magma sediment mingling in an ordovician rift basin the plouezec plourivo half graben armorican massif france the plouezec plourivo basin ppb armorican massif is an ordovician half graben filled with red bed deposits lying on a cadomian metamorphic basement peperites crop out along its northeast coastal sector in connection with diverse magmatic and hydrothermal products combining field structural observations and geochemical analyses of magmatic rocks leads us to propose a model of emplacement of peperites in an extensional setting the ppb peperites result from mingling between magma and initially wet sediment the main corresponding facies consist of rounded magmatic bodies corrugated lava sediment contacts and a remarkable thin sheet of fluidized sediment extending over about 100 m parallel and c 10 cm above the base of a porphyritic lava flow a few ppb peperites formed under a dominant brittle regime are autoclastic breccias at the base of a lava flow and polymict breccias including both fluidal and blocky volcanic clasts sediment fluidization formation of vapor films magma sediment density contrasts and non explosive fragmentation are the main mechanisms invoked to generate the ppb peperites the c 100 m extended sediment derived sheet lying near the base of a lava flow likely formed by diapiric rise of fluidized sediment through molten lava we show that silicic contamination accompanied these peperitic processes

in situ monitoring of frost heave pressure during cross passage construction using ground freezing method the artificial ground freezing method has the dual effect of ground reinforcement and waterproof sealing and the frozen curtain can be designed flexibly it is widely used in the construction of cross passages for shield tunnels in soft ground with high groundwater levels however due to the lack of in situ monitoring data it remains difficult to determine the frost heave pressure acting upon a tunnel in this study based on the use of an anti freezing pad type earth pressure gauge in situ monitoring was carried out to measure the frost heave pressure acting upon tunnel segments during the construction of cross passages for the shanghai yangtze river tunnel the monitoring results show that the earth pressure acting upon the tunnel could decrease dramatically during freezing and this kind of decrease can take place suddenly and unpredictably which can be illustrated using the finite element method the maximum measured frost heave pressure during freezing and cross passage excavation was approximat

magma composition and viscosity as controls on peperite texture an example from passamaquoddy bay southeastern canada silurian mafic lava flows and rhyolite sills in the passamaquoddy bay area southeastern canada interacted with wet sediment to produce peperitic breccia the pahoehoe type mafic flows flowed over and bulldozed into wet unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts soft sediment deformation structures in the siltstone adjacent to the flows irregularly shaped vesicles in the siltstone and siltstone filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts evidence for the rhyolitic magma interacting with wet sediment includes sediment filled vesicles in rhyolite quartz filled vesicles in siltstone and angular non vesicular shards of rhyolite in clastic dykes peperite formed by this process is exclusively blocky the rhyolitic magma in the passamaquoddy bay sequence and other examples of high viscosity magma producing only blocky peperite suggest that in contrast to low viscosity magmas which produce a combination of blocky and fluidal peperite only blocky peperite will form associated with highly viscous rhyolite magma the sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that in this case sediment texture did not control peperite type the change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and or confining pressure and the generation and maintenance of a steam film at the magma wet sediment interface

facies architecture of mineralized submarine volcanic sequences cambrian mount read volcanics western tasmania the middle to late cambrian mount read volcanics western tasmania comprise compositionally and texturally diverse lavas and volcaniclastic rocks most of which were emplaced in submarine environments below wave base the facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes in the northern mount read volcanics the principal volcanic facies are 1 silicic intermediate and mafic lavas 2 juvenile volcaniclastic deposits generated in association with the extrusion of lava flows or else produced by explosive eruptions and 3 synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and or shallow marine volcanic centers are present the volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and or graded bedded sandstone of mixed volcanic and precambrian basement provenance the predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the mount read volcanics mass fiow emplaced pumiceous volcaniclastic facies provide the best framework for correlation because these are produced in large volumes erupted infrequently eraplaced rapidly and are widely distributed distinctive units of this type that occur near hellyer in the southwell subgroup and 40 km to the south at white spur howards road white spur formation could each be part of the same regionally extensive volcaniclastic facies association a similar volcaniclastic facies association hosts the hercules and rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

is plagioclase removal responsible for the negative eu anomaly in the source regions of mare basalts the nearly ubiquitous presence of a negative eu anomaly in the mare basalts has been suggested to indicate prior separation and flotation of plagioclase from the basalt source region during its crystallization from a lunar magma ocean lmo are there any mare basalts derived from a mantle source which did not experience prior plagioclase separation crystal chemical rationale for ree substitution in pyroxene suggests that the combination of ree size and charge m2 site characteristics of pyroxene fo2 magma chemistry and temperature may account for the negative eu anomaly in the source region of some types of primitive low tio2 mare basalts this origin for the negative eu anomaly does not preclude the possibility of the lmo as many mare basalts still require prior plagioclase crystallization and separation and or hybridization involving a kreep component

tectonic erosion and consequent collapse of the pacific margin of costa rica combined implications from odp leg 170 seismic offshore data and regional geology of the nicoya peninsula the convergent margin off the pacific coast of the nicoya peninsula of costa rica exhibits evidence for subduction erosion caused by the underthrusting cocos plate critical evidence for efficacy of this process was recovered at the ocean drilling program odp drilling site 1042 leg 170 positioned 7 km landward of the middle america trench axis off the nicoya peninsula the primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity the so called base of slope sediment boss horizon or surface the boss horizon which can be traced landward from near the trench to the nicoya coastal area and parallel to it for hundreds of kilometers separates a low velocity 2 0 2 5 km s 1 sequence of slope sediment from an underlying sequence of much higher velocity 4 4 5 km s 1 rock site 1042 reached the acoustically defined boss horizon at a below sea level depth of 3900 m and yielded a carbonate cemented calcarenitic breccia of early middle miocene age sedimentological geochemical paleontological and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment on the basis of coastal exposures the boss horizon as a margin wide geologic interface can be temporally and lithostratigraphically correlated to a regional angular unconformity this unconformity known as the mal pais unconformity separates neogene and younger shelf to littoral beds from the underlying mafic units of the mesozoic nicoya complex and cretaceous and early tertiary sedimentary sequences at site 1042 it is inferred that tectonism caused the vertical subsidence of the early neogene breccia from a shallow to a deep water setting the mal pais unconformity of the boss horizon thus connects the rock fabric of the outermost part of margin to that of coastal nicoya and implies that in the early neogene the nicoya shelf extended seaward to near the present trench axis this circumstance requires that the early neogene trench axis was at least 50 km seaward of where it is now located the long term effects of subduction erosion similar to those described for the scientifically drilled japan tonga and peru margins best account for offshore and onshore evidence for a post paleogene history of crustal thinning and landward trench migration of costa rica s pacific margin during the past 16 17 myr the calculated mass removal and landward migration rates are 34 36 km myr 1 km 1 of margin and 3 km myr 1 respectively these values are similar to those found for other pacific margins dominated by nonaccretionary subduction zone processes

how does a monzogranite turn into a trachydacitic extrusion mantled by basinal volcaniclastics and peperites the case of south ouessant armorican variscides france how many volcanic bodies are being confused with plutonic ones worldwide the purpose of this study is not to provide an answer to this question but rather to illustrate this issue through an example from the french armorican variscides it concerns a magmatic body cross cutting highly strained terranes in the ouessant island regarded for decades as a granitoid monzogranite on the basis of both its coarse grained texture and its mineralogy however the volcanic origin of the metamorphosed series flanking this foliated body is here recognized by pillow lavas deposit layers and fiamme bearing volcaniclastics all emplaced onto the soft substrate floor of a fault bounded basin among other things similarities between feldspar megacrysts porphyrocrysts in both the volcaniclastics and the adjoining formerly monzogranitic massive body lead us to reinterpret the latter as a trachydacitic extrusion in our model the corresponding viscous lava progressively flowed in the basin recovering the earlier volcanic formations and inducing load effects on the underlying soft sediments along with compaction of the previously deposited pumices to produce fiamme the interpretation of the south ouessant area as a visean transtensional volcano sedimentary basin provides a new perspective on the distribution of the variscan pull apart basins in the armorican massif

ice lens formation at a silt sand interface a major experiment simulating ground freezing around a buried chilled pipeline in a controlled environment facility provided an opportunity to examine the form and orientation of ice lenses associated with a vertical interface between silt and sand the heave of the silt decreased towards the interface and ice lenses in the silt were found to dip at an increasing angle in the same direction consideration of the thermal regime suggests that the direction of heat flow influences the orientation of the lenses the interface was irregular and ice lenses at the lower part of the interface were closely aligned to it indicating that changes in soil texture also influence ice lens orientation deformation of ice lenses appears to have occurred in the silt near to the interface the arrangement of the lenses can be explained by the different thermal properties and thermodynamic behaviour of the two materials and by the mechanical anchorage of the sand in which there is no ice lens formation cycles of freezing

a comparison of velocity and attenuation between the nicobar and bengal deep sea fans the compressional wave velocity and attenuation measurements from a deep source deep receiver seismic refraction experiment from the nicobar fan indian ocean are presented and compared to a similar set of measurements in the bengal fan the two deep sea fans consist of thick sections of turbidites derived from the same sediment source but have slightly different depositional histories travel time inversion of the data into a velocity depth model uses the tau zeta linear inversion scheme we present a revised reparameterization of the travel time data and improved error analysis for the inversion process although the velocity structures with depth are morphologically similar there exist discrete differences between the two stations in the nicobar fan the initial velocity assumed is 1 513 km s with an initial gradient of 2 32 s 1 the velocity increases smoothly with depth while the velocity gradient decreases rapidly to a constant 0 81 s 1 the bengal fan station showed the same trend although the velocity gradient at depth was only 0 67 s 1 the attenuation profiles are similar only in gross structure with the nicobar fan values of q 1 being a factor of 3 to 4 less than the bengal station the differences in velocity and attenuation structure can be attributed to a number of causes including porosity and sedimentation rates which are related to the distance from the sediment source and the presence of a normal fault near the nicobar fan site this fault may act as a conduit for water removal in the presence of overburden pressure it is believed that porosity changes due to whatever cause dominate the lateral and vertical variations of velocity velocity gradients and together with intrabed multiples attenuation in thick sedimentary sections

olivine basalt and trachyandesite peperites formed at the subsurface surface interface of a semi arid lake an example from the early miocene bigadic basin western turkey abstract miocene successions in western turkey are dominated by lacustrine fluvial and evaporitic sedimentary deposits these deposits include considerable amounts of volcaniclastic detritus derived from numerous ne trending volcanic centres in western turkey as well as in the bigadic region early miocene syn depositional ne trending olivine basalt and trachyandesite bodies that formed as intrusions and lava flows occur within the bigadic borate basin olivine basalts occur as partly emergent intrusions and trachyandesite dykes fed extensive lava flows emplaced in a semi arid lacustrine environment peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features although all the localities include a large variety of clast morphologies from blocky to fluidal fluidal clasts mainly globular ameboidal and pillow like varieties are widespread in the peperite domains associated with olivine basalts apparently due to large volume sediment fluidisation in contrast fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the intrusions and have commonly elongate and polyhedral shapes with digitate margins rather than globular and equant varieties blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration suggesting decreasing temperature and increasing viscosity during fragmentation abundance of blocky clasts with respect to fluidal clasts in the trachyandesite peperite indicates that the fluidal emplacement and low volume sediment fluidisation in the early stages were immediately followed by quench fragmentation due to the high viscosity of the magma size texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly controlled by sediment fluidisation pulsatory magma injection and magma properties such as composition viscosity vesicularity and size abundance and orientation of phenocrysts variously combining these contrasting features to varying degrees may form diverse juvenile clast shapes in peperitic domains

detailed observations on the nature of frost heaving at a field scale observations made using a system of ring magnets have provided a detailed picture of differential soil strain associated with frost heaving at a field scale the results reveal the evolution of heave as freezing advances through the soil and the variation of soil strain with depth and time and with soil temperature conditions the results indicate that soil strain characteristically continues within frozen soil sustained it is proposed by continuing water migration into the frozen soil consequently a considerable thickness of frozen soil appears to be actively involved with moisture movement and ice accumulation during frost heaving the observations support the view that frost heave generally depends in a complex manner on the thermodynamic conditions of temperature and water and ice pressures as they are modified by the rheological properties of the soil key words frost heave differential soil strain frost heave measurement

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

fiamme textures in volcanic successions flaming issues of definition and interpretation abstract fiamme are aligned flame like lenses found in welded ignimbrite fiamme also occur in welded pyroclastic fall deposits secondary welded pumice rich facies diagenetically altered and compacted non welded pumiceous volcaniclastic facies and lavas fiamme can be formed in a variety of ways the common genetic use of the term fiamme for pumice clasts that have undergone welding compaction is too narrow fiamme is best used as a descriptive term for elongate lenses or domains of the same mineralogy texture and composition which define a pre tectonic foliation and are separated by domains of different mineralogy texture or composition this descriptive term can be used regardless of the origin of the texture and remains appropriate for flattened pumice clasts in welded ignimbrite

pipelines in permafrost geotechnical issues and lessons 12010 r m hardy address 63rd canadian geotechnical conference geotechnical input to the design construction and operations of pipelines in permafrost may differ significantly from that for pipelines in temperate terrain the general remoteness and terrain f

crustal structure along the edge transect beneath the kodiak shelf off alaska derived from obh seismic refraction data summary along the edge alaska transect over the kodiak shelf off alaska a deep crustal seismic refraction experiment was carried out in 1994 using an airgun source with a high shot density and closely spaced ocean bottom hydrophones obhs these data clearly define a gently dipping plate boundary that gradually increases from 2 5d at the trench to 5d 250 km landwards of the trench beneath the shelf a landward increase of the velocities in the subducting oceanic crust was found the sedimentary upper plate beneath the continental slope can be divided into a young neogene accretionary prism beneath the lower slope and a more layered structure below the middle and upper slope where a body with elevated velocities 4 2 5 1 km s 1 in the lower part is interpreted as well consolidated sediment of possibly late eocene age acting as the backstop across the inner shelf between kodiak island and kenai peninsula near surface high velocities 6 0 km s 1 document a strong uplift 10 km of the mesozoic and palaeocene accretionary complex and furthermore suggest its seaward extension beneath the outer shelf the most striking feature along the edge transect is the thick low velocity zone lvz that coincides with the arched reflectors in the lower crust beneath the kodiak island kenai peninsula axis on the edge seismic reflection data this lvz is interpreted as indicating underplated low velocity rocks which contribute the same volume of new material as the frontal accretion to the continent the growth of which is rapid totalling 4200 km2 across the kodiak shelf since eocene times a calculation of the material balance shows that an underplating of underthrust sediment alone may not account for the huge volume of the underplated low velocity rocks we speculate that the underplating of continental fragment or more likely seamounts or plateau may be involved in continental growth

the ecology of living stained deep sea benthic foraminifera from the sulu sea the distribution of living rose bengal stained deep sea benthic foraminifera was determined in the upper 20 cm of sediments of eight soutar box cores taken from two depth transects 510 4515 m in the thermospheric 10 c sulu sea despite the uniformity of bottom water temperatures salinities and dissolved oxygen levels below 1000 m significant faunal differences exist at different depths in the low oxygen 1 25 ml l below 1000 m basin the shallowest site 510 m is dominated 10 of the calcareous fauna by cibicidoides uvigerina 150 m and bolivina 63 m while siphonina is codominant with cibicidoides and uvigerina in the 1005 m core the 2000 m cores are dominated by cibicidoides gyroidinoides and oridorsalis while cibicidoides bradyi and oridorsalis umbonatus dominate the 3000 and 4000 m cores infaunal assemblages of valvulineria mexicana are found in the sediments of the 4515 m core relatively low bottom water oxygen values do not necessarily yield typical low oxygen taxa such as bolivina uvigerina chilostomella bulimina and globobulimina changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments an examination of the vertical distributions of foraminiferal assemblages from the 63 m and 150 m fractions reveals that taxa have microhabitat preferences similar to those observed in other regions taxa found in the upper 0 to 1 cm interval epifaunal include cibicidoides wuellerstorfi and hoeglundina elegans while taxa such as chilostomella and globobulimina reach maximum abundances in subsurface sediments and have infaunal microhabitat preferences cibicidoides bradyi and o umbonatus live in sediment depths from 0 to 4 cm and have transitional preferences with both epifaunal and infaunal occurrences intrageneric differences in test morphologies including pore distribution rounded peripheries and variable spire height are observed in cibicidoides and gyroidinoides and are suggested to be related to microhabitat preferences vertical distributions of a number of taxa found in both the 63 to 150 m and 150 m fractions are similar suggesting that juveniles and adults live under similar microhabitat conditions ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effects reported for some taxa in previous studies

blocky versus fluidal peperite textures developed in volcanic conduits vents and crater lakes of phreatomagmatic volcanoes in mio pliocene volcanic fields of western hungary abstract volcanic fields in the pannonian basin western hungary comprise several mio pliocene volcaniclastic successions that are penetrated by numerous mafic intrusions peperite formed where intrusive and extrusive basaltic magma mingled with tuff lapilli tuff and non volcanic siliciclastic sediments within vent zones peperite is more common in the pannonian basin than generally realised and may be also important in other settings where sediment sequences accumulate during active volcanism hajagos hegy an erosional remnant of a maar volcano was subsequently occupied by a lava lake that interacted with unconsolidated sediments in the maar basin and formed both blocky and globular peperite similar peperite developed in kissomlyo a small tuff ring remnant where dykes invaded lake sediments that formed within a tuff ring lava foot peperite from both hajagos hegy and kissomlyo were formed when small lava flows travelled over wet sediments in craters of phreatomagmatic volcanoes at sag hegy a large phreatomagmatic volcanic complex peperite formed along the margin of a coherent intrusion all peperite in this study could be described as globular or blocky peperite globular and blocky types in the studied fields occur together regardless of the host sediment

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

neural network interpretation of lwd data odp leg 170 confirms complete sediment subduction at the costa rica convergent margin abstract the internal structure of a convergent plate boundary was the focus of odp leg 170 in 1996 at the subduction zone off costa rica although the structure of the subduction zone is rather well known from seismic surveys prior to drilling of odp leg 170 it was a matter of discussion whether it is accretionary or non accretionary with a neural network approach we confirm the evidence gained during drilling of leg 170 that at least presently no lower plate sediments are transferred to the upper plate by accretion to supplement lithological information logging while drilling geophysical data have been included in this study and were interpreted in terms of lithology using a genetically trained artificial neural network

recent benthic foraminifera from the eastern south atlantic ocean dependence on food supply and water masses abstract sixty surface sediment samples from the eastern south atlantic ocean including the walvis ridge the angola and cape basins and the southwest african continental margin were analysed for their benthic foraminiferal content to unravel faunal distribution patterns and ecological preferences live stained with rose bengal and dead faunas were counted separately and then each grouped by q mode principal component analysis into seven principal faunal end members then multiple regression technique was used to correlate recent assemblages with available environmental variables and to finally differentiate between four principal groups of environmental agents acting upon the generation of benthic foraminiferal assemblages 1 seasonality of food supply and organic carbon flux rates together with oxygen content in the pore and bottom waters 2 lateral advection of deep water masses 3 bottom water carbonate corrosiveness and 4 energetic state at the benthic boundary layer and grain size composition of the substrate food supply and corresponding dissolved oxygen contents in the pore and bottom waters turned out to be the most important factors which control the distribution pattern of the recent benthic foraminifera at the continental margin in the zone of coastal upwelling and its mixing area benthic foraminiferal assemblages are dominated by stenobathic high productivity faunas characterized by elevated standing stocks low diversities and a large number of endobenthic living species at the continental shelf and upper continental slope the live assemblages are characterized by rectuvigerina cylindrica uvigerina peregrina s 1 uvigerina auberiana and rhizammina spp while the dead assemblages are characterized by cassidulina laevigata bolivina dilatata bulimina costata and b mexicana at the lower continental slope strong influence of high organic matter fluxes on the species composition is restricted to the area off the cunene river mouth where the live assemblage is dominated by uvigerina peregrina s 1 the corresponding dead assemblage by melonis barleeanum and m zaandamae in the adjacent areas of the lower continental slope the biocoenosis is characterized by reophax bilocularis and epistominella exigua which becomes dominant in the corresponding dead assemblage at the walvis ridge and in the abyssal angola and cape basins where organic matter fluxes are low and highly seasonal benthic foraminiferal assemblages reflect both the oligotrophic situation and the deep and bottom water mass configuration the top and flanks of the walvis ridge are inhabited by the rhizammina psammosphaera and r bilocularis live assemblages the corresponding dead assemblages are dominated by g subglobosa on the ridge top and e exigua on the flanks within the highly diverse e exigua dead assemblage several associated epibenthic species coincide with the core of nadw between about 1600 and 3700 m water depth these species include osangularia culter cibicidoides kullenbergi melonis pompilioides bolivinita pseudothalmanni and bulimina alazanensis the assemblages of the abyssal cape and angola basins are characterized by nuttallides umbonifer and a high proportion of agglutinated species these species are adapted to very low organic matter fluxes and a carbonate corrosive environment

seismic attenuation structure of the east pacific rise near 9 30 n we describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9 32 n on the east pacific rise over 3500 estimates of t are obtained from 0 4 to 0 7 s long windows aligned with crustal p phases including diffractions above and below a midcrustal magma chamber and moho reflections which cross the rise axis at a range of lower crustal depths we apply a smoothest model inversion algorithm to the t measurements to derive images of apparent crustal q 1 both 20 km off axis and within a 16 16 km area centered on the rise crest the models resolve regions of high attenuation in the uppermost crust and in a low q zone which extends from midcrustal to lower crustal depths beneath the rise axis off axis q values in the upper 1 km average 35 50 while at depths greater than 2 3 km q is at least 500 1000 the high levels of attenuation in the uppermost crust probably result from the combined effect of factional fluid flow and scattering mechanisms within 1 3 km of the rise axis q increases markedly in the uppermost 1 km to about 65 if the increase in attenuation off axis is entirely due to the 300 m increase in the thickness of layer 2a extrusives required by seismic velocity measurements then q in layer 2a must be about 10 20 no measurements of q are obtained in the immediate vicinity of the 1 6 km deep axial magma lens because no wave paths cross the rise axis through this region the diffractions beneath the magma chamber and the moho reflections require a low q region with minimum q values of 20 50 which extends from no more than 2 5 km depth to the base of the crust these values are similar to laboratory measurements of q obtained at solidus temperatures and constrain the low q region to contain no more than a few percent melt the axial magma chamber which comprises a melt lens and an underlying crystal mush zone must be confined to a narrow 1 km thick region through which no rise crossing paths pass inversions for along axis structure in the low q anomaly show a 20 25 increase in attenuation at 2 3 km depth north of 9 34 n but resolve no such trend at 4 6 km depth the along axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along axis variations in magma supply

relations between basalts and adakitic felsic intrusive bodies in a soft substrate environment the south ouessant visean basin in the variscan belt armorican massif france the metasedimentary and magmatic terranes in the southern part of the ouessant island western brittany france are the offshore prolongation of the leon variscan metamorphic domain they mainly c

the significance of cryoturbation phenomena in environmental reconstruction cryoturbation results mainly from differential heave related to 1 drainage and thermal conditions and 2 frost susceptibility loadcasting and cryostatic pressures are restricted to wet soils such as pure silt or silty sand the translocation of fine particles resulting from repeated freezing and thawing can promote frost susceptibility in a previously non susceptible material as well as a loss in stability of the surface horizons liquefaction small and metric scale cryoturbations can be explained by this process large scale deformations and injections may be related to abnormal liquefaction not necessarily associated with frost regular patterns are associated with fissure nets upturned injections in a downslope direction attest to cryoturbation and frost creep a lowering of the water table promotes the formation of drop like cryoturbations most of which can be related to seasonal frost in imperfectly to poorly drained soils they are not indicative of permafrost except in well drained situations or in the case of flat bottomed involutions

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

planktonic benthonic foraminiferal ratios modern patterns and tertiary applicability abstract the abundance of planktonic specimens in foraminiferal assemblages was determined in numerous bottom samples from inner neritic to deep oceanic depths along the atlantic margin of the northeastern united states the results augment previous studies in other areas that have shown a general increase in percentage of planktonic specimens in total foraminiferal bottom assemblages as water depth increases the patterns found in this area of complex shelf bathymetry and hydrography illustrate the influence on the planktonic benthonic percentages of water depth distance from shore different water mass properties and downslope movement of tests in high energy areas the patterns found in the 661 samples from the atlantic margin were compared with results from 795 stations in the gulf of mexico pacific ocean and red sea the relative abundance of planktonic specimens and water depth correlates positively in all open oceanic areas even though taxonomic composition and diversity of the faunas from different areas is variable the variation of planktonic percentages in bottom samples within most depth intervals is large so that a precise depth determination cannot be made for any given value however an approximate upper depth limit for given percentages can be estimated for open ocean environments a decrease in planktonic percentages is seen in the lower salinity and higher turbidity coastal waters of the gulf of maine planktonic percentages intermediate between the lower values in the less saline coastal waters and the higher values in the normal open oceanic conditions occur in the transitional area between the gulf of maine and the open marine atlantic ocean to the east similarly lowered values in another area of restricted oceanic circulation occur in the high salinity clear but nutrient poor waters of the gulf of aqaba off the red sea a comparison of the similarity of modern planktonic percentage values to those found in earlier tertiary assemblages was made to confirm the usefulness of this measure in the fossil record in some stratigraphic sections in upper paleocene and lower eocene strata of the eastern gulf coastal plain water depths inferred from trends and values of planktonic percentages consistently match paleobathymetry constructed from physical stratigraphic characteristics and paleogeographic relationships

tectonic structure of the convergent pacific margin offshore costa rica from multichannel seismic reflection data the middle america trench between the cocos ridge and a well studied corridor off the nicoya peninsula has a more varied morphology and structure than previously reported the morphological positive features on the lower plate significantly affect the upper plate structure the cocos ridge has uplifted the margin opposite the osa peninsula northwest of cocos ridge numerous seamounts on the oceanic crust sculptured the margin as they subducted a seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted the greater portion of the trench sediment is subducted beneath a lower slope accretionary mass beneath the middle and upper slope is a margin wedge consisting of a high velocity rock with few internal reflections its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area overlying this surface is a low velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred eocene section beneath the shelf the shelf basement is composed of nicoya complex ophiolite with the same acoustic velocity similar structure and no apparent dividing geologic boundary with the margin wedge we favor a seaward continuation of the nicoya complex to the middle slope and emphasize the evidence for a non steady state tertiary tectonic history

combining quantitative textural and geochemical studies to understand the solidification processes of a granite porphyry shanggusi east qinling china igneous rock textures reflect the cooling history of the parental magma combined with chemical data they can provide physical and chemical information about the evolution of a magma body the petrographic textures and chemical compositions of 21 coarse and fine grained granite samples along an 250 m horizontal outcrop of the shanggusi granite porphyry are presented in this case study the coarse grained granite porphyry is an early intrusion and the fine grained granite dykes mostly intruded into the granite porphyry are later intrusions the studied samples have nearly homogeneous major element bulk rock and mineral compositions but show large variations in their trace element compositions and textural characteristics the trace element data suggest the influence of hydrous fluids possibly enriched in co2 f and cl in the evolution of the plutonic body textural analysis of the coarse grained granite porphyry indicates that the crystal size distribution csd slopes intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals lmax average length of the four largest quartz crystals for each sample however most fine grained samples and the groundmass of the coarse grained samples show concave down csds indicating textural coarsening quartz csds in the coarse grained samples are kinked with a steep sloped log linear section representing small crystals 5 1mm and a shallow sloped log linear section representing large crystals 4 1mm these two crystal populations are interpreted as resulting from a shift in cooling regime the straight csds of two fine grained samples may be due to a different cooling history in general the spatial variation of the csd patterns can be attributed to various degrees of overgrowth and mechanical compaction the quartz phenocrysts in several coarse grained samples exhibit a high degree of alignment which may be the result of magmatic flow by integrating the field geology geochemistry and quantitative textural data from the horizontal profile of the shanggusi granite porphyry it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history fluid migration controlled undercooling can explain the solidification processes in the shanggusi intrusion and may also be prevalent in other fluid rich shallow intrusions quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system and provide relatively comprehensive insights into the petrogenesis of granites

overconsolidation effects of ground freezing abstract chamberlain e j 1981 overconsolidation effects of ground freezing eng geol 18 97 110 temporary ground freezing is a valuable technique for stabilizing soft soils during construction it imparts large increases in strength and bearing capacity to most soils however freezing can cause significant changes in soil structure and density which can lead to adverse settlement during thaw settlement of clay soils after freezing and thawing is the result of the suction forces that draw pore water to the freezing front these suction forces cause an increase in the effective stress on the clay beneath the freezing front and thus cause an overconsolidation of the clay as these suction forces often exceed 1 atm their direct measurement is not easy a technique for indirectly determining the maximum suction occurring during freezing is presented which utilizes the apparent memory that clay soils have for maximum past preconsolidation pressures suctions as large as 532 kn m 2 were observed after freezing and thawing a clay soil which was initially consolidated to 128 kn m 2 the volume changes resulting from the freezing and thawing of clays were related to the plastic limit and were observed in the laboratory to be as high as 25 if provisions are not made to account for these volume changes in a ground freezing project considerable damage to structures can occur from settlement and the resulting stresses

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

a late ordovician phreatomagmatic complex in marine soft substrate environment the crozon volcanic system armorican massif france the mafic lavas and the diabases of crozon armorican massif france belong to an anorogenic ordovician volcanic complex emplaced on a rifted passive margin in north gondwana magma passed through syn volcanic soft sedimentary substrate which is today mostly composed of alternating sandstones and mudstones from llanvirn to ashgill in age field observations together with microscopic studies and geochemical analyses of magmatic rocks lead us to propose a model of volcano formation which combines hydromagmatic processes peperitic intrusions a shallow submarine tephra settling eruption fed turbidity currents and a pillow lava effusion the crozon outcrops can be used to reconstruct a complete cross section from the root of the volcanic complex to the lavas and breccias emplaced on the sea floor the sites expose i a hypabyssal breccia containing mud chunks and coarse grained diabase clasts with amoeboidal fine grained magmatic material ii bulbous peperitic sills and pillow like lobes bearing a great quantity of sediment derived enclaves of fluidal morphology iii volcaniclastic breccias containing near spherical magmatic clasts that resulted from the complete fragmentation of sills in the ductile regime iv a rhythmic peperitic breccia interpreted as the product of mingling between thin lava flows and soft calcareous sediment the crozon volcanic form resulting from explosive interaction with subsurface surface water was probably a subaqueous collapsed tuff cone this upper part of the system is synchronous with an ashgill carbonate sedimentation which overlies an ordovician siliciclastic succession deposited in shelf environments

benthic foraminiferal assemblage response to the onset of northern hemisphere glaciation paleoenvironmental changes and species trends in the northeast atlantic abstract the response of the benthic foraminiferal community in the northeast atlantic to the late pliocene onset of northern hemisphere glaciation was investigated using closely spaced samples from dsdp ipod site 548 factor analysis of quantitative species abundance data shows that the replacement of aglobocassidulina subglobosa assemblage first bybulimina mexicana and then bymelonis uvigerina dominated assemblages accompanied the onset of glaciation also the benthic assemblages became much more variable with progressive climatic deterioration in order to examine benthic foraminifera species responses to changing paleoenvironmental variables measurements of planktonic foraminifera species abundances benthic foraminifera stable isotope values and sediment grain size and composition were made the planktonic data and oxygen isotope values were used to quantify paleoclimatic changes and changes in bottom water properties the sediment analyses were used to quantify changes in the substrate and a combination of planktonic assemblage data and benthic carbon isotope values were used to estimate changes in food supply surface ocean productivity canonical correlation analysis of the species data and the independent paleoenvironmental indicators shows a strong relationship between planktonic assemblage diversity and the long term benthic trend of assemblage replacement this trend could be a response to changing bottom water properties but it might also reflect increasing food supply at the sea bed in addition the benthic assemblages show a strong relationship to changes in the carbonate content of the sediments this latter fluctuates due to varying terrigenous input changes in substrate at site 548 may influence the benthic foraminiferal assemblage there remains however a significant fraction of species abundance variability particularly in theuvigerina species that cannot be explained with the paleoenvironmental indicators used in this study

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

paleoproductivity from benthic foraminifera abundance glacial to postglacial change in the west equatorial pacific surface productivity is correlated with the rate of accumulation of benthic foraminifera on the deep sea floor as a rule of thumb for each 1 mg of organic carbon arriving at the sea floor one benthic foram shell 150 m is deposited the correlation can be used to reconstruct organic flux to the sea floor in the past and hence the productivity of past oceans applying the appropriate equations to box core data from the ontong java plateau in the western equatorial pacific we found that productivity during the last glacial maximum exceeded present productivity by a factor of between 1 5 and 2 0 with intermediate values for the mid transition period accumulation of benthic foraminifera was depressed on top of the plateau during the glacial and transitional period presumably because increased winnowing removed part of the food supply

peperitic textures of ordovician dacitic synsedimentary intrusions in argentina s puna highland clues to emplacement conditions abstract synsedimentary crystal rich dacitic sills laccoliths and cryptodomes emplaced in early ordovician submarine thin bedded silt sandstone sequences show a variety of magma water interaction features megapeperites occurring along the igneous contact contain angular dacitic megaclasts that are dispersed in dominantly structureless host sediment they result mainly from quench fragmentation of magma and dynamic stressing of cooling magma bodies on contact with wet sediments combined in some instances with fluidization of the sediments or minor explosive expansion of water incorporated by magma breaching the sediment water interface or stopping of water saturated roof sediments by shallowing intrusions globular peperites having bulbous to globular dacitic clasts immersed in the host sediment with evidence of fluidization along the contact zone are also recognized at the top and base of sills in this case a vapor film might have formed at the magma sediment interface insulating the magma from direct contact with the sediment and suppressing quench fragmentation and steam explosions the diversity of peperite textures observed in the same intrusive and sedimentary rocks appears to be mainly the result of different levels of emplacement of the synsedimentary dacitic intrusions scarcity of peperites related to mafic sills is attributed to their having been emplaced late during the evolution of the basin

melange formation by subduction erosion the case of the osa melange in southern costa rica the convergent plate margin off the osa peninsula in southern costa rica is characterized by the indentation of the cocos ridge at 4 5 ma the indentation causes the uplift of the osa melange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone we present evidence that similar to the nicoya segment of the costa rica convergent margin subduction erosion rather than accretion has been the dominant process along the plate boundary the composition of the osa melange is dominated by tectonized material of the upper plate nicoya ophiolite complex basalt radiolarite limestone strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes which partly experienced temperatures 200 c we thus interpret the osa melange to be a product of subduction erosion at the base of the outer arc wedge structure

seismic attenuation in the costa rica margin wedge amplitude modeling of ocean bottom hydrophone data abstract travel time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles located 10 20 km landward of the middle america trench offshore costa rica these two profiles bracket a region of rapid change in physical properties and structural styles the travel time modeling indicates little structural change along the profiles and therefore the data are suitable for amplitude modeling with the reflectivity method for the profile 10 km from the trench amplitude modeling indicates a margin wedge with velocities of 4 4 4 km s and qp 25 50 for the profile 20 km from the trench the amplitude modeling indicates a margin wedge with velocities of 4 4 4 8 km s and qp 50 75 the primary new result of this analysis is the constraint that qp 25 75 within the margin wedge at both locations these high attenuation values are consistent with fracturing of the material making up the margin wedge either ophiolitic nicoya complex rocks dewatered and cemented sediments or a tectonic mixture of both

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

submarine eruption fed and resedimented pumice rich facies the dogashima formation izu peninsula japan in the izu peninsula japan the pliocene pumice rich dogashima formation 4 55 0 87 ma displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave base marine setting it includes high concentration density current deposits that contain clasts that were emplaced hot indicating an eruption fed origin the lower part of the dogashima 2 unit consists of a very thick sequence 1 m pumice and ash rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension in dogashima 1 and 2 thick cross and planar bedded facies composed of sub rounded pumice clasts are intercalated with eruption fed facies implying inter eruptive mass wasting on the flank of a submarine volcano and reworking and resedimentation by high energy tractional currents in a below wave base environment

subsidence and extension at a convergent plate margin evidence for subduction erosion off costa rica subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the nicoya segment of the convergent plate margin off costa rica based on new results from odp drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the nicoya coastline high seismic velocities in the outermost part of the forearc wedge off the nicoya peninsula below the boss bottom of slope sediment reflector indicate the seaward continuation of the nicoya ophiolite complex into the basement of the forearc wedge odp site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement progradation of the sedimentary succession and landward migration of the coastline tilted block structures are explained by substantial fore arc extension in our interpretation the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation

lidar offshore structural mapping and u pb zircon monazite dating of variscan strain in the leon metamorphic domain nw brittany an exceptional structural picture of the immerged variscan basement offshore the leon metamorphic domain is supplied by high resolution lidar and echosounder data recorded in the molene archipelago western brittany france various types of fabrics are identified and from in situ rock sample analyses further combined with field structural data are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures our onshore offshore study leads us to propose a two phase kinematic model that emphasizes the role of a strike slip duplex in an ew trending relay zone linking the north armorican and pierres noires ductile shear zones nasz pnsz dextral shearing occurred within a transtensional setting synchronously with magmatic intrusions st renan granite and an offshore gabbro diorite complex dated at 314 320 ma by new u th pb ages it post dated an early regional foliation related to top to the ne ductile transpressional shearing our study emphasizes the key role of strike slip tectonics in the nw part of the armorica variscan belt

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

benthic foraminiferal distribution and sedimentary structures suggest tectonic erosion at the costa rica convergent plate margin distribution patterns of benthic foraminiferal faunas from odp leg 170 sites 1041 and 1042 show that the costa rican convergent margin subsided from coastal to abyssal depth from middle miocene to present this favours the model of a margin undergoing active subduction erosion we propose that subduction erosion leads to the removal of material from the base of the forearc wedge and as a consequence to progressive subsidence of the forearc a mean subsidence rate is estimated to be approximately 0 4 mm yr 1

seismic structure of the upper oceanic crust revealed by in situ q logs in situ seismic attenuation is computed through 2 1 km of the upper oceanic crust in the vicinity of odp hole 504b the results strongly tie crustal properties to seismic measurables and observed geological structures we find that the attenuation can be used to define seismic layer boundaries and is closely related to the intensity of vertical heterogeneity the in situ attenuation q 1 consists of both intrinsic and scattering contributions but is dominated by the scattering attenuation unless porosities are near zero when it approaches typical estimates from seismic refraction studies the attenuation is analytically modeled by multiple backscattering from heterogeneities observed in a sonic vp log and is found to decrease step wise in relatively homogeneous layers from q 25 to q 300 between the top of seismic layer 2a and a sharp discontinuity at 1 3 km depth these changes correspond with heterogeneities at 1 0 1 3 m and at 5 6 10 0 m wavelengths that we interpret to be associated with fracturing and structure of pillow basalts and lava flows in seismic layers 2a and 2b although seismic velocity studies suggest that the layer 2 3 boundary also occurs at about 1 3 km the large variation in q 140 to 460 below this depth indicates that a seismically homogeneous and uniform layer 3 has not been reached in hole 504b we derive an empirical relationship between attenuation and porosity q 1 q0 1 e where q0 1 0 004 and 25 that may be applicable at other oceanic crust locations and useful for constraining seismic inversion models

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

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

frost heave and pipeline upheaval buckling frost heave of soils varies greatly with the type of soil and the moisture and thermal conditions of the ground consequently the initially level trench bottom profile of a pipeline can become uneven upheaval buckling is a well known phenomenon in buried pipelines it can lead to large upward movements of a pipeline and is caused by the interaction between the longitudinal compressive force present during operation and overbend irregularities in the profile this paper examines the possibility that frost heave and upheaval buckling can interact adversely and threaten the security of arctic pipelines key words arctic buckling frost heave permafrost pipelines

the miocene of the hatay area s turkey transition from the arabian passive margin to an underfilled foreland basin related to closure of the southern neotethys ocean abstract miocene sediments of the hatay area document the distal southerly part of an underfilled peripheral foreland basin related to the closure of the southern neotethys the basin is underlain by latest cretaceous and eocene shallow marine carbonates interpreted as the youngest part of the arabian passive margin succession unconformably overlying lower miocene conglomerates sandstones and palaeosols up to 250 m thick represent braided river deposits derived from an uplifted area to the south during the middle miocene carbonate sediments accumulated in a variety of shallow marine environments dominated by a northward sloping ramp with non marine peritidal lagoonal and coral reef deposition in local areas the upper miocene succession is mainly deeper water hemipelagic marl with clastic interbeds messinian evaporites were deposited near the depocentre the early miocene coarse fluvial sediments above the regional unconformity reflect the development of a flexural bulge or re activated basement structure the middle miocene carbonates and upper miocene marls are interpreted as the lower and middle units of an underfilled trinity considered typical of peripheral foreland basins similar lower miocene coarse clastic sediments and mid miocene shallow water carbonates unconformably overlie the arabian platform succession c 75 km to the north where the succession continues upwards into thick shallow marine to non marine terrigenous sediments upper unit of the trinity sourced from the overthrust load comparable coarse clastic sediments are absent from the distal foreland basin in the hatay area probably reflecting its more southerly position structural barriers in the foreland and the westward bypassing of sediment towards the mediterranean sea regional convergence halted by the end of the miocene and was followed by strike slip and westward tectonic escape as a result the miocene foreland basin was dismembered to form the plio quaternary transtensional hatay graben this like many other foreland basins shows some features which are not accommodated by ideal models of foreland basin evolution

subsoil investigation of ice lensing at the calgary canada frost heave test facility several frost heave mitigation modes were studied at the calgary canada chilled pipeline frost heave test facility these included deeper burial to increase the pressure on the frost front below the pipe and replacement of the silty soil around the pipe with gravel for a noninsulated pipe frost heave at the deep burial section and at the gravel section was less than the heave at a control section other pipe sections tested the effects of insulation of the pipe on the long term frost heave as well as the effects of replacing the silt around an insulated pipe with gravel summer thawing of the frost bulb around the insulated pipe results in seasonal thaw settlement of the pipe thus reducing the long term pipe heave at least for the warmer ground temperature environment at the calgary facility thermal simulations of the frost bulb growth and predictions of frost heave using the segregation potential model agree well with the observations recent excavation of two frost bulbs in silty soil led to fiel

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

response of northern indian ocean deep sea benthic foraminifera to global climates during pliocene pleistocene abstract the northern indian ocean dsdp site 214 benthic foraminifera have been examined quantitatively to understand their response to the global climatic events and to determine the history of bottom water circulation in the northern indian ocean during pliocene pleistocene uvigerina spp cibicides kullenbergi c wuellerstorfi globocassidulina pacifica g subglobosa oridorsalis umbonatus and bulimina alazanensis are the dominant members of the benthic assemblage and reveal significant changes in distribution patterns based on benthic foraminiferal frequency changes four intervals of major faunal and climatic turnover have been recognized at 5 2 5 1 ma 3 9 3 2 ma 3 2 3 1 ma and 3 1 0 6 ma the interval between 5 2 5 1 ma is marked by widespread antarctic glaciation deep sea hiatuses bottom water cooling increased upwelling high sedimentation rates highest uvigerina abundances and intensified bottom water circulation during 3 9 3 2 ma the bottom waters were warmer and bottom water activity waned with a sharp decline in uvigerina abundance and increase in cibicides the interval between 3 2 3 1 ma was a time of bottom water cooling and coincides with a global benthic positive oxygen isotopic shift antarctic ice volume increase and possible initiation of northern hemisphere glaciation the late pliocene and pleistocene interval 3 1 recent is punctuated by major changes in benthic foraminiferal assemblages warm cold cycles major glacial interglacial intervals increase in polar ice volume and intensification of upwelling on the basis of cumulative percentages of uvigerina and cibicides nine intervals of major bottom water cooling have been inferred in the pliocene and two in the pleistocene at site 214 reflecting the major changes in bottom water circulation

petrogenesis of late variscan high k alkali calcic granitoids and calc alkalic lamprophyres the aber ildut north ouessant complex armorican massif france the aber ildut north ouessant variscan granitoid complex in the armorican massif is an example of high k alkali calcic zoned pluton c 304 ma in age a first magmatic batch intruded through a northern ew trending sinistral transcurrent shear zone before injecting southwards as a huge horizontal zoned sill with moderately peraluminous muscovite free granitoids in the north and strongly peraluminous muscovite bearing leucogranites to the south the second magmatic stage resulted in the intrusion of a large two mica leucogranitic body from a root zone along the same shear zone prior to the end of crystallization of the first injection finally ultrapotassic dykes including calc alkalic lamprophyres leucominettes intrude the complex the strongly peraluminous granites are interpreted as pure melting products of crustal clay rich pelitic material all the other petrographic types including leucominettes are thought to result from mixing of crustal melts and mantle derived mafic liquids highly silicic tourmaline bearing leucogranites are significantly affected by a tetrad effect in relation with ree complexing behavior aber ildut north ouessant granitoids probably formed during the variscan late orogenic stage associated with exhumation and lithospheric thinning at low pressure and by advective heating of a 50 km thick crust from hot metasomatized asthenosphere

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

benthic foraminiferal response to the emergence of the isthmus of panama and coincident paleoceanographic changes abstract late cenozoic benthic foraminiferal faunas from the caribbean deep sea drilling project dsdp site 502 3052 m and east pacific dsdp site 503 3572 m were analyzed to interpret bottom water masses and paleoceanographic changes occurring as the isthmus of panama emerged major changes during the past 7 myr occur at 6 7 6 2 3 4 2 0 and 1 1 ma in the caribbean and 6 7 6 4 4 0 3 2 2 1 1 4 and 0 7 ma in the pacific prior to 6 7 ma benthic foraminiferal faunas at both sites indicate the presence of antarctic bottom water aabw after 6 7 ma benthic foraminiferal faunas indicate a shift to warmer water masses north atlantic deep water nadw in the caribbean and pacific deep water pdw in the pacific flow of nadw may have continued across the rising sill between the caribbean and pacific until 5 6 ma when the pacific benthic foraminiferal faunas suggest a decrease in bottom water temperatures after 5 6 ma deep water to intermediate water flow across the sill appears to have stopped as the bottom water masses on either side of the sill diverge the second change recorded by benthic foraminiferal faunas occurs at 3 4 ma in the caribbean and 4 0 3 2 ma in the pacific at this time the caribbean is flooded with cold aabw which is either gradually warmed or is replaced by glacial bottom water gbw at 2 0 ma and by nadw at 1 1 ma these changes are related to global climatic events and to the depth of the sill between the caribbean and atlantic rather than the rising isthmus of panama benthic foraminiferal faunas at east pacific site 503 indicate a gradual change from cold pdw to warmer pdw between 4 0 and 3 2 ma the pdw is replaced by the warmer poorly oxygenated piw at 2 1 ma although the pdw affects the faunas during colder intervals between 1 4 and 0 7 ma the piw remains the principal bottom water mass in the guatemala basin of the east pacific

precise upb chronometry of 345 340 ma old magmatism related to syn convergence extension in the southern vosges central variscan belt abstract the southern vosges host a large volcano sedimentary basin which contains clastic sediments of famennian late devonian ca 365 ma to late visean ca 340 335 ma age the basin is bordered by an exhumed high grade basement complex and intruded by several granitoid plutons a chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross cutting granitoid rocks by the u pb technique applied to zircon and titanite the basin evolution started with olistolithic deposits and overlying marine shales that have a famennian fauna ca 365 ma and contain detrital zircons with an age of 386 ma the lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 2 ma following a tectonic event at the bottom of the upper visean the rates of sedimentation and volcanism were strongly enhanced leading to basin emersion and were concluded by the extrusion of the uppermost ignimbrite molkenrain rhyolite emplaced at 340 2 ma the whole volcano sedimentary association was subsequently intruded by melts of the ballons and cre tes granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the ballons granite within 3 2 myr 342 1 to 339 5 2 5 ma the zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0 6 and 2 1 ga age the differences in initial nd isotopic composition of the ballons and cre tes granites can be explained by differential uptake of crustal material during magma genesis the precise age data prove that magmatism and basin formation in the southern vosges were extremely short lived and coeval to the exhumation of adjacent high grade gneiss terrains revealing an episode of extension at the end of the lower carboniferous between 345 and 340 ma visean the tectonic regime in the internal part of the variscan orogen is that of extension and strike slip while convergent tectonics were still ongoing in the external part of the variscan orogen

wrench related dome formation and subsequent orogenic syntax bending in a hot orogen variscan ibero armorican arc the ouessant island france