extrusion tectonics of the okhotsk plate northeast asia based on focal mechanism geologic and lineament data faulting in the chersky range northeast asia changes from right lateral transpression between north america and eurasia to left lateral strike slip between north america and the okhotsk plate the location of the okhotsk eurasia boundary is poorly constrained available data suggest this boundary is right lateral the okhotsk plate is being compressed and extruded from between north america and eurasia resulting in high levels of microseismic activity along the southern flanks of the chersky range

oblique convergence and deformation along the kuril and japan trenches the hypothesis that present day deformation within the southern kuril forearc is driven by oblique subduction of the pacific plate is tested using 397 horizontal slip directions derived from shallow thrust earthquakes from the kuril and japan trenches for the period 1963 1991 a simple two plate model fits the 397 slip vectors significantly worse than a model that permits strike slip motion of the southern kuril forearc relative to the overlying plate weighted mean slip directions along the southern kuril trench are systematically rotated toward the direction orthogonal to the trench which implies that the net convergence is partitioned into less oblique subduction and trench parallel displacement of the southern kuril forearc the angular discrepancy between the observed slip direction and the direction predicted by the nuvel 1 pacific north america euler vector implies that the southern kuril forearc translates 6 11 mm yr 1 to the southwest relative to the overlying north american plate these results are consistent with geologically geodetically and seismologically observed convergence at the leading edge of the forearc sliver in southern hokkaido and with previously inferred extension at the trailing edge of the sliver which is located at the bussol strait at 46 n the percentage of the oblique component of the rigid plate convergence vector that is transferred into forearc strike slip motion is only 30 which may be evidence that the hokkaido honshu bend in the trench hinders southwest forearc motion or that the greater strength of oceanic forearcs impedes motion along forearc strike slip faults

imprint of the north american plate in siberia revealed by gps 1 gps observations in east siberia combined with global observations collected 1995 2002 place constraints on the geometry and motions of the eurasian north american and pacific plates in east asia by comparing velocities relative to eurasia and to north america we conclude that east siberia to the east of the cherskiy range belongs to the north american plate hypothesized for three decades but not proven because of uncertainties with the plate boundary arising from the ambiguous seismicity smaller plates in east asia such as okhotsk and amurian can neither be resolved nor excluded by the gps velocities

syn and post eruptive volcanic processes in the yubileinaya kimberlite pipe yakutia russia and implications for the emplacement of south african style kimberlite pipes abstract the yubileinaya kimberlite pipe with a surface area of 59 ha is one of the largest pipes in the yakutian kimberlite province the devonian pipe was emplaced under structural control into lower paleozoic karstic limestone the pipe complex consists of several smaller precursor pipes which are cut by the large round main pipe while the precursor pipes show many features typical for root zones main pipe is younger cuts into the precursor pipes and exposes well bedded volcaniclastic sediments the maximum estimated erosion since emplacement is 250 m open pit mapping of a 180 m thick kimberlite sequence documents the waning phases of the volcanic activity in the kimberlite pipe and the onset of its crater infill by resedimentation three volcanic lithofacies types can be differentiated the deepest and oldest facies type is a massive volcaniclastic rock akb only accessible in drill core it is equivalent to tuffisitic kimberlite in south african pipes and thought to be related to the main volcanic phase which was characterized by violent explosions the overlying lithofacies type comprises primary and resedimented volcaniclastic sediments as well as rock avalanche deposits sourced from the exposed maar crater collar it represents the onset of sedimentation onto the crater floor during the waning phase of volcanic eruptions where primary pyroclastic deposition was contemporaneous with resedimentation from the tephra wall and the widening maar crater ongoing volcanic activity is also testified by the presence of a vertical feeder conduit marking the area of the last volcanic eruption clouds piercing through the diatreme this feeder conduit is overlain by the third and youngest lithofacies type which consists mainly of resedimented volcaniclastic material and lake beds during the sedimentation of this facies primary volcanic activity was only minor and finally absent and resedimentation processes dominated the crater infill the yubileinaya pipe complex exposes root zones contact breccias as well as diatreme and crater infill sediments it has all features typical of large south african style pipes and much can be learned from yubileinaya about the emplacement sequence and behaviour of these pipes emplacement of the pipe occurred over an extended time span with intermittent phases of volcanic quiescence and consolidation the akb reveals little direct evidence of what sort of emplacement process was dominant during the main period of volcanic activity there is neither textural evidence that violent degassing of a juvenile gas phase has caused pipe excavation nor that external water was present during the main phase of volcanic eruptions however there is clear evidence in rock textures that meteoric surface water was present during crater infill base surge deposits forming part of the bedded crater infill sequence indicate that water was present in the eruption clouds and hence the root zone of the pipe there is no reason to assume that groundwater did not also have access to the ascending magma during the main phase of volcanic activity that excavated the pipe and formed the akb

low vp vs ratios in the crust and upper mantle beneath the sea of okhotsk inferred from teleseismic pmp smp and sms underside reflections from the moho 1 teleseismic ground motion recordings of deep earthquakes often contain clear seismic wave reflections from the free surface above the source pp sp ss with isolated precursors pmp smp and sms produced by underside reflections from the moho the crust mantle boundary below the free surface reflection points the waveforms of the depth phases and their precursors are readily modeled providing constraints for steep angles of incidence on integrated two way crustal travel times and moho shear and compressional impedance contrasts above deep earthquake source regions we model surface reflection waveforms in stacks of three component long period wwssn and digital broadband seismograms for signals traversing the crust and upper mantle beneath the sea of okhotsk which overlies deep earthquakes in the subducted kurile slab the crust beneath most of the sea of okhotsk is 19 25 km thick indicating a submerged continental margin with minor crustal thinning toward the kuril back arc basin where thin oceanic crust is found modeling pmp and sms phases that sample the same subregion provides tight constraints on the vp vs ratio in the crust and uppermost mantle assuming common density structure and consistent crustal thickness for p and s waves the absolute velocities are not constrained by the data we find unusually low vp vs ratios in the range 1 6 1 7 for the uppermost mantle under the sea of okhotsk and comparably low average vp vs ratios for the crust in several regions the presence of fluids and extensive silica enrichment possibly involving low temperature veining are viable explanations for the anomalous vp vs ratios this may represent an important process of continentalization taking place landward from the volcanic arc in subduction zones

geodetic imaging of plate motions slip rates and partitioning of deformation in japan 1 interseismic deformation in japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces we use spherical linear block theory constrained by geodetic observations from densely spaced global positioning system gps stations to estimate plate motions fault slip rates and spatially variable interplate coupling on the japan kuril sagami and nankai subduction zones the reference model developed in this paper consists of 20 blocks produces a mean residual velocity magnitude of 1 84 mm yr at 950 stations and accounts for 96 of the observed interseismic deformation signal we estimate fault slip rates in excess of 15 mm yr along the niigata kobe tectonic zone and itoigawa shizuoka tectonic line through central japan confirming their hypothesized roles as major tectonic boundaries oblique convergence across the nankai trough is partitioned with 3 4 of the 30 mm yr of trench parallel motion accommodated by strike slip motion on the subduction interface and the remaining 1 4 accommodated by right lateral slip on the median tectonic line in contrast our models suggest negligible slip partitioning in eastern hokkaido where oblique slip on the japan kuril subduction interface accommodates all of the trench parallel component of relative plate motion inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes

sub volcanic development of kimberlite pipes evidence from the lace and voorspoed group ii kimberlites south africa abstract the lace and voorspoed kimberlites occur on the kaapvaal craton south africa and form part of the kroonstad group ii kimberlite orangeite cluster the lace kimberlite is composed of a main pipe and a satellite blind pipe the latter of which does not reach the current land surface 30 m below the current land surface and is not observed connecting with the main pipe at depth the main pipe increases in size from 100 m to 250 m in diameter at depth the voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers up to 200 m thick of resedimented volcaniclastic kimberlite rvk coherent kimberlite ck identified at all three pipes is described here in order to constrain their formation distinct volcaniclastic kimberlite vk at the lace main pipe is also described here and is characterised by a highly clastic fragmental quartz rich and mud rich interclast material the vk is interpreted to form by resedimentation of extra crater deposits into a deep 1600 m open vent the ck from both lace occurrences is characterised by coherent to globular segregationary textures generally high proportions of crustal xenoliths and the complete absence of karoo basalt xenoliths which are ubiquitous in vk infilling south african cretaceous kimberlites the increase in pipe size with depth associated with the occurrence of ck at the main pipe and the occurrence of ck within the satellite blind pipe strongly favours an interpretation of formation in a hypabyssal setting rather than by pyroclastic processes ck is also described from the voorspoed kimberlite where it occurs at depth along the south western margin of the pipe the ck is xenolith rich the xenolith population is entirely from the immediate sidewall lithologies and has an incipient globular segregationary texture the contact with the rvk in the centre of the pipe is sharp indicating that the ck was cross cut by a period of eruption prior to the formation of spatially associated rvk the pipe margins at similar depths along the northern and eastern sections of the pipe are characterised by the occurrence of contact breccias which have a gradational change to lower degrees of brecciation away from the pipe the ck at voorspoed is interpreted to form by the intrusion of magmatic kimberlite at depth which invades a previously formed contact breccia and incorporates large proportions of crustal xenoliths from the immediate sidewall the ck occurring at the lace and voorspoed kimberlite are interpreted to form in a sub volcanic hypabyssal setting during a stage of pre conditioning of the country rock prior to eruptive emplacement of the pipe a model for the development of a sub volcanic embryonic pipe is presented for the formation of ck at the kroonstad kimberlite cluster

active deformation of the continents whereas relatively narrow bands of deformation define oceanic plate boundaries in continental situations deformation is shown to be distributed over much broader zones the complex and heterogeneous nature of boundary regions between major plates is outlined with reference to the mediterranean and the himalayas relatively aseismic regions have traditionally been defined as microplates but although they are presently acting as rigid blocks undergoing slow deformation it is argued that this need not always have been the case and that deformation may simply have moved elsewhere broad zones of deformation surrounding rigid blocks are highlighted as fundamental to the analysis and interpretation of continental tectonics detailed theoretical consideration is given to the kinematics of distributed formation and the dynamics of continental deformation a j barker

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

paleomagnetism of cretaceous units of the mametchinskiy peninsula kuyul region northeastern russia implications for development and evolution of the northwest pacific basin abstract the northernmost part of the kamchatka peninsula of northeastern russia located along the northwestern margin of the bering sea consists of zones of complexly deformed accreted terranes progressing from the northwestern bering sea inland are the olyutorskiy ukelayat and koryak superterranes which were accreted to the okhotsk chukotsk volcanic plutonic belt ochvb during the campanian maastrichtian koryak to middle eocene olyutorskiy respectively to constrain the accretion paleolatitude of the koryak superterrane we paleomagnetically sampled a sedimentary series on the mametchinskiy peninsula at the mametchinskiy peninsula in the northeastern penzhinskaya guba 61 45 n 163 75 e a gently deformed well bedded section of fine grained lower to lower upper cretaceous turbidites the mametchinskaya and tylakrylskaya formations are exposed these strata which represent the lower part of the sedimentary cover of the terranes in this region and the forearc of ochvb were sampled at 39 sites three to seven samples per site within the ainyn terrane more than 1000 m of section of cenomanian turonian age was sampled at a basal locality sample groups i and ii sites 1 18 19 29 and at an upper locality of valanginian barremian age sample group iii sites 30 39 along the western shore of the peninsula thermal demagnetization and principal component analysis of the demagnetization data show lower temperature a and higher temperature b magnetic components although group iii samples did not display a coherent a component the a component of group i and ii samples was observed as a single polarity lower unblocking temperature component generally removed by 100 400 c this component failed the fold test at the 95 confidence level with respect to direction the a component is similar to both the present day field and axial geocentric dipole directions expected at this site the b component was observed during thermal demagnetization steps up to 580 c and was always of downward directed inclination coherence of bedding corrections within each section do not allow statistically meaningful fold tests within groups i ii or iii assuming the b component represents a cretaceous magnetization two overall models are proposed in the first model preferred with the highest clustering of directions k value 36 7 n sites 36 indicates significant poleward motion of the ainyn terrane observed paleolatitude m1 61 0 6 5 expected north america plate reference site paleolatitude e 74 0 3 5 in the second model no significant poleward displacement is implied m2 72 0 9 6

variations of p wave speeds in the mantle transition zone beneath the northern philippine sea using waveforms and travel times from deep earthquakes we constructed 16 seismic profiles each of which constrains the radial variation in vp over a small area beneath the northern philippine sea taken together the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high vp up to 3 faster than average earth models originally suggested by travel time tomography unlike travel time tomography which relies heavily on arrival times of the direct p phase we utilize the waveforms and move out of later arrivals that mainly sample the mantle transition zone of interest our results identify three important characteristics of the northern philippine sea anomaly that are distinct from previous results first being approximately a subhorizontal laterally uniform feature the anomaly is localized beneath the northwestern corner of the philippine sea within a region of approximately 500 500 km2 immediately east of the ryukyu arc second the anomaly is well constrained to occur in the lower portion of the transition zone extending all the way down to the 660 km discontinuity third the presence of such a distinct anomaly reduces the contrast in vp across the 660 km discontinuity from approximately 6 to 3 such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of subducted slab as negative buoyancy should rest the slab just above the 660 km discontinuity where resistance to subduction is expected from a negative clapeyron slope during the spinel mg fe perovskite transition

simulation of active tectonic processes for a convecting mantle with moving continents summary numerical models are presented that simulate several active tectonic processes these models include a continent that is thermally and mechanically coupled with viscous mantle flow the assumption of rigid continents allows use of solid body equations to describe the continents motion and to calculate their velocities the starting point is a quasi steady state model of mantle convection with temperature pressure dependent viscosity after placing a continent on top of the mantle the convection pattern changes the mantle flow subsequently passes through several stages eventually resembling the mantle structure under present day continents a extension tectonics and marginal basins form on boundary of a continent approaching to subduction zone roll back of subduction takes place in front of moving continent b the continent reaches the subduction zone the extension regime at the continental edge is replaced by strong compression the roll back of the subduction zone still continues after closure of the marginal basin and the continent moves towards the upwelling as a result the ocean becomes non symmetric and c the continent overrides the upwelling and subduction in its classical form stops the third stage appears only in the upper mantle model with localized upwellings

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

deformation of the japanese islands and seismic coupling an interpretation based on gsi permanent gps observations summary the entire area of the japanese islands has been covered by the permanent gps observation network of the geographical survey institute since 1994 in this paper we use a solution for the vectors of motion during 1995 for a selection of 116 stations to discuss the origin of the observed deformation field we refer the displacement field to eurasia using the vlbi determined motion of kashima and demonstrate that other choices such as the okhotsk or north american plates for north japan are not compatible with the data 1 yr gps velocities are much higher than geological constraints would allow because these short term measurements include transient elastic deformation however the good qualitative agreement between the observed geodetic deformation tensors and those inferred from active faults and earthquakes suggests that the quaternary permanent deformation is essentially the result of the transfer of part of the subduction induced elastic deformation into permanent plastic deformation we then compute the elastic deformation of the japanese islands caused by interseismic loading of the pacific and philippine subduction planes the geometry of the coupled zone and its downward extension are determined from the distribution of earthquakes for the pacific slab for the philippine slab we use the geometry proposed by hyndman et al 1995 these elastic models account for most of the observed velocity field if the subduction movement of the philippine sea plate is 100 per cent locked and if that of the pacific plate is 75 85 per cent locked we note that the boundaries of the areas where significant elastic deformation is predicted more than 10 mm yr 1 of motion with respect to eurasia coincide with the main zones of permanent deformation the eastern japan sea deformation zone for the pacific subduction elastic deformation field and the setouchi mtl deformation zone for the nankai field each zone probably accommodates 10 15 mm yr 1 of motion in the long term convergence in the eastern japan sea strike slip in the setouchi mtl zone to account for this deformation the evect of elastic loading from the trench must be combined with 5 10 mm yr 1 of motion of the amur plate with respect to eurasia because loading during the subduction earthquake cycle causes an increase in stress in the eastern japan sea and setouchi mtl deformation zones the probability of earthquake occurrence in these zones may be higher near the end of the cycle

experimental evidence for fluidization processes in breccia pipe formation results of small scale fiuidization spouting and channeling experiments suggest that similar processes may have been involved in the formation of many breccia pipe features various cylindrical parallel plate and boxlike model containers were filled with mixtures of particulate materials ranging from clay sized powdered kaolinitc to 1 cm gravel and each container was attached at its base to a large volume compressed air source features generated during the course of the various experimental runs are analogous to the majority of features observed in breccia pipes complex branching fracture systems developed in models containing abundant cohesive material appear to be similar to incipient crackle breccias steep walled funnel to cone shaped conduits formed readily and in some models inward dipping fractures ring fractures marginal shear folds and or downwarps and or high angle normal faults developed in adjacent material thorough mixing of heterogeneous particulate materials from throughout the stratigraphic column commonly was achieved and blocks of more cohesive materials were readily rounded and comminuted convective particle motion spouting was most effective in the mixing process and was accompanied by downwarping and fragmentation of adjacent strata downward migration of wall rock blocks along conduit margins and generation of annular zonation of conduit filling material in some systems annular size classification resulted in fine layering parallel to conduit walls crude centroclinally nested saucer bedding developed in some experiments where air fall ejecta were generated and these beds commonly were transported essentially intact downward within the pipe during a bubbling fiuidization event a gas streaming or channeling process in which finer material was elutriated or cleared out of coarser material led to the formation of channelways or chimneys which in natural systems would provide excellent avenues for introduction of later magma or hydrothermal solutions in model systems enriched in strongly cohesive clay sized particles accretionary pelletal spherules commonly were generated fluidization and related processes have been defined to describe highly controlled industrial systems problems of scale and strength of materials are significant when comparing experimental with geologic systems but the striking similarity of many features observed in both experimental and natural breccia pipe systems suggests that a similar genetic process is reasonable although many pipes clearly are not formed by fiuidization and related processes these processes may be important in the development of or some developmental stage of some breccia pipes

the economic implications of kimberlite emplacement abstract the economic potential of kimberlite bodies is defined by the volume and diamond content of all their internal units which are summarised in three dimensional geological models canadian geological models reveal distinct types of kimberlite bodies characterised by fundamentally different emplacement processes and products data for representative samples of canadian kimberlites are used to show that qualitative and quantitative macroscopic petrography olivines and xenoliths are powerful and practical techniques in assessing the economic implications of emplacement within individual phases of kimberlite kimberlite bodies result from the near surface emplacement of multiple discrete batches of mantle derived magmas each carrying different diamond populations and in some instances subsequent resedimentation processes magmas containing 25 modal of olivine macrocrysts the average maximum load of solids that can be carried to surface by a kimberlite magma have the greatest potential to be significantly diamondiferous modifications to the abundance and size distribution of the olivine macrocrysts in each batch of magma commonly occur during emplacement the modifications to the olivine macrocrysts which vary with different emplacement processes and products are mirrored in the macrodiamond content dilution by xenoliths is important in predicting diamond contents but it is also reflected in reduced olivine contents thus olivine macrocrysts can act as a proxy for macrodiamonds the abundance and size distribution data for olivine can improve the prediction of diamond grade and distribution within and between kimberlite units summary geological models and macroscopic petrography together improve the evaluation of newly discovered bodies resulting in enhanced resource estimates and increased degrees of confidence

the emplacement of class 1 kimberlites abstract class 1 kimberlites are distinct from class 2 and class 3 in that they are characterized by three zones namely root diatreme and crater class 2 and 3 kimberlites do not form diatreme zones filled with diatreme facies rocks consequently these kimberlites probably form by different processes within the root zones of class 1 kimberlites subvolcanic contact breccias occur at particular horizons within the immediate wall rocks these could be formed by hydraulic fracturing caused by crystallization induced exsolution of juvenile volatiles within the cooling hypabyssal kimberlite this exsolution may subsequently also lead to the formation of transitional facies kimberlites in the uppermost parts of extended columns or plugs of hypabyssal kimberlite from such settings break through to surface and explosive eruption is thought to occur and pipes having characteristic diatremes slope angles of 82 and 500 700 m deep craters are formed through these processes these kimberlites undergo fundamental textural changes to produce a variety of rocks from hypabyssal through transitional to diatreme and crater facies kimberlites

intraplate deformation in the japanese islands a kinematic study of intraplate deformation at a convergent plate margin relative motions within the deforming japanese islands with respect to the sea of japan are determined using earthquake records over the last 414 years slip rates on quaternary faults and angular change rates obtained from triangulation in the last century we use a least squares inversion method to match the strain rates on the surface of the earth with continuous spline functions to recover the velocity gradient tensors the directions of the principal strain axes obtained from seismic geological and geodetic data are in general agreement with each other with the maximum shortening axis oriented in a wnw direction principal strain rate magnitudes obtained from the geodetic data are about 3 5 times larger than the principal strain rates obtained from the seismic and geological data intraplate deformation in southwestern japan determined from the seismic data accommodates a velocity of5 5 2 l mm yr in a direction parallel to the nankai trough which is about 25 of the plate motion velocity component parallel to the nankai trough between the philippine sea and eurasian plates in northern honshu the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the japan trench and the intraplate deformation accommodates about 6 of the total pacific eurasian or north american plate motion velocity the velocity vector at kashima northern honshu calculated from the geodetic data is about 9 12 mm yr in a direction of n55 w n76 w which is about half of the very long baseline interferometry velocity calculated in the eurasian reference frame suggesting that the sea of japan may not be rigidly attached to the assumed rigid eurasian plate

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

satellite thermal observations of the bezymianny lava dome 1993 2008 precursory activity large explosions and dome growth 1 fifteen years worth of advanced very high resolution radiometer avhrr data is presented and used to quantitatively assess processes occurring at bezymianny this andesitic volcano is one of kamchatka s most dangerous volcanoes with 16 eruptions in the last decade that have dispersed ash into north pacific air routes all known episodes of increased activity for which data were available were detected in band 3 3 53 3 93 m avhrr thermal data twenty three peaks can be seen in the data nineteen peaks correspond to known explosions while the remaining three peaks correspond to known phases of dome growth that were not believed to have been accompanied by explosive activity start and end dates of extrusive phases defined by the thermal data are presented repose times between phases of extrusion vary from four months to just over two and half years with an average of just less than a year using rank order statistics a maximum time interval between consecutive explosions of 1288 170 days is determined this could serve as a cut off time for declaring the current dome growth activity over the calculated cumulative erupted volume 0 28 km3 and time averaged extrusion rate 0 6 m3 s 1 from 1993 to 2008 corresponds to values found at bezymianny from 1956 to 1976 showing that the satellite based methodology provides a good way of quantitatively assessing dome growth three different types of precursors to explosive behavior have been identified at bezymianny 1 values that cluster around the mode of the data set prior to explosion potentially due to endogenous dome growth 2 upward trends that commence 15 20 days prior to explosion and reach sensor saturation levels are due to significant extrusion and 3 a gradual upward trend that starts 5 days prior to explosion probably due to ramping up of extrusion this work shows that retrospectively analyzing and modeling of a volcano s thermal signal provides increased insight into its characteristic behavior the methods used in this paper can be used at other dome building volcanoes around the world the insights presented here can be used to improve monitoring capabilities to aid in providing early warnings to large explosions at bezymianny

the amurian plate motion and current plate kinematics in eastern asia we use global positioning system gps velocity data to model eastern asian plate kinematics out of 15 stations in korea russia china and japan studied here three sites considered to be on the stable interior of the hypothetical amurian plate showed eastward velocities as fast as 9 10 mm yr with respect to the eurasian plate they were stationary relative to each other to within 1 mm yr and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity euler vector of the amurian plate the predicted movement between the amurian and the north american plates is consistent with slip vectors along the eastern margin of the japan sea and sakhalin which reduces the necessity to postulate the existence of the okhotsk plate the euler vector of the amurian plate predicts left lateral movement along its boundary with the south china block consistent with neotectonic estimates of the displacement at the qinling fault possibly the southern boundary of the amurian plate the amurian plate offers a platform for models of interseismic strain buildup in southwest japan by the philippine sea plate subduction at the nankai trough slip vectors along the baikal rift the boundary between the amurian and the eurasian plates are largely inconsistent with the gps based euler vector suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies

geology of the gahcho kue kimberlite pipes nwt canada root to diatreme magmatic transition zones abstract the cambrian gahcho kue kimberlite cluster includes four main pipes that have been emplaced into the archaean basement granitoids of the slave craton each of the steep sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite hk to diatreme facies tuffisitic kimberlite breccia tkb the tkb displays many diagnostic features including abundant unaltered country rock xenoliths pelletal lapilli serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate the hk contains common fresh olivine set in a groundmass composed of monticellite phlogopite perovskite serpentine and carbonate a number of separate phases of kimberlite display a magmatic textural gradation from tkb to hk which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity proportion of fresh olivine and the degree of xenolith digestion the pipe shapes and infills of the gahcho kue kimberlites are similar to those of the classic south african pipes particularly those of the kimberley area similar intrusive magmatic emplacement processes are proposed in which the diatreme zone results from the degassing after breakthrough of the intruding magma column the transition zones represent frozen degassing fronts the style of emplacement of the gahcho kue kimberlites is very different from that of many other pipes in canada such as at lac de gras fort a la corne or attawapiskat

double saloon door tectonics in the japan sea fossa magna and the japanese island arc abstract a number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the japanese arc these include i a pair of terranes sw and ne honshu which rotated in opposite directions from 22 21 ma to 14 11 ma ii rotated terranes which comprise a retro arc fold thrust belt attached to an accretionary wedge intruded by a magmatic arc iii contemporaneous backarc extension from 24 to 21 ma which is brought to a halt by progressive collision of the izu bonin and japan arcs from 15 to 5 ma iv isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust during backarc rifting v such isolation may be due to simultaneous rifting or to progressively seaward rifts associated with ridge jumps towards the subduction zone vi opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30 26 ma to 16 15 ma vii concurrent development of a major arc orthogonal rift the fossa magna from 23 18 ma to 14 ma which was thereafter inverted from 15 ma to the recent viii a northeast propagating rift in the northern japan basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest pacific opposite rotational torques leading to opposite terrane rotations may be caused by rollback of a curved trench hingeline or by the divergent slab sinking forces of the pacific and philippine sea plates

canadian kimberlites geological characteristics relevant to emplacement in canada more than 770 kimberlites have erupted in diverse tectonic settings over a period of 1000 ma and over an area at least 5000 km across the kimberlites represent at least 30 comagmatic fields one pipe type dominates each of the different kimberlite fields even with the discovery of a significant number of new kimberlite fields this review of recent data substantiates the previous proposal that there are at least three distinct classes of kimberlite pipes which show a correlation with the nature of the country rocks into which they were emplaced new data show that a variation on the prairies pipe type occurs when kimberlites were emplaced into competent paleozoic sediments resulting in steeper sided pipes infilled with similar pyroclastic kimberlite new data for each pipe type are summarised and the contrasting characteristics of the three pipe types are considered to reflect fundamentally different styles of eruption and deposition apparently uniform near surface olivine and volatile rich kimberlite magmas were modified differently during the contrasting emplacement processes resulting in distinct textural rock types pyroclastic kimberlite pk tuffisitic kimberlite tk and associated hypabyssal kimberlite hk or resedimented kimberlite rvk dominating each type of pipe the substantiated correlation of pipe type with country rock geology could indicate variable constraints on volatile exsolution which affected the nature of the magmatic eruptions in different settings

transient and permanent deformation of central japan estimated by gps 1 interseismic loading and subduction kinematics abstract we analyze continuous global positioning system gps data to estimate the kinematics of the philippine sea plate subduction below southern central japan using a back slip dislocation model we invert the strain rate tensors derived from the gps velocity field to obtain the velocity of the downgoing slab with respect to the tokai and kanto forearcs along the eastern nankai suruga and sagami troughs respectively in both cases these velocities are smaller and slightly rotated with respect to the philippine sea plate subduction velocity in central nankai this result implies strain partitioning between the subduction thrusts and on land and or offshore deformation zones in both the eastern nankai suruga and the sagami area in southern kanto we predict 20 mm yr 1 of dextral shear in agreement with motion observed along the boso strike slip fault in the tokai region the slip deficit accumulated since the last earthquake 1854 ranges between 6 and 3 m considering the extent of the locked zone it would correspond to a magnitude m w 8 15 earthquake if the slip deficit were to be released now during a single event

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

tuffisitic kimberlites from the wesselton mine south africa mineralogical characteristics relevant to their formation abstract tuffisitic kimberlites from the wesselton mine consist in order of formation of the following primary components chloritized olivine macrocrysts and phenocrysts magmaclasts cryptocrystalline diopside phlogopite rich mantles and a smectite chlorite interclast matrix magmaclasts consist of one to several crystals of chloritized olivine set in a microcrystalline groundmass of diopside apatite perovskite spinel and chloritized and fresh phlogopite the latter commonly rimming chloritized olivines magmaclasts have some similarities to holocrystalline hypabyssal kimberlite but lack monticellite carbonate carbonate serpentine segregations and atoll spinels spinels in the magmaclasts show only a limited compositional evolution relative to spinels in spatially associated hypabyssal kimberlite pre existing solids including discrete olivine grains magmaclasts and most xenoliths are mantled by acicular diopside and phlogopite the interclast matrix is now represented by mixed layer phyllosilicates chlorite smectites that are poorer in alumina and iron than chlorite pseudomorphing olivine and microlitic phlogopite and diopside the interclast chlorite smectite is considered to represent former phlogopite which has undergone late stage deuteric hydrothermal like modification the interclast matrix crystallized from the volatile rich remnants of the magma none of the constituents of tuffisitic kimberlites including the chlorite or chlorite smectites represent material formed from externally derived fluids these primary textures are unique to kimberlites and only form in certain circumstances tuffisitic kimberlites formed by progressive crystallization and volatile exsolution induced segregation and or disruption within a kimberlite magma magmaclasts mantles interclast matrix during a continuum of rapidly changing conditions in a subsurface subvolcanic magmatic system this continuum represents a transition from a degassing magmatic system to a magmatic derived hydrothermal like fluid rapid drops in temperature with extensive exsolution of co 2 and incorporation of large amounts of locally derived country rock xenoliths resulted in explosive diatreme formation and within diatreme modification of the crystallization and deuteric replacement sequence of typical kimberlite magma resulting in a distinctive mineralogy and texture

energy release in subvolcanic environments implications for breccia formation mechanical pdelta v energy is released from hydrous magmas in subvolcanic environments by the two generally sequential processes of second boiling and subsequent decompression the second boiling process h 2 o saturated melt crystals h 2 o vapor releases sufficient pdelta v energy in the model system previously established by burnham 1979 and burnham and ohmoto 1980 to cause fracture failure of even the strongest wall rocks tensile strength approximately 3 77 x 10 10 ergs kg super 1 to depths of 4 or 5 km following fracture failure moreover expansion of already exsolved h 2 o and exsolution of additional h 2 o upon decompression into the model fracture system releases additional pdelta v energy 9 7 x 10 10 ergs kg super 1 of magma sufficient to lift an equivalent mass of rock nearly 1 km in the absence of frictional resistance this latter energy although a conservative value is model dependent to the extent that it is affected by changing any one of the following six independent model parameters 1 bulk composition of the magma hornblende biotite granodiorite in the model 2 depth of emplacement of the magma body approximately 2 3 km p t 0 6 kb 3 mass fraction of h 2 o in the initial melt f o w 0 027 2 7 wt h 2 o 4 mass fraction of melt in the magma at the time of emplacement f o m 1 0 100 liquid 5 tensile strength of the wall rocks 3 77 x 10 10 ergs kg super 1 and 6 depth to the top of the fracture once formed approximately 2 3 km p f 0 3 kb although all of these parameters except the first are of major importance in the formation of hydrothermal breccias generally the mass fraction of h 2 o in the initial melt f o w is regarded as the single most important factor in the formation of breccia bodies in typical porphyry type systems f o w not only determines primarily whether or not the second boiling reaction releases enough pdelta v energy to produce fractures it also determines the magnitude of the decompressional energy released into the fractures if formed magmas in which f o w 0 02 to 0 04 at liquidus temperatures generally release sufficient pdelta v energy not only to produce extensive porphyry type fracture systems but also to produce the breccia bodies these systems commonly contain magmas in which f o w 0 02 on the other hand generally do not release sufficient energy for extensive breccia formation except in structurally weak wall rocks at depths shallower than approximately 2 km magmas in which f o w raquo 0 04 at near liquidus temperatures are incapable of emplacement in the subvolcanic environment

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

structure of the kuril trench from seismic reflection records three multichannel seismic reflection records across the kuril convergent margin provide the first deep data in this area the records are located across the southern tip of kamchatka the central kuril arc and the northern extension of hokkaido island platform which show three distinct tectonic regimes the lower slope contains a wedge shaped buttress surrounded by low velocity sediments underplating sediment uplifts the buttress as indicated by faults that displace its upper surface the middle slope is a block of acoustic basement which has a rough surface with significant arcward dipping faults the middle slope is separated from the upper slope along a steep arcward dipping reflection the middle slope boundary the upper slope structure off kamchatka is different from that off hokkaido off kamchatka a regular stratified sediment section has been uplifted tilted and dips seaward off hokkaido stronger uplift has tilted regional blocks seaward and arcward along the southern line north of hokkaido well studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate thrust earthquakes occur under the middle and upper slope whereas tsunami earthquakes occur under the lower slope high amplitude reflections along the lower boundary of the wedge shaped buttress and along the active decollement indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling slow rupture may extend up to the seafloor from a tsunami earthquake

contemporary crustal deformation within the pamir plateau constrained by geodetic observations and focal mechanism solutions we used an updated data set of 192 gps derived surface velocities and 393 earthquake focal mechanisms mw 3 0 hypocenter depths 30 km to evaluate the spatial variations in the surface strain rate and crustal stress regime throughout the pamir plateau and its surrounding regions the strain rate field was estimated using the spline in tension approach that solves for the surface velocity in a rectangular grid and the stress field was predicted from focal mechanism solutions using the damped regional scale stress inversion drssi method of hardebeck and michael journal of geophysical research https doi org 10 1029 2005jb004144 2006 the results show that the crustal stress field around the pamir plateau is predominantly characterized by nnw sse compression and e w extension which is consistent with the principal orientations of the two dimensional surface strain rate tensor this agreement supports the notion that the pamir and southwestern tien shan are uniformly strained blocks in particular the fan shaped rotational pattern between text shmax and the strain rate from the western pamir to the tajik basin shows that the counterclockwise rotation of the text shmax orientation is associated with vertical deformation which is consistent with the idea of schurr et al tectonics 33 8 2014tc003576 2014 concerning the gravitational collapse and westward extrusion of the crust in the western pamir we propose that such a stress strain pattern dominated by nnw ese oriented compression and e w trending extension originated from a combination of the northward push of the indian continent and the southward subduction of the tien shan

distinct kimberlite pipe classes with contrasting eruption processes abstract field and scott smith field m scott smith b h 1999 contrasting geology and near surface emplacement of kimberlite pipes in southern africa and canada proc 7th int kimb conf eds gurney et al 1 214 237 propose that kimberlite pipes can be grouped into three types or classes classical or class 1 pipes are the only class with characteristic low temperature diatreme facies kimberlite in addition to hypabyssal and crater facies kimberlite class 2 and 3 pipes are characterized only by hypabyssal and crater facies kimberlite in an increasing number of class 1 pipes a new kimberlite facies transitional facies kimberlite is being found in most cases this facies forms a zone several metres wide at the interface between the hypabyssal and diatreme facies the transitional facies exhibits textural and mineralogical features which are continuously gradational between the hypabyssal and the diatreme types the textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside both transitional and diatreme facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially co 2 rich volatiles from the volatile rich kimberlite magma the transitional facies is initiated by volatile exsolution at depths of about 3 km below the original surface with subsequent cracking through to the surface and resultant rapid decompression the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies thus diatreme facies kimberlite and class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism distinctly different petrographic features characterize crater facies kimberlite in each of the three pipe classes in crater facies kimberlites of class 1 pipes small pelletal magmaclasts and abundant microlitic diopside are characteristic these features appear to reflect the derivation of the crater facies material from the underlying diatreme zone most class 2 pipes have shallow craters and the crater facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli which are sometimes welded and have vesicles as well as glass possible kimberlite lava also occurs at two class 2 pipes in n angola the possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form class 2 pipes most class 3 kimberlites have very steep craters and crater facies rocks are predominantly resedimented volcaniclastic kimberlites in some cases characterized by the presence of abundant angular magmaclasts which are petrographically very similar to typical hypabyssal facies kimberlite found in class 1 pipes the differences in crater facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition specifically volatile composition kimberlite forming pipe classes 1 and 3 is thought to be relatively water rich and is emplaced by processes involving magmatic exsolution of volatiles the kimberlite magma forming class 2 pipes is co 2 rich can rise to shallow levels and can initiate phreatomagmatic emplacement processes

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

the pimenta bueno kimberlite field rondonia brazil tuffisitic kimberlite and transitional textures abstract the pimenta bueno kimberlite field of rondonia state brazil is located near the edge of the amazon craton and contains more than thirty bodies a late to mid triassic emplacement age has been obtained for two kimberlites the pimenta bueno kimberlite field includes pipes of up to 15 ha and a few apparently flat lying hypabyssal kimberlite sills in the three pipes and one sill examined three textural end member types of kimberlite are recognized hypabyssal kimberlite hk tuffisitic kimberlite tk and resedimented volcaniclastic kimberlite rvk there are gradations in textures between the three types the overall gradation with depth is vkb vk tk tkt hkt to hk where t denotes transitional varieties and the textural gradations are apparent at all scales megascopically to microscopically importantly these gradations appear to occur within single phases of kimberlite tk and the associated transitional varieties observed in the pimenta bueno field have striking similarities to those of many kimberlites of southern africa and canada the pimenta bueno kimberlite field thus represents a newly recognized example of southern african style kimberlite pipes occurring on a third continent the tk bearing pipes of pimenta bueno southern africa and canada are markedly different from those found in the canadian prairies and lac de gras northwest territories which indicates that pipes containing tk and related textures are repeated in space and time but only form in certain circumstances there is an apparent correlation between kimberlite texture and pipe size in the larger pipes 5 ha the tk is overlain by vkb in the small pipes

travel times from earthquakes near southern kuril and their implications for the fate of subducted lithosphere abstract analysis of travel time residuals from deep and intermediate focus earthquakes along the kuril kamchatka arc has been a major impetus in advancing the hypothesis of deep slab penetration the interpretation of travel times however has been controversial the quality of arrivals reported in bulletins has also been debated using both digital and analog seismograms we precisely measured approximately 400 arrival times of teleseismic p phases for seven earthquakes most of which occurred since 1987 near southern kuril travel times from many of these events have not been analyzed by other researchers including a unique pair of shallow and deep focus earthquakes beneath the island of sakhalin in general differences between our precisely measured p arrivals and those reported by the international seismological center isc are within 0 4 s although isolated large differences do exist at common stations large differences in travel time residuals up to 1 5 s are observed between one of the deepest earthquakes in northern kuril and a deep focus event beneath the island of sakhalin located some 150 km farther west of the well defined wadati benioff zone in southern kuril significantly faster travel times up to 1 2 s or more are also observed along north northeast and south southwest azimuths for the deep focus event in northern kuril than for a shallow focus earthquake west of the sakhalin island where the effect of subducted lithosphere should be minimal in both cases the observed differential residuals between earthquakes in northern and southern portions of the arc are too large to be caused by epicenter mislocation since differential travel time residuals largely reflect differences in near source structure seismic wave speeds below depths of approximately 650 km seem to vary along this arc the magnitude of slab like anomalies in the lower mantle beneath sakhalin is probably two to three times smaller than that in northern kuril kamchatka if the pattern of travel time anomalies observed for deep focus earthquakes in northern kuril kamchatka is caused by coherent subducted slab in the lower mantle wholesale slab penetration into the lower mantle probably did not occur beneath the sakhalin there also appears to be a correlation between the state of strain within subducted lithosphere in the upper mantle and the extent of slab penetration into the lower mantle along southern kuril a net down dip extensional strain is observed within the slab at intermediate depths and persists to an unusual depth of approximately 450 km on the other hand in the northern kuril kamchatka down dip compressional strain dominates at all depths in the subducted lithosphere

experimental birth of a maar diatreme volcano abstract maar diatreme eruptions are hazardous to people and infrastructure and are also linked to the formation of the kimberlitic variety of diatremes which is important economically processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood we conducted field scale experiments using analog materials to shed more light on these processes especially the formation of the proto diatreme during the first explosions of a maar eruption specifically a series of buried explosions in a prepared layered substrate pad produced craters extra crater deposits and sub crater deposits analogous to volcanic maar craters tephra rings and incipient diatremes post explosion substrate excavation revealed that single large explosions produce sub crater deposits extending nearly to the crater rim crest the same energy divided into three blasts either co located or at different depths with the same epicenter produced narrower and sometimes deeper sub crater deposits even though the final sizes of the craters were similar to that produced by the single large blast the sub crater deposits have an upper zone with domains from different substrate depths and an underlying zone distinguished primarily by being more loosely packed than the original substrate videos show surface motion extending beyond the post shot crater rim and largely vertical ejection and fallback of material into the footprint of these deposits especially for the explosions that occurred below optimal depth of burial we infer that much of the loosely packed material was disassembled vertically transported to different heights during the explosions then fell back without significant relative lateral movement of grains however subvertical fallback did produce apparent cross cutting structures in shallow sub crater deposits one explosion ejected material from the deepest substrate horizon but it was redeposited only within the crater and is unrepresented in the ejecta rim implications of the experiments for maar diatreme volcanoes including some kimberlite pipes are as follows 1 vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes 2 direct vertical fallback is possibly an important process forming diatreme deposits especially during the earliest stages and 3 even in our limited simulation the number and scaled depth of explosions clearly affect proto diatreme size and structure

extension across the laptev sea continental rifts constrained by gravity modeling the laptev shelf is the area where the gakkel ridge an active oceanic spreading axis approaches a continental edge causing a specific structural style dominated by extensive rift structures from the latest cretaceous to the pliocene extension exerted on the laptev shelf created there several deep subsided rifts and high standing basement blocks to understand syn rift basin geometries and sediment supply relationships across the laptev shelf accurate extension estimates are essential therefore we used 2 d gravity modeling and 3 d gravity inversion to constrain the amount of crustal stretching across the north america eurasia plate boundary in the laptev shelf the latest cretaceous cenozoic extension in that area is partitioned among two rift zones the laptev rift system and the new siberian rift these rifts were both overprinted on the eurasian margin that had been stretched by 190 250 km before the late cretaceous while the laptev rift system connected to the gakkel ridge reveals increasing extension toward the shelf edge 190 380 km the new siberian rift is characterized by approximately uniform stretching along strike 110 125 km the architecture of the laptev rift system shows that the finite extension of about 500 km is sufficient to entirely eliminate crystalline continental crust in the most stretched rift segment continental mantle is exhumed at the base of the late mesozoic basement the example of the laptev rift system shows that extension driven by divergent plate movement is a sufficient cause to produce almost complete continental breakup without an increased heat input from the asthenospheric mantle

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

the 12 july 1993 hokkaido nansei oki japan earthquake effects of source complexity on surface wave radiation abstract we present an analysis of the data dependency of surface wave inversions for source parameters of the 12 july 1993 hokkaido nansei oki japan earthquake this dependency is demonstrated by differences between source mechanisms obtained from different data sets particularly from rayleigh waves or love waves although inversions of various data sets yield unanimously thrust fault mechanisms for the inversion of rayleigh waves the mechanism is characterized by a shallow dipping west plunging nodal plane which is consistent with p wave first motion data and the distribution of aftershocks close to the hypocenter however for the inversion of both rayleigh and love waves the mechanism is characterized by a shallow dipping east plunging nodal plane several numerical experiments were carried out to test a hypothesis that the data dependency is attributed to the effects of source complexity on the surface wave radiation with the source complexity being characterized by tempo spatial changes in focal mechanism and moment release during the rupture process the experiments show that source parameters obtained from inversions of surface waves with periods as long as a few hundred seconds can be largely affected by using various models of source complexity proposed previously for the hokkaido earthquake and the effects on love waves tend to be systematically different from those on rayleigh waves our preferred source model is characterized by significant tempo spatial changes in focal mechanism the experiments based on this model provide a good explanation for the data dependency although our analysis cannot completely reject the alternative explanation that attributes the dependency to the manifestation of errors in modeling the surface wave propagation and or inversion instability the model is consistent with the data of body waves and long period surface waves for a very broad frequency band implying the existence of weak zones with various fault geometries along the eastern margin of the japan sea

palaeomagnetic and chronological study of miocene welded tuffs in the northern part of central japan tectonic implications for the latest stage of arc formation of japan abstract kinematical adjustment of tectonic blocks has occurred in the latest stage of arc formation of japan within central japan miocene welded tuffs have been sampled for chronological and palaeomagnetic studies from the northern part of central japan zircon fission track dating confirms middle to late miocene ages 13 7 5 0 ma for the yatakezawa shiobara kawaji kinugawa katashinagawa okkai and ashio formations characteristic directions with high unblocking temperature component above 560 c are isolated from 36 sites palaeomagnetic directions after tilt correction provide a miocene characteristic palaeomagnetic direction of the northern part of central japan 36 8 n 139 3 e d 7 3 i 50 3 95 6 6 presence of normal and reversed polarities and a fairly long time average exclude the effect of geomagnetic field secular variation comparison with the miocene palaeomagnetic poles of the neighbouring blocks indicates that the northern part of central japan and northeast japan behaved as a unified block since 15 ma whereas the kanto mountain region moved kinematically as an independent block from the northern block comprised of central japan and northeast japan at least between 16 ma and 12 ma the boundary between the northern part of central japan and the kanto mountain was in a convergent regime with dextral strike slip a southward force exerted from the northern part of central japan as well as a northward force by the izu bonin ridge induced the clockwise rotation of the kanto mountain region the northern part of central japan had welded completely with the kanto mountain region by 6 ma

on the welding of pyroclasts from very low viscosity magmas examples from kimberlite volcanoes abstract lithofacies in kimberlite pipes in southern africa exhibit features consistent with welded pyroclastic rocks these include flared conduit filling geometries abundant lithic clasts lithic clast layers subhorizontal clast fabrics gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash and lapilli and melt coated particles the welding dynamics of kimberlite pyroclasts differ from those of glassy vesiculated pyroclasts in silicic volcanic systems low melt viscosity 0 1 pa s results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets the glassy state is difficult to form in silica deficient magmas because the crystallization kinetics are fast in low viscosity melts three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition 1 densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure mel

secular crustal deformation and interplate coupling of the japanese islands as deduced from continuous gps array 1996 2001 abstract data from the nation wide gps continuous tracking network that has been operated by the geographical survey institute of japan since april 1996 were used to study crustal deformation in the japanese islands we first extracted site coordinate from daily sinex files for the period from april 1 1996 to february 24 2001 since raw time series of station coordinates include coseismic and postseismic displacements as well as seasonal variation we model each time series as a combination of linear and trigonometric functions and jumps for episodic events estimated velocities were converted into a kinematic reference frame heki k 1996 horizontal and vertical crustal movements from three dimensional very long baseline interferometry kinematic reference frame implication for reversal timescale revision j geophys res 101 3187 3198 to discuss the crustal deformation relative to the stable interior of the eurasian plate a least squares prediction technique has been used to segregate the signal and noise in horizontal as well as vertical velocities estimated horizontal signals horizontal displacement rates were then differentiated in space to calculate principal components of strain dilatations maximum shear strains and principal axes of strain clearly portray tectonic environments of the japanese islands on the other hand the interseismic vertical deformation field of the japanese islands is derived for the same gps data interval the gps vertical velocities are combined with 31 year tide gage records to estimate absolute vertical velocity the results of vertical deformation show that 1 the existence of clear uplift of about 6 mm yr in shikoku and kii peninsula whereas pattern of subsidence is observed in the coast of kyushu district this might reflect strong coupling between the philippine sea plate and overriding plate at the nankai trough and weak coupling off kyushu 2 no clear vertical deformation pattern exists along the pacific coast of northeastern japan this might be due to the long distance between the plate boundary japan trench and overriding plate where gps sites are located 3 significant uplift is observed in the southwestern part of hokkaido and in northeastern tohoku along the japan sea coast this is possibly due to the viscoelastic rebound of the 1983 japan sea mw 7 7 and the 1993 hokkaido nansei oki mw 7 8 earthquakes and or associated with distributed compression of incipient subduction there we then estimate the elastic deformation of the japanese islands caused by interseismic loading of the pacific and philippine sea subduction plates the elastic models account for most of the observed horizontal velocity field if the subduction movement of the philippine sea plate is 100 locked and if that of the pacific plate is 70 locked however the best fit for vertical velocity ranges from 80 to 100 coupling factor in southwestern japan and only 50 in northeastern japan since horizontal data does not permit the separation of rigid plate motion and interplate coupling because horizontal velocities include both contributions we used the vertical velocities to discriminate between them so we can say there is strong interplate coupling 80 100 over the nankaido subduction zone whereas it is about 50 only over the kurile japan trench

transient and permanent deformation of central japan estimated by gps 2 strain partitioning and arc arc collision abstract deformation of central japan can be divided into two main components elastic deformation due to the interseismic locking of subduction thrusts and deformation associated with intra arc tectonics in a companion paper henry et al earth planet sci lett 2000 this issue we estimated an elastic deformation field based on the modeling of global positioning system gps strain rates in this study we show that the corrected field gps minus predicted elastic deformation is consistent with geological and seismological observations and should represent long term plastic deformation we use this corrected gps deformation field to describe the kinematics of partitioning of the philippine sea plate amurian plate amu convergence in relation with the izu bonin arc collision about one third of the relative philippine sea amur motion is transferred to the central japan crust north of the collision front this transfer induces high strain rates in southern kanto and in the northern japanese alps the latter region corresponds to the eastern boundary of the amu we propose that the independent motion of central japan results from the delamination and the underthrusting of the izu bonin arc lower crust and the upward bent oceanic slab

hanging wall vectoring for buried volcanogenic massive sulfide deposits paleoproterozoic flin flon mining camp manitoba canada the paleoproterozoic flin flon mining district is one of the world s most prolific volcanogenic massive sulfide vms camps and includes a single stratigraphic interval that hosts the 85 5 million tonne mt flin flon 777 and callinan zn cu au deposits rapid seafloor burial of the vms hydrothermal system by a thick succession of pillowed basalt resulted in the hanging wall strata being affected to varying degrees by the still upward migrating fluids this hanging wall alteration hydrothermal fingerprint allows delineation of the regionally metamorphosed paleohydrothermal system and its characterization has the potential to lead to discovery of buried stacked or structurally displaced mineralization evidence for the presence of continued seafloor hydrothermal activity above the flin flon callinan vms horizon is observed in the pillowed flows interlayered hyaloclastite rich flow tops and also within finely bedded interflow volcaniclastic sediment a 30 to 60 metalliferous exhalative component was detected through geochemical and mineral analysis in interpillow volcaniclastic rocks chert and epidosite in the hanging wall sequence the regional distribution of fe and mg rich chlorite epidote clinozoisite biotite annite actinolite hornblende ferrotschermakite and stilpnomelane and albite oligoclase modifies metamorphic isograds and defines discrete vertical fluid pathways controlled by synvolcanic growth faults and associated sill dike swarms silica enriched hanging wall alteration zones are proximal to fe ti basalt sills and occur as discrete hanging wall zones parallel to the plunge of the 62 mt flin flon deposit anomalous concentrations of hg sb ag pb te as au and bi form within these hanging wall halo alteration zones indicating migration of the more volatile metals present in the underlying vms deposits synvolcanic depressions dike swarms and hydrothermal metamorphic fluid corridors are detectable through trace element anomalies trace mineral chemistry and 18o isotope geochemistry oxygen isotope analysis of the flin flon 777 callinan vms hanging wall strata defines a number of high o18 anomalies extending 1 200 m above that indicate that 300 c subseafloor hydrothermal activity continued after burial of the massive sulfide deposits coupled with the geochemical and mineral chemical anomalies this is indicative of the presence of continued relatively low temperature hydrothermal fluid leakage from a robust seafloor hydrothermal event that generated the vms deposits a combination of techniques including mineral chemistry isotope and trace element data is demonstrated to be successful in identifying and delineating zones of hanging wall hydrothermal alteration in greenschist to amphibolite grade metamorphic rocks of the flin flon mining camp use of these coupled with mapping to define periods of quiescence as marked by horizons of sedimentary rocks in the hanging wall basalts of the hidden formation has the potential to lead to discovery of deeply buried deposits on the flin flon horizon or deposits at higher stratigraphic levels our findings indicate that the basaltic hanging wall on the flin flon 777 callinan hydrothermal system was an efficient cap on the system with vestiges of continued hydrothermal fluid flow detected in the interpillow and interflow components these volumetrically minor components are critical sampling media and are pertinent to global exploration for detection of vms mineralization buried beneath thick mafic volcanic sequences

the geology and emplacement of the volcaniclastic infill at the voorspoed group ii kimberlite orangeite pipe kroonstad cluster south africa abstract the voorspoed group ii kimberlite pipe is atypical in terms of south african kimberlite pipes located in the central region of the kaapvaal craton south africa the voorspoed pipe is one of six pipes comprising the kroonstad kimberlite cluster reconstruction of the palaeo stratigraphy at the time of emplacement of the kroonstad kimberlites indicates that significant post emplacement erosion has occurred and the pipes are currently exposed at 1600 m depth from the original land surface the volcaniclastic kimberlite vk infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other south african type kimberlite pipes three textural volcaniclastic kimberlite varieties are observed massive volcaniclastic kimberlite mvk bedded volcaniclastic kimberlite bvk and fine grained volcaniclastic kimberlite fgvk layers the bvk and fgvk are volumetrically insignificant and the bulk of the pipe is infilled with mvk the mvk can be further sub divided into seven varieties the two dominant varieties are described in detail which include main mvk mmvk and the juvenile rich mvk jrmvk the mvks are typically massive clastic poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud rich matrix magmaclasts within the mvks are spherical crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed bvk units are composed of several normally graded beds of volcaniclastic material fgvk layers are well sorted relative to the mvks and all constituents are typically the evidence suggests that the volcaniclastic infill has formed through complex processes involving re working of extra crater tephra back into the vent and coeval pyroclastic deposition it is also likely that there is significant mixing of the resedimented and pyroclastic deposits distinction between deposits formed through re working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re working extra crater tephra from the surface with possible coeval pyroclastic eruption is probably impossible for this reason we interpret the mmvk as being deposited by dominantly resedimentation of tephra whereas the jrmvk has been deposited dominantly by pyroclastic processes the presence of bvk and fgvk deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater 2000 m in depth was periodically open the identification of the complex layered variable internal structure of the volcaniclastic infill at the voorspoed has significant implications for diamond ore evaluation in south africa typical south africa kimberlite pipes are infilled with tuffisitic kimberlite which are characterised by their homogenous nature over large vertical intervals 1000 m this is not the case at the voorspoed pipe where mvk layers are variable in thickness

viscosity of high crystal content melts dependence on solid fraction 1 the rheological properties of suspensions containing high solid fractions are investigated attention is focused on viscosity of silicate and magmatic melt systems a general empirical equation which describes the relative viscosity of suspensions as a function of suspended solid fraction is proposed in the limit of very dilute solid concentrations it reduces to the einstein equation the proposed relationship is satisfactorily applied to reproduce available experimental data relative to silicate melts moreover the extrapolation of the model to very high concentrations is compared with experimental observations on partially melted granite

interseismic horizontal deformation in northern honshu and its relationship with the subduction of the pacific plate in the japan trench we investigate the origin of the geodetically observed interseismic horizontal deformation in northern honshu by comparing shear strain rates principal strain rates and velocity fields determined from geodetic data with those calculated from the elastic dislocation models involving interplate motion at the japan trench the agreement between the observed and predicted directions of the principal strain axes indicates that the geodetic strain field in northern honshu is primarily elastic strain transmitted from the japan trench in order to match the strain rate tensors and velocity magnitudes obtained from the geodetic data the dislocation model requires that 35 to 60 of the nuvel1 a pacific north american plate motion is locked at the plate interface along the japan trench the down dip depth limit of the locked zone is inferred to be 55 km which is consistent with the seismic data in the japan trench

deformation of an island arc rates of moment release and crustal shortening in intraplate japan determined from seismicity and quaternary fault data the historical record of large m 6 9 earthquakes and geologically determined rates of slip on quaternary faults in intraplate japan honshu and shikoku are used to estimate the average rate of seismic moment release m for the last 400 years and during the late quaternary respectively values of m estimated from the two data sets are similar in regions where seismic activity is concentrated on land we interpret this observation to suggest that m in intraplate japan has been constant during the late quaternary and is relatively free from secular variation when averaged over periods of several hundreds of years m in shikoku may be attributed almost solely to right lateral slip of the median tectonic line mtl the easterly strike of the mtl is consistent with a compressive stress field that trends northwest crustal shortening of the izu peninsula taken up on a set of strike slip faults that show north by northwest compression is 1 mm yr northeast honshu is characterized by a set of reverse type faults that trend northerly conversion of m in northeast japan to strain rates suggests that about 5 of the relative plate motion between japan and the pacific plate 9 7 cm yr is accommodated as a permanent east west shortening 5 mm yr of northeast honshu the predominant deformation in central and western honshu takes place as slip on a conjugate system of strike slip faults that strike northeast and southeast and show right lateral and left lateral motion respectively crustal shortening resulting from slip on faults in central and western honshu is 5 and 0 5 mm yr respectively in an easterly direction central and western honshu are in closest proximity to the nankai trough and hence the stress field in these regions cannot simply be attributed to the accommodation of the relative northwesterly convergence of the philippine sea plate the northward impingement of the izu peninsula into honshu may influence stresses in central and western japan but a conclusive explanation of the stress field in central and western honshu remains an enigma

is northern honshu a microplate abstract seismic slip vectors along the japan trench the eastern margin of the japan sea and the sagami trough are compared with global relative plate motions rm2 minster and jordan 1978 to test a new hypothesis that northern honshu japan is part of the north american plate this hypothesis also claims that the eastern margin of the japan sea is a nascent convergent plate boundary kobayashi 1983 nakamura 1983 seismic slip vectors along the japan trench are more parallel to the direction of the pacific north american relative motion than that of the pacific eurasian relative motion however the difference in calculated relative motions is too small avoid to the possibility that a systematic bias in seismic slip vectors due to anomalous velocity structure beneath island arcs causes this apparent coincidence seismic slip vectors and rates of shortening along the eastern margin of the japan sea for the past 400 years are also consistent with the relative motion between the north american and eurasian plates calculated there seismic slip vectors and horizontal crustal strain patterns revealed by geodetic surveys in south kanto beneath which the philippine sea plate is subducting indicate two major directions one is the relative motion between the north american and philippine sea plates and the other that between the eurasian and philippine sea plates one possible interpretation of this is that the eastern margin of the japan sea may be in an embryonic stage of plate convergence and the jump of the north american eurasian plate boundary from sakhalin central hokkaido to the eastern margin of the japan sea has not yet been accomplished in this case northern honshu is a microplate which does not have a driving force itself and its motion is affected by the surrounding major plates behaving as part of either the eurasian or north american plate another possibility is that the seismic slip vectors and crustal deformations in south kanto do not correctly represent the relative motion between plates but represent the stresses due to non rigid behaviors of part of northern honshu

the 2000 mw 6 8 uglegorsk earthquake and regional plate boundary deformation of sakhalin from geodetic data 1 interseismic gps velocities in sakhalin indicate that the island moves to the west at 3 4 mm yr with respect to the eurasian plate which is about half of the relative eurasia north america plate convergence rate gps measurements across the central sakhalin fault system provide evidence of compressive and strike slip strain accumulation at a rate 3 mm yr coseismic vertical displacements produced by the august 4 2000 mw 6 8 uglegorsk earthquake in sakhalin were analyzed by constrained nonlinear inversion which provided evidence for a reverse faulting mechanism on an east dipping fault plane

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

geology of the bk9 kimberlite damtshaa botswana implications for the formation of dark volcaniclastic kimberlite the bk9 kimberlite consists of three overlapping pipes it contains two dark varieties of massive volcaniclastic kimberlite informally termed dark volcaniclastic kimberlite dvk dvk ns is present in the north and south pipes and is interbedded with lenses of basalt breccia at the margins of the pipes dvk c is present within the central pipe where it is overlain by a sequence of basalt breccias with interbedded volcanogenic sediments the features observed within the dvk units of the bk9 kimberlite provide strong evidence for gas fluidisation of the accumulating pyroclastic material these include the massive interior of the pipes marginal epiclastic units well dispersed country rock xenoliths and small scale heterogeneities in lithic clast abundance the upper portions of the central pipe provide a record of the transition from pyroclastic eruption and infill to passive epiclastic infilling of the crater after the eruption has ceased the wall rock of the bk9 kimberlite dips inwards and is interpreted as post pipe fill subsidence of the adjacent country rock the two dvk units contain interstitial silt sized pyroclasts the dvk ns has a higher fraction of former melt and displays evidence of incipient welding as a result of differences in eruption dynamics these units demonstrate that whilst dvk is comparable in many respects to mvk and forms part of a spectrum of volcaniclastic rocks formed by fluidisation it differs in frequently containing silt sized particles and including agglutinated and welded varieties with a high melt fraction the dvk varieties studied here also have a distinctive hydrothermal assemblage resulting from the abundance of low silica accidental lithic clasts both the hydrothermal alteration and the abundance of silt sized particles contribute to the dvks distinctive dark colour

crust and upper mantle of kamchatka from teleseismic receiver functions teleseismic receiver functions rfs from a yearlong broadband seismological experiment in kamchatka reveal regional variations in the moho anisotropy in the supra slab mantle wedge and along the eastern coast ps converted phases from the steeply dipping slab we analyze both radial and transverse component rfs in bin averaged epicentral and backazimuthal sweeps in order to detect ps moveout and polarity variations diagnostic of interface depth interface dip and anisotropic fabric within the shallow mantle and crust at some stations the radial rf is overprinted by near surface resonances but anisotropic structure can be inferred from the transverse rf using forward modeling to match the observed rfs we find moho depth to range between 30 and 40 km across the peninsula with a gradational crust mantle transition beneath some stations along the eastern coast anisotropy beneath the moho is required to fit the transverse rfs at most stations anisotropy in the lower crust is required at a minority of stations modeling the amplitude and backazimuthal variation of the ps waveform suggests that an inclined axis of symmetry and 5 10 anisotropy are typical for the crust and the shallow mantle the apparent symmetry axes of the anisotropic layers are typically trench normal but trench parallel symmetry axes are found for stations apa and ess both at the fringes of the central kamchatka depression transverse rfs from east coast stations kro tum zup and petare fit well by two anisotropic mantle layers with trench normal symmetry axes and opposing tilts strong anisotropy in the supraslab mantle wedge suggests that the mantle lithosphere beneath the kamchatka volcanic arc is actively deforming strained either by wedge corner flow at depth or by trenchward suction of crust as the pacific slab retreats d 2002 elsevier science b v all rights reserved

a test of present day plate geometries for northeast asia and japan alternative geometries for the present day configuration of plate boundaries in northeast asia and japan are tested using nuvel 1 and 256 horizontal earthquake slip vectors from the japan and northern kuril trenches statistical analysis of the slip vectors is used to determine whether the north american eurasian or okhotsk plate overlies the trench along the northern kuril trench sup vectors are well fit by the nuvel 1 pacific north america euler pole but are poorly fit by the pacific eurasia euler pole results for the japan trench are less conclusive but suggest that much of honshu and hokkaido are also part of the north american plate the simplest geometry consistent with the trench slip vectors is a geometry in which the north american plate extends south to 41 n and possibly includes northern honshu and southern hokkaido although these results imply that the diffuse seismicity that connects the lena river delta to sakhalin island and the eastern sea of japan records motion between eurasia and north america onshore geologic and seismic data define an additional belt of seismicity in siberia that cannot be explained with this geometry assuming that these two seismic belts constitute evidence for an okhotsk block two published kinematic models for motion of the okhotsk block are tested the first model which predicts motion of up to 15 mm yr 1 relative to north america is rejected because kuril and japan trench slip vectors are fit more poorly than for the simpler geometry described above the second model gives a good fit to the trench slip vectors but only if okhotsk north america motion is slower than 5 mm yr 1

tertiary geodynamics of sakhalin nw pacific from anisotropy of magnetic susceptibility fabrics and paleomagnetic data abstract sakhalin has been affected by several phases of cretaceous and tertiary deformation due to the complex interaction of plates in the northwest pacific region a detailed understanding of the strain is important because it will provide constraints on plate scale processes that control the formation and deformation of marginal sedimentary basins anisotropy of magnetic susceptibility ams data were obtained from fine grained mudstones and siltstones from 22 localities in sakhalin in order to provide information concerning tectonic strain ams data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred paleomagnetically determined vertical axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation results from southwest sakhalin indicate a n035 e directed net tectonic transport from the mid paleocene to the early miocene which is consistent with the present day relative motion between the okhotsk sea and eurasian plates reconstruction of early late miocene ams fabrics in east sakhalin indicates a tectonic transport direction of n040 e in west sakhalin the transport direction appears to have remained relatively consistent from the oligocene to the late miocene but it has a different attitude of n080 e this suggests local deflection of the stress and strain fields which was probably associated with opening of the northern tatar strait a northward directed tectonic transport is observed in miocene sediments in southeast sakhalin mid eocene sediments in east sakhalin and in late cretaceous rocks of west and northern sakhalin which may be associated with northwestward motion and subduction of the pacific plate in the tertiary period the boundaries of the separate regions defined by the ams data are consistent with present day plate models and therefore provide meaningful constraints on the tectonic evolution of sakhalin

petrogenesis of the 1 9 ga mafic hanging wall sequence to the flin flon callinan and triple 7 massive sulphide deposits flin flon manitoba canada a detailed study of the geochemical and isotopic characteristics of the volcanic rocks of the hidden and louis formations which make up the hanging wall to the volcanogenic massive sulphide deposits at flin flon manitoba was carried out on a stratigraphically controlled set of samples the stratigraphy consists of the lowermost dominantly basaltic hidden formation and the overlying dominantly basaltic louis formation of importance petrogenetically is the 1920 unit a basaltic andesite with nb thmn 0 54 0 62 end 1 9ga 3 6 5 9 ehf 1 9ga 8 5 9 6 and 204pb 206pb 23 9 the basaltic flows that dominate the hidden formation have nb thmn 0 16 0 29 end 1 9ga 1 7 4 4 ehf 1 9ga 7 0 11 8 and 204pb 206pb 16 9 18 6 the carlisle lake basaltic andesite top of hidden formation is characterized by nb thmn 0 16 0 14 and 204pb 206pb 21 4 the rhyodacitic tower member bottom of louis formation has nb thmn 0 23 end1 9ga 4 6 ehf1 9ga 9 3 and 204pb 206pb 22 2 the

seventeenth century uplift in eastern hokkaido japan shores of eastern hokkaido rose by perhaps i m a few centuries ago the uplifted area extended at least 50km along the southern kuril trench it included the estuaries akkeshi ko and hichirippu on

arc continent collision and subduction reversal in the cenozoic evolution of the northwest pacific an example from kamchatka ne russia abstract the principal new feature in the cenozoic evolution of the northwest pacific is based on the fact that two arc terranes of upper cretaceous paleogene age were incorporated in the kamchatka orogen during the early tertiary and at the end of the miocene respectively these two arc terranes so called achaivayam valaginskaya campanian lower paleocene and kronotskaya coniacian eocene arcs are separated by a suture zone that contains deformed slices of young paleocene lower eocene oceanic crust paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic originated in the northwest pacific the arcs were separated by the vetlovka oceanic plate suggested to be a trapped fragment of the kula plate in which a new spreading zone developed in the early tertiary the principal event in the reconstructed evolution is the eocene collision of the extended achaivayam valaginskaya arc terrane with the continental margin of asia the margin is interpreted as the southeastern facing the pacific cretaceous accretionary boundary of the okhotsk microplate docked against the asian continent at 55 65 ma the arc continent collision developed progressively northward from the end of the paleocene early eocene in southern kamchatka to the middle eocene in the olutorka region the different stages of the collision occurred simultaneously along strike of the achaivayam valaginskaya arc according to the geodynamic model based on geological data for southern kamchatka collision included 1 the entrance of the continental margin in the subduction zone deformation of the overriding plate and subduction of fore arc block 2 arc obduction on the continental margin blocking of continental margin subduction and 3 subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc this subduction reversal produced deformation and reverse faulting along the eastern side of the arc thus allowing accretion of tectonic slices of young oceanic crust of the vetlovka plate which began to subduct during the subduction reversal the subducted plate dipping to the southeast was broken off and detached at depth under the arc area the subduction polarity reversal the most characteristic feature of the collision model is strongly supported by structural and geologic data of southern kamchatka and explains the main tectonic metamorphic and magmatic events related to and postdating the collision

tectonic stress state in ne japan as part of the okhotsk plate abstract an existing geodetic flow velocity model obtained by using an internal free network adjustment technique is used to derive estimates for various strain rates parameters in ne japan the greatest shortening rates of the principal strains trending e w are located in regions where much steady internal frame invariant plastic flow deformation is observed to be taking place the internal geodetic adjustment technique yielded the internal deformation in the tohoku arc most of the intraplate deformations including the much folding deformations observed in the inner zone are produced from within an interseismic transient elastic loading at a strongly coupled locked japan trench would not be needed the observed ongoing extensive ductile folding deformation in the inner zone of tohoku may mean that the geodetic strain rates causing shortening at 2 3 cm yr probably reflect the more correct level of the deformation which is steady permanent in ne japan as compared with seismic faulting data which indicate 0 5 cm yr shortening the calculated principal strain rates are used to make an interpretation for the origin of the deviatoric principal stresses within the greater regional plate tectonic framework the tectonic stress state in ne japan as part of the okhotsk plate could mostly be influenced by the okhotsk plate which is extruding southward to lessen the significant accumulated contractional deformation in ne asia in the verkhoyansk cherskii mountains the principal strain rates are n s extensional essentially everywhere in ne japan as a result of the southerly extrusion except in its southernmost leading edge in the uetsu fossa magna province where the japan alps rampart rises in front of the extrusion here an e w compressional stress state prevails a second e w contractional zone is found in north central tohoku extending from the sanriku province in the outer zone to the inner zone in the japan sea side being more prevalent in the latter zone the calculated rotation rates from the geodetic flow model are clockwise cw in both of the e w contractional regions ne japan extruding southward faces buttresses in 1 the oga ojika line ool and or a crustal weakness zone between the northern and the southern halves of tohoku approx at 38 5 n latitude and especially 2 the japanese alps rampart these obstacles cause the northern and southernmost tohoku to veer to its right and rotate cw thereby setting up the e w trending compressional deformation in their respective inner zones between the ool or the 38 5 n boundary and the kanto tectonic line ktl the sense of the differential rotations is counterclockwise ccw towards the ocean to the se the northern tohoku north of the ool and the southernmost tohoku south of the ktl cannot rotate ccw towards the ocean because of the izu block s collision in the south and the relatively strong coupling along the subduction interface beneath the japan trench in the north off sanriku the relatively stronger long term coupling between the northern tohoku and the pacific plate at the sanriku coast with respect to that in off fukushima is due to a flatter subduction of the pacific slab there increasing the plates interface contact area the flattening of the subduction dip angle was caused by ccw rotation and shifting of the northern tohoku along the dextral honjo matsushima line roughly corresponding to the ool towards the pacific and overriding of the subduction zone during the formation of the japan sea

what can we learn from 20 years of interseismic gps measurements across strike slip faults i use gps interseismic velocities and classic 2d elastic half space models with a screw dislocation to estimate the long term fault slip rate locking depth and the offset between the surface fault trace and the location of the dislocation below the seismogenic zone for 13 segments along 8 major strike slip faults using deduced strike slip rates and the position of the dislocation to normalize the interseismic velocities to facilitate comparison of spatial patterns of deformation i show that no substantial differences can be detected ruling out a large asymmetry in interseismic velocities across the 8 faults used in this study only the carrizo plain segment of the san andreas fault shows a significant asymmetry that cannot be explained by shifting the position of the dislocation at depth relative to the fault trace however the resulting perturbation is less than 10 of total strike slip rate fault traces are usually curved defining a concave side when the dislocation at depth is significantly offset from the fault trace the shift is always toward the block on the concave side of the fault trace this suggests that the fault zone in the lower crust may develop a simpler geometry more consistent with relative motion across the fault than its upper seismogenic part constrained by the structural complexity of the brittle crust since the faults used in this study are at different times in their interseismic period comparing the interseismic velocity fields across them allows identification of possible variations of the interseismic velocities with time when normalized by slip rate and dislocation location all the faults show the same interseismic strain with no significant differences between deduced locking depths these comparisons suggest that if temporal variations occur as suggested by some dynamic earthquake cycle models they are small and below the accuracy of the available geodetic measurements

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone