statistical properties of seismic anisotropy predicted by upper mantle geodynamic models 1 we study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation induced lattice preferred orientations lpo of minerals we also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models we show that most of the anisotropy predicted by our convection lpo models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine enstatite lpos however the commonly employed elliptical approximation does not hold in general the orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes correlations between hexagonal anisotropy parameters for p and s waves show simple bilinear relationships such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori the match between our model predictions and observed patterns of anisotropy supports earlier more idealized studies that assumed laboratory derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics the match is evident both in agreement between predicted lpo at selected model sites and that measured in natural samples and in the global pattern of fast seismic wave propagation directions

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

the continental rift system of the rhinegraben structure physical properties and dynamical processes abstract the geophysical investigation of the continental rift system of the rhinegraben by various methods has progressed to such a stage that details of structure physical properties and seismic activity and relations between these features can be recognised differences in the nature of the crust mantle boundary inside and outside the graben proper seem to be a typical property of continental rifts asymmetries of structure seismic activity temperature in the crust and physical properties in the lithosphere are an important clue to the understanding of rift dynamics these asymmetries indicate that the formation of the rift cannot be explained by the updoming of an asthenolith alone but that taphrogenesis took place in the foreland of the continent continent collision in the alpine mediterranean zone a number of future key experiments is proposed

widespread seismic anisotropy in earth s lowermost mantle beneath the atlantic and siberia deep inside the earth just above the core mantle boundary at around 2 700 km depth large scale mantle structures are assumed to play a key role for global geodynamic processes while unusual hot regions are attributed to feed rising mantle plumes and volcanic hotspots the accumulation of subducted lithospheric plates is associated with colder than average features in both environments the appearance of dynamic driven processes such as deformation and mantle flow can directly be inferred by the presence of seismic anisotropy however the geometries as well as the interactions of these massive anomalous structures with the surrounding mantle material are still under debate based on new seismic data from a dense and large aperture recording network in scandinavia we characterize the anisotropic signatures of two so far unexplored regions in the lowermost mantle by using observations of clearly discrepant sks skks shear wave splitting measurements thereby we can demonstrate that anisotropy is located along the northern edges of the large low shear velocity province beneath africa furthermore we recover an anisotropic structure in a region of fast seismic velocity underneath siberia which provides additional evidence for widespread deformation caused by a deeply subducted slab

seismic constraints on a large mafic intrusion with implications for the subsidence history of the danish basin 1 seismic refraction data from the estrid 1 profile are used for seismic velocity modeling along the strike of a large mafic intrusion in the norwegian danish basin central denmark the p wave velocity structure identifies a 8 km thick sedimentary succession with velocities between 1 8 and 5 7 km s the top basement is defined by a step to vp 6 2 km s in the middle to lower crust a high velocity body vp 6 7 km s with its top located at about 10 12 km depth is interpreted as a high velocity and high density gabbroic intrusion of permian age this high velocity body explains the large 50 mgal positive gravity anomaly known as silkeborg gravity high the intrusion has a minimum volume of 40 000 km3 which implies that the magma influx and the consequent cooling of the lithosphere from high temperature could have had profound effects on the subsidence of the danish basin in particular because the magma probably intruded during only a few events and other similar structures cover much of the basin an anomalously high velocity gradient from 7 0 km s in the middle crust to 7 7 km s at 30 32 km depth in the central part of the intrusion coincides with an interval without moho reflections indicative of a gradual transition zone between the crust and the mantle this feature may show the location of the feeder dykes of the intrusion

lowermost mantle anisotropy and deformation along the boundary of the african llsvp shear wave splitting of sk k s phases is often used to examine upper mantle anisotropy in specific cases however splitting of these phases may reflect anisotropy in the lowermost mantle here we present sks and skks splitting measurements for 233 event station pairs at 34 seismic stations that sample d beneath africa of these 36 pairs show significantly different splitting between the two phases which likely reflects a contribution from lowermost mantle anisotropy the vast majority of discrepant pairs sample the boundary of the african large low shear velocity province llsvp which dominates the lower mantle structure beneath this region in general we observe little or no splitting of phases that have passed through the llsvp itself and significant splitting for phases that have sampled the boundary of the llsvp we infer that the d region just outside the llsvp boundary is strongly deformed while its interior remains undeformed or weakly deformed

a stable isotope and fluid inclusion study of the qaleh zaricu au ag deposit khorasan province iran abstract the qaleh zari copper deposit located in south khorasan in the central lut region of iran is a polymetallic vein deposit with major amounts of cu au ag and minor amounts of pb zn and bi mineralization occurs in a series of nw se trending fault planes and breccia zones in paleogene andesitic to basaltic volcanic rocks argillization sericitization and propylitization characterize alteration halos bordering mineral veins the main ore minerals are chalcopyrite pyrite galena and sphalerite with quartz calcite and minor chlorite as the main gangue phases microthermometric measurements of fluid inclusions in cogenetic quartz indicate homogenization temperatures between 160 and 300 c and salinities from 1 to 4 wt nacl equiv boiling occurred in the mineralising fluids at 160 1000 m below the paleo water table at pressures of approximately 15 80 bar at various stages in the formation of the ore body the wide range of pressures and temperatures reflects the multi stage nature of the mineralization at qaleh zari the 18 o values in quartz relative to smow and 34 s values in chalcopyrite and galena relative to cdt range from 6 5 to 7 5 and 0 0 1 5 mean 7 0 respectively at 300 c calculated fluid 18 o values are close to 0 these data suggest a magmatic origin for sulfur and a surficial origin for the mineralizing fluid mineralization at qaleh zari is interpreted as epithermal and low sulfidation in style and was probably related to a deep seated magmatic system ore deposition was the result of boiling cooling and pressure reduction

finite element modelling of a continental rift structure rhinegraben with a large deformations algorithm abstract the formation of continental rifts with small extension values can be simulated over several tens of millions of years by means of a finite element program able to handle large deformations as an example an attempt has been made to model the formation of the northern and southern rhinegraben over the past 40 m y deviatoric extensional stresses of 25 40 mpa were required and to model the moho uplift mass anomalies in the lower lithosphere had to be invoked with density contrasts of 0 15 and 0 30 g cm 3 for the northern and southern rhinegraben respectively trial models without a mass anomaly in the lower lithosphere resulted in crustal thickening beneath the graben and not crustal thinning in the northern rhinegraben the rifting is more passive and uniform and model subsidence values of 4000 m were obtained together with model shoulder and moho uplifts of 800 m and 1500 m respectively in the southern rhinegraben the rifting is more active and episodic and in fact between 13 and 10 m y ago the model graben floor was uplifted in order to simulate the observed graben floor uplift at around 10 m y ago model subsidence values of 2000 m were obtained while the model shoulder and moho uplifts were 1900 m and 7000 m respectively in good agreement with the observed values

a seismic refraction study of the north basin of the dead sea israel a seismic refraction experiment involving the use of 9 obs and 11 portable seismic land stations was conducted along a profile in the north basin of the dead sea the seismometers were deployed along a north south profile in the lake and the adjacent land area the interpretation of the recorded seismograms indicates the presence of two large pliocene salt diapirs in the young basin fill the basement lies at a relatively shallow depth 6 8 km under the north basin further south along the profile a major fault affecting the basement was detected the apparent sense of the faulting is down to the southeast representing the boundary of the south basin this major faulting with a downthrow of some 4 5 km has depressed the crystalline basement and the overlying cretaceous and pre cretaceous sediments the faulting was followed by the deposition of over 8 km of recent and tertiary sediments resulting in a 14 km thick sedimentary sequence in the south basin of the dead sea

a viscoelastic shear zone model of compressional and extensional plate boundaries a model is proposed describing the mechanical evolution of a shear zone along compressional and extensional plate boundaries subject to constant strain rate the shear zones are assumed as viscoelastic with maxwell rheology and with elastic and rheological parameters depending on temperature and petrology stress and strain are computed as functions of time and depth for both kinds of boundaries the model reproduces the existence of a shallow seismogenic zone characterized by a stress concentration the thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone assuming that a fault dislocation takes place the brittle ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values the effects of different strain rates and geothermal gradients on the depth of the brittle ductile transition are studied the model predictions are consistent with values inferred from seismicity data of different boundary zones

subsidence of the paris basin we discuss the main results of a systematic quantitative analysis of the subsidence of the paris basin made on 100 wells backstripping techniques with decompaction as well as sea level and water depth corrections are used to derive tectonic subsidence curves in the absence of water and sediment loading assuming local as well as regional compensation a best fitting sea level variations curve is derived for the deepest central part on the basis of the resulting tectonic subsidence curve the adopted amplitude of the regression since upper cretaceous is 330 m the spatial and temporal distribution of tectonic subsidence is reconstructed and discussed it is shown that rigidity of the lithosphere with an adopted 50 km flexural parameter plays a significant role these results are then quantitatively interpreted in terms of the lithospheric stretching model this interpretation is reasonable provided it is made in terms of volumetric subsidence over the whole basin because of the thermal conduction and the rigidity low pass filtering effects the subsidence can be explained if the axial zone was thinned during permo triassic by a factor of about 1 3 with respect to the adjacent lithosphere it is shown that either the initial altitude was more than 500 m above sea level or the lower part of the lithosphere was lost at the beginning of the permo triassic phase of extension resulting in a rapid uplift of several hundreds of meters the similarity of the permo triassic initiating extensional phase with active continental rifting is shown

the roermond earthquake of 13 april 1992 the netherlands geological aspects the epicentre of the roermond earthquake is located near the western boundary fault of the roer valley trough one of the deepest and the most active in the quaternary part of the lower rhine graben the late pleistocene and holocene activity of the trough is manifested by offsets of the main mindel and the lower riss and late pleistocene terraces along the boundary faults surface fractures have been observed in an area of more than 50 km2 2 5 3 5 km to the north of the town of roermond at 0 8 km to the south of the village of herkenbosch and in the southeastern part of the village of montfort three types of ruptures were differentiated scarps up to 50 cm high along open fractures near the maas river open fractures continued by scarps in some places and open fractures with a liquefaction of the quaternary alluvium sands the last type is predominant the location of the ruptures depends on the landscape and water table of the region while they could be produced solely by hydraulic shock during the earthquake increased by the wet spring season the majority of the ruptures strike n50 w 10 i e parallel to the main trough faults or n55 e 10 along neotectonic lines parallel to the maas valley and deduced from space imagery thus the ruptures could be the secondary surficial effect of the earthquake linked indirectly with the active tectonics of the region

upper lithospheric deformations in the jordan dead sea transform regime abstract seismicity and deep seismic sounding data gathered mainly in the last 15 years have been combined to provide a picture of crustal and uppermost mantle deformations in the jordan dead sea transform and its eastern and western flanks the results show that the main earthquake energy release is connected with the strike slip mechanism of the transform itself the seismicity of the area is concentrated almost exclusively in the upper 21 km of the crust ihus including the upper crust and possibly the top few kilometres of the lower crust relative crustal thinning of 5 8 km with respect to a 40 km thick continental crust regional uplift of 1 2 km extrusion of basalts and possible intrusion of mantle derived material into the crust and possible existence of fluids in the lower crust are the other major manifestations of upper lithospheric deformation in this area

lithospheric breakup in three dimensions necking of a work hardening plastic plate lithospheric breakup is in general oblique to the principal strain axes with shortening in one horizontal axis eh 0 local necking of the plate including the mantle part of the lithosphere is necessary for the creation of a divergent plate boundary the necking develops as a mechanical instability that decreases the resistance of the plate to extension despite the intrinsic hardening of the material we model the flow that precedes breakup stressing two aspects that have been overlooked before 1 three dimensional flow and 2 strain dependent strength of the lithosphere the model mimics spatial and temporal features of the most extensively documented rift systems the neck is oblique to the direction of principal extension and the transition from diffuse stretching to local necking is abrupt at 5 10 strain the critical strain for local necking in the model appears to be independent of the intrinsic strength therefore various rheological layers brittle upper crust ductile lower crust and stiffer but hotter mantle lithosphere may neck concomitantly to form a rift

the role of seismic refraction data for studies of the origin and evolution of continental rifts abstract universal attributes of continental rift systems are crustal extension with associated asthenospheric upwelling or modification of the mantle beneath the rift axis continental rifting is thus greatly controlled if not driven by geodynamic processes in the lower lithosphere seismic refraction profiling is a technique for obtaining high resolution data on structures seismic velocities and petrologic conditions in the deep crust and upper mantle status of presently available seismic refraction data in four major cenozoic continental rift systems rhinegraben of central europe rio grande rift of southwestern united states baikal rift of eastern siberia and kenya rift of east africa is reviewed in the light of extent of coverage in area and depth reliability of velocity measurements and outstanding structural and geodynamical problems that remain unanswered for each system evidence of crustal thinning anomalous mantle at the moho as well as shallow and midcrustal velocity anomalies exist in all four examples the rhinegraben is the best constrained system from the point of view of seismic structure models seismic refraction structure data for the kenya rift are sparse and raise more questions than answers since extensive geological volcanological and petrological data available for the east africa system usually make it the standard for models of continental rifting modern high resolution deep seismic refraction profiles in kenya are urgently needed in order to advance understanding of basic continental rifting processes

the structure of the dead sea basin abstract the dead sea basin is located along the left lateral transform boundary between the arabian and sinai plates its structure and history are known from surface geology drilling seismic reflection and other geophysical data the basin comprises a large pull apart almost 150 km long and mostly 8 10 km wide which is flanked by a few kilometres wide zones of normal faulting the basin formed at about 15 ma or earlier close to the beginning of the transform motion and it reached about half its present length before the end of the miocene a strong negative gravity anomaly records a thick sediment basin fill 5 km under half its length reaching a maximum of 10 km the fill includes a few km of salt ca 6 4 ma which forms several diapirs at any one time large parts of the basin subsided simultaneously but the site of fastest subsidence seems to have shifted northward sedimentation rates reached at least hundreds of metres per million years or more in the miocene and 1 km myr in later periods the basin structure is dominated by longitudinal faults intrabasinal faults which delimit the pull apart and which are the extensions of the major strike slip faults north and south of the basin and normal faults which extend along the basin margins the latter faults express a small component of extension across the basin whereas the pull apart resulted from the much larger lateral motion along the basin in addition transverse faults divide the pull apart into several segments with somewhat different histories the pull apart grew by becoming longer parallel to the transform motion at shallow levels this was probably achieved by normal slip on transverse listric faults while the fill between them was little deformed the crust under the basin was stretched and thinned during basin lengthening which caused its subsidence basin formation was accompanied by uplifting of its flanks by 1 km sparse igneous activity occurred along the basin and its flanks its presence suggests a thermal anomaly in the lower lithosphere beneath the basin and adjacent parts of the transform

multichannel analysis of shear wave splitting a multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of sks and skks this technique utilizes simultaneously a set of records coming from different azimuths the splitting intensity of sks waves measured by the amplitude of the transverse component depends on the angle between the back azimuth of the earthquake and the direction of the symmetry axis and on the delay time t between the two quasi shear waves it is shown that the splitting parameters can be determined from the azimuthal dependence of the splitting intensity which is given by the first right eigenvector of the matrix containing the transverse components of all the records alternatively the splitting intensity can be measured by projecting the transverse components on the radial components derivatives experiments on synthetic seismograms demonstrate that both approaches provide robust estimates of the splitting parameters however the projection approach gives measurements that are closer to the inputs and with smaller error bars which suggests that it should be preferred when the signal to noise ratio is low the new technique is applied to analyze shear wave splitting under stations bcao bangui central african republic and bdfb brasilia brazil the analysis of 75 seismograms recorded by station bcao gives a fast polarization direction in very close agreement with the present day motion direction of the african plate comparisons with the results of previous studies obtained with other techniques and smaller data sets suggest that some of the published measurements may be strongly biased the main limitation of the multichannel analysis is a good azimuthal distribution of the seismicity maps of the azimuthal coverage were determined and it is found that the most favorable regions to study anisotropy with sks waves are a latitudinal band that goes from south america to africa the eastern part of north america and india

crustal and uppermost mantle structure beneath the afro arabian rift system abstract this contribution presents data and results from long range seismic refraction wide angle reflection lines in the afro arabian rift system from the derived crustal structure an evolutionary model is proposed in terms of the amount of crustal thinning in this model the gulf of aden and the axial trough of the southern red sea where the continental crust was thinned so much as to allow thin oceanic crust to form are representative of one end of the spectrum progressing through the northern part of the red sea main trough on the egyptian side with a crustal thickness of 11 14 km to the northern and southern parts of the red sea main trough on the arabian side and the northern afar depression with 15 20 km thick crusts followed by the southern afar depression with a crustal thickness of 22 26 km the members of the other end of the spectrum are reached these include the jordan dead sea and east african rifts which each with a crustal thickness of 30 36 km represent the least thinned closest to normal thickness continental crust in determining the amount and nature of opening of the red sea there remain however many problems argely due to the non unique correspondence of seismic velocity to rock type this may lead to uncertainty in moho depths especially in areas close to the mid ocean ridge system e g the southeastern part of the afar depression in the region around the gulf of tadjura where velocities as low as 6 8 6 9 km s have been interpreted as representirg hot anomalous mantle it also causes uncertainty as to what percentage of the present crust is stretched and thinned original continental crust and what percentage is new crust whether it be truly oceanic or basic intrusiors into continental crust thus it is felt that there is still an urgent need for geoscientific experiments in the afro arabian rift to resolve the details of continental rifting and subsequent oceanic crust emplacement

origin of the tertiary basaltic volcanism in the northern hessian depression volcanic activity started about 20 ma before present with quartz tholeiites qth had a climax with alkali olivine basalts aob 13 to 14 ma ago and ended 7 ma ago with nepheline basanites nb and olivine nephelinites on aob covers 73 of the volcanic area about 250 basalts and peridotite xenoliths were sampled for investigation an upper mantle layer ranging from about 90 to 60 km depth has been conditioned for a preferential alkali basalt production by advection of h2o co2 fluids containing si al ca k na p as major constituents beside numerous incompatible minor elements at the onset of the geodynamically triggered mantle conditioning locally restricted diapirism into shallow depth has caused formation of olivine tholeiite magmas oth at about 1 300 c by partial melting all of these oth primary melts intruded due to a favourable compressibility into granulites of the lower crust the rare qth basalts are their derivative magmas which have been slightly contaminated in the crust magmas of the subsequent alkali basaltic volcanism aob baob nb on mon formed by in situ partial melting at about 75 to 90 km depth after depression of the peridotite solidi by fluids to temperatures 1 200 c except many aob these magmas are primary melts as characterized by olivine melt distribution coefficients of mg fe2 kd 0 29 to 0 34 by ni concentrations 260 to 330 ppm and the occurrence of peridotite xenoliths rapid rise of gas charged melts due to saturation in co2 prevented separation of olivine etc and of xenoliths the sequence of magmas from oth to on or mon is formed from decreasing proportions of orthopyroxene opx and increasing contributions of clinopyroxene cpx and phlogopite ph at almost equal proportions of spinel sp incongruent melting of opx and cpx for oth aob nb and on is correlated with precipitation of olivine the average xenolith composition 73 ol 18 opx 7 cpx 1 1 sp and 1 3 0 5 ph was used to model the sources of the investigated melts by 9 incompatible elements and to calculate degrees of partial melting the occurrence of garnet cannot be reliably excluded by modelling on the basis of hree distribution coefficients the average xenolith composition was used for modelling because of its resemblance with worldwide sampled depleted mantle inclusions for avoiding to exhaust at least one mineral of the model mantle in the support of the norm composition of oth aob nb and mon magmas the degrees of partial melting cannot exceed 12 5 6 6 and 4 respectively mantle containing about 500 ppm k and the correlated incompatible elements like the average of 36 xenoliths allows to explain the formation of oth magmas aob nb and on melts require peridotite with slightly less than 1 500 ppm k 670 ppm p and proportions of the correlated elements lree sr ba zr rb cs ta th hf u which are higher than their abundance in primitive mantle rocks about 20 of the xenoliths have this composition metasomatism of fluids with these elements must have been an immediate precursor of the alkali basaltic volcanism otherwise the preservation of a local disequilibrium in 87sr 86sr ratios between cpx cores and total rock at upper mantle temperatures cannot be explained

finite frequency vectorial tomography a new method for high resolution imaging of upper mantle anisotropy summary amplitude measurements of the transverse component of sks waves the so called splitting intensity can be used to formulate a non linear inverse problem to image the 3 d variations of upper mantle anisotropy assuming transverse isotropy or hexagonal symmetry one can parametrize anisotropy by two anisotropic parameters and two angles describing the orientation of the symmetry axis these can also be written as two collinear pseudo vectors the tomographic process consists of retrieving the spatial distribution of these pseudo vectors and thus resembles surface wave vectorial tomography spatial resolution results from the sensitivity of low frequency sks waves to seismic anisotropy off the ray path the expressions for the 3 d sensitivity kernels for splitting intensity are derived including the near field contributions and validated by comparison with a full wave equation solution based upon the finite element method these sensitivity kernels are valid for any orientation of the symmetry axis and thus generalize previous results that were only valid for a horizontal symmetry axis it is shown that both lateral and vertical subwavelength variations of anisotropy can be retrieved with a dense array of broad band stations even in the case of vertically propagating sks waves

the initiation of the dead sea rift an earlier theory proposed that the dead sea rift started as a shear transform between the arabian and african plates as a result of continental collision newer geological evidence implies a different mechanism for rift initiation mechanical theories and experimental results suggest that fracture initiation propagation and bifurcation along the rift cannot be explained by shear these features can best be explained and simulated by extensile processes recent discoveries of transitional properties of the crust mantle boundary along the rift and considerable thinning of the crust between the dead sea and the sea of galilee on the west side of the rift are interpreted as indications of an active mantle diapir that initiated fracture in this area by propagating horizontally bilaterally towards north and south this fracture reached conditions of bifurcation in several locations along the rift areas of major bifurcation also show characteristic local uplifts and local small grabens oblique to the rif

recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern germany this paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern germany the range of petrological models compatible with the observed azimuthal variation of seismic p wave velocity is explored the azimuthal distribution of amplitudes of mantle phases and the observed increase of p velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation even depletion of the upper mantle in basaltic components as suggested by mantle xenoliths from various parts of germany cannot explain the velocity depth and azimuthal amplitude observations without an increase of anisotropy with depth preferred orientation of olivine is the most likely mechanism for the observed phenomena its fast a axis at the moho level is directed towards n22 5 e the b axis is also required to be horizontal i e the b plane one of the preferred glide planes of olivine is vertical with a strike of n22 5 e therefore this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault plane solutions of earthquakes in western germany this is a strong indication that the preferred orientation of olivine is formed in the recent west european crustal stress field leaking into the upper mantle the distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a axis with depth by 10 towards n32 e and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western germany further experiments are needed to substantiate this suggestion which could lead to a better understanding of the interaction of crustal and upper mantle stress strain fields

lithospheric cross sections of the european cenozoic rift system abstract the lithospheric structure of the european cenozoic rift system ecris is presented in transects through the southern rhine graben and the rhenish massif hessen depression emphasizing the geophysical structure of the lithosphere based on seismic refraction reflection investigations teleseismic tomography electromagnetic depth sounding models and gravity aeromagnetic earthquake uplift subsidence and heat flow data the rift is clearly expressed in the rhine graben but is not evident at the surface in the area of the rhenish massif where its existence is indicated by seismicity it is characterized by abnormal crustal and upper mantle structures which vary considerably in horizontal direction for example under the rhine graben the crust is thinned to 25 km but at 40 km depth anomalously high velocities are observed in contrast beneath the rhenish massif the crust is thickened to 35 37 km and under its eastern part a high velocity thin upper mantle slice is seen at 30 km depth within the lower crust which towards the hessen depression is gradually replaced by normal variscan mantle with the moho near 30 km depth under the western part of the rhenish massif p and s wave velocities are reduced below 50 km depth which is not seen east of the rhine river under the rhine graben the existence of a low velocity upper mantle above 100 km cannot be generalized but is restricted to confined regions

mineral chemistry and h o s pb isotopic compositions of skarn type copper deposits in the jiurui district of the middle lower yangtze river metallogenic belt eastern china abstract the skarn type copper deposits are widespread in the jiurui district in the middle lower yangtze river metallogenic belt this paper reports a detailed study on mineral chemistry and h o s and pb isotopic compositions on skarn silicate and sulfide minerals in the three major skarn dominant deposits wushan dongleiwan and dengjiashan the wushan skarn deposit is characterized with prograde garnet dominated and clinopyroxene limited skarns with average andradite content of 83 and hedenbergite content of 10 whereas the dongleiwan and dengjiashan deposits are featured with retrograde skarn alteration with abundant hydrous minerals such as epidote and chlorite the garnet and clinopyroxene compositions show 59 andradite and 15 hedenbergite for the dongleiwan skarns and 43 and 22 for the dengjiashan skarns respectively the pistacite components ps value defined as fe3 fe3 al and fe3 fe2 value of epidote are 0 12 and 1 63 for the wushan skarns 0 30 and 32 73 for the dongleiwan skarns and 0 17 and 42 85 for the dengjiashan skarns it is suggested that the prograde skarn mineralization in the three deposits was all formed in a relatively oxidizing environment with the wushan showing the highest oxidation potential and the dengjiashan having the least oxidation potential however in the retrograde skarns the dongleiwan and dengjiashan deposits show higher oxidation potential than that of wushan the three deposits show similar sulfur isotopic compositions of 2 9 to 1 4 and similar lead isotopic compositions with 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 17 900 to 18 205 15 538 to 15 649 and 38 170 to 39 025 respectively all the three deposits should have similar magmatic origin for the ore forming materials based on their s and pb isotopes the oxygen isotopic compositions of the prograde and retrograde fluids in the three deposits show some differences with 18ofluid values of 8 13 and 7 81 for the wushan 6 47 and 2 33 for the dongleiwan and 8 27 and 4 43 for the dengjiashan but the hydrogen isotopic compositions are similar for the prograde 65 to 31 and retrograde 64 to 33 fluids therefore the fluid origins and evolution may be different in each deposit the hydrothermal fluids for the prograde skarns in all three deposits were likely derived from magmatic hydrothermal sources but the dongleiwan and dengjiashan show a higher proportion of meteoric water input in the retrograde stage considering the similar average prograde temperatures 574 to 560 c as calculated from coexisting garnet clinopyroxene pairs and similar retrograde temperatures 281 to 246 c as calculated from chlorite chemistry for the three deposits we suggest that the trigger for deposition of sulfide ores in the wushan garnet dominated skarn deposit was mainly caused by system cooling with temperature drop along with magmatic intrusion and crystallization process the dongleiwan and dengjiashan skarn deposits constitute a well developed retrograde skarn system with abundant epidote chlorite quartz and calcite which probably caused by fluid mixing of high temperature saline magmatic hydrothermal fluids with cooler oxidizing and dilute meteoric water

simultaneous inversion for 3 d crustal structure and hypocentres including direct refracted and reflected phases iii application to the southern rhine graben seismic region germany summary the new technique of simultaneous inversion for hypocentres and crustal structure ssh is applied to the southern rhine graben p and s wave first and later arrival times of direct refracted and reflected phases of 350 seismic events are used in the study substantial improvements of over 100 per cent in the non linear residual fit are obtained for the 3 d and less for the 1 d models many of the structural results of the 3 d ssh models are consistent with major findings from other seismic refraction experiments and fit into the general geological petrological and geodynamica1 frame of this area thus the thick sedimentary layer in the rhine graben proper manifests itself by large negative seismic velocity anomalies for the middle crust the ssh models show a strong seismic differentiation between sections of the graben proper and the areas under its western and eastern shoulders of the vosges and the black forest respectively using a new a priori bayesian inversion technique it has been attempted to substantiate the refraction seismic low velocity zone of gajewski prodehl 1987 in the middle crust under the black forest the results demonstrate that such a velocity low is compatible with the observed traveltimes only if one assumes it as a more local crustal phenomenon in this area in the lower crust the ssh models are able to clarify some controversies from earlier refraction experiments in the region the models reveal many seismic features which are typical for continental rift zones such as high transitional velocity anomalies 7 4 km s l under parts of the graben proper rift cushion in the upper mantle the models show a ring like high velocity structure of 8 30 8 40 km s l centred around a low velocity area 7 9 km s l beneath the kaiserstuhl i conjecture this to be a thermal fingerprint of a partially molten upwelling asthenospheric diapir bulging under the moho the simultaneously relocated hypocentres for the 1 d models show some systematic downward movements for the events in the lower crust of the southern black forest area but otherwise do not differ too much from the initial ones to alter the present seismotectonic picture of the southern rhine graben thus the hypothesized phenomenon of the seismic gap along the southern section of the graben black forest wedge can both structurally and seismically be substantiated the relocated hypocentres suggest optically even a widening of this seismic gap the later has thus to be accepted as a possibly more general characteristic of a graben formation

seismic anisotropy in the lowermost mantle near the perm anomaly the lower mantle is dominated by two large structures with anomalously low shear wave velocities known as large low shear velocity provinces llsvps several studies have documented evidence for strong seismic anisotropy at the base of the mantle near the edges of the african llsvp recent work has identified a smaller structure with similar low shear wave velocities beneath eurasia dubbed the perm anomaly here we probe lowermost mantle anisotropy near the perm anomaly using the differential splitting of sks and skks phases measured at stations in europe we find evidence for lowermost mantle anisotropy in the vicinity of the perm anomaly with geographic trends hinting at lateral variations in anisotropy across the boundaries of the perm anomaly as well as across a previously unsampled portion of the african llsvp border our observations suggest that deformation is concentrated at the boundaries of both the perm anomaly and the african llsvp

structure of the crust and upper mantle beneath the central european rift system the crustal structure of the central european rift system has been investigated by seismic methods with varying success only a few investigations deal with the upper mantle structure beneath the rhinegraben the moho is elevated with a minimum depth of 25 km below the flanks it is a first order discontinuity while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20 22 km depth an anomalously high velocity of up to 8 6 km s seems to exist within the underlying upper mantle at 40 50 km depth a similar structure is also found beneath the limagnegraben and the young volcanic zones within the massif central of france but the velocity within the upper mantle at 40 50 km depth seems to be slightly lower here the total crustal thickness reaches only 25 km the crystalline crust becomes extremely thin beneath the southern rhonegraben where the sediments reach a thickness of about 10 km while the moho is found at 24 km depth the pronounced crustal thinning does not continue along the entire graben system north of the rhinegraben in particular the typical graben structure is interrupted by the rhenohercynian zone with a normal west european crust of 30 km thickness evident beneath the north trending hessische senke a single ended profile again indicates a graben like crustal structure west of the leinegraben north of the rhenohercynian zone no details are available for the north german plain where the central european rift system disappears beneath a sedimentary sequence of more than 10 km thickness

full wave multiscale anisotropy tomography in southern california the southern california earthquake data center scedc operates a densely distributed seismic network in southern california the high quality seismic data it provides have enabled seismologists to obtain high resolution 3d isotropic velocity models for this region owing to the limitations of ray theory assumption used in anisotropy analysis the 3d anisotropic structure in southern california is still poorly understood in this chapter we apply our full wave tomography method to the seismically densely covered southern california to obtain a 3d anisotropic model of this region reliable 3d anisotropic model can help us understand the processes of the upper mantle flow and the crustal deformation

fracture interaction in the gregory rift east africa abstract vertical fracture is important and fracture interaction is quite intensive in the central gregory rift crack branching and non coplanar crack interactions in the rift are characterized for major and minor faults these two manifestations follow theoretical and experimental angular relationships of fracture reasonably well mantle diapirism seems to have been responsible for both tensile probably unstable fracture and block tilting ultimately resulting in fault dips deviating from the vertical

horst faulting in continental rifts abstract horsts in continental rifts are classified into three types on the basis of their structural characteristics type 1 large horsts bounded by near vertical bifurcating faults at rift termini or structural nodes a subset of this type includes horsts comprising an exceptionally uplifted rift shoulder type 2 generally smaller than type 1 type 2 horsts are commonly associated with graben formed in broad zones of crustal extension and thinning in particular they are a feature of both volcanic and sedimentary basins where they are bounded by listric faults type 3 small horsts occurring close to the margins of rift zones formed in a transtensional regime and bounded by near vertical faults the genesis of type 2 and some type 3 horsts is explained by the vening meinesz isostatic model but type 1 horsts require an additional hydraulic force acting at the base of thick fractured crust small scale convection in the asthenosphere could provide this force

full wave effects on shear wave splitting in this chapter we develop an approach to the inversion of 3d anisotropy structure using the sensitivity frechet kernels calculated by an efficient and flexible full wave algorithm based on the normal mode theory

upper rhine graben role of preexisting structures during rift evolution 1 the evolution of the cenozoic upper rhine graben was controlled by a repeatedly changing stress field and the reactivation of a complex set of crustal discontinuities that had come into evidence during permo carboniferous times a comparison of the spatial and temporal thickness distribution of synrift deposits with preexisting fault patterns permits to infer a sequence of distinct basin subsidence phases that can be related to changes in the ambient stress field reactivation of a system of late palaeozoic fault systems outlining troughs and highs controlled the nucleation of initially separated middle and late eocene basins the depocenters of which coincided with a preexisting wsw ene trend during oligocene crustal extension the individual basins coalesced resulting in the development of the ssw nne striking upper rhine graben during the late oligocene chattian change in stress field the upper rhine graben was probably reactivated as a dextral strike slip system with the central graben segment forming a releasing bend during the early miocene aquitanian a major reorientation of the regional stress field is held responsible for the main subsidence phase of the northern parts of the upper rhine graben this is reflected by a counterclockwise rotation and northeastward shift of the depocenter axis and later by the middle miocene uplift and erosion of the southern parts of the upper rhine graben during the plio quaternary the upper rhine graben was reactivated as a sinistral strike slip system with the central graben segment forming a restraining bend

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

seismic investigation of crust mantle transition in continental rift systems jordan dead sea rift and rhinegraben abstract the recent acquisition of high quality seismic refraction data in the jordan dead sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic p waves refracted and reflected at the crust upper mantle boundary these waves cause high amplitude arrivals near the outer cusp of the travel time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp it has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth in the case of the jordan dead sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly the interpretation of seismic refraction data in the rhinegraben indicates the existence of a similar transition zone in both rifts the crust mantle boundary outside the rift is represented by sharp velocity discontinuity the comparison of the velocity structure of the crust upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust mantle interaction

the position of continental flood basalts in rift zones and its bearing on models of rifting abstract two models of rifting pure shear and simple shear are at present being widely discussed in this paper attention is drawn to the fact that continental flood basalts cfb associated with rifts usually display a strong asymmetry relative to the rift axis the karroo the parana and the deccan basalts as well as the ethiopian flood basalts and the basalts of the arabian shield are concentrated mainly on one side of the respective rifts structural data indicate that the eruptions occurred on the upper plates if the low angle simple shear model of wernicke 1985 is accepted it is suggested that the asymmetry of eruptions is caused by the asymmetric position of the asthenopheic rise predicted by this model the low angle detachment may serve as a conduit for magmatic fluid this fits well with data on the existence of large magmatic reservoirs of cfb at subcrustal or crustal levels and explains why eruptions are concentrated in the rift and on one of its shoulders while being restricted on the other this asymmetry of volcanism supports models of continental rifting associated with low angle simple shear detachment

high resolution imaging of the deep anisotropic structure of the san andreas fault system beneath southern california summary geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern california with many large earthquakes occurring off the main san andreas fault this unusually complex plate boundary geometry is imposed by the big bend of the san andreas fault and the dynamics of the underlying lithosphere but lithosphere rheology and distribution of applied forces are still poorly constrained geodynamic modelling is thus very uncertain and distribution of lithospheric deformations highly controversial here exploiting the property that deformation orients olivine crystals we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy with a resolution fine enough for detailed geodynamic interpretation this method relies entirely on finite frequency effects in the splitting of sks waves we build an extensive data set of more than 3400 sks splitting measurements from the analysis of sks waves recorded by all the broad band stations in southern california resolution tests demonstrate that using this data set we are able to resolve anisotropic structures smaller than the size of the first fresnel zone of sks waves the good vertical resolution in our tomographic images results from the short inter station spacing in southern california in our 3 d anisotropy model we find no evidence for localized lithospheric shear deformation beneath the san andreas fault instead the lithospheric plate boundary is localized along a broad shear zone beneath the east california shear zone therefore surface and deep deformation patterns are poorly correlated and most likely decoupled active n s convergence in the transverse ranges results in strongly coherent e w alignment of olivine fast axes in the shallow lithosphere at the top of the asthenosphere the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere

european cenozoic rift system abstract the european cenozoic rift system extends from the coast of the north sea to the mediterranean over a distance of some 1100 km it finds its southern prolongation in the valencia trough and a plio pleistocene volcanic chain crossing the atlas ranges development of this mega rift was paralleled by orogenic activity in the alps and pyrenees major rift domes accompanied by subsidence reversal of their axial grabens developed 20 40 ma after beginning of rifting uplift of the rhenish shield is related to progressive thermal lithospheric thinning the vosges black forest and the massif central domes are probably underlain by asthenoliths emplaced at the crust mantle boundary evolution of this rift system is thought to be governed by the interaction of the eurasian and african plates and by early phases of a plate boundary reorganization that may lead to the break up of the present continent assembly

extensional aspects of earthquake induced ruptures determined by an analysis of fracture bifurcation abstract numerical and experimental investigation of symmetric fracture bifurcation kalthoff 1972 has shown that for forks with small branch angles c where c is approximately 14 the propagation of the branches tends to enlarge the angle for forks with larger branch angles c the propagation of the branches tends to diminish the angle forks with the critical angle c will propagate in their original direction kalthoff theorized that the branch angle changes as a function of k i k ii where k i and k ii are the stress intensity factors for tensile and shear sliding modes respectively and k i is considerably larger than k ii in this study i test the hypothesis that this fracture mechanic theory applies to the analysis of fault bifurcation in the crust particularly in cases of rapid fracture fractures produced during the 1968 earthquake at the coyote creek fault in california are intensively branched and an example of rapid rupture the angular behaviour of the branching ruptures in eight forks follows kalthoffs theory unusually well this implies that fracture at the surface was dominated by the tensile mode additional observations that support this implication are series of prominent ruptures which show openings of 20 30 mm per rupture the symmetrical and bilateral forking the high intensity and angular shapes of individual branches the opening of grabens associated with several bifurcations lack of bifurcation in the southern break of the coyote creek fault and the patterns of en echelon fractures which reflect mixed mode surfacial rupture hence contrary to previous interpretations according to field evidence and fracture mechanic theory the fault bifurcation and opening along the coyote creek fault are not compatible with local tension caused by the primary shear fracture probably occurred by different mechanical modes at depth and at the surface while faulting may have originated by shear at depth rupture at the surface was dominated by far field tension associated with ne sw extension in south california the present model predicts the directions of fracture propagation along the fault

book review processes of continental rifting morgan p baker b h editors 1983 elsevier amsterdam 680 pp price hardcover us 160 50 dfl 250 00

seismic anisotropy in the lowermost mantle beneath north america from sks skks splitting intensity discrepancies abstract we examined sks skks splitting intensity discrepancies for phases that sample the lowermost mantle beneath north america which has previously been shown to exhibit seismic anisotropy using other analysis techniques we examined data from 25 long running seismic stations along with 244 stations of the temporary usarray transportable array located in the eastern southeastern and western u s we identified 279 high quality sks skks wave pairs that yielded well constrained splitting intensity measurements for both phases of the 279 pairs a relatively small number 15 exhibited discrepancies in splitting intensity of 0 4 s or greater suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle because only a small minority of sk k s phases examined in this study show evidence of being affected by lowermost mantle anisotropy the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate the discrepant pairs exhibited a striking geographic trend sampling the lowermost mantle beneath the southern u s and northern mexico while other regions were dominated by non discrepant pairs we carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath north america invoking the alignment of post perovskite due to flow induced by the impingement of the remnant farallon slab on the core mantle boundary we found that our measurements are generally consistent with this model and with the idea of slab driven flow but relatively small scale lateral variations in the strength and or geometry of lowermost mantle anisotropy beneath north america are also likely present

geometry and rate of extension across the pajarito fault zone espanola basin rio grande rift northern new mexico the central velarde graben is the active subbasin of the espanola basin section of the rio grande rift in north central new mexico the velarde graben is bounded on the west in the jemez volcanic field by the pajarito fault zone this fault zone has produced a steep fault scarp about 100 m high where it cuts the l l m y old tshirege member of the bandelier tuff detailed mapping along the north trending pajarito fault zone has revealed a fairly simple geometry in the tshirege member the faults follow numerous vertical joints below this member fault dips are 60 and not listric at shallow depths this simple geometry allows calculation of a mean rate of extension of 0 05 mm yr across the pajarito fault zone for the past 1 1 m y if extension is not perpendicular to the fault zone the extension rate could be as great as 0 07 mm yr lack of transverse tilt of the velarde graben wedge implies that the extension rate across the eastern margin is about the same as for the western margin comparison with a published extension rate for the northern albuquerque belen basin just to the south of the jemez mountains of 0 3 mm yr both sides since rifting began 26 m y ago indicates a slower opening for the velarde graben during the past 1 1 m y if extension is localized along the margins of the velarde graben with little activity along other fault zones in the espanola basin then both the mean rate of extension and the width of the actively extending region have decreased with time for this section of the rio grande rift

tectonic processes and geomorphological design abstract deep seated geodynamic processes are the cause of the existence of the geotectonic stress field the influence of the latter and therefore of deep seated processes in surface geomorphology is investigated it is noted that the features caused by deep seated geotectonic processes can be distinguished from those caused by exogenic agents weather erosion etc by their statistical characteristics all statistically systematic features are of geotectonic origin random features are of exogenic origin in this paper various types of morphological features are analyzed with regard to their origin it is shown that lineaments seen on air photographs may not be of recent geotectonic origin specific examples of such lineaments being erosion scarps are given next it is shown that the orientation patterns inherent in fluvial and glacial valleys are indeed affected to a large extent by neotectonic processes the same is naturally the case for rift valleys and chains of volcanoes a further interesting subject is presented by the so called tectonic landforms inselbergs cirques the shapes of mountain summits etc are all shown to be directly or indirectly caused by deep seated processes the mass movements on slopes are also of great importance although it is often thought that these are triggered by meteorological conditions it is shown that their basic design is influenced to a large extent by deep seated processes amongst other things this is borne out by a correlation between landslide and earthquake activity finally it is mentioned that the common joints on rock outcrops are also designed by the neotectonic stress field and that the latter can in fact be determined by an analysis of joint orientations in summary it may be stated that although exogenic agents may be responsible for many morphological features deep seated processes have nevertheless a great influence on the fundamental design of such features

active folding in the algerian earthquake of 10 october 1980 a fold controlling the form of the local topography near el asnam algeria increased in amplitude at the time of the earthquake of 10 october 1980 the deformation occurred rapidly in an elastic brittle manner and not in the gradual ductile fashion in which folds are normally thought to develop

two types of rifting dependence on the condition of extension abstract rifting of continental lithosphere occurs in two ways 1 1 through continuous extension and subsequent sinking 2 2 in a series of pulses separated by quiet periods sometimes of considerable duration in the first case rifting is not accompanied by significant uplifts in the second uplifts of various sizes from 100 to 2000 km are typical rifting of the first type takes place when continental fragments are freely dispersed as was the case with pangea during its late triassic early jurassic break up in the cenozoic rifts developed in the course of continental collision when relative plate motion was restricted and occurred in short pulses uplift is caused by accumulation of anomalous mantle at the base of the crust during the intervals between phases of extension geological evidence is not consistent with the idea of rifting being directly related to a local source of energy in the mantle mantle plumes and diapirs it is produced by regional stresses in the lithosphere which in their turn may have different origins the lithospheric inhomogeneities observed below the present day rifts are not the cause but a consequence of extension

mineral and fluid geochemistry of the hoosier vein chloride mining district sierra county new mexico quartz veins in the chloride mining district sierra county new mexico host an unusual iron poor calc silicate and copper rich iron poor sulfide mineral assemblage the hoosier vein crosscutting paleozoic sedimentary and tertiary volcanic rocks was studied in detail bornite chalcocite chalcopyrite galena sphalerite and traces of pyrite and hematite are present silver occurs in chalcocite less than 32 wt and bornite less than 0 7 wt calc silicates include pyroxene jo sub 18 91 di sub 8 73 hd sub 0 18 garnet ad sub 20 74 epidote ps sub 30 40 prehnite and subcalcic amphibole three paragenetic stages have been recognized stage 1 brecciation cemented by quartz garnet epidote and sulfides cuts pyroxene skarn replacing carbonate rocks and k feldspar alteration of volcanic rocks stage 2 silver rich bornite chalcocite overlaps the brecciation and banded vein stages stage 3 banded quartz veins with minor sulfides or calc silicates cut earlier stages average quartz fluid inclusion t h values range from 284 degrees 200 m below the present surface to 261 degrees c present surface and decrease to 241 degrees c in later shallow veins stage 1 fluids were boiling indicating a paleodepth of 500 m fluid salinity in early stages varied from 0 35 to 5 85 wt percent nacl equiv later stage fluids had low salinities less than 1 wt nacl equiv early skarn fluids evolved into stage 1 fluids with increasing a sub o 2 and ph due to boiling high a sub o 2 and near neutral ph suppressed iron solubility and decreased available reduced sulfur fluids from adjacent red beds possibly supplied metals for stage 2 mineralization

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

lowermost mantle flow at the eastern edge of the african large low shear velocity province abstract observations of seismic anisotropy in the lowermost mantle are plentiful but their interpretation in terms of mantle flow remains challenging here we interrogate the anisotropic structure of the lowermost mantle beneath the afar region just outside the edge of the african large low shear velocity province using a combination of shear wave splitting techniques applied to phases propagating at five distinct raypath orientations we then model the resulting data set by testing various candidate mechanisms for anisotropy the observations are best fit by a model that invokes the lattice preferred orientation lpo of post perovskite with the 100 crystallographic axis oriented either nearly vertically or highly obliquely to the horizontal plane plausible corresponding mantle flow scenarios involve a significant vertical flow component which suggests that the african large low shear velocity province edge may deflect ambient mantle flow upwards or may be associated with a sheet like upwelling

interpretation of a seismic refraction survey across the arabian shield in western saudi arabia abstract in february 1978 seismic refraction profiles were recorded by the u s geological survey along a 1000 km line across the arabian shield in western saudi arabia this report presents a traveltime and relative amplitude study in the form of velocity depth functions for each individual profile assuming horizontally flat layering the corresponding cross section of the lithosphere showing lines of equal velocity reaches to a depth of 60 80 km the crust thickens abruptly from 15 km beneath the red sea rift to about 40 km beneath the arabian shield the upper crust of the western arabian shield yields relatively high velocity material at about 10 km depth underlain by velocity inversions while the upper crust of the eastern shield is relatively uniform the lower crust with a velocity of about 7 km s is underlain by a transitional crust mantle boundary for the lower lithosphere beneath 40 km depth the data indicate the existence of a laterally discontinuous lamellar structure where high velocity zones are intermixed with zones of lower velocities beneath the crust mantle boundary of the red sea rift most probably strong velocity inversions exist here the data do not allow a detailed modelling velocities as low as 6 0 km s seem to be encountered between 25 and 44 km depth

bootstrap inversion for vertical and lateral variations of the s wave structure and the υp υs ratio from shallow earthquakes in the rhinegraben seismic zone germany using s wave arrival times of 400 earthquakes in the southern rhinegraben 1 d and 3 d inversions for the shear s wave crustal structure of that region are performed these models are then combined with previously obtained 1 d and 3 d p wave models to compute vertical and lateral variations of the p s poisson s ratio such an independent p and s wave inversion approach circumvents many of the limitations of the classical wadati diagram method usually used for the determination of crustal p s ratios for the statistical evaluation of the 1 d models we have used a new monte carlo bootstrap resampling technique which has been found useful in many statistical estimation problems where the underlying statistics of the data is poorly known the 1 d bootstrap p s crustal models exhibit significant vertical variations whereas the upper sedimentary and crystalline layer of the crust is characterized by a significant increase of p s over the reference value p s in the upper and middle crust is lowered this correlates well with several findings from wadati diagrams for p s in other seismically active zones and might be explained through the gradual closing of saturated microcracks in that depth range however the p s reduction can also be interpreted in terms of a particular petrological composition such as orthoclase feldspar without the indispensable need for fluids a lowering of p gus is also observed for the lower crust which might reflect the strong lamination found here from previous deep seismic sounding experiments for the upper mantle a strong increase of p s is observed which is primarily triggered by a decrease of s and is best explained by an eclogite petrological assemblage for the 3 d s wave models significant reductions of more than 100 were obtained for the unexplained travel time residuals variations of p s in the upper and middle crust are mostly induced by changes in p rather than s for the upper crust slight decreases of p s are observed for the rhinegraben but increases are detected for the adjacent vosges and black forest areas in the middle crust the refraction seismic deduced reduction of p s under parts of the black forest can neither be corroborated nor rejected however because of some experimental ambiguity on the effects of petrological composition and interstitial pore fluids this does not rule out the presence of the latter in the lower crust a local reduction of p s is observed under the kaiserstuhl and graben black forest shoulder and has been explained in terms of a metamorphic dehydrating phase change of the hydrous amphibolite facies to the anhydrous granulite facies depending on the pore pressure of the dehydrated water and the fractional amount of interstitial co2 granulite facies metamorphism occurs in the 650 800 c temperature range temperatures at the upper end of this interval can be generated by the mechanism of accretionary magmatic underplating as it has been proposed for some extensional rift zones where asthenospheric upwelling as in the rhinegraben occurs

a crustal structure study of jordan derived from seismic refraction data abstract the interpretation of a deep seismic refraction study in jordan performed in may 1984 shows that much of the country is underlain by continental crust 32 35 km thick and normal mantle with a velocity of 8 0 8 2 km s in the aqaba region southwest and central jordan east of wadi araba the crustal thickness is of the order of 32 35 km while in the amman region it is not less than 35 km in southeast jordan the crust thickens to at least 37 km in what is probably the transition to the arabian shield type of crust the boundaries between the upper and lower crust at about 20 km depth and the lower crust and uppermost mantle are probably transition zones the upper crystalline crust has velocities of 5 8 6 5 km s while the lower crust has velocities greater than or around 6 65 km s while the crystalline basement is exposed in southwest jordan and is at a depth of 2 2 5 km north of amman it is at a depth of not less than 5 km in central jordan a comparison of the crustal type and structure of jordan and the adjacent dead sea rift with that of the black forest and the rhine valley yields a striking resemblance the situation of the jordan dead sea rift is explained in terms of the continental crust of arabia rifting in preference to the thin oceanic crust of the mediterranean sea

crustal thinning in relationship to the evolution of the afro arabian rift system a review of seismic refraction data abstract crustal and upper mantle structure of major units of the afro arabian rift system has been progressively investigated by seismic refraction surveys in the jordan dead sea rift the red sea the afar depression and in the east african rift of kenya the two extreme structures are oceanic crust flooring the axial trough of the southern red sea rift and 30 35 km thick continental crust underneath the jordan dead sea rift and its flanks underlain by a mantle with pn velocities of 8 0 8 2 km s the remainder of the units have a crustal structure of transitional to uncertain nature in the southeastern red sea the change from 15 18 km thick transitional crust to the 40 km thick continental crust of the arabian shield is probably very abrupt and is the major lateral change in structure in this region in the northern afar depression the crust is only 15 20 km thick in the southern afar depression anomalous mantle with 7 2 7 3 km s velocities occurs at depths greater than 25 km also here an abrupt change to a 40 km thick shield crust with a velocity of 8 0 km s is evident from seismic and gravity data crustal thickness of the east african rift system amounts to 30 36 km a recent trial experiment in the eastern rift of kenya indicates a 35 36 km thick crust underlain by a low velocity upper mantle with 7 5 km s at 23 24 km depth a thin high velocity lens of 7 1 km s velocity may be present within the 22 km thick lower crustal layer averaging 6 6 km s velocity investigations of local earthquake body waves in the western rift provide information that the crust here is of continental type with a thickness of about 32 km overlying mantle material of 8 0 km s p velocity and 4 6 km s s velocity in conclusion a model of preferential rifting of continents is applied to the afro arabian rift system where rifting seems to progress into areas with thick continental crust as seen for example in the levant and the east african rift systems instead of using paths in thin crust

constraints on the thermal history of the dead sea graben as revealed by coal ranks in deep boreholes abstract coal ranks have been studied by vitrinite reflectance measurements in three boreholes zuk tamrur 1 and zohar 8 on the judea desert plateau adjacent to the graben margin and amiaz 1 on a down faulted block at the southwest dead sea graben in the plateau boreholes time temperature index tti calculations reveal that the coalification profile probably evolved under a late cretaceous thermal event with a thermal gradient of 35 38 c km followed by gradual decay to the present level c 20 c km by miocene time in amiaz 1 the highest coal rank encountered 0 5 ro is marginal for independent tti calculations however some constraints on thermal history can be inferred stratigraphic and structural relationships between the 0 5 ro isoreflectance in amiaz 1 and the plateau boreholes reveals that most of the coalification measured in amiaz 1 pre dates the formation of the graben and that an additional 3 4 km of miocene holocene graben fill barely affected the coalification cessation of coalification despite increasing burial indicates that the post miocene thermal gradient in the amiaz block could not have exceeded 20 23 c km this finding is consistent with present day heat flow and other geophysical information tectonic models for the evolution of the dead sea graben requiring a high thermal regime are inconsistent with the presented data this raises questions as to the mechanism involved in the formation of rhomb shaped grabens in general on the one hand and for the hypothesis of the leaky nature of the dead sea transform north of the gulf of elat on the other

patterns of faulting in the ethiopian rift valley abstract some typical and some unusual fault patterns are described from the ethiopian rift valley they include arcuate faults convex in plan towards the downthrown side lines of obliquely tilted faulted blocks up horsts horizontally stratified rhomb horsts athwart tilted strata faults whose polarity switches in conjunction with development of small horsts intersecting fault sets and re entrant and semi circular faults explanations remain wanting for many of these phenomena which show that crustal extension can be complex at the meso scale

transfer faults and pull apart model in the rhinegraben from analysis of multisource data from analysis of gravimetric data comparisons of levelings seismic profiling paleostress analysis and seismicity together with remotely sensed and morphological data the rhinegraben may be divided into several rhomb shaped basins separated by ene trending basement highs these data express neotectonic movements and slight spatial differences in the subsidence rate these basins are asymmetrical on one side they are bounded by a large several kilometers of vertical throw normal fault zone on the opposite side there is a megaflexure cut by smaller normal faults shaping tilted blocks since the middle miocene extensional directions are subparallel to the ene trending basement highs that consequently may be interpreted as strike slip fault zones of transfer type across the erstein basement high there is a reversal in the rift asymmetry major bordering fault on the western side to the south on the eastern side to the north when considered in the context of the west european rift the rhinegraben is interpreted in terms of a pull apart depression formed during the horizontal movement of blocks guided by large ene trending transcurrent faults the normal faults that are curved shaped in map view may mark the boundaries between areas where the maximum component 1 of the paleostress field developed in pull apart patterns was changing from horizontal to vertical

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation