subsurface tectonic structure between overlapping mid ocean ridge segments our results from seismic anisotropy analyses reveal for the first time the complex spatial variability of the characteristics of subsurface tectonic structures associated with ridge propagation the significance lies in the fact that these variations are found at a locality with few lineaments or fissures at seafloor level the overlap region between mid ocean ridge segments at 98n on the east pacific rise is characterized by aligned cracks that are structurally more closely related to the propagating ridge segment these aligned cracks which are approximately parallel to the ridge segments provide conclusive observational evidence for establishing the nontransform nature of overlapping spreading centers especially those with overlap basins covered by volcanic edifices the aligned cracks of the 98039n overlapping spreading center are more similar to the ridge parallel lineaments observed between overlapping axial summit collapse troughs than those found at larger overlapping spreading centers our results therefore suggest that the lithospheric deformation between overlapping ridge segments depends on ridge offset and that this dependency may be thermally related

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

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

transform zone migration implications of bookshelf faulting at oceanic and icelandic propagating ridges at a propagating ridge a migrating transform zone mtz offsets the propagating and retreating spreading axes we examine several kinematic models of mtz deformation to determine which models can generate observed seafloor fabric orientations vertical tectonics and faulting patterns within the 95 5 w galapagos propagating ridge pr system models that involve transform parallel simple shear within the mtz can fit observed seafloor fabric rotation patterns within the 95 5 w mtz however these models predict pervasive transform parallel strike slip faults within the mtz that are not observed and do not predict the dramatic shear concurrent deepening and post shear uplift that is seen in this propagating ridge system since we only measure the amount of structural rotation of the abyssal hill fabric within the mtz and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills this structural information can only determine the rotational component of the finite strain tensor thus this simple shear family of kinematic deformation models is actually only a small subset of mtz deformation patterns that can produce the same finite rotation patterns but with different amounts of shortening and or extension parallel or normal to abyssal hills there is strong seismic and surface fault break evidence in iceland that the south iceland seismic zone mtz propagates along an array of en echelon transform perpendicular strike slip faults a likely reason for this is that if deformation were accommodated through transform parallel strike slip faults then migration of the transform zone would require the creation of new transform parallel strike slip faults for each increment of transform migration instead if transform migration can be accommodated through slip on preexisting faults then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration we examine the kinematic consequences of finite mtz deformation within this generalized bookshelf faulting deformation scenario we find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95 5 w galapagos pr system vertical tectonics are an almost inevitable by product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation thus local extension within the abyssal hills of de steiguer deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated we conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a mtz since it can explain not only the structural rotations observed at the 95 5 w propagating ridge but can also relate these rotations to the uplift history recorded there

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

southeast indian ocean ridge earthquake sequences from cross correlation analysis of hydroacoustic data summary parameters of earthquake sequences for instance location and timing of foreshocks and aftershocks are critical for understanding dynamics of mid ocean ridge and transform faults whole sequences including small earthquakes in the ocean cannot be well recorded by land based seismometers due to large epicentral distances recent hydroacoustic studies have demonstrated that t waves are very effective in detecting small submarine earthquakes because of little energy loss during propagation in sound fixing and ranging channel for example an mw 6 2 2006 march 6 40 11 s 78 49 e transform fault earthquake occurred at the southeastern indian ocean ridge but national earthquake information center only reported three aftershocks in the first following week we applied cross correlation method to hydroacoustic data from the international monitoring system arrays in the indian ocean to examine the whole earthquake sequence we detected 14 aftershocks and none foreshock for the earthquake and locations of these aftershocks show an irregular pattern from the observation we suggest that the feature could be caused by complicated transcurrent plate boundary dynamics between two overlapped spreading ridges that is possibly explained by the bookshelf faulting model

formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate application to the 95 5 w galapagos propagator the geometry of oceanic spreading centers can change by rift propagation in which a new ridge segment propagates and preempts a preexisting one as the locus of relative motion between the two plates shifts the dying and growing ridge segments are joined by a boundary usually a transform which migrates with time causing seafloor to be transferred from one plate to the other as a result originally ridge parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction an attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics since relative motion occurs only between points on different plates recent surveys of the galapagos propagating rift system at 95 w show seafloor fabric curving away from the growing rift and toward the failed rift these observations have been interpreted as requiring a zone of distributed simple shear between the two rifts such behavior would represent a local deviation from rigid plate tectonics since relative motion would be distributed over a finite region not part of either of the two plates here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation we find that such fabric can be generated by changes either in the velocity rate or direction of rift propagation or in the spreading rate during rift propagation the curvature of various tectonic elements is diagnostic of the different possibilities the geometry observed at the galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate propagator acceleration and spreading rate slowing which produce similar structural trends could in principle be distinguished using spreading rate data at galapagos however since either would have occurred within the present period of normal polarity both are consistent with the magnetic anomaly data we thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics it may however be possible to discriminate between rigid and shear models using fabric observations at other propagators the shear model requires that the sense of curvature at galapagos be a ubiquitous feature of active propagators in contrast the rigid plate tectonic model allows for fabric with a variety of orientations including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

bio optical characterization of the northern antarctic peninsula waters absorption budget and insights on particulate backscattering abstract we utilize a comprehensive set of bio optical properties collected in the northern antarctic peninsula to contribute to the poor documentation of in situ bio optical measurements in the southern ocean the relative contributions of phytoplankton colored dissolved organic matter cdom and detritus to light absorption highlight the importance of phytoplankton but cdom contribution is often important in the blue range of the spectra despite its roughly constant values at 443 nm along the three orders of magnitude of total chlorophyll a concentration tchl a 0 019 2 91 mg m 3 observed in the upper 100 m the particulate backscattering coefficient bbp was remarkably low if compared to other oceanic waters but agrees with previous studies in the southern ocean even with very low absorption detritus was the component better correlated with particulate backscattering in the antarctic peninsula while phytoplankton cells dominant in the particles pool mostly covaried with particulate scattering particulate backscattering ratios bbp divided by the particulate scattering coefficient bp were also below values observed in other oceanic waters the spectral diffuse attenuation coefficient kd was highly correlated to tchl a r2 0 90 at 443 nm and showed no dependence on bbp indeed kd 443 and non water absorption coefficients at 443 nm were related by a 1 to 1 dependence the shape of the spectral remote sensing reflectance varied little responding mainly to variability in tchl a while tchl a vs maximum band ratios dependence deviated from global trends in a very similar fashion as in other studies of the southern ocean likely due to very low bbp

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

present day kinematics of the rivera plate and implications for tectonics in southwestern mexico we model the present day motion of the rivera plate relative to the pacific north america and cocos plates and combine the results with the nuvel i model for cocos north america motion to examine present day deformation in southwestern mexico the pacific rivera data which include 25 three m y average spreading rates determined from original surface ship magnetic data and 22 azimuthal data along the rivera transform are systematically misfit by the nuvel i pacific north america and pacifie cocos euler vectors indicating that the rivera plate is kinematically distinct from north america and cocos an f test shows that the improvement in fit to the pacific rivera data from a model with a distinct rivera plate exceeds that expected solely from adding three model parameters the rivera north america euler vector derived from closure of the rivera pacific north america plate circuit predicts slower and more trenchnormal convergence along the acapulco trench than prior models in addition the model predicts convergence normal to the eastern tamayo fracture zone at a rate 60 slower than the right lateral strike slip predicted by prior models an observed systematic misfit of the pacific rivera euler vector to azimuths from the western rivera transform fault may have several causes there may be a time averaging problem between the 3 0 m y average rates and the shorter time average transform fault azimuths and earthquake slip vectors alternatively rivera transform trends may be biased by deformation within lithosphere adjacent to the transform valley or future detailed mapping of the transform may reveal the misfit to be an artifact of the presently available bathymetry along the entire pacific rivera rise spreading rates averaged over the past 0 7 m y are systematically faster than 3 0 m y average rates with the difference increasing southward along the rise the post 3 ma change in the rate gradient has caused a southward migration of the pacific rivera euler pole since 3 ma possibly in response to the 3 ma completion of a spreading reorganization that doubled the length of the rivera transform fault and halved the area of the rivera plate we hypothesize that deformation in southwestern mexico including opening along the colima rift is related to oblique subduction of the cocos plate along the northern middle america trench three unes of evidence support this hypothesis first the sense of oblique subduction predicted by nuvel i along the middle america trench is consistent with a model in which part of western mexico located southeast of the colima rift moves to the southeast relative to north america second field geologic data and landsat imagery suggest that in the past few million years the chapala oaxaco fault zone has accommodated several kilometers of sinistral motion and sinistral transtension has occurred along the trans mexican volcanic belt either or both of these fault zones could accommodate any southeastward coastal block motion that might result from oblique subduction third the observation that earthquake slip vectors from the northern middle america trench trend systematically counterclockwise from the predicted cocos north america convergence direction is also consistent with southeastward motion the discrepancy between the observed and predicted slip directions along the middle america trench suggests sinistral slip is 0 10 mm yr 1 if oblique subduction is driving southeastward transport of a coastal sliver the colima rift is a passive pull apart zone at the northwestern end of this sliver rather than an incipient spreading ridge that will replace the pacific rivera rise

strain transfer between disconnected propagating rifts in afar we showed before that both the aden and red sea plate boundaries are currently rifting and propagating along two distinct paths into afar through the opening of a series of disconnected propagating rifts here we use new geochronological tectonic and paleomagnetic data that we acquired mostly in the southeastern part of afar to examine the geometry kinematics and time space evolution of faulting related to strain transfer processes it appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected this mechanism implies the rotation about a vertical axis of small rigid blocks along rift parallel faults that are shown to slip with a left lateral component which is as important as their normal component of slip rates of 2 3 mm yr by contrast where rifts do not overlap either a classic transform fault maskali or an oblique transfer zone mak arrasou kinematically connects them the length of the aden red sea overlap has increased in the last 0 9 myr as the aden plate boundary propagated northward into afar as a consequence the first order blocks that we identify within the overlap did not all rotate during the same time span nor by the same amounts similarly the major faults that bound them did not necessarily initiate and grow as their neighboring faults did despite these variations in strain distribution and kinematics the overlap kept accommodating a constant amount of strain 7 to 15 of the extension amount imposed by plate driving forces which remained distributed on a limited number seven or eight of major faults each one having slipped at constant rates 3 and 2 mm yr for vertical and lateral rates respectively the fault propagation rates and the block rotation rates that we either measure or deduce are so fast 30 130 mm yr and 15 38 myr respectively that they imply that strain transfer processes are transient as has been shown to be the case for the processes of tearing rift propagation and strain jumps in afar

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

bubble clouds and temperature anomalies in the upper ocean the processes of mixing in the upper few metres of the ocean are poorly known and largely unqualified1 but are a vital element in ocean atmosphere interaction playing a key part in the transfer and dispersion of heat gases and nutrients here we report results from a towed instrument used to measure simultaneously water temperature and the sound scattered from bubble clouds produced by breaking waves dominant features in the records are correlated intense sound scattering or bubble clouds being associated with warmer water and large positive temperature gradients with the onset of increased scattering the observations provide a more complete picture of the structure and nature of bubble clouds and establish their assocition with dynamical processes of turbulent diffusion in the upper ocean boundary layer

microphotometric characterization of phytoplankton and detrital absorption properties in the sargasso sea microphotometric techniques were used to characterize the spectral absorption properties of individual phytoplankton cells and detrital particles from two sites in the sargasso sea during spring 1985 direct determinations of the absorption efficiency factor qa geometric cross sectional area g and particle type were made for about 50 individual particles for each water sample analyzed large intrasample and intersample variations of qa spectra for both phytoplankton and detrital particles were observed for the mesotrophic northern station mean values of qa and g for phytoplankton cells showed insignificant vertical variations for detrital particles however the mean geometric cross sectional area decreased with depth while their mean qa values increased for the oligotrophic southern station these same relationships were observed although more small scale variability was apparent the statistical comparison of the mean qa values with the total particulate absorption coefficient spectrum ap enabled quantification of the contributions phytoplankton and detrital materials made to ap results showed that phytoplankton abundances accounted for 50 70 of the particulate absorption at 440 nm although instances were observed where phytoplankton cells accounted for 90 of ap 440 nm the reconstructed particulate absorption spectrum p compared favorably with the total ap indicating that nearly all of the optically relevant absorbing particulate material was sampled with the microphotometric technique

tectonic volcanic segmentation and controls on hydrothermal venting along earth s fastest seafloor spreading system epr 27 32 s 1 we have collected 12 khz seabeam bathymetry and 120 khz dsl 120 side scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading 150 km myr part of the global mid ocean ridge system the southern east pacific rise between the easter and juan fernandez microplates this area is presently reorganizing by large scale dueling rift propagation and possible protomicroplate tectonics fracture patterns observed in the side scan data define structural segmentation scales along these ridge segments these sometimes but not always correlate with linear volcanic systems defining segmentation in the seabeam data some of the subsegments behave cohesively with in phase tectonic activity while fundamental discontinuities occur between other subsegments we also collected hydrothermal plume data using sensors mounted on the dsl 120 instrument package as well as ctdo tow yos to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the pacific nazca axis here we report the first rigorous correlation between coregistered hydrothermal plume and high resolution marine geophysical data on similar scales and over multisegment distances major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low these correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest resulting in recent eruptions that have paved over the neovolcanic zone areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas increased crustal permeability as gauged by increased fracture density does not enhance hydrothermal venting in this area axial summit troughs and graben are rare probably because of frequent volcanic resurfacing in this superfast spreading environment and are not good predictors of hydrothermal activity here many of the hydrothermal areas are found in inflated areas near the ends of segments suggesting that abundant magma is being supplied to these areas

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

present day kinematics and fault slip rates in eastern iran derived from 11 years of gps data we analyze new gps data spanning 11 years at 92 stations in eastern iran we use these data to analyze the present day kinematics and the slip rates on most seismogenic faults in eastern iran the east lut west lut kuhbanan anar dehshir and doruneh faults are confirmed as the major faults and are found to currently slip laterally at 5 6 0 6 4 4 0 4 3 6 1 3 2 0 0 7 1 4 0 9 and 1 3 0 8 mm yr respectively slip is right lateral on the ns striking east lut west lut kuhbanan anar and dehshir faults and left lateral on the ew doruneh fault the ns faults slice the eastern iranian crust into five blocks that are moving northward at 6 13 mm yr with respect to the stable afghan crust at the eastern edge of the collision zone the collective behavior of the ns faults might thus allow the arabian promontory to impinge northward into the eurasian crust the ns faults achieve additional ns shortening by rotating counterclockwise in the horizontal plane at current rates up to 0 8 ma modeling the gps and available geological data with a block rotation model suggests that the rotations have been going on at a similar rate 1 0 4 ma over the last 12 ma we identify large strains at the tips of the rotating east lut west lut and kuhbanan faults which we suspect to be responsible for the important historical and instrumental seismicity in those zones

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

the rise flank trails left by migrating offsets of the equatorial east pacific rise axis sea beam and magnetic surveys of the young 1 ma rise flank around nontransform rise crest offsets at 5 5 n 4 9 n 3 4 n 2 0 n and 2 8 s mapped distinctive trails of obliquely lineated highly magnetic crust that spreads from the characteristically curving overlapping rift zones a deep tow survey around the small 1 5 km 3 9 n offset found only normal rise flank terrain near bottom deep tow observations at the medium size 9 km 5 5 n offset and the large 27 km 2 0 n offset provided more detailed information on the formation of crust in the offset trails and of the structure of tectonically modified crust at interrift rise crest sites patches of interrift crust which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets are periodically shed from the axial zone together with abandoned portions of overlapped rift zone to become important elements of the rise flank trails the oblique azimuths of the paired fracture zone trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25 125 mm yr net movement of the 27 km 2 8 s offset has been slower with alternating periods of rapid northward and southward migration information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial east pacific rise epr whose trails were less thoroughly mapped most offsets of the southern pacific cocos boundary have migrated south and most on the northern pacific nazca boundary have migrated north the patterns are not readily explained by the inferred distribution of mantle upwellings along the epr axis

the east ridge system 28 5 32 s east pacific rise implications for overlapping spreading center development abstract we report here on geophysical data from the east ridge and surrounding areas of the large offset overlapping spreading centers oscs that accommodate pacific nazca opening between 28 5 and 32 s the east ridge overlaps and is offset from the west ridge system by 120 km forming the largest known pair of oscs in this area spreading rates reach the fastest currently active on earth of 149 mm yr although the east ridge is composed of 4 morphologically defined segments separated by 3 small oscs other geophysical characteristics imply 1 upwelling segment all the active ridge segments in this area including the propagating tips of the east and west ridges form relative topographic highs with respect to the flanking sea floor however identified abandoned ridge tips form deeps we interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer only partially coupled to the diverging flow of a more broadly distributed ductile deformation zone ddz surrounding the steady state ridges and crossing the offset between the oscs the topographic high of the propagating segment may be maintained primarily by along axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge the large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments cut off from the supply of melt and replaced by a new set the failed rift tips for a period of time overlie the broad ddz and preferentially undergo continued extension and subsidence the ddz surrounding the ridge axes may be very broad in this area because of the very fast spreading rate creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis leading to the formation of the smaller oscs observed

controls on segmentation and morphology along the back arc eastern lau spreading center and valu fa ridge back arc spreading centers increasingly depart from mid ocean ridge mor characteristics with proximity to the arc volcanic front the close association of these departures with slab derived materials in erupted lavas suggests that subduction related chemical effects are their primary cause the eastern lau spreading center elsc and valu fa ridge vfr are type examples of this process together they constitute a first order spreading center in the lau back arc basin that progressively converges on the tofua arc volcanic front from north to south here we use ship multibeam and deep towed side scan sonar data to examine variations in axial morphology and volcanism at the second and third order segment scale along these ridges and develop a model for the processes that control them closest to the arc vfr and the southern segment of the elsc shoal toward second order segment ends in contrast to mors northward and beyond 70 km from the arc the axis becomes abruptly deeper and flatter and no longer shoals toward second order segment ends at vfr along axis topographic highs correlate with the location of arc volcanoes along slab flow lines these correlations are weaker along the southernmost elsc segment and absent along elsc segments farther north the observations show a modulation of back arc segmentation with arc proximity that rapidly diminishes with distance they support a model of the mantle wedge with a strongly hydrous domain within 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc ridge interactions

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

geology and kinematics of the niuafo ou microplate in the northern lau basin we present a revised three plate kinematic model for the lau basin focusing on new interpretations of the geologic characteristics of the niuafo ou microplate boundaries it builds upon the three plate model by zellmer and taylor 2001 while resolving some geologic discrepancies in that model along the southern branch of the mangatolu triple junction and the fonualei rift and spreading center which together form the niuafo ou tonga plate boundary our model predicts slow to ultraslow opening rates 32 8 mm yr southward consistent with axial morphology and magnetic profile modeling our interpretation of the seafloor magnetization pattern and flank morphology along the central lau spreading center clsc indicates that opening rates along the australia niuafo ou boundary are higher than predicted in the previous model thus compensating for slower rates along the niuafo ou tonga boundary lastly we show geologic evidence that peggy ridge pr interpreted previously as an active transform fault and therefore a strong azimuthal constraint on niuafo ou plate motion is inactive and that the active plate boundary is the adjacent lau extensional transform zone extending from the clsc to its intersection with the northwest lau spreading center the removal of the pr azimuthal constraint allows us to determine an euler pole for australia niuafo ou opening that satisfies three plate closure criterion and develop a geologically consistent model of lau basin kinematics while the final model focuses on constraining current plate velocities assuming rigid plate eulerian rotations the geologic analyses presented herein also provide insight into long term basin evolution which appears to involve significant nonrigid plate kinematics

motion of the scotia sea plates earthquake data from the scotia arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the scotia sea region events with magnitude greater than or equal to5 which occurred on or near the boundaries of the scotia and sandwich plates and for which centroid moment tensor cmt solutions are available are examined the newer data fill some of the previous sampling gaps along the boundaries of the scotia and sandwich plates and provide tighter constraints on relative motions variations in the width of the brunhes anomaly on evenly spaced marine magnetic profiles over the east scotia ridge provide new estimates of scotia sandwich plate spreading rates since there are no stable fracture zones in the east scotia sea the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading 18 newrate estimates and four azimuths from the east scotia ridge are combined with 68 selected earthquake slip vectors from the boundaries of the scotia sea in a least squares inversion for the best fitting set of euler poles and angular rotation rates describing the present day motions of the scotia and sandwich plates relative to south america and antarctica our preferred model tlp2003 gives poles that are similar to previous estimates except for scotia plate motion with respect to south america which is significantly different from earlier estimates predicted rates of motion also differ slightly our results are much more robust than earlier work we examine the implications of the model for motion and deformation along the various plate boundaries with particular reference to the north and south scotia ridges where rates are obtained by closure

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

cambrian microbial and silica gel textures in silica iron exhalites from the mount windsor volcanic belt australia their petrography chemistry and origin stratiform quartz hematite or magnetite lenses ironstones occur sporadically throughout the mount windsor volcanic belt at three main stratigraphic positions including along strike from and immediately stratigraphically above the thalanga massive sulfide deposit the area is ideal for examination of the chemical textural and mineralogical changes which occur in silica iron exhalites around ancient hydrothermal submarine vent sites major and trace element chemical data for the ironstones indicate they are essentially composed of sio 2 and fe 2 o 3 generally 99 wt with very low concentrations of other oxides e g al 2 o 3 generally 0 5 wt and trace elements across the length of the mount windsor volcanic belt most of the ironstones have experienced substantial recrystallization at metamorphic grades from greenschist to amphibolite facies however a number of samples from the eastern part of the belt are less recrystallized prehnite pumpellyite facies and retain abundant evidence of microbial fossils and the inorganic maturation of amorphous phases at least three distinct microbial morphologies are recognized 1 approximately 1 mu m avg diam filaments with septa 2 approximately 3 mu m avg diam filaments with no septa and 3 20 to 30 mu m avg diam filaments with no reconnecting branches the range in metamorphic grade along the mount windsor volcanic belt not only permits a rare insight into the primary textures of ancient exhalites but also a useful view of the textural and mineralogical changes which accompany the progressive increase in temperature up to amphibolite facies conditions comparisons with similar modern deposits and experimentally produced synthetic textures suggest that the ironstones formed from the crystallization of silica iron oxyhydroxide gels these gels formed as a result of mixing between hydrothermal fluid and seawater and conductive cooling of the product the rare earth element geochemistry of ironstones along strike from thalanga is compatible with migration of a hydrothermal fluid away from the vent center followed by its accumulation and cooling in topographic depressions on the sea floor microaerophilic chemolithotrophic bacteria and or fungi probably catalyzed the oxidation of fe super 2 to fe super 3 several metamorphosed ironstones also contain pyrite with delta 34 s values approximately 50 per mil lighter than cambrian sulfate suggesting the activity ofsulfate reducing bacteria at some point during their formation the temperature of the original hydrothermal fluid was probably between 10 degrees and 40 degrees c originating either as the in situ product of weak hydrothermal activity in which inorganic sulfate reduction did not occur or as the cooled remnant of a higher temperature fluid which had already deposited sulfides the inferred mode of deposition for the siliceous ironstones as a gel is different from that of many modern iron oxyhydroxide precipitates which form by settling of particles to the sea floor from hydrothermal plumes on the flanks of the thalanga massive sulfide deposit some iron silica exhalites have been encompassed by massive sulfide suggesting that the early low temperature fe si precipitates were subsequently overprinted by hotter fluids this provides good evidence for waxing and waning in the life of the vent system

multimodal function optimization with a niching genetic algorithm a seismological example abstract we present a variant of a traditional genetic algorithm known as a niching genetic algorithm nga which is effective at multimodal function optimization such an algorithm is useful for geophysical inverse problems that contain more than one distinct solution we illustrate the utility of an nga via a multimodal seismological inverse problem the inversion of teleseismic body waves for the source parameters of the m w 7 2 kuril islands event of 2 february 1996 we assume the source to be a pure double couple event and so parametrize our models in terms of strike dip and slip guaranteeing that two global minima exist one of which represents the fault plane and the other the auxiliary plane we use ray theory to compute the fundamental p and sh synthetic seismograms for a given source receiver geometry the synthetics for an arbitrary fault orientation are produced by taking linear combinations of these fundamentals yielding a computationally fast forward problem the nga is successful at determining that two major solutions exist and at maintaining the solutions in a steady state several inferior solutions representing local minima of the objective function are found as well the two best focal solutions we find for the kuril islands event are very nearly conjugate planes and are consistent with the focal planes reported by the harvard cmt project the solutions indicate thrust movement on a moderately dipping fault a source typical of the convergent margin near the kuril islands

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

kinematics of mantle flow beneath a fossil overlapping spreading center the wuqbah massif case oman ophiolite the wuqbah massif central oman ophiolite comprises a well preserved 2 km thick crustal unit over a 6 8 km thick mantle sequence the mantle comprises harzburgites and few dunites cut by various types of dykes with the exception of basal and late fault zones peridotites display granoblastic and porphyroclastic textures acquired by high temperature plastic deformation they locally suffered important recrystallization as shown by neoblastic shapes and arrangements macroscopically the peridotites have well defined foliations and lineations whose features allow the massif to divide into three structural domains it comprises a n110 trending central zone that suffered an anticlockwise rotation with an upwelling zone in its southwestern part bound between two ns areas sheared in right lateral conditions this peculiar geometry is thought to reflect an overlapping spreading center osc where the east and west bordering zones would define the trace in the mantle of two ridge segment terminations and the oblique n110 central zone the axial part of the overlap area it is proposed that the northward migration of the southwest branch and its concomitant ew spreading caused the deformation and rotation of the overlap zone in our osc model flow in the mantle would be continuous from one spreading center branch to the other being curved in the overlap area to accommodate the offset the mantle flow would therefore be more and more decoupled with the brittle crust when approaching the overlap zone where extensional tectonics can occur we consider that some mantle upwelling is possible due to its specific location in between two ridge segments and over the main mantle flow the distance between the two ridge axes is 15 km which means that this paleo osc can be ranked in the class of intermediate oscs illustrating a third order ridge segmentation at the scale of the ophiolite the wuqbah osc is located northwest of a major domain marked by the presence of a nw se trending propagating ridge responsible for the formation of a new oceanic crust it is suggested that the wuqbah osc formed earlier and underlines the trace of a ridge that had later migrated to the east

recent dueling propagation history at the fastest spreading center the east pacific rise 26 32 s the east pacific rise epr 26 32 s located between the easter microplate and the juan fernandez microplate has the world s fastest spreading rate and tectonics characterized by dueling ridge propagation a glori b and sea beam 2000 side scan bathymetry survey together with other geophysical data collection was conducted aboard the r v melville covering an approximately 5 by 5 area centered on this part of epr the side scan and bathymetric data reveal the complete geometry of the propagator system the western and eastern ridges both curve inwardly and are overlapped 120 km with an offset to overlap ratio of nearly one several abandoned ridges and rifts are clearly mapped on the east flank and a few failed rifts are also mapped near the western ridge tip a fracture zone discovered south of easter island indicates a stage of stable ridge transform intersection in the past an inversion with magnetic anomaly vectors was carried out for the area a resultant magnetic boundary strike map was interpreted with the downward component profiles to construct a magnetic isochron map the spreading and propagation rates were estimated from the magnetic isochrons the net southward propagation rate of the western ridge is 120 km m y for the last 1 9 m y the spreading rate of the eastern ridge has been 150 km m y and its current propagation rate is estimated as 500 km m y toward the north intense asymmetric spreading as high as 30 is observed at both ridges together with the side scan and bathymetric data the magnetic lineations provide significant constraints on the evolution of the propagator system and a propagation model with cyclic rift failure was applied to model the tectonic evolution during the last 2 m y a normal ridge transform intersection evolved into a nontransform overlapping offset 1 95 ma and the eastern and the western ridges have propagated alternately the dueling propagation history is characterized by the dominant southward propagation of the western ridge and the transitory propagation events of the eastern ridge with a variety of overlap lengths widths and ridge curvatures in the transition zone

seamarc ii survey of a propagating limb of a large nontransform offset near 29 s along the fastest spreading east pacific rise segment seamarc ii side scan and bathymetric data combined with magnetic anomaly and sea beam bathymetric data define a large 90 120 km nontransform offset of the east pacific rise epr between the easter and juan fernandez microplates near 29 s where one or more transforms had been suggested by the seismicity pattern on the basis of morphology structural and tectonic features three major regions are delineated an eastern spreading axis a western spreading axis and a central transfer zone these two spreading axes overlap by at least 35 km and possibly as much as 100 km the eastern axis is characterized by an axial ridge that narrows and deepens toward the tip 28 38 s 112 08 w approaching the tip the spreading axis azimuth changes from approximately n5 e to n25 w and axial magnetic anomaly values increase to over 700 nt 37 km south of the rift tip a pair of deep 600 m bathymetric troughs on either side of the ridge axis trending n11 w and n44 w converge on the ridge tip and define the outermost boundaries of ridge parallel structures these structures are interpreted as pseudofaults resulting from northward rift propagation the western spreading axis is approximately 2400 m deep and corresponds to a young volcanic area the central lithospheric transfer zone west of the inner pseudofault of the eastern spreading axis is characterized by oblique abyssal hill and magnetic fabric interpreted to result from shear between overlapping spreading centers and a dispersed pattern of teleseismic earthquake epicenters the eastern spreading axis is propagating rapidly northward at up to 265 km m y based on a 158 km m y spreading rate and the 33 angle between the pseudofaults however complex patterns of seafloor fabric which differ from that predicted by the simple propagating rift model suggest that multiple propagation events may have occurred furthermore southward propagation of the western limb may be dominant in the long term the absence of transform faults along this superfast spreading epr segment documents that transforms do not presently exist at such superfast spreading rates

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

construction and destruction of volcanic knobs at the cocos nazca spreading system near 95 w characterization of volcanic knobs observed in sea beam bathymetry near the 95 w galapagos propagator system on the cocos nazca spreading axis provides insight into volcanic and tectonic processes at propagators and mid ocean ridges despite evidence suggesting a higher magma supply rate at the propagator axis crust accreted there contains fewer knobs than crust created at the failing or doomed spreading axes fissure fed flows rather than seamount construction are more important along the propagator the process of transferring lithosphere from one plate to another as this ridge offset migrates through a region destroys about half of the preexisting knobs data tentatively suggest that on axis volcanic cones increase in size and contribution to crustal construction but decrease in abundance with decreasing spreading rate possibly reflecting the magma plumbing systems in differing thermal regimes