nucleation crystal growth and the thermal regime of cooling magmas crystallization at the margin of a quiet cooling magma has been studied numerically taking into account the kinetics of crystallization the variables are the latent heat value the growth and nucleation functions the initial magma temperature and the thermal contrast between magma and country rock we have investigated a wide range of values for these parameters corresponding to natural conditions we show that after a highly transient stage crystallization tends toward an equilibrium between heat production latent heat release and heat loss given the small diffusivity of country rocks latent heat release is the main factor controlling the temperature evolution in order to minimize the latent heat release crystallization occurs at a temperature where nucleation is small this can be close to either the liquidus or the solidus depending on the initial conditions the main process controlling crystallization is nucleation and not crystal growth nucleation occurs as a series of sharp pulses followed by longer periods of crystal growth the nucleation pulses give birth to thermal oscillations these oscillations can be sustained if the interior magma temperature is above the liquidus independently of the heat loss mechanism we show that the phenomenon occurs on the scale of a few centimeters which corresponds to the inch scale layering of many ultrabasic complexes the model allows us to calculate crystal sizes which are in good agreement with geological observations the crucial parameters which determine crystal size variations near the margins of igneous bodies are the initial thermal conditions as well as the nucleation and growth functions in the main cooling regime close to the liquidus significant size variations can be created by small thermal disturbances

seismic discontinuities beneath the southwestern united states from s receiver functions abstract s receiver functions along the colorado plateau rio grande rift great plains transect known as la ristra in the southwestern united states have been utilized to map seismic discontinuities beneath this tectonically active region individual receiver functions were stacked according to ray piercing points with moveout corrections in order to improve the signal to noise ratio of the converted s to p phases a mantle discontinuity which is interpreted as the lithosphere asthenosphere boundary lab is observed along the profile with depth ranging from 80 km beneath the rio grande rift rgr to 100 km beneath the great plains gp and 120 180 km beneath the colorado plateau cp the shallow lab beneath the rio grande rift is indicative of lithosphere extension and asthenosphere upwarp the lab deepens sharply at the rgr cp and rgr gp boundaries providing evidence for edge driven small scale mantle convection beneath la ristra two local discontinuities beneath the southeastern colorado plateau are imaged at 250 km and 300 km and could be the top and base of the eroded lithosphere respectively the s receiver function images suggest that edge driven small scale convection is probably the mantle source for recent extension and uplift in the rio grande rift and the colorado plateau

extrusion tectonics of the okhotsk plate northeast asia based on focal mechanism geologic and lineament data faulting in the chersky range northeast asia changes from right lateral transpression between north america and eurasia to left lateral strike slip between north america and the okhotsk plate the location of the okhotsk eurasia boundary is poorly constrained available data suggest this boundary is right lateral the okhotsk plate is being compressed and extruded from between north america and eurasia resulting in high levels of microseismic activity along the southern flanks of the chersky range

the cooling kinetics of plagioclase feldspar as revealed by electron microprobe mapping in this study we have used electron microprobe mapping to investigate plagioclase compositional evolution due to cooling kinetics we re analyzed five run products from a prior study iezzi et al 2011 crystallized by cooling a natural andesitic melt from 1300 to 800 c at 25 12 5 3 0 5 and 0 125 c min under atmospheric pressure and air redox state as the cooling rate decreases the texture of large plagioclases changes from skeletal to hollow to nearly equant in this study we use x ray map data to obtain a database of 12 275 quantitative chemical analyses the frequency of an rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate at 25 and 12 5 c min the distribution is single mode and narrow at 0 5 and 0 125 c min is single mode but very broad whereas at the intermediate cooling rate of 3 c min two distinct plagioclase populations are present this intermediate cooling rate is fast enough to cause departure from equilibrium for the crystallization of the an rich population but also sufficiently slow that an poor plagioclases nucleate from the residual melt we interpret our findings in the context of time temperature transformation ttt diagrams and infer the crystallization kinetics of plagioclase in the experiments compositional trends and our inferences regarding ttt systematics are consistent with two discrete nucleation events that produced separate populations of plagioclase i e an rich and an poor populations at 3 c min using plagioclase melt pairs as input data for the thermometric reaction between an and ab components we find that plagioclase mirrors very high near liquidus crystallization temperatures with increasing cooling rate these results have important implications for the estimate of post eruptive solidification conditions lava flows and intrusive bodies from centimeters to a few meters thick are characterized by a short solidification time and a significant thermal diffusion under such circumstances it is possible to crystallize plagioclases with variable and disequilibrium chemical compositions simply by cooling a homogeneous andesitic melt x ray element maps enrich the study of plagioclase compositional variations generated under conditions of rapid cooling

crystallization of interstitial liquid and latent heat buffering in solidifying gabbros skaergaard intrusion greenland new compositional profiles across plagioclase grains from the layered series ls marginal border series mbs and upper border series ubs of the skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid the data show that plagioclase crystals display three types of compositional profile over the whole intrusion as follows 1 grains with normal zoning which dominate the mbs and ubs these are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores 2 unzoned crystals which dominate the upper part of the ls also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition 3 crystals with a mantle of decreasing an content followed by a rim of constant composition grains showing this complex zoning mostly occur in the lower parts of the ls depending on the stratigraphic position within the intrusion the composition of the rim can be an56 an51 or an40 in the main magma body these compositions an contents correspond to those of plagioclase primocrysts e g cores at the appearance of cumulus clinopyroxene an56 fe ti oxides an51 and apatite an40 compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush when a new phase saturates the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years under these conditions equilibrium adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition an content efficient thermal buffering of the mush liquid depends on the porosity i e fraction of liquid within the mush and the degree of compositional homogeneity of the mush in a heterogeneous and highly porous mush saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid including the warmest part that is not yet saturated in a new phase as a consequence no thermal buffering occurs and interstitial crystallization produces grains with normal zoning the distribution of the various types of plagioclase grains throughout the skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush such as the residual porosity both at an intrusion wide scale and at a millimetre to centimetre scale

pulsation tectonics as the control of continental breakup abstract new data from the recent ipod drilling of dsdp site 534 in the blake bahama basin give a definitive age for the spreading center shift involved in the breakup of the north american atlantic margin a basal callovian age 155 m y is determined for the blake spur anomaly marking this spreading center shift that signals the birth of the modern north atlantic ocean this is some 20 m y younger than previously thought one implication of this result is that this spreading center shift starting north atlantic breakup is now of an age which could be assigned to the spreading center shift needed to end the rifting in the gulf of mexico it is suggested that this might be one and the same event another implication of this younger age for the blake spur event is that relatively high spreading rates are now required for the jurassic outer magnetic quiet zone along the north american margin this association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the mid cretaceous and implies a link between the processes controlling plate spreading which are in the upper mantle and the processes controlling the magnetic field which are in the outer core the cycles of fast and slow spreading and quiet and reversing magnetic field have a period of 60 100 m y a theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost d layer of the mantle could explain the correlation plumes carry heat away from the core mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading the pulse of fast spreading in the jurassic apparently caused the breakup of the north atlantic other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe implying that this might be a prevalent process i suggest rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics the term pulsation tectonics is appropriate for this new unifying theory explaining variations in the magnetic field plate motions and paleoenvironment the word pulsation was used by grabau 1934 for a long term rhythm nearly the length of a geological time period in which there were widespread transgressions and regressions of seas over whole continents thus pulsations are just the long term cycles of vail et al 1977 grabau s pulsation theory prophetically invoked large scale tectonic changes in the ocean basins as a cause for eustatic changes in contrast to local continental tectonics although he offered no explanation of the ocean changes the cyclic character of crustal movements called for in the theory of pulsation tectonics revives many ideas of these past advocates of non steady state tectonics and provides modern mechanisms that explain the observations on plate motions and the magnetic fields data which were totally unknown to these past authors

near encounter with asteroid 253 mathilde overview abstract on june 27 1997 the near spacecraft carried out the first ever encounter with a c type asteroid flying by 253 mathilde at a distance of 1212 km we summarize findings derived from 330 images obtained by near s msi camera which cover about 60 of the surface of the asteroid the highest resolution achieved was about 160 m pixel mathilde is a low reflectance object geometric albedo 0 047 with principal diameters of 66 48 44 km the mean radius of 26 4 1 3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data mathilde s surface morphology is dominated by large craters at least four of which have diameters comparable to the radius of mathilde the two largest ishikari and karoo have diameters of 29 3 and 33 4 km respectively no evidence of layering is exposed in the crater walls but suggestions of downslope movement are present the surface density of craters in the diameter range from 0 5 to 5 km is close to equilibrium saturation a situation in which as many craters are being destroyed as are being produced observed depth to diameter ratios for craters in this size range are close to those observed on the lunar surface a disruption lifetime of about 4 billion years has been estimated for mathilde based on the mass determination obtained from doppler tracking d k yeomans et al 1997 science 278 2106 2109 and the volume derived from msi images the average density of mathilde is remarkably low 1 3 0 3 g cm 3 a value consistent with a rubble pile structure for the interior assuming that mathilde s rock type is similar to that found in cm meteorites the porosity of the interior must be some 50 shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by mathilde s underdense interior a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events except for the lower geometric albedo 0 047 0 005 the photometric properties of mathilde are closely similar to those of phobos the surface is extremely homogeneous in terms of both color or albedo specifically no color or albedo variations associated with craters have been identified

explosion contamination of the northeast siberian seismicity catalog implications for natural earthquake distributions and the location of the tanlu fault in russia russian regional seismicity catalogs including those in the annual earthquakes of the ussr are contaminated by industrial explosions in northeastern russia explosions occur in tin coal and gold mines as well as in the construction of roads railways and dams most seismically recorded mining and construction related explosions have magnitudes of about 2 0 and occur during local daytime in addition explosions in placer mining areas are concentrated from midwinter to early spring when frozen placers are broken up for the summer processing season we analyzed the temporal variation of over 87 000 events occurring in northeast russia using a newly compiled seismicity catalog to identify areas where there may be explosion contamination areas with temporal biases indicative of mining or other explosions include the yana river delta and chukotka placers the southern amur district coal mining the trace of the baikal amur railroad construction lazo quarry the south yakutian gold fields and the kolyma gold belt the locations and estimates of the level of explosion contamination of the catalog suggest that the natural seismicity may be lower and not as diffuse along the plate boundaries in northeastern russia than previously thought use of only nighttime events from the seismicity catalog which should have a minimum of explosions helps to clarify the extension of the tanlu fault into russia and may ultimately help elaborate tectonics in other areas of eastern russia

disequilibrium dihedral angles in layered intrusions a microstructural record of fractionation the dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre scale crustal layered intrusions the median of a population of these disequilibrium angles cpp varies systematically within a single layered intrusion remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma the stepwise changes in cpp are present in the upper zone of the bushveld complex the megacyclic unit i of the sept iles intrusion and the layered series of the skaergaard intrusion the plagioclase bearing cumulates of rum have a bimodal distribution of cpp dependent on whether the cumulus assemblage includes clinopyroxene the presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid the only parameter that behaves in an exactly analogous manner to cpp is the rate of change in enthalpy with temperature h t during crystallization both h t and cpp increase with the addition of a liquidus phase and decrease with the removal of a liquidus phase the replacement of one phase by another has little effect on h t and no discernible effect on cpp an increase of h t results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization the fractional latent heat it also increases the mass crystallized in each incremental temperature drop the crystal productivity these increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three grain junctions in the mush via thermal buffering of a thickened mushy layer we suggest these are the underlying causes of stepwise increases in cpp stepwise changes in the geometry of three grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions

frictional melt pulses during a 1 1 ma earthquake along the alpine fault new zealand our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine grain size of frictionally fused rock and the lack of detailed high resolution microscopy studies that describe features at the crystal lattice scale in such a study we here report the complex nature of melting and crystallization in a symmetrically layered pseudotachylyte vein from the alpine fault new zealand two melt pulses are recognized attributed to successive but rapid injections of frictionally generated material the initial injection preserved at the vein margins was proximally derived and contains a high concentration of clasts and a si rich glass this was quickly followed by a second generation of a more distally derived melt dominated fraction which was injected into the weak vein center whereas fragments of wall rock biotite are preserved in the marginal zones neocrystallized microlitic biotite characterizes both margins and center the vein biotite is different in composition microstructure and polytypism from the metamorphic biotite of the wall rock in all melt layers newly formed biotite shows notable signs of syn flow crystallization strain features and erosion at crystal glass contacts with breakdown of neocrystallized microlites along both crystal edges and faces these characteristics imply cyclic pulses of heating melting and crystallization occurred during a single large earthquake episode and probably reflects the stick slip propagation properties of coseismic faulting 40 ar 39 ar total gas ages from the vein center give a 1 11 0 04 ma date for cyclic melting which based on current exhumation rates occurred at a crustal of depth v11 km 2003 elsevier science b v all rights reserved

evidence for small scale mantle convection in the upper mantle beneath the baikal rift zone 1 inversion of teleseismic p wave travel time residuals collected along a 1280 km long profile traversing the baikal rift zone brz reveals the existence of an upwarped lithosphere asthenosphere interface which causes a travel time delay of about 1 s at the rift axis central high an area with early arrivals relative to the stable siberian platform of up to 0 5 s is observed on each side of the rift about 200 km from the rift axis flank lows while the location of the central high is approximately fixed in the vicinity of the rift axis those of the flank lows vary as much as 200 km with the azimuth of the arriving rays we use three techniques to invert the travel time residuals for velocity anomalies beneath the profile two of the techniques assume an isotropic velocity structure and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry we use independent geophysical observations such as gravity active source seismic exploration and crustal thickness measurements to compare the applicability of the models other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2 5 lateral velocity reduction and a velocity increase due to transverse isotropy with a vertical axis of symmetry we consider the anisotropy to be the result of the vertical component of a lithosphere asthenosphere small scale mantle convection system that is associated with the rifting

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

compressional wave velocity structure of the upper 350 km under the eastern snake river plain near rexburg idaho relative travel time residuals for teleseismic p and pkikp are used to determine the compressional velocity structure under the eastern snake river plain at rexburg idaho damped least squares inversion of travel time residuals modified from the method of aki indicates a large body of 3 5 2 5 low relative velocity material centered under the northwest edge of the snake river plain less well resolved are high relative velocity bodies one deep under the idaho wyoming border the other 300 km below the idaho batholith comparison with similar velocity mappings of the yellowstone region indicates that the snake river plain low velocity anomaly is an aged extension of the low velocity body under the yellowstone caldera the upper half of the body has vanished in the eastern snake river plain and the lower half is less anomalous than at yellowstone the strong dependence of mantle velocity structure on position under the yellowstone snake river plain volcanic system indicates loss of heat to the surface primarily by advective heat movement by mass movement mechanisms not by conduction no discrimination between propagating rift and upper mantle heat anomaly models of this northeast progressing volcanic system is possible because velocity structures associated with these hot spot models have not been predicted quantitatively upper mantle heat anomaly models thermal plumes chemical plumes gravity anchors explain the observed structure with fewer ad hoc assumptions but propagating rift models cannot be eliminated

ism observation of phobos reinvestigated identification of a mixture of olivine and low calcium pyroxene 1 we reinvestigate the infrared spectral image cube of the martian satellite phobos obtained by the infrared spectrometer for mars ism instrument on board phobos 2 using a band detection method based on the wavelet transform two absorption bands are identified at 1 04 m and 1 9 m the position and profile of these bands are compatible with a mixture of low calcium pyroxene and olivine in specific regions on the surface of the satellite this identification is confirmed by the strong spatial correlation of these two bands which also corresponds to areas with a lower albedo finally we discuss clues pointing toward possible local hydration of phobos

phobos grooves and impact craters a stereographic analysis abstract phobos parallel grooves were first observed on viking images 38 years ago and since then they have been greatly debated leading to several formation hypotheses nevertheless none of them have been favoured and widely accepted in this work we provide a different approach assuming that phobos grooves can be the expression of fracture planes and deriving their spatial distribution and orientation on 3d reconstructions we point out that any origin related only to craters at phobos surface should be ruled out since the majority of the grooves is unrelated to any craters now present at its surface this raises the intriguing possibility that such grooves if expression of fracture planes are remnant features of an ancient parent body from which phobos could have originated such scenario has never been considered for phobos though this origin was already proposed for the formation of 433 eros grooves buczkowski d l barnouin jha o s prockter l m 2008 icarus 193 39 if this idea holds true the observed groove distribution could be explained as the result of possible major impacts on the larger parent body which were inherited by the phobos shard

qp beneath the rio grande and east african rift zones in 1983 teleseismic short period p waves were recorded in the southwest united states along a linear array which stretched 1000 km across the rio grande rift in 1985 a similar experiment was conducted across the east african rift in which stations were installed along a 600 km array which ran east west across kenya we have developed a spectral comparison technique to calculate the attenuation at each station for a given event we simultaneously solve for the effective source spectrum which includes mantle path effects as well as instrument response an amplitude factor and an attenuation operator t at each station we use downward projection of p wave travel time residuals to define the lithosphere asthenosphere boundary which we infer to upwarp beneath both rifts the velocity structure so determined is then used to determine the q of the asthenosphere relative to that of the lower lithosphere when we assume a lower lithosphere q of 100 we obtain asthenospheric q values of 57 beneath the rio grande rift and 26 beneath the east african rift with corresponding velocity contrasts of 8 and 12 respectively the low q values as well as low velocity and density contrasts support the interpretation that the upwarped asthenosphere contains partial melt and that beneath the east african rift the concentration is higher than beneath the rio grande comparison of the results with experiments on peridotite sato et al 1989a b suggests that the asthenosphere is at homologous temperatures of 1 01 and 1 03 with corresponding partial melt fractions of 1 and 3 beneath the rio grande and east african rifts respectively

a lower crustal extension to a midcrustal magma body in the rio grande rift new mexico a prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival repeatedly observed on seismograms recorded in the central rio grande rift from intermediate and deep focus earthquakes at distances near 90 is identified as a probable compressional to shear wave conversion from a lower crustal root of the socorro midcrustal magma body smb this phase is inconsistent with previously determined crustal models containing a simple sill like midcrustal magma body with a negligible conduit system finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19 km deep upper surface of the magma body down into the lower crust of the southwestern portion of the albuquerque belen basin the location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the smb and associated seismicity is occurring via an off axis conduit system located beneath the active margin of the albuquerque belen basin

mars pathfinder spectral measurements of phobos and deimos comparison with previous data the imager for mars pathfinder imp acquired four spectra of parts of the sub mars hemispheres of phobos and deimos the measured region of phobos is expected to be a mixture of the two spectral units identified on that satellite from phobos 2 data and the imp spectra of phobos are intermediate to the two units as expected the derived geometric albedo is consistent with the value for that part of phobos determined from viking imagery the imp spectrum of deimos is generally consistent with previous measurements acquired from the ground and from the hubble space telescope hst but the signal to noise ratio is lower than that of the phobos data the spectral contrast between the two moons is similar to that seen in hst and phobos 2 data mars pathfinder measurements therefore substantiate recent results which indicate that phobos and deimos are not as previously believed analogous to c type asteroids they also provide some indications for an absorption near 700 nm perhaps like that seen in other low albedo asteroids both martian moons are redder than most asteroids and most closely resemble two analog materials believed to have undergone very dissimilar histories primitive d like asteroids and highly space weathered mafic rich assemblages such as are present in lunar mare soils

the imager for mars pathfinder experiment the imager for mars pathfinder imp a stereoscopic multispectral camera is described in terms of its capabilities for studying the martian environment the camera s two eyes separated by 15 0 cm provide the camera with range finding ability each eye illuminates half of a single ccd detector with a field of view of 14 4 14 0 and has 12 selectable filters the 18 optics have a large depth of field and no focussing mechanism is required a mechanical shutter is avoided by using the frame transfer capability of the 512 512 ccd the resolving power of the camera 0 98 mrad pixel is approximately the same as the viking lander cameras however the signal to noise ratio for imp greatly exceeds viking approaching 350 this feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes imp from viking specially designed targets are positioned on the lander they provide information on the magnetic properties of wind blown dust measure the wind vectors and provide radiometric standard reflectors for calibration also eight low transmission filters are included for imaging the sun directly at multiple wavelengths giving imp the ability to measure dust opacity and potentially the water vapor content several experiments beyond the requisite color panorama are described in detail contour mapping of the local terrain multispectral imaging of the surrounding rock and soil to study local mineralogy viewing of three wind socks measuring atmospheric opacity and water vapor content and estimating the magnetic properties of wind blown dust this paper is intended to serve as a guide to understanding the scientific integrity of the imp data that will be returned from mars starting on july 4 1997

microstructural evolution of silicate immiscible liquids in ferrobasalts an experimental study of the microstructural evolution of an immiscible basaltic emulsion shows that the fe rich liquid forms homogeneously nucleated droplets dispersed in a continuous si rich liquid together with droplets heterogeneously nucleated on plagioclase magnetite and pyroxene heterogeneous nucleation is likely promoted by localised compositional heterogeneities around growing crystals the wetting angle of fe rich droplets on both plagioclase and magnetite increases with decreasing temperature droplet coarsening occurs by a combination of diffusion controlled growth and ostwald ripening with an insignificant contribution from coalescence characteristic microstructures resulting from the interaction of immiscible fe rich liquid with crystal grains during crystal growth can potentially be used as an indicator of liquid unmixing in fully crystallised natural samples in magma bodies 10 m in size gravitationally driven segregation of immiscible fe rich droplets is unlikely to be significant

tectonics of the laptev sea and moma rift systems northeastern ussr abstract examination of the topography seismicity focal mechanisms and structure of the extension of the arctic mid ocean ridge into the laptev sea suggests that the present day features of the upper crust are consistent with an asymmetric simple shear extensional system on a large scale the topographically high upper plate side alternates from severnaya zemlya and taimyr peninsula across the rift zone to the new siberian islands and then back across to the kharaulakh range and the omoloi river basin the activity of this system is indicated by moderate size normal faulting earthquakes and the extensive development of extensional structural features a weakly developed axial graben with a few teleseismic earthquakes is developed in the eastern laptev sea further south a nw striking pliocene rift system commonly known as the moma rift extends through the northern cherskii range although there is a broad zone of elevated heat flow and isolated volcanoes this system is transpressional at the present time the zone of major seismicity passes south of the rift system near the indigirka river and indicates that the rift is not acting as the plate boundary the moma rift is also asymmetric but with the lower plate southwest side being topographically higher the moma system is inactive along most of its length and only reached an early stage of development before activity ceased

overview of the mars pathfinder mission launch through landing surface operations data sets and science results mars pathfinder successfully landed at ares vallis on july 4 1997 deployed and navigated a small rover about 100 m clockwise around the lander and collected data from three science instruments and ten technology experiments the mission operated for three months and returned 2 3 gbits of data including over 16 500 lander and 550 rover images 16 chemical analyses of rocks and soil and 8 5 million individual temperature pressure and wind measurements path finder is the best known location on mars having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high resolution orbiter images and in inertial space from two way ranging and doppler tracking tracking of the lander has fixed the spin pole of mars determined the precession rate since viking 20 years ago and indicates a polar moment of inertia which constrains a central metallic core to be between 1300 and 2000 km in radius dark rocks appear to be high in silica and geochemically similar to anorogenic andesites lighter rocks are richer in sulfur and lower in silica consistent with being coated with various amounts of dust rover and lander images show rocks with a variety of morphologies fabrics and textures suggesting a variety of rock types are present rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates although other explanations are also possible which almost definitely require liquid water to form and a warmer and wetter past air borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral interpreted to be most likely maghemite explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains remote sensing data at a scale of a kilometer or greater and an earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods which are relatively dust free the surface appears to have changed little since it formed billions of years ago with the exception that eolian activity may have deflated the surface by 3 7 cm sculpted wind tails collected sand into dunes and eroded ventifacts fluted and grooved rocks pathfinder found a dusty lower atmosphere early morning water ice clouds and morning near surface air temperatures that changed abruptly with time and height small scale vortices interpreted to be dust devils were observed repeatedly in the afternoon by the meteorology instruments and have been imaged

results from the mars pathfinder camera images of the martian surface returned by the imager for mars pathfinder imp show a complex surface of ridges and troughs covered by rocks that have been transported and modified by fluvial aeolian and impact processes analysis of the spectral signatures in the scene at 440 to 1000 nanometer wavelength reveal three types of rock and four classes of soil upward looking imp images of the predawn sky show thin bluish clouds that probably represent water ice forming on local atmospheric haze opacity 0 5 haze particles are about 1 micrometer in radius and the water vapor column abundance is about 10 precipitable micrometers

flexural rift flank uplift at the rio grande rift new mexico like other cenozoic continental rifts the rio grande rift in colorado and new mexico exhibits prominent flanking uplifts several driving stresses and thermal mechanical processes have been proposed to explain the origin of rift flank relief which can be modeled to infer lithospheric structure although we have identified multiple uplift styles at the rio grande rift only one range the sacramento mountains is attributable to flexural upwarping of the lithosphere the process most suitable for geodynamic modeling and interpretation we demonstrate that two common assumptions in such modeling potentially introduce serious errors first presuming only one mechanism acts to uplift the flanks is inappropriate various forces influence flank topography at different depths and wavelengths and no single one is dominant second the end member boundary conditions of complete mechanical continuity or discontinuity broken plate at the range bounding normal fault are in general not applicable at rift flanks we examine alternative analytic plate flexure solutions by comparing them to finite element models of footwall flexure at a normal fault in a two dimensional elastic plate undergoing extension these simulations indicate that broken plate fits to rift flanks underestimate the plate thickness unless the uplift is large at least 1 km which promotes decoupling between the hanging wall and footwall if denudation dominates the flank unloading as may commonly be the case the best fit broken plate thickness error can be even greater our flexural analysis of the sacramento mountains suggests that the pecos river valley originated as a flexural down warp adjacent to the rift flank sensitivity tests of least squares fits to the sacramento mountains imply typical plate thickness errors of 20 although in extreme cases the combined errors may be 50 the average effective elastic lithosphere thickness is 23 km we find that elastic plastic models of rift flank flexure are unable to provide meaningful constraints on the thermal structure of continental lithosphere

the effects of heterogeneity in magma water concentration on the development of flow banding and spherulites in rhyolitic lava abstract this study focuses on the origin of flow banded rhyolites that consist of compositionally similar darker and lighter flow bands of contrasting texture and color infrared radiation was used to obtain fourier transform infrared ftir spectra from which water concentrations were calculated and to map variations in water concentrations across zones of spherulites and glass from the 23 million year old sycamore canyon lava flow of southern arizona lighter colored thicker flow bands consist of gray glass fine grained quartz and large 1 0 to 1 5 mm spherulites darker colored thinner flow bands consist of orange glass and smaller 0 1 to 0 2 mm spherulites the centers of both large and small spherulites are occupied by either 1 a quartz or sanidine crystal 2 a granophyric intergrowth or 3 a vesicle mapping of water concentration dominantly oh in glass and oh and h2o in sanidine crystals illustrates fluctuating water availability during quenching of the host melt textures of large spherulites in the lighter gray bands in some cases indicate complex quenching histories that suggest that local water concentration controlled the generation of glass versus crystals small spherulites in darker orange bands have only one generation of radiating crystal growth both the glass surrounding spherulites and the crystals in the spherulites contain more water in the gray flow bands than in the orange flow bands flow banding in the sycamore canyon lava flow may have originated by the stretching of a magma that contained pre existing zones vesicles or proto vesicles of contrasting water concentration as the magma flowed in the conduit and on the surface variation in the original water concentration in the alternating layers is interpreted to have resulted in differences in undercooling textures in spherulites in the lighter compared to the darker flow bands

a petrographic and isotopic criterion of the state of preservation of precambrian cherts based on the characterization of the quartz veins the coupled o and si isotope variations of precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on earth however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation even pristine samples show evidence of fluid circulation by the presence of various quartz veins here we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples these samples show different quartz habit and texture in their quartz veins which imply various fluid system conditions therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz these observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins we have demonstrated that in some conditions fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo environmental conditions the petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz

upper mantle shear structure beneath the colorado rocky mountains a tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 program for array seismic studies of the continental lithosphere rocky mountain front experiment the shear wave residuals used as data were corrected for known crustal sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the colorado array large differences in shear wave travel time are observed across colorado and into western kansas with a maximum difference of near 5 sec between the central rockies in colorado and a station in western kansas the shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath colorado and kansas large differences in mantle shear velocity are found between the rockies and the kansas great plains the average shear velocity in the upper 200 km is about 9 greater beneath kansas than beneath the rockies at greater depths lateral variations are far smaller and not as well correlated with surface tectonics in a qualitative sense there appear to be three mantle structures present beneath the study area beneath the rockies the upper mantle is extremely slow both to the west and east of the rockies we find intermediate mantle shear velocity there is then an abrupt increase in mantle shear velocity near the colorado kansas border we interpret the abrupt increase in mantle shear velocity near the colorado kansas border to mark the boundary of cratonic lithosphere the slow velocity zone beneath the rockies appears to be upwelled asthenosphere as is seen beneath the rio grande rift to the south to match the seismic contrast between the mantle beneath kansas and the colorado rockies requires a temperature contrast of at least 350 c as well as partial melt beneath the rockies the hot mantle beneath the rockies implies that the mantle density is low and the high topography of the rockies is at least partially due to buoyant mantle

unusually deep earthquakes in east africa constraints on the thermo mechanical structure of a continental rift system shudofsky 1985 has established that earthquakes associated with the east african rift system have well constrained focal depths as great as 25 30 km using published heat flow measurements as a guide to the local geotherm we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong these results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning any model of the rifting process in east africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

the laptev sea rift abstract in north eastern siberia the active mid ocean gakkel ridge interacts with the continental shelf of the laptev sea extension has affected the shelf since at least the early tertiary and has resulted in the formation of a complex horst and graben system we present new seismic data from the laptev sea including deep seismic soundings the most prominent rift basin is the ust lena rift with a minimum e w width of 300 km at latitude 75 n and a cenozoic infill up to 13 km in thickness the asymmetric shape of the basin and conclusive evidence for a detachment imply a simple shear geometry the suggested rift model combines a ramp and flat geometry for the detachment with ductile stretching beneath the detachment a major west dipping hingeline listric fault separates the ust lena rift from the laptev horst the 100 150 km wide laptev horst is subdivided into three units by narrow rift grabens another prominent rift graben is the anisin basin which is located in the northern shelf area though the laptev sea rift formed in interaction with an active mid oceanic ridge there are indications that the laptev sea rift is of the passive rift type the rift was developed east of a sw ne trending transfer zone which links the gakkel ridge to the laptev sea rift

photometric properties of phobos surface materials from viking images abstract clear filter viking images and an accurate numerical model of the shape of phobos have been used to determine this satellite s photometric properties a global average hapke function derived from disk resolved data confirms previous indications that phobos has a strong opposition surge photometrically corrected images were mosaicked into an albedo map most of the resulting normal reflectances are in the range 0 06 0 10 and the brightest region on phobos is the northeast rim of the crater stickney the portion of that rim with the highest concentration of grooves globally there are three albedo classes reasonably separated geographically 1 bright material is to the east and south of stickney corresponding approximately to the locations on phobos having the highest bluest visible near ir ratio murchie et al 1991 2 the darkest material is to stickney s west correlating with material having an intermediate visible nir ratio murchie et al s bluish gray unit 3 intermediate albedo material dominates the anti stickney hemisphere corresponding to material with a lower visible nir ratio murchie et al s reddish gray unit a search for variations in phase behavior across phobos surface shows few such effects overall limited to isolated areas 1 stickney s floor darkens with increasing phase faster than average phobos this crater floor is both slightly more backscattering and significantly rougher than the global average the latter effect may be related to the slumping hinted at in low resolution images of stickney 2 we confirm that in many cases the contrast between the bright rims of small craters and grooves and their surroundings drops noticeably with increasing phase phase angles in use 10 50 however these bright rims overall display a diversity of photometric behavior and are the most heterogeneous areas on phobos in terms of regolith properties 3 we confirm that dark deposits in the floors of smaller craters darken faster with increasing phase than their surroundings cf goguen et al 1978 and find that these deposits are more backscattering than average phobos isolated regions with unusual phase behavior occur on phobos but not on deimos thomas et al 1996 the variable photometric properties of phobos isolated craters and grooves and the association of global albedo features with stickney reinforce interpretations that phobos regolith is emplaced and modified by discrete cratering events and is not mixed horizontally by extensive downslope creep as is apparently the case on deimos

color heterogeneity of the surface of phobos relationships to geologic features and comparison to meteorite analogs multispectral observations of phobos by the vsk videospectrometric tv cameras and krfm combined radiometer and photometer for mars uv visible spectrometer on phobos 2 have provided new determinations of the satellite s spectral reflectance properties at greater spatial and spectral resolutions and over a greater geographic range than have previously been available images of the ratio of visible and nir reflectances covering the longitude range 30o 250ow were constructed from 0 40 0 56 pm and 0 781 1 tm vsk images eight channel 0 3 0 6 pm spectra obtained by the krfm instrument package were used to provide greater spectral resolution of parts of these images the data were calibrated using instrumental parameters measured on ground and in flight and the calibrations were refined and tested using previous spectral measurements of phobos and telescopic spectra of mars which is visible in the background in both data sets the average color ratio of phobos was found to be 0 97 0 14 consistent with previously obtained measurements however the surface is heterogeneous with at least four recognizable spectral units whose absolute color ratios were determined to within 10 a red unit with a color ratio of 0 7 0 8 a reddish gray unit with a color ratio of 0 8 1 0 a bluish gray unit with a color ratio of 1 0 1 1 and a blue unit with a color ratio of 1 1 1 4 the red unit occurs in the interiors of several dark floored craters and as adjacent patches the blue unit composes the interior of stickney as well as a lobate deposit superposed on the crater s rim and extending to the southwest the blue lobe is surrounded by a broad bluish gray aureole that breaks up into patchy outliers in its distal portions intervening surfaces are reddish gray the red reddish gray and bluish gray units were sampled by the krfm spectrometer and the bluish gray unit was found to have a distinct 0 3 0 6 grn spectrum the spatial distributions of the color ratio units and their reflectance systematics are inconsistent with mars shine or particle size differences alone being responsible for color variations but lateral optical or compositional heterogeneity is supported by the units different uv visible spectra the redder and bluer color units are interpreted to have been excavated by impacts from an optically and or compositionally heterogeneous interior overlain by a reddish gray surficial layer the location of the blue lobe emanating from stickney correlates with the location of one of the morphologic classes of grooves as predicted by ejecta reimpact models of groove origin the large color ratio of blue material is inconsistent with a carbonaceous chondfite composition but is comparable to that of an assemblage of mafic minerals like that forming black chondrites qualitative and quantitative comparison of the color ratio and uv visible spectral properties of bluish gray material with those of meteorites indicates that black chondfites are this matefial s closest spectral analog the uv visible spectra of reddish gray and red materials most resemble spectra of black chondrites but are also comparable to spectra of some carbonaceous chondrites

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

cooling and crystallization in sheet like magma bodies revisited abstract in order to characterize the interaction of convection and crystallization during the solidification of magmatic sills a one dimensional model for the solidification of magmatic sills cooling from above has been used the model is based on a stefan type solution for a mushy zone and takes into account the inward moving solidification front whose speed is exclusively controlled by crystal growth below the solidification front the magma cools by convection it may eventually cool below its liquidus and start to partially crystallize a kinetic model has been implemented to account for this crystallization the model calculations lead to three main conclusions 1 crystal growth is not the only factor controlling the advance of magmatic solidification fronts the advance appears to be mainly controlled by nucleation processes and only to a small extent by crystal growth 2 the longevity and vigor of convection inside a magma chamber is strongly dependent on the kinetics of crystallization of magma inside the chamber 3 immediately after the initiation of cooling the melt becomes laced with nuclei

the median valley a result of magma fracture beneath mid ocean ridges the shape of the ocean floor has been successfully modelled using the framework of plate tectonics1 4 lithospheric plates approximately 100 km thick lie above the hotter less viscous asthensophere at the ridges the lithosphere thins plate separation is accompanied by creation of new ocean floor as the asthenosphere flows upwards and cools on approaching the surface as a result heat is concentrated beneath the ridge axes densities are lower due to thermal expansion so that for iso static equilibrium the sea floor sits some 6 km shallower than older sea floor well off axis where the temperature anomaly is absent at fast spreading ridges velocity v 5 cm yr 1 such as the east pacific rise this description fits well a central ridge crest is flanked on either side by bathymetry which deepens in agreement with thermal models3 6 at a rate propor tional to the square root of age of creation however at slow spreading ridges v 5 cm yr 1 a major central graben of order 1 km deep replaces the central ridge crest although the square root of age dependence is observed well away from the axes isostatic equilibrium is not achieved7 since the sea floor lies 1 2 km below the equilibrium level in this report i demonstrate that this graben or median valley can result from steady state dyke intrusion into and brittle failure of the axial lithosphere

quantitative textural analysis of packings of elongate crystals the spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush and in turn is controlled by the rate of crystal growth and accumulation a predominant non spherical habit for igneous minerals brings into question the use of spherical particles in reference packings used for quantification of spatial distribution furthermore variations of crystal clustering ordering with length scale require spatial statistics which take into account the distribution of particles beyond nearest neighbours using random close packings of spherocylinders we demonstrate the importance of aspect ratio for the aggregation index usually known as r and show that packings of spherical particles have more structure than packings of rods the spatial distribution functions demonstrate that the plagioclase grains in the colonnade from the holyoke basalt are clustered on a length scale of 0 5 mm understanding the controls on grain spatial distribution in igneous rocks will depend on the application of these techniques to well understood environments

magma flow pattern of the 1 78 ga dyke swarm of the north china craton during the initial assembly of the supercontinent nuna columbia constraints from rock magnetic and anisotropy of magnetic susceptibility studies abstract the geodynamic origin of the 1 78 ga dyke swarm in the north china craton ncc is long debated it is considered either a post collision process or paleoplume related constraining its origin is essential for the reconstruction of the ncc position within the supercontinent nuna also referred to as columbia here we report on the rock magnetism and anisotropy of magnetic susceptibility ams of thirty three 1 78 ga dykes to constrain the magma center of the igneous event the samples were collected from the entire swarm across the ncc magnetite appears to be the dominant magnetic mineral in all the dykes occasionally with minor maghemite while the magnetite grain size within the dykes is generally increasing from south to north ams results of the dykes portray a magma flow related fabric over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow while a few have been formed by vertical intrusion these observations imply that the 1 78 ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the ncc during dyke propagation some localized magma subchambers may have been formed they could be responsible for the less common vertically intruded dykes and the ew trending dykes therefore a plume related tectonic setting for the 1 78 ga dyke swarm is more compelling with the eruption center along the margin of the ncc it may serve as an essential criterion to search for possible counterparts in other tectonic units within nuna columbia to evaluate the proximity of the ncc with them

three dimensional seismic inhomogeneities in the lithosphere and asthenosphere evidence for decoupling in the lithosphere and flow in the asthenosphere newly discovered three dimensional seismic velocity anomalies in the earth s lithosphere and asthenosphere are interpreted in terms of decoupling in the lithosphere and flow in the asthenosphere a rough estimate of flow in the asthenosphere under california was made using stokes law on the basis of the size of anomalies and density contrast estimated from the velocity contrast the result depends on the cause of velocity anomalies if they are due to any effect which obeys birch s law the flow is as rapid as that of morgan s 1972 plume driving the plate motion on the other hand if they are due to partial melts in thin cracks or at grain boundaries the flow is negligible in accordance with elsasser s 1971 model of plate tectonics driven quietly by the ridge push and slab pull in which the asthenosphere under the base of the plate is only passively dragged the result of cockerham and ellsworth 1979 1980 that the low velocity zone in the mantle under central california appeared to have preserved the shape of the downgoing slab for up to 10 m y supports elsasser s rather than morgan s model the shapes of low velocity bodies found by iyer and his colleagues under several geothermal areas in the western united states may be interpreted in terms of decoupling between the upper brittle lithosphere and the ductile substratum the misalignment of the axis of the low velocity column from the center of the granite outcrop in the roosevelt hot springs area utah and the inclined low velocity column under the geysers clear lake region california indicate a decoupled lithosphere in these areas whereas the upright column found under yellowstone national park suggests an intact lithosphere there

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

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

generation of rhyolitic melt in an artificial magma implications for fractional crystallization processes in natural magmas abstract a large 1 3 10 7 g artificial mafic melt with a bulk composition similar to an unusually calcic basalt 17 wt cao was produced by in situ heating of soil and subsequently cooled and crystallized the final crystalline assemblage consisted dominantly of acicular diopsidic to hedenbergitic pyroxene and anorthitic feldspar with a subordinate amount of potassic feldspar electron microprobe analyses reveal the presence of a small amount 10 vol of rhyolitic glass 71 sio 2 within the crystalline network this glass is the residual material left after crystallization of pyroxene and feldspar analogous to rhyolitic melt which may be generated from fractional crystallization of a basaltic magma ion microprobe imaging indicates that the rhyolitic glass is generally present in isolated triangular to rectangular shaped interstices left by crystallization of acicular and tabular phases and that the glass is enriched in trace elements such as zr and cs which are incompatible within the crystalline phases no evidence of coalescence or migration of the rhyolitic melt is suggested by glass morphology although these observations were made in an artificial magmatic system analogies to a natural system may be drawn the isolated nature and low abundance of the rhyolitic glass highlights the difficulty of extracting and segregating evolved melts produced by high degrees of crystallization of a primitive parent particularly one characterized by elongated rather than equant crystals the distribution of the rhyolitic glass supports the suggestion that extraction of significant amounts of evolved melts from rocks may require repeated partial melting of crystallized material in order to allow the evolved liquid to physically separate from the parent and also suggests that the geometry of crystals may be an important factor in melt segregation

azimuth energy q and temperature variations on p wave amplitudes in the united states mean relative p wave 1 s amplitude characteristics are determined for standardized seismic stations in the united states data are compiled from three azimuths to the north explosions in russia to the northwest earthquakes along the asian pacific arc and to the south southeast earthquakes in south america characteristic receiver amplitudes are found to be azimuthally dependent high amplitude areas occur in the central united states and lowest amplitudes occur along the rocky mountain front range and the rio grande rift zone amplification effects of low velocity sediments are significant in the central united states azimuthal amplitude variations in the eastern united states are consistent with an anomalous attenuating or defocusing body in the lower mantle beneath colombia south america nonazimuthal amplitude studies are tied to the azimuthal data set to increase station amplitude coverage the regional and local stability of amplitudes is analyzed amplitudes are stable with an average variation of a factor of about 1 3 around the regional mean for seismic stations situated on hard bedrock the squared amplitude variations are related to energy variations in the p wave fronts acknowledging the likely importance of elastic focusing defocusing and scattering effects at individual stations variations of anelastic attenuation are derived that are consistent with the observed energy amplitude variations assuming that the variations of anelastic attenuation occur in the upper 400 km of the earth high amplitude stations may be unattenuated low amplitude stations are consistent with q 110 and a mean amplitude implies q 220 240 the range of variation of q is mapped into temperature variations assuming a thermally activated q mechanism and an olivine activation energy for the upper mantle the apparent q variations are consistent with temperature variations of 135 287 c between high and low amplitude seismic station sites geophysical and physiographic correlations with the p wave amplitude variations are discussed accounting for the motion of the north american plate low p wave amplitudes along the rocky mountain front may be related to the uplift of the colorado plateau low amplitudes extending from northern texas to new england lie along the approximate boundary of the continental craton

investigation into the petrogenesis of apollo 14 high al basaltic melts through crystal stratigraphy of plagioclase abstract the petrogenesis of apollo 14 high al basaltic melts was studied using crystal stratigraphy which involves textural crystal size distributions csds and chemical analyses electron microprobe and laser ablation inductively coupled plasma mass spectrometry the samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321 and impact generated crystalline samples 14073 14276 and 14310 plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories plagioclase crystals were analyzed core to rim compositions where possible to test and refine petrogenetic models based upon whole rock compositions groups a b and c designations and to investigate basalt 14072 and impact melt crystallization textural studies have shown that each basalt group has distinctive plagioclase csds which are in turn distinctive from those of the impact melts evolution of the individual basaltic melts was studied by comparing the equilibrium melt compositions calculated from plagioclase compositions using relevant partition coefficients to fractional crystallization fc and assimilation and fractional crystallization afc models petrogenetic modeling of trace elements in group a basalts revealed that petrogenesis continued beyond 40 total crystallization required to model whole rock compositions and that there were open system processes that affected the magma during plagioclase crystallization petrogenetic modeling of pristine high al basalts 14072 and groups a b and c using trace elements shows that the equilibrium melt compositions do not fall on a single afc or fc trajectory this is consistent with fluctuating degrees of assimilation i e variable r values and or variable assimilant compositions during petrogenesis petrogenetic modeling reveals that the impact melts experienced only closed system fractional crystallization this work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis

constraining the recent plumbing system of vulcano aeolian arc italy by textural petrological and fractal analysis the 1739 a d pietre cotte lava flow 1 the 1739 a d pietre cotte lava flow is part of a sequence of low explosive to weak effusion events occurred at la fossa cone the active vent on vulcano island aeolian arc italy this lava is rhyolitic texturally heterogeneous and contains lati trachytic enclaves these compositions are recurrent in the la fossa volcanic products and are representative of the recent vulcano plumbing system the host lava is vesicular relatively phenocryst free and locally contains microlites and millimeter sized spherulites the enclaves are up to 10 cm in size display angular to spherical shapes and can form the core of spherulites enclaves mostly consist of plagioclase and augitic phenocrysts set in a weakly vesicular groundmass characterized by variable abundance of glass and feldspar microlites field textural and fractal data allow us to constrain the rheological features of the rhyolitic and lati trachytic magmas in situ major trace and volatile element analyses provide evidence for heterogeneities in the glassy matrix and zoning of phenocrysts processes of magma evolution have been quantitatively constrained by using the apparent distribution ratios of trace elements measured between mineral phases and glassy matrices the collected data in combination with petrological and fluid inclusion data from the literature provides evidence for 1 a genetic relationship between the two magmas through assimilation fractional crystallization process 2 a mingling mechanism between an uprising rhyolitic magma and a shallower partly crystallized lati trachytic magma plug 3 the desegregation enclaves at variable scales of the lati trachyte within the rhyolite and 4 the possible eruptive scenarios consequent to a future magmatic unrest

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

relationship between plagioclase crystallization and cooling rate in basaltic melts rock textures commonly preserve a record of the near surface crystallization history of volcanic rocks under conditions of simple cooling without convection or mixing textures will reflect sample cooling rate the temperature at which crystallization was initiated and the distribution of mineral phase precipitation across the crystallization interval compilation of plagioclase size and number density data on natural dike sill and lava lake and experimental samples suggests that 1 growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate and 2 the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization evaluation of textural data in the context of recent numerical models of crystallization in simple one and two component systems provides new insight into regularities in the crystallization behavior of basaltic magmas for example the rate of change in crystal size and number density as dictated by mass balance has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems in natural samples plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width solidification time comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes distributions broaden and the grain size at peak frequency increases with increasing crystallization times decreasing cooling rates in contrast partially crystallized makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size measured size distributions in dikes can be explained by late stage modification of makaopuhi type distributions through loss of small crystals possibly the consequence of growth without nucleation finally this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near surface basaltic systems predicted growth rates will be slow and relatively constant 10 10 10 11 cm s for crystallization times expected in most shallow volcanic systems 1000 years

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

magma ascent and eruptive processes from textural and compositional features of monte nuovo pyroclastic products campi flegrei italy geochemical and textural studies were carried out on alkaline products of the ad 1538 monte nuovo eruption due to the integration of the volcanological study with eyewitness reports the dynamics and timing of each phase of the eruption and the volume of emitted magmas are known in detail on this basis unique in campi flegrei the relations between magma chamber mechanisms eruptive styles magma ascent dynamics and volatile exsolution processes have been explored glass and phenocryst compositions indicate that the erupted magma has a homogeneous phono trachytic composition textures and compositions of phenocrysts indicate that they crystallised at equilibrium with the melt in the magma chamber likely as a mushy boundary layer along the chamber wall where the temperature was below the liquidus temperature of the crystal free chamber core the estimated crystallisation temperature is 850 40 c the magma phase relations in petrogeny s residua system suggest that phenocryst crystallisation occurred at text p text h 2 text o between 100 and 200 mpa corresponding to depths ranging from 3 to 8 km the microlite composition and their close genetic relations with vesicles indicate that groundmass crystallisation occurred during the eruption as a consequence of magma degassing and vesiculation induced by decompression during its ascent toward the surface crystal size distributions reveal that microlites grew in two stages of undercooling that we define as 1 magma migration onset upward from the chamber and 2 magma rising through the conduit to the surface possibly lasting tens of days and few days respectively these results provide information on the physical conditions that characterise pre and syn eruptive processes which may be useful in order to define eruptive scenarios and to evaluate short term precursors furthermore the collected data provide for the first time information on degassing induced crystallisation during the eruption of a highly evolved alkaline magma

crystallization processes in an artificial magma variations in crystal shape growth rate and composition with melt cooling history a large 4 8 m3 1 3x107 g artificial mafic melt with a bulk composition similar to that of a basalt but with a high cao content of 17 wt was generated during a demonstration of in situ vitrification and was allowed to cool naturally during the melting process convection was vigorous resulting in a chemically and thermally homogeneous melt body once heating was complete the cooling rate was rapid with the temperature dropping from 1500 c to 500 c in 6 days within the interior of the 3 m diameter 1 5 m thick body a 20 h period of constant temperature 1140 c observed during colling was the result of latent heat released by widespread crystallization the final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar with a subordinate amount of potassic feldspar plus a small amount of evolved glass the compositions and proportions of phases agree well with those predicted by the melts thermodynamic model thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals textural investigation of core samples reveals large up to 1 cm in length acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals 500 m in length crystal shape and pyroxene composition vary as a function of position within the solidified body as a function of cooling rate both crystal size and degree of crystallinity are highest in the central most slowly cooled parts of the rock crystal shape is characterized by tabular equilibrium growth forms in the slowly cooled areas grading to highly skeletal dendritic forms at the rapidly cooled edges of the body the pyroxene crystals are dominantly homogeneous diopside but crystals are characterized by thin fe rich hedenbergitic rims these rims were deposited when mg in the melt was depleted by diopside growth and melt temperature had cooled sufficiently to allow fe rich pyroxene growth crystal growth rates can be calculated based on thermal behavior of the melt reinforced by thermodynamic modelling and are determined to be between 10 7 and 10 8 cm s in the central part of the melt these estimates agree well with growth rates in natural systems with similar cooling rates pyroxene crystals that formed at a higher cooling rates are characterized by higher al and lower mg contents relative to tabular equilibrium crystalline forms presumably as a result of disequilibrium melt compositions at the crystal melt interface

near liquidus growth of feldspar spherulites in trachytic melts 3d morphologies and implications in crystallization mechanisms the nucleation and growth processes of spherulitic alkali feldspar have been investigated in this study through x ray microtomography and electron backscatter diffraction ebsd data here we present the first data on shape preferred orientation spo and crystal preferred orientation cpo of alkali feldspar within spherulites the analysis of synchrotron x ray microtomography and ebsd datasets allowed us to study the morphometric characteristics of spherulites in trachytic melts in quantitative fashion highlighting the three dimensional shape preferred orientation branching of lamellae and crystal twinning providing insights about the nucleation mechanism involved in the crystallization of the spherulites the nucleation starts with a heterogeneous nucleus pre existing crystal or bubble and subsequently it evolves forming bow tie morphologies reaching radially spherulitic shapes in few hours since each lamella within spherulite is also twinned these synthetic spherulites cannot be considered as single nuclei but crystal aggregates originated by heterogeneous nucleation a twin boundary may have a lower energy than general crystal crystal boundaries and many of the twinned grains show evidence of strong local bending which combined with twin plane creates local sites for heterogeneous nucleation this study shows that the growth rates of the lamellae 10 6 10 7 cm s in spherulites are either similar or slightly higher than that for single crystals by up to one order of magnitude furthermore the highest volumetric growth rates 10 11 10 12 cm3 s show that the alkali feldspar within spherulites can grow fast reaching a volumetric size of 10 m3 in 1 s

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

the oslo graben gravity high and taphrogenesis abstract the paleo oslo rift oslo graben system is geophysically characterized by a pronounced gravity high a reinterpretation of gravity data in the region using a flexible inversion scheme indicates that the causative body has a density contrast of 0 06 g cm 3 the inversion results also suggest that this body is most likely located in the upper part of the lower crust and extends eastward well outside the graben area proper estimates of crustal thicknesses before and after the rifting aborted suggest that rifting commenced in the south and then progressed northwards the corresponding pole of rotation was located approximately 240 km nne of the city of oslo the hypocentres of 7 small local earthquakes were found to be located on the periphery of the anomalous body as derived from gravity modeling seismic investigations including 3 d imaging using norsar data have failed to provide conclusive evidence on intrusion of asthenospheric material in the lithosphere as part of the oslo graben taphrogenesis the explanation proposed here is that the geophysical imprints of oslo rift oslo graben evolutionary processes within the upper mantle have been considerably weakened over the associated time span of some 150 200 ma finally it is hypothesized that the asthenospheric injection into the lithosphere took place over a relatively small cross sectional area of the moho the further ascent of the gabbroic magma was partly hindered by the relatively rigid upper crust thus causing extensive lateral flow eastwards outside the oslo graben proper in the 10 20 km depth range

baikal rift zone structure and geodynamics abstract the cenozoic baikal rift zone is superimposed on the caledonian baikal fold belt representing the suture between the precambrian siberian craton and several microcontinents the baikal rift zone consists of a system of disconnected fault bounded basins and extensional and wrench faults that straddles a major arch having a topographic relief of 2 3 km rifting activity commenced during the oligocene and is still active as evident from high seismicity of the baikal rift zone across the baikal rift zone upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8 11 km across the baikal basins as compared to the adjacent unextended areas the discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir indeed geophysical data indicate that the sayan baikal dome coincides with such a diapir the top of which is located at the crust mantle boundary this intrusion is held responsible for oligocene and quaternary doming of the rift zone development of the intra continental baikal rift system is thought to be related to asthenospheric diapirism active rifting however intra plate stresses in conjunction with the himalayan collision may have played some role

crustal thickness of northeast russia abstract the first order crustal structure of the magadan region and northeast sakha republic yakutia northeast russia is obtained by simultaneously inverting for origin times and travel time curves a total of 1210 observations are used as an average a 37 km thick 5 992 0 007 km s crust overlying an 7 961 0 015 km s mantle provides an excellent fit to phase data listed in the materialy po seismichnosti sibiri bulletin travel time curves for individual stations are very close to this average though there are variations in both crustal thickness and velocities upper mantle velocities and crustal thickness appear to increase along the southern edge of the okhotsk chukotka volcanic belt and decrease in the upper kolyma river basin and along the trace of the proposed moma rift crustal thickness is greatest at khandyga on the siberian platform and lowest at yubileniya which may lie within the currently active laptev sea rift system

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

new findings for the equilibrated enstatite chondrite grein 002 we report new petrographic and chemical data for the equilibrated el chondrite grein 002 including the occurrence of osbornite metallic copper abundant taenite and abundant diopside as inferred from low si concentrations in kamacite the presence of ferroan alabandite textural deformation chemical equilibration of mafic silicates and a subsolar noble gas component we concur with grein 002s previous classification as an el4 5 chondrite furthermore the existence of pockets consisting of relatively coarse euhedral enstatite crystals protruding large patches of fe ni alloys suggests to us that this el4 5 chondrite has been locally melted we suspect impact induced shock to have triggered the formation of the melt pockets mineralogical evidence indicates that the localized melting of metal and adjacent enstatite must have happened relatively late in the meteorites history the deformation of chondrules equilibration of mafic silicates and generation of normal zoning in fe zn sulfides took place during thermal alteration before the melting event following parent body metamorphism daubreelite was exsolved from troilite in response to a period of slow cooling at subsolidus temperatures exsolution of schreibersite from the coarse metal patches probably occurred during a similar period of slow cooling subsequent to the event that induced the formation of the melt pockets overall shock features other than localized melting correspond to stage s2 and were likely established by the final impact that excavated the grein 002 meteoroid

pollen and spores date origin of rift basins from texas to nova scotia as early late triassic palynological studies of the nonmarine newark supergroup of eastern north america and of rift basins in the northern gulf of mexico facilitate correlation with well dated marine sections of europe new information emphasizes the chronological link between the newark basins and a gulf of mexico basin and their common history in the rifting of north america from pangea shales from the subsurface south georgia basin are shown to be of late karnian age early late triassic the known time of earliest sedimentation in the culpeper basin is extended from norian late late triassic to mid karnian and the date of earliest sedimentation in the richmond and deep river basins is moved to at least earliest karnian perhaps ladinian the subsurface eagle mills formation in texas and arkansas has been dated palynologically as mid to late karnian the oldest parts of the newark supergroup and the eagle mills formation mostly began deposition in precursor rift basins that formed in ladinian to early karnian time in the southern newark basins sedimentation apparently ceased in late karnian but continued in the northern basins well into the jurassic until genesis of the atlantic ended basin sedimentation

glass forming ability and crystallisation behaviour of sub alkaline silicate melts abstract the intrinsic solidification behaviour and glass forming ability gfa of natural sub alkaline silicate melts have been experimentally quantified via cooling induced solidification approach gfa is measured by the critical cooling rate rc the rate at which a melt solidifies 2 area of crystals cooling rates of 9000 1800 180 60 7 and 1 c h have been run between 1300 c super liquidus region and 800 c quenching temperature at air f o 2 and ambient p for six silicate melts with compositions ranging from basalt b to rhyolite r i e b 100 b 80 r 20 b 60 r 40 b 40 r 60 b 20 r 80 and r 100 and water contents comprised between 53 b 100 and 384 b 20 r 80 ppm the ranges of cooling rates and chemical compositions used in this study are the broadest ever investigated in the earth sciences the phase proportions area were determined by image analysis on about 500 back scattered electron images collected over different magnifications phases are glass clinopyroxene cpx spinel sp and plagioclase plg sp is ubiquitous with abundance of few area and nucleates earlier than silicate crystals cpx solidifies in all runs except in r 100 and its abundance follows asymmetric broad gaussian like trends with tails towards low rates as a function of cooling rate moving from b 100 to b 40 r 60 these trends conserve their shape but shift progressively to lower cooling rates and mineral abundances plg crystallises only at low cooling rates and in sio 2 poor compositions run products with low amounts of crystals 5 area clearly show that cpx preferentially nucleates on surfaces of sp whereas a significant crystallisation of cpx 5 area is observed with decreasing cooling rate and with changing composition from b 100 to b 20 r 80 the crystallisation of silicate crystals is related to the chemical diffusivity of components in the melt also the initial crystallisation of plg occurs preferentially on cpx in general the amount of crystals decreases as the cooling rate increases however in some cases the amount of crystals remains constant or even decreases for b 80 r 20 with decreasing cooling rate rc values change over 5 orders of magnitude being 100 b 20 r 80 b 40 r 60 b 60 r 40 and b 80 r 20 and b 100 respectively the variation of rc can be modelled through nbo t nonbridging oxygen per tetrahedron parameter by the following equation rc a 1 e nbo t b c where a b and c are fitting parameters equal to 9214 0 297 and 0 040 respectively similarly to other glass forming liquids network metallic and molecular systems rc for natural sub alkaline silicate melts is inversely related to the reduced glass transition parameter trg trg tg tm and can be quantified with the equation rc a trg b where a and b are 1 19 10 4 and 28 7 respectively these results may be used to retrieve the solidification conditions of aphyric degassed and oxidised lavas in addition our data provide general constrains on the crystallisation kinetics of natural crystal bearing silicate melts erupted on earth e g lavas with phenocrysts the relationship between crystal content and cooling rate suggests that the solidification path induced by degassing can be also complex and nonlinear the growth of crystals with size up to 1 mm from a nearly anhydrous superheated silicate melt indicates that variable cooling conditions of lavas have to be accounted to discriminate among minerals formed before during and after eruptions moreover our results can be used to design glass ceramics from naturally available easy to find low cost starting materials

tectonic control of triassic sedimentation in southern new brunswick local and regional implications both regional and local tectonics controlled the sediment distribution in the fundy half graben during the triassic locally alluvial fans built out into the basin from the western boundary fault along what is now the south shore of new brunswick the alluvial fan red beds of the honeycomb point formation are covered by fluvial conglomerates of the quaco formation which in turn are buried by a resurgence of alluvial fan deposition represented by the echo cove formation pollen recovered from the upper part of the echo cove formation indicates that regionally the system of triassic jurassic grabens along the eastern seaboard is composed of two separate graben systems one stretching from south carolina to connecticut the other from the gulf of maine to the southern grand banks initial graben formation began at the southern end of each system followed by successive grabens opening toward the north the areal distribution of both graben systems appears to have been controlled by four large transform fault systems from the middle triassic through the jurassic the age and overall distribution of sediments within the fundy basin confirm the existence of a hot spot along the kelvin seamount chain and refines determination of the position and timing of the initial rifting that led to the formation of the present atlantic ocean

geology of the east siberian sea russian arctic from seismic images structures evolution and implications for the evolution of the arctic ocean basin 1 the kinematics and history of the opening of the amerasia basin are closely linked to the geology of the wide shelves surrounding the arctic ocean in this context multichannel seismic reflection data from the virtually unexplored shelf of the east siberian sea russian arctic are discussed in combination with potential field data three seismic marker horizons were defined and mapped their ages were linked to main tectonic and regional events and to onshore findings the data reveal that there is no continuation of the large rift basins from the laptev shelf onto the east siberian shelf and there are no indications for the previously defined several hundred kilometers wide blagoveshchensk basin the east siberian shelf is best described as an epicontinental platform that synsedimentarily subsided continuously since late cretaceous times with stronger subsidence to the northeast resulting in the formation of a large depocenter some form of extensional transtensional stresses affected the area and created relatively small ese wnw striking basins within this depocenter these basins are filled with 6 km thick late cretaceous to tertiary sediments the general dip of the platform of the east siberian shelf toward the northeast may be explained by dip slip movements along a major transform fault that is proposed by the rotation model for the opening of the amerasia basin for the evolution of small sag shaped basins within the east siberian depocenter we suggest a link to the opening of the eurasia basin instead of to the opening of the makarov basin

tectonic stress state in ne japan as part of the okhotsk plate abstract an existing geodetic flow velocity model obtained by using an internal free network adjustment technique is used to derive estimates for various strain rates parameters in ne japan the greatest shortening rates of the principal strains trending e w are located in regions where much steady internal frame invariant plastic flow deformation is observed to be taking place the internal geodetic adjustment technique yielded the internal deformation in the tohoku arc most of the intraplate deformations including the much folding deformations observed in the inner zone are produced from within an interseismic transient elastic loading at a strongly coupled locked japan trench would not be needed the observed ongoing extensive ductile folding deformation in the inner zone of tohoku may mean that the geodetic strain rates causing shortening at 2 3 cm yr probably reflect the more correct level of the deformation which is steady permanent in ne japan as compared with seismic faulting data which indicate 0 5 cm yr shortening the calculated principal strain rates are used to make an interpretation for the origin of the deviatoric principal stresses within the greater regional plate tectonic framework the tectonic stress state in ne japan as part of the okhotsk plate could mostly be influenced by the okhotsk plate which is extruding southward to lessen the significant accumulated contractional deformation in ne asia in the verkhoyansk cherskii mountains the principal strain rates are n s extensional essentially everywhere in ne japan as a result of the southerly extrusion except in its southernmost leading edge in the uetsu fossa magna province where the japan alps rampart rises in front of the extrusion here an e w compressional stress state prevails a second e w contractional zone is found in north central tohoku extending from the sanriku province in the outer zone to the inner zone in the japan sea side being more prevalent in the latter zone the calculated rotation rates from the geodetic flow model are clockwise cw in both of the e w contractional regions ne japan extruding southward faces buttresses in 1 the oga ojika line ool and or a crustal weakness zone between the northern and the southern halves of tohoku approx at 38 5 n latitude and especially 2 the japanese alps rampart these obstacles cause the northern and southernmost tohoku to veer to its right and rotate cw thereby setting up the e w trending compressional deformation in their respective inner zones between the ool or the 38 5 n boundary and the kanto tectonic line ktl the sense of the differential rotations is counterclockwise ccw towards the ocean to the se the northern tohoku north of the ool and the southernmost tohoku south of the ktl cannot rotate ccw towards the ocean because of the izu block s collision in the south and the relatively strong coupling along the subduction interface beneath the japan trench in the north off sanriku the relatively stronger long term coupling between the northern tohoku and the pacific plate at the sanriku coast with respect to that in off fukushima is due to a flatter subduction of the pacific slab there increasing the plates interface contact area the flattening of the subduction dip angle was caused by ccw rotation and shifting of the northern tohoku along the dextral honjo matsushima line roughly corresponding to the ool towards the pacific and overriding of the subduction zone during the formation of the japan sea

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions