structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

imaging a crustal low velocity layer using reflected seismic waves from the 2014 earthquake swarm at long valley caldera california the magmatic system roof

joint inversion of seismic and magnetotelluric data in the parkfield region of california using the normalized cross gradient constraint we present jointly inverted models of p wave velocity vp and electrical resistivity for a two dimensional profile centered on the san andreas fault observatory at depth safod significant structural similarity between main features of the separately inverted vp and resistivity models is exploited by carrying out a joint inversion of the two datasets using the normalized cross gradient constraint this constraint favors structurally similar vp and resistivity images that adequately fit the seismic and magnetotelluric mt datasets the new inversion code tomoddmt merges the seismic inversion code tomodd and the forward modeling and sensitivity kernel subroutines of the mt inversion code occam2dmt tomoddmt is tested on a synthetic dataset and demonstrates the code s ability to more accurately resolve features of the input synthetic structure relative to the separately inverted resistivity and velocity models using tomoddmt we are able to resolve a number of key issues raised during drilling at safod we are able to infer the distribution of several geologic units including the salinian granitoids the great valley sequence and the franciscan formation the distribution and transport of fluids at both shallow and great depths is also examined low values of velocity resistivity attributed to a feature known as the eastern conductor ec can be explained in two ways the ec is a brine filled high porosity region or this region is composed largely of clay rich shales of the franciscan the eastern wall which lies immediately adjacent to the ec is unlikely to be a fluid pathway into the san andreas fault s seismogenic zone due to its observed higher resistivity and velocity values

nature of the continent ocean transition on the non volcanic rifted margin of the central great australian bight a region of 50 120km width defines the continent ocean transition cot in the central great australian bight it is characterized by a thin apron of post break up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the cot and basement ridge modelled gravity and magnetic data and features interpreted from seismic data are consistent with aspects of extensional and break up models proposed for the west iberia margin many of the features and relationships observed beneath the outer margin of the central great australian bight can be explained by extension within a lithosphere scale pure shear environment involving four layers brittle upper crust and upper mantle and ductile lower crust and lower lithospheric mantle the cot is interpreted to be underlain by extended continental lithosphere thus the continentocean boundary is unequivocally defined between oceanic crust and the cot and appears to be associated with sea floor spreading magnetic anomaly 33 indicating that break up and sea floor spreading did not commence until c 83 ma early carnpanian time later than the currently accepted 95ma age the major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting the magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly but one that does not relate to break up and sea floor spreading

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

tomographic vp and vs structure of the california central coast ranges in the vicinity of safod from controlled source seismic data summary a seismic reflection refraction survey across the san andreas fault near parkfield california has refined our knowledge of the upper crustal structure of the central california coast ranges at the san andreas fault observatory at depth safod the survey consisted of a 46 km long line of seismographs 25 50 m spacing and 63 explosions 25 200 kg nominal spacing of 500 m with some gaps the traveltimes of refracted p and s waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce p and s wave velocity models vp vs along the profile extending to as much as 5 km depth the vp and vs models show a prominent lateral drop in velocities a few hundred metres northeast of safod between the drill hole and the san andreas fault the vp model shows particularly well a southwest dipping velocity inversion beneath safod the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below in addition to vp and vs models a vp vs model was derived a vp vs ratio lower than 1 73 is seen only at depth in a narrow zone beginning at the target earthquakes for safod and extending downward and northeastward into the north america plate clusters in the parameter space spanned by vp vs ratios and vp can be identified by two different methods one more intuitive analytical method and one more abstract method based on neural network techniques these clusters are correlated to different rock types based on laboratory and in situ data these clusters are remapped back into x z plane along the profile prominent features mapped this way include salinian granitic rocks beneath and west of safod and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the buzzard canyon fault bcf system these sedimentary rocks extend from this fault to the san andreas fault system unfortunately our cluster analysis shows no significant discontinuity at the san andreas fault owing presumably to the fact that the san andreas fault is located within sedimentary rocks having similar elastic properties this paper is an attempt to downward continue a geological map by geophysical means based on elastic properties of rock samples from the region

crustal thinning and exhumation along a fossil magma poor distal margin preserved in corsica a hot rift to drift transition abstract rift related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults while extensional detachments responsible for the latest stages of exhumation are being increasingly recognized rift related shear zones have never been sampled in odp sites and have only rarely been identified in fossil distal margins preserved in orogenic belts here we report evidence of the jurassic multi stage crustal thinning preserved in the santa lucia nappe alpine corsica where amphibolite facies shearing persisted into the rift to drift transition in this nappe lower permian meta gabbros to meta gabbro norites of the mafic complex are separated from lower permian granitoids of the diorite granite complex by a 100 250 m wide shear zone fine grained syn kinematic andesine mg hornblende assemblages in meta tonalites of the diorite granite complex indicate shearing at t 710 40 c at p 40 ar 39 ar step heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the triassic jurassic boundary and persisted until t final exhumation to the basin floor was accommodated by low angle detachment faulting responsible for the 1 10 m thick damage zone locally capping the mafic complex the top basement surface is onlapped at a low angle by undeformed mesozoic sandstone locally containing clasts of footwall rocks existing constraints from the neighboring corsica ophiolites suggest an age of ca 165 160 ma for these final stages of exhumation of the santa lucia basement these results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting when significant crustal thinning is accommodated in less than 5 myr through the consecutive activity of extensional shear zones and detachment faults high thermal gradients may delay the switch from ductile shear zone to detachment dominated crustal thinning thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod 1 arrival time data from 453 local earthquakes and 6 explosions at parkfield ca are inverted for earthquake locations and three dimensional vp and vp vs structure the structure is dominated by the velocity contrast across the saf with the southwest side about 20 25 faster consistent with previous studies nearly all the earthquakes occur almost directly beneath the fault trace we find high vp vs anomalies that correlate with low resistivity features in a magnetotelluric model that are interpreted to represent fluids we locate a magnitude 2 earthquake that is a potential target event for the final stage of safod drilling yielding a depth of 3 1 km below surface and an epicenter 100 m southwest of the fault trace nonlinear analyses indicate 95 confidence relative and absolute uncertainties on the order of 500 700 m vertically and 200 300 m horizontally for this target earthquake

seismic imaging of the waltham canyon fault california comparison of ray theoretical and fresnel volume prestack depth migration abstract near vertical faults can be imaged using reflected refractions identified in controlled source seismic data often theses phases are observed on a few neighboring shot or receiver gathers resulting in a low fold data set imaging can be carried out with kirchhoff prestack depth migration in which migration noise is suppressed by constructive stacking of large amounts of multifold data fresnel volume migration can be used for low fold data without severe migration noise as the smearing along isochrones is limited to the first fresnel zone around the reflection point we developed a modified fresnel volume migration technique to enhance imaging of steep faults and to suppress noise and undesired coherent phases the modifications include target oriented filters to separate reflected refractions from steep dipping faults and reflections with hyperbolic moveout undesired phases like multiple reflections mode conversions direct p and s waves and surface waves are suppressed by these filters as an alternative approach we developed a new prestack line drawing migration method which can be considered as a proxy to an infinite frequency approximation of the fresnel volume migration the line drawing migration is not considering waveform information but requires significantly shorter computational time target oriented filters were extended by dip filters in the line drawing migration method the migration methods were tested with synthetic data and applied to real data from the waltham canyon fault california the two techniques are applied best in combination to design filters and to generate complementary images of steep faults

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

joint inversion of body wave arrival times and surface wave dispersion for three dimensional seismic structure around safod we incorporate body wave arrival time and surface wave dispersion data into a joint inversion for three dimensional p wave and s wave velocity structure of the crust surrounding the site of the san andreas fault observatory at depth the contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations we find that the trade off between fitting the two data types controlled by the weighting defines a clear optimal solution varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type all the acceptable solutions yield structures with similar primary features but the smaller scale features change substantially when there is a lower relative weight on the surface wave data it appears that the solution over fits the body wave data leading to a relatively rough vs model whereas for the optimal weighting we obtain a relatively smooth model that is able to fit both the body wave and surface wave observations adequately

scandinavia s north atlantic passive margin 1 the tectonics of the eastern passive margin of the north atlantic are reexamined the scandinavian north atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the scandinavian caledonides combined information from structural geology potential field data regional geology basin development and geomorphology made it possible to propose a new definition of the passive margin the rift shoulder is formed by linked faults defining the present rift flank these faults are the extensional structures located furthest to the east onshore norway sweden that can be linked to the rifting that led to the formation of the north atlantic this innermost boundary fault system ibf is formed by a set of normal west dipping crustal faults it extends over a distance exceeding 2000 km from the north sea across the caledonian mountain belt to the barents sea the passive margin width between the continent ocean boundary and the ibf ranges from 550 km in the south to over 700 km in mid norway to 165 km north of lofoten rifting and faulting on the ibf started in permo carboniferous and a succession of rift phases eventually culminated in continental breakup and the formation of the north atlantic basin development between permian and cretaceous was toward the future breakaway fault but faulting was also active on land from mesozoic to present onshore offshore crustal scale cross sections show the geometry of the passive margin the dip orientation of the major faults and the changes in crustal thickness

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

evidence for extensional shear zones in the mantle offshore britain and their implications for the extension of the continental lithosphere deep seismic profiles recorded in the seas around the british isles have revealed bands of seismic reflections in the uppermost mantle on the basis of their appearance geophysical characteristics and association with major crustal faults these features are best interpreted as mantle shear zones furthermore in the north sea the mantle reflectors appear to follow an extensional trend and when interpreted as extensional shear zones help explain the regional stratigraphy of the overlying basin the crosscutting relationships between mantle reflections to the west of scotland and a major strike slip fault the great glen fault ggf suggest that the mantle structure postdates the ggf and hence is probably related to subsequent extension furthermore kinematic analysis of lithospheric deformation to the north of scotland is consistent with the structure observed on the seismic data if the flannan mantle reflection is an extensional shear zone reconstruction of the lithosphere prior to permian extension restores the flannan to the edge of the rockall trough a rift probably initiated in the carboniferous thus the flannan fault may have developed during carboniferous rifting it is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle and the role of such structures during lithospheric extension is considered

seismic imaging using microseismic events results from the san andreas fault system at safod 1 we have developed a new passive seismic imaging approach that consists of two steps first the hypocenter of the microseismic event is precisely located second this event is treated as a pseudo active seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high resolution image of the illuminated subsurface region in this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the san andreas fault observatory at depth main hole results obtained were high resolution 3 d images of different se nw oriented reflectors related to the san andreas fault saf system in the close vicinity of the borehole to support our approach we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology we revealed a predominantly satisfactory agreement for both juxtapositions furthermore the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the saf with much higher resolution than can be obtained from surface seismic reflection data

western mediterranean subcontinental mantle emplacement by continental margin obduction obduction is the tectonic process that results from thrusting of an oceanic lithosphere section ophiolite onto a continent in contrast thrusting of subcontinental mantle observed in several mountain belts remains a major unknown of plate tectonics in the western mediterranean the ronda and beni bousera peridotites are the largest worldwide subcontinental mantle exposure from a geological point of view the ronda peridotites are exhumed by hyperstretching of the continental lithosphere in a back arc immediately followed by thrusting explaining their present day position inside the alboran crust using 2 d and 3 d modeling of new gravimetric data combined with local seismic tomography we show that the ronda peridotites are rooted inside the alboran mantle along the entire gibraltar arc on these bases we propose that the emplacement of the ronda peridotites occurred in a back arc setting and corresponds to the thrusting of an entire hyper stretched continental margin onto a continent a process that we define as continental margin obduction this results from two successive deformation events continental upper plate extension driven by slab roll back immediately followed by upper plate shortening likely triggered when a buoyant continental domain enters the subduction we propose that this process affected the entire western alboran domain with peridotite bodies embedded within the crust along the whole gibraltar arc we suggest that other examples such as the alps ivrea mantle body could likely represent continental margin obduction at the onset of continental collision

a look inside the san andreas fault at parkfield through vertical seismic profiling the san andreas fault observatory at depth pilot hole is located on the southwestern side of the parkfield san andreas fault this observatory includes a vertical seismic profiling vsp array vsp seismograms from nearby microearthquakes contain signals between the p and s waves these signals may be p and s waves scattered by the local geologic structure the collected scattering points form planar surfaces that we interpret as the san andreas fault and four other secondary faults the scattering process includes conversions between p and s waves the strengths of which suggest large contrasts in material properties possibly indicating the presence of cracks or fluids

migration of seismic scatterers associated with the 1993 parkfield aseismic transient event the time varying deformation field within a fault zone particularly at depths where earthquakes occur is important for understanding fault behaviour and its relation to earthquake occurrence1 2 3 but detection of this temporal variation has been extremely difficult although laboratory studies have long suggested that certain structural changes such as the properties of crustal fractures should be seismically detectable4 here we present evidence that such structural changes are indeed observable in particular we find a systematic temporal variation in the seismograms of repeat microearthquakes that occurred on the parkfield segment of the san andreas fault over the decade 1987 97 our analysis reveals a change of the order of 10 m in the location of scatterers which plausibly lie within the fault zone at a depth of 3 km the motion of the scatterers is coincident in space and time with the onset of a well documented aseismic transient deformation event we speculate that this structural change is the result of a stress induced redistribution of fluids in fluid filled fractures caused by the transient event

imaging of structure at and near the core mantle boundary using a generalized radon transform 1 construction of image gathers 1 we introduce a method for imaging heterogeneity at and near interfaces in earth s lowermost mantle with broadband three component seismograms from global seismograph networks our approach is based on inverse scattering and allows the extraction of pertinent signal from large data sets and requires few a priori assumptions about the heterogeneity under study which makes it complementary to the forward modeling of selected waveforms here we construct a generalized radon transform for heterogeneous anisotropic elastic media to map broadband seismogram windows comprising main arrivals with their coda and precursors into multiple images of a target structure the common image point gathers thus produced reveal multiscale variations in elastic properties near deep interfaces the grt can be applied to narrow and wide angle data and the automated extraction of signal from data over a wide range of epicentral distances enables exploration of core mantle boundary cmb regions that cannot with present day data coverage be imaged with the triplicated waveforms used in forward modeling studies tests with synthetic data produced both with idealized and actual source receiver distributions illustrate pertinent aspects of the theory and show that multiple weak interfaces can be detected and located correctly with a radial resolution of 15 km for the frequencies used even in the presence of random noise that would prohibit visual inspection and modeling of the subtle signals we transformed the transverse component scs waveforms into image gathers of a cmb patch beneath central america juxtaposition of stacks of these gathers involving 35 000 seismograms produces a two dimensional 2 d image profile revealing contrasts in elastic properties near the target depth of the cmb and 280 340 km above it the latter may mark the top of the so called d region the images also reveal a richness of structures in between these depths combined with a statistical analysis of these singularities described in paper 2 the approach to imaging presented here paves the way to large scale seismic exploration of the lowermost mantle

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

symmetric alternative to asymmetric rifting models we describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magma poor margins such as the galicia margin west of the iberian peninsula or the apulia margin in the alps 1 distal continental margins consist of fault bounded blocks separated by oceanward dipping normal faults 2 at the tip of the continent lower crust is scarcely preserved or absent and upper crust directly overlies exhumed mantle 3 the base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top to the ocean sense of shear in our model these structures do not reflect asymmetric rift geometry at a lithospheric scale instead they derive from upper crustal collapse over a mid crustal shear zone into the rift center and are present on both sides of the rift axis the model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center we hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well defined rift center

conjugate margins of canada and europe results from deep reflection profiling deep seismic reflection profiles have been collected across the conjugate margins of the north atlantic ocean the eastern north atlantic margin is traversed in the goban spur region and the western north atlantic margin is crossed in the vicinity of flemish cap in the grand banks region these seismic profiles allow us to examine the deep structures and mode of extension in crust that was once contiguous the flemish cap and goban spur margins have different structural styles thick and relatively unfaulted crust is present on the flemish cap margin west of the continent ocean boundary whereas the goban spur margin exhibits a zone of extensional faulting and thinned continental crust the restored rift zone displays overall symmetry in which a bridge of thin crust 15 km thick about 60 km wide joins the two crustal blocks of normal continental thickness 28 km the configuration of the moho the geometry and distribution of the lower crustal reflections and the overall symmetry of the rift zone favor pure shear stretching in the lower lithosphere although either pure or simple shear could accommodate crustal extension

microseismic reflection imaging of the central andean crust the coda of passive seismic recordings is often rich in arrivals that are coherent across several stations if reflections can be extracted then they may be used for seismic reflection subsurface imaging with the objective to image the upper crust of the north chilean precordillera central andes approximate location 21 s 69 w we developed a workflow to process passive seismic data into subsurface reflection images we analysed the waveform recordings of several hundred microseismic events using signal processing and imaging techniques adapted from active controlled source seismic imaging as used in the oil industry key processing steps involved precise arrival time picking and hypocentre determination removing signal amplitude variations due to varying source radiation patterns identification and separation of reflections from coherent noise and transformation of the processed waveforms into images of the subsurface reflectivity when designing our microseismic reflection imaging workflow we took advantage of the fact that the passive seismic recording geometry with the hypocentres located at depth and the receivers positioned at the surface resembles a reverse vertical seismic profiling experiment the resultant p and s wave reflection images reveal several reflective features such as an approximate 15 westward dipping reflector over the 5 25 km depth range that largely coincides with a distinct seismicity boundary we interpret the imaged interface as the brittle ductile transition zone boundary possibly enhanced by a tectonic shear zone for the area of the north chilean precordillera the deduced microseismic reflection sections with horizontal extensions of about 50 km represent the first high resolution images of the shallow crust which could not be obtained from previous active source seismic reflection data

san andreas fault damage at safod viewed with fault guided waves 1 highly damaged rocks within the san andreas fault zone at parkfield form a low velocity waveguide for seismic waves giving rise to fault guided waves prominent fault guided waves have been observed at the san andreas fault observatory at depth safod site including a surface array across the fault zone and a borehole seismograph placed in the safod well at a depth of 2 7 km below ground the resulting observations are modeled here using 3 d finite difference methods to fit the amplitude frequency and travel time characteristics of the data the models require a downward tapering 30 40 m wide fault core embedded in a 100 200 m wide jacket compared with the intact wall rocks the core velocities are reduced by 40 and jacket velocities by 25 based on the depths of earthquakes generating guided waves we estimate that the low velocity waveguide along the fault at safod extends at least to depths of 7 km more than twice the depth reported in pervious studies

imaging the lowermost mantle d and the core mantle boundary with skks coda waves summary in our previous studies we developed a method for imaging heterogeneity at and near the core mantle boundary cmb with a generalized radon transform grt of transverse component broad band scs data and we developed a statistical model for producing images of the d discontinuity with variable confidence levels in these applications the background is smooth and perturbations are represented as contrasts here we extend the theory to allow known discontinuities such as the cmb in the background model the resulting imaging operator which is formally not a grt can be used either alone or along with scs for the imaging of lowermost mantle structure and in particular the d discontinuity with the scattered skks wavefield synthetic seismograms calculated with the wkbj method are used to test the performance of our approach as a proof of concept we transform 38 000 radial component skks waveforms into image gathers of a cmb patch beneath central america the skks image gathers and image traces are in good agreement with the image traces obtained from the grt transform of scs data

ophicalcites from the northern pyrenean belt a field petrographic and stable isotope study brecciated and fractured peridotites with a carbonate matrix referred to as ophicalcites are common features of mantle rocks exhumed in passive margins and mid oceanic ridges ophicalcites have been found in close association with massive peridotites which form the numerous ultramafic bodies scattered along the north pyrenean zone npz on the northern flank of the pyrenean belt we present the first field textural and stable isotopic characterization of these rocks our observations show that pyrenean ophicalcites belong to three main types 1 a wide variety of breccias composed of sorted or unsorted millimeter to meter sized clasts of fresh or oxidized ultramafic material in a fine grained calcitic matrix 2 calcitic veins penetrating into fractured serpentine and fresh peridotite and 3 pervasive substitution of serpentine minerals by calcite stable isotopic analyses o c have been conducted on the carbonate matrix veins and clasts of samples from 12 pyrenean ultramafic bodies we show that the pyrenean ophicalcites are the product of three distinct genetic processes 1 pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity 2 ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity and 3 polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid cretaceous we highlight a major difference between the eastern and western pyrenean ophicalcites belonging respectively to the sedimentary and to the hydrothermal types our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins but a post depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out finally we discuss the significance of such discrepancy in the characteristics of the npz ophicalcites in the frame of the variable exhumation history of the peridotites all along the pyrenean realm

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

steep dip seismic imaging of the shallow san andreas fault near parkfield seismic reflection and refraction images illuminate the san andreas fault to a depth of 1 kilometer the prestack depth migrated reflection image contains near vertical reflections aligned with the active fault trace the fault is vertical in the upper 0 5 kilometer then dips about 70 to the southwest to at least 1 kilometer subsurface this dip reconciles the difference between the computed locations of earthquakes and the surface fault trace the seismic velocity cross section shows strong lateral variations relatively low velocity 10 to 30 high electrical conductivity and low density indicate a 1 kilometer wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

modelling the lithospheric rheology control on the cretaceous rifting in west antarctica small scale analogue models were used to investigate the process of cretaceous orthogonal extension in the west antarctic rift system the models considered the transition from the east antarctic craton to a weaker lithosphere and the results support previous hypotheses about the strong control exerted by lateral variations in lithospheric structures on the process of extension strain was mostly accommodated at the boundary between the two types of lithosphere with a relative uplift of the cratonic block which remained essentially undeformed conversely the weaker lithosphere showed wide rifting style geometry locally associated with core complex like structures in agreement with the natural prototype this tectonic scenario led to a long lasting extension without continental break up and to the absence of relevant surface magmatism 2007 blackwell publishing ltd

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

tomographic images of p wave velocity variation at parkfield california tomographie inversion is applied io delay times from local earthquakes to image three dimensional velocity variations near parkfield california the 25 20 square km region is represented by nearly cubic blocks of 0 5 km per side arrival times of p waves from 551 local earthquakes with depths of 0 to 15 km were used as sources producing 3135 rays covering the target region the data were recorded on low noise downhole seismographs a conjugate gradient method is used to invert the resulting sparse system of simultaneous equations to diminish the effects of noisy data the laplacian of the model parameters is constrained to be small within horizontal layers providing smoothing of the model the resolution of the model is estimated by calculating point spread functions at blocks of interest estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure the results of the inversion show correlation with some of the local geological and geophysical features station corrections removed the long wavelength anomaly associated with the contrast of the salinian block southwest of the san andreas fault versus the franciscan to the northeast a velocity low located a few kilometers northwest of parkfield depth 2 5 3 5 km appears to lie along the gradient of the large bouguer gravity anomaly associated with the parkfield syncline the south southeastward extension of the low velocities may relate to reflections observed on the parkfield consortium for continental reflection profiling cocorp lines we speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment the correlation of higher velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower velocity less competent rocks

three dimensional passive seismic waveform imaging around the safod site california using the generalized radon transform 1 we apply a three dimensional 3d generalized radon transform grt to scattered p waves from 575 local earthquakes recorded at 68 temporary network stations for passive source imaging of near vertical structures close to the san andreas fault observatory at depth safod site the grt image profiles through or close by the safod site reveal near vertical reflectors close to the fault zone as well as in the granite to the southwest and the franciscan melange to the northeast of the main fault although slightly lower in resolution these structures are generally similar to features in 2d images produced with steep dip prestack seismic migration of data from active source seismic reflection and refraction surveys our grt images however also reveal several vertical reflectors to the northeast of the saf that do not appear in the migration images but which are consistent with local geology these results suggest that in a seismically active area inverse scattering of earthquake data for instance with the grt can be a viable and in 3d economic alternative to an active source survey

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

introduction to special section preparing for the san andreas fault observatory at depth 7209 seismology earthquake dynamics andmechanics 7230seismology seismicityandseismotectonics 8010structural geology fractures and faults citation hickman s m zoback and w ellsworth 2004 introduction to specialsection preparing for the san andreas fault observatory atdepth geophys res lett 31 l12s01 doi 10 1029 2004gl020688

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

existence of an s wave reflector near the upper plane of the double seismic zone beneath the southern kanto district japan the amplitude of coda from local shallow earthquakes in the southern kanto district of japan does not decay smoothly with time a clear phase appears in the s coda at lapse time from 15 to 45 s measured from the s wave arrival and it is predominant for frequencies around 8 hz this phase seems to be an s wave reflected from a discontinuity inclined westward at upper mantle depths since it is predominantly on the horizontal components and propagated westward with a high apparent velocity applying an inversion method to the travel times of the phases read from 8 hz band seismograms we locate the s wave reflector near the upper plane of the double seismic zone associated with the subducting pacific plate the s wave reflector is resolved east of the volcanic front at a depth of 80 100 km if we put qs l 10 3 the reflection coefficient is estimated to be around 0 5 however the scatter is large the s wave impedance contrast at the interface seems to be very large it may suggest the existence of a zone containing many liquid bodies

low velocity damaged structure of the san andreas fault at parkfield from fault zone trapped waves 1 we used dense linear seismic arrays across and along the san andreas fault saf at parkfield california to record fault zone trapped waves generated by explosions and microearthquakes in 2002 prominent trapped waves appeared at stations close to the saf main fault trace while some energy was trapped in the north strand at the array site observations and 3 d finite difference simulations of trapped waves at 2 5 hz show evidence of a damaged core zone on the main saf the zone from the surface to seismogenic depths is marked by a low velocity waveguide 150 m wide in which q is 10 50 and shear velocities are reduced by 30 40 from wall rock velocities with the greatest velocity reduction at shallow depth we interpret that this distinct low velocity zone on the main saf is a remanent of damage due to past large earthquakes on the principal fault plane at parkfield a less developed low velocity zone may be evident on the north strand that experienced minor breaks in the 1966 m6 event

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

nature of the s reflector beneath the galicia banks rifted margin preliminary results from prestack depth migration beneath the tilted fault blocks of the galicia rifted margin a bright reflection termed s has been interpreted as the master detachment fault controlling the extension of the margin however on time migrated sections s appears to be locally offset by the steep faults of the upper section we investigate the true geometry of s and its relation to the overlying faults through iterative prestack depth migration we conclude that it is not significantly offset by the faults which instead appear to stop abruptly at s thus we interpret s as the master detachment and consider its role during extension

palaeogeographic consequences of conservational models in the south atlantic ocean conservational models like simple shear pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins these models need large amounts of horizontal movement and have therefore important implications for plate kinematic reconstructions and intraplate deformation in this paper we propose to show these implications in the central segment of the south atlantic ocean in the angola brazilian system these models imply about 240 km of horizontal movement this movement can be com pensated by two end member mechanisms 1 an intraplate deformation located in africa and 2 an intraplate deformation located in south america we detail for each solution the strong geological and geodynamical implications and discuss the consequences for the genesis of passive continental margins

diversity of fault zone damage and trapping structures in the parkfield section of the san andreas fault from comprehensive analysis of near fault seismograms summary we perform comprehensive analyses of trapped waves and signals of damaged fault zone rocks associated with time delays of body waves along the parkfield section of the san andreas fault saf waveforms generated by thousands of earthquakes and recorded by near fault stations in several permanent and temporary deployments are examined with attention to the possible influence of a lithology contrast across the fault on signals of the low velocity damage zone clear candidate trapped waves are identified at only three stations mmnb and two of the other near fault stations flip and pies further to the nw clear candidate trapped waves are not seen at any of the near fault stations to the se of mmnb the locations of the events generating good candidate trapped waves at mmnb and the other two stations are not distributed broadly in space but clustered in a small number of locations moreover events that generate clear candidate trapped waves at one station do not typically generate trapped waves at the other two stations these observations imply that the damage zone is highly variable along strike and that a coherent connected waveguide does not exist for distances along strike larger than at most 3 5 km the distance between stations synthetic waveform fits for observed trapped waves at stations mmnb and flip indicate that the most likely parameters of the trapping structures at these locations are widths of about 150 m depths of about 3 km velocity reductions of 30 40 per cent and q values of 10 40 synthetic calculations of trapped waves demonstrate that if there is a contrast of seismic velocity across the fault the trapped waves are delayed relative to the s wave arrival trapped waves at station mmnb and to a lesser extent also at stations flip and pies in the nw section show this characteristic this suggests a lithology contrast in the top few km at these locations in agreement with results from tomography and studies of head waves in the parkfield area at several mini across fault arrays where trapped waves are not observed a low velocity damage zone is detected from the delay in the arrival time of body wave phases relative to a nearby off fault station the observed delay of the s wave is greater than the p wave delay consistent with the existence of a damage zone with poisson ratio of about 0 33 the observations of time delays without trapped waves indicate that parts of the damage zone are insufficiently coherent to generate trapped waves a broader damage zone may exist in the region between the saf and the south west fracture zone the results highlight the diversity of damage structures along the 40 km of the saf examined in this study and imply that fault imaging based on data at single sites does not necessarily apply to a larger section

ultramafic blocks from the ocean floor southwest of australia abstract samples dredged from the ocean floor near the junction of the naturaliste fracture zone and the diamantina zone 300 km southwest of cape leeuwin western australia contain 3 to 10 cm blocks of cr spinel lherzolites and al spinel olivine clinopyroxenites both assemblages show textural evidence for deformation and recrystallization with extensive development of kaersutite in one clinopyroxenite the blocks are enclosed by clay and fe mn oxide rich fragmental material which also contains a diverse detrital mineral suite and microfossils diopsides in the lherzolites are na and cr rich with marked similarities to those of lherzolite nodules in alkaline basaltic suites the al and ti rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene rich nodules the lherzolites are interpreted as upper mantle residues while the clinopyroxenites probably represent partly recrystallized cumulates from high temperature basaltic magmas traversing the lherzolite mantle both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the cretaceous early tertiary continent ocean boundary off australia

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

iapetus suture located beneath the north sea by birps deep seismic reflection profiling the iapetus suture one of the more fundamental geologic boundaries of the british isles has been located beneath the north sea by deep seismic reflection profiling carried out by the british institutions reflection profiling syndicate birps reflection profiling and magnetotelluric sounding have previously been used to infer the position of the iapetus suture beneath the irish sea and the southern uplands but the present study provides the first direct evidence for the location of the iapetus suture east of britain a comparison of the seismic reflection profiles over the iapetus suture east and west of britain shows variations in the dip of the suture which steepens to the northeast beneath the north sea the comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

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

deep crustal structure of the rhine graben from dekorp ecors seismic reflection data a summary abstract results of two deep reflection seismic lines crossing the northern and southern part of the rhine graben which were recorded in 1988 by the french ecors and the german dekorp teams indicate that the asymmetry of this rift persists to the base of the crust the brittle ductile transition zone approximately coincides with the top of the lower crust ductile shear zones appear to affect the crust mantle boundary upper crustal extension by faulting accounts for a divergence of the graben flanks by 5 7 km whereas the crustal configuration of the rhine graben suggest an extension value of some 17 km

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

high resolution structures of the landers fault zone inferred from aftershock waveform data summary high frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 landers california earthquake were used to determine fault zone structures down to the seismogenic depth we first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low velocity fault zone models we then generated synthetic waveform record sections of a linear array across a vertical fault zone they show that both arrival times and waveforms of p and s waves vary systematically across the fault due to transmissions and reflections from boundaries of the low velocity fault zone the waveform characteristics and arrival time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its p and s wave velocities independently as well as its depth extent therefore the trade off between the fault zone width and velocities can be avoided applying the method to the landers waveform data reveals a low velocity zone with a width of 270 360 m and a 35 60 per cent reduction in p and s velocities relative to the host rock the analysis suggests that the low velocity zone extends to a depth of 7 km the western boundary of the low velocity zone coincides with the observed main surface rupture trace

a new perspective on the geometry of the san andreas fault in southern california and its relationship to lithospheric structure the widely held perception that the san andreas fault saf is vertical or steeply dipping in most places in southern california may not be correct from studies of potential field data active source imaging and seismicity the dip of the saf is significantly nonvertical in many locations the direction of dip appears to change in a systematic way through the transverse ranges moderately southwest 55 75 in the western bend of the saf in the transverse ranges big bend vertical to steep in the mojave desert and moderately northeast 37 65 in a region extending from san bernardino to the salton sea spanning the eastern bend of the saf in the transverse ranges the shape of the modeled saf is crudely that of a propeller if confirmed by further studies the geometry of the modeled saf would have important implications for tectonics and strong ground motions from saf earthquakes the saf can be traced or projected through the crust to the north side of a well documented high velocity body hvb in the upper mantle beneath the transverse ranges the north side of this hvb may be an extension of the plate boundary into the mantle and the hvb would appear to be part of the pacific plate

seasonal evolution of fine scale sea surface temperature fronts in the east china sea abstract this study demonstrates the seasonal evolution of fine scale sea surface temperature sst fronts in the east china sea ecs by applying an entropy based edge detection algorithm to high resolution 1 1 km satellite sst images obtained between 1996 and 2009 five major sst fronts are detected the jiangsu fronts exist all year round along the 20 m isobath near the jiangsu coast in summer the jiangsu fronts exhibit several frontal bands closely corresponding to the shape of radial sand ridges off the jiangsu coast the yangtze bank front forms a ring shaped structure off the yangtze river estuary in winter and creates a boundary between the cold shelf water over the yangtze shoal and the warm kuroshio water the china coastal front evolves in winter along the 50 m isobath from the northern ecs southwestward into the taiwan strait the kuroshio front is enhanced to the northeast of taiwan between the 100 m and 200 m isobaths the cheju island front emerges along the southern coast of the korean peninsula and extends between cheju strait and tsushima strait and is sustained in the summer months thus this study reveals hitherto undescribed fine scale structures of sst fronts and shows their seasonal variations and relation to the bathymetry

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally

kinematics of peridotite emplacement during north atlantic continental rifting galicia northwestern spain abstract the western part of the galicia passive continental margin western edge of spain is bordered by serpentinized peridotites over a n s distance of about 125 km these peridotites were emplaced at the end of continental rifting and or at the very beginning of oceanic accretion they have been observed in situ and sampled in five sites with the nautile submersible during the galinaute cruise 1986 some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge most of the escarpments covered by the nautile on the peridotite ridge have yielded only ultramafic rocks except at dive 10 where the peridotites are overlain by a sheared chlonte bearing schist and at dive 14 where the ultramafic rocks are covered by basalts all the peridotites are plagioclase bearing harzburgites and iherzolites locally cross cut by rare plagioclase rich veins and dioritic intrusives some pyroxenites and gabbros are associated with the peridotites at dive 14 the plagioclase bearing peridotites are similar in composition to those drilled during odp leg 103 primary structures have not been found except in the northern part of the studied zone where a few samples exhibit magmatic textures elsewhere the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high temperature fabric at these localities boudinage structures and ultramylonitic bands referred to as shear bands occur and the rocks have a disrupted mylonitic texture a few dioritic dykes have also suffered this mylonitization shearing occurred in a rotational regime at high and decreasing temperature 1000 850 c and under a high deviatoric stress 102 mpa i e under lithospheric conditions the structural pattern of the peridotite ridge deduced from measurements on oriented samples is complex the trend of the mylonitic foliation appears nearly constant from south to north n125 n105 but its dip increases 20 45 northward from south to north the stretching lineation pitch varies from subvertical to subhorizontal stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area to the north it occurred in a northwesterly direction with a strike slip component stretching in the chlorite bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation this study confirms the uplift of the galicia peridotites during the continental rifting of the north atlantic ocean in cretaceous times during their ascent the peridotites underwent some melting and high temperature low stress ductile deformation followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures the geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress strain field prevailing in the continental lithosphere during the rifting episode it is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point the kinematics of emplacement agree well with an e w opening of the north atlantic ocean

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake