crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

moisture changes and fluctuations of the westerlies in mediterranean central chile during the last 2000 years the laguna aculeo record 33 50 s abstract the multi proxy study of lacustrine sediments from laguna aculeo 33 50 s provides detailed information about moisture changes in the lowlands of mediterranean central chile during the last 2000 years the lake lies just at the northern border of the strong westerlies influence with dry summers and humid winters geochemical sedimentological and diatom analyses provide evidence for an arid period between cal 200 bc and ad 200 and a subsequent increase in moisture after cal ad 200 abundant clastic layers in the core represent flood events periods with a higher frequency of flood events indicate an increased intensity of the westerlies more winter frontal system activity and possibly enso related variability probably comparable to modern conditions in mediterranean central chile periods of high clastic input occur around cal ad 200 400 500 700 especially around cal ad 1300 1700 and around ad 1850 1998 during the last 50 years at least eight flood events were detected correlating mainly with el nino years a very short drier period also occurred in the late 1960s in recent decades human impact has resulted in a eutrophication of the lake

earthquakes and strength in the laminated lower crust can they be explained by the corset model abstract we present a model that may explain deep crustal earthquakes observed in particular in several areas of highly reflective laminated lower continental crust we combine observations from earthquake seismology crustal reflection seismics and tectonic rheological concepts the study concentrates on parts of the northern alpine foreland where many earthquakes occur inside the laminated lower crust which is generally considered to be warm and weak thin mafic ultramafic sill like intrusions and invisible dykes are assumed to form a corset like network with high strength this model can explain the observed strong and multiple reflections and the occurrence of rupture inside a stable structure within a weak lower crust tectonic stress transfer from the alpine collision zone or and the upper rhine graben and its release may follow classical friction concepts in addition the heterogeneity of the laminated lower crust may also favour various viscous instabilities

progress in birps deep seismic reflection profiling around the british isles abstract the results of 4000 km of new birps profiles are briefly outlined as an introduction to other birps papers in this volume highlights of these profiles include the tracing for 900 km of a reflection pattern in the lower crust associated with the lapetus suture and the observation of more than 20 separate reflections from the mantle while progress in solving the problem of deep water multiples has enabled the joining of the continental and oceanic moho reflections off sw england developments in old interpretational problems are reviewed and the new problem of dating reflections is mentioned

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

conditions for flow in the continental crust 1 earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields the lower continental crust can be stronger than the upper mantle and imply that the upper mantle can flow even when the lower crust does not however in other places substantial flow in the lower crust does occur particularly near extensional metamorphic core complexes we suggest that the lower crust only flows when its viscosity is significantly reduced either by heating from igneous intrusions or by the addition of water in both cases the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs the timescale is controlled either by conductive cooling of intrusions or by the separation of melt from its matrix which removes the water

elastic wave velocities in rocks from a lower crustal section in southern calabria italy compressional and shear wave velocities vp and vs have been measured on 14 rock samples recovered from a high grade metamorphic terrane in the southern part of the calabrian massif which is considered to represent a continuous section of a former deep continental crust the experiments have been carried out in a cubic anvil apparatus at constant confining pressures up to 600 mpa and temperatures ranging from 20 to 700 c using the ultrasonic transmission technique at 600 mpa the compressional and shear wave velocities range from 6 33 km s 1 to 7 74 km s 1 and from 3 54 km s 1 to 4 30 km s 1 respectively variations in velocity are strongly related to variations in mineralogy both vp and vs increase with increasing amphibole pyroxene garnet and sillimanite content the range of velocities measured in the granulite facies rocks under conditions of lower crustal depth is similar to the range of seismic velocities for the crust beneath calabria importantly the metapelite rocks with intermediate silica and high alumina contents have in situ compressional and shear wave velocities which are equivalent to the mafic rocks or even significantly higher the detected high velocity layer at a depth between 15 and 20 km can be explained by a sequence of garnet cordierite sillimanite gneisses for which actual velocities between 6 6 and 7 6 km s 1 have been determined experimentally

crustal reflectivity underneath the central scandinavian caledonides abstract we have studied data from a 160 km long explosive vibroseis reflection seismic profile part of the central caledonian transect acquired in the central scandinavian caledonides between 1988 and 1992 extended correlation has been used to increase the record length to 20 s for the vibroseis component the reflectivity interpreted as primarily due to dolerities is strong at depths

unusually deep earthquakes in east africa constraints on the thermo mechanical structure of a continental rift system shudofsky 1985 has established that earthquakes associated with the east african rift system have well constrained focal depths as great as 25 30 km using published heat flow measurements as a guide to the local geotherm we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong these results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning any model of the rifting process in east africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

present day lithospheric strength of the eastern alps and its relationship to neotectonics 1 we calculate the present day lithospheric strength of the eastern alps along the new reflection seismic profile transalp to examine vertical and lateral strength variations and their implications on neotectonic activity of the eastern alps the large scale geometry of the eastern alps and the spatial distribution of upper and lower crustal layers and the lithospheric mantle is constrained by the deep seismic line two rheological models coupled to a kinematic thermal model that accounts for the thermal evolution of the eastern alps for the last 30 myr are investigated for the present day lithospheric configuration in the eastern alps models with strong model a and weak model b crustal rheologies predict the european and the adriatic plates to be stronger than the central zone of the orogen comprising the region between the inntal fault and the periadriatic fault model a is characterized by a brittle ductile boundary between 14 and 9 km depth and strong coupling of the mechanically strong lower crust to the upper mantle whereas model b suggests the presence of a thick decoupling zone between the upper crust and the upper mantle and a shallower brittle ductile boundary 7 10 km of these end member scenarios model a is in better agreement with neotectonic data including seismicity down to the upper lower crust boundary within the adriatic plate uplift of the central zone of the eastern alps and the southern alps and eastward escape of fault bound blocks such deformation pattern is best explained by lateral extrusion upon north south compression supporting a strong weak strong configuration of tectonic units along the transalp line

short scale variations of tectonic regimes in the western european stress province north of the alps and pyrenees the data of the european part of the world stress map wsm are used to study the western european stress province north of the alps and pyrenees the study area is characterised by an almost homogeneous n 145 degrees e orientation of the maximum horizont

impact of current wave interaction on storm surge simulation a case study for hurricane bob 1 hurricane bob moved up the u s east coast and crossed over southern new england and the gulf of maine with peak marine winds up to 54 m s 100 mph on 19 20 august 1991 causing significant damage along the coast and shelf a 3 d fully wave current coupled finite volume community ocean model system was developed and applied to simulate and examine the coastal ocean responses to hurricane bob results from process study oriented experiments showed that the impact of wave current interaction on surge elevation varied in space and time more significant over the shelf than inside the inner bays while sea level change along the coast was mainly driven by the water flux controlled by barotropic dynamics and the vertically integrated highest water transports were essentially the same for cases with and without water stratification the hurricane induced wave current interaction could generate strong vertical current shear in the stratified areas leading to a strong offshore transport near the bottom and vertical turbulent mixing over the continental shelf stratification could also result in a significant difference of water currents around islands where the water is not vertically well mixed

seismic and tectono metamorphic characters of the lower continental crust in phanerozoic areas a consequence of post thickening extension deep seismic profiles of phanerozoic continental crust commonly show a highly reflective lower crust rheological considerations suggest that the seismic fabric of the lower crust can be attributed to the tectonic transposition of various petrological heterogeneities in the main flow plane the variscan provinces of western europe have been affected during phanerozoic times by several extensional and compressional events the geometrical relationships between seismic and geological structures indicate that the layering of the lower crust was acquired during the late carboniferous to permian when the thickened variscan crust was affected by gravitational collapse petrological and geochronological analyses of deep crustal rocks xenoliths and exposed sections indicate that the lower crust has recorded a major high t medium p granulite facies metamorphism during the late variscan extension whereas on the surface upper carboniferous to permian basins were being deposited a similar scenario characterizes other phanerozoic orogenic belts in the caledonian provinces of the british islands the lower crust is seismically reflective it has undergone medium p granulitic metamorphism during the deposition of devonian sedimentary basins at the end of the caledonian orogeny in the same way collapse of the mesozoic belt in the western part of north america is responsible during the cenozoic for pervasive crustal extension whose consequence is a seismic layering of the lower crust accompanied by a low p granulite grade metamorphic event while in the mid and upper crusts low angle ductile normal faults give rise to the basin and range province therefore it is proposed that there is a genetic relationship between 1 post thickening crustal extension 2 low to medium p granulite facies metamorphism of the lower crust and 3 seismic layering of the lower crust

crustal xenoliths from cenozoic volcanic fields of west germany implications for structure and composition of the continental crust crustal xenoliths in three cenozoic volcanic fields of west germany the northern hessian depression nhd the eifel and the urach hegau include medium to high grade meta sedimentary and felsic to mafic meta igneous rocks also present in all three suites are pyroxenites and hornblendites for each volcanic field a model crustal profile is proposed based on calculated or measured p wave velocities of xenoliths and depth vp relationships egt central segment and rhenish massif traverses the xenolith lithologies from the nhd and the eifel show some similarities the middle crust between the depths of about 10 and 25 km consists mainly of meta sediments felsic gneisses and granulites meta sedimentary rock types are particularly abundant in the eifel at depths of between about 5 and 15 km but are less common within the nhd xenolith collection the felsic gneisses range from meta granites to meta tonalites i and s type eifel meta sediments range from meta pelites to meta greywackes and meta quartzites the nhd xenolith suite contains a few highly depleted granulite facies meta sedimentary fragments at depths between 24 and 26 km the increase in vp from about 6 8 to 8 km s 1 28 34 km is correlated with the presence of mafic granulites intercalated with eclogites pyroxenites and hornblendites beneath north hessia the granulite layer problably grades into a composite eclogite peridotite layer at the lower part of this transition zone the crust beneath the urach hegau consists largely of meta sediments with subordinate felsic meta igneous rocks most of the meta sedimentary samples seem to be depleted in felsic components suggesting intra crustal differentiation by partial melting the urach crust contains lithologies which are similar to the outcropping moldanubian type para gneisses of the black forest mafic and ultramafic xenoliths from the urach hegau differ in their mineralogy and chemical composition from the eifel and nhd mafic granulites they represent meta cumulates derived from alkaline magmas which intruded the base of the crust and underwent deformation and recrystallization petrographic and chemical differences between urach hegau and eifel nhd are believed to reflect the contrasting styles of crustal evolution in the rhenohercynian and the moldanubian belts of the variscan orogen

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

messinian climate change and erosional destruction of the central european alps at the end of the miocene the european alps ceased outward expansion and tectonic uplift and exhumation shifted into the orogen interior this shift is consistent with a change from orogenic construction to orogenic destruction reflecting an increase in the ratio of erosional flux to accretionary flux the coincidence of this change with an increase in sediment yield from the alps suggests a climate driven increase in erosional flux the timing of deformation and sediment release from the southern alps indicates that the tectonic change occurred synchronous with the last phase of the messinian salinity crisis we attribute the increase in erosional flux to a climatic shift to wetter conditions throughout europe likely augmented by the base level fall that occurred during the mediterranean dessication this climate change is represented in the stratigraphic record by the lago mare deposits of the mediterranean salinity crisis

first deep seismic reflection images of the eastern alps reveal giant crustal wedges and transcrustal ramps 1 the alps are considered as a classical example for an orogen created by continental plate collision in this study we present new images obtained from deep seismic reflection profiling in the eastern alps between munich and venice which give rise to examine and revise existing concepts the seismic sections exhibit a prominent bi verging reflection pattern at crustal scale a major ramp like structure outcropping at the inn valley fault can be traced southward over 80 km into the mountain root where relics of the subducted penninic ocean are expected new models of the evolution of the eastern alps show an upper lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the european and the adriatic african continents

nature of crustal reflectivity along the dekorp profiles in germany in comparison with reflection patterns from different tectonic units worldwide a review reflectivity of the continental crust displays many different patterns the dekorp lines are used as a basis for comparing and reviewing reflectivity in different tectonic units the brittle upper crust generally exhibits only two types of reflectivity it is either rather transparent preferably in some extensional provinces or and it shows traces of thrust and shear zones of former or present ruptures as these zones have a low impedance interior with few exceptions their first reflection onsets have a negative polarity and evince strong but short signals which sometimes can be correlated over several kilometers the generally ductile lower crust displays a completely different reflectivity in warm extensional and thin crusts the lower part is full of reflecting lamellae it is suggested that this type of reflectivity has a thermo rheological origin the creation of lamellae must take place in a ductile material with contrasting impedance under extensional stresses it can be associated with mineral alignment and corresponding seismic anisotropy destruction of lamellae may take place by a cooling process transforming parts of the lower crust into a brittle regime small stresses might deform or break the lamellae and leave a certain dispersed reflectivity like that in some old and cold shields there are no observations of reflecting lamellae in the upper crust or in the upper mantle in all areas the moho is the last reflecting band reflection moho which most often is identical with the classical refraction moho there are isolated mostly dipping reflections in the uppermost mantle in zones where the last tectonic event a delamination or subduction was not succeeded by a heating process the uppermost mantle is brittle again in most areas and may keep the memory of a cold collision over billions of years

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous