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

the stubenberg meteorite an ll6 chondrite fragmental breccia recovered soon after precise prediction of the strewn field on march 6 2016 at 21 36 51 ut extended areas of upper austria bavaria germany and the southwestern part of the czech republic were illuminated by a very bright bolide this bolide was recorded by instruments in the czech part of the european fireball network and it enabled complex and precise description of this event including prediction of the impact area so far six meteorites totaling 1473 g have been found in the predicted area the first pieces were recovered on march 12 2016 on a field close to the village of stubenberg bavaria stubenberg is a weakly shocked s3 fragmental breccia consisting of abundant highly recrystallized rock fragments embedded in a clastic matrix the texture the large grain size of plagioclase and the homogeneous compositions of olivine fa31 4 and pyroxene fs25 4 clearly indicate that stubenberg is an ll6 chondrite breccia this is consistent with the data on o ti and cr isotopes stubenberg does not contain solar wind implanted noble gases data on the bulk chemistry ir spectroscopy cosmogenic nuclides and organic components also indicate similarities to other metamorphosed ll chondrites noble gas studies reveal that the meteorite has a cosmic ray exposure cre age of 36 3 ma and that most of the cosmogenic gases were produced in a meteoroid with a radius of at least 35 cm this is larger than the size of the meteoroid which entered the earth s atmosphere which is constrained to 20 cm from short lived radionuclide data in combination this might suggest a complex exposure history for stubenberg

electrical resistivity changes in tuffs due to stress certain tuffs sampled near active faults in northern california exhibited the same large electrical resistance changes with stress reported by yamazaki for tuffs from the izu peninsula these effects studied in the laboratory were most pronounced at stresses below 6 mpa for partial saturation and for rocks with porosity less than 20 relative changes of resistance with strain reached 105 for several of the tuffs near a strain of 10 4 a curious and unexplained aspect of the electrical behavior of the tuffs was the recoverability between stress cycles this was not observed for other equally porous rocks like sandstone and for partially saturated granite electrically sensitive tuffs near active faults might be used in earthquake prediction studies provided field sites in partially saturated tuffs can be found

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

slab pull and indentation tectonics insights from 3d laboratory experiments abstract we investigate using 3d laboratory experiments how the dynamics of indentation process are affected by the evolution at depth of the oceanic and continental subductions lithospheric plates are modelled by sand silicone plates floating on glucose syrup and the density contrast between oceanic and continental lithospheric plates and asthenosphere is reproduced analogue experiments model the convergence between two lithospheric plates a small continent indenting a large continental plate we show that the surface deformation in front of the indenter and above the oceanic subduction zone depends on the behaviour of the slab below the collision zone slab break off following the subduction of the small continent favours the indentation process because it results in an increasing compression in front of the indenter and extension above the neighbouring oceanic subduction both of them being responsible for the appearance of the indenter like geometry of the plate boundary when the slab does not deform significantly at depth in contrast the closure of the oceanic domain in front of the indenter is followed by a longer period of continental subduction during which the tectonic regime within the wide continent remains quite homogeneous comparing the presented analogue experiments with the subductions both part of the arabian indenter within eurasia our results suggest that the different tectonic regime on both sides of the arabia indenter may partly result from the probable occurrence of a detachment at depth under eastern anatolia

stress strain state monitoring of the geological medium based on the multi instrumental measurements in boreholes experience of research at the petropavlovsk kamchatskii geodynamic testing site kamchatka russia we present the results of the long term study which make it possible to assess the possibilities of two methods used at the petropavlovsk kamchatskii geodynamic site russia for continuous monitoring of the stress strain state of geoenvironment the methods are based on the borehole geoacoustic measurements and electromagnetic measurements with underground electric antennas combined with hydrogeochemical and hydrogeodynamic borehole measurements these methods allow monitoring the changes in the parameters of geological environment and identifying the stages in the changes of its stress strain state as the examples we present the results of the measurements in the time vicinities of the strongest seismic events that occurred during the period of joint borehole geoacoustic and electromagnetic measurements at the petropavlovsk kamchatskii geodynamic site we also analyze the data obtained in the time vicinities of the tohoku mega earthquake march 11 2011 japan mw 9 1 and a close strong zhupanovskoe earthquake january 30 2016 mw 7 2 that occurred at the epicentral distances r 107 km from petropavlovsk kamchatskii it is shown that the discussed methods are promising for medium and short term forecasting of the earthquakes

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

the north cycladic detachment system and associated mineralization mykonos greece insights on the evolution of the aegean domain 1 in the aegean back arc domain some 30 35 ma ago an increase of the rate of slab retreat led to the initiation of post orogenic extension largely accommodated by large scale structures such as the north cycladic detachment system ncds although this extension is still active nowadays an e w compressional regime developed in the late miocene with the propagation of the north anatolian fault on mykonos island cyclades the ne sw back arc extension is particularly well expressed with the livada and mykonos detachments that belong to the ncds and that are associated with nw se barite veins emplaced during the synkinematic cooling of the mykonos intrusion this study shows that the formation of the mineralization occurred when the pluton crossed the ductile to brittle transition during its exhumation below the ncds at 11 10 ma in addition the kinematics of mineralized structures evolved with time 1 most of the displacement was accommodated by the top to the ne livada and mykonos detachments accompanied by the formation of mineralized normal faults that were 2 reworked in a strike slip regime with an e w direction of shortening and a persistent ne sw stretching and 3 a late post mineralization e w compressional stage with a minor reworking of shallow dipping faults locally including the detachments themselves we interpret this increase of the e w shortening component recorded during the mineral deposition as a consequence of the initiation of the westward motion of anatolia from 10 ma thus 4 ma before the propagation of the north anatolian fault in the dardanelles strait and the localization of the strain on the aegean sea margins

interactions between plutonism and detachments during metamorphic core complex formation serifos island cyclades greece in order to better understand the interactions between plutonic activity and strain localization during metamorphic core complexes formation the miocene granodioritic pluton of serifos cyclades greece is studied this pluton 11 6 9 5 ma intruded the cycladic blueschists during thinning of the aegean domain along a system of low angle normal faults belonging to the south dipping west cycladic detachment system wcds based on structural fieldwork together with microstructural observations and anisotropy of magnetic susceptibility ams we recognize a continuum of deformation from magmatic to brittle conditions within the magmatic body this succession of deformation events is kinematically compatible with the development of the wcds the architecture of the pluton shows a marked asymmetry resulting from its interaction with the detachments we propose a tectonic scenario for the emplacement of serifos pluton and its subsequent cooling during the aegean extension 1 a first stage corresponds to the metamorphic core complex initiation and associated southwestward shearing along the meghalo livadhi detachment 2 in a second stage the serifos pluton has intruded the dome at shallow crustal level piercing through the ductile brittle meghalo livadhi detachment southwest directed extensional deformation was contemporaneously transferred upward in the crust along the more localized kavos kiklopas detachment 3 a third stage was marked by syn magmatic extensional deformation and strain localization at the contact between the pluton and the host rocks resulting in narrow shear zones nucleation which 4 continued to develop after the pluton solidification

continental collision and slab break off a comparison of 3 d numerical models with observations abstract conditions and dynamics of subduction collision and subsequent 3 d slab break off and slab tear propagation are quantified for the first time using fully dynamic numerical models model results indicate that collision after the subduction of old strong subducting oceanic slab leads to slab break off at 20 25 myr after the onset of continental collision and subsequently a slab tear migrates more or less horizontally through the slab with a propagation speed of 100 150 mm yr in contrast young weak oceanic slabs show the first break off already 10 myr after continental collision and can experience tear migration rates up to 800 mm yr slab strength plays a more important role in the timing of slab break off and the speed of a propagating slab tear than negative slab buoyancy does slab break off is viable even for slabs that are supported by the viscosity jump and phase change between the upper and lower mantle the density of the oceanic slab and the subducting continental block is important for the amount of continental subduction and the depth of slab break off a 40 km thick continental crust can be buried to depths greater than 200 km although this maximum depth is significantly less for younger or very weak slabs or thicker continental crust slab break off typically starts at a depth of 300 km mostly independent of mantle rheology but like continental crustal burial can be shallower for young or buoyant plates our 3 d models illustrate how due to the difference in necking in 2 d and 3 d break off has an intrinsic small preference to start as a slab window within the slab s interior rather than as a slab tear at the slab edge however any significant asymmetry in the collision setting e g earlier collision at one end of the subduction zone would override this and leads to slab tearing starting near one edge of the slab these results put important new constraints on the dynamics of the collision and subsequent slab break off for modern collision belts for a proposed timing of the initial arabia eurasia collision at 35 ma break off of the 200 myr old neo tethys slab is unlikely to have occurred before 15 10 ma furthermore our results illustrate that shallow early break off of weak slabs provides a viable explanation for the absence of blueschists and ultra high pressure metamorphism in the precambrian geological record

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

lateral shifts of apenninic foredeep depocentres reflecting detachment of subducted lithosphere abstract the hypothesis of lateral migration of slab detachment was formulated on the basis of seismic tomography results on the 3 d structure of subduction zones in the mediterranean region m j r wortel and w spakman structure and dynamics of subducted lithosphere in the mediterranenan region proc k ned akad wet 95 1992 325 347 the redistribution of slab pull forces associated with the tearing process is taken to affect foredeep development by adding a lateral component to internal external depocentre migrations in the case of the apenninic arc the direction of detachment and associated depocentre migration is expected to be southeast this prediction has been confronted with the tectonostratigraphy of oligocene to recent apenninic foredeeps results show there are two episodes marked by important lateral shifts of foredeep depocentres during the early late miocene tortonian the depocentre of the northern apennines foredeep shifted towards the southeast this is evidenced by the relative positions of the depocenters of the burdigalian serravallian marnoso arenacea and the tortonian umbrian marchean and lazian minor basins a further stepwise southeastward migration can be inferred from the plio pleistocene records of the central and southern apennines in a more general sense our results indicate that slab detachment may play a significant role in the evolution of foredeeps along convergent plate boundaries

itqiy a study of noble gases and oxygen isotopes including its terrestrial age and a comparison with zaklodzie we report noble gas oxygen isotope 14c and 10be data of itqiy as well as noble gas 14c and 10be results for zaklodzie both samples have been recently classified as anomalous enstatite meteorites and have been compared in terms of their mineralogy and chemical composition the composition of enstatite and kamacite and the occurrence of specific sulfide phases in itqiy indicate it formed under similar reducing conditions to those postulated for enstatite chondrites the new results now seem to point at a direct spatial link the noble gas record of itqiy exhibits the presence of a trapped subsolar component which is diagnostic for petrologic types 4 6 among enstatite chondrites the concentration of radiogenic 4he is very low in itqiy and indicates a recent thermal event its 21ne cosmic ray exposure age is 30 1 3 0 ma and matches the most common age range of enstatite chondrites mostly el6 chondrites but not that of zaklodzie itqiy s isotopic composition of oxygen is in good agreement with that observed in zaklodzie as well as those found in enstatite meteorites suggesting an origin from a common oxygen pool the noble gas results on the other hand give reason to believe that the origin and evolution of itqiy and zaklodzie are not directly connected itqiy s terrestrial age of 5800 500 years sheds crucial light on the uncertain circumstances of its recovery and proves that itqiy is not a modern fall whereas the 14c results from zaklodzie suggest it hit earth only recently

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

on late oligocene to pliocene depocentre migrations and the evolution of the carpathian pannonian system abstract the late oligocene to pliocene evolution of foredeep basins of the eastern alps carpathians fold and thrust belt is marked by pronounced internal to external and lateral depocentre shifts the latter shifts covering a present along arc distance of 1700 km portray accelerating rates of foredeep depocentre migration particularly so in the late early and in the middle miocene from about 7 to about 45 cm yr lateral depocentre migration came to a close at the beginning of the late miocene the subsequent late miocene to pliocene foredeep infill history was characterized by exponentially increasing accumulation rates in the intersection area of the east european ukrainian and moesian platforms successive steps in the evolution of the foredeep basins had pronounced counterparts in the intra carpathian area the beginning of acceleration of foredeep depocentre migration in latest early miocene times was coeval with the inception of intra carpathian extensional tectonics the extremely rapid middle miocene depocentre shift coupled with a change in direction of foredeep depocentre migration corresponded with maximum extension in the intra carpathian area the ensuing end of foredeep depocentre migration around the middle late miocene transition was coeval with the end of extension of the intra carpathian area which in turn was followed by the inception of overall thermal subsidence at about 11 5 ma the timing of and spatial relationships between discrete coeval events in arc and intra arc evolution put unambiguous geological constraints on geodynamic modelling of the evolution of the carpathian pannonian system it is speculated that the geological observations may best be understood in terms of the surficial effects of lateral migration of slab detachment such effects would mirror the dominant role of a concentrating slab pull taken to result in e g a time progressive acceleration of foredeep depocentre migration

slab breakoff insights from 3d thermo mechanical analogue modelling experiments abstract the detachment or breakoff of subducted lithosphere is investigated using scaled three dimensional thermo mechanical analogue experiments in which forces are measured and deformation is monitored using high speed particle imaging velocimetry piv the experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs slow subduction experiments with scaled convergence rates 1 cm yr 1 have lower peclet numbers and are characterized by lower tensile strength subducted lithosphere causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low therefore when continental collision is preceded by slow oceanic subduction the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted in contrast if collision is preceded by rapid subduction 10 cm yr 1 breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure the analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary stereoscopic piv reveals a characteristic strain pattern that accompanies the detachment horizontal contraction and subsidence with scaled values up to 1200 m in the trench and forearc area precedes the passage of the detachment and is followed by horizontal extension and uplift up to 900 m high frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr 1 however this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether

early electromagnetic waves from earthquake rupturing i theoretical formulations yongxin gao 1 2 xiaofei chen 1 2 hengshan hu3 and jie zhang1 2 1school of earth and space sciences university of science and technology of china hefei 230026 china e mail gaoyongxin1982 yahoo com cn 2laboratory of seismology and physics of earth s interior university of science and technology of china hefei 230026 china 3department of astronautics and mechanics harbin institute of technology harbin 150001 china

support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages lunar meteorites represent a more random sampling of lunar material than the apollo or luna collections and as such lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis within the lunar meteorite breccias mac 88105 que 93069 dag 262 and dag 400 seven to nine different impact events are represented with 40ar 39ar ages between 2 76 and 3 92 billion years ago ga the lack of impact melt older than 3 92 ga supports the concept of a short intense period of bombardment in the earth moon system at approximately 3 9 ga this was an anomalous spike of impact activity on the otherwise declining impact frequency curve

miocene south directed low angle normal fault evolution on kea island west cycladic detachment system greece 1 new structural petrologic and thermochronologic data from kea west cyclades define a crustal scale ductile shear zone and ductile brittle low angle normal fault lanf system both the greenschist facies shear zone forming the footwall and the overlying lanf zone formed during constrictional strains with a consistent top to sw s shear sense with increasing finite strains toward higher structural levels but decreasing temperatures from footwall to hanging wall the tectonostratigraphy comprises a 450 m thick footwall of shallowly dipping schists and calcite marbles representing the intermediate unit of the attic cycladic crystalline acc above the footwall is a 60 m thick highly strained lanf zone consisting of phyllonites cataclastic schists ultramylonitic calcite marbles proto mylonitic calcite marbles and cohesive cataclasites that arch over the whole island these fault rocks exhibit multistage lanfs evolving from ductile to brittle conditions an up to 50 m thick brecciated limestone and dolostone sequence forms the unmetamorphosed hanging wall which is most probably part of the upper unit of the acc multiequilibrium p t estimates on chlorite white mica pairs in the footwall yield 7 5 5 kbar 360 c 450 c for inclusions in albite and epidote 5 5 3 kbar 400 c 350 c for the main foliation and 3 2 kbar 350 c 280 c in localized shear bands c and c foliations the 40ar 39ar white mica footwall cooling ages demonstrate that greenschist facies retrogression occurred between 21 and 17 ma localized late decimeter thick shear zones were active and dynamically recrystallized before 15 to 13 ma the lanf on kea together with similar structures in south attica and the west cyclades define the west cycladic detachment system characterized by ductile to brittle top sw s shear sense

disruption of the hellenic arc late miocene extensional detachment faults and steep pliocene quaternary normal faults or what happened at corinth 1 extensional faults exposed in the peloponnesus and mainland greece most of which are described here for the first time record a transition from regional extension of the aegean domain to the modern tectonic system the east peloponnesus detachment system trends north northwest from the southern peloponnesus to 30 km north of the gulf of corinth dips gently northeast and is late miocene early pliocene in age it has a minimum displacement of 25 30 km and appears to be the youngest of the regional scale extensional systems with significant displacement that formed parallel to the hellenic arc the partially coeval east sterea extensional system which extends from the gulf of corinth to the aegean sea contains low angle normal faults that both crosscut and trend parallel to older structures of the hellenic arc late miocene to early pliocene displacement within this zone disrupted the arc parallel structures of the hellenides upper pliocene quaternary normal faults which trend approximately east west and generally dip steeply at the surface continue the disruption of the hellenic arc much of the subsidence within the gulf of corinth appears to be unrelated to the younger faults and is instead related to the motion on the east peloponnesus detachment which crosscuts the modern graben

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

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

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

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

bridging onshore and offshore present day kinematics of central and eastern mediterranean implications for crustal dynamics and mantle flow we present a new kinematic and strain model of an area encompassing the calabrian and hellenic subduction zones western anatolia and the balkans using haines and holt s 1993 method we derive continuous velocity and strain rate fields by interpolating geodetic velocities including recent gps data in the balkans relative motion between stable eurasia and the western aegean sea is gradually accommodated by distributed n s extension from southern balkans to the eastern corinth gulf so that the westward propagation of the north anatolian fault naf throughout continental greece or peloponnesus is not required we thus propose that the naf terminates in north aegean and that n s extension localized in the corinth gulf and distributed in southern balkans is due to the retreat of the hellenic slab the motion of the hyblean plateau apulia peninsula south adriatic sea ionian basin and sirte plain can be minimized by a single rigid rotation around a pole located in the sirte plain compatible with the opening the pelagian rifts 2 2 5 mm yr and seismotectonics in libya we interpret the trenchward ultraslow motion of the calabrian arc 2 2 5 mm yr as pure collapse the calabrian subduction being now inactive in the absolute plate motion reference frame our modeled velocity field depicts two toroidal crustal patterns located at both ends of the hellenic subduction zone clockwise in nw greece and counter clockwise in western anatolia we suggest the nw greece toroidal pattern is the surface expression of a slab tear and consequent toroidal asthenospheric flow

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

spreading pulses of the tyrrhenian sea during the narrowing of the calabrian slab the opening of the tyrrhenian sea has been punctuated by short lived episodes of oceanic accretion on separate small backarc basins during early pliocene vavilov basin and early pleistocene marsili basin time these spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow calabrian arc using laboratory models we investigated the long term and transient effects of the reduction of slab width on the subduction kinematics we found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity as the balance between driving and resisting forces acting on the slab is temporarily modified our findings also show that the time scale and amplitude of spreading observed in the tyrrhenian sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between 10 19 and 10 20 pa s

noble gases in 21 saharan ll chondrites exposure ages and possible pairings nineteen ll chondrites and two l ll chondrites adrar 003 and tanezrouft 010 from the saharan desert of algeria and libya have been analysed for their he ne and ar composition as well as for their abundance of 84kr and 132xe calculated 21ne cosmic ray exposure ages vary between 2 and 35 ma the age distribution is consistent with that of modern ll chondrite falls except that no dominant peaks can be observed especially not the one related to a 15 ma collisional event however the lack of young exposure ages of 8 ma is obvious this is a characteristic feature of ll chondrites only one of the 21 ll chondrites namely acfer 066 contains solar gases and is thus considered a regolith breccia three specimens reveal considerable loss of 3he probably due to periods of elevated temperatures in orbits with small perihelion distances furthermore severe loss of 4he and 40ar is found in two samples considering possible pairings we suggest 14 individual falls basically on the basis of the noble gas data the petrographic sub classification and by taking the find location into consideration however there are constraints on confirming pairings solely on the basis of our studies thus we can only exclude individual samples with a unique noble gas fingerprint from paired specimens

numerical simulation of coseismic electromagnetic fields associated with seismic waves due to finite faulting in porous media summary the electrokinetic effect is one mechanism that causes the coupling between seismic and electromagnetic energies this study deals with the numerical simulation of the electrokinetically induced coseismic electromagnetic fields associated with seismic waves created by a finite faulting in porous media the key point of our numerical technique is to combine the point source stacking method with the luco apsel chen generalized reflection and transmission method after some validation test of our simulation method we show several numerical simulation examples through a near field model we show that the point source approximation is not accurate in this configuration as well as that the position of the rupture starting point and rupture speed obviously affect the coseismic em fields then we numerically investigate the influence of different media properties on seismic waves and coseismic em fields we also carry out simulations of multilayer half space models and show the characteristics of the coseismic em fields the preliminary results indicate that our numerical technique provides an effective approach of studying coseismic electromagnetic fields associated with seismic waves due to a finite faulting in porous media

an interdisciplinary study of weathering effects in ordinary chondrites from the acfer region algeria weathering effects on meteorite finds from the acfer region were studied by various analytical techniques and in dependence on the depth of sampling in thin sections of weathered meteorites weathering effects usually decrease from the outside to the interior of the meteorite the results of evolved gas analysis indicate that variation in weathering between surface and core is not significant in respect to the formation of fe oxyhydroxides the secondary alteration effects in the noble gases are distributed unevenly throughout the specimens as seen in the nonsystematic differences observed for the heavy noble gases chemical analyses show significant enrichment of ba and sr in the outer parts of the weathered samples due to element contamination through aqueous solution iron ni and co are partly flushed from the system as the metal oxidation proceeds oxygen isotopes show increases in d180 and d17o with increasing terrestrial age for a set of h3 chondrites the degree of weathering determined from the water content was correlated with terrestrial ages and is discussed with respect to possible weathering mechanisms

new noble gas data of primitive and differentiated achondrites including northwest africa 011 and tafassasset this work reports on the noble gas inventory of 3 new acapulcoites 3 brachinites 2 new eucrites from the dar al gani region in libya the unique achondrite dar al gani dag 896 from the same locality the new eucrite like achondrite northwest africa nwa 011 and the controversial sample tafassasset we determined cosmic ray exposure and gas retention ages evaluated shielding conditions and discuss the trapped noble gas component of the specimens all exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters shielding conditions vary suggesting substantial shielding for all 3 brachinites and tafassasset we cannot exclude however that the mg rich composition of brachinites simply simulates heavy shielding regarding the trapped component we found q like compositions only for the acapulcoite thiel mountains til 99002 the brachinite elephant moraine eet 99402 yields a high subsolar 36ar 132xe ratio of 400 along with a slightly elevated 84kr 132xe ratio indicating minor atmospheric contamination all the other samples particularly the eucrite dag 983 are characterized by clearly elevated ar kr xe ratios due to significant terrestrial alteration tafassasset exhibits noble gas parameters that are different from those of cr chondrites including a relatively high cosmic ray exposure age the absence of a solar component low 132xe concentrations a low trapped 36ar 132xe ratio of 30 and a noticeable amount of radiogenic 129xe similar attributes have been observed for some primitive achondrites these attributes are also consistent with the metamorphic character of the sample we therefore consider tafassassets noble gas record to be inconclusive as to its classification primitive achondrite versus metamorphosed cr chondrite

the kumtag 016 l5 strewn field xinjiang province china the kumtag 016 strewn eld was found in the eastern part of the kumtag desert xinjiang province china in this study 24 recovered meteorites have been ch aracterized by a suite of different analytical techniques to investigate their petrography mineralogy bulk trace elements noble gas isotopic composition density and porosity we attribute to the strewn eld 22 l5 chondrites with shock stage s4 and weathering grade w2 w3 two differen t meteorites kumtag 021 an l4 chondrite and kumt ag 032 an l6 chondrite were recognized within the strewn eld area moreover kumt ag 003 an h5 chondrite was previously found in the same area we infer that the kumtag 016 strewn eld most likely consists of at least four distinct meteorite falls the effects of terrestrial weathering on the studied mete orites involve sul de metal alteration chemical changes sr ba pb and u enrichments and depletion in cr co ni and cs abun dances and physical modi cations decrease of grain density and porosity measurements of the light noble gases indicate that the analyzed kumtag l5 samples contain solar wind implanted noble gases with a ne ne ratio of 12 345 the cosmic ray exposure cre ages of the l5 ch ondrites are in a narrow range 3 6 1 4 ma to 5 2 0 4 ma for l4 cho ndrite kumtag 021 and l6 chondrite kumtag 032 the cre ages are 5 9 0 4 ma and 4 7 0 8 ma respectively

three dimensional numerical modeling of detachment of subducted lithosphere recent seismological studies suggest that slab detachment has occurred in the mediterranean and the new hebrides subduction zones subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km presumably caused by the lateral migration of the tear along the strike of the slab to investigate the physical mechanism of the slab detachment and in particular its migration we constructed a viscoelastic three dimensional finite element model and introduced a small initial tear from one side of the slab we investigated spatio temporal variations in the state of stress within the slab as a function of tear length rheology and a variety of force distribution our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab which is probably sufficient to cause further lateral migration of the tear the stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension from our modeling we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate forming in plate tensional stress at intermediate depths such a condition is indeed observed in the dinarides hellenic and the new hebrides subduction zone

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

thrust or detachment exhumation processes in the aegean insight from a field study on ios cyclades greece 1 whether back arc extension in the aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back arc extension new field observations structural analysis of finite and instantaneous strain and their relationship with metamorphism on ios island cyclades lead us to investigate the significance of the south cycladic shear zone scsz two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the scsz first penetrative top to the south shearing is observed in the upper cycladic blueschists and the lower cycladic basement close to their contact it is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals contemporaneous with overthrusting of the cycladic blueschists over the basement unit second large extensional top to the north shear zones lately crosscut the whole stack these constraints allow us to reinterpret the kinematics of the aegean domain during synorogenic and postorogenic periods when replaced in the reconstructed geometry of the hellenic eocene orogenic wedge the observed top to the south thrusting is coeval with extensional shear zones observed in syros that could represent the roof of the cycladic blueschists an extrusion structure is then proposed for the exhumation of the cycladic blueschists in the subduction zone top to the north asymmetric extension together with the andros tinos mykonos and paros naxos detachments can be correlated to oligo miocene extension in the back arc domain

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

subduction and slab breakoff controls on asian indentation tectonics and himalayan western syntaxis formation 1 we test the link between large scale asian continent deformations and indian slab subduction and breakoff during convergence by means of three dimensional numerical models of the subducting upper plates system we find that the subduction of the buoyant continent results in the reduction of convergence velocity comparable to that observed in the indian motions yet the upper plate deformation remains accommodated in a narrow belt along a straight margin comparable rates are measured when the subducting slab breakoff is modeled although the convergent margin deforms and curves markedly with large underplating contiguous to ongoing subduction similar to what observed along the himalayan range the models support the interpretation of the himalayan western syntaxis evolution the progressive curvature of the indian margin and the underthrusting as a consequence of the indian slab breakoff the modeled slab detachment is followed by short lived large stresses in the upper plate interiors propagating at large distance from the margin with a trend similar to several major asian lithospheric faults such localized stress has likely provided the conditions for the formation of the central asian intracontinental faulting the bangong red river and the altyn tagh faults that followed successive indian slab breakoff episodes continent subduction and breakoff during india asia convergence offer an explanation for the different deformation mechanisms as the long lived understhrusting and the episodic lithospheric faulting in the asian continent and their link to deep processes

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

a thermomechanical model of exhumation of high pressure hp and ultra high pressure uhp metamorphic rocks in alpine type collision belts using a fully coupled numerical thermomechanical model handling strain localization surface processes and ultra high viscosity contrasts 11 orders of magnitude we test a number of possible mechanisms of high pressure hp low temperature lt high temperature ht exhumation in continental collision zones the model considers two end member cases low or high buoyancy of the downgoing crust the first model case predicts three levels of exhumation in the same collisional context the classical corner flow lp lt low pressure low temperature exhumation in the accretionary prism deeper 70 km hp ht exhumation for the thickened subducting crustal sedimentary wedge and ultra hp ht exhumation from the lower crustal chamber forming at the depth of 100 120 km and separated from the upper one by a narrow crustal channel the width of this channel can oscillate in the process of shortening thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber although both zones of crustal accumulation and the narrow channel between them resemble a vortex shaped nozzle this nozzle appears to be too soft to produce any significant overpressures from the upper crustal wedge the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material from the lower crustal chamber the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere in the second modelled case of high buoyancy the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface accelerated or slowed subduction in case of full or partial crustal decoupling respectively and upper plate extension d 2001 elsevier science b v all rights reserved

noble gases in enstatite chondrites ii the trapped component the trapped noble gas record of 57 enstatite chondrites e chondrites has been investigated basically two different gas patterns have been identified dependent on the petrologic type all e chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases q and a subsolar component range of elemental ratios for e4 6 chondrites 36ar 132xe 582 270 and 36ar 84kr 242 88 e3 chondrites usually contain q gases but also a composition with lower 36ar 132xe and 36ar 84kr ratios which we call sub q 36ar 132xe 37 0 18 0 and 36ar 84kr 41 7 18 1 the presence of either the subsolar or the sub q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites we therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in e chondrites trapped solar noble gases have been observed only in some e3 chondrites about 30 of each group eh3 and el3 chondrites amounting to 9 of all analyzed e chondrites show the solar signature notably only one of those meteorites has been explicitly described as a regolith breccia

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

new findings for the equilibrated enstatite chondrite grein 002 we report new petrographic and chemical data for the equilibrated el chondrite grein 002 including the occurrence of osbornite metallic copper abundant taenite and abundant diopside as inferred from low si concentrations in kamacite the presence of ferroan alabandite textural deformation chemical equilibration of mafic silicates and a subsolar noble gas component we concur with grein 002s previous classification as an el4 5 chondrite furthermore the existence of pockets consisting of relatively coarse euhedral enstatite crystals protruding large patches of fe ni alloys suggests to us that this el4 5 chondrite has been locally melted we suspect impact induced shock to have triggered the formation of the melt pockets mineralogical evidence indicates that the localized melting of metal and adjacent enstatite must have happened relatively late in the meteorites history the deformation of chondrules equilibration of mafic silicates and generation of normal zoning in fe zn sulfides took place during thermal alteration before the melting event following parent body metamorphism daubreelite was exsolved from troilite in response to a period of slow cooling at subsolidus temperatures exsolution of schreibersite from the coarse metal patches probably occurred during a similar period of slow cooling subsequent to the event that induced the formation of the melt pockets overall shock features other than localized melting correspond to stage s2 and were likely established by the final impact that excavated the grein 002 meteoroid

noble gases in enstatite chondrites i exposure ages pairing and weathering effects cosmic ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite e chondrites 43 of them were measured for the first time with a total of 62 individual e chondrites literature and this data corrected for pairing the observed spectrum of ages ranges between 0 07 and 66 ma three clusters seem to develop at about 3 5 8 and 25 ma respectively since the uncertainty of ages is estimated to be 20 in contrast to 10 to 15 for ordinary chondrites and the number of examined samples is still comparatively small these peaks have to be confirmed by more measurements regarding the two subgroups eh and el chondrites no systematic trend is apparent in the distribution of cosmic ray exposure ages several e chondrites yield significantly lower 38ar ages compared to those calculated from cosmogenic 3he and 21ne for these e chondrites we suggest a reduction of cosmogenic 38ar as a result of weathering in order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of e chondritic material we simulated terrestrial weathering in an experiment of 12 weeks duration the treatment showed that a significant amount of cosmogenic 38ar is lost on earth by the influence of water

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

exhumation of uhp lt rocks due to the local reduction of the interplate pressure thermo mechanical physical modelling abstract spatial distribution of uhp lt terrains suggests that their exhumation is essentially a three dimensional process that occurs only locally in specific sites along mountain belts on the other hand the continental subduction resulting in the formation of uhp lt rocks takes place along the whole belt the previously performed by the authors 2 d thermo mechanical laboratory modelling of continental subduction has shown that exhumation of deeply subducted continental crust is possible only when the effective interplate pressure p n is lower than the lithostatic pressure low compression subduction regime at the same time this modelling showed that for the deeply subducted continental crust to be preserved at low temperature at great depth the continental subduction should be accompanied by the subduction of the fore arc block or the arc plate the latter process occurs only when p n is high high compression regime to reconcile both processes we suggest that within the background of a generally high compression regime the interplate pressure can be locally reduced in some specific situations which would then allow the local exhumation of uhp lt material using physical modelling technique we investigate one of such situations that occurs when the frontal part of the overriding plate undergoes subduction induced or not extension parallel to the plate boundary with activation or formation of a strike slip transform fault oblique to the plate boundary to the interplate zone the displacement along this fault results in a local reduction of the interplate pressure at the intersection of the fault with the interplate zone this pressure reduction permits the rise of the deeply subducted low density continental crust and sediments submitted to uhp lt conditions under buoyancy force a 10 km thick slice of crust detaches at 150 km depth and moves up along the interplate zone with a starting rate of ca 3 cm yr the ascent rate reduces when the unit reaches crustal depth the deformation of this unit is small when it first detaches and starts rising but increases when reaching the base of the overriding plate the exhuming material is sheared and stretched in a narrow passage at 70 km depth therefore in real conditions the deformation is mainly recorded when the unit is submitted to hp and not uhp conditions

the geological signature of a slab tear below the aegean in this paper we explore the geology of the aegean region in the miocene to identify the geological signature of a first order slab tear observed in all tomographic models from 15 to 8 ma the tear is associated spatially and timely with a fast clockwise rotation of the external hellenides alkaline volcanism high temperature metamorphic domes with a predominance of north dipping detachments and south westward migration of granitoid intrusions these features suggest a warmer geodynamic environment during the rotation and the impact of a hot mantle flow associated with the tear the 8 ma duration between the first high temperature metamorphic domes in the centre of the archipelago and the beginning of the fast rotation may correspond to the time needed for the slab to bend stretch and finally tear

noble gas record and cosmic ray exposure history of the new co3 chondrite moss comparison with lance and other co chondrite falls abstract the moss meteorite is the first co chondrite fall after a time period of 70 yr and the least terrestrially contaminated member of its group its cosmic ray exposure cre age t3 13 5 ma t21 14 6 ma is distinct among co chondrites and within witnessed falls is the shortest after lance which we have reanalyzed gas retention ages are approximately 3 95 109 yr u th he and approximately 4 43 109 yr k ar respectively trapped ar kr and xe are present in moss in abundances typical for co chondrites with planetary elemental and isotopic compositions presence of hl xenon from presolar diamonds is observed in the stepwise release analysis of lance it may also be present in moss but it is difficult to ascertain in single step bulk analyses it follows from our new data combined with a survey of the literature that the abundance of trapped gases in co chondrites is not a good indicator of their petrological subtype

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

exhumation of u hp lt rocks caused by diachronous slab breakoff abstract three dimensional thermo mechanical analogue models investigate how diachronous slab breakoff may lead to the exhumation of subducted continental crust slab breakoff initiates spontaneously in one location and migrates laterally along the plate boundary causing a transient excess downward pull force on the plate boundary in front of the propagating slab tear this pull force locally reduces the pressure between the plates which promotes buoyancy driven exhumation of subducted crust however both the surface area undergoing the pressure reduction and its duration are limited our experiments show that the rate of slab breakoff propagation controls both the duration of the pull force and the magnitude of pressure reduction our results further demonstrate that exhumation occurs where the slab breakoff propagation rate is lowest rather than where the pull force is strongest corresponding to where the slab tear initiates or terminates

non nucleosynthetic heterogeneity in non radiogenic stable hf isotopes implications for early solar system chronology abstract nucleosynthetic heterogeneity and secondary neutron capture reactions may have important implications for 176 lu 176 hf chronology and modelling of early planetary evolution so far the relevance of these phenomena for the lu hf system has not been explored we therefore have analyzed the non radiogenic stable hf isotope composition 177 hf 178 hf 179 hf and 180 hf of meteorites meteorite components and terrestrial rock samples to identify nucleosynthetic or neutron capture induced variations all analyzed chondrites have uniform 178 hf 177 hf and 180 hf 177 hf values that cannot be resolved from the average terrestrial composition thus there is no evidence for nucleosynthetic heterogeneity in chondrites or earth and these data support the use of a chondritic reference value for the hf isotope composition of the bulk silicate earth this homogeneity contrasts with nucleosynthetic heterogeneities found in lighter elements and provides evidence for a separate synthesis of light and heavy r process nuclei various mesosiderite samples and one lunar meteorite display coupled 178 hf 177 hf and 180 hf 177 hf anomalies that are associated with neutron capture induced deviations in 149 sm 154 sm and 150 sm 154 sm however the analyzed chondrites and an aubrite show only sm isotope anomalies and these are the result of neutron capture the hf isotope anomalies require substantial capture of epithermal neutrons whereas sm anomalies result primarily from thermal neutron capture the non radiogenic stable isotope composition of hf is thus a suitable monitor for epithermal neutron capture reactions the data reveal distinct neutron energy spectra mesosiderites are characterized by high epithermal to thermal neutron fluence ratios whereas the remaining samples show low epithermal to thermal ratios secondary neutron capture may significantly increase the measured 176 hf 177 hf in whole rock meteorite samples without causing a resolvable shift in 176 lu 177 hf thus it could potentially induce scatter in lu hf whole rock isochrons and produce spurious initial 176 hf 177 hf values however the slopes of internal i e mineral isochrons cannot be increased significantly by secondary neutron capture this process therefore cannot account for the unrealistically old 176 lu 176 hf ages e g 4 75 ga of some meteorites

the vicencia meteorite fall a new unshocked s1 weakly metamorphosed 3 2 ll chondrite the vicencia meteorite a stone of 1 547 kg fell on september 21 2013 at the village borracha near the city of vicencia pernambuco brazil it was recovered immediately after the fall and our consortium study showed it to be an unshocked s1 ll3 2 ordinary chondrite the ll group classification is based on the bulk density 3 13 g cm 3 the chondrule mean apparent diameter 0 9 mm the bulk oxygen isotopic composition delta 17 o 3 768 pm0 042 text o mkern 5mu mkern 3mu text oo delta 18 o 5 359 pm0 042 text o mkern 5mu mkern 3mu text oo delta 17 o 0 981 pm 0 020 text o mkern 5mu mkern 3mu text oo the content of metallic fe ni 1 8 vol the co content of kamacite 1 73 wt the bulk contents of the siderophile elements ir and co versus au and the ratios of metallic fe 0 total iron 0 105 versus total fe mg 1 164 and of ni mg 0 057 versus total fe mg the petrologic type 3 2 classification is indicated by the beautifully developed chondritic texture the standard deviation 0 09 versus mean cr 2 o 3 content 0 14 wt of ferroan olivine the tl sensitivity and the peak temperature and peak width at half maximum the cathodoluminescence properties of chondrules the content of trapped 132 xe tr 0 317 10 8 cm 3 stp g 1 and the raman spectra for organic material in the matrix the cosmic ray exposure age is 72 ma which is at the upper end of the age distribution of ll group chondrites the meteorite is unusual in that it contains relatively large up to nearly 100 m in size secondary fayalite grains defined as olivine with fa textgreater 75 large enough to allow in situ measurement of oxygen and mn cr isotope systematics with sims its oxygen isotopes plot along a mass dependent fractionation line with a slope of 0 5 and delta 17o of 4 0 pm 0 3 text o mkern 5mu mkern 3mu text oo and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites eet 90161 met 96503 and ngawi these data suggest that secondary fayalite in vicencia was in equilibrium with a fluid with a delta 17 o of 4 text o mkern 5mu mkern 3mu text oo consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites secondary fayalite and the chondrule olivine phenocrysts in vicencia are not in isotopic equilibrium consistent with low temperature formation of fayalite during aqueous alteration on the ll parent body that alteration as dated by the 53 mn 53 cr chronology age of secondary fayalite took place 4 0 1 4 1 1 ma after formation of cv cais when anchored to the quenched angrite d orbigny

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle