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

thermal model of oceanic transform faults a thennal model of the oceanic transform fault which includes effects of the lateral heat conduction across the fault and the shear heating along the fault has been developed the results show that shear heating along the transform fault significantly raises the temperature along the fault the depths to isotherms of the thermal model were compared to the depths of earthquakes along transform faults by including the shear heating the limiting isotherm of the focal depths of atlantic transform earthquakes is 700 c 600 c if not including the shear heating which is close to the limiting temperature of the intraplate oceanic earthquakes if the transform earthquakes are limited by the 750 c isotherm then the transform earthquakes should be shallow events as predicted by our thermal model the model predicts the maximum depth of the 750 c isotherm increases with the length of the transform fault and decreases with the half slip rate both the effects of the lateral heat conduction across the transform fault and the shear heating along the fault generate topographic and geoid anomalies in a 30 to 60 km wide zone at the fault with magnitudes of the order of a few hundred meters and tens of centimeters respectively given the thermal structure of the transform fault the shear traction along the fault can be estimated the average shear traction per unit length of the transform fault is of the order of 1012 n m which is comparable to the magnitude of the major plate driving forces

thermal stresses differential subsidence and flexure at oceanic fracture zones geosat geoid undulations over four pacific fracture zones have been analyzed after correcting for the isostatic thermal edge effect the amplitudes of the residuals are shown to be proportional to the age offset the shape of the residuals seems to broaden with increasing age both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults existing models for flexure at fracture zones cannot explain the observed anomalies a combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented our model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone the best fitting models have mechanical plate thicknesses that are described by the depth to the 600 700 c isotherms

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

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

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

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

geophysical insights and early spreading history in the vicinity of the jan mayen fracture zone norwegian greenland sea abstract using a new high resolution aeromagnetic survey jas 05 that was acquired along the trend of the jan mayen fracture zone jmfz west of the voring volcanic margin we investigated the geodynamic framework of the early spreading evolution of the norwegian greenland sea the tectonic structure main faults and magnetic chrons have been reinterpreted based on new magnetic gridded data and integrated with bathymetry gravity and seismic data the new interpretation reveals more details about the early spreading history of the norwegian greenland sea in the vicinity of the jmfz although anomalous melt production seaward dipping reflectors underplating associated with the breakup of the mid norwegian margin has been described in many studies we present data that suggest that significant magmatism continued episodically during the opening of the norwegian greenland sea along the trend of the jmfz the voring spur vs an anomalous oceanic high lying north of the eastern segment of the jmfz exhibits a contrasting bouguer gravity low and a complex magnetic signature the gravity signature of the vs can be modelled and explained as an abnormal thick oceanic crust which locally can reach up to 15 km we propose that the thick oceanic crust overcrusting was syn rift and formed during mid to late eocene a plate reconstruction at eocene time suggests that the vs could be part of a triple junction initiated during the breakup between the voring marginal high and the greenland part of the traill voring igneous complex now located offshore east greenland mantle upwelling beneath the early spreading ridge and or local stress reorganisation could have induced transtension and lithospheric thinning along the jmfz and magmatic activity would have increased locally along this leaky transform we suggest that the early tertiary tectono magmatic processes that operated in the norwegian greenland sea are similar to the processes involved in the modern triple junction evolution of the azores plateau region

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

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

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

transpression between two warm mafic plates the queen charlotte fault revisited the queen charlotte fault is a transpressive transform plate boundary between the pacific and north american plates offshore western canada previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate the latter has been the preferred model both plates are warm and mafic and have similar mechanical structures new multichannel seismic reflection data show a near vertical queen charlotte fault down to the first water bottom multiple significant subsidence east of the queen charlotte fault a large melange where the fault is in a compressive left step and faulting which involves oceanic basement gravity modeling of profiles indicates that moho varies fairly smoothly across the plate boundary isostatic anomalies indicate that the pacific plate is flexed downward adjacent to the queen charlotte fault upward flexure of north america along with crust thickened relative to crust in the adjacent basin creates topography known as the queen charlotte islands combined with other regional studies these observations suggest that the plate boundary is a vertical strike slip fault and that transpression is taken up within each plate

evaluation of low latitude pi2 pulsations as indicators of substorm onset using polar ultraviolet imagery impulsive pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes it is well documented that there is usually more than one pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm this observational fact poses obstacles to determining substorm onsets with pi2 signals although the pi2 have become one of the most popular indicators for substorm onsets the reliability of using the pi2 in this fashion has not been seriously investigated in this paper we address this question with a statistical approach by using 650 auroral substorm onsets identified with polar ultraviolet images for a time interval from april 1996 to may 1997 a comparison of the low latitude pi2 pulsation onsets from kakioka l 1 07 with the auroral breakups indicates that identifying substorm onset with the pi2 alone is often ambiguous of a total of 119 isolated defined as 10 min of quiet time preceding the onset pi2 bursts seen within 10 min from a magnetic positive bay there were 65 events 55 taking place within 3 min from breakups and 34 events 29 indicating no sign of an auroral breakup within 10 min of the pi2 burst this result suggests that pi2 may not be as a good indicator of the substorm onset as it was thought to be interestingly it is always possible to associate pi2 pulsations with some forms of auroral intensification when compared to auroral breakups pi2 onsets are subject to a small delay of 1 3 min with a peak around l min delays of pi2 onsets are revealed to be a function of location relative to auroral breakup this dependence is found to be consistent with the time of flight for a fast mode wave in a plasmapause cavity mode model propagating in the magnetosphere

surface abrasion of palsas by wind action in finnish lapland palsas at lake ahkojavri 69 35 n 26 11 e western utsjoki finnish lapland display very marked effects of surface erosion mainly westerly winds have caused the removal of a layer of peat in some cases more than 40 cm thick from the palsa surface the surface peat of one eroded palsa is radiocarbon dated to 2710 70 years bp many palsas with heights of 1 5 3 m are almost bare of vegetation cracks typical of vegetated palsa surfaces are either worn away or filled with wind drifted peat erosion takes place in winter by wind drifted snow and ice crystals abrading the peat surface some recently formed palsas have been found on the same palsa mires they are located among the eroded mature palsas and range from 60 to 80 cm in height and 20 to 30 m2 in area the new palsas also imply strong wind activity in winter no unusual development in the depth of the active layer on palsas has been noticed new permafrost is also found in small peat hummocks called pounus

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

the implications of long lived asymmetry of remanent magnetization across the north pacific fracture zones article i nfo large marine magnetic anomalies accompany the pacific fracture zones fzs for thousands of kilometers although the origin of these anomalies is poorly understood their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three fzs from the north pacific cretaceous quiet zone 120 6 to 83 ma that are characterized by a remarkably uniform shape forward and inverse modeling indicate that these anomalies arise from remanent magnetization with enhanced remanence located on one side of each fz along the entire studied area a comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the cretaceous quiet zone as well a more complex magnetization setting is observed where the fzs contain multiple faults there the simple model of one sided enhancements is only partly valid comparison between 3d forward modeling of the quiet zone magnetization and the calculated magnetization contrasts found across the pioneer and pau fzs suggests that the intensity of the geomagnetic field during the cretaceous superchron had less than 50 percent variability about its average value no major trends in the strength of the geomagnetic field during the superchron are observed the presence of long duration 30 m y zones of enhanced magnetization along the young old sides of the pioneer pau fzs both left stepping requires some long lived asymmetry in crustal construction processes near ridge transform intersections although the underlying mechanism that controls this long lived asymmetry remains unclear absolute plate motions might explain this asymmetry shorter period few m y variations in the amplitudes of the enhancements probably result from oscillations in crustal construction

insights into the lurking structures and related intraplate earthquakes in the region of bay of bengal using gravity and full gravity gradient tensor comprehension of subsurface structures buried under thick sediments in the region of bay of bengal is vital as structural features are the key parameters that influence or are caused by the subsurface deformation and tectonic events like earthquakes here we address this issue using the integrated analysis and interpretation of gravity and full gravity gradient tensor with few seismic profiles available in the poorly known region a 2d model of the deep earth crust mantle is constructed and interpreted with gravity gradients and seismic profiles which made it possible to obtain a visual image of a deep seated fault below the basement associated with thick sediments strata gravity modelling along a ne sw profile crossing the hypocentre of the earthquake of 21 may 2014 m w 6 0 in the northern bay of bengal suggests that the location of intraplate normal dip fault earthquake in the upper mantle is at the boundary of density anomalies which is probably connected to the crustal fault we also report an enhanced structural trend of two major ridges the 85 e and the 90 e ridges hidden under the sedimentary cover from the computed full gravity gradients tensor components

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

new observations of the shallow seismic structure of young oceanic crust the results are presented of three new experiments carried out on the mid atlantic ridge mar near latitude 23 n using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust the data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography these 1 to 2 km long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200 300 m of the young igneous crust one experiment was carried out over the site of hole 648b of the ocean drilling program on a small volcano within the median valley of the mar the seafloor velocity was observed to be 2 1 km s 1 underlain by an approximately linear velocity gradient of 4 s 1 given that we know the crust at this location consists of fresh basalt lavas with laboratory measured velocities in excess of 5 8 km s 1 porosities at the seafloor of as high as 30 50 are inferred the two other experiments were located over 7 m y old crust near deep sea drilling project site 395 west of the mar separated by only 14 km laterally but by over 1400 m in water depth the experiment positioned in the 100 m thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable the velocity of the uppermost basaltic basement was 4 1 km s 1 and the velocity gradient was less than 0 5 s 1 this 2 km s 1 difference in velocity between the two sites is assumed to be a consequence of age related modifications to the physical properties of the crust if the primary change is in only the total porosity then a 15 20 reduction is required to explain these observations this seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data e g off axis volcanism secondary mineral deposition tectonic compression a combination of many processes acting in unison would be necessary to produce such a decrease in porosity if the geometry of the cracks and voids were modified in just the right way then no change in total porosity is required to explain the velocity difference these on bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor s geologic features they produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements

a three dimensional gravity study of the pacific antarctic east pacific rise menard transform intersection three dimensional gravity analysis has been carried out over the eastern intersection of the menard transform with the pacific antarctic east pacific rise at 49 8 s typical of the east pacific rise epr the residual mantle bouguer anomalies are nearly constant varying less than 5 mgal along the 60 km of ridge axis surveyed this uniform character of the residual mantle bouguer anomalies suggests that this part of the epr axis has a fairly uniform crustal upper mantle structure although this observation is less constrained by the only three crossings along the ridge axis it is consistent with observations elsewhere along the epr significant features such as thinner crust are not suggested by the residual mantle bouguer anomalies rmba at the ridge transform intersection or along the eastern 75 km of the menard transform which is in marked contrast to what has been reported for the large gravity signatures at the slow spreading mid atlantic ridge the lack of the positive rmba at the intersection could be due to the dual effects of a relatively small thermal contrast across the intersection and the excess lavas from the overshot ridge at the ridge transform intersection fresh lavas from the observed overshot ridge have filled in the transform valley and have subsequently thickened the crust from above possibly compensating the effect of any thinner crust there a large active seamount is present near the inside corner about 40 km west of the ridge axis and fresh lavas were observed surrounding the seafloor and between the seamount and the ridge a large circular low in the residual mantle bouguer anomalies surrounds the seamount suggesting that both a thicker crust due to the active volcanism and hotter less dense material are the source of the gravity low

constraints on melt production rate beneath the mid ocean ridges based on passive flow models we present a model for computing the total melt production rate from the decompression partial melting region beneath a mid ocean ridge and the maximum oceanic crustal thickness created at the ridge axis assuming an ideal melt migration mechanism the calculations are based on a self consistent numerical model for the thermal structure and steady state mantle flow field at a mid ocean ridge the model includes the effect of decreasing the melt production rate within the partial melting region by melt extraction as the residual mantle matrix becomes increasingly difficult to melt thus the melt fraction depends not only on temperature and pressure determined by the location beneath the ridge axis the eulerian description but also on the accumulated melt extraction since the upwelling mantle matrix enters the partial melting region determined by the location along the flow line path the langrangian description this effect has been neglected by previous models the model can predict the size of the melting region and the locations of the boundaries between mantle residual mantle and the partial melting region for a given spreading rate also the distribution of the melt depletion and the mean melting depth given the observed average thickness of oceanic crust 6 km which is relatively independent of spreading rate the model results also provide a constraint on the overall efficiency of melt migration to the ridge axis the efficiency must decrease from 100 at 10 mm yr to about 60 at fast spreading rates 50 mm yr although this reduction may be partially due to the increasing size of the melting region with increasing spreading rate it still requires less efficient melt migration near the ridge axis at fast spreading rate we found that the calculated crustal thickness is very sensitive to the mantle temperature for a normal mantle temperature of 1350 c the model can generate the observed 6 km oceanic crust over the global range of spreading rates while the anomalous thicker crusts of the iceland hotspot and the reykjanes ridge are related to higher mantle temperatures associated with the hotspot finally by comparing our model results with previous ones we found that neglecting variations of the melting relations of the residual mantle matrix with melt removal will overestimate the crustal thickness by at least a factor of 1 7

hydrostatic system palsas at toolik lake alaska field observations and simulation stratigraphic and chemical data from the ice core of an anthropogenic palsa at toolik lake alaska indicate that the mound formed as a consequence of hydrostatic pressure developed in an isolated hydrologic system within the active layer survey data for five palsas over a three year period suggest that growth was essentially complete at the time of the initial survey a net decrease of summit elevation is apparent in all five mounds but complete degradation of the palsas would require several decades at observed rates because accurate field measurements of thermal and hydrologic evolution in such features are extremely difficult simulation of the environmental conditions and events involved in palsa growth is an important supplement to field observation both analytic and finite element models yield results that are in substantial agreement with inferences drawn from observational data

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

modeling of postseismic relaxation following the great 1857 earthquake southern california abstract repeated first order triangulation measurements made from 15 monuments in central and southern california are used to explore the structure of the crust and upper mantle beneath southern california as well as to estimate the distribution of slip during the great 1857 earthquake the monuments are divided into two subnetworks with repeated observations covering the respective time intervals from 1884 to 1923 and from 1898 to 1923 the angle changes occurring within these times are modeled as a sum of two contributions postseismic displacements following the 1857 earthquake and plate tectonic loading the postseismic displacements have been calculated using a elastic viscoelastic coupling model that for any fault geometry yields the exact displacements on a spherically stratified earth the southern california crust and upper mantle are modeled as an upper elastic plate underlain by a viscoelastic asthenosphere for several different combinations of structural parameters inversions are performed for the slip distribution of the 1857 earthquake in general the best fits to the data are achieved with a triangular slip distribution an elastic plate thickness of 16 km a crustal thickness of 16 to 33 km and a mantle viscosity of 0 4 to 0 8 10 19 pa s although these inversions identify the best rheological models they lead to observed residuals that are inconsistent with a normally distributed data set a preferred model of slip is constructed that accounts for the budget of total slip during the earthquake cycle and satisfies a normally distributed data set the preferred slip distribution is similar in shape to the mapped surface slip distribution but is greater in amplitude by a factor of 1 5 to 2 in addition the san jacinto fault which has no known surface slip associated with the 1857 earthquake is inferred to have accommodated 1 to 3 m of slip during the 1857 earthquake and appears to be the favored site for tectonic strain accumulation since 1857 inferred fault slip exceeding known surface slip implies either unrecognized secondary faulting or a slip deficit at the surface relative to the slip at depth

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

observations of soil freezing and frost heave at inuvik northwest territories canada field observations of soil temperature moisture regime and frost heave in silty clay hummocks at inuvik northwest territories over the fall and early winter reveal that a significant amount of moisture migration and frost heave occurs within frozen soil at temperatures down to 2 4 c the field data are analysed using thermodynamic considerations and the apparent hydraulic conductivity is determined as a function of negative temperature the conductivity falls from near 7 10 9 m s 1 above 0 c to about 3 5 10 12 m s 1 at 1 c then remains fairly constant down to 2 4 c the observed decrease in heave with time is explained in terms of a diminishing water supply at the base of the active layer

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

mechanical control of oceanic plate boundary geometry we present a global analysis of oceanic plate boundary geometry based on the mechanics of relative plate motion at mid ocean ridges and transform faults if the observed geometry formed by the first order segmentation of oceanic plate boundaries represents a state of mechanical equilibrium we find the relative strength of spreading ridges and their bounding transform faults to be fundamental to its organisation a consideration of power dissipation along adjacent lengths of spreading ridge and transform fault leads to a simple relationship between spreading obliquity and relative strength increased spreading rate is found to decrease the strength of spreading segments relative to transform faults proximity to an active hotspot reduces the relative strength of spreading ridges

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

centroid depths and mechanisms of mid ocean ridge earthquakes in the indian ocean gulf of aden and red sea we have determined the centroid depths and source mechanisms of 14 earthquakes along spreading centers in the indian ocean gulf of aden and red sea the earthquakes are all characterized by normal faulting on planes dipping at about 45 and generally striking parallel to the local trend of the spreading center for the ridge axis earthquakes in the indian ocean gulf of aden and central red sea seismic moments range from 3 to 9 1024 dyn cm source time functions are all of simple form and centroid depths of events with adequate azimuthal coverage are all very shallow between 1 and 4 km beneath the seafloor if the centroid depth typically marks the mean depth of fault slip then earthquake faulting extended to 2 8 km beneath the seafloor during these earthquakes the centroid depths for two earth quakes near the mouth of the gulf of suez support the view that seismic faulting accompanying continental rifting generally extends to greater depth than does seismic faulting along the axes of oceanic spreading centers when these findings are combined with our earlier results from the mid atlantic ridge the greatest centroid depths of earthquakes with well constrained source mechanisms tend to shoal with increasing spreading rate this result supports a general decrease with spreading rate in the maximum thickness of the zone of brittle behavior for slow spreading ridges

diffuse impact of the mid atlantic ridge with the romanche transform an ultracold ridge transform intersection the romanche is a long offset 950 km slow slip 1 7 cm yr transform thus a hot ridge axis should meet a 50 m y old thick and cold lithosphere at the ridge transform intersection rti a strong thermal topographic transform cold edge effect is therefore predicted a morphobathymetric seismic reflection and petrologic study of the eastern romanche rti shows that as the mid atlantic ridge approaches the transform a well formed axial rift valley disappears about 80 km from the rti and is substituted by short en echelon poorly developed axial ridge segments they too disappear about 30 km from the edge of the transform valley the predicted gradual deepening of the ridge axis toward the transform was not observed an active nodal deep and an inside corner high are also absent these observations and the distribution of earthquake epicenters suggest a poorly developed diffuse rti an inactive rift valley 80 km west of the present rti suggests ridge jumping within the last 4 m y the present poorly developed rti may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform we infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle derived serpentinized peridotites outcrop ubiquitously on the seafloor starting 30 km from the edge of the transform valley the unusually deep 4 km below sea level axial ridge segments the lack of crust and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform these results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold rtis and that ultracold rtis are magma starved short lived and unstable in time and space

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model

la reunion an example of channeled hot spot plume la reunion a large intraplate volcanic system in the mascarene basin indian ocean is widely considered as the expression of a mantle plume or hotspot it lies in a region of the mascarene basin bounded by 2 major transform faults and an extinct rift the bathymetry suggests the presence of a positive topographic anomaly around la reunion we have compared the bathymetry with a theoretical bathymetry based on the thermal subsidence of the oceanic lithosphere as a function of its age a bathymetric anomaly of several hundreds of meters is shown but it is confined to the la reunion zone analogue models have been carried out to explore the deformation resulting from a mantle plume rising beneath a faulted lithosphere they show that the plume and the associated deformation are channeled into the area enclosed by the transform faults and the paleo rift the deformation is virtually nil outside the zone fault bounded zone we have analyzed the surface features and the available gravity and magnetic data in order to find evidence of the deformation predicted by the presence of the bathymetric anomaly and by the analogue models no clear pattern of deformation has been recognized but we demonstrate the complex organization of the oceanic crust in the la reunion zone the fact that the plume head focuses on the restricted area of the zone may amplify the dynamic support of the lithosphere beneath la reunion where not flexure is observed we also suspect that some seamounts near la reunion could correspond to a recent magmatic activity distinct from that of the island the mantle plume lithosphere interaction model presented here is a new to the oceanic domain and it will have to be confirmed by subsequent observations in the la reunion area and possibly in other similar sites on earth

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

the largest recorded earthquake swarm intraplate faulting near the southwest indian ridge the most prolific source of moderate and large earthquakes along the mid ocean ridge system is located near the atlantis ii transform along the southwest indian ridge a remarkable swarm of 48 earthquakes was recorded from 1925 to 1933 which included 29 events with m 6 0 and 4 events with m 7 0 examination of the global earthquake catalog suggests this is the largest known earthquake swarm relocated events cluster around 33 8 s 58 0 e approximately 100 km east of the southern intersection of the atlantis ii transform and the spreading ridge and epicentral uncertainties derived from both standard least squares and resampling techniques preclude locations along the plate boundary multiple event relocation shows that seven 1977 1983 events mb 5 1 5 6 are distributed across a 40 km diameter region at the location of the 1925 1933 swarm events mechanisms determined for three of the recent events using waveform inversion indicate normal faulting with variable strike slip components at depths of 12 17 km neither the nodal planes nor the tensional axes show consistency between the three mechanisms grid search fits to body waveforms from the larger 1925 1933 swarm earthquakes suggest normal faulting mechanisms similar to the recent events with depths of 11 20 km and seismic moments ranging up to 1 8 1027 dyn cm the earthquake depths and tensional mechanisms are consistent with release of thermoelastic stress which may be enhanced at this location by thermal bending moments associated with the transform fault the total moment release of the 1925 1933 swarm 7 1027 dyn cm is greater than the cumulative moment of all known oceanic intraplate seismicity since 1963 3 1027 dyn cm this suggests that intraplate stress release is dominated by infrequent but highly energetic episodes of seismicity and that estimates of intraplate moment release taken from observations over a limited time period underestimate intraplate seismicity levels

an information theory approach to the storm substorm relationship 1 one of the most interesting aspects of the global magnetospheric response to solar wind changes is the relationship between storms and substorms here we present new results on the relationship between these two different classes of magnetospheric phenomena by approaching the problem on the side of information theory using the auroral electrojet al and sym h indices as representative proxies of magnetic substorms and storms we investigate the transfer of information by means of transfer entropy analysis schreiber 2000 the obtained results seem on average to indicate the presence of a net transfer of information from al to sym h on time scales shorter than 10 h on the basis of this result geomagnetic substorms may act as a driver for the occurrence of geomagnetic storms however carrying out a more careful analysis which takes into account the global geomagnetic daily activity we suggest that the direction of information flow between substorms and storms depends on the global activity level indeed if it is true that a sequence of magnetospheric substorms may drive a moderate storm it is also true that very large storms may dominate and drive the occurrence of magnetospheric substorms

a mechanical model for the inside corner uplift at a ridge transform intersection at least part of the inside corner uplift observed at slow ridge transform intersections is shown to be a consequence of the flexure of an elastic plate caused by a twisting moment exerted along the transform fault the frictional drag exerted along the transform increases with depth overburden down to the depth where plastic flow predominates this depth dependent drag results in a moment applied to the edge of the plate an uplift of an inside corner of several hundred meters can be obtained by this mechanism this can be increased to roughly 1 km the observed uplift at some intersections if we assume the elastic thickness of the plate is effectively thinner because of relaxation by long term creep of the stresses resisting plate flexure simple models formulated to test this hypothesis show that the observed inside comer uplift of the vema and oceanographer transforms can be explained if the effective elastic thickness of the plate is about half of the thickness over which drag increases with depth along the transform fault boundary

lithospheric bending at subduction zones based on depth soundings and satellite gravity a global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles original depth soundings and satellite derived gravity profiles a thin elastic plate flexure model was fit to each bathymetry gravity profile by minimization of the l norm the six model parameters were regional depth regional gravity trench axis location flexural wavelength flexural amplitude and lithospheric density a regional tilt parameter was not required after correcting for age related trend using a new high resolution age map estimates of the density parameter confirm that most outer rises are uncompensated we find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches as in previous studies we find that the gravity data favor a longer wavelength flexure than the bathymetry data a joint topography gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model residual scatter obscures the systematic increase of mechanical thickness with age perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength such as extreme inelastic bending and accumulated thermoelastic stress the bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 ma this reflects the increase in saturation bending moment that the lithosphere can maintain using a stiff dry olivine rheology we find that the lithosphere of the gdh1 thermal model stein and stein 1992 is too hot and thin to maintain the observed bending moments moreover the regional depth seaward of the oldest trenches 150 ma exceeds the gdh1 model depths by about 400 m

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

mechanisms for the formation of ridge axis topography atslow spreading ridges a lithospheric plate flexural model the seafloor topography of a slow spreading ridge shows a number of well documented regularities at the ridge segment scale as the result of the complex interplay between ridge axis magmatic and tectonic processes this paper describes the results of a detailed analysis of the seafloor topography of the mid atlantic ridge near the atlantis transform where marine gravity data provide independent although non unique constraints on subseafloor density structure using a combined topography and gravity data set we identified the specific contributions of subseafloor density structure to the seafloor topography we show that the observed along axis deepening 0 3 0 8 km from the midpoint of a ridge segment towards the non transform offsets in the study area can be explained by the vertical deflection of a zero age plate in response to along axis crustal thickness variations however this effect can only account for 50 60 per cent of the observed 1 5 1 7 km deepening towards the atlantis transform suggesting the presence of significant stresses in the lithosphere near a transform results of plate flexural calculations also predict a more elevated rift flank at the inside corner of the ridge transform intersection than at the conjugate outside corner such an asymmetry in rift flank topography is calculated to be greatest near a transform fault with a significant volume of deep transform valley and when adjacent plates across the transform fault are mechanically decoupled or only weakly coupled together these results illustrate the complex interplay between various tectonic processes at a slow spreading ridge

locations of mid oceanic earthquakes constrained by seafloor bathymetry 1 earthquakes associated with the creation of new oceanic crust are difficult to locate precisely because they normally occur far away from seismic stations we assume that epicenters of these events can be constrained by seafloor topography and use the harvard centroid moment tensor cmt focal mechanism of an interplate earthquake to relate the event to either a transform fault a parallel in a coordinate system defined by the pole of rotation between two plates or a mid oceanic ridge a meridian using arrival times from the international seismological center isc bulletin we have relocated about 1500 globally distributed events for the time period 1976 1998 the isc locations can be as much as 70 km away from appropriate topographic features our new locations support that large interplate earthquakes occur on or very close to the primary bathymetric features for earthquakes with unknown mechanism or magnitude below the cmt threshold or predate the cmt catalog we use a simultaneous joint hypocenter determination procedure with respect to the already relocated master events jhdme we invert for earthquakes first for the romanche fracture zone rfz and then for a region extending from 30 n to 30 s seismicity agrees well with the main active ridge or transform however a small number of events might be related to the secondary bathymetry features the improved locations for mid oceanic earthquakes based on bathymetry could help improve our understanding of the oceanic crustal and deep earth structure seismic slip and budget constraint along plate boundaries and dynamics of plate tectonics

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading east pacific rise motivated by recent observations a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading east pacific rise modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros if significant 10 amount of the layer 3 gabbros is emplaced near the moho then the liberated latent heat during crystallization would generate a large molten section of the lower crust due to the inability of cold seawater to penetrate into this hot ductile region a large molten section of the lower crust is not consistent with recent seismic tomographic results of the east pacific rise thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the epr is significantly less than that suggested by oman ophiolite studies such a lower melt lens is short lived or a large portion of the sill intrusions observed at oman was emplaced at a few kilometers off axis 3 4 km

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

on the relation between transform fault resistance and plate motion a simple model for power dissipation due to friction along transform faults is used to examine the relations between relative velocity and power dissipation across transforms transform lengths and plate motions comparison is made between ridge transforms i e those that are part of spreading systems and major boundary transforms such as the san andreas fault power dissipation is calculated using velocity across the transform area above the 400 c oceanic or 350 c continental isotherm transform thermal structure is modeled by averaging the temperatures on either side of the transform whereas ridge transforms across which the relative plate velocity is slow can be of any length fast relative velocities only appear across short transforms when all transforms are used in this comparison a maximum speed as a function of transform length is suggested a minimum speed of 13 5 mm yr across ridge transforms was found in a case study of the pacific plate the transform lengths were found to have varied with time which has minimized the total power dissipation across the plate s transforms

seasat derived gravity constraints on stress and deformation in the northeastern indian ocean we use seasat derived gravity data to investigate crustal deformation in the ne indian ocean gravity highs reflecting crustal undulations vary in orientation from e w in the central indian basin to ne sw in the wharton basin the undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model cloetingh and wortel 1985 and are essentially restricted to the area of predicted compression for both principal horizontal stresses this agreement implies that the stress model describes the basic features of the deformation observed in the gravity as well as the seismicity the gravity data also provide insight into two enigmatic tectonic features the transition in the morphology of the 90 e ridge at 10 s from a continuous high to a complex blocky structure appears related to the deformation since undulations in the central indian basin can be projected eastward to blocks on the ridge the morphology formerly interpreted as a fossil feature may reflect the recent deformation the discrepancy between the trends of the southernmost 85 e ridge and the 90 e ridge previously thought to exclude similar hot spot track origins appears to result from treating a crustal undulation as part of the 85 e ridge earthquakes do not appear to be preferentially located with respect to the peaks and troughs of the undulations

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

determination of the effects of substorms on the storm time ring current using neural networks a controversy exists as to whether the injection of particles into the ring current is caused by the induced electric field associated with the substorm expansive phase or by the enhanced convection electric field that is directly driven by the solar wind to investigate the causal storm substorm relationship artificial neural networks are used to examine the correlation between the westward auroral electrojet index al and the ring current index dst we use the hourly solar wind number density n the solar wind velocity v the interplanetary magnetic field imf magnitude b and the southward component of the imf bs to predict separately the hourly dst and al indices for the first time we show using the prediction residuals the significant ut dependent effects of the substorm current wedge on dst as a consequence of the sparse distribution of only four stations that are used to compute the dst index when none of the dst stations is found beneath a typical substorm current wedge during the main phase of a magnetic storm we find no correlation of dst residuals with al residuals however if a dst station is found beneath a substorm current wedge during the main phase of a magnetic storm then a correlation of dst residuals with al residuals is observed moreover the mean correlation of dst residuals with al residuals over all phases of a magnetic storm is only 2 4 we therefore suggest that the expected effect of substorm expansive phase activity in the development of the storm time ring current is not detectable in the dst index but that the dst index is contaminated whenever a dst station passes beneath the substorm current wedge

earthquake deformation cycle on the san andreas fault near parkfield california six moderate earthquakes are thought to have ruptured the parkfield segment of the san andreas fault since 1857 the similar characteristics of the three seismically recorded events together with the quasi regular 22 year recurrence interval have led to the forecast of a similar event in 1988 5 years in this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent june 1966 parkfield earthquake for a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip although the geodetic data determine only broad spatial averages of the fault slip they do require that slip at depth exceeded the surface offsets measured in 1966 inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14 25 years extreme values range from as little as 6 years to as much as 29 years in comparison historic recurrence intervals of parkfield earthquakes range from 12 to 32 years these results are consistent with the expectation of a magnitude 5 6 earthquake near parkfield by 1993

segmentation of the blanco transform fault zone from earthquake analysis complex tectonics of an oceanic transform fault 1 the blanco transform fault zone btfz forms the 350 km long pacific juan de fuca plate boundary between the gorda and juan de fuca ridges nearby broadband seismic networks provide a unique framework for a detailed long term seismotectonic study of an entire oceanic transform fault otf system we use regional waveforms to determine 129 earthquake source parameters combined with 28 harvard moment tensors they represent the largest waveform derived otf source parameter data set joint epicenter determination removes the northeasterly routine location bias projecting seismicity onto the btfz we determine along fault seismic slip rate variations earthquake source parameters and morphology indicate several transform segments separated by extensional step overs the eastern segment from gorda ridge to gorda depression is a pull apart basin the longest transform 150 km following blanco ridge from the gorda to cascadia depression is seismically very active seismically fully coupled has a wider seismic zone 9 km than other btfz transform segments and accommodates the largest mw 6 4 6 5 btfz earthquakes interpretation of cascadia depression as spreading ridge is supported by plate motion parallel normal faulting t axes spreading is currently tectonic 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling a short transform connects to the pull apart surveyor depression widely spread seismicity along the western btfz reflects complex morphology indicating ongoing plate boundary reorganization along short narrow width subparallel faults seismic coupling is low in extensional 15 compared to transform areas 35 100 implying different mechanical properties centroid depth variations are consistent with seismic slip cutoff near 600 c

timing of transverse ridge uplift along the vema transform central atlantic abstract transverse ridges are large topographic anomalies running adjacent to slow slip oceanic transforms they form due to different processes including thermal stresses hydration dehydration of peridotites non linear viscoelastic rheology of the oceanic crust and vertical tectonic motions of lithospheric slivers induced by changes in ridge transform geometry causing transpression and or transtension along the transform boundary a prominent transverse ridge on the southern side of the vema transform central atlantic rose probably between 12 and 10 ma along the entire length 320 km of the transform exposing a relatively undisturbed section of oceanic lithosphere we used pelagic limestones encrusting serpentinized peridotites sampled from the lower slopes of the uplifted lithospheric section to date this uplift and define mechanisms of its emplacement ages were obtained both by micropaleontology foraminifera and nannofossils and by 87 sr 86 sr isotope ratios no ages older than 12 ma were obtained even in samples recovered at sites with crustal ages determined by magnetic anomalies well over 12 ma on the other side ages as young as 5 6 8 3 ma were found in clusters of samples collected from the eastern part of the transverse ridge probably due to mass wasting episodes that rejuvenated the substratum these results support the hypothesis that the vema transverse ridge rose between 12 and 10 ma due to flexural uplift related to transtension along the transform in line with a general model whereby transverse ridges rise during discrete events as a consequence of changes in ridge transform geometry

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

synthesis of studies of palsa formation underlining the importance of local environmental and physical characteristics this review presents a synthesis of four decades of palsa studies based on field experiments and observations mainly in fennoscandia as well as laboratory measurements palsas are peat covered mounds with a permanently frozen core in finnish lapland they range from 0 5 to 7 m in height and from 2 to 150 m in diameter these small landforms are characteristic of the southern margin of the discontinuous permafrost zone palsa formation requires certain environmental conditions long lasting air temperature below 0 c thin snow cover and low summer precipitation the development and persistence of their frozen core is sensitive to the physical properties of peat the thermal conductivity of wet and frozen peat is high and it decreases significantly as the peat dries and thaws this affects the development of the active layer and makes its response to climate change complex the insulating properties of dry peat during hot and dry summers moderate the thawing of the active layer on palsas in contrast humid and wet weather during the summer causes deep thawing and may destroy the frozen core of palsas ice layers in palsas have previously been interpreted as ice segregation features but because peat is not frost susceptible the ice layers are now reinterpreted as resulting from ice growth at the base of a frozen core that is effectively floating in a mire

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

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

erosion and flexural uplift along transform faults summary in all geodynamic settings intracontinental continent ocean intraoceanic the main morphological feature of transform plate boundaries is a narrow and elongated valley bounded on at least one side by an uplifted shoulder several mechanisms have been proposed to explain this uplift differential thermal subsidence lateral heat transfer extension perpendicular to the transform but none can be used in all settings we propose that the erosion of a lithospheric plate along the transform boundary may be an alternative mechanism that can be active in all geodynamic settings erosional unloading produces flexural uplift along the transform border which does not exceed 35 40 per cent of the difference in height between the lithospheric plate and the transform valley our model perfectly fits the morphology of several examples in the continental lithosphere the uplifted shoulder of the dead sea transform as well as marginal ridges in continental transform margins along an oceanic transform fault the erosion of the transform border only partially explains the uplift of the highest transverse ridges but may amplify other mechanisms

source characterization of two reykjanes ridge earthquakes surface waves and moment tensors p waveforms and nonorthogonal nodal planes well constrained fault plane solutions from p wave first motions for mid ocean ridge normal faulting earthquakes usually require nonorthogonal nodal planes local structural effects and or departures from a double couple source mechanism have been invoked to explain this phenomenon in order to obtain an independent determination of the source mechanisms for the april 24 1970 and april 3 1972 events on the southern reykjanes ridge we invert the rayleigh wave radiation pattern to obtain the source moment tensor the moment tensor formulation should be particularly well suited to this problem because it is not restricted a priori to a double couple source mechanism a potential drawback of the technique however is the requirement that phase velocities along the earthquake station paths be known very accurately in order to obtain the source phase from the observed phase and an objective of this study was to determine whether a regionalized phase velocity model compiled from published dispersion curves is adequate the results of the moment tensor inversion for both events indicate shallow normal faulting with the tension axis approximately horizontal and perpendicular to the local strike of the ridge apparent departures from a pure double couple source seem to result from errors in the data and the poor resolution of the mxz and myz components of the moment tensor for shallow sources after performing the inversion under a series of increasingly more stringent constraints we conclude that the data for both events are compatible with pure double couple sources with moments of 4 8 and 7 5 1024 dyn cm respectively we then show that interference between p pp and sp due to shallowness of the source can account for the observed nonorthogonality and match the observed p waveforms for the april 3 1972 event with theoretical seismograms calculated for a shear fault whose orientation is consistent with the surface wave solution the best fit to the data is obtained for a long narrow fault 13 km by 3 km with rupture initiating near the seafloor the moment indicated by the p waves is 7 5 1024 dyn cm these source parameters give an average displacement of about 60 cm and a stress drop of 30 60 bars taking into account various uncertainties although we might expect attentuation to be high in the mid ocean ridge environment the average attenuation required to fit the teleseismic data is not higher than normal t 1 s the p waves from the april 24 1970 earthquake were too small to be suitable for quantitative modeling by synthetic seismograms but are qualitatively consistent with a shallow fault model similar to that for the larger event we conclude that the faulting process described by these two earthquake mechanisms is directly related to the formation of rift valley topography

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley