inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

seismic structure of the mid atlantic ridge 8 9 s 1 the mid atlantic ridge at 8 9 s is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation we present constraints on the structure of 0 4 ma crust in this region on the basis of the analysis of wide angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers a 350 800 m thick oceanic layer 2a interpreted as high porosity extrusive basalts is underlain by a 2 0 2 5 km layer 2b with velocities which increase with age and decrease in the vicinity of the pseudofaults layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis the crustal thickness varies from 6 7 km near the pseudofaults formed by ridge propagation to 9 10 km at the segment center of the recently 0 3 ma abandoned spreading center seismically determined crustal thickness and density variations and age related lithospheric cooling can plausibly account for all observed variations in gravity across the area and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle the transition from axial valley to axial high occurs at a crustal thickness of 8 km

detachment faulting mantle serpentinization and serpentinite mud volcanism beneath the porcupine basin southwest of ireland the porcupine basin southwest of ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup profiles across the basin image a bright reflection that appears to represent a detachment fault and may in part be a decollement at the top of partially serpentinized mantle although overall the basin appears symmetric the consistent westward structural dip of the detachment implies that at high stretching factors extension was asymmetric farther south the porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

extent of oceanic crust in the labrador sea1 this paper examines the mode of formation of the northern margins of the labrador sea observations of faulted and rotated crustal blocks across the northwest margin of greenland had led to the suggestion that this region is underlain by extended continental crust a detailed analysis of all geophysical data from this region presented here suggests on the contrary the possibility that it could have been formed by an excessively slow seafloor spreading process and ridge axis propagation this gave rise to rotated fault blocks at a time when spreading was mainly confined to the greenland side prior to its shift to the west supporting evidence for such a reconfiguration of the spreading axis comes from gravity and magnetic observations of this and neighbouring regions modelling of magnetic anomalies in the area based on the average rate of spreading for the entire labrador sea shows that a gradual shift of the ridge axis to the west started after the formation of anomaly 32 because the earliest seafloor spreading anomalies are highly attenuated in amplitudes other models like extended and intruded continental crust are possible if one only considers the magnetic anomalies however an extended continental crustal model poses problems in accounting for other geophysical observations based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope with a narrow belt of extended continental crust along the west greenland margin while a much wider zone of perhaps attenuated continental crust extends off labrador breakup of the continental landmass occurred much closer to the west greenland coast than to labrador the applicability of a slow spreading model to other passive margins where similar geophysical observations have been reported is discussed

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

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

high seismic velocities associated with reflections from within the lower oceanic crust near the continental margin of eastern canada abstract a deep seismic reflection profile on the continental rise off eastern canada shows strong dipping reflections apparently from within lower oceanic crust over a region of limited lateral extent the origin of these and similar reflections described elsewhere is uncertain this paper reports new seismic refraction results from the region of these reflections they appear to be associated with a layer of p wave velocity of about 7 4 km s 1 or higher while the surrounding crust has a velocity 7 km s 1 the moho with velocity 8 1 km s 1 lies just below the reflectors and the total crustal thickness is around 8 km the crust is interpreted to be oceanic rather than continental and the high velocities of the dipping reflectors are comparable to those of the mafic ultramafic crust mantle transition zone in the bay of islands ophiolite complex

a 2 d ray based depth migration method for deep seismic reflections summary although accurate depth migration is essential for the correct geometrical interpretation of deep reflection data migration by the standard numerical methods has generally not been satisfactory the alternative method presented in this paper provides an efficient migration for deep reflection profiles in the form of a digitized line drawing assuming a general 2 d velocity structure migration of a digitized line drawing reduces the effects of low signal noise ratios the calculation is rapid allowing migrations to be calculated for a large number of alternative velocity structures and iterative improvement of the velocity structure the migration has been applied to 3 d structures observed on intersecting profiles collected by birps british institutes reflection profiling syndicate

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

new evidence for the origin of the porcupine median volcanic ridge early cretaceous volcanism in the porcupine basin atlantic margin of ireland 1 two dimensional pre stack depth migrated seismic reflection data gravity and velocity models are used to assess the nature and origin of a prominent buried ridge the porcupine median volcanic ridge pmvr within the porcupine basin offshore ireland the debate on the origin of the pmvr during the past 30 years has followed the evolution of the concept of continental margin genesis in this paper the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure revealed by the seismic data implication of the presence of these type of ridges in hyper extensional rifted margins is discussed and compared with other margins the analysis indicates that the ridge is an extrusive volcanic ridge probably tholeiitic in composition constructed by stacked hyaloclastite deltas and topped by carbonate platforms the results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism the extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred but the architecture and velocity of the pmvr demonstrates that it is made of lower velocity materials than serpentinite during the opening of the north atlantic the pmvr represents the northern time equivalent magmatic event expressed along the newfoundland iberia galicia recorded by the j anomaly that originate from cretaceous volcanic deposits

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

crustal structure of the goban spur continental margin northeast atlantic from deep seismic reflection profiling deep reflection data across the uk continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide the origin of the reflections is at present little understood but clearly represents an important crustal process the western approaches margin wam line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension the wam profile across the continental shelf is characterized by bright continuous lower crustal reflections some of which reach 30 km in length and have minimum reflection coefficients of 0 1 deep water multiples in the area of the continental slope partially obscure reflections from the lower crust but detailed analyses show that reflectors are present in the lower crust on the continental slope adjacent to the continent ocean boundary the lower crust on the wam profile is well imaged and free of noise and multiples this very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf i e the upper half of the crust is blank and the lower half contains sub horizontal reflections the lower crustal reflections adjacent to the continent ocean boundary are less continuous than the reflections seen on the shelf and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf the lower crustal reflectors are thus interpreted to pre date the extensional episode that led to the opening of the atlantic the preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the goban spur area

horst and graben structures on the flanks of the mid atlantic ridge in the mark area 23 22 n submersible observations abstract south of the kane fracture zone 23 40 n the mid atlantic ridge is segmented into 20 100 km long spreading cells in which morphology seismic and gravity signatures change along and across the ridge axis the rift valley considered as a typical structure along slow spreading ridges can be symmetric or asymmetric and segment centers experience strong magmatic activity whereas low magma supply results in thinning of the crust at segment tips along the segment centered on the snake pit neovolcanic ridge 23 22 n the rift valley is clearly asymmetric and mantle and lower crustal rocks are commonly exposed on the steepest and highest ridge flanking wall in this area we analyzed submersible observations gravinaute cruise collected up to 50 km on both sides of the axis to discuss the proceses responsible for the axial valley shape its possible relationship with deep rock exposures and to determine the origin of off axis structures our data reveal that the structure and the geology vary on both walls of the axial valley and is different from the volcanic and tectonic organization observed on the neovolcanic ridge itself furthermore the structure of the off axis domain in different from the axial valley and rift mountains are organized in a set of regularly spaced relief about 10 km on both flanks of the ridge scarps which built these topographic highs face either toward the axis or away thus the off axis morphology appears as a set of horsts and grabens which could partly result from off axis tectonic processes and or from a shifting of the volcano tectonic activity out of the axial valley during the last 3 ma off axis data show that serpentinized peridotites more commonly observed on the western rift valley wall crop out on the western flank up to 30 away from the axis models proposed to account for such rock exposure are usually related to the formation of the deep axial valley along slow spreading ridges here we propose that mantle rocks are exposed only in a very particular structural context which is directly controled by the ridge segmentation and is independent of the shape of the axial valley symmetric or not serpentinites crop out within a v shaped structure corresponding with the boundary between two adjacent segments where seismic and gravity data reveal that the crust is thin and the lithosphere is thick here the crust is fed by a lower magma supply than at the segment center snake pit area thus we interpret this segment end to be a region of extremely thin crust where modest normal fault can bring mantle rocks to the surface

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

flemish cap goban spur conjugate margins new evidence of asymmetry we present the combined results of deep multichannel reflection and refraction seismic surveys across the flemish cap goban spur conjugate margin pair north atlantic which we use to infer rifting style and breakup profiles on both margins cross magnetic anomaly 34 and extend into oceanic crust making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust the deep multichannel seismic mcs reflection data have previously been used to support a model of symmetric pure shear extension followed by asymmetric breakup and a sharp continent ocean boundary using both types of seismic data our results indicate instead that asymmetric structures are formed during all stages of rifting breakup and complex transition to oceanic spreading the differing nature of the two ocean continent transition zones is particularly striking for flemish cap our reprocessed image of the mcs profile clearly shows tilted fault blocks beneath back tilted sediment packages consistent with a wide region of highly thinned continental crust inferred from wide angle seismic data in contrast normal incidence and wide angle seismic data for the goban spur transition zone indicate the presence of exhumed serpentinized mantle

viscoelastic behavior of partially molten granites effects of partial melting on complex shear modulus g and internal friction q 1 of granites have been investigated using a torsion deformation apparatus in the frequency range of 2 mhz 20 hz at 650 1250oc granite samples from kirchberg w erzgebirge germany og1 and fine grained quartz feldspar porphyry a

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

a global analysis of mid ocean ridge axial topography summary current views of mid ocean ridges are strongly influenced by extensive mapping of the mid atlantic ridge and east pacific rise the global picture of the mid ocean ridge system particularly in the sparsely surveyed southern oceans is still based primarily on underway bathymetry profiles collected over the past 40 years this study presents a quantitative analysis of global mid ocean ridge morphology based on 156 of these underway bathymetric profiles thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis an empirical orthogonal function eof analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth it is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti symmetric modes the remainder is considered stochastic maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr 1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr 1 the stochastic component of the axial morphology referred to as bathymetric roughness also decreases with spreading rate for rates less than 80 mm yr 1 and remains relatively constant at higher rates although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence they are weakly correlated at slow spreading centres the distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges these observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid ocean ridges

thinning of the goban spur continental margin and formation of early oceanic crust constraints from forward modelling and inversion of marine magnetic anomalies summary the deep seismic reflection profile western approaches margin wam cuts across the goban spur continental margin located southwest of ireland this non volcanic margin is characterized by a few tilted blocks parallel to the margin a volcanic sill has been emplaced on the westernmost tilted block the shape of the eastern part of this sill is known from seismic data but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean continent transition the volcanic body thickens towards the ocean and seems to be in direct contact with the oceanic crust in the contact zone the volcanic body and the oceanic magnetic layer display approximately the same thickness the oceanic magnetic layer is anomalously thick immediately west of the volcanic body and gradually thins to reach more typical values 40 km further to the west the volcanic sill would therefore represent the very first formation of oceanic crust just before or at the continental break up the ocean continent transition is limited to a zone 15 km wide the continental magnetic layer seems to thin gradually oceanwards as does the continental crust but no simple relation is observed between their respective thinnings

seismic properties of layer 2 basalts summary in this paper the physical properties of oceanic layer 2 basalts are examined with the emphasis on micro crack porosity and alteration related changes in physical properties over 160 core samples from some of the more significant deep sea drilling project ocean drilling program holes including 417d 418a 504b and 801c are included in the present investigation overall physical property relationships are discussed in the first half of this study at 200 mpa confining pressure compressionaland shear wave velocities of the sample suite have modal values of 6 1 kms 1 and 3 5 kms 1 respectively a poisson s ratio of 0 28 characterizes the majority of the basalts samples that have experienced extensive low temperature alteration identified through elevated kzo and decreased cao and mgo contents exhibit low velocities and densities and high poisson s ratios the microcrack porosity in the basalt suite is examined using velocity pressure data a pore aspect ratio inversion scheme and scanning electron microscope sem imaging microcracks sealed by alteration minerals such as smectite and calcite or healed by late crystallization are common throughout the sample set rocks from the transition zone and sheeted dyke section of hole 504b are marked by numerous smectite and chlorite filled cracks more significantly the data indicate an increase of low aspectratio open microcracks with increasing depth in samples from several dsdp holes particularly hole 504b these microcracks also observed using the sem are attributed to stress relief as the original drill cores were removed from in situ pressure conditions stress relief micro cracking is apparently more severe in the holocrystalline coarsergrained samples from massive flows than in the phyric basalts with fine grained groundmasses characteristic of pillow units this difference in micro cracking is explained through basic fracture theory

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

deep structure of the armorican basin bay of biscay a review of norgasis seismic reflection and refraction data the bay of biscay is bounded to the north by the north biscay margin which comprises the western approaches and armorican segments in the 1970s and 1980s most researchers considered this margin typical of a non volcanic passive margin it is characterized by a striking succession of tilted blocks beneath which occurs the s reflector and the continent ocean boundary is abrupt this paper examines the armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data norgasis cruise an important result is the discovery of a 80 km wide ocean continent transition zone that coincides with the armorican basin a deep sedimentary basin it is characterized by a high velocity lower crustal layer 7 4 7 5 km s 1 overlain by sediments the other results are 1 the main crustal thinning occurs exclusively under the narrow continental slope 2 the tilted blocks and the s reflector are observed only at the base of the continental slope in the narrow domain called the neck area 3 the north biscay ridge is a large oceanic plateau present only off the nw armorican margin rather than a long ridge elongated off the whole north biscay margin

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

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

poisson s ratio of a seaward dipping reflector series hatton bank summary seismic reflection profiles across the hatton bank continental margin in the eastern north atlantic have imaged thick sequences of seaward dipping reflectors converted shear waves propagating through one of these sequences were recorded on a two ship expanding spread profile just landward of the ocean continent transition synthetic seismogram modelling of these shear waves indicates that poisson s ratio in the sequence is 0 27 0 01 similar to that of normal oceanic layer 2

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

hydrological aspects of geophysical borehole measurements in crystalline rocks of the black forest abstract one of six wells drilled into the crystalline rock of the black forest basement during the german continental deep drilling project ktb site survey has been used to detect hydraulically active fractures and to identify their character by a variety of logging tools such as caliper and televiewer logging vertical seismic profiling vsp and electrical and thermal measurements in addition to standard logging recordings in the 265 m moosengrund well drilled into the triberg granite body these methods were applied successfully to discriminate between fractures and breakouts and to identify open and closed fractures and water consuming or water expelling fractures respectively fracture strike dip and width were determined from televiewer data vsp measurements were used to determine the seismic velocities in the unfractured rock and the fracture zones estimates of permeability 10 55 darcy and transmissivity 0 1 1 0cm 2 s 1 of the fracture zones were obtained by the analysis of p waves and tube waves in the vsp records the summed values of the transmissivity yield an upper estimate of the overall transmissivity determined by a pumping test

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

a seismic refraction and reflexion study of the continent ocean transition beneath the north biscay margin the structure of the northern margin of the bay of biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent ocean boundary these blocks formed by rifting in late jurassic early cretaceous time and are now thinly covered by post rift sediments of aptian to recent age seismic refraction profiles were occupied on the shelf on either side of and across the continent ocean transition to the shelf using p ubs and o bs with explosives and a 4 x 1000 in 3 4 x 16400 cm 3 airgun array two ship expanding spread multichannel 48 trace seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition a two ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf conventional 48 trace single ship multichannel profiles were located along all the refraction and two ship reflexion lines interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms conventional seismic processing techniques have been used to prepare the two ship multichannel seismic data for interpretation the survey is believed to be the first attempt to apply two ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust the results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide the seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone however extension values calculated in the upper crust from the rotation of fault blocks are much less 1 1 1 4 and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust

variations in heat flow across the goban spur and galicia bank continental margins we present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the goban spur and galicia bank continental margins nw atlantic ocean simple extensional models indicate that the heat flow across both these early cretaceous rifted margins should increase from values of 45 50 mw m2 over oceanic crust to 65 80 mw m2 on the continents the rate of this increase should help to constrain the mechanism simple versus pure shear by which the upper more radiogenic continental crust is thinned measurements across goban spur increase from values of 40 45 mw m2 over oceanic crust to 50 55 mw m2 near the continental shelf they follow the predicted trend for pure shear rifting but only if the value of upper crustal radiogenic heating is low 1 2 mu w m3 otherwise they would require the upper crust to thin more rapidly than the total crustal thickness as with a lower plate simple shear margin measurements across galicia bank show a very different pattern with similar values over oceanic crust but much lower values 30 35 mw m2 nearer land this is difficult to reconcile with any simple single rifting event but is more compatible with an origin as a pure shear or lower plate rather than upper plate margin we also note that oceanic values of heat flow require asthenospheric temperatures 100 degrees c lower than normal for both margins this indicates that the triple junction in existence between these margin segments during the breakup of iberia europe and north america was not the site of a major mantle plume

formation of the axial relief at the very slow spreading southwest indian ridge 49 to 69 e the comparison of segment lengths relief and gravity signature along the very slow spreading southwest indian ridge swir between 49oe and 69oe suggests that the marked change in segmentation style that occurs across the melville transform 60o45 e reflects a change in the modes of formation of the axial topography we propose that the axial relief east of melville is largely due to volcanic constructions that load the axial lithosphere from above by contrast the axial relief in segments west of the melville fracture zone appears to be primarily due as proposed for segments of the faster spreading mid atlantic ridge to along axis changes in the depth of the axial valley and to partial compensation of negative loads thicker lower crust and or lighter upper mantle acting within the plate or at the bottom of the plate in terms of geology this means that the contribution of the uppermost effusive part of the crust to along axis crustal thickness variations may be greater east of melville than in other regions of the study area regional axial depths suggest that the ridge east of melville is also characterized by a low melt supply and is underlain by cold mantle a simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region and the shallower region west of the gallieni transform 52o20 e are of the order of 80oc and 4 km respectively

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

the moho depth map of the european plate summary the european plate has a 4 5 gy long and complex tectonic history this is reflected in the present day large scale crustal structures a new digital moho depth map is compiled from more than 250 data sets of individual seismic profiles 3 d models obtained by body and surface waves receiver function results and maps of seismic and or gravity data compilations we have compiled the first digital high resolution map of the moho depth for the whole european plate extending from the mid atlantic ridge in the west to the ural mountains in the east and from the mediterranean sea in the south to the barents sea and spitsbergen in the arctic in the north in general three large domains within the european plate crust are visible the oldest archean and proterozoic crust has a thickness of 40 60 km the continental variscan and alpine crust has a thickness of 20 40 km and the youngest oceanic atlantic crust has a thickness of 10 20 km

the goban spur transect geologic evolution of a sediment starved passive continental margin leg 80 of the dsdp ipod program drilled a transect of four core holes 548 551 across the continent ocean boundary at goban spur a prominent southwest trending structural high on the irish continental slope multichannel seismic reflection profiles show that during rifting continental basement rocks of goban spur were broken up by northwest trending listric normal faults to form a series of half graben basins two of these half grabens were sampled during leg 80 sites 548 and 549 site 550 was located on the adjacent oceanic crust of porcupine abyssal plain site 551 was located on transitional crust at the foot of goban spur the objectives were lo analyze the structural the depositional and the paleoenvironmental development of this sediment starved passive continental margin at sites 548 and 549 basement comprises continental hercynian metasediments of devonian age at sites 550 and 551 the basement is tholeiitic basalt the oldest syn rift sediments barremian age or perhaps late hauterivian were penetrated at site 549 lying uuconformably below aptian strata seismic sequence analysis reveals that aptian strata also overlie this unconformity farther northeastward in the basin an unconformity above the aptian section marks the end of rifting and the beginning of sea floor spreading an albian age for the initiation of sea floor spreading was corroborated at site 550 where abyssal late albian chalks rest upon and are interbedded with oceanic basalts indicating an initial water depth of 2 000 m as sea floor spreading progressed goban spur subsided rapidly so that by cenomanian time bathyal to abyssal chalks were accumulating at deeper sites after two periods of partial stagnation in the aptian albian and in the turonian chalk deposition in well oxygenated environments took place at all sites modified chiefly by shifts in deep circulation patterns and the calcite compensation depth ccd by periodic influx of terrigenous detritus during low stands of sea level especially in the cenozoic and by frequent displacement of older carbonates from the slope to abyssal sites a number of major unconformities correspond to those most often reported from other widespread locations in the north atlantic basin and on surrounding continental shelves and coastal plains several unconformities are preserved undisturbed in our cores and can be correlated with sea level fluctuations with paleoceanographic events and with tectonic movements a thick quaternary section at site 548 records prominent fluctuations of glacial inter glacial paleoclimates an even thicker paleogene section at site 549 provides unusually well preserved and uninterrupted sequences suitable for detailed sedimentological and stratigraphic studies

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

the southern margin of the rockall plateau stratigraphy tertiary volcanism and plate tectonic evolution the geological evolution of the southwest rockall plateau in the area of edoras bank has been clarified using seismic reflection gravity and magnetic data four principal reflectors are observed within the tertiary sedimentary sequence i late miocene ii latest early miocene iii late eocene iv earliest eocene a period of pronounced sediment drift accumulation marks the interval i ii reflector iii the top of a sediment wedge prograding southward from edoras bank marks a change from terrigenous to pelagic sedimentation that is probably related to subsidence of the rockall plateau following the separation of greenland from eurasia in the earliest eocene reflector iv marks the top of a wedge shaped seismically transparent layer that also thins southward away from edoras bank on the basis of its seismic attributes and magnetic signature this layer is interpreted as a volcanic sheet formed as part of the north atlantic tertiary volcanic province during rifting of greenland from eurasia the recognition of voluminous volcanic rocks south of edoras bank extends the known area of the tertiary volcanic province several hundred kilometres to the south gravity anomaly modelling and continental reconstructions suggest that the region south of edoras bank is underlain by thinned continental crust a four stage geological evolution for this region is indicated i initial rifting associated with the separation of labrador from greenland in the late cretaceous is characterized by enhanced crustal thinning and subsidence in the region of a rift triple junction ii passive subsidence and accumulation of late cretaceous and earliest tertiary sediments followed the initiation of seafloor spreading in the labrador sea iii blanketing of the area by palaeocene volcanic rocks masked pre existing magnetic lineations providing an explanation for some of the problems in earlier interpretations based mainly on magnetic data iv post volcanic sedimentation continuing to the present day

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

on the roughness of mesozoic oceanic crust in the western north atlantic summary seismic reflection profiles from mesozoic oceanic crust around the blake spur fracture zone bsfz in the western north atlantic have been widely used in constraining tectonic models of slow spreading mid ocean ridges these profiles have anomalously low basement relief compared to crust formed more recently at the mid atlantic ridge at the same spreading rate profiles from other regions of mesozoic oceanic crust also have greater relief the anomalous basement relief and slightly increased crustal thickness in the bsfz survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation if so the intracrustal structures observed may be representative of an atypical tectonic regime

refraction wide angle reflection investigation of the cadomian crust between northern brittany and the channel islands abstract a seismic refraction experiment has been carried out off the coast of northern brittany nw france to investigate the deep structure of the north armorican block the seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations we present 2d velocity models computed from two crossing seismic refraction profiles labelled p1 and p2 and each about 135 km long oriented ne sw and nnw sse i e parallel and perpendicular respectively to the regional basement structures wide angle reflection profiles were also performed to extend the 2d structures to a 3d crustal interpretation observed and calculated p wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models the model which best fit the data consists of 1 high basement velocities at the top of the crust except in an area with shallow seismic velocity which can be as low as 3 0 km s at the surface and possibly related to the existence of shallow paleozoic sedimentary basins 2 the mean crustal velocity ranges from 5 5 to 6 8 km s at a depth of 15 km depth 3 lower crustal velocities range from 7 0 to 7 5 km s at a depth of 35 38 km and then from 7 5 to 8 1 km s at the crust mantle boundary located between 35 and 38 km depth comparisons with the crustal section imaged by the coincident seismic vertical reflection profile swat 10 show a good correlation between velocity gradients and lower crustal reflectivity when compared to surrounding basement areas the north armorican cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this upper proterozoic block from later paleozoic and mesozoic tectonic episodes the crustal structure of the north armorican block thus appears to be an intercalated feature between the surrounding paleozoic areas and older shield or platform domains

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements