multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

recrystallization and chemical changes in apatite in response to hypervelocity impact despite the wide utility of apatite ca5 po4 3 f cl oh in the geosciences including tracing volatile abundances on the moon and mars little is known about how the mineral responds to the extrem

isotope systematics and shock wave metamorphism iii k ar in experimentally and naturally shocked rocks the haughton impact structure canada this is a report on the first comparative study of the effects of experimental and natural shock on the k ar system first mineral separates from a 451 plus minus 1 ma old gneiss experimentally shocked up to 60 gpa were investigated by stepwise heating sup 40 ar sup 39 ar technique compared with an unshocked samples the shocked samples have lower sup 40 ar sup 39 ar ages and the diffusion properties have changed depending on peak shock pressure and mineralogic composition total resetting of the k ar clock did not occur secondly to compare these results with the effects of natural shock samples from the approximately 23 ma old haughton impact crater devon island canada with different shock stages were investigated up to shock stage 3 no total resetting of the k ar clock was observed but severe argon losses depending on shock pressure result in sup 40 ar sup 39 ar plateau ages which are incorrect in terms of the true crater age yet they provide an upper limit to the time of the cratering event

fission track dating of haughton astrobleme and included biota devon island canada haughton astrobleme is a major extraterrestrial impact structure located on devon island in the canadian arctic archipelago northwest territories apatite grains separated from shocked precambrian gneiss contained in a polymict breccia from the center of the astrobleme yielded a fission track date of 22 4 million 1 4 million years before the present or early miocene aquitanian this provides a date for the impact event and an upper limit on the age of crater filling lake sediments and a flora and vertebrate fauna occurring in them a geologically precise date for these fossils provides an important biostratigraphic reference point for interpreting the biotic evolution of the arctic

geochemistry and neodymium strontium isotope signature of tektite like objects from siberia urengoites south ural glass we report sr nd isotope parameters rare earth element ree and major element data for isolated findings of tektite like objects from western siberia urengoites south ural glass as well as for two indochinites the latter were recovered in vietnam and their overall geochemical characteristics equal those of other tektites from the indochinite subgroup of the australasian strewn field the three urengoites 24 ma are extremely silica rich 89 to 96 wt sic2 and their ree abundances vary between 45 and 76 ppm with lan ybn ranging from 7 6 to 10 4 and eun eu between 0 69 and 0 75 their ree distribution patterns match that of average upper crust the urengoites have present day sr of 155 to 174 and nd ranging from 18 to 23 their model ages in million years are tsrur 1200 up to 4060 and tndchur 1570 up to 2070 data points for the urengoites plot colinearly in the rb sr evolution diagram the age corresponding to the slope is 183 30 ma 2s which is indistinguishable from the intercept age of 211 ma in the tsrurvs l frb diagram rubidium strontium and sm nd systematics of the urengoites indicate a heterogeneous precursor material derived from paleoproterozoic continental crust which underwent rb sr fractionation and partial sr isotope homogenization in jurassic times any relation between the urengoites and the haughton impact crater having within 2s errors an identical age can be excluded on the basis of isotope relationships and geochemical data the only known south ural glass 6 2 ma is characterized by intermediate sio2 65 wt high al2o3 14 wt and cao 12 wt and low feotot 0 4 wt contents this unique tektite like object contains 110 ppm ree displaying a steeply negative c1 normalized distribution with lan ybn of 17 and eun eu1 of 0 71 the rb abundance 10 ppm and rb sr ratio are low and combined with a crustal 87sr 86sr ratio of 0 722 yielding an unrealistic tsrur age of 2 5 ga the rb sr systematics imply a rather recent parent daughter element decoupling the tndchur age of the south ural glass is 1690 ma geochemical data suggest that urengoites and the south ural glass belong to two discrete groups of tektites whose source craters remain to be discovered

haughton impact structure structural and morphological aspects haughton is a complex meteorite impact structure of tertiary age on devon island in the canadian arctic the 20 5 km diameter site was formed in essentially flat lying paleozoic carbonates roughly 1700 m in thickness overlying precambrian gneisses the structure comprises three concentric rings surrounding a central basin with interior peaks the outer ring diameter 20 5 km is an annulus of peaks separating the allen bay dolomites of the regional plateau from the same unit that has been downdropped towards the crater interior along a series of circumferential faults the middle ring of peaks diameter 14 15 km comprises both faulted allen bay dolomites and blocks of the underlying bay fiord formation uplifted 250 m hills of the inner elliptical ring 3 5 5 5 km in diameter are also formed from bay fiord formation and the underlying eleanor river formation brought up 600 m an interconnected series of normal faults occupies most of the annulus between the inner and outer rings within the central b

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

estimation of earthquake hazard parameters from incomplete data files part iii incorporation of uncertainty of earthquake occurrence model most probabilistic seismic hazard analysis procedures require that at least three seismic source parameters be known namely the mean seismic activity rate the gutenberg richter b value and the area characteristic seismogenic source maximum possible earthquake magnitude m max in almost all currently used seismic hazard assessment procedures that utilize these three parameters it is explicitly assumed that all three remain constant over time and space however closer examination of most earthquake catalogs has indicated that significant spatial and temporal variations existed in the seismic activity rate as well as in the gutenberg richter b value in this study the maximum likelihood estimation of these earthquake hazard parameters considers the incompleteness of the catalogs the uncertainty in the earthquake magnitude determination as well as the uncertainty associated with the applied earthquake occurrence models the uncertainty in the earthquake occurrence models is introduced by assuming that both the mean seismic activity rate and the gutenberg richter b value are random variables each described by the gamma distribution this approach results in the extension of the classic frequency magnitude gutenberg richter relation and the poisson distribution of the number of earthquakes with their compounded counterparts benjamin 1968 campbell 1982 1983 the proposed procedure was applied in the estimation of the seismicity parameters in an area that had experienced the strongest and most devastating earthquake in contemporary south african history namely the 29 september 1969 m w 6 3 ceres tulbagh event in this example it was shown that the introduction of uncertainty in the earthquake occurrence model reduced the mean return periods leading to an increase of the estimated seismic hazard additionally this study confirmed that accounting for magnitude uncertainties had the opposite effect that is it brought about increases in the return periods or equivalently a reduction of the estimated seismic hazard

re evaluating the age of the haughton impact event we have re evaluated the published age information for the haughton impact structure which was believed to have formed 23 ma ago during the miocene age and report new ar ar laser probe data from shocked basement clasts this reveals an eocene age which is at odds with the published miocene stratigraphic apatite fission track and ar ar data we discuss our new data within this context we have found that the age of the haughton impact structure is 39 ma which has implications for both crater recolonization models and post impact hydrothermal activity future work on the relationship between flora and fauna within the crater and others at high latitude may resolve this paradox

the ayers rock australia earthquake of 28 may 1989 a temporally isolated mb 5 8 intraplate event abstract at 0255 utc on 28 may 1989 an m b 5 8 earthquake shook the yulara resort and uluru national park in the northern territory of australia the epicenter lay 40 km west of ayers rock in an area where moderate size earthquakes had not previously been recorded within 215 km although many people at yulara and uluru were alarmed damage to buildings was minimal the australian national university anu installed an array of 11 seismographs in the epicentral area 6 days after the m b 5 8 event no aftershocks were detected by the anu network during 2 weeks of recording with an estimated detection threshold of magnitude 1 to 0 another group of seismologists who operated a single recorder near uluru beginning 59 hours after the main shock and continuing through july and 6 other recorders for 6 days in early june also failed to observe any aftershocks within the sensitivity of the arrays the ayers rock event was a temporally isolated earthquake the absence of aftershock activity is in sharp contrast to the seismicity pattern following two magnitude 5 2 earthquakes near tennant creek in january 1987 and to other earthquakes of magnitude 4 2 to 6 3 observed in continental intraplate settings worldwide

spring sapping on the lower continental slope offshore new jersey undersea discharge of ground water during periods of lower sea level may have eroded valleys on part of the lower continental slope offshore new jersey steep headed basins cliffed and terraced walls and irregular courses of these valleys may have been produced by sapping of exposed near horizontal tertiary strata joints in eocene calcareous rocks would have localized ground water movement some karstlike features of the submarine topography and the outcrops suggest that solution of the calcareous rocks also took place

amplitude spectra moment tensor inversion of shallow earthquakes in spain summary the study of focal mechanisms for shallow earthquakes of low to moderate magnitude at regional distances requires a proper inversion technique one of the major problems is the correct separation of path effects from source effects the method used should take into account problems arising from errors in the definition of crustal structure and inclusion of the high frequency content of the waveforms in this study a method of frequency domain moment tensor inversion to determine deviatoric moment tensor solutions from body wave amplitude spectra is described the method has been tested with synthetic data to define instabilities that are due to mismodelling of several parameters the inversion technique is applied to broadband seismic data of shallow 10 km depth earthquakes in spain solutions are obtained for a range of depths and compared with solutions obtained with different methods such as first motion polarities and moment tensor inversion in the time domain a combined fit of amplitude spectra first motion polarities and double couple component is introduced as a method which resolves better the non uniqueness of solution the proposed method can be implemented in fast moment tensor inversion when a green functions database has been created in advance for the studied area

impact induced hydrothermal activity within the haughton impact structure arctic canada generation of a transient warm wet oasis field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot impact generated rocks following formation of the 24 km diameter 23 ma haughton impact structure hydrothermal alteration is recognized in two settings within polymict impact breccias overlying the central portion of the structure and within localized pipes in impact generated concentric fault systems the intra breccia alteration comprises three varieties of cavity and fracture filling a sulfide with carbonate b sulfate and c carbonate these are accompanied by subordinate celestite barite fluorite quartz and marcasite selenite is also developed particularly in the lower levels of the impact breccia sheet the fault related hydrothermal alteration occurs in 1 7 m diameter subvertical pipes that are exposed for lengths of up 20 m the pipes are defined by a monomict quartz carbonate breccia showing pronounced fe hydroxide alteration associated sulfides include marcasite pyrite and chalcopyrite we propose three distinct stages in the evolution of the hydrothermal system 1 early stage 200 c with the precipitation of quartz vapor phase dominated 2 main stage 200 100 c with the development of a two phase vapor plus liquid zone leading to calcite celestite barite marcasite and fluorite precipitation and 3 late stage 100 c with selenite and fibroferrite development through liquid phase dominated precipitation we estimate that it took several tens of thousands of years to cool below 50 c following impact during this time haughton supported a 14 km diameter crater lake and subsurface water system providing a warmer wetter niche relative to the surrounding terrain the results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems

shear wave velocity structure of the southern african upper mantle with implications for the uplift of southern africa summary broad band seismic data from the southern african seismic experiment and the africaarray network are used to investigate the seismic velocity structure of the upper mantle beneath southern africa and in particular beneath the kaapvaal craton a two plane approximation method that includes a finite frequency sensitivity kernel is employed to measure rayleigh wave phase velocities which are inverted to obtain a quasi 3 d shear wave velocity model of the upper mantle we find phase velocities for the kaapvaal craton and surrounding mobile belts that are comparable to those reported by previous studies and we find little evidence for variation from east to west across the namaqua natal belt a region not well imaged in previous studies a high velocity upper mantle lid is found beneath the kaapvaal craton and most of southern africa for the kaapvaal craton the thickness of the lid 150 200 km is consistent with the lid thicknesses reported in many previous studies the cratonic lid is underlain by a 100 km thick low velocity zone with a 3 9 per cent maximum velocity reduction by comparing the velocity model to those published for other archean cratons we find few differences and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern african plateau is supported by an upper mantle thermal anomaly

geological overview and cratering model for the haughton impact structure devon island canadian high arctic the haughton impact structure has been the focus of systematic multi disciplinary field and laboratory research activities over the past several years regional geological mapping has refined the sedimentary target stratigraphy and constrained the thickness of the sedimentary sequence at the time of impact to 1880 m new 40ar 39ar dates place the impact event at 39 ma in the late eocene haughton has an apparent crater diameter of 23 km with an estimated rim final crater diameter of 16 km the structure lacks a central topographic peak or peak ring which is unusual for craters of this size geological mapping and sampling reveals that a series of different impactites are present at haughton the volumetrically dominant crater fill impact melt breccias contain a calcite anhydrite silicate glass groundmass all of which have been shown to represent impact generated melt phases these impactites are therefore stratigraphically and genetically equivalent to coherent impact melt rocks present in craters developed in crystalline targets the crater fill impactites provided a heat source that drove a post impact hydrothermal system during this time haughton would have represented a transient warm wet microbial oasis a subsequent episode of erosion during which time substantial amounts of impactites were removed was followed by the deposition of intracrater lacustrine sediments of the haughton formation during the miocene present day intracrater lakes and ponds preserve a detailed paleoenvironmental record dating back to the last glaciation in the high arctic modern modification of the landscape is dominated by seasonal regional glacial and niveal melting and local periglacial processes the impact processing of target materials improved the opportunities for colonization and has provided several present day habitats suitable for microbial life that otherwise do not exist in the surrounding terrain

landward vergence and oblique structural trends in the oregon margin accretionary prism implications and effect on fluid flow abstract the central oregon margin spans a regional transition in accretionary structures from seaward verging in the south to landward verging in the north new multichannel seismic mcs data image both landward and seaward vergent provinces along the central and northern oregon margin landward vergence is characterized by a deep decollement with deformation distributed across a broad lower continental slope in a coherent structural style in the landward vergent area virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet forming fault bend folds and a distinctive ridge trough morphology this style of landward vergence is not explained by existing models in contrast seaward vergence correlates with a shallower decollement approximately 1 4 km above the oceanic crust and a more intense style of deformation within a narrower slope initial thickening of the trench sediments occurs across a well developed protothrust zone the frontal thrust forms a ramp anticline that is cut by a prominent backthrust previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high amplitude reversed polarity reflections suggestive of enhanced porosity along the faults potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin abrupt changes in structural style occur both along strike and updip and are bounded by two sets of oblique slip faults three nw striking left lateral faults are imaged in both mcs and seabeam data plunging anticlines developed along the nw striking faults are venting fluids and were previously interpreted as mud volcanoes the deformation front is locally disrupted where these faults intersect the prism but they appear to have limited influence on the structural evolution of the prism in contrast the ne striking right lateral faults are confined to the deforming sediments of the upper plate these faults interact with the thrusts within the prism forming a rhomboidal pattern of three dimensional deformation

a left lateral strike slip fault seaward of the oregon convergent margin we have mapped a recently active left lateral strike slip fault the wecoma fault on the floor of cascadia basin west of the oregon convergent margin using seamarc i sidescan sonar seabeam bathymetry and multichannel seismic and magnetic data the fault intersects the base of the continental slope at 45 10 n and extends northwest 293 for at least 18 5 km the fault s western terminus was not identified and the eastern end of the fault splays apart and disrupts the lower continental slope the fault extends to the base of the 3 5 km thick sedimentary section and overlies a basement discontinuity that may be related to movement along the wecoma fault prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left lateral separation allowing the general history of fault motion to be evaluated the fault s average slip rate since 10 24 ka is inferred to be 5 12 mm yr based on the age of an offset submarine channel surficial structural relationships in conjunction with the maximum inferred slip rate indicate that fault movement initiated at least 210 ka and that the fault has been active during the holocene

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

four dimensional tectono stratigraphic evolution of the southeastern peri tyrrhenian basins margin of calabria italy abstract we use seismic reflection profiles of diverse resolutions integrated with high resolution bathymetry and calibrated by deep wells piston cores and field data to derive a structural and stratigraphic interpretation of the southeastern peri tyrrhenian basins the study area is located in the southeastern tyrrhenian sea offshore calabria we document a tectono stratigraphic evolution of the sedimentary basins from the serravallian to the present the basins formation started during the serravallian and the area was successively affected by e w normal faulting and approximately n s transfer faults in the messinian lower pleistocene time range during the lower pleistocene the tectonic depressions completely filled with sediments were affected by folding associated with transcurrent tectonics along a nw trending left lateral fault finally during the middle pleistocene a change in the slip direction produced minor pull apart basins that overprinted the folds the basin analysis revealed the stratigraphic signature of the tectonic events and the uplifting and subsiding areas and the direction from which sediments arrive into the tectonic depression the four dimensional geologic reconstruction of the margin of calabria furnished new constrains for the kinematic evolution of the southeastern peri tyrrhenian basins we suggest that the superposition of different structural styles reflects a variation of the relative velocity between the southern apennine arc and the calabria arc during the formation of the apennine thrust belt

impact induced phase transformations at 50 60 gpa in continental crust an epma and atem study strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 ma old haughton impact crater were studied by optical microscopy electron probe microanalysis epma and analytical transmission electron microscopy atem the study is focused on the substantiation of shock induced phase transformations in rock forming minerals and their implications for post shock thermal regimes and impact age dating in the gneiss fragments the original layering is largerly preserved although all the minerals have experienced dramatic phase transformations chemically the various shock generated decomposition regions still reflect the pre shock mineralogy however significant heterogeneities exist on the 10 100 m scale four different regions of decomposition products can be distinguished epma 1 shocked biotite rich regions with strong variations in si al k fe and mg on the 10 m scale 2 shocked quartz with about 3 al and k and traces of fe and mg 3 a region with al si 1 which does not correspond to any pre shock mineral and 4 shocked k feldspar showing a significant deficit in k and traces of fe and mg on the nanometer scale a uniform bimodal reaction pattern emerges all minerals are transformed to new crystals embedded in a silica rich glass the shock generated crystals are 1 spinel in shocked biotite 2 quartz cristobalite and coesite in shocked quartz 3 an unknown polymorph of corundum al2o3 in feldspar and in the al si 1 region and 4 mullite in association with sio2 glass in shocked sillimanite grains all crystals range in size between 0 05 and 1 0 m taking into account estimated durations and pressure temperature conditions for shock wave passage and post shock temperatures the reactions observed can be ascribed to high temperatures during and after pressure release the different mineral glass parageneses suggest temperatures in excess of 1400 c moreover given the small size of the newly formed minerals very rapid cooling down to 1200 c is required

dating the cooling of exhumed central uplifts of impact structures by the u th he method a case study at manicouagan abstract forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the manicouagan impact structure were dated by the u th he technique a u th he central age of 207 1 6 4 ma 2 standard error se n 40 has been determined with 4 outlier ages removed the central age is refined to 208 9 5 1 ma 2 se both of these ages are within error of the previously determined u pb zircon age of 214 1 ma 2 derived from the impact melt manicouagan s central uplift formed due to rapid elevation from 7 10 km depth as part of the modification stage of the impact process which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the u th he system in titanite correlation with the previous u pb zircon 214 1 ma impact melt crystallization age indicates that the u th he titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets or other melt products the youngest ca 195 ma u th he dates preserved in some titanite fragments are synchronous with early jurassic rift induced lithospheric thinning and associated igneous activity that defines the central atlantic magmatic province camp the u th he titanite data from manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of camp activity

higher degree moment tensor inversion using far field broad band recordings theory and evaluation of the method with application to the 1994 bolivia deep earthquake summary we present a method to estimate parameters of the extended earthquake source using higher degree moment tensors at 27 centroid locations we show that a taylor series expansion of green s functions around a single centroid is not accurate enough when working with seismic wave periods and wavelengths in the range of the rupture duration and spatial extent of the fault respectively introducing a grid of 27 centroid locations on the fault and using higher degree moment tensors we are able to model adequately body and surface waves with periods and wavelengths smaller than the rupture duration and fault dimensions under simplifying assumptions an iterative inversion scheme is coded to estimate parameters of planar haskell type faults realistic inversion examples for deep and shallow earthquakes show that uni and bidirectional rupture models rupture direction fault and auxiliary plane and kinematic source dimensions and can be constrained with teleseismic body and or surface waves the application to the deep bolivia event indicates a subhorizontal fault plane unidirectional rupture to the north is slightly preferred the rupture duration of 25 s and fault dimensions of 47 25 km agree well with the estimates for the main pulse moment release given in other studies

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

the 29 september 1969 ceres south africa earthquake full waveform moment tensor inversion for point source and kinematic source parameters the ceres earthquake of 29 september 1969 is the largest known earthquake in southern africa digitized analog recordings from worldwide standardized seismographic network stations powell and fries 1964 are used to retrieve the point source moment tensor and the most likely centroid depth of the event using full waveform modeling a scalar seismic moment of 2 2 2 4 1018 n m corresponding to a moment magnitude of 6 2 6 3 is found the analysis confirms the pure strike slip mechanism previously determined from onset polarities by green and bloch 1971 overall good agreement with the fault orientation previously estimated from local aftershock recordings is found the centroid depth can be constrained to be less than 15 km in a second analysis step we use a higher order moment tensor based inversion scheme for simple extended rupture models to constrain the lateral fault dimensions we find rupture propagated unilaterally for 4 7 s from east southwest to west northwest for about 17 km average rupture velocity of about 3 1 km s online material waveform and spectral fits

apatite geochronology and chemistry of luanchuan granitoids in the east qinling orogen china implications for petrogenesis metallogenesis and exploration abstract the luanchuan granitoids in the east qinling orogen central china host multiple genetic types of mo w pb zn ag polymetallic deposits here we present results from systematic geochronological and geochemical investigations on apatite with a view to gain insights into the petrogenesis metallogenesis and potential for mineral exploration we analyzed 388 apatite grains from eight late mesozoic granitic plutons in the luanchuan ore district apatite u pb geochronology yields 206pb 238u mean ages in the range of 151 5 102 3 ma representing the crystallization ages through integrating our results with those from previous studies on the multi stage magmatism 160 130 ma 111 108 ma and metallogeny 150 139 ma 138 135 ma 114 ma the newly obtained apatite ages can be correlated with coeval magmatic and metallogenetic events of this ore district in comparison except for the nannihu pluton which is ca 19 ma younger than the corresponding zircon u pb ages the other studied plutons show almost consistent apatite and zircon u pb ages within error ranges suggesting rapid post magmatic cooling below 450 c in response to shallow emplacement apatite chemistry displays ree concentrations in the range of 2379 15 925 ppm characterized by rather high lree concentrations depleted to flat hree profiles and distinctive negative eu anomalies suggesting crust mantle mixing in the magma source prior to apatite crystallization which incorporated different degrees of sediment melting and slab melting components triggered by the subducted paleo pacific slab derived fluids in conjunction with element concentrations and ratios e g sr mn cl y la y eu eu sm nd n la sm n eu eu n ce ce n la yb n in apatite the luanchuan granitoids are classified as moderately oxidized and mineralized i type granites we suggest that the huangbeiling pluton and other igneous rocks in its vicinity likely have relatively higher potential for future w mo mo w exploration and that different degrees of magmatic differentiation might have generated the porphyry skarn mo w deposit in the yuku ore field moreover our study also demonstrates that the apatite ree concentrations and ratios can be used as potential indicators for mineral exploration

impact thermochronology and the age of haughton impact structure canada 1 most successful efforts to determine the ages of impact events are based on the isotope geochronology of crystalline or glassy impact melts studies of impact sites on earth show that many form without significant melt production meaning that traditional geochronologic approaches can yield unsatisfying results we describe here an alternative approach based on theoretical calculations that even brief thermal events related to impact can reset the isotopic systematics of unmelted target rocks thermochronometers based on the production of radiogenic 4he in accessory minerals are particularly amenable to complete resetting by impact and should yield robust impact ages if helium systematics are not further disturbed during post impact thermal events we illustrate the utility of this method through a presentation of a new zircon u th he date for the haughton impact structure canada of 23 5 2 0 ma

geophysical evidence for crustal and mantle weak zones controlling intra plate seismicity the 2017 botswana earthquake sequence abstract large earthquakes away from plate boundaries pose a significant threat to human lives and infrastructure but such events typically occur on previously unknown faults most cases of intra plate seismicity result from compression related to far field plate boundary stresses the april 2017 mw 6 5 earthquake in central botswana and subsequent events occurred in a region with no previously known large earthquakes occurred away from major present day tectonic activity and accommodate extension rather than compression here we present results from an integrated geophysical study that suggests the recent earthquakes may be a sign of future activity controlled by the collocation of a weak upper mantle and weak crustal structure between otherwise strong precambrian blocks magnetotelluric data highlights proterozoic continent accretion structure within the region and shows that recent extension and seismicity occurred along ancient thrust faults within the crust our seismic velocity and resistivity models suggest a weak zone in the uppermost mantle that does not persist to greater depths and is therefore unlikely to represent mantle upwelling the botswana events may therefore be indicative of top down extension as a response to large scale extensional forces

rate of left lateral movement along the easternmost segment of the altyn tagh fault east of 96 e china summary field work carried out in gansu province and complemented with analysis of spot panchromatic scenes allows us to characterize the deformations along the eastern segment of the altyn tagh fault and to place bounds on its holocene left slip rate east of 96 e the long term left lateral offset of stream channels alluvial fans and terrace edges is about 50 m these offsets are most probably of holocene age 12 2 ka and imply that the corresponding derived slip rate is 4 2 mm yr 1 this observation is consistent with a north eastward along strike decreasing slip rate on the altyn fault due to partitioning of slip on multiple more easterly trending splays

microseismic activity and basement controls on an active intraplate strike slip fault ceres tulbagh south africa earthquakes far from plate boundaries are poorly understood because of few well studied examples and uncertainty of what controls the location of these events in 1969 a damaging local magnitude m l 6 3 strike slip earthquake with no surface expression occurred in the ceres tulbagh region in the south african stable continental interior here we present a microseismic study of the ceres tulbagh area conducted over three months in 2012 in which 172 events recorded on at least three stations follow a gutenberg richter relationship for 1 5 m l 0 5 the events delineate a 5 km wide subvertical zone that is microseismically active to a depth of 15 km this fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress we argue that the microseismically active zone is guided by inherited structures in the basement geology this and similar structures may represent significant earthquake hazard in plate interiors online material tables of station and event parameters and p and s wave arrival times

isotope systematics and shock wave metamorphism ii u pb and rb sr in naturally shocked rocks the haughton impact structure canada minerals and whole rocks from crystalline fragments collected in the allochthonous polymict breccia of the 23 ma old haughton impact crater on devon island arctic canada were analysed for u pb k and rb sr in order to constrain degrees of shock wave induced perturbations in these systems the results show that shock wave metamorphism at 50 gpa with onset of total melting does not significantly affect the u pb chronometers in monazite and zircon monazite preserved its age of crystallization during high grade metamorphism at 1928 2 2 1 2 0 ma 2 errors as dated by the upper intercept of 2 9 discordant monazites the same analyses yield a lower intercept age of 277 5 35 1 35 0 ma in the concordia diagram demonstrating that discordancy is not a direct effect of shock wave metamorphism but a product of time integrated pb diffusion prior to the impact event which occurred 23 ma ago this constrains maximal shock induced pb loss to less than 3 including possible alteration related pb diffusion from the shock damaged grains in 1903 1 1 4 1 1 ma old monazite from an unshocked reference gneiss recent pb loss also reaches 3 corroborating the occurrence of weak alteration related upb fractionation the data substantiate that u pb discordancy patterns are not a reliable chronometer to date impact events small rock fragments and a coherent block of shock stage i to iii vi consist of heterogeneous unequilibrated rb sr sub systems on the 10 m scale with drastically varying isr shock related rbsr fractionation is very limited and in the large block stage iii three biotite fractions yield an rb sr isochron age of 1791 34 ma 2 which is undistinguishable from the 1814 8 ma age of biotite in the unshocked basement gneiss in contrast to this m3 sized fragment rb sr systematics in a fist size sample of shock stage iii do not yield any reliable age information due to small scale 1 cm3 isotopic disequilibrium it appears that such small scale disequilibrium is essentially due to a take up of radiogenic 87sr in feldspar glass from biotite during shock wave passage this prevents dating of the impact events by the rb sr internal isochron method and it can be concluded that total rock melting is the first condition necessary to reset the rb sr chronometer in shocked crystalline rocks

automated moment tensor inversion to estimate source mechanisms of hydraulically induced micro seismicity in salt rock abstract two methods for an automated moment tensor inversion are investigated the first is a single source inversion using amplitude spectra of observed body wave phases and theoretical green functions and the second a relative moment tensor inversion using spectral moments of body wave phases an optional general dislocation source constraint is introduced to stabilize the inversions for sparse data both methods are successfully tested on synthetic data and are applied to hydraulically induced micro seismicity in salt rock the estimated moment tensors of 187 events show similar radiation patterns with a major double couple component and a small or zero isotropic component we explain the double couple component with failure on horizontal preexisting planes of weakness within the process zone of circular tension fractures

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

impact generated carbonate melts evidence from the haughton structure canada evidence is presented for the melting of dolomite rich target rocks during formation of the 24 km diameter 23 ma haughton impact structure on devon island in the canadian high arctic field studies and analytical scanning electron microscopy reveal that the s 200 m thick crater fill deposit which currently covers an v60 km 2 area in the center of the structure comprises fragmented target rocks set within a carbonate silicate matrix the silicate component of the matrix consists of si al mg rich glass the carbonate component is microcrystalline calcite containing up to a few wt si and al the calcite also forms spherules and globules within the silicate glass with which it develops microtextures indicative of liquid immiscibility dolomite clasts exhibit evidence of assimilation and may show calcite and rare dolomite overgrowths some clasts are penetrated by calcite and silicate injections along with the carbonate silicate glass textures the presence of pigeonite and spinifex textured diopside suggests that the matrix to the crater fill deposit was originally molten and was rapidly cooled this indicates that the impact event that generated haughton caused fusion of the predominantly dolomitic target rocks it appears that the ca mg component of the dolomite may have dissociated with mg entering the silicate melt phase while the ca component formed a caco3 dominant melt the silicates were derived by the fusion of lower paleozoic sandstones siltstones shales and impure dolomites evidence for melting is corroborated by a review of theoretical and experimental work which shows that caco3 melts at s 10 gpa and s 2000 k instead of dissociating to release co2 this work indicates that carbonate rich sedimentary targets may also undergo impact melting and that the volume of co2 released into the atmosphere during such events may be considerably less than previously estimated fl 2001 elsevier science b v all rights reserved

post break up tectonic inversion across the southwestern cape of south africa new insights from apatite and zircon fission track thermochronometry the south west african margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning recent attention focused on this margin has included theoretical modelling studies of rift processes plate kinematic studies of the opening geometry and timing and empirical studies focused on documenting the crustal structure and offshore sedimentary record here we examine the onshore geomorphic and tectonic response to rifting and breakup with a specific focus on the sw cape of south africa we present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of south africa the data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the early cretaceous 150 130 ma and late cretaceous 100 80 ma respectively both periods of enhanced erosion are regional in extent involved km scale erosion and extend well inland of the current escarpment zone albeit with spatially variable intensity and style the late cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km scale differential displacement and erosion the new aft data do not exclude the possibility of modest surface uplift occurring during the cenozoic but they restrict the depth of regional cenozoic erosion on the western margin to less than c 1 km the inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore orange and bredasdorp basins it is suggested that the late cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the sw cape by local tectonic forces arising from dextral displacement of the falkland plateau along the falkland agulhas fracture zone

east african earthquake body wave inversion with implications for continental structure and deformation summary source parameters for five african intraplate events are obtained from body wave inversion for the moment tensor parameters for the events are as follows location date mb strike dip rake depth ethiopia 4 05 69 5 8 327 1 69 2 45 0 4 zaire 3 20 66 6 3 44 6 44 9 81 5 10 zambia 12 2 68 5 9 20 3 40 2 264 8 6 zambia 5 15 68 5 7 49 1 39 8 263 2 28 s africa 9 29 69 5 6 123 5 89 4 0 58 4 the ethiopia and zaire events exhibit normal faulting as would be expected for their rift zone locations the 1968 may 15 zambia event is unusual in that p pp separation times place its focus in the lower crust 28 km which may be below the brittle ductile transition zone implying high strain rates inversion yielded a high angle normal fault the depth and focal mechanism of this event along with the mechanism for the 1968 december 2 zambia event suggest southwestward rift extension teleseismic long period vertical p waves from the south africa event exhibit distortions that may be attributed to lateral inhomogeneities in the source structure

interrelationship of fluid venting and structural evolution alvin observations from the frontal accretionary prism oregon seismic reflection and sea beam bathymetric data plus submarine geological measurements define a ramp anticline at the deformatoin front of the central oregon subduction zone at its northern termination the ramp anticline is deeply incised by a large 500 m deep submarine canyon and cut by a probable backthrust to the south along the strike of the fold a smaller submarine canyon shallowly erodes the anticline and backthrusting is not apparent in the submersible observations two alvin dives along a transect through the southern canyon show active fluid vents demarked by biological communities at the frontal thrust and at the breached crest of the anticline along a northern transect encompassing the large submarine canyon 10 alvin dives indicated no venting on the formal thrust limited venting in the canyon but numerous biological communities along a scarp interpreted as the surface trace of the backthrust these observations suggest a scenario of vent and structural geomorphic development consisting of 1 frontal thrust faulting and associated venting facilitated by high fluid pressure 2 erosion of the oversteepened seaward flank of the ramp anticline assisted by seepage forces and leading to fluid flow out of stratigraphically controlled conduits in the limbs of the overthrust deposits 3 lockingmore of the frontal thrust due to dewatering or a local decrease in wedge taper associated with development of the large canyon leading to failure along the backthrust and 4 redirection of fluid flow by the backthrust thus within le 0 3 m y deformation of the relatively permeable sediments of the oregon margin results in stratigraphically controlled flow being partially captured by faults less

non double couple earthquakes of 1997 january in west bohemia czech republic evidence of tensile faulting summary a mixed family of double couple dc and non double couple non dc earthquakes was observed during the 1997 january earthquake swarm in west bohemia which is a geothermal region typical for the occurrence of earthquakes swarms the dc and non dc earthquakes occurred within a small area of less than 1 km 3 on faults of different orientations the stress in the area is determined from focal mechanisms and attains values plunge azimuth 1 33 156 2 48 20 3 23 262 with a shape ratio of r 0 69 0 15 the error in the stress directions plunge azimuth is up to 5 10 the absolute stress values were estimated to be 1 350 mpa 2 220 mpa and 3 162 mpa the lithostatic stress is 250 mpa the stress analysis shows that the dc earthquakes occurred on optimally oriented faults with high shear traction but the non dc earthquakes occurred on misoriented faults with low shear traction the percentage of the dc in earthquakes clearly correlates with the magnitude of shear traction high low dc is associated with high low shear traction this indicates that the type of faulting is probably controlled by tectonic stress and that the non dc earthquakes are not related to pure shear faulting but may combine shear and tensile faulting the tensile faulting is manifested by positive isotropic and compensated linear vector dipole components the tensile faulting is conditioned by high pore fluid pressure which was estimated to be approximately 244 mpa the fluid pressure is less than the lithostatic stress by 5 mpa the ratio of the fluid pressure to the maximum compressive stress is 0 7

the esmeraldas canyon a helpful marker of the pliocene pleistocene tectonic deformation of the north ecuador southwest colombia convergent margin

thermochronologic evidence of major tectonic denudation associated with detachment faulting northern ruby mountains east humboldt range nevada fission track ft thermochronology studies on igneous and metamorphic rocks from the ruby mountains east humboldt range metamorphic core complex provide important constraints on the timing and nature of major mid tertiary extension in northeast nevada rocks from within the ruby east humboldt detachment brittle ductile normal sense shear zone were analyzed the dominant lithology studied was variably mylonitic mafic orthogneiss nonmylonitic amphibolite from the top of the structurally lower migmatitic core and porphyritic biotite granodiorite from the oligocene circa 36 ma harrison pass pluton were also studied ft ages on apatite zircon and sphene except one 18 4 ma apatite from the basal portion of the detachment zone are concordant and range in age from 26 5 ma to 23 6 ma all ages overlap at 1 between 25 4 ma and 23 4 ma these data suggest that rocks of the upper and middle portions of the detachment zone cooled rapidly from temperatures above 285 c sphene closure temperature to below 70 c lower temperature limit of track stability in apatite near the beginning of the miocene minimum cooling rate of 40 c ma the length distribution of confined fission tracks in apatites from these exhumed middle crustal rocks is strikingly similar to those of rocks known to have cooled rapidly fish canyon ash flow tuff the lower part of the detachment zone as well as the underlying migmatitic core also cooled through sphene and zircon closure temperatures during this time interval but apparently only cooled to a temperature within the zone of partial annealing of apatite 70 130 c residence of these rocks in this zone led to a partial retention of fission tracks thus resulting in a reduced age confined track length distributions from apatite provide independent evidence of very rapid cooling but also indicate that the rocks of the lower detachment zone experienced a more protracted cooling history remained hotter longer than structurally higher levels of the zone ft data firmly establish the lower limit on the timing of mylonitization during detachment faulting in this area as 23 4 ma rapid cooling of the region is considered to reflect large scale tectonic denudation intracrustal thinning the vertical complement to crustal extension rocks originating in the middle crust 10 15 km were quickly brought near the surface along the ruby east humboldt detachment fault brittle ductile simple shear zone and juxtaposed against brittlely extended rocks deformed under upper crustal conditions

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

seismic moment assessment of earthquakes in stable continental regions ii historical seismicity summary this part i study in conjunction with part 11 develops a method to determine within specified uncertainty bounds the seismic moment and thus moment magnitude of all earthquakes of stable continental regions scr for which instrumental or intensity data exist its basis is polynomial regression analysis using a database of scr earthquakes with direct seismic moment determination the independent variables include modern teleseismic magnitudes and regional magnitudes part i and isoseismal areas or number of recording stations part 11 part i11 is an application of the methodology of parts i and i1 to several major historical earthquakes all data used in the regressions are assigned individual uncertainties estimated from the literature or from experience formal confidence limits 68 per cent or 95 per cent on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis the most complete development is for the teleseismic magnitudes m and mb for both the final regression for log m is a quadratic formula that closely emulates the relationship between amplitude magnitudes and log m expected from dislocation theory and source scaling arguments regressions are also derived for the regional magnitudes mlg and m because there are many scr events mainly pre 1964 that have no teleseismic magnitudes prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ao 18 0 28 range and from regional magnitudes in the ko 23 0 38 range over a wide magnitude band finally the methodology developed here is generic even though the database is specific application to plate boundary oceanic intraplate or active continental intraplate regions should be straightforward

intra crater sedimentary deposits at the haughton impact structure devon island canadian high arctic detailed field mapping has revealed the presence of a series of intra crater sedimentary deposits within the interior of the haughton impact structure devon island canadian high arctic coarse grained well sorted pale gray lithic sandstones reworked impact melt breccias unconformably overlie pristine impact melt breccias and attest to an episode of erosion during which time significant quantities of impact melt breccias were removed the reworked impact melt breccias are in turn unconformably overlain by paleolacustrine sediments of the miocene haughton formation sediments of the haughton formation were clearly derived from pre impact lower paleozoic target rocks of the allen bay formation which form the crater rim in the northern western and southern regions of the haughton structure collectively these field relationships indicate that the haughton formation was deposited up to several million years after the formation of the haughton crater and that they do not therefore represent an immediate post impact crater lake deposit this is consistent with new isotopic dating of impactites from haughton that indicate an eocene age for the impact event sherlock et al 2005 in addition isolated deposits of post miocene intra crater glacigenic and fluvioglacial sediments were found lying unconformably over remnants of the haughton formation impact melt breccias and other pre impact target rock formations these deposits provide clear evidence for glaciation at the haughton crater the wealth and complexity of geological and climatological information preserved as intra crater deposits at haughton suggests that craters on mars with intra crater sedimentary records might present us with similar opportunities but also possibly significant challenges

impact cratering at geologic stage boundaries the largest known cenozoic impact craters with the most accurately measured ages are found to correlate very closely with geologic stage boundaries the level of confidence in this result is 98 99 even under the most pessimistic assumptions concerning dating errors one or more large impacts may have led in at least some cases to the extinctions and first appearances of biotic species that mark many of the geologic stage boundaries

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

a long pollen sequence of neogene age alaska range abstract a thick sediment sequence 590 m of neogene age exposed in the nenana coal field alaska at a latitude of 63 5 n provides an extensive pollen and spore record spanning most of the miocene and probably part of the early pliocene the sediments of the upper usibelli group miocene and overlying nenana gravel pliocene are the basis of this study the flora from both leaves and pollen of the usibelli group connotes a vegetation similar to the mixed northern hardwood forest of northeastern united states and the northern part of east china northern korea and southern hokkaido the warmest pollen and leaf assemblages of the miocene occur in the suntrana formation in the lower third of the unit and these correlate very well with flora of the upper seldovian stage as the reference section on cook inlet which is of late early to early middle miocene age shifts in forest composition from suntrana formation sediments upper seldovian stage into the overlying sediments of the lignite creek and grubstake formations connote a cooling with minor changes of vegetation and climate these cooler phases are correlative with the homerian and clamgulchian stages of cook inlet mean annual temperature mat estimate based on fossil pollen and leaves from the lower suntrana formation is ca 9 c and summers were consistently warm and wet in the cooler phases of the late miocene the mat probably dropped to about 5 c

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

the 2004 mw 4 4 rotenburg northern germany earthquake and its possible relationship with gas recovery we study the 20 october 2004 m w 4 4 rotenburg wumme neuenkirchen earthquake located in a previously aseismic region in the northern german sedimentary basin we constrain the source parameter by using different techniques a possible relationship between this event the regional tectonic setting and local gas recovery is investigated different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km the source mechanism was oblique normal faulting on planes striking roughly north south an inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north consistent with the higher macroseismic intensities found toward the north in the region of hamburg compared with those at a similar distance toward the south in the region of hannover relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals comparison with seismicity triggered in the northern netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the m w 4 4 rotenburg event may be related to gas recovery online material focal mechanism and waveform fit

hydrothermally enhanced magnetization at the center of the haughton impact structure haughton is a 24 myr old mid size apparent diameter 23 km complex impact structure located on devon island in nunavut canada the center of the structure shows a negative gravity anomaly of 12 mgal coupled to a localized positive magnetic field anomaly of 900 nt a field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity datasets as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly petrography rock magnetic and petrophysical measurements were performed on the cores and revealed two different types of clast rich polymict impactites 1 a white hydrothermally altered impact breccia not previously observed at haughton and 2 a grey impact breccia with no macroscopic sign of alteration in the altered core gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts this altered core has a natural remanent magnetization nrm four to five times higher than materials from the other core but the same magnetic susceptibility their magnetization is still higher than the surrounding crater fill impact melt rocks x ray fluorescence data indicate a similar proportion of iron rich phases in both cores and an enrichment in silicates within the altered core in addition alternating field demagnetization results show that one main process remagnetized the rocks these results support the hypothesis that intense and possibly localized post impact hydrothermal alteration enhanced the magnetization of the clast rich impact melt rocks by crystallization of magnetite within the center of the haughton impact structure subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface

late quaternary stream incision and uplift in the forearc of the cascadia subduction zone western oregon documentation of a latest pleistocene earliest holocene episode of strath formation and fluvial aggradation in the oregon coast range provides a datum from which long term bedrock stream incision rates are determined variations in long term incision rates probably reflect cumulative differential uplift in the forearc of the cascadia subduction zone although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision patterns of differential incision are most striking near the latitude of newport where a steep gradient divides a region of higher rates 0 6 0 9 mm yr in the northern coast range from a region of lower rates 0 1 0 3 mm yr in the central coast range the steep incision gradient is nearly coincident with abrupt changes in marine terrace 80 125 kyr uplift rates the locations of quaternary faults and the southern flank of a saddle of low historic 40 70 years uplift the exact causes of these variable patterns of incision uplift are unknown analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the cascadia subduction zone such variations may be related to differences in seismic moment release during individual earthquakes to changes in plate geometry or rates of wedge accretion to segmentation of earthquake ruptures and or to deformation on active structures in the north american plate and accretionary wedge

upward delamination of cascadia basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux the cascadia convergent margin is a first order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion the near trench part of the accretionary prism offshore washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges multichannel seismic reflection data acquired on the orwell project allows us to study the structure and dynamics of the atypical frontal accretion processes we performed a kinematical and mechanical analysis of the frontal accretion structures and developed a dynamic coulomb wedge model for the landward verging backthrust formation backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous cascadia basin sediment succession entering the subduction zone and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation these complex deformation processes cause the passive upward delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits the deformation mechanism at the tectonic front of the cascadia margin is an immediate response to the strongly increased late pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought

relationship between the location of chemosynthetic benthic communities and geologic structure on the cascadia subduction zone chemosynthetic benthic communities which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor are widespread along the cascadia subduction zone these seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments the authors have used deep towed seismic methods including hydrophones mounted on alvin to examine the detailed geologic structure under two of these vent sites at one of the sites located on a seaward dipping thrust zone the benthic communities are associated with a disruption of the subsurface acoustic layering in the thrust zone it appears that at this site dewatering is occurring along fractures in the disrupted sediments which connect to permeable layers in the undeformed sediments and not along the main thrust fault the other site is located near the top of a ridge which has been uplifted by thrusting along a landward dipping thrust most of the benthic communities are found to exist at the outcrop of a steeply dipping unconformity between recent slope basin sediments and the older uplifted sediments with the unconformity serving as the fluid pathway underlying this unconformity at the most active dewatering location is a complexly deformed structuremore which appears to enhance the flow of fluid to the unconformity the source of the fluids could be the older uplifted sediments or the recent slope basin turbidites in either case the source is shallow less than about 1 km less