organic geochemical studies of a recent inner great barrier reef sediment i assessment of input sources the concentration of solvent extractable se and bound hydrocarbons ketones alcohols sterols monocarboxylic acids hydroxyacids and dicarboxylic acids of a surface sample 0 2 5 cm from a recent carbonate tropical sediment taken at 60ft 18m depth north west of the low isles north queensland are reported in detail n alkanes are a minor constituent in the se and bound hydrocarbon fractions with the major component being unresolved complex hydrocarbon material which is not an anthropogenic input 6 10 14 trimethylpentadecan 2 one derived from chlorophyll as well as phytol were identified as major components of the ketone and alcohol constituents no chlorophyll pigments or pigmented degradation products were present implying degradation before incorporation into the sediment this conclusion is consistent with evidence for higher plant input in which all sensitive lipids have been degraded before incorporation se and bound fractions have been studied in detail and show considerable differences probably arising from the importance of viable biomass in the se fraction inputs to the sediment are determined as higher plants algae bacteria fungi and meiofauna in descending order of importance

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

constraints on the moho in japan and kamchatka abstract this review collects and systematizes in one place a variety of results which offer constraints on the depth and the nature of the moho beneath the kamchatka peninsula and the islands of japan we also include studies of the izu bonin volcanic arc all results have already been published separately in a variety of venues and the primary goal of the present review is to describe them in the same language and in comparable terms for both regions we include studies using artificial and natural seismic sources such as refraction and reflection profiling detection and interpretation of converted mode body waves receiver functions surface wave dispersion studies in kamchatka and tomographic imaging in japan the amount of work done in japan is significantly larger than in kamchatka and resulting constraints on the properties of the crust and the uppermost mantle are more detailed japan and kamchatka display a number of similarities in their crustal structure most notably the average crustal thickness in excess of 30 km typical of continental regions and the generally gradational nature of the crust mantle transition where volcanic arcs are presently active

contrasting seismic reflectivity of the lower crust and uppermost mantle between ne japan and sw japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths 1 petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath ichinomegata crater in ne japan and beneath oki dogo island in sw japan in contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath ichinomegata crater relatively large contrasts in acoustic impedance are present beneath oki dogo island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks based on the existing petrologic models of the lower crust and uppermost mantle beneath ichinomegata crater and oki dogo island we then constructed model structures of the lower crust and uppermost mantle beneath these two areas composed of units with randomly arranged 100 m thick foliation parallel horizontal layers of xenolith samples and used these model structures to examine the seismic reflectivity beneath these two areas the results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas although no other recognizable seismic reflector is expected beneath ichinomegata crater many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath oki dogo island thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between ne japan and sw japan

k ar age and geochemistry of the sw japan paleogene cauldron cluster implications for eocene oligocene thermo tectonic reactivation abstract systematic k ar dating and geochemical analyses of paleogene cauldrons in the sanin belt of sw japan have been made to explore the relationship between the timing of their formation and the paleogene subduction history of sw japan documented in the shimanto accretionary complex we also examine the magma sources and tectonics beneath the backarc region of sw japan at the eastern plate boundary of eurasia fifty eight new k ar ages and 19 previously reported radiometric age data show that the cauldrons formed during middle eocene to early oligocene time 43 30 ma following a period of magmatic hiatus from 52 to 43 ma the hiatus coincides with absence of an accretionary prism in the shimanto belt resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench as a common tectono thermal event within a subduction system the cauldrons are composed of medium k calc alkaline basalts to rhyolites and their plutonic equivalents these rocks are characterized by lower concentrations of large ion lithophile elements lile including k2o ba rb th u and li lower la yb n ratios lower initial sr isotopic ratios 0 7037 0 7052 and higher end t values 0 5 to 3 5 relative to late cretaceous to early paleogene equivalents there are clear trends from enriched to depleted signatures with decreasing age from the late cretaceous to the paleogene the same isotopic shift is also confirmed in lower crust derived xenoliths and is interpreted as mobilization of pre existing enriched lithospheric mantle by upwelling depleted asthenosphere relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the sw japan arc trench system during the eocene to oligocene newly identified late eocene low silica adakites and high mg andesites in the sanin belt and early eocene a type granites in the sw korea peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate the backarc region was an extensional tectonic setting and some paleogene rift basins and sanin belt cauldrons occur in linear arrays the eocene oligocene sanin se korea continental arc lies on the ne extension of the east china sea basin the initial stage of which probably formed by continental arc rifting this rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab in response to decreased convergence rate between the pacific and eurasian plates

internal structure of the median tectonic line fault zone sw japan revealed by borehole analysis abstract the internal structure of the median tectonic line mtl fault zone and the processes that prevailed at depths are described based on an analysis of a borehole the fault plane which defines the boundary between the ryoke and sanbagawa derived rocks dips at 56 to the north immediately beneath the boundary approximately 40 m thick fractured rocks form the major strand of the mtl fault zone the hanging wall above the boundary comprises variably deformed ryoke granitoids including several mylonite zones and cataclasite zones the fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 c the mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 c these mylonites and cataclasites represent therefore fault rocks that formed immediately below and above the brittle plastic transition respectively development of dissolution seams in these cataclasites suggests that the cataclasite has low strength the presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle plastic transition on the other hand the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle plastic transition it is therefore likely that the differential stresses immediately below the brittle plastic transition are much higher than those immediately above the transition formation of laumontite in the major strand of the mtl fault zone occurred at temperatures of around 200 c the central slip zone of the major strand is about 30 cm thick and is surrounded by thick gouge zones this situation is favourable for thermal pressurisation during earthquake slips

slump strain indicative of paleoslope in cretaceous izumi sedimentary basin along median tectonic line southwest japan submarine slump folds are common in turbidites of the izumi group upper cretaceous south of osaka in southwest japan structural analysis of the folds indicates predominantly eastward movement of the slump sheet however it is noteworthy that this direction is generally opposite to westward paleocurrents obtained from sole marks but it coincides with the direction of younging of the strata and the plunge direction of macroscopic synclines in the izumi group the paleoslope inferred from the slump folds is believed to have been caused by the eastward migration of the depocenter of the izumi sedimentary basin this slumpi ng is thought to have followed tilting of the beds deposited by the westward current the migration of the depocenters and tilting are associated with left slip faulting on the bounding median tectonic line

structural characteristics of the yilan yitong and dunhua mishan faults as northern extensions of the tancheng lujiang fault zone new deep seismic reflection results abstract the tancheng lujiang fault zone tlfz can be subdivided into three segments that exhibit sharp contrasts in their deep structures a deep seismic reflection profile length 600 km across the north part of the tlfz which provides new constraints on the structural styles of the northern tlfz was recently completed by the chinese sinoprobe project here the tlfz branches into the yilan yitong fault yyf to the west and the dunhua mishan fault dmf to the east the yyf developed as an internal fault in the songnen zhangguangcai massif while the dmf serves as the tectonic boundary between the nadanhada terrane and the khanka massif both faults developed large scale flower structures with that of the yyf being negative and that of the dmf being positive with reverse faults the moho in the profile is at a depth of 25 39 km and is offset by the faults the north part of the tlfz extends into the upper mantle as thin shear zones with the reflectors truncated in the middle lower crust this feature differs from most crustal scale strike slip faults that distribute over a discrete shear zone in the lower crust such as the san andreas fault

tectonic model and fault segmentation of the median tectonic line active fault system on shikoku japan 1 fault segmentation models have been developed by using various types of information about faults including both geological and geophysical data in order to predict the size of future cascade earthquakes in these models however stress conditions within the fault systems have not been considered in sufficient detail major fault systems generally exhibit heterogeneous stress conditions the median tectonic line active fault system mtlafs is the longest and most active arc parallel right lateral strike slip fault system in japan the stress conditions along the mtlafs change from transpression on eastern shikoku island to transtension on kyushu island the change of stress conditions along the fault system is caused by the counterclockwise rotation of the nankai fore arc sliver in response to the relative motion of the philippine sea and eurasian tectonic plates and back arc spreading in the okinawa trough here we propose stress condition segmentation as a concept that deals with the diversity of stress conditions and can be applied in seismic hazard assessment according to this concept the mtlafs is divided into three segments the east shikoku west shikoku and kyushu segments the east shikoku and kyushu segments are under compressional and tensional stress conditions respectively stress in the west shikoku segment which is between the other two segments is transitional from compressional to tensional conditions in particular change in the magnitude of fault normal stresses along strike is one of the key factors that govern the geometry of fault discontinuities rupture propagation and termination and consequently fault segmentation application of our stress condition segmentation concept to the mtlafs suggests that future earthquakes in the west shikoku and kyushu segments will be of lower energy than those in the east shikoku segment

recognition of the shimanto hp metamorphic belt within the traditional sanbagawa hp metamorphic belt new perspectives of the cretaceous paleogene tectonics in japan abstract the sanbagawa metamorphic belt in sw japan has been traditionally referred as a typical subduction related high pressure hp metamorphic belt this belt extends e w direction for more than 800 km from the kanto mountains to kyushu island in part protoliths of many hp metamorphic rocks formed as an accretionary complex after ca 90 80 ma and suffered a blueschist facies metamorphism at 66 61 ma this metamorphic event clearly postdates the sanbagawa metamorphism of 120 110 ma this newly recognized hp rocks occur extensively in the traditional sanbagawa hp metamorphic belt throughout shikoku kii peninsula and kanto mountains thus it was named as the shimanto hp metamorphic rocks thus the traditional sanbagawa metamorphic belt comprises two distinct hp metamorphic belts and the sanbagawa metamorphic rocks cover less than half of the areas for the traditional sanbagawa metamorphic belt importantly the metamorphic grade of the shimanto metamorphic rocks ranges from the pumpellyite actinolite to epidote amphibolite facies just like many sanbagawa metamorphic rocks the similarity between metamorphic rocks of these two belts in spatial distribution metamorphic grades and metamorphic facies series was the main reason why these two belts had not previously been differentiated a pacific type orogenic belt in general comprises a belt of accretionary complex and hp metamorphic belt on the ocean side plus a fore arc basin and a granite batholith belt on the continent side in sw japan the sanbagawa metamorphic belt is chronologically accompanied with the accretionary complex southern chichibu belt lower cretaceous fore arc sediments and the ca 120 70 ma sanyo batholith belt with ryoke low pressure metamorphic rocks likewise the shimanto hp metamorphic belt is accompanied with an upper cretaceous accretionary complex northern shimanto belt campanian to maastrichtian fore arc sediments and the san in batholith belt the opening of a back arc basin japan sea at ca 20 ma extensively modified the primary spatial arrangements of the two cretaceous paleogene orogenic belts in particular the southward thrusting of the ryoke sanyo granitic batholith belt over the sanbagawa metamorphic belt formed the present structural superposition of the ryoke above the sanbagawa and shimanto metamorphic belts

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

dynamic rupture scenarios of anticipated nankai tonankai earthquakes southwest japan 1 we investigated dynamic rupture scenarios of anticipated megathrust earthquakes on the nankai tonankai subduction zone southwest japan to improve the scenario reliability the model parameters should be constrained by available data or derived from their analysis we employed the three dimensional plate interface geometry and the slip deficit rate on the interface accumulated slip deficit was used to obtain the stress drop distribution of anticipated earthquakes the estimated stress drop distribution is consistent with the seismogenic asperity locations known from the analysis of past earthquakes fault friction constitutive parameters however had to be assumed from indirect observations because they cannot be constrained directly by the data based on various geophysical observations we defined three regions where larger fracture energy is required these are the eastern edge of the tonankai area the western edge of the nankai area and the region between the tonankai and nankai areas beneath the kii peninsula such lateral heterogeneity promoted the segmented rupture along the nankai trough with predefined stress drop and constitutive parameters various rupture scenarios for tonankai and nankai asperities were obtained for different initiation locations in some cases a single segment is ruptured while in other cases all the segments are broken due to dynamic linkage at the segment boundary causing a giant earthquake the initiation location is a critical parameter that controls the rupture propagation across the segment boundary these scenarios will be extremely useful to evaluate deterministically the strong ground motions and tsunami hazards caused by the next major earthquakes in southwest japan

crustal structure of southwest japan revealed by the integrated seismic experiment southwest japan 2002 abstract a multi purpose seismic experiment named the 2002 integrated seismic experiment southwest japan was conducted in 2002 along a more than 240 km long line across southwest japan from the pacific coast to the japan sea coast its profile provides the first crustal scale cross section across the japanese island arc which highlights a number of significant points related to the structural development of the arc major outstanding points are that the japanese island arc is composed of two completely different crusts juxtaposed by the median tectonic line mtl and that the mtl started its activity associated with lower crustal thinning and formation of an upper crustal scale half graben in late cretaceous

seismic evidence for active underplating below the megathrust earthquake zone in japan determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes the peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate underplating at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging but direct seismic evidence of this process is lacking by comparing seismic reflection profiles with microearthquake distributions in central japan we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting philippine sea plate this region coincides with the location of slow slip events that may serve as signals for monitoring active underplating

gently north dipping median tectonic line mtl revealed by recent seismic reflection studies southwest japan abstract the median tectonic line mtl with a length of more 1000 km is the most significant fault in japan it juxtaposes the high p t sambagawa metamorphic rocks against the low p t metamorphic rocks of the ryoke belt the mtl was probably formed in the cretaceous with many subsequent reactivations the western segment of the mtl is still active with an almost pure right lateral sense of motion although a great amount of geological information on the mtl has been accumulated information about the subsurface especially the deep seated structure of the mtl is still insufficient it has been generally assumed that the mtl is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion recently new geophysical data have suggested that the mtl dips gently northward at depth we have acquired a complementary set of geophysical profiles seismic reflection and refraction gravity and mt across the mtl in east shikoku our results confirm that the mtl dips northward at about 30 to 40 degrees from the surface to about 5 km depth where it becomes listric this fault geometry more reasonably explains the reactivation history of the mtl the motion has occurred on a listric type fault in so called oblique or lateral ramp manner

evolution of the median tectonic line fault zone sw japan during exhumation abstract like many crustal scale fault zones the median tectonic line mtl fault zone in japan preserves fault rocks that formed across a broad range of physical conditions we examined the architecture of the mtl at a large new outcrop in order to understand fault behaviours under different crustal levels the mtl here strikes almost e w dips to the north and juxtaposes the sanbagawa metamorphic rocks to the south against the izumi group sediments to the north the fault core consists mainly of sanbagawa derived fault gouges the fault zone can be divided into several structural units including two slip zones upper and lower slip zones where the lower slip zone is more conspicuous crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation microstructures and powder x ray diffraction xrd analyses indicate that the four stages of deformation occurred under different temperature conditions the oldest deformation stage 1 was widely distributed and had a top to the east dextral sense of slip at deep levels of the seismogenic zone deformation with the same sense of slip then became localised in the lower slip zone stage 2 subsequently the slip direction in the lower slip zone changed to top to the west sinistral normal stage 3 the final stage of deformation stage 4 involved top to the north normal faulting along the two slip zones within the shallow crust near the surface the widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths the fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels

tectonic implications of early miocene oib magmatism in a near trench setting the outer zone of sw japan and the northernmost ryukyu islands abstract the outer zone of the sw japan and northernmost ryukyu arcs was affected by intense igneous activity during the miocene characterized by morb like basalts alkaline basalts and s type with subordinate i type felsic to intermediate volcano plutonic complexes these igneous rocks are inferred to be the products of near trench magmatism early miocene 18 ma alkaline basalt dikes from the shingu otoyo area in central northern shikoku and an alkaline lamprophyre dike from tanegashima one of the northernmost ryukyu islands pre date the middle miocene felsic to intermediate igneous rocks the basalts and lamprophyre have compositions of basanite basalt trachybasalt and phonotephrite they are characterized by elevated large ion lithophile elements liles e g sr ba and th and high concentrations of high field strength elements hfses e g tio 2 nb and zr the geochemical signatures of the basalts and lamprophyre suggest an ocean island basalt type oib type mantle source the occurrence of alkaline basalts and lamprophyre with oib type intraplate geochemical signatures in a near trench setting is unusual with regard to plate tectonic processes we propose that trench ward motion of the overriding plate during the period around the early miocene resulted in a shallowly dipping slab and interplate coupling between the subducting philippine sea plate psp and the overlying crust beneath most of the outer zone in the western part of sw japan and the northernmost ryukyu islands the oib type magmatism in the near trench environment is most plausibly explained by the upwelling of asthenospheric material from beneath the subducting slab which migrated through fractures and or tears in the slab we envisage two possible scenarios for the formation of these fractures or tears 1 the shallowing dip angle of the subducted psp resulted in concave upwards flexure of the slab generating fractures in the flexed region and 2 differential motion within the subducting slab induced slab tearing and segmentation

lateral variation of the cutoff depth of shallow earthquakes beneath the japan islands and its implications for seismogenesis abstract we estimated the spatial distribution of the cut off depth of shallow seismicity beneath the japan islands based on data from a recently constructed nationwide dense seismic network in order to avoid the effect of heterogeneous seismic velocity structure we determined the 3d seismic velocity structure for the whole of japan which was then used to relocate the hypocenters the cut off depth was determined by calculating d90 the depth above which 90 of the earthquakes occur a spatial distribution of estimated d90 values shows a considerable lateral variation ranging from about 5 km to about 40 km the d90 s are deep along the pacific coast of eastern japan locally shallow d90 s extend along belt like stretches of volcanic areas heat flow is low along the pacific coast in eastern japan and high in the volcanic areas this indicates that the lateral variation of d90 s is formed mainly by lateral geotherm variation cooling by the subducting old very cold pacific plate may cause the deep d90 s along the pacific coast in eastern japan heating by magma supplied from the mantle wedge below is considered to be the main cause of the shallow d90 s in the volcanic areas magma and or aqueous fluids derived from solidified magma contribute not only to locally shallow d90 s but also to the weakening of ambient rocks in the lower crust resulting in stress concentrations in the upper crust just above them large shallow inland earthquakes tend to occur in such areas suggesting that a model of shallow earthquake generation applied to tohoku can be used in other areas in japan as well

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

multivariate statistical analysis of stream sediment geochemistry in the grazer palaozoikum austria the austrian reconnaissance study of stream sediment composition more than 30000 clay fraction samples collected over an area of 40000 km2 is summarized in an atlas of regional maps that show the distributions of 35 elements these maps rich in information reveal complicated patterns of element abundance that are difficult to compare on more than a small number of maps at one time in such a study multivariate procedures such as simultaneous r q mode components analysis may be helpful they can compress a large number of variables into a much smaller number of independent linear combinations these composite variables may be mapped and relationships sought between them and geological properties as an example r q mode components analysis is applied here to the grazer palaozoikum a tectonic unit northeast of the city of graz which is composed of diverse lithologies and contains many mineral deposits