oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

geology and tectonics of the betic zone se spain abstract the southern margin of the iberian continent comprises a complex geological history which can be traced back into the proterozoic when a gneissic basement was formed a cover of nevado filabride rocks was deposited onto this proterozoic basement during the palaeozoic the permian intrusion of the bedar granite suggests that the palaeozoic sedimentary rocks now exposed in the betic cordilleras have been affected by the hercynian orogeny similar to that of palaeozoic rocks in the adjoining ossa morena zone the hercynian basement was probably covered by alpujarride carbonates in the mesozoic when rifting opened tethys and intruded 140 ma old basic dyke swarms the palaeozoic basement of the betic cordilleras comprises a suture of late cretaceous subduction 80 85 ma b p this can be inferred from hp lt metamorphic facies obducted in coeval thrust slices of nevado filabride rocks the nevado filabride nappes include mesozoic carbonates transformed into marbles macael the suture was probably annnealed after the closure of tethys the mesozoic boundary between africa and europe and the rifting in tethys ceased during the palaeogene when the atlas mountains were formed 36 ma b p subsequently relative movements between africa and europe in the neogene were accommodated by strike slip faults these define the iberian microplate moving eastward by atlantic spreading and possibly subducting under italy neogene deformation of iberia is most intensive in southeastern spain due to emplacement of a mantle diapir under the alboran area this occurred early in the neogene aquitanian and caused nappe spreading involving alpujarride and malaguide rocks the subsequent neogene history has now been studied in great detail and is reviewed here it involves clear examples of calc alkaline volcanism most probably not related to subduction seismic motion along strike slip faults within the iberian microplate and syntectonic deposition in intermontane neogene basins

trace elements in ilmenite a way to discriminate provenance or age in coastal sands trace elements and ti percentage in ilmenite grains magnetically separated from modern and late pleistocene coastal sands of southeastern virginia and northwestern north carolina were used to distinguish different deposits multivariate analysis of ilmenite composition ti mn mg cr v ni and cu from coastal deposits and potential source rivers enabled the identification of dominant source rivers using the trace element content of one mineral instead of heavy mineral suites eliminated most of the hydraulic sorting selective weathering and intrastratal solution problems that often obscure heavy mineral provenance determinations most ilmenite grains lacked exsolution or twinning which are common to ilmenite however there were no significant optical differences between river and coastal deposits and thus weathering effects were considered to be negligible in provenance determinations based on ilmenite composition owing to the dynamic mixing of beach sands during deposition they contained more homogeneous ilmenite trace element values than did river or bay sands late pleistocene and modern beach deposits were compositionally similar but different from associated bay sands bay sands were more similar to different source river deposits than were beach sands despite a similar primary or distal provenance subtle differences in the mixture of proximal sources were revealed between the ilmenite composition of samples from a modern and a late pleistocene beach deposit besides aiding in provenance determination ilmenite trace element content thus might be used for distinguishing beach deposits of different ages and for subsurface correlation of discontinuous segments from a barrier island chain or other similarly well mixed sand deposit

a new mesozoic isochron chart of the northwestern pacific ocean paleomagnetic and tectonic implications a new mesozoic isochron chart of the northwestern pacific ocean has been completed by utilizing all the currently available data from 357 cruises our chart provides the first whole view of the mesozoic magnetic anomaly lineations in the northwestern pacific ocean and yields essential information for understanding the problems of the mesozoic magnetostratigraphic time scale and the evolutional history of the pacific plate the mesozoic magnetostratigraphic time scale must be recalibrated using not only the hawaiian lineation set but also other mesozoic lineation sets the lack of lineation m0 in the lineation sets other than the hawaiian lineation set indicates that a reorganization of the ridges except the pacific farallon ridge occurred between chrons m1 and m0 nearly 120 ma the eruption rate of the shatsky hotspot seems to be comparable to that of the reunion hotspot at their initial stage the growth of the pacific plate began in a small triangular region at about 190 ma

controls on focused flow and dispersed flow deepwater carbonates miocene agua amarga basin spain abstract common models for carbonate slope or basin margin sedimentation emphasize the contrasts between linear sediment sources and point sediment sources geologic mapping and high resolution stratigraphic study of a set of miocene outcrops exceptional for their three dimensional exposure and preservation of paleotopography demonstrates that paleotopographic focus or lack thereof may be a dominant control on geometries and sediment character of sediment gravity flow deposits in line sourced systems in the agua amarga basin se spain basin topography is initially modified by subaerial exposure and erosion of volcanic basement to form paleovalley systems after late miocene marine inundation topography is partially modified by infilling of high energy shallow water heterozoan deposits units 1 and 2 that subdued topography in one area to produce a ramp like surface but did not significantly infill a large paleovalley located in another area further relative rise in sea level resulted in an extensive reefal platform that formed along a paleo high adjacent to both features la rellana platform subsequent sediment gravity flow deposits shed from this reefal platform followed two pathways down the ramp like surface dispersed flow or into the large flooded paleovalley focused flow deposits within each system display contrasting lithofacies architectures these deepwater deposits units 3 7 in this study consist of carbonate breccia facies coarse facies graded skeletal packstone facies coarse facies and planktonic foraminifera rich facies fine facies that are interpreted as sediment gravity flow deposits and pelagic sediments relative sea level fluctuations resulted in deepwater facies variations in the large paleovalley as a result of significant backstepping and progradation of the rellana reefal platform on a gently sloping surface fining of basinal deposits and shelfward migration of the locus of deposition for sediment gravity flows is associated with sea level rise coarsening of basinal deposits and basinward migration of the locus of deposition for sediment gravity flows is associated with stillstand and relative fall in sea level both relationships demonstrate a direct link between the locus of reefal platform growth and style of basin sedimentation early parts of relative rises in sea level late parts of highstands and falling stages led to transport of abundant coarse sediment into the basin focused flow deposits were line sourced and funneled into the axis of a large margin parallel paleovalley and confined to a channel volume ratio of coarse facies to fine facies is estimated at 0 7 deposits in proximal paleovalley locations display compensation geometries around positive relief on debrites channelization erosional truncation lapout against paleovalley walls and some drape as a result of lateral confinement and proximity to the sediment source deposits in distal paleovalley locations display increasingly tabular geometries as a result of decreasing lateral confinement and concentration of coarse grained sediment gravity flows down depositional dip dispersed flow deposits were line sourced into the basin along a gently inclined ramp like surface that lacked paleotopographic focus volume ratio of coarse facies to fine facies is estimated at 0 1 the less complex sheet like depositional geometries and locations of these deposits result from the absence of lateral confinement and subtle changes in substrate slope focused flow sediment gravity flow deposits that are confined by topographic elements such as channels are most likely to form sizeable accumulations of coarse facies with characteristics most appropriate for conventional oil and gas exploration there are many paleotopographic configurations that can lead to focusing of flows so this deepwater carbonate system may be more common than previously recognized

geomorphology and neogene tectonic evolution of the palomares continental margin western mediterranean abstract the palomares continental margin is located in the southeastern part of spain the margin main structure was formed during miocene times and it is currently part of the wide deformation zone characterizing the region between the iberian and african plates where no well defined plate boundary occurs the convergence between these two plates is here accommodated by several structures including the left lateral strike slip palomares fault the region is characterized by sparse low to moderate magnitude mw

ecological snapshot of a population of panopea within their traces pliocene agua amarga subbasin se spain abstract pliocene deposits cropping out in the agua amarga subbasin almeria se spain include a composite shell bed made up of variously preserved and densely packed mollusks the characteristics of the shell bed indicate deposition in shallow marine settings under a changeable sedimentation rate the composite shell bed was formed through the amalgamation of several depositional events mostly connected with storm events during relatively slow sedimentation the bivalve panopea colonized the substratum the colonization took place over different stages of the shell bed formation as reflected by differently preserved panopea scattered throughout the shell bed the upper part of the shell bed contains several tens of panopea preserved within their burrows scalichnus cf phiale hanken et al 2001 all bivalves in s cf phiale are articulated and preserved in life position they are confined to a single horizon and most probably represent a single fossil population composed of adult individuals of panopea resulting from anastrophic burial by storm deposits as a consequence the bivalves in their burrows succumbed at the same time that is the study case represents an ecological snapshot or ecological census

tectonic significance of the present relief of the betic cordillera abstract tortonian calcarenites of the betic cordillera were deposited in coastal or very shallow marine environments and represent an ideal marker for estimating vertical movements from the late miocene to the present a map showing the heights at which these tortonian marine rocks are situated has a clear correlation with the present relief indicating that today s relief has been formed since the tortonian there is also a good correlation between present relief and the bouguer anomaly distribution in the betic cordillera as well as with crustal thickness likewise the present relief is directly related to the geodynamic setting of a horizontal n s to nnw sse compression and an almost perpendicular extension along with isostatic readjustment existing in the betic cordillera from the tortonian as a result of these regional stresses faults and folds have produced notable vertical movements the highest rates of uplift of the betic cordillera coincide with large antiforms in particular those of the sierra nevada and the sierra filabres several subsiding sectors also exist for example the granada basin or the guadalquivir basin the foreland guadalquivir basin has a complex history because the uplift in its eastern sector and subsidence in the western sector coexisted during the late tortonian today the whole betic cordillera is characterized by differential regional uplift even in the aforementioned subsiding sectors

the mesozoic tectonic history of the magellan microplate in the western central pacific magnetic lineation mapping in the western central pacific has revealed a pair of opposite sensed fanned lineation patterns that define the accretionary boundaries of the fossil magellan microplate this tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations the entire evolutionary history of the magellan microplate is well constrained to a 9 m y period in the early cretaceous by synchronous spreading patterns and associated geologic data during this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km 600 km with spreading centers on two opposing sides and transform faults on the other two sides the lifetime and size of the magellan microplate are somewhat longer and larger respectively than presently active microplates on the east pacific rise however these modern structures are still evolving and growing and the tectonic behavior of the modern and cretaceous systems appears to be similar study of both active and fossilized microplates should provide additional insights on their common tectonic histories in particular we show that the magellan trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history the magellan trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the pacific farallon and phoenix plates at ml0n time microplate activity ceased when the microplate became welded to the pacific plate at m9 time

the impact of quaternary sea level and climatic change on coastal alluvial fans in the cabo de gata ranges southeast spain conventionally a fall in base level is seen as stimulating incision into the distal zones of alluvial fans in the cabo de gata ranges of southeast spain evidence exists to the contrary two sets of quaternary coastal alluvial fans demonstrate the interaction between climatically driven variations in the supply of sediment and eustatically driven changes in base level the fans are supplied from miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development the evolution of the east coast fans has been affected by variations in sediment supply and changes in sea level the west coast fans were buffered from the effects of changes in sea level by coastal barriers three phases of past sedimentation can be identified on the fans these can be differentiated on the basis of field observations of soil profiles particularly colour of the b horizons and accumulation of caco3 and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties the two earlier major sedimentation phases were coincident with global glacials ca 135 ka and ca 85 10 ka based on the stratigraphy and uranium thorium dating of the coastal sediments high sea levels during the intervening interglacial and during the holocene caused erosion of the distal zones of the east coast fans which led to channel incision into the fan surfaces on the west coast fans no such incision occurred simply proximal incision by small fanhead trenches the youngest relatively minor phase of fan sedimentation has occurred during the holocene these contrasting contexts have produced differing styles of fans with telescopic fan morphology on the east coast and stacked morphology on the west coast fans the differences are reflected in the fan profiles with steeper gradients dominating the east coast fans and extensive lower gradient distal surfaces on the west coast fans fan morphometry based on analysis of the residuals from drainage area to fan area and gradient regressions also differentiates between the fan contexts the fan building phases appear to be controlled proximally by climatically driven pulses of sediment supplied to the fans these occurred during global glacials coincident with low sea levels and caused fan progradation onto the exposed foreshore the intervening global interglacials were times of little fan sedimentation and on the east coast where high sea levels were able to erode the fan toes deep through fan dissection ensued

formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate application to the 95 5 w galapagos propagator the geometry of oceanic spreading centers can change by rift propagation in which a new ridge segment propagates and preempts a preexisting one as the locus of relative motion between the two plates shifts the dying and growing ridge segments are joined by a boundary usually a transform which migrates with time causing seafloor to be transferred from one plate to the other as a result originally ridge parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction an attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics since relative motion occurs only between points on different plates recent surveys of the galapagos propagating rift system at 95 w show seafloor fabric curving away from the growing rift and toward the failed rift these observations have been interpreted as requiring a zone of distributed simple shear between the two rifts such behavior would represent a local deviation from rigid plate tectonics since relative motion would be distributed over a finite region not part of either of the two plates here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation we find that such fabric can be generated by changes either in the velocity rate or direction of rift propagation or in the spreading rate during rift propagation the curvature of various tectonic elements is diagnostic of the different possibilities the geometry observed at the galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate propagator acceleration and spreading rate slowing which produce similar structural trends could in principle be distinguished using spreading rate data at galapagos however since either would have occurred within the present period of normal polarity both are consistent with the magnetic anomaly data we thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics it may however be possible to discriminate between rigid and shear models using fabric observations at other propagators the shear model requires that the sense of curvature at galapagos be a ubiquitous feature of active propagators in contrast the rigid plate tectonic model allows for fabric with a variety of orientations including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities

nearshore temperate carbonate depositional systems lower tortonian agua amarga basin southern spain implications for carbonate sequence stratigraphy abstract the bryozoan rich lower tortonian carbonates of the agua amarga basin in southern spain province of almeria provide an example of sediments formed in a nearshore non tropical depositional setting based on data derived from logging of sections and from field mapping these lower tortonian carbonates form a depositional sequence which is subdivided into several depositional systems a lowstand systems tract which consists of volcaniclastic fan deltas and washover deposits formed on the leeward side of a basement shoal which delimited the basin towards the south a transgressive systems tract which is characterised by a landward encroachment of deposits is represented by submarine bars and dune deposits a highstand systems tract consists of progradational beach deposits owing to strong reworking and re distribution of the particles by currents and waves microfacies differentiation of the deposits in poor these tortonian non tropical carbonates display a style of deposition similar to siliciclastics suggesting that sequence stratigraphic concepts derived from tropical carbonates do not apply to non tropical carbonates

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

compressional tectonic inversion of the algero balearic basin latemost miocene to present oblique convergence at the palomares margin western mediterranean interpretation of new multichannel seismic reflection profiles indicates that the palomares margin was formed by crustal scale extension and coeval magmatic accretion during middle to late miocene opening of the algero balearic basin the margin formed at the transition between thinned continental crust intruded by arc volcanism and back arc oceanic crust deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between n50 e striking reverse faults and associated folds like the sierra cabrera and abubacer anticlines and n10 20 e sinistral strike slip faults like palomares and terreros faults parametric subbottom profiles and multibeam bathymetry offshore structural analysis available gps geodetic displacement data and earthquake focal mechanisms jointly indicate that tectonic inversion of the palomares margin is currently active the palomares margin shows a structural pattern comparable to the north maghrebian margins where africa eurasia plate convergence is accommodated by ne sw reverse faults nnw sse sinistral faults and wnw ese dextral ones contractive structures at this margin contribute to the general inversion of the western mediterranean since 7 ma coeval to inversion at the algerian margin shortening at the alboran ridge and al idrisi faults occurred later since 5 ma indicating a westward propagation of the compressional inversion of the western mediterranean

the monte del casino section northern apennines italy a potential tortonian messinian boundary stratotype abstract results are presented from a high resolution integrated stratigraphic study of the monte del casino section which is considered as a candidate boundary stratotype section for the tortonian messinian t m boundary the section yields a good to excellent cyclostratigraphy tephrastratigraphy and planktonic foraminiferal and nannofossil biostratigraphy it can be correlated in detail to other mediterranean sections using cyclostratigraphic patterns in combination with the planktonic foraminiferal biostratigraphy these correlations indicate that the succession of the monte del casino composite section is continuous in the t m boundary interval and that all sedimentary cycles volcanic ash layers and bio events can be accurately dated by tuning the sedimentary cycles to astronomical target curves the quality of the paleomagnetic signal varies considerably throughout the section a reliable magnetostratigraphy cannot be established for the t m boundary interval because of a secondary magnetisation carried by iron sulphides cyclostratigraphic correlations indicate that discrepancies exist between the position of the reversals recorded here and those found in earlier magnetostratigraphic studies comparison of 40 ar 39 ar ages of ash layers with their astronomical ages show that biotite ages give a good approximation of the age of the section but are not suitable for high resolution chronology despite these short comings the monte del casino section remains a candidate to define the t m boundary but its qualities should be critically weighted against those of other sections

depositional facies of high energy beach to offshore sequence comparison with low energy sequence a study of the nearshore zone and continental shelf of a high energy coast was made in the ventura port hueneme area of california during the summer of 1974 to investigate depositional facies characteristics and compare them with similar facies from a low energy coastal setting analysis of box cores can cores and vibracores permits recognition of three principal zones 1 nearshore facies backshore to 9 m water depth made up primarily of parallel ripple and cross bedded clean sand bioturbation is only locally significant 2 transition facies 9 to 18 m water depth a zone of fine sand and silty sand characterized by an increase in biogenic over physical sedimentary structures that are commonly preserved as laminated to burrowed beds wave ripple bedding and parallel lamination are important structures in this facies 3 offshore facies 18 m water depth sandy silt is the primary texture and bioturbation is the dominant sedimentary structure remnant parallel lamination is the only physical sedimentary structure present comparison of the results of this study with a previous description of a low energy beach to offshore sequence at sapelo island georgia indicates that the two areas do not differ greatly in their facies or overall vertical sequence of sedimentary structures the principal difference is in the thickness of the three facies the california facies are significantly thicker than their georgia counterparts this difference is in direct response to the role of higher wave energy on the california coast

pinning points a method providing quantitative constraints on relative sea level history abstract quantitative constraints on the history of relative sea level allow for a better understanding of the controls on depositional sequence development the constraints are provided by pinning point curves plots of ancient relative sea level elevations pinning points versus time constructing a pinning point curve requires identification of ancient stratigraphic positions of sea level through interpretation of facies and surfaces formed at sea level then their ancient relative elevations are determined through reconstructing aspects of ancient paleotopography upper miocene strata from las negras southeastern spain preserve paleotopography contain ancient surfaces of subaerial exposure and contain facies deposited near sea level the pinning point curve illustrates a complex relative sea level history with large and small scale relative sea level fluctuations defined by 31 pinning points

record of climatic change in neritic carbonates turnover in biogenic associations and depositional modes late miocene southern spain in order to evaluate the geological record of climatic change in neritic carbonates we studied late miocene rock outcrops in southern spain six episodes of reef growth are documented burdigalian to messinian in neogene basins of the betic cordillera which were located close to the margin of the global reef belt the reefs are characterized by various zooxanthellate corals which decrease in diversity with time andhalimeda the youngest reefs of the latest messinian are characterized by the dominance of the genusporites late miocene coral reefs and reef rimmed platforms alternate over time with non reefal carbonate ramps characterized by skeletal calcirudites or with gypsum such as that formed during the messinian salinity crisis the calcirudites lack reef corals calcified green algae and extensive marine cement but exhibit skeletal components described from both modern and fossil nontropical carbonates these include bryozoans mollusks foraminifers echinoderms and minor balanids as well as coralline algae of a bryomol association the presence of some larger foraminifers indicates high temperatures close to the lower temperature threshold of the reef assemblage sea level lowstands and highstands are documented by wedges of bryomol carbonate and chlorozoan patch reefs or prograding platforms thus temperate climate depositional modes correspond to relatively low sea levels and warm water modes to high sea levels the neogene infill of the agua amarga and sorbas basins documents two of these cycles other climate sea level cycles including messinian gypsum in the cool water depositional mode are well established in adjacent neogene basins in southern spain this type of composite sequence seems to occur only along the margin of the global reef belt and indicates an oscillatory latitudinal movement of the margin which is associated with global climatic change the analysis of turnover in neritic depositional carbonate systems may therefore be considered a sensitive tool for reconstructing climatic change from the fossil record however warm water modes and temperate water modes of carbonate sedimentation and diagenesis differ significantly for this reason the interpretation of composite system sequences by sequence stratigraphy requires an extended concept the particular type of mixed bryomolchlorozoan depositional sequence also bears some potential for drowning because sea level rise may be faster than the net production rate of temperate carbonate systems

late jurassic early cretaceous evolution of the eastern indian ocean adjacent to northwest australia over 9700 km of new aeromagnetic data were acquired off the northwest coast of australia and combined with existing magnetic data to map magnetic isochrons in the eastern indian ocean the isochrons were used to constrain a tectonic model of the evolution of the seafloor in the argo cuvier and gascoyne abyssal plains a complete set of anomalies from m26 through m16 was found in the argo abyssal plain trending generally n70 e spreading commenced in the center of the basin at or prior to m26 and propagated outward until at least m24 time anomalies m10 mo recording the separation of australia and india were found in the cuvier and gascoyne abyssal plains with a trend of about n30 e a significant crustal age discontinuity occurs in the vicinity of the joey rise where the two lineation sets converge because there appears to be no overlap of isochron ages in the two groups it is not necessary to postulate that a triple junction existed off northwest australia as has been previously suggested at m4 m5 time a 10 clockwise change in spreading direction occurred on the cuvier gascoyne spreading system this event triggered ridge jumps that transferred two pieces of the indian plate to the australian plate overlapping spreading on the forming and dying ridges curved fracture zones and lineations as well as fanned lineation trends suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1 2 m y

geomorphic analysis of the sierra cabrera an active pop up in the constrictional domain of conjugate strike slip faults the palomares and polopos fault zones eastern betics se spain the nne ssw sinistral palomares and the conjugate dextral wnw ese striking polopos fault zones terminate in the sierra cabrera antiform in order to test the quaternary activity and topographic relief control in the termination of these fault zones here we present new qualitative and quantitative geomorphic analyses supported by a new structural map of the region the main mountain fronts of the cabrera antiform are formed by the north and south cabrera reverse faults that merge laterally into the palomares and polopos faults respectively these faults produce knickpoints stream deflections complex basin hypsometric curves high slk anomalies and highly eroded basins in their proximity furthermore the drainage network shows an s shaped pattern reflecting progressive anticlockwise rotation related to the sinistral palomares fault zone the estimated uplift rates determined by the integration between mountain front sinuosity index and valley floor width to height ratio are larger than those obtained for strike slip faults in the eastern betics these larger uplift rates with our geomorphic and structural dataset indicate that the topographic relief of the sierra cabrera antiform is controlled by reverse faults that form a pop up structure in the constrictional domain between the larger palomares polopos conjugate strike slip faults existing gps geodetic data suggest that the north and south cabrera reverse faults probably accommodate a large part of africa iberia convergence in the region

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

mesozoic radiolarian faunas from the antarctic peninsula age tectonic and palaeoceanographic significance new assemblages of radiolaria including some of the few occurrences of high southern latitude jurassic and cretaceous radiolarian faunas show that several localities in the lemay group of alexander island range in age from latest jurassic earliest cretaceous to at least albian by demonstrating that sedimentation and deformation in the lemay group was diachronous younging oceanwards to the northwest these new age assessments support the model of the lemay group as an accretionary complex the polarity of subduction beneath alexander island was not affected by arc collisions from at least the lower jurassic to the oligocene and such a long period of continuous accretion appears to be unusual deposition of the lemay group spans the kimmeridgian to albian sedimentation in the fossil bluff group fore arc basin thus making the earlier concept of the lemay group as pre jurassic basement untenable allochthonous latest jurassic earliest cretaceous radiolarian assemblages with some supposed tethyan affinities are present in the lemay group in contrast an in situ latest jurassic assemblage from the nordenskjold formation of the back arc basin and a further jurassic assemblage from a probable trench slope basin have characteristics believed diagnostic of high latitudes the biogeographic affinities of radiolarians from cherts in the lemay group accretionary complex suggest that both these cherts and associated basalts are far travelled slices of the phoenix plate rocks from the probable trench slope basin formerly assigned to the younger fossil bluff group fore arc basin sequence now appear to be part of a new previously unrecognized formation

topographic and sea level controls on oolite microbialite coralgal reef sequences the terminal carbonate complex of southeast spain the terminal carbonate complex of southeast spain is a miocene messinian unit of oolite microbialite and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown the relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface four sequences record sea level change with minimum amplitudes of 32 77 m 105 253 ft sequences commonly have local basal stromatolites overlain by local thrombolites ooid grainstone volcaniclastic rich planar bedded ooid grainstone and fenestral ooid grainstone at low substrate positions thrombolite boundstones are thicker and laterally more continuous than at higher positions at intermediate substrate positions relative to sea level history sequences have a build and fill architecture characterized by a relief building phase and a relief filling phase with thin sequences draping paleotopography microbialites dominate during the relative sea level rises and build topographic relief oolites dominate during the relative sea level falls and fill topographic relief at higher substrate positions close to highstand sequences thicken and yield stratigraphic character that is inconsistent with a build and fill model apparently the build and fill model requires an intermediate elevation substrate position and nonoptimal carbonate productivity during rapid sea level change sequences progressively show increasing diversity and more normal marine organisms possibly caused by decreasing aridity lithofacies of the la molata area show evidence of more restricted conditions compared to the la rellana ricardillo area lithofacies likely because la molata was in an embayment these results show that distribution of oolite microbialite and reef facies are predictable given known interaction among sea level paleotopography of the depositional surface substrate and coastline configuration

recalibration of the geomagnetic polarity timescale the quest for improved time control to estimate rates of geologic and paleoenvironmental processes is central to the earth sciences until the middle of this century geochronology was based solely on macrofossil evolution providing the largely unmodified definition of geologic stages in the last few decades with impetus from the deep sea drilling project dsdp and the ocean drilling program odp microfossil biostratigraphy has become increasingly important although new radiometric techniques have resulted in improved absolute age precision the direct correlation of absolute ages to biozonations and hence to geologic stage boundaries remains poor this is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations the bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished particularly for cenozoic and late mesozoic time through the geomagnetic polarity timescale gpts the template for late jurassic to quaternary geomagnetic polarity is derived from oceanic magnetic anomalies as demonstrated by heirtzler et al 1968 the gpts can be constructed using oceanic magnetic anomaly record s assuming constant seafloor spreading rate s and interpolating between available radiometric ages in the last quadrennium there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the gpts

ocean currents along a nearshore frontal zone on the continental shelf of the southeastern united states abstract river and estuarine discharges mix to form a frontal zone along a 400 km length of the coast of the south eastern united states the frontal width extends from the coast outward 10 20 km two ships anchored across the frontal zone during autumn obtained simultaneous hourly profiles of currents and density for five consecutive tidal cycles the frontal zone contained a baroclinic coastal current flowing southward the flow was strongly convergent more water entered on the coastal side than exited on the seaward side much of the inflow was apparently turned southward thus the frontal zone acted as a dynamic barrier that inhibited the advection of mixed river discharge farther offshore a thermal wind relationship suitably predicted vertical shear through the frontal zone bottom friction seemed to play only a secondary role data suggest that the baroclinic coastal current was modified by the presence of a barotropic current seaward of the frontal zone the force responsible for the barotrop

tectonic regime of the southern kurile trench as revealed by multichannel seismic lines the analysis of the two multichannel seismic reflection records p 855 and p 856 reveals the complexity of the tectonic structure of the kurile margin the absence of trench fill and the relatively constant thickness of the subducted sediments on top of the oceanic basement shows that little or no accretion is occurring at present the lower slope is characterized by active thrusting within a small accretionary prism the toe of the middle slope is a more consolidated unit that constitutes an efficient buttress a piggy back basin possibly filled by sediments that have drifted down slope has formed on top of this unit a narrow high marks the transition to the edge of the continental terrace furthermore the upper slope reveals an unconformity marking the top of the basement the similarity of this unconformity and its sedimentary cover with the well documented data acquired around dsdp legs 56 57 suggests that northern japan and the southern kurile forearcs may have experienced a similar cenozoic vertical history 1 regional setting the southwestern part of the kurile arc in the vicinity of the so called hokkaido corner underwent a polyphase tectonic evolution during the tertiary kurile subduction is believed to have begun in the early jolivet 1986 or late kimura and tamaki 1985 cretaceous after the accretion of the okhotsk terrane to the continent of siberia both the nemuro peninsula and the is

the barrier island system abstract barrier islands are part of a major coastal system composed of six interactive sedimentary environments the six environments are also elements needed to impose the designation barrier island on littoral sand bodies the elements are 1 mainland 2 backbarrier lagoon 3 inlet and inlet deltas 4 barrier island 5 barrier platform and 6 shoreface the morphodynamic and sedimentary evolution of each element affects adjacent environments as well as the entire system the interactive elements are defined and described by their morphologic sedimentologic and stratigraphic relationships to the barrier island and the barrier island system the stratigraphic relationships of the various elements of the system may be a valuable tool for paleogeographic reconstruction and sand body mapping

pacific microplate and the pangea supercontinent in the early to middle jurassic comment and reply reply we thank nazario pavoni for pointing out his previous observations of approximate bipolar symmetry in present day global tectonics and deep mantle seismic velocities pavoni 1991 1 the latter observation has been generally confirmed especially by more recent analyses of teleseismic s waves

mountain front migration and drainage captures related to fault segment linkage and growth the polopos transpressive fault zone southeastern betics se spain abstract the polopos e w to ese wnw oriented dextral reverse fault zone is formed by the north alhamilla reverse fault and the north and south gafarillos dextral faults it is a conjugate fault system of the sinistral nne ssw palomares fault zone active from the late most tortonian 7 ma up to the late pleistocene 70 ky in the southeastern betics the helicoidal geometry of the fault zone permits to shift se directed movement along the south cabrera reverse fault to nw directed shortening along the north alhamilla reverse fault via vertical gafarillos fault segments in between since the messinian fault activity migrated southwards forming the south gafarillos fault and displacing the active fault related mountain front from the north to the south of sierra de polopos whilst recent activity of the north alhamilla reverse fault migrated westwards the polopos fault zone determined the differential uplift between the sierra alhamilla and the tabernas sorbas basin promoting the middle pleistocene capture that occurred in the southern margin of the sorbas basin continued tectonic uplift of the sierra alhamilla polopos and cabrera anticlinoria and local subsidence associated to the palomares fault zone in the vera basin promoted the headward erosion of the aguas river drainage that captured the sorbas basin during the late pleistocene

non tropical carbonates related to rocky submarine cliffs miocene almeria southern spain abstract upper miocene deposits in the cabo de gata region se spain provide a unique opportunity to study cliff related temperate carbonates a poorly known type of fossil non tropical carbonates the studied submarine cliffs lie on the western flank of the monte ricardillo volcanic dome two main biocenoses colonised the cliff walls vertical and subvertical walls were either overgrown by vermetid gastropods forming a vermetid framestone or colonised by robust branching bryozoans which were reworked post mortem and accumulated as aprons at the foot of the submarine cliffs coralline algae in the vermetid build up indicate palaeodepths below 15 20 m depressions in front of the cliffs were occupied by nodular bryozoans and bivalves within these depressions barnacles settled on secondary hard substrates with progressive flooding of the depositional area substrate relief was filled in and a carbonate ramp with facies belts following the palaeobathymetric gradient evolved a delicate branching bryozoan facies occurs in the proximal part of the ramp and a nodular bryozoan bivalve facies in its middle part distally these deposits give way to a coralline algal facies these results provide a first insight into temperate water carbonates related to rocky submarine cliffs this work also reveals a major control of substrate relief on temperate carbonate facies and biofacies

coastal staircase sequences reflecting sea level oscillations and tectonic uplift during the quaternary and neogene many coasts feature sequences of quaternary and neogene shorelines that are shaped by a combination of sea level oscillations and tectonics we compiled a global synthesis of sea level changes for the following highstands mis 1 mis 3 mis 5e and mis 11 also we date the apparent onset of sequences of paleoshorelines either from published data or tentatively extrapolating an age for the uppermost purported oldest shoreline in each sequence including the most documented mis 5e benchmark we identify 926 sequences out of which 185 also feature holocene shorelines six areas are identified where elevations of the mis 3 shorelines are known and 31 feature elevation data for mis 11 shorelines genetic relationships to regional geodynamics are further explored based on the elevations of the mis 5e benchmark mean apparent uplift rates range from 0 01 0 01 mm yr hotspots to 1 47 0 08 mm yr continental collision passive margins appear as ubiquitously uplifting while tectonic segmentation is more important on active margins from the literature and our extrapolations we infer ages for the onset of formation for 180 coastal sequences sea level fingerprinting on coastal sequences started at least during mid miocene and locally as early as eocene whether due to the changes in the bulk volume of seawater or to the temporal variations in the shape of ocean basins estimates of eustasy fail to explain the magnitude of the apparent sea level drop thus vertical ground motion is invoked and we interpret the long lasting development of those paleoshore sequences as the imprint of glacial cycles on globally uplifted margins in response to continental compression the geomorphological expression of the sequences matches the amplitude and frequency of glacial cyclicity from middle pleistocene to present day moderately fast 100 000 yrs oscillating sea levels favor the development of well identified strandlines that are distinct from one another pliocene and lower pleistocene strandlines associated with faster cyclicity 40 000 yrs are more compact and easily merge into rasas whereas older cenozoic low frequency eustatic changes generally led to widespread flat lying coastal plains

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

response of plio pleistocene alluvial systems to tectonically induced base level changes vera basin se spain tectonics are perceived to be a major control on the positioning and long term evolution of alluvial systems by increasing slope gradients through uplift and tilting or by changing local base level incision can be stimulated leading to a switch in the active area of sedimentation an example of such a sedimentary response to tectonic activity is provided by well exposed late pliocene early pleistocene alluvial sediments of the salmeron formation from the western margins of the vera basin se spain early stage palaeogeographic reconstructions demonstrate the occurrence of two alluvial fan bodies with distinct palaeocurrent and provenance signatures that suggest sediment source areas from the north sierra lisbona and south sierra de bedar of the study area late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the sierra de bedar in the south proximal parts of this braided river are incised by up to 100 m into underlying fan sediments sourced from the sierra de bedar in distal areas incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced from the sierra lisbona the switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the vera basin during the early pleistocene extensional faulting resulted in uplift and subsidence leading to localized tilting of depositional surfaces in distal areas of the southern fan increased stream power resulted in headward incision through mid and proximal fan areas once the system became fully trenched distal base level controls became effective in generating an enlarged catchment area and continuing incision the resultant changes dramatically increased both sediment and water discharge to the alluvial system and a switch to braided river sedimentation

pre cretaceous tectonic evolution of the pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the jurassic quiet zone abstract linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western pacific ocean new aeromagnetic data allowed the magnetic bight to be more accurately mapped from m21 to m28 and enabled the identification of low amplitude magnetic lineations in the jurassic quiet zone these lineations were formed by magnetic field reversals prior to m29 a revised jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than m29 numbered m30 m38 these reversals extend the record of geomagnetic polarity back in time by approximately 8 ma and are important constraints on the origin of the quiet zone in particular they imply that the jurassic was not a period of constant normal polarity an explanation offered by some authors further they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period the early history of the northern pacific plate and pacific farallon izanagi p f i triple junction was traced by mapped magnetic isochrons the pacific plate seems to have evolved from a small plate that formed near the phoenix farallon izanagi triple junction about 180 188 m y ago at approximately 17 n 160 e in present coordinates until m21 time the evolution of the northern pacific plate was relatively simple and the p f i triple junction migrated north northwest with respect to the pacific

calcareous nannofossil response to the weissert episode early cretaceous implications for palaeoecological and palaeoceanographic reconstructions abstract the causal link between changes in calcareous nannofossil abundances their size and the palaeoenvironmental conditions in the tethyan realm across the valanginian weissert episode was examined through high resolution quantitative analyses of the genera nannoconus and micrantholithus from the angles section vocontian basin se france a set of published mineralogical micropalaeontological and geochemical data from numerous european localities including nannofossil data from italy france and romania and from the atlantic and pacific oceans allows us to propose a coherent palaeoecological and palaeoceanographic model and to discuss the possible forcing factors behind the palaeoenvironmental changes that occurred during the valanginian in the vocontian basin the genus nannoconus appears to be particularly sensitive to global fertility conditions and local continental input while micrantholithus is clearly associated with salinity levels during the first phase of the weissert episode a global increase in nutrients together with a breakdown of the upper water column stratification explains the nannoconid decline elevated freshwater fluxes into the vocontian basin under warmer and more humid conditions account for pentalith decreases low resolution data on pco 2 and ocean ca 2 concentrations cannot highlight large magnitude rapid fluctuations in pco 2 and oceanic ca 2 if they exist however relatively low pco 2 levels and high oceanic ca 2 concentrations probably helped the recovery of nannoconus and micrantholithus from the aftermath of the weissert episode to the early hauterivian the long term changes observed in the valanginian nannolith distribution are consistent with increased hydrothermal activity that influenced fertility levels low to mid latitudinal monsoonal circulation probably contributed to their short term changes by the way of rapid changes in the chemical weathering pattern that triggered significant fluctuations in turbidity fertility and salinity conditions

transport and fate of river discharge on the continental shelf of the southeastern united states the authors analyzed 8 years of wind and salinity data from a frontal zone in a region of the inner continental shelf off the southeastern united states the changes in low salinity water stored in the frontal zone have been parameterized by analyzing the monthly rate of change in freshwater content when the rate of change in freshwater content was negative this was interpreted as a loss of low salinity water from the frontal zone when this parameter was compared with seasonally averaged alongshore wind stress the rate of loss was independent of the alongshore wind stress magnitude until threshold of about 0 1 dyne cm sup 2 was reached above the threshold there was a clear relationship between northward alongshore wind stress and rate of loss of freshwater from the inner shelf experimental evidence suggests that horizontal currents in the inner shelf frontal zone have cyclonic shear with increasing depth when wind stress is northward and offshore near surface low salinity water is transported offshore by ekman transport while near bottom high salinity water is transported shoreward 9 figures 2 tables

late neogene recent uplift of the cabo de gata volcanic province almeria se spain abstract cabo de gata is a miocene volcanic province in the betic cordillera in southeastern spain the distribution of coastal deposits in the successive marine sedimentary units overlying the last volcanic episode about 7 5 ma old has been used to reconstruct the post volcanic palaeogeographic evolution of the region during the late neogene the current elevation of well dated shoreline marker rocks has been used to estimate uplift amount and rates since the late tortonian a n45 e aligned the strike of the carboneras fault system topographic relief was emergent in the cabo de gata region the extension and height of this island increased throughout the late neogene smaller independent islands were emergent and finally became connected to the main island during the messinian the carboneras and agua amarga pliocene sub basins were the last two marine basins prior to the final emergence of the region since the last volcanic eruption ca 7 5 ma maximum uplift of sedimentary rocks in cabo de gata has taken place on the western margin of the n45 e aligned palaeorelief the altitude of the shoreline marker rocks in the successive sedimentary units decreases eastwards to the present day coast and northwards of the rambla del plomo uplift rates since deposition remain nearly constant for the successive messinian rocks and decrease slightly for the lower pliocene outcrops most of the uplift took place before the pliocene while the main island enlarged uplift amounts and rates since deposition of the upper neogene sedimentary units in the cabo de gata area are similar to the ones estimated for laterally equivalent units in the eastern betic basins approximately 50 m ma despite its volcanic nature and the occurrence of the carboneras fault system the cabo de gata probably became elevated as a consequence of regional uplift in connection with the rest of the betic cordillera

depositional sequences and correlation of middle to late miocene carbonate complexes las negras and nijar areas southeastern spain during serravallian through messinian time marine carbonates flanked topographic highs that rimmed neogene basins in the western mediterranean middle to upper miocene carbonate strata in the las negras and nijar areas southeastern spain are 50 150 m thick and display 50 200 m of shelf to basin relief over 1 2 km detailed studies in those areas document the effects of relative sea level change on sedimentation biotic composition and reef development we identify three previously unrecognized regionally correlatable depositional sequences ds1 ds2 ds3 that occur between the underlying basement and the overlying terminal carbonate complex the lower depositional sequences ds1 ds2 are mostly normal marine shelf ramp carbonates deposited on the flanks of basement highs the basal part of ds2 locally contains some megabreccia reef blocks composed of tarbellastraea and porites these blocks are the first evidence of reef growth in the area and represent a previously unrecognized period of reef development prior to the fringing reef development the reef blocks probably formed as upslope patch reefs that were eroded and transported to distal slope locations the upper sequence ds3 is characterized by clinoform strata of a porites dominated fringing reef complex that prograded basinward in a downstepping style with successively younger reefs forming in a topographically lower and more basinward position as a result of a net sea level drop regional correlation of miocene shallow marine strata between basins in spain and elsewhere in the western mediterranean is complicated because basins were semi isolated from adjacent basins making physical correlation impossible in addition age definitive biostratigraphic markers are poorly preserved in most of the miocene shallow water strata basinal sediments that are more easily dated by microfossils do not typically interfinger with the shallow marine strata in outcrop even where datable microfossils are found resolution of dating is poor our studies in the las negras and nijar areas illustrate the usefulness of integrating sedimentological geometric and biotic data with locally derived relative sea level accommodation space curves for correlation the relative sea level curves for each area show remarkable similarities in shape and magnitude of sea level changes these curves indicate several relative sea level fluctuations during miocene carbonate deposition prior to the major sea level drop at the end of ds3 deposition that culminated in the exposure of the basin margin deposits and the deposition of evaporites in basinal areas during the messinian the depositional sequences in the las negras and nijar areas may correlate with depositional sequences of similar age throughout the southern cabo de gata area in mallorca some 600 km to the northeast and possibly in other mediterranean locations the widespread occurrence and possible correlation of the depositional sequences suggest regional processes such as eustacy or tectonism for their formation the integration of sedimentological palaeontological and sequence stratigraphic studies and the construction of relative sea level accommodation space curves may help in the interpretation of depositional histories of shallow marine carbonate complexes and correlation of these strata between isolated areas other dating methods in addition to microfossil dating may allow for better age determination of the sequences and aid in identifying the importance of eustacy and tectonism in sequence development

patterns and average rates of late neogene recent uplift of the betic cordillera se spain abstract the facies distribution in the sedimentary units infilling a series of neogene basins has been used to reconstruct the relief generation and uplift across the internal zone of the betic cordillera in southern spain uplift amounts and average rates can be estimated using the current elevation of the outcrops of well dated deposits indicative of ancient sea level positions coral reefs and coastal conglomerates record the initial development of emergent betic relief during the langhian continental and marginal marine deposits indicate the existence of a large island centred on the present sierra nevada sierra de los filabres chain by the end of the middle miocene the precursor of the sierra nevada sierra de los filabres chain originally part of this large island remained emerged whilst the surrounding areas were re invaded by the sea during the early tortonian at the end of the tortonian the inland basins granada and guadix basins became continental while the sierras de la contraviesa sierra de gador and sierra alhamilla emerged separating the alboran basin from the alpujarra tabernas and sorbas basins which became narrow passages of the mediterranean sea in contrast the sierra cabrera emerged during the late messinian suggesting a progressive uplift from west to east of the sierras south of the sierra nevada sierra de los filabres chain during the pliocene only the low areas closest to the present day coast remained as marine basins and progressively emerged throughout this stage the highest average uplift rate recorded is 280 m ma for the sierra de gador although the average uplift rates of upper neogene coastal marine rocks since depositon have maximum values of approximately 200 m ma most of the uplift of the betic mountains took place before the early pliocene the recorded uplift of neogene rocks was highest at the margins of western sierra nevada where peaks higher than 3000 m occur the average rates of uplift were lower to the east of this major relief the main sierras and depressions in the present day landscape correspond respectively to the emergent land in which uplift was concentrated and to the marine basins that existed before the final emergence of the region the altitude of the sierras reflects the time at which they became emergent the highest mountains being the first to rise above sea level

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

hornblende content of georgia south carolina u s a nearshore sands support for shoreward sediment transport in the nearshore zone core top samples from 51 box cores taken about 1 km apart in several transects off hilton head island south carolina and tybee wassaw and ossabaw islands georgia have been analyzed for heavy mineral composition of acid insoluble 3 2 fractions to determine hornblende content and for acid soluble principally calcium carbonate plus carbonate fluorapatite content and grain size data for acid insoluble sand and gravel fractions 4 and for unacidified samples heavy mineral contents of acid insoluble 3 2 fractions range from 0 2 to 2 3 with hornblende and epidote the most abundant heavy minerals individual hornblende and epidote abundances range from 16 2 to 52 0 hornblende and epidote contents vary antithetically with maxima in hornblende content and minima in epidote content at nearshore and offshore ends of transects and with minima in hornblende content and maxima in epidote content over middle portions of transects ilmenite is the most abundant opaque mineral identified with abundances from 1 3 to 24 5 ilmenite maxima are associated with hornblende minima at several stations off hilton head and tybee islands apparently due to winnowing acid insoluble residues are very fine to fine 4 2 sands with very fine sand recent dominant within 5 km of shore and fine sand relict or palimpsest most abundant farther offshore acid soluble contents in our study area are 25 of total sample weight for nearly all samples analyzed and are often 15 high hornblende contents at seaward ends of transects indicate shoreward transport of hornblende rich sediments from the adjacent inner continental shelf low hornblende contents in middle portions of transects indicate dilution of hornblende rich sands by hornblende poor coastal plain sediments derived from coastal plain rivers and exposed by shoreface erosion 4 8 m deep and by tidal inlet erosion high hornblende contents at nearshore ends of transects reflect contribution of hornblende rich sediment from piedmont rivers to the nearshore zone

hooked and tubular coralline algae indicate seagrass beds associated to mediterranean messinian reefs poniente basin almeria se spain abstract abundant hooked and tubular crustose coralline algae cca record early messinian late miocene seagrass beds on the northern margin of the poniente basin in southeastern spain hooked and tubular shapes in cca result from their epiphytic growth over seagrass leaves and stems the lower messinian deposits consist of a thin retrograding subunit overlain by sigmoidal bed packages that prograded to the southwest at least 650 m from the basin margin seagrass deposits mainly occur as bivalve cca packstone to rudstone and cca floatstone seawards of a conglomerate facies belt in the retrograding subunit and downslope of coral reefs and serpulid packstone to rudstone in the prograding sigmoidal beds fragments of contorted foliose cca algal flakes some hooked or tubular are common components in the seagrass deposits they mainly belong to an unidentified species of mesophyllum like their present day counterparts the epiphytic cca grew attached to seagrass by the initial parts of their thalli and displayed unattached projections or open frameworks of contorted foliose plants in a few cases hooked cca occur attached to halimeda plates small benthic foraminiferal assemblages are further evidence of macrophyte vegetation hooked contorted foliose cca can be a helpful tool for the identification of ancient seagrass beds especially in mediterranean neogene and quaternary deposits in which foraminiferal taxa fully indicative of seagrass ecosystems are scarce

quantitative controls on location and architecture of carbonate depositional sequences upper miocene cabo de gata region se spain abstract sequence stratigraphy pinning point relative sea level curves and magnetostratigraphy provide the quantitative data necessary to understand how rates of sea level change and different substrate paleoslopes are dominant controls on accumulation rate carbonate depositional sequence location and internal architecture five third order 1 10 my and fourth order 0 1 1 0 my upper miocene carbonate depositional sequences ds1a ds1b ds2 ds3 tcc formed with superimposed higher frequency sea level cycles in an archipelago setting in se spain overall our study indicates when areas of high substrate slope 15 are in shallow water independent of climate the location and internal architecture of carbonate deposits are not directly linked to sea level position but instead are controlled by location of gently sloping substrates and processes of bypass in contrast if carbonate sediments are generated where substrates of low slope 15 6 cm ky to 2 cm ky and overall relative sea level rose at rates of 17 21 4 cm ky higher frequency sea level rates were about 111 to more than 260 cm ky producing onlapping fining deepening upward cycles decreasing accumulation rates resulted from decreasing surface area for shallow water sediment production drowning of shallow water substrates and complex sediment dispersal related to the archipelago setting typical systems tract and parasequence development should not be expected in bypass ramp settings facies of onlapping strata do not track base level and are likely to be significantly different compared to onlapping strata associated with coastal onlap basal and upper ds2 reef megabreccias indicating the transition from cool to warmer climatic conditions were eroded from steep upslope positions and redeposited downslope onto areas of gentle substrate during rapid sea level falls 22 7 cm ky of short duration such rapid sea level falls and presence of steep slopes are not conducive to formation of forced regressive systems tracts composed of downstepping reef clinoforms the ds3 reefal platform formed where shallow water coincided with gently sloping substrates created by earlier deposition slow progradation 0 39 1 45 km my is best explained by the lack of an extensive bank top progressively falling sea level and low productivity resulting from siliciclastic debris and excess nutrients shed from nearby volcanic islands although ds3 strata were deposited during a third order relative sea level cycle a typical transgressive systems tract is not recognizable indicating that the initial relative rise in sea level was too rapid 19 cm ky downstepping reefs forming a forced regressive systems tract were deposited during the relative sea level fall at the end of ds3 indicating that relatively slow rates of fall 10 cm ky or less over favorable paleoslope conditions are conducive to generation of forced regressive systems tracts consisting of downstepping reef clinoforms the tcc sequence consists of four shallow water sedimentary cycles that were deposited during a 400 ky to 100 ky time span such shallow water cycles typical of many platforms form only where shallow water intersects gently sloping substrates the relative thicknesses of cycles 2 m to 15 m thick magnitudes of relative sea level fluctuations associated with each cycle 25 30 m high rates of relative sea level fluctuations minimum of 25 120 cm ky and the widespread distribution of similar tcc cycles in the mediterranean and elsewhere are supportive of a glacio eustatic influence with rates of sea level change so high typical systems tracts do not form

block rotation and continental extension in afar a comparison to oceanic microplate systems the reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation to see whether such microplate behavior has implications for continental rifting we investigate the application of a microplate model to the afar region at the nubia somalia arabia triple junction studies of marine magnetic anomalies volcanic ages bathymetry and seismicity suggest that the westward propagating gulf of aden spreading center has propagated into eastern afar within the past 2 m y causing rifting and extension within the continent we derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that pliocene to pleistocene age rocks have undergone a clockwise rotation of 11 we suggest that the history of rifting the rotation and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for afar in this scenario motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift while motion is transferred the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates the rifting history and rotations in eastern afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves provided the constraints we use are appropriate our model better describes the regional kinematics than alternative block models including one based on bookshelf faulting if the tectonics of afar are typical for continental breakup they have interesting implications for the geometry of passive margins in particular asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere additionally if the rate of rift propagation and the rate and location of rift localization are not uniform then along axis structural variations will result

heterogeneous extension and the role of transfer faults in the development of the southeastern betic basins se spain large strike slip faults in the eastern betics are interpreted to have developed in a transcurrent setting in response to 4 6 mm yr of africa iberia nw se convergence however here we show that some of these faults are transfer faults accommodating heterogeneous late miocene extension the north cabrera dextral fault and other e w to ne sw strike slip faults in the sorbas basin were transfer faults produced under sw ne extension these faults together with related normal faults form the main boundaries of two sedimentary depocenters active between the serravallian and the tortonian the older north cabrera depocenter extended between the serravallian and the early tortonian approximately 13 8 to 9 ma while the younger gacia depocenter formed in response to late tortonian extension approximately 9 to 7 5 ma the latter formed to the west of the north cabrera depocenter by a listric fan of normal faults with sw directed transport that are linked by dextral and sinistral transfer fault segments these faults root on a low angle detachment cutting into the exhumed high pressure nevado filabride complex rocks at 0 8 km depth the present work reveals that 1 this extension was partially coeval with and kinematically linked to sinistral displacement along the carboneras fault farther south in the nijar basin 2 this westward directed extension produced elongated core complexes and tilted blocks to the north of the carboneras fault and magmatic accretion upon thinned continental crust to the south probably in response to slab tearing or detachment and associated edge delamination of the iberian continental lithospheric mantle beneath the betics

comment on remanent early cretaceous spreading center in the central pacific basin by kensaku tamaki masato joshima and roger l larson

beaches and beach related facies holocene barrier islands of georgia the mesotidal coast of georgia encompasses diverse sedimentary features and depositional environments most of which typify major facies of the entire georgia bight a broad embayment on the southeastern u s shelf barrier island strandline environments especially include tidal inlets and shoals the beach shoreface foreshore and backshore and dunes and washover fans low energy beach segments may grade directly into small seaside tidal flats relict salt marsh deposits crop out on erosional beaches all deposits are in dynamic equilibrium with fluctuating coastal conditions and a gradual rise in sea level most of these facies are important in interpreting ancient epeiric transgressive regressive coastal sequences pronounced local variations in the overall sequence result from complex latero vertical relationships between 1 the shoreface foreshore and shoals and 2 the shoals low energy beaches and beach related tidal flats washover fans and relict deposits on erosional beaches cause perturbations within present day lateral relationships but are normal in transgressive facies tracts along such coasts close stratigraphic control of outcrops or cores would be necessary for detailed reconstructions of or correlations among ancient analogs

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

meteoric calcite cementation diagenetic response to relative fall in sea level and effect on porosity and permeability las negras area southeastern spain abstract a dolomitized upper miocene carbonate system in southeast spain contains extensive upper and lower zones of calcite cementation that cut across the stratigraphy cement textures including isopachous and circumgranular which are consistent with phreatic zone cementation cements in the upper cemented zone are non luminescent whereas those in the lower cemented zone exhibit multiple bands of luminescent and non luminescent cements in the upper cemented zone isotopic data show two meteoric calcite lines mcl with mean 18 o at 5 1 and 5 8 vpdb whereas no clear mcl is defined in the lower cemented zone where mean 18 o for calcite cement is at 6 7 vpdb 13 c values in both cement zones are predominantly negative ranging from 10 to 2 vpdb suggestive of carbon from soil gas or decayed organics measurements of tm ice in primary fluid inclusions yield a mode of 0 0 c in both zones indicating calcite cementation from fresh water these two zones define the positions of two different paleo water tables that formed during a relative sea level fall and erosional downcutting during the plio pleistocene the upper cemented zone pre dated the lower cemented zone on the basis of known relative sea level history meteoric calcite cementation reduced porosity and permeability but measured values are inconsistent with simple filling of open pore space each texture boundstone grainstone packstone wackestone produces a different relationship between percent calcite cement and porosity permeability distribution of cements may be predictable on the basis of known sea level history and the effect of the cementation can be incorporated into subsurface geomodels by defining surfaces of rock boundaries that separate cemented zones from uncemented zones and applying texture specific relationships among cementation porosity and permeability

spit platform temperate carbonates the origin of landward downlapping beds along a basin margin lower pliocene carboneras basin se spain lower pliocene temperate carbonates exhibit landward downlapping beds at the southern margin of the carboneras basin in south eastern spain this rarely documented stratal geometry resulted from the accumulation of bedded bioclastic carbonate sand and gravel by longshore currents along a spit platform located a few hundred metres from the palaeoshoreline the top of the spit platform was covered by shoals that extended over a gently dipping ramp inclined to the north on the landward slope of the spit sediments washed over from the shoal area were deposited in parallel laminated beds with a southward dip of 8 11 these beds aggraded and retrograded after an increase in accommodation space probably related to an early pliocene eustatic sea level rise as a result the beds downlap onto the underlying unconformity surface in a shoreward direction eventually the depression between the shoreline and the spit platform was filled and a gentle ramp became established these pliocene exposures in the carboneras basin and a similar upper miocene example in southern spain suggest that landward downlapping stratal geometries can be expected in nearshore temperate carbonates along basin margins and demonstrate a similarity in sedimentary dynamics to siliciclastic sands and gravels

physical and biogenic aspects of backbarrier sedimentary sequences georgia coast u s a abstract configuration of the present georgia coast with its broad expanses of salt marsh and intricate tidal drainages behind pleistocene holocene barrier islands implies the filling of an extensive backbarrier lagoonal system however riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures principal sediment textures include clean to muddy coarse and fine sand mud and to a lesser extent shells pebbles and mud and bedrock clasts physical sedimentary structures many modified by bioturbation include large scale cross bedding trough and planar with graded sets small scale crossbedding ripple lamination and flaser wavy and lenticular bedding laminated to interbedded sand and mud and rare pebble or shell beds ebb oriented structures predominate over flood oriented ones except in upper reaches of riverine estuaries biogenic sedimentary structures typically are diverse and abundant characteristic lebensspuren include polychaete dwelling tubes decapod and polychaete burrows ray holes and bioturbate textures imparted by amphipods ophiuroids bivalves and anemones patterns of bioturbation are controlled to some extent by sediment type salinity levels and rates of deposition and physical reworking of sediments although intensity of bioturbation generally increases seaward bioturbate textures are more readily preservable in the lower energy middle reaches of estuaries because of numerous local pleistocene sediment sources and local variations in hydrographic regimes the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries the basic sequence in ascending order typically consists of 1 channel lag and or channel fill deposits 2 estuarine accretionary beds 3 point bar tidal flat and shoal sediments and 4 channel margin and salt marsh deposits

constraints on the movement history of the carboneras fault zone se spain from stratigraphy and 40ar 39ar dating of neogene volcanic rocks

long term and short term global cenozoic sea level estimates backstripping analysis of three continuously cored well dated boreholes from the new jersey coastal plain ocean drilling program odp leg 150x indicates a long term 10 8 n10 7 yr eustatic fall of 100 m since 55 ma early eocene and suggests short term 0 5n3 m y eustatic falls of less than 70 m eustatic estimates are calculated from residuals between the decompacted unloaded and paleodepth corrected records and tectonic subsidence assuming a cooling lithospheric plate because the residuals are similar among the three sites we interpret them as an approximation of the eustatic signal

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

structure and geological history of the carboneras fault zone se spain part of a stretching transform fault system abstract the carboneras fault zone cfz is a major ne sw trending tectonic lineament in se spain active from upper miocene to recent times it separates the volcanic cabo de gata terrain to the se accumulated over 18 6 ma bp from the tract of uplifted alpine metamorphic basement blocks and post orogenic basins that comprise the betic cordilleras lying to the nw new geological mapping and age determinations have been used to constrain the geometry and geological history of the fault zone the cfz consists of left lateral strike slip faults bearing fault gouge formed in the uppermost 5 km of the crust the faults cut metamorphic basement and folded post orogenic sediments and volcanic rocks and acted as a conduit for calc alkaline volcanic rocks rising to the surface nw of the cfz a series of unconformities and deformation episodes affect successive sedimentary formations of upper miocene age the cfz is interpreted as part of a transform fault system separating ne sw stretched and nw se shortened crust deformed above a south westward retreating subducted slab from a less deformed terrain lying to the south east total offset on the cfz may be up to 40 km but is at least 15 km

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

barrier island evolution middle atlantic shelf u s a part ii evidence from the shelf floor where the atlantic coastal gradient is low and the substrate is unconsolidated shoreface processes lead to an equilibrium shoreface surface characterized by a straightened plan view and a concave up profile maintenance of this surface during the post glacial sea level rise may lead to barrier formation by mainland beach detachment where the outer oceanic shoreface is maintained by wave and current processes downwelling storm currents sweep sand from the shoreface of the middle atlantic bight transport it downcoast and seaward and deposit it on the adjacent inner shelf some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather however there is a net loss in most areas the loss is not made good by river sand input since on the atlantic coast all river sand is trapped by estuaries as a result the shoreface in these areas is undergoing erosional retreat mean horizontal retreat rates are on the order of 1 3 m yr 1 but most movement occurs in brief episodes with recurrence rates on the order of several events per century erosional shoreface retreat coupled with aeolian overshoot storm washover and the movement of sand through inlets into the lagoons has lead to landward migration of the atlantic barriers through the holocene period of sea level rise the landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon the resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action the backbarrier deposits eventually re emerge at the shoreface where their upper beds are eroded and recycled the barrier thus migrates over a pavement of its own flood tidal deltas and washover fans sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets which reaches down to the underlying pleistocene the modern shelf sand sheet is the debris blanket resulting from the retreat process the leading edge occurs as a thin veneer of rip current fallout on the shoreface this is periodically stripped off by winter storms to expose the underlying backbarrier strata further seaward shelf sands eroded from the shoreface rest disconformably on backbarrier deposits shoreface beach and dune strata are always missing every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past however the sheet as a whole has been reconstituted its primary structures are those of the inner shelf floor backbarrier sand and mud deposits characterized by channeling and landward dipping reflectors can be traced for over 100 km seaward across the middle atlantic shelf beneath the modern sand sheet the backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the atlantic shelf surface during holocene time the sheet like nature of this record indicates that viewed at the appropriate time scale barrier migration is a continuous process

microplate and shear zone models for oceanic spreading center reorganizations the reorganization of oceanic spreading centers is often accomplished through the process of rift propagation we first examine general rift propagation geometries for the simple two plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously such that the propagating and dying rifts never overlap a velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics we then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation such that a technically independent overlap region like the easter or juan fernandez plates along the east pacific rise is produced we examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers the tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear rigid plate models are suggested by the reasonable success of euler vectors derived from inversions of plate motion data spreading rates transform azimuths and slip vectors both in describing available data and in predicting subsequently acquired data on the other hand recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing rather than rigid behavior in the interior of the easter microplate we use schematic evolutionary simulations for an easter style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models in a microplate model rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region while structures within the overlap zone rotate rigidly in contrast in a shear model relative motions at the boundaries differ from those predicted by rigid plate tectonics and the angular relationships of structures within the overlap region are altered by internal deformation thus inversion of relative motion data yields different results for rigid and shear behavior similarly under favorable circumstances structural trends could be diagnostic of the different cases we find that much of the easter structural data suggestive of shearing are also consistent with rigid plate tectonics we conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics furthermore the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

heterozoan carbonate deposition on a steep basement escarpment late miocene almeria south east spain heterozoan temperate water carbonates mixed with varying amounts of terrigenous grains and muddy matrix azagador limestone accumulated on and at the toe of an inherited escarpment during the late tortonian early messinian late miocene at the western margin of the almeria nijar basin in south east spain the escarpment was the eastern end of an uplifting antiform created by compressive folding of triassic rocks of the betic basement channelized coralline algal bryozoan rudstone to coarse grained packstone together with matrix supported conglomerate are the dominant lithofacies in the higher outcrops comprising the deposits on the slope these sediments mainly fill small canyon shaped half graben depressions formed by normal faults active before during and after carbonate sedimentation roughly bedded and roughly laminated coralline algal bryozoan rudstone to coarse grained packstone are the main lithofacies forming an apron of four small kilometre scale lobes at the toe of the south eastern side of the escarpment almeria area channelized and roughly bedded coralline algal bryozoan rudstone to coarse grained packstone conglomerates packstone and sandy silt accumulated in a small channel lobe system at the toe of the north eastern side of the escarpment las balsas area carbonate particles and terrigenous grains were sourced from shallow water settings and displaced downslope by sediment density flows that preferentially followed the canyon shaped depressions roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope whereas the rest of the carbonate lithofacies were deposited by high density turbidite currents the steep escarpment and related break in slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse grained low transport efficiency skeletal dominated sediment in the apron lobes accelerated uplift of the basement caused a relative sea level fall resulting in the formation of outer ramp carbonates on the apron lobes which were in turn overlain by lower messinian coral reefs this is the first known example of a base of slope apron within temperate water carbonate systems this article is protected by copyright all rights reserved

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

climate and paleoceanography controlled size variations of calcareous nannofossils during the valanginian weissert event early cretaceous abstract strata of valanginian age early cretaceous 139 8 132 9 ma record a 1 5 positive carbon isotope excursion cie the weissert event it coincides with volcanic activity of the parana etendeka large igneous province unlike the mesozoic oceanic anoxic events oaes no organic rich deposits documenting widespread perturbation of the marine carbon cycle are associated with the weissert event to study the impact of paleoceanographic changes in the valanginian on calcareous nannofossils this study analyzes coccolith size data of three species biscutum constans watznaueria barnesiae zeugrhabdotus erectus from northern germany and the western north atlantic further it presents a new valanginian carbon isotope record of organic material 13corg from northern germany the dataset records the weissert event cie in the late valanginian at both locations biometric analyses reveal size decreases of b constans coccoliths during the weissert event a size reduction of 1 m has been observed for northern germany 37 n and of 0 4 m for the north atlantic 23 n the different magnitudes of size decrease possibly reflect the different paleogeographic positions of the sites and the paleoceanographic response to the late valanginian climate variations proxy data support a link between a humid climate with increased weathering and the observed coccolith size variations this impacted nannoplankton communities either through the availability of light or by eutrophication and possibly nutrient limitation in addition to increased terrigenous input emissions of the parana etendeka volcanism possibly acted fertilizing furthermore widely dispersed volcanic emissions possibly caused increased concentrations of toxic trace metals in the surface ocean which affected marine primary producers

factors influencing the locations of hot spots determined by the hot spotting technique seafloor beneath hot spot produced seamounts have crustal flow lines that intersect at the hot spot locations provided there is more than one stage rotation and that hot spots are stationary convolving seamount shapes with their associated flow lines yields an image of cumulative volcano amplitude cva we call the technique of correlating local cva maxima with hot spot locations hot spotting this technique is geometrical and requires no age information for seamounts or seafloor we analyze synthetic and actual seamount data and demonstrate that factors such as across trail scatter in seamount locations inaccurate stage poles and migration of hot spots all exert strong influences on the locations and appearances of cva maxima the interpretation of raw cva images is therefore not straightforward

seafloor spreading history of the western north atlantic basin derived from the keathley sequence and computer graphics abstract an analysis of sea surface magnetic profiles and an aeromagnetic contour chart west of bermuda yields a new tectonic framework for the keathley m series seafloor spreading magnetic anomaly sequence changes in spreading direction derived from the western keathley sequence occur at times corresponding to m24 m21 and m11 of which m11 is a time of continental breakup the models facilitate new identifications of the anomalies between m16 and m4 obviating the need for anomalously slow spreading between m11 and m4 previously suggested by other workers and by us and placing the location of the well defined rough smooth basement boundary at m11 the models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history this strongly suggests that individual ridge axis segments act independently of another in detail our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history among eight profiles collected parallel to seafloor spreading flowlines in the western north atlantic encompassing only 2700 km of normal oceanic crust we can identify more than fourteen ridge jump events there is evidence for a small 30 km ridge jump to the east of many ridge axis segments at m24 this is supported in the eastern north atlantic conjugate crust of the canary basin where m25 and m24 are poorly represented a series of four ridge jumps to the east along one paleoridge axis segment now located at 36 n 65 w is related to the passage of the mesozoic mid atlantic ridge crest over the verde hotspot at 137 m y b p the project magnet magnetic anomaly contour chart for the area between 29 n and 40 n has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation which have a component oblique to spreading between chrons m21 and m11 these trends may relate to the motion of the ridge crest over the mantle

the role of topography and erosion in the development and architecture of shallow water coral bioherms tortonian messinian cabo de gata se spain abstract during the miocene mediterranean shallow water carbonates were rich in scleractinian corals which thrive in various depositional settings a tortonian messinian bioherm belt developing in a heterozoan dominated ramp was investigated along a 1 2 km continuous transect located in the cabo de gata region the interval studied displays four depositional environments from mid to inner ramp dominated by swell waves and storm energy deposited as a single large scale depositional sequence during a 3rd to 4th order transgressive regressive cycle the bioherms grew in three phases and were essentially composed of in place primary frameworks three coral genera were the main framebuilders porites tarbellastrea and rare siderastrea associated with melobesioid and mastophoroid red algae and bryozoans as secondary framebuilders the corals display five morphotypes from a fast growing branched type to slow growing domal to plate morphologies with an uncommon form of mesh porites as the dominant morphotype changes in coral morphotype and composition of micro encrusters communities reveal changes in hydrodynamics detrital influx and perhaps nutrient levels bioherms architecture was driven by sea level palaeotopography and erosion the coral framework was affected during its development by erosion surfaces metres to tens of metres deep and hundreds of metres wide unexpectedly these surfaces are better developed on the inner edges of the bioherms this could indicate the circulation of strong bottom currents between the volcanic palaeohighs and the synoptic relief created by the buildups finally a major sub aerial erosional episode associated with increasing detrital influxes ended bioherm development thus allowing the colonization of the dead coral substratum by red algae

sedimentary model and high frequency cyclicity in a mediterranean shallow shelf temperate carbonate environment uppermost miocene agua amarga basin southern spain uppermost tortonian to lower messinian temperate carbonates crop out in the agua amarga basin se spain they consist of four units the lower three units can be tentatively assigned to the lowstand systems tract of a fourth order sequence constituting in turn the lowstand megatrough unit transgressive breccia unit and highstand bedded unit stages of a higher order cycle all these materials were deposited in a small pull apart basin related to the sinistral carboneras strike slip fault system the best represented is the bedded unit up to 25 m thick which consists of bioclastic bryozoan bivalve dominated calcarenites calcirudites with abundant fragments of echinoids barnacles benthic foraminifers coralline algae brachiopods and solitary corals facies trends within this unit are roughly arranged in an e w direction with the coastline to the north of the basin the depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts seaward of the shoals was the factory area where most organisms lived and maximum carbonate production took place from this area some of the skeletons were washed landwards by waves and or currents during storms and incorporated into the shoals and beaches and others moved downslope along the ramp as mass flows accumulating to form the fan bedded zone the factory area and fan bedded sediments intercalate five well defined thick beds of calcarenites fine grained calcirudites they show bar morphologies single or amalgamated or make up sand waves with very consistent tabular cross bedding pointing landwards these beds formed in a very shallow wave current influenced coastal environment the bars and sand waves in the fan bedded zone developed during lowstands while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages prograding beaches shoals factory facies and fan bedded layers developed during the highstands net skeletal production occurred mainly during the highstands sediment accretion values of these sediments are similar to those of present and ancient shallow marine temperate carbonates considering that the whole bedded unit was deposited in a 100 000 year interval equivalent to the short eccentricity cycle the five cycles inside the bedded unit would therefore correspond to the c 20 000 year precession cycles of the milankovitch band

tectonics and evolution of the juan fernandez microplate at the pacific nazca antarctic triple junction in this study magnetic and bathymetric data from an r v endeavor survey are combined with earthquake mechanism studies to produce a tectonic model for the juan fernandez microplate at the pacific nazca antarctic triple junction magnetic lineations along the east ridge of the microplate fan to the south indicating that the nazca juan fernandez pole is near the northern end of this feature relative motions of the juan fernandez pacific nazca antarctic four plate system are determined using relative plate motion inversions the resulting tectonic model predicts that the northern boundary of the juan fernandez microplate is a zone of compression and that the west ridge and southwestern boundary are spreading obliquely the southeastern boundary is predicted to be obliquely divergent but present data fail to constrain its geometry we also present a schematic evolutionary model which relates the formation of the juan fernandez microplate to differential spreading rates at the triple junction and discuss related complexities the future evolution of the fourplate system depends on whether one or both ridges remain active

the geometry and evolution of a transpressional strike slip system the carboneras fault se spain the carboneras fault system is a 40 km long 1 km wide neogene ne sw trending left lateral transpressional strike slip fault system which is part of the trans alboran shear zone in se spain the carboneras fault system is an anastomosing array of sub vertical individual fault planes or fault zones which surround pods or lenses of less intensely strained rocks displacements along the individual fault planes exhibit reverse components of slips and form positive flower structures faults have either sharp boundaries or wider bands of gouge typically a few metres thick and are hundreds of metres in length second order fault splays are well developed and usually exhibit p shear rather than riedel shear orientations these are interpreted to be related to the transpressional displacement and may also characterize other oblique convergent zones elsewhere the second order faults are interpreted to have formed as shear strain increased along the first order fault and was transferred laterally to branch segments this process produced pods or shear lenses bounded by the fault segments the length to width aspect ratios of the shear lenses were found to be scale independent across five orders of magnitude with the most common values between 3 l and 6 l a model is proposed for the development of the fault zone by incremental finite displacements along the segmented fault surfaces this model is based on field evidence that displacement switched with time from one fault strand to another

tectonic history of the pacific farallon phoenix triple junction from late jurassic to early cretaceous an abandoned mesozoic spreading system in the central pacific basin in the central pacific basin an abandoned ne trending spreading ridge is evident on the satellite gravity map and is confirmed in bathymetric and magnetic data our identification of magnetic anomaly lineations flanking the abandoned ridge suggests that between chrons m21 147 ma and m14 135 ma it was the plate boundary between the pacific plate and a microplate which we here name the trinidad microplate the configuration of the pacific farallon phoenix triple junction until chron m22 149 ma was fault fault ridge ffr after chron m22 the pacific farallon phoenix triple junction changed its configuration the reorganization of the triple junction led to the birth of the trinidad microplate in its vicinity around chron m15 136 ma the spreading ridge between the pacific plate and the trinidad microplate was abandoned and the magellan microplate was born and remained active till chron m9 128 5 ma after welding of the magellan microplate to the pacific plate the triple junction had a ridge ridge ridge rrr configuration until the major reorganization around chron m0 120 ma

mid cretaceous to early tertiary apparent polar wander path of the pacific plate pacific apparent polar wander was delineated for mid cretaceous to early tertiary time using 22 seamount paleomagnetic poles 11 paleocolatitudes from azimuthally unoriented cores five effective inclinations from two dimensional magnetic anomaly inversions and one magnetic lineation relative amplitude datum three of the seamount paleomagnetic poles were new data calculated in this study the apparent polar wander path was defined with six mean poles having ages of 39 66 76 82 85 and 94 ma an alternate path was constructed combining data from two pairs of mean poles 66 and 76 ma 85 and 94 ma that were statistically indistinguishable the apparent polar wander path is hook shaped with a nearly right angle bend at 82 ma its post 82 ma section trends north south the pre 82 ma part east west the path indicates uneven polar wander unresolvable polar motion occurred between 66 76 ma and 85 94 ma suggesting stillstands however the large distances between the 85 and 82 ma poles 15 2 3 8 and 82 and 76 ma poles 11 3 2 8 imply rapid polar wander at rates of 5 1 1 3 m y and 1 9 0 5 m y respectively both the sharp bend and rapid polar wander occurred at the end of the cretaceous normal polarity superchron the mid cretaceous paleomagnetic data show a trend suggestive of southward motion of the pacific plate that may be the end of a period of southward drift postulated by others to have begun in the jurassic the evolution of various features of the apparent polar wander path are discussed in terms of plate motion true polar wander and time varying non dipole geomagnetic fields we argue that the overall shape of the polar wander path resulted from the change of pacific plate motion from southward to northward perhaps brought about by the beginning of the subduction of the plate beneath asia

fast rift propagation along the east pacific rise near easter island during a 1983 seabeam cruise we surveyed the easter nazca spreading center which forms the eastern boundary of the easter microplate we have analyzed all available bathymetry and magnetic anomaly data to determine the present fine scale tectonic configuration and recent tectonic evolution of this spreading center the narrowing of the axial brunhes anomaly is best explained by a young propagating rift growing northward at an average rate of 150 mm yr assuming an average spreading rate of 120 mm yr there are indications of a short episode of faster propagation 350 mm yr prior to 0 4 ma these are the fastest rates of rift propagation yet documented the tip of the propagating rift is at about 25 s 112 25 w 180 km south of the previously hypothesized position the recent spreading direction is n85 w a large well defined sheared zone to the west of the western pseudofault bounding the crust formed by the propagating rift suggests that the easter pacific spreading center 360 km to the west is being replaced by the fast propagator

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans

smooth folds favoring gypsum precipitation in the messinian poniente marginal basin western mediterranean abstract the poniente basin lies in a folded sector located at the boundary between the eastern betic cordillera and the alboran sea we reconstruct its 3d geometry combining depth migrated 2d seismic reflection sections with borehole and surface structural data the folds have been interplaying with sedimentation since late miocene time growth strata allow us to reconstruct both the folding history and the average shortening rate two isolated antiforms began to grow during the late tortonian separated by an antiformal saddle point a rectilinear fold linkage sector between initially isolated short segments developed during the messinian eventually forming a subcylindrical ene wsw trending antiform that has continued growing symmetrically since the pleistocene the antiform formed a barrier that favored messinian evaporite deposition along the contiguous synform to the north the vertical breakup between the lower parts of the synform and the barrier anticline culmination ranges from 500 to 700 m in the last 7 2 ma the analyzed folds accommodated a shortening of 52 m ma along a 20 km wide band at a mediterranean scale our study sheds light on the propagation mode of barriers that favor the precipitation of gypsum in peripheral basins by restricting water exchange with the open sea before the main phase of evaporite deposition occurred in the deep basin