evolution of the eastern volcanic ridge of the canary islands based on new kar data the results of 64 new k ar age determinations together with 32 previously published ages show that after a period of erosion of the basal complex miocene volcanic activity started around 20 ma in fuerteventura and 15 ma in lanzarote forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs this series includes five separate volcanic edifices each one with its own eruptive history in fuerteventura several miocene eruptive cycles have been identified in the central edifice one around 20 17 ma followed by two others centred around 15 and 13 ma in the southern edifice the maximum of activity took place around 16 14 ma whereas in the northern one the main activity occurred between 14 and 12 ma in lanzarote a first cycle of activity took place in the southern edifice between 15 5 and 14 1 ma followed by another between 13 6 and 12 3 ma in the northern edifice three pulses occurred 10 2 8 3 6 6 5 3 and 3 9 3 8 ma an important temporal gap greater in fuerteventura than in lanzarote separates series i from the plio quaternary series ii iii and iv formed by multi vent basaltic emissions in fuerteventura the following eruptive cycles have been identified 5 2 9 2 4 1 8 1 7 0 8 0 4 and 0 1 ma in lanzarote the activity was fairly continuous from 2 7 ma to historic times with a maximum in the lower pleistocene eruptive rates in the series i edifices were on the average 0 1 0 01 km3 ka comparable but slightly smaller than in similar edifices in tenerife and la gomera but much lower than in gran canaria average post miocene eruptive rates were about 0 013 0 027 km3 ka in lanzarote and 0 003 0 007 km3 ka in fuerteventura all these volcanic edifices show a similar general sequence fissural eruptions erosion multi vent volcanism repeated at different periods in different parts of the eastern islands of the canaries the model of growth of the series i edifices is comparable to those in tenerife and la gomera long periods of activity sometimes greater than 6 m y with pulses separated by gaps however salic and intermediate differentiates frequent in tenerife and la gomera are very scarce in these islands the fuerteventura lanzarote ridge shows a decrease in volcanic activity with time and also a certain ssw nne polarity in the temporal development of volcanism

hydrochemical processes in the lower jordan valley and in the dead sea area abstract changes in the concentrations of major some minor and trace elements occurring in both surface and groundwater of the lower jordan river dead sea drainage basin have been investigated in order to identify the characteristics of the regional aquifers and their recharge areas spider patterns of elements and rare earth distribution patterns pinpoint the characteristic chemical features of groundwater as compared to seawater the high br cl ratios in groundwater are caused either by high br cl ratios in precipitation by leaching of br from bituminous matter or by mixing with brines beyond the epsomite stage the locations of groundwater samples with enhanced b contents coincide with the distribution of gypsum in the beds of the lisan formation which produces water with nearly constant b so 4 2 molar ratios aeolian distribution of the unconsolidated lisan sediments influences the b so 4 ratio in the area of lake tiberias and in the jordan highlands the high gd anomaly in the dead sea water is of geologic origin whereas that in the jordan river and in nahal stream qidron is largely anthropogenic the anthropogenic gd input to the dead sea is insignificant compared to the actual amount in the water of the dead sea hot saline water encountered along the western shores of the dead sea with high gd anomalies indicate that they contain large amounts of ancient dead sea water that mix as hot ascending brines with fresh water the recharge area of groundwater in the lower jordan valley extends largely over limestone and dolomite outcrops of the upper cretaceous judea group weathering of locally underlying lower cretaceous volcanics in the area of pezael beqaot and argaman yield groundwater with 34 s so 4 2 values ranging from 2 to 4 the presence of sulphide bearing bodies in this area is attested to magnetic anomalies detected at depths of several kilometres 34 s so 4 2 indicates very deep groundwater circulation high 34 s so 4 2 15 is typical for marine sulphates from the judea and avedat group limestone the springs located along the northwestern shore of the dead sea discharge water replenished on the eastern judea mountains the increase in the salinity of this water is due to brines flushed from sediments and from adjacent sedimentary rocks which host entrapped brines from the precursors of the dead sea formed during the late stages of lake lisan fresher water flushes out these residual brines as a result of falling sea level salinity increase in groundwater is also affected by the ascent of deep seated hot brines from pre sedom periods along the rift faults calculations of mixing between fresh and highly saline end members show that leaching of anhydrite from sediments precipitation of calcite formation of dolomite albitization of plagioclase and ion exchange with abundant clay minerals control the major element composition of the saline groundwater

cone sheet formation and intrusive growth of an oceanic island the miocene tejeda complex on gran canaria canary islands more than 500 trachytic to phonolitic cone sheet dikes hypabyssal syenite stocks and subordinate radial dikes form a 20 km diameter intrusive complex in the volcaniclastic fill of the miocene tejeda caldera 20 35 km on gran canaria canary islands the dikes intruded concentrically around a central axis of radial symmetry and dip uniformly an average of 41 toward the center single crystal 40ar 39ar ages of dikes and syenite stocks ranging from 12 3 0 1 to 7 32 0 05 ma suggest more or less continuous intrusive activity during both volcanically active and inactive phases intrusions were emplaced at average intervals of 5 10 k y while explosive eruptions took place at 50 k y intervals the estimated aggregate volume of evolved cone sheet magma added at shallow level 2000 m below sea level amounts to 250 km3 compared to 500 km3 of evolved extrusive volcanics erupted during the same period formation of the tejeda cone sheets most likely resulted from deformation processes due to resurgent doming initiated by the recurrent replenishment of a flat laccolith like shallow magma chamber magma supply exceeding the volume that could be compensated for by updoming of the overlying caldera fill resulted in the formation of cone shaped fractures

chemical composition of rainfall and groundwater in recharge areas of the bet shean harod multiple aquifer system israel the bet shean and harod valleys are regional recipients and mixing zones for groundwaters draining from a regional multiple aquifer system comprising two different carbonate and two basalt aquifers as well as deep seated reservoirs of confined brines this paper the first of two describes the hydrochemistry of the groundwater bodies related to this multiple aquifer system the paper deals with the chemical composition of rain falling on natural recharge areas the chemical contribution of the aquifer rocks and the chemical evolution of groundwaters in the upper flow courses of all aquifers of this regional system the methodology applied is based on the determination of major dissolved ions and on the examination of their ratios and changes along the upper flow paths rain water falling on natural recharge areas has an average chlorinity of 12mgl 1cl and a calcium bicarbonate composition caused by dust borne terrigenous material this rain water salinity decreases with distance from the sea shore fresh groundwaters flowing through the different aquifers may be identified and differentiated by their characteristic salinity levels and ionic ratios groundwaters flowing through carbonate aquifers are identified by different salinity levels and by distinct rmg rca ratios reflecting dolomite calcite ratios in aquiferr rocks the groundwaters of the two basalt aquifers have typical cation assemblages and high na and mg2 concentrations far in excess of those typical of the recharging rainfall groundwater in the upper flow courses of all aquifers investigated is deficient in so2 4 and k relative to recharging rain water

age and petrology of alkalic postshield and rejuvenated stage lava from kauai hawaii at the top of the waimea canyon basalt on the island of kauai rare flows of alkalic postshield stage hawaiite and mugearite overlie tholeiitic flows of the shield stage these postshield stage flows are 3 92 ma and provide a younger limit for the age of the tholeiitic shield stage the younger koloa volcanics consist of widespread alkalic rejuvenated stage flows and vents of alkalic basalt basanite nephelinite and nepheline melilitite that erupted between 3 65 and 0 52 ma all the flows older than 1 7 ma occur in the west northwestern half of the island and all the flows younger than 1 5 ma occur in the east southeastern half the lithologies have no spatial or chronological pattern the flows of the koloa volcanics are near primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet bearing source that has about two thirds the concentration of p2o5 rare earth elements and sr of the source of the honolulu volcanics on the island of oahu the same lithology in the koloa and honolulu volcanics is generated by similar degrees of partial melting of distinct source compositions the lavas of the koloa volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt respectively phases that remain in the residue of the honolulu volcanics such as rutile and phlogopite are exhausted during formation of the koloa volcanics at all but the smallest degrees of partial melting the mantle source for kauai lava becomes systematically more depleted in 87sr 86sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage at the same time the lavas become systematically more enriched in incompatible trace elements on a shorter timescale the lavas of the koloa volcanics display the same compositional trends but at a lower rate of change the source characteristics of the koloa volcanics considered along with those of the honolulu volcanics support a mixing model in which the source of rejuvenated stage lava represents large percent melts of a plume source mixed with small amounts of small percent melts of a heterogeneous mid ocean ridge source

paleomagnetic test of the emperor fracture zone hypothesis late cretaceous paleomagnetic data from the pacific plate were used to test farrar and dixon s 1981 hypothesis that 1700 km of strike slip accumulated during the early tertiary along the emperor fracture zone system an 8000 km long nw se trending feature which consists mainly of the emperor trough the gardner seamounts and the line islands the data strongly disagree with this hypothesis instead the data are consistent with no motion having occurred between seafloor east of the emperor fracture zone and seafloor west of the emperor fracture zone

deposits related to degradation processes on piton des neiges volcano reunion island overview and geological hazard abstract piton des neiges pn volcano on reunion island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material including extensive debris avalanche deposits pn litho structural units first studied by upton and wadsworth 1965 philos trans r soc lond a 271 pp 105 130 are re examined this review highlights the importance of long volcanic repose periods and erosion processes during pn history volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of pn degradation processes the deposits of these processes have been classified into 1 talus 2 mudflow and debris flow and 3 debris avalanche lithology frequency and estimated volumes of each deposit type imply that the structural evolution of pn can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes the occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of pn which implies a very low risk at present on the contrary mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets moreover if smaller debris avalanches can occur in the cirques of pn another major risk must be evaluated

petrophysical constraints on gravity magnetic interpretation in israel whether exploring for hydrocarbons and minerals or simply studying structural geology gravity and magnetic interpreters prefer to use density and magnetic susceptibility values that are determined for their area of investigation in exploration it is fairly common to have rock properties information from outcrops but it is rare to have real measured values for the subsurface the interpreter extrapolates from the surface bases assumptions on lithologies or uses approximate values from the literature this approach although unavoidable in many instances may lead to errors

evolution of ocean island rifts the northeast rift zone of tenerife canary islands the northeast rift zone of tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone field geology isotopic dating and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the ne rift zone which includes a period of very fast growth toward instability between ca 1 1 and 0 83 ma followed by three successive large landslides the micheque and guimar collapses which occurred approximately contemporaneously at ca 830 ka and on either side of the rift and the la orotava landslide between 690 10 and 566 13 ka our observations suggest that canarian rift zones show similar patterns of development which often includes overgrowth instability and lateral collapses collapses of the rift flanks disrupt established fissural feeding systems favoring magma ascent and shallow emplacement which in turn leads to magma differentiation and intermediate to felsic nested eruptions rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes conversely the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars even if concealed by postcollapse volcanism comparative analysis of the main rifts in the canary islands outlines this general evolutionary pattern 1 growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions 2 flank collapse and catastrophic disruption of the established feeder system of the rift 3 postcollapse centralized nested volcanism commonly evolving from initially ultramafic mafic to terminal felsic compositions trachytes phonolites and 4 progressive decline of nested eruptive activity

a neogene geomagnetic polarity transition record from lavas of the canary islands spain episodic volcanism and or metastable transitional fields summary we present results from a study of a partial magnetic polarity transition record from a 9 7 ma 40ar 39ar dated volcanic section on the island of gomera canary islands spain the record provides results that bear on the debate over whether long term persistent features of field behaviour exist during transitions the sampled section containing more than 20 flows begins after the onset of the transition the nine lowermost flows in the section which are all transitional yield virtually identical directions and similar low paleointensities following the recovery of virtual geomagnetic pole positions vgps to near the spin axis paleointensities progress to very high values transitional directions lay due west reflecting the presence of a non zonal intermediate field vgps for these flows cluster in the west atlantic which according to hoffman would correspond to a zone of fastest p wave propagation in the lower mantle and to a radial flux centre of the present day non dipole field hoffman had noted a similar occurrence at two other locations he found that in a select set of volcanic records vgps tended to group over two areas australia and south america which also corresponded to fast p wave and non dipole field anomalies given the discontinuous and irregular character typical of volcanic sequences it is natural to question whether these clusters are merely artefacts of episodic volcanism or actually reflect standstills of the reversing field we address this question for our record by comparing chemical and magnetomineralogic characteristics of the flows we find that all of the transitional flows have very similar chemical and rock magnetic properties furthermore these flows differ significantly from the overlying reverse flows which give distinctly different values and much greater flow to flow variation from this we conclude that the packet of transitional flows was derived from the same source and had erupted over a relatively short period of time as reflected by a magma source that presumably did not have time to evolve significantly while the vgp cluster in the gomera record is consistent with long term mantle control on transitional fields we cannot conclude that the cluster represents a standstill of the field these results highlight the difficulties of interpreting temporal changes in field behaviour from volcanic sequences in addition they provide a clear incentive for investigating physical characteristics of lavas to assess relative eruptive rates

volcanic and morphological evolution of la gomera canary islands based on new k ar ages and magnetic stratigraphy implications for oceanic island evolution the spatial and chronological evolution of canary islands volcanism is due to eastward progression of the slow moving african plate over a mantle plume la gomera is located between the western shield stage islands 12 ma with 23 k ar ages and palaeomagnetic data we can constrain the timing of the main subaerial volcanic phases of la gomera the shield volcano c 9 4 8 0 ma the postshield peripheral lava flows c 7 4 5 0 ma the postshield horizontal lava flows c 5 4 4 3 ma and late intra canyon lava flows c 1 9 ma the horizontal lava flows fill a 10 km wide depression formed by a giant landslide garajonay the entire structure of la gomera is in relief inversion declining volcanic construction rates from the shield to the postshield stages are correlated with increasing erosion rates whereas the period of volcanic quiescence hiatus has low erosion rates the succession and duration of the hotspot volcanic stages in the canary islands are compared with those in the hawaiian islands thus underlining differences between slow moving and fast moving plate settings

ca chloride brines at common outlets of the bet shean harod multiple aquifer system israel abstract the bet shean and harod valleys are regional recipients and mixing zones for groundwaters draining into them from a multiple aquifer system which includes carbonate and basalt aquifers and deep seated reservoirs of brine the outlets of the three aquifers are located at the mountain border separating the valleys from the gilboa and ramot issakhar mts the aquifers drain through two types of outlets unique and common the latter type is mainly conditioned by the occurrence of fault blocks acting as connecting media between the aquifers upon reaching the interconnecting zones fresh groundwater flowing from carbonate and basalt aquifers intermix losing their hydrochemical identities ca chloride brines originating from deep seated and confined reservoirs leak along the deep seated faults characterizing the common outlet zones and contaminate the relatively fresh mixing products originating from the three regional aquifers two types of dilute ca chloride brines were identified in the study area the devora type is a hot and hypersaline source brine which was identified at great depth and under very high hydrostatic pressures this brine could be the residual product of seawater that evaporated during the cambrian cretaceous continental interval and subsequent prolonged interaction with surrounding carbonate rocks the neve ur type brine seems to be genetically related to the neogene inland sdom sea that preceeded the contemporary dead sea

alluvial fans on volcanic islands a morphometric perspective sao vicente cape verde abstract the distribution morphology and development of volcanic island alluvial fans are investigated using sao vicente from the cape verde volcanic chain east central atlantic ocean this is a tectonically and volcanically inactive dryland setting comprising an eroded plio pleistocene composite volcano overlying a submarine shield volcano we mapped 228 alluvial fans quantifying fan f and catchment c morphological properties area a gradient g relief r length l fans and catchments of varying size and gradient are distributed around the island coastal margins 6 and along interior valley sides 94 regression results i e ve ve slopes of best fit lines conformed to broader fan research but with weak correlations and coefficient exponent values outside or at the lower end of values reported from non volcanic settings analysis of fan area and gradient vs catchment area regression residuals revealed fans were dominated by small steep gradient 28 and large low gradient forms 39 fan building space is a fundamental control specifically linked to volcano structure but with interplay between rock strength base level and climate fans around the volcanic edifice flanks are uncommon due to steep flank slopes where flank margin fans occur they are typically small and steep forms due to marine erosion fans along flank incised valleys are common large low gradient fans are associated with a major volcanic rift aligned valley structure with extensive erosion into altered and weathered edifice core lithologies small steep gradient fans are associated with edifice flank valleys steeper and narrower valley forms reflect incision into stronger lithologies of the composite volcano flanks restricting fan building space fans located within the volcano central depression are amongst the largest and lowest gradient types building into the eroded and unconfined edifice core space coalescence apex toe incision toe erosion and catchment stream channel backfilling displayed by most fans suggest landscape erosion dominates during more arid periods e g interglacials with the dryland setting generating limited sediment and insufficient flood runoff for transport and fan deposition fan building likely occurs during wetter periods linked to quaternary climate variability quaternary eustatic variations are relevant for the volcano edifice margins where sea level changes control fan building space and erosion

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

long term 17 ma turbidite record of the timing and frequency of large flank collapses of the canary islands volcaniclastic turbidites on the madeira abyssal plain provide a record of large volume volcanic island flank collapses from the canary islands this long term record spans 17 ma and comprises 125 volcaniclastic beds determining the timing provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the canary islands especially the western islands of tenerife la palma and el hierro these turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers furthermore the last large volume failures from tenerife coincide with explosive volcanism at the end of eruptive cycles many large volume canary island landslides also occurred during periods of warmer and wetter climates associated with sea level rise and subsequent highstand however these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant

dating of the upper pleistocene holocene volcanic activity of la palma using the unspiked k ar technique abstract combined unspiked k ar dating and detailed mapping have established the eruptive chronology of the cumbre vieja volcano in the canary islands the cv volcano is a large polygenetic volcano subaerial area 220 km2 subaerial volume 125 km3 elevation above sea level 2000 m which rests on the southern flank of the older taburiente cumbre nueva volcano we defined two stratigraphic units by their relationship to the development of coastal cliffs developed around the cumbre vieja volcano a cliff forming series and a scree forming series which is draped over the cliffs and forms the coastal platforms at their bases k ar ages were measured from 20 samples from the two series there are no discrepancies between k ar ages and the stratigraphy the k ar ages of one flow compare well with 14 c determinations 14 c 3 2 0 1 ka k ar 4 2 ka the ages of the scree forming series range from 3 2 ka to 21 2 ka the cliff forming series ages range from 20 2 ka to 123 3 ka but as the base of the sequence is not exposed the growth of the cumbre vieja volcano must have begun substantially earlier the boundary between the two coincides with the low sea level stand in the last glacial maximum which we infer to have been a period of intense coastal erosion dated samples from the upper part of the taburiente cumbre nueva volcano range in age from 853 10 ka to 566 8 ka the age of the youngest taburiente cumbre nueva volcano sample provides an upper bound on the age of the subsequent cumbre nueva giant lateral collapse

the growth collapse and quiescence of teno volcano tenerife new constraints from paleomagnetic data tenerife basically consists of three miocene shield volcanoes the anaga the teno and central shield as well as the pliocene canadas volcano the temporal evolution and structural significance of each volcano with respect to the history of tenerife is still a matter of debate we present paleomagnetic results in order to enhance the view of the volcanic history of the teno volcano by means of magnetostratigraphy it is found that the initial subaerial phase shows reverse magnetizations throughout after two major sector collapses dominantly normally magnetized lavas extruded comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0 4 myr between 6 3 and 5 9 ma significantly younger flows 5 3 myr old according to their radiometric age revealed again normal polarity throughout the absence of inversely magnetized lavas in between the two normal periods indicates a volcanic hiatus or erosional phase the evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano the average geomagnetic field for 6 0 0 2 ma is close to an axial dipole field showing a slight far sided right handed effect the field strength determined by thellier type intensity determinations corresponds to a virtual axial dipole moment of 4 9 1022 a m2 this value is approximately half of the present day field strength but similar to values obtained for the mid miocene it also corresponds to the proposed tertiary low field level of the geomagnetic dipole moment

a new chronostratigraphical and evolutionary model for la gomera implications for the overall evolution of the canarian archipelago a review of the general volcano stratigraphy and geochronology of la gomera one of the lesser known canary islands has led to the establishment of a new evolutionary model the oldest edifice corresponds to the submarine stage built up between 20 and 15 ma the construction of the submarine edifice was followed by an important break in the activity about 4 ma and deep erosion of the edifice about 10 5 ma ago the main present day edifice the old edifice 10 5 6 4 ma emerged which was also submarine in its initial phases two different main stages are distinguishable the first stage was represented by a large some 22 km wide basaltic shield volcano the lower old edifice several lateral collapse events tazo and san marcos avalanches occurred during this time and were responsible for the removal of an important part of its northern flank in the second growth stage the upper old edifice the activity migrated southwards a 25 km wide composite volcano arose covering part of the remaining earlier shield volcano the felsic trachytic to phonolitic activity occurring in two separate episodes formed a significant component of this composite volcano finally one more recent large edifice the young edifice built up from 5 7 to 4 ma the lava flows of this younger edifice covered completely the centre and the south of the island and filled deep ravines in the north more evolved magmas including significant felsic magmas the third and last felsic episode occurred in this phase of activity the growth of la gomera was long lasting separated by an important gap in the activity in the middle miocene with no quaternary activity at all at the same time on tenerife the nearest island east of la gomera three large edifices grew separately roque del conde anaga and teno initially three separated islands from the available data it is inferred that the subaerial activity started earlier in the roque del conde edifice then on la gomera and later in teno in the nw and anaga in ne of tenerife which is the youngest of all these edifices these facts together with the irregular general progress of the volcanic activity support more complex views of the genesis for the canary islands than the simple hotspot model

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

eruptive and structural history of teide volcano and rift zones of tenerife canary islands the teide and pico viejo stratocones and the northwest and northeast rifts are products of the latest eruptive phase of the island of tenerife initiated with the lateral collapse of its northern flank that formed the las canadas caldera and the icod la guancha valley ca 200 ka the eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events a set of 54 new 14 c and k ar ages provides precise age control of the recent eruptive history of tenerife particularly teide volcano the third highest volcanic feature on earth 3718 m above sea level 7 km high and unique in terms of its intraplate setting the development of the teide pico viejo volcanoes may be related to the activity of the northwest and northeast rifts volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth nested within the collapse embayment of an increasingly higher central volcano with progressively differentiated magmas the coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area where the differentiated stratocones developed current volcanic hazard in tenerife is considered to be moderate because eruptive frequency is low explosivity is modest and the eruptive activity of the teide stratocone seems to have declined over the past 30 k y with only one eruption in this period 1150 yr b p

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

hotspot volcanism close to a passive continental margin the canary islands the canarian archipelago is a group of volcanic islands on a slow moving oceanic plate close to a continental margin the origins of the archipelago are controversial a hotspot or mantle plume a zone of lithospheric deformation a region of compressional block faulting or a rupture propagating westwards from the active atlas mountains fold belt have been proposed by different authors however comparison of the canarian archipelago with the prototypical hotspot related island group the hawaiian archipelago reveals that the differences between the two are not as great as had previously been supposed on the basis of older data quaternary igneous activity in the canaries is concentrated at the western end of the archipelago close to the present day location of the inferred hotspot this is the same relationship as seen in the hawaiian and cape verde islands the latter archipelago associated with a well defined but slow moving mantle plume shows anomalies in a plot of island age against distance which are comparable to those seen in the canary islands these anomalies cannot therefore be used to argue against a hotspot origin for the canaries individual islands in both archipelagoes are characterized by initial rapid growth the shield building stages of activity followed by a period of quiescence and deep erosion erosion gap which in turn is followed by a post erosional stage of activity the absence of post shield stage subsidence in the canaries is in marked contrast with the major subsidence experienced by the hawaiian islands but is comparable with the lack of subsidence evident in other island groups at slow moving hotspots such as the cape verdes comparison of the structure and structural evolution of the canary islands with other oceanic islands such as hawaii and reunion reveals many similarities these include the development of triple mercedes star rift zones and the occurrence of giant lateral collapses on the flanks of these rift zones the apparent absence of these features in the post erosional islands may in part be a result of their greater age and deeper erosion which has removed much of the evidence for their early volcanic architecture we conclude that the many similarities between the canary islands and island groups whose hotspot origins are undisputed show that the canaries have been produced in the same way

volcanic complexes in the eastern ridge of the canary islands the miocene activity of the island of fuerteventura abstract fuerteventura has been since early stages of its growth the result of three different adjacent large volcanic complexes southern central and northern the definition of these volcanic complexes and their respective growing episodes is based on volcano stratigraphic morphological and structural criteria particularly radial dyke swarms each complex has its own prolonged history that might be longer than 10 m y during that time several periods of activity alternating with gaps accompanied by important erosion took place the evolution of each volcanic complex has been partially independent but all the three are affected by at least three miocene tectonic phases that controlled considerably their activity the volcanic complexes are deeply eroded and partially submerged in the core of the northern and the central volcanic complexes there is a set of submarine and plutonic rocks intensely traversed by a dyke swarm known as the basal complex the basal complex has been interpreted in different ways but all previous authors have considered it to be prior to the subaerial shield stage of the island here we advance the idea that the basal complex represent the submarine growing stage of the volcanic complexes and the hypabyssal roots plutons and dykes of their successive subaerial growing episodes two seamounts situated nearby southwest of the island might be interpreted as remains of two other major volcanoes these two volcanoes together with those forming the present emerged island of fuerteventura and finally those of famara and los ajaches situated further north on lanzarote constitute a chain of volcanoes located along a lineation which is subparallel to the northwestern african coastline and which may relate to early atlantic spreading trends in the area

hydrological and hydrogeochemical methods for the delineation of complex groundwater flow systems as evidenced in the bet shean valley israel abstract the bet shean valley northern israel is the base level of groundwater draining from three regional aquifers in the natural undisturbed state i e at the beginning of the six ties groundwater drained through many springs with widely different chemical composition and with a total discharge of about 135 million m 3 for the efficient utilization of water resources it was decided to recover groundwater from wells at the cost of a considerable reduction of spring discharges since the sequence and the rate of spring decay diverged from the pattern that had been predicted it became necessary to delineate aquifers and their connections to the various springs three groups of methods were employed for this purpose a analysis of discharge characteristics and their relation to source aquifers b methods for the identification of well fields that affect the discharge of a particular spring and c hydrogeochemical methods for the delineation of flow paths the paper presents the relevant data the methods employed and the results that were achieved the utility and the limitations of the various methods were discussed and general guidelines for the delineation of groundwater flow in multi aquifer systems were formulated

the fault pattern in the northern negev and southern coastal plain of israel and its hydrogeological implications for groundwater flow in the judea group aquifer abstract on the basis of a broadly expanding data base the hydrogeological properties of the judea group sequence in the northern negev and southern coastal plain of israel have been reassessed the updated subsurface model is based on data derived from water and oil wells and on recent large scale geophysical investigations a new regional pattern of the reassessed geological through the subsurface of the study area has been revealed in view of the reassessed geological and hydrological subsurface setting it appears that the judea group aquifer should not be regarded as one continuous and undisturbed hydrological unit owing to the occurrence of regional faults its subaquifers are locally interconnected these subaquifers which contain mainly high quality water are juxtaposed as a result of faulting against kurnub group sandstones containing brackish paleowater the latter group is faulted against late jurassic formations containing highly saline groundwater in the beer sheva area the judea group aquifer is vertically displaced against the senonian and eocene mt scopus and avdat groups which also contain brackish and saline water in the southern coastal plain major faults locally dissect also the pleistocene kurkar group facilitating inflow of mg rich groundwater deriving from judea group dolomites the new geological evidence and its hydrogeological implications provide new solutions for previously unexplained salinization phenomena

slope failures on the flanks of the western canary islands abstract landslides have been a key process in the evolution of the western canary islands the younger and more volcanically active canary islands el hierro la palma and tenerife show the clearest evidence of recent landslide activity the evidence includes landslide scars on the island flanks debris deposits on the lower island slopes and volcaniclastic turbidites on the floor of the adjacent ocean basins at least 14 large landslides have occurred on the flanks of the el hierro la palma and tenerife the majority of these in the last 1 million years with the youngest on the northwest flank of el hierro as recent as 15 thousand years in age older landslides undoubtedly occurred but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments landslides on the canary island flanks can be categorised as debris avalanches slumps or debris flows debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence up to a maximum thickness of 1 to 2 km they are the commonest type of landslide mapped in contrast slumps move short distances and are deep rooted landslides which may affect the entire thickness of the volcanic edifice debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks some landslides are complex events involving more than one of the above end member processes individual debris avalanches have volumes in the range of 50 500 km3 cover several thousand km2 of seafloor and have runout distances of up to 130 km from source overall debris avalanche deposits account for about 10 of the total volcanic edifices of the small relatively young islands of el hierro and la palma some parameters such as deposit volumes and landslide ages are difficult to quantify the key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks broadening into a depositional lobe below 3000 m debris avalanche deposits have a typically blocky morphology with individual blocks up to a kilometre or more in diameter however considerable variation exists between different avalanche deposits at one extreme the el golfo debris avalanche on el hierro has few large blocks scattered randomly across the avalanche surface at the other icod on the north flank of tenerife has much more numerous but smaller blocks over most of its surface with a few very large blocks confined to the margins of the deposit icod also exhibits flow structures longitudinal shears and pressure ridges that are absent in el golfo the primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes observations in experimental debris flows show that the differences between the el golfo and icod landslide deposits are probably controlled by the greater proportion of fine grained material in the icod landslide this in turn relates to the nature of the failed volcanic rocks which are almost entirely basalt on el hierro but include a much greater proportion of pyroclastic deposits on tenerife landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands with landslides propagating perpendicular to the rift orientation however this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a weakness that can be carried forward during island development this may occur because certain island flanks are steeper extend to greater water depths or are less buttressed by the surrounding topography and because volcanic production following a landslide my be concentrated in the landslide scar thus focussing subsequent landslide potential in this area landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight although the actual triggering factors are poorly understood they may include or be influenced by dyke intrusion pore pressure changes related to intrusion seismicity or sealevel climate changes a possible relationship between caldera collapse and landsliding on tenerife is not in our interpretation supported by the available evidence

a pulse in the planet regional control of high frequency changes in relative sea level by mantle convection abstract an explanation for high frequency cycles of sea level in non glacial times has remained elusive despite more than two centuries of research since lavoisier9s seminal observations were published in 1789 in the development of seismic stratigraphy in the 1970s putatively global high frequency changes in relative sea level vail third order cycles were attributed to an unknown eustatic mechanism prompting a search for mesozoic ice ages over the last decade a regional mechanism of sea level control has been developed from studies of the sedimentary record in high quality oil industry data these geological studies have supported the geophysical prediction that significant regional control of sea level is exercised by mantle induced vertical motions of the earth9s surface these vertical motions can occur over time intervals from several tens of million years to less than a million years with amplitudes of tens of metres or more even at the shorter intervals the vertical motions are not confined to regions with major hotspots there are two related controls of surface vertical motion evolution of mantle convection cells and pulsing flow within each cell the effects are evident in the sedimentary record of north atlantic basins mantle convection provides an alternative regional mechanism to eustatic control for explaining medium frequency to high frequency sea level cycles

the evolution of marine evaporitic brines in inland basins the jordan dead sea rift valley marine evaporitic brines frequently display na cl and br concentrations that significantly deviate from seawater evaporation paths yielding markedly conflicting degrees of evaporation calculated for a specific brine here we present 493 new and 33 previously reported analyses of ca chloridic waters of neogene age from the dead sea rift dsr valley to explain such offsets the dsr brines plot along an almost perfect mixing line r2 0 990 on a br cl na cl diagram extending between two end members a and b points a and b are located at na cl 0 804 and br cl 0 00193 and at na cl 0 00773 and br cl 0 0155 respectively within the halite and bischofite stability fields brines a and b originated in a dual mode evaporation basin brine a formed under the classic lagoon scenario mode a with seawater inflow and brine outflow at steady state occasional drops in water level imposed by climatic or tectonic causes resulted in outflow cutoff and in rapid concentration buildup the second mode b initiated upon equilibration of the activity of water in the brine with the overlying relative humidity resulting in composition and salinity approaching that of brine b sustaining it until the next reversal to mode a thick evaporite deposits inhibited infiltration of brines a and b into the subsurface terrain a process that was enabled only when the brine reached the permeable carbonate rock rim and border faults of the basin hence brines that formed during the relatively short shifts from mode a to mode b could not penetrate into the deep subsurface and bittern minerals that were formed during the frequent mode shifts were dissolved and flushed out into the sea upon the next resumption of outflow the proposed model accounts for the deviations of brines from the marine evaporitic evolution curve by brine mixing rather than due to a change in ocean chemistry it also explains the absence of bittern minerals in the thick halite and gypsum anhydrite succession and the compositional gap between the widely different end member hypersaline fluids this model applies directly to the studied dsr brines and evaporites but it may be relevant to other inland evaporitic basins

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

test for bias in paleomagnetically determined paleolatitudes from pacific plate deep sea drilling project sediments paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the pacific plate have been previously used in determining reference poles for the pacific plate apparent polar wander apw path from discordant paleoinclinations and corresponding paleolatitudes determined from these data some workers have inferred tectonic tilting or large horizontal motion of some deep sea drilling sites relative to other parts of the pacific plate here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a pacific plate reference apw path determined from seamount poles skewness and amplitudes of magnetic profiles across magnetically lineated seafloor and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies the reference apw path includes a new determination of the early cretaceous pole from several types of paleomagnetic data which agree well with one another sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference apw path a discrepancy that corresponds to inclinations shallower than expected thus most deep sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference apw paths inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinations

k ar chronology of the volcanic eruptions in the canarian archipelago island of la gomera a geochronological study of the island of la gomera canaries has been carried out by the k ar method the 26 new ages obtained together with the 17 previous determinations show that above the main unconformity of the island separating the basal complex from the later volcanic series there is a unit of lower old basalts more than 10 m y old polymictic volcanic breccias were emplaced between 10 and 9 m y ago the upper old basalts above them were formed between 9 and 6 m y ago with a peak of activity around 7 m y after a period of erosion 6 5 m y a thick series of young basalts associated with trachytic and phonolitic domes and flows were rapidly emplaced between 4 5 4 m y ago finally local basaltic activity took place 2 8 m y ago the age of the basal complex is not well known although three ages 14 6 15 5 and 19 3 m y have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex

rare earths and yttrium hydrostratigraphy along the lake kinneret dead sea arava transform fault israel and adjoining territories rare earth elements and y rey have been analysed in 49 groundwaters from localities in the areas of lake kinneret and the jordan and arava valleys these waters originate from various aquifers and the rey abundances are expected to be controlled by the aquifer rocks the rey pattern allow one to distinguish interaction of waters with basalts basalt limestone interaction zones silicified limestones limestones from the judea and avdat groups upper cretaceous and eocene respectively and sandstones of the lower cretaceous kurnub group groundwater from the quaternary alluvial fill dead sea group are either controlled by judea group limestone or kurnub group sandstone the rey patterns show characteristic features for each group in hydrogeological systems the rocks of natural replenishment areas are usually not the same as rock units from which the waters are collected this becomes evident by comparing the lithostratigraphic groups from which the waters were collected and the hydrochemical grouping according to rey patterns with their characteristic trends and anomalies in many cases there is a correspondence between the lithostratigraphic and the hydrochemical grouping in other cases the 2 groupings disagree this disagreement proves inter aquifer flow of groundwater in some cases the geologically derived aquifers rocks of origin differ from those indicated by rey patterns thus applying the rey grouping new fundamental information for hydrological models can be given and sources of salinisation can be elucidated

gis methods applied to the degradation of monogenetic volcanic fields a case study of the holocene volcanism of gran canaria canary islands spain abstract modeling of volcanic morphometry provides reliable measurements of parameters that assist in the determination of volcanic landform degradation variations of the original morphology enable the understanding of patterns affecting erosion and their development facilitating the assessment of associated hazards a total of 24 volcanic holocene eruptions were identified in the island of gran canaria canary islands spain 87 of these eruptions occurred in a wet environment while the rest happened in a dry environment 45 of holocene eruptions are located along short barrancos s type less than 10 km in length 20 along large barrancos l type 10 17 km in length and 35 along extra large barrancos xl type more than 17 km in length the erosional history of holocene volcanic edifices is in the first stage of degradation with a geomorphic signature characterized by a fresh young cone with a sharp profile and a pristine lava flow after intensive field work a careful palaeo geomorphological reconstruction of the 24 holocene eruptions of gran canaria was conducted in order to obtain the digital terrain models dtms of the pre and post eruption terrains from the difference between these dtms the degradation volume and the incision rate were obtained the denudation of volcanic cones and lava flows is relatively independent both their geographical location and the climatic environment however local factors such as pre eruption topography and ravine type have the greatest influence on the erosion of holocene volcanic materials in gran canaria although age is a key factor to help understand the morphological evolution of monogenetic volcanic fields the gran canaria holocene volcanism presented in this paper demonstrates that local and regional factors may determine the lack of correlation between morphometric parameters and age consequently the degree of transformation of the volcanic edifices evolves in many cases independently of their age

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

geochronology and stratigraphy of the roque nublo cycle gran canaria canary islands the roque nublo cycle of pliocene age is the second main cycle of subaerial volcanic activity in gran canaria during this cycle a stratovolcano at least 2500 m high developed in the central part of the island its volcanic products covered an area of about 540 km 2 the detailed stratigraphy of the roque nublo cycle together with a magnetostratigraphic study and six new k ar dates which complement 22 previously published ones have led to the reconstruction of the volcanic history of this cycle in which six main stages have been distinguished the revised chronology of the roque nublo cycle implies an overlap in age with the earliest rocks of the third cycle of subaerial activity in gran canaria no volcanic hiatus occurred between the second and third cycles as was previously thought to be the case

dike swarms key to the reconstruction of major volcanic edifices the basic dikes of la gomera canary islands the detailed study of the age and distribution of the basic dikes of la gomera canary islands has allowed us to reconstruct the evolution of its main volcanic edifice the dikes of the oldest unit the basal complex have a n70 80 dominant direction at regional scale on the other hand the dikes of the post basal complex subaerial units are the result of local stress fields the oldest swarm is composed of sills dated at 10 2 9 3 ma four younger radial dike swarms have been identified s1 s2 s3 and s4 with ages of 9 1 8 4 ma 8 2 6 7 ma 5 5 4 4 ma and 5 3 4 0 ma respectively the reconstruction of the magmatic focus location using these swarms shows a migration southwards with an average speed of 1 6 mm year this temporal sequence of parallel swarms sills and radial swarms is a pattern frequently repeated during the building of large insular volcanic edifices in the canary islands as well as in other archipelagos

tracing the source of gypsum in new caledonian soils by ree contents and ssr isotopic compositions gypsum minerals forming in pedogenic environments in new caledonia were examined for their s and sr isotope compositions and ree distributions three possible sources were identified volcanic soil derived and marine however differentiation based only on the ree contents and normalized patterns was not possible but the best preserved volcanic sample yields a negative eueu anomaly and the lowest gdnybn ratio due to the high content of organic matter mixing calculations of the 34s values and 87sr86sr ratios confirmed that the marine gypsum has a 34s value close to 21 and an 87sr86sr ratio close to 0 70912 and that the volcanic gypsum has a 34s value of 0 and an 87sr86sr ratio of about 0 70331 the soil derived gypsum samples yield intermediate values as a result of a mixing between the marine and volcanic end members negative 34s values for the volcanic gypsum are explained by low fo2 of the source rocks while high 87sr86sr ratios for the same gypsum are explained by probable atmospheric input the data confirm also that biological fractionation of 34s if any is limited

the felsic complex of the vallehermoso caldera interior of an ancient volcanic system la gomera canary islands abstract a mid to upper miocene felsic cone sheet dike complex caldera collapse and dome intrusion located around the vallehermoso and tamargada areas north of la gomera island is described it constitutes one of the less known volcanic episodes in the canary archipelago and especially in la gomera island based on detailed field and structural evidence it is suggested that the cone sheet shows at the surface an almost circular geometry truncated to the nw sector after the cone sheet dike complex formed decompression of the interior of an assumed shallow magma chamber occurred due to the subsequent migration of felsic magma throughout the dikes of the cone sheet this last event caused the roof of the magma chamber to collapse leading to the formation of a 3 to 4 km wide caldera a breccia deposit formed during the collapse and filled a depressed central area which finally was affected by an episode of magmatic resurgence leading to a later almost circular dome intrusion the association of cone sheet breccia deposits and domes constitutes the so called trachytic phonolitic complex tpc and should be viewed as the root of an ancient intensely eroded volcanic edifice made of trachytes nepheline phonolites and minor hauyne phonolites as the dominant materials a description of the structural petrological and geochemical features of the trachytic phonolitic complex and a discussion about their origin are considered in this paper we also consider the correlation between another felsic intrusion of alkali gabbros and syenites called tamargada syenites that also crops out in the vallehermoso tamargada area with respect to the trachytic phonolitic complex our data suggest that this alkaline intrusion of alkali gabbros and syenites belongs to an earlier intrusive episode on the basis of petrological geochemical and detailed field studies both trachytes and nepheline phonolites included in the trachytic phonolitic complex most likely evolved from alkali mafic magmas from the upper old basalt formation through a dominant fractional crystallization process additionally hauyne phonolites could develop from local and later magmatic processes such as gas transfer that altered and changed a small part of the previously mentioned nepheline phonolites to hauyne phonolites

rejuvenated stage volcanism after 0 6 m y quiescence at west maui volcano hawaii new evidence from k ar ages and chemistry of lahaina volcanics abstract west maui s rejuvenated stage lahaina volcanics were erupted from four discrete sites new k ar ages indicate two pulses of volcanism the older about 0 6 ma and the younger about 0 4 ma compositionally the lava flows are entirely basanitic but each pulse is diverse the underlying postshield stage honolua volcanics were emplaced by about 1 2 ma on the basis of previously published ages therefore the duration of volcanic quiescence prior to rejuvenation is about 0 6 m y at west maui much longer than estimated previously

implications for the early shield stage evolution of tenerife from k ar ages and magnetic stratigraphy abstract the combined use of field geology radioisotopic dating and magnetic stratigraphy applied to the old shield volcanoes of tenerife provides a reliable time framework for the early shield stage evolution of the island the greater part of this new set of ages obtained from sequences of lava flows is in agreement with the astronomical polarity time scale this approach illustrates that previous k ar data collected without a comprehensive stratigraphy should be viewed with caution and in some cases discarded altogether the shield volcanoes of tenerife encompass a relatively small number of magnetozones an observation consistent with the relatively short periods of growth shown by the new ages 1 2 my the island was constructed by the aggregation of three successive shields the roque del conde central shield between about 11 9 and 8 9 ma and the teno 6 2 5 6 ma and anaga 4 9 3 9 ma volcanoes this new oldest subaerial age of tenerife fits with the others obtained in the canaries in a clear west to east monotonous age progression one of the main restrictions for hotspot related island chains

unspiked k ar dating of pleistocene tholeiitic basalts from the deep core soh 4 kilauea hawaii abstract tholeiitic subaerial lava flows from kilauea soh 4 deep core were successfully dated by unspiked k ar with 40ar contents as low as 0 1 ages between 436 and 42 ka were found on lavas between the depths of 1650 and 468 m all subaerial ages are consistent with their stratigraphic position and allow us to derive a lava accumulation rate of 3 07 0 44 mm a which is within the range of rates determined in high 40ar oibs age determinations on dike samples yield ages that are younger than those of the enclosing lavas the validity of the ages was cross checked with u series ages of coral terraces on the northwest flank of hawaii as determined by ludwig et al 1991 dating problems seem to be restricted to a submarine pillow lava sample from level 1808 m which gives an apparent age of 1 ma which is an overestimate with respect both to rate of construction of kilauea and the hawaiian plume history

deformation of the substratum of a large shield volcano triggering factor for past flank collapses in the old volcanic edifice of la gomera canary islands the basal complex of la gomera canary islands has been analyzed with the aim of deciphering its structural evolution spanning the middle to late miocene detailed structural maps and cross sections and the results of a systematic measurement of dike orientation and fault slip data are presented in this work the main structural features found in the basal complex of la gomera are four dike sets and two large normal faults guillama and montana de alcala faults several extensional episodes have been identified the main deformation phase corresponds to a nnw sse to nw se directed extension that generated first a dense swarm of mafic dikes and then a large scale collapse of the basal complex with displacement along the guillama and montana de alcala faults and associated rotation of large blocks of la gomera basement it is interpreted that the supposed staircase geometry of the guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric scale fault bend folds with rounded hinges deformation of the basal complex shows a complete geometric kinematic and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of la gomera subaerial shield volcano the results of this work support the models that invoke the importance of the large scale geodynamic setting on volcano destabilization more attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large catastrophic volcanic landslide events

k ar 40ar 39ar ages and magnetostratigraphy of brunhes and matuyama lava sequences from la palma island abstract we have applied geological mapping magnetostratigraphy and unspiked k ar and 40 ar 39 ar datings to reconstruct the main volcanic stages of the northern part of la palma an oceanic volcanic island of the canaries archipelago based on 57 new k ar and 40 ar 39 ar ages mostly concordant with the astronomic polarity time scale three main volcanic units are distinguished the garafia volcano and the lower and upper taburiente volcanoes this entirely subaerial activity took place between 1722 25 and 440 8 ka the oldest stage corresponding to the garafia volcano encompasses the post olduvai cobb mountain period with isotopic age determinations ranging from 1722 25 to 1208 8 ka the oldest lava flows of the taburiente volcano which was active between 1 08 0 04 and 833 11 ka recorded the jaramillo normal polarity subchron and younger reversed flows were erupted during the upper part of the post jaramillo matuyama period all the upper taburiente lava flows were erupted during the brunhes chron and have ages ranging from 770 11 to 440 8 ka the age of the garafia collapse lies between 1 20 0 05 and 1 08 0 04 ma while the cumbre nueva collapse occurred at about 550 ka

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

an updated and refined holocene uplift history of southern tenerife canary islands and the possible consequences for future volcanic activity various uplift markers suggest asymmetrical uplift of tenerife island with stable conditions in the north but significant uplift of up to 45 m in the south over the past 42 ka fossil shells in beach deposits uplifted by 7 5 9 m were 14 c dated at a holocene age of 2460 35 bp 1 this confirms earlier results and documents very young and probably still ongoing uplift of southern tenerife potentially caused by ascending magma this underlines that southern tenerife is probably undergoing a further cycle of volcanic activity that started 95 ka ago

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

volcanic and geochemical evolution of the teno massif tenerife canary islands some repercussions of giant landslides on ocean island magmatism large scale catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes recent studies however have highlighted a possible feedback relationship between flank collapse or incipient instability and subsequent episodes of structural rearrangement and or renewed volcano growth the teno massif located in northwestern tenerife canary islands is a deeply eroded miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses each removing 20 25 km3 of the volcano s north flank in this paper we use detailed field observations and petrological and geochemical data to evaluate possible links between large scale landslides and subsequent volcanism magmatism during teno s evolution inspection of key stratigraphic sequences reveals that steep angular unconformities relics of paleolandslide scars are marked by polymict breccias near their base these deposits typically include abundant juvenile pyroclastic material otherwise scarce in the region while some of teno s most evolved low density magmas were produced just before flank collapses early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows extremely rich in clinopyroxene and olivine crystals a detailed sampling profile shows transitions from low mg lavas relatively rich in sio2 to lavas with low silica content and comparatively high mg after both landslides long term variations in zr nb normative nepheline and la lu are coupled but do not show a systematic correlation with stratigraphic boundaries we propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation the successive giant landslides modified the shallow volcano tectonic stress field at teno resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage this rapid unloading of several tens of km3 of near surface rocks appears to have upset magma differentiation processes while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after probably reflecting intrinsic plume processes rather than a collapse induced influence on mantle melting our study of teno volcano bears implications for other oceanic volcanoes where short term compositional variations may also directly relate to major flank collapse events

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago we present new conventional k ar and 39ar 40ar age data and geometric observations on dike swarms which intrude the islands of la gomera tenerife and fuerteventura canary archipelago the younger dated dikes of la gomera 5 4 5 2 ma were emplaced by a radial fissural structure independently superimposed on an older one dated between 10 5 and 7 0 ma the dated dike swarms of both the anaga and teno peninsula of tenerife are contemporaneous 5 8 3 6 ma while ages of 0 43 ma were measured on two dikes intruding the pedro gil ridge the dikes of the basal complex of fuerteventura give well defined 39ar 40ar plateaus when they are situated far away from basic and ultrabasic plutons and disturbed 39ar 40ar age spectra when they are situated in the vicinity of the plutons these disturbances are probably due to a thermal metamorphism the results of both the conventional k ar and 39ar 40ar stepwise heating methods show the existence of an important magmatic phase between 24 and 17 ma the dike swarms of the basal complex are partly contemporaneous with other dike systems intruding the basaltic series i of fuster et al 1968a this phase seems to correspond to the final stages of activity on the main nne ssw volcano tectonic axis following this a volcanic edifice was built on the nw part of jandia peninsula mainly between 15 4 and 14 2 ma the similarity of the ages obtained on dikes and lava flows shows the importance of fissural volcanism in these islands this study shows that dating of dike swarms reveals the existence of major eruptive phases an important part of the products of which have been eroded brief episodes of fissural volcanic activity sometimes probably as short as 200 000 years may be correlated with the migration of the lithosphere over a fixed hot spot the north south 3 5 ma old present and the nne ssw 24 17 ma old oriented dike swarms of la palma staudigel et al in prep and fuerteventura may represent the regional paleostress orientation plane 1 2 which affected the lithosphere during their intrusion although other phenomena may control the orientation of dikes these directions are concordant with the known contemporaneous nw african tectonics

mauna loa lava accumulation rates at the hilo drill site formation of lava deltas during a period of declining overall volcanic growth accumulation rates for lava flows erupted from mauna loa as sampled in the uppermost 280 m of the hilo drill hole vary widely for short time intervals several thousand years but overall are broadly similar to those documented elsewhere on this volcano since 100 ka thickness variations and accumulation rates for mauna loa lavas at the hilo drill site have been strongly affected by local paleotopography including funneling and ponding between mauna kea and kilauea in addition gentle submerged slopes of mauna kea in hilo bay have permitted large shoreline displacements by mauna loa flows ages of eruptive intervals have been determined from published isotopic data and from eustatic sea level curves modified to include the isostatic subsidence of the island of hawaii at 2 2 2 6 mm yr prior to 10 ka rates of mauna loa lava accumulation at the drill site varied from 0 6 to 4 3 mm yr for dateable intervals with an overall rate of 1 8 mm yr major eruptive pulses at about 1 3 and 10 ka each probably representing a single long lived eruption based on lack of weathering between flow units increase the overall accumulation rate to 2 4 mm yr the higher rate since 10 ka reflects construction of thick near shoreline lava deltas as postglacial sea levels rose rapidly large lava deltas form only along coastal segments where initially subaerial slopes have been submerged by the combined effects of eustatic sea level rise isostatic subsidence or spreading of volcano flanks overall accumulation of 239 m of lava at the drill site since 100 120 ka closely balances submergence of the hilo area suggesting that processes of coastal lava deposition have been modulated by rise in sea level the hilo accumulation rate is slightly higher than average rates of 1 2 mm yr determined elsewhere along the mauna loa coast based on rates of shoreline coverage and dated sea cliff and fault scarp exposures low rates of coastal lava accumulation since 100 ka near or below the rate of island wide isostatic subsidence indicate that mauna loa is no longer growing vigorously or even maintaining its size above sea level

fluvial dissection isostatic uplift and geomorphological evolution of volcanic islands gran canaria canary islands spain abstract digital analysis of torrential gullies barrancos deeply incised into the volcanic island of gran canaria canary islands allows us to extract the longitudinal profiles and pre incision surfaces for individual basins from which morphometric parameters length elevation area slope have been calculated other derived parameters such as ridgeline profiles maximum incision values volume removed by fluvial erosion geophysical relief and isostatic uplift have also been computed based on k ar ages for the island well constrained incision uplift rates have been calculated by means of the combination of different methodological approaches commonly used in orogens and large mountain ranges the geomorphological and morphometric analyses reveal that the island is clearly divided into four environmental quadrants determined by the combination of a couple of key factors the age of the volcanic surfaces and the climatic conditions these factors determine a young sector covered with plio quaternary platform forming lavas finished at 1 9 1 5 ma evolving under contrasting wet ne to dry se climates and an older sector conserving the residual surfaces of the miocene shield building 14 5 8 7 ma at the ridgelines also subjected to wet nw and dry sw climates incision is related to the age zonation of the island maximum incisions

the subduction of the farallon plate beneath north america as derived from relative plate motions high rates of subduction and near normal convergence of the farallon plate correlate with the duration of the laramide orogeny in the western cordillera the motion of the farallon plate relative to north america is reconstructed back to the late mesozoic using relative plate motion data this reconstruction involves the use of a plate circuit in which each plate is linked to another with seafloor spreading information in order to relate converging plates two models for relative motions are considered the preferred model predicts an increase in the subduction rate of the farallon plate exceeding 150 mm yr with a direction almost due east and perpendicular to the continental margin the duration of this nonoblique normal high rate subduction correlates well with the duration of the laramide orogeny this correlation must be regarded as only suggestive the probable errors when combined and propagated through the relative motion circuit may be of the order of the effect observed however it is encouraging that these reconstructed farallon north american relative motions are similar to the published predictions of others using the alternative more direct approach of hot spot reconstructions

deformation structures associated with the tazo landslide la gomera canary islands deformation structures below the basal plane of gravitational slides can provide useful information about the state of stress undergone by rocks prior to the sliding process and about the triggering forces acting at each particular sliding event in the present work we conducted a structural analysis of the rocks below the surface of the gravitational slide of tazo la gomera canary islands and determined the epigenetic processes involved in the filling of the amphitheatre we also inferred the possible triggering phenomena related to the tazo landslide the rocks located below the surface of the gravitational slide of tazo i e the basaltic lava flows sills and dikes of the lower old edifice and the submarine volcanic rocks gabbros pyroxenites and dikes of the basal complex of la gomera are strongly deformed close to this sliding surface the lava flows and dikes of the lower old edifice are folded with fault breccias and gouges and locally foliated defining the sliding surface the dikes of the basal complex are also folded and the gabbros and pyroxenites are affected by a large number of small faults in the basal complex the sliding surface is defined by a foliated granular gouge in the damage zone the basal complex rocks show an incipient fracture cleavage the sliding amphitheatre has been filled by the debris avalanche or cohesive debris flow generated within the slide as well as by later debris flows hyperconcentrated flows sheet flows and by interspersed lava flows from the upper old edifice we suggest here that the collapse of the north western flank of the lower old edifice at tazo could in part have been triggered by continuous magma injection associated with the emplacement of dikes in a rift zone with an ene wsw direction enhanced by the mechanical weakness of the basal complex unit which was affected by hydrothermal metamorphism under greenschist facies conditions and by the displacement along the montana de alcala and guillama normal faults which are deeply entrenched in the altered rocks of the basal complex

thermomineral waters of ca chloride composition review of diagnostics and of brine evolution ca chloride waters are defined as those in which q rca r so4 hco3 1 rna rcl 0 80 rmg rca 0 5 and wcl wbr 286 r meq l 1 and w mg l 1 throughout the last 50 years different models for the formation of such waters have been suggested these models include 1 filtration through semipermeable membranes under conditions of highly compacted argillaceous sediments 2 deaquation of seawater by evaporation and or by freezing followed by dolomitization 3 hydrolysis of plagioclase and biotites in igneous metamorphic rock masses 4 radiolytic modification of residual metamorphic fluids and 5 dissolution of chalks followed by ion exchange on smectites the better understanding of processes and of natural environments leading to the evolution and natural occurrence of such brines is imperative for the prospection and further sustained exploitation of such waters

k ar ages and magnetic stratigraphy of a hotspot induced fast grown oceanic island el hierro canary islands abstract the combined use of accurate radiometric dating and magnetic stratigraphy can be applied to define the main stages of the building of oceanic volcanic islands this method has been successfully applied on the island of el hierro the westernmost island of the canaries archipelago for the emerged part of this island built in the last 1 2 ma magnetic stratigraphy and mapping show the presence of four consecutive subchrons k ar dating of eighteen samples in stratigraphic sequences using an unspiked technique produced ages consistent with the general volcanic stratigraphy the ages show that the magnetic polarity zones correlate to the upper part of the matuyama and to the brunhes chrons the jaramillo normal polarity subchron of the upper part of the matuyama reverse polarity chron is recorded in one of the lava sequences the lower limit of this event is constrained by two ages of 1 04 0 02 ma and one of 1 05 0 02 ma close to the value deduced from the astronomically tuned magnetic polarity time scale apts the volcanic history can be summarised by two consecutive main basaltic volcanic edifices the el tinor active during the upper part of the matuyama chron and the el golfo edifice constructed during the brunhes chron

polygonal feeder tubes filled with hydroclasts a new volcanic lithofacies marking shoreline subaerial submarine transition this paper describes for the first time a new lithofacies that formed in the passage zone during the construction of pahoehoe lava fed deltas in the north and ne coastal areas of gran canaria canary islands atlantic ocean these lava fed deltas which are pliocene in age present similar lithofacies associations to those previously described in the literature except for two distinct features low abundance of hyaloclastites and a new lithofacies in the passage zone this new lithofacies which we propose to name polygonal feeder tubes filled with hydroclasts comprises metre scale lava feeder tubes with decimetre scale coherent poorly vesicular and aphanitic rinds of polygonal morphology filled with closely packed hydroclasts generated in situ it represents the first stage in the interaction between seawater and thick lava flows entering the sea under the influence of a tidal regime thus marking sea level this lithofacies should be easily recognizable in ancient successions and thus represents a useful marker of palaeoshorelines supplementary material sample preparation and sample geochemistry are available at http www geolsoc org uk sup18785

age distribution of cinder cones within the bandas del sur formation southern tenerife canary islands the quaternary bandas del sur formation in the south of tenerife comprises a complex sequence of pyroclastic rocks and lavas in contrast to the nw and ne rift zone on tenerife the s rift zone comprises a number of characteristics with respect to the morphological features eruption cyclicity and the geochemistry of the volcanic deposits various flank eruptions of the las canadas volcano associated with basaltic lavas and the formation of cinder cones within the bandas del sur are important volcanic units for understanding the explosive volcanic cycles during the pleistocene on tenerife a number of palaeomagnetic studies as well as major and trace element geochemistry and two radio isotope dates k ar have been carried out on prominent cinder cones in order to discover their stratigraphic position combining our results with previous k ar data the cones and lavas can be subdivided into three stratigraphic units the first unit contains cinder cones with reverse magnetization and y nb ratios between 0 37 and 0 41 cinder cones which belong to the second unit show normal magnetization and y nb ratios of c 0 948 0 779 ma and 0 323 0 300 ma these units define volcanic cycles ending in violent plinian eruptions the third and youngest unit possibly marks the beginning of a further volcanic cycle that started c 0 095 ma ago

saline groundwater in israel its bearing on the water crisis in the country abstract one of the major causes for the deterioration of water quality bearing heavily on the water crisis in israel is the ongoing contamination of its water resources by saline water bodies the present paper reviews the geochemical processes forming saline water lists and explains certain chemical and isotopic parameters which enable understanding these processes and describes the saline groundwater bodies and various salinization phenomena occurring in the country s various aquifers deterioration of groundwater in israel is caused by numerous natural processes such as encroachment of sea water migration of connate highly pressurized brines penetrating into fresh groundwater by subsurface dissolution of soluble salts originating in surrounding country rocks and by water rock interaction in addition to sea water two saline water bodies were identified as the main factors causing salinization of fresh groundwater a ca chloride brines encountered in the jordan dead sea rift valley in various parts of the negev and of the coastal plain and b na chloride saline water identified in the subsurface of the negev and in the southern part of the coastal plain intensive exploitation of groundwater in israel has disturbed the natural equilibrium which prevailed between fresh and saline water the newly established groundwater flow regimes have facilitated the migration of saline water bodies their participation in the active hydrological cycle and the progressive contamination of fresh groundwater these processes which were not anticipated by planners and water resources managers emphasize that large scale groundwater exploitation was undertaken without giving sufficient consideration to the occurrence and subsurface migration of saline water and brines

early tertiary rupture of the pacific plate 1700 km of dextral offset along the emperor trough line islands lineament abstract between 67 and 40 ma ago a northwest southeast trending fracture system over 8000 km long split the pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions this system here named the emperor fracture zone efz system consisted of several segments one along the present trace of the emperor trough and another along the line islands joined by short spreading ridges the efz terminated at its northern end against the kula pacific ridge and at its southern end in a ridge transform system called the emperor spreading system which extended to the west north of australia the finite angular velocity vector describing the relative motion between the east and west pacific plates is 0 6 ma about a pole at 36 n 70 w this vector added to the known early tertiary motion of the pacific plate with respect to the global hotspot reference frame accounts in large part for the nnw trend of the emperor seamount chain relative to the wnw hawaiian trend without violation of the integrity of the antarctic plate the meiji emperor and emperor hawaiian bends date respectively the initiation 67 ma ago and cessation 40 ma ago of seafloor spreading on the emperor spreading system the postulated early tertiary relative motion along the efz between the east and west pacific plates explains 1 the present misalignment of the two sets of magnetic bights of the pacific 2 the abrupt truncation of eastern pacific bathymetric lineaments against the emperor trough and line islands 3 the contrast in paleolatitude between the eastern and western pacific as indicated by paleomagnetic and sedimentologic studies and 4 the anomalous gravity signature of the central hawaiian ridge that indicates that the ridge loaded thin hot lithosphere

the felsic dikes of la gomera canary islands identification of cone sheet and radial dike swarms on the northern part of la gomera there exists a great abundance of trachytic phonolitic dikes showing a broad diversity in dip and strike several methods have been applied in order to separate these dikes in different sets localise the area from where they derive and reconstruct the geometry of the swarms the oldest dikes correspond to a radial swarm dated at 8 ma the felsic activity migrated then southwestwards and a second radial swarm and a cone sheet complex were developed between 7 5 and 6 4 ma ago the cone sheet complex is 10 km in diameter and shared its centre with that of the second radial structure the cone sheets exhibit an outward decrease of dip angle whilst every individual sheet maintains a constant inclination this geometry reflects the existence of an ancient single dome shaped shallow magma chamber situated some 1650 m below present sea level the eastern radial swarm represents a felsic episode that could mark the ending of the lower old basalts the earlier subaerial activity of la gomera the two other dike swarms represent a younger episode coeval with the upper old basalts

anomalously shallow palaeomagnetic inclinations and the question of the age of the canarian archipelago summary extensive geological geophysical and geochronological data available from the canary islands establish conclusively that formation of the oldest subaerial volcanic structure of the islands began during the miocene a mid cretaceous age for these volcanic formations has been postulated in previous works on the basis of palaeomagnetic determinations the results obtained in the present palaeomagnetic study of lanzarote include analysis of the record of variations of magnetic inclination in boreholes that penetrate the whole of the oldest volcanic series they show that the excessive age previously assigned to these formations is due to the utilization of volcanic units with abnormally low 15 magnetic inclinations lgis in lanzarote lavas exhibiting lgis appear interbedded in a volcanic series that overall shows a typical miocene inclination 45 the units stratigraphically beneath and above the lgi horizons give in fact directions consistent with the middle upper miocene field direction d 359 i 45 with k 29 a95 6 7 and a palaeopole of 87 n 178 e thereby confirming the miocene age of the oldest subaerial volcanics of lanzarote short excursions of the geomagnetic field seem likely to be the explanation for these lgis since other factors such as tectonic tilting post eruptive modification of the primary remanence or errors in sample orientation can be disregarded the detection in the canary archipelago of volcanic units with abnormally low magnetic inclinations seems to be related to the relatively continuous record of the geomagnetic field in rapidly growing volcanic edifices as seems to be indicated by the presence of a few usually one or two short polarity events in volcanic suites of several hundred metres thickness

geological evolution of the volcanic island la gomera canary islands from analysis of its geomorphology abstract erosion and landslide processes played an important role in shaping the geomorphology of the canary islands but for the heavily eroded la gomera island those processes have been unclear we conducted an integrated study of its offshore extension onshore geology and links between them in order to determine the geomorphological evolution of the island relative to the rest of the archipelago the characteristics of the island s barrancos and its interfluves have led us to differentiate four types of morphological areas whose fluvial networks are in different stages of evolution those barrancos offshore are interrupted by la gomera s shelf and its erosion has been much greater than for the rest of the canary islands such erosion is both a function of time and varied swell action la gomera s insular slope is carved by a system of submarine canyons and channels that have morphology indicative of turbidity current erosion many of the barrancos onshore and the canyons offshore have steps or knickpoints some are the result of varied resistance to erosion and others are explained by other mechanisms a ridge whose crest is covered by cone like shape structures is interpreted as of volcanic origin with the particularity that it lacks onshore continuation the opposite of what has been observed elsewhere in the canary islands also in contrast with the other islands of the archipelago la gomera s margin does not seem to have resulted from destructive mass wasting and the only offshore sign of catastrophic events are the presence of two embayments along la gomera s northern shelf edge and a lobe structure on the insular apron at 3000 m depth these features may be the remains of the catastrophic failures that took place on the island 9 4 8 7 ma ago alternatively they could be due to recent catastrophic failures on the upper slope due to sediment steeping since the conclusion of the main volcanic activity 4 0 ma ago la gomera has been intensively eroded by gradual fluvial denudation and secondary failures la gomera s insular margin reveals much regarding its evolution but more comprehensive studies including seismic and coring are needed to understand its whole geological history

models of hawaiian volcano growth and plume structure implications of results from the hawaii scientific drilling project the shapes of typical hawaiian volcanoes are simply parameterized and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate the dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the hawaiian plume with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit the timescale of volcano growth is t 2 r plate where r is the radius of the melting zone of the circular plume and plate is the velocity of the pacific plate the growth progress of a volcano can be described by a dimensionless time t t plate 2r where t 0 is chosen to be the start of volcano growth and t 1 approximates the end of shield growth using a melt generation rate for the whole plume of 0 2 km 3 yr a plume diameter of 50 km and a plate velocity of 10 cm yr we calculate that the lifetime of a typical volcano is 1000 kyr for a volcano that traverses the axis of the plume the standard dimensions are a volume of 57 000 km 3 a summit thickness of 18 km a summit elevation of 3 6 km and a basal radius of 60 km the volcano first breaches the sea surface at t 0 22 when it has attained only 5 of its eventual volume 80 of the volume accumulates between t 0 3 and t 0 7 typical lava accumulation rates start out over 50 m kyr in the earliest stages of growth from the seafloor and level out at 35 m kyr from t 0 05 until t 0 4 from t 0 4 to t 0 9 the submarine lava accumulation rates decrease almost linearly from 35 m kyr to 0 subaerial accumulation rates are about 30 lower the lava accumulation rate is a good indicator of volcano age a volcano that passes over the plume at a distance 0 4r off to the side of the plume axis is predicted to have a volume of about 60 of the standard volcano a lifetime about 8 shorter and lava accumulation rates about 15 20 smaller the depth age data for mauna kea lavas cored by the hawaii scientific drilling project are a good fit to the model parameters used given that mauna kea appears to have crossed the plume about 15 20 km off axis the lifetime of mauna kea is estimated to be 920 kyr mauna loa is predicted to be at a stage corresponding to t 0 8 kilauea is at t 0 6 and loihi is at t 0 16 the model also allows the subsurface structure of the volcanoes the interfaces between lavas from different volcanoes to be modeled radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross sectional area of the plume this inference is qualitatively born out by available isotopic data the model predicts that new hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano s flank

geology and geochronology of the line islands geological and geophysical studies along the entire length of the line islands were undertaken in order to test the hot spot model for the origin of this major linear island chain volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls volcanic rocks from the line islands are alkalic basalts and hawaiites in addition a tholeiitic basalt and a phonolite have been recovered from the central part of the line chain microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high tio2 1 0 4 0 wt and al2o3 3 4 9 1 wt and are of alkaline to peralkaline affinities major element compositions of the line islands volcanic rocks are very comparable to hawaiian volcanic rocks trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas the line islands lavas are very much unlike typical mid ocean ridge or fracture zone basalts dating of these rocks by 40ar 39ar k ar and paleontological methods combined with deep sea drilling project data from sites 165 315 and 316 and previously dated dredged rocks provide ages of volcanic events at 20 localities along the chain from 18 n to 9 s a distance of almost 4000 km all of these dates define mid cretaceous to late eocene edifice or ridge building volcanic events eocene volcanic events took place from 15 n to 9 s and late cretaceous events took place from 18 n to 9 s in the southern line islands both cretaceous and eocene events took place on the same edifice or ridge indicating recurrent volcanism at a single locality the irregular distribution of atolls in the chain the fact that late cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern line islands and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1 2 related subsidence path magnetic surveys of 11 seamounts show that four of the seamounts from the central line chain give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of cretaceous seamounts these four poles agree with other paleomagnetic data of middle late eocene early oligocene age from the pacific one of these four seamounts yielded a 40ar 39ar total fusion age of 39 ma because the poles of all four seamounts fall into a tight group we infer that they are probably of middle late eocene age to early oligocene age the other three seamounts as well as one seamont from the line islands analyzed previously all give virtual geomagnetic poles which agree with late cretaceous paleomagnetic data from the pacific of these four seamounts three give late cretaceous 40ar 39ar ages ranging from 71 to 85 ma another in the southern line islands is interpreted on paleontological evidence to be of late cretaceous age the origin of the line islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots the single hot spot model cannot account for all of the volcanic edifices in the line islands although it does explain a general age progression of 9 6 0 4 cm yr from north to south along the chain derived from a number of dated edifices the four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data the petrologic data argue against a mid ocean ridge or transform fault origin proposed by earlier workers the complex volcanic province represented by the line islands remains a challenge to existing models for the origin of midplate volcanism in the pacific an atoll drilling program which could determine edifice building histories paleolatitudes of seamount formation and subsidence rates and patterns in the line islands is needed

the development of middle rare earth element enrichments in freshwaters weathering of phosphate minerals abstract many phosphatic minerals of biogenic authigenic and diagenetic origin are distinguished by rare earth element ree compositions which have a well developed middle rare earth element mree enrichment defined by a gradual increase in the shale normalized ratios of ree toward the middle of the ree series la to lu also the ree composition of dissolved matter in the waters of the amazon fly and sepik rivers as well as other river lake and ground waters have well developed mree enrichments the development of mree enrichments was studied through a series of leaching experiments using river bank sediments from the amazon fly sepik and mississippi rivers a well characterized phosphate mineral rich shale with a pronounced mree enrichment in its bulk composition was also leached in order to better assess the relationship between the weathering of phosphate minerals and the formation of mree enriched solutions the two leaching solutions per sediment consisted of filtered lake water at ph 5 and the same water adjusted to ph 2 leaching experiments with the four river sediments result in extensive fractionation whereby the ree compositions of the solutions develop mree enrichments from starting sediments that show no mree enrichments the ph 2 leachates have much higher concentrations of the ree and more fully developed mree enrichments than do the ph 5 leachates the mree enrichments at ph 2 usually form within an hour of starting the experiments the phosphatic shale results show that there is preferential release of large amounts of mree at ph 5 as well as at ph 2 demonstrating the weather ability of the phosphatic minerals at phs of natural freshwaters the sepik river ph 5 experiment also shows significant mree enrichment under near neutral conditions the development and retention of the mree enrichments in the leaching solutions is accompanied by an increase in dissolved phosphate concentrations although the magnitudes of the mree enrichment do not directly correlate with the percent of phosphate released the experimental data suggest that the preferential dissolution of phosphatic minerals during weathering can result in extensive ree fractionation between bulk river sediment and water leading to freshwaters with mree enrichments

paleomagnetism of line islands seamounts evidence for late cretaceous and early tertiary volcanism seamount magnetic anomaly studies have been conducted on eight volcanoes in the line islands seamount chain the magnetic anomalies of these seamounts were inverted to obtain paleomagnetic information four of these gave paleomagnetic pole positions consistent with other pacific cretaceous pole positions of these three have reliable radiometric ages ranging from 72 to 85 ma the remaining four seamounts gave geomagnetic poles that agree with other eocene early oligocene pacific paleomagnetic data and one has a reliable radiometric age of 39 ma all of the apparently eocene volcanoes occur in the central line islands between 8 n and 3 s we suggest that the eocene volcanic event was approximately coincident with the profound change in pacific plate motion recorded by the bend in the hawaiian emperor seamount chain that occurred at approximately 43 ma this volcanism may have resulted from the large change in intraplate stress accompanying the change of plate motion the change in stress might have pulled apart a zone of weakness caused by the original formation of the line islands detailed paleomagnetic modeling of the line islands seamounts was necessary to obtain good results as seven of the eight seamounts appear to be inhomogeneously magnetized to some extent despite these inhomogeneities the high goodness of fit parameters and the excellent agreement of these results with other pacific paleomagnetic data suggest that the paleopoles derived from these seamounts are reliably determined

conceptual hydrogeological model and aquifer system classification of a small volcanic island la gomera canary islands abstract conceptual groundwater models in volcanic island aquifers remain an issue in many regions of the world this study takes advantage of the large number of outcrops exposing the inner structure of la gomera canary islands due to its inactivity during the last 2 5 ma to develop a conceptual groundwater model that helps improving groundwater resource management in the island and increasing knowledge in volcanic aquifers based on a volcanostratigraphic model five hydrostratigraphic units have been defined that include two main aquifers and volcaniclastic deposits located between them that control the vertical movement of the groundwater the island has been classified into four aquifer systems or water management units based on their hydrogeological features that permit a sustainable and independent management of the resources of one aquifer system in relationship with the other ones the consideration of the complexity and heterogeneity of la gomera geology has allowed to develop a conceptual groundwater flow model and to identify four general groundwater situations 1 groundwater in the upper aquifer discharging to the sea 2 groundwater in the upper aquifer springing out along the main ravines 3 groundwater in the basal aquifer impounded by dikes and 4 groundwater in the basal aquifer discharging to the sea

cretaceous plate tectonic reconstructions abstract plate tectonic reconstructions for each cretaceous stage are produced as base maps for cretaceous paleogeography and for oceanic and continental data both geologic and geophysical data are utilized to determine the relative rotations of cretaceous plates with respect to north america a sea floor spreading history is generated from the relative rotations this history is consistent with data from the deep sea drilling project paleolatitude lines are derived using average paleomagnetic pole positions these reconstructions are significantly more comprehensive than previous global reconstructions although there are still many areas which remain controversial

volcano tectonic and geochemical evolution of an oceanic intra plate volcano tahiti nui french polynesia abstract geological mapping accurate k ar dating and geochemical analyses of lavas allow a detailed reconstruction of the geological history of tahiti nui island french polynesia the exposed volcanic activity is first characterized by the construction of a main shield from 1 4 ma to 870 ka with a maximum aerial eruptive rate around 2 km3 kyr lavas from this early building stage are alkaline slightly silica undersaturated with 87sr 86sr and 143nd 144nd compositions rather constant and close to the enriched mantle ii type vent locations were first concentrated along a main e w rift zone which was responsible for the lateral collapse of the northern and southern flanks of the main shield around 0 87 ma ago the subsequent activity was first restricted to the northern depression corresponding to an eruptive rate of about 5 km3 kyr in the period 850 760 ka significant variations in la sm 87sr 86sr and 143nd 144nd occur in lavas erupted immediately after the main northern landslide indicating a sudden increase in the extent of partial melting likely caused by the decompression subsequent to collapse however the later activity declined and the lavas exhibit a gradual change toward strongly silica undersaturated basanites likely indicating a decreasing extent of partial melting of the upper mantle the evolution of radiogenic isotope ratios over time indicates a change in the source toward more depleted compositions until around 500 ka post erosion volcanic activity following an apparent hiatus of 240 kyr exhibits similar major and trace element and isotope compositions the volcano structural and geochemical evolution of tahiti nui and the overall alkaline character of the lavas from other eruptive complexes of the society alignment suggest a relative weakness low temperature and low eruptive rate of the society plume compared to the hawaiian hot spot

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated