mantle reflections beneath the montana great plains on consortium for continental reflection profiling seismic reflection data a systematic search for reflections from the upper mantle has been carried out on 2000 km of consortium for continental reflection profiling seismic reflection data from 14 different sites across the united states the general observation from these data is the striking contrast between the generally pervasive reflectivity of the crust and the relative lack of coherent reflectivity in the upper mantle a contrast that cannot always be attributed to lack of signal penetration in order to image more of the upper mantle section on the seismic data seismic field files were increased from 16 to 28 s two way travel time by extended correlation processing while the results reveal an upper mantle generally devoid of laterally continuous coherent energy pronounced local reflections called here the lonesome lake complex are observed at upper mantle travel times beneath the montana plains reflections at mantle travel times do not ensure mantle reflectors as other mechanisms operating at shallower levels in the crust may be responsible for the observed reflections however multiple reflections and converted waves from the crust are unlikely explanations for the lonesome lake complex although energy from outside the plane of profile cannot be ruled out circumstantial evidence suggests that these reflections are likely due to intramantle heterogeneity the lonesome lake complex joins examples of upper mantle reflectivity observed on other deep seismic data worldwide in establishing the mantle as an productive target for future deep seismic surveys

magnetic fabric in the malanjkhand granite central india implications for regional tectonics and proterozoic suturing of the indian shield abstract in this paper field microstructural and anisotropy of magnetic susceptibility ams data are presented from the 2 48 ga malanjkhand granite and adjacent gneiss with an aim to evaluate the time relationship between fabric development in the granite and regional tectonics the central indian suture cis that demarcates the southern margin of the ne sw to ene wsw trending central indian tectonic zone citz and is a consequence of the accretion between the northern and southern indian shields lies to the north of the granite the northern margin of the granite that has proximity to the cis is mylonitized and shows a dextral top to the ne sense of shear field and magnetic foliation in the gneiss are parallel to each other as well as to the field foliation in the western and northwestern margin of the granite also the field fabrics in the granite and gneiss curve clockwise into the cis the magnetic foliation trajectories curve from a ns orientation in areas remote from the cis to a ne sw orientation in parts that are proximal to the cis in the north the granite shows synmagmatic deformation fabrics such as preferentially oriented zoned feldspar phenocrysts as well as evidence of high t solid state deformation fabric such as chessboard pattern in quartz magnetic anisotropy increases towards the cis which is inferred to support synmagmatic deformation it is concluded that the tectonic evolution of the citz and emplacement fabric development in the malanjkhand granite were synchronous phenomena wnw ese directed convergence led to transpression which is inferred to be responsible for the emplacement of the granite it is concluded that in the malanjkhand granite the development of syndeformational emplacement fabrics to syntectonic cooling was a continuum and the suturing of the northern and southern indian shields along the cis took place during the palaeoproterozoic period thus the study provides an insight into the amalgamation and stabilization of the indian shield during the late archaean palaeoproterozoic time

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

inversion of first break traveltime data of deep seismic reflection profiles first breaks of 2d deep reflection data were used to construct velocity depth models for improved static corrections to a deeper datum level and for geological interpretations the highly redundant traveltime data were automatically picked and transformed directly into a velocity depth model by maximum depth methods such as the giese and the slichter method comparisons with the results of synthetic calculations and a tomographic approach using iterative inversion methods art sirt showed that maximum depth methods provide reliable velocity models as a basis for the computation of static corrections these methods can economically be applied during data acquisition in the field they provide particularly long period static anomalies which are of the order of 20 40 ms 0 5 1 wavelength within cmp gathers of an example of a deep reflection profile in sw germany sited on crystalline basement reprocessing of this profile which was aimed at the comparison between the effects of the originally used and the new statics did not result in dramatically improved stacking quality but showed a subtle influence on the detailed appearance of deep crustal events

the maladeta granite polydiapir spanish pyrenees a detailed magnetostructural study abstract the maladeta granitic complex spanish central pyrenees has been subjected to a detailed magnetostructural study through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites due to the dominant paramagnetic property of the granites a first order correlation can be derived between the susceptibility magnitude and the bulk rock iron content hence the petrographic nature of the rock the maladeta complex is shown to be composed of four principal juxtaposed plutons each one being normally zoned i e becoming more basic towards the periphery the largest pluton namely the colomers unit is itself made of a number of sub plutons due to distinct but more or less coeval batches of magmas the various magnetic data complemented by field and microstructural observations allow us to propose that the maladeta polydiapir probably exploited ne and se trending openings of the brittle crust for its emplacement that accompanied or just preceded a sw verging compressive event a later n s compression marked by inverse dextral and e w trending shears is preferably attributed to late variscan rather than to alpine tectonics

wave field processing of data from a large aperture seismic experiment in southwestern oklahoma data from a wide aperture survey performed in january 1985 in southwestern oklahoma have been processed using prestack imaging and the images are interpreted to reveal a new picture of the crustal structure in the wichita uplift and the anadarko basin as the data were in recording apertures of approximately 90 km it was necessary to develop new processing concepts and software a complete new imaging system based on common source gathers was conceived implemented tested and applied to these data the heart of the system consists of velocity estimation by slant stacking and acoustic prestack reverse time finite difference migration the migration produces reflectivity images in regions of 50 km 100 km has no dip restrictions and operates in an arbitrarily complicated two dimensional velocity distribution for comparison the data were also processed using standard common midpoint processing but this is a less valid approach for wide aperture data the main ambiguities and limitations in processing and interpretation are a consequence of the small number of shots processing and interpretation of the data provide a new picture of the subsurface structure of southwestern oklahoma the environment is clearly compressive with thrust faulting as the main accommodating mechanism in the upper and central crust beneath a possible detachment zone at 30 km depth the lower crust has responded to the compression by thickening by 50 the moho is interpreted to be 45 km in depth beneath the thrust zone shallowing to 40 km beneath the wichita uplift and the anadarko basin

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

anisotropic reflection and transmission calculations with application to a crustal seismic survey from the east greenland shelf a three component refraction data set recorded over the east greenland shelf contained two anomalous shear wave arrivals the direct s phase emerged at the receiver as an apparent sh wave even though the airgun source was purely compressional we have concluded that this arrival is the result of anisotropy in the upper crust by employing a number of fairly simple techniques we were able to highlight a second delayed quasi shear arrival and interpret shear wave splitting modeling of the polarization amplitude and delay time of direct s suggested that the anisotropy is due to aligned subvertical fractures perpendicular to the local seafloor spreading direction the second shear arrival of interest is a p to s conversion from the moho pms the relative strength of this arrival suggests that the moho is a sharp transition in this region pms also contained a significant transverse component but attempts to identify shear wave splitting were inconclusive however modeling revealed that the transverse pms displacement may be due to anisotropy in the upper mantle alignment of olivine crystals with the spreading direction which causes the converted s reflection to have a significant sh transverse component an understanding of and ability to model anisotropic reflection and transmission effects are essential to these interpretations therefore we also outline an efficient method to obtain all of the reflection and transmission coefficients at boundaries between anisotropic media this has been incorporated into a new ray tracing package a trak based on asymptotic ray theory capable of modeling three dimensional inhomogeneous media separated by curved interfaces

continuous deformation ductile thrusting and backfolding of cover and basement in the sierra de guadarrama hercynian orogen of central spain abstract hercynian deformation has affected two different crustal ensembles a crystalline basement and a metasedimentary cover in the sierra de guadarrama the rheological basement is a medium to high grade metamorphic complex composed mostly of granitic orthogneisses with minor amounts of sediments the cover consists of a thick sequence of low to high grade metasediments with minor gneissic bodies ranging in age from late proterozoic to devonian the deformation in the cover was analysed mainly in the eastern part of the sierra de guadarrama whereas the crystalline basement was analysed in the segovia area to the west both cover and basement deformation started with a stage d 1 of continuous penetrative deformation the deformational regime has a strong component of subhorizontal simple shear with east vergence a second phase d 2 of shear deformation gave rise to a broad subhorizontal shear zone and a thrust in the cover and to narrow ductile shear zones in the basement the transition between the two first phases of deformation seems to have been gradual the third phase d 3 is a generalized backfolding episode which induced an intense and penetrative strain in broad areas of both ensembles finally large open folds with steep axial surfaces developed in two mutually perpendicular systems giving rise to dome and basin interference patterns and some subvertical ductile shear zones and faults were also created the structural evolution of the basement and cover points to a thicker skinned model of deformation giving rise to an important crustal thickening a salient feature of the model arose during the second phase when a thick detachment level developed in the lower part of the cover the generalized existence of this level may account for the scarcity of basement outcrops in the spanish hercynian belt

processing of wide angle vibroseis data in 1984 seismic wide angle vibroseis experiments across the rhine graben in eastern france southwest germany were carried out as a joint french german venture signals generated by french vibrators were recorded at the eastern flank of the graben in the black forest with a 200 channel array spread along 16 km group spacing 80 m data were recorded at offsets between 66 and 82 km the data are characterized by a low signal to noise ratio s n and distortions of seismic phases caused by strong topographic variations for wide angle records conventional static corrections based on the assumption of vertical travelpaths are no longer appropriate ray parameter dependent time delays should be used in order to correctly eliminate topographic and near surface influences on the data in addition delineation of reflector segments requires an increase in s n a two dimensional filter based on forward and inverse slant stacking was designed to handle the time delay and s n problems simultaneously stacking al

granulite facies metamorphism in the anatectic complex of toledo spain late hercynian tectonic evolution by crustal extension the anatectic complex of toledo consists of a heterogeneous suite of high grade metamorphic rocks mainly granulitic migmatites of pelitic composition and orthogneisses and different types of syn orogenic granitoids with minor associated basic rocks none of the metamorphic rock types records evidence of the early prograde history and only the peak and retrograde p t conditions can be estimated the intrusion of the syn orogenic granitoids is either slightly before or synchronous with the metamorphic climax these rock types thus can be used to make p t estimates of the peak and retrograde conditions microtextural analysis of reaction textures in conjunction with a petrogenetic grid has enabled construction of an incomplete clockwise p t path that is characterized by an isothermal decompression after or during the metamorphic peak temperatures the peak estimates from different geo thermobarometers are 800 50 c and 4 6 kbar the highest temperatures recorded in this part of the iberian hercynian belt it is proposed that this high t low p metamorphism is at least partially related to an extensional process possibly due to gravitational collapse of previously thickened crust

uplift by thermal expansion of the lithosphere summary green s function solutions are derived for the surface heat flow and the uplift caused by conductive heating when axially symmetric variations in heat flow occur at the base of the lithosphere the solutions determined for different space time dependence of the heat flux into the lithosphere illustrate that 1 when the lateral variations in heat flux occur over short distances in comparison to the lithospheric thickness the amplitude of the uplift and of the anomalous surface heat flow are attenuated in comparison to the changes that would follow uniform variations this effect is slightly more important for the surface heat flow which spreads out over larger distances 2 there is a time lag between the uplift which starts instantly and the change in the surface heat flow it is suggested that the model could be applied to the study of plateaux uplift of moderate amplitude such as the black hills or the adirondacks and possibly to the heating event and uplift that may have preceded the subsidence of intracontinental basins

wide angle vibroseis test across the rhine graben summary as a joint operation of ecors and dekorp a deep wide angle seismic experiment using vibrators was carried out in the autumn of 1984 the object was to get information on the deep crust under the rhine graben without crossing through sedimentary layers offsets were in the range of 50 to 89 km in the first phase two vibration points were executed in the vosges mountains a signal was received in the black forest from solely the farthest vp in the second phase fourteen vps were executed in the black forest no stacking or correlation was performed in the field in france the quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

titanohematite lattice preferred orientation and magnetic anisotropy in high temperature mylonites magnetic and crystallographic fabric studies were performed in mylonitic granulites from a km wide strike slip shear zone in the ribeira belt southeastern brazil in these mylonites a strong compositional layering underlines the tectonic foliation and the elongation of titanohematite amphibole and orthopyroxene crystals defines a mineral stretching lineation magnetic fabric deduced from anisotropy of magnetic susceptibility ams measurements and tectonic fabric compare favorably rock magnetic studies show that both paramagnetic and ferromagnetic minerals can be carriers of ams the anisotropy of isothermal remanent magnetization which is due to the shape preferred orientation of magnetite grains is coaxial with ams lattice preferred orientation lpo measurements using the electron backscattered diffraction technique show that orthopyroxene amphibole biotite and titanohematite have a strong lpo tightly related to the tectonic fabric among these four minerals titanohematite is the only mineral present in relatively large proportions s 1 5 in all studied samples titanohematite lpo is characterized by a strong concentration of 0001 poles c axes sub perpendicular to the foliation and by a distribution of the poles of the 2110 and 1010 prism planes within the foliation with a maximum close to the lineation this characteristic lpo is interpreted as resulting from dislocation creep during the mylonitization magnetic fabrics and titanohematite lpo fit well the axis of minimum susceptibility is aligned with the c axis maximum and the axis of maximum susceptibility coincides with the maximum concentration of poles of the prism planes titanohematite lpo may provide a valuable constraint for the kinematic interpretation of the magnetic fabrics fl 2002 elsevier science b v all rights reserved

magnetic susceptibility of the mont louis andorra ilmenite type granite pyrenees a new tool for the petrographic characterization and regional mapping of zoned granite plutons we have observed remarkably consistent patterns of concentric zonation in the values of low field magnetic susceptibility measurements over the variscan age mont louis andorra granite pluton of the eastern pyrenees of andorra spain and france this zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature as defined by chemical analysis and the magnetic susceptibility magnitude of the rocks it reflects the dominantly paramagnetic nature of the granite carried by fe bearing silicates and this is demonstrated by the fact that low field susceptibilities of representative specimens are almost equal to 1 their high field susceptibilities and 2 their calculated susceptibilities using fe contents assuming a null ferromagnetic contribution we conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons

a decade of progress in earth s internal properties and processes a major component of the inter union commission on geodynamics project labeled internal properties and processes included certain experimental and theoretical research in tectonophysics seismology geochemistry petrology volcanology and planetology this review focuses on a few research areas in which there have been surprises and reversals in particular attention is given to the attempts to quantify the thermal profile in the earth s interior and the material properties of the earth s interior

synoptic interpretation of seismic reflection and refraction data summary the structure of the upper lithosphere beneath southern germany northern switzerland and west central utah u s a has been investigated in detail by various geophysical methods a synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide angle reflection surveys combined with near normal incidence reflection observations now permits the elucidation of the fine structure in a more quantitative and unified manner with this scheme it is possible to unambiguously identify low velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

a comparison of reflection and refraction processing and interpretation methods applied to conventional refraction data from coastal maine abstract in 1984 the u s geological society collected 850 km of refraction data in lines parallel and perpendicular to regional strike of the appalachian orogen in maine and southeastern quebec a 100 km long strike line in coastal maine west of penobscot bay is here interpreted on the basis of both refracted and reflected phases techniques used in seismic reflection processing including normal movement velocity corrections and common mid point stacking were applied to the refraction data in order to enhance the visibility of reflected phases the geometries of the most laterally continuous reflections as defined by these techniques can be correlated with reflections seen on a crossing near vertical reflection profile conventional ray tracing analysis of refracted and wide angle reflected phases in the refraction data constrains the velocity structure to about 14 km depth and provides evidence for high veiocity material 6 5 km sec between 4 and 7 km depth and for lower velocities

emplacement of the aya variscan granitic pluton basque pyrenees in a dextral transcurrent regime inferred from a combined magneto structural and gravimetric study magmatic structure three dimensional shape and emplacement mode of the aya pluton variscan pyrenees have been studied by combining magneto structural and gravimetric studies the aya pluton displays a fabric acquired using the anisotropy of magnetic susceptibility technique comparable to that of several other plutons in the pyrenees with shallow plunging lineations and steep foliations trending e w and ne sw forming an s pattern in map view gravity modelling was used to determine the probable root of the pluton directly below the most basic gabbro dioritic facies this has a funnel shape with an e w trending elliptical horizontal section and a maximum depth of 3 5 km these different data demonstrate that this pluton was synorogenically emplaced during the main d2 variscan tectonic phase in a dextral transcurrent regime

the effect of magnetic anisotropy on paleomagnetic directions in high grade metamorphic rocks from the juiz de fora complex se brazil the juiz de fora complex is mainly composed of granulites and granodioritic migmatite gneisses and is a cratonic basement of the ribeira fold belt which was formed during the last stage of brasiliano orogeny samples widely distributed over the studied region se brazil rio de janeiro state were collected for paleomagnetic and magnetic anisotropy determinations after measurement of anisotropy of low field magnetic susceptibility ams the same specimens were submitted to both alternating field for anisotropy of remanent magnetization arm determinations and thermal demagnetization for paleomagnetic analyses demagnetization processes allowed isolating an eastern steep positive inclination characteristic remanent magnetization chrm direction from the rocks the studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse grained titano magnetite and either fine grained titano magnetite or titanohematite grains respectively these magnetic minerals are responsible for both remanent direction and magnetic anisotropies even though for some specimens paramagnetic minerals are the carriers of ams fabric the chrm direction found for the rocks should represent the cooling phase of the brasiliano event in the area and it was acquired after the magnetic fabric was formed the ams and arm fabrics are coaxial and tectonic in origin and compare favorably with the mesoscopic scale fabrics in the adjacent areas however arm measurements have reached only low coercivity grains and it could be determined in few specimens then only an estimate of arm effect on chrms correction is shown the rocks are strongly magnetically anisotropic 50 and foliated the paleomagnetic directions from each specimen within a site tend to approach its ams or arm foliation kmax kint or armmax armmin plane these directions were corrected specimen by specimen from the sites taking into account the pole of ams or arm foliation kmin or armmin and the value of the degree of anisotropy p results show that even for specimens with high p the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens our data set suggests that ams is a powerful tool to correct the chrm deviation from paleofields in rocks with complex magnetic mineralogy such as those from the juiz de fora complex

mathematical model relationship between the paramagnetic anisotropy and strain in slates abstract a simple mathematical model is used to investigate the relationship between the paramagnetic anisotropy and strain in slates the relationship found is similar to the relationship of whole rock magnetic anisotropy to strain in weakly magnetic slates the anisotropy is probably mostly due to paramagnetic phyllosilicates and the whole rock relationship reflects the paramagnetic fraction

anisotropy of magnetic susceptibility in the montes de toledo area hercynian iberian belt spain and its petrostructural significance

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

crustal origin of hercynian peraluminous granitic batholiths of central spain petrological geochemical and isotopic sr nd constraints abstract in central spain it is possible to distinguish two main types of peraluminous late hercynian granites the pi types and the ps types the distinction between both types is made on the basis of differences in peraluminosity ps types are more peraluminous than pi types and also on the appearance of some characteristic mafic minerals ps types have biotite cordierite and monazite as the most typical accessory phase whereas pi types have biotite amphibole in the less evolved facies and allanite as the accessory mineral both granite types have similar trace element ratios and initial sr nd and pb isotopic signatures initial 87sr 86sr ratios of both types exhibit a large range from 0 7073 to 0 7193 whereas initial end varies in a restricted range from 5 4 to 6 6 the scarce associated basic rocks do not play a significant role in the chemical variability of these peraluminous granites which follow low pressure crystal fractionation trends from granodiorite monzogranite parental magmas mixing and afc modelling of sr and nd isotopic data reveal an unrealistically high mantle contribution based on major and trace element and isotopic data an orthogneissic protolith for both granitic series is proposed nevertheless none of the metamorphic country rocks of central spain has the appropriate sr isotopic composition to satisfy the origin of these granitic rocks and so it is argued that progressive isotopic re equilibration of crustal material during the granulization of the lower hercynian crust together with the possibility of isotopic disequilibrium during melting as has been demonstrated in migmatitic terranes in nearby areas may explain the isotopic differences between the granites and the high level metamorphic country rocks

composite magnetic fabrics from s c mylonites magnetic anisotropy data are presented for samples of s c mylonites from a shear zone developed in the veiga granodiorite a late kinematic pluton located in the northern part of the iberian variscan belt spain the magnetic susceptibility is geometrically represented by an ellipsoid whose principal axes k 2 kint 1 kmi often show a one to one correlation with the principal directions x y and 2 f finite strain we report an exception to this rule since our results reflect composite magnetic fabrics arising from the contribution of both the c and s structures the magnetic foliation defined as the plane normal to k i shows an intermediate orientation between the c and s planes and rotates towards the orientation of the c planes as finite strain increases the magnetic lineation parallel to k and the stretching lineation present a similar trend but different values of plunge copyright 0 1996 elsevier science ltd

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

resolution of ferrimagnetic and paramagnetic anisotropies in rocks using combined low field and high field measurements summary a method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed from the anisotropy measurement of a specimen in a low field susceptibility anisotropy and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present the following anisotropy components can be calculated susceptibility anisotropy field independent of the paramagnetic fraction low field susceptibility anisotropy of the ferrimagnetic fraction and high field anisotropy of the ferrimagnetic fraction errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed

wide angle deep crustal reflections in the northern appalachians summary seismic refraction data collected in the northern appalachians provide an unusual opportunity to use wide angle reflections to examine the lower crust and upper mantle the pmp phase clearly identified on several hundred records has been used to construct an isopach map of the crust which shows a stepwise regional thickening of the crust beneath the axis of the appalachians the data have been examined to times of up to 40 s two way travel time to investigate the possibility of coherent upper mantle reflections such as those recently observed by birps on near vertical data northwest of britain although substantial coherent energy appears after the pmp phase synthetic seismogram modelling shows that all of these arrivals are explicable by s phases converted phases and multiples from within the crust we conclude that in this region of the northern appalachians we have not detected any significant regionally extensive reflecting horizons within the upper mantle

magnetic anisotropy of chloritoid 1 the magnetocrystalline anisotropy of monoclinic chloritoid a relatively common mineral in aluminum rich metapelitic rocks has been determined for the first time by measuring the high field anisotropy of magnetic susceptibility hf ams using two independent approaches i e a directional magnetic hysteresis measurements and b torque magnetometry on a collection of single crystals collected from different tectonometamorphic settings worldwide magnetic remanence experiments show that all specimens contain ferromagnetic s l impurities being mainly magnetite the determined hf ams ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c axis of chloritoid in the basal plane of chloritoid though the hf ams can be considered isotropic the degree of anisotropy is found to be 1 47 which is significantly higher than the anisotropy of most paramagnetic silicates and even well above the frequently used upper limit i e 1 35 for the paramagnetic contribution to the ams of siliciclastic rocks the obtained values for the paramagnetic curie temperature parallel and perpendicular to the basal plane indicate that this pronounced magnetocrystalline anisotropy is related to strong antiferromagnetic exchange interactions in the direction of the crystallographic c axis 0 as a consequence chloritoid bearing metapelites with a pronounced mineral alignment can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic s l minerals the newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid bearing rocks

extensional tectonics of the toledo ductile brittle shear zone central iberian massif abstract the toledo mylonite in the central area of the iberian massif represents a major ductile shear zone only a small part of which has been exposed to view mylonites derived largely from migmatite occupy a 300 350 m thick gently s dipping zone along the southern margin of the toledo crystalline massif the upper plate comprises palaeozoic lower grade metasedimentary rocks intruded by a late hercynian pluton the juxtaposition of the upper and lower plates with different structural and metamorphic conditions indicates that the shear zone is a significant structure with a large displacement progressive simple shear under conditions of a clearly decreasing confining pressure and temperature is supported by a microbrecciated mylonite cataclasite and ultracataclasite sequence overlying the mylonites the top of the ledge of cataclasites corresponds to the detachment fault part of the shear zone structural relations combined with meso and micro structural evidence stretching lineations and asymmetric shear sense indicators support the conclusion that the mylonites and cataclasites associated with the detachment faulting formed during a continuous extensional deformation detachment fault models developed for the formation of metamorphic core complexes can be applied in the toledo massif we propose a model that implies a transition from ductile to brittle deformation associated with extension at greater structural depths during lower permian time

anisotropy of magnetic susceptibility and magmatic structures in the guerande granite massif france a multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the guerande leucogranite in southern brittany western france on one hand extensive field work and microscopic observations and on the other a detailed magnetofabric investigation have been carried out the former showed a magmatic flow plane trending generally to the ene wsw with a moderate dip to the north and a flow direction slightly dipping toward nne these observations suggest a blade shaped body which is not deeply rooted the magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite ilmenite grains with a mean rock susceptibility of about 3 10 6 g oe there is an excellent agreement between the magnetic lineation kmax and the magmatic flow direction the agreement between the magnetic foliation kmax kint plane and the magmatic flow plane is not as good the quantitative analyses of the anisotropy allows one to group the data into two domains the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification that is some deformation on the solid state such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies provided that the microstructural state of the rock is already well known

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

variations in the lithosphere thickness and vertical neotectonic movements abstract thermal changes and mechanical deformation of the lithospheric thickness are presumed to cause vertical neotectonic movements it is established that heterochronous fragments of the continental lithosphere respond differently to cenozoic thermotectonic activation lithospheric segments which had consolidated in the archean were less mobile in the cenozoic those of riphean ma and late phanerozoic age were mobile the archean fragments comprise the stable blocks of ancient and young continental platforms the riphean and late phanerozoic fragments comprise orogenic belts these regional differences may be due to different degrees of cratonization of the continental lithosphere

detachment faulting and late paleozoic epithermal ag base metal mineralization in the spanish central system hydrothermal activity during late hercynian time resulted in epithermal silver base metal pb zn cu vein formation in the eastern part of the spanish central system during the hercynian orogeny the central iberian crust was thickened by compressional tectonics heated weakened and subsequently overthickened by massive late hercynian granitic intrusions subsequently the central iberian crustal welt underwent extensional collapse through lithosphere scale low angle detachment faulting the detachment systems evolved through tectonic denudation isostatic rebound and upward arching to define an extensional province much like the u s basin and range andesitic volcanism and hydrothermal activity occurred during extension inducing epithermal type hydrothermal convecting systems that leached transported and precipitated silver and base metals along fractures crosscutting the hiendelaencina metamorphic core complex

granite emplacement during contemporary shortening and normal faulting structural and magnetic study of the veiga massif nw spain abstract the veiga massif belongs to the calc alkaline series of hercynian granitic rocks of the ibero armorican arc the veiga granodiorite intruded during the upper carboniferous into the core of the wnw ese n verging ollo de sapo antiform formed by precambrian and palaeozoic metasediments internal fabrics show that magma intrusion was contemporary with shortening measurements of feldspars orientations and anisotropy of magnetic susceptibility ams throughout the granite are consistent and indicate a foliation striking wnw ese parallel to folding with a constant dip of 75 85 n the zonation of bulk low field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders the difference in elevation between outcrops more than 600 m allows us to infer the three dimensional attitude of granite fabrics throughout the massif syn magmatic fabric folds are preserved in the inner part of the igneous body the highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion at the scale of the massif foliation is convergent toward the bottom of the intrusion along a line located at its northern border where the magma source is interpreted to be located in the western border of the massif the presence of c and s structures indicates that magma cooling was coeval with movement of the chandoiro fault a n s striking normal fault with a n290e hanging wall displacement direction these results indicate that emplacement of the veiga granite is coeval with nne ssw shortening and with an wnw ese extension direction parallel to the trend of the late folds