unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

evolution of the louisiade triple junction we derived new finite rotations for the opening of the coral sea using revised magnetic anomaly interpretations and fracture zone data from a gravity anomaly grid based on from satellite altimetry these rotations differ from the finite rotations that describe the opening of the tasman sea this confirms the existence of a triple junction between the australian plate the mellish rise and the louisiade plateau active during the opening of the coral sea 62 to 52 ma magnetic anomalies fracture zones visible on the gravity grid and strike slip faults indicate that extension occurred between the mellish rise and the louisiade plateau and extensional and transform motion occurred between australia and the mellish rise attached to the chesterfield and kenn plateaus the configuration of the triple junction from chron 27 to 26 was either ridge ridge ridge rrr or ridge fault fault rff at chron 26 58 ma the triple junction had a rff configuration and migrated southward as the relative motion between the louisiade plateau and the mellish rise was transferred to the boundary between the mellish rise and the kenn plateau the gravity low between the kenn plateau and the mellish rise is interpreted as a strike slip fault active from about 57 to 52 ma this configuration lasted until seafloor spreading ceased in the coral and tasman seas at about 52 ma our model implies extension in the osprey embayment that might explain small areas of oceanic crust west of the coral sea basin the western boundary of the coral sea was a ne sw strike slip fault active between 58 and 52 ma

beach monitoring program of valencia spain the littoral processes and long term shore protection plans are analysed for the coast of valencia spain the breakwaters built during this century by the port authorities have gratly affected the most southern beaches by interrupting the natural longshore sand transport the monitoring program of the beaches of valencia spain has been established for a precise estimation of the evolution in time of the beaches to the north and south by the port of valencia periodic topographic surveys provide the basic timedependent beach description of the area the two main objetives are the following a 3 d description of the beach surface including estimation of reliability and measurement errors of bathymetrics and b stochastic description of wave climate high precision and cost efficient beach surveying techniques have been developed during the monitoring program of the beaches of valencia spain beach profiles and zero shorelines have been obtained with systems and errors similar to the terestrial topography the methods are simple enough to be applied by general land surveyors with a minimum training and may be extended to a variety of beaches for systematic and low cost monitoring programs

the rhone glacier was smaller than today for most of the holocene we present the development and application of the novel in situ cosmogenic 14 c 10 be chronometer to recently exposed proglacial bedrock of the rhone glacier switzerland results show that during the holocene the glacier was smaller than today for 6500 2000 yr and larger than today for 4500 2000 yr this pattern is consistent with limited data from other techniques for glaciers in the alps and scandinavia but in contrast to glaciers from the southern alps of new zealand emphasizing the high sensitivity of large mountain glaciers to small climate changes the 14 c 10 be chronometer also shows that abrasion rates beneath the rhone glacier increased with ice speed

influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides abstract scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field these scaling factors are analogous to those determined in a similar work dunai t j earth planet sci lett 2000 176 157 169 that were accurate for field strengths equivalent to present day field strength but cannot accommodate field strength variations strategies to derive the relevant geomagnetic field parameters are discussed and are tested with existing production rate calibrations for 3 he a production rate of 103 4 2 atoms g olivine yr sea level and for zero cutoff rigidity was derived from calibration sites from hawaii western usa and canary islands covering the time range of 2 281 kyr

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

the petrogenesis of sodic granites in the niujuanzi area and constraints on the paleozoic tectonic evolution of the beishan region nw china abstract ordovician to devonian sodic granites dominate the newly recognized luotuojuan composite granite in the lebaquan luotuojuan niujuanzi region of beishan along the southern margin of the central asian orogenic belt in nw china the granites include sodic k2o na2o 0 5 tonalites with low y 68 that formed during at least two events at c 435 and c 370 360 ma their compositions are consistent with high pressure melting of basaltic crust although relatively non radiogenic nd isotope compositions end t 0 9 require some crustal assimilation the interpretation that these granites reflect melts of a subducted slab i e adakite is supported by independent local and regional geological evidence for an oceanic subduction accretion setting including a long history of calc alkaline magmatism and the identification of a series of early paleozoic ophiolite belts other sodic granites forming the luotuojuan composite granite are mainly quartz diorite and granodiorite formed between c 391 and c 360 ma these rocks are not adakites having sr concentrations and sr y ratios too low and y and yb concentrations too high they are low to medium k calc alkaline rocks more typical of magmas derived through melting in a subduction modified mantle wedge compositional changes from sodic to potassic granites over time frames consistent with subduction processes suggest at least two separate cycles or pulses of hot subduction in the lebaquan luotuojuan niujuanzi region although early paleozoic adakites have been inferred to exist elsewhere in the beishan region many of the reported adakitic rocks have compositions inconsistent with melting of subducted oceanic lithosphere and so tectonic interpretation of hot subduction might not be valid in these cases a study of regional granite data also shows not only that adakite magmatism does not extend into the permian but that if subduction accretion processes extended into the late paleozoic no typical subduction related magmatism was preserved new and published nd isotope data from regional granites also requires at least the local presence of proterozoic basement or microcontinental slivers in the evolution of the beishan region

tectonic evolution and copper gold metallogenesis of the papua new guinea and solomon islands region papua new guinea and the solomon islands are in one of the most prospective regions for intrusion related mineral deposits however because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets the link between mineralization and the regional scale geodynamic framework has not been understood here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region combined with the spatio temporal distribution of intrusion related mineral deposits and six new deposit ages convergence at the pacific australia plate boundary was accommodated from at least 45 ma by subduction at the melanesian trench with related melanesian arc magmatism the arrival of the ontong java plateau at the trench at ca 26 ma resulted in cessation of subduction immediately followed by formation of cu au porphyry epithermal deposits at 24 20 ma throughout the melanesian arc late oligocene to early miocene tectonic reorganization led to initiation of subduction at the pocklington trough and onset of magmatism in the maramuni arc the arrival of the australian continent at the pocklington trough by 12 ma resulted in continental collision and ore deposit formation from 12 to 6 ma this is represented by cu au porphyry deposits in the new guinea orogen and epithermal au systems in the papuan peninsula from 6 ma crustal delamination in papua new guinea related to the prior pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse cu and au porphyry and epithermal deposits within the papuan fold and thrust belt and papuan peninsula from 6 ma to the present day subduction at the new britain and san cristobal trenches from ca 10 ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern papua new guinea and the solomon islands this is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab

rapid change in drift of the australian plate records collision with ontong java plateau earth s largest and thickest oceanic plateau the ontong java plateau is currently colliding with the australian plate but it has been difficult to date the start of this momentous event with much accuracy now a team from the university of queensland presents strong evidence for a collision starting about 26 million years ago the dating comes from geochronological data on hotspot volcanoes in the tasman sea east of australia which reveal a link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain suggesting that immense oceanic plateaus like ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale geochronological data on hotspot volcanoes in eastern australia are presented which reveal a strong link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain and thus provide strong evidence that immense oceanic plateaus can contribute to initiating rapid change in plate boundaries and motions on a global scale the subduction of oceanic plateaux which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood basalt provinces is an important factor in many current models of plate motion1 2 3 4 and provides a potential mechanism for triggering plate reorganization5 to evaluate such models it is essential to decipher the history of the collision between the largest and thickest of the world s oceanic plateaux the ontong java plateau and the australian plate but this has been hindered by poor constraints for the arrival of the plateau at the melanesian trench here we present 40ar 39ar geochronological data on hotspot volcanoes in eastern australian that reveal a strong link between collision of the greenland sized ontong java plateau with the melanesian arc and motion of the australian plate the new ages define a short lived period of reduced northward plate motion between 26 and 23 myr ago coincident with an eastward offset in the contemporaneous tracks of seamount chains in the tasman sea east of australia these features record a brief westward deflection of the australian plate as the plateau entered and choked the melanesian trench 26 myr ago from 23 myr ago australia returned to a rapid northerly trajectory at roughly the same time that southwest directed subduction began along the trobriand trough6 the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the pacific plate including the archetypal hawaiian chain7 and thus provide strong evidence that immense oceanic plateaux like the ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale

from continent to intra oceanic arc zircon xenocrysts record the crustal evolution of the solomon island arc the first u pb ages from a ca 26 24 ma pluton on guadalcanal in the intra oceanic solomon island arc southwest pacific ocean reveal eocene to archean aged zircon xenocrysts xenocryst populations at ca 39 33 ma and ca 71 63 ma correlate with previously obtained ages of supra subduction magmatism within the arc a ca 96 ma zircon population may be derived from cretaceous ophiolite basement crust or region wide continental rift related magmatism xenocryst age populations alternate with periods of oceanic basin formation that fragmented the east gondwana margin early cretaceous to archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust they may have been introduced by eocene interaction of a continental fragment with the arc and concealed by ophiolite obduction the data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra oceanic arc magmas a process likely facilitated by cyclical subduction zone advance and retreat the findings highlight the continuum of arcs that occurs between continental and oceanic end members and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust

sinking of concrete modules into a sandy seabed a case study abstract three submerged coastal structures on the sandy seabed at santa maria del mar smm beach in southwest spain were monitored during the six months after installation starting in november of 2005 the monitoring evaluated the self burial phenomenon of the 40 precast modular concrete elements of the three structures assessment included scouring around the structures their vertical movement the sinking velocity of the modules and the resulting beach profiles pressure sensors and precision topographies permitted continuous monitoring of the self burial process for the first time in a full scale case comparison of two bathymetries one performed immediately prior to installation and another performed six months after installation indicated slight accretion of the seabed whereas intermediate topographic surveys indicated extensive scouring around the structures the three structures began sinking into the sandy bottom platform immediately after placement and continued until they reached the rocky bottom this study implemented a new methodology using pressure sensors attached to the concrete modules to monitor the structural sinking unexpectedly the average sinking speed was extremely rapid at approximately 3 6 cm day 50 of the height of the element was reached in three to six weeks the results of this study are also compared with the results of other full scale cases among other conclusions data from this study indicated that the sinking rate of the modules in smm beach was one order of magnitude greater than the other values

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

new methodology for describing the equilibrium beach profile applied to the valencia s beaches abstract mathematical models used for the understanding of coastal seabed morphology play a key role in beach nourishment projects these projects have become the fundamental strategy for coastal maintenance during the last few years accordingly the accuracy of these models is vital to optimize the costs of coastal regeneration projects planning of such interventions requires methodologies that do not generate uncertainties in their interpretation a study and comparison of mathematical simulation models of the coastline is carried out in this paper as well as elements that are part of the model that are a source of uncertainty the equilibrium profile ep and the offshore limit corresponding to the depth of closure doc have been analyzed taking into account different timescale ranges the results have thus been compared using data sets from three different periods which are identified as present past and future accuracy in data collection for the beach profiles and the definition of the median grain size calculation using collected samples are the two main factors that have been taken into account in this paper these data can generate high uncertainties and can produce a lack of accuracy in nourishment projects together they can generate excessive costs due to possible excess or shortage of sand used for the nourishment the main goal of this paper is the development of a new methodology to increase the accuracy of the existing equilibrium beach profile models providing an improvement to the inputs used in such models and in the fitting of the formulae used to obtain seabed shape this new methodology has been applied and tested on valencia s beaches

tectonic evolution of the paleozoic barluk continental arc west junggar nw china abstract the barluk arc situated in west junggar is an important component of the kazakhstan domain in the central asian orogenic belt caob however the nature of the barluk arc active continental margin or oceanic island arc is in debate the igneous rocks in the shiwu area barluk arc have diagnostic petrologic geochronologic and geochemical signatures that uncover the attributes of the barluk arc in this paper we report detailed petrology whole rock geochemistry and sr nd isotopic zircon u pb dating and hf isotopic data of igneous rocks in the shiwu area our results reveal that 1 except for 302 2 ma gabbro igneous rocks from barluk arc in the shiwu area are dioritic and granitic have ages ranging from 323 8 to 310 5 ma and they have geochemical signatures originated from typical continental arcs including calc alkaline series rocks enrichment of lile depletion of hfse and higher th yb and ta yb values 2 the granitic porphyry and rhyolite in barluk arc have old tdm2 hf ages up to 1091 ma suggesting the existence of precambrian crust or similar materials as their sources and 3 the 302 2 ma gabbro dike has high values of end t 7 54 and ehf t 12 8 to 15 5 and a young t2dm nd age 455 ma and therefore it was possibly related to upwelling of asthenosphere mantle in a post subduction extensional tectonic setting based on these parameters we suggest that the barluk arc is defined as a late paleozoic continental margin arc along the margin of the kazakhstan continent westward subduction of the junggar oceanic plate in the barluk area started in early devonian and continued to the late carboniferous 310 ma subsequent post collisional tectonic processes between the junggar oceanic plate and kazakhstan continent continued to the late carboniferous 302 ma

detrital zircon record from major rivers of luzon island implications for cenozoic continental growth in se asia the significance of continental crustal growth in the phanerozoic has long been debated as an active accretionary orogenic belt se asia has been proposed to be one of the best archives for studying juvenile continental growth in the cenozoic despite the limited availability of data therefore we conducted for the first time a comprehensive u pb and hf isotopic analysis of detrital zircons separated from major rivers of luzon island the largest landmass of the philippine archipelago u pb ages of these zircons are predominantly supplementary material u pb ages and hf isotopic results for detrital zircons from all samples are available at https doi org 10 6084 m9 figshare c 4481882

a tale of two eras pliocene pleistocene unroofing of cenozoic and late archean zircons from active metamorphic core complexes solomon sea papua new guinea u pb ion microprobe analyses of zircons from gneisses and granodiorites exposed in the d9entrecasteaux islands and from conglomerate sections of the goodenough no 1 well in the adjacent trobriand basin provide constraints on the age of magmatism peak metamorphism and nature of rocks unroofed during initial stages of metamorphic core complex formation in the solomon sea the youngest populations of zircons from felsic gneisses and granodiorites indicate late pliocene 206 pb 238 u ages no inherited zircons were identified in the granodiorites and the 206 pb 238 u ages 1 65 0 18 ma 1 98 0 08 ma 2 are interpreted as crystallization ages these synkinematically emplaced granodiorites intruded into actively extending continental crust are some of the youngest known granitoids currently exposed at the earth9 surface zircon ages from felsic gneisses 2 63 0 16 ma 2 72 0 28 ma 2 are interpreted to date zircon growth subsequent to eclogite facies metamorphism felsic gneiss samples also contained zircon xenocrysts from cretaceous miocene protoliths in striking contrast zircons from igneous and metamorphic clasts from the goodenough no 1 well indicate a single population with a 207 pb 206 pb age of 2781 9 ma 2 we speculate that they are derived from basement rocks unroofed during initial stages of development of the d entrecasteaux metamorphic core complexes these results provide the first direct evidence for the existence of archean protoliths in the basement rocks of southeastern papua new guinea

contributions and unrealized potential contributions of cosmogenic nuclide exposure dating to glacier chronology 1990 2010 abstract this paper reviews the application of cosmogenic nuclide exposure dating to glacier chronology exposure dating of glacial landforms has made an outsize impact on this field because the technique filled an obvious need that had already been recognized by glacial geologists by now hundreds of studies have used cosmogenic nuclide exposure dating to date glacial deposits and in fact it is rare to find a study of glacial geology or glacier chronology or any paleoclimate synthesis that makes use of such studies that does not involve exposure dating these developments have resulted in major contributions to glacier chronology and paleoclimate in particular i reconstructing antarctic ice sheet change ii establishing the chronology of late pleistocene and holocene glacier change in mountain regions where it was previously unknown iii establishing the broad chronological outlines of mountain glaciations prior to the last glacial maximum and iv gaining insight into subglacial erosional processes through the observation that many glaciated surfaces preserve cosmogenic nuclide inventories from long past ice free periods as well as the present one an important potential future contribution will be the application of the large data set of exposure dated glacier chronologies to better understand global and regional climate dynamics during lateglacial and holocene millennial scale climate changes however this contribution cannot be realized without significant progress in two areas i understanding and accounting for geologic processes that cause apparent exposure ages on glacial landforms to differ from the true age of the landform and ii minimizing systematic uncertainties in exposure ages that stem from cosmogenic nuclide production rate estimates and scaling schemes at present there exists an enormous data set of exposure ages on glacial deposits but these data cannot be used to their full potential in paleoclimate syntheses due to an inadequate understanding of geologic scatter and production rate uncertainties the intent of this paper is to highlight this situation and suggest some strategies for realizing this potential

melanesian back arc basin and arc development constraints from the eastern coral sea abstract the eastern coral sea is a poorly explored area at the north eastern corner of the australian tectonic plate where interaction between the pacific and australian plate boundaries and accretion of the world s largest submarine plateau the ontong java plateau has resulted in a complex assemblage of back arc basins island arcs continental plateaus and volcanic products this study combines new and existing magnetic anomaly profiles seafloor fabric from swath bathymetry data ar ar dating of e morb basalts palaeontological dating of carbonate sediments and plate modelling from the eastern coral sea our results constrain commencement of the opening of the santa cruz basin and south rennell trough to c 48 ma and termination at 25 28 ma simultaneous opening of the melanesian basin solomon sea further north suggests that a single 2000 km long back arc basin with at least one triple junction existed landward of the melanesian subduction zone from eocene oligocene times the cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the ontong java plateau the correlation between back arc basin cessation and a widespread plate reorganisation event suggests that back arc basins may be used as markers for both local and global plate boundary changes

the petrology geochronology and tectono magmatic setting of igneous rocks in the suckling dayman metamorphic core complex papua new guinea abstract southeastern papua new guinea has hosted magmatism throughout the cenozoic with the latest phase being concurrent with active extension in the woodlark rift there the suckling dayman metamorphic core complex sdmcc has exhumed middle crustal rocks in the past few million years along a still active low angle normal fault the mai iu fault uplift of the sdmcc has exposed metasedimentary and metaigneous rocks of late cretaceous pleistocene age that record an evolution from oceanic spreading to subduction to collision and finally to subduction inversion and extensional exhumation we present new petrographic whole rock geochemical and geochronologic zircon u pb la icp ms data from the sdmcc to reconstruct its long term evolution the dominant footwall lithology of the sdmcc the goropu metabasalt has a morb composition detrital zircons from metasedimentary beds intercalated with these basalts yield u pb based maximum estimates for deposition of 103 and 72 ma suggesting a late cretaceous spreading age of the oceanic protolith bulk compositions of low grade tholeiitic meta gabbroic and tonalitic rocks yau igneous complex that intrude the goropu metabasalt reveal both enriched and depleted light rare earth element patterns zircon u pb ages from the yau igneous complex range between 60 and 57 ma providing a minimum age for the formation of the goropu metabasalt much younger syn extensional granitoids in the mafic footwall of the sdmcc have calc alkaline to high k compositions and u pb ages on zircon between 3 7 and 2 0 ma our data indicate that the mai iu fault had re activated a paleogene thrust as an extensional detachment fault by 3 7 0 2 ma u pb ages of detrital zircons in modern streams draining the footwall of the sdmcc essentially reflect the ages revealed by the paleocene and plio pleistocene intrusions xenocrystic zircons in the plio pleistocene granitoids imply that the crust underlying the metabasaltic carapace of the sdmcc consists chiefly of australian continent derived sedimentary rocks

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

holocene glacier culminations in the western alps and their hemispheric relevance the natural variability of holocene climate defi nes the baseline to assess ongoing climate change greenland ice core records indicate warming superimposed by abrupt climate oscillations in the early holocene followed by a general cooling trend throughout the middle and late holocene that culminated during the little ice age lia tropical precipitation changes correlate with these patterns throughout the holocene here we use mountain glaciers in the european alps to reconstruct the regional holocene climate evolution and to test for a link between mid latitude north atlantic and tropical climate our precise 10 be chronology from tsidjiore nouve glacier western swiss alps indicates a glacier culmination during the earliest holocene 11 4 k y ago likely related to the preboreal oscillation based on our data no holocene glacier advance of similar amplitude occurred until 3 8 k y ago when the glacier reached lia limits the 10 be ages between 500 and 170 yr correspond to the lia while the youngest 10 be ages overlap with the historically recorded post lia glacier positions integrating our data with existing records we propose a hemispheric climate link between the alps north atlantic temperature and tropical precipitation patterns for the holocene supporting the concept of a pervasive climate driver these fi ndings from northern mid latitudes are consistent with the hypothesis formulated for the tropics that the earth s thermal equator responding to north atlantic temperature changes might have migrated southward throughout the holocene reaching the southern turning point toward the end of the lia

geochemistry and u pb hf zircon data for plutonic rocks of the troia massif borborema province ne brazil evidence for reworking of archean and juvenile paleoproterozoic crust during rhyacian accretionary and collisional tectonics abstract rhyacian c 2 30 2 05 ga subduction accretion and collisional processes are thought to represent an important event of crustal growth and reworking often reported in the so called tranzamazonian eburnean granite greenstone terranes of n ne brazil and west african craton in this work we present whole rock geochemistry and u pb hf zircon ages for rhyacian plutonic rocks that intrude paleoproterozoic granite greenstone terranes and archean basement rocks of the troia massif borborema province ne brazil u pb shrimp zircon age of 2185 4 ma was obtained for a metatonalite sample mirador metatonalites the metatonalites show geochemical affinity similar to those of adakites like rocks with all classic slab derived adakitic signature e g high sr y la yb and low hree but more akin to those evolved from partial melting of the metasomatized mantle wedge followed by fractional crystallization zircon hf crustal model ages of the metatonalite sample range between 2800 and 2660 ma evidencing that archean crustal components contributed to its magma genesis probably via subduction of continental derived sediments to the mantle wedge for potassic plutons of the troia massif bananeira suite we obtained u pb shrimp zircon age of 2079 4 ma for a deformed quartz monzonite with geochemical affinities similar to those derived from low pressure partial melting of a k rich mafic protolith for less deformed equigranular granites we obtained a u pb la icpms zircon age of 2068 5 ma they are high k calc alkaline and slightly peraluminous granites that probably derived from low pressure partial melting of tonalitic crustal sources metatonalites and or metagraywackes zircon hf crustal model ages of a granite sample range from 2713 to 2535 ma evidencing archean crustal contribution to magma genesis however two analyzed c 2307 ma inherited zircon grains show ehf t values of c 4 87 indicating that crustal reworking of less radiogenic paleoproterozoic sources also participated in its genesis regional correlations of our results with the paleoproterozoic record of the troia massif borborema province and surrounding cratonic domains suggest that the studied plutons are related to the c 2 2 2 0 ga transamazonian eburnean orogenic cycle the c 2185 ma metatonalites are associated to pre collisional setting while the c 2079 2068 ma quartz monzonites and granites evolved during collisional setting

the building blocks of continental crust evidence for a major change in the tectonic setting of continental growth at the end of the archean abstract oceanic arcs are commonly cited as primary building blocks of continents yet modern oceanic arcs are mostly subducted also lithosphere buoyancy considerations show that oceanic arcs even those with a felsic component should readily subduct with the exception of the arabian nubian orogen terranes in post archean accretionary orogens comprise 50 produced in continental arcs felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs they have lower th yb nb yb sr y and la yb ratios reflecting shallow mantle sources in which garnet did not exist in the restite during melting the bottom line of these geochemical differences is that post archean continental crust does not begin life in oceanic arcs on the other hand the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs during the archean however oceanic arcs may have been thicker due to higher degrees of melting in the mantle and oceanic lithosphere would be more buoyant these arcs may have accreted to each other and to oceanic plateaus a process that eventually led to the production of archean continental crust after the archean oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges hence oceanic lithosphere is more subductable widespread propagation of plate tectonics in the late archean may have led not only to rapid production of continental crust but to a change in the primary site of production of continental crust from accreted oceanic arcs and oceanic plateaus in the archean to primarily continental arcs thereafter

36cl production rate from k spallation in the european alps chironico landslide switzerland the abundant production of in situ cosmogenic 36cl from potassium renders 36cl measurements in k rich rocks or minerals such as k feldspars potentially useful for precisely dating rock surfaces either in single nuclide or in multi nuclide studies for example combined with 10be measurements in quartz however significant discrepancies in experimentally calibrated 36cl production rates from spallation of potassium 36pk sp referenced to sea level high latitude slhl limit the accuracy of 36cl dating from k rich lithologies we present a new 36cl calibration using k feldspars in which k spallation is the most dominant 36cl production pathway 92 of total 36cl thus minimizing uncertainties from the complex multi pathway 36cl production systematics the samples are derived from boulders of an 13 4 ka old landslide in the swiss alps 820 m 46 43 n 8 85 e we obtain a local 36pk sp of 306 16 atoms 36cl g k 1 a 1 and an slhl 36pk sp of 145 5 7 7 atoms 36cl g k 1 a 1 when scaled with a standard scaling protocol lm applying this slhl 36pk sp to determine 36cl exposure ages of k feldspars from 10be dated moraine boulders yields excellent agreement confirming the validity of the new slhl 36pk sp for surface exposure studies involving 36cl in k feldspars in the alps