal belt only near the batholith margin progressive deformation d1 d2 and metamorphism occurred in response to early massive granite generation and emplacement producing a first order plutonic dome that reached full maturity during d2 solid state diapirism d4 followed producing a gneiss dome of similar dimension restriction of early recumbent folds and thrusts in the supracrustal belt to areas within or adjacent to the batholith is consistent with the model elsewhere deformation is minor the late stage diapirism brought a gneiss complex within the batholith into juxtaposition with lowermost greenschist facies rocks of the pilbara supergroup this complex represents in part sialic material that existed below the pilbara supergroup before diapirism began the complex closely resembles other archean high grade gneiss terranes which suggests that some of these terranes are deeper crustal exposures of granite greenstone terranes

crustal and upper mantle structure of stable continental regions in north america and northern europe from an analysis of many seismic profiles across the stable continental regions of north america and northern europe the crustal and upper mantle velocity structure is determined analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques the p wave velocity structure beneath the canadian shield is virtually identical to that beneath the baltic shield to a depth of at least 800 km two major layers with a total thickness of about 42 km characterize the crust of these shield regions features of the upper mantle of these regions include velocity discontinuities at depths of about 74 km 330 km 430 km and 700 km a 13 km thick p wave low velocity channel beginning at a depth of about 94 km is also present

seismic anisotropy of the pacific ocean inferred from long period surface waves dispersion abstract the dispersion of surface rayleigh and love waves in the period range 40 300 s along a large number of paths allows the estimation of both the azimuthal anisotropy and the shear wave polarization anisotropy the regional dispersion is determined taking into account simultaneously its dependence with age and an azimuthal factor the pacific ocean has been divided into 5 regions for rayleigh waves and into 3 regions for love waves this partition discriminates the regions of extreme age which show a fast variation of dispersion with age from the regions of intermediate age where the variation is weak a variation of 2 of rayleigh wave group velocity with the azimuth of the path measured with respect to the direction of spreading is displayed up to very long period on the contrary the azimuthal anisotropy for love waves is difficult to resolve for rayleigh waves the present day direction of plate motion seems to agree best with the direction of maximum velocity on the other hand the isotropic inversion of the regional dispersion curves indicates except for young regions a discrepancy between rayleigh wave and love wave models with this hypothesis sh velocities are higher than sv velocities for the regions older than 23 ma down to a depth of 300 km which is indicative of the presence of polarization anisotropy the latter very weak for the young part of the ocean increases with age and reaches 7 for the oldest region

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

an ocean floor carbonaceous sedimentary sequence in the 3 2 ga dixon island formation coastal pilbara terrane western australia abstract the dixon island formation of the pilbara terrane western australia extends from cleaverville beach to the dixon island coast and is the only example worldwide of a coastal outcrop of a 3 2 3 1 ga low grade greenstone belt the dixon island formation was situated in an immature island arc setting and comprises siliceous carbonaceous deep water sediments that contain evidence for hydrothermal and microbial activity the extensive outcrop along the coastline makes it possible to examine in detail the characteristics of mesoarchean sedimentation in a hydrothermal environment this study focuses on a continuously exposed carbonaceous black chert succession in the central part of the northern coastline outcrop on dixon island at this site a 20 m thick carbonaceous black chert sequence conformably overlies basement rocks of highly altered komatiite rhyolite tuffs the black chert sequence formed well bedded black chert with carbonaceous peloidal matter and fragmented grains and the sequence is homogeneous and finely laminated in this sequence evidence of low temperature hydrothermal fluid sediments and alteration stractures are well preserved in the lowermost section indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition in particular swarms of black chert veins provide evidence of post volcanic hydrothermal activity that released organic matter and silica to the ocean the carbonaceous peloidal textures and 13 c org values of sediments located just above the basement which hosts the vein swarms suggest that the veins were the conduits for hydrothermal fluid which contained organic rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal related sediment the 13 c org values of organic matter in the black cherts range from 42 to 22 average 31 9 n 313 lighter 13 c values 35 to 42 characterize carbonaceous laminated black chert located 5 m above the basement where biogenic structures e g biomat bed microfossil structures are found the lighter 13 c values might be related to methanotrophic micro organic activity within the sediments during hydrothermal activity in summary we reconstructed the sedimentary environment upon a mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

dispersion and attenuation of mantle waves through waveform inversion summary we propose a new approach to the determination of elastic and anelastic parameters of the earth s structure from seismic data instead of measuring such functionals of the medium response as the phase delay or spectral ratios we perturb parameters of the model to satisfy the observed waveforms the advantage of our waveform inversion technique wit is that it uses the properties of the earth as a smoothing filter also because the earth s structure is the common denominator the method allows simultaneous interpretation of different functionals of this structure for example the waveforms of rayleigh and love waves for the same source receiver pair following extensive testing of the method on synthetic and actual data we subject results of our analysis of 37 recordings for various sources and stations to pure path decomposition the period range of analysis extends from 160 to 630s we distinguish four types of regions stable continents areas tectonically active within the last 400myr ocean floors younger than 38 myr and old ocean floors older than 38 myr the results indicate significantly different responses of the oceanic and continental areas at long periods stable continental and tectonic regions have nearly the same dispersion for periods greater than 300 35os and then diverge rapidly with stable continents showing higher velocities the young and old oceans on the other hand become distinct at periods as long as 500 s the old oceans are faster in terms of shear velocity models the data are consistent with a difference of about 4 per cent between the young and old oceans in a depth range from 400 to 670 km while the continental regions are similar and close to the global average at shallower depths our structures are conceptually similar to those inferred from previous pure path analyses and short period surface wave studies while we cannot yet establish statistically significant differences in q for the four regions such differences are obtained if one distinguishes only between continents and oceans the q for the latter is 10 20 per cent lower in the period range of the analysis

structural relationships along a greenstone shallow water shelf contact belingwe greenstone belt zimbabwe the belingwe greenstone belt zimbabwe contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain the nature of the contact between the basalt komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based this contact is a 20 to 200 m thick high strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession in the ultramafic part of the high strain zone tectonites include ductile ultramafic mylonites ultramafic ultramylonites ultramafic phyllonites and serpentinite schists enclosing less deformed ultramafic serpentinite blocks in the part of the shear zone derived from shallow water sedimentary rocks shear zone tectonites include chert c s mylonite phyllonite and iron rich argillaceous shear bands both parts of the shear zone are cut by brittle fault gouge fabrics we suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the mberengwa allochthon contrary to prior suggestions there is no evidence that the tholeiitic basalt and komatiite succession of the belingwe greenstone belt sits conformably over the shallow water shelf rocks documentation of the shear zone at the base of the mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off axis oceanic plateau structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2 6 ga

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

tectonic evolution of the bindura shamva greenstone belt northern zimbabwe progressive deformation around diapiric batholiths abstract the bindura shamva greenstone belt northern zimbabwe and adjacent batholiths are regarded as one of the type areas for granite greenstone tectonics structural observations and three dimensional strain analyses have been used to propose a model for the tectonic evolution of this late archaean greenstone belt the data indicate that pluton emplacement triggered the deformation of the greenstone sequences and that the deformation history is single phase progressive rather than polyphase they do not however lead to an unambiguous interpretation of the emplacement mechanism structural features are best explained by a combination of pluton diapirism and ballooning plutonism criteria used as evidence for pluton related deformation include the pattern of foliation and stretching lineation trajectories the variation in strain type and strain intensity fold types sense of shear at the batholith greenstone interface metamorphic zoning and timing of porphyroblast growth specific structural and strain patterns within cleavage triple points interdomal synclinoria saddle culminations and batholith margins demonstrate interference between strain fields developed around each batholith

faults and folds fact and fiction abstract after reviewing the microscopic and macroscopic texture of fault zones the localisation of rupture and creep is described and the reduced strength of fault zones is investigated simple strength and viscosity models for the whole lithosphere play a major role for the geometry of fault zones compressional faults thrusts show ramp and flat structures often soling in a weak detachment zone extensional faults prefer a listric shape often turning subhorizontal and being invisible in the seismic lamellae of the ductile lower crust which according to the models is also a broad zone of detachment and decoupling faults in the lower crust are rare and restricted to strain hardened fault rocks or intrinsically rigid rocks like oceanic mafic ultramafic rocks of suture zones transient faults in the lower crust are created by rupture processes of large earthquakes some deep faults moderately dipping or flat are again observed in the rather rigid uppermost mantle the observation of folding inside the crust seems to be connected with an intermediate viscosity range and with a broad vertical transition zone where rigid reactions and ductile processes are mixed various methods of detecting faults by seismic studies are critically reviewed and some sequence of reliability is suggested impedance contrasts polarity dip thickness and multiple fault strings have to be deciphered by wavefront modelling origin and survival of faults are considered to be a consequence of tectonic and thermal evolution under specific stress systems

the tectonic evolution of the fennoscandian border zone in denmark abstract the fennoscandian border zone is a complex tectonic zone defining the border between the stable fennoscandian baltic shield and the metastable danish basin and polish trough it forms the northwestern part of the tornquist teisseyre lineament one of the major european tectonic sutures intricate tectonic movements took place within the fennoscandian border zone from early paleozoic to recent time it is postulated that its evolution was dominated during the early palaeozoic by tensional tectonics and during the late paleozoic by dextral wrench movements the latter led to the development of curvilinear faults and the subsidence of pull apart basins transtensional stresses governing the triassic to early cretaceous evolution of the fennoscandian border zone induced the differential subsidence of rift basins during the late cretaceous and early cenozoic dextral transpressional stresses induced the partial inversion of mesozoic basins associated with the fennoscandian border zone this is reflected by the uplift of a major anticlinorium over which cretaceous strata became progressively truncated whilst adjacent areas developed into late cretaceous early tertiary depocentres basin inversion diminishes to the northwest along the fennoscandian border zone and disappears in the skagerrak area

seismostratigraphy and structural framework of the sw baltic sea the sw part of the baltic sea between scania rugen bornholm and mon constitutes a complex crustal transition between the baltic shield and the accreted phanerozoic provinces of the west european platform an integrated interpretation of marine reflection seismic data sets from the babel ac line and two commercial surveys offshore ne germany and s sweden have resulted in a complete view of the structural framework in the area the general seismic picture can best be detected by two characteristic sets of reflection phases the lower set is dominated by slightly dipping and vertically displaced prominent reflectors corresponding to downfaulted lower palaeozoic strata on top of the precambrian basement the upper set represents mesozoic and cenozoic coherent reflection phases including a thick upper cretaceous unit the caledonian deformation front is identified in the southern part of the investigated area as the border against which basement rocks have been affected by caledonian metamorphism and deformation major structural elements also include the n s trending agricola svedala fault and north rugen skurup fault several nw se trending faults are also identified including the nordadler kamien fault jutland mon fault carlsberg fault romeleasen fault zone and the fyledalen fault zone the sedimentary record from ne german offshore wells and scanian boreholes is compared with the seismic data the cambro silurian strata are composed mainly of quartzitic sandstones shales and biomicritic limestones the silurian is dominated by grey micaceous shale and micaceous siltstone deposited in a marginal basin upper palaeozoic strata are merely encountered in the southernmost part of the investigated area these include zechstein strata the mesozoic deposits are dominated by a thick cretaceous sequence of mainly limestones with interbedded sandstones

record of emergent continental crust 3 5 billion years ago in the pilbara craton of australia isotopic data for the earth s oldest rocks1 7 imply that a considerable volume of continental crust existed during the early archaean aeon 3 0 gyr ago but it is not known when this crust first began to form emergent landmasses sedimentary geochemistry suggests8 9that the area of exposed continent was negligible until late in the archaean10 a contention supported by the fact that until now all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of 3 0 gyr here we report the discovery of an angular unconformity an ancient erosion surface beneath rocks of the 3 46 gyr warrawoona group in the pilbara craton of australia currently the oldest known well preserved greenstone succession below the unconformity low grade greenstones older than 3 5 gyr were intruded by voluminous granitoids before erosion as the overlying warrawoona rocks are only mildly metamorphosed slightly deformed and were deposited near sea level we infer that they accumulated on crust that was already rigid cool and buoyant thus by 3 46 gyr ago the sub warrawoona rocks formed an emergent block of continental crust the most ancient known

granitoid diapirism during protracted tectonism in an archaean granitoid greenstone belt yilgarn block western australia abstract the bullfinch parker dome area is an archaean granitoid greenstone terrain in the yilgarn block of western australia affected by a complex protracted tectonic event the majority of the deformation history was progressive rather than multiple discrete events finite strain patterns and structural and metamorphic zonation suggest that deformation in the greenstones was caused by regional ene wsw shortening and broadly simultaneous upward movement of granitoids by solid state diapirism and or ballooning plutonism rather than by core complex formation or multiple folding evidence for pluton related tectonism includes the foliation trends steeply plunging radial mineral lineations indications of upward movement of plutons increase of strain towards the batholith margins concordant structures in the pluton and the surrounding rocks such as lineation and foliation patterns constrictional tectonics near east west trending granitoid greenstone contacts syn to post kinematic peak metamorphism contact metamorphic zoning and pressure differences metamorphism largely of amphibolite facies was probably related to uprise of the granitoids at moderate crustal depths 10 12 km p 3 kbar causing both regional and contact metamorphism as a single thermal event relative and absolute timing defines an overlapping sequence of metamorphism progressive deformation and plutonism in this part of the yilgarn craton during the late archaean

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

middle archean volcano hydrothermal sequence bacterial microfossil bearing 3 2 ga dixon island formation coastal pilbara terrane australia the 3 2 ga dixon island formation in the cleaverville group of the coastal pilbara terrane australia is one of the most complete and best preserved examples of middle archean oceanic stratigraphy and contains possible microbial material field observations and geochemical evidence suggest that this formation contains a low temperature hydrothermal vent system with a biogenic microbial colony from the archean ocean the dixon island formation is 350 m thick and consists of the rhyolite tuff black chert and varicolored chert members in ascending order the rhyolite tuff member contains many vein swarms such as quartz and black chert veins and highly altered rhyolite tuff layers which are identified as an underground bypass zone for circulating hydrothermal fluid many black chert vein swarms in the rhyolite tuff member imply intensive low temperature hydrothermal activity during deposition of the black chert member which is 10 15 m thick the black chert member is composed of massive black chert laminated black chert dark greenish siliceous shale and tuffaceous laminated chert which are mainly composed of very fine quartz abundant pseudomorphs of silica after aragonite barite and gypsum and a distinctly continuous stromatolite like biomat layer 10 20 cm thick are preserved within the laminated black chert bed the stromatolite like biomat bed is formed of fine iron or iron coated quartz pisolite within fine grained silica the absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at 500 2000 m paleodepth and may have been on the slope of an immature island arc microbial material has been preserved well in the lower part of black chert member the massive black chert has carbonaceous peloids 0 3 2 0 mm in diameter which are similar to those in the black chert veins the massive black chert contains spiral rod and dendrite shaped bacterial material the total organic carbon toc value of massive black chert in the lower part of the black chert member is higher toc 0 15 0 45 than that of the overlying laminated chert section toc 0 02 0 15 and the black chert vein toc 0 1 0 13 and the carbon isotope 13 c values of this lithology 33 to 27 are also lighter than for the black chert veins 29 to 26 and the laminated black chert in the upper part of the black chert member and the varicolored chert member 27 to 13 this evidence suggests that the carbonaceous grains and bacteria shaped material in the lower part of the black chert member are of bio genic origin and were formed close to a low temperature hydrothermal vent system the microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity laminated black chert in the upper part of the black chert and the varicolored chert members may have formed by cyanobacterial sedimentation from the ocean surface

remnants of an archean oceanic plateau belingwe greenstone belt zimbabwe stratigraphic and structural data from the archean zimbabwe craton suggest that a major detachment surface exists within the belingwe greenstone belt the surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex older volcanic sedimentary rocks and a shallow water sedimentary sequence in the footwall rocks dated at ca 2 7 ga above the detachment surface form the proposed mberengwa allochthon the regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets not indigenous magmas erupted within autochthonous continental rifts magmatic rocks of the mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts comparison of the zimbabwe craton with the proterozoic birrimian terranes of west africa leads us to suggest that precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere

in situ quadruple sulfur isotopic compositions of pyrites in the ca 3 2 2 72 ga metasedimentary rocks from the pilbara craton western australia abstract the sulfur mass independent fractionation s mif of mesoarchean is distinct from the other archean periods by the dampened magnitudes of 33s and a steeper slope of 36s 33s compared with the archean reference array ara according to previous studies however samples available of this interval are limited owing to poor or no preservation additionally the majority of the data were obtained by analysis of bulk rocks thus are essentially the sulfur isotopic compositions of mixed sulfur components due to multiple stages of post diagenesis overprints and modifications this study systematically measured the multiple sulfur isotopic compositions of pyrites nodular disseminated in ca 3 2 2 72 ga metasedimentary rocks from the pilbara craton western australia where one of the best preserved archean records occur using the upgraded shrimp si after detailed pyrite generation and genesis investigation by bse imaging and naocl etching the results show that 33s values of the diagenetic pyrites in these metasedimentary rocks range from 2 4 to 0 8 such a range is inconsistent with the other archean intervals and any experiment producing s mif so far where the magnitude of the maximum positive 33s value is considerably larger than the corresponding minimum negative 33s value the dampened positive 33s is explained in several ways from the perspective of production and preservation including original photochemical products with larger magnitudes of negative 33s than positive 33s in a different atmospheric composition non zero 33s of initial so2 different mechanism producing s mif chain formation and destruction of positive 33s by subduction the downward 36s 33s deviations from ara in the diagenetic pyrites of 2 72 ga tumbiana formation can be attributed to modifications induced by microbial activities this is based on the negative 0 9 33s 36s values similar to biological products and the large range of negative 34s values the downward 36s 33s offsets in the hydrothermally altered diagenetic pyrites of 2 78 ga mt roe basalt are most likely mixing of primary sulfur on ara with later sulfur disobeying ara the upward 36s 33s deviations in the diagenetic pyrites of 2 93 ga mosquito creek formation likely reflect different atmospheric composition

test of tectonic models by great circle rayleigh waves summary a large set of published great circle rayleigh wave phase velocities in the period range 125 350s is used to compare three recent tectonic models okal leveque jordan prior to any regionalization the symmetry property of the great circle integrals is used to obtain a lower limit of the signal noise ratio in the data it turns out that the signal is responsible for at least 30 per cent of the data variance in the period range 175 300s a standard regression method is applied for computing the pure path velocities and the model efficiency is derived from a variance analysis it is shown that even at great depth none of the three models explains more than 60 per cent of the energy due to the long wavelength lateral heterogeneities 6500 km the three models have nearly the same efficiency for explaining the short period data t 125s between 200 and 300s the higher performance of okal s model indicates that it is important to separate the subduction zones from the other orogenic zones by perturbing the lateral extension of the subduction zones it comes out that they constitute on both sides of the subducting slabs wider anomalies than often assumed suggesting large downgoing flows on the contrary the effect of surface features such as marginal seas are restricted to a close region in front of the trenches finally the anomalous ellipticity values deduced directly from great circle data are partly explained by a coupling between tectonics and ellipticity

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

timing and tectonic significance of late archaean sinistral strike slip deformation in the central pilbara structural corridor pilbara craton western australia a north south striking zone of sinistral strike slip shear 20 100 km and associated folding and thrusting within the central part of the pilbara craton is described this zone referred to herein as the central pilbara structural corridor cpsc was formed during regional d3 deformation and overprints two earlier sets of structures associated with granitoid doming it is bound to the west by a continuous north south striking curviplanar fault at greenschist facies in the east it is bound by a series of ne sw striking fault segments located along sheared fold limbs and by a kilometre wide zone of amphibolite facies mylonite within the western margin of the shaw batholith faults vary from narrow high level brittle structures and retrograde breccia zones in the north to broader more ductile structures with sc mylonitic foliations in the south suggesting a southerly increase in exhumation a set of kilometre scale en echelon folds with ne sw striking upright axial planes and ne plunging axes characterize the interior of the cpsc and are wholly contained within the bounding faults thus indicating coeval development with faulting during sinistral wrenching 1 nw se strike slip deformation in the cpsc is interpreted to have resulted from northwesterly indentation of shaw batholith and contiguous rocks to the east westerly directed tectonic escape of the strelley granite laccolith and associated supracrustal rocks from in between shaw batholith and north pole dome was followed by ramping up the concave eastern margin of the yule batholith and eastward tilting during the transition from westerly to northerly displacement continued northward translation of the strelley granite at the leading edge of the system was terminated by constriction between the carlindi batholith and the north pole dome trailing greenstone successions were crumpled into a series of ne trending en echelon folds and related faults kinematic evidence and structural geometry indicate that sediments of the lalla rookh basin de grey group were deposited in advance of the north moving strelley granite within a basin bounded by faults of opposite displacement rather than in a pull apart basin as previously suggested the age of the d3 deformation is indicated by resetting of the ar ar and rb sr isotopic systems to ca 2950 ma in the western shaw region and by u pb ages of syn kinematic granitoid rocks

shrimp and electron microprobe chronology of uht metamorphism in the napier complex east antarctica implications for zircon growth at 1 000 c zircons in ultra high temperature uht metamorphosed paragneisses from mt riiser larsen in the napier complex east antarctica were dated by using ion microprobe shrimp and electron microprobe emp both shrimp and emp analyses yield consistent 2520 2460 ma age populations for garnet orthopyroxene bearing paragneiss and leucosomes enclosed within the peak uht event was dated at 2480 ma by shrimp analyses on metamorphic zircons from the garnet orthopyroxene paragneiss and those on magmatic zircons from the leucosomes which are interpreted to be formed at syn uht as obtained by shrimp the uht metamorphic event was terminated no later than 2460 ma minor 2520 ma shrimp age suggests either the onset of prograde metamorphism or another high grade metamorphic event unrelated to the uht emp analyses on metamorphic zircons from sapphirine quartz and osumilite bearing magnesian paragneisses give c 2500 2450 ma ages inherited igneous zircon cores of the magnesian paragneisses yield relatively scattered emp ages ranging over c 3000 2650 ma suggesting that igneous materials of these ages sourced the protoliths of the paragneisses and that they were deposited during the interval c 2650 2520 ma

evidence for multiphase deformation in the archean basal warrawoona group in the marble bar area east pilbara western australia abstract the results of a structural analysis of the marble bar greenstone belt in the mcphee reward area of the eastern pilbara block in western australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy cf primary igneous layering around the talga talga anticline early extensional movements were associated with the formation of the warrawoona group chiefly the duffer formation and indicate a wnw ese extensional direction similar to the syn warrawoona extension found in the area of the north shaw batholith extension was followed by thrusting with transport to the east the talga talga anticline is a culmination which formed during this event by thrust stacking a major shear in the area the duffada shear was reactivated during granodiorite emplacement with transport to the ne in their present geometry the shears have thrust movements on the south side of the talga talga anticline but normal movements on the north side after the reactivation came a nw se shortening which was accompanied by conjugate shear activity on north trending sinistral and west trending dextral shears and constrained to 2950 ma in the north shaw area the final movements were associated with a n s shortening which resulted in an e w trending thrust which cuts the northern flank of the talga talga anticline and nw and ne trending conjugate faults there is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the mount edgar batholith were emplaced by a solid state diapiric process the net result of deformation was tectonic disruption of the stratigraphic stacking within the type ara of the talga talga subgroup in the warrawoona group this may offer an additional explanation for anomolously young dates of the talga talga subgroup with respect to the duffer formation and also indicates that the type section for the talga talga subgroup is a lithotectonic sequence

formation of an archean tectonic melange in the schreiber hemlo greenstone belt superior province canada implications for archean subduction accretion process the late archean circa 2750 2670 ma schreiber hemlo greenstone belt superior province canada is composed of tectonically juxtaposed fragments of oceanic plateaus circa 2750 2700 ma oceanic island arcs circa 2720 2695 ma and siliciclastic trench turbidites circa 2705 2697 ma following juxtaposition these lithotectonic assemblages were collectively intruded by synkinematic tonalite trondhjemite granodiorite ttg plutons circa 2720 2690 ma and ultramafic to felsic dikes and sills circa 2690 2680 ma with subduction zone geochemical signatures overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation during d1 circa 2695 2685 ma oceanic plateau basalts and associated komatiites arc derived trench turbidites and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex fragmentation of these sequences resulted in broken formations and a tectonic melange in the schreiber assemblage of the belt d2 circa 2685 2680 ma is consistent with an intra arc right lateral transpressional deformation fragmentation and mixing of d2 synkinematic dikes and sills suggest that melange formation continued during d2 the d1 to d2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating the d2 intra arc strike slip faults may have provided conduits for uprising melts from the descending slab and they may have induced decompressional partial melting in the subarc mantle wedge to yield synkinematic ultramafic to felsic intrusions a similar close relationship between orogen parallel strike slip faulting and magmatism has recently been recognized in several phanerozoic transpressional orogenic belts suggesting that as in phanerozoic counterparts orogen parallel strike slip faulting in the schreiber hemlo greenstone belt played an important role in magma emplacement

evidence for mesoarchean 3 2ga rifting of the pilbara craton the missing link in an early precambrian wilson cycle abstract a sequence of c 3 2 ga low grade sedimentary rocks bimodal volcanic rocks and subvolcanic layered mafic ultramafic sills and dykes occur in greenstone belts that flank the margins of the 3 51 3 24 ga east pilbara terrane of the pilbara craton western australia u pb shrimp zircon data and sm nd model age data suggest that these rocks represent juvenile addition of material to the margins of the craton during a period of major extension accompanied by the local emplacement of granitic rocks this extensional event is also represented in adjacent terranes by thick basalt flows with c 3 2 ga zircon and sm nd model ages and by granitic rocks based on the available age data geochemistry of basaltic rocks and the distribution of this event in space and time within the craton we interpret the c 3 2 ga rocks to reflect widespread rifting of the protocraton and associated thinning of the lithosphere this event resulted in separation of the flanking kurrana and karratha terranes from their parent east pilbara terrane which represents the ancient nucleus of the craton these terranes were then reassembled during subsequent accretion at 3 07 2 905 ga combined the data suggest the preservation of a mesoarchean wilson cycle in the pilbara craton

anelasticity of the crust and upper mantle beneath the pacific ocean from the inversion of observed surface wave attenuation summary backus gilbert inversion theory is applied to a new set of surface wave attenuation data to obtain average q 1 models for the crust and upper mantle beneath the pacific ocean the favoured model is obtained from the inversion of rayleigh wave data alone it includes an increase in q 1 or decrease in q at depths corresponding to the thickness of the litho sphere as determined from surface wave velocity studies a thick zone of high q 1 or low q in the upper mantle and decreasing q 1 deeper in the mantle the mean q 1 values for the lithosphere are lower than those previously determined for the lower crust beneath continents simultaneous inversions of the love and rayleigh wave data yield models which include thin alternating regions of high and low q 1 values independent inversions of the love and rayleigh wave data yield models which differ in their anelastic properties at depths between 20 and 120km these latter two types of models probably reflect systematic errors in the love wave data due to higher mode interference or complexities along the propagation path

late cretaceous intraplate thrusting in central europe effect of africa iberia europe convergence not alpine collision a well established event of intraplate basin inversion and basement thrusting affected central europe in late cretaceous time it is widely accepted to have resulted from the collision of the alpine orogen with europe s margin at that time an early alpine orogen located on the leading edge of the adria microplate still lay far southeast of its present day position and had entered a phase of extension after a first orogenic event characterized by w to nw directed thrusting this configuration is not likely to have induced ssw nne directed thrusting and folding in the future european foreland that was still separated from the alpine wedge by a strip of oceanic lithosphere by contrast the onset of intraplate contraction coincides with an important change in relative motion between the european and african plates at ca 90 ma africa s sse directed sinistral transform motion relative to europe changed to ne directed convergence this agrees well with the timing and kinematics of intraplate thrusting in central europe structures of similar age and kinematics occurring in southern france spain and north africa suggest that the late cretaceous pulse of contraction was caused by pinching west central europe s thin lithosphere between baltica and africa only since the onset of n directed thrusting in the alps in paleocene or eocene time are the kinematics of the alps and their european foreland compatible indicating that mechanical coupling between africa europe and the adria microplate had been achieved

deep seismic survey images crustal structure of tornquist zone beneath southern baltic sea the tornquist zone is europe s longest tectonic lineament and bisects the continent in a nw se direction from the north sea off nw denmark to the black sea new deep seismic reflection and coinci

anisotropic models of the upper mantle long period rayleigh wave and love wave dispersion data particularly for oceanic areas have not been simultaneously satisfied by an isotropic structure in this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy anelastic dispersion sphericity and gravity we assume that the surface wave data represents an azimuthal average of actual velocities thus we can treat the mantle as transversely isotropic the resulting models for average earth average ocean and oceanic regions divided according to the age of the ocean floor are quite different from previous results which ignore the above effects the models show a low velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies the correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive for most of the earth sh sv in the vicinity of the low velocity zone neat the east pacific rise however sv sh at depth consistent with ascending flow anisotropy is as important as temperature in causing radial and lateral variations in velocity the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age this layer defines the lid or seismic lithosphere in the pacific the lid thickens with age to a maximum thickness of 50 km this thickness is comparable to the thickness of the elastic lithosphere the lid thickness is thinner than derived using isotropic or pseudo isotropic procedures a new model for average earth is obtained which includes a thin lid this model extends the fit of a prem type model to shorter period surface waves

structural evolution timing and tectonic style of the sole pit inversion abstract the nw se oriented sole pit high located in the western half of the southern north sea basin is the result of three different structural processes initiated during the late palaeozoic basin subsidence probably related to late hercynian pull apart movements led to continuous sedimentation from the permian to the middle jurassic mid jurassic doming of its eastern flank followed by erosion transtensional movements and differential subsidence until the early cretaceous reflect the mid and late kimmerian crustal distension and fragmentation of the north sea rift basinwide inversion occurred between the turonian and the campanian and was repeated in the oligocene resulting in a total uplift of some 1500 m the present structural style is represented by a complex network of transpressional and transtensional faults at permian rotliegend levels which are thought to represent the upward termination of a series of wrench induced flower structures with a right lateral sense of displacement these faults are detached by zechstein and triassic salt from more orderly transtensional faults systems at shallower levels in areas where salt is absent late kimmerian normal faults became reactivated in a reverse sense during inversion however the relationship of these shallow mappable tectonic components to the deeper pre permian structure of the sole pit high and to the underlying crustal configuration remains speculative

mm a variable period mantle magnitude this paper lays the theoretical groundwork for a variable period mantle magnitude mm based on the measurement of the spectral amplitude x of very long period rayleigh waves we retain the concept of magnitude by restricting ourselves to single station measurements ignoring the focal mechanism and the exact depth of the shallow earthquakes considered our measurements are made at a series of periods in all cases greater than 40 s and the largest value is retained this procedure effectively avoids the well known interference effects leading to saturation of magnitude scales defined at a fixed period two corrections are used a period dependent distance correction cd and a source correction cs also period dependent compensating for the variation of the excitation of rayleigh waves with period both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory the result is a formula of the type mm log10 x cd cs c0 in which all coefficients including the constant c0 are justifiable on sound theoretical grounds the analysis of a data set of 256 records from the broadband seismograph at papeete tahiti the ultra long period system at pasadena and stations of the geoscope network shows that the mean error in the estimation of the seismic moment is on the order of 0 1 0 2 units of magnitude with the standard deviation at each station also on the order of 0 2 units of magnitude no significant trend with either distance period or station can be identified the method can also be transposed to the time domain under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the pacific basin both versions of the method lend themselves well to automation thus by providing a real time estimate of the seismic moment of distant earthquakes mm has considerable potential for tsunami warning purposes its concept can easily be extended to love waves and also to intermediate and deep earthquakes

caledonian and proterozoic terrane accretion in the southwest baltic sea abstract a marine seismic reflection survey was carried out in 1996 by dekorp potsdam and bgr hannover in the sw baltic sea several tectonic lineaments were crossed nearly perpendicularly for example the caledonian deformation front which is assumed to mark the northern boundary of the terrane avalonia which accreted to baltica ca 400 ma here a bivergent collision is clearly observed confirming early ideas from the babel survey the ne dipping reflections in the uppermost mantle are interpreted as signs of the subducted tornquist ocean a similar tectonic style of compression and indentation is observed in the proterozoic crust northeast of bornholm where in addition a remarkable crustal thickening and a strong increase of reflection power is observed the three dekorp lines in this area provide a certain three dimensional control and allows extension of similar observations from the babel line a southward this proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern sweden either to the gothian thrust or the sveconorwegian front

tourmaline in a mesoarchean pelagic hydrothermal system implications for the habitat of early life abstract the rna world hypothesis requires the synthesis of rna to allow the emergence of life on earth hydrothermal systems have been proposed as potential candidates for constructing complex biomolecules however in order to successfully form rna it is necessary to stabilize ribose a rna carbohydrate component borate has been found to stabilize ribose therefore boron rich hydrothermal systems are important environments concerning the origin of life on earth the 3 2 ga dixon island formation of the west pilbara superterrane western australia is a volcano sedimentary sequence the formation represents a mesoarchean pelagic hydrothermal system which formed adjacent to an immature island arc fine grained tourmaline in addition to biogenic carbonaceous matter and spherulitic and tubular bacteriomorphs are found in black chert a boron rich environment was responsible for the formation of these deposits to explore the implications of such a boron enriched environment on microbial activity modes of occurrence and chemical compositions of the tourmaline were examined the tourmaline is schorl or dravite of the alkali tourmaline group and the boron isotope compositions range in 11b from 7 3 to 2 6 the tourmaline occurs in microcrystalline quartz matrix of black chert veins that cross cut a volcanic unit and also in a bedded black chert which overlays the volcanic unit the volcanic unit contains highly altered zones with hydrothermal veins the associated lithologic and stratigraphic features suggest that the black chert veins were the conduits for upward moving hydrothermal fluids which reached the sea floor subsequently the volcanic unit was covered by organic matter rich cherty sediments that in part were fed and or altered by the hydrothermal fluids these results suggest that the origin of boron enrichment to form dixon island tourmaline is not the associated sedimentary mineral assemblage which includes diagenetic clay low grade metamorphic mica and organic matter instead the tourmaline was directly precipitated from hydrothermal fluid enriched in boron furthermore the hydrothermal fluids had already concentrated the boron in the mesoarchean pelagic system prior to the apex of organic matter production and microbial activity our findings support a hypothesis that the boron enriched hydrothermal environment aided the survival and evolution of early life

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

40ar39ar laserprobe ages of metamorphic hornblendes from the coongan belt pilbara western australia abstract 40 ar 39 ar laser step heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the archaean coongan greenstone belt in the south eastern pilbara in western australia provide evidence that these rocks with no previous age constraints belong to the ca 3460 ma old warrawoona group the western margin of the belt is formed by an east dipping ductile shear zone the eastern margin is undeformed and shows a narrow contact metamorphic zone the coongan belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt the rocks in the centre of the belt are transected by north south striking retrograde shear zones one of which has been constrained at ca 2940 ma and may be related to the ca 2940 ma intrusive event in the eastern pilbara foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca 3170 ma which are interpreted as the age of recrystallisation after deformation deformed blue green hornblendes from the adjacent foliated amphibolites however give younger and inconsistent dates of ca 3090 ma and ca 3020 ma these dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone age spectra from blue green hornblendes from the western part of the coongan belt constrain the age of a regional metamorphic event at 3240 ma which compares well with the metamorphic hornblende ages of ca 3200 ma obtained in the western shaw belt by wijbrans and mcdougall the same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks which yield ca 3260 ma maximum ages in the higher temperature steps the age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south western corunna downs batholith is ca 3400 ma which is 70 90 ma younger than the north shaw granodiorites in the shaw batholith but considerably older than previously assumed that is ca 3300 ma all obtained ages fall within the expected range of ca 3 5 2 94 ga and generally increase from west to east through the coongan belt the effects of excess argon on the age spectra seems to be rather considerable in some of the experiments

3 1 ga tuff from the sholl belt in the west pilbara further evidence for d diachronous volcanism in the pilbara craton of western australia abstract an age of 3112 6 2 ma determined by conventional techniques on single zircons from a felsic volcanic rock from the sholl belt in the western part of the archaean pilbara craton of western australia is interpreted as the age of felsic volcanism this is about 100 ma older than felsic volcanic rocks in the nearby but unconnected whim creek belt and is significantly younger than felsic volcanism in the east pilbara which took place during two distinct episodes at 3450 ma and 3300 ma the present results rule out previous correlations between the felsic volcanic rocks of the sholt belt and the 3452 16 ma old duffer formation in the east pilbara

horizontal tectonics pre dating upper gorge creek group sedimentation pilbara block western australia abstract the greenstone belt stratigraphy of the c 3500 ma old pilbara block has been subdivided into the lower dominantly volcanic warrawoona group and the upper mainly sedimentary gorge creek group regional mapping indicated that these units evolved as a near continuous succession with only minor internal unconformities and that both groups suffered their most intense tectonic imprint together as a result of granite diapirism however an upper gorge creek conglomerate a lalla rookh correlate with a basal angular unconformity contains felsic schist clasts derived from highly strained portions of the warrawoona group the folds in the conglomerate are upright as expected for diapirically generated structures and are as complex as any attributed to this process the schist clasts were therefore probably generated in a pre doming event high strain and high grade metamorphism are confined to the lower parts of the greenstone stratigraphy where a polyphase deformation history may be clearly defined the low strain low grade areas are generally regarded as having a simpler tectonic history but some examples contain very tight minor folds that pre date a widespread subhorizontal cleavage and both events pre date the doming in other low grade areas thrust structures are truncated by the angular unconformity at the base of the lalla rookh correlate and the thrust geometry suggests they formed in already folded rocks

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

depth variation of carbon and oxygen isotopes of calcites in archean altered upperoceanic crust implications for the co2 flux from ocean to oceanic crust in the archean abstract middle archean greenstones with mid ocean ridge basalt affinity and overlying bedded chert banded iron formation bif are exposed in the cleaverville area pilbara craton western australia on the basis of the hydrothermal carbonation of these cleaverville greenstones we estimated the potential co2 flux from ocean to oceanic crust and the physical chemical conditions of the subseafloor hydrothermal system for the middle archean the greenstones exhibit various extents of carbonation and the igneous minerals contained in the greenstones are partly or completely replaced by calcite and other secondary minerals the degreeof carbonation correlates with stratigraphy the volume concentration of calcite in greenstones decreaseswith increasing depth below the chert bif horizon our results clearly indicate that the hydrothermal carbonation occurred along the axial zones of a middle archean mid ocean ridge both 13c and 18o values of calcite also change with the depth below the chert bif horizon the 18o value of calcite decreases with increasing depth reflecting increasing temperature while the 13c value of calcite changes from positive to negative stratigraphically downward a model reconstructing the 13c of calcite suggests that the observed isotopic variation reflects the thermal structure and that the carbon source for calcite was derived from seawater these mineralogical geochemical and geological features of calcite provide the amount of co2 in the whole oceanic crust per seafloor unit area 1 2 107 mol m2 based on this value co2 flux from the ocean to the oceanic crust in the middle archean can be estimated to be 1 5 1014 mol yr when the spreading rate m2 yr of archean oceanic crust is assumed to have been three times higher than it is today the estimated co2 flux into the oceanic crust is two orders of magnitude higher than the modern value which points to the significance of sea floor hydrothermal carbonation in the archean carbon cycle

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

archean oceanic flake tectonics assuming that the archean mantle was hotter and the oceanic crust thicker than at present we estimate that the archean oceanic lithosphere had a sandwich rheology with the lower crust behaving as a soft ductile layer accordingly the archean oceanic lithosphere had a rheological structure similar to present day continental lithosphere but unlike present day oceanic lithosphere the thickness of the ductile layer is a decreasing function of age while its strength is an increasing function of age the buoyancy problem in archean subduction could be overcome by delamination of oceanic upper crust but mass balance calculation indicates that most archean oceanic crust must have been recycled where the right dynamic conditions existed for delamination oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in greenstone belts that are characteristic of archean terrains

tectonic settings of archaean 3325 2775 ma crustal supracrustal belts in the west pilbara block abstract the archaean pilbara block comprises six fault bounded tectonostratigraphic domains the west pilbara block contains from east to west domains 4 5 and 6 review of stratigraphic magmatic structural and geochronologic data and new field mapping indicates that the crustal and supracrustal rock records correlate to three megacycles megacycle ivu 3325 3135 ma is recorded by an intra arc basin and arc granitoids in domain 6 and a back arc basin in domains 4 and 5 the domains were an arc continent orogen from 3200 3155 ma during which the outboard are setting was accreted to the back arc continental margin although domain 6 is allochthonous with respect to domains 5 and 4 it was not exotic with respect to the tectonic settings of the pilbara continent megacycle vl 3135 2955 ma is recorded by in decreasing age i a back arc basin and arc granitoids in domain 5 ii a deep marine retroarc or remnant ocean basin in domain 4 and a plutonic belt in domain 5 iii a plutonic belt in domains 5 and 6 and possibly also in domain 4 and iv craton wide deformation belts coeval with the whim creek basin and granitoid plutons in domain 5 the whim creek basin was a half graben that developed at a releasing bend or overstep along the boundary fault to domains 4 and 5 and to the northeast domains 4 and 6 the basin was closed by sinistral transpression as the boundary fault between domains 4 and 6 propagated to the southwest from the releasing bend and thereby generating a restraining bend geometry fundamental strike slip faults which are interpreted to have been tectonic escape responses to a cause external to the preserved pilbara block juxtaposed domains craton wide the initial stage of megacycle vu 2955 2775 ma is recorded by the eroded remnants of a rift province preserved elements include the negri basin domain 5 only mafic ultramafic sub volcanic layered intrusions and granitic plutons and related high level sills two periods of granitoid intrusion are preserved base metal sulphide deposits hosted by the whim creek basin were probably sourced from extensional geofluids in the rift province a plume origin is likely for the komatiitic parent magmas to sub volcanic intrusions and to basalts of the negri basin plume magmatism is interpreted to have been coincident with but not the cause of lithospheric extension because plumerelated volcanism post dates the first period of plutonic intrusion by 25 million years extension was followed by orogenic deformation that converted domains 4 5 and 6 to a hinterland fold thrust belt deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western pilbara margin collision orogeny was the last event of a long history of pacific type tectonics along the margins of the archaean pilbara continent

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

geophysical evidence for early permian igneous activity in a transtensional environment denmark abstract integrated interpretation of gravity magnetic and seismic data in the area of the strong silkeborg gravity high in central denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with permo carboniferous wrench faulting and the development of the oslo horn graben system reinterpretation of the deep seismic data from profile 2 of the eugeno s project in the area of the silkeborg gravity high detected a new thin high velocity layer 6 25 km s at about 7 km deep within the palaeozoic sedimentary sequence in the crystalline basement a separate body of high velocity 6 9 7 0 km s was found at 11 km deep beneath this body the moho reaches its shallowest position in the area the seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6 8 km this layer shows strong remanent and induced magnetization with a southward inclination of about 10 gravity reinterpretation of the area allows the separation of an upper body of density contrast 0 1 g cm3 from a lower body of density contrast 0 2 g cm3 the upper body roughly coincides with the magnetic body the lower body the major one coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area

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

regional dispersion of first order overtone rayleigh waves summary records of the first rayleigh overtone in the 70 110s period range following deep subduction zone events are analysed through standard surface wave techniques to retrieve dispersion characteristics in the oceanic environment our data are in general agreement with published models obtained from regionalization of fundamental rayleigh wave paths a general increase in phase velocity with age is observed with a somewhat smaller effect on group velocities continental and trench area data are processed through a regionalization scheme indicating that the difference between average continental and oceanic velocities both phase and group is about 0 2 km s l at periods of the order of 95 s an investigation of a set of simple models allowing structural heterogeneities between oceans and continents down to variable depth suggests that they probably do not extend past the bottom of the asthenosphere 250 km

phase and group velocities and q of mantle love and rayleigh waves of the first two modes and their azimuthal dependences for the 1963 kurile islands earthquake abstract phase and group velocities and q of mantle love and rayleigh waves from the 1963 kurile islands earthquake m w 8 5 were determined over 37 great circle paths by a time variable filtering technique in a period range 100 500 s for the fundamental modes and 100 275 s for the first higher modes the preliminary reference earth model prem explains reasonably well the average dispersion results for the fundamental love and rayleigh waves there exists a small but significant inconsistency between the observation and the model for the first higher love and rayleigh waves the q structure of prem is inconsistent with the observation for the fundamental love waves but explains other observations reasonably well the dispersion of each mode shows a clear azimuthal dependence from which the four azimuthal windows were established the phase and group velocity measurements for each window were in general shown to be mutually consistent the azimuthal variations are largest for the first higher rayleigh waves indicating strong lateral heterogeneity in the structure of the low velocity zone the first of the four windows is characterized by the largest fraction of precambrian shields and the second window by the largest fraction of normal oceans a comparison of these two windows may give some insight into deep lateral heterogeneity between continents and oceans the observed phase and group velocities of the first window are systematically higher than those of the second window for the fundamental love and rayleigh waves at periods up to 400 s and for the first higher love and rayleigh waves up to 175 s their differences are greatest for the first higher rayleigh waves and least for the fundamental rayleigh waves although the fundamental rayleigh waves show the least velocity differences their persistence up to a period of longer than 300 s is in striking contrast with some of the pure path phase velocities derived earlier for continents and oceans a set of models for continents and oceans pem c and pem o are not consistent with our observation the third azimuthal window is characterized by trench marginal seas and the fourth window by mountainous areas typically the asian high plateaus from northern china to the middle east through tibet a comparison of these two windows gives some information about deep structural differences between subduction zones and continental collision zones both belonging to plate convergence zones for the fundamental and the first higher love waves the phase and group velocities for the third window are markedly low whereas those for the fourth window are somewhat comparable to those for the second window slow rayleigh waves are evident for two windows with the fourth window apparently being the slowest for the fundamental rayleigh above 200 s and for the first higher rayleigh for the fundamental rayleigh waves the third window is very slow below 200 s but becomes progressively fast as the period increases and tends to be the fastest window around 400 s suggesting a deep seated high velocity anomaly beneath trench marginal seas the dispersion characteristics of the fourth window indicate a thick high velocity lid with an extensive low velocity zone beneath it the shield like lithosphere coupled with an extensive low velocity zone may be a characteristic feature of continental collision zones the particle motion of the fundamental love waves was found not to be purely transverse to a great circle connecting the epicenter to a station the departure from the purely transverse motion is systematic among different periods different g arrivals g 2 g 3 and different stations which may be interpreted as being due to lateral refraction

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

on the metamorphic history of an archaean granitoid greenstone terrane east pilbara western australia using the40ar 39ar age spectrum technique abstract age spectrum analyses of blue green hornblendes from amphibolites from the western shaw belt east pilbara western australia indicate an age of at least 3200 ma for early regional metamorphism ages on hornblende and muscovite from the narrow contact zone with the adjacent yule batholith probably date updoming of the granitoid gneiss terranes at 2950 ma hornblendes from within the shaw batholith and from a contact zone of a post tectonic granitoid yield ages of 2840 2900 ma indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post tectonic granitoids the preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the late archaean suggesting that updoming occurred during a period of rapid tectonism

40ar 39ar age constraints on tectonothermal events in the shaw area of the eastern pilbara granite greenstone terrain w australia 700 ma of archean tectonic evolution abstract the shaw area in the eastern pilbara craton consists of granitoids with intrusion ages ranging from 3470 ma to 2850 ma and supracrustal rocks with ages ranging from 3515 ma to 3100 ma as determined by u pb and pb pb dating methods in this terrain several major deformation events have recently been recognized including late strike slip deformation compression and early extension in order to provide time constraints for these events twenty four samples from the shaw area have been used for 40ar 39ar step heating experiments twenty two hornblendes one muscovite one biotite resulting in spectra defining ages between 3520 and 2800 ma the late strike slip event occurred prior to 2944 9 ma the argon cooling ages provide time constraints for the compressional event between 3325 ma and 3200 ma cooling ages between 3300 and 3200 ma cannot be related to granitoid intrusion in this area and are interpreted as cooling after regional metamorphism related to the compressional event the best time constraint from 40ar 39ar ages for the split rock shear zone a major ductile detachment in the shaw batholith associated with early extension is that it was last active prior to 3222 13 ma a 3341 13 ma plateau age of a metabasalt sampled in contact with granites of the northwestern corunna downs batholith provides a minimum age for granitoid intrusion and a minimum age for the warrery shear zone hornblendes from the black range dolerite dyke gave a plateau age of 2796 9 ma providing a constraint on the time of cratonization of the pilbara granitoid greenstone terrain most of the cooling ages including a 3466 13 ma age for the contact zone of the north shaw granodiorite are related to intrusion of granitoids in the shaw batholith a large number but significantly not all of the sample population has been reset during the intrusion of late to post tectonic adamellites between 3050 ma and 2850 ma preservation of such a large range 700 ma in argon isotopic ages in a relatively small area is unusual the resetting of argon ages in the granitoid domain is best explained by a combination of preferential intrusion of younger granites and the difference in heat conductivity of granite versus basalt in the greenstone domain the resetting effect is more localized into a network of fluid flow channels mostly situated in shear zones

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

structure and tectonic evolution of the tornquist zone and adjacent sedimentary basins in scania and the southern baltic sea area abstract southernmost sweden bornholm and the surrounding baltic sea region are located on a large scale releasing bend in the dextral strike slip system of the tornquist zone with its resulting pull apart basins the well constrained geology of scania and bornholm has been combined with detailed on and offshore borehole data and three proprietary marine seismic surveys this in conjunction with supplementary babel deep seismic reflection findings allows a combined 3d interpretation of sediment structure interactions as a result a regional interpretation has emerged which gives a new understanding of the interplay between structural movement on a complex strike slip fault system tornquist zone and its intrazonal depressions vomb trough and colonus shale trough as well as the sedimentation history of associated areas of sediment accumulation ronne and arnager grabens hollviken halfgraben hano bay basin and skurup platform detailed sequential litho and seismo stratigraphic descriptions have been possible by combination of the various data sets this resulted in the clarification or recognition of previously unknown structural limits to sub basins and highs in the study area a 3d chronological 4d model for the development of the region is proposed this model takes into account the long lived structural history combining elements of strike slip extension and inversion tectonics the deep seated faulting controlling these structures is integrated with the deep structure as revealed by the babel line in this area

late archaean crustal extension sedimentary basin formation flood basalt volcanism and continental rifting the nullagine and mount jope supersequences western australia abstract the mount bruce megasequence set formerly the mount bruce supergroup is a succession of volcanic and sedimentary rocks that was deposited on the pilbara craton of western australia in the late archaean to early proterozoic its history can be described and explained in terms of modern style sequence stratigraphy and of phanerozoic style plate tectonics the megasequence set comprises the chichester range megasequence that was deposited on a rifted to divergent continental margin and the overlying hamersley range megasequence that was deposited on a convergent continental margin the chichester range megasequence comprises in order of superposition the nullagine and mount jope supersequences and the marra mamba supersequence package the nullagine and mount jope supersequences are each the rock record of one phase of a protracted two phase late archaean 2770 2700 ma continental break up the nullagine supersequence consists of the mount roe sequence which comprises mostly subaerial basalt and the overlying hardey sequence package which comprises mostly terrigenous sedimentary rocks that were deposited in extensional sedimentary basins the mount jope supersequence consists of in order of superposition the kylena sequence which comprises mostly basalt the tumbiana sequence which comprises mostly tuff and tuffaceous rocks and the maddina sequence package which comprises mostly basalt the nullagine supersequence was deposited during an episode of wnw ese directed crustal extension that possibly led to the development of a rift to the west of the present pilbara craton the mount jope supersequence was deposited during the subsequent development of a wnw ese trending rift the position of which approximates the present southern craton margin regional scale uplift was associated with the development of both supersequences particularly with the nullagine supersequence basement strike slip faults were reactivated as dip slip structures during crustal extension and controlled the locations of some hardey sequence package extensional sedimentary basins nullagine supersequence and mount jope supersequence rifting led to a divergent continental margin bordering an ocean along the south and probably the far west of the craton post rift thermal subsidence was accompanied by the accumulation of mudrock carbonate sedimentary rock and iron formation of the marra mamba supersequence package in a shelf setting the implications of these interpretations are that at 2800 ma the pilbara craton was significantly larger than at present and that it had a sufficiently thick subcontinental lithosphere that divided it in a manner analogous to the break up of phanerozoic continents the nullagine and mount jope supersequences contain three stacked flood basalt successions which were erupted over a time period of between of 55 and 80 m y stacked flood basalt successions covering such a long time period are not known in the phanerozoic and are tangible evidence of a hotter late archaean mantle

structural evolution of the middle archean coastal pilbara terrane western australia 1 the middle archean coastal pilbara terrane is composed of two lithotectonic units the karratha and cleaverville roebourne supercomplexes the karratha supercomplex lies tectonically beneath the cleaverville roebourne supercomplex and consists of 3270 3250 ma granitic batholiths and metamorphosed terrigenous sediments it is identified as a juvenile continental crust with tonalite trondhjemite granodiorite type granites the cleaverville roebourne supercomplex is interpreted as a dismembered immature island arc which has a restored crustal thickness of 20 km this supercomplex consists of 3195 ma bimodal volcanic chemical sedimentary sequences metabasite low k granitic gneiss and peridotite regionally distributed granite porphyries circa 3020 3000 ma have intruded all of the coastal pilbara terrane detailed structural analyses and zircon u pb dating suggest that the coastal pilbara terrane records two phases of deformation the first phase is an immature island arc and micro continental crust collision and is represented by top to the northwest thrusting d1 circa 3100 3020 ma this phase of deformation an arc continent collision appears to represent an important transition in the formation of continental crust the 3020 ma granite porphyry event may be related to syn post collision igneous activity and may reflect a second crustal thickening associated with magmatic underplating the second phase of deformation is characterized by regional scale left lateral strike slip faulting d2 circa 3020 2925 ma and by more localized right lateral strike slip faulting d3 circa 2925 2770 ma this phase of craton scale strike slip faulting may represent stabilization of the newly formed continental crust

variation with age of the deep structure of the pacific ocean inferred from very long period rayleigh wave dispersion the propagation of rayleigh waves along 44 direct paths across the pacific ocean is studied and a regionalization of the dispersion according to the age of the sea floor is performed the group velocity dispersion curves characteristic of each one of four age regions considered are obtained for periods up to 300 s these curves show an increase of group velocities with age over the whole period range considered the shear wave velocity models inferred yield a lithosphere thickening with age and show the presence below the lithosphere of two zones where negative velocity anomalies decrease with increasing age the low velocity zone and a zone at a depth of 350 km

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

the cleaverville group in the west pilbara coastal granitoid greenstone terrain of western australia an example of a mid archaean immature oceanic island arc succession abstract the 4400 m thick volcanosedimentary succession that forms the 3200 3000 yr old cleaverville group is well preserved along the western coastline of the pilbara in northwestern western australia two lithological assemblages namely the volcanic rock assemblage and the chemical volcanosedimentary rock assemblage are identified in the cleaverville group the volcanic rock assemblage is approximately 3300 m thick and consists mainly of pillow basalt and rhyolitic flow rocks the chemical volcanosedimentary rock assemblage which is at least 1100 m thick comprises chemogenic sedimentary rocks chert and banded iron formation and volcanosedimentary rocks coarse and fine grained tuffs three volcanosedimentary cycles have been recognized from stratigraphic patterns displayed by the cleaverville group each cycle involved an initial episode of basaltic volcanism that was followed by rhyolitic volcanism and then by chemogenic sedimentation boundaries between each cyclic package of basalt rhyolite and chemogenic sedimentary rocks are conformable surfaces the thickness of volcanic rocks relative to chemogenic sedimentary rocks within each package decreases upwards within the succession the upward decrease in the proportion of volcanic rocks in each package suggests that bimodal volcanic activity at the depositional site decreased in intensity through time basalts are primarily non vesicular whereas chemogenic sedimentary rocks are very finely laminated and lack evidence of continental derivation these features imply that the cleaverville group was deposited in an offshore probably deep sea environment the lizard hills formation lies above an angular unconformity on the cleaverville group the formation records the first appearance in the west pilbara coastal granitoid greenstone terrain of sedimentary rocks that are of a continental origin the cyclic record of bimodal volcanism and accompanying volcanogenic sedimentation within an oceanic setting suggests that the cleaverville group originated in a tectonic environment similar to a modern immature oceanic island arc furthermore it is possible that continentally derived sedimentary rocks of the lizard hills formation record an arc continent collision event

early tertiary structural inversion in the stord basin norwegian north sea the stord basin is an east sloping extensional basin that lies to the east of the viking graben in norwegian waters of the northern north sea the basin formed by extension with normal faulting beginning in triassic time and continuing episodically into the middle and late jurassic seismic reflection profiles show that the normal faults which affect the sedimentary section are steeply dipping and approximately planar some of these faults were reactivated in the early tertiary this reactivation reversed earlier accumulated normal displacement and created relative structural highs out of previous structural lows the most obvious evidence of fault reactivation and structural inversion is the presence of hanging wall folds that parallel the strike of reactivated faults the inverted structures of the stord basin are part of a widespread field of early tertiary inverted structures in north west europe we suggest that these structures formed in response to horizontal compression that resulted from the interaction of stress fields set up by alpine continent continent collision and the early opening history of the north atlantic ocean

shear velocity and shear attenuation models inverted from the world wide and pure path average data of mantle rayleigh waves 0s25 to 0s80 and fundamental spheroidal modes 0s2 to 0s24 summary global and regional structures of the earth s mantle are studied original phase velocities and q 1 of rayleigh waves are converted to equivalent free oscillation periods and q 1 global average upper mantle shear velocity models are derived from the inversions of the world wide average eigenperiods corrected for attenuative dispersion resolution analysis shows that the thickness and velocity of the lithosphere cannot be determined from our data set inverted models have a well developed low velocity zone in the depth range between 70 and 280 km global average q 1 models for the whole mantle are derived from the world wide average q 1 of mantle rayleigh waves and fundamental spheroidal modes the resolution analysis indicates that q 1 at depths shallower than about 100 km cannot be determined by our data set and that the depth resolution for q 1 of the lower mantle is poor all inverted q 1 models are characterized by a well developed high q 1 zone in the depth range between about 100 and 300 km in the upper mantle q has a minimum value of about 90 at depths around 200 km begins to increase at a depth of about 300 km rapidly increases in the depth range from 300 to 400 km and gradually increases at depths between 400 and 800 km q of the lower mantle is about 330 the boundary between the high q 1 upper mantle and low q 1 lower mantle is sharper than has been suggested in the past inverted q 1 models predict qscs 226 to 250 the earth s surface is divided into mountainous oceanic ridge shield and trench and marginal sea regions and a regionalized shear velocity model of the upper mantle is derived the inversions of the pure path eigenperiods without correction for attenuative dispersion show that strong lateral variations in shear velocity exist at depths shallower than 250 km in the upper mantle a low velocity zone for shear waves exists in the upper mantle beneath all the tectonic regions except perhaps the shield region if the sn velocity of the starting model is equal to or higher than 4 7 km s 1 the shield data require a moderate low velocity zone data from the mountainous region require a much lower shear velocity than the shield region at depths shallower than 250 km the oceanic ridge and trench and marginal sea models have a well developed low velocity zone the shear velocity model for the trench and marginal sea region is characterized by a velocity increase at depths between 250 and 400 km this increase in velocity is interpreted as a consequence of the subducted lithospheric slabs beneath many of our trench and marginal sea regions regional differences in rayleigh wave q 1 are found for modes from 0s25 to 0s45 since the resolving power of this data set is limited we could not locate the depths at which regional variations of q 1 exist since the oceanic q 1 for modes from 0s45 to 0s78 appear to be stable the oceanic q 1 model is derived this model has a well developed high q 1 zone q has a minimum value of about 110 at depths between 200 and 300 km and rapidly increases to a high value of about 250 at a depth of about 400 km

structural development of an archean orogen western point lake northwest territories the point lake orogen in the central archean slave province of northwestern canada preserves more than 10 km of structural relief through an eroded antiformal thrust stack and deeper anastomosing midcrustal mylonites fault restoration along a 25 km long transect requires a minimum of 69 km slip and 53 km horizontal shortening in the western part of the orogen the basal decollement places mafic plutonic volcanic rocks over an ancient tonalitic gneiss complex ten kilometers to the east in the keskarrah bay area slices of gneiss unroofed on brittle thrusts shed molasse into several submerged basins conglomerates and associated thinly bedded sedimentary rocks are interpreted as channel levee and overbank facies of this thrust related sedimentary fan system the synorogenic erosion surface at the base of the conglomerate truncates premetamorphic or early metamorphic thrust faults formed during foreland propagation while other thrusts related to hinterland progressing imbrication displace this unconformity tightening of synorogenic depositional troughs resulted in the conglomerates present localization in synclines to the west of associated thrust faults and steepening of structural dips eastern parts of the orogen consist of isoclinally folded graywackes composed largely of mutti and ricci lucchi turbidite facies b c and d interpreted as submarine fan deposits eroded from a distant volcanic arc thrust faults in the metasedimentary terrane include highly disrupted slate horizons with meter scale duplex structures and recrystallized calcmylonites exhibiting sheath folds and boudin trains with very large interboudin distances the sequence of fabric development and the overall geometry of this metasedimentary terrane strongly resembles younger forearc accretionary prisms conditions of deformation along the thrusts parallel the regional metamorphic zonation amphibolite facies in the basal decollement through greenschist facies shear zones to cataclastic crush zones in the region of emergent thrusts in keskarrah bay depth differences can account for only half of the metamorphic gradient thermal profiles which increased downwards in obducted greenstone belts and synthrusting plutonism explains other high metamorphic gradients a tectonic model involving the collision of an accretionary prism with a continental margin best explains the structural and sedimentological evolution of the orogen

palaeoproterozoic thermal events recorded in the 4 0 ga acasta gneiss canada evidence from shrimp u pb dating of apatite and zircon u pb isotopes of apatites and zircons extracted from a sample of the acasta gneisses that occurs along the western margin of the slave craton in northwest territories of canada have been measured using a shrimp ii ion microprobe eleven apatite analyses 30 m spot give 238u 204pb 206pb 204pb and 204pb 206pb 207pb 206pb isochron ages of 1905 86 ma 2 and 1936 28 ma 2 respectively analyses 20 m spot on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238u 206pb 207pb 206pb concordia plot with concordia intercepts at 4014 25 ma 2 and 1967 93 ma 2 the former agrees with 3 96 4 02 ga protolith ages obtained on the oldest components of the acasta gneisses by other workers the lower concordia intercept for the zircon data is consistent with the u pb age of apatites and may be related to a 500 c thermal event perhaps early in the palaeoproterozoic wopmay orogeny