oceanic crust when earth was young oceanic crust covers two thirds of earth9s rigid outer layer and this was probably already the case during the archean over 2500 million years ago when heat flow from earth9s interior was much higher than today it is believed that oceanic crust generated in the archean differed from that produced today but evidence has been hard to come by in his perspective karson highlights a report by kusky et al who have found the best candidate for archean oceanic crust to date this ophiolite complex appears to include a complete section of ancient submarine lavas to upper mantle material it may allow some of the ideas regarding archean tectonics and the ancient mid ocean ridge biosphere to be tested

geology the start of the world as we know it plate tectonics has created oceans and pushed up mountain ranges but when did the process that shapes the planet get going alexandra witze joins the geologists debating the issue

faulting and fault scaling on the median valley floor of the trans atlantic geotraverse tag segment 26 n on the mid atlantic ridge a quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high resolution dsl 120 sidescan sonar and coregistered bathymetric data from the tag segment near 26 n on the mid atlantic ridge at this location faulting is exposed within a 3 5 km wide ridge parallel zone lying asymmetrically on the eastern half of the median valley floor mapped faults have a normal sense of displacement are 2 km in length and accommodate 1 5 brittle extension evidence of fault linkage within the fault population includes kinked and bent fault traces in map view the development of overlapping fault segments or relay ramps and the presence of multiple local maxima in the displacement distance profiles of some faults faults have a slight tendency to dip to the east or outward relative to the valley axis and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge parallel line faults exhibit a roughly linear relationship between maximum fault throw and fault length with a mean ratio of 0 030 for the population regression of length frequency data indicates a power law distribution with an exponent of 1 64 1 96 demonstrating that fractal populations can exist in the mid ocean ridge environment the fractal nature of this length frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment where the distribution is exponential and the throw to length ratio is 5 times lower these results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and or fault size

complex structure along a mesozoic sea floor spreading ridge birps deep seismic reflection cape verde abyssal plain summary as part of an intensive study of a small area of oceanic lithosphere the british institutions reflection profiling syndicate birps acquired closely spaced deepseismic reflection profiles over the early cretaceous crust of the cape verde abyssal plain off west africa the survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea floor spreading axis isochron profiles and orthogonal dip lines oriented in the original direction of spreading flow profiles a large capacity well tuned airgun source and very quiet shooting conditions ensured a high signal to noise ratio for deep reflection devising a strategy for mitigating contamination from wrap around multiples arriving from previous shots enabled us to use the minimum possible shot point interval 50 m allowed for collecting long 18 s records data processing was oriented towards a medium with low root mean square velocity steeply dipping structure and pervasive low apparent velocity noise from diffraction at the top of the igneous crust the contrast between the isochron and flow profiles is striking isochron profiles are typically highly reflective throughout the igneous crust consisting of bright bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted moho reflection these reflections do not offset intracrustal or top basement structure and thus are not interpreted as faults an igneous intrusive origin seems more likely flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust continuing down in places to the moho dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust the dominant dip of these reflections is to the west towards the spreading ridge axis reflections from the vicinity of the moho while well developed in some places are not particularly prominent across the survey area moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles on isochron profiles dipping reflections occasionally flatten out into and may merge with the moho reflection on flow profiles as dipping crustal reflections approach the moho reflection they are usually abruptly cut off by it without extending deeper this survey shows how oceanic crustal structure can vary rapidly over relatively small areas provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust and gives a conclusive observation of faults that penetrate the entire oceanic crust

the development of divergent margins insights from the north volcanic zone iceland abstract iceland is a natural laboratory where plate separation and mid ocean ridge formation can be studied directly in the field in this setting it is not fully understood how magma interacts with faulting and fissuring and how rift zones propagate in order to improve the understanding of how these processes operate we conducted a detailed field study about the kinematics and propagation of 33 holocene faults along the n s theistareykir rift north volcanic zone and mapped all tension fractures this rift hosts the holocene theistareykir central volcano for which ground deformation data insar and gps indicate an inflating shallow magma chamber throughout 2006 2008 analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault rift propagation the first more developed is directed northwards and affects the area located north of theistareykir volcano whereas south of the volcano the rift propagates southwards tension fractures increase in frequency with respect to faults outwards from the volcano along a n s to nne ssw direction the presence of aligned vents dykes and topographic bulging along some of the n s structures and the active magma chamber below theistareykir volcano suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin the fact that the rift is more developed north of the volcano than south of it is interpreted as the effect of buttressing from another coterminous magma chamber sitting below a neighbouring central volcano other causes for the general observed pattern such as tectonic stresses mechanical interaction of faults and changes in the rheological characteristics of rocks are discussed as well

slow spreading ridge axis tectonics evidence from the lizard complex uk abstract the east coast section of the lizard ophiolite complex cornwall u k preserves a series of interrelated extensional faults and dyke sets which indicate that the formation of this section of obducted oceanic lithosphere resulted from a combination of contemporaneous structural and magmatic events three dolerite dyke sets are evident the first two sets 1 and 2 are cut by and rotated on a series of ridge parallel extensional faults and shear zones later dykes set 3 cross cut the extensional faults these data indicate that both rotation and significant thinning in the region of 40 of this part of the oceanic lithosphere occurred prior to the final period of dyke injection the thinning of the lithosphere was the result of extension along the normal faults all of the dolerite dykes show morb characteristics although the late dykes set 3 are more primitive tending to be aphyric to sparse olivine phyric and show higher cr ni and lower zr values than the typically plagioclase phyric dolerite dykes sets 1 and 2 intruded prior to the ductile shearing event the more primitive dolerites post date the tectonism and indicate the lack of a significant magma chamber following a period of amagmatic extension the data are consistent with observations at present day slow spreading ridge axes and some mafic ultramafic ophiolite sequences and provide insight into the nature of ridge related structures and the interrelationships between tectonism and geochemical evolution of the dykes and record a mechanism listric faulting to account for the presence of peridotites and gabbros commonly dredged at slow spreading centres

structure of tertiary and pleistocene normal faults in iceland normal faults were measured in three selected areas of the late tertiary and pleistocene lava pile in iceland the arithmetic mean throw of 315 faults is 8 2 m the minimum being 0 5 m and the maximum 150 m about 93 of the faults have throws of less than 20 m the mean strike of the faults is n31 e but the mode is n45 e the faults dip from 42 to 89 nearly 80 dip between 65 and 79 and the mean dip is 73 fault breccia is common and varies in thickness from 0 05 m to 2 m there is a positive linear relation between breccia thickness and throw the linear correlation coefficient being 0 78 it is proposed that the normal faults in iceland form by two basic mechanisms one is the development of faults from sets of inclined joints in the lava pile the initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1 for every 150 170 m depth the inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints the other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures the great majority of fractures within the active rift zone are tension fractures but when they attain the length of several hundred meters they normally change into normal faults it is concluded that normal fault development in iceland commonly starts at depths of about 1 km in the crust

morphology and genesis of slow spreading ridges seabed scattering and seismic imaging within the oceanic crust a grid of 32 across axis and five axis parallel multichannel seismic mcs reflection profiles were acquired at an axial volcanic ridge avr segment at 57 45 n 32 35 w on the slow spreading reykjanes ridge mid atlantic ridge to determine the along axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure the along axis continuity and segmentation of melt supply to the crust the development of faulting and the thickness of oceanic layer 2a seismic reflection profiles acquired at mid ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events in this paper we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed mcs profiles the 37 mcs profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2 to the axial magmatic system and to the faults which dismember each avr as it ages through its tectono magmatic life cycle and which form the median valley walls the layer 2a event can be mapped around the entirety of the survey area between 0 1 and 0 5 s two way traveltime below the seabed being thickest at avr centres and thinning both off axis and along axis towards avr tips both avr parallel and ridge parallel trends are observed with the pattern of on axis layer 2a thickness variation preserved beneath relict avrs which are rafted off axis largely intact each active avr is underlain by a mid crustal melt lens reflection extending almost along its entire length similar reflection events are observed beneath the offset basins between adjacent avrs these are interpreted as new avrs at the start of their life cycle developing centrally within the median valley the east west spacings of relict avrs and offset basins is 5 7 km corresponding to a life span of the order of 0 5 0 7 myr during which avrs appear to undergo multiple 20 60 kyr tectono magmatic cycles

causes of variation in fault facing direction on the ocean floor a marked variation in fault facing direction with spreading rate is observed on the flanks of mid ocean ridges on slow and intermediate rate spreading centers inward facing faults predominate whereas at fast spreading centers large numbers of both inward and outward facing faults are observed the distribution and character of inward and outward dipping faults derived from seamarc ii data from the flanks of the east pacific rise and galapagos spreading center are compared with observations from previously published data we suggest that the fault facing direction variation observed may reflect relative differences in the mean stress on potential fault planes through the oceanic lithosphere which might favor inward vs outward facing faults at different spreading rates for example due to the thermal structure of a slow spreading ridge a fault dipping toward the axis extends to shallower depths than one dipping away from the axis thus favoring the occurrence of inward dipping faults on a fast spreading ridge the difference in fault length is less so both inward and outward facing faults are likely to occur

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

discrete characterization of abyssal hills unraveling temporal variations in crustal accretion processes at the 10 30 n segment east pacific rise abstract the morphology of abyssal hills provides important information on the crustal accretion processes acting at mid ocean ridges discretely characterizing abyssal hill shape is challenging and therefore the study of abyssal hill morphology is often approached by examining the average properties of relatively wide regions containing multiple hills however this averaging process removes any spatial trends in abyssal hill shape that may reflect temporal variations in crustal accretion processes here we present an automated approach for analyzing the shape of individual abyssal hills which we detect using the ridgelet transform method of downey and clayton 2007 we first analyze seafloor morphologies as revealed in multibeam bathymetry surveys across 16 mid ocean ridge segments with varying spreading rates the results of this analysis display the known negative correlation between the width and height of abyssal hills and the spreading rate thus validating our approach our results also show that the fraction of inward facing abyssal hills exhibits a simple linear trend across all spreading rates suggesting a property that could be used to determine the spreading rate at which the seafloor formed we then apply our technique to study a flow line transect collected across the fast spreading 10 30 n segment of the northern east pacific rise revealing temporal changes in the shape of the abyssal hills formed during the last 3 8 myr the youngest part of the flow line coincides with a location where basaltic glasses were previously sampled the abyssal hill morphology and mgo content of the glass samples share mutual trends over 105 106 yrs suggesting that abyssal hill morphology is sensitive to the rate at which magma is supplied to the ridge axis finally a 1 myr cyclic variation in seafloor morphology is observed this periodicity is attributed to temporal changes in magma supply at the ridge axis possibly related to upper mantle dynamics or the presence of chemical heterogeneities

models of fracture orientation at oblique spreading centres three models are proposed for the orientation of extensional faults and dykes at mid ocean ridges based on their relationship to the ridge axis and the relative plate separation vector these models predict four ridge geometries allowing them to be tested by orientation data from 17 different sites within the earth9s oceans a transtensional model is shown to be generally applicable to the structure of mid ocean ridges with special conditions required for a departure from it spreading rate influences ridge geometry with intermediate and fast spreading ridges more likely to have an orthogonal spreading geometry which is an end member case of the transtensional model

madagascar s escape from africa a high resolution plate reconstruction for the western somali basin and implications for supercontinent dispersal accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models here we provide a new plate tectonic reconstruction of the opening of the western somali basin during the breakup of east and west gondwana the model is constrained by a new comprehensive set of spreading lineaments detected in this heavily sedimented basin using a novel technique based on directional derivatives of free air gravity anomalies vertical gravity gradient and free air gravity anomaly maps also enable the detection of extinct mid ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the western somali basin the best matching interpretations have basin symmetry around the m0 anomaly these are then used to temporally constrain our plate tectonic reconstruction the reconstruction supports a tight fit for gondwana fragments prior to breakup and predicts that the continent ocean transform margin lies along the rovuma basin not along the davie fracture zone dfz as commonly thought according to our reconstruction the dfz represents a major ocean ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as madagascar drifted southwards and offshore tanzania is an obliquely rifted rather than transform margin new seismic reflection evidence for oceanic crust inboard of the dfz strongly supports these conclusions our results provide important new constraints on the still enigmatic driving mechanism of continental rifting the nature of the lithosphere in the western somali basin and its resource potential

seismic structure of the southern cascadia subduction zone and accretionary prism north of the mendocino triple junction four multichannel seismic reflection profiles collected as part of the mendocino triple junction seismic experiment image the toe of the southern cascadia accretionary prism today 250 600 m of sediment is subducting with the gorda plate and 1500 3200 m is accreting to the northern california margin faults imaged west and east of the deformation front show mixed structural vergence a north south trending 20 km long portion of the central margin is landward vergent for the outer 6 8 km of the toe of the prism this region of landward vergence exhibits no frontal thrust is unusually steep and narrow and is likely caused by a seaward dipping backstop close to the deformation front the lack of margin wide preferred seaward vergence and wedge taper analysis suggests the prism has low basal shear stress the three southern lines image wedge shaped fragments of oceanic crust 1 1 7 3 km in width and 250 700 m thick near the deformation front these wedges suggest shortening and thickening of the upper oceanic crust discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans the southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the gorda basin oceanic crust and overlying sediments these high angle faults bounding basement highs are interpreted as strike slip faults reactivating structures originally formed at the spreading ridge northeast or northwest trending strike slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north south gorda pacific plate convergence

extensional faulting and caldera collapse in the axial region of fast spreading ridges analog modeling the axial high of the east pacific rise epr is bounded by ridge parallel lateral grabens that develop 2 8 km off axis these troughs appear to lengthen away from the ridge crest suggesting that tectonics is active at least 10 km away from the axis along 15 20 of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide these large axial troughs represent elongated collapse calderas that form when the epr magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply under continuous stretching we report the results of analog experiments performed in order to constrain the tectonic magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region we used inflatable elongated balloons filled with water as an analog of the magma reservoir the balloon is capped with a silicone layer representing hot rocks below the brittle ductile transition and is covered by a sand layer representing the brittle crust the sand surface was given a dome shape that approximates the morphology of a fast spreading ridge mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension combination of deflation and extension leads to the creation of two lateral depressions and one axial trough the lateral depressions are controlled by normal faults while the central trough is delineated by reverse faults during balloon deflation tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust this accounts 1 for the development of axial troughs as collapse calderas and 2 for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush

deformation processes at fast to ultra fast oceanic spreading axes mechanical approach abstract the morphology of fast to ultra fast oceanic spreading ridges such as the east pacific rise epr is characterized by an axial dome 5 10 km wide culminating at 300 500 m above the surrounding seafloor this dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis a large summit trough 200 m to 2 km wide locally notches the axial high only where the dome is inflated indicative of a time average robust magma supply this summit trough is thought to represent an elongated axial summit caldera asc created as a result of the subsidence of the top of the axial magma chamber amc such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension analog experiments using small scale modeling have been performed in order to better constrain the tectonic evolution of the axial region the experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table it is capped with a silicone layer representing hot rocks below the brittle ductile transition and is covered by a sand layer representing the brittle crust the experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution extension without withdrawal withdrawal without extension withdrawal and synchronous extension we show that the morphology of the epr axis can be considered as the result of both horizontal and vertical movements two symmetrical lateral grabens develop on both sides of a non deformed axial dome when single extension is applied to a model with a thin silicone layer normal faults of the lateral grabens are rooted on two divergent velocity zones dvzs located on the edges of the groove this situation is regarded as an analog of the natural case where the top of the amc acts as a stress free boundary that fails to transmit the extensional stresses to the upper brittle layer an important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults during synchronous extension and withdrawal the initiation of the lateral grabens is favored by a balloon deflation even if such deflation is unable to generate a superficial collapse this last case is considered as representative of the evolution of epr segments showing little variations in melt supply into the amc higher deflation rates under continuous extension correspond to epr segments undergoing strong variations in melt supply in such experiments the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation finally we show that dvzs located at the brittle ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra fast spreading ridges the distance from axis and the width of the dvzs directly control the location and the distribution of the lateral grabens

segmentation and morphotectonic variations of the central indian ridge 21 10 s 22 25 s bathymetric magnetic and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the central indian ridge cir between 21 10 s and 22 25 s volcanoes abyssal hills and steep slopes corresponding to fault scarps and to volcanic constructional slopes have been obtained by image processing methods applied to sea beam data in the survey area segmentation of the cir is similar to that of a slow spreading ridge the plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions hourglass shaped valleys are bounded by second and third order discontinuities located at local along axis depth maxima 400 700 m depth variations and marked by oblique extensional basins and en echelon jogs respectively large throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places along the axial valley the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs the morphology and structure of the rift valley reflect the three dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center abyssal hills are bounded by both inward and outward facing faults in 1 5 ma crust indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20 40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills reconstructions of the plate boundary geometry through time using abyssal hills as isochrons provide evidence to show that the nontransform discontinuity located at 21 47 s with an offset as small as 2 3 km is associated with v shaped off axis traces in the form of alignments of disrupted abyssal hill lineations these reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery three dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0 780 and 0 984 ma and suggests postaccretionary crustal block rotations within the discordant zones

intraplate deformation of oceanic crust near the rio grande rise in the south atlantic abstract globally oceanic crust has relatively uniform thickness and structure compared to continental crust with typical ocean crust being 5 8 km thick however a substantial proportion of oceanic crust is thickened outside the normal range in the form of oceanic plateaus and crust affected by hotspots the rio grande rise rgr is the most prominent bathymetric feature in the western south atlantic ocean and was thickened by activity of the tristan hotspot we use multichannel seismic data collected along the eastern margin of the rgr to investigate the relationship history between the thickened crust of the rgr and the adjacent normal crust we find numerous normal faults spatially coincident with the region where oceanic crust begins to thicken from normal to rgr crust using crosscutting relationships and superposition our interpretation supports protracted extensional deformation at the eastern margin of the rgr including recent activity that appears to deform the modern seafloor we attribute the faulting to differential subsidence linked to crustal thickness variations between the thickened rgr and normal oceanic crust given the fundamental processes we attribute to the cause of faulting and the proportion of thickened oceanic crust globally it is possible that regions of many oceanic plates experience intraplate deformation after formation

mechanisms of normal fault development at mid ocean ridges 1 slow spreading ridge segments are characterized not only by small closely spaced faults that develop near the segment center but also by large widely spaced faults that develop near the segment ends typically at the inside corner of a ridge offset intersection in this study we investigate the competing effects of stress accumulation in the lithosphere and the yield strength of the lithosphere in controlling the location of normal fault formation and direction of propagation seismic velocity models from the mid atlantic ridge in the oceanographer hayes region and 29 n and the east pacific rise at 9 n were used to estimate the along axis change in dynamic young s modulus corresponding thermal and rheologic models were calculated to estimate the along axis variation in yield strength we then develop a thin plate model to calculate the predicted location of fault initiation or reactivation and the subsequent propagation direction for different combinations of linear along axis gradients in young s modulus and yield strength on the basis of this model we define two modes of normal fault development at slow spreading segments mode c center faults which develop at the segment center and propagate outward and mode e end faults which develop at the segment ends and propagate inward mode c faults are predicted to form at ridges where the along axis variation in yield strength dominates the along axis accumulation of stress conversely mode e faults are predicted to develop at ridges where stress accumulation toward segment ends overcomes the high yield strength in these locations in addition to the accumulation of stress caused by along axis gradients in young s modulus we illustrate that shear stresses resisting relative plate motion along a transform fault will generate higher effective stress at inside corners possibly concentrating mode e faulting in these locations at fast spreading ridges where along axis gradients in stress and lithospheric strength are relatively small more uniform patterns of faulting are predicted the results of this study quantify how the interplay between the along axis variations in stress state and the mechanical properties of the lithosphere controls the style of fault development at mid ocean ridge segments

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

paleomagnetism of tilted dikes in fast spread oceanic crust exposed in the hess deep rift implications for spreading and rift propagation tilted dikes occur within the pillow lava sheeted dike and gabbroic sections of 1 million year old fast spread oceanic crust exposed on the walls of the hess deep rift during recent alvin dives oriented samples from both tilted and vertical dikes were collected from several of the outcrops these are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass remanent magnetic inclinations measured on the dike samples are significantly different from the near horizontal inclinations expected the present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the alvin study areas may be only a local phenomenon or at least not a pervasive characteristic of oceanic crust although tectonic rotations such as those observed may have taken place as a result of the opening of the hess deep rift crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the east pacific rise axis probably as a result of intermittent axial collapse the observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the hess deep rift implications for axial processes at the east pacific rise 1 studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers we present paleomagnetic remanence data from 45 fully oriented samples from dikes gabbros and a small number of basaltic lavas from fast spread crust exposed along the hess deep rift over 25 km along this escarpment dikes and dike subparallel fault zones dip consistently away from the east pacific rise epr while lava flows dip toward the ridge underlying gabbro is less deformed but contains widely spaced low angle fractures tentatively interpreted as shear zones as expected from the crustal age 1 07 1 48 ma most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site overly steep and directionally scattered gabbro remanence and observed low angle shear structures within this unit are tentatively interpreted as the manifestation of three dimensional strain along anastomosing shear zones although some remanence directions are incompatible with any plausible deformation history and thus likely reflect orientation errors the overall data set is consistent with a model involving sequential rotations on 1 outward dipping epr parallel n s normal faults and 2 hess deep rift parallel e w normal faults average rotations for these sequential events are 22 to the east defined by the mean dike attitude and 10 to the south estimated by bathymetry respectively this model best explains the remanence data observed dikes and lava orientations presence of dike parallel fault zones and the observation of steep little deformed dikes cutting both east dipping dikes and faults the data support a structural model for spreading at the epr in which outcrop scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis because these faults form within the neovolcanic zone they are subject to burial and are expected to have subdued to little surface expression

thermal constriction and slab tearing at the origin of a superinflated spreading ridge marsili volcano tyrrhenian sea 1 recently acquired swath bathymetry of the marsili basin has offered an unprecedented opportunity to study the processes of back arc ocean basin development in the tyrrhenian sea in particular the detailed morphology of marsili seamount a large strongly elongated volcano located in an axial position within the 2 ma ocean crust floored marsili basin is a key to understanding the mechanisms governing lithosphere formation in this young basin the basin is near circular in shape with diameter on the order of 120 km and is positioned in the southern tyrrhenian sea above the steeply dipping ionian oceanic slab of mesozoic age it is bounded southward by the aeolian volcanic arc and the calabrian accretionary wedge surface evidence of the northwesterly directed subduction the most outstanding feature of the basin is the elongated 3000 m high marsili volcano which reveals distinctive morphology strikingly akin to the high order segmentation and volcanic landforms described in mid ocean slow spreading ridges on the basis of its distinctive morphology and incremental growth relationship we propose that marsili volcano represents a superinflated spreading ridge resulting from a distinct thermal pulse of increased melt production occurring within the young and immature marsili basin surrounding cooler continental lithosphere thermally constricts ridge propagation and crust production in marsili basin to the finite scale of marsili volcano increased melt production to feed the superinflated marsili ridge is generated by deep lateral asthenospheric mantle flow produced at the edges of tears that bound the subducting ocean crust of the ionian plate slow spreading plate separation outpaced by the increase in magma generation results in vertical accretion to produce the superinflated ridge the existence of dip directed tears delimiting the narrow ionian slab is supported by the geological evolution of the surrounding foreland and apennine maghrebid mountain belt during early middle pleistocene i e the time of formation of the marsili volcano present day structure and volcanism furnish direct and indirect surface evidence of the presence and location of the slab tears

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

the relationships between late ordovician sea level changes and faunal turnover in western baltica geochemical evidence of oxic and dysoxic bottom water conditions abstract the late sandbian to the early katian late ordovician in southeastern norway is dominated by marine mudstones that contain an exceptional spectrum of macrofauna and trace element geochemistry recording both abrupt and gradual faunal changes during major sea level and environmental shifts this study investigates the relationships between sea level changes and their influence on the source of clastic material oxygen levels in the bottom water and faunal changes with special emphasis on the brachiopod fauna trace element ratios indicate that nearly stable upper dysoxic bottom water conditions prevailed in the northwestern part of the baltoscandian sea the exception is during the major shallowing in the earliest katian when there was an abrupt shift to oxic conditions as the sea bottom came within a normal storm wave base nonetheless the pre shallowing epibenthic fauna though not the shelly endobenthic in this area was rich and diverse and two major immigration phases of new brachiopod taxa are seen well before the shallowing this indicates that the immigration of new taxa was not the result of an increase in oxygen content in the bottom water more brachiopod genera stayed during the abrupt shallowing and increased oxic level than did during the following gradual return to deeper dysoxic environments the major brachiopod immigration phases took place markedly earlier in this northwestern part of the baltoscandian sea than in the central part eastern baltic and possibly also the comparable faunal turnover in laurentia the following disappearance of taxa during an early katian transgressive event coincided with the faunal turnover in the shallow water environments of the east baltic the depositional history was consistent within the investigated mudstone dominated offshore facies of the oslo region which are comparable to the central baltoscandian confacies belt of baltoscandia the composition of the siliciclastic material which was deposited in the basin was nearly constant through the sequence though the land area expanded to include ophiolites expressed by cr enrichment coinciding with the major sea level drop the cr enrichment indicates that by that time at least the ophiolite complexes north of the oslo region became subaerially exposed this enrichment thus forms a potential geochemical marker horizon representing the basal katian stage

how to identify oceanic crust evidence for a complex break up in the mozambique channel off east africa abstract the identification of oceanic crust at rifted margins plays a crucial role in academic research understanding rifting mechanisms and the architecture of continent ocean boundaries and is also important for hydrocarbon exploration extending into deeper water in this paper we provide a workflow for the determination of the crustal nature in the mozambique channel east of davie ridge by presenting a compilation of several geophysical attributes of oceanic crust at divergent margins previous reconstructions locate the davie ridge at the trace of a transform fault along which madagascar drifted to the south during the breakup of gondwana this implies a sharp transition from continental to oceanic crust seaward of davie ridge using new multichannel seismic profiles offshore northern mozambique we are able to identify distinct portions of stretched basement east of davie ridge two phases of deformation affecting the basement are observed with the initial phase resulting in the formation of rotated fault blocks bounded by listric faults half grabens are filled with wedge shaped syn extensional sediments overlain by a prominent unconformity that northward merges with the top of highly reflective mildly deformed basement interpreted as oceanic crust the second phase of deformation is associated with wrench faulting and probably correlates with the southward drift of madagascar which implies that the preceding phase affected basement generated or modified prior to the opening of the west somali basin we conclude that the basement is unlikely to consist of normal oceanic crust and suggest that the first extensional phase corresponds to rifting between madagascar and africa we find evidence for a wide area affected by strike slip deformation in contrast to the earlier proposed major single transform fault in the vicinity of davie ridge and suggest that the mozambique channel area to the north of madagascar may be classified as an oblique rather than sheared margin

numerical models of faulting at oblique spreading centers obliquely spreading mid ocean ridges such as the reykjanes ridge display two distinct fault sets distinguishable by orientation and position on axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector while faults on the flanks strike approximately parallel to the ridge axis numerical modeling techniques are used here to simulate the development of faulting on the reykjanes ridge stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast lagrangian analysis of continua flac explicit difference modeling software the predicted stresses from the cross sectional models are imposed as a condition in boundary element models of fracture propagation and linkage on axis fault simulations run under conditions similar to the reykjanes ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges simulations of fractures away from the axis show the development of axis parallel faults by the interaction and linkage of fractures which have been rafted off axis also in accord with observations stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis

quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment mid atlantic ridge 29 n high resolution deep towed side scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading mid atlantic ridge near 29 n sonar data allow us to identify individual fault scarps to measure fault widths and spacing and to calculate horizontal fault displacements heave and tectonic strain we find that over long periods of time 1 myr on average tectonic strain is 10 on average and does not vary significantly along axis there is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment indicated by 50 lower tectonic strain on the east flank than on the west flank these variations in tectonic strain do not correlate directly with changes in fault spacing and heave fault spacing and heave increase from the center of the segment toward the end inside corner on the west flank and from the outside to the inside corner across the axis these parameters remain relatively constant along the segment on the east flank and across the axis at the segment center tectonic strain appears to be decoupled from magmatic accretion at timescales 1 myr as the decrease in magma supply from the segment center toward the end inferred from variations in crustal thickness along the axis is not correlated with a complementary increase in tectonic strain instead tectonic strain remains relatively constant along the axis at 7 on the east flank and at 15 on the west flank these results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis

u pb zircon age constraints on the dongwanzi ultramafic mafic body north china confirm it is not an archean ophiolite abstract in science may 11 2001 p 1142 1145 kusky et al reported an ophiolite complex of late archean age in the dongwanzi area of the north china craton which has subsequently been considered a hallmark for archean plate tectonics however this interpretation has been questioned because no convincing pillow lavas or sheeted dikes have been found in the dongwanzi complex and more importantly as a major part of the ophiolite complex the dongwanzi ultramafic mafic body has not previously been dated and it therefore remains unknown whether it is of archean age in this study we carried out detailed field investigations and shrimp u pb zircon dating on the rocks of the dongwanzi ultramafic mafic body as recognized by kusky et al our field observations reveal that in several places along the boundaries of the dongwanzi body mafic dikes emanating from the main body intrude the country rocks of the unmetamorphosed mesoproterozoic sedimentary successions of the changcheng jixian system one of these dikes yielded a shrimp u pb zircon age of 306 6 ma similar to the shrimp u pb zircon age of 308 4 ma obtained from a plagioclase bearing pyroxenite from the dongwanzi ultramafic mafic body four other samples collected from various parts of the ultramafic mafic body gabbro leucogabbro or pyroxenite record similar shrimp u pb zircon ages around 300 ma therefore both field relationships and shrimp u pb zircon dating results unequivocally demonstrate that the dongwanzi ultramafic mafic body is not part of an archean ophiolite

relationship between polarity of extensional fault arrays and presence of detachments abstract extensional fault arrays are often dominated by a single fault vergence forming domino fault blocks the polarity of faults can be defined as synthetic or antithetic in relation to the overall shear sense across the fault system observations of the geometry of a number of field examples of extensional fault arrays vs the lithologies as a proxy for strength profile show that synthetic extensional arrays from the north sea and bristol channel basin detach on salt layers whereas examples of antithetic arrays from the north sea are found to pin out downwards and no basal detachment is present in the sacramento mountains core complex of the basin and range the major low angle detachment fault post dates antithetic shears that evolved at mid crustal level and are preserved in the granitic footwall these antithetic shears were cut by the main low angle detachment whose hanging wall disintegrated into an array of synthetic faults the kinematics of synthetic vs antithetic arrays dictate that faults within synthetic arrays must branch onto a basal detachment whereas faults within antithetic arrays may die out downwards therefore the results emphasise that fault polarity in domino arrays may be related to the boundary conditions of the fault blocks and therefore may be diagnostic of the strength profile of the faulted stratigraphy

quantitative fault studies on the east pacific rise a comparison of sonar imaging techniques a synthesis is presented of published statistics on the lengths scarp heights and spacing of normal faults on the flanks of the east pacific rise epr between 19 s and 21 n nine data sets are included which were collected at different times employing one of four different sonar mapping systems the scripps deep tow instrument seamarc i seamarc ii and gloria with widely ranging data collection capabilities the epr in this region is morphologically quite similar and in some cases the data sets were obtained from the same area of the seafloor it is assumed therefore that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes we try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made we discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters we also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution the theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution we also discuss the effects that surveying geometry can have on the interpretation of fault lengths the data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3 5 s and 12 n on the epr the correlation which we find to be linear is used to estimate 1 the minimum operational fault target size for each of the four mapping systems and 2 the topography due to faulting in an area where only fault lengths are known

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

faulting and volcanism in the axial valley of the slow spreading center of the mariana back arc basin from wadatsumi side scan sonar images 1 we analyzed in detail the geology of the median valley floor of the mariana basin slow spreading ridge using sea surface geophysical data and a high resolution deep tow side scan sonar survey over one spreading segment analysis of surface magnetic data indicates highly asymmetric accretion with the half spreading rate on the western side of the basin being two to three times larger than on the eastern side surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of 10 km of amplitude deep tow backscatter data indicate along axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end respectively this variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end concerning tectonic processes we find a power law distribution of the fractures with an exponent of 1 74 this suggests that within the inner valley floor fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8 of the strain according to our calculation based on a ratio of 0 02 to 0 03 between the vertical displacement and the length of faults the amount of tectonic strain accommodated in the inner valley floor would consistently be 1 1 3 4 data also show two distinct sets of fractures one trend is parallel to the rift direction at the segment center n160 e and perpendicular to the plate separation direction another set trends 17 oblique to this direction n175 e and is located over the eastern part of the valley in the vicinity of a major bounding fault also trending n175 e that is obliquely to the direction of plate motion we modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three dimensional boundary element approach we found that the orientation of the predicted fissuring near the oblique fault is locally rotated by 15 due to a flexure of the bending plate close to this fault

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

geometry of extensional faults developed at slow spreading centres from pre stack depth migration of seismic reflection data in the central atlantic canary basin we present depth images from portions of profiles that are close to flow lines of cretaceous oceanic crust in the eastern central atlantic compared with post stack time migrations the images illustrate the improvement resulting from the application of pre stack depth migration the images document the scale and geometry of normal faulting in oceanic crust formed over 25 myr at a half spreading rate of less than 10 mm yr 1 and the variation in extensional style with position within the spreading segment away from major fault zones fzs most faults are subplanar dip more than 35 are associated with moderate basement relief 0 2 1 km relief and may penetrate to deep crustal levels these faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains also observed away from fzs are gently dipping to subhorizontal reflections in the upper crust which resemble detachment faults in contrast the inside corner crust is more rugged with basement highs rising up to 2 km above the intervening basins this larger scale topography is associated with a different style of faulting the depth images reveal gently dipping 35 faults that are rooted in the deep crust and that project to the ridgeward flank of the dome shaped large basement highs 1 2 km vertical relief the faults seem to continue as the ridge facing flank of these highs and some may extend over the crest of the high to breakaways beyond in this case the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes these controlling faults are up to 20 km long and have a heave of 10 km sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks we suggest that similar faults can explain the structure and lithologies found at megamullion structures oceanic core complexes at inside corners near the present day spreading ridge

on origin of near axis volcanism and faulting at fast spreading mid ocean ridges at fast and superfast spreading mid ocean ridges such as the east pacific rise a plate boundary is defined by a narrow tens to hundreds of meters wide neovolcanic zone within which the bulk of the upper oceanic crust is created however detailed near bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge axis it has been proposed that off axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis or spatial migration of magmatic sources in the crust and upper mantle we demonstrate that off axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space and the magma delivery is perfectly focussed at the ridge axis theoretical modeling and field observations indicate that off axis volcanism may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills arguably the most abundant relief form on earth fl 2001 elsevier science b v all rights reserved

a one million year history of the 11 45 n east pacific rise discontinuity bathymetric magnetic and side scan sonar data from a june 1987 seamarc ii cruise aboard the r v moana wave have been combined with additional data in an area 140 km by 154 km centered on the overlapping spreading center pair osc located at 11 45 n on the east pacific rise epr the purpose of this study is to correlate and interpret these data sets in order to determine the recent 1 m y history of this osc the data sets include gridded bathymetry and magnetic field anomalies a side scan sonar mosaic and a fault map the bathymetry and anomaly grids were used as inputs in a three dimensional fourier inversion resulting in a crustal rock magnetization distribution solution the most prominent feature of all five maps is a roughly v shaped anomalous zone centered on the osc the zone is characterized by a positive depth anomaly a negative magnetic field anomaly within the central positive anomaly and a zone of high magnetization in the inversion solution a hummock y texture and sparse curved faults mark the zone in the side scan mosaic and fault map the shape of this zone indicates a short period of northward migration of the discontinuity 0 72 0 6 ma at approximately 75 mm yr followed by slow less than 30 mm yr and then fast 140 200 mm yr southward migration for the past 0 6 m y the anomalous zone is not well defined beyond the edge of the central anomaly which is straight and continuous indicating that if this discontinuity of the epr existed 0 72 ma its offset was negligible the present day axis has an orientation of 349 the edge of the central anomaly has an orientation of 353 and jaramillo age crust to the east of the epr has an orientation of 353 355 this variation in magnetic linealion directions suggests a 4 to 6 counter clockwise change in the direction of spreading in the past 1 m y which seems to have been accommodated by asymmetric spreading variations in mean fault trend with age are consistent with a counter clockwise change in the direction of spreading for the same time period

accretional curvature of lithosphere at magmatic spreading centers and the flexural support of axial highs fluid magma should tend to rise up to the level of local isostatic equilibrium at an axis of plate spreading the magma and underlying partial melt accretes to the side of the separating plates and gets denser as it cools and freezes this load bends down the lithosphere and additional accretion forms a plate with concave upward curvature the topography resulting from a curved plate passively moving off axis can be described analytically for accretion on the vertical side of uniform lithosphere when its flexural response is that of a thin elastic plate model predictions are consistent with the shape of the axial highs at fast spreading centers typically 400m high and 20 km wide as well as related gravity anomalies this model requires at least several kilometers of lithospheric thickening within a few kilometers of the ridge axis seismic data appear consistent with this lithospheric structure the magnitude and lateral distribution of faulting on the flanks of axial highs is crudely consistent with formation during plate unbending as lithosphere moves away from the axis according to this model abandoned fast spreading centers should retain an elastically supported axial high while for previous models abandoned highs should collapse

bathymetric gradients of lineated abyssal hills inferring seafloor spreading vectors and a new model for hills formed at ultra fast rates abstract abyssal hill morphology provides a preliminary measure of the direction and rate of seafloor spreading however additional information e g magnetic anomaly data or a nearby mid ocean ridge is usually required to verify these estimates previous attempts to identify a unique spreading rate proxy from abyssal hill dimensions e g height length width have largely failed due to the relatively large scatter of data or the non linear character of spreading rate trends we present a new stand alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis the spreading rate is determined with the peak height parameter defined as the difference in the height maximum frequency of the two dominant modes observed in the azimuthal histograms this parameter exhibits a clear nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10 20 km myr the spreading direction is determined with the peak width parameter which compares the average width of the two dominant modes in the azimuthal histograms the wider distribution of slope azimuths is oriented away form the paleo ridge axis for all spreading rates and thus spreading direction can be determined the trends in the peak height and width parameters are used to constrain a new model of abyssal hill formation at ultra fast spreading rates which require greater off axis extensional faulting resulting in a few large throw faults on the outward facing hillsides and many smaller throw faults on the inward facing hillsides

fault strain and seismic coupling on mid ocean ridges the contribution of extensional faulting to seafloor spreading along the east pacific rise epr axis near 3 s and between 13 n and 15 n is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data it is found that faulting may account for of the order of 5 10 of the total spreading rate which is comparable to a previous estimate from the epr near 19 s given the paucity of normal faulting earthquakes on the epr axis a maximum estimate of the seismic moment release shows that seismicity can account for only 1 of the strain due to faulting this result leads us to conclude that most of the slip on active faults must be occurring by stable sliding laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding while increasing temperature promotes stable sliding by applying a simple frictional model to mid ocean ridge faults it is shown that at fast spreading ridges 90 mm yr the seismic portion of a fault ws is a small proportion of the total downdip width of the fault w the ratio ws w interpreted as the seismic coupling coefficient x and in this case x 0 in contrast at slow spreading rates 40 mm yr ws w and therefore x 1 which is consistent with the occurrence of large magnitude earthquakes mb 5 0 to 6 0 occurring for example along the mid atlantic ridge axis

topographic controls on dike injection in volcanic rift zones abstract dike emplacement in volcanic rift zones is often associated with the injection of blade like dikes which propagate long distances parallel to the rift but frequently remain trapped at depth and erupt only near the tip of the dike over geologic time this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface in this study we investigate the evolution of faulting topography and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2 d elastic viscoplastic layer we show that the intrusion of blade like dikes focuses deformation at the rift axis leading to the formation of an axial rift valley however flexure associated with the development of the rift topography generates compression at the base of the plate if the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far field extension further dike injection will be inhibited in general this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50 60 of the far field extension rate these results indicate that over geologic timescales the injection of blade like dikes is a self limiting process in which dike generated faulting and topography result in an efficient feedback mechanism that controls the time averaged distribution of magma accretion within the crust

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