sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

successive overprinting granulite facies metamorphic events in the anmatjira range central australia the anmatjira range and adjacent reynolds range central australia comprise early proterozoic metasediments and othogneisses that were affected by three and possibly four temporally distinct metamorphic events m1 4 and deformation events d1 4 in the period 1820 1590 ma the north western portion of the range around mt stafford preserves the effects of 1820 ma m1 d1 and shows a spectacular lateral transition from muscovite quartz bearing schists to interlayered andalusite bearing migmatites and two pyroxene granofelses that reflect extremely low pressure granulite facies conditions over a distance of less than 10 km orthopyroxene cordierite garnet k feldspar quartz bearing gneisses occur at the highest grade implying peak conditions of 750 c and 2 5 0 6 kbar an anticlockwise p t path for m1 is inferred from syn to late d1 sillimanite overprinting andalusite petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages the effects of m1 have been variably overprinted to the south east by a c 1760 ma m2 d2 event much of the central anmatjira range around ingellina gap comprises orthogneiss deformed during d2 and metapelites that have m1 andalusite and k feldspar overprinted by m2 sillimanite and muscovite the south eastern portion of the range around mt weldon comprises metasediments and orthogneisses that were completely recrystallized during m2 d2 with metapelitic gneisses characterized by spinel sillimanite k feldspar quartz bearing assemblages that suggest peak m2 conditions of 750 c and 5 5 1 kbar overprinting parageneses in metapelitic gneisses imply that d2 occurred during essentially isobaric cooling a third granulite facies event m3 affected rocks in the reynolds range immediately to the south of the anmatjira range at c 1730 ma a possible fourth event m4 with a minimum age of c 1590 my affected both ranges but resulted in only minor overprinting of m1 3 assemblages the superimposed effects of m1 4 mapped for the entire anmatjira reynolds range area indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding events although the immediate cause of each of the metamorphic events involved advection the ultimate causes were external to the metasediments and most probably external to the crust

atmosphere sea ice interactions during a cyclone passage investigated by using model simulations and measurements abstract a high resolution atmosphere sea ice model is used to investigate the interactions between cyclones and sea ice cover in polar regions for this purpose a cyclone passage observed during the 1999 fram strait cyclone experiment framzy is simulated for two consecutive days the results of the coupled mesoscale transport and stream model mesoscale sea ice model metras mesim are compared with aircraft and ice drift measurements with the exception of temperature all atmospheric parameters are well simulated main reasons for discrepancies were found in large differences between the measurements and the forcing data taken from the results of the regional model remo in addition advection was slightly wrong as a result of a 17 deviation in wind directions the altogether well simulated wind field is interactively used to force the sea ice model mesim results agree well with drift buoy measurements average deviations of simulated and measured ice drift are smaller than 8 for direction and s

neotectonics in central mexico regional neotectonic activity in central mexico was mapped through analysis of eleven landsat thematic mapper images the study area stretches from mexico city on the east to the gulf of california on the west between 17 and 21 n latitude fault scarps of plio quaternary age define several large scale tectonic features within the mexican volcanic belt dominated by rifting transtension and shear with probable sinistral components three large crustal blocks south of the mexican volcanic belt are separated from the north american plate and from each other by zones of neotectonic activity that express the relative motions between the adjacent blocks a simple vector diagram for plates and crustal blocks in central mexico has been constructed using information from the literature combined with the orientations of fault zones as mapped in this study the relative motion between the blocks varies from about 2 5 to 8 mm year 1 which is comparable to motions of crustal blocks in areas of active rifting in other parts of the world in the mexican volcanic belt there is a clear relationship between volcanism and neotectonic activity fault zones that traverse nearly the entire region have the same location and orientation as the volcanic belt this may indicate that deformation within the crust of the overriding slab plays a significant role in determining the unusual orientation of the mexican volcanic belt relative to the trench the ultimate factor controlling the location of plio quaternary volcanism and tectonism in central mexico may be large zones of crustal weakness formed during major mesozoic and cenozoic tectonic events

constraints and deception in the isotopic record the crustal evolution of the west musgrave province central australia abstract the hf and nd isotopic evolution of the musgrave province central australia is used to constrain the timing of crust formation and lithospheric organisation of proterozoic australia the dataset from this region challenges two widely held tenets of hf and nd isotope systematics namely that crust formation events can only be identified as periods when crystallisation ages correspond to model ages and that linear hf evolution arrays away from depleted mantle along crustal lu hf or sm nd slopes reflect reworking of the same source hf isotopes in musgrave province zircon crystals indicate two major crust formation events at c 1900 ma and at 1600 1550 ma although no juvenile rocks or crystals are known from c 1900 ma radiogenic addition into the crust at this time is required to account for consistent nd and hf evolution patterns which show no indication of an initially heterogeneous source oxygen isotopes in zircon grains confirm that much of the c 1900 ma hf isotopic signal is not compromised by mixtures furthermore the correspondence between mantle extraction and the commencement of reworking of archean material supports new crust generation at c 1900 ma and a coupling between lower and upper crustal processes the c 1900 ma timing of juvenile addition is dissimilar to that in the albany fraser and arunta orogens and may reflect continental arc development on the margin of a southern continent the general hf isotopic evolution trend of the musgrave province apparently reflects reworking from a dominant c 1900 ma source with some additional unradiogenic and radiogenic input through time however in the 1220 1050 ma interval this apparent isotopic evolution contrasts with geological observations that indicate input of voluminous mantle derived material intracontinental rifts and other regions with sustained very high temperature crustal recycling processes generate magmatic provinces with extreme hfse enrichment this can have a profound influence on isotopic evolution trends suppressing typical juvenile addition patterns isotopic mixture modelling indicates that a significant volume of mantle derived material can be accommodated within hfse enriched magmas without diverging isotopic signatures from apparent reworking trends in the musgrave province the crust had become so hfse enriched during the prolonged musgrave orogeny 1220 1150 ma that it was insensitive to mantle input which is estimated to have been as much as 85 during this event

proterozoic evolution and tectonic setting of the northwest paterson orogen western australia abstract the proterozoic igneous deformation and metamorphic histories of the palaeoproterozoic rudall complex in the northwestern paterson orogen can be linked to those of the arunta inlier in central australia and in part with the capricorn orogen in central western australia the similarities in deformation and metamorphic histories for these widely separated regions indicate a palaeoproterozoic continent continent collisional event between the palaeoproterozoic west australian and north australian cratons between c 1830 and 1765 ma in the paterson orogen this palaeoproterozoic collisional event resulted in the yapungku orogeny which included thrust stacking of clastic sedimentary and volcanic rocks deposition of the protoliths for the c 1790 ma siliciclastic paragneiss succession contemporaneous with granitic intrusion and metamorphism up to granulite facies during this 65 million year period the arunta inlier and capricorn orogen were deformed metamorphosed at medium to high grades and intruded by granitoids during the strangways orogeny in the arunta inlier and the capricorn orogeny in the capricorn orogen the neoproterozoic tarcunyah throssell and lamil groups are clastic sedimentary sequences that were deposited after 1070 ma in the northwestern paterson orogen and deformed by the miles orogeny before 678 ma the miles orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults the orogeny may have been coincident with the c 750 720 ma areyonga tectonic movement affecting the arunta inlier and the lower neoproterozoic part of the amadeus basin in central australia at c 550 ma the paterson orogeny which is most likely equivalent to the petermann orogeny in the musgrave complex of central australia deformed the northwestern paterson orogen and was preceded by local intrusion of granites the similarities of styles and timing of deformation in the northwestern paterson orogen arunta inlier and capricorn orogen indicate that these three regions were probably linked during most of the proterozoic

impact of a fram strait cyclone on ice edge drift divergence and concentration possibilities and limits of an observational analysis 1 this study aims at the determination of a fram strait cyclone track and of the cyclone s impact on ice edge drift divergence and concentration a 24 h period on 13 14 march 2002 framed by two radarsat images is analyzed data are included from autonomous ice buoys a research vessel special sensor microwave imager ssm i and quikscat satellite and the operational european centre for medium range weather forecasts ecmwf model during this 24 h period the cyclone moved northward along the western ice edge in the fram strait crossed the northern ice edge made a left turn loop with 150 km diameter over the sea ice and returned to the northern ice edge the ecmwf analysis places the cyclone track 100 km too far west over the sea ice a deviation which is too large for representative sea ice simulations on the east side of the northward moving cyclone the ice edge was pushed northward by 55 km because of strong winds on the rear side the ice edge advanced toward the open water but by a smaller distance because of weaker winds there the ice drift pattern as calculated from the ice buoys and the two radarsat images is cyclonically curved around the center of the cyclone loop ice drift divergence shows a spatial pattern with divergence in the loop center and a zone of convergence around ice concentration changes as retrieved from ssm i data follow the divergence pattern such that sea ice concentration increased in areas of divergence and decreased in areas of convergence

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

grenvillian aged reworking of late paleoproterozoic crust of the southern north australian craton central australia implications for the assembly of mesoproterozoic australia abstract la icp ms u pb monazite and zircon geochronology from metapelites and migmatitic orthogneiss along the central southern margin of the north australia craton nac central australia reveal the presence of a regional scale grenvillian aged ca 1130 ma deformation system grenvillian aged deformation extends over a strike length of over 110 km from the southern aileron province in the vicinity of alice springs to the teapot granite complex within the warumpi province there is also evidence for grenvillian aged migmatisation and resetting of monazite further west in the mount liebig area which may extend the grenvillian aged footprint to a strike length of at least 250 km deformation associated with grenvillian aged reworking produced map scale east west trending folds and strongly foliated steeply dipping shear zones that define the structural architecture of the interface region between the aileron and warumpi provinces shallow westerly plunging folds associated with partial melting have evolved into shear zones and mylonites that record south side up movement phase equilibria modelling of age constrained garnet biotite sillimanite cordierite bearing metapelites from the southern aileron and warumpi provinces suggest grenvillian aged metamorphism reached temperatures in the range of 775 820 c and pressures of 5 5 5 kbar corresponding to thermal gradients of 130 165 c kbar 1 the extensive grenvillian aged system reworks and overprints late paleoproterozoic ca 1650 1630 ma high grade metamorphic rocks the grenvillian aged system occurs above a south dipping lithospheric scale interface that has been geophysically imaged to depths of at least 200 km this feature was interpreted to represent a fossil subduction zone of late paleoproterozoic age however the presence of regional scale grenvillian aged deformation in the crust above this feature suggests that it may instead be grenvillian aged if that is the case deformation along the southern margin of the nac may record suturing of the nac with the grenvillian aged musgrave province to the south this would effectively place the long lived late mesoproterozoic ultrahot orogen that is the musgrave province in southern central australia in an upper plate tectonic setting linked to the convergence of the nac with the south australian craton

a mesoscale cyclone over the fram strait and its effects on sea ice the ice export from the arctic ocean through the fram strait into the atlantic ocean is steered by atmospheric processes on the large scale as well as on the local scale a case study of a mesoscale cyclone over the fram strait and its effects on the sea ice drift is presented the cyclone developed on march 13 1993 at the ice edge of the greenland sea and moved northward over the ice and over the position of the research icebreaker polarstern and an array of six meteorological ice buoys the cyclone had a radius of about 150 km and was essentially restricted to the lowest 2 km of the atmosphere near the surface the temperature difference between the air in the warm sector and the surrounding cold air was 25 k resulting in a pressure difference of about 15 hpa between center and the surrounding with low level wind speeds of up to 20 m s the passage of the cyclone caused a full loop of the ice drift otherwise oriented from northeast to southwest the ratio of ice drift speed to wind speed increased from 5 before to 9 after the passage extreme values of ice deformation and large variations of the ice strain rate tensor occurred during the cyclone passage it is concluded that passing cyclones can affect the sea ice drift through the bottleneck of the fram strait and may have integral effects on larger time scales and space scales

paleoproterozoic gold deposits in the bald hill and coyote areas western tanami western australia significant gold deposits in the western tanami region of western australia include deposits in the bald hill and coyote areas the ca 1 864 ma bald hill sequence of turbiditic and mafic volcanic rocks hosts the kookaburra and sandpiper deposits and a number of smaller prospects the ca 1 835 ma turbiditic killi killi formation hosts the coyote deposit and several nearby prospects the kookaburra deposit forms as a saddle reef within a syncline and the sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline gold in these deposits is hosted by anastomosing quartz pyrite arsenopyrite veins within quartz sericite schist with disseminated arsenopyrite pyrite and marcasite after pyrrhotite based on relative timing relationships with structural elements the auriferous veins are interpreted to have been emplaced before or during the ca 1 835 1 825 ma tanami orogeny regional d1 gold deposition is thought to have been caused by pressure drops associated with saddle reef formation kookaburra and chemical reactions with graphitic rocks sandpiper the coyote deposit the largest in the western tanami region consists of a number of ore lenses localized along the limbs of the coyote anticline which formed during the tanami orogeny the largest lenses are associated with the gonzalez fault which is located along the steeply dipping southern limb of this fold gold was introduced at ca 1 790 ma into dilatant zones that formed in local perturbations along this fault during later reactivation regional d5 towards the end of a period of granite emplacement gold is associated with quartz chlorite pyrite arsenopyrite galena sphalerite veins with narrow 5 mm chloritic selvages a quartz muscovite biotite k feldspar tourmaline actinolite arsenopyrite assemblage which is interpreted to relate to granite emplacement overprints the regional greenschist facies metamorphic assemblage the mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the coyote deposit are associated with the granite related metamorphic metasomatic assemblage gold deposition is thought to have been caused by pressure drops within dilatant zones

rate of arunta inlier evolution at the eastern margin of the entia dome central australia abstract zircon dating in the entia dome of the arunta inlier helps to temporally define the multiphase successive strangways and harts range orogenies it appears that the entire activity of both orogenies took place between 1767 2 ma and 1731 1 ma with a major shearing and thrusting event associated with plutonic activity at 1747 2 3 ma these comparatively short time spans indicated are quite sufficient for the large horizontal movements required for the intra continental basin formation and deformation that has been postulated by james and ding the earlier of these events are later than those currently proposed for equivalent stages of other middle proterozoic inliers of the north australian orogenic province which raises difficulties with detailed correlations in grouping the inliers together monazite u pb muscovite rb sr and biotite rb sr isotopic systems had not closed until 360 ma 325 ma and 308 ma respectively indicating uplift and cover removal from 20 km to

an estimate of the mean field of arctic sea ice motion the observational basis for our understanding of sea ice motion comes from the trajectories of particular ice floes these ice floes may have been marked by a manned research station or more recently by an automatic data buoy since 1893 almost 100 station years of drift data have been recorded using the principles of optimal linear estimation these data are used to obtain a quantitative estimate of the field of mean motion

fracture control on type morphology and distribution of parasitic volcanic cones an example from mt etna italy abstract this paper uses a coupled morphometric and stratigraphic analysis of several tens of parasitic pyroclastic cones of the se flank of mt etna volcano italy to explain the structure of a poorly understood actively deforming zone of the volcano moreover we present a new general classification of pyroclastic cones based on the parameterisation of coeval eruption points along the same magma feeding fracture each cone has been studied in detail in order to define its stratigraphic position morphometric characteristics growth evolution in relation to fracture distribution and orientation stress field and morphology of the substratum end members of this classification are at one side a simple circular or elongated edifice that grew around a single eruption point and can be related to a magma feeding fracture constrained by a relatively high confining stress in the substratum or a low magma pressure the other end member is a multiple rifted strongly elongated cone formed along multiple eruption points of a magma feeding fracture constrained by a relatively low confining stress or a high magma pressure lithostratigraphic and historical analyses demonstrate that the cones in this area developed between 15 ka bp and historical times the most recent one in 1669 ad testifying to a high rate of lateral eruption occurrence in the area these cones show n s elongated maximum crater axes n s trending axis of the depressed points of the crater rim and southward and southeastward breaching directions while coeval cones are n s aligned these morphometric characteristics reflect the geometry of n s magma feeding fractures the area can be regarded as a transitional weakness zone between the n s rift and the releasing system represented by the transtensional trecastagni and mascalucia faults indicating an e w oriented local component of extension possibly related to the interplay between the regional timpe fault system and the eastward gravitational spreading of the volcano the azimuth of breaching is controlled by magma feeding fracture orientation and substrate morphology especially in the eastern part of the area where it is influenced by the scarps related to the timpe regional fault system but it is also sensitive to cone development

shrimp ionprobe dating of short lived proterozoic tectonic cycles in the northern arunta inlier central australia structural and metamorphic evidence from low p high t metamorphic terranes within the northern arunta inlier central australia indicates that two discrete tectonic cycles affected the region each cycle was associated with compressive deformation coincident with peak granulite facies metamorphism and granite intrusion shrimp u th pb analysis of zircons from early syn and late tectonic granitoids allows the age of the two tectonic cycles to be directly measured the age of the first cycle d1 is determined by emplacement of the mount stafford granite dated at or before 1818 15 ma after the first two foliations of the cycle had formed but before the axial plane cleavage associated with later upright folding the regional extent of this tectono magmatic event is confirmed by an emplacement age of 1818 8 ma measured on the large discordant harverson granite 40 km away the second cycle d2 consisted of four distinct deformation stages d2a 2d the ages of which are constrained by 1785 1775 ma old early syn and late tectonic granitoids the warimbi granite emplaced at or before 1785 22 ma and the granitic protolith to the napperby gneiss inferred to be 1780 10 ma pre date or formed during the earliest foliation s2a zircon considered to have grown at peak granulite facies metamorphic conditions during generation of the dominant s2b foliation gives an age of 1774 6 ma in the possum creek charnockite leucogranite in the napperby gneiss which post dates s2b and large scale upright nw se trending f2d folds yields an age of 1775 12 ma and confirms that the entire tectonic cycle d2a 2d formed in a rapid 10 ma period younger zircons which formed at 1659 6 ma in the napperby gneiss cannot be related to any local structural event but are similar in age to a major magmatic metamorphic episode in the southern arunta black and shaw 1992 150 km away they may be the peripheral manifestation of that event further zircon growth at 1600 ma in the napperby gneiss and possum creek charnockite was caused by hydrothermal fluid fluxing associated with post tectonic pegmatite emplacement these dykes were deformed by amphibolite facies retrograde shear zones ion probe dating of zircon stratigraphy from the granitoids has confirmed the discrete successive tectonic cycles identified by earlier structural and metamorphic studies for the northern arunta inlier at least those cycles or orogenies are associated with short lived 10 ma thermal pulses related to granite magmatism not prolonged thermal perturbations spanning 100 ma or more cf dirks and wilson 1990 dirks and hand 1991 such short lived pulses are intimately associated with granite intrusion and may be a general feature of low p high t metamorphism

volcano tectonic activity along structures of the unstable ne flank of mt etna italy and their possible origin abstract we describe the prehistoric historic activity of the main and minor volcano tectonic structures of the northeastern flank of mt etna giving new data which provide a clearer view of how destabilized volcanoes react to magma pressure and gravity forces data are mostly based on new lithostratigraphic surveys and on offset measurements of late pleistocene holocene deposits and historic man made features the north east rift ne rift is a volcanic rift that has guided mostly explosive but also secondary effusive activity since the end of the pleistocene monogenetic cones and explosive craters are strongly elongated and aligned along 66 main surface fractures and several others hidden but inferred from sinkhole alignments the opening direction is locally oblique with a left lateral component in the north eastern segment of the rift at the ne tip of the rift the movements are relieved by the normal left lateral pernicana fault pf an 11 km long structure comprising e w fault segments connected by right stepping en echelon faults our study demonstrates that the very high slip rates at a centennial scale of these structures are well constrained and similar ne rift 0 6 1 8 cm a pf 0 5 2 2 cm a and have been also recognized in the latest pleistocene holocene 2 7 cm a the e w fiumefreddo fault ff has normal left lateral kinematics and a historic slip rate of 0 1 cm a ripa della naca faults strike ne and are covered by unfaulted

importance of gravitational spreading in the tectonic and volcanic evolution of mount etna the interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude from small cones to the oceanic crust1 3 although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth it is rarely recognized as such here we describe a striking example of this process at etna volcano in italy the volcano and its clay rich substratum are slowly spreading towards the east and south driven by gravity spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity as gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions an appreciation of its effects could become an essential element of future volcanic hazard evaluation our model proposed here for etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes

structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite an example from the reynolds range central australia three types of zircon occur in a complexly deformed and variably migmatized quartzofeldspathic gneiss from the reynolds range central australia the oldest type is inherited from the granitic precursor of the gneiss and is overgrown by a second group of zircon grains that formed during prograde granulite facies metamorphism partial melting of the gneiss resulted in solution of both the inherited and metamorphic zircon no new zircon growth accompanied crystallization of the partial melt suggesting loss of zirconium rich residual fluids hydrous amphibolite facies retrogression of the gneiss and its migmatized variants during late shearing produced new idiomorphic zircon in both the shear zone and its wall rocks important implications of this study are that i zircon has a tendency to dissolve if it comes into direct contact with a melt produced from anhydrous biotite breakdown in a quartzofeldspathic granulite ii melt crystallization is not necessarily accompanied by zircon growth and iii euhedral zircon can grow from a hydrous fluid phase under subsolidus amphibolite facies conditions e g within shear zones

location and orientation of eruptive fissures and feederdykes at mount etna influence of gravitational and regional tectonic stress regimes abstract lava extrusion from fissures on the flanks of the etna volcano is a common feature of recent activity as evidenced by the eruptions of 1971 1981 1983 and 1985 and was of similar importance during prehistoric times we propose here a model within which the location and orientations of the large majority of historic eruptive fissures on etna s flanks together with their feeder dykes are explained within the framework of a gravitational stress regime stress orientations within this regime are controlled largely by the gross morphology of the volcano but are significantly modified by local topographic anomalies we propose that the gravitational stress regime largely supercedes an earlier developed regional tectonic stress regime which still operates in the sub volcanic basement and may remain the dominant control of eruptive fissure and feeder dyke disposition at lower altitudes on the flanks the proposed model is supported by 1 detailed examination of fissure locations and orientations and the relationship between fissure azimuths and topography 2 precise horizontal distance measurement and vertical ground deformation studies of a recent eruption and 3 qualitative physical modelling experiments in which coloured water is injected into solidified gelatine solutions representing variously shaped volcanic constructs a cautious approach is advocated when extrapolating trends of spatially limited fissure segments for the purpose of locating potential sites of magma ascent and implications of the model for hazard analysis are outlined

a curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites new constraints on p t estimates in the antarctica segment of the musgrave albany fraser wilkes orogen the windmill islands region in wilkes land east antarctica preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable p t estimates from conventional thermobarometry and mineral equilibria modelling this is uncommon in granulite facies terranes where conventional thermobarometry and phase equilibria modelling generally produce conflicting p t estimates because peak mineral compositions tend to be modified by retrograde diffusion processes in situ u pb monazite geochronology and calculated metamorphic phase diagrams show that the windmill islands experienced two phases of high thermal gradient metamorphism during the mesoproterozoic the first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c 1305 ma this event was regional in extent involved crustally derived magmatism and reached conditions of 3 2 5 kbar and 690 770 c corresponding to very high thermal gradients of 150 c kbar the elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back arc setting that existed prior to c 1330 ma the first metamorphic event was overprinted by granulite facies metamorphism at c 1180 ma that was coeval with the intrusion of charnockite this event involved peak temperatures of 840 850 c and pressures of 4 5 kbar a phase of granitic magmatism at c 1250 1210 ma prior to the intrusion of the charnockite is interpreted to reflect a phase of compression within an overall back arc setting existing conventional thermobarometry suggests conditions of 4 kbar and 750 c for m1 and 4 7 kbar and 750 900 c for m2 the apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly or that the p t ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of p t conditions obtained in this study however without phase equilibria modelling the veracity of existing conventional p t estimates cannot be evaluated the calculated phase diagrams from this study allow the direct comparison of p t conditions in the windmill islands with phase equilibria models from other regions in the musgrave albany fraser wilkes orogen this shows that the metamorphic evolution of the wilkes land region is very similar to that of the eastern albany fraser orogen and musgrave province in australia and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the 5000 km strike length of this system this article is protected by copyright all rights reserved

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

shrimp u_pb zircon geochronology of granites in the arunta inlier central australia implications for proterozoic crustal evolution abstract new shrimp u pb zircon isotopic ages are reported for ten granitic suites in the eastern arunta inlier these data combined with previous u pb zircon studies in the region constrain the timing of major palaeoproterozoic igneous events within the arunta inlier and establish a geochronological framework for understanding lithostratigraphic relationships tectonic events and geochemical and isotopic evolution of the granites in the region most granites in the arunta inlier including both syn tectonic and post tectonic types yield zircon ages in the range of 1770 1710 ma which are significantly younger than the 1880 1850 ma orogenic barramundi igneous association recognised throughout many other proterozoic terrains in northern australia no rocks of the barramundi association have been identified in this study the previously proposed barramundi type suites including the jervois and dneiper granites yield ages of 1771 6 and 1771 15 ma respectively many of the granite suites contain inherited zircons with ages in the range of 1890 1830 ma similar to the age of the barramundi association this and the age of the atnarpa igneous complex zhao and cooper 1992 which is coeval with but geochemically distinct from the barramundi association suggest that the 1880 1850 ma orogenic event affected the arunta inlier but has been obscured by the younger tectonic events the youth of tectonic events in the arunta inlier as compared with other proterozoic terrains of northern australia may have resulted from the inlier being located on the southern margin of the northern australian orogenic province during the palaeoproterozoic with the generation of granites in the arunta inlier possibly being related to subduction the combined data indicate that major granitic activity occurred at 1880 1850 1820 1770 1750 1730 1710 1660 1650 and 1615 1590 ma this result is broadly consistent with earlier summaries of the other proterozoic terrains in northern australia although the intensity of each episode varies in different regions

active spreading and regional extension at mount etna imaged by sar interferometry abstract large flank instability has been proposed by most recent authors as a major process in the dynamics of mount etna however several aspects of the models are still disputed such as the boundaries of the moving area and the driving force for the instability in this paper we present synthetic aperture radar interferometric data which allow us to identify two main sectors of active instability where the deformational process differs an eastern sector is bounded to the north by the pernicana provenzana fault to the west by the north rift zone and the south rift zone and to the south by the mascalucia tremestieri trecastagni fault system interferograms provide new data on the activity and extent of the southern boundary which was previously underestimated in this sector the structural features indicate an eastward sliding driven by e w regional extension which can be interpreted as a result of the retreating slab of the ionian subduction a second collapsing sector exists along the southern flank of mount etna where the interferograms show the presence of an active anticlinal ridge this ridge is interpreted as the result of southward gravity spreading over a basal decollement between etna and its plio quaternary basement and the hyblean platform the two sectors first became active in summer 1996 after the beginning of a new cycle of eruptive activity at the summit the activity was ongoing until january 1998 with the deformation rate ranging from 4 6 mm yr for the mascalucia tremestieri trecastagni faults to 12 mm yr for the anticlinal ridge

an assessment based on u_pb zircon data of rb_sr dating in the arunta inlier central australia abstract the chronology of the arunta inlier was originally based mostly on rb sr dating although some of the ages derived from that technique are now shown to be erroneous those which were constrained by structural observations and which relied only on whole rock and not mineral analyses have proven to be remarkably realistic ion microprobe data show that even conventional multigrain u pb zircon analysis is not a particularly appropriate dating technique in a region of this complexity three new u pb zircon ages and others previously derived from the same isotopic system date three of the main tectonic events that were imposed on the arunta inlier the widespread strangways orogeny of the northern and central provinces occurred at 1760 1750 ma the argilke event is not older than 1680 ma demonstrating that the southern province is the youngest in the inlier the dominantly thermal ormiston event is dated at 1140 ma

the palaeozoic history of an unusual intracratonic thrust belt in central australia based on 40ar 39ar k ar and fission track dating intraplate thrusting involving reactivation of a mid proterozoic province boundary within the basement metamorphic rocks of the arunta block occurs at the northern margin of the late proterozoic to mid palaeozoic amadeus basin in central australia earlier geological and geophysical studies show that the thrust system referred to as the alice springs orogen is 9thick skinned9 in the central part of the thrust belt and was dominated by the crust cutting redbank thrust zone several episodes of thrusting defined stratigraphically began about 400 ma ago analysis and modelling of 40 ar 39 ar age spectra on k feldspar from the basement rocks combined with regional geological constraints point to cooling rates of 3 10 c ma and moderate overall exhumation rates of 100 400 m ma during the alice springs orogeny without appreciably increased heat flow however final uplift along the thrust front at 300 320 ma was rapid as the time of final closure of small domains in k feldspar approximates apatite fission track ages by using 40 ar 39 ar age data as a constraint together with available geological and geophysical data crustal shortening across the thrust belt is thought to be approximately 20 to 30 overall convergence rates of 0 6 3 km ma are slow compared with rates of 5 km ma for continent continent collision zones

high resolution aeromagnetic data over central australia assist grenville era 1300 1100 ma rodinia reconstructions 1 high resolution aeromagnetic data covering the polydeformed and poorly exposed musgrave province of australia reveals the grenville aged crustal architecture a combination of upward continuation and the potential field tilt filter out magnetic anomalies relating to a late neoproterozoic orogenic event and emphasize the more subtle magnetic structural grain formed during the ca 1320 1150 ma musgravian orogeny the resulting images indicate that crustal architecture is defined by the distribution of ca 1150 ma magnetic granitoids within less magnetic basement and is dominantly northeast trending continuous beneath the amadeus and officer basins and defines an orogenic belt that connects mesoproterozoic provinces the orogenic belt is truncated within australia and precludes a direct east trending connection between the musgrave province and contemporaneous orogens in laurentia clockwise rotation of the south australian craton and subsequent collision with the north and west australian cratons is our preferred model to explain the grenville aged architecture observed in australia

the mangaroon orogeny synchronous c 1 7 ga magmatism and low p high t metamorphism in the west australian craton abstract the capricorn orogen records nearly one billion years of intraplate orogenesis within the west australian craton although the processes responsible for this protracted punctuated reworking remain unclear of the major tectonic events that affected the region the 1680 1620 ma mangaroon orogeny is one of the least well understood mainly due to a lack of direct ages for metamorphism an absence of pressure p temperature t constraints and uncertainty regarding the duration of granitic magmatism that is spatially and possibly temporally associated with deformation in this study we define the p t time t conditions associated with the mangaroon orogeny based on in situ shrimp u pb monazite and xenotime geochronology and calculated p t pseudosections data from a pelitic migmatite constrain the timing of low p high t metamorphism to 1691 7 ma at conditions of 665 755 c and 2 7 4 3 kbar 175 240 c kbar data from a garnet bearing sillimanite biotite pelitic gneiss suggests higher pressure during the clockwise prograde history at one locality furthermore the onset of the metamorphism coincides with the oldest granites in the region 1695 9 ma which constrain the onset of the mangaroon orogeny our results also show that deposition and burial of the precursor sediments occurred at most c 70 million years before the onset of partial melting at c 1695 ma therefore there was no long incubation period before the onset of orogenesis we conclude that unlike many of the proterozoic orogenic events in australia the c 1 7 ga low p high t metamorphism recorded in the upper crust in the capricorn orogen cannot be explained by a thermal lid model but rather was synchronous with granitic magmatism

the age of the ormiston pound granite implications for mesoproterozoic evolution of the arunta inlier central australia the ormiston pound granite southern arunta inlier intruded during the terminal stages late to post d2 of a major n directed overthrust event called the chewings orogeny which deformed supracrustal rocks of the simpsons and chewings assemblages ion probe zircon u pb analysis of the granite indicates an emplacement age of 1603 10 ma more than 500 ma older than the rb sr isochron age timing of emplacement is confirmed by an age of 1575 20 ma for zircon mantles from a gneissic xenolith in the granite zircon cores from the xenolith yield an age of 1678 14 ma which is taken to represent the protolith age of the surrounding gneisses the shrimp age is similar to but more precise than the 1586 69 ma rb sr isochron age for the chewings orogeny determined by marjoribanks and black 1975 penecontemporaneous 1600 ma magmatism is evident as pegmatites and associated fluid flow zones in the anmatjira and reynolds ranges northern arunta inlier 1100 1030 ma rb sr mica ages from the 1600 ma ormiston pound granite coeval pegmatites and 1680 ma host rock gneisses are similar to a sm nd isochron age for the mafic stuart dyke swarm of the southern arunta inlier and the contemporaneous alcarra dyke swarm in the musgrave inlier some 500 km farther south the rb sr mica ages reflect isotopic resetting by a thermal perturbation either associated with regional crustal extension during 1080 ma dyke intrusion or with the 1150 ma magmatic event in the southern arunta cf black and shaw 1992 irrespective the 1050 900 ma ormiston event described by shaw et al 1984 as granite intrusion associated with high grade metamorphism migmatization and folding needs to be re defined or abandoned

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

a barometric response to late compression in the strangways metamorphic complex arunta block central australia evidence for four major deformation events d1 d4 and two metamorphic events m1 m2 is shown by granulite and amphibolite facies rocks of the strangways metamorphic complex central australia which is divided into two domains the ongeva granulites to the east and the anamarra granite domain to the west d1 and d2 produced layer parallel foliations with many of the effects of d1 overprinted by intense non coaxial shear during d2 low pressure granulite facies metamorphism m1 accompanied d1 2 at conditions of p 5 3 1 2 kbar and t 750 c the effects of m1 are characterized by s1 spinel quartz bearing assemblages with overprinting s2b sillimanite biotite magnetite assemblages implying approximately isobaric cooling effects of m2 are characterized by s1 2 cordierite pseudomorphed by orthopyroxene sillimanite biotite magnetite symplectites at conditions of p 7 5 0 8 kbar and t 800 c this retrograde increase in pressure was probably the result of crustal thickening during d3 which produced ne plunging f3 and f4 folds in the ongeva granulites and a widespread shear foliation in the anamarra granite domain the increase in pressure with m2 may reflect the beginning of a clockwise p t t path typical of collisional style tectonics sillimanite and orthopyroxene bearing ultramylonites were produced during d4 an extension event parallel to the d3 transport axis and probably related to crustal collapse after thickening during d3 wide n dipping shear zones d5 containing sillimanite kyanite staurolite and overprinting greenschist facies assemblages bound the strangways metamorphic complex these zones indicate a s directed sense of shear and were probably responsible for bringing the strangways metamorphic complex to higher crustal levels during both a probable late proterozoic event and the mid carboniferous alice springs orogeny

the role of the pernicana fault system in the spreading of mt etna italy during the 2002 2003 eruption flank instability and collapse are observed at many volcanoes among these mt etna is characterized by the spreading of its eastern and southern flanks the eastern spreading area is bordered to the north by the e w trending pernicana fault system pfs during the 2002 2003 etna eruption ground fracturing along the pfs migrated eastward from the ne rift to as far as the 18 km distant coastline the deformation consisted of dextral en echelon segments with sinistral and normal kinematics both of these components of displacement were one order of magnitude larger 1 m in the western previously known portion of the pfs with respect to the newly surveyed 9 km long eastern section 0 1 m this eastern section is located along a pre existing but previously unknown fault where displaced man made structures give overall slip rates 1 1 9 cm year only slightly lower than those calculated for the western portion 1 4 2 3 cm year after an initial rapid motion during the first days of the 2002 2003 eruption movement of the western portion of the pfs decreased dramatically while parts of the eastern portion continued to move these data suggest a model of spreading of the eastern flank of etna along the pfs characterized by eruptions along the ne rift instantaneous short lived meter scale displacements along the western pfs and more long lived centimeter scale displacements along the eastern pfs the surface deformation then migrated southwards reactivating one after the other the nnw sse trending timpe and trecastagni faults with displacements of 0 1 and 0 04 m respectively these structures along with the pfs mark the boundaries of two adjacent blocks moving at different times and rates the new extent of the pfs and previous activity over its full length indicate that the sliding eastern flank extends well below the ionian sea the clustering of seismic activity above 4 km b s l during the eruption suggests a deep decollement for the moving mass the collected data thus suggests a significant movement volume 1 100 km3 of the eastern flank of etna both on shore and off shore

geochronological constraints on orogenic events in the arunta inlier a review early workers subdivided the arunta inlier central australia into three major tectonic provinces classified the principal tectonostratigraphic units into three divisions and separated the tectonomagmatic history into five discrete events ranging in age from 1800 ma to 300 ma a review of the evidence for tectonic boundaries within the arunta inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces but that the redbank thrust zone is at least in part a province boundary the tectonostratigraphic division concept also requires revision as division 3 rocks of the northern and southern arunta cannot be time equivalents division 1 rocks grade into division 2 in the northern region and division 3 of the northern region may be equivalent to division 1 in the central region rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made compilation of rb sr and 40ar39ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 ma to 1000 ma but age clusters exist at 1400 ma 1100 ma and 400 300 ma which have geological significance subdivision of older ages into discrete tectonic events was not justified from this data alone but more recent u pb isotopic analysis of zircons in rocks designated to particular structural metamorphic events has revealed a complex tectonic history in the 1880 1600 ma interval the northern and central provinces underwent major tectonism between 1780 1730 ma but much of the southern province was not deformed until 1600 ma the revised tectonic history nomenclature is proposed for the arunta inlier following deposition of a widespread turbiditic unit at or before 1880 ma possibly on thinned archaean crust the northern and central provinces underwent localized tectonism magmatism at 1880 ma yuendumu tectonic event and 1820 ma stafford tectonic event prior to the widespread strangways orogeny which is subdivided into an early phase at 1780 1770 ma and late phase at 1745 1730 ma it is not clear whether the strangways orogeny is a single arunta wide event or a series of separate short lived smaller scale events although the major deformation in all areas appears to have involved w to sw directed thrusting in the southern province the major events occurred at 1680 ma argilke tectonic event and 1600 ma chewings orogeny the latter involving n directed thrusting that was penecontemporaneous with post tectonic pegmatite intrusion in the northern province suggesting linkage of both regions at this stage the younger history of the inlier is characterised by several stages of uplift and isolated thermal magmatic events major mesoproterozoic crustal exhumation anmatjira uplift phase has been interpreted from widespread 1500 1400 ma ages including regional scale closure of 40ar 39ar systems in hornblende from mafic granulite outcrop scale rb sr total rock isotopic systems in mylonitized granitoids and sm nd garnet hornblende pairs in mylonitized amphibolites a 1150 ma thermal event in the southern part of the inlier teapot magmatic event is recorded by intrusion of granite pegmatites and the alkaline mordor igneous complex and by isotopic resetting of rb sr mineral systems dolerite dyke injection at 1050 ma reflects limited extensional tectonism that may have related to formation of the amadeus basin the second major uplift event was the alice springs orogeny at 400 300 ma an unusual intracratonic thick and thin skinned thrust event that involved limited retrograde amphibolite facies but extensive greenschist facies metamorphism and restricted pegmatite intrusion

magma ascent and the pressurization of mount etna s volcanic system after a period of deflation during the 1991 1993 flank eruption mount etna underwent a rapid inflation seismicity and ground deformation show that since 1994 a huge volume of magma intruded beneath the volcano producing from 1998 onward a series of eruptions at the summit and on the flank of the volcano the last of these started on 27 october 2002 is still in progress and can be considered one of the most explosive eruptions of the volcano in recent times here we show how geodetic data and seismic deformation between 1994 and 2001 indicate a radial compression around an axial intrusion consistent with a repressurization of mount etna s plumbing system at a depth of 6 to 15 kilometers which triggered most of the seismicity and provoked the dilatation of the volcano and the recent explosive eruptive activity

post orogenic 1500 ma thermal history of the palaeo mesoproterozoic mt isa province ne australia abstract we present hornblende white mica biotite and alkali feldspar 40 ar 39 ar data from paleo mesoproterozoic rocks of the mt isa inlier australia which reveal a previously unrecognised post orogenic non linear cooling history of part of the northern australian craton plateau and total fusion 40 ar 39 ar ages range between 1500 and 767 ma and record increases in regional cooling rates of up to 4 c ma during 1440 1390 and 1260 1000 ma forward modelling of the alkali feldspar 40 ar 39 ar arrhenius parameters reveals subsequent increases in cooling rates during 600 400 ma the cooling episodes were driven by both erosional exhumation at average rates of 0 25 km ma and thermal relaxation following crustal heating and magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing in the arunta block and gawler craton late mesoproterozoic cooling could have either been driven by increased rates of exhumation or a result of thermal relaxation following a heat pulse that was synchronous with dyke emplacement in the arunta musgrave and mt isa province as well as grenville aged orogenesis in the albany fraser belt latest neoproterozoic cambrian cooling and exhumation was probably driven by the convergence of part of the east antarctic shield with the musgrave block and western australia petermann ranges orogeny as well as collisional tectonics that produced the delamerian ross orogen major changes in the stress field and geothermal gradients of the australian plate that are synchronous with the assembly and break up of parts of rodinia and gondwana resulted in shearing and repeated brittle reactivation of the mt isa inlier probably via the displacement of long lived basement faults within the northern australian craton

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

upb and rbsr geochronology and geological evolution of the harts range ruby mine area of the arunta inlier central australia abstract supracrustal and meta igneous rock units from the ruby mine area of the harts range eastern arunta inlier central australia have been dated by the zircon u pb and rb sr total rock tr and mica methods a well defined zircon discordia for a weakly deformed specimen of the bruna granite gneiss yields an age of emplacement of 1748 4 5 ma thereby constraining the minimum age of the irindina supracrustal assemblage metapelitic gneiss within the supracrustals and a meta igneous ultramafic boudin from the associated harts range meta igneous complex yield highly discordant zircon data revealing a strong early palaeozoic overprint rb sr tr data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed also apparently during the lower palaeozoic however rb sr model age calculations and the zircon u pb data suggest a maximum age of about 2000 ma for the supracrustal and meta igneous rocks and argue for new proterozoic crust formation zircon u pb data from a deformed pegmatite emplaced in the meta igneous complex yield an emplacement age of 520 4 5 ma further pointing to lower palaeozoic magmatism and deformation correlations of u content and calculated 206 pb 238 u age for the ultramafic boudin zircons suggest that new growth of low u zircons occurred during retrogression associated with this event the sr isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of palaeozoic sr our data suggest lower palaeozoic possibly delamerian tectonothermal activity to be more important in the evolution of the harts range area than previously recognised on the other hand rb sr mica ages for deformed and undeformed pegmatites and tr isochrons for the latter show that pervasive tectonothernal activity had ceased by about 315 ma and that regional cooling occurred between about 345 and 325 ma local shear zone biotite resetting may have persisted to about 300 ma consistent with the previously recognised alice springs orogeny possible dilational pb loss in the bruna zircons occurred at about 103 ma

synoptic and mesoscale atmospheric features over the ice covered portion of the fram strait in spring data obtained during the coordinated eastern arctic experiment cearex from march 31 to april 20 1989 from two ice camps in the pack ice region in the fram strait have been analyzed the data show that the total period can be divided into two periods or regimes divided by a transition period the first period was a radiative type characterized by very low surface temperatures and low wind speeds fairly regular oscillations between southerly and northerly winds were caused by small scale disturbances in the surface pressure field associated with upper level waves of large amplitude during a transition period toward an advective weather type on april 11 a sharp frontal zone passed both ice camps the front was associated with a small scale low which had formed over the ice on the shallow but intense baroclinic zone along the northeastern greenland coast mesoscale vortices of this type may if numerous enough affect the horizontal transfer of heat from lower latitudes toward arctic regions as well as the ice drift through the fram strait

metamorphic and tectonic evolution of granulites arunta block central australia petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern strangways range arunta block central australia nsr in fig 1 beginning with migmatization followed by dry granulite metamorphism at high temperatures t and moderate pressures p estimated at 850 920 c 8 1 kbar1 this occurred at 1 800 myr refs 2 3 very shortly after deposition of the protolith4 a bimodal volcanic suite5 and associated sediments granulite metamorphism was followed by isobaric cooling during which there was an episode of pervasive but partial hydration later retrogression and hydration localized along shear zones occurred at the normal geothermal gradient in the kyanite stability field a tensional rift like regime best describes the geological environment in which the protolith formed was deeply buried metamorphosed with high heat flow and then remained in isostatic equilibrium metamorphic parageneses recording uplift may be matched with mid palaeozoic tectonic events in central australia if this correlation is correct the rocks of the northern strangways range may have remained deeply buried for 1 400 myr during which the shear zones were reactivated several times fig 2

the duration of the strangways event in central australia evidence for prolonged deep crust processes abstract protoliths of the strangways metamorphic complex smc in the arunta region of central australia are dated to the 1810 1800 ma stafford magmatic event by shrimp u pb dating of relict igneous zircon within granulite facies metamorphic rocks detrital zircons preserved in associated metasediment have a provenance age spectrum similar to that in the lander package sedimentary rocks which are widespread in the region the onset and duration of granulite facies metamorphism is recorded differentially across the smc depending on lithology varying mechanisms of metamorphic zircon crystallisation and the specific p t t paths of individual rocks in different sectors of the complex the earliest recorded thermal process is partial melting in the kanandra granulite at 1734 3 ma zircon ages indicate that melts at edwards creek 1716 3 ma and coles hill 1708 6 ma formed later overgrowths indicate renewed metamorphic zircon precipitation in the kanandra granulite probably from fluids as late as ca 1690 ma a 50 myr continuum of metamorphic zircon ages from ca 1740 to 1690 ma is recorded in a single rock at utnalanama and attributed to prolonged solid state metamorphic zircon formation the strangways event was not a transient and discrete process with a single age there is evidence for prolonged residence at granulite facies conditions with both discrete crystallisation events and prolonged crystallisation responses in different locations termination of the high grade metamorphism coincided with local extension indicated by the intrusion of a dolerite dyke swarm at 1689 8 ma the strangways event coincides with the segment between prominent bends b0 and b1 of the australian palaeomagnetic polar wander path relating the thermal process to major direction changes in the movement of the north australian plate the geographic distribution of high grade metamorphism within the smc shallow crust emplacement of magmas in the tennant region and a low temperature ar isotope record in the tanami region is consistent with effects propagating northwards from a plate boundary south of the smc

exhumation of the lower crust during crustal shortening an alice springs 380 ma age for a prograde amphibolite facies shear zone in the strangways metamorphic complex central australia foliated garnet bearing amphibolites occur within the west bore shear zone cutting through granulite facies gneisses of the strangways metamorphic complex in the amphibolites large euhedral garnet up to 3 cm occurs within fine grained recrystallized leucocratic diffusion haloes of plagioclase quartz the garnet and their haloes include a well developed vertical foliation also present in the matrix this foliation is the same as that cutting through the unconformably overlying neoproterozoic heavitree quartzite the textures indicate syn to late kinematic growth of the amphibolite facies mineral assemblages all mineral assemblages record an arrested prograde reaction history noteworthy is the growth of garnet at the expense of hornblende and plagioclase and the breakdown of staurolite hornblende to give plagioclase gedrite these dehydration reactions indicate increasing p t conditions during metamorphism and suggest heating towards the end of a period of intense deformation temperature estimates for the garnet amphibolite and related staurolite hornblende assemblages from the shear zone are about 600 c pressure is estimated at about 5 kbar an sm nd isochron gives an age of 381 7 ma for the peak metamorphism and associated deformation this age determination confirms that amphibolite facies conditions prevailed during shear zone development within the strangways metamorphic complex during the alice springs orogeny these temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient the alice springs orogeny was associated with significant crustal thickening allowing exhumation of the granulite facies palaeoproterozoic lower crust along strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the alice springs orogeny

a fram strait cyclone properties and impact on ice drift as measured by aircraft and buoys 1 atmospheric cyclones in the fram strait affect the sea ice transport from the arctic ocean into the atlantic ocean during the field experiment framzy in april 1999 a fram strait cyclone and its impact on the ice drift was measured using a research aircraft and an array of 15 ice buoys the synoptic scale cyclone moved from the south into the area it was discernible up to 500 hpa in the pressure field but the horizontal temperature contrast of up to 16 k between the warm and cold sides was confined to the lowest 500 m the average ice drift was 0 21 ms 1 toward 200 but increased to 0 6 ms 1 during the cyclone passage the ice drift amounted to 1 6 of the geostrophic wind with a turning angle of 51 on the average comparisons between the aircraft measurements and operational weather model analyses show an insufficient representation of the temperature inversion and indicate an underestimate of wind speed and thus momentum transfer to the sea ice