ridge offsets normal faulting and gravity anomalies of slow spreading ridges we develop a model relating mid ocean ridge normal faulting and crustal structure by examining recently available high resolution gravity and multibeam bathymetric data of the mid atlantic ridge and other spreading centers results of the analysis reveal a consistent pattern of positive residual gravity anomalies along the crust paralleling all major atlantic offsets studied especially along the inside corner side of the offsets individual residual gravity anomaly spikes local peaks which have amplitudes of up to 20 mgal and typical across axis spacing of 10 30 km often coincide with individual major fault scarps suggesting crust thinned by normal faulting theoretical calculations indicate that the amplitude and spacing of the observed residual gravity spikes are consistent with the presence of successive ridge parallel low angle faults that originate episodically at inside corners of ridge offset intersections fault scarp heights and the amounts of crustal thinning as inferred from gravity anomalies are consistently larger at inside corners than at outside corners supporting a model in which tectonic extension near ridge offsets is asymmetric with low angle faults occurring preferentially at inside corners these results on spatial variations in seafloor morphology and gravity anomalies further support a three dimensional tectonic faulting model at oceanic spreading centers with three major characteristics 1 low angle faults form preferentially at inside corners where the mantle lithosphere is the strongest and the lithospheric plates are sufficiently decoupled across ridge axis offsets 2 low angle faults decrease in throw toward midpoints of long ridge segments where large low angle faults may not be sustained by a weak lithosphere and 3 the residual gravity peaks are statistically larger at transform than near nontransform offsets indicating that the length of a ridge offset and its tectonic style control the development of low angle faults

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

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

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

quantitative analysis of abyssal hills in the atlantic ocean a correlation between inferred crustal thickness and extensional faulting a recent cruise to the office of naval research atlantic natural laboratory obtained 100 hydrosweep bathymetrie coverage 200 hawaii mrl hmrl side scan coverage gravity and magnetics over an area spanning three ridge segments along axis 25 25 n to 27 10 n and crustal ages from 0 to 26 30 ma 400 km on the west flank of the mid atlantic ridge this data set represents a first opportunity for an extensive regional analysis of abyssal hill morphology created at a slow spreading ridge the primary purpose of this work is to investigate the relationship between abyssal hill morphology and the properties of the ridge crest at which they were formed we apply the method of goff and jordan 1988 for the estimation of two dimensional statistical properties of abyssal hill morphology from the gridded hydrosweep bathymetry important abyssal hill parameters derived from this analysis include root mean square rms height characteristic width and plan view aspect ratio the analysis is partitioned into two substudies 1 analysis of near axis 7 ma abyssal hills for each of the three segments and 2 analysis of temporal variations 2 29 ma in abyssal hill morphology along the run of the south segment the results of this analysis are compared and correlated with analysis of the gravity data and preliminary determination of faulting characteristics based on hmrl side scan data principal results of this study are 1 abyssal hill morphology within the study region is strongly influenced by the inside outside corner geometry of the mid ocean ridge segments abyssal hills originating at inside corners have larger rms height and characteristic width and smaller plan view aspect ratio than those originating at outside corners 2 the residual mantle bouguer gravity anomaly is positively correlated with intersegment and along flow line variations in rms height and characteristic width and it is negatively correlated with plan view aspect ratio from this result we infer that lower relief narrower and more elongated abyssal hills are produced when the crust being generated is thicker 3 intersegment variations in near axis rms height negatively correlate with average fault density as determined from analysis of hmrl side scan imagery

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

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

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

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

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

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

a simple model for the fault generated morphology of slow spreading mid oceanic ridges we postulate that fluctuations in magmatic activity at mid oceanic ridges perturb the horizontal least principal stress across rift bounding normal faults leading to alternating phases of magmatic accretion which increases valley width and tectonic extension which results in the growth of inner rift wall topography fine scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow spreading ridges are moderately dipping approximately 45 planar features throughout the seismogenic oceanic lithosphere a simple quantitative model that includes flexural deformation of a 10 km thick elastic plate by slippage on 45 dipping normal faults can match the bathymetrie profiles across several slow spreading ridge segments comparison among dip distributions of normal faulting earthquakes at mid ocean ridges in the trench outer rise region and on continents suggests that most events from these three tectonic environments initiated at dips close to 45 raising unanswered questions about the mechanical conditions under which the faults originated

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

hydrology of lake eyre australia el nino link abstract all major ephemeral rivers of australia lie in the lake eyre basin dry for years on occasions they carry enormous floods with annual volumes of 40 km 3 and instantaneous discharges of the order of 30 000 m 3 s 1 mean annual runoff of the basin is 4 km 3 or 3 5 mm significantly less than the 57 mm for the whole of australia this paper examines lake eyre and its drainage basin describing the historical hydrology of the catchment both in terms of instrument records and inflows hindcast using a model based on the technique of coral fluorescence palaeohydrological reconstruction coral fluorescence is a function of nearby river discharge and can be used to reconstruct annual river outputs reconstruction coral fluorescence is a function of nearby river discharge and can be used to reconstruct annual river outputs beyond the instrument period the burdekin river catchment runoff history is strongly paralleled by the volumes of lake eyre inflows during the period of common reliable instrument gaugings the burdekin runoff data have been extended back to 1735 a d using the correlation between river discharge and annual growth band fluorescence in an old coral from pandora reef off the north queensland coast for the instrument period 1952 to 1980 73 of the variance between coral fluorescence and burdekin discharge is common during the period 1949 1980 41 of the variance between coral fluorescence and lake eyre inflow data is common modelled inflows to lake eyre for the period 1885 1948 are based on the latter data set

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

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

three dimensional gravity study of the mid atlantic ridge evolution of the segmentation between 28 and 29 n during the last 10 m y during the high resolution survey sara segmentation ancienne de la ride atlantique sea beam bathymetry magnetic gravity and seismic reflection data were collected on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to provide off axis information up to approximately 10 m y and to complement a detailed on axis survey carried out in 1988 and 1989 sempere et al 1990 between the atlantis and kane fracture zones a previous gravity study had revealed the existence of bulls eye shaped gravity lows centered on the axial segments and gravity highs centered on the non transform discontinuities lin et al 1990 we carried out a three dimensional calculation of the mantle bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center the off axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities after correcting for the gravity effect of lithospheric cooling away from the ridge the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry off axis bathymetry is characterized by southward pointing v shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously however the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows northward on the western flank and southward on the eastern flank the sense of offset between the gravity and bathymetric traces appears to be related to the right stepping nature of the axial discontinuities

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

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

kinematics of the north american african plate boundary between 28 and 29 n during the last 10 ma evolution of the axial geometry and spreading rate and direction abstract a high resolution survey sara segmentation ancienne de la ride atlantique collected sea beam bathymetry magnetic gravity and seismic reflection data on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to complement a portion of an on axis study carried out between the atlantis and kane transform faults which revealed the configuration of the spreading axis the sara survey extends this coverage to an age of approximately 10 ma on either flank a sinuous isochron pattern was established from anomaly identification made using a two dimensional model the plate tectonic reconstruction based solely on this isochron pattern provides an accurate kinematic description of the relative motion between the african and north american plates within the zone our results confirmed by data from the famous area and a flowline comparison with the trend of the kane fracture zone show that a change in axial orientation spreading direction and spreading rate occurred at approximately anomaly 4 time 7 01 ma this was coincident with the appearance of a new axial segment all the axial segments observed have changed length continously during the last 10 ma by advancing or retreating at one or both ends in any combination so as to leave non parallel trails on the flanks that do not align with either the spreading direction or with the absolute plate motion factors contributing to the sinuous magnetic anomaly pattern are discussed

generation of morphotectonic fabric on the mid atlantic ridge flanks 28 to 29 n implications for the limits of tectonic deformation and abyssal hill formation 1 high resolution multibeam bathymetry is used to analyze the morphotectonic fabric of a 35 000 km2 zone of the mid atlantic ridge crest between 28 and 29 n interpolated isochrons at 1 myr intervals derived from a kinematic reconstruction provide a temporal framework for the structural analysis the axial valley within the survey area is offset by three non transform discontinuities dividing the axis into en echelon segments the flanks display a predominantly ridge parallel fabric of elongate abyssal hills crosscut by several discordant zones throughout the zone faulting is the predominant factor in generating the morphotectonic fabric analysis of fault scarp distribution places the limit of active tectonic deformation between 2 and 3 ma 25 to 35 km on either side of the spreading axis fault scarp relief on the ridge flanks varies with a period of approximately 2 myr which corresponds to the wavelength of abyssal hills within the area we propose an interpretation in which abyssal hill formation occurs periodically 2 myr within the ridge mountains as the lithosphere transitions from the dynamic supported regime near the axis to the isostatic regime of the flanks at 2 3 ma 25 35 km from the axis

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

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

fault structure and detailed evolution of a slow spreading ridge segment the mid atlantic ridge at 29 n we present preliminary results of a detailed near bottom study of the morphology and tectonics of the 29 n broken spur segment on the slow spreading mid atlantic ridge using principally the tobi deep towed instrument the survey covered two thirds of the segment length including all of its southern non transform boundary and extended off axis of 40 km 3 3 ma on either side we obtained nearly complete near bottom sidescan sonar coverage and deep towed three component magnetic observations along 2 km spaced e w tracks sidescan data reveal new details of fault structure and evolution faults grow by along axis linkage in the inside corner they also link in the axis normal direction by curving to meet the next outer older fault this leads to wider spaced faults compared to segment centre or outside corner outward facing faults exist but are rare the non transform offset is characterised by faults that are highly oblique not parallel to the spreading direction and show cross cutting relations with ridge parallel faults to the north suggesting along axis migration of the offset almost all volcanic activity occurs within 5 km of the axis most fault growth is complete within 15 km of the axis 1 2 ma though large scarps continue to be degraded by mass wasting beyond there crustal magnetisation is strongly three dimensional the current neovolcanic zone is slightly oblique to earlier reversal boundaries and its magnetisation rises to a maximum of 30 a m 1 near its southern tip the central magnetisation high tapers southwards and is asymmetric with a sharp western but gradual eastern boundary we infer a highly asymmetric accretion of layer 2 near the segment end older magnetic anomalies are kinked and sometimes missing we interpret these observations as evidence of a rapid 18 km southward migration of the segment boundary during the past 1 8 ma and present a series of reconstructions illustrating this tectonic history

a quantitative study of the axial topography of the mid atlantic ridge some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults for example the presence of an axial depression coincides in most cases with slow spreading rates and the overall depth of the ridge axis increases toward ridge transform intersections rtis on the other hand it is also well known that the relief and width of the axial valley on say the mid atlantic ridge mar vary along strike in an unpredictable manner the purpose of the present study is to quantify how much of the observed variation in the first order topography at the axis is related to changes in other parameters such as spreading rate and distance from rtis to carry out this test the zero age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the mar between the equator and 50 n full spreading rates 22 36 km ma zero age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a gaussian bell the measured axial valley relief varies between 600 and 2100 m average 1300 m while the valley width varies between 16 and 62 km average 35 km the correlation between zero age depth axial valley relief and width latitude of axial crossing spreading rate distance from nearest rtt and offset on the nearby transforms has been investigated using linear regression techniques the main results of the present study are that 1 zero age depth significantly correlates with latitude of crossing distance from nearest rti and offset on the nearby transforms and 2 the variation in axial valley relief and width is essentially uncorrelated with spreading rate zero age depth distance from nearest rtt and offset on nearby transforms the preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation in fact if the rough abyssal hill topography typical of the mar flanks is created within the axial valley the shape of the axial valley cannot be steady state although the existence of an axial valley may be a steady state phenomenon this hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography measured as the residual on the thermal subsidence trend

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

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

detailed geological mapping of two contrasting second order segments of the mid atlantic ridge between oceanographer and hayes fracture zones 33 30 n 35 n we present detailed geological mapping of the axial valley floor and ridge flanks of two neighboring but contrasting spreading segments oh1 and oh3 of the mid atlantic ridge between oceanographer and hayes fracture zones new in situ observations from the submersible nautile correlated with swath bathymetry and acoustic backscattering data from these segments reveal that the along and across axis volcanic tectonic variability within second order segments is larger than commonly acknowledged segment oh1 is a long and robust segment with a narrow and shallow axial valley the most intense magmatic activity is found at the segment center where the thickest crust has been imaged suggesting focused magma supply away from this part of the segment faulting and sedimentary cover predominate in contrast the center of segment oh3 is dominated by extensive sedimentary cover fissuring and faulting furthermore the most recent constructional volcanism is located away from the segment center in a region of greater depth and thinner crust this relocation of volcanism suggests either a recent shift in the magmatic source or the last vestige of a centrally located source fed by lateral dike injection segment tip magmatic oscillations are suggested by the distribution of rock types at both segment ends serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform offsets ntos whereas only basaltic rocks crop out on the nodal basin floors we suggest that the combination of low magmatic budget and extension taking place at the ntos during a segment retreat favors the uplift and exposure of ultramafic massifs

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

magmato tectonic cyclicity at the ultra slow spreading southwest indian ridge evidence from variations of axial volcanic ridge morphology and abyssal hills pattern on axis deep tow side scan sonar data are used together with off axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra slow spreading southwest indian ridge differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments we interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development including periods of volcanic construction and periods of tectonic dismemberment using off axis bathymetric data we identify numerous abyssal hills with a homogeneous size for each segment these abyssal hills all display an asymmetric shape with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away we suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment i e a magmato tectonic cycle we observe that large abyssal hills are in ridge sections of thicker crust whereas smaller abyssal hills are in ridge sections of thinner crust this suggests that the magma supply controls the size of abyssal hills the abyssal hills in ridge sections of thinner crust are regularly spaced indicating that the magmato tectonic cycle is a pseudoperiodic process that lasts 0 4 m y about 4 to 6 times shorter than in ridge sections of thicker crust we suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long lived segments by contrast when magma supply strongly decreases and becomes highly discontinuous regular abyssal hills patterns are no longer observed