seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

dynamic source inversion of the m6 5 intermediate depth zumpango earthquake in central mexico a parallel genetic algorithm we introduce a method for imaging the earthquake source dynamics from the inversion of ground motion records based on a parallel genetic algorithm the source model follows an elliptical patch approach and uses the staggered grid split node method to simulate the earthquake dynamics a statistical analysis is used to estimate errors in both inverted and derived source parameters synthetic inversion tests reveal that the average rupture speed vr the rupture area and the stress drop may be determined with formal errors of 30 12 and 10 respectively in contrast derived parameters such as the radiated energy er the radiation efficiency r and the fracture energy g have larger errors around 70 40 and 25 respectively we applied the method to the mw 6 5 intermediate depth 62 km normal faulting earthquake of 11 december 2011 in guerrero mexico inferred values of 29 2 6 2 mpa and r 0 26 0 1 are significantly higher and lower respectively than those of typical subduction thrust events fracture energy is large so that more than 73 of the available potential energy for the dynamic process of faulting was deposited in the focal region i e g 14 4 3 5 1014j producing a slow rupture process vr vs 0 47 0 09 despite the relatively high energy radiation er 0 54 0 31 1015 j and energy moment ratio er m0 5 7 10 5 it is interesting to point out that such a slow and inefficient rupture along with the large stress drop in a small focal region are features also observed in both the 1994 deep bolivian earthquake and the seismicity of the intermediate depth bucaramanga nest

the structure of archean ketilidian crust along the continental shelf of southwestern greenland from a seismic refraction profile the velocity structure of the continental crust on the outer shelf of southwestern greenland is determined from dense wide angle reflection refraction data obtained with large air gun sources and ocean bottom seismometers along a 230 km seismic line this line crosses the geological boundary between the archean block and the ketilidian mobile belt although the data have high noise levels p and s wave arrivals from within the upper intermediate and lower crust and at the moho boundary can be consistently identified and correlated with one dimensional wkbj synthetic seismograms in the archean p and s wave velocities in the upper crust are 6 0 and 3 4 km s while in the intermediate crust they are 6 4 and 3 6 km s these velocities match for the upper crust a quartz feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite in the ketilidian mobile belt p and s wave velocities are 5 6 and 3 3 km s for the upper crust and 6 3 and 3 6 km s for the intermediate

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

synthetic seismograms for a complex crustal model the algorithm of the original reflectivity method has been vectorized and implemented on a cdc cyber 205 computer calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second mflop the rapid calculation of synthetic seismograms for complex models high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield as an example we model refraction data of the black forest southwest germany and are able to derive rather tight constraints on the physical properties of the lower crust

roots of the labradorian orogen in the grenville province in southeast labrador evidence from marine deep seismic reflection data marine deep seismic reflection data obtained as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe project from offshore southeast labrador are evaluated using geodynamic and gravity models and a tectonic interpretation is developed the geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore as there is little evidence of subsequent severe structural modification apart from strike parallel grenvillian dextral transpression which does not alter the crustal geometry along the line of the seismic transect the seismic reflectors are considered to represent labradorian structures inferred from onshore geochronological data to be mostly related to a 1665 ma labradorian accretionary event the southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward labradorian subduction coast parallel gravity and magnetic anomalies are interpreted as an expression of a neoproterozoic to early phanerozoic extensional basin the existence of which had little effect on the underlying crystalline basement other than to downfault it relative to that exposed onshore

wide angle seismic transect across the torngat orogen northern labrador evidence for a proterozoic crustal root a refraction wide angle reflection seismic transect across the labrador peninsula covers the core zone of the se churchill province the paleoproterozoic torngat orogen and the archean nain province including a portion of the labrador continental margin an airgun array was used as source and 11 ocean bottom seismometers and 16 land stations recorded the shots forward modeling of travel times and amplitudes reveals a deep asymmetric crustal root beneath the torngat orogen with a crustal thickness of 49 km and with p wave velocities of 6 9 7 0 km s the geometry of the velocity model and the root can be best explained by either westward dipping subduction or westward underthrusting of the nain crust gravity modeling suggests a correlation of the crustal root with a gravity low that extends 100 km in an e w direction and 200 km from north to south the preservation of the crustal root is attributed to the lack of postorogenic heating and ductile reworking consistent with the lack of abundant postcollisional magmatism in the se churchill province a discontinuity possibly cutting through the entire crust is interpreted as a zone of major transcurrent shearing associated with the main phase of deformation west of the torngat orogen the crustal thickness in the core zone of the churchill province varies between 35 and 38 km p wave velocities of 5 8 7 0 km s east of the orogen the crystalline crust in the nain province is 38 km thick velocities from 5 8 to 6 9 km s but thins to 9 km in the shelf area of the labrador margin where it is covered with up to 8 km of sediments no high velocity zone was found beneath the thinned continental crust at the margin

the seismic velocity structure of northern appalachian crust around western newfoundland the crustal velocity structure beneath western newfoundland is constrained by a reanalysis of older regional refraction profiles together with an analysis of one new profile initial interpretation of the older data gave inconsistent and discontinuous structures that are difficult to reconcile with more recent deep reflection profiles we also show that traveltimes predicted by the earlier models often do not yield acceptable fits to the original observations our reinterpretation reveals a simpler pattern in which the crust is characterized by a persistent high velocity lower crustal hvlc layer this layer has velocities of 7 0 7 9 kmls and thicknesses of 5 23 km it is thickest beneath the grenville crustal block east of the appalachian structural front and thins or is possibly absent within the central block analysis of the new much higher resolution profile off western newfoundland confirms the existence of the hvlc layer with a velocity of 7 2 kmls and thicknesses of 11 19 km increasing to

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

estimation of ground motion in mexico city from a repeat of the m 7 0 acambay earthquake of 1912 abstract the acambay earthquake of 1912 m 7 0 which occurred in the central trans mexican volcanic belt ctmvb about 100 km west northwest of mexico city has been thought to be a critical scenario event in the estimation of seismic hazard of the city we use seismograms of two small earthquakes located near actopan 15 december 2003 18 may 2010 and recorded at station cuig a hill zone site in mexico city as empirical green s functions egfs for the acambay region because actopan like acambay is situated in the ctmvb and both are located at about the same distance from cuig the use of the recordings of the actopan earthquakes as egfs is partly justified we first analyze data of the two small earthquakes at a local station dhig to estimate their seismic moment and stress drop as there is considerable uncertainty in the estimated stress drops of the two events we choose a reasonable range of values for them and apply a technique of random summation of egfs to simulate ground motion at cuig from a postulated m w 7 earthquake the estimated geometric mean horizontal peak ground acceleration pga and peak ground velocity pgv at cuig range from 2 7 to 9 7 cm s 2 and from 1 1 to 3 0 cm s respectively ground motion maps for the entire city are presented using a simulated trace at cuig and the known transfer functions of many sites within the city the results are consistent with reported seismic intensities in mexico city and pga and peak ground displacement pgd at a 2 s period at the seismic station of tacubaya during the acambay earthquake estimated ground motions suggest that a repeat of the event does not present significant hazard to mexico city

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

tectonic model for crustal seismic reflectivity patterns in compressional orogens a pattern of crustal seismic reflectivity tentatively dated as ordovician silurian recurs along strike in the newfoundland appalachians comparable fault patterns are produced in sandbox models of asymmetric compressional systems where oppositely dipping step up shear zones are localized above a discontinuity in basal velocity the reflectivity pattern may be produced by a mechanically analogous process in which a stress discontinuity results from detachment and underthrusting of mantle and possibly lower crustal lithosphere during con tinent continent collision similar seismic reflectivity patterns exist elsewhere in the appalachian caledonide mobile belt as well as in older and younger compressional orogens suggesting that detachment and underthrusting leading to asymmetric crustal deformation may be a common feature of compressional orogens

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

seismic reflections from the mantle represent relict subduction zones within the continental lithosphere in many areas of the world the continental lithospheric upper mantle contains bright continuous regionally extensive seismic reflectors despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles their significance and geologic origin remain obscure we report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region these new seismic data reveal regionally extensive dipping and subhorizonal slabs of high velocity 8 4 km s high density 3500 kg m 3 material several kilometres 2 km thick entrained within otherwise unremarkable upper mantle the geometry physical properties and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust a relict of pre caledonian oceanic subduction now preserved within the lower continental lithosphere such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

seismic anisotropy in northern and central gulf of california region mexico from teleseismic receiver functions and new evidence of possible plate capture 1 we present anisotropic models for the shallow lithosphere in the northern and central gulf of california obtained from receiver function rf analysis this region has undergone intense plate reorganization over the past 25 ma that led to the transfer of the plate boundary east of the modern baja california peninsula posterior to the cease of the subduction of the farallon plate beneath the north american plate our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary we calculate rfs from teleseismic p waves recorded by the network of autonomously recording seismographs nars baja array and we stack them in 20 wide back azimuthal bins to enhance coherent information these rfs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping moho therefore we interpret the conspicuous rfs energy of the transverse component as convincing evidence of anisotropy we present forward models obtained by fitting the radial and transverse rfs with synthetic seismograms calculated with a reflectivity code our analysis suggests substantial variations in the structure of the shallow lithosphere along the baja california peninsula and the presence of stalled fragments of the extinct farallon plate beneath its central part the results at station ne81 in sonora are interpreted as frozen crustal anisotropy inherited from miocene basin and range extension consistent with previous observations of the upper mantle anisotropy beneath this station

coincident seismic wave velocity and reflectivity properties of the lower crust beneath the appalachian front west of newfoundland we have determined the seismic velocity structure of the crust in the vicinity of the appalachian deformation front off western newfoundland and the adjacent gulf of st lawrence these measurements

structural evolution of the makkovik province labrador canada tectonic processes during 200 myr at a paleoproterozoic active margin in the makkovik province the structures and plutonic rocks of the reworked southern edge of the archean nain craton parautochthon and an adjacent juvenile domain form a spatially concentrated record of 200 ma at a paleoproterozoic active margin d1 thin and thick skinned structures formed during terrane collision before ca 1896 ma were nearly erased by d2 transpressive structures present across the entire parautochthon that had been thermally softened by emplacement of arc plutons at ca 1895 1870 ma d3 transpressive inversion of a back arc rift and d4 strike slip shearing formed structures at the oceanward margin of the parautochthon and were responses to events in the active arc that was now widely separated from the parautochthon by accreted terranes the tectonic isolation of the parautochthon ended by the time of d5 ca 1740 1710 ma and d6 with the reappearance there of granite and low grade shear zones structural and plutonic style varied at each stage primarily controlled by the crustal level attained

anatomy and orogenic history of a paleoproterozoic accretionary belt the makkovik province labrador canada the makkovik province is a segment of a paleoproterozoic accretionary belt the makkovik ketilidian orogen that developed on the southern margin of laurentia at 1 9 1 7 ga in contrast to coeval laurentian orogenic belts that mainly resulted from collision of archean plates makkovikian ketilidian orogenesis was dominated by active margin processes including continental margin arc plutonism and juvenile terrane accretion both of which were accompanied by regional transpression in the makkovik province earliest deformation and amphibolite facies metamorphism of paleoproterozoic rift drift assemblages post hill and moran lake groups and the archean foreland nain province occurred at 1 9 ga in response to accretion of a paleoproterozoic island arc following this collision cratonward dipping subduction was established resulting in the formation of the 1895 1870 ma island harbour bay plutonic suite a calc alkaline magmatic arc built on reworked archean crust crust formation continued between ca 1

ramp flat geometry within the central kapuskasing uplift evidence from potential field modeling results potential field data constrained by seismic reflection seismic refraction geologic and physical properties data have been used to study the deep crustal structure of the central kapuskasing uplift the val rita block is interpreted as a thrust sheet of granulites detached at mid crustal depths and uplifted along a ramp producing an arched double granulite layer a thin thrust sheet of southward thickening granulites best describes the groundhog river block whereas simple thrusting along a ramp characterizes the chapleau block from this interpretation a sequential evolution of the kapuskasing uplift is proposed at about 2450 ma northwest southeast compression 1 resulted in variable thrusting along the ivanhoe lake fault zone this was followed by a period of normal faulting resulting in the juxtaposition of high and low pressure rocks across the lepage and saganash lake faults with possible truncation of the val rita block arch further rotation of 1 to an east west direction resulted in the

geochemistry and age of the aillik group and associated plutonic rocks makkovik bay area labrador implications for tectonic development of the makkovik province the aillik domain of the makkovik province is dominated by deformed and metamorphosed sedimentary and bimodal volcanic rocks of the redefined aillik group and abundant unfoliated late to post orog

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

tomography and thermal state of the cocos plate subduction beneath mexico city 1 the geometry and thermal state of the subducting cocos plate beneath mexico city has been enigmatic because of the absence of a deep wadati benioff zone we present a tomographic image of the slab based on inversion of 8869 teleseismic p wave travel times measured on a portable broadband seismic network the images combined with receiver function analysis show that the slab runs flat from the coast to near mexico city where it dives into the mantle just before the trans mexican volcanic belt with a dip of 75 it continues down to a depth of 500 km at a distance of 400 km from the trench where the tomography reveals that the dipping portion ends as well as standard block tomography we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the 15 ma age of the cocos plate the combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older thicker nontruncated plate

geology and geochemistry of the mendejin plutonicrocks mianeh iran abstract the mendejin pluton is located in the mianeh area nw iran 550 km from tehran this pluton is probably of oligo miocene age and is the result of extensive magmatism which occurred during and after the alpine orogeny similar plutons are common in the alborz azarbaijan structural zone of iran and it is likely that there are concealed plutons related to this extensive cenozoic magmatism but due to their youth and low rates of erosion they have not yet been exposed the mendejin pluton is a composite body made up of four types of plutonic rocks pink tonalite grey tonalite diorite and aplite the pink tonalite is porphyritic and contains phenocrysts of plagioclase k feldspar and hornblende in a groundmass consisting of quartz plagioclase k feldspar hornblende zircon monazite leucoxene apatite and hematite the grey porphyritic tonalite has more biotite pyroxene and pyrite and less accessory phases compared with the pink tonalite the diorite has a microporphyritic texture with phenocrysts of plagioclase hornblende and augite this rock also occurs as xenoliths in the mendejin pluton the aplitic dykes are the youngest magmatic products at mendejin the mendejin tonalite contains more cl as s cu ni and zn than the global granite these rocks are of i type peraluminous and calc alkaline with medium to high potassium and were formed as part of a volcanic arc the mendejin pluton contains up to 8 ppb gold and could potentially have been the source of an economic gold deposit by leaching of au from wall rocks and deposition in extensive hydrothermally altered marginal zones

moho depth and poisson s ratio in the western central alps from receiver functions summary current knowledge about deep crustal structure of the alpine orogen has mainly been derived from p wave velocity models obtained from active and passive seismic experiments a complementary s wave model to provide lithological constraints necessary for unique structural interpretation has been missing to date in this paper we present important new information on s wave velocity model in the alps we applied the receiver function method using 6 yr of high quality data from 61 permanent and temporary stations sampling the western central alps we determined first order crustal features moho depth h and average vp vs ratio with the h stacking technique that uses timing of direct and multiple p to s converted phases from the moho interface synthetic tests reveal a dipping moho interface expected beneath an orogen causes a systematic bias of h and potentially leading to misinterpretation we thus applied corrections determined from synthetic data to remove the bias providing better fit of recovered moho depths with active seismic estimates for each site we also obtained independent h and estimates based on the timing of the strong ps phase our results show a gently south southeast dipping european moho at a depth of 24 30 km beneath the northern alpine foreland steepening rapidly towards the europe africa suture zone to reach a maximum depth of 55 km south of the suture the moho of the adriatic crust promontory of the african plate is at 35 45 km depth in the previously ill constrained western alps we found the european moho at 30 km depth beneath the more external units dipping east northeast to reach 50 55 km in the inner core of the alps the poisson s ratio clearly correlates with the tectonic units that comprise the alps average crustal values in the european alpine foreland are close to 0 25 in the alps we observe low values 0 22 in the highly deformed nappes of the mesozoic helvetic and southern alps indicating a thickening of felsic upper crustal material in contrast the poisson s ratio is significantly higher 0 26 in the penninic and austroalpine units near the suture zone this rapid and significant change marks a clear rupture between the alpine forelands and the suture domain we assign this high poisson s ratio to doubling of mafic lower crust consistent with results from previous active seismic experiments a continuation of the lower crustal wedge into the central part of the western alps however seems unlikely based on low observed poisson s ratios

the double mer formation and the lake melville rift system eastern labrador the double mer formation is a sequence of redbed arkosic sandstone conglomerate siltstone and shale in the grenville province of eastern labrador the formation is closely associated with a rift system extending inland for at least 300 km from the labrador coast substantial thicknesses of strata of double mer formation are confined to two basins the lake melville graben and the double mer half graben strata correlated with the double mer formation are found elsewhere in southeast labrador one isolated outcrop occurs within the sandwich bay graben a separate smaller basin 100 km to the southeast that trends parallel to the lake melville graben the only definite age constraint on the double mer formation is that it postdates grenvillian deformation previously the formation was assigned a latest precambrian to early cambrian age based on lithological similarity with the lower cambrian bradore formation of the southeast coast of labrador on the basis of a structural model proposed here that sugges

proterozoic metamorphism in the grenville province a study in the double mer lake melville area eastern labrador a regional metamorphic gradient from upper greenschist to granulite facies is identified south of the grenville front in the double mer lake melville area of eastern labrador mineral assemblages in politic granitic gneiss amphibole bearing quartzo feldspathic gneiss and coronitic metagabbro allow three major metamorphic domains to be established these are collectively divisible into 11 subdomains geothermobarometry applied to the higher grade domains suggests that each is characterized by specific p t conditions which achieved 1000 1100 mpa and 700 800 c in the deepest level rocks the problem of reconciling geochronological data which record a major orogenic event at 1650 ma with the occurrence of high grade mineral assemblages in 1426 ma metagabbro which suggests a pervasive grenvillian event is discussed in terms of three models the preferred model envisages crustal stabilization at 1650 1600 ma to give high grade mineral assemblages seen in the host rocks and with which mineral assemblage

nd isotope evidence for crust mantle interaction in the generation of a type granitoid suites in labrador canada early proterozoic a type granite suites in labrador northeastern canada have initial nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks one of these a type granite suites consists of a linear array of discrete plutons that crosses the boundary between an archean craton and an adjoining early proterozoic crustal block the nd of these granites at formation t 1720 ma changes from 8 to 4 across this boundary reflecting changes in the nature and antiquity of the continental crust however in both domains the granites cannot be derived entirely by anatexis of the local basement rocks and apparently require significant amounts of juvenile mantle derived material a type granites have been widely interpreted as anatectic derivatives of lower crustal rocks that were depleted by previous melt extraction the nd isotopic data suggest that these labrador examples were instead mixtures of mantle derived magmas and overlying lower crustal rocks and that new material may have been the volumetrically dominant component 50 90 in most cases as an alternative to the prevalent residual source model it is proposed that the distinctive geochemical characteristics of these a type granites are a function of the high liquidus temperatures and primitive mg rich character of subcrustal magma fluxes in post orogenic and anorogenic environments where such magmas are emplaced into stabilized continental crust further nd isotope studies of other a type granite suites are required to ascertain if models invoking crust mantle interaction are more widely applicable

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

tomographic search for missing link between the ancient farallon subduction and the present cocos subduction summary a striking feature of the tomographic images of the earth s mid mantle is the long high velocity belt extending in a north south direction under the north and south american continents which is believed to be the remnant subduction of the farallon plate in the oligocene epoch the north farallon plate subduction terminated off baja california and the south farallon plate broke into the cocos and nazca plates this important period of the farallon subduction history is not clearly understood due in part to the lack of high resolution tomographic images our p wave tomographic image of the mantle below mexico indicates that the currently subducting slab of the cocos plate is torn apart from the already subducted slab of the ancient farallon plate in a region behind the slab window or slab gap off baja california we suggest that the southeastward advance of this slab tearing was synchronous with the counter clockwise rotation of the cocos plate against the eastward to northeastward subduction of the ancient farallon plate the cocos slab torn apart from the deeper farallon slab dips to the north to northeast with its strike oblique to the trench axis this slab configuration delineates well with the intermediate depth earthquake activity and the volcanic activity known as the trans mexican volcanic belt further to the south of the slab tearing of the subducted slab continues from the deeper farallon part to the shallower cocos part but with considerable distortion in the shallower part

gold in birrimian greenstone belts of burkina faso west africa the early proterozoic birrimian greenstone belts of boromo and hound burkina faso were studied by the geology and mining projects of the united nations development program in burkina faso the southern extremities of both belts are north northeasterly trending but farther north in the center of the country the trend changes to northeasterly the belts consist of steeply dipping volcanic and volcano sedimentary units intruded by eburnean granitoids ca 2 000 m y all of which have undergone incipient to low greenschist facies metamorphism many former gold prospects and mines are known from both greenstone belts including the poura vein gold deposit the dossi shear zone prospects the silicified gold bearing zone of kiere and the gold and scheelite bearing quartz veins of bagassi in addition more recently gold anomalies have been found at kwademen zoulou semapoum koupela nagsene and koudma most of the primary gold occurrences were considered to be of metamorphogenic origin having been derived from auriferous source rocks of the older volcanic and volcano sedimentary units studies were made of the distribution of gold in birrimian basalts andesites rhyodacites or rhyolites and exhalites and in eburnean granites and granodiorites samples weighing 1 5 to 2 0 kg were collected from 109 bed rock exposures of these rock types from beyond the alteration areas and zones of favorable gold mineralization these were then analyzed by atomic absorption spectrometry to determine the background gold content of the unmineralized rocks of the greenstone belts copper nickel and zinc contents of the birrimian volcanic rocks and of the eburnean granitoids were also determined the analytical data showed that gold was more abundant in the birrimian volcanic rocks and exhalites than in the eburnean granites and granodiorites furthermore the gold content was shown to increase from basalts to andesites to rhyodacites to exhalites among different types of exhalites including chert and tuffaceous exhalites rich in manganese and iron and in quartzite the gold content varied from 0 1 to 9 7 ppb mean 2 97 ppb because of the petrographic variability of these rocks further detailed studies of their gold contents are required the gold contents of rocks surrounding zones of confirmed gold mineralization were generally higher than elsewhere ranging from 3 to 10 ppb except in the mineralized zones themselves where it exceeded 10 ppb

crustal structures of grenville makkovik and southern nain provinces along the lithoprobe ecsoot transect regional seismic refraction and gravity models and their tectonic implications crustal structures of the eastern grenville makkovik and southern nain provinces are determined using seismic reflection refraction and gravity data along the lithoprobe eastern canadian shield onshore offshore transect ecsoot within the grenville province the velocity model contains a 5 km thick upper crust and a variable thickness middle to lower crust the total crustal thickness varies from 25 to 43 km with the thickest crust in the south and thinnest crust in the north a high velocity lower crustal wedge is coincident with a strong band of northward dipping reflectors the two dimensional velocity structure is compatible with modelling of a 60 mgal gravity high over the hawke river terrane in the makkovik province the thickness of upper crustal velocities increases to 17 km the velocity decrease in the upper to middle crust from the grenville province to the makkovik province is similar to that of refraction models across the grenville front in ontario and quebec it is possibly related t

size of popocatepetl volcano explosions 1997 2001 from waveform inversion several volcanic explosions have been recorded since april 1997 at broadband seismic stations located around the popocatepetl volcano mexico we have inverted waveforms of ten of these explosions to estimate the following source parameters depth duration magnitude and direction of the single force f the crustal structure used in generating green s function at nearest stations is derived from the inversion of teleseismic receiver functions at the broadband permanent station ppig located 5 km north of the volcano this inversion reveals a low velocity zone at 8 km beneath the summit with high poisson ratio possibly related to the magma chamber we find that f scales with the duration of the source time function as f tau based on this relationship we determine an impulse magnitude scale mk this magnitude is tied to the mount saint helens initial explosive phase of may 18 1980 whose magnitude is estimated as 4 6 mk of the ten popocatepetl explosions ranges between 1 8 and 3 2 finally we also propose an equivalent formula for rapid estimation of magnitude of future popocatepetl explosions which requires filtered amplitudes at ppig

seismic imaging of the cocos plate subduction zone system in central mexico broadband data from the meso america subduction experiment mase line in central mexico were used to image the subducted cocos plate and the overriding continental lithosphere beneath central mexico using a generalized radon transform based migration our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental north america the converted and reverberated phase image shows complete horizontal tectonic underplating of the cocos oceanic lithosphere beneath the north american continental lithosphere with a clear image of a very thin low velocity oceanic crust 7 8 km which dips at 15 20 degrees at acapulco then flattens approximately 300 km from the middle america trench farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the trans mexican volcanic belt tmvb where the slab bends underneath the tmvb the migrated image depicts the transition from subducted oceanic moho to continental moho at 230 km from the coast neither of which were clearly resolved in previous seismic images the deeper seismic structure beneath the tmvb shows a prominent negative discontinuity fast to slow at 65 75 km within the upper mantle this feature which spans horizontally beneath the arc 100 km may delineate the top of a layer of ponded partial melt

geophysical anomalies along strike of the southern appalachian piedmont bouguer gravity anomalies and aeromagnetic anomalies display a northward increase of values in the charlotte and carolina slate belts from georgia to virginia analysis of these data with constraints imposed by seismic reflection seismic refraction conductivity measurements and surface geology imply that this increase is due to a corresponding increase in the percentage of mafic rocks in the upper crust a regional trend in the bouguer field is determined for an observed eastward decrease in crustal thickness based on seismic refraction measurements a comparison of the calculated regional with surface boundaries shows that in places the crustal thinning occurs more than 50 kilometers west of the inner piedmont charlotte belt transition the comparison also shows that the crustal thinning may take place west of exposed grenville basement the evidence thus indicates that the southern appalachian gravity gradient is not likely related to a crustal or lithospheric suture the evidence further indicates that because the upper crustal and deep crust upper mantle variations are independent in terms of spatial position the upper crust of the charlotte slate belts is decoupled from the lower crust perhaps at subhorizontal boundaries seen on reflection data

crustal structure across the nain makkovik boundary on the continental shelf off labrador from seismic refraction data a detailed seismic refraction profile was shot along the continental shelf off labrador across the boundary between the archean nain province to the north and the proterozoic makkovik orogenic zone to the south a large air gun source was used with five ocean bottom seismometers as receivers the data were analysed by forward modelling of traveltimes and amplitudes and provided a well determined seismic velocity structure of the crust along the profile within the nain province thin postrift sediments are underlain by crust with a p wave velocity of 6 1 km s which increases with depth and reaches 6 6 km s at about 8 km moho is at around 28 km and there is no evidence for a high velocity 7 km s lower crust the p and s wave velocity structure is consistent with a gneissic composition for the archean upper crust and with granulites becoming gradually more mafic with depth for the intermediate and lower crust in the makkovik zone the sediments are thicker and a basement layer of p wave velocity

lithoprobe onshore seismic reflection transects across the newfoundland appalachians vibroseis seismic reflection data have been recorded to 18 s two way traveltime along three transects across the island of newfoundland the upper crust has both steep and subhorizontal reflectors consistent with a ramp flat style of deformation whereas the middle and lower crust are largely free of regional flats reflectors descend through ca 20 km of vertical section in the middle and lower crust to flatten into the moho or perhaps cut through it in places the moho is interpreted to be no younger than the dipping reflectors reflection fabrics interpreted to be indicators of dominantly mid ordovician to mid silurian strain show consistent orientations among the transects and divide the crust into two blocks a northwestern block is characterized by upper and middle crustal reflectors dipping mostly southeast at variable angles this block is underlain to the southeast by supposedly younger and dominantly northwesterly dipping reflectors that define a northwest tapering wedge shaped block floored

proterozoic orogens of the northeastern canadian shield new information from the lithoprobe ecsoot crustal reflection seismic survey as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe acquired 1250 km of deep seismic reflection data along the coast of labrador and across ungava bay to image evidence of proterozoic crustal accretion to the archean nuclei of the nain and superior provinces of the canadian shield the relatively pristine archean crust of the nain province has low reflectivity and generally lacks systematic reflector orientations reworking of archean crust on the margins of the makkovik province has little effect on this weak signature in contrast the archean crust in the eastern churchill rae province appears to have been overprinted by a strongly developed whole crustal easterly dipping reflection fabric interpreted to result from proterozoic collision of the nain and superior provinces in the paired new quebec and torngat orogens juvenile proterozoic crust in the makkovik and grenville provinces also shows strong whole crustal dipping reflection fabrics interpretable as outw

the seismic velocity structure of the newfoundland appalachian orogen the deep structure of the newfoundland appalachian orogen is investigated by analyses of three intersecting seismic refraction wide angle reflection profiles which traverse the gander and dunnage zones or central mobile belt of newfoundland a simultaneous travel time inversion for velocity and interface was applied to the in line seismic refraction wide angle reflection data and constrained by synthetic amplitude models the results of the modeling procedure show a subhorizontally layered crust with upper crustal velocities ranging from 5 4 to 6 2 km s a midcrustal velocity of 6 25 6 35 km s and a lower crustal velocity of 6 7 0 2 km s the top of the lower crust is marked by a series of prominent reflections between 18 and 23 km depth which suggest a complex layered velocity interface strong laterally coherent moho reflections indicate a sharp crust mantle transition at 35 3 km the uppermost mantle has a velocity of 8 0 0 2 km s and a reflecting horizon at 55 km depth suggests an increase to velocities approaching 8 5 km s normal moveout corrections applied to fan profiles provide constraining evidence for the reflecting horizon at the top of the lower crust and laterally continuous moho reflections at 11 12 s two way travel time comparisons with a coincident deep seismic reflection profile show that the refraction and reflection mohos match to better than 2 3 km bulk poisson s ratios of 0 23 0 24 for the whole crust calculated from pmp sms travel times suggest a crust dominated by quartzofeldspathic lithologies and a notable absence of voluminous mafic additions to the lower crust the absence of a deep crustal root coupled with the bulk intermediate composition inferred for the lower crust from the seismic refraction wide angle data implies that the crust beneath central newfoundland has undergone multiple periods of reactivation and equilibration following successive orogenic episodes

the moho of north america a brief review focused on recent studies abstract recent national programs in canada lithoprobe and the u s earthscope are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution more modest but significant results are also being produced in and about mexico this paper primarily focuses on results published since 2005 and presents an updated moho map of north america this overview of crustal structure does reveal a number of interesting observations for example two buried distinct crustal blocks have been discovered in the past 20 years one was caught in a precambrian suture zone in canada and the other is buried under the gulf coast region and is still of unknown origin the precambrian assembly of the continent has been documented by the mapping of numerous suture zones and shallow portions of subducted slabs have been preserved in several cases not surprisingly the internal structure of these suture zones is complex and highly variable isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations specifically the correlation of crustal thickness with elevation is only approximate at best several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast vp 7 0 km s rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the mid continent rift system to the broad region of extension in the basin and range province crustal structure along the active modern western margin of north america is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism we clearly still have a lot to learn about the structure and evolution of this continent

a fluid inclusion study of au bearing quartz vein systems in the central and north deborah deposits of the bendigo gold field central victoria australia the central and north deborah gold deposits with a production of about 7 tons au are located in one of 15 domes in the bendigo gold field central victoria australia an important mesothermal lode gold province the gold bearing quartz vein systems are hosted predominantly by reverse faults in lower ordovician turbidites which underwent lower greenschist facies metamorphism in the central and north deborah mines there are six stages of quartz veins that developed in the following sequence 1 laminated veins with a metal association of au as sb pb zn ni 2 spurs au 3 massive barren veins main dilation stage 4 brecciated veins with au as 5 quartz ankerite veinlets and 6 late pure quartz or calcite veinlets of the three mineralized vein stages 1 2 and 4 only the first one predated or was coeval with the main deformation event whereas stages 2 and 4 postdated the main deformation the vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing fluid inclusions in vein quartz contain c o h fluids of variable compositions three main types of fluid inclusions are recognized at room temperature type i two phase primary ch4 v h2o l fluid inclusions type ii two or three phase primary pseudosecondary co2 v or l ch4 v or l h2o l fluid inclusions including iia ch4 co2 h2o and iib co2 h2o fluid inclusions and type iii h2o v h2o l fluid inclusions type iii fluid inclusions are further divided into three groups iiia primary pseudosecondary vapor rich iiib primary pseudosecondary liquid rich and iiic secondary liquid rich data from fluid inclusion distribution microthermometry and raman spectroscopy indicate that fluids associated with au mineralized quartz veins stages 1 2 and 4 have moderate salinity ranging from 0 to 12 wt percent nacl equiv modeled salinities around 7 8 wt nacl equiv these veins formatted at temperatures from 325 to 375 c and pressures of 200 to 300 mpa with emplacement depths of about 8 to 12 km fluids associated with barren quartz veins stages 3 5 and 6 have a low salinity of about 0 to 3 wt percent nacl equiv modeled salinities about 2 3 wt nacl equiv and lower temperatures there is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce but differing fo2 may also have influenced gold deposition cox et al 1995 hydrothermal ore forming fluids responsible for gold mineralization are in the ch2 co2 h2o nacl system whereas those of massive barren veins stage 3 and veinlets stages 5 and 6 are in the co2 h2o nacl and h2o nacl systems respectively the carbonic phases of type i and ii fluid inclusions range from pure ch4 through mixed ch4 and co2 to pure co2 gold ore forming fluids were weakly acidic with f o2 estimated to have been 10 25 to 10 37 bars and generally above the co2 ch4 and below the so2 h2s buffer boundaries throughout gold mineralization the relatively reduced mineralizing fluids are similar to those of other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes

analytical estimation of short period ground motions in mexico city for engineering applications in previous studies short period 0 2 5 0 s ground motions in mexico city were investigated by a systematic analysis of surface and downhole accelerograms and it was found that short period love waves were very dominant at the predominant periods of grounds although the average vertical amplification of ground motions with long duration was interpreted fairly well at soft soil sites the large time variations in the vertical amplification remain unexplained the present study addresses two issues on the vertical amplification at two soft soil sites which are important for engineering applications the first issue is the interpretation of the large time variations in the vertical amplification of ground motions and the influence of the depth of the structure in which surface waves are trapped is examined the second issue is the interpretation of inertial force excitation which is usually used in the engineering community in terms of seismic waves the second issue is also estimated by the vertical amplification the conclusions are as follows 1 the large time variations in the vertical amplification of ground motions can be explained sufficiently by both the mixture of s waves and love waves and the depth of the structure in which love waves are trapped the structure corresponds to the mexico city basin 2 although inertial force excitation is considered to be valid for s waves it proves to be invalid for love waves

s wave velocity structure below central mexico using high resolution surface wave tomography shear wave velocity of the crust below central mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense 500 km long linear seismic network vertical components of regional records from 90 well located earthquakes were used to compute rayleigh wave group velocity dispersion curves a tomographic inversion with high resolution in a zone close to the array obtained for periods between 5 and 50 s reveals significant differences relative to a reference model especially at larger periods 30 s a 2 d s wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions the results show large differences especially in the lower crust among back arc volcanic arc and fore arc regions they also show a well resolved low velocity zone just below the active part of the trans mexican volcanic belt tmvb suggesting the presence of a mantle wedge low densities in the back arc inferred from the low shear wave velocities can provide isostatic support for the tmvb

evidence of low q below popocatepetl volcano and its implication to seismic hazard in mexico city seismograms recorded in mexico city reveal that the amplitudes of seismic waves whose wavepaths pass below popocatepetl presently an volcano before reaching the city are diminished by a factor of about one third at frequencies greater than 1 hz as compared to those which do not cross the volcano the high attenuation of seismic waves below the volcano may be attributed to the presence of magma and partial melting of rocks q of shear waves below the volcano is roughly estimated as 60 a consequence of the large attenuation of high frequency seismic waves is a decrease in the seismic hazard to low rise buildings in mexico city from intraplate earthquakes whose wavepaths cross the volcano

destructive strong ground motion in mexico city source path and site effects during great 1985 michoacan earthquake simultaneous consideration of source path and site effects on ground motion during the michoacan earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in mexico city by the rupture process the mode of propagation of the waves between the epicentral zone and mexico city and the local amplification in contrast to the horizontal component which showed dramatic amplification for the 2 to 3 sec motion at lake sediment sites we observe almost identical vertical displacement seismograms containing ripples with 2 to 3 sec period throughout the mexico city valley whether the recording site is on the lake sediments or on hard rock we therefore conclude that the 2 to 3 sec motion responsible for the destruction of mexico city was present in the incident field after performing a phase analysis we interpret the signal as the superposition of long period rayleigh waves and short period lg with a dominant period of about 3 sec the analysis of the teleseismic records indicates that the radiation of this event is enhanced for waves around the 3 sec period except in the case of stations for which an anomalous path effect is suspected the records present ripples appearing a few seconds after the beginning of the signal the characteristics of near fault records show that the rupture process consists of the growth of a smooth crack the numerical simulation indicates that the 3 sec period ripples can be explained by a series of changes of the rupture front velocity we examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in mexico city our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in mexico city this interpretation leads to a very coherent scenario of what happened from the start of the failure on the fault up to the destruction in mexico city this example illustrates the need to consider simultaneously source path and site effects in order to understand strong ground motions

a collisional crustal fabric pattern recognised from seismic reflection profiles of the appalachian caledonide orogen abstract deep seismic reflection profiles across the appalachians and across the once contiguous caledonides around britain and ireland show a broadly similar pattern of crustal reflection fabrics the pattern is of two adjacent fabric domains each characterized by dips towards the other bounded by an interface which dips in conformity with one of the fabrics the domain interface strikes parallel to the orogen suggesting a genetic relationship between the fabrics and the orogen from the north sea to the long island platform the domain boundary dips to the northwest in the southern appalachians the pattern appears with less consistency between the charlotte and carolina slate belts the pattern appears with the same polarity as further north but below the coastal plain it occurs with opposite polarity the interface dips to the southeast an attractive but not exclusive hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

crustal structure beneath the eastern part of the coast ranges diablo range of central california from explosion seismic and near earthquake data abstract seismic refraction and near earthquake data of the u s geological survey for central california have been compiled into record sections along profiles and interpreted in terms of crustal structure the profiles are located northeast of the san andreas fault of central california and run parallel to the general structures for the explosion seismic line through the centre of the diablo range an uppermost layer franciscan formation with p velocities of 3 6 5 0 km s 1 decreases in thickness towards the northwest the lower boundaries of layers with constant velocities of 5 75 and 6 8 km s 1 are found at almost constant depths of 12 and 21 km respectively between 21 and 26 km depth a well defined low velocity zone appears whose velocity is estimated as 5 3 km s 1 with the aid of a hedgehog inversion and the calculation of amplitudes this zone is underlain by a layer 3 5 km thick with a velocity of 7 6 km s 1 the upper mantle velocity beneath the moho at 29 30 km depth is 8 2 km s 1 the near earthquake profiles located 20 km southwest and parallel to the explosion seismic line follow more or less the hayward and calaveras fault systems the velocity depth distribution derived for the earthquake data is very similar to that found beneath the diablo range however the low velocity zone at 21 26 km depth does not seem to exist everywhere along the line the moho is not disturbed beneath the calaveras hayward and silver creek faults it rises slightly from the diablo range towards the southwest

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

crustal structure deduced from receiver functions via single scattering migration an investigation of the teleseismic p wave coda is performed using the single scattering approximation the method allows one to image short wavelength scale 2 km velocity and density heterogeneities and structures that are barely detected by traveltime tomography source effects are removed by using receiver functions for data interpretation but the amplitude especially the sign of the signal is synthesized both broad band seismological stations in the southwestern alps france and campanian plain italy are used for the illustration of the proposed method because of the large aperture of our arrays laterally small scale heterogeneities are difficult to image and we must assume lateral continuity of the detectable structure if so we show that this depth migration based on a single scattering approach can recover both the depth and the geometry of the main discontinuities below the studied areas in the southwestern alps we underline a complex crustal structure and moho dipping topography the moho depth increases from 20 to 30 km between the coastline and the mercantour range in the campanian plain surrounding mount vesuvius we also find a southeastwards dipping moho furthermore the three main discontinuities are imaged showing a relatively shallow mantle crust discontinuity and a deeper one near the coastline

crustal structure below the valley of mexico estimated from receiver functions abstract receiver functions rfs from two permanent stations cuig and zaig of the mexican broadband network were calculated to infer the crustal structure at both sites the data were stacked to allow uncertainty estimations and inverted using two global optimization algorithms simulated annealing and genetic algorithms strong and systematic azimuthal dependence was observed in the stacked functions at cuig site which is located in the mexican volcanic belt mvb assuming a one dimensional five layer model per azimuthal stack these variations were explained with the same upper crustal structure including a superficial 2 km thick low velocity layer associated with the mvb and a well defined conrad interface around 15 km depth different depths for the moho discontinuity and variations in both the s wave velocity and poisson s ratio for the upper mantle our inversion of observed rfs at cuig and theoretical considerations suggest an eastward crustal thickening of at least 5 km below the valley of mexico we tested the sensitivity of the modeling procedure by applying it to the recordings at zaig station in this case no azimuthal dependence was noticed in the stacked functions after a simultaneous inversion of three different azimuthal stacks a single four layer model satisfactorily explained all data thus revealing a flat one dimensional crustal model below the mexican mesa central where zaig station is located these results support the resolution and robustness of the applied method at both sites

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

some aspects of source characteristics of the 19 september 1985 michoacan earthquake and ground motion amplification in and near mexico city from strong motion data abstract the fourier amplitude ratio of the 19 september m s 8 1 to the 21 september 1985 m s 7 6 michoacan earthquakes obtained from data in and near mexico city increases from about 3 5 at 0 5 hz to about 10 at 0 2 hz this increase is neither seen in the coastal data nor in the teleseismic broadband gdsn p wave spectra it is also not predicted by 2 or gusev9s scaling laws we tentatively attribute the increase at frequencies of less than 0 5 hz to a special path and or a depth of energy release effect for the 19 september earthquake the damage to mexico city is mostly related to 0 5 hz energy since at f 0 5 hz the fourier amplitude ratios in and near mexico city along the coast and at teleseismic distances are nearly equal we attribute this energy to the source characteristics of the 19 september earthquake the evidence suggests that the 19 september was about twice more energetic at f 0 5 hz than expected from scaling laws and from spectral regression of coastal earthquakes recorded in mexico city the analysis of strong motion data recorded in mexico city shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in ciudad universitaria the frequency at which the maximum relative amplification occurs varies from site to site and lies between 0 2 and 0 7 hz the data suggest that f o 4 h where f o frequency at which maximum relative amplitude occurs and and h are the shear wave velocity and the thickness of the clay layer respectively relatively small if any nonlinear behavior of the clay is found at strains of about 0 2 per cent during the great 19 september earthquake for a given lake bed site maximum relative amplitude and f o are roughly constant independent of magnitude depth and azimuth of sources located more than about 200 km ground motion at lake bed sites in mexico city with f o 0 5 hz appear to be amplified 75 times with respect to hard rock coastal sites at equal distances from the source this is also confirmed by m l calculation of the 19 and 21 september earthquakes for the 19 and 21 september earthquakes the average values of m l at hard rock coastal sites 100 distance 250 km are 7 4 and 6 7 whereas they are 8 7 and 8 4 in the lake bed zone distance 300 km respectively

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

crustal evolution in eastern labrador constraints from precise u pb ages abstract new u pb ages for rocks of eastern labrador complement previous u pb dates for the grenville and makkovik provinces and substantiate the following conclusions eastern labrador consists of two distinct crustal blocks that are different in age as well as magmatic metamorphic and structural overprint the northern block includes archaean to lower proterozoic basement rocks 1900 ma and two distinct episodes of rhyolitic volcanism at 1860 and 1807 ma high grade metamorphism deformation and anatexis affected this block between 1860 and 1760 ma ago makkovikian orogeny the southern block comprises the trans labrador batholith and the various terranes of the grenville province in contrast to the northern region the southern block shows no proof for the existence of continental crust older than 1710 ma this suggests that large amounts of juvenile material were added between 1710 and 1630 ma ago as dated by a large series of precise rock ages at least towards its end this crust forming event was associated with high grade metamorphism deformation and anatexis labradorian orogeny the 1710 1630 ma period of activity was followed by three distinct phases of anorogenic plutonism causing the emplacement of pegmatites 1500 ma a widespread suite of cotonitic metagabbros michael gabbro 1426 ma and granitic syenitic bodies 1296 ma during the grenvillian orogeny the region achieved its present structural configuration including the emplacement of granitoid plutons the structurally highest regions the mealy mountains and the hawke river terranes were thrust over the lake melville terrane and part of the groswater bay terrane and they were only marginally affected by grenvillian metamorphism the more deeply buried lake melville and groswater bay terranes however were significantly modified during tectonothermal events at 1030 ma and 980 960 ma respectively