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

deformation of the oceanic crust between the north american and south american plates fracture zone trends and magnetic anomalies in the atlantic ocean indicate that the north american plate must have moved with respect to the south american plate during the opening of the atlantic a comparison of plate tectonic flow lines with fracture zones identified from geosat and seasat altimeter data suggests that the north american south american plate boundary migrated northward from die guinea demarara shear margin to the vema fracture zone before chron 34 84 ma to north of the doldrums fracture zone before chron 22 51 9 ma and to north of the mercurius fracture zone between chron 32 72 5 ma and chron 13 35 5 ma the paleoridge offset through time identified from magnetic anomalies and the computed cumulative strike slip motion in the plate boundary area indicate that the triple junction may have been located between the mercurius and the fifteen twenty fracture zones after 67 ma chron 30 plate reconstructions indicate a late cretaceous phase of transtension followed by transpression in the tertiary for the tiburon barracuda ridge area south of the fifteen twenty fracture zone the ocean floor in this area is characterized by a series of ridges and troughs with large bouguer gravity anomalies up to 135 mgal we use smoothing spline estimation to invert bouguer anomalies for crustal layer structure our model results suggest that the moho is uplifted 2 4 km over short wavelengths 70 km at the barracuda and tiburon ridges and imply large anelastic strains the severely thinned crust at the two ridges implies that crustal extension must have taken place before they were uplifted we propose that the north south american plate boundary migrated to the latitude of the tiburon ridge bounded by the vema and marathon fracture zones before chron 34 84 ma post chron 34 crustal thinning during a transtensional tectonic regime may have been localized at preexisting structural weaknesses such as the vema marathon mercurius and fifteen twenty fracture zone troughs but reaching the fifteen twenty fracture zone and future barracuda ridge area only after chron 32 72 5 ma this interpretation concurs with our crustal structural model which shows stronger crustal thinning underneath the tiburon ridge than at the barracuda ridge subsequent transpression may have continued along the existing zones of weakness in the tertiary creating the presently observed crustal deformation and uplift of the moho accompanied by anelastic failure of the crust middle eocene upper oligocene turbidites on the slope of the tiburon ridge now located 800 m above the abyssal plain suggest that most of its uplift occurred at post oligocene times the unusually shallow moho underneath the tiburon and barracuda ridges represents an unstable density distribution which may indicate that compressive stresses are still present to maintain these anomalies and that the north american south american plate boundary may still be located in this area

a model for the simulation of combined major and trace element liquid lines of descent abstract variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems application of a two lattice melt model greatly reduces compositional dependence for trace elements this allows the temperature dependence of isobaric trace element partitioning to be evaluated independently thus making it possible to model an expanded range of conditions and compositions the first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes by combining trace element partitioning with a major element phase equilibrium model nielsen 1985b the second goal was the application of this model to the simulation of magma chamber recharge assimilation eruption and fractional crystallization these simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system recharge or assimilation will drive a magma composition off the cotectic along which it is evolving however fractionation will drive the liquid back towards the cotectic in a recharged magma chamber as the recharge flux increases the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma the bulk partition coefficient for an open system can be considerably different 5 10 or more for some elements compared to a closed system paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high al basalts and andesites however the addition of an al rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that al in the derived liquids will be effectively buffered or even decrease the assimilation of forsteritic olivine by reaction keleman and ghiorso 1986 increases the proportions of pyroxene and olivine thus reducing the fe enrichment and al depletion characteristic of low pressure basalt fractionation

magnetic field and susceptibility logging in gpf3 borehole at couy france between 0 and 3500 m summary the aim of the couy experiment in the gtologie profonde de la france programme was to reach the source body of the magnetic anomaly of the bassin de pans the drilling stopped at 3500 m magnetic field logging and susceptibility logging were undertaken to obtain as complete a description as possible of the magnetic parameters of rocks crossed by the borehole remanent magnetization and susceptibility measurements on the core were also carried out the entire set of results is presented in this paper for both sedimentary layers and the basement the values obtained for magnetic field variations and for susceptibility lead to the conclusion that the magnetic source body of the ambp was not reached

slow subduction of old lithosphere in the lesser antilles abstract the lesser antilles subduction zone is an extreme case of the subduction of old 90 m y lithosphere at a slow 2 cm y convergence rate focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data during the time period 1950 1978 studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation all three large magnitude seven earthquakes were from intraplate normal faults no large thrust faulting earthquakes and few small ones occurred these observations suggest that the plate boundary is largely decoupled that subduction is at least partially aseismic and that the downgoing slab is in a state of extension

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

in situ magnetostratigraphy interpretation of magnetic logging in sediments abstract the interest of magnetic logging in different geological settings oceanic crust sedimentary basins and deep continental crust is discussed in all these cases a good estimate of both the magnetic susceptibility and the remanence of the surrounding rocks could yield invaluable information about the geological formations penetrated in boreholes a calculation of all the components of the magnetic field produced in a borehole by horizontal magnetized layers is presented the variations of these components are calculated along the axis and off axis through simple geological formations and particularly through a zone of transition of polarity of the earth s magnetic field both susceptibility and remanence are involved in the interpretation of the field variations recorded when a magnetometer is passed through a borehole drilled in sediments the separation of these variations is envisaged using magnetic field and susceptibility logs recorded simultaneously we present an example of magnetic logging in sediments by the measurement of total field intensity the results are compared with a model calculated from laboratory measurements of the natural remanent magnetization and magnetic susceptibility of cores

historical seismicity of the southeastern caribbean and tectonic implications we have relocated the twenty eight largest magnitude 4 3 ms 7 3 historical 1922 1963 earthquakes of the southeastern caribbean we also present new focal mechanisms for seven of these events the relocations are based on reported issp ands arrival times that we analyzed using generalized linear inversion techniques the new focal mechanisms were constrained by first motionp polarities as reported by the iss and as picked by us where records were available and by the polarities and ratios ofsh andssh andsv andssv arrivals that we determined from seismograms the results of the relocations are commensurate with the distribution of seismicity observed in the recent era hypocenters are shallow and intermediate in depth 0 200 km and the events occur almost exclusively in areas known to be currently seismic the frequent seismic activity in the vicinity of the paria peninsula venezuela is clearly a persistent feature of the regional earthquake pattern intermediate depth earthquakes indicative of subduction beneath the caribbean plate occur here and along the lesser antilles arc the grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era trinidad and the eastern gulf of paria were also largely quiescent

modeling magnetostratigraphy in a borehole the magnetic field along a hole bored through a sequence of dipping layers with varying magnetization and planar interfaces is calculated the emphasis is on recovering remanent magnetization polarity transitions for magnetostratigraphic applications dating correlations although intensity of remanent magnetization can in itself be a useful rock property results are presented for various cases of geologic interest in the form of axial vertical profiles and section maps of the holes at varying distances from polarity reversal interfaces the vertical profiles demonstrate a resolving power of about six times the hole radius meaningful magnetostratigraphies can be expected for rocks with a magnetization larger than 10 3am 1 for instruments with a sensitivity of 0 1 nt in a number of natural occurrences it may not be necessary to resort to progressive demagnetization to recover the polarity sequence depending on which magnetic field and magnetization component one looks at the section maps display c

middle jurassic magnetopolarity stratigraphy of a well core from the brent group north sea abstract a magnetopolarity stratigraphy has been constructed by detailed palaeomagnetic investigations of a well core covering the middle jurassic brent group north sea these high time resolution results have provided a chronostratigraphic framework which has been correlated tentatively with the reported magnetopolarity stratigraphy based on a pelagic limestone sequence from caracabuey southern spain biostratigraphic datings suggest the same age span as the magnetopolarity results the sediments which range from mudstone to coarse sandstone have a very low content of ferrimagnetic minerals remanent coercivity analysis suggests that magnetomineralogy is facies and grain size independent and pyrrhotite is the most likely main magnetic remanence carrying mineral palaeomagnetic analysis of the core specimens indicates mostly multicomponent magnetization the characteristic remanent magnetization is inferred to be a combination of post depositional detrital remanent magnetization and a dominant early chemical remanent magnetization

petrology of the tekirova antalya ophiolite southern turkey evidence for diverse magma generations and their tectonic implications during neotethyan subduction the sw antalya complex is an assemblage of mesozoic carbonate platform margin and ophiolitic rocks which record the formation and tectonic emplacement of a small mesozoic ocean basin the late cretaceous ophiolitic rocks are located at two localities namely the relatively intact tekirova ophiolite to the east of kemer zone and the dismembered godene ophiolite to the west of kemer zone the tekirova antalya ophiolite comprises harzburgitic tectonites ultramafic to mafic cumulates isotropic gabbros and sheeted dikes numerous isolated dikes ranging in thickness from 5 cm to 10 m intruded the crustal rocks at different structural levels the isotropic gabbros are represented by gabbro diorite and quartz diorite rocks with granular to ophitic subophitic textures the isolated dikes are characterized by dolerite diabase and microdiorite with ophitic intersertal and microgranular textures these rocks exhibit tholeiitic to alkaline compositions new geochemical data presented in this paper from the isolated dikes and isotropic gabbros suggest that there are three main types of parental basic magmas that form the oceanic crustal rocks of the tekirova antalya ophiolite these are 1 iat series which can be referred to the group i isolated dikes and isotropic gabbros 2 low ti boninitic series characterized by the group ii isolated dike and isotropic gabbros and 3 oib type including the group iii isotropic gabbros the geochemical evidence suggests that the crustal rocks of the tekirova antalya ophiolite were generated from a progressive source depletion from island arc tholeiites iat to boninites therefore a fore arc tectonic setting seems likely for the generation of the crustal rocks from the tekirova antalya ophiolite in the southern branch of neotethys during the late cretaceous the oib type alkaline isotropic gabbros are thought to have resulted from either 1 a late stage magmatic activity fed by melts that originated within an asthenospheric window due to slab break off or 2 subduction of a ridge system which generated oib source across the asthenospheric window that has been no influence of fluids from the subducted slab into the overlying mantle wedge shortly before the emplacement of the tekirova antalya ophiolite onto the tauride platform

the early miocene 25 ma volcanism in the northern kyushu palau ridge enriched mantle source injection during rifting prior to the shikoku backarc basin opening the northern kyushu palau ridge kpr remnant conjugate arc of the izu ogasawara bonin mariana ibm active arc is dominated by basalt andesite except for the komahashi daini seamount where acidic plutonic rocks of 38 ma were recovered these mafic to intermediate volcanics are produced by the rifting volcanism in the proto ibm arc associated with spreading of the shikoku basin the hfse and hree contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front the lile ratios exhibit similar characteristics to reararc volcanism of the recent izu arc and some enriched volcanics exhibit high abundance of sediment melt inputs based on these observations and compilations of the published data set the replacement event of the wedge mantle under the ibm arc occurred two times the first event occurred between 45 and 38 ma with pacific type mantle being replaced by depleted indian type mantle the second event occurred between 36 and 25 ma enriched mantle flowed from reararc side the slab component during the proto ibm arc rifting was a similar characteristic to recent reararc volcanism of the izu arc and sediment melt added in a local area

paleomagnetism of the bonin islands and its tectonic significance abstract paleomagnetic samples were collected from six of the bonin islands located on the northeastern margin 27 n 142 e of the philippine sea these volcanic islands are of eocene age both lava flows and dikes were collected at each of 34 sites with chichi jima the largest island being sampled most extensively mean directions of the 27 sites on chichi jima are divisible into two groups showing antipolar relationships at the 95 confidence level a between site mean direction after tilt correction of 22 of the 27 sites is i 7 d 110 95 14 while that of the remaining sites is i 10 d 273 95 32 the seven sites on the other three bonin islands as a whole seem to cluster well around the mean of i 3 d 213 with a95 19 which differs significantly from both mean directions determined for chichi jima the low inclinations and deflected declinations of the bonin islands are interpreted as being due to tectonic rotation the islands have undergone a northward migration of at least 30 from the equatorial region together with possibly clockwise rotation of 30 to over 90 around the vertical axis since the northward drift and clockwise rotation are consistent with the tectonic movement inferred for the west philippine basin by previous studies the bonin islands appear to have been part of the basin before opening of the shikoku and parece vela basins during the miocene

a cause of rupture segmentation and synchronization in the nankai trough revealed by seismic imaging and numerical simulation 1 a giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes however it is still unknown if a giant earthquake coincidentally occurs or if there is a causal factor to control its generation in this study we show a cause and a growth process of a giant earthquake which may occur along southwestern japan on the basis of seismic images obtained from wide angle seismic data and a numerical simulation incorporating the structural images the wide angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 tonankai moment magnitude mw 8 1 and the 1946 nankai mw 8 4 earthquakes the seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary the numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire nankai trough a distance of over 600 km long mw 8 7 the growth process revealed from the simulated slip history in and around the strongly coupled patch is 1 prior to the giant earthquake a small slow event or earthquake occurs near the segmentation boundary 2 this accelerates a very slow slip slower than the plate convergent rate at the strong patch which reduces a degree of coupling and 3 then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary consequently growing into a giant earthquake

seismic activity offshore martinique and dominica islands central lesser antilles subduction zone from temporary onshore and offshore seismic networks this work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the central lesser antilles island arc from martinique to guadeloupe islands during the whole recording period extending from january to the end of august 2007 more than 1300 local seismic events were detected in this area a subset of 769 earthquakes was located precisely by using hypoellipse we also computed focal mechanisms using p wave polarities of the best azimuthally constrained earthquakes we detected earthquakes beneath the caribbean forearc and in the atlantic oceanic plate as well at depth seismicity delineates the wadati benioff zone down to 170 km depth the main seismic activity is concentrated in the lower crust and in the mantle wedge close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed we propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense thick and rigid crustal block and the lighter and more flexible outer forearc seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

magnetostratigraphy of the middle jurassic brent group in the oseberg oil field northern north sea abstract magnetopolarity stratigraphy of the brent group and the uppermost part of the dunlin group has been established in two well cores from the oseberg field well 30 6 7 and well 30 9 2 northern north sea palaeomagnetic analysis of thermal demagnetization results defines a series of normal and reverse magnetozones in both wells remanent coercivity analysis of these middle jurassic clastic sediments indicates a magnetomineralogy independent of grain size interpreted to be of diagenetic origin thus suggesting that the magnetization is of early post depositional origin integration with the existing palynostratigraphic framework results in the establishment of several isochronous horizons especially in the ness formation which suggests lateral continuity of magnetostratigraphic datum planes between the two wells at the oseberg field although the exact dating of the magnetozones is not established the combined application of magnetopolarity and biostratigraphy provides a stratigraphic framework of improved resolution for these rock formations

k ar dating on volcanic rocks in the bonin islands and its tectonic implication abstract the k ar dating method was applied to the volcanic rocks the boninite and related rocks from the bonin islands four fresh samples gave k ar ages of about 40 ma these ages suggest that the main volcanism of the bonin islands occurred about 40 ma ago as a result of the initiation of subduction of the pacific plate at its western margin results of the k ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 10 ma ago at least in chichijima

the effects of weathering on rare earth element y and ba abundances in tertiary basalts from southeastern australia tholeiitic basalts showing anomalously high ba y and rare earth element ree concentrations occur within the tertiary newer volcanic province of victoria and these geochemical characteristics are related to incipient alteration accompanying weathering ree and y released during weathering of basalts are retained either in secondary phosphate or clay minerals and in rocks showing very high ba concentrations the ba is present as baso4 leaching experiments suggest that glassy or fine grained rocks are more likely to release ree y and ba while more extensively altered rocks are likely to hold these elements probably because they substitute in or are adsorbed on clay minerals or because secondary baso4 and ree phosphates are resistant to acid leaching samples taken sequentially across a weathering profile in a tholeiitic basalt flow from fresh to intensively altered rock illustrate that in the very earliest stages of alteration extreme variations in the abundances of ba y and ree can occur this is reflected in the broad patterns of trace element behaviour manifested by an extensive analysed sample suite from the victorian newer volcanic province where there is no correlation between anomalous enrichments in ba y or ree and the level of incipient alteration ba anomalies are not necessarily accompanied by abnormal y and ree behaviour and this is probably because the mobility of each trace element is a function of a the ionic radius and charge of the ionic species it forms in solution b local groundwater parameters such as eh ph groundwater flux and concentrations in the groundwater of species such as so42 and c the secondary mineral assemblages developed in a particular situation sr isotopic compositions are not strongly affected by incipient weathering and do not show significant changes until alteration has progressed to the point where plagioclase begins to break down the study illustrates that relatively slight alteration at the earliest stages of weathering can produce drastic changes in the concentrations of ree y and ba and petrographic assessment of the freshness of samples is not precise enough to eliminate prior to analysis cases where these effects may be manifested where weathering has caused changes in abundance patterns these may be recognised by careful comparisons of appropriate elemental ratios e g ba rb throughout a particular analysed sample suite

microseismicity and stresses in the lesser antilles dipping seismic zone abstract microearthquake data from a telemetered array have been used to study the central antilles arc system the zone lying between lat 14 17 n and long 59 5 62 w has been investigated during the period 1979 1980 no shock with a magnitude larger than 4 5 was recorded results are well correlated with previous conclusions derived from teleseismic records the dip of the subducting plate is 60 everywhere in this area no composite focal mechanism can be produced for the region north of dominique for the southern region composite focal mechanisms reveal two zones for depths between 45 and 80 km the stress field seems mainly related to the interplate action beneath 80 km the slab dominantly neither in compression nor in tension earthquakes are more likely related to local stress fields a double layered structure has not been found in the earthquake distribution in the subducted plate nor in the stress field deduced from focal mechanisms

relative motion between the caribbean and north american plates and related boundary zone deformation from a decade of gps observations global positioning system gps measurements in 1986 1994 and 1995 at sites in dominican republic puerto rico cuba and grand turk define the velocity of the caribbean plate relative to north america the data show eastward motion of the caribbean plate at a rate of 21 1 mm yr 1 standard error in the vicinity of southern dominican republic a factor of 2 higher than the nuvel 1a plate motion model prediction of 11 3 mm yr independent measurements on san andres island and an euler vector derived from these data also suggest a rate that is much higher than the nuvel 1a model available data combined with simple elastic strain models give the following slip rate estimates for major left lateral faults in hispaniola 1 the north hispaniola fault offshore the north coast of hispaniola 4 3 mm yr 2 the septentrional fault in northern dominican republic 8 3 mm yr and 3 the enriquillo fault in southern dominican republic and haiti 8 4 mm yr the relatively high plate motion rate and fault slip rates suggested by our study combined with evidence for strain accumulation and historical seismicity imply that seismic risk in the region may be higher than previous estimates based on low plate rate low fault slip rate models and the relatively low rate of seismicity over the last century

can high alumina arc basalt be derived from low alumina arc basalt evidence from kanaga island aleutian arc alaska the coexistence of both high mgo low alumina basalt lab and low mgo high alumina basalt hab magmatic inclusions within a single andesitic host lava on kanaga island provides the opportunity to rigorously evaluate the presence or absence of a fractionation relation between the two basalt magma types traditional mass balance calculations pearce ratio arguments comparative phenocryst modes and compositions and incompatible element rb ba zr abundances all indicate that selective clinopyroxene and orthopyroxene fractionation from the plagioclase clinopyroxene orthopyroxene titanomagnetite saturated lab magma yielded the plagioclase phyric hab magma the same data are consistent with a model of low pressure hab generation calling upon selective mafic crystal fractionation along the margins of a crystallizing and convecting lab magma chamber the ability to demonstrate that nonliquid hab magma can be produced from lab through crystal fractionation eliminates many of the arguments used in support of a primary eclogite melting origin for hab

segmentation of the lesser antilles subduction zone abstract precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the benioff zone beneath the lesser antilles island arc there are two distinct segments to the zone one to the north of martinique which trends nnw and one to the south of st lucia which trends nne the northern zone dips at 60 50 the southern zone has a dip of 50 45 in the north but is vertical in the south pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the benioff zone mid miocene swarms of dykes in martinique and st lucia have nw and ne azimuthal modes respectively we interpret this to mean that this kink in the subduction zone existed as long ago as the miocene the kink may represent the deformation of a single subducting plate or two separate north and south american plates subducting beneath the caribbean plate with a triple junction in the region of martinique st lucia

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

control of forearc density structure on megathrust shear strength along the chilean subduction zone abstract unveiling how the interaction of friction pore pressure p and normal stress n controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains an existing density model of the chilean margin 18 45 s allows the computation of the so called vertical stress anomaly vsa that accounts for the component of normal stress due to the weight of the overlying crustal column a spatial correlation between vsa surface geology and several earthquake parameters suggests a significant control exerted by the geologically dominated density structure of the crust on megathrust shear strength the very seismogenic forearc in front of the high central andes 18 34 s has positive vsa values up to 60 mpa and a mafic dominated crust which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column in addition to a likely dry subduction channel characterized by high and low p in contrast the southern andean forearc 34 45 s shows neutral to negative vsa down to 30 mpa correlated with a felsic dominated crust which added to a presumably wet subduction channel low and high p configures a rather weak megathrust below a light forearc earthquakes here are concentrated around the arauco peninsula a first order morpho structural anomaly characterized by comparatively higher values of vsa than the rest of this segment which is the nest of the giant mw 9 5 valdivia 1960 earthquake and the southern rupture termination of the recent mw 8 8 maule earthquake the ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative vsa values the correlation between vsa and the along strike segmentation of the andean orogen indicates that the geologically inherited density structure of the forearc also rules long term mechanical coupling between converging plates and prescribes the possible structure of the orogen because central and southern andes represent two extremes in terms of orogenic structure and seismogenic behavior results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins

a trapped philippine sea plate origin for morb from the inner slope of the izu bonin trench basement outcrops sampled by submersible and dredge from the inner slope of the izu bonin trench at 32 n and 6200 6700 m water depth have a distinct mid ocean ridge basalt morb chemistry unlike any other rocks previously sampled in the izu bonin arc they are low k tholeiites with moderate tio2 0 7 1 8 wt extremely low ba 1 5 7 parts per million low ba la 1 2 3 and are depleted in light rare earth elements these samples could represent either an accreted piece of subducting pacific plate or a trapped remnant of philippine sea plate on which the izu bonin arc was built although their major and trace element chemistry do not help to distinguish their source the sr nd and pb isotopes clearly support a philippine sea plate origin the isotopic signature of the inner trench slope samples matches that of philippine sea plate lavas with 87 sr 86 sr 0 70321 0 70373 143 nd 144 nd 0 513057 0 513077 and 206 pb 204 pb 18 2 18 5 the samples have elevated 207 pb 204 pb 15 3 15 5 and 208 pb 204 pb 38 0 38 2 values compared to the northern hemisphere reference line nhrl and their isotopic signature is distinct from the mesozoic pacific morb being subducted these are the first samples of trapped philippine sea oceanic crust discovered in the izu bonin mariana arc they require that models for the formation of intra oceanic arc crust account for pre existing oceanic crust and that estimates of arc magma production rates are lowered accordingly fl 1999 elsevier science b v all rights reserved

evolution from fore arc oceanic crust to island arc crust a seismic study along the izu bonin fore arc 1 petrological studies have suggested that oceanic crust is formed during the initial stage of subduction however there is little geophysical evidence for the formation of oceanic crust in the fore arc region we conducted an active source seismic survey in the fore arc region of the izu bonin intraoceanic arc to examine processes of crustal formation associated with the initiation of subduction we used seismic refraction tomography to obtain a detailed seismic velocity model and diffraction stack migration of picked reflection travel times to derive reflectivity images these data show a remarkably thin crust 10 km thick along the northern half of the bonin ridge and abrupt crustal thickening to 20 km toward the south of the ridge comparison of velocity depth profiles of the thin fore arc crust of the bonin ridge with those of typical oceanic crusts showed them to be seismologically identical boninitic magmatism is evident in the area of thin crust and tholeiitic calcalkaline andesitic volcanism in the area of thick crust high precision dating of the volcanic rocks showed that the thin fore arc oceanic crust was created soon after initiation of subduction 48 45 ma and that the nonoceanic thick crust was created with tholeiitic calcalkaline andesitic magmatism after the boninitic magmatism was ceased our seismological image strongly supports the view that fore arc oceanic crust was formed by fore arc spreading during the initial stage of subduction along the izu bonin intraoceanic arc

deep structure of an island arc backstop lesser antilles subduction zone 1 we present the results from a coincident seismic reflection refraction grid conducted at the lesser antilles subduction zone near 16 n this paper focuses on the seismic refraction data and constraints these data place on the three dimensional structure of the island arc backstop we find that the backstop in this region contains considerable topography in both the strike and dip directions two ridges each 25 35 km in length and 10 km in width rise 1 6 km above the adjacent basement the eastern edge of one of the ridges deepens by 4 6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop in contrast to the complex nature of the backstop the adjacent accretionary wedge displays little lateral variability at large scales this may be a consequence of the spatial scales involved the backstop topography is 10 35 km in width while the accretionary wedge extends 125 km from the deformation front to the backstop the top of the subducting oceanic crust as identified by an increase in velocities to 6 6 5 km s intersects the backstop at a depth of 14 15 km the updip limit of plate boundary seismicity is located 75 100 km west and downdip of the backstop however two earthquake clusters are observed at the intersection of the subducted barracuda ridge and tiburon ridge with the backstop suggesting active deformation associated with the backstop edge at these locations

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

petrology and geochemistry of boninite series volcanic rocks from the mariana trench boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the mariana trench these hauls included olivine boninites boninites boninitic andesites and boninitic dacites as well as island arc tholeiitic basalts and andesites the boninite series volcanics range from 52 to 68 sio2 and are characterized by very low abundances of high field strength cations and heavy rare earth elements boninites and olivine boninites have phenocrysts of olivine and orthopyroxene the andesites phenocrysts of orthopyroxene and clinopyroxene and the dacites orthopyroxene clinopyroxene and plagioclase most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine orthopyroxene clinopyroxene and plagioclase in the proportions 2 5 4 1 2 leaving 47 residual liquid the fractionation must be in part open system reverse zoned phenocrysts resorbed olivine and plagioclase xenocrysts and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation two boninitic magma stems can be identified with similar high field strength element abundances but different amounts of ca na al and light rare earth elements there is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites the compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid the mariana forearc boninite series lacks a strong iron enrichment but produces andesites with lower ti al and y zr and higher mg ni and cr than typical calcalkaline arc andesites and dacites boninites in the mariana system were erupted only in the earliest phases of subduction zone activity

light rare earth enrichment of supra subduction zone mantle evidence from the troodos ophiolite cyprus geochemical data including rare earth elements ree are presented for the primitive high mgo glassy lavas from the upper sections of the troodos ophiolite al2o3 tio2 ratios are used as an index of degrees of mantle melting and or successive depletion events the sources of the lavas have undergone both a ubiquitous introduction of low field strength elements and a selective addition of light ree suggesting a two stage mantle contamination model correlations between al2o3 tio2 ratios depletion index and the amount of light ree enrichment present in the lavas suggest a component induced melting model in this model progressive addition of a light ree component induced higher degree batch melts or enabled melting of highly refractory source material the light ree enrichment event is shown to be of the same character across the ophiolite but it is limited to the older ocean crust and a transform influenced regime

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

continuous record of geomagnetic field intensity between 4 7 and 2 7 ma from downhole measurements abstract a continuous record of the geomagnetic field intensity from 4 7 to 2 7 ma has been obtained together with a precise magnetostratigraphy from downhole magnetic measurements at site 884 of odp leg 145 in the north pacific the record confirms the saw tooth pattern of geomagnetic field intensity proposed by valet et meynadier 10 reversals are characterized by a steep intensity decrease followed by a quick regeneration over each polarity interval rapid variations are superimposed over a progressive decrease in the mean intensity we find that the duration of each polarity interval is inversely proportional to the mean rate of decrease in the field intensity over this period and thus this duration seems to be pre determined

direct determination of the natural remanent magnetization effect in a hole drilled in layered ground from magnetic field and susceptibility logs a new method as presented allows the joint interpretation of both electromagnetic em and magnetic logs in layered ground based on the fact that the susceptibility responses for these two measurements are linear thus we can make use of the classical properties of the fourier transform to extract from these two signals the magnetic field due to remanent magnetization theoretical models show that for a sufficient sample step this remanent magnetization can be recovered even if the koenigsberger ratio is of the order of 0 2 and if the thickness of the magnetized layer is of the order of 1 m the results for two case examples in a sedimentary context are also shown despite the difficulties due to experimental procedures we show that the amplitude of the extracted information is significant in these two cases even if its variations are somewhat structureless and cannot be easily explained by the geology

supraslab earthquake clusters above the subduction plate boundary offshore sanriku northeastern japan seismogenesis in a graveyard of detached seamounts 1 thousands of offshore repeating earthquakes with low angle thrust focal mechanisms occur along the subduction plate boundary of ne japan double difference relocation methods using p and s wave arrivals reveal clusters of events above these repeating events to assure good depth control we restrict our study to events that are close to seismic stations these supraslab earthquake clusters are regional features at depths of 25 to 50 km and most of these clusters are below the depth of the forearc moho which we determined from converted waves seismicity over this depth range does not occur under the inland area of ne japan except just below the vicinity of the arc volcanoes re entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the pacific plate during slab descent driven by the resistance of seamounts to subduction detachment during slab descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth high fluid pressure produced during dehydration of clay minerals and other low temperature hydrous minerals could enable detachment at depths seamount crust is thus accreted to forearcs possibly leading to a long term component of near coastal uplift supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system

borehole magnetic surveys in weakly magnetic sediments chicxulub impact crater versus strongly magnetic volcanics bathurst mining camp borehole navigation surveys performed using a triaxial fluxgate magnetometer record the change in orientation of the magnetic vector versus depth variations in the orientation of the magnetic vect

borehole magnetostratigraphy absolute age dating and correlation of sedimentary rocks with examples from the paris basin france total and cea have conceived and developed two new logging tools which allow continuous precise and accurate in situ magnetic measurements within weakly magnetized sedimentary rocks encountered in uncased industry drilled wells the combination of these magnetic measurements permits for the first time continuous determination of natural remanent magnetization nrm polarities occurring in the logged section the resulting well magnetostratigraphic sequence wms which may be recognized from well to well magnetostratigraphic correlations may also be correlative to a geomagnetic polarity time scale gpts or reference magneto stratigraphic scale rms given adequate dating tie points and lack of noticeable remagnetizations application of this method to the upper jurassic formation in four wells of the paris basin has yielded promising results for both stratigraphic and near absolute age correlations as well as determination of facies diachronisms the comparison of results obtained from magnetic measurements and from sequence stratigraphy analysis shows that third order sequence boundaries may be correlative to geomagnetic reversals but may also be diachronous in wells 80 km or less apart the potential of borehole magnetic measurements as presented in this paper is still under investigation however these encouraging initial results show that this new investigating and dating method could be of great help in the analysis of subsurface stratigraphy by extending the concepts of magnetostratigraphy from the outcrops and laboratory directly into borehole logging

magnetostratigraphy from downhole measurements in odp holes abstract the geological high resolution magnetic tool ghmt was used on 17 odp legs to obtain downhole logs of magnetic field strength and magnetic susceptibility the remanent magnetization of the sediment around the borehole is derived from these measurements and used to produce magnetostratigraphies this review of the odp ghmt data indicates that reliable magnetostratigraphies can usually be obtained from reasonably well magnetized above approximately 10 3 a m fine grained sediments additionally the ghmt data indicates that a drill string overprint common in data collected on cores does not significantly affect the sediments surrounding the borehole this allows us to use the remanent and induced magnetization data to obtain estimates of relative paleointensity and when averaged over time in homogenous sediment the paleomagnetic recording efficiency of that sediment

geology and volcanic stratigraphy of the canatuan and malusok volcanogenic massive sulfide deposits southwestern mindanao philippines the canatuan and malusok massive sulfide deposits are located near siocon zamboanga del norte in southwestern mindanao philippines the canatuan malusok area is underlain by the jurassic cretaceous tungauan schists which form much of the zamboanga peninsula the volcanic strata at canatuan and malusok can be traced for 7 km along strike and is host to at least three discrete massive sulfide bodies canatuan malusok and se malusok basal basaltic andesite volcanic rocks are generally chemically uniform and show only moderate alteration the massive sulfide deposits occur in overlying rhyolitic to rhyodacitic volcanic rocks that are altered to a schistose assemblage of quartz sericite chlorite and pyrite the alteration is texturally destructive but graded clastic beds are locally observed despite tropical saprolitic weathering four lithogeochemical subunits of the felsic package are identified stratigraphic interleaving however has made correlation of these units over any significant distance difficult the sulfide lenses are overlain by a few metres of felsic schists which locally contain manganese bearing silicates and oxides that serve as a stratigraphic marker hangingwall andesitic volcaniclastic rocks are discontinuously preserved although where present they consist of regularly bedded mafic volcanic sandstones the lateral continuity of a manganese bearing marker and flanking felsic volcaniclastic intervals indicate that locally the volcanic strata form a homoclinal sequence the canatuan au ag cu zn deposit consists of a gossan overlying a massive sulfide lens the sulfides and gossan are flat lying and hosted within felsic volcanic rocks the gossan is gold silver rich and was formed by a combination of oxidation and volume collapse of the original sulfide lens the sulfide minerals present below the current water table are auriferous massive pyrite with base metal sulfides with some supergene chalcocite the transition from gossan to sulfides is very sharp occurring at the water table massive sulfide deposits at malusok are hosted in the same felsic sequence as canatuan and they have similar base and precious metal contents only limited gossan has been found at malusok the bimodal nature of the volcanic rocks at canatuan together with their low hfse contents near flat ree patterns and tholeiitic affinities suggest that they formed in an intra oceanic arc setting above a depleted mantle source mafic and felsic volcanic rocks of similar composition have been recovered from the tonga kermadec and izu bonin marianas island arc systems in the western pacific mafic rocks at canatuan show no evidence for lile enrichment that characterizes melts derived from metasomatized mantle under more mature arcs suggesting that they are the product of a nascent rather than a mature arc there is no evidence from the ree or other incompatible trace elements that continental crust or evolved arc crust was involved in the generation of the canatuan malusok volcanic rocks although it has been proposed that the zamboanga metamorphic complex comprises microcontinental fragments of eurasian affinity our data do not support an evolved crustal setting for the canatuan malusok volcanic rocks which we suggest were derived from an intra oceanic arc and subsequently accreted to the eastern mindanao terrane

isotopic characteristics of subduction fluids in an intra oceanic setting izu bonin arc japan new radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active izu bonin arc the oligocene izu arc and their associated rift basins as with many intra oceanic island arcs the pliocene recent izu bonin frontal arc lavas are highly depleted in zr nb and the rare earth elements relative to typical mid ocean ridge basalt morb indicating that the mantle wedge source has undergone a previous episode of melting ratios between these elements such as nb zr and la sm as well as 143nd 144nd do not vary significantly along the length of the frontal arc these parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge however ba zr ba rb and 87sr 86sr increase along the frontal arc to the north this leads us to propose that a variable enrichment in ba and radiogenic sr is superimposed on the mantle wedge sr nd and pb nd isotope variation indicate that both sr and pb become more radiogenic after fluid addition however pb isotope ratios do not correlate with increases in pb concentration or ratios such as ba zr and nb pb in other words the pb isotopic composition of the arc lavas appears to be independent of the amount of pb introduced by subduction fluids into the mantle source this buffering of pb isotopes along the frontal arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities despite this we are able to determine that the mantle wedge has isotopic characteristics similar to indian ocean morb and that the subduction fluid solute is primarily derived from subducted oceanic basalt with a 2 contribution from subducted sediment lavas in the oligocene izu arc and fore arc basin were derived from a mantle wedge of similar composition to the active arc despite levels of pb enrichment comparable to those of the modern arc the pb isotopes of the oligocene volcanics indicate a lower sediment input into the melting region

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

rare earth mobility in young arc type volcanic rocks from northern new zealand abstract samples of late cenozoic arc type volcanic rocks from the hauraki volcanic region in northern new zealand show evidence for enrichment of the rare earth elements rees these samples have variable to high la and y abundances together with ree abundance patterns which are up to an order of magnitude higher than those considered typical of the suite as a whole scanning electron microscope imaging and analysis shows the presence of micron scale veins and patches containing ree bearing phases these observations are explained as the result of cryptic alteration processes which enrich the rees but not other chemical components and which are probably the result of low temperature

downhole magnetostratigraphy in sediments comparison with the paleomagnetism of a core the magnetostratigraphy of weakly magnetized sediments is determined for the first time from direct downhole measurements experimental results were obtained in both scientific and oil exploration drillholes penetrating jurassic sediments of the paris basin france the magnetic polarity of the remanence is deduced from the total magnetic induction combined with the magnetic susceptibility the magnetic induction is measured in depth by a proton precession magnetometer with overhauser effect designed by the laboratoire d electonique de technologie et instrumentation commissariat a i energie atomique this method gives a continuous record with an accuracy of 0 1 nt and an excellent stability in order to cancel out the transient component of the earth s magnetic field the magnetic induction is computed as the difference between the induction measured in the borehole and that obtained at a surface reference point located near the drillhole the dipole dipole susceptibility probe gives a continuous record with an accuracy of a few 10 6 the four total magnetic induction logs recorded in oil exploration holes show a tight correlation although distant from up to 80 km this technique is therefore excellent for precise stratigraphic correlation data processing allows us to isolate the component of induction due to remanent magnetization and to find the magnetic polarity a sequence of magnetic polarity is proposed for kimmeridgian and oxfordian stages the variation of the sedimentation rate is analyzed with a very high resolution in time and compared with the mean sedimentation rate deduced from a lithologic log cores were collected in the lowermost part of two holes the correlation between the magnetostratigraphy deduced from paleomagnetic measurements performed on 240 standard plugs and that obtained by logging is good a study of the components of magnetization found in the core has shown that the remagnetization occurred mainly during the coring operation itself or during the storage of the core consequently downhole magnetostratigraphy proves to be reliable besides providing the advantage of a continuous investigation performed over a large volume of rock scrutinized in situ

supra subduction zone ophiolites from inner mongolia north china implications for the tectonic history of the southeastern central asian orogenic belt abstract the recently discovered diyanmiao ophiolite in the eastern part of the erenhot hegenshan ophiolite belt forms part of the southeastern domain of the central asian orogenic belt caob the ophiolite suite is composed of the northern baiyinbulage ophiolite and the southern naolaiketu ophiolite each extending for nearly 100 km and a width of around 6 km the suite preserves a well exposed and a nearly complete ophiolite stratigraphy and includes from bottom to top harzburgite bedded gabbro isotropic gabbros anorthosite spilite pillow basalt keratophyre baschtauite with overlying chert and plagiogranite veins locally intruding the pillow basalt and gabbro the pillow basalts possess the geochemical characteristics of hahajima island arc tholeiite iat indicating that the diyanmiao ophiolite formed in the incipient arc setting of the supra subduction zone la icp ms u pb dating of zircon grains from three gabbro samples yield ages of 361 13 ma 356 7 3 8 ma and 340 14 ma indicating an early carboniferous age for the diyanmiao ophiolite we suggest that the intra oceanic subduction initiation of the erenhot hegenshan paleo ocean during the early carboniferous resulted in the formation of an incipient arc oceanic crust and that the diyanmiao ophiolite represents an important north dipping subduction event of the paleo asian oceanic crust beneath the southern margin of the siberian plate after early carboniferous and early permian our study provides further insights into the evolution of the paleo asian ocean and eventual construction of caob the globe s largest phanerozoic orogen

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern

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

arc volcanism in an extensional regime at the initiation of subduction a geochemical study of hahajima bonin islands japan hahajima represents an eocene forearc volcanic centre produced during the initiation of subduction within an intra oceanic domain of the western pacific the eocene oligocene izu bonin arc forearc is envisaged as a broad region of volcanism generated in an extensional tectonic regime the forearc is likely to have consisted of a series of small rift basins punctuated and bordered by coherent volcanic edifices hahajima represents one such edifice at the southern termination of the chichijima mukojima rift basin major element trace element and isotopic data are presented for representative volcanic rocks from hahajima the volcanic sequence comprises porphyritic basalt and andesite breccias and flows intercalated with tuffaceous sandstones all lavas contain high proportions of plagioclase phenocrysts 20 36 and comparatively low mafic mineral contents the andesite suite is typified by the presence of magnetite and orthopyroxene phenocrysts and pseudomorphs after olivine basalts are dominated by an assemblage of plagioclase clinopyroxene and olivine nodules and xenocrysts typical of the andesites are found in the basalts and vice versa suggesting an intimate relationship between the two magmas during volcanism the basaltic suite follows a tholeiitic fractionation path iron enrichment while the andesites are more representative of calc alkaline magma evolution these trends are found to be clearer when corrections are made for the excess proportions of plagioclase in the whole rock trace element differences between the suites can not be reconciled by a straightforward crystallization model but can be related to a similar parental magma if the shallow level magma evolution involves a combination of open and closed system fractionation processes comparisons of volcanic suites within and between each site sampled along the izu bonin forearc indicate that each eruptive centre has an individual trace element signature this suggests that variations within each site can be explained by melting and or shallow level fractionation processes while differences between localities are related to heterogeneities in the composition trace element and isotopic enrichment and thermal characteristics of the source the hahajima volcanic rocks contrast with the more recent arc magmas of the izu bonin system in that they share some features such as high mg and elevated zr y with the contemporaneous boninitic lavas