structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

analogue modeling of overlapping spreading centers insights into their propagation and coalescence abstract the propagation and segmentation of mid ocean ridges is studied using centrifuged analogue models built with non linear materials the deformation of the brittle ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force this linear upwelling laterally stretches the model mantle that in turn induces failure in the upper layer simulating the brittle crust arrays of fractures initiate within zones of high stress concentration above the diapir fractures propagate laterally in a direction perpendicular to the maximum tensile the minimum principal stress secondary tension cracks initiate in the vicinity of parent fracture tips through going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns the overlap overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence non overlapping sub parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture if overlapping fractures are parallel they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material fracture curvature in this case results from crack crack interaction and is similar to that of overlapping spreading centers oscs observed along mid ocean ridges overlapping non parallel fractures tend to coalesce by one of their tips propagating sub parallel to the spreading direction toward the other fracture such offsets can serve as models for the development of the orthogonal ridge transform fault patterns common along mid ocean ridges

evolution of vertical faults at an extensional plate boundary southwest iceland vertical faults having both opening and vertical displacements are common in southwest iceland and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward we address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest iceland individual fracture segments are commonly rotated out of the general trend of the fracture zone suggesting oblique motion along subsurface normal faults these faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side based on these field observations and the results of numerical models we propose that 60 758 dipping normal faults in the subsurface propagated to the surface from below vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m 25 50 of the fault length in response to stress concentrations along the tip line model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures which subsequently breach the monoclines along the upper hinge line forming vertical fault scarps and open fissures at the surface if vertical fractures utilize pre existing cooling joints in basalt to connect directly to the surface the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace q 2003 elsevier ltd all rights reserved

the 1928 1929 eruption of kammourta volcano evidence of tectono magmatic activity in the manda inakir rift and comparison with the asal rift afar depression republic of djibuti there are two rifts zones in the republic of djibuti the active asal rift birthplace of the ardoukoba basaltic volcano in 1978 and the poorly known manda inakir rift described here the most recent volcanic event in the manda inakir rift was the formation of the kammourta basaltic cone probably in 1928 accompanied by strong seismic activity this historic eruption and related tectonic features show that the manda inakir rift like asal is presently active the kammourta basalt of transitional alkaline type belongs to the manda inakir differentiated series which ranges from basalt to rhyolite in contrast volcanic rocks of the asal rift are entirely transitional tholeiitic basalt the differences in magmatic affinity and tectonics between these two rift zones reflect the more advanced evolution of rifting in the asal zone than in manda inakir

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

submarine venting of magmatic volatiles in the eastern manus basin papua new guinea abstract the susu knolls and desmos hydrothermal fields are located in the back arc extensional transform zone of the eastern manus basin in 2006 highly acidic and so4 rich vent fluids were collected at both sites and analyzed for the chemical and isotopic composition of major and trace species fluids exiting the seafloor have measured temperatures from 48 to 215 c and are milky white in appearance due to precipitation of elemental s0 vent fluid concentrations of na k and mg are depleted by as much as 30 relative to seawater but have the same relative abundance in contrast the fluids are highly enriched in dissolved co2 cl sio2 aq fe and al relative to seawater measured ph 25 c ranged from 0 95 to 1 87 and aqueous so4 ranged from 35 to 135 mmol kg the chemical and isotopic composition points to formation via subsurface mixing of seawater with a na k mg and ca free volatile rich magmatic fluid exsolved from subsurface magma bodies during a process analogous to subaerial fumarole discharge estimates of the magmatic end member composition indicate a fluid phase where h2o so2 co2 cl f the hydrogen and oxygen isotopic composition of h2o and carbon isotopic composition of co2 in the vent fluids strongly suggest a contribution of slab derived h2o and co2 to melts generated in the mantle beneath the eastern manus volcanic zone abundant magmatically derived so2 undergoes disproportionation during cooling in upflow zones and contributes abundant acidity so42 and s0 to the venting fluids interaction of these highly acidic fluids with highly altered mineral assemblages in the upflow zone are responsible for extensive aqueous mobilization of sio2 aq fe and al temporal variability in the speciation and abundance of aqueous s species between 1995 and 2006 at the desmos vent field suggests an increase in the relative abundance of so2 in the magmatic end member that has mixed with seawater in the subsurface results of this study constrain processes responsible for the formation of hot spring fluids in magmatically active back arc environments and the resulting chemical exchange between the lithosphere and water column

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

seismotectonics and present day relative plate motions in the tonga lau and kermadec havre region abstract an updated compilation of shallow seismicity in the tonga kermadec arc and back arc region including especially 304 focal mechanisms from centroid moment tensor solutions files provides new insights on the present day tectonics and allows the relative motions between the pacific indo australian and tonga kermadec plates to be quantified azimuths of slip vectors along the trench change at 19 s in the southern domain 35 19 s the mean azimuth ranges from n280 e to n285 e while in the northern domain it trends n276 277 e this 19 s boundary also exits in the back arc domain south of 19 s from 35 s to 20 s the direction of back arc extension is respectively n135 e n122 e and n111 e within the southern havre trough northern havre trough and southern lau basin in the southern lau basin the n111 e opening direction is compatible with the trend of the newly mapped valu fa ridge north of 19 s in the northern lau basin 16 18 s a n93 e trending crustal extension occurs along the n135 e peggy ridge and along an inferred n5 10 e spreading zone between these two extensional features a fourth plate the northern lau microplate is present at 18 19 s a ridge ridge ridge triple junction accommodated by n95 115 e right lateral strike slip fracture zones is proposed at the complex transitional zone between the southern and the northern parts of the lau basin the fiji north tonga fracture zone which bounds northward the lau back arc basin is composed of two segments a n95 e eastern segment extending from the northern end of the tonga trench to 178 30 w and a n75 e western segment lying north of the fiji platform a ridge fracture fracture triple junction is inferred at 178 30 w 15 45 s between the northern end of the peggy ridge and the two segments of the fiji north tonga fracture zone using the model rm 2 of minster and jordan 1978 and the directions of back arc spreading and convergence at the trench relative plate motions are estimated the extension rates in the back arc domain increase northward and are respectively 0 8 2 1 and 8 cm y at 33 s 28 s and 24 s farther north parallelism between directions of different motions does not allow the calculation of any accurate result therefore north of 24 s we used a spreading rate of 8 cm y which corresponds to the 24 s result and also to the value deduced from magnetic anomalies the consumption rates along the tonga kermadec trench are respectively 7 2 9 6 16 4 17 1 and 17 8 cm y at 33 s 28 s 24 s 20 s and 17 s both the back arc opening rate and the consumption rate of the pacific plate sharply increase between the northern havre trough and the southern lau basin this limit coincides with the louisville ridge trench junction along the n95 e eastern segment of the fiji north tonga fracture zone a pure left lateral strike slip motion parallel to the n93 e opening tectonics within the northern lau basin occurs in contrast along the n75 e western segment of the fiji north tonga fracture zone a left lateral strike slip motion of 7 cm y should be accompanied by a n135 e extensional motion of 3 5 cm y

off ridge alkaline magmatism and seamount volcanoes in the masirah island ophiolite oman abstract the masirah ophiolite offers an unique opportunity to study well preserved small seamount structures obducted seamounts have not been described up to now and from the present day ocean floor they are almost exclusively known from bathymetric studies the thin oceanic crust of the masirah ophiolite was formed at a ridge transform intersect in upper jurassic time it was overprinted and reworked by a major intra oceanic tectono magmatic event at mid cretaceous time that has been well dated owing to the presence of interstratified sedimentary rocks late hauterivian to early barremian c 130 125 ma this mid cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites volcanism occurred in a nw se extensional regime small elongate submarine volcano structures seamounts developed within widespread alkalibasaltic pillow lava and pillow breccia deposits which are interfingered with deep marine pelagic sediments the volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height the seamounts are built up of basic to acid subvolcanic stock or sheet like intrusions several generations of dikes vent agglomerates and pyro to epiclastic deposits the latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment in the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place the alkaline rocks show a pronounced ocean island basalt oib character indicating the considerable contribution of a mantle plume source hotspot as cause of the volcanism we propose a combination of original transform setting followed by drift past the marion hotspot during the major plate tectonic reorganization between greater india madagascar and africa starting in mid cretaceous time

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

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

interactions between a transform fault and arc volcanism in the bismarck sea papua new guinea 1 we present a new interpretation of the geological evolution of the western branch of the bismarck sea seismic lineation bssl region offshore the northwestern coast of papua new guinea from mapping and interpretation of bathymetry and backscatter data acquired aboard r v kilo moana in 2004 at present the schouten islands parallel the papua new guinea coast but this distribution results from the left lateral displacement of some of these volcanic islands by the bismarck sea seismic lineation the trend of the islands and seamounts was initially aligned n s and then displaced by the bssl wei island lies on a large submarine edifice that may have formed as part of a leaky transform subsequent to forming wei island was bisected and its pieces were displaced 45 km using this distance together with the strike slip rate for the bismarck sea seismic lineation predicted from the north bismarck plate south bismarck plate pole the division occurred approximately 385 000 years ago

relationships between tectonics and magmatism in a transtensive transform setting an example from faial island azores portugal the relationships between tectonics and hotspot related magmatism in transform transtensive settings are poorly known the azores archipelago lying where the transform plate boundary between the nubian and eurasian plates meets the mid atlantic ridge is a rare site to investigate these relationships the distinct tectono magmatic features of faial make it the ideal island to focus the study here we analyze the relationships between tectonics and magmatism using remote sensing field and paleomagnetic analyses dominant wnw ese trending lineaments correspond to major faults with transtensive dextral motion and ne sw opening probably related to the nearby transtensive terceira rift most 60 dikes vent elongations and alignments are parallel to this system the lavas forming the basement of the distinctive wnw ese trending pedro miguel graben strike parallel to the graben axis dipping outward with a tilt consistent with that of paleomagnetic data these lavas are dissected by the wnw ese trending transtensive faults therefore the graben consists of outward tilted fault bounded blocks forming two opposite verging dominoes its estimated stretching factor 1 35 and mean extension rate between 3 4 and 8 2 mm yr are similar or slightly larger than those of terceira rift to the east part of the graben extension may be magma induced via diking we suggest that 1 faial along with the nearby pico island is a major locus of extension within the azores directly above the imaged hotspot and 2 the faial pico magmatic segment constitutes the offset westward magmatic continuation of the terceira rift the segment opening highlights the importance of extensional strain along magmatic transtensive plate boundaries

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

two dimensional sph simulations of landslide generated water waves water waves generated by landslides have been of interest to ocean and coastal engineers as well as to dam engineers the present study uses a meshless and pure lagrangian method known as smoothed particle hydrodynamics sph to simulate nonlinear water waves due to landslides with the aim of an accurate numerical prediction of the generation and propagation of such water waves validation is carried out by comparison between the computed prediction and experimental data of water waves generated by a two dimensional triangular rigid body sliding into water the calculated results show that the simulated water waves agree well with those observed in the experiment confirming the effectiveness and the accuracy of the proposed scheme

earthquakes associated with the back arc opening in the eastern bismarck sea activity mechanisms and tectonics abstract to make clear the precise locations of earthquakes and their tectonic implications for the active back arc extension in the eastern bismarck sea we deployed a network of ocean bottom seismometers obss for a period of 26 29 days in late 1983 in an area that is the intersection of a previously postulated transform fault zone and spreading center we located the hypocenters of 200 earthquakes with high resolution using arrival times for p and s waves at five or more obs stations and using the station corrections their focal depths were estimated to be generally in the western survey area area 1 a linear zone of obs located earthquakes trending n65 w defines the locus of the major transform fault to within a linear zone in addition south of the two major seismic zones mentioned above we identified two isolated spots with microearthquakes shallower than 10 km the composite focal plane solution of the isolated earthquakes is similar to those teleseismically obtained for shallow events south of the linear seismic segments in the western bismarck sea suggesting a systematic pattern of the intraplate stress regime of the bismarck plate we finally suggest that the isolated intraplate events partly reflect the fundamental tectonic origin of the extension in the bismarck sea

evidence for an 18o depleted mantle plume from contrasting 18o 16o ratios of back arc lavas from the manus basin and mariana trough abstract back arc basin glasses from the mariana trough and manus basin display contrasting oxygen isotope characteristics that require differences in their mantle sources in both basins the lavas that are most depleted in high field strength elements possess 18o values of around 6 0 that are elevated with respect to mid ocean ridge basalt morb this characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere the nature of the more fertile mantle component differs between the two basins the lowest 18o values in the mariana trough are similar to morb and suggest that the ambient upper mantle interacts with a subduction modified mantle to produce mariana trough back arc basin basalts oxygen isotope ratios of basaltic glasses from the manus basin display a negative correlation with helium isotope ratios the subduction modified component is associated with 3he 4he ratios typical of the upper mantle glasses with 3he 4he ratios greater than average morb characteristic of a deep mantle plume have 18o values that are lower than expected for upper mantle melts this suggests that the manus basin plume taps a reservoir that is 18o depleted relative to the source of morb two mechanisms are identified that might generate this reservoir deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large 18o depleted domains in the deep mantle alternatively we propose that interactions between silicate and iron alloy during the segregation of the earth s core may have the potential to generate such reservoirs resolution between these mechanisms requires further experimental investigation of oxygen partitioning between silicates and iron alloy each of these mechanisms has distinct implications for the origins and dynamics of the manus basin plume

first observations of high temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of iceland high temperature 250 c hydrothermal vents and massive anhydrite deposits have been found in a shallow water sediment filled graben near 66 36 n in the tjornes fracture zone north of iceland the site is located about 30 km offshore near the small island of grimsey the main vent field occurs at a depth of 400 m and consists of about 20 large diameter up to 10 m mounds and 1 3 m chimneys and spires of anhydrite and talc a north south alignment of the mounds over a 1 km strike length of the valley floor suggests that their distribution is controlled by a buried fault widespread shimmering water and extensive white patches of anhydrite in the sediment between the mounds indicates that the entire 1 km2 area occupied by the vents is thermally active a 2 man research submersible jago was used to map the area and to sample vent waters gases and chimneys actively boiling hydrothermal vents occur on most of the mounds and extensive two phase venting indicates that the field is underlain by a large boiling zone 200 300 m the presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring the highest temperature pore fluids were encountered in talc and anhydrite rich sedimentary layers that occur up to 7 m below the mounds baked muds underlie the talc and anhydrite layers and pyrite is common in stockwork like fractures and veins in the hydrothermally altered sediments however massive sulfides pyrite marcasite crusts were found in only one relict mound subseafloor boiling has likely affected the metal carrying capacity of the hydrothermal fluids and deposition of sulfides may be occurring at greater depth although the mounds and chimneys at grimsey resemble other deposits at sedimented ridges e g middle valley escanaba trough guaymas basin the shallow water setting and extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system

a comparison between phase diagram modelling of metamafic rocks and experimental and independent thermobarometric data abstract the use of equilibrium phase diagrams such as pseudosections has become widespread in recent years but few studies tested the phase relations predicted by these models this work aims at comparing predicted phase relations with mineral assemblages observed in experimental products and in natural samples in this latter case using as reference metamorphic conditions pressures and temperatures determined for metapelites metamorphosed under the same conditions as the mafic rocks pseudosections were calculated with thermocalc for six mafic bulk rock compositions in the range 425 700 c and 2 8 kbar the resulting diagrams can be divided into two groups one in which a classic sequence of mineral assemblages occur and another where quartz is unstable and diopside is stable throughout the results compare well to the independent data in three of the six studied cases the main inconsistencies found in the pseudosections are underestimation of the stability field of garnet and overestimation of the stability fields of diopside and hornblende which is at odds not only with data specific to the rocks studied here but with the literature on metamafic rocks in general in the case of garnet the most likely case of discrepancy is the absence of mn in the activity composition models available on the other hand the issue with diopside and hornblende is much harder to evaluate as it may be related to any of the activity composition models or to relationships between two or more of them nonetheless we can affirm that the overestimation of the diopside stability field is related to sio2 content although there are other factors that influence it

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

oligocene miocene spreading history of the northern south fiji basin and implications for the evolution of the new zealand plate boundary a tectonic model of the evolution of the northern half of the south fiji basin including the minerva triple junction and cook fracture zone is developed from regional gravity multibeam bathymetry and a new interpretation of magnetic anomalies pinned to radiometric dates of oceanic crust in the basin the geometry and age of a portion of the minerva triple junction and the cook minerva spreading center the connection from the triple junction to the cook fracture zone which accommodated coeval opening of the norfolk basin are resolved with multibeam bathymetry and magnetics the south fiji basin opened from about 34 to 15 ma in an anticlockwise sweep about an euler pole located at the northern end of the present lau ridge this rotation and a rigidly straight southeastward motion of the three kings ridge were accommodated by the configuration of the triple junction changing from ridge fault fault to ridge ridge fault to ridge ridge ridge during this evolution the southeastern arm of the system the julia fracture zone underwent several transformations and the cook minerva spreading center experienced repeated ridge jumps the kinematics of the northern south fiji basin dictate to a large extent the evolution of the southern south fiji basin and the norfolk basin this in turn leads to the interpretation of a complex trench trench double transform fault framework at the northern new zealand margin which explains most aspects of the geology structure and arc volcanic history of the margin and provides a radical new setting for the origin of the northland allochthon

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

extensional transform zones and oblique spreading centers extensional transform zones etzs are plate boundary segments of order 100 km long that strike at angles between 15 and 45 to the extension direction they are characterized by neovolcanic tectonic zones comprising overlapping en echelon volcanic systems and or faults that trend 30 75 to the extension direction sometimes accompanied by a riedel shear below these surficial en echelon structures the deformation is aseismic and ductile and the plate boundary is probably continuous etzs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years etzs link oblique spreading segments to transform faults in the manus and probably the lau backarc basins the reykjanes peninsula and tjornes fracture zone in iceland and the mak arrasou in afar are etzs that link subaerial to submarine spreading or rifting segments the brawley and cerro prieto seismic zones appear to be etzs in the imperial and mexicali valleys that link the san andreas imperial and cerro prieto transform faults experimental and analytical models of transtensional deformation in brittle ductile systems match many of the observed characteristics of etzs and oblique spreading centers including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction the contrasting fault patterns reflect the rheology of the models and lithosphere they are more sigmoidal when the strain in the lower ductile layer is more focused causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins the angle o between the faults and the extension direction decreases with the angle between the strike of the zone and the extension direction etzs occur in the range 15 45 whereas oblique spreading centers have 45 90 and transform relay zones have 0 15 oblique fast spreading segments exhibit ridge parallel faults and volcanic systems o presumably reflecting locally rotated stress fields whereas at oblique slow spreading centers o is closer to orthogonal o 90

geochemistry of hydrothermal fluids from the pacmanus northeast pual and vienna woods hydrothermal fields manus basin papua new guinea abstract processes controlling the composition of seafloor hydrothermal fluids in silicic back arc or near arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic hydrothermal activity in such environments we conducted a survey of vent fluid compositions from two contrasting sites in the manus back arc basin papua new guinea to examine the influence of variations in host rock composition and magmatic inputs both a function of arc proximity on hydrothermal fluid chemistry fluid samples were collected from felsic hosted hydrothermal vent fields located on pual ridge pacmanus and northeast ne pual near the active new britain arc and a basalt hosted vent field vienna woods located farther from the arc on the manus spreading center vienna woods fluids were characterized by relatively uniform endmember temperatures 273 285 c and major element compositions low dissolved co 2 concentrations 4 4 mmol kg and high measured ph 4 2 4 9 at 25 c temperatures and compositions were highly variable at pacmanus ne pual and a large newly discovered vent area fenway was observed to be vigorously venting boiling 358 c fluid all pacmanus fluids are characterized by negative d h 2 o values in contrast to positive values at vienna woods suggesting substantial magmatic water input to circulating fluids at pual ridge low measured ph 25 c values 2 6 2 7 high endmember co 2 up to 274 mmol kg and negative 34 s h 2 s values down to 2 7 in some vent fluids are also consistent with degassing of acid volatile species from evolved magma dissolved co 2 at pacmanus is more enriched in 13 c 4 1 to 2 3 than vienna woods 5 2 to 5 7 suggesting a contribution of slab derived carbon the mobile elements e g li k rb cs and b are also greatly enriched in pacmanus fluids reflecting increased abundances in the crust there relative to the manus spreading center variations in alkali and dissolved gas abundances with cl at pacmanus and ne pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents in further contrast to vienna woods substantial modification of pacmanus ne pual fluids has taken place as a result of seawater ingress into the upflow zone consistently high measured mg concentrations as well as trends of increasingly non conservative so 4 behavior decreasing endmember ca cl and sr cl ratios with increased mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment decreased ph and endmember fe mn ratios in higher mg fluids indicate that the associated mixing cooling gives rise to sulfide deposition and secondary acidity production several low temperature 80 c fluids at pacmanus ne pual also show evidence for anhydrite dissolution and water rock interaction fixation of b subsequent to seawater entrainment hence the evolution of fluid compositions at pual ridge reflects the cumulative effects of water rock interaction admixing and reaction of fluids exsolved from silicic magma phase separation segregation and seawater ingress into upflow zones

oblique back arc rifting of taupo volcanic zone new zealand taupo volcanic zone tvz is a back arc rift innorth island new zealand its geometry andkinematics are investigated using shaded reliefimages field examination of faults and offset streamchannels the results show that tvz trends nne is 250 km long by hikurangi margin in the north island of new zealand 20 km wide and consists of fivesegments extension is principally manifest as steeplydipping 60 90 normal faults parallel to tvz these in the last 300 kyr have experienced acomponent of dextral shear tvz is therefore anoblique back arc rift the dextral shear is 37 of thetotal displacement which for previously estimatedspreadingrates 7mm yr correspondsto 2 6mm yr thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in thenorthisland distributeddextralshearacrosstvzthussuggests that strain partitioning across the plateboundary at latitudes of tvz is less significant thanpreviously thought

tectonic interpretation of the andrew bain transform fault southwest indian ocean 1 between 25 e and 35 e a suite of four transform faults du toit andrew bain marion and prince edward offsets the southwest indian ridge swir left laterally 1230 km the andrew bain the largest has a length of 750 km and a maximum transform domain width of 120 km we show that currently the nubia somalia plate boundary intersects the swir east of the prince edward placing the andrew bain on the nubia antarctica plate boundary however the overall trend of its transform domain lies 10 clockwise of the predicted direction of motion for this boundary we use four transform parallel multibeam and magnetic anomaly profiles together with relocated earthquakes and focal mechanism solutions to characterize the morphology and tectonics of the andrew bain starting at the southwestern ridge transform intersection the relocated epicenters follow a 450 km long 20 km wide 6 km deep western valley they cross the transform domain within a series of deep overlapping basins bounded by steep inward dipping arcuate scarps eight strike slip and three dip slip focal mechanism solutions lie within these basins the earthquakes can be traced to the northeastern ridge transform intersection via a straight 100 km long 10 km wide 4 5 km deep eastern valley a striking set of seismically inactive ne sw trending en echelon ridges and valleys lying to the south of the overlapping basins dominates the eastern central section of the transform domain we interpret the deep overlapping basins as two pull apart features connected by a strike slip basin that have created a relay zone similar to those observed on continental transforms this transform relay zone connects three closely spaced overlapping transform faults in the southwest to a single transform fault in the northeast the existence of the transform relay zone accounts for the difference between the observed and predicted trend of the andrew bain transform domain we speculate that between 20 and 3 2 ma an oblique accretionary zone jumping successively northward created the en echelon ridges and valleys in the eastern central portion of the domain the style of accretion changed to that of a transform relay zone during a final northward jump at 3 2 ma

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

the manda inakir rift republic of djibouti a comparison with the asal rift and its geodynamic interpretation abstract asal ghoubbet and manda inakir two axial zones and the mak arrassou n s strike slip zone are located between the afar depression where significant crustal thinning is shown by geophysical data and the relatively thicker continental crust of the danakil horst these three active structures were formed in response to the counterclockwise motion of the danakil horst the two modes of response rift and strike slip faults are related to the orientation of the horst boundaries the boundaries perpendicular to the opening direction lead to extensional zones and rifts asal and manda lnakir the parallel boundaries give strike slip faults mak arrassou the two narrow subsided axial structures rift in rift located on the crustal discontinuity seem to represent a stage where after crustal thinning between 4 and 1 ma when the stratoid series was erupted the breaking point was reached these two axes though not genuine oceanic crust the older continental series are found under the axial series represent a proto oceanic stage and foreshadow the future ridges they also indicate the southwest border of the arabian plate including the danakil horst

contrasting evolution of hydrothermal fluids in the pacmanus system manus basin the sr and s isotope evidence ocean drilling program odp leg 193 investigated two sites of hydrothermal activity along the crest of the pual ridge in the eastern manus basin a site of low temperature diffuse venting snowcap site 1188 and a high temperature black smoker site roman ruins site 1189 were drilled to depths of 386 and 206 m below seafloor mbsf respectively although the two sites are 1000 m apart the 87sr 86sr and 34s signatures of anhydrite recovered at both sites are very different the data suggest a complex interplay among hydrothermal fluid magmatic fluid and seawater during alteration and mineralization of the pacmanus papua new guinea australia canada manus system these new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems

source properties of earthquakes near the salton sea triggered by the 16 october 1999 m 7 1 hector mine california earthquake we analyze the source properties of a sequence of triggered earthquakes that occurred near the salton sea in southern california in the immediate aftermath of the m 7 1 hector mine earthquake of 16 october 1999 the sequence produced a number of early events that were not initially located by the regional network including two moderate earthquakes the first within 30 sec of the p wave arrival and a second approximately 10 minutes after the mainshock we use available amplitude and waveform data from these events to estimate magnitudes to be approximately 4 7 and 4 4 respectively and to obtain crude estimates of their locations the sequence of small events following the initial m 4 7 earthquake is clustered and suggestive of a local aftershock sequence using both broadband trinet data and analog data from the southern california seismic network scsn we also investigate the spectral characteristics of the m 4 4 event and other triggered earthquakes using empirical green s function egf analysis we find that the source spectra of the events are consistent with expectations for tectonic brittle shear failure earthquakes and infer stress drop values of 0 1 to 6 mpa for six m 2 1 to m 4 4 events the estimated stress drop values are within the range observed for tectonic earthquakes elsewhere they are relatively low compared to typically observed stress drop values which is consistent with expectations for faulting in an extensional high heat flow regime the results therefore suggest that at least in this case triggered earthquakes are associated with a brittle shear failure mechanism this further suggests that triggered earthquakes may tend to occur in geothermal volcanic regions because shear failure occurs at and can be triggered by relatively low stresses in extensional regimes

segmentation volcanism and deformation of oblique spreading centres a quantitative study of the reykjanes ridge abstract new multi beam swath bathymetry charts and high resolution side scan sonar imagery provide detailed evidence for volcanic and magmatic segmentation along the reykjanes ridge by combining these data sets we have quantified some relationships between different orders of segmentation various characteristics of volcanism and changes in tectonism along the ridge axis three orders of segmentation along the reykjanes ridge have been recognised 1 a long wavelength depth variation or swell that is related to the shoaling of the ridge towards iceland 2 intermediate wavelength rises of the order of 40 120 km and 3 a short wavelength volcanic segmentation of the order of 5 30 km the long wavelength depth variation which is probably controlled by the temperature of the underlying asthenosphere has a maximum positive deviation from a predicted depth curve that forms a break in slope at 59 60 n this feature migrated southward along the ridge at between 5 and 10 cm yr at least four times in the past 10 ma the intermediate wavelength rises which are also temporally and spatially unstable are probably related to local rayleigh taylor style asthenospheric instabilities the short wavelength segments have an evolutionary life cycle starting with eruption via fissure volcanoes and progressing by increasing magma flux to the formation of conical and then shield like volcanoes these segments are tectonically dismembered by extensional faulting following a rapid decrease in magma supply we propose a model for the construction of the volcanic layer of the reykjanes ridge which involves a cyclicity of magmatic and tectonic activity

complex fault patterns transtension and structural segmentation of the lofoten ridge norwegian margin using digital mapping to link onshore and offshore geology an integrated onshore offshore study involving regional to outcrop scale fault analysis is used to develop a self consistent structural model for transtension along the lofoten ridge the lofoten vesteralen archipelago lva is a segmented basement high showing distinct lateral variations in trend deformational style and structural complexity this study investigates whether segmentation can be linked to differences in the obliquity of preexisting structures relative to plate movement vectors regional analysis of fault lineament patterns using geographic information systems gis reveals that the lva can be subdivided into a series of distinct lineament domains these domains are closely coincident with changes in ridge trend and variations in structure within offshore models derived from seismic reflection studies digital field mapping and spatial analysis of faulting in the north lofoten reveal that multimodal faulting is dominated by transtensional dip slip and oblique slip movements which are comparable to analogue models where the ridge axis is 30 oblique to regional extension the overall change in fault orientation fault geometry and deformation style are consistent with models for transtension where the ridge bounding structure becomes increasingly oblique to regional extension previously identified transfer zones simply reflect segment domain boundaries and are not reactivating basement structures this model is a possible analogue for other orthogonal and oblique rift structures on the norwegian and other margins

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

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

application of b mg li and sr isotopes in acid sulfate vent fluids and volcanic rocks as tracers for fluid rock interaction in back arc hydrothermal systems

microearthquakes and tectonics in an active back arc basin the lau basin abstract an ocean bottom seismograph obs array was deployed for 20 22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back arc opening in the northern lau basin using p and s wave arrival times from four or more obss the hypocenters of 300 shallow earthquakes were located with a high confidence level the magnitudes of most obs located earthquakes were estimated to be less than four in the northern half of the survey area a narrow linear zone of microearthquakes trending nnw sse has been identified the northern part of the narrow seismic zone is within a central axial depression at the southern end of the peggy ridge further south the trend of the seismic zone becomes more n s the narrow seismic zone seems to be composed of at least six seismic segments offset by short aseismic zones most of the seismic segments trend nnw sse suggesting a system of left stepping en echelon spreading ridges where the spreading ridge segment is seismically inactive and the transform fault is active the spreading ridges appear to strike n s or nnw sse but the direction of the back arc opening is considered to be nw se no hypocenters were located with a high level of precision in the area south of latitude 18 s except a small isolated zone of shallow earthquakes at the southeastern part of the survey area we suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the tonga platelet this platelet is separated from the major indo australian plate by the back arc opening system in the lau basin

focused and diffuse extension in controls of ocean ridge segmentation in analogue models 1 this work aims to provide insights into the initiation and evolution of segments along ocean ridges as they propagate and mature it is based on the results of analogue modeling performed in a centrifuge with applied acceleration of gravity force the effect of the varied degree of extension allowed during propagation of analogue ridge results in segments being focused along the ridge axes or widely distributed within the entire axial zone three distinct types of segment interaction are documented controlled by the relative distribution and orientation of interacting segments and leading to the development of linear ridges overlapping spreading centers and oblique transform zones the importance of the initial distribution of segments in the ridge evolution inferred in the models suggests its significance for formation of natural ridges whose mature configurations must be largely inherited from that of spreading centers formed in the early stages of focused or diffuse extension

geologic setting of pacmanus hydrothermal area high resolution mapping and in situ observations abstract this study presents a systematic analysis and interpretation of autonomous underwater vehicle based microbathymetry combined with remotely operated vehicle rov video recordings rock analyses and temperature measurements within the pacmanus hydrothermal area located on pual ridge in the bismarck sea of eastern manus basin the data obtained during research cruise magellan 06 and so 216 provides a framework for understanding the relationship between the volcanism tectonism and hydrothermal activity pacmanus is a submarine felsic vocanically hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries vent temperatures and morphologies the total area of hydrothermal activity is estimated to be 20 279 m2 the microbathymetry maps combined with the rov video observations allow for precise high resolution mapping estimates of the areal extents of hydrothermal activity we find the distribution of hydrothermal fields in the pacmanus area is primarily controlled by volcanic features that include lava domes thick and massive blocky lava flows breccias and feeder dykes spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field we define a three stage chronological sequence for the volcanic evolution of the pacmanus based on lava flow morphology sediment cover and lava sio2 concentration in stage 1 sparsely to moderately porphyritic dacite lavas 68 69 8 wt sio2 erupted to form domes or cryptodomes in stage 2 aphyric lava with slightly lower sio2 concentrations 67 2 67 9 wt sio2 formed jumbled and pillowed lava flows in the most recent phase stage 3 massive blocky lavas with 69 to 72 5 wt sio2 were erupted through multiple vents constructing a volcanic ridge identified as the pacmanus neovolcanic zone the transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot mantle derived melts

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

wilkes transform system and nannoplate a recent hydrosweep survey of the wilkes transform system the second fastest slipping transform on the mid ocean ridge system shows it to be a highly complex and continually evolving plate boundary an area 50 km 2 north of the eastern part of the wilkes transform appears to be rotating counterclockwise in accordance with the theory of edge driven microplate kinematics we have called this region a nannoplate to distinguish it both as a separate small area of lithosphere between the pacific and nazca plates and as a smaller and less stable phenomenon than a microplate

morphology and segmentation of the western galapagos spreading center 90 5 98 w plume ridge interaction at an intermediate spreading ridge author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 8515 doi 10 1029 2003gc000609

oblique rifting at oceanic ridges relationship between spreading and stretching directions from earthquake focal mechanisms the relationship between spreading and stretching directions is investigated at oblique spreading oceanic ridges using earthquake focal mechanisms the stretching direction at ridge axes corresponds to the direction of the greatest principal strain 31 taken as the mean trend of the seismic t axes of extensional earthquake focal mechanisms it is compared with the spreading direction provided by global plate motion models we find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend a result in line with analogue experiments of oblique rifting this result is satisfactorily accounted for with an analytical model of oblique rifting for which the direction of 31 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics for low stretching factors typical of incremental seismic deformations 31 obliquity is two times lower than rifting obliquity for higher stretching factors the stretching and spreading directions become parallel 2006 elsevier ltd all rights reserved

propagation of an oblique spreading centre the western gulf of aden abstract the east west trending gulf of aden rift between arabia and somalia is spreading along a n35 e direction it propagates westward inside the african continent toward the afar triple junction oblique to the spreading direction these combined propagation and oblique rifting generate a special faulting pattern associated with an irregular distribution of volcanism we analyse this pattern using a structural map derived from the analysis of multibeam bathymetric data and backscattering images from the 1995 tadjouraden cruise in the western gulf of aden the rift valley is bounded by faults trending from n110 e to n130 e in the eastern part of the surveyed area where the valley trends n90 e and by large faults in the central western part where the valley is oriented n70 e the direction of extension deduced from the fault histogram is n37 e 10 over the whole area whatever the rift trend therefore this bend of the plate boundary does not result from a change in the stretching direction this bending increases the rifting obliquity westward and therefore modifies the faulting pattern faults display two distinct patterns either en echelon on the walls indicating a right lateral component of shear or purely normal and parallel to the rift trend inside the valley volcanoes are mostly located in the eastern part of the rift where they are aligned into swarms trending roughly perpendicular to the extension direction deduced from fault directions backscattering images show that recent lava flows fill the axial valley of the eastern part whereas in the central and western parts volcanoes are scattered in the middle part of the valley this distribution is interpreted as a westward propagation of the volcanism and as the direction of deformation migration the thickness of the upper brittle layer of the lithosphere is estimated from the width if the inner graben is inside the rift it indicates a deepening of the brittle ductile transition from east 6 km to west 16 km these values are typical of oceanic and continental systems respectively and consistent with the depth of earthquakes in the western part this points out the existence of a transitional area between 44 e and 45 20 e characterised by the lack of recent volcanism the nature of this mechanical transition is discussed in terms of propagation of an spreading centre into a continental or transitional afar type lithosphere

magnetic surveys of iceland abstract data from a 1968 1980 aeromagnetic survey of iceland and a 1972 1973 marine magnetic survey of a part of the shelf around iceland have been digitized and upgraded additional aeromagnetic survey lines were flown in 1985 1986 after appropriate low pass filtering and regridding a new multicolor map of geomagnetic field intensity anomalies over the island and its surroundings was produced the main features of the anomaly field are of two types the first 1 a comprises broad lineations subparallel to currently active or extinct spreading zones these are believed to be mostly due to tilled basalt lava flows of alternating remanence polarity in quaternary regions other extrusives of subglacial origin also contribute to the lineations the second 2 b comprises localized anomalies of the order of 10 km or less at individual volcan c centers we describe some of these features in terms of current knowledge of the age structure and magnetic properties of the uppermost crust in iceland

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

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

fracture analysis near the mid ocean plate boundary reykjavik hvalfjordur area iceland abstract the geometry and thermal history of fractures have been determined at 59 stations from reykjavik to hvalfjordur in southwestern iceland the data provide information on crustal stress regimes in the vicinity of mid ocean ridges two major generalized fracture orientations are present 1 1 a northeast system trend 010 030 except on akranes where the orientation is 040 060 2 2 a broad east west system containing one or more sets with strike between 070 130 thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures the thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting some of the east west trending fractures also opened at this time but many formed later after the area had begun to cool and drift from the active zone the northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern iceland they have been interpreted as extension fractures resulting in about 0 4 maximum extension forming generally from the same stress field associated with normal faulting and dike injection in the active zone fracturing in an east west direction estimated 0 1 maximum extension mainly near the edge and outside the active zone indicates a reorientation of this stress field the dominant mechanism related to the origin of the east west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates both fracture systems are inferred to have formed in the griffiths idealization under nearly biaxial effective compressive loading on the order of 200 bar the discrepancy between this value and the kilobar order strengths of short time laboratory tests reflects such factors as high temperature stress corrosion and fatigue fracture propagation is assumed to have been stable but governed primarily by lateral load diminishing mechanisms rather than by progressive loading these relaxation mechanisms may have been episodic northeast system fissure swarm activity or steady state thermoelastic contraction in time

structure and development of the midocean ridge plate boundary in the gulf of aden evidence from gloria side scan sonar a gloria side scan sonar survey along the crest of the midocean ridge in the gulf of aden has provided information on the small scale morphology of its spreading centers and fracture zones spreading center offsets increase in length to the east this is thought to result from changes in the behavior of the mantle with increasing distance from the afar hot spot a 300 km long spreading center in the western gulf of aden is oriented obliquely to the spreading direction nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller elongated basins each oriented approximately normal to the spreading direction close to the axis of spreading volcanic and closely spaced tectonic structures run approximately normal to the spreading direction higher on the walls of the median valley more widely spaced normal faults have formed subparallel to the valley in the western gulf of aden there are no indications of sonar targets that might represent transform faults parallel to the nne spreading direction in the east spreading center offsets of 15 25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading offsets of greater than 25 km are achieved on single transform faults and the associated tectonic fabric has a spreading direction trend the boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps these scarps are situated in the older crust and face the younger this is the opposite of the situation predicted on the basis of simple plate cooling models at some places in the gulf of aden the fracture zone topography is very subdued and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall there is evidence of a change of spreading direction 3 5 m y ago from about 040 to 022 it is argued that the small scale morphology of the transform fault spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection it is concluded that the transform fault must be weaker than the spreading center

hydrothermal activity in the lau back arc basin sulfides and water chemistry the submersible nautile completed 22 dives during the nautilau cruise r v nadir april 17 may 10 1989 for a detailed investigation of the southern lau basin near tonga the objective of the scientific team from france germany and tonga was to understand the process of sea floor ore formation associated with hydrothermal circulation along the valu fa back arc ridge behind the tonga kermadec trench the four diving areas between lat 21 25 s and 22 40 s in water 2000 m deep were selected on the basis of results from cruises of the r v jeancharcot and r v sonne the nadir cruise provided proof of hydrothermal activity in all four areas over more than 100 km as indicated by the widespread occurrence of hydrothermal deposits and by heat flow conductivity and temperature measurements near the sea bottom the most spectacular findings were high temperature white and black smokers and associated fauna and ore deposits hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges this difference is seen in the water chemistry of the hydrothermal fluid ph 2 and high metal content and the chemical composition of sulfides enrichment in ba as and pb

back arc extension in the andaman sea tectonic and magmatic processes imaged by high precision teleseismic double difference earthquake relocation 1 the geometry kinematics and mode of back arc extension along the andaman sea plate boundary are refined using a new set of significantly improved hypocenters global centroid moment tensor cmt solutions and high resolution bathymetry by applying cross correlation and double difference dd algorithms to regional and teleseismic waveforms and arrival times from international seismological centre and national earthquake information center bulletins 1964 2009 we resolve the fine scale structure and spatiotemporal behavior of active faults in the andaman sea the new data reveal that back arc extension is primarily accommodated at the andaman back arc spreading center absc at 10 which hosted three major earthquake swarms in 1984 2006 and 2009 short term spreading rates estimated from extensional moment tensors account for less than 10 of the long term 3 0 3 8 cm yr spreading rate indicating that spreading by intrusion and the formation of new crust make up for the difference a spatiotemporal analysis of the swarms and coulomb stress modeling show that dike intrusions are the primary driver for brittle failure in the absc while spreading direction is close to ridge normal it is oblique to the adjacent transforms the resulting component of e w extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the west andaman fault system after the mw 9 2 sumatra andaman earthquake of 2004 a possible skew in slip vectors of earthquakes in the eastern part of the absc indicates an en echelon arrangement of extensional structures suggesting that the present segment geometry is not in equilibrium with current plate motion demands and thus the ridge experiences ongoing re adjustment

structure and petrology of the reykjanes ridge between 62 55 n and 63 48 n geophysical and petrologic data are presented on the northern extent of the reykjanes ridge as it nears iceland the crest of the reykjanes ridge is a small belt of rough topography superimposed on an elevated plateau the rough topography consists of a series of en echelon ridges immediately to the southwest of the reykjanes peninsula the ridges strike about n35 e and apparently represent a rotational adjustment toward the n40 e trends on iceland to the south the en echelon ridges strike about n25 e magnetic profiles place the peak of the magnetic anomaly either directly below or along the eastern side of the topographic highs eight distinctive en echelon volcanic systems swarms are identified and are structurally and petrologically indistinguishable from those of the western reykjanes ridge petrologically three rock types picrite basalts olivine tholeiites and tholeiites were recovered these form distinctive groups which match those described from the western reykjanes peninsula

spreading mode of backarc basins in the western pacific abstract a number of phases are suggested in backarc spreading extension of the arc lithosphere is followed by spreading and formation of oceanic crust in the backarc basin in some basins a third phase of rifting with a reorientation of the spreading direction occurs during the second and third phases spreading is governed by rotation about a nearby pole and spreading axes are oblique to the transform faults this contrasts with the geometry of mid oceanic ridges where the ridges are on great circles perpendicular to the transform faults three spreading models are defined according to the various angular relations between transform faults and spreading axes backarc basins facing the pacific plate tend to open with axes spaced among adjacent ridge segments on projections parallel to the spreading axes type i spreading mode in some cases new spreading occurs in the countiguous area forming overlapped spreading axes type ii spreading mode initial spreading to form a backarc basin tends to spread with both spaced and overlapped axes type iii spreading mode inner backarc basins intercalated with other basins between the pacific plate also tend to spread with type iii spreading mode spreading modes and possible mechanisms for these formations in the backarc basins in the western pacific are discussed