crustal versus mantle core complexes deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures metamorphic core complexes oceanic core complexes and magma poor passive margins using available analogue and numerical models and their comparison with natural examples the present paper reviews the mechanical processes involved in these different types of extensional setting three main aspects are considered i the primary role of lithosphere rheology ii the lithosphere scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii the initiation and development of detachment zones crustal core complexes develop in continental lithospheres whose moho temperature is higher than 750 c with upper crust dominated strength profiles contrary to what is commonly believed it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface in models convex upward detachments result from a rolling hinge process mantle core complexes develop in either the oceanic lithosphere at slow and ultra slow spreading ridges or in continental lithospheres whose initial moho temperature is lower than 750 c with sub moho mantle dominated strength profiles it is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment except if strong strain localization could occur in the lithosphere mantle distributed crustal extension by upper crust faulting above a decollement along the ductile crust increases toward the rift axis up to crustal breakup mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis during increasing extension

the bright moho reflection in the 1986 nevada passcal seismic experiment abstract detailed study of the reflected phases from the moho identified in the seismic reflection recordings of the 1986 nevada passcal lithospheric seismic experiment place important constraints on the fine structure and geological nature of the mohorovicic discontinuity two different spectral ratio techniques are used to estimate the reflectivity of the moho we evaluate the ratio of the reflected pmp phase to the incident source amplitude we also calculated the spectral ratio of the pmp phase with a reference reflection reflectivity estimates calculated using both spectral ratio techniques are very consistent high reflectivity within the range 0 28 0 39 was obtained the analysis of the spectral characteristics suggests a layered structure for the moho with an average thickness between 85 and 250 m these results require a layered moho structure with high impedance contrasts amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1 0 1 5 km thick layered structure produces amplitudes and waveforms comparable to the data to achieve such high reflectivities 20 25 of the moho consists of layers with velocities below 6 5 km s the remaining layers have velocities between 7 7 and 8 1 km s geologically the low velocities are consistent with velocities characteristic of basaltic magmas partial melts the high velocities are characteristic of mantle material the partial melts may represent an expression of present day underplating

comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the basin and range province direct comparison of low fold high energy explosive and high fold lower energy vibroseis methods for acquiring deep crustal seismic reflection data in the basin and range province suggests that the high fold common midpoint cmp method there does not provide the best possible image of lower crustal structure during the recent acquisition of a vibroseis profile in the basin and range province we fired single deep shot holes to obtain a coincident single fold explosive section within the upper crust upper 3 s the explosive source and vibroseis records are nearly equivalent for record times below 3 s however comparison of the explosive source gathers and the coincident final 60 fold vibroseis section demonstrates that low fold explosive profiling provides a higher quality image of the midcrust to lower crust 3 10 s the higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives making them less sensitive to common noise sources whereas deeper than 4 5 s the vibroseis energy levels on individual source efforts fall to that of ambient noise levels the explosions provide signal generated energy exceeding ambient noise levels down to 18 19 s although individual reflections can be correlated on explosion and vibroseis shot gathers reflection events on the 60 fold vibroseis stack do not correlate to those on the single fold explosion profile suggesting that the high fold cmp method in our study did not maintain the integrity of the weak lower crustal reflected arrivals reasons why the high fold cmp method apparently failed include complex even time varying statics nonhyperbolic moveout at long offsets and the difficulty in resolving stacking velocities with data having low signal to noise ratios reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower energy vibroseis section

nature and origin of seismic reflection fabric ruby east humboldt metamorphic core complex nevada seismic reflection profiling across exposed upper and middle crustal rocks of the ruby east humboldt metamorphic core complex delineates important characteristics of the crustal structure developed during tertiary extensional deformation the goals of this study were to trace a tertiary extensional shear zone from mylonitic surface outcrops into a seismic section and to characterize the deeper crustal fabric associated with the polyphase deformational history of the complex reflections in the shallow surface correlate with a plastic to brittle shear zone that formed during the tectonic unroofing of the middle crustal rocks constructive interference from strong planar layering in the mylonitic shear zone is considered chiefly responsible for generating the reflections east and southwest dipping reflectors in the seismic section appear to correlate with exposed opposing dipping mylonitic foliation domains the opposing dips of the mylonitic layering may reflect warping of the normal sense shear zone during tectonic exhumation coupled with an overall anastomosing character deeper in the section a heterogeneous reflection character correlates with increases in velocity shown from wide angle measurements in this region and is interpreted as penetrative fabric originating from extensional flow facilitated by broad scale pure shear and localized simple shear lower crustal rocks apparently achieved granulite facies metamorphism during extension a maximum of 6 8 km of mafic material could have been added to the crust during cenozoic extension the moho reflection has such a high amplitude that it may be caused by partially molten rocks interleaved with peridotite reflections from the moho show no significant upwarping of the base of the crust beneath the core complex this suggests that the lower crustal configuration is at least as young as the period of extensional activity that lasted from approximately 40 to 20 ma which was responsible for the exhumation of the metamorphic core complex

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

resolution properties and 3 d reconstruction from multi azimuth wide angle data in the baltic region abstract recent development of geotomography methods and wider use of seismic array observations have improved conditions for studying the 3 d velocity structure of the earth the solution to this problem has a number of specific features which ranges from the typical irregular geometry of the observation sites and seismic sources to the possibilities for seismic tomography at different scales global regional and local the international babel survey provided one such spatial array with irregular geometry many sources and a relatively small number of observation points the seismic ray coverage is not uniformly dense and sufficient enough for reconstructing the media within the framework of tomography approach this article presents results of a 3 d velocity reconstruction using data from shots along babel lines 1 6 and 7 recorded by all available land stations estimation of the accuracy of reconstruction is made on the basis of mathematical modeling model parametrization and starting model selection modeled velocities of 7 45 7 55 km s between 46 and 57 km are transitional between those traditionally assigned to lower crust and mantle and are interpreted to represent high grade metamorphic crustal rocks or intermixing of crust and mantle at a seismically small scale 100 m

contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

structure and development of the southeast newfoundland continental passive margin derived from screech transect 3 new seismic reflection data from the grand banks of newfoundland and the newfoundland basin add to the growing knowledge of the composition structure and history of this nonvolcanic margin geophysical imaging is now approaching the extent of that done previously on the conjugate margin along iberia providing a valuable database for the development of rifting models two parallel profiles over the shelf platform image deep crustal fabric representing precambrian or possibly appalachian deformation as well asmesozoic extension progressively more intense extension of continental crust is imaged oceanwards without the highly reflective detachments frequently seen on profiles off galicia a landward dipping event l is imaged sporadically and appears to be analogous to a similar event on the iberian iam9 profile the transition zone is probably exposed serpentinized mantle as interpreted off the iberian margin although there appears to be a difference in the character of ridge development and reflectivity the distinctive u reflection identified previously at the base of the newfoundland basin deep water sedimentary section and recently identified as one or more thin basalt sills is imaged on newly presented profiles that connect previously published profiles scr3 and scr2 showing that u is highly regular and continuous except where interrupted by basement highs u is also seen to have a major impact on the ability to image underlying basement a full transect beginning over completely unextended continental crust through to oceanic crust has provided a data set from which estimates of extension and the pre rifting location of the present continental edge can be made two estimates were obtained 85 km based on faulting and 120 km based on crustal thickness

crustal structure and tectonics of the northern part of the southern granulite terrane india deep seismic reflection studies investigating the exposed archean lower continental crust of the southern granulite terrane india yield important constraints on the nature and evolution of the deep crust including the formation and exhumation of granulites seismic reflection images along the kuppam bhavani profile reveal a band of reflections that dip southward from 10 5 to 15 0 s two way time twt across a distance of 50 km the bottom of these reflections beneath the dharwar craton is interpreted as the moho further south another reflection band dipping northward is observed these bands of reflectivity constitute a divergent reflection fabric that converges at the moho boundary observed at the mettur shear zone reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks which we infer resulted in crustal thickening and the formation of granulites anomalous gravity and magnetic signatures are also observed across the mettur shear zone the gravity model derived from the bouguer gravity data corroborates seismic results the tectonic regime and seismic reflection profiles are combined in a 3 d representation that illustrates our evidence for paleo subduction at a collision zone the structural dissimilarities and geophysical anomalies suggest that the mettur shear zone is a suture between the dharwar craton in the north and another crustal block in the south this study contributes significantly to our understanding of the operation of archean plate tectonics here inferred to involve collision and subduction furthermore it provides an important link between the gondwanaland and global granulite evolution occurring throughout the late archean

evidence of an imbricate crustal thrust belt in the southern british variscides contributions of southwestern approaches traverse swat deep seismic reflection profiling recorded through the english channel and the celtic sea the southwestern approaches traverse swat seismic reflection profiles recorded through the celtic sea and the english channel provide unexpected data concerning notably the deep structure of the irish and south british variscan crust one of the most significant results is the recognition of prominent deep southerly dipping reflectors regarded as large scale foreland directed variscan thrusts which constitute in the southern innermost zones a crustal stacking wedge the resulting overthickened crust may have induced the petrogenesis of the cornubian granitic batholith by an anatectic melting process the roots of this crustal duplex are not imaged on the swat lines because of their subsequent assimilation by the more recent layering of the present day lower crust upward the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone not revealed by the swat profiles and which underlies the thick devonian allochthonous high strained units of cornwall northward this shallow northerly verging shear zone is overstepped by the tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as caledonian features southward the imbricate crustal thrust unit is overridden by the lizard ophiolitic suture which appears as a 8 km thick gently southerly dipping sheared zone crosscuttting the entire crust and rooting deep beneath the present day moho its hanging wall is constituted by the channel cadomian block characterized by an almost seismically featureless upper crust which is not involved in the main variscan thrust stacking northward the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat lying decollement level this frontal ramp emerges along a broadly 110 n trending discontinuous thrust front located above the northern downfaulted margins of the munster ireland and pembroke wales devonian basins the swat deep seismic data relate the overall variscan crustal shortening undergone by the south british domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous channel block and the northern autochthonous foreland this crustal shortening occurred during a carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust floored basin lizard

results of nfp 20 seismic reflection profiling along the alpine section of the european geotraverse egt summary recent deep crustal seismic reflection profiling across the central alps of eastern and southern switzerland has provided a detailed image of this continental collision zone as part of the swiss national science project nfp 20 a series of reflection profiles was recorded during two phases along n s transects crossing the alpine edifice the eastern and southern profiles coincide with the alpine segment of the european geotraverse egt and in part with the earlier swiss geotraverse these data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the alps on a larger scale the new reflection data also add to a growing set of profiles crossing the alpine fold belt which includes an additional traverse carried out by nfp 20 across the western swiss alps and the ecors crop profile across the western alps the initial results of the reflection data across the central alps outline a crustal framework involving northward indentation of the adriatic hinterland into the subducting european foreland this occurs beneath the collapsed oceanic basins of the penninic allochthon which is defined by its highly reflective crystalline nappes the present crustal thickness within the alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the adriatic wedge the south plunging european foreland implies that portions of the thickened crust have been subducted into the upper mantle this general framework of flake tectonics is laterally consistent with the ecors crop results from the western alps however the western alps show a higher degree of crustal imbrication which may be related to the presence of the rigid ivrea body and an overall greater amount of crustal shortening compared to the central alps

multistage remobilization of the southwestern margin of the south china plate insights from zircon u pb geochronology and hf isotope of granitic rocks from the yao shan complex southeastern tibet plateau

on the dynamics of extensional basin geological and geophysical data from the north china bohai basin and basin and range province were examined and compared they are similar to each other in many respects surficial geological structures are characterized by a series of half grabens with their one flank constituted by normal fault those extensional structures usually extend to a depth of 6 8 km therefore the stress condition in the upper 8 km can be written as sigma 2 sigma x sigma y where x y denote the directions of maximum compression and maximum tension on the horizontal plane while z signifies the vertical direction

deep seismic reflection profiling and the growth of the continental crust abstract deep seismic reflection profiling provides evidence for the style and magnitude of phanerozoic crustal growth by both lateral and vertical accretion profiles across convergent zones have been interpreted as showing tectonic underplating of the continental crust by fragmented oceanic lithosphere profiling across extensional zones provides evidence for magmatic underplating of the crust in addition to studies over active areas deep reflection profiling over large areas of western europe north america and australia shows that the lower crust is strongly reflective and seismically distinct from the upper crust and upper mantle the layering is detected beneath many different terranes of varying crustal type and basement age from archaean to cenozoic and is often apparently younger than the tectonic or metamorphic age of the exposed upper crust thus continental growth curves estimated from upper crustal measurements alone may underestimate phanerozoic crustal additions and mean crustal ages similarly estimated may be too high

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

origin of deep crystal reflections seismic profiling across high grade metamorphic terranes in canada abstract in an attempt to better understand the origin of deep crustal reflections lithoprobe has sponsored or co sponsored seismic reflection surveys across tracts of high grade metamorphic rock in the archean superior craton the proterozoic grenville orogen and the phanerozoic cordilleran orogen common to these three diverse terranes are near surface zones of prominent seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology at some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks stratified assemblages sills etc whereas in other regions it was generated by extreme attenuation stretching and ductile flow of weakly layered or irregularly organized rocks it seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust with reflections originating at lithological boundaries and zones of mylonite

tectonic uplift mechanism of the goodenough and fergusson island gneiss domes eastern papua new guinea constraints from seismic reflection and well data 1 the d entrecasteaux island dei gneiss domes are fault bounded domes with 2 5 km of relief exposing ultrahigh pressure uhp and high pressure hp metamorphic gneisses and migmatites exhumed in an oligocene miocene arc continent collision and subduction zone subject to late miocene to recent continental extension to study the style of continental extension accompanying exhumation of the dei gneiss domes a grid of 1518 km of 2 d multichannel seismic mcs reflection data and well data is interpreted from the offshore areas surrounding the dei including the trobriand basin and the goodenough basin the offshore study is combined with onshore geologic information to constrain the oligocene to recent tectonic evolution of the basins mcs and well data are consistent with the trobriand basin forming as a forearc basin caused by southward miocene subduction at the trobriand trench at 8 ma the margin transitioned to an extensional tectonic environment since then the trobriand basin has subsided 1 2 5 km with few normal faults deforming the basin fill south of the dei the goodenough rift basin developed after extension began 8 ma with the hanging wall of the north dipping owen stanley normal fault bounding the southern margin of the basin the lack of evidence of upper crustal extension accompanying subsidence in the trobriand and goodenough basins suggests depth dependent lithospheric extension from 8 to 0 ma has accompanied uplift of the dei gneiss domes and supports schematic model of uplift of the dei domes involving vertical exhumation of buoyant postorogenic lower crust far field extension from slab rollback and an inverted two layer crustal density structure

magma in layering at the moho of the basin and range of nevada layering at the moho as determined from seismic reflection data is a commonly observed phenomenon in the basin and range for which we propose a porous flow melt model analysis of seismic reflection data from the 1986 passcal nevada experiment indicates that the moho in the nevada basin and range is layered and has unusually high reflectivity this reflectivity is difficult to explain in terms of a velocity contrast caused by different rock types and in fact a lower velocity of less than 6 5 km s suggests the presence of partially molten material at the moho in a zone containing 20 to 25 of low velocity layers to explain the seismic observations we have developed a viscous porous flow model for basaltic melt ascent such a model produces systematic variations in volumetric melt flow with depth and close correspondence between seismic interpretation and modeling of partial melt ascent this suggests that such layering involving viscous porous flow of partial melt is a plausible explanation of the layering of the moho in the basin and range and may have more general application

tectonic and magmatic evolution of the northwestern basin and range and its transition to unextended volcanic plateaus black rock range nevada the seismically active eastern and western margins of the northern basin and range have been extensively studied yet the northwestern margin of the province remains incompletely understood the black rock range of northwestern nevada straddles the transition from the basin and range province to the south and east and flat lying volcanic plateaus to the west this poorly understood range preserves a remarkably complete record of cenozoic magmatism and provides an important window into the pre miocene history of the unextended volcanic plateaus of northeastern california and southern oregon geologic mapping and 40ar 39ar geochronology from the northern black rock range document three significant episodes of eocene to middle miocene volcanism eocene 35 ma basalts directly overlie mesozoic granites and arc related volcanic and sedimentary rocks locally erupted oligocene to early miocene 27 21 ma bimodal volcanic rocks comprise the bulk of the cenozoic section and conformably overlie the eocene basalt flows these bimodal units include rhyolitic lavas variably welded rhyolitic ash flows unwelded ash fall deposits and thin basalt flows in the neighboring pine forest range 20 km to the north similar oligocene to early miocene units are overlain by more than 500 m of ca 16 4 ma steens equivalent basalt flows and are capped by ca 16 ma rhyolitic ash flow tuffs in the northern black rock range the ca 16 4 ma middle miocene basalts are absent from the section and a 16 2 ma rhyolitic ash flow tuff directly overlies the early miocene flows basaltic and rhyolitic volcanic products in the northern black rock range span 35 16 ma with many of the oligocene volcanic units derived from local vents and dikes despite the map scale complexities of locally derived lava flows the cenozoic section is broadly conformable and dips gently 5 10 to the northwest the region experi enced no significant tilting between 35 and 16 ma with moderate tilting 5 10 and concomitant uplift occurring after 16 ma this tectonic history is consistent with that of the nearby pine forest and santa rosa ranges where low temperature thermochronology documents footwall exhumation along the range bounding normal faults after 12 ma the velocity structure of the crust beneath the northern black rock range is constrained by a recent geophysical survey seismic reflection refraction and gravity and contains gradients that correspond to basin depths predicted by our geologic mapping together with recently completed geological and geophysical studies from the surrounding region our results suggest that the evolution of the northwestern margin of the basin and range was characterized by long lived and voluminous volcanism without significant tectonism followed by low magnitude 20 extension along high angle normal faults

structural characteristics of the yilan yitong and dunhua mishan faults as northern extensions of the tancheng lujiang fault zone new deep seismic reflection results abstract the tancheng lujiang fault zone tlfz can be subdivided into three segments that exhibit sharp contrasts in their deep structures a deep seismic reflection profile length 600 km across the north part of the tlfz which provides new constraints on the structural styles of the northern tlfz was recently completed by the chinese sinoprobe project here the tlfz branches into the yilan yitong fault yyf to the west and the dunhua mishan fault dmf to the east the yyf developed as an internal fault in the songnen zhangguangcai massif while the dmf serves as the tectonic boundary between the nadanhada terrane and the khanka massif both faults developed large scale flower structures with that of the yyf being negative and that of the dmf being positive with reverse faults the moho in the profile is at a depth of 25 39 km and is offset by the faults the north part of the tlfz extends into the upper mantle as thin shear zones with the reflectors truncated in the middle lower crust this feature differs from most crustal scale strike slip faults that distribute over a discrete shear zone in the lower crust such as the san andreas fault

gravity interpretation of a unified 2 d acoustic image of the central alpine collision zone summary four individual deep seismic reflection profiles across the eastern and southern swiss alps have been combined along the trend of the bouguer gravity anomalies onto the european geotraverse the profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide angle profiles running parallel to the strike of the alpine arc the contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles the resulting acoustic image depicts the subduction of parts of the lower crust of the european plate beneath the adriatic promontory of the african plate at a low angle of 15 based on the interpretation of the seismic data orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the european plate and wedging of the european and adriatic middle and lower crusts the latter being ill constrained by the seismic evidence alone the south vergent thrusting of the southern alps can be accounted for by the observed downbending of the adriatic moho and the lower crust in conjunction with the inferred wedging at mid crustal levels using the geometric constraints provided by seismic data gravity modelling of the alpine lithosphere asthenosphere system relative to stable central european platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth such a gently dipping subduction zone is at odds with prominent models on alpine geodynamics which favour a near vertical orientation of the subducting lithosphere verschluckung in agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen both seismic and gravimetric evidence therefore suggest that late orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper middle and lower parts of the crust the amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab

cenozoic crustal scale duplexing and flat moho in southern tibet evidence from reflection seismology abstract although the lhasa terrane of southern tibet is an integral part of the tibetan plateau how the area has achieved its current high altitude of 5000 m remains poorly understood this is largely due to the fact that little is known about deep 15 km crustal structures and the depth and geometry of the moho below the terrane in this study we address this question by acquiring and analyzing high resolution seismic reflection data gathered from four high energy dynamite shots along a 137 km north south traverse spanning the southern lhasa terrane our new seismic data reveal the presence of north dipping reflectors in the crust a seismically weak reflection layer at the lowermost crust and a sub horizontal and locally discontinuous moho at depth of 69 72 km with a crustal velocity of 6 30 km s the discordant relationship between the crustal dipping reflectors the subhorizontal lower crustal layer and the flat moho suggests that the 12 km basal section of the lhasa terrane crust is a decoupling zone accommodating different styles of lithospheric scale deformation because the north dipping reflectors can be projected and or connected with the reflectors in the cenozoic himalayan thrust system and the southernmost lhasa terrane we propose that the dipping crustal reflectors are parts of a south directed crustal scale thrust duplex created by the cenozoic india asia collision the relatively discontinuous and horizontal geometry at the base of the crust indicates that the moho was reconstituted through ductile deformation in the lowermost crust caused by abnormally high temperature

vertical deformation associated with normal fault systems evolved over coseismic postseismic and multiseismic periods vertical deformation of extensional provinces varies significantly and in seemingly contradictory ways sparse but robust geodetic seismic and geologic observations in the basin and range province of the western united states indicate that immediately after an earthquake vertical change primarily occurs as subsidence of the normal fault hanging wall a few decades later a 100 km wide zone is symmetrically uplifted the preserved topography of long term rifting shows bent and tilted footwall flanks rising high above deep basins we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting we replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust a 60 dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width 10 km which is predicted to take place within 1 2 decades after each large earthquake thus the best preserved topographic signature of rifting is expected to occur early in the postseismic period the relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades

post critical sspmp and its applications to virtual deep seismic sounding vdss 3 back projection imaging of the crust mantle boundary in a heterogeneous lithosphere theory and application

long wavelength undulations of the seismic moho beneath the strongly stretched western anatolia summary western turkey provides spectacular examples of the two end member models of deformation of the continental lithosphere with strain localization along the north anatolian fault and distributed north south extension along the aegean coast to provide constraints on the mechanisms of continental deformation we present a new high resolution image of lithospheric structurealonga 650kmtransectcrossingwesternanatoliaat28 elongitudefromtheblack sea to the mediterranean more than 2600 receiver functions are computed from records of teleseismic earthquakes at 40 broadband seismic stations with an average spacing of 15km lateral variations of crustal thickness and vp vs ratio are inferred from both h k and common conversion point stacks we observe long wavelength variations of moho depth from 31km in the thrace basin to 25km beneath the marmara sea 32km beneath the izmir ankara suture 25km beneath the menderes massif and 20km on the coast of the mediterranean no mid or lower crustal interface is visible in the migrated depth section and seismic discontinuities are confined to the shallow crust the small amplitude and long wavelength lateral variations of the moho topography suggest that viscous flow in a hot lower crust has smoothed out the lateral variations of crustal thickness induced by cenozoic continent continent collision thecrust mantleboundaryisflatbeneaththecentralandsouthernmenderesmassif the rougher moho topography and more heterogeneous crust of the marmara region likely result from transtension in the north anatolian fault zone superimposed onto aegean extension the moho of the subducted african lithosphere is observed dipping northward between 40 and 60km depth at the southern end of the profile the abrupt termination of the subducted slab only 50km to the north of the mediterranean coast confirms the slab tear inferred from previous tomographic studies

shear wave velocity structure in the northern basin and range province from the combined analysis of receiver functions and surface waves a new method based on the joint inversion of receiver functions and surface wave phase velocities results in well determined shear velocity structures that are consistent with the compressional wave structure gravity heat flow and elevation data in the northern basin and range this new inversion method takes advantage of average velocity information present in the surface wave method and differential velocity information contained in the receiver function method thus min imizing the nonuniqueness problem that results from the velocity depth trade off an unusually thick 38 kin and relatively faster crust and upper mantle are found in central and eastern nevada compared to the thin 28 to 34 km and slower crust and upper mantle of the western basin and range we interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less cenozoic ex tension relative to the surrounding regions to the west and north the thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern nevada simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface indicating a near classical airy type of compensation in the province we analyze in detail the shear wave s wave velocity model derived from the receiver functions at station bmn and compressional wave p wave velocity models derived from the 1986 passcal experiment in northwestern nevada the most interesting feature of these models is the presence of negative velocity gradients in the s wave model with no corresponding velocity decrease in the p wave models between depths of 10 and 24 km this combined velocity model may be explained by high pore fluid pressures at these depths this model favors a layered fluid porosity model proposed in the literature to explain extensive middle to lower crust conti nental seismic reflections and high electrical conductivity an upper mantle grada tional low velocity zone is present between 32 and 38 km in the s wave model this upper mantle shear wave low velocity zone is consistent with partial melt which may be the source material for magmatic underplating in this region

crustal structure of the midcontinent rift system in eastern lake superior from controlled amplitude analysis of glimpce deep reflection seismic data the reprocessing of line f of the glimpce deep marine reflection seismic survey according to controlled amplitude principles provides new insights on the crustal structure of the midcontinent rift

crustal scale duplexing beneath the yarlung zangbo suture in the western himalaya the fate of the indian plate during collision with asia is debated seismic images of the western himalaya reveal large scale thrust faults that transfer indian crust upwards into the overriding asian plate

imaging the crustal structure of the central iberian zone variscan belt the alcudia deep seismic reflection transect alcudia is a 230 km long vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern central iberian zone part of the pre mesozoic gondwana paleocontinent of the variscan orogen of spain the carefully designed acquisition parameters resulted in a 20 s twtt deep 60 90 fold high resolution seismic reflection transect the processed image shows a weakly reflective upper crust the scarce reflectivity matching structures identified at surface a thick highly reflective and laminated lower crust and a flat moho located at 10 s twtt 30 km depth the transect can be divided into three segments with different structural styles in the lower crust in the central segment the lower crust is imaged by regular horizontal and parallel reflectors whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust north and tectonic wedging involving the mantle south the alcudia seismic image shows that in an intracontinental orogenic crust far from the suture zones the upper and lower crust may react differently to shortening in different sectors which is taken as evidence for decoupling the interpreted structures as deduced from surface geology and the seismic image show that deformation was distributed homogeneously in the upper crust whereas it was concentrated in wedge thrust structures at specific sectors in the lower crust the seismic image also shows the location of late variscan faults in spatial association with the lower crustal thickened areas

the effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low angle normal faults to investigate the formation of core complexes and low angle normal faults we devise thermo mechanical simulations on a simplified wedge like orogenic hinterland that has initial topography moho relief and a preexisting mid crustal shear zone that can accommodate shear at very low angles 20 we mainly vary the strength of the lower crust and the frictional strength of the preexisting mid crustal shear zone we find that the strength of the lower crust and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes with increasing lower crustal strength we recognize varying extensional features with decreasing exhumation rate these are characterized by bivergent metamorphic massifs classic cordilleran metamorphic core complexes multiple consecutive core complexes or boudinage structures and a flexural core complex underlined by a large subsurface low angle detachment fault with a small convex curvature topographic loading and mantle buoyancy forces together with divergent boundaries drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading the detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust though the regional stress is largely andersonian we find non andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve

deep seismic reflection evidence for the role of extension in the evolution of continental crust summary deep seismic reflection profiling as well as geologic studies indicate that extensional basins are a common feature of continental crust the wide spread occurrence of extensional basins combined with published models on the effects of extension on lower crustal rocks suggests that extensional processes play an important role in the evolution of continental cmst extension is now recognized to have been the last major tectonic event to affect approximately 50 of the united states this observation and the preservation of extensional features in areas which have experienced subsequent episodes of compression suggests that extensional processes may lead to a strengthening of continental crust in areas of active extension such as the western united states seismic and petrologic data as well as theoretical modeling of heat flow data suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present mohorovicic discontinuity formed during extension although the interpretation of the various data is somewhat speculative we suggest that the volume of continental material in some areas has doubled as a result of extension thus extension may result in the addition of a significant amount of new material to the continents

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

processing and inversion of refraction and wide angle reflection data from the 1986 nevada passcal experiment we derive averaged one dimensional velocity models for the crust beneath the basin and range in western nevada by inverting p composites extracted from slant stacks of refraction and wide angle reflection data a combination of high cut filtering in the frequency domain with coherency filtering in the p domain has proved effective in smoothing severe static shifts due to lateral heterogeneities and suppressing artifacts due to spatial aliasing we supplement the low frequency information provided by the refraction records with higher frequency arrivals picked from the normal incidence and wide angle reflection sections generalized least squares inversion of data from eight shots for p and s wave velocity structure beneath two perpendicular profiles yields consistent estimates for total crustal thickness p wave 30 31 km s wave 30 32 km and average crustal velocity 6 2 km s 3 4 3 5 km s average poisson s ratio beneath the n s profile is 0 27 0 28 p wave velocities near the base of the crust 7 4 0 3 km s are estimated by inverting precritical reflections from the moho standard errors in depth for these models range from 1 to 3 km extremal depth bounds in general are about twice as large uncertainties in velocity estimated from resolving kernels range from 0 1 to 0 3 km s estimates of poisson1s ratio as a function of depth are characterized by large standard errors reflecting the poor resolution of shear wave velocities the combination of low average p and s wave velocities with a high average poisson s ratio in the upper half of the crust however is consistent with the presence of fluid filled cracks these results highlight the need for three component recording to better resolve shear wave velocity structure

the deep structure of northern england and the iapetus suture zone from birps deep seismic reflection profiles the nec deep seismic reflection profile images structures in the lower crust which represent the iapetus suture zone beneath southern scotland and northern england on the basis of reflectivity characteristics of the lower crust and reflection moho we recognize four different crustal zones which correspond to four crustal terrane types midland valley zone a a sub continental subduction complex zone b lake district zone c and midland platform zone d the junctions between each of these zones are interpreted as being tectonic a and b originate from the northern continental margin of the iapetus ocean and are separated from c and d which are derived from a southern continent by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust the structure responsible for these reflections does not displace the reflection moho north of the ab cd junction the base of zone b and base of the reflective crust is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km these reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust at their northerly limit the deepest of the two parallel reflectors transects the moho to a depth of 6 7 km into the uppermost mantle we suggest that their termination delimits the northerly extent of a decollement or shear zone at the present continental moho the shear zone was active during the late stages of caledonian collision when mantle originally from beneath the southern continent underthrust old and newly created crust of the collision zone

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

crustal structure of the celtic sea and western approaches from gravity data and deep seismic profiles constraints on the formation of continental basins a free air anomaly map of the celtic sea and the western approaches is presented including all available marine gravimetric data and bouguer anomalies on land n60 features are predominant in the area they correspond to the n60 so called caledonian grain which played a major role in the variscan orogeny and in the formation of the permo triassic sedimentary basins the fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of n105 features which we regard as late carboniferous transpressional structures in the eastern pan of the celtic sea thrusts are in the same n105 direction west of 6 w longitude however folds and thrusts trend preferentially in a n60 direction following the older grain there the n105 lineaments may be regarded as dextral shear structures with a possible compressive component the hercynian shortening of the celtic sea domain located between the variscan front and the lizard suture resulted from the displacement along the n160 direction of the two adjacent rigid domains north of the variscan front the n105 and n135 features were possibly active in more recent times then both the n60 and n105 trends controlled the formation of permo triassic basins the n105 directions being transfer faults two dimensional gravity modelling of the swat profiles in the celtic sea shows that the base of the reflective lower crust may be associated with the moho discontinuity everywhere in the celtic sea except below the saint george s channel basin the lower crust does not change significantly in depth and thickness beneath the permo triassic basins none of the available models simply satisfies such conditions they all require at least a modification of the geometry of the lower crust after the formation of the celtic sea basins nevertheless if the simple shear model can be ruled out mainly from structural arguments the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models however two additional conditions are necessary 1 the preservation of crustal seismic features formed before the creation of basins must be explained and 2 a significant portion of the volume of the stretched lower crust about 40 of this volume for the north celtic sea basin must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

some unsolved birps problems summary birps has 8000 km of records around britain that are relevant to the rheology of continents and to faulting of the lithosphere problems include the need to measure seismic velocities with better resolution and to eliminate multiples in deep water the question are blank bits of records due to noise increase or to geology and the need to know the physics of the origin of deep reflections in order to answer geologists questions

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

folding in regions of extension summary many extension zones have been subjected to folding and shortening in a direction perpendicular to the stretching such deformation can be accounted for by the extension of a thin superficial elastic layer overlying a substrate that has small elastic moduli or that deforms in a viscous regime laboratory experiments are used to document the wavelength and amplitude of the folds for a range of geometrical configurations folding is observed even for very small amounts of extension less than 1 per cent with characteristics that are consistent with finite amplitude scaling laws because of the intrinsically 3 d nature of the deformation field the size of the region affected by folding and the direction of the fold axes depend on the orientation of the extension with respect to the rigid blocks that bound the deforming region for regions of extension where the elastic thickness is about 10 km as in the basin and range province for example it is predicted that folding occurs with wavelengths in a 20 40 km range such that it induces little deformation in the lower crust and maintains a flat moho discontinuity these predictions are consistent with the observations the characteristics of faulting that is associated with such deformation are discussed

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

deep structure of the continental lithosphere in an unextended orogen an explosive source seismic reflection profile in the urals urals seismic experiment and integrated studies urseis 1995 explosive source near vertical seismic reflection data from the urals seismic experiment and integrated studies urseis profile displays an image of orogenic lithosphere to depths of 225 km the reflective crustal section is characterized by dipping crustal scale shear zones and a pronounced crustal root that show no evidence of overprinting by extension and are preserved since late paleozoic collision toward the east the uralian crustal root is composed of a partially subducted volcanic arc while to the west it has affinity with the east european platform the reflection character of the moho varies across tectonic strike and implies that the moho 1 acted as a structural detachment beneath the trans uralian zone 2 forms a tectonic boundary between terranes beneath the east uralian zone 3 consists of a transitional zone from eclogitized lower crust to peridotitic upper mantle in the crustal root 4 is a 3 9 km thick reflective zone beneath the fold and thrust belt and 5 is a boundary 200 m thick in the west mantle reflections at 80 100 km depth 22 24 s may represent a continuation of the main uralian fault into the subcrustal lithosphere alternatively these reflectors may signify paleozoic or younger mantle shear zones or the top of a zone of partial melting in the upper mantle deeper mantle reflections at 140 160 km 35 45 s and 225 km 55 s may image mafic intrusions at the base of the lithosphere or along localized shear zones in the upper mantle

constraints on residual topography and crustal properties in the western united states from virtual deep seismic sounding we use virtual deep seismic sounding vdss and data from 1 000 broadband seismic stations to provide high resolution estimates of crustal structure in the western cordillera of the united states us the most robust result is the geographic distribution of residual topography that is the difference between observed elevation and that expected from crustal buoyancy alone and by implication thermal or petrologic anomalies in the mantle overall residual topography of the western us cordillera varies considerably with contrasts of up to about 3 km across distances of 200 km or less high residual topography indicating large mantle effects is evident along the periphery of the colorado plateau and the surroundings of the great basin in contrast the central colorado plateau and the wyoming basin show low residual topography close to what is expected of a geologically stable lithosphere overall in regions to the east of the wasatch hinge line the eastern limit of significant extension in the north american cratonic basement patterns of high residual topography and anomalies of low seismic wave speeds in the upper mantle are similar suggestive of a common thermal origin in contrast such a similarity is absent in regions to the west of the hinge line suggesting substantial effects of petrological heterogeneities in the mantle finally joint analyses of vdss and conventional receiver functions reveal a wide range of crustal p wave speeds locally as high as 6 7 km s perhaps indicating magmatic modification of the crust

crustal velocity structure beneath the eastern flank of the ruby mountains metamorphic core complex results from normal incidence to wide angle seismic data abstract p wave velocity structure along the eastern side of the ruby mountains metamorphic core complex determined using 2 d ray inversion modeling shows that 1 the crust can be divided into three layers corresponding to the upper middle and lower crust 2 from east of ruby dome to the northern end of the seismic profile velocities of the upper crust range from 6 12 0 1 to 6 20 0 1 km s and near surface velocities range from 1 90 0 1 to 4 80 0 1 km s from this point to the southern end of the profile upper crustal velocities range from 5 80 0 1 to 6 25 0 1 km s and near surface velocities range from 3 01 0 1 to 4 80 0 1 km s 3 middle crustal velocities range from 6 35 0 15 to 6 45 0 15 km s 4 lower crustal velocities range from 6 60 0 15 to 6 80 0 15 km s and 5 depths to the moho vary irregularly between 30 5 and 33 5 km from interpretation of these results we conclude that 1 the transition in metamorphic grade from deep upper crustal rocks south of harrison pass to middle crustal rocks north of harrison pass does not correlate with an increase in seismic velocity near surface basement seismic velocities increase in the central part of the range near ruby dome well north of the major metamorphic transition 2 depths to the moho do not reflect local surface relief isostatic balance is achieved predominantly in the crust rather than by formation of a local crustal root and 3 the crust has been modified during extension by intracrustal processes rather than by large scale magmatic underplating

dating the source of lower crystal reflectivity using birps deep seismic profiles across the lapetus suture abstract six birps deep seismic reflection profiles around the british isles that cross the trend of the lapetus suture allow a geophysical definition of the suture zone for 900 km along its strike and demonstrate that specific reflective structures have survived since the mid palaeozoic caledonian orogen in an area northeast of england spanning 70 km along caledonian strike and 200 km across strike three profiles show very considerable similarities in the reflectivity patterns of the lower crust when these similarities are mapped the boundaries separating regions of lower crust with the same reflective character run parallel to caledonian strike this strongly suggests that the distribution of reflectivity in the lower crust in this area is directly controlled by caledonian structures geologic models for the area imply that many of the reflectors themselves may also be of caledonian age however though an age for the imposition of patterns of reflectivity can be inferred the age of an individual reflector cannot be determined unless it can be traced to outcrop or unless there is independent evidence as to the physical nature of the reflector

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

does the great glen fault really disrupt moho and upper mantle structure many previous interpretations of deep seismic reflection profiles across the great glen strike slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the moho discontinuity such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the british isles reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the moho step interpretation diffraction analysis and seismic migration applied to reflection data north of shetland across the northward continuation of the great glen fault walls boundary fault indicate a narrow synform or keel developed on the moho directly beneath the fault this keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault north and west of ireland analyses of amplitudes frequencies and the geometrical behavior of reflections upon migration show that structures previously interpreted as moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin in one location north of ireland where some three dimensional control exists the vertically downward projection of the great glen fault intersects without disrupting dipping structure in the upper mantle this observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp or blind thrust in the uppermost mantle somewhat analogous to models developed for the san andreas fault that indicate displacement along the fault to be laterally offset within the middle crust one of the principal conclusions of this study that major vertical steps on the moho beneath the great glen fault are difficult to support from the available seismic data is consistent with theologically based studies which predict that moho topography such as vertical steps is unlikely to be preserved over long periods of geologic time

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

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

receiver function imaging of the lithosphere at the kunlun qaidam boundary northeast tibet abstract we studied the crustal structure and tectonics in the north tibetan plateau from the songpan ganzi terrane to the qaidam basin using teleseismic receiver function imaging across a major lithospheric boundary the kunlun qaidam boundary where previous studies suggest a 15 20 km change in crustal thickness from thicker crust in the kunlun mountains to thinner crust in the qaidam basin we report p receiver functions for 70 stations largely the international deep profiling of tibet and the himalaya indepth phase iv experiment our most dense station coverage is located along the roughly north south indepth iv active source seismic profile at approximately 95 e longitude azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and vp vs from across the study area receiver functions show strong converters that we interpret as the moho at 70 km depth beneath the qiangtang songpan ganzi terranes and kunlun mountains and at 50 km depth beneath the central qaidam basin this large change in crustal thickness occurs 50 km north of the north kunlun strike slip fault on which the 2001 m8 1 kunlun earthquake occurred receiver functions for some of the stations north of the thickness change at the kunlun qaidam boundary also show a deeper 70 km bright converter in addition to the 50 km converter the two converters appear to overlap by up to 30 km in some locations along the south qaidam basin we combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north tibetan plateau including crustal flow and crustal injection at depths imaged here shallower than 100 km we see no evidence of southward subduction of eurasian lithosphere

deep seismic reflectivity beneath an intracratonic basin insights into the behavior of the uppermost mantle beneath the illinois basin abstract reflectivity images of the lower crust and uppermost mantle beneath the paleozoic illinois basin have been derived from reprocessing of several hundred kilometers of industry seismic reflection data using extended vibroseis recorrelation the recorrelation was based on extending an originally 4 s correlated record acquired with a 16 s sweep from 14 to 126 hz to the absolute limit of the full 20 s 70 km listening traveltime sub moho reflections are recorded to 18 s two way traveltime unmigrated and are observed on intersecting profiles apparently dipping to the southwest and striking northwest southeast occasional moho reflections are also observed across the profiles 12 s or 38 km while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short gently dipping reflections and diffraction segments the presence of newly observed mantle reflectivity beneath the illinois basin indicates significant uppermost mantle heterogeneity relative to other parts of the usa studied using reflection methods the mantle reflectivity effectively defines an area of anomalous mantle that rests beneath the depocenter of the early paleozoic illinois basin which is itself superimposed over a deeper proterozoic basinal depocenter filtering and inversion of geopotential field data reveal that this anomalous mantle also partially correlates with a positive density anomaly in the upper mantle these correlations suggest that the anomalous mantle zone exerted an influence on the early paleozoic subsidence of the basin perhaps as a buried load due to eclogitization of former lower crust the relatively isolated occurrence of sub moho reflections beneath the basin makes it difficult to uniquely infer their origin however available geologic and geophysical constraints especially from geochemical and geochronological studies of drilled and exposed basement rocks limit the possibilities to 1 remnants or scars of sub crustal processes associated with lithospheric extension or delamination related to the melting of the proterozoic crust that led to the emplacement of the granite rhyolite province that underlies much of usa midcontinent or 2 deformation caused by plate subduction associated with the hypothetical accretion of a juvenile island arc to the pre 1 6 ga southern margin of the laurentian continent

seismic images of the continental moho of the indian shield abstract deep seismic reflection profiling has imaged different patterns of the moho across the indian shield with a variety of tectonic environments from archean to recent the character of the moho varies from a discrete strong event the base of strong coherent lower crustal sub horizontal reflections the base of dipping lower crustal reflections into mantle to no clear reflection boundary the seismic reflection data suggest a laminated lower crust in several places and offsets in the moho at others kinematic and dynamic modeling of wide angle reflection data across the mesoproterozoic south delhi fold belt and the central indian tectonic zone suggests a laminated lower crust for these regions in general the lower crust of the indian shield is heterogeneous a clear moho is identified in some of the precambrian orogenic belts and sedimentary basins along with reflective lower crust whereas the cratonic areas exhibit a diffused moho post collisional extensional process such as orogenic collapse delamination magmatic intrusions low viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger moho the termination of lower crustal reflectivity at the moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle

deep seismic image enhancement with the common reflection surface crs stack method evidence from the aravalli delhi fold belt of northwestern india summary imaging of deep crustal features from narrow angle deep seismic reflection data especially from fold belt region has been a challenging task the common reflection surface crs stack is an alternative seismic imaging technique for multicoverage reflection data it is an automatic stacking process which does not require explicit knowledge of stacking velocity this crs stack is especially useful when the data quality is poor and foldage is low in this paper we demonstrate an application of the crs stack to a deep seismic reflection data set acquired across the aravalli delhi fold belt of the northwestern india which provides a seismic stack section with much improved signal to noise ratio comparing the conventional common midpoint cmp stack with the crs stack we find that the moho and other crustal reflections have been resolved clearly and the continuity of the reflectors has also been enhanced with the crs stack method the major findings from our crs processing include clear image of the moho discontinuity below the marwar basin and sandmata complex and prominent upper and mid crustal reflections our study for the first time images an extension of crustal scale jahazpur thrust below the sandmata complex which becomes listric at the moho some of the crustal features derived in this study were not identified in the earlier investigations using the cmp stack our study clearly demonstrates that the crs stacking method is more appropriate for imaging the crustal and subcrustal structures of the thrust fold belt region than the conventional cmp method where limited velocity information is available crustal thickness across the proterozoic orogenic aravalli delhi fold belt varies between 38 and 50km global correlation of the seismic results suggests no relation between crustal thickness and age of the crustal block but it depends on the thermorheological and tectonic history of the region palaeosignatures of the proterozoic subduction and collision processes are still preserved in major portion of the crust in this study area as well as in several parts of the world

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

crust mantle boundary reflectors in nevada from ambient seismic noise autocorrelations summary the depth of reflector layers in the earth s crust is usually estimated using controlled sources or earthquake signals ambient seismic noise however can also be used for this purpose we develop and apply a new method based on continuous waveform analysis to estimate the two way p wave reflection component of the green s function beneath each station the green s functions are retrieved from continuous record autocorrelation stacks at broad band sensor locations within the usarray earthscope transportable array in the western great basin and the sierra nevada in a region with complex crustal and upper mantle structure in this paper we show evidence of a reflector at the crust mantle boundary moho discontinuity derived for the first time from ambient noise autocorrelations using short period 1 s data our results compare well with earthquake and controlled source investigations and with tomography findings in the region moho depth is difficult to resolve seismically because of the lack of favourable spatial distribution of source and receiver geometries in contrast our method can be applied at any desired sensor spacing to estimate earth reflector depth beneath surface located sensors providing unprecedented resolution

deep crustal seismic reflection profiling across two major tectonic zones in southern sweden abstract a seismic reflection profile has been shot in varmland southwestern sweden across two major tectonic zones the protogine zone and the mylonite zone the crustal bedrock units separated by the tectonic zones are clearly distinguished in the seismic profile by changes in the reflection character in the upper 5 6 km the protogine zone is represented by a 15 20 km wide band of dipping structures the mylonite zone dips too steeply to be seen directly and a lack of clear reflectors at depth makes a continuation down through the crust hypothetical in the uppermost 5 6 km between the two tectonic zones stronger and more continuous reflections can be seen and are interpreted as being connected with hyperite intrusions which have been mapped at the surface the lower crust shows generally little reflectivity with only short and weak reflectors an exception occurs at mid crustal depths where a number of clearly recognizable bands of reflected energy are suggested to be major shear zones formed during sveconorwegian grenvillian thrusting towards the east a slight increase in reflectivity in the depth range 42 48 km correlates well with the moho depth determined from refraction seismic studies the possible relationships between data acquisition parameters and the observed scarcity and low strength of reflections from the deeper crust have been considered in some detail it is shown that the use of 28 hz single geophones instead of 10 hz geophone strings along a major part of the profile did not reduce the signal to noise ratio for deep reflections however the varying source characteristics in combination with the different quality of the recordings made computation of the residual static corrections difficult which may have resulted in a partly destructive stacking of seismic traces in this way the low reflectivity of the deeper crust in this area is further brought out in the stacked seismic sections

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

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

array analysis of the large aperture array of the 1988 89 passcal basin and range passive source seismic experiment summary the 1988 89 passcal basin and range passive source seismic experiment consisted of two array deployments the large and small aperture arrays the large aperture array consisted of seven stations with three component mid period seismometers and independently triggered prototype passcal data recorders with goes clocks the array spatial distribution and the synchronized data have permitted us to employ simple beamforming operations to improve receiver function estimation from single teleseismic events the beamforming reinforces the coherent teleseismic p wave and the deterministic portions of the p wave coda primarily generated by interactions at major lithospheric interfaces microseismic background noise is incoherent across the array and is attenuated by beamforming beamforming also diminishes those portions of the coda due to scattering by inhomogeneities that appear random on the scale of the array aperture the enhanced signal to noise ratio of the teleseismic recording dramatically improves the single event receiver function deconvolution results

sn velocities in western and eastern north america we have investigated the use of the new delay time data set of engdahl et al 1997 for the construction of tomographic images of sn velocity in north america the delays show a significant difference in average delay between paths located west and east of 105 w this difference is larger when paths longer than 10 are included indicating a positive gradient for the s velocity in eastern north america discarding outliers two separate regions in the western and eastern united states and canada where imaged low velocities were found under the colorado plateau extending westwards towards the gulf of california in the eastern u s the michigan basin stands out with low sn velocity and the grenville province is characterized by a positive anomaly

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

crustal decoupling and intracrustal flow beneath domal exhumed core complexes betics se spain the sierra nevada core located in the betic hinterland features a n s large scale open antiform with a central relatively uplifted highly extended domain placed between two less extended domains in the east and in the west dipping eastwards and westwards respectively the core bounding detachment system formed during the serravallian 15 11 ma in an episode of ene wsw extension the esci beticas 2 deep seismic reflection profile a transect through the core shows a highly reflective deep crust overlying a subhorizontal moho and a fairly transparent upper crust and upper mantle the lack of moho relief beneath this area with differential values for supra crustal thinning suggests a mechanism of intracrustal isostatic compensation surface geology data together with seismic imaging indicate intracrustal flow and upward doming as a response to footwall unloading accompanying the middle miocene supracrustal extension a prominent mid crustal reflector mcr is deemed to represent a decoupling zone between the upper and the deep crust subsequent n s shortening and associated folding occurred in the late miocene the interference pattern of this folding over the middle miocene core produced the current e w dome shaped tectonic windows where the deepest complex of the betic hinterland crops out

cocorp deep seismic reflection traverses of the u s cordillera summary cocorp seismic reflection traverses of the u s cordillera at 40 n and 48 5 n latitude reveal some fundamental similarities as well as significant differences in reflection patterns on both traverses autochthonous crust beneath thin skinned thrust belts of the eastern part of the cordillera is unreflective immediately to the west the cordilleran interior is very reflective above a flat prominent reflection moho mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns in constrast to more easily deciphered patterns beneath areas of cenozoic accretion the prominent reflection moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during cenozoic extension the main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the cordilleran interior these differences are probably related to the way cenozoic extension was accommodated at depth laminated middle and lower crust above the reflection moho in the western basin and range 40 n may be related to magmatism ductile pure shear and large scale transposition during cenozoic extension by contrast beneath the eastern basin and range 40 n and the orogenic interior in the nw united states 48 5 n cenozoic extension was probably accommodated along dipping deformation zones throughout the crust

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

thin viscous middle crust and evolving fault distribution during continental rifting insights from analog modeling experiments abstract analog modeling of continental rifting assuming a crustal scale jelly sandwich like rheology was carried out to test the mechanical effect of varying the absolute thickness of a weak viscous middle crust silicone layer interbedded between a brittle upper crust sand layer and a strong lower crust results consistently show a delocalization of the brittle deformation i e a uniform scattering of the faults throughout the upper brittle layer this effect is interpreted to be associated with pressure driven flow in the viscous layer caused by the tectonic collapse of upper brittle fault blocks into the viscous substratum a reduction of the overall viscous layer thickness increases its resistance to accommodate internal thickness variations which promotes delocalization of the fault pattern in the upper brittle layer our results contribute to the understanding of the mechanics of the so called upper plate paradox a large scale structure often recognized at non volcanic rifted margins a thin viscous middle crust provides means of decoupling the deformation affecting upper and lower crust during rifting on one hand this promotes a uniform scattering of faults throughout the upper brittle crust on the other hand it allows for a strong localization of the deformation in lower crust and upper mantle expressed by the lithospheric necking in the rift center

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

an 1800 km cross section of the lithosphere through the northwestern north american plate lessons from 4 0 billion years of earth s history the lithoprobe slave northern cordillera lithospheric evolution snorcle study across northwestern north america in combination with related crustal studies has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high resolution geophysical data seismic reflection refraction electromagnetic potential fields and detailed bedrock geology the cross section offers one of the longest continuous profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships the primary conclusion of the study is that during all major orogenic episodes recorded from archean to present in that part of earth s lithosphere the crust and perhaps much of the mantle was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis the observed subsurface geometries of relict subduction zones accretion

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results

implications of transient deformation in the northern basin and range western united states transient deformation events observed in global positioning system gps data from the basin and range extensional province may illuminate qualitatively similar transient events observed in subduction zones and other tectonic environments we model gps time series at 22 sites using a combination of hyperbolic tangent function analysis and elastic load deformation estimated from climatological data we identify two transient events 2000 4 and 2004 4 with roughly similar timing and displacement to those described previously by other researchers the first few years of gps observations adopted as a reference state in earlier studies are found to be anomalous our results differ from previous studies in two respects first a significant component of northward transient motion occurs during both events despite a reversal of sign in east component motion second sites move coherently in the eastern as well as the western basin and range surface mass loading the largest source of transient stress forcing in the region exhibits no evidence of a simple relationship to the deformation transients prior studies inferred slip on a single megadetachment at the moho but that hypothesis assumes negligible ductile deformation of the lower crust and a dry olivine rheology for the uppermost mantle recent measurements of crustal quartz abundance and effective elastic thickness suggest both assumptions are unlikely basin and range transients can be reconciled with the frictional slip mechanism widely accepted for subduction zone transients provided that slip is occurring on discontiguous detachment surfaces at midcrustal depths