surface waves tomography and non linear inversion in the southeast carpathians abstract a set of shear wave velocity models of the lithosphere asthenosphere system in the southeast carpathians is determined by the non linear inversion of surface wave group velocity data obtained from a tomographic analysis the local dispersion curves are assembled for the period range 7 150 s combining regional group velocity measurements and published global rayleigh wave dispersion data the resolution of the tomography data is improved using a priori information about the shallow crustal velocity structure the lithosphere asthenosphere velocity structure is reliably reconstructed to depths of about 250 km for 27 cells with size 1 1 the adopted non linear inversion method computes a set of acceptable velocity models for each cell local smoothness optimization lso is applied to select the representative cellular structures and the three dimensional shear wave velocity model of the studied region is assembled interpretation of the obtained structure shows that the thickness of the lithosphere in the region varies from about 90 km to 170 km and the asthenosphere can be as deep as about 150 km mantle seismicity concentrates where the high velocity lid is detected just below the moho the obtained results are in agreement with recent seismic refraction receiver function and travel time p wave tomography investigations in the region the similarity among the results obtained from different kinds of structural investigations including the present work highlights some new features of the lithosphere asthenosphere system in southeast carpathians such as the relatively thin crust under transylvania basin and in the vrancea zone

charge rearrangement by sprites over a north texas mesoscale convective system 1 charge rearrangement by sprites is analyzed for a mesoscale convective system mcs situated in north texas and east new mexico on 15 july 2010 during the thunderstorm electric field data were recorded by the langmuir electric field array lefa while magnetic field data were recorded by the charge moment network near duke university a high speed 12500 fps video system operated at langmuir laboratory recorded telescopic images of the sprites data from the national lightning detection network nldn show that each sprite was preceded by a series of cloud discharges and cloud to ground discharges the triggering event preceding the sprite was typically a positive cloud to ground cg stroke for one out of the 10 sprites that were recorded there was a positive hump in the electric field a few milliseconds after the cg return stroke the size and shape of the hump roughly matched the light intensity emitted from the sprite the electric field hump is fit by a sprite current that originates in the ionosphere and propagates downward producing the same effect as a downward moving positive current the integral under the current hump was 23 9 c when the velocity of the current pulse was between 0 25 c and 0 55 c the large sprite current was followed by impulsive electromagnetic radiation which has not been previously reported and could be a recoil effect similar to what is called a k change when it is observed in a lightning flash

geometrical spreading and attenuation of lg waves a comparison between western anatolia turkey and southern spain abstract we compared the attenuation and geometrical spreading coefficient of lg waves for crustal paths in western anatolia and southern spain at source receiver distances between 80 and 400 km we used the coda normalization method which is based on the elimination of the source and site effects from observed spectra of lg waves q for lg waves in the crust was determined as a function of frequency in the range 1 5 10 0 hz for western anatolia and in the range 1 5 12 hz for southern spain we found that qlg shows a strong frequency dependence in both regions in western anatolia the frequency dependence of qlg is qlg f 82 f1 0 and the geometrical spreading coefficient ranges from 0 6 to 0 9 we found qlg 83 f0 88 and a geometrical spreading coefficient close to 0 5 in southern spain comparing our results with coda q values obtained in both regions qlg qc are similar qlg is close to qi in all frequency bands we interpret the geometrical spreading coefficient higher than 0 5 for western anatolia as due to a strong layered and heterogeneous crustal structure

regional seismic wave propagation lg and sn and pn attenuation in the arabian plate and surrounding regions an edited version of this paper was published by blackwell publishing copyright 2004 blackwell publishing see also http www blackwell synergy com doi abs 10 1111 j 1365 246x 2004 02246 x http atlas geo cornell edu middleeastnorthafrica publications al damegh2004 htm

a continent wide map of 1 hz lg coda q variation across eurasia and its relation to lithospheric evolution 1 we present new maps of lg coda q qo and its frequency dependence at 1 hz that cover virtually all of eurasia qo is relatively high up to 650 or more in the east european siberian indian and kazakh platforms but is surprisingly low 300 500 in the arabian platform and western portion of the siberian platform as well as in great britain and the northwestern portion of mainland europe it is generally low throughout the tethysides orogenic belt but there too it displays substantial regional variations 150 400 most qo anomalies appear to be related to the tectonic history of the eurasian crust the four regions with lowest values coincide with four of eurasia s most active concentrations of earthquake activity observed eurasian qo values are consistent with a previously determined empirical plot of global values in which qo in any region is proportional to the time elapsed since the most recent episode of tectonic or orogenic activity there comparisons of the new qo map with maps of rayleigh wave velocity temperature subducted lithosphere and seismicity lead us to infer that lg coda wave attenuation is most easily explained by energy lost in moving fluids through permeable rock or in being scattered from fluid enhanced zones the fluids are likely to have originated by hydrothermal release from subducting lithosphere or other upper mantle heat sources anomalies in qo and other lithospheric properties in some cases extend well outside the tethysides belt and other regions where upper mantle heat sources are known to occur possible explanations include earlier episodes of subduction activity crustal deformation due to collision of the afro arabian and indian plates or broader than expected back arc activity beneath eurasia

mapping crustal heterogeneity using lg propagation efficiency throughout the middle east mediterranean southern europe and northern africa in this paper we describe a technique for mapping the lateral variation of lg characteristics such as lg blockage efficient lg propagation and regions of very high attenuation in the middle east north africa europe and the mediterranean regions lg is used in a variety of seismological applications from magnitude estimation to identification of nuclear explosions for monitoring compliance with the comprehensive nuclear test ban treaty ctbt these applications can give significantly biased results if the lg phase is reduced or blocked by discontinuous structure or thin crust mapping these structures using quantitative techniques for determining lg amplitude attenuation can break down when the phase is below background noise in such cases lg blockage and inefficient propagation zones are often mapped out by hand with our approach we attempt to visually simplify this information by imaging crustal structure anomalies that significantly diminish the amplitude of lg the visualization of such anomalies is achieved by defining a grid of cells that covers the entire region of interest we trace lg rays for each event station pair which is simply the great circle path and attribute to each cell a value equal to the maximum value of the lg p coda amplitude ratio for all paths traversing that particular cell the resulting map from this empirical approach is easily interpreted in terms of crustal structure and can successfully image small blockage features often missed by analysis of raypaths alone this map can then be used to screen out events with blocked lg prior to performing q tomography and to avoid using lg based methods of event identification for the ctbt in regions where they cannot work for this study we applied our technique to one of the most tectonically complex regions on the earth nearly 9000 earthquake station raypaths traversing the vast region comprised of the middle east mediterranean southern europe and northern africa have been analyzed we measured the amplitude of lg relative to the p coda and mapped the lateral variation of lg propagation efficiency with the relatively dense coverage provided by the numerous crossing paths we are able to map out the pattern of crustal heterogeneity that gives rise to the observed character of lg propagation we observe that the propagation characteristics of lg within the region of interest are very complicated but are readily correlated with the different tectonic environments within the region for example clear strong lg arrivals are observed for paths crossing the stable continental interiors of northern africa and the arabian shield in contrast weakened to absent lg is observed for paths crossing much of the middle east and lg is absent for paths traversing the mediterranean regions that block lg transmission within the middle east are very localized and include the caspian sea the iranian plateau and the red sea resolution is variable throughout the region and strongly depends on the distribution of seismicity and recording stations lg propagation is best resolved within the middle east where regions of crustal heterogeneity on the order of 100 km are imaged e g south caspian sea and red sea crustal heterogeneity is resolvable but is poorest in seismically quiescent northern africa

seismic lg wave q tomography in and around northeast china 1 we investigate regional variations in the lg wave quality factor q in northeast china and its vicinity with a tomographic method digital seismic data recorded at 20 broadband stations from 125 regional events are used to extract lg wave spectra tomographic inversions are independently conducted at 58 discrete frequencies distributed log evenly between 0 05 and 10 0 hz we simultaneously invert for the lg wave q distribution and source spectra at individual frequencies without using any a priori assumption about the frequency dependence of the q model and source function the best spatial resolution is approximately 1 1 in well covered areas for frequencies between 0 4 and 2 0 hz the lg q shows significant regional variations and an apparent relationship with regional geology we use a statistical method to investigate the regional variations of lg q and their frequency dependence the average q0 1 hz lg q in the entire investigated region is 414 sedimentary basins are usually characterized by lower average q0 values from 155 to 391 while volcanic mountain areas have relatively high average q0 values from 630 to 675 lg q generally increases with increasing frequency however the frequency dependence has complex nonlinear features on a double logarithmic scale indicating that the commonly adopted power law relationship may be oversimplified in a broad frequency band the frequency dependence varies in different geological areas with larger variations seen at lower frequencies

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

lg coda q and its relation to the geology and tectonics of the middle east regional seismograms were collected to image the lateral variations of lg coda q at 1 hz q 0 and its frequency dependence in the middle east using a back projection method the data include 124 vertical component traces recorded at 10 stations during the period 1986 1996 the resulting images reveal lateral variations in both q 0 and in the turkish and iranian plateaus a highly deformed and tectonically active region q 0 ranges between about 150 and 300 with the lowest values occurring in western anatolia where extremely high heat flow has been measured the low q 0 values found in this region agree with those found in other tectonically active regions of the world throughout most of the arabian peninsula a relatively stable region q 0 varies between 350 and 450 being highest in the shield area and lowest in the eastern basins all values are considerably lower than those found in most other stable regions low q values throughout the middle east may be caused by interstitial fluids that have migrated to the crust from the upper mantle where they were probably generated by hydrothermal reactions at elevated temperatures known to occur there low q 0 values about 250 are also found in the oman folded zone a region with thick sedimentary deposits varies inversely with q 0 throughout most of the middle east with lower values 0 4 0 5 in the arabian peninsula and higher values 0 6 0 8 in iran and turkey q 0 and are both low in the oman folded zone and western anatolia

strong lg wave attenuation in the northern and eastern tibetan plateau measured by a two station two event stacking method 1 the regional seismic phase lg which involves shear wave reverberations in the crust is strongly attenuated along propagation paths in northern and eastern tibet robust estimates of the attenuation quality factor qlg in these regions are obtained using a two station two event stacking method for broadband vertical component lg data in the passband 0 2 1 0 hz we find average 1 hz qlg values qo of 79 4 in northern central tibet and 119 17 in eastern tibet for the 0 2 0 5 hz passband localized regions of eastern tibet have qo estimates ranging from 66 to 121 these results confirm and augment the spatial coverage of recent estimates of low qo values in north central and south central tibet and support the notion of widespread partial melting of the crust throughout the plateau

lateral variations of crustal seismic attenuation along the indepth profiles in tibet from lg q inversion 1 fourier spectra of crustal guided lg waves are collected to study seismic attenuation or q along the indepth profiles the 1 hz lg q q0 values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying q0 models along the indepth iii profile in central tibet the q0 model consists of low and nearly constant values of 90 these low values are consistent with abnormally high temperature as well as partial melts in the crust along the indepth ii profile in southern tibet the q0 values increase southward they are extremely low 60 over the northernmost segment but increase by a factor of 2 over a distance of 100 km between the indus yalong suture and the kangmar dome farther south and into the high himalayas q0 values are higher than 300 regional rayleigh waves observed along the northern indepth ii profile are used to infer a low velocity low q layer at midcrustal depths between 15 and 30 km in southern tibet the aqueous fluid trapped in the upper crust and a midcrust partial melting zone associated with the underthrusting of the indian lithosphere are the likely causes of the high lg attenuation in southern tibet low q rather then a systematic lg conversion along a dipping moho is the main cause of the previously observed lg blockage over paths crossing the southern tibetan boundary

tomographic pn and sn velocity beneath the continental collision zone from alps to himalaya 1 we have obtained vp and vs velocity images of the uppermost mantle beneath the continental collision zone from the alps to the himalaya by performing tomographic inversion using both pn and sn travel times 654 999 pn arrivals and 121 838 sn arrivals were selected from the joint database including isc ehb iran bulletin and the annual bulletin of chinese earthquakes average pn and sn velocities are 8 04 km s and 4 60 km s respectively and maximum velocity perturbations are about 6 pn velocity correlates well with topography in general mountains with high elevations show low velocity while the seas basins and plains with low elevations show high velocity because the mountains are collision zones with strong tectonic activity and the low elevation areas are stable plates the large tectonic lines are boundaries between high and low pn velocity such as plate boundaries sutures and faults between plates and orogens sn velocity shows a very similar pattern to pn velocity a geodynamic cartoon is proposed to show the relationship between velocity and tectonics indicating that the sutures are boundaries with high dip angles and the plate boundaries are low angle reverse faults in the region

volcanic belts as markers of the mesozoic cenozoic active margin of eurasia abstract from the triassic to the present linear belts of calc alkaline volcanism developed along the southern margin of eurasia in the triassic the calc alkaline belt can be traced in turan fore caucasus crimea and dobrogea its development is related to the closure of the paleo tethys when fragments of gondwana iran afghanistan etc were approaching eurasia since the early jurassic calc alkaline volcanism has been related to subduction of the oceanic crust of the mesozoic tethys five stages of calc alkaline volcanic activity can be distinguished early jurassic middle jurassic early cretaceous late cretaceous paleogene and late miocene quaternary the configuration of the volcanic belts the intensity of the volcanism and the chemistry of the erupted products change considerably from stage to stage mesozoic belts were most pronounced in the pontian transcaucasian segment of the margin starting from the late cretaceous and especially in the cenozoic the volcanic belts can be continuously traced from the balkans to afghanistan most of the belts were actually island arcs formed on the fragments of the hercynian of baikalian basement and in this respect similar to the island arc of japan only the incipient island arcs of senonian age formed close to the passive margin of gondwana are the exceptions they were built on oceanic crust and are geochemically close to the mariana arc at certain times early jurassic eocene parallel chains of island arcs existed in general the situation resembled the present day convergent boundary in the western pacific as closure of the tethys proceeded island arcs gave way to marginal continental volcanic belts with wide spread development of high potassium particularly shoshonitic series peaks of volcanic activity in the bajocian late cretaceous eocene and late miocene present alternated with quiet periods and corresponded to the times when the convergence rate between africa and eurasia increased the opening of the back arc basins is reflected in outbursts of basaltic rift type volcanism these outbursts occurred in the toarcian aalenian bathonian late jurassic aptian turonian and early middle eocene

strong lg attenuation in the tibetan plateau the widespread existence of strong lg attenuation in the tibetan plateau is further demonstrated by analysis of lg spectra on many paths within the plateau and quantitative estimates of the lg attenuation from low frequency lg signals for new localized path geometries strong path length dependent shifts of the lg spectra to lower corner frequencies with increasing distance are observed across the plateau consistent with a low regional average 1 hz lg attenuation value q 0 of about 125 there are clearly lateral variations within the plateau found in this and other recent studies with localized areas having q 0 values of 60 90 low enough to eliminate high frequency lg energy over path lengths of just several hundred kilometers while some localized areas may have higher q 0 values of up to 147 or higher a q 0 value of 103 is found in south central tibet compatible with recent work by others for higher frequencies on very localized scales and values from 83 to 147 are found in eastern tibet the lowest q 0 estimates found in tibet tend to be in areas for which there is evidence of volcanism and or partial melting within the crust however the strong regional attenuation may have a contribution from scattering by small scale crustal heterogeneity the strong lg attenuation in tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

pg attenuation tomography within the northern middle east abstract a seismic attenuation map of regional phase pg is constructed for the northern middle east using two station frequency dependent q measurements and seismic attenuation tomography pg is widely used in source discrimination but its attenuation behavior is not well understood we investigate the functional form of the geometrical spreading of pg for both uniform and more complex seismic velocity structures we find that there is significant trade off between attenuation and geometrical spreading of pg and we quantify this trade off to interpret the observed pg attenuation in the middle east the northern middle east attenuation tomographic images show significant lateral q pg variations which may be caused by lateral variations of crustal rheology if intrinsic attenuation dominates the seismic attenuation mechanism of this region we observe very large variations in the q pg model such as low q pg within the anatolian plate and high q pg within much of the stable arabian plate the low q pg anomalies generally coincide with active fault systems or quaternary volcanisms in order to perform a systematic comparison with lg propagation we have also updated the q lg map of zor et al 2007 these two new maps provide estimates for crustal q pg and q lg for the northern middle east

characteristics of lg attenuation in the tibetan plateau 1 lg a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties in stable continental regions lg propagates with a group velocity of about 3 5 km s and can often be observed at distances up to 4000 km to better understand the absence of high frequency lg arrivals for paths traversing the northern boundary of the tibetan plateau we investigate spatial variations of broadband 0 15 5 0 hz energy in the lg group velocity window 3 6 3 0 km s using regional waveforms recorded at the chinese digital seismic network station wmq vertical component seismograms are analyzed for 90 events with magnitudes of 4 4 mb 6 4 that occurred between 1987 and 1999 in the tibetan plateau and around its margins the lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1 2 hz nearly identical to those for comparable size events at similar distances outside the plateau high frequency 1 hz lg energy recorded at wmq decreases rapidly as a function of source distance into the plateau a path length of 300 400 km within the northern plateau suffices to eliminate 2 5 hz lg energy for events in southern tibet with paths crossing the central portion of the tibetan plateau almost total lg extinction occurs even for energy in the low frequency band of 0 2 1 hz corresponding apparent corner frequencies of the lg amplitude spectra range between 0 2 and 0 4 hz the corner frequency shift is found to vary systematically with path length across the plateau linear regressions demonstrate that the shift in apparent corner frequency of lg amplitude spectra is negatively correlated with features of the tibetan plateau such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds the systematic variations in the amplitude and frequency content of energy in the lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in lg extinction for paths traversing central and northern tibet superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau spectral ratios of many event pairs along great circle paths give estimates of frequency dependent lg attenuation for paths crossing western central and eastern sectors of tibet the region of strong sn attenuation in northern central tibet also has strong lg attenuation with qlg for 1 hz on the order of qo 80 90 while in southern central tibet qo increases to about 316 and in eastern and western tibet qo is on the order of 120 200 for paths traversing the entire plateau the strong lg attenuation in northern central tibet is responsible for the so called lg blockage and may be associated with partial melting in the crustal low velocity layer in northern tibet our qlg values for tibet are significantly lower than most earlier estimates primarily as a result of not excluding blocked observations along with allowing for lateral variations within tibet index terms 7203 seismology body wave propagation 7255 seismology surface waves and free oscillations 7205 seismology continental crust 1242 keywords lg tibetan plateau nuclear event discrimination crustal attenuation lg blockage crustal partial melting

high resolution regional seismic attenuation tomography in eastern tibetan plateau and adjacent regions 1 the q of regional seismic phases lg and pg within the crust is assumed as a proxy for crustal q and q which is used as a constraint of crustal rheology we measure regional phase q of the eastern tibetan plateau and adjacent areas this method eliminates contributions from source and site responses and is an improvement on the two station method tsm we have generated tomographic images of crustal attenuation anomalies with resolution as high as 1 in general we observe low q in the northernmost portions of the tibetan plateau and high q in the more tectonically stable regions such as the interior of the qaidam basin the calculated site responses appear to correlate with topography or sediment thickness furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the rtm method effectively removes the source response and may be used as an alternative to source magnitude

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

seismic imaging of the lithospheric structure of the zagros mountain belt iran we present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across central and northern zagros the receiver function analysis of teleseismic earthquake records gives a high resolution image of the moho beneath the seismic transects on both cross sections the crust has an average thickness of 43 2 km beneath the zagros fold and thrust belt and the central domain the crust is thicker beneath the hanging wall of the main zagros reverse fault mzrf with a larger maximum moho depth in central 69 2 km than in northern zagros 56 2 km the thickening affects a narrower region 170 km beneath the sanandaj sirjan zone of central zagros and a wider region 320 km in northern zagros we propose that this thickening is related to overthrusting of the crust of the arabian margin by the crust of central iran along the mzrf considered as a major thrust fault cross cutting the whole crust the fault is imaged as a lowvelocity layer in the receiver function data of the northern zagros profile moreover the crustal scale thrust model reconciles the imaged seismic moho with the bouguer anomaly data measured on the central zagros transect at upper mantle depth p wave tomography confirms the previously observed strong contrast between the faster velocities of the arabian margin and the lower velocities of the iranian micro blocks our higher resolution tomography combined with surface wave analysis locates the suture in the shallow mantle of the sanandaj sirjan zone beneath central zagros the arabian upper mantle has shield like shear wave velocities while the lower velocities of the iranian upper mantle are likely due to higher temperature but these velocities are not low enough and the low velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath iran the lack of a high velocity anomaly in the mantle beneath central iran suggests that the neotethyan oceanic lithosphere is probably detached from the arabian margin

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

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

strong lg wave attenuation in the middle east continental collision orogenic belt abstract using lg wave q tomography we construct a broadband crustal attenuation model for the middle east the q lg images reveal a relationship between attenuation and geological structures strong attenuation is found in the continental collision orogenic belt that extends from the turkish and iranian plateau to the pamir plateau we investigate the frequency dependence of q lg in different geologic formations the results illustrate that q lg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions an average q lg value between 0 2 and 2 0 hz q lg 0 2 2 0 hz may be a critical index for crustal attenuation and is used to infer the regional geology low q lg anomalies are present in the eastern turkish plateau and correlate well with low pn velocities and cenozoic volcanic activity thus indicating possible partial melting within the crust in this region very strong attenuation is also observed in central iran the afghanistan block and the southern caspian sea this in line with the previously observed high crustal temperature high conductivity layers and thick marine sediments in these areas suggests the high lg attenuation is caused by abnormally high tectonic and thermal activities

midcrustal low velocity layer beneath the central himalaya and southern tibet revealed by ambient noise array tomography 1 ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the earth using 2 months of ambient noise data from 20 stations of the himalayan nepal tibet seismic experiment we investigate the upper and middle crustal structure in the central himalaya and southern tibet about 120 interstation rayleigh wave empirical green s functions with sufficient signal to noise ratio are obtained and used for group velocity dispersion analysis in the period range 6 25 s using frequency time analysis technique the obtained dispersion data are then used to construct 2 d group velocity maps at the short periods from 9 to 15 s the distribution of rayleigh wave velocities delineates several distinct low and high velocity zones separated mainly by geological boundaries the high group velocity zone is located mainly around regions with plutonic rocks and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks finally we invert for the shear velocity structure in the upper and middle crust along a n s trending cross section at the longitude 86 5 e we observe a clear low velocity layer in the middle crust about 10 25 km depth distributed on both sides of the indus yarlung suture zone the existence of this midcrustal low velocity zone suggests a mechanically weaker middle crust beneath the central himalaya and southern tibet which might decouple the upper crustal deformation from that of the lower crust in the tibetan himalayan orogenic processes

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

amplitude ratio tomography for regional phase q 1 we image regional seismic phase attenuation q by extending amplitude ratio techniques which are widely used to determine average q within a sampled region to obtain laterally varying q structure using tomographic techniques the method assumes that source radiation is isotropic and that the amplitude ratios can be modeled by a two dimensional q map and relative site terms application to 2286 1 hz lg amplitudes from 816 events recorded at 35 central and east asia stations yields a misfit of 0 09 log10 units a variance reduction of 73 relative to the uniform q model that best fits the data and laterally varying q from 100 to 1000 with patterns closely following regional geology the image is resolved on average to 2 5 with resolution peaking at 1 5 in the best covered areas which is remarkable given the sparse station geometry employed the high resolution results from the dense distribution of earthquake sources and is aided by the increased confidence in source locations provided by merging catalogs from many different agencies

the quasi 2 day wave observed in the polar mesosphere comparison of the characteristics observed at tromso and poker flat 1 a comparison of the quasi 2 day wave q2dw observed at tromso 69 6 n 19 2 e and poker flat 65 2 147 6 w is presented at four heights of 70 76 82 and 88 km using wind data taken for 4 years from 1 november 1998 to 7 november 2002 the characteristics of the q2dw such as seasonal variation occurrence of period of maximum amplitude ratio of meridional to zonal amplitudes shape of altitude profile of phase and modulation of amplitude at a 4 10 days rate found at poker flat are very similar to those found at tromso reported by nozawa et al 2003 the activity of the q2dw is higher in winter than in summer for meridional and zonal components at the two sites long term variation i e seasonal variation of the q2dw is found to be similar between the two sites while short term variations i e over several days do not synchronize well with each other the ratio of amplitudes of the q2dw between the two sites varies mainly between 0 5 and 2 and there is no significant preference toward either site phase differences of q2dws between the two sites are examined and it is found that in phase like events are more frequently seen than out of phase like events at 76 82 and 88 km in winter and at 88 km in summer this can be interpreted to mean that the zonal wave number of the q2dw appears to be 2 or 4 more often than 3 in the polar upper mesosphere these results suggest that observed q2dws have features consistent with the rossby gravity wave mode but the amplitude and phase of the q2dw are affected significantly by local sources a possibility that the observed q2dw is an eastward moving wave is also discussed