the geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites abstract using hygromagmatophile element abundances normalized to primordial mantle composition it is possible to discriminate between the tectonic settings of tholeiitic basalts and basaltic andesites 47 0 53 5 sio2 based on field and chemical evidence data from provinces or grouped localities have been compiled in order to establish chemical reference data sets for ocean island continental ocean ridge and floor subduction zone and back arc environments the disturbing effects of crustal contamination and hydrothermal alteration on the hygromagmatophile h element patterns are of minor significance in the recognition of plate tectonic settings on the basis of the data sets magmas from a number of ambiguous tectonic settings have been recognized using their h element patterns attention is directed to problems concerning rifting and drifting related basalts in an apparent continental setting and to the origin of ophiolite complexes using available data the pillow lavas of the troodos ophiolite complex cyprus are recognized as basalts erupted in a subduction zone environment

crustal large scale serpentinized mantle peridotite body in the sulu ultrahigh pressure metamorphic belt eastern china evidence from gravity and magnetic anomalies abstract from analysis of the geological and geophysical data gravity magnetic seismic and petrophysics we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body smpb situated in the middle to lower crust in the sulu belt the smpb was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh pressure uhp metamorphism our finding suggests an emplacement mechanism for the serpentinized mantle wedge smw early in the subduction process this is different from the classic view which holds that the serpentinized forearc mantle is formed by in situ hydration processes blakely et al 2005 the petrophysical properties of the smpb are similar to those of the serpentinized forearc mantle or smw in modern subduction zones worldwide but the formation mechanisms for smpb and smw are different this observation is important for understanding the geodynamic processes that operated in the large uhp metamorphic belt in the dabie sulu area eastern china

transition from a singly vergent to doubly vergent wedge in a young orogen the greater caucasus the greater caucasus mountains due to their youth 5 ma provide an opportunity for insight into the early stages of orogen development during continent continent collision however their recent tectonic evolution and first order architecture remain unclear here we investigate the evolution of the orogen by integrating new observations of the fluvial geomorphology and neotectonics of the range with prior work on seismicity geodetic strain bedrock geology and foreland basin structure we find that the range contains four zones along strike that differ in structural architecture topography and first order tectonic history in particular two south directed singly vergent zones at the western and eastern tips of the orogen are separated by both a central doubly vergent zone that is dominated by north directed deformation and an eastern doubly vergent zone in which south directed thrusting dominates we hypothesize that the along strike changes in vergence and locus of deformation reflects different stages in the development of a doubly vergent orogen with the tips of the range preserving an early singly vergent form and the center recording a more advanced orogen the differences between the two doubly vergent zones seem to be driven by the initial stages of collision between the structurally thickened crust of the greater and lesser caucasus orogens which initiated at 5 ma

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

neodymium isotope systematics of the mantle beneath the baltic shield evidence for depleted mantle evolution since the archaean abstract more than 300 published and unpublished nd isotopic analyses of mantle derived rocks from the baltic shield have been compiled the rocks range in age from archaean to phanerozoic within any given age interval the mantle derived rocks range in nd t from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of depaolo 1981 and depaolo et al 1991 to negative values initial neodymium isotopic compositions below the de paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants the data now available lend no support to the existence of isolated less depleted or undepleted mantle domains beneath the baltic shield as was suggested by mearns et al 1986 and valbracht 1991a b

nature and distribution of deep crustal reservoirs in the southwestern part of the baltic shield evidence from nd sr and pb isotope data on late sveconorwegian granites late sveconorwegian postorogenic granites c 1 0 0 93 ga make up a voluminous and widespread suite of intrusions across south norway from radiogenic isotope data three groups of late sveconorwegian granites can be distinguished 1 granite with more than 150 ppm sr 87 rb 86 sr 87 sr 86 sr 0 93 ga nd 87 rb 86 sr 5 87 sr 86 sr 0 93 ga 0 710 and e nd 87 sr 86 sr 0 93 ga nd 0 granite plutons belonging to group 1 normal sr concentration granite occur all over south norway and include the largest batholiths ostfold fla herefoss granite plutons of group 2 low sr concentration granites are restricted to north central telemark rjukan rift and are associated with c 1 5 ga rjukan group rhyolite group 3 is represented by one intrusion only but still suggests input of mantle derived magma or the presence of young mantle derived rocks in the deep crust in the region at c 0 93 ga the group 1 granites are similar in sr and nd characteristics to some of the older 1 05 ga sveconorwegian granitic intrusions augen gneisses in the region in terms of radiogenic isotope systematics but pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss the nd sr and pb isotopic systematics of the late sveconorwegian granites indicate mixing between a depleted mantle derived component and two or more major components with an extended crustal history one of the crustal end members is present throughout south norway and has an isotopic signature similar to older granitic rocks of the trans scandinavian igneous belt tib the other crustal end member is indistinguishable from rjukan group rhyolites and slightly younger intrusions associated with these and is restricted to areas within the mid proterozoic rjukan rift the available data suggest that the deep continental crust of south norway both east and west of the permian oslo rift is an integral part of the baltic shield with a common history back to 1 7 1 9 ga that is to the end of the svecofennian orogeny and the tib magmatism

u pb zircon ages of basement gneisses and discordant felsic dykes from vestranden westernmost baltic shield and central norwegian caledonides four new u pb zircon ages are reported from northern vestranden the northernmost part of the western gneiss region of norway scandinavian caledonides the age of a basement tonalite 1819 6 ma together with earlier datings on granites indicates that the dominantly granitoid basement in vestranden began to form more than 1800 ma years ago the minimum age of a subsequent precambrian migmatization event is given by a u pb age of 1630 80 ma found a granite dyke which cuts migmatitic veining in the basement gneisses while age and lithology suggest general similarity and thereby favour a correlation between the basement granites of the northern part of vestranden and svecofennian granites farther to the east no detailed chronological information is at present available that support such a correlation

heavy metals in a contaminated australian estuary dispersion and accumulation trend very high concentrations of zn pb cu and cd occur in the muddy bottom sediments of lake macquarie a saline coastal lagoon in southeastern australia the trace metals emanate from industrial sources especially a lead zinc smelter at the northern end of the lake individual metal concentrations decrease progressively away from the source area but at differing rates zn is most mobile and cd appears to be deposited first they approach natural background levels in the southern part of the lake mn shows a reverse trend but ni co ag and fe rarely rise above background levels shallow cores in the lake bed penetrated a metal enriched surface zone 15 35 cm thick underlain by uncontaminated sediments with natural background metal concentrations sedimentation rates determined from radiocarbon ages on shells in the cores mainly range between 0 15 and 0 5 mm yr 1 over the 85 years since industrialization commenced less than 5 cm of mud has accumulated on the lake bed bioturbation is invoked to account for the depth to which the sediment has been enriched in heavy metals

the deep layers of a paleozoic arc geochemistry of the copley balaklala series northern california abstract ree zr nb concentrations and sr nd isotope compositions have been measured in copley basalts and andesites balaklala rhyolites and mule mountain trondhjemites northern california which represent the deep layers of a well preserved intra oceanic island arc of siluro devonian age 87sr 86sr is shifted towards high values up to 0 707 whereas ce is preferentially removed from rhyolites a large proportion of the analyzed samples including some acidic rocks shows a pronounced depletion in light ree the end t values of most copley balaklala and mule mountain rocks fall in the range 6 to 8 which suggests that they originated from a normal morb type source end t 9 contaminated with either sediments or an oib type component in modern island arcs only the shallow levels are accessible comparison with the copley balaklala mule mountain series suggests that at depth an immature island arc is likely to comprise thick layers of lile depleted tholeiites and rhyolites intensely altered by pervasive circulation of seawater least square solutions of trace element models suggest that rhyolites and trondhjemites represent remelting of mafic volcanics from the arc basement rather than residual melts of basalt andesite differentiation

isotopic constraints on the timing of crustal accretion of the bamble sector norway as evidenced by coronitic gabbros abstract this paper presents the first sm nd data for coronitic gabbros hyperites from the high grade proterozoic bamble sector norway sm nd whole rock isochron ages of the vestre dale gabbro 1 11 0 14 ga the jomasknutene gabbro 1 77 0 19 ga and the flosta gabbro 1 64 0 23 ga indicate that contrary to earlier ideas intrusion of these gabbros is not exclusively bound to the sveconorwegian orogeny 1 2 0 9 ga some of the gabbros were already emplaced at the onset of or during the gothian orogeny 1 75 1 5 ga initial nd values of about 5 5 of jomasknutene and flosta gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the southwest scandinavian domain this highly positive value differs from the nd isotope evolution curve for the southwest scandinavian mantle as postulated by mearns et al 1986 which assumes that depletion of the mantle beneath this part of scandinavia started at c 1 74 ga however the initial nd value of the vestre dale gabbro plots close to this curve whereas 143nd 144nd ratios of the mantle sources of the tromoy and arendal gabbros evolved with approximately chondritic 144sm 144nd ratios their nd 1 1 ga values range from 5 1 0 3 to 6 0 0 3 gothian metasomatism of a depleted mantle source by lile and lree bearing fluids provides a mechanism to obtain low nd isotope ratios in magmas represented by the vestre dale gabbro examples of low nd isotope magmas have also been found in the telemark sector and in southern sweden the marked depletion in nb indicates that the metasomatic fluids were possibly related to gothian subduction

phosphorus speciation and changes with depth in the sediment of lake illawarra new south wales australia in order to better understand phosphorus p cycling and origins in the sediment of the lake illawarra two sediment cores were extracted in november 2010 and a modified sequential extraction scheme sedex was used to profile the exchangeable p pex reactive fe al bound p preac reductive fe al bound p predu authigenic apatite p pauth detrital p pdet organic p porg and residual p presi the total sedimentary p tp ranged from 93 to 437 g g 1 and was dominated by inorganic p the average percentage of each fraction of p in the sediment followed the sequence preac 28 6 presi 23 5 pauth 19 1 predu 17 0 porg 4 9 pex 4 7 pdet 2 2 the profiles of tp and porg showed two peak values with depth which were matched to land use history in the lake illawarra catchment the sediment depth profiles indicated that fe oxyhydroxides play a predominant role in the p cycle in the sediments of the lagoon this is supported by significant positive correlation between preac and reactive fe and a negative correlation between pauth and fe pauth and preac concentrations were also well negatively correlated possibly a result of competitive equilibrium between fe and ca for p the estimated p burial efficiency was up to 82 for this lagoon which is likely related to the high sediment accumulation rate and the high value of rfe p in addition the bioavailable p which consists of pex preac and porg represented a significant proportion of the sedimentary p pool accounting on average for 38 of the tp this result indicates that the sediment is a potential internal source of p for this lake ecosystem

softening trigerred by eclogitization the first step toward exhumation during continental subduction direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn exhumation deformation most ultrahigh pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces low viscosity in the subduction channel overburden removal by erosion and normal faulting a basic question is seldom considered why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface this event seminal to exhumation processes must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it our field observations in the bergen arc show that caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the caledonian subduction thus the sequence of deformation preserved in the bergen arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation this observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions i e underthrusting of the subducting slab in this context of simple shear metamorphic reactions assisted by fracturating fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab these tectonic units can then be incorporated into the channel circulation and start their upward travel

exhumation mechanisms of melt bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates a series of 2d petrological thermomechanical numerical experiments was conducted to i characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic uhpm rocks during collision of spontaneously moving plates and ii study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high temperature ultrahigh pressure metamorphic ht uhpm rocks to this end the models include fluid and melt induced weakening of rocks five distinct modes of exhumation of u hpm rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows vertical crustal extrusion large scale crustal stacking shallow crustal delamination trans lithospheric diapirism and channel flow the variation in exhumation mechanisms for u hpm rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and or did not include fluid and melt induced weakening of rocks this difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid and melt related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments such as slab breakoff vertical crustal extrusion large scale stacking shallow crustal delamination and relamination and eduction of the continental plate consequently the significance of channel flow for the exhumation of uhpm rocks may have been overstated based on the results of the earlier numerical experiments in addition the results from this study extend over a larger proportion of the high temperature range of p t conditions documented from uhpm rocks including those retrieved from ht uhpm rocks than the results of experiments from previous numerical models in particular the highest peak metamorphic temperatures up to 1000 degrees c are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab nonetheless some extreme temperature conditions which have been suggested for the kokchetav and bohemian massifs perhaps up to 1100 1200 degrees c are still to be achieved in experiments using numerical models

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

high pressure rocks of the acatlan complex southern mexico large scale subducted ordovician rifted passive margin extruded into the upper plate during the devonian carboniferous abstract high pressure hp rocks in the paleozoic acatlan complex of southern mexico occur in a median belt extruded into the complex which is the root zone of a klippe to the west these hp rocks are predominantly clastic metasedimentary rocks and include a bimodal suite of amphibolites and granitoids u pb la icpms zircon dating indicates that one of the amphibolites was intruded at or before 461 2 ma and a megacrystic granitoid yielded an intrusive age of 488 8 7 ma which is within the 490 450 ma range of other dated hp megacrystic granitoids the amphibolites have a tholeiitic transitional to alkalic within plate chemistry derived from a heterogeneous mantle by various degrees of partial melting the host metasedimentary rocks contain detrital zircons derived both from the acatlan granitoids and the ca 1 0 1 3 ga oaxacan complex and are inferred to represent rifted passive margin sediments an age of 348 4 1 ma from zircons in a migmatized amphibolite is interpreted as the time of migmatization and dates decompression melting following peak pressure at depths of 45 55 km retrogression from 530 c to 370 c occurred between 343 and 335 ma the predominance of reasonably coherent rifted passive margin rocks over a surface area of 10 90 km suggests removal of a large slice of the upper plate during flat slab subduction this slice was subducted to depths of ca 45 55 km where it was underplated and transferred to the overriding plate the absence of oaxacan complex rocks in the hp belt suggests the oaxacan basement was subducted to greater depths underplating was followed by extrusion facilitated by extension in the upper plate that produced mississippian rift basins east of the median hp belt westward overthrusting of the hp rocks produced a klippe that overrode mississippian clastic wedge sediments

catchment clearing accelerates the infilling of a shallow subtropical bay in east coast australia abstract understanding processes that govern the transport and distribution of terrestrial sediments to and within bays is critical for interpreting the drivers of long term changes in these ecosystems on the east coast of australia increased soil erosion and sediment delivery following extensive land clearing in the contributing catchments associated with european settlement is highlighted as a key driver of the decline of numerous nearshore habitats including seagrass meadows and in shore coral reefs here we use optical radiocarbon and radionuclide dating to estimate mass accumulation rates and type of terrestrial sedimentation in central moreton bay during the holocene we compare the long term rates of infilling within the central basin with the recent past and show a 3 9 fold increase in sediment accretion over the last 100 years compared to the long term last 1500 to 3000 yrs average infilling during the holocene is not spatially uniform with preferential deposition occurring within the now submerged palaeochannels of the brisbane and pine rivers we suggest that modern turbidity regimes in moreton bay are the result of the compounded effect of both a historical increase in fine sediment supply and a rapid decline in the effective storage capacity of the basin

middle proterozoic anorthosite rapakivi granite complexes an example of within plate magmatism in abnormally thick crust evidence from the east european craton abstract large bimodal and multistage anorthosite rapakivi granite complexes argcs of middle proterozoic age 1 8 1 5 ga are distinct magmatic assemblages in the western part of the present day european craton argcs formation commenced after stabilization of the svecofennian orogen and relics of its abnormally thick crust occur in many places in the eastern part of the kola karelian domain and the volga urals domain where crustal thickness is normal 40 km argcs are practically absent geological data evidence that melting processes occurred both in the crust and in the mantle during formation of the complexes geochemical peculiarities of argc rocks include enrichment in alkalis mainly in k ti zn pb zr be sn in y nb rb f cu w mo sometimes li and u the negative nd values and relatively high th and zn concentrations that are most frequently observed in the rocks imply that parental magmas were considerably contaminated by crustal components according to the gravity and seismic profiles argcs represent upper parts of large transcrustal magmatic systems composed of alternating basic and silicic rocks layers they are located over mantle uplift structures 10 20 km high interpreted to represent former mantle plume heads origin of transcrustal systems was probably linked with penetrating sill like bodies of fe ti basalts which after intruding into sialic crust caused large scale re melting of the granitic material above them forming two layer magma chambers solidification of such chambers occurred in two stages lower basic layers solidified first while heated from below whereas the upper silicic layer crystallized later co existing neighboring chambers led to gravitational instability overturn and mass redistribution in large volumes of crust furthermore multistage formation of the argcs led to important petrological consequences contamination of basic magmas by crustally derived sio 2 and al 2 o 3 causing preferred precipitation of plagioclase and formation of plagioclase cumulates anorthosites diffusion of na into the basic melt and k into the silicic melt led to the appearance of potassic rapakivi granites argcs on all precambrian shields were developed in places with stabilized paleoproterozoic orogens with atypical unusually thick crust compared with the phanerozoic and associated with large mesoproterozoic belts of within plate felsic volcanism under these conditions the majority of mantle plume derived mafic magmas were not able to reach the surface and were lost within the crust in the form of sill like intrusions with zones of intense melting of a sialic roof evidently argcs illustrate structure and processes that are not repeated in the phanerozoic

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

the petrogenesis of massif anorthosites a nd and sr isotopic investigation of the proterozoic of rogaland vest agder sw norway sm nd and rb sr isotopic analyses of charnockitic migmatite augen gneiss anorthosite leuconorite and two acid plutons from the rogaland and vest agder districts of southwest norway constrain their crustal residence ages origin and evolution the charnockitic migmatites which are a major component of the metamorphic basement complex represent the oldest and largest episode of accretion in which new crust was derived 1 5 1 9 ga ago from a mantle source of depleted nd isotopic composition the basement complex was intruded by a number of large anorthositic to granitic plutons during and after the sveconorwegian orogenic period samples from the ca 1050 ma old synorogenic haland anorthosite leuconorite massif exhibit substantial variation of initial nd of 2 1 to 4 4 at an anorthosite locality and 0 5 to 2 3 at a leuconorite locality but display significant variation of initial 87sr 86sr ratio only between the localities anorthosite mean 0 70369 leuconorite mean 0 70560 a model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of and fractional crystallization from a picritic magma derived from isotopically depleted mantle two younger acid intrusions the 950 ma old lyngdal granodiorite and the 930 ma old farsund charnockite both have initial sr and nd isotope ratios consistent with massive contamination of depleted mantle derived magma by old continental crustal material

anthropogenic marker evidence for accelerated sedimentation in lake illawarra new south wales australia the chronology of near surface sediments in lake illawarra has been investigated using radiocarbon dating and anthropogenically derived substances including trace metals ash and137cs sediments at depths about 1 m below the water sediment interface ranged in age from modern to 786 calendar years bp on the basis of radiocarbon dating ofnotospisula trigonella valves multiple marker for example ash trace metals depth concentration sediment profiles yielded estimates of sedimentation rate ranging from 3 to 5 mm yr 1 at griffins bay to more than 16 mm yr 1 at macquarie rivulet sedimentation rates of approximately 10 mm yr 1 appeared to be typical of the western and southwestern portions of the lagoon rates of sediment accretion prior to catchment clearing urbanization and industrialization have been estimated at less than 1 mm yr 1 thus indicating a general tenfold increase in sediment accumulation adjacent to the western foreshore caused by catchment development accelerated sedimentation in shallow coastal lagoons constitutes significant environmental impacts including shoaling degradation of seagrass beds and increased turbidity with consequent loss of aesthetic appeal management policy should be directed at attempts to reduce the amount of sediment input by the construction of strategically placed sediment retention ponds siltation within the shallow embayments of lake illawarra could be ameliorated by a carefully planned program of dredging

analysis of digital elevation data for palaeosurfaces in south western sweden a 3873 km large rectangular area of the precambrian basement in south western sweden has been investigated regarding gross morphology of palaeosurfaces gis and digital elevation data were used to

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

smnd evidence for late archaean crust formation in the lapland kola mobile belt kola peninsula russia and norway abstract a sm nd model age traverse across the kola peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early archaean saamian crust in the region granitoid gneisses and subordinate metasediments have been sampled from the main archaean geological units i e from northeast to southwest the murmansk domain model ages of 2 68 2 96 ga central kola domain including samples from the superdeep borehole 2 74 2 92 ga belomorian domain including the eastern part of the tersk area 2 70 2 93 ga the narrow range of depleted mantle model ages from 2 68 to 2 96 ga for archaean granitoid orthogneisses close to the oldest crystallization ages from the region indicate a major episode of late archaean crustal growth and reveal no indication of ancient saamian material in keeping with other geochronological evidence similar results 2 86 to 2 93 ga for archaean metasediments support this conclusion proterozoic model ages for supracrustals from the western part of the tersk area 2 16 to 2 23 ga indicate that the crust in this area is much younger than previously recognized initial nd values for late archaean samples with reasonable age constraints fall within a range of 2 3 to 2 5 two well dated samples from the belomorian domain have the highest initial nd values nd 2 81 ga 2 0 and nd 2 78 ga 2 3 and suggest that a depleted mantle source was present in the kola region in late archaean times dated samples from the central kola domain have lower initial nd between 2 5 and 1 2 suggesting a possible regional difference in mantle character in contrast to the depleted signature of the archaean orthogneisses between 2 7 ga and 2 8 ga previous studies of earliest palaeoproterozoic rift related majic intrusives in the kola and karelia regions report negative nd values at 2 4 2 5 ga possibly these data reflect a change to more enriched mantle values

on the origin of orogens in order to understand how orogens work a quantitative approach demonstrating proof of concept is essential our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of working numerical models that are consistent with both the underlying physics and first order geological constraints we present a simple conceptual temperature magnitude t m framework for orogenesis in terms of the progression from small cold to large hot orogens and we use forward numerical models to test hypotheses corresponding to specific stages along the t m spectrum small cold orogens are analyzed using crustal scale singularity s point models in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics the transition from oceanic subduction to continental collision and the subsequent evolution of large hot orogens has been investigated using both crustal and upper mantle scale models the latter including dynamic subduction of suborogenic mantle lithosphere large hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot weak lower crustal infrastructure beneath plateaus tectonic processes are dominated by ductile flow of weak crust in response to differential pressure while plateau flanks form external thrust sense wedges we discuss four topical issues in orogenic tectonics including the response of the suborogenic mantle lithosphere to convergence the interaction of climate and tectonics the current debate concerning wedge versus channel flow models to explain the himalayan tibetan system and the interpretation of metamorphic architecture in terms of orogenic processes we conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins accompanied by shortening thickening and heating of deformed crust

age and lead isotope systematics of uranium enriched cobalt mineralization in the modum complex south norway abstract in the modum complex in the precambrian kongsberg sector of south norway cobalt sulphides and arsenides chalcopyrite and pyrite occur in stratiform bands and lenses the cobalt ore minerals are intimately associated with thorian uraninite both on a regional and on a microscopic scale due to the presence of coeval inclusions and intergrowths of uraninite in the cobalt minerals hand picked separates of ore minerals show a large range of u pb ratios this has led to a very large range of present day lead isotopic composition 206 pb 204 pb 18 1 to 277 207 pb 204 pb 15 6 to 36 208 pb 204 pb 37 to 57 two groups of samples can be discerned group 1 defines an imperfectly fitted pb pb lead isochron with an age of 1434 29 ma group 2 defines a pb pb correlation line with an age of 1146 66 ma these results indicate that the uranium bearing cobalt ore was deposited during the orogenic interlude commonly thought to separate the gothian kongsbergian ca 1 5 ga and sveconorwegian 1 2 0 9 ga orogenies in the baltic shield multi stage lead modelling indicates that the initial isotopic composition of the lead in the ores is indistinguishable from that of lead in the country rock metasediments suggesting that it was extracted from such a source the group 2 leads were affected by a strongly localized hydrothermal reworking process during the sveconorwegian orogeny which led to small scale partial pb isotopic homogenization and to strong fractionation of uranium thorium and lead the 1 43 ga age for primary mineralization defines a definite lower age limit for the onset of clastic sedimentation in the kongsberg sector the crustal precursor of the sediments must have retained a high lile upper continental crust character for several hundred million years prior to sedimentation

the alps 2 controls on crustal subduction and ultra high pressure rock exhumation in alpine type orogens building on our previous results we use 2 d upper mantle scale thermomechanical numerical models to explore key controls on the evolution of alpine type orogens and the alps per se focusing on ultra high pressure u hp metamorphic rocks the models show that uhp rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit here we test the sensitivity of the models to surface erosion rate crustal heat production plate convergence divergence rates geometry of the subducting continental margin and strength of the retrocontinent surface erosion affects crustal exhumation but not early buoyant exhumation metamorphic temperatures increase with crustal radioactive heat production maximum burial depth prior to exhumation increases with plate convergence rates but exhumation rates are only weakly dependent on subduction rates onset of absolute plate divergence does not trigger exhumation in these models we conclude that contrasting peak pressures exhumation rates and volumes of u hp crust exhumed in the alps orogen primarily reflect along strike contrasts in the geometry thermal structure and or strength of the subducting microcontinent brianconnais and continental european margin crust the experiments also support the interpretation that the western alps u hp internal crystalline massifs exhumed as composite stacked plumes and that these plumes drove local crustal extension during orogen scale shortening for weak upper plate retrocrusts postexhumation retrothrusting forms a retrowedge overall these results are consistent with predictions using the exhumation number ratio of buoyancy to side traction forces in the conduit which expresses the combined parameter control of the depth volume of crustal subduction and the transition to buoyant exhumation

age tectonic setting and provenance of ostfold marstrand belt supracrustals westward crustal growth of the baltic shield at 1760 ma the ostfold marstrand paragneiss belt is the oldest part of the sw scandinavian domain of the baltic shield in the swedish part of the belt minor mafic volcanics now amphibolites in the metagreywacke dominated stora le marstrand slm supracrustals are chemically similar to primitive modern oceanic arcs or back arc regions a wide compositional range is attributed mainly to fractional crystallisation yielding fe rich andesitic amphibolites diverging lree patterns between different localities are consistent with variations in the degree of partial melting in the source a nine point sm nd errorchron of 1758 78 ma mswd 9 4 for the amphibolites is interpreted to date the volcanism and coeval sedimentation and hence directly dates crustal growth in this part of the baltic shield the exclusion of two pillow lava samples which may have been contaminated yields an isochron with an identical though more precise age an nd value of 4 3 indicates a depleted mantle source similar to that of canada sw u s a s greenland and parts of finland but rather more depleted than that reported hitherto from eastern sweden at the same time nd isotopic data suggest a two component provenance for the slm sediments involving acid derivatives from the mafic volcanics and a continental source similar to the exposed 2 0 1 75 ga old svecofennian crust of eastern sweden with minimal contribution from substantially older crust the mafic volcanism took place during subduction of oceanic crust outboard of the svecofennian part of the baltic shield after substantial addition of 1650 ma old granitoids accretion to the baltic shield possibly took place at 1580 ma

paradigms new and old for ultrahigh pressure tectonism regional ultrahigh pressure uhp metamorphic terranes exhibit a spectrum of lithological structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them at least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths i continental margin subduction ii microcontinent subduction iii sediment subduction iv intracontinental subduction v subduction erosion and vi foundering of a crustal root most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of uhp rocks at least six geodynamic processes can be envisioned to have exhumed uhp continental rocks i eduction ii microplate rotation iii crustal stacking iv slab rollback v channel flow and vi trans mantle diapirs most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one uhp terrane more detailed and systematic field investigations are warranted to assess the predictions of numerical models and more sophisticated and realistic numerical models are required to replicate and explain the petrological structural and chronological data obtained from uhp terranes

retroward extrusion of high pressure rocks an example from the hellenides pelion blueschist nappe nw aegean current tectonic models interpret the hellenides as a unidirectional sw vergent orogenic belt new micro structural and amphibole chemistry analyses show instead that the exhumation of the pelion blueschist nappe pbn of the internal hellenides was achieved by retroward ene directed ductile extrusion which opposes the principal proward orogenic vergence retroward extrusion occurred between two sub parallel major ductile shear zones basal thrust and upper detachment with opposite shear senses which operated simultaneously under blueschist to greenschist facies conditions during the late eocene oligocene because the pbn is tectonically sandwiched between pelagonian basement rocks we suggest that the pbn protolith was accumulated during the late cretaceous in an incipient backarc basin developed above the ne dipping subduction of the pindos ocean beneath the pelagonian microcontinent subsequent apulian pelagonian collision triggered basin inversion and the proward dipping intracontinental subduction that caused the early middle eocene blueschist facies metamorphism followed by the retroward extrusion of the pbn this article is protected by copyright all rights reserved

partitioning of crustal shortening during continental collision 2 d thermomechanical modeling partitioning of crustal shortening between the colliding continental plates is highly variable in nature physical controls of such variability remain largely enigmatic and require quantitative understanding in this study we employ 2 d thermomechanical numerical modeling to investigate the influence of the rheological properties of the continental crust on the dynamics and distribution of crustal shortening during continental collision three major physical parameters i the mechanical strength of the upper crust ii the moho temperature and iii the convergence rate are investigated and their influences on crustal shortening partitioning between the lower and upper plates are systematically documented numerical modeling results suggest that a strong upper crust of the lower plate high moho temperature and slow convergence rate favor migration of crustal shortening from the lower to the upper plate our numerical modeling results compare well with natural observations from the alpine orogenic system where variable partitioning of crustal deformation between the plates is documented

crustal residence ages of clastic sediments orogeny and continental evolution abstract the relationships between additions to the continental crust and periods of orogenesis have been investigated by means of sm 1bnd isotopic analyses of fine grained clastic sediments from the western canadian and baltic shields the crustal residence ages of these sediments indicate that proterozoic crustal additions occurred in both regions but in canada preceded the main magmatic stage of the wopmay orogeny demonstrating that proterozoic orogenic events are not always accompanied by identifiable crustal addition a review of sm 1bnd data for clastic sediments world wide suggests that the major sediment sources have undergone broadly similar evolutionary histories in quite separate continental areas and together yield a mean age estimate for the entire sedimentary mass of 2 0 0 2 ga a minimum crustal growth curve can be derived from these data which requires that 90 or more of the continental crust existed at the end of the proterozoic

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

nd isotope data on 1 9 1 2 ga old basic rocks and metasediments from the bothnian basin central sweden abstract sm nd data are presented for basic rocks and metagreywackes from the early proterozoic bothnian basin in central sweden the basic rocks include synsedimentary metabasalts early orogenic intrusions and anorogenic dolerites covering the age range 1 9 1 2 ga the samples thus span the entire period from the early part of the svecokarelian thermo tectonic episode to the time when this region of the baltic shield had been a stable craton for about 0 4 ga all these samples have slightly positive nd values up to 2 for synorogenic rocks and up to 3 5 for anorogenic dolerites if these values are representative for the contemporaneous mantle they suggest a less depleted character for this mantle than that shown for several other proterozoic shield areas three metagreywacke samples have nd values of 2 4 to 3 1 indicating a significant influx of detrital older crustal material into the bothnian basin

crustal contaminants in the permian oslo rift south norway constraints from precambrian geochemistry abstract during the late paleozoic oslo rifting event the sw part of the baltic shield was penetrated by mantle derived magmas from a depleted lithospheric or sublithospheric source along the way to their final emplacement these magmas may have interacted with a heterogeneous continental crust consisting of a mosaic of continental terranes each with its unique composition and internal crustal history information on radiogenic isotope ratios and trace element distributions in the precambrian terranes surrounding the rift can be used to define characteristic crustal components these components may be used as endmembers in petrogenetic modelling of the oslo rift magmatic system based on available data six endmember components can be identified and semi quantitatively characterized in terms of sr nd and pb isotopes and selected trace elements data on the distribution of rock types along the rift flanks allow estimates to be made of the relative importance of the components in different parts of the rift combining these data with petrological information may allow a realistic understanding of crust magma interaction in the oslo rift magmatic system

diagnosing non meteorically induced variations in the chemistry of saline springs of the vrancea seismic area romania a monitoring operation conducted over more than seven years has been addressing chemical fluctuations displayed by several cool low discharge springs located close to the highly seismic vrancea area romania those outflows compositions proved to be strongly controlled by binary essentially isochemical mixing between a deep origin brine and meteoric freshwater by taking advantage of this particular setting there have been constructed diagrams aimed at investigating how the groundwater discharges na k mg geothermometric parameters fluctuated as a function of the chloride content taken to be an estimator of the freshwater induced dilution we made use of the reciprocal algebraic relationships existing between the equations describing the na k mg geothermometric parameters na k temperature and k mg temperature on the one hand and the so called maturity index mi on the other in an accordingly derived plot of mi against the logarithm of the chloride concentration a series of data points being quite uniformly off set from the mi dilution curve constructed for an apparently regular period suggested that episodically all concerned springwaters were simultaneously subject to some similar changes in the controlling geochemical processes one such modification intervened 3 4 months before the occurrence of the strongest earthquake m text w 5 8 of the hydrochemical monitoring period the consequently derived interpretation was that then the numerical values of certain geothermometric coefficients were likely altered such a process could be consistent with changes in the alkali feldspars solubility relationships possibly in response to episodic al si complexing which might develop within a hypothesised still active exhumation channel

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

crustal emplacement of exhuming ultra high pressure rocks will that be pro or retro side ultra high pressure u hp rocks form and exhume from deep within subduction channels but subsequent horizontal transport in the shallower orogenic crust makes it difficult to reconstruct their tectonic histories we use a conceptual framework and numerical models to show that buoyant exhumation from within a subduction conduit formed during one sided subduction may lead to emplacement of u hp rocks into either the lower plate prowedge or upper plate retrowedge of an orogen depending on whether the upper plate crust deforms or acts as a backstop during exhumation both modes may operate at different positions or different times within an orogen leading to emplacement of u hp rocks into both plates without changing subduction geometry we propose that retrotransport during exhumation may explain some u hp rocks e g liverpool land situated in the upper plate of the greenland norwegian caledonides

chronological studies of the quaternary marine sediments of northern spencer gulf south australia abstract four dating methods have been used to obtain chronological information about holocene and pleistocene coastal marine sediments in south australia and to check the stratigraphic reliability of each dating technique estimates of accumulation rates for holocene seagrass bank deposits vary according to the time period over which they are averaged the estimates vary from 2 0 to 2 7 mm yr 1 for measurements by the pb 210 method averaged over about 100 years to 1 4 0 2 mm yr 1 for c 14 derived measurements averaged over a period of 1000 7000 yrs radiocarbon age determinations on carbonate and organic fractions do not date the true sediment age shells shell fragments and calcareous fines predate the sediment whereas organic and seagrass detritus postdate it by dating sample pairs their age can be adequately defined thermoluminescence age estimates correlate well with available c 14 ages where the sediment grains have experienced good exposure to sunlight prior to deposition amino acid racemization and thermoluminescence measurements indicate a late pleistocene age of ca 110 000 yrs for the mambray formation of spencer gulf amino acid racemization data also support a correlation with the late pleistocene glanville formation of adelaide

petrology geochemistry and tectonic implications of volcanics dredged from the intersection of the yap and mariana trenches abstract rocks dredged from the forearc very close to the intersection of the yap and mariana trenches include a suite of highly depleted arc tholeiites and several samples of transitional to slightly alkaline basalt the tholeiites range from magnesian quartz tholeiites with 0 46 0 6 tio2 to andesites with up to 62 sio2 and 8 2 feo all show pronounced lree depletion and have very low contents of ba and sr they are postulated to have been produced by partial melting of upper mantle peridotite residual after morb extraction following influx of hydrous fluids from the subducted slab while these fluids were responsible for small enrichments in ba k rb and sr in melts generated lree were not involved in the metasomatism and the strong lree depletion probably reflects the unmodified depleted source peridotite the second lava suite includes slightly ne normative ti augite bearing basalts with convex upward ree patterns showing slight lree depletion la sm n 0 76 the chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening a k ar age on one sample 7 8 1 3m y is in good agreement with the initial opening of the mariana trough the tectonic significance of the dredged arc tholeiite suite is less obvious a k ar age of10 8 0 4 my on one andesite and the occurrence of similar lavas in dredges from at least 300 km along the length of the yap arc suggest that subduction was occurring beneath the yap arc in the late miocene after overthrusting of the yap greenschist allochthon and while calc alkaline arc magmatism was occurring further north on the west mariana ridge we suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the yap forearc following subduction beneath this area of young hot sorol trough crust these arc tholeiites represent a magma type transitional between more typical arc tholeiites e g tongan and high mg andesites and boninites

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

the alps 1 a working geodynamic model for burial and exhumation of ultra high pressure rocks in alpine type orogens abstract eocene ultra high pressure u hp rocks exposed in the western alps are generally interpreted to result from subduction of european continental crust beneath adria and its subsequent exhumation however the roles of extension either from plate divergence or internal to the orogen and erosion during exhumation remain controversial here we use 2d numerical geodynamic models to explain the formation and exhumation of u hp rocks in an alpine type orogen and interpret the results in the conceptual prowedge uplifted plug retrowedge conduit purc framework u hp metamorphism of oceanic and microcontinent crust in the models results from burial and accretion to a subduction channel conduit formed beneath an advancing retrocontinent rapid exhumation from u hp conditions is achieved by the buoyancy driven transport of a composite plume of stacked u hp oceanic and microcontinent crust from the subduction conduit to the overlying orogenic prowedge accommodated by coeval thrusting and normal sense shearing subsequent trans crustal exhumation is achieved by a combination of doming internal extension and later retrotransport of the u hp plume through the uplifted plug during underthrusting of the thick continental margin crust coupled with increased erosion our proposed mechanism implies that exhumation related normal sense shearing in the western alps per se was driven from below by the buoyancy of the ascending plume and that extension owing to plate divergence is not required to explain u hp rock exhumation the efficiency of the exhumation mechanism depends strongly on the buoyancy and strength of the u hp plume suggesting that in order to exhume rapidly it must achieve a critical size by implication the multiple small eocene u hp complexes within the internal crystalline massifs may have been exhumed as part of a single composite body comprising diverse units aggregated from different levels of the subduction conduit during burial or ascent rather than as individual small bodies exhumed in separate pulses

radiogenic isotope systematics of the herefoss granite south norway an indicator of sveconorwegian grenvillian crustal evolution in the baltic shield abstract a new pb pb mineral isochron dates the crystallization of the herefoss granite south norway to 926 8 ma confirming earlier rb sr evidence of a late sveconorwegian grenvillian age initial sr and nd isotopic compositions recalculated to esr 926 ma 40 end 926 ma 3 2 to 0 8 indicate that the magma has originated from a mixture of two distinct source components 1 a crustal component which has resided in a moderately lile enriched reservoir f rb 7 to 8 238 u 204 pb 17 22 since 1 6 1 9 ga this crustal component is clearly distinct from the extremely lile enriched metasedimentary country rocks of the granite f rb 21 238 u 204 pb 24 2 a mantle derived component e nd 4 6 to 5 9 e sr 12 to 9 238 u 204 pb 7 96 younger than 1 5 ga the ranges of initial sr nd and pb composition observed in the intrusion indicate that the magma was isotopically heterogeneous at the time of emplacement and that the crustal contribution amounted to 35 55 by the end of the sveconorwegian orogeny the southwestern part of the baltic shield had acquired a compositionally layered structure where an uppermost zone of the crust with extreme lile enrichment was underlain by a heterogeneous layer consisting of rocks with normal upper crustal degree of lile enrichment and mafic intrusions younger than 1 5 ga there is no evidence of involvement of lile depleted deep continental crust source material in the petrogenesis of the herefoss granite rejuvenation of the lower crust by injection of mantle derived magmas underplating has been an important process in the evolution of the southwestern part of the baltic shield the absence of a young mantle derived component from 1 12 ga charnockitic intrusions in the bamble sector and from the 989 ma grimstad granite suggests that this process may have been contemporaneous with late sveconorwegian mafic magmatism in the rogaland vest agder sector further to the west

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

constraints on the subduction erosion extrusion cycle in the paleozoic acatlan complex of southern mexico geochemistry and geochronology of the type piaxtla suite abstract the type high pressure hp piaxtla suite in the acatlan complex of southern mexico consists of retrogressed eclogite amphibolite megacrystic granitoids and high grade meta sedimentary rocks exhumation of these hp rocks has recently been interpreted as the result of extrusion into the upper plate rather than by return flow up the subduction zone geochemical analyses of the retrograde eclogites indicate that they have a rift tholeiitic transitional alkalic composition these are closely associated with a megacrystic meta granitoid that has yielded an intrusive age of 452 6 ma concordant u pb zircon analyses with inherited zircon populations at ca 800 950 ma and 1000 1200 ma derived from the underlying basement probably the oaxacan complex which borders the acatlan complex to the east the bimodal nature of these igneous rocks and their spatial and temporal close association with continentally derived sedimentary rocks is similar to most hp rocks in the acatlan complex derived from a rifted passive margin the youngest detrital zircon population in a meta psammite sample yielded a u pb age of 365 15 ma with older analyses distributed along a chord with an upper intercept of 1287 29 ma the ca 365 ma age provides a maximum age for the time of deposition of this sample 40 ar 39 ar ages from the retrogressed eclogites provided hornblende plateau ages of 342 2 ma and 344 2 ma whereas muscovite from the granitoid and meta psammite yielded 334 2 ma plateau ages these data constrain the subduction erosion extrusion cycle to 35 ma during which the rocks were taken to a depth of ca 40 km at a rate of 2 7 km ma and back to the surface at 2 4 km ma such exhumation rates are slower than those in continent continent collision zones but similar to those in the iberia czech variscan belt where tectonic interpretation also suggests extrusion into the upper plate

late archaean smnd model ages from the voche lambina area implications for the age distribution of archaean crust in the kola peninsula russia abstract although an early archaean age for the saamian gneisses has been generally accepted in the former soviet union partly on the basis of their commonly granulite facies metamorphic grade and partly on account of their structural complexity and complex intrusion history these rocks have yielded no precise isotopic ages in excess of 2 9 ga sm nd isotopic analyses are used to test the possible presence of early archaean 3 5 ga saamian gneisses at voche lambina in the kola peninsula here tonalitic trondhjemitic and granodioritic gneisses presumed to be early archaean saamian in age are overlain by a sequence of late archaean lopian supracrustals a sample of the structurally oldest trondhjemitic gneiss with a minimum age of 2 64 ga has a depleted mantle model age tdm of 2875 ma tonalitic gneiss also assumed on structural grounds to be early archaean has a tdm age of 2808 ma identical to its u pb zircon age of 2807 9 ma this suggests that a depleted mantle model may be appropriate for at least this part of the kola region in the late archaean lopian rhyodacite with a u pb zircon age of 2800 ma and a rb sr isochron age of 2540 83 ma has a tdm age of 2756 ma in each case the close agreement between the sm nd model age and the time of intrusion or eruption leaves little scope for a significantly older crustal history for these rocks the data provide no evidence for a saamian component in this part of the kola peninsula

crustal structure a key constraint on the mechanism of ultra high pressure rock exhumation abstract the distribution of ultra high pressure uhp metamorphic rocks demonstrates that burial to 100 km and rapid exhumation 1 cm a 1 of continental crust is a normal part of early 10 ma continental collision currently there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper crustal structures resulting from early collisional uhp rock exhumation characteristic features requiring explanation include structural domes that are cored by uhp nappes associated medium to high pressure nappes displaying a distinct pressure gap overlying lower grade rocks including suture zone ophiolites and coeval foreland directed thrust faults and syn exhumation normal faults we present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge competition between down channel shear traction and up channel buoyancy forces expressed as the exhumation number e controls burial and exhumation leading to rapid up channel flow when e 1 exhuming uhp material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge driving thrusting and extension this solution is compelling because it explains both the geology and the petrology of the tso morari and other uhp complexes and because it demonstrates that pulse like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures this places constraints on the exhumation mechanism and provides a test of alternative models other proposed mechanisms such as continuous circulation in a lithospheric scale wedge or overpressured subduction channel predict different types of upper crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of uhp terranes

development of joint controlled rock basins in bohus granite sw sweden the granite area of bohuslan along the swedish west coast is characterised by an abundance of rock basins of different sizes within a 163 km2 large part of the area over 400 basins occur ranging

aspartic acid racemisation dating of mid holocene to recent estuarine sedimentation in new south wales australia a pilot study abstract the degree of aspartic acid racemisation measured in radiocarbon dated specimens of fossil estuarine molluscs collected from lake illawarra a holocene barrier estuary in new south wales australia is evaluated in the context of results of laboratory induced racemisation established in heating simulated ageing experiments the general kinetic trend of aspartic acid racemisation in both heating experiments and fossil molluscs anadara trapezia and notospisula trigonella conforms to a model of apparent parabolic kinetics using the apparent parabolic kinetic model numeric ages based on the degree of aspartic acid racemisation in the fossil molluscs have been calculated an aminostratigraphy of the lake illawarra holocene basin fill has been established based on the extent of aspartic acid racemisation measured in 29 specimens of fossil molluscs aspartic acid d l ratios in a trapezia and n trigonella range from 0 079 0 005 to 0 436 0 004 representing an age range from

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

precambrian multi stage crustal evolution in the bamble sector of south norway pb isotopic evidence from a sveconorwegian deep seated granitic intrusion abstract the precambrian ubergsmoen augen gneiss in the bamble sector of south norway is a dry syntectonic charnockitic intrusion which was emplaced at amphibolite to granulite facies conditions in the sveconorwegian orogeny 1120 ma the present day 206 pb 204 pb and 207 pb 204 pb ratios show restricted variation 206 pb 204 pb 17 566 17 938 207 pb 204 pb 15 459 15 590 in whole rocks and 206 pb 204 pb 17 159 17 350 207 pb 204 pb 15 451 15 487 in feldspar whereas the 208 pb 204 pb ratio has a wider range of variation in whole rocks 36 571 42 200 but not in feldspars 36 487 37 054 these values reflect a 238 u 204 pb of 2 6 since the sveconorwegian and a variable th u ratio 2 20 at the time of final u pb differentiation homogenization of the pb isotopes was imperfect the data therefore do not allow pb pb isochron dating but are well suited for model calculations the isotopic evolution of pb can be reproduced by a three stage model involving 1 a mantle stage ending at 1 9 1 6 ga 2 extraction of a crustal precursor at 1 9 1 6 ga and differentiation of a lile enriched component 14 22 and 3 anatexis igneous differentiation emplacement and subsequent metamorphism at 1 12 ga the granitic intrusion acquired its low u pb ratio by magmatic differentiation processes rather than by fluid induced u loss at high grade metamorphic conditions