zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

characteristic features of pockmarks on the north sea floor and scotian shelf this paper presents the results of a comparative study of pockmarks and associated features appearing on both sides of the north atlantic on the scotian shelf off nova scotia and in the northern north sea pockmarks are formed in seabed material consisting of soft silty clay the seismic sonar and lithologic characteristics of the sediments on the scotian shelf are remarkably similar to those found in the northern north sea sediment clouds suspended in the water column immediately over the seabed have previously been observed on side scan records associated with gas charged sediments on corresponding shallow seismic records these and similar observations strongly suggest that most pockmarks are caused by gas efflux through the seafloor however the detailed mechanism of formation and the origin of the gas in the sediments is still unknown

lift off moraines markers of last ice flow directions on the scotian shelf lift off moraines are acoustically incoherent subparallel ridges observed on sidescan sonograms and high resolution reflection seismic profiles on the southeastern continental margin of canada they are up to 3 m high 20 80 m wide and are commonly overlain by stratified proglacial sediments although little is known about them detailed study of high resolution seismic profiles from the emerald basin and the lahave basin on the scotian shelf show that their height width ratio varies with the sounder seabed separation suggesting that the ridges may be narrower than they appear their morphology is similar to degeer moraines or cross valley moraines which form perpendicular to ice flow direction as their orientations can be estimated at the intersection of seismic lines they can be used to estimate ice flow directions since proglacial sediments are draped directly over top of them they are assumed to record the direction of last ice flow this directional data suggests that ice retreated not only

variational inverse parameter estimation in a cohesive sediment transport model an adjoint approach 1 parameter estimation in the sediment deposition and resuspension process is an important issue in numerical modeling of suspended sediment transport the sediment settling velocity and resuspension rate are two critical parameters controlling the sediment exchange process between the water column and sediment bed in this paper a variational inverse data assimilation scheme for estimation of the sediment settling velocity and resuspension rate is developed and tested with a three dimensional cohesive sediment transport model the sediment settling velocity and resuspension rate are treated as poorly known parameters in the model and are estimated by the variational inverse scheme using an adjoint approach a limited memory quasi newtonian conjugate gradient algorithm is used in the minimization process the variational inverse model is tested in the james river estuary in virginia by identical twin experiments numerical experimental results show that variational inverse data assimilation is a useful tool for retrieving poorly known parameters in a cohesive sediment transport model

the deglaciation of atlantic canada as reconstructed from the postglacial relative sea level record the post wisconsinan relative sea level record from atlantic canada is used to reconstruct the morphology of late wisconsinan age ice cover during its retreat from the atlantic region the proposed reconstruction has little or no grounded ice in the southern gulf of st lawrence an ice dome over the north shore of the st lawrence and thin ice often less than 1 km thick over much of the rest of the area a sensitivity analysis shows that the proposed reconstruction is not unique in its ability to account for the relative sea level record but that the thickness of ice in any individual area of the reconstruction is unlikely to be in error by more than a factor of two the exact position of the ice margin in some areas is not well constrained by the model an example is in southeastern newfoundland the numerical model used to relate ice morphology to postglacial relative sea level assumes that the ice sheets are isostatically equilibrated at the glacial maximum and therefore that load changes associat

zircon u pb geochronology and hf isotopes of major lithologies from the jiaodong terrane implications for the crustal evolution of the eastern block of the north china craton abstract as an important component of the western shandong complex in the eastern block of the north china craton the yishui terrane consists of neoarchean high grade supracrustal rocks granitoid gneisses and charnockites la icp ms zircon u pb dating and hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the neoarchean crustal evolution of the eastern block of the north china craton new zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2 54 2 53 ga and 2 57 2 55 ga respectively and the charnockites were emplaced contemporaneously around 2 56 2 53 ga single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at 2 50 ga suggesting that the yishui terrane experienced a high grade metamorphic event at the end of the neoarchean detrital zircons from pelitic gneisses yield 207pb 206pb ages of 2 89 2 65 ga with a major age peak at 2 53 ga and a subordinate age peak at 2 70 ga implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from 2 53 ga rocks in the yishui terrane and 2 70 ga rocks in the adjacent luxi granite greenstone terrane hf isotopic compositions show that the neoarchean magmatic zircons have positive ehf t values ranging from 1 4 to 7 8 and depleted mantle model ages of 2 92 2 60 ga with a prominent peak at 2 8 2 7 ga suggesting that the neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the yishui area combined with previous data from the adjacent luxi granite greenstone terrane and other complexes in the eastern block it is concluded that the neoarchean was an important period of crust accretion in the eastern block of the north china craton

three dimensional numerical model for flow through natural rivers this paper presents a three dimensional numerical model for simulating flow through natural river reaches the model solves the reynolds averaged navier stokes rans equations closed with the standard ik e turbulence model large scale roughness and multiple islands are directly resolved by employing boundary fitted curvilinear coordinates in conjunction with a multiblock approach small scale bed roughness is accounted for using a two point wall functions approach calculations are carried out for flow through a 4 km stretch of the columbia river downstream of the wanapum dam for which detailed field and laboratory measurements were collected for a range of power plant operating conditions measurements at one operating discharge are employed to calibrate the small scale roughness distribution in the numerical model subsequently the calibrated model is validated by comparing the computed results with laboratory and field measurements for other discharge combinations these comparisons demonstrate the ability of the model to capture most experimental trends with remarkable accuracy

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

evidence for wisconsinan glaciations in the verrill canyon area scotian slope a composite thickness of about 25 m of sediment has been cored from the verrill canyon on the scotian slope it is interpreted that the majority of this sequence was deposited in a glaciomarine environment during oxygen isotopic stage 2 and the top of stage 3 these sediments as seen in high resolution seismic reflection profiles are well stratified become thicker upslope are laterally variable in thickness and pass upslope into possible outer shelf tills three wedge shaped units of incoherent reflections interfinger with the parallel reflections and terminate in water depths greater than 700 m these wedge shaped units are interpreted as slumped diamict and outwash deposits the age of the uppermost wedge shaped unit is 26 000 21 000 yr based on extrapolation of radiocarbon dates this unit documents a late wisconsinan glacier readvance on the outer scotian shelf the underlying wedge shaped unit estimated to be 70 000 yrs old extends further west along the continental slope and may represent a more extensive early wisconsinan ice advance a third wedge shaped unit inferred to have formed during isotopic stage 6 is possibly a remnant of the first glaciation in the study area

foraminiferal distribution on the continental margin off nova scotia abstract q mode factor analysis of total foraminiferal abundance data living plus dead from 250 grab samples taken from the continental margin off nova scotia allows the determination of eight factor assemblages on the northeastern shelf an exclusively agglutinated assemblage dominated byadercotryma glomerata occupies both banks and basins central shelf basins contain a predominantly calcareous assemblage dominated byglobobulimina auriculata andnonionellina labradorica transitional between these two factor assemblages is an agglutinated assemblage dominated bysaccammina atlantica consistently present along the shelf edge is atrifarina angulosa assemblage in northeastern bays and a few samples near sable island an agglutinatedeggerella advena assemblage is found a relict and transport affected assemblage dominated byelphidium excavatum occurs in the southwestern approaches to emerald basin sandy gravelly areas of the inner shelf and outer bank regions are characterized by acibicides lobatulus assemblage the statistical relationships of these defined assemblages to various aspects of the marine environment depth temperature salinity percent gravel sand and mud were investigated through multiple regression techniques results indicate that the present foraminiferal distribution patterns off nova scotia are influenced by the prevailing watermass characters and substrate theadercotryma glomerata assemblage is influenced by the presence of cold 0 4 c low salinity waters 32 33 of arctic labrador current origin the central basin assemblage g auriculata is related to warmer 8 12 c more saline waters 35 of slope origin the transition between these two bottom waters is marked by the opportunisticsaccammina atlantica assemblage preferred substrate character possibly determines the occurrence of thecibicides lobatulus islandiella islandica andeggerella advena assemblages trifarina angulosa shows a significant relationship to salinity and depth although the surficial sediments on the nova scotian shelf are largely the product of reworking of glacial deposits during late glacial and early holocene times all but thee excavatum factor assemblage appear to be in equilibrium with the modern oceanographic regime

submarine moraines in newfoundland coastal waters implications for the deglaciation of newfoundland and adjacent areas abstract submarine moraines near modern coasts of newfoundland show regional variations in architecture off the west coast arcuate moraines in shallow water were altered to varying degrees by postglacial sea level lowering off the southwest coast arcuate moraines on deep fiord mouth sills interdigitate with glaciomarine deposits whose maximum age is 14 17 ka bp off the northeast coast submarine moraines are absent at the mouths of the deep fiords morainal ridges outside the fiords are coast parallel it is proposed that the moraines formed during deglaciation of the region the late wisconsinan ice sheet extended to the edge of the continental shelf and was divided into catchments with fast flow concentrated in deep channels ice calved back along channels isolating the newfoundland ice cap and ice caps on the offshore banks ca 14 ka drawdown from the interior was localized in fiords and ice margins stabilized just offshore forming the submarine moraines described here off the southwest coasts the ice margin was maintained for up to 1 5 ka at fiord mouths off the northeast coast many of the deep wide fiords are without sills and retreat was nearly continual until the margin stabilized at fiord heads just before 13 ka

till tongue stratigraphy seismostratigraphic studies on the scotian and mid norwegian continental shelves have led to the development of a conceptual model for glacial deposition and erosion by ice sheets in a marine environment till tongue model some of the terminal deposits of the marine ending glaciers are composed in part of acoustically incoherent wedge shaped bodies the wedges are thought to be part of a grounding line system associated with a floating terminus according to the model the sedimentary wedges are formed through the accumulation of glacial debris by subglacial processes proximal to the grounding line together with a penecontemporaneous proglacial contribution from sediment gravity flows sedimentation rates appear to be highest near and proximal to the grounding line that is within a grounding line zone ice proximal stratified glaciomarine sediment accumulates from plumes within the water column and rain out of ice rafted debris the glaciomarine sediment interfingers with the till tongues resulting from oscillations of the grounding line till tongues represent discrete stratigraphic units formed in both vertical and lateral successions marginal to massive till deposits major till tongues can be correlated regionally for hundreds of kilometers and thus can provide a basis for stratigraphic correlation on the mid norwegian shelf lateral and vertical successions of till tongues delineate several large scale till units which represent major advances and retreats of the ice that probably were of both stadial and glacial rank we refer to this type of succession as a till tongue stratigraphy it appears that till tongue formation is limited to intermediate water depths which range from 75 to 1 000 m present water depth but those formed in water depths 600 700 in may not be strictly subglacial in origin we speculate that processes leading to the development of till tongues possibly give rise to the formation of subglacial undermelt till together with thin flow tills near the grounding line it is also possible that another type of subglacial till embracing concepts inherent in the expression deforming basal tills as described by alley and others 1989 may contribute to the formation of till tongues boundaries between the major till units of the mid norwegian shelf are mainly depositional in the zone of maximum glaciation near the shelf edge and upper slope proximal to this zone of major deposition intraformational erosional processes dominate and give rise to smooth widespread time transgressive erosional surfaces erosion progressively increases in a proximal direction beveling the earlier glacial deposits and ultimately reaches and modifies the bedrock surface these regional surfaces are analogous to the ross sea unconformity which was also formed by glacial erosion morainic deposits within a till tongue stratigraphy are termed grounding zone moraines and are subdivided into advance and retreat varieties with linear tabular and hummocky attributes fields of small morainic ridges lift off moraines form in basinal areas during recession

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

hydrodynamic simulations of charlotte harbor and its major tributaries in florida using a dynamically coupled 3d 2dv model abstract hydrodynamics salinity transport processes and thermodynamics in charlotte harbor and its major tributaries in florida usa were simulated using a coupled three dimensional 3d and laterally averaged two dimensional 2dv hydrodynamic model named unless in support of the re evaluation of minimum freshwater inflows for two water bodies in the system namely the lower peace river lpr and lower shell creek lsc the unless model involves a dynamic and two way coupling of the 3d unstructured cartesian grid model unless3d chen 2011 with the laterally averaged model lamfe chen 2003a using a semi implicit scheme named the free surface correction fsc method chen 2003b the simulation period was january 2013 through august 2014 during which best real time data of water elevation current salinity and temperature at five measurement stations within the simulation domain were available comparisons of model results with these real time field data show that unless is capable of simulating hydrodynamics salinity transport processes and thermodynamics well in the charlotte harbor estuary good matches of model results with measured data suggest that both estimated ungauged flows and downstream boundary conditions were reasonable major physical characteristics in the charlotte harbor system are correctly simulated by the unless model it is demonstrated that the dynamically coupled 3d 2dv model unless is robust and very efficient for complex systems such as charlotte harbor which has a large waterbody with islands and several narrow and meandering tributaries

structurally controlled hydrocarbon seeps on a glaciated continental margin hecate strait offshore british columbia abstract this paper describes structurally controlled gas seeps and their influence on seafloor morphology at sites within queen charlotte basin qcb a sedimentary basin situated offshore british columbia western canada numerous surficial and shallow sediment features related to both ice cover and shallow gas have been identified in high resolution multichannel seismic huntec deep towed seismic multibeam bathymetry and backscatter and piston core data that were acquired over a 10 km by 10 km grid in an eastern part of qcb pockmarks seafloor mounds and iceberg ploughmarks are visible in the multibeam data small scale folds vertical blank zones and acoustic turbidity are observed in the huntec data the multichannel seismic data show a faulted anticline phase reversed bright spots zones of low instantaneous frequency and velocity pull ups one seafloor mound lies directly above a shallow fault that offsets an asymmetric fold the remaining mounds lie either above or in close proximity to asymmetric folds that are faulted bright spots are observed along some fault traces and within a cut and fill sequence at two way travel times between 0 42 and 1 0 s velocity pull ups are observed in the multichannel data below gas expansion voids present in piston cores backscatter data show that seafloor mounds are highly reflective compared to the surrounding seafloor these observations indicate that gas has been migrating upward through the rock column and expelled into the shallow sediments the shallow gas has been biologically mediated to form carbonate sediments within and around seafloor mounds shallow gas has also deformed surficial sediments into positive relief mounds and negative relief pockmarks

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

rainstorm induced event sedimentation at the tidewater front of a temperate glacier runoff from a late summer storm drained rapidly to a subglacial stream of mcbride glacier southeast alaska the stream transported large volumes of sediment that discharged from the glacier s tidewater front directly into fjord water at 40 m depth measurements of salinity temperature and suspended sediment concentration indicate that the sediment was transported by interflows within brackish fjord water sedimentation rates were exceptionally high up to five times normal laminated fine sand accumulated in proximal sediment traps and silty mud accumulated in distal traps the low frequency of large storms late in the melt season indicates that the sedimentary products would be rare but significant marker horizons in temperate glaciomarine sequences

relationship among submarine instabilities sea level variations and the presence of an ice sheet on the continental shelf an example from the verrill canyon area scotian shelf sea level low stands are associated with the growth of glacial ice sheets during maximum stages of glaciation ice sheets may extend to the outer continental shelf these glacial maxima normally correspond to sea level low stands consequently ice sheets may be supported directly by the seafloor inducing increased pore pressures in the sediment and increasing the overall bearing stresses on the margin these stresses trigger undrained failure in upper slope sediment and bearing capacity failure in the middle to lower slope after complete ice melting and dissipation of excess pore pressure the inter glacial sediments become overconsolidated shear resistance is higher and no gravity or earthquake induced failure occurs the gravity events can only occur in underconsolidated and normally consolidated glacial and postglacial deposits this explains the variation in volume and activity of mass wasting events during low and high sea level periods in areas where a glacial ice exists analyses of maximum ice loading events are presented for the scotian slope in contrast with traditional thought these analyses suggest that ice loading rather than high sedimentation rates is the dominant mechanism for slope instability on glaciated margins

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

late quaternary sedimentation on the southwestern scotian slope eastern canada relationship to glaciationgeological survey of canada contribution 20070176 the sedimentary record on continental slopes has the potential to preserve a record of glacial retreat on the adjacent continental shelf the glacial history of the southwestern part of the scotian shelf is poorly known air gun and high resolution sparker profiles and numerous sediment cores up to 10 m long have been used to determine the character of sedimentation on the southwestern scotian slope since the last glacial maximum lgm seismic reflection profiles show that glacial till was deposited at shallow depths on the upper continental slope and correlation to dated piston cores farther downslope show that this till dates from the lgm slope sedimentation at this time was dominated by local ice and deposited as plume fallout and turbidites progressively increasing importance of red brown sediment derived from glacial supply to laurentian channel indicates retreat of ice from the shelf edge and diminishing supply of proglacial sediment from the calving embayment in the mid scotian shelf with the

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

glacial tunnel valleys and quaternary history of the outer scotian shelf high resolution seismic reflection data indicate the presence of huge sub surface channel networks on the outer nova scotian continental shelf channel axes extend to over 450 m below present sea level b s l channel walls average 2 3 km in width mechanisms capable of producing such channels include fluvial submarine canyon or glacial erosion considerable debate has focused on the positions of the tertiary quaternary t q and pleistocene holocene p h boundaries1 4 in this region and the relationship of the t q boundary to the unconformity at the base of the channel networks uncertainty also surrounds the extent of pleistocene ice sheets on the south east canadian continental margin4 7 here we present a new stratigraphy for the sable island region fig 1 based on seismic profiles with lithologic and biostratigraphic control provided by two strategically placed boreholes this strati graphic analysis establishes the positions of the t q and p h boundaries outside the valleys at 51 m and 220 m b s l respectively our analysis also implies that the large channel systems are tunnel valleys cut by a sub ice meltwater process under a pre to early wisconsinan ice sheet which extended close to the shelf edge

a synthesis of geochemistry and sm nd isotopes of archean granitoid gneisses in the jiaodong terrane constraints on petrogenesis and tectonic evolution of the eastern block north china craton abstract the jiaodong terrane is situated in the northeastern part of the shandong province in eastern china experiencing multi stage magmatism during the mesoarchean 2 9 ga and neoarchean 2 7 ga and 2 5 ga it consists dominantly of granitoid gneisses with minor amphibolite enclaves or lenses this study presents new whole rock geochemical and sm nd isotopic data for the archean granitoid gneisses from the jiaodong terrane in order to constrain their petrogenesis and tectonic setting geochemical studies and petrological features suggest that the protoliths of most granitoid gneisses are typical archean tonalitic trondhjemitic granodioritic ttg suites which are high in sio2 60 9 76 8 wt al2o3 12 6 16 8 wt na2o 3 3 5 3 wt sr 135 604 ppm and sr y ratios 13 9 84 but low in mgo 0 4 4 5 wt k2o 0 7 3 4 wt tio2 0 2 0 9 wt cr

late wisconsinan deglaciation of emerald basin scotian shelf emerald basin is a glacially overdeepened basin located on the central scotian shelf where quaternary sediments form a thin veneer over tertiary bedrock nineteen piston cores and 1200 line km of seismic data were obtained from the basin from which seven acoustic facies were recognized i cambro ordivician metasediments ii tertiary sediments iii scotian shelf drift till iv acoustically stratified emerald silt v acoustically unstratified emerald silt vi lahave clay vii diffused gas six depositional sequences were recognized in the emerald silt and lahave clay facics and were correlated with sediments recovered in piston cores depositional sequence 0 is a strongly stratified package which drapes over the scotian shelf drift it is correlated with stiff unbioturbated slightly gravelly sandy muds with no in situ molluscs and is characterised by an ice marginal foraminiferal assemblage depositional sequence 1 drapes over depositional sequence 0 and shows onlap on topographical highs it is correlated with slightly gravelly muds which are characterised by alternating bioturbated and unbioturbated hands and monospecific molluscan and foraminiferal assemblages depositional sequence 2 asymmetrically drapes over and onlaps onto depositional sequence 1 and is correlated with bioturbated slighty gravelly muds with monospecific molluscan and foraminiferal assemblages depositional sequence 3 infills the basin deeps erosionally truncating reflections of depositional sequence 2 it is correlated with bioturbated muds which are characterised by a monospecific foraminiferal assemblage and a slightly diverse molluscan assemblage depositional sequence 4 drapes over the erosional upper surface of depositional sequence 3 and is correlated with bioturbated muds which are characterised by a diverse molluscan assemblage depositional sequence 5 onlaps onto depositional sequence 4 infilling the basin deeps and is correlated with soft bioturbated sandy silts which are characterised by diverse molluscan and foraminiferal assemblages the present day seafloor in emerald basin is marked by small gas escape structures intepreted as pockmarks buried pockmards are observed in depositional sequences 4 and 5 buried iceberg scours are observed on the upper surface of depositional sequence 0 and within depositional sequences 1 and 2 small ridges interpreted as lift off moraines are formed on the upper surface of the scotian shelf drift and are oriented transverse to ice flow directions during glacial retreat wedge shaped acoustically unstratified features interpeted as till tongues are rooted in moraines on the basin flanks and were deposited simultaneously with depositional sequences 0 1 and 2 detailed interpretation of the seismic and core data suggest that glacial ice advanced across emerald basin reaching the shelf edge at approximately 24 ka melting of the ice sheet resulted in deglaciation of emerald basin at 17 5 ka by lift off of the grounded ice sheet to form a floating ice shelf of 10 100 yr duration sediment beneath the ice shelf was primarily supplied by subglacial meltwater streams prior to lift off till was squeezed into basal crevasses in the ice to form features known as lift off moraines the ice shelf calved at approximately 17 4 ka causing intense scouring of the seafloor by in situ icebergs and isolating ice rises on the banks surrounding emerald basin these ice rises disintegrated rapidly except for the rise on emerald bank which contributed sediment to emerald basin until 12 ka sediment was primarily supplied by subglacial meltwater and deposited out of suspension except near the ice margin where ice rafting and gravity flow mechanisms were important iceberg scouring occurred until about 14 ka the main ice margin remained on the northern flank of the basin until 15 4 ka at which time it retreated landward a widespread erosive event caused by storm driven currents occurred at 13 ka rain out from suspended sediment plumes was the dominant depositional mechanism until 12 ka lahave clay is primarily derived from inner shelf sediments reworked by rising sea level storm generated currents have created an erosional surface at the present day seafloor around the flanks of the basin

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

zircon u pb hf isotopes and geochemistry of two contrasting neoarchean charnockitic rock series in eastern hebei north china craton implications for petrogenesis and tectonic setting abstract the zunhua qinglong microblock in eastern hebei north china craton exposes two contrasting charnockitic rock series in the yuhuzhai taipingzhai and cuizhangzi areas charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks and charnockite series showing tonalitic and granodioritic components petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks medium fine grained anhedral orthopyroxenes are preserved in both series and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series in situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene whereas the medium fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements ree and no obvious eu anomalies resulting in left inclined ree patterns combined with petrographic features these new data indicate that the medium fine grained anhedral orthopyroxenes were formed by granulite facies metamorphism caused by dehydration reactions whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 17 and 2523 12 ma and those of the charnockite series were emplaced between 2527 28 and 2515 22 ma we conclude that these magmatic precursors are contemporaneous emplacements of two neoarchean granitoid magmatic events which were followed by regional granulite facies metamorphism at 2 45 ga the charnockitic plagioclase gneiss series and charnockite series exhibit similar hf t values of 2 4 to 5 3 and 3 2 to 5 5 indicating either depleted mantle or juvenile crust affinity and involvement of ancient crustal components the charnockitic plagioclase gneiss series exhibit relatively low sio2 contents 54 1 65 2 with various mgo contents 1 93 5 36 mg 0 36 0 55 and la yb n values 3 49 56 00 the charnockite series exhibit higher sio2 contents 60 1 69 2 with similar la yb n values 14 61 42 69 mgo contents 2 10 3 99 and mg 0 29 0 52 these geochemical and zircon lu hf isotopic features together with chemical modeling suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab derived melts fluids and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases the aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab and it was strongly contaminated by mantle peridotites these new data combined with the previous studies of neoarchean basement rocks in the zunhua qinglong microblock in eastern hebei suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back arc related tectonic environment at a convergent plate margin

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

timing of metamorphism in the paleoproterozoic jiao liao ji belt new shrimp u pb zircon dating of granulites gneisses and marbles of the jiaobei massif in the north china craton abstract the paleoproterozoic jiao liao ji belt lies in the eastern block of the north china craton with its southern segment extending across the bohai sea into the jiaobei massif high pressure pelitic and mafic granulites have been recently recognized in the paleoproterozoic jingshan group jiaobei massif new shrimp u th pb geochronology combined with cathodoluminescence cl imaging of zircon has been applied to the determination of the timing of the metamorphism of the high temperature and high pressure granulites and associated gneisses and marbles metamorphic zircons in these high pressure granulites gneisses and marbles occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores metamorphic zircons are all characterized by nebulous zoning or being structureless with high luminescence and relatively low th u values metamorphic zircons from two high pressure mafic granulites yielded 207pb 206pb ages of 1956 41 ma and 1884 24 ma one metamorphic zircon from a garnet sillimanite gneiss also gave an apparent 207pb 206pb age of 1939 15 ma these results are consistent with interval of ages of c 1 93 1 90 ga already obtained by previous studies for the north and south liaohe groups and the laoling group in the northern segment of the jiao liao ji belt metamorphic zircons from a high pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207pb 206pb ages of 1837 8 ma 1821 8 ma and 1836 8 ma respectively two diopside olivine phlogopite marbles yielded weighted mean 207pb 206pb ages of 1817 9 ma and 1790 6 ma these paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c 1 85 ga produced from the ji an group in the northern segment of the jiao liao ji belt and c 1 86 1 80 ga obtained for the high pressure pelitic granulites from the jingshan group in the southern segment as this metamorphic event was coeval with the emplacement of a type granites in the jiao liao ji belt and its adjacent areas it is interpreted as having resulted from a post orogenic or anorogenic extensional event

qingyuan high grade granite greenstone terrain in the eastern north china craton root of a neoarchaean arc abstract the qingyuan high grade granite greenstone terrain is one of the first recognized granite greenstone terrains in the eastern north china craton similar to other archaean terrains its tectonic environment is debated was it an intra continental rift or a modern analogy of arc occurrence and petrogenesis of major igneous series the ultramafic mafic and felsic volcanic rocks 2510 ma and the plutons of the quartz diorite 2570 2510 ma ttg 2570 2510 ma and quartz monzodiorite 2510 2490 ma series in xinbin area are investigated the mafic intrusives and volcanic rocks have high mgo content 5 4 7 5 wt and mg number 48 61 they show slightly depleted to flat trace element patterns the ultramafic rocks serpentinite could be genetically related to the mafic rocks the meta dacite rhyolite is adakitic with enriched light and middle rees and liles but significantly depleted hfses the quartz diorite has high mg number 60 64 moderately enriched light and middle rees and liles the ttg shows distinct light but moderate middle ree enrichment prominent nb ta depletion but zr hf enrichment zr sm 100 the quartz monzodiorite has moderate light and middle ree enrichment significant nb ta depletion and negative eu sr anomalies the ttg has more depleted sr nd isotopes endt 2 6 87sr 86srt 0 700 than all the others endt 0 2 87sr 86srt 0 701 703 their petrogenesis can be explained by an archaean style subduction defined as a mantle wedge absent flat hot subduction with significant vertical tectonism in the overriding slab the ultramafic mafic rocks were originated from primitive mantle the meta dacite rhyolite was originated from the eclogite facies overriding crust the quartz diorite was a mixture of melts from mantle and the overriding crust the ttg was from the subducting slab under amphibolite to amphibole bearing eclogite facies and the quartz monzodiorite was from the subducting slab after the derivation of the ttg the qingyuan terrain represents remnants of the root of a neoarchaean arc its anticlockwise p t path recorded the initiation of the arc to its cession and exhumation cratonization at 2480 ma

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

latest pleistocene holocene paleoceanographic trends on the continental margin of eastern canada foraminiferal dinoflagellate and pollen evidence micropaleontological studies were made of cores from four shelf basins on the eastern canadian margin emerald and canso basins on the scotian shelf 44 46 n notre dame channel newfoundland shelf 50 n and cartwright saddle labrador shelf 55 events were correlated using a combination of14c dates and pollen stratigraphies surface and bottom water changes were compared on the basis of dinoflagellates and benthic foraminifera respectively the results indicate significant paleoceanographic shifts along a north south gradient both prior to and during the holocene distinct late pleistocene holocene paleoceanographic events were distinguished in the emerald canso and notre dame basins these events are less obvious in cartwright saddle which is in deeper water and further off shore pleistocene glaciomarine sediments in all basins contain a fauna dominated byelphidium excavatum f clavata dinoflagellates and pollen are rare or absent the widespreadelphidium fauna probably reflects turbid glacial meltwater and or a permanent ice shelf cover from 20 000 10 000 yrs bp the notre dame core also penetrates older sediment with an outer labrador current fauna which may represent a late wisconsinian interstade at about 23 000 yrs bp from 7 000 10 000 yrs bp a cold water fauna occurred which is similar to modern outer labrador current faunas from about 5000 7000 yrs bp a warm interval is indicated by a relatively warm water calcareous benthonic foraminiferal fauna and increased representation of typical gulf stream dinoflagellates the most recent change occurred in the last 2000 years with an abrupt cooling associated with stronger flow of the arctic inner labrador current this cooling event is marked by an increase in arctic dinoflagellates and by an exclusively agglutinated benthonic foraminiferal fauna at two sites canso and notre dame these holocene paleoceanographic changes are not clearly seen in the benthic fauna of the deep northern basin cartwright saddle although dinoflagellate data at this site indicate that surface water changes have occurred that are similar to those found in shallower basins shifts in the zonal position of the gulf stream and changes in the relative mass transports of the west greenland and labrador currents are mechanisms which may account for the paleoceanographic events the glacial interstadial glacial sequence recorded in the notre dame channel in conjunction with other theories on glacial triggering mechanisms provides biostratigraphic evidence which suggests the onset of a glacial stage in the near future

the near surface geology of st georges bay nova scotia implications for the hollow fault a formline contour map which depicts the near surface structural configuation of the strata underlying st georges bay northeastern nova scotia was made from bedding attitude data compiled in the coastal areas apparent dips measured from single channel seismic reflection data and true strikes and dips calculated at survey track intersections the geology interpreted from the formline map is characterized by northeast striking faults and fold axes the folds in the bay comprise broad open synclines and narrow tightly folded or faulted anticlines gravity and deep seismic reflection data suggest that the faulted anticlines are intruded by salt correlations from offshore to onshore suggest that the structures mapped offshore in the bay extend onshore the onshore extensions of the faulted anticlines are mapped as faults and their antiformal nature is subdued they are locally associated onshore with carboniferous windsor group outcrop the offshore extension of the hollow fault which is interprete

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

chronology of late wisconsinan glaciation emerald basin scotian shelf eight new accelerator mass spectrometry ams radiocarbon dates from proglacial sediments in emerald basin indicate that the previously proposed chronology based on dates of total organic matter is much too old the new chronology indicates that grounded ice occupied emerald basin in the late wisconsinan

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

simulating hydrodynamics in a spring fed estuary using a three dimensional unstructured cartesian grid model abstract this paper presents an application of a three dimensional unstructured cartesian grid model chen 2011 to a real world case namely the crystal river kings bay system located on the gulf coast of the florida peninsula of the united states crystal river kings bay is a spring fed estuarine system which is believed to be the largest natural refuge in the united states for manatees during the coldest days in winter because of the existence of a large amount of discharge out of numerous spring vents at the bottom of kings bay the unstructured cartesian grid model was used to simulate hydrodynamics including salinity transport processes and thermodynamics in the estuary during a 34 month period from april 2007 to february 2010 although there are some unidentified uncertainties in quantifying flow rates from the spring vents and salinity variations in spring flows simulated water elevations salinities temperatures and cross sectional flux all match well or very well with measured real time field data this suggests that the unstructured cartesian grid model can adequately simulate hydrodynamics in a complex shallow water system such as crystal river kings bay and the numerical theory for the unstructured cartesian grid model works properly the successful simulation of hydrodynamics in the estuarine system also suggests that an empirical formula that relates the spring discharge with the water level in kings bay and the groundwater level measured in a nearby well is reasonable

the quaternary sediments of banquereau scotian shelf the acoustic stratigraphy of quaternary sediments on banquereau scotian shelf interpreted from regional and site survey seismic lines is correlated with sedimentary units determined from undisturbed borehole samples the informal litho stratigraphic formations of the quaternary sequence on the scotian shelf are subdivided into members and litho chronozones diagnostic of depositional environments a late wisconsinan glacial advance dated by c 14 analysis of shell material took place ca 18 000 to 26 000 yr b p it formed a widespread gravel rich unit barren of fauna and capped by an erosional surface the unit occurs within stratified pro glacial marine sediments known locally as the emerald silt formation the minimum stand of sea level 110 m below present sea level occurred ca 14 000 yr b p and is confirmed by the presence of channel deposits at the top of the pleistocene section a marine transgression across banquereau took place ca 8 000 yr b p and is defined by a widespread erosion surface at the base of the holocene section this section known locally as the sable island sand and gravel formation comprises as much as 40 m of reworked sand which is divided into 3 members diagnostic of sand ridge generation eastern shoal an offshore storm ridge situated on banquereau was formed during holocene time created by modern day storm driven and tidal currents the intensity of these currents is the result of water mass acceleration over the shallow banks of the scotian shelf eastern shoal appears to have moved 30 km across the continental shelf at a rate of 3 5 m yr the regional setting of this sand body is similar to that postulated for the shannon and tocito sandstones of the western interior of the united states and shows that sand transport and large scale ridge development can take place in a shelf edge setting on present day storm dominated continental shelves

crustal evolution of the eastern block in the north china craton constraints from zircon u pb geochronology and lu hf isotopes of the northern liaoning complex abstract the northern liaoning complex in northeastern china constitutes an important component of the eastern block in the north china craton the major lithologies consist of archean granitoid gneisses with minor supracrustal rocks occurring as tectonic lenses this study presents zircon lu hf isotopic data for the first time as well as new shrimp zircon u pb data of the major lithologies from the northern liaoning complex in order to elucidate the crustal evolution of the complex and provide new constraints on the neoarchean crustal evolution of the eastern block in the north china craton magmatic zircon u pb data from this study show that the protolith magmas of the supracrustal rocks and granitoid gneisses were generated during 2 55 2 50 ga metamorphic zircons document consistent metamorphic ages at 2 49 2 48 ga suggesting a regional metamorphic event immediately after the magmatism at the end of the neoarchean in the northern liaoning complex inherited detrital zircons of 2 79 2 60 ga suggest possible existence of ancient crust in this region zircon lu hf isotopic compositions show that the magmatic zircons have variable hf t values from 4 0 to 9 0 with depleted mantle model ages of 3 6 2 5 ga of which most hf t values are positive with a model age peak at 2 9 2 7 ga these zircon hf signatures reveal major juvenile crustal growth with additions of older crustal materials during 2 9 2 7 ga and a crustal reworking event with minor juvenile additions at 2 6 2 5 ga in the studied area integrated with previous data from other neoarchean complexes in the eastern block both the major juvenile crustal growth during 2 9 2 7 ga and the strong crustal reworking at 2 6 2 5 ga contribute to the extensive neoarchean crust formation of the eastern block in the north china craton the north china craton share similar neoarchean continental crustal evolution to other cratons in the world though it is distinctively featured by intensive tectonothermal overprinting at the end of the neoarchean

redefinition of depositional ages of neoarchean supracrustal rocks in western shandong province china shrimp u pb zircon dating abstract western shandong province is an important terrane in the eastern part of the north china craton where 2 75 2 5 ga tectono thermal events are well recognized supracrustal rocks named the taishan group experienced greenschist to amphibolite facies metamorphism and occur as lenses and strips in granitoids and are subdivided into the yanlingguan shancaoyu and liuhang formations the yanlingguan formation is mainly composed of amphibolite but also contains fine grained biotite hornblende gneiss komatiitic rocks and schist the liuhang formation is similar in rock association to the yanlingguan formation but contains more metasedimentary rocks the shancaoyu formation is mainly composed of fine grained biotite gneiss leptynite preserving bedding and is interlayered with schist its protoliths are mainly sedimentary rocks with some mafic to felsic volcano sedimentary varieties we undertook shrimp zircon dating on eighteen metasedimentary and volcanic samples mainly from the shancaoyu formation and the upper part of the liuhang formation ages of detrital and volcanic zircons vary from 2 75 to 2 54 ga and 2 55 to 2 525 ga respectively and the age distribution patterns vary in different samples we conclude that the shancaoyu formation and the upper part of the liuhang formation were deposited in the late neoarchean 2 55 2 525 ga and not in the early neoarchean 2 8 2 7 ga as thought before western shandong is the only terrane in the north china craton where both early and late neoarchean supracrustal rocks have been identified a magmatic arc model may be applied to explain the tectono thermal evolution at the end of the neoarchean

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

dynamic coupling of a three dimensional hydrodynamic model with a laterally averaged two dimensional hydrodynamic model 1 an efficient method to dynamically couple a three dimensional 3d hydrodynamic model with a laterally averaged two dimensional 2dv hydrodynamic model is described the method is a useful tool to incorporate narrow tributaries that have vertically 2d flow patterns into a three dimensional hydrodynamic simulation for a shallow and large water body that has a 3d flow pattern the coupling of the two models is facilitated with a free surface correction fsc method that is unconditionally stable with respect to gravity waves wind and bottom shear stresses and vertical eddy viscosity terms the coupled model solves laterally averaged 2dv reynolds averaged navier stokes rans equations for the narrow tributaries and three dimensional rans equations for the larger water body the coupled 3d 2dv model was used to simulate hydrodynamics and salinity transport processes in an idealized estuary with a large water body and two narrow tributaries in the 3d 2dv simulation the entire simulation domain was split into one 3d subdomain and two 2dv subdomains to illustrate the efficiency of the coupled 3d 2dv model a 3d simulation was also conducted for the entire simulation domain it was found that the coupled 3d 2dv model can generate the same model results as those by the 3d model but uses less than half of the computer time of the 3d simulation for this particular case an application of the coupled 3d 2dv model to the lower peace river upper charlotte harbor lpr uch system in southwest florida usa shows that the dynamically coupled model retains the efficiency of both the 3d model and the 2dv model

the orientation of buried iceberg scours and other linear phenomena abstract the average orientations of surficial iceberg scours can be obtained from sidescan sonograms using certain assumptions the average orientation of similar buried phenomena can be estimated from a pattern of intersecting seismic profiles two possible solutions are generated at each intersection of two seismic profiles each of which is oriented at an angle i from the profile which encounters the fewer scours where i tan 1 sin i r cos i and r is the ratio of the number of scours observed on the two seismic lines several pairs of solutions are required to determine which solution of each pair is significant iceberg scours observed on aerial photographs of the floor of glacial lake agassiz demonstrate that the estimates provided by this technique correspond with the average orientation of the population applying this method to buried iceberg scours in emerald basin scotian shelf gives orientations which are parallel to paleo bathymetric contours

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

probable late wisconsinan ice margin on the upper continental slope off st pierre bank eastern canada seismic reflection profiles from the continental slope off st pierre bank show a distinctive acoustic facies characterized by incoherent reflections unit e overlain by 5 20 m of stratified sediments units d a cores from unit e include poorly sorted silty diamict locally overconsolidated and including in places some foraminifera stratified sediments also occur in places in this facies the overlying sediments of units d a except for the topmost metre of unit a have a foraminiferal fauna dominated by elphidium excavatum forma clavata and cassidulina reniforme which has been interpreted elsewhere as indicating ice margin sedimentation the sediments contain turbidites and rare ice rafted detritus and are bioturbated accelerometer mass spectrometer radiocarbon dating of shells from the stratified sediments yielded dates between 3 3 and 11 8 ka facies e the top of which has an extrapolated age of 11 5 12 0 ka is interpreted as slumped morainal diamict and proglacial sediment resulting from a lat

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

petrographic evidence from the eastern canadian margin of shelf crossing glaciations abstract it is difficult to evaluate when and where glaciers extended across the outer continental shelf off eastern canada the outer shelf and upper slope are commonly hard bottomed or have a great thickness of post glacial sediment in either case making direct sampling and dating of ice margin sediment impossible the timing and extent of glacier ice is important for understanding continental slope sedimentation and ice sheet dynamics we have therefore determined the lithologic components of the 1 4 mm granule fraction of over 100 well dated samples from 40 cores from the more distal parts of the deep water margin off nova scotia and newfoundland this fraction is demonstrably ice rafted in these cores and geographic variation in component results from input of icebergs at different calving margins the samples show that heinrich layers have a distinctive granule fraction that shows some areal variation as a result of dilution by sources other than hudson strait heinrich event 2 ca 21 ka is widespread as far southwest as the west scotian slope but by the last glacial maximum lgm numerous discrete sources of ice rafted granules are recognised implying that calving ice margins were present in several places off newfoundland and nova scotia including off banquereau a distinctive purplish lgm gravelly mud is recognised on the eastern newfoundland margin derived from a calving margin on the ne newfoundland shelf at the time of heinrich event 1 ca 14 ka there was a calving ice outlet somewhere on the grand banks and another in halibut channel retreat of ice from the gulf of st lawrence can be tracked by changing granule petrology from the lgm to 13 ka these data suggest that final ice retreat from many parts of the outer shelf was later than is shown in many recent ice margin reconstructions