deep structure of the central lesser antilles island arc relevance for the formation of continental crust oceanic island arcs are sites of high magma production and contribute to the formation of continental crust geophysical studies may provide information on the configuration and composition of island arc crust however to date only few seismic profiles exist across active island arcs limiting our knowledge on the deep structure and processes related to the production of arc crust we acquired active source wide angle seismic data crossing the central lesser antilles island arc north of dominica where the oceanic tiburon ridge subducts obliquely beneath the forearc a combined analysis of wide angle seismics and pre stack depth migrated reflection data images the complex structure of the backstop and its segmentation into two individual ridges suggesting an intricate relation between subducted basement relief and forearc deformation tomographic imaging reveals three distinct layers composing the island arc crust a three kilometer thick upper crust of volcanogenic sedimentary rocks and volcaniclastics is underlain by intermediate to felsic middle crust and plutonic lower crust the island arc crust may comprise inherited elements of oceanic plateau material contributing to the observed crustal thickness a high density ultramafic cumulates layer is not detected which is an important observation for models of continental crust formation the upper plate moho is found at a depth of 24 km below the sea floor upper mantle velocities are close to the global average our study provides important information on the composition of the island arc crust and its deep structure ranging from intermediate to felsic and mafic conditions in this study we model the deep structure of the lesser antilles island arc we use a hybrid analysis of refraction and reflection seismic data we image the complex structure of two ridges forming the backstop island arc crust composition ranges from intermediate to felsic to mafic conditions we discuss the formation of island arc and continental crust

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

the dykes and structural setting of the volcanic front in the lesser antilles island arc the orientations of dykes from many of the islands of the lesser antilles island arc have been mapped most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age with distinct preferred orientations dykes are known from all cenozoic epochs except the palaeocene but are most common in pliocene miocene and oligocene rocks a majority of the sampled dykes are basaltic intrude volcaniclastic host rocks and show a preference for widths of 1 1 25 m locally dyke swarms dilate their hosts by up to 9 over hundreds of metres and up to 2 over distances of kilometres the azimuths of dykes of all ages show a general ne sw preferred orientation with a second nw se mode particularly in the miocene rocks of martinique the regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments saba montserrat guadeloupe martinique st lucia grenada the spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart this magma is normally intercepted at crustal depths by dioritic plutons and andesitic dacitic magma generated there plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps throughout its history the volcanic front as a whole has migrated episodically towards the backarc at an average velocity of about 1 km ma the local direction of plate convergence is negatively correlated with the local preferred orientation of dykes the dominant ne sw azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded a generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the barracuda rise perturbs the stress field there is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics

constraints on the composition of the aleutian arc lower crust from vp vs 1 determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust we present new constraints on the composition of high p wave velocity vp 7 3 7 6 km s lower crust of the aleutian arc from best fitting average lower crustal vp vs ratio using sparse converted s waves from an along arc refraction profile we find a low vp vs of 1 7 1 75 using petrologic modeling we show that no single composition is likely to explain the combination of high vp and low vp vs our preferred explanation is a combination of clinopyroxenite 50 70 and alpha quartz bearing gabbros 30 50 this is consistent with aleutian xenoliths and lower crustal rocks in obducted arcs and implies that 30 40 of the full aleutian crust comprises ultramafic cumulates these results also suggest that small amounts of quartz can exert a strong influence on vp vs in arc crust

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

displacement and strain patterns from naturally occurring shear zone terminations abstract strain distribution patterns are described from several naturally occurring ductile shear zone terminations and a two fold classification is suggested type i termination patterns show a symmetrical decrease in strain area and intensity and are associated with very low shear strain values in the main shear zone the more common type ii termination patterns show a symmetrical increase in strain area with decrease in strain intensity some brittle ductile examples of type ii terminations contain secondary shears that resemble splay faults all the available evidence suggests that the deformation near the terminations of these shear zones is not plane strain nor do the observed strain patterns readily fit the existing theoretical models

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

experimentally derived high pressure cumulates from hydrous arc magmas and consequences for the seismic velocity structure of lower arc crust 1 this contribution explores the consequences of experimentally derived ultramafic plutonic rocks for estimates of continental crust composition the formation of high pressure cumulates from primitive to evolved hydrous arc magmas was investigated in the range 0 8 1 5 gpa and 1350 800 c to provide constraints on solid assemblages that form the roots of island arc crust at pressures exceeding 0 8 gpa 25km 40 60 of ultramafic cumulates are produced to obtain andesite to dacite compositions that are typical for evolved island arc volcanic and plutonic rocks calculated cumulate densities are comparable or higher than upper mantle values while seismic velocities vp vary continuously between 7 3 and 8 km s resulting in a broad crust mantle transition the moho represents a plagioclase saturation boundary our results are consistent with seismic imaging from continental margins and island arcs revealing that beneath an active volcanic front a sharp geophysical moho signature is sometimes absent

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

fractional crystallization of primitive hydrous arc magmas an experimental study at 0 7 gpa differentiation of mantle derived hydrous basaltic magmas is a fundamental process to produce evolved intermediate to sio2 rich magmas that form the bulk of the middle to shallow continental and island arc crust this study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0 7 gpa for hydrous calc alkaline to arc tholeiitic magmas fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous higher temperature experiment temperatures ranged from near liquidus at 1 170 c to near solidus conditions at 700 c h2o contents varied from 3 0 to more than 10 wt the liquid line of descent covers the entire compositional range from olivine tholeiite 1 170 c to high silica rhyolite 700 c and evolves from metaluminous to peraluminous compositions the following crystallization sequence has been established olivine clinopyroxene plagioclase spinel orthopyroxene amphibole titanomagnetite apatite quartz biotite anorthite rich plagioclase and spinel are responsible for a marked increase in sio2 content from 51 to 53 wt at 1 040 c at lower temperatures fractionation of amphibole plagioclase and fe ti oxide over a temperature interval of 280 c drives the sio2 content continuously from 53 to 78 wt largest crystallization steps were recorded around 1 040 c and at 700 c about 40 of ultramafic plutonic rocks have to crystallize to generate basaltic andesitic liquids and an additional 40 of amphibole gabbroic cumulate to produce granitic melts andesitic liquids with a liquidus temperature of 1 010 c only crystallize 50 over an 280 c wide range to 730 c implying that such liquids form mobile crystal mushes 50 crystals in long lived magmatic systems in the middle crust allowing for extensive fractionation assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs

assimilation of sediments embedded in the oceanic arc crust myth or reality abstract arc magmas are commonly assumed to form by melting of sub arc mantle that has been variably enriched by a component from the subducted slab although most magmas that reach the surface are not primitive the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle this jeopardises the integrity of mass balance recycling calculations here we use sr and o isotope data in minerals from a suite of volcanic rocks from st lucia lesser antilles arc to show that assimilation of oceanic arc basement can be significant analysis of 87sr 86sr in single plagioclase phenocrysts from four soufriere volcanic complex svc st lucia hand samples with similar composition 87sr 86sr 0 7089 0 7091 reveals crystal isotopic heterogeneity among hand samples ranging from 0 7083 to 0 7094 with up to 0 0008 difference within a single hand sample o 18 measurements in the svc crystals show extreme variation beyond the mantle range with 7 5 to 11 1 for plagioclase n 19 10 6 to 11 8 for quartz n 10 9 4 to 9 8 for amphibole n 2 and 9 to 9 5 for pyroxene n 3 while older lavas pre soufriere volcanic complex with less radiogenic whole rock sr composition 87sr 86sr 0 7041 0 7062 display values closer to mantle range 6 4 to 7 9 for plagioclase n 4 and 6 to 6 8 for pyroxene n 5 we argue that the 87sr 86sr isotope disequilibrium and extreme o 18 values provide compelling evidence for assimilation of material located within the arc crust positive correlations between mineral o 18 and whole rock 87sr 86sr 143nd 144nd and 206 207 208pb 204pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in st lucia but also in the whole lesser antilles since st lucia encompasses almost the whole arc range of isotopic compositions this highlights the need for detailed mineral scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes

foundering of lower island arc crust as an explanation for the origin of the continental moho a long standing theory for the genesis of continental crust is that it is formed in subduction zones however the observed seismic properties of lower crust and upper mantle in oceanic island arcs differ significantly from those in the continental crust accordingly significant modifications of lower arc crust must occur if continental crust is indeed formed from island arcs here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc kohistan and talkeetna the kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs in contrast the lower talkeetna crust is density sorted preserving only relicts about ten to a hundred metres thick of rock with density exceeding that of the underlying mantle specifically the foundering of the lower talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres characteristic of the continental mohorovicic discontinuity the moho dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years moreover because foundering will continue after arc magmatism ceases this process ultimately results in the formation of the continental moho

lateral variation in crustal structure along the lesser antilles arc from petrology of crustal xenoliths and seismic receiver functions abstract we reconstruct crustal structure along the lesser antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions xenoliths show considerable island to island variation in xenolith petrology from plagioclase free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole indicative of changing magma differentiation depths xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km we use xenolith mineral modes and compositions to calculate seismic velocities v p v s and density at the estimated equilibration depths we create a five layer model of crustal structure for testing against receiver functions rf from island seismic stations along the arc lowermost layer 5 comprises peridotite with physical characteristics of mantle xenoliths from grenada uppermost layer 1 consists of 5 km of volcaniclastics and sediments whose physical properties are determined via a grid inversion routine the three middle layers 2 to 4 comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island by inversion we obtain a petrological best fit for the rf on each island to establish the nature and thicknesses of layers 2 to 4 along the arc we see variations in the depth and strength of both moho and mid crustal discontinuity mcd on length scales of tens of km moho depths vary from 25 to 37 km mcd from 11 and 32 km the moho is the dominant discontinuity beneath some islands st kitts guadeloupe martinique grenada whereas the mcd dominates beneath others saba st eustatius along arc variability in mcd depth and strength is consistent with variation in estimated magmatic h2o contents and differentiations depths that in turn influence xenolith lithologies a striking feature is steep along arc gradients in v p similar to those observed at other oceanic arcs these gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle we find no evidence for large interconnected bodies of partial melt beneath the lesser antilles instead the crustal velocity structure is consistent with magma differentiation in vertically extensive crystal mush dominated reservoirs along arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge itself driven by variation in slab derived h2o fluxes

extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling a recent extensive seismic wide angle experiment revealed a new image of crustal and uppermantle structure across northern honshu arc japan the western part of the arc recorded the crustal deformation by the miocene back arc spreading of the sea of japan the crust is composed of highly deformed tertiary sedimentary layers a relatively low velocity 5 75 5 9 km s crystalline basement and a 15 km thick lower crust with a velocity of 6 6 7 0 km s clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading the crust attains the maximum thickness 32 35km east of the backbone range for which the magmatic intrusion underplating since 10 15 ma is a predominant factor the eastern part of the arc has a less deformed upper crust and a reflective middle lower crust probably remaining a stable block since the time of the backarc spreading

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

upper and middle crustal deformation of an arc arc collision across hokkaido japan inferred from seismic refraction wide angle reflection experiments abstract the hidaka collision zone hcz central hokkaido japan is a good target for studies of crustal evolution and deformation processes associated with an arc arc collision the collision of the kuril arc ka with the northeast japan arc nja which started in the middle miocene is considered to be a controlling factor for the formation of the hidaka mountains the westward obduction of middle lower crustal rocks of the ka the hidaka metamorphic belt hmb and the development of the foreland fold and thrust belt on the nja side the hokkaido transect project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical geological research including seismic refraction reflection surveys and earthquake observations an e w trending 227 km long refraction wide angle reflection profile found a complicated structural variation from the ka to the nja across the hcz in the east of the hcz the hinterland region is covered with 4 4 5 km thick highly undulated neogene sedimentary layers beneath which two eastward dipping reflectors were imaged in a depth range of 10 25 km probably representing the layer boundaries of the obducting middle lower crust of the ka the hmb crops out on the westward extension of these reflectors with relatively high vp 6 0 km s and vp vs 1 80 consistent with middle lower crustal rocks beneath these reflectors more flat and westward dipping reflector sequences are situated at the 25 27 km depth forming a wedge like geometry this distribution pattern indicates that the ka crust has been delaminated into more than two segments under our profile in the western part of the transect the structure of the fold and thrust belt is characterized by a very thick 5 8 km sedimentary package with a velocity of 2 5 4 8 km s this package exhibits one or two velocity reversals in paleogene sedimentary layers probably formed by imbrication associated with the collision process from the horizontal distribution of these velocity reversals and other geophysical geological data the rate of crustal shortening in this area is estimated to be greater than 3 4 mm year which corresponds to 40 50 of the total convergence rate between the nja and the eurasian plate this means that the fold and thrust belt west of the hcz is absorbing a large amount of crustal deformation associated with plate interaction across hokkaido island

the nature of the crust beneath the afar triple junction evidence from receiver functions the afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading here we present receiver function results from new and legacy experiments crustal thickness ranges from 45 km beneath the highlands to 16 km beneath an incipient oceanic spreading center in northern afar the crust beneath afar has a thickness of 20 26 km outside the currently active rift segments and thins northward it is bounded by thick crust beneath the highlands of the western plateau 40 km and southeastern plateau 35 km the western plateau shows vp vs ranging between 1 7 1 9 suggesting a mafic altered crust likely associated with cenozoic flood basalts or current magmatism the southeastern plateau shows vp vs more typical of silicic continental crust 1 78 for crustal thicknesses 2 0 can only be explained by significant amounts of magmatic intrusions in the lower crust this suggests that melt emplacement plays an important role in late stage rifting and melt in the lower crust likely feeds magmatic activity the crust between the location of the miocene red sea rift axis and the current rift axis is thinner 2 0 than beneath the eastern part of afar 26 km vp vs 1 9 this suggests that the eastern region contains less partial melt has undergone less stretching extension and has preserved a more continental crustal signature than west of the current rift axis the red sea rift axis appears to have migrated eastward through time to accommodate the migration of the afar triple junction

along arc variation in seismic velocity structure related to variable growth of arc crust in northern izu bonin intraoceanic arc the izu bonin arc is an intraoceanic island arc where the pacific plate subducts beneath the philippine sea plate along arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the izu bonin arc we have imaged three dimensional 3 d seismic velocity structures in the northern izu bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations our 3 d velocity model indicates heterogeneous structure in the mantle wedge along the arc low velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front low velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of aoga shima and south of sumisu jima this coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust along topographic highs in the forearc low velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc providing further evidence of this view of arc crust formation

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates

using unsupported lead 210 measurements to investigate soil erosion and sediment delivery in a small zambian catchment abstract traditional techniques used to assemble information on rates of erosion and soil redistribution possess many important limitations as a result the use of environmental radionuclides and more particularly 137 cs measurements has attracted increasing attention in recent years as a means of obtaining spatially distributed information on rates of erosion and deposition the application of the 137 cs approach is however hampered in some areas of the world where 137 cs inventories are low and the low concentrations of 137 cs found in soils and sediments cause problems for laboratory analysis these problems will increase as time progresses due to the radioactive decay of the existing inventory most of which was deposited as fallout ca 40 years ago this contribution explores the potential for using another fallout radionuclide namely unsupported 210 pb as an alternative to 137 cs in the small 63 km 2 upper kaleya catchment in southern zambia where 137 cs inventories are already very low the approach employed with unsupported 210 pb is similar to that used for 137 cs although the essentially constant fallout of unsupported 210 pb through time means that the resulting estimates of erosion and soil redistribution rates reflect a longer period of time ca 100 years rather than ca 40 years the estimates of erosion and deposition rates derived from the unsupported 210 pb measurements are used to construct typical sediment budgets for the three main land use types in the upper kaleya catchment namely commercial cultivation communal cultivation and bush grazing the results obtained from the unsupported 210 pb are compared with equivalent results based on 137 cs measurements provided by a previous investigation undertaken in the study catchment the two sets of results are highly consistent the study reported confirms the viability of using unsupported 210 pb as an alternative to 137 cs in this environment and demonstrates that conjunctive use of both radionuclides can provide additional information on the erosional history of a study area