japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

extreme mantle uplift and exhumation along a transpressive transform fault earth s crust diverges and extends along mid ocean ridges analyses of gravity and seismic data from the equatorial atlantic show that propagation of ridge segments can compress the crust and create sufficient uplift to create small islands

exhumation of the ultrahigh pressure continental crust in east central china cretaceous and cenozoic unroofing and the tan lu fault the orogenic architecture of the world s largest ultrahigh pressure exposure the hong an dabie mountains of the triassic qinling dabie orogenic belt is dominated by cretaceous and cenozoic structures that contributed to its exhumation from 30 km depth cretaceous magmatic crustal recycling 50 for the entire dabie and heating 250 to 700 c were most prominent in dabie and exhumation magmatism and cooling were all controlled by cretaceous transtension exhumation was accomplished principally by an asymmetric cordilleran type extensional complex in the northern dabie northern orthogneiss unit between 140 and 120 ma at rates as fast as 2 mm yr and average horizontal stretching rates of up to 6 mm yr cretaceous reactivation occurred within a regional transtensional strain field as a result of far field collisions and pacific subduction the onset of crustal extension was preceded and possibly facilitated by a reheating of the hong an dabie crust 140 ma coeval with the onset of voluminous magmatism in eastern china 145 ma which resulted from a change in pacific subduction from highly oblique to orthogonal the tan lu continental scale fault was a normal fault zone in the mid cretaceous 110 90 ma and underwent 5 4 km dip slip and 4 km throw in the cenozoic during the india asia collision the qinling dabie belt acted as the structural discontinuity between the strike slip dominated escape tectonics south of the qilian qinling dabie belt and the rifting dominated tectonism north of it the most prominent cretaceous and cenozoic structures of the hong an dabie the xiaotian mozitang and the jinzhai fault zones respectively reactivated major lithospheric structures of the triassic orogen i e the huwan detachment zone and the suture

sea beam deep tow investigation of an active oceanic propagating rift system galapagos 95 5 w we have tested and corroborated the propagating rift hypothesis with high resolution sea beam and deep tow data collected over the galapagos 95 5 w propagating rift system the propagating rift is continuously breaking through the cocos plate at a velocity of about 50 km m y with an azimuth of about 273 away from the galapagos hotspot this process transfers cocos lithosphere to the nazca plate including the preexisting spreading center which fails as it is gradually preempted by the propagating rift the spreading center azimuth is being changed by about 8 clockwise by this rift propagation the active propagating and failing rift axes overlap by about 15 km and are joined by a broad and anomalously deep zone of nonrigid plate deformation rather than by a transform fault although rift propagation is continuous on a scale of a few kilometers 50 000 years and lithospheric transferral from one plate to the other occurs mostly continuously as the preexisting rift fails it breaks into discrete segments the geometry of seafloor fabric and tectonic elements such as active and failed rifts the v shaped pattern of pseudofaults and the oblique tectonic fabric in the sheared zone of transferred lithosphere are observed as predicted this quantitative verification provides strong support for the propagating rift hypothesis

systematics of ridge propagation south of 30 s abstract new high resolution geosat altimetry data south of 30 s reveal numerous propagating ridge wakes along intermediate and slow spreading ridges these new examples provide a large enough database to establish systematics of ridge propagation almost all active propagating ridges propagate down a regional along axis gravity or bathymetry gradient the sense of the propagating ridge offset right lateral vs left lateral is related to recent changes in spreading direction we find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than 50 of their full spreading rate and ridges with a median valley morphology which usually propagate at 25 of their spreading rate the axial high propagators leave behind an asymmetric wake the outer pseudofault appears as a continuous linear trough step while the sheared zone appears as a chain of small gravity bumps while we clearly see the propagating ridge wakes from offsets greater than 10 km at slow and intermediate spreading ridges at ridges spreading faster than 75 mm yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in geosat altimetry data the systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation

spatial and temporal distribution of seismicity along the northern mid atlantic ridge 15 35 n 1 a detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern mid atlantic ridge between 15 and 35 n is presented two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones the seismically active zone extends 20 km to either side of the ridge axis consistent with earlier results from studies of fault morphology along the axis hydrophone recorded activity shows important variations areas with intense and persistent seismic activity stripes stand in sharp contrast to areas that lack seismicity gaps the regions of persistent activity are a new observation at mid ocean ridges in general the patterns of seismically active inactive regions are also recognized in the 28 year teleseismic record implying that these patterns are maintained at timescales between a few years and a few decades we find no simple relationship between individual segment variables e g length or trend of the segment maximum offset of discontinuities or along axis change in mantle bouguer anomaly mba and water depths and number of hydrophone recorded events there does appear to be a correlation between axial thermal structure and seismicity regions of low and high numbers of events would thus correspond to thinner hotter and thicker colder lithosphere respectively seismicity may reflect thermal structure at short timescales decadal or longer while relief and inferred crustal thickness may integrate this structure over longer periods of time order of 1 myr

an ocean bottom gravity study of the southern juan de fuca ridge we use seafloor and sea surface gravity data to model density structure along the southern juan de fuca ridge we determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models of 63 seafloor gravity measurements obtained 42 observations were distributed over the 3 km left lateral overlapping rift zone orz located at latitude 45 03 n and separating the juan de fuca ridge into the cleft and vance segments a 21 measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44 52 n a region of linear ridge geometry on the cleft segment these seafloor gravity measurements which are sensitive to shallow crustal density variations were augmented by roughly 800 km of sea surface gravity measurements using a bathymetry bouguer anomaly analysis of the seafloor gravity data we determined the average density of the shallow roughly upper 2 km oceanic crust at these two locations to be 2630 kg m 3 50 kg m 3 within the uncertainties of the density determinations there is no difference between the average shallow oceanic crustal density at the linear cleft segment and at the cleft vance orz using the seafloor measurements we modeled the study area s underlying density to provide constraints on its fine scale structure localized porosity of up to 17 within the upper 500 m of oceanic crust layer 2a provide a possible explanation for the observed gravity anomalies at the cleft segment two dimensional density models of the crust underlying the linear cleft segment 44 52 n show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis for the cleft vance orz three dimensional modeling suggests low density material between the rifts in agreement with thickened layer 2a from seismic observations in contrast layer 2a variations do not explain the low density inferred at the northern end of the cleft segment since seismic observations suggest a thin layer 2a the magmatic activity recently observed in this region may have a distinct deeper source

three dimensional gravity and bathymetry analysis of the mid atlantic ridge between 20 n and 24 n flow geometry and temporal evolution of the segmentation bathymetry and gravity data covering 10 m y on both flanks of the mid atlantic ridge south of the kane fracture zone reveal a complex segmentation pattern well correlated with short scale temporal variations in crustal production segment and paleosegment centers are associated with crustal thickening while their extremities appear as elongated bands of thin crust subsidence rates vary between centers and extremities of a segment with higher than world average values at segmert midpoints and closer to average values at the edges this is consistent with segment centers being the site of focused mantle upwellings with associated thin lithosphere and with thick crust a pronounced circular minimum in the long wavelength of the mantle bouguer anomaly associated to a maximum in the long wavelength of the bathymetry marks the site of a mantle upwelling associated with a particularly robust segment located north of a small fracture zone 21 20 n an offset is often observed between extreme values of topography and crustal thickness resulting in a lack of isostatic equilibrium for off axis short wavelength features an obliquity between maxima of crustal thickening associated with active segments and the topographic axial domain suggests that the shift between topography and crustal structure may arise partly from a decoupling between crust and topography formation processes

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

small scale structure of the kane oceanic core complex mid atlantic ridge 23 30 n from waveform tomography of multichannel seismic data 1 frequency domain waveform tomography applied to deep sea multichannel seismic data collected across the kane oceanic core complex occ reveals the small scale structure of this section of oceanic lithosphere formed at the mid atlantic ridge 3 3 2 1 myr ago that has been uplifted and exhumed at the seafloor along an oceanic detachment fault geological and geophysical studies indicate that the central eastern area of the kane occ is formed by a large gabbroic intrusion the new high resolution seismic velocity models show that this gabbroic core is 1 000 1 350 m thick and it is underlain by a 300 m thick layer of low velocity material interpreted as serpentinized peridotite the models also reveal the presence of a deeper 200 m thick by 1 700 m long high velocity body interpreted as a small gabbroic sill embedded in serpentinzed peridotite

mode of cenozoic east west extension in tibet suggesting a common origin of rifts in asia during the indo asian collision cenozoic rifts in tibet were traditionally interpreted as a result of topographic collapse of the tibetan plateau reaching the maximum elevation that can be supported by its mechanical strength recent studies have emphasized possible similarities between rifting in tibet and extension in the basin and range of the western united states however when examined in detail one finds that spacing of long 100 km rifts in tibet 100 300 km is significantly greater than that in the basin and range 20 40 km from south to north rift spacing decreases systematically from 191 67 km in the himalaya south of the indus yalu suture to 146 34 km in south tibet between the indus yalu and bangong nujiang sutures and farther north to 101 31 km in central tibet between the bangong nujiang and jinsha sutures instability analysis suggests that the mantle lithosphere must have been involved in east west tibetan extension specifically the widely spaced rifts in the himalaya and tibet may have been related to the presence of a relatively light crust density 2 90 g cm 3 and a strong mantle lithosphere 40 km thick and a factor of 5 stronger than the upper crust the observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in tibet from 70 80 km in the himalaya in the south to 50 55 km in central tibet in the north a regional comparison of rifts in east asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for tibetan rifts the baikal rift and the shanxi graben this implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the tibetan rifts a regional boundary condition applied throughout east asia must be required

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

structural geology to fault or not to fault most of the oceanic crust has a simple layered structure the discovery that slow spreading ridges exhibit a comparatively complex crustal structure and some of the largest extensional faults on earth is leading to the recognition of a new mode of seafloor spreading

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

a new model for the dynamic evolution of chinese lithosphere continental roots plume tectonics abstract chinese continental lithosphere comprises three tectonic domains 1 eastern china a region characterized by rifting extensional basins and voluminous basaltic volcanism 2 central china a plexus of cratonic terrains with low heat flow 40 50 mw m2 including the tarim erdos and yangtze blocks welded by pre cenozoic orogenic belts and 3 western china a region comprising the qinghai tibet himalaya orogen the relatively thin crust 35 km and lithosphere 70 km of eastern china is believed to reflect mantle upwelling while near normal crust 45 km and thickened lithosphere 200 km in central china suggest a mantle lithosphere root resembling those inferred for the kaapvaal and siberian cratons thickened crust 70 km and lithosphere 150 km in western china reflect the advanced development of an orogen root the lower density and relative buoyancy of central cratonic mantle roots appear to have contributed to the long term tectonic stability of the central chinese lithospheric blocks in contrast gravitational instability produced by thickening of the denser western chinese lithosphere and the resulting subsidence and eventual delamination of orogenic roots are believed to have led to postorogenic extensional collapse although the geological relationships suggest compressional forces caused both mountain building and orogen root development extensional stress resulting from gravitational collapse is believed to have induced lithospheric and crustal thinning with concomitant reduction of topography the term continental roots plume tectonics has been adopted to describe the configuration and dynamic condition of subcontinental lithosphere and upper mantle beneath china accordingly it is proposed that supracrustal tectonic forms represent the surface expressions of and responses to deep continental roots plume tectonics while the prevailing view is that the western orogenic belt is not genetically related to eastern continental rifting such a relationship is inherent to the continental roots plume tectonics model it is further proposed that the formation of orogenic lithosphere roots triggered eastward extrusion of the asthenosphere along the 400 km depth mantle interface and in response to subduction at the eastern margin produces plume like upwelling beneath eastern china we suggest that processes involved in continental roots plume tectonics are directly responsible for the formation and evolution of the eurasian supercontinent

tectonic evolution of 200 km of mid atlantic ridge over 10 million years interplay of volcanism and faulting we reconstruct the history of the mode of accretion of an area of the mid atlantic ridge south of the kane fracture zone using bathymetric morphology the area includes 200 km of the spreading axis and reaches to 10 ma on either side we distinguish three tectonic styles 1 volcanic construction with eruption and intrusion of magma coupled with minor faulting 2 extended terrain with abundant large offset faults 3 detachment faulting marked by extension on single long lived faults over 40 of the seafloor is made of extended terrain and detachment faults the area includes products of seven spreading segments the spreading axis has had detachment faulting or extended terrain on one or both sides for 70 of the last 10 ma in some parts of the area regions of detachment faulting and extended terrain lie close to segment boundaries regions of detachment faulting initiated at 10 ma close to the adjacent fracture zones to the north and south and then expanded away from them we discuss the complex evidence from gravity seismic surveys and bathymetry for the role of magma supply in generating tectonic style overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting but the relationship is not strong other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension perhaps through the lubrication of active detachment faults by the formation of low friction materials talc serpentine on detachment fault surfaces

seismic velocities of the uppermost igneous crust versus age summary seismic refraction velocity data from the acoustic basement called layer 2a have been compiled for different mid ocean ridges the data from post 1970 studies show a strong correlation between velocity and basement age importantly velocities double in less than 10 myr for all ridges but for older crust up to 160 myr in age velocities do not increase significantly

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

modelling the thermal evolution of slow spreading ridge segments and their off axis geophysical signature systematic studies conducted between 15 n and 40 n over ridge segments along the slow spreading mid atlantic ridge mar have shown that segment characteristics are related to the thermal state of the segments and gradually vary with their length this paper presents further developments of a 3 d model based on the presence of a hot zone located under the segment centres gac et al 2003 to 1 quantify the thermal structures and the geophysical signatures of segments of various lengths considered as representative of the various mar segments 2 test if a simple and single model of thermal evolution can account for the characteristics of all segments and 3 explain the past evolution of the segmentation as is observed off axis along the mar the modelled thermal structure and three simulated geophysical outputs crustal structure along axis variations of the earthquake maximum depth and the mantle bouguer gravity anomalies mba are found to be directly controlled by the shape geometry and dimensions of the hot zone a consistent fit between model outputs and along axis variations of the geophysical observables over the various segments is obtained by varying solely the length of the hot zone this result shows that segments of different length may in fact constitute the different stages of a single evolution process the axial geophysical characteristics of the segment would progressively evolve from those of shorter segments to those of longer ones as the hot zone lengthens along axis a subsequent shortening of the segment would result from a simultaneous shortening of the hot zone segment characteristics reverting back from those of longer segments to those of shorter ones three geophysical fields topography gravity and magnetic anomalies are simulated as the results of the thermal evolution of aligned and offset segments the length of which evolves through time these simulations succeed in fitting observations for the entire range of observed axis offsets between adjacent mar segments the segment evolution produces peculiar off axis isostatic topography and gravity anomaly mba the rhomb shaped patterns our simulations which model adjacent offset segments having evolved through several cycles of lengthening and shortening yield a good fit to the isostatic topography and mba patterns observed in the off axis region finally the distribution of magnetization depends on the magnetic properties of each type of rocks and on the petrological structure of the lithosphere which in turn results from its thermal structure and evolution modelled magnetic anomalies are shown to be in good agreement with off axis observations along the n21 40 segment tammar of the mar

40ar 39ar dating constraints on the high angle normal faulting along the southern segment of the tan lu fault system an implication for the onset of eastern china rift systems abstract high angle normal faulting in eastern china was an important tectonic process responsible for the rifting of the eastern asian continental margin along the southern segment of the tan lu fault system part of the eastern china rift system 55 70 east dipping normal faults are the oldest structures within this rift system chlorite pseudotachylite and fault breccia are found in fault zones which are characterized by microstructures and syn deformation chlorite minerals aligned parallel to a down dip stretching lineation 40ar 39ar dating of syn deformation chlorite and k feldspar from the fault gouge zone yields cooling ages of 75 70 ma interpreted as the timing of slip along the normal faults this age is older than that of opening of the japanese sea and back arc extension in the west pacific but similar to the onset of the indo asian soft collision

cretaceous deformation history of the middle tan lu fault zone in shandong province eastern china abstract based on field analysis of fault slip data from different rock units of the cretaceous basins along the middle part of the tan lu fault zone shandong province eastern china we document polyphase tectonic stress fields and address the changes in sense of motion of the tan lu fault zone during the cretaceous the cretaceous deformation history of the tan lu fault zone can be divided into four main stages the first stage during the earliest cretaceous was dominated by n s extension responsible for the formation of the jiaolai basin we interpret this extension to be related to dextral strike slip pull apart opening guided by the tan lu fault zone the second stage during the middle early cretaceous was overwhelmingly rift dominated and characterized by widespread silicic to intermediate volcanism normal faulting and basin subsidence it was at this stage that the tan lu parallel yi shu rift was initiated by e w to wnw ese extension the tectonic regime then changed during the late early cretaceous to nw se oriented transpression causing inversion of the early cretaceous rift basin and sinistral slip along the tan lu fault zone during the late cretaceous dextral activation of the tan lu fault zone resulted in pull apart opening of the zhucheng basin which was subsequently deformed by ne sw compression this deformation chronology of the tan lu fault zone and the associated cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of asia or related to collision in the tibet region

cracked media poisson s ratio and the structure of the upper oceanic crust summary theoretical models of water filled cracks within a solid are in general agreement and indicate that the poisson s ratio of the composite material will typically increase for thin cracks with aspect ratios less than a but will decrease for thick cracks with aspect ratios between and i near spherical pores cause little change in poisson s ratio this is true both for randomly oriented isotropic crack models and at certain directions for aligned crack models which predict seismic anisotropy observed anomalous values of poisson s ratio in oceanic layer 2 can often be explained using these models as an effect of cracks and do not necessarily require the presence of rocks of unusual composition in particular the low poisson s ratios reported by spudich orcutt 1980 and au clowes 1984 can approximately but not always exactly be fitted with a model consisting of thick cracks within higher poisson s ratio rock more typical of the upper oceanic crust however this interpretation is near the limit of existing theories of cracked material so other explanations for the anomalous observations must also be considered

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

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

the mesozoic cenozoic east china rift system abstract the mesozoic cenozoic rift system of eastern china extends in a n s direction over a distance of some 4000 km and has a width of 600 1000 km to the east it reaches into the coastal areas of the east and south china sea and extends onto the continental shelf this very large rift system evolved on a heterogenous basement consisting of a number of terranes which were amalgamated during the indosinian orogeny a first rifting phase spanned late jurassic early cretaceous times it was followed by a period of relative tectonic quiescence and thermal relaxation a latest cretaceous phase of compressional tectonics giving rise to partial basin inversion was followed by a renewed rifting cycle spanning eocene early neogene times development of the east china rift system involving vertical as well as lateral crustal movements was probably induced by physico chemical changes within the earth s mantle that can be related to activity along the pacific subduction zones the mesozoic cenozoic east china rifts contain some important hydrocarbon provinces which rely to a large extent on syn rift sediments for source and reservoir rocks

the mid atlantic ridge between 29 n and 31 30 n in the last 10 ma abstract the segmentation of the mid atlantic ridge between 29 n and 31 30 n during the last 10 ma was studied within our survey area the spreading center is segmented at a scale of 25 100 km by non transform discontinuities and by the 70 km offset atlantis transform the morphology of the spreading center differs north and south of the atlantis transform the spreading axis between 30 30 n and 31 30 n consists of enechelon volcanic ridges located within a rift valley with a regional trend of 040 south of the transform the spreading center is associated with a well defined rift valley trending 015 magnetic anomalies and the bathymetric traces left by non transform discontinuities on the flanks of the mid atlantic ridge provide a record of the evolution of this slow spreading center over the last 10 ma migration of non transform offsets was predominantly to the south except perhaps in the last 2 ma the discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 ma south of 30 30 n in contrast north of 30 30 n the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently the orientation of the bathymetric traces suggests that the migration of non transform offsets is not controlled by the motion of the ridge axis with respect to the mantle the evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes and by local processes perhaps related to the amount of melt delivered to spreading segments relative plate motion changes over the last 10 ma in our survey area have included a decrease in spreading rate from 32 mm a 1 to 24 mm a 1 as well as a clockwise change in spreading direction of 13 between anomalies 5 and 4 followed by a counterclockwise change of 4 between anomaly 4 and the present interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time these differences as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments indicate that magmatic and tectonic activity are in general not coordinated between adjacent spreading segments

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

an early miocene transition in deformation regime within the red river fault zone yunnan and its significance for indo asian tectonics recent work has tended to support the view that lateral extrusion of indochina along the red river fault zone was a significant factor in accommodating the convergence of india with asia to better understand the nature and timing of this feature 40ar 39 ar age spectrum analyses were undertaken on minerals from the ailao shan red river metamorphic belt in southern yunnan china interpreted using diffusion domain theory these results yield thermal histories in the temperature range 550 150 c discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early miocene at least in the vicinity of our samples left lateral strike slip ductile deformation appears to have ceased by about 20 ma the mylonitic gneisses in the shear zone were subsequently obliquely unroofed probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief locally 100 c m y cooling episode this transition may have resulted from a component of northeast southwest extension attributable to greater clockwise rotation of indochina relative to south china after 20 ma than that consistent with pure strike slip along the fault k ar ages on clay minerals from gouge within the normal faults bounding the ailao shan and diancang shan yield a range of ages between 20 and 185 ma but show a significant clustering at 20 25 ma these results suggest that early miocene movement on a precursor to the range front normal fault in the ailao shan caused oblique unroofing of the shear zone this early miocene event corresponds in time with a transition from low to very high denudation rates throughout the himalaya and southern tibet this coincidence may reflect that once left lateral strike slip motion on the red river fault zone slowed or was terminated accommodation of much of the continued northward convergence of india occurred on the main central thrust resulting in rapid uplift in the tethyan himalaya and gangdese belt

changes in direction of seafloor spreading revisited rotation and propagation have both been proposed as mechanisms by which spreading centers can reorient following changes in direction of seafloor spreading the rotation hypothesis predicts a smooth asymmetric fanning of magnetic anomaly and abyssal hill orientations whereas the propagation hypothesis predicts that the orientation changes will occur at abrupt discontinuities which are either failed rifts or pseudofaults to test between these hypotheses we conducted a detailed sea beam magnetics seismic reflection survey of the area where the direction of pacific farallon spreading changed between anomalies 24 and 22 just north of the major bend in the surveyor fracture zone the change in trend of seafloor fabric is abrupt supporting the rift propagation hypothesis for spreading center reorientation

cretaceous and cenozoic cooling history across the ultrahigh pressure tongbai dabie belt central china from apatite fission track thermochronology abstract the crystalline terrane of the tongbai dabie region central china comprising the earth s largest ultrahigh pressure uhp exposure was formed during triassic collision between the sino korean and yangtze cratons new apatite fission track aft data presented here from the uhp terrane extends over a significantly greater area than reported in previous studies and includes the eastern dabie the hong an northwestern dabie and tongbai regions the new data yield ages ranging from 44 3 to 142 36 ma and mean track lengths between 10 and 14 4 m thermal history models based on the aft data taken together with published 40 ar 39 ar k ar apatite and zircon u th he and u pb data exhibit a three stage cooling pattern that is similar across the study region commencing with an early cretaceous rapid cooling event followed by a period of relative thermal stability during which rocks remained at temperatures within the aft partial annealing zone 60 110 c and ending with a possible renewed phase of accelerated cooling during pliocene to recent time the first cooling phase followed large scale transtensional deformation between 140 and 110 ma and is related to early cretaceous eastward tectonic escape and pacific back arc extension between this phase and the subsequent slow cooling phase a transition period from 120 to 80 ma to 70 to 45 ma along the tan lu fault was characterised by a relatively low cooling rate 3 5 c ma this transition is likely related to a tectonic response associated with the mid cretaceous subduction of the izanagi pacific plate as well as lithospheric extension and thinning in eastern asia the present regional aft age pattern is therefore basically controlled by the early cretaceous rapid cooling event but finally shaped through active cenozoic faulting following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence modelling of the aft data suggests a possible pliocene recent cooling episode which may be supported by increased rates of sedimentation observed in adjacent basins this cooling phase may be interpreted as a response to the far field effects of the frontal india eurasia collision to the west approximate estimates suggest that the total amount of post 120 ma denudation across the uhp orogen ranged from 2 4 to 13 2 km for different tectonic blocks and ranged from 0 8 to 9 7 km during the cretaceous to between 1 7 and 3 8 km during the cenozoic

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

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

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

fast rift propagation at a slow spreading ridge bathymetric magnetic gravity and morphologic data from the flank of the slow spreading mid atlantic ridge reveal obliquely oriented features that offset magnetic isochrons and morphological patterns within individual ridge segments these features form angles of 10 40 with the isochrons and are inferred to result from rift propagation at rates several times the spreading rate representing the fastest propagators yet observed at a slow spreading ridge these fast propagators appear to have formed as a result of tectonic extension migrating along ridge segments as the segments change from more magmatic to less magmatic periods of spreading

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

deformation of chlorite in naturally deformed low grade rocks the intracrystalline deformation of chlorite in naturally deformed low grade rocks was investigated with transmission electron microscopy tem as in other phyllosilicates the deformation of chlorite is dominated by the 001 slip plane slip along this plane is very easy through the generation and movement of partial dislocations with burgers vector b 1 n uv0 after initial bending of the 001 planes through a flexural slip mechanism accommodated by slip along 001 the misorientation is concentrated in tiltwalls subnormal to 001 which are formed by partial dislocations with b 1 n uv0 tiltwalls subparallel to 001 are formed by arrays of dislocations with a total burgers vector 001 these boundaries are either initiated at the grain boundary and move inwards by dislocation climb or form by movement of free dislocations towards the tip of propagating boundaries by glide along planes inclined to 001 once a boundary is formed the increasing misfit is accommodated by climb of these dislocations along 001 due to the combination of very easy slip along 001 and the ability to form subgrain boundaries subparallel to 001 by dislocation climb and activation of hard slip systems chlorite deforms plastically even at temperatures below 300 c

crustal structure and kinematics of the tammar propagating rift system on the mid atlantic ridge from seismic refraction and satellite altimetry gravity the tammar segment of the mid atlantic ridge forms a classic propagating system centred about two degrees south of the kane fracture zone the segment is propagating to the south at a rate of 14 mm yr 1 15 per cent faster than the half spreading rate here we use seismic refraction data across the propagating rift sheared zone and failed rift to investigate the crustal structure of the system inversion of the seismic data agrees remarkably well with crustal thicknesses determined from gravity modelling we show that the crust is thickened beneath the highly magmatic propagating rift reaching a maximum thickness of almost 8 km along the seismic line and an inferred from gravity thickness of about 9 km at its centre in contrast the crust in the sheared zone is mostly 4 5 6 5 km thick averaging over 1 km thinner than normal oceanic crust and reaching a minimum thickness of only 3 5 km in its nw corner along the seismic line it reaches a minimum thickness of under 5 km the pmp reflection beneath the sheared zone and failed rift is very weak or absent suggesting serpentinisation beneath the moho and thus effective transport of water through the sheared zone crust we ascribe this increased porosity in the sheared zone to extensive fracturing and faulting during deformation we show that a bookshelf faulting kinematic model predicts significantly more crustal thinning than is observed suggesting that an additional mechanism of deformation is required we therefore propose that deformation is partitioned between bookshelf faulting and simple shear with no more than 60 per cent taken up by bookshelf faulting

geological investigation of a lineated massif at the kane transform fault implications for oceanic core complexes recently available new perspectives on the morphology of the sea floor near the mid atlantic ridge reveal the widespread development of dome like massifs with lineated upper surfaces in form and dimensions these features resemble continental metamorphic core complexes developed in regions of large magnitude extension and thus may be oceanic core complexes excellent examples of these features occur south of the kane transform fault where high resolution geological data from side looking sonar deeply towed cameras and manned submersibles are available the geological data from two different massifs representing somewhat different types of crustal structure provide important constraints on the development of these oceanic core complexes inferences from these direct geological investigations of oceanic core complexes are a first step toward understanding the processes by which these features form in slow spread crust specific relationships in these massifs and nearby areas suggest that these structures have formed over significant lengths of ridge axis tens of kilometres and are not necessarily confined to any specific setting with respect to morphologic segmentation patterns in some cases they are developed in thick gabbroic crust but elsewhere have formed in crust composed serpentinite riddled with gabbroic intrusions ductile stretching and thinning of these sections is recorded in extensive mylonites and shear zones syntectonic gabbroic intrusions are widespread and contradict the notion that the massifs develop during intervals of amagmatic spreading cataclastic fault zones in gabbros and serpentinites developed during uplift cooling and hydration of the stretched crust and may have been localized as major detachment faults sparse diabase dikes and basaltic lavas are the latest magmatic additions to these areas numerous joints and steeply dipping small offset faults are the youngest observed structures across part of the top surface of one massif small scale faults appear to bound elongated ridges of fractured basement rocks creating the lineated terrain thus the extensive lineated surfaces nearby need not reflect prolonged displacement along a continuous detachment fault surface geological relationships developed in the dome like massifs along the kane transform fault hint at a complex array of processes involved in the evolution of these features and raise new questions regarding their origin and analogies with continental core complexes

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

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

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

wide angle seismic imaging of two modes of crustal accretion in mature atlantic ocean crust we present a high resolution 2 d p wave velocity model from a 225 km long active seismic profile collected over 60 75 ma central atlantic crust the profile crosses five ridge segments separated by a transform and three nontransform offsets all ridge discontinuities share similar primary characteristics independent of the offset we identify two types of crustal segment the first displays a classic two layer velocity structure with a high gradient layer 2 0 9 s 1 above a lower gradient layer 3 0 2 s 1 here pmp coincides with the 7 5 km s 1 contour and velocity increases to 7 8 km s 1 within 1 km below we interpret these segments as magmatically robust with pmp representing a petrological boundary between crust and mantle the second has a reduced contrast in velocity gradient between the upper and lower crust and pmp shallower than the 7 5 km s 1 contour we interpret these segments as tectonically dominated with pmp representing a serpentinized alteration front while velocity depth profiles fit within previous envelopes for slow spreading crust our results suggest that such generalizations give a misleading impression of uniformity we estimate that the two crustal styles are present in equal proportions on the floor of the atlantic within two tectonically dominated segments we make the first wide angle seismic identifications of buried oceanic core complexes in mature 20 ma atlantic ocean crust they have a 20 km wide domal morphology with shallow basement and increased upper crustal velocities we interpret their midcrustal seismic velocity inversions as alteration and rock type assemblage contrasts across crustal scale detachment faults

crustal structure of the propagating tammar ridge segment on the mid atlantic ridge 21 5 n active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time the mechanisms controlling ridge propagation however are poorly understood at the slow spreading mid atlantic ridge at 21 5 n a seismic refraction and wide angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply thus crust is thickest 8 km at the domed segment center and decreases in thickness toward both segment ends however thicker crust is formed in the direction of ridge propagation suggesting that melt is preferentially transferred toward the propagating ridge tip further while seismic layer 2 remains constant along axis seismic layer 3 shows profound changes in thickness governing variations in total crustal thickness this feature supports mantle upwelling at the segment center thus fluid basaltic melt is redistributed easily laterally while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply the onset of propagation seems to have coincided with the formation of thicker crust suggesting that propagation initiation might be due to changes in the melt supply after a rapid initiation a continuous process of propagation was established the propagation rate seems to be controlled by the amount of magma that reaches the segment ends the strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

reflective oceanic crust formed at a fast spreading center in the pacific we present new seismic images of cretaceous crust formed at a fast spreading center in the pacific the high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast spreading centers are not seismically detectable subhorizontal reflections can be traced at 600 800 ms two way time below the top of basement for tens of kilometres suggesting the presence of a widespread seismic boundary possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes lower crustal reflections dipping dominantly toward the paleo spreading center may represent mafic ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections

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

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

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

enhanced mantle upwelling melting caused segment propagation oceanic core complex die off and the death of a transform fault the mid atlantic ridge at 21 5 n crustal structure provides the key to understand the interplay of magmatism and tectonism while oceanic crust is constructed at mid ocean ridges mor at slow spreading rates magmatic processes dominate central areas of mor segments whereas segment ends are highly tectonised the tammar segment at the mid atlantic ridge mar between 21 25 n and 22 n is a magmatically active segment at 4 5 ma this segment started to propagate south causing the termination of the transform fault at 21 40 n this stopped long lived detachment faulting and caused the migration of the ridge offset to the south here a segment centre with a high magmatic budget has replaced a transform fault region with limited magma supply we present results from seismic refraction profiles that mapped the crustal structure across the ridge crest of the tammar segment seismic data yield crustal structure changes at the segment centre as a function of melt supply seismic layer 3 underwent profound changes in thickness and became rapidly thicker 5 ma this correlates with the observed bull s eye gravimetric anomaly in that region our observations support a temporal change from thick lithosphere with oceanic core complex formation and transform faulting to thin lithosphere with focused mantle upwelling and segment growth temporal changes in crustal construction are connected to variations in the underlying mantle we propose there is a link between the neighbouring segments at a larger scale within the asthenosphere to form a long highly magmatically active macro segment here called the tammar kane macrosegment

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

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