seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting pacific plate beneath the northeastern japan arc a triple planed deep seismic zone the northeastern japan arc is located in one of the most seismically active subduction zones in the world in this study we relocated hypocenters and determined focal mechanisms of small earthquakes m 5 beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate in the hypocenter relocation we adopted a source region station correction method in which station corrections vary with hypocenter locations we have developed a new focal mechanism determination technique named master solution method which is analogous to the master event method in hypocenter determination we applied the method to p and sh wave amplitude data to obtain 1106 focal mechanism solutions from the new mechanism solutions and relocated hypocenters we found that there occur both low angle thrust fault lt type and downdip compression dc type earthquakes at depths from 40 to 70 km near the aseismic front the dc type events are underlying the lt type events almost all the earthquake clusters are composed of lt type events the western limit of the region where lt type events have occurred is subparallel to the trench axis although it undulates considerably it delineates the westernend of the active region of interplate seismicity furthermore we found that normal fault nf type events also occur at depths from 70 km in the upper plane of the double planed deep seismic zone which is characterized mainly by dc type event these nf type events are distributed only in a thin uppermost portion of the slab close to the plate boundary below these in the lower plane are downdip extension de type events this result indicates that the deep seismic zone in the northeastern japan arc is not double planed but triple planed even beneath the land area which cannot be explained by any simple models

the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

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

on the origin of the drumlins of the south german alpine foreland abstract the drumlins formed during the last wurmian glaciation in the south german alpine foreland are situated in distinct topographic positions these drumlins are found on the gently ascending molasse socles between the glacially incised basins and behind the terminal moraines of the in the rhein glacier area so called inner young terminal moraines the former position has been explained as a result of differential ice thicknesses between the basins and the molasse interfluves the latter position remained problematical recent investigations in the frontal area of the past iller glacier indicate that 1 the formation of the drumlins date from one distinct glacier re advance the luiblings phase and 2 the permafrost table within near surface sediments at the time of drumlin formation was at a sufficient depth not to have any influence upon the formative processes at surface level the drumlins therefore formed because the glacier re advanced over decaying permafrost within non lithified pleistocene sediments whereas in the past the ice had advanced over frozen ground and later over unfrozen ground this situation as found in front of the iller glacier is not an exceptional case in comparing three other areas the western rhein glacier area the inn chiemsee area and the eastern flank of the iller glacier a general conformity of both landforms and glacial processes was found to exist however where slight variations do occur adequate explanations can be made

mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

metamorphosed boninitic basalts arc tholeiites and cryptic volcanic stratigraphy from the elzevir terrane of the grenville province calumet mine quebec amphibolites hornblende plagioclase rocks with tpeak metamorphism 650 700 c are abundant near the old calumet zn pb ag mine and au prospect 90 km northwest of ottawa they lack primary structu

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

intraslab earthquakes dehydration of the cascadia slab we simultaneously invert travel times of refracted and wide angle reflected waves for three dimensional compressional wave velocity structure earthquake locations and reflector geometry in northwest washington state the reflector interpreted to be the crust mantle boundary moho of the subducting juan de fuca plate separates intraslab earthquakes into two groups permitting a new understanding of the origins of intraslab earthquakes in cascadia earthquakes up dip of the moho9s 45 kilometer depth contour occur below the reflector in the subducted oceanic mantle consistent with serpentinite dehydration earthquakes located down dip occur primarily within the subducted crust consistent with the basalt to eclogite transformation

an early proterozoic volcanic arc succession in southeastern wyoming the green mountain formation of early proterozoic age in the sierra madre range of southeastern wyoming comprises a bimodal mafic and felsic volcanic assemblage the rocks which are chiefly breccias agglomerates flows and volcaniclastic sediments represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments volcanic rocks are clearly calc alkaline in character and share a large number of geochemical features in common with continental margin arcs or evolved oceanic arc systems the low mg numbers and ni contents of the basalts require 30 40 olivine fractional crystallization and the high contents of the most incompatible elements high la sm n ratios and low zr nb ratios require an undepleted or enriched mantle source geochemical data are consistent with an origin for the felsic volcanics and associated encampment river granodiorite by shallow fractional crystallization of calc alkaline basalt in a tectonic setting similar to modern ar

high resolution image of the geometry and thickness of the subducting nazca lithosphere beneath northern chile 1 results obtained from s and p receiver functions produced a clear image of the top and bottom of the subducting nazca lithosphere beneath northern chile using data from the teleseismic events recorded at 15 permanent integrated plate boundary observatory chile ipoc stations we obtained new constraints on the geometry and thickness of the descending nazca lithosphere we observed the subducted crust of the nazca plate at depths ranging from 50 km beneath the coastal cordillera down to 110 km beneath the western cordillera we found significant along strike variations in the geometry of the nazca plate beneath northern chile on closer inspection it appears that the oceanic nazca plate is divided into two distinct segments as it descends beneath the continental south american plate the transition from the relatively steeper 23 and deeper slab to the north of 21 s to the flatter southern segment 19 is shown reasonably clearly by our data this feature could well be associated with variations in the curvature of the plate margin and the geometry of the chile trench which is mainly curved to the north of 21 s we have also mapped the continental moho of the south american plate at depths ranging between 60 and 70 km to the east of the longitudinal valley beneath the coastal cordillera this boundary becomes invisible probably due to the serpentinization of the forearc mantle wedge that reduces the velocity in the uppermost mantle the base of the subducted nazca plate was clearly identified as a sharp boundary in the results obtained from the p and s receiver functions the thickness of the subducted oceanic nazca plate which has an age of 50 my is estimated to be 50 km although this thickness is consistent with that predicted by thermal gradients the explanation of the sharpness of the lithosphere asthenosphere boundary may require another mechanism such as hydration or melting

origin of the active drumlin field at mulajokull iceland new insights from till shear and consolidation patterns abstract stratigraphic and morphologic data previously collected from the forefield of mulajokull iceland suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them we have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier s forefield patterns of till strain indicated by the anisotropy of magnetic susceptibility ams of 2300 intact till samples indicate that till was deposited during shear deformation with shearing azimuths and planes that conform to the drumlin morphology thus till deposition occurred as drumlins grew in agreement with lidar data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges previously described unconformities on the drumlin flanks however indicate that drumlin relief at mulajokull has resulted in part from erosion given that the last surge deposited a till layer both on and between drumlins a reasonable hypothesis is that erosion between drumlins occurred during normal quiescent flow of the glacier between surges densities of till samples analyzed in conjunction with laboratory consolidation tests indicate that effective stresses on the bed during such periods were on the order of 100 kpa larger between drumlins than within them an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins the observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation moreover the lack of ams fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till flux gradients in a deforming bed

imaging the source region of cascadia tremor and intermediate depth earthquakes the subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface trigger earthquakes and regulate transient phenomena such as episodic tremor and slip ets it is not clear how dehydration can separately induce earthquakes within the subducting plate and ets partly because few data exist on their relationship to subduction zone structures we present results of a seismic experiment in the washington cascades united states that images a region producing both earthquake types migration of scattered teleseis mic waves provides images of low velocity subducting crust at depths 40 45 km with sharp boundaries above and below it the sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40 45 km depth regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is 95 km deep but ets only exists where the sharp upper boundary occurs the ets location supports models of slow slip that require near lithostatic fluid pressure whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration very low shear stresses on the plate boundary may limit seismicity to ets and similar phenomena

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

anomalous deepening of a seismic belt in the upper plane of the double seismic zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials abstract hypocenter relocations of earthquakes in the pacific slab have shown an anomalous deepening of a seismic belt in the upper plane of the double seismic zone at depths of 80 120 km beneath the hokkaido corner while it is located at depths of 70 90 km in the surrounding tohoku and eastern hokkaido areas seismic tomographic inversions performed beneath the hokkaido corner have shown that a low velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the pacific slab and makes direct contact with the upper surface of the pacific slab these observations suggest that 1 the low velocity zone just above the pacific slab is part of the subducted kuril forearc sliver 2 the contact with the subducted and so relatively cold sliver materials prevents the mantle wedge from heating the pacific slab and causes a lower temperature condition in the pacific slab crust beneath the hokkaido corner than in the surrounding areas 3 as a result a delay of eclogite forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there

intraslab stresses in the cascadia subduction zone from inversion of earthquake focal mechanisms at the cascadia subduction zone intraslab earthquakes occur mostly in the northern part of the margin and its southern end the mendocino triple junction mtj we determine intraslab stress orientations by inverting earthquake focal me chanisms and develop working hypotheses to explain the inferred intraslab stresses and observed seismicity our inversion results show that the subducting juan de fuca jdf slab in northern cascadia is primarily under compression normal to the slab surface and tension in the downdip direction most likely controlled by the net slab pull an exception is a northernmost shallow region near the nootka fault zone where the state of stress is dominated by nearly east west tension we hypothesize that the shear force on the nootka fault zone and margin parallel mantle resistance to slab motion induce the east west tension in this region near the mtj stresses in the jdf plate are dominated by north south compression down to about 20 km depth consistent with a strong push by the pacific plate from south of the mendocino trans form fault but the deeper part of the slab shows downdip tension similar to northern cascadia deviatoric stresses in the jdf slab appear to be very low resulting in very low intraslab seismicity in comparison with northern cascadia the stresses in most of southern cascadia are even lower resulting in nearly no intraslab seismicity

geochemical criteria for a refined tectonic discrimination of orogenic andesites abstract minor trace element and sr isotope data are compiled for tertiary recent orogenic andesites of three tectonic settings 1 oceanic island arcs divided into low k and other andesites 2 continental island arcs and thin continental margins and 3 thick continental margins andean from 1 to 3 there is a steady overlapping increase in la ce ree la yb la y th th u zr y hf yb and ni co and a decrease in k la p la sc cr and sc ni optimum discriminationis achieved with a plots of la yb vs th and la yb vs sc ni and b cluster analysis using the critical parameters listed above shoshonitic andesites and high mg andesites boninites are considered separately orogenic andesites are distinguished from non orogenic andesites by high contents of al low contents of ti zr ree y nb ta and ga and the absence of pronounced negative eu anomalies

nd isotope mapping of a pinwarian age composite arc belt in the quebecia terrane of the central grenville province canada abstract there continues to be confusion in the geological community concerning the ages of crustal formation and crustal accretion within the central grenville province of quebec to clarify the geological history of crustal growth in the eastern part of the quebecia terrane sixty new nd isotope analyses are presented from a 60 000 km2 area northwest of baie comeau most samples yield pinwarian nd model ages that suggest them to be derived from juvenile oceanic arc terranes however two large slivers with paleoproterozoic tdm ages attest to the existence of hidden blocks of old crust which divide the area of juvenile pinwarian crust into a composite arc belt of smaller accreted terranes the old crustal blocks probably do not reach the present surface but represent an igneous crustal protolith of younger i type magmatism by comparison with a mesozoic tectonic analogue it is proposed that accreted juvenile arc crust with attached slivers of older crust were rifted away from an active margin to the west and translated eastwards until blocked by the previously accreted labradoria terrane although the quebecia composite arc belt consists of over 95 of pinwarian juvenile arc crust the age of arc accretion was post pinwarian the proposed oblique nature of the accretion process may explain the lack of a major collisional orogen at that time this paper shows the usefulness of large scale nd isotope mapping of precambrian crustal terranes to clarify complex processes of crustal formation and arc accretion

crustal assimilation in the burnt lake metavolcanics grenville province southeastern ontario and its tectonic significance three intimately interbedded suites of volcanic rocks are identified geochemically in the burnt lake area of the belmont domain in the central metasedimentary belt and their petrogenesis is evaluated the burnt lake back arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back arc basins formed in continental crust the burnt lake continental tholeiitic suite comprises basalts and andesites similar in trace element composition to continental tholeiitic sequences the burnt lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts rare earth element models suggest that the burnt lake back arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5 partial melting of an amphibole bearing depleted mantle enriched in light rare earth elements by a subduction component the modelling also suggests that t

existence of interplane earthquakes and neutral stress boundary between the upper and lower planes of the double seismic zone beneath tohoku and hokkaido northeastern japan abstract using data from the recently constructed nationwide dense seismic network we determined the hypocenters and focal mechanisms of many intermediate depth intraslab earthquakes within the pacific slab beneath northeastern japan the results show that in addition to the upper and lower planes of the double seismic zone a considerable number of intraslab earthquakes also occur between the two planes this interplane earthquake activity is not homogeneously distributed in space being high beneath eastern hokkaido and the fore arc regions of southeast and central tohoku the focal mechanisms of the interplane earthquakes tend to be of the down dip compressional type dc in tohoku and the hokkaido corner but down dip tensional type de in eastern hokkaido upper plane earthquakes are characterized by dc type stress while lower plane earthquakes are de type in both tohoku and eastern hokkaido the existence of interplane earthquakes enables estimation of the position of the neutral plane between the upper plane dc stress and the lower plane de stress we did so by applying stress tensor inversions to focal mechanism data obtained in the present study and japan meteorological agency jma data the results show that the neutral plane is located about 22 km beneath the upper plate interface beneath tohoku but only about 10 km beneath the upper surface beneath eastern hokkaido this difference in the location of the neutral plane may be due to the difference in buoyancy force exerted by the less dense metastable olivine wedge that is a result of the oblique plate subduction beneath hokkaido comparison of large intraslab earthquakes beneath the two regions shows that their aftershock areas are limited by the neutral plane suggesting that large earthquake ruptures are confined to either the dc or de stress field and do not go beyond the neutral plane

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

the petrogenesis of mafic minor intrusions and volcanics of the central metasedimentary belt grenville province canada morb and oib sources abstract numerous separate suites containing mafic igneous rocks each characterized by its own trace element signature have been identified and described in the western part of the elzevir terrane of the central metasedimentary belt of the grenville province in canada the average compositions of the mafic rocks from the tudor and turriff volcanics and the low zirconium lzs intermediate zirconium izs high zirconium hzs calc alkaline cas highly fractionated tholeiite hft and ultra potassic uks suites of minor intrusions all show negative nb and ti anomalies on chondrite normalized multi element plots this series of rock suites with their characteristic anomalies have been interpreted as being emplaced in a marginal basin formed on attenuated continental crust similarities in multi element plots and the results of petrogenetic calculations suggest that the turriff lavas lzs izs and tudor lavas and hzs suites were all formed from melts derived from the same spinel lherzolite source the composition of this source is considered to be similar to that from which mid ocean ridge basalts are derived modified by crustal contamination the hft cas and uks rocks were formed from melts derived from various garnet lherzolite sources these sources may be modelled as having a trace element composition similar to that from which ocean island basalts are derived modified to different extents by subduction derived components

drumlins and catastrophic subglacial floods shaw j kvill d and rains b 1989 drumlins and catastrophic subglacial floods in j menzies and j rose editors subglacial bedforms drumlins rogen moraine and associated subglacial bedforms sediment geol 62 177 202 a number of lines of evidence are shown to be consistent with drumlin formation by catastrophic meltwater floods drumlins are associated with landforms related to such floods and landscapes produced by outburst floods in extragiacial areas contain drumlin like landforms some drumlins which contain mainly residual materials are compared to similar forms covering a wide range of scales and are interpreted to be scour remnant ridges produced by the action of horseshoe vortices on subgiacial materials these vortices are a product of flow separation in turbulent fluids and this interpretation implies drumlin formation by meltwater other drumlin calied cavity fill drumlins have distinctive shapes that are analogous with infilled erosional marks produced by turbulent separated flows these drumlins are interpreted as the infills of cavities eroded upwards into glacier beds by broad subgiacial meltwater sheetflows the sedimentology and internal structure of the cavity fill drumlins indicate rapid deposition from hyperconcentrated flow clast shape surface texture and lithology are consistent with the short distance of giaciofhivial transport predicted by the cavity fill hypothesis estimates of flow discharge 0 6 6 x 107 m3 s total volume of flow 84 1 103 km3 and flow duration 16 162 days are given for the catastrophic flood proposed to have produced the livingstone like saskatchewan drumlin field subgiacially derived floods are used to explain the sudden drainage of extraglacial lakes and major depositional events in proglacial lakes and it is suggested that they might have contributed to the lid of cold freshwater over the north atlantic held to have caused a late glacial cooling event the flood hypothesis requires extensive subgiacial lakes to have occurred beneath the laurentide and fennoscandian ice sheets which if they existed must have influenced the form and behaviour of these major bodies of ice

the queensborough mafic ultramafic complex a fragment of a meso proterozoic ophiolite grenville province canada abstract the queensborough mafic ultramafic complex occurs as a fault bounded block up to 10 km wide and having an area 220 km2 it lies in the grimsthorpe domain of the bancroft elzevir mazinaw sharbot lake terrane in the central metasedimentary belt of the grenville province it has been suggested without adequate supporting data that the complex may represent oceanic crust a fragment of an ophiolite or even a metavolcanic sequence made up of basaltic and komatiitic flows the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre scale compositional layering represented by several different metamorphic assemblages of talc chlorite carbonate anthophyllite and actinolite tremolite these assemblages indicate that the original rocks were cumulate peridotites and pyroxenites the ultramafic rocks are overlain structurally by a series of mafic rocks predominantly massive to highly sheared gabbros the gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics major and trace element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros lavas and mafic dykes and that the ultramafic cumulates are co genetic the regional geological setting and the trace element signatures of the mafic rocks suggest that they were formed in a back arc basin comparison of the queensborough complex with proterozoic and phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a mesoproterozoic ophiolite in addition the rocks of the queensborough complex are petrographically and geochemically similar to those of the vavilov basin which occurs in the deepest part of the tyrrhenian sea by analogy with this neogene back arc basin we suggest that the central metasedimentary belt which contains the queensborough complex formed by attenuation of the continental crust the complex is preserved along a suture and is interpreted as having formed during the collision of a continental island arc with the continental margin of laurentia the characteristics and preservation of the queensborough mafic ultramafic complex are very similar to those of many phanerozoic ophiolites and suggest that mesoproterozoic thermotectonic processes are also similar to those presently operating

geochemistry of two metavolcanic arc suites from the central metasedimentary belt of the grenville province southeastern ontario canada samples of two subalkaline metavolcanic suites the tudor formation ca 1 28 ga and the overlying kashwakamak formation have been analysed for major elements and 27 trace elements including rare earth elements the tudor formation is tholeiitic and contains mainly basaltic flows whereas the kashwakamak formation is calc alkaline and contains mainly andesitic rocks with minor felsic rocks the succession has been regionally metamorphosed to upper greenschist lower amphibolite facies trace element abundances and ratios indicate that rocks of the tudor and kashwakamak formations are island arc type geochemical modelling using rare earth elements zr ti and y indicates that the tudor volcanic rocks are not derived from a single parental magma through simple fractional crystallization equilibrium partial melting of a heterogeneous proterozoic upper mantle can explain the trace element abundances and ratios of tudor formation volcanic rocks the intermediate to felsic rocks of the kashwakamak formation appear to have been derived from a separate partial melting event the data are consistent with an origin of the arc either on oceanic crust or on thinned continental crust and with accretion of the arc to a continental margin between the time of extrusion of tudor volcanic rocks and that of kashwakamak volcanic rocks

cordierite gedrite rocks from the central metasedimentary belt boundary thrust zone grenville province ontario mesoproterozoic metavolcanic rocks with affinities to the composite arc belt cordierite gedrite rocks in the southern grenville province occur near the base of the central metasedimentary belt boundary thrust zone interpreted by some as a crustal suture between the 1 29 1 24 ga composite arc belt and 1 4 ga rocks of laurentia major and trace element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite these cordierite gedrite rocks have low cao average 1 2 wt and major element and oxygen isotope ratios suggestive of hydrothermal alteration before metamorphism rare earth element ree compositions also indicate igneous protoliths although some ree patterns have been modified by local melt extraction the trace element compositions of cordierite gedrite rocks and neodymium isotope systematics are similar to those of metavolcanic rocks in the composite arc belt and are consistent with the extension of the composite arc belt to the base of the boundary thrust zone

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

exhumed subglacial landscape in uruguay erosional landforms depositional environments and paleo ice flow in the context of the late paleozoic gondwanan glaciation abstract a well exposed glacial surface sculpted on precambrian crystalline basement rocks occurs below the glacial succession of the san gregorio formation on the eastern border of the chaco parana basin in uruguay and was formed in the context of the late paleozoic gondwana ice age on the glacial surface are asymmetric parallel streamlined bedrock landforms interpreted as whalebacks the downglacier lee side faces of the whalebacks have gentle slopes dipping nnw with striated and sometimes polished surfaces on crystalline rocks these landforms are covered by 10 100 cm thick layers of tillites and shear laminated siltstones suggesting glacial abrasion produced mainly by subglacial till sliding the subglacial facies are ice molded and exhibit meso scale glacial lineations such as ridges and grooves up to 30 m long and 30 cm deep the subglacial association is directly overlain by proglacial fine grained facies rhythmites with dropstones indicating a subaqueous depositional environment following ice margin retreat the fine grained facies are erosively cut by a succession of sandstones with wave generated stratification resting on a basal conglomerate intraformational striated surfaces nne oriented were found on four distinct bedding planes within the sandstone package and interpreted as ice keel scour marks produced by floating ice the san gregorio deposits are partially confined in a wide and shallow subglacial trough and the stratigraphic succession is interpreted as the record of a glacial advance retreat cycle comparable to deglacial sequences from other late paleozoic localities the paleo ice flow to the nnw indicated by subglacial lineations is parallel to that verified in the southernmost parana basin located north of the study area suggesting a paleogeographic scenario in which glaciers advanced northward into a glaciomarine environment the proposed palaeogeography does not confirm the previous hypothesis of an ice center on the sul riograndense shield but instead it corroborates a south derived uruguayan ice lobe advancing to the north probably with provenance far afield in terranes of the present day southern african

variations of fluid pressure within the subducting oceanic crust and slow earthquakes 1 we show fine scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high pressure fluids released by progressive metamorphic dehydration reactions in the subducting philippine sea plate in tokai district japan these zones have a strong correlation with the distribution of slow earthquakes including long term slow slip ltss and low frequency earthquakes lfes overpressured fluids in the ltss region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore arc and impede intraslab earthquakes therein in contrast fluid pressures are reduced in the lfe zone which is deeper than the centroid of the ltss because there fluids are able to infiltrate into the narrow corner of the mantle wedge leading to mantle serpentinization the combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary which in turn control the occurrence of slow earthquakes

thermal petrological controls on the location of earthquakes within subducting plates we find that in young and warm subducting plates earthquakes occur just below the moho in older plates earthquakes occur throughout the subducting oceanic crust as well as the subducting mantle we document this behavior in several subduction zones where there are independent constraints on earthquake locations and slab structure specifically for northern and southern japan alaska and cascadia the differences in earthquake depth relative to subducting crust may reflect large differences in temperature and thus locations of major dehydration reactions in colder slabs the crust passes through blueschist facies dehydration reactions while in cascadia and nankai the major dehydration reactions in crust may be due to zoisite and amphibole breakdown or associated melting the cold paths allow more mineral bound h2o to be retained within the crust at shallow depths eventually released upon dehydration over shorter time intervals than warm paths the cold path dehydration reactions also result in net positive volume changes of solid fluid with solid volume decreasing less than the volume of h2o produced on hot paths the net volume changes are negative with solid volumes decreasing more than the volume of h2o produced the difference in behavior could drive a net increase in pore pressure upon dehydration for the cold but not the hot crustal paths the difference in rate of release in h2o and difference in sign of net system volume change may promote seismogenesis in cold subduction zones but inhibit it in the crust of warm slabs within the mantle of the downgoing plate earthquakes mostly occur where serpentine is stable or breaks down in both settings

new geophysical insight into the origin of the denali volcanic gap summary volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins they are regions where the necessary conditions to produce melt may appear favourable but where volcanoes are surprisingly absent from the surface in this study we present a new model that can explain the occurrence of such volcanic gaps it is based on seismic imaging and geodynamic modelling of the denali volcanic gap a 400 km wide region at the eastern end of the alaska aleutian subduction zone here the thick crust of the pacific plate and yakutat terrane subduct at shallow angle beneath north america a high resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge we interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate in steadystate subduction models melt accumulates at the apex of a vaulted mantle wedge the pinch zone from where it may break through the overlying lithosphere to the surface beneath the denali volcanic gap the pinch zone is absent or greatly reduced because shallow subduction of the yakutat terrane progressively cools the system and causes the slab to advance and replace the hot core of the mantle wedge this regime can be seen as the opposite of subduction rollback it prevents the formation of a pinch zone reduces the length of the melting column and causes melt to pool at the base of the overriding plate thus inhibiting magma generation and extraction

what controls interplate coupling evidence for abrupt change in coupling across a border between two overlying plates in the ne japan subduction zone article i nfo interplate coupling in the southernmost extent of the northeast japan subduction zone east off kanto japan is investigated in detail using a large data set of small repeating earthquakes in this region of the trench trench trench type triple junction the subducting pacific plate is in contact with two different overlying plates the north american and philippine sea plates the border between the two overlying plates extending northwestward from the triple junction is identified along the upper surface of the pacific plate based on the slip vectors of many interplate events including small repeating earthquakes the interplate coupling coefficient estimated from the cumulative slip of small repeating earthquakes for the period from 1993 to 2007 reveals a distinct change from south ca 0 3 to north ca 0 7 across the border this border also corresponds to the southern limit of the source areas of mn7 interplate earthquakes in the last 80 years along the japan trench again indicating the stronger coupling to the north we also investigate hypocenter distribution and seismic velocity structure along the plate boundary from a large number of travel time data obtained from the nationwide seismograph network the results reveal a distinct low velocity zone just above the pacific plate and low seismicity along the plate boundary in the region overlain by the philippine sea plate whereas there are no low velocity zones and the seismicity is high in the region overlain by the north american plate these observations imply that the geological difference of overlying plate controls large scale coupling at the plate interface

tomographic imaging of hydrated crust and mantle in the subducting pacific slab beneath hokkaido japan evidence for dehydration embrittlement as a cause of intraslab earthquakes abstract we estimate detailed three dimensional seismic velocity structures in the subducting pacific slab beneath hokkaido japan using a large number of arrival time data from 6902 local earthquakes a remarkable low velocity layer with a thickness of 10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust the layer gradually disappears at depths of 70 80 km suggesting the breakdown of hydrous minerals there we find prominent low velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 kushiro oki earthquake m7 8 since seismic velocities of unmetamorphosed peridotite are much higher than the observations hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake on the other hand regions between the upper and lower planes where seismic activity is not so high compared to the both planes show relatively high velocities comparable to those of unmetamorphosed peridotite our observations suggest that intermediate depth earthquakes occur mainly in regions with hydrous minerals which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

lithology and stratigraphy of some rhine glacier drumlins south german alpine foreland abstract various exposures and core drillings in drumlins of the southwestern part of the bodanruck drumlin field were examined sequences start with fine grained till and lake deposits followed by marginal gravels and end upwards with an upper till which is often deformed and sheared the whole sequence is of wurm age gravels and upper till are deposits of the last major re advance of the piedmont glacier innere jungendmorane ije which is also the time of drumlin formation clastic dykes within the gravels also date from this time they are younger than gravels but older than the upper till thickness of permafrost during dyke intrusion is estimated to be 50 m drumlins consisting of gravel and till are described an early erosional phase of drumlin formation is contemporaneous to the erosion of the mindelsee glacial basin also upper parts of one of the clastic dykes were deformed indicating that the permafrost surface may have started to sink down during deformation facies transitions of till drumlins may reflect ongoing erosion of the mindelsee basin or local paleotopography of the gravel surface

an automated strategy for calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions we formulate an algorithm for the calculation of stable phase relations of a system with constrained bulk composition as a function of its environmental variables the basis of this algorithm is the approximate representation of the free energy composition surfaces of solution phases by inscribed polyhedra this representation leads to discretization of high variance phase fields into a continuous mesh of smaller polygonal fields within which the composition and physical properties of the phases are uniquely determined the resulting phase diagram sections are useful for understanding the phase relations of complex metamorphic systems and for applications in which it is necessary to establish the variations in rock properties such as density seismic velocities and volatile content through a metamorphic cycle the algorithm has been implemented within a computer program that is general with respect to both the choice of variables and the number of components and phases possible in a system and is independent of the structure of the equations of state used to describe the phases of the system

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

u pb geochronology of the raglan gabbro belt central metasedimentary belt ontario implications for an ensialic marginal basin in the grenville orogen the raglan gabbro belt of the ontario grenville orogen is coincident with the top of the central metasedimentary belt boundary thrust zone a major mid crustal shear zone separating the central gneiss belt in the footwall from the central metasedimentary belt in the hanging wall it has been suggested that the gabbros making up the belt are coeval that they formed in a marginal basin within the central metasedimentary belt and that they formed a horizon of theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca 1190 ma u pb zircon dating of plutons within the raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt magmatic crystallization ages for three of the gabbros fall in the range 1246 1227 ma and a fourth yields a minimum age of ca 1175 ma the results are permissive of a common origin for the gabbros and allow that the raglan gabbro belt may have been related to

a new view into the cascadia subduction zone and volcanic arc implications for earthquake hazards along the washington margin in light of suggestions that the cascadia subduction margin may pose a significant seismic hazard for the highly populated pacific northwest region of the united states the u s geological survey usgs the research center for marine geosciences geomar and university collaborators collected and interpreted a 530 km long wide angle onshore offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation we observed 1 an increase in the dip of the juan de fuca slab from 2 7 to 12 where it encounters a 20 km thick block of the siletz terrane or other accreted oceanic crust 2 a distinct transition from siletz crust into cascade arc crust that coincides with the mount st helens seismic zone supporting the idea that the mafic siletz block focuses seismic deformation at its edges and 3 a crustal root 35 45 km deep beneath the cascade range with thinner crust 30 35 km east of the volcanic arc beneath the columbia plateau flood basalt province from the measured crustal structure and subduction geometry we identify two zones that may concentrate future seismic activity 1 a broad because of the shallow dip possibly locked part of the interplate contact that extends from 25 km depth beneath the coastline to perhaps as far west as the deformation front 120 km offshore and 2 a crustal zone at the eastern boundary between the siletz terrane and the cascade range

the geochemical nature of the igneous rocks of the sharbot lake domain central metasedimentary belt ontario two bimodal mafic silicic suites of igneous rocks the sharbot lake volcanic rocks and the lavant igneous complex are identified geochemically in the sharbot lake domain of the central metasedimentary belt in ontario and their genesis and thermotectonic environment are evaluated the sharbot lake volcanic rocks comprise a series of basalts characterized by light rare earth element lree depletion and relatively high concentrations of fe2o3 tio2 mno v and y together with rhyolites and silicic pyroclastic rocks they are intruded by rocks of the lavant igneous complex which comprises tholeiitic gabbros characterized by lree enrichment and low concentrations of fe2o3 tio2 mno v and y and granitoid rocks the trace element signatures of the mafic rocks of the sharbot lake volcanic sequences are most like those of back arc tholeiitic basalts and those of the lavant igneous complex are comparable to those of low k tholeiitic basalt suites the trace element signatures of the silicic rocks ass

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

seismic structure and seismogenesis off sanriku region northeastern japan summary in 1997 a seismic experiment using an airgun array and ocean bottom seismographs obss was performed in the forearc region of the northeastern japan nej arc the objectives of this experiment were to clarify seismic structures of the forearc region off sanriku japan using airgun obs data and to understand the nature of the japan trench seismogenic zone fundamental features of the structure are as follows i the subduction angle becomes steeper from east to west from 3 to 8 to 11 with the top of the plate located at depths of 11 km 40 km west from the trench 21 km 120 km west from the trench and 28 km 140 km west from the trench respectively ii the nej moho beneath the forearc is defined at a depth of approximately 20 km iii p wave velocity of the island arc upper mantle is approximately 8 km s 1 iv the p wave velocity of the cretaceous sedimentary layer exhibits significant lateral variability 4 0 5 5 km s 1 i nthe e w direction v the upper crust of nej forearc also has the significant lateral heterogeneity 5 0 6 3 km s 1 i nthe e w direction but not in the n s direction vi the middle crust defined on land extends in some locations 40 km east of the coastline vii lateral variability in the p wave velocity of the lower crust 6 7 7 0 km s 1 i ssmall compared with that of the upper crust according to these results it is suggested that the rupture areas of large earthquakes m7 in this region are limited within the contact zone between the nej arc crust and the subducting oceanic crust although a thin layer with low velocity is probably present within this contact zone the serpentinization of uppermost mantle wedge of the nej arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited this might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km

archean mafic metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 1 mobility of the major elements reply in a suite of archean mafic pillows from the rouyn noranda region of quebec s abitibi greenstone belt including both tholeiitic and calc alkaline varieties spanning the prehnite pumpellyite to upper greenschist metamorphic facies three types of alteration can be defined i chlorite epidote actinolite ii chlorite epidote and iii chlorite sericite the number of mineral phases decreases as a result of progressive hydration from type i to type iii alteration albitization resulting from substitution of in calcic plagioclase is highly variable in type i alteration but in types ii and iii the plagioclase is totally albitized and in some cases silicified chloritization is closely linked to increasing hydration and ca leaching with mgo and feo substituting for cao in ferromagnesian minerals calcium was mobilized and carried by solutions as evidenced by the variable concentration of epidote at the margins of pillows this calcium leaching generated an excess of al2o3 with respect to the combine

rupture process and near source shaking of the 1965 seattle tacoma and 2001 nisqually intraslab earthquakes 1 we inverted teleseismic data for the 1965 seattle tacoma earthquake slip and rake the earthquake had 2 asperities of 12 and 16 km2 near the hypocenter with average slip of 2 and 2 8 m respectively mo 9 4 1025 dyne cm mw 6 6 we also inverted strong motion and teleseismic data for the 2001 nisqually earthquake this earthquake had 2 subevents with an asperity area of 45 km2 between 59 and 62 km depth with an average slip of 1 7 m mo 1 7 1026 dyne cm mw 6 8 shake maps of peak ground acceleration pga and spectral acceleration sa were generated using a time domain approach including slip model site and basin depth corrections based on california geology corrections reduced the residuals between observed and predicted pga and sa the simulated pattern of shaking from the 1965 earthquake was twice as high as in the 2001 earthquake due to the proximity of the 1965 epicenter to sedimentary basins

geochemistry of the dubois greenstone succession an early proterozoic bimodal volcanic association in west central colorado abstract the dubois greenstone succession an early proterozoic supracrustal succession in west central colorado is composed of fine grained felsic volcaniclastic sediments ash flow tuffs and tholeiitic flows sills and dikes the volcanic rocks comprise a bimodal suite of tholeiite and rhyolite the tholeiites are divided into two groups group i exhibiting nearly flat ree patterns 20 30 chondrites and group ii with light ree enriched patterns 40 70 chondrites these groups can be produced by two stage melting of an undepleted or enriched garnet lherzolite source in which garnet remains in the residue of only the first melting stage which produces group ii magmas the felsic magmas can be produced by partial melting of a high grade gneiss in the lower crust in which zircon is entrained in the magma the dubois greenstone succession appears to have formed by felsic and mafic subaqueous eruptions in which some ash flows upon entering water underwent phreatomagmatic explosions giving rise to detritus which formed the volcaniclastic sediments although tectonic setting cannot be assigned to the succession with a high degree of confidence an immature back arc basin developed on sialic crust in consistent with most data

coupled stratigraphic and structural evolution of a glaciated orogenic wedge offshore st elias orogen alaska 1 the st elias orogen is the result of 10 myr of oblique convergence and flat slab subduction in the gulf of alaska between north america and the yakutat microplate extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge reflection seismic profiles across the offshore pamplona zone fold thrust belt the frontal st elias orogenic wedge provide constraints for quantifying pleistocene deformation recorded in the glaciomarine yakataga formation the total amount of pleistocene shortening observed varies from 3 to 5 mm yr compared to the current gps derived yakutat north america convergence rate across the st elias orogen of 45 mm yr growth strata and kinematic fold analysis allow comparison of relative timing of fault activity which reveals temporal and spatial shifting of active deformation during the glacial period faulting localized adjacent to the coastline and at the current submarine deformation front the abandoned currently inactive region is colocated with the major glacial depocenter in the region the bering trough these observations imply that glacial processes such as sediment loading and focused erosion during advance retreat cycles has a direct effect on the evolution of individual faults within the pamplona zone and the overall deformation pattern in the offshore st elias margin this information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during pleistocene glacial interglacial periods

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

existence of a seismic belt in the upper plane of the double seismic zone extending in the along arc direction at depths of 70 100 km beneath ne japan 1 we relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the pacific slab beneath ne japan we found a seismic belt parallel to the iso depth contour of the plate interface that is beneath the forearc area at depths of 70 100 km indicating that the distribution of the upper plane earthquakes is non uniform the location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where h2o contents change in the slab crust events in the upper seismic plane have mainly down dip compression type focal mechanisms but several events have normal fault type nf type versions whose spatial distribution appears to correspond to these boundaries these nf events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions

seismic evidence for thermally controlled dehydration reaction in subducting oceanic crust 1 we perform travel time tomography to estimate detailed seismic velocity structures in the crust of the pacific slab from northeastern ne japan to the kanto district japan and reveal that the depth extent of the low velocity hydrated oceanic crust varies along the arc the low velocity oceanic crust is subducting to depths of 120 150 km beneath kanto which is 40 70 km deeper compared to ne japan such deeper preservation of the low velocity oceanic crust beneath kanto can be explained by lower temperature conditions in the pacific slab as a result of the subduction of the philippine sea slab immediately above it these observations suggest that dehydration reactions accompanied by large velocity changes are controlled principally by temperatures not by pressures we also find spatial correspondence between intensive seismicity in the oceanic crust and the disappearance depth of the low velocity oceanic crust suggesting that breakdown of hydrous minerals triggers earthquakes in the oceanic crust

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

petrogenesis of oceanic andesites the andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49 50 sio2 to about 76 sio2 the most primitive compositions having contents of about 50 sio2 are the basaltic andesites which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust the oceanic andesites cannot have assimilated material from the continental crusts but their petrogenesis bears on the origin of the continental crust the fractionation of andesitic magmas occurs at relatively low pressures within the stability range of plagioclase basaltic andesites fractionate mainly orthopyroxene plagioclase while andesites fractionate clinopyroxene orthopyroxene plagioclase the petrogenesis of the continental andesites is more complex as the primary andesites contain sedimentary material from the continental crusts

the geochemistry and tectonic significance of pre metamorphic minor intrusions of the central metasedimentary belt grenville province canada abstract more than 250 samples collected from dykes and sills intruded into the grenville supergroup have been analysed by xrf techniques for their major and trace element contents the ree contents of 43 of the samples have been determined by inaa using these data the rocks have been identified and placed in seven suites each characterized by its own trace element signature there are four tholeiitic suites the low zirconium suite which comprises a bimodal series of basalts and minor dacites the intermediate zirconium suite which is made up of basalts andesites dacites and rhyolites the high zirconium suite consisting of high iron basalts and rare highly fractionated tholeiites which comprise basalts characterized by high mg number high ni and cr contents and with strongly fractionated trace element patterns in addition a calc alkaline suite made up of basalts and andesites a transitional suite of andesites and trachyandesites and an ultra potassic suite of leucitites have been identified on chondrite normalized multi element plots all of the suites show negative nb and ti anomalies similar ranges of rock types with similar anomalies occur within volcanic arcs and back arc basins formed at collisional plate margins where the nb and ti anomalies are interpreted as subduction signatures and also in some continental flood basalt sequences in which the anomalies are commonly suggested to result from crustal contamination the geology of the central metasedimentary belt leads us to favour an interpretation in which the minor intrusions were emplaced in supracrustal rocks deposited upon attenuated continental crust in a back arc basin during a collisional orogeny

three dimensional seismic velocity structure and configuration of the philippine sea slab in southwestern japan estimated by double difference tomography 1 three dimensional seismic velocity structure in and around the philippine sea plate subducting beneath southwestern sw japan is determined by applying double difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in japan a region of low s wave velocity and high vp vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from tokai to kyushu this characteristic layer dips shallowly in the direction of slab subduction compared with the upper surface of the philippine sea slab based on seismic reflection and refraction surveys on seven survey lines we interpret that the low vs and high vp vs layer corresponds to the oceanic crust of the philippine sea slab on the basis of the position of the low vs and high vp vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes the upper surface of the philippine sea slab is reliably determined for the entire area of sw japan nonvolcanic deep low frequency earthquakes that occurred associated with the subduction of the philippine sea slab are distributed along the isodepth contour of 30 km in sw japan except for the tokai district where the depth of deep low frequency earthquakes becomes gradually deeper toward northeast

geochemistry and origin of early proterozoic supracrustal rocks dos cabezas mountains southeastern arizona early proterozoic supracrustal rocks in the dos cabezas mountains in southeastern arizona include felsic volcanic and hypabyssal rocks amphibolite feldspathic quartzite arkose and conglomerate quartzite and phyllite felsic volcanics are chiefly water laid tuff and associated volcaniclastic sediments feldspathic quartzites arkoses and conglomerates are derived from granitic and felsic volcanic sources felsic and basaltic igneous rocks define a bimodal suite and felsic rocks can be divided into three populations rhyodacites quartz latites and rhyolites volcanics share major and trace element distributions in common with modern volcanics from continental rift systems and few in common with arc or oceanic volcanics dos cabezas basalts can be produced by 20 melting of an incompatible element enriched iherzolite source in the mantle followed by as much as 40 fractional crystallization of olivine rhyodacites can be produced by partial melting of an undepleted granulite source in the middle crust and quartz latites and rhyolites by fractional crystallization of rhyodacite magma the proterozoic succession in the dos cabezas mountains appears to have formed in a continental rift system similar to or part of early proterozoic rift systems in central and southern new mexico and perhaps in central arizona

drumlins products of controlled or uncontrolled glaciodynamic response abstract two schools of thought on the process of drumlin formation have recently developed that have created a controversy within this ara of glacial geology one hypothesis contends that drumlins are in the main developed within a soft mobile subglacial debris zone the other school argues that for those drumlins that contain primary deposited stratified sediment formation during catastrophic subglacial flood events is necessary a classification of subglacial bed types is presented that allows for the accommodation of both schools it is proposed that a new paradigm for subglacial research may be needed in order that a closer relationship between glaciodynamics and sedimentological processes be established in future attempts at explaining glacial processes and froms

geochemistry of early mesozoic tholeiites from morocco abstract mesozoic dolerites from two areas of morocco the high atlas fold belt between marrakech and demnat and the anti atlas belt in the area of foum zquid are most high ti quartz normative tholeiites whichi in many respects resemble mesozoic dolerite dikes from eastern north america the dolerites display a wide range of major and trace element compositions some of which are due to fractional crystallization the doleritic sequences from high atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top this trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source it is suggested that the continental upper mantle source for dolerites of morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites

the tekapo glacier new zealand during the last glacial maximum an active temperate glacier influenced by intermittent surge activity abstract quaternary glaciations have created impressive landform assemblages that can be used to understand palaeo glacier extent character and behaviour and hence past global and local glacier forcings however in the southern hemisphere and especially in new zealand the quaternary glacial landform record is relatively poorly investigated with regard to glaciological properties in this study a 1 m digital elevation model dem was generated from airborne lidar data and supplemented with aerial imagery and field observations to analyse the exceptionally well preserved glacial geomorphology surrounding lake tekapo new zealand we describe a rich suite of last glacial maximum lgm and recessional ice marginal subglacial supraglacial glaciofluvial and glaciolacustrine landform assemblages these represent two landsystems comprising i fluted till surfaces with low relief push moraine ridges and ii crevasse squeeze ridges zig zag eskers and attenuated lineations the former landsystem records the behaviour of an active temperate glacier and the latter landsystem which is superimposed upon and inset within the former strongly suggests intermittent surge phases the two landsystem signatures indicate a sequential change in ice marginal dynamics during recession that was likely to have been partially non climatically driven overall we present the first evidence of surge type glacier behaviour in new zealand

the geochemistry of the turriff metavolcanics grenville province southeastern ontario fifty seven samples of the turriff metavolcanic sequence from the hastings region of the grenville province have been chemically analyzed ten major element oxides and ten trace element values for each sample were determined by standard x ray fluorescence techniques the sampled sections consist predominantly of mafic flows subordinate intermediate flows and minor felsic flows and pyroclastics the sequence has been regionally metamorphosed to lower amphibolite facies the geochemical data indicate that the metavolcanics represent a tholeiitic mafic silicic bimodal sequence immobile element discriminant diagrams indicate that the turriff metabasalts are most similar to recent oceanic ridge basalts whereas the metabasalts from other published studies of grenville metavolcanics have an island arc tholeiite affinity however because of the many chemical similarities between continental and oceanic ridge tholeiites the bimodal nature of the sequence and the field relationships it is most likely that t

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

petrology and geochemistry of cambrian volcanic rocks from the avalon zone in new brunswick lower to middle cambrian volcanic rocks occur within the avalon zone of southern new brunswick at beaver harbour and in the long reach area the beaver harbour rocks are intensely altered but the major and trace element geochemistry indicates that they could be highly evolved basaltic andesites within plate basalts the mafic flows from the long reach area form two chemically and petrologically distinct groups 1 basalts with feldspar phenocrysts that represent evolved continental tholeiites with some oceanic characteristics and 2 a group of aphyric basalts showing extremely primitive continental tholeiite compositions also with oceanic affinities and resembling some rift related jurassic basalts on the eastern seaboard felsic pyroclastic rocks in the long reach area make the suite bimodal this distribution of rock types supports conclusions from the mafic rocks that the area experienced tension throughout the early to middle cambrian