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

anomalies in global carbon cycling and extinction at the triassic jurassic boundary evidence from a marine c isotope record this study investigates whether the end triassic biotic crisis was coupled with a perturbation of the marine c isotope budget the marine c isotope signature serves as a proxy of the marine carbon reservoir and ultimately of the global c cycle a continuous shallow water marine limestone succession from the western southern alps bergamasc alps northern italy provides information on the end triassic biotic crisis and on the evolution of the marine carbon reservoir across the triassic jurassic t j boundary the established carbonate c isotope curve is marked by a negative c isotope pulse coinciding with the disappearance of the end triassic benthic faunal assemblage and a widespread rhaetian carbonate platform drowning event the negative spike is followed by a positive c isotope excursion starting at the palynological t j transition the negative c isotope pulse may have resulted from the sudden release of gas hydrates the positive isotope excursion records a global change in organic carbon burial rates probably in response to elevated atmospheric co2 levels at a time of massive volcanic activity in the central atlantic magmatic province high co2 levels were responsible for the end triassic biocalcification crisis carbonate platform collapse and possibly the sudden release of methane from gas hydrate

cemented sands under static loading cemented sands are found in many areas of the world one of their distinguishing characteristics is their ability to stand in steep natural slopes large deposits are located along the california coast and in a number of areas intense urban development has occurred near the crest of high steep slopes because of the hazards posed by slope failures in the cemented sands a test program was undertaken to define the nature of the cementation and its effect on behavior of the soils a total of 137 laboratory compression and tension load tests were performed on undisturbed samples of naturally cemented sands and artificially prepared cemented sands the materials exhibited friction angles similar to uncemented sands but had a cementation strength intercept and a tensile strength the primary cementing agents of the sands tested were silicates and iron oxides the strength of the cemented sands were found to be a function of density amount of cementing agent grain shape and grain arrangement

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

short and long term erosional processes affecting the stability of the mediterranean coastal cliffs of israel abstract the destructive processes affecting the coastal cliffs of israel are seen as time dependent processes acting in two independent zones at the top and bottom of the cliffs at the top of the cliff these processes are classed as short term phenomena that may be seen as seasonal changes or those initiated by man two main phenomena are recognized as multiple sliding at the contact of hamra clay and kurkar sandstone horizons and collapse of an overlying hard calcarenite layer high water content is by far the most significant contributor to instability which is aggravated by excessive watering of gardens near the cliff edge other factors contributing to instability are grain size and lack of cementation at the bottom of the cliff these processes are classed as long term phenomena where several climatic seasons to a number of years may pass between events these processes are mainly dependent on the abrasive action of waves and to a lesser extent on wind stability is dependent on factors such as grain size distribution of particles cross bedding and degree of cementation of the sandstone the factor of safety f for kurkar slopes ranges from f 1 29 1 78 and that of hamra slopes from f 0 59 0 60 recognition of the active processes causing instability in the different parts of the cliffs and the factors controlling them is essential for the design and remedial measures required for controlling the continued destruction of the cliffs

characterisation of collapsing loess by seismic dilatometer abstract structural collapse and sudden volume changes represent a major geotechnical issue particularly in loess soils there are different criteria for assessing collapse potential based on laboratory test results which require the collection of undisturbed samples from test pits this can be a complicated and costly procedure often financially unjustified for smaller projects this paper presents the results of seismic dilatometer tests sdmt performed at a single location in a loess soil in belgrade in addition to the results of single oedometer collapse tests performed on high quality samples from test pits after comparing laboratory test results it was possible to determine that in samples collected from boreholes dry unit weight is approximately 20 higher than in block samples taken from test pits while moisture content is between 1 and 4 higher unit weight estimated from dmt is on average 15 higher than the unit weight obtained from block samples the constrained modulus mdmt determined by dmt are highly compatible with oedometer modulus eoed obtained from samples at natural moisture content this paper presents two possible ways of identifying collapsing loess based on intermediate dmt parameters and the ratio of g0 mdmt the tests results indicate that if the ratio between the material index id and the horizontal stress index kd is greater than 5 the danger of collapse is imminent simultaneous observation of the changes of id and kd with depth on a semi log graph in the same scale is recommended in this way the relative distance between them may be clearly noticed the larger the distance becomes the greater the risk of collapse in the collapsing loess ratio g0 mdmt is found to be higher than 21 for kd less than 0 6 while in non collapsing loess g0 mdmt is less than 21 for higher values of kd in terms of dmt loess is considered as underconsolidated soil which is one of the most common definitions of loess

a constitutive model for soft clayey rocks that includes weathering effects microstructural and mineralogical observations of clay stones reveal the presence of clay aggregates bonded by a skeleton of inert minerals such as calcium carbonate the behaviour of these natural materials evolves from a rock like behaviour when undisturbed to clay like soil when weathered or when subjected to straining a model has been developed to simulate the constitutive behaviour of these transitional materials following the proposal made by vaunat gens the material is conceived as a composite medium made of a clay matrix and a quasi brittle bonding microstructure the basic model has been modified and extended to reproduce the expansive behaviour of the clay matrix a fundamental aspect to simulate weathering effects induced largely by drying wetting cycles the clay matrix reacts to stress and suction changes whereas the bond component is not affected by suction changes an elasto plastic double structure model for expansive clay soils describes the clay matrix the bonding structure fol

on the role of bond breakage due to unloading in the behaviour of weak sandstones a laboratory investigation was made of the effects of isotropic unloading on the mechanical behaviour of cemented sandstones modelling in a simple way the influence of sampling on hydrocarbon reservoir sandstones that have been cemented at depth artificial samples of sandstone were created by allowing a cement to hydrate while a sand sample was under a high confining stress in a triaxial apparatus comparisons were then made between the behaviour of samples that had been unloaded to zero confining stress with that of samples tested at stress levels around that during cementing it was found that the isotropic unloading damaged the cement which had a significant effect on the small strain and yielding behaviour but not on the strength and large strain behaviour comparisons were also made with tests on natural samples of reservoir and analogue sandstones and it was found that the artificial sandstone had reproduced the key elements of their behaviour the intrinsic properties of all of the sandstones t

first alkaline magmatism during iberia newfoundland rifting the age and origin of alkaline rocks emplaced into the sediments of the rift related continental lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between iberia and newfoundland the u pb titanite ages are 146 5 1 6 2 sterr 145 3 1 4 and 142 3 1 0 ma and initial pb isotopic ratios of feldspars lie at 18 418 18 978 for 206pb 204pb at 15 594 15 925 for 207pb 204pb and at 37 105 39 216 for 208pb 204pb initial 87sr 86sr ratios measured in the same feldspar fractions lie at 0 705409 0 706462 this episode of magmatic activity lasting for at least 4 2 2 6 myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface such zones are preferentially re activated palaeozoic faults of the iberian plate the isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental iberian mantle

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

stability of steep slopes in cemented sands the analysis of steep slope and cliff stability in variably cemented sands poses a significant practical challenge as routine analyses tend to underestimate the actually observed stability of existing slopes the presented research evaluates how the degree of cementation controls the evolution of steep sand slopes and shows that the detailed slope geometry is important in determining the characteristics of the failure mode which in turn guide the selection of an appropriate stability analysis method detailed slope profile cross sections derived from terrestrial lidar surveying of otherwise inaccessible cemented sand cliffs are used to investigate failure modes in weakly cemented unconfined compressive strength ucs30 kpa and moderately cemented 30ucs400 kpa sands and their role in the evolution of the geometry of the slopes the results show that high resolution slope topography such as can be obtained with terrestrial lidar is essential for identifying altogether different failure modes in weakly cemented shear mode and moderately cemented tensile mode sand slopes analyses show that the standard culmann method for steep slopes is inappropriate for modeling the stability of cemented sand slopes since it tends to overpredict expected crest retreat and underestimate failure plane angle instead a simplified analysis using infinite slope assumptions but applied to a slope with finite dimensions subject to changing geometric conditions such as toe erosion and slope steepening is suggested for analysis of weakly cemented sand slopes for moderately cemented sand slopes a limit equilibrium analysis directly comparing the cliff tensile stress and cemented sand tensile strength is shown to reasonably predict failure conditions and timing as a result of either slope steepening or tensile strength loss presumably from wetting in most cases doi 10 1061 ascegt 1943 5606 0000396 ce database subject headings slope stability sand soil type limit equilibrium tensile strength failure modes author keywords slope stability cemented sand limiting equilibrium tensile strength lidar cliffs failure mode

hydraulics and basin morphometry of the largest flash floods in the conterminous united states abstract the maximum rainfall runoff floods measured by indirect methods in small basins 0 39 370 km2 in the conterminous united states are examined this analysis identified twelve floods that were the largest ever measured these floods all occurred in semiarid to arid areas for eleven of the twelve largest rainfall runoff floods measured in small basins by the slope area method values of hydraulic depth hydraulic radius width depth ratio n value mean velocity velocity head coefficient froude number water surface energy and channel slopes shear stress and unit stream power are tabulated estimated composite n values weighted by subsection conveyance range from 0 028 to 0 048 with a mean of 0 038 mean velocities ranged from 3 47 to 9 97 m s 1 froude numbers ranged from 0 81 to 2 49 with 9 of 12 floods having values greater than 1 00 water surface energy and channel slopes vary considerably for each flood energy slope always was less than water surface slope by values of 1 104 channel slope was greater than energy slope in eight floods shear stresses ranged between 61 and 855 n m 2 and unit stream power from 212 to 8131 w m 1 floods in these small basins produced shear stresses and unit stream powers several hundred times greater than floods in large rivers floods on other small streams with smaller unit discharges produced greater shear stresses and stream powers this indicates that the force of a flood is controlled by the depth slope product not absolute discharge in the twelve watersheds studied basin relief ranged from 165 to 1280 m elongation ratios ranged from 0 55 to 0 80 the number of first order streams basin magnitude ranged from 10 to 4297 drainage density ranged from 4 1 to 10 9 km km 2 basin slope ranged from 0 0043 to 0 2486 relief ratio ranged from 0 0097 to 0 34 ruggedness number ranged from 0 69 to 7 17 and first order channel frequency ranged from 5 1 to 38 6 km 2 elongation ratios were larger and drainage density and first order channel frequency lower than other small flash flood prone basins in the united states these twelve basins have neither the most favorable morphometric characteristics which contribute to flash flood peaks nor did the storms causing these flash floods have the greatest short duration intensities maximum flood peaks originate from an optimal combination of basin morphology and physiography and storm intensity

paleoflood hydrology origin progress prospects from an origin in diverse studies of flood geomorphology and quaternary geology paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s since acquiring its formal name in 1982 the most productive approach in paleoflood hydrology has become energy based inverse hydraulic modeling of discrete paleoflood events recorded in appropriate settings as slackwater deposits and other paleostage indicators swd psi or as various threshold indicators of non exceedence technological advances particularly in hydraulic modeling and geochronology were instrumental in moving the discipline to its present status the most recent advances include 1 new techniques for the accurate geochronology of flood sediments notably tams radiocarbon analyses and osl dating and 2 the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies from its initial demonstration in the southwestern united states swd psi paleoflood hydrology proved its widespread applicability to various landscape environments particularly important studies have been accomplished in australia china india israel south africa spain and thailand paleoflood hydrology has also generated its share of controversy in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged quaternary geology geomorphology versus applied hydrologic hydraulic engineering nevertheless the future growth of the discipline is assured given the rapid pace of discoveries that it engenders indeed so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change

stiffness evolution in frozen sands subjected to stress changes abstractsampling affects all soils including frozen soils and hydrate bearing sediments the authors monitor the stiffness evolution of frozen sands subjected to various temperature and stress con

reservoir rock chemo mechanical alteration quantified by triaxial tests and implications to fracture reactivation abstract co 2 injection into geological formations shifts the geochemical equilibrium between the minerals and resident brine the induced mineral brine co 2 reactions can alter the reservoir rock strength and deformational behavior which subsequently affects co 2 storage mechanical integrity this study attempts to investigate quantitatively the effect of mineral cement and particle dissolution through numerical modeling and validation of triaxial tests performed on unaltered and geologically altered entrada sandstone we utilize a numerical model that couples the discrete element method dem and the bonded particle model bpm to perform simulations of triaxial tests on synthetic rocks that mimic tested rock samples under various confining stresses numerical results in agreement with experimental evidence show that both cement and particle dissolution significantly contribute to rock weakening in sandstone samples with carbonate hematite cements and pore filling carbonate sensitivity analyses show that the brittleness of dem numerical samples is mostly conditioned by the cement bond size among all cement microscopic parameters an alteration path that mimics the mineral dissolution under subsurface boundary conditions leads to 1 vertical compaction and horizontal stress relaxation in the reservoir rock and 2 transfer of stresses to adjacent strata that results in bond breakage and potentially natural fracture reactivation at a large scale

the maximum discharge of outburst floods caused by the breaching of man made and natural dams the sudden release of water impounded in natural and man made reservoirs has been responsible for some major disasters in mountainous regions of the world recent natural damming events and failures of natural dams have illustrated the need to examine the nature and magnitude of outburst floods and the behaviour of debris dams in general an empirical relationship between maximum discharge qmax and volume of water released during the outburst event vmax is established qmax 0 72vmax0 53 for man made dams and the relationship is thought applicable to the breaching of natural debris dams landslides and moraines this relationship allows a first order estimate to be made of qmax in the vicinity of the breach for a given vmax during the failure of a man made dam or a natural debris dam key words outburst floods man made dams natural dams landslide dam failure

controls on aggradation and incision in the ne negev israel since the middle pleistocene we investigated the mid pleistocene to recent aggradation incision pattern of two drainage systems nahal peres and nahal tahmas in the hyperarid north eastern negev desert southern israel although these drainage systems drain into the tectonically active dead sea basin lake level fluctuations cannot account for the aggradation incision pattern as bedrock knickpoints disconnect the investigated parts of these drainage systems from base level influence we applied geomorphic mapping soil stratigraphy optically stimulated luminescence osl and cosmogenic in situ 10be exposure dating to reconstruct cycles of aggradation and incision of alluvial terraces and to study their temporal association with regional periods of humidity and aridity and global glacial interglacial cycles the spatial and temporal relationships between the alluvial units suggest changes in the drainage system behavior since the middle pleistocene and show a pattern in which prolonged periods of sediment aggradation alternated with short periods of rapid and intense degradation through erosion and incision into sediment and bedrock we obtain ages for several pleistocene holocene periods of incision 1 1 ma 300 ka 120 ka 20 ka 12 ka and 2 ka although broadly synchronous the nahal peres and nahal tahmas systems exhibit temporal differences in aggradation and incision hyperarid conditions have persisted in the region at least since the middle pleistocene as evidenced by gypsic salic soils that ubiquitously cap the investigated alluvial terraces this observation is consistent with other observations throughout the negev indicating prolonged aridity thus alternation between sediment aggradation and degradation cannot be correlated in a simple and straightforward way to climatic changes we explain the temporal differences in aggradation and incision between nahal peres and nahal tahmas as resulting from the differences in stream gradient basin hypsometry and drainage basin area which influences the amount of water passing through the system regardless of humidity source tropical plumes red sea trough or mediterranean fronts and the frequency and intensity of rainfall in the region our study shows that 1 unlike other climatic proxies such as speleothems the evolution of alluvial terraces which involves many stages sediment generation transport deposition and subsequent incision results in a significant time gap between the actual ages of terraces and the timing of external triggers such as climatic fluctuations and 2 such time gaps may vary even within nearby drainage systems due to differences in drainage systems specific characteristics and thus result in asynchronicity or at least poor synchronicity of alluvial terrace deposition and incision

palaeoflood hydrology of the salt river arizona geomorphic evidence along bedrock confined reaches of the salt river in east central arizona provides a record of the river s largest flood events fine grained flood slackwater deposits accumulated at channel margin irregularities several metres above the low flow channel discharges associated with flow events responsible for the deposits were estimated by computer flow modelling these estimates document flood magnitudes in excess of gauged historic streamflows relative and radiocarbon dating suggest that a flood record in excess of 600 y is preserved in the slackwater sequences a prominent flood scar cut into grussy hillslope soils allows the extension of the prehistoric flood record to several thousand years a maximum discharge estimate of 4600 m3s 1 affixed to the flood scar represents the largest flood event in the record and is given a minimum recurrence interval of 1000 2000 y the 1952 flood is the largest historic flow event experienced along the study reach and is estimated at 2900 m3s 1 two palaeoflood events preserved in the slackwater stratigraphy exceed the 1952 event and are given recurrence intervals of 300 and 600 y the latter flood event had an estimated discharge of 3200 m3s 1 it is apparent that discharge estimates affixed to these infrequent large magnitude flood events approach a maximum with decreased probabilities large recurrence intervals this suggests that a physical limit on discharge may exist within the salt river drainage basin and is perhaps directly related to drainage basin size

radiocarbon dating of flood events katherine gorge northern territory australia radiocarbon analyses of organic materials provide accurate dating of flood events in the katherine gorge northern territory australia selected flood events in the period 1955 1980 can be very precisely dated essentially to the calendar year through the use of curves showing the time variation of anomalously high 14 c activity in the troposphere that was generated by nuclear testing conventional radiocarbon analysis of alluvial charcoal and organic litter is used to establish dates for the largest katherine river floods of the past several centuries the improved geochronologic techniques demonstrated here will help improve paleoflood hydrologic interpretations

late archaean early proterozoic source ages of zircons in rocks from the paleozoic orogen of western galicia nw spain abstract u pb data are reported for nine suites of zircons and three monazites from the paleozoic orogen in western galicia nw spain one paragneiss and six orthogneisses from the early paleozoic basement and two carboniferous ca 310 ma old intrusions of two mica granite new whole rock rb sr analyses along with earlier data indicate an age of ca 470 440 ma ordovician for the emplacement of the granitic precursors of the orthogneisses monazite from the paragneiss also yields an u pb age of ca 470 ma for all nine investigated suites of zircons the u pb systematics signal the presence of a minor proportion of precambrian zircon the zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3 0 ga and 2 0 ga and lower intercepts between ca 480 and 460 ma the former are interpreted as approaching the age of the old zircon component and the latter as reflecting either the time of crystallization of new zircons from the magma orthogneisses or the time of radiogenic lead loss from the old zircons paragneiss the suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470 440 ma ago in most cases leading to false discordias without geochronological significance similarly the zircons of the two investigated two mica granites do not produce meaningful discordias because of post paleozoic disturbance the monazite u pb systems of the latter granites indicate sub recent lead loss from the high initial 87 sr 86 sr ratios an involvement of precambrian continental crust material is evident in the generation of the early paleozoic suite of granites while the zircon u pb data give evidence of the presence of about 3 0 2 0 ga old late archaean early proterozoic components in the source material zircons from the oldest sedimentary rocks in the area now present as catazonal paragneisses and a likely source for the granites likewise reveal a provenance age of 3 0 2 0 ga the late archaean early proterozoic source rocks in western galicia probably formed part of an archaean continental crust underlying much of western europe but mostly modified beyond recognition by younger events

40ar 39ar ages and duration of the central atlantic magmatic province volcanism in morocco and portugal and its relation to the triassic jurassic boundary the central atlantic magmatic province camp is the world s largest known continental igneous province and is 15 contemporaneous with the triassic jurassic t j boundary that marks one of the five largest biological extinctions in the last 16 600 ma although constraints on the duration of the camp volcanism remain yet poorly defined the assumption that the camp 17 may have had a causal relationship with the climatic and biotic crisis was suggested marzoli a bertrand h knight k cirilli 18 s buratti n verati c nomade s renne p r youbi n martini r allenbach k neuwerth r rapaille c zaninetti l 19 bellieni g 2004 synchrony of the central atlantic magmatic province and the triassic jurassic boundary climatic and biotic 20 crisis geology 32 973 976 in this paper we present new 40ar 39ar ages on basaltic flows from moroccan central high atlas 21 oujda and argana and portuguese algarve and santiago do cacem basins where camp lava flows are interlayered with t j 22 sedimentary sequences 23 in the moroccan basins where the basalts were erupted across the t j boundary well defined 40ar 39ar plateau ages n 12 24 for more than 90 of the total erupted volume of camp basalts range from 197 8 0 7 ma to 201 7 2 4 ma and display a main 25 restricted peak at 199 1 1 ma providing a precise estimate of the 40ar 39ar age of the t j boundary concordant with the current 26 u pb age 199 6 0 3 ma palfy j mortensen j k carter e s smith p l friedman r m tipper h w 2000 timing the 27 end triassic mass extinction first on land then in the sea geology 28 39 42 the topmost lava flows recurrent unit yield a 28 significantly younger mean age of 196 6 0 6 ma highlighting late episodic eruption of camp magmas these late basalts 29 represent a small magma volume b10 of the total which was erupted most likely due to asthenospheric upwelling and 30 lithospheric extension within the t j central high atlas basins 31 in the portuguese basins stratigraphical and palynological studies of the associated continental sedimentary sequence 32 underlying the volcanic units suggest an age close to the t j boundary for the camp basaltic flows this is confirmed by new 33 40ar 39ar data for the portuguese basalts which provide the first accurate radio isotopic ages for lava flows in the european 34 camp with two plateau ages yielding a mean value of 198 1 0 4 ma these plateau ages plus three concordant mini plateau 35 ages suggest synchronous rifting and volcanism both in south western europe and morocco central high atlas and oujda basins in summary the eruptive history of the camp with an early extremely widespread and intense peak at 199 1 ma and a tail of 37 episodic eruption events until 196 6 ma is consistent with the biotic turnover at the t j boundary which was likely caused by 38 devastating effects of extensive volcanic eruptions

static and dynamic properties of a lightly cemented silicate grouted sand this paper describes the results of an experimental investigation on the behaviour of a lightly grouted loose medium sand under both monotonic and cyclic loading the experimental programme in the present study was carried out on both untreated and treated sand specimens stabilized with a silica based grout and comprises isotropic triaxial compression tests equipped with bender elements drained triaxial monotonic shearing tests and undrained cyclic tests in a simple shear ss apparatus the results highlight that the weak cementation level induced by chemical treatment was sufficient to moderately increase small strain stiffness and stress dilatancy of the grouted sand during drained monotonic shear a small cohesion intercept was observed in the analyzed failure envelope while cyclic liquefaction resistance exhibited a much more significant increase with a different pattern of behaviour between low and high stress levels finally undrained cyclic ss test results evidenced that silicate grout emplo

138 121 ma asthenospheric magmatism prior to continental break up in the north atlantic and geodynamic implications along the galicia and gorringe banks and in the iberia abyssal plain of the north atlantic unroofed sub continental mantle fills the gap between true oceanic crust and the continental crust margin these lithospheric peridotites are intruded by gabbros and dolerites and locally covered by basalts primary magmatic zircons extracted from gabbros and meta gabbros of the two banks were dated by the u pb chronometer and initial hafnium isotope signatures hfi were determined on the same grains for mt gettysburg at gorringe gabbro emplacement ages of 137 5 0 5 2 ma and 135 7 0 8 ma are obtained and corresponding hfi lie at 20 5 0 3 2 and 19 5 0 4 substantiating magma formation from severely lile depleted mantle domains gabbro zircons from mt ormonde at gorringe yield a much younger age of 77 1 0 4 ma and the hf isotopes document an intermediately lile depleted mantle source having a hfi of 7 6 0 4 given its age and hf signature emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the iberian continent in upper cretaceous time concerning the galicia section zircons from a meta gabbro yield an emplacement age of 121 7 0 4 ma and a hfi of 14 0 0 2 and a hfi of 14 6 0 2 is obtained for zircons from a previously dated meta gabbro of identical age these results indicate magma extraction from mantle reservoirs that are slightly less lile depleted than those sampled by the about 20 myr older gorringe gabbros the data demonstrate that magmatism occurring prior to complete separation of europe from america was essentially of asthenospheric origin both the 138 135 ma ages for the gorringe gabbros and 122 ma ages for the galicia gabbros are at least 5 myr older than the oldest sediments on gorringe and the break up unconformity at the galicia bank respectively magma source signatures of the syn rift gabbros are in agreement with values expected for differently depleted cretaceous morb type mantle reservoirs and the age difference for magmatism between the gorringe and galicia banks suggests a rate of 4 4 0 3 cm yr for northward progression of continental rifting based on the new results a structural model for iberia america rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying lithospheric mantle

evidence from ocean drilling program leg 149 mafic igneous rocks for oceanic crust in the iberia abyssal plain ocean continent transition zone leg 149 of the ocean drilling program explored the ocean continent transition oct on the iberia abyssal plain and its role in the opening of the atlantic ocean approximately 130 ma mafic igneous rocks recovered from holes 899b and 900a have mid ocean ridge basalt morb trace element and isotopic characteristics indicating that a spreading center was active during the opening of the iberia abyssal plain oct the hole 899b weathered basalt and diabase clasts have transitional to enriched morb rare earth element characteristics and the hole 900a metamorphosed gabbros have morb initial epsilon nd values between 6 and 11 during the opening event the iberia abyssal plain oct is envisioned to have resembled the central and northern parts of the present red sea with localized spreading centers and magma chambers producing localized patches of morb mafic rocks the lack of a normal ocean floor magnetic anomaly pattern in the iberia abyssal plain means that a continuous spreading center similar to that observed in the present southern red sea was not formed before spreading ceased in the iberia abyssal plain oct and jumped to the present mid atlantic ridge

evolution of deep water rifted margins testing depth dependent extensional models 1 a general understanding of rifted margins which form by thinning of the continental lithosphere exists nevertheless the exact form of thinning is unclear this debate has been stimulated by acquisition of dense seismic wide angle and deep reflection surveys from atlantic ocean margins a central issue concerns the way in which thinning changes with depth we have tackled this issue by developing a generalized inverse model this model attempts to fit subsidence and crustal thinning observations by varying strain rate as a function of time and space depth dependent thinning is permitted but we do not prescribe its existence or form here the algorithm is applied to six margins including two of the most contentious conjugate margins newfoundland iberia and brazil angola calculated strain rate histories predict thinning estimates which broadly match estimates inferred from normal faulting the eastern indian and beaufort sea margins formed by largely uniform lithospheric thinning in contrast the newfoundland iberian conjugate margins formed by a pattern of strongly depth dependent strain rate to account for the paucity of syn rift decompression melting of the underlying asthenosphere the lithospheric mantle close to oceanic continent transition must thin more slowly than the overlying crust this form of depth dependency is not common for example the brazil angolan conjugate margin could have formed by uniform lithospheric thinning provided thick layers of salt were deposited in a preexisting 400 m deep topographic depression depth dependent thinning is not required to account for rapid subsidence of presalt strata

40ar 39ar dating and geochemistry of tholeiitic magmatism related to the early opening of the central atlantic rift tholeiitic effusive and intrusive magmatism from iberia morocco algeria and mali related to the early opening of the central atlantic rift was dated by the 40ar 39ar step heating method four plateau ages ranging from 203 7 2 7 to 197 1 1 8 ma were obtained on plagioclase from dykes from the taoudenni area mali and two lava flows from morocco the messejana dyke iberia which previously yielded discrepant conventional kar dates did not furnish any 40ar 39ar plateau dates however there is a clear inverse relationship between apparent age and k ca ratio for gas fractions from a plagioclase separate proportional to the alteration degree which combined with dates obtained on amphibole biotite and pyroxene allows us to determine an age of around 200 ma for this body these data and those obtained on the foum zguid morocco and the ksi ksou algeria dykes give evidence of a brief magmatic event between 206 and 195 ma ago which affected a large area ca 2500 km long trace element modelling shows that most of these formations originated from a homogeneous enriched source material such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces e g the deccan traps and the red sea rift precisely dated by the 40ar 39ar method as well

cross anisotropic deformation characteristics of natural sedimentary clays the cross anisotropic elasticity parameters were experimentally determined for six sedimentary clays of different ages and origins at both natural and reconstituted states by using high precision triaxial apparatus all the parameters and their dependency on the effective stress are satisfactorily described by a simple conventional power law model the patterns of the inherent anisotropy exhibited close agreement between the natural and reconstituted states suggesting that the development of natural microstructures due to ageing and diagenesis does not play a primary role in determining the anisotropy patterns in clays despite different degrees of stiffness anisotropy observed in the six clays relatively well defined correlations encompassing all the tested clays were found between certain stiffness parameters while fully characterising the drained elasticity moduli is still a challenging laboratory task these correlations will help estimate some of them based on more readily measurable undrained mo

parametric study of the physical properties of hydrate bearing sand silt and clay sediments 2 small strain mechanical properties v and sediment specific surface the s velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness particularly when hydrate saturation exceeds shyd 0 4 at very high hydrate saturations the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress the p velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore filling phases during fluid to hydrate conversion small strain poisson s ratio varies from 0 5 in soft sediments lacking hydrates to 0 25 in stiff sediments i e subject to high vertical effective stress or having high shyd at shyd 0 5 hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress meaning that hydrate rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field

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

effect of bio cementation on geophysical and cone penetration measurements in sands microbially induced calcite precipitation micp is a potentially environmentally conscious ground improvement method that can improve the engineering properties of granular soils through the precipitation of calcite in this study an experiment involving two 1 7 m diameter tank specimens was completed to investigate the effect of bio cementation on cone penetrometer and geophysical measurements in sands following nonuniform bio cementation treatments specimens achieved calcite contents ranging from 0 5 to 5 3 by mass shear wave velocity vs values between 131 and 967 m s and mid depth cone penetration resistances qc ranging between 3 6 and 32 1 mpa at calcite contents exceeding 5 0 qc and vs improvements were as high as 527 and 686 respectively although cone penetration resistance sleeve friction and friction ratio measurements exhibited limited sensitivity to bio cementation at calcite contents of less than 3 0 vs measurements successfully detected bio cementation at calcite content

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

assessment of dam failure flood and a natural high magnitude flood in a hyperarid region using paleoflood hydrology nahal ashalim catchment dead sea israel 1 a dam failure flood in 1993 with a peak discharge of 600 700 m3 s 1 volume of 0 5 106 m3 and a natural rainfall runoff flood with a peak discharge of 420 m3s 1 volume of 2 0 106 m3 in 2004 in the nahal ashalim catchment 75 km2 in the hyperarid dead sea region were comparatively analyzed the two floods which caused serious damage to the dead sea works industries were analyzed for magnitude and frequency using regional analysis extrapolation from the short systematic record rainfall runoff modeling and paleoflood hydrology the 7600 year paleoflood record was reconstructed at the ashalim cave at the upper nahal ashalim peak discharges in this high gradient mountainous wadi are highly sensitive to the roughness coefficient and the flow regime the dam failure peak discharge is lower than the envelope curves of measured floods of the dead sea region and of paleofloods of the negev and the dead sea deserts the probability of the dam failure flood and the natural flood were estimated by various methods at 1 to 0 2 and 0 5 respectively adding the peak discharge of the october 2004 flood increased the probability from 0 2 to 0 5 0 2 the long paleoflood record encompasses different hydrological regimes related to climatic fluctuations which are not relevant to the present hydrological regime therefore the chosen period for frequency analysis was a d 1673 1996

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

a 4500 year record of large floods on the colorado river in the grand canyon arizona a sequence of flood deposits left by the colorado river in the grand canyon arizona provides evidence of at least 15 floods with peak discharges greater than 5500 m 3 sec 1 over the last 4500 yr ten floods during the last 2000 2300 yr had discharges greater than 6800 m 3 sec 1 one flood 1600 1200 yr ago had a discharge exceeding 14 000 m 3 sec 1 a flow rate more than twice the largest gaged flood this record of flooding is one of the longest for a major u s river and combined with the gaged record of twentieth century floods allows determination of the frequency and history of large floods that have affected key aspects of colorado river geomorphology

geotechnical properties of cemented sands in steep slopes an investigation into the geotechnical properties specific to assessing the stability of weakly and moderately cemented sand cliffs is presented a case study from eroding coastal cliffs located in central california provides both the data and impetus for this study herein weakly cemented sand is defined as having an unconfined compressive strength ucs of less than 100 kpa and moderately cemented sand is defined as having ucs between 100 and 400 kpa testing shows that both materials fail in a brittle fashion and can be modeled effectively using linear mohr coulomb strength parameters although for weakly cemented sands curvature of the failure envelope is more evident with decreasing friction and increasing cohesion at higher confinement triaxial tests performed to simulate the evolving stress state of an eroding cliff using a reduction in confinement type stress path result in an order of magnitude decrease in strain at failure and a more brittle response tests aimed at examining the influence of wetting on steep slopes show that a 60 decrease in ucs a 50 drop in cohesion and 80 decrease in the tensile strength occurs in moderately cemented sand upon introduction to water in weakly cemented sands all compressive cohesive and tensile strength is lost upon wetting and saturation the results indicate that particular attention must be given to the relative level of cementation the effects of groundwater or surficial seepage and the small scale strain response when performing geotechnical slope stability analyses on these materials

effect of cementation on the small strain parameters of sands natural cementation affects the properties of soils the interpretation of in situ and laboratory test results and the selection of criteria for geotechnical design in this paper published exper

gravimetric and magnetic fabric study of the sintra igneous complex laccolith plug emplacement in the western iberian passive margin the geometry and emplacement of the 96 km2 late cretaceous sintra igneous complex sic ca 80 ma into the west iberian passive margin is presented based on structural data gravimetric modeling and magnetic fabrics a granite laccolith 76 km2 1 km thick according to gravimetric modeling surrounds a suite of gabbro diorite syenite plugs 20 km2 4 km deep and is encircled by cone sheets and radial dykes anisotropy of magnetic susceptibility was interpreted from 54 sites showing fabrics of para and ferro magnetic origin most fabrics can be interpreted to have a magmatic origin according to the scarcity of solid state deformation in most part of the massif magnetic foliations are shallowly dipping in the granite laccolith and contain a sub horizontal ene wsw lineation the gabbro syenite body displays concentric magnetic foliations having variable dips and steeply plunging lineations the sic can be interpreted to be intruded along an nnw sse 200 km long fault perpendicular to the magnetic lineation within the laccolith and was preceded by the intrusion of basic sills and plugs the sic intruded the mesozoic series of the lusitanian basin during the post rift passive margin stage and its geometry was only slightly modified during the paleogene inversion that resulted in thrusting of the northern border of the intrusion over the country rocks

assessment of the self desiccation process in cemented mine backfills during the placement of fine grained cemented mine backfill the high placement rates and low permeability often result in undrained self weight loading conditions when assessed in the conventional manner however hydration of the cement in the backfill results in a net volume reduction the volume of the hydrated cement is less than the combined volume of the cement and water prior to hydration though the volume change is small it occurs in conjunction with the increasing stiffness of the cementing soil matrix and the result in certain circumstances can be a significant reduction in pore water pressure as hydration proceeds in this paper the implications of this phenomenon in the area of cemented mine backfill are explored an analytical model is developed to quantify this behaviour under undrained boundary conditions this model illustrates that the pore water pressure change is dependent on the amount of volume change associated with the cement hydration the incremental stiffness change of the s

the evolution of holocene reg gravelly soils in deserts abstract reg soil is an aridisol developed upon a stable alluvial surface composed of sand to coarse gravel the objectives of the present study were 1 to examine the initial evolution of reg soil properties with time 2 to assess and explain the rates of development of different properties 3 to establish the suitability of different soil properties for relative age dating the time dependent soil properties examined were 1 mechanical weathering of surficial rocks and desert pavement evolution 2 chemical and biological weathering of surficial rocks 3 the thickness of soil profiles and soil horizons 4 soil texture in the different horizons 5 salts and gypsum distribution and concentration in different soil horizons 6 degree of mechanical weathering of gravel in c horizons fifteen successive surfaces of the nahal wadi ze elim sequence of alluvial fans were selected for study dead sea region israel under an extremely arid climate the sequence is of holocene age 14 000 years b p to recent the main conclusions are 1 reg soils develop rather fast in several 10 3 years since their initiation however the various soil properties developed at different rates 2 the rates of change of three soil properties decrease with time these are the texture in different horizons change in added airborne and rain washed dust salinity within the soil profile change in added salts and gypsum and the amount of mechanically shattered rocks 3 soil thickness is established rather rapidly young and older holocene soil have practically the same depth surficial weathering rates appear to remain fairly constant with time 4 in coarse gravel the evolution of reg soils is dependent on added fines and salts of airborne origin and the resultant changing hydrologic regime in the soil profile the run off infiltration ratio changes as more fines and salts are added to the soil profile and as desert pavement develops 5 surficial features and fine clay in the soil profile may be used for approximate dating of otherwise undatable holocene alluvial surfaces salts and gypsum content should be cautiously used in a rather general way

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

kinematics of peridotite emplacement during north atlantic continental rifting galicia northwestern spain abstract the western part of the galicia passive continental margin western edge of spain is bordered by serpentinized peridotites over a n s distance of about 125 km these peridotites were emplaced at the end of continental rifting and or at the very beginning of oceanic accretion they have been observed in situ and sampled in five sites with the nautile submersible during the galinaute cruise 1986 some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge most of the escarpments covered by the nautile on the peridotite ridge have yielded only ultramafic rocks except at dive 10 where the peridotites are overlain by a sheared chlonte bearing schist and at dive 14 where the ultramafic rocks are covered by basalts all the peridotites are plagioclase bearing harzburgites and iherzolites locally cross cut by rare plagioclase rich veins and dioritic intrusives some pyroxenites and gabbros are associated with the peridotites at dive 14 the plagioclase bearing peridotites are similar in composition to those drilled during odp leg 103 primary structures have not been found except in the northern part of the studied zone where a few samples exhibit magmatic textures elsewhere the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high temperature fabric at these localities boudinage structures and ultramylonitic bands referred to as shear bands occur and the rocks have a disrupted mylonitic texture a few dioritic dykes have also suffered this mylonitization shearing occurred in a rotational regime at high and decreasing temperature 1000 850 c and under a high deviatoric stress 102 mpa i e under lithospheric conditions the structural pattern of the peridotite ridge deduced from measurements on oriented samples is complex the trend of the mylonitic foliation appears nearly constant from south to north n125 n105 but its dip increases 20 45 northward from south to north the stretching lineation pitch varies from subvertical to subhorizontal stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area to the north it occurred in a northwesterly direction with a strike slip component stretching in the chlorite bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation this study confirms the uplift of the galicia peridotites during the continental rifting of the north atlantic ocean in cretaceous times during their ascent the peridotites underwent some melting and high temperature low stress ductile deformation followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures the geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress strain field prevailing in the continental lithosphere during the rifting episode it is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point the kinematics of emplacement agree well with an e w opening of the north atlantic ocean

cyclic response of calcareous soil from the north west shelf of australia this paper reports the results from 23 cyclic ciu and cau triaxial tests on undisturbed samples of calcareous soil the samples which include both uncemented and cemented material have heen subjected to a range of mean and cyclic shear stresses results from the tests on the uncemented soil display many of the characteristics of the cyclic response of other cohesionless soils the more cemented soils sustained higher cyclic stresses and depending on the load level showed either elastic behaviour with no pore pressure build up eventual stabilization after some pore pressure build up or steady state pore pressure build up and eventual failure the effect of frequency on the response in the range 0 1 to 10 hz has heen studied since no dramatic differences in performance could he detected no firm conclusions could be drawn regarding the effect of testing rate l article preente les reultats obtenus a partir de 23 essais triaxiaux cycliques de type ciu et cau effectues sur des echantillons de sol calca

frequency of boulders transport during large floods in hyperarid areas using paleoflood analysis an example from the negev desert israel abstract direct measurements of boulder entrainment in desert wadis are not available the 2004 flood peak discharge 470 m3 s 1 recurrence interval 120 years in the hyperarid ungauged nahal hatzera ephemeral stream 45 km2 transported and deposited 0 85 2 1 m concrete boulders and slabs detached from infrastructure upstream and natural boulders edm and drone air photographic surveys documented the geometry of the study reach and the location of boulders analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200 760 m3 s 1 during the last 600 years 1 d hec ras hydraulic analysis provided water surface profiles discharges and hydraulics along the study reach and velocity shear stress and stream power for each boulder max program and pearson 3 distribution were used for flood frequency analysis most of the concrete boulders were deposited in the sub critical backwater of channel constrictions where velocities were 1 5 2 1 m s 1 the largest boulders were deposited in super critical flow where velocity was 8 9 2 m s 1 the alluvial channel enabled to transport these concrete boulders reflecting the unstable active sandy layer of the channel bed over which the boulders moved the maximum flood shear stress and stream power characterize medium large floods with return period of 20 120 years and not for the largest floods as expected boulders about 2 1 m and weighing about 15 t can be transported at least once in 120 years the shear stress and stream power indicate that the moderate large floods are the most geomorphically effective floods rather than the largest floods in nahal hatzera basin nevertheless the geomorphic effectiveness of the 2004 flood a typical desert flash flood with high peak and short duration was small based on the minor changes along the channel and banks indicating that their resistance thresholds were not exceeded and energy expenditure was mainly on boulders entrainment and transport

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

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

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

sedimentology of high stage flood deposits of the tagus river central spain abstract this paper details the sedimentology of high stage flood deposits with the definition of sedimentary environments and their characteristic sequences along two bedrock reaches of the tagus river central spain high stage flood deposits accumulated in bedrock canyons include slackwater flood deposits swd and other types of deposits located at flow separation zones and associated with slow moving flow 1 these flood deposits are common indirect indicators of flood stages used in palaeoflood studies for estimating the discharges associated with quaternary floods depositional environments of flood deposits include 1 channel widening 2 canyon expansion 3 bedrock obstacles and 4 backflooded areas along tributary streams these flood deposits can be found associated with other non fluvial environments namely aeolian reworked and slope washflow facies channel widening due to flood stage variations comprises internal and external zones of the channel margins and their characteristic sequences contain similar facies to those of alluvial floodplains canyon expansion environments favour vertical accretion of slackwater units and the development of flood deposit benches which contain four sequences related to bench elevation and distance from the channel s main thread of flow at the lee side of bedrock obstacles characteristic sedimentary sequences are dominated by reverse flow structures e g climbing ripples migrating upstream due to eddies with a high sand concentration flood deposits located within tributary mouths contain typical sequences of reworked floodplain deposits backflooding of tributaries during flood stages produces deposition from suspension of sand silt and clay within three sequences characterised by non structure or parallel lamination and intense bioturbation a better understanding of the flood deposit sequences may contribute to the characterisation of flood magnitudes and flood hydraulics and can also be applied to some ancient depositional environments

late holocene climates of the near east deduced from dead sea level variations and modern regional winter rainfall the dead sea is a terminal lake of one of the largest hydrological systems in the levant and may thus be viewed as a large rain gauge for the region variations of its level are indicative of the climate variations in the region here we present the decadal to centennialresolution holocene lake level curve of the dead sea then we determine the regional hydroclimatology that affected level variations to achieve this goal we compare modern natural lake level variations and instrumental rainfall records and quantify the hydrology relative to lake level rise fall or stability to quantify that relationship under natural conditions rainfall data pre dating the artificial dead sea level drop since the 1960s are used in this respect jerusalem station offers the longest uninterrupted pre 1960s rainfall record and jerusalem rains serve as an adequate proxy for the dead sea headwaters rainfall principal component analysis indicates that temporal variations of annual precipitation in all stations in israel north of the current 200 mm yr 1 average isohyet during 1940 1990 are largely synchronous and in phase 70 of the total variance explained by pc1 this station also represents well northern jordan and the area all the way to beirut lebanon especially during extreme drought and wet spells we a determine the modern and propose the past regional hydrology and eastern mediterranean em climatology that affected the severity and length of droughts wet spells associated with multiyear episodes of dead sea level falls rises and b determine that em cyclone tracks were different in average number and latitude in wet and dry years in jerusalem the mean composite sea level pressure and 500 mb height anomalies indicate that the potential causes for wet and dry episodes span the entire em and are rooted in the larger scale northern hemisphere atmospheric circulation we also identified remarkably close association within radiocarbon resolution between climatic changes in the levant reflected by level changes and culture shifts in this region

characterization of sills associated with the u reflection on the newfoundland margin evidence for widespread early post rift magmatism on a magma poor rifted margin author posting the authors 2010 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 182 2010 113 136 doi 10 1111 j 1365 246x 2010 04635 x

behavior of hydrocarbon reservoir sands and sandstones most new oil and gas reservoirs are discovered in sands and weak sandstones and the petroleum industry has therefore generated an extensive amount of data from laboratory tests on these materials some of these data are reexamined in this paper within the context of recent soil mechanics research of key importance to petroleum engineers is the compressibility of the sands and this is shown to be related both to the current density and stress level of the soil and to the location of its normal compression line when present the cement bonds between the particles appear to be weaker than the particles themselves so that the major yield point during compression is still associated with breakage of the particles rather than of the cement bonds the paper also examines the behavior of the sands and sandstones in shearing allowing a more general understanding of the mechanics of the materials to be developed

isotropic yielding in an artificially cemented soil cured under stress the work simulates in the laboratory the formation of a cemented sedimentary deposit in which cement bonding occurs after burial and under geostatic stresses isotropic compression tests were carried out on artificially cemented specimens made with variable cement contents after consolidating the samples to the uncemented normal compression line the specimens were allowed to cure for 48 h before testing the curing confining stresses ranged from 50 to 2000 kpa and were intended to represent soil elements at different depths in the fictitious sedimentary deposit when the cementing occurred the contribution of the cement bonds to soil compression and the changes in the isotropic yield stress as a function of void ratio and cement content were analysed the results showed the importance of the void ratio during the formation of cement bonds and also of the degree of cementation for the compressive behaviour of the cemented soil and demonstrated that the variation in yield stress with void ratio and ce

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

recent advances in rock physics the need to extract more information about the subsurface from geophysical and petrophysical measurements has led to a great interest in the study of the effect of rock and fluid properties on geophysical and petrophysical measurements rock physics research in the last few years has been concerned with studying the effect of lithology fluids pore geometry and fractures on velocity the mechanisms of attenuation of seismic waves the effect of anisotropy and the electrical and dielectric properties of rocks understanding the interrelationships between rock properties and their expression in geophysical and petrophysical data is necessary to integrate geophysical petrophysical and engineering data for the enhanced exploration and characterization of petroleum reservoirs the use of amplitude offsets s wave seismic data and full waveform sonic data will help in the discrimination of lithology the effect of in situ temperatures and pressures must be taken into account especially in fractured and u

the holocene climatic record of the salt caves of mount sedom israel mount sedom is a salt diapir on the southwestern shore of the dead sea which has been rising above the local base level throughout the holocene karst development within the salt body has kept pace with the rise forming subhorizontal cave passages with vertical shafts wood fragments found embedded in flood sediments that were deposited in the cave passages yielded 14c ages ranging from c 7100 to 200 bp a palaeoclimatic sequence was constructed based on parameters that include relative abundance of plant types or floral communities the elevations of the corresponding cave passages and the ratio of their width to present passage width the results were correlated to the holocene sedimentary sequence of the dead sea basin and other features associated with shifting lake levels moister climatic stages are indicated by relatively abundant wood remains by wide cave passages and by elevated outlets indicating high dead sea level arid periods are marked by a scarcity of wood remains by narrow cave pa

the palaeoflood record of a hyperarid catchment nahal zin negev desert israel the palaeohydrology of nahal zin a 1400 km2 catchment in the hyperarid negev desert is inferred from slackwater deposits and palaeostage indicators in a canyon near its lower end the palaeoflood record augmented by the instrumental and historical records of the last decade includes 28 floods ranging from 200 to 1500 m3s 1 over the last 2000 years this helps to reanalyse the frequency of floods in this drainage system the clusters of floods around 1000 years bp and again during the last 60 years are characterized by high flow magnitudes periods with many floods correspond well to periods with high dead sea levels and are probably relatively wet periods while periods with few floods correspond well to low dead sea levels indicating a drier climate fluctuations in the frequency of floods are typical of periods of transition from one climate regime to another copyright 2000 john wiley sons ltd

formation history and physical properties of sediments from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract the synthesis of available geological information and surface temperature evolution in the alaska north slope region suggests that biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse grained layers that have relatively high hydraulic conductivity and low gas entry pressures hydrate started forming before the beginning of the permafrost eventually the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations the single tested specimen depth 620 47 620 62 m from the d unit consists of uncemented quartzitic fine sand with a high fraction of fines 56 by mass finer than sieve 200 the as received specimen shows no evidence of gas present the surface texture of sediment grains is compatible with a fluvial deltaic sedimentation environment and shows no signs of glacial entrainment tests conducted on sediments with and without thf hydrates show that effective stress porosity and hydrate saturation are the major controls on the mechanical and geophysical properties previously derived relationships between these variables and mechanical geophysical parameters properly fit the measurements gathered with mount elbert specimens at different hydrate saturations and effective stress levels we show that these measurements can be combined with index properties and empirical geomechanical relationships to estimate engineering design parameters volumetric strains measured during hydrate dissociation vanish at 2 4 mpa therefore minimal volumetric strains are anticipated during gas production at the mount elbert well however volume changes could increase if extensive grain crushing takes place during depressurization driven production strategies if the sediment has unexpectedly high in situ porosity associated to the formation history or if fines migration and clogging cause a situation of sustained sand production

reconciling cretaceous paleomagnetic and marine magnetic data for iberia new iberian paleomagnetic poles 1 the cretaceous paleogeography and the kinematic evolution of the iberian plate are poorly constrained especially problematic is to reconcile iberian paleomagnetic data with paleomagnetic data of the neighboring plates and with euler poles derived from seafloor magnetic anomalies the first limitation arises from the cretaceous normal polarity superchron where paleogeographic reconstruction using marine magnetic anomalies is handicapped the second arises from the paucity of reliable paleomagnetic poles with satisfactory statistical criteria and age in order to address these shortcomings and provide new high quality paleomagnetic poles for iberia we conducted a detailed rock magnetic and paleomagnetic study of two cretaceous magmatic sills the paco de ilhas pi and foz da fonte ff sills from the lusitanian basin portugal recently dated at about 88 and 94 ma respectively our results show that the magnetic mineralogy of the sills is primary i e acquired during magma cooling and essentially represented by titanomagnetite the corresponding paleomagnetic poles match the synthetic apwp from the african plate at 80 and 100 ma on the basis of a rigorous selection of iberian cretaceous poles we then calculated mean paleomagnetic poles for different time intervals and found that iberian paleomagnetic data fit well the global apwp between 70 and 120 ma but move far away from the apwp at pre rift times our approach shows that new and better constrained paleomagnetic poles can aide in solving part of the contradiction between iberian and african apwps

deformation characteristics of two cemented calcareous soils the effect of cementation on the deformation characteristics of two cemented calcareous soils was investigated through a series of undrained triaxial tests performed under both monotonic and cyclic

paleofloods and a dam failure flood on the virgin river utah and arizona abstract flood deposits on the virgin river in southern utah and northern arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since ad 900 in the virgin river gorge several of the natural paleoflood discharges were comparable with or even greater than the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m3 s 1 in this reach of the virgin river to estimate the paleoflood discharges the heights of slackwater flood deposits in the lower bedrock gorge of the virgin river were compared with water surface elevations calculated with the hec 2 step backwater hydraulic model the timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits the estimates of both the 1989 flood and the paleofloods were carried out in the virgin river gorge about 60 km downstream of the dam break site no natural floods of this magnitude have occurred within the relatively short period of the gaged record this study illustrates how incorporating rare extreme events from the long term paleoflood record improves the database for flood frequency analysis the 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower virgin river

flood frequency analysis on the ardeche river using french documentary sources from the last two centuries fitting statistical laws from a short time series does not give any guarantee of reliability on extreme flood estimation historical investigation through documentary sources can enlarge the record period this paper presents a case study on the ardeche river based on collaboration between historians archivists hydraulic engineers and hydrologists for a better assessment of the flood risk a list of historical flood levels from 1644 to the present has been drawn up and converted into discharge using hydraulic modelling a sensitivity analysis provides error intervals on discharge estimates taking into account uncertainties on water level roughness coefficient and channel geometry and the impact of a nonpermanent discharge or the backward effect a flood frequency analysis using systematic and nonsystematic data gives a practical example on how historical information can improve flood knowledge it reduces the sampling uncertainty and shows also a good agreement with a hydrometeorological approach gradex model

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

paleoclimate reconstruction based on the timing of speleothem growth and oxygen and carbon isotope composition in a cave located in the rain shadow in israel high resolution th 230 u 234 ages and delta 18 o and delta 13 c compositions of speleothems in ma ale efrayirn cave located to the east of the central mountain ridge of israel enable us to examine the nature of the rain shadow aridity during glacial and interglacial intervals speleothem growth occurred during marine glacial isotopic periods with no growth during the two last marine isotope interglacial intervals and during the peak of the last glacial maximum this contrasts with speleothem growth in caves located on the western flank of the central mountain ridge in the eastern mediterranean semiarid climatic zone which continued throughout the last 240 000 yr thus during glacial periods water reached both sides of the central mountain ridge a comparison of the present day rain and cave water isotopic compositions and amounts at the ma ale efrayim cave site with those on the western flank shows that evaporation and higher temperatures on the eastern flank are major influences on isotopic composition and the lack of rainfall the delta 18 o and delta 13 c profiles of the speleothems deposited between 67 000 and 25 000 yr b p match the general trends of the isotopic profiles of soreq cave speleothems suggesting a similar source eastern mediterranean sea and similar climatic conditions thus during glacial periods the desert boundary effectively migrated further south or east from its present day location on the eastern flank whereas interglacial periods appear to have been similar to the present with the desert boundary at the same position the decrease in overall temperature and a consequent reduction in the evaporation to precipitation ratios on the eastern flank are viewed as the major factors controlling the decay of the rain shadow effect during glacial periods c 2003 elsevier science usa all rights reserved

the nahal oz reservoir dam break flood geomorphic impact on a small ephemeral loess channel in the semi arid negev desert israel abstract the nahal oz reservoir in the coastal semi arid southwestern israel was designed to enhance local irrigation of crops using reclaimed sewage water during the dry summer months on march 2001 part of the western dike of the reservoir was breached and generated a flow release of 3 5 106 m3 of secondary irrigation water that was channeled down the 1st order ephemeral loess stream nahal yare akh the consequent 12 hour flood surge with an estimated peak discharge of 1000 m3s 1 inflicted severe loess erosion agricultural property and infrastructural damage downstream 6 million post flood mapping documented the geomorphic response to the flood which included channel scour and widening along the initial 2 km downstream of the reservoir where a spillway channel was formed the increase in the cross sectional area was about 60 and had an estimated 170 000 m3 of sediment bed bank and floodplain erosion calculated maximum shear stress and stream power along this section are estimated at 300 nm 2 and 900 wm 2 respectively the peak discharge at the end of this segment was estimated at 800 m3s 1 indicating only minor attenuation along this segment two km downstream of the breach a wide braided fan indicated deposition of the eroded sediments at the end of this segment the floodwater diverged into several watercourses and inundated tilled agricultural fields and neighborhoods downstream 9 km from the reservoir the discharge attenuated to 100 m3s 1 slightly above bankfull further downstream and upon reaching the large shikma stream the flow was already very low this reduction in discharge is attributed to the anthropogenic infrastructure roads neighborhoods and agricultural fields and the large transmission losses typical of sandy ephemeral streams the study shows that channels within erodible materials respond to high peak discharges very locally erosional thresholds that severely incised the channel are only maintained for 2 km below the breached reservoir and as distance from the source area increases available energy is substantially reduced due to high transmission losses gentler valley gradients very wide shallow flow and the ability of the drainage network to accommodate large discharges consequently the geomorphic impact was limited to the first 2 6 km with only minor erosional or depositional evidence downstream the current natural flow regime has only minor impact on the newly formed channel geometry below the breached dam accordingly the channel geometry may be preserved for a relatively long time however channel recovery in this segment was rapid and within a few years all bare loess surfaces exposed during the flood have been covered by dense vegetation the analysis also shows that nahal oz reservoir s dam break flood transmission losses and flood attenuation rates were extreme in comparison to other case studies this artificial flood event in a desert environment offers a rare opportunity to quantify channel and bank erosion evolution processes during an extremely high magnitude flood within highly erodible fine sediment as well as to detect the recovery processes a few years later

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

a high magnitude storm and flood in a hyperarid catchment nahal zin negev desert israel in october 1991 a high magnitude rainstorm flood estimated return period 40 years occurred in nahal zin a 1400 km2 catchment in the hyperarid negev desert the meso scale structure of the storm was a curved squall line that developed from a thunderstorm in accordance with the topography of the catchment divide by which it was strongly affected tropical moisture reached the area via the subtropical jet stream in conjunction with a lower level northward intrusion of the red sea trough rst n into the mediterranean sea rainfall as measured at the few and sparse gauging stations was much too small to account for the resulting large flood peak flow and other hydraulic characteristics of the flood were indirectly reconstructed the techniques of palaeoflood hydrology used were based on sedimentological evidence of fine grained flood sediments deposited in back flooded tributaries as well as on other stage indicators the hec 2 procedure was employed to determine water surface profiles the spatial and temporal characteristics of the event were studied through a combination of rainstorm analysis remote sensing hydrological and sedimentological data they jointly explain the magnitude and timing of tributary contributions producing the integrated flood in the main channel the flood as reconstructed reveals a three peak hydrograph two peaks were generated by the same storm but had different floodwave arrival times in the main channel the third resulted from a local rainstorm which occurred on the following day and covered only one tributary the curved structure of the storm and its dynamics in relation to catchment orientation resulted in storm move ment in tandem with the floodwave the synchronous contribution from all main tributaries preserved evidence of the floodwave both in stage and volume by replacing the transmission losses in the sections with thick alluvium other high magnitude floods on record for the large negev desert catchments are caused by a cold upper air incursion associated with the rst n most of them occur in the autumn and are caused by storms with high intensity rainfall this is in stark contrast with the flooding behaviour of the semi arid zone further north which is linked primarily to the core of the mediterranean winter the complexities involved in the generation of a large desert flood as revealed by this study illustrate the fallacy of applying routine hydrological modelling to such events and underline the need to study the processes involved in adequate detail 1998 john wiley sons ltd

on the behaviour of thanet sand an example of an uncemented natural sand an extensive laboratory investigation was made of the mechanics of thanet sand testing the soils from a number of locations and depths in the triaxial apparatus over a wide range of stress levels comparisons between tests on intact and reconstituted samples and between vertically and horizontally trimmed intact samples highlighted the effects of structure on the behaviour of this uncemented sand in particular a significant effect of anisotropy two distinct substrata have previously been identified with different fines contents and the key difference in their behaviour has been found here to be the location of their critical state line in the v lnp plane which has a significant effect on the engineering behaviour of the two substrata and can explain recent unusual observations of decreasing pile capacity with depth tests in which the glauconite content was removed from the sand showed that it did not affect its behaviour

small strain stiffness shear wave velocity and soil compressibility abstractthe small strain shear modulus depends on stress in uncemented soils in effect the shear wave velocity which is often used to calculate shear stiffness follows a power equation with the mean effective stress in the polarization plane vs m 1 kpa where the factor is the velocity at 1 kpa and the exponent captures the velocity sensitivity to the state of stress the small strain shear stiffness or velocity is a constant fabric measurement at a given state of stress however parameters and are determined by fitting the power equation to velocity measurements conducted at different effective stress levels so changes in both contact stiffness and soil fabric are inherently involved therefore the and parameters should be linked to soil compressibility cc compiled experimental results show that the factor decreases and the exponent increases as soil compressibility cc increases and there is a robust inverse relationship between and for all sediments 0 73 0 27 log

exhumed mantle forming transitional crust in the newfoundland iberia rift and associated magnetic anomalies transitional zones located between iberia and north america formed during continental rifting and mostly consist of exhumed mantle in this study we show that ages of exhumed mantle at ocean drilling program odp sites 1068 and 1070 in the iberia abyssal plain and site 1277 in the newfoundland basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation on the basis of paleomagnetic and geological data and a comparison with a fossil ocean continent transition in the alps we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater and which only affects the upper tens of meters of the exhumed mantle the inversion of magnetic data euler deconvolution shows that magnetic sources are n s trending horizontal cylindrical bodies located within the highly serpentinized upper crust therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading within transition zones sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust but not on its nature oceanic versus exhumed mantle this discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting during rifting the deep distal continental margins and the adjacent transitional zones in the newfoundland iberia rift system were formed by ultraslow extension from early berriasian to late valanginian early hauterivian and by slow extension from early hauterivian to the late aptian early albian boundary therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern

methods for predicting peak discharge of floods caused by failure of natural and constructed earthen dams floods from failures of natural and constructed dams constitute a widespread hazard to people and property expeditious means of assessing flood hazards are necessary particularly in the case of natural dams which may form suddenly and unexpectedly we revise statistical relations derived from data for past constructed and natural dam failures between peak discharge qp and water volume released v0 or drop in lake level d but assert that such relations even when cast into a dimensionless form are of limited utility because they fail to portray the effect of breach formation rate we then analyze a simple physically based model of dam breach formation to show that the hydrograph at the breach depends primarily on a dimensionless parameter kv0 gl 2d7 2 where k is the mean erosion rate of the breach and g is acceleration due to gravity the functional relationship between qp and takes asymptotically distinct forms depending on whether 1 relatively slow breach formation or small lake volume or 1 relatively fast breach formation or large lake volume theoretical predictions agree well with data from dam failures for which k and thus can be estimated the theory thus provides a rapid means of predicting the plausible range of values of peak discharge at the breach in an earthen dam as long as the impounded water volume and the water depth at the dam face can be estimated