mapping upper mantle anisotropy beneath se france by sks splitting indicates neogene asthenospheric flow induced by apenninic slab roll back and deflected by the deep alpine roots the presence of two regional seismic networks in southeastern france provides us high quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy the 10 three component and broadband stations installed in corsica provence and western alps efficiently complete the geographic coverage of anisotropy measurements performed in southern france using temporary experiments deployed on geodynamic targets such as the pyrenees and the massif central teleseismic shear waves mainly sks and skks are used to determine the splitting parameters the fast polarization direction and the delay time delay times ranging between 1 0 and 1 5 s have been observed at most sites but some larger delay times above 2 0 s have been observed at some stations such as in northern alps or corsica suggesting the presence of high strain zones in the upper mantle the azimuths of the fast split shear waves define a simple and smooth pattern trending homogeneously wnw ese in the nice area and progressively rotating to nw se and to ns for stations located further north in the alps this pattern is in continuity with the measurements performed in the southern massif central and could be related to a large asthenospheric flow induced by the rotation of the corsica sardinia lithospheric block and the retreat of the apenninic slab we show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the gulf of lion 30 22 ma to the drifting of the corsica sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the liguro provencal basin 22 17 ma in the external and internal alps the pattern of the azimuth of the fast split waves follows the bend of the alpine arc we propose that the mantle flow beneath this area could be influenced or perhaps controlled by the alpine deep penetrative structures and that the alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

anisotropy beneath the pyrenees range from teleseismic shear wave splitting results from a test experiment we have investigated the seismic anisotropy from sks and skks wave splitting along a n s profile perpendicular to the pyrenean mountain belt the line crossed the north pyrenean fault npf which probably represents the plate boundary between the european and the iberian plates we observed large delay times t 1 3 to 1 5 s and uniform fast polarization directions n100e at three stations located on the iberian plate on the other hand t are smaller 0 5 1 0 s and the scattered at stations located north of the npf the small scale variability the relation with surface geology and the correlation of the t with lithospheric thickness suggest that the anisotropy could be predominantly of lithospheric origin however an asthenospheric contribution cannot be ruled out in the north pyrenean zone an important thermal event before the pyrenean compression has likely erased the former mantle fabric the observed anisotropy is then likely related to the pyrenean orogeny on the iberian plate our splitting parameters are similar to those obtained at two stations located on hercynian structures in central spain this suggests that two contributions are likely superposed to that of the pyrenean built up one related to the hercynian orogeny and the other one related to the rotation of the iberian plate

models of seismic anisotropy in the deep continental lithosphere seismological observations sks wave polarizations systematic p residual variations azimuthal dependence of pn and surface wave velocities or a dispersion of surface waves are not consistent with isotropic if laterally heterogeneous upper mantle structure therefore an anisotropy should be considered as an a priori aspect of future large scale studies of mantle structure most studies of anisotropy however have assumed horizontal or vertical axes of symmetry but such orientations cannot explain bipolar patterns of spatial variations of p residuals which we have observed at many seismological stations on the basis of the petrophysical properties of real upper mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine orthopyroxene and clinopyroxene rotations of the aggregates about vertical and horizontal axes allow us to find the three dimensional orientations of symmetry axes that fit combinations of both p and s seismological observations in central europe and in western north america the orientations with plunging symmetry axes velocity extremes seem to be consistent across large spatially uniform tectonic units and change abruptly at important suture zones

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

topography of the variscan orogen in europe failed not collapsed the variscan orogenic collage consists of three subduction collision systems rheno hercynian saxo thuringian and massif central moldanubian devonian to early carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts but also in post tectonic basins within these foreland belts and on the cadomian crust of peri gondwanan microcontinental fragments which represent the upper plates of the subduction collision zones these marine basins preclude high elevations in the respective areas and also in their neighbourhood widespread late carboniferous intra montane basins with their coal bearing sequences are likewise incompatible with high and dry plateaus while narrow belts with high elevations remain possible along active margins within the orogen comparison of the variscides with the himalaya tibetan plateau is unfounded plausible reasons for the scarcity of high variscan relief include subduction of oceanic and even continental crust subduction erosion orogen parallel extension and most important lithospheric thinning accompanied by high heat flow and magmatism in many areas timing and areal array of magmatism and ht metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse it is suggested instead that lithospheric thinning and heating are due to mantle activities caused by the tethys rift the lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift in terms of topography the variscides represent a failed orogen the ht regime also explains the abundance of granitoids and ht lp metamorphic rocks typical of the variscides melting in the ht regime extracted mafic components from variscan and cadomian crust as well as from cadomian metasomatized lithospheric mantle thus mimicking subduction related magmatism the onset of the ht regime at c 340 ma may also have triggered the final ascent of hp uhp felsic metamorphic rocks

tectono thermal evolution magma emplacement and shear zone development in the caruaru area borborema province ne brazil abstract the neoproterozoic borborema province bp in northeastern brazil is characterized by a regional flat lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones combined field micropetrographic thermobarometric and magmatic fabric studies carried out on plutons country rocks and shear zones in the eastern domain of the province the caruaru area reveal that 1 an early episode of regional deformation with a top to the ne displacement was followed by the development of conjugate strike slip shear zones 2 similar low pressure 650 c metamorphic conditions occurred during the two events 3 plutons intruded the flat lying foliation 4 magma emplacement slightly predated strain localization in transcurrent shear zones but 5 plutons underwent strike slip deformation before complete crystallization therefore regional deformation pluton emplacement and shear zone development were successive events occurring over a relatively short time span 40ar 39ar laser dating of amphibole 584 ma and biotite 545 553 ma single grains supports slow cooling 5 c ma of country rocks through the argon closure temperatures of these minerals complex sometimes discordant amphibole ages bracketed between 552 and 575 ma for shear zones may be attributed to episodic activity and or dike swarm intrusions into them which disturbed the ar system biotite ages for the shear zones 533 545 ma also imply a relatively low cooling rate these data show that the massive injection of magmas in the continental crust of the bp during the brasiliano pan african orogeny produced a long lived thermal anomaly of regional extent the 40ar 39ar ages in the caruaru area are older than in other segments of the bp with similar geological characteristics this indicates a tectono thermal history for the bp that is more complex than previously recognized with either diachronous deformation magmatism and associated low p high t metamorphism or contrasting cooling of the different domains possibly due to differential vertical movements

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

tectonic switching of the trans north china orogen in the middle paleoproterozoic insights from mafic magmatism in the luliang complex

is there a remnant variscan subducted slab in the mantle beneath the paris basin implications for the late variscan lithospheric delamination process and the paris basin formation abstract the paris basin northern france is a late paleozoic mesozoic intracratonic basin that settled upon the collapsed variscan collisional belt the lithospheric roots of the variscan orogenic system below the paris basin have been investigated using a european scale p wave velocity tomographic model tomography points out the existence of a significant high velocity anomaly in the upper mantle below the western part of the basin at 150 200 km depth the anomaly extends with a nw se trend along the buried northern france trace of the northern variscan suture zone i e the bray segment of the upper carboniferous lizard rhenohercynian lrh suture moreover the high velocity anomaly is spatially correlated with the prominent paris basin magnetic anomaly its downdip extent reaches depths greater than 200 km below the southern margin of the paris basin as suggested in previous tomographic studies below ancient suture zones these data argue for such anomaly being the remnant of a variscan subducted slab that escaped the extensive late orogenic delamination process affecting the lithospheric roots by late carboniferous early permian times and that was preserved stable over 300 ma at the base of the lithosphere on a general geodynamical perspective these results provide a new insight into the long term evolution of subducted lithosphere into the mantle in the case of the western european variscan orogenic belt they suggest that the subduction of the lrh slab below the previously thickened variscan crust and its final detachment from the orogenic root have played an important role in the collapse of the belt inducing thermal erosion and extension of the overriding lithosphere the spatial evolution of late orogenic extension across the belt and of subsequent thermal subsidence in the paris basin is suggested to result from the heterogeneous delamination of the lithospheric roots along strike and from the resultant pattern of asthenospheric rise

seismic anisotropy and velocity variations in the mantle beneath the saxothuringicum moldanubicum contact in central europe we report on results of a passive seismic experiment undertaken to study the 3 d velocity structure and anisotropy of the upper mantle around the contact zone of the saxothuringicum and moldanubicum in the western margin of the bohemian massif in central europe spatial variations of p wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere a joint interpretation of p residual spheres and shear wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the moldanubicum

frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations campillo m and plantet j l 1991 frequency dependence and spatial distribution of seismic attenuation in france experimental results and possible interpretations phys earth planet inter 67 48 64 we processed digital records from 431 earthquakes at 25 stations of the laboratoire de detection geophysique network in france we computed spectral amplitudes of pg and lg in different group velocity windows this large data set aliowed us to compute a map of mean crustal attenuation through simultaneous consideration of source amplitude site response and apparent q our inversion procedure combines an iterative reconstruction technique with an adjustment of source amplitudes and site responses at each step the stability of the result is tested with different subsets of data and the resolution is evaluated maps of apparent attenuation at frequencies between 1 5 and 10 hz were computed from pg lg and early coda of lg these results are in a good agreement with the predictions of the singie scattering model both for frequency dependence and for qs qp ratios the comparison between the attenuation anomaly found in the yariscan belt and results from a deep reflection seismic profile confirms the prominent part played by scattering in the apparent attenuation of seismic waves in the crust

pn velocities beneath continental collision zones the turkish iranian plateau summary tomographic imaging of the uppermost mantle velocity in the middle east shows normal pn velocities 8 0 8 2 kms beneath the black sea and the southern caspian sea in contrast low pn velocities 7 9 km s i are found beneath the aegean sea and much of the turkish iranian plateau a region of exceptionally low pn velocities c7 6 km s beneath the lesser caucasus mountains and along the turkish iranian border coincides with a region of high sn attenuation and extensive neogene volcanism all these features suggest that near solidus conditions exist within the uppermost mantle beneath the turkish iranian plateau such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above the presence of a partially melted uppermost mantle weakens the lithosphere beneath the turkish iranian plateau thus allowing it to become the locus of deformation in the arabian eurasian collision zone

physical chemical properties and metal budget of au transporting hydrothermal fluids in orogenic deposits we use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits the most common ore fluid is a low salinity 0 4 6 5 wt nacl twoto three phase aqueous carbonic fluid its th total range is 200 400 8c and total homogenization occurs by bubble or liquid disappearance a less common aqueous fluid of similar bulk salinity and t h total is also documented microchemical data show that the ore fluid in the five deposits is chemically uniform and made of na subordinate k and b and minor to trace amounts of cu as li sr rb ba cs sb and au range 0 5 5 mg g thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition and that its vapour phase was distinctly enriched in au b as and sb the proposed mechanism of gold precipitation is a combination of fluid decompression and boiling combining these results with 3d geometrical reconstructions of one of the studied deposits sigma we estimate that a minimum ore fluid volume of c 0 1 1 0 km is sufficient to generate a world class orogenic deposit orogenic deposits provide about a third of the global gold production goldfarb et al 2001 frimmel 2008 these deposits are formed within active continental margins in orogenic belts where major tectonic boundaries juxtapose metamorphosed volcanic plutonic and sedimentary terranes the ore bodies consist of au mineralized vein networks filled with quartz carbonates and other silicates that in places crosscut the stacked terranes of the continental margins the veins are dilated portions of vertically extensive brittle ductile to ductile fracture networks that form due to the advection of aqueous hydrothermal fluids within the continental crust ridley et al 1996 groves et al 2003 these ore fluids are generated in still undefined source regions deep in the crust and transported to shallower levels 3 15 km depth through the vein networks along which they deposit gold a number of studies kerrich et al 2000 ridley diamond 2000 groves et al 2003 showed that although these networks do not occur systematically in all orogens they have a set of important geological features in common which include 1 formation during accretionary orogenic events 2 occurrence relatively close to regional tectonic lineaments especially along lower order splays of these structures 3 spatial association with granitoid intrusions 4 occurrence within some metamorphic belts with gold precipitation being synto postmetamorphic in age 5 relatively low sulphide abundance within the veins 6 carbonate sericitealbite sulphide hydrothermal alteration of host rocks postdating metamorphism 7 poorly developed mineral and metal zoning within the ore bodies and 8 genesis from low to moderate salinity ore fluids over a large temperature range 250 700 8c but mostly within 300 350 8c these common traits occur systematically in deposits from different terrains having distinct ages suggesting that a common geological process and au transporting fluid are responsible for their formation here we present a database of fluid inclusion properties from five typical orogenic deposits which show most of the geological characteristics listed above the database was constructed with the aim of determining as accurately as possible the properties of the au transporting fluid in this geological environment in particular laser ablation inductively coupled plasma mass spectrometry la icp ms of single fluid inclusions paragenetically coeval with vein gold provides the concentration of a suite of metals including au in this fluid this dataset when combined with 3d geometrical reconstructions of one of these deposits offers new constraints on the volume of the ore from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 8 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics by guest on april 3 2014 http sp lyellcollection org downloaded from

impact melt sheet zircons and their implications for the hadean crust impacts may have been important mechanisms of crustal redistribution and differentiation particularly during intense postaccretionary bombardment between 4 5 ga and 3 9 ga ago evidence of crustal processes during this period is largely provided by detrital zircons from the yilgarn craton australia trace element compositions crystallization temperatures and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas implying widespread felsic continental crust on the early earth however there is little knowledge of zircons formed in impact melt sheets a potential source for the hadean zircons here we present ti thermometry trace elements and inclusion populations of zircons from the 1 85 ga sudbury impact melt sheet ontario canada our results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems we also show that zircons in mafic rocks crystallize in residual liquids of granitic composition producing inclusion assemblages that are remarkably similar to those reported for the ancient yilgarn grains thus we conclude that the trace element compositions and inclusion populations of the hadean zircons are consistent with crystallization from more mafic melts than previously recognized although high crystallization temperature distributions of sudbury zircons indicate that impact melt sheets were not a dominant source for the grains older than 3 9 ga

massif central france new constraints on the geodynamical evolution from teleseismic tomography the massif central the most significant geomorphological unit of the hercynian belt in france is characterized by graben structures which are part of the european cenozoic rift system ecris and also by distinct volcanic episodes the most recent dated at 20 ma to 4000 years bp in order to study the lithosphere asthenosphere system beneath this volcanic area we performed a teleseismic field experiment during a six month period a joint french german team operated a network of 79 mobile short period seismic stations in addition to the 14 permanent stations inversion of p wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3 d velocity structure beneath the massif central down to 180 km depth the upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations the 3 d model reveals two channels of low velocities interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from hercynian time and reactivated during oligocene times the teleseismic inversion model yields no indication of a low velocity zone in the mantle associated with the graben structures proper the observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60 100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere thus giving an idea of the lithosphere thickness a broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the massif central this body is likely to be the source responsible for the volcanism it could be interpreted as the top of a plume type structure which is now in its cooling phase

compressional velocity structure and anisotropy in the uppermost mantle beneath italy and surrounding regions travel times of about 39 000 pn arrivals recorded from regional earthquakes by the italian telemetered seismic network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in italy and surrounding regions this method allows simultaneous imaging of variations of pn velocity and anisotropy as well as crustal thickness variations the po plain the adriatic sea and the ionian sea have normal to high pn velocities in contrast lower velocities 7 9 8 0 kin s are imaged in italy beneath the western alps the northern apennines and eastern sicily and nearby calabria as well as in northern albania and beneath the pannonian basin low pn velocities beneath the northern apennines correlate with present day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes low velocities are consistent with the high attenuation zone inferred in the uppermost mantle beneath the internal apennine units and the tyrrhenian margin of the peninsula by mele et al 1996 1997 on the contrary low velocities beneath the western alps may be an apparent effect due to the abrupt thickening of the crustal roots pn velocity is anisotropic in the study area with a maximum amplitude of 0 2 km s the largest anisotropic velocity anomalies are observed along the major arc structures of italy i e the northern apennines and the calabrian arc indicating that these features are controlled by uppermost mantle processes the anisotropy anomaly along the calabrian arc extends as far as albania but ends abruptly north of this area suggesting that a lithospheric discontinuity is present along the northern albanian border

fluid chemical evolution as a factor in controlling the distribution of gold at the archean golden crown lode gold deposit murchison province western australia the golden crown lode gold deposit is one of a number of northwest southeast to north south trending quartz veins hosted by a steeply dipping 500 m thick differentiated dolerite sill the great fingall sill and adjacent graphitic shale in the day dawn district of the murchison province in the archean yilgarn craton western australia quartz veins of the golden crown deposit are shear zone hosted crosscut the dominant host rock stratigraphy at a high angle and are laminated partly boudinaged and generally bifurcate at their extremities veins are dominated by dark gray or milky white quartz with subordinate calcite and ankerite and in order of abundance arsenopyrite pyrrhotite pyrite galena chalcopyrite and minor sphalerite the vein sulfides coexist with native gold and are most abundant at vein margins textural evidence indicates that precipitation of the majority of these minerals overlapped and that most phases can be considered to have been in equilibrium during vein genesis mineral chemistry fluid inclusions alteration and ore assemblages show that there are significant systematic gradients in intensive variables within the golden crown deposit these are recorded over the approximately 450 m of vertical exposure of the vein system and over up to 30 m into the wall rocks based on proximal alteration assemblages and zoning patterns zoned wall rock alteration haloes are divisible into four sequences three of which are closely related to gold mineralization specific alteration sequences occur within different parts of the vein system biotite alteration predominates in the deeper portions whereas muscovite alteration characterizes the upper and southern portions of the system higher grade gold zones within the golden crown deposit are limited to the northern end of the system in the deeper portion of the veins and expand along strike to the south in the upper portions fluid inclusions interpreted to be trapped during gold mineralization are low salinity 10 wt nacl equiv h2o co2 ch4 inclusions with variable carbonic to aqueous contents that are interpreted to be due to fluid immiscibility inferred methane concentrations in the carbonic phase vary significantly from 19 7 mole percent in the vicinity of the shale contact to 5 2 mole percent in the shallower southern end of the deposit carbon and oxygen isotope compositions of carbonates 13ccalcite 10 5 to 1 9 13cankerite 5 3 to 4 1 18ocalcite 7 0 18 3 and 18oankerite 7 7 9 2 and oxygen isotope compositions of vein quartz 18oquartz 10 2 12 5 are well within the range of known archean orogenic lode gold deposits combining the results of all geothermometers and geobarometers a temperature of 300 to 400 c and pressure of 2 5 0 5 kbars are determined with temperature variations over the exposed quartz vein of less than 40 c the as content at of arsenopyrite and au ag ratios of native gold show significant compositional variation which appears to be controlled by parameters such as a h2s aq and f o2 not temperature gradients on the basis of the variations in fluid composition determined in the reefs the two major processes responsible for high grade gold mineralization are interpreted to have been phase separation in the deeper portions of the ore forming system approximately below 400 m level and above these levels mixing between two modified ore fluids one reduced fluid that had reacted with graphitic shale and a relatively oxidized fluid that had partly equilibrated with dolerite

upper mantle deformation beneath the pyrenean domain inferred from sks splitting in northern spain and southern france we performed shear wave splitting analysis on 203 permanent french rlpb cea and cat alonian networks and temporary pyrope and iberarray experiments broad band stations around the pyrenees these measurements considerably enhance the spatial resolution and coverage of seismic anisotropy in that region in particular we characterize with different shear wave splitting analysis methods the small scale variations of splitting parameters and t along three dense transects crossing the western and central pyrenees with an interstation spacing of about 7 km while we find a relatively coherent seismic anisotropy pattern in the pyrenean domain we observe abrupt changes of splitting parameters in the aquitaine basin and delay times along the pyrenees we moreover observe coherent fast directions despite complex lithospheric structures in iberia and the massif central this suggests that two main sources of anisotropy are required to interpret seismic anisotropy in this region i lithospheric fabrics in the aquitaine basin probably frozen in hercynian anisotropy and in the pyrenees early and late pyrenean dynamics ii asthenospheric mantle flow beneath the entire region imprint of the western mediterranean dynamics since the oligocene

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

cenozoic volcanism in the bohemian massif in the context of p and s velocity high resolution teleseismic tomography of the upper mantle

mapping seismic anisotropy of the lithospheric mantle beneath the northern and eastern bohemian massif central europe abstract we evaluate 3d body wave anisotropic parameters shear wave splitting fast s polarizations and delay times of the split slow shear waves and directional terms of relative p wave residuals from data of passive seismic experiment bohema ii 2004 2005 data from 34 temporary stations increased substantial density of observations in the bohemain massif bm and allowed us to scan in detail the upper mantle structure of its northern and eastern parts for the first time we delimit domains of the mantle lithosphere in this part of the bm and infer 3d self consistent models of their fabrics by a joint inversion of the body wave anisotropic parameters fabrics of the northern and north eastern parts of the bm are approximated best by peridotite aggregates with the a c foliations dipping approximately to the n and ne respectively whereas a model with the westerly dipping a lineation fits best the fabric of the south eastern domain the saxothuringian fabric nw of the eger rift extends to the east across the elbe fault zone and continues along this zone to the southeast beneath the cretaceous basin the south eastward continuation of the elbe fault zone seems to be related to the boundary between two different fabrics of the northern and southern brunovistulian domains below the moravo silesian zone this study shows an underthrusting of the brunovistulian micro plate beneath the eastern rim of the bm and indicates that its northern and southern fragments might have originally belonged to baltica and to gondwana respectively according to a zone of distinctly decreased anisotropic signals the brunovistulian micro plate extends at least about 100 km westward beneath the moldanubian with these new findings we update the domain like mantle structure of the bm and compare the results with inferences from the upper mantle velocity tomography and depth changes of the lithosphere asthenosphere boundary retrieved in a detailed model from relative residuals and from the receiver functions inferences on anisotropic structure beneath the permanent observatories included in different experiments are stable and are validated by the bohema ii experiment

gold abundance and an aluminosilicate rich alteration zone in the val d or mining district abitibi quebec examples of the fossil geothermal fields abstract investigation of gold abundance in a gold rich mining district by accurate analytical techniques e g instrumental neutron activation and or radiochemical neutron activation shows that gold is anomalously distributed in some parts of the val d or mining district of quebec it is possible to distinguish 1 background values 1 4 3 5 ppb au 2 zones of primarily anomalous gold values around the lamaque sigma mines median 15 ppb au 3 enrichment halos around gold orebodies median 70 ppb au 4 secondary gold enrichment in shear zones and 5 high background values for the east sullivan monzonitic stock the bourlamaque batholith is considered to be a gold enriched intrusion because of areas shear zones that are anomalously enriched in gold median 10 ppb au it is concluded that there are auriferous geological units in the val d or mining district such as the bourlamaque batholith and val d or formation the latter contains the lamaque sigma gold mines representing 49 6 of the total gold production in the district it is interpreted that the val d or formation is part of a central volcanic complex within an island arc system that the centre of this complex is located in the main lamaque plug and that this environment may be compared to high temperature active geothermal systems which are commonly responsible for the formation of epithermal gold deposits the lamaque sigma area is considered to be a fossil geothermal field potentially explaining the anomalous gold values in addition a dumortierite bearing aluminosilicate rich zone of hydrothermal alteration in the val d or mining district is considered as for other similar occurrences to be the result of hydrothermal alteration within a geothermal field producing an al and si rich leached protholith which has subsequently been metamorphosed and deformed a model is proposed for the formation of the gold deposits within the primary gold rich environment by mobilisation of gold in the volcanic rocks due to metamorphism and deformation therefore two distinct and successive events are postulated 1 gold rich synvolcanic geothermal activity and 2 late remobilisation from the host rocks followed by deposition of gold ore within favourable structures

carbonate sulfide equilibria and stratabound disseminated epigenetic gold mineralization a proposal based on examples from alleghany california u s a abstract stratabound disseminated pyritic au ore bodies were produced by reactions between wall rocks and through flowing fluids in mesozoic epigenetic au quartz vein systems in the sierra nevada metamorphic belt equilibrium relations among fe bearing carbonate and sulfide minerals were critical in determining which rock types were likely to host disseminated mineralization along portions of discordant veins the compositions of metasomatic carbonates in hydrothermally altered wall rocks at alleghany california u s a were larely predetermined by the relative proportions of fe mg and ca in the unaltered wall rocks thus coexisting solid solutions in the magnesite siderite and dolomite ankerite series from a variety of different wall rocks yield an empirical phase diagram for a large part of the ca co 3 mgco 3 feco 3 system at the temperature of metasomatism 325 50 c because fe mg silicates were unstable in alteration zones adjacent to the veins wall rock fe was partitioned between carbonates and sulfides pyritization and disseminated au mineralization occur in a variety of igneous and metasedimentary wall rocks in which the initial molar fe fe mg 0 5 in altered wall rocks with initial molar fe fe mg 0 5 fe was incorporated almost entirely within mg rich carbonates x feco 3 0 6 in magnesite siderite solutions it is proposed that the co 2 rich vein fluid responsible for the alteration and mineralization was partially buffered with respect to h 2 s co 2 h 2 ratios by equilibrium between pyrite and mg 0 4 fe 0 6 co 3 graphite as it traversed and altered intermediate volcanic and sedimentary rocks this fluid then locally reacted with lower fe fe mg rocks to form fe bearing dolomite magnesite assemblages and reacted with higher fe fe mg rocks to form ankerite pyrite assemblages gold precipitated from saturated solutions of bisulfide complexes partly in response to fluid desulfidation and reduction caused by the pyritization reactions in terranes dominated by intermediate metavolcanic and metasedimentary rocks favorable host rocks for this type of mineralization need not have high fe contents but do require high fe fe mg ratios they may include felsic volcanic and plutonic rocks fe rich tholeiitic differentiates banded fe formations and a variety of siliceous and argillaceous sedimentary rocks rocks which tend not to be heavily sulfidized because they have low initial fe fe mg ratios include ultramafic and mafic igneous rocks and some argillaceous sedimentary rocks exploration guidelines based on these principles may be useful elsewhere in the sierra nevada and in other comparable heterogeneous metamorphic terranes if modified to reflect the dominant buffering rock types in a given fluid flow path carbonate sulfide equilibria are capable of approximately buffering the carbonate sulfide ratios of co 2 rich vein fluids f co 2 10 2 8 at 325 c 200mpa or 2000 bar the alleghany fluid f co 2 10 3 2 or 10 mol co 2 had a molar co 2 h 2 s ratio of approximately 10 3 assuming graphite saturation at lower co 2 fugacities fe bearing silicates entered the buffering assemblages carbonatization reactions could potentially de sulfidize some wall rocks releasing s and associated metals to the fluid this would be most likely to occur in pyrite bearing mafic and ultramafic rocks and some argillites

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

the silidor deposit rouyn noranda district abitibi belt geology structural evolution and paleostress modeling of an au quartz vein type deposit in an archean trondhjemite late orogenic gold bearing quartz vein deposits within the rouyn noranda mining district occur mainly within archean tonalitic plutons of the blake river group southern abitibi belt the silidor deposit is a representative example of a pluton hosted lode gold deposit the silidor mine contained 2 95 million tons mt mined and estimated reserves grading 5 1 g t au 15 t au the mineralized zone is 900 m in length and has a vertical extent of 900 m an average thickness of 3 5 m and trends northwest southeast with a dip of 50 to 70 ne its alteration envelope consists of a red hematite altered trondhjemite measured 18 o values for quartz veins 7 7 10 9 and 34 s values for pyrite 6 1 to 9 4 imply oxidizing conditions during gold deposition the mineralized zone comprises vein quartz white gray and smoky varieties beige mineralized trondhjemite and green carbonate sericite fuchsite breccia which resulted from shearing and metasomatism of an early northwest southeast dioritic dike a quantitative microtectonic study 400 measurements was carried out on the powell tonalitic sill in the silidor mine area to reconstruct the deformation before during and after the mineralizing events the reduced stress tensors display large variations in 1 orientation trending successively northwest southeast northeast southwest and finally north south the silidor deposit is the result of several vein filling events which occurred during evolution from strike slip faulting to reverse faulting regimes with 1 remaining northeast southwest but with exchange of orientation of 2 and 3 such variations may reflect an oblique collision and processes of stress permutation paleostress mapping of archean terranes can be used as a targeting tool for mesothermal lode gold deposits reduced stress tensors were used for the computation of paleostress maps using a distinct element model the silidor au quartz mineralization appears on these paleostress maps in an area characterized by low mean stress throughout the deformation history of the area near the horne creek fault this study emphasizes the role of second order faulting in the location of low and high pressure zones in the archean crust and the possible role of a tectonic indentor in the location of au mineralization

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

the deep lithosphere in the alps a model inferred from p residuals abstract the tomographic studies of the alps and their surroundings based on the relative p residuals indicated two lithosphere roots attaining depths over 200 km separated by an upwelling of the lithosphere asthenosphere transition beneath the central part of this mountain range the directional dependence of relative p velocities indicates an anisotropic wave propagation in the subcrustal lithosphere beneath the alps and their surroundings the presented model of the deep lithosphere structure suggests several phases of the tectonic development of this region collision of a tip of the adriatic plate with europe in the eastern alps and dextral strike slip movement of the adriatic ending with underthrusting of europe beneath the western alps and counterclockwise rotation the latter contributes to a steep plunge of both plates beneath the eastern alps and to stress accumulation in the friuli region a model with delamination of the rigid upper crust and its rotation resolves some of the inconsistencies between the near surface and deep tectonics and also explains the distribution of seismicity

coupled magmatism mantle convection system with variable viscosity abstract numerical models are presented for the thermal and chemical evolution of the mantle when it is controlled by both magmatism and mantle convection the mantle is modeled as a fluid with temperature and pressure dependent viscosity in a two dimensional square box heated by incompatible radioactive elements that decay with time the influence of phase transitions at 660 km between the upper and lower mantle is also included magmatism is modeled as permeable flow of magma produced upon pressure release partial melting of the convecting mantle materials it is shown that the mantle may exist in one of two regimes at first when the internal heating rate is higher than a threshold the mantle is on a regime where episodic but active magmatism takes place to make the mantle chemically stratified with the upper mantle occupied by magma residue harzburgite and the deeper part of the lower mantle occupied by basaltic materials there is a chemical discontinuity along the 660 km phase boundary mantle convection occurs as a layered convection punctuated by flushing events and is sluggish except at the time of flushing events owing to the compositional buoyancy from the chemical stratification the flushing events induce vigorous magmatic activity as the internal heating rate falls below the threshold owing to the decay of radioactive elements the thermal and chemical state jumps to another regime in this regime magma does not erupt and mantle convection occurs as layered thermal convection in spite of the stirring due to the thermal convection the chemical discontinuity and a portion of the chemical stratification formed in the earlier regime remains for billions of years the numerical models suggest that a mantle convection was much less active than expected from the strong internal heating and chemically distinct reservoirs are formed in the early earth and once formed b the reservoirs have survived convective stirring for billions of years

joint interpretation of upper mantle anisotropy based on teleseismic p travel time delays and inversion of shear wave splitting parameters abstract we inverted shear wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self consistent three dimensional 3 d image of the anisotropic upper mantle beneath the continents efficiency of the simultaneous 3 d analyses of p residual spheres and shear wave polarizations is demonstrated on data from two regions southern sweden and the western usa the anisotropic inferences of the subcrustal lithosphere are to a first approximation represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes

seismic anisotropy of the french massif central and predisposition of cenozoic rifting and volcanism by variscan suture hidden in the mantle lithosphere 1 we model the structure of the lithosphere its thickness and seismic anisotropy caused mainly by systematic orientation of olivine crystals from spatial variations of p wave delay times and the shear wave splitting observed at a dense network of mobile and permanent stations within the french massif central mc three major lithosphere domains with different seismic anisotropy are distinguished a suture in the deep lithosphere limits the thick lithosphere of the limousin western mc characterized by a consistent anisotropic pattern with high velocities dipping to the west the eastern mc comprising two different domains with a weakened mantle fabric in the south shows the high velocities dipping systematically to the east the suture between the western and eastern mc parallels the major crustal boundary the late variscan sillon houiller transfer fault in the south and the tauve aigueperse fault in the north with an offset of 10 20 km to the east the offset of the crustal and mantle parts of the same suture indicates that the rigid upper crust might be detached from the mantle lithosphere we suggest that the mantle suture hidden beneath an allochthonous crust was reactivated during the cenozoic extension of the weakened and thinned lithosphere in the south of the eastern mc and predestined a space for the major volcanism mont dore cantal the rigid northern domain characterized by a thicker lithosphere with a well preserved mantle fabric reacted to the extension predominantly by rifting e g limagne and roanne grabens

european mantle lithosphere assembled from rigid microplates with inherited seismic anisotropy abstract we have modelled three dimensional seismic anisotropy of the mantle lithosphere from anisotropic parameters of teleseismic body waves we invert jointly shear wave splitting parameters and p residual spheres based on data from dense networks of temporary and permanent stations in four european regions ranging from the variscan belt to the baltic shield changes in orientation of the large scale anisotropy caused by systematic preferred orientation of olivine identify boundaries of domains of mantle lithosphere individual domains are characterized by a consistent large scale orientation of anisotropy approximated by hexagonal or orthorhombic symmetry with generally inclined symmetry axes the domains are separated by mapped tectonic boundaries sutures which cut the entire lithosphere besides the change of anisotropy orientation at domain boundaries we often observe a change of the lithosphere and or crust thicknesses we do not detect any fabric of the mantle lithosphere which could have been produced by a collision of microcontinents in a volume detectable by large scale seismic anisotropy the observations of consistent anisotropy within individual blocks of the mantle lithosphere reflect frozen in olivine preferred orientation most probably formed prior to the assembly of microcontinents that created the modern european landmass therefore our findings support a plate tectonic view of the continental lithosphere as a mosaic of rigid blocks of the mantle lithosphere with complicated but relatively sharp contact zones these contacts are blurred by the easily deformed overlying crust terranes

the role of geoscience breakthroughs in gold exploration success yilgarn craton australia abstract australia experienced a major increase in gold production during the 1980s and this is reflected in the production of 250 tonnes of gold per annum since then much of the production has come from the archean yilgarn craton of western australia and mostly from gold only deposits discovery is the factor best accounting for the outperformance of yilgarn gold production in a global context and both greenfield and brownfield exploration success have been essential scientific input to gold exploration evolved rapidly from 1980 and is implicated in the exploration success in the yilgarn six scientific breakthroughs had changed the nature of almost all yilgarn gold exploration thinking by 1990 and appear to have been important components of the exploration success breakthroughs relate to primary gold geology timing and structural control favourable host rocks gold related alteration and gold in the regolith environment landscape evolution gold geochemistry targeted sampling these six breakthroughs changed paradigms from which the western australian industry has not reverted these breakthroughs were not necessarily a key part of the original aims of the researchers common features in the development of the breakthroughs were small but persistent research groups with an understanding of industry a willingness to challenge dogma development of informal industry linkages and the ability to formulate and tackle key questions breakthroughs led to applications not considered when the research was conceived the breakthrough ideas were eventually available globally but they were not equally applicable and not equally accepted and adopted an openness to new ideas may correlate with the massive and ongoing exploration success witnessed in the yilgarn goldfields

boundaries of mantle lithosphere domains in the bohemian massif as extinct exhumation channels for high pressure rocks abstract five domains microplates have been recognized by seismic anisotropy in the mantle lithosphere of the bohemian massif the mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their variscan assembly the present day mantle fabric indicates that this process consisted of at least three oceanic subductions each followed by an underthrusting of the continental lithosphere the seismic anisotropy does not detect remnants of the oceanic subductions but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions the most robust continent continent collision was followed by westward underthrusting of the brunovistulian mantle lithosphere still detectable by seismic anisotropy more than 100 km beneath the moldanubian mantle lithosphere major occurrences of the high pressure ultra high pressure hp uhp rocks follow the ene and nne oriented sutures and boundaries of the mantle lithosphere domains mapped from three dimensional modeling of body wave anisotropy the hp uhp rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries the close relation of the mantle sutures and occurrences of the hp uhp rocks near the paleosubductions testifies for models interpreting the granulite garnet peridotite association by oceanic continental subduction underthrusting followed by the exhumation of deep seated rocks our findings support the bivergent subduction model of tectonic development of the central part of the bohemian massif the inferences from seismic anisotropy image the bohemian massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric the collisional mantle boundaries blurred by tectonometamorphic processes in easily deformed overlying crust served as major exhumation channels of the hp uhp rocks

lateral displacement of crustal units relative to underlying mantle lithosphere example from the bohemian massif abstract we propose a mechanical model of deformation of the entire lithosphere of the bohemian massif bm whose core is formed by an asymmetric block of the tepla barrandian tb unit in between the saxothuringian st and moldanubian md units for the modelling we have re processed p wave travel times recorded during the last two decades at dense networks of seismic stations installed in the bm during several passive seismic experiments we also use previous results of anisotropic studies based on splitting of teleseismic shear waves this allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains the domains are rigid enough to preserve pre orogenic olivine fabrics differently oriented in each of the units shapes and dips of the mantle boundaries representing major zones of weakness inherited from the variscan amalgamation of independent microplates indicate that north westward subductions beneath the tb unit dominated tectonic development of the core of the bm two mantle lithosphere domains with different fabric orientations separated by a wsw ene striking shear zone underlie the tb crust the nw domain is the tb mantle lithosphere while the se domain is the md mantle lithosphere thrust under the tb crust lithosphere of the north western tb domain compressed between early variscan subductions of the st continental lithosphere from the northwest and the md continental lithosphere from the southeast was pushed south westward by about 50 km though the crust of the south westerly tb promontory is commonly attributed to the md unit apparently it preserves the tb mantle lithosphere the shifted tb lithosphere provides compelling evidence in support of older views suggesting that the zone erbendorf vohenstrauss zev originally belonged to the tilted western rim of the tb unit during the final phase of the assemblage of the bm the rigid tb lithosphere was disrupted by the southward pushing st lithosphere along the newly formed nw se striking jachymov fault zone jfz this lithosphere scale process most likely changed the tectonic regime released subduction related forces and started the gravity dominated tectonics

a multidisciplinary study of a syntectonic pluton close to a major lithospheric scale fault relationships between the montmarault granitic massif and the sillon houiller fault in the variscan french massif central 2 gravity aeromagnetic investigations and 3 d geologic modeling 1 new gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric scale fault this paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and anisotropy of magnetic susceptibility ams studies on the same pluton in the northern part of the variscan french massif central fmc the montmarault massif crops out along the sillon houiller fault shf bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the shf and suggest laccolitic spreading northwestward on the basis of petrophysical measurements direct two dimensional 2 d joint gravity and magnetic modeling has been performed along 10 cross sections in order to quantitatively constrain the 3 d pluton geometry and its relationships with surrounding geologic units these geophysical cross sections new structural information field and ams measurements and petrophysical data have been integrated into a regional 3 d geological and geophysical model altogether the results obtained from geochronology petromagnetic fabrics part 1 gravity and aeromagnetic investigations as well as 3 d modeling part 2 demonstrate that the montmarault pluton was emplaced during the namurian period along the shf our results further show that at that time in response to a nw se regional extension if the shf existed it behaved as a normal fault mylonites attesting for synmagmatic normal motion on the northeastern part of the montmarault pluton strengthen this tectonic scenario during the late carboniferous the fmc experienced ne sw extension along the shf by 80 km of brittle left lateral wrench offset this second tectonic event is well recorded in the stephanian coal basins which were formed along nw se listric brittle faults and constrain the present day shape of the montmarault pluton

seismic tomography of the upper mantle beneath the north eastern bohemian massif central europe abstract we present a 3 d velocity model of the upper mantle beneath the northern and eastern parts of the bohemian massif bm based on data from passive seismic experiment bohema ii may 2004 june 2005 consisted of 35 temporary stations complemented by data of permanent observatories the resulting tomographic model with a variance reduction of 84 shows relatively small velocity variations both in size and in amplitude a small size high velocity perturbation in the eastern part of the model can be related to the complex structure beneath the sudetes moravo silesian unit contact similarly to results from the bohema i experiment in the western bm the presented model from bohema ii data confirms the north southward thickening of the lithosphere though in the upper mantle the low perturbations prevail some inconsistencies among the isotropic velocity perturbation model sp receiver function inferences and anisotropic models of the mantle lithosphere domains indicate that both the topography of the lithosphere asthenosphere boundary and internal velocity structure of the upper mantle beneath the bm can be complex we tested two regularization techniques truncated singular value decomposition and damped least square method results of which lead us to prefer the damped least square method the truncated singular value decomposition seems to be more sensitive to uneven ray geometry

seismic anisotropy beneath the northern apennines italy mantle flow or lithosphere fabric shear wave splitting estimates from recordings of 10 portable seismographic stations during the first year of the retreat seismic deployment in combination with broadband data from the italian national seismic network are associated with seismic anisotropy within the upper mantle beneath the northern apennines anisotropic parameters derived from both shear wave splitting and p travel time residuals vary geographically and depend on event back azimuth reflecting complexity in the underlying mantle strain field variations of the splitting time delays and fast polarization seem to exclude a 2 d sublithosphere corner flow associated with the apennines subduction as the main source of the inferred anisotropy the anisotropic signal may be generated by a frozen in fabric of the adriatic and tyrrhenian lithosphere domains or by flow variations induced by episodic and fragmentary slab rollback

how robust is isotropic delay time tomography for anisotropic mantle using isotropic inversion of teleseismic p travel times computed for laterally varying anisotropic structures we investigate how anisotropy may show up in classical tomographic studies based on isotropic 3 d inversions anisotropic bodies of several tens of kilometers in the upper mantle may significantly bias the estimation of the isotropic part of a p wave velocity image the largest artifacts are introduced by anisotropic bodies with a dipping olivine a axis which could be associated with fossil subduction zones however anisotropy induced artifacts are negligible for anisotropy caused by pure shear deformation of the lithosphere due to horizontal extension or compression the negative p wave velocity anomaly arising from isotropic inversion beneath a rift zone may be enhanced if the olivine crystal s a axes are preferentially oriented parallel to the asthenospheric flow along the rift axis examination of recently published tomographic models shows that the image of the rhine graben may contain significant anisotropic artifacts

refraction seismic investigations of the northern massif central france abstract new refraction seismic data have been obtained in the northern french massif central seven shots were recorded simultaneously on two parallel nnw sse i e perpendicular to the surface structures oriented profiles allowing for inline and fan interpretations the results of a 2d interpretation reveal a large scale updoming of the moho of about 2 km compared to average western europe corresponding to the cenozoic uplift of the massif central local crustal thinning of up to 20 is restricted to the cenozoic grabens as seen at the surface beneath the volcanic area of cantal one of the largest cenozoic european shield volcanoes which was active in miocene to recent times significant crustal thickening of 2 3 km is observed velocities in the upper mantle are generally in the range of 7 9 8 1 km s beneath the limagne graben and surrounding undisturbed areas underneath the volcanic area of cezallier however velocity is reduced to about 7 7 km s corresponding to the area of crustal thickening therefore the low velocity in this zone is explained by the remainders of cooling magma chambers and heating by ascending hot plume material the cenozoic tectonic evolution of the massif central has affected crust and upper mantle in different ways oligocene rifting corresponds well to local crustal thinning but no correlation is detected to velocities of the upper mantle in contrast the volanic event occurring since miocene times is related to upper mantle velocity reduction and crustal thickening due to underplating

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

a global study of pn anisotropy beneath continents we apply a two station method of velocity determination to p n arrival times reported in the international seismological centre bulletin during the period 1964 1993 the azimuthal variation of these velocities provides evidence of p n anisotropy as large as 9 6 anisotropic regions are identified by examination of the azimuthal variation of the p n velocities within circular caps of radius 1 5 and 3 spaced at 1 intervals across the earth this procedure leads to 255 geographically distributed estimates of anisotropy the length scale over which the anisotropy varies indicates that uppermost mantle anisotropy beneath continents is controlled by regional orogenic strains and is inconsistent with a general interpretation in terms of plate scale mantle flow in addition the correlation between p n anisotropy and surficial tectonic features indicates that the anisotropy is primarily the result of the most recent deformation event we compare our p n anisotropy results fast directions and magnitudes with those of sks splitting studies to attempt to constrain the depth extent of the anisotropy this comparison suggests that in some areas where the fast directions agree sks splitting observations may arise from a layer beginning just below the moho whereas in others it must have a deeper origin in these areas the relative magnitudes of the measured anisotropy are consistent with a thin 100 150 km lithospheric layer of anisotropy

multi stage growth of the trachytic lava dome of the puy de dome chaine des puys france field geomorphological and petro geochemical evidence abstract understanding lava dome eruptions is a major concern in volcanology regarding the assessment of associated hazards this question is usually addressed through the study of active calc alkaline domes however alkaline domes remaining poorly known the puy de dome is the most prominent volcano of the chaine des puys quaternary intraplate volcanic field in the french massif central it is a complex trachytic lava dome ca 11 ka old field investigations geomorphological and petro geochemical studies were performed to understand the magmatic system of this dome and reconstruct its volcanic evolution the different lava facies were fully characterised and their relationships with the volcano building stages have been established a new four step evolution is proposed for the growth of this edifice mineralogical and geochemical variations document magmatic differentiation with self mixing and minor crustal contamination in a zoned reservoir within the upper crust magma mixing and crustal contamination through fluids interactions during magma ascent and the involvement of fumarolic activity throughout the edifice building are also documented further this work highlights the specific features of the puy de dome compared to calc alkaline domes lack of lava flows and almost complete magma degassing during ascent resulting in the lack of significant explosive activity until the last eruption step

an array study of lithospheric structure across the protogine zone varmland south central sweden signs of a paleocontinental collision abstract a small seismological array was installed on both sides of the protogine zone pz in varmland south central sweden to study the structure of the mantle lithosphere and lateral variations of its anisotropy no distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the pz the observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture the best solution of a joint analysis of anisotropic inferences from teleseismic p residual spheres and an inversion of shear wave splitting parameters resulted in 3d self consistent anisotropic models of blocks of the subcrustal lithosphere the anisotropic structures within the lithosphere are approximated by hexagonal models k p 5 with low velocity symmetry axes the high velocity planes dip to the e in a region westward of the pz and to the nw eastward of the pz the pz can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin

lateral variations of pn wave velocity in northwestern italy the pn arrival times recorded from seismic networks operating throughout northwestern italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station by means of careful data selection a quality data set was obtained removing statistical outliers poorly recorded events and scantily sampled stations moreover synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique the agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete a low velocity zone was found beneath the western and northwestern side of the alps it may be related to an increase in depth of the moho as supported by other geophysical data the high pn velocities found in the eastern side of the western alps indicate the presence of high velocity and density lower crust rocks of the so called ivrea body the high pn velocity underlying the ligurian sea could be related to a high velocity structure existing in the upper mantle the shape of the velocity anomalies matches not only the major tectonic features but also the bouguer gravity anomalies of the area

crust mantle relationships in the french variscan chain the example of the southern monts du lyonnais unit eastern french massif central garnet lherzolite from the lyonnais area eastern french massif central occurs as several lenses elongated within the regional foliation of garnet biotite sillimanite gneisses within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses petrological and thermobarometric studies emphasize a tectonometamorphic re equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution during the burial stage interpreted as lithospheric subduction the peridotites underwent their mylonitic deformation under high pressure conditions 23 30 kbar in contrast the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions 6 8 kbar 700 c our thermobarometric investigations allow us to interpret the granulitic ultramafic association from the monts du lyonnais area as a lithospheric section buried into a palaeozoic subduction zone laminated during continental collision and uplifted by erosion processes

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

imprints of diverse mantle deformational episodes in the cauvery suture zone south india abstract this study attempts to understand the upper mantle deformation pattern beneath the northern part of southern granulite terrain sgt located to the south of archean dharwar craton in south india the study region comprises the tectonic blocks nilgiri biligiri rangan shevaroy and madras blocks separated from the northern part of madurai block by the cauvery suture zone csz associated with subduction accretion collision tectonics shear wave splitting observed on high quality xk k s sks skks pks waveforms from 119 teleseismic events recorded by 19 broadband seismic stations mostly within the csz and the northern part of madurai block is utilized in this study the 122 splitting measurements obtained show an average delay time of 0 8 s between the fast and slow waves strong lateral variations in the fast polarization azimuths fpa are observed with the splitting parameters at the majority of stations showing a dependence on back azimuth importantly excluding the madras block a poor correlation between the fpa and the present day absolute plate motion apm direction is observed this dominance of non apm strain reflects complex anisotropy preserved in the lithosphere by plotting the fast polarization trends based on the ray piercing points at a depth of 150 km three distinct splitting patterns associated with the i biligiri rangan block and the eastern part of the nilgiri block ii eastern part of the csz and the northern madurai block nmb and iii madras block and its vicinity within the csz are observed the strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions two episodes of ocean closure and the collision that followed plate motion related strain or a combination of these similar deformation patterns in the northern part of madurai block might be related to remnants of subducted oceanic lithosphere

a tertiary asthenospheric flow beneath the southern french massif central indicated by upper mantle seismic anisotropy and related to the west mediterranean extension abstract upper mantle flow beneath the french massif central is investigated using teleseismic shear wave splitting induced by seismic anisotropy about 25 three component stations short period intermediate and broadband were installed during the period 1998 1999 in the southern massif central from the clermont ferrand volcanic area to the mediterranean sea teleseismic shear waves sks skks and pks were used to determine the splitting parameters the fast polarization direction and the delay time delay times ranging between 0 7 and 1 5 s have been observed at most of the sites the azimuths of the fast split shear waves trend homogeneously nw se in the southern massif central suggesting a homogeneous mantle flow beneath this area the observed nw se direction differs from the n100 e pyrenean anisotropy further south it does not appear to be correlated to hercynian structures nor to the present day motion of the plate but is well correlated to the tertiary extension direction we propose that the opening of the western mediterranean induced by the rotation of the corsica sardinia lithospheric block and the roll back to the se of the tethys slab may have generated a large asthenospheric mantle flow beneath the southern massif central and a deflection of the up going plume centered beneath the northern massif central toward the se

lithosphere structure of the ne bohemian massif sudetes a teleseismic receiver function study in 2004 and 2005 a passive seismic experiment was carried out in the northern and northeastern part of the bohemian massif sudetes to study the lithospheric structure we present results from ps and sp receiver function analyses with one exception moho depth at stations in the northwestern part of the study area varies between 28 and 32 km thicker crust up to 35 km was mapped toward the south moldanubian unit and toward the east moravo silesian and brunovistulian units confirming results from previous active seismic measurements there exists a relatively sharp step in moho depth between units of the central sudetes 30 km and the moravo silesian unit 35 km the vp vs ratios inverted from primary and multiple moho ps conversions hint for different crustal compositions of the units toward the carpathian thrust we have no clear indications for any crustal root or slab beneath the western carpathians however our data suggests a deepening of the moho or at least a complicated crust mantle transition in this area additional ps phases were observed between 6 and 10 s delay time in the sudetes these phases cannot be explained by moho reverberations but are most probably caused by low velocity zones in the middle crust or lithospheric mantle as shown by modeling of theoretical receiver functions the stations showing these abnormal phases are located in the area of permo carboniferous basins on probably tepla barrandian crust therefore we assume that the phases hint at a mid crustal low velocity zone between 16 and 20 km depth which is interpreted as a felsic solidified magma reservoir of the permo carboniferous volcanism beneath the sudetic basins sp receiver functions show phases with negative polarity at 9 to 12 s lead time on average which we interpret as lithosphere asthenosphere boundary at about 80 to 110 km depth

a new interpretation of stephanian deformation in the decazeville basin massif central france consequences on late variscan tectonism five stages of faulting were observed in and around the stephanian decazeville basin in the sw french massif central at the southern edge of the sillon houiller fault the older stage ends during middle stephanian time and corresponds to a strike slip regime with n s shortening and e w extension before the end of the middle stephanian three other stages were recorded two strike slip regimes with nw se then e w compression and ne sw then n s extension and finally a nne ssw extensional regime during the main subsidence of the basin from the end of the middle stephanian to late stephanian based on mining documents a new interpretation of the n s striking folds of the decazeville basin is proposed folding may not be associated with e w compression but with diapirism of coal seams along syn sedimentary normal faults during the extensional phase a last strike slip regime with n s compression and e w extension may be related to cainozoic pyrenean orogeny at a regional scale it is suggested that from the end of the middle stephanian to the late stephanian the main faults in the decazeville basin may represent a horsetail splay structure at the southern termination of the sillon houiller fault

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

anisotropic measurements in the rhinegraben area and the french massif central geodynamic implications as part of an integrated seismic study polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband semi permanent long and short period and temporary short period seismological stations located in two geodynamically important areas in western europe namely the rhinegraben urach area and the french massif central volcanic field

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central fennoscandia abstract we analyze splitting of shear waves recorded during the svekalapko passive seismic experiment in south central finland to study fabrics of the mantle lithosphere of the precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central fennoscandian mantle lithosphere including the proterozoic archean transition and to model their fabrics by joint inversion of body wave anisotropic parameters fabrics of the archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the ne on the other hand anisotropy of the proterozoic mantle lithosphere is weaker and we model its fabric by the a c foliations dipping to the se we present a 3d self consistent anisotropic model of the proterozoic and archean upper mantle along the sw ne profile in the south central finland boundaries of inter growing wedges of the proterozoic and archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization mantle xenolith ages surface wave tomography and location of the upper mantle reflectors we interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro continents assembled

the variscan tectonic inheritance of the upper rhine graben evidence of reactivations in the lias late eocene oligocene up to the recent processing of gravity and magnetic maps shows that the basement of the upper rhine graben area is characterized by a series of ne sw trending discontinuities and elongated structures identified in outcrops in the vosges black forest and the odenwald mountains they form a 40 km wide n30 40 striking sinistral wrench zone that in the visean shifted the variscan and pre variscan structures by at least 43 km to the ne wrenching was associated with emplacement of several generations of plutonic bodies emplaced in the time range 340 325 ma the sub vertical ne sw trending discontinuities in the basement acted as zones of weakness susceptible to reactivation by subsequent tectonism the first reactivation marked by mineralizations and palaeomagnetic overprinting along ne sw faults of the vosges mountains results from the liassic nw se extension contemporaneous with the break up of pangea the major reactivation occurred during the late eocene n s compression and the early middle oligocene e w extension the ne sw striking basement discontinuities were successively reactivated as sinistral strike slip faults and as oblique normal faults elongated depocenters appear to form in association with reactivated variscan wrench faults some of the recent earthquakes are located on ne sw striking variscan fault zones and show sinistral strike slip focal mechanisms with the same direction suggesting also present reactivation

southern rhine graben small wavelength tomographic study and implications for the dynamic evolution of the graben summary inversion of p wave traveltime residuals of teleseismic events recorded by a dense network of short period stations yielded a detailed image of the 3 d velocity structure of the lithosphere asthenosphere system beneath the southern rhine graben area the upper part of the lithosphere marked by velocity anomalies orientated along the graben strike can be related to the development of the graben these upper 50 km display stronger lateral heterogeneities than the weak anomalies observed at deeper levels which show roughtly an e w orientation the teleseismic inversion models yield no indications for significantly reduced seismic velocities in the upper mantle as could be expected from an actively upwelling mantle diapir located beneath the southern rhine graben area the observation of generally weak velocity variations at greater depth indicates a rather smooth transition from the lower lithosphere to the upper asthenosphere deeper parts of the lithosphere were probably altered by metasomatic processes as a consequence of the intrusion of melts during middle and late miocene times these melts cooled and solidified from the end of the miocene when rifting and thermal doming ceased in the southern rhine graben area

propagation anomalies in northwestern italy by inversion of teleseismic residuals travel time residuals of teleseismic p waves were analysed in order to elucidate the crust upper mantle structure in northwestern italy and the western alpine arc using digital data obtained from both fixed seismograph networks operating in nw italy notably liguria piedmont and temporary arrays with the aid of cross correlation techniques reliable travel time residuals were calculated which were then inverted to obtain models of propagation anomalies the reliability of the inversion procedure was tested using synthetic data the model thus obtained appears to be stable and shows strong lateral heterogeneities at a litho asthenospheric level in particular it confirms the high velocity contrast caused by the ivrea body in the shallower layers and the presence of alpine roots reaching down to at least 200 km a statistical analysis performed on the propagation times of rays crossing the resulting four layered model reveals rms below 0 1 s

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

upper mantle beneath the eger rift central europe plume or asthenosphere upwelling summary we present the first results of a high resolution teleseismic traveltime tomography and seismic anisotropy study of the lithosphere asthenosphere system beneath the western bohemian massif the initial high resolution tomography down to a depth of 250 km did not image any columnar low velocity anomaly which could be interpreted as a mantle plume anticipated beneath the eger rift similar to recent findings of small plumes beneath the french massif central and the eifel in germany alternatively we interpret the broad low velocity anomaly beneath the eger rift by an upwelling of the lithosphere asthenosphere transition we also map lateral variations of seismic anisotropy of the mantle lithosphere from spatial variations of p wave delay times and the shear wave splitting three major domains characterised by different orientations of seismic anisotropy correspond to the major tectonic units saxothuringian moldanubian and the tepla barrandian and their fabrics fit to those found in our previous studies of mantle anisotropy on large european scales

investigations of sn and pn phases in the area of the upper rhine graben and northern switzerland summary 22 regional earthquakes in sw germany and northern switzerland recorded with the southern black forest digital seismic network are studied for pn and sn phases in contrast to the results of some seismic refraction studies a large number of these earthquake recordings show clearly correlatable sn phases so far there are two remarkable trends concerning the appearance or disappearance of sn 1 earthquakes with large focal depths z 20 30 km preferentially show sn phases 2 earthquakes from the sw with rays crossing the southern rhine graben show no sn energy regardless of focal depth to investigate the amplitude behaviour of the sn phase synthetic seismograms for varying focal parameters as well as for different structural parameters submoho velocity gradient and attenuation of s waves are calculated using the reflectivity method the synthetics show that the sn amplitude is sensitive especially to the fault plane orientation and the submoho velocity gradient after correction for the radiation pattern the missing of the sn phase from earthquakes in the south western part of the studied area seems to be associated with structural effects in the uppermost mantle a small submoho s wave gradient or a high upper mantle attenuation are the most likely explanations another very interesting observation relates to the pn velocities the azimuthal variation of the velocity as well as the high maximum pn velocity v 8 6 8 8 km s 1 itself point to an anisotropic uppermost mantle the velocity variation of 0 6km s 1 and the directions of minimum and maximum velocities are in accordance with earlier results from seismic refraction experiments and their petrological interpretation the derived sn velocities have a mean value of 4 71 km s 1 up to now the quality and number of the sn recordings do not allow us to resolve a possible s wave anisotropy of the uppermost mantle

two dimensional anisotropic tomography of lithosphere beneath france using regional arrival times this paper presents an anisotropic tomography study beneath france travel times of pn waves were inverted in order to simultaneously retrieve lateral variations of both seismic velocity and seismic anisotropy within the subcrustal lithosphere the parameterization scheme uses a classical approach the subcrustal lithosphere is modeled using a two dimensional horizontal grid of nonoverlapping cells 0 5 0 5 each cell being characterized by an average slowness the isotropic term of slowness and an azimuthal anisotropy term the velocity and anisotropy perturbation amplitudes do not exceed 2 in the paris basin and in the aquitaine basin more significant perturbations can be observed in the tectonically active regions the images of the modeled subcrustal lithosphere emphasize the not recently reactivated areas of the hercynian range by high velocities 2 to 5 from the armorican massif to the west part of the massif central and from the northeast massif central to the vosges mountains and black forest mountains regions these areas are also characterized by an anisotropy of 2 5 with the fast directions parallel to the hercynian structures low velocities 2 to 4 are observed in the neogene volcanic areas of the massif central which confirm the results obtained from a short wavelength teleseismic tomography these low velocities are most probably caused by high temperatures another low velocity region 3 to 4 roughly follows the alpine belt and is interpreted as the image of the crustal root of the range in the inner part of the alps fast pn directions lie perpendicular to the belt with an anisotropy amplitude of 3 while in the outer part they are parallel to the range with amplitudes ranging from 3 to 5 to the east and from 2 to 3 to the west the anisotropy feature of the pyrenean range follows a simple pattern one observes high amplitudes up to 5 and a fast direction which is parallel with the strike of the range the observations show seismic anisotropy for most of the tectonically active zones across france recently formed structural units and old hercynian structures as well and the amplitudes reach the same order as the amplitudes of the velocity perturbations

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

the lithosphere in central europe seismological and petrological aspects abstract the lithosphere thickness in the variscan belt of central europe varies between about 60 and 150 km with typical values of 100 120 km our estimates derived from directionally independent representative p wave residuals are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle the large scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative p residuals vary between 6 5 and 15 2 for p velocities the s wave anisotropies determined from sks polarizations vary between 2 2 and 6 7 these values are in reasonable agreement with the anisotropies of peridotites determined in laboratory systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere the residual patterns at most stations in the saxothuringicum rhenohercynicum and in the massif central imply northwesterly orientated dips of the anisotropic structures while stations in the moldanubicum the alpine foredeep and most of the alps north of the insubric line suggest southeasterly orientated dips in our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere the variscides of central europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the moldanubicum and the saxothuringicum

distinguishing barren and auriferous veins in the sigma mine val d or quebec the sigma mine is an archean lode gold deposit located in the abitibi greenstone belt quebec barren quartz veins occur subparallel to auriferous veins within the mine and also occur north of the

images of lithospheric heterogeneities in the armorican segment of the hercynian range in france the armorican massif is located in western france it is a part of the hercynian range formed in several phases of se nw compression from devonian to carboniferous time ca 400 300 ma the main tectonic features are the north and south armorican shear zones with wnw ese strike in the framework of the geofrance3d program we have installed 35 seismological stations along two ns lines crossing the main hercynian wnw ese fault systems the data set collected during the experiment allows us to compute a 3d p velocity model and to map s wave seismic anisotropy at depth using teleseismic shear waves splitting measurements the south armorican shear zone sasz is characterized by a strong 4 5 velocity contrast and a fast shear wave azimuth parallel to its strike the north armorican shear zone nasz does not show any significant lithospheric signature the tomographic image and anisotropy results suggest that the armorican lithosphere consists of the juxtaposition of distinct lithospheric blocks assembled in a subduction type process predating the hercynian orogeny d 2002 elsevier science b v all rights reserved