ocean crust formation processes at very slow spreading centers a model for the mohns ridge near 72 n based on magnetic gravity and seismic data the mohns ridge in the norwegian greenland sea is one of the slowest spreading centers of the mid ocean ridge system 8 mm yr half rate sea beam data acquired with r v jean charcot near 72 n show that its rift valley floor is characterized by en echelon volcanic ridges oriented obliquely relatively to the average strike of the ridge axis these ridges are regularly spaced along the axis about every 40 km and are separated by nontransform discontinuities sharp positive magnetic anomalies centered over the topographic highs suggest that they are eruptive centers considered as the surficial expression of active spreading cells over the rift valley bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface this body if it exists probably corresponds to the zone of low viscosity that can be inferred from the model of chen and morgan 1990b which predicts the existence of a decoupling region between the upper crust and the asthenophere below its width varies rapidly along strike from less than about 5 km to more than 15 km in plan view it has a pinch and swell form which defines a series of spreading cells the center of one cell being where the bouguer anomaly is widest short wavelength less than 10 to 20 km along strike variations such as bouguer anomaly lows centered on the topographic highs reflect local effects associated with the presence of the eruptive centers seismic tomography data from a 20 10 km active oblique volcanic ridge near 72 22 n tend to indicate that the links between the main low velocity body at depth and the magma injections centers which lie within the rift valley inner floor are probably complex

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

influence of rift obliquity on fault population systematics results of experimental clay models abstract we use clay models to simulate how fault population systematics vary as a function of rift obliquity rift obliquity is related to the acute angle between the rift trend and the displacement direction so that the value of is inverse to the degree of obliquity the range of azimuths in a fault population increases as rift obliquity increases i e as decreases the length of the longest faults the sum of fault lengths and the width of the deformed zone all increase as rift obliquity decreases i e as increases the majority of faults in our models are segmented and have highly tortuous traces tortuosity is maximum when 30 as segments of widely varying azimuth link during fault growth significant changes occur in the fault patterns between 30 and 45 at 30 two fault populations of equal importance develop in the center of the rift zone one approximately rift parallel and the other displacement normal between 30 and 45 the number of faults more than doubles and between 45 and 60 summed fault length more than doubles fault patterns for all models are fractal with fractal dimensions that increase with increasing and that are comparable to those found in the field fault populations are not multi fractal because the cumulative frequency distributions of fault lengths do not generally follow a power law relationship an exponential distribution best describes the data for whole faults with the characteristic length increasing with increasing segment lengths also follow an exponential distribution with characteristic length varying very little with the fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid ocean ridge segments and are similar in geometry to those in oblique continental rift basins at the rift margins tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions resulting in the formation of two distinct sub populations of faults during oblique rifting

width of continental rifts and rheological layering of the lithosphere abstract a review of some well known continental rifts shows that rift features such as maximum basin depth are dependent on the amount of extension conversely the initial rift width appears to be independent of the amount of extension a comparison is made with laboratory models of rifts developed in a brittle ductile lithosphere experimental models show that normal faults which bound a graben initially meet at the brittle ductile interface a relation between initial rift width and the depth of the brittle ductile transition is thus suggested implications for natural examples are discussed

bouvet triple junction in the south atlantic geology and evolution the south american african and antarctic lithospheric plates meet in the bouvet triple junction btj located in the south atlantic near the island of bouvet multibeam magnetic gravimetric and seismic reflection data have been used to understand the evolution of the three accretionary transform boundaries that converge in the btj the easternmost segments of the american antarctic ridge aar have a spreading full rate of 19 5 mm yr for the last 8 m y they are oriented n s except for some ne sw segments probably created by magma poor extension the southernmost portion of mid atlantic ridge mar spreading full rate of 30 5 mm yr for the last 9 m y has elevated topographic anomalies it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation the mar axial valley bifurcates at its southern tip in two branches oriented 115 o and 180 o that are or have been up to recently loci of crustal accretion the bifurcation represents a former ridge ridge ridge triple junction the westernmost segments of the southwest indian ridge swir are anomalously high the segment adjacent to the island of bouvet spreading rate 14 5 mm yr is shallower than normal by almost 1 km due to the influence of the bouvet hot spot the westernmost swir segment spiess ridge consists of a swollen volcanic ridge that reaches 320 rn below sea level and has a deep caldera on its summit spiess ridge narrows and deepens to the nw v shaped topographic and magnetic lineations suggest that it propagates nw at a rate of 40 to 50 mm yr the spiess magmatic event started at roughly 1 ma when it caused deactivation of the 115 o spreading branch therefore the antarctic south american and african plates meet presently not in a triple point but in a broad zone of diffuse deformation an area of extensional deformation observed east of spiess ridge may be caused by excess crustal formation at spiess ridge that cannot be accommodated by motion of rigid plates the evolution of the btj since 10 ma involves stages of rrr rff and rrf configurations with highly variable geometry of the accretionary transform boundaries topographic anomalies anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread strong thermal anomalies that have influenced the configuration of the btj and determined indirectly intraplate deformation in wide areas of the btj region the thermal anomaly that gave rise to the swir spiess excess magmatism is the prime cause of the recent disruption of a former rrr configuration and of the imminent establishment of a new rrr triple junction to the north

extensional tectonics in the gorringe bank rocks eastern atlantic ocean evidence of an oceanic ultra slow mantellic accreting centre the gorringe bank corresponds to an upper mantle peridotite ridge enclosing a 500 m thick 50 km long laccolith like body of gabbro locally cut and poorly covered by tholeiitic rocks strain and kinematic analysis of orientated gabbros and peridotites sampled during the gorringe diving cruise 1996 provides new kinematic constraints on extensional high temperature deformation recorded at deep levels during stretching near an accreting centre axis of a mantle dominant oceanic lithosphere it is argued that the gorringe bank lithosphere formed at an oceanic ultra slow n010 020 trending accreting centre mostly by passive tectonic denudation of the mantle without any synchronous large magmatism this peculiar lithosphere may be representative of the iberia oceanic domain located between the continent and the j anomaly ridge which likely marks the beginning of true spreading at an oceanic spreading ridge

role of basin width variation in tectonic inversion insight from analogue modelling and implications for the tectonic inversion of the abanico basin 32 34 s central andes

obliquity along plate boundaries most of the plate boundaries are activated obliquely with respect to the direction of far field stresses as roughly only 8 of the plate boundaries total length shows a very low obliquity ranging from 0 to 10 sub orthogonal to the plate displacement the obliquity along plate boundaries is controlled by i lateral rheological variations within the lithosphere and ii consistency with the global plate circuit indeed plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influence strain localization geodynamical evolution controls large scale mantle convection and plate formation consumption and re organization thus triggering plate kinematics variations and the adjustment and re orientation of far field stresses these geological processes may thus result in plate boundaries that are not perpendicular but oblique to the direction of far field stresses this paper reviews the global patterns of obliquity along plate boundaries using gplate we provide a statistical analysis of present day obliquity along plate boundaries within this framework by comparing natural examples and geological models we discuss deformation patterns and kinematics recorded along oblique plate boundaries

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

deformation partitioning inside a fissure swarm of the northern icelandic rift abstract extension in an oceanic rift domain is accommodated by fissuring and faulting processes the fissuring including dike injection involves extension and an increase in crustal volume consistent with magma supply as in oceanic ridge and hot spot contexts the faulting mainly involves thinning and extension the relationship between these two deformation modes is analysed in the northern rift of iceland in the active rift zone deformation and volcanism occur within 2 3 km wide fissure swarms separated by 10 km wide zones where deformation is minor based on topographic and geological mapping the local structure in a major fissure swarm was analysed the fracture density increases near the faults but also beyond their tips the geometrical analysis of structures revealed that for the last 10 000 years block faulting and tilting accommodated about one half of the extension the other half resulting from fissuring this distribution shows that even in an active volcanic zone with widespread magmatism the stretching resulting from normal shear can play an important role as estimated from the ages of eroded surfaces and lava flows the deformation rates range between 1 5 and 15 cm year these values are compared with other rates estimated for different time intervals in the same area

structural expression of oblique seafloor spreading in the macquarie island ophiolite southern ocean the macquarie island ophiolite is an uplifted block of oceanic crust formed at the australia pacific spreading center between 12 and 9 ma the sense of motion and geological processes across this plate boundary reflect an evolution from orthogonal spreading through progressively more oblique spreading to the present day transpressional regime the crust that makes up the island was formed during an interval of oblique spreading along east trending spreading segments punctuated by a series of northwest trending discontinuities the discontinuities are accommodation zones marked by oblique slip dextral normal faults localized dikes and lava flows and extensive hydrothermal alteration indicating that these zones were active near the spreading axis these features provide a window into the internal structure of oceanic crust generated by oblique spreading

fracture populations on the reykjanes peninsula iceland comparison with experimental clay models of oblique rifting 1 we have used high resolution scanned air photos and field measurements to analyze fracture population systematics for the reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the reykjanes peninsula sw iceland the peninsula is oriented approximately 30 oblique to the direction of plate motion data show significant differences between the strike length and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism along the margin fractures are long highly segmented extension fractures and normal faults with offset on the order of several meters and a predominant strike 20 oblique to the trend of the plate boundary rift axis in the center of the zone of active volcanism fractures are generally shorter straighter fewer in number and strike approximately perpendicular to the direction of maximum horizontal extension and parallel to the trend of eruptive fissures right lateral oblique slip faults striking approximately perpendicular to the spreading direction are present in the center of the rift zone scaled experimental models of oblique extension predict a significant change in fracture strike at the rift margins due to the presence of a secondary stress field on the reykjanes peninsula this corresponds to the boundary between thicker and thinner brittle crust the models confirm that right lateral oblique or strike slip faults accommodate significant left lateral shear where deformation is distributed in a rift zone

structural analysis of fault populations along the oblique ultra slow spreading knipovich ridge north atlantic ocean 74 30 n 77 50 n abstract the knipovich ridge 73 30 78 40 n is an extreme end member of the mid ocean ridge spreading system both in terms of spreading rate fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems for the ridge as a whole faults in the axial region are short straight isolated not linked into complex fault zones and exhibit length scaling relationships characteristic of young and active fault systems faults are generally oblique to both the ridge axis and the spreading direction and orientations vary systematically with angle between ridge and spreading direction along axis analysis reveals the influence of axial segmentation on fault population characteristics segment centers are dominated by faults perpendicular and sub perpendicular to plate motion with longer characteristic length and generally lower fracture density conversely segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density we infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion while faulting in segment ends is controlled by the mechanics of oblique rifting non transform discontinuities and or accommodation zones the contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra slow spreading rate of this spreading system

resource assessment of undiscovered seafloor massive sulfide deposits on an arctic mid ocean ridge application of grade and tonnage models abstract norway has started ongoing initiatives related to deep sea mining these include the evaluation of mineral resources within its sovereign territorial waters on the deep ocean floor of the arctic region i e the mohns ridge 71 73 n among the variety of possible seabed resources seafloor massive sulfides sms attract particular interest owing to the significant development potential and the general lack of knowledge of the sms deposits the present study aims at assessing mineral resources i e cu zn au and ag from a number of undiscovered sulfide deposits with the help of a revised model for mafic hosted volcanogenic massive sulfide vms deposits that adequately corresponds to geological settings of slow spreading ridges application of the model requires a priori knowledge of the seafloor terrain to depict favorable geologic environments for the occurrence of sms deposits in such context estimates are conducted for the easily accessible volcanically active parts of the ridge where 11 undiscovered deposits are expected in these areas total metal endowments are calculated to be on average 447 000 tonnes using a monte carlo simulation that combines the probabilistic estimates of number of undiscovered deposits with the grade and tonnage models estimates of in place metal resources are generated in cumulative distribution form to present expected amount of undiscovered metals at 90 confidence intervals

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

fault architecture in the main ethiopian rift and comparison with experimental models implications for rift evolution and nubia somalia kinematics abstract the main ethiopian rift mer offers a complete record of the time space evolution of a continental rift we have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models and implemented with field controls this analysis has been compared with the results of lithospheric scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along axis variation of the rift development and southward decrease in rift evolution the northernmost mer sector is in a mature stage of incipient continental rupture with deformation localised within the rift floor along discrete tectono magmatic segments and almost inactive boundary faults the central mer sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase the southernmost mer sector is instead in an early continental stage with the largest part of deformation being accommodated by boundary faults and almost absent internal faults the mer thus records along its axis the typical evolution of continental rifting from fault dominated rift morphology in the early stages of extension toward magma dominated extension during break up the extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along axis variations in rift architecture and evolutionary stage being oblique rifting conditions controlling the mer evolution since its birth in the late miocene in relation to a constant post ca 11 ma n100 e nubia somalia motion

analogue experiments of propagation of oblique rifts abstract we present the results of analogue models of oblique rifting which were carried out using a isostatically compensated two layered small scale models the experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular rifted basins set en echelon along the axial zone of the experiment box in time the basins propagate lengthways to merge and form a unified composite rift however traces of the initial en echelon rifts expressed in deeper axial basins separated by shallower threshold zones are clearly discernible in the mature rifts both the basins and the threshold zones are located within the composite rift so that the rift margins are abrupt and steep along the basins and more diffuse along the threshold zones the thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor model experiments of nearly orthogonal extension showed a single unified rift which differed conspicuously from the composite oblique rifts the results of the experiments suggest that oblique extension forms en echelon rift segments the axes of which are perpendicular to the extension direction separated by threshold zones therefore there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension

morphology and tectonics of the mid atlantic ridge 25 27 30 s from sea beam and magnetic data high resolution sea beam bathymetry and sea surface magnetic data have been collected between 25 s and 27 30 s on the mid atlantic ridge analysis of the morphology and structure of the ridge axis and flanking terrain as well as the results of a three dimensional inversion of sea surface magnetic data are presented this 227 km long portion of the mid atlantic ridge is offset 42 km by the right lateral rio grande transform at 25 40 s and 9 5 km by the left lateral nontransform moore discontinuity at 26 30 s a third nontransform discontinuity the midway discontinuity exists near 26 12 s and is defined by a small displacement of the axial magnetization distribution and the disruption in the along strike continuity of ridge parallel terrain elements although the detailed survey presented in this paper extends out to only 5 to 7 m y old crust a regional compilation of magnetic data from this area by cande et al 1988 indicates that the relative positions and dimensions of the spreading cells have characterized this part of the mid atlantic ridge for at least the past 55 m y the offsets dimensions and geometries however have changed markedly because of prolonged differential asymmetric spreading between segments between and within the accretionary units that are spreading at uniform rates 35 mm yr total opening rate the morphology of the ridge axis varies dramatically from a well defined rift valley 20 25 km wide 1 1 5 km deep to an axial swell that rises 1500 m above the adjacent valley floor the axial magnetic structure is also highly variable along strike the inversion solution shows a series of generally linear short wavelength 10 km magnetization highs within the central brunhes anomaly over each of the four segments in the survey area these anomalies are interpreted to be the central anomaly magnetization high that is observed over the neovolcanic zones of many of the world s mid ocean spreading centers the distinct axial magnetization highs can extend out to 2 m y old crust at ridge discontinuity intersections suggesting that in some instances they may also be the product temporal variations in the geochemistry of the extrusive rocks e g highly fractionated and strongly magnetized basalts enriched in iron and titanium the magnetic inversion shows that the central anomaly is locally attenuated at the minimum depth points of three of the four spreading segments in the survey area both inversion and forward modeling of magnetic data show that topography alone cannot produce the anomalously low values observed in the case of an axial swell centered at 26 s attenuation of the central anomaly is attributed to the effective thinning of the source layer by demagnetization effects of a crustal thermal anomaly and or a zone of substantial alteration taken together the bathymetric data and magnetic modeling document strong along axis gradients in crustal and upper mantle properties

recurrence intervals of characteristic m4 8 0 1 earthquakes off kamaishi ne japan comparison with creep rate estimated from small repeating earthquake data abstract we examined the cause of the fluctuation in recurrence intervals of an m4 8 0 1 characteristic earthquake sequence off kamaishi ne japan by comparing the recurrence intervals with creep quasi static slip rates around the asperity estimated from small repeating earthquake data the eight recurrence intervals 5 52 yrs on average of the sequence are almost constant but have small fluctuations standard deviation of 0 68 yrs characterized by a longer recurrence interval following a shorter one from the analysis of small repeating earthquakes from 1990 to 2003 we found a significant acceleration of creep on the plate boundary during and after an earthquake swarm in 1992 to the east of the m4 8 0 1 event the acceleration seems to have migrated from east shallower part of the plate boundary to west deeper part over about 80 km in 2 yrs to reach the asperity of the m4 8 0 1 events the 1995 event that occurred with the shortest recurrence interval was probably advanced by this acceleration of creep in around 1994 we also found similar earthquake swarms prior to the 1962 and 1973 characteristic events that occurred after the second and third shortest recurrence intervals respectively therefore it is possible that the 1962 and 1973 events were also advanced by creep accelerations the three pairs of longer and shorter recurrence intervals we observed suggest the existence of creep deceleration following the acceleration we conclude that the recurrence interval fluctuations of the m4 8 0 1 events are mainly controlled by temporal changes of creep around its asperity

evolution pattern and partitioning of deformation during oblique continental rifting inferences from lithospheric scale centrifuge models oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone during extension this weakness localizes deformation and different rift obliquity is obtained by varying its trend with respect to the stretching direction model results show that deformation is mostly controlled by the obliquity angle a defined as the angle between the orthogonal to the rift trend and the extension direction for low obliquity alpha 45 degrees no boundary faults form and the extensional deformation affects the rift depression since early stages of extension dominance of the strike slip motion over extension leads to the development of oblique slip and nearly pure strike slip faults oblique to both the rift trend and the orthogonal to the extension direction with no strain partitioning between the margins and the rift floor these results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution architecture and strain partitioning findings that have a significant relevance for natural oblique rifts

tectonics at the axis of the very slow spreading southwest indian ridge insights from tobi side scan sonar imagery we present the analysis of the deformation in the axial valley of two contrasted regions of the very slow spreading southwest indian ridge based on side scan sonar images our objective is to investigate how the obliquity is accommodated along the system we show that the robust magmatic segments have axial valleys and major faults subperpendicular to spreading the other sections show fault populations with various degrees of obliquity often arranged in left stepping echelons accommodating part of the strike slip deformation side scan sonar reveals the presence of a corrugated surface near 59 e interpreted to be an incipient detachment fault we show that the large width of the swir oblique sections and the difference in tectonic style between the robust volcanic segments and the magma starved sections is accounted for by large variations in the thickness of the brittle lithosphere we suggest that the emplacement of mantle rocks in the surveyed amagmatic ridge sections can occur by alternating conjugate faults serpentinization of outcropping peridotites might also play a significant role in the development of faults in thin crust regions and the distribution of deformation in space and time

deformation partitioning in a slow spreading ridge undergoing oblique extension mohns ridge norwegian sea although oceanic spreading is often perpendicular to the ridge trends in some cases the angle between these two directions can be significantly less than 90 40 50 this occurs because of either a bend of the ridge trend or a change of the spreading direction we here describe oblique spreading in the mohns ridge resulting in deformation partitioning between the valley walls which are dominantly affected by strike slip displacements and the axial valley which is subject to nearly pure extension the axial valley walls are characterized by en echelon normal faults affecting the walls while the axial valley is affected by parallel faults grouped into oblique sets these fault sets define different structures horst or tilted blocks that are regularly spaced inside the axial valley moreover some ridge segments mainly undergo pure extension whereas others are affected by oblique extension we explain this faulting pattern including the along strike and transverse variations as a consequence of depth variations of the brittle ductile transition

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

seismoacoustic recordings of a spreading episode on the mohns ridge a period of very active seismicity near 72 7 n 4 e marks an episode of seafloor spreading on the mohns ridge the earthquakes were recorded from november 1995 to january 1996 by onshore seismic stations and by u s navy hydrophone arrays in the north atlantic both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events the hydrophone arrays recorded 2 3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed during the most intense phase of activity a level of 50 200 events per day was sustained throughout the episode initial locations of the events were obtained from the seismic bulletin further refinement of the epicenters was possible using p s converted to an acoustic phase at the seafloor and t waves in the hydrophone data analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm a few events occurred at a more distant location the activity tends to concentrate in one area or another for short periods a few days but at times it is clear that events occur simultaneously at more than one location we have not found evidence of steady migration of activity such as might accompany propagation of a magma filled dike we thus infer that despite the 50 70 km length of ridge involved in the spreading episode rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas

preliminary analysis of the knipovich ridge segmentation influence of focused magmatism and ridge obliquity on an ultraslow spreading system abstract bathymetry gravity and deep tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow spreading knipovich ridge in the norwegian greenland sea northeast atlantic ocean discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle bouguer anomaly mba lows generally resemble those of the gakkel ridge and the easternmost southwest indian ridge these magmatically robust segment centers are regularly spaced about 85 100 km apart along the ridge and are characterized by accumulated hummocky terrain high relief off axis seamount chains and significant mba lows we suggest that these eruptive centers correspond to areas of enhanced magma flux and that their spacing reflects the geometry of underlying mantle upwelling cells the large scale thermal structure of the mantle primarily controls discrete and focused magmatism and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge segment centers along the southern knipovich ridge are characterized by lower relief and smaller mba anomalies than along the northern section of the ridge this suggests that ridge obliquity is a secondary control on ridge construction on the knipovich ridge as the obliquity changes from 35 to 49 from north to south respectively while spreading rate and axial depth remain approximately constant the increased obliquity may contribute to decreased effective spreading rates lower upwelling magma velocity and melt formation and limited horizontal dike propagation near the surface we also identify small magmatically weaker segments with low relief little or no mba anomaly and no off axis expression we suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller discrete mantle upwelling centers with short lived melt supply

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

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

development of en echelon magmatic segments along oblique spreading ridges en echelon magmatic segments commonly develop along obliquely spreading oceanic ridges to clarify some of the dynamic aspects of this plate boundary we performed a series of thermo mechanical numerical tests when extension of oceanic lithosphere becomes oblique deformation within the axial region localizes into distinct upwelling centers temperatures are elevated in the upwelling cells which are shallow mantle features that form the new plate boundary the predicted features are similar to the axial volcanic ridges documented at mohns and reykjanes ridges and we conclude that they become the new loci of extensional deforma tion upwelling and magmatic activity these ridges suborthogonal to the plate spreading direction only develop when the axis rift zone is weak the subsegment length and spacing depend primarily on obliquity and axial width predicted crustal thickness along the subsegmented axis varies discernibly this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges

nucleation growth and linkage of faults in oblique rift zones results from experimental clay models and implications for maximum fault size we use scaled clay models to study the temporal evolution of fault populations in experiments of moderately oblique a5 608 and highly oblique a5 308 distributed extension where a is the angle between the rift axis and the direction of displacement faults nucleate at random heterogeneities enhancing nucleation of diffuse clusters of new faults in the highly oblique model clusters of displacement normal faults form parallel arrays leading to fault growth dominated by tip propagation and along strike linkage until maximum length is achieved subsequent growth of rift subparallel faults leads to a phase of growth characterized by linkage and formation of branching faults in the moderately oblique model clusters form in a stepping geometry leading to growth dominated by linkage faults nucleate and grow more rapidly and their growth is less restricted than in the highly oblique model our results have implications for the maximum size earthquake to be expected in an oblique rift setting

oscillatory spreading explanation of anomalously old uplifted crust near oceanic transforms rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some atlantic transform zones rocks of palaeocene 55 58 myr and possibly even of cretaceous age are found near the vema transform on tectonically uplifted crust which should not be older than 30 myr we present here a model which might explain the presence of uplifted anomalously old crust near the vema transform zone tz the proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field during transform migration a crustal wedge formerly part of the african plate is transferred to the south american plate the ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift

extension direction re orientation in the oceanic rift of iceland and comparison with continental rifts abstract in this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of iceland namely 1 the northern part of the eastern rift segment northern volcanic zone nvz and 2 the western rift segment that include the western volcanic zone thingvellir area and the reykjanes peninsula in the n s trending nvz mainly krafla volcanic area the obtained extension directions trend around ese at thingvellir the extension directions trend ca nw se and are perpendicular to the main ne sw trending faults in the ene wsw trending tectono magmatic segments of the reykjanes peninsula our data show a marked difference with the gps data and spreading directions the extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors extension shows an overall southward increase in divergence from the regional directions indicated by gps vectors this behavior has been compared with the continental main ethiopian rift of east africa where structural studies show a re orientation of the extension direction between the rift margins and the internal sectors resulting in a divergence between local and regional extension directions aimed at investigating the causes of these differences we performed a new series of analogue models by varying different parameters such as rift obliquity and width our findings suggest that rift width exerts a primary control on the boundary axis variation of the extension direction in particular significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys e g continental rift of ethiopia conversely fault orientation and local extension direction are much more homogeneous in narrower rift valleys e g rifts of iceland where a variations in the direction of extension from the margins to the rift axis are not observed

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

the axial topographic high at intermediate and fast spreading ridges abstract an axial topographic high is commonly observed at both fast spreading ridges and some segments of intermediate spreading ridges at fast rates the axial high is primarily created by the buoyancy of hot rock and magma beneath the rise as newly formed crust is transported off axis little vestige of an axial high is observed on the ridge flanks in contrast at intermediate rates a significant component of the positive topography may be a volcanic construction preserved on the ridge flanks as abyssal hills which are split axial volcanoes we suggest this difference in the nature of the axial high reflects a lithosphere strong enough to support construction of a volcanic crestal ridge at intermediate spreading rates but only rarely at fast rates relict overlap ridges found within the discordant zones left by overlapping spreading centers is one class of ridge flank topography which appears to have a significant volcanic constructional component even at fast spreading ridges unlike topography away from these discontinuities the relief and shape of overlapping spreading centers is preserved as relict ridge tips are rafted onto the ridge flanks reduced magma supply at these discontinuities may give rise to an axial lithosphere strong enough to support volcanic construction of overlap ridges low axial lithospheric strength may also account for the lack of normal faults within the innermost 1 2 km of fast and some intermediate spreading ridges with a thin weak brittle layer at the ridge crest tensile failure will predominate and few normal faults will form depths to the axial magma chamber reflector observed in multi channel seismic data limit the thickness of the brittle layer on axis to less than 1 2 km for much of the epr this depth is comparable to depths over which tensile failure within the oceanic crust will predominate estimated from the griffith criteria for fracture initiation 0 5 1 5 km as the brittle layer thickens strengthens away from the ridge shear failure begins and the large scale normal faults associated with abyssal hill relief develop

late amagmatic extension along the central and eastern segments of the west philippine basin fossil spreading axis the spreading processes within the west philippine basin wpb remain partly unknown this study presents an analysis of the tectono magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33 30 ma we demonstrate that the late episode of n s opening from an e w trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin this event was responsible for transtensional strain accommodated along the nw se faults cutting through the former e w rift valley in the central part of the basin in its eastern part the same event occurred at a larger extent and led to the creation of a new nw se axis obliquely cutting the older e w spreading segments and their associated spreading fabrics at this location several tens of kilometers of slightly oblique amagmatic extension occurred following a v60 direction we propose that this late event is associated with the onset of e w opening of the parece vela basin located along the eastern border of the wpb at 30 ma extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the wpb the newly created nw se axis most likely propagated from east to west being responsible for scissors opening within the wpb ne sw extension ceased when well organized spreading started at 26 ma in parece vela basin accommodating entirely the global extensive stress pattern 2002 elsevier science b v all rights reserved

faulting and volcanism in the axial valley of the slow spreading center of the mariana back arc basin from wadatsumi side scan sonar images 1 we analyzed in detail the geology of the median valley floor of the mariana basin slow spreading ridge using sea surface geophysical data and a high resolution deep tow side scan sonar survey over one spreading segment analysis of surface magnetic data indicates highly asymmetric accretion with the half spreading rate on the western side of the basin being two to three times larger than on the eastern side surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of 10 km of amplitude deep tow backscatter data indicate along axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end respectively this variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end concerning tectonic processes we find a power law distribution of the fractures with an exponent of 1 74 this suggests that within the inner valley floor fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8 of the strain according to our calculation based on a ratio of 0 02 to 0 03 between the vertical displacement and the length of faults the amount of tectonic strain accommodated in the inner valley floor would consistently be 1 1 3 4 data also show two distinct sets of fractures one trend is parallel to the rift direction at the segment center n160 e and perpendicular to the plate separation direction another set trends 17 oblique to this direction n175 e and is located over the eastern part of the valley in the vicinity of a major bounding fault also trending n175 e that is obliquely to the direction of plate motion we modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three dimensional boundary element approach we found that the orientation of the predicted fissuring near the oblique fault is locally rotated by 15 due to a flexure of the bending plate close to this fault

volcano tectonic events and sedimentation since late miocene times at the mohns ridge near 72 n in the norwegian greenland sea abstract the combined analysis of structural and magnetic data from the mohns ridge near 72 n in the norwegian greenland sea shows the existence of different phases of seafloor spreading from anomaly 5a 12 ma which represents the limit of the magnetic survey onwards for the period comprised between anomaly 5 9 ma and anomaly 3a 6 5 ma most particularly a dramatic drecrease of spreading rate is observed by as much as 50 between anomaly 3a and 2a 3 2 ma an acceleration of spreading is observed together with a global uplift of the acoustic basement this latter period may correspond to a local enhancement of tectono volcanic activity the existence of ridges located at 25 and 50 km from the ridge axis and parallel to it suggests that both the beginning 5 4 5 ma and the end 3 2 5 ma of this phase were marked by intense volcanism in the sedimentary basins of the survey area two at least intra sedimentary horizons appear at respectively 140 160 ms and 340 360 ms two way travel time beneath the sediments surface using an inferred value of sedimentation rate of 50 70 m myr the age of the first horizon between 3 and 2 ma corresponds to the beginning of mid pliocene glaciations in the northern hemisphere the age of the second one between 6 5 and 4 5 ma to the pliocene miocene boundary our data and other data from other areas indicate that the three major sources of sediments western spitzbergen margin bear island trough and storfjord trough which supply the mohns ridge area near 72 n have been dramatically more active after 5 5 ma in addition local unconformities recorded in the single channel seismic sections indicate that some tectonic event occurred off axis little after the date corresponding to the deposition of the second horizon s2 if one accepts that the knipovich ridge migrated eastward 6 5 ma ago it can be hypothesized that the tectonic release of the lena trough knipovich ridge mohns ridge system forced the sediment production to increase at each one of the three sediment sources after the miocene pliocene transition

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

normal faulting block tilting and decollement in a stretched crust extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas if we except continental and oceanic rifts stretched portions of the crust exhibit complex faulting patterns at upper levels with high angle and low angle normal faults very little is known concerning the geometry of structures and physical processes at depth it is therefore difficult to extrapolate surface observations to the crustal scale five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed two natural examples the west armorican atlantic margin and eldorado mountains basin and range province are presented they are characterized by an association of high angle normal faults tilted blocks and low angle normal faults field and geometrical arguments are given to demonstrate that low angle faults are controlled by decollement surfaces along preexisting interfaces geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults in such cases the amount of stretching cannot be related to bulk thinning of the crust

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

transtensional rifting in the proto gulf of california near bahia kino sonora mexico continental rifts require focused strain to rupture and form an ocean basin in oblique rifts such as the gulf of california focused transtensional strain associated with strike slip faulting may serve as a catalyst for rupture to test this hypothesis we analyzed structural mapping geochronology paleomagnetism and fault kinematics of pre and synrift rocks exposed in an 200 km 2 coastal mountain belt flanking the eastern rift margin of the northern gulf of california this coastal sonora region hosts the onshore portion of the transform boundary between the upper tiburon and adair tepoca marine basins two early formed oblique rift segments extension commenced here between 11 5 ma and 7 ma resulting in 25 40 of e ne tilting initiation of clockwise vertical axis rotation of fault bounded blocks and minor basin sedimentation rates of deformation prior to 7 ma are unconstrained due to a lack of exposed syntectonic deposits deformation after 7 ma was associated with rapid tilting and the majority of observed clockwise vertical axis rotation and strike slip faulting nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older tilted strata by 5 6 ma deformation in coastal sonora must have largely ceased and migrated westward into the upper tiburon marine basin we document up to 120 total extension and total clockwise block rotations up to 53 in portions of the study area extension and rotation were supplanted by strike slip faulting as deformation proceeded we develop a tectonic model for this coastal sonora fault zone which is bounded by major nw striking transform faults with 10 km of displacement internal to the coastal sonora fault zone the majority of an estimated 6 2 1 1 km of dextral deformation associated with up to 5 7 km of wnw directed extension occurred over the final 1 2 m y of its life span at a strain rate approaching 10 14 s 1 this activity occurred as the plate boundary localized along nascent pull apart basins in the northern gulf of california consistent with the hypothesis that late proto gulf of california dextral shear zones such as the coastal sonora fault zone acted to focus lithospheric scale strain and promoted continental rupture in the wide rift setting of the mexican basin and range

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

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

thermoluminescence and the shock and reheating history of meteorites ii annealing studies of the kernouve meteorite abstract samples of the unshocked equilibrated chondrite kernouve h6 have been annealed for 1 100 hours at 500 1200 c their thermoluminescence sensitivity measured and na k mn ca and sc determined by instrumental neutron activation analysis the tl sensitivity decreased with temperature until by 1000 c it had fallen by 40 the process responsible has an activation energy of 8 kcal mole and probably involves diffusion samples annealed 1000 1200 c had tl sensitivities 10 2 times the unannealed values most of the decrease occurring 1100 c this process has an activation energy of 100 kcal mole and is probably related to the melting of the tl phosphor feldspar with some decomposition and loss of cs na and k meteorites whose petrography indicates healing 1100 c by natural shock heating events shock facies d f have tl sensitivities similar to samples annealed 1100 c our own and literature compositional data indicate that tl is more stable to annealing than ag in tl bi zn and te and less stable than na k mn ca se and co while the tl decrease resembles very closely the pattern of cs loss on annealing

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

thickness of extrusive basalts dominating the magnetic structure along the ultraslow spreading mohns ridge axis 71 8 73 7 n abstract the equivalent magnetization em and mantle bouguer anomaly mba were calculated along the ultraslow spreading mohns ridge axis in the norwegian greenland sea the magnetic anomaly and the associated em were compared with the bathymetry mba seismically determined crustal structure and geochemical data at both the inter segment scale 60 km and the intra segment scale 20 60 km at the inter segment scale the magnetic highs at the segment centers are independent of the mba of the 13 segments 9 with magnetic anomalies 700 nt coincide with axial volcanic ridges identified from multibeam bathymetry maps which suggests that the magnetic highs at the segment centers may be more associated with the extrusive lavas rather than the amount of magma supply with few exceptions the magnetic anomaly lows associated with mba highs at the segment ends increase from south to north this trend might be explained by thickened extrusive basalts and or more serpentinized peridotites at the segment ends in the north at the intra segment scale the most prominent features are the decreases in the magnetic anomalies and associated ems from the segment centers to the ends the intra segment magnetic anomalies have positive and negative correlations with the bathymetry and mba respectively the magnetic signal modeled by the seismically determined layer 2a with an assumed constant magnetization is remarkably consistent with the observed magnetic anomaly which strongly suggests that the thickness of the extrusive basalts dominates the magnetic structure in each segment along the mohns ridge in general the thickness of the extrusive basalts dominates the magnetic structure along the mohns ridge whereas the contributions from serpentinized peridotites may be significant at the segment ends and may produce long wavelength magnetic variations the magnetic data can be used as an indicator of the thickness of the extrusive basalts within segments along the ultraslow spreading mohns ridge

three dimensional thermomechanical modeling of oceanic spreading initiation and evolution abstract this work employs high resolution 3d thermomechanical numerical models of the incipient oceanic spreading to investigate nucleation and long term evolution of ridge transform spreading patterns the eulerian lagrangian visco plastic model allows for large strains and accounts for plate cooling by both heat conduction and hydrothermal circulation as well as for partial melting of the asthenosphere and oceanic crust growth by magmatic accretion according to the numerical experiments the oceanic spreading pattern depends strongly on the initial offset of spreading centers and the magnitude of fracture healing rate three different characteristic long term spreading modes are obtained 1 ridge transform patterns 2 single ridges and 3 spreading centers with an intermediate plate ridge transform oceanic spreading patterns form gradually from moderate initial offsets of 40 60 km and become fully established several million years after the plate breakup moreover it is demonstrated on the basis of simple analyses that the ridge transform system is a long term plate growth pattern that is generally different from an initial plate rifting pattern geometry of the ridge transform system is governed by geometrical requirements 180 rotational symmetry for open space occupation for simultaneous accretion and displacement of new plate material within two offset spreading centers connected by a sustaining rheologically weak transform fault according to these requirements the characteristic spreading parallel orientation of oceanic transform faults is the only thermomechanically consistent steady state orientation results of numerical experiments compare well with both incipient and mature ridge transform systems observed in nature