new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms japan this study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade graphitization can be parameterized using crystal structure and chemical and isotopic compositions the extent of graphitization could be characterized mainly by temperature duration of metamorphism and rock composition we compared the graphitization trends for two metamorphic terrains a contact aureole of the kasuga area and a regional metamorphic terrain of high temperature low pressure type of the ryoke metamorphic terrain in northern kiso area central japan and for two different lithologies carbonate and pelite using x ray diffractogram dta tg analysis and chemical and stable isotope analyses during contact metamorphism graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks the decreases in basal spacing d 002 of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400 c furthermore carbon isotopic ratios of graphite in carbonate rocks also change to 13c enriched values implying exchange with carbonates the beginning of this enrichment of 13c in the carbonaceous matter coincides with an abrupt increase of the graphitization processes carbon isotopic shifting up to 5 in pelites could be observed as metamorphic temperature increased probably by about 400 c carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well crystallized graphite detritus dta tg analysis is an effective tool for the distinction of detrital graphitic material two sources for the original carbon isotopic composition of carbonaceous matter in pelites in the kasuga contact aureole can be distinguished about 28 and 24 regardless of the presence of detrital graphite and were mainly controlled by depositional environment of the sediments graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole in the low temperature range the differences in extent of graphitization between the two metamorphic regions is large however at temperatures higher than 600 c the extent of graphitization in both regions is indistinguishable the degree of graphitization is different in limestones and pelitic rocks from the ryoke metamorphic terrain we demonstrate that the graphitization involves a progressive re construction process of the crystal structure the sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three dimensional structured graphite

cadomian terranes wrench faulting and thrusting in the central europe variscides geophysical and geological evidence sixty five per cent of the paleozoic basement of western and central europe is hidden by a sedimen tary cover and or sea this work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the variscan or ogeny it is based on the interpretation of various forms of data a published gravity maps corrected for the ef fects of the crust mantle boundary topography and light sedimentary basins b aeromagnetic maps c measurements of densities and d induced and rema nent magnetizations on rocks from paleozoic outcrops of the upper rhenish area from the northern b ohem ian massif to the eastern paris basin the saxothurin gian is characterized by a 500 km long belt of gravity highs the most important being the kraichgau high most of the corresponding heavy bodies are buried un der a post early visdan cover they are interpreted as relics of late proterozoic terranes overlain by an early to middle paleozoic sequence equivalent to the bo hemian t errane in the bohemian massif the most probable continuation of these dense bohemian ter ranes toward the west is the southern channel north ern brittany cadomian terrane the gravity lows are correlated with variscan granites and pre and early variscan metagranites gravity and magnetic maps demonstrate large scale displacement in devonian early carboniferous times along the parallel and equidistant nw se striking vistula elbe bavarian bray and south armorican dextral wrench faults in the vosges schwarzwald and central massif the faults continue with the east west striking lalaye lubine ba den baden and marche faults and with south vergent thrusts the bavarian faults shift the kraichgau terrane by 150 km relative to the bohemian terrane whereas the offset of the north ern brittany cadomian relative to the northern vosges kraichgau terranes is estimated at 400km along the bray fault sinistral wrench faults are the ne sw striking sillon houiller rheingraben rodl vitis and diendorf faults the southern vosges schwarzwald devonian dinanti an basin is interpreted as a pull apart basin at the south easterly extremity of the bray fault the bohemian and kraichgau body form allochthonous terranes which were thrust over the saxothuringian crust thrusting to the north west was accompanied by back thrusting and led to the forma tion of pop up structures contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision the zonal structure of the varis cides in the sense of kossmat 1927 is relevant only to the rhenohercynian foreland belt kossmat 1927 al ready spoke of a moldanubian region because it dis plays no real zonal structure the saxothuringian zone was formed by terrane accretion their apparent zonal structure is not a pre collisional feature but only the result of accretion and collision

marine geophysical investigation of the chain fracture zone in the equatorial atlantic from the pi lab experiment the chain fracture zone is a 300 km long transform fault that offsets the mid atlantic ridge we analyzed new multibeam bathymetry backscatter gravity and magnetic data with 100 multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the passive imaging of the lithosphere asthenosphere boundary pi lab experiment analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 myr we model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic magmatic history of the region within the active transform fault zone we observe four distinct positive flower structures with several en echelon fault scarps visible in the backscatter data we find up to 10 mgal residual mantle bouguer anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the atlantic the extensional compressional features observed in the chain transform are less pronounced than those observed further north in the vema st paul and romanche and may be due to local ridge segment adjustments

carbo permian pteridophyll leaf fragments from an amphibolite facies basement tauern window austria graphitic schists from the pennine basement tauern window austria contain fragments of pteridophyll leaves they are preserved despite the overall strong deformation and metamorphism at amphibolite facies conditions 525 c 750 100 mpa due to a particular strain path with volume loss by pressure solution they were determined as cf callipteridium sp indicating a permo carboniferous age this depositional age is similar to radiometric age data obtained for the intrusion of nearby metagranitoids indicating either tectonic contacts between the metagranitoids and the graphite schists or a very active and rapidly changing tectonic regime during the hercynian event

recent vertical movements from high precision leveling data in northeast spain abstract historical and modern high precision leveling data are analyzed to measure recent vertical movements in northeast spain the recent vertical movements are deduced by comparing the original height differences taken from the original logbooks measured in the field in two surveys carried out by the instituto geografico nacional in the periods 1871 1922 and 1925 1974 an exhaustive study of the errors involved in the measurements indicates that some of the observed movements are significant we present the recent vertical movements along eight profiles with significant uplift rates a southern anomaly of about 1 mm yr observed in two independent profiles associated with the southern part of the catalan ranges frontal thrust one anomaly between caldetes and arenys with an average velocity of about 4 mm yr which can be related to the seismic activity of this century two anomalies of about of 2 5 mm yr in the north and of about 4 mm yr in the south associated with the nw se striking amer brugent fault system which has been related to a great seismic crisis in the fifteenth century and one anomaly of about 0 8 mm yr associated with the emporda basin dynamics other observed recent vertical movements are mainly related to sediment compaction masking possible tectonic deformations all the northern anomalies are associated with the nw se system of faults considered as a transfer zone of the ne sw striking faults which controlled the formation of the neogene european rifting the former shows plio quaternary tectonic activity folds faults volcanism hydrothermalism and historical seismicity the low seismicity of this century around the amer brugent fault system contrasts with both the high average velocities estimated here and with the fifteenth century seismic crisis therefore if these high uplift rates are tectonic in origin they may represent an accumulation of elastic strain which would be released in a future large earthquake rather than in continual small ones according to this hypothesis a large part of the stress accumulated since the fifteenth century has yet to be released further leveling and neotectonic surveys are needed to test this hypothesis

gravity driven deformation of damavand volcano iran detected through insar time series the detection and monitoring of gravity driven volcano deformation are vital for understanding volcanic hazards such as landslides lateral blasts and debris avalanches although deformation has been detected at several large active volcanoes e g mount etna vesuvius kilauea these systems also exhibit persistent magmatic activity obscuring the gravity driven signals of ground motion in this study we present a first interferometric synthetic aperture radar insar deformation time series at the dormant damavand volcano in northern iran over the period a d 2003 2008 the high resolution data show a lateral extension of the volcano at the relative rate of as much as 6 mm yr accompanied by subsidence at the rate of as much as 5 mm yr at the volcano summit we find that lateral motion of the east flank is more significant than that of the west flank on the basis of past understanding and modeling of deforming volcanoes elsewhere we interpret this new evidence to reveal long term slow gravity driven deformation possibly in the form of gravitational spreading at damavand this persistent deformation activity is well expressed although no volcanic activity was ever reported in history this finding shows that magmatic activity is not required for spreading and highlights the importance of identifying long lived gravity driven deformation for hazard assessment at dormant or inactive volcanoes

extreme mantle uplift and exhumation along a transpressive transform fault earth s crust diverges and extends along mid ocean ridges analyses of gravity and seismic data from the equatorial atlantic show that propagation of ridge segments can compress the crust and create sufficient uplift to create small islands

evidence of suppression of the unequal refraction error in geodetic leveling analyses of historic data indicate that procedures in force since the nineteenth century have effectively suppressed the unequal refraction error ure in geodetic leveling although theoretical considerations argue that large refraction errors should accumulate where levelings traverse gentle slopes permitting long sight lengths extensive examination of routinely developed results lends little support to this argument our analyses of the historic record consider 1 height differences based on otherwise similar levelings over the same route but characterized by substantially different sight length 2 modeled refraction error differences and actually measured elevation changes on both a section by section and a line by line basis 3 misclosures produced from levelings identified with significantly different procedural constraints in technically stable areas of comparable variation in terrain and climate and 4 alternatively and sequentially developed heights in areas of tectonic stability and areas of persisting instability all of these analyses indicate that the ure is generally small and unpredictable and hence inconsistent with both theoretical expectations and the results of two conceptually different experimental studies this inconsistency probably is attributable to the inability of the theoretical models and the failure of the experiments to incorporate in their design those procedures especially the rejection criteria at the setup section and circuit level that normally minimize refraction error traditionally invoked procedural specifications may have suppressed the ure through biasing of observations toward less refractive conditions whereas all of the theoretical models assume an averaged sampling of the atmosphere alternatively a variety of real world complexities that are disregarded in the models may take on added importance in normally constrained leveling thus refraction corrections based on simplified models and mean atmospheric and terrain conditions tend to overcompensate substantially for refraction errors associated with routinely constrained surveys

evolution of the oceanic lithosphere in the equatorial atlantic from rayleigh wave tomography evidence for small scale convection from the pi lab experiment

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

spreading rate dependence of gravity anomalies along oceanic transform faults gravity and bathymetry data from transform fault systems associated with mid ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate and fast spreading ridges if confirmed this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid ocean ridges mid ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge slow spreading mid ocean ridge segments exhibit significant crustal thinning towards transform and non transform offsets1 2 3 4 5 6 7 8 9 10 11 12 which is thought to arise from a three dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1 2 4 5 6 7 9 10 12 in contrast fast spreading mid ocean ridges are characterized by smaller segment scale variations in crustal thickness which reflect more uniform mantle upwelling beneath the ridge axis13 14 15 here we present a systematic study of the residual mantle bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate previous studies have shown that slow slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1 2 4 6 but our analysis reveals that intermediate and fast slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments this finding indicates that there is a mass deficit at intermediate and fast slipping transform faults which could reflect increased rock porosity serpentinization of mantle peridotite and or crustal thickening the most negative anomalies correspond to topographic highs flanking the transform faults rather than to transform troughs where deformation is probably focused and porosity and alteration are expected to be greatest indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed this finding in turn suggests that three dimensional magma accretion may occur near intermediate and fast slipping transform faults

structural analysis of the gachsar sub zone in central alborz range constrain for inversion tectonics followed by the range transverse faulting analysis of the gachsar structural sub zone has been carried out to constrain structural evolution of the central alborz range situated in the central alpine himalayan orogenic system the sub zone bounded by the northward dipping kandovan fault to the north and the southward dipping taleghan fault to the south is transversely cut by several sinistral faults the kandovan fault that controls development of the eocene rocks in its footwall from the paleozoic mesozoic units in the fault hanging wall is interpreted as an inverted basin bounding fault structural evidences include the presence of a thin skinned imbricate thrust system propagated from a detachment zone that acts as a footwall shortcut thrust development of large synclines in the fault footwall as well as back thrusts and pop up structures on the fault hanging wall kinematics of the inverted kandovan fault and its accompanying structures constrain the n s shortening direction proposed for the alborz range until late miocene the transverse sinistral faults that are in acute angle of 15 to a major magnetic lineament which represents a basement fault are interpreted to develop as synthetic riedel shears on the cover sequences during reactivation of the basement fault this overprinting of the transverse faults on the earlier inverted extensional fault occurs since the late miocene when the south caspian basin block attained a ssw movement relative to the central iran therefore recent deformation in the range is a result of the basement transverse fault reactivation

gps and gravity constraints on continental deformation in the alborz mountain range iran a network of 54 survey gps sites 28 continuous gps stations and three absolute gravity ag observation sites have been set up in the alborz mountain range to quantify the present day kinematics of the range our results allow us to accurately estimate the motion of the south caspian block scb for the first time and indicate rotation of the scb relative to eurasia accounting for the left lateral motion in the alborz range in light of these new results it clearly appears that deformation rates vary along the range the eastern part accommodating mainly left lateral strike slip 2 mm yr 1 south of the range and 5 mm yr 1 north of the range with a very low range normal shortening rate on the khazar thrust fault similar to 2 mm yr 1 and the western part accommodating range normal shortening similar to 6 mm yr 1 on the khazar thrust fault with a left lateral component of similar to 2 mm yr 1 north of the range and 1 mm yr 1 south of the range these present day kinematics agree with geomorphologic estimated slip rates but not the long term deformation corroborating the idea that the kinematics of the range have changed recently due to the change of scb motion modelling of the interseismic deformation suggests a deep locking depth on the central western segment of the khazar fault similar to 30 km in agreement with the baladeh earthquake rupture and aftershock ranging between 10 and 30 km given this unusual deep locking depth and the 34 degrees dip of the thrust a large part of the alborz range is located above the seismically coupled part of the fault based on our ag measurements this part of the range seems to uplift at a rate of 1 5 mm yr 1 in agreement with terrace uplift

oceanic broad multifault transform plate boundaries oceanic transform plate boundaries consist of a single narrow a few kilometers wide strike slip seismic zone offsetting two mid ocean ridge segments however we define here a new class of oceanic transform boundaries with broad complex multifault zones of deformation similar to some continental strike slip systems examples are the 750 km long 120 km wide andrew bain transform on the southwest indian ridge and the romanche transform where the mid atlantic ridge is offset by a lens shaped 900 km long 100 km wide sliver of deformed lithosphere bound by two major transform valleys one of the valleys is seismically highly active and constitutes the present day principal transform boundary however strike slip seismic events also occur in the second valley and elsewhere in the deformed zone some of these events may be triggered by earthquakes from the principal boundary numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of 30 m y i e in extra long 500 km slow slip transforms multiple boundaries develop so that strike slip ruptures avoid very thick and strong lithosphere

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

empirical estimates of cumulative refraction errors associated with procedurally constrained levelings based on the gaithersburg tucson refraction tests of the national geodetic survey

hydrography of the romanche and chain fracture zones the romanche and chain fracture zones rfz and cfz are known as pathways for the lower north atlantic deep water lnadw and the antarctic bottom water aabw from the western trough to the eastern trough of the equatorial atlantic ocean across the mid atlantic ridge we report hydrographic observations describing the evolution of the lnadw and aabw properties during their transit through the equatorial fracture zones to the west of the rfz and cfz sills the lnadw and aabw cores are separated by a deep thermocline pycnocline which marks the transition between the two water masses the deep thermocline is eroded eastward and vanishes in the eastern basin this zonal asymmetry is the signature of a zonal baroclinic pressure gradient associated in each fracture zone with eastward flows of lnadw and aabw below 3800 m depth during the eastward transit through the rfz the near bottom aabw becomes less dense saltier 0 07 and warmer 0 7 c richer in oxygen 0 3 ml l 1 but less rich in silicate 30 mol l 1 simultaneously the lnadw core becomes fresher and colder at a given density and the oxygen maximum characterizing the lnadw core weakens by 0 2 ml l 1 we argue that the property changes within the aabw and lnadw are due to intense vertical mixing in addition to vertical mixing with the aabw it seems likely that isopycnal mixing of the lnadw with the eastern basin deep water contributes to the property evolution of the lnadw exiting the rfz in the cfz evidence is also given of mixing occuring within the aabw and the lnadw a sudden change in the aabw properties observed across the cfz main sill suggests the blocking of the eastward progression of the densest aabw by the cfz main sill however this dense water can reach the eastern basin by flowing northeastward through valleys the sills of which are deeper than thoses of the cfz finally a simple hydraulic control model applied to the rfz and cfz flows predicts a transport of 1 4 0 1 106 m3 s 1 of water colder than 1 9 c across the rfz cfz main sill

flexural uplift of a lithospheric slab near the vema transform central atlantic timing and mechanisms abstract the vema transverse ridge vtr is a prominent long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the vema transform at 11 n in the atlantic it rises abruptly about 140 km from the axis of the mid atlantic ridge mar in 10 myr old crust and runs continuously up to 25 myr old crust it reaches over 3 km above the predicted lithospheric thermal contraction level it is absent in crust younger than 10 myr thus the uplift of the vtr must have ended roughly 10 ma the vtr is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long mar segment based on satellite gravimetry imagery this mar segment was born roughly 50 ma and increased its length at an average rate of 1 6 mm yr multibeam data show that the mar parallel sea floor fabric south of the vtr shifts its orientation by 5 to 10 clockwise in 11 12 myr old crust indicating a change at that time of the orientation of the mar axis and of the position of the euler rotation pole this change caused extension normal to the transform followed between 12 and 10 ma by flexure of the edge of the lithospheric slab uplift of the vtr at a rate of 2 to 4 mm yr and exposure of a lithospheric section vema lithospheric section or vls at the northern edge of the slab parallel to the vema transform ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the vls at different distances from the mar axis suggest that the entire vtr rose vertically as a single block within the active transform offset a 50 km long portion of the crest of the vtr rose above sea level subsided was truncated at sea level and covered by a carbonate platform subaerial and submarine erosion has gradually removed material from the top of the vtr and has modified its slopes spreading half rate of the crust south of the transform decreased from 17 2 mm yr between 26 and 19 ma to 16 9 mm yr between 19 and 10 ma to 13 6 mm yr from 10 ma to present the slowing down of spreading occurred close in time to the change in ridge transform geometry suggesting that the two events are related a numerical model relates lithospheric flexure to extension normal to the transform suggesting that the extent of the uplift depends on the thickness of the brittle layer consistent with the observed greater uplift of the older lithosphere along the vtr

the impact of refraction correction on leveling interpretations in southern california recent investigations show that the refraction corrections in leveling are large but can be adequately modeled the corrections are proportional to the temperature differences and the square of the sight lengths changes in sight length with time have resulted in apparent crustal movements when non refraction corrected data were used applying refraction corrections to leveling data in southern california significantly alters the size and configuration of the southern california uplift uplift at palmdale and gorman lebec relative to los angeles bm v32 during the 1953 1978 time period is no greater than 10 cm the bulk of the motion about 6 cm occurs as localized motion along the san gabriel fault during the 1953 1964 time period this motion along the san gabriel fault and uplift of 8 cm directly north of the san fernando fault between 1955 and 1968 may represent precursors to the 1971 san fernando earthquake

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

saugus palmdale california field test for refraction error in historical leveling surveys a field test designed to measure atmospheric refraction error in historical and modern leveling was conducted in may june 1981 on a 50 km long grade from saugus to palmdale california during 1955 1971 the length of sights made between the level instrument and rods systematically decreased from 60 m to 26 m the difference in height near palmdale measured by single run long sight 42 m and short sight 22 m leveling during the test was 6 times larger than expected random error correction for refraction by using either the observed or modeled vertical temperature gradient in kukkamaki s balanced sight equation reduced the height difference to the level of random error uncertainty the observed temperature gradient obeyed a power law relation t a bzc where z is the height and a b and c are constants that depended on atmospheric conditions and the ground surface beneath the line of sight the refraction corrected leveling satisfies the specifications and meets all standards of first order control surveys the six historical surveys of the saugus palmdale grade were corrected for refraction error using the results of the experiment and for rod scale errors and nontectonic subsidence considered in previous investigations the corrected uplift near palmdale reached 56 16 mm with respect to saugus during the period 1955 1965 this amount of uplift is about one third that obtained before removal of refraction error the corrected displacement profiles also reveal previously unrecognized deformation in the epicentral region of the 1971 san fernando ml 6 4 earthquake during the decade before the main shock

u pb investigations on zircons from pre variscan gneisses i a study from the schwarzwald west germany abstract field observations in the schwarzwald indicate that pre variscan rocks were formed by the following events 1 1 intrusion of granitic to granodioritic magma into sediments 2 2 a sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity the absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved we collected several samples of a coarse grained homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the einetwald forest north of steinach kinzig valley the metasedimentary rock corresponds to the biotitepoor paragneisses in the schwarzwald the zircons in the latter sample are detrital and only very small specimens 45 m are euhedral the zircons from the orthogneiss samples are euhedral some are zoned and some have dark inclusions analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 myr and 524 20 myr respectively the 2000 myr value is interpreted as the average age of the sedimentary source rocks for this material the 524 myr intercept defines an event responsible for most of the lead loss in the zircons as well as new zircon growth analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons stepwise leaching of two zircon fractions shows newly grown u rich zircon rims around old cores a 207 pb 206 pb age of 517 15 myr was found for the first dissolution steps about 50 of the zircons dissolved in subsequent steps with progressive dissolution of the residue 207 pb 206 pb age increased to 950 myr the u pb zircon data for the paragneiss and the orthogneiss establish an age of 520 15 myr for the magma intrusion

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

transport of bottom water in the romanche fracture zone and the chain fracture zone abstract two moored arrays deployed in the romanche fracture zone and chain fracture zone in the equatorial atlantic ocean provide two year long time series of current and temperature in the lower north atlantic deep water and the antarctic bottom water total time averaged transport of antarctic bottom water potential temperature 1 9 c across the mid atlantic ridge amounts to 1 22 106 m3 s 1 eastward with a standard deviation of 0 25 106 m3 s 1 a time averaged transport of 0 36 0 23 106 m3 s 1 eastward is found for the lower north atlantic deep water in the 1 9 2 1 c temperature range but this may represent only a fraction of the total flow of this water mass across the ridge contributions of the romanche fracture zone and chain fracture zone to the antarctic bottom water transport are similar while the chain fracture zone has the greater share of lower north atlantic deep water transport semiannual and annual periods are detected in the transport time series and together expl

chitinozoan contribution to the ordovician and lower silurian paleobiogeography relationships between various lower silurian chitinozoan assemblages are established using statistical methods such as calculation of the number of species in common and similarity coefficient conclusions clearly indicate that eastern canadian chitinozoan fauna are distinct from those of the north gondwana margin the proposed relationships are refined by the use of more rigorous analytical techniques such as correspondence analysis and cluster analysis the results are compared to the paleogeographic reconstructions available for the ordovician and the lower silurian they show that chitinozoans are useful not only for stratigraphic correlations but also as their paleogeographic distribution seems to be linked to paleolatitudes they can be used to document plate tectonic dynamics

active transtension inside central alborz a new insight into northern iran southern caspian geodynamics the tectonic activity in the alborz mountain range northern iran is due both to the northward convergence of central iran toward eurasia and to the northwestward motion of the south caspian basin with respect to eurasia inducing a left lateral wrenching along this range these two mechanisms give rise to a nne ssw transpressional regime which is believed to have affected the entire range for the last 5 6 2 m y in this paper we show that the internal domain of central alborz is not affected by a transpressional regime but by an active transtension with a wnw ese extensional axis we show that this transten sion is young middle pleistocene it postdates an earlier n s compression and may have been initiated when the south caspian basin started moving consequently our results suggest that the south caspian basin motion may have taken place more recently than previously proposed

influence of a transverse basement fault on along strike variations in the geometry of an inverted normal fault case study of the mosha fault central alborz range iran abstract inverted normal faults show variations in structural geometry from footwall shortcut thrusts to hanging wall break back thrusts in the present study we evaluated along strike variations in the geometry of the mosha fault central alborz range iran detailed structural mapping within the taleqan mountains revealed that the mosha fault in the eastern part of the study area is associated with a large basement cored hanging wall anticline thrust over tertiary rocks that occur as a syncline or cut by footwall shortcut thrusts in the western part of the study area however the hanging wall consists of break back thrust sheets of paleozoic mesozoic rocks these thrust sheets occur as a duplex system and have allochthonous masses gravitationally emplaced in the foothills of the taleqan mountains curved fold axial traces and scattered intrusive bodies are observed in the valian valley area where the mosha fault is displaced across a transverse basement fault this basement fault controlled along strike variations in the geometry of the mosha fault during its evolution from a basin bounding fault to an inverted normal fault inversion of the fault in the eastern part of the study area where the fault is relatively steep involved basement rocks in the western part of the study area where the fault is relatively shallowly dipping deformation in the hanging wall is accommodated by thin skinned break back thrust sheets

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

rb sr and u pb isotope studies on migmatites from the schwarzwald germany constraints on isotopic resetting during variscan high temperature metamorphism mineral and isotope studies were undertaken on migmatites from the schwarzwald moldanubian zone of the variscan belt the aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre scale migmatite profiles respectively suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation deformation enhanced mass transport rather than by partial melting anatexis the results of rb sr thin slab dating on these profiles indicate that sr isotopes were not completely reset during migmatization no true isochron ages but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87sr 86sr ratios back to various stages in their evolution the coincidence of these rb sr data with u pb ages of monazites from migmatites and non migmatitic gneisses shows that gneisses and migmatites were formed during the same high temperature event in the carboniferous 330 335 ma the observation that high temperature metamorphism failed to equilibrate sr isotopes on the centimetre scale imposes limitations on the use of conventional whole rock isochron techniques in dating migmatites

global sediment thickness data set updated for the australian antarctic southern ocean we present a new 5 min sediment thickness grid for the australian antarctic region 60 e 155 e 30 s 70 s new seismic reflection and refraction data have been used to add detail to the conjugate australian and antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously on the margins sediment thickness estimates were computed from velocity depth functions from sonobuoy refraction velocity solutions ground truthed against seismic reflection data for the southeast indian ridge abyssal plains sediment thickness contours from geli et al 2007 were used the new regional minimum sediment thickness grid was combined with the global national geophysical data center ngdc sediment grid to create an updated global grid even using the minimum estimates sediment accumulations on the extended australian and antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the ceduna basin compared to the global ngdc compilation of sediment thickness data

deposition and deformation of late archaean sediments and preservation of microfossils in the harris greenstone domain gawler craton south australia abstract late archaean sediments felsic mafic and ultramafic volcanics in the harris greenstone domain central gawler craton south australia are interpreted to have been deposited at 2520 ma in back arc settings and were metamorphosed at 2440 ma sedimentary evidence suggests that the metasediments in the domain might have been deposited in fluvial estuarine to deltaic shelf environments and deformed by intermediate amphibolite facies metamorphism accompanying the sleafordian orogeny at 2440 ma microfossils are preserved in metachert layers that had been boudinaged into lenses these microfossils including spinose acritarchs are organic walled as evidenced by laser raman microscopy fossil bearing off cuts were then treated by hf and similar microfossils degraded organic matter and acid resistant minerals such as zircon were exposed in situ on the etched surface their morphologically complex forms and detailed wall structures suggest that these microfossils are of primary biological origin the microfossils in foliated metachert lenses either from original deposition or from post depositional quartz veins are older than the 2440 ma age of the sleafordian orogeny which provides a minimum age for the host rocks in either case these spinose acritarchs are the oldest protists known to date

determining sedimentation ages of high grade metamorphic gneisses by their palynological record a case study in the northern schwarzwald variscan belt germany in the northern schwarzwald moldanubian zone of the variscan belt paragneisses drilled from approximately 2500 m depth at the buhl 1 drilling site have experienced amphibolite facies metamorphism and were later partly transformed to mylonites under greenschist facies conditions completely graphitised acritarchs and chitinozoans were recovered from these paragneisses and mylonitic gneisses whereas both groups of palynomorphs give evidence of a marine depositional environment only the chitinozoans are stratigraphically useful in this case and indicate a paleozoic age previous findings of acritarchs in a different high grade metamorphic gneiss unit within the schwarzwald indicate an upper proterozoic marine sedimentation age the spectrum of palynomorphs found thus far in the schwarzwald in combination with petrological data suggests that within the moldanubian domain of the armorican microplate rocks with different sedimentary records later experienced diverging prograde metamorphic histories in general the results reveal that palynological studies in combination with petrological data are a promising tool for reconstructing pre orogenic settings in collisional belts

new insights into the effect of the quaternary fault activity on river knickpoints in the central alborz iran abstract this study examines the knickpoints in a region to provide new insights for assessing the tectonically formed knickpoints tfks in this paper we have used a quantitative method to recognize tfks using two different river steepness methods the stream gradient gd and the normalized steepness index ksn analysis of the extracted tfks with the longitudinal profile logarithm slope area plots and natural logarithm gradient distance plots suggests that these knickpoints correlate with the active segments of faults in addition evaluation of the tfks indicates that the different length and height of the tfks can be related to the different features of each fault the investigation of tfks was implemented in the central alborz located at the southern margin of the caspian basin in northern iran which is a part of an active deformation region of the arabian and eurasian plate collision zone the investigation of tfks suggests that the quaternary faults activities are related to the recent tectonic activity in the northern flank of the central alborz as well as the central part of the central alborz which is consistent with global positioning system gps studies and increased uplift rate the results are in agreement with field observations in terms of geomorphic features and geology

synchronous deformation on orogenic plateau margins insights from the arabia eurasia collision abstract the temporal and spatial distribution of deformation during the early phases of plateau formation is poorly known the iranian plateau is developing in response to arabia eurasia convergence and is a potential analog to the early phases of tibetan plateau deformation previous studies document the onset of the iranian plateau growth based on the kinematic history of the zagros to the south and the alborz mountains to the north here we present new detrital apatite fission track aft data from 13 modern rivers of the talesh mountains northwestern iranian plateau these mountains lie immediately south west of the rigid south caspian block a total of 1233 single grain cooling ages were measured and range between 185 and 4 ma deconvolution of grain age data documents three main phases of exhumation including the oligocene 30 23 ma early to middle miocene 18 12 ma and early pliocene 5 ma comparison between cooling ages from the talesh mountains and elsewhere across the iranian plateau indicates widespread plateau formation in the oligocene earlier than previously suggested for the northwestern plateau margin these results highlight deformation on both the northern and southern margins of the plateau over almost the entire history of iranian plateau formation a similar pattern of synchronous deformation on the north northeastern and southern margins of the tibetan plateau has recently been documented and highlights the significance of rigid basement blocks focusing far field deformation for both plateaus

cenozoic exhumation history of the alborz mountains iran new constraints from low temperature chronometry the alborz mountains in the north of the turkish iranian plateau mainly developed in the cenozoic as a consequence of the closure of neo tethys and continental collision between arabia and eurasia cenozoic growth of the fold and thrust belt exploited an older palaezoic mesozoic crustal fabric but the extent to which this governed the overall form of the mountain belt is unclear to determine when and how the alborz mountain belt has grown apatite fission track aft and u th he ahe thermochronometry were performed on 46 bedrock samples collected along 8 transects across the range aft central ages range from 157 24 ma to 10 1 ma with most ages falling between 40 ma and 10 ma ahe ages range from 17 2 ma to 6 1 ma the data revealed enhanced exhumation ca 35 5 ma minor exhumation between ca 30 20 ma and an increase in exhumation thereafter this pattern matches the tripartite mid cenozoic stratigraphy of central iran where upper oligocene lower miocene carbonates are sandwiched between terrestrial clastic formations the most intriguing thermochronometric signal found is a major acceleration of exhumation that initiated in the late miocene to early pliocene recorded by the ahe data there does not appear to be a direct tectonic cause for pliocene intensified erosion as convergence rates between arabia and eurasia have remained relatively constant enhanced exhumation at this time may reflect a climatically induced intensification of erosion during caspian isolation and base level fall or a regional tectonic re organization of the arabia eurasia collision or both

quantification of vertical movements in the eastern betics spain by comparing levelling data comparison of precision levelling data carried out at diverent times is a technique that allows the quantification of recent vertical movements we used the precision levelling data from the instituto geografico nacional of spain to improve our understanding of the recent tectonic evolution of some areas in the southeastern iberian peninsula eastern betics where significant deformation has occurred during the last century the resulting movements are presented in recent vertical movement profiles profiles with significant movements are discussed taking into account the geologic and seismic features of each zone these profiles are alicante albacete alicante almeria larva near jaen almeria and almeria el palo near malaga the results obtained point to the existence of important tectonic deformation rates near the main tectonic structures crossing the profiles the main tectonic anomalies expressed in constant rates of vertical velocity are a 2 mm year step near the ene wsw trending valldigna jumilla fault zone between alicante and albacete a 0 9 mm year step located in the nne ssw trending cocon terreros fault zone between murcia and almeria a subsidence of about 1 5 mm year of the almeria basin zone that can be related to nw se and e w faults a step of 1 mm year related to the ene wsw trending guadahortuna fault between granada and jaen a 1 mm year step associated with the ene wsw trending cadiz alicante fault zone near the granada area and a 1 4 mm year step related to the tectonic activity of the nw se and ne sw trending system of faults that avect the iberian coast of the alboran sea almeriaa nd granada zone the anomalies related to the ene wsw to e w trending structures are associated with reverse faults whereas the anomalies related to the faults oriented in a ne sw or nw se direction are associated with strike slip faults with a normal component of vertical movement these movements are consistent with the present day betics stress tensor determined with diverent methodologies such as the focal mechanism analysis and microstructural and geomorphological studies that indicate the existence of a maximum horizontal compression in a nnw sse direction together with a perpendicular extension if we consider the rates of vertical deformation to be constant in time and if we consider that all the accumulated stress results in big earthquakes m 6 0 with no ductile deformation an estimate of the minimum return periods for major earthquakes can be made the return periods which range between 500 and 1200 years depending on the zone should be regarded as new information in the assessment of seismic hazard these values are in agreement with those deduced from the seismic catalogues of the areas where there is suycient information 2000 elsevier science b v all rights reserved

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

thermal histories from the central alborz mountains northern iran implications for the spatial and temporal distribution of deformation in northern iran we integrate new and existing thermochronological geochronological and geologic data from the western and central alborz mountains of iran to better constrain the late cenozoic tectonic evolution of northern iran in the context of the arabia eurasia collision new data are presented for two granitic plutons north of the alborz range crest additional new apatite u th he data are also presented for volcanic intrusive and detrital apatite grains from two transects south of the range crest our most definitive results include zircon and apatite u th he and limited k feldspar 40 ar 39 ar thermal history data from the cretaceous ca 98 ma nusha pluton that reveal that the alborz basement underwent generally slow denudation 0 1 km m y as late as 12 ma with more accelerated exhumation 0 45 km m y that likely began shortly after 12 ma the lahijan pluton a late neoproterozoic cambrian basement exposure near the caspian shore records apatite u th he closure at 17 13 ma additional u th he results from detrital apatites sampled along two separate horizontal transects all consistently yielded latest miocene to pliocene apparent ages that imply that even supracrustal cover rocks within the alborz have undergone significant regionally extensive exhumation overall our data are consistent with 5 km of regionally extensive denudation since ca 12 ma the onset of rapid exhumation in the alborz at ca 12 ma appears to be consistent with other timing estimates that place the onset of the arabia eurasia collision between 14 and 10 ma

496 my age of plagiogranites in the chamrousse ophiolite complex external crystalline massifs in the french alps evidence of a lower paleozoic oceanization genuine ophiolitic series are extremely rare in the variscan orogenic belt and the chamrousse formation appears as an exception the main lithological and geochemical characters of the ophiolite are summarized and a special attention is paid to the field relationships and the magmatic filiation of the leucocratic rocks the time evolution of the mafic magmas is considered to express the transition of attenuated continental lithosphere to oceanic lithosphere the silicic rocks represents the final products from a multisequence fractional crystallization of a tholeiitic lil enriched magma and their compositional diversity results from the local increase of the h2o partial pressure during syn accretion tectonics dating the plagiogranites using theup b method on zircons also give the date of the oceanization the resulting age of496 6ma is closely comparable 1 to the accretion age of ophiolites from peri atlantic paleozoic belts and 2 to the protolith crystallization age of the leptyno amphibolitic groups lags from western europe but these relics ophiolites and lags of the cambro ordovician distensional event later undergo very different tectonometamorphic evolution the distinct post magmatic evolutions may be related to their rather distinct original geodynamic significance and to their primitive location with respect to the subsequent zonation of the hercynian belt

geochemistry of metasediments in the hercynian and pre hercynian crust of the schwarzwald the vosges and northern switzerland abstract several units in the pre hercynian gneiss basement and hercynian schists of the schwarzwald southwestern germany and northern switzerland and the vosges france were characterized geochemically the samples are attributed to possible sedimentary protoliths by means of their main constituents groups of analyses belonging to the defined protoliths are then compared by means of statistical procedures the mann whitney test and multiple discriminant analysis which reveal significant differences so that every area is characterized by a specific pattern of major and trace elements furthermore correlations between some units of the schwarzwald and the vosges are possible the gneisses of ste marie aux mines vosges can be closely correlated with those of the central schwarzwald whereas there is a distinct contrast between gneisses of the central schwarzwald and gneisses of the southern schwarzwald the schists of baden baden schwarzwald most probably have their counterpart in the schists of ville in the vosges

evidence from zircon dating for existence of approximately 2 1 ga old crystalline basement in southern bohemia czech republic zircon ages are reported for three moldanubian amphibolite grade orthogneisses from the southern bohemian massif obtained by conventional u pb analyses for two of these orthogneisses conventional u pb data are supported by ion microprobe single zircon ages or single grain evaporation data the amphibolite grade orthogneisses occurring in three small tectonic lenses within the varied group close to the south bohemian main thrust are of tonalitic granodioritic or quartz dioritic composition

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

exhumation of the west central alborz mountains iran caspian subsidence and collision related tectonics crystallization and thermal histories of two plutons in the west central alborz also elburz elburs mountains northern iran are combined with crosscutting relations and kinematic data from nearby faults to determine the cenozoic tectonic evolution of this segment of the youthful euro arabian collision zone u pb 40 ar 39 ar and u th he data were obtained from zircon biotite k feldspar and apatite the akapol pluton intruded at 56 2 ma cooled to 150 c by ca 40 ma and stayed near that temperature until at least 25 ma the nearby alam kuh granite intruded at 6 8 0 1 ma and cooled rapidly to 70 c by ca 6 ma these results imply tectonic stability of the west central alborz from late eocene to late miocene time consistent with miocene sedimentation patterns in central iran elevation correlated u th he ages from the akapol suite indicate 0 7 km m y exhumation between 6 and 4 ma and imply 10 km of alborz uplift that was nearly synchronous with rapid south caspian subsidence suggesting a causal relation uplift south caspian subsidence and subsequent folding reversal of alborz strike slip from dextral to sinistral and eastward extrusion of central iran coarse zagros molasse deposition dead sea transform reorganization red sea oceanic spreading and north and east anatolian fault slip all apparently began ca 5 2 ma suggesting a widespread tectonic event that we infer was a response to buoyant arabian lithosphere choking the neo tethyan subduction zone

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

accommodation of late cenozoic oblique shortening in the alborz range northern iran abstract the alborz range northern iran deforms by strain partitioning of oblique shortening onto range parallel left lateral strike slip and thrust faults deformation is due to the north south arabia eurasia convergence and westward motion of the adjacent south caspian relative to iran roughly north south shortening occurs on thrusts that dip inwards from the range margins precambrian basement is not exposed because of detachment along upper proterozoic evaporites and the thickness of overlying strata 10 km other detachments occur within phanerozoic sediments active left lateral strike slip faults trend ene in the east of the range wnw in the west the eastern mosha fault has a left lateral offset of 30 35 km shortening across the range is 30 km 25 30 at the longitude of tehran this is hard to reconcile with the crustal thickness of 35 km which is similar to adjacent basins percentage shortening across the alborz is similar to the adjacent kopet dagh but because the latter is twice as wide finite shortening is roughly half the difference may be accommodated by subduction of the adjacent south caspian basin northwards under the middle caspian south caspian sediments began folding in the pliocene the present kinematics of the alborz may have begun at this time including a reversal from range parallel right lateral to left lateral strike slip faulting but the regional stratigraphy indicates uplift as early as the mid tertiary

crustal scale thrusting and extension in the hercynian schwarzwald and vosges central europe the hercynian outcrop areas in the schwarzwald of sw germany and vosges of easternmost france consist from north to south of three major lithotectonic complexes very low to medium grade metamorphics of the saxothuringian belt the polymetamorphic central gneiss complex and the tectonic melange of the southern complex juxtaposition of these three complexes occurred by oblique convergent motions in early carboniferous time the saxothuringian belt is part of a major se dipping accretionary wedge that consists of lower paleozoic meta sedimentary and metaigneous rocks and superimposed upper devonian to lower carboniferous synorogenic volcanics and elastics the central gneiss complex consists of polymetamorphic paragneisses orthogneisses metabasites and meta ultramafics dipping to the nw the southern complex consists of an imbricated melange of north to nw dipping synorogenic elastics volcanics and blocks of ultramafics and gneissic basement the central gneiss complex cgc experienced pervasive hercynian low pressure high temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins this convergence 340 330 ma produced mylonitic cataclastic shear zones in the cgc the most significant shear zones are the se verging todtnau fault zone between the cgc and the southern complex and the nw verging lubine fault zone between the central gneiss complex and the rocks of the saxothuringian belt these two fault zones enclose a bivergent pop up of the polymetamorphic basement the todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km crustal thickening during early carboniferous convergence led to an extensional phase of crustal reordering between about 330 and 280 ma that was accompanied by the massive rise of posttectonic granitic plutons from middle to high crustal levels simultaneous creation of a structural lamination in the refractory lower crust neglected the main convergent tectonic boundaries it produced a basin and range type lower crust and mantle crust transition in early permian time and involved a major advective input of heat

sediment characterization at the equatorial mid atlantic ridge from p to s teleseismic phase conversions recorded on the pi lab experiment accurate marine sediment characteristics for example thickness and seismic velocity are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments we analyze p to s seismic phase conversions from the sediment crust boundary recorded by the passive imaging of the lithosphere asthenosphere boundary pi lab experiment to infer the sediment thickness across the mid atlantic ridge covering 0 to 80 myr old seafloor we find p d s p delay times of 0 04 0 37 s or 5 to 82 m thickness sediment thickness increases with age thickness agrees with global estimates for young 15 20 myr seafloor but is thinner on older lithosphere our result may represent a lower limit on sediment thickness given that several of our stations are on topographic highs the sedimentation rate decrease observed from 5 to 1 2 mm kyr at 10 myr suggests a recent increase in productivity related to climate change eolian dust fluxes and or biogenic marine activity

present day crustal deformation and plate kinematics in the middle east constrained by gps measurements in iran and northern oman summary a network of 27 gps sites was implemented in iran and northern oman to measure displacements in this part of the alpine himalayan mountain belt we present and interpret the results of two surveys performed in 1999 september and 2001 october gps sites in oman show northward motion of the arabian plate relative to eurasia slower than the nuvel 1a estimates e g 22 2m m yr 1 at n8 5 e instead of 30 5 mm yr 1 at n6 e at bahrain longitude we define a gps arabia eurasia euler vector of 27 9 0 5 n 19 5 1 4 e 0 41 0 1 myr 1 the arabia eurasia convergence is accommodated differently in eastern and western iran east of 58 e most of the shortening is accommodated by the makran subduction zone 19 5 2m m yr 1 and less by the kopet dag 6 5 2m m yr 1 west of 58 e the deformation is distributed in separate fold and thrust belts at the longitude of tehran the zagros and the alborz mountain ranges accommodate 6 5 2m m yr 1 and 8 2m m yr 1 respectively the right lateral displacement along the main recent fault in the northern zagros is about 3 2m m yr 1 smaller than what was generally expected by contrast large rightlateral displacement takes place in northwestern iran up to 8 mm yr 1 the central iranian block is characterized by coherent plate motion internal deformation 2m m yr 1 sites east of 61 e show very low displacements relative to eurasia the kinematic contrast between eastern and western iran is accommodated by strike slip motions along the lut block to the south the transition zone between zagros and makran is under transpression with right lateral displacements of 11 2m m yr 1

thermal structure of oceanic transform faults we use three dimensional fi nite element simulations to investigate the temperature structure beneath oceanic transform faults we show that using a rheology that incorporates brittle weakening of the lithosphere generates a region of enhanced mantle upwelling and elevated temperatures along the transform the warmest temperatures and thinnest lithosphere are predicted to be near the center of the transform previous studies predicted that the mantle beneath oceanic transform faults is anomalously cold relative to adjacent intraplate regions with the thickest lithosphere located at the center of the transform these earlier studies used simplifi ed rheologic laws to simulate the behavior of the lithosphere and underlying asthenosphere we show that the warmer thermal structure predicted by our calculations is directly attributed to the inclusion of a more realistic brittle rheology this temperature structure is consistent with a wide range of observations from ridge transform environments including the depth of seismicity geochemical anomalies along adjacent ridge segments and the tendency for long transforms to break into small intratransform spreading centers during changes in plate motion

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

c o h s fluid composition and oxygen fugacity in graphitic metapelites c 0 h fluid produced by the equilibration of hzo and excess graphite must maintain the atomic h o ratio of water 2 1 this constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric isothermal conditions the o h20 and co fugacities j fhto and fc of such fluids have been estimated from equations of state and fit as a function of pressure and temperature these fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration e g pelitic lithologies because there are no compositional degrees of freedom for graphite saturated fluids produced entirely by dehydration the variance of the dehydration process is not increased in comparison with that in non graphitic systems thus compositional buffering of c 0 h fluids by dehydration equilibria a common petrological model requires that redox reactions decarbonation reactions or external h o 2 fluid sources perturb the evolution of the metamorphic system such perturbations are not likely to be significant in metapelitic environments but their tendency will be to increase the fo of the fluid phase at high metamorphic grades pyrite desulphidation reactions may cause a substantial reduction of fhz0 and slight increases info and fcs relative to sulphur free fluid at low metamorphic grade sulphur solubility in h o s 2 fluids is so low that pyrite decomposition must occur by sulphurconserving reactions that cause iron depletion in silicates a common feature of sulphidic pelites with increasing temperature and sulphur solubility pyrite desulphidation may be driven by dehydration reactions or infiltration of h o rich fluids the absence of magnetite and the assemblages carbonate aluminosilicate or pyrite pyrrhotite ilmenite from most graphitic metapelites is consistent with an h o 2 model for gcoh s fluid for graphitic rocks in which such a model is inapplicable a phase diagram variable that defines the h o ratio of gcoh s fluid is more useful than the conventional

a newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere asthenosphere boundary the marine magnetotelluric mt method is a useful tool for offshore studies aimed at for example hydrocarbon exploration and the understanding of earth s tectonics marine mt data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land at mid ocean ridges the resistivity of earth s structure can be assumed to be two dimensional which allows mt data to be decomposed into a transverse electrical te mode with electric current flowing approximately along the ridge and a transverse magnetic tm mode with electric current flowing perpendicular to the ridge we collected marine mt data at the middle atlantic ridge which exhibited highly negative tm mode phases as large as 180 at relatively high frequencies 0 1 0 01 hz similar negative phases have been observed in other marinemtdata sets but have not been the subject of study we show here that these negative phases are caused by a newly distinguished coast effect the tm mode coast effect is not only a galvanic effect as previously understood but also includes inductive distortions tm mode negative phases are caused by the turning of the poynting vector the phase change of electromagnetic fields and vertically flowing currents in the seafloor the findings provide a new understanding of the tm mode coast effect which can guide our ability to fit the field data with the inclusion of coastlines and reduce misinterpretation of the data in offshore studies the study also shows that the tmmode coast effect is sensitive to the depth and conductivity of the asthenosphere an important feature of the earth s interior that was the object of our atlantic ocean study

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

the geology of damavand volcano alborz mountains northern iran damavand volcano located in northern iran is a large 400 km 3 composite cone that is currently dormant it shows fumarolic activity near the summit but no evidence of eruption in the past 1000 yr the volcano represents an isolated focus of magmatism of uncertain tectonic affinity although geophysical and geochemical constraints point toward a local hotspot plume origin possibly associated with lithospheric delamination rather than any association with subduction new u th he and 40 ar 39 ar geochronological constraints indicate that the present cone young damavand has been constructed over 600 k y on an older eroded edifice of indistinguishable composition younger than 1 8 ma damavand activity has been characterized by the eruption of radially directed trachyandesite lava flows almost exclusively from summit vents limited pyroclastic activity has yielded thin fallout pumice lapilli layers and a few pyroclastic flows only one significant pyroclastic event is recognized in the remnants of a welded ignimbrite ponded and preserved along the haraz river drainage relatively short periods of volcanic eruptive activity were interspersed with longer periods of erosion in which volcanic products were transported particularly as hyperconcentrated flows into the surrounding drainage systems to be further reworked into epiclastic deposits occasional catastrophic events punctuated this interplay between volcanism and erosion at least one sector collapse is signified by the presence of a large debris avalanche deposit and the regional drainage systems appear to have been frequently dammed by incursions of volcanic material

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

carbonaceous material in the ryoke metamorphic rocks kinki district japan abstract carbonaceous material in the ryoke pelitic metamorphic rocks in the wazuka area kinki district japan has been studied by x ray diffraction xrd analysis detrital graphite in the lower grade rocks is recognized in both x ray diffractograms and transmission electron micrographs progressive graphitization is considered to have proceeded continuously on the basis of the xrd data of the bulk concentrates of carbonaceous material and a conspicuous asymmetric shape of xrd peaks in lower grade samples is ascribed to the mixture of carbonaceous materials with different crystallinities fully ordered graphite does not occur until the highest grade part of the chlorite biotite zone the variation of degree of crystallinity of carbonaceous materials inferred from xrd data is consistent with the prograde mineral zones the temperatures for the development of fully ordered graphite are estimated to be between 410 and 440 c in regional metamorphism through comparing the xrd data from low medium and high pressure types of metamorphic terrains