the mesozoic margin of the maghrebian tethys in the rif belt morocco evidence for polyphase rifting and related magmatic activity

stratigraphic numerical modelling of a carbonate platform on the romanche transverse ridge equatorial atlantic abstract the romanche transverse ridge equatorial atlantic is located in the northern flank of the fracture zone opposite the eastern ridge transform intersection rti it constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve a series of e w aligned peaks located on the crest of the transverse ridge were at sea level during early and middle miocene times they are presently capped by a 300 m thick shallow water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level these structures are now about 1 km below sea level high resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower miocene ages derived from microfossils suggest that the base of the carbonate platform dates from 17 25 m y ago and the sinking of the platform started between 18 and 13 m y ago these data were included in a numerical simulation model that takes into account thermal and tectonic subsidence growth potential of the carbonates subaerial and submarine erosion rates and mio pliocene absolute sea level fluctuations the results outline the subsidence history of the romanche transverse ridge and suggest post miocene subsidence faster than that predicted by the thermal cooling model

diffusion kinetics of lutetium in diopside and the effect of thermal metamorphism on lu hf systematics in clinopyroxene abstract we report new experimental data for lu diffusion in diopside from 1150 to 1250 c at 1 bar pressure and oxygen fugacity corresponding to the iron wustite buffer the primary experimental dataset defines the following arrhenius relation for lu diffusion parallel to the b crystallographic axis d lu b 1 37 1 2 10 13 exp 208 7 15 2 kj mol 1 rt m 2 s where t is in kelvin we find that lu diffusion in diopside is significantly faster than that of hf which could lead to spurious lu hf ages via decoupling of 176lu from 176hf if the system was subjected to protracted periods of high temperature the extent to which lu hf ages are compromised during initial cooling depends on the peak temperature grain size cooling rate and the nature of lu partitioning between clinopyroxene and its surrounding phase s however in geologically realistic scenarios the effects of diffusion on lu hf ages during initial cooling do not produce lu hf ages that differ from the age corresponding to the closure temperature of hf by more than a few million years lu hf systematics in clinopyroxene are far more susceptible to diffusion during reheating events that occur after radiogenic 176hf has already accumulated in the host phase for a given thermal pulse the lu hf age varies as a function of grain size temperature the timing and duration of reheating and the partitioning of lu between clinopyroxene and its surrounding phase s the data collected and numerical model developed in this study have been used to simulate the effect of thermal metamorphism on the internal lu hf systematics of the eucrites millbillillie and piplia kalan although our model results do not offer a comprehensive solution to the apparent pre solar lu hf ages observed in many ancient meteorites we argue that the 4 9 ga pyroxene whole rock lu hf ages of these specific samples are at least partially if not primarily due to diffusive loss of lu from the constituent minerals to a partial melt phase during high temperature metamorphism

cenozoic deformation and mesozoic abandoned spreading centre in the tagus abyssal plain west of portugal results of a multichannel seismic survey the seismic stratigraphy of the sedimentary section that fills in the tagus abyssal plain is specified by means of new multichannel seismic data six sedimentary sequence units are identified these ranging from late jurassic to recent have been dated by extrapolation of deep sea drilling project dsdp and ocean drilling program odp drilling results and from sampling by dredging and submersible surveys based on this stratigraphic calibration and other geophysical data a geodynamic evolution can be established for the tagus abyssal plain faulting and folding in these sedimentary units clearly indicate that both the portuguese margin and the adjacent oceanic crust were deformed by a miocene compressive event an oxfordian kimmeridgian rifting in the lusitanian basin and on the portuguese margin may herald a sea floor spreading in the tagus abyssal plain the continent ocean boundary is marked by landward dipping reflectors similar to those observed along the canadian margin the oceanic structure of

the importance of melt tio2 in affecting major and trace element partitioning between fe ti oxides and lunar picritic glass melts abstract lunar mare basalts and picritic glasses have tio2 abundances ranging from less than 1 wt to over 16 any high ti mare basalt or picritic glass petrogenetic model must include fe ti oxides in the mantle source or invoke fe ti oxide assimilation during magma ascent to the lunar surface we conducted partitioning experiments to investigate high field strength element hfse rare earth element ree and transition metal distribution between fe ti oxides and lunar picritic glass melts over a range of melt compositions our results suggest that ilmenite melt and armalcolite melt hfse cr and v partition coefficients dhfse dcr dv are strongly dependent on melt tio2 content whereas ilmenite melt ree partition coefficients appear to be insensitive to melt composition as tio2 increases in picritic glass melts hfse cr and v activities in melt also increase and fe ti oxide melt dhfse dcr and dv decrease the effect of ti on partitioning behavior can be attributed to the formation of fe o ti melt species in high ti melts ilmenite dhfse range from compatible in oxides in equilibrium with low ti melts to incompatible in oxides in equilibrium with depolymerized high ti picritic glass melts dhfse are inversely correlated with tio2 abundance in the melt and become nearly constant for melts with more than 6 8 tio2 we present simple partitioning models that utilize the solubility of ilmenite and armalcolite in melt to effectively predict hfse partition coefficients across a wide range of picritic glass melt compositions the hfse budget of ilmenite cumulates that crystallize from the lunar magma ocean strongly depends on the composition of the magma ocean low ti and high ti lunar basalts can be produced by an ilmenite or armalcolite bearing hybridized mantle source or by assimilation of late stage magma ocean cumulates the dependence of dhfse and dcr on melt tio2 is consistent with the formation of lunar type 1 armalcolite from high tio2 picritic glass melt however lunar type 2 and 3 armalcolite have a more complicated yet undetermined history of formation

facies associations and evolution of pelagic carbonate platform basin systems examples from the italian jurassic abstract during the structural and sedimentological evolution of rifted continental margins in carbonate dominated settings a characteristic stage is commonly reached when pelagic carbonate sedimentation occurs in association with complex fault block systems an integrated study of the ammonite biostratigraphy and sedimentology of jurassic pelagic rocks in the umbria marche apennines central italy and in the sila mountains north east calabria southern italy has revealed three distinctive facies associations which can be related to specific structural and depositional settings the condensed pelagic facies association is the constituent of condensed usually discontinuous sequences with numerous hiatal surfaces the normal and resedimented pelagic facies association is typical of thicker sequences comprising gravity flow and rockfall deposits the composite pelagic facies association is one in which rockfall deposits are found associated with condensed sequences containing minor or no gravity flow deposits palaeotopographical highs are dominated by the condensed pelagic facies association and constitute the pelagic carbonate platform pcp structurally the pcp can be coincident with the top of a horst or central high block or the crestal area of a tilted block in the latter case the sediment surface of the pcp slopes gently maximum of 1 3 in this the shallowest area of a pelagic carbonate ramp pcps can have erosional bypass rare and depositional margins stepped margins also exist as subparallel rows of erosional and bypass margins trends from erosional to depositional margins are usually seen as a response to basin filling and burial of submarine tectonic escarpments by pelagic deposits in late synrift or early post rift times this is apparent from progradation of pcp facies deposits beyond the structural boundaries of the pcp palaeoenvironmental analysis in pelagic carbonate platform deposits of the umbria marche apennines suggests that drowning of ancestral peritidal carbonate platforms in pliensbachian time was followed first by a moderate deepening and then by shallowing in the tithonian

extensional tectonics in the gorringe bank rocks eastern atlantic ocean evidence of an oceanic ultra slow mantellic accreting centre the gorringe bank corresponds to an upper mantle peridotite ridge enclosing a 500 m thick 50 km long laccolith like body of gabbro locally cut and poorly covered by tholeiitic rocks strain and kinematic analysis of orientated gabbros and peridotites sampled during the gorringe diving cruise 1996 provides new kinematic constraints on extensional high temperature deformation recorded at deep levels during stretching near an accreting centre axis of a mantle dominant oceanic lithosphere it is argued that the gorringe bank lithosphere formed at an oceanic ultra slow n010 020 trending accreting centre mostly by passive tectonic denudation of the mantle without any synchronous large magmatism this peculiar lithosphere may be representative of the iberia oceanic domain located between the continent and the j anomaly ridge which likely marks the beginning of true spreading at an oceanic spreading ridge

petrology and chemistry of mil 03346 and its significance in understanding the petrogenesis of nakhlites on mars antarctic meteorite miller range mil 03346 is a nakhlite composed of 79 clinopyroxene 1 olivine and 20 vitrophyric intercumulus material we have performed a petrological and geochemical study of mil 03346 demonstrating a petrogenetic history similar to previously discovered nakhlites quantitative textural study of mil 03346 indicates long 1 x 101 yr residence times for the cumulus augite whereas the skeletal fe ti oxide fayalite and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling probably as a lava flow from the relatively high forsterite contents of olivine up to fo43 compared with other nakhlites and compositions of augite cores wo38 42en35 40fs22 28 and their hedenbergite rims we suggest that mil 03346 is part of the same or a similar martian cumulate rich lava flow as other nakhlites however mil 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date calculated trace element concentrations based upon modal abundances of mil 03346 and its constituent minerals are identical to whole rock trace element abundances parental melts for augite have ree patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients this parallelism reflects closed system crystallization for mil 03346 where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization a model for the petrogenesis of mil 03346 and the nakhlites nakhla governador valadares lafayette yamato 000593 northwest africa nwa 817 nwa 998 would include 1 partial melting and ascent of melt generated from a long term lree depleted mantle source 2 crystallization of cumulus augite olivine magnetite in a shallow level martian magma chamber 3 eruption of the crystal laden nakhlite magma onto the surface of mars 4 cooling crystal settling overgrowth and partial equilibration to different extents within the flow 5 secondary alteration through hydrothermal processes possibly immediately succeeding or during emplacement of the flow this model might apply to single or multiple flow models for the nakhlites ultimately mil 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks on mars with analogous petrogenetic histories to pyroxene rich terrestrial lava flows and to komatiites

iberian plate kinematics a jumping plate boundary between eurasia and africa the rotation of iberia and its relation to the formation of the pyrenees has been difficult to decipher because of the lack of detailed sea floor spreading data although several models have been proposed1 7 here we use detailed aeromagnetic measurements from the sea floor offshore of the grand banks of newfoundland to show that iberia moved as part of the african plate from late cretaceous to mid eocene time with a plate boundary extending westward from the bay of biscay when motion along this boundary ceased a boundary linking extension in the king s trough to compression along the pyrenees came into existence finally since the late oligocene iberia has been part of the eurasian plate with the boundary between eurasia and africa situated along the azores gibraltar fracture zone

a new view of lunar ferroan anorthosites postmagma ocean petrogenesis 1 new melting experiments in the range of 0 1 mpa to 4 7 gpa provide the basis for calculations of polybaric crystallization which show that both a 300 km deep magma ocean mo enriched in refractory elements and a 600 km deep magma ocean with refractory element concentrations similar to the earth s upper mantle produce sufficient amounts of differentiated residual liquid capable of crystallizing the troctolitic noritic mineral assemblages that are prevalent in lunar ferroan anorthosites lfa however the both oceans become saturated with augite after 20 30 crystallization of the plagioclase saturated residuum and thereafter are no longer capable of producing noritic assemblages a second set of melting experiments reveals the existence of a previously unrecognized equilibrium augite liquid pigeonite plagioclase and a thermal minimum on the pigeonite plagioclase liquidus surface at 0 6 gpa these equilibria will produce melts of mafic cumulates with lower wo content than those from which the cumulates originally formed in a postmagma ocean environment of overturning mafic cumulates melt thus formed and entrained in a buoyant segregation of plagioclase would then form diapiric masses that would crystallize troctolitic noritic assemblages after intruding upward diapiric intrusions could also provide the local deformational environment needed to produce the apparently high 95 plagioclase modes of the lfa an unstable density profile in the cumulate pile following crystallization of most of the mo should provide the impetus for overturn and the heat for partially remelting mafic cumulates

contribution a la datation de la croute oceanique de l atlantique central age valanginien inferieur des basaltes oceaniques et âge neocomien des calcaires maiolica de maio iles du cap vert abstract the discovery of calpionellids and radiolarians in thin sedimentary layers intercalated in the upper part of morb tholeiitic pillow basalts allows us to date the oceanic crust of maio island capo verde islands as early valanginian instead of as was hitherto believed late jurassic this new dating fits better with reconstructions of the geological history of the central atlantic based on magnetic anomalies the overlying light coloured pelagic limestones maiolica facies with radiolarians aptychi and ammonites are valanginian barremian in age these limestones are lithologically similar to the white limestones drilled at several dsdp sites in the central atlantic

kreep cumulates in the western lunar highlands ion and electron microprobe study of alkali suite anorthosites and norites from apollo 12 and 14 alkali suite anorthosites and norites are the second most common plutonic rock association in the western lunar highlands after the magnesian suite but their origin poses an enigma for most petrologic models of lunar crustal evolution some models suggest that the alkali and magnesian suites formed from distinct unrelated parental magmas whereas other models propose that both suites formed from the same parental magma the contrast in major element chemistry of the cumulus phases in each suite is difficult to reconcile with their similar incompatible element chemistry we present herein a detailed ion microprobe sims and electron microprobe study of seven alkali suite rocks our data show that most alkali suite anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks and that these characteristics can be used to reconstruct the parental magma composition the data indicate that cumulates of the alkali suite crystallized from magmas with rare earth element ree contents approximately 0 6 2 0xhigh k kreep and small but consistently positive eu anomalies eu eu super approximately 2 relative to kreep snyder and others 1995a have proposed that the alkali suite parental magma is similar to apollo 15 pristine kreep basalt our model suggests that the major element composition of cumulus plagioclase in most alkali suite rocks is too sodic for the calculated crystal line of descent of pristine kreep basalt and that assimilation of pre existing calcic anorthosite is required this conclusion is supported by the ree patterns of the alkali suite parental magma determined here we propose that alkali suite anorthosites formed as flotation cumulates in kreepy plutons that may have formed norites as complementary bottom cumulates the alkali flotation cumulates reflect fractional crystallization of their parental pluton local equilibrium crystallization assimilation of plagioclase rich roof rock and episodic magma mixing during convective overturn of the crystallizing magma bodies texturally pristine alkali anorthosites exhibit petrographic characteristics that are consistent with their origin as cumulates in a kreepy pluton including abundant modal plagioclase post cumulus pyroxenes both augite and pigeonite that generally lack exsolution lamellae and that have equilibration temperatures of 950 1100 degrees c relict igneous textures and in some cases igneous lamination the lack of cumulus mafic phases in rocks that should be pyroxene saturated suggests separation of the plagioclase by flotation not sinking assimilation of plagioclase from older anorthositic highlands crust is indicated by the high eu contents of the cumulates and by the positive eu anomalies in their calculated parental melts relative to high k kreep mixing of the evolved alkali suite parental magma with primitive melt occurred episodically as shown by reverse zoning profiles in some cumulus plagioclase injection of this primitive hot magma into the crystallizing pluton may have induced convective overturn of the magma chamber

clinopyroxene ree distribution coefficients for shergottites the ree content of the shergotty melt abstract ree distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred shergotty intercumulus liquid by in situ analysis with the electron microprobe using samples doped to per cent concentration levels these coefficients exhibit a strong positive correlation with pyroxene wollastonite content recommended values for augites similar in composition to the shergotty cumulus augite cores wo32en48 are d la 0 023 d ce 0 039 d nd 0 10 d sm 0 17 d eu 0 16 d gd 0 20 d yb 0 29 and d lu 0 30 recommended values for pigeonites similar in composition to the shergotty cumulus pigeonite cores wo12en58 are d la 0 002 d ce 0 004 d nd 0 019 d sm 0 031 d yb 0 13 and d lu 0 13 these values resemble distribution coefficients measured for terrestrial basalts of higher mg fe than the shergotty melt rather than those of more evolved siliceous rocks of similar mg fe to the shergotty melt thus it appears that mg fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes ree abundances were computed for the shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions these computed abundances display lree depletion such lree depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the lree enriched patterns required for a two stage sm nd evolution history this result implies that either 1 the melting process was complex e g continuous melting 2 the source region had a more complex history than that of the simple 2 stage model or 3 shergotty does not satisfy the closed system assumption of the calculation and has suffered post magmatic lree enrichment

lithospheric structure of the gorringe bank insights into its origin and tectonic evolution 1 the gorringe bank is a 5000 m high seamount near the atlantic coast of iberia characterized by a 9 m high geoid anomaly and a 120 mgal bouguer anomaly relative to the surrounding abyssal plains it has been linked to a nw directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed down eurasian oceanic crust along the tagus abyssal plain however estimations of crustal shortening have yielded dissimilar results and the deep structure of the ridge remains highly unknown we present a restored cross section and a new model of the lithospheric structure based on gravity geoid elevation and the presence of serpentinized peridotites at least 20 km of shortening took place along a flat ramp flat thrust fault and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70 at the surface to 20 at 14 km depth and 0 at 40 km the topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness te of 30 km the evolution of the gorringe bank since the late jurassic is interpreted in relation to eurasia africa north america plate motion in four stages 1 transtension between newfoundland iberia and africa which generated small oceanic basins and mantle exhumation 2 opening of the north atlantic and seafloor spreading at the nw side of the exhumed gorringe which produced gabbro intrusions and serpentinization 3 a quiescent tectonic period dominated by subsidence and sediment accumulation and 4 a transpressional plate boundary between eurasia and africa with nw directed subcrustal thrusting and generation of the present gorringe relief

morphotectonics of the gorringe bank summit eastern atlantic ocean based on high resolution multibeam bathymetry abstract high resolution swath bathymetry of the gorringe bank located along the europe africa obliquely convergent plate boundary offshore sw portugal allowed investigation of its shallow depth morphotectonic structure as a contribution to understanding the interplay of sea level changes and active deformation of the region dominant ene wsw to nne wsw and subordinate nw se trending morphologic alignments were mapped on the digital terrain model dtm built from multibeam data for both the gettysburg and ormonde summits of the bank based on appraisal of regional structural and seismological data the dominant and subordinate sets are interpreted to reflect dip slip to left oblique reverse faults and normal faults or dyke like joints respectively several terrace remnants were mapped on the dtm and were grouped into five orders at depths ranging from 40 to 120 m morphological features suggest that the terraces were formed in a coastal environment likely during middle late pleistocene sea level changes the terraces are probably polycyclic implying minor vertical tectonic motion however the whole terrace set except the deeper and younger one which was mainly shaped during the lgm lowstand is consistently up to 10 m lower at ormonde in the east relative to gettysburg in the west suggesting displacement across a nne ssw striking fault mapped in the saddle between the two seamounts the structure assemblage is argued to represent the shallow splay system of a crustal scale fault ramp underlying the bank the ene wsw reverse faults have a southward polarity and together with the overall northwest tilt of the bank point to displacement toward the horseshoe abyssal plain to the south at least at upper crustal levels

paired nakhlites mil 090030 090032 090136 and 03346 insights into the miller range parent meteorite abstract miller range mil 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase the discovery of three new nakhlites mil 090030 mil 090032 and mil 090136 in the miller range antarctica geographically close to the location of mil 03346 suggests that they may come from the same parent meteorite in this study we investigate the mineralogy and texture of cumulus and intercumulus phases in situ major and trace element compositions for the cumulus phases as well as pyroxene crystal size distribution patterns and spatial distribution patterns of mil 090030 090032 and 090136 using these combined results we conclude that the three nakhlites studied here are paired with mil 03346 however modal mineral abundances of mil 090030 090032 090136 and 03346 exhibit variations indicating that a single sample of a miller range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system

origin and evolution of the nakhla meteorite inferred from the sm nd and u pb systematics and ree ba sr rb and k abundances analyses of sm nd and u th pb systematics ree ba sr rb and k concentrations were carried out for whole rock and mineral separates from the nakhla meteorite the 1 26 07 b y sm nd age obtained in this work is in good agreement with those previously obtained by the rb sr and ar ar methods the high initial nd value of 16 suggests that nakhla was derived from a light ree depleted old planetary mantle source u th pb data after correction for pre analytical terrestrial pb contamination assuming an age of 1 26 b y suggest that the age of the nakhla source is 4 33 b y the agreement in the age determined by three independent radiometric methods and the high initial nd value strongly suggest that the 1 3 b y age dates one thorough igneous event in the parent body which not only reset these isotopic systems but also established the chemical and petrologic characteristics observed for the nakhla meteorite using a three stage sm nd evolution model in combination with lil element data and estimated partition coefficients we have tested partial melting and fractional crystallization models to estimate lil element abundances in a possible nakhla source our model calculations suggest that partial melting of the light ree depleted source followed by extensive fractional crystallization 50 of the partial melt could account for the ree abundances in the nakhla constituent minerals the estimated source is depleted in the light ree ba rb and k and therefore may resemble the morb source in the earth s upper mantle or the upper 60 300 km of the moon the significantly younger age of nakhla than the youngest lunar rock the young differentiation age inferred from the u th pb data and the estimated lil element abundances including those of k u and th in the source suggest that the nakhla meteorite may have been derived from a relatively large well differentiated planetary body such as mars

discovery of late jurassic fossils inside modern sediments at gorringe bank eastern atlantic ocean and some geological implications introductionin the horseshoe chain mainly com prising volcanic seamounts the gor ringe bank 36 n 11 w hereafterreferred to as gb is an exception because it consists of suboceanic man tle peridotites and gabbros the otherpeculiarity of this seamount is thegreat bathymetric gradient betweenthe summit of the bank at 30 m and the surrounding abyssal plains tagus and horseshoe at 5000 m fig 1 the gb is a n60 trendingridge 200 km long and 80 km wide with two summits the gettysburg inthe south west and the ormonde inthe north east beauzart et al 1979 the two summits separated by a deep 800 m saddle consist of igneousrock outcrops with very thin or absentsedimentary cover the rst exploration of the gb wasmade by the glomar challenger in1970 120 dsdp sites ryan et al 1973 this and some subsequentexplorations with manned submersi bles along the anks of the ridge in the0 3000 m bathymetric range auzen de et al 1978 lagabrielle andauzende 1982 and more recently atgreater depths girardeau et al 1998 have indicated that the gbrepresents a remarkable example of aseamount comprising lower oceaniccrust upper mantle rocks in december 2002 to january 2003duringthegorringe 2003 expedition on board r v urania nationalresearch council italy swath bathy metry seismo magnetic survey exten sive bottom samplings and some rov remotely operating vehicle dives overthe top of the gb were carried out dealteriis et al 2004 a further aim ofthe cruise was to study the bioclasticsediments recorded during previousexpeditions auzende et al 1978 gir ardeau et al 1998 at shallow depths the location and physiography of thegb guaranteed that these sedimentswere totally autochthonous anduncontaminated by other sources the only previous analyses availablefromthesesedimentswereprovidedbyavila and malaquias 2003 geological and stratigraphic resultsthe multibeam and geophysical sur vey extended to 500 m depth overboth summits revealed that gettys burg seamount has an almost subcir cular summit elongated n s for8 10 km the shelf area is somewhatconvex from 60 to 175 m at thecentre of the summit the igneousbasement crops out between depthsof 29 and 60 m the rocky outcropconsists of roughly n s trending rid ges consisting of foliated serpentinitesand gabbros completely surroundedby a blanket of bioclastic sands andgravels down to 140 m where a cleardepositional shelf break appears figs 1 and 2 pockets of these sandsare also hosted in the inter ridgedepressions sands consist of skeletalcalcareous algae forams bryozoansand molluscs the heterozoan associ ation james 1997 is dominated byforamol facies lees and buller 1972 over the entire shelf area withthe typical absence of allochthonousnon skeletal grains some exhumedearly pliocene foraminifers globoro talia margaritae bolli bermudez are also present in the ne fraction lithoclasts are generally rare andmainly consist of serpentinized ultra ma cs and gabbros samplings and rov observationscon rm that modern bioclastic mate rial is continually produced by thebenthic ecosystem that characterizesthis and other atlantic seamountsas temperate carbonate factories detailed bathymetry and subbottompro ling allow us to establish that theaverage thickness of this material doesnot exceed a few metres in the shelfareas and in the pockets theseabstract

alternating crustal architecture in west iberia a review of its significance in the context of ne atlantic rifting the crustal architecture of the western iberian margin is investigated so that the relationship between synrift intra plate segmentation magmatic events and subsequent margin convergence can be discussed in the context of the evolution of the central north atlantic ocean the evidence in this paper indicates distinct crustal architectures for nw iberia lower plate and sw iberia upper plate as shown by 1 the different geometries of the rotated tilt blocks across thinned continental crust 2 the distinct dips of deep crustal detachments and 3 the presence of first order strike slip zones bounding different crustal segments we demonstrate that a switch in the dip direction of the crustal detachment is accomplished by accommodating persistent transcurrent deformation along first order transfer zones and associated uplifted hinges both of which accommodated synrift displacement and post rift margin inversion this alternating architecture explains why recurrent magmatism is conspicuously located on the upper plate margin which we consider to have favoured in discrete pulses the emplacement of magma finally the crustal detachment underlying the upper plate is interpreted to have been reactivated since the eocene as the weak zone accommodating ocean continent convergence and the putative onset of margin subduction supplementary material summary of the three main magmatic events on the onshore and offshore west iberia and their associated occurrences location and magmatism type in relation with the syn to post rift evolution of the margin are available at https doi org 10 6084 m9 figshare c 3591209

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

an experimental study of trace element partitioning between augite and fe rich basalts abstract aluminous titanium rich hedenbergite and fe rich augite were experimentally produced at 1050 1220 c and 0 8 2 2 gpa major element compositions are analogous to clinopyroxene from layered intrusions angrites nakhlites and late stage lunar magma ocean cumulates trace element concentrations in pyroxene and coexisting melt were measured by laser ablation inductively coupled plasma mass spectrometry for rare earth elements ree high field strength elements hfse transition metals and large ion lithophile elements ree hfse and transition metal partition coefficients ds are tightly correlated with the cation abundance of fe on the pyroxene m1 site x fe m1 and also correlated with al on the tetrahedral site x al t parameterized lattice strain models were developed to predict ree and hfse partition coefficients as functions of temperature and pyroxene composition x al t or x fe m1 the parameterized models can be used to calculate ree and hfse partition coefficients for fe rich high ca pyroxene we calculated partition coefficients for clinopyroxene derived from modeled end stage lunar magma ocean cumulate compositions and observe a factor of 3 variation in clinopyroxene melt ree partition coefficients using representative nakhlite clinopyroxene core and rim compositions we calculated partition coefficients and observed a factor of 4 variability in clinopyroxene melt ree partition coefficients hfse partition coefficients are even more sensitive to composition showing variations of a factor of 4 for lunar late cumulates and an order of magnitude for nakhlites the strong dependence of ree and hfse partitioning on composition necessitates careful selection of appropriate partition coefficients for fe rich systems

the jurassic evolution of the africa iberia conjugate margin and its implications on the evolution of the atlantic tethys triple junction abstract the central atlantic north atlantic and maghrebian tethys formed a triple junction during the mesozoic break up of pangea plate reconstructions of this triple junction have concentrated on the atlantic branches for which there are abundant geophysical data over their conjugate margins and associated oceanic domains this is not the case for the maghrebian tethys where both the mesozoic iberian and african margins and the intervening ocean are mostly overthrust by the betic rif tell orogenic systems the resulting lack of constraints on the evolution of the africa iberia system has led most authors to treat the evolution of this system as a secondary problem in the more global atlantic and tethys systems in this paper the evolution of the basins of iberia and northwest africa is discussed and geological constraints are used to propose a new late triassic fit of iberia with north america and africa this late triassic fit and its implications for jurassic evolution are compared with the evolution of the sedimentary basins of the atlantic tethys triple junction and in particular those of the africa iberia margins the geological evidence and the resulting plate reconstructions are consistent with a jurassic evolution of the africa iberia system that is dominated by three major events 1 the early to middle jurassic transition in which oblique rifting gives way to transtensional rifting 2 the middle to late jurassic transition marking the termination of most rifting in the system and interpreted to coincide with exhumation of sub continental mantle and formation of oceanic crust in a setting dominated by transcurrence between africa and iberia and 3 the late jurassic to early cretaceous transition in which the rate of relative motion between and iberia and africa decreased significantly as rifting in the north atlantic gained momentum

the lunar magma ocean reconciling the solidification process with lunar petrology and geochronology abstract the moon is thought to have originated with a magma ocean that produced a plagioclase flotation crust as solidification proceeded ages of anorthositic crust range over at least 200 million years the model for solidification presented here integrates chemical and physical constraints of lunar magma ocean solidification to determine 1 the final thickness of flotation crust generated by a fractionally solidifying magma ocean 2 the timescale of crystallization before plagioclase is a stable phase 3 the timescale of solidification after the formation of the plagioclase flotation crust and 4 the post overturn lunar mantle composition and structure we find that magma oceans of as much as 1000 km depth are consistent with creating an anorthositic crust 40 to 50 km in thickness solidification of the magma ocean prior to formation of the flotation crust may occur on the order of 1000 years and complete solidification would require additional ten to tens of millions of years reconciling these short model timescales with radiometric dates of crustal samples requires either a very late forming moon combined with finding older crustal ages to be incorrect or calling on tidal heating of the crust by the early earth to prolong solidification gravitationally driven overturn of cumulates during tidal heating provides a mechanism for creating the compositions and ages of the lunar mg suite of crustal rocks further we find that upon crystallization the moon likely began with an azimuthally heterogeneous gravitationally stable mantle after magma ocean cumulate overturn this result may help explain the experimentally determined origin of picritic glasses at similar depths but from different source materials

the effect of fluorine on clinopyroxene melt trace element partitioning the impact of fluorine on phase relations and clinopyroxene melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the ree and hfse during crystallisation of alkaline and peralkaline magmas clinopyroxene was grown from three hbox h 2 hbox o saturated synthetic glasses of tephriphonolite to phonolite composition in a rapid quench internally heated pressure vessel at 650 800 circ c and 200 mpa with delta log f hbox o 2 fixed to ca fmq 1 the fluorine content in the charges was varied and produced quenched melts from fluorine free to 1 6 wt f the experiments yield an assemblage of melt fluid sodic clinopyroxene biotite magnetite k feldspar titanite fluorite and hiortdahlite a na ca ti f sorosilicate addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene relative to fluorine free compositions experiments with 1 6 wt fluorine in the melt show a strong decrease in clinopyroxene melt partition coefficients for the trivalent ree la dy and y with a lesser decrease for the d mathrm hree and d mathrm hfse 4 the diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase consistent with the formation of ree f complexes and with modifications to the medium range structural environment around hbox hfse 4 ions in the melt an increase in the fluorine content of the melt will thus make the ree and hbox hfse 4 progressively less compatible and therefore available for residual enrichment

eastern segment of the azores gibraltar line central eastern atlantic an oceanic plate boundary with diffuse compressional deformation new seismic reflection images across the eastern segment of the azores gibraltar line west of the collisional area between the african and iberian plates have revealed a complex pattern of compressional deformation involving the mesozoic oceanic lithosphere the compressional deformation developed in a region of slow plate convergence and is diffused at different lithospheric levels across an area spanning 200 km from the gorringe ridge to the seine plain the convergence between the african and iberian plates has been active since tertiary time and our results indicate that no subduction zone exists across this part of the plate boundary

a chemical model for generating the sources of mare basalts combined equilibrium and fractional crystallization of the lunar magmasphere abstract it is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early moon wide magma ocean or magmasphere however the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal crystal and crystal liquid separation in a convecting magma chamber crystal liquid and crystal crystal separations were similarly inefficient during evolution of the lunar magma ocean lmo allowing for the trapping of interstitial liquid and entrainment of a small proportion of less dense plagioclase into the denser mafic cumulate mush indeed petrography of lunar highlands samples demonstrates this for anorthosites with 1 10 olivine the residual liquid after 80 90 crystallization was very evolved in fact kreepy and even in small proportions 1 5 would have a noticeable effect on the trace element chemistry of melts generated from these cumulates this trapped residual liquid would elevate total ree abundances in the cumulate pile while synchronously deepening the already negative eu anomaly essentially this trapped liquid will make the cumulate more fertile for melting to generate both kreep basalt and mare basalt magmas plagioclase entrained in the mafic cumulate pile adds an essential al component to the high ti basalt source and will moderate the requisite negative eu anomaly in the cumulate early in the evolution of the lunar mantle when the lmo still was largely liquid it is likely that vigorous convection was an important factor in crystallization such convection would allow crystals to remain suspended and in equilibrium with the lmo liquid for relatively long periods of time this extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust previous authors have proposed a three component model for the evolution of high ti mare basalt source regions this model includes kreep early olivine rich high mg cumulates and late ilmeniterich low mg cumulates in various proportions however we propose a model for high ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions this model for the high ti source includes trapped instantaneous residual liquid tirl 1 3 and entrainment of a small 2 5 proportion of plagioclase into the late stage cumulate pile in order to account for both the observed al compositions and trace element characteristics of high ti mare basalts melting of this relatively shallow ilmenite and clinopyroxene bearing late stage cumulate can generate high ti mare basalt magmas furthermore we are in agreement with other workers that only through a process of nonmodal melting will the high ti values for the parent magmas be realized large scale convective overturn of the cumulate pile and mixing of kreep with early and late stage cumulates is not required however localized overturn of the upper tenth of the cumulate pile is likely and in fact required to achieve an appropriate major element balance for the high ti mare basalt source region

active in situ disaggregation of oceanic crust and mantle on gorringe bank analogy with ophiolitic massives the cyagor ii cruise may 1981 using submersible cyana sp 3000 on gorringe bank off south west portugal confirmed the scheme proposed at the end of cyagor i cruise1 2 that during the middle cretaceous 110 myr bp gorringe bank an old transform fault of triassic liassic age appears to have tilted to the north east leading to the exposure of a section of oceanic mantle and crust due to its location at the boundary between european and african plates gorringe bank suffered tectonic reactivation and uplifting during compressive and shearing type events that occurred at the cretaceous tertiary boundary and during the tertiary eocene miocene during the 12 dives of the cyagor ii cruise fig 1 phenomena linked to the disintegration of serpentinites and gabbros in the deep sea domain have been studied in situ with more detail than before3 such processes have been suggested from studies on samples dredged or cored in oceans or from observations in ophiolitic belts but direct observation had not enabled the magnitude of such phenomena to be measured the observations on gorringe bank suggest that the processes described here are active in many oceanic sites they help us to understand better the significance of many ophiolitic detritic beds more or less closely associated with pelagic sediments we now compare the observations made at sea especially on gorringe bank with similar observations in ophiolitic zones in mountain belts which suggests that the ophiolite complexes have formed in a similar way to the breccias seen on the ocean floor

repeated events of hardground formation and colonisation by endo epilithozoans on the sediment starved pontine continental slope tyrrhenian sea italy abstract rock samples dredged offshore the pontine archipelago 400 2000 m water depth were studied by palaeontological analyses of planktic foraminifera and endo epilithozoan faunas colonising fe mn crusts and by petrographic analyses of volcanic samples palaeontological and petrographic analyses have revealed a complex plio pleistocene depositional history controlled by hemipelagic sedimentation and volcanic activity with episodic non depositional phases the sedimentary deposits are associated with ferromanganese hardgrounds more or less colonised by endolithic and epilithozoan organisms the presence of foraminifera typical of tropical and subtropical waters such as globigerinoides ruber globigerinoides trilobus pulleniatina obliquiloculata and sphaeroidinella dehiscens indicates that the micritic deposition occurred during warm climatic periods whilst micrite cementation and hardground formation are here attributed to colder climates the depositional processes recognised on the pontine continental slope reveal a sediment starved condition that locally resemble those of seamount environment the slope sedimentation found in the pontine archipelago can be helpful in the interpretation of ancient deposits from seamounts and continental slope depositional environments characterised by hardground and hiatuses

revised lower palaeogene lithostratigraphy for the outer moray firth north sea abstract the lower palaeogene lithostratigraphy of the outer moray firth has been revised using wireline log and biostratigraphic data a two fold subdivision into the montrose and moray groups is recognized the montrose group consists of the maureen and lista formations whereas the younger moray group contains the sele balder dornoch and beauly formations the lista formation contains the arenaceous andrew and balmoral members separated by the tuffaceous glamis member the dornoch and beauly formations are prograding shelfal sandstone units that pass basinwards into the age equivalent dominantly argillaceous sele and balder formations the sele formation contains the forties and cromarty members which are locally developed sand rich units stratigraphic analysis has led to the identification of a number of consistently developed high gamma log markers that are interpreted as condensed mudstone horizons these markers are believed to record clastic sediment starvation during periods of maximum marine transgression they often have a strong biostratigraphic fingerprint coinciding with influxes of rich concentrations of microfossils and microflora they are interpreted as the product of synchronous regional transgressive events and have been an important aid to the development of a confident lithostratigraphic framework for the lower palaeogene of the outer moray firth

trace and minor elements in minerals of nakhlites and chassigny clues to their petrogenesis abstract ion microprobe measurements of ree rare earth element and other selected trace and minor element concentrations were made in clinopyroxene olivine feldspar and chlorapatite in chassigny and the three nakhlites nakhla governador valadares and lafayette in chassigny we have found pigeonite occurring in a poikilitic habit it is the first reported occurrence of this mineral in this meteorite the most significant implication of this finding is that estimates of the major element composition of the chassigny parent melt will have to accommodate pigeonite as a primary mineral crystallizing before plagioclase chlorapatite contains the highest ree abundances in all meteorites studied here the ree patterns of chlorapatite plagioclase and augite in nakhlites and chassigny are similar although the downward trends of ree patterns are somewhat steeper for these minerals in chassigny than in the nakhlites pyroxenes in nakhlites and chassigny are extensively zoned in rees and other trace and minor elements such as y zr ti and al which appear to be resistant to diffusive reequilibration thus primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystallization histories the calculated ree patterns of the parent melts of nakhlites and chassigny are lree enriched and parallel to their whole rock ree patterns indicating that once crystal accumulation occurred the intercumulus trapped melt fractionally crystallized in an essentially closed system and that infiltration i e metasomatism by lree enriched magmas need not be invoked to explain the lree enrichment in bulk samples of these meteorites fractional crystallization model calculations for nakhla also confirm these conclusions the distributions of trace and minor elements in augites in nakhlites and chassigny suggest that all three nakhlites could be samples from different horizons of the same lithologic unit but that chassigny was not comagmatic with the nakhlites