shallow seismic reflection profiling over the brevard zone south carolina shallow reflection profiles over crystalline rocks of the brevard zone show events that correlate with projections of mapped lithologic contacts in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering saprolite the profiles which were collected in western south carolina about 5 km from appalachian ultra deep core hole adcoh regional lines 1 and 3 straddle the contact between the brevard fault zone and the wider brevard mylonitic shear zone to the southeast the principal goal of these experiments was to test the feasibility of using a hammer source and 24 channel seismograph to image near surface brittle and ductile structures to a depth of several hundred meters we recorded two dip lines and one strike line with a total common midpoint cmp coverage of 1 km processing focused on eliminating static shifts and separating reflections from overlapping refracted s waves a statics approach based on the alignment of first arrivals on common offset gath

possible mid crustal shears at the edge of the london platform summary a reflection survey across part of the southern north sea has revealed sw dipping bands of reflection segments in the midcrust under the edge of the london platform the upper limit of each group of reflection segments has been contoured in twt to give a three dimensional impression of the shape of the features the shape position and orientation of these groups together with the reflection character within each group suggest that they represent large scale extensional probably dilatant shear zones it is proposed that they developed at the edge of the north sea basin due to relative movement between an undeformed brittle upper crust and a more ductile lower crust which has been stretched towards the basin to the ne the shears are thus caused by heterogeneous crustal stretching

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

velocity modeling of a complex deep crustal structure across the mesoproterozoic south delhi fold belt nw india from joint interpretation of coincident seismic wide angle and near offset reflection data an approach using unusual reflections in wide angle records 1 seismic wide angle record sections from favorably placed source and limited aperture recording spreads particularly across collision boundaries reveal bright unusual high velocity contrast reflections of limited lateral coherence the arrival times of the unusual reflections for recording updip of the reflecting plane align as high apparent velocity phases at wide angle offsets coincident near offset reflection images from similar structural features provide additional control for modeling the unusual phases on the wide angle record sections we propose an approach for joint interpretation of these coincident seismic data sets and modeling the unusual phases on the processed wide angle records to delineate both the velocity stratification and the geometry of the deep crustal structures consistent with the near offset reflection images processed and migrated near offset reflection images reveal several steeply dipping isolated reflections on the western margin of the south delhi fold belt dfb nw india interpreted as the south delhi thrust fault coincident limited aperture wide angle records reveal bright unusual reflections which are also found to be associated with the same thrust fault observations of similar dipping reflection patterns in various other regions lend support to consider them as genuine in plane reflections resulting from a structural fabric imprinted on horizontally oriented geological structures we use the reflectivity structural model from the near offset data set that provides necessary constraints for modeling and simulating the travel times and amplitudes of the unusual reflections recognized in the wide angle records the unusual reflections are effectively used as part of the wide angle data set for gaussian beam synthetic seismogram modeling thus delineating for the first time a complex 2 d model of the seismic velocity structure of the deep crust and uppermost mantle across the marwar basin mb and the dfb the dipping events revealed in the wide angle data set are successfully modeled and interpreted as reflections from the thrust fault consistent with the near offset reflection image the 2 d velocity model parameterized as a stack of isolated dipping reflectors embedded in deep crustal layers is successively refined by including the structural features recognizable both in the near offset reflection image and in the wide angle records the unusual reflections are modeled as a set of discrete reflectors representing local velocity variations imprinted on the background of a deep crustal velocity structure the lamellar structure revealed by short isolated dipping and the subhorizontal middle to lower crustal reflections suggests heterogeneities at scale lengths on the order of at least the fresnel zone diameter as resolved here the local velocity variations in the deep crust can be considered analogous on a different scale to the models in which velocity heterogeneities on scale lengths of the seismic wavelength constitute a medium of random distribution of scatterers the crustal scale south delhi thrust fault on the western margin of the dfb delineated as the stack of reflectors dipping 25 30 se probably developed during the mesoproterozoic collisional episode related to the delhi orogeny contemporaneous with the formation of rodinia it is concluded that complex structural features developed during collisional episodes can survive through geological periods longer than generally expected depending on the accretionary characteristics at the time of orogenic evolution the 2 d velocity model further reveals significant variations of crustal seismic structure with a relatively sharp and shallow moho at 39 40 km in the mb and a deeper moho at 45 47 km underlying a 12 km thick subhorizontal lamella of high velocity 6 9 7 5 km s layers in the dfb regions

geophysical studies of the structure of the appalachian orogen in the atlantic borderlands of canada results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the appalachian orogen in eastern canada wide angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins high velocity lower crust attributed to underplating is found below the former laurentian continental margin in newfoundland and below the magdalen basin potential field data are used to trace the surface zones of the orogen from the northeast newfoundland shelf to cape breton but extrapolation to new brunswick and quebec is unclear because of late paleozoic basin development the central mobile belt of the orogen is only a few tens of kilometres wide in southwest newfoundland and cape breton but broadens substantially to around 200 km elsewhere reflection images show a strong deep crustal fabric that runs along the orogen with a margin that crosses into the avalon zone in southern newfoundland but coincides with the avalon gander zone boundary el

seismic reflection models of continental crust based on metamorphic terrains summary laboratory seismic velocity measurements on rock samples from metamorphic terrains coupled with geologic cross sections provide the basis for synthetic seismic reflection profiles for various types of continental crust results from greenstone belts mylonite zones and partial cross sections of continental crust indicate that lithologic heterogeneity and geometrical factors control crustal reflection characteristics

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

p wave crustal structure of the lake vanern area sweden eugeno s profile 6 abstract to investigate the deep structure of the precambrian basement airgun shots in lake vanern southern sweden were recorded on land by two 100 km lines of mars stations to the south two geostore stations east and west of the lake and a 7 km multichannel reflection array on the varmlandsnas in the centre of the lake modelling of two profiles running roughly n s parallel to the major tectonic grain and on either side of the mylonite zone shows variation in crustal seismic velocity structure from east to west comparison of these models with near vertical incidence stacked sections from the varmlandsnas experiment and with an e w profile suggests continuity of reflecting structures in a n s direction this e w profile shows shallowly west dipping strongly reflective structures probably associated with the mylonite zone and also a small scale moho structure tentatively interpreted as the western limit of a wedge of overriden older precambrian crust

grain sorting processes grain sorting affects river bed aggradation through downstream fining and erosion processes through static and mobile armor formation a fractionwise sediment transport calculation procedure is presented it has been calibrated on rotational erosion tests and sediment feeding experiments under equilibrium conditions in the mobile armor case equal mobility must be reproduced to simulate the rotational erosion process application to flow ranges for which a static armor layer forms shows that transport under equilibrium conditions with identity of subsurface and bedload mixture is then not possible a unique sediment function exists only for higher flows

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

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

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

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

seismic imaging in the frontal part of the sveconorwegian orogen south western sweden reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the sveconorwegian orogen south western sweden receiver spacing was 25 m and nominal shot spacing 100 m shots ranging in size from 0 5 to 1 0 kg were fired in 3 m deep shot holes the data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km the profile crosses the central axis of an inferred fan like structure indicated from earlier detailed surface mapping results deformational fabrics within this structure dip approximately 15 40 e in the western part of the profile and 60 80 w in its eastern part the seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault protogine zone in earlier studies surface structural data predict the hinge axis to be located about 3 km farther east despite this discrepancy there is good general agreement with the structure inferred from the surface data the image obtained from the reflection seismic data establishes that the fan like structure is a major upper crustal feature the present data in combination with previous deep seismic data sets exclude a single stage extensional model with an east dipping extensional deformation zone protracting eastwards from the western part of the profile the combined seismic data provide support for a two stage compressional model for the development of the fan like structure with crustal thickening and stacking prior to 955 ma and later compression with a stronger horizontal component of displacement during 930 905 ma the latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the sveconorwegian frontal deformation zone sfdz a tectonic analogy to the grenville front in the eastern part of north america the sfdz dips westwards may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile the kinematics and geometry of the sfdz suggest that reverse displacements thrusting may be responsible at least in part for the exhumation of the medium to high grade rocks in the western part of the fan like structure both the seismic data and the surface structural geology show many similarities to the grenville front

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

geological significance of seismic reflections in collision belts geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well documeted references some of these problems are illustrated by examples coming from various tectonic settings in france and adjacent areas and tentative interpretations are proposed

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

crustal evolutionary model for the variscides of ireland and wales from swat seismic data the swat seismic reflection profiles recently acquired across the celtic sea demonstrate that the crust which underlies the munster and pembroke variscan foldbelts contains extensive south dipping reflections interpreted as hercynian foreland directed ramp thrusts largely involving the pre devonian basement these large scale crustal ramps developed without any intervening flat lying decollement levels and are thought to have originated in a deep infra crustal decoupling zone not imaged on the swat profiles because it lies within the present day layered lower crust at approximately 25 km depth the pre existing extensional basinal pattern is assumed to have highly influenced the geometry of these deeply rooted thrusts and in particular that of the frontal ramp which emerges along a discontinuous n100 trending thrust zone located above the faulted extensional margin of the munster pembroke and mendips devonian basins shortening of the irish and welsh external zones is hence assigned to dominantly crustal stacking achieved along steep ramps dying out upwards and giving way to mainly undisrupted folds in the thick overlying devonian infill in addition a shallow thin skinned fold thrust system developed above an intra namurian master decollement still preserved in the footwall of the frontal ramp where it either stays blind ireland or emerges pembroke above a dominantly wrench faulted caledonian basement compiling swat seismic data and structural geology leads to a crustal evolutionary model for the irish and south welsh external variscides from a devonian basin template to a final deformation pattern

physical properties of crustal and upper mantle rocks with regards to lithosphere dynamics and high pressure mineralogy this paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes it addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation the major themes to be discussed will be the seismic signature of crustal and mantle shear zones the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity in particular seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures because the measured seismic anisotropy has a structural origin emphasis will be placed on the relations between shear wave splitting and fabric anisotropy fabric anisotropy and strain and strain and geologic tectonic processes the second part of this paper discusses the effect that metamorphic phase transitions in crustal and mantle rocks may have on rock physical properties velocity density specifically the possible role that the gabbro eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered

seismic profiling of archean crust crustal structure in the morton block minnesota river valley subprovince abstract a 58 km seismic reflection profile recorded by the university of wyoming over the early archean gneisses 3 6 3 8 ga of the minnesota river valley subprovince superior province shows more complicated structure and rock variation with depth than the limited surface geology suggests wide angle data show a high average velocity of 6 8 0 1 km s and a crustal thickness of 49 1 km from a zero offset time of 14 2 0 2 s upper crustal 0 10 km reflections are consistent with the gentle dips from surface folding while middle crustal 10 24 km reflections are interpreted as a 40 km long se dipping thrust or fold structure 6 km thick lower crustal 24 48 km reflections are short and low amplitude consistent with small reflection coefficients and short reflectors modeling suggests that this reflectivity is consistent with small

seismic reflectivity of detachment faults of the iberian and tethyan distal continental margins based on geological and petrophysical data abstract low angle detachment faults are key to our understanding of the tectonic evolution of magma poor rifted continental margins in seismic images of present day rifted margins the identification and interpretation of such features is however notoriously difficult and ambiguous we address this problem by studying the structure and seismic response of such faults through a synoptic interpretation of petrophysical data and geological evidence from the distal segments of the present day west iberian and the ancient tethyan margins on the basis of the geologically well constrained remnants of the tethyan margins which are spectacularly preserved and exposed in the alps of eastern switzerland vertical profiles at four key geological settings of a typical magma poor rifted margin are constructed and their synthetic seismic responses are compared to the observed seismic data from corresponding locations in the present day iberian margin the seismic structure of these profiles is considered as the sum of deterministic large scale and the stochastic small scale components both components are analyzed for all pertinent lithologies the large scale structures are derived from laboratory measurements on samples from both the west iberian and tethyan margins whereas the small scale fluctuations are constrained predominantly on the basis of well log data from the iberian margin different realizations of the simulated stochastic small scale velocity fluctuations illustrate the potential variability of impedance contrasts and its impact on the seismic response from lithological interfaces and fault structures our results indicate that the nature of the seismic response from low angle detachment faults is largely determined through the fracture healing behavior of the surrounding rocks geological evidence from the exposed fragments of the tethyan margins indicate that fracture healing is generally well developed in crustal lithologies but largely absent in mantle lithologies it is for this reason that low angle intra crustal detachment faults tend to be seismically undetectable conversely crust mantle detachments have a complex and variable seismic response depending on the nature of the damaged zone and on the frequency content of the seismic data these model based inferences are consistent with the available evidence from the present day iberian passive margin and thus open new perspectives for the interpretation of the corresponding seismic images

p wave velocity across a noncoaxial ductile shear zone and its associated strain gradient consequences for upper crustal reflectivity in order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone we determined p wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile 27 cm long through a transition zone and its associated strain gradient the ductile shear zone developed within an aplitic leucogranite was sampled from a kilometer wide ductile transcurrent fault in the northern french massif central strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation shear strain systematically increases from zero in the undeformed protolith to 30 in the mylonite the transition zone thickness t is about 30 cm and the mylonite thickness m is about 10 cm the amount of quartz and mica increases relative to feldspar toward the mylonite indicating that a mineralogical composition change accompanied mylonitization mica and quartz developed a strong crystallographic preferred orientation cpo in the least strained domain seismic anisotropy is low and mean vp is 6 km s at 600 mpa anisotropy increases up to 10 and vp decreases up to 5 35 km s for propagation normal to the mylonite foliation through the transition zone this systematic velocity change correlates with the increasing through the transition zone and can be directly related to the cpo of mica and the increase in volume percent mica within the mylonite zone these results indicate that velocity and anisotropy gradients may in some cases be associated with ductile shear zones and that mylonite boundaries may not represent first order discontinuities the reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength and on the t m ratio synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio t m increases we show that layers with second order boundaries velocity gradients in transition zones are only seismically detectable within a narrow thickness range extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is to a first approximation scale independent in granitic domains ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low temperature conditions in the granitic upper crust indeed low temperature strain gradient may explain the high seismic reflectivity of the upper crust in the scandinavian caledonides whereas high temperature strain gradient may explain in part the relative transparency of the european variscan upper crust

upper crustal structure of the larderello geothermal field as a feature of post collisional extensional tectonics southern tuscany italy abstract reflection seismic profiles crossing the larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust the upper 3 4 km of the crust is characterized by low reflectivity its base corresponding to a reflector referred to as the k horizon and recognized in many other areas of southern tuscany beneath this reflector a seismic facies of 0 8 1 0 twt thickness is characterized by strong convergent and divergent reflections forming a lozange shaped geometry the lowermost seismic facies is characterized by almost horizontal reflections based on the distribution of the hypocentral microearthquakes temperature data recorded in deep boreholes and the geometry of normal faults the k horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part according to this hypothesis the k horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior it is inferred that this horizon acts as a decoupling plane in the upper continental crust it developed as a consequence of the extensional tectonics and the high heat flow which characterize southern tuscany

high resolution reflection seismic imaging of the ullared deformation zone southern sweden the ullared deformation zone udz is one of a few structures worldwide known to contain decompressed eclogite facies rocks of precambrian age given the unique nature of the ullared eclogites a 1

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

the effects of mylonitization and fractures on elastic wave velocities in crystalline rock examples from the cajon pass scientific drillhole sonic compressional and shear wave velocities were calculated from full waveforms recorded in the cajon pass drillhole from 250 to 1829 m depth a complete suite of schlumberger logs was also recorded over the same interval compressional velocities in the sediments 250 495 m range from 3 3 to 4 0 km s shear velocities are unreliable shallow crystalline basement has low velocities vp 4 8 vs 2 8 km s and resistivities due to minor alteration below 708 m vp is 5 5 to 6 0 km s and vs is 3 2 to 3 5 km s velocities are generally lower in more felsic rocks in gneisses sonic waveforms are more distorted semblance coherence is lower and physical properties are more variable mylonitized zones and fractured intervals have distinctly lower velocities but different aggregate log response lower velocities are found within a zone of fractures within the gneiss at about 840 m porosity increases only at one large fracture within the interval in a mylonitized shear zone at 1440 m the velocity reduction and the porosity increase occur across the zone

a quantitative approach to bedrock velocity resolution and precision the lithoprobe vancouver island experiment bedrock velocities obtained from the inversion of first break picks of multichannel seismic reflection data have been used to aid the interpretation of shallow reflectors the trade off between resolution and stability of inversion results has been examined models have been optimized by minimizing the variability of static corrections and a monte carlo method has been used to estimate their precision a velocity of 4 6 0 4 km s for the leech river complex lrc compares with velocities of 5 9 0 6 and 5 8 0 3 km s for the neighboring geological units velocity contrasts associated with the change in lithologies may therefore be a contributing factor for generating reflections from the faults that bound the lrc velocities of 5 5 0 4 and 4 6 0 6 km s on either side of the san juan fault suggest that an event identified on several shot gathers may be a reflection from a steeply dipping fault

reflection characteristics of a mylonite zone based on compressional wave velocities of rock samples summary a combined field and laboratory study of the seismic reflection response and seismic properties of the mylonite zone capping the kettle dome metamorphic core complex provides important constraints on reflectivity of mylonites because of its structural and petrologic characteristics this mylonite zone can be regarded as a model for mid to deep crustal mylonite zones cdp reflection profiling revealed that the mylonite zone is reflective laboratory measurements of compressional wave velocities v at high confining pressures in samples collected directly up dip from the cdp reflection line revealed a wide variation of v in common mid crustal lithologies mafic and carbonate rock mean v values range from 6 67 to 7 31 km s at 600 mpa and quartzite and quartzo feldspathic values range from 6 16 to 6 52 km s seismic anisotropy at low pressures can be related to open microcracks and preferred mineral orientation whereas the latter effect controls anisotropy at high pressures the laboratory data provide the basis for a seismic model which indicates that the kettle dome mylonite zone can generate reflections from zones where high and low velocity lithologies are interlayered these reflections correlate with those observed on the cdp data thus demonstrating that the reflectivity of the mylonite zone is a direct result of its lithologic diversity and layered nature in this case it is not necessary to explain the reflections by the effects of anisotropy retrogression high pore pressure or constructive interference

vertical seismic profile results from the kola superdeep borehole russia multi offset vertical seismic profiles vsps from the kola superdeep borehole sg 3 as part of a larger seismic study of the kola region conducted during the spring of 1992 sample the dipping pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and precambrian environments from the surface to 6000 m the sg 3 borehole penetrates interlayered proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade respectively the kola vsps display a 6 velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the sg 3 borehole seismic interfaces are identified by mode converted energy ps and sp transmissions and reflections in addition to primary seismic phases the vsp shear wave energy is generated at or near the source by vertical vibrators p wave and s wave reflections are generally detected from the same reflecting horizons but increases in relative s wave and sp reflection amplitudes originate at 1900 m 3800 m 4500 m and 5100 m depths these depths coincide with zones of elevated vo vs and may support the presence of free pore fluid which is reported from initial drilling for the proterozoic lithologies sampled by the vsp reflection events result from five mylonitic shear zones and three lithologic contrasts

seismic exploration of continental strike slip zones abstract seismic exploration of both palaeozoic and active strike slip zones shows strike slip faults that penetrate all or most of the crust offsets on the moho are evident particularly at young and active zones with a component of compression such as the alpine fault of new zealand where a change in crustal thickness of about 20 km is observed moho offsets for the old palaeozoic strike slip zones are usually much less prominent careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets the moho structure consists of a localized keel type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide the large moho offsets of young strike slip zones may in some cases partially decay with time active strike slip zones are becoming an important focus of study but seismic exploration is hampered by complex near surface geology 3d structure and the difficulty of imaging steeply dipping structure in the subsurface in order to meet this challenge a wide range of seismic techniques is now being deployed these include wide angle seismic reflection refraction p wave delays and the study of guided s waves results from california give geophysical images of vertical strike slip faults that penetrate to the moho in contrast the alpine fault of new zealand appears to be a surface manifestation of an inclined 40 ramp extending down to the lower crust and along which uplift and exhumation of the continental crust and possibly strike slip motion is taking place

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

seismic reflectivity of a finely layered granulite facies ductile shear zone in the southern grenville province quebec abstract lithoprobe reflection profile 54 across the granulite facies morin shear zone msz in the grenville tectonic province quebec canada reveals a west dipping zone of strong reflections the origin of these reflections has been investigated by detailed field mapping laboratory measurements of p wave velocities and densities of representative rock samples from the main lithological units and forward synthetic modeling petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin layer clusters likely resulted from intensive tectonic transposition and metamorphic differentiation lattice preferred orientation lpo induced anisotropy of granulite facies mylonites except for those containing abundant sillimanite biotite or amphibole is very low and hence unlikely to be the primary cause for reflections confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 mpa since composite reflections of thin layer clusters can cause high reflectivity the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin layer mafic clusters within dominantly felsic rocks

reactivation of intrabasement structures during rifting a case study from offshore southern norway abstract pre existing structures within crystalline basement may exert a significant influence over the evolution of rifts however the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood using borehole constrained 2d and 3d seismic reflection data from offshore southern norway we identify and constrain the three dimensional geometry of a series of enigmatic intrabasement reflections through 1d waveform modelling and 3d mapping of these reflection packages we correlate them to the onshore caledonian thrust belt and devonian shear zones based on the seismic stratigraphic architecture of the post basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events reactivation preferentially occurs along relatively thick c 1 km relatively steeply dipping c 30 structures with three main styles of interactions observed between them and overlying faults i faults exploiting intrabasement weaknesses represented by intra shear zone mylonites ii faults that initiate within the hangingwall of the shear zones inheriting their orientation and merging with said structure at depth or iii faults that initiate independently from and cross cut intrabasement structures we demonstrate that large scale discrete shear zones act as a long lived structural template for fault initiation during multiple phases of rifting

evidence for shear zones in the lower crust offshore britain lower crustal reflections seen on british institutions reflection profiling syndicate birps deep seismic reflection data recorded offshore the united kingdom are commonly short convex up and crosscutting consequently the lower crust should not be described as layered as much of the reflecting structure is not fully resolved and may be quite complex moreover the lower crustal reflective zone lcrz is not a uniformly reflective layer but contains reflective bands that wrap around nonreflective transparent zones because transparent zones probably correspond to a lack of a reflecting fabric understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust although transparent zones may have a variety of explanations e g large shear pods or frozen magma bodies the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip line field theory and by drawing analogies with structures developed in highly tectonised high grade metamorphic terrains this interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms in particular extension may be accommodated by the relative movement of low strain lozenges along intervening zones of simple shear the lower crust may be stretched during extension but it must also transfer deformation between the upper crust and the mantle the observation on birps data of mantle reflections interpreted as mantle shear zones implies that deformation in the mantle is localised however the manue reflections are never colinear with faults in the upper crust requiring at least locally a component of sub horizontal bulk strain within the lower crust which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the lcrz

reflectivity of the crystalline crust hypotheses and tests summary the nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms but most geological information still comes from speculative interpretations of seismic experiments run in different regimes pronounced lower crustal reflectivity detected worldwide is ascribed in various geological hypotheses to primary lithologic layering to ductile strain banding or to trapped fluids a birps deep crustal profile across the atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone study of worldwide crustal profiles shows that exposed high pressure terranes are not as reflective as in situ lower crust at high pressure suggesting either that these granulite terranes are not representative of the lower crust or that physical properties possibly the presence of fluids or thermally controlled ductile strain banding are more likely responsible for observed reflectivity than are simple lithologic boundaries the argument for the importance of physical properties in causing observed lower crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat flow 1 geophysical constraints the notion that geometric effects thin layer interference and focussing in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature because many reflections are observed on deep crustal profiles velocity and density cannot increase monotonically with depth while remaining within bounds set by refraction experiments implying the existence of a funtion of velocity with depth that is oscillatory while the concept thus developed of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity for normal incidence no structure embedded in a constant impedance medium gives a stronger reflection than a quarter wavelength layer which produces an amplitude twice that due to a simple interface complex structures can increase the total energy reflected but the energy is spread over longer times and the peak reflected amplitude is not increased observed reflection amplitudes from the lower crust vary by factors u2 so that the presence or absence of thin layer interference cannot be the only effect causing lower crustal

reflections from a mylonitized zone in central sweden dipping reflections imaged on a deep seismic reflection profile acquired within the baltic shield can be indirectly linked to strain induced acoustic impedance contrasts within a regional brittle ductile shear zone the singo shear zone measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis velocity measurements include some made on cored field samples at confining pressures of 0 4 gpa in a laboratory 10 mhz sensor frequencies and some made in situ using ultrasonic 54 khz sensors on exposures within the shear zone laboratory measurements show a 10 change in seismic anisotropy and a contrast of 0 5 km s 1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite the in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes including zones where strain and associated volume loss of silica remain elevated without mylonitization synthetic seismograms generated using a one dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6 25 hz frequencies ray trace two dimensional forward modeling of a high density wide angle reflection section demonstrates that deep dipping reflections could result from strain induced acoustic impedance contrasts within the singo or similar nearby shear zones

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

an andean type retro arc foreland system beneath northwest south china revealed by sinoprobe profiling abstract in the mesozoic south china was situated along the convergent margin between the asian and pacific plates providing an excellent laboratory to understand the interactions between deformation sedimentation and magmatism in a retroarc environment the crustal architecture of northwest south china is displayed along the 600 km long sinoprobe deep seismic reflection profiles and reveals from east to west 1 highly folded and truncated reflectors in the upper crust of the yangtze fold zone which correspond to thin and thick skinned thrust systems and document large scale intraplate structural imbrication and shortening 2 a crustal scale flat ramp flat structure termed the main yangtze decollement which forms a weak viscous layer to accommodate strain decoupling and material transport in the thin and thick skinned systems and 3 nearly flat lying reflectors in the sichuan basin which support interpretation of the basin as a weakly deformed depocentre the yangtze fold zone and the sichuan basin represent a retro arc foreland basin system that is 800 km away from the continental margin magmatic arc we suggest that tectonic processes across the arc and retro arc systems including arc magma flare up basin sedimentation retroarc thrust propagation lithosphere underthrusting root foundering and extension related magmatism were interrelated and governed mass transfer age data and geological relations link the tectonic processes to evolving geodynamics of the subducting paleo pacific plate

an axial view of a metamorphic core complex crustal structure of the whipple and chemehuevi mountains southeastern california a 135 km long nw se trending seismic refraction wide angle reflection profile provides a unique along strike view of the crustal structure of a belt of metamorphic core complexes in southeastern california the whipple chemehuevi and sacramento mountains metamorphic core complexes interpretation of the seismic data was done by two dimensional forward modeling of travel times and amplitudes the final model consists of 1 a thin 1 5 km veneer of upper plate and fractured lower plate rocks velocities of 1 5 5 3 km s 1 overlying a fairly homogeneous basement with velocities of 6 0 km s 1 2 a localized high velocity 6 4 km s 1 body situated directly beneath the whipple mountains 3 a 6 3 6 4 km s 1 middle crust that is thickest beneath the core complexes 4 a 6 65 0 15 km s 1 lower crust 5 crustal thickness of 27 km with a deeper crustal root 3 km beneath the whipple mountains metamorphic core complex and 6 a pn velocity of 8 0 0 10 km s 1 the crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust upper crustal velocities are higher beneath the whipple mountains where velocities increase to 6 4 km s 1 at 5 km depth than beneath the chemehuevi and sacramento mountains in addition midcrustal discontinuities rise 2 5 km beneath the whipple complex compared to the other complexes these observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the whipple mountains compared to rocks in the chemehuevi and sacramento mountains despite the enhanced uplift and extension in the whipple mountains the crust is thicker here 30 km than anywhere else along the colorado river extensional corridor this may be in part a relic of compressional and magmatic thickening during the mesozoic however we suggest that inflation of the crust during tertiary extension was the dominant mechanism both mantle derived magmatism and lateral ductile inflow in the crust are proposed

the hardangerfjord shear zone in sw norway and the north sea a large scale low angle shear zone in the caledonian crust the hardangerfjord shear zone is a more than 600 km long low angle extensional structure that affects the south norway and north sea caledonides the ductile shear zone which shows total maximum onshore displacement of the order of 10 15 km is primarily a basement structure with an associated passive monoclinal fold structure of the overlying caledonian nappes deep seismic data indicate that the shear zone continues down to the lower crust 20 25 km at a dip of 22 23 where it appears to flatten and merge with the general lower crustal deformation fabric onshore the hardangerfjord shear zone consists of a system of hard linked ductile shear zone segments brittle faults the l rdal gjende fault system occur in the folded caledonian allochthons in the ne part of the hardangerfjord shear zone and reappear in the north sea these may represent a high level brittle response to the devonian development of the hardangerfjord shear zone but were reactivated during permo triassic and late jurassic extensional events a c 5 km thick package of seismic reflectors along the hardangerfjord shear zone is presumed to represent a mylonite zone which is too thick to be formed entirely by 10 15 km of devonian displacement hence the hardangerfjord shear zone is likely to be a proterozoic shear zone reactivated during devonian extension

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

a multidisciplinary geophysical study in the betic chain southern iberia peninsula abstract the combined analysis of magnetotelluric measurements tomographic velocity models and deep seismic reflection images confirms that the betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties at depth these data sets show evidence for a non coincidence of the petrological and the seismic moho beneath the betics chain the data sets reveal the geophysical properties of the alboran domain internal betics and the iberian massif external betics according to this the iberian crust features a relatively high seismic velocity is seismically transparent in the seismic reflection images and is electrically resistive the alboran domain crust is characterized by a low average velocity displays high reflectivity in the seismic reflection images and is electrically conductive the outcrops of the metamorphic complexes alpujarride and nevado filabride showing relatively high velocities coupled with low v p v s values 1 67 derived from the wadati slopes suggest the existence of rocks rich in silica beneath the alboran domain crust an interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the iberian upper crust partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels these would account also for the reflectivity and the low v p v s ratios mapped beneath the alboran domain

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

a new approach for reconstructing glacier variability based on lake sediments recording input from more than one glacier abstract we explore the possibility of building a continuous glacier reconstruction by analyzing the integrated sedimentary response of a large 440 km2 glacierized catchment in western norway as recorded in the downstream lake nerfloen n61 56 e6 52 a multi proxy numerical analysis demonstrates that it is possible to distinguish a glacier component in the 8000 yr long record based on distinct changes in grain size geochemistry and magnetic composition principal component analysis pca reveals a strong common signal in the 15 investigated sedimentary parameters with the first principal component explaining 77 of the total variability this signal is interpreted to reflect glacier activity in the upstream catchment an interpretation that is independently tested through a mineral magnetic provenance analysis of catchment samples minimum glacier input is indicated between 6700 5700 cal yr bp probably reflecting a situation when most glaciers in the catchment had melted away whereas the highest glacier activity is observed around 600 and 200 cal yr bp during the local neoglacial interval 4200 cal yr bp until present five individual periods of significantly reduced glacier extent are identified at 3400 3000 2700 2100 2000 1700 1500 and 900 cal yr bp

fracture related reflectors in basement rock from vertical seismic profiling at cajon pass multi azimuth vsp data acquired between 500 and 2000 meters deep in crystalline rock penetrated by the cajon pass deep scientific borehole show several reflectors intersecting the hole the reflectors correlate with vsp amplitude data drilling and cutting logs and various geophysical logs most appear to be related to fracture zones or lithologic transitions accompaanied by fracturing mineral alteration and fluid circulation amplitude systematics from two azimuthally different offsets are consistent with the presence of a high angle fault in the neighborhood of the well

lithospheric growth at margins of cratons abstract deep seismic reflection profiles collected across proterozoic archean margins are now sufficiently numerous to formulate a consistent hypothesis of how continental nuclei grow laterally to form cratonic shields this picture is made possible both because the length of these regional profiles spans all the tectonic elements of an orogen on a particular cratonic margin and because of their great depth range key transects studied include the lithoprobe snorcle 1 transect and the babel survey crossing the slave and baltic craton margins respectively in most cases the older archean block appears to form a wedge of uppermost mantle rock embedded into the more juvenile proterozoic block by as much as 100 200 km at uppermost mantle depths and archean lithosphere is therefore more laterally extensive at depth than at the surface particularly bright reflections along the moho are cited as evidence of shear strain within a weak low viscosity lower crustal channel that lies along the irregular top of the indenting wedge the bottom of the wedge is an underthrust subduction zone and associated late reversal in subduction polarity beneath the craton margin emerges as a common characteristic of these margins although related arc magmatism may be minor

a link between deformation history and the orientation of reflective structures in the 2 68 2 83 ga opatica belt of the canadian superior province true three dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked this is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth dip and depth of a reflector and the medium velocity results from lithoprobe line 48 located in the opatica belt of the archean superior province differentiate reflectors with two distinct orientations which coincide with the attitudes of two deformational fabrics mapped at surface assuming a connection between reflectivity and strain induced by tectonic processes the reflectors with nne strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the opatica belt other reflectors which strike ene wsw and dip shallowly to the north and to the south indicate that most of the reflectors in the southern opatica and beneath the abitibi greenstone belt at middle and lower crustal levels formed during a later approximately n s shortening event mantle reflections previously interpreted as a relict suture of an archean subduction zone dip to the north at around 30 45 and are also associated with this n s event the distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the opatica crust was reworked during the n s shortening event although a region of the middle and lower crust characterized by the earlier d1 reflectors is preserved in the central part of the belt

seismic image of the deep crust at the eastern margin of the alps austria indications for crustal extension in a convergent orogen abstract a 39 km long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the eastern alps austria the ese wnw trending low fold seismic line crosses austroalpine basement units and extends approximately from 20 km west of the penninic window group of rechnitz to 60 km sse of the alpine thrust front the explosive source seismic data reveals a transparent shallow crust down to 5 km depth a complexly reflective upper crust and a highly reflective lowermost crust the upper crust is dominated by three prominent west dipping packages of high amplitude subparallel reflections the upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low angle normal shear zones related to the miocene exhumation of the rechnitz metamorphic core complex in the western portion of the upper crust east dipping and less significant reflections prevail the lowermost package of these reflections is suggested to represent the overall top of the european crystalline basement along the western portion of the line the lower crust is characterised by a 6 8 km thick band of high amplitude reflection lamellae typically observed in extensional provinces the moho can be clearly defined at the base of this band at approximately 32 5 km depth due to insufficient signal penetration outstanding reflections are missing in the central and eastern portion of the lower crust we speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower moho in the east preferable at about 30 5 km depth the high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust the latter interpreted to represent broad extensional mylonite zones emphasises the latest extensional processes in accordance with eastward extrusion

shallow seismic reflection profiling over a mylonitic shear zone ruby mountains east humboldt range metamorphic core complex ne nevada seismic reflection profiling carried out with a sledgehammer source has imaged tertiary extensional structures over a depth range of 45 500 m within lower plate rocks of the ruby mountains east humboldt range metamorphic core complex the 400 m cmp profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low angle ductile brittle normal faulting subhorizontal reflections from layering within the tectonic slices give way at 160 ms 160 220 m depth to reflections that dip 15 45 to the east in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up plunge exposures of a late proterozoic early cambrian sequence of metaquartzites marbles schists and granitic rocks that forms the bulk of the underlying shear zone differences with the regional profile are attributed to the higher frequencies 30 100 hz generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot receiver offsets 46 157 m contributing to the stack for each cmp the results suggest that the near surface east dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted rough estimates of interval velocities 1500 4500 m s inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account peaks in amplitude spectra of stacked traces suggest long wavelength components of layering resolved at scales from 5 8 m depth 50 m to 15 25 m depth 500 m

precambrian basement structure and laramide deformation revealed by seismic reflection profiling in the laramie mountains wyoming the university of wyoming conducted seismic reflection profiling in the laramie mountains of wyoming to investigate the nature of precambrian crustal structure and laramide deformation in the region here the imaging of layering in the laramie anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events moreover the presence of prominent primary layering in the complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush imaging of laramide thrusts in the laramie mountains adds credence to the idea that brittle faults can be reflective in some situations migrated apparent dips on these faults are 30 westward basement involved compressive structures consisting of monoclinal folding and a fault triple junction demonstrate that compression controlled the style of deformation during the laramide orogen in wyoming west dipping precambrian structures imaged in these data have a similar attitude to the laramide structures suggesting that laramide deformation was influenced by precambrian features these precambrian structures may be subthrust slices of exotic proterozoic terranes

a collisional crustal fabric pattern recognised from seismic reflection profiles of the appalachian caledonide orogen abstract deep seismic reflection profiles across the appalachians and across the once contiguous caledonides around britain and ireland show a broadly similar pattern of crustal reflection fabrics the pattern is of two adjacent fabric domains each characterized by dips towards the other bounded by an interface which dips in conformity with one of the fabrics the domain interface strikes parallel to the orogen suggesting a genetic relationship between the fabrics and the orogen from the north sea to the long island platform the domain boundary dips to the northwest in the southern appalachians the pattern appears with less consistency between the charlotte and carolina slate belts the pattern appears with the same polarity as further north but below the coastal plain it occurs with opposite polarity the interface dips to the southeast an attractive but not exclusive hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis

seismic reflectivity of the whipple mountain shear zone in southern california the whipple mountains shear zone of southeastern california comprises part of the whipple mountains metamorphic core complex of the north american cordillera the 3 9 km thick shear zone displays excellent exposures of ductilely deformed mid tertiary mylonitic gneisses as well as kinematically related but younger cataclasites which underlie a major low angle normal fault the whipple detachment fault prominent crustal reflections beginning at 3 4 s were recorded on calcrust seismic profiles southwest of the whipple mountains in order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others we collected oriented rock specimens from all major structural units in the whipple mountains and determined p wave velocities for these samples parallel to three principal fabric directions under laboratory induced confining pressures up to 500 mpa using a geologic section through the mylonitic gneisses from a previous investigation we then constructed a detailed acoustic impedance section for the whipple mountain shear zone based upon laboratory results this section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the calcrust records significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses the finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the seismic fabric of strong laterally discontinuous but subparallel reflections as seen in the calcrust profiles southwest of the whipple mountains chloritic breccias beneath the whipple detachment fault wdf have much lower seismic impedance than that of rocks occurring both structurally above and below however the significant thinning of the chloritic breccia zone to the southwest of the whipple mountains may have caused the poor image of the wdf on the calcrust profiles the absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone

numerical models of ductile rebound of crustal roots beneath mountain belts crustal roots formed beneath mountain belts are gravitationally unstable structures which rebound when the lateral forces that created them cease or decrease significantly relative to gravity crustal roots do not rebound as a rigid body but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust 2 d numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound numerical models of root rebound show three main features which may be of general application first with a low viscosity lower crust the rheology of the mantle lithosphere governs the rate of root rebound second the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere and third redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images

deep crustal structure and genesis from contrasting reflection patterns an integrated approach summary an integrated approach including technique development into data acquisition processing and modeling is used to aid the final interpretation of crustal reflection data by minimizing the many unknowns thus a project may start out with a series of experiments involving data acquisition designed to attack a particular problem and conclude with modeling as a test to the interpretation of a problem

bright seismic reflections and fluid movement by porous flow in the lower crust abstract the origin of high amplitude lower crustal bright seismic reflections in a phanerozoic area is investigated by modeling a distribution of metamorphic fluid flow through pores as a possible explanation a numerical model of metamorphic fluid flow in a viscous porous medium is developed a model is applied to a tectonically active area such as the basin and range where viscous behavior of the lower crust is suggested calculations show that the characteristic vertical dimension of relative volumetric flow variations corresponds to the thickness of seismic lamellae as determined from reflection frequency spectra for the basin and range this result shows that a dynamic effect which was produced by fluid pressure and viscous matrix resistance could be responsible for the origin of the seismic lamellae in the lower crust in an area of active tectonics

seismic reflection images of high angle faults and linked detachments in the trans hudson orogen postcollisional 1 8 1 7 ga intracontinental deformation in the trans hudson orogen canada produced a series of orogen parallel high angle faults and folds in seismic reflection profiles the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4 8 s 12 24 km the folded and faulted upper part of the crust is underlain by laterally coherent shallow dipping reflections that are locally bounded by discrete highly reflective zones these zones are interpreted as detachments shear zones and can be traced from the upper to lower crust where some of them appear to pass into laterally continuous reflections that define the moho two distinct regimes of postcollisional crustal deformation are inferred from the seismic images high angle faulting and lateral block extrusion in the upper crust and low angle ductile shearing in the mid lower crust the surface geology indicates that the faults resulted in southwest orogen parallel extrusion of the orogen s internal zone relative to the bounding archean hearne and superior cratons faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction the seismic images suggest that the high angle fold fault structures are kinematically linked to low angle detachments represented by laterally coherent highly reflective zones the detachment shear zones are inferred to have a top to the southwest sense of shear associated with a subhorizontal northeast southwest extension direction parallel to those observed for 1 83 1 80 ga collisional shear zones exposed in major postcollisional fold culminations long lived orogen parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the superior and hearne margins

integrating seismic reflection and geological data and interpretations across an internal basement massif the southern appalachian pine mountain window usa the southern appalachian pine mountain window exposes 1 1 ga grenvillian basement and its metasedimentary paleozoic cover through the allochthonous inner piedmont the issue of whether the crustal block inside the window was either transported above the master appalachian late alleghanian decollement or is an autochthonous block that was overridden by the decollement has been debated for some time new detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from gondwana but docked with laurentia before the alleghanian event reprocessed deep seismic reflection data from west central georgia pr and poststack noise reduction amplitude variation analysis and prestack depth migration indicate that a significant band of subhorizontal reflections occurs almost continuously beneath the window collinear with the originally recognized decollement reflections north of the window a marked variation in the decollement image from strong and coherent north of the window to more diffuse directly beneath the window is likely a partial consequence of the different geology between the inner piedmont and the window the more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover i e signal to noise ratio two alternative tectonic models for the pine mountain window can partially account for the observed variation in the decollement reflectivity 1 the pine mountain block could be truncated below by a relatively smooth continuation of the decollement the window would thus expose an allochthonous basement duplex or hors block thrust upward from the south along the late proterozoic rifted continental margin 2 the window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window either model is viable if only reflector geometry is considered model 1 is favored if both geometry and kinematics of blue ridge piedmont thrust sheet emplacement are incorporated in either model the southern margin of the window merges to the west with the iapetan early alleghanian central piedmont suture which juxtaposes north american affinity piedmont rocks to the north and exotic panafrican rocks of the carolina avalon terrane to the south immediately south of the window this suture dips southward and merges in the lower crust with the late alleghanian suture joining the appalachians with gondwana

nature of the lower crust and moho in eastern nevada from wide angle reflection measurements wide angle recordings at offsets between 20 and 40 km in the ruby mountains consistently show 5 strong reflections between 4 and 11 s with enough moveout to estimate velocities to the base of the crust the uppermost layer with a temperature corrected velocity of 6 2 km s and thickness of 9 km corresponds to quartzofeldspathic rocks such as metasedimentary rocks migmatites and deformed granites and is underlain by a 7 km thick layer with velocities of 6 4 km s which corresponds to quartzofeldspathic material interlayered with amphibolites the lower crust consists of a heterogeneous 9 km thick zone with velocities of 6 7 to 6 8 km s corresponding to mafic rocks interlayered with smaller amounts of quartzofeldspathic rock the lowermost crust is marked by the x reflection overlying a 3 km thick layer with velocities between 7 4 7 8 km s corresponding to anomolous material possibly layered cumulates the subhorizontal layered structure of the crust is caused primarily by ductile extension which may well be superposed on material added to the crust by underplating at most approximately one third of the present crust could have been added by underplating in the cenozoic

lower crustal deformation during terrane dispersion along strike slip faults abstract after accretion terranes caught between obliquely converging plates are often displaced several hundreds of kilometers how this motion is accommodated at depth is not well known spurred by our trans alaskan crustal transect tact seismic studies of the yukon tanana terrane alaska i propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin parallel translation the locations of these ductile regions are dependent on temperature composition and strain rate for geotherms based on moderate surface heat flow 60 90 mwm 2 and reasonable geological strain rates 100 in 3 ma typical lower crustal rocks can deform ductilely the resultant subhorizontal layered fabric should be detectable with seismic techniques two terranes that have experienced hundreds of kilometers of margin parallel translation the yukon tanana terrane sampled by our seismic refraction wide angle reflection profiles and the salinian block california sampled by both seismic refraction and reflection have reflective lower crust both of these terranes have moderate to high surface heat flows have relatively thin crust and are interpreted seismically to be underlain by a quartzo feldspathic lower crust conditions favorable for ductile deformation in the lower crust interpretations of lower crustal layering for these two terranes is based on an increase in lower crustal reflectivity this layering is attributed to tectonic banding consistent with ductile deformation

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites