paleomagnetism of archean rocks from northwestern ontario iii rock magnetism of the shelley lake granite quetico subprovince the shelley lake granite of northwestern ontario contains five magnetic phases deuteric and post crystallization hematites which are relatively abundant but carry only 1 4 of the natural remanent magnetization nrm primary magnetite in coarse 50 500 m grains both optically homogeneous and subdivided by hematite lamellae micrometre size secondary magnetite in chloritized biotites and submicrometre size magnetite whose presence is inferred from low blocking temperatures in thermal decay curves of the nrm the nrm is a composite of type 1 and type 2 remanences which differ in direction by about 90 see companion paleomagnetic paper both nrm components occur in normal n and reverse r polarities type 1 remanences 1n 1r have the hallmarks of multidomain md behaviour high blocking temperatures but low coercivities exponential alternating field af decay curves generally md results of the lowrie fuller test and md to transitional values 0 3 10 of the koenigsberger qn ratio further

geochronology of type santacrucian middle tertiary land mammal age patagonia argentina mammal bearing lacustrine and tuffaceous sediments from three localities of the santa cruz formation type fauna of the santacrucian land mammal age in patagonia southern argentina are calibrated by radioisotope dating with the aid of magnetostratigraphy the strata range from about 17 6 ma to perhaps 16 0 ma and are thus of late early miocene age the santacrucian land mammal age ranges from about 18 0 ma to about 15 0 ma

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

formation of high grade ignimbrites part ii a pyroclastic suspension current model with implications also for low grade ignimbrites analogue experiments in part i led to the conclusion that pyroclastic flows depositing very high grade ignimbrite move as dilute suspension currents in the thermo fluid dynamical model developed the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation initial conditions of the currents are derived from column collapse modeling for magmas with an initial h2o content of 1 3 wt erupting through circular vents and caldera ring fissures the flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content temperature bottom slope and coarser initial grain size progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance the ash plumes reach stratospheric heights and distribute 30 80 of the erupted material as widespread co ignimbrite ash pyroclastic suspension currents with initial mass fluxes of 107 1012 kg s can spread for tens of kilometers with only limited cooling although they move as supercritical strongly entraining currents for the eruption conditions considered here with increasing eruption mass flux cooling during passage through the fountain diminishes while cooling during flow transport increases the net effect is that eruption temperature exerts the prime control on emplacement temperature pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is to 1 3 tmw the minimum welding temperature high eruption rates a large fraction of fine ash and a ring fissure vent favor the formation of extensive high grade ignimbrite for very hot eruptions producing sticky partially molten pyroclasts analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation which reduces runout as a result very high grade ignimbrites cannot spread more than a few tens of kilometers from their source in cooler pyroclastic currents particles do not aggregate and the sedimentation process may involve re entrainment of particles which potentially leads to more extensive cooling and longer runout such effects however are only significant when net erosion of substrate occurs model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios the model also provides an alternative explanation of the observed decrease in h lratios with ignimbrite mass

paleomagnetism of the uairen formation roraima group southeastern venezuela evidence for one of the oldest middle proterozoic depositional remanent magnetizations this study reports paleomagnetic results from nine localities 22 sites of the uairen formation the basal and oldest unit of the roraima sedimentary sequence roraima group in the precambrian guayana shield southeastern venezuela three distinct natural remanent magnetizations nrm s u1n u1r and u2 were isolated by alternating field and thermal demagnetization u1n and u1r nrm s have mean directions d 342 i 6 k 14 95 10 n 14 sites and d 155 i 10 k 8 95 17 n 8 sites respectively because of their high coercivities hc and high 580 c laboratory unblocking temperatures tub it is believed that these nrm s are carried mainly by fine grained magnetite and or hematite the u2 nrm has d 338 i 7 k 5 95 25 n 6 sites u2 is a low hc 20 mt and low tub 300 c nrm carried by coarse grained magnetite despite its low fisher s k value the u2 grand mean closely resembles that for u1n directional similarities between u1n u1r and u2 to g

the altitude dependence of magnetic remanence in the slowly cooled precambrian plutonic terrain of west greenland abstract to evaluate the change in magnetic remanence with altitude through a slowly cooled precambrian basement terrain three vertical sections have been sampled in west greenland the study employs the principle that higher structural levels passed through their blocking temperatures earlier than lower levels and therefore record earlier pole positions and it utilises dolerites and diorites intruded late in or after the tectonic history to minimise anisotropy effects in the amphibolite facies terrain at qa qatoqaq 1400 m highly stable magnetite held remanences move on demagnetisation progressively along small circles interpreted to record younger to older apparent polar wander a p w motions during cooling through the blocking temperature ranges although the raw data show no systematic variation with altitude when account is taken of the blocking temperature spectra as defined by thermal demagnetisation there is a systematic change in palaeofield direction in the same sense as that recorded by the demagnetisation trends granulite facies terrain at igdlu nguit qula t 600 m again shows systematic variation with altitude when the sites are divided into those with a remanence dominated by hemo ilmenite and those dominated by magnetite a third section at praestefjeldet 250 m yields a palaeofield reversal and a high blocking temperature component the age evidence is evaluated to suggest that the a p w path defined by 5 mean palaeopoles between 318 e 1 n and 247 e 38 n represents up to 50 ma of palaeofield motion recorded by the uplift and cooling of this basement terrain at crustal depths of the order of 10 km the calculated rate of a p w motion is 1 2 ma and the rate of crustal uplift 10 20 m ma these rates are respectively up to an order higher and at least an order lower than phanerozoic rates the collective data from greenland agree closely with post hudsonian poles from the laurentian shield and represent part of a very widespread uplift event following this mobile episode they show that altitude sections can yield a systematic record of the magnitude and direction of precambrian a p w motions provided that the blocking temperature spectra are taken into account

volcanic plume electrification experimental investigation of a fracture charging mechanism although ashfall from particulate volcanic plumes is known to be highly electrically charged little is known about the charging mechanism we describe experiments designed to investigate the particle charges generated from the fracture of pumice small silicate particles were produced in the laboratory during collisions between two samples cut from pumice clasts the net charge magnitudes detected on these particles are similar to those previously measured on ashfall from volcanic plumes 10 5 to 10 6 c kg 1 this net charge is also shown to be the result of a small imbalance between the sums of individual particle charges of both polarities which are up to several orders of magnitude larger than the net charge the magnitude of both the net and single polarity specific charges were only weakly affected by changes of relative humidity but single polarity charges increased steadily with increasing sample impact velocities the dominant charging process during the experiments was that of material fracture the charging mechanism is thus interpreted to be fractoemission the release of nuclear particles from fresh crack surfaces occurring during the production of the silicate particles this implies that the electrification of volcanic plumes could be the result of brittle fragmentation of magma or pumice clasts within the upper regions of the conduit and in the jet region of the plume

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

characterization of shape and terminal velocity of tephra particles erupted during the 2002 eruption of etna volcano italy in this paper we present a complete morphological characterization of the ash particles erupted on 18 december 2002 from etna volcano italy the work is based on the acquisition and processing of bidimensional digital images carried out by scanning electron microscopy sem to obtain shape parameters by image analysis we measure aspect ratio ar form factor ff compactness cc and rectangularity rt of 2065 ash particles with size between 0 026 and 1 122 mm we evaluate the variation of these parameters as a function of the grain size ash particles with a diameter of 0 250 mm are subelongate we find that on average particles with a diameter of 0 50 mm are angular using this morphological analysis and an empirical relation between the drag coefficient cd and reynolds number re of wilson and huang earth planet sci lett 44 311 324 1979 we calculate the terminal settling velocities vwh the comparisons between these velocities and those calculated with the formula of kunii and levenspiel fluidization engineering wiley new york pp 97 1969 vkl which considers ash particles as spheres show that vkl are in average 1 28 greater than vwh hence we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out by tephra dispersal models when the particles are assumed to be spherical

paleomagnetism of the red lake greenstone belt northwestern ontario possible evidence for the timing of gold mineralization we report paleomagnetic results from 1 largely unaltered batholiths and 2 sheared and altered units in the archean red lake greenstone belt a major gold producing region in the uchi subprovince of northwestern ontario group 1 includes the hammell lake killala baird trout lake and little vermilion lake batholiths which were intruded between 2730 and 2700 ma ago u pb zircon group 2 includes the howey diorite the dome stock and dikes and andesites from the dickenson mine these units from the mineralized deformation zones dzs between the batholiths were sheared and hydrothermally altered in the same event that caused gold mineralization in supracrustal units the batholiths carry a stable reversed remanence rlr withd 159 i 74 k 20 95 16 n 4 units 14 sites rlr has high coercivities unblocking temperatures and remanence intensities and appears to be a primary thermal remanence dating from 2700 ma the altered units from the gold bearing dzs carry a different remanence rlg withd 180 i 57 k 23 95 3 n 3 units 6 sites rlg also has relatively high coercivities and unblocking temperatures but its magnetization intensities are 1 2 orders of magnitude lower than those of rlr opaque minerals and silicates observed in thin section are highly altered rlg is therefore believed to be a chemical or thermochemical remanence acquired 2580 ma ago during the late stages of cooling and gold mineralization seeing as rlg characterizes the shear zone rocks but is absent from unaltered rocks outside the dzs it could be used as a rapid prospecting tool in delineating areas of alteration and possible mineralization the paleopoles for rlr 2700 ma and rlg 2580 ma link with those of the shelley lake granite 2580 ma 40ar 39ar hornblende and biotite and the matachewan hearst dikes 2450 ma u pb zircon and baddeleyite the sense of motion on the late archean early proterozoic apparent polar wander path for laurentia as defined by this sequence of paleopoles is opposite to that previously accepted

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of el chichon volcano mexico the detailed stratigraphic study of the pyroclastic surge units s1 iu and s3 produced during the most violent phases of the 1982 eruption of el chichon volcano contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates based on their morphology and internal structure four different types of aggregates were distinguished a accretionary lapilli b armored lapilli c irregular aggregates and d cylindrical aggregates the first three types have been described in the volcanological literature field and experimental studies cylindrical forms are reported here for the first time these hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer grained matrix they formed around millimeter size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges sem analyses suggest different mechanisms of formation for the four types of aggregates irregular aggregates and armored lapilli formed nearly instantaneously whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

magnetic susceptibility magnetic mineralogy and magnetic fabrics in a late archean granitoid gneiss belt abstract the opatica belt is a late archean granitoid gneiss terrane situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt quebec this belt is composed of several suites of tonalite granodiorite granite composition variably deformed under epidote amphibolite to upper amphibolite grade conditions specimens from 87 sampling sites within the opatica belt show a wide range in variation of magnetic susceptibility k the large variations are due essentially to varying proportions of ferromagnetic trace minerals detailed study of the opaque mineralogy by reflected light petrography electron microprobe analysis and sem observations as well as alternating field and thermal demagnetization experiments confirm that multi domain very pure magnetite is the dominant ferromagnetic phase which along with ferrosilicates contribute to the magnetic susceptibility in some cases hematite may also make a significant contribution to k in rocks with low k the susceptibility may be dominated by the ferrosilicates hornblende and biotite intensity of anisotropy of magnetic susceptibility ams is also highly variable and appears to be controlled by the mineralogy of the rock rather than by strain intensity or deformation state the shapes of ams ellipsoids are also controlled by mineralogy detailed comparisons are made of ams fabrics with field measurements of magmatic and metamorphic foliations and mineral and stretching lineations and with mineral lineations determined by image analysis of polished slabs there is very good agreement between the average orientations of poles to structural foliations and the average orientation of the k3 principal axes of the susceptibility ellipsoids in all outcrops in most cases there is also good agreement between average structural lineations and the average k1 axis in quartz monzodiorites disagreement between the average lineation and average k1 may be due to complex textural occurrences of magnetite and the presence of abundant hematite in gneisses broad scatter of the orientations of k1 for individual samples from an outcrop quantified by statistical confidence regions about the principal axes locally occurs within the foliation plane and may be attributed to the partial overprinting of earlier fabrics by subsequent deformation

paleomagnetic and rock magnetic evidence for inverse zoning in the parguaza batholith southwestern venezuela and its implications about tectonics of the guyana shield abstract we report paleomagnetic and rock magnetic data from the rapakivi granites of the parguaza batholith guyana precambrian shield southwestern venezuela these results suggest that the pluton is inversely zoned with respect to the cooling ages in order to explain such an age pattern tentative structural settings are proposed placing the parguaza intrusions in a plate tectonic context six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith thermomagnetic curves x ray diffraction and fluorescence hysteresis loops thermal and alternating field af intensity plots transmission tem and scanning sem electron microscope analyses and konigsberger ratios qn values were used to identify the different magnetic mineralogies and their distribution of grain sizes magnetite titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations nrms in these rocks magnetic granulometry indicators such as konigsberger ratios qn values suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect the paleomagnetic results reveal the existence of two primary thermoremanent and or thermochemical magnetizations trms tcrms for sites csp 3 pi 2 and pi 4 decl 328 incl 21 k 15 95 11 4 and site csp 2 pi 1 and pi 3 decl 16 incl 87 k 10 95 13 respectively there is also a poorly defined g3r2 magnetization decl 284 incl 86 k 6 95 27 found in sites csp 2 pi 1 and pi 3 the overlap of coercivities and the unblocking temperatures spectra probably resulting from the coexistence of primary single domain magnetic mineralogies with secondary exsolutions of single domain like ti poor fe rich regions almost pure magnetite in multidomain titanomagnetite grains in most cases precludes the complete resolution of hybrid g1 n r or g3 n r the relative ages for these components were determined using a map of rb sr model age chrontours g1 and g3 are the older and the younger trms tcrms respectively and were acquired at two discrete moments of the batholith s geological history the final map resulting from integrating the paleomagnetic and rb sr data shows an age pattern for this batholith and the rest of the intrusions that belong to the parguaza igneous complex that could be explained either as the effect of inverse cooling of a single intrusive body the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of ne trending horsts and grabens cutting through the pluton because of its feasibility and agreement with the most recent theories about the tectonic evolution of the guyana shield we favor the latter hypothesis

magmatic evolution of the austral patagonian andes abstract new geological evidence and geochronological data obtained within the patagonian cordillera between lat 47 and 49 s allow the redefinition of an andean region with different and characteristic geological evolution compared to the adjacent segments an acidic middle jurassic volcanism covered a metamorphic basement formed by low grade metasediments and metabasites of upper paleozoic age dike swarms of basic alkaline rocks were emplaced since the cretaceous in a marginal basin developed on the jurassic sialic crust the outcrop density of such bodies seems to indicate an attenuation of the sialic crust towards the south where oceanic crust of upper jurassic lower cretaceous age floored the marginal basin the cretaceous volcanic activity is well developed in the adjacent sectors but almost absent in the studied segment several magmatic pulses lead to the final emplacement of the patagonian batholith which yields its peak activity around 98 m y and presents typical geochemical variations the upper cretaceous and cenozoic magmatism are mainly exposed to the east of the cordillera and are represented by a series of alkali plateau basalts different pulses of activity have been recorded during eocene upper miocene pliocene and even during quaternary times associated with deep crustal fractures and probable upper mantle origin as evidenced by the low 87sr 86sr ratios and the ultramafic and mafic xenoliths of the lavas granitic stocks in the andean domain are post orogenic in relation to a compressive tectonic phase during miocene times the upper cenozoic calc alkaline volcanism presents a gap in this segment of the andean belt probably related with the post miocene migration of the chile rise

paleomagnetic evidence for post 2 55 ga tectonic tilting and 1 1 ga reactivation in the southern kapuskasing zone ontario canada we report paleomagnetic results from two traverses in the southern part of the kapuskasing structural zone ksz of north central ontario which is hypothesized to be an oblique cross section through upper and middle archean crust exposed by uplift and tilting anorthosites and granulite grade gneisses were sampled to the east in the shawmere traverse while amphibolite grade tonalitic gneisses were sampled to the west in the racine traverse three distinct remanent magnetization rm components were isolated by alternating field and thermal demagnetization the a rm has mean directions d 50 i 62 k 138 95 5 n 5 sites 23 samples for the shawmere traverse and d 23 i 53 k 49 95 7 n 8 sites 46 samples for the racine traverse because of the high 500 c laboratory unblocking temperatures of the a rm in the shawmere granulite grade rocks we believe this magnetization was acquired in early slow uplift without tilting from about 24 km to about 16 km depths and has survived the later main ksz uplift and tilting event the racine a rm has laboratory unblocking temperatures down to 400 c and was blocked prior to uplift from a shallower crustal level about 8 km with tilt corrections of 30 and 15 respectively about the 35 striking axis of the ksz the shawmere and racine a paleomagnetic poles fall on the laurentian apparent polar wander path around 2 55 ga dating the time of initial uplift crustal tilting in the ksz occurred after 2 55 ga and was greater in the east than in the west the b rm found mainly in shawmere anorthosites has d 115 i 68 k 17 95 17 n 4 sites 14 samples with a paleopole at 144 w 49 n near 1 1 ga keweenawan reverse poles and the normal pole of the nearby 1 0 ga nemegosenda carbonatite the b rm is probably a thermochemical overprint related to reactivation of the ksz at the time of keweenawan rifting and volcanism in the lake superior region a novel c rm with d 64 i 2 k 18 95 13 n 6 sites 10 samples is either an unresolved hybrid of the a and b rms or an earliest keweenawan 1 3 ga overprint

probabilistic tephra fall simulation for the auckland region new zealand abstract the auckland region new zealand is at significant risk from tephra falls originating both from the local auckland volcanic field avf and several distant large volume centres we use geological data and observations of historical eruptions to develop a catalogue of simulated tephra dispersal patterns for the auckland region using the ashfall model simulated patterns relate to individual eruptive phases and can be combined to simulate events that vary in volume and duration avf eruption parameters were chosen to replicate fine tephra fall dispersed from small volume phreatomagmatic eruptions modelling of eruptions from distant volcanoes was carried out in two stages first the sensitivity of modelled tephra thickness to various model parameters was tested by far the largest thickness variations were the result of varying wind conditions followed by eruption volume the second stage involved modelling volumes that best replicated thicknesses of preserved events within the auckland region allowing the catalogue to be created only about 1 of simulations from distant volcanoes reached the auckland region suggesting that preserved tephra layers are either the result of reasonably long lived eruptions or a number of eruptive phases for eruptions originating within the avf mean modelled tephra thickness on land is small approximately 1 6 mm near the coast and thinning rapidly inland mean andesitic thicknesses are dominated by eruptions from egmont volcano which typically deposit tephra over a narrow section of the auckland region with a maximum thickness of approximately 1 7 mm rhyolitic eruptions are more widespread with mean thickness varying from 30 mm in the south of the region to 9 mm in the north when all centres are combined and weighted by their conditional likelihood of occurrence mean thickness ranges from 7 mm in the south to 1 5 mm in the north however maximum thicknesses of 150 mm for the avf 12 mm for andesitic centres and 830 mm for rhyolitic centres all have the potential to cause much greater damage within parts of the auckland region

the long life of samba connection in columbia a paleomagnetic study of the 1535 ma mucajai complex northern amazonian craton brazil abstract in recent years there has been a significant increase in the paleomagnetic data of the amazonian craton with important geodynamic and paleogeographic implications for the paleo mesoproterozoic columbia supercontinent a k a nuna hudsoland despite recent increase of paleomagnetic data for several other cratons in columbia its longevity and the geodynamic processes that resulted in its formation are not well known a paleomagnetic study was performed on rocks from the 1535 ma amg anorthosite mangerite rapakivi granite mucajai complex located in the roraima state brazil in the northern portion of the amazonian craton the guiana shield thermal and af treatments revealed northwestern southeastern directions with upward downward inclinations for samples from twelve sites this characteristic remanent magnetization is mainly carried by ti poor magnetite and in a lesser amount by hematite site mean directions were combined with previous results obtained for three other sites from the mucajai complex producing the dual polarity mean direction dm 132 2 im 35 4 n 15 95 12 7 k 10 0 and a paleomagnetic pole located at 0 1 e 38 2 s a95 12 6 k 10 2 the mucajai pole favours the samba south america baltica link in a configuration formed by amazonia and baltica in columbia also there is geological and paleomagnetic evidence that the juxtaposition of baltica and laurentia at 1 76 1 26 ga forms the core of columbia the present paleomagnetic data predict a long life 1 78 1 43 ga samba connection forming part of the core of the supercontinent

timing and nature of volcanic particle clusters based on field and numerical investigations aggregation processes are known to play an important role in volcanic particle dispersal and sedimentation they are also a primary source of uncertainty in ash dispersal forecasting since fundamental questions such as the timing and deposition dynamics of volcanic aggregates still remain unanswered here we applied a state of the art combination of field and numerical strategies to characterize volcanic aggregates we introduce a new category of aggregates observed with high speed high resolution videos namely cored clusters cored clusters are mostly sub spherical fragile aggregates that have never been observed in the deposits nor on adhesive tape as they typically break at impact with the ground they consist of a core particle 200 500 m fully covered by a thick shell of particles 90 m the low preservation potential of cored clusters in ash deposits explains the poor documentation in the literature and the low consideration attributed so far cored clusters can also better explain the deposition of fine ash in proximal and medial regions and the polymodality observed in many ash deposits in addition numerical inversions show how cored clusters can rapidly form within 175s from eruption onset finally our observations represent the first field based evidence of the so called rafting effect in which the sedimentation of coarse ash in cored clusters is delayed due to aggregation

the crustal structure of the guayana shield venezuela from seismic refraction and gravity data abstract we present results from a seismic refraction experiment on the northern margin of the guayana shield performed during june 1998 along nine profiles of up to 320 km length using the daily blasts of the cerro bolivar mines as energy source as well as from gravimetric measurements clear moho arrivals can be observed on the main e w profile on the shield whereas the profiles entering the oriental basin to the north are more noisy the crustal thickness of the shield is unusually high with up to 46 km on the archean segment in the west and 43 km on the proterozoic segment in the east a 20 km thick upper crust with p wave velocities between 6 0 and 6 3 km s can be separated from a lower crust with velocities ranging from 6 5 to 7 2 km s a lower crustal low velocity zone with a velocity reduction to 6 3 km s is observed between 25 and 25 km depth the average crustal velocity is 6 5 km s the changes in the bouguer anomaly positive 30 mgal in the west and negative 20 mgal in the east cannot be explained by the observed seismic crustal features alone lateral variations in the crust or in the upper mantle must be responsible for these observations

the immediate environmental effects of tephra emission the earth s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere these tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents the interaction of tephra with or in receiving environments may induce an array of physical chemical and biological effects the consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent small eruptions while larger and rarer events may have acute regional scale impacts it is therefore necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems we collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post eruptive effects of tephra emission into the atmosphere onto vegetation soil or ice snow surfaces and in aquatic systems we further consider the repercussions that may be induced in the weeks to years afterwards in the atmosphere tephra can influence cloud properties and air chemistry by acting as ice nuclei in or by offering sites for heterogeneous reactions respectively tephra on vegetation causes physical damage and sustained coverage may elicit longer term physiological responses tephra deposits on soils may alter their capacity to exchange gas water and heat with the atmosphere or may have a specific chemical effect such as nutrient input or acidification on sensitive soils tephra deposition onto snow or ice may affect ablation rates rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged months or years erosion from the tephra covered catchment in the first weeks after deposition tephra leaching may affect river chemistry the abundance and speciation of phytoplankton populations in lakes may be altered by tephra induced changes in water chemistry or sediment water nutrient cycling in the oceans tephra deposition may fertilise fe limited waters with potential impacts on the global carbon cycle embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies combined with consideration of the interconnectivity of induced impacts within and between different receiving environments

fall out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of st vincent abstract the 1979 explosive activity of soufriere volcano st vincent commenced on april 13th and continued for two weeks producing eleven eruption columns the activity produced air fall ash with abundant accretionary lapilli plus minor pyroclastic flows and base surge deposits the dispersal of the columns was observed by an sms 1 geostationary satellite all but two of the plumes were dispersed entirely to the east the last event on april 26th formed a minor lobe to the east and a major lobe to the south southeast the april 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour the magma discharge rate was greater than 3000 m3 s the plume expanded at a rate of 4000 km2 hr and was dispersed at a mean velocity of 15 m s the ash deposits show bimodal grain size distribution with 95 of the ash collected being sub 1 mm diameter modelling of the ash fall out has allowed interpretation of grainsize parameters in a quantitative manner the fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density the coarse population of the april 26th deposit displayed a systematic change in md and away from source the fine mode displays uniform md and everywhere and is interpreted as having been deposited in the form of accretionary lapilli

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

sulfur as a binding agent of aggregates in explosive eruptions understanding the mechanisms that govern ash aggregation is of critical importance in volcanology aggregation reduces the residence time of ash 2 mm in the atmosphere strongly by enhancing the sedimentation of finer grained material generated during explosive eruptions to date experimental studies have focused on the expectation that water provides the strongest bonds between particles to form spherical to oblate aggregates typically less than or equal to a few mm occasionally up to several cm preserved in pyroclastic deposits under water rich conditions individual accreted particles rarely exceed 1 mm in pyroclastic density current deposits produced during the 1982 eruption of el chichon mexico which emitted 7 5 tg of so2 in the atmosphere aggregate particles one to several millimeters strongly cemented by a s rich film are common they exhibit similarities with aggregates found in sulfur cones at poas volcano costa rica we propose that sulfur is the binder between the silicate grains such a binding capacity implies a relatively fluid behavior of sulfur such as might be expected in the low viscosity temperature regime just above its melting point if so then the explosive ejection of sulfur during eruptions combined with its ability to act as a cement for particles 2 mm would imply that size fractions of lapilli can be efficiently removed from eruptive clouds a few kilometers from the vent such an aggregation mechanism would have important implications for pyroclast dispersal models and hazard assessment

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

the aerodynamic behaviour of volcanic aggregates a large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 m in diameter the majority of these particles become incorporated into a wide range of aggregate types the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory in the absence of such data theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities larger volcanic ejecta often consists of pumice of lower than bulk density experimental data are presented for the fall velocities of porous aggregates and single particles determined in systems analogous to that of ejecta falling from a volcanic plume it is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity and a size corresponding to an enclosing sphere particles incorporated into aggregates attain a substantially higher fall velocity than single particles this is due to the larger physical dimensions of the aggregate which overcomes the effect of lower aggregate density additionally the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity the increase in fall velocity of particles incorporated into aggregates rather than falling individually results in the enhanced removal of fine material from volcanic plumes

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles such as following explosive eruptions and meteorite impacts we identify successive stages in the growth history of particle aggregates based upon well preserved ash aggregate bearing pyroclastic layers on tenerife the layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper widespread coignimbrite ash fall layer the upper part of each ignimbrite contains whole and fragmented concentric laminated accretionary lapilli whereas the overlying coignimbrite ash fall layer lacks accretionary lapilli and is composed of framework supported smaller and nonlaminated ash pellets sometimes slightly deformed or partly disaggregated the pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer these field relations are repeated numerous times in several different successions and they indicate that ash pellets not accretionary lapilli form within the coignimbrite ash plumes some pellets fell directly to the ground producing coignimbrite ash fall layers but others settled into pyroclastic density currents where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli the fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli numerous ignimbrite veneers on tenerife have the form of ash layers a few centimeters thick that drape topography and locally contain matrix supported accretionary lapilli most volcanoes lack laterally continuous field exposure and such accretionary lapilli bearing layers might be mistaken for ash fall deposits we highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits for example during hazard assessments

aggregation dominated ash settling from the eyjafjallajokull volcanic cloud illuminated by field and laboratory high speed imaging the recent eyjafjallajokull iceland eruption strikingly underlined the vulnerability of a globalized society to the atmospheric dispersal of volcanic clouds from even moderate size eruptions ash aggregation controls volcanic clouds dispersal by prematurely removing fine particles from the cloud and depositing them more proximally physical parameters of ash aggregates have been modeled and derived from ash fallout deposits of past eruptions yet aggregate sedimentation has eluded direct measurement limiting our ability to predict the dispersal of volcanic clouds here we use field based high speed video analysis together with laboratory experiments to provide the first in situ investigation and parameterization of the physical features and settling dynamics of ash aggregates from a volcanic cloud in may 2010 high speed video footage was obtained of both ash particles and aggregates settling from the eyjafjallajokull volcano eruption cloud at a distance of 7 km from the vent fallout samples were collected simultaneously experimental laboratory determinations of the density morphology and settling velocity of individual ash particles enable their distinction from aggregates the combination of field and experimental analyses allows a full characterization of the size settling velocity drag coefficient and density distributions of ash aggregates as well as the size distribution of their component particles we conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud aggravating the ash hazard locally and modifying cloud dispersal regionally this study provides a valuable tool for monitoring explosive eruptions capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecast

experimental investigation of volcanic particle aggregation in the absence of a liquid phase 1 understanding the dispersal and deposition of fine grained silicate particles from volcanic plumes is key to interpreting ash fall deposits and predicting hazards for future eruptions it is known that many of these particles fall incorporated into delicate dry aggregates whose sedimentation characteristics have not been previously investigated here we present the results of laboratory experiments on aggregates of small dry silicate particles produced by the fragmentation of pumice collected from the 18 may 1980 mount st helens fall deposit the aggregation process is driven by electrostatic charges naturally imparted to the particles during the fracture process for particle fall distances of 1 m images of the in flight aggregates show that they commonly have irregular shapes and are up to 800 m in size strobe photography was used to determine aggregate fall velocities and by representing aggregates as falling spheres suggested that they had densities of 100 200 kg m 3 comparable densities were obtained from experiments where equivalent fall velocities were assumed for aggregates and single particles which had been transported similar distances within a horizontal airflow these dispersal experiments produced bimodal particle size distributions similar to those observed in the 18 may 1980 mount st helens deposits and suggest that the aggregates were composed mainly of particles 70 m in diameter our experimental results are in agreement with aggregate size and density estimates previously used within several theoretical plume sedimentation models in order to explain some features of natural ash deposits

volcanic particle aggregation in explosive eruption columns part i parameterization of the microphysics of hydrometeors and ash abstract the aggregation of volcanic ash particles within the eruption column of explosive eruptions has been observed at many volcanoes it influences the residence time of ash in the atmosphere and the radiative properties of the umbrella cloud however the information on the processes leading to aggregate formation are still either lacking or very incomplete we examine the fate of ash particles through numerical experiments with the plume model atham active tracer high resolution atmospheric model in order to determine the conditions that promote ash particle aggregation in this paper we describe the microphysics and parameterization of ash and hydrometeors in a companion paper this issue we use this information in a series of numerical experiments the parameterization includes the condensation of water vapor in the rising eruption column the formation of liquid and solid hydrometeors and the effect of latent heat release on the eruption column dynamics are considered the interactions of hydrometeors and volcanic ash within the eruption column that lead to aggregate formation are simulated for the first time within a rising eruption column the microphysical parameterization follows a modal approach the hydrometeors are described by two size classes each of which is divided into a liquid and a frozen category by analogy with the hydrometeor classification we specify four categories of volcanic ash particles we imply that volcanic particles are active as condensation nuclei for water and ice formation ash can be contained in all categories of hydrometeors thus forming mixed particles of any composition reaching from mud rain to accretionary lapilli collisions are caused by gravitational capture of particles with different fall velocity coalescence of hydrometeor ash aggregates is assumed to be a function of the hydrometeor mass fraction within the mixed particles the parameterization also includes simplified descriptions of electrostatics and salinity effects

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

paleoproterozoic 2155 1970 ma evolution of the guiana shield transamazonian event in the light of new paleomagnetic data from french guiana abstract we present a comprehensive paleomagnetic study on paleoproterozoic 2173 2060 ma plutonic and metamorphic rocks from french guiana representative of the full range of the main transamazonian tectonothermal steps twenty seven groups of directions and poles were obtained from combination of 102 sites 613 samples based on age constraint similar lithology and or geographical proximity paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases this is also reinforced by positive field tests baked contact and reversal tests recent u pb and pb pb on zircon and complementary 40ar 39ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages estimated magnetic ages based on amphibole dating as a proxy enable us to construct a guiana shield apparent polar wander path for the 2155 1970 ma period it is also possible to present paleolatidudinal evolution and continental drift rates related to specific transamazonian tectonic regimes french guiana and probably the guiana shield were located at the equator from ca 2155 to 2130 ma during the meso rhyacian d1 magmatic accretion phase related to subduction of eorhyacian oceanic crust after closure of the eorhyacian ocean and collision of west african and amazonian plates the guiana shield moved the first evolution towards 60 latitude occurs after 2080 ma during the neorhyacian d2a post collisional sinistral transcurrent phase during the late rhyacian d2b phase up to 2050 ma the guiana shield reaches the pole and starts to move to lower latitudes on an opposite meridian by the orosirian d2c phase from ca 2050 to 1970 ma the guiana shield reaches the equator based on the amphibole 40ar 39ar dates we estimate the continental drift between 12 and 16 cm y for the meso to late rhyacian period followed by a lower rate between 9 and 14 cm y up to orosirian time this study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100 200 ma time period our results confirm the possibility of apwp construction on paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi method radiometric dating

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km