analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

hf w chronology of ordinary chondrites abstract fifteen h l and ll ordinary chondrites of petrologic types 4 6 have been analyzed for hf w isotope systematics to constrain the chronology internal structure and thermal history of their parent bodies for most samples coarse grained metals plot below the isochrons defined by silicate dominated fractions which consist of variable mixtures of silicate minerals with tiny metal inclusions this offset results from an earlier hf w closure in the large metal grains and provides a new means for simultaneously determining cooling rates and hf w closure ages for individual samples for most type 5 and 6 samples cooling rates and hf w ages are inversely correlated indicating that these samples derive from concentrically zoned bodies in which more strongly metamorphosed samples derive from greater depth these data therefore provide strong evidence for a common onion shell structure for the h l and ll chondrite parent bodies the cooling rates and hf w ages of some type 5 and 6 chondrites overlap indicating that the hf w systematics provide a more robust measure of the thermal history and burial depth of a given sample than the simple petrographic distinction between types 5 and 6 two type 6 samples deviate from the correlation between cooling rates and hf w ages and cooled much faster than expected for their hf w age these samples likely were excavated by impacts that occurred during high temperature metamorphism and prior to complete closure of the hf w system at 10 ma after cai formation as these impacts would have disturbed the asteroid s cooling history these samples likely derive from different bodies than samples with undisturbed cooling histories implying that ordinary chondrites derive from more than just three parent bodies the hf w data reveal that metal silicate fractionation among the h l and ll groups occurred between 2 and 2 7 ma after cai formation and hence was about coeval to chondrule formation as both metal silicate fractionation and chondrule formation occurred prior to chondrite parent body accretion there should be no ordinary chondrite chondrules that are younger than 2 7 ma finally ordinary chondrite precursors had lower hf w ratios than carbonaceous chondrites suggesting that inner and outer solar system materials respectively were chemically distinct even for refractory elements

rift structure evolution and earthquakes in the gulf of corinth from reflection seismic images abstract continental extension is forming the gulf of corinth across the strike of earlier alpine evolution here we present the first deep reflection sections with pre stack depth migration processing across the deep basin of the corinth active rift which image structures unpredicted by current models resolving the infill as a pile of layers that are broken by faults allows one to follow their subsidence and deformation history variation among the profiles suggests that southern normal faults control the rift in a time progression from the east towards its western tip on the central derveni itea transect a 3 km widening of the basin accrued since the initiation of this control that is marked by an unconformity between the two main sedimentary units this is estimated to have occurred 0 5 0 6 myr ago assuming the glacio eustatic sea level changes have controlled the stratigraphy of sediments deposited as a succession of layers on the subsiding hangingwall as they did on the southern footwall in forming the famous flight of marine terraces of corinth a roll over anticline and crestal collapse graben are diagnostic of the control by a normal fault of dip varying with depth the deeper low angle part of this bi planar fault is indeed imaged as a reflector in the basement the occurrence of the collapse with a breakaway at the steep southern basin bounding fault of the hangingwall slab can be estimated 0 12 0 2 myr ago with a marked increase in extension rate that brought it to its present fastest value over 10 mm yr the low angle part of the active fault might also have controlled earlier evolution upslope and in the basin when compared with inferences from earthquake studies this low angle active fault may not appear to be seismogenic but may participate to the seismic cycle possibly in a conditionally stable regime active faults seen as sea bottom scarps merely accommodate deformation of its subsiding hangingwall the footwall of the low angle faults which current seismicity shows to be in extension appears then as being pulled out from beneath the rift in a motion towards the rolling back slab that causes the hellenic subduction retreat

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

waveform inversion of the s reflector west of spain fine structure of a detachment fault the s reflection west of iberia has been interpreted as a low angle detachment fault separating crustal fault blocks from partially serpentinized mantle we apply full waveform inversion to investigate the fine structure of s our results confirm that s is largely a step increase in velocity and density probably from crustal rocks to partially serpentinized mantle peridotites a 50 m thick low velocity zone above s might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone both interpretations imply focused fluid flow along s raising the possibility that low angle movement along s was aided by the development of local transient high fluid pressures

was the 1952 tokachi oki earthquake mw 8 1 a typical underthrust earthquake plate interface reflectivity measurement by an air gun ocean bottom seismometer experiment in the kuril trench 1 the kuril trench subduction zone is one of the most seismogenic regions where underthrust earthquakes with m 8 recur along the trench the seismic gap between the source areas of the 1973 nemuro oki and 2003 tokachi oki earthquakes which are typical underthrust earthquakes faulting with rupture velocities of 3 km s has been ruptured by the 1952 tokachi oki earthquake the seismic gap has also slipped incidental to neighboring asperities the difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface such as a reflectivity of the plate interface we estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun ocean bottom seismometer experiment on the along trench profile across the seismic gap strong reflections from the plate interface were observed in the 1952 tokachi oki source area including the seismic gap rather than in the 1973 nemuro oki source area the strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable the coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes such as the 2003 tokachi oki and 1973 nemuro oki earthquakes our results suggest that the 1952 tokachi oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake

a jurassic cretaceous island arc in the ladakh himalayas abstract the ladakh mesozoic ophiolite belt western himalaya contains a pile of volcanic thrust sheets dras unit which differ significantly in structure and composition from the ophiolitic melange zones the dras unit is composed of pillow lavas doleritic sills very irregular basaltic basaltic andesites and dacitic flows intercalated with pyroclastics volcanoclastic sediments and radiolarian cherts according to fossil evidence this volcanism must have been active between upper jurassic and upper cretaceous the presence of relict primary minerals such as magnesiochromite clinopyroxene hastingsitic hornblende and ti magnetite as well as distinctive bulk chemistries suggests that the volcanics belong to island arc tholeiite and to calc alkaline rock series typical of present island arcs in the caribbean and pacific model calculations incorporating probed phenocryst phases indicate that in addition to olivine clinopyroxene and plagioclase amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt the latter process must have taken place at about 1000 c and at moderate depth of 5 15 km within or underneath the island arc today hornblende bearing mafic cumulates appear in the vicinity of kargil within and close to the dras volcanics in a sr evolution diagram the dras volcanics have yielded a pseudo isochron with a low initial 87 sr 86 sr ratio of 0 7035 0 0003 which is in the same range as the mean of modern island arc volcanics however a geologically unrealistic age of 263 m y is obtained from the slope of this isochron the upper mantle is regarded as the source material for the island arc tholeiitic magmas enrichment in k ba sr and lree supports the involvement of components derived from dehydration or incipient melting of subducted tethyan oceanic crust in the mantle

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

earthquake relocation and three dimensional vp and vp vs models along the low angle alto tiberina fault central italy evidence for fluid overpressure summary the active extension in the central portion of the northern apennines is believed to be accommodated by an ene dipping low angle normal fault cutting the upper crust called alto tiberina fault atf the physical properties and mechanics of such a severely misoriented fault are still unknown and debated in this study we constrain the deep geometry of the atf and investigate the material properties of the fault system by using microseismicity recorded by a dense local seismic network the inversion of local earthquake p and s wave arrival times yields high resolution v p and v p v s images of the fault system we find positive v p anomalies that define the eastward verging thrust and fold belt developed in the sedimentary cover which inner bulk consists of triassic evaporites remains of the older compression microseismicity reveals the 20 ne dipping atf nearly planar from 3 to 14 km depth no sharp velocity contrasts are observed across the fault but only a gentle warping of the velocity anomalies and the dislocation of the triassic evaporites top at shallow depth on the fault hangingwall normal v p and strong high v p v s indicate the presence of a fractured high fluid pressure volume within which steep normal fault segments are located this evidence suggests that fluid pressure plays a dominant role in triggering the shallow background seismicity along the atf low v p v s anomalies are observed suggesting a different level of cracking and pore saturation and that the fault acts as impermeable barrier we speculate that the microseismic activity on the atf is favoured by the entrapped and locally overpressured co2 testified by the low v p v s volumes

dynamics of outer rise faulting in oceanic continental subduction systems 1 during subduction bending of downgoing oceanic lithosphere gives rise to normal faulting due to the extensional stress state generated in the upper plate as deformation patterns inherently reflect a material s state of stress and rheology extensive global observations of outer rise faulting patterns and subduction dynamics provide a unique opportunity to examine the factors controlling outer rise deformation despite a wide range of observed oceanic plate ages convergence rates and slab pull magnitudes across modern subduction systems however measured outer rise faulting patterns show effectively no correlation to variations in these parameters this lack of correlation may reflect that outer rise faulting patterns are strongly sensitive to all of these parameters are dependent on additional parameters such as downgoing overriding plate coupling or that existing faulting measurements require additional analysis in order to provide a basis for future analysis of outer rise faulting patterns we build on previous thermo mechanical numerical models of outer rise deformation and explore the relationship between outer rise faulting patterns subduction dynamics and brittle rheology in an oceanic continental subduction system analysis of time averaged outer rise faulting patterns indicates that downgoing plate age and velocity downgoing overriding plate coupling and slab pull all significantly affect faulting patterns while variations in brittle rheology have a significantly smaller impact these relationships reflect that the sensitivity of outer rise faulting patterns to the frictional properties of the oceanic crust and mantle is small compared to variations in the overall stress state and deformation rate of subduction systems in order to gain additional insight into the origin outer rise faulting patterns future numerical studies should focus on specific regions in order to place constraints on the structure of the downgoing plate and dynamics of the subduction system

evidence for fault weakness and fluid flow within an active low angle normal fault determining the composition and physical properties of shallow dipping active normal faults dips 35 with respect to the horizontal is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere seismic reflection images1 and earthquake source parameters2 show that a magnitude 6 2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25 30 located ahead of a propagating spreading centre in the woodlark basin here we present results from a genetic algorithm inversion of seismic reflection data which shows that the fault at 4 5 km depth contains a 33 m thick layer with seismic velocities of about 4 3 km s 1 which we interpret to be composed of serpentinite fault gouge isolated zones exhibit velocities as low as 1 7 km s 1 with high porosities which we suggest are maintained by high fluid pressures we propose that hydrothermal fluid flow possibly driven by a deep magmatic heat source and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low angle normal fault

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

variation of physical properties beneath a fault observed by a two ship seismic survey off southwest japan 1 we conducted a two ship seismic reflection survey at kumano nada off the southwest japan arc in 2002 in order to clarify physical properties along a splay fault that branches from the plate boundary of the nankai trough subduction zone during the survey seismic reflection data were collected out to maximum offset of 20 km using multiple passes of a shooting and a receiving vessel separated by a different constant offset on each pass we conducted analyses on reflection polarity and amplitude variation with offset avo effects to examine physical properties of layers immediately beneath the splay fault reflections from the splay fault showed varying polarities along the fault in the present study we assumed that the poisson s ratio of the layer above the fault was 0 30 0 40 where polarity is reversed the poisson s ratio of the layer immediately below the fault was estimated to be 0 33 0 445 11 increase on average where polarity was normal the ratio was estimated to be 0 263 0 365 11 decrease on average if the reverse polarity indicates the existence of fluid as many authors suggest the polarity analysis demonstrates inhomogeneous distribution of fluid in the splay fault shear zone the results of avo analysis also support the possibility of the spatial variation of fluid distribution under some assumptions because the poisson s ratio changes with content of fluid in rocks the variation in reflection polarity and corresponding avo effects may signify the possibility of localized fluid migration in the splay fault shear zone

tectonic evolution of a low angle extensional fault system from restored cross sections in the northern apennines italy 1 broad geological and geophysical documentation is available on regional extensional systems driven by low angle normal faults however little information exists about the three dimensional geometry and the offset distribution of such extensional structures we present a new set of balanced geological sections across the extensional fault system driven by the altotiberina low angle normal fault in the northern apennines of italy we document this extensional system throughout a large set of surface field surveys and geological maps and subsurface data seismic reflection profiles and boreholes the subsurface data allowed us to define the fault deep geometry and to obtain its structural contours the fault geometry is characterized by both along dip and along strike irregularities in cross section the fault displays a staircase trajectory with the shallowest part being dome shaped and flattened to horizontal this bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 mpa the sequential restoration of five geological cross sections yields a maximum extension of about 10 km accumulated over approximately 3 ma the resulting long term slip rate is about 3 mm yr which is of the same order as the present day extensional rate measured by gps 2 5 3 0 mm yr suggesting an almost steady state extension over the last 3 ma the distribution of the extension values along the fault strike is bell shaped as expected for a continuous surface

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

earthquake supercycles as part of a spectrum of normal fault slip styles like thrust boundaries in subduction zones some normal faults show evidence of temporally transient slip patterns such as temporally clustered earthquakes or periods of elevated creep rate we create rate and state friction numerical models to assess whether typical distributions of frictional parameters can explain transient slip on normal faults we constrain our models with geometry from the wasatch fault zone where 5 10 kyr timescale earthquake clusters are recorded on fault scarps we use numerical models to test whether these earthquake clusters could be the geologic representation of supercycles which we define as long period stress cycles containing multiple seismic cycles field observations laboratory data and analogue experiments suggest that the ratio of velocity weakening to velocity strengthening material varies with the ratio of brittle to ductile material in the shear zone we therefore employ a linear transition of frictional parameters with depth to simulate bulk frictional properties of the shear zone we find that for generalized normal faults rate and state friction predicts a spectrum of slip behaviors including steady aseismic creep aseismic transients clustered earthquakes and regular seismic cycle earthquakes the dominant control on slip behavior is the degree of weakening material in the potentially seismogenic fault patch earthquake clusters occur with higher proportions of velocity weakening material which correspond to more competent less ductile shear zone assemblages we find that rate and state friction predicts a wider spectrum of slip behavior on active normal faults than has been previously recognized potentially due to geologic preservation and identification issues

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

3d development of detachment faulting during continental breakup abstract the developing asymmetry of rifting and continental breakup to form rifted margins has been much debated as has the formation mechanics and role of extensional detachments bespoke 3d seismic reflection data across the galicia margin west of spain image in unprecedented detail an asymmetric detachment the s reflector mapping s in 3d reveals its surface is corrugated proving that the overlying crustal blocks slipped on s surface during the rifting crucially the 3d data show that the corrugations on s perfectly match the corrugations observed on the present day block bounding faults demonstrating that s is a composite surface comprising the juxtaposed rotated roots of block bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward changes in the orientation of the corrugations record the same oceanward migration however in contrast to previous rolling hinge models the slip of the crustal blocks on s occurred at angles as low as 20 requiring that s was unusually weak consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust as the crust only becomes entirely brittle once thinned to 10 km the asymmetric s detachment and the hyper extension of the continental crust only developed late in the rifting process which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs the 3d volume allows analysis of the heaves and along strike architecture of the normal faults whose planes laterally die or spatially link together implying overlaps in faults activity during hyper extension our results thus reveal for the first time the 3d mechanics and timing of detachment faulting growth the relationship between the detachment and the network of block bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins

signature of fault zone deformation in near surface soil visible in shear wave seismic reflections small throw seismogenic fault segments hidden in the holocene sediments are crucial but difficult targets in seismic exploration we report here the detection of the deformation pattern and a concealed fault segment in the unconsolidated sediments at vila franca xira portugal through identification in shear wave reflection data of multiple signatures of ductile deformation associated with faulting we find step like changes in the stacking velocity along a shallow subsoil layer boundary indicating synsedimentary faulting we also recognize a consistent distortion in the moveout of the reflection events in the raw shear wave data synthetic modeling of seismic data helps in interpreting these observations and identifying backscattered energy from a steeply dipping shallow fault zone prior to this finding there was no evidence for holocene activity of this fault although the fault is considered to be the most probable source for the disastrous 1531 earthquake

seismotectonics of mid ocean ridge propagation in hess deep hydroacoustic data from the eastern equatorial pacific reveal low magnitude seismicity concentrated at the propagating tip of the galapagos rise in hess deep the patterns of seismicity and faulting are similar to those observed in the process zone of laboratory scale propagating tensile cracks because the fracture energy required for propagation scales with crack length and process zone size it follows that ridges can propagate stably in the brittle crust without exceptional resisting forces as proposed by previous models based on linear elastic fracture mechanics

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin