lidar observations of polar stratospheric clouds at the south pole 1 stratospheric unperturbed conditions 1990 observations of polar stratospheric clouds pscs carried out at the amundsen scott south pole station by lidar in the winter of 1990 are reported echoes attributable to pscs began to appear in late may were a persistent feature till early september and from that time until the end of october were sporadically observed values of the backscatter ratio as high as 16 were recorded in late august at 13 km analyses based on the sensitivity of the backscattering to temperature confirm that the attribution of the echoes to psc type i or ii is possible in some cases those that could be ascribed to type ii were observed mostly during july and august while those attributable to type i appeared only at the beginning of the winter the comparison of water vapor mixing ratios derived from psc type ii occurrence temperatures and frost point hygrometer measurements indicates a fast dehydration in the beginning of the winter

volcanic markers of the post subduction evolution of baja california and sonora mexico slab tearing versus lithospheric rupture of the gulf of california the study of the geochemical compositions and k ar or ar ar ages of ca 350 neogene and quaternary lavas from baja california the gulf of california and sonora allows us to discuss the nature of their mantle or crustal sources the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement nine petrographic geochemical groups are distinguished regular calc alkaline lavas adakites magnesian andesites and related basalts and basaltic andesites niobium enriched basalts alkali basalts and trachybasalts oceanic morb type basalts tholeiitic transitional basalts and basaltic andesites peralkaline rhyolites comendites and icelandites we show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting three successive stages are distinguished between 23 and 13 ma calc alkaline lavas linked to the subduction of the pacific farallon plate formed the comondu and central coast of the sonora volcanic arc in the extensional domain of western sonora lithospheric mantle derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the basin and range province the end of the farallon subduction was marked by the emplacement of much more complex middle to late miocene volcanic associations between 13 and 7 ma calc alkaline activity became sporadic and was replaced by unusual post subduction magma types including adakites niobium enriched basalts magnesian andesites comendites and icelandites the spatial and temporal distribution of these lavas is consistent with the development of a slab tear evolving into a 200 km wide slab window sub parallel to the trench and extending from the pacific coast of baja california to coastal sonora tholeiitic transitional and alkali basalts of subslab origin ascended through this window and adakites derived from the partial melting of its upper lip relatively close to the trench calc alkaline lavas magnesian andesites and niobium enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot pacific asthenosphere through the window during the plio quaternary the no slab regime following the sinking of the old part of the farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic transitional basalts of deep asthenospheric origin in baja california and sonora the lithospheric rupture connected with the opening of the gulf of california generated a high thermal regime associated to asthenospheric uprise and emplaced quaternary depleted morb type tholeiites this thermal regime also induced partial melting of the thinned lithospheric mantle of the gulf area generating calc alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

the age petrogeneis and tectonic significance of the frontenac formation basalts northern new hampshire and western maine within plate basalts are present across the connecticut valley gaspe trough in new england and canada proposed mechanisms for the extensional magmatism include back arc basin extension slab breakoff and extensional collapse the age and composition of the bimodal frontenac formation of northern new england and southern quebec has bearing on the topic we analyzed zircons from the meta felsic rocks of the formation and determined a silurian age of 432 8 ma we also analyzed the frontenac rocks for major and trace elements and compared their compositions to the other mafic rocks along strike in the connecticut valley gaspe trough these include the comerford intrusive suite and the standing pond volcanic member of the waits river formation of vermont the silurian chaleur group and the early devonian dalhousie group of new brunswick and the gaspe peninsula and to the piscataquis belt rocks of northern maine all these rocks are post taconic within plate tholeiites that span an age range from 432 ma frontenac formation to 419 ma comerford intrusive suite in spite of their similar tectonic setting subtle differences in trace element compositions are evident indicating a multiplicity of sources in the production of the siluro devonian magmas ce yb versus la ta values suggest that the frontenac magmas were derived from spinel peridotite sources in the asthenospheric mantle we suggest that slab rollback and back arc basin extension permitted asthenospheric mantle upwelling to the spinel peridotite stability field causing the frontenac partial melting event at depths as low as 35 km sediments of the frontenac formation were deposited as turbidite successions into a developing trough resulting from extension the coeval influx of sediment and intrusion of basalt produced a sediment sill complex because the upward migration of the basaltic magmas was hindered by the rapidly thickening sediment overburden subsequently slab breakoff may have opened an asthenospheric window generating the younger within plate magmas that occurred from 425 to 418 ma across the connecticut valley gaspe trough

arc rift transition volcanism in the puertecitos volcanic province northeastern baja california mexico the neogene puertecitos volcanic province of northeastern baja california records a transition from arc related volcanic activity to rift volcanism associated with opening of the gulf of california the eastern puertecitos volcanic province is divided into three volcanic sequences based on mapping petrology and 40 ar 39 ar geochronology the lowest sequence comprises early to middle miocene 20 16 ma arc related andesitic lava flows volcanic necks and proximal pyroclastic and epiclastic deposits up to 400 m in thickness with minor basaltic lava flows following the initiation of crustal extension in the region 11 6 ma synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc related rocks the older synrift sequence 6 4 5 8 ma is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick the upper sequence 3 2 2 7 ma consists of ash flow tuffs and pumice lapilli pyroclastic flows collectively up to 200 m thick minor andesite eruptions followed each episode of silicic synrift volcanism synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection rhyolite domes are aligned parallel to the predominant north northwest to north northeast fault pattern all three volcanic sequences are calc alkaline however the synrift andesite is characterized by lower k 2 o lower incompatible element concentrations and less fractionation of light rare earth elements than the arc related basalt and andesite this suggests that the primary melts were more primitive for synrift andesite than for the arc related rocks

the petrology and geochemistry of cretaceous to recent volcanism in colombia the magmatic history of an accretionary plate margin the pacific margin of colombia records a complex history of subduction related magmatism from at least cretaceous times cretaceous lavas from the western cordillera and part of the central cordillera of the colombian andes exhibit some chemical characteristics typical of island arc tholeiites including low lil element abundances and flat to light depleted ree patterns but also have chemical and geological affinities with deposits from marginal basins and with transitional type ocean floor basalts an appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent a suite of basic rocks from the coastal cordillera of upper cretaceous to tertiary age has been interpreted elsewhere as abyssal tholeiites following accretion of the cretaceous sequences on to cratonic s america volcanism continued in the upper miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the western cordillera this tholeiitic suite is lree and lil element enriched compared with the cretaceous lavas and represents the trenchward portion of an ensialic island arc sequence subduction related magmatism continued from the miocene with the formation of composite calc alkaline strato volcanoes mostly along the axis of the central cordillera these miocene to recent lavas are dominantly andesitic with strong lil and lree enrichment and 87 sr 86 sr ratios of 0 7042 0 7047

the composition and emplacement of basaltic magmas produced during the development of continental margin basins the gulf of california mexico the gulf of california is a region of active ensialic basin development and transform faulting legs 64 and 65 of the deep sea drilling project have demonstrated the diversity of basement stratigraphy drilling at sites 474 482 483 and 485 at the mouth of the gulf recovered massive basaltic flows and occasional pillow lavas and sills whereas drilling in the central part of the gulf in the guaymas basin recovered a sequence of sediments and massive doleritic sills very high sedimentation rates over 1000 m ma are characteristic of the guaymas basin and prevent extrusion of the basaltic magma onto the seafloor thus resulting in at least partial intercalation of oceanic layers 1 and 2 such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development when sedimentation rates may be very high over 200 samples of basalts and dolerites from the gulf of california have been analysed by x ray fluorescence and neutron activation techniques basalts from the mouth of the gulf are tholeiitic and contain very low abundances of the more hygromagmatophile more hyg elements k rb ba th ta nb and light ree relative to the hyg elements y zr ti and heavy ree thus resembling depleted n type morb from elsewhere along the e pacific rise epr dolerites from the guaymas basin also have a tholeiitic chemistry and some epr basalt characteristics e g la n sm n

queen charlotte area cenozoic tectonics and volcanism and their association with relative plate motions along the northeastern pacific margin cenozoic relative plate interactions along the queen charlotte margin based on published models for the north america and northeast pacific plates are related to the tectonic and igneous history of the region plate motion models indicate convergence and subduction prior to the eocene since that time there has been transcurrent motion with varying small amounts of oblique extension or compression assuming that the pacific america farallon triple junction was to the south of the region a late miocene or early pliocene about 4 ma to present period of oblique convergence is well resolved the present queen charlotte fault zone along the west coast of the islands may have initiated more recently within the cenozoic transcurrent regime a period of oblique extension in the mid tertiary 36 20 ma in one model is supported by a variety of volcanic and tectonic evidence a small plate motion change at about 20 ma may have resulted in the transfer of the yakutat terrane from the north america plate to the pacific plate and the resulting motion of the terrane northwestward to its present position along the margin of alaska the onset of tertiary masset volcanism that is extensive on the queen charlotte islands corresponds within a few million years to the time of major plate reorganization at 43 ma that has tectonic expression around the entire pacific basin the period of most extensive masset volcanism and plutonism appears to correlate with the model oblique extension in the mid tertiary for one model the geochemistry and physical volcanology of the masset volcanic s are indicative of an extension regime the main syntectonic deposition in the queen charlotte basin skonun formation as inferred from seismic reflection and well data also appears temporally correlated with this time interval of oblique extension a variety of data including plate models dikes normal faults basin subsidence crustal thickness from seismic refraction and present and paleo heat flow indicate mid tertiary crustal extension of at least 20 in the queen charlotte region with up to 150 up to 2 5 in the main queen charlotte basin posttectonic basin subsidence and deposition may correlate with the model time interval of general transcurrent or oblique convergence motion from 20 to 4 ma more recent shortening deformation observed in outcrop and seismic sections of the northern parts of the basin may correlate with plate model oblique convergence from 4 ma to the present the latter convergence is associated with underthrusting that formed a trough or trench and an accretionary sedimentary prism off the west coast of the queen charlotte islands and with inferred uplift and erosion of the western part of the islands

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

geochemistry of back arc basin volcanism in bransfield strait antarctica subducted contributions and along axis variations 1 bransfield strait is a quaternary ensialic back arc basin at the transition from rifting to spreading fresh volcanic rocks occur on numerous submarine features distributed along the rift axis including a discontinuous neovolcanic ridge similar to the nascent spreading centers seen in some other back arc basins smaller edifices near the northeast end of the rift yielded basalts with the most arc like compositions e g high large ion lithophile element high field strength element and 87 sr 86 sr the most mid ocean ridge basalt morb like basalts are from a large caldera topped seamount and a 30 km long axial neovolcanic ridge toward the southwest end of the rift but these two features also yielded andesite and rhyolite respectively the volcanic and geochemical variations are not systematic along axis and do not reflect the unidirectional propagation of rifting suggested by geophysical data the most depleted basalts have major and trace element characteristics indistinguishable from morb except for slightly higher cs and pb concentrations pb isotopic ratios show little variation compared to sr and nd isotopic ratios and do not extend to the depleted pb isotopic ratios found in other back arc basins either the depleted mantle beneath bransfield strait has higher than normal pb isotopic ratios or the subducted component beneath bransfield strait has such high pb concentrations that it dominates the pb isotopic composition of the bransfield strait mantle without significantly affecting the sr and nd isotopic compositions metalliferous sediments and fluids extracted from a subducting slab may have the necessary high concentrations of pb index terms 1040 geochemistry isotopic composition chemistry 3640 mineralogy and petrology igneous petrology 3655 mineralogy and petrology major element composition 3670 mineralogy and petrology minor and trace element composition keywords back arc basin bransfield strait antarctica geochemistry volcanism recycling

strontium and neodymium isotope geochemistry of igneous rocks from the north east pacific and gulf of california abstract thirty three measurements of the 87sr 86sr ratios on dredged and drilled whole rock samples of basalts diabases and glasses from the ne pacific and gulf of california vary from 0 7024 to 0 7057 the 143nd 144nd ratios measured on twelve selected whole rock samples range from 0 5129 to 0 5133 the 87sr 86sr ratios measured on thirty acid leached samples are also widely distributed range 0 7023 0 7058 but most of the values fall below 0 7030 the 87sr 86sr ratios generally decrease on leaching i e 87 86sr an alteration parameter defined later in the text shows generally positive values weight loss on leaching as well as sr content of the leached samples are also measured and show that in the altered samples most of the sr resides in the acid leachable fraction the concentration of sr in the leached samples is positively correlated with that in the whole rock samples the study of the core margin glass or fresh glass palagonitized glass splits shows that both the sr content and the 87sr 86sr ratio may suffer significant changes upon low temperature seawater alteration a plot of the whole rock 143nd 144nd ratio vs the leached sample 87sr 86sr ratio shows that most of the samples analyzed plot within or close to the morb field the results from an altered diabase and a considerably less altered glass from d s d p site 469 in the ne pacific and from an altered margin and less altered glass splits of a basalt sample dredged from the mouth of the gulf of california show that the 143nd 144nd ratio may also suffer a very slight but significant decrease upon seawater alteration

lower palaeozoic convergent plate margin volcanism on bomlo sw norway and its bearing on the tectonic environments of the norwegian caledonides the island of bomlo presents a geological section through a caledonian convergent plate margin the lykling ophiolite and the overlying ensimatic are sequence of the geitung unit were emplaced onto the baltic shield probably in arenig times after a period of deep erosion the basin and range type volcanics of the siggjo complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism possibly due to the successful subduction of a spreading ridge at the top of the mainly sedimentary ashgill llandovery vikafjord group the basin and range type volcanism resumed this was probably preceded immediately by the calc alkaline subaerial volcanism at the base of the langevag group which passes stratigraphically upwards into basin fill lithologies such as submarine volcaniclastics cherts turbidites and submarine greenstones of chemistry compatible with the early stages of ensialic back arc basin development the stratigraphy and volcanism are discussed in context with recent views on the rest of the norwegian caledonides

volcanic aerosol layers observed by lidar at south pole september 1991 june 1992 during 1991 the terrestrial stratosphere went through a substantially increased aerosol load due mainly to the eruptions of mt pinatubo in the philippines and to a lesser extent to those of mt hudson in chile this paper reports lidar observations of the stratospheric aerosols at south pole two layers were present at different altitudes during the austral summer but only the higher one persisted in the stratosphere until the onset of the polar stratospheric cloud psc phenomenon data have been analyzed in terms of the integrated backscattering coefficient and the aerosol mass content has been estimated

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

the ellsworth terrane coastal maine geochronology geochemistry and nd pb isotopic composition implications for the rifting of ganderia the ellsworth terrane is one of a number of fault bounded blocks that occur along the eastern margin of ganderia the westernmost of the peri gondwanan domains in the northern appalachians that were accreted to laurentia in the paleozoic geologic relations detrital zircon ages and basalt geochemistry suggest that the ellsworth terrane is part of ganderia and not an exotic terrane in the penobscot bay area of coastal maine the ellsworth terrane is dominantly composed of bimodal basalt rhyolite volcanic sequences of the ellsworth schist and unconformably overlying castine volcanics we use new u pb zircon geochronology geochemistry and nd and pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the ellsworth terrane and its relationship with ganderia u pb zircon geochronology for rhyolites indicates that both the ellsworth schist 508 6 0 8 ma and overlying castine volcanics 503 5 2 5 ma are middle cambrian in age two tholeiitic basalt types are recognized type tb 1 basalt present as pillowed and massive lava fl ows and as sills in both units has depleted la and ce la nd n 0 53 0 87 values fl at heavy rare earth element ree values and no positive th or negative ta anomalies on primitive mantle normalized diagrams in contrast type tb 2 basalt present only in the castine volcanics has slightly enriched lree la yb n 1 42 2 92 values and no th or ta anomalies both basalt types have strongly positive e nd 500 values tb 1 7 9 8 6 tb 2 5 6 7 0 and relatively enriched pb isotopic compositions 206

geochemical constraints on the origin of the annieopsquotch ophiolite belt newfoundland appalachians the early ordovician annieopsquotch ophiolite belt occurs immediately west of the main iapetus suture zone and imposes important constraints on the tectonic processes associated with closure of the peri laurentian portion of iapetus the annieopsquotch ophiolite the most prominent ophiolite within the annieopsquotch ophiolite belt exposes a 5 5 km thick section of gabbros sheeted dikes and pillow basalts in which three magmatic episodes have been recognized based on field and geochemical data the first phase is composed of layered troctolites which are preserved as enclaves within the gabbro zone trace element modeling suggests the troctolites crystallized from boninitic melts the troctolite substrate was intruded by the dominant second tholeiitic magmatic phase which formed a gabbro sheeted dike basalt sequence all tholeiites have suprasubduction zone chemical characteristics but the suprasubduction zone signature decreases toward the top of the basalt sequence the third magmatic episode is composed of primitive dikes which are interpreted as off axis intrusions other ophiolites within the annieopsquotch ophiolite belt have comparable geochemical signatures suggesting they may have constituted a single piece of oceanic lithosphere based on geochemical and regional tectonic constraints the annieopsquotch ophiolite belt is interpreted to have formed during initiation of west directed subduction fast rollback of the subducting slab would have induced volatile fluxed decompression melting of previously depleted mantle yielding boninitic melts the suprasubduction zone tholeiite sequence would have formed from ascending fertile mantle fluxed with sub duction related fluids as rollback continued this suggests that the annieopsquotch ophiolite belt does not represent the remnants of normal oceanic crust or backarc basin crust as previously thought our model constrains the initiation and early evolution of a west dipping peri laurentian subduction zone that was responsible for formation of several arc backarc complexes currently preserved in the annieopsquotch accretionary tract

stratospheric clouds at south pole during 1988 1 results of lidar observations and their relationship to temperature an optical radar lidar has been operational at the amundsen scott south pole station since summer 1987 1988 the observations were specially directed to the detection of aerosol layers and polar stratospheric clouds pscs the lidar utilized a nd yag laser followed by a second harmonic generator and a 0 5 m diameter cassegrain receiving telescope results obtained during the period may october 1988 are summarized some 10 000 profiles of the lidar echoes each the result of 1 min averaging were obtained data sets consisting of profiles of the scattering ratio and of the backscattering cross section ba based on half hour averaging are presented the data can be related to profiles of the atmospheric temperature t usually obtained on a daily basis at south pole stratifications appear to have two distinct types of structures one structure shows only a modest variation with height the other is characterized by sharp features with large changes of the cross section with height the basic results the relationship between ba and t and their statistical relevance are considered in this paper the microphysical interpretation the attribution of these structures to psc type i and type ii respectively involving the condensation of nitric acid trihydrate and of water ice and the seasonal evolution of the phenomena are treated in a companion paper

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

felsic a type basic plume induced early palaeozoic bimodal magmatism in the maures massif southeastern france the maures massif forms an important piece of the southernmost part of the variscan belt of western europe this massif exhibits high grade bimodal felsic basic volcanic complexes a distinctive lithological feature documented elsewhere in similar domains of the european variscides and referred to the cambro ordovician extensional episode two major alkalic and tholeiitic compositional groups and subordinate transitional metabasites have been identified occurring at several distinct horizons or in bimodal complexes this chemical diversity is interpreted in terms of variable degrees of partial melting of progressively depleted mantle source s which experienced melting at different depths from garnet to spinel stability domains during a progressive mantle upwelling associated with intracontinental rifting this setting is reinforced by the presence of metabasites with compositions similar to continental flood basalts showing slightly humped ree patterns and interpreted as resulting from the melting of a partially depleted source at a relatively low degree of melting in the garnet spinel transition zone the metafelsites from the tholeiitic bimodal complex exhibit the distinctive major and trace element characteristics of a type rhyolites their elemental variations are consistent with fractional crystallization of major and accessory phases but some discontinuous ree profiles result from a hydrothermal fractionation mechanism the modelling of common anhydrous fractionating assemblages suggests that these a type compositions may be derived from the associated tholeiites by extensive degrees of fractionation 90 with some continental crust involvement or by anhydrous partial melting 30 of an underplated mafic parent of tholeiitic composition the bimodal character of the late cambrian maures magmatism together with the chemistry of the various metabasites and metafelsites suggests plume induced intracontinental magmatic activity resulting in lithospheric thinning prior to the onset of rifting and break up of the palaeozoic continental lithosphere at this northern margin of gondwana

last glacial maximum and hypsithermal in the southern hemisphere abstract a synthesis of the two contrasting climates of the upper quaternary in the southern hemisphere based on continental geological indicators suggests that the antarctic anticyclone is the most important climatic system in the hemisphere according to glaciological and oceanographic studies the antarctic anticyclone covered an area in the lgm that was double of that during the hypsithermal this phenomenon produced a consequent shifting of the climatic belts by approximately 10 of latitude to the north in the lgm and a migration to the south during the hypsithermal oceanic and continental anticyclones were active systems in shaping climates in south america australasia and southern africa these structures were enhanced throughout the lgm provoking generalized droughts and weakened at the hypsithermal permitting more rains in tropical latitudes the results presented here were obtained by integrating our own results with regional and continental syntheses of other authors the paleoclimatic proposals are amply validated by more than 1500 dates in the four continents geological and geomorphological features such as dunefields paleosols fluvial terraces and lake levels were the main tools employed in the work

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

early mesozoic oceanic subduction related volcanic rocks pindos basin greece abstract small outcrops of volcanic rocks of mid triassic to early jurassic age in the deep water mesozoic pindos basin have been systematically sampled and analysed geochemically to interpret the tectonic environment in which these rocks formed associated sediments indicate a rifting environment the volcanic rocks include all the main types found in modern back arc basins shoshonites with clinopyroxenes of calc alkaline affinity occur at the basin margin minor enriched type morb contains clinopyroxene typical of morb minor basalts that resemble oib with some subduction related characteristics contain clinopyroxene typical of alkahne rocks thick andesites and pyroclastic rocks with morb like trace elements except for some lile enrichment are associated with cumulate gabbro and oceanic type plagiogranite these thick andesites and pyroclastic rocks formed at an intra backarc ocean ocean subduction zone the supra subduction zone ophiolites of the eastern margin of the pindos basin which are of similar age are probably related to the same subduction zone the overall volcanic assemblage is related to formation of a back arc basin associated with the subduction of paleo tethys the hypothesis that late triassic eariy jurassic rifting formed only small pull apart basins by transtension is inconsistent with the range of volcanic products and the evidence for significant intra back arc subduction

geochemistry and tectonic implications of two ordovician bimodal igneous complexes southern french massif central two bimodal meta igneous complexes from the southern massif central provide important tectonic constraints on the initial stage of the palaeozoic orogen the metabasites may be subdivided into two groups according to the distribution of the large ion lithophile lil and high field strength hfs elements n type morbs with low lil hfs ratios and subduction type basalts with high lil hfs ratios these associations suggest a complex tectonic setting such as an extensional zone or a back arc basin above a subduction zone the great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust the association of terrigeneous sediments with n type morbs subduction type basalts and high silica peraluminous rhyolites argue against an intra oceanic setting and suggest that the area was one of ensialic crustal tension during middle ordovician the two complexes are identified as remnants of ensialic back arc basins rather than relicts of major oceanic sutures such marginal basins could have been located in a mainly continental domain such as the armorican microplate

late pleistocene vertebrates from northern uruguay tools for biostratigraphic climatic and environmental reconstruction abstract an updated list of vertebrates from quaternary sediments of northern uruguay is provided including new reports a detailed study of equus amerhippus neogeus was performed due to its biostratigraphic value in correlating the fossil assemblage with the lujanian stage late pleistocene early holocene the faunal association is referred to the sopas formation the only stratigraphic unit recognized three stratigraphic sections reveal the predominance of subaquous environmental deposition yucutuja and malo creeks and the development of paleosols on fluvial flood plains sopas creek the mammal content and the 14c and thermoluminescence optically stimulated luminescence ages indicate a late pleistocene age information indicating a relationship with ois3 is discussed the mammals of the sopas formation seem to be different and older than the fauna of the guerrero member of the lujan formation ecological and biogeographic information provided by the fauna is used to discuss paleoenvironments and climatic processes involved this reinforces the hypothesis of the presence of a variety of habitats composed dominantly of open areas plus some forested zones with permanent lacustrine and fluvial contexts brazilian and some patagonian vertebrates occurring together in the sopas formation lead to discussion of the nature of the fossil assemblage

volcanostratigraphy and petrogenesis of the nemrut stratovolcano east anatolian high plateau the most recent post collisional volcanism in turkey abstract the nemrut stratovolcano with its well defined summit caldera is the most recent volcanic eruption center 1441 a d in the east anatolian high plateau one of the best examples of an active colllision zone widespread volcanism has been active in this region for the last 10 ma producing large volumes of lavas and pyroclastics covering a broad belt across the bitlis suture zone from the anatolian plate in the north to the arabian plate in the south three major evolutionary stages have been identified in the evolution of the nemrut stratovolcano pre caldera post caldera and late stages the pre caldera stage is further differentiated into effusive extrusive and explosive phases the latter being responsible for the caldera formation the products of this stage cover a broad compositional range from basalts to rhyolites and consist of lava flows domes large volumes of ignimbrites and associated pyroclastics the post caldera stage consists of three phases comprising phreatomagmatic eruptions vitrophyric rhyolitic lava flows and dykes the late stage consists of the explosive and the following effusive phases with vitrophyric rhyolitic and basaltic lava flows respectively the post caldera and late stages show a compositional gap between the younger mafic and the older felsic members the products of all three stages are silica saturated and display a gradual transition in nature from sodic in the mafic to potassic in the felsic members the overlaps in the compositional spectra and the geochemical trends on bivariate plots suggest the derivation of the products of all three stages from a single parental magma or similar parental magmas yet the stratigraphic position of the mafic and felsic members points to a periodically replenished magma chamber the major and trace element systematics along with petrographic evidences suggest clinopyroxene plagioclase olivine amphibole as the major fractionating phases governing the liquid evolutionary paths with a progressive increase in the proportions of plagioclase and amphibole joined by k feldspar biotite and apatite in the fractionating assemblages during the course of magmatic differentiation the volcanics of all three stages exhibit enrichment in light rare earth elements lree over heavy rare earth elements hree and large ion lithophile elements lile over high field strength elements hfse the multi element patterns and low th y and nb y ratios imply a within plate type mantle asthenospheric source for this volcanics trace element modelling suggests derivation of parental magma s from 10 to 30 partial melting of a spinel lherzolite source assimilation fractional crystallization afc modelling reveals significant but variable degrees of crustal contamination in the evolution of the volcanics with a range of r values assimilation rate crystallization rate from 0 2 to 0 8 these interpretations are in conformity with the recent findings from seismic studies showing that lithospheric mantle is absent in eastern anatolia the crust in the region is about 45 km thick and directly underlain by the asthenosphere possibly due to slab breakoff following the collision of the arabian and the anatolian plates along the bitlis suture zone

tectonic setting and regional significance of the port aux basques gneiss sw newfoundland the port aux basques gneiss of southwest newfoundland comprises three paragneiss and schist assemblages the grand bay and port aux basques complexes and the harbour le cou group they are intruded by large volumes of basic sheets and coeval middle ordovician granitoids the margaree and kelby cove orthogneiss which also cut a tectonic slice of earlier ultrabasic to basic rocks within the grand bay complex the big barachois assemblage the grand bay and port aux basques complexes are juxtaposed by the late silurian isle aux morts fault zone against the harbour le cou group to the east which dominantly comprises sillimanite and garnet bearing pyritiferous metasediments with amphibolitic intrusions and meta pillow basalts in its lower stratigraphic units geochemical studies have shown that metabasites of the grand bay complex port aux basques complex margaree and kelby cove orthogneiss comprise fractionated and crustally contaminated morb like and within plate tholeiites suggestive of an ensialic okinawa type back arc basin these have comparable characteristics to metabasites in the harbour le cou group and suggest that these units comprise remnants of an extensive mid ordovician marginal basin developed within a promontory on the peri gondwanan continental margin in contrast the big barachois assemblage in part comprises a distinct suite of pre mid ordovician island arc tholeiites

isotopic and trace element constraints on mixing and melting models of marginal basin volcanism bransfield strait antarctica bransfield strait is a narrow marginal basin separating the south shetland islands from the northern end of the antarctic peninsula quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting and as two subaerial off axis volcanoes on the northern margin of the strait the bransfield strait lavas are similar to published analyses from other marginal basins ranging from basalts and basaltic andesites to trachytes they exhibit moderate enrichments in alkali and alkaline earth elements relative to high field strength elements that are typical of many back arc basin basalts the seamount basalts have trace element chemistry similar to enriched mid ocean ridge basalts e morb but with variously higher alkali and alkaline earth element concentrations and frequently lower nb low pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active melting of depleted mantle mixed with 0 5 2 crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes the off axis volcanoes are the products of less partial melting than the on axis volcanoes one of the off axis volcanoes also contains more of an enriched component in its source some of the lavas have rb sr too low to account for their 87sr 86sr perhaps due the loss of rb during mantle metasomatism or interaction between slab derived fluids and the mantle source of the basalts strontium neodymium and 207pb 204pb isotopic signatures remained constant during the transition from tertiary island arc volcanism to quaternary marginal basin volcanism but 206pb 204pb increased high cen smn of basalts from some volcanoes requires residual garnet in the source thus the young 15 25 ma at the trench subducted slab is either deeper than 60 km beneath the volcanoes or it has fractured and is no longer a coherent barrier to melts generated below it

a quantitative analysis of transtensional margin width abstract continental rifted margins show variations between a few hundred to almost a thousand kilometres in their conjugated widths from the relatively undisturbed continent to the oceanic crust analogue and numerical modelling results suggest that the conjugated width of rifted margins may have a relationship to their obliquity of divergence with narrower margins occurring for higher obliquity we here test this prediction by analysing the obliquity and rift width for 26 segments of transtensional conjugate rifted margins in the atlantic and indian oceans we use the plate reconstruction software gplates www gplates org for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until breakup our rift width corresponds to the distance between the onshore maximum topography and the last identified continental crust we find a weak positive correlation between the obliquity of rifting and rift width highly oblique margins are narrower than orthogonal margins as expected from analogue and numerical models we find no relationships between rift obliquities and rift duration nor the presence or absence of large igneous provinces lips

petrology geochemistry and tectonic implications of volcanics dredged from the intersection of the yap and mariana trenches abstract rocks dredged from the forearc very close to the intersection of the yap and mariana trenches include a suite of highly depleted arc tholeiites and several samples of transitional to slightly alkaline basalt the tholeiites range from magnesian quartz tholeiites with 0 46 0 6 tio2 to andesites with up to 62 sio2 and 8 2 feo all show pronounced lree depletion and have very low contents of ba and sr they are postulated to have been produced by partial melting of upper mantle peridotite residual after morb extraction following influx of hydrous fluids from the subducted slab while these fluids were responsible for small enrichments in ba k rb and sr in melts generated lree were not involved in the metasomatism and the strong lree depletion probably reflects the unmodified depleted source peridotite the second lava suite includes slightly ne normative ti augite bearing basalts with convex upward ree patterns showing slight lree depletion la sm n 0 76 the chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening a k ar age on one sample 7 8 1 3m y is in good agreement with the initial opening of the mariana trough the tectonic significance of the dredged arc tholeiite suite is less obvious a k ar age of10 8 0 4 my on one andesite and the occurrence of similar lavas in dredges from at least 300 km along the length of the yap arc suggest that subduction was occurring beneath the yap arc in the late miocene after overthrusting of the yap greenschist allochthon and while calc alkaline arc magmatism was occurring further north on the west mariana ridge we suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the yap forearc following subduction beneath this area of young hot sorol trough crust these arc tholeiites represent a magma type transitional between more typical arc tholeiites e g tongan and high mg andesites and boninites

petrology of alarcon rise lavas gulf of california nascent intracontinental ocean crust 1 the alarcon basin in the southern gulf of california originated by intracontinental rifting of the southwestern margin of north america and has evolved into an intercontinental ocean basin by 3 7 m y of seafloor spreading lava samples collected on the axial ridge along alarcon rise two of the many near axis seamounts on the northwest flank of the rise and an abyssal hill on 0 6 ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and he sr nd pb isotope composition most samples are typical mid ocean ridge basalt morb but rocks at the axial ridge site closest to continent are more differentiated rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it this is contrary to the postulated transform fault effect alarcon basin differs from the salton trough and guaymas basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component we propose that pacific morb source mantle underlies the gulf of california because the western north american margin has overridden the pacific suboceanic mantle miocene rifting and pliocene spreading in alarcon basin have created a window allowing the most recent morb melts to rise uncontaminated to the surface

mafic rocks in a deep crustal segment of the variscides the gory sowie sw poland evidence for crustal contamination in an extensional setting coronitic metagabbronorites so called hyperites and metabasites occur within gneisses migmatites and minor granulites in the high grade metamorphic gory sowie block in the sudetes sw poland incompatible trace element and end data combined with field and petrographic evidence suggest that three groups may be distinguished 1 enriched amphibolites zr nb ca 8 end500 1 2 5 2 depleted amphibolites zr nb 30 end500 5 6 and 3 metagabbronorites zr nb 13 19 end500 scattered between 5 3 and 1 these characteristics favour an extensional within plate setting consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses in that setting enriched mantle sources were partially melted to produce a range of basic magmas showing affinities with continental tholeiites within plate type basalts presumably in a more advanced stage of rifting more depleted varieties transitional to n morb were emplaced these mafic magmas were variably contaminated during their ascent in the crust the metagabbronorites with their well preserved igneous textures were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism some authors suggesting a relation to the circum gory sowie ophiolites however the geochemical characteristics and p t estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the gory sowie block and in contrast to them have experienced a complex polybaric p t path

the recent isla san luis volcanic centre petrology of a rift related volcanic suite in the northern gulf of california mexico abstract isla san luis one of the most recent eruptive centres in the gulf of california presents a complete magma evolution trend from basaltic andesites to rhyolites the less evolved lavas are palagonite tuffs related to surtseyan type activity which characterized the emergent stage of the island subaerial lava flows and later high energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing quenched bombs with an obsidian crust and a pumiceous core basaltic andesites contain olivine fo87 80 calcium rich plagioclase and sparse clinopyroxene microphenocrysts the typical mineralogy of dacitic and rhyolitic lavas is plagioclase two pyroxenes major and trace elements vary regularly with mgo taken as a differentiation index isla san luis lavas are enriched in lile and depleted in nb ta and ti their characteristics are intermediate between tholeiitic and calc alkaline lavas they are also lree enriched and the la yb n ratios steepen slightly with differentiation such an evolution is better explained by combined fractional crystallization and assimilation processes

structure of the rift basins in the central gulf of california kinematic implications for oblique rifting abstract we use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central gulf of california our analysis reveals that oblique strain in the central gulf formed two tectono sedimentary domains during distinct deformation stages the eastern domain offshore sonora is bounded by the east and west pedro nolasco faults that may constitute the southernmost segments of the tiburon fault system within this domain the dip slip yaqui fault controlled deposition of 3 9 km of sediments in the half graben yaqui basin the western domain offshore baja california is bounded by the guaymas transform fault which controlled the accumulation of 1 45 km of sediments within a half graben that formed the early guaymas basin the tectono sedimentary activity offshore sonoran likely ranges from late miocene pliocene to late pliocene time while activity in the guaymas basin commenced in late pliocene time extinction of the main faults offshore sonora was nearly coeval to the initiation of the guaymas transform fault our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central gulf the guaymas basin is now a nascent spreading center but prior to this it evolved as a half graben controlled by the guaymas transform fault such drastic transition is not constrained but likely occurred during the pleistocene time and must be localized

lower palaeozoic convergent plate margin volcanism on bomlo southwest norwegian caledonides geochemistry and petrogenesis major and trace element analyses of a lower palaeozoic metavolcanic sequence of convergent plate type from bomlo southwest norwegian caledonides are presented and discussed this sequence ranges in age from the upper cambrian through the lower silurian petrogenetic models for the lavas in terms of partial melting and crystal fractionation are discussed two models are presented for the metabasalts in order to explain their different trace element abundances and ratios 1 ree modelling assuming a mantle source with ree abundances twice chondritic suggests progressively more varied sources with time thus the metabasalts from the oldest upper cambrian lower ordovician geitung unit of primitive island arc type and those of the mid ordovician siggjo complex of basin and range type can be modelled in terms of high around 25 and moderate around 5 degrees of partial melting of spinel lherzolite respectively the metabasalts of the post ashgillian vikafjord group of typical continental flood basalts are compatible with moderate c 5 10 degrees of partial melting of spinel and garnet lherzolite sources the supposed lower silurian langevag group of calc alkaline andean type metabasalts grading into alkaline to tholeiitic metabasalts of early marginal basin youngest character require low 2 source heterogeneity produced by subduction zone derived enrichment of lil elements and contemporaneous stabilization of minor phases which accommodate hfs elements this process combined with possible continental contamination may possibly yield the trace element concentrations and ratios of the different metabasalts by partial melting of modally similar mantle sources

the existence of the edge region of the antarctic stratospheric vortex 1 new evidence from models ozone measurements and balloon trajectories is presented that confirms the existence of a broad cohesive region of air at the edge of the antarctic stratospheric vortex that is only weakly mixed with the core of the vortex comprehensive measurements by antarctic ozonesondes in 2003 show quite different evolution in the edge region than in the core with one exception long duration balloons launched from antarctica in spring 2005 remained confined to either the edge region of the vortex or its core calculations of effective diffusivity for 2003 and 2005 show similarly weak mixing in the edge region as earlier calculations for 1996 they again show that the edge region is a significant proportion of the area of the ozone hole its importance lies in the possibility that unmixed it can have more polar stratospheric clouds during the course of the 21st century thereby delaying the recovery of the ozone hole

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization