zircon u pb ages of the metamorphic supracrustal rocks of the xinghuadukou group and granitic complexes in the argun massif of the northern great hinggan range ne china and their tectonic implications abstract the basement of the argun massif in the northern great hinggan range consists of the metamorphic supracrustal rocks of the xinghuadukou group and associated granitic complexes the metamorphic supracrustal rocks were previously interpreted as paleoproterozoic while the granitic complexes were considered mesoproterozoic in age this paper presents new zircon la mc icp ms u pb ages of biotite plagioclase leptynite and biotite schist from the xinghuadukou group in the lulin forest area heilongjiang province zircon shrimp u pb ages of biotite plagioclase gneiss from the xinghuadukou group in lulin forest and quartz diorite and monzogranite from the granitic complexes in mohe county heilongjiang province new geochronological data from the three metasedimentary rock samples of the xinghuadukou group can be preliminarily divided into five groups 1 2017 2765 ma 2 1736 1942 ma 3 1359 1610 ma 4 749 1239 ma and 5 448 716 ma except for the zircons of the 448 716 ma group belonging to a metamorphic origin the other age groups had the youngest age of 749 17 ma indicating that the xinghuadukou group formed during the neoproterozoic era at least

mineralogy and geochemistry of devonian ultramafic minor intrusions of the southern kola peninsula russia implications for the petrogenesis of kimberlites and melilitites minor magmatic intrusions of kimberlite melilitite and cpx melilitite occur in the southern part of the kola peninsula russia on the terskii coast and near the town of kandalaksha they yield k ar ages of 382 14 ma and 365 16 ma similar to the magmatic rocks from the kola alkaline province the terskii coast kimberlites have mineralogical and geochemical affinities with group 1 kimberlites whereas the kandalaksha monticellite kimberlite more closely resembles calcite kimberlites the lower al2o3 content in the kola kimberlites indicates a strongly depleted harzburgitic source while higher al2o3 in the melilitites suggests a lherzolitic source the terskii coast kimberlites are anomalously potassic and significantly enriched in p and ba compared to other group 1 kimberlites in contrast the melilitites are sodic and are anomalously depleted in p compared to worldwide melilitites trace element patterns of the kola kimberlites and melilitites indicate the presence of k and p rich phases in the mantle source to account for the k troughs shown by both magma types a k rich phase such as phlogopite is thought to be residual in their sources however the anomalous k enrichment in the terskii coast kimberlites may indicate that an additional metasomatic k rich phase e g k richterite and or a complex k ba phosphate existed in the kimberlite source the p depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual in the melilititic source however anomalous p enrichment in the kimberlites cannot be explained by complete melting of the same phase because the kimberlites are a smaller degree melt thus it is most likely that another metasomatic phosphate mineral existed in the source of the kimberlites the kola kimberlites and melilitites are all strongly lree enriched but the kimberlites have a steeper ree pattern and are significantly more depleted in hree indicating a higher proportion of garnet in their source higher nb y ratios and lower sio2 values in the kimberlites indicate that they were a smaller degree partial melt than the melilitites the presence of diamonds in the terskii coast kimberlites indicates a relatively deep origin while the melilitites originated from shallower depth the non diamondiferous kandalaksha monticellite kimberlite has lower abundances of all incompatible trace elements suggesting a higher degree of partial melting and or a less enriched and shallower source than the terskii coast kimberlites the 87sr 86sri 143nd 144ndi and pb isotope compositions confirm that the terskii coast kimberlites have close affinities with group 1 kimberlites and were derived from an asthenospheric mantle source while the kandalaksha monticellite kimberlite and terskii coast melilitites were derived from lithospheric mantle impact of a devonian asthenospheric mantle plume on the base of the archaean proterozoic lithosphere of the kola peninsula caused widespread emplacement of kimberlites melilitites ultramafic lamprophyres and other more fractionated alkaline magmas the nature of the mantle affected by metasomatism associated with the plume and in particular the depth of melting and the stability of the metasomatic phases gave rise to the observed differences between kimberlites and the related melilitites and other magmas

the significance of paleoproterozoic zircon in carbonatite dikes associated with the bayan obo ree nb fe deposit sensitive high resolution secondary ion mass spectrometry shrimp ii laser ablation inductively coupled plasma mass spectrometry icp ms and laser raman spectroscopy have been used to determine the ages in zircons from carbonatite dikes associated with the bayan obo giant ree nb fe deposit inner mongolia china analyses of small amounts of zircon extracted from large samples of three carbonatite dikes yield late palaeoproterozoic 207pb 206pb ages of 1894 27 1944 20 and 1956 9 ma one sample also contained inherited grains with ages up to 2 55 ga zircon grains even from a single rock have a wide range of ree patterns suggesting that they are probably inherited zircons trapped during magma genesis not zircons crystallized from a single magma none of the zircon grains has the extremely high th u characteristic of the bayan obo ore deposit or of the associated carbonatites further mineral inclusions in the zircon identified by raman spectroscopy are all silicate minerals quartz and feldspars consistent with crystallization of the zircon from silicate not from carbonate magmas the palaeoproterozoic zircons in the bayan obo carbonatite dikes did not crystallize from carbonatite magma at the time of dike emplacement but were derived from wall rock contamination the ages obtained from the zircons are consistent with ages measured on basement igneous rocks elsewhere in the bayan obo region

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

the saamian of the belomorian mobile belt new geochronological constraints abstract most gneisses and amphibolites of the belomorian belt in the white sea region have previously been considered to represent the oldest saamian crustal component in the baltic shield which has been referred to the early archaean new u pb zircon ages of some of the most typical saamian rocks from the northern segment of the belomorian belt are reported these are 1 2 65 2 60 ga for the main rebolian event of folding high p metamorphism and migmatization 2 2 8 ga for the protolith of an orthogneiss in a granulite facies relic area within the region of rebolian folding on the eastern shore of lake kovdozero 3 2 74 ga for tonalitic orthogneisses that dominate the belomorian gneiss terrain around kandalaksha bay of the white sea 4 2 4 ga for a gabbro granite intrusion on the tolstik peninsula of the white sea this intrusion has previously been considered to be one of the type rock units of the early archaean these and other age determinations previously only reported in russian constrain the age of the assumed saamian in the belomorian belt in conjunction with new field observations they place most of the dated rock units within the same age group as the late archaean lopian rocks as a result of the age determinations the concept of a single array of late archaean geodynamic events in the baltic shield is advanced the belomorian belt is interpreted as an edge of the late archaean granite greenstone terrain in karelia which had been involved in strong structural reworking during an event of tectonic stacking and thrusting 2 65 2 60 ga ago part of the belomorian belt was subsequently also affected by palaeoproterozoic deformation the new results throw doubt on the previously assumed late archaean lopian age of some metavolcanic rock units occurring within the belomorian belt but overlying the presently dated orthogneisses these metavolcanics which comprise for instance the so called kolvitsa belt and its continuation towards the northwest are assigned tentatively to the palaeoproterozoic they may be age equivalents of lapponian sumian 2 5 2 4 ga old rocks underlying the lapland granulite belt

fractures as collections of cracks abstract a single fracture can be thought of as two rough surfaces in partial contact observations show that the noncontacting regions are patches of widely varying size and shape which are characterized by an aperture which is much less than any of the in plane dimensions a further idealization is therefore to assume that the regions of contact are welded and the noncontacting patches are replaced by cracks the deformation of cracks arranged in simple two dimensional 2 d geometries can be described analytically solutions for noninteracting and interacting cracks are reviewed and the analogy with solutions for ubiquitously cracked intact rock is discussed using the model of a fracture as a two dimensional collection of cracks expressions for fracture stiffness flow of a single phase fluid through a fracture and propagation of elastic waves across a fracture are formulated comparison with observations shows that this simple model provides a consistent description of many first order attributes of the mechanical deformation and fluid flow in fractures under normal stress and the velocity and amplitude of seismic waves propagating in fractured rock

anisotropy in seismic velocities and amplitudes from multiple parallel fractures many rock structures include multiple near parallel planar discontinuities such as bedding planes or joints the effects of these nonwelded interfaces on seismic wave propagation are often analyzed using effective moduli in terms of which seismic wave propagation is independent of frequency and without loss unless the moduli include imaginary terms an alternative approach is to treat these interfaces as a boundary condition in the seismic wave equation across which seismic stress is continuous but seismic particle displacements are discontinuous the ratio of the stress to displacement is called the specific stiffness of the interface and characterizes the elastic properties of a fracture for a completely elastic system this results in frequency dependent reflection and transmission coefficients for each interface as well as a frequency dependent group time delay using multiple parallel displacement discontinuities and ignoring converted and reflected waves expressions derived for transmitted wave amplitudes and group velocities show that these depend on frequency angle of incidence and polarization in the case of shear waves measurements on a laminated steel block show that shear pulses propagating parallel to the laminations and polarized parallel and perpendicular to the plane of the laminations both travel at the velocity for solid steel although the spectra of these pulses differ considerably however the energy of the pulse polarized perpendicular to the laminations may propagate as an interface wave between each pair of laminations predictions of the displacement discontinuity model have features quite distinct from many crustal observations to date we suggest that we are able to model dense populations of coplanar cracks that cannot be treated by effective moduli methods which require a dilute concentration of cracks

early mesozoic deformations of the eastern yanshan thrust belt northern china the yanshan thrust belt ytb is located at the northern edge of the north china plate because of the intense thicking and subsequent delamination of the lithosphere in north china geologists have been focused on the late mesozoic deformation in the yanshan belt the yanshan belt has been regarded as part of a stable craton from the proterozoic to the early mesozoic in this paper the authors present that the yanshan area was deformed during the early mesozoic this deformation could be related to ocean basin closure along the northern margin of north china or related to the collision between the north china and yangtze plates along the qinling dabie ultrahigh pressure belt three stages of early mesozoic deformation are identified in the eastern yanshan at lingyuan county the first stage is characterized by westward thrusting d1 the second stage comprises a top to east thrust system d2 and the third stage comprises extensional gravity induced collapse and landsliding d3 the timing of these evens is constrained by both the crosscutting relationships of faults and the isotopic dating of volcanic rocks and gravels the d1 and d2 events took place in the late triassic and early jurassic whereas d3 event occurred at the end of the middle jurassic the dengzhangzi formation was deposited during the d1 d2 period and recorded a rapid uplift erosion and deposition sequence these early mesozoic contractional deformations in the ytb were probably related to the closure of ancient asian ocean and ancient qinling ocean the later crustal extension was caused by gravitational collapse of the eastern china plateau during early mesozoic

molybdenite re os and u pb zircon dating and genesis of the dayana w mo deposit in eastern ujumchin inner mongolia abstract the dayana w mo deposit in eastern ujumchin of inner mongolia is a quartz vein type deposit in the mid western part of the central asian orogenic belt caob biotite monzogranite quartz porphyry and hornfels host w mo in quartz veins based on spatial relationships molybdenite was deposited first followed by wolframite this contribution presents precise laser ablation inductively coupled plasma mass spectroscopy la icp ms u pb zircon dating and geochemical analysis of the biotite monzogranite the u pb dating shows that the monzogranite is 134 1 ma major and trace element geochemistry shows that the monzogranite is characterized by high sio 2 and k 2 o contents a right inclined shape of the chondrite normalized ree patterns enrichment of large ion lithophile elements liles and depletion of high field strength elements hfses such as nb p ba the monzogranite is high k calc alkaline has a strong negative eu anomaly eu eu 0 04 0 45 low p 2 o 5 content high a cnk of 1 2 enriched in large ion lithophile elements liles such as rb th u nd and hf and notably depleted in ba sr p ti and nb these characteristics define the dayana monzogranite as a highly fractionated peraluminous granite re os isotopic analysis of seven molybdenite samples from the deposit yield an isochron age of 133 3 ma mswd 2 2 which indicates that the monzogranite and ore have the same age within error are probably genetically related and related to a major early cretaceous mineralizing event in china known as the yanshanian

compressional wave anisotropy in fractured rock abstract compressional wave anisotropy produced by a single fracture was measured using an acoustic imaging system for the fractured sample the compressional wave amplitude was highest and the group velocity the slowest when waves were propagated parallel to the fracture these observations are contrary to predictions based on the displacement discontinuity or effective moduli theories for the simulation of wave propagation across a fracture

new precise timing constraint for the keketuohai no 3 pegmatite in xinjiang china and identification of its parental pluton abstract the keketuohai pegmatite field is one of the pegmatite rich regions in the altay orogen with abundant pegmatites and voluminous granite magmatism the keketuohai no 3 pegmatite is renowned for its deposits of rare metals but despite a lot of in depth research the geochronology of the pegmatite remains the subject of dispute and there is a lack of precise geochronological evidence to support the view that the aral granite represents the parental magma of the pegmatite to further our understanding of this pegmatite field one sample was collected from the aral granite in the northern part of the ore district and six molybdenite samples were taken from the marginal zone of the keketuohai no 3 pegmatite the geological context of the molybdenite samples indicates an origin by hydrothermal metasomatism in the zone between the margin of the pegmatite and the wall rock and the time of its formation should coincide with or immediately follow the formation of the pegmatite margin this allows the lower time limit of the pegmatite formation to be set la mc icp ms dating of zircons from the aral granite gives 206 pb 238 u ages that range from 209 0 to 215 6 ma from 16 spot analyses and the plotted data are mainly distributed on or around the concordant line the weighted mean age is 211 4 0 8 ma mswd 1 4 which may represent the crystallization age of the granite the re os isotopic model ages for the six molybdenite samples range from 207 8 to 212 2 ma the weighted mean age of 209 9 1 3 ma mswd 0 69 and the re os isochron age of 208 8 2 4 ma mswd 0 89 are consistent within error if decoupling of the re os isotope system in molybdenite is neglected this consistency in the age data may indicate the time at which molybdenite was deposited from fluids after researching in detail the previously published chronological data we propose that earlier views on the ages of the keketuohai no 3 pegmatite and the aral granite may be incorrect and we now suggest that the pegmatite started to form around 209 210 ma in the late triassic the agreement between the newly determined u pb ages for the aral granite and the re os ages of the molybdenite in the pegmatite strongly indicates a close relationship between the crystallization of the aral granite and the formation of the pegmatite and its rare metals deposits it also points to the aral granite being the parent of the no 3 pegmatite

effects of non welded interfaces on guided sh waves the effects of non welded interfaces on guided sh waves are examined analytically and numerically using the displacement discontinuity model to describe the mechanical shear stiffnesses of the interfaces a modal solution for guided sh waves is developed for a layer in non welded contact with underlying and overlying half spaces for low interface stiffnesses the modal solution degenerates to the sh plate wave solution and for high interface stiffnesses to the love channel wave solution it is demonstrated that a single non welded interface separating two half spaces is incapable of supporting an sh interface wave the effects of interface stiffness on the phase velocities and displacement profiles of guided sh waves are examined for a low velocity layer in non welded contact with bounding half spaces using the modal solution and a numerical boundary integral equation code the results of this analysis demonstrate that the magnitude of the interface stiffness can have a first order effect on the velocities amplitudes and displacement profiles of guided sh waves in a low velocity layer which should make these waves highly detectable in crosshole seismic surveys

poroelastic modeling of seismic boundary conditions across a fracture permeability of a fracture can affect how the fracture interacts with seismic waves to examine this effect a simple mathematical model that describes the poroelastic nature of wave fracture interaction is useful in this paper a set of boundary conditions is presented which relate wave induced particle velocity or displacement and stress including fluid pressure across a compliant fluid bearing fracture these conditions are derived by modeling a fracture as a thin porous layer with increased compliance and finite permeability assuming a small layer thickness the boundary conditions can be derived by integrating the governing equations of poroelastic wave propagation a finite jump in the stress and velocity across a fracture is expressed as a function of the stress and velocity at the boundaries further simplification for a thin fracture yields a set of characteristic parameters that control the seismic response of single fractures with a wide range of mechanical and hydraulic properties these boundary conditions have potential applications in simplifying numerical models such as finite difference and finite element methods to compute seismic wave scattering off nonplanar e g curved and intersecting fractures

u pb zircon ages from a devonian carbonatite dyke kola peninsula russia a record of geological evolution from the archaean to the palaeozoic abstract zircon from a xenolite rich carbonatite dyke on telyachi island near kandalaksha on the kola peninsula northwest russia have been analysed for u th pb to determine the age of the dyke and study the geological evolution of the belomorides zircon from the ankeritic carbonatite analysed by conventional isotope dilution yielded a concordant u pb age of 380 7 ma this age is confirmed by sims u pb analysis of zircon from thin carbonatite veinlets in xenoliths which gave a concordant 238 u 206 pb age of 378 8 ma these results demonstrate that the dyke belongs to the 360 380 ma old kola alkaline province the xenoliths represent a variety of rock types we have analysed zircon from xenoliths of amphibolite gneissic granite and granite most of these crystals have distinct core rim structures which are clearly shown by cathodolumenescence cl imaging the majority of sims analysed spots in cores are close to concordant at c 2 65 2 75 ga which reflects the ages of the rocks from which the xenoliths were derived some cores lost a large fraction of their pb during the carbonatite magmatism one analysed cl dark rim appears to reflect a high grade metamorphic event which is slightly older than 2 75 ga this result requires that the xenoliths also include pre 2 75 ga rocks most rims developed during high grade metamorphism 1 7 1 9 ga ago these are cl bright and characterized by very low th u ratios and formed by a rejuvenation process in solid state which resulted in partial or total loss of radiogenic pb most rims have also lost pb during the devonian carbonatite magmatism the ages obtained from xenolith zircons are in excellent agreement with the independently known geological evolution of the belomorides

elastic wave propagation in fractured media using the discontinuous galerkin method abstractwe have formulated and implemented a discontinuous galerkin method dgm for elastic wave propagation that allows for discontinuities in the displacement field to simulate fractures or faults the approach is based on the interior penalty formulation of dgm and the fractures are simulated using the linear slip model which is incorporated into the weak formulation by including an additional term that is similar to the penalty term but uses the fracture compliance instead of an arbitrary penalty parameter we have calibrated our results against an analytic solution of fracture induced anisotropy for a set of elongated horizontal fractures and we have evaluated numerical examples that simulate the reflection and transmission of waves at a fracture and at fracture interface waves this method can further be used with models containing intersecting fractures and multiple fracture sets in 2d or 3d domains

polarization of crack waves along an artificial subsurface fracture we analyze three dimensional 3d particle motions of crack waves which propagate along a fracture to determine fracture orientation crack waves were measured at an artificial subsurface fracture at a depth of approximately 370 m in the higashihachimantai hot dry rock model field in japan dispersion was observed in crack waves velocities of crack waves below 100 hz are 100 m s 150 m s along this artificial fracture therefore crack waves arrived after compressional wave and shear wave a coherence matrix is used to analyze the 3d particle motions in the frequency domain noise which is outside of the band of the detector is excluded in the coherence matrix analysis we examine the polarization of 3d particle motions of crack waves using principal component analysis the longest axis of an ellipsoid which approximates the 3d particle motions shows a direction consistent with estimates obtained by core sample measurements and tectonic stress measurements the differences in fracture orientation obtained from the crack wave analysis and the core sample analysis are less than 16 in azimuth and less than 20 in dip

evolution of fault interface rayleigh wave speed over simulated earthquake cycles in the lab observations interpretations and implications abstract to image fault zone elastic and mechanical properties is of great importance for the understanding of fault mechanics and earthquake physics in this study we show how a previously less known wave fault interface rayleigh wave firw can be used to probe the feedback between fault zone properties and earthquake ruptures we conducted meter scale rock friction experiments to simulate earthquake cycles in the lab three tests were performed under a fixed normal stress of 6 7 mpa negative for compression and a constant loading rate sequentially at 0 01 0 1 and 1 mm s we installed a strain gauge array near the fault to monitor the local dynamics focusing on mode ii rupture breaking the free edge of the fault and the breakout induced firw propagating backward along the fault by applying a waveform correlation method we systematically measured the firw speed over an 800 mm long interval in the central fault portion the results show that a relative reduction of firw speed by 1 4 can be detected near the end of the test loaded at 1 mm s such wave speed reduction can be explained by two mechanisms rupture induced brittle damage that softens fault zone rocks and some dissipative processes related to fault re rupturing viscous damping etc that take energy away from firw both mechanisms are also supported by other independent observations e g distribution of fault surface wear behavior of macroscopic friction implying that they may be coupled these observations and interpretations reveal an application potential of firw for constraining the elastic and mechanical properties of fault zones we suggest several directions for future research aimed at turning that application potential into reality

isotope geochemistry and genesis of the liyuan gold deposit shanxi north china abstract the liyuan lode gold deposit is located in the shanxi province central north china craton gold orebodies are mainly hosted in archean metamorphic rocks and structurally controlled by the nne trending faults gold occurs in disseminated and auriferous quartz sulfide veins and veinlets within hydrothermally altered rocks mineralization can be divided into three intervals 1 quartz pyrite stage 2 quartz polymetallic sulfides stage and 3 quartz carbonate stage gold formed mainly in the middle stage sericite sample associated with the middle stage pyrite from phyllic alteration zones yields an well defined 40ar 39ar plateau age of 133 3 1 2 ma which is remarkably consistent with zircon u pb age 133 4 1 1 ma of the quartz porphyry dikes in the liyuan mine indicating a close relationship between gold mineralization and granitic magmatism in the area the sulfur isotopic compositions of pyrite py1 2 from the early stage have a narrow range from 0 3 to 4 1 indicating a deep seated magmatic source however the sulfides py3 5 sphalerite galena from the middle stage have lower 34sv cdt values of 7 2 to 3 0 which is thought to be a result of fluid oxidation during gold mineralization the pb isotopic compositions of sulfides from liyuan ores have 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 16 671 16 860 15 236 15 255 and 41 452 44 159 respectively indicting a high thorium lower continental crust source of ore forming materials the 13cvpdb and 18ovsmow values of late stage calcite ranging from 7 1 to 5 8 and 12 2 to 13 7 respectively illustrating a deep seated magmatic source of carbon bearing portion given the absence of granite bodies at the present mining levels we speculate that the ore forming fluids may have been exsolved from a concealed granite pluton at greater depth based on the regional tectono magmatic events ore geology geochronologic and isotopic data obtained in this study together with the previous published fluid inclusion data we suggest that the liyuan deposit is an orogenic gold deposit that is probably related to the early cretaceous granite magmatism in the interior of the north china craton

two dimensional finite difference seismic modeling of an open fluid filled fracture comparison of thin layer and linear slip models within the context of seismic wave propagation fractures can be described as thin layers or linear slip interfaces in this paper numerical simulations of elastic wave propagation in a medium with a single fracture represented by these two models are performed by 2d finite difference codes a variable grid isotropic code for the thin layer model and a regular grid anisotropic code for the linear slip model numerical results show excellent agreement between the two models for wavefields away from the fracture the only discrepancy between the two is the presence of a slow wave traveling primarily within the fracture fluid of the thin layer model the comparison of the computational cost shows that modeling of the linear slip model is more efficient than that of the thin layer model this study demonstrates that the linear slip model is an efficient and accurate modeling approach for the remote seismic characterization of fractures

compressional waves guided between parallel fractures abstract we have performed experiments on the laboratory scale to image seismic wavefronts of compressional waves that are guided between parallel fractures the wavefronts are detected using water coupled 1 mhz piezoelectric transducers the stiffness of the fractures is controlled by applying radial stress in a hydraulic stress apparatus that leaves the observation face of the fractured sample open to measurement the two fractures form a waveguide in isotropic homogeneous aluminum and act as parallel displacement discontinuity boundary conditions separated by a distance d 2 the compressional wave is a leaky guided mode that sheds energy into the medium beyond the fractures however the fractures provide sufficiently strong energy confinement inside the central waveguide for a sample length l of approximately l 15 to observe the guided wave fractures with low fracture specific stifffness produce strong energy confinement in the guided compressional mode wave

fracture channel waves the properties of guided waves which propagate between two parallel fractures are examined plane wave analysis is used to obtain a dispersion equation for the velocities of fracture channel waves analysis of this equation demonstrates that parallel fractures form an elastic waveguide that supports two symmetric and two antisymmetric dispersive rayleigh channel waves each with particle motions and velocities that are sensitive to the normal and tangential stiffnesses of the fractures these fracture channel waves degenerate to shear waves when the fracture stiffnesses are large to rayleigh waves and rayleigh lamb plate waves when the fracture stiffnesses are low and to fracture interface waves when the fractures are either very closely spaced or widely separated for intermediate fracture stiffnesses typical of fractured rock masses fracture channel waves are dispersive and exhibit moderate to strong localization of guided wave energy between the fractures the existence of these waves is examined using laboratory acoustic measurements on a fractured marble plate this experiment confirms the distinct particle motion of the fundamental antisymmetric fracture channel wave a sub 0 mode and demonstrates the ease with which a fracture channel wave can be generated and detected copyright 1999 american geophysical union

on the elastic anatomy of heterogeneous fractures in rock abstract this study examines the feasibility of the full field ultrasonic characterization of fractures in rock to this end a slab like specimen of granite is subjected to in plane o 10 4 hz excitation while monitoring the induced 2d wavefield by a scanning laser doppler vibrometer sldv with sub centimeter spatial resolution upon suitable filtering and interpolation the observed wavefield is verified to conform with the plane stress approximation and used to i compute the maps of elastic modulus in the specimen before and after fracturing via a rudimentary application of the principle of elastography ii reconstruct the fracture geometry iii expose the fracture s primal traction displacement jump contact behavior and iv identify its profiles of shear and normal specific stiffness through the use of full field ultrasonic data the approach provides an unobscured high resolution insight into the fracture s contact behavior foreshadowing in depth laboratory exploration of interdependencies between the fracture geometry aperture interphase properties and its seismic characteristics

elastic interface waves along a fracture non welded interfaces can be treated as a displacement discontinuity characterized by elastic stiffnesses applying this boundary condition to a generalized rayleigh wave it is shown that a fast and a slow dispersive wave can propagate along the fracture even when the seismic properties of the rock on each side are identical

the kola alkaline province of the cis and finland precise rbsr ages define 380 360 ma age range for all magmatism abstract twenty one new rb sr mineral and biotite whole rock isochrons are presented for intrusive rock complexes of the palaeozoic kola alkaline province in the cis and finland samples from the ultrabasic alkaline and nepheline syenite complexes khibina lovozero afrikanda turiy mys ozernaya varaka sokli and iivaara all yield ages in the very narrow range from 361 7 to 377 3 ma i e late middle devonian to upper devonian these data represent crystallization rather than intrusion ages but are nevertheless argued as marking very closely the time of intrusion there is no indication of bimodal age distribution the emplacement of the alkaline magmas is related temporally to the subsidence of the kontozero graben of the central kola peninsula sr initial ratios of the agpaitic nepheline syenites of the giant khibina and lovozero intrusions scatter between 0 703 and 0 704 because of high rb sr ratios of the rocks these isotope ratios exclude long term pre enrichment of alkalis in the sources of the magmas prior to segregation and intrusion and thus episodic enrichment during earlier rifting events in the northern baltic shield the magma formation may be related to wrench movements in the surroundings of the major trollfjord komagelv fault which forms part of the late caledonian arctic north atlantic megashear system

observation of a distinct compressional mode interface wave on a single fracture a compressional mode interface wave propagating along a fracture has been observed in a fracture in limestone this compressional mode interface wave co propagates with the bulk compressional wave is localized to the region surrounding the fracture and has particle motions containing both transverse and longitudinal displacements as stress across the fracture increases the wave energy shifts to earlier arrival times and is accompanied by a decrease in the transverse displacement and an increase in the longitudinal displacement of the particle motion

interface waves propagating along tensile fractures in dolomite elastic interface waves have been observed in induced tensile fractures in dolomite rock cores multiscaling wavelet analysis distinguishes the interface wave from bulk shear waves quantifies the interface wave spectral content and determines the arrival time of the peak energy the dominant seismic energy is concentrated in the slow interface wave with little or no detectable energy in the fast wave as the stress across the fracture increases the slow interface wave velocity increases and the frequency of the spectral peak shifts to higher frequencies the shear dynamic specific stiffness of the fracture was calculated from the peak energy arrival time as a function of stress

enhanced characterization of fracture compliance heterogeneity using multiple reflections and data driven green s function retrieval the spatial heterogeneity along a fracture is a key determinant for fracture associated mechanical and hydraulic properties of the subsurface laboratory experiments have been performed to test the applicability of the nonwelded interface representation to predict the frequency and angle dependent elastic response of a single fracture the observation that nonwelded interface model can represent quite well the frequency and angle dependent reflection response of a fracture has led us to develop a new methodology for estimating the spatially heterogeneous fracture compliance from the reflection response along a fracture surface a data driven approach based on marchenko equation coupled with inverse scattering to solve the nonwelded interface boundary condition has been formulated the approach estimates the elastic wavefield along a fracture accurately including the multiple reflections as an extension it offers the possibility to estimate fracture compliance using the multiple reflections we illustrate the concept by numerically modeling 2 d sh waves sensing the heterogeneous tangential compliance of a fracture the stationary phase method is applied to single and double spatial integrals to analyze the effect of source and receiver aperture on the green s function retrieval our results show that the use of multiple reflections allows a better estimation of the heterogeneous fracture compliance than using primary reflections alone especially for the far offsets on the fracture plane

seismic focusing by a single planar fracture 1 a single plane fracture with an axially symmetric stress distribution behaves as a seismic lens that focuses seismic energy to a beam waist at a focal plane both phase and amplitude effects on a seismic wave propagating across the fracture contribute to the lensing behavior radial gradients in the fracture specific stiffness cause wave refraction through a radially varying group time delay while the fracture transmission amplitude approximates a fresnel zone plate this work demonstrates that a two dimensional planar fracture contrasted with three dimensional geologic structures such as basins and domes can focus seismic waves focusing of seismic waves by fractures should be considered in the interpretation of seismic data from fractured strata with heterogeneous stress distributions

transmission of seismic waves across single natural fractures fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation seismic stress is continuous across such a boundary but seismic particle displacement and seismic particle velocity are not the complete solutions for seismic wave reflection conversion and transmission across a displacement and velocity discontinuity between two half spaces with different densities and elastic properties are derived for all angles of the incident wave the ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity respectively a displacement discontinuity results in frequency dependent reflection and transmission coefficients and a frequency dependent group time delay the velocity discontinuity results in frequency independent coefficients and zero delay results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described measurements were made at different effective stresses under dry and saturated conditions at room temperature it is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress as does the static specific stiffness of these fractures changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor q which is found to be a function of frequency