composition of 298 baptistina implications for the k t impactor link bottke et al 2007 suggested that the breakup of the baptistina asteroid family baf 160 30 20 myr ago produced an asteroid shower that increased by a factor of 2 3 the impact flux of kilometer sized and larger asteroids striking the earth over the last 120 myr this result led them to propose that the impactor that produced the cretaceous tertiary k t mass extinction event 65 myr ago also may have come from the baf this putative link was based both on collisional dynamical modeling work and on physical evidence for the latter the available broadband color and spectroscopic data on baf members indicate many are likely to be dark low albedo asteroids this is consistent with the carbonaceous chondrite like nature of a 65 myr old fossil meteorite kyte 1998 and with chromium from k t boundary sediments with an isotopic signature similar to that from cm2 carbonaceous chondrites to test elements of this scenario we obtained near ir and thermal ir spectroscopic data of asteroid 298 baptistina using the nasa irtf in order to determine surface mineralogy and estimate its albedo we found that the asteroid has moderately strong absorption features due to the presence of olivine and pyroxene and a moderately high albedo 20 these combined properties strongly suggest that the asteroid is more like an s type rather than xc type moth diniz et al 2005 this weakens the case for 298 baptistina being a cm2 carbonaceous chondrite and its link to the k t impactor we also observed several bright v mag 16 8 baf members to determine their composition

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

the formation of asteroid satellites in large impacts results from numerical simulations abstract we present results of 161 numerical simulations of impacts into 100 km diameter asteroids examining debris trajectories to search for the formation of bound satellite systems our simulations utilize a 3 dimensional smooth particle hydrodynamics sph code to model the impact between the colliding asteroids the outcomes of the sph models are handed off as the initial conditions for n body simulations which follow the trajectories of the ejecta fragments to search for the formation of satellite systems our results show that catastrophic and large scale cratering collisions create numerous fragments whose trajectories can be changed by particle particle interactions and by the reaccretion of material onto the remaining target body some impact debris can enter into orbit around the remaining target body which is a gravitationally reaccreted rubble pile to form a smashed target satellite smats numerous smaller fragments escaping the largest remnant may have similar trajectories such that many become bound to one another forming escaping ejecta binaries eebs our simulations so far seem to be able to produce satellite systems qualitatively similar to observed systems in the main asteroid belt we find that impacts of 34 km diameter projectiles striking at 3 km s 1 at impact angles of 30 appear to be particularly efficient at producing relatively large satellites around the largest remnant as well as large numbers of modest size binaries among their escaping ejecta

upper cretaceous paleomagnetic data from shikotan island kuril arc implications for plate kinematics abstract maastrichtian tuffaceous sandstones and siltstones were sampled from four sites in the northern part of shikotan island in the lesser kuril islands a characteristic component chrm isolated from most samples passes the reversal and fold tests and is most probably a primary remanence the chrm mean direction d 334 i 56 k 36 95 3 2 is 14 3 5 steeper than the pacific reference direction and 13 4 more gentle than the eurasian reference direction we assume that the lesser kuril islands together with their southwestern extension into the nemuro peninsula hokkaido were originally an island arc situated at about 36 n in the central west pacific soon afterwards probably by the end of the late cretaceous the nemuro shikotan island arc became inactive and started moving with the pacific plate in the miocene about 15 m y ago the nemuro shikotan island arc collided with the eurasian plate overrode the subduction zone and occupied its present day position on the pacific side of the kuril island arc although this scenario fits the available paleomagnetic data and kinematics of the pacific plate itself it disagrees somewhat with the kinematic models for the north pacific as a whole in particular by the end of the late cretaceous the northern margin of the pacific plate together with the extinct nemuro shikotan island arc should have been in an area close to the transform or ridge type kula pacific boundary the available paleomagnetic data from northeast asia however indicate that active islands arcs existed in mid northern latitudes of the modern pacific ocean in the cretaceous

mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex example from southern sakhalin abstract four modes of mixture of oceanic materials such as cherts limestones and basalts and terrigenous trench fill sediments are recognized in the cretaceous accretionary complexes in southern sakhalin far eastern pacific margin of the u s s r the first mode is a thin skinned stacking of pelagic bedded cherts with black shale due to duplex formation in association with shallow underplating or off scraping the second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt with overlying pelagic sediments of chert and limestone olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench and finally trenchfill sediments of turbidite this sequence is imbricated by thrusting which appears to have occurred at the time of underplating the third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate a reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust the fourth mode is in situ basalt volcanism at the trench where thick baked margins of terrigenous shale are formed around the injected basalts this mode appears to represent a ridge trench encounter these four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts

taconian orogeny in the new england appalachians collision between laurentia and the shelburne falls arc tectonic models of the ordovician taconian orogeny in western new england usually invoke a collision between the laurentian margin and a magmatic arc identified as the bronson hill arc however in central massachusetts and southern new hampshire rocks in the bronson hill arc are 454 to 442 ma and therefore younger than much of the taconian deformation and metamorphism in western new england and eastern new york which began by 470 ma u pb and single grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc the shelburne falls arc that formed west of the bronson hill arc at 485 to 470 ma the shelburne falls arc formed above an east dipping subduction zone by the early ordovician the taconian orogeny was the result of the collision between laurentia and the shelburne falls arc beginning ca 475 to 470 ma the younger bronson hill arc formed above a west dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the taconian orogeny the taconian orogeny ended when plate convergence between laurentia and iapetus was accommodated by the newly developed west dipping subduction zone instead of by crustal shortening in the taconian thrust belt the tectonic history of the new england appalachians is inconsistent with a middle ordovician collision between laurentia and the proto andean margin of gondwana

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

late cretaceous paleomagnetism of the east ranges island arc complex kamchatka implications for terrane movements and kinematics of the northwest pacific a campanian lower paleocene island arc complex was sampled for paleomagnetic studies at 12 sites in the east ranges tectonic zone of kamchatka after thermal demagnetization a reversed polarity characteristic remanent magnetization chrm was isolated from most volcanoclastic and basaltic units as well as from lava debris from intraformational conglomerates the fold and conglomerate tests are positive and the chrm in the studied rocks is likely primary the formation mean inclination of 66 3 3 7 corresponds to a paleolatitude of 48 7 5 0 n which is about 20 lower than the late cretaceous north american reference values because northward displacement of the studied terrane is indicated by the paleomagnetic data we examine several models of intraoceanic transport with the pacific and or kula plates and coastwise transport after terrane accretion far to the south of the present day position of kamchatka our preferred interpretation is that the studied island arc complex accumulated at about 83 79 ma the island arc to which the studied terrane had originally belonged was active between this time and 65 60 ma according to geological data the docking time nearly coincided with cessation of volcanic activity and northward movement of the island arc took place simultaneously with the volcanic activity the absolute motion of a subduction zone should have the same direction as the overriding plate therefore the subduction zone related to the east ranges island arc is inferred to have moved northward with the kula plate or with the kula and pacific plates successively consuming either the oceanic periphery of a continental plate or some unknown minor oceanic plate this process went on until 65 55 ma when the island arc and related subduction zone approached the continental margin and became extinct the proposed models also place additional constraints on kinematics of the kula pacific transform plate boundary

archean protolith and accretion of crust in kamchatka shrimp dating of zircons from sredinny and ganal massifs abstract we used an ion microprobe shrimp rg for u pb dating of individual zircons in high grade metamorphic basements of the felsic sredinny and mafic ganal massifs in kamchatka thirty percent of zircons from paragneisses of the sredinny massif contain archean 2800 2500 ma early proterozoic 1700 2100 ma and late proterozoic 1000 ma detrital cores in addition a broad and almost uninterrupted range of phanerozoic detrital cores 460 175 ma is present with jurassic to lower cretaceous 150 96 ma zircons forming the youngest detrital age group ubiquitous documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

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

kinematics and rheology of a major landslide based on signal analysis a detailed geodetic record which is over 23 years old and describes the kinematics of the mandria or left bank upstream landslide a major deep seated landslide close to the polyphyton dam greece was analysed on the basis of advanced signal analysis techniques without any geotechnical constraints it was found that all points share the same pattern of displacement which characterises both short and longer records the overall behaviour of the landslide can be described as viscoelastic and viscous or elastoviscous with multi annual periodic effects superimposed on the creep as the results of spectral analysis indicate the landslide seems insensitive to seasonal variations of precipitation and to annual fluctuations of the reservoir level although the first filling of the reservoir is likely to have had an important role in its movement there is a lack of subsurface information but the available data indicate that the landslide corresponds to a rigid mass dominated by brittle behaviour and int

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

tectonic collision processes after plate rupture abstract rupture of a continental plate subducted below a forearc produces a fold and thrust mountain belt with fast overthrusting of nappes post rupture plate unflexing leading to reflexure provides a mechanism for foreland basin formation application to the australia banda arc collision accounts for the origin of the timor trough its imbrication and contemporaneous extension in outer arc as well as reversal of subduction direction after the emplacement of nappes

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

evidence for cenozoic crustal extension in the bering sea region geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the bering sea have been created or altered by crustal extension and back arc spreading these processes have occurred during and since early eocene time when the aleutian ridge developed and isolated oceanic crust within parts of the bering sea the crust in the aleutian basin previously noted as presumably early cretaceous in age m1 m13 anomalies is still uncertain some crust may be younger vitus arch a buried 100 to 200 km wide extensionally deformed zone with linear basement structures and geophysical anomalies crosses the entire west central aleutian basin we suggest that the arch and the inferred fracture zones in the aleutian basin are early cenozoic structures related to the early entrapment history of the bering sea these structures lie on trend with known early cenozoic structures near the bowers shirshov aleutian ridge junction and on the beringian continental margin with possible continuation into alaska the structures may have coeval and cogenetic histories for early cenozoic and possibly younger times cenozoic deformation within parts of the bering sea region is principally extensional although the total amount of extension is not known as examples the komandorsky basin formed by back arc seafloor spreading the aleutian ridge has been extensively sheared and extensional block faulting is common sedimentary basins of the bering shelf have formed by extension associated with wrench faulting the cenozoic deformation throughout the bering sea region probably results from the interaction of major lithospheric plates and associated regional strike slip faults we present models for the bering sea over the past 55 m y that show oceanic plate entrapment back arc faulting and spreading along vitus arch breakup of the oceanic crust in the aleutian basin at fracture zones and back arc spreading in bowers basin

the latest cretaceous early paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern pacific margin episodic growth of accretionary complexes along the northwestern pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks the latest cretaceous to early paleogene rapid growth of the accretionary complexes observed in hokkaido and sakhalin islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone the mongolia okhotsk zone in the northern asian continent rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks now exposed as the kamuikotan complex in hokkaido and the susunai complex in sakhalin the forearc basin shallowed as a result of the uplift the uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region magmatic activity along the continental margin was also active at the same time subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time a large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge

slab breakoff as a mechanism for flipping of subduction polarity in taiwan we propose a model that involves breakoff of the east dipping eurasian slab beneath the taiwan orogen to account for the flipping of subduction polarity in northern taiwan the breakoff was initiated in southern ryukyu in the early pliocene and then propagated southwestward into taiwan like an opening zipper detachment of the eurasian slab has created a mantle window for the north dipping philippine sea plate to move in laterally causing a switch in the subduction polarity slab breakoff not only provides a testable model for the lithospheric structure of taiwan but also accounts for the high heat flow rapid uplift synorogenic extension deep focus earthquakes and north diminishing crustal contraction in the taiwan mountain belt

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

late cretaceous paleomagnetic data from the median range of kamchatka russia tectonic implications abstract a late cretaceous island arc complex was sampled for paleomagnetic study in the northern part of the median range of kamchatka in most samples thermal demagnetization in 12 18 steps revealed either a reversed characteristic remanent magnetization chrm or a remagnetization circle combining these yielded a positive fold test k s k g 5 8 and we interpreted the chrm to be primary the tilt corrected overall mean direction of dec 105 5 inc 67 0 95 4 1 for these rocks corresponds to a paleolatitude of 49 7 5 6 n this value is 20 to 22 lower than the late cretaceous north american reference values and northward transport of the subject tectonostratigraphic terrane and island arc to which it had originally belonged is strongly indicated the observed paleolatitudes are approximately similar for all late cretaceous island arc complexes in kamchatka and the southern part of the koryak highlands we conclude that these complexes were originally part of the same island arc which originated 20 to 25 south of the continental margin in the campanian and then was transported northward until its emplacement in the paleocene northward drift of the island arc coincided with its activity and the related subduction zone also had to drift with a rapidly moving oceanic plate most probably the kula plate this relationship implies that the subduction zone dipped southward and was consuming oceanic lithosphere presumably the oceanic periphery of the north american plate

tomographic imaging of the p wave velocity structure beneath the kamchatka peninsula summary a total of 5270 shallow and intermediate depth earthquakes recorded by the 32 stations of the regional seismic network of the geophysical service of russia are used to assess the p wave velocity structure beneath the kamchatka peninsula in the western pacific the tomographic inversion is carried out in three steps first a 1 d tomographic problem is solved in order to obtain an initial velocity model based on the 1 d velocity model 3 d tomographic inversions with homogeneous and heterogeneous starting models are obtained the conrad 15 km depth and moho 35 km depth discontinuities determined from the 1 d tomographic inversion and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray path determinations both velocity structure and hypocentral locations are determined simultaneously in the inversion the spacing of the grid nodes is a half degree in the horizontal direction and 20 50 km in the vertical direction a detailed p wave tomographic image is determined down to a depth of 200 km the resulting tomographic image has a prominent low velocity anomaly that shows a maximum decrease in p wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes at depth low velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km these anomalies are apparently associated with the volcanic activity the sedimentary basin of the central kamchatsky graben to the west of the volcanic front and the accretionary prism at the trench correlate with shallow low velocity anomalies highvelocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the ganalsky valaginskoe uplift and upper crust of shipunsky cape the results also suggest that the subducted pacific plate has p wave velocities approximately 2 7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

palaeomagnetism of palaeogene volcanic series of the kamchatskymys peninsula east kamchatka the motion of an active island arc abstract the paleocene tarkhov and eocene baklan formation are the lower and upper members respectively of a continuousisland arc sequence at the kamchatsky mys peninsula on the pacific side of the kamchatka peninsula we studied paleocene tuffs at five localities and eocene tuffs and basalts at nine localities from beds of various attitude thermal demagnetization revealed consistent directions of the characteristic component chrm of natural remanent magnetization in both formations normal and reversed chrm directions in the paleocene rocks are roughly antiparallel thus constituting a positive reversal test at the same time the fold test mcfadden and jones 1981 is not straightforward as locality means display a banana shaped distribution most probably because of local rotations inclination only data however show considerable decrease in dispersion after tilt correction thus constituting a positive fold test and implying a prefolding and most prprobably primary age of the chrm in these rocks the formation mean inclination of 57 5 3 8 calculated with the aid of inclination only statistics corresponds to a palaeolatitude of 38 1 4 1 n chrm directions isolated from the eocene baklan formation are well grouped and pass both the fold and reversal tests the formation mean inclination of 65 0 4 9 corresponds to a palaeolatitude of 47 0 6 4 n geological data point to uninterrupted accumulation of the studied island arc complex and hence to continuous activity of the subduction zone and related island arc the difference between the paleocene and eocene palaeolatitudes of 8 9 6 3 is statistically significant thus implying an absolute motion of the active subduction zone and related island arc during the paleocene to eocene no other active subduction zone existed between the studied area and eurasia we think that the studied island arc belonged to the pacific plate and was moving northwestward while the oceanic part of the eurasian or north american plate was subducted southward underneath this arc this island arc became inactive in the late eocene and was transported with the pacific plate which started subducting under the eurasia margin since that time

signature of remnant slabs in the north pacific from p wave tomography summary a 3 d ray tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesofthenorthpacific thedatareportedbythegeophysical surveyofrussia 1955 1997 wereusedtogetherwiththecataloguesoftheinternational seismologicalcenter 1964 1991 andtheusgeologicalsurveynationalearthquake information center 1991 1998 and the recompiled catalogue was reprocessed the final data set used for following the inversion contained 523430 summary ray paths the whole of the earth s mantle was parametrized by cells of 2 2 and 19 layers the large and sparse system of observation equations was solved using an iterative lsqr algorithm asubhorizontalhigh velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththealeutiansubductionzone thishigh velocityfeatureisobservedatlatitudes of up to 70 n and is interpreted as a remnant of the subducted kula plate which disappeared through ridge subduction at about 48ma a further positive velocity perturbation feature can be identified beneath the chukotka peninsula and okhotsk sea extending from 300 to 660km depth and then either extending further down to 800km chukotka or deflecting along the 660km discontinuity okhotsk sea this high velocity anomaly is interpreted as a remnant slab of the okhotsk plate accreted to siberia at 55ma

assembly of the annieopsquotch accretionary tract newfoundland appalachians age and geodynamic constraints from syn kinematic intrusions abstract the annieopsquotch accretionary tract aat comprises several ophiolites and arc back arc igneous complexes that were accreted to the dashwoods microcontinent during the ordovician taconic orogeny the lloyds river fault zone which separates the aat from the dashwoods microcontinent yielded 40ar 39ar hornblende ages of ca 470 ma the fault zone was intruded syn kinematically by the shoshonitic portage lake monzogabbro and the pierre s pond suite which gave u pb zircon ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document lan

analogue models for the transpressional docking of volcanic arcs in the western pacific abstract many convergent zones involve oblique subduction of oceanic crust the subduction often ends with oblique collision of a microcontinent island arc or oceanic plateau against a larger continental area we have conducted a series of experiments in which scaled multilayered models of the lithosphere made of sand and silicone were subjected to horizontal shortening oblique to lithospheric boundaries the aim was to investigate tectonic phenomena occurring at such boundaries for example subduction reversal partitioning of dip slip and strike slip motions lateral translation of continental fragments block rotations and entrapment of oceanic material the physical models ranged in complexity from a simple uniform oceanic lithosphere to configurations involving a volcanic arc against a continental margin our models explore the importance of rheology and width of continental indenters rather than the obliquity of convergence the models confirm that intra oceanic deformation is likely prior to complete subduction beneath both the arc and the continental margin they further explain the presence of extensional features in a convergent setting subduction zones resulting from oblique convergence may be associated with large strike slip faults by partitioning of the convergence vector when a model volcanic arc collides against a model continent a subduction reversal occurs after suturing and the strike slip fault which accompanies the subduction reversal initiates with a horse tail pattern the resulting plate boundary is somewhat different from the one that existed prior to the docking of the allochthonous belt these experiments tested many of the mechanisms proposed for terrane formation and accretion the models have been used to interpret the late neogene geodynamic setting of the western pacific with emphasis on the southern philippines and the island of new guinea

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

upper plate deformation induced by subduction of a volcanic arc the snellius plateau molucca sea indonesia and mindanao philippines abstract the northern molucca sea shows the incipient subduction of a composite oceanic and arc volcanic block the snellius halmahera block shb multi beam reflectivity seismic and gravity data obtained during the modec marine survey showed that the shb disappears beneath the accretionary wedge and the outer ridge of the sangihe arc to the north in mindanao island ongoing convergence generated shortening of the forearc basin and the backthrusting of the shb meanwhile a classical system of paired subduction philippine trench and strike slip fault philippine fault was installed the transition from lithospheric subduction to crustal overthrusting is located where the philippine trench sensu stricto begins and also coincides with the off shore extension of the philippine fault we observe a reversal of the thrusts from an eastward vergence in the molucca sea to a westward vergence in mindanao island this reversal takes place at the latitude where the forearc area emerges by uplift and the downgoing crust shb deepens resulting in a strong gravity low centered above the accretionary wedge the philippine fault initiated in a place where the crust was sliced off by a transfer zone which marked the northern termination of the molucca sea and drags northward a sliver of the previously accreted shb the northward drifting of this sliver created an extension which however cannot account for the gravity low we propose that the shortening and the uplift of the upper plate were induced by the buoyancy of the subducted unit shb and triggered the thrust reversal

role of preexisting topography and overburden on strain partitioning of oblique doubly vergent convergent wedges 1 scaled sandbox modeling was used to analyze the three dimensional aspects of strain partitioning along obliquely convergent margins that underwent a subduction polarity reversal event in the experiments the first phase of shortening produced a preexisting topography that affected wedge kinematics during the second phase increasing degrees of obliquity during the first shortening phase produced different amounts of inherited topography and overburden giving way to different effects on wedge response during the second phase of shortening during the second phase the models with an inherited heavy overburden show an anomalous orientation for riedel shears predating the development of a margin parallel strike slip fault these shears are oriented at an angle that is higher than kinematically predicted with respect to the subduction slot suggesting a transient rotation of the stress applied by the obliquity of subduction in contrast experiments with a lighter overburden do not show this inconsistency in addition all the models displayed an unexpectedly high accretion rate during the second phase of shortening which we interpret to be dependent on topographical slope breaks inherited after the first phase of shortening and the critical asymmetrical architecture of doubly vergent wedges

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

tertiary tectonics of the sea of okhotsk russia far field effects of the india eurasia collision a new structural map of the western sea of okhotsk based on grids of multichannel seismic data provides several new insights into the tertiary deformation of asia especially in regard to the extension of extrusion tectonics linked to the india eurasia collision into northeastern asia the sedimentary basins in this offshore region are the result of two regional shear systems in the west the north trending sakhalin hokkaido dextral fault system transects sakhalin island accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts the accessory normal faults are predominantly eocene to lower miocene and indicate transtensional shear whereas the folds and thrusts are mostly younger and indicate late miocene and pliocene transpression in the north an east west trending sinistral shear zone enters the sea at its northwest corner this sinistral shear system links farther west to extensional faults of the early tertiary baikal rift extending northeastward from the terminus of this sinistral shear zone are a series of long predominantly northwest dipping listric normal faults that form a large lazy s shaped pull apart basin the shantar liziansky basin slb normal faults in this 500 140 km basin accommodate 15 to 20 extension extension and related sinistral shear appear to be largely eocene to oligocene in age with lesser later activity to the northeast of slb a further extension of the sinistral shear zone appears to bend northeastward transecting pustorets and penzhina basins and following the course of the older mesozoic mongol okhotsk chukotsk active margin and suture it possibly reaches as far as the arctic as suggested by previous workers the baikal rift and its associated sinistral shear zone may be interpreted as by products of the india eurasia collision in eocene time mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast the sinistral shear zone is transected by the dextral sakhalin hokkaido fault zone in the kashevarov region of central slb where a fan of north to northwest striking late tertiary transpressional fault splays marks the northern termination of the dextral fault zone older slb normal faults are locally deformed and half grabens are partly inverted by the slightly younger dextral splays the two regional fault systems are essentially conjugate and the pattern of intersection closely resembles that predicted by recent modelling studies of india eurasia collision tectonics

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

the formation of the baptistina family by catastrophic disruption porous versus non porous parent body in this paper we present numerical simulations aimed at reproducing the baptistina family based on its properties estimated by observations a previous study by bottke et al 2007 indicated that this family is probably at the origin of the k t impactor is linked to the cm meteorites and was produced by the disruption of a parent body 170 km in size due to the head on impact of a projectile 60 km in size at 3 km s 1 this estimate was based on simulations of fragmentation of non porous materials while the family was assumed to be of c taxonomic type which is generally interpreted as being formed from a porous body using both a model of fragmentation of non porous materials and a model that we developed recently for porous ones we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head on impact of a projectile 54 km in size at 3 km s 1 thus our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated but the difference remains small enough to not affect the proposed scenario of this family history we then find that the break up of a porous body leads to different outcomes than the disruption of a non porous one the real properties of the baptistina family still contain large uncertainties and it remains possible that its formation did not involve the proposed impact conditions however the simulations presented here already show some range of outcomes and once the real properties are better constrained it will be easy to check whether one of them provides a good match

paleomagnetism of paleogene rocks of the central east kamchatka and komanorsky islands tectonic implications abstract paleomagnetic investigation of paleocene eocene to miocene rocks has been carried out at five localities in the eastern kamchatka and komandorsky islands after thermal and af cleanings consistent paleomagnetic directions were isolated for a number of formations data from the kronotsky peninsula and medny island passed both the fold and reversal tests and thus can be considered as reliable other results are of lower quality all the results were derived from sedimentary rocks and thus the corresponding paleolatitudes may be biased due to inclination errors a limited collection of basalts yielded inclination values systematically lower than those in sediments this anomaly was tentatively explained by shape anisotropy the correctness of paleolatitude estimations was thus left unproved several models of the northern pacific tectonic evolution were suggested and analyzed but the authors failed to find a model that was fully compatible with paleomagnetic kinematic and geological data

origin transport and emplacement of an exotic island arc terrane exposed in eastern kamchatka russia the regional stratigraphy of eastern kamchatka includes an exotic early late cretaceous ophiolite and late cretaceous island arc volcanic sequence integrating the existing geologic and geophysical data we examine the origin transport emplacement and postemplacement deformation of the island arc terrane which is named the olyutorsky island arc results from several paleomagnetic studies consistently indicate that the island arc terrane originated 1000 km to the south of where it is presently exposed although the formative paleolatitudes of the island arc rocks approximately correspond to the location of the izanagi farallon subduction zone the age of the volcanic rocks postdates the cessation of izanagi farallon convergence thus indicating that an unnamed plate or back arc basin existed in the northwest pacific during late cretaceous time we examine two possible models for northward transport of the island arc terrane to kamchatka 1 infra oceanic transport with the pacific or kula plates and 2 coastwise translation of the island arc terrane after accretion to the eurasian margin far to the south of kamchatka for both models the dominant eocene and miocene deformation ages observed in eastern kamchatka are used as two possible age limits for the cessation of northward transport although the observed paleolatitudes from paleomagnetic data correspond best with the infra oceanic transport model the provenance of the paleogene transport stratigraphy indicates a near shore sediment supply our preferred interpretation is that the island arc terrane 1 accreted onto the eurasian margin concurrent with cessation of island arc volcanism maastrichtian danian and 2 underwent northward coastwise translation along a major strike slip fault zone ending by middle late eocene time 43 50 ma it is unclear whether the ophiolite was exposed during arc continent collision or whether the ophiolite was obducted onto the island arc prior to collision a consequence of either infra oceanic transport or coastwise translation is that an open corridor between the western terminus of the aleutian arc and kamchatka must have existed until middle to late eocene time spreading within the komandorsky basin subduction of sea mounts and collision of the aleutian arc with kamchatka are proposed to have instigated the second miocene phase of deformation which uplifted and reexposed the island arc terrane

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

velocity field of around the sea of okhotsk and sea of japan regions determined from a new continuous gps network data to investigate the current crustal movements in and around the sea of okhotsk and sea of japan regions we have established a continuous gps network by the end of 1997 the network had been expanded to include 12 new stations data for the period from july 1995 to november 1997 were analyzed together with data from international gps service for geodynamics igs global stations to fix the estimated coordinates to the terrestrial reference frame the tsukuba igs station was assumed to be moving westward relative to the stable eurasian continent at 2cm yr according to heki s l996 estimate we find that 1 stations in the western margin of the sea of japan have eastward velocity vectors 2 the pole position of the okhotsk plate is located near okha which reconfirms the okhotsk micro plate 3 a plate boundary of the okhotsk and amurian plates between southen sakhalin and hokkaido is suggested

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