revisiting the age and paleomagnetism of the modipe gabbro of south africa abstract the modipe gabbro is a precambrian intrusive unit hosted by the kaapvaal craton of south africa and provides one of the oldest records of geomagnetic field behavior in the late archean earlier paleomagnetic research into the modipe gabbro provided valuable insight into its magnetic properties and mineralogy but predate the development of paleomagnetic tools that are important in unraveling potentially complex histories of precambrian rocks further a lack of information about the precise age of the intrusion prevented this earlier paleomagnetic study from being used to evaluate the tectonic history of the kaapvaal craton here we present a comprehensive paleomagnetic and geochronologic study of the modipe gabbro that complements and builds upon the existing work the emplacement age of the gabbro 2784 0 1 0 ma established using the u pb id tims method on baddeleyite links the intrusion to nearby igneous bodies in the kaapvaal craton establishing one of the oldest fully differentiated magmatic events in the geologic record the precise age of the modipe gabbro and field observations about its strike allow us to perform a tilt correction using the both the attitude and paleomagnetism of the nearby derdepoort basalts the tilt corrected data allow a comparison of kaapvaal craton s paleolatitude with that of the pilbara craton at 2 78 ga with implications for the existence of the supercraton vaalbara

2 6 ga high si rhyolites and granites in the kursk domain eastern sarmatia petrology and application for the archaean palaeocontinental correlations abstract a 2 61 ga intraplate magmatism took place locally within the kursk domain of the eastern sarmatia forming a volcanic plutonic association of rhyolites and granites the zircon lu hf and sm nd whole rocks isotope data indicate the crustal source heterogeneity of these neoarchaean intraplate granitoids their protoliths were dominated by palaeoarchaean metapelites and mesoarchaean ttg associations the neoarchaean rhyolites of the kursk domain are characterized by the high crystallization temperatures of c 900 c according to the data of mineral geothermobarometry and geochemistry of zircons tiq and tizr the magmatic chambers were located at a depth of no more than 7 km the silicic melt parental for these 2 61 ga old rhyolites and granites could be formed as a result of 20 dehydration melting of the metapelite ttg mixture in equilibrium with the ilm bt opx pl restite at a pressure of no more than 4 kbar the atamansky complex granites crystallized at deeper levels and contain an admixture of opx pl restite or cumulus component this caused their geochemical differences from rhyolites when these phases were removed during the melts migration to higher levels of the crust the atamansky granites are considered as intermediate chambers where differentiation of the initial silicic magmas took place with the formation of the residual rhyolitic melts the heat source could be a mafic plume magmatism known in the eastern sarmatia there in intraplate settings hot deep plume generated mafic magmas intruded into the crust forming shallow interim chambers the latter initiated melting of the felsic host rocks predominantly metapelites and ttg in the upper parts of the crust producing the silicic volcanic plutonic association the simultaneous 2 61 ga old granitoid intraplate magmatism in the eastern sarmatia and dharwar craton caused by the mafic heat source adds one more argument in favor of an assumption that the sarmatia pilbara kaapvaal and possibly dharwar cratons composed a single supercontinent vaalbara in the neoarchaean the dharwar craton perhaps was at the periphery of the vaalbara supercontinent next to the eastern sarmatia

zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

3d modeling of crustal shortening influenced by along strike lithological changes implications for continental collision in the western and central alps abstract the european alps show major along strike variations in the partitioning of crustal shortening between the upper and the lower plates either explained by lateral variation in temperature and rheology or by along strike changes in crustal scale lithology here we present the first 3d thermomechanical geodynamic model to test the potential impact of along strike lithological and associated rheological changes and lateral temperature variations on crustal shortening during continental collision in the western and central alps numerical modeling results suggest a minor impact of lateral thermal variations and lithological changes in the lower plate for the alpine orogeny in contrast lateral lithological changes in the upper plate structure defined by the presence of the buried ivrea body in the western alps should play a major role in determining the present day architecture of the alpine belt our models suggest that the ivrea body acts as a strong rigid buttress constraining crustal shortening to the lower plate promoting the formation of the jura fold and thrust belt and supporting the higher surface elevation observed in the western alps on the other hand in the central alps where the ivrea body is absent much broader distribution of crustal shortening results in lower surface elevation numerical modeling results also suggest that crustal shortening in the central alps is focused within the upper plate during the final stages of continental collision favored by the indentation of thickened upper crust of the lower plate and by detachment of the adriatic upper crust from the underlying lithosphere

precambrian mafic dyke swarms in the singhbhum craton eastern india and their links with dyke swarms of the eastern dharwar craton southern india discussion

prefolding and overprint magnetic signatures in precambrian 2 9 2 7 ga igneous rocks from the pilbara craton and hamersley basin nw australia a total of 446 oriented samples have been collected from an archaean intrusion and the fortescue group volcanics and intrusions in the pilbara and hamersley regions of northwestern australia the paleomagnetism and rock magnetism of these samples have been investigated using susceptibility and hysteresis properties and both alternating field af and thermal demagnetization the most consistent paleomagnetic results were achieved by first applying af demagnetization and then stepwise thermal demagnetization evidence of the effects of lightning are widespread which accounts for the efficacy of this particular demagnetization strategy application of the fold test has resulted in placing significant constraints on the acquisition ages of some of the magnetizations and their corresponding pole positions a prefolding pole position from the millindinna complex is latitude 11 9 s longitude 161 3 e dp 6 8 dm 8 4 and is correlated with a radiometric age of 2 86 0 02 ga folding in the millindinna complex occurred prior to the deposition of the fortescue group new geochronological data indicate that the extrusion of the flood basalts commenced about 2 8 ga this provides a minimum age for the magnetization of the millindinna complex deformation responsible for the formation of the mobile belt separating the pilbara and yilgarn blocks during the ophthalmian orogeny has affected the fortescue group and produced a moderate degree of folding in the southern part of the hamersley basin in the region of millindinna complex outcrop the fortescue group is generally flat lying prefolding pole positions from the fortescue group are for the mount roe basalt latitude 52 4 s longitude 178 0 e dp 6 4 dm 9 1 and for the mount jope volcanics latitude 40 5 s longitude 128 7 e dp 19 9 dm 20 8 the overprint magnetization in the mount jope volcanics yields a pole position latitude 35 0 s longitude 211 5 e dp 2 8 dm 5 5 and is interpreted as having been acquired during folding relating to the formation of the ophthalmian fold belt

a crustal scale cross section of the south western alps combining geophysical and geological imagery a geotransect across the south western alps from the pelvoux massif external french alps to the dora maira massif internal italian alps through the monviso ophiolitic complex was investigated in the framework of the geo france 3d alpes programme a new interpretative crustal scale section across the south western alps is proposed combining geological and geophysical 2d 3d data the apulian mantle i e the ivrea body might be divided into two rigid pieces separated by the downward prolongation of the penninic frontal thrust these mantle indenters drive the decoupling of the european crust beneath the high to ultra high pressure metamorphic nappes the deep structure results from the stacking of crustal slices extracted from the european lithosphere the proposed struc tural model provides a basis for discussing the evidence of the crustal scale partitioning of the current strain pattern as well as the location of the seismicity

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

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

emplacement ages of paleoproterozoic mafic dyke swarms in eastern dharwar craton india implications for paleoreconstructions and support for a 30 change in dyke trends from south to north abstract large igneous provinces lips and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents the dharwar craton of southern peninsular india represents a substantial portion of archean crust and has been considered to be a principal constituent of superia sclavia nuna columbia and rodinia supercontinents the craton is intruded by numerous regional scale mafic dyke swarms of which only a few have robustly constrained emplacement ages through this study the lip record of the dharwar craton has been improved by u pb geochronology of 18 dykes which together comprise seven generations of paleoproterozoic dyke swarms with emplacement ages within the 2 37 1 79 ga age interval from oldest to youngest the new ages integrated with u pb ages previously reported for the hampi swarm define the following eight swarms with their currently recommended names ne sw to ese wnw trending ca 2 37 ga bangalore karimnagar swarm n s to nne ssw trending ca 2 25 ga ippaguda dhiburahalli swarm n s to nnw sse trending ca 2 22 ga kandlamadugu swarm nw se to wnw ese trending ca 2 21 ga anantapur kunigal swarm nw se to wnw ese trending ca 2 18 ga mahbubnagar dandeli swarm n s nw se and ene wsw trending ca 2 08 ga devarabanda swarm e w trending 1 88 1 89 ga hampi swarm nw se ca 1 79 ga pebbair swarm comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features such as regional dharwar greenstone belts and the late archean ca 2 5 ga closepet granite batholith have led to the hypothesis that the northern dharwar block has rotated relative to the southern block by restoring a 30 counter clockwise rotation of the northern dharwar block relative to the southern block we show that pre 2 08 ga arcuate and fanning dyke swarms consistently become approximately linear two possible tectonic models for this apparent bending and concomitant dyke rotations are discussed regardless of which deformation mechanisms applies these findings reinforce previous suggestions that the radial patterns of the giant ca 2 37 ga bangalore karimnagar dyke swarm and probably also the ca 2 21 ga anantapur kunigal swarm may not be primary features

geochronological constraints on evolution of singhbhum mobile belt and associated basic volcanics of eastern indian shield abstract the singhbhum mobile belt smb of the eastern indian shield represents a roughly east west trending arcuate belt of folded supracrustals overlying the granite greenstone basement of the singhbhum orissa craton along its northern eastern and western margins and is bounded by the chotanagpur gneissic complex to further north the radiometric ages of the basement singhbhum and equivalent granites and the intrusive anorogenic mayurbhanj granite pluton constrain the time of evolution of this mobile belt between 3 12 and 3 09 ga hence the smb supracrustals also known as singhbhum group is late mesoarchaean in age and not proterozoic as thought earlier the evolution of the smb was followed by emplacement of some major basic igneous rocks within or adjacent to the supracrustals these include simlipal volcanics at 3 09 ga on the smb mayurbhanj gabbro along with mayurbhanj granite at 3 09 ga along the marginal part of the craton near the smb and the dalma volcanics on the smb along with the dhanjori volcanics adjacent to smb at 2 80 ga the 2 80 ga old basic volcanics is also associated with emplacement of some small granite plutons occurring along the marginal part of the craton one of them the tamperkola granite intrudes the smb the 3 09 ga onward igneous activities along the marginal part of singhbhum orissa craton took place essentially under anorogenic tectonic setting before being affected by a major metamorphism at 2 50 ga which is recorded on the dalma volcanics and on some small granite pluton occurs along the marginal part of the craton the jagannathpur and stratigraphically equivalent malangtoli volcanics occurring within the singhbhum orissa craton at the west were erupted at 2 25 ga the boundary between the smb supracrustals and the singhbhum orissa craton is demarked by a prominent shear zone known as the singhbhum shear zone which shows multiple reactivation the oldest being at 3 09 ga followed by subsequent reactivation during palaeo and mesoproterozoic periods at 2 2 1 8 1 6 1 5 1 4 and 1 0 ga respectively the singhbhum group and the adjacent chotanagpur gneissic complex appear to have evolved from a near shore syn rift and a distal post rift stable shelf sedimentary assemblages respectively which were deposited without any stratigraphic break in a marine basin existed in the present north of the singhbhum orissa craton both of these assemblages were deformed and metamorphosed together during proterozoic at 2 5 to 2 3 ga 1 6 ga and 1 0 ga

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

paleomagnetic study of 1765 ma dyke swarm from the singhbhum craton implications to the paleogeography of india abstract we report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765 3 1 0 ma wnw ese trending dyke swarm from singhbhum craton this pole has been used in this study to propose the paleogeographic reconstruction of india with baltica craton and north china craton incremental alternating field af and thermal demagnetization isolated high coercivity components with north to north westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes the primary origin of the chrm is supported by the positive baked contact test the wnw ese trending dykes yield a mean paleomagnetic direction with a declination 329 2 and an inclination 22 8 k 31 6 95 9 3 the positive bake contact test proves the primary nature of remanence the pole position of singhbhum craton at 1765 ma is 45 n 311 e dp 5 2 and dm 9 9 paleogeographic reconstruction at ca 1770 ma supported by geological tectonic and metallogenic evidences indicate that the baltica craton and india linkage can be stable for at least 370 ma 1770 1400 ma there is also reasonable evidence in support of india north china craton spatial proximity at 1770 ma

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

u pb hf isotope systematics of detrital zircons in high grade paragneisses of the ancient gneiss complex swaziland evidence for two periods of juvenile crust formation paleo and mesoarchaean sediment deposition and 3 23 ga terrane accretion abstract we present the first results of combined u pb and lu hf isotope analyses of detrital zircon grains in high grade paragneisses luboya kubuta granulites from the ca 3 66 3 22 ga ancient gneiss complex agc in swaziland uranium pb ages demonstrate that sedimentation along the se margin of the proto kaapvaal craton took place during two time intervals during the paleoarchaean ca 3 53 and 3 43 ga and mesoarchaean ca 3 22 and 3 16 ga the hf isotope data show that the hinterland to the paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca 3 53 ga hf 3 53ga values between 0 7 and 4 1 and by reworking of more ancient crust t dm 3 8 3 6 ga in contrast detrital zircons in mesoarchaean metasediments record juvenile crust formation at 3 32 ga hf 3 32ga values between 1 5 and 6 0 t dm 3 45 3 32 ga followed by internal reworking of this crust until 3 22 ga a small number of detrital zircons indicate limited input from older paleoarchaean agc crust compared to sediments of the barberton greenstone belt i e fig tree and moodies group the mesoarchaean metasediments of swaziland are more juvenile in character and show greater complexity in their detrital zircon age spectra pointing towards a different provenance our new u pb hf isotope data together with field and petrological information from the study area suggest that the paleoarchaean metasediments were largely derived from the agc whereas the mesoarchaean metasediments were derived from a ca 3 45 3 32 ga juvenile island arc terrane located se of the agc which became accreted onto the kaapvaal craton at ca 3 23 ga i e the onset of burial and high grade metamorphism in the area

burial and exhumation across the alps apennines junction zone constrained by fission track analysis on modern river sands terra nova 24 221 226 2012 abstract we applied fission track analysis on modern river sands to constrain the cenozoic evolution of the alps apennines junction zone west of the villalvernia varzi ottone fault samples yielded major peaks at 33 36 and 62 65 ma showing that apatites in the eroded wedge top successions were largely unreset to the east samples yielded prominent peaks at 4 0 4 7 6 5 6 7 and 19 ma probably formed by reset ages our results indicate that the alpine orogenic wedge originally tapered out to the south the northern apennines were buried beneath a thick and widespread epiligurian cover now almost completely eroded and experienced major uplift and erosional exhumation since early pliocene times at average rates of 1 km ma 1 a large amount of alpine derived detritus previously stored on top of the orogenic wedge was transferred into the po plain sediment recycling thus overwhelmed any eventual effect of climate change on plio quaternary sedimentation rates south of the alps

archean granitoids at the contact of eastern ghat granulite belt and singhbhum orissa craton in bhuban rengali sector orissa india abstract the small granite plutons occurring at the contact of the singhbhum orissa iron ore craton ioc to the north and the eastern ghat granulite belt eggb to the south in eastern indian shield are characterised by the presence of enclaves of the granulites of eggb and the greenschist facies rocks of ioc these granites also bear the imprints of later cataclastic deformation which is present at the contact of the ioc and the eggb in situ pb pb zircon dating of these granites gives minimum age of their formation 2 80 ga a whole rock three point rb sr isochron age of this rock is found to be 2 90 ga therefore the true age of formation of these granites will be around 2 90 2 80 ga these granitic rocks also contain xenocrystic zircon components of 3 50 ga and show a later metasomatic or metamorphic effect 2 48 ga obtained from the analyses on overgrowths developed on 2 80 ga old zircon cores the presence of granulitic enclaves within these contact zone granite indicates that the granulite facies metamorphism of the eggb is 2 80 ga or still older in age the cataclastic deformations observed at the contact zone of the two adjacent cratons is definitely younger than 2 80 ga and possibly related to 2 48 ga event observed from the overgrowths as 2 80 ga granite plutons of small dimensions are also observed at the western margin of the ioc it can be concluded that a geologic event occurred 2 80 ga over the ioc when small granite bodies evolved at the marginal part of this craton after its stabilisation at 3 09 ga

geological environment of the cigar lake uranium deposit the cigar lake uranium deposit occurs within the athabasca basin of northern saskatchewan canada like other major uranium deposits of the basin it is located at the unconformity separating helikian sandstones of the athabasca group from aphebian metasediments and plutonic rocks of the wollaston group the athabasca group was deposited in an intra continental sedimentary basin that was filled by fluviatile terrestrial quartz sandstones and conglomerates the group appears undeformed and its actual maximum thickness is about 1500 m on the eastern side of the basin the detrital units correspond to the manitou falls formations where most of the uranium deposits are located the lower pelitic unit of the wollaston group which lies directly on the archean basement is considered to be the most favourable horizon for uranium mineralization during the hudsonian orogeny 1800 1900 ma the group underwent polyphase deformation and upper amphibolite facies metamorphism the hudsonian orogeny was followed by

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

neoarchean tectonothermal imprints in the rengali province eastern india and their implication on the growth of singhbhum craton evidence from zircon u pb shrimp data a detailed zircon u pb shrimp geochronological study of the amphibolite to granulite grade rocks of the rengali province of eastern india records the growth history of the southern margin of the singhbhum craton pelitic and mafic granulites from the gneissic belt exhibit contrasting styles of metamorphism zircon of the pelitic granulites from the eastern segment yields c 3528 3064 ma detrital ages charnockitic gneiss from the eastern segment has protolith age of 3058 15 ma while that from the central segment has protolith age of 2861 30 ma the latter rock records high grade metamorphism at 2818 15 ma hornblende gneiss from the central sector has a protolith age of 2828 9 ma deformed leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807 13 and 2809 13 ma respectively the protolith of felsic gneiss from the central sector was emplaced at 2776 24 ma most of the zircon samples contain overgrowths of c 2500 ma inferred to be the age of reworking of the gneissic belt our data suggest that the rengali province evolved as an orogenic belt in the neoarchean time c 2800 2500 ma during southward growth of the singhbhum craton these tectonothermal imprints at the margin of the singhbhum craton are possibly related to its inclusion within the supercontinent ur

precise pb baddeleyite geochronology by the thermal extraction thermal ionization mass spectrometry method abstract the ability to easily obtain precise 0 1 age determinations on mafic rocks can facilitate identification of contemporaneous magmatic events across continents and hence open new avenues of research investigations related to large scale mantle processes we present high precision pb age determinations on baddeleyite zro 2 a common accessory mineral in mafic rocks using the recently developed thermal extraction thermal ionization mass spectrometer te tims procedure for dating of zircon in order to constrain the conditions for thermal extraction of pb from baddeleyite a sequence of sem back scattered electron images were acquired along with energy dispersive x ray spectroscopy spot analysis of baddeleyite grains embedded in silica beads at progressively increasing temperature between 1200 and 1590 c replicate analyses of twelve preheated at 1250 c and silica embedded baddeleyite grains from the paleoproterozoic phalaborwa carbonatite using this procedure have yielded a 207 pb 206 pb weighted mean age of 2060 3 0 4 ma consistent with previously reported 207 pb 206 pb ages for this carbonatite occurrence determined by the conventional u pb isotope dilution thermal ionization mass spectrometry id tims technique fourteen baddeleyite fractions from a large paleoproterozoic n s striking mafic dyke swarm in the dharwar craton yielded a 207 pb 206 pb weighted mean age of 2215 9 0 3 ma also consistent with previously determined conventional ages all data were corrected for mass fractionation of 0 18 amu as determined previously on zircon both data sets scatter outside measurement errors implying an external 2 sigma error of 0 055 in the case of the carbonatite and 0 043 for the mafic dyke which we suggest represents the reproducibility of the fractionation correction our results demonstrate that thermal extraction 207 pb 206 pb ages are comparable with the conventional u pb ages in both accuracy and precision this procedure does not need ultra clean laboratory conditions

formation of the jinchuan ultramafic intrusion and the world s third largest ni cu sulfide deposit associated with the 825 ma south china mantle plume we report here shrimp u pb geochronological geochemical and nd isotopic data for the jinchuan ultramafic intrusion gansu province china which hosts the world s third largest magmatic ni cu sulfide deposits u pb baddeleyite analyses yield an age of 812 26 ma for the ultramafic intrusion this age is indistinguishable within analytical uncertainties from the u pb zircon ages of 827 8 ma and 828 3 ma for the sulfide bearing ultramafic rocks and the dolerite dykes that cut the ultramafic intrusion respectively these u pb dating results show beyond doubt that the jinchuan ultramafic intrusion and associated ni cu sulfide deposit were formed at 825 ma rather than 1500 ma as has been widely believed the ultramafic rocks exhibit large negative nd t values 8 9 to 12 0 that decrease with increasing la sm suggesting that their parental magmas were derived from a long term enriched lithospheric mantle and experienced crustal contamination mineralogical petrological and geochemical data all indicate that the jinchuan intrusion was generated by melting of the enriched lithospheric mantle heated by an anomalously hot plume the u pb ages of 825 ma for igneous baddeleyites and zircons and 900 880 ma for inherited zircons in the jinchuan mafic ultramafic rocks are comparable with those in the qaidam block and qilian belt the western extension of the qinling belt that was likely derived from northern yangtze craton the jinchuan ni cu sulfide bearing intrusion along with coeval regional plume related mafic dykes and tholeiites and mafic ultramafic complexes with associated v ti and ni cu pge mineralization is interpreted to be genetically related to the 825 ma south china mantle plume

a seismotectonic picture of the inner southern western alps based on the analysis of anomalously deep earthquakes abstract the anomalously deep seismicity beneath the western po plain is here analyzed to shed light on the complex and still poorly understood tectonic configuration of the internal side of the western alps area the original dataset including 590 earthquakes deeper than 20 km recorded during the last 25 years has been accurately relocated with hypodd using both catalog and cross correlation differential times we found that the distribution of seismic events faithfully mirrors the presence of two distinct tectonic domains axial belt domains 1 and 2 originally belonging to the paleogene alpine wedge and now anomalously juxtaposed beneath the sedimentary infill of the western po plain shallow low magnitude earthquakes 8 km and accommodated in correspondence of the fault could be linked to its neogene activity coeval with the northward translation of the retreating adriatic slab

strain partitioning in the axial nw alps since the oligocene 1 this work analyzes the postmetamorphic evolution of the northwestern alps thanks to a multiscale field geology study integrated by fission track analysis it illustrates how the fission track method constraining throws along major faults and age of mesoscale deformation provides interpretation keys to detangle complex structural data sets results are discussed within the framework of the alps apennines knot and give new insights on strain partitioning within transpressional orogenic belts the axial nw alps acted since the late oligocene as an orogen scale shear zone accommodating in the aosta valley dextral strike slip coeval with forward propagation of external thrusts within this transcurrent belt right lateral deformation was partitioned between the fast exhuming western block and the already unroofed eastern block these blocks show contrasting structural characters the former is cut by first order longitudinal structures bounding major tectonostratigraphic units the latter is fragmented by complex patterns of lower order faults interpreted as subsidiary shears the onset of mesoscale extension widespread in the eastern block is diachronous across the belt and migrated through time toward the european foreland transition from syncollisional shortening to orogen scale transcurrence observed in the nw alps corresponds to a regional strain field variation documented in the mid oligocene all around the alps apennines knot which is possibly related to a change of the adria trajectory relative to europe since the late oligocene deformation associated with oblique plate convergence was partitioned between strike slip in the axial belt and thrusting in external areas most of the shortening was accommodated in the helvetic chains 102 km deformation was limited elsewhere to an order of 101 km

geomorphological thermo mechanical modeling application to orogenic wedge dynamics abstract coupled geomorphological thermo mechanical modeling is presented in a new implementation that combines two established thermo mechanical and landscape evolution models a finite difference marker in cell technique is used to solve for the thermo mechanical problem including complex visco plastic rheologies in high resolution each timestep is synchronously solved with a fluvial landscape evolution model that includes numerical solution of fluvial incision and analytical hillslope processes for both diffusive and slope limited processes on an adaptive grid the implementation is successful in modeling large deformation at different scales we demonstrate high degrees of coupling through processes such as exhumation of rocks with different erodibilities sensitivity of the coupled system evolution to surface parameters and mechanical parameters is explored for the established case of development of compressive wedges the evolution of wedge models proves to be primarily sensitive to erodibility and the degree of river network integration relief follows deformation in propagating forward with wedge growth we apply the method to a large scale model of continental collision in which a close relationship between deep tectonics fluvial network evolution and uplift and erosion can be demonstrated

temporal distribution of granitoid plutonic rocks in the archean slave province northwest canadian shield granitoid rocks in the slave province consist of 4 0 2 8 ga granitoid gneisses that predate the 2 71 2 65 ga volcanic and turbiditic rocks of the yellowknife supergroup and 2 70 2 58 ga granitoid plutons that intrude the yellowknife supergroup u pb zircon ages and nd and pb isotopic data indicate that the older granitoids are restricted to the western part of the slave province granitoid gneisses in the eastern slave province previously suspected to predate the yellowknife supergroup are similar in age to the volcanic rocks in this paper the results of a detailed geochronological study of plutonic rocks of the contwoyto lake nose lake area of the northcentral slave province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the slave province the data indicate that the timing of the later 2 7 ga plutonism is bimodal consisting of two distinct magmatic periods separated by approximately 20 ma of apparent magmatic quiescence the first period was synvolc

neoarchean paleoproterozoic stratigraphy of the dhanjori basin singhbhum craton eastern india and recording of a few u pb zircon dates from its basal part abstract the stratigraphy of the dhanjori basin placed at the northeastern margin of the paleo mesoarchean singhbhum craton has been revised a mainly quartz pebble conglomerate qpc and volcani clastics bearing unit at the base of the dhanjori basin and intruded by the mayurbhanj granite has been named phuljhari formation the dhanjori volcani sedimentary sequence with quartzite polymictic conglomerate at its base disconformably overlies the phuljhari formation and non conformaby over the mayurbhanj granite the dhanjori group is inferred to broadly range in age from 2 6 2 1 ga detrital zircons from qpc belonging to the phuljhari formation have yielded oldest chemical ages of 3 09 3 04 ga thus fixing the upper age limit of the formation the 3 09 ga old zircons recovered earlier from the mayurbhanj granite which is intrusive into the relatively younger phuljhari fm therefore appears to be xenocrystic in nature a fine grained variety of the mayubhanj granite close to its intrusive contact with the phuljhari fm was sampled during the present study it has recorded strong impress of acid magmatic event around 1 0 0 8 ga by its lower intercept and an upper intercept age of 2 8 ga the latter might record the inherited age of protolith to the mayurbhanj granite and correspond to widespread granite magmatic activity affecting the margin of the singhbhum craton signature of 1 0 0 08 ga old thermal perturbation also affected lower parts of the dhanjori basin and the singhbhum shear zone detrital au u bearing qpc in the phuljhari formation represent neoarchean sedimentation at the margin of the peneplained craton the dhanjhori group comprising mafic ultramafic volcanics and siliciclastics was hosted in rift induced intracratonic basin around neoarchean palaeoproterozoic threshold

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

a review of hadean to neoarchean crust generation in the singhbhum craton india and possible connection with pilbara craton australia the geochronological perspective abstract the precambrian singhbhum craton sc of the eastern india holds a prolonged record of sporadic crust generation and evolution starting from as ancient as the hadean till late archean like every craton episodic nature of crust generation in the sc has been indicated by several workers based on numerous stratigraphic and geochronological evidence long before however it is only in the past decade that a growing zircon u pb hf database has resolved its evolution in detail such as the timing of juvenile crust versus reworking events integration of over a thousand geochronological data n 1558 from hadean 4 ga till the end of archean 2 5 ga exhibits multiple prominent crustal growth peaks during c 3320 ma 3300 ma and 2780 ma combined with moderate peaks at c 3460 ma 3380 ma and 2820 ma integral crust modelling of hf isotopezircon indicates major juvenile crust addition during the periods c 3760 3640 ma 3620 3440 ma while prominent age frequency peak at c 2780 ma defined by u thbaddeyelite ages of regional mafic ultramafic dyke swarm provides a direct proxy of large scale juvenile crust generation the interplay between the growth of new crust and reworking of older crust combined with multiphase deformation and metamorphic events till the end of archean constructed the critical juxtaposition of diverse crustal components of this important craton constraining crust generation and deformation events from an updated geochronological database of this craton reveal remarkable similarity in crustal growth pattern with the archean pilbara craton australia moreover paleomagnetic studies of the neoarchean mafic dyke swarms of both the cratons indicate similar paleo positions therefore compiled field geochronological paleomagnetic parameters invoke the possibility that the archean singhbhum and pilbara cratons might have been connected and shared similar crustal history at least till the neoarchean era

petrogenesis and geochronology of the arkasani granophyre and felsic dalma volcanic rocks implications for the evolution of the proterozoic north singhbhum mobile belt east india the north singhbhum mobile belt nsmb is a 200 km long curved proterozoic fold thrust belt that skirts the northern margin of the archean singhbhum craton of ne india the singhbhum shear zone ssz developed between the dhanjori and chaibasa formations near the southern margin of the nsmb and represents an important cu u p metallotect a shrimp u pb zircon date of 1861 6 ma obtained for the syn to post kinematic arkasani granophyre that has intruded the ssz provides a minimum age for the prolonged tectonic activity and mineralization along the ssz and for the time of closure of the chaibasa and dhanjori sub basins the dalma volcanic belt a submarine rift related bimodal mafic felsic volcanic suite forms the spine of the nsmb a shrimp u pb zircon igneous crystallization date of 1631 6 ma was obtained for an unfoliated felsic volcanic rock from the base of the dalma volcanic sequence these new findings suggest that the different sub basins in the nsmb evolved diachronously under contrasting tectonic environments and were juxtaposed during a later orogenic movement

precambrian mafic dyke swarms in the singhbhum craton eastern india and their links with dyke swarms of the eastern dharwar craton southern india abstract based on trend cross cutting relationships and u pb dating precambrian mafic dykes in the singhbhum craton earlier collectively identified as newer dolerite swarm have been separated into seven distinct swarms which are thought to be the plumbing systems for large igneous provinces lips these singhbhum swarms range in age from 2 80 ga to 1 76 ga and include the 2 80 ga ne sw trending keshargaria swarm 2 75 2 76 ga nne ssw to ne sw trending ghatgaon swarm the 2 26 ga ne sw to ene wsw trending kaptipada swarm based on a new u pb id tims age 2256 6 ma the 1 77 ga wnw ese trending pipilia swarm the early paleoproterozoic e w to ene wsw trending keonjhar swarm the middle paleoproterozoic nw se to nnw sse trending bhagamunda swarm and the late paleoproterozoic n s to nne ssw trending barigaon swarm two of the singhbhum swarms the 2 26 ga kaptipada and 1 77 ga pipilia are closely matched with the 2 26 2 25 ga ippaguda dhiburahalli and 1 79 ga pebbair swarms respectively of the eastern dharwar craton the correlations suggest that the singhbhum and dharwar cratons were close enough at these times to share two reconstructed lips a 2 26 2 25 ga kaptipada ippaguda dhiburahalli lip and a 1 79 1 77 ga pipilia pebbair lip and if so both swarms must be present in the intervening bastar craton candidates are proposed also the 2 76 2 75 ga ghatgaon swarm of the singhbhum craton can be provisionally correlated with 2 7 ga keshkal swarm of the bastar craton the 2 26 2 25 ga kaptipada ippaguda dhiburahalli lip of the singhbhum bastar dharwar reconstruction has age matches in the vestfold hills of antarctica 2 24 ga dykes the kaapvaal craton the 2 25 2 23 ga hekpoort lavas and perhaps the zimbabwe craton 2 26 ga chimbadzi troctolite intrusions the 1 76 1 79 ga pipilia pebbair lip of the singhbhum bastar dharwar reconstruction has age matches in the north china australian shield amazonian rio de plata and sarmatia cratons the relevance of these matches for reconstructions will require future testing using paleomagnetic studies while there are 2 7 2 8 ga lip type greenstone belts in many crustal blocks there are no precise matches with the 2 76 2 75 ga ghatgaon swarm of the singhbhum craton however the 2 80 ga keshargaria swarm can be potentially linked with units in the pilbara and yilgarn cratons

feeding the aneurysm orogen parallel mass transport into nanga parbat and the western himalayan syntaxis the nanga parbat haramosh massif nphm western himalayan syntaxis requires an influx of mass exceeding that in the adjacent himalayan arc to sustain high topography and rapid erosional exhumation rates what supplies this mass flux and feeds this tectonic aneurysm we show using a simple 3 d model of oblique orogen convergence that velocity strain partitioning results in horizontal orogen parallel op crustal transport and the same behavior is inferred for the himalaya with op transport diverting converging crust toward the syntaxis model results also show that the op flow rate decreases in the syntaxis thereby thickening the crust and forming a structure like the nphm the additional crustal thickening over and above that elsewhere in the himalayan arc sustains the rapid exhumation of this aneurysm normally velocity strain partitioning would be minimal for the himalayan arc where the convergence obliquity is no greater than 40 however we show analytically that the himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone which accommodates op transport are very weak corresponding numerical results confirm this requirement and demonstrate that a nanga parbat type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active these results lead us to question whether the position of nphm aneurysm is localized by river incision as previously suggested or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models

magmatic and metamorphic history of paleoarchean tonalite trondhjemite granodiorite ttg suite from the singhbhum craton eastern india texturally controlled dating of zircon from paleoarchean tonalite trondhjemite granodiorites of the older metamorphic tonalitic gneisses and the singhbhum granite batholith phases i ii and iii from the singhbhum craton in eastern india reveals a polycyclic evolution of the archean crust the granitoid suites were emplaced in two pulses at 3 45 3 44 ga and 3 35 3 32 ga tonalites and trondhjemites of the older metamorphic tonalitic gneisses were emplaced at ca 3 45 3 44 ga together with phase iii of the singhbhum granite pluton while granites belonging to the older metamorphic tonalitic gneisses were emplaced at ca 3 35 3 32 together with phase i and phase ii of the singhbhum granite pluton both crustal units underwent an early phase of relatively high grade metamorphism at 3 30 3 28 ga followed by extensive fluid induced alteration during low grade metamorphism at 3 19 3 12 ga and 3 02 2 96 ga the two units have also been marginally affected at ca 2 52 ga and 1 06 ga by major metamorphic events in the north singhbhum mobile belt and the singhbhum shear zone at the northern margin of the craton the zircon grains in granites have inherited cores with ages of ca 3 61 ga and 3 46 3 41 ga and with well developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source the emplacement of the expansive granitoids belonging to the older metamorphic tonalitic gneisses and the singhbhum granite was synchronous with the amphibolite facies metamorphism ca 3 32 ga of older meta igneous and metasedimentary rocks belonging to the older metamorphic group major felsic crust formation in the craton occurred in a narrow time interval between 3 46 and 3 32 ma with minor contributions of material as old as 3 6 ga the complex polycyclic evolution of the paleoarchean crust in the singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism

textural and geochemical discrimination between xenotime of different origin in the archaean witwatersrand basin south africa abstract xenotime ypo4 of detrital diagenetic and hydrothermal origin within siliciclastic rocks of the archaean witwatersrand basin south africa has been identified on the basis of petrography and in situ ion microprobe shrimp age data the chemical composition of xenotime determined by in situ electron microprobe analysis can be correlated with its origin this allows the origin of any xenotime grain to be assessed by a non destructive microanalytical method prior to ion microprobe geochronology the main chemical discriminators are mree hree abundance normalised hree slope and eu anomaly and in some cases u and th contents igneous detrital xenotime 2800 ma is distinguished from diagenetic 2780 ma and hydrothermal

evidence for a neoarchean lip in the singhbhum craton eastern india implications to vaalbara supercontinent abstract we report eight new pb pb baddeleyite ages and paleomagnetic results on a series of hitherto unknown neoarchean nne ssw trending mafic dyke swarms intruding the paleoarchean basement rocks in the singhbhum craton eastern india being the most dominant dyke swarms in the region they occur over the entire singhbhum craton with an areal distribution extending to about 30 000 km 2 of the eight dyke samples analysed across the entire dyke swarm six were emplaced at 2762 4 2 0 ma weighted mean of 6 dyke ages two with similar strike directions yielded older and younger ages of 2800 2 0 7 ma and 2752 0 0 9 ma respectively the older 2800 2 0 7 ma dyke event appears to be coeval with several other magmatic events reported earlier from the singhbhum craton emplacement of dacite tuffs within the malaigiri basin on the southern margin of the craton the budhapal and rengali granite from the south western and southern parts of the craton along with the temperkola granite activity and associated acid volcanics in the western part of the craton and perhaps the mayurbhanj granite fine grained phase are also contemporaneous suggesting a fairly wide spread thermal event in the region at this time well defined craton wide magmatic events contemporaneous to the 2762 4 2 0 ma singhbhum dyke activity have been reported from the pilbara the younger 2752 0 0 9 ma age dyke event occurring in the singhbhum is also know from the eastern pilbara an older event at 2783 1 2 ma gaborone kanye plantation porphyry derdepoort modipe episode was identified in the kaapvaal craton comparison of paleomagnetic data obtained on the singhbhum neoarchean dyke swarms with the pilbara and kaapvaal data show all these continents were located at steep latitudes and could have been proximal to each other during this time if pilbara and kaapvaal formed supercraton vaalbara during the neoarchean time we propose singhbhum with similar archean geology could also have had its ancestry in the same supercraton

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

2 8 ga old anorogenic granite acid volcanics association from western margin of the singhbhum orissa craton eastern india abstract the tamperkola granite acid volcanics association occurring at the western margin of the archaean singhbhum orissa iron ore craton soc eastern india is intrusive into the darjing group which represents a sequence of mobile belt metasediments in this part of the soc the darjing group rests unconformably on the bonai granite 3 2 ga old absence of any deformational imprints of the country rock metasediments on the tamperkola granite acid volcanics together with its undeformed and unmetamorphosed nature its alkali feldspar dominant mineralogy and its high sio 2 and na 2 o k 2 o and low mgo and cao contents suggest that this granite acid volcanics association is anorogenic in nature two representative samples one each from the granite and the acid volcanics have been dated by in situ 207 pb 206 pb zircon dating method using a small ion microprobe minimum age of crystallisation of the acid volcanics is found to be 2 8 ga strong peak in the 207 pb 206 pb frequency diagram and equality of the observed and expected errors in radiogenic 207 pb 206 pb ratios suggest that this age probably represents the true age of formation of the volcanics the age data place the deposition and metamorphism of the mobile belt metasediments of the darjing group in between 3 2 and 2 8 ga occurrence of 2 9 2 8 ga old small granitoid plutons alkali feldspar granite to syenogranite in composition is also known from the southern margin of the soc therefore it appears that around 2 9 2 8 ga small alkali granite bodies formed at the marginal part of this cratonic block after its stabilisation at 3 1 ga

generation and evolution of palaeoarchaean continental crust in the central part of the singhbhum craton eastern india abstract palaeoarchaean granitoids are exposed over wide area in the singhbhum craton whose origin and role in crustal evolution are not well constrained this study presents whole rock and mineral chemical data coupled with zircon u pb dating and hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis tectonic setting and role in continental crustal evolution the first phase of granitoid magmatism in this area is represented by a 3 47 ga na rich gneissic tonalite belonging to the archaean ttg tonalite trondhjemite granodiorite suite this rock is characterized by high hree heavy rare earth elements negative eu anomaly low sr y ratio and positive zircon hf isotope signature ehft 2 1 to 4 8 it is interpreted to be formed by shallow melting of a juvenile mafic source at 3 35 ga a silicic ferroan porphyritic biotite granite formed it shows variable k na low y and high sr y moderately fractionated hree and positive zircon ehft 1 8 to 4 0 and is explained as a product of high temperature melting of a heterogeneous juvenile source consisting of tonalites and mafic rocks at lower crustal depth the final phase of granitoid magmatism is marked by a 3 30 ga non porphyritic ferroan silica rich biotite granite geochemical characteristics like moderately high k moderately fractionated hree low ca and sr y and zircon ehft 0 8 to 3 7 suggest that the granite was formed by high pressure melting of a tonalite dominated source with short crustal residence all the three granitoid phases display low mg ni and cr contents and magnesium number mg precluding direct involvement of mantle in their origin rather crustal reworking caused by episodic plume related mafic ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved k rich granitoids it appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during palaeoarchaean

interactions between thin and thick skinned tectonics at the northwestern front of the jura fold and thrust belt eastern france 1 this study investigates spatial and temporal interactions of thin and thick skinned tectonics in a classical foreland setting located at the front of the jura fold and thrust belt in eastern france the working area coincides with the intracontinental rhine bresse transfer zone and represents the most external front of the deformed alpine foreland the investigation combines analyses of largely unpublished and newly available subsurface information with our own structural data including an exhaustive paleostress analysis and geomorphologic observations results are provided in the form of a new tectonic map and a series of regional cross sections through the study area the besancon zone forming the most external part of the thin skinned fold and thrust belt encroached onto the eo oligocene rhine bresse transfer fault system until early pliocene times thrust propagation was largely controlled by the late paleozoic to paleogene preexisting fault pattern that characterizes the rhine bresse transfer zone thick skinned deformation dominant throughout the avant monts zone located farther to the west was associated with compressional to transpressional reactivation of such faults overprinting and crosscutting criteria of fault slip data allow distinguishing between systematically fanning maximum horizontal stress axes that define the front of the thin skinned jura fold and thrust belt and consistently nw se directed maximum horizontal stress axes that characterize deformation of the autochthonous cover of the foreland which is affected by thick skinned tectonics tectonic and geomorphic analyses indicate that thick skinned tectonics started at a very late stage of foreland deformation post early pliocene geomorphic observations imply that deformation between mesozoic cover and basement is locally still decoupled however overprinting relationships and recent seismicity suggest that present day tectonic activity is thick skinned which probably reflects ongoing tectonic underplating in the alpine foreland

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

geochronology of a late archaean flood basalt province in the pilbara craton australia constraints on basin evolution volcanic and sedimentary accumulation and continental drift rates abstract eleven high precision 2 5 million years shrimp zircon u pb ages have been obtained from felsic rocks within a single stratigraphic section of late archaean volcanic and sedimentary rocks in the east pilbara of western australia the stratigraphic succession nullagine and mount jope supersequences in sequence stratigraphic terminology fortescue group in lithostratigraphic terminology is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional rift related environment between about 2772 and 2715 ma the geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy field mapping geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs while flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession felsic volcanism is now known to have been active periodically through each geotectonic cycle the succession covers a time period of about 57 million years the lower 1400 m of a thick 1700 m clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years in contrast a younger 150 m thick cogenetic tuff basalt unit accumulated in less than 3 million years and others probably accumulated at similar rates comparable to those of phanerozoic flood basalts unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time gap of more than 10 million years the unconformity based stratigraphic framework the new geochronology and palaeomagnetic studies j geophys res 108 2003 b12 2551 emp 2 1 to 2 21 have been combined to determine a possible late archaean continental drift rate for one part of the succession implying a period of motion as fast as or up to five times faster than any known from the phanerozoic

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

precise u pb baddeleyite age dating of the usushwana complex southern africa implications for the mesoarchaean magmatic and sedimentological evolution of the pongola supergroup kaapvaal craton the usushwana complex of the south eastern kaapvaal craton south africa and swaziland intrudes the ca 3 6 3 1 ga basement of the craton as well as the mesoarchaean volcanic and sedimentary cover succession of the pongola supergroup new high precision u pb dating of gabbros belonging to the piet retief suite of the usushwana complex yield ages of 2989 1 ma 2990 2 ma and 2978 2 ma the piet retief suite represents part of an intricate magmatic feeder to a major volcanic event which gave rise to the oldest known continental flood basalts on earth the nsuze volcanic rocks broadly coeval se trending dolerite dykes of the barberton badplaas dyke swarm in the larger region of the south eastern kaapvaal craton formed along the same structural trend as the usushwana complex one such dyke is dated herein to 2980 1 ma using the high precision u pb geochronological data the nsuze volcanic rocks can now be resolved into at least two magmatic episodes which can be correlated with parts of the pongola supergroup the first episode at ca 2 99 2 98 ga is broadly coeval with the pypklipberg nhlebela volcanic rocks whereas the second at ca 2 97 2 96 ga was near synchronous to the agatha volcanic rocks a dolerite sill intruding into the mozaan group of the pongola supergroup thought to be part of the usushwana complex was dated to 2869 5 ma and is instead coeval with the hlagothi complex further to the south and provides a new minimum age for deposition of the mozaan group c 2015 elsevier b v all rights reserved less

alpine orogenic evolution from subduction to collisional thermal overprint the 40ar 39ar age constraints from the valaisan ocean central alps 1 the investigated hp lt metasedimentary units of the valaisan and adjacent european domains occupy a key position in the alpine belt for understanding the transition from early subduction related hp lt metamorphism to collision related barrovian overprint and the evolution of mountain belts in general the timing of high pressure metamorphism subsequent retrogression and following barrow type overprint was studied by 40ar 39ar dating of biotite and several white mica generations that are well characterized in terms of mineral chemistry texture and associated mineral assemblages four distinct age populations of white mica record peak pressure conditions 42 40 ma and several stages of subsequent retrograde metamorphic evolution 36 25 ma biotite isotopic analyses yield consistent apparent ages that cluster around 18 16 ma for the barrow type thermal overprint the recorded isotopic data reveal a significant time gap in the order of some 20 ma between subduction related hp lt metamorphism and collision related barrovian overprint supporting the notion of a polymetamorphic evolution associated with a bimodal p t path

vaalbara earth s oldest assembled continent a combined structural geochronological and palaeomagnetic test the only remaining areas of pristine 3 6 2 7 ga crust on earth are parts of the kaapvaal and pilbara cratons general similarities of their rock records especially of the overlying late archean sequences suggest that they were once part of a larger vaalbara supercontinent here we show that the present geochronological structural and palaeomagnetic data support such a vaalbara model at least as far back as 3 1 ga and possibly further back to 3 6 ga vaalbara fragmented prior to 2 1 ga and possibly as early as 2 7 ga suggesting supercontinent stability of at least 400 myr consistent with neoproterozoic and phanerozoic analogues

relating orogen width to shortening erosion and exhumation during alpine collision we investigate along strike width changes of the thickened accreted lower plate talp in the central and in the eastern alps we set the width of the talp in relation to the inferred amount of collisional shortening and exhumation along six orogen scale cross sections taking the present day along strike gradients in the amount of collisional shortening to represent the temporal evolution of the collisional wedge it may be concluded that the cross sectional area of the talp diminishes during ongoing shortening indicating that the erosional flux outpaced the accretionary flux higher amounts of collisional shortening systematically coincide with smaller widths of the talp and dramatic increases of the reconstructed eroded rock column higher amounts of shortening also coincide with larger amplitudes of orogen scale upright folds with higher exhumation and with higher exhumation rates hence erosion did play a major role in reducing by 30 km the vertical crustal thickness in order to accommodate and allow shortening by folding long term climate differences cannot explain alternating changes of width by a factor of almost 2 along straight segments of the orogen on length scales less than 200 km as observed from the western central alps to the easternmost eastern alps sedimentary or paleontological evidences supporting such paleo climatic differences are lacking suggesting that erosional processes did not directly control the width of the orogen

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

geochemical and ion microprobe u pb zircon constraints on the archaean evolution of singhbhum craton eastern india abstract geochemical and shrimp u pb zircon analyses were obtained for nine samples of the singhbhum craton to investigate major regional granite intrusion volcanism sedimentary deposition metamorphism and deformation episodes detrital zircons within a recrystallized sandstone enclave within tonalitic gneiss provided a maximum time for deposition of 3375 3 ma all uncertainties are at 95 confidence unless otherwise specified for the host sandstone igneous crystallization dates between 3380 and 3299 ma were obtained for tonalite gneiss and foliated trondhjemite samples from widely dispersed sites whereas unfoliated granitic samples provided igneous crystallization dates