concentrations en uranium et desequilibres radioactifs iou en mediterranee orientale discussion sur l origine des sapropeles abstract sedimentation rates of the oxidized sediments from the eastern basin of the mediterranean sea are about a few centimetres per 1000 years not very different from the sedimentation rates of the western basin the sapropels levels show strong u enrichment from 10 to 30 times and ionium 230 th daughter of 238 u is in radioactive equilibrium with u at these levels these facts together with the high surficial productivity and or conservation factor implied by the strong organic material concentration of the sediments are hardly explained if sapropels are formed in the sea it seems that they are largely made up by continental material debris accumulated during propitious epochs of vegetation growth they would be expelled massively from the continent lagoons or closed seas when the sea level goes down causing the ablation of continental or coastal formations by the running waters then deposited onto the sea floor sapropel s 5 for instance could be made up by material from the last interglacial rich in organic matter but dispatched to the sea at the beginning of the glacial era

investigation of the ingrowth of radioactive daughters of 238u in mediterranean sapropels as a potential dating tool abstract five different sapropels s1 s3 s6 from two sites 2750 m and 3308 m water depth in the eastern mediterranean were analysed for corg content and specific activities of 238 u 234 u 230 th and 232 th anoxia in the sapropel sediments during or shortly after deposition leads to u enrichment and the observed excess of 234 u over 238 u activity is consistent with a seawater source for authigenic u in certain cases the u profile shape in and around the sappropels is interpreted to indicate that oxidation shortly after sapropel formation has caused a post depositional downwards migration of both u isotopes systematic variability of the 234 u 238 u activity ratio across the sapropel units indicates that a different diagenetic process has also led to preferential relocation of 234 u produced in situ from the 238 u parent the construction of quasi isochrons in successively older sapropel units demonstrate that radioactive secular equilibrium is approached by a systematic ingrowth of 230 th over time from the marked initial disequilibrium between 230 th and the u isotopes the effects of post depositional diagenesis violate the isochron model boundary conditions however and lead to a deviation of the isochron ages from the expected stratigraphic ages following assessment of each data point some improvements in the calculated ages were achieved in one case by exclusion from the calculations of those points most severely affected by open system behaviour

panafrican basement and mesozoic gabbro in the zagros orogenic belt in the dorud azna region nw iran laser ablation icp ms zircon ages and geochemistry abstract the dorud azna region of the sanandaj sirjan metamorphic zone ssmz in nw iran is part of the zagros orogenic belt and exposes an amphibolite grade metamorphic succession of mixed continental e g the granitic galeh doz orthogneiss and oceanic or rift e g various amphibolites origin which is intruded by the darijune gabbro laser ablation icp ms u pb zircon ages of 608 18 ma and 588 41 ma of the granitic galeh doz orthogneiss indicate a panafrican basement similar to that of the yazd block of central iran we therefore interpret this part of the ssmz to represent a previously subducted part of the panafrican basement of the iranian microcontinent based on geochemistry amphibolites and metagabbros from the amphibolite unit representially potentially the cover on the panafrican galeh doz orthogneiss are interpreted to have their origin in a likely upper paleozoic to mesozoic rift or e morb ophiolite succession one of the metagabbros here termed dare hedavand metagabbro yield a 206pb 238u mean age of 314 6 3 7 ma talc bearing greenschists to epidote amphibolites of the triassic june complex formed in a similar but younger environment furthermore the metamorphic complex is intruded by the darijune gabbro which yields a mean icp ms u pb zircon age of 170 2 3 1 ma the final stage of likely jurassic greenschist facies grade metamorphism postdates the gabbro intrusion we interpret the darijune gabbro to have resulted from the initial subduction of neotethyan oceanic lithosphere in a continental arc setting the new data combined with data from literature indicate that the ssmz represents a tectonic element of a mixed continental and oceanic origin likely accreted to central iran during a jurassic tectonic process the new data also constrains thick skinned nappe stacking within the sanandaj sirjan zone

3 d lithospheric structure and regional residual bouguer anomalies in the arabia eurasia collision iran this research has been partly funded by projectsatiza cgl2009 09662 bte and tecla cgl2011 26670 and consolider ingenio 2010 topo iberia csd2006 00041

source parameters and scaling relations for small earthquakes in the matsushiro region southwest honshu japan abstract using near field broadband data from a suite of crustal earthquakes which occurred in a small homogeneous source region we examine the scaling relationships among source parameters for earthquakes in the critical 1 5 to 4 5 magnitude range we take special precautions to insure that our conclusions are not biased by local structure seismic moment fault radius and stress drop relations were determined for 32 small shallow strike slip earthquakes 1 6 m l 4 3 in the matsushiro region of southwest honshu japan spectral and cepstral analyses of short period arrivals exhibit structure interpreted as resulting from reverberations within thin low velocity surface layers below the recording site furthermore calculated crustal transfer functions for sh predict a peak over the range of 2 to 4 hz this type of site response can bias estimates of spectral parameters particularly corner or characteristic frequencies for simple sources such a bias can be reduced by filtering in the cepstral domain the effect of cepstral filtering on corner frequency determinations is generally less than 20 per cent and tends to decrease them particularly for larger events where the structurally induced spectral enhancement is at frequencies above the source related corner frequencies estimates of the seismic moment brune stress drop and fault radius for each event were calculated from sh displacement amplitude spectra the seismic moments range from 10 18 to 10 22 dyne cm the stress drops range from 0 008 to 16 bars the relationship between log stress drop and log moment is approximately linear with a slope of 1 up to a moment of 2 10 21 dyne cm but for higher moments the slope is not significantly different from zero fault radii range from 0 3 to 1 2 km there is a physical scatter in radius for any given moment up to a factor of 1 5 for moments less than 2 10 21 dyne cm there is no significant trend in the radius as a function of moment but for higher moments the radius increases with moment the main conclusion is that the stress drop is the dominant scaling factor for moments less than 10 21 dyne cm for higher moment the source dimension becomes the controlling factor

uranium and radium diffusion in organic rich sediments sapropels among the late quaternary mediterranean sapropels the s5 125 ka is one of the best preserved due to its high organic carbon content that has limited postdepositional oxidation the high uranium content in this sapropel 40 dpm g 1 makes this layer interesting for studying uranium series disequilibrium in organic rich sediments for this reason the present work provides isotopic measurements of the u decay series in a s5 sapropel by applying more precise mass spectrometric methods tims mc icpms and gamma spectrometry assuming that u in the sapropel mostly originated from seawater the 234u 238u 230th 238u 226ra 230th and 231pa 235u activity ratios show systematic deviations from the theoretical values for a closed system evolution of the u series over the 125 ka since sapropel formation the radiogenic 234urad and 226ra show clear evidence of migration in the sapropel with modeled diffusion coefficients of 7 1 1 1 10 12 cm2 s 1 and 1 6 0 2 10 10 cm2 s 1 respectively the diffusion of 234urad cannot explain the high 230th 238u and 231pa 235u activity ratios observed in the sapropel two possible mechanisms or a combination of both are proposed for explaining the irregular 230th 238u and 231pa 235u activity profiles in sapropel s5 the first one is an enhanced export flux of 230thxs and 231paxs excesses exceeding the production rate in seawater during the time of sapropel formation and the second one is diffusion of authigenic uauth in the sapropel however the ambiguous determination of 230thxs and 231paxs in the sapropel and the poorly understood processes that might lead to uauth migration in anoxic sediments still limit a final explanation for the deviation of 230th 238u and 231pa 235u activity ratios from their expected theoretical values

active deformation and relief evolution in the western lurestan region of the zagros mountain belt new insights from tectonic geomorphology analysis and finite element modeling 2 d finite element modeling of both coseismic and interseismic deformation was performed along a transect across the seismogenic fault of the mw 7 3 november 2017 lurestan earthquake zagros mount

middle and late quaternary depositional sequences and cycles in the eastern mediterranean although most of the quaternary sediments of the eastern mediterranean appear reworked extensive areas of hemipelagic sediments occur mainly in the central part of the basin mediterranean ridge the most obvious character of these muddy sediments is the numerous changes in colour they result from the organic carbon content as well as from concentration and chemical form of metallic elements mn fe cu zn the succession of coloured layers is arranged into sequences developed between two sharp contacts a complete sequence involves five lithofacies from an oxidized layer at the base to a reduced sapropel at the top and displays progressive internal changes of colour from ochre beige greyish beige grey and black layers several sequence variants top truncated base truncated recurrent are identified these sequences are a consequence of changes in oceanographic conditions i e from oxidizing to reduced environments resulting in different organic and mineral contents based on detailed studies of quaternary sediments collected by coring a synthesized standard succession representative of the ordered sequences in the eastern mediterranean is proposed from about 650 000 yr as deduced from the calculated rate of sedimentation of 1 7 mm 1000yr 1 this succession appears modulated by cyclic events large scale periods 200 000 yr of reduced and oxidized sequences correlate with major alternate phases of flooding and reduced flow of the nile river during the quaternary subsequent sequences of shorter time scale 20 000 50 000 yr results from two factors heaviest monsoons leading to an increase of the influx of the nile river and global climatic changes eurasian ice sheet meltwater these two factors act synchronously or independently along the studied time span these interplaying factors result in periodic stages of increasingly restricted conditions in the basin and explain the complexity of the eastern mediterranean succession at least since 650 000 yr ago

geochemistry of tuscan archipelago granitoids central italy the role of hybridization processes in their genesis this paper reports major trace and rare earth element data for selected samples from the elba giglio and monte cristo plutons which lie in the tuscan archipelago italy on the basis of field and petrologic constraints all the rocks can be divided into five facies and range in composition from granodioritic to alkali granitic with a strong predominance of monzogranites tuscan archipelago granitoids are clearly collision related rocks and they straddle the i and s types of chappell and white however mineralogical and chemical characteristics indicate that the more basic rocks are broadly i type while the high silica rocks are s type genetic models that relate these magmas through the segregation of their major and accessory mineral phases are inconsistent with major trace and ree element abundant data geological petrographical and geochemical constraints show that the microgranular mafic enclaves found within the rocks are blobs of mafic high temperature magma chilled within a cooler m

long term seismic observations along myanmar sunda subduction margin insights for 2004 m w 9 0 earthquake the present study investigates the temporal variation of few seismic parameters between the myanmar zone i andaman nicobar northwest sumatra zone ii southeast sumatra west indonesia zone iii and east indonesia zone iv converging boundaries in reference to the generation of 26 december 2004 mw 9 0 off sumatra mega earthquake event the four segments are distinguished based on tectonics parameters distinct geological locations great earthquake occurrences and the wadati benioff zone characteristics two important seismic parameters such as seismic energy and b values are computed over a time window of 6 month period during the entire 1976 2013 period for these segments the b values show a constant decrease in zones ii iii and iv whereas the zone i does not show any such pattern prior to the 2004 mega event the release of seismic energy was also gradually decreasing in zones ii and iii till the 2004 event and little similar pattern was also noted in zone iv this distinct observation might be indicating that the stress accumulation was dominant near the sumatra java area located towards southeast of zone ii and northwest of zone iii the released strain energy during the 2004 event was subsequently migrated towards north rupturing 1300 km of the boundary between the northwest sumatra and the north andaman the occurrence of 2004 mega event was apparently concealed behind the long term seismic quiescence existing near the sumatra and nicobar margin a systematic study of the patterns of seismic energy release and b values and the long term observation of collective behaviour of the margin tectonics might have had given clues to the possibility of the 2004 mega event

humic substances in an arabian shelf sediment and the s1 sapropel from the eastern mediterranean abstract the molecular size distribution of humic substances has been investigated in sediment from two different geographical sites two depth horizons in a core sample taken from the arabian sea upwelling region age limits of 700 3000 yr have been analysed together with samples of the s 1 sapropel and the underlying grey marl deposit from the eastern mediterranean the extractable humic substances account for 20 of the total organic c in the sapropel 50 in the grey marl and 76 and 58 in the arabian sea sediments the humic compounds are described on the basis of their molecular weight distribution elemental composition and ir spectra results are considered in relation to the likely sources of organic c at each site the age of the sediment and the sedimentary environment the results obtained indicate that in both areas during the formation of the organic rich deposits the major source of organic c was planktonic the humic and fulvic acid distributions being dominated by high molecular weight humic acids

a method for multidimensional analysis of earthquake frequency magnitude distribution with an application to the vrancea region of romania abstract a novel method is proposed for the time space analysis of the frequency magnitude distribution of earthquakes fmd the fmd is calculated using a non parametric technique based on the nearest neighbourhood statistics and the fmd parameters namely the occurrence rate at a reference magnitude and the b value are subsequently estimated by a parametric log linear regression the method s potential is examined by using a microearthquake catalogue of the intermediate depth h 60 km seismicity in the vrancea region romania spanning 18 yr for the magnitude intervals of ml 2 8 4 0 and 4 0 5 2 the calculated occurrence rates exhibited different linear decays with increasing magnitude on a log linear scale quasi linear correlations were found between the fmd parameters over selected time space intervals in several cases the results showed that changes in the b values were inversely related to changes in the occurrence rates of magnitudes at the limits of the first magnitude interval the three large events mw 7 reported in the catalogue were preceded by time space regions of relatively low b values lasting for a few years at the depth of these events three episodes of distinct behaviour were identified i b values are inversely related to changes in the occurrence rates of magnitudes at the limits of the first study range ml 2 8 and 4 0 ii a decrease in the b value occurs when an increase in the occurrence rate of relatively larger magnitude events ml 4 0 is observed iii the occurrence rate of larger events tends to saturate whereas the rate of small events tends to fluctuate the results suggest that multi dimensional b values and occurrence rates over selected magnitudes have the potential to outline characteristic trends in the recorded seismicity and provide relative constraints for the modelling of the earthquake cycle

constraints on palaeodrainage evolution induced by uplift and exhumation on the southern flank of the zagros iranian plateau foreland sedimentary rocks from the northern fars region of iran contain a record of deformation associated with the cenozoic collision between arabia and eurasia that resulted in formation of the zagros orogen the timing of the deformation associated with this event is poorly known to address this we conducted a study of miocene foreland sedimentary rocks 19 7 14 8 ma of the chahar makan syncline using clast composition clay mineralogy and low temperature fission track dating the results showed that most of the sedimentary rocks were sourced from ophiolitic rocks detrital apatite fission track aft age signatures of miocene sedimentary rocks record exhumation in the hanging wall of the main zagros thrust and confirm that the change from underthrusting of the stretched arabian margin to widespread crustal thickening and deformation in the zagros region is no younger than 19 7 ma a transition from late oligocene to mesozoic eocene aft detrital age signatures between 19 7 16 6 ma and 16 6 13 8 ma is interpreted to reflect a possible rearrangement of palaeodrainage distribution that resulted from folding and expansion uplift of the zagros iranian plateau region supplementary material details of the succession of the study area a complete presentation of sample preparation and fission track age measurements of apatite grains and examples of x ray diffractograms are available at http www geolsoc org uk sup18502

magnitude frequency relation for small earthquakes a clue to the origin of ƒmax of large earthquakes in order to test the hyothesis that the max of large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3 we constructed the frequency magnitude relation for earthquakes with magnitude range from 1 3 to 4 recorded at a borehole seismograph station operated by the university of southern california at baldwin hills in the middle of the newport inglewood fault a procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time the cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit for ts p for each magnitude we found a clear depature of the observed log frequency magnitude relation for m 3 the observed frequency of earthquakes with m 0 5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3 we conclude that the observed frequency magnitude relation departs from self similarity for earthquakes with magnitude smaller than about 3 this departure coincides with the widely observed departure of the moment corner frequency relation from self similarity these observations are consistent with the interpretation of max of large earthquakes in terms of the slip weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers

eocene gold ore formation at muteh sanandaj sirjan tectonic zone western iran a result of late stage extension and exhumation of metamorphic basement rocks within the zagros orogen a multidisciplinary study including field geology microstructure analysis 40ar 39ar dating and fluid inclusion microthermometry and raman spectrometry has been carried out on the muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the sanandaj sirjan tectonic zone zagros orogen iran the muteh gold deposit has been previously interpreted as genetically related to precambrian granitic intrusions as an exhalative hot spring deposit related to paleozoic rhyolitic acidic tuffs and to local metamorphic processes host rocks of the gold deposit are predominantly schist and gneiss and subsidiary amphibolite and quartzite and are intruded by leucogranites in the vicinity of the gold deposit the metamorphic and granitic rocks display a subhorizontal mylonitic foliation containing a northeast oriented stretching lineation field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest oriented normal faults and joints dipping to the northeast and the southwest and crosscutting the ductile fabric of the host rocks both the ductile fabric of the host rocks and the gold ore controlling brittle structures are interpreted to have formed within a single continuous extensional event which started with ductile deformation and gradually changed into brittle deformation hydrothermal alteration associated with ore formation consists of quartz muscovite pyrite dolomite ankerite and albite which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks pyrite is the dominant opaque mineral and is the major phase associated with gold chalcopyrite marcasite bismuth galena sphalerite and pyrrhotite are subsidiary to rare phases 40ar 39ar incremental heating and in situ laser ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the muteh gold deposit muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40ar 39ar ages between 55 7 and 38 5 ma and show that gold mineralization is the youngest among the different dated events this eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults which can be correlated with tertiary extensional tectonic events reported by previous field investigations the data also indicate that gold ore formation is coeval with magmatism in the adjacent tertiary urumieh dokhtar magmatic belt and with intrusive activity within the sanandaj sirjan zone with a 40ar 39ar age of 54 ma near muteh according to our study the remaining 40ar 39ar data reveal a cretaceous to early tertiary metamorphic magmatic and deformation history of the complex hosting the muteh gold deposit the fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later eocene gold ore formation early regional fluids consist of an h2o co2 nacl fluid with subsidiary n2 and ch4 that underwent local unmixing the salinities of the liquid rich inclusions resulting from unmixing range between 2 2 and 17 5 wt percent nacl equiv and those of the co2 rich vaporrich inclusions range between 2 8 and 7 2 wt percent nacl equiv the high salinity of the regional liquid rich fluids is attributed to metamorphism of evaporite bearing sedimentary rocks magmatic intrusions retrograde hydration reactions metamorphism of impure marbles and migmatization the fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low salinity aqueous fluid with a dilute low density co2 component and maximum salinities between 2 6 and 5 8 wt percent nacl equiv gold deposition at muteh occurred during or after late stages of eocene brittle extension as a result of exhumation of the host metamorphic complex possibly during magmatic activity in the sanandaj sirjam zone it is most likely linked to the introduction of a distinct low salinity low co2 content aqueous fluid during crustal extension these fluids are younger than and therefore unrelated to the highly saline and co2 rich regional metamorphic magmatic fluids our results refute previous models advocating precambrian to paleozoic ages of the muteh gold deposit

zagros orogeny a subduction dominated process this paper presents a synthetic view of the geodynamic evolution of the zagros orogen within the frame of the arabia eurasia collision the zagros orogen and the iranian plateau preserve a record of the long standing convergence history between eurasia and arabia across the neo tethys from subduction obduction processes to present day collision from 150 to 0 ma we herein combine the results obtained on several geodynamic issues namely the location of the oceanic suture zone the age of oceanic closure and collision the magmatic and geochemical evolution of the eurasian upper plate during convergence as testified by the successive sanandaj sirjan kermanshah and urumieh dokhtar magmatic arcs the p t t history of the few zagros blueschists the convergence characteristics across the neo tethys kinematic velocities tomographic constraints subduction zones and obduction processes together with a survey of recent results gathered by others we provide lithospheric scale reconstructions of the zagros orogen from 150 to 0 ma across two sw ne transects the evolution of the zagros orogen is also compared to those of the nearby turkish and himalayan orogens in our geotectonic scenario for the zagros convergence we outline three main periods regimes 1 the mid to late cretaceous 115 85 ma corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper plate fragmentation 2 the paleocene eocene 60 40 ma witnesses slab break off major shifts in arc magmatism and distributed extension within the upper plate and 3 from the oligocene onwards 30 0 ma collision develops with a progressive sw migration of deformation and topographic build up sanandaj sirjan zone 20 15 ma high zagros 12 8 ma simply folded belt 5 0 ma and with partial slab tear at depths 10 ma to present our reconstructions underline the key role played by subduction throughout the whole convergence history we finally stress that such a long lasting subduction system with changing boundary conditions also makes the zagros orogen an ideal natural laboratory for subduction processes

u pb zircon ages field geology and geochemistry of the kermanshah ophiolite iran from continental rifting at 79ma to oceanic core complex at ca 36ma in the southern neo tethys abstract the geodynamic evolution of the zagros mountains of iran remains obscure in particular the time of formation of the zagros ophiolites and the closure of the neo tethys ocean are highly controversial here we present new precise zircon u pb ages that show that the younger part sahneh kamyaran of the kermanshah ophiolite formed at 35 7 0 5 ma and the older part harsin at 79 3 0 9 ma field relations and geochemical evidence show that the younger sahneh kamyaran part is probably a fossil oceanic core complex and the older sahneh part is probably a continental oceanic transition complex both the sahneh kamyaran and sahneh parts were later emplaced into an accretionary complex we conclude and infer that the final closure time of the southern neo tethys ocean was after the late eocene our data and tectonic model have crucial implications for the geodynamic evolution of the zagros region

late quaternary mediterranean sapropels ii organic geochemistry and palynology of s1 sapropels and associated sediments six s1 sapropel samples from the eastern mediterranean were investigated organic geochemically to study the lateral variation within this organic rich layer five of these samples were also analyzed palynologically in addition some sediment samples from below and overlying the sapropel intervals were investigated for their organic geochemical characteristics organic matter of s1 sapropels is of a mixed marine terrigenous and bacterial origin a trend of relatively increasing amounts of continentally derived organic matter towards more seaward and deeper realms can be observed from both palynological and organic geochemical data this trend is supported to some extent by 13c values of the organic matter the sapropelic intervals deposited on the nile cone are characterized by expanded thicknesses and a diluted organic carbon content due to a higher sedimentation rate the environmental conditions in terms of preservation during sapropel formation over the eastern mediterranean were probably not uniform at site 29 the conditions were favourable for the deposition of sapropel with a higher organic carbon content than at the other locations this might have been caused by better preservation conditions an increasing discharge from the river nile seems to be the driving force for formation of the s1 sapropels based on this assumption a model for sapropel formation is proposed although not all the data could be explained properly by this model most phenomena can be explained

nonlinear frequency magnitude relationships for the hokkaido corner japan abstract frequency magnitude relationships are determined for a source region defined by a circular epicentral area of 50 km radius centered on the carnegie broadband seismograph at kmu in the southeast corner of hokkaido island japan within this region bounded by a maximum depth of 125 km more than 11 100 earthquakes were detected and cataloged by the hokkaido university network during the period july 1976 through january 1987 the seismicity is divided into two effectively decoupled suites crustal eurasian plate and subduction pacific plate the frequency magnitude recurrence distributions exhibit departures from linearity which are statistically significant below m d 2 5 for the crustal data suite and m d 3 5 for the subduction data suite three independent tests show the nonlinearity is not caused by incompleteness the catalog is complete down to magnitude 2 0 or less for both data suites these observations are therefore inconsistent with the linear gutenberg richter relationship which is assumed to be valid over any magnitude range for which the data are complete there are relatively more large events and fewer small events in the subduction suite the logarithm of the ratio of the number of crustal events to the number of subduction events versus magnitude is monotonically decreasing and remarkably linear over the magnitude range 1 0 to 5 0 this suggests that the observed nonlinearity of these two frequency magnitude relationships has a common origin

precursory seismicity of the 1994 eruption of popocatepetl volcano central mexico popocatepetl volcano is located in the central mexican volcanic belt within a densely populated region inhabited by over 20 million people the eruptive history of this volcano indicates that it is capable of producing a wide range of eruptions including plinian events after nearly 70 years of quiescence popocatepetl reawakened in december 21 1994 the eruptive activity has continued up until the date of this submission and has been characterized by a succession of lava dome growth and destruction episodes similar to events that have apparently been typical for popocatepetl since the fourteenth century in this regime the episodes of effusive and moderately explosive activity alternate with long periods of almost total quiescence in this paper we analyze five years of volcano tectonic seismicity preceding the initial eruption of the current episode the evolution of the v t seismicity shows four distinct stages which we interpret in terms of the internal processes which precede an eruption after a long period of quiescence the thermal effects of a magma intrusion at depth the fracturing related to the slow development of magma related fluid pathways the concentration of stress causing a protracted acceleration of this process and a final relaxation or redistribution of the stress shortly before the initial eruption are reflected in the rates of v t seismic energy release a hindsight analysis of this activity shows that the acceleration of the seismicity in the third stage asymptotically forecast the time of the eruption the total seismic energy release needed to produce an eruption after a long period of quiescence is related to the volume of rock that must be fractured so imposing a characteristic threshold limit for polygenetic volcanoes limit that was reached by popocatepetl before the eruption

timing of uplift in the zagros belt iranian plateau and accommodation of late cenozoic arabia eurasia convergence the motion of arabia was stable with respect to eurasia over the past 22 ma deformation and exhumation in the zagros is seen to initiate at the same time as argued by new detrital thermochronologic constraints and increasing accumulation rates in synorogenic sediments a recent magnetostratigraphic dating of the bakhtyari conglomerates in the northern fars region of the zagros further suggests that shortening and uplift in the zagros folded belt accelerated after 12 4 ma available temporal constraints from surrounding collision belts indicate that shortening and uplift focused in regions bordering the iranian plateau to the south between 15 and 5 ma as boundary velocity was kept constant this requires concomitant decreasing strain rates in the iranian plateau slab detachment has been proposed to explain the observed changes as well as mantle delamination but the insignificant change in the arabian slab motion and lack of unambiguous constraints make both hypotheses difficult to account for it is proposed based on a review of shortening estimates provided throughout the arabia eurasia collision that the total 440 km of convergence predicted by geodesy and plate reconstruction over the past 22 ma can be accounted for by distributed shortening i suggest that the topography and expansion of the iranian plateau over late miocene pliocene time can be reproduced by the progressive thickening of the originally thin iranian continental lithosphere presumably thermally weakened during the eocene extensional and magmatic event

the magnitude frequency distribution of earthquakes recorded with deep seismometers at cajon pass southern california abstract the cumulative b value the slope of the gutenberg richter relationship between earthquake occurrence rate and magnitude is commonly found to be constant 1 network catalogues however reveal a decrease at small magnitudes b value is not just an artifact of catalogue incompleteness but that small earthquakes are really not as numerous as a constant b value extrapolated from larger events would predict in the cajon pass area southern california the b value of seismicity recorded by the local network scsn appears to decrease below about m l 1 6 in order to investigate whether this decrease is real or simply represents the network detection threshold we use seismicity recorded by the deep 1 5 and 2 5 km seismometers deployed in the cajon pass scientific drillhole between april 1992 and october 1994 the maximum amplitudes recorded downhole are compared to scsn magnitudes for events recorded by the network to determine the relationship between amplitude and m l as a function of hypocentral distance from the borehole magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole magnitude frequency curves are calculated for those events within 18 km of the borehole and a constant b value is observed to m l 0 5

modeling the flexural evolution of the amiran and mesopotamian foreland basins of nw zagros iran iraq the evolution of the amiran and mesopotamian flexural basins of the zagros belt is approached by coupled 2 d forward modeling of orogenic wedge formation lithospheric flexural isostasy and stream power erosion transport sedimentation thrust geometries and sequence of emplacement derived from geometric and kinematic models presented here are the inputs to our evolutionary model constrained by basin geometry sediment volume and topography modeling results confirm that the zagros flexural basins evolution is consistent with two stages of deformation 1 the obduction stage involving the kermanshah accretionary complex and a basement unit and 2 the collision stage emplacing the gaveh rud and sanandaj sirjan domains in the hinterland and forming a basement duplex in the outer part results provide quantitative insights into processes involved in mountain and basin building the lithospheric equivalent elastic thickness te changed from 20 km during the amiran stage 90 50 ma to 55 km during the mesopotamian subsidence stage last 20 myr the amiran basin results from flexure of the arabian plate below the load of the kermanshah cover and basement thrust sheets during this stage material eroded in the inner parts was enough to fill the flexural trough the mesopotamian basin formed in front of the outermost basement units flexing the arabian plate during this latter stage material eroded from the orogenic wedge was not enough to fill the mesopotamian basin an additional longitudinal sediment supply of up to 200 m myr is required to fill the flexural basin

orogenic gold mineralization at the chah bagh deposit muteh gold district iran abstract the chah bagh gold deposit in the muteh gold district is located in the central part of the sanandaj sirjan zone ssz iran gold mineralization at chah bagh is hosted by a paleozoic sequence of rocks that is dominated by deformed schist metarhyolite and amphibolite that exhibits a greenschist to lower amphibolite facies metamorphism three deformation events are recognized in the chah bagh area d1 d2 and d3 the major nw trending n280 n290 dextral strike slip shear zone in the area was formed during d2 ductile events gold mineralization at chah bagh occurred over a prolonged deformation history but is closely related to alteration retrograde greenschist facies assemblages and ductile and brittle deformation during d2 and d3 the geometry of the au bearing quartz veins indicates that they are temporally related to the s2 foliation and therefore to the d2 flattening and shearing some veins however are spatially and temporally related to d3 brittle normal faults and are brecciated and boudinaged during the associated shear movement the presence of deformed au bearing quartz veins and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone indicates continuous mineralization during the d2 and d3 episodes the au hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones this gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein hosted au mineralization the epigenetic au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for chah bagh deposit similar to qolqoleh and kervian in the northwestern part of the ssz this model is consistent with a dextral transpressional deformation kinematic model along the ssz and refutes the previous intrusion related model suggested for the muteh gold deposits metamorphic devolatilization and fluid flow within a ca 30 m y long period in late cretaceous and early tertiary is suggested for the genesis of the gold occurrences in both ductile and post transpressional brittle structures

mineral chemistry of isotropic gabbros from the manamedu ophiolite complex cauvery suture zone southern india evidence for neoproterozoic suprasubduction zone tectonics abstract the dismembered units of the neoproterozoic manamedu ophiolite complex moc in the cauvery suture zone southern india comprises a well preserved ophiolitic sequence of ultramafic cumulates of altered dunites pyroxenites mafic cumulates of gabbros gabbro norites and anorthosites in association with plagiogranites isotropic gabbros metadolerites metabasalts amphibolites and thin layers of ferruginous chert bands the isotropic gabbros occur as intrusions in association with gabbroic anorthosites plagiogranite and metabasalts amphibolites the gabbros are medium to fine grained with euhedral to subhedral orthopyroxenes clinopyroxenes and subhedral plagioclase together with rare amphiboles mineral chemistry of isotropic gabbros reveal that the clinopyroxenes are diopsidic to augitic in composition within the compositional ranges of en 42 59 fs 5 12 wo 31 50 they are ca rich and na poor na2o

implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates abstract increasingly fault slip rates are being used to constrain earthquake recurrence relationships for site specific probabilistic seismic hazard ground motion assessments this paper shows the sensitivity of seismic hazard assessments to variations in recurrence models and parameters that incorporate fault slip rates two models are considered to describe the partitioning of the slip rate or seismic moment rate into various magnitude earthquakes an exponential magnitude distribution and a characteristic earthquake distribution assuming an exponential distribution the activity rate n m 0 is constrained by the upper bound magnitude m u the b value for the region and the fault slip rate s for a given s variations in m u and b value have significant effects on recurrence and computed hazard depending on whether the assumption is made that the seismicity rate is constant or the moment rate is constant there is increasing evidence that the characteristic earthquake model is more appropriate for individual faults than the exponential magnitude distribution based on seismicity data from areas having repeated large earthquakes a generalized recurrence density function is developed and the resulting recurrence relationship requires only m u b value and s a comparison of the recurrence relationships from this model with the historical seismicity and paleoseismicity data on the wasatch and san andreas faults shows a good match the computed hazard based on the characteristic earthquake model differs from that obtained for the exponential model as a function of the fault to site distance and the acceleration level one check on the recurrence models using slip rate is to compare over large regions activity rates based on seismicity data and slip rate data such a comparison in the western transverse ranges shows a reasonable match for both the exponential and characteristic earthquake models in the moderate to large magnitude range

frequency magnitude distribution of 3 7 mw 1 mining induced earthquakes around a mining front and b value invariance with post blast time we investigated frequency magnitude distribution fmd of acoustic emissions ae occurring near an active mining front in a south african gold mine using a catalog developed from an ae network which is capable of detecting aes down to m w 5 when records of blasts were removed fmds of aes obeyed a gutenberg richter law with similar b values not depending on post blasting time from the initial 1 min interval through more than 30 h this result denies a suggestion in a previous study richardson and jordan bull seismol soc am 92 1766 1782 2002 that new fractures generated by blasting disturb the size distribution of background events which they interpreted as slip events on existing weak planes our ae catalog showed that the gr law with b 1 2 was valid between m w 3 7 and 0 for aes around the mining front further using the mine s seismic catalog which covers a longer time period of the same area we could extend the validity range of the gr law with the same b value up to m w 1

spatial distribution and b value of intermediate depth earthquakes beneath central peru we have investigated the spatial distribution of microseismicity at intermediate depth in the wadati benioff w b zone beneath central peru within a 100 to 150 km depth range in the subducting nazca plate 67 event hypocenters define a horizontally subducting zone horizontal zone 450 to 600 km from the trench and a zone of increasing dip resubduction zone 600 to 700 km from the trench although the shape of the w b zone is generally consistent with previous observations from teleseismic data the distribution of seismicity is not in a 550 km wide band extending from 400 to 750 km from the trench the density of teleseismicity mb 4 1971 1981 increases as a function of distance from the trench while the density of microseismicity 1 6 mb 4 2 decreases combining local and teleseismic data the b value changes from 84 in the horizontal zone to 57 in the resubduction zone this result suggests that the minimum earthquake size and or stress drop increases in the resubduction zone low b values may also be intrinsic to proximity to the seismic cut off at 150 km depth

the neotethyan sanandaj sirjan zone of iran as an archetype for passive margin arc transitions the sanandaj sirjan zone of iran is a northwest trending orogenic belt immediately north of the zagros suture which represents the former position of the neotethys ocean the zone contains the most extensive best preserved record of key events in the formation and evolution of the neotethys from its birth in late paleozoic time through its demise during the mid tertiary collision of arabia with eurasia the record includes rifting of continental fragments off of the northern margin of gondwanaland formation of facing passive continental margins initiation of subduction along the northern margin and progressive development of a continental magmatic arc the latter two of these events are critical phases of the wilson cycle that elsewhere in the world are poorly preserved in the geologic record because of superimposed events our new synthesis reaffirms the similarity between this zone and various terranes to the north in central iran late paleozoic rifting preserved as a type granites and accelerated subsidence was followed by a phase of pronounced subsidence and shallow marine sedimentation in permian through triassic time marking the formation and evolution of passive margins on both sides of the suture subduction and arc magmatism began in latest triassic early jurassic time culminating at 170 ma the extinction of arc magmatism in this zone and its shift northeastward to form the subparallel urumieh dokhtar arc occurred diachronously along strike in late cretaceous or paleogene time post cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid tertiary time

geochemical signatures preserved in sediments of the semaforo and vrica sections calabria italy and their relations with variations of the sedimentary regime abstract the well known semaforo and vrica sections in calabria italy represent a sequence of homogeneous marls with intercalations of laminated marls these marine sediments are of pliocene to pleistocene age and were originally deposited at a water depth of about 500 m the homogeneous marls have been deposited under oxic conditions the laminated marls under oxygen poor or anoxic conditions samples collected from present day outcrops of the homogeneous and laminated marls were analyzed for major minor and trace elements the laminated marls have a lower caco3 content as compared to the homogeneous marls which is primarily caused by dilution through increased input from land in the case of one semaforo and all but one vrica laminates the caco3 contents could have been diminished further by conversion into gypsum which was formed after oxidation of pyrite variations in the organic carbon corg content of the marls are probably caused by changes in sedimentation rates the small average corg enrichment in the laminates may be attributed to better preservation under euxinic conditions the high cl content of some samples is probably caused by evaporation of porewater from the sampled outcrops the profiles of the al ti fe k mg and p contents of the terrigenous components are practically equal throughout the sections the laminateshave systematically lower na zr and mn contents the lower na and zr contents indicate changes in the sedimentation regime whereas the lower mn content can be attributed to reductive dissolution under suboxic to anoxic conditions in comparison with the homogeneous marls the laminates are relatively enriched in redox sensitive and or chalcophile trace elements in the decreasing order mo cd sb se u cu v ni cr syn sedimentary or early diagenetic hydrogenous enrichment from overlying seawater is the only plausible explanation for their enrichment for most of these trace elements the enrichment is more pronounced when the laminates were deposited during bottom water anoxia a condition inferred from the presence of pyrite derived gypsum for reasons not understood two trace elements behave in opposite fashion laminated marls are depleted in as and zn relative to the homogeneous marls

building the zagros collisional orogen timing strain distribution and the dynamics of arabia eurasia plate convergence he zagros mountains are the result of the arabia eurasia collision initiated at 35 ma as the rifted arabian lithosphere was underthrusted beneath the iranian plate due to its negative buoyancy the onset of crustal thickening started at 25 ma as recorded by the hinterland exhumation and foreland clastic deposition deformation throughout the arabia eurasia collision zone and the uplift of the iranian plateau occurred after 15 12 ma as a result of shortening thickening of the thin iranian crust we emphasize that only 42 of the post 35 ma convergence is partitioned by shortening within central iran tomographic constraints show ongoing slab steepening or breakoff in the nw zagros whereas underthrusting of the arabian plate is observed beneath central zagros the current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in nw zagros and underthrusting in central zagros critical wedge approach applied to the zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust in contrast the weak sedimentary cover deforms by buckling of a thick multilayered cover thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes there is evidence that the role of the slab pull force in driving the arabian plate motion was reduced after 12 ma large scale mantle flow induced by mantle upwelling at the afar plume appears to be the main driver of the arabia plate motion we stress that the main kinematic change in the zagros region occurred at 15 12 ma as the zagros uplifted before the arabian slab detached the zagros appears key to investigate coupling between continental rheology plate driving forces and mountain building in which the role of rift inheritance appears to be central

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

mineralogical geochemical and isotopic evolution of two miocene mafic intrusions from the zagros iran abstract in the zagros mountain belt which represents in iran the collision zone between eurasian and arabian african plates large plutonic intrusions were emplaced during miocene times these massifs are composed of layered cumulates with sequences of fine grained texturally isotropic rocks from dominant gabbros to subordinated felsic rocks for this study two intrusions have been selected according to the geotectonic site of their emplacement the tah intrusion is located in the ophiolitic zone representing the remnants of the tethyan seafloor whilst the kharsareh intrusion is emplaced in the sanandaj sirjan metamorphic domain which corresponds to the border of the central iran plate starting with equivalent cumulates and gabbros the evolution of the two intrusions are clearly different the tah rock series comprises syenogabbros syenodiorites and syenites whereas silica oversaturated rocks appear in the karsareh series with the sequence of monzodiorites quartz monzonites and monzogranites the two intrusions show regular variations in their mineralogical and geochemical characters suggesting a direct genetic relationship within each rock series sr and nd isotopic compositions show smaller variations in the tah intrusion with sr and nd values respectively in the 6 7 to 12 4 and 3 3 to 5 4 ranges than in the karsareh intrusion with equivalent ranges of 16 to 99 and 0 4 to 5 2 the na k ba sr rb and ree contents of the gabbros although not truly representative of the primary magmas are typical of transitional basalts a two step combined wall rock contamination and crystal fractionation process would account for the evolution of the two series isotopic modelling suggests that during the first step which affects both intrusions the wall rocks would be basaltic rocks whereas during the second one only acting in the kharsareh intrusion the wall rock would be a hydrous continental crust the major differences between the two intrusions have been related to the geotectonic site of their emplacement

frequency magnitude distribution of earthquakes in vrancea relevance for a discrete model a high resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes h 60 km extending over an interval of 9 3 years emphasizes a significant deviation at low magnitudes ml 4 in a linear frequency magnitude distribution it appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve this is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity like earthquakes and the transition zone ml 3 3 3 9 from crack like earthquakes background seismicity to asperity like events these features are better pointed out on well individualized active zones the background seismicity shows a tremendeous decrease in its b slope from 1 03 to 0 57 during a 6 year interval before the occurrence of a major earthquake in 1986 mw 7 3 followed by a fast recovery to 1 03 in 2 years after this event such a behavior could be correlated to the continuous growth of the shear stress free surface on the fault as in a percolation process followed by its sudden diminution due to the locking of the fault the above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

crustal scale cross sections across the nw zagros belt implications for the arabian margin reconstruction quantified balanced and restored crustal cross sections across the nw zagros mountains are presented in this work integrating geological and geophysical local and global datasets the balanced crustal cross section reproduces the surficial folding and thrusting of the thick cover succession including the near top of the sarvak formation 90 ma that forms the top of the restored crustal cross section the base of the arabian crust in the balanced cross section is constrained by recently published seismic receiver function results showing a deepening of the moho from 42 2 km in the undeformed foreland basin to 56 2 km beneath the high zagros the internal parts of the deformed crustal cross section are constrained by new seismic tomographic sections imaging a 50 ne dipping sharp contact between the arabian and iranian crusts these surfaces bound an area of 10800 km 2 that should be kept constant during the zagros orogeny the arabian crustal cross section is restored using six different tectonosedimentary domains according to their sedimentary facies and palaeobathymetries and assuming airy isostasy and area conservation while the two southwestern domains were directly determined from well constrained surface data the reconstruction of the distal domains to the ne was made using the recent margin model of wrobel daveau et al 2010 and fitting the total area calculated in the balanced cross section the arabian continental oceanic boundary at the time corresponding to the near top of the sarvak formation is located 169 km to the ne of the trace of the main recent fault shortening is estimated at 180 km for the cover rocks and 149 km for the arabian basement including all compressional events from late cretaceous to recent time with an average shortening rate of 2 mm yr 1 for the last 90 ma