calibration of an ice core glaciochemical sea salt record with sea ice variability in the canadian arctic correlation between glaciochemical time series from an ice core collected on devon ice cap nunavut canada and gridded time series of sea ice concentrations reveals statistically significant inverse relationships between sea salt concentrations mainly na m g 2 and cl in the ice core and sea ice cover in baffin bay over the period 1980 97 an empirical orthogonal function eof analysis performed on all major ions shows that the dominant mode of glaciochemical variability eof1 represents a sea salt signal which correlates best with sea ice concentration in baffin bay on a seasonal basis the strongest and most spatially extensive anticorrelations are found in baffin bay during the fall followed by spring summer and winter these results support the notion that increased open water conditions in baffin bay during the stormy seasons fall and spring promote increased production transport and deposition of sea salt aerosols on devon ice cap comparison of ice core time series of eof1 d 18 o and melt percentage with air temperatures recorded in upernavik greenland suggests that ice cover variations in baffin bay over the past 145 years were dynamically rather than thermodynamically controlled with periods of strengthened cyclonic circulation leading to increased open water conditions and a greater sea salt flux on devon ice cap

himalayan ice core dating with snow algae snow algae in shallow ice cores 7 m long from yala glacier in the lang tang region of nepal were examined for potential use in ice core dating ice core samples taken at 5350 m a s l in 1994 contained more than seven species of snow algae in a vertical profile of the algal biomass 11 distinct algal layers were observed seasonal observation in 1996 at the coring site indicated most algal growth occurred from late spring to late summer pit observation in 1991 1992 and 1994 indicated that algal layer formation takes place annually 18 o chemical ions na cl so 4 2 and no 3 and microparticles failed to show any clear seasonal variation particularly at depths exceeding 2 m possibly due to heavy meltwater percolation snow algae in ice cores would thus appear to be accurate boundary markers of annual layers and should prove useful for ice core dating in himalayan type glaciers

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

the geometry of aleutian subduction three dimensional kinematic flow model we construct a steady state kinematic model of slab flow along the aleutian arc assuming the slab s geometry is known the slab geometry is constrained by local network data where the slab is seismically active and by p wave residual sphere analysis of 11 earthquakes and one nuclear explosion where it is aseismic the slab is approximated by a thin newtonian sheet subducting in a less viscous and incompressible mantle particles enter the trench at known rates of relative plate motion and are required to subduct along the predefined slab geometry for discrete but small 0 1 m y time steps we determine the speed and direction of each slab particle that minimizes dissipation power as the spherical shell oceanic lithosphere deforms to obtain the observed slab shape because the dip of the slab in the central aleutians is steep relative to the radius of curvature of the trench the slab must stretch along arc to obtain its present configuration our kinematic model represents one internally consistent flow that results in the observed geometry furthermore it provides a lower bound on the total amount of internal in plane deformation required by the shape and an estimate of stream lines and of the orientation and spatial distribution of the deformation rate the predicted in plane strain rate field is dominated by along arc extension reaching values as large as 10 15s 1 in the central aleutians and decaying by an order of magnitude to the east and west the slab accumulates an in plane strain of 10 by the time it reaches the depth of the seismicity cutoff along arc variations in both the magnitude and orientation of calculated strain rates are consistent with the spatial distribution of seismic moment release and source mechanism orientations of intermediate depth earthquakes suggesting that in order to understand intermediate focus earthquakes one must consider along arc deformation three dimensional analyses of subduction in a spherical geometry such as the present study are required to model along arc effects relative plate motions along the trench vary from nearly normal in the eastern aleutians to transform farther west coincident with this change the maximum depth of seismicity within the descending slab decreases from 250 to 50 km and active andesitic volcanism ceases the kinematic flow model predicts that the slab subducted obliquely along the central portion of the aleutian arc is transported laterally westward producing cold slab material to a depth of at least 300 km along the entire arc after only 15 m y of subduction the lack of volcanic activity in the western aleutians is apparently related to the lateral transport of the slab perhaps indicating that the slab is dehydrated and incapable of producing the volatiles necessary for arc magmatism the along arc variation in the maximum depth of seismicity can be explained thermally we construct temperature estimates within the slab using a quasi three dimensional finite difference scheme including entrained mantle flow stream lines from the kinematic model and the time history of the age of the subducting lithosphere as a function of position along the trench the theoretical temperature at the seismicity cutoff depth is remarkably close to a constant fraction of the depth dependent mantle solidus along the entire arc this observation is consistent with the hypothesis that seismicity terminates because of the initiation of high temperature steady state creep and provides an explanation for the lack of intermediate focus earthquakes in the western aleutians even though slab material exists there

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

continuous high velocity aseismic zone beneath the izu bonin arc since 1904 eleven unusually deep earthquakes have been reported near the southern end of the izu bonin arc about 24 n 142 e these isolated events are separated from ordinary deep earthquakes in the wadati benioff w b zone of the izu bonin i b arc by a distance of about 300 km parallel to the trench axis they also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them using 3 d ray tracing a detailed analysis is performed of the difference in travel time residuals between the anomalous events and the deep events within the w b zone to the north this analysis yields higher reliability and resolution than can be obtained from ordinary tomographic imaging we find that these unusual events occurred in a high velocity zone which is connected to the northern w b zone of the i b arc the aseismic high velocity zone has a p velocity at least 3 faster than the surrounding mantle both our result and previous evidence for a horizontally lying slab west of the w b zone at 27 n e g okino et al 1989 are consistent with the existence of a prominent contortion of the subducted slab in the i b region

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

transition from interplate slip to double seismic zone along the kuril kamchatka arc during subduction the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth to characterize this transition we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the kuril kamchatka arc precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances at depths of 30 50 km our results delineate a transitional thrust zone characterized by earthquakes showing thrust faulting on moderately dipping nodal planes the northwesterly dipping nodal plane dips approximately 40 some 10 15 steeper than the dip of low angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km for earthquakes along the transitional thrust zone secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds in the former case the transitional thrust zone is interpreted as the deep seated portion of the plate interface whose dip increases with depth the alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km the inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region however a locked interface does not seem to affect the state of strain in the double seismic zone the upper compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km at this depth the two layers of seismicity in the double seismic zone are separated by as much as 30 40 km too large a distance to be accommodated by models of seismogenesis invoking either the gabbro eclogite transition or dehydration of subducted oceanic crust

a double seismic zone in new britain and the morphology of the solomon plate at intermediate depths we have studied the spatial configuration of earthquakes in the new britain subduction zone by inverting regional and teleseismic p pkp and pp arrival times using a hypocentroidal decomposition method a previously unreported intermediate depth double seismic zone exists along a small 70 km long segment of the slab in this region centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal along strike compression and the lower zone shows downdip tension this contrasts with most previously studied double seismic zones which generally show downdip compression in the upper zone we propose that along strike rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the new britain trench which undergoes a 40 change in strike between 148 and 152 longitude the double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo these results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics and that along strike membrane and bending stresses resulting from slab morphology can be significant in producing double seismic zones

impact of glacier changes on ecosystem of proglacial lakes in high mountain regions of east siberia russia mountain lakes in east siberia have been studied for recent changes in water chemistry turbidity and diatom assemblages preserved in bottom sediments we performed a regional analysis of the relative effect of climate and glacier changes on change in diatom diversity and supply of meltware in proglacial lakes we analysed sediment records from east siberian glacier lakes using geochemical and diatoms proxies we found that dramatic changes in communities and abundance of diatoms and biogenic proxy could be induced by low nutrient concentrations in glacial lakes and high turbidity due to active degradation of glaciers and snow patches as a result of the global increase in temperature in the northern hemisphere our evidences show that diatoms have been gradually decreased since ca the 1880s a significant tendency towards diatom reducing occurred at high summer regional temperatures this tendency may be attributed to the fact that glaciers and snow patches thawed actively in east siberia during ca 1880 1958 which was induced by the beginning of the recent warming ca 1850 1860 and a long period of relatively warm regional climate from ca 1900 to 1960

stress in the contorted nazca plate beneath southern peru from local earthquakes we study earthquake focal mechanisms in a region of highly contorted subducting lithosphere to identify dominant sources of stress in the subduction process we observe a stress pattern in the contorted nazca plate beneath southern peru from an analysis of hypocentral trend and focal mechanisms of intermediate depth earthquakes expanding on previous studies we examine the hypocentral trend using 1673 of 2178 well located local events from the nine station arequipa network the dip of the plate beneath southern peru averages 25 30 from 25 to 100 km depth below this depth there is an 80 to 100 km wide contortion between a zone of increasing dip convex to the southeast and a flat lying concave zone to the northwest using more than 6000 p wave first motions of events deeper than 50 km we derive stress orientations from a moving average of composite focal mechanisms across a 200 by 350 km region including the contortion the in plate distribution of tension t and compression p axes reveals a coherent stress pattern the trend is most clear beneath south central peru nw section and below 100 km depth in southernmost peru se section both t and p axes tend to be dominantly in plate especially below 100 km depth t axes orient toward the contortion in a fan shaped trend which suggests that the deepest part of the seismic zone within the convex se section is sinking and pulling the more buoyant nw section we conclude that from 50 to 200 km depth slab pull forces are dominant in the observed stress our results suggest that a significant amount of plate extension occurs in this region of intermediate depth subduction

trench curvature and deformation of the subducting lithosphere summary the subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation the downdip component of strain is associated with external forces that are applied to the slab during its sinking namely the gravitational force and the mantle resistance to penetration here we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone independently from the long term physical processes that have determined the actual curvature of the subduction zone in particular we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters such as trench curvature dip function and subduction velocity we also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths and may include large extensional lateral deformation at intermediate depths whereas a state of lateral mechanical equilibrium can only represent a localized exception our formulation overcomes the flaws of the classic ping pong ball model for the bending of the lithosphere at subduction zones which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs a study of the geometry and seismicity of eight modern subduction zones is performed to assess the validity of the theoretical relationship between trench curvature slab dip function and lateral strain rate the strain pattern within the eight present day slabs which is reconstructed through an analysis of harvard cmt solutions shows that tectonic plates cannot be considered as flexible inextensible spherical caps whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

seismicity and focal mechanisms at the new britain trench related to deformation of the lithosphere abstract the focal mechanisms and distribution of earthquakes at the new britain trench show evidence for both large scale contortion and segmentation of the subducted slab although trench morphology is continuous around the bend in the new britain trench the subducted slab is continuous only to about 100 km depth a v shaped notch in the seismicity extending from 150 km downward may indicate a tear in the lithosphere along the fold hinge which accomodates northeast subduction beneath bougainville and northwest subduction beneath new britain deep seismicity probably originates within segments of lithosphere detached from the presently subducting slab at about 250 km depth

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

seismotectonics of the central segment of the indonesian arc abstract in this paper a revision of seismicity affecting the central segment of the indonesian island arc ranging between 110 and 126 e is presented using the areal and in depth distribution of seismic activity strain release maps and focal mechanisms lateral changes in the wadati benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries the seismicity pattern indicates that the sumbawa flores wetar sector shows different forms of behaviour with respect to the adjacent sectors these include driving mechanism inclination and continuity of the subducting slab and subduction features this area therefore seems to be isolated from the sunda and banda arcs by two principal boundaries these having a nearly n s trend in the bali region and with a nearly e w trend in the region ranging between wetar northern timor and tanibar the first boundary characterized by an absolute minimum of seismic activity at all ranges of depth has been interpreted in terms of subduction of the roo rise aseismic bathymetric ridge for the second boundary dividing a northwardly steeply dipping slab from an e w subducting slab dipping with an angle of 30 40 a tear in the upper part depth less than 300 km and a hinge fault system in the deepest part of the lithosphere have been proposed from the analysis of focal mechanisms of shallow earthquakes it was inferred that the central part of the indonesian arc is subject to a vortex shaped stress field centred on the savu basin in this model the compressive axes appear to rotate counterclockwise from sw to nne in the sumba sumbawa western flores region and clockwise from w to nnw in the timor eastern flores zone to interpret these features on the basis of seismological evidence a lateral discontinuity in the arc trench system close to sumba a collision between sumba and sumbawa and a rotation towards the north northeast of sumba have been suggested the proposed structural discontinuity trending nw se may represent a major transcurrent fault zone through which the australian continental lithosphere comes into contact with the indian oceanic lithosphere

changes in glaciers in hidden valley mukut himal nepal himalayas from 1974 to 1994 glaciological investigations were carried out in 1994 on the glaciers in hidden valley mukut himal nepal himalayas in order to make a comparison with observations made in 1974 most of the glaciers were found to have retreated by 30 60 m in terminus elevation over the 20 years between the two studies rikha samba glacier the longest glacier in the valleys has retreated by about 200 m the areal average of the amount of surface lowering and the volume loss of the glacier was estimated to be 12 6 m ice equivalent and 13 of the total mass respectively the annual mass balance of 0 55 m a 1 water equivalent was obtained as an average for 20 years which is one of the largest negative values amongst small glaciers of the world

nival glacial dynamics of geosystems in eastern siberia russia for the last 50 years it is presented the results of studying of small glaciers located in the eastern siberia along transect the kodar mountains baikalskiy and barguzinskiy ridges the eastern sayan mountains some glaciers from this transect have not been described before the inventorying of these glaciers is based on high resolution satellite images and fieldworks the most numbers of glaciers are located in the kodar mountains and few are in the baikalskiy and the barguzinskiy ridges most likely these glaciers remained mainly due to coldest winters and geomorphologic features the main parameters of a glacier s data the dynamics the current state cartographic representation remote sensing data and others are selected by analogy of the international glaciological database

evidence for propagation of cold adapted yeast in an ice core from a siberian altai glacier 1 cold environments including glacier ice and snow are known habitats for cold adapted microorganisms we investigated the potential for cold adapted yeast to have propagated in the snow of the high altitude belukha glacier we detected the presence of highly concentrated yeast over 104 cells ml 1 in samples of both an ice core and firn snow increasing yeast cell concentrations in the same snow layer from july 2002 to july 2003 suggests that the yeast cells propagated in the glacier snow a cold adapted rhodotorula sp was isolated from the snow layer and found to be related to psychrophilic yeast previously found in other glacial environments based on the d1 d2 26s rrna domains 26s rrna clonal analysis directly amplified from meltwater within the ice core also revealed the presence of genus rhodotorula analyses of the ice core showed that all peaks in yeast concentration corresponded to the peaks in indices of surface melting these results support the hypothesis that occasional surface melting in an accumulation area is one of the major factors influencing cold adapted yeast propagation

glacier fluctuation in northern and high asia historical and methods perspective glaciers in asia occupy nowadays much smaller extents than those of the pleistocene but they still play an important role in alpine ecosystems e g shahgedanova 2002 glaciers also pose hazards to society and infrastructure with avalanches and glacial lake outburst floods being the most destructive of glacier related natural events richardson and reynolds 2000 jansk et al 2010 nie et al 2013 wang and zhang 2014 since the end of the little ice age lia glaciers have been retreating almost everywhere around the world and this process has dramatically accelerated during the recent decades fig 1 wgms 2013 however individual glaciers are behaving differently and some have advanced while most have been in retreat ipcc 2013 yao et al 2012 mountain glaciers are sensitive indicators of climate change e g dyurgerov and meier 2000 haeberli et al 2004 and to fully understand the connection between changing climate and glacier fluctuations all the variables of the glacier mass balance need to be assessed at the local and regional scale dowdeswell et al 1997 oerlemans 2005 one of the regions where information about glacier changes is still scarce is northern and high asia fig 1 largely because glaciers of the region are located relatively far from human settlements and some have only been found after the 1940s for example ivanov et al 2015 have provided the first inventory of glaciers in the barguzinsky ridge east of lake baikal russia in this issue at the same time continental glaciers of northern and high asia are located far from the main sources of moisture and are thus sensitive to climate change fujita 2008 kaser et al 2010 in this issue we have collected information about glaciers situated along a southwestnortheast transect from 77 e 32 n the western himalaya india to 128 e 65 n the orulgan range russia fig 2 and fed from different sources of moisture the north atlantic the arctic and the indo pacific air masses because glaciers in our focus area belong to different climate systems their systematic study across the whole region has the potential not only to assess the climate change at local and regional scales but also to reveal climate change patterns at a larger continental scale the papers of this issue cover a variety of glacier related topics methods studies glacier inventories current glacier dynamics and paleo reconstructions and are based on a range of data such as satellite imagery synoptic maps biogeochemical proxies from proglacial lake bottom sediments and meltwater streams pollen and oxygen isotope records from snow ice cores and high resolution seismic reflection data

lateral stretching of the philippine sea plate subducting along the nankai suruga trough focal mechanisms are investigated for shallow earthquakes h 60 km of central japan by using telemetered seismic data of microearthquakes shallow earthquakes in central and southwestern japan can be distinguished into three groups upper crustal earthquakes subcrustal earthquakes and interplate thrust earthquakes the first and the second ones being subjects of the present study most of the upper crustal earthquakes are of strike slip type or reverse faulting with e w to se nw compression whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with e w to ne sw extension the subcrustal seismic zone subparallel to the nankai trough becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the izu peninsula which has been interpreted as a microcontinent of the philippine sea plate colliding against the eurasian plate on the basis of hypocentral distribution three dimensional seismic velocity structure and focal mechanisms subcrustal earthquakes are considered to occur within the subducting philippine sea plate that extends down to a depth of at least 60 km on the basis of this and other focal mechanism studies stress trajectories are drawn for the subducting philippine sea plate and for the overriding eurasian plate the overriding plate is on the whole under the e w compression which coincides with the moving direction of the pacific plate relative to the eurasian plate the subducting plate on the other hand is under lateral extension along its strike we interpret that the latter is a stress regime unique to an incipient subduction zone where the subducting plate at subcrustal depths is forced to warp downward being constrained laterally at the arc arc junctions and thereby comes under lateral stretching the lateral constraints arise either from apparent increase in flexural strength of the plate at the arc arc junction or from buoyancy of a microcontinent that resists subduction

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

fluctuation of glaciers in the southeast russian altai and northwest mongolia mountains since the little ice age maximum the present state and dynamics since the maximum of the lia of the glaciers of southeast russian and north mongolian altai have been studied data were obtained from 15 areas of glaciation the research is based on multiyear in situ observations and analysis of aerial photos of mid 1960s and high resolution space imagery 553 modern glaciers with total area 313 4 km2 have been mapped and put down into catalogues including glaciated areas and glaciers that have not been studied before glacial complexes with united accumulation zone and flat summit glaciers in the center and valley glaciers on the sides are typical for the area of research the firn line level rises from 3200 3300 m in the west to 3600 3700 m in the east this tendency reflects the reduction of nourishment of the glaciers in this direction due to decrease of precipitation the largest valley glaciers exist on the northeastern leeward slopes as a result of two to threefold concentration of snow the last glacial advance in the maximum of lia according to dendrological data finished in the first half of the 19th century from the maximum of the lia the glaciation of the region lost about 51 ela average uplift was 89 m on the background of general trend of glacial retreat there were several periods of stabilization of glacial fronts the most distinct in mid 1960s in the last 50 years there were two warm and dry intervals 1972 1981 and 1992 2008 and three cool and moist intervals prior to 1972 during 1982 1994 and subsequent to 2009 it caused fluctuations of firn line altitude with about 300 m range and abrupt changes of glacial area maximal changes were observed between 1995 and 2008 when areas of the glaciers decreased with average rate 1 2 year determination of retreat of glacial snouts after the lia by geomorphic features is described by vi phase scheme

rapid earth expansion an eclectic view abstract paleobiogeography plate evolution and minor amounts of subduction require a rapidly expanding earth during post triassic time not plate tectonics an asteroid impact at the p tr boundary in the congo basin ruptured the lithosphere and together with another impact in the carnian of arizona caused earth s volume to subsequently expand the increase in volume was likely due to inner core and lower mantle transformation growth at the expense of the fluid outer core a modified pacific reconstruction is proposed that closes up the pacific basin in the triassic and allows continents to cover a 55 radius earth some serious weaknesses in the plate tectonics theory are noted and tectonics by high energy impacts is discussed

variations of the glacier mass balance and lake water storage in the tarim basin northwest china over the period of 2003 2009 estimated by the icesat glas data accurately estimating the changes in glacier mass balance and water storage in lakes and reservoirs is critical to studying the water cycle in the inland river basin in northwest china we used high resolution satellite images to analyze the changes in water surface area of lakes and reservoirs in tarim basin and used the icesat glas altimeter data to estimate their water level changes and the glacier mass balance change over the period 2003 2009 the results showed the average glacier thinning in the entire basin was at a rate of 0 34 0 25 m w e year equivalent height of water which means that the glacier mass balance occurred 6 8 1 2 km3 water equivalent over the study period however the mean water level of nearby lakes decreased by 0 41 0 2 m even with the influx of glacial melt water indicating that the lake level declines were caused by the withdrawal of lake water for human activities

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

tearing of stagnant slab subducted slabs of oceanic lithosphere below the western pacific tend to be stagnant in the transition zone with poorly known mechanical properties typical examples are the izu bonin and japan slabs that meet each other to form a cusplike junction beneath southwest japan here we show that these two slabs are torn apart at their junction when they bend to flatten over the 660 kilometer discontinuity as is expected from a simple geometric argument we present three lines of evidence for this ongoing slab tear

geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics tectonic features at the earth s surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow a model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations because of the progressive shrinkage of the pacific averaging 0 5 km2 yr over the last 180 m y this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the pacific caribbean drake passage australian antarctic gap in this model upper mantle return flow streams originating at the western pacific trenches and at the java trench meet south of australia filling in behind this rapidly northward moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the australian antarctic discordance the long continued tectonic movements toward the east that characterize the caribbean and the easternmost scotia sea may be produced by viscous coupling to the predicted pacific outflow through the gaps and the caribbean floor slopes in the predicted direction if mantle outflow does pass through the gaps in the pacific perimeter it must pass beneath three seismic zones central america lesser antilles scotia sea none of these seismic zones shows foci below 200 km mantle material flowing through the caribbean and drake passage gaps would supply the mid atlantic ridge while the java trench supplies the indian ocean ridges so that deep mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations the analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents the second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move this hypothetical driving mechanism has four aspects 1 the lower mantle convects 2 this convection drives the continents by drag on their deep roots 3 return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them and 4 oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull this mechanism accounts for the opening of the atlantic the ability of spreading ridges to stay on the midlines of oceans and the penetration of india into asia following their collision continental motions strongly imply lower mantle upwelling and divergence beneath the atlantic and southeast indian oceans and convergence and subsidence along the tethyan belt this picture disagrees with the concept derived from hot spot studies of an undeforming absolute reference framework at depth but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature if the present model is generally correct a fairly simple pattern of lower mantle convection with as few as four cells may explain much of the tectonic complexity of the earth

seismicity gaps and the shape of the seismic zone in the banda sea region from relocated hypocenters 1 we relocate hypocenters for more than 800 earthquakes deeper than 50 km with mb 5 0 along the banda arc using several thousand handpicked direct depth and core reflected phases in addition to phases reported by the international seismological centre the seismicity distribution is found to be very nonuniform both along the arc and in depth gaps in the relocated hypocenters exist along depth in most places of the arc with the upper edge of the gaps varying from 100 to 450 km depth and the lower edge varying from 350 to 670 km in different portions of the arc the seismic zone between 129 and 131 e in the 100 200 km depth range is the widest along the arc both in strike and downdip this region near the highest arc curvature has the highest seismic activity and is the only part of the arc with earthquakes continuously occurring from the surface down to below 600 km the very deep earthquakes under sulawesi are shown to be part of the west southwest dipping seram slab in the westernmost part of the banda arc the slab is under downdip tension in the 50 250 km depth range while the deepest portion of the slab in this region is under compression from 128 to 131 e the slab between 100 and 200 km depth is under mainly horizontal compression our study supports the two slab model for the banda arc the depth of the wadati benioff zone below the volcanoes is 60 100 km for the five volcanoes between 128 and 130 e and 150 km for the 23 volcanoes between 118 and 124 e

numerical modelling of historical front variations and dynamic response of sofiyskiy glacier altai mountains russia the recent fluctuation of the central asian climate and its effect on the region s glaciers is poorly known largely because of a lack of knowledge of the dynamic behaviour of so called summer accumulation type glaciers in this study a one dimen sional numericalglaciermodelisusedtosimulatethedynamicresponseof sofiyskiyglacier altai mountains russia to climate forcing a successful simulation of the observed histor ical front variations was accomplished by dynamic calibration this resulted in a recon struction of the recent mass balance history of the glacier showing a distinct decline in surface massbalance in the second half of the19th century a slightly higher massbalance at the beginning of the 20th century followed by a steady decline towards present condi tions the future response of sofiyskiy glacier was projected for six 21st century climate scenarios undera no change scenario the glacier will retreat 42kmby 2100 if airtem perature gradually risesby 45 c during this century the glacier will vanish around 2100 basic response characteristics of sofiyskiy glacier were determined these indicate rather low mass balance sensitivity to temperature change but a strong front reaction due to geo metric conditions

membrane strain rates in the subducting plate beneath south america geometric constraints associated with the concave oceanward bend in the trench and andes mountains known as the bolivian orocline force along strike compression in the subducting slab our calculations of membrane strain rates in an assumed continuous slab demonstrate that this can be accommodated by either 10 along strike compressive strain or by geometric buckling in nearly all other subduction zones exhibiting a similar bend in the trench the compression is accommodated by slab arching landward of the trench bend the slab geometry is markedly different in south america two regions of nearly flat lying slab north and south of the bolivian orocline result in the 150 km slab depth contour being much straighter than the trench profile our calculations show that this geometry helps to relieve along strike compression along the deeper part of the subduction zone the along strike compression seems to be accommodated by pronounced along strike buckling we infer the approximate geometry of this buckle in an otherwise aseismic region from the great mw 8 3 bolivian deep earthquake on june 9 1994 and its aftershocks the deep slab appears to have a nearly east west strike in the vicinity of this earthquake whereas the strike is more nearly north south to the north and south the focal mechanisms of deep earthquakes exhibit consistent down dip compression and along strike null axes tracking the contortions in slab geometry however our calculations indicate that the particle paths themselves are not down dip everywhere instead where the deep slab strike is oriented nearly east west the particle flow is more nearly along strike than down dip in general this is not a major problem but if the slab is anchored in the lower mantle and moving very slowly this produces large deformation rates near the base of the upper mantle where the slab is assumed to move at near plate rates this increased complex deformation in the deep slab may contribute to the conditions for very large earthquakes in two ways first this may dramatically increase deformation rates and secondly the increased deformation may lead to anomalous advective thickening of a proposed overdriven olivine wedge this provides a mechanism for locally thickening the olivine wedge and allowing transformational faulting to occur over a much larger volume than allowed for normal subduction

reconstruction of deglaciation of northern mongolia for the last 330 ka bp inferred from ostracod stable isotope records from lake khubsugul we have compared 18o and 13c compositions of ostracod valves with elemental and grain size records of bottom sediments from the drill core kdp 01 northern mongolia nm formed for the last 330 ka bp the increase in the 18o values a high content of br and a low percentage of clay particles were found to positively correspond with high lake bio productivity and warm marine isotope stages mis 3 5 7 and 9 and visa versa during cold mis stages positive 13c values have been linked with high inflowing dissolved inorganic carbonates and forest steppe vegetation seems to be evidenced during cold mis stages mis 2 4 6 and 8 sharp depletions in the 18o values on 1 5 2 5 occurring during the ends of mis 6 and mis 4 could be induced by glacier meltwater discharge into lake khubsugul however shifts in the 18o values were not significant in mis 2 and mis 8 based on our assumption that the inflow rates of glacier meltwater were low due to the relatively small sizes of glaciers in the nm during these stages during mis 2 northern mongolian glaciers were the smallest in sizes due to high regional aridity the climate condition in mis 3 was not typically favorable for a warm mis stage while glaciers or snow patches formed in mis 4 most likely were still in the nm as a result isotopically depleted 18o meltwater was supplied into lake khubsugul at the period

mantle flow and deformation of subducting slab at a plate junction abstract at junctions between plate boundaries there are several general characteristic features these include trench normal fast polarization direction of s wave splitting in the mantle wedge along arc variation of the angle of subduction and the different behavior of slab stagnation along the strike of the trench in this paper we focus on the junction of the tohoku kurile arc we show the dynamical effects of the junction using a numerical model of mantle convection with a realistic curved shape of the trench we obtain 3d flow in the mantle wedge which is consistent with the observation of seismic anisotropy even with the oblique subduction we also find the along arc variation of subduction angle which agrees with the observations although the match becomes worse when we include the effect of dislocation creep that is non linear viscosity along arc variation of the angle of subduction partly contributes to the different behavior of slab stagnation in the tohoku kurile arc our results show that the shape of the trench is an important factor which considerably affects mantle flow and deformation of subducting slabs thus 3d modeling is necessary to constrain the dynamics of subduction zones near the junction

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

why subduction zones are curved 1 we give an explanation for the polarity localization shape size and initiation of subduction zones on earth by considering a soft thin curved lithospheric cap with either elastic or viscous rheology supported by a thick nearly incompressible mantle we find two different characteristic subduction geometries arise depending on boundary conditions 1 plate boundaries where subduction results primarily from the gravitational body force free subduction have characteristic plate lengths and form arc shaped dimpled segments resulting from the competition between bending and stretching in edge buckling modes of thin spherical shells and 2 subduction zones due to localized applied loads that push one slab of thin positively buoyant lithosphere beneath an overriding plate forced subduction form localized straight segments consistent with the deformation of indented spherical shells both types of subduction are nonlinear subcritical instabilities so small perturbations in the mechanical properties of the lithosphere have pronounced effects on subduction initiation and evolution yet in both cases geometric relationships determined by the shape of the earth itself play the most critical role in controlling the basic morphology and characteristic length scales of subduction zones

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

a 25 000 year tropical climate history from bolivian ice cores ice cores that were recovered from the summit of sajama mountain in bolivia provide carbon 14 dated tropical records and extend to the late glacial stage lgs oxygen isotopic ratios of the ice decreased 5 4 per mil between the early holocene and the last glacial maximum which is consistent with values from other ice cores the abrupt onset and termination of a younger dryas type event suggest atmospheric processes as the probable drivers regional accumulation increased during the lgs during deglaciation and over the past 3000 years which is concurrent with higher water levels in regional paleolakes unlike polar cores sajama glacial ice contains eight times less dust than the holocene ice which reflects wetter conditions and extensive snow cover

high resolution determination of the benioff zone geometry beneath southern peru the benioff zone geometry beneath southern peru is determined using 32 months of arrival time data from a local seismic network our data set includes 2476 located events of which 205 are chosen to be master events earthquakes with the most reliable locations all the events are then relocated using sourceregion dependent station corrections derived from the station residuals of the master events we find that the benioff zone in the more southerly region of our study area dips at a nearly constant 30 to a depth of at least 200 km while in the more northerly region this trend is apparent down to a depth of only 100 km at which depth the benioff zone becomes nearly horizontal there are enough earthquakes in our data set in the transition zone between the regions to show conclusively that the deformation of the benioff zone is continuous i e there is no discontinuity in the seismicity that might suggest a tear in the subducting plate these results corroborate those of hasegawa and sacks 1981 which used only about a third of the data now available

the potential influence of subduction zone polarity on overriding plate deformation trench migration and slab dip angle abstract a geodynamic model exists the westward lithospheric drift model in which the variety of overriding plate deformation trench migration and slab dip angles is explained by the polarity of subduction zones the model predicts overriding plate extension a fixed trench and a steep slab dip for westward dipping subduction zones e g mariana and predicts overriding plate shortening oceanward trench retreat and a gentle slab dip for east to northeastward dipping subduction zones e g chile this paper investigates these predictions quantitatively with a global subduction zone analysis the results show overriding plate extension for all dip directions azimuth 180 to 180 and overriding plate shortening for dip directions with 90 to 110 the wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west dipping slabs west dipping subduction zones are never fixed irrespective of the choice of reference frame while east to northeast dipping subduction zones are both retreating and advancing in five out of seven global reference frames in addition westward dipping subduction zones have a range in trench migration velocities that is twice the magnitude of that for east to northeastward dipping slabs finally there is no recognizable correlation between slab dip direction and slab dip angle east to northeast dipping slabs 30 to 120 have shallow 0 125 km slab dip angles in the range 10 60 and deep 125 670 km slab dip angles in the range 40 82 while west dipping slabs 60 to 120 have shallow slab dip angles in the range 19 50 and deep slab dip angles in the range 25 86 local mean deep slab dip angles are nearly identical for east and west dipping slabs while local mean shallow slab dip angles are lower by only 4 7 8 1 for east to northeast dipping slabs it is thus concluded that overall there is no observational basis to support the three predictions made by the westward drift model and for some sub predictions the observational basis is very weak at most alternative models which incorporate and underline the importance of slab buoyancy driven trench migration slab width and overriding plate motion are better candidates to explain the complexity of subduction zones including the variety in trench migration velocities overriding plate deformation and slab dip angles

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

glaciers of the levaya sygykta river watershed kodar ridge southeastern siberia russia modern morphology climate conditions and changes over the past decades kodar ridge 57 n 118 e southeastern siberia is a glaciarized mountain area of inner asia here we have studied areal changes of glaciers and mid summer snow cover in the levaya sygykta river watershed area 650 km2 60 of total ice cover we used 1 50 000 1 100 000 topographic maps landsat tm 1995 etm 2001 2002 high resolution cartosat 1 2009 and worldview 2 2013 imagery two digital elevation models dems and field survey data to quantify the areal changes of the glaciers since the end of the little ice age lia to 2013 in 2013 the exposed area of 18 small glaciers from 0 02 to 0 88 km2 in the study area was 3 917 0 064 km2 and volume 255 106 m3 between the lia maximum and 2013 the area has been decreased by 61 with rate of 0 377 a 1 the data available today suggest the areal shrinkage of kodar glaciers from the lia seems to be larger than elsewhere in siberia the rate of area decrease in 1995 2001 was five times greater than in 1850 2013 and in 1850 1995 1 7 times less than in 1850 2013 using regional climatic data we have found that climate conditions obviously stimulated glacier mass loss over the past at least 60 years the increased variability of absolute and relative changes in 1995 2013 was due to influence of local factors on the overall climate driven deglaciation trend comparison of two dems has showed that between 1970 and 2009 ice surfaces of 12 tested glacier tongues have been lowered on average by 29 15 m with a rate of 0 75 m a 1

seismicity in the outer rise forearc region and configuration of the subducting lithosphere with special reference to the japan trench along nine subduction zones where the largest mw or ms 7 6 normal faulting earthquakes have occurred beneath the outer rise there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc in six cases a gentle inner trench is also observed taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data we investigate the nature of this spatial correlation between seismicity and bathymetry along the honshu forearc in some detail an aseismic region in the forearc lies directly west of the rupture zone of the 1933 sanriku earthquake mw 8 4 a great normal faulting earthquake that most likely has ruptured the entire brittle portion of the lithosphere in the outer rise along this particular cross section of the japan trench large to moderate sized interplate earthquakes span a short interval of depths between approximately 14 and 31 km about half the interval observed along an adjacent profile to the south 12 45 km the aseismic portion of the plate interface between depths of approximately 30 and 45 km has persisted for over 100 years the occurrence of lithospheric normal faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab limited by yield strength of the lithosphere has nearly reached an asymptotic value moment saturation at this state the curvature of the plate can increase with little increase in bending moment as such any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate s curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise forearc region and configuration of the subducting lithosphere

source process of the great 1971 solomon islands doublet abstract large underthrusting earthquakes in the solomon islands tend to occur as multiplets separated by a few hours to several days in time and 30 100 km in space the largest sequence a doublet on july 14 1971 m w 8 0 and july 26 1971 m w 8 1 is particularly unusual in that it occured at the junction of two trenches with the july 14 event in the solomon islands trench preceding the july 26 rupture in the new britain trench the location and large moment release of this doublet as well as an incompatibility of published body wave and surface wave focal mechanisms for the july 14 event motivated a detailed study of the source process of the 1971 sequence to satisfy both the surface wave and body wave data we explored the possibility of a change in the faulting geometry during rupture for the july 14 event a fault model that changes orientation from a northwest southeast striking shallow dipping plane similar to the mechanisms of neighboring underthrusting events in the solomon islands trench to a more north south striking plane 70 s after rupture initiation is consistent with the observed rayleigh and love wave amplitude patterns this change in mechanism is suggested by systematic variations in the mechanisms of smaller earthquakes near the cusp in the solomon islands plate a simultaneous inversion of wwssn p wave seismograms for the time location seismic moment and focal mechanism of individual subevents gives results in agreement with the surface wave model although the precise change in mechanism is not well resolved the spatiotemporal distribution of subevents reveals two major pulses of moment release the first occurred on the initial fault near the epicenter at the origin time of the earthquake the second occurred on the north south striking fault 50 70 km northwest of the epicenter 70 s after the origin time the change in mechanism appears to reflect contortion of the slab near the trench junction inversion of wwssn p wave seismograms for the rupture history of the july 26 event reveals a westerly propagating rupture extending 60 km along the new britain trench for both of the 1971 earthquakes regions of high moment release are located in the vicinity of the solomon islands new britain trench junction contortion of the subducting lithosphere as it bends around the arc arc junction may have enhanced the degree of mechanical coupling between the subducting and overriding plates concentrating stress in this region and in turn promoting efficient triggering across the trench junction

spherical shell tectonics on the buckling of the lithosphere at subduction zones abstract the buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method fem and the effects of geometrical nonlinearity are examined the subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge in scaling the geometry of the subducting lithosphere two parameters are employed one is the thickness of the hemispherical shell i e the thickness of the lithosphere and the other is the length of its deformable portion i e the length of the descending slab to simulate the bending of the lithosphere at the trench uniform inward load is applied to the free circumferential edge of the deformable portion which corresponds to the leading edge of the subducting lithosphere at the opposite edge of the deformable portion i e at the trench a built in boundary condition is imposed no displacements and no rotations are allowed with a load greater than some critical value the deformable portion of the shell buckles and undulates azimuthally the investigation of this buckling phenomenon can be summarized as follows 1 1 buckling with a shorter wavelength is associated with a thinner and shorter shell mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much after appropriate scaling the lengths of subduction zones coincide with the buckling wavelength calculated by f e m 2 2 sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load the shape of this cuspidal undulation is quite similar to that actually observed at an arc arc junction 3 3 buckling can take place in the stress environment likely to exist in the earth these results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

the relationship between wadati benioff zone geometry and p t and b axes of intermediate and deep focus earthquakes for 865 earthquakes with focal depths exceeding 90 km this paper presents a concise global summary of the distributions of p t and b axes of their focal mechanisms with respect to the downdip along strike and normal axes of wadati benioff zones the focal mechanisms are best double couple solutions for the centroid moment tensors reported by the harvard group with the p t and b axes corresponding to the principal axes of the individual moment tensors we find as did previous investigators that events below 300 km have predominantly downdip p axes with 50 of all p axes of deep focus events lying within 28 of downdip for events between 90 and 300 km depth there are regional differences but most regions possess events with nearly downdip p axes or events with nearly downdip t axes in almost all regions having downdip p or t axes bingham statistics show that the greatest concentration of p or t axes lies somewhat below the plane of the wadati benioff zone for events of all focal depths in nearly all regions b axes cluster along the strike direction with 50 occurring within 38 of the strike from these regional results we define a typical focal mechanism as one having the b axis within 30 of the along strike and having either the t or p axis within 30 of the downdip such a focal mechanism might occur in response to a stress tensor having its greatest or least principal stresses channeled approximately along the downdip direction of the subducting slab with the intermediate principal stress lying horizontally within the slab controlled possibly by slab bending however by this criterion only 29 of the focal mechanisms for deep and intermediate events are typical thus the majority of the individual events cannot be explained simply as occurring in response to a downdip and slab normal greatest and least principal stress system with a horizontal along strike intermediate principal stress a few regions such as java and the marianas are clearly anomalous in that they possess significantly fewer typical focal mechanisms than do most other regions

dating of seasonal snow firn accumulation layers using pollen analysis reliable chronologies in ice cores and snow pits from many alpine glaciers in latitudes between 60 n and 60 s are often difficult to establish owing to problems with annual layer counting problems arise from melting wind erosion and the negligible amount of precipitation in some seasons all of which tend to obscure the seasonal variations in 18 o and chemical concentrations that are typically used to date ice cores however alpine glaciers contain many species of pollen grains that peak at particular times of the year we used the peaks in betulaceae pinus artemisia and a combination of abies and picea pollen species to determine the four seasonal layers of a snow pit on belukha glacier in russia s altai mountains comparing the pollen dated profiles with wind and precipitation records allows us to determine where a seasonal layer is missing thus the pollen dating method described here may be a useful tool to measure the annual snow deposition on alpine glaciers even when some seasonal layers are eroded by wind or missing due to negligible precipitation

role of lateral mantle flow in the evolution of subduction systems insights from laboratory experiments summary we present 3 d laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions in particular we present a set of experiments characterized by different conditions imposed at the trailing edge of the subducting plate that is plate fixed in the far field plate detached in the far field imposed plate motion experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle for each set we show differences between models that do or do not include the possibility of out of plane lateral flow in the mantle by varying the lateral boundary conditions our results illustrate how a subducting slab vertically confined over a 660 km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways on the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction

variations in p wave speeds and outboard earthquakes evidence for a petrologic anomaly in the mantle transition zone we investigate the nature of laterally varying p wave speeds vp and their relationship to a group of unusual deep earthquakes beneath the backarc of tonga this subhorizontal swath of outboard earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined wadati benioff zone surprisingly the seismogenic region is not associated with fast vp it follows that the effect of cold temperature as implied by the presence of large earthquakes must have been counteracted by other factors such as compositional or mineralogical effects our results are based on a sequence of seismic profiles in a fan shot geometry for a given azimuthal sector we seek the simplest model of radially varying vp that explains observed broadband waveforms and travel times the lateral extent of anomalous vp is then constrained by comparing the results from a wide range of azimuths this approach fully utilizes information from triplicate p arrivals that are sensitive to seismic wave speeds at the depths of interest our results identify prominent anomalies of fast vp 3 over average earth models both in the transition zone and in the lower mantle beneath the tonga backarc however the region of fast vp in the transition zone is offset by 300 km to the south of that in the lower mantle and the region of offset coincides with the outboard earthquakes a straightforward interpretation is that a large flux of subducted pacific lithosphere lingers in the transition zone before descending into the lower mantle within this broad remnant of subducted lithosphere the outboard earthquakes occur in a region where impounding of subducted material is most significant a condition favorable for the accumulation of met astable olivine or volatiles such materials reduce vp and are candidates for triggering earthquakes thus accounting for the absence of fast vp in the source zone of deep earthquakes

depositional and summer snow melting features in 2007 2011 on the upstream side of potanin glacier mongolian altai reconstructed by pollen and oxygen isotope analysis this study analyzed pollen in snow pits dug in late summer 2008 2011 on the upstream side of potanin glacier a summer accumulation type glacier in the mongolian altai to reconstruct the recent snow depositional environment particularly melting conditions annual snow pit observations were performed at sites 0 and 4 3752 and 3890 m a s l respectively seasonal layers in the pits were identified according to the presence of pollen from three taxa betulaceae pinus and artemisia which have different pollen seasons typically between may and early september the pollen dating method was successful in snow pits where conventional methods such as ice layer dating and use of 18o records were unreliable due to significant melting depths of pollen concentration peaks for the three taxa differed annually at both sites in some years concentration peaks of the three pollen taxa were found at different depths but in other years peaks associated with each taxon were found at the same depth this difference was explained by summer temperature snow melting during the relatively warm summers of 2007 and 2008 caused pollen grains to concentrate on the glacier surface producing peaks at the same depth in contrast snow melting during the relatively cool summer of 2009 did not substantially alter pollen grain location and each peak thus retained its original depth the median temperatures in 2010 and 2011 corresponded to pollen concentrating on the glacier surface at site 0 but retaining their original position at site 4

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

three dimensional miniature of earthquake distribution abstract we present a simple method to create a high resolution three dimensional miniature of hypocentral distribution a pile of transparent thin acryl plates to each of which are attached spherules with hypocentral marks are put into an acryl box filled with a certain liquid the coincidence of the refractive indices between the acryl plates and the liquid provides high transparency and high resolution of the miniature and enables us to observe it from every direction we modeled the distribution of crustal and subcrustal earthquakes beneath central japan into a miniature with dimensions of 36 28 12 cm observation of this miniature makes it possible to comprehend the complex three dimensional distribution of the hypocenters both clearly and intuitively

lithospheric deformation within the molucca sea arc arc collision evidence from shallow and intermediate earthquake activity this paper presents the results of a local earthquake survey in the molucca sea arc arc collision zone of eastern indonesia although intermediate depth earthquakes define zones dipping to the west beneath the sangihe arc and to the east beneath the halmahera volcanic arc only shallow activity less than 60 km depth occurred beneath the central part of the collision zone where the majority of activity is observed the concentration of earthquake foci in the 10 to 50 km depth range in a limited region beneath the talaud mayu ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening molucca sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs published focal mechanism solutions suggest that high angle reverse faulting beneath the talaud mayu ridge generates almost all of the seismic energy release within the collision zone the predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting molucca sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs the geometry and state of stress within the molucca sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting molucca sea plate at least at the present stage of collision

evolution of the slab bending radius and the bending dissipation in three dimensional subduction models with a variable slab to upper mantle viscosity ratio abstract three dimensional laboratory subduction models are presented investigating the influence of the slab upper mantle viscosity ratio sp um on the slab bending radius r b with sp um 66 1375 here r b is non dimensionalized by dividing it by the upper mantle thickness t um the results show that r b t um varies with time reaching a maximum when the subduction velocity is maximum furthermore r b t um increases approximately linearly with increasing sp um for the investigated viscosity range the model results show that the slab bending force f be and the energy dissipation during bending be are small compared to the negative buoyancy force of the slab f bu and the potential energy release during sinking bu maxima in be bu f be f bu are reached in the early stage of subduction when r b t um is minimum and the slab tip is at 220 440 km depth maximum be bu increases with increasing sp um with be bu max 0 06 0 11 0 18 and 0 22 for sp um 66 217 709 and 1375 respectively for subduction depths 220 440 km be bu 0 02 0 11 for all viscosity ratios assuming that in nature sp um r b t um and be bu to natural subduction zones implies that in nature sp um 1 7 10 2 and um 0 8 2 7 10 20 pa s finally the laboratory models which use glucose syrup and silicone oil as modelling materials highlight the importance of accurate control on temperature during an experiment new material investigations show that the viscosity of these two materials decreases exponentially with temperature in the range 3 33 c their density decreases approximately linearly with temperature and their coefficient of thermal volumetric expansion is 3 8 4 2 10 4 c 1 glucose syrup and 9 2 10 4 c 1 silicone oil

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

application of pollen analysis to dating of ice cores from lower latitude glaciers 1 ice cores from temperate regions have long been expected to be useful for understanding local climate trends but a reliable dating method has proved difficult here we show that measurements of pollen using samples with only 10 ml of water can give an accurate measure of the annual ice accumulation in addition two major types of pollen allow us to estimate the summer ice accumulation as well as the annual accumulation

seismological evidence for a slab detachment in the tonga subduction zone abstract the tonga wadati benioff zone is characterized by a large seismicity gap beneath the lau basin that raises the question of the slab continuity between the shallow to intermediate part 60 300 km and the deep part 400 700 km to address this problem we investigated the wadati benioff zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor using the global seismicity engdahl e van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seism soc am 88 722 743 and the centroid moment tensor solutions cmt catalogs dziewonski a m chou t woodhouse j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 the stress tensors were computed using the gephart s program gephart j w forsyth d 1984 an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence j geophys res 89 9305 9320 the stress inversion results indicate that between 21 s and 27 s and depths down to 700 km the slab is under homogeneous down dip compressional stress regime while north of 21 s we found strong variations of the stress orientations between the intermediate and deep portions of the slab we also show that between 14 and 19 s the stresses at intermediate depth 60 300 km can be resolved into two slab parallel domains a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension this pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670 km depth boundary these results together with the large seismicity gap within the slab argue for a slab detachment