imaging mapping and modelling continental lithosphere extension and breakup an introduction abstact this special publication is a direct outcome of a small but dedicated group of researchers who met in pontresina switzerland to review and define the fundamental observations characterizing extensional systems and their application in guiding and constraining modelling efforts and results the various summaries of the keynote addresses give an objective overview of the state of the art in modelling lithospheric extensional systems both from the regional scale using dynamic models to individual basins using kinematic models with an emphasis on capturing the extensional history of the iberia and newfoundland margins at the heart of all of these efforts is a simple question exactly what mechanisms allow the continental lithosphere to be thinned to the point of rupture related questions are 1 do crustal and mantle faults play a major role in this thinning process if so what is their geometry and does their importance and geometry change with time 2 are there other mechanisms of lithospheric and crustal thinning that cannot be imaged on seismic sections 3 how is deformation accommodated in space and time 4 what role do inherited mechanical thermal and or chemical heterogeneities play in controlling strain distribution and localization 5 when how and to what degree does magma production affect the distribution and localization of extension and 6 what is the stratigraphic record of continental extension and how does it document the extension of the crust and thinning of the lithospheric mantle the aim of this special publication is to address many of these fundamental questions concerning the extreme extension and thinning of continental lithosphere

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

u th dating of striated fault planes direct dating of brittle fault activity is of fundamental importance to tectonic reconstructions and paleoseismic studies one way to address this issue is by constraining the timing of fault striations but this requires a better understanding of the striation formation mechanism and associated mineralization we present results from a microstructural geochemical and geochronological study of calcite precipitates associated with striated fault planes from the dead sea fault zone in northern israel we recognize four types of coexisting calcite precipitates including calcite cement in dilation breccia calcite in striated groove morphology calcite gouge associated with hydraulic fracturing and pressure solution and calcite coating of the fault surface carbon oxygen isotopes 87sr 86sr ratios and rare earth element and yttrium rey patterns indicate various precipitation mechanisms associated with formation of syntectonic calcite cement and striations coseismic calcite gouge and interseismic calcite coating precipitates in the fault zone using u th dating of samples from three adjacent fault planes we delineate four welldefined deformation ages in the period from 220 to 60 ka we conclude that these ages constrain the timing of activity along the dead sea fault zone in northern israel and argue that a similar methodological approach could potentially shed light on the timing of deformation in other brittle fault zones

thermochronological investigation of fault zones abstract the timing of faulting episodes can be constrained by radiometric dating of fault zone rocks fault zone material suitable for dating is produced by tectonic processes such as 1 fragmentation of host rocks followed by grain size reduction and recrystallization to form mica and clay minerals 2 secondary heating melting of host rocks by frictional fault motions and 3 mineral vein formation as a result of fluid advection associated with the fault motions the thermal regime of fault zones consists primarily of the following three factors a regional geothermal structure across the fault zone and background thermal history of studied province bounded by fault systems b frictional heating of wall rocks by fault motions and c heating of host rocks by hot fluid advection in and around the fault zone thermochronological methods widely applied in fault zones are k ar 40ar 39ar fission track and u th methods for which methodological principles as well as analytical procedures are briefly described the thermal sensitivities of individual thermochronological systems are then reviewed which critically control the response of each method against the thermal processes based on the knowledge above representative examples as well as key issues are highlighted to date fault gouges pseudotachylytes mylonites and carbonate veins placing new constraints upon geological geomorphological and seismological frames finally the nojima fault is presented as an example for multiple applications of thermochronological methods in a complex fault zone

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

u pb geochronology of calcite mineralized faults absolute timing of rift related fault events on the northeast atlantic margin constraining the timing of brittle faulting is critical in understanding crustal deformation and fluid flow but many regional scale fault systems lack readily available techniques to provide absolute chronological information calcite mineralization occurs in crustal faults in many geological settings and can be suitable for u pb geochronology this application has remained underutilized because traditional bulk dissolution techniques require uncommonly high u concentration because u and pb are distributed heterogeneously throughout calcite crystals high spatial resolution sampling techniques can target domains with high u and variable u pb ratios here we present a novel application of in situ laser ablation inductively coupled plasma mass spectrometry la icp ms to basaltic fault rock geochronology in the faroe islands northeast atlantic margin faults that are kinematically linked to deformation associated with continental break up were targeted acquired ages for fault events range from mid eocene to mid miocene and are therefore consistently younger than the regional early eocene onset of ocean spreading highlighting protracted brittle deformation within the newly developed continental margin calcite geochronology from la icp ms u pb analysis represents an important and novel method to constrain the absolute timing of fault and fluid flow events

location of shallow subduction zone earthquakes inferred from teleseismic body waveforms this article proposes to use the teleseismic waveform of body waves to locate shallow subduction zone earthquakes when the laterally heterogeneous structure near the trench axis is incorporated in calculating synthetic body waveforms not only source depth but also horizontal distance from the source to the trench axis can be well constrained by comparing the synthetic and observed waveforms this analysis is possible because the waveforms are highly sensitive to the shift of the source position an example of this waveform method for an interplate thrust earthquake will be described and gives a well constrained and reasonable location

vein calcite dating reveals pyrenean orogen as cause of paleogene deformation in southern england an improved method of u pb dating of vein calcite formed during deformation is used to determine the age and cause of folding along the south coast of england fractures arising from folding of late cretaceous chalk of southern england occurred 34 7 1 7 myr ago underlying jurassic strata have veins within fractures with ages of 55 48 42 39 37 34 31 and 25 ma with 34 31 ma being the tectonic culmination folding was slightly younger than the age of the youngest strata in the overlying solent group suggesting that folding terminated basin sedimentation this age of north south shortening is inconsistent with attribution to intraplate forces from the mainly younger alps but is plausibly a result of the pyrenean orogen which evolved from 50 to 28 myr ago with a late eocene culmination a modified method for carbonate laser ablation inductively coupled plasma mass spectrometry u pb dating is presented that uses measured 232 th and 208 pb as a better monitor of common pb and it has distinct advantages over existing methods initial common lead determined on samples conforms closely to model compositions calculated by earlier workers with free regressions giving the most robust dates the modified method is applicable to structural geology carbonate diagenesis and to dating of carbonate relevant to palaeo environmental and archaeological studies supplementary material details of sample locations and characteristics photographs of sample localities microscopic images of veins details of the experimental procedure and u pb data for all samples and reference materials are available at https doi org 10 6084 m9 figshare c 3997086

isotopic evidence b c o of deep fluid processes in fault rocks from the active woodlark basin detachment zone we report results from boron carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during odp leg 180 in the western woodlark basin off papua new guinea in this area of active continental extension crustal break up and incipient seafloor spreading a shallow dipping seismically active detachment fault accommodates strain defining a zone of mylonites and cataclasites vein formation and fluid infiltration syntectonic microstructures and vein fill mineralogy suggest frictional heating during slip during extension and exhumation of moresby seamount low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids 13cpdb down to 17 18opdb down to 22 formed by both dehydration and decarbonation boron contents are low 7 ppm indicating high grade metamorphic source rock for the fluids some of the 11b signatures 17 35 parent solutions to calcite vein fills are low when compared to deep seated waters in other tectonic environments likely reflecting preferential loss of 11b during low grade metamorphism at depth pervasive devolatilization and flux of co2 rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip multiple rupture and healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and hence seismogenic fault movement

linking hematite u th he dating with the microtextural record of seismicity in the wasatch fault damage zone utah usa techniques directly dating fault slip are few limiting the ability to interpret the rock record of seismicity hematite is commonly found in fault zones amenable to u th he dating and slip surface hematite may be reset by shear heating events and or recrystallization glossy hematite coated fault surfaces in the wasatch fault footwall damage zone utah usa exhibit evidence of hematite cataclasis and preserve pliocene hematite u th he dates apatite u th he and fission track data from the host gneiss indicate footwall unroofing through 2 km by ca 4 5 ma internally reproducible but disparate hematite u th he dates 4 5 ma and younger from isolated locations on a single fault surface do not reflect ambient cooling we hypothesize that these dates and associated iridescence and annealed crystal texture document rapid cooling from friction generated heat during small seismic slip events between ca 4 5 and 2 5 ma thus hematite u th he dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 10 6 yr time scales

effects of sedimentary structure and bathymetry near the source on teleseismic p waveforms from shallow subduction zone earthquakes summary the effects of sedimentary layers and non flat bathymetry on teleseismic p waveforms from shallow subduction zone earthquakes are examined by calculating synthetic seismograms the actual bathymetry and shallow sedimentary layers revealed by the seismic refraction studies are incorporated in 2 d velocity models of the oceanic trench region 2 d finite difference calculation is employed to get a complete response of the assumed non flat model structures the result of the modelling of wwssn long period seismograms shows that not only the non flat bathymetry but also thick sedimentary layers are essential to model the synthetic seismograms the lack of non flat bathymetry or sedimentary structure in the structural model often results in unrealistic synthetics in general the paths going across the trench axis are most affected this is caused by the steeply dipping ocean bottom and the thick sedimentary layers at the trench slope observed seismograms of the 1968 kurile thrust event mb 5 5 and the 1967 java normal event mb 6 0 show remarkable large amplitude later arrivals despite their moderate magnitudes a realistic model of the trench structure however reproduces remarkably well this character of later arrivals which shows that it is essential to incorporate the structure our results also indicate that we need precise knowledge on the trench structure to model the structural effects in detail on the gdsn long period seismograms the effect of the sedimentary layers is negligible because the rather narrow band response of the gdsn seismograph lies in a frequency range lower than the affected band however the effect of the non flat bathymetry is significant and can not be neglected

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

evidence for seismogenic normal faults at shallow dips in continental rifts several recent observations indicate that normal faulting earthquakes occasionally occur on faults dipping 35 dips often considered shallow most of these occur in the woodlark and aegean rifts the woodlark and aegean rifts are found to generate significantly more earthquakes than others and are the most rapidly extending so display the widest variety of fault behavior even within the woodlark rift system extension rates vary along strike with the shallowest dipping faults confined to the most rapidly rifting segment here several events mw 6 0 6 8 feature nodal planes dipping 23 35 these planes are subparallel to shear zones bounding nearby metamorphic core complexes including one imaged to 8 9 km depth by seismic reflection profiling in the western gulf of corinth at least one large event mw 6 4 occurred on a fault dipping 33 as the woodlark example this rift segment exhibits a high opening rate 10 20 mm yr several other cases elsewhere based on older historic data microseismicity or geologic inference suggest seismic slip at similar or shallower dips still no documented large earthquake exhibits seismic slip on sub horizontal surfaces dip 10 15 stress rotation may explain the 23 35 dips but thus far no realistic mechanism has been found more likely these faults represent surfaces somewhat weaker than surrounding rock through some combination of modest cohesion of the surrounding rock and slightly lower frictional coefficients on the fault such weakening may be a consequence of high slip rates which rapidly generate large offsets and mature fault systems

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

inclined k ar illite age spectra in brittle fault gouges effects of fault reactivation and wall rock contamination k ar clay fraction ages of brittle faults often vary with grain size decreasing in the finer size fractions producing an inclined age grain size spectrum k ar ages and mineralogical characterization of gouges from two normal faults in the kongsberg silver mines southern norway suggest that inclined spectra derived from brittle fault rocks reflect the mixing of inherited components with authigenic mineral phases the ages of the coarsest and finest fractions constrain faulting at c 260 270 ma and reactivation around 200 210 ma respectively this study demonstrates how wall rock contamination influences the k ar age of the coarsest size fractions and that authigenic illite and k feldspar can crystallize synkinematically under equivalent conditions and thus yield the same k ar ages

the onset of the dead sea transform based on calcite age strain analyses the onset and evolution of the dead sea transform are re evaluated based on new in situ u pb dating and strain analyses of mechanically twinned calcites direct dating of 30 syn faulting calcites from 10 different inactive fault strands of the transform indicates that the oceanic to continental plate boundary initiated between 20 8 and 18 5 ma within an 10 km wide distributed deformation zone in southern israel ages from the northern dead sea transform 17 1 12 7 ma suggest northward propagation and the establishment of a well developed 500 km long plate bounding fault in 3 m y the dominant horizontal shortening direction recorded in the dated twinned calcites marks the onset of left lateral motion along the evolving plate boundary the observed changes in the strain field within individual fault strands cannot be simply explained by local weakening effects along strands of the dead sea transform or by gradual changes in the euler pole through time

rapid full wave centroid moment tensor cmt inversion in a three dimensional earth structure model for earthquakes in southern california summary a central problem of seismology is the inversion of regional waveform data for models of earthquake sources in regions such as southern california preliminary 3 d earth structure models are already available and efficient numerical methods have been developed for 3 d anelastic wave propagation simulations we describe an automated procedure that utilizes these capabilities to derive centroid moment tensors cmts the procedure relies on the use of receiver side green s tensors rgts which comprise the spatial temporal displacements produced by the three orthogonal unit impulsive point forces acting at the receivers we have constructed a rgt database for 219 broad band stations in southern california using a tomographically improved version of the 3 d scec community velocity model version 4 0 cvm4 and a staggered grid finite difference code finite difference synthetic seismograms for any earthquake in our modelling volume can be simply calculated by extracting a small source centred volume from the rgt database and applying the reciprocity principle the partial derivatives needed for the cmt inversion can be generated in the same way we have developed an automated algorithm that combines a grid search for suitable focal mechanisms and hypocentre locations with a gauss newton optimization that further refines the grid search results using this algorithm we have determined cmt solutions for 165 small to medium sized earthquakes in southern california preliminary comparison with the cmt solutions provided by the southern california seismic network scsn shows that our solutions generally provide better fit to the observed waveforms when applied to a large number of earthquakes our algorithm may provide a more robust cmt catalogue for earthquakes in southern california

orogenic pulses in the alberta rocky mountains radiometric dating of major faults and comparison with the regional tectono stratigraphic record radiometric ages from regionally distributed thrust fault gouge show that the eastward propagation of the southern rocky mountain fold and thrust belt in canada occurred in four orogenic pulses that correlate with tectonic events of the cordilleran interior and with depositional patterns in the adjacent foreland in the main ranges the pyramid 163 0 ma simpson pass 161 7 ma and johnson creek 145 7 ma thrusts were related to the initiation of thin skinned deformation from jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin in the front ranges the emplacement of the greenock thrust 103 1 ma and broadview snake indian thrust 99 2 ma was contemporaneous with development of cenomanian deltaic deposits in the immediate foreland three thrusts in the front ranges rocky pass 74 8 ma sulfur mountain 75 6 ma and clearwater 74 2 ma thrusts define a campanian phase of tectonic loading that led to the last major transgression in the southern portion of the alberta foreland along the eastern margin of the front ranges the mcconnell thrust 54 0 ma together with the muskeg 52 4 ma brule 53 9 ma and nikanassin 52 1 ma thrusts in the foothills recorded the last phase of regional contraction the late jurassic mid cretaceous late cretaceous and early eocene deformation pulses are separated by relatively long periods 40 m y 20 m y and 10 m y of tectonic quiescence these spatially and temporally restricted fault motion pulses contrast with gradual forward fault propagation while regional eastward progression of deformation is preserved

three dimensional dynamical model of continental rift propagation and margin plateau formation we investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three dimensional finite element implementation of the yield stress envelope model in its initial configuration the three dimensional model represents a 1000 km by 1000 km area of continental lithosphere an initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates the model region beyond the weak trend is uniform model conditions are varied by changing the thickness and composition of the crust the geotherm and extension rate the strength of the initial weakness and the dimensions of the preweakened and uniform regions for each model breakup nucleates at a point on the weak trend in response to an applied constant extension rate the break propagates rapidly along the weak trend and then slows as it crosses the uniform region this produces a locked zone in which breakup is delayed resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus the total extension required to tear across the 1000 km length of the model is 210 km for 40 km thick crust 400 km for 45 km thick crust and 710 km for 50 km thick crust smaller increases in total extension occur for elevated geotherms and reduced extension rates this indicates that prior extension estimates based on subsidence which exceed 600 km for margin plateaus can be explained by rift propagation in three dimensions differential extension along the model margin is linearly related to the length of the locked zone the slope of the line which we call is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in from 0 2 to 1 3 we compare values computed from our model to those estimated from the subsidence of bonavista platform and rockall plateau conjugate margins in the north atlantic basin and georges bank and blake plateau and their conjugate margins in the central atlantic basin these results indicate that the differential extension associated with these locked zones which ranges from 170 km to 550 km can be predicted in terms of their initial crustal thickness and their length along the margins

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

illite and hydrocarbon exploration illite is a general term for the dioctahedral mica like clay mineral common in sedimentary rocks especially shales illite is of interest to the petroleum industry because it can provide a k ar isotope date that constrains the timing of basin heating events it is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap anticline etc that is to hold the oil illite also may precipitate in the pores of sandstone reservoirs impeding fluid flow illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial k ar ages are apparent ages of mixtures of detrital and diagenetic end members and what we need are the ages of the end members themselves this paper describes a methodology based on mineralogy and crystallography for interpreting the k ar ages from illites in sedimentary rocks and for estimating the ages of the end members

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

focal depth determination of central aleutian earthquakes focal depths of suboceanic subduction zone earthquakes best determined by the careful identification and analysis of teleseismic depth phases on short period vertical component seismograms arc crustal structure can cause water surface reflections pwp and swp and sediment basement reflections defined here as pp and sp with amplitudes comparable to those of water sediment reflections pp and sp however changes in source mechanism and local variations in arc structure make the observation of these short period depth phases highly unpredictable digital and analog vertical component seismograms were examined from nearly 400 teleseismically recorded central aleutian earthquakes that occurred during the period 1964 to 1984 a data set consisting of the travel times relative to p of short period depth phases was assembled for 54 shallow depth earthquakes and 33 earthquakes deeper than 50 km across the arc in addition long period waveforms for the larger events were examined magnitude mb 4 8 was approximately the lower limit for useful depth phase arrivals short period reflected phases from shallow depth earthquakes were analyzed by matching observed travel times relative to p to theoretical values computed for a two dimensional model of arc crustal structure depths resolved in this fashion provide a sharper depth profile of the shallow seismic zone in the central aleutians than previously available outer rise and trench earthquakes occur over a narrow range of subcrustal depths north of the trench the plate dips gently beneath the terrace and appears to descend gradually into the mantle the thickness of the interplate thrust earthquake zone is no more than 10 to 15 km earthquakes near the volcanic arc and above the plate interface occur at depths beneath the sediment basement interface estimation of the focal depths of earthquakes deeper than 50 km poses a special problem because an accurate specification of slab structure is needed to correct for path effects the phases pwp and swp are ordinarily only weakly recorded on long period vertical component seismograms however long period waves reflected from boundaries with sharp long period impedance contrasts such as the sea bottom and the sediment basement interface are strongly recorded moment tensor depths for shallow earthquakes based on the inversion of long period waveforms using a standard earth model once transformed into true depths within a model of arc crustal structure for the central aleutians agree with depths determined from short period reflected phases alone to within a few kilometers reported depth phases are routinely used for depth determination by the preliminary determination of epicenters pde and the international seismological centre isc services however the phases pp and sp are often obscured in the p coda and for suboceanic earthquakes e g pwp is misidentified and reported as pp this often results in the overestimation of the focal depths of routinely determined hypocenters reported by these services

possible seismogenic shallow dipping normal faults in the woodlark d entrecasteaux extensional province papua new guinea active normal faults dipping less than 30 have not been documented in the seismic record but they are commonly observed in geologic structures one interpretation is that seismogenic low angle faulting is physically impossible and that the geologic structures do not form at shallow dips to better examine this problem teleseismic body waves were inverted to determine source parameters for five large earthquakes in the woodlark d9entrecasteaux region a rare example of active continental extension associated with pliocene quaternary metamorphic core complexes the inversions show that at least two and possibly four events are consistent with normal dip slip on shallow dipping faults the largest earthquake m w 6 8 is located near the mapped core complexes the shallow dipping plane for this event determined to dip between 10 and 25 is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low angle structures although slip on the conjugate high angle plane cannot be ruled out observations of quaternary uplift and the similarity to the adjacent core complex bounding structures favor motion on the low angle plane similar earthquake mechanisms have not yet been documented in most other extensional provinces perhaps because instances of active core complex exhumation are rare

40ar 39ar ages of fallout tephra layers and volcaniclastic deposits in the sedimentary succession of the western woodlark basin papua new guinea the marine record of miocene pleistocene volcanism

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

synchronous reorientation of the woodlark basin spreading center abstract a sidescan and multibeam bathymetry survey of the woodlark basin reveals that its 500 km long spreading center reoriented synchronously without propagation about 80 ka there is no evidence of the v shaped pseudofault geometry typical of spreading center propagation nor of the progressive fanning of seafloor fabric characteristic of spreading center rotation the reorientation is recognized by a sharp contact between two seafloor fabric trends and ruptured off axis lithosphere formed up to 0 7 myr previously the length of the reoriented spreading segments and the tendency to fault pre reorientation seafloor fabric are controlled by the strength of the lithosphere the angle of the reorientation and the length of pre existing spreading and transform segments we document the process of synchronous reorientation in the woodlark basin and propose that it may occur in other ocean basins

extensional tectonics in the jeanne d arc basin offshore newfoundland implications for the timing of break up between grand banks and iberia

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

directly dating geologic events u pb dating of carbonates 1 carbonates are ubiquitous forming in a variety of settings including caves soils the oceans during burial from hydrothermal fluids and along faults u pb dating of carbonates is applicable to almost the entire geologic time scale with results from pleistocene speleothems showing great promise for dating records 1 ma old here we review studies of u pb dating of carbonates from a range of depositional and diagenetic environments and consider published results with regard to stratigraphic associations and the behavior of u and pb in various fluids we detail the steps required to select suitable samples summarize current laboratory techniques and provide directions for improvement of analytical protocols the u pb dating of carbonates has tremendous potential for dating events of geologic interest and is virtually untapped

a detailed subduction structure in the kuril trench deduced from ocean bottom seismographic refraction studies abstract in 1983 we conducted an ocean bottom seismograph obs experiment in the southernmost part of the kuril trench beneath which the pacific plate is subducting under hokkaido island japan the aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling using explosives and airguns the total length of the profiles was 560 km along which ten obs s were deployed the observed data were of good quality which enabled us to obtain a detailed velocity structure of the active margin down to 20 30 km we constructed a velocity structure model by ray tracing and amplitude modeling in the oceanic basin the crust has a typical oceanic structure characterized by three layers with p wave velocities of 1 8 3 8 6 5 and 6 5 7 0 km s the velocity gradient in layer 3 increases downward from 0 075 to 0 10 s 1 the moho discontinuity was well constrained by clearly observed p n and p m p phases the total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin the p wave velocity is 7 9 km s beneath the moho discontinuity which increases downward with a rather small velocity gradient 0 015 0 03 s 1 beneath the continental slope we found relatively low velocity 2 5 to 5 5 5 8 km s material the oceanic moho discontinuity associated with the subducting plate was traced down to a depth of 25 km our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure this result is in remarkable contrast with the velocity structure in the active margin of the ryukyu trench area about 1000 km south of the present experimental area where we found a 12 km thick prominent low velocity wedge is situated 50 150 km landward from the trench axis on the seaward side of the wedge the surface of the igneous basement undulates severely the wedge was probably formed by materials of oceanic origin such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern pacific

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

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

gps and seismological constraints on active tectonics and arc continent collision in papua new guinea implications for mechanics of microplate rotations in a plate boundary zone 1 the island of new guinea is located within the deforming zone between the pacific and australian plates that converge obliquely at 110 mm yr new guinea has been fragmented into a complex array of microplates some of which rotate rapidly about nearby vertical axes we present velocities from a network of 38 global positioning system gps sites spanning much of the nation of papua new guinea png the gps derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in png we simultaneously invert gps velocities earthquake slip vectors on faults and transform orientations in the woodlark basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region the data are best explained by six distinct tectonic blocks the australian pacific south bismarck north bismarck and woodlark plates and a previously unrecognized new guinea highlands block significant portions of the ramu markham fault appear to be locked which has implications for seismic hazard determination in the markham valley region we also propose that rapid clockwise rotation of the south bismarck plate is controlled by edge forces initiated by the collision between the finisterre arc and the new guinea highlands

emplacement ages and exhumation rates for intrusion hosted cu mo sb au mineral systems at freegold mountain yukon canada assessment from u pb ar ar and u th he geochronometers to decipher the thermal history of mineralized systems across the freegold mountain area yukon canada a combined geochronology zircon u pb and hornblende biotite and whole rock ar ar and thermochronology apatite and zircon u th he study was carried out previous u pb data combined with new u pb and ar ar data show that intrusive bodies across the freegold mountain were emplaced during two protracted episodes the first spanning from 109 6 to 98 ma and the second between 79 and 68 ma overprinting of the first intrusive event by a second magmatic hydrothermal event is suggested by a zircon u pb age of 108 7 0 4 ma for a chlorite altered dyke and a whole rock ar ar plateau age of 76 25 0 53 ma zircon u th he data are between 66 and 89 ma whereas apatite u th he data are scattered 38 7 109 9 ma and bracket the two magmatic emplacement events our combined data reveal a complex history of reheating that led to resetting of numerous chronometers in most of the investigated magmatic hyd

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

examination of slow and late moment release of the 1988 spitak and 1991 racha caucasus earthquakes summary two large shallow earthquakes occurred in the caucasus part of the collisional belt caused by the northward movement of the arabian plate with respect to the eurasian plate the 1988 december 7 spitak armenian m 6 7 and 1991 april 29 racha m 6 9 earthquakes in the present study slow and late moment releases of the earthquakes which were previously proposed based on the body wave analyses are examined using long period surface waves results from spectral inversions using fundamental mode surface waves show that the source durations of both earthquakes are about 20 s although additional investigations utilizing the long period surface waves which are optimized to detect slow and late moment release were employed no evidence of slow and late moment release was found for either earthquake for the racha earthquake the obtained source duration is consistent with the result from body wave analyses within the resolution of the body wave data the simple p and sh waveforms are well modelled by a source time history model with a duration of 20 s however for the spitak earthquake the source duration obtained from the surface waves is about 30 s shorter than the source model suggested in the body wave analysis this discrepancy is attributed to a slow and late thrust fault slip in the body wave source model which was proposed in order to interpret the late arrivals of p waves as the result of source complexity numerical experiments predict that rayleigh wave radiation is sensitive to the radiation from the proposed slow and late slip a comparison between the observed and predicted rayleigh wave radiation patterns however provides no indication of the surface wave radiation from the slow and late slip it is unlikely that the discrepancy in the source model is only due to errors in modelling the propagation of long period surface waves because for the racha earthquake the source models obtained from body and surface waves are consistent the present results thus indicate that the late arrivals of p waves are probably caused by something other than source complexity for example complex local structure near the earthquake source in the continental collisional belt for the spitak earthquake a large non double couple component is observed in the moment tensor solution obtained at long periods if the non double couple component is due to source complexity it should be produced within the 20 s rupture process

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

effects of near source seafloor topography on long period teleseismic p waveforms in the study of the shallow earthquakes in the subduction zones reverberations inside the water layer near the source are known to affect very significantly the far field body waveforms the usual correction is to take a horizontally layered model as the near source structure but it is not realistic for the trench geometry therefore finite difference simulation combined with far field ray theory is employed in order to examine the effects of the realistic non flat seafloor topography as a realistic trench model the japan trench is selected the response of the structure near the source is calculated by two dimensional finite difference simulation then teleseismic p waveforms are calculated by using reciprocal theorem our results suggest that 1 around the trench bottom effects of non planar structure are remarkable and the realistic seafloor cannot be approximated by any model of planar structure however 2 effects of the continental slope are well approximated by a model of planar dipping seafloor of appropriate dip therefore the near source structure must include the realistic seafloor topography or the waveform analyses may lead to spurious subevents in a later stage of rupture process and mislocation of events our results also show that present knowledge about the seafloor topography is enough to incorporate the complicated geometry

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

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

teleseismic synthetics obtained from 3 d calculations in 2 d media summary at subduction zones the laterally heterogeneous medium such as the steeply dipping sea bottom and thick sedimentary layers causes large effects on the teleseismic body waveforms from the shallow events occurred in this region we develop a new method to incorporate these effects from near source structure in calculating the teleseismic body waveforms to incorporate the structural effects we simulate 3 d elastic wave propagation in a two dimensionally heterogeneous structure although the simulated wavefield is 3 d our method does not require huge computer storage this is because we assume a 2 d structure to reduce the dimension of the equations of motion by spatial fourier transform because of the strong laterally heterogeneous features such as steeply dipping sea bottom and thick sedimentary layers and because of the 2 d geometry of the oceanic trench our method is particularly applicable to the shallow subduction zone earthquakes in our formulation we assume a 2 d model structure in which the physical properties are constants in the direction parallel to the trench axis then we apply the spatial fourier transform to the equations of motion in that direction the resultant wavenumber domain equations can be solved as independent sets of 2 d equations on a computer with a moderate sized storage we solve these wavenumber domain equations by the staggered grid finite difference method finally the inverse fourier transform provides the 3 d displacement field the reciprocal principle is used to obtain the teleseismic displacement first the response of the near source structure to an input plane p wave is calculated by the 3 d method introduced here then based on the reciprocal principle convolution of the response strain at the source position with the moment tensor is calculated to get far field p wave displacement as an example of the application of the present method we calculated the waveforms from a shallow thrust event that occurred at the kurile trench 1964 october 16 ms 6 5 the synthetics calculated by the new method reproduce the large amplitude later arrivals in the observed seismograms furthermore a reasonable source position is obtained from the waveform analysis

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

effects of near source bathymetry on teleseismic p waveforms teleseismic p waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms this study suggests that considerable misfit results from the non horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water crust interface geometric ray theory synthetics calculated for a double couple source show that p waveforms are extremely sensitive to seafloor orientation because of the large impedance contrast across the boundary seafloor dips as small as 0 5 significantly modify the ray paths affecting the take off angle at the source reflection and transmission coefficients at the interface geometric spreading and travel time to test the applicability of such modeling p waveforms from the march 29 1976 cocos ridge intraplate earthquake were studied the waveforms many of which show prominent water multiples poorly modeled with flat lying bathymetry were inverted for the best fitting planar ocean bottom orientation using a grid search technique synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure and reproduce the observed azimuthal dependence of water multiple amplitudes the derived ocean bottom dip angle and direction are in agreement with bathymetric charts suggesting p waveforms provide constraints on the seafloor orientation in the epicentral region green s functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes

large scale coast parallel displacements in the cordillera a granitic resolution to a paleomagnetic dilemma abstract resolution of the paleomagnetic dilemma the discrepancy between large paleomagnetically determined dextral displacement of outboard portions of the northern cordillera and much smaller offsets implied by mapping and stratigraphic correlations is fundamental to understanding the tectonic evolution of the cordillera this paper presents structural orientation data from the middle cretaceous dawson range batholith of west central yukon and its wallrocks and suggests that some of the missing displacement may be found in intrusions the elongate northwest trending batholith has a margin parallel foliation a sub horizontal stretching lineation and records syn intrusive dextral shearing in country rocks adjacent to the batholith north trending lineations are deflected clockwise into near parallelism with the batholith s margins lineations from wallrock screens within the batholith are all aligned parallel with the batholith s long axis the big creek strike slip fault forms the north margin of the batholith and accommodated a minimum of 20 km of dextral slip these observations imply that the batholith invaded an active dextral shear zone accommodated shearing while crystallizing and focused post crystallization fault development the batholith is conservatively estimated to have accommodated 45 km of syn intrusive shearing collectively middle cretaceous intrusions of the northern cordillera may account for 400 km of previously unrecognized dextral displacement

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

source depth dependence of micro tsunamis recorded with ocean bottom pressure gauges the january 28 2000 mw 6 8 earthquake off nemuro peninsula japan abstract micro tsunami waves with a maximum amplitude of 4 6 mm were detected with the ocean bottom pressure gauges on a cabled deep seafloor observatory south of hokkaido japan following the january 28 2000 earthquake m w 6 8 in the southern kuril subduction zone we model the observed micro tsunami and estimate the focal depth and other source parameters such as fault length and amount of slip using grid searching with the least squares method the source depth and stress drop for the january 2000 earthquake are estimated to be 50 km and 7 mpa respectively with possible ranges of 45 55 km and 4 13 mpa focal depth of typical inter plate earthquakes in this region ranges from 10 to 20 km and stress drop of inter plate earthquakes generally is around 3 mpa the source depth and stress drop estimates suggest that the earthquake was an intra slab event in the subducting pacific plate rather than an inter plate event in addition for a prescribed fault width of 30 km the fault length is estimated to be 15 km with possible ranges of 10 20 km which is the same as the previously determined aftershock distribution the corresponding estimate for seismic moment is 2 7 10 19 nm with possible ranges of 2 3 10 19 3 2 10 19 nm standard tide gauges along the nearby coast did not record any tsunami signal high precision ocean bottom pressure measurements offshore thus make it possible to determine fault parameters of moderate sized earthquakes in subduction zones using open ocean tsunami waveforms

propagating rifts crustal extension during continental rifting propagating rifts may not be incompatible with plate tectonic tenets which are considered at the appropriate scale propagation of a rift is a natural consequence of relative motion about a pole of rotation it is not necessarily valid that 1 rifts propagate in one direction or 2 crustal stretching increases with progressive propagation of a rift

geochemistry of basalts from the western woodlark lau and manus basins implications for their petrogenesis and source rock compositions abstract geochemical compositions are reported for back arc and marginal basin basalts sampled during the 1990 cruise of r v akademik mstislav keldysh in the lau basin north lau ln king s triple junction lk central lau lc east lau le manus basin red star region mrs and western woodlark basin dobu seamount region wd continental rifting segment east basin region we the boundary between continental and spreading segments and franklin seamount region wf spreading rifting segment two main geochemical types of basalts have been distinguished within the lau and manus back arc marginal basins i depleted basalts comparable with n morb lavas lc le mrs and ii enriched basalt lavas ln lk vesicular pillow basalts lk vpb correspond to typical babb the sources of melts were depleted spinel or plagioclase lherzolite n morb type for lc le mrs basalts the n morb source was metasomatized by adding lile and volatiles arc source components for enriched lavas direct observations from mir submersibles in the area of the king s triple junction lau basin established that lile depleted massive sheet flow basalts overlie vesicular pillow basalts the western woodlark basin is a tectonic transition zone between typical oceanic spreading in the central part of the basin and continental rifting in easternmost papua new guinea the wd basalts show strong lile lree p and zr enrichment and are spatially associated with alkaline rhyolites the we and wf lavas demonstrate moderate enrichment in these elements the western woodlark lavas formed from multiple sources including n morb mantle arc source components and enriched mantle like oib

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

ten million years of activity within the eastern california shear zone from u pb dating of fault zone opal abstract reconstructions of long term fault activity are essential for understanding both the mechanisms controlling fault behavior and accurate earthquake hazard assessments increasing evidence for temporal variations in strain accumulation suggests non uniform strain rates over a range of historic to geologic timescales the paucity of long term records of fault activity has limited our ability to resolve these variations we present a method for constraining long term fault activity based on u pb dating of fault related opal from secondary fault segments within the eastern california shear zone ecsz the presence of sheared and breccia cemented opaline silica within well exposed faults at near surface conditions suggest that opal formation is associated with high magnitude earthquakes capable of surface rupture 6 m temporal constraints from massive and sheared syntectonic opal n 74 on related secondary faults from this study provide new insights on the timing of fault initiation reactivation and longevity the oldest dates obtained indicate that ecsz activity commenced at or before 10 ma multiple deformation events dated within a single structure on episodically deposited and sheared opal up to six generations demonstrate that fault reactivation occurred over 105 yr timescales 0 7 0 1 myr relative probabilities of dated deformation events can be used to evaluate changes in fault activity in the past 2 5 ma n 60 this analysis indicates enhanced fault activity starting at 2 ma and peaking around 1 ma possibly due to fault interactions and distribution of deformation between the ecsz and the san andreas fault

inhibiting magmatism on nonvolcanic rifted margins the conditions under which continental rifting may occur without an accompanying protracted period of synextensional magmatism are explored with a series of finite element models that dynamically simulate continental extension the models demonstrate that 5 7 km of new oceanic crust can be generated at the time of breakup while restricting synextensional magmatism to less than 2 m y prior to the end of the rifting episode if the extension rate exceeds 15 mm yr and the crust contains only minor lateral variations in strength under these conditions extensional strain in the mantle is localized during the early stages of rifting preventing diffuse lithospheric thinning melting does not begin until lithospheric necking is well established resulting in a narrow zone of melt production that limits magmatism to within 30 km of the ocean continent boundary once melting begins rifting progresses rapidly to breakup restricting the period of synextensional magmatism to as little as 1 6 m y prior to the onset of sea floor spreading because the models do not require either a cool mantle prior to rifting or pronounced conductive cooling of the mantle during rifting they are able to account for the generation of new oceanic crust at the time of breakup the model predictions are consistent with the structure and magmatic history of nonvolcanic rifted margins in the north atlantic ocean

timing of brittle deformation within the nojima fault zone japan abstract k ar ages from clay fault gouges of outcrop and core samples ug500 well from the nojima fault zone awaji island japan are consistent internally with established field constraints and with apatite and zircon fission track ages demonstrating the suitability of this method for dating of brittle deformation six unspiked and thirty conventional k ar data of clay minerals separated from gouge zones were analysed characterized by protoliths of high grade metamorphic and magmatic rocks the samples from the nojima fault zone offer the opportunity to distinct identify newly grown illite in the fault planes all dated sample fractions were characterized by xrd sem and tem k ar ages of 29 separates have an age range from 63 4 1 3 ma early palaeocene danian to 42 2 1 0 palaeogene middle eocene lutetian some

influence of fault plane heterogeneity on the seismic behavior in the southern kurile islands arc the seismic behavior in the southern kurile islands arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958 1963 1969 and 1973 provide a first order image of the major regions of enhanced strength or asperities hypocenters of these and an additional 83 smaller earthquakes with mb 5 3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns nearly all of the smaller magnitude earthquakes locate outside of the principal asperities of the great earthquakes this observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture an along dip depth dependent component of strength heterogeneity appears to influence the seismicity patterns the asperities of the 1958 and 1973 great earthquakes and hypocenters of most of the ms 7 earthquakes occur near the downdip edge of the 100 km wide seismically active plate interface this would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle ductile or stick slip stable slip transition that defines the base of the seismogenic zone the asperities of the 1963 and 1969 earthquakes however appear to be located well above the downdip edge of the coupled plate interface the occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along dip changes in other parameters besides lithostatic pressure that control shear resistance along strike segmentation of the kurile islands thrust zone may be a consequence of along strike alternation in the location of the largest asperities between the deep interface and the midinterface

geophysical investigation and reconstruction of lithospheric structure and its control on geology structure and mineralization in the cordillera of northern canada and eastern alaska a reconstruction of the tintina fault is applied to regional geophysical and topographic data facilitating the definition of west trending lineaments within the lower crust and or mantle lithosphere oblique to the nw trending structure of the cordilleran terranes the lineaments which exhibit a range of geophysical and geological signatures are interpreted to be related to the liard transfer zone continuous to the denali fault that divided lower and upper plates during late proterozoic cambrian rifting of the laurentian margin three dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north the transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north these conclusions suggest that extended north american basement related to laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth continuously underlies a thin carapace of accreted terranes in western yukon and eastern alaska the interpreted continuity of north american basement reaffirms that if oroclinal bending of the intermontane terranes occurred then it was prior to its emplacement upon the rifted basement examination of the spatial relationships between mineral occurrences and postaccretionary cretaceous lithospheric lineaments from their manifestation in geophysical geological and topographic data suggest that the late proterozoic lineaments influenced mesozoic mineralization through influence on the development of the shallow crustal structure intrusion and exhumation and erosion

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems