comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

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

coastal dynamics under conditions of rapid sea level rise late pleistocene to early holocene evolution of barrier lagoon systems on the northern adriatic shelf italy this multidisciplinary case study of two preserved barrier systems combined the analysis of radiocarbon datings grain size distributions high resolution seismics and shelf bathymetry with reconstructions of palaeo environmental conditions tides waves sea level change and forward modelling of barrier lagoon systems to provide an integrated view of the coastal transgressive evolution of a large sector of the northern adriatic shelf between 15 and 8 ka bp palaeoenvironmental reconstructions point to increased tidal amplitude low energy wave climate and high rates of sea level rise up to 60 mm a during the formation of the oldest preserved barrier system not vert similar90 m water depth 14 3 cal ka bp a younger barrier system 42 m water depth 10 5 cal ka bp formed under conditions of lower tidal amplitude higher wave energy and a lower rate of sealevel rise 10 mm a forward modelling suggests that the probability of barrier island overstepping during transgression is inversely proportional to tidal amplitude if all other factors are assumed equal the oldest barrier lagoon system developed under conditions of large tidal amplitude which permitted rapid transgression however this system apparently failed to keep up with the anomalously high rate of sea level rise resulting from melt water pulse 1a the youngest barrier system appears to have drowned in place due to antecedent topography as the barrier system transgressed over an ancient pleistocene alluvial plain the rapid increase in backbarrier accommodation caused an abrupt disequilibrium between shoreface and backbarrier sedimentation which led to barrier overstepping although barsim modelling indicates that tidal deposition can reduce the probability of barrier overstepping there are other driving mechanisms in our case extremely rapid sea level rise and antecedent topography which are more determinative in explaining the transgressive coastal evolution of barrier lagoon systems in the northern adriatic sea grain size analysis of shoreface deposits sampled above the transgressive ravinement surface across the northern adriatic shelf indicate a distinct relation between the sediment grain size and the rate of sea level rise during deposition which implies that progressive sorting must have been highly effective

size fractionation by suspension transport a large scale flume experiment with shoaling waves abstract laboratory simulation of wave shoaling over a medium sand bed mean size 250 m for a range of wave conditions a natural spectrum symmetrical and asymmetrical wave groups showed that a distinct horizontal fractionation of sediment by size occurred sediment became progressively coarser down wave onshore from the test section and progressively finer up wave offshore from the same source this spatial sorting is explained by suspension transport alone and controlled by two factors a a vertical profile of grain size in the suspension in which there is a distinct increase in the proportion of fines with elevation and a decrease in the mean size by approximately 18 between 0 04 and 0 24 m and b a vertical profile of the mean mass transport velocity which revealed a down wave onshore directed current close to the bed and a current reversal at higher elevations the average elevation for the reversal was 0 14 m under the range of wave conditions simulated h s 0 22 0 78 m t pk 2 25 3 78 s these two factors were enhanced by a frequency dependent transport in the wave field the primary waves produced a maximum net transport close to the bed which was directed down wave at higher elevations as a response to a change in the phase coupling between concentration and velocity the net transport was reversed in contrast the net transport associated with the group bound long wave was directed up wave at all elevations and the decay rate with elevation was significantly less than that associated with the primary waves thus near the bed transport by primary and secondary waves was roughly equal and opposite resulting in little wave induced net transport at higher elevations an up wave transport by the secondary wave was dominant complementing the net transport due to the mean current

lithospheric extension from rifting to continental breakup at magma poor margins rheology serpentinisation and symmetry the symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years with various authors proposing asymmetric simple shear models others advocating more symmetric pure shear models and some combinations of the two the unroofing of vast expanses of sub continental mantle at non volcanic margins has led some authors to argue in favour of simple shear models but supporting evidence is lacking subsidence evidence from conjugate margin pairs is equivocal and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions in the porcupine basin where the final stages of break up are preserved the development of structural asymmetry is demonstrable and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust this agrees with theoretical modelling results which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled however whether such asymmetry is maintained during continued exhumation of the mantle is unclear

the equilibrium distribution of grain size fractions and its implications for cross shore sediment transport a conceptual model abstract the relative distribution of individual grain size fractions across the cross shore coastal profile of terschelling north sea displays a constant shape when average conditions are considered the shape of the distribution curve depends on the hydrodynamic processes acting in the littoral zone and if the curve is contemplated in a normalized relative form it can be considered as a permanent feature of the nearshore zone the normalized curve is an equilibrium distribution curve and it can be used in the prediction of the cross shore redistribution of sediments when the textural distribution of the supplied sediment is known sediment may be imported into the active profile by either natural or man induced processes such as e g nourishments the application of the conceptual model in the prediction of the sediment redistribution from the shoreface nourishment carried out in front of the island of terschelling indicates a predominant sediment transport from the nourished area towards the shoreline 58 and a subordinate cross shore transport in an offshore direction 17 about 25 of the supplied sediment remains in the nourished area comparison of these results with the volumetric estimations of the sediment transport based on the morphological evolution shows a good agreement and suggests that the method gives reliable results the method may become a simple additional and useful tool in predicting the behaviour and dispersion of naturally or artificially imported volumes of sediment the conceptual model is a typical black box model and it cannot replace the existing and future generations of generic coastal morphodynamic models or process related models such as 2dv coastal profile models in addition the application of this model is definitely limited to coastal environments that satisfy certain hydrodynamic and geological conditions that are related to e g a longshore uniformity in processes and the lack of influence of sedimentary outcrops

mode of lithospheric extension conceptual models from analogue modeling comparison of analogue experiments at crustal and lithospheric scale provides essential information concerning the mode of deformation during lithospheric extension this study shows that during extension lithospheric deformation is controlled by the development of shear zones in the ductile parts at lithospheric scale the global deformation is initiated by the rupture of the brittle mantle lithosphere this failure generates the formation of conjugate and opposite shear zones in the lower crust and the ductile mantle lithosphere the analysis of the internal strain of the ductile layers suggests that the two opposite shear zones located below the asymmetric graben in the lower crust and the ductile mantle lithosphere prevail experiments show that from a similar initial stage the relative predominance of these shear zones originates two different modes of deformation if the crustal shear zone prevails a major detachment like structure crosscuts the whole lithosphere and controls its thinning in this model named the simple shear mode the resulting geometry shows that crustal and lithospheric thinning are laterally shifted if the mantle shear zone predominates the lithospheric thinning is induced by the coeval activity of the two main shear zones this process called the necking mode leads to the vertical superposition of crustal and mantle lithospheric thinning applied to natural laboratories west european rift red sea rift and north atlantic this conceptual model allows a plausible explanation of the different geometries and evolutions described in these provinces the north atlantic and the red sea rift systems may result from a simple shear mode whereas the necking mode may explain part of the evolution of the west european rift especially in the massif central and the eger graben

experimental simulation of shoreface nourishments under storm events a morphological hydrodynamic and sediment grain size analysis this study focuses on barred beach shoreface nourishments physically simulated in a wave flume the attack of a schematic storm on three different nourishments is analysed the apex and waning storm phases lead respectively to offshore and onshore sediment transports nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore the bar acts as a wave filter and reduces shore erosion lee effect in contrast nourishment on the beach face leads mostly to shore feeding and reconstruction feeder effect with successive nourishments the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates the surface grain size analysis reveals marked variations coarser sediments are sorted on the bar and the upper beach face these locations correspond to large wave dissipation zones during the storm apex

central city colorado the upper part of an alkaline porphyry molybdenum system the distribution of molybdenite fluorite rhodochrosite enargite and telluride minerals together with anomalous quantities of cu pb zn and mo in rocks and variations in stable isotope ratios define a complex zoned hydrothermal system in the central city mining district the molybdenite mineralization is centered on a laramide quartz bostonite or alkali rhyolite breccia pipe that occurs at the intersection of the dory hill fault and the idaho springs ralston shear zone stream sediment geochemistry suggests that significant amounts of w may be present in the system uranium base and precious metal mineralization of a similar age but with a distribution different from the molybdenite mineralization is also present and is considered to be a part of the same hydrothermal system fluid inclusion data for the molybdenite and precious and base metal mineralization indicate that fluid inclusions associated with early molybdenite veins are of two types co 2 rich and rare halite bearing with respective homogenization temperatures of 340 degrees to 420 degrees c and 240 degrees to 340 degrees c inclusions associated with precious metal bearing pyrite and base metal veins are aqueous two phase types with homogenization temperatures in the range of 220 degrees to 380 degrees c those from late molybdenite veins are aqueous two phase inclusions with homogenization temperatures of 200 degrees to 280 degrees c salinities as equiv wt nacl are between 34 to 42 for halite bearing inclusions and 2 to 12 for all other types no evidence of boiling has been found trapping pressures of co 2 rich inclusions are 0 46 to 0 75 kb which suggest an emplacement depth of about 2 km for the molybdenite mineralization respective quartz delta 18 o and fluid inclusion delta d values are 1 early molybdenite veins 9 3 to 14 3 per mil and 87 to 66 per mil 2 precious metal bearing pyrite and base metal veins 8 4 to 15 9 per mil and 90 to 47 per mil and 3 late molybdenite veins 10 1 to 15 4 per mil and 77 to 69 per mil with associated fluorite having fluid inclusion delta d values of 70 to 57 per mil a late telluride vein has a quartz delta 18 o value of 11 2 per mil and fluid inclusion delta d value of 86 per mil these data suggest that all except the telluride ore fluids contained large proportions of a d rich magmatic fluid the distribution of fluid inclusion delta d values defines two distinct anomalies each zoned with delta d enrichment occuring inward toward centers which correspond to the areas of known molybdenite occurrences and precious and base metal mineralization likewise delta d values for sericites developed during early hydrothermal activity increase inward toward the quartz bostonite pipe various intrusive breccias and molybdenite mineralization in the district there is no indication at central city of negative delta 18 o anomalies in the precambrian country rock within or peripheral to the centers of mineralization suggesting that ore fluids were thus essentially confined to discrete fractures it is concluded that molybdenite and other mineralization in this classic gold mining district may be related and may represent the upper part of an alkaline porphyry molybdenum system

diachronous evolution of late jurassic cretaceous continental rifting in the northeast atlantic west iberian margin 1 regional 2 d seismic reflection profiles outcrop and borehole data are used to characterize the evolution of deep offshore sedimentary basins in southwest iberia alentejo basin the interpreted data indicate the bulk of late jurassic earliest cretaceous subsidence occurred in the present day continental slope area as shown by 1 significant thickening of synrift strata basinward from a slope bounding fault system sfs west of which the total thickness of sediment can reach more than 9 0 km and 2 relatively thin mesozoic strata east of the sfs where thickening of synrift units against principal faults is limited five principal regressive events and their basal unconformities reflect tectonic uplift and relative emersion in proximal basins which were located on the rift shoulder to subsiding tilt blocks west of the sfs these regressive events are correlated with major rift related events occurring on the deeper margin direct comparisons with the peniche basin of northwest iberia reveal that significant portions of the iberian lower plate margin were uplifted and eroded during the last stages of continental rifting this process was repeated at different times and in different areas as the locus of rifting and continental breakup migrated northward as a result two distinct rift axes are recognized in west iberia a first axis extending from the porto basin to the alentejo basin and a second axis located on the outer proximal margin north of 38 30n in addition the sfs delimited 1 prograding deposits of cretaceous paleogene age and 2 late cenozoic deposits draping the modern continental slope these latter facts demonstrate that on lower plate passive margins the relative position of the continental slope is established during the final rifting episode s preceding continental breakup

basin evolution of the northern part of the northeast german basin insights from a 3d structural model abstract a 3d structural model for the entire southwestern baltic sea and the adjacent onshore areas was created with the purpose to analyse the structural framework and the sediment distribution in the area the model was compiled with information from several geological time isochore maps and digital depth maps from the area and consists of six post rotliegend successions the upper permian zechstein lower triassic middle triassic upper triassic jurassic cretaceous and cenozoic this structural model was the basis for a 3d backstripping approach considering salt flow as a consequence of spatially changing overburden load distribution isostatic rebound and sedimentary compaction for each backstripping step in order to reconstruct the subsidence history in the region this method allows determination of the amount of tectonic subsidence or uplifting as a consequence of the regional stress field acting on the basin and was followed by a correlation with periods of active salt movement in general the successions above the highly deformed zechstein evaporites reveal a thickening trend towards the gluckstadt graben which also experienced the highest amount of tectonic subsidence during the mesozoic and cenozoic two periods of accelerating salt movement in the area has been correlated with the e w directed extension during the late triassic early jurassic and later by the late cretaceous early cenozoic inversion suggesting that the regional stress field plays a key role in halokinesis the final part of this work dealt with a neotectonic forward modelling in an attempt to predict the future topography when the system is in a tectonic equilibrium the result reveals that many of the salt structures in the region are still active and that future coastline will run with a wnw ese trend arguing that the compressional stresses related to the alpine collision are the prime factor for the present day landscape evolution

characterization of controls on high resolution stratigraphic architecture in wave dominated shoreface shelf parasequences using inverse numerical modeling abstract a new inverse numerical modeling method is used to constrain the environmental parameters e g relative sea level sediment supply and wave climate histories that control stratigraphic architecture in wave dominated shallow marine deposits the method links a process response forward stratigraphic model that simulates wave and storm processes barsim to a combination of inverse methods formulated in a bayesian framework that allows full characterization of uncertainties this method is applied for the first time to a real geologic dataset collected at outcrop from two shoreface shelf parasequences in the aberdeen member blackhawk formation of the book cliffs east central utah usa the environmental parameters that controlled the observed stratigraphic architecture are quantified and key aspects of stratigraphic architecture are successfully predicted from limited data stratigraphic architecture at parasequence stacking and intra parasequence scales was driven principally by relative sea level varying by up to about 55 m and sediment supply varying by up to 70 m2 yr whose interplay determines the shoreline trajectory within zones of distinctive shoreline trajectory variations in wave climate of up to about 3 m in fairweather wave height controlled superimposed variations in sandstone and shale content e g the development of upward coarsening and upward fining bedsets the modeling results closely match the observed stratigraphic architecture but their quality is limited by 1 the formulation and assumptions of the forward modeling algorithms and 2 the observed data distribution and quality which provide poor age constraint

pooled subsidence records from numerous wells reveal variations in pre break up rifting along the proximal domains of the iberia newfoundland continental margins the iberia newfoundland continental margin is one of the most studied conjugate margins in the world however many unknowns remain regarding the nature of rifting preceding its break up we analyse a large dataset of tectonic subsidence curves created from publicly available well data to show spatial and temporal trends of rifting in the proximal domains of the margin we develop a novel methodology of bulk averaging tectonic subsidence curves that can be applied on any conjugate margin with a similar spread of well data the method does not rely on the existence of conjugate deep seismic profiles and specifically attempts to forego the risk of quantitative bias derived from localized anomalies and uncertain stratigraphic dating and correlation results for the iberia newfoundland margin show that active rift driven tectonic subsidence occurred in the central segment of the conjugate margin from c 227 ma early norian to c 152 1 ma early tithonian in the southern segment from c 208 5 ma early rhaetian to c 152 1 ma early tithonian and in the northern segment from c 201 3 ma early hettangian to c 132 9 ma early hauterivian this indicates that rifting in the stretching phase of the proximal domain of the iberia newfoundland margin does not mirror hyperextended domain rifting trends south to north that ultimately led to break up the insights into broad scale three dimensional spatial and temporal trends produced using the novel methodology presented in this paper provide added value for interpretation of the development of passive margins and new constraints for modelling of the formation of conjugate margins

the sweet home rhodochrosite specimen mine alma district central colorado the porphyry molybdenum fluorine connection intermediate sulfidation veins containing quartz sphalerite tetrahedrite rhodochrosite fluorite in the sweet home mine alma district colorado were originally mined for silver starting in 1873 for the last 13 years up until 2004 however the mine has produced world class rhodochrosite specimens some of these specimens are considered to be among the finest mineral specimens ever produced and the finest of their species with values well over 1 million us dollars the extraction preparation and marketing techniques pioneered at the sweet home operation have revolutionized the minerals specimen industry the sweet home deposit is interpreted as a single pulse variant of a climax type hydrothermal system evidence for this includes 1 an age of mineralization 25 8 0 3 ma that coincides with the age of the end stages of mineralization of the climax molybdenum deposit approximately 7 5 km to the northeast 2 a geochemical mn w f and mineralogical topaz fluorite hubnerite greisen muscovite signature typically associated with climax type systems 3 the presence of porphyry rhyolite dikes a breccia dike and local quartz molybdenite veins in the nearby area 4 a small pegmatite within the mine with an age 25 9 0 3 ma coincident with mineralization which also contains minor amounts of disseminated molybdenite and 5 the presence of similar appearing gemmy red rhodochrosites at climax and other high silica rhyolite systems a significant difference is that unlike climax type systems the sweet home hydrothermal system appears to have consisted of a single relatively small pulse of magmatic fluid that slowly cooled and diluted with groundwater this is inferred to have occurred at moderate depths in the order of 1 5 2 5 km below the surface the fluids that formed the sweet home veins were dilute salinity in the order of 2 4 wt nacl equivalent high temperature temperatures of homogenization up to 370 c and initially of magmatic origin gem quality ruby red rhodochrosite at the sweet home mine is nearly pure manganese carbonate with minimal solid solution with fe 2 ca or mg it formed at higher temperatures and salinities in comparison to lower value pink rhodochrosite gemmy ruby red rhodochrosite is distinctly associated with highly evolved silica rich igneous hydrothermal systems the high fluorine content typical of such systems suggests that mn was transported in solution as fluoride complexes which in turn favored rhodochrosite deposition at above average temperatures and with minimal cation contamination

reconstruction of tertiary progradation and clinoform development on the new jersey passive margin by 2 d backstripping we have reconstructed the oligocene to middle miocene paleobathymetry and stratigraphy of the new jersey margin using a modified backstripping technique by analyzing the geometry of the margin through time we investigate its response to fluctuating sea level changing climate and variable sediment supply during the tertiary the reconstructions reveal a change in the margin morphology from a more steeply dipping 1 300 to 1 500 carbonate ramp in the eocene to a flatter shelf with a sharp shelf edge at present this was accomplished by an increase in the terrigenous sediment supply that filled available accommodation and caused progradation across the margin we link the increase in sediment flux with climatic cooling rather than tectonic processes the progradation is evidenced by a series of clinoforms whose formation was modulated by sea level and which extend over 100 km across the shelf the height and dip of the clinoforms increased as they extended onto the deeper parts of the earlier ramp the miocene clinoform rollovers at the new jersey margin had water depths of60 130 m and are interpreted as the edge of a new continental shelf built over the older ramp sea level fall was probably insufficient to drive the miocene shorelines past the shelf breaks thus measurements of sea level amplitude based upon coastal onlap over the clinoforms are not reliable 1999 elsevier science b v all rights reserved

simulation of grain size abundances on a barred upper shoreface abstract using the barred upper shoreface of long island new york as an example this study provides a semi quantitative demonstration of the concept that the distribution of grain sizes in a closed depositional system is the product of the reworking and redistribution by the physical process and the availability of the sediment source in the particular case of long island the physical process is represented by the agitation of sediment grain sizes by the shoaling waves under the average fair weather wave conditions as a first order approximation a nearby subaerial morainal deposit represents the drowned glacial deposit on the shoreface as the sediment source the probability of abundance of four representative sediment grain sizes across the upper shoreface of the study site was simulated the shoaling wave field over a segment of the upper shoreface of long island was first simulated by a numerical wave and circulation model using deepwater incident waves under the conditions of the average fair weather remote ocean swells and a local storm the model results were then used to estimate sediment entrainment across the upper shoreface of the study site by using an empirical formulation developed for sediment movement under progressive waves the probability of the occurrence of each grain size on the upper shoreface is characterized by a non dimensional retention index the results indicate that the fair weather wave climate has the highest probability for sediment retention on the seabed the cross shore distribution of a grain size which is represented by the probability of abundance abundance index can be simulated by multiplying the probability of occurrence retention index as a result of the differential wave agitation with the availability grain size frequency distribution of the source sediment deposit the grain sizes representing the fine sand group have the best fit between the observed and simulated distributions suggesting a group of grain sizes that is in dynamic equilibrium with shoaling waves for the rest of the grain sizes the combined effects of shoaling waves and other secondary hydrodynamic processes are important in determining their cross shore distributions

the climax alma granite batholith of oligocene age and the prophyry molybdenum deposits of climax colorado u s a abstract the porphyry molybdenum deposits of climax colorado are stockworks of quartz molybdenite veinlets they are apical to intrusive cupolas of leucocratic rhyolite granite porphyry of oligocene age 33 to 24 ma the upper parts of the orebodies are overprinted by overlying zones of phyllically altered rocks and by stockworks of greisen like veinlets containing quartz pyrite topaz and huebnerite the phyllic and or greisen like zones are surrounded by a propylitic zone characterized by quartz chlorite hematite veinlets and chlorite after biotite late veins radial and peripheral to the intrusive center are common but widely scattered and discontinuous such veins typically contain quartz fluorite rhodochrosite pyrite dark sphalerite galena tetrahedrite and traces of huebnerite molybdenite argentite native gold and or electrum the alma district 4 to 10 km southeast of climax contains widely scattered clusters of veins and replacement deposits the veins cut chloritized to sericitized silicate host rocks and the replacement deposits are in carbonate host rocks the ore and gangue mineral assemblages of the veins of the alma district resemble those of veins peripheral to the climax molybdenum deposits veins of the alma district cut dikes of late white rhyolite 35 ma which resemble the rhyolite porphyries of climax a swarm of such dikes is parallel to the long axis of an elliptical drainage pattern which bounds a structural dome here called the outer alma dome the outer alma dome 8 12 km surrounds an inner alma dome 6 8 km veins of the alma district generally are axial and or marginal to the outer alma dome and or radial to the inner alma dome veins and alteration minerals associated with the outer alma dome formed between 35 and 27 ma ago those associated with the inner alma dome formed about 27 ma ago as indicated by fission track age determinations on thermally annealed zircon grains from hydrothermally altered rocks within and around the inner alma dome negative bouguer gravity anomaly patterns indicate 1 a regional northeast trending gravity low associated with the composite monzonite granodiorite granite b batholith of the colorado mineral belt average density about 2 62 g cm3 2 a semi regional gravity trough within the gravity low of the colorado mineral belt extending northeastward across the mosquito range 3 an oval pattern of gravity lows coincident with the alma domes and 4 a closed gravity low associated with the climax stock density about 2 56 g cm3 tweto and case 1972 behrent and bajwa 1974 corry 1981 the semi regional gravity trough is interpreted as the gravimetric expression of a granite batholith here called the climax alma batholith the gravity lows of the climax and alma districts are interpreted as expressions of cupolas on this granite batholith the highly differentiated rhyolite granite intrusions of climax plunge toward the alma domes the climax intrusions are interpreted as products of apical differentiation of the inferred climax alma granite batholith magmatic hydrothermal fluids associated with highly differentiated apical magmas produced the climax molybdenum orebodies most of the veins and replacement deposits of the alma district are associated with lower broader cupolas of the inferred granite batholith the shape of the composite climax alma magmatic hydrothermal system resembles that of a teapot with climax above the spout and the alma district above the lower broadly domed lid

the geodynamic province of transitional lithosphere adjacent to magma poor continental margins two different types of transitional lithosphere have been documented along magma poor rifted margins one consists of apparently subcontinental mantle that has been exhumed brit tlely deformed and serpentinized during late stages of rifting a second is thinned 10 km con tinental crust which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth dependent extension in both cases it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter and thus that transitional lithosphere has relatively limited width here three representa tive cases of transitional lithosphere are examined one in the newfoundland iberia rift and one at goban spur both exhumed mantle and one off the angola congo margin thin continental crust flanked seaward by apparently exhumed lower continental crust exhumed mantle considering the geological and geophysical evidence it appears that depth dependent extension riftward flow of weak lower continental crust and or upper mantle may be a common phenomenon on magma poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed transitional lithosphere in this wide zone may consist of subcontinental mantle lower continental crust or some combination thereof depending on the strength profile of the pre rift continental lithosphere transitional lithosphere ceases to be emplaced i e final break up occurs only when emplacement of heat and melt from the rising asthenosphere becomes domi nant over lateral flow of the weak lower lithosphere this model implies a two stage break up first the rupture of the brittle continental crust and second the eventual separation of the ductile sub continental lithosphere which is coincident with emplacement of normal oceanic crust well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpenti nized exhumed mantle and such anomalies therefore are not diagnostic of oceanic crust where present the anomalies can be helpful in interpreting and dating the rifting history

opening of the central atlantic ocean implications for geometric rifting and asymmetric initial seafloor spreading after continental breakup study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust in this study the central atlantic conjugate margins are compared based on compilation of wide angle seismic profiles from nw africa nova scotian and u s passive margins the patterns of volcanism crustal thickness geometry and seismic velocities in the transition zone suggest symmetric rifting followed by asymmetric oceanic crustal accretion conjugate profiles in the southern central atlantic image differences in the continental crustal thickness while profiles on the eastern u s margin are characterized by thick layers of magmatic underplating no such underplate was imaged along the african continental margin in the north two wide angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric a region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the canadian side corresponding on the african side to an oceanic crust with slightly elevated velocities these might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the moroccan margin after isochron m25 a large scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides

structure of tendaho graben and manda hararo rift implications for the evolution of the southern red sea propagator in central afar 1 the red sea and aden rifts or propagators meet in afar here we use remote sensing and field analyses to define the geology and structure of the southern part of the red sea propagator in central afar this consists of the nw se trending tendaho graben tg and the younger and active nw se trending manda hararo rift mhr partly within tg tectonic and volcanic activity within tg developed mostly between 1 8 and 0 6 ma with a stretching factor 1 1 an extension rate 3 6 mm yr and the fissural eruption of part 7000 km3 ma of the afar stratoid sequence mainly basaltic lava flows and ignimbrites mhr before terminating southward has a 1 04 and an extension rate 1 2 mm yr and is associated with the emission of 600 km3 ma of basalts in the last 0 2 ma these data suggest that after the exceptional amount of magma erupted between 1 8 and 0 6 ma magmatic and tectonic activity significantly decreased along the southern part of the red sea propagator in the last 0 2 ma this decrease coincides with the on land development and migration of the more active inferred extension rate in the order of 10 mm yr as proposed in previous studies aden propagator suggesting that spreading in central afar mainly occurred along one active propagator at any one time

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

multigrain sedimentation erosion model based on cross shore equilibrium sediment distribution application to nourishment design abstract in the light of global warming and sea level rise there are many coastal beaches that suffer from erosion beach nourishment has become a common practice to maintain the sediment balance on a shore face in this paper a three dimensional numerical model for evaluating long term impact of beach nourishment projects has been developed the model addresses the longstanding complex issue of coastal morphology and sediment grain size distribution from an unconventional angle which exploits the strong links between grain size distribution and the prevailing transport direction of each sediment constituent under average wave and storm action the present model predicts the redistribution of nourished sediment according to the subtle clues implied by equilibrium distribution curves and latest coastal wave transformation theories after verification against recent field observations in terschelling the netherlands the model was used to predict long term effects of different beach nourishment strategies it was found that a given the source sediment available in terschelling the tactics of large volume and less frequent implementation are better than otherwise and b from a pure engineering point of view waterline nourishment outperforms offshore trough nourishment the model offers an additional tool for coastal engineers to evaluate the feasibility effectiveness and the optimization of dumping locations for beach nourishment projects it is also a useful tool for stratigraphic modelling of shallow marine sedimentation in conjunction with sea level changes

a phase of transient subsidence sediment bypass and deposition of regressive transgressive cycles during the breakup of iberia and newfoundland abstract seismic outcrop and well data from west iberia and newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence in west iberia two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present day continental shelf this character is similar to newfoundland where correlative strata comprise lower cretaceous cenomanian coarse grained siliciclastics accumulated around local sediment source areas the interpreted data reveal that the two breakup sequences 1 materialise sediment bypass onto continental slope depocentres that experienced important tectonic subsidence during continental breakup but without showing typical syn rift growth packages 2 generate specific forced regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin subsidence and sediment stacking patterns in both west iberia and newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere to their mantle breakup stages on both margins coarse grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep water basins this work also confirms that in a setting dominated by a significant sediment influx yet lacking the burial rates of continental slope basins in newfoundland west iberia comprised accommodation driven basins during continental breakup not necessarily sediment starved as a corollary of our analysis we classify breakup sequences around the world based on the characteristic lithologies of their regressive transgressive depositional cycles

quantification and restoration of extensional deformation along the western iberia and newfoundland rifted margins 1 many recent papers describe the structure of the iberia and newfoundland rifted margins however none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading in our study we use two pairs of cross sections considered as conjugate lines one across the galicia bank flemish cap and the other across the southern iberia abyssal plain flemish pass both transects have been imaged by reflection and refraction seismic methods and have been drilled during ocean drilling program legs 103 149 173 and 210 drilling penetrated parts of the rift stratigraphy and the underlying basement the cross sections can therefore be considered as the best documented conjugate transects across present day hyperextended magma poor rifted margins the aim of this paper is threefold 1 provide a detailed description of the crustal architecture of the two conjugate sections 2 define the extensional structures and their ages and 3 quantify the amount of strain and strain rate accommodated along these lines this paper proposes a quantitative description of extension along the iberia newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

timing of initial seafloor spreading in the newfoundland iberia rift broad areas of subcontinental lithospheric mantle are commonly exposed along ocean continent transition zones in magma poor rifts and are thought to be exhumed along lithospheric scale detachment faults during the final stages of rifting however the nature of the transition from final rifting to seafloor spreading is controversial we present the first high precision u pb zircon geochronologic and hf isotopic data from gabbros that intrude exhumed mantle at ocean drilling program odp sites 1070 and 1277 in the newfoundland iberia rift north atlantic the sites are conjugate to one another within crust that is commonly considered to have been emplaced during early seafloor spreading magnetic data suggest that crustal accretion occurred at both sites during magnetic polarity chrons m3 m0 130 126 ma however our data indicate that asthenospheric melts were emplaced over brief intervals 1 m y prior to or coeval with mantle exhumation at 124 ma at odp site 1070 and 115 ma at odp site 1277 we suggest that this discrepancy is the result of continued mantle exhumation along large west dipping detachment faults until lithospheric breakup the breakup location is likely coincident with the large amplitude magnetic j anomaly and our 115 ma date for magmatism within this anomaly provides the best available age constraint for breakup along the studied transect

the behaviour of a multiple bar system in the nearshore zone of terschelling the netherlands 1965 1993 abstract the behaviour of a multiple bar system in the nearshore zone of the island of terschelling the netherlands is investigated on the time scale of years using a data set of soundings the data set covers the period from 1965 to 1993 the behaviour is analysed in terms of bar crest position and in terms of morphometric parameters such as bar crest depth height width and volume two or three breaker bars are permanently present in the study area each individual bar passes through three stages during its existence 1 generation close to the shore 2 seaward migration and 3 degeneration at the outer margin of the nearshore bar zone the key factor controlling the behaviour of the inner bars is the crest depth of the outer bar three different couplings between the behaviour of the individual bars have been observed 1 bar stage changes due to the degeneration of the outer bar 2 prevention of the transition of an inner bar from the first to the second stage caused by the appearance from alongshore of a new outer bar having a small crest depth and 3 difference in mean annual seaward migration rate caused by the possible presence of a more seaward positioned bar it is hypothesized that the crest depth of the outer bar governs the relative importance of processes inducing shoreward and seaward migration in the inner nearshore bar zone and therefore controls the behaviour of the nearshore bar zone on the time scale of years

magmatic breakup as an explanation for magnetic anomalies at magma poor rifted margins during continental breakup the onset of seafloor spreading is thought to be marked by the first occurrence of a magnetic anomaly analysis of seismic and magnetic data from the iberia newfoundland continental rift system suggests that the first magnetic anomaly observed here instead represents a magmatic event that pre dates seafloor spreading

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

morphodynamic response of nearshore bars to a shoreface nourishment abstract the autonomous nearshore bar behaviour along the barrier island of terschelling the netherlands is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline in 1993 a perturbation of the cyclic bar system was introduced by the implementation of a 2 mm 3 shoreface nourishment supplied to the nearshore bar zone filling up the trough between the middle and outer bar the morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth height width and volume along with a pronounced development of a three dimensional bar system unseen in the autonomous behaviour the nearshore bars exhibited a 6 7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre nourishment morphometric values at the subsequent onset of bar movement bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends it is shown that the observed onshore transport of nourished sediment in the 6 7 year arrest results from a gradual deepening of troughs cross shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore short wave nonlinearity and offshore undertow sediment transport rates the gradual reappearance of the pre nourishment bar trough morphology is shown to engender a normalisation in the cross shore distribution of sediment transport rates to pre nourishment rates

a geochemical study of the sweet home mine colorado mineral belt usa hydrothermal fluid evolution above a hypothesized granite cupola deposition of quartz molybdenite pyrite topaz muscovite fluorite and subsequent hubnerite and sulfide fluorite rhodochrosite mineralization at the sweet home mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world class climax molybdenum deposit about 26 to 25 m y ago the mineralization occurred at depths of about 3 000 m and is related to at least two major fluid systems 1 one dominated by magmatic fluids and 2 another dominated by meteoric water the sulfur isotopic composition of pyrite strontium isotopes and rey distribution in fluorite suggest that the early stage quartz molybdenite pyrite topaz muscovite fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400 c as indicated by co2 bearing moderately saline 7 5 12 5 wt nacl equiv fluid inclusions la icpms analyses of fluid inclusions in quartz demonstrate that fluids from the sweet home mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry cu au mo or climax type deposits the ore forming fluid exsolved from a highly differentiated magma possibly related to the deep seated alma batholith or distal porphyry stock s sulfide mineralization marking the periphery of climax type porphyry systems with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low salinity co2 bearing fluids with low metal content at temperatures below 400 c the sulfide mineralization is characterized by mostly negative 34s values for sphalerite galena chalcopyrite and tetrahedrite highly variable 18o values for rhodochrosite and low ree contents in fluorite the pb isotopic composition of galena as well as the highly variable 87sr 86sr ratios of fluorite rhodochrosite and apatite indicates that at least part of the pb and sr originated from a much more radiogenic source than climax type granites it is suggested that the sulfide mineralization at the sweet home mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids the migration of the latter fluids that were major components during late stage mineralization at the sweet home mine was probably driven by a buried magmatic intrusion

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

sediment sorting at the sand motor at storm and annual time scales bed sediment composition with a focus on the median grain size d50 was investigated at a large scale nourishment the sand motor at the dutch coast 21 5 million m3 sand considerable alongshore heterogeneity of the bed composition d50 was observed as the sand motor evolved over time with 1 coarsening of the exposed part of the sand motor 90 to 150 m and 2 a depositional area with relatively fine material 50 m finer just north and south of the sand motor the alongshore heterogeneity of the measured d50 values was most evident outside the surfzone i e seaward of msl 4 m coarsening of the bed after construction of the sand motor was attributed to hydrodynamic sorting processes because the alongshore heterogeneity of the d50 showed a similar spatial pattern as the mean bed shear stresses the observed alongshore heterogeneity of the d50 and correlation of d50 with modelled mean bed shear stresses suggest that preferential erosion of the finer sand fractions has taken place the selective transport of finer sand fractions results in a coarser top layer of the bed at the sand motor the preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport the measurements also show the impact of a storm which consists of a 40 m finer d50 of the offshore bed composition in front of the sand motor i e where a considerably coarser bed was in place additionally storms may generate a temporary zone with fine bed material at the toe of the deposition profile this means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate

zoned base metal mineralization in a porphyry system origin and evolution of mineralizing fluids in the morococha district peru porphyry related base metal vein and replacement mineralization i e cordilleran polymetallic mineralization in the morococha district central peru is part of a large magmatic hydrothermal system associated with the emplacement of several late miocene porphyry intrusions and formation of important cu mo mineralization zn pb ag cu veins overprint the giant toromocho porphyry cu mo deposit in the center of the district and display a typical concentric base metal zonation cu zn pb ag covering an area of approximately 50km2 a detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal rich intermediate density porphyry type fluids in early stages of cordilleran base metal vein formation fluid inclusions have low salinities of 2 to 5 wt nacl equiv co2 contents of 3 to 10mol and homogenization temperatures th of 380 to 340 c they are similar to intermediate density fluid inclusions trapped in a milky quartz vein predating cordilleran polymetallic mineralization with similar low salinities 3 0 3 8 wt nacl equiv and low co2 contents 6 5 8 mol but higher th of 420 to 410 c during cooling of the intermediate density fluids from 400 to 300 c the lithostatic pressure regime changed to a hydrostatic one the fluids underwent pressure drop as well as phase separation i e unmixing and lost most of their co2 they acquired moderate salinities in some cases intermediate up to 16 wt nacl to brine compositions however the bulk of the magmatic fluid retained low salinity while it continued to cool under open system conditions and precipitated tennantite tetrahedrite chalcopyrite enargite sphalerite and galena upon cooling below 270 c the fluids deposited abundant rhodochrosite and quartz while following the boiling curve toward lower p t conditions these data record an evolution of mineral precipitation from deep minimum depth of 2 1 5 km to shallow environments 300 800 m oxygen hydrogen and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage copper 5 000 18 000 g g sulfur up to 12 000 g g and iron 2 100 6 000 g g concentrations in the intermediate density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate density inclusions of the early cordilleran base metal veins the base metals zn pb and mn have comparable concentrations between 100 and 1 000 g g for both types of fluid these findings suggest that the fluids identified in cordilleran polymetallic veins are compositionally similar to the porphyry type fluids and could have derived from the latter after precipitation of cu and fe bearing sulfides in a deeper porphyry environment the new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry centered districts e g bingham butte and show that both porphyry and polymetallic mineralization can precipitate from similar magmatic hydrothermal fluid pulses

abrupt plate accelerations shape rifted continental margins rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved it is well known that strain rate dependent processes control rift evolution yet quantified extension histories of earth s major passive margins have become available only recently here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions we find that rifted margins feature an initial slow rift phase less than ten millimetres per year full rate and that an abrupt increase of plate divergence introduces a fast rift phase plate acceleration takes place before continental rupture and considerable margin area is created during each phase we reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions the extension models suggest that the two phase velocity behaviour is caused by a rift intrinsic strength velocity feedback which can be robustly inferred for diverse lithosphere configurations and rheologies our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force this mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes offering new insights into the balance of plate driving forces through time

modelling of bed sediment composition changes at the lower shoreface of the sand motor abstract large perturbations in the coastline such as the sand motor nourishment 21 million m3 at the holland coast can initiate considerable spatial and temporal changes in the median grain size d 50 of the sea bed on the lower shoreface the relevance of hydrodynamic conditions for the development of the heterogeneity in d 50 at large scale nourishments was assessed with a numerical model delft3d which required a validation against 2 5 years of d 50 measurements a good representation of the observed spatial pattern of d 50 was obtained independent of a 2dh or 3d approach and initial condition for the d 50 of the bed five sediment size fractions and a multi layer administration of the bed composition were used the extent and magnitude of the coarsening of the bed is related to the velocity of the horizontal tide while a far less pronounced coarsening takes place during energetic conditions i e h m 0 3 m differential suspension behaviour between the size fractions which are all mobilized at the bed causes a preferential transport of fine sediment in alongshore direction away from the sand motor at the lower shoreface i e seaward of msl 6 m storm conditions may induce a partial removal of the coarse top layer due to mobilization of all of the size fractions and mixing with the relatively fine substrate material simulations also show that transport of the fine sand fraction extents to much deeper water than for the medium and coarse sand fractions models with multiple sediment fractions are therefore required for the assessment of environmental impacts of large scale coastal structures or land reclamation s and sediment transport on the lower shoreface

quantitative subsidence analysis of the mesozoic evolution of the lusitanian basin western iberian margin abstract quantitative subsidence analysis of 26 wells in the lusitanian basin provides new constraints on the western iberian mesozoic passive margin development backstripped tectonic subsidence curves show a three fold subdivision of vertical motions from late triassic onward continental rifting was initiated during the late triassic and early jurassic from middle jurassic onward a distinct different behaviour is expressed in the subsidence curves for the north and the south lusitanian basin during the middle jurassic the south lusitanian basin records a stretching episode with stretching factors of about 1 08 while the north lusitanian basin typically has a middle jurassic hiatus this different development is also expressed in the late jurassic and early cretaceous sedimentary sequence when both the north and south lusitanian basin are subjected to another stretching episode with a more pronounced development of the southern than of the northern part of the basin the stretching factors for this last phase are about 1 03 for the northern part and 1 08 for the southern part of the area this north south difference during the middle jurassic to early cretaceous for which the transition roughly coincides with the location of the nazare fault zone is probably a result of differences in pre rift crustal composition or thickness late cretaceous sediments are mostly absent in the analysed wells in the southern part of the basin the absence of the cretaceous record is a consequence of erosion due to cenozoic inversion of the basin the generally low estimates for stretching factors suggest that the analysed eastern part of the lusitanian basin forms the distal part of the mid cretaceous continental breakup a comparison with subsidence curves of neighbouring basins of iberia reflects general patterns in mesozoic basin development and confirms the generally held view that the extension leading to continental breakup migrated from south to north during the middle jurassic to early cretaceous in west iberia

aspects of the structural evolution of the lusitanian basin in portugal and the shelf and slope area offshore portugal abstract the study provides a regional seismic interpretation and mapping of the mesozoic and cenozoic succession of the lusitanian basin and the shelf and slope area off portugal the seismic study is compared with previous studies of the lusitanian basin from the late triassic to the cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence the triassic rifting was concentrated in the central part of the lusitanian basin and in the southernmost part of the study area both as symmetrical grabens and half grabens the evolution of half grabens was particularly prominent in the south the triassic fault controlled subsidence ceased during the latest late triassic and was succeeded by regional subsidence during the early early jurassic hettangian when deposition of evaporites took place a second rift phase was initiated in the early jurassic most likely during the sinemurian pliensbachian this resulted in minor salt movements along the most prominent faults the second phase was concentrated to the area south of the nazare fault zone and resulted here in the accumulation of a thick sinemurian callovian succession following a major hiatus probably as a result of the opening of the central atlantic resumed deposition occurred during the late jurassic evidence for late jurassic fault controlled subsidence is widespread over the whole basin the pattern of late jurassic subsidence appears to change across the nazare fault zone north of the nazare fault fault controlled subsidence occurred mainly along nnw sse trending faults and to the south of this fault zone a nne ssw fault pattern seems to dominate the oxfordian rift phase is testified in onlapping of the oxfordian succession on salt pillows which formed in association with fault activity the fourth and final rift phase was in the latest late jurassic or earliest early cretaceous the jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones however only salt pillow development can be demonstrated the extensional tectonics ceased during the early cretaceous during most of the cretaceous regional subsidence occurred resulting in the deposition of a uniform lower and upper cretaceous succession marked inversion of former normal faults particularly along ne sw trending faults and development of salt diapirs occurred during the middle miocene probably followed by tectonic pulses during the late miocene to present the inversion was most prominent in the central and southern parts of the study area in between these two areas affected by structural inversion fault controlled subsidence resulted in the formation of the cenozoic lower tagus basin northwest of the nazare fault zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed the miocene compressional stress was mainly oriented nw se shifting to more n s in the southern part

plate tectonics and basin subsidence history tectonic setting exerts fi rst order control on basin formation as refl ected in basin subsidence history while our approach ignores the effects of fl exural loading and eustatic sea level change consistency of backstripped subsidence histories i e with local loading effects of sediment removed suggests consistent tectonic driving mechanisms in each tectonic setting with the possible exception of forearc basins based on published subsidence curves and open fi le stratigraphic data we show the subsidence characteristics of passive margins strike slip basins intracontinental basins foreland basins and forearc basins passive margin subsidence is characterized by two stages rapid initial synrift subsidence and slow post rift thermal subsidence with increasing subsidence rates toward the adjacent ocean basin subsidence of intracontinental basins is similar in magnitude to that seen in passive margin settings but the former is generally slower longer lived and lacks initial subsidence long lived subsidence for many intracontinental basins is consistent with cooling following thermal perturbation of thick lithosphere found beneath old parts of continents basins associated with strike slip faults are usually short lived with very rapid subsidence changes in local stress regimes as strike slip faults evolve and migrate over time coupled with three dimensional heat loss in these small basins likely explain this subsidence pattern foreland basin subsidence rates refl ect the fl exural response to episodic thrust loading resultant subsidence curves are punctuated by convex up accelerating segments forearc basins have the least consistent subsidence patterns subsidence histories of these basins are complex and may refl ect multiple driving mechanisms of subsidence in forearc settings second order deviations in subsidence suggest reactivation or superimposed tectonic events in many basin settings the effects of eustatic sea level change may also explain some deviations in curves for many of these settings subsidence histories are suffi ciently distinctive to be used to help determine tectonic setting of ancient basin deposits

mineralogy and fluid inclusion study of the southern amethyst vein system creede mining district colorado ag mineralization occurs as vein fillings and disseminations confined to areas of high fracture density and brecciation in the hanging wall of the amethyst fault and vein paragenesis is divided into two main stages 1 is a sulfide poor assemblage characterized by rhodochrosite quartz barite and an absence of silver 2 represents a complex event made up of three substages disseminated mineralization in the study area appears to be related to stage 2 analysis of fluid inclusions in stage 2 vein quartz showed that average homogenization temperatures and salinities decrease in a regular manner from lower to upper levels of the mine inclusions in stage 2 quartz from the lower mine levels had an average t h 238 degrees c and an average salinity of 9 8 equivalent weight percent nacl those in the uppermost level had an average t h 170 degrees c and an average salinity of 6 5 equivalent weight percent nacl the rapid drop in temperature and dilution of hydrothermal solutions which was a result of mixing with near surface waters caused the deposition of the high grade ag ores in the southern amethyst vein system modified journal abstract

gradation effects in sediment transport to determine the effects of grain size and density gradation in oscillatory sheet flow experiments are conducted in an oscillating water tunnel a formal derivation of a schematised transport model shows that the transport rates per sediment fraction can be determined with and without the assumption of an active layer a technique that measures sediment composition from natural occurring radionuclides is used to determine the transport rates without that assumption of an active layer the measurements on total sediment transport rates and suspended sediment concentrations show that effects of gradation are present the effects of size gradation are mainly in the increased transport rates of the coarse fraction with respect to the uniform coarse material and the availability at the bed under similar conditions the experiments on density graded sediments indicate that total mass transport rates are larger than the mass transport rates of quartz material whilst the transport rates of the quartz fraction show that this fraction is not hindered by the availability at the bed

jurassic tectono sedimentary evolution of the northern lusitanian basin offshore portugal abstract the lusitanian basin is one of a series of rift related basins developed during the rifting that led to the break up of iberia and canada the strong influence of halokinesis on the depositional evolution of its northern sector provides an analogue for the development of other evaporite prone rift basins on atlantic type passive margins in this paper the pre valanginian evolution of the northern lusitanian basin is documented based on regional seismic stratigraphy isochron and time structure maps tied with sonic and stratigraphic data from 10 exploration wells the pre salt structure of the study area is marked by two sets of faults i a n s to ne sw striking set active throughout the jurassic period and ii a nw se striking set active during the late jurassic the latter defines five late jurassic sub basins northeast of a basin margin structure the marinha grande fault the development of half graben basins in the northern lusitanian basin during the jurassic was overprinted by halokinesis over the marinha grande fault and other major marginal basement faults halokinesis climaxed during the oxfordian kimmeridgian rifting with the formation of a 10 km long salt ridge over the marinha grande fault the ridge limited the southward progradation of fluvial deltaic units derived from hanging wall drainage systems and restricted the syn rift sediments to a 15 km long 5 km wide bowl shaped depocentre located 10 km west of the present coastline in order to better characterise the jurassic tectono sedimentary evolution of the northern lusitanian basin the interpreted seismic data is compared with physical models for extensional forced folds above active normal faults and with stratigraphic information from well and outcrop

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

radio frequency identification rfid technology applied to the definition of underwater and subaerial coarse sediment movement abstract in this paper radio frequency identification technology has been applied to track both underwater and subaerial displacement of pebbles along an artificial coarse beach at marina di pisa italy several preliminary laboratory tests have been performed to adapt the rfid technique for underwater use which has been the primary impediment to this promising approach to the study of coarse sediment transport and movement tests showed the reliability of low frequencies for this kind of work since they enable good signal transmission and reception through water passive abs plastic transponders were inserted into about 100 pebbles and released onto the beach in march 2009 a core 125 reader was chosen as the operating antenna to continuously transmit low frequency 125 khz signals an acoustic signal toned whenever a pebble was detected while the unambiguous identification code of the pebble is shown immediately on the screen of a laptop connected to the reader the positions of the pebbles were recorded with a total station after two months may 2009 74 marked pebbles were retrieved 77 of the total the positions of the retrieved pebbles were also recorded with the total station thus allowing calculation of the coarse sediment transport tendency about 60 of the recovered pebbles 44 out of 74 were found on the upper shoreface the analysis of the marked pebble trajectories revealed a divergent transport movement in the northernmost sector of the beach this movement was probably triggered by an irregularity of the submerged breakwater fronting the shoreline the southern sector is characterised by chaotic pathways related to the formation and evolution of beach cusps this outcome highlights and confirms the importance of a complete definition of the beach system with no separation between the underwater and the subaerial portion of the shore when it comes to sediment transport and movement this successful application of rfid technology to the underwater environment provides a chance to broaden understanding of a topic in need of further study

sultan mountain mine western san juan mountains colorado a fluid inclusion and stable isotope study the sultan mountain mine in the western san juan mountains of colorado has produced cu pb zn ag au ores from the mid 1870s until the 1950s production was from veins filling faults and fissures along the southern margin of the silverton caldera the principal host rock to the veins is a quartz monzonite stock five periods of hypogene mineralization have been recognized i early quartz pyrite ii quartz pyrite iii rhodochrosite siderite iv main ore stage chalcopyrite tetrahedrite galena sphalerite and gold and v quartz fluorite evidence of open space filling banding crustification vugs is widespread heating studies of fluid inclusions in quartz rhodochrosite siderite sphalerite and fluorite indicate temperatures were approximately 200 degrees c for stages i to iv and 186 degrees c for stage v stages i ii and iv show evidence of boiling crushing studies indicate that high pressure gas probably co 2 is present in the fluid inclusions freezing point depression estimates of salinity corrected for co 2 indicate a range of 13 6 to 1 3 wt percent nacl equiv these data together with p v t data for saline solutions and p sub co 2 38 bars give a minimum depth of formation of 600 m the delta 18 o values at 200 degrees c of water calculated for oxygen isotope composition of quartz range from 5 63 to 8 8 per mil and the values for rhodochrosite siderite are 0 05 and 0 35 per mil the delta 18 o values of whole rocks adjacent to the champion and alletha veins range from 2 65 to 6 10 per mil these data suggest hydrothermal fluids were predominantly composed of meteoric water which underwent isotopic exchange with the quartz monzonite the delta 13 c values for rhodochrosite siderite range from 6 9 to 7 3 per mil carbon isotope data are inconclusive as to the source of the carbon the delta 34 s values for sulfides range from 2 9 to 6 1 per mil and these limited data suggest a paleozoic evaporite source for most of the sulfur stable isotope data indicate isotopic control of the fluid by the surrounding country rocks geologic and geochemical studies suggest a model of sultan mountain ore deposition in which predominantly meteoric hydrothermal solutions restricted mostly to the sultan mountain area scavenged metals from the precambrian basement and the tertiary volcanic and intrusive rocks sulfur was derived mostly from solution of paleozoic evaporites however the sulfur isotope data indicate some of the sulfur may have been derived from the quartz monzonite stock the primary mechanism of precipitation of the ore was an increase in ph as a result of boiling

equilibrium shoreface profiles a sediment transport approach abstract large scale coastal behaviour models use the shoreface profile of equilibrium as a fundamental morphological unit that is translated in space to simulate coastal response to for example sea level oscillations and variability in sediment supply despite a longstanding focus on the shoreface profile and its relevance to predicting coastal response to changing environmental conditions the processes and dynamics involved in shoreface equilibrium are still not fully understood here we apply a process based empirical sediment transport model combined with morphodynamic principles to provide new insight into equilibrium shoreface profile development and shape the quantitative model is based on field measurements and shows that a balance between onshore sediment transport due to oscillatory wave motion and offshore sediment transport due to gravity produces profile shapes that are similar to natural shoreface profiles from the danish north sea coast and the southeast australian coast model predictions of equilibrium profile response to the effects of climate change such as rising sea level or changing wave climates are examined implementation of the model is straightforward there is no tuning or calibration and computation times are short it is therefore easily implemented with repeated iterations to manage uncertainty

distal alluvial fan sediments from the upper jurassic of portugal controls on their cyclicity and channel formation the lusitanian basin of west central portugai is an iberian atlantic margin basin during a late jurassic rifting event a series of alluvial fans distributed coarse basement derived clastics southeastward from the western margin of the basin the distal part of this sequence which is about 140 m thick consists of heterolithic massive sandstone and massive mudrock facies arranged in crude fining upwards cycles 1 15 m thick these form discrete packets of sediment in a strongly aggrada tional sequence and are interpreted as the products of sheetflooding as the active area of sedimenta tion on the fan surface switched channelized flow was not necessarily an early feature of cycle development considerable fluctuations in discharge occurred and were probably related to climatic factors large quantities of tractional and suspended sediment were transported at high stages and were rapidly deposited as flows waned caliche soil profiles indicate a seasonal semi and climate however other features such as widespread soft sediment deformation the lack of desiccation structures and the absence of evaporites suggests that areas of active sedimentation often remained wet three fining upward megacycles are distinguished these indicate rapid advance of fan toes triggered by tectonic activity followed by gradual retreat

fallen arches dispelling myths concerning cambrian and ordovician paleogeography of the rocky mountain region high resolution sedimentologic biostratigraphic and stable isotope data from numerous measured sections across colorado reveal a complex architecture for lower paleozoic strata in the central cordilleran region a lack of precise age control in previous studies had resulted in misidentification and miscorrelation of units between separate ranges corrections of these errors made possible by our improved data set indicate the following depositional history the quartz rich sandstone of the sawatch formation was deposited during onlap of the precambrian erosion surface in the early late cambrian the overlying dotsero formation a regionally extensive carbonate and shale rich succession records blanket like deposition with only minor facies changes across the state an extremely widespread meter scale stromatolite bed the clinetop bed caps the dotsero formation in most areas however a latest cambrian erosional episode removed 9 11 m of the upper dotsero formation including the clinetop bed from just east of the homestake shear zone in the sawatch range eastward to the mosquito range the overlying manitou formation differs in character and thus in member stratigraphy on the east vs west sides of the state these differences were previously interpreted as the result of deposition on either side of a basement high that existed within the central colorado embayment or colorado sag a region of major breaching across the transcontinental arch this paleogeographic reconstruction is shown herein to be an artifact of miscorrelation biostratigraphic data show that the northwestern members of the manitou formation are older than the members exposed in the southeastern part of the state and that there is little or no overlap in age between the two areas this circumstance is the result of 1 removal of older manitou formation strata in the southeast by an unconformity developed during the rossodus manitouensis conodont zone and 2 erosion of younger manitou strata in central and western colorado along middle ordovician and devonian unconformities deciphering these complex stratal geometries has led to invalidation of long held views on western laurentian paleogeography during the cambrian and earliest ordovician specifically the existence of the colorado sag and a northeast trending high within the sag that controlled depositional patterns on either side the mid rossodus uplift and resultant unconformity eliminated any and all upper cambrian and lower ordovician deposits in southern colorado and northern new mexico and thus their absence should not be misconstrued as evidence for earlier nondeposition in this region lithofacies distribution patterns and isopach maps provide no evidence that highlands of the transcontinental arch existed in colorado prior to the mid rossodus age uplift event in fact regional reconstructions of earliest paleozoic paleogeography along the entire length of the purported transcontinental arch should be reevaluated with similarly precise biostratigraphic data to reconsider all potential causes for missing strata and to eliminate topographic elements not supported by multiple stratigraphic techniques this study illustrates how seriously paleogeographic reconstructions can be biased by the presumption that missing strata represent periods of nondeposition rather than subsequent episodes of erosion particularly in thin cratonic successions where stratigraphic gaps are common and often inconspicuous

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

the betic orogen and the iberian african boundary in the gulf of cadiz geological evolution central north atlantic abstract the study of the gulf of cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics the evolution of the southern iberian margins was more complex than in most north atlantic margins since it entailed several phases of rifting convergence and strike slip motions three main tectonic provinces surround the internal zones of the gibraltar arc orogenic belt these include in the iberian margin of the gulf of cadiz the flysch units of the campo de gibraltar complex the betic external zones and the neogene basins of the guadalquivir valley fault bounded blocks of flysch and subbetic units crops out over large areas of the southeastern iberian shelf the basement of the northwestern area in contrast is represented by the paleozoic rocks of the hercynian massif of iberia half graben structures determined the main structural trends of the margin during the mesozoic which were affected by inversion structures during the neogene compressional stages the mesozoic and lower cenozoic units are best observed in wells and seismic profiles from the northern area these units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the gulf of cadiz seven lithoseismic units from triassic to upper oligocene and another seven neogene and quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome the first tectonic phase was characterized by a passive margin which was controlled by the development of half graben extensional structures and carbonate platforms this evolution comprises the mesozoic and early cenozoic ocean spreading in the north atlantic induced extensional tectonics which deformed the cretaceous syn tectonic post aptian deposits increased amounts of terrigenous materials were supplied to the margin from aptian to albian times controlling depositional patterns while terrigenous siliciclastic facies replaced the jurassic carbonate platforms from middle eocene to early miocene the margin was influenced by the relative motions of iberia and africa and the development of the alpine orogeny the iberian african boundary in the gulf of cadiz experienced transpression and the mesozoic basins probably underwent subduction the emplacement of an olistostrome took place in the gulf of cadiz towards the central atlantic basin plains during the tortonian the end of the olistostrome emplacement during the late miocene coincides with accelerated tectonic subsidence while thick progradational and aggradational depositional sequences were developed the occurrence of closely juxtaposed regions of compression and extension during the miocene may reflect the westwards progression of the gibraltar arc mountain front over a subducting thinned tethys crust when the motion between iberian and africa was n s to nnw sse oriented the migration of the arcuate mountain front into the eastern gulf of cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere in contrast to the betic rif belts however there is no geophysical evidence to postulate that continental collision took place in the gulf of cadiz during the cenozoic

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

process based modelling of a shoreface nourishment abstract a state of the art process based model is applied to hindcast the morphological development of a shoreface nourishment along the barrier island of terschelling the netherlands the applied morphodynamic model is delft3d for fully three dimensional flow and sediment transport in coastal environments owing to a complex geomorphological setting of the study area the curvi linear model includes adjacent tidal inlets and covers 40 70 km with an increasing grid size resolution towards the nourishment site in the center of the island in 1993 a total of 2 mm3 of sand was supplied to the nearshore bar zone off terschelling filling up the trough between the middle and outer bar by spring 1994 most of the nourished sediment had been redistributed onshore and welded onto the middle bar where it remained in the following years the morphodynamic model is applied to the prediction of this rapid nearshore profile behaviour the calibration of the model against an extensive set of full scale hydrodynamic data at several locations throughout the nearshore bar zone shows a good representation of the measured hydrodynamics morphodynamic results show a dependency on spatial scale on the scale of precise bed level evolution with respect to bar migration and growth model predictions are poor as the nearshore bars are predicted to flatten out resorting to bulk volumes integrated over larger spatial scales the model clearly has skill in predicting the overall effects of the nourishment the lack of phase shift between sediment transport and bathymetry is identified as the key controlling factor for the poor sandbar predictions

observations and modeling of steep beach grain size variability novel observations of surface grain size distributions are used in combination with intra wave modeling to examine the processes responsible for the sorting of sediment grains on a relatively steep beach slope 1 7 5 the field observations of the mean grain size collected with a digital camera system at consecutive low and high tides for a 2 week period show significant temporal and spatial variation this variation is reproduced by the modeling approach when the surf zone flow circulation is relatively weak showing coarse grain sizes at the location of the shore break and finer sediment onshore and offshore of the shore break the model results suggest that grain size sorting is dominated by the wave breaking related suspended sediment transport which removes finer sediment from the shore break and transports it both on shore and offshore the transport capacity of wave breaking related suspended sediment is controlled by the sediment response time scale in the advection diffusion equation where small large values promote onshore offshore transport comparisons with the observed beach profile evolution suggest a relatively short time scale for the suspended sediment response which could be explained by the vigorous breaking of the waves at the shore break

event based stratigraphic simulation of wave dominated shallow marine environments abstract the presence of event beds in the shallow marine stratigraphic record indicates formation is governed by high magnitude low frequency processes rather than low magnitude high frequency processes in this paper a 2 d multiple grain size process response model barsim is presented which uses event deposition to simulate stratigraphic response to changes in sea level and sediment supply barsim uses variable time steps to simulate individual storm event and fair weather periods deposition during storm conditions solely consists of reworked shoreface sediment while fair weather deposits consist of reworked shoreface sediment combined with sediment supplied by littoral drift and suspension simulations using variations in sea level sediment supply both amount and grain size and wave height regime result in distinct depositional patterns stratal geometry and bed characteristics model results indicate that both wave height regime and grain size of supplied sediment have considerable effect on shoreface morphology and stratal characteristics unraveling coastal evolution from the shallow marine stratigraphic record may therefore be more difficult than previously assumed as both variables are expected to vary over geologic time scales