characterization of quartzites in the southern korean peninsula abstract in the southern korean peninsula twelve quartzite strata occur in the gyeonggi massif and okcheon belt their geologic ages range from precambrian to upper paleozoic all quartzites in the gyeonggi massif are of precambrian in age and are characterized by high grade metaquartzites they are seosan anyang yongmunsan and uiam quartzites from west to east quartzite types occurring in the okcheon belt are diverse from orthoquartzite to medium grade metaquartzite orthoquartzites are all paleozoic in age and are distributed mainly in the eastern okcheon belt taebaeksan basin jangsan dongjeom and jeongseon quartzites with one in the central okcheon belt mungyeong quartzite low grade metaquartzite is hwasan quartzite in the western part and medium grade metaquartzites are daehyangsan and geumsusan quatzites in the central part and yongamsan quartzite in the southwestern part of the okcheon belt distribution of quartzite types in the southern korean peninsula is not related to the geologic age of quartzites as a case study quartzite characteristics were applied to a provenance study of quartzite clasts in the northwestern part of the cretaceous gyeongsang basin quartzite clasts in the study area are interpreted to have been mostly derived from source quartzites in the okcheon belt which is consistent with the results of previous studies

a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

early middle paleozoic intraplate orogeny in the ogcheon belt south korea a new insight on the paleozoic buildup of east asia the polycyclic ogcheon belt of south korea represents the boundary between two precambrian blocks investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the asian continent we report here structural evidence showing that the earlier tectonic event dates back to early middle paleozoic the ogcheon tectonism or orogeny the ductile piling up of nappes of the ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area the nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement field evidence includes the presence of s1 and s1 2 foliations bearing an extensive stretching lineation perpendicular to the belt associated with a strongly noncoaxial deformation regime the f1 isoclinal folds are commonly a type and sheath folds whose axes are parallel to the stretching lineation whereas the southeast facing f2 recumbent folds have subhorizontal axes parallel to the belt all lines of evidence unconformity superposed deformations age of the metamorphism etc imply that the d1 2 ogcheon tectonism is pre middle carboniferous late silurian early devonian the subsequent indosinian middle triassic f3 upright synfolial folds that trend nne ssw and later structures clearly overprint the middle paleozoic ductile thrusts and isoclinal folds the early paleozoic ogcheon supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin on the basis of the geochemical characteristics of the metavolcanics it appears that oceanization did not occur and ogcheon rift aborted soon after it was formed we suggest that middle paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features during the ductile stacking episode the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event middle carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks a tentative correlation with early paleozoic imjingang qinling and cathaysian belts within the asian continent reveals that ogcheon rift was emplaced within the south china plate and that imjingang belt should represent the limit between north and south china blocks in the korean peninsula

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

geodynamics of the ogcheon belt south korea abstract the ogcheon belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early palaeozoic rift which was affected by polyphase tectonics its pre cretaceous structure results from three main orogenic events the ogcheon late silurian devonian indosinian late permian triassic and yenshanian middle jurassic early cretaceous tectonic phases the external autochthonous or para autochthonous units represent platform zones with shallow water deposits whereas rift facies characterize the internal domain the yeongweol sequence forming an outer shelf on the southeastern margin of the rift displays affinities with the jiangnan basin of south china it is in wrench fault contact with the easternmost duwibong sequence correlated with the north china north korea platform in the internal metamorphic units post cambrian early ordovician pelitic and turbiditic rift metasediments overlie a thin cambrian or cambrian ordovician early platform sequence two subdomains are distinguished the upper units chung ju and pibaryeong units representing the deep axial part of the basin contain distal flyshoids and large olistoliths derived from the northwestern narrow margin the lower units turung san poeun and iwharyeong are related to the wide margin located between the deep rift domain and the stable southeastern platform the latter transitional domain is characterized by thick submarine debris flows and volcanics of alkali or transitional tholeiitic affinity the volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the cambrian ordovician platform sequence in one single basin the post middle silurian pre mid carboniferous ogcheon ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area it gave rise to an intracontinental fold thrust belt without ophiolites this tectonic event may have been contemporaneous with early palaeozoic tectonism in the quinling belt in central china and in the southeast china fold belt the definitive constitution of the sino korean craton took place before the mid carboniferous and molasse sedimentation was initiated on a single platform located on the western edge of the yakuno oceanic basin of southwest japan in late permian and early triassic times the closure of the oceanic area and the subsequent westward collision of the honshu block are marked in korea by the intracratonic indosinian tectonism since late triassic times predominantly transcurrent movements have occurred along nne ssw faults associated with syntectonic intermontane troughs and important granitization

detailed geomorphology and neotectonics of the endeavour segment juan de fuca ridge new results from seabeam swath mapping a 36 km wide corridor of seabeam bathymetry was collected on the juan de fuca ridge north of 48 00 n in may 1983 these data merged with previous seabeam bathymetric compilations for the cobb offset as well as with sea marc i and sea marc ii side scan sonar seismic reflection magnetometer and deep tow photography data have been used to identify morphological variations along the spreading center of the endeavour segment of the juan de fuca ridge between the cobb offset and sovanco fracture zone the endeavour segment consists of two ridge sections separated by the 13 km wide endeavour offset which formed when spreading jumped from middle valley to west valley 200 000 yr ago the locus of spreading occurs along the entire endeavour segment as a narrow 1 2 km wide shallow 10 30 m deep inner rift which is superimposed upon larger structures that range from broad 5 10 km wide deep as much as 3 000 m median valleys to narrow 5 km wide shallow 2 100 m volcanic ridges an abrupt discontinuity in the tectonic fabric occurs at about 48 05 n south of this latitude spreading occurs within an axial high endeavour ridge which gradually deepens to the south south endeavour valley as it approaches the zone of overlap with the propagating rift at the cobb offset north of 48 05 n the axis is characterized by broad deep median valleys north endeavour valley and west valley these large scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism waning magma supply at the distal ends of ridge axes thermal contrasts across ridge offsets propagating rifts and an unstable triple junction the pronounced axial deep of west valley is interpreted as reflecting the youth of the structure 200 000 yr and may be primarily due to collapse with little extension and magmatism the rift axis jump to west valley fortuitously isolated endeavour seamount the youngest edifice in the heck seamount chain from the pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed endeavour offset development of a remarkably simple overlapping conjugate rift pair at endeavour offset has resulted in spite of the complex and highly variable crustal thickness magma supply and pre existing structural grain rifting in south west valley has caused pre existing topography generated at the endeavour ridge to be destroyed by subsidence and burial thereby creating the apparent discontinuity in morphology north of 48 05 n

upper mantle dynamics revealed by helium isotope variations along the southeast indian ridge helium isotope variations in igneous rocks are important for relating isotopic heterogeneity to convective mixing in the earth s mantle high 3he 4he ratios at many ocean islands along with lower and relatively uniform values in mid ocean ridge basalts morbs are thought to result from a well mixed upper mantle source for morb and a distinct deeper mantle source for ocean island basalts at finer scales 3he 4he variations along mid ocean ridges have been related to underlying mantle heterogeneity but relationships between the scales of geochemical segmentation and mantle convection remain enigmatic here we present helium isotope data for morb glasses recovered along approximately 5 800 km of the southeast indian ridge and develop an approach to quantitatively relate spatial variations in geochemical and geophysical parameters at the earth s surface a point to point correlation analysis reveals structure in the helium isotope data at length scales of approximately 150 and approximately 400 km that appears to be related to secondary convection in the underlying mantle

convective upwelling in the mantle beneath the gulf of california in the past six million years baja california has rifted obliquely apart from north america opening up the gulf of california between transform faults seafloor spreading and rifting is well established in several basins other than hotspot dominated iceland the gulf of california is the only part of the world s seafloor spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments such resolution is needed to address the long standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres here we use rayleigh wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle low shear velocities similar to those beneath the east pacific rise oceanic spreading centre underlie the entire length of the gulf but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart these anomalies are 40 to 90 kilometres beneath the surface at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin we interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle

relationship between axial morphology crustal thickness and mantle temperature along the juan de fuca and gorda ridges we analyze gravity data over the juan de fuca and northern gorda ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate 28 mm yr half rate the cleft segment at the south end of the juan de fuca ridge has an axial high morphology while the northern segment of the gorda ridge has a rift valley residual mantle bouguer anomalies rmba on the northern gorda ridge are elevated relative to the cleft segment by 10 20 mgal indicating thinner crust and or a colder mantle the minimum value 50 mgal of the rmba along the juan de fuca ridge is over axial seamount and gradually increases south toward the blanco transform the observed rmba are interpreted to result from along axis variations in crustal thickness and mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs we estimate that mantle temperatures are elevated by 30 40 c beneath axial seamount resulting in an excess crustal thickness of 1 7 km the cleft segment is associated with crust that is estimated to be only 300 700 m thicker and mantle temperatures are only 10 15 c higher than beneath the northern gorda ridge however even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology these results are consistent with the predicted sensitivity of recent thermo mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates we attribute the systematic differences in axial morphology crustal thickness mantle temperature and lithospheric strength along the juan de fuca gorda ridge system to the presence of the cobb thermal anomaly at axial seamount the hotter mantle beneath the juan de fuca ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner weaker lithosphere than along the gorda ridge

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

evidence for slab rollback in westernmost mediterranean from improved upper mantle imaging this work is funded by the picasso project nsf grant ear 0808939 the deployment and data processing for spanish stations was funded by consolider ingenio 2010 project topo iberia csd2006 00041 as well as alert es cgl2010 19803 c03 02

geomorphology and coastal erosion of a quartzite island hongdo in the yellow sea off the sw korean peninsula abstractas a dense metamorphic rock with silica grains that are strongly fused quartzite exposed on rocky shores is generally resistant to marine erosion the longevity of quartzite coasts compared to those formed by other rock types is underscored by the preservation of many former islands that retain notable topographic relief as inliers of archean and proterozoic quartzite surrounded by paleozoic marine strata in north america and western australia among the few places where modern rocky shores composed principally of quartzite may be studied with regard to marine erosion hongdo in the yellow sea off the southwestern korean peninsula is a natural laboratory where intersection of structural geology coastal geomorphology oceanography sedimentology and climate converge to demonstrate how a quartzite island evolves through time although small in size 6 87 km2 hongdo is open to the ocean on all sides the island s structure as an asymmetrical anticline permits a wide range of attack angles for se

ironstones and green marine clays in the dongjeom formation early ordovician of korea abstract in the sino korean north china block the occurrence of ironstone is reported for the first time from the lower ordovician dongjeom formation korea the dongjeom formation comprises much quartzose sediment deposited on the extensive carbonate platform and is composed of three coarsening upward sequences ironstone beds are distributed on the tops of the lower two sequences suggesting that they mark approximately maximum flooding events during the relative lowstand of sea level when the dongjeom formation was deposited extensive weathering in the source areas coupled with higher order sea level rises resulted in the formation of the dongjeom ironstones the dongjeom ironstones include much green marine clays ironstones of the lowest sequence are characterized by glaucony facies and those of the middle sequence by chamosite facies chemical compositions of dongjeom glauconies reveal that they were originally nascent to slightly evolved 3 to 5 k 2 o wt and that they were affected by illitization after burial their immature compositions suggest that glauconies were formed in about 10 3 to 10 4 years or less on the seafloor the substrates for glauconitization are interpreted as detrital mineral grains regressions after glauconitization introduced iron oxides to the green grains eventually forming the iron oxide rich band under the most oxidizing conditions the distribution of glauconies in limited horizons suggests that the dongjeom glauconies are autochthonous in origin chamosites have granular shapes and are mixed with quartzose sand grains suggestive of reworking during storms the dongjeom chamosites were transformed from the precursor berthierines the occurrence of glauconies in correlative horizons in korea and north china may indicate that glauconitization in the host sediments which were deposited in far northern gondwana occurred under the influence of a coeval transgressive event

provenance analysis of lower cretaceous sindong group sandstones in the gyeongsang basin korea using integrated petrography quartz sem cathodoluminescence and zircon zr hf analysis the sindong group aptian albian in southeastern korea is a 2 3 km thick fluvio lacustrine sedimentary package deposited in an elongated basin the nakdong trough which was formed by extension in an active continental margin setting the provenance of the sindong group was studied to understand spatial and temporal variation in composition in three different parts of the basin by using integrated data on petrography quartz sem cathodoluminescence cl analysis and zircon zr hf analysis although sindong group sandstones display wide variation in the composition of framework grains they generally have increasing amounts of feldspar and decreasing amounts of quartz up sequence significant amounts of volcanic rock fragments and volcanic quartz are observed in the late stage sediments metamorphic quartz is predominant in all sindong group sandstones indicative of the exposure of metamorphic rocks in the source terrane mostly from precambrian basement and triassic granites the occurrence of significant amounts of plutonic quartz from early stage sediments suggests that jurassic granites were widely exposed in the source terranes in addition the episodic increase in volume of plutonic quartz content in the sequence especially in the northern part of the basin suggests that episodic tectonic activity occurred in the catchment the increasing feldspar content up sequence could support the occurrence of tectonic activity in the catchments the zr hf analysis of detrital zircons revealed that the majority of zircons are of continental crust origin formed in orogenic settings but zircons in the lowest strata in the northern part of the basin were derived largely from anorogenic magmatic rocks of mantle origin our results demonstrate that source terranes for the sindong group have heterogeneous spatial and temporal distribution and were composed mainly of precambrian basement and triassic to jurassic granitic rocks with minor meta sedimentary rocks and syndepositional volcanic rocks the detritus derived from syndepositional volcanic rocks became significant in the late stage of basin filling representing the transition from extensional tectonism to continental arc magmatism the differences in characteristics of quartz sem cl and zircon zr hf ratios in different parts of the basin are best explained by deposition on different alluvial fans and river systems

upper mantle seismic wave attenuation effects of realistic partial melt distribution frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated we consider the contribution of the melt squirt mechanism through which pressure differences between disk shaped inclusions are equalized by melt passing through connecting tubes the pressure differences arise as a result of shear strain compressing disk shaped pores differently on the basis of disk orientation with respect to the applied shear the frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores the pressure equalization process is modeled by a system of first order linear differential equations whose eigenvalues are the characteristic frequencies for melt squirt relaxation it is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity as well as distribution of melt inside pores and conduits use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation some conceivable melt distributions however would result in detectable attenuation in the seismic band in all cases investigated attenuation increases with frequency indicating that melt squirt is not responsible for global upper mantle q observations

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

possible eastward extension of chinese collision belt in south korea the imjingang belt structural petrological and geochronological data from the middle korean peninsula indicate that the qinling dabie sulu collisional belt of east central china crosses the yellow sea and extends into the imjingang belt the yeoncheon complex first identified as the western imjingang belt comprises primarily north dipping metamorphic sequences 1 the northern jingok unit consisting of barrovian type metapelites and 2 the southern samgot unit consisting of calc silicate and amphibolitic rocks south vergent structures with reverse sense shearing are dominant in the jingok unit whereas late normal sense shearing is pervasive in the samgot unit and the deformed granitoid to the south these structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt pressure temperature p t estimates from the amphibolites suggest a high p amphibolite facies metamorphism 8 13 kbar and 630 790 c possibly evolving from eclogite facies conditions along a clockwise p t path sm nd and rb sr geochronological data suggest that the amphibolites emplaced in late proterozoic time were metamorphosed during permian triassic time

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

geochemistry of the boil mountain ophiolitic complex northwest maine and tectonic implications a coherent ophiolitic complex of pyroxenite serpentinite metagabbro mafic volcanics felsic volcanics and sediments crops out in nw maine adjacent to the chain lakes massif the complex here informally referred to as the boil mountain ophiolitic complex is about 500 m y old the volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics the mafic volcanics are divided into two geochemical groups a stratigraphically lower group is depleted in ti zr y cr and ree contents similar to basalts from supra subduction zone ophiolites an upper mafic group has trace element contents similar to normal mid ocean ridge basalts the felsic volcanics are mostly rhyolitic and similar to low k rhyolites found in the forearc of the marianas trench and in an island arc sequence in the klamath mountains california the flat ree patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the oman ophiolite the presence of thick sequences of felsic volcanics the abundance of pyroxenite the low ti zr and ree contents of some mafic rocks the flat ree patterns of the felsic volcanics and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone the complex may be correlated with ophiolitic fragments in the eastern part of the dunnage zone in newfoundland rather than the main ophiolite belt of the western appalachians

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

provenance of quartzite clasts in the eocene oligocene sespe formation paleogeographic implications for southern california and the ancestral colorado river quartzite clasts in fluvial sespe formation conglomerates of the santa monica mountains were compared petrographically with potential source rocks in the basin and range and transition zone provinces to determine their provenance modal analysis indicates that sespe orthoquartzite clasts are mainly quartzofeldspathic and derived from source rocks with craton interior provenance such clasts were probably derived from stirling quartzite and wood canyon formation sources in the northern mojave desert the mazatzal peak quartzite of central arizona is the probable source of quartzolithic orthoquartzite clasts containing abundant detrital jasper grains sespe metaquartzite clasts are mainly from sources in the southern mojave desert but some may be from the yavapai terrane of central arizona the dual provenance of orthoquartzite clasts suggests that the sespe paleoriver was a bifurcating system tapping both mojave desert and sonora desert sources after compensating for neogene displacement along the san andreas fault a paleogeographic reconstruction shows that there is a general spatial coincidence between sespe and other cenozoic paleodelta deposits around los angeles that contain exotic clasts and the inferred location of the mouth of the colorado river assuming that it existed during the eocene thus the colorado river may be much older than previously thought as old as late paleocene and perhaps was once part of an ancient fluvial connection with the mojave desert interior

fluvial plain lacustrine facies transition in the cretaceous sindong group south coast of korea abstract along the south coast of korea a 250 m stratigraphic interval corresponding to the uppermost hasandong and lowermost jinju formations of the cretaceous sindong group has been studied in terms of facies transition the interval displays a passage from a fluvial plain facies association consisting of floodplain levee crevasse splay and channel facies to a lacustrine facies association composed of crevasse splay channel overbank sheet splay marginal lake open lake and stream mouth bar facies between these two associations a transitional facies association occurs in which some facies of both associations alternate in the lacustrine association shallow marginal lake deposits including channel facies are gradually replaced upwards by deeper open lake deposits made up of open lake and stream mouth bar facies during the hasandong jinju period small channels which originated in the uplifted areas beyond the fault s bounded western margin of the gyeongsang basin flowed through alluvial fans developed along the fault s and a fluvial plain to a lake which occupied the central part of the basin as a result the lake shoreline was non coincident with the basin margin sinuosity of the streams increased downstream reaching a maximum near the shoreline but decreased in the marginal lake areas the fluvial plain was progressively submerged by the transgression of a non deltaic shoreline resulting in a deepening of the depositional sites from a fluvial plain through a marginal lake to an open lake setting intermittent channel overflows deposited levee and crevasse splay facies in the fluvial plain and crevasse splay and overbank sheet splay facies in the marginal lake stream mouth bars were developed at the ends of the sublacustrine channels by hyperpycnal density flows operated mainly during floods the non deltaic shoreline was formed due to 1 maintenance of shallow and wide marginal lake areas 2 dispersal of sediments by small channels 3 continuous shifting of the channels and their mouths and 4 direct transport of coarser sediments to the open lake and their subsequent wide dispersions

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

imaging the seismic lithosphere asthenosphere boundary of the oceanic plate 1 the seismic lithosphere asthenosphere boundary lab or g discontinuity a seismologically characterized abrupt drop in wave speed in the uppermost mantle is one of the key issues in current geodynamics although plate tectonics started as a theory for the ocean reports on lab for normal oceanic regions are scarce due to paucity of seismic data and whether or not the oceanic lab grows with age is the key issue to be resolved we conduct a systematic survey for the oceanic lab using s to p converted seismic waves along three margins of oceanic plates whose crustal age ranges from 10 myr to 130 myr and we observe laterally continuous oceanic lab images the thickness of the oceanic plate estimated from labs increases with the plate age though scattered suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic lab represents a boundary that grows with age

the chemical signature of source rocks in size fractions of holocene stream sediment derived from metamorphic rocks in the wet mountains region colorado u s a eight stream sediments draining primarily mixed silica rich and silica poor metamorphic rocks in the wet mountains colorado were analyzed at their source for their mineralogic and selected elemental composition the low silica source rocks include amphibolite hornblende schist and muscovite biotite schist the high silica source rocks include granodioritic to leucocratic granitic gneisses the high silica source rocks tend to contain higher concentrations of ree th hf and ba lower concentrations of fe sc and cr higher la sc and thsc ratios and more negative eu anomalies than the low silica source rocks especially those low silica source rocks derived from basic sources rather than weathered silicic sources the stream sediments draining this mixed metamorphic rock terrain are dominated by sands and gravels and they are composed of feldspar quartz and 25 heavy minerals the silts contain more total biotite hornblende pyroxene and opaque minerals and less quartz and feldspar than the sands and gravels sediment especially the silts derived from mixed low and high silica sources tends to be intermediate in elemental composition between those of the end member sources so there is some hope that source rock signatures may be preserved in the sediment elemental ratios of la sc th sc la co th co and ba co and th la sc plots most clearly distinguish sediment derived from the most silicic and most basic source rocks in addition finer sediment near the source can potentially distinguish silicic sources higher ree and more negative eu anomalies from those of basic sources the clay and silt fractions contain negative eu anomalies normalized to the mcps north american mid continent platform sediment and the sands and gravels have positive eu anomalies normalized to the mcps most size fractions are similar to more enriched in la sc th sc la co and th co ratios than the mcps this suggests local sources in the wet mountains are on the average more silica rich than those that formed the mcps

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

an allostratigraphic approach to a non marine basin the north western part of cretaceous kyongsang basin se korea the north western part of the cretaceous kyongsang basin south east korea comprises alluvial deposits of conglomerate gravelly sandstone sandstone and mudstone which can be grouped into four allomembers bounded by stratigraphic discontinuities the discontinuities trend nw se and are marked by distinct facies transitions abrupt emplacement of conglomerate and thin but persistent mudstone beds sedimentary facies and architectural analyses reveal that each allomember formed a depositional system of fluvial channel networks draining toward the south east with alluvial fans on the northern margin each allomember can be characterized by distinctive architecture of channel fills clast composition of conglomerate and sandstone mudstone ratio successive units show an eastward shift in the locus of deposition suggesting basinward relocations of alluvial systems such variations with time and space are interpreted to reflect changes in accommodation space and sediment supply during basin evolution probably caused by fault movements this study shows that detailed mapping combined with architectural analysis and the establishment of alluvial allostratigraphy can help assess changes in alluvial systems and structural development of the basin

provenance of quartzarenite clasts in the tetori group middle jurassic to early cretaceous japan paleogeographic implications abstract there are many strata containing quartzite clasts in the japanese islands but the on land geology of the japanese islands has no candidate for the source rocks of the quartzite clasts this study deals with the provenance of quartzite clasts of the tetori group middle jurassic and early cretaceous distributed in northern central japan quartzarenite clasts from conglomerate beds of the tetori group are texturally subdivided into three types the most abundant is matrix bearing quartzarenite type 3 followed by poorly sorted feldspar bearing quartzarenite type 2 and by well sorted pure quartzarenite type 1 to identify the provenance of quartzarenite clasts in the tetori group characteristics of quartzarenite sequences distributed in north china and korea were studied and compared with those of the tetori quartzarenite clasts texturally and geochemically the quartzarenite clasts of the tetori group are very similar to pre mesozoic quartzarenites distributed in the south central and east central korean peninsula considering the previous results of depositional environments paleomagnetism and paleocurrent studies on the tetori group this study suggests that during deposition of the tetori group the northern part of central japan was probably land connected with the central region of eastern korea

explorer deformation zone evidence of a large shear zone and reorganization of the pacific juan de fuca north american triple junction recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the explorer juan de fuca ridge systems the boundaries between the pacific and northern juan de fuca plates the bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the sovanco fracture zone to include the heck and heckle seamounts and potentially as far south as the springfield seamounts and cobb offset along the juan de fuca ridge this indicates that the triple junction between the pacific juan de fuca north american plates may be reorganizing southward to establish at the cobb offset

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

rayleigh wave phase velocities in the atlantic upper mantle phase velocity in the period range 30 130 s is measured for approximately 10 000 fundamental mode rayleigh waves traversing the atlantic basin in order to isolate the signal of the oceanic upper mantle paths with 30 of their length through continental upper mantle are excluded the lateral distribution of rayleigh wave phase velocity in the atlantic upper mantle is explored with two approaches one phase velocity is allowed to vary only as a function of seafloor age two a general two dimensional parameterization is utilized in order to capture perturbations to age dependent structure in both scenarios phase velocity shows a strong dependence on seafloor age at all periods with higher velocity associated with older seafloor removing age dependent velocity from the 2 d phase velocity maps highlights areas of anomalously low velocity almost all of which are proximal to locations of hotspot volcanism the age dependent phase velocities for the atlantic are not consistent with a half space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400 c in contrast age dependent phase velocities for the pacific basin determined by nishimura and forsyth 1989 can be fit reasonably well by a half space cooling model with mantle temperature approximately 50 c warmer than the atlantic comparison of rayleigh wave phase velocity and fractionation corrected na concentrations in mid ocean ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period as expected if along ridge variations in mantle potential temperature are controlling both quantities

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

detrital zircon u pb ages of the jangsan formation in the northeastern okcheon belt korea and its implications for material source provenance and tectonic setting abstract the jangsan formation distributed in the taebaeksan basin central eastern korea is unfossiliferous and is composed of quartz arenite this formation is conventionally believed to be the lowermost stratigraphic unit of the lower paleozoic sequence deposited on the yeongnam massif in south korea corresponding to early cambrian in age u pb dating of detrital zircon grains using a laser ablation inductively coupled mass spectrometer yields ages ranging from archean to middle proterozoic 1738 67 ma to 3058 53 ma detrital zircons show no ages younger than 1 8 ga suggesting that the jangsan formation was deposited at some time after this age but much prior to ca 520 ma the depositional age of the immediately overlying myobong formation this interpretation is supported by the recent report on the presence of the unconformity between the jangsan and the myobong formations kim and lee 2006 zircon ages from jangsan sandstones define two major groups archean age grains with a maximum frequency at about 2 5 ga and paleoproterozoic age grains with maximum frequencies at about 2 1 and 1 8 ga the observed zircon age distribution in the jangsan sandstones may represent that of the yeongnam massif on which the jangsan formation was deposited such an age distribution of the yeongnam massif matches well with that the north china block suggestive of the yeongnam massif being a part of the sino korean craton during the precambrian

resolution uncertainty and whole earth tomography 1 using the arrival times of 10 distinct phases from the international seismological center catalogue p s pp pcp ss scs pkpab pkpbc pkpdf sksac and two sets of differential times ss s410s and ss s660s we estimate the p and s velocity structure of the whole earth mantle outer core and inner core boundary topography of four interfaces within the earth hypocentral relocation parameters and station corrections the entire catalogue is relocated and reassociated using the radial velocity model ak135 both p and s phases are used in the relocation of over 41 000 events in depth isotropic velocity variations within the earth are parameterized by 22 layers lateral variations in shear and compressional velocity within the layers are represented by one of two grids in the mantle each layer is subdivided into 4560 equal volume cells of dimension 3 3 at the equator however in the outer and inner core the cell size is increased to 6 6 this change in grid spacing accounts for decreasing cell volumes with depth and variations in sampling between the mantle and core previous attempts using a fine grid throughout the earth resulted in poor resolution of core structure with the current grid the resolution of velocity heterogeneity in the mantle and outer core is comparable in the upper and midmantle the well resolved cells lie beneath eurasia and a narrow zone surrounding the mantle beneath the pacific basin in the lowest mantle the resolution decreases significantly an examination of the averaging kernels associated with six locations near the base of the mantle reveals the significant lateral and vertical averaging inherent in estimates of compressional velocity in well sampled regions of the outer core we are able to resolve compressional velocity variations in our 6 6 cells however velocity variations in the inner core are poorly constrained at the scale of our blocks in well sampled regions of the mantle and outer core model parameter standard deviations remain below 0 25 for much of the inner core the standard deviations are quite large exceeding 0 5 narrow high velocity anomalies are imaged in the upper mantle associated with subduction zones in a number of locations such as beneath the americas these features extend well into the lower mantle in the outer core heterogeneity is generally lower in magnitude of the order of 0 5 the most notable feature in the outer core is a systematic shift to slightly higher velocity in addition we find a roughly symmetric distribution of heterogeneity at the base of the outer core compressional velocities are generally higher in the equatorial regions and lower at the poles

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

mantle dynamics beneath the discrete and diffuse plate boundaries of the juan de fuca plate results from cascadia initiative body wave tomography we use the delay times of teleseismic s phases recorded by ocean bottom seismometers during the plate scale cascadia initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere our study area covers the entire juan de fuca jdf and gorda plates from their creation at the jdf and gorda ridges to their subduction beneath the north american continent and the entire length of the blanco transform fault the range of the observed vs anomalies requires variations in the melt fraction of the asthenosphere the data require that low vs anomalies extend to depths of at least 200 km which is within the carbonatite melting regime in the upper 200 km of the mantle vs increases rapidly to the east of the jdf ridge while there is no clear relationship with the age of the lithosphere in the gorda region the distribution of melt is asymmetric about both the jdf and gorda ridges dynamic upwelling due to the buoyancy of the mantle and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the jdf ridge the asymmetry about the jdf ridge and the sinuous pattern of upwelling near the blanco transform fault finally mantle flow beneath the diffuse gorda and explorer plate boundaries is distinct from that beneath the discrete plate boundary of the jdf ridge in particular shear between the pacific and jdf plates appears to dominate mantle deformation over seafloor spreading beneath the gorda ridge