collisional history of h chondrites the cosmic ray exposure histories of ordinary chondrites were recently reviewed marti and graf 1992 here we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual h chondrites we further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records in addition to the stochastic events 7 to 8 m y ago a peak at 33 ma represents a statistically significant deviation from an exponential age distribution and signals a major collisional event at this time the h6 subgroup also recorded a similar event 24 m y ago these records present significant arguments against a continuous supply of h chondrites from the asteroid belt we show that the records of fall times for h5s are distinct from other subgroups and that a different orbit for the h5 parent is indicated in collisional events 7 to 8 m y ago the systematics of noble gas losses in the h5 subgroup are consistent with this interpretation the evidence for stochastic fragmentation and subsequent delivery to earth of h chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution

tectonic patterns of the panama block deduced from seismicity gravitational data and earthquake mechanisms implications to the seismic hazard abstract an integration of seismicity parameters gravitational data focal mechanisms other geophysical data and geological information is used to determine boundaries of the panama block pb and tectonic regimes which control them the panama fracture zone represents a clear boundary between the nazca and cocos plate and is undergoing subduction beneath the pb its interaction with the cocos ridge at the western boundary of the block from the panamanian shelf break and inland adds a higher degree of tectonic complexity to this area in the north the boundary with the caribbean plate lies on the panama deformed belt along the caribbean margin of the isthmus of panama the eastern boundary of the pb is the panama south america suture zone in which the shear motion between the pb and the nazca plate is coupled with major compressive forces associated with the convergence of the nazca and south american plates the southern boundary consists of a combination of a left lateral transform fault south of the gulf of panama and a broad left lateral shear zone between the coiba ridge and the azuero peninsula the values of the cumulative seismic hazard index are greater in zones with more complex tectonics

cosmogenic nuclides in the kirin chondrite abstract radioactive and stable cosmogenic nuclides have been determined in the various fragments of the kirin chondrite h5 experimental results obtained from our samples are described and compared with similar data obtained in other laboratories cosmogenic 54 mn 22 na 60 co 26 al 53 mn 40 k in the metal phase and light noble gases were measured based on these data the irradiation history of this meteorite can be explained in terms of 1 a multi stage exposure which involves a first stage irradiation for 10 7 years and a second stage for about 4 10 5 years and 2 depth effects in the productions of the nuclides in 4 second stage and 2 first stage geometries these conclusions are consistent with those of our previous work which was based on a limited number of samples

reply to comment of d w g sears f a hasan b m myers and h sears on update on terrestrial ages of antarctic meteorites

a model for meteoritic and lunar 40 ar 39 ar age spectra addressing the conundrum of multi activation energies abstract results of whole rock 40 ar 39 ar step heating analyses of extra terrestrial materials have been used to constrain the timing of impacts in the inner solar system solidification of the lunar magma ocean and development of planetary magnetic fields despite the importance of understanding these events the samples we have in hand are non ideal due to mixed provenance isotopic disturbances from potentially multiple heating episodes and laboratory artifacts such as nuclear recoil although models to quantitatively assess multi domain diffusive 40 ar loss have long been applied to terrestrial samples their use on extra terrestrial materials has been limited here we introduce a multi activation energy multi diffusion domain model and apply it to 40 ar 39 ar temperature cycling step heating data for meteoritic and lunar samples we show that age spectra of extra terrestrial materials the jilin chondrite k 4 and apollo 16 lunar breccia 67514 43 yielding seemingly non ideal behavior commonly interpreted as either laboratory artifacts or localized shock heating of pyroxene are meaningful and can be understood in context of the presence of multi diffusion domains containing multiple activation energies internally consistent results from both the meteoritic and lunar samples reveal high temperature short duration thermal episodes we interpret as due to moderate shock heating

a new look at evidence for a wadati benioff zone and active convergence at the north panama deformed belt the nature of the northern boundary of the panama microplate often referred to as the north panama deformed belt npdb has been the subject of much speculation previous studies have used a variety of data including teleseismic gravity bathymetric marine magnetic and field studies of uplift and tsunami deposits as well as modified mercalli intensity distributions from historic earthquakes to reveal the nature of the npdb data have been collected for over 30 yrs and yet the character of the npdb remains unclear current analyses and interpretation provide a number of mutually exclusive options and much controversy in this article we examine local and regional seismicity combined with teleseismic observations and historic earthquake data to present an alternative analysis for the npdb using small earthquakes recorded by a local network we image a well defined wadati benioff zone dipping southward beneath the panama microplate these data provide new evidence regarding the nature of panama microplate caribbean plate boundary kinematics and demonstrate the existence of an active subduction zone a more complete understanding of the nature of the seismicity and plate interactions along the npdb offshore panama including a subduction zone capable of producing earthquakes of m 7 should be included in future earthquake hazard assessments

noble gases in h chondrites and potential differences between antarctic and non antarctic meteorites abstract thirty one antarctic h chondrites from the allan hills have been analyzed for their noble gases seven of the specimens contain solar gases thirteen have a cosmic ray exposure age around 8 ma both fractions are as observed for h chondrites from the rest of the world scrutiny of the data for paired samples yields a minimum number of 17 independent falls with 6 falls required for the 8 ma peak of exposure age the distribution of exposure ages as well as that of radiogenic 4 he and 40 ar yield no evidence that h chondrites from the allan hills region represent a mixture of populations that is different from that brought to moderate latitudes of the earth in more recent times

spallogenic10be in the jilin chondrite abstract the 10 be contents measured in eight bulk samples of the jilin meteorite range from 5 05 0 55 to 16 3 1 7 dpm kg more than three fold and correlate with the 21 ne contents a least squares fit of these data has a positive intercept on the 10 be axis which can be explained in terms of a two stage irradiation the second more recent stage lasted 0 57 0 14 myr model calculations give first stage 10 be 21 ne ages ranging from 6 2 to 7 1 myr

seismicity and tectonics of southern central america and adjacent regions with special attention to the surroundings of panama abstract ten new focal mechanisms are derived for earthquakes in southern central america and its adjacent regions these are combined with a study of seismicity and data of previous workers to delineate the position and nature of the plate boundaries in this complex region the middle america subduction zone may be divided into four or five distinct seismic segments the plate boundary between north america and the caribbean near the trench might be located more towards the south than previously suspected subduction has basically stopped south of the underthrusting cocos ridge there is not much evidence for a seismically active strike slip fault south of panama but its existence cannot be ruled out more activity reveals the zone north of panama which is identified as a subduction zone with normal fault events shallow seismicity induced by the interaction of the nazca plate extends from the colombia panama border south along the pacific coast to meet a high angle continental thrust fault system subduction with a pronounced slab starts only south of that point near a hot region which offsets the seismic trend at the trench the carnegie ridge and or the change of direction of subduction in ecuador produce a highly active zone of seismicity mainly at the depth of 200 km the area in the pacific displays a termination of activity at a propagating rift west of the galapagos islands the main eastern boundary of the cocos plate the panama fracture zone is offset towards the west at the southern end of the malpelo ridge its northern end consists of two active branches as defined by large earthquakes a strike slip mechanism near the southeastern flank of the cocos ridge was previously believed to be the site of an extended fracture zone this paper proposes submarine volcanic activity as an alternative explanation

light noble gases in jilin more of the same and something new results are reported for documented samples from two drill cores and for specimens from the strewn field of the largest known stone meteorite the h chondrite jilin core b was found to have been parallel to the surface of jilin during its first stage of irradiation in 2 geometry core a was normal to the 2 surface in it the mean attenuation length for the production by galactic cosmic rays of 38ar from metal and of 21ne from mg al si in silicates was found to be the same a numerical value for the mean attenuation length of 71 4 cm or 246 14 g cm 2 follows if corrections for the contribution from the second stage of exposure are based on t2 0 32 ma agreement with the lower values of 180 g cm 2 obtained from lunar studies and target data requires t2 to be about twice as long previous results are confirmed that in specimens with high contents of stable spallogenic gases the ratio 21nebulk 38armetal is low the suggestion had been and is mat this is a transition effect in near surface samples where the secondary cascade of nuclear active particles and hence the production of 21ne was not yet fully developed this suggestion is borne out by the present results on two samples that based on cosmic ray tracks and 60co content are certified near surface samples although strictly speaking this is true only for the depth of burial during the second stage of irradiation cosmic ray produced 60co is positively correlated with 4he content indicating that significant losses of 4he occurred when the jilin meteoroid had acquired already its final size and shape and that the losses were more severe for near surface samples than for such from the 4 interior presumably the losses were caused by a thermal spike associated with the excavation of jilin from its parent body the same event caused losses of part of the 3he produced during the first irradiation stage from the systematics of the 3he 21ne vs 4he correlation we derive for the second 4 irradiation a production ratio 3he 21ne 3 2 0 4

the l3 6 chondritic regolith breccia northwest africa nwa 869 ii noble gases and cosmogenic radionuclides abstract we measured cosmogenic radionuclides and noble gases in the l3 6 chondrite breccia northwest africa nwa 869 one of the largest meteorite finds from the sahara concentrations of 10be 26al and 36cl in stone and metal fractions of six fragments of nwa 869 indicate a preatmospheric radius of 2 0 2 5 m the 14c and 10be concentrations in three fragments yield a terrestrial age of 4 4 0 7 kyr whereas two fragments show evidence for a recent change in shielding most likely due to a recent impact on the nwa meteoroid approximately 105 yr ago that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic ray fluxes this scenario is supported by the low cosmogenic 3he 21ne ratios in these two samples indicating recent loss of cosmogenic 3he most nwa samples except for clasts of petrologic type 4 6 contain significant amounts of solar ne and ar but are virtually free of solar helium judging from the trapped 4he 20ne ratio of approximately 7 trapped planetary type kr and xe are most clearly present in the bulk and matrix samples where abundances of 129xe from decay of now extinct 129i are highest cosmogenic 21ne varies between 0 55 and 1 92 10 8 cm3 stp g 1 with no apparent relationship between cosmogenic and solar ne contents low cosmogenic 22ne 21ne c ratios in solar gas free specimens are consistent with irradiation in a large body combined 10be and 21ne concentrations indicate that nwa 869 had a 4 cosmic ray exposure cre age of 5 1 myr whereas elevated 21ne concentrations in several clasts and bulk samples indicate a previous cre of 10 30 myr on the parent body most probably as individual components in a regolith unlike many other large chondrites nwa 869 does not show clear evidence of cre as a large boulder near the surface of its parent body radiogenic 4he concentrations in most nwa 869 samples indicate a major outgassing event approximately 2 8 gyr ago that may have also resulted in loss of solar helium

isotopic composition of xenon and krypton in silicate graphite inclusions of the el taco campo del cielo iab iron meteorite noble gases extracted by temperature stepped pyrolysis from acid resistant silicate schreibersite and graphite residues of inclusions from the iab iron meteorite el taco campo del cielo have been analyzed for their isotopic composition the concentrations of spallogenic light gases agree with previous data on adjacent specimens nominal feldspar and pyroxene fractions have lost up to 80 of their 3he during or towards the end of the exposure of el taco to the cosmic radiation in olivine the 3ne21ne and nepne ratios are as low as 2 18 0 01 and 1 09 0 03 respectively presumably because the silicates were irradiated within a feni matrix under extremely heavy shielding argon kr and xe in the silicates are dominated by a trapped planetary component with 100 36ar132xe 700 and 1 7 84kr132xe 2 2 xenon released from the silicates at extraction temperatures 1000 c is isotopically close to ureilite xe while the high t xe from graphite and schreibersite is unlike any of the presently well established types of xe the isotopic composition of this xe can be generated as a mixture of mass fractionated u xe with light isotopes depleted by 2 2 0 1 per atomic mass unit and 244pu xe the fission component 10 in the case of 136xe is not due to in situ production it must have been added prior to incorporation of the xe into the el taco parent body and presumably underwent the mass fractionation process together with u xe from the silicates 127i n produced 128xe and radiogenic 129xe are released together with the same 128xe 129xe ratio in nominal olivine pyroxene and feldspar although the maximum release from pyroxene occurs about 300 c higher than from feldspar from this the cooling rate of silicates for the temperature range 1500 1200 c is deduced to be 100 c per million years the 80kr82kr ratio of 79 81br n produced 80 82kr is 2 86 0 04 in agreement with the value found in inclusions of the iab linwood but higher than observed in almost all stone meteorites

on the influence of stratification and continental shelf and slope topography on the dispersion of subinertial coastally trapped waves abstract the behavior of subinertial coastally trapped free waves in a continuously stratified ocean is examined using a two slope topography in which the continental shelf and slope are each represented by a single uniform slope surface intensified stratification is assumed in which the squared buoyancy frequency profile is of the form n2e z where n is the buoyancy frequency at the surface z 0 and the vertical decay scale normalized by the deep sea depth the continental slope is typically assumed to be steeper than the shelf two qualitatively different types of dispersive behavior are distinguished when n f a2 e h 2 where f is the coriolis parameter a2 the bottom slope of the continental slope and h the shelf break depth normalized by the deep sea depth then free waves may occur at any subinertial frequency and each dispersion curve rises to f at some finite alongshore wavenumber as in the case of baroclinic kelvin waves if n f a2 e h 2 then all of the free wave dispersion curves a

thorium and uranium abundances in the jilin h5 chondrite abstract thorium and uranium abundances have been measured in the jilin h5 chondrite by radiochemical neutron activation analysis although the abundances of 35 8 1 5 ppb th and 14 9 0 9 ppb u are within the range of literature values the ratioth u is about 25 lower than the average value for h chondrites terrestrial addition of uranium appears to be the most likely explanation

cosmic ray tracks and cobalt 60 in near surface samples of the jilin chondrite cosmic ray track densities crtds have been measured on several spot samples from two jilin specimens from the strewn field vi 21 057 and vi 42 04 along with 60co activity in three and four sections respectively of the same specimens the observed crtd gradient agrees remarkably well with that predicted by bhattacharya et al s calculations 1973 of crtd variation as a function of shielding depth which indicates a negligible contribution from the first 2 irradiation stage the shortest distance of each specimen to the preatmospheric surface is 3 6 cm and 6 8 cm respectively the distance scale deduced from these measurements allows the determination of the variation of 60co activity with shielding depth at low depth values which is found to be much less steep than model predictions two slightly different 60co profiles are proposed that correspond to the two specimens studied the mean preatmospheric radius of the jilin chondrite is 85 8 cm the uncertainty reflecting in part the meteoroid surface roughness

exposure history of the mocs l6 chondrite a study of strewn field samples we measured cosmogenic radionuclides 10be 26al and 36cl and noble gases he ne and ar in 10 specimens of the mocs l6 chondrite to determine the exposure history and preatmospheric relationship among fragments from known locations in the strewn field cosmogenic noble gas contents alone are consistent with a simple irradiation exposure of 15 2 ma however mocs has very low 22ne 21ne ratios indicative of deep burial in a large meteoroid but radionuclide levels at saturation values typical for much smaller meteoroids this paradox suggests a possible complex exposure for the latter case we propose a two stage exposure history in which mocs initially was deeply buried in a large object for 110 ma followed by exposure in a 65 cm object for 10 5 ma relative shielding was inferred from the measured 22ne 21ne ratios assuming constant 22ne 21ne production for all samples during the first stage these shielding levels which are supported by estimates based on 36cl production by neutron capture indicate a possible relationship between depth of samples in the mocs meteoroid and fall location in the strewn field

exposure history of the regolithic chondrite fayetteville ii solar gas free light inclusions abstract noble gases and cosmic ray tracks were determined on light solar gas free inclusions of the solar gas rich h4 chondrite fayetteville 10 be 26 a1 major and trace element concentrations as well as oxygen isotope ratios complete the data set one exceptional inclusion is an l chondrite xenolith identified by its oxygen isotope signature and the concentration of metallic fe ni oxidized fe of this inclusion equilibrated with the host early in the history of fayetteville s parent body track density variations indicate that the majority of fayetteville s inclusions were irradiated as pebbles in the parent body by solar flare particles for about 10 4 10 6 years the l chondritic inclusion contains an unequivocal excess of gcr produced he and ne relative to the remaining inclusions this clast as well as two more solar gas free samples suffered a gcr exposure on the parent body lasting some 10 million years or longer this exposure is comparable in length to the residence times of solar gas rich matrix samples in the gcr active zone of the fayetteville parent regolith probably the other inclusions also were irradiated by gcr during at least several million years prior to the separation of the meteoroid from the parent body

long term earthquake prediction in central america and caribbean sea based on the time and magnitude predictable model abstract instrumental and historical information on strong mainshocks for 12 seismogenic sources in central america and caribbean sea has been used to show that the interevent time t t in years between two strong earthquakes and the magnitude m f of the following mainshock are given by the relations log t t 0 30 m m i n 0 48 m p 0 69 log m 0 13 70 m f 0 98 m m i n 0 31 m p 0 28 log m 0 4 55 where m min is the surface wave magnitude of the smallest mainshock considered m p the magnitude of the preceding mainshock and m 0 the moment rate per year in each source on the basis of these relations the probability of the occurrence of a mainshock during the decade 1993 to 2002 as well as the magnitude of this expected mainshock in each seismogenic source has been calculated the highest probability p 10 0 67 was estimated for the seismogenic source of el salvador a 1 for the occurrence of a large earthquake with expected magnitude m f 7 5 and high probabilities p 10 0 55 were estimated for the seismogenic sources of jamaica c 3 and puerto rico c 5 for earthquakes with expected magnitudes m f 7 5 in both sources

noble gases and the history of jilin meteorite abstract potassium and noble gases have been determined in more than twenty specimens from the largest known stone meteorite the h5 chondrite jilin thirteen specimens came from the surface of the present main mass the remainder from various locations in the strewn field the average k content is 802 ppm 29 samples from 23 specimens maximum deviations from the mean are 7 and 11 whole rock gas retention ages of different specimens are distinctly different they vary between 2 22 and 3 90 ae for40ar 40k and between 0 44 and 2 0 ae for4he u th severe losses of4he and40ar up to 95 and 80 respectively must have occurred up to less than 440 ma ago they cannot have happened during the fall of the meteorite however differences in concentration of cosmic ray produced3he 21ne and38arme by factors of five six and seven respectively reflect a complex irradiation history they are compatible with a short 4 irradiation and an extended one in 2 geometry at shallow depth top samples were most probably located near the transition maximum of nuclear active particles at around 15 cm depth they definitely cannot have been buried deeper than 4 m 3he 21ne ratios in bulk samples are lowered by diffusion losses of3he 25 61 while22ne 21ne ratios appear to be unaffected 22ne 21ne values range between 1 060 and 1 086 with a mean of 1 069 which is at variance with predictions for the particular irradiation conditions of jilin low36ar 38ar ratios down to 0 553 in clean metal samples are interpreted as the combined effect of large size and the transient lowering of this ratio because of a sudden increase in production rates upon going from 2 to 4 irradiation

nuclear track study of jilin chondrite twenty one specimens of jilin sampling the whole spectrum of21ne concentrations and coming from the main mass as well as from various locations in the strewn field were studied for their nuclear track record in olivines exceedingly low track densities were found in 19 of these specimens 300 cm 2 including 10 in which the low60co t1 2 5 3 years activities were seemingly indicative of a low shielding during the second 4 irradiation stage in two specimens vi 42 04 and vi 21 057 which have high21ne contents and the largest40ar and4he losses track densities of 750 and 1500 cm 2 clearly emerge from the background these tracks are attributed to very heavy cosmic ray nuclei 1 they were produced during the first 2 stage exposure it follows that the two samples were a the least shielded during the first irradiation period 28 cm of chondritic matter b not subjected to a last environmental temperature higher than 350 c for a few days in contradiction with the thermal model of muller and jessberger otherwise the cosmic ray tracks would have been totally erased c located at a depth of at least 20 cm within the meteoroid during the second 4 irradiation episode of 0 4 myr otherwise crtds produced during this time would have been detected this depth implies that the60co activities of these two specimens not known until now should be around 130 140 dpm kg it remains then to explain why 10 other specimens with60co activities in the range of 50 100 dpm kg defining depths of 6 15 cm within the meteoroid show no evidence at all of crtds above the track density background of 150 100 cm 2 2 the crtds were registered during the second 4 irradiation stage of 0 4 myr this requires the samples to have been located at depths of 15 and 12 cm from the pre atmospheric surfaces within a meteoroid with a radius of 85 cm at such depths the predicted60co activities for specimens vi 21 057 and vi 42 04 would range between 70 and 100 dpm kg the major problem with our results is to understand why in 10 locations with low 50 100 dpm kg 60co activitiesno crtds have been registered during the second irradiation stage a possible answer would be that depths inferred from60co activities are in error at least in the case of large meteoroids and for depths 15 20 cm this has indeed been found to be the case in allende where at shallow locations 15 cm depths of burial derived from crtds are larger by factors of 2 5 than those inferred from60co activities while at larger depths the agreement between the two methods appears to be good the ubiquitous presence of a track density background found in most samples is best explained by the spontaneous fission of238u since 4 0 gyr this explanation allows one to define a strict upper limit to the temperature of the last thermal event responsible for the losses of radiogenic and spallogenic gases a temperature of 500 c 1 hour or 465 c 24 hours would have annealed the internal fission tracks in olivines these limiting temperatures are not in real disagreement with the thermal model of muller and jessberger provided the crtds found in the key specimens vi 21 057 and vi 42 04 were registered during the second 4 irradiation stage and that shielding depths derived from crtds are right the jilin meteoroid had an original mass of about 15 tons r0 1 m of which about 4 tons were recovered

exposure history and terrestrial ages of ordinary chondrites from the dar al gani region libya we measured the concentrations of noble gases in 32 ordinary chondrites from the dar al gani dag region libya as well as concentrations of the cosmogenic radionuclides 14c 10be 26al 36cl and 41ca in 18 of these samples although the trapped noble gases in five dag samples show ratios typical of solar or planetary gases in all other dag samples they are dominated by atmospheric contamination which increases with the degree of weathering cosmic ray exposure cre ages of dag chondrites range from 1 myr to 53 myr the cre age distribution of 10 dag l chondrites shows a cluster around 40 myr due to four members of a large l6 chondrite shower the cre age distribution of 19 dag h chondrites shows only three ages coinciding with the main h chondrite peak at 7 myr while seven ages are 5 myr two of these h chondrites with short cre ages dag 904 and 908 show evidence of a complex exposure history five of the h chondrites show evidence of high shielding conditions including low 22ne 21ne ratios and large contributions of neutron capture 36cl and 41ca these samples represent fragments of two or more large pre atmospheric objects which supports the hypothesis that the high h l chondrite ratio at dag is due to one or more large unrecognized showers the 14c concentrations correspond to terrestrial ages 35 kyr similar to terrestrial ages of chondrites from other regions in the sahara but younger than two dag achondrites despite the loss of cosmogenic 36cl and 41ca during oxidation of metal and troilite concentrations of 36cl and 41ca in the silicates are also consistent with 14c ages 35 kyr the only exception is dag 343 h4 which has a 41ca terrestrial age of 150 40 kyr this old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr a possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods such as the current one which started 3000 years ago finally based on the 26al 21ne and 10be 21ne systematics in 16 dag meteorites we derived more reliable estimates of the 10be 21ne production rate ratio which seems more sensitive to shielding than was predicted by the semi empirical model of graf et al 1990 but less sensitive than was predicted by the purely physical model of leya et al 2000

allan hills 88019 an antarctic h chondrite with a very long terrestrial age we have measured the concentrations of the cosmogenic radionuclides loge 26al and 36cl half lives 1 51 ma 716 ka and 300 ka respectively in two different laboratories by accelerator mass spec trometry ams techniques as well as concentrations and isotopic compositions of stable he ne and ar in the antarctic h chondrite allan hills alh 88019 in addition nuclear track densities were measured from these results it is concluded that the meteoroid alh 880 19 had a preatmospheric radius of 20 5 cm and a shielding depth for the analyzed samples of between 4 and 8 cm using calculated and experimentally deter mined production rates of cosmogenic nuclides an exposure age of 40 ma is obtained from cosmogenic 21ne and 38ar the extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 f 0 4 ma a similarly long terrestrial age was found so far only for the antarctic l chon drite lewis cliff lew 86360 such long ages establish one boundary condition for the history of mete orites in antarctica

earthquake focal mechanisms and tectonic processes along the southern boundary of the caribbean plate the focal mechanisms of five small earthquakes 4 9 mb 5 6 that occurred in northwestern venezuela have been determined using teleseismically recorded rayleigh waves the rayleigh wave data were also used to constrain the depths of the five events studied these results suggest that the southern boundary of the caribbean plate is presently characterized by a broad zone of internal deformation where the eastward displacement of the caribbean plate with respect to the south american plate is being accommodated along variously oriented strike slip faults this strike slip faulting appears to occur along pre existing zones of weakness such as the bocono fault and the earthquakes studied here are all consistent with an east west trending compressive stress field being released along these zones of weakness these focal mechanisms therefore reflect the tectonic stress field generated by the eastward motion of the caribbean plate as opposed to relative plate motion per se thus the southern boundary of this plate cannot be classified as a simple plate boundary i e transform fault spreading center or subduction zone instead relative plate motion in this region appears to be accommodated by internal deformation within a broad zone extending from the interior of the caribbean plate into northern south america

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

nitrogen in chondritic metal we report new nitrogen isotopic data in metals of h l and one ll chondrites with n abundances in the range of 0 3 to 3 3 ppm and half of these 1 ppm nitrogen isotopic signatures in metals with low indigenous n concentrations are modified by cosmic ray spallation components corrections are required to determine the indigenous n signatures the metals of type 4 and 5 show uniform indigenous nitrogen 15n 6 8 0 5 and confirm a reported possible genetic association of chondritic metal with metal in iie and iva iron meteorites distinct isotopic signatures are observed in two metal samples of the portales valley h6 meteorite which both are inconsistent with signatures in h4 and h5 chondrites but possibly reveal a record of impact induced melting and metamorphism on the parent asteroid anomalous nitrogen signatures in metals of type 3 chondrites on the other hand may reflect residues of surviving presolar isotopic signatures

the record of cosmogenic radiogenic fissiogenic and trapped noble gases in recently recovered chinese and other chondrites abstract we performed a comprehensive study of the noble gas isotopic abundances in thirty six chondrites including twenty seven chondrites recovered in china the large data base allows us to recognize some new characteristics of the nuclear record in chondritic matter the comparison of the trapped noble gas release pattern for ordinary and carbonaceous chondrites shows that the planetary trapped noble gases in ordinary chondrites are released mainly above 1200 c whereas 85 of these gases in carbonaceous chondrites are degassed at 200 c there exists a clear correlation of the fraction of trapped xe released at 1200 c and petrologic type of chondrites it thus appears that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same the ngawi ll3 chondrite is solar gas rich the solar gases in meteorites and lunar surface material are mixtures of solar wind sw and solar energetic particles sep we show that the variations of the 3 he 4 he and 20 ne 22 ne ratios for solar gas rich meteorites and lunar surface material could be either due to preferential diffusive losses of the lighter isotopes of he and ne or due to a change of the sw sep flux ratio with time furthermore we demonstrate that the 81kr concentration is a function of the shielding depth of a sample within the meteoroid deviations from this depth dependency curve are observed for jilin h5 lishui l5 suizhou l6 and dongtai ll6 a complex exposure history for jilin is thus confirmed and is possible for the other three chondrites cosmic ray exposure ages are calculated based on six different nuclides 3he 21ne 38ar 83kr 126xe and 81kr kr in most cases good agreement is observed for the ages derived from the different methods quality classes are assigned to the exposure ages and the age distributions are discussed in some meteorites we observe effects induced by secondary cosmicray produced neutrons epithermal neutron fluxes jn 30 300 ev and fast neutron fluxes jn 5 mev are derived based on the reactions 79br n 80kr and 24mg n 21ne respectively and preatmospheric masses of the meteoroids are estimated we show that the ratio j n 30 300 ev j n 5 mev increases with increasing preatmospheric mass we introduce a 3he exposure age 21ne exposure age vs 4he gas retention age 40ar gas retention age diagram that is a powerful tool for distinguishing different thermal histories of meteoroids finally in some chondrites we observe xe produced by 244pu fission and calculate the time span between fission xe retention in these chondrites and that of the angra dos reis achondrite we find that the ordinary chondrites started to retain fission xe 48 30 ma earlier than angra dos reis we do not observe systematic differences between h l and ll or type 5 and 6 chondrites with respect to the time of fission xe retention

seismicity of the panama block ii spatial attenuation of seismic intensities abstract on the basis of a general consideration of seismicity and tectonics the panama block is divided into four seismic source zones the attenuation of observed modified mercalli mm intensity from 15 earthquakes with epicentral intensity i 0 larger than iv was studied fitting the observational data to different equations published in the seismological literature the effect of subdividing the total macroseismic area is discussed using the root mean square values and the coefficients in each regression formula an attempt is made to determine the rate of absorption of the seismic energy released in each seismic source zone as well as the influence of the seismotectonic characteristics on the propagation of the seismic hazard at different distances from the epicentre a possible explanation for the differences in spatial attenuation of the mm intensity within the panama block is that the younger structure of the crust in the panama fracture zone pfz and the panama deformed belt pdb contributes substantially to the absorption of seismic waves

conditions of the cosmic ray exposure of the jilin chondrite abstract nine documented samples from the main mass jilin 1 and three samples from different locations of the strewn field have been analyzed for cosmogenic 26 al t 1 2 0 716 myr 60 co t 1 2 5 27 yr and 22 na t 1 2 2 6 yr 53 mn t 1 2 3 7 myr has been determined in seven of the same documented samples and in two further documented samples the pre atmospheric size of jilin was reconstructed from the documented sample positions and from the measured 60 co activities jilin was almost spherical and had a pre atmospheric radius of about 85 cm the cosmogenic nuclides in jilin can be explained by a two stage exposure history the first stage occurred near the surface of the parent body in 2 geometry and lasted about 10 myr jilin was then expelled from its parent body and exposed to cosmic rays in a second 4 stage lasting approximately 0 4 myr the measured concentration profiles of spallogenic nuclides are compared to calculated production rates

thermoluminescence studies on jilin meteorite abstract the thermoluminescence tl of jilin meteorite was measured as a function of depth up to 21 7 mm below the fusion crust using the 50 100 m size fraction up to 6 mm depth the natural tl increases steeply due to thermal draining during atmospheric passage resulting in a temperature gradient of 45 c mm below 7 mm depth a constant natural tl signal is observed corresponding to an equivalent dose of 3300 gy this quantity represents a lower limit for the total natural dose received by the meteorite assuming a dose rate of 0 1 gy a a minimum age of 33 000 years is calculated for the last thermal event 7 cm below the fusion crust

cosmogenic radionuclides in l5 and ll5 chondrites from queen alexandra range antarctica identification of a large l ll5 chondrite shower with a preatmospheric mass of approximately 50 000 kg the collection of approximately 3300 meteorites from the queen alexandra range que area antarctica is dominated by more than 2000 chondrites classified as either l5 or ll5 based on concentrations of the cosmogenic radionuclides 10 be 26 al 36 cl and 41 ca in the metal and stone fraction of 16 que l5 or ll5 chondrites we conclude that 13 meteorites belong to a single meteorite shower que 90201 with a large preatmospheric size and a terrestrial age of 125 kyr members of this shower have properties typical of l e g pyroxene composition and ll chondrites e g metal abundance and composition as well as properties intermediate between the l and ll groups e g olivine composition and is thus best described as an l ll5 chondrite based on comparison with model calculations the measured radionuclide concentrations in the metal and stone fractions of que 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150 cm representing one of the largest chondrites known so far based on the abundance of small l5 and ll5 chondrites at que and their distinct mass distribution we conclude that the que 90201 shower includes up to 2000 fragments with a total recovered mass of 60 70 kg 1 of the preatmospheric mass of approximately 50 000 kg the mass distribution of the que 90201 shower suggests that the meteoroid experienced catastrophic atmospheric fragmentation s either because it was a fragile object or it had a high entry velocity

recent large earthquakes along the middle american trench and their implications for the subduction process several large shallow earthquakes ms 7 0 have occurred along the middle american trench since the installation of the wwssn network included are the 1965 1968 and 1978 oaxaca events the 1970 chiapas event the 1973 colima event and the 1979 petatlan event these earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes the events have seismic moments ranging from 1 0 1027 dyne cm for the 1968 event to 3 2 1027 dyne cm for the 1978 event all events are of predominantly thrust type consistent with subduction to the northeast of the cocos plate body waves for the 1965 1968 1978 and 1979 events along the trench indicate rather simple faulting processes these events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars the 1970 and 1973 events the easternmost and westernmost respectively of the events studied here are located close to triple junctions for the cocos north america caribbean plates 1970 and the cocos north america rivera plates 1973 these two events generated more complex body waves than the oaxaca and petatlan quakes this source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters this may offer a new perspective to viewing source complexity in relation to subduction zones

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

mobile trace element contents in jilin chondrite abstract we determined ppm ppt levels of co se ga rb cs te bi ag in tl zn and cd arranged empirically in order of increasing mobility at 1000 c by radiochemical neutron activation analysis in eight jilin samples of known40ar content and a large vein from one of these the trace element contents of the vein do not differ markedly from those of jilin whole rock samples only rb correlates with40ar possibly due to chance all elements but cd are present at levels similar to those in h4 6 chondrites with longk ar age i e presumably mildly shocked these levels are low relative to those in analogous l4 6 chondrites suggesting that h chondrites formed and or evolved under higher pre shock temperatures than did mildly shocked l chondrites time temperature conditions during shock loading of jilin parent material were mild relative to those in strongly shocked l chondrites being sufficient at most only to mobilize cd

aluminum 26 sodium 22 and cobalt 60 in two drill cores and some other samples of the jilin chondrite two drill cores have been taken from the main mass of the jilin chondrite jilin i one was bored perpendicular to the 2 surface of the first exposure stage the other parallel to this surface and both intersected at the presumed 4 center of jilin s second exposure stage the probed depths were 65 cm and 105 cm respectively with a diameter of 26 5 mm fifteen drill core samples 2 documented samples and 14 strewn field samples have been analysed for 26al 60co and 22na the reconstruction of the preatmospheric jilin via60co resulted in slight deviations for the center position and for the radius now r 80 10 5 cm compared to an earlier study in 1985 the 60co production rate profile is now almost completely reproduced a constant production rate of 22na and of 26al during the 4 irradiation is confirmed also for the center region the good linear correlation of 26al and 21ne yields slightly revised irradiation ages for both stages of t1 7 1 ma and t2 0 32 0 06 0 03 ma

cosmic ray effects in the antarctic meteorite allan hills a 78084 abstract in the antarctic h4 chondrite alha 78084 measurements of 26 al 53 mn noble gases and cosmic ray tracks have been performed on documented samples to investigate depth dependences of cosmic ray induced effects 26 al activities vary between 224 and 246 dpm kg si equ 53 mn activities range from 299 to 359 dpm kg fe in agreement with the kohman and bender model 1 22 ne 21 ne ratios between 1 145 and 1 176 are observed the 3 he 21 ne vs 22 ne 21 ne correlation shows a trend line similar to those found for st severin and keyes track data indicate ablation depths between 1 3 and 6 5 cm with a mean ablation of 3 8 cm this leads to a preatmospheric mass of about 40 kg and an ablation loss of about 66 by mass preatmospheric size and shielding calculated from measured 22 ne 21 ne ratios using the semi empirical curves of bhandari and potdar 2 agree with those derived from track measurements the combined data of spallation nuclides and cosmic ray tracks indicate a simple one stage irradiation as a small object with a preatmospheric radius of 14 1 cm for about 32 m y and a terrestrial age of less than 170 000 years

noble gases 81krkr ages and10be of chrondrites from china abstract a comprehensive study of the cosmic ray exposure history of five ordinary chondrites from china was carried out using measurements of the noble gas isotopic abundances and 10 be concentrations the following average cosmic ray exposure ages based on cosmogenic 21 ne and on 81 kr kr dating were obtained zhaodong l4 15 7 3 0 m y nan yang pao l6 48 10 0 m y guangrao l6 16 8 3 5 m y and lunan h6 26 7 5 0 m y the h5 chondrite zaoyang was exposed for only 0 90 0 12 m y to galactic cosmic rays as calculated from the 10 be activity and from the low amounts of cosmic ray produced noble gases the zhaodong chondrite contains large amounts of 80 kr and 82 kr produced by neutron capture of bromine from the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite

cosmic ray records in the ll chondrite dhurmsala abstract the radionuclides 10be 26al and 53mn the noble gas isotopic composition of he ne and xe and cosmic ray particle track densities were determined in six documented samples of a large fragment of the dhurmsala meteorite fall using the 22 ne 21 ne ratios and the 21ne concentrations the cosmic ray exposure age of dhurmsala is deduced to be 13 0 1 0 my the results suggest a simple exposure history the six samples covering a distance of 13 cm do not show any significant isotopic variations due to depth effects as expected in a large meteorite consistent with the model calculations of reedy 1985 and bhandari and potdar 1982 the results obtained in this study indicate that the dhurmsala samples investigated here originate at a depth of 20 to 24 cm in a meteoroid with a preatmospheric radius of r 50 cm

dating jilin and constraints on its temperature history abstract as part of the consortium effort to study the jilin h5 meteorite two documented samples from the main mass and one from a smaller strewn field fragment have been dated with the 40 ar 39 ar technique in addition 4 he and argon release patterns from a non irradiated sample were determined to obtain diffusion parameters concordant 40 ar 39 ar ages of 4 0 ae indicate that jilin was totally degassed at that time using also the results from other consortium studies 1 2 it is shown that largely varying secondary 40 ar and 4 he losses are related to the position of the samples within the pre atmospheric meteoroid this sets rather stringent limits to the temperature history of jilin the surface of the meteoroid was heated probably 400 000 years ago at the separation from its parent body model calculations suggest that the heating event was moderate initial surface temperature 500 600 c and brief surface cooling rate 1 2 c h heating of meteoroids at separation from their parent bodies might be a common phenomenon it could explain late disturbances in some isotope systems and could be the driving force for hydrothermal alterations the age of the reheating does not at all become apparent in the 40 ar 39 ar data calling in question the use of low temperature 40 ar 39 ar ages to date multi collisional events in meteorite parent bodies

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

the non trivial problem of meteorite pairing pairing is the procedure of identifying fragments of a single meteorite fall that were separated during atmospheric passage or during terrestrial history by establishing the similarity of two or more meteorite fragments we argue that pairing is governed by two principles that only a single mismatch of properties is required to refute a proposed pairing and that virtually all pairings bear some degree of uncertainty using data distributions for modern falls we take a probability approach to estimate degrees of certainty associated with proposed pairings emphasizing the importance of unusual features for new pairing criteria or new analytical additions to old criteria the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified criteria for pairing can be divided into 1 parent body history indicators 2 meteoroid space history indicators and 3 terrestrial history indicators included in these categories are 11 specific criteria including petrographic textures mineralogy and mineral composition terrestrial age estimates cosmic ray exposure ages and natural thermoluminescence tl levels not all criteria are applicable to all meteorite types about 2275 pairings suggested in the literature have been subjected to this analysis many literature pairings especially those involving common meteorite types bear large uncertainties due to lack of data

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

cosmic ray constancy and cosmogenic production rates in short lived chondrites abstract five chondrites with short cosmic ray exposure ages 26al and 53mn saturation values deduced from short lived chondrites alone agree within 20 with those deduced from all chondrites values of 21p based on 53mn and 26al are 0 30 0 03 and 0 46 0 05 respectively in h chondrites possible causes for the difference include half life errors data selection and a recent 2 5myr increase in the cosmic ray intensity

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

cosmic ray exposure ages of six chondritic almahata sitta fragments the almahata sitta strewn field is dominated by ureilites but contains a large fraction of chondritic fragments of various types we analyzed stable isotopes of he ne ar kr and xe and the cosmogenic radionuclides 10be 26al and 36cl in six chondritic almahata sitta fragments el6 breccia el6 el3 5 cb ll4 5 r like the cosmic ray exposure cre ages of five of the six samples have an average of 19 2 3 3 ma close to the average of 19 5 2 5 ma for four ureilites the cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with almahata sitta this corroborates that almahata sitta chondrite samples were part of the same asteroid as the ureilites however ms 179 has a lower cre age of 11 0 1 4 ma further analysis of short lived radionuclides in fragment ms 179 showed that it fell around the same time and from an object of similar size as almahata sitta making it almost certain that ms 179 is an almahata sitta fragment instead its low cre age could be due to gas loss chemical heterogeneity that may have led to an erroneous 21ne production rate or perhaps most likely ms 179 could represent the true 4 exposure age of almahata sitta or an upper limit thereof while all other samples analyzed so far experienced exposure on the parent body of similar lengths finally ms 179 had an extraordinarily high activity of neutron capture 36cl 600 dpm kg 1 the highest activity observed in any meteorite to date related to a high abundance of the cl bearing mineral lawrencite

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

noble gases and cosmogenic radionuclides in the gold basin l4 chondrite shower thermal history exposure history and pre atmospheric size we measured the concentrations of the cosmogenic radionuclides 10be 26al 36cl and 41ca in the stone and metal fractions of 15 fragments of the gold basin l4 chondrite shower as well as noble gases in 18 gold basin fragments a comparison of 10be 26al and 41ca concentrations with calculated production rates from two different models indicates that the gold basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3 5 m as was predicted by recent model calculations the noble gases show a reversal of the 22ne 21ne ratio at very high shielding the 21ne 10be and 21ne 26al ratios in most samples are constant and correspond to a 4 exposure age of 18 2 myr however three gold basin samples show a 30 120 excess of 21ne implying that they were previously exposed close to the surface of the parent body whereas the other samples were buried several meters deeper concentrations of neutron capture 36ar in most samples are consistent with measured concentrations of neutron capture 36cl and an exposure age of 18 myr large excesses of neutron capture 36ar were found in those samples with an excess of 21ne providing additional evidence of a first stage exposure on the parent body the excess of spallation produced 21ne and neutron capture produced 36ar in these samples indicate a first stage exposure of 35 150 myr on the parent body the radiogenic 4he and 40ar concentrations indicate a major impact on the parent body between 300 and 400 myr ago which must have preceded the impacts that brought the gold basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 myr ago