late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

implications of the carbonaceous chondrite mn cr isochron for the formation of early refractory planetesimals and chondrules abstract an excellent 53 mn 53 cr isochron for bulk ci cm co cv cb and ungrouped c3 chondrites seems to suggest that each carbonaceous chondrite group acquired its mn cr ratio 4568 1 myr ago this age is indistinguishable from the age of ca al rich inclusions cais which is considered to be the start of the solar system t 0 however carbonaceous chondrites were not assembled until at least 1 5 5 myr after t 0 to judge by the 207 pb 206 pb and 26 al 26 mg ages of the chondrules within them and by the fact that they were not melted by heat from the decay of 26 al presumably therefore these meteorites inherited their bulk mn cr isochron from precursor materials which experienced mn cr fractionation at t 0 as a possible physical mechanism for how the isochron was established initially and later inherited by the carbonaceous chondrites we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements and hence had variably low mn cr the planetesimals and the undepleted high mn cr primitive dust from which they were made shared the same initial e 53 cr and therefore evolved on an isochron we suggest that later impact disruption of the planetesimals produced dusty debris which became mixed in various proportions with unprocessed high mn cr dust before accreting to the carbonaceous chondrite parent bodies with mixing in a closed system the isochron was unchanged we infer that some debris rich material was converted to chondrules prior to accretion the chondrules could have been formed by flash melting of the mixed dust or could instead have been made directly by the impact splashing of molten planetesimals or by condensation from impact generated vapor plumes

subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

age of the eucrite caldera from convergence of long lived and short lived chronometers abstract the long lived 147 sm 143nd chronometer has been used in conjunction with the short lived 146 sm 142 nd and 53 mn 53 cr chronometers to constrain the time of formation for the eucrite caldera the combination of these three chronometers has produced results that are consistent with one another and converge on a precise high resolution age of 4537 12 ma within 95 confidence limits so far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite thus this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body most likely between 17 and 41 million years after the formation of the first condensates in the early solar system from these age constraints it is evident that caldera either formed within a magma body on a large 500 km asteroid or in an extensive impact melt on the crust of its parent body

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

high spatial resolution ion microprobe measurements refine chronology of carbonate formation in orgueil aqueous activity on meteorite parent bodies is indicated by the presence of carbonates high spatial resolution ion microprobe analyses of nine individual carbonate grains four dolomites five breunnerites from the orgueil meteorite reveal linear correlations between 53cr excesses and mn cr ratios in all grains indicative of in situ decay of radioactive 53mn half life 3 7 million years the well defined isochrons appear to have chronological significance if this is the case then 53mn 55mn ratios between 2 1 and 4 7 x 10 6 are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 ma are calculated systematic uncertainty of 2 ma dolomites tend to have formed slightly earlier than the breunnerites our data imply extensive aqueous activity on the orgueil parent body over a period of several million years starting 3 4 myr after formation of the solar system that most likely was the result of impact heating and latent heat from the decay of radioactive 26al and 60fe

paleogene evolution of the kodiak islands alaska consequences of ridge trench interaction in a more southerly latitude the kodiak islands of alaska are principally composed of a late cretaceous and paleogene complex of accreted deep sea rocks an early paleocene magmatic event invaded the accretionary complex with mid ocean ridge basalt morb andesite and granodiorite geochemical evidence suggests both the andesite and granodiorite are mixtures of morb and sediment the volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter the granodioritic intrusions with k ar ages as old as 62 ma postdate emplacement of the early paleocene sedimentary and volcanic rocks interaction of the subduction complex with the kula farallon ridge most simply explains this early paleocene magmatic episode paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40 3 6 north latitude which is 25 3 9 south of the expected position of the kodiak islands relative to north america the magmatic arc presently northwest of the kodiak accretionary complex was active during the late cretaceous and early paleocene diminished after the ridge trench interaction and resurged from late eocene into miocene time the magmatic resurgence is coeval with the obductive offscraping of eocene to oligocene submarine fan deposits in the kodiak islands the petrology of these fan deposits indicates they are the most likely proximal equivalent for the zodiac fan thereby limiting its relative motion with respect to the kodiak islands possible models accounting for the northward motion of the kodiak accretionary complex include 1 coupling the accretionary complex and possibly related arc to the northward moving kula plate and closing an ocean basin north of the arc in paleogene time 2 emplacing the kodiak accretionary complex at the northward limit of the paleomagnetic data with subsequent northward motion accomplished by closure of a small back arc basin and or intracontinental shortening and 3 transcurrent faulting and northward motion of the kodiak accretionary complex without the presently adjacent arc

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

live iron 60 in the early solar system isotopic analyses of nickel in samples from the differentiated meteorite chervony kut revealed the presence of relative excesses of 60ni ranging from 2 4 up to 50 parts per 104 these isotopic excesses are from the decay of the now extinct short lived nuclide 60fe and provide clear evidence for the existence of 60fe over large scales in the early solar system not only was 60fe present at the time of melting and differentiation that is fe ni fractionation of the parent body of chervony kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal the inferred abundance of 60fe suggests that its decay alone could have provided sufficient heat to melt small diameters of several hundred kilometers planetary bodies shortly after their accretion

the duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal abstract precise ratios of w and re relative to ir determined by laser ablation icp ms reveal that matrix metal in equilibrated ordinary chondrites eoc exhibits a correlated variation of these ratios in unequilibrated ordinary chondrites uoc the w ir ratio varies by over two orders of magnitude w ir 0 003 0 6 and shows no correlation with any other siderophile element most matrix metal in uoc is depleted in w relative to metal in eoc thus w must enter metal during metamorphism the correlation between w ir and re ir is evident in type 4 chondrites this implies that the abundance and isotopic composition of w is set in ordinary chondrite metal by petrologic type 4 and that the reductant must be exhausted to limit further isotopic exchange as indicated by the 182 hf 182 w ages of ordinary chondrite metal lee and halliday science 274 1996 1876 1879 this offers a means of dating the onset of metamorphic heating of oc parent bodies a discrepancy in ages between 182 hf 182 w lee and halliday science 274 1996 1876 1879 and 207 pb 206 pb gopel et al earth planet sci lett 121 1994 153 171 or 129 i 129 xe brazzle et al geochim cosmochim acta 63 1999 739 760 dating is real and measures the interval of metamorphism the time difference between reduction of w during the onset of heating and isotopic closure of pb or xe by cooling e g t m 2 12 myr for h chondrites the 187 re 187 os and 182 hf 182 w ages are set by the same process metamorphic equilibration establishing an important link between an absolute chronometer t 1 2 187 re 41 6 billion years and a short lived radioisotope system t 1 2 182 hf 9 myr

formation conditions of plagioclase bearing type i chondrules in co chondrites a study of natural samples and experimental analogs abstract about ten percent of type i feo poor chondrules in unequilibrated co chondrites contain plagioclase that appears to be igneous in origin crystallizing at a late stage of solidification of host chondrule melts we have studied plagioclase bearing chondrules in detail and compared them with plagioclase free chondrules in order to determine the formation conditions of plagioclase and the constraints that the presence of plagioclase places on the conditions of chondrule formation plagioclase bearing chondrules have similar textures mineral compositions and bulk compositions to plagioclase free chondrules the only possible chemical difference that might control the presence or absence of plagioclase in a given chondrule is that most plagioclase bearing chondrules have a slightly higher bulk al ca ratio than plagioclase free chondrules we carried out dynamic cooling experiments on a type iab chondrule analog in order to investigate chondrule formation conditions our experiments at slow cooling rates

26al in plagioclase rich chondrules in carbonaceous chondrites evidence for an extended duration of chondrule formation abstract the 26al 26mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase rich chondrules prcs from 13 carbonaceous chondrites ccs one ungrouped acfer 094 six cr five cv and one co reveal large variations in the initial 26al 27al ratio 26al 27al 0 well resolved 26mg excesses 26mg from the in situ decay of the short lived nuclide 26al t1 2 0 72 ma were found in nine chondrules two from acfer 094 five from the cv chondrites allende and efremovka and one each from the paired cr chondrites eet 92147 and eet 92042 with 26al 27al 0 values ranging from 3 10 6 to 1 5 10 5 data for seven additional chondrules from three cv and two cr chondrites show evidence suggestive of the presence of 26al but do not yield well defined values for 26al 27al 0 while the remaining chondrules do not contain excess radiogenic 26mg and yield corresponding upper limits of 11 2 10 6 for 26al 27al 0 the observed range of 26al 27al 0 in prcs from ccs is similar to the range seen in chondrules from unequilibrated ordinary chondrites uocs of low metamorphic grade 3 0 3 4 however unlike the uoc chondrules there is no clear trend between the 26al 27al 0 values in prcs from ccs and the degree of thermal metamorphism experienced by the host meteorites high and low values of 26al 27al 0 are found equally in prcs from both ccs lacking evidence for thermal metamorphism e g crs and ccs where such evidence is abundant e g cvs the lower 26al 27al 0 values in prcs from ccs relative to most cais are consistent with a model in which 26al was distributed uniformly in the nebula when chondrule formation began approximately a million years after the formation of the majority of cais the observed range of 26al 27al 0 values in prcs from ccs is most plausibly explained in terms of an extended duration of 2 3 ma for the formation of cc chondrules this interval is in sharp contrast to most cais from ccs whose formation appears to be restricted to a narrow time interval of less than 105 years the active solar nebula appears to have persisted for a period approaching 4 ma encompassing the formation of both cais and chondrules present in ccs and raising important issues related to the storage assimilation and mixing of chondrules and cais in the early solar system

the tectonic history of drake passage and its possible impacts on global climate this study provides an integrated review of plate tectonic models of the evolution of the antarctica patagonia connection compared to geological records collected on land in patagonia and tierra del fuego and offshore along the northern edge of the scotia sea a temporal framework for the sedimentary and tectonic events of the north scotia ridge and tierra del fuego is constructed with additional data compiled from entire patagonia and the austral basin this review provides robust correlations of seaways and tectonic events along the scotia and south america plates and indicates that the opening of the drake passage was not steady state since ca 30 ma rather the regions forming the present day northern limit of this gateway experienced important paleogeographic changes from deep marine basins to shallow ridges and emerged regions during the late oligocene and early middle miocene time our compilation of geological data shows that emergence along the north scotia ridge and tierra del fuego was achieved at 23 22 ma and has been followed by elimination of the patagoniano sea in patagonia starting at 22 23 ma and achieved at 20 ma this transition towards more continental sedimentation in southern south america is correlated with more shallow marine conditions in the austral basin this succession of events had a strong influence on the general geometry of the drake passage corresponding to a constriction of its northern limit starting in the window 29 22 ma and achieved at 21 ma this period of active deformation in southern south america also corresponds to a period of the global climate having two anomalies well known from the isotopic records the late oligocene warming around 26 ma and the mid miocene climatic optimum which ended between 15 and 14 ma the possible effects of the post oligocene tectonic evolution of the drake passage region on general oceanic circulation are discussed causes for the synchronicity between tectonic events and these global warming events are examined

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

evidence of a laterally variable lower mantle structure from p and s waves summary examples of grafenberg array data showing anomalous p waves which typically arrive 3 5s after the direct p wave and which have a slowness 0 7 0 8 sdeg smaller than direct p are presented this additional phase is most frequently observed for events located in the ne portion of the southern kurile island subduction zone 73 80 from grafenberg but systematically disappears for events in the sw portion of this zone because of the magnitude of the slowness difference these observations cannot be attributed to a complex source rupture process nor to multipathing through the descending slab likewise they may not be accounted for by near receiver structure because these phases are not seen for all kurile events if present they appear at all stations of the array but they follow direct p too closely to be a multiple from the moho therefore we conclude they are very likely caused by lower mantle velocity structure the most likely explanation is the presence of a p velocity jump of about 3 per cent approximately 290 km above the core mantle boundary since such a reflector in the lowermost mantle not only gives a good fit of traveltimes and slowness but is also able to model the waveform and the amplitudes of this additional p phase the distribution of bounce points on this reflector for the kurile events indicates a lateral extension of this velocity anomaly under northern siberia of about 150 km by at least 200 km the best fitting s wave model has a reflector in the same depth but the velocity contrast seems to be only about 2 per cent suggesting a different behaviour of the p and s velocity in d few events from other regions in this distance range are suitable for a definitive analysis of this kind from among this group some observations indicate a lower mantle anomaly under the lomonosow ridge and under northern greenland but since the lower mantle under western siberia northern novaya zemlya the azores islands region and the ussr afghanistan border region does not produce an additional phase in the grafenberg recordings it is very unlikely that such a velocity anomaly in the lowermost mantle is a global feature

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

seismic ridge subduction and topography foreland deformation in the patagonian andes abstract the patagonian andes recorded several episodes of active ridge subduction in the last 80 million years an analysis of the spatial and temporal relation between the present segment of collision of the chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the patagonia fold and thrust belt several magmatic episodes related to the collision such as near trench magmatism adakite emplacement oib plateau basalts in the retroarc and the arc volcanic gap are associated with the uplift and deformation of the patagonian cordillera based on these correlations a collision of the aluk or phoenix farallon ridge during paleogene times south of 43 30 is identified changes in magmatic patterns molasses deposits deformation and uplift of the patagonian cordillera constrain the region affected by the collision similar evidence implies a third period of collision in the late cretaceous based on the occurrence of adakitic rocks arc magmatic gap and deformation along the southern patagonian andes this earliest hypothesized collision would require the existence of a new oceanic microplate between the pacific and the aluk plates during late cretaceous times present rapid isostatic rebound related to the continental ice cap retreat in the patagonian andes is restricted to the region south of chile triple junction 46 30 s the uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the northern hemisphere and requires an abnormally hot mantle with low viscosity this abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat slab subduction proposed by some authors to explain the arc volcanic gaps

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

short period body wave constraints on properties of the earth s inner core boundary in this study we model short period waveform data in an attempt to constrain the p and s velocity structure at the earth s inner core boundary icb the data set consists of recordings from 10 events in the south pacific and the data selection criteria as well as methods of analysis were designed to avoid problems with receiver response often associated with short period waveform data these data are modeled using a calculation technique that facilitates the consideration of a wide variety of models for the icb results suggest that q in the inner core has the frequency dependence proposed by doornbos 1983 the data determine the p velocities above and below the icb to within a trade off that is well constrained by the data for example with a p velocity structure above the icb given by prem the p velocity below the icb is 11 03 0 03 km s similarly for the data analyzed here the estimate of the s velocity at the top of the inner core trades off with the estimate of q but we use the q model of doornbos 1983 to estimate the jump in s velocity to be 3 1 km s

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

magmatism and tectonics in continental chiloe chile 42 42 30 s the chiloe chonos region seems to preserve the oldest depositional events in the fore arc accretionary complex of the southeast pacific margin there are isolated occurrences of low grade metamorphic rocks including slates with a devonian trilobite fauna and schists that give rb sr evidence of a ca 290 ma metamorphism pillow basalts and ultramafic rocks may represent parts of the pan thalassic ocean floor on which the palaeozoic sediments were laid down emergence of a magmatic arc is indicated by jurassic to early cretaceous volcanogenic and marine deposits during the mid cretaceous climax of plutonic activity these were intruded by monzogranites which here constitute the eastern portion of the north patagonian batholith they give rb sr isochron ages of 120 100 ma barremian albian initial 87sr 86sr ratios of 0 7040 0 7045 and ndt values of 0 5 to 1 5 indicate a simple petrogenesis with a mantle source the western part of the batholith is petrologically more primitive being composed predominantly of tonalite diorite and gabbro and initial 87sr 86sr ratios are more variable late cenozoic movement of the liquine ofqui fault zone lofz generated deep pull apart basins to the west of the uplifted batholith basement complex these were filled by thick marine sequences of volcanogenic debris indicating the wide extent of a mainly rhyolitic volcanic field during miocene times pliocene tonalite and granodiorite plutons dated by a rb sr whole rock isochron at 4 7 0 5 ma and holocene andesite basalt stratovolcanoes are located along the lofz the latter feature has thus been a major influence on the tectonic evolution of the area there is no evidence for major post palaeozoic compression or crustal shortening

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

tectonics in the eastern central patagonian cordillera 45 30 47 30 s a subhorizontal angular erosional unconformity that separates subhorizontal upper palaeocene lower eocene fluvial deposits of the ligorio marquez formation from folded and faulted lower cretaceous tuffs and sedimentary rocks in the area south of chile chico in the northern patagonian fold and thrust belt of southern chile 46 45 s indicates tectonism prior to the previously recognized post early miocene contractional event similar observations between latitudes 45 30 and 47 30 s in argentina support the existence of a pre palaeocene eocene contractional tectonic phase and locally of a late early cretaceous event this indicates that the northern patagonian fold and thrust belt includes structures earlier than previously thought probably masked by the later neogene ones the timing of this earlier tectonism is not precisely known beyond being pre upper palaeocene lower eocene and post barremian one possibility is that it may be the earlier stages of a progressive deformation that in its later stages is represented by the post lower miocene tectonism previously documented in the region in this case it may represent the latest cretaceous palaeocene major contractional tectonism and uplift well documented along the southernmost patagonian cordillera south of latitude 51 s the upper palaeocene lower eocene ligorio marquez formation may represent the first accumulation in a foreland wedge top basin developed east of the orogenic front subsequently and probably during the middle miocene these beds were folded and faulted during the tectonism that generated the foreland basin where the fluvial deposits of the galera formation accumulated different speculative scenarios for the origin of this compression can be invoked depending on its age

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

the origin of chondrules and chondrites debris from low velocity impacts between molten planetesimals we investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin shelled largely molten planetesimals collided at low speed during accretion this scenario here dubbed splashing stems from evidence that such planetesimals intensely heated by 26al were abundant in the protoplanetary disk when chondrules were being formed approximately 2 myr after calcium aluminum rich inclusions cais and that chondrites far from sampling the earliest planetesimals are made from material that accreted later when 26al could no longer induce melting we show how splashing is reconcilable with many features of chondrules including their ages chemistry peak temperatures abundances sizes cooling rates indented shapes relict grains igneous rims and metal blebs and is also reconcilable with features that challenge the conventional view that chondrules are flash melted dust clumps particularly the high concentrations of na and feo in chondrules but also including chondrule diversity large phenocrysts macrochondrules scarcity of dust clumps and heating we speculate that type i feo poor chondrules come from planetesimals that accreted early in the reduced partially condensed hot inner nebula and that type ii feo rich chondrules come from planetesimals that accreted in a later or more distal cool nebular setting where incorporation of water ice with high 17o aided oxidation during heating we propose that multiple collisions and repeated re accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties and led to so called complementarity and metal depletion in chondrites we suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation

pliocene near trench magmatism in southern chile a possible manifestation of ridge collision eight shallow level intrusions and an ophiolite complex have recently been discovered in a remote region along the pacific coast of southern chile known as the taitao peninsula the magmatic rocks are found only 10 to 15 km from the buried extension of the peru chile trench the ophiolite body includes serpentinized ultramaflc rocks gabbro dikes and an interbedded sequence of volcanic and marine sedimentary rocks more regionally distributed granodiorite plutons and porphyritic stocks and sills intrude pre late jurassic basement as well as tertiary marine sequences of the south american forearc k ar ages of 3 5 to 4 0 ma of the ophiolite and other intrusions suggest that these rocks represent a short lived pulse of pliocene magmatism directly seaward of the taitao peninsula a segment of the chile rise bounded by the tres monies and taitao fracture zones collided with the chilean forearc and was subducted about 2 5 to 4 0 ma because of this good spatial and temporal correlation the magmatic activity of the taitao peninsula is likely to be a direct result of ridge subduction

manganese chromium isotope systematics of enstatite meteorites abstract we present results of a study of the 53 mn 53 cr isotope systematics in the enstatite chondrites and achondrites aubrites the goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short lived radionuclide 53 mn our earlier work lugmair and shukolyukov 1998 shukolyukov and lugmair 2000a has shown that the asteroid belt bodies are characterized by essentially the same initial 53 mn abundance however we have found the presence of a gradient in the abundance of the radiogenic 53 cr between the earth moon system mars and the asteroid vesta if this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated this can be explained either by an early volatility controlled mn cr fractionation in the nebula or by an original radially heterogeneous distribution of 53 mn the enstatite chondrites are suggested to form in the inner zones of the solar nebula much closer to the sun than the ordinary chondrites therefore their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53 mn we have studied the bulk samples of the eh4 chondrites indarch and abee and the el6 chondrite khairpur although these meteorites have essentially the same mn cr ratio as the ordinary chondrites the relative abundance of the radiogenic 53 cr is three times smaller than in the ordinary chondrites because these meteorites are primitive undifferentiated and no mn cr fractionation had occurred within their parent bodies this difference is a strong argument in favor of an initially heterogeneous distribution of 53 mn in the early inner solar system this finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula using the characteristic 53 cr excess of the enstatite chondrites and the observed gradient their place of origin falls at about 1 4 au or somewhat closer to the sun i e 1 0 1 4 au we also present chronological results for the enstatite chondrites and achondrites the absolute 53 mn 53 cr ages of the eh4 chondrites are old 4565 ma the el6 chondrite khairpur is 4 5 ma younger which is in good agreement with the 129 i 129 xe data from the literature the age of the aubrite pena blanca spring appears to be similar to those of the enstatite chondrites while that of the aubrite bishopville is at least 10 ma younger which is also in agreement with the 129 i 129 xe data the results from bulk samples of aubrites indicate that the last mn cr fractionation in their parent body occurred 4563 ma ago and imply an evolution of the mn cr isotope system in an environment with an higher than chondritic mn cr ratio for several millions of years

chondrule textures and precursor grain size an experimental study chondrule formation appears to have been a major event in the early solar system but chondrule properties do not allow us to distinguish between several possible formation mechanisms the physical nature of the precursors especially grain size must affect the textures of the chondrules they yield when heated we melted precursors of different grain sizes including extremely fine grained crystalline aggregates analogous to nebular condensates to see whether objects resembling most natural chondrules can be crystallized with one minute heating and moderate cooling rates the grain size of the charges depends directly on the grain size of the starting material for temperatures up to very close to the liquidus temperature a single rapid heating of condensate like material thus produces very fine grained chondrules like dark zoned chondrules for a very wide range of peak temperatures it is incapable of generating the observed textural distribution of chondrules which are predominantly porphyritic the simplest model for chondrules a single heating of unmodified condensate material therefore appears unrealistic coarse grained chondrules might be formed from fine grained precursors by extended heating with evaporation leading to coarsening or by multiple reheating events with higher temperatures in subsequent events otherwise an origin from annealed condensates planetary rocks or by condensation of liquid and crystals is required

53mn 53cr and 26al 26mg ages of a feldspathic lithology in polymict ureilites abstract we report 53 mn 53 cr and 26 al 26 mg isotopic data obtained by in situ sims analysis for feldspathic clasts in polymict ureilites dag 319 and dag 165 the analyzed clasts belong to the albitic lithology the most abundant population of indigenous feldspathic materials in polymict ureilites and are highly fractionated igneous assemblages of albitic plagioclase fe rich pyroxenes phosphates ilmenite silica and fe mn k p ti enriched glass glass in dag 165 clast 19 has extremely high and variable 55 mn 52 cr ratios 500 58 000 and shows correlated 53 cr excesses up to 1500 clearly indicating the presence of live 53 mn at the time of formation the slope of the well correlated isochron defined by glass and pyroxenes from this clast corresponds to 53 mn 55 mn 2 84 0 10 10 6 2 data for less 55 mn 52 cr enriched glasses from dag 319 clast b1 as well as phosphates from several other clasts are consistent with this isochron the 53 mn 55 mn ratio obtained from the isochron implies that these clasts are 0 70 0 18 ma younger than the d orbigny angrite corresponding to the absolute age of 4563 72 0 22 ma plagioclase in dag 319 clast b1 has a fairly constant 27 al 24 mg ratio of 900 and shows resolvable 26 mg excesses of 2 the slope of the isochron defined by pyroxene and plagioclase in this clast is 3 0 1 1 10 7 2 corresponding to a time difference of 5 4 0 3 0 5 ma after cai assuming the canonical initial 26 al 27 al ratio of 5 10 5 and an age 0 5 0 3 0 5 ma younger than d orbigny its absolute age relative to d orbigny is 4563 9 0 4 0 5 ma in agreement with the 53 mn 53 cr age from clast 19 these data provide the first high precision age date 5 4 ma after cai for ureilites giving a minimum estimate for the age of differentiation of their parent asteroid interpretation of this age for the thermal and physical history of that asteroid depends on a number of currently unknown or model dependent parameters including its size bulk composition and oxidation state and the petrologic relationship between the feldspathic clasts and main group ureilites

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

widespread magma oceans on asteroidal bodies in the early solar system our solar system formed about 4 6 billion years ago and within 4 million years small planetary bodies had formed some melting to form volcanic and related rocks two families of meteorites the heds and angrites are thought to have originated from asteroids that melted at this time new oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large scale melting in the early solar system these new results show that early global scale melting was a feature of all the differentiated planets earth moon and mars and asteroids so far sampled immediately following the formation of the solar system small planetary bodies accreted1 some of which melted to produce igneous rocks2 3 over a longer timescale 15 33 myr the inner planets grew by incorporation of these smaller objects4 5 through collisions processes operating on such asteroids strongly influenced the final composition of these planets4 including earth5 currently there is little agreement about the nature of asteroidal igneous activity proposals range from small scale melting to near total fusion and the formation of deep magma oceans2 here we report a study of oxygen isotopes in two basaltic meteorite suites the heds howardites eucrites and diogenites which are thought to sample the asteroid 4 vesta6 and the angrites from an unidentified asteroidal source our results demonstrate that these meteorite suites formed during early global scale melting 50 per cent events we show that magma oceans were present on all the differentiated solar system bodies so far sampled magma oceans produced compositionally layered planetesimals the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features such as earth s high mg si ratio

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

chondrule formation by repeated evaporative melting and condensation in collisional debris clouds around planetesimals a synthesis of previous work leads to a model of chondrule formation that involves periodic melting of dispersed dust in debris clouds that were generated by collisions between chondritic planetesimals i suggest that chondrules formed by the passage of nebular shock waves through these dust clumps which temporarily surrounded disrupted planetesimals type i chondrules formed by more intense evaporative heating of fewer particles in tenuous clumps or at the edges of dense clumps and type ii chondrules formed by less intense evaporative heating of more particles deeper within dense clumps chondrules reaccreted by self gravity into the planetesimals mixing with less heated dust and rock this process of disruption melting and reaccretion could have repeated many times in this way chondrite components of various origins and thermal histories could remain preserved in planetesimals as a distinctive mix of materials for extended periods of time while still allowing for a repetitive melting process that converted some of the planetesimal debris into chondrules i also suggest that during chondrule formation the inner solar nebula gas was evolving by the gradual incorporation and heating of icy bodies depleted in 16 o causing a general increase in gaseous d 17 o with time in most places especially close to the snow line in this model early formed type i chondrules in c chondrites with lower d 17 o values were produced inside the snow line and later formed type i and type ii chondrules in c and o chondrites with higher d 17 o values were created nearer the snow line after it had moved closer to the young sun

continental response to active ridge subduction 1 apatite fission track ages from a 2000 m elevation transect from the patagonian fold and thrust belt 47 5 s allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction accelerated cooling started at 17 ma predating the onset of ridge collision 14 10 ma and was followed by reheating between 10 and 6 ma thermal modeling favors reheating on the order of 60 c at 28 c ma due to east migration of a slab window after the ridge trench collision final rapid cooling since 4 ma of 18 c ma geothermal gradient of 14 c km correlates with the presence of an orographic barrier and 1 km rock uplift in this region between 17 1 and 6 3 ma increased precipitation and erosion since 4 ma caused asymmetric exhumation with 3 4 km on the leeside repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the chile triple junction

u pb and 207pb 206pb ages of zircons from basaltic eucrites implications for early basaltic volcanism on the eucrite parent body we have undertaken petrologic and shrimp u th pb isotopic studies on zircons from basaltic eucrites yamato y 75011 y 792510 asuka a 881388 a 881467 and padvarninkai with different thermal and shock histories eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene and typically have rounded to subrounded shapes due to recrystallization superchondritic zr hf ratios of eucritic zircons indicate that they crystallized from incompatible element rich melts after crystallization of ilmenite concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite y 75011 are higher than those in metamorphosed eucrites the u pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance radiogenic lead loss up to 48 from zircons is observed in the shock melted eucrite padvarninkai the 207pb 206pb ages of zircon in y 75011 4550 9 ma n 5 are nearly identical within analytical uncertainty to the ages of zircons from the metamorphosed eucrite y 792510 4545 15 ma n 13 the highly metamorphosed eucrites a 881388 4555 54 ma n 5 and a 881467 4558 13 ma n 8 and the shock melted eucrite padvarninkai 4555 13 ma n 18 the averaged 207pb 206pb age of zircon from five eucrites analyzed in this study is 4554 7 ma 95 confidence limits n 49 indistinguishable from the averaged u pb age 4552 9 ma of the same samples because of the high closure temperature of lead in zircon tclosure 1050 c with a cooling rate of 0 2 c yr the 207pb 206pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas this fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the solar system history 7 20 ma after cai formation 4567 2 0 6 ma thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body the u pb ages of eucritic zircons are older than the u pb sm nd and rb sr ages of some basaltic eucrites which is consistent with differences in closure temperatures of each isotopic system and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene plagioclase and phosphates

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

abundance major element composition and size of components and matrix in cv co and acfer 094 chondrites abstract the relative abundances and chemical compositions of the macroscopic components or inclusions chondrules and refractory inclusions and fine grained mineral matrix in chondritic meteorites provide constraints on astrophysical theories of inclusion formation and chondrite accretion we present new techniques for analysis of low count pixel si mg ca al ti and fe x ray intensity maps of rock sections and apply them to large areas of co and cv chondrites and the ungrouped acfer 094 chondrite for many thousands of manually segmented and type identified inclusions we are able to assess pixel by pixel the major element content of each inclusion we quantify the total fraction of refractory elements accounted for by various types of inclusion and matrix among co chondrites both matrix and inclusion mg si ratios approach the solar and bulk co ratio with increasing petrologic grade but si remains enriched in inclusions relative to matrix the oxidized cv chondrites with higher matrix inclusion ratios exhibit more severe aqueous alteration oxidation and their excess matrix accounts for their higher porosity relative to reduced cv chondrites porosity could accommodate an original ice component of matrix as the direct cause of local alteration of oxidized cv chondrites we confirm that major element abundances among inclusions differ greatly across a wide range of co and cv chondrites these abundances in all cases add up to near chondritic solar bulk abundance ratios in these chondrites despite wide variations in matrix inclusion ratios and inclusion sizes chondrite components are complementary this complementarity provides a robust meteoritic constraint for astrophysical disk models

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

development of coarse grained facies in lacustrine rift basins examples from east africa detailed studies of high resolution and multifold seismic reflection data from the two largest east african rift lakes malawi and tanganyika reveal a complex suite of coarse grained depositional facies these facies occur within specific regions of the controlling half grabens that compose the rift lakes sand prone environments include subaqueous channels and small drowned fluvial complexes channel systems range from large erosional canyons to deep water turbidite channel levee systems lowstand and highstand deltas of axial and shoaling side rivers are volumetrically important coarse grained facies fan deltas develop along the base of major border faults during lake lowstands subaqueous talus fan deposits occur along the base of the border faults during lake highstands lowstand deltas are the best preserved progradational facies in these rift lakes in addition to simple tectonic control drastic tectonically or climatically induced lake level change significantly regulates the production of coarse grained lacustrine synrift deposits

automatic 1 d waveform inversion of marine seismic refraction data summary full waveform synthetic seismograms are now used as standard in the interpretation of marine refraction data but only with a laborious trial and error fitting procedure we show how the matching of observed waveforms with wkbj synthetic seismograms can be efficiently and automatically performed for 1 d earth models automation of the inversion is difficult because of the multi modal irregular form of the misfit of data and synthetics a sequence of three steps is required for a complete inversion location of the deepest valley of the misfit function descent to the global minimum and description of the neighbourhood of the minimum for error analysis the first step is accomplished with a monte carlo search through a very large model space defined by poor prior knowledge of velocities and gradients of the solution weak traveltime constraints are used to eliminate poorly fitting models from waveform calculations a comparison of the traveltime and waveform misfits of the monte carlo models clearly illustrates that waveforms are providing more information than traveltimes alone bayesian statistics are used to construct marginal probability distributions and the covariance matrix which give a rough preliminary error analysis in the second step damped least squares linearized inversion makes small adjustments to the best fitting monte carlo model as it descends to the global minimum finally the immediate neighbourhood of the global minimum is explored with constrained least squares inversion realistic error bounds on each parameter are defined from the resulting slices through the misfit function these bounds are much narrower than traveltime inversion provides correlations between parameters are obtained from the covariance matrix constructed from models examined during this error analysis the inversion methods are illustrated on the ff2 refraction data set of the scripps institution of oceanography the monte carlo search successfully locates the valley of the global minimum as well as a nearby secondary minimum the error analysis puts realistic error bounds on the detailed inversion of this data set by spudich orcutt

subducting seamounts and deformation of overriding forearc wedges around japan abstract two subducting seamounts under inner trench slopes have been identified around japan on the basis of magnetic anomalies morphology and geological structure the first one is located under the foot of the inner trench slope at the junction between the japan trench and the kuril trench another one occurs beneath the slope slightly seaward of the tosabae the basement high at the trench slope break along the nankai trough off shikoku the magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i e a swell landward and a depression seaward the seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions which suggests that the subducting seamounts have kept roughly their original shapes the morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward deformation of the forearc wedge by the subducting seamount extends to the forearc basin the seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes two other examples of deformed inner trench slopes around japan which can be explained by subduction of topographic highs are presented one example is a depression on the foot of the inner trench slope northeast of the junction between the kyushu palau ridge and the nankai trough another one is an area of complex morphology of the inner trench slope along the japan trench around the daiichi kashima seamount

structural and magmatic responses to spreading ridge subduction an example from southwest japan subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries the late oligocene early miocene rocks of the shimanto accretionary prism at cape muroto shikoku island record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms these accreted strata have been imprinted with an unusual late stage event that involved 1 regional cusp like flexing of structural trends in the prism 2 near trench magmatism and 3 pervasive faulting this event overlaps in time with the opening of the shikoku back arc basin circa 26 14 ma the shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of cape muroto the regional flexing of structural trends in the prism affects rocks as old as cretaceous although the flexure is most tightly developed in the late oligocene early miocene portion of the prism where it is steeply plunging kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature at cape muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid ocean ridge basalts morb from the shikoku back arc basin preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development these data combined with plate reconstructions for the area at 15 ma indicate that indentation of the prism most likely resulted from early miocene orthogonal subduction of the topographically elevated shikoku basin spreading ridge in the muroto area morb magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction

subduction of the south china sea axial ridge below luzon philippines abstract the scarborough seamount chain present at the axis of the extinct south china sea spreading center is being subducted obliquely along the manila trench a detailed seabeam survey of this convergent zone reveals that the fabric of the ridge is characterized by n60 e trending normal faults and n130 e transform faults this ridge can be traced into the forearc area this could be interpreted as the result of underplating of ridge fragments which are accreted to the upper plate

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

are high temperature fractionations in the solar nebula preserved in highly siderophile element systematics of the earth s mantle the relative abundances of the highly siderophile elements hse os ir ru pt rh and pd in relatively pristine lherzolites differ from solar abundance ratios and are several orders of magnitude higher than predicted for equilibrium distribution between metal silicate core mantle the samples are characterized by a mean ca al ratio of 1 18 0 09 m and a mean ca si ratio of 0 10 0 01 m overlapping with a mean ca al of 1 069 0 044 m and a mean ca si of 0 081 0 023 m found in chondrites wasson and kallemeyn 1988 interestingly the ci normalized abundance pattern shows decreasing solar system normalized abundances with increasing condensation temperatures the abundance of the moderately volatile element pd is about 2x higher than those in the most refractory siderophiles ir and os thus the hse systematics of upper mantle samples suggest that the late bombardment which added these elements to the accreting earth more closely resembles materials of highly reduced eh or el chondrites than carbonaceous chondrites in fact the hse in the earth mantle are even more fractionated than the enstatite chondritesan indication that some inner solar system materials were more highly fractionated than the latter

the i xe chronometer and the early solar system we review the development of the i xe technique and how its data are interpreted and specify the best current practices individual mineral phases or components can yield interpretable trends in initial 129i 127i ratio whereas whole rock i xe ages are often hard to interpret because of the diversity of host phases many of which are secondary varying standardizations in early work require caution only samples calibrated against shallowater enstatite or bjurble can contribute reliably to the emerging i xe chronology of the early solar system although sparse data for which i xe and mn cr can be compared suggest that the two systems are concordant among ordinary chondrite samples we derive a new age for the closure of the shallowater enstatite standard of 4563 3 0 4 myr from the relationship between the i xe and pb pb systems this yields absolute i xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system absolute i xe dates for aqueous and igneous processes are consistent with other systems consideration of the i xe host phases in cais and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre existing material the ranges of chondrule ages deduced from the al mg and i xe systems in semarkona ll3 0 and chainpur ll3 4 are consistent chainpur i xe data exhibit a greater range of ages than semarkona possibly reflecting a greater degree of parent body processing however individual chondrules show little or no evidence of such processing determining the host phase s responsible for high temperature correlations may resolve the issue

the scale size of chondrule formation regions constraints imposed by chondrule cooling rates meteoritic data strongly suggest that most chondrules reached maximum temperatures in a range of 1650 2000 k and cooled at relatively slow rates of 100 1000 k h implying a persistence of external energy supply the presence of fine grained rims around chondrules in most unequilibrated chondrites also indicates that a significant quantity of micron sized dust was present in chondrule formation regions here we assume that the persistent external energy source needed to explain chondrule cooling rates consists primarily of radiation from surrounding heated chondrules fine dust and gas after the formation event using an approximate one dimensional numerical model for the outward diffusion of thermal radiation from such a system the scale sizes of formation regions required to yield acceptable cooling rates are determined for a range of possible chondrule dust and gas parameters results show that the inferred scale sizes depend sensitively on the number densities of micron sized dust and on their adopted optical properties in the absence of dust scale sizes lo00 km are required for plausible maximum chondrule number densities and heated gas parameters in the presence of dust with mass densities comparable to those of the chondrules and with absorptivities and emissivities of 0 01 calculated for mie spheres with a pure mineral composition scale sizes as small as 100 km are possible if dust absorptivities and emissivities approach unity as may occur for particles with more realistic shapes and compositions then scale sizes as small as 10 km are possible considering all uncertainties in model parameters it is concluded that small scale sizes 10 100 km for chondrule formation regions are allowed by the experimentally inferred cooling rates

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

aluminum 26 in h4 chondrites implications for its production and its usefulness as a fine scale chronometer for early solar system events in order to investigate whether or not 26al can be used as a fine scale chronometer for early solar system events we measured with an ion microprobe mg isotopes and al mg ratios in separated plagioclase olivine and pyroxene crystals from the h4 chondrites ste marguerite forest vale beaver creek and quenggouk and compared the results with the canonical 26al 27al ratio for ca al rich inclusions cais for ste marguerite sm and forest vale fv pb pb and mn cr ages have previously been determined gpel et al 1994 polnau et al 2000 polnau and lugmair 2001 plagioclase grains from these two meteorites show clear excesses of 26mg the 26al 27al ratios inferred from these excesses and from isotopically normal mg in pyroxene and olivine are 2 87 0 64 x 10 7 for sm and 1 52 0 52 x 10 7 for fv the differences between these ratios and the ratio of 5 x 10 5 in cais indicate time differences of 5 4 0 1 ma and 6 1 0 2 ma for sm and fv respectively these differences are in agreement with the absolute pb pb ages for cais and sm and fv phosphates but there are large discrepancies between the u pb and mn cr system for the relative ages for cais sm and fv for example mn cr ages of carbonates from kaidun are older than the pb pb age of cais however even if we require that cais are older than these carbonates the time difference between this adjusted cai age and the mn cr ages of sm and fv require that 26al was widely distributed in the early solar system at the time of cai formation and was not mostly present in cais a feature of the x wind model proposed by shu and collaborators gounelle et al 2001 shu et al 2001 from this we conclude that there was enough 26al to melt small planetary bodies as long as they formed within 2 ma of cais and that 26al can serve as a fine scale chronometer for early solar system events

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

oxygen silicon and mn cr isotopes of fayalite in the kaba oxidized cv3 chondrite constraints for its formation history the iron rich olivine end member fayalite occurs in the matrix chondrules ca al rich inclusions cais silicate aggregates and dark inclusions in the kaba and mokoia oxidized cv3 chondrites in most occurrences fayalite is associated with magnetite and troilite to help constrain the origin of the fayalite fa98 100 we measured oxygen and silicon isotopic compositions and mn cr systematics in fayalite from two petrographic settings of the kaba meteorite one setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine grained magnetite and sulfide while the other setting consists of small fayalite magnetite sulfide assemblages within or at the surface of type i barred or porphyritic olivine chondrules oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line and is distinct from that in chondrule forsterites which are enriched in 16o 17o 4 oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites silicon isotopes suggest that forsterite is 1 amu heavier than adjacent fayalite within kaba chondrules however we were unable to confirm large silicon isotopic differences among fayalites reported previously the mn cr data for big kaba fayalites give an initial 53mn 55mn ratio of 2 07 0 17 10 6 consistent with literature results on mokoia chondrule fayalites the combined data suggest that fayalites in both petrographic settings formed at about the same time 9 7 ma after the formation of cais our data indicate that those fayalite magnetite troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body the formation site and mechanism for the big fayalite laths is less clear but the petrographic setting indicates that they did not form in situ none of the models that have been suggested for formation of these fayalites is entirely satisfactory

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

the crustal structure from teleseismic p wave coda i method summary the crustal structure beneath a broad band station has a strong influence on the recorded body wave coda conversely it is evident that crustal receiver effects can be employed to obtain information about the crustal structure at the receiver site in this paper we present a method to estimate the crustal receiver response and we invert the response to obtain a model of the most distinct features of the crustal structure beneath the station the crustal receiver response of a station is determined from the p wave coda of deep teleseisnic events by deconvolution with an empirically determined source wavelet this source wavelet is assumed to incorporate source attenutation and instrument effects responses obtained from clusters of events are stacked to enhance the signal to noise ratio of the data and to obtain an estimate of the variance of the data the crustal p and s velocity structure at the station is determined by non linear waveform inversion of its crustal responses an inversion strategy was developed to search efficiently for the global minimum in the misfit between data and synthetics the inversion appears to be most sensitive to large velocity constrasts in the velocity structure which give rise to high amplitude reflected and converted phases the method is illustrated by the inversion of the crustal responses of nars station ne05 it is shown that a thin layer 1 2 km of unconsolidated sediments and a strong mid crustal discontinuity at 11 km depth constitute the most prominent features of the crustal structure at this station they cause an apparent delay of the first arrival on the radial component of the seismograms and produce high amplitude sv multiples in the p wave coda

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

on early solar system chronology implications of an heterogeneous spatial distribution of 26al and 53mn abstract early solar system chronology is usually built with the assumption that the distribution of short lived radionuclides was homogeneous through the solar accretion disk at present there is no unambiguous evidence for a homogeneous distribution of short lived radionuclides in the solar accretion disk while some data point to a heterogeneous distribution of short lived radionuclides in this paper we explore a possible chronology based on a heterogeneous distribution of 26 al and 53 mn in the accretion disk our basic assumption is that the different abundances of extinct short lived radionuclides in calcium aluminium rich inclusions cais and chondrules are due to spatial rather than temporal differences we develop a simple model where cais and chondrules form contemporaneously in different spatial locations and are characterised by distinct initial 26 al and 53 mn abundances in this model all evolved bodies are supposed to be originally chondritic i e to be made of a mixture of cais chondrules and matrix this mixture determines the initial content in 26 al and 53 mn of a chondritic parent body as a function of its cai and chondrule abundance fraction this approach enables us to calculate coherent 26 al and 53 mn ages from the agglomeration of the parent body precursors cais and chondrules until the isotopic closure of 26 al and 53 mn thereafter called 26 al 53 mn age we calculate such 26 al 53 mn ages for a diversity of evolved objects with the constraint that they should be found for realistic chondritic parent body precursors i e objects having similar or identical petrograpy to the existing chondrite groups the so defined age of the d orbigny angrite is 4 3 1 1 myr for the asuka 881394 eucrite 2 8 1 0 myr for the h4 chondrite sainte marguerite 3 myr and for h4 forest vale 5 myr the calculated 26 al 53 mn ages give timescales for the evolution of the respective parent bodies meteorites that can be investigated in the light of further petrographic studies we anchor the calculated relative chronology to an absolute chronology using absolute pb pb ages and relative hf w ages of the objects under scrutiny the precursors of sainte marguerite and forest vale agglomerated at the same time 4565 8 1 2 ma ago the precursors of eucrites asuka 881394 agglomerated 4564 8 1 2 ma ago the precursors of angrites agglomerated late 4561 5 1 8 ma ago our model provides a fully compatible al mg mn cr pb pb chronology and is shown to be robust to reasonable changes in the input parameters the calculated initial 26 al 27 al ratios are high enough to have 26 al as a possible heat source for differentiation

high precision u pb zircon dating of the chalten plutonic complex cerro fitz roy patagonia and its relationship to arc migration in the southernmost andes 1 we report new high precision u pb ages and geochemical data from the chalten plutonic complex to better understand the link between magmatism and tectonics in southern patagonia this small intrusion located in the back arc region east of the patagonian batholith provides important insights on the role of arc migration and subduction erosion the chalten plutonic complex consists of a suite of calc alkaline gabbroic to granitic rocks which were emplaced over 530 kyr between 16 90 0 05 ma and 16 37 0 02 ma a synthesis of age and geochemical data from other intrusions in patagonia reveals a striking similarities between the chalten plutonic complex and the neogene intrusions of the batholith and differences to other back arc intrusions such as torres del paine b a distinct e w trend of calc alkaline magmatic activity between 20 and 17 ma we propose that this trend reflects the eastward migration of the magmatic arc and the consistent age pattern between the subduction segments north and south of the chile triple junction suggests a causal relation with a period of fast subduction of the farallon nazca plate during the early miocene previously proposed flat slab models are not consistent with the present location and morphology of the southern patagonian batholith we advocate alternatively that migration of the magmatic arc is caused by subduction erosion due to the increasing subduction velocities during the early miocene

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

nickel and chromium isotopes in allende inclusions high precision nickel and chromium isotopic measurements have been carried out on nine allende inclusions 62ni 64ni excesses are present in at least three of the samples the magnitudes of the excesses are about 3 e units 1 e unit 1 part in 10 000 for64ni and about 1 e unit for62ni chromium isotopes measured for some of the same samples show general excesses on54cr of about 7 e units while small deficits are observed for53cr considering these isotope anomalies except those for53cr together with the excesses present in the samples of other neutron rich isotopes of the iron group elements the most likely mechanism responsible for these anomalies is a neutron rich statistical equilibrium process in addition there is an indication of elevated60ni in almost every inclusion measured however for alk 111 this excess was clearly resolved by our experimental technique this effect may be related to the decay of now extinct60fe but the present data are not sufficient to clearly demonstrate its existence in the early solar system an upper limit of 1 6 10 6 can be calculated for60fe 56fe at the time these allende inclusions crystallized

significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core mantle differentiation of terrestrial planets abstract seven bulk chondrites with 57 fe 54 fe values between 0 1 and 0 relative to irmm 14 tend to be slightly lighter than 11 bulk iron meteorites which have 57 fe 54 fe values ranging from 0 04 to 0 2 at the mineral scale taenite from two iron meteorites cranbourne and toluca shows 57 fe 54 fe values heavier by up to 0 3 than their kamacite counterpart thus calling into question the significance of bulk iron meteorite data on three pallasites esquel marjalahti and springwater we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites eagle station and imilac but the range of 57 fe 54 fe differences from 0 32 to 0 07 is larger than that originally found troilite from two pallasites appears to be even heavier than the metal fraction whereas schreibersite is lighter than its olivine counterpart there is thus a general tendency for minerals within a given rock to show a heavier fe isotope composition as the coordination number of fe increases although troilite is an exception to this rule iron meteorites are classically considered as remnants of asteroid cores and pallasites as core mantle interfaces the simultaneous finding that the metal fractions of pallasites have a higher 57 fe 54 fe signature than the coexisting olivines and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core mantle differentiation is accompanied by sizeable iron isotope fractionation in this hypothesis mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material that is not observed however since all planetary mantles analyzed so far have 57 fe 54 fe values equivalent to or heavier than those of chondrites it thus appears that the moderate temperature and pressure metal silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as fe isotopes are concerned instead these mantle signatures could reflect equilibrium fractionation at a higher temperature or the lack of a global core mantle equilibration at the planetary scale overall these new results show that the mass dependent isotopic scatter observed among inner solar system bodies from the bulk rock to the planetary scale 0 3 57 fe 54 fe is more restricted than previously thought this likely confirms a homogenization process that occurred in the protoplanetary accretion disk between refractory inclusion condensation and chondrule formation

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region