shallow virtual source redatuming by multi dimensional deconvolution recently new on shore acquisition designs have been presented with multicomponent sensors deployed in the shallow sub surface 20 m 60 m virtual source redatuming has been proposed for these data to compensate for surface statics and to enhance survey repeatability in this paper we investigate the feasibility of replacing the correlation based formalism that undergirds virtual source redatuming with multi dimensional deconvolution offering various advantages such as the elimination of free surface multiples and the potential to improve virtual source repeatability to allow for data driven calibration of the sensors and to improve robustness in cases with poor sensor spacing in the shallow sub surface resulting in a relatively high wavenumber content we propose a new workflow for this configuration we assume a dense source sampling and target signals that arrive at near vertical propagation angles first the data are preconditioned by applying synthetic aperture source filters in the common receiver domain virtual source redatuming is carried out for the multi component recordings individually followed by an intermediate deconvolution step after this specific pre processing we show that the downgoing and upgoing constituents of the wavefields can be separated without knowledge of the medium parameters the source wavelet or sensor characteristics as a final step free surface multiples can be eliminated by multi dimensional deconvolution of the upgoing fields with the downgoing fields

textural and thermal history of partial melting in tonalitic wallrock at the margin of a basalt dike wallowa mountains oregon columbia river basalt group dikes invade biotite hornblende tonalite to granodiorite rocks of the wallowa mountains most dikes are strongly quenched against wallrock but rare dike segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal melting at maxwell lake the 4 m thick wallrock partial melt zone contains as much as 47 vol melt glass plus quench crystals around mineral reaction sites and along quartz feldspar boundaries bulk compositional data indicate that melting took place under closed conditions excepting volatiles with progressive melting hornblende biotite and orthoclase were consumed but plagioclase quartz and magnetite persisted in the restite clinopyroxene orthopyroxene plagioclase and fe ti oxides were produced during dehydration melting reactions involving hornblende and biotite reacting phases became more heterogeneous with progressive melting crystallizing phases were relatively homogeneous progressive melting produced an early clear glass followed by light high k and dark high ca brown glass domains overprinted by devitrification melts were metaluminous and granitic to granodioritic thermal modeling of the partial melt zone suggests that melting took place over a period of about 4 years thus rare dikes with melted margins represent long lived portions of the columbia river basalt dike system and may have sustained large flows

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

adaptive decomposition of multicomponent ocean bottom seismic data into downgoing and upgoing p and s waves with wavefield decomposition the recorded wavefield at a certain depth level can be separated into upgoing and downgoing wavefields as well as into p and s waves the medium parameters at the considered depth level e g just below the ocean bottom need to be known in order to be able to do a decomposition in general these parameters are unknown and in addition measurement related issues such as geophone coupling and crosstalk between the different components need to be dealt with in order to apply decomposition to field data an adaptive five stage decomposition scheme was developed in which these issues are addressed in this study the adaptive decomposition scheme is tested on a data example with a relatively shallow water depth 120 m consisting recordings from of a full line of ocean bottom receivers although some of the individual stages in the decomposition scheme are more difficult to apply because of stronger interference between events compared to data acquired over deeper water

virtual source method applied to crosswell and horizontal well geometries the virtual source method is a useful tool for redatuming a seismic survey below complicated overburden by creating virtual sources at downhole receiver locations hence generating data independent of the overburden and the time lapse changes therein in this article we first apply this technique to crosswell geometries whereby a virtual crosswell survey is simulated by shooting a line of surface shots passing through two boreholes instrumented with downhole sensors using this acquisition geometry receivers in one of the two wells are turned into virtual sources by correlating the wavefield recorded by those receivers with the recording at receivers in the other well and summing the correlated data over the surface shots

influence of climate change and uplift on colorado plateau paleotemperatures from carbonate clumped isotope thermometry the elevation history of earth s surface is key to understanding the geodynamic processes responsible for the rise of plateaus we investigate the timing of colorado plateau uplift by estimating depositional temperatures of tertiary lake sediments that blanket the plateau interior and adjacent lowlands using carbonate clumped isotope paleothermometry a measure of the temperature dependent enrichment of 13 c 18 o bonds in carbonates comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate and therefore assess the contribution of changes in elevation to the variations of surface temperature on the plateau analysis of modern lake calcite from 350 to 3300 m elevation in the southwestern united states reveals a lake water carbonate temperature lct lapse rate of 4 2 0 6 c km analysis of miocene deposits from 88 to 1900 m elevation in the colorado river drainage suggests that the ancient lct lapse rate was 4 1 0 7 c km and temperatures were 7 7 2 0 c warmer at any one elevation than predicted by the modern trend the inferred cooling is plausible in light of pliocene temperature estimates off the coast of california and the consistency of lapse rates through time supports the interpretation that there has been little or no elevation change for any of the samples since 6 ma together with previous paleorelief estimates from apatite u th he data from the grand canyon our results suggest most or all of the plateau s lithospheric buoyancy was acquired 80 60 ma and do not support explanations that ascribe most plateau uplift to oligocene or younger disposal of either the farallon or north american mantle lithosphere

target oriented interferometric tomography for gpr data an interferometric form of fermat s principle and traveltime tomography is used to invert ground penetrating radar gpr data for the subsurface velocity distribution the input data consist of gpr traveltimes of reflections from two buried interfaces a reference and b target where the data are excited and recorded by gpr antennas at the surface fermat s interferometric principle is then used to redatum the surface transmitters and receivers to interface a so the associated reflection traveltimes correspond to localized transit times between interfaces a and b the overburden velocity model above interface a is not required the result after tomographic inversion is a high resolution estimate of the velocity between interfaces a and b that does not depend on the velocity model above interface a a motivation for introducing interferometric traveltime tomography is that typical layer stripping approaches will see the slowness error increase with depth as the layers are inverted this suggests that

crosswell monitoring using virtual sources and horizontal wells time lapse crosswell seismic provides an efficient way to monitor the migration of a c o2 plume or its leakage after c o2 injection into a geologic formation recently crosswell seismic has become a powerful tool for monitoring underground variations using the concept of a virtual source with virtual sources positioned at the receivers installed in the well and thus the positions of sources and receivers can be invariant during monitoring however time lapse crosswell seismic using vertical wells and virtual sources has difficulty in describing the front of a c o2 plume which usually is parallel to the vertical wells and in obtaining sufficient ray coverage for the first arrival tomography these problems arise because of the theoretical downward illumination directivity limitation of the virtual source we have developed an effective monitoring method that uses virtual sources and two horizontal wells one above and one below the c o2 sequestration reservoir in our method we redatum the traces th

source receiver interferometric redatuming using sparse buried receivers to address complex near surface environments a case study of seismic imaging quality and time lapse repeatability

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

should felsic magmas be considered as tectonic objects just like faults or folds abstract integrated studies that correlate gravity data with magmatic structures or geochemical analysis reveal that the organization of granitic plutons shape and feeder varies according to the tectonic regime brittle or ductile and type extension strike slip or compression under which magma was generated deformation clearly controls granite emplacement it also has influence during the ascent segregation of melt and indirectly during the phase of initial melting because of the contrast of viscosity between melt and its plastic matrix strain partitioning develops during magma ascent that facilitates melt flow which bears consequences for the chemical evolution of the magma fast rate of melt extraction out of the source may lead to chemical disequilibrium at a larger scale petrochemical zoning of plutons is described as a dynamic process that results from the competition between the rate of magma emplacement the rate of the room provided by deformation and the rate of magma delivery

decoding whole rock plagioclase zircon and apatite isotopic and geochemical signatures from variably contaminated dioritic magmas zircon and apatite grains from three types of variably contaminated quartz diorite were analyzed in the gesiniec intrusion sw poland the zircon isotopic compositions ehf300 5 5 0 and 18o 5 6 9 3 are consistent with contamination processes zircon is characterized by an increase in u th pb and ree from the least to the most evolved quartz diorite type apatite is characterized by ranges in trace element concentrations and with the progress of contamination its composition evolves from those typical for i type granites toward those typical for s type isotopic and trace element variations in zircon and apatite correlate with sr isotopic composition of plagioclase 87sr 86sr300 0 7069 0 7084 pietranik and waight 2008 however both sr isotopic composition of plagioclase and trace element composition of apatite could be homogenized by late stage resorption leaving zircon as the only record of contamination processes that predate resorption comparison between plagioclase and zircon isotopic composition is consistent with both plagioclase cores and zircons recording similar stages of magma contamination comparison between plagioclase and apatite composition indicates that crystallization of apatite was contemporaneous with crystallization of plagioclase rims the preferred interpretation of isotopic and trace element variations observed in different grains is a two stage contamination by crustally derived magmas early contamination was probably dominated by material typical of the lower crust i e with none or only small amounts of a sedimentary component as indicated by low 18o in zircon 5 6 6 5 in both the least and the most contaminated types later contamination was dominated by crustally derived magmas containing more than 20 of a sedimentary component as the most contaminated sample includes zircon with 18o up to 9 3 partial melts generated from an s type granite source are the most probable contaminant for this second stage contamination as indicated by apatite trace element composition in general isotopic and chemical composition of plagioclase zircon and apatite provides complementary information on magmatic processes but records a longer time span than a single mineral only due to their diachronous crystallization histories

isotopic disequilibrium during melting of granite and implications for crustal contamination of magmas the assumption that the isotopic ratios of anatectic melts generated in response to crust magma interaction are equivalent to the bulk source rock from which they are derived was tested experimentally via dehydration melting of a proterozoic biotite granite heating of the coarse grained granite at 1250 c for durations of 1 to 48 h produced extreme disequilibrium among simultaneously generated mafic and felsic melts quenched to glass and whole rock 87sr 86sr ratios in situ microdrill sr isotopic analyses across gradational contacts between mafic high 87sr 86sr and felsic low 87sr 86sr glass register significant isotopic and chemical heterogeneity on the submillimetre scale our experiments suggest that during the early stages of wall rock melting when the rate of heating may exceed melting rates isotopic equilibrium may not be maintained thus producing contaminant liquids having isotopic compositions different from the bulk source rock if isotopic equilibrium is not attained during partial melting in the crust current geochemical models of open system magmatic processes may require modification

insights from low temperature thermochronometry into transpressional deformation and crustal exhumation along the san andreas fault in the western transverse ranges california 1 the san emigdio mountains are an example of an archetypical transpressional structural system bounded to the south by the san andreas strike slip fault and to the north by the active wheeler ridge thrust apatite u th he and apatite and zircon fission track ages were obtained along transects across the range and from wells in and to the north of the range apatite u th he ages are 4 6 ma adjacent to the san andreas fault and both u th he and fission track ages grow older with distance to the north from the san andreas the young ages north of the san andreas fault contrast with early miocene u th he ages from mount pinos on the south side of the fault restoration of sample paleodepths in the san emigdio mountains using a regional unconformity at the base of the eocene tejon formation indicates that the san emigdio mountains represent a crustal fragment that has been exhumed more than 5 km along the san andreas fault since late miocene time marked differences in the timing and rate of exhumation between the northern and southern sides of the san andreas fault are difficult to reconcile with existing structural models of the western transverse ranges as a thin skinned thrust system instead these results suggest that rheologic heterogeneities may play a role in localizing deformation along the big bend of the san andreas fault as the san emigdio mountains are compressed between the crystalline basement of mount pinos and oceanic crust that underlies the southern san joaquin valley

data driven adaptive decomposition of multicomponent seabed recordings dual sensor hydrophone and three component geophone data recorded on the seafloor allow the elastic wavefield to be decomposed into its upgoing and downgoing p and s wave components most decomposition algorithms require accurate knowledge of the elastic properties of the seafloor in the vicinity of the receivers and properly calibrated sensors in order for the data to be a faithful vector representation of the ground motion we present a multistep adaptive decomposition scheme that provides the necessary information directly from the data by imposing constraints on intermediate decomposition results the proposed scheme requires no a priori information and only a minimal amount of user defined input thus allowing multicomponent data to be decomposed in an automated data driven fashion the performance of the technique is illustrated using seabed data acquired in the north sea with prototype single sensors multicomponent geophones individually sampled realistic seafloor properties and sensor calibration operators are obtained and elastic decomposition of the calibrated data generally yields good results dominant water layer reverberations are successfully attenuated and primary reflections are substantially enhanced in the computed upgoing p wave potential just below the seafloor in contrast the result for the upgoing s wave potential is somewhat less convincing although the energy of water layer multiples is substantially reduced notable amounts of undesired multiple energy remain in this section after decomposition particularly at high offsets these imperfections may point to inaccuracies in the parametrization of the seafloor or remaining inaccuracies in the vector fidelity of the horizontal geophone recordings nevertheless the results obtained with the extended data driven decomposition scheme are at least comparable to previously published results

the augen gneisses of the peloritani mountains ne sicily granitoid magma production during rapid evolution of the northern gondwana margin at the end of the precambrian abstract the medium to high grade polymetamorphic basement rocks of the peloritani mountains northern sicily include large volumes of augen gneiss of controversial age and origin by means of a geochemical and shrimp zircon study of representative samples the emplacement age of the original granitoid protoliths of the augen gneisses and the most likely processes and sources involved in that granitoid magmatism have been determined u pb dating of three samples from widely spaced localities in the peloritani mountains yielded igneous protolith ages of 565 5 545 4 and 545 4 ma respectively these late ediacaran early cambrian ages are much older than was previously assumed on geological grounds and are typical of the peri gondwanan terranes involved in the geodynamic evolution of the northern gondwana margin at the end of the avalonian cadomian orogeny major and trace element compositions and sr nd isotopic data in combination with zircon inheritance age patterns suggest that the granitoid protoliths of the sicilian and coeval calabrian augen gneisses were generated by different degrees of mixing between sediment and mantle derived magmas the magmas forming the ca 545 ma inheritance rich granitoids appear to have had a significant contribution from partial melting of paragneiss that is the dominant rock type in the medium to high grade peloritanian basement the closeness of the inferred deposition age of the greywacke protoliths of the paragneisses with the intrusion age of the granitoids indicates rapid latest precambrian crustal recycling involving erosion burial metamorphism to partial melting conditions and extensive granitoid magmatism in less than ca 10 ma

the sr nd and o isotopic studies of the 1991 1995 eruption at unzen japan abstract the magma generation at unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization afc the magma in the unzen volcano is estimated to consist of about 50 80 of residual magma f and about 30 70 assimilated crustal material a relative to the original magma concerning the 1991 1995 eruption it is estimated that the magma formed as the result of mixing of about 50 60 crustal material and about 55 65 of residual magma an alternative magma eruption model for the 1991 1995 eruption is proposed here in the early stage the isotopic characteristics of 1991 eruption are defined by afc process in the deeper magma chamber later the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption the minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber the crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent therefore the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber in contrast the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history the 1991 1995 eruptive rocks of the unzen volcano show their characteristics in sr and nd isotopic values independent of their two previous eruptions however the isotopic values of early eruptive product could represent the original magma value this result also supports the previous work of chen et al 1993 chen c h depaolo d j nakada s shieh y n 1993 relationship between eruption volume and neodymium isotopic composition at unzen volcano nature 362 831 834 that suggested the end of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption

model based radiation pattern correction for interferometric redatuming in 4d seismic interferometric virtual source vs redatuming converts surface triggered source records into the equivalent records as if they originated from buried receiver locations by crosscorrelating downgoing waves with the corresponding upgoing waves theory suggests that when the receivers are surrounded by an enclosing boundary of sources then the vs has an isotropic radiation pattern and yield accurate response the resultant records should show improving the seismic repeatability and image quality comparing with non vs however in the presence of complex near surface intricate shallow structure and highly variable weathering layers can severely distort the ray path such that it produces the uneven angle coverage to the buried vs additionally near surface reverberations surface multiples and other mode converted waves may leak into the time gated early arrivals and further corrupt the direct wavefields abovementioned issues can result in distorted radiation patterns and contaminated responses of the v

dual sensor obc technology before the invention and successful implementation of the towed streamer in the late 1940s the ocean bottom cable obc method and variations thereof represented our only means for acquiring seismic data in marine areas the marine streamer facilitated more rapid acquisition of 2 d data its widespread adoption relegated obc to shallow waters where streamers could not be towed

structural model of the san bernardino basin california from analysis of gravity aeromagnetic and seismicity data 1 the san bernardino basin is an area of quaternary extension between the san jacinto and san andreas fault zones in southern california new gravity data are combined with aeromagnetic data to produce two and three dimensional models of the basin floor these models are used to identify specific faults that have normal displacements in addition aeromagnetic maps of the basin constrain strike slip offset on many faults relocated seismicity focal mechanisms and a seismic reflection profile for the basin area support interpretations of the gravity and magnetic anomalies the shape of the basin revealed by our interpretations is different from past interpretations broadening its areal extent while confining the deepest parts to an area along the modern san jacinto fault west of the city of san bernardino through these geophysical observations and related geologic information we propose a model for the development of the basin the san jacinto fault related strike slip displacements started on fault strands in the basin having a stepping geometry thus forming a pull apart graben and finally cut through the graben in a simpler bending geometry in this model the san bernardino strand of the san andreas fault has little influence on the formation of the basin the deep central part of the basin resembles classic pull apart structures and our model describes a high level of detail for this structure that can be compared to other pull apart structures as well as analog and numerical models in order to better understand timing and kinematics of pull apart basin formation

genesis of aluminous and intermediate granulites a case study in the eastern sierras pampeanas argentina abstract we report petrologic and geochemical features of anhydrous granulites from the eastern sierras pampeanas argentina aluminous grt crd bearing granulite is the most abundant rock type in the study region and hosts intermediate opx grt bearing granulites as interlayered lenses both granulite types crystallized under similar p t conditions of t 850 900 c and p c 7 5 kbar the formation of grt crd granulites by subsolidus dehydration conflicts with the absence of k feldspar in them moreover the aluminous granulites have an ree pattern that precludes postulating they derived by close system metamorphism of an extensively weathered protoliths comparing mineral and rock chemistry with experimental results in simple systems acfm akf and afm permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified this work demonstrates that orthopyroxene is a primary phase in opx grt granulites however never crystallized within aluminous granulites the aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke like sedimentary precursor compared with the composition of the average continental crust aluminous granulites are depleted in most lithophile trace elements and lree have somewhat low k rb ratios and high contents of elements having high partition coefficients for garnet and or ilmenite hree y cr ni however aluminous granulites have initial sr isotope ratios 0 713 we discuss the possibility that an intense but short lived melting event led to isotopic disequilibrium during melting of biotite bearing protoliths leaving a granulitic residuum with lower initial sr isotope ratios in contrast opx grt granulites have trace element features e g high sr contents and ree patterns with marked positive eu anomalies that do not support the idea that they are simply residues left after biotite dehydration melting and melt extraction rather these rocks display several petrological major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites

comparison between interferometric migration and reduced time migration of common depth point data one of the difficulties in seeing beneath salt is that the migration velocity in the salt and above it is not well known this can lead to defocusing of migration images beneath the salt in this paper we show that reduced time migration rtm and interferometric migration im can partly mitigate this problem rtm time shifts the traces with the time difference between the calculated arrival time sg ref and the natural arrival time sg ref of a reference reflection where s and g denote the source and receiver locations on the surface respectively we use the terms natural and calculated to represent respectively the arrival times that are velocity independent traveltimes directly extracted from the data without knowledge of the velocity model and velocity dependent traveltimes calculated by ray tracing through a given velocity model the benefit of rtm is a significant reduction of defocusing errors caused by errors in the migration velocity im on the other hand requires extrapolation of

isotopic disequilibrium in migmatitic hornfels of the gennargentu igneous complex sardinia italy records the formation of low 87sr 86sr melts from a mica rich source isotopic disequilibrium is increasingly recognized as a common feature of magmatic systems but the details of the mechanism s underpinning the development of isotopic disequilibrium during partial melting processes are not fully understood partial melting of mica rich lithologies may be predicted to generate melts enriched in radiogenic sr compared to the bulk protolith compositions due to the typically high rb sr ratio coupled with low melting temperature of mica in crustal rocks here we report a puzzling case study where the sr isotope composition of the melt fraction leucosome of partially molten metapelites migmatites is instead less radiogenic than the restitic component melanosome the examined migmatites fringe 50 m wide zone a low pressure 200 mpa high temperature 1050 c quartz dioritic intrusion which was emplaced in the gennargentu igneous complex sardinia italy at 306 26 ma bulk rock rb sr dating the migmatites derive from anatexis of the muscovite rich metapelitic wall rocks they include a quartzo feldspathic leucosome and a melanosome containing cordierite k feldspar plagioclase biotite fe ti oxide minerals and both corundum and hercynite the leucosome has a less radiogenic sr and more radiogenic nd isotope composition than the melanosome 87sr 86sr 306 ma 0 71068 and 0 71536 end 306 ma 6 4 and 9 2 respectively with bulk migmatite samples having intermediate compositions the significantly lower content of mica in the migmatites compared to the protolith indicates that muscovite and to a lesser extent biotite largely contributed to melt formation however the leucosome volume 50 estimated through mass balance calculations is considerably higher than the amount of melt 10 vol generally produced by mica dehydration melting in the crust suggesting that partial melting was enhanced by an external hydrous fluid the o isotope composition of the migmatites is lower than the typical metapelite values 10 but overlaps with the 18o range of the quartz diorites 8 8 9 9 suggesting that such a hydrous fluid was released from the quartz dioritic intrusion we put forward a model whereby the anatexis temperature conditions t 800 c favored the preservation of isotopic disequilibrium of micas and plagioclase in the protolith in this context the leucosome formed by the preferential melting of less radiogenic plagioclase rather than more radiogenic micas the melt was then efficiently separated from the melanosome containing restitic biotite and peritectic k feldspar and magnetite derived from mica breakdown during the anatectic process the quartz diorite provided not only the heat budget but also the fluid amount responsible for i the hydration of the pelitic country rock ii the increase of melt fraction and iii the higher mobility of anatectic magma

a spectrum of disequilibrium melting preserved in lava hosted partially melted crustal xenoliths from the wudalianchi volcanic field ne china abstract disequilibrium melting has been established as a common process occurring during crustal anatexis and thus demonstrates that crustal assimilation is not a closed system observations of extreme compositional heterogeneity within partial melts derived from crustal xenoliths have been documented in several recent examples however the retention or transfer of elements to and from residues and glasses and their relative contributions to potential crustal contaminants warrants further investigation recent eruptive products of the huoshaoshan volcano in the holocene wudalianchi volcanic field of northeast china contain crustal xenoliths which preserve a spectrum of partial melting both petrographically and geochemically thus providing an excellent natural example of crustal anatexis correlations exist between the volume of silicic glass preserved within the xenoliths and bulk rock sio2 70 83 wt al2o3 8 16 wt glass 87sr 86sr 0 715 0 908 abundances of elements common in feldspars and micas sr ba rb and elements common in accessory minerals y zr nb these correlations are likely associated with the consumption of feldspars and micas and the varying retention of accessory phases during partial melting the xenoliths which contain greater volumes of silicic glass and residual quartz interpreted as being the most melted were found within pahoehoe lava whilst the least melted xenoliths were found within scoria of the summit cone of huoshaoshan thus it is interpreted that the extent of melting is linked to the immersion time in the lava which we estimate to be on the order of days however the least partially melted samples display geochemical evidence in support of melt loss evidenced by the relatively depleted nature of their whole rock compositions and the larger difference between whole rock and glass yet the more melted samples appear to have experienced less melt loss thus the extent of melting cannot be linked linearly to the volume of melt loss which occurs small scale mm mingling and transfer of material from the enclosing lava to the xenolith are observed however modelling of potential contaminant compositions is inconsistent with crustal contamination during lava petrogenesis it is thus more likely that any crustal contamination in these eruptives is extremely localized within this open magmatic system and most likely occurs at the contact zone between the xenolith and host lava

modal expansion of one way operators in laterally varying media one of the main benefits of prestack depth migration in seismic processing is its ability to handle complicated medium configurations when considerable lateral variations in the acoustic parameters are present in the subsurface prestack depth migration is necessary for optimal lateral resolution however most migration algorithms still deal with lateral variations in an approximate manner because these variations are in many cases moderate compared to the profound variations in the depth direction from other areas of science e g optics oceanography and seismology it is known that lateral variations can be dealt with by a decomposition of the wavefield into wave modes in this paper we explore the possibility of applying this concept to the construction of one way wavefield operators for depth migration we expand the helmholtz operator on an orthogonal basis of wave modes and obtain one way wavefield operators that are unconditionally stable and significantly increase the lateral resolution of the result

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

on the use of multicomponent streamer recordings for reconstruction of pressure wavefields in the crossline direction three component measurements of particle motion would bring significant benefits to towed marine seismic data if processed in conjunction with the pressure data we show that particle velocity measurements can increase the effective nyquist wavenumber by a factor of two or three depending on how they are used a true multicomponent streamer would enable accurate data reconstruction in the crossline direction with cable separations for which pressure only data would be irrecoverably aliased we also show that conventional workflows aimed at reducing these aliasing effects such as moveout correction applied before interpolation are compatible with multicomponent measurements some benefits of velocity measurements for deghosting data are well known we outline how the new measurements might be used to address some long standing deghosting challenges of particular interest specifically we propose methods for recovering de ghosted data between streamers and for 3d deghosting of seismic data at the stream

target enclosed seismic imaging seismic reflection data can be redatumed to a specified boundary in the subsurface by solving an inverse or multidimensional deconvolution problem the redatumed data can be interpreted as an extended image of the subsurface at the redatuming boundary depending on the subsurface offset and time we retrieve targetenclosed extended images by using two redatuming boundaries which are selected above and below a specified target volume as input we require the upgoing and downgoing wavefields at both redatuming boundaries due to impulsive sources at the earth s surface these wavefields can be obtained from actual measurements in the subsurface they can be numerically modeled or they can be retrieved by solving a multidimensional marchenko equation as output we retrieved virtual reflection and transmission responses as if sources and receivers were located at the two target enclosing boundaries these data contain all orders of reflections inside the target volume but exclude all interactions with the part of the medium outside this volume the retrieved reflection responses can be used to image the target volume from above or from below we found that the images from above and below are similar given that the marchenko equation is used for wavefield retrieval if a model with sharp boundaries in the target volume is available the redatumed data can also be used for two sided imaging where the retrieved reflection and transmission responses are exploited because multiple reflections are used by this strategy seismic resolution can be improved significantly because target enclosed extended images are independent on the part of the medium outside the target volume our methodology is also beneficial to reduce the computational burden of localized inversion which can now be applied inside the target volume only without suffering from interactions with other parts of the medium

combination of surface and borehole seismic data for robust target oriented imaging a novel application of seismic interferometry si and marchenko imaging using both surface and borehole data is presented a series of redatuming schemes is proposed to combine both data sets for robust deep local imaging in the presence of velocity uncertainties the redatuming schemes create a virtual acquisition geometry where both sources and receivers lie at the horizontal borehole level thus only a local velocity model near the borehole is needed for imaging and erroneous velocities in the shallow area have no effect on imaging around the borehole level by joining the advantages of si and marchenko imaging a macrovelocity model is no longer required and the proposed schemes use only single component data furthermore the schemes result in a set of virtual data that have fewer spurious events and internal multiples than previous virtual source redatuming methods two numerical examples are shown to illustrate the workflow and to demonstrate the benefits of the method one is a synthetic model and the other is a realistic model of a field in the north sea in both tests improved local images near the boreholes are obtained using the redatumed data without accurate velocities because the redatumed data are close to the target

a model that helps explain sr isotope disequilibrium between feldspar phenocrysts and melt in large volume silicic magma systems feldspar phenocrysts of silicic volcanic rocks are commonly in sr isotopic disequilibrium with groundmass in some cases the feldspar is more radiogenic and in others it is less radiogenic several explanations have been published previously but none of these is able to accommodate both senses of disequilibrium we present a model by which either more or less radiogenic feldspar or even both within a single eruptive unit can originate the model requires a magma body open to interaction with biotite and feldspar bearing wall rock magma is incrementally contaminated as wall rock melts incongruently biotite preferentially melts first followed by feldspar such melting behavior which is supported by both field and experimental studies first contaminates magma with a relatively radiogenic addition followed by a less radiogenic addition feldspar phenocrysts lag behind melt groundmass of volcanic rock in incorporating the influx of contaminant thus resulting in sr isotopic disequilibrium between the crystals and melt the sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process this model is testable by isotopic fingerprinting of individual feldspar crystals for a given set of geologic boundary conditions specific core to rim sr isotopic profiles are expectable moreover phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history initial results of sr isotopic fingerprinting of sanidine phenocrysts from the taylor creek rhyolite are consistent with the model more tests of the model are desirable

isotope disequilibrium during anatexis a case study of contact melting sierra nevada california the geochemical consequences of contact melting were investigated in the sierra nevada batholith where trachyandesite magmas have intruded and melted the host granitic rocks partial melting of the granite varies from 10 20 at contact regions to 50 70 in a granite block within the trachyandesite thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyandesite intrusion reached a maximum of 1000 c after ca 1 5 yr and remained above 500 c for ca 40 yr sr nd and pb isotope data of granitic melts record marked sr isotope and to a lesser extent pb isotope disequilibrium both within different glass fractions and between glasses and their source rb sr isochron calculations on residual mineral glass pairs fail to yield the age of melting obtained by40ar 39ar dating 11 9 ma sr and pb isotope data of glasses establish that major rock forming minerals of the granite had not attained isotope equilibrium before the onset of melting consequently the isotopic composition of the successive liquid fractions originating from granite melting depended on the relative contribution of each mineral to the elemental budget of the melts the extrapolation of these results to the petrogenesis of crustal melts and more generally to studies of crustal evolution suggests that isotope disequilibrium during anatexis and high grade metamorphism may be a common characteristic of many geologic settings if future studies substantiate these results some geochemical models of crustal melting and assimilation may need revision in addition marked sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises concerns about the reliability of radiometric dating in metamorphic terranes

virtual source applications to imaging and reservoir monitoring the virtual source method is a breakthrough that allows us to image and monitor the subsurface in cases where surface seismic or vsp fail to deliver in settings where the overburden is complex or changing traditional time lapse signals are weak or nonrepeatable leading to ineffective seismic input to reservoir modeling and little of value generated by repeated seismic surveys on the other hand the virtual source method allows us to image under complex overburden yields repeatable data for reservoir monitoring enables shear seismic and may help us look ahead as we drill

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

scale of pluton wall rock interaction near may lake yosemite national park ca usa interaction of magma with wall rock is an important process in igneous petrology but the mechanisms by which interactions occur are poorly known the western outer granodiorite of the cretaceous tuolumne intrusive suite of yosemite national park california intruded a variety of metasedimentary and igneous wall rocks at 93 1 ma the may lake metamorphic screen is a metasedimentary remnant whose contact zone exhibits a variety of interaction phenomena including xenolith incorporation disaggregation and partial melting the chemical contrast of these metasedimentary rocks with the invading pluton provides an excellent measure of pluton wall rock interactions wall rock xenoliths mostly pelitic quartzite are predominantly located in an elongate horizon surrounded by a hybridized fine grained granodiorite initial sr and nd isotopic ratios of the hybridized granodiorite indicate significant local incorporation of crustal material major and trace element geochemical data indicate that contamination of the granodiorite occurred via selective assimilation of both high k and low k high silica partial melts derived from pelitic quartzite although the hybridized granodiorite shows significant amounts of contamination adjacent to xenoliths the proportion of contamination is undetectable more than a meter away these results indicate that the chemical and isotopic variability of the tuolumne intrusive suite is not caused by magma contamination via in situ wall rock assimilation

reconciling disparate estimates of total offset on the southern san andreas fault late cenozoic tectonic reconstructions of southern california united states have been hindered for decades by disagreement over the total displacement on the southern san andreas fault disparate estimates of dextral offset range from 160 km to 240 km prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers we present a new reconstruction that integrates published lithologic structural and paleocurrent data with the effects of clockwise rotation in the eastern transverse ranges etr this model yields 200 14 km of cumulative offset on the southern san andreas fault system in the salton trough the disparity between our estimate and a lower estimate of 160 km for the mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the etr transfer of strain to the eastern california shear zone and diffuse crustal shortening in the big bend region this reconstruction reconciles previously incompatible and long debated models and places an important new constraint on the tectonic evolution of the pacific north america plate boundary in southern california

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

extreme mineral scale sr isotope heterogeneity in granites by disequilibrium melting of the crust abstract the broadest ranges of initial sr isotopic ratios 87 sr 86 sr i ever reported within a single igneous rock 2 10 2 are preserved within the late miocene laccolith pluton dyke felsic complex of elba island italy for these units the integration of textural and crystal scale isotope data allows tracing the evolution of the 87 sr 86 sr i of the melt from the emplacement level back to the earliest pre emplacement crystallization stage the rock matrix minerals record the 87 sr 86 sr i composition of the magma at the emplacement level 0 715 0 716 k feldspar megacrysts representing an earlier phase crystallized at depth record a rim to core increase of sr isotopic ratios from values similar to those of the matrix to significantly higher ones 0 719 remarkably biotites hosted within megacrysts representing the first crystallization stage have extreme and contrasting 87 sr 86 sr i values in the different intrusive units biotites within megacrysts from the laccolith record the lowest ratio in the intrusive complex 0 710 while those in the megacrysts from the pluton and associated felsic dyke have the highest 87 sr 86 sr i 0 732 this time transgressive record of isotopic variation in the magma reflects episodic recharge and mixing of magma batches formed by disequilibrium melting of crustal sources that produced melts through different reactions as temperature was increasing the progression from muscovite to biotite dominated fluid absent melting generates melts with increasing 87 sr 86 sr while at higher temperatures the progression from biotite to hornblende dominated melting reactions results in a decrease in the 87 sr 86 sr of the melt

strengthening the virtual source method for time lapse monitoring time lapse monitoring is a powerful tool for tracking subsurface changes resulting from fluid migration conventional time lapse monitoring can be done by observing differences between two seismic surveys over the surveillance period along with the changes in the subsurface differences in the two seismic surveys are also caused by variations in the near surface overburden and acquisition discrepancies the virtual source method monitors below the time varying near surface by redatuming the data down to the subsurface receiver locations it crosscorrelates the signal that results from surface shooting recorded by subsurface receivers placed below the near surface for the mars field data redatuming the recorded response down to the permanently placed ocean bottom cable obc receivers using the virtual source method allows one to reconstruct a survey as if virtualsources were buried at the obc receiver locations and the medium above them were a homogeneous half space separating the recorded wavefields

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

early silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system the case of the xuehuading panshanchong pluton south china block

synthetic aperture controlled source electromagnetics controlled source electromagnetics csem has been used as a de risking tool in the hydrocarbon exploration industry although there have been successful applications of csem this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs csem can detect in this paper we apply the concept of synthetic aperture to csem data synthetic aperture allows us to design sources with specific radiation patterns for different purposes the ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna consequently the types of hydrocarbon reservoirs that csem can detect are significantly extended because synthetic apertures are constructed as a data processing step there is no additional cost for the csem acquisition synthetic aperture has potential for simplifying and reducing the cost of csem acquisition we show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture csem source

wavelet crosscorrelation based interferometric redatuming in 4d seismic interferometric virtual source redatuming crosscorrelates downgoing waves with the corresponding upgoing waves to convert records from surface source gathers to virtual gathers at the buried receiver locations the assumption is that the virtual source records show reduced effects of overburden complexity and therefore provide improved seismic repeatability and image quality in practice however the repeatability and quality of the redatumed data can be degraded by one sided illumination and inadequate preprocessing in the presence of complex near surface structure for land redatuming intricate shallow structure and highly variable weathering layers introduce surface multiples ground roll scattering noise interferences among different wave modes which additionally contaminate both down and upgoing wavefields to address these issues we propose a new interferometric redatuming method based on crosscorrelation in the wavelet domain specifically by transforming time offset data into local time sca

active faulting in the walker lane 1 deformation across the san andreas and walker lane fault systems accounts for most relative pacific north american transform plate motion the walker lane is composed of discontinuous sets of right slip faults that are located to the east and strike approximately parallel to the san andreas fault system mapping of active faults in the central walker lane shows that right lateral shear is locally accommodated by rotation of crustal blocks bounded by steep dipping east striking left slip faults the left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area including rattlesnake and garfield flats teels columbus and rhodes salt marshes and queen valley the benton springs and petrified springs faults are the major northwest striking structures currently accommodating transform motion in the central walker lane right lateral offsets of late pleistocene surfaces along the two faults point to slip rates of at least 1 mm yr the northern limit of northwest trending strike slip faults in the central walker lane is abrupt and reflects transfer of strike slip to dip slip deformation in the western basin and range and transformation of right slip into rotation of crustal blocks to the north the transfer of strike slip in the central walker lane to dip slip in the western basin and range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long lived feature of the deformation field the complexity of faulting and apparent rotation of crustal blocks within the walker lane is consistent with the concept of a partially detached and elastic brittle crust that is being transported on a continuously deforming layer below the regional pattern of faulting within the walker lane is more complex than observed along the san andreas fault system to the west the difference is attributed to the relatively less cumulative slip that has occurred across the walker lane and that oblique components of displacement are of opposite sense along the walker lane extension and san andreas contraction respectively despite the gross differences in fault pattern the walker lane and san andreas also share similarities in deformation style including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike slip and dip slip faults

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

source controlled 87sr 86sr isotope variability in granitic magmas the inevitable consequence of mineral scale isotopic disequilibrium in the protolith abstract the broad sr isotopic variability exhibited by granitoid rocks is commonly interpreted to reflect the mixing of magmas from different sources however evidence from granites and migmatites indicates that melting and magma extraction from crustal sources can occur sufficiently rapidly that trace element and isotopic equilibration between liquid and residual phases is commonly not achieved additionally evidence from unmelted high grade metamorphic rocks indicates that major reactant minerals in the fluid absent melting process principally biotite and plagioclase do not always attain equilibrium during regional metamorphism when these two circumstances occur in combination the melt does not inherit its radiogenic isotopic signature from the bulk source in a simple way in such situations the isotopic composition of the melt will be dependent on the isotopic compositions of the reactant phases and the stoichiometry of the melting reaction this study has used information from experimental studies of fluid absent partial melting in metapelites and metagreywackes to investigate the consequences of sr isotopic disequilibrium between the reactant minerals for magma composition the study demonstrates that a range of isotopically distinct magmas can arise from progressive melting of a single source that is able to undergo melting through different reactions as temperature increases when translated to the typically layered sources constituted by sedimentary and volcano sedimentary rocks this process will produce magmas characterized by sr isotope variability that reflects the differences in melting reaction stoichiometry within the different layers even with no bulk rock isotopic variability between layers this study demonstrates that the sr isotope variability commonly observed within granitic suites as well as at the grain and sub grain scale within individual magmatic bodies can be primary reflecting differences in composition between magma batches produced from the progressive melting of a single source