quantitative bedrock geology of alaska and canada 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock on the basis of data from the most recent geologic maps of alaska beikman 1980 1 2 500 000 and canada douglas 1969 updated 1 5 000 000 made available in digital form by the u s geologic survey and the geologic survey of canada national resources canada respectively sediments cover 72 9 and 52 4 of the surface of alaska and canada volcanic rocks comprise 11 6 and 6 of the surface area in alaska and canada respectively whereas intrusive rocks cover 6 6 and 24 of the surface respectively ultramafic rocks account for 0 20 and 0 08 of the bedrock area whereas metamorphic rocks cover 3 4 and 16 1 the average ages of major lithologic units weighted according to bedrock area are significantly younger in alaska than in canada marine sediments stratigraphic age of 206 myr in alaska versus 599 myr in canada continental sediments stratigraphic age of 81 myr versus 150 myr volcanic rocks 126 myr versus 1377 myr intrusive rocks 114 myr versus 2109 myr ultramafic rocks 261 myr versus 874 myr and metamorphic rocks 474 myr versus 2545 myr this age difference reflects the contrast between the young active margin influence on the geology of alaska and the old cratonic nature of much of the bedrock exposed in canada glacial erosion and the resulting exposure of precambrian bedrock primarily in canada contributes to this age contrast the average temporal and spatial resolution of the digital data is sufficiently high to perform age area analyses on individual river basins larger than 20 000 km2 and to evaluate the relationship between bedrock geology and river chemistry

records of the nd isotope composition of seawater from the bay of bengal implications for the impact of northern hemisphere cooling on itcz movement abstract this study presents a record of planktonic foraminiferal neodymium isotopic gradients along a north south transect in the bay of bengal during time slices of late holocene and last glacial maximum lgm age together with a record of planktonic foraminiferal variation in the northern bay of bengal 15 n over the last 195 ky in late holocene core top planktonic foraminifera the north south end gradient rises from nonradiogenic values of 12 at 20 n to 10 at 5 n in parallel with the modern surface salinity gradient controlled by discharge of himalayan rivers in the northern bay during the lgm end increased throughout the bay the contrast between northernmost and southernmost sites decreased and maximum end values of 6 5 occurred between 12 and 15 n a small part of the shift to higher mean end throughout the bay during the glacial may arise from a uniform increase in deposition of far field dust from arabian and persian gulf regions however the spatial pattern of end variation between lgm and late holocene also suggests a shift from modern dominance of nonradiogenic nd sources from the ganges brahmaputra basin to lgm dominance of more radiogenic nd sources from arakan coastal rivers over the last 195 ky at 15 n the most radiogenic end values of 7 occur at glacial maxima and the most nonradiogenic values of 11 occur during interglacials end values are highly correlated with glacial interglacial variations in planktonic foraminiferal 18o in this record shifts in river sources from the more northerly ganges brahmaputra watershed to the more southerly arakan coastal river systems respond dominantly to inter tropical convergence zone itcz movement driven by northern hemisphere cooling during 100 ky glacial interglacial cycles with a small component of variation on precessional timescales the nonlinear correlation of end with ice volume suggests that itcz movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume these data add support to recent general circulation models of itcz response to northern hemisphere ice sheets which simulate decreased glacial precipitation in the ganges brahmaputra basin and increased glacial precipitation in the southern arakan coastal basin

global chemical erosion during the last glacial maximum and the present sensitivity to changes in lithology and hydrology geographically based calculations for 18 000 years ago 18 ka and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial interglacial timescales runoff fields were derived from general circulation model predictions of precipitation minus evaporation then global lithologic maps were prepared so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates assuming that significant chemical erosion does not occur beneath ice sheets we find that chemical erosion in ice free areas at 18 ka was only slightly greater than today this result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff the small 20 increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates which weather faster than average data on glacial interglacial movements of the calcite compensation depth ccd are inconclusive but seem to indicate little change whereas even a 20 increase in the riverine bicarbonate flux should cause an observable deepening if the ccd did not indeed change then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means such as increased carbonate productivity about half of the observed decrease in atmospheric pco2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice free areas consideration of the maximum possible effect of meltwater at ice margins leads to an 80 increase in the global chemical erosion rate at 18 ka such an increase would lead to a larger and faster drop in atmospheric pco2 but also to an excessive deepening of the global ccd this scenario seems unrealistic our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of quaternary marine sr isotopic ratios therefore if these records are indeed globally representative and entirely due to increased silicate chemical erosion as opposed to changes in source areas or hydrothermal inputs then other factors such as a shift in the global weathering regime which we do not explicitly consider here may be involved sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non spatially resolved calculation

preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the bushveld layered intrusion abstract the preferred orientation of plagioclase and pyroxene in gabbronorites from the bushveld layered igneous intrusion south africa was investigated using electron backscatter diffraction in two localities 200 km apart belfast and rustenburg strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane defined by 010 planes of plagioclase and 100 planes of pyroxene the pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure primary magmatic oxides are also influenced by the preferred orientation of plagioclase generating an oblate magnetic susceptibility fabric parallel to the foliation plane plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p waves traveling 4 8 faster in the vertical than in the horizontal direction

subsidence in magma chamber and the development of magmatic foliation in oman ophiolite gabbros abstract in the oman ophiolite the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified starting in the root zone of the sheeted dike complex rzsdc located above this horizon 18 sections down to the upper gabbros unit have been mapped in great detail in selected areas of the southern massifs of this ophiolite they are complemented by 133 sites located throughout the entire ophiolite where the transition from the rzsdc to the uppermost foliated gabbros is well exposed altogether half the sites and 11 cross sections display within a few tens of meters beneath the rzsdc a magmatic foliation which is parallel to the overlying sheeted dikes in the other sites and cross sections the gabbro foliation is either flat lying or steep but not parallel to the sheeted dikes compared to the rzsdc isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths in the topmost steeply foliated gabbros clinopyroxene is idiomorphic becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende moving down from these uppermost gabbros and over one hundred meters the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized granular gabbros of the gabbro unit these repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying main magma chamber the topmost gabbros beneath rzsdc which were expelled from the melt lens by drifting very soon after settling on the melt lens floor display in plagioclase a spectacular zoning pointing to a fast cooling moving downwards stronger foliation increased compaction and recovery recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude 50 to 500 yr over this vertical distance this field based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis

quantitative bedrock geology of the continents and large scale drainage regions we quantitatively analyze the area age distribution of sedimentary extrusive volcanic and endogenous plutonic and or metamorphic bedrock on the basis of data from the most recent digital geological map of the world at a scale of 1 25 000 000 the spatial resolution of the digital bedrock data averages 13 905 km2 per polygon comparison of certain regions of the world previously analyzed at higher spatial resolution with the low resolution world data reveals general consistency in the areal exposure of major rock types as well as a minor systematic bias toward older average bedrock ages in the global data set application of the global bedrock data to 19 large scale drainage regions and three large internally drained regions reveals considerable regional variability of earth s bedrock geology that is consistent with the dominant geotectonic setting of the respective drainage region

microstructures and crystallographic preferred orientation of anorthosites from oman ophiolite and the dynamics of melt lenses 1 microstructures and crystallographic preferred orientation cpo of anorthosite samples interlayered in the upper and lower gabbro sections in the oman ophiolite were analyzed in this paper in the anorthosites registering the dynamics of the melt lenses foliation is flat lying and starts to develop a few meters below the root zone of the sheeted dike complex rzsdc microstructures and cpo of these rocks were developed in response to four different mechanisms 1 density controlled settling of plagioclase on the lens floor 2 deposition of anorthosites related to convection currents 3 melt compaction and 4 uncompacted melt accumulation in these anorthosites the poles to 010 of plagioclase are parallel to the flow plane of convection whereas the 100 axes and poles to 001 express the convection flow direction and the axis of convection rolls respectively the effect of subsidence of melt lens floor is recorded immediately below the rzsdc and is characterized by the rapid but progressive development of dipping foliation and lineation reflecting the increase of deformation downsection the degree of foliation and cpo development in the anorthosites is directly related to the distance of the center of the melt lenses before the subsidence starts despite the uncertain origin of the anorthosites from the lower gabbro section all the samples lost the magmatic microstructural characteristics and presently are reequilibrated aggregates however they still preserve plagioclase cpo where some of these patterns present similarities with the anorthosites from the upper gabbro section but no evidence of intracrystalline deformation under high temperatures

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

cumulate recycling and crustal evolution in the bay of islands ophiolite the lower crustal section of the north arm mountain massif of the bay of islands ophiolite is composed of interlayered gabbroic and ultramafic cumulates despite pervasive ridge related deformation key contact relationships are preserved in low strain domains or late kinematic intrusions angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts olivine gabbro gabbro gabbro norite norite and fe oxide amphibole gabbros the ultramafic intrusions are composed of dunite wehrlite harzburgite lherzolite and olivine websterite clino to ortho pyroxenite reaction rims line many peridotitic gabbroic contacts the voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros fine grained harzburgitic sills crosscut the foliation of wehrlitic host rocks with orthopyroxenite reaction rims

quantitative bedrock geology of the conterminous united states of america 1 we quantitatively analyze the area age distribution of bedrock based on data from the most recent geologic map of the conterminous united states of america king and beikman 1974a 1974b made available in digital form by the united states geologic survey the area age distribution agrees surprisingly well with older data higgs 1949 but provides much higher temporal resolution the mean stratigraphic age of all sedimentary bedrock is 134 myr that of tertiary cambrian sediments is 104 myr the analysis also reveals area coverage of some minor lithologies such as ultramafic rocks that cover 0 15 of the conterminous united states area coverage of 162 lithostratigraphic units is made available as an excel data sheet

inside the magma chamber of a dying ridge segment in the oman ophiolite for the first time the crystallized remnant of an oceanic ridge magma chamber is documented in the oman ophiolite it exists in the centre of a 40 km long monoclinal ridge jebel dihm wadi tayin massif exposing a full crustal section perpendicular to the spreading direction new detailed mapping supported by u pb zircon geochronology suggests that the active fast spreading ridge that died just prior to detachment of the ophiolite is preserved and largely intact our observations provide insights into the crystallizing mush zone of a magma chamber before it crosses the external walls and solidifies as deformed gabbros our data provide new constraints on the shape and internal dynamics of a magma chamber including gabbro subsidence from the floor of a perched melt lens and the limited contribution of sills to crustal accretion by locating precisely the palaeo ridge axis prior full spreading rate estimates can be increased to 140 km ma 1

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

the skaergaard trough layering sedimentation in a convecting magma chamber the upper parts of the floor cumulates of the skaergaard intrusion east greenland contain abundant features known as troughs the troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase rich material the origin of the troughs and layering is contentious attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls or to in situ development by localised recrystallisation during gravitationally driven compaction they are characterised by outcrop scale features such as mineral lineations parallel to the trough axis evidence of erosion and layer truncation associated with migration of the trough axis and disruption of layering by syn magmatic slumping a detailed microstructural study of the modal trough layers using electron backscatter diffraction together with geochemical mapping demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution reprecipitation instead the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning we argue that the lineations and foliations are therefore a consequence of grain alignment during magmatic flow post accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid with upwards migration of the si rich conjugate into the plagioclase rich upper part of the layers whereas the fe rich immiscible conjugate remained in the mafic base both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal rich currents descending from the nearby chamber walls

experimental and theoretical constraints on melt distribution in crustal sources the effect of crystalline anisotropy on melt interconnectivity in partially molten systems the equilibrium distribution of melt at the grain scale is governed by the principle of interfacial energy minimization in ideal sources i e partially molten rocks that are monomineralic have single valued solid liquid and solid solid interfacial energies and are subject to hydrostatic stress the wetting angle is known to be a unique characteristic which specifies the melt configuration for a given melt fraction crustal rocks cannot be modelled as ideal sources because of their polymineralic nature the moderate to high anisotropy of interfacial energies which characterizes common refractory minerals and the possible presence of a crystallographic preferred orientation that partially molten crustal rocks depart from ideal sources is documented by a series of high p high t experiments illustrating the textural relationships of biotite and amphibole with silicic melts the melt distributions observed in these experiments differ significantly from those expected in ideal sources 1 crystal melt interfaces are commonly planar rational faces rather than smoothly curved irrational surfaces and 2 the concept of a unique wetting angle does not hold as shown in the biotite silicic melt system these textural features demonstrate that anisotropy of crystal melt interfacial energy is a factor of primary importance in modelling the grain scale distribution of partial melts the petrological implications of our study are the following 1 1 at high degrees of anisotropy and low melt fractions melt is predicted to form isolated plane faced pockets at grain corners the overall shape of these pockets and therefore the value of the connectivity threshold c are expected to be very sensitive to the ratio of solid solid to solid liquid interfacial energies ss sl c is the melt fraction at which melt interconnectivity is established melt pockets with low volume to surface ratio and low but non constant wetting angles should prevail at high ss sl resulting in very low values of c 1 to a few vol higher values of c a high volume to surface ratio of melt pockets and high wetting angles are expected at low ss sl 2 2 the wetting angle at hornblende hornblende melt junctions at 1200 mpa 975 c is 25 a review of existing data indicates that quartz melt and feldspar melt wetting angles are also low to moderate 12 60 a very low value of c should therefore be the general rule during crustal anatexis in particular a connectivity threshold lower than 3 4 vol is predicted for partially molten amphibolite 3 3 in biotite rich rock types such as melanosomes in migmatites the combination of a pronounced crystalline anisotropy and a marked preferred orientation of mica flakes leads to a very low permeability normal to layering biotite rich melanosomes should therefore impede chemical interactions between neighbouring leucosomes and mesosomes

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

textural character of gabbroic rocks from pito deep a record of magmatic processes and the genesis of the upper plutonic crust at fast spreading mid ocean ridges

structure and dynamics of ridge axial melt lenses in the oman ophiolite 1 the oman ophiolite is regarded as derived from a fast spreading oceanic ridge as in its present day marine analogs expectedly the gabbro unit crystallized in a large magma chamber underneath a small melt lens in ophiolites this melt lens is reduced to a horizon mapped in the field where the floor and the roof of the melt lens joined the magmatic activity of the former melt lens has been studied in gabbros located below the melt lens horizon beneath a stable melt lens gabbros subside and drift through the steep walls of the magma chamber as steeply foliated gabbros in contrast in unstable melt lenses more specifically considered here flat lying gabbros are exposed beneath the lens these gabbros were settling on the floor while the melt lens was retreating their strong magmatic foliations and lineations point to a dynamic deposition on the floor and lineation trends record the structure of convection rolls within the lens time constraints suggest a 100 year periodicity of melt recharge by short and powerful melt pulses followed by a shorter time of convection activity after recharge and a longer period of static magma settling as recently proposed for the east pacific rise at each new melt pulse the lens nearly doubles its volume by surface expansion within a short time lapse before slowly and progressively retreating to feed crustal accretion large horizontal surface inflation of the lens better explains the seawater hydration of the accreting crust than melt lens vertical motion through the lid

quantitative bedrock geology of east and southeast asia brunei cambodia eastern and southeastern china east timor indonesia japan laos malaysia myanmar north korea papua new guinea philippines far eastern russia singapore south korea taiwan thailand vietnam 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock based on data from the most recent digital geologic maps of east and southeast asia coordinating committee for coastal and offshore geosciences programmes in east and southeast asia ccop and the geologic survey of japan 1997 1 2 000 000 published as digital geoscience map g 2 by the geological survey of japan sedimentary rocks volcanic rocks plutonic rocks ultramafic rocks and metamorphic rocks cover 73 3 8 5 8 8 0 9 and 8 6 of the surface area respectively the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 123 myr median age of 26 myr volcanic rocks 84 myr 20 myr intrusive rocks 278 myr 195 myr ultramafic rocks unknown and metamorphic rocks 1465 myr 1118 myr the variability in lithologic composition and age structure of individual countries reflects the complex tectonic makeup of this region that ranges from precambrian cratons e g northeast china and north korea to mesozoic cenozoic active margins e g japan the philippines indonesia and new guinea the spatial resolution of the data varies from 44 km2 per polygon japan to 1659 km2 per polygon taiwan and is on average 490 km2 polygon similar to our previous analyses of the united states of america and canada the temporal and spatial resolution is sufficiently high to perform age area analyses of individual river basins larger than 10 000 km2 and to quantitatively evaluate the relationship between bedrock geology and river chemistry as many rivers draining tropical mountainous islands of east and southeast asia have a disproportionate effect on the dissolved and particulate load delivered to the world oceans bedrock geology in such river drainage basins disproportionately affect ocean chemistry

structural contribution from the oman ophiolite to processes of crustal accretion at the east pacific rise a comprehensive model for the activity of the elementary accretion segment at fast spreading ridges relies on integration of structural data from the oman ophiolite and geophysical results from the east pacific rise epr around 9 n which are of comparable size and spreading rates the axial melt lens at shallow crustal level provides a link between deval segmentation at the seafloor and a lower melt sill at moho level imaged at the epr as a crustal melt zone cmz and mapped in oman as the moho transition zone mtz both are attached to a mantle upwelling at the epr and to a frozen diapir in oman the physical link between diapiric mantle uprising at the moho and devals segmentation at the seafloor is the melt being injected from the mantle into the lower mtz ponding there and then being released by powerful injections into the upper melt lens the magma chamber covers the diapir at a distance of 5 km from the ridge axis

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

axial melt lenses at oceanic ridges a case study in the oman ophiolite abstract the oman ophiolite a fossil fast spreading oceanic ridge had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing in ophiolites this melt lens is reduced to a horizon that has been identified in the field the gabbros that subsided from the lens horizon and drifted out of the magma chamber while developing a steep magmatic foliation provide insights into the active melt lens processes a gabbro sampled right below the horizon where the lens closed has virtually not subsided and it offers a direct glance at the floor of the lens below the deeper the gabbros are sampled the more they have subsided before drifting out of the magma chamber and the closer they derive from the center of the lens floor cross sections in these gabbros record successive events occurring within the lens in particular anorthosite layers and lenses that are interlayered within the foliated gabbros record the intrusion of a new melt where plagioclase is the first mineral to crystallize on liquidus detailed mapping of anorthosite swarms suggest that melt intrusions are massive lasting less than 4 yr and that their frequency is 100 yr their large volume controls the stability of melt lenses a new melt surge induces rapid and large inflation of the lens melt lens deflation follows that is due to the ongoing process of crustal accretion both in the gabbro unit by continuous subsidence of a thick gabbroic mush from the lens floor and in the lid above the lens roof by sheeted dike intrusions feeding basalt extrusions at the higher frequency of 10 yr

seismic analysis of the axial magma chamber reflector along the southern east pacific rise from conventional reflection profiling the thickness and internal properties of the magma sill located at the top of the axial magma chamber amc along the southern east pacific rise epr have been investigated through a combination of waveform modeling the near vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source receiver offset or slowness our results show that the amc reflector observed along the southern epr is best modeled by a thin 100 m thick sill of partial melt vs 0 km s sandwiched between higher velocity material and that the thickest sills are generally associated with the lowest p and s wave velocities the comparatively high p wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content this may have important implications for eruption mechanisms and along axis mixing of magma at the epr there is no simple relationship between morphologic indicators of magma supply e g axial depth or volume and sill thickness depth or velocity magma sill properties may be closely tied to the eruption and replenishment cycle of the amc and thus may vary on a much shorter spatial and temporal scale than axial morphology which reflects longer term variations in magma supply to the ridge

detection of the subducting crust of oceanic plates beneath the kanto district japan abstract low velocity crustal layers on the top of the subducting slabs beneath the kanto district central japan were detected by a travel time inversion using 40 763 p wave arrival times from 3038 local earthquakes recorded by a local network of 41 seismic stations the inversion covered 188 228 110 km volume of the target region and the model was parameterized by 47 57 18 rectilinear blocks measuring 4 0 km per side horizontally and 4 10 km varying thickness in depth a prominent e w directed low velocity band of about 25 km width appeared in the central kanto region at a depth of 30 km moving north or northwestward gradually with depth this band can be traced to the depth of about 60 km at a depth of 60 km a slightly lower amplitude nearly n s directed low velocity band was observed at the east coast of the kanto region moving westward with depth these low velocity bands are thin layers attached to the top of the inclined thick layers with relatively high velocity in the kanto district the philippine sea phs plate subducts under the eurasian eur plate and the pacific pac plate also underthrusts beneath both the eur and phs plates some authors already suggested low velocity anomalies near the surface of those descending plates in this region mainly from the analyses of later phases and focal mechanisms of local earthquakes the shapes of the low velocity bands are similar to the depth distribution of the upper boundary of the phs and pac plates proposed by other authors the thrust earthquakes that occur at the boundaries between the eur and phs plates and between the phs and pac plates are almost located in the low velocity layers described above we conclude that these low velocity layers are the crusts that exist at the top of the subducting phs and pac plates while high velocity layers are subducting slabs themselves for the phs plate low density crust of the izu bonin arc is probably stacking at the range from 139 25 e to 139 55 e at 35 55 n which is observed as a thick low velocity region to the south of the range and in a steep inclination to the north of it this is also recognized from the gravity anomaly

sediment discharge and export of fluvial carbon and nutrients into the arafura and timor seas a regional synthesis abstract the arafura and timor seas ats are a crucial link between the indian and pacific oceans and play a vital role in global ocean circulation and climate some high standing islands in the ats are globally significant sources of sediment here we derive a synthesis of river catchments and their role as sources of water sediment carbon and nutrients to estimate the pathways magnitude and fate of exported riverine materials the edge of the australian continental shelf and slope receives riverine sediment from asia as isotopes suggest a mixing of sediment sources with some overlap between the australian and non australian material including evidence of significant input of volcanic material from indonesia the catchments bordering both the arafura and timor seas account for 12 of sediment and 35 of water discharged from tropical asia northern australia discharges a volume of freshwater comparable to southwest new guinea but 50 65 of total sediment 754 mt dic 61 1 mt poc 7 9 mt doc 3 5 mt tn 2 88 mt and tp 254 264 t delivered annually to the ats come from new guinea the island of timor discharges much smaller amounts of water 170 km 3 yr 1 and sediment 133 mt yr 1 but area specific rates of dic 1150 t km 2 yr 1 poc 238 t km 2 yr 1 doc 94 t km 2 yr 1 tn 61 t km 2 yr 1 and tp 4 4 t km 2 yr 1 are higher reflecting very high rates of deforestation and land degradation compared to other tropical rivers carbon export into the ats is dominated by dic the ats catchments are being increasingly affected by human activities and material discharge to the continental margins will likely increase impacting shelf communities and highly diverse reef mangrove and seagrass habitats

the isotopic composition of particulate organic carbon in mountain rivers of taiwan small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon poc to the oceans this input has implications for the geochemical stratigraphic record we have investigated the stable isotopic composition of poc 13corg in rivers draining the mountains of taiwan in 15 rivers the suspended load has a mean 13corg that ranges from 28 1 0 8 to 22 0 0 2 on average 37 samples per river over the interval of our study to investigate this variability we have supplemented suspended load data with measurements of poc in bedrock and river bed materials and constraints on the composition of the terrestrial biomass fossil poc in bedrock has a range in 13corg from 25 4 1 5 to 19 7 2 3 between the major geological formations using coupled 13corg and n c we have found evidence in the suspended load for mixing of fossil poc with non fossil poc from the biosphere in two rivers outside the taiwan central range anthropogenic land use appears to influence 13corg resulting in more variable and lower values than elsewhere in all other catchments we have found that 5 variability in 13corg is not controlled by the variable composition of the biomass but instead by heterogeneous fossil poc in order to quantify the fraction of suspended load poc derived from non fossil sources fnf as well as the isotopic composition of fossil poc 13cfossil carried by rivers we adapt an end member mixing model river suspended sediments and bed sediments indicate that mixing of fossil poc results in a negative trend between n c and 13corg that is distinct from the addition of non fossil poc collapsing multiple fossil poc end members onto a single mixing trend as an independent test of the model fnf reproduces the fraction modern fmod in our samples determined from 14c measurements to within 0 09 at the 95 confidence level over the sampling period the mean fnf of suspended load poc was low 0 29 0 02 n 459 in agreement with observations from other mountain rivers where physical erosion rates are high and fossil poc enters river channels the mean 13cfossil in suspended poc varied between 25 2 0 5 and 20 2 0 6 from catchment to catchment this variability is primarily controlled by the distribution of the major geological formations it also covers entirely the range of 13corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous poc if land sourced poc is preserved in marine sediments then changes in the bulk 13corg observed offshore taiwan could instead be explained by changes in the onshore provenance of sediment the range in 13corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans care should be taken in accounting for fossil poc in marine deposits supplied by active mountain belts

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

quantitative bedrock geology of brazil 1 we quantitatively analyze the area age distribution of sedimentary igneous metamorphic and ultramafic bedrock on the basis of data from the digital geologic map of brazil published as a gis map by the brazilian geological survey bedrock units exclusively encompassing sedimentary rocks igneous rocks or metamorphic rocks cover 40 4 31 5 and 17 7 respectively of the total bedrock area these numbers have to be considered minimum estimates of the areal abundance of sedimentary igneous and metamorphic bedrock because polygons defined by mixed lithologies cover 8 5 9 5 of the total bedrock area these mixed units are sedimentary rocks with igneous and or metamorphic contributions 1 4 metamorphic rocks with sedimentary contributions 1 2 metamorphic rocks with igneous contributions 1 5 igneous rocks with sedimentary and or metamorphic contributions 4 4 and ultramafic units with sedimentary igneous and or metamorphic contributions 1 2 the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 248 5 1 myr median stratigraphic age of 87 5 myr igneous rocks 1153 13 1 myr metamorphic rocks 1678 30 1 myr and ultramafic rocks 1227 25 1 myr the average bedrock age of brazil is 946 7 1 myr the range in lithologic composition and age structure of the various bedrock units reflects the complex tectonic makeup of brazil that ranges from neogene sedimentary cover in the amazon basin to precambrian cratons guyana and brazilian shields and transamazonian greenstone belts the average spatial resolution of the data is 232 km2 polygon 1 and is sufficient to perform area age analyses of individual river drainage basins larger than 5 000 km2

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

single crystal plagioclase feldspar dissolution rates measured by vertical scanning interferometry here we introduce a technique for simultaneous measurement of surface normal retreat rates of specific cleavage faces by vertical scanning interferometry and the bulk dissolution rate of a mineral powder a hydrothermal reactor is used to contain both a well characterized powder and oriented single crystals with a masked reference surface at elevated temperatures we show examples using both anorthite and albite reacted at temperatures between 150 and 200 c in the case of albite dissolution rates of fine grained powders are substantially enhanced compared to those prevailing on large single crystal cleavage surfaces rates developed on the 010 albite cleavage surface are also substantially faster than those on the 001 face where etch pit development was relatively modest and surface normal retreat was not detectable within the time frame of the experiment the reasons for this difference are not immediately clear but may be related to anisotropy in the distribution of al o si vs si o si bonds in the albite structure 010 twinning expressed on the 001 surface and possible disruption of kink propagation across the twin plane