translation and docking of an arc terrane geological and geochemical evidence from the southern zambales ophiolite complex philippines abstract the zambales ophiolite complex is made up of three massifs the masinloc cabangan and san antonio massifs field petrographic and geochemical analyses show that the cabangan and san antonio massifs are genetically related to the coto transitional mid ocean ridge island arc and acoje island arc blocks of the masinloc massif respectively the subic bay fault zone a left lateral fault zone separates the san antonio massif island arc terrane from the transitional mid ocean ridge island arc like sheeted diabase dikes pillow basalts of the cabangan massif the san antonio massif is a rifted terrane from the acoje block which was translated southward to its present position through the west luzon shear subic bay fault zone tectonized clinopyroxenite and gabbronorite hills which mimic the physical and geochemical characteristics of the acoje block and the san antonio massif ultramafic mafic cumulate rocks were left behind along the western side of the cabangan massif during the translation of the arc massif southward this scenario can account for the present day configuration of the zambales ophiolite complex

aeolian dune migration along the ceara coast north eastern brazil aeolian dune dimensions and migration rates are analysed along the ceara coast north east brazil dunes that are currently mobile along the ceara coast are composed of barchans and sand sheets the results show that barchans maintain an equilibrium form which can be characterized by values of dimensionless shape parameters h w and w l where h is the dune height w is the wing to wing width and l is the dune length dunes are highly mobile with average migration rates of 17 5 m year 1 for barchans and 10 m year 1 for sand sheets the calculated migration rates were found to depend strongly on dune dimensions for both barchans and sand sheets i e the larger the dune is the lower the migration rate will be this size dependence was associated with the existence of a representative common transport rate along the dune fields which induces a different dune migration rate dependent on dune size finally from the observed dune evolution an aggregated scale aeolian sediment transport was inferred this bulk transport rate of the order of 90 100 m3 m 1 year 1 is only valid for a timescale of years to decades which is the timescale used in dune evolution analysis

conceptual models of the evolution of transgressive dune field systems abstract this paper examines the evolutionary paths of some transgressive dune fields that have formed on different coasts of the world and presents some initial conceptual models of system dynamics for transgressive dune sheets and dune fields various evolutionary pathways are conceptualized based on a visual examination of dune fields from around the world on coasts with high sediment supply dune sheets and dune fields tend to accumulate as large scale barrier systems with little colonization of vegetation in arid hyper to arid climate regimes and as multiple active discrete phases of dune field and deflation plain couplets in temperate to tropical environments active dune fields tend to be singular entities on coasts with low to moderate sediment supply landscape complexity and vegetation richness and diversity increases as dune fields evolve from simple active sheets and dunes to single and multiple deflation plains and basins precipitation ridges nebkha fields and a host of other dune types associated with vegetation e g trailing ridges slacks remnant knobs gegenwalle ridges and dune track ridges tree islands and bush pockets three principal scenarios of transgressive dune sheet and dune field development are discussed including dune sheets or dune fields evolving directly from the backshore development following foredune and or dune field erosion and development from the breakdown or merging of parabolic dunes various stages of evolution are outlined for each scenario knowledge of evolutionary patterns and stages in coastal dune fields is very limited and caution is urged in attempts to reverse change and or modify dune fields to restore some perceived loss of ecosystem or dune functioning

metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

source parameters of march 10 and september 13 2007 united arab emirates earthquakes abstract on march 10 and september 13 2007 two earthquakes with moment magnitudes 3 66 and 3 94 respectively occurred in the eastern part of the united arab emirates uae the two events were widely felt in the northern emirates and oman and were accompanied by a few aftershocks ground motions from these events were well recorded by the broadband stations of dubai uae and oman seismological networks and provide an excellent opportunity to study the tectonic process and present day stress field acting in this area in this study we report the focal mechanisms of the two main shocks by two methods first motion polarities and regional waveform moment tensor inversion our results indicate nearly pure normal faulting mechanisms with a slight strike slip component we associated the fault plane trending nne ssw with a suggested fault along the extension of the faults bounded bani hamid area the seismicity distribution between two earthquake sequences reveals a noticeable gap that may be a site of a future event the source parameters seismic moment moment magnitude fault radius stress drop and displacement across the fault were also estimated from displacement spectra the moment magnitudes were very consistent with waveform inversion the recent deployment of seismic networks in dubai and oman reveals tectonic activity in the northern oman mountains that was previously unknown continued observation and analysis will allow for characterization of seismicity and assessment of seismic hazard in the region

solutions in chronostratigraphy the paleocene eocene boundary debate and aubry vs hedberg on chronostratigraphic principles abstract in several recent papers m p aubry et al have argued that hedbergian principles of chronostratigraphy are being violated by the international commission on stratigraphy ics when selecting global stratotype sections and points gssps for the formal divisions of the geological time scale the current debate over the definition of the paleocene eocene p e boundary has been a major focus of their arguments unfortunately aubry et al have obscured matters by misusing the term unit stratotype and by equivocally using the term stage for the very different concepts of synthem and global chronostratigraphic stage the p e boundary option most repugnant to aubry et al carbon isotope excursion cie p e thanetian ypresian boundary is perfectly compatible with h d hedberg s views in contrast another option preferred by aubry et al recognition of new 1 m y duration age stage between thanetian and ypresian is inconsistent with hedberg s views additional problems with the p e boundary arguments of aubry et al include the fact that a ypresian unit stratotype does not exist the fact that the base of the ypresian synthem is not immutable and the fact that the nannofossil tribrachiatus digitalis is of dubious relevance to the boundary debate as for chronostratigraphy in general aubry et al have misrepresented hedberg s views by 1 falsely claiming that the content of a stage is what determines its boundaries 2 misunderstanding the general concept of the boundary stratotype 3 distorting the base defines boundary principle 4 falsely claiming that traditional pre gssp chronostratigraphic boundaries cannot be changed 5 falsely implying that traditional stage unit stratotype boundaries can be adjusted by no more than 300 000 years when defining formal age stage boundaries with gssps 6 falsely claiming that the definition of a geochronologic chronostratigraphic boundary should precede its correlation 7 claiming that traditional unconformable stage boundaries may be suitable horizons for gssps 8 distorting the meaning of arbitrariness in regard to the definition of geochronologic chronostratigraphic boundaries and 9 claiming that gssps are inherently unstable in that they are subject to redefinition whenever a more powerful element of correlation is discovered if taken seriously the unit stratotype sanctifying chronostratigraphic philosophy of aubry et al would require the creation of perhaps dozens of new phanerozoic ages stages of relatively very short duration wherever there was a significant gap between two successive historical stage unit stratotypes for all of these reasons the arguments of aubry et al have no merit

a guide to stratigraphical procedure the continuing development of stratigraphical concepts and the application of new methods has led the stratigraphy committee of the geological society of london to present a new guide to stratigraphical procedure some of the principal types of stratigraphy are outlined and guidelines and recommendations are given on procedure practice and terminological usage

three new upper cambrian stages for north america the existing upper cambrian stages for north america are obsolete three new stratotype defined stages are equivalent to and replace three successive biomeres marjuman stage replaces marjumiid biomere lower boundary at base ptychagnostus atavis zone steptoean stage replaces pterocephaliid biomere lower boundary at base aphelaspis zone sunwaptan stage replaces ptychaspid biomere lower boundary at base irvingella major subzone the base of the overlying ibexian series is placed at the base of the eurekia apopsis subzone

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

synchronous seafloor spreading and subduction at the paleo convergent margin of semail and arabia the semail ophiolite is the largest and best exposed ophiolite having preserved intact record of a precollision system related to neotethyan ocean closure breaking with the alternative models of ophiolite exhumation and emplacement this paper builds on integrating the joined contribution of tectonic evolution of the continental margin in the premise of the ophiolite obduction connecting the along strike segmentation of the ophiolite recorded in the lithospheric flow structure and recent high precision dating results in a 3 d model of evolution of the ophiolite and its continental margin during jurassic cretaceous time among the relevant characteristics newly accounted 1 the contamination of the ophiolite by fluids and magmas of continental origin 2 the discrimination of different types of metamorphics associated to the ophiolite and conventionally classified metamorphic soles 3 the instability of the arabian margin during the late jurassic cretaceous times 4 the new precise ages relating to overlapping of spreading and detachment and 5 the inferred vergence of neotethyan closure during late mesozoic are stressed the complex stem revealed may relate to processes associated with spreading center subduction at ridge trench transform rtt triple junction as documented along a convergent margin the semail ophiolite emplacement on the arabia continental margin would result from a temporary arabian platform directed subduction zone that consumed the southern tethyan plate during late jurassic cretaceous time

structures in podiform chromite from the maqsad district sumail ophiolite oman about 15 chromite bodies have been recognized in the maqsad area of the oman ophiolite the occurrence in this area of three chromite bodies within the cumulate sequence must be integrated into the classification of cassard et al 1981 which presently explains only those pods lying in the uppermost mantle sequence plastically deformed harzburgites and dunites the occurrence of chromite bodies within the cumulates and the abundance of chromite in the maqsad area are related to the exceptional magmatic activity and the unusual plastic flow pattern particular to this area it was probably a feeding zone along the oceanic spreading center sitting on top of a mantle diapir

the march 11 2002 masafi united arab emirates earthquake insights into the seismotectonics of the northern oman mountains a moderate m approx 5 earthquake struck the northeastern united arab emirates uae and northern oman on march 11 2002 the event was felt over a wide area of the northern emirates and was accompanied by smaller felt events before and after the march 11 main shock the event was large enough to be detected and located by global networks at teleseismic distances we estimated focal mechanism and depth from broadband complete regional waveform modeling we report a normal mechanism with a slight right lateral strike slip component consistent with the large scale tectonics the normal component suggests relaxation of obducted crust of the semail ophilite specifically the khor fakkan block while the right lateral strike slip component of the mechanism is consistent with shear across the oman line felt earthquakes are rare in the region however no regional seismic network exists in the uae to determine local seismicity this event offers a unique opportunity to study the active tectonics of the region as well as inform future studies of seismic hazard in the uae and northern oman

does the golden spike still glitter a modus operandi for precambrian stratigraphy is suggested which combines the geochronometrical scale and late proterozoic chronostratigraphy the development of phanerozoic stratigraphical classification is followed from the pioneering days of murchison through the growing understanding of fossils in relation to time and the later distinction of rock and time rock to the advent of the golden spike criticisms of chronostratigraphy global standard stratigraphy are assessed and its value upheld stratigraphical procedures such as those of event stratigraphy are useful in particular situations but correlative methods in the phanerozoic are dominated by biostratigraphy examples are taken of its use and potential in north american cordilleran palaeozoic stratigraphy in the triassic system as a whole and in the caledonides of britain and ireland

formation of parabolic dunes from barchan and transverse dunes along israel s mediterranean coast the mediterranean coastal dunes of israel underwent a land use change during the second half of the 20th century due to intense agricultural and pastoral activity the coastal dunes were stripped of natural vegetation until the end of the first half of the 20th century the barchan and transverse dunes were shaped by strong southwesterly winter winds a decrease in human activity during the second half of the 20th century brought about a renewal of natural vegetation on the dune crest the only area with neither erosion nor deposition the establishment of vegetation on the crest changes the dynamics of these barchan and transverse dunes so that not all of the sand eroded from the windward side is carried to the lee slip face some is trapped by plants consequently there is a change in the shape of the windward slope from convex to concave and the dune gradually becomes parabolic in this study we trace the morphodynamics of the dunes by analysing 12 sets of aerial photographs which were taken from 1944 to 1995 the average rate of advance of 15 dunes has decreased from 3 4 m a 1 to 1 9 m a 1 while the vegetation cover has increased from 4 3 to 17 per cent during this period copyright 2002 john wiley sons ltd

model for the genesis of coastal dune fields with vegetation vegetation greatly affects the formation and dynamics of dune fields in coastal areas in the present work we use dune modeling in order to investigate the genesis and early development stages of coastal dune fields in the presence of vegetation the model which consists of a set of coupled equations for the turbulent wind field over the landscape the saltation flux and the growth of vegetation cover on the surface is applied to calculate the evolution of a sand patch placed upwind of a vegetated terrain and submitted to unidirectional wind and constant sand influx different dune morphologies are obtained depending on the characteristic rate of vegetation growth relative to wind strength barchans transverse dunes with trailing ridges parabolic dunes and vegetated alongshore sand barriers or foredunes the existence of a vegetation free backshore is found to be important for the nucleation timescale of coastal dune generations the role of the sand influx and of the maximum vegetation cover density for the dune shape is also discussed

stratigraphy and structure of the schoonover sequence northeastern nevada implications for paleozoic plate margin tectonics radiolarian biostratigraphy and detailed geologic mapping have been used to resolve the complex structure and stratigraphy of part of the golconda allochthon in the independence mountains nevada here the schoonover sequence is latest devonian to early permian in age spanning the time interval from the emplacement of the roberts mountains allochthon onto the shelf in the earliest mississippian antler orogeny to the inception of the sonoma orogeny the history of the schoonover basin is tied to that of the adjacent shelf in nevada and provides important insights into the upper paleozoic paleogeographic framework of the continental margin in the schoonover sequence latest devonian famennian chert overlies basaltic and andesitic greenstone and is in turn overlain by early mississippian kinderhookian chert interbedded with tuff and volcaniclastic rocks derived from an island arc the kinderhookian volcaniclastic rocks grade upward with no obvious depositional break into siliciclastic turbidites and pebbly mudstones that contain debris derived primarily from erosion of the antler orogenic belt although in some beds volcanic rock fragments and greenschist grade metasedimentary clasts are also common the age of underlying kinderhookian and interbedded and overlying osagian to meramecian chert sequences indicates that the siliciclastic turbidites are synchronous with the early deposits of the autochthonous foredeep basin of the antler orogenic belt the dual or composite source terranes represented by the schoonover siliciclastic rocks place the basin between an arc and the antler orogen on the edge of the shelf meramecian age basalt flows in the schoonover are coeval with subsidence and basaltic volcanism in autochthonous shelf sequences in northern nevada suggesting that a rifting event may have occasioned the end of antler age compression in nevada the onset of limestone turbidite deposition in the schoonover corresponds to the reestablishment of carbonate shelf conditions on the continental margin in latest mississippian earliest pennsylvanian time the late devonian earliest mississippian part of the schoonover depositional history has not yet been documented elsewhere in the golconda allochthon but late mississippian to permian rocks in the schoonover sequence are analogous to those of the havallah sequence suggesting deposition in the same basin thrust faults in the schoonover repeat the late devonian to permian section detaching the basinal sequence from its depositional basement thrust faults extend along strike for at least 10 km but thrust plates are the stratigraphic relations documented in the schoonover sequence are compatible with a back arc thrusting model for the formation of both the roberts mountains and golconda allochthons but they are more difficult to reconcile with models that interpret the allochthons as accretionary prisms developed in front of farther traveled arcs that collided with a passive margin in a broader context the stratigraphy of upper paleozoic allochthonous rocks in nevada records short lived episodes of crustal shortening along the continental margin separated by longer episodes of extensional and or transcurrent tectonics this data suggest that the western united states was a southwest pacific style active margin at least as far back as the devonian

the southern uplands controversy report of a discussion meeting held at burlington house on 12 february 1986 the idea for a discussion meeting followed from remarks made by professor c h holland 1986 in his presidential address to this society in 1985 when he expressed concern about the uncritical application of the accretionary prism model to the southern uplands of scotland it has since become apparent that divergent views are held both among those against the model and those in favour of it this meeting served to air some of these different opinions even if it has not immediately resolved many of the arguments most of the contributors to the meeting see the society newsletter for january 1985 have produced written papers for this volume and so far only one or two have modified their views between writing their abstract and submitting their paper the concept of an accretionary prism model developed gradually a comparison of scotland with burma suggested that the southern uplands were like western burma where trench and oceanic sediments are being underthrust by subduction processes mitchell mckerrow 1975 with the development of modern analogues from the pacific seeley et al 1974 karig sharman 1975 the model was applied to the southern uplands with more emphasis mckerrow et al 1977 leggett et al 1979 a b 1982 moseley 1977 1978 was one of the first to express doubts about this model he believed that the iapetus ocean had closed by the end of the ordovician and thus like many others

obduction related remagnetization at the base of an ophiolite paleomagnetism of the samail nappe lower sequence and of its continental substratum southeast oman mountains this paper considers the effect of obduction tectonics on the magnetic properties of a fast spreading ridge ophiolite the paleomagnetism of the ultramafics seven sites and layered gabbros two sites of the samail wadi tayin ibra and muscat massifs of oman has been investigated along with that of the permian metabasites from the subophiolitic saih hatat continental window five sites of the 202 studied samples 183 exhibit stable components with a dominant nnw trend and a shallow positive or less frequently negative inclination superposition of normal and reverse polarities was observed in one gabbro three peridotite and two metabasite sites the in situ characteristic directions are similar in both the nappe and metamorphic window except for a lone low grade metabasite with a component d 168 i 35 referred to a preserved permian magnetization the moderate directional scatter can be explained by postnappe tilting along the nne trending axis the high grade metabasites in the window were completely remagnetized during the metamorphism associated with nappe emplacement and subsequent exhumation tectonics between 80 and 70 ma since the overlying ophiolites have similar normal or reverse directions they were remagnetized contemporaneously the magnetic mineralogy of the ultramafic layered gabbro sequence was likely modified extensively at that time by serpentinization from the upward metamorphic fluid advection in the faulted ophiolite therefore the magnetic properties of the lower part of the southern samail ophiolite cannot be used for modeling magnetic anomalies of oceanic lithosphere

structural evolution of the northern end of the oman ophiolite and enclosed granulites abstract a combined structural field mapping microstructural and petrographic study has been undertaken to better understand the origin and evolution of the northernmost part of the oman ophiolite khawr fakkan massif in the framework of an oceanic spreading centre emplacement of a sheet of granulite facies metamorphic rocks into the peridotites occurred at temperatures at which the peridotites deformed ductily the metamorphic rocks define the southern extension of a block boundary which separates an eastern peridotite part displaying high temperature fabrics from a western one with porphyroclastic to mylonitic fabrics granitoid intrusions representing partial melts from the metamorphic rocks occur only in the ophiolitic part overlying the metamorphic rocks thus delineating the same boundary between the two mantle blocks this high temperature boundary is not visible on satellite or aerial photographs the granulite facies foliations and lineations in the metamorphic rocks can be correlated with porphyroclastic textures developed at decreasing temperatures in the associated peridotites and to the obduction of the ophiolite the final emplacement of the metamorphic rocks in their present position was accompanied by retrograde hydration decompression and continuous deformation at temperatures ranging from 800 to 300 c the hydration is extended along the wadi ham fault two models are proposed for the emplacement of the metamorphic rocks in the first model the protolith of the metamorphic rocks was deposited in a pull apart basin along a transform fault and then subducted along the transform fault after reversal from transtension to transpression in this model the ophiolite obduction occurred later along another thrust plane in the second model the protolith material descends along an intraoceanic subduction zone and is emplaced into the ophiolite hanging wall along a secondary thrust caused by transpressive or rotational movements during the ophiolite obduction in this case the ophiolite obduction occurred along the same intraoceanic subduction plane

subdivision of precambrian time recommendations and suggestions by the subcommission on precambrian stratigraphy abstract the subcommission on precambrian stratigraphy consisting at the time of its last formal meeting in 1982 of 12 titular members and 30 correspondents has been actively working toward an acceptable subdivision of precambrian time for more than a decade comprehensive time rock charts have been prepared for major precambrian shields and this composite record has been analysed for patterns and events common to several or to all a geochronologic subdivision scheme has evolved that reflects these patterns of major events in earth history while still recognizing that those events are typically varied in intensity and timing from one region to another time unit boundaries have been selected so as to enclose or delimit principal cycles of sedimentation orogeny and magmatism but the boundaries are defined in years without specific reference to any bodies of rock several recommendations developed sequentially at formal meetings held at intervals of 2 4 years and announced in geologic journals with calls for comment and criticism are proposed 1 1 the recommendation to establish two major time units of eon rank proterozoic and archaean archean separated by a time boundary at 2500 ma is reaffirmed 2 2 the provisional recommendation to subdivide the proterozoic eon into three units of era rank provisionally labelled proterozoic i ii and iii with time boundaries at 2500 1600 900 and about 570 ma respectively is confirmed and 3 3 further subdivision of the proterozoic into eight time units of period rank as yet unnamed with time boundaries at 2500 2100 1800 1600 1400 1200 900 700 and about 570 ma respectively is provisionally recommended the nature of the subcommission s deliberations and the guidelines developed are described herein comment and criticism both on the procedure followed and on the recommendations themselves are invited

the petrogenesis of leucogranitic dykes intruding the northern semail ophiolite united arab emirates field relationships geochemistry and sr nd isotope systematics the khawr fakkan block of the semail ophiolite united arab emirates exhibits a suite of 10 100 m scale metaluminous to peraluminous granitic intrusions ranging from cordierite andalusite biotite monzogranites to garnet tourmaline leucogranites which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto ophiolite in an intra oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite moho compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a h2o and pressures of 3 to 5 kbar the lile sr rb and ba covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments initial 87sr 86sr ratios of analysed dykes vary between 0 710 and 0 706 at initial nd values of between 6 3 and 0 5 cogenetic units of a composite sill from ra s dadnah yield a sm nd isochron age of 98 8 9 5 ma mswd 1 18 geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill haybi complex sediments preserved as greenschist asimah area to granulite facies bani hamid area ophiolitic metamorphic sole terranes the sr nd isotope systematics suggest that hybrid granitic melts were derived from pre magmatic mixing of two contrasting subduction zone sources

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

kinoshitalite ba mg 3 al2si2 o10 oh f 2 a brittle mica from a manganese deposit in oman paragenesis and crystal chemistry kinoshitalite ideally ba mg 3 al 2 si 2 o 10 oh f 2 was studied in a granulite facies manganese ore associated with calcium bearing magnesian tephroite hausmannite calcite and manganoan clinochlore and in a second assemblage together with hausmannite altered tephroite manganoan diopside and calcite the metamorphic rocks occur enclosed in peridotites of the semail ophiolite sultanate of oman and were metamorphosed during ophiolite obduction single crystal x ray data collected on an inclusion free kinoshitalite of ba 0 99 k 0 06 na 0 01 mg 2 64 mn 0 31 al 2 01 si 2 03 o 10 oh 1 61 f 0 37 cl 0 02 composition yielded a c centered lattice of monoclinic symmetry with a 5 316 1 b 9 230 2 c 10 197 2 a 100 06 1 v 492 6 a 3 and z 1 characteristic of the 1 m polytype the structure was refined in two models assuming complete si al ordering in the space groups c 2 and c 1 allowing for additional twinning the data clearly suggest that in spite of the si al ratio of 1 assumption of complete si al ordering can be rejected two models both in agreement with space group c 2 m causing si al disorder are discussed three dimensional si al disorder would lead to violation of the al avoidance rule and substantial ba displacement the rather well defined ba position capped by two six membered rings of si 3 al 3 o 18 composition suggests one dimensional disorder where completely si al ordered layers exist parallel to 001 the disorder occurs perpendicular to 001 and may be interpreted as random stacking faults

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

development and setting of the alcudia bay beach dune system mallorca spain abstract the beach dune system of alcudia bay is located in the north of the island of mallorca the system includes both simple and compound parabolic dunes formed by n to nne winds and is made up of two dune areas with different dynamics 1 the northern area where despite the fact that the source of beach sediment is from the south dunes are formed by northerly winds and develop a narrow and linear barrier which separates a marsh from the bay and 2 the southern area where complex parabolic dunes formed by north northeasterly winds prograde south southwest landward extending several kilometers inland the broad pattern of the dunefield size is limited to the southeast by a mountain range and to the west by the effects of a topographic corridor oriented north south which channels the prevailing northerly wind causing a southward dune progression and limiting the westward extension of the dunefield

nebkha formation implications for reconstructing environmental changes over the past several centuries in the ala shan plateau china abstract sediments of chinese tamarisk tamarix chinensis lour nebkhas in china s arid ala shan plateau document both the formation processes of these vegetated dunes and the environmental changes and geomorphological evolution in this region dating of the nebkhas showed that after surface water disappeared the wadis and playas evolved into gobi deserts desert pavements within two centuries the 13c trends in the litter deposited within the nebkhas revealed three stages of fluctuating moisture conditions during the past five centuries before the 1580s between the 1580s and the late 1990s and from the early 2000s to the present however changes in the coarse fraction of the nebkha sediments show that this region experienced at least five phases of aeolian sediment transport during the same period the early stages of nebkha formation coincided with the initial stage when the wadis and playas evolved into gobi deserts and nebkha sizes increased rapidly due to the abundant sediment supply provided by the playas however after the playa surfaces stabilized the nebkha sediments documented environmental changes in adjacent regions our results also show that the formation of the tamarix nebkhas in this region was not controlled by the groundwater table because in most periods the water table depth remained above the depth that limits tamarix survival in the ala shan plateau nebkha formation was mainly controlled by aeolian processes and revealed the integrated effects of environmental changes thereby providing possibilities for climate and environment reconstructions with a relatively high resolution in extremely arid areas

paleomagnetic results from oman ophiolites related to their emplacement abstract the paleomagnetic results from the cretaceous oman ophiolites suggest large rotations of the ophiolitic nappe during the intraoceanic thrusting and continental obduction which preceded its last emplacement onto the arabian margin and they confirm that the ophiolites were formed and thrust at equatorial latitudes furthermore they may indicate the occurrence of short reversals during the cretaceous long normal polarity period our paleomagnetic study was mainly conducted on metalliferous sediments interbedded within the volcanics of the ophiolites ninety nine cores from 31 samples from fifteen outcrops distributed along the nappe were analysed alternating field and stepwise demagnetisation revealed complex multicomponent magnetisations carried by hematite and magnetite among the various components a stable component was commonly found in each hand specimen from the same outcrop with varying unblocking temperature or field spectra because of varying composition from one sample to another after tilt correction the inclinations of these stable components were all found to be low and the declinations grouped around nw se and nne ssw axes each group containing roughly antipodal directions the occurrence of antipodal directions is inconsistent with the ages of the sediments situated in the middle of the cretaceous long normal polarity period but is consistent with previously published findings of dual polarity in gabbros and dikes the two nw se and nne ssw paleomagnetic groups may correspond to two successive episodes of magnetisation acquired during diagenesis or during high or low temperature hydrothermal activity the large rotations of the nappe that these magnetisations suggest would be consistent with the transport directions recorded within the metamorphic soles in further studies the magnetic mineralogy and the paleomagnetic results will have to be related to hydrothermal activity and the particular magnetic properties of our manganese rich sediments will need to be better understood

a model for the crustal evolution of southern scotland in the late silurian southern scotland was a mature accretionary active margin of sumatra type on the southeastern edge of north america while england was on a passive margin on the opposite side of the iapetus ocean southward subduction below england during the middle ordovician produced a magmatic arc which now lies close to the iapetus suture and thus argues for a missing forearc terrane tectonic erosion or strike slip removal may be responsible for some of this missing terrane to the north of the lake district but we now suggest that underthrusting below southern scotland may also be an important cause if the ordovician forearc terrane of northern england is now below the southern uplands we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the iapetus suture velocity structure from the lispb study sr u pb isotopic signatures and geophysical attributes of the c 400 ma granites the under thrusting model still leaves room for considerable modification of both margins by large strike slip faults seismic refraction profiles of modern accretionary active margins sumatra sw japan allow us to propose a likely velocity profile for late silurian southern scotland we choose a mesozoic forearc truncated by strike slip faulting the queen charlotte islands margin of the ne pacific as an analogue for the late silurian southern iapetus margin following previous suggestions of highly oblique and slow convergence during the final stages of closure of iapetus we combine the two profiles by under thrusting the southern margin to produce a model which involves minimal shortening in the collision zone the crustal profile resulting from such a collision would be very similar to the modern southern scotland profile both in terms of the scale of the tectono stratigraphic terranes involved and of the resulting crustal velocity structure the cessation of accretion after the mid wenlock may be related to impingement of the english continental crust against the scottish trench subsequent uplift of the southern uplands may be connected with the appearance of coarse detritus in the lake district during the silurian

peraluminous potassium rich granitoids in the semail ophiolite potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the semail ophiolite predominantly in the northern and less frequently in the central and southern part new chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites occurring as late intrusives in the gabboro and mantle sequence based on mineral chemistry the p t conditions of andalusite cordierite bearing pegmatoidal granites were calculated giving pressures of 1 8 2 2 kbar and a solidus within the stability field of andalusite hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675 c at 2 kbar at 700 c excess h2o there is already 70 80 melt from the granitic starting compositions potassic granites in the ophiolite can be the result of extreme fractional crystallization in the late intrusive series granitic melts can also be produced by a two stage process in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series two mica granites and pegmatites in the underlying metamorphic sole possibly melting products of amphibolites and biotite gneisses are contemporaneous these escaping fluids could also explain the pb and sr isotopic compositions of the leucocratic dikes indicating variable crustal contamination

holocene development of parabolic dunes in the central st lawrence lowland quebec abstract stabilized parabolic dunes in the central st lawrence lowland are oriented ne sw in the postulated direction of dune building winds coming from anticyclonic air circulation induced by the retreating laurentide ice sheet about 10 000 yr ago the eolian chronology reconstructed from several sections in mixed dune peatland environments indicates that postglacial plant colonization characterized by a fortuitous assemblage of arctic subarctic and boreal elements preceded dune formation during champlain sea regression around 10 000 yr b p confined peatlands and small forests were buried by eolian sands between 10 000 and 7500 yr b p under dry and temperate conditions this eolian episode lasted about 2500 14 c yr and ended when cyclonic air circulation similar to the present humid climatic regime was established following the breakup and disappearance of the laurentide ice sheet over hudson bay and peripheral areas dune stabilization through paludification of well drained eolian sands about 7500 yr b p suggests a major shift in climate toward wetter conditions that have been characteristic during most of the holocene in eastern north america minor eolian erosion induced by wildfire was recorded during late holocene time about 1250 yr b p anthropogenic perturbation logging and agriculture practice was also responsible for recent very local eolian activity

worldwide correlation of telychian upper llandovery strata using graptolites the formal boundaries to the telychian stage have been stated to correlate with the bases of the monograptus turriculatus biozone at the bottom and the cyrtograptus centrt ugus biozone at the top the evidence for this however is far from conclusive the telychian has been divided mainly using monograptids into eight or more biozones although fewer have been recognized in britain due to the general paucity of recent studies of the graptolite faunas of this age and to the existence of barren beds high within most british telychian sections each biozone has a duration of only a few hundred thousand years but most are at present difficult to recognize in the absence of the index species or where this species stratigraphical range extends beyond the limits of its biozone intercontinental correlation of telychian strata using graptolites is hampered to a limited extent by provincialism and water mass specificity and considerably by taxonomic problems a thorough taxonomic revision of published species would provide a firm foundation for future biostratigraphical subdivision and correlation of the telychian stage knowledge of telychian graptolite biostratigraphy would also be enhanced by restudies of two classic sections the trannon river section in wales and the bajiaokou section ziyang china the graptoloids offer the potential for some of the finest resolution biostratigraphical subdivision and correlation of strata of any fossil group at any time in the stratigraphical column being rapidly evolving planktic in many cases particularly in the silurian cosmopolitan to a certain degree facies independent and in theory at least readily identifiable to specific level the graptoloids possess the majority of the features required of the ideal zonal fossil this paper outlines the current state of telychian upper llandovery graptolite biostratigraphy the degree of precision possible in international and intercontinental correlation and highlights those areas in which future research might profitably be focused the formal boundaries to the telychian

mapping in the oman ophiolite using enhanced landsat thematic mapper images abstract the large areal extent and complex nature of the terrain has meant that previous largely field based mapping programmes in the oman ophiolite have produced maps of internally inconsistent quality recognition of subtle changes and minor outcrops across the area of a 1 1 map sheet is possible in the field only by unrealistically exhaustive mapping by using parts of the short wavelength infrared spectrum landsat thematic mapper data have the capacity to capitalize on distinctions in the spectral reflectances of different rock types and their weathering products and to allow consistent mapping across the whole ophiolite decorrelation stretching of these data produces images in which it is possible to recognize variations in gabbro composition to identify small acidic gabbroic and ultramafic intrusions to discriminate the uppermost mantle from the deeper mantle to locate the moho precisely and to delineate gossans and areas subject to chloritic epidotic alteration off axis plutonism is more widespread and dispersed than previously appreciated the significance of certain faults is reassessed in the light of new insights provided by the imagery such data could be a valuable mapping tool in similarly exposed terrains in arid and semi arid regions even where already mapped at the 1 100 000 scale

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

relationship between vegetation dynamics and dune mobility in an arid transgressive coastal system maspalomas canary islands abstract this paper explores the relationship between vegetation dynamics and dune mobility in an arid transgressive coastal dune system specifically the dune field of maspalomas gran canaria canary islands the aim is to understand the strategies of colonization and survival that plant communities have developed in slacks that face dune advance the relationship between plant colonization and dune migration was performed by following tamarix canariensis and traganum moquinii plants for several years morphological data about each individual as well as the distance of each plant to the dune were measured a study of the colonization patterns developed by t moquinii t canariensis cyperus laevigatus and launaea arborescens communities was performed by analyzing the evolution of consolidated plant patches and adult plants in relation to the dune advance this was achieved using digital orthophotos and spatial analysis from geographic information systems initiation of plant colonization over transgressive dunes occurs on both wet and dry slacks the results show that both plant colonization and development of adult plants are largely related to dune mobility thus survival of t moquinii and t canariensis plants under dune migration conditions is related to both distance to the dune front and plant height at the moment of burial distance from the dune front and plant height increases chance of survival the dynamics of adult plants is also related to dune displacement rates thus each community has different thresholds of resistance to mobility rates the t canariensis community withstands average rates higher than 3 m year its arboreal structure allows this species to grow high enough to resist the advance of the dunes and burial for the t moquinii community the population decreases gradually to eventually disappear when dune mobility rates exceed 4 m year the c laevigatus community develops at dune mobility rates lower than 3 m year decreasing its surface area at higher rates the l arborescens community endures dune migration rates of at least 1 8 m year however different distances between the dune front and the vegetated area also significant factor because these can compensate for the effects of displacement rates thus the closer a vegetated area is to a dune front the lower the rates of displacement must be to produce a greater reduction in the surface vegetation plant communities present two patterns of plant colonization to resist burial by sand one vertical and the other horizontal the horizontal pattern is employed by c laevigatus and l arborescens communities and consists of locating new generations of plants in progressive alignment with the dune front migration the vertical pattern is employed by the t canariensis community and consists of increasing the heights of the plants the t moquinii community can utilize both patterns because it reacts positively to some degree of burial since it is located in areas where the dunes reach different heights

k ar ages of the metamorphic sole of the semail ophiolite implications for ophiolite cooling history k ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the semail ophiolite the hornblende ages of 99 0 5 and 102 0 8 ma and the 90 ma ages of coexisting micas from the high grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism t 800 850 c p 6 5 9 kbar the new pressure estimates are based on findings of kyanite in garnet amphibolite and cordierite in quartzitic rocks these data indicate a cooling rate of 10 30 c ma the oldest mica ages of 95 1 ma are observed in the lowest grade greenschists these also largely represent cooling ages but might in part also include formation ages the pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low grade metamorphic zone through the high grade rocks and into the base of the overlying ophiolite radiometric ages of hornblendes 92 3 0 5 and 94 8 0 6 ma indicate that the crustal gabbro sequence cooled below 500 c later than the base of the ophiolite sequence metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate the peak of metamorphism must have been contemporaneous with the main magmatism in the semail ophiolite one of the dated muscovites yields an age of 81 3 0 8 ma but this is related to discrete deformation zones that were active during late stage emplacement of the ophiolite

transgressive dunefield landforms and vegetation associations dona juana veracruz mexico transgressive dune fields often comprise a multiplicity of landforms where vegetation processes largely affect landform dynamics which in turn also affect vegetation processes these associations have seldom been studied in detail this paper examines four separate landform types in a complex coastal transgressive dunefield located in the central gulf of mexico in order to assess the relationships between dunefield habitat local environmental factors vegetation associations and landform evolution topographic surveys using tape and clinometer were conducted in conjunction with vegetation survey transects at four locations across the dona juana dunefield vegetation surveys allowed the estimation of relative plant cover of each plant species found along the transects a large variety of landforms were found at the dona juana dunefield deflation plains gegenwalle counter ridges transverse dune trailing ridges blowouts and parabolic dunes akle fish scale shaped dunefields and precipitation ridges with plant species associations developing on these different landforms equally variable flood tolerant species were located in the lower parts deflation plain and gegenwalle ridges whereas the older and dryer parts were covered by coastal matorral shrubs burial tolerant species were dominant in the most mobile areas blowouts and akle dunefield and margin the dune trailing ridge with relatively milder conditions showed the highest richness with no dominant species a dual interaction was found such that colonizing species both create and affect topography and in turn topography determines vegetation association and succession patterns in coastal dunes the vegetation and abiotic environment namely the different landforms and the inherent micronevironmental variability interact tightly and generate a complex and highly dynamic biogeomorphic system where substrate mobility and colonization processes reinforce one another in positive feedback copyright 2010 john wiley sons ltd

notes on geochronologic and chronostratigraphic units there is no need to regard the term geochronology as a redundant synonym of either geologic time scale or radiometric dating this term may reasonably be used in reference to all principles and methods employed in geology to determine the relative and numerical ages of all kinds of rocks the term chronostratigraphy would best be restricted to the process of defining the formal subdivisions of the geologic time scale geochronologic unit is best used as a general category term it includes aurichronologic units spans of time defined operationally by golden spikes geochronometric units spans of time defined theoretically in terms of absolute numerical age before present and biochronologic units spans of time defined theoretically by paleobiological events a chronostratigraphic unit is a set of material existing stratified rock that was formed during a given span of time geochronologic unit therefore a geochronologic unit must be defined before the corresponding chronostratigraphic unit can even be conceptualized chronostratigraphic units are generally not isochronous bodies of rock because owing to the unconformities that exist over any appreciable geographic area their upper and lower material boundaries are not everywhere synchronous thus particular systems series and stages are material stratigraphic units of geographically variable time value within the maximum temporal limits of the corresponding periods epochs and ages geochronologic boundaries that have been formally ratified serve a conventional function and will rarely if ever need to be redefined we can continue to discover and debate important events in earth history without insisting that such events must be used to constantly redefine the boundaries of our global temporal pigeonholes

problems in chronostratigraphy stages series unit and boundary stratotypes global stratotype section and point and tarnished golden spikes abstract stratigraphy is a magnificent subject magnificent in its breadth and scope magnificent in its importance to so many lines of human endeavor it is essentially that branch of geology which deals with the arrangement the distribution and the chronological succession of rock strata and other associated rock bodies with respect to any or all of the various characters properties and attributes which rocks may possess hedberg 1958 p 1881 chronostratigraphy the temporal ordering of geologic strata is at the heart of earth history in reviewing chronostratigraphic theory from d orbigny to hedberg we see that concepts and practices have evolved under pressure of constant improvements in time control and data quality such stimuli are naturally felt more strongly in the youngest part of the stratigraphic record where dating is most precise and the biostratigraphic data are most abundant and well preserved it is not surprising therefore to find that chronostratigraphy is conceptually more advanced in the cenozoic erathem than in the mesozoic and paleozoic erathems where biostratigraphy and chronostratigraphy are widely confused evidently out of necessity taking the five inter series boundaries in the cenozoic erathem as case histories we review their current status in relation to the lower boundary of the lowest standard stage above the boundary in light of the hierarchical principle recommended in the international stratigraphic guide hedberg 1976 salvador 1994 in each instance what should be a simple nested relationship turns out to be complex and difficult and in the cases of the proposed global stratotype section and point gssps for the eocene oligocene series boundary and the paleocene eocene series boundary there are discrepancies so large with regard to the base of the rupelian and ypresian standard stages respectively that some essential relaxation of the hierarchic rule seems to be required if the historic value of these long known stages is not to be violated since stage boundaries should not be defined a posteriori to fit series boundaries we propose here to reconsider the role of the standard stage as the obligate elemental subdivision in a rigidly hierarchical chronostratigraphic scale and suggest that series and subseries should be formalized and their boundaries defined by gssps we do not suggest that stages be abandoned on the contrary we propose to integrate them in a two step hierarchical chronostratigraphic framework so as to use them in complementary fashion with series and subseries however the boundary concept is more appropriate for stage than the gssp we recognize the need for precise and globally valid chronostratigraphic correlations based on gssps in attempts to understand the earth system while at the same time we emphasize the need for conceptual continuity with regard to the central place that the stage has played in chronostratigraphy over several decades

emplacement of semail emirates ophiolite at ridge trench collision the oman emirates is the largest and best exposed ophiolite consequently it has attracted significant interest among scientists together with serious conflicts most geologists regard this ophiolite as having formed in an intra oceanic subduction zone before being accreted to the arabian continent here we propose an alternative scenario supported by detailed field observations and integrated geophysics the smaller emirates part of the ophiolite was forced into a nearby continent in the pre collision stage of tethyan closure the contraction led to the exhumation of the mantle floor of segmented basins accreted in a rifted system similar to the present day gulf of california the implied high temperature high pressure metamorphism and the range of geochemical signatures were introduced during the process of rifting whereas the larger oman ophiolite was emplaced by obduction onto and along the subducting continental shore this ridge trench transform system might call for a new process to obduct over continents to obduct over continents in particular tethyan ophiolites tethyan ophiolites

a new approach to an absolute timescale from measurements of orbital cycles and sedimentary microrhythms when the sedimentary microrhythms of two groups of jurassic zones forming part of the chronostratigraphical scale are analysed against recent radiometric age scales there is good agreement with present day cycles resulting from the obliquity of the ecliptic sedimentary microrhythms resulting from orbital forcing can be used to establish either the relative or the true length of chronostratigraphical zones and thus provide a new approach towards an absolute timescale such a scale will be powerful in the analysis of other cycles and perturbations in the geological record

transverse dune trailing ridges and vegetation succession abstract we describe the evolution of and vegetation succession on a previously undescribed landform transverse dune trailing ridges at el farallon transgressive dunefield in the state of veracruz mexico three dimensional clinometer compass and tape topographic surveys were conducted in conjunction with 1 m2 contiguous percent cover and presence absence vegetation survey transects at eight locations across two adjacent trailing ridges at the study site and elsewhere the transverse dune trailing ridges are formed by vegetation colonization of the lateral margins of active transverse barchanoidal transverse and akle or network dunes for simplicity all trailing ridges formed from these dune types are referred to as transverse dune trailing ridges because there are several transverse dunes in the dunefield multiple trailing ridges can be formed at one time two adjacent trailing ridges were examined the shortest length ridge was 70 m long and evolving from a 2 5 m high transverse dune while the longer ridge was 140 m long and evolving from an 8 m high dune trailing ridge length is a proxy measure of ridge age since the longer the ridge the greater the length of time since initial formation with increasing age or distance upwind species diversity increased as well as species horizontal extent and percent cover in turn the degree of bare sand decreased overall the data indicate a successional trend in the vegetation presence and cover with increasing age upwind those species most tolerant to burial croton and palafoxia begin the process of trailing ridge formation ipomoea and canavalia are less tolerant to burial and also are typically the next colonizing species trachypogon does not tolerate sand burial or deposition very well and only appears after significant stabilization has taken place the ridges display a moderately defined successional sequence in plant colonization and percentage cover with time and upwind distance they are significant geomorphologically as a unique landform in transgressive dunefields and also because they may be the only remaining indication of transverse dune presence and net dune migration direction once the dunefield is stabilized and in a final evolutionary state

storm controlled oblique dunes of the oregon coast the large mean height 25 m spacing 300 m relatively straight crested dunes of the central oregon coast migrate an average of 3 8 m yr toward an azimuth of 26 the dunes are transverse to the strong south southwesterly winter storm winds that are responsible for their basic form orientation and migration the dry moderate north northwesterly summer winds modify the dune form but not the dune trend comparison of the sand transport calculated from wind data and the transport measured from dune migration indicates that the actual transport by the wet southerly winds is only one third of the amount calculated assuming dry conditions the resultant vector mean transport rate as recalculated by comparison of the measured and initially calculated rates is 34 m 3 m yr toward an azimuth of 45 the dunes are thus oblique by our definition of an oblique dune angle between dune trend and resultant transport direction between 15 and 75 the internal structures of the dunes confirm northward migration during wet conditions evidence for deposition during wet conditions includes slipface deposits deformed mostly by sliding and various structures formed by the adhesion of sand grains to wet surfaces most summer deposits are not preserved but those on the basal apron the gentle north slope at the base of the winter slipface have a high preservation potential a depositional model based on dune climbing predicts that the preserved record of oblique dunes formed by an obtuse bimodal wind regime would consist of tabular sets of crossbeds in which the dip angles increase upward from the base of each set

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

seismic velocity structure at the southeastern margin of the arabian peninsula summary the lithospheric structure in oman has been determined by analyzing teleseismic p receiver functions recorded at broad band and short period seismic stations of the oman seismological network lithospheric structure is obtained by jointly inverting receiver functions and rayleigh wave group velocities derived from continental scale surface wave tomography of pasyanos we observe relatively thick crust 40 48 km within the ophiolite formed mountains in northern oman the crustal thickness is about 35 km within the passive continental margin of the southern oman region uppermost 5 km crustal shear wave velocities are faster in the northern ophiolite regions compared to the southern oman region while shear velocities in the middle crust are faster in the southern oman region compared to the ophiolite region this observation coincides well with cretaceous to eocene marine platform sequences overlying precambrian to cambrian basement of the southern part joint inversion analysis shows that the moho depth of oman varies from 34 km in the southern region to 48 km in the northern part

grain size distribution and rheology of the upper mantle abstract the physical processes that govern the grain size of rocks in the upper mantle are examined the analysis is based on the experimental data on creep recrystallization and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere the grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate above this depth the microstructures record the thermomechanical history of rocks rather than the in situ stress in the case of the oceanic lithosphere where the thermomechanical history is best known the following features of grain size distribution are found at the uppermost mantle where the amount of grain growth is limited the grain size is determined by the initial value and the growth rate and where the effect of grain growth dominates it increases with depth when the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform the grain size is then determined by the applied stress this grain size is however frozen when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization thus at the intermediate depth region the grain size records the fossil frozen stress at which the microstructures of rock have been frozen since the frozen stress increases with age the grain size in this depth interval decreases with depth finally the grain size below this level reflects the in situ stress and increases with depth its extent being dependent on the nature of return flow in the deep mantle thus the grain size versus depth relation may show a sigmoid curve the qualitative features of this curve may be similar also in the case of the continental lithosphere if a similar thermal event i e the intrusion of hot material and subsequent cooling occurs the results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules ave lallemant et al 1980 the present model suggests that the depth of minimum grain size 65 and 150 km at the continental rift zone and the shield region respectively corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels 1 mpa and in the geological time scale this result leads to a new definition of the thickness of lithosphere in terms of its rheological properties this thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness the model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy

rotation of the oman ophiolite and initial location of the ridge in the hotspot reference frame abstract the volcanic units of the oman ophiolite were paleomagnetically studied in order to better constrain the large rotation of the nappe during emplacement suggested by previous paleomagnetic results the volcanic sequence going from pillow basalts extruded at the mid oceanic ridge up to submarine lavas extruded during the oceanic thrusting before the end of the obduction displays components of magnetization which were acquired at various stages during the movement of the nappe on the sea floor paleomagnetic euler pole analysis shows that the movement can be modeled by a single large rotation of about 145 150 around an euler pole located close to the nappe less than 200 km away combining plate tectonic concepts a mantle anchored hotspot reference frame and paleomagnetic data we were able to determine the initial paleolatitude around 10 n the initial paleolongitude around 45 e as well as the paleorientation nnw sse to n s of the ridge in hotspot coordinates the linear velocity of the nappe between the period of accretion at the ridge and the end of the obduction onto the arabian peninsula would have been slightly less than 2 cm per year