the neogene recent hatay graben south central turkey graben formation in a setting of oblique extension transtension related to post collisional tectonic escape structural data and a regional tectonic interpretation are given for the ne sw trending hatay graben southern turkey within the collision zone of the african arabian and eurasian anatolian plates regional gps and seismicity data are used to shed light on the recent tectonic development of the hatay graben faults within upper cretaceous to quaternary sediments are categorized as of first second and third order type depending on their scale location and character normal oblique and strike slip faults predominate throughout the area the flanks of the graben are dominated by normal faults mainly striking parallel to the graben that is 045 225 in contrast the graben axis exhibits strike slip faults trending 100 200 together with normal faults striking 040 060 and 150 190 a subset strikes 110 130 similarly orientated normal faults occur throughout upper cretaceous to pliocene sediments whereas strike slip faults are mostly within pliocene sediments near the graben axis stress inversion of slickenline data from mostly pliocene sediments at ten suitable locations all near the graben axis show that 3 directions minimum stress axis extension direction are uniform in the northeast of the graben but orientated at a high angle to the graben margins more variable 3 directions in the southwest may reflect local block rotations during miocene times the arabian and anatolian plates collided forming a foreland basin associated with flexurally controlled normal faulting during the late miocene there was a transition from extension to transtension oblique extension the neotectonic hatay graben formed during the plio quaternary in a transtensional setting in the light of modern and ancient comparisons it is suggested that contemporaneous strain was compartmentalized into large scale normal faults on the graben margins and mainly small scale strike slip faults near the graben axis overall the graben reflects plio quaternary westward tectonic escape from a collision zone towards the east to a pre or syn collisional zone to the west in the mediterranean sea

influence of pre existing pervasive fabrics on fault patterns during orthogonal and oblique rifting an experimental approach abstract pervasive mechanical anisotropy e g metamorphic foliation in the pre rift basement rocks often appears to control the orientation of new faults in natural rift systems and thus tend to influence formation of fault bounded sedimentary basins however the extent to which this influence works in nature has remained debatable and contrasting ideas exist regarding the issue of structural inheritance in natural rifts like the east african rift system earlier experimental works have investigated different kinematic aspects of faulting in simulated orthogonal and oblique rift systems including the role of a crustal scale pre existing zone of weakness on continent scale rifting these experiments have tested the influence of discrete pre existing weak zones e g pre existing faults or an ancient orogenic belt on localization of subsequent rifting but have not considered the control of pervasive fabric anisotropy e g metamorphic foliation etc in pre rift basement rocks on the fault patterns during rifting here we present a series of simple roughly scaled analog model experiments where pervasive strength anisotropy in the basement was created by brushing plaster of paris on a viscous pitch substratum this way a transverse anisotropy of tensile strength was generated in the plaster of paris layer due to thickness variation across the brush marks the basement was overlain by a dry sand pack simulating weak sedimentary rocks overlying a foliated crystalline basement orthogonal and oblique extension of the models at various angles and with differently oriented strength anisotropy exhibited the control of the anisotropy on the orientations of new faults in the sand layer in general anisotropy could significantly control the fault pattern only when it was at a high angle 45 with respect to the maximum instantaneous horizontal stretching direction e h1 otherwise they largely disregarded the anisotropy and formed orthogonal to e h1 faults never formed orthogonal to the displacement vector d in oblique rifting unless the fault controlling anisotropy was perpendicular to d faults strongly following the anisotropy were much longer than those not following that trend faults oblique to the anisotropy were short segmented and tortuous in nature and mainly grew by segment linkage the experimental results support many field observations made in the east african rift system and are likely to be applicable to other rift systems like gondwana basins of india where field data on such structural inheritance are meager

evaluation of the applicability of acid leaching for the 238u 230th internal isochron method abstract the 238 u 230 th internal isochron method has great potential for determining eruption ages of young volcanic materials if the separated groundmass phases have a sufficiently wide range of u th ratios we examined the fractionation behavior of u and th in aged i e 0 5 ma basaltic samples by 6 m hydrochloric acid leaching to evaluate the applicability of acid leaching treatment for the 238 u 230 th internal isochron method acid leaching of aged basaltic rocks in 238 u 234 u 230 th secular equilibrium at the bulk rock scale results in 230 th 238 u and 234 u 238 u radioactive disequilibria for both leachates and residues these radioactive disequilibria can be explained by redistribution of 234 th parent of 234 u and 230 th between acid soluble and acid resistant phases due to recoil the number of 230 th atoms redistributed by recoil can be calculated by using a mass conservation equation for 234 u atoms and by the relative amount of recoiled 230 th and 234 th the latter proportional to the kinetic energy of the recoiled nuclide when the fraction of daughter nuclide 234 u remaining in either the residue or leachate after recoil redistribution of 238 u is large enough 95 the corrected 230 th 238 u values of leachate and residue show radioactive equilibria this result demonstrates that preferential fractionation between u and th does not occur during acid leaching for basaltic samples if there is no selective etching of the recoil track this study implies that acid leaching can be used in conjunction with the 238 u 230 th internal isochron method for dating young volcanic rocks by evaluating the degree of the recoil redistribution of 234 u

alkenone molecular stratigraphy in an oceanic environment affected by glacial freshwater events the sedimentary record of a ratio uk37 of long chain c37 unsaturated alkenones is a useful indicator of glacial interglacial climatic change in the late quaternary northern gulf of mexico where a planktonic foraminiferal 18o caco3 record is complicated by meltwater and or fluvial events williams and kohl 1986 application of a laboratory temperature calibration of the uk37 ratio prahl and wakeham 1987 to a pigmy basin hydraulic piston core record deep sea drilling project core 619 suggested that the minimum glacial surface mixed layer sml temperature was 8 1 c colder than the holocene high sml temperature of 25 6 0 5 c this predicted glacial interglacial temperature difference was significantly larger than the differences predicted by either the foraminiferal 18o or foraminiferal assemblage temperature methods 0 8 2 0 c this large difference may be caused by local prymnesiophyte assemblage changes in response to climatically induced hydrographic changes interglacial periods may be dominated by pelagic prymnesiophyte assemblages while glacial periods may be dominated by neritic assemblages a combined mechanism of both climatically varying prymnesiophyte species assemblage and depth of alkenone biosynthesis may account for the large difference between the temperature methods

the mid proterozoic mporokoso group of northern zambia stratigraphy sedimentation and regional position abstract the mporokoso group formerly plateau series pro parte redefined is a sequence of fluvial aeolian and lacustrine sedimentary rocks consisting predominantly of sandstones with conglomerate mudstone and tuff intercalations the group overlies granites and volcanics of the crystalline basement of the bangweulu block in northern zambia dated at ca 1820 ma and is intruded by the lusenga syenite dated at 1134 ma it was probably deposited during a period close to the older age limit the source area of clastics was to the south of the sedimentary basin and the ubendian belt did not contribute sedimentary material to the mporokoso group which is subdivided into four formations the older subdivisions lower and upper plateau series and the abercorn sandstone were established in a non typical local area and cannot be followed across the whole basin and their use is discontinued similarities of the mporokoso group and the coeval waterberg group of southern africa are discussed the sedimentary basins of both groups have a similar relation to centres of extrusive volcanism preceding subsidence

assimilation and fractional crystallization controlled by transport process of crustal melt implications from an alkali basalt dacite suite from rishiri volcano japan mechanisms of fractional crystallization with simultaneous crustal assimilation afc are examined for the kutsugata and tanetomi lavas an alkali basalt dacite suite erupted sequentially from rishiri volcano northern japan the major element variations within the suite can be explained by boundary layer fractionation that is mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor systematic variations in sio2 correlate with variations in the pb sr and nd isotopic compositions of the lavas the geochemical variations of the lavas are explained by a constant and relatively low ratio of assimilated mass to crystallized mass r value in the magma chamber in which the kutsugata and tanetomi magmas evolved a strong thermal gradient was present and it is suggested that the marginal part of the reservoir was completely solidified the assimilant was transported by crack flow from the partially fused floor crust to the partially crystallized floor mush zone through fractures in the solidified margin formed mainly by thermal stresses resulting from cooling of the solidified margin and heating of the crust the crustal melt was then mixed with the fractionated interstitial melt in the mushy zone and the mixed melt was further transported by compositional convection to the main magma causing its geochemical evolution to be characteristic of afc the volume flux of the assimilant from the crust to the magma chamber is suggested to have decreased progressively with time proportional to t 1 2 and was about 3 10 2 m year at t 10 years and 1 10 2 m year at t 100 years it has been commonly considered that the heat balance between magmas and the surrounding crust controls the coupling of assimilation and fractional crystallization processes i e absolute value of r however it is inferred from this study that the ratio of assimilated mass to crystallized mass can be controlled by the transport process of the assimilant from the crust to magma chambers

climatically controlled marker layers in santa barbara basin sediments and fine scale core to core correlation varved sediments with annual layering can provide a detailed history of environmental conditions of marginal ocean basins and of lakes to reconstruct this history requires fine scale correlation and correct age assignment in the santa barbara basin a combination of x radiography water content profiles and microfossil stratigraphy yields correlations of the desired resolution x radiography permits identification of individual layers 1 2 mm thick that differ in porosity or pore water content and hence x ray absorption microfossils and sedimentological parameters provide the clues to the presence for el nino years e g in a d 1984 1959 and 1942 varves allowing the tie in to absolute years within this century el nino layers once identified are readily recognized on the radiographs and within the porosity profiles counting of laminae in x radiographs and core to core correlation via water content then allow establishing age scales for other cores a prominent shell layer in the first half of the 19th century is dated at a d 1838 this event macoma i is of supraregional significance

molecular stratigraphy of the santa barbara basin comparison with historical records of annual climate change abstract the molecular record of 20th century climatic variability in laminated sediments of the santa barbara basin has been compiled distributions and abundance profiles of alkenones and selected sterols reveal annual oscillations in sea surface temperatures and phytoplankton productivity positive excursions in alkenone temperature profiles coincide with all major 20th century el nino events recording oceanic warming downcore abundance profiles for dinosterol brassicasterol cholesterol and alkenones reflect productivity trends in phytoplankton and illustrate the comparative resilience of their molecular records series of subsurface concentration maxima for dinosterol a dinoflagellate derived sterol appear synchronous with regional historical reports of dinoflagellate blooms

oblique slip deformation in extensional terrains a case study of the lakes tanganyika and malawi rift zones the east african rift system ear is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins the three dimensional structure of parts of the ear has been recently elucidated by a multifold seismic mfs survey of lakes tanganyika and malawi project probe analysis of fault geometries displayed on the probe mfs data coupled with the more extensive 28 khz echosounder data has improved understanding of the geometry and kinematics of the linked fault systems that underlie the lakes in particular it has been recognized that profiles across i e at high angles to the rift elongation commonly display fault geometries that are not readily retrodeformable there must therefore be significant displacement out of the plane of what would conventionally be regarded as dip lines that is perpendicular to rift elongation and parallel to the tectonic transport or extension direction careful determination of fault plane dip and the dip of synrift reflectors in the hanging wall demonstrates that the dip directions of the shallowest faults and the steepest hanging wall sediment dips are oriented either nw or se this defines the direction of maximum fault block rotation and therefore the direction of extension or tectonic transport as nw se oblique to the trend of the regional rift axis and to the apparent strike of many of the major border faults in the survey area the border faults and many other faults imaged on the probe mfs data must therefore have significant components of strike slip motion and the tanganyika and malawi rift zones have undergone extension oblique to and not perpendicular to their axes near dip slip normal faults steeply dipping oblique slip faults and subvertical strike slip or transfer faults have all been recognized in a complex linked fault system that accomplishes the extension the overall orientation of the rift lakes and their internal segmentation are influenced by major preexisting subvertical fault zones within the basement complex accommodation zones act principally as transfer zones that allow switches in gross polarity of the border fault system the detailed geometries of the accommodation zones result from the specific relations between juxtaposed half graben and the strike and internal geometry of the influencing basement structure the variations in fault geometry subsidence water depth and basin or rift morphology can be better explained by an oblique slip extensional model influenced by basement structures than by a simple orthogonal extensional model

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

a multiscale approach to model the anisotropic deformation of lithospheric plates the association of experimental data showing that the plastic deformation of olivine the main constituent of the upper mantle is highly anisotropic and the ubiquitous seismic anisotropy in the upper mantle which indicates that olivine crystals show coherent orientations over scales of tens to hundreds of kilometers implies that the long term deformation in the upper mantle is anisotropic we propose a multiscale approach based on a combination of finite element and homogenization techniques to model the deformation of a lithospheric plate while fully considering the mechanical anisotropy stemming from a strain induced orientation of olivine crystals in the mantle this multiscale model explicitly takes into account the evolution of crystal preferred orientations cpo of olivine and of the mechanical anisotropy during the deformation we performed a series of numerical experiments simulating the uniaxial extension of a homogeneous 100 olivine but anisotropic plate to test the role of the olivine cpo on the plate mechanical behavior and the link between cpo and mechanical anisotropy evolution even for this simple solicitation different orientations and intensity of the initial olivine cpo result in variable plate strengths and deformation regimes a plate with an initial cpo where the olivine 100 and 010 axes are concentrated at 45 degrees to the extension direction has high resolved shear stresses on the easy 010 100 and 001 100 slip systems of olivine this results in low strength and in deformation by transtension plates with an initial cpo where the maximum of 100 axes is parallel or normal to the extension direction show a high initial strength isotropic plates have an intermediate behavior the progressive rotation of olivine 100 axes toward the imposed stretching direction results in hardening in all models except in those characterized by an initial concentration of olivine 100 axes normal to the imposed extension in which softening is followed by hardening

aspects of the kinematic history and mechanisms of superposition of the proterozoic mobile belts of eastern central africa northern malawi and southern tanzania abstract commonly the proterozoic tectonic evolution of the central african basement is subdivided into three major events 1 the ubendian 2300 1800 ma 2 the irumide 1350 950 ma and 3 the pan african 900 450 ma orogenic cycles relics of the ubendian event are granite intrusions and possibly an amphibolite to granulite facies metamorphism the ubendian orogeny was followed by deposition of clastic sediments muva supergroup in northern malawi these sediments and the underlying basement were then thrust to the east southeast southeast during the irumide orogeny horizontal shearing along subvertical zones i e the ubendian belt was associated with subhorizontal crustal shortening across the irumide belt metamorphism reached the amphibolite facies in the basement and the upper greenschist facies in the mafingi sediments subsequently the metamorphic parageneses were retrograded during ongoing shearing in neo proterozoic time clastic sediments of the bukoban buanji and upper kundelungu groups were deposited the basement of easternmost malawi and southern tanzania was in turn folded and thrust to the northeast onto the bukoban sediments early pan african orogeny deformation occurred under greenschist facies conditions in northern malawi and southern tanzania and reached amphibolite grade in central malawi eventually conjugate sets of strike slip zones cut through the basement strike slip deformation occurred along the retrograde path of metamorphism late pan african orogeny the structural investigation of the proterozoic mobile belts shows that these belts represent zones of structural weakness between stable cratons and are very susceptible to various kinds of reactivation at the subvertical ubendian belt no structures could be identified which can unequivocally be attributed to the ubendian orogeny major tectonism in the ubendian belt is expressed by lateral shearing during the later irumide orogeny in the course of the pan african orogeny and also during cenozoic rifting the ubendian belt was again reactivated

crustal shear zones and thrust belts their geometry and continuity in central africa the precambrian orogenic belts of africa are often defined by ductile shear zones which developed in response to large displacements and which mark orogenic fronts between mobile and stable parts of the crust they are thought to represent the major crustal reflectors seen by seismic reflection profiling in younger orogenic belts these orogenic fronts are connected by shear zones that transfer displacement or accommodate different displacements between orogenic segments smaller shears within an orogenic belt occur as a result of differential movements these shear zones are seen to pass from flat lying to steep structures and may have a thrust or strike slip sense they compare with the staircase trajectories characteristic of foreland thrust belts in common with thrust belts the geometry of the shear zones can be used to estimate displacement direction as can regional extensional fabrics developed in the associated high strain tectonites central africa has been previously described as a complex network of late proterozoic mobile belts the recognition of similar displacements and time equivalence in these belts allows their reinterpretation in terms of a linked thrust and strike slip shear zone system an example is the damaran lufilian zambezi and ukingan system these orogenic belts share a similar displacement picture and broad time equivalence and were apparently linked in a lower crustal shear zone of continental dimensions this shear zone system appears to have developed under a single tectonic framework

timing of exhumation of a high pressure mafic granulite terrane of the paleoproterozoic ubende belt west tanzania abstract the ubendian belt west of the archean tanzanian craton is known as a prominent crustal structural weakness the belt is composed of different terranes set in a nw se oriented lateral shear pattern sharply outlined by steeply inclined inter terrane shear fault zones its predominant mafic terranes ubende and mbozi display different and more complex evolutionary characteristics than their adjacent felsic terranes ufipa metamorphic studies reveal that mafic and ultramafic high pressure granulites and rare eclogites subsist as rigid blocks and lenses within the highly sheared amphibolite facies country rock p t studies suggest two succeeding decompressional stages for the uplift of the high pressure mafic granulite terranes the second stage being identified on high pressure blastomylonites included in the latter are amphibolites mafic tectonites for which barroisite constitutes a p t marker near the southwestern boundary of the nw se oriented mafic terranes these barroisite bearing tectonites display a pronounced extensional shear banding considering that this tectonite has been formed during the second stage of uplift of the ubende high pressure mafic granulites timing of that stage is pursued ar ar stepwise heating analyses on different barroisite separates from this mafic tectonite have yielded a weighted average cooling age of 1848 6 ma for the argon fractions released at intermediate temperatures this age falls in the time range 1950 1850 ma associated with the second phase of the evolution of the ubende belt characterized by amphibolite facies metamorphism the isotopic composition of argon released at higher temperature steps from compositionally different phases in the separates yields apparent ages tentatively considered only as indicative for a possible older paleoproterozoic event based on geochronological and structural data the uplift of the high pressure granulite ubende terrane is interpreted as related to a late paleoproterozoic dextral lateral shear deformation in ductile amphibolite facies conditions this appears to be associated with the emplacement of granites subsequent proterozoic strike slip reactivation of the paleoproterozoic ubende shear belt did not fundamentally contribute to the near surface exhumation of the high pressure mafic granulites

backflow and margin parallel shear within an ancient subduction complex lineated and foliated tectonites of the teslin suture zone yukon represent the deep seated part of a west dipping subduction complex that formed seaward of western north america in permian triassic time the tectonites were progressively underplated onto the hanging wall plate during b type subduction with continued subduction and underplating the boundary between the hanging wall and subduction channel migrated oceanward resulting in growth of the hanging wail plate at depth these tectonites preserve a progressive record of backflow dextral margin parallel shear and downflow within the subduction channel early jurassic a type subduction of the leading edge of western north america resulted in eastward overthrusting of the tectonites as a coherent block onto north american strata these tectonites provide empirical evidence that backflow margin parallel displacement and downflow can all occur within the deeper parts of subduction zones

biogeochemistry of lipids in surface sediments of the peru upwelling area at 15 s abstract organic carbon lycopane a monoene of 2 6 10 trimethyl 7 3 methyl butyl dodecane c37 alkenones and 20 5 and 22 6 polyunsaturated fatty acid concentrations in two box cores from the peru continental margin area at 15 s in 90 and 268 m water depth have depth profiles that are synchronous and appear to indicate an historical record of fluctuating higher productivity lower productivity enso periods the u37k ratio calculated using c37 2 and c37 3 alkenones in the 268 m water depth core in the center of the upwelling zone varies with depth in the core in a manner consistent with the u37k functioning as an historical recorder of euphotic zone temperature concentrations and cpi for n alkanes c25 c31 are reported for both cores a c27 anthrasteroid has been identified and quantified in the 268 m water depth core

structural controls on orogenic au mineralization during transpression lupa goldfield southwestern tanzania au mineralization in the western lupa goldfield southwestern tanzania was associated with transpression and reverse sinistral slip along a network of steeply s dipping shear zones with non andersonian geometries slip was accommodated by 1 frictional failure and sliding during emplacement of quartz au bearing veins and 2 crystal plasticity and fluid assisted diffusive mass transfer the kenge mineral system is situated along a nw se trending shear zone and is characterized by 10 m thick au bearing fault fill veins hosted by well developed phyllosilicate rich mylonites the broadly contemporaneous porcupine mineral system is situated along an ene wsw to e w trending shear zone which is characterized by narrow discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith au mineralization at porcupine occurs within steeply dipping fault fill and subhorizontal extension oblique extension veins three dimensional frictional reactivation theory provides a self consistent explanation for the different vein styles at kenge and porcupine and extends the classic fault valve model to the general case of oblique slip along multiple arbitrarily oriented shear zones analysis of the differential stress required for frictional reactivation suggests the following 1 the kenge shear zone was intrinsically weaker than the porcupine shear zone consistent with the lack of well developed mylonites at porcupine and 2 frictional reactivation of the kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures whereas frictional reactivation of the porcupine shear zone occurred under near lithostatic fluid pressures we hypothesize that near lithostatic pore fluid pressures relieved effective normal stresses at grain grain contacts helping to preserve intragranular and fracture porosity at the porcupine orebody as such these pore spaces may be important microstructural sites for au mineralization low effective normal stresses can also explain the poorly developed phyllosilicate rich mylonites and limited degree of shear zone weakening at porcupine

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

magma plumbing system of the 2000 eruption of miyakejima volcano japan during the 2000 eruption at miyakejima volcano two magmas with different compositions erupted successively from different craters magma erupted as spatter from the submarine craters on 27 june is aphyric basaltic andesite 5 vol phenocrysts 51 4 52 2 wt sio2 whereas magma issued as volcanic bombs from the summit caldera on 18 august is plagioclase phyric basalt 20 vol phenocrysts 50 8 51 3 wt sio2 the submarine spatter contains two types of crystal clots a type and a type andesitic type the phenocryst assemblages plagioclase pyroxenes and magnetite and compositions of clinopyroxene in these clots are nearly the same but only a type clots contain ca poor plagioclase an 70 we consider that the a type clots could have crystallized from a more differentiated andesitic magma than the a type clots because feo mgo is not strongly influenced during shallow andesitic differentiation the summit bombs contain only b type basaltic type crystal clots of ca rich plagioclase olivine and clinopyroxene the a type and b type clots have often coexisted in miyakejima lavas of the period 1469 1983 suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas according to the temporal variations of mineral compositions in crystal clots the andesitic magma became less evolved and the basaltic magma more evolved over the past 500 years we conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period causing the andesitic magma to become less evolved with time the mineral chemistries in crystal clots of the submarine spatter and 18 august summit bombs of the 2000 eruption fall on the evolution trends of the a type and b type clots respectively suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters the 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber as the collapse occurred it may have caused disruption of crustal cumulates which then contaminated the ascending deeper basalt thus porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera the historical magma plumbing system of miyakejima was probably destroyed during the 2000 eruption and a new one may now form

the organic geochemistry of peru margin surface sediments i a comparison of the c37 alkenone and historical el nino records abstract the alkenone u k 37 paleothermometer has potential as a sedimentary marker for el nino southern oscillation enso events in the peru upwelling regime we assessed this potential by comparing the historical enso record with detailed u k 37 profiles for 210 pb dated cores from the peru margin oxygen minimum zone omz sediments with the greatest potential for holding a sedimentary u k 37 record of el nino events are sediments from the center of the omz sectioned at intervals the yearly sedimentation rate the u k 37 signals of individual el nino events were substantially attenuated in the sediments we examined and periods of frequent enso activity e g 1870 1891 were more readily identified than isolated enso events in periods of less frequent enso activity detailed depth profiles of the c 37 alkenones in a core from 253 m o 2 k 37 values for the five 2 mm sections from 0 1 cm suggests that the u k 37 may be unaffected by the alkenone loss correlation between the c 37 alkenone concentration profiles in two cores from 15 s collected nine years apart are consistent with the use of these compounds for molecular stratigraphy

role of late jurassic intra oceanic structural inheritance in the alpine tectonic evolution of the monviso meta ophiolite complex western alps

sand box modelling of basement controlled transfer zones in extensional domains the interaction modalities of transfer zones connecting rift segments may be influenced by several factors amongst these the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies sand box models were made to investigate how the orientation geometry and kinematics of transfer zones depend upon pre existing basement anisotropies analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults strike slip faults are present inside the transfer zone provided that angle between the extension direction and the axis of the transfer zone is less than 50 narrower transfer zones striking oblique to the extension direction occur for angle between the direction perpendicular to the extension direction and the direction of the basement anisotropy less than 90 wider transfer zones subparallel to the extensional direction occur for 90 increasing the overstep induces narrower transfer zones striking subparallel to the extension direction similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts the comparison between experimental and natural data shows how the geometry kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies transfer zones along inherited basement structures set at lower angles to the extension direction and display lower aspect ratios than transfer zones where inherited anisotropies are absent

case studies of plagioclase growth and residence times in island arc lavas from tonga and the lesser antilles and a model to reconcile discordant age information abstract the time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved however crystals and liquids can have different ages leading to apparently contradictory information from different dating methods two case studies are presented from island arc lavas that highlight such dichotomies plagioclase crystal size distributions and u th ra isotope data are presented for lavas from late and metis shoal in tonga and from soufriere st vincent in the lesser antilles in all three lavas mineral data yield apparent u th ages of 30 50 kyr yet 226 ra excesses are preserved in both the minerals and the whole rocks the crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations furthermore a cumulate from soufriere yields the same u th age as its host lava on 226 ra ba versus time evolution diagrams curves for melts in equilibrium with plagioclase provide apparent ages of 1700 3300 yr for tonga but for soufriere they are displaced below that of the whole rock our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age the apparent u th ages reflect mixtures of old cores and young rims and indicate a volume integrated age of 10s of kyr for the time for initial crystal growth crystal liquid segregation the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption the 226 ra excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma because crystal growth segregation and entrainment can all appear to occur in 100s to a few thousand years at most the bulk of the u th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches plagioclase growth rates of 3 9 10 13 cm s 1 would reconcile the combined ra th and crystal size distribution data magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times indicating complex phenocryst liquid behaviour beneath island arc volcanoes

the growth of non colinear normal fault systems what can we learn from 3d seismic reflection data abstract many rift zones exhibit a range of fault orientations rather than simple colinear faults that strike orthogonal to the inferred least principal stress the formation of non colinear fault sets has implications for assessing rift zone kinematics as well as determining palaeo stress state in extensional basins using 3d seismic reflection data we deduce the likely mechanisms responsible for the formation of a population of non colinear faults in the maloy slope area of the northern north sea three basement displacing fault populations exist on the maloy slope i large 1 km throw n s striking faults ii smaller

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

recycling of crystal mush derived melts and short magma residence times revealed by u series disequilibria at stromboli volcano abstract the presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts representing the crystalline portion of old magmas recycled in the juvenile material but very little is known about the fate of interstitial liquid hosted within the crystal rich mush i e antemelt u series disequilibria measured in magmas erupted in the past 18 years at stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano despite almost constant major and trace element composition significant variations in isotope ratios are observed 230 th 232 th decreases with time whilst 238 u 232 th 226 ra 230 th and 87 sr 86 sr are different in the two types of magma erupted magma with low phenocryst content lp is erupted as pumices during paroxysm and is thought to belong to a deep reservoir highly porphyritic magma hp is erupted during the normal strombolian activity as scoria and during the effusive events as lavas and it is considered to derive from the former one within a shallow reservoir through degassing driven crystallisation mixing and incorporation of antecrysts the distinct 238 u 232 th of lp and hp magma requires the involvement of a component with high 87 sr 86 sr and 238 u 232 th deriving from older magmas erupted earlier in the volcano history up to 2 5 ka the incompatibility of u and th in major mineral phases limits the possible effect of antecrysts hence requiring the involvement of a u and th rich antemelt the decrease of 226 ra excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old antemelt the variation with time of 230 th 232 th within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to 55 yrs inferred reservoir volume 0 5 km 3 and 2 10 yrs inferred reservoir volume 0 02 0 09 km 3 respectively our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so called steady state volcano such as stromboli we show that while most of the magma is erupted within few years a portion of it made of both crystals antecrysts and residual liquid antemelt can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir the presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes hence favouring their remobilisation

geochemical evolution of a shallow magma plumbing system during the last 500 years miyakejima volcano japan constraints from 238u 230th 226ra systematics abstract in order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes we investigated u series disequilibria of rocks erupted over the past 500 years 1469 2000 ad from miyakejima volcano izu arc japan miyakejima volcanic rocks show 238u 230th 226ra disequilibria with excess 238u and 226ra due to the addition of slab derived fluids to the mantle wedge basaltic bombs of the 2000 ad eruption have the lowest 230th 232th ratio compared to older miyakejima eruptives yielding the youngest 238u 230th model age of 2 kyr this reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath miyakejima occur episodically on a several kyr timescale in the last 500 years miyakejima eruptives show 1 a vertical trend in a 230th 232th 238u 232th diagram and 2 a positive linear correlation in a 226ra 230th 0 1 230th diagram which is also observed in lavas from some of the single eruptions e g 1940 1962 and 1983 ad the variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230th and 226ra but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab derived fluids a much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust subsequently modifying the original magma composition and producing variations in 230th 232th and 226ra 230th 0 ratios via assimilation and fractional crystallization afc the assimilant of the afc process would be a volcanic edifice of previous miyakejima magmatism due to the relatively short timescales involved the interaction between the assimilant and recent miyakejima magmatism has not been recorded by the sr nd pb isotopic systems in such cases th isotopes and 226ra 230th ratio are excellent geochemical tracers of magmatic evolution

long chain unsaturated ketones and esters in living algae and marine sediments abstract series of c37 c39 di and triunsaturated ketones and methyl and ethyl esters of hexatriacontadienoic acid c36 2 have been previously identified in marine sediments and the coccolithophorid emiliania huxleyi these compounds have now been found in four further species of the isochrysidales prymnesiophyta which are not known to bear coccoliths suggesting that they are more widely distributed in living algae than previously thought in addition the ketones have been recognised in sediments of eocene age ca 45 ma which greatly extends their known geological time range and increases their potential as molecular markers for algal input heptatriacontatetraen 2 one c37 4 octatriacontatetraen 3 one c38 4 methyl and ethyl esters of hexatriacontatrienoic acid c36 3 and the ethyl ester of hexatriacontatetraenoic acid c36 4 have also been identified in varying amounts in these algae by gas chromatography and computerised gas chromatography mass spectrometry

re os molybdenite pyrite and chalcopyrite geochronology lupa goldfield southwestern tanzania tracing metallogenic time scales at midcrustal shear zones hosting orogenic au deposits a detailed re os molybdenite pyrite and chalcopyrite geochronology at five shear hosted au occurrences kenge mbenge porcupine konokono and dubwana in the lupa goldfield southwestern tanzania is reported in this paper au occurrences within the lupa goldfield share many geologic similarities with the orogenic au deposit type and are situated within a paleoproterozoic magmatic arc that intruded the archean tanzanian cratonic margin pyrite chalcopyrite molybdenite bearing fault fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest au grades re os sulfide ages are deemed a suitable proxy to constrain the timing of au based on the occurrence of au bearing minerals as inclusions within pyrite and chalcopyrite whereas au bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event molybdenite at kenge occurs as ultrafine disseminations within fault fill veins 1953 6 ma n 3 that possess nominally older weighted average re os ages than molybdenite hosted by stylolite like veins 1937 8 ma n 7 both sample sets are ca 70 m y older than a weighted average re os pyrite age from the mylonitic shear zones at kenge and mbenge 1876 10 ma n 13 which contain fault fill veins and record the timing of mylonitization molybdenite at porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples 1886 6 ma n 4 that are broadly equivalent in age to weighted average re os ages of molybdenite occurring as stylolite like veins 1873 5 ma n 6 and pyrite within oblique extension veins 1894 45 ma n 2 weighted average re os pyrite model ages at konokono 1880 14 ma n 9 and dubwana 1905 25 ma n 2 are also consistent with the ca 1 88 ga event observed at kenge mbenge and porcupine gold occurrences in the lupa goldfield therefore record a protracted hydrothermal history 1 95 1 87 ga comprising at least three temporally distinct hydrothermal events ca 1 95 1 94 and 1 88 ga which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the re os method the sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca 1 88 ga that permitted the transportation and deposition of gold comparison between re os sulfide and high precision u pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism however the range of re os ages argues against an intrusion related deposit model whereby metallogenic fluids are solely derived from an individual intrusion the regional ca 1 88 ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the ubendian orogenic cycle and provides one of earth s earliest temporal links between subduction zone processes and orogenic au deposit formation during the paleoproterozoic

long lived continent ocean interaction in the early proterozoic ungava orogen northern quebec canada the early proterozoic ungava orogen contains evidence for the development of a longlived 80 m y and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc continent collision a magmatic arc was built on 2 00 ga oceanic crust above a north dipping subduction zone between about 1 90 and 1 86 ga development of a south verging continental thrust belt containing units associated with prior rifting of the archean superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca 1 87 ga subduction of the superior province continental crust is inferred to have led to a reversal in subduction polarity a younger south dipping subduction zone is suggested by a suite of 1 85 1 83 ga quartz diorite to granite plutons that intrude the upper plate oceanic crust and the older arc the arc continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement

rapid timescales of differentiation and evidence for crustal contamination at intra oceanic arcs geochemical and u th ra sr nd isotopic constraints from lopevi volcano vanuatu sw pacific abstract the extent and geochemical impact of crustal contamination during magmatic evolution in intra oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained acquiring such information is crucial if meaningful timescales of magma generation and crustal residence beneath volcanoes are to be determined high mgo basalts and differentiates erupted over the last 100 years at lopevi volcano display a strong negative correlation between 87sr 86sr isotope ratio and indices of differentiation e g sio2 even though these lavas have a restricted sr nd isotopic range compared to other vanuatu arc lavas this trend provides compelling evidence for the interaction of rising mafic magmas with primitive sub arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of crustal interaction using u series data quantitative geochemical modelling of whole rock trace element ratios 87sr 86sr isotope compositions and u series data shows that assimilation of a relatively small degree partial melt of 380 ka mafic oceanic crust similar to pacific or indian morb in 87sr 86sr isotopic composition during fractional crystallisation of magma exerts major control on the 230th 232th and 226ra 230th activity ratios of the lavas the incorporation of higher 230th 232th and lower 226ra 230th assimilated material drives compositions closer to secular equilibrium than simple closed system differentiation reducing calculated apparent timescales of closed system differentiation from th isotope composition 104 105 by orders of magnitude modelling suggests that assimilation occurs extremely rapidly at lopevi with timescales for magma generation differentiation and eruption on the order of 102 years

geology and geophysics of the rukwa rift east africa exploration in the rukwa rift using gravity and seismic reflection surveys fieldwork and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the western rift the stratigraphy comprises precambrian basement karroo sandstones shales and coals upper miocene red beds and miocene recent lacustrine and fluvial sediments during miocene recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast the southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the miocene recent cenozoic age rift structures have a dominant nw se and a subordinate n s trend the nw se trend tends to follow a precambrian basement and later karroo structural trend ne sw seismic lines indicate up to 10 km extension of the tertiary section in a direction oblique to the probable e w regional extension direction in the southeastern portion of the basin both karroo and tertiary quaternary sediments expand into the lupa fault zone reaching thicknesses of up to 3 and 7 km respectively tertiary quaternary sediment thicknesses decrease northwestward accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the lake tanganyika rift

the role of structural inheritance in the development of high displacement crustal faults in the necking domain of rifted margins the klakk fault complex froya high offshore mid norway abstract the role of inherited structures during the development of normal faults in continental rifts and proximal domains of passive margins have been extensively studied few studies however have a focus on deciphering the role of inheritance in the development of high displacement 10 km low angle 25 km is present during rifting

u th systematics of dispersed young volcanoes in ne china asthenosphere upwelling caused by piling up and upward thickening of stagnant pacific slab abstract young 300 to 5500years old dispersed intraplate volcanism characterized by small volume of melt and large compositional variability occurred over ne china at jingbohu long gang tianchi and wudalianchi these young volcanic rocks provide ideal opportunities for investigating the mechanism of dispersed continental volcanism using short lived uranium series isotopes we present systematic u series nd sr pb isotope and trace element analyses of the young lavas from ne china jingbohu lavas display variable extents of 230th excesses 10 to 28 and moderately depleted nd isotopic compositions end 1 5 to 3 3 long gang lavas have pronounced 33 to 35 230th excesses and slightly depleted nd isotopic compositions end 0 5 to 0 7 the tianchi lavas display moderate 12 230th excesses and slightly enriched nd isotopic compositions end 1 0 to 1 1 230th enrichments in all these volcanic rocks are uncharacteristic of melts generated by subduction the strongest evidence against the involvement of subduction related fluids is derived from the long gang lavas having more than 30 230th excesses low u th sr th and ba th ratios and positive nb and ta anomalies the fluids released from the subduction of the pacific plate under the eurasian plate might have already been completely lost before the slab reached the mantle beneath ne china we postulate that in addition to the convective circulation process the continuous piling up and upward thickening of stagnant pacific slab in the mantle transition zone may help drive a broad asthenosphere upwelling far from the trench leading to mantle partial melting and the formation of the young volcanoes in ne china in addition our u th disequilibrium data provide constraints on magma evolution time the magma evolution time from low sio2 47 5 basalt to higher sio2 50 4 basalt at long gang is within 10 thousand years 10ka the magma evolution time from trachyte to rhyolite at tianchi volcano is also less than 10ka if the tianchi silicic rocks were evolved from long gang basaltic magmas with minor crustal assimilation then the magma evolution time from basaltic magma to silicic magmas is less than 110 thousand years

architecture of the continental lithosphere the continental lithospheric plates are traversed by numerous large scale disclocations many of which date back to late archean or proterozoic times these major dislocations often extending for 1000 km or more are commonly characterized not only by the presence of fault rocks and by significant misfits between the adjacent crustal blocks but also by the presence of localized basins of deposition or by anomalous igneous suites or mineral deposits although to a first approximation they may be regarded as narrow linear features most consist of complex arrays of faults ductile shear zones and fractures some with associated deformation spread over tens and hundreds of kilometres lineaments that originated as whithin plate transcurrent shear zones or rift systems are generally steep and many of these probably extend down through the base of the crust into lithospheric mantle lineaments that iriginated as sutures along which formerly separated crustal blocks have been welded together are on the other hand sometimes expressions of low angle dislocations not continuous with mantle structures directly beneath the lineament observed at the surface geological records show that displacements or magmatic activity or both along many lineaments were repeated over long periods spanning successive changes in the global tectonic regime modern lineament movements can be fitted to plate tectonic hypotheses however pre phanerozoic movements are more conjectural for example early proterozoic lineaments appear to be associated with more extensive internal deformation within large crustal blocks the siting of fracture systems activated in response to regional plate motions has on occasion been determined by the occurrence of pre existing deep dislocations the scale and longevity of the deep dislocations raise many interesting general questions with respect to the coordination of mantle and crustal activities and to the evolution of the continental lithosphere

regional setting and kinematic features of the needle falls shear zone trans hudson orogen needle falls shear zone is the southern part of a major northeast trending ductile shear system within the paleoproterozoic trans hudson orogen in saskatchewan throughout its exposed length of 400 km the shear zone separates reworked archean continental crust and infolded paleoproterozoic supracrustals of the cree lake zone to the northwest from mainly juvenile paleoproterozoic arc terrains and granitoid plutons of the reindeer zone to the southeast it also defines the northwest margin of the ca 1855 ma wathaman batholith which forms the main protolith to shear zone mylonites although not precisely dated available age constraints suggest that the shear zone formed between ca 1855 and 1800 ma toward the end of peak thermotectonism in this part of the orogen in the needle falls study area shear zone mylonites exhibit varied sequentially developed ductile to brittle fabric features including c s fabrics winged porphyroclasts especially delta type small scale compressional and extensional

the effects of magma replenishment processes on 238u 230th disequilibrium short lived isotope data are increasingly used to investigate magma chamber residence times arc lavas have excellent potential as tracers of magma evolution because their magmas are often out of u series isotope equilibrium and some arc magmas have yielded surprisingly old pre eruption u series ages from mineral separates a model is presented to examine the behaviour of u th isotopes for a range of conditions between closed system and steady state magmatic systems magmas can be maintained out of secular equilibrium for long periods of time by periodic mixing events with new magma batches the latter results in oscillations in 230th 232th consistent with the relatively constant values observed in lavas of different ages from some centres the effects of periodic reinjection of magma and mixing of old and young crystals on u series mineral isochrons are also considered mixtures of crystals produced by either replenished or closed system processes can readily result in linear arrays in many cases it may be valid to interpret these as minimum average residence times for the crystals whereas the liquid residence times calculated from u series data for periodically replenished systems are maximum values

rift nucleation rift propagation and the creation of basement micro plates within active rifts abstract in this contribution we study the dynamics of rift nucleation and the interaction of propagating rift segments we use the east african rift system ears as a case study with special emphasis on the albertine rift system situated within the western branch of the ears with the 5000 m high rwenzori horst a basement block that was captured by two rift segments fieldwork in the rwenzori mountains shows that the range is not only affected by range parallel faults but that major fault sets cut across the rwenzoris in order to understand these complex fault patterns and the dynamics of the process that leads to the capturing of basement blocks within the extending rift system we developed a two dimensional visco elasto plastic spring model the model shows that propagating rift faults commonly capture micro plates with shapes similar to the rwenzori mountains these micro plates rotate during successive rift opening in a clockwise or anti clockwise sense depending on the direction of the rift segment overstep a left lateral overstep as is the case for the albertine rift system produces a clockwise rotation of the associated micro plate indicating that the rwenzori mountains rotate clockwise during the rotation of the micro plate the local stress field at the rift segment tips is perturbed so that the rifts turn 70 to 90 and propagate towards each other to capture the plates completely this perturbed stress field produces an extensional fault set that is oriented almost parallel to the far field extension direction and explains the range cutting faults that we find in the rwenzori mountains the large scale geometry of the rift system seems to be strongly controlled by rigid cratons in the lithosphere leading to a curvature of the western branch of the ears and a sinistral wrench component in the lake albert rift north of the rwenzori mountains capturing of micro plates of various sizes within the rift system is a common phenomenon and leads to strong perturbations of the local stress field during micro plate rotation the resulting structures are normal and strike slip faults that cut through the micro plates and local reverse faults

magma plumbing system for the millennium eruption at changbaishan volcano china constraints from whole rock u th disequilibrium abstract a caldera forming eruption in 946 ce known as the millennium eruption me at changbaishan or baekdusan volcano located on the border between china and north korea was one of the largest explosive eruptions on earth in the last 2000 years in this study we obtained new whole rock sr nd and pb isotopic data and 238u 230th disequilibrium data of representative volcanic products of the me as well as those of a pre me 4 5 ka eruption to understand the evolution of the magma plumbing system related to the caldera forming eruption the volcanic products of the me have sio2 contents ranging from 54 to 75 wt and the whole rock 230th 232th ratios tend to decrease systematically from basaltic trachy andesite through trachyte to comendite the relatively high 230th 232th ratios of the felsic trachytic and comenditic volcanic products suggest that they were formed primarily from fractional crystallization of mafic magmas rather than from partial melting of crustal materials the basaltic trachy andesite sample has a distinctly high 230th 232th ratio and the least radiogenic pb isotopic ratios among me volcanic products suggesting that the mafic magma was recently introduced into the main felsic magma system and is hypothesized to have triggered the eruption the lower 230th 232th ratios of the comendite samples than those of the trachyte samples as well as published u series disequilibrium data suggest that the comendite magmas were produced by mixing old 23 ka and young

mechanisms of inheritance of rift faulting in the western branch of the east african rift tanzania the western branch of the east african rift system is commonly cited as a result of phanerozoic reactivation of the paleoproterozoic ubendian belt in western tanzania geological evidence is provided to show that prominent mechanical anisotropies successively appeared during proterozoic evolution of the precambrian basement and that their different reactivation behavior contributed to the phanerozoic rift pattern the ubende belt 1950 1850 ma is a nw oriented amphibolite facies ductile lateral shear belt in which older 2100 2025 ma and complex granulite facies terranes are included along trend retrograde multiphase sinistral strike slip mylonites developed along the nw oriented ductile shear belt they reflect persistent proterozoic wrench fault reactivation of the latter shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the proterozoic wrench fault reactivation neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone which appears as an elongate and nw trending positive flower structure locally enhanced by late proterozoic contraction phanerozoic rifting is demonstrated by others to occur in three distinct episodes during which the complex rift segment formed upon the multiphase proterozoic wrench fault zone the evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered the proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern which became selectively reactivated by sinistral transpressional ductile brittle mylonites proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults according to the position of these mylonites in the center or in the external parts of their nw oriented neoproterozoic transpression they reactivate as complex and multiphase rift faults or as normal and recent faults respectively the paleoproterozoic nw oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy its reactivation in phanerozoic stress fields is likely dextral oblique transtension considered as a leading mechanism of the pluriphase and nw oriented deep rift basins

rates of thermal and chemical evolution of magmas in a cooling magma chamber a chronological and theoretical study on basaltic and andesitic lavas from rishiri volcano japan rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from rishiri volcano northern japan by dating of these lavas using u th radioactive disequilibrium and c dating methods in combination with theoretical analyses we obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by c dating of charcoals the eruption age of the andesitic lavas was estimated to be 20 2 3 1ka utilizing a whole rock isochron formed by u th fractionation as a result of degassing after lava emplacement because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber the difference of the ages i e 9 kyr is a timescale of magmatic evolution the thermal and chemical evolution of the rishiri magma chamber was modeled using mass and energy balance constraints as well as quantitative information obtained from petrological and geochemical observations on the lavas using the timescale of 9 kyr the thickness of the magma chamber is estimated to have been about 1 7 km the model calculations show that in the early stage of the evolution the magma cooled at a relatively high rate 40 18c year and the cooling rate decreased with time convective heat flux from the main magma body exceeded 2w m when the magma was basaltic and the intensity diminished exponentially with magmatic evolution volume flux of crustal materials to the magma chamber and rate of convective melt exchange compositional convection between the main magma and mush melt also decreased with time from 0 1m year to 10 3 m year and from 1m year to 10 2 m year respectively as the magmas evolved from basaltic to andesitic compositions although the mechanism of the cooling i e thermal convection and or compositional convection of the main magma could not be constrained uniquely by the model it is suggested that compositional convection was not effective in cooling the main magma and the magma chamber is considered to have been cooled by thermal convection in addition to heat conduction

structure and stratigraphy of the rukwa rift abstract combining recently acquired multifold seismic data with well and gravity information and field mapping a comprehensive picture of the structure and stratigraphy of the rukwa rift has emerged the rukwa rift lies between the tanganyika and nyasa malawi rifts in the western branch of the east african rift system in southwest tanzania the rukwa rift is a nw trending half graben basin that is 350 km long and 40 km wide unlike the neighboring tanganyika and nyasa rifts there is no evidence of half graben polarity reversals in the rukwa rift the nw trending boundary fault system lies on the northeastern side of the basin and comprises a series of listric faults most internal faults also show listric forms and trend n s oblique to the boundary fault the basal sedimentary section is the permo triassic karroo sequence this is overlain by the red bed sandstone sequence in which both mesozoic and tertiary fauna have been found the cenozoic lake bed sequence is the highest unit and covers nearly all of the present basin sediment thicknesses commonly reach 7 km and attain a maximum of 12 km at the southeastern end of the basin the lake bed sequence is the thickest unit in the main depocentre but the karroo sequence is often the thickest unit on the shoaling side of the half graben the rukwa rift is here interpreted to have evolved as a strike to oblique slip pull apart basin based on numerous indications of nw trending strike slip faulting

geodynamic significance of the trm segment in the east african rift w tanzania active tectonics and paleostress in the ufipa plateau and rukwa basin abstract the tanganyika rukwa malawi trm rift segment in western tanzania is a key sector for understanding the opening dynamics of the east african rift system ears in an oblique opening model it is considered as a dextral transfer fault zone that accommodates the general opening of the ears in an nw se direction in an orthogonal opening model it accommodates pure dip slip normal faulting with extension orthogonal to the rift segments and a general e w extension for the entire ears the central part of the trm rift segment is well exposed in the ufipa plateau and rukwa basin within the paleoproterozoic ubende belt it is also one of the most seismically active regions of the ears we investigated the active tectonic architecture and paleostress evolution of the ufipa plateau and adjacent rukwa basin and in order to define their geodynamic role in the development of the ears and highlight their pre rift brittle tectonic history the active fault architecture fault kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the trm segment two pre rift brittle events are evidenced an older brittle thrusting is related to the interaction between the bangweulu block and the tanzanian craton during the late pan african early paleozoic it was followed by a transpressional inversion during the early mesozoic this inversion stage is the best expressed in the field and caused dextral strike slip faulting along the fault systems that now control the major rift structures it has been erroneously interpreted as related to the late cenozoic ears which instead is characterized by pure normal faulting our third and last stress stage

normal vs strike slip faulting during rift development in east africa the malawi rift kinematic analysis of neogene and quaternary faults demonstrates that the direction of extension in the malawi rift rotated from east northeast to southeast rift development commenced with the formation of half grabens bounded by northwest north and northeast striking normal faults owing to slightly oblique rifting the northwest striking faults in the northernmost rift segment show a small dextral oblique slip component whereas north and northeast oriented faults in the central part of the rift display a sinistral oblique slip component this first event resulted in block faulting and basin subsidence which is largely responsible for the present day basin morphology of lake malawi a major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike slip faulting assumed a major role the strike slip regime amplified uplift of basement ridges within the rift in regions of local transpression but it also created alluvial basins because of local transtension this new kinematic style is compatible with the recent seismicity older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift toward the center the faults depict an increase in strike slip components of movement suggesting deformation propagation toward the rift center which results in a narrowing of the active rift environments with time

caldera collapse during the 2000 eruption of miyakejima volcano japan a collapsed caldera 1 6 km in diameter and 450 m in depth was formed at the summit of miyakejima volcano during the 2000 eruption the collapsed caldera appeared on 8 july with a minor phreatic eruption 12 days after seismic activity and magma intrusion occurred northwest of the volcano growth of the caldera took from 8 july to the middle of august with seismic swarms associated with the continuous intrusion of magma northwest of the volcano the growth rate of the caldera was about 1 4 107 m3 day and the final volume of the collapsed caldera was about 6 108 m3 major phreatomagmatic eruptions produced a total of about 1 6 1010 kg 1 1 107 m3 of volcanic ash after caldera growth the caldera structure and the nature of the eruptive materials of the first collapse on 8 july suggest that the surface subsidence was caused by the upward migration of a steam filled cavity with stoping of the roof rock above the magma reservoir the diameter of the stoping column was estimated to be 600 700 m from circumferential faults that developed in the caldera floor and the collapse of the caldera wall enlarged the diameter of the caldera to 1 6 km the total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1 6 2 1 km

alkali metal and rare earth element evolution of rock forming minerals from the gatumba area pegmatites rwanda quantitative assessment of crystal melt fractionation in the regional zonation of pegmatite groups abstract this study presents a general model for the evaluation of rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare element pegmatites the magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element ree analyses of rock forming minerals feldspars micas and tourmaline which represent a whole sequence of regional pegmatite zonation the gatumba pegmatite field rwanda central africa is chosen as case study area because of its well developed regional zonation sequence the pegmatites are spatially and temporally related to peraluminous g4 granites 986 10 ma the regional zonation is developed around a g4 granite and the proximal pegmatites grade outwardly into biotite two mica and muscovite pegmatites rare element nb ta sn pegmatites occur most distal from the granite alkali metal fractionation trends in pegmatitic k feldspar rb 350 6000 ppm cs 2 160 ppm and muscovite rb 670 7000 ppm cs 10 150 ppm define a single and continuous trend which is modelled by rayleigh fractional crystallisation starting from a parental granite composition g4 composition k 3 1 wt rb 222 ppm cs 11 ppm the fractionation model shows moreover that the pegmatites adjacent to the parental pluton are the least fractionated and the distal pegmatites are the most fractionated biotite pegmatites form from 0 to 69 crystallisation two mica pegmatites from 69 to 92 and muscovite pegmatites from 92 crystallisation onwards the extreme rb and cs enrichment in rare element pegmatites requires at least 98 fractionation of the initial g4 granite composition mathematical derivation of the k rb versus cs relationship in k feldspars confirms rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation moreover 1 it demonstrates the continuity of the fractionation process from biotite pegmatites to rare element pegmatite and indicates a genetic link among them and 2 it allows a general evaluation of pegmatite fields in terms of system parameters i e the initial element concentration in the granitic melt and partition coefficients ree patterns of the rock forming minerals feldspars muscovite biotite and schorl show a distinct transition along the regional zonation they evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites minerals from the rare element pegmatites feldspars muscovite and elbaite show again more fractionated heavy ree depleted patterns the observed evolution in the ree patterns corresponds to early crystallisation of light ree enriched monazite and a late crystallisation of mainly cogenetic heavy ree enriched phases such as apatite columbite group minerals and beryl modelling of rayleigh fractionation starting from an initial parental granite g4 composition lan ybn 10 and ree 83 shows that this evolution in mineral ree pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of ree incorporating minerals such as monazite and apatite consequently trace element modelling indicates that rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare element pegmatites this mechanism shows that granitic pegmatites are extremely fractionated residual melts which are genetically and directly connected to a main granite body

magma differentiation rates from 226ra 230th and the size and power output of magma chambers we present a mathematical model for the evolution of the 226ra 230th activity ratio during simultaneous fractional crystallization and ageing of magma the model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization these are tholeiitic basalt from ardoukoba djibouti morb from the east pacific rise alkali basalt to mugearite from vestmannaeyjar iceland and basaltic andesites from miyakejima izu bonin arc in all cases 226ra 230th correlates with indices of fractional crystallization such as th and the data fall close to model curves of constant fractional crystallization rate the best fit rates vary from 2 to 6 10 4 yr 1 consequently the time required to generate moderately evolved magmas f 0 7 is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs these rates and timescales are argued to depend principally on the specific power output i e power output per unit volume of the magma chambers that are the sites of fractional crystallization equating the heat flux at the epr to the heat flux from the sub axial magma chamber that evolves at a rate of ca 3 10 4 yr 1 implies that the magma body is a sill of ca 100 m thickness a value which coincides with independent estimates from seismology the similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca 10 to 50 mw km 3 their differentiation rates are some two orders of magnitude slower than that of the basaltic makaopuhi lava lake hawaii that cooled to the atmosphere this is consistent with the two orders of magnitude difference in heat flux between makaopuhi and the east pacific rise 226ra 230th data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and from this the thermal budget of the magma chamber addressed and where an independent measurement of heat flux exists to place constraints on the size of the magma chamber such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas

el nino in santa barbara basin diatom radiolarian and foraminiferan responses to the 1983 el nino event abstract santa barbara basin box cores collected in august of 1985 were sub sampled and processed in search of microfossil evidence of the 1983 el nino microplankton and microbenthos indicate the presence of that el nino bounded by non el nino conditions specifically the event is recognized by an increase in diatom diversity and the presence of several warm water taxa e g coscinodiscus plicatoides hemidiscus cuneiformis nitzschia kolaczekii pseudoeunotia doliolus thalassiosira lineata thalassiosira nodulolineata an increase in polycystine radiolarian flux and the presence of the eastern tropical pacific species spongaster pentas an increase in the diversity of planktonic foraminifera and the occurrence of the warm water species globoturborotalita rubescens there is also evidence for an oxygenation of the basin during the 1983 el nino as suggested by the increase in the abundance of the benthic foraminifera bolivina vaughani and by the decrease in the preservation of phaeodarian radiolarian spines which are best preserved under anoxic conditions a new onboard procedure for sampling of box cores and freezing of subcores is described

modeling two sequential coaxial phases of shortening in a foreland thrust belt abstract analog sandbox models are used to simulate two sequential coaxial phases of shortening in a foreland thrust belt a sufficient hiatus is considered so that erosion and sedimentation after the first phase create an angular unconformity that is subsequently deformed the effect of variation in thickness of post erosional sediment package and presence of a weak layer at the unconformity level are analyzed during the second phase some first phase thrusts are reactivated and new thrusts are also initiated thrust reactivation results in a structure spacing that is smaller than the expected spacing for a thicker sediment package reactivation of pre existing structures prevents the weak layer from acting as an intermediate decollement an increase in thickness ratio tends to weaken reactivation of pre existing thrusts model results also show that total displacement along individual reactivated thrusts generally increases downwards across the unconformity which could be used to distinguish thrust reactivation in the field two regional examples from the northern eastern cordillera in colombia and from the variscan frontal zone in western europe respectively where multiphase coaxial shortening occurred are compared with model results both natural cases show features such as partially eroded first generation folds and truncated first generation thrusts that are indicators for two sequential phases of deformation as observed in the models

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

isobaric cooling of proterozoic high temperature metamorphites in the northern arunta block central australia implications for tectonic evolution abstract protoliths of metamorphites in the proterozoic reynolds range region northern arunta block central australia included mafic and felsic igneous rocks pelites calcareous rocks and quartzites metamorphism began with an early high t low p facies stage followed by early rehydration in higher grade areas isobaric cooling and late high t low p facies hydration in and near shear zones metamorphic grade increases southeastwards shown by the change from andalusite in metapelites in the northwest to sillimanite in the southeast and by the incoming of cordierite cordierite biotite cordierite k feldspar sillimanite and cordierite garnet migmatites kornerupine quartz spinel and orthopyroxene k feldspar assemblages occur in the highest grade rocks downwarping at near peak temperatures is shown by garnet sillimanite replacing cordierite and by sillimanite pseudomorphing andalusite early in the near isobaric cooling that followed peak temperatures freezing of partial melts released water and biotite formed the hydrated retrograde assemblages formed after temperatures had fallen considerably are also zoned from northwest to southeast with andalusite sillimanite or kyanite bearing assemblages developed in appropriate bulk compositions the early high t metamorphism is best explained by tectonic models based on extension crustal delamination and underplating the hydration and deformation were caused by processes unrelated to the early history occurring at least 200 ma later possibly as late as the mid palaeozoic

tectonic evolution of the cuvette centrale zaire the cuvette centrale central basin of zaire is an infra cambrian to recent basin with up to 9 km of sedimentary section basin subsidence can be largely explained by a postulated late proterozoic failed rift and subsequent thermal relaxation seismic reflection profiles and well data reveal a palaeozoic development of the basin punctuated by two regional deformational episodes one in the early palaeozoic late cambrian and a second in the late palaeozoic late permian triassic both episodes result from ne sw contraction with the late palaeozoic episode being largely a reactivation of the early palaeozoic structures the timing of the two deformations correlates with collisional events at continental margins some distance from the congo basin in the early palaeozoic the deformation appears to be related to the late stages of the west congolian fold and thrust belt and coincides regionally with the pan african event sensu stricto the late palaeozoic deformation coincides with collisional tectonics along the southern margin of gondwana and the generation of the cape and sierra ventana foldbelts it is postulated that in both the early and late palaeozoic africa and gondwana experienced widespread intracontinental deformation generated by distant collisional processes much as central asia is experiencing today

geodynamic evolution of the bangweulu block northern zambia abstract the bangweulu block is a cratonic unit underlying most of northern zambia and adjacent parts of tanzania and zaire its geodynamic evolution is shown to be entirely proterozoic except for cainozoic rifting and related sedimentation the block has a crystalline basement of schists acid metavolcanics and granitoids which formed during a deformational and magmatic event of eburnian age between 2000 and 1800 ma ago the metavolcanics have affinities with high k calc alkaline suites from modern subduction controllled tectonic regimes suggesting an eburnian plate margin to the northwest of the block the polyphase sedimentary cover of the bangweulu block comprises the mporokoso group a 5000 m thick sequence of fluvial aeolian and lacustrine sediments deposited in the intracratonic mporokoso basin in response to the final phase of the eburnian tectogenic cycle the kasama formation a sequence of fluvial sediments which is 100 300 m thick on the bangweulu block but increases in thickness to nearly 400 m in the downfaulted pericratonic irumide basin formed in the initial phase of the kibaran tectogenic cycle the luitikila beds a 3000 m thick unit of post orogenic molasse of the kibaran irumide belt which encroaches with reduced thickness on the bangweulu block the luapula beds deposited on the margins of the bangweulu block and representing parts of the kundelungu sedimentary sequence accumulated in the pan african pericratonic basin faults and shear zones bordering the bangweulu block developed in sequence during the eburnian kibaran and pan african orogenies the older ones are invariably reactivated during the younger tectogenetic events resulting in substantial lateral and rotational movement of the bangweulu block with respect to adjacent structural terranes

shift and rotation of composition trends by magma mixing 1983 eruption at miyake jima volcano japan levels of ascent of the fountain in the host andesite magma analysis of compositional zoning in titanomagnetite crystals revealed that the eruption of the 1983 magmas was initiated soon after the replenishment of the basaltic magma in the 1kbar magma chamber

crystal and magma residence at kilauea volcano hawaii 230th 226ra dating of the 1955 east rift eruption abstract previous estimates of crustal storage time of magmas at kilauea volcano hawaii range from a few years to a few thousand years leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation we present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226ra 230th disequilibria and apply this approach to the early phase of the 1955 east rift eruption at kilauea when fractionation of ra from ba a proxy for initial ra in the crystals during crystal growth is considered along with the effects of inclusions in the minerals the data are consistent with co precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000 300 400 a unless a significant fraction 30 of the crystals are remnants from an earlier batch of evolved magma in the system these data constrain the minimum magmatic residence time to be 550 yr considerably longer than most previous estimates of storage time at kilauea as well as those for some other basaltic systems for the temperature interval of augite plagioclase growth in the early 1955 magma a maximum constant cooling rate of 0 1 c yr 1 10 5 c h is derived from the minimum magmatic residence time of 550 yr the total magma storage time would be 2500 yr if this cooling rate applied to the entire thermal history of the magma although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr the disparate estimates of magma residence at kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma more work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and or of kilauean lavas in general

the role of strike slip fault systems at plate boundaries a new analysis shows that most 59 plate boundaries have a relative velocity vector that is markedly oblique greater than 22 to the boundary normal a significant proportion 14 have vectors that are nearly 22 parallel to the boundary accommodation of the oblique motion usually involves strike slip faulting but the kinematic role of these faults differs at divergent and convergent boundaries four main types of plate boundary related strike slip faults are distinguished ridge transforms boundary transforms trench linked strike slip faults and indent linked strike slip faults discrimination of the four types should be possible in ancient orogenic belts but is complicated by the common reactivation of the strike slip zones in other roles plate boundary related strike slip faults form major lineaments at the present day ridge transforms have a low preservation potential in continents boundary transforms and indent linked faults often re use old lineaments but trench linked strike slip faulting is an effective method of forming new lineaments in continental crust strike slip faulting in general is less commonly recognized in ancient orogenic belts than its abundance in present plate boundary orogens requires this under recognition results both from poor understanding of strike slip kinematics and from deeper prejudices about the way in which orogenic belts form

a new method for u th pa ra separation and accurate measurement of 234u 230th 231pa 226ra disequilibria in volcanic rocks by mc icpms abstract a new method for the chemical separation and mc icpms measurements of u th pa ra disequilibria in volcanic samples from a single sample aliquot is presented the accuracy and precision of our techniques is assessed by replicate analyses of the synthetic u and th isotope standards irmm 184 and irmm 35 and the secular equilibrium rock standards bcr 2 w 2a and tml a measure of true sample reproducibility including the errors related to sample processing and the chemical separation is obtained from analysis of multiple digestions of the rock standards 234u 238u 230th 238u 231pa 235u and 226ra 230th activity ratios on ten separate dissolutions of usgs rock standard bcr 2 for example reproduce to 0 4 1 4 1 1 and 2 4 2 sd respectively all elements u th pa ra are separated from a single spiked sample aliquot and the elemental concentrations are measured on the same solutions as the isotopic compositions analytical artefacts from determination of elemental and isotopic concentrations on different sample aliquots due to sample heterogeneity are therefore avoided in addition our new chemical separation protocol is time efficient and allows for rapid processing of volcanic samples with less than 25 fg of pa and ra