acoustics variability of air gun signals recorded at intermediate ranges within the lau basin 1 during january february 2009 an active source seismic survey was performed over the eastern lau spreading center in the lau back arc basin 21 s 176 s acoustic signals generated by ther v langseth s 36 gun pneumatic source array were recorded within the deep sound channel at offsets of 29 416 km the local ocean acoustic environment is everywhere bottom limited with seafloor depths within the study domain ranging from 1700 2800 m low frequency 4 125 hz sound levels are monitored using root mean square energy flux density and zero to peak measurement techniques from these field data transmission loss is found to exceed the predictions of a geometric spherical spreading model at similar ranges arrival amplitudes vary by up to 20 db and durations vary by a factor of three to six the depth of the seafloor beneath the air gun source exhibits a positive correlation with arrival duration and a negative correlation with range corrected amplitude explaining up to 30 of the observed variation in both parameters the strength of this correlation however varies for stations lying at different azimuths highlighting the importance of seafloor aspect and slope in the coupling of bottom interacting acoustic energy into the sound channel range dependent ray tracing shows that shots deployed over shallower seafloor are more likely to produce sound channel trapped signals that propagate with limited bottom interaction this results in arrivals that are more impulsive with shorter durations and higher amplitudes shots deployed in deeper water typically undergo a larger number of bounces and are characterized by more emergent longer duration and smaller amplitude arrivals

global distribution of large lunar craters implications for resurfacing and impactor populations by using high resolution altimetric measurements of the moon we produced a catalog of all impact craters 20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics the most densely cratered portion of the highlands reached a state of saturation equilibrium large impact events such as orientale basin locally modified the prebasin crater population to 2 basin radii from the basin center basins such as imbrium orientale and nectaris which are important stratigraphic markers in lunar history are temporally distinguishable on the basis of crater statistics the characteristics of pre and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the orientale basin event

a monte carlo study of parameters affecting computer simulations of crater saturation density computer models of cratered surfaces often use inputs of uncertain nature and importance this work evaluates the sensitivity of the resulting crater saturation estimates to the input parameters principally applicable to the study of craters upward from 8 km diameter in order of decreasing importance crater saturation simulations are found to be sensitive to 1 the dynamic range of crater diameters used 2 the effectiveness of ejecta blanket obliteration assumed and 3 the number of points taken to describe the crater rim the size of the largest crater in proportion to the size of the simulated surface has no effect on the results when the edges of the simulated surface are correctly treated and craters are not counted simply by integers craters should be counted by their fractions lying within the simulated area a similar procedure is recommended when gathering crater size density data from images

bombardment history of asteroid 4 vesta recorded by brecciated eucrites large impact event clusters at 4 50 ga and discreet bombardment until 3 47 ga abstract the thermal impact histories of sixteen eucrite meteorites were investigated three monomict eucrites nwa 999 a 87272 87 and stannern five polymict eucrites nwa 1000 nwa 1666 nwa 5601 y 980066 100 and y 980255 100 three quench textured eucrite melt rocks y 981646 21 y 981651 105 and mil 0766214 one eucrite dominantly comprised of quench textured clasts que 99005 11 three unclassified eucrite breccias lap 031316 9 lar 06870 5 que 99799 4 and one unbrecciated eucrite eet 92004 17 included here due to its shock features we have measured fifteen high precision new 40 ar 39 ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias these ages range from 4534 56 ma to 4491 16 ma resulting in a concordant age population p 0 16 the fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components leads us to propose that those ages recorded a single heating event on a large scale and is interpreted here as a high energy impact event early in the history of vesta at 4500 4 ma we propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material holsapple et al 2002 and references therein the other analyses define a spread of five plateau ages ranging from 3851 21 ma to 3469 35 ma over 380 ma an additional apparent plateau age of 4288 38 ma but with a diffusion profile of cumulative 39 ar release along with published u pb apatite age of 4 14 ga suggests that the data might either define a true continuum normal background bombardment from 4 5 ga to 3 47 ga or cluster between 3 85 ga and 3 47 ga excavation of a fresh surface at 3 85 ga continuously bombarded until 3 47 ga both scenarios are compatible with a final ejection age of 3 47 ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid where the brecciated eucrites stayed relatively well protected from subsequent major impacts based on these results and recent crater counting measurements we propose that the excavation and bulk ejection were caused by the rheasilvia ca 3 47 ga basin forming impact diffusion models on plagioclase crystals with different ar age spectrum signatures from a single breccia nwa1666 4501 7 ma suggest that either 1 the formation of the breccia is very young or 2 different plagioclase crystals have different diffusion characteristics and or 3 the porosity caused heterogeneous temperatures during an impact heating event particularly likely if the 4 5 ga brecciated eucrites were stored in a rubble pile asteroid many or possibly most large asteroids being re accumulated rubble piles with potential large porosity holsapple et al 2002 scenarios 2 and 3 preclude the usage of multi grain aliquots to decipher the time temperature history of most impact breccia using 40 ar 39 ar thermochronology

the geologically recent giant impact basins at vesta s south pole since 17 july 2011 nasa s spacecraft dawn has been orbiting the asteroid vesta the second most massive and the third largest asteroid in the solar system see the cover russell et al p 684 1 use dawn s observations to confirm that vesta is a small differentiated planetary body with an inner core and represents a surviving proto planet from the earliest epoch of solar system formation vesta is also confirmed as the source of the howardite eucrite diogenite hed meteorites jaumann et al p 687 2 report on the asteroid s overall geometry and topography based on global surface mapping vesta s surface is dominated by numerous impact craters and large troughs around the equatorial region marchi et al p 690 3 report on vesta s complex cratering history and constrain the age of some of its major regions based on crater counts schenk et al p 694 4 describe two giant impact basins located at the asteroid s south pole both basins are young and excavated enough amounts of material to form the vestoids a group of asteroids with a composition similar to that of vesta and hed meteorites de sanctis et al p 697 5 present the mineralogical characterization of vesta based on data obtained by dawn s visual and infrared spectrometer revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle the global color variations detailed by reddy et al p 700 6 are unlike those of any other asteroid observed so far and are also indicative of a preserved differentiated proto planet 1 lookup doi 10 1126 science 1219381 2 lookup doi 10 1126 science 1219122 3 lookup doi 10 1126 science 1218757 4 lookup doi 10 1126 science 1223272 5 lookup doi 10 1126 science 1219270 6 lookup doi 10 1126 science 1219088

geologic mapping of ejecta deposits in oppia quadrangle asteroid 4 vesta abstract oppia quadrangle av 10 288 360 e 22 is a junction of key geologic features that preserve a rough history of asteroid 4 vesta and serves as a case study of using geologic mapping to define a relative geologic timescale clear filter images stereo derived topography slope maps and multispectral color ratio images from the framing camera on nasa s dawn spacecraft served as basemaps to create a geologic map and investigate the spatial and temporal relationships of the local stratigraphy geologic mapping reveals the oldest map unit within av 10 is the cratered highlands terrain which possibly represents original crustal material on vesta that was then excavated by one or more impacts to form the basin feralia planitia saturnalia fossae and divalia fossae ridge and trough terrains intersect the wall of feralia planitia indicating that this impact basin is older than both the veneneia and rheasilvia impact structures representing pre veneneian crustal material two of the youngest geologic features in av 10 are lepida 45 km diameter and oppia 40 km diameter impact craters that formed on the northern and southern wall of feralia planitia and each cross cuts a trough terrain the ejecta blanket of oppia is mapped as dark mantle material because it appears dark orange in the framing camera clementine type color ratio image and has a diffuse gradational contact distributed to the south across the rim of rheasilvia mapping of surface material that appears light orange in color in the framing camera clementine type color ratio image as light mantle material supports previous interpretations of an impact ejecta origin some light mantle deposits are easily traced to nearby source craters but other deposits may represent distal ejecta deposits emplaced 5 crater radii away in a microgravity environment

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

mega ejecta on asteroid vesta 1 asteroid 4 vesta sometimes called the smallest terrestrial planet will be orbited next july by nasa s dawn mission this will be the first time a small planet is visited by a spacecraft and novel geological structures are expected a key issue regarding vesta mean diameter 530 km is to what extent its geology is dominated by the 460 km diameter impact basin on its southern hemisphere we model the basin s formation using a very high resolution 3d smooth particle hydrodynamics simulation to establish some of the major impact related aspects of vesta s geology the goal is to provide a framework for interpreting anticipated observations of landforms a collision of this magnitude a 50 km diameter impactor at 5 km s exposes many deep strata from within the planet and offsets the center of mass by 10 km from the center of figure vesta spins every 5 3 hr so that a hemispheric scale impact evolves in a non inertial frame and deposits variably shaped multiply folded and abruptly terminated ejecta sequences of regional scale since little of this ejecta would have been molten these massive sequenced deposits could be mistaken in images of vesta for other geologic forms such as thrusts and folds detailed mapping and the piecing together of mega ejecta via impact models will enable an informed understanding of the interior geology of vesta

vesta surface thermal properties map the first ever regional thermal properties map of vesta has been derived from the temperatures retrieved by infrared data by the mission dawn the low average value of thermal inertia 30 10 j m 2 s 0 5 k 1 indicates a surface covered by a fine regolith a range of thermal inertia values suggesting terrains with different physical properties has been determined the lower thermal inertia of the regions north of the equator suggests that they are covered by an older more processed surface a few specific areas have higher than average thermal inertia values indicative of a more compact material the highest thermal inertia value has been determined on the marcia crater known for its pitted terrain and the presence of hydroxyl in the ejecta our results suggest that this type of terrain can be the result of soil compaction following the degassing of a local subsurface reservoir of volatiles

madagascar a continental fragment of the paleo super dharwar craton of india the morphological kinship of madagascar to its immediate neighbors on the west african continent and east indian subcontinent during the early and middle cretaceous has been debated for the past two decades on the basis of available geologic tectonic and paleomagnetic information most of the paleoreconstructions of madagascar have shown its attachment to the east african continent we present magnetic satelite and gravity data and morphological geophysical and geotectonic similarities to hypothesize that in the period before the breakup of gondwana madagascar was a continental fragment of the paleo super dharwar craton of india

emplacement ages of paleoproterozoic mafic dyke swarms in eastern dharwar craton india implications for paleoreconstructions and support for a 30 change in dyke trends from south to north abstract large igneous provinces lips and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents the dharwar craton of southern peninsular india represents a substantial portion of archean crust and has been considered to be a principal constituent of superia sclavia nuna columbia and rodinia supercontinents the craton is intruded by numerous regional scale mafic dyke swarms of which only a few have robustly constrained emplacement ages through this study the lip record of the dharwar craton has been improved by u pb geochronology of 18 dykes which together comprise seven generations of paleoproterozoic dyke swarms with emplacement ages within the 2 37 1 79 ga age interval from oldest to youngest the new ages integrated with u pb ages previously reported for the hampi swarm define the following eight swarms with their currently recommended names ne sw to ese wnw trending ca 2 37 ga bangalore karimnagar swarm n s to nne ssw trending ca 2 25 ga ippaguda dhiburahalli swarm n s to nnw sse trending ca 2 22 ga kandlamadugu swarm nw se to wnw ese trending ca 2 21 ga anantapur kunigal swarm nw se to wnw ese trending ca 2 18 ga mahbubnagar dandeli swarm n s nw se and ene wsw trending ca 2 08 ga devarabanda swarm e w trending 1 88 1 89 ga hampi swarm nw se ca 1 79 ga pebbair swarm comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features such as regional dharwar greenstone belts and the late archean ca 2 5 ga closepet granite batholith have led to the hypothesis that the northern dharwar block has rotated relative to the southern block by restoring a 30 counter clockwise rotation of the northern dharwar block relative to the southern block we show that pre 2 08 ga arcuate and fanning dyke swarms consistently become approximately linear two possible tectonic models for this apparent bending and concomitant dyke rotations are discussed regardless of which deformation mechanisms applies these findings reinforce previous suggestions that the radial patterns of the giant ca 2 37 ga bangalore karimnagar dyke swarm and probably also the ca 2 21 ga anantapur kunigal swarm may not be primary features

vesta s shape and morphology vesta s surface is characterized by abundant impact craters some with preserved ejecta blankets large troughs extending around the equatorial region enigmatic dark material and widespread mass wasting but as yet an absence of volcanic features abundant steep slopes indicate that impact generated surface regolith is underlain by bedrock dawn observations confirm the large impact basin rheasilvia at vesta s south pole and reveal evidence for an earlier underlying large basin veneneia vesta s geology displays morphological features characteristic of the moon and terrestrial planets as well as those of other asteroids underscoring vesta s unique role as a transitional solar system body

airgun inter pulse noise field during a seismic survey in an arctic ultra shallow marine environment offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and or behavioral disturbances however few studies have investigated the resulting multipath propagation and reverberation from airgun pulses this research uses continuous acoustic recordings collected in the arctic during a low level open water shallow marine seismic survey to measure noise levels between airgun pulses two methods were used to quantify noise levels during these inter pulse intervals the first based on calculating the root mean square sound pressure level in various sub intervals is referred to as the increment computation method and the second which employs the hilbert transform to calculate instantaneous acoustic amplitudes is referred to as the hilbert transform method analyses using both methods yield similar results showing that the inter pulse sound field exceeds ambient noise levels by as much as 9 db during relatively quiet conditions inter pulse noise levels are also related to the source distance probably due to the higher reverberant conditions of the very shallow water environment these methods can be used to quantify acoustic environment impacts from anthropogenic transient noises e g seismic pulses impact pile driving and sonar pings and to address potential acoustic masking affecting marine mammals

preliminary anisotropy of magnetic susceptibility studies on 2367 ma bangalore karimnagar giant dyke swarm southern india implications for magma flow abstract anisotropy of magnetic susceptibility ams fabrics have been studied for the first time on a well constrained paleoproterozoic bangalore karimnagar giant mafic dyke swarm at ca 2367 ma in the dharwar craton south india a total number of 43 samples from nine sites representing five n e trending dykes have been subjected to ams and conventional rock magnetic studies the magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are 10 3 si which is further supported by isothermal remanent magnetization irm studies hysteresis loops isothermal remanent magnetisation irm thermomagnetic properties and optical observations indicate that pseudo single domain psd low titanium magnetite particles dominate the low field susceptibility of samples the magnetic fabrics results correspond well with the comprehensive field studies however the degree of anisotropy results ranges from 0 4 to 12 8 with a mean total anisotropy of 4 7 the principal ams axes for samples from all the sites are tightly clustered on equal area plots and two main fabrics were revealed in the bangalore karimnagar giant dyke swarm in the first the maximum susceptibility axes is sub horizontal and parallel to the dyke walls minimum susceptibility axes plot near the pole of the dyke it is interpreted as the primary magma flow direction a second fabric with maximum susceptibility axis is sub horizontal and perpendicular to the dyke walls and is interpreted as a result of vertical compaction at a late stage of crystallization preliminary results suggest that dyke propagation in the bangalore karimnagar giant dyke swarm preserve a flow induced fabric slightly upwards to the ne and magma flow is sub horizontal

the violent collisional history of asteroid 4 vesta vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids the dawn spacecraft extensively imaged vesta s surface revealing a collision dominated history results show that vesta s cratering record has a strong north south dichotomy vesta s northern heavily cratered terrains retain much of their earliest history the southern hemisphere was reset however by two major collisions in more recent times we estimate that the youngest of these impact structures about 500 kilometers across formed about 1 billion years ago in agreement with estimates of vesta asteroid family age based on dynamical and collisional constraints supporting the notion that the vesta asteroid family was formed during this event

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

magnetic versus crystallographic fabrics in a basaltic lava flow combined anisotropy of magnetic susceptibility ams and crystallographic studies were performed within a quaternary lava flow for which we have a thorough knowledge of the flow direction microscope examinations microprobe analyses and studies of rock magnetic properties show that ams is carried by homogeneous multidomain ti rich titanomagnetite xc0 6 this mineral forming either subhedral isolate individuals or grouped in various elongated aggregates represents a late crystallized interstitial phase ams determined from samples collected in the lower the middle and the upper part of the flow is characterized by a well defined magnetic foliation whereas magnetic lineation is generally poorly grouped and therefore meaningless the foliation plane of the lower and upper part of the flow show an obliquity which is interpreted as an opposite imbrication indicating a westward flow in agreements with the known flow direction in the lower part of the lava flow the tight correlation between ams and plagioclase lattice preferred orientation suggests that a silicate framework control the titanomagnetite crystallization and thus the orientation of the ams ellipsoid d 2005 elsevier b v all rights reserved

impact excavation on asteroid 4 vesta hubble space telescope results hubble space telescope images of asteroid 4 vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole color measurements within the 13 kilometer deep crater are consistent with excavation deep into a high calcium pyroxene rich crust or olivine upper mantle about 1 percent of vesta was excavated by the crater formation event a volume sufficient to account for the family of small vesta like asteroids that extends to dynamical source regions for meteorites this crater may be the site of origin for the howardite eucrite and diogenite classes of basaltic achondrite meteorites

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

smnd pbpb and rbsr geochronology and petrogenesis of the mafic dyke swarm of mahbubnagar south india implications for paleoproterozoic crustal evolution of the eastern dharwar craton abstract the sm nd rb sr and pb pb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the mahbubnagar swarm have been investigated this dyke swarm intrudes the granitic basement of the cuddapah basin in the eastern dharwar craton edc the sm nd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 64 ma and an initial nd value of 0 4 0 3 the sm nd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 123 ma with a negative initial nd value of 1 8 1 4 rb sr and pb pb internal isochron ages of three gabbros and a pb pb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 ma and within the limits of experimental error broadly conform to the sm nd isochron age by contrast rb sr internal isochron ages of three other samples range between 1474 and 1952 ma and one sample yields a pb pb internal isochron age of 3446 ma showing the effects of partial isotopic resetting possibly due to hydrothermal alteration these data suggest that mafic dykes of the mahbubnagar swarm were emplaced at c 2170 ma marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the cuddapah basin different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and or crystallization of the progressively evolved batches of magma during a relatively short time interval the initial nd values calculated at 2170 ma range from 0 6 to 2 0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of edc the initial 87 sr 86 sr ratios 0 7027 0 7064 and model 1 values 8 3 8 5 of apparently undisturbed samples are also consistent with this suggestion

anisotropy of magnetic susceptibility in the ponta grossa dyke swarm brazil and its relationship with magma flow direction abstract measurements of anisotropy of magnetic susceptibility ams in 95 mafic dykes mainly tholeiites 10 200 m in width from the mesozoic ponta grossa swarm southern brazil revealed two main types of magnetic fabric type i fabric plane k 1 k 2 parallel to the dyke plane represents magma flow within the dykes whereas type ii plane k 1 k 3 parallel to the dyke plane is compatible with a fabric pattern reflecting vertical compaction of the magma column fabric type i dominates 51 of the dykes within the swarm whereas type ii 38 of the dykes concentrates mainly in the western region where the dykes intrude sediments considering the dykes with type i fabrics it is concluded that 58 of the dykes were fed by horizontal or sub horizontal k 1 inclinations less than 30 magmatic flow and 42 were fed by inclined to vertical k 1 inclinations more than 30 magma flow the latter are more frequent in the southeastern part of the swarm suggesting a magma source close to this area although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

rock magnetic stratigraphy of a mafic layered sill a key to the karoo volcanics plumbing system abstract the insizwa sill is an 1 km thick subhorizontal layered mafic intrusion and part of the karoo large igneous province in south africa this well exposed intrusion consists of several superimposed petrologically and geochemically distinct units magnetic methods were used to study the intrusion in order to constrain the physical processes active in these types of bodies during crystallization rock magnetism studies indicate that within different petrologic units bulk susceptibility is controlled by primary magnetite with minor pyrrhotite and or paramagnetic minerals olivine pyroxene new magnetic data based on 659 specimens obtained from 3 vertical borehole cores each spaced 5 km apart confirm the prominent vertical zonation in low field magnetic susceptibility k lf degree of anisotropy p j and orientation of the anisotropy of magnetic susceptibility ams axes the magnetic susceptibility correlates very well with petrographic units and the lateral continuity of magnetic units between boreholes is very consistent petrologic units with high but variable k lf also show moderate anisotropy and dominantly vertical foliations we interpret these patterns to result from inverse fabrics from single domain magnetite the degree of anisotropy is low in petrologic units with low k lf which also show shallowly dipping magnetic foliations we interpret that the magnetic properties of these units are dominated by the paramagnetic minerals these low k lf petrologic units also show no systematic increase in k lf suggesting that only minor differentiation is occurring in these units the dataset derived from 2 surface sampling traverses are consistent with borehole core ams data showing a pattern of dominantly steep magnetic foliation and variably plunging magnetic lineation with a nw se trend

paleomagnetic study of french guyana early jurassic dolerites hypothesis of a multistage magmatic event a detailed paleomagnetic and anisotropy of magnetic susceptibility ams study was carried out on 34 sites of early jurassic dolerite dykes from french guyana which formed during the initial opening of the central atlantic ocean four types of ams fabrics are recognized i normal fabric 21 dykes defined by clustering of k1 k2 axes on the dyke plane whereas the k3 axis is nearly perpendicular to it this fabric is interpreted as due to magma flow the subhorizontal inclination of the k1 axis permitted to suggest that the french guyana dykes could be fed by horizontal magma fluxes from a distant magma source ii reversal fabric 8 dykes is characterized by the k2 k3 plane close to the dyke plane and the k1 perpendicular to dyke orientation such fabric was attributed to the local shearing stress iii intermediate fabric 1 dyke is defined by k1 k3 axes close to the dyke plane and k2 axis is perpendicular to this plane it was interpreted as due to vertical compaction of a static magma column iiii other fabric 4 dykes does not show any preferential orientation scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers two magnetic components were isolated ages of magnetic remanences are estimated at 198 3 2 0 ma to 192 3 1 5 ma their virtual geomagnetic poles are calculated pole a v 73 2 n p 15 3 e k 288 8 a95 3 4 n 8 and pole b v 81 6 n p 89 1 e k 69 8 a95 4 2 n 18 these two groups probably correspond to two distinct magmatic events which occurred in a short period this hypothesis is consistent with published 40 ar 39 ar radiometric ages though with mini plateau spectra these paleomagnetic results suggest the presence of magmatic pulses led to the construction of the central atlantic magmatic province in french guyana during the early jurassic fl 2000 elsevier science b v all rights reserved

spectroscopic characterization of mineralogy and its diversity across vesta the mineralogy of vesta based on data obtained by the dawn spacecraft s visible and infrared spectrometer is consistent with howardite eucrite diogenite meteorites there are considerable regional and local variations across the asteroid spectrally distinct regions include the south polar rheasilvia basin which displays a higher diogenitic component and equatorial regions which show a higher eucritic component the lithologic distribution indicates a deeper diogenitic crust exposed after excavation by the impact that formed rheasilvia and an upper eucritic crust evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta this is broadly consistent with magma ocean models but spectral variability highlights local variations which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene cumulates overall vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle

pb and nd isotope constraints on the origin of high mg and tholeiitic amphibolites kolar schist belt south india the late archean north south trending kolar schist belt in south india 4 km wide by 80 km long is thought to be a suture between two gneiss terranes krogstad et al 1989 within this volcanics dominated belt are recognized both tholeiitic and high mg komatiitic and picritic amphibolites which make up some 70 and 5 respectively of the exposed outcrops a massive tholeiitic amphibolite separates the belt into western and eastern parts a pb pb whole rock age of 2732 155 ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock samples of this rock have nd values at 2700 ma that range between 3 8 and 6 8 1 initial 238u 204pb of 7 5 and k1 initial 232th 238u of about 4 two different types of high mg amphibolites are recognized from the western part of the belt a picritic or p type and a komatiitic or k type the p type have highly variable ce al ratios all greater than chondritic nd yb ratios greater than chondritic nd at 2700 ma of 1 5 to 8 and pb isotope compositions variable in 207pb 204pb with 1 of about 8 0 and k1 of about 4 the trace element data suggest that the light ree enrichment is a character of the mantle source and is not due to residual garnet the k type amphibolites have near chondritic ce al and nd yb ratios nd at 2700 ma of 1 5 to 8 and 1 of about 8 and k1 of about 4 although the p type is light ree enriched compared to the k type both types have similar ce nd ratios as well as initial pb and nd isotopes if the 2696 136 ma age for the sm nd isochron which includes both types of high mg amphibolite has any significance it dates the time of light ree enrichment of the mantle source for the p type komatiitic amphibolites the high mg amphibolites in the eastern part of the belt are light ree enriched have pb isotopic compositions that are variable in 207pb 204pb with a 1 about 8 5 and nd at 2700 ma of 1 8 to 4 5 hydrothermal fluids associated with metamorphism and shearing prior to about 2400 ma ago were responsible for the introduction of gold quartz carbonate veins into the kolar schist belt the pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous pb from adjacent older granitoid gneisses into the amphibolites which could be responsible for the variability in the 207pb 204pb ratios of the samples this extraneous pb probably is not responsible for the distinct pb isotope character of each type of amphibolite

a petrogenetic relationship between 2 37 ga boninitic dyke swarms of the indian shield evidence from the central bastar craton and the ne dharwar craton abstract the indian shield is cross cut by a number of distinct paleoproterozoic mafic dyke swarms the density of dykes in the dharwar and bastar cratons is amongst the highest on earth globally boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the southern indian shield geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes sio2 51 5 to 55 7 wt mgo 5 8 to 18 7 wt and tio2 0 30 wt to 0 77 wt from the central bastar craton bhanupratappur and the ne dharwar craton karimnagar a single u pb baddeleyite age from a boninitic dyke near bhanupratappur yielded a weighted mean 207pb 206pb age of 2365 6 0 9 ma that is within error of boninitic dykes from the dharwar craton near karimnagar 2368 5 2 6 ma and farther south near bangalore 2365 4 1 0 ma to 2368 6 1 3 ma rhyolite melts modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel garnet transition zone the nd isotopes end t 6 4 to 4 5 of the bhanupratappur dykes are more variable than the karimnagar dykes end t 0 7 to 0 6 but they overlap the variability of sr nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source the chemical and temporal similarities of the bhanupratappur dykes with the dykes of the dharwar craton karimnagar penukonda chennekottapalle indicate they are members of the same giant radiating dyke swarm moreover our results suggest that the bastar and dharwar cratons were adjacent but likely had a different configuration at 2 37 ga than the present day it is possible that the 2 37ga dyke swarm was related to a mantle plume that assisted in the break up of an unknown or poorly constrained supercontinent

olivine on vesta as exogenous contaminants brought by impacts constraints from modeling vesta s collisional history and from impact simulations the survival of asteroid vesta during the violent early history of the solar system is a pivotal constraint on theories of planetary formation particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts as this constrains both the interior structure of vesta and the number of major impacts the asteroid suffered during its life the nasa dawn mission revealed that olivine is present on vesta s surface in limited quantities concentrated in small patches at a handful of sites not associated with the two large impact basins rheasilvia and veneneia the first detections were interpreted as the result of the excavation of endogenous olivine even if the depth at which the detected olivine originated was a matter of debate later works raised instead the possibility that the olivine had an exogenous origin based on the geologic and spectral features of the deposits in this work we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years to perform this study we took advantage of the impact contamination model previously developed to study the origin and amount of dark and hydrated materials observed by dawn on vesta a model we updated by performing dedicated hydrocode impact simulations we show that the exogenous delivery of olivine by the same impacts that shaped the vestan surface can offer a viable explanation for the currently identified olivine rich sites without violating the constraint posed by the lack of global olivine signatures on vesta our results indicate that no mantle excavation is in principle required to explain the observations of the dawn mission and support the idea that the vestan crust could be thicker than indicated by simple geochemical models based on the howardite eucrite diogenite family of meteorites

predictions for impactor contamination on ceres based on hypervelocity impact experiments new experiments predict that ceres should be extensively contaminated with meteoritic debris derived from the asteroid belt all types of impactors likely contribute to the contamination ceres may accrete debris more efficiently if it is ice rich because of enhanced projectile survival and retention in porous ice targets experiments indicate that if a silicate regolith lag protects subsurface ice then some of the projectile should be injected into the regolith during high angle impacts thereby hiding part of the projectile component from view if impacts excavate ice sublimation will gradually concentrate projectile relics into a surficial lag in contrast if the near surface lacks ice then accreted meteoritic debris should be distributed throughout a vertically mixed regolith high resolution images may reveal pristine projectile relics lining some crater floors moreover we predict that the surface of ceres is not exclusively endogenic and may be dominated by delivered exogenic debris

the onset of the lunar cataclysm as recorded in its ancient crater populations abstract the earliest bombardment history of the moon potentially provides powerful constraints for solar system evolution models a major uncertainty however is how much of this history is actually recorded in lunar craters for example some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes others believe that most lunar craters and large basins were formed in a narrow time interval between 3 8 and 4 0 ga the so called lunar cataclysm in the light of recent improvements in our understanding of early solar system evolution it is possible that the contributions from both scenarios could be represented in the lunar crater record if so when did the declining bombardment end and the lunar cataclysm begin here we show using new counts of 15 150 km diameter craters on the most ancient lunar terrains that the craters found on or near nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various pre nectarian era terrains this dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits which may have strongly modified the source of lunar impactors this work also suggests that the lunar cataclysm may have started near the formation time of nectaris basin this possibility implies that south pole aitken basin spa the largest lunar basin and one of the oldest by superposition was not created during the cataclysm this view is strengthened by our interpretation that a substantial fraction of ancient craters on spa were made by low velocity impactors finally we believe these results shed new light on the impact history of the primordial earth

a 2082 ma radiating dyke swarm in the eastern dharwar craton southern india and its implications to cuddapah basin formation abstract using consistent paleomagnetic data together with precise pb pb baddeleyite ages on a series of mafic dykes occurring over an area of at least 70 000 km2 a 2081 8 1 1 ma weighted mean of 4 dykes dyke swarm was identified intruding the archean basement rocks in the eastern dharwar craton skirting the cuddapah basin on its north northwest and western flanks the geometry of these dykes collectively due to their progressive variation in trend from n134 w to n28 e defines a fan angle of about 162 and forms a spectacular radiating swarm converging toward a focal point beneath the cuddapah basin anisotropy of magnetic susceptibility ams investigations on these dykes based on the orientation of principal eigenvectors indicate magma flow was vertically upward in them suggesting the magma source of these dykes to be proximal to the sampling sites these features together with reported geophysical evidence for high density material below the cuddapah basin suggest that this mafic volcanic province probably formed due to the impact of an asthenospheric mantle upwelling perhaps triggered by a plume head or other causal mechanisms like global warming of mantle or small scale instability like edge driven convection this may have resulted in the domal uplift of the continental lithosphere large scale crustal extension and thinning followed by thermal relaxation and subsidence that may have been responsible for the formation of the intra cratonic cuddapah basin shortly after 2082 ma dyke swarms of this age fort frances dykes 2076 5 4 ma or of slightly younger lac esprit dykes 2069 1 ma and older cauchon lake dykes 2091 1 1 8 2 1 ma ages are fairly wide spread in the superior province however a reconstruction of the paleopositions of dharwar cuddapah dykes 38 n 180 e a95 4 and superior at 2080 ma using paleomagnetic data fort frances dykes p 43 n lp 184 e does not suggest a close proximity for these provinces at that time their disparate locations could therefore suggest these were two distinct nodes of wide spread magmatism between 2080 ma and 2065 ma

precise pb baddeleyite geochronology by the thermal extraction thermal ionization mass spectrometry method abstract the ability to easily obtain precise 0 1 age determinations on mafic rocks can facilitate identification of contemporaneous magmatic events across continents and hence open new avenues of research investigations related to large scale mantle processes we present high precision pb age determinations on baddeleyite zro 2 a common accessory mineral in mafic rocks using the recently developed thermal extraction thermal ionization mass spectrometer te tims procedure for dating of zircon in order to constrain the conditions for thermal extraction of pb from baddeleyite a sequence of sem back scattered electron images were acquired along with energy dispersive x ray spectroscopy spot analysis of baddeleyite grains embedded in silica beads at progressively increasing temperature between 1200 and 1590 c replicate analyses of twelve preheated at 1250 c and silica embedded baddeleyite grains from the paleoproterozoic phalaborwa carbonatite using this procedure have yielded a 207 pb 206 pb weighted mean age of 2060 3 0 4 ma consistent with previously reported 207 pb 206 pb ages for this carbonatite occurrence determined by the conventional u pb isotope dilution thermal ionization mass spectrometry id tims technique fourteen baddeleyite fractions from a large paleoproterozoic n s striking mafic dyke swarm in the dharwar craton yielded a 207 pb 206 pb weighted mean age of 2215 9 0 3 ma also consistent with previously determined conventional ages all data were corrected for mass fractionation of 0 18 amu as determined previously on zircon both data sets scatter outside measurement errors implying an external 2 sigma error of 0 055 in the case of the carbonatite and 0 043 for the mafic dyke which we suggest represents the reproducibility of the fractionation correction our results demonstrate that thermal extraction 207 pb 206 pb ages are comparable with the conventional u pb ages in both accuracy and precision this procedure does not need ultra clean laboratory conditions

magma flow inferred from ams fabrics in a layered mafic sill insizwa south africa the insizwa sill is a 25 km diameter 1000 m thick layered mafic intrusion part of the karoo igneous province in south africa the peridotitic and gabbronoritic rocks are undeformed and mineral fabrics demonstrably result from magma flow a horizontal centimeter scale model layering is visible in numerous outcrops plagioclase crystals are both tabular and elongated their preferred orientation parallel to the layering forms a foliation and a nw se lineation respectively interpreted as the magma flow plane and flow direction throughout the 78 stations of this study 699 specimens magnetic susceptibilities km range from 750 to 10 000 10 6 si the magnetic anisotropy pj ranges from 1 03 to 1 08 magnetic ellipsoids are both prolate and oblate average tjc0 anisotropy of magnetic susceptibility ams fabrics are dominated by multidomain to pseudo single domain magnetite high field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50 for low susceptibility rock types the anisotropy results from magnetite grain shape solely as shown by no significant increase in pj with increasing km the magnetic lineation 305j 0 5j is consistent throughout the sill at various scales and coincides with the mineral lineation in average in contrast the magnetic foliation 125j ne 10j is generally perpendicular to the mineral foliation and to the layering several explanations for this odd configuration are discussed the variations of magnetic parameters across the layering and field observations point to a multiple injection the magnetic lineation is consistent with the presence of a single feeder dike situated to the se of the sill d 2002 elsevier science b v all rights reserved

vesta vestoids and the howardite eucrite diogenite group relationships and the origin of spectral differences spectra of asteroid 4 vesta and 21 small estimated diameters less than 10 km asteroids with vesta like spectral properties vestoids were measured at visible and near infrared wavelengths similar to0 44 to similar to1 65 mum all of the measured small asteroids except for 2579 spartacus have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from vesta none of the observed asteroids have spectra similar to diogenites we find no spectral distinction between the 15 objects tabulated as members of the vesta dynamical family and 6 of the 7 sampled non family members that reside just outside the semi major axis a eccentricity e and inclination i region of the family the spectral consistency and close orbital a e i match of these non family objects to vesta and the vesta family imply that the true bounds of the family extend beyond the subjective cut off for membership asteroid 2579 spartacus has a spectrum consistent with a mixture of eucritic material and olivine spartacus could contain olivine rich material from vesta s mantle or may be unrelated to vesta altogether laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes finer particle sizes lead to an increase in the slope which is usually referred to as reddening this range of spectral variation for the best known meteoritic analogs to the vestoids regardless of whether they are actually related to each other suggests that the extremely red spectral slopes for some vestoids can be explained by very fine grained eucritic material on their surfaces

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

geologic map of the northern hemisphere of vesta based on dawn framing camera fc images abstract the dawn framing camera fc has imaged the northern hemisphere of the asteroid 4 vesta at high spatial resolution and coverage this study represents the first investigation of the overall geology of the northern hemisphere 22 90 n quadrangles av 1 2 3 4 and 5 using these unique dawn mission observations we have compiled a morphologic map and performed crater size frequency distribution csfd measurements to date the geologic units the hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to 200 km in diameter the most widespread unit is a plateau cratered highland unit similar to although of lower elevation than the equatorial vestalia terra plateau large scale troughs and ridges have regionally affected the surface between 180 e and 270 e these tectonic features are well developed and related to the south pole veneneia impact saturnalia fossae trough unit elsewhere on the hemisphere they are rare and subdued saturnalia fossae cratered unit in these pre rheasilvia units we observed an unexpectedly high frequency of impact craters up to 10 km in diameter whose formation could in part be related to the rheasilvia basin forming event the rheasilvia impact has potentially affected the northern hemisphere also with s n small scale lineations but without covering it with an ejecta blanket post rheasilvia impact craters are small

the quest for regolithic howardites part 2 surface origins highlighted by noble gases we report noble gas data of helium he neon ne argon ar krypton kr and xenon xe cosmic ray exposure cre ages and nominal gas retention k ar u th he ages for seven howardites cre 01400 eet 87513 eet 87518 eet 99400 gro 95535 gro 95602 san 03472 in continuing research to identify regolithic samples and better understand the vestan regolith in our previous work we found little correlation between suggested regolith parameters of ni 300 g g al 2o 3 8 9 wt and eucrite diogenite e d ratio of 2 1 warren et al 2009 and trapped solar wind sw fractionated solar wind fsw or planetary noble gas components from impacted material noble gas indicators of a regolithic origin here we have expanded our data set to include samples outside of these parameters to further explore composition and the differences in ni content as indicators for the presence of non vesta material in addition our sample set includes two potentially paired meteorites from the gro suite finally in our petrographic studies the samples selected showed no evidence for carbonaceous chondrite fragments which should reduce the effect of contamination by planetary noble gas components and will allow us to better identify sw fsw components where present of the samples studied here three howardites gro 95535 gro 95602 and eet 87513 show evidence for a regolithic origin with both isotopic and element noble gas ratios clearly pointing to the presence of trapped components similar to sw fsw or planetary the two gro howardites gro 95535 and gro 95602 show similar noble gas ratios to our previously defined sw fsw dominated regolithic group lew 85313 and met 00423 suggesting a surface origin for these samples however interestingly the gro samples show vastly different cosmogenic noble gas abundances and thus different cre ages which suggests that they are not paired for howardite eet 87513 the data hint to the presence of cm material with a neon release pattern similar to our defined planetary fsw dominated regolithic group cm rich samples pra 04401 sco 06040 our petrological investigations found no evidence for cm fragments within eet 87513 though a single clast was reported previously buchanan et al 1993 aside from the ne release pattern the remaining noble gas data show more similarity with sw fsw components the remaining four howardites cre 01400 eet 87518 eet 99400 and san 03472 and are dominated by cosmogenic noble gases and show no evidence for a regolithic origin our data suggest that a cm composition is likely present in all samples to some degree but that this can be overprinted by sw components or cosmogenic components obtained in situ on the vestan surface or during transit to earth respectively the presence of cm material is an important parameter for understanding the evolution of vesta s surface while we have uncovered three further regolithic howardites 13 regolithic total of 41 analysed further noble gas analysis of hed meteorites is needed to not only determine regolithic origins but to better characterise the abundance of carbonaceous chondrite material and its effect on the noble gas signatures of such samples

a crater and its ejecta an interpretation of deep impact we apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the deep impact event an important question is whether the cratering event was gravity or strength dominated the answer gives important clues about the properties of the surface material of tempel 1 gravity scaling was assumed in pre event calculations and has been asserted in initial studies of the mission results because the gravity field of tempel 1 is extremely weak a gravity dominated event necessarily implies a surface with essentially zero strength the conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution we address that feature here and conclude that even strength dominated craters would result in a plume that appeared to remain attached to the surface we then calculate the plume characteristics from scaling laws for a variety of material types and for gravity and strength dominated cases we find that no model of cratering alone can match the reported observation of plume mass and brightness history instead comet like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few hour time frame with such mechanisms and to within the large uncertainties either gravity or strength craters can provide the levels of estimated observed mass thus the observations are unlikely to answer the questions about the mechanical nature of the tempel 1 surface

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

palaeoproterozoic indian shield in the global continental assembly evidence from the palaeomagnetism of mafic dyke swarms abstract palaeomagnetic study of palaeoproterozoic mafic dykes in the basement along the margins of the cuddapah basin the largest precambrian sedimentary basin in south india is presented in detail for a general discussion of palaeoproterozoic igneous activity in india the results are compared with all earlier published data on mafic dykes in india and are integrated with recently published high precision u pb baddeleyite ages to provide a comprehensive account of palaeoproterozoic igneous activity in india the analysis consolidates palaeomagnetic poles for six age divisions between 2 45 and 1 85 ga with robust statistical criteria our best estimates of overall mean poles from 241 dykes are situated at 1 17 7 n 106 0 e a95 9 0 7n 24 at c 2 45 ga 2 7 1 n 57 2 e a95 4 5 n 69 at c 2 37 ga 3 41 6 s 5 5 e a95 5 1 n 34 at c 2 22 ga 4 4 7 n 343 0 e a95 4 4 n 31 at 2 18 ga 5 49 2 n 332 9 e a95 4 8 n 24 at 1 99 1 89 ga and 6 73 7 n 282 6 e a95 2 9 n 39 at 1 86 ga the data permit us to construct an apparent polar wander path for the indian shield for an 600 ma interval of the palaeoproterozoic eon 2 45 1 85 ga testing and evaluation of continental reconstructions for this interval which are mostly based on geological correlations reveal many inconsistencies between 2 45 and 2 37 ga the indian shield was situated at higher latitudes similar to the yilgarn craton of australia it was subsequently located near the equator at 2 22 2 18 1 99 and 1 86 ga thus an india australia connection is supported during these times but a proposed australia kaapvaal link in zimvaalbara and a dharwar india slave connection in sclavia or a superior zimbabwe india connection in superia are inconsistent with indian data in addition the close palaeomagnetic comparison between the palaeoproterozoic dykes of dharwar bastar bundelkhand cratons in india indicates an age of 2 45 ga for orogenic activity along the central indian tectonic zone hence matching this zone with the 2 0 1 8 ga trans north china orogenic belt or positioning north china adjacent to india or juxtaposition of the indian shield along the western margin of laurentia in the columbia reconstructions is not supported the indian data appear to be in accord with the essential features of the refined protopangaea model and the original ur configuration finally the results are interpreted in terms of four dyke emplacement events in the age range 2 45 2 18 ga linked to short lived 5 10 ma lips developed over mantle plumes the dykes of 1 99 1 89 ga age probably relate to continued long lived igneous activity while the 1 86 ga dykes are relatively fewer in number and may represent waning stage of a large igneous event related to a major mantle plume

paleomagnetism of ca 2 3 ga mafic dyke swarms in the northeastern southern granulite terrain india constraints on the position and extent of dharwar craton in the paleoproterozoic abstract the pole positions reported for the mafic dykes occurring in the northeastern southern granulite terrain and easterly trending dykes of dharwar craton are alike while their ages differ by ca 700 ma confounding tectonic reconstruction to resolve the discordance and to constrain the position of the indian continent in the proterozoic paleomagnetic and geochronological studies have been carried out on the mafic dyke swam in the northeastern southern granulite terrain two distinct pole positions are obtained one from the group of tiruvannamalai dykes as 27 7 s 231 5 e dp dm 12 14 with corresponding mean declination dm and inclination im as 125 and 73 8 95 22 7 6 respectively and second from the east coast dykes as 2 32 s 188 2 e dp dm 5 8 with corresponding mean dm and im as 88 9 and 33 8 95 48 7 1 respectively the remanent efficiency of magnetization values between 0 01 and 0 08 suggest that the dykes have been cooled through curie temperature of magnetite which is the carrier of magnetization the baked contact test confirms that the characteristic remanent magnetization is the primary magnetization the sm nd mineral whole rock isochron from one of the tiruvannamalai dykes yielded an age of 2318 60 ma mswd 1 9 this age is the time of intrusion of the tiruvannamalai dykes and is similar within errors to the age of dykes in the dharwar craton thus the contiguity of dharwar craton with the northern block of southern granulite terrain as early as 2 3 ga ago is established the pole position thus obtained corresponds to a high latitudinal position of dharwar craton during early paleoproterozoic

broadband calibration of the r v marcus g langseth four string seismic sources 1 the r v marcus g langseth is the first 3 d seismic vessel operated by the u s academic community with up to a four string 36 element source and four 6 km long solid state hydrophone arrays this vessel promises significant new insights into earth science processes the potential impact of anthropogenic sound sources on marine life is an important topic to the marine seismic community to ensure that operations fully comply with existing and future marine mammal permitting requirements a calibration experiment was conducted in the gulf of mexico in 2007 2008 results are presented from deep 1 6 km and shallow 50 m water sites obtained using the full 36 element 6600 cubic inches seismic source this array configuration will require the largest safety radii and the deep and shallow sites provide two contrasting operational environments results show that safety radii and the offset between root mean square and sound exposure level measurements were highly dependent on water depth

the dharwar craton southern india interpreted as the result of late archaean oblique convergence abstract the dharwar craton comprises two distinct parts separated by a steep n s sinistral shear zone in the western part a pre 2900 ma complex of orthogneisses granodiorites and narrow tracts of supracrustal rocks sargur group forms the basement to volcanic and sedimentary basins of the dharwar supergroup ca 2800 2550 ma late archaean deformation is characterised by ne sw crustal shortening and steep n s sinistral shear zones the eastern part is underlain by parallel steep n s or nw se linear belts of calc alkaline anatectic and juvenile granites and granodiorites dharwar batholith ca 2750 2510 ma with intervening volcanic and sedimentary schist belts ca 2800 2550 ma the plutonic belts are 15 25 km wide up to 150 km long and bounded by steep nw se high strain zones up to 2 km wide with sinistral shear sense except one which is dextral magmatic state fabrics and structures in the plutonic rocks are parallel to solid state sinistral shear fabrics in the high strain zones but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear magmatic state structures swarms of vertical nw se dykes of granite and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress upright folds and schistosity steep reverse faults and effects of regional ht lp metamorphism show that deformation was partitioned into ne sw shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime the western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts intra arc basins in the east ne sw shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc normal and arc parallel displacements during oblique convergence analogous to mesozoic cenozoic convergent settings

estimating shallow water sound power levels and mitigation radii for the r v marcus g langseth using an 8 km long mcs streamer for seismic surveys in shallow water environments the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species this is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins one potential solution to this problem is to measure received levels using the ship s multichannel seismic mcs streamer which could allow for the dynamic real time determination of sound levels and mitigation radii while a survey is underway we analyze r v langseth streamer data collected on the shelf and slope near the washington coast during the cascadia open access seismic transects coast and ridge2trench projects to measure received levels up to a distance of approximately 8 km from the sound source array we establish methods to filter clean and process streamer data to accurately determine received power levels and confidently establish mitigation radii we show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued the establishment of mitigation radii based on local conditions may help to maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

the dharwar craton and the assembly of peninsular india peninsular india can be separated into five discrete crustal areas the bhandara singhbhum and aravalli bundelkhand cratons the eastern ghats and a block in southern india containing the dharwar craton and adjoining granulite terrain these areas are separated by four major joins eastern ghats front godavari rift mahanadi rift sukinda thrust and narmada son lineament and associated thrusts all joins except the godavari rift are former orogenic belts where thrusts juxtapose rocks of different metamorphic grade these thrusts and the absence of clear correlation of suites among different cratons suggest that the indian shield formed by accretion of separate continental fragments epicontinental late archean early pro terozoic sediments throughout india however may indicate that the joins are intracontinental formed within a coherent block the term dravidian shield is used for the southernmost of the five crustal blocks including all of india south of the deccan basalts and godavari rift

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

r v marcus g langseth seismic source modeling and calibration this paper presents analyses of calibration data of the r v marcus g langseth seismic sources that were collected in 2007 2008 the analysis includes measurements of the levels and azimuthal directivity of the langseth two string source which is used for 3 d seismic surveys and compares these data with multichannel seismic hydrophone array data in shallow water the analysis also shows the important contribution of seafloor and subseafloor reflected and refracted arrivals and the associated effects of water depth and seafloor slope as expected and predicted by modeling azimuthal directivity depends greatly on the footprint dimensions of the seismic array in shallow water where the acoustic field is dominated by near vertically traveling reflected and refracted waves data recorded using a towed mcs array are a useful proxy for single hydrophone calibration data in deeper water the easily modeled direct arrivals are exceeded in amplitude by seafloor reflected and subseafloor refracted energy at offsets which depend upon the water depth limiting the applicability of a priori modeling that does not include these interactions in addition in a seafloor sloping environment amplitudes depend greatly on whether the receivers are upslope or downslope from the source

fugitives from the vesta family abstract spectroscopic observations of asteroid 4 vesta and numerous members of the vesta family located in the inner asteroid belt have determined that these objects have reflectance properties of basaltic material a plausible hypothesis is that the surface of vesta was punctured by large impacts in the past which dispersed fragments of its basaltic crust into space and produced one of the most prominent asteroid families ever created in the belt until recently vesta was the only known object in the asteroid belt which underwent differentiation and survived to the present epoch since 2000 many new small basaltic asteroids have been discovered in the inner and outer parts of the asteroid belt possibly representing fragments from distinct differentiated bodies these discoveries may help us to better understand the number and nature of objects in the inner solar system that underwent geological differentiation to investigate these issues we performed extensive numerical simulations whose aim was to reproduce as precisely as possible the dynamical evolution of vesta s ejected fragments over timescales comparable to the family s age specifically we numerically integrated the orbital evolution of 6600 test bodies with orbits that started within the vesta family and dynamically evolved over 2 gy our model included gravitational perturbation of all planets except mercury and the yarkovsky effect the results show that a relatively large fraction of the original vesta family members may have evolved out of the family borders defined by clustering algorithms and are now dispersed over the inner asteroid belt we compared the orbital distribution of our model fragments with the orbital locations of known basaltic asteroids in various parts of the inner main belt to find that i most basaltic asteroids with semimajor axis a 2 3 au located outside the vesta family s borders in the inner main belt including 809 lundia and 956 elisa are most likely fugitives from the vesta family that have evolved to their current orbits via various identified dynamical pathways our results also suggest that the vesta family is at least 1 gy old ii interestingly orbits of many basaltic asteroids with 2 3 a 2 5 au like those of 4796 lewis and 5379 abehiroshi are displaced from the vesta family to low inclinations and are not obtained in our simulations with sufficient efficiency we propose that i these small basaltic asteroids may be fragments of differentiated bodies other than 4 vesta or ii they were liberated from the vesta s surface before or during the late heavy bombardment epoch 3 8 gy ago and their orbital inclinations separated from that of vesta when secular resonances swept through the region

sound source localization technique using a seismic streamer and its extension for whale localization during seismic surveys marine seismic surveys are under increasing scrutiny because of concern that they may disturb or otherwise harm marine mammals and impede their communications most of the energy from seismic surveys is low frequency so concerns are particularly focused on baleen whales extensive mitigation efforts accompany seismic surveys including visual and acoustic monitoring but the possibility remains that not all animals in an area can be observed and located one potential way to improve mitigation efforts is to utilize the seismic hydrophone streamer to detect and locate calling baleen whales this study describes a method to localize low frequency sound sources with data recoded by a streamer beamforming is used to estimate the angle of arriving energy relative to sub arrays of the streamer which constrains the horizontal propagation velocity to each sub array for a given trial location a grid search method is then used to minimize the time residual for relative arrival times along the streamer estimated by cross correlation results from both simulation and experiment are shown and data from the marine mammal observers and the passive acoustic monitoring conducted simultaneously with the seismic survey are used to verify the analysis

magnetic fabric and its significance in the 1400 ma mealy diabase dykes of labrador canada an anistropy of magnetic susceptibility ams study of the relatively fresh mealy diabase dyke swarm indicates a correlation between the ams and the deformational effects in the rocks several ams components were revealed during experimental work on 211 samples that had been heated to 640 c during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields seventeen sites with slight or no deformation have a magnetic fabric composed of two major components the stable component attributed to a primary shape anisotropy has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike n 60 e respectively this configuration is consistent with an origin due to a post emplacement stress field whereby horizontal extension acted on a vertical sheet of partially crystallized magma the orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement the unstable component removed by fields up to 250 mt is attributed to induced domain anisotropy produced by mild tectonic stress during the grenville orogeny the approximate orientation of its principal components as found by the method of substitutional ellipsoids suggests an east west directed maximum stress four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed but which may be largely composed of domain and stress induced anisotropies developed during the grenville orogeny

paleomagnetism and geochronological studies on a 450 km long 2216 ma dyke from the dharwar craton southern india abstract paleomagnetic and geochronological studies were carried out on a 450 km long from 17 sites n s striking paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km increasing from north to south in the dharwar craton to study the characteristics of paleomagnetism and geochronology in vertical dimension u pb pb pb dating on baddeleyite gives a crystallisation age of 2216 0 0 9 ma for long dyke akld paleomagnetic data from this well dated 2216 ma dyke swarm in dharwar craton are of excellent quality high coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions combined normal and reverse polarity remanences on akld and other n s dykes define the most reliable paleomagnetic pole for 2216 ma at latitude 36 s and longitude 312 e a95 7 though paleomagnetic data is unavailable on other n s dykes below the cuddapah basin high precision geochronology suggest that they are of similar age within errors though there is a variation in the crustal depth of dharwar craton from north to south consistent pb pb u pb baddeleyite geochronology and paleomagnetic studies along the akld established its continuity and preservation along its entire strike length the virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions updated apparent polar wander path of dharwar craton indicates relatively low drift rate during 2 21 2 08 ga interval magnetogranulometry and sem studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals