seismic source study of the racha dzhava georgia earthquake from aftershocks and broad band teleseismic body wave records an example of active nappe tectonics summary the racha dzhava earthquake ms 7 0 that occurred on 1991 april 29 at o9 12 48 1 gmt in the southern border of the great caucasus is the biggest event ever recorded in the region stronger than the spitak earthquake ms 6 9 of 1988 a field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was deployed to record the aftershock activity a very precise image of the aftershock distribution is obtained showing an elongated cloud oriented n105 with one branch trending n310 in the western part the southernmost part extends over 80 km with the depth ranging from o to 15 km and dips north the northern branch which is about 30 km long shows activity that ranges in depth from 5 to 15 km the complex thrust dips northwards a stress tensor inversion from p wave first motion polarities shows a state of triaxial compression with the major principal axis oriented roughly n s the minor principal axis being vertical body waveform inversion of teleseismic seismograms was performed for the main shock which can be divided into four subevents with a total rupture time duration of 22s the most important part of the seismic moment was released by a gentle northerly dipping thrust the model is consistent with the compressive tectonics of the region and is in agreement with the aftershock distribution and the stress tensor deduced from the aftershocks the focal mechanisms of the three largest aftershocks were also inverted from body wave records the april 29th ms 6 1 and may 5th ms 5 4 aftershocks have thrust mechanisms on roughly e w oriented planes similar to the main shock surprisingly the june 15th ms 6 2 aftershock shows a thrust fault striking n s this mechanism is explai ned by the structural control of the rupture along the eastdipping geometry of the dzirula massif close to the borzhomi kazbeg strike slip fault in fact the orientation and shape of the stress tensor produce a thrust on a n s oriented plane nappe tectonics has been identified as an important feature in the caucasus and the source mechanism is consistent with this observation a hidden fault is present below the nappe and no large surface breaks were observed due to the main shock the epicentral region is characterized by sediments that are trapped between two crystalline basements the dzirula massif which crops out south of chiatoura and the caucasus main range north of oni most if not all of the rupture is controlled by the thrusting of overlapping deformed and folded sediments over the dzirula massif this event is another example of blind active faults with the distinctive feature that the fault plane dips at a gentle angle the racha range is one of the surface expressions of this blind thrust and its growtb is the consequence and evidence of similar earthquakes in the past ii 5

transition from a singly vergent to doubly vergent wedge in a young orogen the greater caucasus the greater caucasus mountains due to their youth 5 ma provide an opportunity for insight into the early stages of orogen development during continent continent collision however their recent tectonic evolution and first order architecture remain unclear here we investigate the evolution of the orogen by integrating new observations of the fluvial geomorphology and neotectonics of the range with prior work on seismicity geodetic strain bedrock geology and foreland basin structure we find that the range contains four zones along strike that differ in structural architecture topography and first order tectonic history in particular two south directed singly vergent zones at the western and eastern tips of the orogen are separated by both a central doubly vergent zone that is dominated by north directed deformation and an eastern doubly vergent zone in which south directed thrusting dominates we hypothesize that the along strike changes in vergence and locus of deformation reflects different stages in the development of a doubly vergent orogen with the tips of the range preserving an early singly vergent form and the center recording a more advanced orogen the differences between the two doubly vergent zones seem to be driven by the initial stages of collision between the structurally thickened crust of the greater and lesser caucasus orogens which initiated at 5 ma

seismic imaging of melt in a displaced hawaiian plume the precise location of the mantle plume upwelling beneath hawaii is debated seismic data reveal a thick layer of melt in the mantle beneath western hawaii implying that the upwelling plume may be deflected around an ancient resistive root beneath the island

guyot and seamount morphology and tectonics of the hawaiian emperor elbow by multi beam sonar abstract five guyots and five seamounts are shown from the southernmost end of the emperor chain an area called the hawaiian emperor bend or the milwaukee seamount group in the north central pacific ocean the data are from the u s naval oceanographic office navoceano multi beam survey files the bend is shown to occur over an area encompassing 260 km questions are raised as to the relative proximity of different flat top depths and formational possibilities in the light of the differing geomorphology on the features in addition there are two new features taish and go shirakawa seamounts and four additional new names of features are introduced antoku guyot toba guyot and genji and go sanj seamounts

the pacific lithosphere asthenosphere boundary seismic imaging and anisotropic constraints from ss waveforms 1 the lithosphere asthenosphere boundary lab separating the rigid lid from the underlying weaker convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics however the exact depth and defining mechanism of the lab interface remain poorly understood the ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70 of earth s surface area and have a relatively simple geological history seismically imaging the oceanic lab at high resolution has proved challenging yet several studies have recently increased resolution with provocative results we summarize recent imaging of discontinuity structure beneath much of the pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean continent margins ss precursors and waveform modeling of multiple phases including multiple bounce s waves scs reverberations and surface waves overall there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the lab some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths yet when the results are considered together no single age depth relationship is illuminated there are also puzzling discrepancies in where the discontinuity is detected which again suggests greater complexity here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure we stack ss waveforms with bounce points in the central pacific into azimuthal bins we use two methods one that inverts for discontinuity structure based on subtle variations in the character of the ss waveform and another that considers ss at higher frequency we find azimuthal variation in the amplitude of the waveform including a polarity reversal we suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate and must be carefully considered to constrain the age depth dependence and defining mechanism of the oceanic lithosphere

the role of rheology in lithospheric thinning by mantle plumes lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on venus the tharsis rise on mars and several hotspot swells on earth we present a suite of time dependent three dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature dependent viscosity mantle efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume lithosphere boundary layer these instabilities are non axisymmetric time dependent and have horizontal scales of a few tens of kilometers these instabilities depend on the temperature dependence of viscosity and occur when the plume s viscosity is about an order of magnitude less than the background mantle as predicted by boundary layer theory thus in planetary mantles plumes with excess temperatures of 100 to 200 k will efficiently thin the lithosphere via small scale convective instabilities

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

the armenian earthquake of 1988 december 7 faulting and folding neotectonics and palaeoseismicity summary the spitak earthquake of 1988 december 7 is the first well documented event directly associated with surface breaks in the transcaucasian regions of the ussr the earthquake was located within the ese wnw oriented pambak sevan thrust and fold zone corresponding to the southern front of the lesser caucasus the mechanism of the earthquake is consistent with the nearly ns compressive tectonics due to the active continental collision between the arabian block and the russian platform the rupture is composed by several branches two of which reach the surface the first branch oriented n140 begins near the village of alavar has a length of 11 km and disappears at about 4 km se of spitak it consists of a right lateral en echelon system of strike slip faults with a maximum offset of 50 cm the second branch is the main one and breaks for about 8 km between spitak and gekhasar with a general orientation n120 showing reverse faulting dipping to the north with a right lateral component surface ruptures between gekhasar and spitak show either a narrow band of pressure ridges in alluvial deposits and soil or a fault scarp in the bed rocks when soil is absent maximum displacements observed between spitak and gekhasar attain 160 cm of vertical motion and 90 cm of horizontal dextral offset the displacement also varies within this branch showing finer segmentation few secondary deformations are observed some normal faults near gekhasar correspond to the collapse of the uplifted block an anticline fold oriented parallel to the fault scarp and situated along its northwestern prolongation emphasizes the regional compressive tectonics landslides were activated on its flanks a 200 m long reverse fault break observed along the hinge of a secondary fold suggests the occurrence of a blind thrust at depth and that the hidden branches which continue the main central segment towards the nw are associated to surface folding therefore the total length of the fault and fold zone is about 40 km in agreement with the rupture length and seismic moment obtained on the basis of surface wave modelling well developed uplifted terraces in the northern block and subsiding valleys in the southern block indicate past quaternary activity in the fault region palaeoseismological evidence of ancient earthquakes has been recognized in trenches across the spitak fault one old event occurred between 17 000 years bp and the beginning of the formation of present day soil

structures in southwest asia and evolution of the eastern tethys abstract southwest asia comprises the following 1 a two major orogenic belts a northern belt derived from the paleo tethys and a southern belt derived from the neo tethys 2 b minor orogenic belts derived from neo tethyan marginal basins 3 c continental blocks that originated from gondwana by splitting and stripping and subsequently drifted and collided with continental asia in its process of construction and 4 d residual oceanic basins which are assumed as being not yet closed neo tethyan marginal basins the entire region appears as the product of tethyan evolution

uncertainties in plate reconstructions relative to the hotspots pacific hotspot rotations and uncertainties for the past 68 million years summary a method previously used for estimating the uncertainty in rotation of a plate relative to the hotspots is shown to be insufficient because of the neglect of the uncertainty in one of the three degrees of freedom of a rotation two new methods for estimating best fitting rotations and associated uncertainties for the rotation of a plate relative to the hotspots are presented both methods require a priori estimates of the uncertainties in the locations of individual hotspots and their ancient tracks for a particular age the use of a priori uncertainties permits the hypothesis of hotspot fixity to be tested the first method the two hotspot method leads to simple geometrical interpretations of best fitting rotations and of the eigenvectors and eigenvalues of the appropriate covariance matrix this simplicity comes at the cost of using only two hotspots and their associated tracks with assumed equal circular uncertainties the second method the n hotspot method allows use of an arbitrary number of hotspots 2 with unequal elliptical uncertainties this generality and power comes at the cost of losing the simple geometrical interpretation of the first method elliptical uncertainties are generally more appropriate because hotspot tracks are less well known along strike than across strike because of gaps and uncertainties in ages along the tracks when both methods are applied to the estimate of the pacific hotspot rotation since the age of the hawaiian emperor elbow using the tracks of both the hawaiian and louisville hotspots they produce similar rotations and similar uncertainties the n hotspot method is further applied to estimate pacific hotspot motion at six additional ages from 10 9 ma to 67 7 ma corresponding to the ages of key magnetic anomalies used in global plate reconstructions the goodness of the resultant fit shows that the assigned uncertainties and assumption of hotspot fixity are mutually consistent the pre elbow i e pre 47 9 ma or emperor pole of rotation is shown to differ significantly from the post elbow i e post 47 9 ma or hawaiian pole of rotation moreover rotation over the past 47 9 ma is shown to have occurred in at least three statistically distinct stages from 47 9 to 20 1 ma from 20 1 ma to 6 ma and from 6 ma to the present the change in pole position at 20 1 ma is accompanied by a highly significant 50 per cent increase in the rate of rotation of the pacific plate relative to the hotspots these encouraging results suggest that the new methods can be used to improve and to quantify plate motions relative to the hotspots for a wide variety of applications

paleoproterozoic high sulfidation mineralization in the tapajos gold province amazonian craton brazil geology mineralogy alunite argon age and stable isotope constraints the brazilian tapajos gold province contains the first evidence of high sulfidation gold mineralization in the amazonian craton the mineralization appears to be in large nested calderas the tapajos parima or ventuari tapajos geological province consists of a metamorphic igneous and sedimentary sequence formed during a 2 10 to 1 87 ga ocean continent orogeny the high sulfidation mineralization with magmatic hydrothermal alunite is related to hydrothermal breccias hosted in a rhyolitic volcanic ring complex that contains granitic stocks ranging in age from 1 89 to 1 87 ga cone shaped hydrothermal breccias which flare upward contain vuggy silica and have an overlying brecciated cap of massive silica the deposits are located in the uppermost part of a ring structure volcanic cone drill cores of one of the hydrothermal breccias contain alunite natroalunite pyrophyllite andalusite quartz rutile diaspore woodhouseite svanbergite kaolinite and pyrite along with inclusions of enargite luzonite chalcopyrite bornite and covellite the siliceous core of this alteration center is surrounded by advanced argillic and argillic alteration zones that grade outward into large areas of propylitically altered rocks with sericitic alteration assemblages at depth several occurrences and generations of alunite are observed alunite is disseminated in the advanced argillic haloes that envelop massive and vuggy silica or that underlie the brecciated silica cap coarse grained alunite also occurs in branching veins and locally is partly replaced by a later generation of fine grained alunite silicified hydrothermal breccias associated with the alunite contain an estimated reserve of 30 tonnes of gold in rock that grades up to 4 5 g t 1 au seven alunite samples gave 40ar 39ar ages of 1 869 to 1 846 ga with various degrees of apparent minor ar loss stable isotopic data require a magmatic hydrothermal origin for the alunite typical for high sulfidation mineralization the 34s values of most samples of alunite range from 14 0 to 36 9 sulfur isotopic alunite pyrite and oxygen isotopic alunite so4 oh temperatures range from 130 to 420 c the dh2o and 18oh2o values for alunite forming hydrothermal fluids suggest a predominance of magmatic water with a small meteoric contribution a rare sample of supergene alunite has a 34s value of 4 1 and an 40ar 39ar age of 51 3 0 1 ma other than local foliation in the volcanic rocks and recrystallization of alunite near faults the mineralization and associated alteration appears to have been remarkably undisturbed by later metamorphism and by supergene alteration the au mineralization was preserved because of burial by sediments and tuffs in taphrogenic basins that probably developed shortly after mineralization and were probably first exhumed at about 60 ma because high sulfidation mineralization forms at relatively shallow crustal levels the discoveries in tapajos province provide new perspectives for mineral exploration for the amazonian and perhaps for other precambrian cratons

north arch volcanic fields near hawaii are evidence favouring the restite root hypothesis for the origin of hotspot swells a pronounced regional bathymetric swell is a common feature of oceanic hotspot volcanism recently the hypothesis of a buoyant sublithospheric swell root has been favoured the root being either a refracted plume of hot buoyant and hence low viscosity plume material embedded within surrounding higher viscosity asthenosphere or a restite root composed of hot and more viscous residues to hotspot melt extraction from the hottest central portion of the upwelling plume in this article we present numerical experiments that show that these two scenarios predict different flow and melting patterns whenever the plume passes beneath an obliquely oriented fracture zone where the base of the overriding lithosphere changes in thickness we find that the restite root hypothesis predicts an asymmetric spatial pattern of persistent arch volcanism strikingly similar to that found in the hawaiian chain surrounding the molokai fracture zone in contrast the refracted plume hypothesis predicts more chaotic patterns quite different from the off chain pattern of hawaiian volcanism

diversity dynamics of bajocian middle jurassic ammonites and transgressions regressions in the caucasian sea northern neo tethys ocean a case high resolution test of probable dependence abstract ammonites commonly occur in the bajocian deposits of the greater caucasus basin from where 180 species and 52 genera are reported this back arc basin was located on the northern neo tethyan margin embraced by the caucasian sea an evaluation of diversity patterns for 7 bajocian ammonite zones documents high diversity in the beginning middle and end of the stage with a maximum in the humphriesianum zone and two diversity minima the mid early bajocian impoverishment of ammonite assemblages was caused by both an increase in the rate of disappearance and a decrease in the rate of appearance whereas the only latter was responsible for the mid late bajocian diversity drop an analysis of dominating facies types in the greater caucasus basin documents that the total diversity rose during regressions and fell during transgressions shoreline shifts corresponded better with changes in the number of appearances and worse with those of disappearances a comparison of results from this study with those obtained in the betic cordillera does not clarify the observed dependence changes in the exterior connections of the caucasian sea its depth and oxygen depletion of its waters are unlikely explanations of the regionally documented coincidence of ammonite diversity dynamics and transgressions regressions

three dimensional simulations of plume lithosphere interaction at the hawaiian swell rapid lithospheric thinning by mantle plumes has not been achieved in numerical experiments performed to date efficient thinning depends on small scale instabilities that convectively remove lithospheric material these instabilities are favored by hotter plumes or stronger temperature dependence of viscosity and a simple scaling independent of rheology controls their onset this scaling allows extrapolation of the results of numerical experiments to the earth s mantle mantle plumes between 100 and 150 kelvins hotter than the background mantle should exhibit small scale convective rolls aligned with the plate motion the unusual variation in heat flow across the hawaiian swell may be due to such instabilities it was found that the spreading of the plume creates a downwelling curtain of material that isolates it from the rest of the mantle for distances of at least 1000 kilometers from the plume origin this isolation has important consequences for the geochemical heterogeneity of the lithosphere and upper mantle

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

northern caucasus basin thermal history and synthesis of subsidence models abstract burial histories of the eastern central and western parts of the northern caucasus basin are reconstructed on the basis of well data and seismic sections subsidence began in the early triassic after the late carboniferous permian orogeny triassic sediments were mainly removed during late triassic early jurassic uplift and erosion platform cover began to form in the middle jurassic and albian sediments covered the whole territory of the basin thermal modelling shows that jurassic eocene subsidence was mainly controlled by late triassic early jurassic intrusive warming this heating event induced thermal uplift of the whole territory followed by exponentially decelerating subsidence due to cooling of the lithosphere in the southern areas adjacent to great caucasus subsidence was also affected by caucasian extensional and compressional events in the oligocene early miocene the eastern and the central basins underwent rapid long wavelength subsidence maikopian subsidence the geodynamic cause of this subsidence is probably associated with the mantle flow appearance after cessation of the tethyan subduction due to reequilibration of subducted slab while in the late miocene quaternary times the eastern and the western basins underwent foreland type asymmetrical subsidence due to loading of the great caucasus orogen the central basin was uplifted according to flexural modelling the main component of orogen loading was the lithospheric root load delamination of the latter under the central caucasus caused rapid uplift of the orogen and adjacent basin

geodynamics seismicity and seismic hazards of the caucasus abstract being a part of ongoing continental collision between the arabian and eurasian plates the caucasus region is a remarkable site of moderate to strong seismicity where devastating earthquakes caused significant losses of lives and livelihood in this article we survey geology and geodynamics of the caucasus and its surroundings magmatism and heat flow active tectonics and tectonic stresses caused by the collision and shortening gravity and density models and overview recent geodetic studies related to regional movements the tectonic development of the caucasus region in the mesozoic cenozoic times as well as the underlying dynamics controlling its development are complicated processes it is clear that the collision is responsible for a topographic uplift inversion and for the formation of the fold and thrust belts of the greater and lesser caucasus tectonic deformations in the region is influenced by the wedge shaped rigid arabian block indenting into the relatively mobile region and producing near n s compressional stress and seismicity in the caucasus regional seismicity is analysed with an attention to sub crustal seismicity under the northern foothills of the greater caucasus which origin is unclear whether the seismicity associated with a descending oceanic crust or thinned continental crust recent seismic tomography studies are in favour of the detachment of a lithospheric root beneath the lesser and greater caucasus the knowledge of geodynamics seismicity and stress regime in the caucasus region assists in an assessment of seismic hazard and risk we look finally at existing gaps in the current knowledge and identify the problems which may improve our understanding of the regional evolution active tectonics geodynamics shallow and deeper seismicity and surface manifestations of the lithosphere dynamics among the gaps are those related to uncertainties in regional geodynamic and tectonic evolution e g continental collision and associated shortening and exhumation lithosphere structure deformation and strain stress partitioning and to the lack of comprehensive datasets e g regional seismic catalogues seismic gravity and geodetic surveys

mantle upwelling and melting beneath slow spreading centers effects of variable rheology and melt productivity abstract we examine the effects of non uniform rheology and melting models on the 3d structure of mantle upwelling beneath spreading centers our numerical models identify a range of conditions for which 3d upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges in a highly viscous shallow region due to dehydration of the solid by melt extraction flow results essentially from the plate spreading this contrasts with a deeper buoyant low viscosity region where melt is present but upwelling solid is not yet dehydrated and where both solid and melt flows acquire their 3d nature the thickness of this buoyant region depending mainly on the temperature interval between the wet and dry solidi results in a specific characteristic length scale for the upwelling smaller segmentation wavelengths occur for smaller values of this thickness in contrast to isoviscous models where minimum segmentation wavelengths were large 150 km minimum wavelengths obtained in models with non uniform rheology and melt production 40 70 km are comparable to smallest observed segment lengths a single wavelength may be preferred for a given set of parameters but such a preferred wavelength is difficult to achieve because a range of initially prescribed wavelengths can persist for long times the persistence of 3d solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid ocean ridges may be inherited from the initial stages of spreading

variation of the apparent compensation depth of hotspot swells with age of plate abstract most oceanic hotspot swells are associated with anomalously shallow seafloor and broad geoid anomalies these depth and geoid anomalies are generally positively correlated and linearly related the linear relationship can be used to infer an apparent depth of isostatic compensation of the swell we have analysed geoid and depth anomalies over 23 oceanic hotspot swells almost the majority of oceanic hotspots and deduced the apparent depth of compensations of these features the data indicate a linear increase of the compensation depth with the square root of seafloor age t on the order of 7 km m y 1 2 this trend is comparable to that predicted by conductive cooling models for the deepening of the isotherms inside the oceanic lithosphere

constraints on the exhumation and erosion of the high himalayan slab nw india from foreland basin deposits petrography sr nd isotope compositions and single grain laser 40ar 39ar ages of detrital white mica from early middle miocene molasse of the dharamsala and lower siwalik formations of northern india dated by magnetostratigraphy determine the sediment sources their metamorphic grade and exhumation rates in the himalayan palaeo hinterland deposition of the lower dharamsala member 21 17 ma occurred during the period of rapid isothermal decompression and crustal anatexis 24 18 ma of the metamorphic core this episode of decompression is thought to be coeval with thrusting on the main central thrust and normal faulting on the south tibetan detachment system the sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the high himalayan crystalline series deposition of the upper dharamsala member 17 13 ma and basal siwalik group 13 12 5 ma spanned the period in which thrusting transferred south from the main central thrust the sediment composition and detrital mica ages contrast strongly with those of the lower dharamsala indicating erosion from sedimentary and low grade rocks the isotopic composition indicates that these rocks were part of the high himalayan series unaffected by tertiary metamorphism i e from upper structural levels of the high himalayan slab this suggests that a major reorganisation of the orogenic wedge occurred at 17 ma involving forward propagation of the mct and cessation of rapid exhumation of the metamorphic slab

petrology and k ar ages of dredged samples from laysan island and northampton bank volcanoes hawaiian ridge and evolution of the hawaiian emperor chain summary the hawaiian emperor volcanic chain is made up of at least 107 indentifiable shield volcanoes chase and others 1970 bargar and jackson 1974 clague and others 1980 and is the longest linear island and seamount chain in the pacific fig 1 beginning at the active volcanoes kilauea and mauna loa on the island of hawaii this chain of volcanoes extends west northwestward along the hawaiian ridge for a distance of 3 500 km where it turns northward and continues another 2 300 km as the emperor seamounts ending near the intersection of the aleutian and kurile trenches it is now thought that the hawaiian emperor chain was formed as the pacific lithospheric plate moved first northward and then northwestward relative to the hawaiian hot spot a more or less stable melting anomaly in the asthenosphere and that the bend in the chain reflects a major change in pacific plate motion several mechanisms have been proposed to explain the hawaiian and other hot spots including a propagating fracture betz and hess 1942 jackson and wright 1970 diapiric upwelling along a line of structural weakness green 1971 mcdougall 1971 thermal morgan 1972a 1972b or chemical anderson 1975 plumes of gravitationally unstable material rising from the deep mantle shear melting coupled with thermal feedback shaw 1973 and geographically stabilized by gravitational sinking of refractory residua shaw and jackson 1973 and extrusion induced by lateral motion along plate boundaries handschumacher 1973 despite the variety of dynamic models proposed there has yet to be devised a definitive experiment to test any of them all these mechanisms are however encompassed by the general or kinematic hot spot hypothesis which when used in the broadest sense implies neither mechanism nor excess heat

a reexamination of the flexural deformation beneath the hawaiian islands seismically derived depth estimates to the top of the oceanic crust beneath the hawaiian islands indicate that the curvature of the deflected lithosphere is much larger than commonly believed the conservative and model independent curvature estimates exceed 10 7 m 1 and are comparable in magnitude to curvatures at trenches and outer rise systems the depth estimates are used to constrain both two dimensional 2 d and three dimensional 3 d flexural models the curvature constraints require a 2 d variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load in an attempt to understand the nature of the yielding beneath the hawaiian islands we introduce two new 3 d models the first model combines a realistic yield strength based rheology with a new technique for 3 d flexure calculations in which the elastic plate thickness is curvature dependent the new variable rigidity model predicts an undeformed mechanical plate thickness of 44 km decreasing to 33 km beneath the big island of hawaii the best fitting mechanical thickness corresponds approximately to the depth to the 600 c isotherm in 90 m y old lithosphere the second model uses a broken plate but here the crack is oriented along the weak molokai fracture zone rather than along the island chain trend this unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts

late cenozoic burial history and dynamics of the northern caucasus molasse basin implications for foreland basin modelling abstract the collisional history of the caucasus segment of the alpine himalayan fold belt started at the end of the eocene the associated development of the northern caucasus foreland basin occurred during two syn collisional stages each displaying different subsidence patterns the 34 16 ma maikopian or pre foreland stage displays a long wavelength subsidence of a broad area whereas the 16 0 ma foreland stage displays asymmetrical foreland subsidence the first is correlated with the termination of subduction in the southern area and can be associated with mantle flow induced by the re equilibration of the subducted slab the along strike configuration of the second molasse basin stage contradicts the hypothesis in which topographical loading is considered to be the main control on foreland subsidence there is a clear anti correlation between basin depths and orogen heights the deepest parts of the basin are at the tips of the orogen where mountain heights are negligible while the area adjacent to the highest mountains central caucasus is uplifted the influence of other types of loading in foreland basin development has therefore been investigated and it has been possible to find a good fit with both gravity and basin architecture by including only two additional model parameters into the model these are crustal and lithospheric thickening thinning the results demonstrate that crustal thickening and removal of lithospheric roots are responsible for supporting the high central caucasus mountains and uplift of adjacent areas the subsidence of the basins at the orogen tips is explained by loading of lithospheric roots both effects are important in the geographically intermediate areas in general it is concluded that topography should not be considered as the main control on foreland subsidence but only as one of several counterbalancing mechanisms the existing flexural model therefore needs improvements such that it can be related directly to subsurface loading by real structures and processes during collision

effect of lithospheric root on decompression melting in plume lithosphere interaction models summary global distribution of large igneous provinces lips shows that volcanic events associated with diapiric plume heads have often occurred near the margins of cratonic lithosphere previous numerical models of plume lithosphere interaction and decompression melting assumed the lithosphere as a rigid lid of mostly uniform thickness here we present a 3 d numerical model of a starting mantle plume consisting of a diapiric head and a trailing conduit which interacts with a moving lithospheric plate of non uniform thickness we study in particular the effect of a lithospheric root on plume flow and partial melting the plume is deflected away from the root preferentially in the upstream direction the presence of a root facilitates more than three fold enhancement in the melting rate by restricting the horizontal spreading of plume material and the diffusion of thermal anomaly in comparison to the model having no root the large melting rate is sustained for longer duration in the presence of depletion buoyancy of the residue our model does not require an excessively high plume temperature or compositionally heterogeneous plume head and reproduces the characteristics of melting rates and melt volumes seen in flood basalt volcanic provinces and the trailing hotspot tracks the model also supports occurrence of volcanism of different ages at a given location as a result of initial melting in the upstream region and its subsequent passage over the plume conduit due to plate motion inferences for the occurrence of such late stage volcanism are discussed for the deccan volcanic province cenozoic volcanism of eastern australia is also discussed as an example of deflection of a plume due to the presence of thick lithosphere

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

evolution of the north caucasus foredeep constraints based on the analysis of subsidence curves abstract using a database of more than 130 wells the alpine evolution of the north caucasus foredeep can be described in three main periods 1 the early jurassic to middle late cretaceous including the cenomanian relates to the initial rifting phase and was characterised by succession of comparatively high rates of subsidence and uplift during this stage many events in the eastern and western parts were synchronous whereas some of them appeared to be smoothed in the stavropol high the southern border of the area had a more complex behaviour often moving in the opposite direction relative to the rest of the area the main peculiarities of evolution of the great caucasus region can be explained if we adopt the hypothesis of stamply and pillevuit 1993 and suppose that the early jurassic extension of the trough was accompanied by a strong left lateral transform movement as a result the central part of the great caucasus trough formed as a pull apart basin analysis of the style of movements of the southern border of the scythian plate as well as data on tectonics and volcanism in the great and lesser caucasus showed that other regional compressional and extensional mesozoic events could also have a transform component shear stresses within the lithosphere of the great caucasus can be due to oblique subduction or even transform movements at the plate boundary to the south of the caucasus region dercourt et al 1993 2 the period from the late cretaceous to the middle eocene from the turonian to the bartonian relates to the oceanic suture of the lesser caucasus and was characterised in the great caucasus area by alternating subsidence and uplift events of considerably lower amplitude at least up to the maastrichtian beginning in the late paleocene subsidence curves for almost all the area reflect the same events but in the western part and also in the north of the central part the rate of movements was considerably higher and since the late paleocene short term events in this area took place on the background of rather fast subsidence at nearly constant rate we believe that this subsidence and formation of the east black sea depression have the same origin we consider strong differences in evolution of the eastern western and central parts during the second stage to be due to closure of the lesser caucasus oceanic basin and arrival of the nakhichevan block this led to changes of configuration of the plate boundary and resulted in reorganisation of stress and displacements within the caucasus region this reorganisation was a reason for the opening of the east black sea depression and rapid subsidence of the western and central parts of the area at the end of the second stage 3 the period from the middle eocene to the present relates to the development of a foreland basin coeval with shortening and uplift in the adjacent great caucasus range it was characterised by alternation of relatively short uplift and longer subsidence events an important feature of this stage is that although the amplitude of movements varied from place to place these events were synchronous in all parts of the foredeep at the stavropol region sediments preserved only for the beginning of this stage formation and evolution of the foredeep during the third stage can not be explained exclusively by elastic flexure to do this we used the model of a small scale convection within the asthenosphere mikhailov et al 1996 mikhailov et al 1999 by the comparison of numerical results with the data on the evolution of the north caucasus foredeep we concluded that there were four main stages of compression in the processes of formation of the great caucasus mountain belt the first compression took place before the maykopian between 39 5 and 36 0 ma the other three were in the tarkhanian 16 6 15 8 ma konkian early sarmatian 14 3 12 3 ma and pontian 7 0 5 2 ma the different width of the great caucasus trough by the beginning of the compression as well as variations in thickness of the lithosphere and a different thermal state can cause interruption of the formation of the foredeep at the stavropol high

volcaniclastic deposits from the north arch volcanic field hawaii explosive fragmentation of alkalic lava at abyssal depths submarine explosive eruptions are generally considered to become less likely with increasing depth due to the increasing hydrostatic pressure of the overlying water column volcaniclastic deposits from the north arch volcanic field north of oahu have textural characteristics of explosive fragmentation yet were erupted in water depths greater than 4 200 m

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

active convergence between the lesser and greater caucasus in georgia constraints on the tectonic evolution of the lesser greater caucasus continental collision abstract we present and interpret newly determined site motions derived from gps observations made from 2008 through 2016 in the republic of georgia which constrain the rate and locus of active shortening in the lesser greater caucasus continental collision zone observation sites are located along two 160 km long profiles crossing the lesser greater caucasus boundary zone one crossing the rioni basin in western georgia and the other crossing further east near the longitude of tbilisi convergence across the rioni basin profile occurs along the southern margin of the greater caucasus near the surface trace of the north dipping main caucasus thrust fault mctf system and is consistent with strain accumulation on the fault that generated the 1991 m w 6 9 racha earthquake in contrast convergence along the tbilisi profile occurs near tbilisi and the northern boundary of the lesser caucasus near the south dipping lesser caucasus thrust fault approximately 50 70 km south of the mctf which is inactive within the resolution of geodetic observations 0 5 mm yr at the location of the tbilisi profile we suggest that the southward offset of convergence along strike of the range is related to the incipient collision of the lesser greater caucasus and closing of the intervening kura basin which is most advanced along this segment of the collision zone the identification of active shortening near tbilisi requires a reevaluation of seismic hazards in this area

submarine lava fields in french polynesia abstract shipboard multibeam survey is powerful tool to locate submarine volcanoes especially having small volume small submarine volcanoes may represent the initial stages of hotspot activity but they may also form via lithospheric flexing regional convection of the mantle and the presence of fracture zones here we describe several volcanoes flood lavas and volcanic clusters in french polynesia using data from archives of multibeam data the clusters of small volcanoes are similar to petit spots and they are not considered to represent the initial stages of a hotspot as they are composed of both young and old edifices and because the sites are located far from any known hotspot these newly discovered submarine volcanoes are located in areas with low velocity seismic shear waves at depths of 60 and 100 km these lava fields will therefore facilitate geochemical mapping of the mantle in areas unrelated to hotspots because these lavas may have developed from melts in the shallow mantle beneath french polynesia

active stress field and fault kinematics of the greater caucasus abstract this work contributes to depict the current seismicity fault kinematics and state of stress in the greater caucasus territories of georgia azerbaijan and russia we merged and homogenized data from different earthquake catalogues relocated 1000 seismic events created a database of 366 selected focal mechanism solutions 239 of which are new and performed a formal stress inversion preferential alignments of crustal earthquake foci indicate that most seismic areas are located along the southern margin of the belt and in the north eastern sector this is consistent with the presence of dominant active wnw ese faults parallel to the mountain range in the entire greater caucasus a dominant nne ssw oriented greatest principal stress 1 controls the over all occurrence of earthquakes of minor and major magnitude main earthquakes are characterized by a vertical least principal stress 3 corresponding to reverse kinematics reverse slip is more common along the southwestern and north eastern foothills of the greater caucasus although in these areas there are also scattered strike slip events this suggests the presence of local stress fields with horizontal 1 and 3 in the central southern part of the mountain belt in correspondence of the local collision between the lesser and the greater caucasus 1 rotates to nnw sse the strike slip events instead dominate along the southern flank of the central eastern mountain range this is interpreted as the effect of the collision that promotes eastward escape of the tectonic blocks located to the east

the caucasus an actual example of the initial stages of continental collision abstract a field study of the neotectonics of the georgian caucasus is combined with geophysical geodetic and geological information in order to obtain a mechanical model of actual deformation the analysis of satellite photos together with topographical and geological maps shows that the great caucasus is bisected by a nne trending sinistral strike slip fault zone the western great caucasus is thrusted to the south over the rioni basin while on the other hand its northern flank is a monocline that descends gently into the russian platform active volcanism is present from elbruz to kazbeg the eastern great caucasus is limited by thrusts to the north and to the south and intermediate structures are disposed in a fan like shape no recent volcanism is observed the eastern limit of the chain is a nnw trending dextral strike slip fault zone seismicity from a local network illustrates the activity of the thrusts and gives evidence for the strike slip faults microtectonic data confirm the recent activity and the sinistral character of the borjomi kazbeg strike slip fault the strike slip faults bordering the eastern caucasus may be seen as the limits of a plastic wedge in front of the arabian plate at an early stage of continental collision

geology and zircon u pb geochronology of the mtkvari pyroclastic flow and evaluation of destructive processes affecting vardzia rock cut city georgia abstract late cenozoic subaerial volcanic highland crop out in eastern anatolia and the western part of the lesser caucasus part of which located in georgia is known as samtskhe javakheti the samtskhe javakheti highland 4500 km2 is cut by the mtkvari river canyon where into the thick pyroclastic flow in twelfth century is hewn the unique city vardzia despite this characteristics of the pyroclastic flow such as scale source type genesis isotopic age and evaluation of destructive geological processes of the vardzia rock cut city have not been well studied our research has shown that the mtkvari pyroclastic flow is exposed at a distance of about 35 km from the karzameti fortress to the khertvisi fortress it is inclined northward by 2 4 and its thickness increases to the north from 40 m up to 80 m these rocks represent welded weakly welded and non welded ignimbrites of the andesitic dacitic composition isotopic parameters of these rocks nd varies between 3 and 4 and 87sr 88sr between 0 70341 and 0 70450 indicate that they originated as a result of fractionation of mantle derived melts as a result of field work we interpret the mtkvari ignimbrites as products of megacaldera collapse modern niala fields which nowadays is filled by postcaldera domes of andesitic composition and quaternary sediments the result of zircon dating using the u pb method with la icp ms technology is 7 52 0 21 ma which corresponds to the late miocene epoch geological research of the vardzia rock cut city has revealed that it is hewn into weakly welded ignimbrites and for which reason these rocks are under the intensive affecting of weathering and erosion processes at the same time the city is completely included in a tectonic block of 900 m length which is detached from the main body of rocks and is gradually subsiding towards the mtkvari canyon nowadays vardzia block is lowered by 30 6 m in comparison with bordering blocks in addition this block is split into several micro blocks by a joint set thus reducing its stability the situation is worsened by the active deep fault running along the vardzia rock cut city which represents a potential earthquake source

new observational and experimental evidence for a plume fed asthenosphere boundary layer in mantle convection abstract the textbook view is that the asthenosphere is the place beneath the tectonic plates where competing temperature and pressure effects on mantle rheology result in the lowest viscosity region of earth s mantle we think the sub oceanic asthenosphere exists for a different reason that instead it is where rising plumes of hot mantle stall and spread out beneath the strong tectonic plates below this plume fed asthenosphere is a thermal and density inversion with cooler underlying average temperature mantle here we show several recent seismic studies that are consistent with a plume fed asthenosphere these include the seismic inferences that asthenosphere appears to resist being dragged down at subduction zones that a sub oceanic thermal inversion 250 350 km deep is needed to explain the seismic velocity gradient there for an isochemical mantle that a fast halo of shear wave travel times surrounds the hawaiian plume conduit and that an apparent seismic reflector is found 300 km beneath pacific seafloor near hawaii we also present 2d axisymmetric and 3d numerical experiments that demonstrate these effects in internally consistent models with a plume fed asthenosphere if confirmed the existence of a plume fed asthenosphere will change our understanding of the dynamics of mantle convection and melting and the links between surface plate motions and mantle convection

active tectonics of central western caucasus georgia abstract this work contributes to a better knowledge of potentially seismogenic faults of the georgia greater and lesser caucasus by evaluating the distribution of earthquake foci active tectonic stress field kinematics and geometry of main fault planes we consider all the information coming from field structural geology geomorphology seismological data from historical and instrumental catalogues seismic reflection sections as well as new focal mechanism solutions these data enable recognizing some active ene wsw reverse faults in the core of the greater caucasus that are parallel to the mountain range at the southernmost front of the greater caucasus a series of main thrusts dipping towards nne are active with up to hundreds km long segments along this thrust zone a potentially locked segment is present about 90 km long the studied section of the lesser caucasus has active structures along the northern front given by south dipping thrusts as well as in the central core where strike slip and oblique faults coexist the transcaucasian depression between the two mountain ranges shows an ongoing inversion tectonics of the central part of the rioni basin where active n to ne dipping reverse faults are present accompanied by clear evidence of uplift of a wide area the data are coherent with a n s to nne ssw contraction of the central western greater caucasus and lesser caucasus although in general the seismicity decreases westward in terms of number of earthquakes and magnitude seismological and geological structural data in the rioni basin indicate here a quaternary propagation of deformation towards the west

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

cenozoic recent tectonics and uplift in the greater caucasus a perspective from azerbaijan the greater caucasus is europe s highest mountain belt and results from the inversion of the greater caucasus back arc type basin due to the collision of arabia and eurasia the orogenic processes that led to the present mountain chain started in the early cenozoic accelerated during the plio pleistocene and are still active as shown from present gps studies and earthquake distribution the greater caucasus is a doubly verging fold and thrust belt with a pro and a retro wedge actively propagating into the foreland sedimentary basin of the kura to the south and the terek to the north respectively based on tectonic geomorphology active and abandoned thrust fronts the mountain range can be subdivided into several zones with different uplift amounts and rates with very heterogeneous strain partitioning the central part of the mountain range defined by the main caucasus thrust to the south and backthrusts to the north forms a triangular shape zone showing the highest uplift and fastest rates and is due to thrusting over a steep tectonic ramp system at depth the meridional orogenic in front of the greater caucasus in azerbaijan lies at the foothills of the lesser caucasus to the south of the kura foreland basin

new constraints on the origin of the hawaiian swell from wavelet analysis of the geoid to topography ratio abstract analyzing the formation mechanism of hotspot swells enhances our understanding of intraplate volcanism and the underlying geodynamical processes the two main hypotheses for the origin of the archetypal hawaiian swell are thermal lithospheric thinning and dynamic support by an ascending plume any successful model would have to be able to simultaneously explain the swell topography and the corresponding geoid anomaly in simple models of isostatic compensation the geoid to topography ratio gtr is linearly related to the depth of the compensating mass therefore it is often considered a fundamental parameter to assess swell support mechanisms previous estimates for the geoid to topography ratio gtr of the hawaiian swell however are biased towards low values by incomplete removal of the effects of volcanic loading and lithospheric flexure in order to resolve these issues we here apply a continuous wavelet transform which allows resolution of lateral variations of the gtr at various spatial scales in a series of synthetic tests the robustness of this approach and its power to identify the origin of hotspot swells are established with 8 m km on the youngest part of the chain the recovered gtr agrees well with the predictions for dynamic support therefore ruling out thermal rejuvenation as an important mechanism we also find that the depth of the compensating mass decays by 20 km over a distance of 500 km from hawaii to kauai and identify sublithospheric erosion by small scale convection in the ponded plume material as a viable mechanism to support this decay

diversity dynamics of toarcian aalenian jurassic ammonites and transgressions regressions in the greater caucasus basin caucasian sea northern neo tethys ocean an evidence of partial dependence abstract possible dependence of ammonite diversity dynamics on shoreline shifts i e transgressions and regressions is yet to be understood data from the greater caucasus basin which was located on the northern active margin of the neo tethys ocean in the mesozoic permit to analyze the noted dependence at the toarcian aalenian time interval distribution of 161 species and 40 genera in each regional ammonite based biozone of this interval allows to evaluate the diversity dynamics of ammonites the total number of both species and genera changed cyclically the diversity maxima occurred in the h bifrons g thouarsense and l murchisonae zones the diversity minima occurred in the d semicelatum h variabilis p aalensis and g concavum zones semi quantitative analysis of facies distribution in 28 areas of the basin indicates a stepwise early toarcian middle aalenian transgression followed by a late aalenian regression the general trend towards species diversification through the study interval coincided with the noted transgression this is an evidence of partial dependence but no dependence between the generic diversity dynamics and shoreline shifts is visible it is hypothesized that transgressions facilitated the growth of the ammonite diversity but when they stopped the diversity was controlled by other factors changes in the spatial distribution of deep marine palaeoenvironments might also affect the ammonite diversity dynamics but no regional evidence of importance of oxygen depletion and seawater temperature fluctuations is found the minimal ammonite diversity in the d semicelatum zone could be a consequence of the early toarcian mass extinction in the greater caucasus basin the latter occurred at a peak of regression

receiver function imaging of lithospheric structure and the onset of melting beneath the galapagos archipelago abstract the galapagos archipelago represents an opportunity to investigate the properties of young oceanic lithosphere the effects of a hotspot anomaly on lithospheric thickness and melting dynamics in a hotspot ridge interaction here we use data recorded by the signet array and permanent station payg on the islands santa cruz and isabela respectively we used p to s ps and s to p sp receiver functions to constrain crust and mantle structure a simultaneous deconvolution method was used to constrain 1 d structure and also for the modeling of robust features a migrated extended multitaper method was used to investigate 3 d structural variations ps images a velocity increase with depth at 11 7 km probably the base of the pre plume crust or old moho sp imaging and modeling images a second deeper velocity increase at 37 7 km depth a velocity decrease with depth is imaged on average at 75 12 km likely associated with the lithosphere asthenosphere boundary this discontinuity is imaged deeper 82 km in the southwest and shallower 66 km in the northeast near the spreading ridge although the trend is consistent with lithospheric thickening with age the thickness is much larger than predicted by conductive cooling models of 0 10 my oceanic lithosphere we infer a compositional contribution to velocity variations finally a velocity increase with depth is imaged at 125 to 145 15 km depth that is likely associated with the onset of melting the discontinuity is imaged deeper in 3 sectors of the galapagos platform ridge region all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km one is located in the southwest in a hypothesized plume location the other two are to the northwest and northeast possibly illuminating multiple plume diversions related to complex plume ridge interactions

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

rapid formation of the small isles tertiary centre constrained by precise 40ar 39ar and u pb ages the relative chronology of magmatic and tectonic events is key to an understanding of the influence of the iceland plume on the north atlantic in particular the location and duration of magmatism is of fundamental importance initial widespread flood basalt formation occurred in baffin island greenland and britain before complete plate break up at 56 ma after which time magmatism became concentrated in the active rift zone historically the british tertiary igneous province btip has been instrumental in advancing many concepts of igneous petrology however the absolute age and duration of the province remains unresolved here we present new internally consistent 40ar 39ar ages that help to constrain the volcanic activity in the small isles centre to within 2 my this short duration has implications for the onset of magmatism in the larger north atlantic province the rapid unroofing of the rum volcano and more significantly some of the evidence used to propose pulsing of the iceland plume

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

the greater caucasus a galatian or hanseatic terrane comment on the formation of pangea by g m stampfli c hochard c verard c wilhem and j von raumer tectonophysics 593 2013 1 19 abstract a set of new global plate tectonic reconstructions sheds light on the evolution of the earth s oceans continents and individual terranes in the palaeozoic triassic the greater caucasus is interpreted as a hanseatic terrane that separated from laurussia in the devonian and then collided with the gondwana derived galatian superterrane however this contrasts with the hypothesis according to which the greater caucasus terrane is galatian in its nature the reasons for the interpretation of the greater caucasus as a hanseatic block are unclear and this interpretation does not explain some observations further research is necessary in order to understand the plate tectonic position of the greater caucasus terrane in the palaeozoic

the impossible galapagos connection geometric constraints for a near american origin of the caribbean plate geometric constraints derived from the present plate configuration and from plate motion vectors of the caribbean as well as the north and south american plates within a hotspot reference frame indicate that the thickened caribbean oceanic crust was formed in a near american position rather than at the galapagos hotspot a lateral displacement of more than 1000 km between the caribbean plate and the north and south american plates is related to differences in plate motion velocities during the cenozoic era the differential motion between the caribbean and the american plates results from trench parallel mantle flow as a response to the westward motion of the american plates

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

tectonic nappes and horizontal layering of the earth s crust in the mediterranean belt carpathians balkanides and caucasus abstract the morphological and kinematic features and mechanism of nappe formation of the pre alpine and alpine tectonic nappes of the carpathians balkanides and caucasus are discussed detached basement nappes cover nappes and ophiolite obduction nappes are distinguished all detached nappes are shown to be genetically compressional squeezed out as a result of tangential compression at depth accompanied by active underthrusting of the autochthone under the allochthone amstutz type subduction ophiolite nappes are formed by squeezing out of oceanic crust and the occurrence of bilateral or unilateral obduction simultaneously with subduction this formation of ophiolite nappes is a very transient process occurring at lightning speed in the geological sense compared to subduction data on large horizontal displacements at various levels in the crust entirely confirm the idea of a proposition that horizontal crustal and lithospheric layering has recently been substantiated by geophysical data too

depth and geoid anomalies over oceanic hotspot swells a global survey the broad depth and geoid anomalies associated with 23 hotspot swells in oceanic areas have been analyzed maximum height and geographical extent of the topographic swell and of the geoid anomaly as well have been measured for each hotspot the results indicate a clear increase of the topographic swell height with age of the underlying lithosphere from values in the range 300 500 m at young ages to values in the range 1 500 2 000 m at ages larger than 100 ma the geoid anomaly amplitude also increases with plate age from nearly zero close to mid ocean ridges to 6 8 m over old plates on the other hand the geographical extent of the swell does not show any clear relationship with plate age the mean lateral extent of swells range from 1 000 to 1 500 km swells located close to spreading ridges show a significant non zero depth anomaly but are associated with negligible geoid signal these results complete those of a previous study where the apparent compensation depth of oceanic hotspot swells was showed to increase linearly with the square root of plate age and coincide roughly with the base of the thermal lithosphere this trend may either be interpreted in terms ofmore lithospheric thinning or dynamical support besides both seem necessary to explain the observed bathymetry in proportion evolving with aging of the lithosphere less

spatial heterogeneity of the early middle toarcian jurassic ammonite diversity and basin geometry in the northwestern caucasus southwestern russia northern neo tethys abstract paleontological and geological information from the northwestern caucasus a large region in southwestern russia allows to evaluate the spatial heterogeneity of the early middle toarcian ammonite diversity and its possible controls the total number of ammonite species and genera is calculated for four time slices which correspond to the dactylioceras semicelatum harpoceras falciferum hildoceras bifrons and haugia variabilis zones in each of the ten areas distinguished within the study region these areas differ by the ammonite diversity which indicates heterogeneity the latter persisted through the entire early middle toarcian this heterogeneity was relatively low in the beginning of the toarcian when the total regional diversity of ammonites was minimal long and short term landward shoreline shifts facilitated spatial distribution of ammonites and increase in their taxa number shallow marine paleoenvironments tended to sustain a higher diversity than those deep marine and therefore changes in the basin depth were also responsible for the observed spatial heterogeneity of the ammonite diversity interestingly a more or less similar diversity dynamics is established in the areas of the northwestern caucasus the distribution of ammonites in the study region indicates that these invertebrates migrated there from the open sea s stretched along the northern margin of the neo tethys ocean results of the present analysis also imply that the mass extinction might have been responsible for the low diversity of ammonites observed in the entire region and its particular areas in the beginning of the toarcian

tectonic paleostress fields and structural evolution of the nw caucasus fold and thrust belt from late cretaceous to quaternary abstract the nw caucasus fold and thrust belt essentially corresponds to the inverted western flysch zone of the great caucasus mountains a deep basin that developed from late jurassic to eocene times between the scythian plate to the north and the transcaucasian terranes to the south the shatsky ridge sw of the nw caucasus zone the flysch basin was strongly affected by compression in late eocene times when the characteristic wnw trending folds and thrusts of the nw caucasus belt developed some authors regard the main compressive deformation as miocene in age by means of remote sensing analysis we elucidate the geometry of major structures in the belt wnw trending south vergent thrusts and folds and major vertical and transverse nne ssw to ne sw deep fault zones the later structures are interpreted as ancient faults that were active during the development of the flysch basin paleostress investigations reveal seven main tectonic episodes in the evolution of the nw caucasus since late cretaceous combining structural interpretation remote sensing analysis and paleostress field reconstruction we propose a model for the structural evolution of the belt during the late cretaceous paleocene the western caucasus zone was under transtensional regime with an e w to ne sw trending 3 that generated oblique normal movements along nne ssw transverse faults and wnw ese margins of the flysch basin this tectonism could correspond to rifting related to the formation of the eastern black sea basin at the paleocene eocene boundary a transpressional event with an e w to nw se trending 1 developed and the nne ssw to ne sw trending faults could have been inverted this event could correspond to an attenuation in the eastern black sea basin formation or to the incipient accretion of the transcaucasian terranes during the eocene another e w to nw se oblique extension transtensional event affected the flysch basin that could be related to a known rifting phase in the eastern black sea basin strong nne ssw to ne sw compression characterises late eocene tectonism the fold and thrust belt developed at this time as a result of the direct collision of the shatsky ridge with the scythian plate a ne sw extension followed the late eocene event related to basin development around the newly formed fold belt a wnw ese oblique contraction affected the belt during the early miocene as the result of arabian plate convergence with the caucasian system the latest inferred event is a compressional regime with nnw sse trending 1 that is affecting the nw caucasus belt from sarmatian times until the present under this oblique compression the belt has deformed as in a dextral shear zone and the thrust surfaces have acquired lenticular shapes this study highlights the occurrence of oblique movements in the nw caucasus area prior to and after the dominant late eocene compression from the late cretaceous until the eocene the structural development of the nw caucasus was closely related to the evolution of the eastern black sea basin from the late eocene until quaternary times it was rather related to the arabia eurasia plate convergence

geodynamic evolution of the caucasus and adjacent areas in alpine time abstract the alpine geodynamics of the caucasus and adjacent areas is discussed palinspastic reconstructions are given on the basis of new geologic paleomagnetic and paleobiogeographic data the dynamic unity of extension and compression processes is shown to take place within the central segment of the mediterranean belt the closure of the paleotethys oceanic basin coincides approximately with the opening of the mesotethys owing to this the opening between afro arabia and eurasia persists as is confirmed by paleomagnetic and other data the closure of the mesozoic tethys is on the whole synchronous with the opening of the central and south atlantic the formation of the present structure of the caucasus and adjacent areas is connected with the unidirectional compression of this area and the rotation of the compressional axis from a northeastern to a sub meridional direction in the course of the cimmerian alpine cycle

40ar 39ar geochronology of santo antao cape verde islands santo antao the northernmost island of the cape verde archipelago consists entirely of silica undersaturated volcanic products and minor intrusions 40ar 39ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence the oldest lavas 7 57 0 56 ma representing an older volcanic basement are exposed about 620 m above mean sea level after an interval of quiescence of up to 4 3 ma the volcanic activity resumed and continued at low eruption rates the older basement is unconformably overlain by a ca 810 m thick lava sequence that spans an age range from 2 93 0 03 to 1 18 0 01 ma this sequence is cut by many dykes and sills simultaneous volcanic activity occurred in the northeastern central and eastern part of the island a phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0 20 ma this predates volcanic activity that formed the highest point of the island tope de coroa which has an age of 0 17 0 02 ma the most recent eruption on the island formed nephelinitic lavas in the porto novo region at 0 09 0 03 ma the oldest volcanism exposed on santo antao which took place about 7 6 ma ago was simultaneous with waning activity on maio at the eastern end of the cape verde archipelago

numerical modeling of neotectonic movements and the state of stress in the north caucasus foredeep 1 this paper examines modern tectonic processes and the state of stress in the north caucasus along three profiles that cross its eastern central and western parts using the models of crustal structure along these profiles we solved the following inverse problem to find boundary conditions in this study velocity at side boundaries and bottom of the model that provide the best fit of calculated vertical movements at the top of the model and vertical neotectonic movements during the late quaternary our modeling shows that during the late quaternary geodynamics of the eastern and central parts has been controlled by regional intraplate compression normal to the greater caucasus belt the western part has been strongly influenced by tectonic processes taking place in the eastern black sea the calculated slow flow patterns and stress fields showed good agreement with the distribution of seismic events results of focal mechanisms inversions and gps data

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

heat flow and the thermal origin of hot spot swells the hawaiian swell revisited we present 150 new heat flow measurements obtained at eight sites along a 1230 km long profile across the hawaiian swell about 700 km ese of midway island most of the measurements include in situ thermal conductivity determinations which helped to reduce the statistical uncertainties 95 confidence at all sites to 59 mw m 2 about 20 higher than the predicted heat flow for 100 ma seafloor based on simple lithospheric cooling models thus even though there is a small increase in heat flow with age along the swell when compared with the off swell heat flux the anomalous heat flow associated with the hawaiian swell is probably of the order of 5 10 mw m 2 and arguably may not exist at all a previous investigation von herzen et al 1982 may have overestimated the magnitude of the heat flow anomaly associated with the hawaiian swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models the heat flow anomalies associated with the bermuda rise and the cape verde rise may also be smaller than previously estimated given the uncertainties in the heat flux on the normal seafloor surrounding these swells the lack of a significant heat flow anomaly associated with the bathymetric expression of the hawaiian swell is inconsistent with simple models of lithospheric reheating dynamic support accompanied by modest temperature increases 80 ma oceanic lithosphere

seismic imaging of thickened lithosphere resulting from plume pulsing beneath iceland ocean plates conductively cool and subside with seafloor age plate thickening with age is also predicted and hot spots may cause thinning however both are debated and depend on the way the plate is defined determining the thickness of the plates along with the process that governs it has proven challenging we use s to p sp receiver functions to image a strong persistent lab beneath iceland where the mid atlantic ridge interacts with a plume with hypothesized pulsating thermal anomaly the plate is thickest up to 84 6 km beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 6 km beneath regions formed during colder intervals we performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere instead periods of increased plume temperatures likely increased the melting depth causing deeper depletion and dehydration and creating a thicker plate this suggests plate thickness is dictated by the conditions of plate formation

jurassic transgressions and regressions in the caucasus northern neotethys ocean and their influences on the marine biodiversity abstract in the jurassic the caucasus presently located in the southwest of russia georgia armenia and azerbaijan was located on the northern active margin of the neotethys ocean facies interpretation in all 62 areas distinguished by differences in facies allows to semi quantitatively evaluate jurassic regional transgressions and regressions for this region major transgressive regressive cycles took place in the hettangian aalenian bajocian bathonian and callovian tithonian each transgression was more extensive than the previous the same cycles are established in the greater caucasus basin deep marine environments were common in the pliensbachian late aalenian and late bathonian whereas they were very restricted in the late jurassic the jurassic transgressions and regressions in the caucasus coincided with the proposed global eustatic changes however some differences were caused by the regional tectonic activity although transgressions and regressions cause some changes in marine biodiversity it seems that only ammonites might have been directly influenced by them diversity of bivalves brachiopods and belemnites was driven by other factors however global changes in marine biodiversity were more closely related to the eustatic fluctuations than it was documented on a regional scale

rayleigh wave dispersion along the hawaiian swell a test of lithospheric thinning by thermal rejuvenation at a hotspot summary in this paper we present a seismological test of the thermal rejuvenation model for the formation of the hawaiian swell the archetype of a midplate hotspot swell twostation measurements of rayleigh wave group and phase velocities between midway atoll and o ahu provide the basis for the test comparison of the observed dispersion curves with age dependent regionalized dispersion curves suggests that the swell has the seismic velocity structure of 50 110 myr old lithosphere rather than that of 20 50 myr old lithosphere as suggested by the rejuvenation model joint inversion of the group and phase velocity curves for isotropic velocity models yields an estimate of 100 km for the seismic lithosphere s thickness along the swell this value is difficult to reconcile with the 40 50 km thickness demanded by the rejuvenation model preliminary measurements of the coefficients of anelastic attenuation are also presented although the accuracy of these values is difficult to assess they too appear to be consistent with thick lithosphere

petroleum geology of the black sea abstract the black sea comprises two extensional basins formed in a back arc setting above the northward subducting tethys ocean close to the southern margin of eurasia the two basins coalesced late in their post rift phases in the pliocene forming the present single depocentre the western black sea was initiated in the aptian when a part of the moesian platform now the western pontides of turkey began to rift and move away to the south east the eastern black sea probably formed by separation of the mid black sea high from the shatsky ridge during the palaeocene to eocene subsequent to rifting the basins were the sites of mainly deep water deposition only during the late miocene was there a major sea level fall leading to the development of a relatively shallow lake most of the margins of the black sea have been extensively modified by late eocene to recent compression associated with closure of the tethys ocean gas chromatography mass spectrometry and carbon isotope analysis of petroleum and rock extracts suggest that most petroleum occurrences around the black sea can be explained by generation from an oil prone source rock of most probably late eocene age although a wider age range is possible in the basin centres burial history modelling and source kitchen mapping indicate that this unit is currently generating both oil and gas in the post rift basin a palaeozoic source rock may have generated gas condensate in the gulf of odessa in bulgarian waters the main plays are associated with the development of an eocene foreland basin kamchia trough and in extensional structures related to western black sea rifting the latter continue into the romanian shelf where there is also potential in rollover anticlines due to gravity sliding of neogene sediments in the gulf of odessa gas condensate has been discovered in several compressional anticlines and there is potential in older extensional structures small gas and oil discoveries around the sea of azov point to further potential offshore around the central azov high in offshore russia and georgia there are large culminations on the shatsky ridge but these are mainly in deep water and may have poor reservoirs there are small compressional structures off the northern turkish coast related to the pontide deformation these may include eocene turbidite reservoirs the extensional fault blocks of the andrusov ridge mid black sea high are seen as having the best potential for large hydrocarbon volumes but in 2200 m of water

louisville seamount chain petrogenetic processes and geochemical evolution of the mantle source the louisville seamount chain is a 4300 km long chain of submarine volcanoes in the southwestern pacific that spans an age range comparable to that of the hawaiian emperor chain and is commonly thought to represent a hot spot track dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 myr of the chain s history all samples are alkalic similar to previous dredge and drill samples providing no evidence for a hawaiian type tholeiitic shield volcano stage major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting isotopic values define only a narrow range in agreement with a surprising long term source homogeneity relative to the length scale of melting and overlap with proposed fields for the c and fozo mantle end members trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism except for a small number of lavas from the westernmost louisville seamounts built on young 20 ma old oceanic crust the louisville hot spot has been postulated to be the source of the 120 ma ontong java plateau but the louisville isotopic signature cannot have evolved from a source with isotopic ratios like those measured for ontong java plateau basalts on the other hand this signature can be correlated with that of samples dredged from the danger islands troughs of the manihiki plateau which has been interpreted as a rifted fragment of the greater ontong java plateau

tests of fixity of the indo atlantic hot spots relative to pacific hot spots rates of inter hot spot motion have been debated for decades herein we present updated predictions for the tracks of the tristan da cunha reunion and iceland hot spots assuming them to be fixed relative to pacific hot spots uncertainties in pacific hot spot rotations which include uncertainties in the current locations of hot spots of 100 200 km are combined with uncertainties in relative plate motions accumulated through the plate circuit to obtain the final uncertainty in the predicted positions including uncertainties of 150 200 km in the current locations of the indo atlantic hot spots improvements to reconstruction methods to relative plate reconstructions to age dates along the tracks and to the geomagnetic reversal timescale lead to significant changes from prior results when compared with the observed tracks the predicted tracks indicate nominal rates of motion of only 2 6 mm a 1 of these indo atlantic hot spots relative to pacific hot spots over the past 48 ma within the uncertainties the rates range from no motion to rates as high as 8 13 mm a 1 for reconstructions prior to 48 ma b p however the apparent rates of inter hot spot motion are much larger 46 55 20 mm a 1 if the motion occurred entirely between 68 ma b p and 48 ma b p either hot spots moved rapidly before 48 ma b p and slowed drastically at 48 ma b p or global plate circuits through antarctica become less reliable as one goes increasingly further into the past most paleomagnetic data favor the latter explanation

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

geoid height versus topography for a plume model of the hawaiian swell abstract we use a three dimensional variable viscosity convection model of a stationary plume beneath a drifting lithosphere to study the factors that control the geoid to topography ratio gtr of the hawaiian swell the rate of melting in the plume is predicted using a batch melting parameterization and the melt is assumed to migrate to the surface where it builds a volcanic edifice equivalent to the hawaiian island chain viscous stresses elastic deformation of the lithosphere and optionally the volcanic material deposited on the ocean floor are included in the calculation of surface topography and the corresponding geoid the derivation of the gtr from the model imitates methods that have previously been used to estimate the observed gtr for the hawaiian swell the first method we use here is that of marks and sandwell j geophys res 96 1991 8045 8055 which applies bandpass filters to retain only wavelengths from 400 to 4000 km as most characteristic of the swell topography and geoid and the gtr is estimated from the slope of the regression line of geoid versus topography another group of methods analyzes the data along profiles drawn across the hotspot swell and eliminates the unwanted signals e g the volcanic islands and the flexural moat around them by cutting out parts of the sections the gtr is then calculated from curves which best fit the topography and geoid profiles on the swell flanks only in our plume model when the effects of the volcanic surface loading are included marks and sandwell s method yields 4 4 m km for the gtr while profile fitting on the swell flanks gives 7 8 5 m km ignoring the volcanic load leads to 7 8 m km in all processing methods the observed gtr for the hawaiian swell has been reported to lie between 4 and 5 m km analysis of the data processing methods shows that the applied bandpass filters cannot completely remove the signal due to the volcanic edifice and lithospheric flexure and this causes apparent gtrs around 4 m km the pure swell model containing no volcanic load on the surface demonstrates that the hawaiian swell may have a proper gtr near 7 m km

late cenozoic deformation of the kura fold thrust belt southern greater caucasus although the geometry and kinematics of the first order structures accommodating arabia eurasia convergence are relatively well known in turkey and iran major shortening structures remain poorly understood within the central portion of the collision zone in eastern anatolia and the caucasus new remotely sensed neotectonic mapping synthesis of regional geologic and stratigraphic data and balanced cross sections suggest that the kura fold thrust belt has accommodated the majority of arabia eurasia convergence since the early pliocene between the longitudes of 45 e and 49 e this belt lies southeast of the n80 w striking greater caucasus mountains and forms an eastward narrowing band of elevated topography that roughly parallels the range front for 400 km along strike the belt is separated from the greater caucasus to the north by the 10 to 25 km wide alazani basin and comprises a series of predominantly south verging folds deforming eocene quaternary flysch and molasse to document structural geometries within the kura fold thrust belt we have used the real time interactive mapping system rims software to analyze advanced spaceborne thermal emission and reflection radiometer aster visible to near infrared vnir and digital elevation model dem data this neotectonic mapping indicates an along strike eastward decrease in both structural complexity and the degree to which deformed geomorphic surfaces are dissected existing geologic maps indicate a corresponding eastward decrease in the depth of exposure by integrating the structural geometries determined in our analysis of remote sensing data with existing geologic data we have constructed two balanced cross sections which suggest these systematic along strike variations result from a west to east decrease in total shortening within the kura fold thrust belt we interpret this variable shortening to stem from eastward propagation of the kura fold thrust belt comparison of our preliminary total shortening estimates with those predicted by current plate motions suggest that the kura fold thrust belt has accommodated 30 40 25 km of total arabia eurasia convergence since 5 ma and thus forms a first order structural system within the central portion of the collision zone