seismic investigations along the european geotraverse and its surroundings in central europe since 1975 several high resolution seismic refraction and reflection surveys have been carried out in western germany to investigate the structure of the earth s crust and uppermost mantle the investigation culminated in the seismic refraction survey along the 825 km long central part of the european geotraverse egt in 1986 this contribution summarizes the main results of the more recent crustal investigations along and around the egt the internal crustal structure throughout the area of the variscides is very complex and changes laterally considerably distinct crustal blocks differing in their internal structure can be assigned to geologically defined units of the variscan and caledonian orogeny in spite of local deviations in general a more or less transparent and low velocity upper crust contrasts with a highly reflective lower crust a subdivision of upper and lower crust by a well defined boundary conrad discontinuity is not always seen towards the alps the average velocity of the lower crust is as low as 6 2 km s 1 in contrast to the area north of the swabian jura where the velocities above moho vary between 6 8 and 7 2 km s 1 in northern germany the elbe line separates the lower crust into two regions with 6 4 km s 1 average velocity in the south and 6 9 km s 1 in the north the total crustal thickness under the variscan part of germany is fairly constant between 28 and 30 km except under the rhine graben area with 25 26 km and beneath the central part of the rhenish massif where an anomalous crustal thickening to 37 km is observed under northern germany the moho rises to about 26 km depth and the data indicate at least one fault like step of 1 km before the crust thickens toward the ringkobing fyn basement high the synthesis of seismic velocity structure and petrological information from xenolith studies allows us to propose a mafic composition for the deeper levels of the crust and uppermost mantle which may be valid at least for the central part of the variscan crust along the european geotraverse in central europe

miocene jiali faulting and its implications for tibetan tectonic evolution the karakoram jiali fault zone kjfz comprises a series of right lateral shear zones that southerly bound the eastward extrusion of northern tibet relative to india and stable eurasia here we present new 40 ar 39 ar age data from the puqu and parlung faults two easternmost branches of the jiali fault zone which indicate a main phase of the kjfz shearing from v18 to 12 ma thus the tibetan eastward extrusion bounded by principal strike slip fault zones started and was probably most active around the middle miocene an interval marked also by active east west extension in southern tibet the coincidence of these two tectonic events strongly suggests a common causal mechanism which is best explained as oblique convergence between india and asia under the framework of this mechanism the extension in southern tibet is not a proxy for the plateau uplift the kjfz activity was furthermore coincident with right lateral displacements along the gaoligong and sagaing faults in southeast asia this defines a miocene deformation record for the regional dextral accommodation zone that in response to the continuing india asia collision may have accounted for the initiation and prolonged history of clockwise rotation of the tibetan extrusion around the eastern himalayan syntaxis 2002 elsevier science b v all rights reserved

sequence stratigraphy and the evolution of the ganges brahmaputra delta complex 1 a seismic stratigraphic analysis of 2600 km of data covering part of the ganges brahmaputra delta has shown that it evolved in three stages 1 the proto ganges delta developed immediately after the breakup of gondwana at approximately 126 ma the indian plate then moved rapidly northward ultimately crossing more than 50 degrees of latitude because the catchment area was small sedimentation was relatively restricted and the early delta consists of relatively small scale prograding clastic bodies alternating with mixed carbonate clastic and carbonate platform assemblages 2 the transitional delta developed following a major eustatic sea level low at 49 5 ma initially sedimentation was relatively restricted but at about 40 ma the flux of water and clastic sedime ts dramatically increased and the delta grew rapidly with the development of major prograding clastic depositional sequences abundant evidence of lobe switching suggests that the delta was stream dominated at that time the abrupt change in the morphology of the delta appears to be a direct response to the collision of the indian plate with the eurasian plate and the rise of the himalayas the delta remained stream dominated some time after the major plate collision and until the bay of bengal became a well developed deep embayment at about 21 ma the bengal fan began to accumulate during this interval as turbidity currents carried the increased sediment load into deeper water 3 the modern delta began to appear in its present form following a major eustatic sea level low and erosio at 10 5 ma by this time the delta had become tidally dominated and had prograded well beyond the study area into the now deeply embayed bay of bengal

deep crustal reflections from a vibroseis survey in northern switzerland abstract in 1982 a vibroseis survey comprising 180 km of reflection profiles was run in northern switzerland in order to investigate the suitability of the crystalline basement for the deposition of highly radioactive waste a configuration was chosen with 144 channels 25 m of geophone spacing 20 s sweeps ranging from 11 to 61 hz and stacking of 4 or 8 sweeps of 3 simultaneous vibrators at twice the geophone spacing the listening time was generally 4 s and at 4 sites it was extended to 11s for the detection of deeper crustal reflectors this survey unravelled the complicated fault and thrust system beneath the swiss folded jura mountains the stack from 4 s to 11 s reveals clearly a strong sloping reflector between 3 0 and 3 5 s which is strong evidence for a pronounced differentiation in the upper crust a series of reflections is observed between 5 8 and 7 2 s the top of which can be correlated with the conrad discontinuity a strong layered signal between 9 0 and 9 5 s is interpreted as reflections from the m discontinuity the main features are compatible with results from nearby refraction surveys in the southern rhinegraben rift system which show a distinct velocity increase of about 0 5 km s in the lower crust at a depth ranging from 15 to 20 km followed by an inversion zone or a laminated structure before reaching the moho at about 27 km depth the correlation of the field recordings with the first 10 s of the up sweep only shows some loss of resolution in the uppermost 3 s because of the lower frequency content of the signal however the lower parts of the sections are nearly identical the fact that the deeper reflectors in the sections can consistently be traced laterally is a strong argument for using this processing technique thus high coverage vibroseis surveys utilizing up sweep can be processed for deep crustal reflections even if the recording time is restricted to the standard 4 s provided the surface static corrections are carried out with high precision

weak faults rotten cores earthquakes are triggered when crustal deformation produces displacements along fault zones the rock in these zones is found to be weaker than adjacent material but what are the mechanisms of this loss of strength in his perspective holdsworth 1 discusses how recent studies of exposed ancient fault zones combined with laboratory analyses are revealing that rock properties at the grain scale can determine the large scale behavior of crustal deformation 1 http www sciencemag org cgi content full 303 5655 181

deep structure of the southern ural mountains as derived from wide angle seismic data p and s wave modelling of the data obtained during the seismic refraction wide angle reflection experiment of the urseis95 project demonstrate the presence of a 15 18 km thick crustal root beneath the magnitogorsk tagil zone in the central part of the urals orogen however the centre of this crustal root is displaced by 50 80 km to the east of the present day maximum topography also beneath the magnitogorsk tagil island arc zone an upper crustal body with a high p wave velocity of 6 3 km s 1 at 4 9 km depth can be interpreted as consisting of mafic and or ultramafic rocks this in turn would help to explain the positive bouguer gravity anomaly and the surface heat flow minimum associated with the zone and would also be consistent with the known surface geology of the zone another major feature of the seismic model is the presence of high p and s wave velocities 7 5 and 4 2 km s 1 respectively at the base of the crustal root if the deeper parts of the thickened crust also have high densities small density contrast of about 0 1 g cm 3 with respect to the uppermost mantle then this helps to explain the absence of a pronounced gravity minimum associated with the root these high velocities and densities can be most easily explained by mafic rocks or a mix of mafic and ultramafic rocks within the structural framework of berzin et al 1996 these rocks would belong to the lower russian plate which was being subducted beneath the siberian plate during the uralian orogeny it is possible that the crustal root is formed from the remnants of oceanic crust or a mix of oceanic crust and mantle attached to the russian plate this would mean that little or no continental crust has been subducted or that subduction and hence the uralian orogeny stopped when there was no more oceanic crust or when an attempt was made to subduct lighter continental crust

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

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

crustal evolution of the rhinegraben area 1 exploring the lower crust in the rhinegraben rift by unified geophysical experiments abstract unified geophysical investigations of the lithosphere in the rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process the new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust 1 1 the lower crust as the layer beneath the conrad discontinuity with a p wave velocity of about 6 5 km s or greater refraction seismics 2 2 the laminated band of reflections as seen in the near vertical reflection seismic experiments in many parts of the continents 3 3 the ductile part of the crust below the brittle ductile transition devoid of earthquakes in seismically active regions 4 4 the electrical conductivity of the lower crust indicative of dry or wet conditions or still unknown conduction phenomena in the rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process in which the crust of the rhinegraben rift system has been subjected to three different natural dynamic processes 1 uplift by 2 to 3 km with subsequent erosion of the rhinegraben shoulders black forest and vosges mountains caused decompression possibly leading to the formation of a low velocity high electrical conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the black forest 2 the brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust which became about 5 to 7 km thinner below the graben than below the shoulders 3 the deepest hypocentres in the black forest dinkelberg area if projected onto the neighbouring reflection profile would be located 7 to 8 km within the laminated lower crust beneath the southern black forest indicating a discrepancy between the top of the lower crust as defined by the brittle ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band the rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations

regional three dimensional gravity investigations in the black forest southwest germany abstract the gravimetric pre surveys of the continental deep drilling project ktb yielded results different to those hoped for at the beginning of the program firstly 2 d calculations based on a refraction seismic interpretation allowed a comparison of gravity effects on a 160 km long profile but despite the fact that in the 2 d model granitic bodies were added at the earth s surface we only obtained a regional gravitational fit with strong deviations on a local scale secondly a gravimetric field survey in the region around haslach yielded rather poor indications of the geological structures in the 5 15 km depth interval which is in the context of the ktb of significant interest therefore all available information derived from geological and tectonic hypotheses and from reflection and refraction seismic interpretation were combined in a regional 3 d density model for the local anomalies the distribution of granite and gneiss bodies known from surface geology was added model bodies from an earlier regional study in southern germany which combined gravimetric and seismic results the moho undulations and the density of the lower crust and upper mantle were taken as additional model parameters in order to discuss regional features the kraichgau anomaly in the north of the black forest is modelled from a 2 d hypothesis as a body with an unusually high density and with a considerable lateral extent this approach demonstrates that evidence concerning the density contrast of anomalous masses can be found if boundary conditions are considered in the construction of the model

high times on the tibetan plateau paleoelevation of the thakkhola graben nepal east west extension in the tibetan plateau is generally assumed to have resulted from gravitational collapse following thickening and uplift on the basis of this assumption several studies have dated east west extensional structures to determine when the plateau attained its current high elevation however independent estimates of elevation are needed to determine whether extension occurred before during or after the plateau achieved its current elevation because the isotopic composition of meteoric water decreases with increasing elevation significant change in local elevation throughout the thakkhola graben depositional history should be recorded by change in 18 o values of fluvial and lacustrine carbonates the 18 o values of 16 to 23 of thakkhola graben carbonates reflect meteoric water values similar to modern values and suggest that the southern tibetan plateau attained its current elevation prior to eastwest extension initiation of thakkhola graben extension is constrained between 10 and 11 ma based on magnetostratigraphy of the older tetang formation the 13 c values of soil carbonates suggest an age younger than 8 ma for the base of the thakkhola formation

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

a new constraint on the composition of the topmost continental mantle anomalously different depth increases of p and s velocity summary compared to clear crustal p and s waves as well as a strong refracted pn wave propagating in the topmost mantle i e 10 to 15 km below the moho an anomalously weak appearance of the corresponding refracted shear wave sn is observed on refraction seismic profiles in sw germany france coastal maine ne united states and fennoscandia it is possible that the observation of pn pmp ratios close to unity combined with small sn sms ratios i e small sn amplitudes is typical for the continental uppermost mantle since this was observed for quite different types of continental crust it is shown that the observed phenomenon is not a source effect several possible causes to explain the observations are studied also with the aid of synthetic seismograms the effects of temperature attentuation and anisotropy have been investigated their influence cannot explain the observations the most likely candidate is a depth increasing vp vs ratio an increase in the order of 0 027 over a depth range of 10 km forcing p and s waves to propagate on different paths in the topmost mantle the observed phenomenon provides new constraints on the change of composition with depth for the crust mantle transition zone with basalt depletion as the most likely explanation

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

seismic images and evolution of the iberian crust in the pyrenees abstract crustal images of the pyrenean range based on coincident wide angle and vertical deep seismic profiles are presented in this study wide angle experiments in the thickened iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust in contrast reversed sounding on a line across the pyrenean range shows wide angle reflectivity its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust this observation is consistent with a new image of vertical incidence reflectivity obtained by coherency weighted migration of the coincident ecors traverse this profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust the reflections dip towards the axis of the range the upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple continuous three layered model previously used for geodynamical reconstructions the reflective zones cannot be brought back to the horizontal image of a layered lower crust of proposed extensional origin by plate unbending to the pre alpine stage the dipping reflections may result from compressional deformation as the south pyrenean deep crustal domain has undergone both hercynian and alpine orogenic deformation if part of the present reflectivity pattern was due to alpine remobilization the corresponding lower crustal deformation would have spread out thereby accomodating convergence by thickening even outside the axial zone where the upper crustal deformation appears to be concentrated

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

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

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

lower crustal thinning in the rhinegraben implications for recent rifting seismic near vertical and undershooting experiments carried out in the southern upper rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties significant differences in the apparent thickness of the reflective lower crust and the transparent upper crust appear to be related to different structural settings a 12 to 14 km thick reflective lower crust beneath a 15 km thick transparent upper crust of the eastern graben shoulder black forest probably originated during permo carboniferous reequilibration of thickened variscan orogenic crust thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder the dinkelberg block is interpreted to be related to cenozoic extensional faulting a discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupling at depth during extension congruent modification of other physical properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust in the transitional dinkelberg block such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity it is interpreted to be the presently active zone of decoupling which in time shifted from the rift axis to the eastern transition area

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

the krisp 90 seismic experiment a technical review abstract on the basis of a preliminary experiment in 1985 krisp 85 a seismic refraction wide angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the kenya rift down to depths of greater than 200 km this report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main krisp 90 effort the seismic refraction wide angle reflection survey was carried out in a 4 week period in january and february 1990 it consisted of three profiles one extending along the rift valley from lake turkana to lake magadi one crossing the rift at lake baringo and one located on the eastern flank of the rift proper a total of 206 mobile vertical component seismographs with an average station interval of about 2 km recorded the energy of underwater and borehole explosions to distances of up to about 550 km during the teleseismic survey an array of 65 seismographs was deployed to record teleseismic regional and local events for a period of about 7 months from october 1989 to april 1990 the elliptical array spanned the central portion of the rift with nakuru at its center and covered an area about 300 200 km with an average station spacing of 10 30 km major scientific goals of the project were to reveal the detailed crustal and upper mantle structure under the kenya rift to study the relationship between deep crustal and mantle structure and the development of sedimentary basins and volcanic features within the rift to understand the role of the kenya rift within the afro arabian rift system and to answer fundamental questions such as the mode and mechanism of continental rifting

petrologic case for eocene slab breakoff during the indo asian collision comment and reply comment kohn and parkinson 2002 1 proposed a conceptual model invoking eocene slab breakoff to explain the generation of eocene eclogites and miocene leucogranites in the himalaya and late eocene k rich magmas in southeastern tibet kohn and parkinson further argued that the slab breakoff and its

seismicity and structural complexity of the dinkelberg block southern rhine graben summary a detailed analysis of the dinkelberg seismicity is presented the seismic activity of the central dinkelberg block starts at about 10 km and increases with depth to a maximum between 18 and 20 km joint hypocentre determinations clearly prove that no hypocentres are located within the lower crust which starts at a depth of about 20 km according to results from recent reflection seismic surveys the seismic activity is concentrated just above the top of the lower crust fault plane solutions of the dinkelberg events show that the strike of the nodal planes is parallel to the directions of near surface tectonic features a correlation also exists between the main fault directions and the spatial distribution of the hypocentres in the pratteln earthquake cluster at the southern end of the dinkelberg block the occurrence of negative p residuals from local earthquakes for special propagation directions of waves crossing the dinkelberg block is modelled by a westward dipping layer this postulated upper crustal structure together with observations of preferred s wave polarizations are discussed in regard to recent tectonic models and geological findings

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

seasonal stable isotope evidence for a strong asian monsoon throughout the past 10 7 m y o of wet season rainfall was signi cantly morenegative 29 5 smow prior to 7 5 ma than after 26 5 smow if this change is attributable to a lessening of the amounteffect in rainfall this agrees with oral and soil geochemical datathat indicate increasing aridity beginning at 7 5 ma keywords tibetan plateau monsoon stable isotopes paleohydrology seasonality introductionthe tibetan plateau is the engine that drives the modern asianmonsoon by generating a high altitude region of low pressure in thesummer as the plateau heats and a region of high pressure in the winteras the plateau cools hastenrath 1991 during the summer warm airrises from the plateau pulling moist air off the ocean across the indiansubcontinent and into the highlands this results in heavy summer rain fall on the subcontinent the opposite occurs in the winter resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent thus the presence of a strong wet season dry season alternation implies the presence of a plateau broad and highenough to drive the monsoon the timing of the uplift of the plateau remains a matter of con siderable debate because there are few direct indicators of paleotopog raphy in the geologic record consequently past workers in tibet re lying on indirect indicators of uplift have proposed dates ranging from40 to 3 4 ma on the basis of initiation of potassic volcanism chunget al 1998 turner et al 1993 or extension on the plateau harrisonet al 1995 coleman and hodges 1995 changes in marine sedimen tation rates burbank et al 1993 sediment types rea et al 1998 or biota nigrini and caulet 1992 kroon et al 1991 and changesin stable carbon isotope and palynological patterns on the indian sub continent quade et al 1989 chen 1981 although different areasof the plateau may have risen at different times many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7 8ma by a process such as lithospheric delamination molnar et al 1993 this inference was based on the following approximately coevalphenomena a major change in plant communities of the indian sub continent quade et al 1989 and shifts in marine upwelling patternsthat are linked to an intense monsoon kroon et al 1991 althoughthere is strong evidence for signi cant climate change at 7 8 ma it isunclear whether this is the onset of the monsoon the oral transitionseems to have been a global rather than local phenomenon cerling etal 1997 and monsoonally driven upwelling may have already beenpresent by 10 12 ma nigrini and caulet 1992 kroon et al 1991 because there is an intimate association between the intense sea sonality of the modern monsoon and a high tibetan plateau and be cause evaporation can be unambiguously recognized in the d

seismic refraction investigation of the black forest abstract within the framework of site surveys of the deep continental drilling program ktb deep seismic sounding experiments were carried out in the black forest southwest germany in august 1984 these site surveys comprised seismic refraction as well as seismic reflection measurements this paper will concentrate on the interpretation of the refraction data which were observed along a 240 km long line approximately following the morphological axis of the black forest in a n s direction and extending in the south into the swiss molasse basin from the available seismic refraction data a detailed velocity depth model of the crust and uppermost mantle was derived a clear phase refracted in the basement with a velocity of 5 9 km s is observed for all shots this phase is followed by a reflection from the upper boundary of a low velocity zone this zone is very accentuated under the northern and central black forest and is characterized by a velocity decrease from 6 2 to 5 4 km s between depths of about 8 and 14 km towards the south the velocity inversion gradually loses its intensity under the center of the southern black forest velocities of about 6 km s prevail throughout the upper crust the lower crust which starts at the lower boundary of the low velocity zone shows a very complicated response in the record sections due to long reverberations between travel times of 1 5 3 5 s reduced by 6 0 km s at an epicentral distance of approximately 30 70 km a classical phase correlation is not possible based on a combined interpretation of near angle and wide angle reflection data the lower crust is characterized by a stack of thin layers with alternating high and low velocities the average velocity of the laminated zone is 6 7 km s the reflection from the moho at a depth of 25 26 km is the most prominent phase at epicentral distances greater than 60 km

the refraction seismic experiment granu95 in the saxothuringian belt southeastern germany summary in this paper we present newly acquired high quality wide angle seismic data of the granu95 project and first models of the crustal structure along two profiles 95 a and 95 b beneath the saxonian granulites a major ellipse shaped exposure of lower crustal material within the mid european variscan belt in southeastern germany saxony the crust is subdivided into four layers the crystalline basement with velocities higher than 6 0 km s 1 is generally reached at shallow depths with three major sedimentary structures as prominent exceptions where velocities considerably lower than 6 0 km s 1 as low as 5 1 km s 1 reach as deep as 4 km the highest upper crustal velocities up to 6 5 km s 1 are not seen below the exposed granulites themselves but at shallow depths 4 km sw of the exposure these shallow high velocities correlate well in depth with highly reflective zones observed on three seismic reflection lines of dekorp 85 4n 95 01 95 02 at their intersection with the seismic refraction line 95 b where they appear as a set of nw se trending dome shaped reflections on profile 95 a this high velocity uppercrustal layer 6 3 km s 1 dips from 5 to 9 km beneath the se margin of the exposed granulites these results suggest that the granulite dome and its western continuation are widely underlain by a ne trending antiformal structure probably a sheet of metabasic rocks where the exposed felsic granulites form just a local cap on top below the upper crustal high velocities a layer with decreased velocity 6 2 6 25 km s 1 extends down to an average depth of 15 km along the variscan strike 95 b and to 11 16 km depth slightly dipping towards the se perpendicular to the terrane boundaries 95 a at mid crustal levels a weak reflection from a layer with a velocity of 6 4 6 6 km s 1 may indicate the classical conrad discontinuity at the depth range 22 24 km the velocity jumps to an average value of 7 0 km s 1 thus defining a prominent high velocity layer in the lower crust which may be viewed as the well known laminated lower crust typical of variscan structures but with higher average velocity than usually detected the crust mantle boundary at about 30 31 km is typical for western europe and confirms the extensional signature of the west european crust below the moho poorly constrained upper mantle velocities of about 7 9 8 0 km s 1 are derived the high velocities observed in the lower crustal layer would not exclude the possibility of mantlederived intrusions but the lack of any sign of an updoming moho favours the interpretation of a more passively driven extension

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

a three dimensional crustal model of southwest germany derived from seismic refraction data abstract from densely covered seismic refraction data obtained in 1978 urach experiment and 1984 schwarzer zollern wald experiment and from seismic reflection data and results from previous refraction investigations a three dimensional crustal model of southwest germany was derived travel time and amplitude information of seismic refraction data were interpreted with two dimensional forward modeling ray tracing to calculate two crustal cross sections in southwest germany these results fill a gap in the existing data and enabled the construction of a detailed three dimensional crustal model while seismically the upper crust is laterally homogeneous 5 9 6 0 km s throughout the area the middle and lower crust show pronounced lateral variations in thickness velocity and reflectivity the moho is a flat surface at a relatively shallow depth 25 26 km we classify the middle and lower crust of southwest germany into two characteristic crustal types type i consists of a mid crustal low velocity zone 5 4 5 8 km s overlying a thick 10 km high velocity 6 6 6 8 km s lower crust type ii has no prominent mid crustal low velocity zone and a thin

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

petrologic case for eocene slab breakoff during the indo asian collision comment and reply reply we thank chung et al 2003 1 for their comment which permits further discussion of indo asian tectonics in our paper kohn and parkinson 2002 2 we presented three petrologic lines of evidence for slab breakoff including 1 himalayan eclogites 2 partial melting of the greater

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

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

scales of structure in the lithosphere images of processes advances in deep seismic sounding observations of the lithosphere on continents allow to identify various scales of heterogeneities in the crust mantle transition in this paper a fresh look is taken at the dynamic properties of seismic waves propagating through this zone ranging from the lower crust well into the lithospheric mantle as observed in near vertical reflection and wide angle refraction experiments the abrupt termination of near vertical reflections at the moho and the coincident presence of a strong coda accompanying the supercritical pmp reflection phase in record sections of continental seismic wide angle refraction experiments reveal a new property of the crust mantle boundary the moho forms a sandwiched mix of crust mantle material between lower crust and topmost mantle in combination with a step in mean velocity a significant change in scale of the structural dimensions and of velocity variance this view is strongly supported by the recognition of the subcrustal lithosphere as a high frequency 5 10 hz scattering wave guide for long distance propagation of the pn phase over more than 3000 km detected during seismic experiments in russia using peaceful nuclear explosions evidence is brought forward that this high frequency wave guide is likely to be present also in the phanerozoic upper mantle of western europe it is furthermore hypothesized that the global propagation of teleseismic sn or corresponding pn waves observed on continents and oceans combined with the recording of strong pmp codas for wave paths on continents points to a world wide change of scale of structure and velocities near the moho a change in scale of heterogeneities under almost all physical and chemical conditions indicates possibly a differential horizontal movement or decoupling of crust and upper mantle at the level of the moho the global presence of the scattering pn wave guide with a preferred horizontal elongation is hypothetically described as a seismic image of processes in the lithospheric mantle locations of present day tectonic activity where horizontal decoupling between crust and mantle are likely to occur e g subduction at continent continent collision zones change of tectonic regimes and continental transform faults are discussed

lower crustal petrology from wide angle p and s wave measurements in the black forest abstract the lower crust of the black forest in southwest germany is strongly reflective for compressional waves but almost non reflective for shear waves if the reflectivity for p waves is caused by alternating high and low velocity rocks the s wave observations require that the low velocity rocks have a low poisson s ratio and vice versa a purely compositional petrological model for lower crustal rocks can fit the observed p and s wave velocity data provided a rather high quartz content 50 is acceptable for the lower crust

seismic anisotropy within the uppermost mantle of southern germany summary this paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern germany the dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the p phase that could be effectively used in a time term analysis to determine horizontal velocity distribution immediately below the moho for 12 crossing profiles amplitude ratios of the p phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth a p wave anisotropy of 3 4 per cent in a horizontal plane immediately below the moho at a depth of 30 km increasing to 11 per cent at a depth of 40 km was determined for the axis of the highest velocity of about 8 03 kms at a depth of 30 km a direction of n31 e was obtained the azimuthal dependence of the observed p amplitude is explained by an azimuth dependent sub moho velocity gradient decreasing from 0 06 s in the fast direction to 0 s in the slow direction of horizontal p wave velocity from the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived