magma mingling and hybridization in the kuh e dom pluton central iran abstract the upper eocene kuh e dom granitoids in central iran comprise mafic microgranular enclaves ranging from a few centimeters to meters in size the spherical to ellipsoidal enclaves consist of diorite quartz diorite monzodiorite and quartz monzodiorite whereas the more felsic host intrusions mainly comprise monzogranite and granodiorite most enclaves show sharp contacts with the host granites and distinct chilled margins in the small enclaves indicate rapid cooling after entering the felsic host magma disequilibrium textures include 1 poikilitic and antirapakivi textures in feldspar megacrysts 2 ocelli quartz rimmed by fine grained mineral aggregates 3 rounded and corroded plagioclase 4 patchy plagioclase 5 mafic clots 6 blade biotite 7 small lath shaped plagioclase in larger tabular plagioclase 8 acicular apatite and 9 crenulated and cuspate contacts between enclaves and host rocks these textures indicate mingling mixing of two different magmas microprobe data show variable plagioclase compositions ranging between oligoclase and andesine in granodiorite and between andesine to labradorite in the enclaves with normal zoning and oscillatory zoning geochemically the enclaves are na rich whereas the host rocks are k rich in addition the enclaves are depleted in lree and lile and enriched in hree and ti p nb and y relative to their host granodiorites average initial nd sr isotopic compositions for the host rocks are end 47 ma 5 09 and sri 0 7068 compared to enclave sri 0 7060 and end 47 ma 2 89 fractionation of both enclaves and host rocks from the same melt cannot explain these petrological geochemical and isotopic differences instead enclaves and host rocks more likely originated from two separate magmas which subsequently mingled and mixed the mafic enclaves are modified mantle derived melts while the granodioritic host rocks are partial melts of lower continental crust the mafic enclave magma was at near liquidus conditions when injected into the granitic host melt and the kuh e dom intrusion reflects a hybrid product from mingling and partial incomplete mixing of these two melts

evidence of isotopic equilibration between microgranitoid enclaves and host granodiorite warburton granodiorite lachlan fold belt australia abstract microgranitoid enclaves in the s type warburton granodiorite lachlan fold belt australia display a range in mineralogical and isotopic compositions many show obvious similarities to the host granodiorite they contain biotite as the sole ferromagnesian phase and have isotopic signatures similar to the host other enclaves contain cummingtonite actinolite and rare orthopyroxene mainly in the core of the enclaves these enclaves typically have higher nd i and lower sol 87 sr 86 sr i than the host rocks reversely zoned plagioclase megacrysts in some enclaves contain anhedral cores which are similar in composition to an poor plagioclase crystals in the host rock an45 these megacrysts are jacketed by an rich plagioclase an70 an80 the enclaves are interpreted to be globules of a more mafic magma which mingled with the host while it was still partially liquid megacrysts within the enclaves are thought to have been derived from the granodioritic magma the mafic enclave magma is likely to have been derived from a mantle source but equilibration between enclaves and host rock has changed the mineralogy and isotope geochemistry of all enclaves the equilibration process was incomplete for some enclaves resulting in mineralogical zoning and distinctions in isotopic signature between these enclaves and the host rock this equilibration process is likely to have been arrested as a result of rapid cooling of the pluton due to its small size and shallow emplacement level it is uncertain whether the compositions of the least equilibrated enclaves are truly representative of the original mafic enclave magma or whether their trace element signature has been affected by diffusional processes within a stratified magma chamber

preeruption conditions and timing of dacite andesite magma mixing in the 2 2 ka eruption at mount rainier analytical field and experimental evidence demonstrate that the mount rainier tephra layer c 2 2 ka preserves a magma mixing event between an andesitic magma whole rock sio 2 content of 57 60 wt and a dacitic magma whole rock sio 2 content of 65 1 wt the end member andesite a mix of an injected and chamber andesite and dacite can be characterized on the basis of the homogeneity of the matrix glass and phenocryst rim compositions many pumices however contain mixtures of the end members the end member dacite contains a microlite free matrix glass with 74 77 wt sio 2 orthopyroxene rims of mg 57 64 clinopyroxene rims of mg 66 74 and plagioclase rim anorthite contents of an 45 65 the temperature and oxygen fugacity from fe ti oxide compositions are 930 10 c and 0 5 0 75 log units above nno the mixed andesite contains mg 73 84 orthopyroxene rims mg 73 78 clinopyroxene rims an 78 84 plagioclase rims and mg 67 74 amphibole rims the temperature from fe ti oxides hornblendeplagioclase and two pyroxene geothermometry is 1060 15 c and the oxygen fugacity is approximately one log unit above nno for the injected andesite the chamber andesite is estimated to be a magma with a 64 65 wt sio 2 melt at 980 c and a nno oxygen fugacity we conclude that the andesitic and dacitic magmas are from separate magma storage regions at 7 km and 2 4 km due to differences in the bimodal whole rock matrix glass and phenocryst compositions and the presence or absence of stable hornblende the time involved from the mixing event through the eruption is limited to a period of 4 5 days based on fe ti oxide reequilibration phenocryst growth rates and hornblende breakdown the eruption sequence is interpreted as having been initiated by an injection of the 1060 15 c andesitic magma into the 980 c 7 km andesite storage region the mixed andesitic magma then intersected a shallow 2 4 km dacitic storage system on its way toward the surface the eruption became more dacitic over time and the final products some show evidence of partial reequilibration between the andesite and dacite

angular velocities of nubia and somalia from continuous gps data implications on present day relative kinematics abstract this study focuses on the break up of the african tectonic plate into separate nubian and somalian blocks based on recent global positioning system gps data a new unique velocity field has been obtained by processing all available observations of permanent gps stations on africa since 1996 the quantity and distribution of the stations and the length of the time series of observations exceed that of previous studies by a considerable margin allowing one to derive a reliable estimate of the differential motion between the nubia and somalian plates which are considered as a single african block in the prevailing global tectonic plate models the estimated relative pole of rotation of somalia with respect to nubia is located at 54 8 s 37 0 e with magnitude 0 069 ma implying distinct opening in the ethiopian rift of magnitude 7 mm year and azimuth n94 e whereas in southeastern south africa this value is reduced to 2 mm year in almost the same direction this is in accordance with some of the independent geological and geophysical tectonic models of the nubia somalia plate boundary region however the spatial density of the current tracking network is still not optimal to establish the exact location of the entire somalia nubia plate boundary in particular the possible branch east of lake victoria and heading towards the mozambique channel is impossible to confirm or reject at this moment

rift kinematics during the incipient stages of continental extension evidence from the nascent okavango rift basin northwest botswana high resolution aeromagnetic data from the nascent okavango rift in northwest botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension crosscutting relationships between west northwest trending 180 ma karoo dikes and reactivated northeast trending proterozoic basement faults are used to document the kinematics of cenozoic faulting during the initial stages of rifting depth estimates to the top of the dikes using three dimensional euler deconvolution solutions have produced the following interpretations 1 the okavango rift is a half graben with a downthrow of 200 300 m 2 the width of the okavango rift 100 20 km is similar to that of more mature continental rifts such as the tanganyika and baikal rifts this suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin nor the amount of extension it is suggested that the cumulative downthrow sediment infill included and subsidence may be a better indicator of the relative maturity of rift basins 3 preexisting basement faults exert a major control during rifting and reactivation processes do not occur synchronously along the entire length of preexisting faults 4 the okavango rift is defined by normal faults there is no evidence of major strike slip faults thus excluding a pull apart tectonic model for this nascent continental rift stage 5 the preexisting sekaka shear zone terminates the okavango rift to the south suggesting that such shear zones represent major barriers during longitudinal propagation of rifts this probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts

upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

experimental interaction of granitic and basaltic magmas and implications for mafic enclaves we have performed time series experiments for periods ranging from 3 min to 44 h on the interaction of granite melt and partially molten basalt at 920 c and 10 kbar in the presence of 5 wt water with time the assemblage of the basalt domain changes from predominantly amphibole plagioclase to clinopyroxene garnet the melt fraction increases from 2 5 to 40 and between the two domains the melt compositions progressively equilibrate initially in each run melts of the basalt domain have uniform plateau concentrations for sio 2 al 2o 3 cao mgo and feo because the activities of these components are regulated by the mineral assemblage but at advanced stages of reaction no such control is evident we have derived analytical expressions to describe and simulate the diffusion profiles the concentration profiles for sio 2 al 2o 3 cao and na 2o in the granite emanating from the basalt granite interface have been used to estimate effective diffusivities the values from the shorter runs are compared with those of the experiment of longest duration for which we assumed finite couples in our calculations in the diffusion calculations for k2o the difference in melt fraction between the two domains is accounted for the resulting values in cm 2 s are d na 2o 6 10 7 d k 2o 3 10 7 d mgo 9 10 8 d cao 4 6 10 8 and d sio 2 and d al 2o 3 3 0 6 10 8 they are in reasonable agreement with values from other studies on the basis of our experiments we calculate that mafic enclaves of magmatic origin should equilibrate to a large degree with their host magma in slowly cooling non convecting granitic plutons enclaves approaching complete re equilibration retain distinctly higher modal amounts of mafic minerals they do not compositionally resemble binary magma mixtures but are more like host magma with accumulated crystals we show that the modal differences between enclave and host are indicative of the temperature of homogenization and that in principle this temperature can be deduced from equilibrium phase diagrams

late mesozoic intrusive complexes in the coastal area of fujian se china the significance of the gabbro diorite granite association abstract the pingtan and daiqianshan igneous complexes southeastern china are typical of calc alkaline series developed at active continental margins these two complexes are dominated by granites or gabbros with minor diorites and granodiorites and these rock types closely coexisted both temporally and spatially the major and trace element signatures of gabbros indicate they were crystallised from high aluminium basaltic magmas which were generated from metasomatised upper mantle the typical metaluminous calc alkaline composition trace element patterns and sr nd isotopes indicate the granites in the complexes are i type granites which may have been generated from the deeper crust by a thermal contribution from the mantle field relationships petrography and geochemistry favour a mechanical mingling together with a limited chemical mixing between high aluminium basaltic magma and granitic magma resulting in the formation of diorites or granodiorites the essentially bimodal character of these complexes reflects the general character of back arc extensional magmatism which was induced by the subduction of the pacific plate beneath the eurasian continent in late mesozoic time

earthquakes in the davie ridge madagascar region and the southern nubian somalian plate boundary the largest earthquake sequence mb up to 6 4 and m0 up to 5 1018 nm to have struck eastern africa within the past 60 years occurred in may and june of 1985 off the coast of the tanzania mozambique border the epicenters are located on the northern extension of the davie ridge a prominent north south trending bathymetrie feature accompanied by distinct gravity anomalies a joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation al teleseismic distances p and sh waves generated by the two largest events of this sequence have nearly identical waveforms on the long period records of the worldwide standard seismograph network a formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north at least two point sources are necessary to model each event one at a depth of 40 10 km and the other at about 20 5 km on a regional scale normal faulting seems to characterize the scattered seismicity throughout the region between madagascar and the eastern arm of the east african rift zone even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified since normal faulting events of up to 25 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude we propose that the southern termination of the nubian somalian plate boundary consists of a diffuse zone of east west extension up to 2000 km wide

fault kinematics and stress fields in the rwenzori mountains uganda the rwenzori mountains in western uganda form an active rift transfer zone in the western branch of the east african rift system here we quantify local stress fields in high resolution from field observations of fault structures to shed light on the complex polyphase tectonics expected in transfer zones we apply the multiple inverse method which is optimized for heterogeneous fault slip data to the northern and central rwenzori mountains observations from the northern rwenzori mountains show larger heterogeneity than data from the central rwenzori including unexpected compressional features thus the local stress field indicates polyphase transpressional tectonics we suggest that transpression here is linked to rotational and translational movements of the neighboring victoria block relative to the rwenzori block that includes strong overprinting relationships stress inversions of data from the central rwenzori mountains indicate two distinct local stress fields these results suggest that the rwenzori block consists of smaller blocks

detection of upper mantle flow associated with the african superplume a continental scale low seismic velocity anomaly in the mid to lower mantle beneath africa is a robust feature of global tomographic models assuming the low velocities are associated with warm less dense material the african seismic anomaly has been ascribed to a long lived thermal upwelling from the lower mantle such a large scale upwelling should also affect the regional horizontal flow field in the upper mantle to test this model we compare seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography we calculate splitting parameters at 13 ocean island stations surrounding africa splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy we tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions we find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations 500 km from a ridge axis they are a poor fit to the data at stations located farther off axis thus we conclude that far from a ridge axis the observed anisotropy is dominated by asthenospheric flow to test for an active component of mantle upwelling we considered several models with varying assumptions about the velocity at the base of the asthenosphere that it is 1 stationary below plates moving in the no net rotation nnr or hotspot reference frames 2 driven by plate motions at the earth s surface or 3 driven by a combination of plate motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction we find that the best fitting flow field is generated by plate motions and density heterogeneity associated with large scale upwelling originating in the lower mantle beneath southern africa and is manifest as a radial pattern of flow at the base of the asthenosphere this model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction the resulting sub asthenospheric flow field is estimated to have velocities of 0 3 cm year and an asthenospheric viscosity of 3 1019 pa s is found to be most consistent with the regional anisotropy geoid height and dynamic topography

mineralogy and geochemistry of microgranular enclaves in palaeoproterozoic malanjkhand granitoids central india evidence of magma mixing mingling and chemical equilibration palaeoproterozoic ca 2 480 ma felsic magmatism of malanjkhand region of central indian precambrian shield referred to as malanjkhand granitoids mg contain xenoliths of country rocks and mesocratic to melanocratic fine grained porphyritic microgranular enclaves me the shape of me is spheroidal ellipsoidal discoidal elongated and lenticular varying in size from a few centimeters to about 2 m across the contact of me with the host mg is commonly sharp crenulate and occasionally diffuse which we attribute to the undercooling and disaggregation of me globules within the cooler host mg the me as well as mg show hypidiomorphic texture with common mineral hbl bt kfs pl qtz assemblage but differ in modal proportions the variation in minerals composition presence of apatite needles elongated biotites resorbed plagiclase ocellar quartz and other mafic felsic xenocrysts strongly oppose the restite and cognate origins of me compositions of plagioclases an3 an29 amphiboles mg mg fe2 0 55 0 69 and biotites mg mg fe2 0 46 0 60 of me are slightly distinct or similar to those of mg which suggest partial to complete equilibration during mafic felsic magma interactions al in amphibole estimates the mg pluton emplacement at ca 3 4 0 5 kbar and therefore magma mixing and mingling must have occurred at or below this level the hbox fe rightleftharpoons hbox mg substitution in biotites of me and mg largely suggests subduction related calc alkaline metaluminous i type nature of felsic melts most major and trace elements against sio2 produce near linear variation trends for me and mg probably generated by the mixing of mafic and felsic magmas in various proportions trace including rare earth elements patterns of me mg pairs however show partial to complete equilibration most likely governed by different degrees of elemental diffusion the available evidence supports the model of me origin that coeval mafic enclave and felsic mg magmas produced a hybrid me magma layer which injected into cooler partly crystalline mg and dispersed mingled and undercooled as me globules in a convectively dynamic magma chamber

itrf2000 a new release of the international terrestrial reference frame for earth science applications 1 for the first time in the history of the international terrestrial reference frame the itrf2000 combines unconstrained space geodesy solutions that are free from any tectonic plate motion model minimum constraints are applied to these solutions solely in order to define the underlying terrestrial reference frame trf the itrf2000 origin is defined by the earth center of mass sensed by satellite laser ranging slr and its scale by slr and very long baseline interferometry its orientation is aligned to the itrf97 at epoch 1997 0 and its orientation time evolution follows conventionally that of the no net rotation nnr nuvel 1a model the itrf2000 orientation and its rate are implemented using a consistent geodetic method anchored over a selection of itrf sites of high geodetic quality ensuring a datum definition at the 1 mm level this new frame is the most extensive and accurate one ever developed containing about 800 stations located at about 500 sites with better distribution over the globe compared to past itrf versions but still with more site concentration in western europe and north america about 50 of station positions are determined to better than 1 cm and about 100 sites have their velocity estimated to at or better than 1 mm yr level the itrf2000 velocity field was used to estimate relative rotation poles for six major tectonic plates that are independent of the trf orientation rate a comparison to relative rotation poles of the nuvel 1a plate motion model shows vector differences ranging between 0 03 and 0 08 m y equivalent to approximately 1 7 mm yr over the earth s surface itrf2000 angular velocities for four plates relative to the pacific plate appear to be faster than those predicted by the nuvel 1a model the two most populated plates in terms of space geodetic sites north america and eurasia exhibit a relative euler rotation pole of about 0 056 0 005 m y faster than the pole predicted by nuvel 1a and located about 10 n 7 e more to the northwest compared to that model

influence of the tectosphere upon plate motion this paper tests two hypotheses for the influence of thick lithosphere the tectosphere upon plate motion the first is that the tectosphere reaches down into high viscosity regions of the asthenosphere effectively slowing the motion of continental plates the second is that the tectosphere reaches down below the slow moving convective boundary layer into regions of rapid mantle convection and effectively speeds or slows the motion of continental plates our data imply that the tectosphere speeds plate motion rather than impedes plate motion and are thus most consistent with the second hypothesis

the somalia plate and the east african rift system present day kinematics summary the motion of the somalia plate relative to the nubia africa arabia and antarctica platcs is re evaluated using a new inversion method based on a monte carlo technique and a least absolute value misfit criterion a subset of the nuvel 1 data set with additional data along the levant fault and in the red sea is used the results confirm that the motion of arabia with respect to africa is significantly different from the motion relative to somalia it is further shown that the data along the sw indian ridge are compatible with a pole of relative motion between africa and somalia located close to the hypothetical diffuse triple junction between the ridge and thc east african rift the resulting africa somalia motion is then compatible with the geological structures and seismological data along the east african rift system assuming a separate somalia plate thus solves kinematic and geological problems around the afar triple junction and along the east african rift

anomalous seafloor mounds in the northern natal valley southwest indian ocean implications for the east african rift system abstract the natal valley southwest indian ocean has a complicated and protracted opening history as has the surrounding southwest indian ocean recently collected multibeam swath bathymetry and 3 5 khz seismic data from the natal valley reveal anomalous seafloor mounds in the northern natal valley the significance of these domes as recorders of the geological history of the natal valley and se african margin has been overlooked with little attempt made to identify their origin evolution or tectonic significance this paper aims to describe these features from a morphological perspective and to use their occurrence as a means to better understand the geological and oceanographic evolution of this basin the seafloor mounds are distinct in both shallow seismic and morphological character from the surrounding seafloor of the natal valley between 25 km and 31 km long and 16 km and 18 km wide these features rise some 400 m above the sedimentary deposits that have filled in the natal valley such macro scale features have not previously been described from the natal valley or from other passive margins globally they are not the result of bottom water circulation salt tectonics rather igneous activity is favoured as the origin for these anomalous seafloor features we propose a hypothesis that the anomalous seafloor mounds observed in the natal valley are related to igneous activity associated with the ears the complicated opening history and antecedent geology coupled with the southward propagation of the east african rift system creates a unique setting where continental rift associated features have been developed in a marine setting

a new velocity field for africa from combined gps and doris space geodetic solutions contribution to the definition of the african reference frame afref 1 we analyzed 16 years of gps and 17 years of doppler orbitography and radiopositioning integrated by satellite doris data at continuously operating geodetic sites in africa and surroundings to describe the present day kinematics of the nubian and somalian plates and constrain relative motions across the east african rift the resulting velocity field describes horizontal and vertical motion at 133 gps sites and 9 doris sites horizontal velocities at sites located on stable nubia fit a single plate model with a weighted root mean square residual of 0 6 mm yr maximum residual 1 mm yr an upper bound for plate wide motions and for regional scale deformation in the seismically active southern africa and cameroon volcanic line we confirm significant southward motion 1 5 mm yr in morocco with respect to nubia consistent with earlier findings we propose an updated angular velocity for the divergence between nubia and somalia which provides the kinematic boundary conditions to rifting in east africa we update a plate motion model for the east african rift and revise the counterclockwise rotation of the victoria plate and clockwise rotation of the rovuma plate with respect to nubia vertical velocities range from 2 to 2 mm yr close to their uncertainties with no clear geographic pattern this study provides the first continent wide position velocity solution for africa expressed in international terrestrial reference frame itrf2008 a contribution to the upcoming african reference frame afref except for a few regions the african continent remains largely under sampled by continuous space geodetic data efforts are needed to augment the geodetic infrastructure and openly share existing data sets so that the objectives of afref can be fully reached

experiments on the kinetics of partial melting of a leucogranite at 200 mpa h 2 o and 690 800 c compositional variability of melts during the onset of h 2 o saturated crustal anatexis we have experimentally investigated the kinetics of melting of an aplitic leucogranite quartz sodic plagioclase of ab90 k feldspar traces of biotite at 690 740 and 800 c all at 200 mpa h2o leucogranite cylinders 3 5 mm in diameter and 7 mm in length were run in the presence of excess h2o using cold seal pressure vessels for 11 2 925 h at 690 and 740 c and any experimental time and 800 c and short run times silicate glass melt at run conditions occurs as interconnected films along most of the mineral boundaries and in fractures with the predominant volume occurring along quartz feldspars boundaries and quartz plagioclase k feldspar triple junctions glass film thickness is roughly constant throughout a given experimental charge and increases with experimental temperature and run duration the results indicate that h2o saturated partial melting of a quartzo feldspathic protolith will produce an interconnected melt phase even at very low degrees 5 vol of partial melting crystal grain boundaries are therefore completely occluded with melt films even at the lowest degrees of partial melting resulting in a change in the mechanism of mass transport through the rock from advection of aqueous vapor to diffusion through silicate melt at 690 and 740 c the compositions of glasses are homogeneous and at both temperatures close to but not on the h2o saturated 200 mpa haplogranite eutectic glass compositions do not change with run duration at 800 c glasses are heterogeneous and plot away from the minimum although their molar ratios asi mol al2o3 cao na2o k2o and al na are constant throughout the entire charge at any experimental time glass compositions within individual 800 c experiments form linear trends in wt normative quartz albite orthoclase space the linear trends are oriented perpendicular to the 200 mpa h2o haplogranite cotectic line reflecting nearly constant albite orthoclase ratio versus variable quartz feldspar ratio and have endpoints between the 800 c isotherms on the quartz and feldspar liquidus surfaces with increasing experimental duration the trends migrate from the potassic side of the minimum toward the bulk rock composition located on the sodic side due to more rapid and complete dissolution of k feldspar relative to plagioclase the results indicate that partial melting at or slightly above the solidus 690 740 c is interface reaction controlled and produces disequilibrium melts of near minimum composition that persist metastably for up to at least 3 months relict feldspars show no change in composition or texture and equilibration between melt and feldspars might take from a few to tens of millions of years partial melting at temperatures well above the solidus 800 c produces heterogeneous disequilibrium liquids whose compositions are determined by the diffusive transport properties of the melt and local equilibrium with neighboring mineral phases feldspars recrystallize and change composition rapidly partial melting and equilibration between liquids and feldspars might take from a few to tens of years h2o saturated conditions at these temperatures well above the solidus

location of the nubia somalia boundary along the southwest indian ridge the northern boundary between the nubian west african and somalian east african plates is marked by the existence of the east african rift which is the locus of roughly east west stretching due to the separation of these plates south of 20 s any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle although the boundary must continue until it intersects another plate boundary until recently there were no observational limits on the location of that intersection and the recent limits were very broad being thousands of kilometers wide here from an analysis of the locations of the old edge of magnetic anomaly 5 which is over 11 ma seafloor we show that the main locus of deformation between nubia and somalia near the southwest indian ridge over the past 11 m y can be no wider than a few hundred kilometers from 100 km west of the du toit fracture zone to 50 km east of the andrew bain fracture zone complex deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the andrew bain fracture zone complex which is only 100 km wide the displacement along the nubia somalia boundary over the past 11 m y is 23 km 6 km 95 confidence limits indicating a displacement rate of 2 mm yr much slower than typical rates of seafloor spreading and subduction but comparable to that along the owen fracture zone and dalrymple trough which separate the arabian plate from the indian plate and along the azores gibraltar line which separates the eurasian plate from the nubian plate

erosion of the rwenzori mountains east african rift from in situ produced cosmogenic10be 1 high relief and steep topography are thought to result in high erosion rates in the rwenzori mountains of the albert rift east africa where more than 3 km of relief have formed during uplift of the rwenzori fault block overall low denudation rates prevail we measured in situ derived cosmogenic denudation rates of 28 2 to 131 mm kyr in mountainous catchments and rates of 7 8 to 17 7 mm kyr on the adjacent low relief east african plateau these rates are roughly an order of magnitude lower than in other settings of similar relief we present an extensive geomorphological analysis and find that denudation rates are positively correlated with relief hillslope gradient and channel steepness indicating that river incision controls erosional processes in most upper headwater reaches above quaternary ela levels 4500 m a s l glacial imprinting inherited from several older and recent minor glaciation stages prevails in regions below 4500 m a s l however mild climatic conditions impede frost shattering favor dense vegetation and minimize bare rock areas and associated mass wasting we conclude that erosion of the rwenzori mountains is significantly slower than corresponding rates in other mountains of high relief due to a combination of factors extremely dense mountain cloud forest vegetation high rock strength of gneiss and amphibolite lithologies and low internal fracturing due to the extensional tectonic setting this specific combination unique to this extensional tropical setting leads to unexpected low erosion rates that cannot outpace post pliocene ongoing rock uplift of the rwenzori fault block

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

the genesis of i and s type granitoid rocks of the early ordovician oledo pluton central iberian zone central portugal abstract the early ordovician oledo pluton consists of four distinct granodioritic to granitic phase g1 g4 which intruded a cambrian schist metagraywacke complex but were themselves intruded by a late carboniferous pluton id tims u pb ages for zircon and monazite from these granitic rocks indicate emplacements within a short period of time at 479 480 ma granodiorite g1 is the most deformed rock with shear zones and deformation at the border g1 and g3 contain fine grained biotite tonalite and biotite granodiorite microgranular enclaves which are darker and richer in mafic minerals than the host granodiorites the geological mineralogical geochemical and sr nd and o isotopic data show that tonalitic and granodioritic enclaves and host g1 are of i type and were related predominantly by a fractional crystallization process least square analysis of major elements and modelling of trace elements indicate that granodioritic enclaves and host g1 could be derived from the tonalitic enclave magma by fractional crystallization of plagioclase grunerite biotite and ilmenite granodiorite g2 is of hybrid origin most variation diagrams for granodioritic enclaves and host g3 granodiorite and their biotites show linear trends modelling of major and trace elements of granodioritic enclaves indicate that they result from mixing of relatively primitive granodiorite magma with magma derived from crustal melting tonalitic enclaves correspond to globules of a more mafic relatively primitive magma granite g4 has the most pronounced crustal signature and is of s type

formation of composite dykes by contact remelting and magma mingling the shaluta pluton transbaikalia russia abstract a unique opportunity to study the source areas from which composite dykes were injected occurs in the shaluta pluton transbaikalia russia the major quartz syenite pluton was intruded by several synplutonic gabbro bodies of various sizes investigations of the contact zones between gabbro and host syenite showed that liquid basalt magma intruded the incompletely crystallized coarse grained quartz syenite with t 700 720 c and caused contact remelting of the silicic rock at about 900 950 c mechanical interaction between newly formed silicic melt and partially crystallized mafic magma resulted in extensive magma mingling chemical interaction was exhibited by migration of mgo cao feo sr h2o and cl from the basalt magma whereas silica alkalis rb and ba migrated from the silicic refusion zone into the crystallized gabbro presence of melt inclusions with homogenization temperature ranging from 640 to 790 c in quartz and attaining 850 900 c in late clinopyroxene indicates that at least part of newly formed minerals crystallized from the hybrid melt mingled magmatic material was squeezed out inwards into the host solid quartz syenite pluton and formed dyke like apophyses that can be traced for a distance of 60 70 m from the contact zone apophyses have the same dimensions structure and composition as typical composite dykes that are common in the roof pendant over the gabbro bodies and nearby the gabbro exposures

progressive hybridisation between late devonian mafic intermediate and felsic magmas in the meguma zone of nova scotia canada two felsic plutons of late devonian 385 370 ma age in the meguma zone of southwestern nova scotia contain three circa 376 ma synplutonic mafic intermediate intrusions that collectively record progressive stages of in situ hybridisation a 5 m wide spessartite dyke in the port mouton pluton probably underwent rapid cooling and crystallisation immediately after intrusion which heated and coarsened the adjacent tonalite an 85 m long sheet of pillowed kersantite also in tonalite of the port mouton pluton presumably contained residual magma after quenching and obtained k p ba rb more radiogenic sr zr nb and light ree from the tonalite during magma mingling the third synplutonic body a 100 m wide diorite sheet intrudes granodiorite of the adjacent shelburne pluton and has a circa 45 m wide gradational contact of metaluminous hornblende tonalite this tonalite dominantly records magma mixing by the transfer of ti mg fe ca and v in hornblende biotite plagioclase and at least in part apatite xenocrysts derived from dioritic pillows that were originally disaggregated in the granodiorite probably in response to convection scattered data points unusual concave down variation trends for al2o3 p2o5 and sr and non hyperbolic sr nd isotopic characteristics in the tonalite apparently reflect syn or post mixing fractionation and accumulation of xenocrysts from residual magma phosphorus may have assisted diffusion of sr zr nb and light ree and caused premature quenching of the hybrids at mcleods cove and birchtown during magma mingling and mixing

water transfer during magma mixing events insights into crystal mush rejuvenation and melt extraction processes many plutons preserve evidence of magma mixing between hydrous mafic magmas and resident felsic crystal rich mushes to investigate water transfer processes in such systems following thermal equilibration we conducted 24 h experiments to establish the petrological evolution of a water undersaturated 4 wt h 2 o in the interstitial melt quartz bearing dacite crystal mush 0 5 0 8 in crystal fraction intruded by a water saturated 6 wt h 2 o initially crystal free andesite magma at 950 c and 4 kbar 12 km depth our results show isothermal undercooling resulting from a change in liquidus temperatures of the interacting magmas due to their changing water content specifically mafic samples dramatically crystallize during water escape into the felsic end members and consequent increase in liquidus temperature conversely the addition of water to the felsic mush reduces the liquidus temperature leading to an increase in melt fraction the experiments provide insights into how volatiles contribute to crystal mush rejuvenation i e increase of melt fraction however h 2 o diffusion alone is not sufficient to promote melt extraction from short and long lived mushes in the earth s crust

geochemistry and u pb shrimp zircon chronology of granitoids and microgranular enclaves from jhirgadandi pluton of mahakoshal belt central india tectonic zone india abstract the northern part of central india tectonic zone citz is delineated by an arc shaped supracrustal belt commonly referred to as mahakoshal belt which is considered as a product of intense rifting of sialic crust that occurred at ca 2400 2600 ma several granitoid plutons intrude the parsoi formation of mahakoshal belt among these an elliptical small stock like granitoid body trending e w is exposed in and around jhirgadandi region of mahakoshal belt referred herein as jhirgadandi pluton it is composed of minor amount of mafic rocks diorite and predominant granitoids country rock pelitic xenoliths and microgranular enclaves me are commonly hosted in granitoids but are absent in diorite the me exhibit typical magmatic texture with a bt cpx hbl pl kf qtz mag ap assemblage similar to that in host granitoids but with contrasting mineral proportions whole rock molar al 2 o 3 cao na 2 o k 2 o a cnk ratios of diorite 0 63 0 72 me 0 69 1 21 and granitoids 0 83 1 05 suggest their nature largely metaluminous i type to rarely peraluminous s type granitoids on most binary plots involving silica two distinct compositional paths can be recognized one formed by an array of differentiating diorite and me and another by fractionating granitoids gradually depleting in compatible elements it is most likely that me were generated by progressive and concurrent mixing of coeval pristine mafic diorite and granitoid magmas and fractionation processes however coherent and identical trace elements except for sr th y and ni and ree patterns for me granitoid pairs most likely suggest partial to near complete chemical equilibration through varying degrees of diffusion process across the me partly crystalline host granitoid boundary high precision u pb shrimp zircon 206 pb 238 u ages for me 1758 19 ma and host granitoid 1753 9 1 ma from jhirgadandi pluton further support the notion that they were coeval the obtained age 1750 ma of jhirgadandi pluton also points to the existence and role of super columbian continental component in the evolution of mahakoshal belt of the citz

growth and demise of cenozoic isolated carbonate platforms new insights from the mozambique channel seamounts sw indian ocean although long term evolutions of isolated shallow water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades their driving processes are still the subject of active debates the mozambique channel sw indian ocean is characterized by several flat topped seamounts ranging from 11 s to 21 s in latitudes based on a comprehensive geomorphologic study and on dredged samples analysis we show that these features correspond to tropical isolated shallow water carbonate platforms coupling strontium isotopy and foraminifera biostratigraphy well constrained chronostratigraphy results indicate that shallow water carbonate production started in the mozambique channel during distinct cenozoic periods ranging from paleocene to early miocene our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism while some of them kept developed until present days forming modern carbonate systems some others were drowned during late neogene and subsided to form guyots although different factors can be discussed tectonic and volcanism appear as good potential triggers for demise episodes during late miocene early pliocene times chronology and location of this geodynamical activity tend to emphasize influence of east african rift system until southern mozambique channel

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

mozambique earthquake sequence of 2006 high angle normal faulting in southern africa 1 we report source mechanisms for the six largest shocks of the mozambique earthquake sequence of 22 february 2006 the main shock of this sequence is one of the largest mw 7 0 to occur in africa over the past 100 years and its p waveforms alone are sufficient to show that north south trending normal faulting near the surface continues to depths of more than 15 km along an exceptionally steep dip of 76 4 this new result shows that globally seismogenic normal faulting spans a wide range of dips from about 30 to 75 s waveforms when combined with a minor component of left lateral slip observed in the field indicate the rake to be between 80 and 89 which in turn places a new constraint on relative plate motions complications from a nonspecific rovuma microplate notwithstanding our results favor the euler pole between the somalia and nubia plates to lie southward of the epicenters

the loch scridain xenolithic sill complex isle of mull scotland fractional crystallization assimilation magma mixing and crustal anatexis in subvolcanic conduits

enclaves in the cadillac mountain granite coastal maine samples of hybrid magma from the base of the chamber the cadillac mountain intrusive complex is dominated by the introduction cadillac mountain granite and a 2 3 km thick section of interlayered enclaves occur in nearly all granites and have recently gabbroic dioritic and granitic rocks which occurs near the base of been the focus of many studies see reviews by vernon the granite the layered rocks record hundreds of injections of basaltic 1983 and didier barbarin 1991 although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma magmatic enclaves ranging in different sources there has been a growing consensus composition from 55 to 78 wt sio2 are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks the most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments their compositions can be best explained by periodic vernon 1984 1990 these can appropriately be termed replenishment mixing and fractional crystallization of basaltic magmatic enclaves their compositions are commonly magma at the base of the chamber the intermediate to silicic intermediate but can vary from basaltic to highly silicic enclaves formed by hybridization between the evolved basaltic magma magmatic enclaves occur in all types of granite and are and resident silicic magma there is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite instead alkaline i type granites in spite of partial reequilibration hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have nd isotopic commagmas at the base of the chamber enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics suggesting support their formation from mantle derived magmas they were derived from a discrete batch of hybrid magma enclaves holden et al 1987 were probably dispersed into a localized portion of the granitic the compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton and they can only layer rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma there is ample petrographic and chemical evidence in many enclaves for hybridization mixing of liquids and

re evaluation of the kinematics of victoria block using continuous gnss data the divergent boundary between the somalia and nubia plates is a complex tectonic domain where extensional processes are localized along narrow rift structures isolating small plates imbedded within the east african rift one of these tectonic units is the victoria plate which is the subject of this study here we process space geodetic data for 37 permanent gnss stations distributed along the nubia somalia and victoria plates to 1 compute the motion of the three plates in the itrf2008 reference frame and 2 deduce the relative motion of victoria with respect to its neighboring plates the nubia plate motion is computed from a set of 25 stations the somalia plate motion from a set of 7 stations and the victoria plate motion from a set of 5 stations the instantaneous relative euler poles for the nubia victoria and somalia victoria pairs are now evaluated as 10 6582on 32 9752eo 0 1196o myr and 8 0164os 32 2869oe 0 1589o myr respectively the computation of the relative inter plate velocities along victoria s plate boundary is straightforward in most situations because the western and northeastern boundary segments correspond to well developed rift basins where extension is mostly normal to rift basin flanks and seismicity concentrates along narrow structures this is particular evident on the western branch between victoria and nubia the southeastern limit of the victoria plate is poorly defined and geodetic data indicates that differential motion between somalia and victoria may be accommodated by a complex plate boundary area which roughly encompasses the masai terrain geodetic observations of the victoria somalia boundary along the eastern branch particularly in the manyara rift reveal highly oblique horizontal extension in this region seismicity is sparse which suggests that strain is accommodated by magmatic processes

the pan african toro complex northern nigeria magmatic interactions and structures in a bimodal intrusion the toro complex is one of the pan african older granites of nigeria first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim it has been thoroughly reinvestigated both from the petrographic and structural points of view with the help of systematic anisotropy of magnetic susceptibility ams measurements the granite main body is a hornblende biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations stage 1 this fabric is overprinted by a later one due to solid state strain along north south subvertical dextral shear zones stage 2 in the vicinity of the diorite an evengrained granite displays magmatic structures that are contemporaneous with this strike slip event the diorite granite contact is a complex zone where field petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma tonalites cropping out within this contact zone are interpreted as hybrid rocks the reverse zonation of the diorite itself is also the result of some hybridization process magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass the detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric before full crystallization of the granitic core intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters the second stage structures whether characterized by magmatic fabric near the diorite or by solid state strain features in north south shear zones elsewhere in the granite are related to late pan african dextral strike slip tectonics in the basement of northern nigeria the bimodal toro complex is therefore considered as a late pan african syntectonic pluton

experimental simulation of magma mixing at high pressure abstract magma mixing features are observed in many plutonic and volcanic environments they result from the juxtaposition of two chemically contrasted magmas usually not only during the replenishment of a magmatic reservoir but also syn eruptively within the conduit despite its ubiquity only a few experimental studies have explored mixing between magmas existing data have been mostly acquired at atmospheric pressure and high shear rates 10 1 s 1 which differ from those accompanying magma mixing in reservoirs to fill this gap we performed high pressure mixing experiments at strain rates ranging from 4 10 4 to 1 10 3 s 1 layers of a synthetic crystal free haplotonalite and a natural partially molten basalt were juxtaposed in a paterson apparatus at 300 mpa and deformed between 900 and 1200 c the experiments shed light on the first stages of magma mixing and illustrate the role and behaviour of crystals either pre existing or newly grown experiments evidence a rheological threshold for mafic material disruption which sets in abruptly as its melt fraction exceeds 50 which in the experiments occurs in the narrow temperature interval 1160 1170 c below this threshold plagioclase crystals in the mafic magma form a rigid touching network and all the deformations are accommodated by the less viscous felsic layer above it the crystal network collapses allowing typical mingling mixing features to appear altogether such as enclaves melt filaments and single xenocrysts isolated into the felsic end member coexisting with newly grown phases plagioclase and pyroxene whose compositions spread out over considerable ranges the pre existing fabric of the mafic magma is only slightly affected by deformation altogether providing few clues on either the regime or geometry of applied deformation during the magmatic stage

seismicity and seismotectonics of madagascar revealed by the 2011 2013 deployment of the island wide macomo broadband seismic array abstract the seismicity and seismotectonics of madagascar have been studied using an island wide distribution of broadband seismic stations the 28 station madagascar comoros mozambique macomo array was deployed for a 23 month period between 2011 and 2013 macomo data were supplemented by seven temporary stations from the seismological signatures in the lithosphere asthenosphere system of southern madagascar selasoma project ten temporary stations from the reunion hotspot and upper mantle reunions unterer mantel project rhum rum and 11 permanent stations a total of 695 earthquakes with magnitudes between ml1 and ml5 3 located within madagascar were recorded a new local magnitude scale was developed and focal mechanisms were determined for 23 well recorded events most of the seismicity is clustered within central madagascar forming nw se trends in the ankaratra region and ne sw trends in the alaotra and ankay regions that roughly align with extensional tectonic features the northern and southern parts of madagascar also show seismicity clusters that align parallel to existing tectonic features primarily precambrian shear zones focal mechanisms exhibit a wide orientation of nodal planes show predominantly normal faulting throughout madagascar and provide no evidence for a sharp east west striking plate boundary between the lwandle and somalian plates in the middle of the island however a diffuse plate boundary cannot be excluded we suggest that topographically generated extensional stresses is the cause of most seismicity given that significantly fewer earthquakes are located in the lower elevation areas of the island compared to regions of higher elevations the frequency magnitude distribution has a b value of 1 2 and indicates that events with magnitude ml5 or greater should occur with an average repeat time of 1 4 years however no earthquake exceeding ml6 has been recorded in the last century suggesting that the frequency magnitude distribution could be truncated

the mozambique ridge a document of massive multistage magmatism the mozambique ridge a prominent basement high in the southwestern indian ocean consists of four major geomorphological segments associated with numerous phases of volcanic activity in the lower cretaceous the nature and origin of the mozambique ridge have been intensely debated with one hypothesis suggesting a large igneous province origin high resolution seismic reflection data reveal a large number of extrusion centres with a random distribution throughout the southern mozambique ridge and the nearby transkei rise intrabasement reflections emerge from the extrusion centres and are interpreted to represent massive lava flow sequences such lava flow sequences are characteristic of eruptions leading to the formation of continental and oceanic flood basalt provinces hence supporting a large igneous province origin of the mozambique ridge we observe evidence for widespread post sedimentary magmatic activity that we correlate with a southward propagation of the east african rift system based on our volumetric analysis of the southern mozambique ridge we infer a rapid sequential emplacement between 131 and 125 ma which is similar to the short formation periods of other large igneous provinces like the agulhas plateau

convective crystal dissolution the effect of free and forced convection on crystal dissolution is examined both theoretically and experimentally well established relationships for heat and mass transfer are applied to obtain approximate expressions for the dissolution velocity and the associated thickness of the compositional boundary layer these expressions are found to be in good agreement with experimental observations of the dissolution of quartz crystals in basalt and nacl crystal in water when applied to light felsic crystals in basaltic magmas the expressions predict that forced convection will produce a boundary layer thickness of about 100 m and a dissolution velocity of order 10 6 cm s 1 these velocities are too slow for xenocrysts to be dissolved significantly during magma ascent in dykes but are sufficient for cm size crystals to dissolve in the interior of a convecting magma chamber larger crystals are likely to accumulate at the chamber s roof where free convection is predicted to dissolve them at velocities of order 10 7 cm s 1 in an appendix the dissolution of the chamber s walls is also considered and a velocity of order 10 8cm s 1 is predicted

middle miocene to pleistocene sedimentary record of rift evolution in the southern albert rift uganda this study presents an almost complete middle miocene to pleistocene sequence of synrift sediments in the western branch of the east african rift the studied succession is exposed in several patches on an eastward tilted block between the northern tip of the rwenzori block and the eastern shoulder of the albert rift in this position it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across east africa the synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations lithofacies and architectural analysis indicate alluvial plain delta plain nearshore delta front or lacustrine depositional environments based on the vertical stacking pattern prograding and retrograding trends of the depositional environments and climatic indicators e g conservation of feldspar gypsum and or iron hydroxide precipitation four evolutionary phases can be distinguished i a first phase between ca 14 5 and 10 0 ma is characterised by bedload dominated fluvial environment with massive sandy to gravelly bedforms feldspar rich sands rare iron impregnations and relatively low accommodation space this phase is interpreted as pre and early synrift sedimentation under a semiarid climate ii from ca 10 0 to 4 5 ma predominantly fine grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space fluctuation of thin beds dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity iii during the third phase between 4 5 and 2 0 ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred indicating a humid period with lake level highstands and accelerated subsidence iv during the final sedimentary interval between 2 0 and 1 5 ma gravel units reoccurred with less iron but more carbonate and gypsum impregnations and arkosic sandstones this phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the rwenzori mountains together with accelerated rift flank uplift and strong subsidence of the rift floor the results of this study are of particular importance for delineating key controls on sedimentation in the albert rift

magma generation and evolution and global tectonics an issue in honour of peter j wyllie for his life long contributions by means of experimental petrology to understanding how the earth works foreword

anatexis of interlayered amphibolite and pelite at 10 kbar effect of diffusion of major components on phase relations and melt fraction we have conducted h2o undersaturated melting experiments on charges consisting of a layer of a sillimanite bearing metapelite and a layer of garnet bearing amphibolite with the goal of studying how layered metamorphic rocks interact during anatexis experiments with the layered charges were performed at 10 kbar 1 gpa at 900 c 334 h 925 c 341 h and 950 c 50 161 332 h dehydration melting of the amphibolite alone and of the pelite alone were studied at the same p t conditions when run alone at 900 c the pelite and the amphibolite yielded ca 40 vol and 15 vol melt respectively corresponding melt fractions at 950 c were ca 50 vol and 30 vol when run side by side melt abundance in the pelite is fairly uniform and ca 20 vol higher than in the solo runs at all temperatures studied in contrast complex zoning patterns parallel to the interface develop in the amphibolite layer far from the interface the melt fraction is 5 10 vol lower than in the amphibolite solo runs closer to the interface the melt fraction in the amphibolite layers are comparable to or somewhat higher than in the amphibolite solo runs and both clinopyroxene and plagioclase react out and are replaced by orthopyroxene and quartz melt fraction closest to the interface reaches 65 70 vol and the only crystalline phase present is garnet with hollow cores the increase in melt fraction in the pelite layers of the layered runs is caused by transfer of na from the amphibolite to the pelite which forces biotite to be eliminated at lower temperature along the biotite garnet reaction boundary k is transferred from the pelite to the amphibolite and the lower melt fraction in the distal end of the amphibolite layers is caused by faster diffusion of na compared to k which results in a net depletion of alkalies ca is transferred from the amphibolite to the pelite and the resulting changes in cao concentrations affect the stabilities of plagioclase pyroxenes garnet and quartz and cause the observed zoning in the amphibolite layer ca entering the pelite layer induces crystallization of large plagioclase neoblasts radiating from the interface into the pelite layer these results show that diffusion of components between contiguous lithologies undergoing anatexis can cause profound changes in phase relations melt fractions and restite compositions

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

seismic anisotropy and mantle dynamics beneath the malawi rift zone east africa sks skks and pks splitting parameters measured at 34 seismic stations that we deployed in the vicinity of the cenozoic malawi rift zone mrz of the east african rift system demonstrate systematic spatial variations with an average splitting time of 1 0 0 3 s the overall ne sw fast orientations are consistent with absolute plate motion apm models of the african plate constructed under the assumption of no net rotation of the global lithosphere and are inconsistent with predicted apm directions from models employing a fixed hotspot reference frame they also depart considerably from the trend of most of the major tectonic features these observations together with the results of anisotropy depth estimation using the spatial coherency of the splitting parameters suggest a mostly asthenospheric origin of the observed azimuthal anisotropy the single layered anisotropy observed at 30 and two layered anisotropy observed at four of the 34 stations can be explained by apm related simple shear within the rheologically transitional layer between the lithosphere and asthenosphere as well as by the horizontal deflection of asthenospheric flow along the southern and western edges of a continental block with relatively thick lithosphere revealed by previous seismic tomography and receiver function investigations this first regional scale shear wave splitting investigation of the mrz suggests the absence of rifting related active mantle upwelling or small scale mantle convection and supports a passive rifting process for the mrz

dissolution of quartz albite and orthoclase in h2o saturated haplogranitic melt at 800 c and 200 mpa diffusive transport properties of granitic melts at crustal anatectic conditions we have conducted experiments on dissolution of quartz albite orthoclase and corundum into h2o saturated haplogranite melt at 800 c and 200mpa over a duration of 120 1488h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures cylinders of anhydrous starting glass and a single mineral phase quartz or feldspar were juxtaposed along flat and polished surfaces inside gold or platinum capsules with 10 wt added h2o concentration profiles in glass quenched melt perpendicular to the mineral glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384h and with longer durations the concentration profiles are controlled only by diffusion of components in the melt the evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space sio2 n a2o and k2o axes si na and k eigenvectors respectively and a direction between the al2o3 na2o and k2o axes al eigenvector such that the al na molar ratio is equal to that of the bulk melt and the al na k molar ratio is equal to the equilibrium asi mol al2o3 na2o k2o of the melt experiments in which a glass cylinder was sandwiched between two mineral cylinders quartz and albite quartz and k feldspar or albite and corundum tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients the application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data provides diffusivities along the directions of the si eigenvector and al eigenvector of 2 0 2 8 10 15 m 2 s and 0 6 2 4 10 14 m 2 s respectively minimum diffusivities of alkalis 3 9 10 11 m 2 s are orders of magnitude greater than the tetrahedral components of the melt the information provided here determines the rate at which crustal anatexis can occur when sufficient heat is supplied and diffusion is the only mass transport mixing process in the melt the calculated diffusivities imply that a quartzo feldspathic source rock with initial grain size of 2 3mm undergoing hydrostatic h2o saturated melting at 800 c infinite heat supply could produce 20 30 vol of homogeneous melt in less than 1 10 years slower diffusion in h2o undersaturated melts will increase this time frame

origins of igneous microgranular enclaves in granites the example of central victoria australia to investigate their genesis and relations with their host rocks we study igneous microgranular enclaves imes in the c 370 ma post orogenic high level felsic plutons and volcanic rocks of central victoria australia the imes are thermally quenched magma globules but are not autoliths and they do not form mixing series with their host magmas these imes generally represent hybrids between mantle derived magmas and very high t crust derived melts modified by fractionation ingestion of host derived crystals and to a lesser extent by chemical interactions with their hosts isotopic and elemental evidence suggests that their likely mafic progenitors formed by partial melting of subcontinental mantle but that the ime suites from different felsic host bodies did not share a common initial composition we infer that melts of heterogeneous mantle underwent high t hybridisation with melts from a variety of crustal rocks which led to a high degree of primary variability in the ime magmas our model for the formation of the central victorian imes is likely to be applicable to other occurrences especially in suites of postorogenic granitic magmas emplaced in the shallow crust however there are many different origins for the mingled magma globules that we call imes and different phenomena seem to occur in differing tectonic settings the complexity of ime formation means that it is difficult to unravel the petrogenesis of these products of chaotic magma processes nevertheless the survival of fine grained non equilibrium mineralogy and texture in the imes suggests that their tenure in the host magmas must have been geologically brief

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

rift nucleation rift propagation and the creation of basement micro plates within active rifts abstract in this contribution we study the dynamics of rift nucleation and the interaction of propagating rift segments we use the east african rift system ears as a case study with special emphasis on the albertine rift system situated within the western branch of the ears with the 5000 m high rwenzori horst a basement block that was captured by two rift segments fieldwork in the rwenzori mountains shows that the range is not only affected by range parallel faults but that major fault sets cut across the rwenzoris in order to understand these complex fault patterns and the dynamics of the process that leads to the capturing of basement blocks within the extending rift system we developed a two dimensional visco elasto plastic spring model the model shows that propagating rift faults commonly capture micro plates with shapes similar to the rwenzori mountains these micro plates rotate during successive rift opening in a clockwise or anti clockwise sense depending on the direction of the rift segment overstep a left lateral overstep as is the case for the albertine rift system produces a clockwise rotation of the associated micro plate indicating that the rwenzori mountains rotate clockwise during the rotation of the micro plate the local stress field at the rift segment tips is perturbed so that the rifts turn 70 to 90 and propagate towards each other to capture the plates completely this perturbed stress field produces an extensional fault set that is oriented almost parallel to the far field extension direction and explains the range cutting faults that we find in the rwenzori mountains the large scale geometry of the rift system seems to be strongly controlled by rigid cratons in the lithosphere leading to a curvature of the western branch of the ears and a sinistral wrench component in the lake albert rift north of the rwenzori mountains capturing of micro plates of various sizes within the rift system is a common phenomenon and leads to strong perturbations of the local stress field during micro plate rotation the resulting structures are normal and strike slip faults that cut through the micro plates and local reverse faults

mantle upwellings melt migration and the rifting of africa insights from seismic anisotropy the rifting of continents and eventual formation of ocean basins is a fundamental component of plate tectonics yet the mechanism for break up is poorly understood the east african rift system ears is an ideal place to study this process as it captures the initiation of a rift in the south through to incipient oceanic spreading in north eastern ethiopia measurements of seismic anisotropy can be used to test models of rifting here we summarize observations of anisotropy beneath the ears from local and teleseismic body waves and azimuthal variations in surface wave velocities special attention is given to the ethiopian part of the rift where the recent eagle project has provided a detailed image of anisotropy in the portion of the ethiopian rift that spans the transition from continental rifting to incipient oceanic spreading analyses of regional surface waves show sub lithospheric fast shear waves coherently oriented in a north eastward direction from southern kenya to the red sea this parallels the trend of the deeper african superplume which originates at the core mantle boundary beneath southern africa and rises towards the base of the lithosphere beneath afar the pattern of shear wave anisotropy is more variable above depths of 150 km analyses of splitting in teleseismic phases sks and local shear waves within the rift valley consistently show rift parallel orientations the magnitude of the splitting correlates with the degree of magmatism and the polarizations of the shear waves align with magmatic segmentation along the rift valley analysis of surface wave propagation across the rift valley confirms that anisotropy in the uppermost 75 km is primarily due to melt alignment away from the rift valley the anisotropy agrees reasonably well within the pre existing pan african lithospheric fabric an exception is the region beneath the ethiopian plateau where the anisotropy is variable and may correspond to pre existing fabric and ongoing melt migration processes these observations support models of magma assisted rifting rather than those of simple mechanical stretching upwellings which most probably originate from the larger superplume thermally erode the lithosphere along sites of pre existing weaknesses or topographic highs decompression leads to magmatism and dyke injection that weakens the lithosphere enough for rifting and the strain appears to be localized to plate boundaries rather than wider zones of deformation

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

mafic injections in situ hybridization and crystal accumulation in the pyramid peak granite california located in the northern sierra nevada the middle jurassic pyramid peak granite and associated dioritic rocks display superbly exposed interlayered sheets of mafic felsic and hybrid rocks the mafic hybrid layers show a consistent asymmetry sharply chilled on one side and gradational through hybrids to granite on the other and dip to the southwest load casts and silicic pipes along the chilled mafic northeastern margins of the sheets indicate that depositional way up is to the southwest and that the adjacent mount tallac roof pendant was the original floor of the intrusion orientations of the pipes indicate that the layers were tilted after deposition and solidification we interpret the sheets to be intramagmatic flows that formed by injections of mafic magma into a silicic magma chamber followed by flow across the crystal rich base of that chamber the granitic and hybrid rocks interlayered with the mafic sheets represent material that accumulated on the chamber floor between episodes of mafic replenishment most contamination of the mafic sheets probably occurred because of flow front instabilities generated as the mafic magma entered and spread across the chamber floor the upward gradation from diorite to granite with decreasing abundance and size of mafic enclaves can best be explained by convective stirring at the upper boundaries of the dioritic replenishments the pyramid peak granite was probably initiated as a thin concordant sheet intruding into gently dipping jurassic supracrustal rocks the pluton was constructed incrementally with many replenishments of felsic and mafic magma and dominantly solidified by accumulation of crystals on the floor of the chamber the present steep dips of the layers suggest that downward warping of the chamber floor may have been a factor in accommodating the incrementally constructed pluton

evidence for a post 3 16 ma change in nubia eurasia north america plate motions we combine updated gps velocities from the nubian nu eurasian eu and north american na plates with 500 new 3 16 myr average seafloor spreading rates and nine transform fault azimuths from the northern atlantic and arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates the numerous new seafloor spreading rates and gps velocities improve our ability to detect recent changes in the relative motions of these plates the angular velocity vector that best fits the eu na gps velocities lies significantly north of the 3 ma average pole in accord with previously published geologic evidence that the eu na pole has migrated northward since v3 ma although we also find evidence for a significant post 3 ma change in nu na motion it is less compelling because the nubian plate gps velocity field is sparse and nu na seafloor spreading rates appear to have remained steady within the 1 mm yr 31 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations the nu eu pole derived from gps site velocities lies more than 30 angular degrees south of the tightly constrained 3 ma average estimate and predicts significantly slower and more oblique present day nu eu convergence in the mediterranean both models for nu eu motion pass a key test for their accuracy namely they correctly predict strike slip motion along the well mapped gloria fault east of the azores the change to more oblique nu eu motion may reflect increasing difficulty in maintaining margin normal convergence within this continent continent collision zone 8 2003 published by elsevier b v

experimental mixing of hydrous magmas deformation experiments involving hydrous magmas of different compositions basalt and haplotonalite have been performed in a paterson press at 300 mpa in the temperature range 600 c 1020 c with water saturated melts during 2 4 h prior to deformation the two end member magmas were annealed at either 950 c or 1000 c yielding magmas with crystal contents in the range 31 53 wt and 2 sets of viscosity contrasts under the experimental conditions investigated i e moderate shear rates 950 c in the temperature range 950 985 c a few mixing and mingling textures occur though both end members essentially retain their physical integrity it is only at or above 1000 c that a dramatic jump in mingling efficiency happens corresponding to a crystal fraction of 45 vol textures include entrainment of mafic crystals into the felsic magma mafic felsic banding enclave formation and diffusion induced interface the latter only over limited distances 300 m due to the short run durations in the most strained parcels of interacting magmas complex mixing mingling textures were produced similar to those observed in volcanic and plutonic rocks in arc settings the experiments show that mixing between hydrous felsic and mafic magmas takes place at around 1000 c a temperature which is almost 200 c lower than mixing under dry conditions magma mixing is commonly invoked as a trigger for volcanic eruptions our experiments suggest that such eruptions can be driven by small 15 c temperature fluctuation in the reservoir our results also suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result in stratification between end members rather than in a homogeneous mixture

petrology of an unusual orthopyroxene bearing minette suite from southeastern peru eastern andean cordillera al rich lamproites contaminated by peraluminous granites abstract the puno minettes eastern andean cordillera southern peru are phlogopite phyric rocks with a groundmass consisting mainly of phlogopite alkali feldspar and apatite microphenocrysts and or microlites they display the strong large ion lithophile element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan rb ba sr up to 103 primordial mantle ree up to 6 102 ci chondrites in addition to their close association with granitoids the rocks of this series show many features of minettes occurring in arc related collisional zones i e absence of mantle xenoliths up to 30 vol felsic xenocrysts and strong negative nb and ti anomalies however they are distinguished by unusual phenocryst phases whole rock compositions and groundmass mineralogy some minettes contain only a few per cent of biotite quartz plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite biotite quartz plagioclase alkali feldspar cordierite zircon pyrite in a high k latitic to trachydacitic groundmass field relations radiometric ages whole rock data and felsic xenocrysts all indicate that the puno minettes result from a mixing process involving al rich lamproites and peraluminous monzogranites the most common phenocryst assemblage highly magnesian phlogopite olivine titanian cr spinel ca rich enstatite magnesian ilmenite al and ti poor diopside is similar to that reported in al rich phlogopite lamproites from southeastern spain likewise the host rocks have diagnostic features of spanish phlogopite lamproites e g k2o al2o3 ratios 9 cao contents

calibration of soil constitutive models with spatially varying parameters soil constitutive models are frequently calibrated from laboratory tests that utilize global boundary measurements which necessarily relegate soil response to that of a homogenized equivalent medium this paper demonstrates the applicability of advanced experimental technologies to enhance the state of model based predictions in soil mechanics by taking into account the possibility of material heterogeneity during model calibration by utilizing the full field displacement measurement technique of three dimensional digital image correlation displacements of the surfaces of deforming triaxial sand specimens are measured throughout deformation these displacements are assimilated into finite element fe models of the test specimen through solution of an inverse problem during optimization in which the difference between measured and predicted displacements across the specimen surface form the basis for the objective function model parameters are allowed to vary spatially throughout the specimen volume fe models allowing three different levels of spatial variability are tested results indicate that accommodating consideration of material heterogeneity during calibration leads to more accurate predictions of global stress strain behavior that are more faithful to observed full field response

effects of diffusion of water and migration of melts in crustal rocks an experimental study abstract water and melts are crucial in controlling the chemistry architecture and dynamics of the earth s crust however how water and melts influence chemical and textural changes of pervaded rocks during transport and how they chemically react with surrounding minerals remain cryptic we present experimental data illustrating how water undersaturated and potassium rich melt in partially molten felsic granulite or melt lost from such granulite affects the chemistry and texture of an initially nearly anhydrous eclogite at high pressure and high temperature 1 2 gpa 950 c for 20 h the first experiment simulates interaction of eclogite with granulite having a small proportion of melt melt 2 vol solids 98 vol with h2o 0 4 wt in the system granulite melt called in this paper granulite the second experiment simulates interaction of eclogite with melt melt 90 vol solids 10 vol h2o 4 wt in the system granulite melt called in this paper granulite partial melt our results show that during granulite eclogite interaction i the melt in the granulite crystallises as a consequence of water diffusion into the eclogite ii this water causes partial replacement of garnet by plagioclase orthopyroxene magnetite amphibole symplectite whose volumetric proportion correlates with water content in the granulite and iii a small proportion of melt from the granulite 2 vol infiltrates into the eclogite as documented by mineral overgrowths in granulite and by glass pools with increased potassium content in symplectites and along grain boundaries in the eclogite water and melt transfer appears to be effective in causing textural changes but appears to be ineffective in changing the bulk chemistry of the eclogite even if local enrichment in potassium and water occurs such chemically stealthy diffusion of water and migration of limited amount of melt is able to cause major textural changes accompanied by localised chemical changes at grain scale that would not be otherwise observed at the scale of the bulk rock composition

magmatic cycles pace tectonic and morphological expression of rifting afar depression ethiopia the existence of narrow axial volcanic zones of mid oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain as a result of repeated diking and faulting axial volcanic zones therefore represent a spectacular topographic expression of plate divergence however the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space in this study we use the dabbahu manda hararo dmh magmatic rift segment afar ethiopia to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries the dmh magmatic rift segment is considered an analogue of an oceanic ridge exhibiting a fault pattern extension rate and topographic relief comparable to intermediate to slow spreading ridges here we focus on the northern and central parts of dmh rift where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent september 2005 dike intrusion the data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism suggestive of reduced magmatic activity probably in association with changes in strain distribution in the crust our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment and thus to the magmatic cycles of replenishment differentiation 100 kyr our findings are also consistent with magma induced deformation in magma rich rift segments the record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the dmh segment

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

the interplay between crystallization replenishment and hybridization in large felsic magma chambers abstract while hybridized granitoid magmas are readily identifiable the mechanisms of hybridization in large crustal magma chambers are so not clearly understood characteristic features of hybrid granitoids are 1 both the granitoid and included enclaves are commonly hybrids as shown by mineralogy geochemistry and isotopes 2 mixing seen in zoned plutons and synplutonic dykes and enclaves occurred early 3 zoned plagioclase phenocrysts commonly show very complex life histories of growth and dissolution 4 mafic end members in hybrids are commonly fractionated magmas and 5 stratification in subvolcanic granitoid magma chambers is not uncommon and stratification has been identified in some deeper level plutons hybridization must overcome the tendency to form a stable stratification of dense mafic magma underlying less dense felsic magma experimental work with magma analogues and theoretical considerations reveal very severe thermal rheological and dynamical limitations on mixing only very similar composition temperature magmas are likely to mix to homogeneity and only moderately silicic hybrids are likely to be produced however impossibly silicic hybrids do exist synchronous interactive fractional crystallization and hybridization may provide a mechanism for hybridization of magmas in the following manner a mafic magma intrudes into the base of a stratified felsic magma and is cooled against it crystallization of the upper boundary layer of the mafic magma yields an eventually buoyant residual melt that overturns and mixes with an adjacent stratum of the felsic magma chamber subsequently melt released by crystallization pf this now hybrid zone mixes with adjacent more felsic zones thus a suite of hybrid magmas are progressively formed density inhibitions are overcome by the generation of relatively low density residual melts as crystallization proceeds later injections are preserved as dykes and enclaves composed of hybrid magma in this process only physically adjacent and dynamically thermally similar magmas directly interact and so may mix to homogeneity finally not simply felsic and mafic endmembers mix but a whole suite of intermediate endmembers participate ranging from relatively mafic through to felsic pairs of magmas direct mixing between the primary magmas only occurs at the beginning

instantaneous global plate motion model from 12 years of continuous gps observations 1 we estimate a global plate motion model for 17 major and minor tectonic plates solely on the basis of analysis of data from 106 globally distributed continuous gps stations spanning the period from january 1991 to july 2003 site positions estimated from 24 hour segments of data are aligned day by day to the international gps service igs realization of the itrf2000 reference frame using a similarity transformation thereby ensuring that the itrf2000 no net rotation condition is preserved linear velocities of a carefully selected set of stations are estimated from the position time series along with annual and semiannual fluctuations and position offsets due to gps instrument changes a white noise plus flicker noise model is applied to estimate realistic uncertainties for the site velocities which are then propagated into the derived plate motion model parameters we also examine the vertical velocities in the site selection process at the scripps orbit and permanent array center we have implemented a procedure whereby the plate motion model is updated on a regular monthly basis to improve its precision and reliability as new data become available and as a baseline against which anomalous motions can be detected