intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

performances of ground penetrating radars in arid volcanic regions consequences for mars subsurface exploration a gpr field experiment in the republic of djibouti provides evidence for very low radar penetration in arid volcanic materials in the range 100 500 mhz this phenomenon is attributed to the high iron oxide and evaporite concentration in soils which significantly increases the conductivity thus leading to poor subsurface imaging performances the geologic context in djibouti is shown to provide a good terrestrial analogue to mars geology results of this study show that the future sounding radar missions to mars may not reach the penetration depths previously anticipated

low frequency radar sounding investigations of the north amargosa desert nevada a potential analog of conductive subsurface environments on mars 1 theoretical estimates of low frequency radar sounding performance and its potential for mapping moist subsurface interfaces in conductive environments on mars are controversial with predictions of ultimate penetration depth ranging from a few meters to kilometers to address this issue we conducted a broadband electromagnetic field survey in which we combined ground penetrating radar gpr operating at multiple low frequencies with the transient electromagnetic method tem to investigate the dependence of radar penetration depth on ground resistivity surveys were performed in the frequency range 16 100 mhz at two locations on the northwest margin of the amargosa desert nevada where numerous mars analog investigations have been performed the surveys were conducted on a 20 m high homogenous sand dune and on the flanks of a 20 m high scoria cone and above a buried lava flow a wet alluvial interface was located at the bottom of each structure gpr detected the wet alluvium contact at the base of the sand dune but failed to penetrate to the same depth at the scoria cone under similar residual moisture content depths of investigation for both the scoria cone and the buried lava flow were limited to approximately 10 m owing to the presence of conductive inclusions in the first few meters which are below the radar resolution but dramatically decreased the dynamic of the radar backscattered echoes and hence the penetration depth absorption models constrained by the tem data are in good agreement with these observations depths of investigation varied weakly with frequency owing to substantial frequency independent absorption

excess 40ar in biotite and hornblende from the noril sk 1 intrusion siberia implications for the age of the siberian traps the siberian traps represent the largest known phanerozoic manifestation of continental flood volcanism the general agreement is now that the volcanism was of short duration 1 m y and was synchronous with the permian triassic boundary discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism several alkalic mafic intrusions associated with the siberian traps represent subvolcanic feeders for the trap lavas the noril sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows biotites from the noril sk 1 intrusion have previously yielded 40ar39ar plateau dates 254 251 ma that are significantly older than seemingly reliable 40ar39ar plateau dates 250 247 ma on plagioclase and whole rock samples from trap basalts that are geologically constrained to be older than the intrusion possible explanations of this discrepancy include 1 plagioclase and whole rock samples are anomalously young due to 40ar loss via alteration or episodic outgassing and or 2 the biotites contain excess 40ar detailed laser heating 40ar39ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of ar through isotope correlation patterns an isochron age of 250 1 1 5 ma 2 and an excess ar component with 40ar36ar 565 39 are obtained by regression of the 115 highest temperature steps step heating of a bulk hornblende sample also reveals excess ar with 40ar36ar 327 10 and an isochron age of 249 3 1 6 ma neglecting uncertainties in the ages of neutron fluence monitors the two dates just overlap at 2 but are distinctly younger than the previous mean plateau date 253 9 1 6 ma for biotite the different initial 40ar36ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination

el mayor cucapah mw 7 2 earthquake early near field postseismic deformation from insar and gps observations el mayor cucapah earthquake occurred on 4 april 2010 in northeastern baja california just south of the u s mexico border the earthquake ruptured several previously mapped faults as well as some unidentified ones including the pescadores borrego paso inferior and paso superior faults in the sierra cucapah and the indiviso fault in the mexicali valley and colorado river delta we conducted several global positioning system gps campaign surveys of preexisting and newly established benchmarks within 30 km of the earthquake rupture most of the benchmarks were occupied within days after the earthquake allowing us to capture the very early postseismic transient motions the gps data show postseismic displacements in the same direction as the coseismic displacements time series indicate a gradual decay in postseismic velocities with characteristic time scales of 66 9 days and 20 3 days assuming exponential and logarithmic decay respectively we also analyzed interferometric synthetic aperture radar insar data from the envisat and alos satellites the main deformation features seen in the line of sight displacement maps indicate subsidence concentrated in the southern and northern parts of the main rupture in particular at the indiviso fault at the laguna salada basin and at the paso superior fault we show that the near field gps and insar observations over a time period of 5 months after the earthquake can be explained by a combination of afterslip fault zone contraction and a possible minor contribution of poroelastic rebound far field data require an additional mechanism most likely viscoelastic relaxation in the ductile substrate

liquefaction resistance of soils from shear wave velocity a simplified procedure using shear wave velocity measurements for evaluating the liquefaction resistance of soils is presented the procedure was developed in cooperation with industry researchers and practitioners and evolved from workshops in 1996 and 1998 it follows the general format of the seed idriss simplified procedure based on standard penetration test blow count and was developed using case history data from 26 earthquakes and 70 measurement sites in soils ranging from fine sand to sandy gravel with cobbles to profiles including silty clay layers liquefaction resistance curves were established by applying a modified relationship between the shear wave velocity and cyclic stress ratio for the constant average cyclic shear strain suggested by r dobry these curves correctly predicted moderate to high liquefaction potential for 95 of the liquefaction case histories and are shown to be consistent with the standard penetration test based curves in sandy soils a case study is provided to illustrate application of the procedure additional data are needed particularly from denser soil deposits shaken by stronger ground motions to further validate the simplified procedure

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

parana flood basalt volcanism primarily limited to 1 myr beginning at 135 ma new 40 ar 39 ar ages for rocks from rio grande do sul and critical evaluation of published radiometric data abstract 40 ar 39 ar step heating analyses were carried out on seven rocks five basalts an andesite and a rhyolite from the southern parana province 28 s 30 s they yield plateau isochron ages of 135 134 ma in good agreement with published step heating data on rocks from the same area critical review of laser spot isochron ages for rocks from the province ranging from 140 to 130 ma are shown to be unreliable estimates of crystallization ages as the rocks were substantially altered step heating results on three of these rocks thought to yield good plateau ages are shown to be incorrect as a result of a technicality in dating procedures followed u pb ages on zircon and baddeleyite separated from a variety of rock types 30 s 23 s fall in the range 135 to 134 ma all reliable 40 ar 39 ar and u pb ages indicate volcanism was sharply focused initiated at 135 ma and 1 myr in duration no variation of age with either latitude or longitude is noted scrutiny of published 40 ar 39 ar ages on the florianopolis dykes shows they cannot be used as reliable crystallization ages u pb work shows that this dyke swarm was formed coevally with the main part of the parana province most of the published 40 ar 39 ar ages on the ponta grossa dyke swarm are unreliable a few ages appear reliable and suggest the magmatic event in this area may have postdated the main parana pulse by 1 2 myr a single 40 ar 39 ar age from a high nb basalt in the southernmost part 34 s of the parana at 135 ma highlights the need for further radiometric work on other areas of this flood basalt province the parana province postdates the time of the jurassic cretaceous bound ary by 10 myr

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

late quaternary geomorphology and soils in crater flat yucca mountain area southern nevada crater flat is an alluvium filled structural basin on the west side of yucca mountain nevada which is under consideration for a high level nuclear waste repository north trending late quaternary faults offset alluvium in crater flat both along the canyons of the western flanks of yucca mountain and out on the piedmont slope we believe the initial lack of young offsets at yucca mountain was in part due to unrecognized late quaternary stratigraphy we hypothesize that alluviation in the yucca mountain region was more active during the late quaternary than previously thought several techniques were tried to test this hypothesis results are compared with previous soils and surface exposure dating studies and correlated to stratigraphy of other late quaternary units in the southern nevada death valley and mojave desert areas and provide new stratigraphic data relevant to understanding climatic alluvial processes in the basin and range province during the late quaternary 76 refs 7 figs 6 tabs

two case histories demonstrating the effect of past earthquakes on liquefaction resistance of silty sand abstractthe paper compares two liquefaction case histories in california 1 the response of the wildlife site in the imperial valley to the 2010 el mayor cucapah earthquake mw 7 2 amax 0 15 g

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

earthquake monitoring in southern california for seventy seven years 1932 2008 the southern california seismic network scsn has produced the scsn earthquake catalog from 1932 to the present a period of more than 77 yrs this catalog consists of phase picks hypocenters and magnitudes we present the history of the scsn and the evolution of the catalog to facilitate user understanding of its limitations and strengths hypocenters and magnitudes have improved in quality with time as the number of stations has increased gradually from 7 to 400 and the data acquisition and measuring procedures have become more sophisticated the magnitude of completeness m c of the network has improved from m c 3 25 in the early years to m c 1 8 at present or better in the most densely instrumented areas mainshock aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470 000 events the earthquake frequency size distribution has an average b value of 1 0 with m 6 0 events occurring approximately every 3 yrs the three largest earthquakes recorded were 1952 m w 7 5 kern county 1992 m w 7 3 landers and 1999 m w 7 1 hector mine sequences and the three most damaging earthquakes were the 1933 m w 6 4 long beach 1971 m w 6 7 san fernando and 1994 m w 6 7 northridge earthquakes all of these events ruptured slow slipping faults located away from the main plate boundary fault the san andreas fault their aftershock sequences constitute about a third of the events in the catalog the fast slipping southern san andreas fault is relatively quiet at the microseismic level and has not had an m 6 earthquake since 1932 in contrast the slower san jacinto fault has the highest level of seismicity including several m 6 events thus the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation

on the origin of an amphibole rich vein in a peridotite inclusion from the lunar crater volcanic field nevada u s a abstract late cenozoic alkali basaltic lavas of the lunar crater volcanic field lcvf located in the center of the great basin of the western u s a contain a diverse suite of nodule samples of the lower crust and upper mantle this paper documents a composite nodule from the marcath flow in which an amphibole bearing wehrlite 59 olivine 30 clinopyroxene 6 amphibole is cut by a 6 9 mm wide vein of andesine amphibolite 80 kaersutite 15 andesine 3 ilmenite aside from nodule basalt reaction at the nodule exterior there is little chemical variation either within or between individual grains of hydrous and anhydrous phases in the vein and host wehrlite furthermore there is no systematic compositional zoning in the wehrlite relative to vein proximity the whole rock major and trace element composition of the vein is similar to a primitive mg mg fe 0 692 basaltic liquid and has al fe mg ca mn na k zr y and sr contents similar to basalts observed in the lcvf in contrast to the sr isotopic equilibrium displayed by vein feldspar and vein amphibole sr isotopic disequilibrium is exhibited between the vein 0 70318 4 wehrlite 0 70322 4 and host basalt 0 70357 5 n 3 however the sr isotopic ratios of older lcvf basalts 0 7030 0 7038 n 14 overlap those of the vein and wehrlite and the magmatic activity leading to vein and wehrlite formation could be related to this older phase of lcvf volcanism petrographic and geochemical evidence is not consistent with a metasomatic origin for the vein and instead supports the view that the vein originated by the intrusion into a wehrlite mass and subsequent crystallization of a relatively primitive alkali basaltic magma in the lower crust or upper mantle the wehrlite contains olivine of fo71 and probably originated by crystal separation and accumulation from a relatively differentiated basaltic magma in the lower crust or upper mantle

on water detection in the martian subsurface using sounding radar several radar experiments are planned to map the martian subsurface down to several kilometers searching for subsurface liquid water reservoirs using different concepts and techniques all based on the penetration property of radio frequency waves in arid soils the penetration depth of low frequency radar is mainly related to the electromagnetic properties of the investigated medium thus a good knowledge of the martian subsurface dielectric profile along the first few kilometers is necessary for future water identification and data interpretation in this work we have investigated the electrical and magnetic properties of the martian surface and subsurface using terrestrial laboratory analogues in the frequency range 1 500 mhz covering the frequency domain of the mars advanced radar for subsurface and ionosphere sounding marsis experiment on board the mars express mission esa 2003 the netlander ground penetrating radar gpr cnes 2007 and future sounding radar that may be updated to the mars exploration program in the follow the water strategy in our approach we constructed experimentally the most common dielectric profile representative of the martian subsurface by measuring the electric permittivity and magnetic permeability of well defined mixtures of basaltic volcanic and sedimentary materials that have been reported for mars we also considered iron oxides hematite and maghemite and evaporites that may be present such as gypsum and their mixtures with representative amounts of the martian geological context under the most common petrophysical and geophysical conditions along the subsurface profile this led to synthetic representative samples of the martian subsurface materials under adequate conditions of porosity and temperature that should exist in the first 2 5 km of the upper crust dielectric measurements show that the first layers of the martian subsurface a few hundred meters which are mainly composed of volcanic iron rich materials could dramatically decrease the radar penetration depth initially foreseen thus limiting deep subsurface exploration we also investigated the constraints on subsurface water detectability in a radar lossy medium and its dielectric identification among surrounding geological materials

liquefaction potential of recent fills versus natural sands located in high seismicity regions using shear wave velocity abstractthe liquefaction potential of clean and silty sands is examined on the basis of the field measurement of the shear wave velocity vs the starting point is the database of 225 case histories supporting the andrus stokoe vs based liquefaction chart for sands silts and gravels only clean and silty sands with nonplastic fines are considered resulting in a reduced database of 110 case histories which are plotted separately by type of deposit a line of constant cyclic shear strain cl 0 03 is recommended for liquefaction evaluation of recent uncompacted clean and silty sand fills and earthquake magnitude mw 7 5 the geologically recent natural silty sand sites in the imperial valley of southern california have significantly higher liquefaction resistance as a result of preshaking caused by the high seismic activity in the valley a line of constant cyclic shear strain cl 0 1 0 2 is recommended for practical use in the imperial valley additional research including revisiting available vs

a minimum upb age for siberian flood basalt volcanism abstract establishing an accurate and precise age for siberian flood basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the permian triassic boundary we report a new minimum u pb age obtained from zircon and baddeleyite from the mineralized noril sk i intrusion that cuts the lower third of this rapidly deposited 3500 m thick volcanic sequence near noril sk this 251 2 0 3 2 ma age is within analytical error of the shrimp u pb age for zircon from the permian triassic boundary at meishan south china 251 1 3 6 ma 2 and confirms siberian basaltic volcanism as a possible contributor to the mass extinction

the timing of late cretaceous alkalic igneous activity in the northern gulf of mexico basin southeastern usa upper cretaceous igneous rocks in the northern gulf of mexico basin are poorly understood due both to the paucity of material recovered from drillholes in the sedimentary basins and lack of geochronological data rocks from the alkalic igneous complexes at magnet cove and granite mountain arkansas the monroe uplift and the jackson dome louisiana and mississippi yielded 40ar 39ar ages ranging from 94 ma to 65 ma high precision plateau ages reveal that the magnet cove complex 94 ma predates granite mountain by 4 myr igneous activity in the monroe uplift may have been contemporaneous with that at magnet cove and continued sporadically for 30 myr a single specimen from the jackson dome indicates volcanism at 65 ma coincident with waning igneous activity at the monroe uplift based on geochemical and geochronological data the source for volcanoclastic material in the woodbine formation arkansas are rocks belonging to the magnet cove phase of magmatism cretaceous alkalic rocks of central ark

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by discussion abstract barry et al 2010 presented 40 ar 39 ar ages to argue for a very limited duration 0 4 m y for the eruption of the grande ronde lavas critical examination of their data sets shows 1 a number of their plateau ages are statistically invalid and 2 many of their groundmass matrix samples were altered and did not yield valid estimates of the time of crystallization in general their ages err on the young side reflecting partial 40 ar loss due to alteration and there is no compelling evidence therein to argue for very limited duration for the grande ronde basalt the imnaha grande ronde and wanapum basalt erupted within 0 7 m y starting at 16 4 ma by contrast the ages of barry et al 2010 span the interval 16 6 to 14 7 ma and in some instances are out of stratigraphic sequence

a new jurassic recent apparent polar wander path for south america and a review of central andean tectonic models abstract the jurassic to recent palaeomagnetic data from the stable cratonic areas of south america and africa are reviewed and evaluated from the combined data set seven mean reference pole positions are calculated for the early middle and late jurassic early and late cretaceous palaeogene and neogene most of these new palaeopoles are well defined and indicate a complex course of apparent polar wander for the south american plate comparison of the palaeomagnetic data from the andean margin with the reference poles indicates vertical axis rotations defining a broad pattern of anticlockwise rotations north of the arica deflection at approximately 19 s and clockwise rotations to the south further analysis indicates however that the distribution and magnitude of rotations is more complicated than this broad division tectonic models proposed to explain the pattern of rotations and the possible origins of the arica deflection are discussed it is suggested that the rotations probably result from localised in situ rotations controlled by strike slip fault systems or thrust sheets rather than large scale rotation mechanisms the broad rotation pattern may result from the influence of pre existing structures in the margin or variations in the flexural rigidity of the brazilian shield behind the deformation zone

centrifuge modeling of the effect of preshaking on the liquefaction resistance of silty sand deposits abstractfield observations suggest that preshaken natural sands in some seismic regions have high liquefaction resistance as a result of geologic aging and or preshaking this paper focuses on the young silty sand deposits located in the imperial valley of california recent deposition and intense seismic activity in the valley suggest that preshaking is the main cause of their increased liquefaction resistance the first part of the paper examines the liquefiable layer at the wildlife site which may have been deposited by flooding approximately between 1905 1907 the site was instrumented with accelerometers and piezometers in 2005 providing data over the last 10 years the following conclusions are reached from this and from the catalog information on earthquakes before 2005 1 since 1907 the wildlife layer has been subjected to approximately 60 70 earthquakes having amax 0 1 g at the site which caused pore pressure buildup in the layer 2 most of these earthquakes generated excess pore pressur

age of jurassic continental tholeiites of french guyana surinam and guinea implications for the initial opening of the central atlantic ocean abstract a detailed 40ar 39ar study of mineral separates from the jurassic atlantic continental tholeiites jact of guyana french guyana and surinam south america and guinea west africa related to the initial opening of the central atlantic has been carried out in french guyana plateau ages of 196 0 5 7 ma and 196 1 7 5 ma were obtained on single small amphibole grains from nnw sse trending dykes in guinea single biotite grains from intrusive formations from the kakoulima and fouta djalon areas yielded plateau ages of 200 4 0 2 ma and 194 8 0 5 ma concordant with high temperature apparent ages on other biotites the bulk plagioclase samples display disturbed age spectra due to alteration and excess argon however intermediate temperature weighted mean plagioclase ages are similar in both regions of guyana and guinea ranging from 200 2 2 4 ma to 188 7 1 9 ma partly in agreement with the amphibole and biotite data these data combined with previous 40ar 39ar and u pb results from the northern part of the central atlantic margins indicate intense magmatic activity distributed over a large area from iberia to liberia ca 4500 km long for a short period of time 204 195 ma perhaps less for the bulk of the magmatism during the initial break up of pangea continent these data do not support an initiation of the magmatism from a radial volcano tectonic system centred in the south of the region as suggested by may 1 and the initial break up seems to affect the whole central atlantic during a period of 9 ma

liquefaction potential map of charleston south carolina based on the 1886 earthquake a liquefaction potential map of the peninsula of charleston s c is presented in this paper liquefaction potential is expressed in terms of the liquefaction potential index developed by iwasaki et al and calculated using 44 cone penetration test profiles the cone profiles are supplemented with information from the 1 24 000 scale geologic map by weems and lemon several first hand accounts of liquefaction and ground deformation that occurred during the 1886 charleston earthquake and liquefaction probabilities determined by elton and hadj hamou based on standard penetration tests nearly all of the cases of liquefaction and ground deformation occurred in the holocene to late pleistocene beach deposits that flank the higher ground sediments of the wando formation to match the observed field behavior a deposit resistance correction factor of 1 8 is applied to cyclic resistance ratios calculated for the 100 000 year old wando formation no corrections are needed for the younger deposits in additional to 1886 field behavior the deposit resistance corrections are supported by ratios of measured to predicted shear wave velocity

gpr a ground penetrating radar for the netlander mission in the coming decade several missions are planned that will land on the surface of mars landers or instrumented geophysical stations among the scientific objectives of these projects one of the most important will be to unravel the many unknowns in the geological and hydrological history of the planet the netlander mission offers a unique opportunity to explore the interior of mars its subsurface its atmosphere and its distant environment from four landing sites that will be selected to offer a variety of different geophysical conditions we have thus proposed to fly on these four landers a ground penetrating radar gpr to explore the geological characteristics of the subsurface and search for water reservoirs down to a depth which may be sufficient to allow a possible detection of liquid water we provide in this paper a short description of this radar which is based on a new concept to allow a 3 d imaging of the subsurface by determining the range and direction of the underground reflectors in order to access to deep layers it will operate at a low frequency of 2 mhz some results obtained by a numerical modeling of the radar operation in an electromagnetic model of the martian subsurface are presented in order to illustrate the main capabilities of the radar in the last section preliminary results from an initial field test are reported in addition to its primary goal as a ground penetrating radar the gpr will also be operated on mars as an ionospheric sounder and in a passive mode as a hf receiver to measure the radio electric background

discussion of examination and reevaluation of spt based liquefaction triggering case histories by ross w boulanger daniel w wilson and i m idriss

genesis of poikilotopic zeolite in aeolianites an example from the parnaiba basin ne brazil abstract zeolite cement is one of the main components of the jurassic to lower cretaceous fluvial aeolian succession in the parnaiba basin northeastern brazil core and outcrop based stratigraphic analyses permitted the paleo environmental and diagenetic interpretation of aeolian sandstone and showed the influence of the volcanic substrate in the origin of the zeolite cement the substrate is represented by an extensive volcanic plain developed in the last stages of the central atlantic magmatic province camp in west gondwana the siliciclastic deposits consist of dune field sand sheet and wadi deposit facies associations the zeolite cement of the aeolian succession forms poikilotopic crystals with anomalous up to 50 modal content x ray diffraction and sem eds cl analyses showed laumontite and ca stilbite exhibiting respectively massive and radial textures during eodiagenesis the main processes included clay infiltration linked to water table fluctuations and flash flood events producing clay coatings and displacive stilbite cementation massive laumontite cementation and smectite chlorite conversion occurred in the mesodiagenesis while in the final stages the zeolite was partially replaced by calcite cement telodiagenesis is recorded by iron exsudation and the corrosion of clay films several processes can be inferred to explain the origin of the zeolite cement in the jurassic lower cretaceous sandstone of the parnaiba basin after the first magmatic pulse triassic magmatism the zeolite bearing basalts were submitted to chemical weathering causing cation leaching afterwards the deposition of aeolian sands related to a wet desert system occurred above this weathered substrate fractured basaltic substrate favored the release of ions to the surface with fluids enriched in sodium and calcium that percolated the permeable aeolian sandstone beds during the diagenesis furthermore zeolite cement is geographically associated with the volcanic substrate in the western parnaiba basin however it is absent in the eastern part where the aeolianites overlie sedimentary rocks we suggest that the system was thermally reactivated by the last thermal camp phase with basalt emplacement in the eastern parnaiba basin linked to the opening of the equatorial atlantic ocean cretaceous magmatism the progressive heating towards the western part of the basin triggered fluids convection and the enrichment in ions such as ca2 si4 na al3 hco 3 and so42 promoting optimal conditions for zeolite crystal growth 100 c 300 c sandstone textural aspects as well as extensional faults and fractures allowed fluids migration resulting in the massive poikilotopic zeolite cementation characterizing one of the most singular features of the diagenetic history of the west gondwana record

experimental constraints on magma ascent rate for the crater flat volcanic zone hawaiite phase equilibria and isothermal constant rate decompression experiments conducted on a hawaiite from the crater flat volcanic zone near yucca mountain nevada provide insight into the dynamics of magma transport for this magmatic province h 2 o contents up to 4 6 wt in melt inclusions the lack of plagioclase phenocrysts and the presence of amphibole all suggest that phenocrysts and melt in the crater flat volcanic zone were equilibrated at near water saturated conditions during ascent comparison of decompression induced crystallization of plagioclase microlites in experiments with natural tephra implies that magma ascent rates for the region are 0 04 m s if the magmas are water saturated estimates of minimum ascent rates increase if the magmas are water undersaturated a two stage process is needed to generate reaction rims found on natural amphibole suggesting that small batches of magma were briefly stored 3 5 days at depths as shallow as 800 m prior to eruption

a geomagnetic polarity time scale for the period 0 17 ma based on 40ar 39ar plateau ages for selected field reversals utilizing 40ar 39ar plateau ages for selected field reversals at 1 2 10 16 and 34 ma a geomagnetic polarity time scale gpts is constructed following the technique of cande and kent 1992 this differs markedly from previous gpts especially in middle late miocene time the ages for the chrons in post miocene time are in agreement with values derived by the astrochronological technique refinements in this gpts await acquisition of 40ar 39ar plateau ages for field reversals in the time frames 3 8 ma and 11 15 ma

pliensbachian environmental perturbations and their potential link with volcanic activity swiss and british geochemical records abstract the mechanisms leading to environmental and climate change prior to and anticipating the well studied toarcian oceanic anoxic event are not completely understood specifically the role of the continuing break up of pangea associated palaeogeographic rearrangements and the influence of volcanic activity are less well known therefore we studied the pliensbachian interval in the open marine hemipelagic section of the breggia gorge in southern switzerland to elucidate the potential impact of these major changes on the environment and climate we identified carbon isotope excursions towards lower values in whole rock carbonates dated from the sinemurian pliensbachian boundary davoei margaritatus and spinatum zones and possibly from the pliensbachian toarcian boundary using the whole rock oxygen isotope record and phosphorus content we associated these events with phases of increased runoff possibly related to climate warming we also established the first mercury hg record for the entire pliensbachian and in order to verify its validity we analysed an additional section located along the dorset coast in southern england the presence of hg spikes common in both sections may indicate the influence of episodes of increased volcanic activity the hg spike during the earliest pliensbachian may coincide with a late phase of the central atlantic magmatic province and or increased rifting linked with the opening of the hispanic corridor further significant maxima in hg contents are observed within the davoei and margaritatus zones and both phases may correspond to early volcanic activity of the karoo ferrar large igneous province further analyses are however needed to corroborate this interpretation and evaluate the importance of organic matter scavenging and detrital input in comparison to volcanic activity

absorption and scattering in ground penetrating radar analysis of the bishop tuff 1 ground penetrating radar gpr signals are attenuated by both absorption and scattering we performed low frequency 100 mhz gpr surveys at the volcanic tableland of the bishop california tuff to evaluate the factors that control gpr depth of investigation and to develop insight into the capabilities of such radars for mars the subsurface reflection character was very different for two different commercial systems used together they revealed both internal welding contacts in the tuff and an abundance of discrete scatterers attenuation coefficients were computed from profiles that showed distributed scattering the semilogarithmic signal decay is directly analogous to seismic coda the absorption intrinsic loss was determined to be 1 db m from low frequency vertical electric soundings the residual attenuation that is the attenuation in the absence of absorption is attributed to scattering scattering attenuation of 1 db m at 25 50 mhz corresponds to mean free paths as short as 4 m a fraction of the two way propagation distances of 20 40 m therefore the bishop tuff is formally a strong scatterer to gpr the mean free path is also comparable to the subsurface radar wavelength in this case maximizing scattering loss the scatterers themselves likely originate as welding heterogeneities contrasts in dielectric constant due to density differences may be supplemented by moisture variations on mars scattering is likely to contribute significant losses to gpr signals in all but the most uniform materials and unfrozen thin films of water in the lower cryosphere could influence both absorption and scattering

fate of mantle plume material trapped within a lithospheric catchment with reference to brazil 1 local regions of thin lithosphere act as catchments of hot buoyant plume material unless replenished the trapped plume material cools by convection to the mantle adiabat by several tens of million years in particular currently hot material from the 130 ma parana starting plume head is unlikely to supply 85 ma to recent volcanism on the mainland of brazil and the martin vaz and fernando hot spots rather a plume tail may now underlie southern brazil where tomographic studies detect conduit shaped velocity anomaly through the upper mantle if the tomographic study in fact found a plume tail the track crossed the amazon rift at 85 ma and since then lateral flow along thin regions of the lithosphere from the tail fed widespread feeble volcanism including the flow line hot spots of martin vaz and fernando

radar sounding of temperate permafrost in alaska analogy to the martian midlatitude to high latitude ice rich terrains 1 radar detection of subsurface ice on mars has been widely debated in part because the dielectric signature of ice as deduced from the dielectric constant can be confused with dry silicate rich materials to identify the ice dielectric signature it is crucial to estimate the imaginary part of the dielectric permittivity inferred from the dielectric attenuation after removing the scattering loss unfortunately the latter remains poorly quantified at both mars advanced radar for subsurface and ionospheric sounding marsis and shallow subsurface radar sharad frequencies to address this ambiguity we conducted multiple frequency ground penetrating radar and resistivity investigations in well characterized temperate permafrost in fairbanks alaska the area shows several geomorphologic similarities to midlatitude and high latitude terrains on mars this approach allowed us to quantify the dielectric and scattering losses in temperate permafrost over the 10 to 1000 mhz frequency band at 20 mhz our results suggest an average dielectric loss rate of 0 25 0 03 db m whereas the corresponding average scattering loss rate is 0 94 0 37 db m the scattering loss was found to represent 69 of the total signal attenuation considering this result and the study by heggy et al 2006a in volcanic environments we revised the interpretation of the attenuation coefficient calculated from sharad data over the deuteronilus mensae region and amazonis planitia we then used the reevaluated dielectric loss to estimate the imaginary part of the dielectric permittivity our results suggest that even if deuteronilus mensae deposits and the vastitas borealis formation may have similar dielectric constants their imaginary parts are different this implies that the two regions have different bulk compositions with the former being ice rich sediments and the latter being nonconsolidated volcanic deposits

surface rupture and slip distribution of the 1940 imperial valley earthquake imperial fault southern california implications for rupture segmentation and dynamics we analyzed high resolution aerial photography taken soon after the 1940 imperial valley earthquake to provide a higher resolution distribution of displacement with which to test variation in lateral slip to compare the difference in making slip measurements parallel to local or regional fault strike and to compare to the seismo logical properties of the 1940 earthquake we performed 648 new measurements of displacement along the 15 km section of rupture for which imagery exists nearly 7 m of maximum displacement occurred within 2 km of the border which was higher than previous estimates with an average slip of about 5 5 m for this section there is considerable variation along the strike on the order of 1 m of variability across hundreds of meters of lateral distance in the section of high slip areas of larger offset tend to have greater variations in displacement from these observations we conclude that lateral variability of displacement typically varies by as much as 30 along a rupture section the difference in measured displacement at a specific point along the fault varied considerably when using local fault strikes versus that of regional or average fault however the overall average difference between measurements for different azimuths at a specific point is only 0 01 0 19 m indicating that this is not a major issue if one is consistent finally abrupt changes in displacement along a strike are apparently related to segmentation and the rupture process and mimic the subevents as documented through seismological methods the largest moment pulse corresponds to the section south of the border whereas the greatest displacements occurred north of the border where the average moment release is inferred to be less we explain this by inferring a strong shallow as perity in the border region consistent with 1979 rupture event beneath it online material high resolution imagery of the 1940 rupture with locations of offset measurements and tabulated offset values

thermal history of potential gas reservoir rocks in the eastern parnaiba basin brazil the parnaiba basin is a major intracratonic sedimentary basin in brazil with unconventional petroleum systems as a potential natural resource formed by the influence of igneous intrusions to constrain the thermal history of unexplored potential reservoir rocks in the eastern part of the parnaiba basin sedimentary rocks near intrusions were analyzed by petrography and thermochronology apatite fission track aft and zircon fission track zft dating petrography shows grain dissolution and carbonate pore filling generated by thermal destabilization of feldspars the aft results indicate partial annealing by the last magmatic event in the basin and zft results show ages of maximum paleotemperature compatible with the sardinha magmatic event 120 130 ma in thermal history models rocks close to intrusions experienced maximum paleotemperatures above 300 c which is higher than what is considered favorable for reservoir rocks hydrothermal fluids modified the diagenetic evolution of the succession by dissolving and precipitating carbonate cement in pore spaces in the studied area at distances greater than 50 m 164 ft from the intrusion the rocks were not substantially altered and in terms of paleotemperature they can be considered potentially viable reservoirs the thermal history analysis of potential tight gas sandstone reservoirs affected by intrusive bodies is valuable for characterizing the thermal enhancement or destruction of the reservoir s qualities

exploring morphology layering and formation history of linear terrestrial dunes from radar observations implications for titan abstract understanding the morphology and internal layering of large aeolian dune fields through radar observations can provide unique insights into the climatic and geophysical conditions that led to their formation in this study we perform a large scale characterization of the morphology and internal layering of linear dunes in hyper arid areas on earth through utilizing multiple complementary radar datasets sir c imaging srtm interferometry derived elevations and radar sounding or ground penetrating radar gpr linear dune fields in the egyptian desert are of special interest due to their significance as planetary analogs to dunes on mars and saturn s largest moon titan satellite radar imagery and elevation data of the region show significant variance in the geomorphology of different dune fields in egypt in addition gpr probing of the first few meters suggests different inner settings in the layering of dunes of different ages in eastern and western egypt reflecting different paleo climatic regimes that led to their formation furthermore our radiometric analysis suggests that dunes with different inner layering arrangement also exhibit different radar backscatter returns as a function of their heights for relatively younger dunes with a homogeneous inner structure like the ones in eastern egypt in the qattaniya dune field we observe that sigma0 does not change as a function of the dune height for relatively older dunes in western egypt like the great sand sea northern siwa and southern dune fields we observed a linear correlation between sigma0 and the dune height thus surface properties of dunes like morphology and backscatter variation with height are related to inner characteristics like arrangement of internal layering relative ages and can be used to infer their depositional history linear dunes discovered in the equatorial regions of titan by the cassini huygens mission are morphologically very similar to these linear dune fields in the egyptian sahara hence assessing the variability of morphology and radar backscatter properties of titan s dunes as a function of their heights can help constrain the ambiguities associated with their internal structure and formation history and provide insights into titan s paleo wind regimes

the anari and tapirapua jurassic formations western brazil paleomagnetism geochemistry and geochronology the anari and tapirapua formations are very similar from the point of view of paleomagnetic geochronological and geochemical results they date from around 197 ma and the flows are mainly tholeiitic basalts with a low tio2 and incompatible element content the magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites maghemites and probably titanomaghemites paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95 confidence level the calculated mean for these poles is 250 3 e 65 5 s n 15 a95 3 6 ksc 1578 this pole is compatible with jurassic poles which have been determined for south america or transposed from africa

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

liquefaction potential assessment of pleistocene beach sands near charleston south carolina abstract liquefaction potential of four pleistocene beach sand deposits in the greater charleston area south carolina is assessed the assessment is based on a review of 51 sites of conspicuous craterlets and horizontal ground displacement that occurred in beach sand deposits during the 1886 charleston earthquake and an analysis of 82 seismic cone penetration tests with pore pressure measurements of the 51 ground failure sites 23 are associated with the ten mile hill beds 13 with the wando formation 13 with the silver bluff terrace and younger deposits that lie adjacent to the harbor rivers and creeks and two with the ladson formation liquefaction potential is analyzed using the seismic cone data with and without correction for age related processes diagenesis and then expressed in terms of the liquefaction potential index lpi probability curves are developed from the lpi calculations for different earthquake ground shaking parameters the probability curves for the wando formation overpred

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

influence of nonplastic fines on shear wave velocity based assessment of liquefaction many false positives no liquefaction detected when the normalized shear wave velocity cyclic stress ratio vs1 csr combination indicated that it should have been are observed in the database used in the simplified liquefaction assessment procedure based on shear wave velocity two possible reasons for false positives are the presence of a thick surface layer of nonliquefiable soil and the effects of fines on cyclic shear resistance crr and vs1 about 67 of the false positives that could not have been caused by an overlying thick surface layer are associated with silty sands with less than 35 fines the effects of fines on the liquefaction resistance of silty sands and on the shear wave velocity are analyzed theoretical crrfield versus vs1 curves for silty sands containing 0 to 15 nonplastic fines are established they show that the theoretical crr vs1 correlations for silty sands with 5 to 15 nonplastic fines are all located to the far left of the semi empirical curves that separate liquefac

cyclic shear strain needed for liquefaction triggering and assessment of overburden pressure factor kσ abstractthis paper has two objectives the first is to evaluate the cyclic shear strain needed to trigger liquefaction cl in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio crr the second objective is to explore the effect of a high effective overburden pressure v0 on the value of crr the first objective is accomplished mainly by using an equation relating cl and crr valid for shear wave velocity based liquefaction charts this equation is supplemented with laboratory results from undrained cyclic strain controlled tests as well as large scale and centrifuge model shaking experiments it is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude mw 7 5 cl 0 03 with this value increasing to cl 0 3 in some cases to 0 6 in denser and overconsolidated preshaken and compacted sands these small values of cl in the field are controlled by two factors excess pore pressure buildup in the

implications of seismic reflection and potential field geophysical data on the structural framework of the yucca mountain crater flat region nevada seismic reflection and gravity profiles collected across yucca mountain nevada together with geologic data provide evidence against proposed active detachment faults at shallow depth along the pre tertiary tertiary contact beneath this potential repository for high level nuclear waste the new geophysical data show that the inferred pre tertiary tertiary contact is offset by moderate to high angle faults beneath crater flat and yucca mountain and thus this shallow surface cannot represent an active detachment surface the reflection lines reveal that the amargosa desert rift zone is an asymmetric half graben having a maximum depth of about 4 km and a width of about 25 km the east dipping bare mountain fault that bounds this graben to the west can be traced by seismic reflection data to a depth of at least 3 5 km and possibly as deep as 6 km with a constant dip of 64 degree 5 degree along the profile the transition from east to west dipping faults occurs at or just west of the solitario canyon fault which bounds the western side of yucca mountain the interaction at depth of these east and west dipping faults having up to hundreds of meters offset is not imaged by the seismic reflection profile understanding potential seismic hazards at yucca mountain requiresmore knowledge of the subsurface geometry of the faults near yucca mountain since earthquakes generally nucleate and release the greatest amount of their seismic energy at depth the geophysical data indicate that many fault planes near the potential nuclear waste facility dip toward yucca mountain including the bare mountain range front fault and several west dipping faults east of yucca mountain thus earthquake ruptures along these faults would lie closer to yucca mountain than is often estimated from their surface locations and could therefore be more damaging less

evidence of magmatic activity related to middle jurassic and lower cretaceous rifting from northeastern brazil ceara mirim k ar age palaeomagnetism petrology and sr 1bnd isotope characteristics the mesozoic magmatic activity in the easternmost part of ne brazil ceara mirim is mainly represented by two pyroxene tholeiitic dykes only few dykes have alkaline character k ar ages and palaeomagnetism data indicate that ceara mirim dykes are of middle jurassic 175 160 ma and early cretaceous 140 130 ma age both middle jurassic and lower cretaceous dykes have high incompatible element concentrations and tio2 with the lower contents generally confined to the middle jurassic dykes rare tholeiitic dykes low in tio2 and incompatible elements are present sr 1bnd isotopic and other chemical data do not support appreciable crustal contamination and in a 87sr86sr vs 143nd144nd diagram almost all the dykes plot in a narrow area of the enriched quadrant of the mantle array tholeiitic and alkaline ceara mirim dykes are related to different parent magmas probably originating by different degrees of melting of garnet peridotite mantle sources characterized by distinct sr 1bnd isotope compositions and small scale chemical heterogeneities ceara mirim tholeiitic dykes show close mineralogical chemical and isotopic similarities with the lower cretaceous high tio2 tholeiites from maranhao sardinha intrusives and parana northern province basins in general chemical and sr 1bnd isotope data for high and low tio2 brazilian mesozoic tholeiites appear to be related to time integrated enriched and depleted mantle components respectively subcontinental mantle heterogeneity is believed to be due to variable enriched components related to metasomatic processes the ceara mirim jurassic dykes correlate well in terms of composition and tectonic setting with the analogue tholeiitic dyke swarms from the benue trough and can be related to the opening of the central atlantic ocean the lower cretaceous ceara mirim magmatic activity corresponds to the coeval one of the benue trough and can be related to the early rifting events responsible for the opening of the equatorial domain of the south atlantic whose oceanic crust formed between 120 and 100 ma

low frequency electromagnetic exploration for groundwater on mars 1 water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near ideal exploration target on mars for lowfrequency diffusive electromagnetic methods models of the temperature and frequency dependent electrical properties of rock ice water mixtures are used to predict the electromagnetic response of the martian subsurface detection of ice is difficult unless it is massively segregated in contrast liquid water profoundly affects soundings and even a small amount of adsorbed water in the cryosphere can be detected subcryospheric water is readily distinguishable at frequencies as low as 100 hz for fresh water to 10 mhz for brines these responses can be measured using either natural or artificial sources ulf signals from solar wind and diurnal heating perturbations of the ionosphere are likely and disturbances of regional crustal magnetic fields may also be observable spherics or elf vlf signals from lightning discharge would provide optimal soundings however lightning may be the least likely of the possible natural sources among the active techniques only the time domain electromagnetic tdem method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more a ground or aircraft based tdem system of several kilograms can detect water to a depth of several hundred meters and a system of tens of kilograms featuring a large fixed rover or ballistically deployed loop can detect water to several kilometers depth index terms 0694 electromagnetics instrumentation and techniques 0925 exploration geophysics magnetic and electrical methods 6225 planetology solar system objects mars 5494 planetology solid surface planets instruments and techniques keywords mars water electromagnetic magnetotelluric tdem

evidence for errors in the geomagnetic polarity time scale at 17 15 ma 40ar 39ar dating of basalts from the pacific northwest usa 40ar 39ar incremental heating studies have been carried out on whole rock basalt samples from the columbia river basalt crb and the steens basalt to obtain crystallization ages with a standard error 1 these high precision accurate ages are utilized with the known magnetostratigraphy of the crb to test the accuracy of the early to middle miocene section of the geomagnetic polarity time scales currently in use we conclude that these time scales underestimate the ages of various chrons within this time range by 0 9 m y

on the age and morphology of the reunion event data pertinent to the age of the normal polarity reunion event or subchron and whether it is a split feature are critically examined new 40 ar 39 ar age determinations on lavas from two sequences on reunion island along with available potassium argon k ar 40 ar 39 ar and astrochronological analyses indicate an age of 2 15 2 13 ma although the possibility cannot be completely ruled out at present there is no convincing evidence supporting the contention of a short duration of normal polarity some 70 90 kyr later

miocene magmatism of sinai related to the opening of the red sea extension associated with the initial early miocene stages of opening of the red sea resulted in intrusion of a widespread system of dikes and smaller bodies primarily along the northeastern margin of the red sea gulf of suez axis dikes some up to more than 800 km in length were emplaced dominantly parallel to the red sea gulf of suez from the sinai peninsula to yemen intrusive rocks are mainly basaltic in composition but minor granophyres are also present in the sinai this magmatic event is about 20 m y old dikes of the southern and southeastern sinai crop out in precambrian terrane in the central and northern sinai they occur in phanerozoic terrane where they may be associated with linear grabens and cryptovolcanic features basaltic dikes of the sinai underwent variable degrees of alteration alteration phases include seriate quartz smectite chlorite and serpentine x ray diffraction and petrographic analysis indicate that in spite of extensive alteration primary minerals still control the major element compositions of the rocks basalts are tholeiitic in composition with sio2 48 5 to 50 and na2o k2o 3 to 3 5 rb and sr concentrations are probably dominated by alteration and by introduction of secondary material primarily carbonate zr tio2 which is relatively unchanged during alteration substantiates that the basaltic rocks are subalkaline initial 87sr 86sr determined after leaching of samples is approximately 0 7043 0 7063 much higher than red sea axial trough basalts 0 703 we infer that these sinai basalts equilibrated at depths of 35 50 km possibly within an enriched mantle whether the main trough of the red sea is underlain by extended and modified continental crust or by oceanic crust is controversial since miocene dikes were intruded along both coasts but primarily the saudi arabian coast of the red sea gulf of suez axis in a zone up to 150 km wide they may also underlie the main trough of the red sea and result at least partially in the discontinuous magnetic and gravity anomalies observed there possibly the abundance of dikes beneath the main trough is much greater than adjacent to the red sea giving rise to properties intermediate between those of continental and oceanic crust downwarping and widespread intrusion of dikes along the red sea 18 26 m y ago probably signified crustal dilation and the initiation of a continental rift because tholeiitic basalts of the sinai are higher in 87sr 86sr than red sea axial trough basalts we infer that the conditions of their origin must also be very different we suggest that the sinai basalts may have been generated within but near the base of subcrustal continental lithosphere perhaps indicating thermal thinning of lithospheric mantle above upwelled asthenosphere associated with continental rifting this inference if correct would indicate that the lithosphere was at least 35 50 km thick during the early miocene phase of rifting but possibly not much thicker since dikes farthest from the red sea are apparently identical in composition to those closest asthenospheric upwarping must have been broad enough that all magmas were derived from approximately the same depth in the yemen ethiopia areas volcanism began earlier about 30 m y ago and was much more voluminous than elsewhere along the red sea gulf of suez axis suggesting the presence of underlying mantle upwelling accompanying rifting of the red sea in spite of differences in age and volume the presence of large volumes of tholeiites among the miocene basalts of the yemen ethiopia areas suggests that the lithosphere was similarly thinned along the entire length of the red sea

clast size controls and longevity of pleistocene desert pavements at lathrop wells and red cone volcanoes southern nevada formation of desert pavement and accretionary soils are intimately linked in arid environments well sorted fallout scoria lapilli at lathrop wells 75 80 ka and red cone ca 1 ma volcanoes southern nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular a av horizons variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for av development contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes this indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

40ar 39ar ages of flood basalt provinces in russia and china and their possible link to global faunal extinction events a cautionary tale regarding alteration and loss of 40ar abstract the hypothesis that the permo triassic boundary ptb mass extinctions were caused by flood basalt volcanism in russia siberian traps and or china the emeishan traps is investigated from the point of view of time of occurrence 40 ar 39 ar ages numerous published ages in the literature are rejected as good estimates of the time of crystallization the filters applied in this respect are a statistical reliability of plateau isochron sections of stepheating data and b the alteration state of the material that was dated alteration appears to be ubiquitous unsurprising since most of the material dated was used without acid leaching a procedure that is effective in yielding fresh er samples of 70 ages in the literature for the main pulse of siberian trap volcanism less than ten prove to be reliable ages similar techniques applied to 40 ar 39 ar for the emeishan traps leaves only a single reliable age for the magmatic episode these ages are compared to both published and new 40 ar 39 ar ages for the ptb as based on analysis of minerals from critical ash beds in china there is good overlap in the ages ptb 250 0 0 1 ma siberian trap lavas 250 1 0 4 ma lending credence to a genetic link between the formation of the siberian traps and the faunal extinction event at the ptb a similar link for the formation of the viluy traps russia and the late devonian extinction event is investigated only a single reliable 40 ar 39 ar age is available for the viluy traps and falls close to the interpolated age for the frasnian fammenian boundary the use of the unspiked k ar technique to yield accurate ages for such altered samples is questioned a review of u pb data pertinent to these problems suggests a close temporal link between the formation of the siberian traps and the ptb comparison of u pb and 40 ar 39 ar ages for the ptb raises questions about the accuracy of high precision sanidine ages possibly resulting from very slow leakage of 40 ar from this mineral

quaternary basin evolution and basaltic volcanism of crater flat nevada from detailed ground magnetic surveys of the little cones ground magnetic surveys of the quaternary 0 77 0 98 ma little cones and surrounding areas of southern crater flat nevada reveal detailed images of buried basaltic lava flows not evident from surface exposures magnetic maps derived from these surveys show an apron of buried lava extending radially up to 300 m from the center of the two cones modeling of the anomalies indicates burial of the lava apron by up to 15 m of alluvium on top of approximately 10 m thick lava flows from these results we estimate a volume 0 01 km3 of volcanic material for the little cones comparable to the more completely exposed and similarly aged red and black cones of central crater flat partial burial of the little cones by alluvium in contrast to the exposed flows associated with red and black cone suggests differential subsidence of the southern part of the basin in the quaternary this pattern of subsidence is consistent with recent structural studies that show increased slip toward the south on major normal fault

2 d geoelectrical model for the parnaiba basin conductivity anomaly of northeast brazil and tectonic implications abstract a magnetometer array study in the north northeast of brazil has revealed a roughly ne sw trending conductive structure in the southeastern part of the intracratonic parnaiba basin the magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation the 2 d numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body the anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data the graben structure in the basement is corroborated by the aeromagnetic data the formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the brasiliano orogeny or with the jurassic cretaceous magmatic events the diabase dikes intruded in the basin in association with the jurassic cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting paleozoic sediments the thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon saturated sediments

a reconnaissance 40ar39ar geochronologic study of ore bearing and related rocks siberian russia abstract 40 ar 39 ar age spectra of biotite from a mineralized vein in the ore bearing noril sk i intrusion and from picritic like gabbrodolerite from the weakly mineralized lower talnakh intrusion show that these bodies were emplaced at 249 2 ma which is not significantly different from the age of the permian triassic boundary the ore bearing intrusions postdate the lower third of the flood basalt sequence in the noril sk area and on the basis of geochemistry can best be correlated with lavas slightly younger than those which they cut thus flood basalt was erupted at the time of the permian triassic mass extinction event although its role in this event is as yet ill defined additional new 40 ar 39 ar age data for a group of intrusive and extrusive rocks on the western margin of the siberian craton indicate that mafic magmatism extended over a period of several tens of million years whereas paleomagnetic data suggest that the bulk of the siberian flood basalt sequence near noril sk has been erupted in only a million years or so 40 ar 39 ar ages of plagioclase from early flood basalt flows are about 2 younger than those obtained for biotite from the crosscutting noril sk i intrusion probably because of slight alteration and argon loss from the plagioclase

multiphase magmatic flows at yucca mountain nevada 1 the proposed yucca mountain radioactive waste repository is sited in southern nevada in a region that has experienced sporadic basaltic volcanism since the late miocene volcanic risk assessment for the proposed repository requires estimating the consequences of a new monogenetic volcano intersecting the underground facility during its 104 106 year performance period we report numerical studies aimed at understanding the range of processes and dynamic parameter values that could accompany intersection of an open repository drift by a volatile rich trachybasaltic magma as it ascends in a dike we focus on one end member type of magmatic behavior namely a fragmented magmatic mixture under pressure interacting with an underground cavity initial and boundary conditions are based upon field data and previous modeling studies of the interaction between vertically propagating dikes and a repository opening the calculations are two dimensional and time dependent and are conducted with the multiphase hydrodynamics code gmfix calculations indicate that gas particle mixtures as they rise from below and interact with horizontal openings form complex flow patterns involving varying degrees of recirculation and deposition of pyroclasts dynamic pressures are up to 106 pa but are more typically on the order of 103 to 104 pa the geometry and number of outlets play a key role in determining the types of flow patterns as do volatile contents and the degree of fragmentation the detailed numerical simulations provide information that will be used to confirm the adequacy of simplified probabilistic consequence models used in risk assessments

40ar 39ar age of the lathrop wells volcanic center yucca mountain nevada paleomagnetic and 40 ar 39 ar analyses from the lathrop wells volcanic center nevada indicate that two eruptive events have occurred there the ages 136 8 and 141 9 thousand years ago for these two events are analytically indistinguishable the small angular difference 4 7 between the paleomagnetic directions from these two events suggests they differ in age by only about 100 years these ages are consistent with the chronology of the surficial geological units in the yucca mountain area these results contradict earlier interpretations of the cinder cone geomorphology and soil profile data that suggest that at least five temporally discrete eruptive events occurred at lathrop wells approximately 20 000 years ago

a short interval of jurassic continental flood basalt volcanism in antarctica as demonstrated by 40ar39ar geochronology a continental flood basalt province the ferrar group kirkpatrick basalt and ferrar dolerite crops out along 3000 km of the transantarctic mountains in antarctica and is temporally related to the break up of gondwanaland although a wide range of dates between 90 and 193 ma have been published for the kirkpatrick basalt it is now recognized that the young dates reflect non ideal behavior of ar in the matrix in order to refine the geochronology feldspar separates have been analyzed by the 40ar39ar incremental heating method the main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt basalt samples have been studied from the three principal outcrop areas yielding the following apparent ages central transantarctic mountains 176 8 0 5 ma south victoria land 176 4 0 4 ma north victoria land 176 6 0 7 ma ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct the data imply that the eruptive activity which produced the kirkpatrick basalt occurred within a short interval of less than about 1 m y at 176 6 1 8 ma over an area which included more than 1200 km of the transantarctic mountains the jurassic volcanism in antarctica represents a short episode of magmatism comparable in duration with other well dated continental flood basalt provinces the linearly extensive outcrop of the ferrar province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism the poorly constrained age of the bajocian bathonian boundary makes the previously suggested connection between ferrar volcanism and an extinction event at that boundary uncertain

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

mafic dykes intrusive into pre cambrian rocks of the sao luis cratonic fragment and gurupi belt parnaiba province north northeastern brazil geochemistry sr nd pb o isotopes 40ar 39ar geochronology and relationships to camp magmatism abstract dykes of diabase and microgabbro intruded into pre cambrian rocks of the sao luis cratonic fragment and gurupi belt which are tectonic and erosive windows of the parnaiba basin in north northeastern brazil ar ar ages were determined and major trace element and nd sr pb o isotopic compositions of these dykes were measured to provide insights into their age and into the nature of their mantle sources and petrogenetic processes the data have also been used to compare the chemical and isotopic signatures of the dykes with those of the central atlantic magmatic province camp four chemical groups of mafic dykes have been identified these comprise two subtypes of high ti rocks i hti 1 tio 2 2 47 wt ii hti 2 tio 2 2 7 wt sio 2 47 wt in addition to iii evolved high ti tio 2 4 wt sio 2 of 46 wt and iv low si tio 2 2 2 wt sio 2 40 ar 39 ar geochronology of plagioclase returned ages of 201 4 ma and 193 10 ma for the hti 2 subtype and of 201 2 ma and 207 9 ma for the evolved high ti group the hti 1 and low si groups presented highly disturbed age spectra and did not allow the definition of their emplacement ages the argon data indicate an age 200 ma for the low si group and are dubious with respect to the age of thehti 1 subtype if coeval with i e 200 ma or older than the hti 2 and evolved high ti types all groups present 18 o values of pyroxene that are compatible with uncontaminated mantle derived magmas the hti 1 subtype average 143 nd 144 nd 200 0 512644 87 sr 86 sr 200 0 7035 206 pb 204 pb of 17 86 shows the less enriched and less fractionated more primitive trace element distribution of all groups the hti 2 subtype shows enriched trace element pattern and depleted nd sr signature 143 nd 144 nd 200 0 512610 87 sr 86 sr 200 0 7037 and average 206 pb 204 pb ratios of 17 23 the evolved high ti chemical group shows average ratios of 143 nd 144 nd 200 0 512558 87 sr 86 sr 200 0 7035 and 206 pb 204 pb of 16 88 and the more enriched trace elements signature among the four groups the chemical and isotopic compositions and trends of the hti 1 hti 2 ehti types are consistent with their derivation from an asthenosphere derived parental magma further modified by differentiation and minimal crust contamination higher in the hti 2 and ehti types and by the derivation of one type from another via fractional crystallization these high and evolved high tio 2 types show ages and some chemical and isotopic features that are consistent with those of the camp magmatism some differences found are ascribed to petrogenetic processes such as magma differentiation a combination of warming of the mantle and edge driven convection beneath the pangea supercontinent after the closure of the neoproterozoic brasiliano pan african orogenies in the ediacaran cambrian boundary might have triggered the magmatic event the low si type shows paired ta nb and zr hf depletions and depleted sr nd average 143 nd 144 nd 200 0 512687 87 sr 86 sr 200 0 703 and enriched pb 206 pb 204 pb of 18 66 isotopic compositions that may be interpreted to result either from interaction of a subcontinental lithospheric mantle with products of an earlier subduction or by contamination of the mantle derived magma during ascent and emplacement in the continental crust it is hypothesized that these dykes were emplaced in the ediacaran cambrian boundary after the neoproterozoic orogeny that built up the gurupi belt and in the early extensional stages that preceded the formation of the parnaiba basin

stratigraphic profiling with ground penetrating radar in permafrost a review of possible analogs for mars 1 we review our past and ongoing use of ground penetrating radar to investigate permafrost in alaska and in the dry valleys of antarctica the results may be relevant to radar efforts on mars because of arid conditions and the presence of ice the pulses were centered at 50 100 and 400 mhz we interpret profiles from two sites in the eastern taylor valley to show glaciolacustrine and glaciofluvial stratigraphy the maximum depth of stratigraphy profiled there was about 33 m near fairbanks alaska the depth of penetration at 50 mhz was near 80 m in marginally frozen and stratified alluvial sands at the fairbanks sites supplementary drilling was required to differentiate between reflections from conductive bedrock a graphitic schist and those from the water table at depths of 20 25 m at a site on the north slope of alaska we profiled present and remnant freezing fronts in an alluvial floodplain the relative permittivity at most sites ranged between about 4 and 5 5 which is consistent with dry conditions the mineralogy and low ice content weak interface reflectivity or the lack of further interfaces may have limited the interpretation of maximum penetration where no water table was present because signal absorption should have been low and scarce diffractions imply that scattering was weak the interface reflectivities beneath taylor valley may be a function of only density contrasts since free water and possibly ice is absent

trace element variations in atlantic ocean basalts and proterozoic dykes from northwest scotland their bearing upon the nature and geochemical evolution of the upper mantle abstract trace element data on ocean floor and ocean island basalts from the atlantic and from early proterozoic dyke swarms are considered in relation to the nature and causes of upper mantle heterogeneity studies of dredged and drilled basalts from the north atlantic have enabled the compositions of erupted ridge basalts to be monitored in space and time and have revealed the following features a that while melting processes may give rise to appreciable ree variation at a particular site ratios of the more incompatible elements remain remarkably consistent in basalts erupted at a particular ridge segment for tens of millions of years and b that there are major differences in incompatible element ratios in basalts erupted at different ridge segments the relationships suggest that the volumes of mantle with these characteristic incompatible element ratios must be large but that the processes associated with basalt generation are not capable of significantly changing incompatible element ratios nevertheless the processes which have produced these incompatible element variations must be incorporated into any model for mantle evolution our preferred model invokes both crustal extraction and incipient mantle melting as the major mechanisms for changing incompatible element mantle source regions both represent very small degrees of mantle melting a depleted upper mantle variably veined with incompatible element enriched liquids or fluids derived through incipient melting at greater depths is most consistent with the trace element and isotopic compositions of north atlantic basalts many of which are not normal mid ocean ridge basalts n type morb ocean island basalts may represent more intensely veined and in some cases at least more refractory sources sampled at lower degrees of partial melting mantle from which incipient melts have been removed probably represents the n type morb source extraction of crustal material having high contents of the large ion lithophile elements but low contents of the high field strength ions ie ta and nb from a veined metasomatized mantle source leaves the upper mantle residue enriched in ta and nb relative to the large ion lithophile elements this might explain the enrichment of ta and nb in most mantle sources of continental and ocean island basic lavas mantle sources enriched by incipient melting processes are continually dissipated by mantle convection but may be preserved for a significant time beneath stable continents trace element data on early proterozoic tholeiitic to picritic dyke swarms from scotland suggest that short term mantle heterogeneities may have existed beneath the region at that time and could have been related to the processes of crustal generation 600 m y earlier

mars a water rich planet abstract mars had outgassed at least 0 5 to 1 km of water 10 to 20 bar of co2 and 0 1 to 0 3 bar of n2 the volatiles that have been retained are mostly in the cratered uplands terrain softening fretted channels debris flows and closed depressions indicate that at least the upper 2 km of the cratered uplands at high latitudes 30 contain ice in amounts that exceed the porosity estimated to be 10 20 theoretical studies and lack of these features in the cratered uplands at low latitudes suggest that the upper 1 km of the uplands at low latitudes is ice poor however valley networks indicate that water was present near the surface early in the planet s history although in amounts smaller than at high latitudes the entire upper 1 km planetwide is estimated to have contained 75 125 m of water at the end of heavy bombardment the largest sink for water is the megaregolith below 1 km episodic eruption of water from the deep megaregolith cut many of the large outflow channels from the volume of water needed to cut the circum chryse channels and assuming uniform planetwide distribution of water the deep megaregolith is estimated to have contained at least 350 m of water at the end of heavy bombardment thereby giving a total minimum inventory of 424 475 m planetwide most of the water lost from the low latitude uplands by diffusion and in cutting the valley networks is now believed to be in the polar layered terrains most of the water involved in cutting the outflow channels is in the low lying northern plains where a variety of features that have been attributed to ground ice is present a large fraction of the planet s surface has been overplated with water poor volcanics of which we have samples in the snc meteorites the younger volcanics have reacted extensively with the old volatile rich basement some of the co2 and n2 outgassed was lost during heavy bombardment by impact erosion of the atmosphere and other processes the remaining was fixed carbonates and folded deep into the megaregolith during heavy bombardment

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

investigation of the age and migration of reversing dunes in antarctica using gpr and osl with implications for gpr on mars gpr provides high resolution images of aeolian strata in frozen sand in the mcmurdo dry valleys of antarctica the results have positive implications for potential gpr surveys of aeolian strata on mars within the lower victoria valley seasonal changes in climate and a topographically constrained wind regime result in significant wind reversals as a consequence dunes show reversing crest lines and flattened dune crests ground penetrating radar gpr surveys of the dunes reveal sets of cross strata and low angle bounding surfaces produced by reversing winds summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation solar radiation which heats the valley floor melts ice cements making sand available for transport at the same time solar heating of the valley floor generates easterly winds that transport the sand contributing to the resultant westward dune migration the location of the dune field along the northern edge of the lower victoria valley provides some shelter from the powerful foehn and katabatic winds that sweep down the valley topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation migration and preservation of the dune field optically stimulated luminescence osl ages from dune deposits range from 0 to 1 3 kyr showing that the dune field has been present for at least 1000 yr the osl ages are used to calculate end point migration rates of 0 05 to 1 3 m yr which are lower than migration rates reported from recent surveys of the packard dunes and lower than similar sized dunes in low latitude deserts the relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment

3 d 40ar39ar geochronology in the parana continental flood basalt province new 40ar39ar analyses of borehole samples from the parana etendeka continental flood basalt province provide critical evidence of eruption rates in unexposed regions when combined with surface samples the new data clarify the duration of this major volcanic province new ages from both surface and borehole samples confirm a 10 12 million year duration of magmatism which post dates the tithonian mass extinction and provide a unique 3 dimensional picture of the spatial and temporal eruption of the lava pile chemically defined magma types are diachronous and the onset of magmatism occurred 500 1000 km inland migrating southeast towards the incipient south atlantic ocean calculated eruption rates increased from 0 03 km3 yr 1 between 138 and 135 ma through 0 13 km3 yr 1 between 135 and 133 ma to 0 21 km3 yr 1 between 133 and 131 ma supporting the notion that the rate of magmatism increased with time and proximity to the developing south atlantic after 131 ma the continental eruption rates dropped dramatically to 0 01 km3 yr 1 presumably because rifting had occurred and eruption centred in the oceanic rio grande rise with eruption rates of around 0 3 0 5 km3 yr 1 given the available constraints on the potential temperature of the plume thickness of the continental lithosphere and degrees of extension through time predicted melt production rates within the plume seriously underestimate the calculated eruption rates on the continent however the eruption rates can be reconciled if melting occurred by conductive heating of volatile enriched mantle the diachronous nature of the magma types supports the idea that melting occurred over a wide area and that the different magma types reflect different source regions rather than the temporal evolution of magmas from a single source given that 10 million years is appropriate to the timescales for conductive heating above the mantle plume it is suggested that these distinct source regions were located within the lithospheric mantle consistent with geochemical studies

critical evaluation of the age of the deccan traps india implications for flood basalt volcanism and faunal extinctions attempts to evaluate the possible link between flood basalt volcanism and the faunal extinctions observed at the k t boundary must rely on precise knowledge of the amount of such volcanism that took place 65 ma the current geochronological data do not permit close resolution of either the timing or the duration of deccan volcanism the age of the bulk of the material is placed in the range 70 60 ma further resolution of this hypothesis awaits acquisition of high quality 40 ar 39 ar plateau ages on key sections of the deccan traps

centrifuge and large scale modeling of seismic pore pressures in sands cyclic strain interpretation abstractcentrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large scale experiment in large scale test sg 1 a 5 6 m thick mildly sloping deposit of hydraulic fill clean ottawa sand of dr 40 was subjected to 5 s of low intensity base shaking 0 02g that induced excess pore pressures short of liquefaction three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large scale test one of these centrifuge simulations used the same ottawa sand and dr 40 of the prototype a viscous pore fluid and dry pluviation deposition which created a soil fabric stiffer than the prototype the other two centrifuge simulations used silty sand saturated with water the pore pressure buildup in one of the silty sand tests was in good agreement with the prototype while the other two centrifuge deposits did not develop any excess pore pressure the various responses in the four tests are expla

shear wave velocity based probabilistic and deterministic assessment of seismic soil liquefaction potential abstractshear wave velocity vs offers a means to determine the seismic resistance of soil to liquefaction by a fundamental soil property this paper presents the results of an 11 year international project to gather new vs site data and develop probabilistic correlations for seismic soil liquefaction occurrence toward that objective shear wave velocity test sites were identified and measurements made for 301 new liquefaction field case histories in china japan taiwan greece and the united states over a decade the majority of these new case histories reoccupy those previously investigated by penetration testing these new data are combined with previously published case histories to build a global catalog of 422 case histories of vs liquefaction performance bayesian regression and structural reliability methods facilitate a probabilistic treatment of the vs catalog for performance based engineering applications where possible uncertainties of the variables comprising both the seismic demand an

the deccan trap cretaceous paleogene boundary connection new 40ar 39ar ages and critical assessment of existing argon data pertinent to this hypothesis abstract the cretaceous paleogene boundary kpgb was dated by the 40ar 39ar method herein from the western interior of north america at 65 48 0 12 ma 1 in good agreement with other recent published estimates for the deccan traps india new argon ages as well as others available in the literature are assessed for reliability based on a statistical reliability of plateau isochron sections and b freshness of material dated utilizing the alteration index method from tholeiitic lavas from the composite western ghats section cwgs only six ages are found to be reliable estimates of the time of crystallization these ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale gpts at 67 64 ma alkaline rocks from the anjar area of kutch provide three reliable ages that suggest a hiatus in lava extrusion around kpgb for the rajahmundry basalts the upper flow s age defines its formation during chron 29n a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the gpts the reliable lava ages indicate the most voluminous reversed polarity sections of the cwgs were extruded at a time statistically indistinguishable from that of the kpgb the deccan trap kpgb faunal extinction hypothesis remains plausible but must compete with the latest report favoring a very close temporal connection 0 03 m y between the chixculub impact crater and the kpgb

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

lithosphere asthenosphere interaction and the origin of cretaceous tholeiitic magmatism in northeastern brazil sr nd pb isotopic evidence abstract rio ceara mirim swarm forms a 400 km long mainly e w trending dykes which have been emplaced in northeastern brazil during the opening of south and equatorial atlantic oceans at early cretaceous this swarm is dominated by high and low ti tholeiitic dykes showing em type chemical characteristics such as i strong to moderate enrichment in large ion lithophile and high field strength elements and ii radiogenic sr 0 70713 to 0 70473 and non radiogenic nd 0 512494 to 0 512 and pb 206pb 204pb 18 77 to 18 34 isotopic compositions they are compatible with the melting of an old tdm ages ca 1 0 ga enriched lithospheric mantle source indicating that both proterozoic and mesozoic related mantle processes accounted for the genesis of continental tholeiitic magmatism conversely a distinct chemical component comparable to himu type has been identified in local alkaline sub swarm such dykes have depleted sr 0 70367 0 7031 and nd 0 5129 0 512772 and strongly radiogenic 206pb 204pb 19 95 ratios the source of the rio ceara mirim basalts appears to be related to melting of an enriched lithospheric mantle triggered by a long lived thermal anomaly it could be related either to deep st helena mantle plumes or convective cells induced by lithospheric discontinuities

liquefaction resistance of a silty sand deposit subjected to preshaking followed by extensive liquefaction abstractthe effect of extensive liquefaction on the liquefaction resistance of heavily preshaken saturated silty sand is studied using a centrifuge experiment the base of a 6 m homogeneous deposit was subjected to a total of 91 shaking events of different horizontal base accelerations and durations three event types were used in alternating patterns mild preshaking events a stronger preshaking events b and strong liquefying events c the experiment was divided in two stages in stage one reported in a previous publication 66 preshaking events a and b were applied in stage two which is the focus of this paper 25 additional shakings were applied which included two events c in addition to continuing the same pattern of events a and b stage one resulted in a significant increase in liquefaction resistance of the deposit in stage two extensive liquefaction produced by events c resulted in a dramatic immediate reduction in liquefaction resistance of the deposit to a level comparable to that before

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the

ground penetrating radar geologic field studies of the ejecta of barringer meteorite crater arizona as a planetary analog 1 ground penetrating radar gpr has been a useful geophysical tool in investigating a variety of shallow subsurface geological environments on earth here we investigate the capabilities of gpr to provide useful geologic information in one of the most common geologic settings of planetary surfaces impact crater ejecta three types of ejecta are surveyed with gpr at two wavelengths 400 mhz 200 mhz at meteor crater arizona with the goal of capturing the gpr signature of the subsurface rock population in order to ground truth the gpr characterization subsurface rocks are visually counted and measured in preexisting subsurface exposures immediately adjacent to and below the gpr transect the rock size frequency distribution from 10 to 50 cm based on visual counts is well described by both power law and exponential functions the former slightly better reflecting the control of fragmentation processes during the impact ejection event gpr counts are found to overestimate the number of subsurface rocks in the upper meter by a factor of 2 3x and underestimate in the second meter of depth 0 6 1 0x results attributable to the highly scattering nature of blocky ejecta overturned ejecta that is fractured yet in which fragments are minimally displaced from their complement fragments produces fewer gpr returns than well mixed ejecta the use of two wavelengths and division of results into multiple depth zones provides multiple aspects by which to characterize the ejecta block population remote gpr measurement of subsurface ejecta in future planetary situations with no subsurface exposure can be used to characterize those rock populations relative to that of meteor crater