oceanic transverse ridges a flexural response to fracture zone normal extension one of the most puzzling characteristics of sea floor morphology is the occurrence of anomalously shallow fracture zone parallel oceanic transverse ridges a model is proposed for the formation of transverse ridges near lat 21 and 24 n on the mid atlantic ridge in which the differential responses of large offset and small offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off axis traces of fracture zones numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension 5 km along low angle faults 45 are responsible for the transverse ridges

mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

a nonlinear rheology model for mid ocean ridge axis topography this paper describes a mechanical model of the mid ocean ridge axis based on a nonlinear rheology the oceanic lithosphere is modeled as a strongly temperature dependent power law material with the byerlee s friction law describing the brittle behavior of the upper oceanic lithosphere we examined two simple models a mantle only model predicted an axial rift valley 1 2 km deep and 10 15 km wide at slow half rates of 5 25 mm yr which is in fair agreement with observations however the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller thus the mantle only model failed to explain the observed transition in axial topography with spreading rate by including a uniform thickness oceanic crust with theological properties different from the mantle our second model produced the first order variation of axial topography with spreading rate a pronounced rift valley with 1 2 km relief at slow rates a smaller relief 500 m rift valley at intermediate rates and no rift valley at fast rates there are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models first horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis i e the stresses are concentrated in a very narrow zone for the nonlinear rheology this conclusion is remarkably independent of the boundary conditions at the seafloor second the computed width of the axial plastic failure zone decreases with spreading rate a result very different from that of a uniform viscosity model this is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate beyond some distance from the ridge axis the brittle plate roughly the region above the 750 c isotherm deforms very little both the horizontal strain rate 11 and the maximum shear strain rate max are less than 10 17 s 1 which is in good agreement with the 10 18 s 1 average for oceanic brittle plate deformation inferred from seismic moment data within the ductile region area beneath the 750 c isotherm the strain rates are in the range of 10 14 10 16 s 1 except within a narrow band near the ridge axis where higher strain rates are located we found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern or the rheology but is very sensitive to the cooling by hydrothermal circulation near the ridge axis axial structure is also influenced by factors other than the spreading rate within the range of slow and intermediate rates our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature in other words a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley in the vicinity of hotspots the absence of an axial rift valley even at slow spreading rates such as the reykjanes ridge can be well explained by the anomalously thicker crust and higher mantle temperatures in contrast at the australian antarctic discordance a deeper than normal section of an intermediate spreading ridge has very rough topography not present on the adjacent sections of the ridge this can be explained by our model as due to an anomalously low mantle temperature as has been suggested or perhaps also due to a thinner oceanic crust

along axis variations in oceanic crustal density and their contribution to gravity anomalies at slow spreading ridges gravity data from mid ocean ridges are routinely used to infer crustal thickness variations commonly the contribution to the gravity anomaly from mantle density variations are taken into account but variations in crustal density are rarely considered wide angle seismic experiments conducted in the same locations as detailed gravity surveys on the mid atlantic ridge show that the crustal thickness inferred from gravity data can deviate substantially from that derived from seismic data particularly in the vicinity of ridge offsets this discrepancy may be partly attributed to unmodeled crustal density variations at slow spreading ridges crust formed near ridge offsets has consistently lower velocity than crust formed near segment centers and a corresponding density reduction of up to 20 may be inferred

a different pattern of ridge segmentation and mantle bouguer gravity anomalies along the ultra slow spreading southwest indian ridge 15 30 e to 25 e abstract the results of a recent bathymetric and geophysical investigation of a 650 km long portion of the very slowly opening 16 mm yr full rate southwest indian ridge swir between 15 30 e and 25 e are presented bathymetry and mantle bouguer gravity anomalies mba caused by variations in crustal thickness and or crustal and upper mantle densities show different characteristics from those observed at faster spreading centers like the mid atlantic ridge mar 20 30 mm yr full rate with the exception of the du toit transform none of the ridge axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off axis traces the mba patterns associated with individual segments are much more complex than the simple circular bull s eyes lows reported along the mar while the short wavelength ridge segment length is comparable to that of the mar there is little correlation with mba amplitude and segment length and axial relief furthermore an eastward propagating magma source and an 84 km long zone of oblique spreading appears to define a fundamental boundary along the swir between two 250 300 km long sections characterized by distinctly different axial morphology and gravity signatures we interpret these results to indicate a long wavelength segmentation pattern of the underlying upwelling mantle melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes fissure eruptions within the rift valley build linear ridges defining a short wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle our results and interpretation are quite different than that predicted by extending current models of the faster spreading mar to these ultra slow spreading rates

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

the structure of three dimensional convection beneath oceanic spreading centres summary buoyant flow beneath oceanic spreading centres can be driven by density gradients due to thermal expansion and compositional gradients arising from the extraction of melt numerical experiments are used to examine the pattern of mantle flow and melting produced by these two sources of buoyancy compositional buoyancy leads to three dimensional flow beneath the spreading axis with along axis variations in upwelling and melt production thermal buoyancy drives axis parallel circulation and convective rolls aligned perpendicular to the spreading axis that originate at a distance off axis controlled by the spreading rate and mantle viscosity hie distance from the axis to the onset of observed sea floor gravity lineations may show a similar dependence on spreading rate compositional density stratification suppresses the formation of thermally driven rolls near the axis and when it is large causes the thermal rolls to become unstable leading to time dependent behaviour the amount of crust produced is less sensitive to spreading rate than in passive flow models however it has a maximum at intermediate spreading rates due to the presence of thermally driven upwelling in the melting region along axis variations in upwelling and melt production suggest that buoyant flow may have an important effect on the segmentation of spreading centres particularly at slow spreading rates

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

seamount volcanism at the super slow spreading southwest indian ridge between 57 and 70 using high resolution bathymetric data of the southwest indian ridge axial valley floor 523 seamounts were identified between the atlantis ii fracture zone 57 and the rodrigues triple junction 70 we find that the seamount population has a much larger spatial density 103 per 10 3 km 2 and shorter characteristic height 48 m to the west of 62 than to the east 8 per 10 3 km 2 and 105 m respectively these densities of seamounts are much lower than those found on the mid atlantic ridge suggesting that magma production is attenuated under the southwest indian ridge compared with faster spreading oceanic ridges we propose that cooler mantle temperatures which induce lower degrees of partial melting and less magmatic supply to the ridge axis result in construction of taller and fewer seamounts between 62 and 70

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n

geophysical evidence for a transform margin offshore western algeria a witness of a subduction transform edge propagator for the first time a deep seismic data set acquired in the frame of the algerian french spiral program provides new insights regarding the origin of the westernmost algerian margin and basin we performed a tomographic inversion of traveltimes along a 100 km long wide angle seismic profile shot over 40 ocean bottom seismometers offshore mostaganem northwestern algeria the resulting velocity model and multichannel seismic reflection profiles show a thin 3 4 km thick oceanic crust the narrow ocean continent transition less than 10 km wide is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with miocene to quaternary sediments this fault system as well as the faults organized in a negative flower structure on the continent side marks a major strike slip fault system the extremely sharp variation of the moho depth up to 45 3 degrees beneath the continental border underscores the absence of continental extension in this area all these features support the hypothesis that this part of the margin from oran to tenes trending n65 n70 degrees e is a fossil subduction transform edge propagator fault vestige of the propagation of the edge of the gibraltar subduction zone during the westward migration of the alboran domain

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

buoyant mantle upwelling and crustal production at oceanic spreading centers on axis segmentation and off axis melting numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers buoyancy results from 1 the depletion of residual mantle in fe relative to mg mantle depletion buoyancy 2 the presence of low density melt melt retention buoyancy and 3 thermal expansion thermal buoyancy thermal buoyancy drives off axis convective rolls that develop closer to the spreading axis at slower spreading rates melt retention buoyancy drives focused upwelling into isolated melting centers with increasing spreading rate the amplitude of on axis crustal segmentation decreases the melting region being essentially two dimensional at full spreading rates greater than 100 km m y these numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations this discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow at fast spreading rates when the lithosphere is thinner melt retention buoyancy gives rise to isolated focused centers of off axis upwelling and melt production off axis melting centers begin to form at about 300 km from the axis with a characteristic along axis spacing of 200 km successive off axis melting centers are shifted along axis so that crustal thickness far from the axis is increased at a 100 km wavelength

seismic and gravity constraints on the nature of the basement in the africa eurasia plate boundary new insights for the geodynamic evolution of the sw iberian margin we present a new classification of geological domains at the africa eurasia plate boundary off sw iberia together with a regional geodynamic reconstruction spanning from the mesozoic extension to the neogene to present day convergence it is based on seismic velocity and density models along a new transect running from the horseshoe to the seine abyssal plains which is combined with previously available geophysical models from the region the basement velocity structure at the seine abyssal plain indicates the presence of a highly heterogeneous thin oceanic crust with local high velocity anomalies possibly representing zones related to the presence of ultramafic rocks the integration of this model with previous ones reveals the presence of three oceanic domains offshore sw iberia 1 the seine abyssal plain domain generated during the first stages of slow seafloor spreading in the ne central atlantic early jurassic 2 the gulf of cadiz domain made of oceanic crust generated in the alpine tethys spreading system between iberia and africa which was coeval with the formation of the seine abyssal plain domain and lasted up to the north atlantic continental breakup late jurassic and 3 the gorringe bank domain made of exhumed mantle rocks which formed during the first stages of north atlantic opening our models suggest that the seine abyssal plain and gulf of cadiz domains are separated by the lineament south strike slip fault whereas the gulf of cadiz and gorringe bank domains appear to be limited by a deep thrust fault located at the center of the horseshoe abyssal plain

spreading rate dependence of gravity anomalies along oceanic transform faults gravity and bathymetry data from transform fault systems associated with mid ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate and fast spreading ridges if confirmed this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid ocean ridges mid ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge slow spreading mid ocean ridge segments exhibit significant crustal thinning towards transform and non transform offsets1 2 3 4 5 6 7 8 9 10 11 12 which is thought to arise from a three dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1 2 4 5 6 7 9 10 12 in contrast fast spreading mid ocean ridges are characterized by smaller segment scale variations in crustal thickness which reflect more uniform mantle upwelling beneath the ridge axis13 14 15 here we present a systematic study of the residual mantle bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate previous studies have shown that slow slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1 2 4 6 but our analysis reveals that intermediate and fast slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments this finding indicates that there is a mass deficit at intermediate and fast slipping transform faults which could reflect increased rock porosity serpentinization of mantle peridotite and or crustal thickening the most negative anomalies correspond to topographic highs flanking the transform faults rather than to transform troughs where deformation is probably focused and porosity and alteration are expected to be greatest indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed this finding in turn suggests that three dimensional magma accretion may occur near intermediate and fast slipping transform faults

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

the relative importance of plate driven and buoyancy driven flow at mid ocean ridges the dynamical interaction between three dimensional 3 d buoyant flow and plate driven mantle flow beneath a mid ocean ridge is examined using a combination of laboratory and numerical experiments in a unique laboratory setup a layer of strongly temperature dependent viscous fluid is heated from below and cooled from above to drive thermal convection forced plate driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface uniform viscosity 3 d numerical models have been designed to simulate the laboratory runs to provide additional information on the flow and to attempt to isolate the effects of variable viscosity in one type of experiment we modeled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis in a second set of experiments the entire base of the tank is heated in order to investigate the interaction between plate driven flow and larger scale upper mantle convection the pattern of segment scale 100 150 km convection beneath a spreading center is found to be a strong function of the spreading rate and the rayleigh number ra of the buoyant flow purely two dimensional 2 d flow exists only in the case of low ra 105 and faster spreading rates 4 6 cm yr half rate three dimensionality is strongly enhanced during transient pulses of upwelling convection on the scale of the upper mantle can contribute significant long wavelength spatial 600 1000 km and temporal 20 40 m y variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume ridge interaction for a given ra the upwelling near the spreading axis can essentially be described by three regimes weakly 3 d at the slow spreading rates strongly 3 d at slow to intermediate rates and 2 d at fast spreading rates the regime boundaries shift toward higher plate velocities with increasing ra comparison of the laboratory and numerical experiments indicates that temperature dependent viscosity may strengthen the position of focused upwelling centers and cause a sharper transition between 2 d and 3 d upwelling patterns taken together these results suggest that buoyancy driven dynamic flow is an important element in the geodynamics of mid ocean ridges

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

constraints on crustal structure of adjacent occs and segment boundaries at 13 n on the mid atlantic ridge the 13 n segment of the mid atlantic ridge is an example of a morphologically well studied slow spreading ridge segment populated with oceanic core complexes occs in this paper we present the results of a 200 km long 2d seismic and gravity transect through this segment the bounding fracture zones to the south and the ridge discontinuity to the north we use this transect to consider the two end member models of occ evolution in which one referred to as the segment scale model implies they are interconnected with their detachments being part of a single segment long feature and the other the localised model that each occ is structurally isolated we show using the 7 5 km s 1 velocity contour as the base of crust marker that the crust is consistently relatively thin ridge parallel at 5 km thick on average and that beneath the occs the moho marks the top of a velocity gradient transition into the mantle rather than a distinct velocity discontinuity although each occ is not traversed in an identical structural location they show a different crustal velocity density structure with depth with along axis variations in this structure mirrored by the bathymetric deeps between them older occs have a contrasting velocity depth signature to the currently active 13 20 n occ the 13 20 n occ is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours while the 13 30 n occ has an apparently thinner crust our combined p wave seismic travel time tomography and gravity forward modelling suggests that the occs of the 13 n segment are not interconnected at depth to the north of the 13 30 n occ our modelling also suggests that the crust is being magmatically refreshed or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge the profile also crosses the marathon and mercurius fracture zones that mark the southern limit of the 13 n segment and the southern ridge transform intersection outside corner along profile marathon fracture zone offsets younger 1 my vs 8 my oceanic crust than mercurius fracture zone 8 my vs 11 my when considered in combination both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of marathon fracture zone coupled with a region of low density that most likely reflects serpentinisation of the uppermost mantle in addition the crust captured between fracture zones appears relatively rotated about an e w axis and uplifted to the north with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre both the outside corner and the crust bounded by fracture zones have velocity depth characteristics similar to that of the 13 n segment occs rather than normally accreted oceanic crust particularly in the upper to middle crust overall our results support the localised model of occ evolution and suggest that fracture zones do not become locked immediately on transform to fracture transition as current models dictate

rates of continental breakup magmatism and seafloor spreading in the norway basin iceland plume interaction 1 in year 2000 an ocean bottom seismometer obs profile was acquired across the more margin to the aegir ridge an extinct seafloor spreading axis the margin is an early eocene volcanic passive margin located between the faeroe iceland ridge fir and the east jan mayen fracture zone ejmfz the p wave data were modeled by ray tracing to give a crustal transect showing a 10 11 km thick igneous crust created by breakup magmatism tapering off to magma starved seafloor spreading by c23 time 51 4 ma the location of the ejmfz was reinterpreted from a satellite derived gravity map and spreading direction in the norway basin reevaluated no other fracture zones were confirmed and both thin oceanic crust 4 5 km and lack of fracture zones resemble ultraslow spreading on the arctic gakkel ridge magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the obs profile and half spreading rates were derived early seafloor spreading was slow 15 32 mm yr 1 approaching ultraslow 6 8 mm yr 1 by c20 time 42 7 ma a v shaped pattern seen in the gravity field located only around the northern part of the aegir ridge corresponds to increased crustal thickness in the seismic model recording northeast transport 3 6 mm yr 1 of more melt fertile asthenosphere zones the magma starved character of the norwegian basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the iceland plume constructed the fir to the south as the asthenosphere is subsequently transported into the norway basin

continental hyperextension mantle exhumation and thin oceanic crust at the continent ocean transition west iberia new insights from wide angle seismic hyperextension of continental crust at the deep galicia rifted margin in the north atlantic has been accommodated by the rotation of continental fault blocks which are underlain by the s reflector an interpreted detachment fault along which exhumed and serpentinized mantle peridotite is observed west of these features the enigmatic peridotite ridge has been inferred to delimit the western extent of the continent ocean transition an outstanding question at this margin is where oceanic crust begins with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies here we present results from a 160 km long wide angle seismic profile western extension 1 travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the s reflector the s reflector correlates with the 6 0 7 0 km s 1 velocity contours corresponding to peridotite serpentinization of 60 30 respectively west of the peridotite ridge shallow and sparse moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer which increases in thickness from 0 5 km at 20 km west of the peridotite ridge to 1 5 km 35 km further west p wave velocities increase smoothly and rapidly below top basement to a depth of 2 8 3 5 km with an average velocity gradient of 1 0 s 1 below this velocities slowly increase toward typical mantle velocities such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth

morphology and crustal structure of the kane fracture zone transverse ridge the kane fracture zone kfz transverse ridge is an anomalously shallow ridge which parallels the kfz for over 200 km east of its intersection with the mid atlantic ridge rift valley sea beam bathymetry and gravity data have been used to determine the morphology and density structure of the ridge and travel time data from two seismic refraction experiments have been modeled to constrain its seismic velocity structure the transverse ridge first appears on older lithosphere opposite the eastern ridge transform intersection it rises to a maximum height of about 2000 m above the bordering kfz trough 40 km east of the intersection area although bathymetry data along the western limb of the kfz are relatively sparse a ridge of similar dimensions is not present the kfz transverse ridge has a decidedly flexural shape in cross section with a high steep south facing wall and a gently sloping northern side the grain of the ridge parallel topography north of the kfz continues undisrupted across the transverse ridge suggesting that the ridge has formed by vertical uplift of this crust seismic ray tracing and gravity analyses indicate that the kfz transverse ridge is not locally compensated by an overthickened crust but is underlain by crustal thicknesses and velocities similar to that of normal oceanic crust on either side of the kfz anomalously thin crust is only present immediately beneath the kfz trough and along the south facing wall of the transverse ridge these constraints and the formation of the kfz transverse ridge on the older plate preclude several previously proposed models for its origin including flexural bending due to differential subsidence across the fracture zone constructional volcanism and serpentine diapirism although no single mechanism provides a completely satisfactory explanation for the origin of the kfz transverse ridge the most likely mechanism appears to be a combination of thermal and viscodynamic forces operating near the ridge transform intersection

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

erosional processes as a control on the structural evolution of an actively deforming fold and thrust belt an example from the pamir tien shan region central asia the foreland of the pamir mountains displays an unusual orogenic topography in which an active north vergent thin skinned fold and thrust belt shows no topographic foredeep in advance of the thrust sheets instead the thrusts are impinging on a topographic barrier formed by the southern edge of the tien shan the deformation front of this thrust belt is coincident with a deeply incised river valley occupied by a large river the surkhob we suggest that this river played a key role in the evolution of this orogenic system by acting as a lateral conveyor belt for the transport of mass delivered to the deformation front by thrusting on the basis of reconnaissance geologic mapping it is clear that the present system developed in the late miocene or pliocene when the tien shan were uplifted within an existing foreland basin as the tien shan highland developed a longitudinal river within the foreland basin was trapped between the north directed thrusts and the tien shan highland with continued deformation the river was squeezed into a narrow canyon and since that time the system has been in an unusual equilibrium where thrusting acts to push the deformation front northward toward the tien shan but fluvial erosion and transport act to keep it in a constant position geologic relationships indicate that the vakhsh thrust the frontal thrust of the north directed system is now emerging on an erosion surface beneath a thin quaternary cover throughout the canyon suggesting that the river has been unable to keep pace with the tectonic mass flux and the thrust front as well as the river have been pushed northward across the beveled edge of what was originally the southern edge of the tien shan we suggest that the original deformation front of the tien shan now lies 10 km south of the present day topographic front of the tien shan beneath the north vergent vakhsh thrust as indicated by 1 a near vertical zone of intense basement seismicity the petrovsky seismic zone 2 an en echelon array of backthrusts along the south flank of the peter the first range and 3 footwall cutoff relationships of the vakhsh thrust system order of magnitude mass balance considerations based on sediment budgets support this conclusion but suggest that the mass imbalance may be small thus large volumes of material may have passed through the thrust fron and been carried out of the system by erosion a relationship that may account for the narrow 30 50 km wide band of deformed sedimentary rock between the pamir salient and the tien shan the present near equilibrium is maintained by long term tectonic transport rates and fluvial transport rates the latter closely linked to climate particularly precipitation thus although there is apparently a mass pile up within this system other systems of this type might show grossly different structural and stratigraphic relationships depending on the amount of rainfall and corresponding variations in rates of fluvial sediment transport similar settings can be recognized in the active thick skinned thrust systems of the tien shan and ancient systems like the laramide belt of western north america

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

thin crust beneath ocean drilling program borehole 735b at the southwest indian ridge abstract a wide angle seismic experiment at the atlantis ii fracture zone southwest indian ridge together with geochemical analyses of dredged basalt glass samples from a site conjugate to ocean drilling program hole 735b has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735b the measured na8 composition of 3 3 0 1 corresponds to a melt thickness of 3 1 km a result consistent with rare earth element inversions which indicate a melt thickness of between 1 5 and 4 5 km the seismic crustal thickness to the north and south of the atlantis platform on which hole 735b is located is 4 1 km and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty beneath hole 735b itself the moho is at a depth of 5 1 km beneath the seafloor the seismic model suggests that on average about 1 km of upper crust has been unroofed on the atlantis platform however allowing for the inferred local unroofing of 2 km of upper crust at 735b the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor and is underlain by a 2 3 km thick layer of serpentinised mantle peridotite the p wave velocity of 6 9 km s for the serpentinised peridotite layer corresponds to a 35 10 vol serpentine content the moho beneath hole 735b probably represents a serpentinisation front

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

crustal structure of the southwest indian ridge at 66 e seismic constraints the southwest indian ridge represents a slow spreading end member of the global mid ocean ridge system and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges we present results from a wide angle seismic experiment conducted at the ridge axis at 66 e in a region that has comprehensive swath bathymetric gravity and magnetic data coverage and where the full spreading rate is 12 mm yr 1 based on these data the experiment traversed four spreading segments crustal thickness and velocity structure were determined along three intersecting profiles each 100 km long using shots from a 10 gun 71 l tuned airgun array towed at 15 m depth and fired at 40 s intervals recorded on three ocean bottom hydrophones on each profile obh data show high amplitude arrivals from oceanic layer 2 lower amplitude arrivals from layer 3 and wide angle reflections from the moho forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1 5 2 5 km thick layer 2 with a high velocity gradient and a 0 5 3 0 km thick layer 3 with a low velocity gradient and a crustal thickness of 2 2 5 4 km additional constraints on the models come from 2 d modelling of gravity data along the profiles corrected for 3 d effects of off line bathymetry along axis the thickness of layer 2 varies little but layer 3 is thick at segment centres and very thin at segment boundaries along a flowline profile crustal thickness varies by up to 75 per cent from its minimum value in 3 myr the reduced crustal thickness is consistent with observations from very slow spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle the large along axis variations in layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation flowline variations in crustal thickness may result from episodicity of melt supply on timescales of 3 myr and by tectonic extension during amagmatic periods velocities at the top of layer 2 are poorly correlated with crustal age based on magnetic anomalies suggesting also that episodicity is decoupled between adjacent segments

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

three dimensional seismic structure of the dragon flag oceanic core complex at the ultraslow spreading southwest indian ridge 49 39 e the southwest indian ridge swir is an ultraslow spreading end member of mid ocean ridge system we use air gun shooting data recorded by ocean bottom seismometers obs and multibeam bathymetry to obtain a detailed three dimensional 3 d p wave tomographic model centered at 49 degrees 39 e near the active hydrothermal dragon flag vent results are presented in the form of a 3 d seismic traveltime inversion over the center and both ends of a ridge segment we show that the crustal thickness defined as the depth to the 7 km s isovelocity contour decreases systematically from the center similar to 7 0 8 0 km toward the segment ends similar to 3 0 4 0 km this variation is dominantly controlled by thickness changes in the lower crustal layer we interpret this variation as due to focusing of the magmatic activity at the segment center the across axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting a locally corrugated oceanic core complex dragon flag occ on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient we infer that this occ may be predominantly made of gabbros we suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections hydrothermal activity at the dragon flag vents is located next to the detachment fault termination we infer that the detachment fault system provides a pathway for hydrothermal convection

dynamic accretion beneath a slow spreading ridge segment iodp hole 1473a and the atlantis bank oceanic core complex 809 deep iodp hole u1473a at atlantis bank swir is 2 2 km from 1 508 m hole 735b and 1 4 from 158 m hole 1105a with mapping it provides the first 3 d view of the upper levels of a 660 km 2 lower crustal batholith it is laterally and vertically zoned representing a complex interplay of cyclic intrusion and ongoing deformation with kilometer scale upward and lateral migration of interstial melt transform wall dives over the gabbro peridotite contact found only evolved gabbro intruded directly into the mantle near the transform there was no high level melt lens rather the gabbros crystallized at depth and then emplaced into the zone of diking by diapiric rise of a crystal mush followed by crystal plastic deformation and faulting the residues to mass balance the crust to a parent melt composition lie at depth below the center of the massif likely near the crust mantle boundary thus basalts erupted to the seafloor from 1 550 mbsf by contrast the mid atlantic ridge lower crust drilled at 23 n and at atlantis massif experienced little high temperature deformation and limited late stage melt transport they contain primitive cumulates and represent direct intrusion storage and crystallization of parental morb in thinner crust below the dike gabbro transition the strong asymmetric spreading of the swir to the south was due to fault capture with the northern rift valley wall faults cutoff by a detachment fault that extended across most of the zone of intrusion this caused rapid migration of the plate boundary to the north while the large majority of the lower crust to spread south unroofing atlantis bank and uplifting it into the rift mountains

the effect of a shallow low viscosity zone on the mantle flow the geoid anomalies and the geoid and depth age relationships at fracture zones summary using a 2 d finite element method we studied the flow that is driven by the horizontal temperature gradient at a fracture zone and calculated the resulting geoid and topography anomalies assuming a three layered viscosity structure for the upper mantle consisting of a conducting lid overlying a low viscosity channel which in turn overlies a unit viscosity layer extending to the base of the upper mantle we studied the effect of varying 1 the viscosity contrast between the fluid layers 2 the rayleigh number based on the viscosity of the bottom layer and 3 the thickness of the low viscosity channel the flow at first downwells immediately adjacent to the fracture zone on the older side its time scale and characteristic wavelength depend on the viscosity and depth of the top layer which is nearest the largest temperature gradient in the fluid after the initiation of the driven flow additional flows are generated away from the fracture zone through shear and thermal coupling and boundarylayer instabilities eventually however the flow extends throughout the fluid so that the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers when the rayleigh number that is based on the viscosity of the top layer and the depth of both fluid layers is less than lo6 the geoid anomalies are dominated by the convective signal when this rayleigh number exceeds lo6 the geoid anomalies retain a step across the fracture zone out to large ages we compared our results with geoid anomalies over the udintsev fracture zone and found that the predicted geoid anomalies are in good agreement with the observed anomalies when the viscosity of at least the top layer is greater than one order of magnitude less than post glacial rebound values 102 pas however we also compared the calculated topographic steps with those predicted by the average depth age relationships observed in the oceans and found that a viscosity contrast of at least one order of magnitude at a depth in the uppermost mantle is required to produce realistic topographic steps

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

nonlinear viscoelasticity and the formation of transverse ridges transverse ridges are regions of anomalously high uplift that run parallel to many of the transform fattits at mid ocean ridges previous models of the formation of these ridges generally fail to explain the magnitude of uphft between 1 and 8 km or account for geophysical observations which indicate that the ridges are dynamically uphfted by neutral or heavy but not buoyant material however the potentially large maxwell relaxation time of the lithosphere and the high strain rates and finite deformation of transform faults imply that nonlinear viscoelasticity may play an important role in the formation of these ridges we present simple theoretical and experimental models of viscoelastic flow beneath a transform fattit to show that the purely horizontal motion of the fattit can generate vertical uplift typical of transverse ridges the theoretical model is of an infinite half space of fluid driven from above by two plates moving parallel and in opposite directions to one another the constitutive relation for the viscoelastic rheology is for a second order fluid i e nonlinear viscoelastic effects are treated as perturbations to newtonian purely viscous flow the theoretical model not only predicts the two transverse ridges that are often observed on either side of the transform fault but the central transform valley that separates them as well for typical hthospheric flexural rigidity and deformation associated with transverse ridges the viscoelastic effect provides sufficient vertical stress to produce the observed uplift further the dynmnic uplift is a purely mechanical effect i e it does not involve buoyant materim which corresponds to geophysical observations the experimental model is comprised of a viscoelastic fluid 2 aqueous solution of high molecular weight carboxymethylcellulose driven by a rotating plate this is analogous to a t ransk rm fattit because the boundary of a spiroting circular tectonic plate surrounded by stationary plates is one continuous transform fattit the experiment produces a large ridge of fluid along the edge of the plate with a shght trough outside of the ridge this result is qualitatively described by a cyhnch ical version of of the above theoretical model in conclusion both experimental and theoretical models show that when nonlinear

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

thin crust on the flanks of the slow spreading southwest indian ridge summary the southwest indian ridge marks an end member of mid ocean ridge spreading rates with a current full rate of only 13 15 mm a most thermal models of mid ocean ridges suggest a decrease in crustal thickness at such slow spreading rates because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression seismic measurements of the thickness of crust formed at very slow spreading ridges are sparse we have reanalysed data from a twoship split spread seismic refraction experiment conducted in 1962 on the southern flank of the southwest indian ridge francis raitt 1967 we used synthetic seismograms to model amplitude variations which were carefully recorded by the original investigators 1 and 2 d modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust with an unusually high velocity of 6 0 km s near the top of the crust increasing to 7 0 km s just above the moho and a crustal thickness of 5 km converted shear waves yield a poisson s ratio of 0 30 0 01 in the crust which is intermediate between values for gabbros and serpentinized upper mantle the crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid ocean ridges the unusual velocity structure may indicate the presence of gabbroic rocks near the seabed unroofed by extension

contrasted surface mass balances of debris free glaciers observed between the southern and the inner parts of the everest region 2007 15 three debris free glaciers with strongly differing annual glaciological glacier wide mass balances mbs are monitored in the everest region central himalaya nepal the mass budget of mera glacier 5 1 km 2 in 2012 located in the southern part of this region was balanced during 2007 15 whereas pokalde 0 1 km 2 in 2011 and west changri nup glaciers 0 9 km 2 in 2013 30 km further north have been losing mass rapidly with annual glacier wide mbs of 0 69 0 28 m w e a 1 2009 15 and 1 24 0 27 m w e a 1 2010 15 respectively an analysis of high elevation meteorological variables reveals that these glaciers are sensitive to precipitation and to occasional severe cyclonic storms originating from the bay of bengal we observe a negative horizontal gradient of annual precipitation in south to north direction across the range 21 mm km 1 i e 2 km 1 this contrasted mass balance pattern over rather short distances is related i to the low maximum elevation of pokalde and west changri nup glaciers resulting in years where their accumulation area ratio is reduced to zero and ii to a steeper vertical gradient of mb for glaciers located in the inner arid part of the range

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

continental breakup and the onset of ultraslow seafloor spreading off flemish cap on the newfoundland rifted margin prestack depth migrated seismic reflection data collected off flemish cap on the newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system within continental crust there is no clear evidence for detachment surfaces analogous to the s reflection off the conjugate galicia bank margin demonstrating a first order asymmetry in final rift development anomalously thin 3 4 km magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized this indicates that initial accretion of the oceanic crust was in a magma limited setting similar to present day ultraslow spreading environments seaward oceanic crust thins to 1 3 km and exhibits an unusual highly reflective layering we propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep marine sheet flows to form this layering subsequent seafloor spreading formed normal 6 km thick oceanic crust this interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow spreading system was established

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

gravity anomalies and crustal thickness variations along the mid atlantic ridge between 33 n and 40 n multibeam bathymetry and gravity data have been obtained along an 800 km long section of the mid atlantic ridge from just south of the hayes fracture zone at 33 n to the northern edge of the azores platform near 40 n a three dimensional analysis of these gravity and topography data combined with results from earlier seismic refraction studies in this area reveal two different scales of crustal heterogeneity a systematic along axis segment scale 8 9 km near the middle of spreading segments and the thinnest crust 3 4 km near segment offsets the magnitude of this along axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset there also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs inferred to be thinner crust beneath the inside corner highs adjacent to these offsets this segment scale crustal heterogeneity is similar to that reported from the kane to atlantis section of the mid atlantic ridge and from parts of the intermediate spreading southern mid atlantic ridge it is superimposed on a second longer wavelength variation in gravity and crustal thickness associated with the azores hot spot the most pronounced effect of the azores hot spot on the mid atlantic ridge occurs between 38 n and 40 n where the ridge axis rapidly shoals by more than 1000 m the crust thickens by over 2 km and the rift valley largely disappears the absence of a deep axial rift valley on the azores platform and near the midpoints of some ridge segments along the mid atlantic ridge to the south agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate the transition from a rift valley to an axial high morphology at these spreading rates 11 mm yr half rate occurs at a crustal thickness of about 9 1km

segmentation and melt supply at the southwest indian ridge the segmentation of mid ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust we present seismic refraction results from three segments of the very slow spreading southwest indian ridge from the absence of oceanic layer 3 beneath segment boundaries and from significant variations in maximum crustal thickness between segments we infer that melt generation and or delivery is focused at segment midpoints and varies between segments anomalously thin crust 2 0 2 5 km beneath the nontransform segment boundaries and 3 5 6 0 km beneath the segment mid points is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

seismic structure of an oceanic core complex at the mid atlantic ridge 22 19 n we present results from a seismic refraction and wide angle experiment surveying an oceanic core complex on the mid atlantic ridge at 22 19 n oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections tomographic inversion of our seismic data yielded lateral variations of p wave velocity within the upper 3 to 4 km of the lithosphere across the median valley a joint modeling procedure of seismic p wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle a gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks caused by un roofing lower crust rocks under the core complex crust is approximately 40 thinner than in the median valley and under the conjugated eastern flank clear pmp reflections turning under the western ridge flank suggest the creation of a moho boundary and hence continuous magmatic accretion during core complex formation

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

earthquakes in the orozco transform zone seismicity source mechanisms and tectonics as part of the rivera ocean seismic experiment a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the orozco transform fault in the central eastern pacific we present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network all epicenters are within the transform region of the orozco fracture zone and clearly delineate the active plate boundary about half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone region 1 most focal depths for these events are very shallow within 4 km of the seafloor several well determined focal depths however are as great as 7 km no shallowing of seismic activity is observed as the rise transform intersection is approached to the contrary the deepest events are within 10 km of the intersection first motion polarities for most of the earthquakes in region 1 are compatible with right lateral strike slip faulting along a nearly vertical plane striking parallel to the spreading direction another zone of activity is observed in the central part of the transform region 2 the apparent horizontal and vertical distribution of activity in this region is more scattered than in the first and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions the effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three dimensional velocity grid while findings for events in region 1 are not significantly affected in region 2 epicentral mislocations of up to 10 km and azimuthal deflections of up to 45 may result from assuming a laterally homogeneous velocity structure when corrected for the effects of lateral heterogeneity the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending nw se the focal depths however are poorly constrained these results suggest an en echelon spreading center or leaky transform regime in the central transform region

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

crustal structure of atlantic fracture zones i the charlie gibbs fracture zone summary a seismic refraction experiment was carried out in 1982 over and in the vicinity of the southern transform valley of the charlie gibbs double fracture zone to measure the rate of crustal thinning normal to the southern transform valley axis three reversed lines were shot with obs two were east west one parallel to the fault on normal crust and one along the fault the third line was north south normal to and intersecting the first two lines travel times were modelled by ray tracing thin 4 km thick anomalously low velocity crust was detected in the fracture zone along the n s line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge transform intersection the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment a gravity profile along the n s line was also analysed the bouguer anomaly was modelled subject to the constraints of the seismic ray tracing after applying corrections for the three dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone the gravity models supported the seismic interpretation and also indicated low densities within the fracture zones two alternative models were obtained for the structure under hecate bank an east west ridge which lies within the crust generated at the short central spreading segment of the cgfz one model features dense material within the crust the other normal but uplifted crust the evidence favours the latter mode but the cause of the uplift is uncertain the uplift may have

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

along and across axis variations in crustal thickness and structure at the mid atlantic ridge at 5 s obtained from wide angle seismic tomography implications for ridge segmentation two end member styles of crustal accretion are observed at two adjacent spreading segments at the mid atlantic ridge at 5 s focused accretion to the segment center with rapid crustal thinning toward the transform in the northern segment and crustal thickening toward the transform at an oceanic core complex in the southern segment our results were obtained by tomographic inversion of wide angle seismic reflection and refraction data collected along three intersecting profiles the segment north of the 5 s fracture zone is characterized by a well developed median valley with a pronounced seafloor bulge in the segment center a discrete portion of anomalously low velocities 0 4 to 0 5 km s relative to average off axis structure at depths of 2 5 km beneath this bulge is possibly related to the presence of elevated temperatures and perhaps small portions of partial melt this suggests that this segment is currently in a magmatically active period which is confirmed by the observation of fresh lava flows and ongoing high temperature hydrothermal activity at the seafloor close to the current spreading axis the crust thins rapidly from 8 5 km beneath the segment center to less than 3 km beneath the transform fault which indicates that melt supply here is strongly focused to the segment center the reduction in crustal thickness is almost exclusively accommodated by the thinning of velocity portions indicative of seismic layer 3 the transform fault is characterized by more uniform velocity gradients throughout the entire crustal section and very low upper mantle velocities of 7 2 7 3 km s indicating that serpentinization could be as much as 25 at 3 5 km depth in contrast 4 1 ma old crust of the northern segment shows only minor thinning from the segment center toward the segment end here the transform is characterized by a normal seismic layer 2 3 transition suggesting robust melt supply to the segment end at those times in the adjacent southern segment the crust thickens from 2 5 km beneath the flank of an oceanic core complex to 5 0 km at the segment boundary the observed changes in crustal thickness show a significant temporal and lateral variability in melt supply and suggest a more complex crustal emplacement process than predicted by models of focused melt supply to the segment centers